Kinetic studies of secondary alcohol photo-oxidation on ZnO and TiO2 at 348 K studied by gas-chromatographic analysis

Abstract

Kinetics of conversion of propan-2-ol and butan-2-ol to the corresponding ketones by photoassisted dehydrogenation, (–H2)*, to lower aldehydes by a photoassisted Cα—Cβ bond cleavage process, (α–β)*, and to trace quantities of alkene by photoassisted dehydration (–H2O)* have been investigated at oxygen partial pressures in the range 0–700 Torr and at alcohol partial pressures of 0–60 Torr, utilising dynamic flow photoreactors and gas-chromatographic analysis. Qualitatively similar results were obtained either with a ‘continuous reactant flow plus continuous u.v. illumination’ procedure allied to intermittent sampling, or with ‘pulsed reactant and analysis’ procedure. Both procedures yielded dependence upon alcohol pressure indicative of two parallel photoassisted pathways to (–H2)* product, one being Langmuir–Hinshelwood (LH*) and the other Eley–Rideal (ER*) in character. A decline in activity of the metal oxide surfaces as photoassisted conversion of alcohol increased was observed with both procedures and affected the LH* process for (–H2)* more strongly. It is proposed that this LH* process involved alcohol chemisorption and hole localisation at coordinatively unsaturated O2–cus ions. The ER* process is envisaged to have involved encounter of alcohol (from the gas phase or from a reversibly adsorbed weakly bound state) with a much more numerous type of surface location capable of being photoactivated by hole capture. The proposed mechanisms account for observed dependences of the (–H2)* process upon square root of the incident light intensity, upon alcohol pressure and upon oxygen pressure.