Mechanistic insights into the partial oxidation of acetic acid by O2 at the dual perimeter sites of a Au/TiO2catalyst

Abstract

The unique interfacial sites of Au nanoparticles supported on TiO₂ are known to catalyze the activation of oxygen and it's addition to small molecules including H₂, CO, NO and propylene. Herein we extend these ideas and show that the unique Au–Ti dual perimeter sites that form at the Au/TiO₂ interface can also catalyze more demanding C–H and C–O bond activation reactions involved in the deoxygenation organic acids such as acetic acid. We have shown previously that acetic acid can be partially oxidized on a Au/TiO₂ catalyst to form a novel gold ketenylidene (Au₂CCO) intermediate. In the present work we use in situ infrared spectroscopy and first-principle density functional theory (DFT) to examine the mechanism and the kinetics by which this reaction proceeds. The reaction was found to be localized at the dual perimeter sites of the Au/TiO₂ catalyst, where O₂ was activated. In contrast to Au/TiO₂, no ketenylidene formation was observed on a similar Au/SiO₂ catalyst or a TiO₂ blank sample. The reaction involves the activation of multiple C–H bonds as well as the C–O bond in the adsorbed CH₃COO species. C–O bond scission is postulated to occur at the TiO₂ sites, while C–H bond scission occurs on Au sites, both near the active Au–Ti⁴⁺ dual perimeter sites. ¹⁸O₂ isotopic labeling indicated that the O moiety of the ketenylidene species originates from the acetic acid during the oxidation process involving molecular O₂. The rate-limiting step was found to be the C–O bond scission resulting in an apparent overall activation energy of 1.72 eV as determined from DFT calculations. This is in very good agreement with the experimentally measured apparent activation energy of 1.7 ± 0.2 eV. A deuterium kinetic isotope effect of ∼4 indicates that C–H bond activation is kinetically involved in the overall acetate oxidation reaction.