Plasma technology for tail pipe reduction of NO{sub x} in diesel exhaust

In the conventional application of plasmas to the treatment of power plant flue gases, the plasma is used to oxidize NO{sub x} to nitric acid; the final product us ammonium nitrate, which is obtained by addition of ammonia. A critical issue in the application of plasmas to cars and trucks is whether the plasma can remove the NO{sub x} by direct chemical reduction to N{sub 2}. We have found that gas-phase reactions in the plasma alone cannot lead to true chemical reduction of NO{sub x}. Any reduction of NO{sub x}to N{sub 2} can only be accomplished through heterogeneous reactions of NO{sub 2} with surfaces or particulates. This paper discusses how the plasma oxidation of NO to NO{sub 2} can be utilized to enhance the heterogeneous catalytic reduction of NO{sub x} to N{sub 2}. Plasma- assisted heterogeneous catalysis can enhance the NO{sub x} reduction under conditions that normally make it difficult for either the plasma or the catalyst to function by itself. The combination of a plasmas with a catalyst opens the opportunity for a new class of catalysts that are potentially more durable, more active, more selective and more sulfur-tolerant compared to conventional lean-NO{sub x} catalysts