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.