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Electrocatalytic reduction of nitrogen containing compounds on platinum surfaces
Marta Costa Figueiredo
Electrocatalytic reduction of nitrogen containing
compounds on platinum surfaces
Doctoral Thesis
Under supervision of:
Prof. Juan M. Feliu
Dr. Víctor Climent
Marta Costa Figueiredo
Alicante, 2012
Memoria presentada para optar al grado de doctor
Universidad de Alicante
2012
Fdo.: Marta Costa Figueiredo
DIRECTORES
Fdo.: Dr. Juan Feliu Fdo.: Dr Víctor Climent
Acknowledgements
I would like to acknowledge to all that contributed for the successful ending of this
thesis:
- My supervisors, Prof. Juan Feliu and Dr. Víctor Climent. Thanks for receiving
me in your laboratory, for teaching me the beauty of electrochemistry,
particularly of platinum single crystals and all the wise advices both for science
and life than allowed me to grow up as a scientist and as a person.
- ELCAT project for the financial support, and all the “ELCAT people” for the
good meetings and training courses. From professors to students, this was a
great group where we had the opportunity of share and exchange knowledge.
- Prof. Enrique Herrero for all the suggestions, comments and discussions that
contributed to this and other works.
- Prof. Antonio Rodes, for all the help and teaching on Infra red spectroscopy.
- Dr. José Solla and Dr. Fran Vidal for nanoparticles synthesis, for showing me
how to work with them, for all the suggestions and all the help in the problems
that appear in this long way.
- Prof Richard Nichols, for receiving me in his laboratory in Liverpool, and to the
entire Richard´s group for the good environment and for teaching me the
secrets of STM.
- Prof Marc Koper, for allowing me to spend some time in his group doing
OLEMS experiments. To all the colleagues in the group that taught me and
helped me.
- Prof Daniel Scherson and his group for introducing me to non linear optical
methods and receiving me so friendly.
- To the present and past colleagues on the group as well to the technician,
administrative and maintenance staff.
- All my friends, the closer ones in distance and all the others in the other side
of the border. Each one of you has directly or indirectly contributed for this
thesis to happen.
- My family, I´m very lucky for having you all by my side.
- To my parents, for my life, for making me happy all this years, for supporting
me, for all the love that I receive from you and that I hope I can give you back
some day. You are the best parents!
- Tiago, thanks for being by my side even when it was not easy, for being my
best friend, for listening about my work hours and hours, for given me
motivation when things were hard, from being happy for me when things were
good. Thank you helping me on another hard stage of my life.
Table of Contents
ABBREVIATION LIST I
RESUMEN III
1. INTRODUCTION 3
1.1. NITROGEN CYCLE 3
1.2. ELECTROCHEMISTRY OF NITROGEN COMPOUNDS 8
1.3. SCOPE OF THIS THESIS 17
2. EXPERIMENTAL 25
2.1. STRUCTURE OF PLATINUM SINGLE CRYSTAL SURFACES 25
2.2. CRYSTAL’S PREPARATION AND CLEANING 31
2.3. GENERAL EXPERIMENTAL CONDITIONS 33
2.4. NANOPARTICLES 35
2.5. EXPERIMENTAL TECHNIQUES 38
2.6. CHEMICALS 55
3. ELECTROCHEMICAL SURFACE CHARACTERIZATION 63
3.1. BASAL PLANES IN ACID MEDIA 63
3.2. ALKALINE MEDIA 67
3.3. NEUTRAL PHOSPHATE BUFFERED MEDIA 72
3.4. IRREVERSIBLE ADSORBED BISMUTH ADATOMS 77
4. NITRATE REDUCTION AT PT(100) SINGLE CRYSTALS AND
PREFERENTIALLY ORIENTED NANOPARTICLES IN NEUTRAL MEDIA 91
4.1. CONCEPTS 91
4.2. NITRATE REDUCTION ON PT(100) IN NEUTRAL MEDIA 92
4.3. NO STRIPPING ON PT(100) IN PHOSPHATE BUFFER 96
4.4. NITRATE REDUCTION ON PT STEPPED SURFACES WITH (100)
TERRACES 98
4.5. SPECTROELECTROCHEMICAL RESULTS – FTIRS ON PT(100) 103
4.6. NANOPARTICLES 111
4.7. CONCLUSIONS 113
5. NITRITE ELECTROREDUCTION ON PT(100) AND (100) STEPPED
SURFACES IN ALKALINE MEDIA 119
5.1. CONCEPTS 119
5.2. ELECTROCHEMICAL EXPERIMENTS 120
5.3. SPECTROELECTROCHEMICAL EXPERIMENTS: FTIRS OF PT(100) AND
STEPPED SURFACES 125
5.4. OLEMS EXPERIMENTS WITH PT(100) AND STEPPED SURFACES 130
5.5. TRANSFER EXPERIMENTS WITH NO AND NHX MASS SPECTROMETRY
AND ELECTROCHEMICAL RESULTS 132
5.6. MECHANISM AND STRUCTURE SENSITIVITY OF NITRITE REDUCTION
AT PT (100) ELECTRODES 135
5.7. CONCLUSIONS 138
6. NITRATE REDUCTION ON PT(111) SURFACES MODIFIED WITH
BISMUTH ADATOMS: FROM SINGLE CRYSTALS TO NANOPARTICLES 143
6.1. CONCEPTS 143
6.2. VOLTAMMETRIC RESULTS FOR NITRATE REDUCTION ON PT(111)/Bi 144
6.3. SPECTROSCOPIC STUDY OF NITRATE REDUCTION ON PT(111)/Bi 147
6.4. ON LINE MASS SPECTROSCOPY 152
6.5. QUANTIFICATION OF THE CATALYSIS PROMOTED BY BI ADATOMS 153
6.6. CONCLUSIONS 168
7. NITRITE REDUCTION ON BISMUTH MODIFIED PT (111) SURFACES IN
DIFFERENT ELECTROLYTIC MEDIA 173
7.1. CONCEPTS 173
7.2. CYCLIC VOLTAMMETRY RESULTS OF NITRITE REDUCTION ON
PT(111)/Bi 174
7.3. FTIR RESULTS 183
7.4. CONCLUSIONS 188
8. NO ADSORPTION ON PT(111) BISMUTH MODIFIED SURFACES 193
8.1. CONCEPTS 193
8.2. CYCLIC VOLTAMMETRY FOR NO AND Bi COADSORPTION ON PT(111) 194
8.3. IN SITU IR SPECTROSCOPY RESULTS 200
8.4. IN SITU STM EXPERIMENTS 204
8.5. CONCLUSIONS 206
9. FINAL REMARKS 211
LIST OF PUBLICATIONS 215
Abbreviation list
Abbreviation Name
Fcc face centered cubic
CV Cyclic Voltammetry
RE Reference electrode
CE Counter electrode
WE Working electrode
RHE Reference Hydrogen Electrode
SCE Standard Calomel Electrode
UPD Under potential deposition
IRRAS Infrared reflection adsorption
spectroscopy
DEMS Differential Electrochemistry Mass
Spectroscopy
ATR Attenuated Total Reflection
SEIRAS Surface-enhanced infrared absorption
spectroscopy
UHV Ultra High Vacuum
TEM Transmission Electron Microscopy
HRTEM High Resolution Transmission Electron
Microscopy
OLEMS On-Line Electrochemical Mass
Spectrometry
STM Scanning tunneling microscopy
ECSTM Electrochemical scanning tunneling
microscopy (EC-STM)
LEED Low energy electron diffraction
Resumen
El estudio de la electroquímica de compuestos que contienen nitrógeno inorgánico es,
hoy en día, un tema muy importante en Electrocatálisis. Los métodos electroquímicos
pueden ofrecer soluciones alternativas para el problema del desequilibrio causado por
la influencia humana en el ciclo del N. Sin embargo, la electroquímica de compuestos
nitrogenados es complicada por el gran número de estados de oxidación estables en N
y, hasta hoy, ningún proceso artificial es tan eficiente e inofensivo como los existentes
en la naturaleza.
El objetivo principal del trabajo realizado en esta tesis fue dar una contribución, desde
el punto de vista fundamental, para el desarrollo y la comprensión del Pt como
catalizador para la electrorreducción de compuestos nitrogenados, con la esperanza
de que el conocimiento de los aspectos mecanísticos de estos procesos ayude, en el
futuro, al desarrollo de métodos electroquímicos competitivos para la eliminación de
estos compuestos en medios acuosos.
El nitrógeno es un componente vital de muchas moléculas orgánicas esenciales como
las proteínas, el material genético, la clorofila, etc. Todos los organismos vivos
requieren nitrógeno para realizar sus funciones biológicas. Este elemento está en
cuarto lugar entre los elementos químicos más comunes en los tejidos vivos, detrás del
oxígeno, carbono e hidrógeno [1].
Aunque el nitrógeno se encuentre en la atmósfera en cantidades significativas (78%),
no puede ser utilizado por las plantas y animales directamente desde el aire, como
sucede con el dióxido de carbono y el oxígeno. En su lugar, el nitrógeno debe ser fijado
mediante el ciclo del nitrógeno antes de que pueda ser utilizado por las plantas y todos
los otros organismos vivos, desde los depredadores a los descomponedores.
El ciclo de nitrógeno es un proceso natural a través del cual el nitrógeno se convierte
entre sus diversas formas químicas. Aunque esta transformación pueda llevarse a cabo
IV Resumen
tanto por procesos biológicos como no biológicos, de entre todos los ciclos
biogeoquímicos, el del nitrógeno es el que más estrecha y sistemáticamente, se
encuentra asociado con microorganismos. Los procesos más importantes en el ciclo
del nitrógeno son la fijación, mineralización, nitrificación y desnitrificación [2].
La primera forma disponible de nitrógeno es orgánica. Cuando una planta o animal
muere, o un animal expulsa residuos, las bacterias (u hongos en algunos casos)
convierten el nitrógeno orgánico en amonio (NH4+), mediante un proceso llamado
amonificación o mineralización.
La fijación de nitrógeno es el proceso por el cual este elemento es recolectado desde
el aire y enlazado al hidrógeno y oxígeno para formar compuestos inorgánicos,
principalmente amonio (NH4+) y nitrato (NO3
-), que pueden ser utilizados por los
animales y plantas.
En la nitrificación se lleva a cabo la oxidación de amonio (NH4+) a nitrito (NO2
-). La
conversión se realiza principalmente por bacterias como las Nitrosomonas, que
convierten el amoniaco a hidroxilamina (intermediario), y luego, en una segunda
reacción, la hidroxilamina se convierte en nitrito. Después de este proceso, otras
especies bacterianas, como la Nitrobacter, oxidan nitritos (NO2-) en nitratos (NO3
-) para
concluir esta fase del ciclo.
El ciclo se termina con la desnitrificación. Aquí, los nitratos son reducidos de vuelta a
nitrógeno gas (N2) por acción bacteriológica.
El incremento de acciones humanas como la utilización de combustibles fósiles, el uso
de fertilizantes nitrogenados o el cultivo de leguminosas fijadoras de nitrógeno han
ocasionado alteraciones graves en el ciclo global del N [3]. La velocidad de los cambios
globales causados por humanos ha aumentado fuertemente en los últimos siglos, pero
ninguno tan rápidamente como la producción industrial de fertilizantes de nitrógeno,
Resumen V
que ha crecido exponencialmente desde la década de 1940. El aumento de la
disponibilidad de nitrógeno aumenta significativamente la acumulación y producción
de biomasa [1], lo que hace que los cambios en el ciclo del nitrógeno también
conduzcan a cambios en el ciclo global del carbono, generando un aumento de dióxido
de carbono en la atmósfera [4].
Además, el aumento de la disponibilidad de N también, generalmente, reduce la
diversidad biológica de los ecosistemas afectados e influye en las rutas y velocidades
del ciclo [5, 6]. Los nitrato que se infiltran en los suelos, llegando a los cursos de agua
corriente y las aguas subterráneas, son responsables de problemas como el
agotamiento de los minerales en los suelos, el aumento de la acidez y alteración de
aguas de manantial y de ecosistemas marinos costeros [7, 8]. Por otra parte, los óxidos
de nitrógeno son importantes precursores de lluvias ácidas y de la niebla fotoquímica
[9] y el óxido nitroso contribuye al desarrollo del efecto invernadero [2].
Un aspecto muy particular de los efectos sobre el medio ambiente, de compuestos que
contienen N, es que su acción nociva puede producirse en serie. Este fenómeno ha
sido nombrado como cascada de nitrógeno [10], lo que significa que un átomo de
nitrógeno, en secuencia, puede: aumentar el ozono atmosférico (impacto en la salud
humana), aumentar las partículas finas (impacto en la visibilidad), alterar la
productividad de los bosques, acidificar las aguas superficiales (pérdida de la
biodiversidad), aumentar la productividad de los ecosistemas costeros, promover la
eutrofización costera y aumentar el potencial de efecto invernadero de la atmósfera (a
través de la producción de N2O) [9]. La magnitud de las consecuencias, junto con la
magnitud de la generación actual de compuestos nitrogenados, hace que la cuestión
de la acumulación de NOx sea un tema de mucha importancia en la actualidad.
Junto con todos los desequilibrios nombrados hasta ahora, existen, además, otras
fuentes para la acumulación de nitratos en sistemas de aguas subterráneas. Algunos
procesos industriales como la producción de celofán, fabricación de explosivos e
VI Resumen
industrias de acabado metálico producen aguas residuales con una concentración de
nitratos muy elevada. Otra fuente de contaminación con nitratos es la producción de
energía nuclear [11]. El exceso de nitratos en el agua potable puede causar
enfermedades graves como la metahemoglobinemia. La toxicidad de los nitratos a los
seres humanos es debido a la reducción en el cuerpo de nitrato a nitrito. Este último
tiene un papel muy importante como un precursor de la cianosis clínica (síndrome del
bebé azul) y de nitrosaminas carcinogénicas. Por estas razones, la cantidad máxima de
nitratos en el agua potable es limitada por regulaciones gubernamentales (actual
límite por Reglamento de la UE: 12 mg/L) aunque en muchas regiones del mundo la
concentración de nitratos en las aguas potables esté en gran medida por encima de
estos límites. En consecuencia, el contenido de nitratos en aguas potables debe
reducirse, necesariamente, a fin de evitar riesgos para la salud. Aunque las tecnologías
en este ámbito estén aumentando, todavía es necesario optimizar las actuales técnicas
de tratamiento y desarrollar los procesos emergentes basados en nuevas técnicas de
eliminación de nitrato [12].
Las técnicas más esperanzadoras para la eliminación de nitrato, sin producción de
aguas residuales paralelas, son la digestión biológica y la desnitrificación catalítica con
uso de catalizadores de metales nobles [12]. Los procesos de desnitrificación biológica
tienen un gran potencial para el tratamiento de aguas residuales municipales e
industriales, pero hay algunas preocupaciones sobre la posible contaminación
bacteriana del agua tratada, la presencia de residuos orgánicos y el posible aumento
de las cantidades de cloro del agua purificada, siendo estas las principales razones para
la lenta transferencia y aplicación de estas tecnologías.
La reducción de las soluciones acuosas de nitrato utilizando hidrógeno sobre un
catalizador sólido, ofrece un proceso alternativo y económicamente ventajoso cuando
se compara con los tratamientos biológicos. En estos sistemas, los nitratos se
Resumen VII
convierten selectivamente a través del hidrógeno vía intermedios de nitrógeno en
reactores de dos o tres fases que operan bajo condiciones de reacción suaves. Los
catalizadores bimetálicos como Pd–Cu, Pd–Sn, Pd–In y Pt–Cu exhiben alta actividad
para la reducción de nitrato y buena resistencia química, pero insuficiente selectividad
hacia la producción de nitrógeno. La principal desventaja de estos catalizadores es la
formación de amoniaco como producto paralelo, que no es deseable en aguas
potables. Ya que, la hidrogenación catalítica de fase líquida aun se encuentra en sus
etapas iníciales, se necesitan más estudios para desarrollar nuevos catalizadores más
eficaces para la purificación del agua potable [13].
La electroquímica desempeña un papel importante en las investigaciones actuales
sobre el desarrollo de nuevas tecnologías de desnitrificación debido a su
compatibilidad medioambiental, versatilidad, eficiencia energética, selectividad y bajos
costes asociados. Además, el uso de un electrocatalizador apropiado puede
proporcionar una selectividad optimizada y completa para lograr productos
inofensivos como N2. Por estas razones, se han hecho varios estudios electroquímicos
de nitrato y otros intermediarios del ciclo del nitrógeno (tales como óxido nítrico o
nitrito) en las últimas décadas [14-22]. Un conocimiento detallado de la electroquímica
de estos compuestos es indispensable para alcanzar el objetivo final. La complejidad
asociada a la reducción electroquímica de compuestos nitrogenados es debida,
esencialmente, a la existencia de un gran número de especies estables con estados de
oxidación diferentes de entre -3 a + 5.
Como se ha referido hasta ahora, los desequilibrios en el ciclo del nitrógeno y los
problemas ambientales a él asociados confieren especial importancia al estudio de los
compuestos participantes en el ciclo electroquímico del nitrógeno tales como el
nitrato o el nitrito.
El estudio de estas reacciones, en esta tesis, se ha hecho bajo dos perspectivas
distintas. La primera (capítulos 4 y 5) aborda la reducción de nitratos y nitritos en
VIII Resumen
superficies bien orientadas, con especial atención a la superficie Pt(100), en medio
neutro para el nitrato y alcalino para el nitrito. La segunda perspectiva adoptada fue el
uso de adátomos metálicos para modificar la composición de superficies de Pt bien
definidas, de forma que se mejore la actividad catalítica de los electrodos hacia a la
reducción de estos compuestos (capítulo 6, 7 y 8).
En los párrafos siguientes se dará un breve resumen del contenido de esta tesis.
El capítulo 2 se ha dedicado a la descripción de los detalles experimentales de los
trabajos presentados en este manuscrito. En primer lugar, se dan algunas nociones
sobre cristalografía de superficies con especial énfasis en la nomenclatura y la
notación típica de las superficies monocristalinas. La nomenclatura descrita se ha
aplicado a los capítulos posteriores de la tesis. Para finalizar el capítulo, se describen
las técnicas empleadas, así como las configuraciones experimentales utilizadas para el
desarrollo de este trabajo.
La caracterización de las superficies monocristalinas es de suma importancia en
estudios de electrocatálisis. Por esa razón, el capítulo 3 incluye la caracterización por
voltametría cíclica de las superficies en los distintos medios utilizados como electrolito
suporte. Una parte de este capítulo se ha dedicado a la descripción sucinta del estado
del arte del conocimiento de los procesos asociados a los perfiles voltamétricos
característicos obtenidos en monocristales de platino en los electrolitos de suporte
utilizados. En la última parte de este capítulo de presentan los conceptos y
caracterización de superficies modificadas con adátomos irreversiblemente
adsorbidos, particularmente para la adsorción irreversible de bismuto en Pt(111).
El capitulo 4 es dedicado a la exposición de los resultados obtenidos para la
electrorreduction de nitrato en electrodos monocristalinos Pt(100) en medio tampón
fosfato, pH 7,2. La sensibilidad de la reacción a la orientación cristalográfica de la
Resumen IX
superficie fue probada a través de la introducción controlada de defectos, mediante el
uso de superficies escalonadas con terrazas (100) de escalón monoatómico de simetría
(111) o (110).
Como se ha señalado anteriormente, los desequilibrios del ciclo del nitrógeno tienen
una especial importancia en lo que respecta a la seguridad de los recursos de agua,
como ríos, mares y aguas subterráneas [1]. Por estas razones, con el objetivo de
descontaminación de los recursos hídricos, los estudios electroquímicos de moléculas
nitrogenadas en medios neutros tienen una relevancia especial. Sin embargo, los
pocos estudios en pH neutro que existen en la actualidad en este sentido están
relacionados con la reducción de nitrato en electrodos de cobre [23]. En lo que
respecta a electrodos monocristalinos de platino, de entre los tres planos basales, el
Pt(100) ha surgido como la superficie más activa para romper el enlace N-O
condiciones de UHV [24]. También se ha demostrado la capacidad del Pt(100) para
catalizar la reducción electroquímica de compuestos de nitrógeno, con la reacción de
reducción de nitrito en medios alcalinos, donde el N2 se encontró entre los productos
de reacción [20].
Por lo tanto, en el capítulo 4 se discuten los resultados sobre la reducción
electrocatalítica de nitrato en disoluciones neutras con electrodos de Pt(100),
utilizando voltametría cíclica y espectroscopia infrarroja in situ. La aplicabilidad real de
este estudio se ha demostrado mediante el uso de catalizadores dispersos
(nanopartículas de platino).
Los resultados muestran que la reducción de nitrato se produce principalmente en
terrazas (100) bien definidas en la región de potencial donde la adsorción de
hidrógeno empieza a disminuir, permitiendo al anión nitrato acceder a la superficie. Se
ha detectado NO adsorbido como un intermedio estable en este medio. Se ha
observado un proceso de oxidación a 0.8 V (vs RHE), que se ha atribuido a la formación
de NO adsorbido. Este proceso está ligado a una reducción secundaria observada en el
X Resumen
barrido negativo posterior. Utilizando FTIRS in situ, se ha identificado amonio como el
principal producto de la reducción de nitrato. Esta especie es, posteriormente, a
potenciales altos, oxidada a NO adsorbido y nitrato (probablemente con nitrito como
intermediario). La introducción de escalones en la superficie con simetría (111) o
(110), disminuye su capacidad para la reducción de nitrato, aunque la forma de los
voltagramas no sufre cambios substanciales cuando el número de átomos en la terraza
decrece. La disminución de las densidades de corriente es, probablemente, debida a
que la concentración de intermediarios activos es menor en terrazas más cortas,
siendo substituidos por sitios donde la adsorción es más fuerte. Las corrientes de pico
son siempre más grandes para las superficies con escalones de simetría (111). Sin
embargo, el efecto de la simetría del escalón es pequeño en comparación con el
decrecimiento de la longitud de la terraza, y está, probablemente, relacionado con la
disponibilidad de los distintos tipos de sitios de escalón.
El uso de nanopartículas de Pt, preferentemente orientadas, para la reducción de
nitrato, en este medio, ha contribuido para evidenciar la sensibilidad de la reacción a
la estructura superficial del catalizador y se han encontrado comportamientos
similares a las superficies con terrazas cortas.
En el Capitulo 5, se describe la reducción de aniones nitrito en superficies Pt(100) en
medio alcalino. Este capítulo está dedicado a una comprensión más profunda del
origen mecanistico de la reactividad única de los sitios (100) hacia la reducción de
nitrito a N2. La conversión altamente selectiva de nitrito en N2 en electrodos de Pt(100)
en medio alcalino ha sido investigada con un énfasis particular en la sensibilidad de la
reacción a la estructura superficial y su mecanismo.
Como se ha dicho anteriormente, el N2 es el producto deseado para la reducción de los
compuestos nitrogenados. El N2 se ha encontrado como producto de la reducción de
nitrito en procesos totalmente distintos, como la reducción selectiva catalítica (SCR, en
Resumen XI
condiciones de alto vacio) [25] y los procesos de tratamiento bacteriológicos de aguas
residuales (“ammamox”) [17, 25, 26].
Para optimizar la actividad catalítica y dirigir la selectividad hacia a N2 son necesarias
facetas (100) de alta calidad: los defectos de cualquier simetría reducen drásticamente
la generación de N2 en las superficies [n(100)x(110)] y [n(100)x(111)]. Combinando
experimentos de espectroelectroquímica y experimentos de espectrometría de masas
con reactivos marcados isotópicamente, se ha demostrado que la generación de
nitrógeno en esta reacción tiene como especies clave el NHx y NOads. Estos
experimentos implican la generación de especies adsorbidas marcadas
isotópicamente, NO y NHx, y su posterior transferencia a una célula que contiene
reactivos con distintonisotopo de N. La monitorización de la distribución isotópica en
los productos generados proporciona claves importantes para entender el mecanismo
de esta reacción. Los resultados mostrarán que la reducción de nitrito es similar a los
otros procesos que generan N2: como la de oxidación bacteriana de amoníaco
("anammox") [25, 26] y la reacción de NO + NH3 a alta temperatura en monocristales
de Pt(100), bajo condiciones de alto vacío [27-30]. Así, la combinación de estas dos
especies nitrogenadas parecer conformar una vía universal (baja temperatura) para la
obtención de N2.
La evidencia experimental apoya un esquema mecanístico basado en una
recombinación de Langmuir-Hinshelwood de dos especies adsorbidas en la superficie
(NOads y NHx), que surgen de la anterior reducción del nitrito y que puede esperarse
que coexistan en la región de potenciales en que la evolución de N2 tiene lugar. Estos
hallazgos, destacan una vía única, totalmente selectiva de reducción de nitrito a N2 en
metales y sistemas biológicos y que pueden ayudar a orientar en el diseño de nuevos
catalizadores, con el propósito de lograr aplicaciones prácticas en el campo de
tratamiento de aguas residuales.
XII Resumen
Con el capitulo 6, se empieza la segunda parte de esta tesis, destinada al estudio de los
compuestos nitrogenados con superficies monocristalinas de Pt modificadas con
adátomos adsorbidos irreversiblemente.
En el capítulo 6, el efecto de la modificación de los electrodos monocristalinos de
Pt(111) con adátomos de Bi en la electrorreducción de aniones nitrato se ha estudiado
mediante voltametría cíclica y espectroscopía FTIR in situ.
Se sabe que los adátomos irreversiblemente adsorbidos se pueden utilizar para
cambiar la composición de la superficie de una forma controlada. El electrodo
modificado, tiene a menudo una reactividad electroquímica mejorada [31], como se
demuestra con la oxidación de HCOOH [32]. La actividad catalítica de estas superficies
bimetálicas, puede ser distinta dependiendo tanto del adátomo como de la simetría
del sustrato [33, 34]. En el caso particular de la reducción de nitrato, los electrodos de
platino han sido modificados con diferentes adátomos como germanio [35], paladio
[36] o estaño [37] para promover su reducción electrocatalítica. Sin embargo, en
cualquiera de estos casos, no se ha encontrado N2 como producto final. Se han
obtenido Hidroxilamina y NO con adcapas de germanio y, N2O y NO adsorbido con
paladio. Para Pt modificado con Sn, los productos de reducción dependen del
recubrimiento del adátomo. N2O se encuentra como el principal producto para
recubrimientos de estaño intermedios, mientras que el NO es el producto dominante
para altos recubrimientos.
En Pt(111), la reducción de nitrato se produce en potenciales inferiores a 0.35 V, pero
con Pt (111)/Bi este proceso se desplaza a potenciales significativamente más elevados
(0,6 – 0,7 V). El rango de potenciales en que se produce la catalisis coincide con el
rango de estabilidad del Bi oxidado. Los resultados espectroscopicos muestran que los
productos de la reducción de nitrato en estos electrodos modificados son N2O y NO,
aunque el NO sea observado también en la superficie Pt(111) sin modificar. El IR y los
Resumen XIII
resultados de OLEMS demostraron que el único producto de esta reducción catalítica
promovida por la presencia de Bi es N2O.
El efecto catalítico se ha cuantificado mediante el análisis de las corrientes
voltamétricas para la reducción de nitrato en función del recubrimiento de Bi en la
superficie de Pt(111). La magnitud del efecto catalítico se cuantifica mediante la
integración de la carga involucrada en la reducción en función del recubrimiento del
adátomo, revelando que la actividad aumenta con la cantidad de Bi hasta
recubrimientos cercanos a la mitad del bloqueo máximo de la superficie. La actividad
disminuye abruptamente para recubrimientos más elevados, resultando una curva con
forma de volcán. La dependencia de la actividad catalítica con el recubrimiento de Bi y
su comparación con el comportamiento observado en estudios similares de oxidación
de pequeñas moléculas orgánicas [34] sugieren la participación de un efecto de tercer
cuerpo, lo que significa que el Bi impide la formación de NO en la superficie que actúa
como un veneno para la reducción de nitrato. La presencia de Bi en las superficies
disminuye el envenenamiento con NO permitiendo al nitrato reducirse a potenciales
superiores. En bajos recubrimientos, la probabilidad de formar un ordenamiento capaz
de evitar la formación de veneno es muy baja y, por esa razón, la adsorción de NO es
todavía significativa y el efecto catalítico es bajo. Es interesante observar que el Bi
oxidado parece ser el responsable de esta reducción una vez que el proceso empieza a
potenciales altos en el barrido negativo, y la reacción es inhibida después de que el Bi
haya sido totalmente reducido. En el barrido positivo, la reducción vuelve a empezar
repentinamente cuando el Bi se oxida nuevamente. Sin embargo, el hecho de que la
actividad catalítica está estrechamente relacionada con el proceso redox del Bi, con
una pérdida repentina de la actividad tras su reducción es un claro indicio de la
existencia de un efecto catalizador adicional al mencionado efecto de tercer cuerpo.
Además, la sensibilidad de la reacción, que depende del medio usado como electrolito
suporte (debido tanto al pH como a la adsorción especifica de aniones), de la
estructura superficial del substrato (otros planos de base modificados con Bi no
XIV Resumen
presentan reactividad) y del adátomo (otros adátomos con procesos redox en el
mismo rango de potenciales del Bi tampoco son activos) demuestran el carácter
específico de este sistema y que el Bi tiene, además del efecto de tercer cuerpo,
efectos electrónicos y un carácter catalítico específico.
También se estudió el efecto de envenenamiento por adsorción de NO, formado
espontáneamente por el contacto del electrodo con soluciones de nitrato, en
superficies con diferentes recubrimientos de Bi. Estos estudios se han extendido a
superficies a vecinales al Pt(111) y a nanopartículas con orientación preferencial {111}.
El análisis del efecto del orden bi-dimensional de los sitios de Pt se ha hecho bajo estos
resultados. Los resultados obtenidos en monocristales y nanopartículas se encuentran
de acuerdo con el efecto de tercer cuerpo antes mencionado, aunque para las
nanopartículas con altas coberturas de Bi se observan desviaciones del
comportamiento lineal esperado de un efecto de tercer cuerpo. Comparando estos
resultados con los obtenidos con las superficies escalonadas con terrazas de 9 y 5
átomos de ancho, con orientación (111) es posible concluir que este comportamiento
en altas coberturas está relacionado con la disminución de los sitios de la terraza y no
a la existencia de defectos que no han demostrado tener contribución en el efecto
catalítico.
Los datos obtenidos en el capítulo 6, llevaron a que se ampliara el estudio de
superficies de Pt(111) modificadas con adátomos a la reducción de otros compuestos
nitrogenados como el nitrito. Los resultados obtenidos en medios ácido y neutro
utilizando voltametría cíclica y espectroscopía infrarroja in-situ para este estudio se
reportan en el capítulo 7.
La reducción electroquímica de nitrito ha recibido atención constante durante las
últimas décadas [17, 38, 39]. Su interés está principalmente relacionado con el
tratamiento de los residuos nucleares y la síntesis de algunos compuestos que
Resumen XV
contienen nitrógeno, ya que es uno de los compuestos más reactivos en el ciclo del
nitrógeno.
El estudio de la reducción de nitrito se ha hecho casi exclusivamente mediante
electrodos de metales puros. Una excepción es la publicación de Da Cunha y Nart [40],
que estudiaron la reducción de nitrito en electrodos de platino con 10% de rodio. Los
resultados fueron similares a los obtenidos en Pt puro por otros autores [39], es decir,
se han encontrado NO y N2O, pero no N2 como producto de la reacción.
A similitud de lo que ocurre con el nitrato, la presencia del adátomo desplaza la
reducción de nitrito a potenciales tan altos como 0.80–0.60 V vs RHE, coincidiendo con
el potencial en que el Bi sufre su reacción superficial de redox. Los resultados
obtenidos son similares a los de la reducción de nitrato en la superficie modificada. Las
curvas obtenidas para la dependencia de las cargas de reducción en función del
recubrimiento del Bi también tienen forma de volcán y la actividad máxima ha sido
obtenida para recubrimientos cercanos a la mitad del bloqueo máximo de la
superficie. Estos resultados indican que los sitios libres de Pt también son necesarios
para el proceso catalítico a alto potencial, sugiriendo que la catálisis del Bi es
producida a través de efectos electrónicos cambiando la reactividad de los átomos
vecinos de platino. Además de en medio acido, los experimentos se realizaron también
en medio neutro (pH 7) donde la estabilidad del Bi en las superficies es mayor. En
medio neutro, las curvas de dependencia de la carga de reducción con los
recubrimientos de Bi en el Pt(111) son ligeramente distintas; las cargas crecen más
rápido para cantidades de Bi menores. Este efecto es debido probablemente, a que, en
este medio, el grado de descomposición del nitrito es menor, mientras en medio acido
el nitrito descompone casi totalmente en NO. El hecho de que la cantidad de NO sea
menor, disminuye el grado de envenenamiento de la superficie permitiendo que la
superficie Pt(111)/Bi, reduzca el nitrito por un proceso catalítico directo.
XVI Resumen
Las mediciones de IR demostraron que el N2O es el principal producto detectable
asociado con la reducción a potenciales altos en los electrodos de Pt (111)/Bi, en
ambos pHs estudiados. El NO se ha observado en ambos medios de soporte y para
superficies modificadas y no modificadas.
Los resultados presentados en este capítulo, ayudan a realzar la importancia del papel
de los adátomos en la mejora de la reactividad del platino para la reducción de
especies nitrogenadas en un rango de potenciales donde el platino no modificado es
totalmente inactivo.
El capítulo 8 trata de explicar el papel del óxido nítrico (NO) en superficies de Pt(111)
modificadas con adátomos de Bi adsorbidos irreversiblemente.
El NO es un importante intermediario en reacciones industriales y ambientales tales
como reducción de nitrato y oxidación de amoníaco o producción de hidroxilamina
[17]. Junto con monóxido de carbono (CO), el óxido nítrico es uno de los
contaminantes más comunes en la actualidad [4, 41]. Se ha demostrado que la
acumulación de estos compuestos puede ser potencialmente más peligrosa que el de
CO2. Además, en los capítulos 6 y 7 se ha demostrado que el NO actua como veneno
para la reducción de nitratos y nitritos en electrodos de Pt. Por estas razones, el
estudio de la adsorción NO en superficies de Pt(111)/Bi se ha considerado como una
cuestión importante para entenderse en el marco de esta tesis.
El estudio de la coadsorcción de moléculas con adátomos adsorbidos
irreversiblemente apenas cuenta con un pequeño número de publicaciones,
referentes esencialmente al CO [42-45], aunque también se puedan encontrar algunos
estudios de NO en superficies bimetálicas Pt/Rh y Pt/Pd [46, 47]. En estos últimos, se
ha verificado que los adátomos también pueden adsorber NO y se ha propuesto la
formación de islas entre el adátomo y el NO como conclusión de la constancia de las
Resumen XVII
frecuencias de las bandas de IR en diferentes recubrimientos. Para el caso específico
de Bi irreversiblemente adsorbido, su coadsorcción con CO reveló la formación de una
adcapa mixta en Pt(111) [42-44]. Para sistemas como Pt(111)/Cu-CO [43] o capas de
Pt/S-CO [44] se encontraron evidencias de la formación de capas segregadas.
En el capítulo 8, se reportan los resultados obtenidos para la coadsorción de Bi y NO
en superficies de Pt(111). Las técnicas empleadas para caracterizar las adcapas
formadas por el NO después de la modificación de la superficie de Pt(111) con capas
parciales de Bi fueron voltametría cíclica, espectroscopia FTIR y STM in situ.
Los resultados voltamétricos revelan la interacción entre los dos compuestos
coadsorbidos. En presencia de NO, los picos redox del Bi se encuentran desplazados
negativamente 30 mV desde el valor de potencial habitual. Se obtuvieron espectros
infrarrojos in situ en presencia de NO coadsorbido y Bi. No se han encontrado
diferencias significativas en las frecuencias de vibración características de NO cuando
el Bi se encuentra presente en la superficie, lo que sugiere la formación de una adcapa
segregada en la superficie Pt(111). La presencia de la adcapa segregada y la formación
de islas de Bi fueron confirmadas mediante los resultados de microscopía túnel. Se ha
demostrado que la adsorción de NO sobre la superficie de Pt(111) modificada con Bi,
conduce a la formación de islas de Bi que no se observan cuando el NO no está
presente.
Como observación final, es importante reforzar la complejidad de la electroquímica
con compuestos de nitrógeno. La reacción depende de varios factores y los cambios de
la estructura o composición superficial son suficientes para la pérdida de respuesta
catalítica. Sin embargo, los hallazgos presentados en esta tesis, sin duda, contribuirán
a una mejor comprensión de estos procesos en superficies de Pt.
