Direct Decomposition of NO Activated by Microwave Discharge

Res. , 2003, 42 (24), pp 5993–5999. DOI: 10.1021/ie0304208. Publication Date (Web): November 1, 2003. Copyright © 2003 American Chemical Society...
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Ind. Eng. Chem. Res. 2003, 42, 5993-5999

5993

Direct Decomposition of NO Activated by Microwave Discharge Junwang Tang, Tao Zhang,* Lei Ma, and Ning Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

The environmentally friendly removal of NO has been investigated using continuous microwave discharge (CMD) at atmospheric pressure. In these experiments, conversions of NO to N2 as well as NO2 were mainly observed for both dry and wet feed gas, which showed a great difference from those observed with other discharge methods. The effects of a series of reaction parameters, including microwave input power, O2 concentration, NO concentration, and gas flow rate, on the product distribution and energy efficiency were also studied. Under all reaction conditions, the conversions of NO to N2 were higher than those to NO2. The highest conversion of NO to N2 was 88%. The reaction rate of NO removal and the effects of the different discharge modes on NO conversion and product distribution are also discussed. Through comparison of the results of different discharge modes, it was found that the addition of CH4 apparently increased the conversion of NO to N2 as well as the energy efficiency. A possible reaction process is suggested. 1. Introduction Although considerable progress has been achieved in the reduction of NO emissions, the release of NO from the burning of fossil fuels is still a major environmental problem.1-3 Especially under conditions of a high air/ fuel ratio in which the fuel efficiency is higher, engines emit comparatively greater amounts of NO because of the higher combustion temperature.4 For the removal of NO emitted from fossil-fuel vehicles, two techniques are commonly used, namely, modification of engines and treatment of exhaust gases. However, in recent years, techniques for diesel engine modification have reached their limit for reducing NO emission levels. Engine modification is still insufficient to meet recent regulations for NO emission levels. Hence, an effective exhaust gas treatment process for NO removal is urgently needed. In principle, the direct decomposition of NO is the most attractive way to reduce NO pollution.5 For the decomposition of NO, zeolite Cu-ZSM-5 has been found to be the most suitable catalyst.5-7 Unfortunately, even the overexchange capacity of Cu-ZSM-5 is active only in a limited interval of temperatures and in the absence of O2.8 In addition, other molecules such as H2O or SO2 act as inhibitors or cause rapid deactivation of catalysts.9 Meanwhile, emission regulations for NO are becoming increasingly strict. To satisfy the requirements of future environmental protection legislation, many new de-NO technologies have been developed in the past decades, among which the discharge process turns out to be one of the promising routes for direct decomposition of NO. Many interesting results have been observed in chemical reactions using discharge technology, such as methane oligomerization10,11 and the transformation of methane and carbon dioxide to higher hydrocarbons.12 * To whom correspondence should be addressed. Address: State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China. Tel.: +86-411-4678404. Fax: +86-4114691570. E-mail: [email protected].

Discharges are capable of activating gaseous species, and in certain cases, nonthermodynamic equilibrium can be attained during the selective activation of stable small molecules.13 Various types of energy and radiation, such as microwave,14-16 radio frequency,17 and electricity,1,18-24 can be used to generate discharge. Mok and Ham, Kim et al., and Broer and Hammer20,23,24 investigated NO conversion using a pulse discharge in the first two cases and nonthermal plasma in the latter. They found that NO was mainly converted to NO2. However, only a limited number of studies in microwave and radio-frequency discharge are available because of problems associated with the use of low pressure, the erosion of electrodes, and so on.18 We have developed a continuous microwave discharge (CMD) process at atmospheric pressure that does not require the use of any electrodes.15,16 Applying this CMD process in environmental chemistry, we are aiming at the direct decomposition of NO, which is a novel and environmentally friendly process free from the drawbacks of conventional catalytic processes, such as the use of high temperatures and the deactivation of catalysts. CMD is a nonequilibrium discharge process characterized by a low gas temperature but a high electron temperature, i.e., it produces high-energy electrons in the gas while leaving the temperature of the bulk gas very low.15,16 Compared to other discharge technologies, such as AC (alternating current) and DC (direct current) discharge, CMD is energy-efficient because a large amount of energy goes into the production of energetic electrons rather than into gas heating.14 In the present work, we explored NO decomposition with three kinds of feed gas, i.e., NO + O2, NO + O2 + He (denoted as “dry gas”), and NO + O2 + H2O + He (denoted as “wet gas”). Furthermore, we investigated the reaction of NO with CH4 using a Y-type reactor. The effects of selective excitation of the reactants, input power, reactant concentrations, and flow rate on product distributions were also investigated. The reaction rate of NO conversion, the energy efficiencies of these reactions, and possible mechanisms are discussed as well.

10.1021/ie0304208 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/01/2003

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2.3. Calculations. The main reaction investigated in this work is

2NO f O2 + N2 or

2NO + CH4 + O2 f 2H2O + N2 + CO2 The flow rate was considered to be constant because the numbers of moles before and after the reaction were equal. The conversions of CH4 and NO are defined separately as follows

XCH4 (%) ) Figure 1. CMD reaction system schematic diagram: 1, microwave generator; 2, cooling water and circulator; 3, waveguide; 4, reflectometer; 5, gas passage; 6, Y-type reactor; 7, tuning plunger; 8, resonant cavity; 9, NOx analyzer; 10, gas chromatograph.

