Elucidating the Role of the Metal Catalyst and Oxide Support in the Ru/CeO-Catalyzed CO Methanation Mechanism.

This study addresses the yet unresolved CO methanation mechanism on a Ru/CeO 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 H to CO + H mixture oxidizes both the Ru and CeO phases at low temperatures, which is explained by the CO adsorption modes assessed by means of DFT on each representative surface. CO 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, CO can be strongly adsorbed as carbonates on ceria surface oxygen sites or on the reduced ceria at oxygen vacancies as carboxylates (CO ), 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 CO 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 CO methanation process. Altogether, this study highlights the noninnocent role of the ceria support in the performance of Ru/CeO toward CO methanation.