© 2009 ahmed moghieb - university of...

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ELECTROCHEMICAL OXIDATION OF ETHANOL USING NAFION ELECTRODES

MODIFIED WITH HETEROBIMETALLIC CATALYSTS

By

AHMED MOGHIEB

A THESIS PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2009

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© 2009 Ahmed Moghieb

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To my God who created us for the betterment of the entire world

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ACKNOWLEDGMENTS

First of all, I thank my GOD for all his blessings without which nothing of my work would

have been done. I would like to take the opportunity to express my deep appreciation to my

research advisor Prof. Lisa McElwee-White for her strong support and encouragement. I always

admire and respect her as a research mentor, under her supervision, I have learned many

scientific and precious aspects of research and this will be a good guideline for my future career.

I would also like to thank the Institute of International Education (IIE) for the award of the

Ford Foundation Fellowship, which has supported me during my two years of research. I cannot

end without thanking my family for their constant encouragement and love that I have relied on

throughout my time at the University. Finally, I am especially thankful for the patience and

support of my wife, Marwa.

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TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...............................................................................................................4

LIST OF TABLES ...........................................................................................................................7

LIST OF FIGURES .........................................................................................................................8

ABSTRACT ...................................................................................................................................10

CHAPTER

1 LITERATURE REVIEW .......................................................................................................11

Electro-Oxidation of Alcohols ................................................................................................11

Anode Catalysts ...............................................................................................................13 Mechanism of Ethanol Electro-Oxidation Using Heterogeneous Catalysts ....................14

Transition Metal Complex Catalysts ......................................................................................16 General Principle of Electrocatalysis ..............................................................................16 Ruthenium Oxo Complexes as Electrochemical Catalysts for Alcohol Oxidation .........16

Heterobimetallic Catalysts for the Electrooxidation of Alcohols ...........................................18 Synthesis ..........................................................................................................................19

Cyclic Voltammetry ........................................................................................................21 Product Detection ............................................................................................................23

Bulk Electrolysis with Complexes 1, 2, and 6 .................................................................24

2 CHEMICALLY MODIFIED ELECTRODES CONTAINING IMMOBILIZED

HETEROBIMETALLIC COMPLEXES................................................................................26

Introduction .............................................................................................................................26 Chemically Modified Electrodes ............................................................................................26

Electrocatalysis Using Chemically Modified Electrodes (CMEs) ..................................27 Polymer Modified Electrodes ..........................................................................................27

Electrodes Modified with Heterobimetallic Complexes .........................................................30 Preparation of Polymer Modified Electrodes ..................................................................30

Cyclic Voltammetry of Nafion Modified Electrodes ......................................................31

Effect of Electrolyte and Solvent ....................................................................................33

Electrolyte-solvent concentration ....................................................................................37 Effect of Ethanol Concentration ......................................................................................39 Effect of Scan Rate ..........................................................................................................39 Lifetime of the Electrode .................................................................................................40 Applications .....................................................................................................................41

Bulk Electrolysis .............................................................................................................47

3 EXPERIMENTAL SECTION ................................................................................................51

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General Methods .....................................................................................................................51

Synthesis ..........................................................................................................................51 Preparation of the Nafion Modified Electrode ................................................................51 Electrochemistry ..............................................................................................................51

Product Analysis ..............................................................................................................52

LIST OF REFERENCES ...............................................................................................................53

BIOGRAPHICAL SKETCH .........................................................................................................59

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LIST OF TABLES

Table page

1-1 Formal Potentials of Complexes 1-7..................................................................................21

1-2 Product Distributions and Current Efficiencies for the Electrochemical Oxidation of

Methanol by 1, 2, and 6. ....................................................................................................25

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LIST OF FIGURES

Figure page

1-1 A schematic representation of Direct Ethanol/Oxygen Fuel Cell . ....................................12

1-2 Mechanism of ethanol electro-oxidation at a platinum surface in acid medium. ..............15

1-3 Cyclic voltammograms of 1 under nitrogen in 2.5 mL of DCE/0.7 M TBAT

(tetrabutylammonium triflate); glassy carbon working electrode; Ag/Ag+ reference

electrode. ............................................................................................................................22

1-4 Cyclic voltammograms of 2 under nitrogen in 2.5 mL of DCE/0.7 M TBAT


electrode. ............................................................................................................................22

1-5 Cyclic voltammograms of 6 under nitrogen in 3.5 mL of DCE/0.1 MTBAT


electrode. ............................................................................................................................23

2-1 Generic illustration of a modified electrode . ....................................................................26

2-2 An ion-exchange polymer filled with free redox molecules (adapted from ref 88). .........28

2-3 (a): Structure of Nafion ......................................................................................................29

2-3 (b): Three-phase model of Nafion . ...................................................................................29

2-4 Heterobimetallic Ru/Pd complex 8. ...................................................................................31

2-5 Cyclic voltammograms in 0.1 M TBACF3SO3/ DCE, Ag/Ag+ reference electrode;

245 mM Ethanol; 50 mV/s a) Nafion blank, b) Nafion modified electrode with 10

mM of complex 8, and c) The difference between the voltammogram in the presence

and absence of ethanol for both Nafion blank and Nafion modified. ................................32

2-6 Cyclic voltammograms of Nafion blank and Nafion modified electrodes with 10 mM

of complex 8, in 0.1 M TBACF3SO3,TBAPF6,and TBABF4/ DCE, DCM, and AN,

245 mM of ethanol, 50 mV/s , and Ag/Ag+ reference electrode .......................................34

2-6 Continued. ..........................................................................................................................35

2-7 a) Cyclic voltammograms and b) plot of oxidation peak currents against electrolyte

concentration of Nafion modified electrode with 10 mM of complex 8 using 0.05-

0.5M TBACF3SO3/DCE, 245 mM of ethanol, 50 mV/s , and Ag/Ag+ reference

electrode. ............................................................................................................................37

2-8 a) Cyclic voltammograms of Nafion modified electrode with 10 mM of complex 8,

b) plot of oxidation peak currents against ethanol concentrations (98-489 mM), and

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c) Cyclic voltammograms of Nafion blank electrode using 0.1M TBACF3SO3/DCE,

50 mV/s , and Ag/Ag+ reference electrode. ......................................................................38

2-9 a) Cyclic voltammograms, b) plot of oxidation peak currents against the square root

of the scan rates of Nafion modified electrode with 10 mM of complex 8 using 0.1M

TBACF3SO3/DCE, 245 mM of ethanol, and Ag/Ag+ reference electrode. .......................40

2-10 a) Cyclic voltammograms, b) plot of oxidation peak currents against the number of

times using the same Nafion modified electrode with 10 mM of complex 8 using

0.1M TBACF3SO3/DCE, 245 mM of ethanol, 50 mV/s ,and Ag/Ag+ reference

electrode. ............................................................................................................................41

2-11 Structures of compounds 8-11. ............................................................................................42

2-12 Cyclic voltammograms with 10 mM of complex 8 a) glassy carbon working

electrode in 0.7 M TBACF3SO3/DCE; 50µl methanol b) Nafion modified electrode

of the complex in 0.1M TBACF3SO3/DCE, 245 mM (50µl) ethanol, 50 mV/s ,and

Ag/Ag+ reference electrode. ...............................................................................................44

