Role of Methyl Nitrate in Plasma Exhaust Treatment

There is growing interest in the use of a nonthermal plasma combined with a catalyst for NO x removal from diesel engine exhaust streams. Such exhaust streams contain excess oxygen (typically 6−10%), low concentrations of hydrocarbons (typically 100−1000 ppm), and significant concentrations of water (typically 5−12%). Conversion of NO x to environmentally acceptable compounds, without requiring a scrubber or an added reductant, is the desired end result. In our research we observe the formation of substantial amounts of methyl nitrate (CH3ONO2) by the plasma discharge. Since similar compounds have been proposed as reaction intermediates in NO x removal, tests were performed to elucidate the effect of CH3ONO2 in the plasma−catalyst system. CH3ONO2 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 arriving at the catalyst is converted to methanol and NO2. While methyl nitrate was shown to be formed in this system, it is not a significant intermediate in the mechanism of conversion of NO x to nitrogen.