Post-print of Journal of Physical Chemistry C, 2015, 1119, 16708-16723

Ruthenium Effect on Formation Mechanism and

Structural Characteristics of LaCo1-xRuxO3

Perovskites and its Influence on Catalytic

Performance for Hydrocarbon Oxidative Reforming

Noelia Motaa, Laura Barrioa†*, Consuelo Alvarez-Galvána, François Fauthb, Rufino M.

Navarroa*, Jose Luis G. Fierroa

a Institute of Catalysis and Petrochemistry, CSIC, Marie Curie 2, Cantoblanco, 28049 Madrid,

Spain

b Experiments Division CELLS-ALBA, 08290 Cerdanyola del Vallès, Barcelona, Spain

DOI: 10.1021/acs.jpcc.5b04287

KEYWORDS LaCoO3, Ruthenium, Perovskites, X-ray diffraction, Raman, EXAFS.

1

ABSTRACT This work deals with the formation mechanism of LaCo1-xRuxO3 perovskites (x = 0,

0.05, 0.1, 0.2 and 0.4). In situ characterization of perovskite during formation were monitored

with X-ray diffraction and Raman spectroscopy techniques, revealing that perovskite formation

occurs via an oxo-lanthanum carbonate intermediate phase. Structural characterization of

perovskites showed structural changes in the perovskite as the Ru inserted in the structure

increases. It was observed that the insertion of Ru affects the bulk structure by creating rotational

and Jahn-Teller distortions in the perovskite structure. Raman spectroscopy completed the

description, proving the strong distortions of the lattice oxygen and the La-O coordination

induced by the presence of ruthenium. Such distorted configuration gave rise to a weakening of

metal-oxygen bonds, maximizing anionic mobility and reactants adsorption. Surface changes

were also observed with the insertion of Ru in the perovskite structure. XPS showed that there

are cobalt spinel species, unaltered by ruthenium, and lanthanum oxide species that become more

carbonated when Ru is present. The formation of carbonate-like structures is enhanced by

ruthenium, which must be interacting with lanthanum entities, loosening La-O bonds in order to

facilitate the adsorption of CO2. Relating these structural effects with catalytic performance in

hydrocarbons reforming, we can conclude that the structural distortion induced by ruthenium

favours catalytic stability, probably by stabilizing metallic Co and Co-Ru sites, increasing metal

dispersion and by making oxygen mobility easier in the disturbed La2O3 support.

1. INTRODUCTION

Wet impregnation of different supports is the commonly procedure employed to deposit metal

nanoparticles on a catalyst surface. This method is rather simple, but is not completely

reproducible as the distribution of the metal component across the surface is not homogeneous.

In this scenario, solid catalysts can be prepared using the metal ion precursors perfectly

2

distributed in a crystalline structure, which develops upon reduction highly dispersed and stable

metal particles on the substrate surface. Perovskite oxides (ABO3) have been extensively studied

in heterogeneous catalysis because they are a promising alternative to traditional supported

catalyst formulations, on account of their controllable physical and chemical properties due to

the wide range of ions and valences which this simple crystal structure can accommodate.1-3

Perovskites could act as precursors of catalysts containing perfectly distributed active metal (B)

in the perovskite structure (ABO3), which upon reduction develops highly dispersed and stable

active metal particles (B-site cations) on the surface of the oxide on element A.4-6 LaCoO3 is

particularly attractive as precursor of catalysts for hydrocarbon reforming because it is one of the

most reducible ABO3-type perovskites. After reduction, it forms highly dispersed Co particles in

close contact with La2O3, which has an important role in catalyst stability by favouring coke

gasification during hydrocarbon reforming. A way to achieve more active and stable catalysts is

to tailor catalytic properties by producing structural and electronic modifications through partial

substitution of Co sites with another cation in the perovskite lattice. Among the transition metals

for Co replacement in the perovskite lattice, ruthenium was particularly effective in catalytic

reforming of hydrocarbons for hydrogen production as shown in our previous works.7,8 Besides

the higher intrinsic activity of ruthenium, its better catalytic behaviour is attributed to the partial

distortion of the rhombohedral phase associated to high Co replacement, which manifests a

higher exposition of active phases formed during reaction. The synthetic route and formation

mechanism of perovskite-structured mixed oxides is of vital importance to determine the origin

of the reactivity obtained in catalysts derived from perovskites. The formation mechanism of

perovskites is scarcely studied in the literature. Perovskite formation has been studied by Ivanova

et al.,9,10 who performed a thermal analysis on a series of LaCoO3, LaCo1-xNixO3 and LaCo1-

xFexO3 samples prepared by a Pechini-like synthetic route. According to this study, the formation

of perovskite oxides starts from an amorphous hydrated lanthanum carbonate, goes through an

oxo-carbonate intermediate that finally decomposes to give rise to the perovskite oxide.

3

Taking into account the importance of the formation mechanism of perovskites in the origin of

the reactivity of the catalysts derived from them, we present herein an extensive characterization

analysis of the structural and chemical changes upon incorporating Ru to the LaCoO3 perovskite

lattice. Advanced in situ characterization by synchrotron-based wide-angle XRD and Raman

spectroscopy has been applied in order to unravel the formation mechanism of the perovskite

oxide. In line with this, we have studied the mechanism of Ru incorporation into the LaCo1-

xRuxO3 (x = 0.05-0.4) perovskite during annealing steps and how the resulting structures affect

catalyst performance in the hydrocarbon reforming reaction. Furthermore, characterization of the

formed perovskites has been performed by high resolution XRD, Raman, EXAFS and XPS in

order to relate structural changes with performance in the hydrocarbon oxidative reforming.

2. EXPERIMENTAL SECTION

2.1 Perovskite preparation

LaCo1−xRuxO3 perovskite oxides (x = 0, 0.05, 0.1, 0.2 and 0.4) have been prepared by a modified

citrate sol–gel method (Pechini method). 1 M aqueous nitrate solutions containing the precursor

cations La(NO3)3·6H2O (99.9% Alfa Aesar), Co(NO3)2·6H2O (97.7% Alfa Aesar) and RuCl3

(40.49% Ru Johnson Matthey) were added to a solution of citric acid (Alfa Aesar) and ethylene

glycol (99.5% Riedel-de Haën) (molar ratio ethylene glycol/citric acid = 1 and citric acid/(La +

(Co + Ru)) = 2.5). The mixture was stirred and heated at 70 ºC for 5 h in order to evaporate the

excess of solvent and promote polymerization. After some hours, a purple or black, highly

viscous gel was obtained. The resulting resin, which contains the metal cations inside a

polymeric network, was charred at 300 ºC for 2 h to remove the organic matter in order to obtain

the perovskite precursor. After that, the resin was milled to obtain a fine powder. For the

formation of the perovskite, the samples were calcined under air at 750 ºC for 4 h.

4

2.2 Physicochemical characterization

Formation of the perovskite structures was followed by Time Resolved X-ray diffraction (TR-

XRD), acquired at beamline X7B (λ = 0.3184 Å) of the National Synchrotron Light Source at

Brookhaven National Laboratory. Two-dimensional XRD patterns were collected with an image

plate detector (Perkin-Elmer). Each diffraction pattern was acquired in 3 min. The powder rings

were integrated using the FIT2D code. The sample (5-10 mg) was loaded into a quartz capillary

cell (1 mm diameter), which was attached to a flow system. A small resistance heater was

wrapped around the capillary, and the temperature was monitored with a 1.0 mm chromel-alumel

thermocouple that was placed straight into the capillary near the sample.11 Samples were heated

in a O2/He (5% vol. O2) flow up to 800 ºC. The relative product concentrations from the TR-

XRD experiments were measured with a 0–100 amu quadruple mass spectrometer (QMS,

Stanford Research Systems). A portion of the exit gas flow passed through a leak valve and into

the QMS vacuum chamber. QMS signals at mass-to-charge ratios of 2 (H2), 4 (He), 16 (O, CH4),

17 (OH), 18 (H2O), 28 (CO), 32 (O2) and 44 (CO2), were monitored and recorded during the

experiments.

