Nearly 30 years ago, Haruta and co-workers discovered that Au nano-particles deposited onto the right metal oxide supports are active low temperature CO oxidation catalysts [1]. This discovery has led to extensive research into determining the nature of the active site(s) and elucidating the reaction mechanism. Recent advances have established that the reaction centers for CO oxidation are likely the low-coordinated, Au sites at the periphery of the Au nanoparticles and at the metal-support interface [3, 4]. The presence of water has been shown to increase CO2 formation rates by an order of magnitude; [2] however, the precise role of water in the catalysis and the overall reaction mechanism have remained elusive. In a recent mechanistic study, we reported kinetic measurements, isotopic labeling, and in situ IR spectroscopy results for CO oxidation over Au/TiO2[5]. A hydrogen/deuterium kinetic isotope effect of nearly 2.0 implicates O-H(D) bond breaking in the rate determining step. A Michaelis-Menten kinetic model indicated that the presence of water altered the number of active sites, but did not change the inherent activity of the active sites. Careful reaction kinetics studies, paired with in situ IR spectroscopy measurements clearly showed that weakly adsorbed water (not support hydroxyl groups) on the TiO2 surface is the key proton donor in the catalysis. Our experimental results provide the first and strongest direct evidence for the active involvement of water in the CO oxidation mechanism over Au/TiO2. Based on our results, we are able to propose a new reaction mechanism, but could not readily evaluate the relative rates of individual elementary steps. In particular, although the KIE clearly establishes proton donation in a kinetically important step, the experimental kinetics could not clearly elucidate exactly where this step occurs. At the same time, the new mechanistic information provided critical information for new DFT studies, highlighting the necessity of including both support hydroxyls and water adsorbed onto the support in the DFT model. References [1] Haruta M, Kobayashi T, Sano H, Yamada N (1987) Chem Lett 2:405–408. [2] Kung MC, Davis RJ, Kung HH (2007) J Phys Chem 111:11767–11775. [3] Chandler BD, Kendell S, Doan H, Korkosz R, Grabow LC, Pursell CJ (2012) ACS Catal 2:684–694. [4] Widmann D, Behm RJ (2011) Angew Chem Int Ed Engl 50:10241–10245. [5] Saavedra J, Doan HA, Pursell CJ, Grabow LC, Chandler BD (2014) Science 345:1599-1602.

Critical role of water in catalytic CO oxidation over Au/TiO2 catalysts: Part 1 - experiment

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.