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In the discharge-catalyst treatment of diesel exhaust the discharge chemistry is known to oxidize NO to NO2 as well as to produce partially oxidized hydrocarbons for the heterogeneous reduction step. We find NO2 to be much more easily reduced to N2 on our catalysts, as long as there is a sufficient supply of reductant present. Unfortunately we typically find that a fraction of the NO2 is only partially reduced back to NO. Since much of the original hydrocarbon survives both the plasma and our catalyst, a subsequent stage of plasma will oxidize NO back to NO2 while at the same time replenishing the supply of partially oxidized hydrocarbon for another stage of heterogeneous catalysis. We present experimental evidence illustrating the advantages of multi-step discharge-catalyst treatment of NOx in simulated diesel exhaust.

We present here a phenomenological analysis of a cascade of two-step discharge-catalyst reactors. That is, each step of the cascade consists of a discharge reactor in series with a catalyst bed. These reactors are intended for use in the reduction of tailpipe emission of NOx from diesel engines. The discharge oxidizes NO to NO2, and partially oxidizes HC. The NO2 then reacts on the catalyst bed with hydrocarbons and partially oxidized HCs and is reduced to N2. The cascade may be essential because the best catalysts for this purpose that we have also convert significant fractions of the NO2 back to NO. As we show, reprocessing the gas may not only be necessary, but may also result in energy savings and increased device reliability.

Recent work has shown that energy efficiencies as well as yields and selectivities of the NOx reduction reaction can be enhanced by combining a plasma discharge with select catalysts. While analysis of gas phase species with a chemiluminescent NOx meter and mass spectrometer show that significant removal of NOx is achieved, high background concentrations of nitrogen preclude the measurement of nitrogen produced from NOx reduction. Results presented in this paper show that N2 from NOx reduction can be measured if background N2 is replaced with helium. Nitrogen production results are presented for a catalyst system where the catalyst is in the plasma region and where the catalyst is downstream from the plasma. The amount of N2 produced is compared with the amount of NOx removed as measured by the chemiluminescent NOx meter. The measured nitrogen from NOx reduction accounts for at least 40% of the total NOx removed for both reactor configurations.