Heterogeneous photocatalytic oxidation of trichloroethylene and toluene mixtures in air: Kinetic promotion and inhibition, time-dependent catalyst activity

Photocatalyzed degradation of trace level trichloroethylene (TCE) and toluene in air were carried out over near-UV-illuminated titanium dioxide (anatase) powder in a flow reactor using a residence time of about 5-6 ms. Concentration ranges for TCE and toluene were 0-800 mg/m(3). TCE photooxidation was very rapid under our experimental conditions, and similar to 100% conversion was achieved for TCE concentration examined up to 753 mg/m(3) as a single air contaminant. Initial photodegradation rates for toluene in humidified air were fitted by a Langmuir-Hinshelwood rate form. Toluene photooxidation sole contaminant rates were of the same order of magnitude as reported previously for m-xylene and acetone. No significant intermediates were detected by GC/FID during TCE or toluene photooxidation reactions. Humidification had significant influence: Toluene photooxidation rate increases with the water concentration up to about 1650 mg/m(3) 20% relative humidity) and decreases thereafter, The presence of sufficient TCE (225-753 mg/m(3)) promoted the toluene photooxidation reaction rate to achieve 90-100% toluene conversion in 5.6 ms. When feed toluene levels measured below similar to 90 mg/m(3). Higher toluene feed levels ''quenched'' this TCE promotion effect and also depressed TCE conversion very strongly, but the toluene conversion fell just to the toluene-only levels observed in single contaminant experiments. Photooxidation kinetics of TCE and toluene mixtures in air are thus shown to exhibit strong promotion and inhibition behavior vs that expected from the single-species kinetic degradation data. A previously suggested chlorine radical oxidation of TCE is modified to rationalize the TCE enhancement of the toluene photooxidation rate and the corresponding toluene ''quench'' of TCE destruction, Time-dependent catalyst activation (by TCE) and deactivation (by toluene or toluene oxidation products and, eventually, even by TCE products) were observed. Carboxylate formation and carboxylic acid accumulation postulated by previous investigators could be a major cause of such catalyst deactivation. (C) 1996 Academic Press, Inc.