XVIII Resumen
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1Introduction
1. Introduction
1.1. Nitrogen Cycle
Nitrogen is a vital component of many essential organic molecules like proteins,
genetic material, chlorophyll, etc. All living organisms require nitrogen to perform their
biological functions. This element is in fourth place among the most common chemical
elements in living tissues behind oxygen, carbon and hydrogen [1].
Although nitrogen is present in significant amounts in the atmosphere (78%), it cannot
be used directly from the air by plants and animals as it happens with carbon dioxide
and oxygen. Instead, nitrogen needs to be fixed through the nitrogen cycle before it
can be used by plants and all the other living organisms, from predators to the
decomposers.
The nitrogen cycle is a natural process through which nitrogen is converted between
its various chemical forms. Although this transformation can be carried out by both
biological and non-biological processes, over all the biogeochemical cycles, nitrogen is
the one most closely and systematically associated with microbes. The most important
processes in the nitrogen cycle are the fixation, mineralization, nitrification, and
denitrification [2].
The first available form of nitrogen is organic. So, when a plant or animal dies, or an
animal expels waste, bacteria (or fungi in some cases) convert the organic nitrogen
back into ammonium (NH4+), through a process called ammonification or
mineralization (Fig.1-1).
The nitrogen fixation (Fig.1-1) is the process whereby N is collected from the air and
bonded to hydrogen or oxygen to form inorganic compounds, mainly ammonium
4 Chapter 1
(NH4+) and nitrate (NO3
-) that can be used by animals and plants. The formation of
ammonium is normally done by bacteria associated with leguminous crops that reduce
N2 by the following equation:
N� � 8H� � 6e � 2NH � (1)
On the other hand, nitrate formation takes places through N2 oxidation with O2. This
process can also by abiotic occurring by lightning.
Figure 1-1– Nitrogen cycle and formal nitrogen states for NOx species taken from [3].
In the first step of nitrification, the oxidation of ammonium (NH4+) to nitrites (NO2
-)
takes place. The conversion is performed mainly by bacteria such as the Nitrosomonas
and can be represented by the equations:
NH � � O� � 2e � NH�OH � H�O (2)
NH�OH � H�O � NO� � 5H� � 4e (3)
Introduction 5
Where first, bacteria convert ammonia to the intermediate, hydroxylamine, and then,
in a second reaction, hydroxylamine is converted to nitrite.
After this process, other bacterial species, like the Nitrobacter, oxidize nitrites (NO2-)
into nitrates (NO3-) concluding this step.
To end up the cycle, denitrification occurs. In this process nitrates are reduced back
into nitrogen gas (N2). By the action of bacterial species, such as Pseudomonas and
Clostridium, under anaerobic conditions, using nitrate as electron acceptor during
respiration, instead of oxygen.
Another biological process for denitrification is that called anammox, by which
ammonium is anaerobically oxidized by nitrite to N2 gas. Anammox bacteria oxidize
ammonium by using nitrite as the electron acceptor to produce gaseous nitrogen,
according with the equation:
NH � � NO�
� N� � 2H�O (4)
The amount of gaseous nitrogen being fixed by natural processes is only a small
addition to the already fixed nitrogen that cycle among the Earth ecosystems.
The increasing of actions like combustion of fossil fuels, production of nitrogen
fertilizers, cultivation of nitrogen-fixing legumes are inducing severe alterations in the
global N cycle (Fig. 1-2) [4]. The speed of many human-caused global changes has
increased severely in last centuries, but none so rapidly as industrial production of
nitrogen fertilizer, which has grown exponentially since the 1940s (Fig. 1-2). The
increasing of the availability of N also increases biomass production and accumulation
significantly [5]. Consequently, changes in nitrogen cycle can also lead to changes in
the global carbon cycle, generating an increase of carbon dioxide in the atmosphere
[6].
6 Chapter 1
Figure 1-2 – Comparative timing of a number of global changes [1]
In addition, increasing N availability also generally reduces the biological diversity of
affected ecosystems, and changes the rates and pathways of N cycling and loss [7, 8].
Nitrate leaches through soils to stream water and groundwater are responsible of
problems like soils depletion from minerals, increasing acidity and alteration of
downstream freshwater or coastal marine ecosystems [9, 10]. Moreover, nitrogen
oxides are important precursors of both acid rain and photochemical smog [11] and
nitrous oxide contributes to development of the greenhouse effect [2].
A very particular aspect of the impact of N-containing compounds on the environment
is that their effect can occur in series. This phenomenon has been named as nitrogen
cascade [12], meaning that one atom of nitrogen can, in sequence, increase
atmospheric O3 (human health impact), increase fine particulate matter (visibility
impact), alter forest productivity, acidify surface waters (biodiversity loss), increase
coastal ecosystem productivity, promote coastal eutrophication, and increase
greenhouse potential of the atmosphere (via N2O production) [11]. The magnitude of
the consequences, coupled with the magnitude of current rates of nitrogen
compounds generation, makes the issue of NOx accumulation an important one to
address.
Introduction 7
Along with the nitrogen cycle imbalance itself there are other sources for the nitrates
accumulation in ground water systems. Some industrial processes like cellophane
production, explosive manufacture and metal finishing industries produce waste
waters with elevated nitrate concentration. Nitrate wastes are also produced in
nuclear energy production [13]. Excess of nitrate in drinking water can cause serious
diseases such as methemoglobinemia. It can also lead to nitrosamine formation in the
stomach which is a carcinogenic substance. For these reasons, governmental
regulations limit the maximum amount of nitrate in drinking water (current limit by EU
regulation: 12 mg/L) [14].
In many regions of the world [15], nitrate concentration in drinking waters is largely
above this limit. In consequence nitrate content should be necessarily reduced in order
to avoid health risks. The toxicity of nitrates to humans is due to the body’s reduction
of nitrate to nitrite. The role of the latter as a precursor of clinical cyanosis (blue baby
syndrome) and carcinogenic nitrosamines as well as to other N-nitroso compounds is
firmly established [15].
Removal of nitrates from drinking water is an important and developing area of
research. Although technology in this area is rising, there is still a need to further
optimize the current treatment techniques and to develop the emerging processes for
nitrate remediation [16].
The most hopeful techniques for nitrate removal, without production of parallel
wastewater, are biological digestion and catalytic denitrification by using noble metal
catalysts [16]. Biological denitrification processes are known to have a great potential
for the treatment of municipal and industrial wastewater streams but there are some
concerns over possible bacterial contamination of treated water, the presence of
residual organics, and the possible increase in chlorine demand of purified water.
8 Chapter 1
These are the main reasons for the slow transfer of this technology to drinking water
purification.
The reduction of aqueous nitrate solutions by using hydrogen over a solid catalyst,
offers an alternative and economically advantageous process to biological treatments.
In these systems, nitrates are selectively converted through hydrogen via
intermediates to nitrogen in two- or three-phase reactors operating under mild
reaction conditions. Supported Pd–Cu, Pd–Sn, Pd–In and Pt–Cu bimetallic catalysts
exhibit high activity for nitrate reduction and chemical resistance, but inadequate
selectivity toward nitrogen production. The main disadvantage of these catalysts is the
formation of ammonia as parallel product, which is undesirable in drinking water [17].
Because catalytic liquid-phase hydrogenation of aqueous nitrate solutions is in its
initial stages, more studies are required in order to develop an effective catalytic
process for purifying drinking water streams.
1.2. Electrochemistry of nitrogen compounds
Electrochemistry plays an important role on the actual investigations concerning the
development of new denitrification technologies due to its environmental
compatibility, versatility, energy efficiency, selectivity and low associated costs.
Moreover, the use of an appropriated electrocatalyst can provide an optimized and full
selectivity to achieve harmless products like N2.
For these reasons, several electrochemical studies of nitrate and other nitrogen cycle
intermediates (such as nitric oxide or nitrite) have been done in the last decades [18-
26]. A detailed knowledge of the electrochemistry of these compounds is
indispensable in order to achieve the final goal.
Introduction 9
The general complexity of the electrochemical reduction of nitrogen containing
compounds is due to the existence of a continuous set of stable species with different
oxidation states (fig. 1-3) from -3 to +5.
Figure 1-3 - Oxidation state diagram of inorganic nitrogen compounds in acidic solution
from ref [21].
The existence of such wide range of possible stable intermediate compounds becomes
even more complicated by the fact that some of them can eventually lead to surface
poisoning or even to the inhibition of the catalyst activity [21, 27]. This complication
increases the difficulty to reach an overall understanding of the different mechanisms
participating in N electrochemistry. In addition, the high overpotentials usually
required to reach significant activities are also a challenge.
The state of the art for the electrochemistry of the nitrogenated compounds studied in
this thesis would be given in following pages separately.
10 Chapter 1
1.2.1. Nitrate
Nitrate reduction can lead to the formation of a high number of stable intermediates
[21]. The product distribution and the reaction mechanism depend on several factors
like nitrate concentration, pH, electrode material, and presence of anions in the
electrolyte solution.
The list of products to which nitrate can be reduced and the corresponding standard
potentials are given below.
NO� � 2H� � 2e � NO�
� H�O E� � 0.835 V �1�
NO� � 4H� � 3e � NO��� � 2H�O E� � 0.958 V �2�
2NO� � 10H� � 8e � N�O��� � 5H�O E� � 1.116 V �3�
2NO� � 12H� � 10e � N���� � 6H�O E� � 1.246 V �4�
NO� � 8H� � 6e � NH�OH� � 2H�O E� � 0.727 V �5�
NO� � 10H� � 8e � NH
� � 3H�O E� � 0.875 V �6�
The electrochemical reduction of nitrate has been studied on several metal electrodes
[28, 29]. Among transition metals, Pt, Pd, Rh, Ru and Ir have been tested. For this
reaction, rhodium was found as the most active catalyst with the activity decreasing in
the order Rh> Ru> Ir> Pt >Pd [28]. Some coinage metals like Cu, Ag and Au were also
studied, in acidic media [28]. Copper showed to be the most active metal for this
reaction having ammonia as main product. In addition to transition and coinage
metals, there are a few studies, in acidic media, for other metals like mercury [30],
indium [31]; cadmium [32] and tin [33]. For these metals a linear correlation between
the overpotential for hydrogen evolution and nitrate reduction was found. Although Pt
Introduction 11
is not the most active metal for nitrate reduction it has been the most studied catalyst
for this reaction [22, 34-38].
On polycrystalline platinum surfaces [34, 35] the electrocatalytic reduction of nitrate is
strongly hindered by the presence of specifically adsorbing anions, such as sulfate, as
well as other anions. FTIR measurements [39] and transfer experiments followed by
stripping in a nitrate-free solution [28] demonstrated that the main surface-bonded
intermediate formed from nitrate is nitric oxide (NO). Rotating ring-disk and online
electrochemical mass spectrometry measurements showed that during nitrate
reduction on platinum, hydroxylamine and gaseous products are not formed [28, 40,
41] leaving ammonia as the only possible product. A more recent ATR-SEIRAS study of
nitrate reduction at polycrystalline platinum [38] argued that the main species
observed spectroscopically at 1547-1568 cm-1
is adsorbed nitrate, more specifically a
chelating bidentate nitrate chemisorbed on the Pt surface.
The number of publications concerning nitrate electrocatalytic reduction on platinum
surfaces is more limited [23, 28, 37, 39, 42-44]. The reaction on the three basal planes
of platinum [37] as well as on some stepped surfaces [22] was studied in acidic media,
showing to be sensitive to the surface structure of the electrode and inhibited in the
potential region where H is adsorbed. It was also suggested that, in acidic media, NO is
the main stable intermediate and ammonia is the final product of this reaction.
The most active platinum surface for nitrate reduction is Pt(110), but only in perchloric
acid. In sulfuric acid, the Pt(110) surface is strongly blocked, and the nitrate reduction
activity is lower than on Pt(111) revealing the sensitivity of this reaction to the
presence of adsorbed anions. The studies made on stepped surfaces [22] showed that
the electrocatalytic activity for nitrate reduction depends on the step density in a
nonlinear way, the activity strongly increasing when the terrace length is shorter than
5 atoms.
12 Chapter 1
In a recent review, Rosca et al [21] summarized important information about the
electrocatalytic reduction of nitrate on Pt surfaces concluding that the N2O and N2
formation is prohibited by the strong adsorption of NO on Pt and that ammonia is the
only significant product of nitrate reduction.
The use of surfaces modified with foreign adatoms constitutes an adopted strategy for
enhancing the catalytic properties of chosen surfaces toward nitrate reduction with
the aim of achieving higher selectivity for the formation of hydroxylamine or
dinitrogen. de Vooys et al [45] performed electrochemical studies of palladium-copper
electrodes for the reduction of nitrate that showed that the presence of copper on the
Pd surface significantly enhances nitrate reduction. The highest selectivity toward N2
(ca. 40%) was obtained for low Cu coverage while higher Cu coverage lead to higher
reduction currents but showed lower selectivity toward N2.
Platinum electrodes modified with Cu underpotential deposition were also accessed
[45] but revealed different behavior than Pd-Cu surfaces, because Pt is less active in
the reduction of N2O to N2 and hence less selective in the reduction of NO to N2.
Therefore, it is understandable that PtCu catalysts produce more ammonia and less N2.
Another promoter studied for the reduction of nitrate on platinum or palladium that
also leads to the enhancement of the selectivity toward N2 formation was tin [46-48].
The maximum activity was obtained for surfaces with Sn coverage of ca. 0.35-0.4
monolayer, that showing a product distribution (determined from prolonged
electrolysis) of 62% ammonia, 30% N2, and 8 %N2O. The authors suggest that the role
of tin in catalyzing this step is to provide a site to which one of the oxygen atoms of
the nitrate ion can coordinate, on the basis of the higher oxophylicity of Sn.
Germanium [41] and palladium [49] adlayers on Pt were also used to promote the
electrocatalytic reduction of nitrate. For these adatoms, hydroxylamine and NO, in the
first case, and N2O and adsorbed NO, in the second case, were obtained as products.
Introduction 13
1.2.2. Nitrite
The electrochemical reduction of nitrite has also received continued attention over the
past decades [21, 34, 50, 51]. Nitrite can be present in three different electroactive
forms in solution, NO+ in highly acidic media, HNO2 in moderately acidic media and
NO2- in alkaline and neutral media [21]. Another important point is that, in solution,
HNO2 decomposes, and can disproportionate into NO and NO2 or into NO and HNO3.
Thermodynamically, the preferred product of nitrite reduction is N2:
2NO� � 8H� � 6e � N���� � 4H�O E� � 1.520 V �7�
However, other products such as NO(g), N2O(g), hydroxylamine, and ammonia are also
formed:
NO� � 2H� � e � NO��� � H�O E� � 1.202 V �8�
2NO� � 6H� � 4e � N�O��� � 3H�O E� � 1.396 V �9�
2NO� � 12H� � 4e � 2NH�OH � 2H�O E� � 0.673 V �10�
NO� � 8H� � 6e � NH
� � 2H�O E� � 0.897 V �11�
The nitrite reduction reaction was reported by Gadde and Bruckenstein on Pt rotating
disc in HClO4 solution [50]. Essentially two HNO2 reduction features were observed: a
prewave at lower overpotentials (ca. 0.6-0.3 V vs RHE) and a main wave at higher
overpotential (<0.3 V vs RHE) corresponding to the potential region in which the NO
adlayer is being reduced. Using online differential electrochemical mass spectrometry
(DEMS), N2O formation was detected from ca. 0.6 V, with a maximum in the prewave
region. Some NO was detected, but it was ascribed to the decomposition of HNO2. No
N2 was detected in the online mass spectrometry. Nishimura et al. [34] studied the
same reaction on a porous platinum electrode in sulphuric acid with DEMS. In contrast
14 Chapter 1
with previous reports, they observed the formation of NO and N2 in the DEMS during
voltammetry experiments, simultaneously with the formation of N2O.
Bae et al. [52] also reported N2O formation during nitrite reduction on polycrystalline
Pt using FTIR spectroscopy in acidic media and at low potentials. The authors also
reported that at a very negative potential (just at the beginning of the hydrogen
evolution) the nitrite reduction can proceed via multielectron transfer process to yield
hydroxylamine as product.
Recently, Duca et al [27] showed that the electrochemical reduction of nitrous species
on polycrystalline Pt can follow different pathways as a function of the electrode
potential and the solution pH. In acidic media, the process is a combination of the
nitrite/nitrous acid electroreactivity with a homogeneous phase reaction producing
additional reactive species (NO) from the decomposition of nitrous acid. N2O is the
major product at higher potentials (0.3–0.4 V), while NH2OH dominates at lower
potentials (0.05–0.10 V). In alkaline media, the absence of aqueous-phase reactions
causes a decrease on the activity of platinum towards the nitrite reduction. However,
it can still be reduced and the main products are non-volatile species. Rima et al [53]
reported the reduction of adsorbed nitrite at a platinum electrode, studied by surface-
enhanced infrared absorption spectroscopy (SEIRAS). On polycrystalline platinum the
potential dependence of the spectra revealed that adsorbed nitrite is converted to NO
adsorbed at on-top, bridge, and defect sites via IR-inactive surface nitrite species.
These three adsorbed NO species were also formed during the adsorption process of
NO from the solution, as that formed by the disproportionation of nitrite. Rodes et al
[20] have studied HNO2 reduction on platinum single-crystal electrodes. No strong
structure sensitivity was observed, and the reaction was strongly controlled by the
reduction of adsorbed NO.
Introduction 15
In alkaline media, NO, N2O, N2 and NH3 were the products detected by DEMS on
polycrystalline platinum [ 54]. The results are very similar to acidic media. Ye et al. [55]
investigated the reduction of nitrite at low-index Pt single-crystal electrodes in alkaline
solutions (pH 13) and, contrarily to the situation in acidic media, marked structure
sensitivity for the reduction of nitrite was reported.
1.2.3. Nitric Oxide
The adsorption and reactivity of NO on transition metal surfaces in an electrochemical
environment are also of considerable technological and scientific interest. NO is also
key intermediate in environmental and industrial important reactions such as nitrate
reduction and ammonia oxidation or hydroxylamine production [21]. Vibrational
spectroscopic studies of NO adsorbed on well-defined surfaces [56-58] led to the
conclusion that, similarly to adsorbed CO, the NO molecule could be used as probe to
test surface morphology.
On nitrate and nitrite reduction reactions, NO is one of the most important
intermediates and it is generally assumed that it acts as poison in a similar way to CO
for the oxidation of small organic molecules [59].
As with the other nitrogen compounds, the thermodynamically preferred reaction of
NO is its conversion to N2:
2NO��� � 4H� � 4e � N���� � 2H�O E� � 1.678 V �12�
However, in practice, other products such as N2O, hydroxylamine, and ammonia are
also formed:
2NO��� � 2H� � 2e � N�O � 2H�O E� � 1.590 V �13�
NO��� � 4H� � 3e � NH�OH� E� � 0.490 V �14�
16 Chapter 1
NO��� � 46 � 5e � NH � � H�O E� � 0.836 V �15�
The reduction of nitric oxide on platinum has been studied in two different ways:
either its reductive stripping of surface bonded NO (in the absence of NO in solution)
or a continuous NO reduction (in the presence of NO in solution). NO reduction on
platinum single crystals is well reported in the literature [60-63]. The reaction has
revealed as structure sensitive on platinum surfaces and very dependent on the
presence or not of NO in solution as well as the NO coverage on the surface [63].
Rodes et al [60, 64] showed that NO adlayer can be generated upon immersion of the
electrode in an acidic solution of nitrite. NO remains adsorbed on the platinum surface
at potentials between 0.40 and 0.95 V, forming adlayers whose spectral properties are
similar to those previously observed under ultrahigh vacuum conditions for NO dosed
in the gas phase at high coverages.
Using a combination of voltammetric and FTIR studies, Rosca et al [63] studied the
relationship between the NO adsorption modes (atop, bridge, 3-fold hollow) and the
reactivity of the NO species. The voltammetric features observed for NO(ads)
reduction on Pt(100), Pt(111), and Pt(110) were found to be determined by the
reduction of NO molecules occupying different adsorption sites and not by consecutive
reaction steps.
It has been shown that NO adsorbs on Pt(111) forming a stable adlayer on the surface
in the potential range between 0.9 and 0.4 V vs RHE [61], linearly bonded (atop) and
face-centered cubic 3-fold-hollow species coexist and can be reduced consecutively
and independently [63]. Ammonia has been identified as the only product of NOads
reduction since no formation of gases or hydroxylamine were detected along the
reduction.
Introduction 17
The effect of steps on a series of stepped platinum single-crystal electrodes,
Pt(S)[n(111)×(110)], for the NO reductive stripping was reported by Beltramo et al [62].
The authors could not demonstrate a clear effect of the step density on the catalytic
activity. Therefore, they concluded that NO reductive stripping is not a markedly
structure sensitive reaction.
The continuous reduction of nitric oxide was also studied on polycrystalline platinum
electrodes [19, 42]. In this case, two different processes were observed. The first one,
at high potentials (-0.2 V vs SCE), is attributed to N2O formation and the second one, at
lower potential (-0.5 V vs SCE), essentially coincides with the potential range in which
the NO adsorbate reduction takes place. In this potential range hydroxylamine,
ammonium, and nitrous oxide are the main products.
The presence or not of NO in the solution represents big differences on the reaction
mechanism on Pt. When NO is not in solution N2O is not formed and, in the low-
potential reduction wave, the reduction of the NO adsorbate gives only ammonia.
When NO is in solution both ammonia and hydroxylamine are observed as reduction
products.
1.3. Scope of this Thesis
As shown in the previous pages, the imbalance on the nitrogen cycle and associated
environmental problems give special importance to the electrochemical study of the
participating compounds (like nitrate or nitrite) in the cycle. Although the studies
carried out in the development of this thesis use always a fundamental approach, the
main aim is to contribute to the detailed knowledge and better understanding of the
nitrate and nitrite reduction reactions. The study of electrocatalytic reduction
reactions of nitrate and nitrite on platinum electrodes is addressed under two
18 Chapter 1
different perspectives. In the first one (reported in Chapters 4 and 5), both nitrate and
nitrite reduction on well oriented surfaces, with special attention to the Pt(100), was
done in neutral media for nitrate, and alkaline media for nitrite. The second adopted
perspective was the use of adatoms to modify the composition of well-defined Pt
surfaces to enhance the reduction of these compounds (Chapter 6, 7 and 8). In the
following paragraphs, a brief outline of the thesis will be given.
The Chapter 2 was dedicated to give a description of the experimental details of the
work presented in this manuscript. First of all, some notions about surface
crystallography with special emphasis on the nomenclature and the typical notation
for single crystals surfaces are given. This description will be used in the following
chapters. Next, the details on the techniques employed as well as the experimental
setup used for the development of the experimental work are given.
Chapter 3 includes the voltammetric characterization of the surfaces used for this
study. A succinct description of the state of the art is devoted to the understanding of
the processes associated to the characteristic voltammetric profiles given by the Pt
single crystals in the different supporting electrolytes used. The last part of this
Chapter is dedicated to the concepts and characterization of surfaces modified with
irreversible adsorbed adatoms, particularly to bismuth irreversible adsorption on
Pt(111).
On Chapter 4, the electroreduction of nitrate on Pt(100) electrodes in phosphate
buffer neutral solution, pH 7.2, is reported. The sensitivity of the reaction to the
crystallographic order of the surface was tested through the controlled introduction of
defects, by using stepped surfaces with (100) terraces of different length separated by
monoatomic steps, either with (111) or (110) symmetry. The products of the reduction
reaction were identified by spectroelectrochemical techniques and the application of
nanoparticles was also studied.
Introduction 19
The results obtained for the reduction of nitrite anions in alkaline media at Pt(100)
surfaces is shown in Chapter 5. Using in situ infra-red and mass spectroscopy
techniques, a mechanism was suggested for the dinitrogen formation from nitrite
reduction on Pt(100) in this media. The effect of the surface defects of any symmetry
on this reaction is also reported.
In Chapter 6, the effect of Bi modified Pt(111) electrodes on the electroreduction of
nitrate anions by using voltammetric and FTIR experiments is shown. The
quantification of the catalytic enhancement observed in the presence of different
coverages of Bi for Pt(111) and preferentially {111}Pt oriented nanoparticles was made
using cyclic voltammetry. The behavior of the nanoparticles will be compared with that
of platinum stepped surfaces of 9 and 5 atoms-width terraces with (111) orientation
and the effect of the poisoning role of NO for this reaction is also presented in this
Chapter.
Similarly to the previous Chapter, Chapter 7 shows the electrocatalytic enhancement
of the Pt(111) surface modified with Bi adatoms towards nitrite reduction and the
quantification of the catalytic effect of different adatom coverages. The results were
obtained in acidic and neutral media using cyclic voltammetry and in-situ infrared
spectroscopy measurements.
Chapter 8 tries to explain the role of nitric oxide (NO) on Pt(111) surface modified with
bismuth irreversible adsorbed adatoms with the voltammetric results obtained in the
co-adsorption of the two compounds (NO and bismuth). In situ infrared spectroscopy
and scanning tunnelling microscopy were used to access the structure of the formed
adlayer.
Finally, in Chapter 9 the major conclusions of the results obtained in the development
of this thesis are drawn.
20 Chapter 1
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22 Chapter 1
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2Experimental
2. Experimental
2.1. Structure of platinum single crystal surfaces
The essential characteristic of a single crystal is the periodic nature of its atomic
structure that can be related to a network of points in space called lattice. The
coordinates of a given point in a lattice (or atom in a structure) are given in terms of
three lattice vectors that define the edges of a parallelepipedic arrangement called
unit cell, which is the fundamental building block of the crystal. The unit cell has a
definite atomic arrangement with lattice points at each corner and, in some cases,
lattice points at the center of the face or at the center of the volume.
Metals like Au, Ag, Cu or Pt crystallize in the cubic system (face centered cubic metals,
fcc). The typical unit cell for the face centered cubic metals is shown in figure 2-1.
Figure 2-1 – Typical unit cell for the face centered cubic metals [1].
The angular relationships among crystal faces cannot be accurately displayed by
perspective drawings. Therefore, it is more convenient to project them on a two
dimensional plane using the stereographic projection. Due to the high symmetry of the
cubic system, it is often sufficient to represent crystal surface planes of interest on a
minimum triangle (figure 2-2), while all other possible surfaces are related with
another one inside such triangle by a symmetry operation.
26 Chapter 2
Figure 2-2 – Stereographical Triangle.
The most common notation to designate the geometrical orientation of different
crystals faces are the Miller indices. Miller indices can be calculated from the
intersection of the plane that defines the crystal surface with the coordinated axis of
the crystallographic system. There are two kinds of Miller indices for a given direction
in a crystal lattice, and the Miller indices for a plane in the lattice. By convention Miller
indices enclosed in parentheses (for example (111)) are used for the Miller indices of a
plane, while when they are enclosed in square brackets (for example [111]) are used
for designated crystallographic directions. The use of curly brackets is made to
designate an equivalent group of planes. For example, in a cubic lattice, {111} would
include the (111), (1�11), etc [2]. In this thesis, curly brackets are used to designate the
group of particles that enclose a nanoparticle, because, on them, several different
planes belonging to a common family are present while in single crystals there is one
only plane present and parentheses are used.
For a cubic crystallographic system, the Miller indices are the coordinates of a vector
perpendicular to the plane, referred to the coordinated system of the lattice unit. In
addition, for an fcc crystalline structure (like Pt) the simplest surface structures are
those given by the lower Miller indexes: (100), (111) and (110). These surfaces are
called basal planes and they are characterized by a single type of symmetry (Figure 2-
3). They are located at the three corners of the stereographic triangle.
Figure 2-3 – Unit cell representation and hard spheres model for the basal planes in an fcc
metal taken from
The number of surface atoms per unit area,
determined geometrically for the different planes of Pt single crystals from the
geometric relationships of the unit
atom density is given by the reciprocal of the unit cell area, a
has to decide if the second atom below the top most layer is considered as surface
atom or not.
With the surface atomic density, another important parameter can be calculated
charge that would be involved in the transference of one electron per superficial Pt
atom. This charge would correspond to the theoretical charge of adsorbing one
monolayer of hydrogen involving 1 electron per Pt atom. These charges are 241, 209
and 149 µC/cm2 for Pt(111), Pt(100) and Pt(110)
An atomic flat surface is the surface of an ideal crystal when cut by a plane. The flat
surfaces nearly parallel to a particular basal plane are called its vicinal surfaces and
they have higher Miller indices. Six particular groups of
stepped surfaces which have ideal structures composed of flat terraces separated by
monoatomic steps. They are located at the edges of the stereographic triangle.
Experimental
Unit cell representation and hard spheres model for the basal planes in an fcc
taken from [1] with modifications.
The number of surface atoms per unit area, the surface atomic density, can be
geometrically for the different planes of Pt single crystals from the
cell. For the basal planes for example, the surface
eciprocal of the unit cell area, although for Pt(110) one
cide if the second atom below the top most layer is considered as surface
another important parameter can be calculated:
charge that would be involved in the transference of one electron per superficial Pt
correspond to the theoretical charge of adsorbing one
monolayer of hydrogen involving 1 electron per Pt atom. These charges are 241, 209
for Pt(111), Pt(100) and Pt(110), respectively [3].
An atomic flat surface is the surface of an ideal crystal when cut by a plane. The flat
parallel to a particular basal plane are called its vicinal surfaces and
. Six particular groups of these faces are those called
which have ideal structures composed of flat terraces separated by
teps. They are located at the edges of the stereographic triangle.
27
Unit cell representation and hard spheres model for the basal planes in an fcc
density, can be
geometrically for the different planes of Pt single crystals from the
for example, the surface
lthough for Pt(110) one
cide if the second atom below the top most layer is considered as surface
the
charge that would be involved in the transference of one electron per superficial Pt
correspond to the theoretical charge of adsorbing one
monolayer of hydrogen involving 1 electron per Pt atom. These charges are 241, 209
An atomic flat surface is the surface of an ideal crystal when cut by a plane. The flat
parallel to a particular basal plane are called its vicinal surfaces and
these faces are those called
which have ideal structures composed of flat terraces separated by
28 Chapter 2
To more easily relate the Miller indices to the structure of the crystal face, a notation
was devised that specifies the crystallographic orientation of the steps and terraces on
stepped high Miller index surfaces. The naming system was described many years ago
[4] but a recent revision is due to by Somorjai and co-workers [5] and includes
specification of the number of atomic rows in a terrace (n), the Miller index for the
terrace (hkl), and the Miller index for the step (h’k’l’). In describing a stepped Pt
surface, the general notation is Pt(S)-[n(hkl) × (h’k’l’)].
Table 2-1 summarizes the relation between Miller indices and Steps terrace
nomenclature. Other significant properties of the surface that can be calculated by this
model are the step density, or number of steps per unit length, and the area of the
unit cell.
Table 2-1: Relationships between the step-terrace notation and the Miller indexes for
stepped surfaces in the cubic system.
Terrace step notation Miller Indexes
n (111) x (111) (n, n, n-2)
n (111) x (100) (n+1, n-1, n-1)
n (100) x (111) (2n-1, 1, 1)
n (100) x (110) (n, 1, 0)
n (110) x (111) (2n-1, 2n-1, 1)
n (110) x (100) (n, n-1, 0)
Along the zone from the (111) to the (100) pole, a series of high index planes
consisting of (111) oriented terraces, separated by single atom high (100) steps, is
encountered up to the (311) pole. The (311) surface has one terrace and one step
atoms per unit cell and therefore has two equivalent step-terrace notations [2(111) x
(100)] or [2(100) x (111)]. Moving from (311) toward the (100) pole, the step and
terrace orientations reverse. The (311) pole is called the “turning point” of the zone.
Experimental 29
For example, the Miller index designation for the surface in the figure is (433) (Figure
2-4 B, and the step-terrace notation is [7(111) × (100)]. In the case of the (775) surface
two different terrace x step notation can be used, [7(111) × (111)] or [6(111) × (110)]
depending on the symmetry of the step considered (Figure 2-4 A).
Figure 2-4 – Hard sphere model for the (775) and (443) surfaces (n=7), the unit cell of the step
has been marked with a darker color.
It is notable that a (110) plane of an fcc crystal has structure equivalent to a stepped
surface involving two atom wide (111) oriented terraces, separated by single atom
(111) oriented steps [6] (figure 2-5).
Figure 2-5 – Side view of Pt(110) from reference [1].
For stepped surfaces, surface atom densities can be decomposed into the sum of
contributions of step and terrace atoms. To calculate such decomposition into both
contributions, it helps to consider the geometrical relations depicted in figure 2-6. The
30 Chapter 2
area of the unit cell for the high Miller index surface planes is easily determined from
its projection onto the terrace plane (figure 2-6).
Figure 2-6 – Drawing showing a step and terrace on the unreconstructed Pt(443) - Pt(s)[8(111)
× (111)] surface plane. Unit cells for the (111) (dotted line) and (443) (dashed line planes are
indicated. The inset to the right shows the (443) plane lies at an angle, β, of 7.3 from the
(111) plane. [1]
In the Figure the distance between nearest neighbor Pt atoms is indicated by d
(2.77 × 10−8
cm), while ι represents the distance between rows of Pt atoms on the
(111) plane. The distances d and ι are related through the angle θ indicated in the unit
cell (θ = 120°). For the particular surface in the figure, the center-to-center distance
between the row of step atoms and the row of atoms on the plane immediately below
projected onto the (111) plane is ι /3. Therefore, the distance between steps is (n-1) ι +
1/3 ι and, the projected area(S) for the unit cell is given by:
� � �� √� � � �
(1)
As a result, the unit cell area is S/cos β where β is the angle between the planes of the
surface and the terrace. As the unit cell contains one step, the number of step atoms
per unit surface area is cos β/S. The number of terrace atoms depends on the
geometry chosen to describe the step sites. When the step plane is defined as the
Experimental 31
(110) plane, (n-2) atoms can be counted on the terrace giving a contribution to the
atomic site density of ((n − 2) cos β)/S. Taking the sum of the step and terrace atom
contributions and substituting for S from (1) the number of surface atoms per unit
area on the Pt(S) [n(111) x (110)] is given by:
������������� � ������ ��� ��� √ ����
� (2)
The general expressions for the surface atom densities for the planes Pt(S) [n(111) x
(100)] and Pt(S) [n(100) x (111)] can be also obtained, and are presented in (3) and (4)
respectively.
������������� � � � ��� ��� √ � �� ��
(3)
������������� � � ��� ��� �� ��
(4)
2.2. Crystal’s preparation and cleaning
The single crystals used in this work were prepared using Clavilier´s method [7].
In the early 1980s, Clavilier described a simpler and less expensive (compared to
previous methods based on X-ray diffraction) approach to prepare single crystals by
fusion and subsequent slow crystallization of a platinum wire. Typically, single crystal
beads are grown from high purity (>99.995%) Pt wire, although zone refining and
further purification takes place during bead formation. The process starts with a piece
of Pt wire that was carefully positioned in a fuel-O2 flame until a spherical bead of
molten Pt is formed at the wire tip [1]. When carefully cooled, a single crystal
displaying facets with (100) and (111) orientations is formed from the molten Pt [8]
(Figure 2-7). After bead formation, the facets are inspected to determine whether or
not their positions and crystallographic orientations are consistent with those of a
single crystal.
32 Chapter 2
For the orientation step, the crystal is supported in a goniometer with four rotation
axes. This goniometer is placed in the extreme of an optical rail with a length of 2.5m.
In the other extreme of the bench, opposite to the goniometer, there is a low power
laser that is used to create the reflection-diffraction patterns specific for the (100) and
(111) facets. Light reflected from the facets produces bright spots on the walls and
ceiling around the room. Reflections from (100) facets tend to be circular and diffuse,
while larger (111) facets produce more sharply focused spots. After assigning the spots
to reflections from either (111) or (100) planes on the crystal, the goniometer can be
used to measure the angle between any pair of spots and confirms the quality of the
single crystal bead. With the natural facets present on the bead and by knowing the
angles that they form with a particular orientation that we want to obtain, a scalar
product problem, the crystal can be oriented and fixed in the optical bench for
obtaining the desired plane.
To produce an electrode, a single crystal bead is mechanically cut and polished parallel
to the chosen surface plane. For that, first the electrode is fixed in the goniometer
using epoxy. Usually two steps are used in fixing the crystal with epoxy, letting then
during overnight to harden. To polish the crystal a polishing wheel mounted on the
optical rail is used. Coarse sandpaper is used initially to remove material up to close to
a hemisphere of the crystal. Successively finer grades, down to at least 0.25 μm
alumina or diamond paste, are used subsequently. After polishing, a “mirror finishing”
quality is obtained. The epoxy resin is removed with a proper solvent (chloroform) and
the crystal is annealed in a fuel–air flame for approximately 20 minutes.