2-13 Cyclic voltammograms with 10 mM of complex 9 a) 5 mM, glassy carbon working

electrode; 50µl methanol b) 10 mM, Nafion modified electrode, 245 mM (50µl)

ethanol, in 0.1M TBACF3SO3/DCE, 50 mV/s ,and Ag/Ag+ reference electrode. .............45







2-16 Product distribution for the electrolysis of methanol using 10 mM of Nafion/complex

8 vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF3SO3/DCE, 350 mM

methanol, and Ag/Ag+ reference electrode. .......................................................................48

2-17 Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex 8

vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF3SO3/Ethanol, 245

mM ethanol, and Ag/Ag+ reference electrode. ..................................................................49

2-18 Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex 9

vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF3SO3/Ethanol, 245


2-19 Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex

11 vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF3SO3/Ethanol, 245


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Abstract of Thesis Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Master of Science

ELECTROCHEMICAL OXIDATION OF ETHANOL USING NAFION ELECTRODES

MODIFIED WITH HETEROBIMETALLIC CATALYSTS

By

Ahmed Moghieb

December 2009

Chair: Lisa McElwee-White

Major: Chemistry

This work describes the preparation and the use of chemically modified electrodes with a

Nafion /catalyst film on the electrode surface. The electrochemical oxidation of ethanol has been

studied at a Nafion/ Cp(PPh3)Ru(μ-I)(μ-dppm)PdCl2 coated glassy carbon electrode using cyclic

voltammetry. The Nafion/ heterobimetallic complex modified glassy carbon electrode displays

significant improvement of the catalytic activity for electrochemical oxidation of ethanol

compared to that of the homogeneous process. The results obtained affirm that the

immobilization of the heterobimetallic complex on the surface of the glassy carbon electrode is

connected with higher catalytic activity.

The electrolyte-solvent combination, electrolyte concentration, scan rate, ethanol

concentration and lifetime of the electrode were optimized for the cyclic voltammetry

measurements. Other previously prepared Ru/Pd, Ru/Pt, and Fe/Pd heterobimetallic complexes

have been examined for electrochemical oxidation of ethanol. The alcohol oxidation products

(acetaldehyde, acetic acid, diethoxyethane, and ethyl acetate) were analyzed by gas

chromatography (GC).

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CHAPTER 1

LITERATURE REVIEW

Electro-Oxidation of Alcohols

Whereas higher electric efficiency and performance can be obtained by the use of pure

hydrogen or a hydrogen-rich gas as a fuel rather than alcohols for polymer electrolyte membrane

fuel cells (PEMFC), there are some limitations for the development of such techniques like the

clean production, storage and distribution of hydrogen.1-3

The use of the low-molecular weight

alcohols (methanol, ethanol, etc.), in Direct Alcohol Fuel Cells (DAFCs), has advantages,

compared to pure hydrogen because they are liquids (easily handled, stored and transported).

Moreover, their theoretical mass energy densities (6.1 and 8.0 kWh kg-1

corresponding to 6 and

12 electrons per molecule for the total oxidation of for methanol and ethanol, respectively to

carbon dioxide) are comparable to that of gasoline (10-11 kWh kg-1

).4-6

DAFCs are attractive as power sources for mobile, stationary, and portable applications

due to their high energy-conversion efficiency and low operating temperature.7-10

Discovered in

England in 1839 by Sir William Grove, the fuel cell is an electrochemical device, which converts

the heat of combustion of a fuel (hydrogen, natural gas, methanol, ethanol, hydrocarbons, etc.)

directly into electricity. The electrochemical cell consists of two electrodes, anode and cathode,

which are electronic conductors, separated by an electrolyte, protonic conductor. At the anode

the fuel is electrochemically oxidized, without producing any pollutants (only water and/or

carbon dioxide for complete oxidation), while at the cathode the oxidant (oxygen from the air) is

reduced. A schematic representation of a Direct Ethanol/Oxygen Fuel Cell (DEFC) is shown in

Figure 1-1 as an example for fuel cells.

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Figure 1-1. A schematic representation of Direct Ethanol/Oxygen Fuel Cell (adapted from ref 4).

The direct oxidation of methanol in fuel cells has been widely investigated, as methanol

has been considered the most promising fuel because it is more efficiently oxidized than other

alcohols. However, there are still some development challenges, such as it is relatively toxic,

inflammable, and it is not a primary fuel, nor a renewable fuel.5

Therefore, other alcohols have began to be considered as alternative fuels. Among organic

small molecule alcohols, ethanol has been subjected to electro-oxidation in numerous studies.11-

13 Ethanol offers an attractive alternative as a fuel in low-temperature fuel cells because it can be

produced in large quantities from agricultural products and it is the major renewable biofuel from

the fermentation of biomass. Also, after evaluation of ethanol, 1-propanol, and 2-propanol as

alternative fuels for direct alcohol/oxygen fuel cells, Wang et al.14

found that ethanol is a

promising alternative to methanol by using on-line mass spectrometry for determination of the

relative product distribution for the electro-oxidation of these alcohols under fuel cell operating

conditions, since ethanol has a higher electrochemical activity to oxidize to CO2.

However, the complete oxidation of ethanol to CO2 is more difficult than that of methanol

due to the difficulty of breaking the C–C bond at low temperatures. High yields of partial

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oxidation products, CH3CHO and CH3COOH are produced at Pt catalysts15

and decrease the

performance of the fuel cell. So, there is a need to improve the activity of anode electrocatalysts

for alcohol electro-oxidation.

Anode Catalysts

Platinum electrocatalysts are well-known to be the best for the electrochemical oxidation

of organic molecules in acid media.16

However, the formation of strongly adsorbed CO

intermediates as the result of the dissociative chemisorption of methanol/ethanol poisons the

platinum anode catalysts and causes high overpotentials that drastically reduce the activity of

pure platinum as shown by infrared reflectance spectroscopy.17,18

Considerable efforts have been devoted towards the improvement of the activity and

endurance of platinum electrodes by decreasing the CO poisoning. Addition of a second element

to form a bimetallic catalyst promotes the electrocatalytic activity of pure platinum by lowering

the overpotential for alcohol electro-oxidation significantly. Most of the literature data agree that

the onset of the oxidation reaction of alcohols on PtRu is about 0.2 V less positive than on the

single Pt catalyst.19

This improvement is attributed to two mechanisms:

1) The bifunctional mechanism, 20-25

in which the Pt anode sites adsorb and dehydrogenate

alcohol (methanol, ethanol) and produce carbon-containing species, while the second metal sites

which are more oxophilic, adsorb the OH species and supply the oxygen atom to the adjacent

metal at lower potential than pure Pt surface.

2) The intrinsic mechanism,26

which is based on the modification of Pt electronic structure

by the presence of the second metal (Ru) which makes the Pt atoms more capable of the

adsorption of oxygen-containing species at lower potential relative to pure Pt.

