Oxygen Vacancy and Adsorbed Hydride Formation on Metal-Substituted Anatase TiO ₂ and Generalization Across Doped Metal Oxide Surfaces

The formation of surface oxygen vacancies (Ovac) and the availability of metal (M)-bound hydride species are essential for using reducible metal oxides to catalyze hydrogenation reactions. Hydride species can form through H–M binding within Ovac, and they participate in the C–H bond formation reaction steps. Thus, improving the reducibility of oxide surfaces without undermining hydride formation is important for creating Ovac active sites for metal oxide hydrogenation catalysts. The reducibility of surfaces can be modulated by replacing one metal cation on a pure oxide surface with a heterometal cation (i.e., doping). However, the optimal choice of cation and the potential trade-offs in enhancing reducibility versus promoting hydride formation are undetermined. In this study, a comparative analysis elucidates the effects of different dopants on the Ovac and hydride formation on surfaces for anatase TiO2 (A-TiO2) using density functional theory (DFT) methods. A substantial enhancement of surface reducibility is observed when dopants exhibit greater stability at oxidation states lower than those of the original Ti cation. Importantly, this enhancement can occur without compromising hydride formation, suggesting that cation doping may be an effective strategy to increase hydrogenation rates on reducible oxide catalysts. Insights developed from cation-doped A-TiO2 are generalized across doped oxides. Predictive models for the surface reducibility and hydride formation energies of metal-substituted oxides are established using descriptors available in open databases. These models provide a framework for designing metal-substituted oxides tailored to selective hydrogenation reactions.