Transient studies of 2-propanol photocatalytic oxidation on titania

Transient reaction techniques were used to study the room-temperature, photocatalytic oxidation (PCO) of 2-propanol on a thin catalyst film of titania (Degussa P25) in an annular reactor. Adsorbed 2-propanol was photocatalytically oxidized in the absence of gas-phase 2-propanol, and the species remaining on the TiO2 surface were characterized by temperature-programmed desorption (TPD) and oxidation (TPO). Nonphotocatalytic decomposition (TPD) and oxidation (TPO) of 2-propanol and acetone were also studied. The initial PCO of 2-propanol at room temperature rapidly forms acetone and water; water can displace acetone into the gas phase. Adsorbed acetone is subsequently oxidized photocatalytically to CO2 and H2O at a slower rate than 2-propanol photocatalytically oxidizes to acetone and it may form a surface intermediate before complete oxidation, Thus, at steady-state, the TiO2 surface is expected to be covered with H2O and strongly bound acetone or an intermediate. The surface reaction steps require near-UV light, but desorption does not. Formation of gas-phase acetone and water are desorption limited, but CO2 and CO formation are reaction limited. The rate of PCO to form acetone is essentially independent of O-2 pressure, and acetone forms at room temperature even in 30 ppm O-2, apparently utilizing lattice oxygen. In contrast, complete oxidation to CO2 is first order in O-2 at low concentrations and zero order at higher O-2 concentrations. The coverage of photoadsorbed oxygen is low (less than 1 mu mol/ g TiO2), and the oxygen is strongly bound to the surface (similar to 200 kJ/mol binding energy). There are no indications that photoadsorbed oxygen was important for PCO; gas phase O-2 is needed to replace lattice oxygen, but this may or may not be photoadsorbed. In the absence of near-UV light, titania is an oxidation catalyst only at much higher temmperatures. Thermal oxidation of 2-propanol also proceeds through acetone formation, but acetone thermally oxidizes faster than 2-propanol. Methanol and ethanol also undergo PCO on TiO2 at room temperature to form aldehydes, CO2, and H2O. (C) 1995 Academic Press, Inc.