Several binary catalysts were proposed for methanol and ethanol oxidation, most of them

based on modification of Pt with some other metal. Examples of these catalysts are Pt-Rh,15

Pt-

14

Ru,27,28

Pt-Sn,28-30

Pt-Mo,31

Pt-Ni,32 Pt-Co,

33 Pt Os,

34 and Pt-Fe

35 which were reported as alcohol

electro-oxidation catalysts. The Pt-Ru binary catalysts are known to be the most effective anode

material for methanol and ethanol oxidation.36

However, it has been reported that some ternary and quaternary alloy catalysts based on

the Pt/Ru have a higher catalytic activity than Pt/Ru. Examples of these materials are Pt-Ru-W

and Pt-Ru-Mo,37

Pt-Ru-Rh,38

Pt-Ru-Co,39

Pt-Ru-Ir/C,40

Pt-Ru-Ni/C,41

Pt-Ru-Fe/C,42,43 Pt-Co-

Cr/C,44

Pt-Ru-Sn-WO3,45

Pt-Ru-Os-Ir,46

and Pt-Pd-Ru-Os.47

Mechanism of Ethanol Electro-Oxidation Using Heterogeneous Catalysts

Numerous studies on the electro-oxidation of ethanol have been carried out to identify the

adsorbed intermediates on the electrode and to examine the reaction mechanism. Identification

of the adsorbed intermediates and the reaction products for the electro-oxidation of ethanol were

provided using various techniques, such as chromatography (HPLC,GC),6,48,49

differential

electrochemical mass spectrometry (DEMS), 12,18,50

and in situ Fourier transform infrared

spectroscopy (FTIRS).51-53

For the electrocatalytic oxidation of isotopically labeled ethanol on

Pt electrodes in acid medium, DEMS studies identified acetaldehyde and CO2 as primary

reaction products. It showed that by cleavage of the C-H bond on the α-carbon and the O-H

bond in the hydroxyl group, the acetaldehyde is formed, whereas CO2 is formed via a multistep

pathway involving strongly bonded intermediates. Also, at low potentials methyl radical is

formed by C-C bond cleavage, and methane and ethane are detected as desorption products.

FTIRS studies showed that adsorbed carbon monoxide is a strongly adsorbed intermediate at the

Pt surface. Based on previous studies (electrochemical and spectro-electrochemical), the

reaction mechanism of electro-oxidation of ethanol on platinum electrode can be postulated as

follows.

15

Generally, it is known that ethanol has two reactive sites which can interact with the metal

catalyst during the adsorption processes, the α-carbon atom and the OH group. The ethanol can

be adsorbed through any of these groups after the breaking of a particular C-H or O-H bond as

shown from the previous work of Iwasita and Pastor.18

The ethanol can then adsorb on Pt

following two modes:

Figure 1-2. Mechanism of ethanol electro-oxidation at a platinum surface in acid medium.

According to one of the generally accepted reaction mechanisms given in Fig. 1-2,54

at a

potential less than 0.8 V/NHE, the ethanol is adsorbed by the α-carbon at the surface of

platinum. This adsorption involves a C-H cleavage with the transfer of one electron (1), and then

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the adsorbed intermediate desorbs to produce acetaldehyde with the transfer of another electron

(2). The second hydrogen of the acetaldehyde can be eliminated by the nucleophilic attack of

H2O oxygen, and leads to adsorbed acetyl which completes the oxidation into acetic acid (3).

The adsorbed acetyl also leads to adsorbed methyl, which is a reactive intermediate, and will

desorb as CH4 at E < 0.2 V/NHE (4) or oxidize to carbon dioxide via the surface reaction of

adsorbed CO species and adsorbed OH (coming from the dissociation of H2O) at E > 0.5 V/NHE

(5)

Transition Metal Complex Catalysts

General Principle of Electrocatalysis

For electrochemical reactions, it is required to find an electrocatalyst that lowers the

overpotential and increases the reaction rate. Transition metal complexes are used to catalyze

many redox reactions, working as electrocatalysts. In the electrochemical cell, the electrode

oxidizes the transition metal complexes to generate an active species, which then catalyzes the

oxidation/reduction of the substrate. There are two basic different types of electrocatalysis:

heterogeneous and homogeneous. The heterogeneous electrocatalytic process includes the

immobilization of the electrocatalyst on the electrode surface, so that the substrate transfers from

the bulk solution and exchanges the electron with the electrode. In homogeneous

electrocatalysts, the electron transfer and chemical reactions occur in the bulk solution, and the

substrate diffuses to the electrode surface.55

Ruthenium Oxo Complexes as Electrochemical Catalysts for Alcohol Oxidation

The redox chemistry of ruthenium oxo complexes has attracted particular interest, due to

the wide range and the stabilization of the high oxidation states. Among different types of the

metal complexes, poly(pyridyl)oxo ruthenium complexes have been applied with success in

electrocatalytic oxidation of different organic compounds.56-58

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Meyer and co-workers reported the first example of using the ruthenium-oxo complexes

for the oxidation of various substrates including alkenes. The strong oxidant Ru(IV) oxo

complex ion, (bpy)2pyRuO2+

, (bpy = 2,2-bipyridine, py = pyridine), can be generated from the

corresponding Ru(II) aquo complex via proton-coupled electron transfer as shown in Scheme

1.56

Scheme 1-1.

The system involves a net 2e− oxidation in aqueous solution and the resulting Ru

IV=O

complex in a buffered solution is rapidly reduced to [RuII(bpy)2(py)(OH2)]

2+ when an

unsaturated organic compound is added. Both the RuII/Ru

III and Ru

III/Ru

IV couples are reversible,

thus the oxidation reaction can be made for the substrates (organic compounds) catalytically by

using an external source of oxidation (electrode potential at which the redox reaction occurs)

sufficient to regenerate the [RuIV

(bpy)2(py)O]2+

in aqueous solution.

One of the attractive applications of these complexes is as electrocatalysts in fuel cells that

utilize organic fuels such as methanol or formaldehyde.59

There are two ways to use the Ru-oxo

complex oxidation catalysts: in solution (homogeneous) or by incorporation of a metal catalyst

into the electrode (heterogenized homogeneous) which could give a significant improvement.

