Role of Water in the Photocatalytic Degradation of Trichloroethylene Vapor on TiO2 Films

The focus of this study was to investigate the role of water vapor in the photocatalytic degradation of trichloroethylene (TCE) on anatase titanium dioxide films immobilized on the surfaces of ring-roughened annular reac tors. Experimental variables included TCE concentration (0.7−7 parts per million by volume or ppmv), oxygen concentration (13−120 000 ppmv), residence time (2.77−9.81 s), relative humidity (0−100%), and reactor length. TCE conversion was not affected by relative humidities up to 20%, but it deteriorated as the gas mixture approached saturation with respect to water vapor. Major intermediates and products from TCE degradation were the same as those previously reported for dry conditions: carbon tetrachloride, chloroform, hexachloroethane, pentachloroethane, and tetrachloroethylene. The formation rates for these compounds increased with increasing water vapor concentrations at relatively low humidities as a result of a stronger deteriorating rate effect of water vapor on atomic oxygen oxidation reactions as compared to that on competing chlorine atom attack reactions. The presence of moderate concentrations of water vapor resulted in greater conversions of chloroform and pentachloroethane as compared to dry conditions due to rate enhancement of hydrogen extraction reactions. In contrast, tetrachloroethylene conversions decreased with increasing humidity primarily because of a deteriorating rate effect of water vapor on chlorine extraction reactions.