Unraveling Hydrogenation Kinetic Behavior of Transition Metal Oxides via Decoupling Dihydrogen Dissociation and Substrate Activation

Both noble metals and transition metal oxides are recognized as active centers for alkyne hydrogenation. However, it is still a "black box" how the catalytic behavior of oxides evolves upon the catalytic intervention of noble metals. Herein, we report a modularized strategy to track the hydrogenation mechanism of oxides (e.g., TiO2, CeO2, and ZrO2) using a core-shell micromesoporous zeolite as a structure model, in which the oxide and noble metal (Pt) are functionally separated within a mesopore shell and a micropore core (TS-1 zeolite), respectively. The Pt species are atomically distributed and stabilized by the oxygen atoms of five-membered rings in TS-1 zeolite, which facilitates the heterolytic activation of dihydrogen over Pt delta+O2- units. The active hydrogen species, i.e., H+ and H delta-, migrate to the oxide surface, where the adsorbed reactants are activated for hydrogenation. Mechanistic studies reveal that TiO2, CeO2, and ZrO2 possess efficient hydrogenation properties at near-room temperature with the assistance of spillover hydrogen species, demonstrating dihydrogen dissociation as the main rate-limiting step for pure oxide. Impressively, the adsorbed H2O molecule on TiO2, ZrO2, and CeO2 not only acts as a bridge of hydrogen spillover in reducing the proton diffusion barrier but also forms H3O+ species on the TiO2 (100) surface and endows TiO2 with extraordinary hydrogenation properties. This work opens the "black box" for the hydrogenation behavior of transition metal oxides and develops a molecule-assisted strategy for the rational design of hydrogenation catalysts.