Figure 2-7 – Image of a pollyoriented bead [9].
Experimental 33
Immediately prior to each electrochemical measurement, the surface of a Pt single
crystal is treated to remove contaminants and order the top-most atoms. Flame
annealed Pt electrodes must be cooled under conditions that protect the surface from
contact with atmospheric contaminants and O2. After removal from the flame, the
crystal is moved into a reductive atmosphere obtained by purging by an ultrapure Ar+
H2 (3:1 ratio) gas mixture [10-14] in a flask containing ultra pure water (Figure 2-8).
Figure 2-8 – Apparatus for cooling single crystal electrodes in a controlled atmosphere of
Ar+H2 taken from reference [1]
The H2 in the Ar+ H2 gas mixture reduces the likelihood atoms on the Pt surface will
become oxidized and disordered as the crystal cools down [10, 14]. The crystal is held
above the water surface as it cools. After the redness disappears the crystal is
submerged in the ultrapure water saturated with the Ar + H2 gas mixture. Then, the
electrode can be transferred from the cooling flask to the electrochemical cell under
the protection of a water droplet.
2.3. General experimental conditions
In this work, conventional electrochemical cells were used (figure 2-9). The cells are
made of Pyrex glass, with entries for the working and counter electrodes, and also for
the purging with argon (in the bulk solution or in the liquid surface) and the Luggin
34
capillary. The Luggin allows putting the reference electrode in a
compartment and the electric connection to the solution is done by the thin layer of
solution that covers the key. As counter electrode, a platinum wire was used, and as
reference a reversible hydrogen electrode (RHE) is generally used although o
reference electrodes can be considered.
Figure 2-9 – Scheme a typical electrochemical cell
To perform voltammetric experiments, the surface plane of the single crystal working
electrode is typically brought in contact with the electrolyte solution and positioned
slightly above the liquid level such that a meniscus hangs from the face of the crys
(Figure 2-10) [10, 15, 16]. This so
electrochemical reactions from taking place along the sides of the electrode.
Figure 2-10 – Scheme of meniscus with bead single crystal electrodes
Due to high reactivity of platinum single crystals, systems using them as working
electrodes need to be extremely clean. For this reason all the glass material was,
previously to each experiment, submerged over night in an acidic solution of
Chapter 2
capillary. The Luggin allows putting the reference electrode in a separated
compartment and the electric connection to the solution is done by the thin layer of
solution that covers the key. As counter electrode, a platinum wire was used, and as
reference a reversible hydrogen electrode (RHE) is generally used although other
reference electrodes can be considered.
Scheme a typical electrochemical cell.
To perform voltammetric experiments, the surface plane of the single crystal working
electrode is typically brought in contact with the electrolyte solution and positioned
slightly above the liquid level such that a meniscus hangs from the face of the crys
. This so-called “meniscus configuration” prevents
electrochemical reactions from taking place along the sides of the electrode.
Scheme of meniscus with bead single crystal electrodes taken from reference
[1].
platinum single crystals, systems using them as working
electrodes need to be extremely clean. For this reason all the glass material was,
previously to each experiment, submerged over night in an acidic solution of
separated
compartment and the electric connection to the solution is done by the thin layer of
solution that covers the key. As counter electrode, a platinum wire was used, and as
ther
To perform voltammetric experiments, the surface plane of the single crystal working
electrode is typically brought in contact with the electrolyte solution and positioned
slightly above the liquid level such that a meniscus hangs from the face of the crystal
called “meniscus configuration” prevents
taken from reference
platinum single crystals, systems using them as working
electrodes need to be extremely clean. For this reason all the glass material was,
previously to each experiment, submerged over night in an acidic solution of
Experimental 35
concentrated KMnO4. After this, the material was rinsed first with water, then with an
acidic solution of H2O2 and after that rinsed again with a significant amount of
ultrapure water. Finally, the cell was mounted and filled with ultrapure water. This
water was boiled for several minutes to desorb impurities more effectively and to
increase the cleanness. Rinsing and boiling is repeated several times.
2.4. Nanoparticles
The use of nanoparticles has become a very interesting topic in catalysis and
electrocatalysis aiming at the development of more active and more selective Pt
catalysts [17, 18].
It is well known that for a great number of electrochemical reactions, the surface
structure is a fundamental parameter for the catalytic properties of the platinum
surfaces [19, 20]. For these structure sensitive reactions (like nitrate electrocatalytic
reduction) the surface structure of the nanoparticle will strongly control their final
reactivity.
The application of shape-controlled metal nanoparticles in Electrocatalysis is of great
importance because not only allows to deliberately tune both reactivity and selectivity
but also because these systems are ideal candidates to point out the experimental
correlations ‘‘from single-crystals to nanoparticles’’ [21].
In analogy with the unit stereographic triangle, there is also an intrinsic triangle that
coordinates the crystal surface index and the shape of the nanoparticle, [22] as shown
in figure 2-11 in which the three apex represent the coordinates of polyhedral
nanocrystals bounded by basal facets, i.e. cube essentially covered by (100),
octahedron by (111), and rhombic dodecahedron by (110) [22-24].
36 Chapter 2
Figure 2-11 - The stereographic triangle of polyhedral nanocrystals bounded by different
crystal planes.
The methodology for synthesizing the nanoparticles used in this thesis is described
below.
Pt nanoparticles with preferential cubic shape were synthesized with a colloidal
method using sodium polyacrylate (PA, Mw = 2100) as a capping agent and K2PtCl4 as a
metallic precursor (10-4
M aqueous aged solution) [18, 25, 26]. The ratio of K2PtCl4 to
PA was (1:5). Then, this colloidal suspension was purged with Ar gas for 20 min and
finally bubbled with H2 gas for 5 min to reduce the Pt precursor. The reaction vessel
was then sealed and the solution was left overnight. After complete reduction (12-14
hours) these Pt NPs were cleaned with strong basic aqueous solution followed by
several water washes to finally achieve a water suspension with clean cubic
nanoparticles.
Pt nanoparticles with preferential octahedral and tetrahedral shape were synthesized
by a colloidal method using PA as a capping agent and H2PtCl6 as a metallic precursor
(10-4
M aqueous aged solution) [18, 25, 26]. The ratio of H2PtCl6 to PA was (1:5). The
suspension initial pH (around 8) was adjusted to 7 with 0.1 M HCl solution. Then, this
colloidal suspension was purged with Ar gas for 5 min and finally bubbled with H2 gas
for 1 min to reduce the Pt precursor. The reaction vessel was then sealed and the
Experimental 37
solution was left overnight. After complete reduction (12-14 hours) these Pt NPs were
cleaned with strong basic aqueous solution followed by several water washes to finally
achieve a water suspension with clean octahedral-tetrahedral nanoparticles. It can be
seen that small composition changes deeply influence the shape of the nanoparticles.
Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron
Microscopy (HRTEM) have been employed to investigate the shape of the
nanoparticles at the atomic scale (figure 2-12). TEM experiments were performed with
a JEOL, JEM-2010 microscope working at 200 kV whereas HRTEM experiments have
been carried out on a JEOL 3010 microscope (LaB6, Cs=1.1 mm) operated at 300 kV,
providing a point-to-point resolution of 0.19 nm. The sample was obtained by placing a
drop of the dispersed solution onto a Formvar-covered copper grid and evaporating it
in air at room temperature. For each sample, usually more than 200 particles from
different parts of the grid were used to estimate the mean diameter and size
distribution of the nanoparticles.
Figure 2-12 - Representative TEM image of nanoparticles used in this study A) cubic B) octo-
tetrahedrical C) HRTEM of octo-tetrahedrical particles.
The procedure used for the electrochemical study has been previously reported [27-
29]. As current collector, a polycrystalline gold disc electrode (3 mm diameter) was
used, onto which nanoparticles were deposited by putting a drop (generally 2–5 μL) of
the nanoparticle suspension in water on the surface of the gold disc and dried; this
deposition would represent a maximum amount of ≈ 6 μg of Pt. Before each
38 Chapter 2
experiment, the gold collector was mechanically polished with alumina and rinsed with
ultra-pure water to completely eliminate the nanoparticles from previous
experiments. This was checked by investigating the support ability to oxidize hydrogen
at low potential because this reaction should not take place on pure gold. The
experiments for characterization and cleaning of the nanoparticles were performed in
a 0.5 M H2SO4 solution at room temperature. Following the procedure previously
described, prior to the use of the nanoparticles for nitrate reduction, they were
cleaned by CO adsorption and stripping, after which the base voltammograms were
recorded in 0.5 M H2SO4 not only to calculate the real surface area of the catalyst but
also to assess the surface cleanliness. The active surface area of the Pt nanoparticles
was determined by the charge involved in the so-called hydrogen UPD region assuming
0.23 mC·cm-2
for the total charge after the subtraction of the double layer charging
contribution [30].
2.5. Experimental techniques
2.5.1. Cyclic Voltammetry
Cyclic voltammetry is the most common technique used in electrochemistry to gain
wide information about the electrode surface reactions. This technique allows a fast
localization of the redox potentials of the electroactive species, in the polarizable zone
and a convenient evaluation of the effects from this redox process.
In cyclic voltammetry [31], the potential applied in the working electrode is scanned
linearly in time at constant rate. The scanning is usually done between two potentials,
V1 and V2 and when V2 is reached the scan direction is reversed towards V1 at the
same constant sweep (dE/dt) rate (Figure 2-13).
Figure 2-13 – Schematic representation of potential scan in cyclic voltammetry.
The most common way to represent CV results is in the form of i
voltammograms. In one CV experiment, one or several consecutive scans can be
registered, depending on the information that we want to obtain.
Figure 2-14 shows the typical cyclic voltammogram for a reversible reaction of 1
electron transfer in a solution containing one electroactive specie.
Figure 2-14 – Typical cyclic voltammogram for a reversible reaction of 1 electron transfer
In the figure, the scan is started from negative to more positive potentials in the
positive going sweep, starting at an initial potential were no reaction occurs. When the
applied potential becomes close to the equilibrium potential for the redox process,
Experimental
Schematic representation of potential scan in cyclic voltammetry.
The most common way to represent CV results is in the form of i-E curves, called cyclic
voltammograms. In one CV experiment, one or several consecutive scans can be
registered, depending on the information that we want to obtain.
shows the typical cyclic voltammogram for a reversible reaction of 1
n transfer in a solution containing one electroactive specie.
Typical cyclic voltammogram for a reversible reaction of 1 electron transfer [32
In the figure, the scan is started from negative to more positive potentials in the
positive going sweep, starting at an initial potential were no reaction occurs. When the
applied potential becomes close to the equilibrium potential for the redox process,
39
E curves, called cyclic
voltammograms. In one CV experiment, one or several consecutive scans can be
shows the typical cyclic voltammogram for a reversible reaction of 1
32].
In the figure, the scan is started from negative to more positive potentials in the
positive going sweep, starting at an initial potential were no reaction occurs. When the
the
40 Chapter 2
anodic current starts to increase and a peak is observed. The maximum in the
oxidation current reflects the fact that more positive potentials make the oxidation
reaction easier but, as time goes, less active material reaches the electrode surface as
the diffusion layer becomes thicker. After the maximum current is attained, the
reaction becomes diffusion controlled and currents decreases with time, potential and
time are equivalent in this situation. When the scanning direction is reversed, after the
maximum of the oxidation current, the reduction of the species oxidized in the
previous sweep takes place. This current also reaches a maximum for the same
reasons explained above.
An important characteristic of this kind of voltammograms is that the maximum
currents in the two sweeps appear at slightly different potentials, ΔEp, that would be
separated by approximately by 60 mV for reversible systems.
For reversible systems the peak currents in Amperes are given by:
Ip = -2.69x105 n
3/2 A D0
1/2 C v
1/2 (5)
Where, A is the electrode area (cm2), D0 the diffusion coefficient of the species (cm
2/s),
C the concentration (mol/cm3) and v the scan rate.
The ΔEp is given by the equation:
59a c
p pE E E mV
n∆ = − = (6)
And the peak position (potential) is maintained for different scan rates, while the
maximum current of the anodic and cathodic peaks increases proportionally in a way
that the relation between them is approximately 1:
1
a
p
c
p
i
i= (7)
Experimental 41
The peaks maximum intensity, in this case (reversible process), are proportional to the
square route of the scan rate.
,a
pic
pi v∝ (8)a
The reversible systems can be considered as reference of reactions that would not be
catalyzed. Reactions with different characteristics reflect kinetic limitation and should
be studied carefully and the model adjusted for each case. For example, for an
irreversible process, the characteristic parameters of the curves are different.
The ΔEp is:
59a c
p pE E E mV
n∆ = − > (5)
The potential difference between the two peaks depends on the scan rate. It increases
when the process is slower and the scan rate higher. The maximum current intensity of
the peaks is no longer 1.
1
a
p
c
p
i
i≠
(6)
The maximum current intensity of the peaks is, like for the reversible process,
proportional to the scan rate.
,a
pic
pi v∝ (7)
For cases that the electrode process is controlled by the adsorption of the
electroactive species (fast electron transfer), the characteristics of the voltammetric
42 Chapter 2
peaks are significantly different. The superficial processes have well defined peaks and
the voltammogram is symmetric.
For a reversible system the potential difference between the voltammetric peaks is
given by:
0a c
p pE E E∆ = − =
(8)
The peak position is not affected by the scan rate and the maximum current peaks also
increases proportionally in a way that the relation between them is approximately 1
1
a
p
c
p
i
i=
(9)
Another important difference in respect to the other process is that the maximum
intensity of the peaks current is proportional to the scan rate.
,a
pic
pi v∝ (10)
In addition, the equations should take in account the adsorption isotherm of the
involved species. The application of Langmuir isotherm [31, 33-35] requires the
assumption of three main considerations: first, the homogeneity of the surface;
second, that at high concentrations of adsorbates in solution, the surface reached the
monolayer saturation. Another important aspect is that this isotherm does not include
the lateral interactions between adsorbed species [31].
Taking the simpler example of a reversible adsorption of a species with charge
transfer:
O ne� # R%&� (11)
Experimental 43
The mass balance will be given by
� '()'* � +
�,- (12)
Where Γ/ is the superficial concentration of R (normally in mol/cm2). This
concentration is directly related with the electrode potential and the bulk
concentration through the adsorption isotherm. Introducing this relation in the
equation 12 and assuming that c0 is higher enough than the adsorbed amount for
being considered constant, gives:
+�,- � � '()
'0 12
�0�* � � '()
'0 12
3 (13)
Where v is negative or positive depending on the direction of the sweep rate. This
equation shows that the current will be directly proportional to the scan rate.
Taking into account these ideal conditions of the adsorption process, the surface
concentration (Γi) of the adsorbed specie is given by:
Γ+ � 4 +�,-5
006 �7 Γ� �76� (14)
Where Γs can be either zero or the maximum concentration of the monolayer,
depending on the choice of E*.
The Langmuir isotherm is applicable for reversible electronic transfer reactions with
reversible (in potential and in charge) voltammetric peaks. When lateral interactions
between the adsorbed species exist, other models need to be used, like Frumkim
isotherms. In general, processes with attractive interaction between the adsorbed
species will be represented by sharp voltammetric peaks. On other hand, repulsive
interaction will rise in broad peaks in the voltammograms [31]. For irreversible process
involving adsorption, peak separation is no longer zero but depend on the rate
constant of the electron transfer process.
44 Chapter 2
In summary, CV gives an account of surface process with peak potential giving a
measure of adsorption energies, peak area being related with amount of adsorbed
species and peak width with interactions between species.
In this work, cyclic voltammetry was used as main technique for characterization both
the electrode surface as well as the catalytic properties toward the reactant under
study. The measurements were performed with an EG&G PARC 175 signal generator,
an eDAQ EA161 potentiostat and an eDAQ e-corder ED401 recording system.
2.5.2. In situ Infrared Reflection Adsorption Spectroscopy
Infrared spectroscopy is employed in spectroelectrochemical studies to obtain
structural information related with to the electrochemical interface. It became one of
the most useful techniques for in situ characterization at molecular level because it
can provide information about the nature of adsorbed species, adsorbate bonding
geometry, adsorbate–adsorbate interactions and, indirectly, the surface adsorption
sites.
The first in situ IR spectroelectrochemical experiments used the internal reflection
mode [36, 37]. Under these conditions, the penetration of the infrared beam into the
solution side of the interface is limited to a fraction of a micron. In proper setups, the
infrared window is also used as the substrate for the deposition of a thin metal film
acting as the working electrode [38-42]. Major limitations of the internal reflection
experiments are related to the stability of the thin film electrodes (thickness typically
around 20 nm) and to the control of the surface structure.
Later, the already used specular external reflectance spectroscopy using UV-visible
radiation was adapted to infrared studies [43]. In this case, the reflecting surface of a
bulk electrode is pushed against an infrared window with a low refractive index in such
a way that the thickness of the solution layer sampled by the radiation is reduced to a
few microns [44, 45]. The biggest disadvantages for the external reflection method are
Experimental 45
the high electric resistance in the thin layer solution, hindered mass transport
conditions [42, 46] and interference due to infrared absorption from bulk water. To
circumvent this problem, spectra are usually taken at two potentials and then
subtracted to remove water contributions. Besides, spectra accumulation is used to
minimize random noise signals. Typically, 100 or 200 scans (interferograms) are
averaged to obtain the final spectrum. External reflection experiments are
advantageous in many cases because they allow the spectroscopic detection of the
consumption and/or formation of reactants or intermediates allowing the detection of
even submonolayers quantities of species at the electrode- electrolyte interface.
The IR adsorption reflection spectroscopy is based on the analysis of the intensity of
the reflection from the metallic surface in contact with a solution as function of the
wavenumber of the incident radiation. The adsorption of the radiation occurs from the
metallic surface and also from the molecules on the path of the beam. The adsorption
from the molecules is given by the interaction of the electric field from the beam and
the dynamic dipolar moment of the molecule.
On the other hand, when the beam is reflected on a metal, the amplitude of the
electric field vector on the surface is the sum of the vectorial amplitudes of the
incident and reflected light and depends on both polarization of the incident beam and
the incidence angle. While the component perpendicular to the incidence plane (s
polarization) suffers a phase transition around 180o for almost all the incident angles,
the phase transition of the component parallel to the incident plan (p polarization)
remains near zero for a wide range of incidence angles. For this reason, for a radiation
with p polarization, the resulting electric field has only one component perpendicular
to the surface, while that for a radiation with polarization s the electric field is almost
zero on the surface. This is the origin of the surface selection rule, fundamental for this
technique: with p polarization, just the vibrational modes that imply a change in the
dipole moment perpendicular to the surface are active in Infrared reflection
46 Chapter 2
adsorption spectroscopy (IRRAS). With s polarization, only bands resulting from
vibrations of species in solution far from the electrode surface are visible.
Another important issue in the IR experiments is that the electric field on the surface
depends on the incidence angle. The optimal angle is near 90o with respect to the
normal, slightly dependent on the metal. In electrochemical conditions the incidence
angle is limited as consequence of the refraction of the incident beam on both faces of
the window. Higher angles can be obtained by using, instead of planar windows,
prismatic windows beveled 60 or 65º.
The detection of these infrared adsorbances by submonolayer quantities requires a
high level of sensitivity. As mentioned before, one of the biggest inconveniences in the
IRRAS in situ experiment is the strong adsorption of the electromagnetic radiation by
the solvent. In most of the cases the number of molecules of reactant is low and
transmittance values should be in the range of 0.1-0.01%. To reach these sensitivity
values, the adsorption from the solvent should be minimized. As pointed out, one way
to achieve it is by decreasing the liquid layer that the radiation needs to go through.
For this purpose the thin layer configuration is used, pressing the electrode against the
prismatic window (CaF2) in a way to get a 1 to 5 µm liquid layer (Figure 2-15).
Figure 2-15 – Schematic representation of the cell used for the in situ FTIR experiments.
Experimental 47
The alternation of the electrode potential between two values (E1 and E2) produces a
modulation of the composition in the interfacial region, and the potential difference
infrared spectroscopy monitors the concomitant changes in the absorbance of the
infrared radiation at both potentials. Usually, a constant potential (E1) is selected as
reference and the other potential (E2) varies in the whole range of interest. In practice
the alternating infrared signal is measured as a change in reflectivity of the electrode
surface (ΔR), and the difference spectrum is obtained by rationing ΔR against the total
reflectivity, R. For the small changes that are typically observed ΔR/R is equivalent to
the absorbance change.
8 � �9:; //2
� �9:; 1 /�/2/2
< � ∆//2
(15)
Since changes only occur in the interfacial region, other adsorbances in the path of the
beam do not give a ΔR signal [47].
Figure 2-16 – Resulting spectrum obtained after
subtracting two spectra obtained for different
potentials.
4000 3500 3000 2500 2000 1500 1000
∆R
/ R
(ER
ef)
W avenum bers/cm-1
R(E
Ref)
R
(Es)
48 Chapter 2
This subtraction procedure is schematized in figure 2-16: a) at the sample potential,
bands corresponding to the adsorbed species and species in solution are observed.
These bands are in addition to the water bands at 1640 (bending) and between 3000
and 4000 cm-1
(stretching). At the reference potential only the band corresponding to
the species in solution is present. Since the species desorbed is accumulated in the
thin layer, the magnitude of this band is larger at the reference potential than at the
sample potential. When the two spectra are subtracted, water bands cancel and
negative bands results for species that are present at the sample potential and positive
bands corresponds to species present with higher concentration at the reference
potential. If absorbance is used instead of transmittance, the sign of the bands is
opposite. Monopolar bands are also observed when the coverage in the surface
increases from one potential to other. Bipolar bands can arise if the band position
changes with the electrode potential for adsorbed species without change in coverage.
In this figure, the relative magnitude of the bands of interest has been exaggerated in
comparison with the water bands and in a real spectrum these bands are usually
indiscernible from the background spectrum.
Real spectra are presented in figure 2-17 from an experiment with CO adsorbed on a
Pt(111) surface. As is it shown on panel A just slight differences are observed on the
spectra (insert panel A). When reference spectrum is subtracted from the sample
spectrum, all the bands due to water are eliminated and only the CO bands remain due
to their change with potential (panel B).
Experimental 49
4000 3500 3000 2500 2000 1500 1000
0
6
12
18
Wavenumbers cm-1
Inte
nsi
ty a
.u. Single Beam at 0.1 V
Single Beam at 0.3 V
A
2000 1950 1900 1850 1800 1750 1700
8
9
10
Inte
nsity
a.u
.
wavenumbers cm-1
2800 2400 2000 1600 1200-0.004
-0.002
0.000
0.002
0.004
on top CO
log (R0.3 V
/R0.1V
)
bridge CO
B
Figure 2-17 – Spectra obtained for CO oxidation on Pt(111) at different potential.
For this work spectroelectrochemical experiments were performed with a Nicolet
Magna 850 or a Nexus 8800 spectrometer equipped with a MCT detector and specular
reflectance system Veemax from Spectra - Tech. The spectroelectrochemical cell was
provided with a prismatic CaF2 window bevelled at 60o. Unless otherwise is specified,
spectra shown were collected with a resolution of 8 cm-1
and p polarized light. They
are presented as absorbance, according to A= - log (R/R0) where R and R0 are the
reflectance corresponding to the single beam spectra obtained at the sample and
reference potentials, respectively.
50 Chapter 2
2.5.3. On line Electrochemical Mass Spectroscopy (OLEMS)
Using mass spectroscopy, volatile chemical species generated at the electrode
interface can be detected by mass spectrometry with very little time delay. The use of
this technique applied to electrochemical systems was started by Bruckenstein and
Gadde [48] who collected gaseous electrochemical reaction products in a vacuum
system before detecting them by mass spectrometry. Due to a proper design of the
vacuum system including two pumping stages, product formation rates were
measured; to distinguish the technique from product sampling, i.e., integrating
approaches, the method was called “differential”. Even at the beginning, the
technique was sensitive enough to detect desorption products corresponding to about
one monolayer of adsorbed species at porous electrodes.
It was subsequently extended and improved in such a way that products from bright
surfaces could be analyzed, by the groups of Heitbaum and Baltruschat [49-51], and it
is now a routine technique utilized by many groups worldwide.
Almost 10 years ago, Koper group [52] developed an On-Line Electrochemical Mass
Spectrometry (OLEMS) system for detecting volatile products formed during
electrochemical reactions at a single-crystal electrode in hanging meniscus
configuration.
Experimental 51
Figure 2-18 - Schematic drawing of the on line electrochemical mass spectrometer, WE –
working electrode, RE – reference electrode, CE – counter electrode from reference [52].
In figure 2-18 the OLEMS setup is schematically represented. The system consists of a
mass spectrometer, a measuring tip, a micrometer positioning system, a video camera,
an electrochemical cell, a potentiostat and software for measuring the masses
simultaneously with the electrochemical parameters.
The electrode-tip assembly is positioned with the aid of a micrometer system mounted
on two Teflon blocks, which can be placed on the cell. The Teflon blocks have two
holes for the working electrode and the glass vacuum tube, both fixable with plastic
screws. During positioning, the distance between the tip and the electrode is
monitored using a black and white camera with magnifying lenses and a periscope. It is
mounted on a stand, which is adjustable in all directions over a range of a few cm, and
which can overturn slightly to place the camera view in the same plane of the
electrode. Any experiment starts by placing the single-crystal electrode in meniscus
mode and measuring the voltammetry in ‘‘tip-retracted’’ mode. Next, the electrode is
dropped into solution and the tip is placed at 10–20 µm distance from the electrode
surface with the help of the micrometer system and the video camera, without
52 Chapter 2
touching the electrode surface. Finally the electrode and tip are fixed and are placed
simultaneously in hanging meniscus configuration.
More detailed description of the setup can be found in previous publications [52]. In
the particular case of the experiments done for this thesis the solution was not stirred
during the experiments, and a flow of blanketing Ar was maintained to protect the
solution from oxygen. All OLEMS experiments were carried out at a scan rate of 1
mV/s. The OLEMS set-up does not allow a quantitative analysis of the signals.
However, if the experiments are repeated with the same PTFE tip and at a comparable
pressure (measured with a full-range pressure gauge), the relative magnitudes of ion
currents measured prove to be highly reproducible. An internal, semi-quantitative
calibration can also be carried out, and further details will be reported below when the
results are presented.
2.5.4. Scanning Tunneling Microscopy (STM)
Scanning tunneling microscopy (STM) belongs to a group of techniques (scanning
probe microscopies) in which sharp tips are scanned over the sample (or the sample is
moved under the probe) providing local information for every single image point. All
scanning probe microscopes involve very precise mechanical movements of the probe
(or sample) by means of piezoelectric translators. The first of these microscopes that
was developed was the STM. It was developed by Gerd Binnig and Heinrich Rohrer
while working at IBM Zurich Research Laboratories in Switzerland. This experimental
approach would later lead Binnig and Rohrer to be awarded the Nobel Prize in physics
in 1986 [53].
The STM works by scanning a very sharp metal wire tip over a surface (figure 2-19). By
bringing the tip very close to the surface, and by applying an electrical voltage to the
tip or sample, the surface can be imaged at an extremely small scale, down to atomic
resolution. The STM is based on several principles and one is the quantum mechanical
effect of tunneling. According to this effect, current can flow crossing the potential
Experimental 53
barrier imposed by a non-conducting material between the tip and the sample. The
large sensitivity of this current to the distance between tip and surface is the reason
for the extremely high resolution of the technique. Another principle with
fundamental application is the piezoelectric effect that allows to precisely scan the tip
with angstrom-level control. Lastly, a feedback loop is required, which monitors the
tunneling current and coordinates the current and the positioning of the tip [54, 55].
Figure 2-19 – Scheme of a scanning tunnel microscope [56].
The electrochemical scanning tunneling microscope (EC-STM) is designed to operate
with a small electrochemical cell and a four-electrode bipotentiostat enabling
independent control over the tip and substrate electrochemical potentials. The
reference electrode (RE) provides a fixed reference potential to the electrolyte
solution while the counter electrode (CE) completes the circuit with the working
electrode (WE). In combination, these electrodes can be used to ensure control over
electrochemical processes occurring under potential control at the working electrode.
The STM tip is the fourth electrode in the EC-STM setup. Control of the
electrochemical potential of both tip and substrate is achieved and the bias potential
between the tip and the substrate is therefore defined.
The precise control of the redox processes within an EC-STM set-up is achieved by
independent control of the potentials of the tip and sample relative to a reference
electrode. The current measured by STM is the sum of the tunneling current and
54 Chapter 2
faradaic or capacitive background currents. Uncoated STM tips may typically exhibit
electrochemical currents in the order of μA, which overwhelms typical tunneling
currents in the region of nA. To avoid the influences of the electrochemical tip current
on tunneling current, the tip should be properly isolated coating most of its surface.
There are a number of coating methods depending on the tip materials employed.
Ideally, the insulated tip has an active surface area as small as possible, but still the
outermost end of the tip should be exposed to enable tunneling to the substrate.
One of the most commonly used methods for isolating STM tips is Apiezon wax
coating. Melt coating with Apiezon wax produces a coating which is less fragile than
glass coated tips and the wax is relatively inert in aqueous electrolytes.
In situ STM experiments were carried with a Molecular Imaging (Agilent) STM in
conjunction with the Picoscan 5.3.3 software. Tunnelling tips from Au or PtIr were
used, both prepared by electrochemical etching before each experiment. Au tips were
etched in a solution of 50% ethanol - 50% HCl and PtIr in a concentrated solution of
CaCl2/HCl. When the experiments were made in situ (under electrochemical
conditions) the tips were coated with Apiezon wax. All STM images were recorded in
constant current mode with tunnelling currents ranging from 1.0 to 0.1 nA. They are
conventionally presented as top view images with darker colors corresponding to
lower surface areas. The electrochemical cell employed during in situ STM can be seen
in the figure 2-20. The analysis of the images was made with the software WSxM from
Nanotech [57].
Figure 2-20 – Electrochemical cell used in the EC
2.6. Chemicals
Due to the high reactivity of Pt electrodes all the reagents used in this thesis were high
purity chemicals and were used as received (table 2.2).
Table 2-2 – Characteristics of reagents employed.
Reagent Formula
Perchloric Acid HClO
Sulphuric Acid H
Sodium hydroxide NaOH
Sodium Dihydrogen phosphate H2
Disodium hydrogen phosphate HNa
Sodium Nitrate NaNO
Sodium Nitrite NaNO
Bismuth oxide Bi
Labeled sodium nitrite Na
Experimental
Electrochemical cell used in the EC-STM setup.
reactivity of Pt electrodes all the reagents used in this thesis were high
purity chemicals and were used as received (table 2.2).
Characteristics of reagents employed.
Formula Grade Company
HClO4 suprapur® Merck
H2SO4 suprapur® Merck
NaOH suprapur® Merck
2NaPO4 suprapur® Merck
HNa2PO4 suprapur® Merck
NaNO3 suprapur® Merck
NaNO2 99.999% Sigma
Bi2O3 Extra pure Merck
Na15
NO2 98% Cambridge Isotope Laboratory
55
reactivity of Pt electrodes all the reagents used in this thesis were high
Cambridge Isotope Laboratory
56 Chapter 2
Water from Elga Purelab Ultra, 18.2 MΩ cm was used for rinsing the cell and to
prepare the solutions. The electrolyte was purged with argon (N50, Air Liquid) and the
solution was kept under the argon blanket throughout the duration of the experiment.
Hydrogen Alphagaz B50 was used for the electrode cooling and for the reference
electrode. For experiments with NO gas, NO N30 from Air liquid was used.
Experimental 57
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Experimental 59
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3Electrochemical surface
characterization
3. Electrochemical surface characterization
Platinum can be considered one of the most important electrode materials due to its
high catalytic activity toward important fuel cells type reactions such as hydrogen and
oxygen reduction and oxidation, and the oxidation of small organic molecules. The
characterization of the electrochemical behavior of platinum single crystal electrodes
has been the subject of many studies during the last decades, starting with an iconic
experiment by J. Clavilier and the introduction of the flame annealing in 1980 [1].
Previous experiments lack sufficient control of the surface quality and/or cleanliness.
During this time it has been extensively proved that most electrocatalytic reactions are
sensitive to the atomic-level structure of the electrode surface [2-4].
In this chapter, a brief description about the characteristic electrochemical features of
some of the surfaces used in this thesis will be done. The chapter is divided by the
behavior on the different media in which nitrate reduction has been studied with
platinum single crystals. The use of irreversible adsorbed adatoms on Pt will also be
addressed as well as the corresponding characterization in perchloric acid and neutral
phosphate solutions.
3.1. Basal planes in acid media
In acidic media, the platinum single crystal voltammetric response is well described in
the literature [5-10]. Figure 3-1 shows the typical cyclic voltammograms for the 3 basal
planes in both perchloric (A) and sulphuric (B) acid. In both media, for Pt(111) it is
possible to observe that currents are nearly constant between 0.05 V and 0.25 V, in
the positive and negative sweep directions. It is accepted that the process responsible
for the current in this region is the adsorption/ desorption of atomic hydrogen. If the
64 Chapter 3
charge between 0.05 and 0.32 V is integrated (subtracting the double layer capacitive
charge) a charge of 160 μC/cm2 is obtained. This gives a measure of the amount of
adsorbed hydrogen in this potential range. [11]. Taking into account the Pt atomic
density on the (111) surface plane, the hydrogen coverage inferred from this charge
would correspond to 2/3 of a monolayer. The remaining 1/3 of the monolayer is
probably not attained because hydrogen evolution starts before the adlayer can be
completed [12].
When the electrode potential is increased, the differences between the voltammetric
profiles in perchloric and sulphuric acid start to appear. In sulphuric acid, in the
positive scan, hydrogen is replaced by SO42−
(or HSO4−) on the surface. The oxidative
desorption of Pt-H creates Pt sites, which become occupied by anions as soon as an
excess of positive surface charge develops [11, 13]. The pair of symmetric spikes near
0.45 V corresponds to a reversible phase transition in the SO42−
adlayer between
disordered and ordered states [13]. For potentials above the sharp peak a √3x√7
structure has been identified by STM [14]. The integrated charge obtained from the
voltammogram in this potential range amounts to 80 μC cm−2
after traditional double
layer correction. The sulphate adsorption peaks have been called in the literature as
“butterfly” region [4, 13].
Additional peaks are observed around 0.8 V. The coincidence of these peaks with the
main adsorption region in HClO4 (see below) is taken by some groups [15, 16] as an
indication that this feature is related to OH adsorption also in this media. However, it
has been clearly demonstrated that these peaks are related with sulphate adsorption
[17, 18].
In absence of superficially adsorbed anions, it is now generally accepted that current
through the butterfly region of Pt(111) in HClO4 mainly results from the adsorption of
OH− [19, 20]. It is known that ClO4
− ions weakly adsorb on Pt so it is believed that these
ions do not contribute to anion adsorption charge in cyclic voltammograms [19].
Electrochemical surface characterization 65
Moreover, the voltammogram largely coincides with that measured in HF [21] and
CF3SO3H [22] solutions which contains also weakly adsorbing anions, indicating that
these features should be ascribed to some common species, namely, the water.
0.0 0.2 0.4 0.6 0.8 1.0
-150
-100
-50
0
50
100
150
Pt(111)
Pt(100)
Pt(110)
j (µ
A/c
m2)
E (V) vs RHE
0.0 0.2 0.4 0.6 0.8 1.0
-300
-200
-100
0
100
200
300 Pt(111)
Pt(100)
Pt(110)
Figure 3-1 – Characteristic cyclic voltammograms of Pt(111), Pt(110) and Pt(100) in A) 0.1M
HClO4 and B) 0.1M H2SO4, 50 mV/s.
Contrarily to Pt(111), the surfaces of the other low index surface planes, Pt(100) and
Pt(110), may suffer large structural changes associated with reconstruction that takes
place during annealing and subsequent cooling steps [23, 24]. This causes that a
significantly smaller number of studies have been performed on these surfaces when
compared with Pt(111). However, by controlling the crystal annealing and subsequent
cooling step it has been proved that the unreconstructed (1 × 1) surfaces of Pt(110)
and Pt(100) can be stabilized [25-27].
For Pt(110), the voltammetric responses are relatively similar for sulphuric and
perchloric acid. In sulphuric, one peak is observed near 0.15 V while this peak splits
66 Chapter 3
into two peaks at 0.14 V and 0.24 V in perchloric acid as consequence of the lower
adsorption strength of the electrolyte anion. The peaks in this region arise from the
coupling of hydrogen desorption or/and anion adsorption on the scan toward positive
potentials, and the reverse of these processes on the scan toward negative potentials
[28-30].