The ruthenium-aquo species [Ru(4,4’-Me2bpy)2(AsPh3)(H2O)](ClO4)2 (4,4’-Me2bpy =

4,4’-dimethyl-2,2’-bipyridine) was reported by De Giovani and co-workers to undergo 2e-/2H

+

transfer in the oxidation process at pH 7.0 in the homogeneous electro-oxidation of benzyl

alcohol, 1-phenylethanol and cyclohexene.60

Also, the electrocatalytic oxidation of benzyl

alcohol, cyclohexanol, cyclohexene, 1-pentanol, 1,2-butanediol and 1,4-butanediol was

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performed using the ruthenium(II) [Ru(cis-L)(totpy)(H2O)](PF6)2 and [Ru(trans-

L)2(totpy)(H2O)](PF6)2 (2) (L = 1,2-bis(diphenylphosphino)ethylene; totpy = 4’-(4-tolyl)-

2,2’:6’,2’-terpyridine) complexes in solution and immobilized in carbon paste electrodes.61

The

ruthenium aquo complexes containing phosphines and polypyridine as ligands

[Ru(L)(totpy)(H2O)](PF6)2 (L = 1,2-bis(dimethylphosphino) ethane and 1,2-

bis(dichlorophosphino)ethane; totpy = 4’-(4-tolyl)-2,2’:6’,2’’-terpyridine) were tested with

benzyl alcohol, cyclohexanol, cyclohexene, ethylbenzene, 1,2-butanediol and 1,4-butanediol in

solution and immobilized in carbon paste electrode and ITO/Nafion electrodes.62

The proposed mechanism of using the Ru-oxo complexes in the oxidation of organic

compounds proposes the formation of an electron deficient carbon in the transition state, for

which the substituent group induces the delocalization of the positive charge via the

hyperconjugation, inductive, or resonance effect, which decreases the reaction activation

energy.61,63

The mechanistic details usually involve a two electron hydride transfer, such as the

oxidation of 2-propanol to acetone by a hydride transfer pathway (Scheme 2 )64

Scheme 1-2.

Heterobimetallic Catalysts for the Electrooxidation of Alcohols

Heterobimetallic catalysts, bimetallic complexes that have two different metals, are of

interest for catalysis.65-68

Such mixed–metal complex systems have attracted much attention

because of their superior properties in organic synthesis. They are effective catalyst precursors

for the aerobic Heck coupling,69

the oxidation of secondary alcohols,70

ethylene

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polymerization,71

and olefin hydroformylation.72

The presence of different types of metal

centers close to each other within the same molecule in the heterobimetallic catalysts affects their

reactivity, gives significant properties different from their mononuclear analogues, and can lead

to higher catalytic activity.73 Also, the different electronegativities between metal ions in the

mixed bi- and polymetallic complexes are known to mediate the electrocatalytic reactions and

lead to the cooperative effects between the two different metal centers.74

Previous studies in the McElwee-White research group that focused on the development of

heterobimetallic catalysts for electro-oxidation of methanol resulted in the synthesis of the dppm

(bis(diphenylphosphino)methane) bridged Ru/Pd, Ru/Pt, and Ru/Au heterobimetallic complexes

Cp(PPh3)Ru(μ-Cl)(μ-dppm)PtCl2 (1),75

Cp(PPh3)Ru(μ-Cl)(μ-dppm)PdCl2 (2),76

and

Cp(PPh3)RuCl(μ-dppm) AuCl (3).76

Synthesis

Complexes 1-3 were synthesized at room temperature by reacting CpRu(PPh3)(η1-dppm)Cl

(4) with Pt(COD)Cl2, Pd(COD)Cl2 and Au(PPh3)Cl, respectively (Scheme 1-3). Due to the poor

solubility of complexes 1-3 in water, and in order to compare the behavior of these compounds

with ruthenium-oxo catalysts that are soluble in water, several attempts have been carried out by

the McElwee-White research group to modify these complexes and synthesize water soluble

heterobimetallic, ammonium substituted cyclopentadienyl complexes.

Examples, include the heterobimetallic Ru/Pd complexes [η5-C5H4CH2CH2N(CH3)2

HI]Ru(PPh3)(μ-I)(μ-dppm)PtI2 (6),77

and [η5-C5H4CH2CH2N(CH3)2 HI]Ru(PPh3)(μ-I)(μ-

dppm)PdCl2 (7).77

Complexes 6 and 7 were synthesized by reaction of [η5-

C5H4CH2CH2N(CH3)2 HI]Ru(PPh3)(I)(Κ1-dppm) (5) with Pd(COD)Cl2 in dichloromethane at

room temperature (Scheme 1-4).

20

Scheme 1-3.

Scheme 1-4.

Dppm was used as a bridging ligand due to the presence of the strong metal-phosphorus

bonds which give the chance for the two metal centers to be in close proximity and facilitate the

electronic interaction between them. The metal-metal interactions, electron donation between

21

the metals takes place through the halide bridge, and can be detected by the shifts of the redox

potentials.

Cyclic Voltammetry

Cyclic voltammetry can be used to explore the metal-metal interactions in the

heterobimetallic complexes, by comparing the redox potential of the heterobimetallic systems to

that of monomeric ones.78

For example, the cyclic voltammogram of Cp(PPh3)Ru(μ-Cl)(μ-

dppm)PdCl2 (2) shows a 730 mV positive shift for the Ru(II/III) waves compared to the

monomeric compound CpRu(PPh3)(Κ1-dppm)Cl (4) as a result of electron donation from the Ru

to Pd through the chloride bridge. However, the cyclic voltammogram for Cp(PPh3)RuCl(μ-

dppm)AuCl (3) shows only a 30 mV positive shift for the Ru(II/III) waves compared to

CpRu(PPh3)(Κ1-dppm)Cl, indicating that the dppm bridge has no effect in the electronic

interaction between the two metals in contrast to the halogen one.76

The cyclic voltammograms

for the Ru/Pd compounds (6) and (7) each exhibit irreversible oxidation at 1.35 V and 1.00 V vs.

NHE, respectively, while the mononuclear complex (5) is at 1.05 V. This potential shift

indicated the electron donation through the iodide bridge. The CV data for complexes 1-7 are

listed in Table 1-1.75-77

Table 1-1. Formal Potentials of Complexes 1-7. Complex Couple E1/2(V)

a Couple E1/2 (V)

a Couple E1/2(V)

a

Ru/Pt (1) Ru(II/III) 1.13b Pt(II/IV) 1.78

c Ru(III/IV)

Ru/Pd (2) Ru(II/III) 1.29 Pd(II/IV) 1.45c Ru(III/IV)

Ru/Au (3) Ru(II/III) 0.89 Au(II/IV) 1.42c Ru(III/IV)

Ru (4) Ru(II/III) 0.56 b Ru(III/IV)

Ru (5) Ru(II/III) 1.05 Ru(III/IV) 1.95

Ru/Pd (6) Ru(II/III) 1.35 Pd(II/IV) 1.67c Ru(III/IV) 2.04

Ru/Pd (7) Ru(II/III) 1.00 Pd(II/IV) 1.57c Ru(III/IV) 2.09

a All potentials obtained in DCE/TBAT (tetrabutylammonium triflate) and reported vs NHE and referenced to

Ag/Ag+.

bPerformed in CH2Cl2/TBAH (tetrabutylammonium hexafluoro phosphate).

cIrreversible wave, Epa

reported.

22

Cyclic voltammetry of complexes 1,2 and 6 in the presence of methanol results in a

significant increase in the oxidation current of the second metal redox waves Pt(II/IV) or

Pd(II/IV ) that indicate the catalytic process of the complexes in electrocatalytic oxidation of

methanol (Figures 1-3, 1-4 and 1-5).

Figure 1-3. Cyclic voltammograms of 1

75 under nitrogen in 2.5 mL of DCE/0.7 M TBAT


electrode.




electrode.

23




electrode.