Raman spectra were acquired with a Renishaw inVia spectrophotometer, equipped with Leica

optics, a CCD detector cooled at -70 ºC and super-Notch holographic filters to get rid of the

elastic dispersion. A red laser (785 nm and maximum power of 300 mW) was chosen as an

excitation source. Photons dispersed by the sample were grated trough a 1200 lines/mm

monochromator before reaching the detector. The spectrometer was calibrated with a Si standard

using a Si band position at 520.3 cm-1. The ex situ Raman spectra of the annealed perovskites

along with the reference compounds La2O3, Co3O4 y RuO2, were recorded in a static mode

(centered at 700 cm-1) with a laser power of 0.3 mW, 10 s of exposure time, 20 accumulations, 1

cm-1 of spectral resolution and a 50x objective. For the in situ experiments during perovskite

formation, precursor samples were placed in a Linkam CCR 1000 cell. A 50 mL/min flow of

5

O2/N2 (21% vol. O2) was employed with a heating rate of 10 ºC/min up to 750 ºC holding this

temperature for 30 min. Temperature was raised in 100 ºC intervals and Raman spectra were

recorded at room temperature under the same conditions than the ex situ analysis.

High resolution XRD patterns were performed at beamline MSPD (λ = 0.4246 Å) at ALBA

Synchrotron Light Facility with the collaboration of ALBA staff.12 The powder samples were

loaded into thin (1/16’) kapton capillaries. One dimensional XRD patterns were collected by

continuous scanning of the so called MAD26 detector setup which is composed of 13 silicon

analyser crystals (Si 111 reflection) + scintillator/PMT detectors separated by ~1.5º angular

offsets.13 Each pattern was collected over 48 minutes in a 0-48 degree 2theta range. Rietveld

refinement was accomplished by the use of GSAS software.14 The instrument parameters

(Thompson-Cox-Hastings and asymmetry profile coefficients) were derived from the fit of a Si

reference pattern.15-18 The obtained patterns were compared with the Inorganic Crystal Structural

Database (ICSD) data for phase identification.

X-ray absorption measurements of annealed perovskites were carried out at beam line X18B of

the National Synchrotron Light Source at Brookhaven National Laboratory. A Si (111) double

crystal monochromator was used for energy selection. The monochromator was detuned by 20%

to suppress higher harmonic radiation. The intensities of the incident and transmitted X-rays

were monitored by ionization chambers. EXAFS spectra were acquired in transmission mode.

The energy resolution employed was 0.5 eV. Finely grounded powder samples were

homogeneously spread over kapton tape that was folded 3 to 4 times to achieve an optimal

energy jump. Sample data was acquired simultaneously with that of a 7 mm thin Co foil (for

energy calibration) at room temperature. Data analysis and background subtraction was

performed using Athena suite of programs.19

X-ray photoelectron spectra of the annealed perovskites were recorded on a VG Escalab 200R

spectrometer equipped with a hemispherical electron analyser and an Mg Kα (1253.6 eV), X-ray

6

source (12 kV and 10 mA). The powder samples were packed into small aluminium cylinders

and mounted on a sample rod placed in the pre-treatment chamber and degassed at 500 ºC for 1 h

before being moved into the analysis chamber. The base pressure of the ion-pumped analysis

chamber was maintained below 3.10-9 mbar during data acquisition. Charge effects on the

samples were corrected by fixing the binding energies of C1s peak at 284.8 eV due to

adventitious carbon. This reference gave binding energies values with accuracy of ±0.1 eV. Data

treatment was performed using “XPS peak” software. The spectra were decomposed with the

least square fitting routine using Gaussian/Lorentzian function and after subtracting a Shirley

background. Peak intensities were estimated by calculating the integral of each peak after

smoothing and subtracting a Shirley-type background.20 Atomic surface contents were estimated

from the areas of the peaks, corrected using the corresponding sensitivity factors.21

2.3 Activity tests

Catalytic tests for oxidative reforming of diesel were carried out in a fixed-bed continuous-flow

stainless steel reactor. The catalytic bed, 100 mg of catalysts, was placed in a tubular reactor (8

mm i.d.) with a coaxially centred thermocouple in contact with the catalytic bed. Prior to

reaction, perovskite precursors were flushed in H2/N2 (10% vol. H2, 50 mL/min) at 700 ºC for 1 h

before admission of feed mixture. The flow rates of diesel and water feeds were controlled by

liquid pumps and were preheated (200 ºC) in an evaporator before passing through the catalyst

bed in the reactor. Diesel fuel was provided by CEPSA (R&D Center, C14,4H27,4) and its sulphur

amount was 22 ppmw. Nitrogen gas was also fed to the evaporator to facilitate the evaporation

and passage of both the hydrocarbon and water. For the oxidative reforming of diesel, the

reactants were introduced into the reactor in a molar ratio of H2O/O2/C= 3/0.5/1. The total gas

flow rate was kept at 75 mL/min (GHSV= 20000 h-1). Activity was measured at atmospheric

pressure and 750ºC maintaining the reaction for 24 h at this temperature. The products were

analysed periodically by an on-line gas chromatograph (Varian 450-GC) equipped with a TC

7

detector and programmed to operate under high-sensitivity conditions. A 5A (CP7538) molecular

sieve column is used for H2, O2, N2, CO and CH4 separation and a PoraBOND Q (CP7354) for

CO2, C2H6, C2H4, C3H8 y C3H6.

The diesel conversion and hydrogen yield are defined as follows:

Diesel conversion (%):

mole C (CO2+CO+CH4+C2 H4+C2 H 6+C3 H6+C3 H 8) in reformatemole C (C14 . 4 H 27 .4 ) feed

×100

Hydrogen yield (%):

mole of H2 in reformatemaximum theoretical mole of H2

×100

3 RESULTS AND DISCUSSION

3.1 Characterization of the evolution of perovskites precursors during calcination

In order to follow the influence of Co substitution by Ru in the formation of LaCo1-xRuxO3

perovskite, we have studied the structural evolution of perovskite precursors during the

calcination process up to 800 ºC under oxidant atmosphere by in situ characterization using X-

ray diffraction and Raman spectroscopy. Figure 1 shows the time-resolved XRD patterns

obtained during annealing from room temperature to 800 ºC under a 5% O2/He flow of the

LaCo0.8Ru0.2O3 perovskite precursor. The diffraction pattern recorded at room temperature

showed a wide featured profile indicating the low crystallinity of the perovskite precursor. At

room temperature, two diffraction peaks at 7.5º and 9.0º matched the cobalt spinel Co3O4 phase

with a cubic structure (Fd3m). Also found in this sample is a wide diffraction feature, centred at

4.3º. The most probable composition associated to this signal is a lanthanum carbonate hydroxide

structure of the type La2(OH)6-2x(CO3)x.22 This structure could be generated during the synthesis

of the perovskite precursor. In the synthesis of the precursor, the organic citrate groups form an

8

organometallic complex, while the ethylene glycol gives rise to a resin-like polyester. When

treating this gel in an oxidizing atmosphere the organic ligands decompose, generating carbonate

species. These CO32- groups leave the sample during annealing, mainly in the form of CO2. Due

to the basic nature of La3+ cations, the CO2 is adsorbed on the surface, leading to the formation of

carbonate structures. At room temperature these carbonate phases are heavily hydroxylated, and

could therefore be responsible for the abovementioned XRD signal detected in the perovskite

precursor.22 Ruthenium oxides can also form amorphous hydroxylated species at room

temperature that can similarly contribute to this wide diffraction feature.

1817161514131211109876543

2

800

700

600

500

400

300

200

100

Temperature /ºC

800

600

400

200

Co3O4 (ICSD-27497) LaCoO3 (ICSD-201767) La2(OH)6-2x(CO3)x

La2O2(CO3) hexagonal

Figure 1. Time-resolved XRD patterns of LaCo0.8Ru0.2O3 sample during calcination in 5% O2/He

flow

XRD patterns evolve smoothly during calcination until temperature reaches 400 ºC. At this

temperature, the peak at 4.3º disappears, followed by the growth of a new broad peak at 6.0º,

while the signal of the cobalt spinel Co3O4 phase remains constant. It is difficult to make a

precise assignment with only one broad diffraction peak. However, the position of this peak

matches the most intense peak of a lanthanum oxo-carbonate (La2O2CO3) phase in a hexagonal

9

structure; therefore, it seems feasible to assign this new peak to such a lanthanum oxo-carbonate

compound. An analogous assignment to a lanthanum oxo-carbonate nanocrystallized compound

has already been made in bibliography by both thermogravimetric analysis and diffraction

experiments.23 Another way visualizing the structural changes at 400 ºC is to see the compression

of the hydroxocarbonate structure (peak at 2 = 4 º, d-spacing = 4.5 Å) by the loss of water and

CO2 molecules to a more compacted structure with an interplanar spacing of 3 Å.

