Environ. Sci. Technol. 1999, 33, 3427-3431
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 Company 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 NOx 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 NOx 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 NOx removal, tests were performed to elucidate the effect of CH3ONO2 in the plasmacatalyst 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 NOx to nitrogen.
Introduction Increasingly stringent air quality standards will require future diesel engines to be equipped with exhaust aftertreatment systems capable of decreasing the emission of nitrogen oxides (NOx). Among systems proposed for diesel NOx reduction are those based on a nonthermal plasma discharge combined with a catalyst. Penetrante et al. (1) report testing of a plasma discharge upstream of a γ-Al2O3 catalyst. Their results indicate roughly 50% conversion of NOx to N2 in diesel exhaust 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 and 30 J/L energy deposition. Balmer et al. (3) report testing of a plasma 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 * Corresponding author phone: (313) 594-1316; fax: (313) 5942923; e-mail:
[email protected]. 10.1021/es9813010 CCC: $18.00 Published on Web 08/18/1999
1999 American Chemical Society
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 mechanism 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 conventional catalysts do not work well with a plasma, and some catalysts which work well 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 sites activates the HC in some poorly defined manner. The rate-limiting step in this process is the pairing of N atoms to form nitrogen gas (N2). Several steps have been proposed, including chemisorption of NO or NO2 on the catalyst, followed by migration of the N atoms along the surface. Reactions which have been postulated include intermediates such as nitromethane or methyl nitrate formed by the gasphase addition reaction of NO2 to methyl or methoxy radicals (in the following reactions “M” denotes a third body needed to remove the excess energy associated with bond formation):
CH3 + NO2 + M f CH3NO2 + M
(1)
CH3O + NO2 + M f CH3ONO2 + M
(2)
These intermediates may then react with NO2 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 has significant amounts of oxygen and water present, O and OH radicals are common. Input hydrocarbons react with these radicals, generating a large number of HC radicals and compounds. While numerous 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.. A summary of 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 temperature and passed through measurement instrumentation. The principal analytical instrument is an FTIR spectrometer operated at a spectral resolution of 0.125 cm-1 and equipped with a long path length (20.7 m) sampling cell (8). In addition VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic of test lab configuration (FTIR, Fourier transform infrared spectrometer; FID, flame ionization detector for hydrocarbons; CLA, chemiluminescence NOx analyzer).
TABLE 1. Nominal Input Gas Composition, N2 Carrier gas
nominal concn
gas
nominal concn
NO CO CO2 C3H6 C3H8
250 ppm 400 ppm 7% 1575 ppm (C1) 525 ppm (C1)
Ar H2 H 2O O2 SO2
1% 133 ppm 5% 8% 90 ppm
tion with Pacific Northwest National Lab and the U.S. Department of Energy. Samples of the catalyst can be provided free of charge to other investigators for testing upon request. At 2 L/min, space velocity (the reciprocal of residence time) is 4600 h-1. Gas residence time in the oven is approximately 10 s. The methyl nitrate was prepared by the nitration of methanol in concentrated sulfuric acid as described by Blatt (9). The organic phase was dried over CaCl2 and was used without further purification. FTIR analysis indicated that this sample has a purity greater than 95%. A small sample (1-2 mL) was transferred to an evacuated metal gas cylinder (size 300) and pressurized to 200 psi with pure nitrogen gas. The final concentration of CH3ONO2 in the steel cylinder was measured to be 580 ppm by FTIR. Control experiments were performed to determine the stability of CH3ONO2/N2 mixtures; there was no observable (
