CATALYTIC DESTRUCTION OF HALOGENATED AIR TOXINS AND THE EFFECT OF ADMIXTURE WITH VOCS

Supported platinum group metal catalysts have been successfully commer cialized for complete destruction of environmentally harmful halogenat ed volatile organic pollutants. Catalyst properties and oxidative dest ruction mechanisms were further investigated by evaluating the effects of admixture with nonhalogenated organic compounds. Catalyst composit ion and organic species were found to control the light-off characteri stics. Destruction mechanisms for saturated chlorocarbons were found t o depend on catalyst support. Surface Bronsted acidity appears to play an important role on alumina-based catalysts; the light-off curves ar e 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. U nsaturated chlorocarbons are more stable than saturated ones. As a res ult, their oxidation generally requires high temperatures. The light-o ff characteristics of unsaturated chlorocarbons were also found to be determined by the mechanism of the destruction The light-off temperatu res were 200 degrees C lower on a titania-based catalyst than on an al umina-based catalyst. Oxidation temperature of volatile organic compou nds (VOC) is also a function of the chemical nature of the compound. I n contrast to chlorinated hydrocarbons, temperatures of activation for oxidation of unsaturated hydrocarbons are lower than saturated ones. Therefore, admiring unsaturated hydrocarbons with chlorinated hydrocar bons 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 t he tolerance of catalyst supports to bromide poisoning at low temperat ure. Bromide was strongly adsorbed on an alumina-based catalyst and be came a catalyst poison. As a result, toluene light-off in the admixtur e was shifted to a much higher temperature. The oxidation of toluene c an be restored by removing Br from the catalyst by heating. On the tit ania-based catalyst, however, the Light-off temperature of methyl brom ide was shifted to a higher temperature in the presence of toluene. To luene may either compete for the same adsorption site or suppress the formation of radicals from methyl bromide.