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 (CH 3 ONO 2 ) 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 CH 3 ONO 2 in the plasma−catalyst system. CH 3 ONO 2 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. CH 3 ONO 2 arriving at the catalyst is converted to methanol and NO 2 . 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.