The charge measured under the peak, after subtraction of the capacitive component is
215 μC/cm2. The contribution from adsorbed hydrogen, determined by performing
charge displacement measurements at 0.08 V, is 150 μC/cm2 which corresponds to a
monolayer of hydrogen on the (1 × 1) surface [30]. The rest of the charge should be
attributed to the adsorption of anion species.
A representative cyclic voltammogram for Pt(100) is also shown in figure 3-1 on both
perchloric and sulphuric acid. Similarly to Pt(110) responses for Pt(100) in perchloric
acid and sulphuric acid are also closely similar and the voltammetric currents coincide
at low potentials. The main difference is that, in perchloric acid, the peak near 0.4 V is
broader and the side features have greater intensity. It was described in the literature
that the region between 0.2 V and 0.7 V on Pt(100) can be attributed to the coupling
of hydrogen desorption with anion adsorption/desorption [31]. It was also suggested
that current below 0.2 V may originate from hydrogen adsorption on (111) or (110)
oriented step defects and the peak at 0.25 V was assigned to contributions from
hydrogen adsorption on the (100) terrace sites at the step edge. These defect are
probably originated during the elimination of the hexagonal reconstruction that takes
place on the (100) surface during the flame annealing. The charge measured between
0.06 and 0.5V, in this surface, after double layer corrections is 240 µC/cm2. This value
is slightly higher than the expected 210 µC/cm2
for the adsorption of one H per surface
Pt atom, suggesting that, in this potential range, there is some contribution from anion
adsorption. The charge displacement experiments showed that, at 0.1 V, the charge is
very similar in perchloric and sulphuric acid (197 µC/cm2) and very close to a complete
monolayer of H. At higher potential (0.4 V) the displaced charges are negative
Electrochemical surface characterization 67
supporting that there is anion adsorption at this potential. This shows that the
voltammetric process observed between 0.2 and 0.4 V in Pt(100) corresponds to the
desorption of the H layer and subsequent adsorption of the anion.
3.2. Alkaline media
3.2.1. Basal planes
The number of publications about the voltammetric characterization of platinum single
crystals in alkaline solution electrolytes is much smaller as compared to those in acidic
media [32-36].
For Pt(111), the CV (figure 3-2) shows a platform-like flat hydrogen wave in the
potential region from 0.05 to 0.4 V similar to that obtained in acid media. The process
corresponds to hydrogen underpotential deposition and is directly followed by the so-
called double layer region (0.4 V < E < 0.6 V). It is worth mentioning that the charge of
hydrogen adsorption decreases with pH increasing in Pt(111) (from 160 to 135 µC/cm2
from acid to neutral pH) what can be observed on the CV with the decrease of the
voltammetric currents above 0.3 V. The most striking difference between acidic an
alkaline media appears in the potential region between 0.6 and 0.9 V. In NaOH
solution, the butterfly is replaced by a couple of reversible broad peaks, which are
commonly assumed to represent the discharge of water or OH- to form hydroxyl
adlayer.
68 Chapter 3
0.0 0.2 0.4 0.6 0.8 1.0
-200
-100
0
100
200 Pt(111)
Pt(100)
Pt(110)
j (µ
A/c
m2)
E (V) vs RHE
Figure 3-2 - Characteristic cyclic voltammograms of Pt(111), Pt(110) and Pt(100) in 0.1 M
NaOH, 50 mV/s.
The hydrogen adsorption wave on Pt(110) in NaOH solution shows a simple behavior,
as in H2SO4 solution (figure 3-2). This similarity is entirely different from those of
Pt(111) and (100). In NaOH, the CV for Pt(110) shows just a couple of reversible peaks
where hydrogen and anion (OH) adsorption are overlapped.
In the case of Pt(100), there is not a general agreement on the correct voltammetric
profile for the ordered Pt(100) electrode (figure 3-2). In general, four peaks can be
observed, but their relative magnitude depends on the cooling conditions, the
supporting electrolyte and the quality of the crystal.
When compared to the profile obtained in acid media for Pt(100) significant
differences can be observed. In the absence of specific adsorption, two main broad
features can be distinguished in acid media centered at approximately, 0.40 and 0.55 V
and are assigned to the adsorption of hydrogen and OH on the terraces, respectively
[31, 35]. The small peak at 0.30 V has been assigned to hydrogen adsorption on terrace
edges as deduced from the evolution of this peak after the deliberate introduction of
Electrochemical surface characterization 69
steps on the surface [31]. The presence of four different contributions in alkaline
media indicates that the adsorption processes of hydrogen and OH in alkaline media
are more complex. Additionally, some of the states corresponding to the adsorption of
hydrogen or/and OH have been displaced to more positive potentials. However, the
measured charge between 0.2 and 0.7 V without any double layer correction is 323
µC/cm2, only slightly higher than that measured in acid (303 µC/cm
2 [31]) which
suggests that the hydrogen and OH coverages are similar. Another remarkable
difference between the voltammetric responses recorded in both media is related to
the adsorption processes below 0.2 V. In the case of alkaline solutions, the current
measured in this potential range features a shape typically assigned to double layer
contributions whereas in acid media some adsorption processes are clearly visible [31,
34].
3.2.2. Stepped surfaces vicinal to the (100) pole
In order to shed some additional light on the factors that affect the voltammetric
profile of Pt(100) in alkaline media, the voltammetric response from stepped surfaces
of the type Pt(S)[n(100)×(111)] has been also investigated. The aim is to report the
voltammetric characteristics of these surfaces, to analyze trends in the evolution of
the voltammetric profiles and finally determinate the nature of the different species
involved in the charge transfer.
The voltammetric profiles measured for Pt (S) [n(100)×(111)] surfaces in 0.1 M NaOH
are presented in figure 3-3.
70 Chapter 3
-70
0
70
0.0 0.2 0.4 0.6 0.8
E vs. RHE/V
j/µ
A c
m-2
Pt(100)
Pt(39,1,1)
Pt(29,1,1)
Pt(23,1,1)
Pt(15,1,1)
0.0 0.2 0.4 0.6 0.8
-140
-70
0
70
140 Pt(11,1,1)
Pt(711)
Pt(311)
Pt(100)
j/µ
A c
m-2
E vs. RHE/V
Figure 3-3 - Voltammetric profile of Pt(S) [n(100)×(111)] surfaces in 0.1M NaOH. Scan rate: 50
mV/s.
The addition of (111) steps to the surface induces significant changes on the CV
profiles, especially on the peaks at 0.395 and 0.465 V. The behavior of the (100) vicinal
surfaces can be divided in two different groups, the surfaces with wide terraces (n≥7)
and the surfaces with narrow terraces. For the surfaces with wide terraces, the
Electrochemical surface characterization 71
changes are quite small. The peak at 0.465 V decreases and shifts towards higher
potential values when the step density increases. From this behavior, it is clear that
this peak is associated to adsorption processes on terrace sites. On the other hand, the
current density for the peak at 0.395 V increases and the peak potential shifts to
higher values. It should be mentioned that this signal for the Pt(100) electrode is just a
very small shoulder in the broad feature. The broad feature remains almost constant
with the step density for surfaces with wide terraces and the peak at 0.395 V
increases. Thus, the small peak that develops as the step density increases can be
assigned to a response from species adsorbed on step sites. The other two peaks,
those at 0.290 and 0.570 V, are not significantly affected by the increasing presence of
steps on the surface for n>7; in fact, the peak shape and current is almost not affected
by the step density. Also at potentials lower than 0.2 V changes after introduction of
steps are very small.
For the surfaces with narrow terraces (Figure 3-3, bottom panel) changes in the
voltammogram are more dramatic. The contributions above 0.46 V previously
discussed completely disappear and the one at 0.395 V associated to the presence of
step sites becomes the predominant feature of the voltammogram. This peak becomes
broader and shifts to higher potentials. Additionally, the introduction of steps causes
similar effects as those observed in acidic media for the potentials below 0.2 V [31].
The current of this region increases as the step density increases.
Since the differences between acid and basic media are significant, it is important to
study intermediate pH values. Concomitantly, information about the effect of
adsorbing anions can be also obtained. For this propose, basal planes and stepped
surfaces of (100) terraces were studied at neutral pHs and will be discussed in the next
section.
72 Chapter 3
3.3. Neutral phosphate buffered media
3.3.1. Basal planes
In neutral media, the number of studies concerning the voltammetric characterization
of platinum single crystals is rather scarce [37-39]. The voltammograms of the three
basal planes in phosphate buffered media (pH 7.20) show some similarities with the
corresponding ones in acidic media, having an intermediate behavior on those
observed in perchloric and sulphuric acid. In figure 3-4, the CV obtained for the three
basal planes in sodium phosphate buffer pH 7.2 are plotted.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-200
-150
-100
-50
0
50
100
150
200
j (µ
A/c
m2)
E (V) vs RHE
Pt(111)
Pt(100)
Pt(110)
Figure 3-4 - Characteristic cyclic voltammograms of Pt(111), Pt(110) and Pt(100) in 0.05M
NaH2PO4 + 0.05 M Na2HPO4, pH7.2 and 50 mV/s.
The behavior of Pt(111) surfaces in phosphate buffer solutions has been previously
reported in the literature [38, 39]. The voltammetric profile of this surface was found
to be dependent not only on the buffer pH but also on the nature of the cation. In
potassium phosphate at pH 7, at potentials higher than 0.4 V (RHE) only one peak is
observed in each of the going sweeps. Meanwhile, if sodium is the cation on the
Electrochemical surface characterization 73
phosphate salt, for the same pH, this single peak at high potential splits into three
peaks. For pH= 8.8, peak splitting is also observed in K+ containing buffer solutions.
This suggests that the co-adsorption of cations and protons with the phosphate
adlayer plays an important role in the voltammetric shape [39].
For Pt(111), for E > 0.4 V the behavior is very similar to that observed in a lower pH
window, with a charge of 147 µC/cm2 with double layer correction. At 0.53 V the
typical butterfly-type sharp reversible peak is related to a disorder-phase transition in
the adlayer of hydrogen phosphate HPO4 2−
, which is the main species in solution [39].
At more positive potentials, two more anodic peaks are observed, which are
tentatively associated to the combination of deprotonation of hydrogen phosphate to
phosphate PO4 3−
and a concomitant change in coordination of phosphate to the (111)
surface [40].
The stationary voltammetric profile for the Pt(110) electrode is also shown in Fig. 3-4.
It exhibits a sharp peak at 0.23 V in the positive scan. This peak is also present in the
negative scan but at a slightly lower potential (0.17 V). At 0.35 V another couple of
reversible small peaks are also observed. From 0.35 V to 0.85 V no more features are
observed in the CV.
For Pt(100) two pairs of reversible peaks are observed between around 0.3 and 0.4 V.
The main peak at 0.4 V is preceded by a splitted peak at 0.3 V. Another broad
reversible adsorption state is visible between 0.45 and 0.6 V, similar to that observed
for this surface electrode in perchloric acid solutions. It was shown that the first peak
(0.35 V) corresponds to hydrogen adsorption on the terrace border/defects of the
surface similarly to alkaline electrolytes. The main peak (0.4 V) is due to
adsorption/desorption of H and concominant desorption/adsorption of anion
(phosphate) [34].
74 Chapter 3
3.3.2. Stepped surfaces vicinal to (100) pole
Since the differences between acid [31] and basic media (addressed before) are
significant, it is important to study intermediate pH values. In addition, information is
obtained about the role of adsorbing anions.
The voltammograms of the stepped surfaces on the (100) pole at pH 9 and 7 in
phosphate buffered solutions are presented in figure 3-5. In panel C, the
voltammograms of the Pt(100) electrode at three different pH’s are compared. As can
be seen, the major differences are located in the region above 0.4 V, whereas the
voltammetric profile remains almost constant below that potential. Additionally, the
peaks at 0.465 and 0.570 V shift in opposite directions. The peak at 0.570 V (pH 13)
shifts towards higher potential values when the pH is decreased and eventually
disappears at pH 7, whereas that at 0.465 V moves in the opposite direction and its
charge increases. From the shifts described above and the comparison with the
behavior observed for the Pt(100) electrode when sulfate anions are added to
perchloric acid solutions [40], it can be proposed that the peak at 0.465 V corresponds
to the competitive adsorption of hydrogen/anions and the peak at 0.570 V
corresponds to the adsorption of OH. It should be stressed that the reference
electrode used in this work (RHE) is sensitive to the proton concentration and,
therefore, the shifts just described are different from the simple nernstian
displacement of the adsorption potential and reflect the competitive behavior
between hydrogen / OH and anions. Finally, it is also interesting to note that the two
peaks at 0.395 and 0.29 V, visible in the voltammogram recorded in 0.1 M NaOH,
merge into one single peak at 0.3 V when pH is decreased to 7. An intermediate
situation is observed at pH 9.
Electrochemical surface characterization 75
-150
0
150
0.0 0.2 0.4 0.6 0.8
b
E vs. RHE/V
Aa
c
-150
0
150
j/µ
A c
m-2
Pt(100)
Pt(39,1,1)
Pt(29,1,1)
Pt(23,1,1)
Pt(15,1,1)
Pt(711)
Pt(311)
B
0.0 0.2 0.4 0.6 0.8
-100
0
100
pH=13
pH=9
pH=7
C
E vs. RHE/V
Figure 3-5 - Voltammetric profile of Pt(S) [n(100)×(111)] surfaces in 0.1 M phosphate buffer
pH=7 (A) and pH=9 (B), comparison between Pt(100) at pH 7, 9 and 13 (0.1M NaOH) (C). Scan
rate 50 mV/ s.
76 Chapter 3
For the stepped surfaces, the changes upon increasing the step density are similar to
those observed in 0.1 M NaOH. In all the cases, the current at potentials below 0.2 V
increases with the step density of the surface, revealing that, independently from the
pH, hydrogen adsorption on the steps occurs in this lower potential window, as has
been also proposed for acid media [41]. At pH 7, fewer peaks are observed in the CV,
probably as a consequence of the adsorption of phosphate anions. Regarding the pH
dependence of peak potentials, for processes associated with terrace sites (peaks at
0.465 and 0.570 V), the evolution is equivalent to that observed for the Pt(100)
electrode. Figure 3-6 shows the variation of peak potentials with the pH of the solution
for the Pt(15 1 1) surface. The peaks at 0.32-0.39 V and 0.42-0.47 V move less that
60 mV per pH unit in the SHE scale (around 50 mV). Since anion adsorption is expected
to be pH independent in the SHE scale at pH values in which the acid is completely
dissociated (i.e., shift 59 mV per pH unit in a RHE scale) and hydrogen adsorption
should shift 59 mV per pH unit (i.e., should be pH independent in the RHE scale), we
conclude that hydrogen as well as anion competitive adsorption are involved in those
peaks, since intermediate shifts are observed. On the other hand, the peak at 0.295 V
does not change its potential in the RHE scale, that is, the peak shifts 59 mV per pH
unit in the SHE scale. Thus, it can be suggested that this process is related only to
hydrogen adsorption. It should be noted that the changes in this region between acid
and basic media are very small (there is only a slight shift in the signals), which also
suggest that the process is mainly related to hydrogen adsorption.
Electrochemical surface characterization 77
7 8 9 10 11 12 13
0.30
0.35
0.40
0.45
0.50
0.55
Ep
ea
k vs.
RH
E/V
pH
Figure 3-6 – Peak potential for main peaks in the voltammogram of Pt(15 1 1) surface in
phosphate buffer solutions and NaOH plotted as a function of the pH.
As a summary, it can be said that the processes below 0.3 V correspond to the
adsorption of hydrogen, whereas those occurring above 0.55 V correspond to the
adsorption of OH at pH=13. In the region between both values, the adsorption of
hydrogen and anions contribute to shape the different peaks in the voltammogram.
3.4. Irreversible Adsorbed Bismuth adatoms
3.4.1. Adatoms as catalysis promoters
Irreversibly adsorbed adatoms on well defined single crystal surfaces can be used to
modify the surface composition in a controlled way that leads many times to an
enhancement of the overall reactivity of the electrodes [42, 43]. Different effects were
described, depending on the platinum substrate and the adatom modifier, as
explained below. One of the particular characteristics of a group of these irreversibly
adsorbed adatoms is that they undergo a surface redox reaction in which the oxidation
78 Chapter 3
state of the adatom changes. In this way, the composition of the adatom layer is also
changing at both sides around the potential region in which the redox process takes
place although the adatom coverage remains constant.
A successful example of the enhancement of the catalytic properties of platinum well
defined surfaces by the deposition of irreversible adsorbed adatoms is found on the
oxidation of small organic molecules like HCOOH [44]. Surface modifications with sp
elements have greater effects on the electrocatalysis of formic acid [43-45] than on
methanol oxidation [46]. Most of these reactions proceed through a dual-path
mechanism. This means that at least two parallel pathways exist: one leading to the
formation of a poisoning intermediate and another producing a stable and soluble
oxidized compound, ideally CO2. Adatom modification can affect each of these
pathways independently, the effect normally sought being inhibition of the poisoning
reaction and enhancement of the direct oxidation reaction.
The effect of these surface modifiers on the electrocatalytic activity of the material can
be divided in three main general categories [47-50]:
- Change in the Electronic Properties of the Substrate. In this case the new
electronic properties induced by the adatom adsorption change the adsorption
energy and intramolecular bond energy in adsorbed reactants and intermediates.
In addition, some strain affects can also occur when either the adatoms or the
host metal atoms are forced to adopt positions different from the equilibrium
position in the bulk materials.
- Ensemble or third body effect arising from the selective blockage of a particular
adsorption site by the deposited adatoms. This phenomenon is particularly
interesting when the reaction contains parallel paths that involve adsorption of
species that can block the surface and inhibit the desired reaction but requires
more neighbor surface sites than the direct path.
Electrochemical surface characterization 79
- Bifunctional Catalysis. It happens when the adatoms provide suitable adsorption
sites for a second reactant necessary for the reaction to proceed, while the main
reactant still adsorbs on the free sites of the substrate.
More recently, Norskov and co-workers [51], described the modification on the
electronic properties of bimetallic surfaces as result of the changing on the strain and
ligant effects. The average bond lengths between the metal atoms in the supported
monolayer surface are typically different from those in the bulk metal constituents
resulting in changes due to strain. In addition, in heterometallic bonding interactions,
called “ligant effects”, between the surface atoms and the substrate can result in
modification of the surface electronic structure. The combination of this two effects,
the strain and ligant effects, in the formation of the bimetallic surface, would lead into
changes in the surface d-band width manifestated in the interatomic matrix element
describing bonding interactions between the atoms and its nearest-neighbors.
3.4.2. Procedure for Irreversible Adsorption of the Adatom
The spontaneous irreversible adsorption of many elements of the p-block of the
periodic table on the surface of a platinum electrode can be done just by immersing
the electrode in a solution containing the soluble salt of the corresponding element,
without an external supply of electricity [52-57]. The electrode can then be rinsed and
transferred to an electrochemical cell that does not contain the corresponding ion of
the deposited element, which remains on the surface, irreversibly adsorbed.
The process that causes the irreversible adsorption in the surface is not totally clarified
and there are several possibilities proposed in the literature. One of the possibilities is
the formation of local cells, with the ion of the adatom being reduced and either
hydrogen [58] or platinum [52] being oxidized. However in this case, if the local cell
formation is spontaneous, the anodic reaction should take place at a potential lower
than that required for the reduction of the adatom. For hydrogen oxidation, this
condition is plausible, although spontaneous deposition of adatoms also occurs even
80 Chapter 3
when the surface is not expected to be pre-covered by hydrogen. On the other hand,
the possibility that Pt-OH is formed at low enough potentials is unexpected, especially
in the case of Pt(111) surfaces. For this reason, it has also been proposed that the
surface oxidation could take place at defect sites [52]. In the case of sulfur, for
example, oxidative adsorption has also been reported [59, 60]. A disproportionation
reaction was, alternatively proposed for the case of Sn [61]. Another possibility is an
initial physisorption (adsorption without charge transfer) of the adatom when the
electrode is put in contact with the solution. The physisorbed species will remain on
the surface after rinsing and the reduction would take place in the electrochemical
cell.
3.4.3. Voltammetric Characterization of the Modified Electrode
The presence of an adatom on the surface can be accessed easily by using cyclic
voltammetry. Its presence induces some changes on the characteristic profile of the
surface. In figure 3-7, the typical cyclic voltammograms for Bi adsorption on Pt(111)
are shown for both perchloric and sulphuric acids. The modification on the electrode
can be observed by the suppression of the hydrogen and anion adsorption processes
characteristic of clean Pt and by the presence of two new peaks at 0.63-0.67 V. The
voltammetric profile remains stationary over a wide potential range (normally from 0
to 0.8–1.0 V), indicating that the adatom remains stable on the surface. However,
desorption of the adsorbed adatom takes place if the upper potential limit is increased
further.
Electrochemical surface characterization 81
0.0 0.2 0.4 0.6 0.8 1.0
-210
-140
-70
0
70
140
210
E (V) vs RHE
j (µ
A/c
m2)
0.1M HClO4
0.0 0.2 0.4 0.6 0.8 1.0
0.1M H2SO
4
Figure 3-7 – CVs of Pt(111) modified with Bi adatoms in perchloric and sulphuric acid at 50
mV/s. θBi = 0.19.
The new redox peak has been attributed to the oxidation/reduction of the deposited
adatoms [52] according to
Pt� � M � nHO � Pt� � M OH�� � nH� � ne
or
Pt� � M �n
2HO � Pt� � MO�/ � nH� � ne
where m is the number of Pt sites blocked by the adatom and n is the number of
electrons transferred in the oxidation of one adatom. For Bi adatoms it has been
proposed that each adsorbed adatom blocks three Pt(111) sites and that there are 2
electrons involved in the reaction [52, 55].
82 Chapter 3
The Bi redox peaks shift 60mV per pH unit what supports that this process corresponds
to the oxidation of adsorbed Bi either by OH or by oxide formation without desorption
to the solution [52].
The charge of the redox processes undergone by the modified electrodes are related
with the amount of deposited bismuth. By comparing the charge of this redox process
with the blockage of hydrogen adsorption, it is possible to calculate the ratio between
the number of electrons transferred in this process and the number of blocked sites on
the Pt substrate. The Bi coverage is given by:
� ����
��� ���� (1)
where qAd is the charge density involved in the adatom oxidation process and qPt(hkl) is
the charge density corresponding to the transfer of one electron per Pt atom on the
surface [241 µC/cm2 for Pt(111)].
The charge density corresponding to hydrogen adsorption on the free Pt sites is given
by
�� � ��
!" 1 � 3�� (2)
where �� !" is the maximum hydrogen charge density attained in the absence of the
adatom. From the two latest equations it is possible to eliminate �, obtaining the
following relation between hydrogen and adatom charge densities:
�� � ��
!" � %
�&'()
��� ���� �*+ (3)
Then, the maximum adatom charge density corresponding to full blockage of hydrogen
adsorption is
�*+ �
% �,- ./0� (4)
Electrochemical surface characterization 83
From equation 2, �max for Bi is 0.33. This maximum coverage is in agreement with
identified structures characterized by UltraHigh Vacuum (UHV) measurements on
adlayers prepared both in UHV [62] and electrochemical environments.
In figure 3-8, the qBi vs qH for several Bi coverages are plotted. In agreement with
previous publications [63, 64] the plot is a straight line. When qBi =0 the charge is ≈ 160
µC/cm2 which is the well-accepted value for ��
!" for Pt(111) in 0.1 M H2SO4. On the
other hand, the extrapolated value for �12 345 is ca. 160 µC/cm
2, also in agreement
with the values published previously [65].
-10 0 10 20 30 40 50 60 70 80 90
60
80
100
120
140
160
qH (
µC
/cm
2)
qBi
(µC/cm2)
Figure 3-8 – Plot of the charge integrated under the Bi redox process as a function of the
hydrogen remaining charge for a Pt(111)/Bi electrode in 0.1M H2SO4.
The stoichometry of Bi adsorption on Pt(111) do not seems to depend on the
adsorption of anions on the surface or supporting electrolyte pH. The same values of
maximum coverage and charge had been found in perchloric [64] and sulphuric acids
[65].
84 Chapter 3
However to assure that the relations are also valid for neutral phosphate buffers the
system was also evaluated in 0.1M phosphate buffer pH 7.2. The cyclic voltammetry
results are presented in figure 3-9.
Similarly to acid supporting electrolytes, increasing Bi coverage on the Pt(111) surface
causes a decrease on the currents observed on the hydrogen adsorption region
(>0.4 V) together with the increasing of the Bi redox peaks at 0.63 V (figure 3-9).
0.0 0.2 0.4 0.6 0.8 1.0-15
-10
-5
0
5
10
15
E (V) vs RHE
j (µ
A/c
m2)
Figure 3-9 – CV´s obtained for Pt(111)/Bi with different coverages in 0.05 M of H2NaPO4 +
0.05 M HNa2PO4 at 50 mV/s.
When the qBi vs qH for several Bi coverages is plotted, a straight line is obtained (Figure
3-10). When no Bi is on the surface the hydrogen charge is ≈ 130 µC/cm2. This value is
in a good agreement with the results reported for Pt(111) in phosphate buffer
electrolyte at pH 7.20. The value obtained for the �12 345 is ≈150 µC/cm
2 suggesting
that the Bi stoichometry for this surface at pH 7 is the same than the reported
previously in acidic solutions [64, 65].
Electrochemical surface characterization 85
20 40 60 80 100 120 140
0
20
40
60
80
100
120
q B
i / µ
C/c
m2
q H µC/cm
2
Figure 3-10 - Plot of the charge integrated under the Bi redox process as function of the
hydrogen remaining charge for a Pt(111)/Bi electrode in 0.05 M of H2NaPO4 + 0.05 M
HNa2PO4.
86 Chapter 3
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4Nitrate reduction at Pt(100)
single crystal and
preferentially oriented
nanoparticles in neutral
mediamedia
4. Nitrate reduction at Pt(100) single crystals and
preferentially oriented nanoparticles in neutral
media
4.1. Concepts
As pointed out in the Introduction of this thesis, the imbalance of nitrogen cycle has a
special importance in what concerns to the safety of water resources such as rivers,
seas and groundwaters.
For decades, nitrate concentrations in many rivers and drinking water supplies have
been closely monitored in developed regions of the world, and analysis of these data
confirms a historic rise in nitrogen levels in surface waters [1]. Increased
concentrations of nitrate have also been observed in groundwater in many agricultural
regions. This increase brings important problems since levels of nitrates in drinking
water raise significant human health concerns, especially for infants [1].
For these reasons, with the aim of decontamination of water resources,
electrochemical studies of nitrogenated molecules in neutral media have a special
relevance. However, the only few studies at neutral pH that exist at present in this
regard are related to nitrate reduction on Cu electrodes [2].
Among the three basal planes of platinum, Pt(100) has emerged as the most active
surface for breaking the N-O bond under UHV conditions [3]. For NO and
hydroxylamine reduction on this surface, in acidic media, HNOads (or alternatively
NOHads) has been proposed as the intermediate species [4]. The ability of Pt(100) to
catalyze the electrochemical transformation of nitrogen containing compounds was
also demonstrated with the nitrite reduction reaction in alkaline media. Interestingly,
N2 was found in this case among the reaction products [5].
92 Chapter 4
In this Chapter, results on the electrocatalytic reduction of nitrate in neutral solutions
on Pt(100) electrodes using cyclic voltammetry and in situ infrared spectroscopy will
be discussed. The sensitivity of the reaction to the crystallographic order of the surface
is studied through the controlled introduction of defects by using stepped surfaces
with (100) terraces of different length separated by monoatomic steps, either with
(111) or (110) symmetry. The real applicability of this study will be shown by using
dispersed catalysts. In this case, the use of preferentially oriented Pt nanoparticles
evidences the sensitivity of this reaction to the surface structure of the catalyst.
The results show that nitrate reduction occurs mainly on well defined (100) terraces in
the potential region where H adsorption starts to decrease, allowing the nitrate anion
to access the surface. Adsorbed NO has been detected as a stable intermediate in this
media. An oxidation process observed at 0.8V has been identified as leading to the
formation of adsorbed NO and being responsible for a secondary reduction process
observed in the subsequent negative scan. Using in situ FTIRS, ammonium was found
to be the main product of nitrate reduction. This species can be oxidized at high
potentials resulting in adsorbed NO and nitrate (probably with nitrite as intermediate).
4.2. Nitrate reduction on Pt(100) in neutral media
Nitrate reduction in neutral media turns out to be an extremely structure sensitive
reaction and, among the three basal planes of platinum, significant nitrate reduction
currents were only obtained for Pt(100) surface. The other basal planes, Pt(111) and
Pt(110) were also studied but no significant differences were observed between the
voltammograms with or without nitrate (fig. 4-1). Therefore, they were not
investigated further.
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 93
0.0 0.2 0.4 0.6 0.8
-200
-150
-100
-50
0
50
100
150
200
absence of nitrate
presence of nitrate
E (V) vs RHE
j (µ
A/c
m2)
Pt(111)A
0.0 0.2 0.4 0.6 0.8
-200
-150
-100
-50
0
50
100
150
200
B
absence of nitrate
presence of nitrate
Pt(110)
Figure 4 -1 - CVs obtained for A) Pt(111) and B) Pt(110) in 0.05 M NaH2PO4 + 0.05 M Na2HPO4
blank solution (thin lines) and in the solution containing 0.1 M NaNO3 (thick lines), 50 mV s-1
.
Figure 4-2 shows the CVs obtained for nitrate reduction on Pt(100) in 0.1 M phosphate
buffer solution, pH 7.2, at 50 mV s-1
. Several features can be identified in the
continuous CV of nitrate reduction on Pt(100). In the positive going sweep, after the
double peak of H adsorption on the terrace edges at 0.30V [6] a sharp reduction peak
(0.41 V) can be observed. Following this first peak, an oxidation process is observed
centred at 0.85 V. In the negative scan, different reduction peaks can be observed. The
first and less intense is centred at 0.53 V. This peak was revealed to be associated with
the oxidation at 0.80 V in the positive scan, suggesting that it corresponds to the
reduction of the product formed in the oxidation at high potentials. This same redox
couple has been observed, also for this surface but in acidic media, after partial NO
stripping [7]. This surface redox process has not been identified and was tentatively
assigned to reactions involving hyponitrous acid [7].
94 Chapter 4
At lower potentials, between 0.50 and 0.20 V at least three additional reduction
features can be observed at 0.40, 0.36 and 0.20 V.
0.0 0.2 0.4 0.6 0.8 1.0
-320
-240
-160
-80
0
80
160
absence of NO-
3
presence of NO-
3 (higher E 0.9V)
presence of NO-
3 (higher E 0.85V)
j (µ
A c
m-2)
E (V) vs RHE
Figure 4-2 - CVs obtained for Pt(100) in 0.05 M NaH2PO4 + 0.05 M Na2HPO4 blank solution
(dotted line) and in the solution containing 0.1 M NaNO3 (solid lines), 50 mV s-1
. Two
voltammograms in the presence of nitrate are shown corresponding to two different high
potential limits.
If the CV is recorded at a slower scan rate, 2 mV s-1
(Figure 4-3), the peaks around 0.4 V
in both scans become dominant, and the main peak in the positive going sweep is
splitted into two peaks (0.39 and 0.42 V).
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 95
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
-50
-40
-30
-20
-10
0
10
j (µ
A c
m-2
)
E (V) vs RHE
Figure 4-3 - Nitrate reduction on Pt(100) in 0.05 M NaH2PO4 + 0.05 M Na2HPO4 + 0.1 M NaNO3
at 2 mV s-1
The characteristic voltammetric profile depicted for nitrate reduction in neutral
solutions at Pt(100) is very similar to those reported previously in acidic media [8-10].
In both perchloric and sulphuric acids, the sharp reduction peak in the positive sweep
is observed, although slightly shifted to a lower potential (0.32 V vs RHE), in
comparison with both the RHE reference and the main hydrogen adsorption peak. This
may suggest that either the mechanism of the reduction change with the pH or that it
shifts with the pH with a slope smaller than 59 mV/decade. This contrasts with the
thermodynamic shift expected for nitrate reduction to ammonium (see below)
according to:
NO3- + 10 H
+ + 8 e � NH4
+ + 3 H2O.
From the coincidence of the peak potential in acid media with the hydrogen
desorption from the steps this process was attributed to nitrate reduction at specific
defect sites [8]. However, it was reported recently [11] that in acidic media, nitrate
reduction on Pt stepped surfaces with (100) terraces showed that the reduction
96 Chapter 4
process decreases with the introduction of steps on the surface, suggesting that this
reduction is associated with terraces sites and not with defects. Moreover, it was
shown in the previous chapter that the first voltammetric feature at 0.28 V in the blank
voltammogram of Pt(100) in phosphate buffer corresponds to H adsorption on terrace
edges of the surface [6] and remain unaffected by the presence of nitrate in solution.
The main peak at 0.40 V is due to H desorption and anion adsorption on the (100)
terraces. These results suggest that, in phosphate media, nitrate reduction is occurring
not on defect sites but on the well-ordered terraces when the decreasing H coverage
attains a sufficiently low value. This conclusion will be confirmed with the results from
the stepped surfaces reported below.
Regarding the broad features in the negative going sweep, they were also observed in
acidic media, but the oxidation peak at high potentials and consequently the reduction
peak at 0.5 V were only observed in perchloric acid and not in sulphuric acid. This
suggest that it is a process strongly influenced by competitive anion adsorption and
was attributed to NO formation and subsequent adsorption [8].
In order to obtain more information about the possible presence of adsorbed NO
some NO stripping experiments were performed on Pt(100) in this media.
4.3. NO stripping on Pt(100) in phosphate buffer
The behaviour of Pt(100) covered with a saturated layer of NO in neutral solutions has
been previously reported [12]. In that case, NO was adsorbed either from nitrite
solutions in acid media or from NO saturated solutions. As a difference with the nitrite
anion, nitrate does not spontaneously decompose in solution. Therefore, NO
concentration in a nitrate solution should be negligible and NO coverage on the
surface is expected to be low. For this reason, the study of submonolayers of NO at
low coverage is more relevant for the present case and we complement here previous
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 97
work on saturated NO adlayers with the study of the stripping of adsorbed NO at low
coverage on Pt(100). Representative voltammetric results for two different NO
coverages are shown in Figure 4-4.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-60
-40
-20
0
20
E (V) vs RHE
j (µ
A c
m-2
)
θNO
= 0.37
θNO
= 0.49
Figure 4-4 - NO reductive stripping on Pt(100) in 0.05 M NaH2PO4 + 0.05 M Na2HPO4 , 2 mV s-1
,
and subsequent CV of the clean surface (positive scan) for a high NO coverage (0.49, dotted
line) and a low NO coverage (0.37, solid line). NO coverage is calculated from the charge
under the stripping peak with consideration of the hydrogen charge involved.
The results show that when the Pt(100) surface is saturated with a NO layer (Fig. 4-4),
the surface is totally blocked in the potential region between 0.5-0.7 V. When the NO
is stripped (main peak below 0.10 V), the blank CV is almost completely recovered in
the positive going sweep. The same experiment was repeated with a NO adlayer of
lower coverage (Fig. 4-3, solid line). Although, the surface still remains fully blocked
between 0.50 and 0.70 V, a significant shift of the main NO reduction peak is observed
that now appears at 0.3 V. As before, the blank voltammogram is almost recovered in
the positive going sweep.
98 Chapter 4
These results suggest that most of the redox peaks observed during nitrate reduction
on this surface can be attributed to the presence of a low coverage NO adlayer. These
broad peaks at different potentials can be explained as being the stripping of NO
patches with different local coverage, since the experiment in figure 4-4 demonstrates
that the potential of the NO stripping peak depends strongly on the coverage of the
adlayer.
4.4. Nitrate reduction on Pt stepped surfaces with (100)
terraces
The reduction of nitrate in neutral media on stepped surfaces containing (100)
terraces will be described in this section with the aim of characterizing the effect of
surface defects on the reaction. The effect of the Pt(100) terrace width was studied
with stepped surfaces having both (111) and (110) monatomic steps in order to point
out long range order effects, as well as the modification of its reactivity due to the
symmetry of the step.
Figure 4-5 shows the characteristic voltammograms for several Pt(S)[n(100)x(111)]
stepped surfaces in 0.05 M NaH2PO4 + 0.05 M Na2HPO4, pH 7.2, in the absence (A) and
in the presence (B) of 0.1 M NaNO3. These stepped surfaces have Pt(2n-1,1,1) Miller
indices, where n is the number of atoms in the terrace (terrace width).
