HETEROGENEOUS PHOTOCATALYTIC OXIDATION OF DILUTE TOLUENE-CHLOROCARBON MIXTURES IN AIR

At 1-2 ms residence time, the photocatalytic oxidation of dilute (10-750 mg m(-3)) mixtures of toluene and 1,1,3-trichloropropene (TCP) or perchloroethylene (PCE) in air increased the toluene conversion to 100% from the pseudo-steady single-component toluene feed levels of 10%-20%. A chain mechanism involving chlorine radical attack of the toluene and halogenate, previously proposed by Luo and Ollis (J. Catal, 00 (1994) 00-00) for toluene-trichloroethylene mixtures, is extended to rationalize the current results. The conversion enhancement occurred at low toluene concentrations (up to 22 mg m(-3)) and high TCP/PCE levels. Increasing the toluene concentration to 43 mg m(-3) completely quenched the TCP oxidation chain mechanism, giving the same toluene conversion as observed for the toluene-only feeds. The possible enhancement of toluene conversion in toluene-halomethane (methylene chloride, chloroform, carbon tetrachloride) mixed feeds was examined. Carbon tetrachloride and chloroform were ineffective as toluene promoters, and were not photo-oxidized themselves, either alone or in mixtures. Methylene chloride photo-oxidation occurred at very modest rates and conversion, and toluene-methylene chloride mixtures exhibited only a slight enhancement of toluene conversion. Catalyst deactivation was observed with all toluene feeds. Both chlorocarbon addition (TCP, PCE) and humidification slowed, but did not eliminate, deactivation.