Time‐Resolved, In Situ DRIFTS/EDE/MS Studies on Alumina‐Supported Rhodium Catalysts: Effects of Ceriation and Zirconiation on Rhodium–CO Interactions

Abstract

The effects of ceria and zirconia on the structure–function properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al2O3) during CO exposure are described. Ceria and zirconia are introduced through two preparation methods: 1) ceria is deposited on γ-Al2O3 from [Ce(acac)3] and rhodium metal is subsequently added, and 2) through the controlled surface modification (CSM) technique, which involves the decomposition of [M(acac)x] (M=Ce, x=3; M=Zr, x=4) on Rh/γ-Al2O3. The structure–function correlations of ceria and/or zirconia-doped rhodium catalysts are investigated by diffuse reflectance infrared Fourier-transform spectroscopy/energy-dispersive extended X-ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time-resolved, in situ conditions. CeOx and ZrO2 facilitate the protection of Rh particles against extensive oxidation in air and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts prepared by CSM are observed and compared with Rh/γ-Al2O3 samples, whereas supported Rh particles are easily disrupted by CO forming mononuclear Rh geminal dicarbonyl species. DRIFTS results indicate that, through the interaction of CO with ceriated Rh particles, a significantly larger amount of linear CO species form; this suggests the predominance of a metallic Rh phase.

Time-resolved, in situ DRIFTS/EDE/MS studies on alumina supported Rh catalysts: effects of ceriation on the Rh catalysts in the process of CO oxidation

“…These results differ from those reported for a 2 wt% Rh/Al 2 O 3 which have been interpreted in terms of a stepwise binding of CO to metallic rhodium, fragmentation of Rh-CO, and then binding of a second CO to the Rh(I) sites in five-coordinate centres [34]. Neither in this study which includes oxygen in the cycle, nor for the direct adsorption of CO [16] do we observe complete fragmentation or obtain any evidence for an isolated Rh-CO centre. Also, periodic DFT calculations suggest that the most stable coordination sites for the Rh I (CO) 2 are square-planar, in a variety of surface locations [35].…”

“…On another hand, the ceriated Rh catalysts (CSM) show clearly a larger mean particle size. When studying the CO chemisorption on a series of Rh catalysts by a combined array of XAS/diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)/mass spectrometry (MS) in the lower temperature regime (323-423 K) geminal dicarbonyl species adsorbed on Rh are dominant entities formed on the surface of unpromoted Rh/Al 2 O 3 and Rh/CeO x /Al 2 O 3 (OSM) while the EXAFS analysis suggests reduction of Rh particle size and elongation of RhRh bonds [16]. In contrary, only for the Rh/CeO x /Al 2 O 3 (CSM) catalysts, CO was found to mainly be adsorbed as linear and bridged species on the surface of supported Rh.…”

Structure–function relationship for CO2 methanation over ceria supported Rh and Ni catalysts under atmospheric pressure conditions

“…The promoting effect of ceria is mainly related to its ability to undergo changes in its oxidation state (Ce 3+ -Ce 4+ redox couple) with the consequent formation/annihilation of surface defects (O vacancies). 15,49 Some previous reports have suggested Ce 3+ as the active site for CO 2 adsorption and conversion along with a valence change from Ce 3+ to Ce 4+ . 38,[50][51][52] A recent study by Wang et al 53 on Ru supported on ceria showed also that the surface oxygen vacancies on ceria rather than on Ru are more likely to be the active sites in CO 2 methanation.…”

Understanding the role of promoters in catalysis: operando XAFS/DRIFTS study of CeOx/Pt/Al2O3 during CO oxidation

“…The study showed that small amounts of CeO x (0.35 wt%) were able to dramatically change the reaction profile, with Ce exhibiting facile redox switching from Ce 3+ to Ce 4+ at room temperature under reducing and oxidising atmospheres, respectively. This method has also been used elsewhere to improve the properties of Rh/Al 2 O 3 catalysts for CO oxidation [6,7]. To understand these complex structure function relationships between metal nanoparticles and interfacial metal oxide clusters it is crucial to characterise the catalyst whilst it is operating under normal process conditions, this is to say, operando.…”