Structural Reversibility of LaCo1-xCuxO3 Followed by In Situ X-ray Diffraction and Absorption Spectroscopy

Combinations of perovskite-type oxides with transition andprecious metals exhibit a remarkable self-regenerable propertythat could be exploited for numerous practical applications. Theobjective of the present work was to study the reversibility ofstructural changes of perovskite-type oxides under cyclicreducing/oxidizing atmosphere by taking advantage of thereducibility of LaCoO3. LaCoO3 dand LaCo0.8Cu0.2O3 dwereprepared by ultrasonic spray combustion and were character-ized by scanning electron microscopy (SEM), X-ray diffraction(XRD), X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). XRD and XAS data confirmed thatcopper adopted the coordination environment of cobalt at theB-site of the rhombohedral LaCoO3under the selected synthesisconditions. The structural evolution under reducing atmospherewas studied byin situXRD and XANES supporting the assign-ment of the observed structural changes to the reduction of theperovskite-type oxide from ABB’O3(B’=Cu) to B’0/ABO3and toB’0B0/A2O3. Successive redox cycles allowed the observation of anearly complete reversibility of the perovskite phase, i.e. copperwas able to revert into LaCoO3upon oxidation. The reversiblereduction/segregation of copper and incorporation at the B-siteof the perovskite-type oxides could be used in chemicalprocesses where the material can be functionalized bysegregation of Cu and protected against irreversible structuralchanges upon re-oxidation.Ministerio de Investigación de España (MINECO)-CTQ2014-60524-RUniversidad de Sevilla-IV PPIT-U

Understanding the differences in catalytic performance for hydrogen production of Ni and Co supported on mesoporous SBA-15

Three mono and bimetallic Nix Co1−x /SBA-15 catalysts (x = 1, 0.5 and 0) with a total metallic content of 10 wt% have been prepared by a deposition–precipitation (DP) method. The catalytic performances on the dry reforming of methane reaction (DRM) have been determined and correlated with their physical and chemical state before and after the catalytic reaction. So, while the nickel monometallic system presents a high activity and stability in the DRM reaction, the Co/SBA-15 catalytic system turns out completely inactive. For its part, the Ni0.5Co0.5/SBA-15 has initially a catalytic performance similar to the Ni/SBA- 15 monometallic system, but rapidly evolving to an inactive system, therefore resembling the behavior of the cobalt-based catalyst. The characterization by TEM and in situ XPS techniques has allowed us to ascribe these differences to the initial state of metallic particles after reduction and their different evolution under reaction conditions. So, while after reduction both nickel containing Nix Co1−x /SBA-15 catalysts (x = 1 and 0.5) present a well dispersed metallic phase, the cobalt monometallic catalyst yields big metallic particles with a heterogeneous distribution of sizes. Additionally, unlike the Ni/SBA-15, the NiCo/SBA-15 system increases during reaction the metallic particle sizes. Besides indicating that the particle size is a major reason determining the catalytic performances, these results suggest that in the Ni–Co system both metals form after reduction a bimetallic phase mainly located inside the mesoporous channels of SBA-15 support. Under DRM reaction conditions, the cobalt is segregated to the surface of the bimetallic particles, which seems to determine the interaction with the support surface SBA-15. This feature gives rise to a much less stable metallic phase which suffers an important sintering process under DRM catalytic conditions. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Supported nickel catalytic systems are currently one of the most important industrial heterogeneous catalysts because its remarkable performance in a number of economically strategic processes [1–5]. Among them, the steam reforming of methane (SRM, CH4 + H2O ↔ 3H2 + CO) can be outlined as the main industrial process for obtaining hydrogen and synthesis gas, used to syn- thesize various important chemicals and fuels [6–9]. Although it is not yet commercially exploited, the dry reforming of methane (DRM, CH4 + CO2 ↔ 2H2 + 2CO) is an especially interesting reac- tion that transforms two of the most harmful greenhouse gases, methane and carbon dioxide, into a mixture of hydrogen and car- bon monoxide [10–12]. Once again, Ni-based catalysts are the most ∗ Corresponding author. E-mail address: [email protected] (A. Caballero). widely tested in the literature for this reforming reaction, even though noble metal based catalysts such as Pt, Ru and Rh are much more performance toward methane conversion. The principal issue comes from the fact that Ni typically undergoes severe deactivation processes, mainly due to coke formation, but also due to sinter- ing of the metallic phase, generating big metallic particles which at the same time, favors the coke formation processes [13–17]. As an alternative to overcome these issues, a number of publications have shown as the use of bimetallic systems, as the combination of nickel and cobalt modified the catalytic performance in steam and dry reforming of methane [18–23]. But, depending on the support or the preparation methods both effects, improvements and wors- ening of the efficiency, have been reported. Main reasons explaining these contradictory findings are probably related with differences in the interaction of metals with support surface, which has been recognized as an essential factor affecting the stability of metal [24–26]. So, a strategy to avoid the growth of metallic particles is the use of special supports, and in particular mesoporous supports.Ministerio Economía y Competitividad de España (MINECO) y fondos FEDER de la Unión Europea-ENE2011-24412 y CTQ2014-60524-

Preferential oxidation of CO on a La-Co-Ru perovskite-type oxide catalyst

A Ru-containing perovskite-type oxide La(Co,Ru)O3 of nominal composition LaCo0.8Ru0.2O3 was prepared by ultrasonic spray combustion and tested for the preferential oxidation of CO (PROX). EXAFS indicated that Ru adopted the coordination environment of Co in LaCoO3 while Co was present as LaCoO3 and Co3O4. PROX activity was replaced by CO hydrogenation activity above 250 °C. Short oxidation at 500 °C between temperature programmed reaction ramps did not restore the initial La(Co,Ru)O3 structure but generated a catalyst with improved PROX activity compared to the initial La(Co,Ru)O3. Under reductive PROX conditions the material experienced structural changes that improved its overall catalytic activity only if the catalyst was oxidized after each temperature programmed ramp.Spanish Ministry of Research (Project CTQ2014-60524-R)University of Seville for a scholarship for R.P. and the Swiss National Science Foundation (SNF)(Project nr. 200021_159568

Structural and chemical reactivity modifications of a cobalt perovskite induced by Sr-substitution. An in situ XAS study

LaCoO3 and La0.5Sr0.5CoO3-δ perovskites have been studied by in situ Co K-edge XAS. Although the partial substitution of La(III) by Sr(II) species induces an important increase in the catalytic oxidation activity and modifies the electronic state of the perovskite, no changes could be detected in the oxidation state of cobalt atoms. So, maintaining the electroneutrality of the perovskite requires the generation of oxygen vacancies in the network. The presence of these vacancies explains that the substituted perovskite is now much more reducible than the original LaCoO3 perovskite. As detected by in situ XAS, after a consecutive reduction and oxidation treatment, the original crystalline structure of the LaCoO3 perovskite is maintained, although in a more disordered state, which is not the case for the Sr doped perovskite. So, the La0.5Sr0.5CoO3-δ perovskite submitted to the same hydrogen reduction treatment produces metallic cobalt, while as determined by in situ XAS spectroscopy the subsequent oxidation treatment yields a Co(III) oxide phase with spinel structure. Surprisingly, no Co(II) species are detected in this new spinel phase.Ministerio de Ciencia y Educación ENE2011-2441