Mechanistic insights into the catalytic oxidation of carboxylic acids on Au/TiO₂: Partial oxidation of propionic and butyric acid to gold ketenylidene through unsaturated acids

The partial oxidation of model C₂-C₄ (acetic, propionic, and butyric) carboxylic acids on Au/TiO₂ catalysts consisting of Au particles ∼3 nm in size was investigated using transmission infrared spectroscopy and density functional theory. All three acids readily undergo oxidative dehydrogenation on Au/TiO₂. Propionic and butyric acid dehydrogenate at the C2-C3 positions, whereas acetic acid dehydrogenates at the C1-C2 position. The resulting acrylate and crotonate intermediates are subsequently oxidized to form β-keto acids that decarboxylate. All three acids form a gold ketenylidene intermediate, Au₂C=C=O, along the way to their full oxidation to form CO₂. Infrared measurements of Au₂C=C=O formation as a function of time provides a surface spectroscopic probe of the kinetics for the activation and oxidative dehydrogenation of the alkyl groups in the carboxylate intermediates that form. The reaction proceeds via the dissociative adsorption of the acid onto TiO₂, the adsorption and activation of O₂ at the dual perimeter sites on the Au particles (Au-O-O-Ti), and the subsequent activation of the C2-H and C3-H bonds of the bound propionate and butyrate intermediates by the weakly bound and basic oxygen species on Au to form acrylate and crotonate intermediates, respectively. The C=C bond of the unsaturated acrylate and crotonate intermediates is readily oxidized to form an acid at the beta (C3) position, which subsequently decarboxylates. This occurs with an overall activation energy of 1.5-1.7 ± 0.2 eV, ultimately producing the Au₂C=C=O species for all three carboxylates. The results suggest that the decrease in the rate in moving from acetic to propionic to butyric acid is due to an increase in the free energy of activation for the formation of the Au₂C=C=O species on Au/TiO₂ with an increasing size of the alkyl substituent. The formation of Au₂C=C=O proceeds for carboxylic acids that are longer than C₂ without a deuterium kinetic isotope effect, demonstrating that C-H bond scission is not involved in the rate-determining step; the rate instead appears to be controlled by C-O bond scission. The adsorbed Au₂C=C=O intermediate species can be hydrogenated to produce ketene, H₂C=C=O(g), with an activation energy of 0.21 ± 0.05 eV. These studies show that selective oxidative dehydrogenation of the alkyl side chains of fatty acids can be catalyzed by nanoparticle Au/TiO₂ at temperatures near 400 K.