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 99
0.0 0.2 0.4 0.6 0.8 1.0-300
-200
-100
0
100
200
300
Pt(100)
Pt(29,1,1)
Pt(23,1,1)
Pt(15,1,1)
Pt(711)
j (µ
A c
m-2
)
A
0.0 0.2 0.4 0.6 0.8 1.0
Pt(100)
Pt(29,1,1)
Pt(23,1,1)
Pt(15,1,1)
Pt(711)
E (V) vs RHE
B
Figure 4-5 - Voltammetric profiles of Pt(100), Pt(29,1,1), Pt(23,1,1), Pt(15,1,1), and Pt(711) in
A) 0.05 M NaH2PO4 + 0.05 M Na2HPO4; B) 0.05 M NaH2PO4 + 0.05 M Na2HPO4 + 0.1 M NaNO3.
Scan rate 50 mV s-1
.
The results show that decreasing the width of the (100) terraces also decreases the
surface activity towards nitrate reduction. The main features in both scans are mostly
the same for the different surfaces, with the only effect of the introduction of the
steps being the decrease of current intensities of most of the voltammetric peaks.
Clearly, the main peak in the positive sweep (0.40 V) corresponds to the nitrate
reduction on the well-ordered (100) terraces, and, when the terrace width is equal or
lower than 4 atoms, the reduction current in this potential region is almost negligible
at this scan rate (50 mV s-1
).
The disruption of long-range order with steps, even for wide (100) terraces, reduces
the activity of the Pt surface towards nitrate reduction. This effect can be tentatively
explained considering the progressive diminution of large atomic domains [13] due to
the presence of steps that would stabilize adsorbed intermediates, which in turn
100 Chapter 4
would not be able to react further. A similar explanation of the role of two-
dimensional domains was proposed for ammonia oxidation [14]. It is also clear from
this figure that nitrate reduction in the positive sweep starts always after the peak at
0.28 V which corresponds to H desorption on terrace edges. According with the results
presented in Chapter 3 [6] at this pH, the peak at 0.40 V on the blank CV of Pt(100) has
both contributions from the H desorption and anion adsorption on the terraces. This
suggests that nitrate reduction occurs when the H coverage starts decreasing (and
anion adsorption starts increasing) and that the balance between these two processes
is the reason for the characteristic reduction peak at 0.40 V in the positive sweep.
The oxidation peak at 0.80 V seems to be less affected by the decrease of the terrace
length except for the shortest studied terrace (n=4) where the peak almost completely
disappears. It was observed in acidic media for other stepped surfaces [15], that NO is
stabilized by increasing the step density on the surface and, for this reason, the redox
couple at high potentials is no longer observed for high step densities.
A similar general result is observed in Figure 4-6, which shows the voltammograms
recorded for Pt(S)[n(100)x(110)] stepped surfaces, having Pt(n,1,0) Miller indices,
under the same conditions as in the previous series.
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 101
0.0 0.2 0.4 0.6 0.8 1.0-300
-200
-100
0
100
200
Pt(100)
Pt(15,1,0)
Pt(10,1,0)
Pt(710)
Pt(410)
j (µ
A c
m-2
)
A
0.0 0.2 0.4 0.6 0.8 1.0
Pt(100)
Pt(15,1,0)
Pt(10,1,0)
Pt(710)
Pt(410)
E (V) vs RHE
B
Figure 4-6 - Voltammetric profiles of Pt(100), Pt(15,1,0), Pt(10,1,0), Pt(710), and Pt(410) in A)
0.05 M NaH2PO4 + 0.05 M Na2HPO4; B) 0.05 M NaH2PO4 + 0.05 M Na2HPO4 + 0.1 M NaNO3.
Scan rate 50 mV s-1
.
Similarly to what was observed for the stepped surfaces with (111) steps, the
introduction of (110) steps on the surface decreases their capacity to reduce nitrate
anions. Moreover, no significant changes in the CV shape are observed when the
terrace width decreases.
The voltammetric results suggest that the nitrate reduction reaction on Pt(100) vicinal
surfaces in neutral media is not affected by the symmetry of the steps (fig.4-7), thus
reinforcing the conclusion that this reduction process takes place only on (100) terrace
sites.
102 Chapter 4
0.0 0.2 0.4 0.6 0.8 1.0-200
-150
-100
-50
0
50
100
150
200
j (µ
A/c
m2)
E (V) vs RHE
Pt(1511)
Pt(710)
A
0.0 0.2 0.4 0.6 0.8 1.0-200
-150
-100
-50
0
50
100
150
200
B
Pt(1511)
Pt(710)
Figure 4-7 - Voltammetric profiles of Pt(15 1 1) and Pt(710) in A) 0.05 M NaH2PO4 + 0.05 M
Na2HPO4; B) 0.05 M NaH2PO4 + 0.05 M Na2HPO4 + 0.1 M NaNO3. Scan rate 50 mV s-1
.
All of these results are summarized in Figure 4-8, which shows the plots of peak
current density values measured at 50 mV s-1
versus the step density for both the
Pt(S)[n(100)x(111)] (solid squares) and Pt(S)[n(100)x(110)] (open circles) series. As the
number of step sites increases, the current density diminishes probably because the
concentration of active intermediates on the terraces decreases, being substituted by
the stronger adsorption on step sites. Peak currents are always higher for stepped
surfaces with (111) step symmetry. However, the effect of step symmetry is small in
comparison to the decrease due to shortening the terrace length, and is probably
related to the availability of the terrace sites adjacent to the different type of step
sites.
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 103
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35-300
-250
-200
-150
-100
-50
0
j (µ
A c
m-2
)
Step density (nm-1)
Figure 4-8 - Correlation between the current density at 50mV s-1
involved in the reduction
peak at 0.4 V in the positive scan and step density for Pt(S)[n(100)x(111)] (solid squares) and
Pt(S)[n(100)x(110)] (open circles) surfaces in 0.05 M NaH2PO4 + 0.05 M Na2HPO4 + 0.1 M
NaNO3.
In conclusion the reduction of nitrate on platinum is clearly a structure sensitive
process that takes place almost exclusively on Pt(100) terrace sites.
4.5. Spectroelectrochemical results – FTIRS on Pt(100)
In order to identify the chemical nature of the species involved in nitrate reduction on
(100) terraces in neutral media some in situ IR experiments were made on the (100)
basal plane for this electrode, which should exhibit the greatest reactivity.
In the following spectra, positive bands correspond to the products formed at the
sample potential, during the nitrate reduction, while negative bands are due to the
consumption of species present at the reference potential. The contact of the
electrodes with the nitrate solution was made at a controlled potential of 0.10 V
where, apparently, no adsorption or reaction process occurs. This potential was
maintained until the electrode was pressed against the CaF2 window. After collecting
104 Chapter 4
the reference spectrum, the potential was stepped to progressively higher sample
potential values, where the corresponding sample spectra were collected. Those
potentials are labelled in the respective figures and captions.
To verify the vibrational frequencies of the possible stable compounds involved in the
nitrate reduction, vibrational spectra of solution species were acquired with an ATR
configuration. The spectra obtained for nitrite, nitrate, ammonium and hydroxylamine
in neutral media, using the phosphate buffer as reference, are given in Fig 4-9. In this
figure, negative bands corresponds to phosphate vibrations in the reference spectra
[16]. Positive bands around 1100 cm-1
in the spectra of hydroxylamine and ammonium
salts are due to the sulfate present in the salt as the counterion. In the nitrate spectra,
the band at 1351 cm-1
is related to the asymmetric N–O stretching of uncoordinated
nitrate anions [17]. For nitrite the band at around 1280 cm−1
could be related to the
symmetric ν(NO2) [18]. In neutral media, almost all the ammonia is protonated and the
ammonium cation is largely predominant, the ratio of the ammonium to the ammonia
concentration is equal to 100:1 at pH 7. According to this, it is not surprising that the
main band in the ammonia spectra is the N–H bending mode for ammonium
(1460 cm-1
). Hydroxylamine did not present any further band in the region under
study. Although literature data [19] report bands at 1500 and 1180 cm-1
for solid
hydroxilamine, no such bands are observed under the present conditions. The band at
1180 cm-1
could be redshifted and overlapped with the sulphate band at 1100 cm-1
,
since this band appears slightly blue shifted and with a shoulder at higher frequencies.
What is most important is that clearly, no bands are observed around 1500 cm-1
and
therefore bands observed in this region from the products of nitrate reduction should
be better assigned to ammonium and not to hydroxylamine formation.
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 105
2500 2000 1500 1000
1115
Wavenumbers (cm-1)
1237
1351
ammonia
nitrate
nitrite
hydroxylamine
1451
Figure 4-9 - Characteristic spectra of solution species measured in ATR configuration of 0.1 M
sodium nitrite, 0.1 M sodium nitrate, 0.1 M ammonium sulphate and 0.1 M
hydroxylamonium sulphate in 0.05 M Na2HPO4 + 0.05 M NaH2PO4, 100 interferograms and 8
cm−1
.
In figure 4-10, the results obtained for nitrate reduction on phosphate buffer pH 7.2 at
Pt(100) are shown. From the spectra, it is possible to identify 4 major bands at 1640,
1590, 1477 and 1355 cm-1
. The band at 1640 cm-1
corresponds to water bending, as a
result of water depletion from the thin layer. The other bands can be attributed
respectively to NO stretching mode, NH vibrations and NO3 vibrations.
As expected, in the positive going sweep, a negative band at 1355 cm-1
can be
observed. This band corresponds to the reduction of nitrate, in the major peak of the
CV presented previously, and thus corresponds to its depletion from the thin layer. A
small positive band at 1477 cm-1
can be observed at the same time as nitrate is being
reduced. This band is due to ammonia (ATR from fig. 4-9) formation from the
reduction of nitrate. Unfortunately, no other bands are observed in the spectra
suggesting that any other product of nitrate reduction on Pt(100) in phosphate buffer
106 Chapter 4
pH 7.2 is not IR active. At 0.5 V the negative band is still observed but less intense. It
should be borne in mind that due to the thin layer configuration employed for the
acquisition of the spectra, diffusion in and out of the thin layer is very slow and the
observation of the nitrate band at 0.5 V does not mean that nitrate is still being
reduced but that the depletion from the thin layer has not been totally recovered.
2400 2200 2000 1800 1600 1400 1200
0.4 V
1355
1477
negative going sweep
Ab
sorb
ance
a.u
.
0.8 V
0.7 V
0.5 V
0.3 V
0.001 a.u.
positive going sweep
1590
2400 2200 2000 1800 1600 1400 1200
0.3 V
1355
1477
0.1 V
Wavenumbers (cm-1)
0.8 V
0.7 V
0.5 V
0.4 V
1590
0.001 a.u.
Figure 4-10 - Spectra for nitrate reduction on Pt(100) in 0.05 M NaH2PO4 + 0.05 M Na2HPO4 +
0.1 M NaNO3, 200 interferograms, 8 cm-1
. The same reference spectrum, taken at Eref 0.1V at
the beginning of the positive scan was used for both sweeps.
When the potential reaches 0.8 V, two positive bands are observed. The first at 1590
cm-1
corresponds to adsorbed NO [12] and the second to nitrate (1355 cm-1
). It can be
suggested that the peak at 0.8 V on the CV corresponds to the oxidation of a product
formed during nitrate reduction, which leads to NO formation. On the other hand, the
origin of the positive nitrate band observed in the spectra collected at this potential is
less clear. Nitrate can be formed from the oxidation of one of the reduction products
trapped in the thin layer, but it can also be transported into the thin layer by migration
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 107
as a consequence of the increasing value of the electrode potential. Under the present
experimental conditions, the transport number of nitrate is not negligible since its
concentration is the same as that of the supporting electrolyte.
In the negative going sweep, NO and nitrate bands are observed again at high
potentials (0.7-0.8 V). When the electrode potential reaches the lowest limit (0.4-0.1
V) the nitrate reduction is observed without any IR active product.
With the aim of distinguishing between nitrate migration and oxidative formation at
high potentials, the same experiment was repeated after increasing the supporting
electrolyte concentration to 0.5 M. The experimental parameters were identical
except the number of interferograms, which was raised to 500 scans to increase the
peak definition, and the sequence of applied potentials that was selected to better
elucidate the nature of the band at high potentials. The obtained results are presented
in Figure 4-11.
Figure 4-11 - Spectra for nitrate
reduction on Pt(100), 0.25 M NaH2PO4
+ 0.25 M Na2HPO4 + 0.1 M NaNO3, 500
interferograms, 8cm-1
. Different Eref
as indicated in the figure. The spectra
were collected consecutively starting
from the one at the top.
The spectra show that in the positive scan (top spectra in fig. 4-11), nitrate reduction
leads to ammonia formation. When the potential increases from 0.5 to 0.9 V, two
positive bands are observed, attributed to NO and nitrate formation. The intensity of
2400 2200 2000 1800 1600 1400 1200
0.2 V Eref
= 0.4 V Ab
sorb
ance
a.u
.
Wavenumbers (cm-1)
0.4 V Eref
= 0.2 V
0.9 V Eref
= 0.5 V
0.5 V Eref
= 0.9 V
1590 cm-10.0005 a.u
1355cm-1
1477 cm-1
108 Chapter 4
the latter is not affected by the increase of the concentration of the supporting
electrolyte. These results allow us to say that nitrate is really being produced and the
positive band and it is not just due to migration. It is possible that nitrite is formed
from ammonia oxidation and that this one is further oxidized to nitrate. The fact that
nitrate IR band is broad and that phosphate band appears around 1200 cm-1
makes
almost impossible to verify the presence of nitrite with this measurements. Formation
of nitrate from the reoxidation of its reduction products (ammonium or
hydroxylamine) has been also suggested in a recent study of nitrate reduction on
Pt(100) in acid media [11]. In the negative scan (bottom spectra in fig. 4-11), the
results reveal that when the potential is stepped from 0.9 to 0.5 V, the NO and nitrate
bands became negative, suggesting that these compounds are being consumed by
some chemical or electrochemical reaction. At lower potential, the same negative
bands appear but no other bands can be observed in the spectra.
Because it has been shown previously that the reduction of a NO partial adlayer on
Pt(100) leads to the same features than those observed when nitrate is present, the
NO stripping at low coverage was also examined by IR measurements, being the
results reported in Figure 4-12.
Figure 4-12 - Spectra for NO partial
layer stripping on Pt(100), 0.05 M
NaH2PO4 + 0.05 M Na2HPO4, 200
interferograms, 8 cm-1
. Eref
indicated in the figure. Spectra
were collected consecutively with
the same adlayer, starting with the
spectrum shown at the top.
2400 2200 2000 1800 1600 1400 1200
Abso
rban
ce a
.u.
Wavenumbers (cm-1)
0.2 V Eref
= 0.5 V
0.5 V Eref
= 0.9 V
0.9 V Eref
= 0.5 V
1590 cm-1
0.0005 a.u
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 109
The obtained results show that, apart from the water and the NO bands, no other
species can be identified in the spectra. In order to preserve the stability of the NO
adlayer, low potentials were initially avoided and the potential was first stepped
between 0.5 and 0.9 V. As a result, a positive band is observed in the positive scan,
which becomes negative when the potential is decreased, corresponding to adsorbed
NO. No nitrate band is observed, what allows us to say that the nitrate formation on
the previous conditions is due to the oxidation of a precedent product of nitrate
reduction at lower potentials, likely ammonium. When NO is stripped no IR active
products can be observed, either because their concentration is below the detection
limit or because they are not IR active.
The ammonium oxidation on neutral media with Pt(100) was also investigated with the
aim to understand the observed nitrate formation at high potentials. The results are
presented in figure 4-13. The spectrum obtained for ammonium oxidation after
adsorbing a partial layer of NO on the electrode is also shown in the figure.
The results clearly show that ammonium oxidation on neutral pH produces nitrate as
marked by the positive band at 1355 cm-1
. The negative band at 1477 cm-1
is due to the
NH4+ consumption. The voltammetry recorded in these conditions reveals a strong
oxidation peak above 0.7 V, in the same potential region where the nitrate band is
observed. Remarkably no NO band is observed under these conditions indicating that
NO is not formed from ammonium oxidation in neutral media.
110 Chapter 4
2200 2000 1800 1600 1400 1200
B
Ammonium + NO
partial layer
1355cm-1
1477cm-1
Wavenumbers (cm-1)
0.005 a.u.
1590cm-1
Ammonium
A
0.0 0.2 0.4 0.6 0.8 1.0-100
-50
0
50
100
150
200
250
j(µ
A c
m-2)
E (V) vs RHE
Figure 4-13 - A) Spectra obtained for a potential step from 0.5 (Eref) to 0.9 V (Esample) in 0.01 M
(NH4)2SO4 in 0.05 M NaH2PO4 + 0.05 M Na2HPO4 in the presence and absence of low
coverages of NO on the surface, 100 interferograms at 8 cm-1
; B) CV for ammonium oxidation
on Pt(100) under de same conditions at 50 mV/s.
The top spectrum in the figure 4-13A was obtained after predosing NO from acidic
nitrite solution and clearly shows that, if present, NO band should be observed at 1590
cm-1
. These results allow us to say that nitrate formation at high potential, observed in
figures 4-10 and 4-11, comes from the oxidation of the ammonium produced at lower
potentials when nitrate is previously reduced on the Pt(100) terraces. The redox
couple at 0.5/0.8 V in the CVs of the figures 4-2 and 4-3 seems to have a different
origin, since it is associated with the presence of adsorbed NO at low coverages on the
surface but, unfortunately, the involved species can not be identified by IR.
Summarizing, nitrate on neutral media is reduced to ammonium at 0.40 V. This latter
one can be oxidized again at high potentials to nitrate. In the potential region between
0.5 and 0.9 V two processes overlap, the ammonium oxidation and the redox couple
associated with low NO coverages.
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 111
4.6. Nanoparticles
The use of nanoparticles has become a very popular topic in catalysis and
electrocatalysis aiming at the development of more active and more selective metal
catalysts [20, 21]. The present system is an interesting example of a structure sensitive
reaction that can benefit from the use of nanoparticles with preferential shapes. It has
been previously shown that cubic platinum nanoparticles can be synthesised and used
in electrochemical experiments [22]. Since it has been shown above that nitrate
reduction takes place preferentially on (100) terrace sites, this reaction is specially
suited to be tested with cubic nanoparticles.
The morphology of the nanoparticles and the synthesis procedures has been described
in Chapter 2. In addition to the microscopic characterization, to quantify the amount of
{100} domains present at the surface of the nanoparticles, Ge irreversible adsorption
analysis has been also performed, as described elsewhere [23]. The results showed
that the used nanoparticles had 52% of {100} domain sites.
A comparison between preferential {100} nanoparticles and polycrystalline
nanoparticles was also made. To prepare the polycrystalline nanoparticles, the same
sample of {100}Ptnano was taken to a high potential (1.45 V) for 10 cycles at 50 mV/s
in order to destroy the {100} well-ordered domains. This procedure was preferred
instead of using a sample of polycrystalline nanoparticles synthesised by a different
methodology to avoid the influence of electrode preparation on the results and also to
maintain almost the same particle size [24].
The blank CVs obtained for the Pt nanoparticles in 0.5 M H2SO4 as well as the CVs in pH
7.2 in the presence of nitrate are presented in Figure 4-14. In Figure 4-14A, the loss of
the {100} domains on the nanoparticles after high potential cycling can be easily
detected as a significant decrease of the current between 0.31 and 0.44 V, which is
characteristic of {100} terrace domains.
112 Chapter 4
0.0 0.2 0.4 0.6 0.8
-100
-50
0
50
100
Ptnano (100)
Disordered (100)Ptnano
E(V) vs RHE
j (µ
A/c
m2)
0.5M H2SO
4
50mV/s
A
0.0 0.2 0.4 0.6 0.8 1.0
-6
-4
-2
0
2
4
2mV/s
Ptnano (100)
Disordered (100)Ptnano
0.05M Na2HPO
4 +0.05M NaH
2PO
4
0.1M NaNO3
B
Figure 4-14 - CVs for {100}Ptnano and disordered {100}Ptnano in A) 0.5 M H2SO4 at 50 mV s-1
and B) 0.05 M NaH2PO4 + 0.05 M Na2HPO4 +0.1 M NaNO3 at 2 mV s-1
.
The results in figure 4-14B show that when there are enough {100} domains in the
nanoparticles, nitrate reduction processes are observed at 0.4 V in the positive and
negative going sweeps as well as the oxidation peak at 0.8 V. Again, this result
supports the idea that this reaction preferentially occurs at these specific domains.
Interestingly, if the surface order is destroyed by cycling to high potentials, the
reduction currents remarkably decrease as a consequence of the electrochemical
perturbation of the {100} terrace domains present at the surface of the nanoparticles.
Similarly to the results obtained for the stepped surfaces, it is possible to observe that
the reaction occurs after hydrogen adsorption at the terrace border (0.37 V). In
addition, as previously reported in figure 4-6, the peak current density on the
reduction process at 0.4 V is almost linear with the step density of the surface.
Although the comparison between well defined surfaces and nanoparticles is not
straightforward, due to several factors like relative size dimensions or the presence of
other surface orientation sites, the current density obtained in the {100}Ptnano was
compared with that obtained with the stepped surfaces (after current density
Nitrate reduction at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media 113
normalization for the scan rate). Thus, the current density obtained with the
{100}Ptnano would fit with that of a surface containing 4 atoms wide terraces. This
estimation is much lower than that expected from Ge irreversibly adsorption and
points out that size limited two-dimensional terraces on nanoparticles may have
different reactivity than that of the terraces on stepped surfaces [25] which have, at
least formally, an unlimited dimension in the direction parallel to the steps.
4.7. Conclusions
In this paper, the nitrate reduction in neutral media on Pt single crystals surfaces is
studied. The reaction has revealed as structure sensitive in this media, being the
Pt(100) surface the only basal plane catalytic for this reaction. The voltammetric
profile is very similar to that previously reported for acidic solutions. The results
suggest that the reaction occurs on the (100) well-defined terraces and when steps are
introduced on the surface, the catalytic activity decreases. The effect of step
introduction is nearly independent of its geometry.
The FTIRS study showed that ammonium is the main product of the nitrate reduction
at this pH, which can be later oxidized to nitrate in the subsequent positive scan
identified with this technique.
Preferentially oriented {100} nanoparticles were also used to reduce nitrate in this
media, showing a similar reactivity than that of some surfaces with short (100)
terraces.
114 Chapter 4
References
[1] P.M. Vitousek, J.D. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.H.
Schlesinger, D.G. Tilman, Eco. Applications, 7 (1997) 737.
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Electrochim. Acta, 58 (2011) 184.
[7] A. Rodes, R. Gómez, J.M. Orts, J.M. Feliu, A. Aldaz, J. Electroanal. Chem., 359 (1993)
315.
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555.
[10] F. ElOmar, R. Durand, J. Electroanal. Chem., 178 (1984) 343.
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[12] A. Rodes, V. Climent, J.M. Orts, J.M. Pérez, A. Aldaz, Electrochim. Acta, 44 (1998) 1077.
[13] A. Cuesta, ChemPhysChem, 12 (2011) 2375.
[14] F.J. Vidal-Iglesias, J. Solla-Gullón, V. Montiel, J.M. Feliu, A. Aldaz, J. Phys. Chem. B, 109
(2005) 12914.
[15] G.L. Beltramo, M.T.M. Koper, Langmuir, 19 (2003) 8907.
[16] F.C. Nart, T. Iwasita, J. Electroanal. Chem., 308 (1991) 277.
[17] G. Socrates, Infrared Characteristic group frequencies, John Wiley & Sons, Chichester,
1994.
[18] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination compounds,
John Wiley & Sons, New York, 1986.
[19] R.E. Nightingale, E.L. Wagner, The Vibrational Spectra and Structure of Solid
Hydroxylamine and Deutero?Hydroxylamine, AIP, 1954.
[20] R. Rioux, H. Song, M. Grass, S. Habas, K. Niesz, J. Hoefelmeyer, P. Yang, G. Somorjai,
Top. Catal., 39 (2006) 167.
[21] J. Solla-Gullón, F.J. Vidal-Iglesias, E. Herrero, J.M. Feliu, A. Aldaz, Electrochem.
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136.
5Nitrite electroreduction on
Pt(100) an (100) stepped
surfaces in alkaline media
5. Nitrite electroreduction on Pt(100) and (100)
stepped surfaces in alkaline media
5.1. Concepts
Among the noble metals, Pt has long been recognized for its high activity towards the
reduction of nitrogen-containing molecules [1]. These reactions have been studied in
detail, more recently also on well-defined monocrystalline electrodes [2-5]. Within this
field of study, the Leiden group has recently demonstrated a unique reactivity and
selectivity at Pt (100) surfaces for nitrite reduction to N2 [6]. The peculiar ability of Pt
(100) surfaces in reactions involving bond breaking and bond making [7] has also been
demonstrated for the selective oxidation of NH3 to N2 [7-12].
The reduction of nitrite to harmless N2 is particularly important for wastewaters
treatment. Valuable clues to unraveling the steps leading to N2 can be obtained by a
comparison of nitrite reduction with two processes belonging to completely different
research fields: the so-called “anammox” bacterial sewage treatment [1, 13, 14] and
the selective-catalytic reduction (SCR) [15] of NO by NH3 to N2 under UHV conditions at
a Pt(100) surface [16-18]. SCR conversion of NO to N2 at Pt(100) at high temperature
has attracted much interest in the literature [15] both concerning the experimental
behavior of this system under UHV conditions [15-18] and its theoretical simulation
[19]. Adsorbed NO plays a very important role: there is compelling evidence that the
hexagonal reconstruction of Pt(100) can be lifted upon NO adsorption, creating the
(1x1) unreconstructed surface, which is the only active structure for NO reduction to
N2 because it offers a favorable surface for the stabilization of NHx fragments [16, 19].
Although θNO>θNHx in all cases, due to the higher heat of adsorption of NOads on Pt
(100), evidence of mutual stabilization of NOads and NHx,ads was found [16, 17], with the
ensuing formation of NOads-NHx,ads (x= 1-3) complexes at the periphery of the NO
islands, where the N2 evolution preferentially occurs [17].
120 Chapter 5
This Chapter is devoted to a deeper understanding of the mechanistic origin of the
unique reactivity of (100) sites towards nitrite reduction to N2. The highly selective
conversion of nitrite to N2 at a quasi-perfect Pt(100) electrode in alkaline media has
been investigated with a particular emphasis on its structure-sensitivity and its
mechanism. High-quality (100) facets are required to optimize the catalytic activity and
steer the selectivity to N2: defects of any symmetry dramatically reduce the N2
evolution at [n(100)x(110)] and [n(100)x(111)] surfaces. By combining
spectroelectrochemical studies and mass spectrometry experiments with isotope
labeling, it will be shown that a low-temperature path of the SCR mechanism is
responsible for the high selectivity to N2. In particular, NHx,ads and NOads will be
identified as the key surface species that take part in a Langmuir-Hinshelwood
recombination, which is the defining step of the overall mechanism leading to N2.
The results will show that nitrite reduction is similar to other processes generating N2:
from bacterial anoxic ammonia oxidation (“anammox”) [13, 14] and the high-
temperature NO + NH3 reaction at Pt (100) crystals under ultrahigh-vacuum conditions
[15-18].Thus, the combination of these two nitrogen-containing species is a (low-
temperature) universal pathway to N2.
5.2. Electrochemical experiments
In Figure 5-1 the blank cyclic voltammetry for a well-ordered Pt (100) surface in 0.1 M
NaOH along with the voltammetric response in the presence of nitrite anions for
successively increased values of the reverse potential (Eup) is presented.
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 121
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-1000
-800
-600
-400
-200
0
200
R1
R2
E (V) vs RHE
i (µ
A/c
m2)
R3
O1
Figure 5 -1 - Cyclic voltammetric profiles for Pt (100) surface in 0.1 M NaOH, in the absence
(dashed line) and in the presence of 2 mM NaNO2. The grey line refers to Eup = 0.6 V, the black
line to Eup = 0.8 V, and the thin lines to three selected intermediate potentials. Scan rate,50
mV/s.
Several features can be identified in the broad voltammetric profile recorded from
0.06 V to 0.8 V at 50 mV/s; for these values of Estart and Eup, negligible faradaic currents
were measured at the two extremes of the potential window. In the first positive-
going sweep, a major reduction peak at 0.4 V can be observed (R1), followed by a
broad oxidation peak at 0.55-0.75 V (O1). Upon reversal of the scan direction at E = 0.8
V, several reduction signals can be observed in the negative-going scan. A minor peak
at 0.63 V (R3) is followed by a more intense peak at 0.55 V (R2); the largest signal is still
the broad peak centered at 0.4 V (R1). On the basis of previous studies [2, 6], the
following assignment of the peaks can be suggested: R1 arises from direct nitrite
reduction to ammonia [2], whereas R2 is ascribed to selective nitrite reduction to
dinitrogen [6]. O1 falls in the region where ammonia (generated at R1) reoxidation to
122 Chapter 5
dinitrogen was reported before (Chapter 4). The modification of the upper potential
limit allows us to investigate the relationship between the various voltammetric
features. As depicted in figure 5-1, there is a correlation between the growths of O1
with the increase of R2. The growth of the two signals is maximal when Eup is increased
between 0.6 and 0.75 V, and at higher potentials they both level off to an almost
constant value. Therefore, within the timescale of this specific electrochemical
experiment, the oxidation of a particular intermediate, or surface species involved in
O1, can enhance the ensuing reduction process R2 (dinitrogen formation).
In order to probe the importance of long-range (100) facets for the selective reduction
of NO2-, the effect of the introduction of steps of known orientation into the (100)
structure have also been studied. The voltammetric profiles of nitrate reduction for
some of these stepped surfaces are shown in figure 5-2A and 5-2B for the (111) and
(110) step orientation, respectively.
The peak pattern of the (100) electrode is largely conserved, but the magnitude of all
signals decreases with increasing step density, regardless of the orientation of the
step. This effect affects all peaks, although to a different extent. The large reduction
peak at 0.4 V, associated with the formation of ammonia, does not decrease
remarkably when surfaces with long terraces are used. The oxidation signal above 0.6
V does not shrink appreciably when (111) steps are introduced unless a high step
density is reached. The trend of the peak charge of R2 with respect to the step density
is shown in figure 5-3. The step density has been calculated as (1/(n-0.5)) for [n(100) x
(111)] surfaces and (1/n) for [n(100) x (110)] surfaces (Chapter 2). The introduction of
steps dramatically reduces the corresponding charge, with a noticeable decrease even
for low step densities. We conclude that only surfaces with long-range ordered (100)
domains are able to reach the maximum catalytic activity towards N2 evolution from
nitrite, and the interruption of such long-range ensemble with defects, even if (100)
terraces are very wide between the steps, reduces the activity of the Pt surface.
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 123
-900
-600
-300
0
300
j ( µ
A/c
m2)
Pt(100)
Pt(2911)
Pt(1511)
Pt(711)
A
-900
-600
-300
0
300 B
Pt(100)
Pt(1010)
Pt(710)
Pt(410)
0.0 0.2 0.4 0.6 0.8
-900
-600
-300
0
300C
E(V) vs RHE
Ar + H2
Air
Figure 5-2 – A) Nitrite reduction on Pt [n(100)x(111)] surfaces. Thick line (100), dashed line
(29 1 1), dotted line (15 1 1), thin line (7 11). B) Nitrite reduction for Pt [n(100)x(110)]
surfaces. Thick line (100), dashed line (10 1 0), dotted line (710), thin dotted line (410). C)
Comparison between the voltammetric responses of a Pt (100) electrode cooled in argon +
hydrogen atmosphere (thick line) and in ambient air (thin line). v = 50 mV/s, NaNO2
concentration 2 mM in 0.1 M NaOH.
124 Chapter 5
0.00 0.05 0.10 0.15 0.20 0.25 0.30
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
(111) step
(110) step
Step Density (nm-1)
(1/n-0.5)
(1/n)
Q (
mC
/cm
2)
Figure 5-3 - Peak charge of R2 plotted as a function of the step density.
The importance of high-quality (100) facets was further corroborated by an additional
experiment involving a non-optimal procedure of electrode pretreatment, which will
prevent the surface from reaching an ideal (100) orientation by inducing various types
of defects [20, 21]. Using Pt (100), we performed flame-annealing as described in the
experimental details (Chapter 2), followed by cooling in ambient air rather than in a
controlled oxygen-free atmosphere. The activity towards nitrite reduction of the
electrode was then checked and compared to that of a well-ordered (100) electrode.
Figure 5-2C shows the voltammetric profile of such an “air-cooled” Pt (100). The peak
related to N2 evolution has completely disappeared, along with the oxidation signal
recorded above 0.6 V. The reduction peak associated with ammonia formation is still
present, although it features a much lower peak current and a less positive peak
potential with respect to the crystal cooled in argon + hydrogen atmosphere.
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 125
5.3. Spectroelectrochemical experiments: FTIRS of (100)
and stepped surfaces
In-situ IR spectroelectrochemistry has been employed for the identification of possible
reaction intermediates. IR active vibrations assigned to (adsorbed) NO [22-24], NO2-
[22], NO3-
[25, 26] and NHx fragments [27] have previously been observed with a
comparable configuration. In the following spectra, the positive bands correspond to
the products formed during the reaction, while negative bands arise due to the
consumption of species present at the reference potential. The electrodes were
always brought into contact with the nitrite solution at controlled potential (0.1 V or
0.8 V) where negligible faradaic current is measured. After collecting the reference
spectrum, the potential was progressively stepped to the sample potentials: these
values are reported in the corresponding figures and captions (see below).
As an additional remark, the IR experiments were performed in thin layer
configuration, and it was observed that the gas formation – in this case N2 evolution –
disturbed the acquisition of the optical signal through the thin layer. Therefore, no IR
spectra could be acquired in the region corresponding to gas formation, more
precisely 0.5 <E< 0.65 V. However, selected experiments were performed with a
sample potential just outside this window, in order to compare the electrode surface
state at the two extremes of the N2-forming region. Additionally, the reference
potential was selected as close as possible to the sample potential in order to minimize
the influence of the water bands on the spectra (potential-driven modification of the
water configuration in the thin layer). This is especially important when a continuous
reaction is studied, for which reactant depletion may arise in the thin-layer
configuration.
126 Chapter 5
Figure 5 - 4 – Spectra for nitrite reduction at Pt(100) in
2mM NaNO2 + 0.1 M NaOH in solution in: A) H2O; 1000
interferograms at resolution of 8 cm-1
. Thin line:
reference potential 0.1 V and sample potential 0.4 V.
Thick line: reference potential 0.4 V and sample potential
0.7 V. B) D2O; 200 interferograms with a resolution of 8
cm-1
. Thin line: reference potential 0.5 V and sample
potential 0.8 V. Thick line: reference potential 0.8 V and
sample potential 0.5 V.
Figure 5-4A displays two results of FTIR spectra for Pt (100) in the presence of NO2-.
Four bands are clearly observed in the spectra: the signal at 1640 cm-1
corresponds to
water bending [28], and arises from the water depletion from the thin layer. The other
bands can be attributed respectively to the N-Oads stretching mode (1553 cm-1
) [22], N-
H bending mode (1442 cm-1
) [27], and to the asymmetric NO2- bending mode (1234
cm-1
) [22, 28, 29]. If the potential is stepped from a reference starting potential of 0.1
V to 0.4 V, we observe that the band assigned to NHx species is positive, which means
that NHx species are produced at 0.4 V with a simultaneous depletion of solution-
phase nitrite, a process highlighted by the negative band at 1234 cm-1
. Subsequently,
when the potential is stepped from 0.4 to 0.7 V across the N2-formation region, the
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 127
band at 1442 cm-1
becomes negative, indicating that NHx species are removed from
the thin layer at 0.7 V with respect to 0.4 V. This process is accompanied by the
appearance of a positive band related to adsorbed NO. Therefore, this experiment
shows that various processes take place at the Pt (100) electrode along the sequence
0.1 – 0.4 – 0.7 V: first conversion of nitrite to “N-H” species, presumably involving also
NH3, then removal of these N-H species and appearance of surface NO.
Although the band related to adsorbed NO is unambiguous, the interference of the
neighboring water band was removed by repeating the experiment in the presence of
D2O. In this way, the water bending signal is displaced to lower wavenumbers [30]. The
experiment in D2O adds little information about other bands, for example the nitrite
bending, which is now masked by the D2O signal. Figure 4B displays the results of a
potential-step FTIRS experiment in D2O. When the reference potential is chosen as
close as possible to the N2-region (0.5 V) and then stepped to the working potential
(0.8 V), a positive band corresponding to the appearance of NOads is observed,
consistent with the results displayed in figure 5-4A. If a new reference spectra is
acquired and the potential is then returned to 0.5 V, the NO band changes sign,
showing removal of this species. Moreover, the disappearance of the band at
1442 cm-1
in the spectra acquired in D2O confirms that this band corresponds to a
hydrogenated species.
