Role of Reduced CeO2(110) Surface for CO2 Reduction to CO and Methanol

Density functional theory (DFT) calculations were performed to study the mechanism of carbon dioxide (CO2) reduction to carbon monoxide (CO) and methanol (CH3OH) on CeO2(110) surface. CO2 dissociates to CO on interacting with the oxygen vacancy on reduced ceria surface. The oxygen atom heals the vacancy site and regenerates the stoichiometric surface via a redox mechanism with intrinsic activation and reaction energies of 259.2 and 238.6 kJ/mol, respectively. Lateral interaction of oxygen vacancies were studied by the generation of two oxygen vacancies per unit of CeO2 surface. Compared to a single isolated vacancy, the activation and reaction energies of CO2 dissociation on a divacancy were approximately reduced to half of its value. Hydrogen atom coadsorbed on the surface was observed to assist CO2 dissociation by forming a carboxyl intermediate, CO2+H -> COOH (Delta E-act = 39.0 kJ/mol, Delta H = -69.2 kJ/mol) which on subsequent dissociation produces CO via the redox mechanism. On hydrogenation, CO is likely to produce methanol. The energetics of CO hydrogenation to produce methanol showed exothermic steps with activation barriers comparable to the DFT calculations reported for Cu (111) and CeO2x/Cu(111) interface. While on the stoichiometric surface, COOH dissociation COOH -> CO+OH (Delta E-act = 55.6 kJ/mol, Delta H = 5.7 kJ/mol) is likely to be difficult as compared to rest of the elementary steps, whereas on the reduced surface the energetics of the same step were significantly lowered (Delta E-act = 47.4 kJ/mol, Delta H = -80.4 kJ/mol). In comparison, hydrogenation of methoxy, H3CO+H -> H3COH, appears to be relatively difficult (Delta E-act = 58.7 kJ/mol) on the reduced surface.