Photoreaction of Ethanol on TiO2(110) Single-Crystal Surface

The adsorption, thermal, and UV reactions of ethanol over a TiO2(110) single-crystal surface have been studied in the presence and the absence of molecular oxygen. Adsorption of ethanol is dissociative at room temperature and gives rise to two C1s peaks of equal intensities (at 285.2 and 286.5 eV) attributed to -CH3 and -CH2O- groups, respectively. The surface coverage at saturation (of the dissociative adsorption mode at 300 K) is close to 0.5 with respect to Ti atoms. Thermal annealing resulted in the disappearance of the C1s signal attributed to both groups (-CH3 and -CH2O-), with negligible oxidation of the ethoxide groups. The decrease of both peaks is not symmetric, it is attributed to water desorption consuming bridging surface oxygen followed by migration of ethoxide species into these defects in the process of healing surface oxygen atoms. Exposures to UV irradiation (3.2 eV) of the ethoxide covered surface in the presence of oxygen at 300 K resulted in considerable decrease of the ethoxide C1s peaks with irradiation time and the formation of a carboxylate peak at about 290 eV. This XPS C1s signal, attributed to both CH3COO(a) and HCOO(a) species, is most likely due to oxidation by the photoactive O-2(-) or O-2(2-) species, formed by capture of the photoexited electrons at the conduction band (Ti 3d). The dependence of the rate of ethoxide decomposition on the O-2 pressure follows the expected Langmuir-Hinshelwood kinetics. The photoreaction cross section was estimated from the decay of the XPS C1s signal (starting from surface saturation) and was found equal to ca. 2 x 10(-18) cm(-2) at 1 x 10(-6) Torr of O-2. This figure compares well with that obtained for acetate decomposition under similar conditions on this same surface.