This nano-composite of lanthanum carbonate grows in intensity during calcination until the

temperature reaches 600 ºC. Finally, around 700 ºC the perovskite diffraction peaks start to

grow, as the Co3O4 peaks (at 7.5º and 9.0º) diminish in intensity. All LaCo1-xRuxO3 perovskite

precursors showed a similar behaviour during annealing to that described for the LaCo0.8Ru0.2O3

sample (Figure 1). The detailed temperatures for each transition for each of the LaCo1-xRuxO3

perovskite precursors are depicted in Table 1, while the evolution of the XRD peak intensities for

phases La2(OH)6-2x(CO3)x (4.0º), La2O2CO3 (6.0º) and perovskite (6.6º) during the annealing of

perovskite precursors can be followed in Figure 2A. From the results presented in this figure, the

degree of ruthenium replacement in the perovskite precursor has no effect on the formation

temperature of the perovskite phase. Analyses of gas evolved during annealing of the perovskite

precursors were followed by MS (signals of CO2 displayed in Figure 2B). An initial CO2

formation was observed at around 400 ºC, which accounts for the decarbonation and

dehydroxylation of the hydroxy-carbonates into oxo-carbonate structures, and a second, much

weaker, at high temperatures of 700 ºC that originates from the decomposition of the lanthanum

carbonate to form the final perovskite oxide.

SampleLa2O2CO3

Peak 2θ=6º

Perovskite

Peak 2θ=6.6º

CO2 decomposition (ºC)

Low Temp High Temp

x = 0 530 659 383 661

x = 0.05 527 679 377 689

10

x = 0.1 442 691 364 685

x = 0.2 421 686 353 698

x = 0.4 363 688 356 663

Table 1. Temperature of the maximum formation rate for lanthanum carbonate and perovskite

phases along with the temperature of CO2 decompositions during calcination of perovskite

precursors

100 200 300 400 500 600 700 8001

2

3

4

5

6

100 200 300 400 500 600 700 800

3.00E-009

6.00E-009

9.00E-009

1.20E-008

1.50E-008

1.80E-008

A)

x = 0.4

peak @ 2=4º peak @ 2=6º peak @ 2=6.6º

Nor

mal

ized

pea

k in

tens

ity

Temp /ºC

B)

x = 0.2

x = 0.1

x = 0.05

x = 0

Temp /ºC

x = 0.4

x = 0.2

x = 0.1

x = 0.05

CO

2 Par

tial p

ress

ure

(atm

)

x = 0

x10 MS

Figure 2. A) Evolution of normalized XRD peak intensities of the phases La2(OH)6-2x(CO3)x (4º),

La2O2CO3 (6º) and perovskite (6.6º) during the annealing of perovskite, B) MS CO2 signal versus

temperature during the calcination of perovskite precursors

In parallel with diffraction experiments, vibrational Raman spectroscopy was used for structural

determination and phase transitions. Figure 3 shows the evolution of the Raman spectra of the

perovskite precursors obtained during calcination from room temperature to 750 ºC under a 5%

11

O2/He flow. All samples showed a similar evolution of Raman spectra during calcination. The

Raman spectra obtained at room temperature is governed by the cobalt spinel signals at 690, 480

and 196 cm-1. The evolution of the spinel phase towards the perovskite phase is observed on

increasing the calcination temperature. This accounted for a diminishing intensity of the peak at

690 cm-1. Only after annealing at 750 ºC were the signals ascribed to the perovskite phase at 619

cm-1 observed. Both diffraction and the Raman data prove that annealing at high temperature (>

700 ºC) for a long time is needed to obtain a well-structured perovskite mixed oxide.

100 300 500 700 900 1100

689617480196

160

195689

520620

481750 ºC 30 min

600 ºC500 ºC

400 ºC

300 ºC

Cou

nts

/ a.u

.

Raman shift / cm-1

30 ºC

2000

x = 0.05

100 300 500 700 900 1100

689629

560196

689

522618

481196

750 ºC 30 min

600 ºC

500 ºC

400 ºC

300 ºC

Cou

nts

/ a.u

.

Raman / cm-1

30 ºC

2000

x = 0.1

100 300 500 700 900 1100

690619483196

692622481195

692481195

691483195

654691

537483196

686

542

479

195

750 ºC 30 min600 ºC500 ºC400 ºC

300 ºC

Cou

nts

/ a.u

.

Raman shift /cm-1

30 ºC

2000

x = 0.2

100 300 500 700 900 1100

688667611

480

392

195

620521

688478

195 750 ºC 30 min

600 ºC

500 ºC

400 ºC

300 ºC

Cou

nts

/ a.u

.

Raman shift / cm-1

30 ºC

500

x = 0.4

12

Figure 3. Evolution of Raman spectra during calcination of the LaCo1-xRuxO3 perovskite

precursors

3.2 Effect of ruthenium on the formation mechanism of LaCo1-xRuxO3 perovskite

Throughout the in situ measurements performed during calcination of perovskite precursors, only

crystalline phases corresponding to Co3O4 spinel and LaCo1-xRuxO3 perovskite oxides were

observed. For lanthanum entities, wide diffraction features assigned to lanthanum carbonate

species (hydrated and de-hydrated) appeared, and no contribution was detected from any

ruthenium phases (oxides, hydroxides or carbonates), neither in diffraction nor in Raman. The

absence of Ru phases is startling, particularly for the samples with high Ru content, in which a

segregated oxide should be easily distinguished by XRD or Raman. Compared to their Ru-free

LaCoO3 counterpart, those containing ruthenium show no peak shifts in the Co3O4 spinel phase

(neither in XRD nor in Raman signal), a fact that excludes the possibility of the Ru atoms

becoming incorporated into the spinel structure forming a Co2RuO4 oxide.24 At low temperature,

both Ru and La are known to form amorphous hydroxide and hydroxycarbonate phases.

Ru(OH)n decomposition to RuO2 occurs between 300-400 ºC,25 but even at much higher

temperatures no diffraction or Raman peaks from RuO2 are identified. Only the diffraction lines

from low crystallinity La2(OH)6-2x(CO3)x and La2O2CO3 phases are distinguished. The highly

disordered and quasi-amorphous nature of these structures makes them ideal candidates to

conceal ruthenium species in inter-laminar positions. Another less likely option is that ruthenium

is forming a segregated or independent amorphous oxide.

For all the samples, the transition from the perovskite precursor to the perovskite structure occurs

through an oxo-lanthanum-carbonate intermediate phase. This intermediate phase decomposes

mostly at around 400 ºC into an oxo-carbonate structure and, finally, this oxycarbonate forms the

final perovskite at around 700 ºC. Table 1 summarizes the temperatures at which the transitions

13

between the different phases are observed during the annealing of perovskite precursors. From

the evolution of the peak at 6.6º due to the formation of the perovskite, two main features are

derived. One is that its intensity is growing continuously until quenching the experiment, which

suggests that the kinetics of the formation is slow. Secondly, the position of the perovskite

diffraction peak does not change sharply with time, pointing to single-step perovskite oxides

formation. If the Ru were incorporated at a later stage than Co on the structure, there would

appear peak shifts in the diffraction patterns of the ruthenium-containing samples during

annealing (see Figure 2). There is no correlation between Ru content and the temperatures of

formation of either carbonate or perovskite phases (data in Table 1). The temperature for the

transition from La2(OH)6-2x(CO3)x to La2O2CO3 is between 420 ºC to 530 ºC, whereas the

temperature range for perovskite formation is 660-690 ºC. Ruthenium presence does not seem to

alter the formation mechanism of the perovskite oxides. The sample with highest Ru content (x =