Product Detection

The electro-oxidation process of methanol produces formaldehyde and formic acid, as

detected by on-line FTIR spectroscopy analysis of the product of on a platinum – ruthenium

catalyst in a prototype direct –methanol fuel cell.79

However, neither was observed in the

reaction mixtures. Dimethoxymethane (DMM) and methyl formate (MF) were detected instead.

It is reported that oxidation of methanol takes place via multistep mechanism. Methanol

oxidizes to formaldehyde, then in excess methanol forms dimethoxymethane or is completely

electrochemically oxidized to CO2 (Eq. 1.1- 1.3)

24

Also, methanol oxidation can go through a four – electron electro-oxidation to formic acid,

which in excess methanol forms methyl formate or is further electrochemically oxidized to CO2

(Eq. 1.4-1.6)

Finally, methanol can go through a six – electron electro-oxidation to form CO2 (Eq.1.7)

Bulk Electrolysis with Complexes 1, 2, and 6

The evolution of the product distributions as the reaction progresses is shown in Table

1.2.75-77

For the complexes 1 and 2, at low quantities of charge passed, they give a much higher

amount of DMM. As the reaction progresses, water is formed in situ during the condensation of

the formaldehyde and formic acid with excess of methanol. The tendency toward production of

(MF) increases more than that for DMM because the presence of water shifts the product ratios

toward the four-electron oxidation product, MF. For the complex 6, the behavior is different as

throughout the reaction, the two – electron oxidation product DMM is favored.

The current efficiencies for the oxidation processes are the ratio of the charge necessary to

produce the observed yield of DMM and MF to the total charge passed during the bulk

electrolysis (Eq. 1.8).

25

Table 1-2. Product Distributions and Current Efficiencies for the Electrochemical Oxidation of

Methanol by 1, 2, and 6.

Charge / C

Product ratio [μmoles of CH2(OCH3)2 (DMM)/ HCOOCH3(MF)]c

Ru/Pt (1) a Ru/Pd (2)

a Ru/Pd (6)b

25

50

75

100

130

150

200

2.45 3.18

2.35 2.41 1.66

1.51 1.54 1.76

1.23 0.97 2.34

1.20 0.87

3.17

3.82

Current Efficiency (%)d

after 130 C

18.6 24.6

Current Efficiency (%)d

after 200 C

44

Electrolyses were performed at 1.7a

and 1.5 b V vs. NHE. A catalyst concentration of 10 mM was used. Methanol

concentration was 0.35 M. c

Determination by GC with respect to n-heptane as internal standard. Each ratio is

reported as an average of 2-5 experiments. dAverage current efficiencies after 75-130 C of charge passed.

26

CHAPTER 2

CHEMICALLY MODIFIED ELECTRODES CONTAINING IMMOBILIZED

HETEROBIMETALLIC COMPLEXES

Introduction

Modification of the surface of the electrodes by coating with a thin film of selected

chemical produces improvements of the electrochemical function over conventional electrodes.

The electrodes with modified surfaces get a large usable potential window, increasing the

sensitivity, selectivity and electrochemical stability. There is a rapidly growing need for the

improvement of the electrode performance chemically in many disciplines of science due to the

possibility of transferring the desirable properties of a chemical site into the electrode surface.80

Chemically Modified Electrodes

Chemically modified electrode (CME)81

is: a conducting or semiconducting conventional

electrode that has been coated with a monomolecular, multi-molecular, ionic, or polymeric film

(see Figure 2-1, the film or adlayer is anchored to a conventional electrode to enhance

performance or achieve a specialized function)80

to enhance the electrochemical properties of the

interface.

Figure 2-1. Chemically Modified Electrodes (adapted from ref 80).

27

Electrocatalysis Using Chemically Modified Electrodes (CMEs)

In electrocatalysis, immobilization of the electroactive molecule to the electrode surface

produces an interface between the electrode surface and the analyte in solution that assists its

reduction or oxidation.82

Using CMEs in the electrocatalysis lowers the overpotential of the

heterogeneous electron transfer of the target analyte between the electrode that is derivatized by

the electrocatalyst and the solution. This amplifies the detected signal with respect to that at a

bare electrode.81

The porous structure and high surface area of conducting polymers provide the opportunity

for the immobilization of metal catalysts and development of new catalytic and electrocatalytic

materials. High electrical conductivity of some polymers gives these composite materials great

chance to achieve electrocatalysis by electron-transfer through the polymer chains between the

electrode and the immobilized materials.83

Considerable interest has been devoted in recent years to development of suitable electrode

materials for alcohol oxidation for possible application in fuel cells. Conducting polymers

possess both protonic and electronic conductivity. B. Rajesh84

and co-workers reported that the

use of Pt nanoparticles-loaded conducting hybrid material based on polyaniline and V2O5 as an

electrode exhibited high electrocatalytic activity and stability for electrooxidation of methanol.

Also, Maiyalagan85

reported that poly(o-phenylenediamine) (PoPD) nanotube electrodes give

significant improvement for the catalytic activity of platinum nanoparticles for oxidation of

methanol.

Polymer Modified Electrodes

The network nature of the polymer provides the electrode surface with a high number of

the active sites which give the possibilities for studying many interfacial phenomena, for

example, charge transport or electron transfer mediation reactions.86

28

At least three processes occur during the electrocatalytic conversion of the analyte at the

modified polymer electrode:83

1) the heterogeneous electron transfer between the electrode and

the conducting polymer layer, and electron transfer within the polymer film. The rate of the

heterogeneous electron transfer process depends on the electrical conductivity of the polymer. 2)

The diffusion of the analyte through the conductive layer to the electrode. This process depends

on the interaction between the analyte and the polymer film. 3) The heterogeneous reaction

occurs between the conducting polymer and the solution.

One of the polymer modifier types is ion-exchange polymer that has the ability to serve as

the electron transfer site within and on the surface of the film on the modified electrode. By

changing the counter-ion for the charged electroactive ion, the result is a layer with the ability to

transport electrons.87

For example, Nafion® can be immobilized on the electrode surface through

drop-casting, spin-coating, or dip-coating. Then the desired redox ion (the charged electroactive

ion) is ion-exchanged into the polymer (see Figure 2-2, Filled circles depict the reduced form of

the couple and open circles the oxidized form. Smooth arrows show electron-transfer events

between the two forms, and wavy arrows indicate diffusional processes).88

Figure 2-2. An ion-exchange polymer filled with free redox molecules.

29

Nafion®1

, discovered in the late 1960s by Walther Grot of DuPont, is a perfluorosulfonated

polymer consisting of poly-tetrafluoroethylene (PTFE) backbone and perfluoroether side chain

with a sulfonate group. Due to its high proton conductivity, thermal and mechanical stability, it

is commonly used as solid polymer electrolyte for fuel cells.89

As seen in Figure 1.6,90

the three phase model is based on a three phase clustered system.

The three regions consist of the hydrophobic fluorocarbon (PTFE) backbone (A), an interfacial

zone containing some pendant side chains, some water, sulfate or carboxylic groups (B) and the

hydrophilic ionic zone where the ionic exchange sites, counterions and absorbed water exist in

majority (C)

Figure 2-3. (a): Structure of Nafion

Figure 2-3. (b): Three-phase model of Nafion (adapted from ref 90).

