Mechanistic Insights into CO₂ Methanation over Ru-Substituted CeO₂

CO₂ methanation is an important probe reaction to understand CO₂ interactions with catalytic surfaces. The importance of this reaction is further increased by its association with CO₂ utilization. This study reports the mechanistic aspects of CO₂ methanation over combustion synthesized Ru-substituted CeO₂ catalyst. Temperature-programmed reaction experiments were carried out to understand the interaction of CO₂, H₂, and their stoichiometric mixture with the catalyst surface. In situ FTIR spectroscopy was used to identify the intermediates of the reaction. It was observed that CO₂ adsorption took place on the surface of Ce_0.95Ru_0.05O₂ and the formation of surface carbonate intermediates took place only when H₂ was present in the gas phase. In the absence of H₂, CO₂ did not show any indication for chemisorption. This behavior was explained in terms of the reaction between CO₂ and the surface hydroxyls leading to the formation of a vacancy. Upon dissociation, carbonates led to chemisorbed CO which eventually formed methane upon reaction with gas-phase H₂. The exact identity of carbonate species and the pathway for the methanation step were ambiguous following purely experimental studies. Density functional theory calculations were carried out to augment the experimental observations. Complete energy landscapes developed on the basis of differentiation of oxidized and reduced forms of the catalyst showed that the reaction followed a pathway consisting of surface carbonate species formed by the interaction of oxide surface and chemisorbed CO, and a sequential methanation via the surface methoxy species formation. The study provides physical insights into the role of the oxidation state of the catalyst and the surface anionic vacancies in governing the reaction pathway