0.4) has a different behaviour than the rest of the samples. First of all, the position of the most

intense peak of the perovskite phase is shifted toward smaller values. As shown in previous

sections, this sample crystallizes in a monoclinic phase. The position shift causes the perovskite

peak at 6.6º to overlap with the signal of the La2O2CO3 phase at 6.0º, owing to which, unlike in

the other samples in the series, we cannot observe the disappearance of the lanthanum oxo-

carbonate phase. Besides, the formation temperature of the perovskite oxide is the highest of the

series. Such a high temperature is needed to obtain Ru3+ species.26

Based on these analyses we can propose two alternative mechanisms for the formation of LaCo1-

xRuxO3 perovskites (the * marks the crystalline phases that are clearly distinguished in the

diffraction patterns and in the Raman spectra):

T > 350 ºC: 2La2(OH)6-2x(CO3)x ∆→ La2O2CO3 + xCO2 + 3H2O [1a]

T > 700 ºC La2O2CO3 + 2/3Co3O4* + xRuO2 + nO2∆→ 2LaCo1-xRuxO3* + CO2 [2a]

14

T > 350 ºC 2La(OH)6-2y(CO3)y-Rux ∆→ La2O2CO3-Rux + yCO2 + 3H2O [1b]

T > 700 ºC 3La2O2CO3-Rux + 2Co3O4* + ½O2 ∆→ 6LaCo1-xRuxO3* + 3CO2 [2b]

The first mechanism involves the segregation of Ru species in the form of amorphous ruthenium

oxide phase (mechanism a), while the second requires the incorporation of Ru species in the

lanthanum oxycarbonate phase (mechanism b). In the first, the formation of the perovskite phase

can only take place if a solid-state reaction between three different phases, La 2O2CO3, Co3O4 and

RuO2 takes place simultaneously. In the second, a very disordered carbonate phase, containing

La3+ ions and Ru4+ species, reacts with the cobalt spinel to form the perovskite oxide as shown

schematically in Figure 4. The highly unlikely event of a solid-state reaction between three

different phases makes mechanism b statistically and kinetically preferential over mechanism a.

3 La2(CO3)3(H2O)8 Rux

O2, D

H2O + CO2

3 La2O2(CO3)2Rux

O2, D

H2O + CO2

Ste

p1

T

350º

C

3LaCo1-xRuxO3Co3O4 3 La2O2(CO3)2Rux

O2, D

CO2

+

O2, D

CO2

+

Ste

p2

T >

700

ºC

Figure 4. Proposed scheme for the formation of LaCo1-xRuxO3 perovskite oxide

3.3 Characterizacion of perovskites after calcination

The high-resolution X-ray diffraction patterns of the LaCo1−xRuxO3 calcined samples, along with

the refined data, are displayed in Figure 5 A. The diffraction pattern of LaCoO3 sample showed

15

strong reflexions at 8,97º and 9,05º corresponding to the rhombohedral (R3c) structure 8 of

perovskite with a minor contribution at 7.5º and 9.0º of cobalt spinel (Co3O4) indicative of the

high degree of incorporation of the La and Co oxides into the perovskite structure. The

diffraction patterns of the LaCo1−xRuxO3 samples show profiles corresponding to single

perovskite structures without peaks attributable to ruthenium oxides (Figure 5A). The partial

substitution of Co by Ru evidences changes in the rhombohedral structure of the perovkite as the

changes the diffraction lines characteristic of the of LaCoO3 sample indicated in Figure 5B. In

this figures it is observed that the diffraction lines of the Ru-substituted perovskites shifted to

lower angles respect to the diffraction lines characteristic of the rhombohedral LaCoO3

perovskite phase that result in a modification of the structure of pure LaCoO3. As the degree of

Ru replacement increased, the rhombohedral perovskite structure became increasingly more

distorted, which can be accounted for by a shift in the peaks toward smaller angles and by a

lesser splitting of the peaks at 9º. The maximum distortion in rhombohedral structures is

achieved for the sample with 20% of the atomic substitution of Co by Ru. For a degree of Ru

atomic substitution greater than 20%, the rhombohedral phase is no longer stable and the

LaCo0.6Ru0.4O3 sample crystallizes in a double perovskite structure with monoclinic

symmetry.27,28 . No diffraction peaks were observed from any crystalline phase associated to

ruthenium oxides Table 2 summarizes the results of X-ray Rietveld refinement: lattice

dimension, crystallite size as obtained by the Scherrer equation, occupancy of B site and weight

fraction of each crystallographic phase. Rietveld refinement shows that as the Ru substitutes Co

positions in the lattice, a cell expansion is observed. This expansion could be caused by the

higher ionic radii of the Ru3+ (0.68 Å) as compared to the Co3+ ions (0.55 Å). The cell expansion

may also be caused by charge redistribution between Ru and Co ions. Ru ions are very stable as

+4 cations and they may incorporate as such into the perovskite structure. If that is the case,

some Co3+ ions must get reduced ion order to compensate charges. The substitution of cobalt by

ruthenium is confirmed by an increased occupancy of B sites as obtained in the Rietveld

16

refinement. Another important parameter, also affected by the presence of Ru, is the crystallite

size determined by the Scherrer equation. As Ru is incorporated in the perovskite lattice, the

particle size diminishes reaching its minimum for the LaCo0.8Ru0.2O3. Results from the Rietveld

refinement show that the presence of ruthenium hinders phase segregation since cobalt spinel

phase fraction diminishes when ruthenium is added. Additionally, the distortion induced by the

presence of ruthenium also allows the formation of smaller crystallite sizes.

120x103

100

80

60

40

20

0

Cou

nts

/a.u

.

161514131211109876

2

Observed Rietveld Refinement Co3O4 (ICSD-27497) LaCoO3 (ICSD-201767)

x = 0

x = 0.05

x = 0.1

x = 0.2

x = 0.4

A)

120x103

100

80

60

40

20

0

Cou

nts

9.29.08.88.6

2

x = 0

x = 0.05

x = 0.1

x = 0.2

x = 0.4

B)

Figure 5. High resolution XRD patterns and Rietveld refinement of the calcined LaCo1-xRuxO3

perovskites (x=0, 0,05, 0,1, 0,2 and 0,4) (A), Inset of the doublet peak at 9º showing the peak

shift and symmetry loss with increasing Ru substitution in the perovskite (B)

Sample Space group a (Å) b (Å) c (Å)size

(nm)

Occ

. B

site

%weight

LaCoO3 La2CoRuO6 Co3O4

x = 0 R3c

rhombohedral

5.442 5.442 13.102 42.5 1.05 97.77 0.00 2.23

x = 0.05 5.457 5.457 13.142 32.7 1.09 98.13 0.00 1.87

x = 0.1 5.478 5.478 13.193 31.4 1.10 98.56 0.00 1.44

17

x = 0.2 5.505 5.505 13.393 28.3 1.20 98.35 0.00 1.65

x = 0.4P21/n

monoclinic5.571 5.608 7.870 30.8 1.18 11.78 87.69 0.53

Table 2. Structural parameters from XRD Rietveld refinement of the calcined LaCo1−xRuxO3

perovskites

Figure 6 shows the XANES spectra of the calcined LaCo1−xRuxO3 perovskites analysed in

comparison with the CoO and Co3O4 reference oxides. The XANES spectra of the parent

LaCoO3 and the Ru-substituted samples are similar to those previously published for LaCoO3 by

Thornton et al.29 The XANES region of the Co K-edge deals with the electronic transition

between the core 1s electrons and the empty states of the 4s and 4p shell, where the 1s4p

transition is the one allowed by selection rules while the pre-edge features are governed by the

symmetry-forbidden Co3+ 1s3d transitions. In order to better analyse the subtle variations in the

XANES signal, we will also study the 1st and 2nd derivatives of the absorption spectra (Figure 6).

The maximum for the first derivative provides the position of the absorption edge (E0) of each

sample as summarized in Table 3. The absorption edge for the pure LaCoO3 sample is 7724.4 eV

in agreement with the reported value.30 Upon substituting the samples with increasing Ru

amounts, a shift towards smaller energies of the adsorption peak is observed. This effect

accounts for a partial reduction of the Co3+ ions to Co2+. This is followed by all the samples in the

series except for the one with the highest Ru content (x = 0.4), for which cobalt species are fully

oxidized to +3. The partial reduction of the Co3+ ions to Co2+ when ruthenium is incorporated is

in full agreement with the Rietveld refinement data showed above which point to the reduction

of Co to 2+ oxidation state to compensate the introduction of Ru4+ ions in the lattice of

perovskite. Ru K-edge XANES data corroborates this face showing a Ru4+ oxidation state in all

the samples.

