Transient Studies of 2-Propanol Photocatalytic Oxidation on Titania

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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 O2 pressure, and acetone forms at room temperature even in 30 ppm O2, apparently utilizing lattice oxygen. In contrast, complete oxidation to CO2 is first order in O2 at low concentrations and zero order at higher O2 concentrations. The coverage of photoadsorbed oxygen is low (less than 1 μmol/ g TiO2), and the oxygen is strongly bound to the surface (∼ 200 kJ/mol binding energy). There are no indications that photoadsorbed oxygen was important for PCO; gas phase O2 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.