Kinetics and Thermodynamics of H2O Dissociation on Reduced CeO2(111)

We use density functional theory to investigate the reaction between reduced CeO 2– x (111) and water. H 2 O dissociation to hydroxyl is facile on surface vacancies and lattice oxygen, while subsequent decomposition of hydroxyl into H 2 has a high barrier, which results in reversible adsorption of H 2 O under ultra-high-vacuum conditions. The barrier to H 2 formation through hydroxyl decomposition decreases by 0.2 eV, while H 2 O formation becomes more difficult at high hydroxyl coverage. However, on isolated oxygen vacancies on a hydroxyl covered surface, H 2 may be produced through a CeH intermediate with a 1.14 eV barrier. Oxygen vacancies are found to be more stable in the subsurface than in the surface layer at all vacancy coverages and for hydroxyl coverages less than 25–50%. The competition of H 2 O desorption and vacancy diffusion from the subsurface to the surface may prevent formation of hydroxyl from H 2 O dosing at low temperature, while the highly stable hydroxyl phase may provide a thermodynamic driving force for further surface reduction in the presence of water. On the basis of our calculations we suggest substitutional doping with a cation that binds H stronger than Ce may improve the decomposition of hydroxyls into hydrogen.