18

7700 7710 7720 7730 77407700 7720 7740 7760 7780

1st d

eriv

ativ

e of

abs

orpt

ion

data

Energy /eV

Co2+ LaCoO3

x = 0.2

x = 0.1

x = 0.05

x = 0.4

x = 0.2

x = 0.1

x = 0.05

x = 0

x = 0.05

x = 0.1

x = 0.2

CoO

Co3O

4

x = 0

x = 0.4

Co3O

4

Nor

m. a

bs

Energy /eV

CoO

x = 0

Co3O4

CoO

7700 7710 7720 7730 7740

x = 0.4

2nd d

eriv

ativ

e of

abs

orpt

ion

data

Energy /eV

Co(1s)Co(4p)

LMCT

Figure 6. Normalized Co K-edge XANES spectra of calcined LaCo1-xRuxO3 perovskites and

their 1st and 2nd derivatives

Sample E0 Oxidation state

CoO7720.

5+2

Co3O4 7721. +2, +3

19

7

x = 07724.

4+3

x = 0.057723.

4+2, +3

x = 0.17722.

7+2, +3

x = 0.27722.

2+2, +3

x = 0.47724.

0+3

Table 3. Co K-edge energies and main oxidation states of calcined LaCo1−xRuxO3 perovskites

along with CoO and Co3O4 reference samples

Figure 7A shows the k2-weighted Co K-edge EXAFS spectra of the analysed samples. As Ru is

incorporated in the structure, oscillations are broadened and signal is flattened at high k values.

This behaviour can easily be ascribed to an increased disorder due to the distortion induced by

the presence of Ru in the coordination of Co atoms. The sample with the highest Ru content, x =

0.4, shows a different behaviour as it regains order in its structure. Moreover, the oscillations in

k-space of this sample are very similar to those of the LaCoO3 parent structure, suggesting

similar chemical first coordination shells in both samples. This similar short-range ordering

(octahedral oxygen coordination around Co atoms) for the LaCo0.6Ru0.4O3 sample as compared to

the LaCoO3 is consistent with evolution to a different long-range structure, as observed in the

distinct phase obtained by XRD. Figure 7b shows phase uncorrected interatomic distance

obtained by Fourier transformation of (k) over the k-space range between 2-12.5 Å-1. For

the parent LaCoO3 perovskite, the first peak in the radial distribution at 1.5 Å is assigned to the

20

first shell Co-O distance. Weak peaks at 2.3 and 2.6 Å correspond to Co-O-O and Co-O-La

distances. The strong peak at 3.1 Å is attributed to Co-O-Co(Ru) distance in adjacent octahedra.

It was noted that the absolute values of the interatomic distances obtained by EXAFS are too

short, as compared to the real distances, due to the inherent phase shift of the uncorrected

EXAFS dispersion data. For Ru-containing samples, the appearance of a shoulder is observed at

short distances (0.95 Å) indicating a Co-O bond shortening. Such distortion in the Co-O bond

distance may be due to a Jahn-Teller distortion in the CoO6 octahedra, arising from the different

ionic radii of Co3+, Co2+ and Ru4+ cations occupying B sites. The Jahn-Teller distortion causes the

splitting of the first shell Co-O bond distance, elongating some bonds and shortening others. At

high R-values the disorder created by the incorporation of Ru leads to a flattening of the signal.

The different bond distances at R > 4 Å due to the variable second coordination shell of Co-O-

Ru paths are caused by the combined Jahn-Teller and rotational distortions of MO6 octahedra.

The EXAFS signal shows that the disorder induced by Ru incorporation affects the local

environment of Co in addition to the long-range effects observed by XRD.

21

Figure 7. The k2-weighted Co K-edge EXAFS spectra of the calcined LaCo1-xRuxO3 perovskites

(x=0, 0,05, 0,10, 0,20, 0,40) (A) and the derived radial distribution function for each sample (B)

The Raman spectra of the LaCo1−xRuxO3 annealed samples, along with the Co3O4 and RuO2

reference oxides, are displayed in Figure 8. The spinel Co3O4 (cubic structure)31 shows Raman

modes at 196, 481 (Eg), 521 (F2g), 619 (F2g) and 690 (A1g) cm-1, whereas the Raman spectrum of

RuO2 (tetragonal phase) possesses three vibrational modes at 523 (Eg), 644 (A1g) and 710 cm-1

(B2g).32 The LaCoO3 with perovskite structure displays bands at 157, 196, 416, 480, 521, 619,

647 and 689 cm-1.2,33 The region from 400 to 700 cm-1 is assigned to Co-O bending and stretching

modes. At low wavenumbers we can also observe a rotational Raman band of the Co-O

octahedral at 196 cm-1 and the mode of La-O bending at 157 cm-1. The most intense Raman

modes of the LaCoO3 perovskite overlap with the Co3O4 signal, making it difficult to quantify

phase segregation from the Raman spectra. This is due to a similar local environment around the

CoO6 octahedra in both phases. In the region from 400 to 700 cm-1, characteristic of the Co-O

stretching and bending local vibrations, the LaCoO3 sample presented a Raman peak at around

690 cm-1, followed by a wide band centred at 647 cm-1 and a small peak at 635 cm-1. In this

region, the Ru-substituted samples presented the characteristic peak at 690 cm-1 slightly shifted

towards a higher wavenumber while the signal at 600-650 cm-1, arising from oxygen mobility in

the structure, became broader and more intense. The general trend is for the oxygen mobility to

improve with Ru incorporation. The shift in the position of the 690 cm -1 is ascribed to different

bond distances in the vibration modes, caused by distortion in the M-O octahedral as Ru is

incorporated. The sample with highest Ru content, x = 0.4, crystallized in the double perovskite

structure La2CoRuO6 with a monoclinic symmetry and originated an intense sharp Raman mode

at 665 cm-1 with a weaker peak at 689 cm-1 and a wide band at 597 cm-1. As observed from XRD

results, there were no signals arising from ruthenium oxides in segregated phases, confirming the

absence of segregation of ruthenium phases from the perovskite structure. The presence of

ruthenium not only alters the coordination of cobalt atoms, but it also has strong effect on the

22

lanthanum environment. For the low wavenumber region (below 200 cm-1), assigned to La-O

roto-vibrations there also are variations dependent on Ru content. For the Ru-substituted

perovskites, this Raman mode is broadened and split into two components, suggesting a

distortion on La-O bonds.

100 125 150 175 200 400 500 600 700 800 900

x = 0.4

x = 0.2

x = 0.1

x = 0.05

Co-Ostretching

Co-ObendingRotationalLa bending

RuO2

x = 0

Inte

nsity

Raman shift /cm-1

x 0,03 Co3O

4

23

Figure 8. Raman spectra of the calcined LaCo1-xRuxO3 perovskites along with Co3O4 and RuO2 as

reference

The surface composition and oxidation state of the calcined LaCo1−xRuxO3 perovskites was

determined by XPS. The Co 2p level of all LaCo1−xRuxO3 perovskites (Figure 9) shows the BE of

the most intense Co 2p3/2 peak of the Co 2p doublet centred at 780.1 eV with a satellite line at

790.2 eV. These values are consistent with the presence of Co3O4 species on the perovskite

surface.34,35 Cobalt signal was unaltered in the Ru-substituted samples and no peak shift or

broadening was observed upon the incorporation of ruthenium, except for the sample with the

highest Ru content (x = 0.4). For this sample the Co 2p doublet appeared somewhat broadened,

the satellite peak being less intense. This behaviour points to a surface enrichment in Co3+

species.34

765 775 785 795 805 815

790.2

780.1

c.p.

s. /

a.u.

B.E. / eV

500 x = 0

765 775 785 795 805 815

790.1

779.7

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.05

765 775 785 795 805 815

789.6

780.1

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.1

765 775 785 795 805 815

790,1

780,4

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.2

24

Figure 9: Co 2p XP spectra of the calcined LaCo1-xRuxO3 (x=0, 0.05, 0.1, 0.2 and 0.4) perovskite

oxides.

