The Sabatier reaction (CO 2 + H 2 → CH 4 + H 2 O) can contribute to renewable energy storage by converting green H 2 with waste CO 2 into CH 4 . Highly dispersed Ru on CeO 2 represents an active catalyst for the CO 2 methanation. Here, we investigated the support effect by considering a single atom of Ru and a small Ru cluster on CeO 2 (Ru 6 /CeO 2 ). The influence of doping CeO 2 with Ru was investigated as well (Ru 6 /RuCe x –1 O 2 x –1 ). Density functional theory was used to compute the reaction energy diagrams. A single Ru atom on CeO 2 can only break one of the C–O bonds in adsorbed CO 2 , making it only active in the reverse water–gas shift reaction. In contrast, Ru 6 clusters on stoichiometric and Ru-doped CeO 2 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 /CeO 2 and Ru 6 /RuCe x –1 O 2 x –1 . A higher CO 2 methanation rate is predicted for Ru 6 /RuCe x –1 O 2 x –1 . Electronic structure analysis clarifies that the lower activation energy for HCO dissociation on Ru 6 /RuCe x –1 O 2 x –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 CO 2 methanation activity of Ru clusters on CeO 2 compared to a Ru step-edge surface, representative of Ru nanoparticles, where the H coverage is low due to stronger competition with adsorbed CO.