Reduction of NOx in synthetic diesel exhaust via two-step plasma-catalysis treatment

Significant reduction of NOx in synthetic light duty diesel exhaust has been achieved over a broad temperature window by combining atmospheric plasma with appropriate catalysts. The technique relies on the addition of hydrocarbon reductant prior to passing the simulated exhaust through a non-thermal plasma and a catalyst bed. The observed chemistry in the plasma includes conversion of NO to NO2 as well as the partial oxidation of the hydrocarbon. The overall NO, reduction has a maximum of less than 80%, with this maximum obtained only at high-energy input into the plasma, high concentration of hydrocarbon reductant and low space velocity. We present data in this paper illustrating that a multiple-step treatment strategy, whereby two or more plasma-catalyst reactors are utilized in series, can increase the maximum NO, conversion obtainable. Alternatively, this technique can reduce the energy and/or hydrocarbon requirements for a fixed conversion efficiency. When propene is used as the reductant, the limiting reagent for the overall process is most likely acetaldehyde. The data suggest that acetaldehyde is formed in concert with NO oxidation to NO2 in the plasma stage. The limited NO, reduction efficiency attained in a single step, even with excess energy, oxygen content and/or hydrocarbon-to-NO, ratio is well explained by this hypothesis, as is the effectiveness of the multiple-step treatment strategy. We present the data here illustrating the advantage of this approach under a wide variety of conditions. (C) 2002 Elsevier Science B.V. All rights reserved.