When taken together, the FTIRS experiments in normal and heavy water allow us to
identify the surface species involved in nitrite reduction (NHx,ads and NOads) and their
stability potential ranges. Between 0.5 V and 0.7 V both NOads and NHx,ads are
presented in the surface and react forming N2.
Further FTIRS experiments were aimed at determining how the surface structure
influences the behavior of the adsorbed species. The FTIR spectra of a few selected
stepped surfaces are shown in figure 5-5.
128 Chapter 5
2000 1600 1200
0.4-0.1V
0.7-0.4V
0.4-0.7V
Pt(510)Pt(711)
Inte
nsi
ty a
.u.
Pt(100)
0.0004 u.a
1560
1449
0.1-0.4V
A
2000 1600 1200
Wavenumber (cm-1)
B
1449
1560
0.4-0.1V
0.7-0.4V
0.4-0.7V
0.1-0.4V
2000 1600 1200
C
1357
1449
1560
0.4-0.1V
0.7-0.4V
0.4-0.7V
0.1-0.4V
Figure 5 - 5 - Spectra for nitrite reduction on Pt (100), Pt (711) and Pt (510), in 0.1M NaOH and
2mM NaNO2 in H2O. 200 interferograms with a resolution of 8 cm-1
. The potential program
involved the following steps: 0.1 – 0.4 – 0.7 – 0.4 – 0.1 V. The thick lines represent the
positive-going direction (from 0.1 to 0.7 V) while thin lines represents the negative-going
direction (0.7 to 0.1 V).
The experiment shown in figure 5-5 involves a complete sweep between 0.1 and 0.7 V,
performed by means of the following stepwise potential program: 0.1 – 0.4 – 0.7 – 0.4
– 0.1 V. For every step, a new reference potential was acquired at the starting value.
As an additional difference with the experiment reported in figure 5-4a, the number of
interferograms acquired is much smaller: this choice is advantageous because it allows
a faster data acquisition but it suffers from the drawback that vibrations related to NHx
and nitrite are somewhat less intense than in figure 4, and the very intense vibration
of NOads dominates the spectra of figure 5-5.
The spectroelectrochemical analysis of the Pt (711) surface, displayed in figure 5b,
reveals that no NHx bands at 1449 cm-1
are observed upon stepping the potential from
0.1 to 0.4 V, suggesting a more limited nitrite consumption, in agreement with the
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 129
voltammetric data of figure 5-2, where Pt (711) features a much lower reduction
current at R1 than Pt (100). The second step to 0.70 V shows that the stepped surface
generates less NO than the basal plane, as demonstrated by the weaker NO band
observed.
In the case of the Pt (510) surface (having 110 steps), a small band related with NHx
species can still be observed after the first step to 0.40 V: a partial contribution of the
fairly reactive (110) sites [2, 6] for the reduction of nitrite to ammonia cannot be
excluded. At 0.70 V, the NO band can be observed, along with another band at
1357 cm-1
. This band can be assigned to the asymmetric bending of adsorbed nitrite in
a “nitro” configuration (M-N coordination) [25, 31] or to solution-phase nitrate [29].
When the potential is stepped back to 0.40 V, no NO consumption was observed; on
the contrary, NO is still present and adsorbed at the surface as indicated by the bipolar
band. The NO is only stripped from the surface at potentials lower than 0.40 V and can
only be removed in the last step to 0.10 V. This is most likely related to the high affinity
of NO towards step sites of (110) geometry, as reported in previous works [1].
The spectroelectrochemical experiments on short-terrace stepped surfaces shed
additional light on the origin of the decrease in reactivity as a result of the introduction
of steps on (100) terraces: (111) steps cause a decrease on the conversion of nitrite
both into NHx,ads and NOads, thus acting as inert surface domains. On the other hand,
(110) steps do not depress the formation of NHx while displaying a high affinity
towards both NO2- and NO, the latter residing at the surface across the potential
region of N2 formation, suggesting that (110) steps could be described as self-poisoned
as long as NO is bound.
130 Chapter 5
5.4. OLEMS experiments with Pt(100) and stepped
surfaces
The activity and selectivity trends obtained by cyclic voltammetry was further
corroborated by a study of the influence of the step density on the MS ion current
related to N2 (m/z =28). During the measurements, m/z = 28 was the only mass
recorded, in order to optimize the signal intensity: we should however remark that a
preliminary Online Mass Spectroscopy - OLEMS - experiment including also m/z = 30
(NO, N2O) and m/z = 44 (N2O) confirmed that for a (711) surface no gaseous product
other than N2 is formed during nitrite reduction, which can confidently be assumed to
be true for surfaces with larger terraces as well (previous studies [6] on Pt (100) also
excluded other gaseous side-products). Although the OLEMS setup has some intrinsic
limitations to a quantitative analysis, a semi-quantitative comparison of different
experiments can still be carried out, and a correction was introduced by measuring the
steady-state ion current related to hydrogen evolution, m/z = 2 at E = - 0.1 V for all
electrodes at the end of the experiment, without changing the geometry (i.e. tip-
electrode distance) of the system.
Figure 5-6 shows the m/z = 28 normalized peak heights (I* = I / Inormalization) and areas
(A* = A / Inormalization) for the positive-going (from 0.1 V to 0.8 V) and the negative going
half-cycles, as a function of step density of selected surfaces, from Pt(100) down to
large, medium and small terrace widths separated by both (111) and (110)
monoatomic steps.
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 131
Figure 5 - 6 - OLEMS data for N2 evolution on the stepped surfaces investigated in this
paper. Squares refer to the (100) electrode, triangles to surfaces featuring (111) steps, and
circles to those with (110) steps. v = 1 mV/s.
The figure shows that, irrespective of the parameter employed (peak height or area)
the amount of N2 formed during the potentiodynamic cycle is strongly dependent on
the step density: for surfaces with wide terraces such as (39 1 1) and (20 1 0) the
magnitude of N2 evolution is already decreased by more than 50% with respect to the
(100) surface. An additional increase in the step density causes a further decrease of
the intensity of the N2 MS peak until the lowest values are reached for (711) and (510)
surfaces. The trend reported in figure 6, which can be qualitatively fitted by an
exponential decay, can be positively compared to the plot in figure 5-3, although the
electrochemical analysis and the OLEMS experiments were carried out in different
conditions. Consequently, it is by no means surprising that the trends of figures 5-3
and 5-6 are not identical, the latter showing a much more abrupt decrease of the MS
peak parameters for N2 evolution even with very small step densities. However, both
techniques agree on the limiting terrace width for a detectable N2 evolution (4 atoms),
and the mass spectroscopy data corroborate the observation that steps –regardless of
their symmetry – have a deleterious effect on the reaction pathways leading to N2.
132 Chapter 5
5.5. Transfer experiments with NO and NHx Mass
spectrometry and electrochemical results
Since NO was observed with FTIRS during nitrite reduction at Pt (100), electrochemical
“transfer” experiments were performed to check the influence of the adsorbed NO on
N2 evolution. To do that, a saturated NO adlayer was adsorbed on the surface outside
the electrochemical cell and then put in contact with the cell containing nitrite ions. In
this experiment, the presence or the absence of the R2 peak was considered as the
only evidence of generation of this gas at this stage. The NO-covered electrode was
contacted with a nitrite-containing 0.1 M NaOH solution at E = 0.8 V, and a potential
sweep in the negative direction was started. The two subsequent voltammetric scans
are shown in figure 5-7.
Figure 5 - 7 - Voltammetric profile of an NO-saturated Pt (100) electrode in a 0.1 M NaOH
solution containing 2 mM NaNO2. First scan thick line, second scan thin line. Estart = 0.8 V, v =
50 mV/s.
The reduction profile of the electrode is clearly different in the first and in the second
cycle. During the first sweep, with the electrode fully covered with NO, no reduction
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 133
current is detected until 0.25 V are attained, which can be ascribed to NOads reductive
stripping [22]. Upon reversal of the potential scan, the broad reduction peak R1
reappears and, in the following second cycle, R2 can be observed again. The removal of
NO from the surface is not complete with the fast scan-rate employed. Hence, a
residual, very low coverage of NO may still be present, but it was not found to be
deleterious for the reaction leading to N2.
A more detailed analysis of the role of adsorbed NO was carried out using labeled
Na15
NO2. The aim was to determine the role of adsorbed 15
NO at Pt (100) during
reduction of Na14
NO2 dissolved in 0.1 M NaOH. Figure 5-8 shows the ion current traces
for m/z = 28 (14
N2), m/z = 29 (14
N15
N), along with the voltammetric profile recorded
during OLEMS experiments.
Figure 5-8- OLEMS measurements in a 0.1 M
Na14
NO2 solution following adsorption of 15
NO at a Pt (100) electrode. A) Cyclic
voltammogram; B) ion current profiles for
m/z = 28. C) ion current profiles for m/z = 29.
The arrows indicate the direction of the
potential sweep. Estart = 0.8 V, v = 1 mV/s.
The 15
NO-covered electrode is not completely inhibited towards N2 evolution, in
contrast with the behavior observed during standard electrochemical measurement
134 Chapter 5
shown in figure 8: this fact is ascribed to the known tendency of NOads to desorb from
the Pt (100) surface in alkaline media during long experimental timescales [22].
However, here it is important to emphasize that there is still a residual degree of
electrode poisoning, evidenced by the smaller ion current for 14
N2 recorded in the first
(negative-going) half cycle, with respect to the positive-going sweep. In spite of this
residual poisoning, N2 can be observed and a certain amount of 14
N15
N is also detected,
which amounts to ca. 10% of 14
N2. This fact testifies that the surface 15
NO takes part in
a reaction process involving a recombination with a 14-N species which must have
originated from the solution-phase nitrite. All labeled NO is consumed in the first
sweep, because no 14
N15
N is measured in the positive-going scan. m/z = 30 ion current
was found to be zero throughout the experiment, which shows that 15
N15
N potentially
arising from a recombination of two 15
NO molecules cannot take place (or is below the
instrumental detection limit).
A second experiment was designed to probe the role of adsorbed NHx, which we
detected during FTIRS experiments. It is known [8] that such a fragment can be
adsorbed onto a Pt (100) electrode from an alkaline solution of ammonia in the
potential range of NH3 oxidative adsorption (which proceeds via a dehydrogenation
step to adsorbed NHx fragments). The potential window of stability of NHx fragments is
roughly 0.35 V <E< 0.5 V. In addition, a previous publication has shown that transfer
experiments involving NHx species are viable [32]. NHx fragments were adsorbed from
a 14
NH3 solution at a constant potential (E = 0.45 V) and transferred to a second
electrochemical cell containing labeled Na15
NO2. Figure 5-9 shows the ion current
traces for m/z = 29 (14
N15
N), m/z = 30 (15
N2), along with the voltammetric profile
recorded during the OLEMS experiment.
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 135
Figure 5 - 9 - OLEMS measurements in a 0.1 M
Na15
NO2 solution following adsorption of 14
NHx at a Pt (100) electrode. A) Cyclic
voltammogram; B) ion current profiles for m/z
= 29; C) ion current profiles for m/z = 30. The
arrows indicate the direction of the potential
sweep. Estart = 0.45 V, v = 1 mV/s.
During the voltammetric scan, started at E = 0.45 V, evolution of 15
N2 is observed in
both cycles, without poisoning effect. However, the formation of 14
N15
N is detected
only in the first (positive-going) half cycle, whereas no signal related to this molecule is
recorded in the negative-going sweep. Therefore, the 14
NHx fragments adsorbed on
the surface recombine with another labeled moiety originated from the solution-phase
labeled nitrite, thus giving rise to 14
N15
N. It must be pointed out that no 14
N2 is
detected: the recombination of 14
NHx fragments does not take place, or is below the
sensitivity of the instrument.
5.6. Mechanism and structure sensitivity of nitrite
reduction at Pt (100) electrodes
The mechanistic analysis of nitrite reduction at Pt (100) is intimately correlated with
the structure sensitivity of this reaction.
136 Chapter 5
FTIR evidence presented above (section 5.3) supports the presence of two key surface
species at potentials vicinal to the potential window where N2 evolution occurs: NOads
and NHx,ads. The direct involvement of both surface species in N2 formation was further
corroborated by OLEMS experiment with labeled compounds (section 5.5).
Rosca et al. presented evidence for the presence of NH2,ads as the dominant
intermediate during NH3 oxidation at Pt(100) in alkaline media [8]. DFT calculations
have also shown that this ammonia fragment is characterized by a larger adsorption
energy at Pt(100) surfaces than other NHx species, providing extra stabilization to NH2
on Pt(100) compared to other basal planes [33, 34]. In addition, previous
electrochemical experiments evidenced that this fragment is stable from 0.35 V to ca.
0.55 V. At 0.55 V a large oxidation signal ascribed to bulk NH3 oxidation predominates
in the voltammogram [8, 11], and all NH2,ads is likely to be oxidatively removed.
The second surface species, NOads, has been previously studied as adsorbate at Pt(100)
in clean 0.1 M NaOH [22] and the reported results showed that the potential window
of stability of NOads features a lower limit at E = 0.35 V. In addition, the adsorbate is
not stable during long-term experiments, showing a tendency to desorb over time,
which testifies that NO is a fairly labile adsorbate at Pt (100) in alkaline media.
Combining this information with experimental results reported before, it seems
reasonable to assume that NOads exists during the N2 formation region up to 0.5 V: this
is the potential value where we obtained clear FTIRS evidence of the removal of this
species on the negative going sweep.
The presence of a central potential region, satisfactorily close to the observed R2 peak
potential (>0.55 V), where the co-existence of NOads and NH2,ads is expected, suggests
that a Langmuir-Hinshelwood recombination may be the fundamental step leading to
N2.
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 137
NO��� � NH,��� � N � HO (1)
Although this reaction cannot be a truly elementary step, is the most likely process
responsible for N2 evolution, also in the light of the similar recombination that occurs
between NO and NH3 under UHV conditions. The suggestion of x=2 for the NHx
fragment can be supported by experimental observations in previous reports [8, 11]
concerning NH2 stability on Pt (100) surfaces. When the potential is changed in the
positive- or negative-going direction, either NH2,ads or NOads, respectively, are already
available at the surface from previous processes and N2 formation can take place.
The correlation between O1 and R2 showed in figure 5-1 suggests the strong influence
of the presence of adsorbed NO on the N2 formation. In fact, as the upper potential
limit is extended upwards, the potential excursion will cross an increasingly broader
section of the potential window where NOads formation is higher. In turn, this can cover
the surface with a larger amount of NOads which, upon potential inversion, will be
readily available to take part in N2 evolution as soon as some NH2,ads has formed (from
nitrite reduction at lower potential).
The structure-sensitivity of nitrite reduction, concentrating on the most structure-
sensitive process, R2 peak (N2 formation) should also be addressed. It has already been
mentioned above, that (100) planes stabilize NH2,ads fragments. For this reason, steps
of any symmetry would simply remove productive (100) sites and destabilize the
NH2,ads adlayer. NO, on the other hand, is known to be fairly strongly adsorbed at
Pt(110) sites, and so we could suppose that the reaction of N2 formation will happen
on this surface, too. However, NO is possibly a strongly adsorbed intermediate and
thereby prevented from taking part in recombination. Indeed, the (510) surface gave
rise to clear FTIRS evidence of the presence of NO in a broader potential region, as low
as E = 0.4 V. On the other hand, (111) sites seem to be the least able to form adsorbed
NO, as indicated by the decreased FTIRS band of NO (figure 5-5). Consequently, the
138 Chapter 5
(100) surface possibly offers the “ideal” surface structure, by performing NO
adsorption but without excessive stabilization, and without achieving a saturated
adlayer which would poison the surface (figure 5-7). A combination of these factors
must contribute to determine the special behavior of Pt(100), which is the only Pt
basal plane able to feature the coexistence of the two nitrogen moieties responsible
for N2 formation.
5.7. Conclusions
Nitrite conversion to N2 was studied at Pt (100) and related [n(100)x(111)],
[n(100)x(110)] stepped electrodes with in situ techniques (FTIRS, OLEMS). The increase
of defects of any symmetry on the surface causes a rapid decrease in the catalytic
activity to N2 formation: well-ordered Pt(100) was found to be the ideal surface for this
reaction. Experimental evidence supports a mechanistic scheme based on a Langmuir-
Hinshelwood recombination of two surface species, which ultimately arise from nitrite
(NOads and NHx,ads) and which can be expected to co-exist in the potential region in
which N2 evolution takes place. These findings, highlighting the only known fully
selective pathway leading from nitrite to N2 for metals and biological systems, will help
to guide in the design of practical catalysts, with the purpose to achieve practical
applications in the field of wastewater treatment.
Nitrite electroreduction on Pt(100) and (100) stepped surfaces in alkaline media 139
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6Nitrate reduction on
Pt(111) surfaces modified
with bismuth adatoms: from
single crystals to
nanoparticlesnanoparticles
6. Nitrate reduction on Pt(111) surfaces modified with
bismuth adatoms: from single crystals to
nanoparticles
6.1. Concepts
It is known that irreversibly adsorbed adatoms can be used to change the surface
composition in a controlled way. The modified electrode often shows an enhanced
electrochemical reactivity [1], as demonstrated with HCOOH oxidation [2]. Different
enhancement mechanisms operate in such bimetallic surfaces and the same adatom
can behave differently depending on the substrate symmetry [3, 4] (see Chapter 3). In
the specific case of nitrate reduction, platinum electrodes have been modified with
different adatoms like germanium [5], palladium [6], or tin [7] to promote its
electrocatalytic reduction. However, in any of these cases, N2 was produced as final
product. Hydroxylamine and NO were obtained with germanium adlayers and N2O and
adsorbed NO, with palladium. For Pt modified with Sn the reduction products were
dependent on the Sn coverage. N2O is found as the main product for intermediate tin
coverages whereas NO is the dominant product for high Sn coverages (detailed
information was previously described in Introduction).
In this Chapter, the enhancement of nitrate reduction on Pt(111) electrodes achieved
through irreversible adsorbed bismuth will be presented.
Cyclic voltammetry, FTIR and OLEMS were used for characterizing the effect of Bi
modification of Pt(111) electrodes on the electroreduction of nitrate anions. On
Pt(111), nitrate consumption occurs at potentials lower than 0.35 V, but with
Pt(111)/Bi this process is shifted to significantly higher potentials (0.6 – 0.7 V). The
spectroelectrochemical results have shown that the product on the surface of these
modified electrodes is N2O.
144 Chapter 6
The catalytic effect is quantified by analysing the voltammetric charges for nitrate
reduction as function of the amount of Bi. A third body effect was found, meaning that
Bi impedes the NO formation on the surface that acts as a poison for the nitrate
reduction. However, evidences for an electronic and true catalytic behavior for Bi
adatoms are also given by the results.
The poisoning effect was also studied by stripping NO spontaneously formed by
contact with nitrate solutions with different Bi coverages. These studies are extended
to (111) vicinal surfaces and {111} preferentially shaped nanoparticles. The effect of
two dimensional order is analyzed under the light of these results.
6.2. Voltammetric results for nitrate reduction on
Pt(111)/Bi
Figure 6-1A compares the reference voltammograms for Pt(111) and Bi modified
Pt(111) (Pt(111)/Bi) (θBi = 0.34) in 0.1 M HClO4. The presence of adsorbed Bi is
characterised by the redox process at 0.67 V. The contribution of the Pt (111) sites can
still be seen at E<0.4 and E>0.75 V, indicating that the surface is not completely
covered by Bi (see chapter 3 for more details). Figure 6-1B shows the nitrate reduction
on the same surfaces, in 0.1 M HClO4 + 10 mM KNO3. For both surfaces, two reduction
peaks can be seen at potentials lower than 0.4 V. Despite the fact that the peaks are
slightly displaced to more negative potentials when Bi is present on the electrode
surface, is very reasonable to assume that they correspond to the same processes,
related to nitrate reduction on Pt(111) sites. In addition to those processes, for
Pt(111)/Bi a net reduction current can be seen in the same potential range where Bi
redox process takes place. The peaks appear at 0.63 V and 0.66 V, in the negative and
positive going sweeps, respectively, e.g. nitrate reduction is shifted more than 200 mV
in the positive direction. The reduction charge in the negative going sweep is higher in
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 145
-150
0
150
Pt(111)
Pt(111)/Bi
j (µ
A/c
m2) 50mVs
-1
(A)
0,0 0,2 0,4 0,6 0,8
-10
-5
0
(B)
Pt(111)
Pt(111)/Bi 2mVs-1
E (V) vs RHE
Einitial
the presence of nitrate, clearly indicating that not only Bi is being reduced in this case.
Moreover, in the positive going sweep a reduction process dominates, overlapped
with the surface oxidation peak of Bi.
Figure 6 - 1 - Cyclic voltammograms of
Pt(111) (dashed) and Pt(111)/Bi
(solid).(A) 0.1M HClO4 at 50 mVs-1
and
(B) 0.1M HClO4 + 0.01M KNO3, at 2
mVs-1
. (θθθθBi =0.33)
These results suggest that nitrate electroreduction occurs in two main potential
ranges: below 0.4 V the reduction occurs on the Pt(111) free sites; and between 0.60
and 0.70 V the process clearly takes place on the sites covered by, or next to, bismuth
adatoms. In the presence of bismuth the available number of free platinum sites is
lower, causing the decrease of the current density in the low potential range. In this
potential region the presence of Bi on the surface inhibits the reduction of nitrate.
It is very important to mention that catalytic nitrate reduction on Pt(111)/Bi surfaces is
not observed on sulphuric acid or neutral pH phosphate electrolytes. It was observed
146 Chapter 6
by infra red measurements that sulphate also adsorb ion Bi, suggesting that at the
potential range were the catalytic effect of Bi on the nitrate reduction should be
observed, both Pt and Bi sites are blocked by the anion. In the case of sulphate
electrolytes (Figure 6-2 A), probably a double effect is inhibiting the reaction at high
potential – pH and anion adsorption. As it is possible to see in the figure 6-2 A, nitrate
reduction is almost absente at this pH in both surfaces Pt(111) and Pt(111)/Bi. These
results suggest an effect of the adsorbed anion on the reaction or, in the case of
phosphate neutral solutions, a mixed effect of pH and anion adsorption.
Other sp adatoms, also showing a similar surface redox process, were tested toward
nitrate reduction (As, Sb, Te). However, none of them gave promising results. In figure
6-2 B the results obtained for nitrate reduction on perchloric acid electrolyte, with
Pt(111)/As are shown. On Pt(111) arsenic redox peaks are in a potential range very
close to those of Bi, around 0.56 V vs RHE. However, just a small reduction current is
observed at 0.58 V that can be attributed to nitrate reduction. This current is negligible
when compared with the reduction given by Pt(111)/Bi under the same conditions. All
the other features in the CV are due to nitrate reduction on the free Pt sites (low
potential peaks) or to the redox process of the As (high potential peaks). These results
reinforce the specificity of the Pt(111)/Bi as catalyst for nitrate reduction.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 147
0.0 0.2 0.4 0.6 0.8 1.0
-12
-8
-4
0
4
8
12
5 mV/s
0.05 M H2NaPO
4 + 0.05M HNa
2PO
4
0.01M NaNO3
j (µ
A/c
m2)
E (V) vs RHE
Pt(111)
Pt(111)/Bi
0.1M HClO4 + 0.01M NaNO
3
2 mV/s
A
0.0 0.2 0.4 0.6 0.8 1.0-15
-12
-9
-6
-3
0
3
6 B
Pt(111)
Pt(111)/As
Figure 6 – 2 – Cyclic voltammograms of nitrate reduction (0.01M NaNO3) on: A) 0.05 H2NaPO4
+ 0.05 HNa2PO4 on Pt(111) (thin line) and Pt(111)/Bi (thick line), θBi 0.07, 5 mV/s . B) 0.1M
HClO4 on Pt(111) (thin line) and Pt(111)/As (thick line), θAs 0.09, 2 mV/s.
6.3. Spectroscopic study of nitrate reduction on
Pt(111)/Bi
In order to understand the catalytic activity of bismuth adatoms on the
electroreduction of nitrate, several spectroscopic measurements were made. In the
following spectra the positive bands correspond to the products formed during the
nitrate reduction, while negative bands arise because of the consumption of species
present at the reference potential. The contact of the electrodes with the nitrate
solution was made at controlled potential (0.9 V). This potential was maintained until
the electrode was pressed to the CaF2 window. After collecting the reference
spectrum, the potential was stepped to progressively lower potentials, down to 0.05 V,
148 Chapter 6
and then increased again back to 0.9 V. This is referred below as negative and positive
scans, respectively.
Experiments with both water and heavy water were done, but only the results in D2O
are presented here, because no significant differences were observed in both solvents.
The only expected product that would be identifiable in H2O but not in D2O is NH4+,
(1470 cm-1
). However, in the potential region in which ammonium is produced through
NO reduction [8], nitrate consumption also takes place. Then, the two bands overlap
and no conclusions can be drawn.
Figures 6-3 and 6-4 show the spectra collected at several potential during nitrate
reduction on Pt(111) and Pt(111)/Bi, respectively, in D2O and HClO4 0.1M + KNO3 0.1M.
Nitrate consumption can be identified with the band at 1370 cm-1
corresponding to the
E mode of free nitrate ions in solution [9].
For Pt(111), as observed in Figure 6-3, nitrate consumption only occurs in the low
potential range, below 0.15 V, both in the negative (Figure 6-3 A) and the positive scan
(Figure 6-3 B), in agreement with the reduction peaks observed in Figure 6-1B. In the
positive going sweep, at 0.90 V, the nitrate consumption band can still be seen
because nitrate depletion from the thin layer has not been fully compensated.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 149
Figure 6 - 3 - in situ FTIR spectra for nitrate reduction on Pt(111). Test solution: 0.1M HClO4 +
0.1M KNO3 in D2O. Reference potential: 0.9 V for main figure and 0.05 V for insets. Sample
potential indicated in figure. A and B: positive and negative scans, respectively. Spectra taken
with 200 interferograms and resolution of 8 cm-1
.
Besides nitrate consumption, the spectra show the presence of bands between 1550
and 1690 cm-1
. According to Gómez et al. [8], , bands in this region can be attributed to
low coverage NO adsorbed in two different ways (“on top” and “bridge”). These bands,
related with the presence of NO, appear in the spectra of Figure 6-3 with positive sign
at potentials between 0.75 and 0.4 V, revealing that NO is formed before starting the
negative potential scan. At 0.35 V the NO bands disappear, suggesting that this species
has been reduced, likely in the most positive reduction peak observed on Pt(111). The
insets show some of these spectra, but using 0.06 V as reference potential. At this
2250 2000 1750 1500 2250 2000 1750 1500
absorb
ance (
a.u
.)
wavenumber (cm-1)
0.05V
0.15V
0.35V
0.45V
0.65V
0.85V
Pt(111)
5E-4 a.u.
negative scan
(A)
1370cm-1
1652cm-1
(B)
positive scan
0.90V
0.25V
0.35V
0.45V
0.65V
0.85V
1370cm-1
1700 1600 1500
0.90V
0.75V
0.55V
0.45V
1700 1600 1500
0.35V
0.45V
0.85V
0.65V
150 Chapter 6
latter potential the surface is free from NO and then the NO bands collected at the
different working potentials can be more clearly seen. It is remarkable that nitrate
consumption is only observed below 0.15 V in the negative-going sweep. Thus, the
band at 1450 cm-1
in the spectra collected between 0.65 – 0.35 V can be quite
confidently assigned to adsorbed NO present at the reference potential. In the positive
going sweep (Figure 6-3 B) the band around 1550 cm-1
appears again at 0.55 V; but at
0.9 V only the band at 1698 cm-1
is observed. All the bands between 1400 – 1700 cm-1
can be attributed to adsorbed NO, as none of them can be seen with s polarized light.
The spectra for nitrate reduction on Pt(111)/Bi in HClO4 and D2O are presented in
Figure 6-4. Nitrate consumption, in this case, takes place at higher potentials (0.7 V),
proving that the net reduction current observed in the voltammetric profile
corresponds to nitrate reduction. Simultaneously to nitrate consumption, a positive
band appears at 2232 cm-1
, related to the production of N2O. The same behavior is
observed in the positive going sweep (Figure 6-4 B), indicating that nitrate is always
reduced to N2O, irrespectively of the sweep direction. It is known that N2O is weakly
adsorbed at the electrode surface [10], and this agrees with the results obtained with s
polarized light, that show the presence of this species in solution. Other possible
soluble products such as N2 are not IR active and its presence, if produced, cannot be
detected with the actual experimental setup. At lower potentials, nitrate reduction can
be observed again, with less intensity, but N2O formation is not observed. Here the
nitrate consumption is related to the remaining free platinum sites, in agreement with
that obtained with the unmodified Pt(111) surface.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 151
Figure 6 - 4 - in situ FTIR spectra for nitrate reduction on Pt(111)/Bi. Test solution: 0.1M HClO4
+ 0.1M KNO3 in D2O. Reference potential 0.9 V for main figure and 0.05 V for insets. Sample
potential indicated in figure. A and B: positive and negative scans, respectively. Spectra taken
with 200 interferograms and resolution of 8 cm-1
.
At 0.60 V, in both negative (Figure 6-4 A) and positive (Figure 6-4 B) sweeps, a bipolar
band of bonded NO can be observed at 1694 – 1677 cm-1
. The negative band at 1694
cm-1
suggests that NO is already present at the reference potential (0.90 V). The
bipolar shape reflects the effect of the electrode potential on the N-O stretching
frequency, which shifts to lower wavenumbers as the electrode potential decreases
[11]. At lower potentials this species is no longer present, and the negative band at
1695 cm-1
reflects its consumption. The inset shows the results by using 0.06 V as
2250 2000 1750 1500 2250 2000 1750 1500
1694cm-1
1370cm-1
(A)
negative scan Pt(111)/Bi
absorb
ance (
a.u
.)
wavenumber (cm-1)
0.05V
0.15V
0.30V
0.50V
0.60V
0.80V
1E-3 a.u.
2232cm-1
(B)
positive scan
0.25V
0.35V
0.50V
0.60V
0.75V
0.90V
2232cm-1
1370cm-1
1700 1600 1500
0.90V
1E-3 a.u.
0.65V
0.50V0.30V
0.15V
1700 1600 1500
1E-3 a.u.
0.90V
0.75V
0.65V
0.50V
0.35V
152 Chapter 6
reference potential. It can be seen that the NO band shifts to higher wavenumbers
with the increasing potential [8].
6.4. On line mass spectroscopy
Although in situ infrared spectroscopy is a very useful technique to gain very important
molecular information about the species involved in the reaction, N2 formation cannot
be detected with this technique. For this reason OLEMS experiments for the nitrate
reduction on Pt(111)/Bi were made in order to check the possibility of having
dinitrogen as product of the reaction. Unfortunately, no N2 was detected and N2O was
found to be the only gaseous product of nitrate reduction on Pt(111) modified with Bi
adatoms. Results are plotted in fig. 6-5 and because no changes in the mass m/z =28
were observed this mass was not considered in figure. Small differences observed in
the voltammetric shape are due to the change of the electrochemical conditions. It
should be kept in mind that during all the experiment products are being pumped
from the surface, changing the hydrodynamic conditions from those obtained in static
cyclic voltammetry experiments.
Figure 6 - 5 - OLEMS measurements in a 0.01 M
NaNO3 + 0.1M HClO4 solution with a Pt (111)/Bi
surface (θBi 0.22). A) Cyclic voltammogram,
1mV/s; B) ion current profiles for m/z = 44.
-0.3
-0.2
-0.1
0.0
i (m
A)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
m/z = 44 (N2O)
E (V) vs RHE
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 153
6.5. Quantification of the catalysis promoted by Bi
adatoms
6.5.1. Pt(111)
In Fig 6-6 the cyclic voltammograms for Pt(111) with different Bi coverages, in the
absence (A) and in the presence of nitrate (B) are presented. The increase of Bi
coverage on the surface is reflected (Fig 6-6 B) in the increase of the charge under the
characteristic Bi redox process at 0.67 V vs RHE [12]. The increasing amount of surface
Bi also causes a decrease of the hydrogen adsorption region as a consequence of the
blockage of the free platinum sites.
0.0 0.2 0.4 0.6 0.8 1.0
-400
-300
-200
-100
0
100
200
300
400
E (V) vs RHE
j (µ
A/c
m2)
Increasing
Bi coverage
50mV/s
0.1M H2SO
4
A
0.0 0.2 0.4 0.6 0.8 1.0
-40
-30
-20
-10
0
10
20
Increasing
Bi coverage
5mV/s
0.1M HClO4 + 0.01M NO
-
3
B
Figure 6 - 6 – CV´s for Pt(111) /Bi with different coverages, A) in 0.1 M H2SO4, 50 mV/s and B)
on 0.01 M NaNO3 0.1 M HClO4, 5 mV/s for Bi coverages of 0, 0.08, 0.23, 0.26, 0.31.
154 Chapter 6
As explained previously (section 6.2), the presence of Bi adatoms allows nitrate
reduction at potentials as high as 0.6-0.7 V, overlapping with the surface redox
reaction undergone by Bi (Fig 6-6 B). The increase of Bi coverage on the surface
decreases the reduction currents in low potentials, as expected, because there are less
free platinum sites and the reaction at this potential range is due to nitrate reduction
on Pt atoms. Note that Bi redox process in perchloric acid takes place at 0.67 V
(instead of 0.63 V), the shift being a consequence of anion adsorption [13].
Fig. 6-7 plots the charge involved on nitrate reduction at 0.6-0.7 V in the cathodic scan
as a function of the adatom coverage on Pt(111). The charge corresponding to the
bismuth redox process has been subtracted from the total charge, integrated in the
high potential region in absence of solution nitrate, in order to obtain the net charge
of the nitrate reduction.
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
0
50
100
150
200
250
300
θ Bi
Qre
d (
µC
/cm
2)
Figure 6 - 7 - Reduction charge between 0.7 and 0.6 V with the Bi coverage of the Pt(111)
electrode in a 0.1 M HClO4 with 0.01M NaNO3.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 155
The electrocatalytic activity of the surface increases with the presence of Bi until its
coverage is significantly higher than one half (0.27) of the maximum coverage on this
surface (0.33). After that, the activity markedly decreases. This result suggests that the
reduction reaction needs Pt-Bi pairs and Bi adatoms are not active by themselves.
This type of catalytic behavior for bimetallic electrodes is similar to that observed in
the case of formic acid oxidation [4]. For this process, it was also observed that Bi
adsorption on Pt(111) enhances the activity of the platinum sites for direct oxidation
of HCOOH, suppressing almost completely the poison (CO) formation reaction. In that
case, activity was shown to be proportional to the amount of Pt- Bi pairs. One
important characteristic in that case is that the electrode activity increases initially
linearly with the amount of Bi, until a critical coverage is attained, since, at low
coverage, number of Pt-Bi pairs is simply proportional to number of Bi adatoms. This
behavior is not observed in the present situation, since at low coverage, increase of
activity with the amount of Bi on the surface is maintained very low, and activity starts
only to take off when Bi coverage exceeds 0.15. This behavior is nevertheless
comparable to the electrocatalytic effect of Bi adsorbed on Pt(100) for formic acid
oxidation. The key difference between the behavior of Pt(111) and Pt(100) for this
reaction is the existence of poison formation, which is virtually absent for Pt(111) [14].
In this framework, shape of the obtained curve (fig. 6-7) can be explained by
considering that adatoms break the existence of a particular ensemble of sites on the
surface in which poison formation can take place [4]. The remaining free Pt sites on
the surface, which are not available for poison formation, would be still active for the
direct reduction of nitrate. For low coverages, the activity remains very low, since the
probability of having enough number of adjacent Pt atoms to allow poison formation
would still be high. The current only starts to increase for a surface blockage higher
than half of the monolayer (0.15-0.20). It can be assumed that Bi is avoiding the NO
poisoning of the surface through a third body effect, allowing nitrate to be reduced at
higher potentials.
156 Chapter 6
Moreover, the fact that catalytic activity is linked to the Bi redox process, with a
sudden loss of activity after Bi reduction takes place is a clear indication of the
existence of a true catalytic effect in addition to the third body effect mentioned
above. In addition, the fact that the reaction has such a big specificity (other adatoms
are not active or the inhibition of the reaction in other electrolytes media) also support
the idea that Bi as a special character as catalyst, not only as a third body.
In order to have a better understanding of the role of Bi on the poisoning step on the
modified surfaces, the stripping of NO poison spontaneously formed at open circuit
potential on the surface of Pt(111) single crystal electrode was done in a series of
experiments as shown in fig. 6-8.
Similarly to early poisoning studies from formic acid and methanol [15], the poison
adsorption was performed by putting the electrode surface in contact with a 0.01 M
nitrate solution in 0.1 M HClO4 at open circuit potential during 1 minute. After that,
the electrode is rinsed with water and transferred to an electrochemical cell in the
absence of nitrate or NO (supporting electrolyte only).
