Role of chlorine and oxygen in the photocatalytic degradation of trichloroethylene vapor on TiO₂ films

The photocatalytic degradation of trichloroethylene (TCE) was investigated with ring-roughened annular reactors. Anatase titanium dioxide films were immobilized by chemical vapor deposition using titanium tetraisopropoxide as the source of titanium. Experimental variables included TCE concentration (0.1-10 parts per million by volume or ppmv), oxygen content (20-200 000 ppmv), residence time (2.46-9.57 s), and reactor length. In general, TCE conversion increased with increasing values of all of these variables up to a maximum of 99.4% within the range investigated. Ten chlorinated organic intermediates/products were observed primarily at low oxygen concentrations. Among these compounds, carbon tetrachloride, chloroform, hexachloroethane, pentachloroethano, and tetrachloroethylene were present at quantifiable levels. Additional experiments revealed that pentachloroethane and tetrachloroethylene also decomposed with degradation efficiencies (up to 98 and 94%, respectively) being somewhat lower than that for TCE. Chloroform conversions were low (up to 30%), and no appreciable degradation of carbon tetrachloride was observed for the range of operating conditions investigated. Chlorinated organic intermediate/product formation was the result of various reactions involving chlorine and hydrogen atom extractions and additions. Chlorine atom attack reactions were inhibited in the presence of relatively high oxygen concentrations including that in ambient air due to the predominance of competing oxidation reactions involving unidentified reactive oxygen species.