Photocatalytic oxidation of trichloroethylene on zinc oxide: characterization of surface-bound and gas-phase products and intermediates with FT-IR spectroscopy

The gas-phase heterogeneous photooxidation of trichloroethylene (TCE) on ZnO has been investigated. In the presence of ZnO and molecular oxygen, trichloroethylene photooxidizes upon irradiation with wavelengths of light between 300 and 390 nm. Surface-bound and gas-phase products and intermediates formed during the photocatalytic oxidation of TCE were characterized with infrared spectroscopy. The gas-phase product distribution was found to be dependent on TCE pressure. At low TCE pressures, CO2 and CO were produced as the predominant carbon-containing, gas-phase products. At high TCE pressures, additional carbon-containing products were detected. These products include phosgene, and dichloroacetylchloride. It is postulated that phosgene and dichloroacetylchloride form only after the ZnO surface becomes saturated with adsorbed products. These adsorbed photoproducts have been identified as water, bidentate formate and dichloroacetate. These adsorbed photoproducts block sites so that partial oxidation products cannot undergo further oxidation on the ZnO surface. (C) 1998 Elsevier Science B.V.