Role of methyl nitrate in plasma exhaust treatment

There is growing interest in the use of a nonthermal plasma combined with a catalyst for NOx removal from diesel engine exhaust streams. Such exhaust streams contain excess oxygen (typically 6-10%), low concentrations of hydr ocarbons (typically 100-1000 ppm), and significant concentrations of water (typically 5-12%). Conversion of NOx to environmentally acceptable compound s, without requiring a scrubber or an added reductant, is the desired end r esult. In our research we observe the formation of substantial amounts of m ethyl nitrate (CH3ONO2) by the plasma discharge. Since similar compounds ha ve been proposed as reaction intermediates in NOx removal, tests were perfo rmed to elucidate the effect of CH3O NO2 in the plasma catalyst system. CH3 ONO2 was prepared and added to the gas blend on test equipment simulating a diesel exhaust gas. A dielectric barrier plasma discharge was followed by a zeolite-based catalyst. Methyl nitrate introduced upstream of the plasma discharge is largely unreacted upon passing through the plasma. CH3ONO2 arr iving at the catalyst is converted to methanol and NO2. While methyl nitrat e was shown to be formed in this system, it is not a significant intermedia te in the mechanism of conversion of NOx to nitrogen.