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-supported noble metal catalysts (Pt and Pd) and in the presence of hydrogen-rich compounds, i.e. water, hexane and toluene was evaluated. Experiments were performed at conditions of lean TCE concentration (around 1000 ppm) in air, between 250 and 550°C in a conventional fixed-bed reactor. Hexane and toluene were added to the feedstream in a concentration of around 1000 ppm and water concentration varied from 1000 to 15 000 ppm. TCE oxidation 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°C) but inhibited it at higher temperatures (>400°C). Selectivity to HCl was much improved by feeding water as a hydrogen-supplying 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 chlorination of the feed, and Cl2 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 enhanced. Hexane showed higher inhibition ability than toluene over both catalysts for the C2Cl4 and Cl2 formation. Unlike in the presence of water, selectivity to CO increased, as a consequence of partial oxidation of both hydrocarbons.