(677c) Wet Metal-Support Interfaces Control Paths of H2 and O2 Activation over Au Nanoparticles

Au-based catalysts readily oxidize and reduce small molecules (e.g., CO, H₂, O₂) and organic substrates at metal-support interfaces, often with the assistance of water molecules. Herein, we investigate how the support identity and size of Au nanoparticles affect transformations of H–H, O–O, and O–H bonds during reactions of H₂ and O₂ at the solid-liquid-support interfaces of Au-based catalysts.

Figure 1a shows that rates of H₂ activation are 1-2 orders of magnitude greater on Au nanoparticles supported on metal oxides (e.g., TiO₂) compared to more inert materials like carbon and BN, suggesting oxygen functions (e.g., O-H) assist H–H activation. Similarly, basic and reducible metal oxides (e.g., Au-La₂O₃) favor O–O bond cleavage (16-22 kJ mol^-1), while more inert and acidic interfaces (e.g., Au-SiO₂) obstruct O–O dissociation (72-85 kJ mol^-1) and improve selectivity to H₂O₂. Moreover, H₂O₂ selectivities increase as the fraction of sites at the Au-support interface decrease relative to metallic sites far from this interface, improving H₂O₂ selectivity on larger Au nanoparticles (2-25 nm).

Figure 1b shows that rates of H₂O₂ formation increase linearly with H₂ pressure and are independent of O₂ pressure on Au-TiO₂, suggesting O₂-derived species saturate active sites. Moreover, rates of HD scrambling are one order of magnitude lower than rates of H₂O₂ formation, indicating irreversible adsorption and activation of H₂ and D₂. Such findings agree with decreased oxygen reduction rates when using D₂ instead of H₂ (kH/kD = 1.5), implicating kinetically relevant H–H dissociation during H₂O₂ formation. Still, Figure 1c shows that H₂O₂ only forms within protic solvents, indicating H₂O₂ forms by proton transfer steps. Yet, rates are equal within H₂O and D₂O, implying proton transfer is kinetically irrelevant. These findings suggest that solid-liquid-support interfaces catalyze proton-electron transfer reactions of H₂, O₂, and H₂O, limited by H–H activation steps.