For the LaCoO3 sample, the La 3d spectra (Figure 10) shows a characteristic doublet for each La

3d5/2 and La 3d3/2 component at 833.3 and 835.3 eV. Taking into consideration the position and

shape of the peaks, the first contribution at 833.3 eV is assigned to La3+ in a perovskite

environment35,36 where the second contribution at 835.3 is assigned to La3+ combined with

hydroxyl37 or carbonate groups (La2(CO3)3, La2O2CO3).37,38 The Ru-containing samples show a

poorly resolved La 3d doublet profile as well as a shift towards higher BE values, which suggest

the increasing content in lanthanum hydroxide or carbonate species induced by the presence of

ruthenium. The O 1s spectra of the annealed perovskites show differences depending upon the

degree of Ru substitution (Figure 11). The O 1s spectra of all perovskites show three different

contributions due to lattice oxygen (529 eV), hydroxide/carbonate species (531 eV) and a

characteristic tail around 533.5 eV related to oxygen from molecular water strongly adsorbed on

the surface. The Ru-substituted perovskites presented higher percentages of surface oxygen in

765 775 785 795 805 815

c.p.

s. /

a.u.

B.E. / eV

1000779.9

789.5

x = 0.4

765 775 785 795 805 815

789.6

780.1

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.1

765 775 785 795 805 815

790,1

780,4

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.2

25

the form of hydroxide/carbonate than in the form of lattice oxygen. This fact could be related to

the higher concentration of lanthanum hydroxide or carbonate surface species previously

observed in the analysis of La 3d levels that could be associated to the Co substitution by Ru in

the perovskite lattice.

The surface composition of the calcined LaCo1−xRuxO3 perovskites (Table 4) was determined

from XPS data. The comparison of nominal and surface concentration of Co and Ru calculated

from XPS intensities are presented in Figure 12. It was observed that the surface concentration of

Co was lower than the nominal value in the case of the samples with higher Ru substitution

(LaCo0.8 Ru0.2O3 and LaCo0.8 Ru0.4O3), indicating a loss of cobalt at surface level. The Ru surface

exposure varied with the Ru loading in the perovskite precursor. As it is shown in Fig. 12, the

relative surface concentration of ruthenium proportionally increased with the Ru loading in the

perovskite precursor except for the sample with higher Ru substitution for which a strong surface

concentration was detected.

825 830 835 840 845 850 855 860 865

835.3833.3

c.p.

s. /

u.a.

B.E. / eV

2000 x = 0

825 830 835 840 845 850 855 860 865

835.4833.4

c.p.

s. /

a.u.

B.E. / eV

2500 x = 0.05

825 830 835 840 845 850 855 860 865

835.4833.6

c.p.

s. /

a.u.

B.E. / eV

2500 x = 0.1

825 830 835 840 845 850 855 860 865

835.7833.8

c.p.

s. /

u.a.

B.E. / eV

2500 x = 0.2

26

Figure 10: La3d XP spectra of the calcined LaCo1-xRuxO3 (x=0, 0.05, 0.1, 0.2 and 0.4) perovskite

oxides.

524 526 528 530 532 534 536 538

532.3

530.6

528.7

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0

524 526 528 530 532 534 536 538

532.1

530.6

528.6

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.05

825 830 835 840 845 850 855 860 865

835.4833.6

c.p.

s. /

a.u.

B.E. / eV

2500 x = 0.1

825 830 835 840 845 850 855 860 865

c.p.

s. /

a.u.

B.E. / eV

5000

837.7

834.2

x = 0.4

825 830 835 840 845 850 855 860 865

835.7833.8

c.p.

s. /

u.a.

B.E. / eV

2500 x = 0.2

524 526 528 530 532 534 536 538

532.8

531.1529.1

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.2

524 526 528 530 532 534 536 538

533.5

530.8528.8

c.p.

s. /

u.a.

B.E. / eV

1000 x = 0.1

27

Figure 11: O1s XP spectra of the annealed LaCo1-xRuxO3 (x=0, 0.05, 0.1, 0.2 and 0.4) perovskite

oxides.

Figure 12 XPS surface Co/La and Ru/La atomic ratio of calcined LaCo1−xRuxO3 (x=0, 0.05, 0.1,

0.2 and 0.4) perovskites

Sample Co/La* Ru/La* (Co+Ru)/La* O(OH-/CO32-)/Ored O/(Co+Ru+La)*

524 526 528 530 532 534 536 538

532.8

531.1529.1

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.2

524 526 528 530 532 534 536 538

533.5

530.8528.8

c.p.

s. /

u.a.

B.E. / eV

1000 x = 0.1

524 526 528 530 532 534 536 538

533.8

530.7

528.5

c.p.

s. /

a.u.

B.E. / eV

1000 x = 0.4

28

x = 01.03

(1.00)-

1.03

(1.00)1.03

2.53

(1.50)

x =

0.05

0.95

(0.95)

0.08

(0.05)

1.03

(1.00)1.53

2.58

(1.50)

x = 0.11.09

(0.90)

0.11

(0.10)

1.20

(1.00)1.92

2.21

(1.50)

x = 0.20.65

(0.80)

0.22

(0.20)

0.87

(1.00)1.21

2.77

(1.50)

x = 0.40.35

(0.60)

0.20

(0.40)

0.56

(1.00)3.31

2.54

(1.50)

*In brackets nominal atomic ratio

Table 4. XPS surface atomic ratio of calcined LaCo1−xRuxO3 (x=0, 0.05, 0.1, 0.2 and 0.4)

perovskites

3.4 Influence of Ru incorporation in the catalytic performance of perovskites for hydrocarbon

reforming

Characterization of perovskites showed that the partial substitution of Co by Ru led to

differences in their structure, crystallite size and surface characteristics. The structural effects

observed on the LaCo1-xRuxO3 perovskites have strong influence on their catalytic behaviour in

the oxidative reforming of diesel. The activity of the catalysts derived from LaCo1-

xRuxO3perovskite precursors for the oxidative reforming of diesel was measured in terms of

diesel conversion and hydrogen yield. Figure 13 shows the evolution of diesel conversion with

the reaction time for each catalyst. It is observed that each catalyst evolved in a different way

with time-on-stream, depending on the catalyst precursor. Figure 14 shows the hydrogen yield

29

obtained in the oxidative reforming of diesel on the catalysts derived from LaCo1-xRuxO3

perovskites. The catalysts displayed differences in the initial activity (0-8 h) and stability with

time-on-stream. These differences are indicative of differences in the initial concentration and

stability of the Co, Ru and La phases present in catalysts that are affected by the ruthenium

incorporation in the perovskites precursors.

Figure 13. Evolution of diesel conversion with time on stream during the oxidative reforming of

diesel over catalyst derived from LaCo1-xRuxO3 perovskite precursors (x = 0, 0.05, 0.1, 0.2 and

0.4)

30

60

55

50

45

40

35

30

% H

2 y

ield

4035302520151050

% at. Ru

0-8 h 16-24 h

Figure 14. Initial and steady-state hydrogen yield from oxidative reforming of diesel over

catalyst derived from LaCo1-xRuxO3 perovskite precursors as a function of ruthenium content on

perovskite (x = 0, 0.05, 0.1, 0.2 and 0.4)

It is known that the initial activity of the reforming catalysts is related with the surface exposure

of metal active Co and Ru sites.8 In this sense, the observed changes in the initial reforming of

samples should be related with differences in the dispersion of Co and Ru developed after the

reduction of the perovskite precursors. XRD, EXAFS and Raman analysis on LaCo1-xRuxO3

perovskite precursors indicated that the partial substitution of Co by Ru into LaCoO3 perovskite

led to structural changes associated with the ruthenium incorporation into the perovskite lattice.

