Identification of adsorbed species during steady-state photocatalytic oxidation of ethanol on TiO2

The identification and concentrations of species on a TiO2 surface during photocatalytic oxidation (PCO) of ethanol were determined by combining transient and steady-state PCO with temperature-programmed desorption (TPD) and oxidation (TPO). Ethanol and its partial oxidation intermediates (acetaldehyde, acetic acid, formaldehyde, and formic acid) are on the catalyst surface, and their concentrations depend on the ethanol, O-2, and water feed concentrations. The rate of PCO was greater initially than at steady state for all experimental conditions, and this initial deactivation may be due in part to the accumulation of acetaldehyde on the surface. Weakly bound ethanol preferentially forms acetaldehyde whereas the more strongly bound ethoxide species preferentially produces CO2. Increasing the gas-phase ethanol concentration produces more of the weakly bound ethanol and therefore a greater rate of acetaldehyde production. Increasing the O-2 concentration from 0.2 to 20% only increased the steady-state rate of reaction by 50% and did not change the selectivity. Although ethanol and water compete for sites, adding 0.5% water (13% relative humidity) to the feed stream did not change either the rate or selectivity and only marginally increased the amount of adsorbed water. (C) 1998 Academic Press.