Plasmacatalytic processes for environmental problems

Abstract

The application of non-thermal electrical discharges instead of thermal energy has been shown to be a suitable alternative for the treatment of exhaust gases, especially for low concentrations (<100 ppmv) of contaminants. In the present paper, we describe the synergetic application of plasma and catalytic treatment for the oxidative removal of volatile organic compounds (VOCs) and the low-temperature conversion of NO x to N 2 in excess oxygen. The catalytic oxidation of butyl acetate and dichloroethene (DCE) as typical VOC has been studied at 50–200°C in combination with pulse corona or dielectric barrier discharges (DBDs). The best results with a significant synergetic effect were obtained with mixed transition metal oxide catalysts, which are able to decompose the ozone at low (<120°C) temperature. With respect to treatment by the plasma alone, the combination shifts the process towards total oxidation. The amount of reaction by-products such as formyl chloride (in case of chlorocarbons), CO, nitric oxides and ozone is lowered. The synergetic effect has been verified at technical conditions in the plasmacatalytic removal of VOC from stripper air in a groundwater cleaning plant. The NH 3-SCR of NO x in excess oxygen using a NH 4-loaded zeolite at 100°C can be significantly accelerated if combined with a plasma discharge. The mechanism of the reaction has been verified by labeling NO with 15NO and by a double-labeled experiment using 15NO and 15NH 4-zeolite. Experiments with a diesel engine have shown that the plasmacatalytic conversion of NO x to N 2 is also effective at real technical conditions.