1 Nafion

® is a registered trademark of E.I. DuPont de Nemours & Co

http://en.wikipedia.org/wiki/DuPont

30

Electrodes Modified with Heterobimetallic Complexes

Homogeneous electrocatalysts such as transition metal complexes have the ability to

mediate electron transport when immobilized on the surface of the electrode.82

The

immobilization of a monolayer of a redox active metal complex to an electrode surface offers the

advantages of combining the homogeneous catalysis advantages (high activities and selectivities)

and heterogeneous catalysis (easy separation of catalyst from the products) by fixing the

homogeneous catalysts in a heterogeneous matrix.91

Preparation of Polymer Modified Electrodes

As classified by Drust,81

polymer film-coated electrodes may be subdivided into seven

categories based on the process used to apply the film:

(1) Dip-coating- Immersing the electrode material in the solution of the polymer for a

certain time; the film is formed by adsorption.

(2) Droplet evaporation- Applying a droplet of the polymer solution to the surface of the

electrode; the film is formed by evaporation of the solvent.

(3) Spin coating- Applying a droplet of the polymer solution to the surface of a rotating

electrode; the film is formed after excess solution is spun off the surface

(4) Electrochemical deposition–Applying the electrochemical deposition; the film is

formed irreversibly when the polymer is oxidized or reduced to its less soluble state.

(5) Electrochemical polymerization- The polymerization occurs on the surface of the

electrode by using the solution of a monomer that is oxidized or reduced to an active

form.

(6) Radiofrequency polymerization- A radiofrequency plasma discharge is exposed to

the vapor of the monomer.

31

(7) Cross-linking- A copolymerization of bifunctional and polyfunctional monomers to

give a film on the electrode with desired properties.

In this work, the droplet evaporation method was used to make Nafion film- coated

electrodes that have the advantage of known concentration and droplet volume. The compound

that was used in this work to make Nafion supported heterobimetallic complexes is

Cp(PPh3)Ru(μ-I)(μ-dppm)PdCl2 (8).78

Figure 2-4. Heterobimetallic Ru/Pd complex 8.

Cyclic Voltammetry of Nafion Modified Electrodes

Cyclic voltammetry was used to study the electrochemistry of the glassy carbon electrode

(GCE) Nafion modified electrodes under different conditions. Various attempts were done to

optimize the experimental conditions for the electro-oxidation of ethanol using Nafion electrodes

modified by the previously prepared heterobimetallic complex Ru/Pd 8, by changing the

electrolyte-solvent combination, and concentration, lifetime of the modified electrode, heat,

ethanol concentration and scan rate. Cyclic voltammograms were obtained using 0.1 M

electrolyte, 245 mM ethanol and 50 mV/s scan rate, and GCE coated with 10 mM Nafion

catalyst layer as a working electrode. High catalyst concentration (15 or 20 mM) does not

dissolve completely in the Nafion solution, and thus has a lower catalytic activity for

electrochemical oxidation of ethanol.

Figure 2-5 shows the cyclic voltammograms (CVs) of ethanol oxidation on Nafion blank

and Nafion modified with 10 mM complex 8 electrodes a) and b) respectively. The ethanol

32

Figure 2-5. Cyclic voltammograms in 0.1 M TBACF3SO3/ DCE, Ag/Ag

+ reference electrode;

245 mM Ethanol; 50 mV/s a) Nafion blank, b) Nafion modified electrode with 10

mM of complex 8, and c) The difference between the voltammogram in the presence

and absence of ethanol for both Nafion blank and Nafion modified.

33

oxidation current peak is recorded at 1.659V vs NHE for the modified electrode. However, this

peak does not appear when using the Nafion blank electrode, which indicates the ethanol

oxidation activity of the complex 8. To explore the activity of the heterobimetallic complex 8

for electro-oxidation of ethanol, the cyclic voltammograms in presence and absence of ethanol

for both Nafion blank and Nafion modified electrodes a) and b) were subtracted to give c). This

method is applied for all the CVs in this work to differentiate between the influencing factors,

and to obtain the best conditions for the activity of the heterobimetallic compounds in

electrochemical oxidation of alcohols and specifically ethanol.

Effect of Electrolyte and Solvent

Supporting electrolyte, a solvent containing dissolved mobile ions, is the medium that all

electrochemical reactions occur in, and able to support the current flow between the electrodes in

the electrochemical cell. The electrolyte maintains the charge balance and carries 97% of the

current in the bulk solution by providing the pathway for the ions to flow in the cell instead of

the ones produced or consumed at electrodes during the electrochemical reaction. Therefore, it

eliminates the electroactive species migration and decreases the uncompensated resistance drops

between the working and reference electrode and thus improves the accuracy for the potential

measurements at the electrode. The main required properties of this medium are high solvating

power to dissolve the reactants and products from the electrode reaction, high conductivity (the

ability of ions to move in an electric field), low viscosity for rapid transport, and low reactivity

with the reactants and the products.80

34

Figure 2-6. Cyclic voltammograms of Nafion blank and Nafion modified electrodes with 10 mM of complex 8, in 0.1 M

TBACF3SO3,TBAPF6, and TBABF4/ DCE, DCM, and AN, 245 mM of ethanol, 50 mV/s, and Ag/Ag+ reference electrode

a c b

d e f

35

Figure 2-6. Continued.

.

g h i

36

Figure 2-6 shows the cyclic voltammograms of the 10 mM Nafion complex 8 modified

GCE using 0.1 M of different electrolyte-solvent combinations, tetrabutylammonium

trifluoromethanesulfonate (TBACF3SO3), tetrabutylammonium tetrafluoroborate (TBABF4), and

tetrabutylammonium hexafluorophosphate (TBAPF6) with 1,2 dichloroethane (DCE),

dichloromethane (DCM), acetonitrile (AN). As shown in Figure 2-6a, d, and g, DCE is the best

solvent for the electro-oxidation of ethanol (245mM), because there is large increase in the

oxidation current of ethanol displayed by the Nafion modified electrode indicating oxidation

activity for ethanol, while the Nafion blank does not yield significant oxidation activity. The

ethanol oxidation current peak using TBACF3SO3 is located at 1.65V vs. NHE, while for both of

TBABF4 and TBAPF6, oxidation occurs at potentials more anodic than 2.0V vs. NHE. The

higher potentials for oxidation of ethanol are not favored; they signify higher overpotentials for

the electro-oxidation. For the CVs of Figure 2-6b, and e using DCM, both Nafion blank and

modified have oxidation peaks, so it is impossible to differentiate which one comes from the

catalytic activity of the complex, while Figure 2-6h has a distinct oxidation peak for ethanol by

the modified electrode, but at high potential. Finally, in Figure 2-6c, f, and i, acetonitrile has a

very high dielectric constant (35.9) as compared to 1,2-dichloroethane, (10.37) or

dichloromethane (8.93) which makes the conductivity of the electrolyte high and does not give

the complex the chance to show its catalytic activity for the electro-oxidation of ethanol. In

acetonitrile the CVs for both the Nafion and Nafion modified electrodes are the same.