Figure 15 shows the change in the crystallite size of perovskite and the reduction of Co3+ to Co2+

sites as function of the Ru content in the perovskite. The figure shows that as ruthenium is

incorporated in the perovskite, the Co reduction degree increases and the perovskite crystallite

31

size diminishes. The LaCo0.6Ru0.4O3 sample does not follow the trend and partially recuperates

both crystallite size and Co oxidation state. The abovementioned changes in size and reduction

degree of Co associated with the ruthenium incorporation in the perovskite coincide with the

trend in the initial reforming activity of the samples as observed in Figure 14. The lower size and

higher reduction degree of Co sites in perovskites may facilitate both the reduction of the

perovskite and the surface exposure of active Co and Ru sites that could be in the origin of the

differences in the initial activity observed among the samples. In addition, the partial substitution

of Co by Ru in LaCoO3 perovskite led to the distortion of the rhombohedral structure as a

consequence of the insertion of ruthenium cations into the Co position. This distortion makes

easier the oxygen mobility that facilitates the reduction of perovskites to form the catalysts.

7724.5

7724.0

7723.5

7723.0

7722.5

7722.0

7721.5

7721.0

7720.5

Abs

orpt

ion

Edg

e /e

V

4035302520151050

% at. Ru

42

40

38

36

34

32

30

Crystallite size /nm

E0 LaCo1-xRuxO3 Crystallite size

Co3O4 (Co+2

+ Co+3

)

CoO (Co+2

)

Figure 15. Co reduction degree (from absorption edge position) along with crystallite size of the

LaCo1-xRuxO3 perovskite as a function of ruthenium content

All catalysts show a decrease in activity after the first hours on stream indicating some

deactivation (Figure 14). However, the deactivation is lower as the Ru content in perovskite

increases (Figure 14). Catalyst deactivation by carbonaceous deposits is the main factor to

analyse in order to justify the evolution of catalysts under reforming conditions. Higher

carbonaceous deposits on reforming catalysts imply higher deactivation rates. Dispersion of

32

metal particles and the nature of supports strongly affect the formation of carbonaceous deposits

on reforming catalysts. Carbonaceous deposits are favoured on metal particles of larger size

while supports as La2O3 assist in coke removal from catalyst surfaces. In literature, positive

effect of lanthanum as support of noble metals applied to steam reforming of hydrocarbons are

explained by the ability of lanthanum to adsorb CO2 forming lanthanum oxycarbonates which

participate in coke gasification.39,40 Taking these facts into account, the evolution of catalysts

deactivation under reforming conditions could be related with the differences in cobalt and

ruthenium dispersion as well as their contact with lanthanum entities associated with the

ruthenium incorporation in the perovskite precursor. As commented previously, the partial

substitution of Co by Ru in the perovskite precursor probably generated a better Co and Ru

dispersion that could contribute to lower the carbon deposits on catalysts. In addition, the

deactivation differences observed on catalysts may also be related to the differences in

lanthanum coordination and chemistry derived from the structural distortion induced by

ruthenium substitution in the perovskite. As it was observed in the XPS and Raman spectra, the

cobalt substitution by ruthenium in the perovskite modifies the lanthanum characteristics

favouring the formation of carbonated species at the surface of the catalytic precursors. The

formation of carbonate-like structures is enhanced by ruthenium, which must be interacting with

lanthanum entities, loosening La-O bonds in order to facilitate the adsorption of CO2. Therefore,

the lower deactivation observed on catalysts with higher ruthenium substitution could be also

related with the enhancement of the capacity of the catalysts to form lanthanum oxycarbonates

which participate in coke gasification.

CONCLUSIONS

The in situ studies during the calcination process by time-resolved synchrotron X-ray diffraction

and Raman spectroscopy demonstrated that ruthenium content in LaCo1-xRuxO3 perovskites did

33

not affect the kinetics or formation temperature of the perovskite oxide. The formation

mechanism of LaCo1-xRuxO3 likely involves a solid-state reaction between cobalt spinel and

lanthanum oxide-carbonate in close contact with ruthenium species. Structural characterization

of perovskites showed structural changes in the perovskite with the insertion of Ru in the

structure. It was observed that the insertion of Ru affects the bulk structure by creating rotational

and Jahn-Teller distortions in the perovskite structure. In this way, the formation of a single

perovskite phase was observed, either in a rhombohedral (x = 0), distorted rhombohedral (x =

0.05, 0.1 and 0.2) or monoclinic (x = 0.4) structure, depending on the amount of Ru incorporated

into the perovskite structure. EXAFS analysis of Co K-edge showed strong Jahn-Teller

distortions around the CoO6 octahedra, in addition to the rotational distortions observed in the

diffraction patterns. Raman spectroscopy completed the description, proving the strong

distortions of the lattice oxygen and the La-O coordination induced by the presence of

ruthenium. Such distorted configuration gave rise to a weakening of metal-oxygen bonds,

maximizing anionic mobility and reactants adsorption. Surface changes were also observed with

the insertion of Ru in the perovskite structure. XPS showed that there are cobalt spinel species,

unaltered by ruthenium, and lanthanum oxide species that become more carbonated when Ru is

present. The formation of carbonate-like structures is enhanced by ruthenium, which must be

interacting with lanthanum entities, loosening La-O bonds in order to facilitate the adsorption of

CO2. Relating these structural effects with catalytic performance in hydrocarbons reforming, we

can conclude that the structural distortion induced by ruthenium favours catalytic stability,

probably by stabilizing metallic Co and Co-Ru sites, increasing metal dispersion and by making

oxygen mobility easier in the disturbed La2O3 support.

AUTHOR INFORMATION

Corresponding Author

34

*e-mail: r. navarro@icp.csic.es; phone +34915854774

Present Addresses

† Velocys, Milton Park, OX14 4SA United Kingdom.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support provided by the Spanish Ministry of Economy

and Competitiveness (MINECO) under grant CTQ2013-48669-P and by CAM under grant

P2013/MAE-2882. Our appreciation goes to I. Peral and C. Popescu for their help in the

acquisition of HR-XRD at MSPD ALBA beamline. We thank J. Hanson and W. Xu for their

help in performing the TR-XRD experiment at X7B beamline (NSLS). We also wish to thank N.

Marinkovic for his help in the XAFS measurements at X18B (NSLS). Use of the National

Synchrotron Light Source, BNL, was supported by the US DoE, Office of Basic Energy

Sciences, under Contract No. DE-AC02-98CH10886.

REFERENCES

35

1. Tejuca, L. G.; Fierro, J. L. G.; Tascón, J. M. D. Structure and reactivity of perovskite-type

oxides. Adv. Catal. 1989, 36, 237-328.

2. Iliev, M. N.; Abrashev, M. V. Raman phonons and Raman Jahn-Teller bands in perovskite-

like manganites. J. Raman Spectrosc. 2001, 32, 805-811.

3. Peña, M. A.; Fierro, J. L. G. Chemical structures and performance of perovskite oxides. Chem.

Rev. 2001, 101, 1981-2017.

4. Pietri, E.; Barrios, A.; Gonzalez, O.; Goldwasser, M. R.; Pérez-Zurita, M. J.; Cubeiro, M. L.;

Goldwasser, J.; Leclercq, L.; Leclercq, G.; Gingembre, L. Perovskites as catalysts precursors for

methane reforming: Ru based catalysts. Stud. Surf. Sci. Catal. 2001, 136, 381-386.

5. Tomishige, K.; Kanazawa, S.; Suzuki, K.; Asadullah, M.; Sato, M.; Ikushima, K.; Kunimori,

K. Effective heat supply from combustion to reforming in methane reforming with CO2 and O2:

comparison between Ni and Pt catalysts. Appl. Catal., A 2002, 233, 35-44.

6. Goldwasser, M. R.; Rivas, M. E.; Pietri, E.; Pérez-Zurita, M. J.; Cubeiro, M. L.; Gingembre,

L.; Leclercq, L.; Leclercq, G. Perovskites as catalysts precursors: CO2 reforming of CH4 on Ln1-

xCaxRu0.8Ni0.2O3 (Ln = La, Sm, Nd). Appl. Catal., A 2003, 255, 45-57.

7. Navarro, R. M.; Alvarez-Galvan, M. C.; Villoria, J. A.; González-Jiménez, I. D.; Rosa, F.;

Fierro, J. L. G. Effect of Ru on LaCoO3 perovskite-derived catalyst properties tested in oxidative

reforming of diesel. Appl. Catal., B 2007, 73, 247-258.

