H 2 Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H 2 Activation at the Metal-Support Interface

• Published in: Journal of the American Chemical Society Vol. 140; no. 48; pp. 16469 - 16487
• Main Authors: Whittaker, Todd, Kumar, K B Sravan, Peterson, Christine, Pollock, Meagan N, Grabow, Lars C, Chandler, Bert D
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society (ACS) 05-12-2018
• Subjects: Chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.8b04991

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/Al
O
catalysts, reaching the following mechanistic conclusions: (i) O
activation follows a similar mechanism to that proposed in CO oxidation catalysis; (ii) weakly adsorbed H
O 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.