Role of Methyl Nitrate in Plasma Exhaust Treatment JOHN W. HOARD,* TIMOTHY J. WALLINGTON, JAMES C. BALL, MICHAEL D. HURLEY, AND KENNETH WODZISZ Ford Motor Com pany Research Laboratory, Dearborn, Michigan 48121-2053 M. LOU BALMER Pacific Northwest National Laboratory, Richland, Washington 99352 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. Introduction Increasinglystringent air qualitystandardswillrequire future diesel engines to be equipped with exhaust aftertreatment systemscapable ofdecreasingthe emission ofnitrogen oxides (NOx). Among systems proposed for diesel NOx reduction are those based on a nonthermalplasma discharge combined with a catalyst.Penetrante et al.(1)reporttestingofaplasma discharge upstream ofa γ-Al2O3 catalyst.Theirresultsindicate roughly50%conversion ofNOx to N2 in dieselexhaust at 370 °C, 30 J/L energy deposition, and 18 000 h -1 space velocity (SV). Hoard and Balmer (2) report that use of a dielectric barrier discharge plasma followed by a proprietary catalyst results in roughly 50% NOx conversion at 180 °C an d 30 J/ L energydeposition.Balmer et al.(3)reporttestingofaplasma with the same proprietary catalyst used previously (2). The catalyst was located either in the plasma or downstream of it. Evidence of N2 formation has been reported during the operation of the plasma (3). Shimizu and Oda (4), using plasma with a variety of catalysts and catalyst substrates, showed NOx removal from 20 to 80%. All of these systems are based on selective catalytic reaction (SCR) of NOx with a hydrocarbon (HC) reductant. The HC is present in the exhaust stream due to incomplete combustion and/or addition of fuel to the exhaust for the purpose of increasing NOx reduction efficiency. The mech- anism of NOx removal may be similar to lean NOx catalysts operating without a plasma discharge. However, research such as that of Tonkyn et al. (5) indicates that at least some catalysts which are effective as conventionalcatalysts do not workwellwith a plasma,and some catalysts which workwell with a plasma do not work without it. Nonetheless, it is worth noting the reaction schemes postulated as being significant to nonplasma SCR with HC reductant (6). It is known that both oxygen and HC must be present to achieve NOx removal.It is often proposed that the first step is oxidation of NO to NO2. A catalyst with acidic surface sitesactivatesthe HCin some poorlydefined manner. The rate-limitingstep in this process is the pairingofN atoms to form nitrogen gas (N2).Severalsteps have been proposed, including chemisorption of NO or NO2 on the catalyst, followed by migration of the N atoms along the surface. Reactionswhich have been postulated include intermediates such as nitromethane or methyl nitrate formed by the gas- phase addition reaction ofNO2 to methylor methoxyradicals (in the following reactions “M” denotes a third body needed to remove the excess energyassociated with bond formation): These intermediates may then react with NO 2 yielding N2. The chemistry of a plasma discharge in exhaust gas is complicated. Reference 1 gives a summary of potential reactions and their underlying kinetics. In the plasma, electron impact causes dissociation and radical formation; the radical chemistry creates a mixture of chemical species in the plasma effluent.Since exhaust hassignificant amounts ofoxygen and water present,Oand OH radicals are common. Input hydrocarbons react with these radicals, generating a large number of HC radicals and compounds. While numer- ous papers have described simplified systems such as NO in dry air, or NO-H2O-O2 in N2, a detailed description of the chemistry occurring in a plasma treatment system with a realistic exhaust gas blend has yet to appear.. Asummaryof a system with NO, O2,H2O, and HC, has been reported by Penetrante et al. (7). The work reported here was undertaken to investigate the role of methyl nitrate in a plasma based diesel exhaust treatment system. Experimental Section The work was carried out on a flow bench which blends gases to simulate engine exhaust. Figure 1 is a schematic of the system. NO, CO, CO2,O2, SO2,H2, Ar, C3H6, and C3H8 gasses are mixed in N2 carrier. Liquid water is injected into the gas in heated lines. The test plasma and catalyst are in an oven maintained at 180 °C. The effluent is mixed with extra N2 to prevent condensation of water at room temper- ature and passed through measurement instrumentation. The principalanalyticalinstrument is an FTIRspectrometer operated at a spectralresolution of0.125cm -1 and equipped with a longpath length (20.7m)samplingcell(8).In addition *Corresponding author phone: (313) 594-1316; fax: (313) 594- 2923; e-mail: jhoard@ford.com. CH 3 + NO 2 + M f CH 3 NO 2 + M (1) CH 3 O + NO 2 + M f CH 3 ONO 2 + M (2) Environ. Sci. Technol. 1999, 33, 3427-3431 10.1021/es9813010 CCC: $18.00  1999 American Chemical Society VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3427 Published on Web 08/18/1999