It is possible to observe (fig. 6-8) that the amount of NO adsorbed decreases when the
Bi coverage increases, as evidenced by the decreasing magnitude of the reductive NO
stripping charges. These charges were corrected to account for the recovery of the H
adsorption charge that takes place after elimination of NO [8]. For the surfaces
modified with Bi, the hydrogen charge that has to be subtracted corresponds only to
the free Pt sites.
It had been proposed by Koper [16] that the two reduction peaks on the NO stripping
process correspond to two types of adsorbed NO. The peak at 0.3 V would be due to
the NO adsorbed in linear position while that at 0.1 V would correspond to bridge
bonded NO. It is interesting to remark that these peaks have different behavior when
Bi coverage increases: the peak corresponding to the bridge NO decreases faster than
that of the linear NO contribution. Assuming that bridge NO is bonded in a three-
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 157
hollow site position involving three platinum atoms, it is not surprising that, when less
free Pt sites are available, bridge NO decreases faster.
0.0 0.2 0.4 0.6 0.8 1.0
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
j (µ
A/c
m2)
E (V) vs RHE
Increasing Bi coverage
Figure 6 - 8 –Stripping of NO poison formed spontaneously on Pt(111)/Bi modified surfaces in
0.1M HClO4 , 5 mV/s.
In fig. 6-9, the charges related with the spontaneous NO poisoning formation for
different Bi amounts on the Pt(111) surface are plotted.
As mentioned before, linear and bridge NO show different behavior with the change
on the Bi coverage. The decrease of the charge involved under the peak at 0.1 V is
linear while the charge for the 0.3 V peak needs a higher Bi coverage to start noticing
it. For θBi < 0.1, Bi presence does not affect the amount of NO adsorption on linear
position.
158 Chapter 6
0.0 0.1 0.2 0.3 0.4
0
100
200
300
400
500
600
NO total
NO linear (0.3 V)
NO bridge (0.1 V)
QN
O (
µC
/cm
2)
θBi
Figure 6 - 9 – Charges involved in the NO stripping for different Bi coverages on Pt(111)
surfaces.
The overall charge of adsorbed NO decreases quite linearly with the presence of Bi.
This effect is expected when the adatom plays a third body effect on the catalytic
reaction, blocking adsorption sites of the poisoning compound, in this case NO.
6.5.2. Stepped surfaces Pt(554) and Pt(332)
As shown in previous reports [17] the use of well defined stepped surfaces has a
special interest when the aim is to study the behavior of more real catalyst surfaces
with defects. So, in order to study the effect caused by surface defects on the
electrocatalysis promoted by Bi for nitrate reduction on Pt(111), experiments with
stepped surfaces were done. The surfaces used have {111} terraces and {110}
monoatomic steps. From the hard-sphere model, Pt(554) has 9 atom-width terraces
whereas Pt(332) has terraces with 5 atoms-width.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 159
The presence of irreversibly adsorbed bismuth was estimated voltammetrically, by
inspection of the characteristic adsorption states related to the steps and terraces as a
whole, in 0.1 M sulfuric acid, which is a convenient electrolyte for characterization
proposes. The evaluation of the coverage of Bi on more complex surfaces is difficult
because the maximum coverage value depends on the basal plane, e.g. 0. 33 Bi/Pt at
Pt(111) and 0.5 Bi/Pt at Pt(100) [18]. In this respect, when the surface contains sites
with different symmetry, as happens with stepped surfaces [19], each type of site
would have a different maximum coverage. Moreover, for surfaces with steps/defects,
Bi adsorb preferentially on them. Since Bi on steps does not show a voltammetric
redox process, the contribution at 0.63 V is not observed until the steps are fully
blocked and Bi starts adsorbing on the terrace. For this reason, the determination of Bi
coverage on stepped surfaces and nanoparticles is not straightforward. This difficulty is
overcome by relating Bi coverage with the amount of blocked H charge
��� � 1 � ��� � ��
� � ����
���
where qPto and qPt
Bi are the hydrogen and anion adsorption charges for a clean
electrode and bismuth modified electrode, respectively. In this scale, maximum
coverage is normalized to 1. For the sake of comparison, an axis with a parallel scale
for θBi will also be presented in the graphics.
Experiments for nitrate reduction were made with Pt(554) and Pt(332) surfaces
modified with Bi. The voltammetric response reveals that the surface just becomes
catalytic for the reduction at high potential when Bi starts depositing on the terraces
(Fig. 6-10). Bi on the steps is not active towards nitrate reduction. It is possible to see
that, when no Bi is adsorbed on the terraces, there is no reduction process at high
potentials (red curves). At low potentials the reduction peaks are less intense,
suggesting small nitrate reduction on the steps. As soon as some Bi is adsorbed on the
terraces the catalytic behavior at high potentials is observed (green curves).
160 Chapter 6
The curves measured for the nitrate reduction charges at 0.6-0.7 V for electrodes with
different bismuth coverage on Pt(554) and Pt(332) are shown in figure 6-11. The shape
of the resulting curves is very similar to those obtained for Pt(111), revealing a third
body effect of the Bi adatoms. However, there is now an additional reason for this
shape. At low coverages, activity remains low because Bi tends to deposit
preferentially on steps and such Bi is not active for nitrate reduction.
0.0 0.2 0.4 0.6 0.8 1.0-210
-140
-70
0
70
140
210
θBi
= 0
θBi
= 0.15
θBi
= 0.48
j (µ
A/c
m2)
E (V) vs RHE
Pt(554)
0.1M H2SO
4
0.0 0.2 0.4 0.6 0.8 1.0-30
-20
-10
0
10
θBi
= 0
θBi
= 0.15
θBi
= 0.48
0.1M HClO4 + 0.01M NaNO
3
Figure 6 - 10 – CV´s for Pt(554) with θBi of 0 and 0.05 (low coverage) in A) 0.1M H2SO4 50 mV/s
and B) 0.1M HClO4 + 0.01M NaNO3, 5 mV/s.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 161
0.0 0.2 0.4 0.6 0.8 1.0
0
40
80
120
160
200
240
QR
ed(µ
C/c
m2)
(1-θH)
Pt(554)
0.0 0.2 0.4 0.6 0.8 1.0
0
40
80
120
160
200
240
Pt(332)
0.0 0.1 0.2 0.3 θ Bi
0.0 0.1 0.2 0.3
Figure 6 - 11 - Reduction charge between 0.7 and 0.6 V with the Bi coverage of the Pt (554)
and Pt(332) in 0.1 M HClO4 with 0.01M NaNO3
The spontaneous poisoning formation from nitrate solution was also performed on the
stepped surfaces. The results are plotted in Fig 6-12 for Pt(554) and Pt(332).
It is possible to observe a significantly different behavior when different atoms-width
terraces are considered in experiments of poison formation. For Pt(332) two linear
segments are observed in the plot that result in a change of slope observed for (1- θPt)
=0.4 (θBi = 0.15). For Pt(554) however, the decreasing of the NO charge for the
different Bi coverages is linear in almost all the range. At low coverages, Bi will adsorb
preferentially on the steps and the blockage for the poison formation is less significant.
This gives rise to a linear behavior with a lower slope for low coverages. This behavior
is more pronounced in the Pt(332) surface because the step density is higher in this
surface. For higher Bi coverages, both surfaces behave like Pt(111) (for the 3 surfaces
the slope is 1300±200 µC/cm2 per coverage unit). When Bi covers the steps, NO
formation also diminishes although in a lesser extent than that measured when the
adatom starts covering the terraces. This supports the conclusion that poisoning from
162 Chapter 6
nitrate is a structure-sensitive reaction that takes place in all surface sites at different
rate. On the other hand, the electrocatalytic effect for nitrate reduction at high
potential (0.6-0.7 V) only takes place on the (111) terraces.
0.0 0.2 0.4 0.6 0.8 1.0100
200
300
400
500
θBi
(1-θPt)
QN
O(µ
C/c
m2)
Pt(554)
0.0 0.2 0.4 0.6 0.8 1.0100
200
300
400
500Pt(332)
0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.3
Figure 6 - 12 – Charges involved in the NO stripping for different Bi coverages on Pt(554) and
Pt(332).
In summary, Bi on steps decreases NO formation although it is less effective than Bi on
terraces. On the other hand, for the direct nitrate reduction, Bi on steps is not
effective, probably because it is reduced and the catalysis requires oxidized Bi
adatoms. A significant difference with formic acid poison formation becomes apparent
from this study. For the latter, poison formation takes place preferentially at steps,
and, when these are blocked, almost no poison formation takes place. For nitrate
decomposition, however, terrace sites seem more active, since introduction of step
decreases the amount of NO formed.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 163
6.5.3. Preferentially {111}Pt oriented nanoparticles
It is known that the use of shape controlled nanoparticles can be particularly
interesting in electrocatalysis. In order to test the possible use of platinum
nanoparticles for nitrate reduction, preferentially oriented {111}Pt nanoparticles will
be used as substrate. These particles have high contribution of {111} sites where Bi can
be adsorbed and nitrate be reduced, in a similar way as that observed for Pt(111)
single crystals. In fig. 6-13 the cyclic voltammograms for nitrate reduction on
preferentially oriented {111}Pt nanoparticles modified with Bi are given.
In general, all the features observed with Pt basal planes are also observed on these
nanoparticles although contribution from other orientations is also present. For that
reason, a direct estimation of the bismuth coverage cannot be easily achieved. As for
the stepped surfaces, the blockage of the charge associated to hydrogen adsorption
will be used as an evaluation of the fraction of sites covered by bismuth (see section
6.5.2 for more details).
Fig 6-13 shows the cyclic voltammograms obtained for the preferentially oriented
{111}Pt nanoparticles with different Bi coverages: panel (A) shows the blanks in
sulfuric acid and panel (B) reports the catalytic reduction of nitrate.
164 Chapter 6
0.0 0.2 0.4 0.6 0.8 1.0
-150
-100
-50
0
50
100
150
j (µ
A/c
m2)
E (V) vs RHE
Increasing Bi
coverage
A
0.1M H2SO
4
50mV/s
0.0 0.2 0.4 0.6 0.8 1.0-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
0.1M HClO4+ 0.01M NO
-
3
50mV/s
Increasing Bi
coverage
B
Figure 6 - 13 - CV for Bi modified platinum preferential {111} oriented nanoparticles, A) in 0.1
M H2SO4, 50 mV/s and B) for nitrate reduction on 0.1M HClO4 5 mV/s.
The Bi deposition on the Pt nanoparticles follows similar behavior to that of Pt stepped
surfaces with {111} domains [17, 20]. The first effect of the bismuth deposition is the
diminution of the peaks on the hydrogen adsorption region without a significant
diminution of the specific signal for anion adsorption on the terraces. This means that
bismuth deposits initially on the step/defects sites. Furthermore, increasing amounts
of bismuth on the electrode surface lead to the diminution of both the hydrogen and
(bi)sulfate adsorption on the {111} two dimensional domains. Additionally, the signal
from Bi redox process at 0.63 V starts increasing.
The nitrate reduction on Pt(111)/Bi is expected to occur on the well-defined {111}
domains. As said before, the Bi will deposit first on the steps/defects of the
nanoparticles, what means that for small coverages the catalytic activity at high
potentials will be negligible as shown before with the stepped surfaces. Reduction
currents at 0.6-0.7 V only appear when Bi starts to be deposited on the terraces.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 165
The results shown in Fig. 6-13 (B), demonstrate a similar behavior for nitrate reduction
on the preferentially oriented {111}Pt nanoparticles as that described for Pt(111)
single crystals when both are modified with Bi. At high potential the characteristic
features related with the reduction of nitrate are observed in both substrates. In the
same way, the low potential reduction taking place at the free Pt atoms decreases as
the Bi coverage increases. Increasing the Bi coverage increases the currents at high
potential, where Bi redox and nitrate reduction are overlapped. When the current
under the nitrate reduction peak in the negative sweep is plotted against the coverage
(fig. 6-14) the shape of the curve is almost the same as can be observed for single
crystals. It is important to remind that the Bi charge corresponding to the Bi redox
process has been subtracted from the total integrated charge to obtain the net charge
corresponding to nitrate reduction. The maximum activity is again obtained when
more than half of the surface is covered by Bi (0.85), the same value obtained on
Pt(111), and the shape of the curve is in agreement with that expected for a third body
effect. The small deviations observed at very low Bi coverage may indicate the role of
sites other than {111}.
Figure 6 - 14 - Reduction charge between
0.7 and 0.6 V with the Bi coverage of the
Pt preferential (111) oriented
nanoparticles in a 0.1 M HClO4 with
0.01M NaNO3.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.080
100
120
140
160
180
200
θBi
Qre
d (
µC
/cm
2)
(1-θH)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
166 Chapter 6
The poison formation step was investigated as before with the preferentially oriented
{111}Pt nanoparticles as a function of Bi coverage, observing a similar effect as
previously described for the single crystals (fig. 6-15). The voltammetric profile of the
NO stripping on preferentially oriented nanoparticles is slightly different from that
obtained with the well oriented surfaces. It should be born in mind that other
orientations are present in the nanoparticles sample, not only the {111} contribution.
All these other orientations may have minor contributions on the final CV and it is not
straightforward to evaluate its contribution to the final response. We should also keep
in mind that size and bidimensional order also play a role on electrocatalysis and these
effects are obviously different when considering single crystals or nanoparticles. These
reasons may explain that, on the reductive stripping of adsorbed NO on nanoparticle
samples only one peak is observed at 0.2V, overlapping different contributions from
atop and bridge NO. A small shoulder can be distinguished in the high potential side of
this peak. Another small peak is observed at 0.1 V. The latter is probably related with
the stripping of NO adsorbed on other contributions different from {111} [8].
When the coverage of Bi increases on {111}Pt nanoparticles, the stripping of adsorbed
NO also decreases. In the same way as reported for Pt(111) single crystal surfaces,
atop and bridge NO show different behavior as it is observed by the different evolution
of the shoulder and the main peak. For higher Bi coverages, the shoulder at 0.3 V (atop
NO) becomes more pronounced and finally becomes the main peak contribution in the
CV.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 167
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
-15
-10
-5
0
E(V) vs RHE
j (µ
A/c
m2)
Increasing Bi coverge
Figure 6 - 15 – Stripping of NO poison formed spontaneously on Pt preferentially {111}
oriented Bi modified nanoparticles, 0.1M HClO4 5 mV/s.
When the charges involved in the adsorbed NO stripping are plotted against the Bi
coverage (fig. 6-16), it is possible to observe that the linear decrease just occurs for
small coverages and that a slope change appears at higher coverage. It is also
remarkable that the amount of NO formed is much smaller for the nanoparticles,
probably because the size of (111) terraces is relatively small. This break in linearity
can be due to several factors, like the existence of smaller {111} domains or the
existence of other type of crystallographic symmetry on the surface [21].
The slope obtained for high coverages is 227 while that for the single crystal is 1407
µC/cm2
per coverage unit. This difference can be understood if we accept that the two-
dimensional domains are much smaller in the surface of the nanoparticles and hence
the total charge of NO poison formation already decreases even on the modified
nanoparticles in the absence of Bi. There are less available sites for the poisoning
168 Chapter 6
reaction, and there are more defect sites to adsorb Bi what makes the blockage from
Bi less marked on the terraces.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
24
30
36
42
48
54
60
66
72
θBi
QN
O (
µC
/cm
2)
(1-θPt)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Figure 6 - 16 - Charges involved in the NO stripping for different Bi coverages on Pt
preferentially {111} oriented nanoparticles.
6.6. Conclusions
The presence of irreversible adsorbed bismuth on Pt(111) electrodes catalyzes nitrate
reduction at potentials as high as 0.63 - 0.67 V. The spectroelectrochemical results
show that the products of the nitrate reduction on the surface of these modified
electrodes are N2O and NO, although NO was already observed to be present on the
unmodified Pt(111) surface. The IR and the OLEMS results showed that the only
product of this catalytic reduction promoted by the presence of Bi is N2O.
Nitrate reduction on Pt(111) surfaces modified with Bi adatoms 169
The quantification of the catalytic effect as function of different Bi coverage was made
for Pt(111), (111) vicinal surfaces and preferentially {111} oriented nanoparticles using
cyclic voltammetry. The shape of the obtained curves can be explained with a third
body effect. The presence of Bi on the surfaces decreases NO poisoning allowing
nitrate to be reduced at higher potentials. At low coverages, the probability of forming
an ensemble capable of avoiding poison formation is very low and, for this reason, NO
is still significant and the catalytic effect is low.
However, the catalytic reduction showed to be closely related with the redox process
of the Bi, with a sudden loss of activity when Bi is reduced. Moreover, the reaction
showed to be very dependent on the supporting electrolyte (both pH and anion
adsorption), structure of the substrate or adatom. These results give a very specific
role to Bi, suggesting in addition to the third body effect, some electronic and pure
catalytic effects.
The poisoning effect was also achieved by stripping NO spontaneously formed by
contact with nitrate solutions with different Bi coverages. The results of both single
crystal and nanoparticles agree with the third body effect, although for nanoparticles
at high coverages the linearity is lost. Comparing these results with those obtained
with Pt stepped surfaces of 9 and 5 atoms-width terraces with {111} orientation is
possible to conclude that this behavior at high coverages is related to the decrease of
the terrace sites and not to the existence of defects/steps that do not have
contribution on the catalysis.
170 Chapter 6
References
[1] V. Climent, N. García-Araez, J.M. Feliu, in: M.T.M. Koper (Ed.) Fuel Cells Catalysis. A
Surface Science Approach, John Wiley & Sons, Inc., Hoboken, New Jersey, 2009, pp.
209.
[2] J. Clavilier, A. Fernández-Vega, J.M. Feliu, A. Aldaz, J. Electroanal. Chem., 261 (1989)
113.
[3] V. Climent, E. Herrero, J.M. Feliu, Electrochim. Acta, 44 (1998) 1403.
[4] E. Leiva, T. Iwasita, E. Herrero, J.M. Feliu, Langmuir, 13 (1997) 6287.
[5] G.E. Dima, V. Rosca, M.T.M. Koper, J. Electroanal. Chem., 599 (2007) 167.
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2094.
[7] J. Yang, M. Duca, K.J.P. Schouten, M.T.M. Koper, J. Electroanal. Chem., 662 (2011) 87.
[8] R. Gómez, A. Rodes, J.M. Orts, J.M. Feliu, J.M. Perez, Surf. Sci., 342 (1995) L1104.
[9] M.C.P.M. daCunha, M. Weber, F.C. Nart, J. Electroanal. Chem., 414 (1996) 163.
[10] G.A. Attard, A. Ahmadi, J. Electroanal. Chem., 389 (1995) 175.
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4392.
[12] J.M. Feliu, A. Fernandez-Vega, J.M. Orts, A. Aldaz, J.Chim.Phys.Phys.-Chim.Biol., 88
(1991) 1493.
[13] V. Climent, R. Gómez, E. Herrero, J.M. Orts, A. Rodes, J.M. Feliu, Colloids Surf., A, 134
(1998) 133.
[14] J. Clavilier, J.M. Feliu, A. Fernández-Vega, A. Aldaz, J. Electroanal. Chem., 269 (1989)
175.
[15] E. Herrero, A. Fernández-Vega, J.M. Feliu, A. Aldaz, J. Electroanal. Chem., 350 (1993)
73.
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[17] Q.S. Chen, F.J. Vidal-Iglesias, J. Solla-Gullón, S.G. Sun, J.M. Feliu, Chem. Sci., 3 (2012)
136.
[18] J. Clavilier, J.M. Feliu, A. Aldaz, J. Electroanal. Chem., 243 (1988) 419.
[19] Q.S. Chen, A. Berna, V. Climent, S.G. Sun, J.M. Feliu, Phys. Chem. Chem. Phys., 12
(2010) 11407.
[20] V. Climent, E. Herrero, J.M. Feliu, Electrochem. Commun., 3 (2001) 590.
[21] J. Solla-Gullón, P. Rodríguez, E. Herrero, A. Aldaz, J.M. Feliu, Phys. Chem. Chem. Phys.,
10 (2008) 1359.
7Nitrite reduction on
bismuth modified Pt(111)
surfaces in different
electrolytic media
7. Nitrite reduction on Bismuth modified Pt (111)
surfaces in different electrolytic media.
7.1. Concepts
As already emphasized in previous chapters the electrochemical reduction of nitrite
has received continued attention over the past decades [1-3]. Its interest is mainly
connected with the treatment of nuclear waste and the synthesis of some nitrogen
containing compounds since it is one of the most reactive molecules in the nitrogen
cycle. In addition, the presence of nitrite in drinking waters is responsible of serious
diseases, like baby blue syndrome [4].
The electrocatalytic properties of well-defined surfaces modified with foreign adatoms
have been already stressed in previous chapters. It has been shown that this can be a
very efficient approach to achieve, in a controlled way, an enhancement of the
electrocatalytic properties, in particular on well-defined platinum surfaces [5-8]. In the
case of nitrate (Chapter 6) Pt(111)/Bi catalyze its reduction in a potential window
much higher (0.6-0.7 V vs RHE) than that found in the clean surface (0.2-0.1 V vs RHE),
forming N2O as main product. Nitrite is one of the most stable intermediates of nitrate
reduction and, for this reason, its reduction on modified Pt(111)/Bi surfaces was also
examined and will be reported in the present chapter.
The study of nitrite reduction has been almost exclusively made using pure metal
electrodes. One exception is the work of Da Cunha and Nart [9], who studied nitrite
reduction on platinum based electrodes with 10% rhodium by using DEMS and FTIR.
The results were similar to those obtained for pure Pt by Nishimura et al. [2], that is,
evolution of NO and N2O, but not of N2. They also observed a spectroscopic feature at
ca. 1820 cm-1
, which they ascribe to a dinitrosyl species (i.e., dimeric NO), presumably
as a precursor of N2O formation.
174 Chapter 7
In this Chapter, the catalytic activity of a Pt(111)/Bi modified electrode for nitrite
reduction will be reported for electrolyte solutions with different pH. Different
coverages of adatom were also prepared in order to quantify the catalytic properties
of the adatoms for the reduction reaction. Results reported here stress the important
role of adatoms to enhance the reactivity of platinum for reduction of nitrogenated
species in a potential range where unmodified platinum is totally inactive. The
presence of the foreign adatom shifts nitrite reduction to potentials as high as 0.80–
0.60 V vs RHE, coinciding with the potential at which Bi undergoes its redox surface
reaction. The magnitude of the catalytic effect is quantified through charge integration
as a function of adatom coverage, revealing that the activity increases with the
amount of Bi until its coverage approaches half the saturation of the maximum surface
blockage. The activity decreases steeply, for higher coverages resulting in a Volcano-
like curve. Apart from the acidic media, the experiments were also performed at
neutral media (pH 7) where Bi stability on the surfaces is higher. The
spectroelectrochemical experiments show that the main product of nitrite reduction at
these high potentials is N2O.
7.2. Cyclic Voltammetry results of nitrite reduction on
Pt(111)/Bi
As mentioned above, the catalytic effect of adsorbed bismuth adatoms on the Pt(111)
surface towards nitrate reduction in 0.1 M HClO4 media (Chapter 6) shows that for
Pt(111)/Bi a net reduction current can be seen at potentials significantly higher (0.62-
0.67 V) than on the unmodified electrode [7]. In order to test the possible catalytic
activity of this modified electrode toward more reduced nitrogen compounds, nitrite
reduction was also investigated.
Nitrate reduction on Bi modified Pt(111) surfaces 175
In Fig. 7-1, the cyclic voltammograms (CVs) used for the characterization of both the Bi
modified (θBi = 0.22) and the Pt(111) unmodified surfaces in 0.1 M HClO4 are shown. As
previously discussed in Chapters 3 and 6 the presence of Bi is characterized by a redox
process at 0.67 V. The contribution of uncovered Pt (111) sites is reflected in the
remaining hydrogen adsorption / desorption charge at potentials below 0.3 V,
indicating that the surface is not fully covered by Bi.
0.0 0.2 0.4 0.6 0.8 1.0
-200
-100
0
100
200
E (V) vs RHE
Pt(111)
Pt(111)/Bi
j (µ
A/c
m2)
Figure 7 - 1 – Cyclic voltammograms corresponding to Pt(111) (dashed) and Pt(111)/Bi (solid)
electrodes in 0.1 M HClO4 at 50 mVs-1
.
Fig. 7-2 shows the nitrite reduction on both, Pt(111) and Pt(111)/Bi surfaces in 0.1 M
HClO4. Similarly to nitrate reduction, the presence of Bi on the surface enhances nitrite
reduction at high potentials (0.8-0.6 V). This reduction peak has the onset at 0.80 V
and the maximum at 0.62 V, being superimposed with the Bi reduction in the cathodic
sweep. Clearly, this current corresponds to reduction of species coming from solution,
and not only to surface Bi reduction, since the charge under the peak (2290 µC/cm2) is
much higher than the charge of Bi on the blank voltammogram (107 µC/cm2).
176 Chapter 7
Moreover, clear reduction currents are observed on the positive going sweep,
overlapped with the surface Bi oxidation.
0.0 0.2 0.4 0.6 0.8 1.0
-400
-300
-200
-100
0
E (V) vs RHE
Pt(111)
Pt(111)/Bi
j (µ
A/c
m2)
Figure 7 - 2 - Nitrite reduction on Pt(111) (dotted line) and Pt(111)/Bi (solid line) electrodes,
in 0.1 M HClO4 and 0.002 M NaNO2. Sweep rate 10 mVs-1
. θBi 0.22.
For the unmodified Pt(111) surface, the reduction of nitrite coincides with two main
features of the voltammogram. Both peaks were reported before in the literature [10,
11]. The first one is a very broad peak, starting at 0.6 V and showing a maximum
around 0.4 V. This peak has been related with N2O formation. The lower potential
feature in the hydrogen adsorption region was also described in the literature, for
polycrystalline Pt, and has been attributed to hydroxylamine formation [1, 10, 11]. In
the modified electrode, Pt(111)/Bi, the broad peak between 0.6 and 0.3 V almost
completely disappears, and the peak at potentials lower than 0.3 V decreases as a
consequence of the partial blockage of the surface by Bi adatoms, indicating that Bi
doesn’t catalyze these processes. This low potential process needs clean Pt sites to
occur, and even small amounts of adsorbed NO can inhibit the reaction, as it was
previously reported [12].
Nitrate reduction on Bi modified Pt(111) surfaces 177
In the positive-going sweep for Pt(111)/Bi a small positive feature can be observed
around 0.6 V, overlapped with the negative currents due to nitrite reduction. The small
oxidation current in this potential region corresponds to Bi surface oxidation, which is
almost totally masked by the reduction of the nitrogen containing species.
One important observation is that the stability of Bi on the surface is reduced by the
presence of nitrite in acidic media. This point is illustrated in Fig 7-3 (A), where two
consecutive sweeps for the reduction of nitrite on a Pt(111)/Bi (θ=0.18) surface are
included. On the other hand, Figure 7-3(B) reports the characterization of the modified
surface in acidic media before and after the experiment in the nitrite containing
solution.
In Fig 7-3A it is possible to verify that the reduction currents at high potentials,
associated to the presence of bismuth on the surface, decrease from the 1st
to the 2nd
cycle while the low potential currents associated with the free platinum domains
increase with cycling. In fig 3B it is possible to observe that the surface coverage of Bi
decreases from 0.18 to 0.12 after performing only two cycles in the acidic nitrite
solution. The results show that the effect of cycling the Bi modified electrode in nitrite
containing solution removes up to 30% of the Bi layer. The loss of activity in the
potential range where Pt(111)/Bi surface presents catalytic activity is about 52%
revealing that the changes in the adatom coverages have important effect on the
catalytic ability of the surface. This effect will be discussed again below. Regarding the
increasing activity of the Pt active sites, the current of the peaks at potentials lower
than 0.4 V only increases around 20% suggesting the possibility that some poisoning
species remain on the available sites. Those results strongly suggest that nitrite in
acidic media can remove the Bi from the surface. The stability of bismuth on the
surface improves at higher coverages, but these high covered surfaces have poor
electrocatalytic properties.
178 Chapter 7
0.0 0.2 0.4 0.6 0.8 1.0-300
-250
-200
-150
-100
-50
0
50
100
1st cycle
2nd cycle
E (V) vs RHE
j (µ
A/c
m2)
A
0.0 0.2 0.4 0.6 0.8 1.0
-150
-100
-50
0
50
100
150 Before NO-
2
After NO-
2
B
Figure 7 - 3 - (A) Nitrite reduction on Pt(111)/Bi surface (θBi 0.18) in 0.1 M HClO4 and 0.002 M
NaNO2 at 10 mV/s, first (solid line) and second (dotted line) cycles. Sweep rate 10 mVs-1
; (B)
Pt(111)/Bi modified electrode in 0.1 M HClO4 before (solid line) and after (dashed line) cycling
in nitrite containing solution.
In Fig.7-4 the CVs obtained for nitrite reduction in phosphate buffer (pH 7.2) for
Pt(111) and Pt(111)/Bi electrodes are given. The nitrite reduction on Pt(111) at neutral
pH shows slightly different behavior in comparison to that observed in acidic media.
Two main features can be observed, one at 0.1V in the negative going sweep and the
other at 0.2 V in the positive going sweep. Interestingly, no “prewave” (reduction
current between 0.6 and 0.4 V) is observed at this pH on the unmodified electrode.
Nitrate reduction on Bi modified Pt(111) surfaces 179
0.0 0.2 0.4 0.6 0.8 1.0-300
-200
-100
0
j (µ
A/c
m2)
Pt(111)
Pt(111)/Bi
E (V) vs RHE
Figure 7 - 4 - Nitrite reduction on Pt(111) (dotted line) and Pt(111)/Bi (solid line) electrodes,
in 0.05 M NaH2PO4 + 0.05 M Na2HPO4 (pH 7.2) and 0.002 M NaNO2, 10 mVs-1
. θBi 0.18.
In the case of the Pt(111) surface modified with Bi adatoms, catalytic reduction
currents at high potential are observed, in a similar potential range (vs the RHE) to
those observed in acidic pH (0.8-0.6 V). In this case, the reduction process in the
negative-going sweep amounts to 504 µC/cm2, while in the blank recorded in the
absence of nitrite the Bi charge is only 82 µC/cm2. Moreover, both processes (Bi
reduction and nitrite reduction) can be clearly distinguished since the latter appears as
a shoulder on the high potential side of the Bi reduction peak. It should be reminded
that nitrate reduction was catalyzed by Bi on Pt(111) only in acid media but not in the
neutral phosphate buffered solution. Three features are observed at potentials lower
than 0.4 V, the main one being a peak at 0.1 V in the negative going sweep. In the
positive going sweep two peaks are observed at 0.1 V and 0.2 V. The current of the
peaks at lower potentials decreases in the presence of Bi on the surface, although the
peak potentials are the same on both electrodes probably suggesting that they are
180 Chapter 7
related with nitrite reduction on platinum sites. No stability problem of the bismuth
adlayer is detected at this higher pH, contrarily to the observation in 0.1 M HClO4,
what likely suggests the absence of the species that is capable of removing the
irreversibly adsorbed Bi from the surface. In this respect, it is known that nitrous acid
can disproportionate into NO (NO and NO2 or NO and HNO3) in acidic media but in
neutral media NO2- is stable [3]. So the stability of the Bi layer can be related with the
presence of NO in solution.
In order to evaluate the influence of NO on the stability of the adlayer, some
experiments were performed as shown in Figure 7-5. The characterization cell was
prepared with phosphate buffer at pH 7.2 (0.05 M Na2HPO4 + 0.05 M NaH2PO4) in the
absence of nitrite. The Pt(111)/Bi electrode was prepared as described above and
characterized in this solution (Fig. 7-5 – solid line). After recording the corresponding
CV, NO was adsorbed on the electrode, immersing it for 20 s in a solution prepared by
bubbling NO gas through phosphate buffer solution at pH 7.2. This ensures that the
main effect on the surface composition is due to NO adsorption and not to the acidity
of the media. The NO layer was reduced from the surface (Fig 7-5 – dashed line) and a
final blank CV of the Pt(111)/Bi electrode was acquired (Fig 7-5 – dotted line). This
experiment demonstrates that the adsorption of NO almost completely removes the Bi
from the surface even at a neutral pH. This result suggests that the loss of Bi from the
surface is related to the presence of NO in the solution and most likely to its
concentration, and not directly related to the pH of the solution.
Nitrate reduction on Bi modified Pt(111) surfaces 181
0.0 0.2 0.4 0.6 0.8 1.0-200
-150
-100
-50
0
50
100
150 Pt(111)/Bi
NO reduction
Final surface
E (V) vs RHE
j (µ
A/c
m2)
Figure 7 - 5 – Pt(111)/Bi surface in 0.05 M Na2HPO4 + 0.05 M NaH2PO4 (pH 7.2), 50 mV/s (solid
line), NO layer reduction in the same surface, 5 mV/s (dashed line) and final CV 50 mV/s
(dotted line).
Fig.7-6 shows the charges involved on the reduction peak at 0.8-0.6 V measured on the
negative scan, plotted against the Bi coverage on the surface in both pHs under study.
The charge corresponding to the bismuth redox process has been subtracted from the
total charge, integrated in the high potential region, to obtain the net charge
corresponding to nitrite reduction. In the two cases, the electrocatalytic activity of the
surface increases with the presence of Bi until its coverage is higher than one half of
the maximum coverage. After that, the activity decreases dramatically. This result
suggests that the reduction reaction occurs on the platinum sites on the borders of the
Bi islands formed on the surface, and not on the bismuth adatoms itself. In other
words, Bi-Pt pairs are necessary for the catalytic enhancement similarly to the
behavior observed for nitrate reduction (Chapter 6).
182 Chapter 7
0.07 0.14 0.21 0.28 0.350
500
1000
1500
2000
2500
θBi
Qre
d(µ
C/c
m2)
θBi
0.1M HClO4
A
0.07 0.14 0.21 0.28 0.35
200
250
300
350
4000.05M Na
2HPO
4 + 0.05M NaH
2PO
4
B
Figure 7 - 6 - Variation of the reduction charge between 0.8 and 0.6 V with the initial Bi
coverage on the Pt(111) electrode in A) 0.1 M HClO4; B) 0.05 M NaH2PO4 + 0.05 M Na2HPO4
(pH 7.2).
As discussed in Chapter 6, this type of catalytic behavior for bimetallic electrodes is
similar to that observed previously in the case of formic acid oxidation [6]. For Bi on
Pt(111), it was also observed that the adsorption of Bi enhances the activity of the
platinum sites for the direct oxidation, suppressing almost completely the poison
formation (CO). The most active sites were those close to a neighbor Bi covered site.
In Chapter 6, it was observed that the enhancement of the catalytic activity of
Pt(111)/Bi electrodes for nitrate reduction was due to a combination of a third body
and a true catalytic effect. As well as for nitrate, nitrite reduction dependence on the
Bi coverage in acidic media (Figure 7-6 A) has the typical shape of the third body effect,
as deduced from the existence of an inflection in the behavior at low coverages (θBi =
0.13). The maximum of the curve is obtained for coverages higher than 0.20. The
observed dependence of the catalytic activity on Bi coverage can be explained in term
Nitrate reduction on Bi modified Pt(111) surfaces 183
of the poisoning behavior of NO, just as in the case of nitrate and also in the same way
as CO is a poison for organic molecules oxidation [6]. In this case, Bi avoids the NO
poisoning of the surface through a third body effect, allowing nitrite to reduce at
higher potentials. At low coverages, the probability of forming an ensemble capable of
avoiding poison formation is very low and, for this reason, the catalysis is lower at
small Bi coverages. In this regard, the curve in figure 7-6A resembles the behavior
observed for formic acid oxidation on Sb covered Pt(100) electrodes, where such a
third body effect has been proposed [6]. However, this explanation should be taken
with caution, since, as previously discussed, the stability of bismuth on the surface is
compromised by the presence of nitrite in solution, making the measurement of the
coverage at low values uncertain. It should be stressed that this complication was not
present for nitrate reduction and still the shape of the curves was very similar.
In neutral media, the shape of the curve is slightly different and charges increase faster
for small Bi coverages. This different behavior is possibly related with the smaller
amount of NO in solution at this pH. Nitrite decomposition in neutral media is
practically absent what means that NO presence is very low, while in acidic media
nitrite decomposes into NO. The fact that NO amounts are smaller decreases the
poisoning degree of the surface allowing Pt(111)/Bi surface, to catalyze nitrite
reduction through a direct catalytic effect.
7.3. FTIR results
In order to identify the species involved in the catalytic reduction at high potentials
some IR experiments were made.
