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

The Sabatier reaction (CO2 + H-2 -> CH4 + H2O) can contribute to renewable energy storage by converting green H-2 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 (Ru-6/CeO2). The influence of doping CeO2 with Ru was investigated as well (Ru-6/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, Ru-6 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 Ru-6/CeO2 and Ru-6/RuCex-1O2x-1. A higher CO2 methanation rate is predicted for Ru-6/RuCex-1O2x-1. Electronic structure analysis clarifies that the lower activation energy for HCO dissociation on Ru-6/RuCex-1O2x-1 is caused by stronger electron-electron repulsion due to its closer proximity to Ru. Strong H-2 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.