Adsorption and photocatalytic oxidation of acetone on TiO2: An in situ transmission FT-IR study

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 f ollowed as a function of coverage on clean TiO2 surfaces (dehydrated TiO2). Infrared spectra at low acetone coverages (theta < 0.05 hit) show absorpti on bands at 2973, 2931, 1702, 1448, and 1363 cm(-1) which are assigned to t he vibrational modes of molecularly adsorbed acetone. At higher coverages, the infrared spectra show that adsorbed acetone can undergo an Aldol conden sation reaction followed by dehydration to yield (CH3)(2)C=CHCOCH3, 4-methy l-3-penten-2-one or, more commonly called, mesityl oxide. The ratio of surf ace-bound mesityl oxide to acetone depends on surface coverage. At saturati on coverage, nearly 60% of the adsorbed acetone has reacted to yield mesity l oxide on the surface. In contrast, on TiO2 surfaces with preadsorbed wate r (hydrated TiO2), very little mesityl oxide forms. Infrared spectroscopy w as also used to monitor the photocatalytic oxidation of adsorbed acetone as a function of acetone coverage, oxygen pressure, and water adsorption. Bas ed on the dependence of the rate of the reaction on oxygen pressure, aceton e coverage, and water adsorption, it is proposed that there are potentially three mechanisms for the photooxidation of adsorbed acetone on TiO2. In th e absence of preadsorbed H2O, one mechanism involves the formation of a rea ctive O-(ads) species, from gas-phase Or, which reacts with adsorbed aceton e molecules. The second mechanism involves TiO2 lattice oxygen. In the pres ence of adsorbed H2O, reactive hydroxyl radicals are proposed to initiate t he photooxidation of acetone. (C) 2000 Academic Press.