Adsorption, Dissociation, and Spillover of Hydrogen over Au/TiO₂ Catalysts: The Effects of Cluster Size and Metal-Support Interaction from DFT

The effects of cluster size and metal-support interaction on the catalytic activity of Au nanoparticles supported on anatase TiO₂(101) and (001) surfaces for H₂ adsorption, activation and dissociation were investigated by periodic density functional theory (DFT) calculations. On the stoichiometric TiO₂ surfaces, it was found that the adsorptions of both Au clusters and H₂ molecules are sensitive to the cluster size of gold, and the (001) facet with "soft" lattice and coordination unsaturated atoms on the surface is superior for Au adsorption stability, but H₂ adsorption does not show apparent distinction on the two catalysts. The Au atoms active for H₂ activation should be neutral in charge and located at the edge or corner of the Au nanoparticles in lower coordination. The metal-support interaction plays an important role for H₂ activation and dissociation, and the O^2--H⁺-H^--Au structure was identified in the transition state through which H₂ dissociation occurred via a heterolytic dissociation process at the Au-TiO₂ interface for both facets. H∗ spillover from the Au site to the bridging -O_2cH site can generate H₂O molecules on the two facets, and O-vacancy formation is energetically more favorable on (101) than (001). The presence of O-vacancy defects significantly impacts the adsorption stability of Au clusters and H₂ molecule but has small effect on the energy barrier for H₂ dissociation, which proceeds fast on both the stoichiometric and reduced anatase TiO₂(101) and (001) surfaces.