Elucidating the Role of the Metal Catalyst and Oxide Support in the Ru/CeO₂-Catalyzed CO₂Methanation Mechanism

Abstract

This study addresses the yet unresolved CO2 methanation mechanism on a Ru/CeO2 catalyst by means of near-ambient-pressure X-ray photoelectron spectroscopy (NAP–XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) complemented with periodic density functional theory (DFT) calculations. NAP–XPS results show that the switch from H2 to CO2 + H2 mixture oxidizes both the Ru and CeO2 phases at low temperatures, which is explained by the CO2 adsorption modes assessed by means of DFT on each representative surface. CO2 adsorption on Ru is dissociative and moderately endergonic, leading to polybonded Ru-carbonyl groups whose hydrogenation is the rate-determining step in the overall process. Unlike on Ru metal, CO2 can be strongly adsorbed as carbonates on ceria surface oxygen sites or on the reduced ceria at oxygen vacancies as carboxylates (CO2 –δ), resulting in the reoxidation of ceria. Carboxylates can then evolve as CO, which is released either via direct splitting at relatively low temperatures or through stable formate species at higher temperatures. DRIFTS confirm the great stability of formates, whose depletion relates with CO2 conversion in the reaction cell, while carbonates remain on the surface up to higher temperatures. CO generation on ceria serves as an additional reservoir of Ru-carbonyls, cooperating to the overall CO2 methanation process. Altogether, this study highlights the noninnocent role of the ceria support in the performance of Ru/CeO2 toward CO2 methanation.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

“…This low activation energy for formate formation, is consistent with these species being observed by DRIFTS at temperatures as low as 60 • C. In fact, formates are very stable species with relative Gibbs energies of −1.56, −2.22 and −3.02 eV at 350 • C when surrounded by one, two or three OH groups, respectively. Even at high OH coverages, formates are significantly stabilized as their evolution towards CO(ads) + *OH is very energy demanding, as recently showed in our mechanistic study of the CO 2 methanation reaction on ceria [80]. Hence, we predict formates to accumulate on ceria for a broad temperature window, as observed by DRIFTS.…”

Computational Study of CO2 Methanation on Ru/CeO2 Model Surfaces: On the Impact of Ru Doping in CeO2

“…COOH* can be readily dissociated into CO* and OH*. The dissociation of HCOO* leads to HCO* and O*. , Another CO 2 dissociation pathway via a Mars-van Krevelen mechanism involves an oxygen vacancy in the support. The formation of oxygen vacancies in the CeO 2 surface can be facilitated by spillover H from metal particles …”

“…The reaction predominantly follows the carbonyl route at low temperatures, while the formate route was dominant at higher temperatures . As activity appears to benefit from a higher concentration of oxygen vacancies, doping of CeO 2 with other metals has also been explored. − One such case pertains to the doping of Ru in the CeO 2 lattice. ,, Besides the role of oxygen vacancies, the size of the supported Ru particles will also affect the catalytic performance. It has been shown that single atoms of Ru on CeO 2 are highly selective to CO, i.e., they do not produce CH 4 . , Guo et al speculated that the size-dependent selectivity is due to strong metal–support interactions and H spillover being affected by the size of Ru .…”

Progress in reaction mechanisms and catalyst development of ceria-based catalysts for low-temperature CO2methanation

“…Recently, more details of methanation behavior and mechanism over Ru/CeO 2 have been provided by López-Rodríguez et al 130 The (NAP-XPS) results indicated the reduced Ru/CeO 2 can be slightly oxidized below 300 °C under reaction conditions of CO 2 methanation (CO 2 + H 2 ). Combined with the in situ DRIFTS results, it can be seen that the chemisorbed CO 2 on Ru sites was dissociated into Ru-CO* and O atoms, which could potentially oxidize the surface of Ru 0 and CeO 2 .…”