Distinct Site Motifs Activate O₂ and H₂ on Supported Au Nanoparticles in Liquid Water

Au nanoparticles catalyze the activation and conversion of small molecules with rates and kinetic barriers that depend on the dimensions of the nanoparticle, composition of the support, and presence of catalytically culpable water molecules that solvate these interfaces. Here, molecular interpretations of steady-state rate measurements, kinetic isotope effects, and structural characterizations reveal how the interface of Au nanoparticles, liquid water, and metal oxide supports mediate the kinetically relevant activation of H₂ and sequential reduction of O₂-derived intermediates during the formation of H₂O₂ and H₂O. Rates of H₂ consumption are 10-100 fold greater on Au nanoparticles supported on metal oxides (e.g., titania) compared to more inert and hydrophobic materials (carbon, boron nitride). Similarly, Au nanoparticles on reducible and Lewis acidic supports (e.g., lanthana) bind dioxygen intermediates more strongly and present lower barriers (<22 kJ mol^-1) for O-O bond dissociation than inert interfaces formed with silica (>70 kJ mol^-1). Selectivities for H₂O₂ formation increase significantly as the diameters of the Au nanoparticles increase because differences in nanoparticle size change the relative fractions of exposed sites that exist at Au-support interfaces. In contrast, site-normalized rates and barriers for H₂ activation depend weakly on the size of Au nanoparticles and the associated differences in active site motifs. These findings suggest that H₂O aids the activation of H₂ at sites present across all surface Au atoms when nanoparticles are solvated by water. However, molecular O₂ preferentially binds and dissociates at Au-support interfaces, leading to greater structure sensitivity for barriers of O-O dissociation across different support identities and sizes of Au nanoparticles. These insights differ from prior knowledge from studies of gas-phase reactions of H₂ and O₂ upon Au nanoparticle catalysts within dilute vapor pressures of water (10^-4 to 0.1 kPa H₂O), in which catalysis occurs at the perimeter of the Au-support interface. In contrast, contacting Au catalysts with liquid water (55.5 M H₂O) expands catalysis to all surface Au atoms and enables appreciable H₂O₂ formation.