Mechanistic insights into CO2 methanation over ru-substituted CeO2

Abstract

CO2 methanation is an important probe reaction to understand CO2 interactions with catalytic surfaces. Importance of this reaction is further increased by its association with CO2 utilization. This study reports 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 Ce0.95Ru0.05O2 and the formation of surface carbonate intermediates took place only when H2 was present in the gas phase. In absence of H2, CO2 did not show any indication for chemisorption. This behavior was explained is terms of the reaction between CO2 and the surface hydroxyls leading to formation of 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 methanation step was ambiguous following purely experimental studies. Density functional theory calculations were carried out to augment the experimental observations. Complete energy landscape 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 role of oxidation state of the catalyst and the surface anionic vacancies in governing the reaction pathway is demonstrated.