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The critical role of water at the gold-titania interface in catalytic CO oxidation

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Science  26 Sep 2014:
Vol. 345, Issue 6204, pp. 1599-1602
DOI: 10.1126/science.1256018

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Easier oxidation over gold with added water

Gold adsorbed on metal oxides is an excellent catalyst for the room-temperature oxidation of CO to CO2. However, there has been continuing disagreement between different studies on the key aspects of this catalyst. Saveeda et al. now show through kinetics and infrared spectroscopy that the presence of water can lower the reaction activation barrier by enabling OOH groups to form from adsorbed oxygen (see the Perspective by Mullen and Mullins). The OOH then reacts readily with CO. It thus seems that the main role of oxide support and its interface with the metal is in activating water, but that the steps of the reaction that involve CO occur on gold.

Science, this issue p. 1599; see also p. 1564

Abstract

We provide direct evidence of a water-mediated reaction mechanism for room-temperature CO oxidation over Au/TiO2 catalysts. A hydrogen/deuterium kinetic isotope effect of nearly 2 implicates O-H(D) bond breaking in the rate-determining step. Kinetics and in situ infrared spectroscopy experiments showed that the coverage of weakly adsorbed water on TiO2 largely determines catalyst activity by changing the number of active sites. Density functional theory calculations indicated that proton transfer at the metal-support interface facilitates O2 binding and activation; the resulting Au-OOH species readily reacts with adsorbed Au-CO, yielding Au-COOH. Au-COOH decomposition involves proton transfer to water and was suggested to be rate determining. These results provide a unified explanation to disparate literature results, clearly defining the mechanistic roles of water, support OH groups, and the metal-support interface.

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