Therefore, DCE is the best solvent that can be used for electrochemical oxidation of ethanol

using complex 8. Due to the CV of TBACF3SO3/DCE exhibiting the highest current peak at low

potential, this electrolyte was selected for the electrochemical studies for complex 8.

37

Electrolyte-solvent concentration

Figure 2-7a shows the influence of the concentration of the TBACF3SO3/DCE on the

intensity of the anodic oxidation current, using a 0.05 – 0.5 M concentration range at 1.659 mV

vs. NHE. At the low concentration range, the anodic peak current increases as the concentration

of the electrolyte increases until 0.2 M and then a decrease is observed. Addition of excess

supporting electrolyte does improve the conductivity of the electrolyte solution. However, at

concentrations more than 0.2 M, the activity of the complex 8 for the electro-oxidation of ethanol

decreased.

Figure 2-7. a) Cyclic voltammograms and b) plot of oxidation peak currents against electrolyte

concentration of Nafion modified electrode with 10 mM of complex 8 using 0.05-

0.5M TBACF3SO3/DCE, 245 mM of ethanol, 50 mV/s , and Ag/Ag+ reference

electrode.

b

a

38

So, in the highly conductive medium, the behavior of the complex 8 for the

electrochemical oxidation of ethanol changes and decreased currents are observed. This is

similar to the CVs using the electrolyte/acetonitrile combination that did not exhibit oxidation

activity because of the high conductivity of acetonitrile

Figure 2-8. a) Cyclic voltammograms of Nafion modified electrode with 10 mM of complex 8,

b) plot of oxidation peak currents against ethanol concentrations (98-489 mM), and c)

Cyclic voltammograms of Nafion blank electrode using 0.1M TBACF3SO3/DCE, 50

mV/s, and Ag/Ag+ reference electrode.

a

c

b

39

Effect of Ethanol Concentration

For the quantitative evaluation of the electrocatalytic activity of the complex 8 for the

electrochemical oxidation of ethanol, the dependence of the current peak height on the bulk

concentration of ethanol was studied. The CVs of Nafion modified and Nafion blank electrodes

were performed using different concentrations of ethanol (Figure 2-8a and c). In the potential

range where the electrocatalytic oxidation of ethanol occurs, the anodic currents grow as the

concentration of ethanol increases. By plotting the plot of oxidation peak currents against

different ethanol concentrations, a nearly linear dependence was obtained (correlation coefficient

= 0.9596), indicating that the electrolysis process is pseudo-first-order in this ethanol

concentration range. The bulk ethanol concentrations have no influence on the Nafion blank

electrode as shown in Figure 2-8b.

Effect of Scan Rate

The effect of scan rate on the electrocatalytic peak current was evaluated over the 20-70

mV/s range (Figure 2-9) to study the kinetics of the oxidation reactions. The anodic peak current

increases non-linearly upon changing the scan rate. It was found that the resulting plot of

oxidation peak currents against the square root of the scan rates displays linearity up to 40 mV/s

and then a downward deviation at higher scan rates is observed. We can conclude that rapid

diffusion-controlled oxidation for the ethanol occurs only at slow scan rates, however at higher

scan rates, the fast potential sweep does not gave the modified electrode time to oxidize the

ethanol.

40

Figure 2-9. a) Cyclic voltammograms, b) plot of oxidation peak currents against the square root

of the scan rates of Nafion modified electrode with 10 mM of complex 8 using 0.1M

TBACF3SO3/DCE, 245 mM of ethanol, and Ag/Ag+ reference electrode.

Lifetime of the Electrode

Figure 2-10 demonstrates the decay of the Nafion modified electrode with the number of

scans. The lifetime of a Nafion modified electrode is depend on the binding of the

heterobimetallic complex to the electrode surface and the stability of Nafion modified layer in

the electrolyte. The Nafion modified layer on the electrode has an effective life after which the

activity for the electrocatalytic process will decrease. CVs were performed using Nafion

modified electrodes and repeated for the same electrode. During the first five scans, the Nafion

was dissolving and while the electrode was still working, performance was decaying.

b

a

41

Figure 2-10. a) Cyclic voltammograms, b) plot of oxidation peak currents against the number of

times using the same Nafion modified electrode with 10 mM of complex 8 using

0.1M TBACF3SO3/DCE, 245 mM of ethanol, 50 mV/s, and Ag/Ag+ reference

electrode.

Applications

Generally, the cyclic voltammograms of the heterobimetallic complexes Cp(PPh3)M1(μ-

I)(μ-dppm)M2X2 (M1= Ru or Fe, M2= Pd or Pt, X= Cl, or I) in homogeneous electrocatalytic

oxidation of alcohols exhibit three redox waves within the solvent window. The first and the

third are attributed to the first metal M1(II/III) and M1(III/IV) couples, respectively, while the

second wave is assigned to the redox couple of the second metal M2(II/IV). The first anodic

potential M1(II/III) depends on the extent of electron donation to the second metal through the

halogen bridge, however, the M1(III/IV) potentials from one complex vary. Current at the

b

a

42

second metal wave M2(II/IV) is increased in presence of alcohols, indicating catalytic activity of

the complex for electro-oxidation of alcohols.76,77,92

Figure 2-11 Structures of compounds 8-11.

The heterobimetallic Ru/Pd, Ru/Pt, and Fe/Pd complexes (Figure 2-11) Cp(PPh3)Ru(μ-

I)(μ-dppm)PdCl2 (8),92

[η5-C5H4CH2CH2N(CH3)2•HI]Ru(PPh3)(μ-I)(μ-dppm)PtCl2 (9).

77

CpFe(CO)(μ-I)(μ-dppm)PdI2 (10)93

and CpRu(CO)(μ-I)(μ-dppm)PtI2 (11)93

were previously

prepared as catalysts for the electrochemical oxidation of methanol. These complexes were

studied as examples of the main types of catalysts previously studied in the McElwee-White

group.

Figures 2-12-15a show the cyclic voltammograms for the electrochemical oxidation of

methanol by complexes 8-11. Complexes 8 and 10 display three waves, the first and third ones

for the oxidation of the first metal (Ru and Fe), while the second one for the oxidation of the

second metal (Pd) in complexes 8 and 10, respectively. Complex 9 has one more wave observed

due to the oxidation of the amino moiety and complex 11 displays only two waves because the

43

second and third waves overlap. The formal potentials for the Ru and Fe(II/III) and Ru and

Fe(III/IV) couples are in the range 1.07-1.36V and 1.76-2.25V, respectively, while those for the

redox couple of the second metal Pd and Pt(II/IV) is 1.56-1.91V.

Nafion modified electrodes exhibit different CVs than those in homogeneous solutions.

