(176b) Interfacial Sites Determine Paths of O2 and H2 Activation on Au Nanoparticles: Effects of Nanoparticle Size and Support Identity on O2 Reduction in Aqueous Media

Au-based catalysts readily oxidize and reduce small molecules and organic substrates through water-assisted paths. The barriers of such paths depend strongly on the morphology of Au nanoparticles and the chemical moieties of support materials, suggesting sites at Au-support interfaces co-catalyze redox reactions. Here, we examine rates of H₂ and O₂ activation as a function of the support identity and mean diameter of Au nanoparticles during the formation of H₂O₂ and H₂O within aqueous media. Specifically, rates of H₂ activation are 1-2 orders of magnitude greater on Au nanoparticles supported on metal oxides (e.g., TiO₂, Al₂O₃) compared to unfunctionalized materials (e.g., Carbon, BN), suggesting oxygen functions (e.g., O-H) assist the kinetically relevant activation of H-H bonds. Similarly, Au-support interfaces that bind dioxygen intermediates strongly (e.g., Au-La₂O₃) show lower barriers (16-22 kJ mol^-1) to dissociate O-O bonds compared to weakly binding interfaces like Au-SiO₂ (72-85 kJ mol^-1). Still, H₂O₂ only forms in protic solvents, suggesting solvent molecules co-catalyze oxygen reduction on Au surfaces.

Moreover, selectivities of H₂O₂ formation increase as the fraction of sites at the Au-support interface increases relative to metallic sites far from this interface. Infrared spectra of adsorbed CO estimate the relative fractions of Au atoms exposed at interfacial sites and metallic regions of Au nanoparticles, corroborating the increase of H₂O₂ selectivity with larger Au nanoparticles (2-25 nm). These findings also explain differences in reactivity between PdAu nanoparticles on different supports. Alloying Pd significantly lowers barriers of hydrogen activation and increases oxygen reduction rates; however, unselective interfaces lower barriers of H₂O formation. Thus, unreactive support materials favor the formation of H₂O₂. Furthermore, this understanding guides the design of selective and reactive Au-based catalysts for H₂ and O₂ activation, applicable to other redox reactions in liquid media.