Enhancement of the catalytic oxidation of hydrogen-lean chlorinated VOCs in the presence of hydrogen-supplying compounds

The complete catalytic oxidation of trichloroethylene (TCE) over alumina-su pported noble metal catalysts (Pt and Pd) and in the presence of hydrogen-r ich compounds, i.e. water, hexane and toluene was evaluated. Experiments we re performed at conditions of lean TCE concentration (around 1000 ppm) in a ir, between 250 and 550 degrees C in a conventional fixed-bed reactor. Hexa ne and toluene were added to the feedstream in a concentration of around 10 00 ppm and water concentration varied from 1000 to 15 000 ppm. TCE oxidatio n occurred faster in the presence of hexane and toluene over both catalysts . Over palladium catalysts, water did not alter catalytic activity, whereas over platinum catalysts water enhanced TCE oxidation at low temperatures ( <400 degrees C) but inhibited it at higher temperatures (>400 degrees C). S electivity to HCl was much improved by feeding water as a hydrogen-supplyin g reactant; 7500 ppm of water enhanced HCl outputs from 39.4 to 78.0% with Pd, and from 37.5 to 58.9% with Pt. Selectivities to C2Cl4, formed by chlor ination of the feed, and Cl-2 were greatly reduced. On the other hand water promoted complete oxidation of TCE to CO2, and thus reduced selectivity to CO. In the presence of hexane and toluene, formation of HCl was also enhan ced. Hexane showed higher inhibition ability than toluene over both catalys ts for the C2Cl4 and Cl-2 formation. Unlike in the presence of water, selec tivity to CO increased, as a consequence of partial oxidation of both hydro carbons. (C) 2000 Elsevier Science B.V. All rights reserved.