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

Abstract

The Sabatier reaction (CO2 + H2 → CH4 + H2O) can contribute to renewable energy storage by converting green H2 with waste CO2 into CH4. Highly dispersed Ru on CeO2 represents an active catalyst for the CO2 methanation. Here, we investigated the support effect by considering a single atom of Ru and a small Ru cluster on CeO2 (Ru6/CeO2). The influence of doping CeO2 with Ru was investigated as well (Ru6/RuCex-1O2x-1). Density functional theory was used to compute the reaction energy diagrams. A single Ru atom on CeO2 can only break one of the C-O bonds in adsorbed CO2, making it only active in the reverse water-gas shift reaction. In contrast, Ru6 clusters on stoichiometric and Ru-doped CeO2 are active methanation catalysts. CO is the main reaction intermediate formed via a COOH surface intermediate. Compared to an extended Ru(11-21) surface containing step-edge sites where direct C-O bond dissociation is facile, C-O dissociation proceeds via H-assisted pathways (CO → HCO → CH) on Ru6/CeO2 and Ru6/RuCex-1O2x-1. A higher CO2 methanation rate is predicted for Ru6/RuCex-1O2x-1. Electronic structure analysis clarifies that the lower activation energy for HCO dissociation on Ru6/RuCex-1O2x-1 is caused by stronger electron-electron repulsion due to its closer proximity to Ru. Strong H2 adsorption on small Ru clusters explains the higher CO2 methanation activity of Ru clusters on CeO2 compared to a Ru step-edge surface, representative of Ru nanoparticles, where the H coverage is low due to stronger competition with adsorbed CO.