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 wa s photocatalytically oxidized in the absence of gas-phase 2-propanol, and the species remaining on the TiO2 surface were characterized by te mperature-programmed desorption (TPD) and oxidation (TPO). Nonphotocat alytic decomposition (TPD) and oxidation (TPO) of 2-propanol and aceto ne were also studied. The initial PCO of 2-propanol at room temperatur e rapidly forms acetone and water; water can displace acetone into the gas phase. Adsorbed acetone is subsequently oxidized photocatalytical ly to CO2 and H2O at a slower rate than 2-propanol photocatalytically oxidizes to acetone and it may form a surface intermediate before comp lete oxidation, Thus, at steady-state, the TiO2 surface is expected to be covered with H2O and strongly bound acetone or an intermediate. Th e surface reaction steps require near-UV light, but desorption does no t. Formation of gas-phase acetone and water are desorption limited, bu t CO2 and CO formation are reaction limited. The rate of PCO to form a cetone is essentially independent of O-2 pressure, and acetone forms a t room temperature even in 30 ppm O-2, apparently utilizing lattice ox ygen. In contrast, complete oxidation to CO2 is first order in O-2 at low concentrations and zero order at higher O-2 concentrations. The co verage of photoadsorbed oxygen is low (less than 1 mu mol/ g TiO2), an d the oxygen is strongly bound to the surface (similar to 200 kJ/mol b inding 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 l ight, titania is an oxidation catalyst only at much higher temmperatur es. Thermal oxidation of 2-propanol also proceeds through acetone form ation, but acetone thermally oxidizes faster than 2-propanol. Methanol and ethanol also undergo PCO on TiO2 at room temperature to form alde hydes, CO2, and H2O. (C) 1995 Academic Press, Inc.