Elucidation of the Reverse Water–Gas Shift Reaction Mechanism over an Isolated Ru Atom on CeO2(111)

Ru/CeO2 single-atom catalysts (SAC) are highly selective for the hydrogenation of CO2 to CO. In this study, we developed a molecular-level understanding of the reverse water–gas shift (rWGS) reaction over Ru/CeO2 SAC using density functional theory in conjunction with microkinetic modeling. A reaction mechanism network involving different Ru states was constructed. Starting from a Ru single atom coordinating to three lattice oxygen atoms of ceria (denoted as RuO3), RuO3 can be hydrogenated to Ru(OH)3-(OH) through hydrogen spillover or reduced to RuO2(Ov). Direct and H-assisted CO2 dissociation mechanisms are taken into account. Microkinetics simulations indicate that Ru(OH)3-(OH) is the dominant active site in the low-temperature regime. The presence of hydroxyl species in Ru(OH)3-(OH) promotes the dissociation of CO2 and water formation. The promoting effect of hydroxyl groups is caused by enhanced charge donation of Ru to antibonding orbitals of CO2. At elevated temperatures, a Mars-van Krevelen mechanism is preferred due to the facile formation of oxygen vacancies. Overall, our findings provide insight into the role of spillover H species in the rWGS reaction on Ru/CeO2 SAC and the change in active sites with reaction temperature.