Effect of Irradiation Sources and Oxygen Concentration on the Photocatalytic Oxidation of 2-Propanol and Acetone Studied by in Situ FTIR

Abstract

The photocatalytic oxidation (PCO) of 2-propanol and acetone was studied on TiO2 powder at high coverage (>2 monolayers) at various oxygen concentrations using in situ FTIR. Two UV irradiation sources, either a continuous-wave UV lamp or an excimer laser, were applied to investigate the effect of the UV source on the PCO. It was found that higher oxygen concentrations increased the PCO rate of 2-propanol and acetone. The excimer laser source accelerated the PCO rate of 2-propanol while decelerating that of acetone as compared to the continuous-wave source. This effect was found to be larger at higher oxygen concentrations. Factors such as multiphoton adsorption or a pulsed irradiation were excluded as possible explanations for the irradiation source effect. More likely, the laser source changes the reaction mechanism by introducing an abrupt temperature increase, and hence the thermal decomposition, that is, dehydration of 2-propanol to propene, is effectively initiated on the surface. The PCO of 2-propanol was found to proceed along two routes: one was through the chemisorbed species, 2-propoxide, to form CO2 directly; the other was through conversion of H-bonded 2-propanol to acetone, followed by thermal fragmentation of mesityl oxide (aldol condensation product of acetone) to form formate species, and finally to CO2. It was also found that the acetone decay rate during acetone PCO was much faster than that of acetone formed as an intermediate during 2-propanol PCO at the same oxygen concentration. This was likely due to the presence of other species competing for reaction sites on the catalyst surface.