Ruthenium Effect on Formation Mechanism and Structural

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Article

Ruthenium Effect on Formation Mechanism and Structural Characteristics of LaCo RuO Perovskites and Its Influence on Catalytic Performance for Hydrocarbon Oxidative Reforming 1-x

x

3

Noelia Mota, Laura Barrio, Maria Consuelo Alvarez-Galvan, François Fauth, Rufino M. Navarro, and Jose Luis Garcia Fierro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04287 • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on July 4, 2015

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The Journal of Physical Chemistry

Ruthenium Effect on Formation Mechanism and Structural

Characteristics

Perovskites

and

its

of

Influence

LaCo1-xRuxO3 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

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

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

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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 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

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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 LaCo1xFexO3

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. 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 LaCo1xRuxO3

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

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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. 2.2 Physicochemical characterization Formation of the perovskite structures was followed by Time Resolved X-ray diffraction (TRXRD), 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

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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 TRXRD 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 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

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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 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

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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 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 (%):

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mole C (CO 2 + CO + CH 4 + C 2 H 4 + C 2 H 6 + C 3 H 6 + C 3 H 8 ) in reformate × 100 mole C (C14.4 H 27.4 ) feed Hydrogen yield (%): mole of H 2 in reformate × 100 maximum theoretical mole of H 2

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 Xray 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 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,

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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.

Co3O4 (ICSD-27497) LaCoO3 (ICSD-201767) La2(OH)6-2x(CO3)x La2O2(CO3) hexagonal 800 800

600

700 400

600 500

200

400 300 200

Temperature /ºC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

2Θ

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 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

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compression of the hydroxocarbonate structure (peak at 2θ = 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. La2O2CO3

Perovskite

CO2 decomposition (ºC)

Sample Peak 2θ=6º Peak 2θ=6.6º Low Temp High Temp x=0

530

659

383

661

x = 0.05

527

679

377

689

x = 0.1

442

691

364

685

x = 0.2

421

686

353

698

x = 0.4

363

688

356

663

10


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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

peak @ 2Θ=4º peak @ 2Θ=6º

A)

B)

peak @ 2Θ=6.6º

6

x10 MS

1.80E-008

x = 0.4

x = 0.4

5 1.50E-008

x = 0.2

CO2 Partial pressure (atm)

Normalized peak intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

x = 0.1 3

x = 0.05

1.20E-008

9.00E-009

x = 0.2

x = 0.1 x = 0.05

6.00E-009

2 3.00E-009

x=0

x=0

1 100 200 300 400 500 600 700 800

Temp /ºC

100 200 300 400 500 600 700 800

Temp /ºC

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% 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

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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.

x = 0.1

160 196

x = 0.05 480

617

560

196

689

629 689 689

689

750 ºC 30 min

750 ºC 30 min

195 481 520

196 620

481 522

500 ºC 400 ºC

2000

618

Counts / a.u.

Counts / a.u.

600 ºC

600 ºC 500 ºC 400 ºC

2000 300 ºC 300 ºC

30 ºC

30 ºC

100

300

500

700

900

1100

100

300

-1

619 622

690

667

692

481

692

483

691 691

196

1100

483 537

688

611

750 ºC 30 min 600 ºC 500 ºC

500 195

480

400 ºC

392

654 688

195

686

300 ºC

195

2000

478

Counts / a.u.

481

195

900

x = 0.4 483

195 195

700

Raman / cm

x = 0.2 196

500

-1

Raman shift / cm

Counts / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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750 ºC 30 min

620

521

600 ºC 500 ºC 400 ºC

479

542

300 ºC

30 ºC

100

300

500

700

900 -1

Raman shift /cm

1100

30 ºC

100

300

500

700

900

1100

-1

Raman shift / cm

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

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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 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

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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 oxocarbonate 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 LaCo1xRuxO3

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

T > 700 ºC

La2O2CO3 + 2/3Co3O4* + xRuO2 + nO2 2LaCo1-xRuxO3* + CO2

T > 350 ºC

2La(OH)6-2y(CO3)y-Rux La2O2CO3-Rux + yCO2 + 3H2O

T > 700 ºC

3La2O2CO3-Rux + 2Co3O4* + ½O2 6LaCo1-xRuxO3* + 3CO2

∆

∆

∆

[1a]

[2a]

[1b]

[2b]

14


Page 15 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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, La2O2CO3, 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.

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

15



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Page 16 of 40

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 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

16


Page 17 of 40

phase fraction diminishes when ruthenium is added. Additionally, the distortion induced by the presence of ruthenium also allows the formation of smaller crystallite sizes.

A)

B)

120x10

3

x = 0.4

120x10

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

100

80

x = 0.2

60

3

x = 0.4

100

80

Counts

Counts /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


x = 0.1

x = 0.2 60

x = 0.1

40

40

x = 0.05

x = 0.05

20

20

x=0

x=0

0 6

7

8

9

10

11

12

13

14

15

16

0 8.6

2Θ

8.8

9.0

9.2

2Θ

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)

Occ.

