H Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H Activation at the Metal-Support Interface.

Water adsorbed at the metal-support interface (MSI) plays an important role in multiple reactions. Due to its importance in CO preferential oxidation (PrOx), we examined H oxidation kinetics in the presence of water over Au/TiO and Au/AlO catalysts, reaching the following mechanistic conclusions: (i) O activation follows a similar mechanism to that proposed in CO oxidation catalysis; (ii) weakly adsorbed HO is a strong reaction inhibitor; (iii) fast H activation occurs at the MSI, and (iv) H activation kinetics are inconsistent with traditional dissociative H chemisorption on metals. Density functional theory (DFT) calculations using a supported Au nanorod model suggest H activation proceeds through a heterolytic dissociation mechanism, resulting in a formal hydride residing on the Au and a proton bound to a surface TiOH group. This potential mechanism was supported by infrared spectroscopy experiments during H adsorption on a deuterated Au/TiO surface, which showed rapid H-D scrambling with surface hydroxyl groups. DFT calculations suggest that the reaction proceeds largely through proton-mediated pathways and that typical Brønsted-Evans Polanyi behavior is broken by introducing weak acid/base sites at the MSI. The kinetics data were successfully reinterpreted in the context of the heterolytic H activation mechanism, tying together the experimental and computational evidence and rationalizing the observed inhibition by physiorbed water on the support as blocking the MSI sites required for heterolytic H activation. In addition to providing evidence for this unusual H activation mechanism, these results offer additional insight into why water dramatically improves CO PrOx catalysis over Au.