Catalytic destruction of halogenated air toxins and the effect of admixture with VOCs

Supported platinum group metal catalysts have been successfully commercialized for complete destruction of environmentally harmful halogenated volatile organic pollutants. Catalyst properties and oxidative destruction mechanisms were further investigated by evaluating the effects of admixture with nonhalogenated organic compounds. Catalyst composition and organic species were found to control the light-off characteristics. Destruction mechanisms for saturated chlorocarbons were found to depend on catalyst support. Surface Bronsted acidity appears to play an important role on alumina-based catalysts; the light-off curves are not dependent on the Cl/C ratio. Radical initiation is suggested on titania supported catalyst; light-off temperature is directly related to the energy required for a radical dissociation of a chlorocarbon. Unsaturated chlorocarbons are more stable than saturated ones. As a result, their oxidation generally requires high temperatures. The light-off characteristics of unsaturated chlorocarbons were also found to be determined by the mechanism of the destruction The light-off temperatures were 200 degrees C lower on a titania-based catalyst than on an alumina-based catalyst. Oxidation temperature of volatile organic compounds (VOC) is also a function of the chemical nature of the compound. In contrast to chlorinated hydrocarbons, temperatures of activation for oxidation of unsaturated hydrocarbons are lower than saturated ones. Therefore, admiring unsaturated hydrocarbons with chlorinated hydrocarbons significantly decreased the light-off temperatures of the latter. The effect of saturated hydrocarbons such as ethane had little effect on the Light-off characteristics of the chlorinated hydrocarbons. The destruction of brominated compounds was also evaluated to determine the tolerance of catalyst supports to bromide poisoning at low temperature. Bromide was strongly adsorbed on an alumina-based catalyst and became a catalyst poison. As a result, toluene light-off in the admixture was shifted to a much higher temperature. The oxidation of toluene can be restored by removing Br from the catalyst by heating. On the titania-based catalyst, however, the Light-off temperature of methyl bromide was shifted to a higher temperature in the presence of toluene. Toluene may either compete for the same adsorption site or suppress the formation of radicals from methyl bromide.