In the spectra below the positive bands correspond to the products formed during
nitrite reduction, while negative bands account for the consumption of species present
at the reference potential. The contact of the electrodes with the nitrite solution was
184 Chapter 7
made at controlled potential (0.90 V). This potential was maintained until the
electrode was pressed against the CaF2 window. After collecting the reference
spectrum, the potential was stepped to progressively lower potentials down to 0.05 V
and then increased again back to 0.90 V. We only show here the negative going sweep
since no significant differences were observed between both scans. The sample
potential of each spectrum is noted in the figures.
Figure 7-7 shows IR spectra obtained for nitrite reduction in acidic media in D2O for the
surfaces under study. The spectra in H2O were also collected but no significant
differences with those presented here were observed. The use of D2O was chosen here
since it allows better definition in the region around 1600 cm-1
. From the CV results it
was expected an enhancement of the N2O formation and a shift to high potentials, but
the IR spectra are very similar for Pt(111) and Pt(111)/Bi. Both surfaces show bipolar
bands at 1700-1649 cm-1
, corresponding to adsorbed NO already present at the
reference potential. To explain the absence of features associated to the presence of
bismuth on the Bi/Pt(111) electrode we must recall that bismuth is unstable when
nitrite is present in solution. Therefore, bismuth coverage associated to the FTIR
experiment and stabilization of the thin layer is greatly reduced after the relatively
longer times of experiment and the vibrational features associated to the catalytic
currents corresponding to bismuth are probably below the detection limit. Bismuth
coverage was checked after the spectra collection by recording a new voltammogram
and it was found not possible to achieve conditions for an intermediate coverage
(where catalytic effect is maximum) stable during the FTIR measurements.
Nitrate reduction on Bi modified Pt(111) surfaces
Figure 7 - 7 – In situ FTIR spectra for nitrite reduction on Pt(111) and Pt(111)/Bi electrodes.
Test solution: 0.1 M HClO4 + 0.002 M NaNO
potential indicated in the figure.
The positive bands at 1280 cm-1
observed on both surfaces at low potential correspond
to the D2O bending vibration and are due to changes in the composition of the t
layer (e.g. pH variation) associated with nitrite reduction. These bands appear in the
same frequency region where nitrite bands are expected (1239 cm
why nitrite consumption bands are not observed in this medium. Another small band
can be observed at 1469 cm-1
, likely due to changes in DOH concentration, formed in
the vicinity of the electrode from H2
bismuth adsorption.
Nitrate reduction on Bi modified Pt(111) surfaces 185
In situ FTIR spectra for nitrite reduction on Pt(111) and Pt(111)/Bi electrodes.
+ 0.002 M NaNO2 in D2O. Reference potential 0.9 V. Sample
potential indicated in the figure.
observed on both surfaces at low potential correspond
O bending vibration and are due to changes in the composition of the t
pH variation) associated with nitrite reduction. These bands appear in the
rite bands are expected (1239 cm-1
) and may explain
why nitrite consumption bands are not observed in this medium. Another small band
, likely due to changes in DOH concentration, formed in
2O traces that remain on the electrode surface after
185
In situ FTIR spectra for nitrite reduction on Pt(111) and Pt(111)/Bi electrodes.
O. Reference potential 0.9 V. Sample
observed on both surfaces at low potential correspond
O bending vibration and are due to changes in the composition of the thin
pH variation) associated with nitrite reduction. These bands appear in the
) and may explain
why nitrite consumption bands are not observed in this medium. Another small band
, likely due to changes in DOH concentration, formed in
traces that remain on the electrode surface after
186 Chapter 7
2000 1600 1200
0.9
0.8
0.7
0.6
0.1
0.3
0.5 A
bso
rban
ce u
.a
0.005 u.a
0.7
2230 cm-1
1238 cm-1
2000 1600 1200
1341 cm-1
Pt(111)/Bi (θ = 0.20)
2230 cm-1
1239 cm-1
Wavenumbers cm-1
0.1
0.3
0.5
0.7
Pt(111)
0.9
0.8
0.7
0.6
1800 1700 1600 1500 1400
1680-1640 cm-1
Abso
rban
ce u
.a.
Wavenumbers cm-1
1800 1700 1600 1500 1400
Ab
sorb
ance
u.a
.
Wavenumbers cm-1
1680-1640 cm-1
Figure 7 - 8 - In situ FTIR spectra for nitrite reduction on Pt(111) and Pt(111)/Bi electrodes.
Test solution: 0.05 M Na2HPO4 + 0.05 M NaH2PO4 and 0.002 M NaNO2 in water. Reference
potential 0.9 V. Sample potential indicated in the figure. (Insets show the corresponding
spectra in the NO stretching region collected in D2O, reference potential at 0.1 V).
The spectra obtained for Pt(111) and Pt(111)/Bi at neutral pH are presented in Fig. 7-8.
Experiments with both water and heavy water were also done at this pH. Figure 7-8
shows the results in H2O, since this media allows a better definition in the region of the
bands associated with nitrite consumption, that are otherwise overlapped with the
D2O bending vibration. The inset shows the NO region of the spectra collected in D2O,
at different potentials and using 0.1V as reference potential to obtain absolute bands
of the adsorbed NO.
The spectra for Pt(111)/Bi show a negative band (1239 cm-1
) at high potentials (0.7 V)
corresponding to nitrite consumption. Simultaneously, another negative band can be
Nitrate reduction on Bi modified Pt(111) surfaces 187
observed at 1341 cm-1
. These bands were reported in the literature [13] and assigned
to nitrite: the asymmetric stretching wavelength at 1237 cm-1
and the weak shoulder
at 1340 cm-1
attributed to the symmetric stretching mode. Still, the wavenumber of
this shoulder normally overlaps with the nitrate signal. To verify the vibration
frequencies of these two species in neutral media, vibrational spectra of solution
species were acquired with an ATR configuration. The spectra obtained for nitrite and
nitrate in neutral media, using the phosphate buffer as reference, are given in fig. 7-9.
The results show that in phosphate media at pH = 7, the nitrate band is observed at
1351 cm-1
while for nitrite, two bands can be observed, the main band at 1238 cm-1
and a small shoulder at 1341 cm-1
. These results allow to attribute the bands observed
after nitrite reduction in neutral media on the Pt(111)/Bi modified electrode to a
higher consumption of nitrite.
At high potentials (0.5-0.7 V), where the Bi modified Pt(111) surface showed catalytic
activity in the CV experiments, it is possible to identify N2O (positive band at
2232 cm-1
) as a product of the nitrite reduction (Fig 7-8). On the unmodified electrode,
the nitrite consumption and consequent N2O production is only observed at potentials
lower than 0.3V. Adsorbed NO was identified on both surfaces, but was more clearly
visible in the spectra collected in D2O and using 0.1 V as reference potential (Fig 7-8
inset).
188 Chapter 7
1400 1200
Wavenumber (cm-1)
NO-
3
NO-
2
1351
1238
1341
0.001 a.u.
Figure 7 - 9 – ATR spectra of 0.1 M nitrite (solid line) and 0.1 M nitrate (dashed line) in 0.05 M
Na2HPO4 + 0.05 M NaH2PO4, 100 scans and 8 cm-1
.
7.4. Conclusions
This Chapter reports the catalytic activity of the Pt(111)/Bi modified surface towards
nitrite reduction, and the quantification of this catalytic behavior with different
adatom coverages. The CVs for nitrite reduction in the presence of Bi on the Pt(111)
surface reveal a significant reduction process at high potentials (0.8-0.6 V), both in
acidic and neutral media. This reduction coincides with the potential region where Bi
undergoes its surface redox reaction. To quantify the effect of the adatoms on this
process, different Bi coverages on Pt(111) were studied. These results showed that the
catalytic activity in the potential region 0.8-0.6 V increases with the presence of Bi on
the surface until the coverage reaches a value about 0.2, and then starts to decrease.
These results indicate that free Pt sites are also necessary for the catalytic process at
Nitrate reduction on Bi modified Pt(111) surfaces 189
high potentials, suggesting that Bi catalysis is produced through electronic effects
changing the reactivity of neighboring platinum atoms. Conversely the reduction
process around 0.2 V decreases when the surface becomes covered by Bi because the
free Pt(111) domains also decrease.
IR measurements showed that N2O is the main detectable product associated with the
reduction at high potentials on the Pt(111)/Bi surface, on both pHs studied. NO was
observed in both media and for both modified and unmodified surfaces.
190 Chapter 7
References
[1] R.R. Gadde, S. Bruckenstein, J. Electroanal. Chem., 50 (1974) 163.
[2] K. Nishimura, K. Machida, M. Enyo, Electrochim. Acta, 36 (1991) 877.
[3] V. Rosca, M. Duca, M.T. de Groot, M.T.M. Koper, Chem. Rev., 109 (2009) 2209.
[4] P.M. Vitousek, J.D. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.H.
Schlesinger, D.G. Tilman, Eco. Applications, 7 (1997) 737.
[5] V. Climent, E. Herrero, J.M. Feliu, Electrochim. Acta, 44 (1998) 1403.
[6] E. Leiva, T. Iwasita, E. Herrero, J.M. Feliu, Langmuir, 13 (1997) 6287.
[7] M.C. Figueiredo, J. Souza-Garcia, V. Climent, J.M. Feliu, Electrochem. Commun., 11
(2009) 1760.
[8] V. Climent, N. García-Araez, J.M. Feliu, in: M.T.M. Koper (Ed.) Fuel Cells Catalysis. A
Surface Science Approach, John Wiley & Sons, Inc., Hoboken, New Jersey, 2009, pp.
209.
[9] M.C.P.M. da Cunha, F.C. Nart, Phys. Status Solidi A, 187 (2001) 25.
[10] I.T. Bae, R.L. Barbour, D.A. Scherson, Anal. Chem., 69 (1997) 249.
[11] M. Duca, V. Kavvadia, P. Rodriguez, S.C.S. Lai, T. Hoogenboom, M.T.M. Koper, J.
Electroanal. Chem., 649 (2010) 59.
[12] R. Gómez, A. Rodes, J.M. Perez, J.M. Feliu, J. Electroanal. Chem., 393 (1995) 123.
[13] R.T. Yang, M.J.D. Low, Anal. Chem., 45 (1973) 2014.
8NO adsorption on Pt(111)
bismuth modifies surfaces
8. NO adsorption on Pt(111) bismuth modified
surfaces
8.1. Concepts
Together with carbon monoxide (CO), nitric oxide is one of the most common
pollutants nowadays [1, 2]. It has been shown that the accumulation of these
compounds can be potentially more dangerous than that of CO2.
As referred in the Introduction, NO is also an important intermediate in environmental
and industrial important reactions such as nitrate reduction and ammonia oxidation or
hydroxylamine production [3]. In previous Chapters (6 and 7) it was shown that NO
seems to act as poison for nitrate and nitrite reduction on Pt electrodes. The behavior
of NO at well defined surfaces acquires also special importance because its use as
probe molecule to test surface morphology [4-6].
For these reasons, the study of the NO adsorption on Pt(111)/Bi surfaces appears as an
important issue to be understood in the framework of this thesis.
The study of the coadsorption of adsorbed molecules with irreversible adsorbed
adatoms has only been reported previously essentially for CO [7-10]. Some studies
with NO on bimetallic surfaces were also published [11, 12]. In these cases the foreign
adatoms were also able to adsorb NO and island formation was proposed as
conclusion from of the constancy of the IR band frequencies on both metals at
different adatoms coverage. For the specific case of irreversible adsorbed Bi, its
coadsorption with CO revealed a formation of an intermixed adlayer on Pt(111) [7-9].
For systems like Pt(111)/Cu-CO [8] or Pt/S-CO [9] segregated layers were found.
In this Chapter, the results obtained for the coadsorption of Bi and NO on Pt(111) are
reported. Cyclic voltammetry, FTIR Spectroscopy and in situ STM were used to
194 Chapter 8
characterize the adlayer formed by NO after the modification of the Pt(111) surface
with partially covered Bi, in order to draw a more comprehensive picture of this
system. The results showed that when NO is adsorbed on the Pt(111) surface modified
with Bi, a segregated adlayer is formed. The coadsorption of NO leads to the formation
of Bi islands that are not observed when NO is not present.
8.2. Cyclic voltammetry for NO and Bi coadsorption on
Pt(111)
The reduction of adsorbed NO on Pt(111) surfaces modified with irreversible adsorbed
Bi was evaluated by voltammetry. The results obtained for the NO stripping on clean
Pt(111) and Pt(111) modified with Bi adatoms are compared in fig 8-1. The
voltammetric profile obtained for the unmodified surface is similar to those reported
earlier on the literature [13-16]. The Pt(111) surface (Figure 8-1, solid line) is blocked in
the presence of adsorbed NO between 0.8 and 0.45V. At lower potentials, reduction
currents due to NO stripping can be observed. Two main features can be distinguished
in the potential range between 0.45 and 0.05V. The first at 0.35V has been attributed
to atop NO reduction and the second at 0.26V to the reduction of bridge NO [16]. After
the total reduction of the NO adlayer on the surface, the blank CV is totally recovered.
When Bi is present on the surface no significant changes are observed on the NO
reduction profile. While OH adsorption on Pt is absent on the NO covered surface, the
Bi redox peaks are still observed at 0.67-0.68V. In the lower potential region, both atop
and bridge NO reduction peaks are visible. As expected, these peaks are less intense
than on Pt(111), because they are due to NO reduction on the free Pt sites that are left
after Bi adsorption.
An interesting result worth noting is the negative shift on the Bi redox peaks occurring
when NO is coadsorbed at the surface. As previously stated, the redox process of
NO adsorption on Pt(111) Bi modified surfaces 195
adsorbed Bi in perchloric acid is observed at 0.679 V, while it shifts to 0.652 V (27 mV
more negative) when NO is coadsorbed. After reducing the NO layer, the Bi
characteristic peaks are displaced back to the usual potential. The results suggest
some interaction between the two coadsorbed species. The interactions could be
explained by the formation of mixed or segregated adlayers of Bi and NO on the
Pt(111). In order to obtain more information about the structure of the coadsorbed
adlayer, more experiments were done such as IR and STM that will be presented
below.
0.0 0.2 0.4 0.6 0.8 1.0-15
-10
-5
0
5
10
0.679V
Pt(111)
Pt(111)/Bi
j (µ
A/c
m2)
E (V) vs RHE
0.652V
Figure 8-1 – NO reductive stripping on Pt(111) (thin line) (θNO = 0.38) and Pt(111)/Bi (solid
line, θBi = 0.10, θNO = 0.22 ) in 0.1M HClO4 , 2 mV/s.
Another important result is that the coverage of Bi on the surface always decreases
after contacting the electrode with the solution containing NO (Figure 8-2). The
decrease of Bi is reflected in the decrease of the Bi peak and the recovery of the
hydrogen adsorption region.
196 Chapter 8
0.0 0.2 0.4 0.6 0.8 1.0
-200
-100
0
100
200
E (V) vs RHE
j (µ
A/c
m2)
before NO
after NO
Figure 8-2 – CV for Pt(111)/Bi modified surface before (solid line, θBi = 0.15) and after (dashed
line, θBi = 0.08) 1 min in contact with NO containing solution, in 0.1M HClO4 , 50mV/s.
The loss of Bi is higher when the initial coverage is smaller (eg.20% is lost at a high
coverage - θBi 0.30 - while 55% lost at a low coverage - θBi 0.06) for the same time of
immersion on the NO containing solution. If the time that the electrode is in contact
with the NO solution is long enough, the blank CV can be fully recovered. This
behaviour was described before for the nitrite reduction on Pt(111)/Bi in acid media
(Chapter 7) [17], where it was shown that the presence of NO is able to remove Bi
from the surface. Similar behaviour was also reported by Smith et al [18] in the
presence of different anions. The authors reported that, when the strength of anion
adsorption increases, the stability of the Bi adlayer decreases, and, for example, the
bromide affinity for Pt(111) causes the complete displacement and dissolution of the
Bi adlayer.
NO adsorption on Pt(111) Bi modified surfaces 197
From the Bi peak recorded before NO stripping it can be confirmed that Bi desorption
take place during NO adsorption and not during NO reduction. For this reason all the
coverages reported are referred to the final coverage of Bi (after NO adsorption).
On figure 8-3, the CV obtained for the NO stripping on the Pt(111)/Bi surface with
sequentially lower potential limits is presented. If the NO is partially stripped
(decreasing NO coverage) from the surface, the potential of the Bi redox peaks move
sequentially to higher potentials to reach the usual value of 0.67V when all NO is
completely removed. The charge involved on the Bi redox peak is also affected by the
amount of NO on the surface (Figure 8-4).
0.2 0.4 0.6 0.8
-100
0
100
1st cycle (Elower
=0.50V)
2nd cycle (Elower
=0.35V)
3rd cycle (Elower
=0.25V)
4th cycle (Elower
=0.06V)
j (µ
A/c
m2)
E (V) vs RHE
Decreasing NO coverage
Figure 8-3 – CV´s obtained for NO reductive stripping on Pt(111)/Bi (θBi = 0.07) surface with
limiting potentials sequentially lower in 0.1M HClO4, 2 mV/s.
198 Chapter 8
0.0 0.2 0.4 0.6
0.648
0.656
0.664
0.672
0.680
E Bi
EB
i (V
) vs
RH
E
Decreasing NO coverage
0.0 0.2 0.4 0.6
36
38
40
42
44
46
Decreasing NO coverage
E lower (V) vs RHE
QB
i (µ
C/c
m2)
Q Bi
Figure 8-4 – Evolution of A) peak potential and B) Charge for the redox peaks of Bi after
partial desorption of the NO coadsorbed adlayer as shown in figure 8-3.
Another way to access to different NO coverages on the surface is by stripping the NO
at high scan rate. Under these conditions, the time at low potentials is not enough to
reduce the entire NO adlayer and some molecules remain on the surface. Figure 8-4
shows that the trends observed on the Bi redox peak are similar to those obtained
after the partial reduction experiments shown in figure 8-5. As the NO is being
stripped the redox peaks related to the adsorbed Bi (C) increase in potential. Moreover
the entire surface seems to go through a reorganization process; the free platinum
sites left by the NO reduction are gradually replaced by H (A) and OH (D) adsorption as
it is clearly seen by the increasing currents on the H region and in the butterfly peaks.
A couple of adsorption peaks (B), can be observed at 0.55V (just before the redox
peaks of Bi) after the first stripping cycle. These peaks are probably due to the
NO adsorption on Pt(111) Bi modified surfaces 199
adsorption of some product from the NO reduction reaction that is stripped from the
surface when cycling.
0.0 0.2 0.4 0.6 0.8 1.0
-100
-50
0
50
100
D
C
B
j (µ
A/c
m2)
E (V) vs RHE
NO stripping
A
Figure 8-5 – NO reductive stripping on Pt(111)/Bi surface on 0.1M HClO4, 50 mV/s.
The results obtained by cyclic voltammetry might be taken as an indication of the
existence of some interaction between Bi and NO when both molecules are
coadsorbed on the Pt(111) surface. The Bi redox behaviour on the surface is modified
when NO is present while it recovers the normal behaviour when NO is completely
eliminated from the surface. Interaction between Bi and other coadsorbed molecules
were previously observed, as in the case of CO [8, 9]. However, as it will be shown
below, FTIR and STM results indicates that no mixed adlayers are formed. An
alternative explanation for the influence of NO on Bi redox peak will be offered below.
200 Chapter 8
8.3. In situ IR spectroscopy results
The application of electrochemical in situ infrared spectroscopy offers interesting
opportunities for understanding the main changes in the structure and binding forms
of molecular adsorption on metal surfaces.
The NO IR adsorption bands have been described to be dependent of several factors
like NO coverage and the type of bonding. For Pt (111), at high NO coverages, there
are normally two bands in the spectra at ca. 1400-1500 cm-1
for the bridge bonded NO
and ca. 1700 cm-1
for atop NO [19-21].
1800 1600 1400
θ 0
Ab
sorb
ance
u.a
.
Wavenumber cm-1
1700
1685
1666
1600
1600
1574
1612
0.005 u.a
0.3 V
0.1 V
0.5 V
0.7 V
0.9 V
1700
1682
1661
1604
1597
1571
1612
ν 80cm-1/V
1800 1600 1400
ν 77cm-1/V
0.005 u.a
0.3 V
0.1 V
0.5 V
0.7 V
0.9 V
1800 1600 1400
ν 75cm-1/V
0.005 u.a
0.5 V
0.7 V
0.9 V
0.3 V
0.1 V
1700
1686
1669
1612
1599
1574
1612
0.1M HClO4 in D
2O
Pt(111)_Bi θ 0.27 Pt(111)_Bi θ 0.06 Pt(111)
Figure 8-6 – Spectra for NO adsorption on Pt(111)/Bi : A) θ =0.27 ; B) θ =0.06; C) θ =0, in 0.1M
HClO4 in D2O, 100 scans, 8cm-1
, Ref spectra 0.05V.
Some IR experiments were performed for NO adsorption on Pt(111) surfaces modified
with Bi. In figure 8-6 the spectra obtained for NO adsorption on Pt(111) with two Bi
coverages are presented. For the sake of comparison the spectra obtained for the
clean surface are also shown. The spectra were obtained at different potentials and
the reference spectra were taken at 0.05V, after reduction of the NO layer, in order to
provide absolute bands.
NO adsorption on Pt(111) Bi modified surfaces 201
The spectra show that there is no difference on the bands shape or frequency with the
increasing Bi coverage. For all the Bi coverages two bands are observed. The typical
bands at 1680-1700 cm-1
are due to NO adsorbed at high coverage [20, 21] as well as a
less intense one at 1612 cm-1
the wavenumber of this band is too high for bridge
bonded NO. Since no other species are expected on the electrochemical cell this band
can be due to (100) defects on the surface (NO band is expected at this wavenumber
on Pt(100) electrodes [20]) or to some tilted NO. It was demonstrated by LEED [22]
that adsorbed NO can exist on the surface atop tilted. This tilt will affect the energy of
the N-O bond, increasing its length and decreasing the wavenumber for the NO
vibration. It might also affect the corresponding band intensity, since this is
proportional to the variation of the dipole component perpendicular to the surface.
As it was referred before, no significant changes on the vibrational spectra were
obtained in the presence of Bi for adsorbed NO. If the coadsorption of NO and Bi
would form a mixed layer on Pt (111) it would be expected a difference on the
vibration frequency of NO, as observed before for the Bi coadsorption with CO [8, 9].
The vibration frequency of NO is strongly dependent on the applied potential due to
the Stark effect or/and potential (charge) dependence of the metal adsorbate bonding
[23]. The small vibration frequency increase observed for NO with different coverages
of Bi on Pt(111) is most likely related with the decrease of the NO coverage [21] than
with the presence of the adatom.
This kind of behaviour, when the vibrational spectra is unaffected by the presence of
the adatom suggests that the coadsorption of Bi and NO forms segregated adlayers,
where the Bi layer is expected to be in islands. Segregated adlayers were also observed
for systems like Pt with S and CO coadsorbed [9], Rh(100)/Cu – CO [10] or Pt(111)/Cu -
CO [8] .
It has been previously argued [24] that, even at coverages lower than 0.33, bismuth
adatoms are likely to be grouped in islands instead of in dispersed structures. The
202 Chapter 8
mean compactness of these islands should be the same as that proposed for the full
layer (0.33) following a (√3x√3)R30º distribution. It is interesting to remark that, in
H2SO4 electrolytes, even for considerable coverages of Bi, (bi)sulphate adsorption and
phase transition still occurs [24] as it can be observed by the presence of the
adsorption peaks on the CVs, suggesting the existence of a bidimensional order on the
surface. However, in the case of HClO4 electrolytes, this is not observed and even small
amounts of Bi inhibit the occurrence of the voltammetric features related with the
anion (OH) adsorption [25]. Therefore, it seems that the strength of anion adsorption
plays a role in the structure of bismuth adlayers. Under the light of those observations,
it can be proposed that NO coadsorption with Bi force the latter to aggregate to form
compact islands. There is still one issue that need further discussion: the effect of NO
coadsorption on the Bi peak potential. This effect could seem contradictory with the
existence of segregated domains since it seems to point towards the existence of an
intimate interaction between both adlayer. One explanation for this effect can be
given based on the existence of more compact structures on the Bi layer [26]. The low
surface temperature deposition of Bi on Pt(111) has been characterized using low-
energy electron diffraction (LEED). This study showed that submonolayers of Bi
progress with increasing coverage from a p(2x2) pattern, a diffuse (√3x√3)R30º, a “12-
spot ring”, a sharp c(4x2) to a “split p(2x2). LEED patterns for p(3x3) and p(4x4)
structures were also obtained after annealing at high temperatures. The models
presented by these authors are shown in fig 8-7.
NO adsorption on Pt(111) Bi modified surfaces 203
Figure 8-7 – Proposed structural models for submonolayers Bi on Pt(111) in UHV from
reference [26]. Platinum atoms are represented by dots, Bi by large spheres, dotted lines
show unit cells.
The possible existence of more compact Bi adlayers have also been proposed in
electrochemical environment. It was reported in previous works [18, 24, 27] that for
the formation of a second layer or the transition to a more compact arrangement of Bi
in Pt(111) two peaks can be observed in the voltammograms for the redox process.
The characteristic peak at 0.67 V is masked by a new peak that appears at lower
204 Chapter 8
potentials (0.61 V vs RHE). This layer was found to be quite unstable and it was
possible to observe it only after forcing the Bi deposition electrochemically. Compact
layers were also tentatively proposed for Pt(111)/Bi systems at higher pH and for some
stepped surfaces containing (111) terraces [28]. According to the literature data and
the results obtained, we can suggest that the displacement of the Bi redox peak in the
presence of NO on Pt(111) can be due to the formation of a more compact layer when
both species are coadsorbed on the surface.
It can be suggested that, in a similar way to what happens for the layer generated by
UPD of Bi [27] the decrease on the redox potential peaks is associated to a conversion
from a (√3x√3)R30º diluted layer to a more compressed phase top (3x3) (also observed
in UHV). After NO is stripped off, Bi adatoms are allowed to reorder again to the dilute
adlayer configuration, causing the changes on the redox potentials of Bi.
8.4. In situ STM experiments
In situ STM experiments were made to evaluate the influence of coadsorbed NO on
the Bi layer on Pt(111) surfaces.
In situ STM experiments were carried out in 0.1M HClO4 electrolyte in single crystals
beads similar to those described in Chapter 2. Au STM tips were freshly prepared for
each experiment by electrochemical etching and coated with Apiezon wax. The use of
Au tips was preferred because of the possible interactions between the adsorbed NO
on the surface and the PtIr tips that could end up in the tips passivation. All STM
images were recorded in constant current mode with tunnelling currents ranging from
1.0 to 0.1 nA. The analysis of the images was made with WSxM from Nanotech [29].
From the best of our knowledge there is only one report concerning STM studies of Bi
modified Pt(111) surfaces. This paper shows the results obtained for Pt(111)/Bi
systems when CO is coadsorbed on the surface [7]. The reported results show the
NO adsorption on Pt(111) Bi modified surfaces 205
presence of an ordered layer having a (3x√3) unit cell, and that these domains are
composed of an intermixed adlayer. In the absence of CO, the authors suggest that Bi
forms an ordered layer near the terrace edges with a (3x3) unit cell, and a disordered
layer on the terraces. The presence of this compact layer on the terrace edges was
justified by a significantly higher local coverage at these regions. In our experiments,
Pt(111)/Bi systems were also tested but no differences between a clean Pt(111)
surface and Pt(111)/Bi were found (Fig 8.8).
Figure 8-8 - In situ STM images for Pt(111)/Bi systems at 0.8V.
For the system Pt(111)/Bi-NO the results are presented in the fig. 8-9. The results were
obtained under potential controlled condition (0.8 V vs RHE) in 0.1M HClO4 and the
surface was prepared as described before. The images were obtained with a set point
of 0.1nA and a potential bias of 0.1V in 100x100 nm of area and at 3.4 lines/s.
In agreement with the results from cyclic voltammetry and FTIR spectroscopy, the STM
image (fig.8-9) shows clearly the formation of compact Bi islands on the surface when
NO is also present.
206 Chapter 8
The islands appear very uniformly distributed across the surface, with an island
diameter ranging from 4 to 6 nm, with an average value of 5.0 nm. The islands seem to
nucleate without preference of terrace or steps. The data analysis for the pseudo
three-dimensional formations showed the presence of a layer with 1, 2 or 3 Bi atoms
high. The monolayer islands are more frequent and the 3 layer islands are normally the
bigger ones (6 nm large). Unfortunately no atomic resolution images were obtained.
Figure 8-9 – In situ STM images for Pt(111)/Bi-NO systems at 0.8V.
8.5. Conclusions
The adsorption of nitric oxide (NO) on Pt(111) surface modified with bismuth
irreversible adsorbed adatoms is reported. Voltammetric results reveal interaction
between the two coadsorbed compounds. In presence of NO, Bi redox peaks appear to
be 30 mV negatively displaced from the usual potential value. In situ infrared spectra
were obtained in the presence of coadsorbed NO and Bi. No significant differences
NO adsorption on Pt(111) Bi modified surfaces 207
were found on the characteristic vibrational frequencies of NO when Bi was present on
the surface, suggesting the formation of a segregated adlayer on the Pt(111) surface.
The presence of the segregated adlayer and the formation of Bi islands was confirmed
using scanning tunnelling microscopy. The obtained results showed that when NO is
adsorbed on the Pt(111) modified with Bi surface, a segregated adlayer is formed and
that the coadsorption of NO leads to the formation of Bi islands that are not observed
when NO is not present.
208 Chapter 8
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9Final remarks
9. Final Remarks
As discussed along this work, the study of the electrochemistry of nitrogen containing
inorganic compounds is nowadays a very important subject in electrocatalysis.
Electrochemical methods can offer alternative solutions for the problem of human
caused imbalance in the N cycle. However, electrochemistry of nitrogenated
compounds is complicated by the large number of stable oxidation states in N and, till
nowadays, no artificial process was found to be as efficient and harmless as nature.
The main objective of the work done in this thesis was to give a contribution from a
fundamental point of view, for the development and the understanding of Pt as
electrocatalyst for the electroreduction of nitrogen containing compounds with the
hope that knowledge of mechanistic aspects of these processes will help in the future
in the production of competitive electrochemical methods for elimination of harmful
nitrogenated compounds.
In the first part of the thesis (Chapters 4 and 5), nitrate and nitrite reduction were
studied on Pt(100) and its vicinal stepped surfaces. Both reaction turned out to be
extremely structure sensitive and a special behavior of (100) domains was found. The
catalytic activity was showed to decrease with the introduction of steps on the surface,
independently of the symmetry of the step, both for nitrate in neutral and nitrite in
alkaline media. In the case of nitrate in neutral media, the main product found was
ammonium that later can be oxidized to nitrate. On the other hand, nitrite reduction
produces N2 in a certain potential range. This is important since N2 is a desired
harmless product from nitrate and nitrite elimination. It has been demonstrated that
NOads and NHx,ads (from previous nitrite reduction) recombination constitute the
determining step for N2 formation.
212 Chapter 9
The most important conclusion from these two chapters, is the special ability of the
(100) surface for nitrogen containing compounds reduction. It has been revealed to be
a surface with unique ability to promote reactions involving bond-breaking or bond-
making. The key to the high catalytic activity of Pt (100) seems to be its capacity to
stabilize important intermediates as the case of NO and NHx that in alkaline media lead
to the formation of N2.
The second part of the thesis was devoted to study the effect of the presence of Bi on
Pt (111) surfaces on the electrocatalytic reduction of inorganic nitrogenated
compounds (nitrate – Chapter 6, nitrite – Chapter 7 and NO Chapter 8). The presence
of irreversible adsorbed bismuth on Pt(111) electrodes catalyzes nitrate and nitrite
reduction at potentials as high as 0.60 - 0.80 V, where N2O is the main detectable
product. It was shown that free Pt sites are also necessary for the catalytic process at
these potentials, and Bi is not responsible for the catalysis itself. It is interesting to
observe that oxidized Bi looks to be the responsible for this reduction process since in
the negative going sweep, the reaction is inhibited at high potentials after Bi has been
totally reduced and, in the positive going sweep, start suddenly when Bi is oxidized
again.
The quantification of the effect of adatoms on these processes was achieved by
measuring the catalysis provided by different Bi coverages on Pt(111) surfaces
(Chapters 6 and 7). This study, together with the characterization of the poison
formation on the modified surface, suggests a third body effect to describe Bi role on
the surface. In this case, the adatom impedes the NO poisoning from the solution,
resulting likely in a higher concentration of free Pt sites available for the reduction at
high potentials. The activity remains very low, for small Bi amounts on the surface,
since the probability of having enough number of adjacent Pt atoms to allow poison
formation would still be high. The current only starts to increase for a surface blockage
higher than half of the monolayer in all the cases (nitrate and nitrite at different pHs).
However, the fact that catalytic activity is closely related to the Bi redox process, as
Final remarks 213
mentioned above, with a sudden loss of activity after Bi reduction, is a clear indication
of the existence of an additional catalytic effect.
To end up this study, the adsorption of nitric oxide (NO) on Pt(111) surface modified
with bismuth irreversible adsorbed adatoms was also invetigated (Chapter 8). Since
the role of NO as a poisoning intermediate in nitrate and nitrite reductions
demonstrate in previous chapters, it is relevant to conclude the thesis with a study of
the coadsorption of this intermediate with the Bi layer. The formation of a segregated
adlayer was found between NO and Bi when both are coadsorbed on the Pt(111)
surface. It was found (Chapter 7) that NO was responsible for removing Bi from the
Pt(111) surface. At low dosages, the adlayer reorganise to form segregated islands. It
can be suggested that NO plays a double role on this catalytic reaction: it can act as
poison on the surface, impeding the reduction of nitrate and nitrite, but it also leads to
a Bi organization on the surface that may be the responsible for the catalysis. It was
observed that, for the catalytic effect to occurs, at least a small amount of NO need to
be formed on the surface or be present on the solution.
As final remark, it is important to stress once more the complexity of nitrogen
containing compounds reduction. The reaction depends on several factors, and the
changes of the adatom or the surface used as substrate are enough for the loss of
catalytic response. However, the findings presented in this thesis will certainly help to
a best understanding of the nitrate and nitrite reduction.
Publication list
Publications on the scope of the thesis
Marta C. Figueiredo, Janaína Souza-Garcia, Victor Climent, Juan M. Feliu, Nitrate
reduction on Pt(1 1 1) surfaces modified by Bi adatoms, Electrochemistry
Communications, 2009, Volume 11, Issue 9, Pages 1760-1763
Marta C. Figueiredo, Victor Climent, Juan M. Feliu, Nitrite Reduction on Bismuth
Modified Pt(111) Surfaces in Different Electrolytic Media, Electrocatalysis, 2011,
Volume 2, Number 4, Pages 255-262
Marta C. Figueiredo, José Solla-Gullón, Francisco J. Vidal-Iglesias, Víctor Climent, Juan
M. Feliu, Nitrate reduction at Pt(100) single crystals and preferentially oriented
nanoparticles in neutral media, Catalysis Today, 2012, doi:
10.1016/j.cattod.2012.02.038
Marta C. Figueiredo, José Solla-Gullón, Francisco J. Vidal-Iglesias, Víctor Climent, Juan
M. Feliu, Nitrate reduction on Platinum (111) surfaces modified with Bi: single
crystals and nanoparticles, Submited to Zeitschrift für Physikalische Chemie, 2012.
Matteo Duca, Marta C. Figueiredo, Victor Climent, Paramaconi Rodriguez, Juan M.
Feliu, and Marc T. M. Koper, Selective Catalytic Reduction at Quasi-Perfect Pt(100)
Domains: A Universal Low-Temperature Pathway from Nitrite to N2, J. Am. Chem.
Soc., 2011, Volume 133, Pages 10928-10939
Rosa Arán, Marta C. Figueiredo, José Solla-Gullón, Francisco J. Vidal-Iglesias, Víctor
Climent, Enrique Herrero, Juan M, On the behavior of the Pt(1 0 0) and vicinal
surfaces in alkaline media, Electrochimica Acta, 2011, Volume 58, Pages 184-192
Other publications:
216 Publication list
Marco Suárez; Marta C. Figueiredo, Juan Feliu, Voltammetry of Basal Plane Platinum
Electrodes in Acetonitrile Electrolytes: Effect of the Presence of Water, Langmuir,
2012, Volume 28, Pages 5286-5294.
Marco Suárez; Marta C. Figueiredo, Juan Feliu, Electrochemical and Electrocatalytic
Properties of Thin Films of Poly(3,4-Ethylenedioxythiophene) grown on Basal Plane
Platinum Electrodes, submited to Journal of Physical Chemistry, 2012.