There is only one wave that is significantly increased in the presence of alcohol, which is ethanol

in this work. The cyclic voltammograms of complexes 8-11 using the Nafion modified electrode

(Figures 2-12-15b) show a catalytic anodic current in 1.5-2.0V range. Here the advantages of

using the heterogeneous electrocatalysts in the electrochemical oxidation process are evident, as

the heterobimetallic complex in the Nafion modified film is present in a concentration 350 times

less than the homogeneous system. Approximately the same catalytic activity appears in the

anodic current density for the modified electrode and the solution. In the heterogeneous process,

the substrate transfers from the bulk solution and exchanges the electron with the immobilized

electrocatalyst on the electrode surface, resulting high heterogeneous electron transfer rate,

amplified detection signal, and high electrocatalytic activity. However, in homogeneous

electrocatalysts, the electron transfer and chemical reactions occur in the bulk solution, and the

substrate diffuses to the electrode surface, which costs more overpotential, and results in less

electrocatalytic activity.

44

Figure 2-12. Cyclic voltammograms with 10 mM of complex 8 a)92

glassy carbon working

electrode in 0.7 M TBACF3SO3/DCE; 50µl methanol b) Nafion modified electrode of

the complex in 0.1M TBACF3SO3/DCE, 245 mM (50µl) ethanol, 50 mV/s ,and

Ag/Ag+ reference electrode.

a

b

45


5 mM, glassy carbon

working electrode; 50µl methanol b) 10 mM, Nafion modified electrode, 245 mM

(50µl) ethanol, in 0.1M TBACF3SO3/DCE, 50 mV/s ,and Ag/Ag+ reference electrode.

b

a

46


5 mM, glassy carbon



b

a

47


5 mM, glassy carbon



Bulk Electrolysis

As previously mentioned, the oxidation of methanol (Eq. 1.1- 1.6) results in two and four –

electrons oxidation products, formaldehyde and formic acid. In excess methanol, these products

condense and form dimethoxymethane (DMM), and methyl formate (MF), respectively. In the

presence of water, the product ratio shifts more forward (MF) than (DMM) as shown in Figure 2-

b

a

48

16. The bulk electrolysis for methanol is carried out outside the glove box, then the humidity is

enough to shift the electrolysis products to MF.

Figure 2-16. Product distribution for the electrolysis of methanol using 10 mM of

Nafion/complex 8 vitreous carbon working electrode in 3.5 mL of 0.1 M

TBACF3SO3/DCE, 350 mM methanol, and Ag/Ag+ reference electrode.

For electrolysis of ethanol, (Figure 1-2), the oxidation products are acetaldehyde, acetic

acid, methane, and CO2. To obtain methane and CO2, the C-C bond has to break, which requires

higher activation energy than C-H bond cleavage. Also, in excess ethanol, the two and four –

electron oxidation products, acetaldehyde and acetic acid, will form 1,1-diethoxyethane (acetal,

and ethyl acetate, respectively (Eq. 2.1- 2.2).

By using the complexes 8-11 for the electrolysis of ethanol, (Figure 2-17, 18, and 20),

acetaldehyde and acetic acid are low concentration products because they condense in the

presence of excess ethanol to the acetal and ethyl acetate, respectively. However, the retention

time in Gas Chromatography (GC) is the same, so I could not know the ratio of acetal to ethyl

acetate.

49

Figure 2-17. Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex

8 vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF3SO3/ethanol, 245

mM ethanol, and Ag/Ag+ reference electrode.

.




50




51

CHAPTER 3

EXPERIMENTAL SECTION

General Methods

Synthesis

Cp(PPh3)Ru(μ-I)(μ-dppm)PdCl2 (8) was synthesized according to literature procedure.78

Other catalyst compounds were obtained from Daniel Serra and Marie Correia.

Preparation of the Nafion Modified Electrode

The glassy carbon electrode (GCE), 3 mm diameter, was polished with 0.05 µm Al2O3

paste and washed ultrasonically in distilled water. The working electrode was coated with the

catalyst layer from a 10 mM Nafion/catalyst solution with 5% wt Nafion ionomer, and 10µl of

catalyst solution were pipetted onto the glassy carbon electrode and the solvent was allowed to

evaporate.

Electrochemistry

Electrochemical experiments were performed using an EG&G PAR model 263A

potentiostat/galvanostat (Princeton Applied Research) and a three-compartment H-cell separated

by a medium-porosity sintered glass frit. The Nafion modified electrode was the working

electrode, a platinum flag was used as the counter electrode, and all potentials are referenced to

Ag/Ag+, and reported versus NHE. The reference electrode consisted of a silver wire immersed

in an acetonitrile solution containing freshly prepared 0.01 M AgNO3 and 0.1 M TBACF3SO3.

The Ag+ solution and silver wire were contained in a 75 mm glass tube fitted at the bottom with

a Vycor tip.

The reference electrode was standardized against a 10 mM solution of ferrocene. The

FeCp2 +/FeCp2 potential is 0.558 V vs. NHE in 0.1M (n-Bu)4NClO4 in 1,2-dichloroethane, with a

52

scan rate of 100 mV/s.94

Cyclic voltammograms were recorded in 3.5 mL of 0.1 M

TBACF3SO3/DCE at ambient temperature over the potential range 0-2.5 V vs. Ag/Ag+.

Product Analysis

Bulk electrolysis was performed using a three-compartment H-cell in 3.5 mL of 0.1 M

TBACF3SO3/DCE, for electrolysis of 350 mM methanol, and in 3.5 mL of 0.1 M

TBACF3SO3/ethanol, for electrolysis of 245 mM M ethanol inside a glove box at room

temperature under nitrogen. Nafion modified vitreous carbon (0.5mL of 10 mM Nafion/catalyst

solution with 5% wt Nafion ionomer was pipetted onto the vitreous carbon electrode and the

solvent was allowed to evaporate) was used as a working electrode. Platinum foil was used as

the counter electrode, and an Ag/Ag+ electrode was the reference. Electrolyses were allowed to

continue until the desired number of coulombs was reached, the alcohol oxidation products from

the bulk electrolysis were analyzed by gas chromatography on a Shimadzu GC-17A

chromatograph. For methanol, the column was a 15 m × 0.32 mm column of AT-WAX (Alltech,

0.5 μm film) on fused silica which was attached to the injection port with a neutral 5 m × 0.32

mm AT-Wax deactivated guard column, and n-heptane was used as an internal standard. For

ethanol, the column was a 15 m × 0.45 mm of ECTM-WAX (Alltech, 1.0 μm film) attached to

the injection port with a universal guard column. Dodecane was used as an internal standard.

Product identification was confirmed by comparison of the retention times of the oxidation

product with authentic samples.

53

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BIOGRAPHICAL SKETCH

Ahmed Moghieb was born in Kuwait in 1977 but was raised in Egypt, where his family is

from. In 2000, he received Bachelor of Science in chemistry from Cairo University, Egypt.

Then, he took master courses in inorganic chemistry at Beni-sueif University, Egypt. These

courses have broadened his understanding of inorganic chemistry. As a reward from the

Egyptian government for graduating at the top of his class, Ahmed was nominated to work at the

National Research Centre (NRC), Egypt as a chemical specialist in 2001. Then, due to his hard

work and the recommendations of his supervisors, he was promoted to be a research assistant in

the Electro-Analytical Research Lab in 2003. In 2007, he was granted a Ford Foundation

Fellowship for two years to pursue his master’s degree in University of Florida, Florida, USA.

© 2009 ahmed moghieb - university of...

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