A theoretical study of surface reduction mechanisms of CeO₂(111) and (110) by H₂

Reaction mechanisms for the interactions between CeO₂(111) and (110) surfaces are investigated using periodic density functional theory (DFT) calculations. Both standard DFT and DFT+U calculations to examine the effect of the localization of Ce 4f states on the redox chemistry of H₂- CeO₂ interactions are described. For mechanistic studies, molecular and dissociative local minima are initially located by placing an H₂ molecule at various active sites of the CeO₂ surfaces. The binding energies of physisorbed species optimized using the DFT and DFT+U methods are very weak. The dissociative adsorption reactions producing hydroxylated surfaces are all exothermic; exothermicities at the DFT level range from 4.1 kcal mol^-1 for the (111) to 26.5 kcal mol^-1 for the (110) surface, while those at the DFT+U level are between 65.0 kcal mol^-1 for the (111) and 81.8 kcal mol^-1 for the (110) surface. Predicted vibrational frequencies of adsorbed OH and H₂O species on the surfaces are in line with available experimental and theoretical results. Potential energy profiles are constructed by connecting molecularly adsorbed and dissociatively adsorbed intermediates on each CeO₂ surface with tight transition states using the nudged elastic band (NEB) method. It is found that the U correction method plays a significant role in energetics, especially for the intermediates of the exit channels and products that are partially reduced. The surface reduction reaction on CeO₂(110) is energetically much more favorable. Accordingly, oxygen vacancies are more easily formed on the (110) surface than on the (111) surface.