%weight

size Sample

Space group

a (Å)

b (Å)

c (Å)

B (nm)

LaCoO3 La2CoRuO6 Co3O4 site

x=0

5.442

5.442 13.102

42.5

1.05

97.77

0.00

2.23

x = 0.05

R3 c

5.457

5.457 13.142

32.7

1.09

98.13

0.00

1.87

x = 0.1

rhombohedral

5.478

5.478 13.193

31.4

1.10

98.56

0.00

1.44

5.505

5.505 13.393

28.3

1.20

98.35

0.00

1.65

5.571

5.608

30.8

1.18

11.78

87.69

0.53

x = 0.2 P21/n x = 0.4

7.870

monoclinic

17



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Page 18 of 40

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 1s→4p transition is the one allowed by selection rules while the pre-edge features are governed by the symmetry-forbidden Co3+ 1s→3d 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


Page 19 of 40

Co(1s) Co(4p)

x = 0.4

Co

2+

LaCoO 3

LMCT x = 0.4

x = 0.05 x=0 Co 3 O 4

CoO

7700

7720

7740

7760

7780

x = 0.2 x = 0.1 x = 0.05

x=0

Co 3O 4 CoO

7700

Energy /eV

nd

1st derivative of absorption data

x = 0.1

2 derivative of absorption data

x = 0.4

x = 0.2

Norm. abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7710

7720

7730

x = 0.2 x = 0.1 x = 0.05 x=0

Co 3 O 4

CoO

7740 7700

Energy /eV

7710

7720

7730

7740

Energy /eV

Figure 6. Normalized Co K-edge XANES spectra of calcined LaCo1-xRuxO3 perovskites and their 1st and 2nd derivatives

Sample

E0

Oxidation state

CoO

7720.5

+2

Co3O4

7721.7

+2, +3

x=0

7724.4

+3

x = 0.05 7723.4

+2, +3

x = 0.1

7722.7

+2, +3

x = 0.2

7722.2

+2, +3

x = 0.4

7724.0

+3

19



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Page 20 of 40

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 κ2χ (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 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

20


Page 21 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.

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 CoO 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

21



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Page 22 of 40

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 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.

22


Page 23 of 40

La bending

Rotational

Co-O Co-O bending stretching

RuO2

x = 0.4

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


x = 0.2

x = 0.1 x = 0.05

x=0 Co3O4

x 0,03

100

125

150

175

200

400 500 600 700 800 900

Raman shift /cm

-1

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

23



x=0

500

x = 0.05

1000

c.p.s. / a.u.

779.7

c.p.s. / a.u.

780.1

790.2

765

775

785

795

805

815

790.1

765

775

B.E. / eV

815

x = 0.2

c.p.s. / a.u.

c.p.s. / a.u.

805

780,4

789.6

775

795

1000

780.1

765

785

B.E. / eV x = 0.1

1000

785

795

805

815

790,1

765

775

785

795

805

815

B.E. / eV

B.E. / eV

x = 0.4

1000

779.9

c.p.s. / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

789.5

765

775

785

795

805

815

B.E. / eV

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

24


Page 25 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 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.

25



x=0

2000

x = 0.05

2500

833.3

c.p.s. / a.u.

c.p.s. / u.a.

835.3 835.4 833.4

825 830 835 840 845 850 855 860 865

825 830 835 840 845 850 855 860 865

B.E. / eV

B.E. / eV x = 0.1

2500

x = 0.2

2500

835.4

c.p.s. / u.a.

c.p.s. / a.u.

833.6

835.7 833.8

825 830 835 840 845 850 855 860 865

825 830 835 840 845 850 855 860 865

B.E. / eV

B.E. / eV x = 0.4

5000

c.p.s. / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

837.7 834.2

825 830 835 840 845 850 855 860 865

B.E. / eV

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

26


Page 27 of 40

528.7

530.6 528.6

c.p.s. / a.u.

c.p.s. / a.u.

530.6

532.3

524

526

528

530

x = 0.05

1000

x=0

1000

532

534

536

538

532.1

524

526

528

B.E. / eV

530

532

534

536

538

B.E. / eV x = 0.1

1000

x = 0.2

1000

530.8

531.1

529.1

c.p.s. / a.u.

528.8

c.p.s. / u.a.

533.5

524

526

528

530

532

534

536

538

532.8

524

526

528

B.E. / eV

530

532

534

536

538

B.E. / eV

1000

530.7

x = 0.4

528.5

c.p.s. / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


533.8

524

526

528

530

532

534

536

538

B.E. / eV

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

27



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Page 28 of 40

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)* 1.03

x=0

1.03 -

2.53 1.03

(1.00)

(1.00)

(1.50) 2.58

0.95

0.08

1.03

(0.95)

(0.05)

(1.00)

(1.50)

1.09

0.11

1.20

2.21

x = 0.05

1.53

x = 0.1

1.92 (0.90)

(0.10)

(1.00)

(1.50)

0.65

0.22

0.87

2.77

x = 0.2

1.21 (0.80)

(0.20)

(1.00)

(1.50)

0.35

0.20

0.56

2.54

x = 0.4

3.31 (0.60)

(0.40)

(1.00)

(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

28


Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 LaCo1xRuxO3perovskite

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 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.

29



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Page 30 of 40

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


Page 31 of 40

60

55

0-8 h 16-24 h 50

% H2 yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45

40

35

30 0

5

10

15

20

25

30

35

40

% at. Ru

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 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

31



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

42 E0 LaCo1-xRuxO3 Crystallite size

7724.0

40

Absorption Edge /eV

7723.5 38 7723.0 36

7722.5 7722.0 Co3O4 (Co+2+ Co+3)

34

7721.5

32

7721.0

Crystallite size /nm

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30 +2

7720.5

CoO (Co )

0

5

10

15

20

25

30

35

40

% at. Ru

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 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

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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 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

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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 *e-mail: r. [email protected]; phone +34915854774 Present Addresses † Velocys, Milton Park, OX14 4SA United Kingdom.

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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.

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t Table of Contents Graphic

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Ruthenium Effect on Formation Mechanism and Structural

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