Role of Low-Coordinated Ce in Hydride Formation and Selective Hydrogenation Reactions on CeO2 Surfaces

Catalysts based on ceria exhibit high activity toward selective hydrogenation reactions. There has been much debate on the catalytic mechanisms, especially on the production of hydride (H–) species, which serve as the key species for hydrogenation reactions. Previous studies illustrated that the hydride species are usually formed at oxygen vacancy sites of reduced CeO2 surfaces, and the stoichiometric surfaces are believed to be inactive. In this work, we performed extensive density functional theory calculations corrected by on-site Coulombic interaction (DFT + U) to investigate the mechanisms of H2 dissociation on the various stoichiometric CeO2 surfaces, including the low-index (111) and (100) surfaces and the high-index (221), (223), and (132) ones. We find that the H– species can be generated via H2 heterolytic dissociation on the various CeO2 surfaces, and the stability of the hydride species increases with the decrease of the coordination number of the surface Ce. This is mainly because the repulsive electrostatic interaction between the H– species adsorbed at the low-coordinated Ce species and its surrounding species is much less and it is, therefore, more favorable to occur than the H– species adsorbed at the relatively high-coordinated Ce. In addition, the low-coordinated Ce3+ species can have a relatively high-lying energy level of the localized 4f electron and tend to donate the electron to the adsorbed H to produce a hydride. Moreover, through calculations of the key reaction steps, we showed that the as-formed metastable H– species can regulate the catalytic activity and selectivity for CO2 hydrogenation by preferentially producing HCOO* intermediates.