8. Mota, N.; Álvarez-Galván, M. C.; Al-Zahrani, S. M.; Navarro, R. M.; Fierro, J. L. G. Diesel

fuel reforming over catalysts derived from LaCo1-xRuxO3 perovskites with high Ru loading. Int.

J. Hydrogen Energy 2012, 37, 7056-7066.

9. Ivanova, S.; Senyshyn, A.; Zhecheva, E.; Tenchev, K.; Nikolov, V.; Stoyanova, R.; Fuess, H.

Effect of the synthesis route on the microstructure and the reducibility of LaCoO 3. J. Alloys

Compd. 2009, 480, 279-285.

36

10. Ivanova, S.; Senyshyn, A.; Zhecheva, E.; Tenchev, K.; Stoyanova, R.; Fuess, H. Crystal

structure, microstructure and reducibility of LaNixCo1-xO3 and LaFexCo1-xO3 perovskites (0 < x <

0.5). J. Solid State Chem. 2010, 183, 940-950.

11. Chupas, P. J.; Qiu, X.; Hanson, J. C.; Lee, P. L.; Grey, C. P.; Billinge, S. J. L. Rapid-

acquisition pair distribution function (RA-PDF) analysis. J. Appl. Crystallogr. 2003, 36, 1342-

1347.

12. Fauth, F.; Peral, I.; Popescu, C.; Knapp, M. The new material science powder diffraction

beamline at ALBA synchrotron. Powder Diffr. 2013, 28, S360-S370.

13. Peral, I.; McKinlay, J.; Knapp, M.; Ferrer, S. Design and construction of multicrystal

analyser detectors using Rowland circles: application to MAD26 at ALBA. J. Synchrotron Rad.

2011, 18, 842-850.

14. Toby, B. H. EXPGUI, a graphical user interface for GSAS. J. Appl. Crystallogr. 2001, 34,

210-213.

15. Howard, C. J. The approximation of asymmetric neutron powder diffraction peaks by sums

of Gaussians. J. Appl. Crystallogr. 1982, 15, 615-620.

16. Thompson, P.; Cox, D. E.; Hastings, J. B. Rietveld refinement of Debye–Scherrer

synchrotron X-ray data from Al2O3. J. Appl. Crystallogr. 1987, 20, 79-83.

17. Finger, L. W.; Cox, D. E.; Jephcoat, A. P. A correction for powder diffraction peak

asymmetry due to axial divergence. J. Appl. Crystallogr. 1994, 27, 892-900.

18. Stephens, P. W. Phenomenological model of anisotropic peak broadening in powder

diffraction. J. Appl. Crystallogr. 1999, 32, 281-289.

19. Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data analysis for X-ray

absorption spectroscopy using IFEFFIT. J. Synchrotron Rad. 2005, 12, 537-541.

20. Shirley, D. A. High-resolution X-ray photoemission spectrum of the valence bands of gold.

Phys. Rev. B: Condens. Matter Mater. Phys. 1972, 5, 4709-4714.

37

21. Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H.

Empirical atomic sensitivity factors for quantitative analysis by electron spectroscopy for

chemical analysis. Surf. Interface Anal. 1981, 3, 211-225.

22. Bernal, S.; Díaz, J. A.; García, R.; Rodríguez-Izquierdo, J. M. Study of some aspects of the

reactivity of La2O3 with CO2 and H2O. J. Mater. Sci. 1985, 20, 537-541.

23. Laberty-Robert, C.; Fontaine, M. L.; Mounis, T.; Mierzwa, B.; Lisovytskiy, D.; Pielaszek, J.

X-ray diffraction studies of perovskite or derived perovskite phase formation. Solid State Ionics

2005, 176, 1213-1223.

24. Mandrus, D.; Keppens, V.; Chakoumakos, B. C. Spin-glass formation in Co2RuO4. Mater.

Res. Bull. 1999, 34, 1013-1022.

25. Balek, V.; Labhsetwar, N. K.; Mitsuhashi, T.; Haneda, H.; Šubrt, J.; Zeleňák, V. Study of the

preparation of ruthenia based catalytic materials by heating their precursors. J. Mater. Sci. 2004,

39, 3095-3103.

26. Labhsetwar, N. K.; Watanabe, A.; Mitsuhashi, T. New improved syntheses of LaRuO3

perovskites and their applications in environmental catalysis. Appl. Catal., B 2003, 40, 21-30.

27. Thornton, G.; Tofield, B. C.; Hewat, A. W. A neutron diffraction study of LaCoO3 in the

temperature range 4.2 < T < 1248 K. J. Solid State Chem. 1986, 61, 301-307.

28. Bos, J. W. G.; Attfield, J. P. Crystal and magnetic structures of the double perovskite

La2CoRuO6. J. Mater. Chem. 2005, 15, 715-720.

29. Thornton, G.; Orchard, A. F.; Rao, C. N. R. A study of LaCoO3 and related materials by X-

ray photoelectron spectroscopy. J. Phys. C: Solid State Phys. 1976, 9, 1991-1998.

30. Haas, O.; Struis, R. P. W. J.; McBreen, J. M. Synchrotron X-ray absorption of LaCoO3

perovskite. J. Solid State Chem. 2004, 177, 1000-1010.

31. Batista, M. S.; Santos, R. K. S.; Assaf, E. M.; Assaf, J. M.; Ticianelli, E. A. High efficiency

steam reforming of ethanol by cobalt-based catalysts. J. Power Sources 2004, 134, 27-32.

38

32. Devadas, A.; Baranton, S.; Napporn, T. W.; Coutanceau, C. Tailoring of RuO2 nanoparticles

by microwave assisted "Instant method" for energy storage applications. J. Power Sources 2011,

196, 4044-4053.

33. Motin Seikh, M.; Sudheendra, L.; Narayana, C.; Rao, C. N. R. A Raman study of the

temperature-induced low-to-intermediate-spin state transition in LaCoO3. J. Mol. Struct. 2004,

706, 121-126.

34. Goldwasser, M. R.; Rivas, M. E.; Lugo, M. L.; Pietri, E.; Pérez-Zurita, J.; Cubeiro, M. L.;

Griboval-Constant, A.; Leclercq, G. Combined methane reforming in presence of CO2 and O2

over LaFe1-xCoxO3 mixed-oxide perovskites as catalysts precursors. Catal. Today 2005, 107-108,

106-113.

35. Giraudon, J.-M.; Elhachimi, A.; Wyrwalski, F.; Siffert, S.; Aboukaïs, A.; Lamonier, J.-F.;

Leclercq, G. Studies of the activation process over Pd perovskite-type oxides used for catalytic

oxidation of toluene. Appl. Catal., B 2007, 75, 157-166.

36. Armelao, L.; Bandoli, G.; Barreca, D.; Bettinelli, M.; Bottaro, G.; Caneschi, A. Synthesis and

characterization of nanophasic LaCoO3 powders. Surf. Interface Anal. 2002, 34, 112-115.

37. Glisenti, A.; Galenda, A.; Natile, M. M. Steam reforming and oxidative steam reforming of

methanol and ethanol: The behaviour of LaCo0.7Cu0.3O3. Appl. Catal., A 2013, 453, 102-112.

38. Lima, S. M.; Assaf, J. M.; Peña, M. A.; Fierro, J. L. G. Structural features of La 1-xCexNiO3

mixed oxides and performance for the dry reforming of methane. Appl. Catal., A 2006, 311, 94-

104.

39. Zhang, Z. L.; Verykios, X. E. Carbon dioxide reforming of methane to synthesis gas over

Ni/La2O3 catalysts. Appl. Catal., A 1996, 138, 109-133.

40. Slagtern, A.; Schuurman, Y.; Leclercq, C.; Verykios, X.; Mirodatos, C. Specific features

concerning the mechanism of methane reforming by carbon dioxide over Ni/La2O3 catalyst. J.

Catal. 1997, 172, 118-126.

39

t Table of Contents Graphic

3 La2(CO3)3(H2O)8 Rux

O2, D

H2O + CO2

3 La2O2(CO3)2Rux

O2, D

H2O + CO2

Ste

p1

T

350º

C

3LaCo1-xRuxO3Co3O4 3 La2O2(CO3)2Rux

O2, D

CO2

+

O2, D

CO2

+

Ste

p2

T >

700

ºC

40