Adsorption and Photocatalytic Oxidation of Acetone on TiO₂: An in Situ Transmission FT-IR Study

Abstract

In situ transmission Fourier-transform infrared spectroscopy has been used to study the mechanistic details of adsorption and photocatalytic oxidation of acetone on TiO₂ surfaces at 298 K. The adsorption of acetone has been followed as a function of coverage on clean TiO₂ surfaces (dehydrated TiO₂). Infrared spectra at low acetone coverages (θ<0.05 ML) show absorption bands at 2973, 2931, 1702, 1448, and 1363 cm⁻¹ which are assigned to the vibrational modes of molecularly adsorbed acetone. At higher coverages, the infrared spectra show that adsorbed acetone can undergo an Aldol condensation reaction followed by dehydration to yield (CH₃)₂CCHCOCH₃, 4-methyl-3-penten-2-one or, more commonly called, mesityl oxide. The ratio of surface-bound mesityl oxide to acetone depends on surface coverage. At saturation coverage, nearly 60% of the adsorbed acetone has reacted to yield mesityl oxide on the surface. In contrast, on TiO₂ surfaces with preadsorbed water (hydrated TiO₂), very little mesityl oxide forms. Infrared spectroscopy was also used to monitor the photocatalytic oxidation of adsorbed acetone as a function of acetone coverage, oxygen pressure, and water adsorption. Based on the dependence of the rate of the reaction on oxygen pressure, acetone coverage, and water adsorption, it is proposed that there are potentially three mechanisms for the photooxidation of adsorbed acetone on TiO₂. In the absence of preadsorbed H₂O, one mechanism involves the formation of a reactive O⁻(ads) species, from gas-phase O₂, which reacts with adsorbed acetone molecules. The second mechanism involves TiO₂ lattice oxygen. In the presence of adsorbed H₂O, reactive hydroxyl radicals are proposed to initiate the photooxidation of acetone.