Heterogeneous photocatalytic oxidation of 1-butene on SnO2 and TiO2 films

Three types of films were prepared by a-dip-coating process for the photocatalyzed decomposition of l-butene in a gas-solid reaction. Under UV illumination (lambda > 300 nm, 352 nm peak intensity), Itrasmall SnO2 with a diameter of 5 nm exhibited initial photoactivity as high as 3 times that of P-25 TiO2 (30 nm) in the absence of water vapor whereas SnO2-1 (22 nm) did not show photoactivity. Quantum size effects were mainly responsible for the high photoactivity achieved by SnO2. Inactivity of SnO2-1 film was due to the absence of active hydroxyl groups on the catalyst surface and low surface areas. By investigation of the effects of humidity, water has two different functions: maintaining constant oxidation rates at low water levels by replenishing hydroxyl groups and decreasing the photoactivity at high water levels by competitive adsorption with butene on active sites. Compared with TiO2, SnO2 cannot withstand high humidity since it is very sensitive to water concentration. Hydroxyl groups on catalyst surfaces are the active centers for the reaction. The occurrence of obvious deactivation on SnO2 films was:due to the depletion of hydroxyl groups and the accumulation of carbonate species on particle surfaces, i.e., M-OCOOR species in place of M-OH. The kinetic data correlate with a Langmuir-Hinshelwood single-site model. XRD, UV-vis spectroscopy, and FTIR techniques were employed to characterize the particle size, band gaps, and surface properties of the catalysts.