2. Experimental Section 2.1. Apparatus. The experimental diagram is presented in Figure 1. The CMD reaction system consisted of a microwave generator, a rectangular waveguide, a circulator, a resonant cavity, and a tuning plunger. The microwave energy was supplied by a 2.45-GHz microwave generator, the power of which could be varied continuously in the range of 0-200 W. The effective input power for the experiments ranged from 5 to 60 W. The power reflected by the reactor was monitored by a reflectometer. For these experiments, a Y-type reactor was designed to enable CMD to be carried out stably.16 The reactor was aligned vertically at the center of the single-mode resonant cavity, so that the discharge region was located in the microwave field of maximum intensity. Microwave discharge was induced by the cooperative action of the wall of the quartz reactor and a mass of quartz wool placed in the reactor arm. CH4 and NO were introduced into the two arms separately for the convenience of activating either of the reactants selectively. When a reactant was activated by CMD in one arm (D arm), the other reactant was introduced along the other arm (G arm) to the region downstream of the discharge at the junction of the two arms. 2.2. Preparation and Analysis of Materials. Ultrapure NO, CH4, O2, and He were used in the present work. Each gas flow rate was controlled by a mass flowmeter. NO, O2, and He were first mixed in a gasmixing cylinder and then were introduced along one reactor arm. CH4 diluted with He was introduced along the other reactor arm when desired. The reactants and products were analyzed by an on-line NOx analyzer (model FGA 4000/4005, Foshan Analytic Instruments Co. Ltd., Foshan, China) and a gas chromatograph (model GC-8800, Shanghai Kechuang Analytic Instruments Co., Shanghai, China, equipped with a thermal conductivity detector with 13X and PQ columns). The concentrations of NO, O2, N2, and CH4 were determined by the external standard calibration method. The total flow rate ranged from 20 mL/min (1.2 L/h) to 180 mL/ min (10.8 L/h) according to the different reaction requirements.

XNO (%) )

[CH4]o - [CH4]e [CH4]o [NO]o - [NO]e [NO]o

× 100%

× 100%

(1)

(2)

To calculate the degree of environmentally friendly removal of NO, the conversions of NO to N2 and to NO2 were mainly considered, which are defined as follows

Conversion of NO to N2: XN (%) )

2[N2]e [NO]o

× 100% (3)

Conversion of NO to NO2: XNO2 (%) )

[NO2]e [NO]o

× 100% (4)

where [i]o is the concentration of reactant i in the total reactants before the CMD reaction and [i]e is the concentration of reactant (such as CH4 and NO) or product (such as N2 and NO2) i after the CMD reaction. The nitrogen mass balance of the reaction is defined as follows

BN )

2[N2]e + [NO2]e [NO]o - [NO]e

(5)

If the value of BN equals approximately 1, NO is essentially completely converted to N2 and NO2; otherwise, NO is possibly converted to N2, NO2, N2O, and so on. The energy efficiencies for NO removal and NO conversion to N2 are defined as follows

ENO )

30νg[NO]oXNO 24 500Pin

(6)

EN 2 )

30νg[NO]oXN 24 500Pin

(7)

where the units of ENO and EN2 are grams of NO per kilowatt-hour [g(NO)/kWh]. Pin is the input microwave power (in kilowatts). The number of milliliters in 1 mol of a gas at room temperature is 24 500, and the molecular weight of NO is 30. νg is the total gas flow rate (in milliliters per hour).

Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 5995 Table 1. Removal of NO by CMD under Different Conditions group 1a

group 2b

group 3c

input power (W):

38 ( 4

44 ( 4

38 ( 4

44 ( 4

38 ( 4

44 ( 4

conversion of NO (%) conversion of NO to N2 (%) conversion of NO to NO2 (%) BN

100 86.10 13.84 0.999

100 88.03 11.17 0.992

77.23 59.00 17.62 0.992

80.54 63.58 16.25 0.991

82.04 54.28 27.05 0.991

84.41 60.30 22.71 0.983

a 2000 ppm NO, balance He; total flow rate ) 60 mL/min. b 2000 ppm NO, 2% O , balance He; total flow rate ) 60 mL/min. c 2000 ppm 2 NO, 2% O2, 5% H2O, balance He; total flow rate ) 60 mL/min.

The rates of NO removal are N2 and NO2 formation are calculated as follows

removal rate of NO (mol/h) )

XNO[NO]oνg 24 500

(8)

[N2]eνg 24 500

(9)

formation rate of N2 (mol/h) ) formation rate of NO2 (mol/h) )

[NO2]eνg (10) 24 500

3. Results and Discussion 3.1. Removal of NO by CMD (without CH4). 3.1.1. Removal of NO under Different Conditions. Here, the conversion of NO was carried out in the Y-type reactor under CMD without CH4 injection. First, a gas mixture designated as the group 1 gas (2000 ppm NO, balance He) was introduced in one arm of the Y-type reactor with a flow rate of 60 mL/min, and the other arm was sealed. By the cooperation of the quartz wool and the inner wall of the quartz reactor arm, the group 1 gas was induced to discharge, and the flame spread out evenly in the reactor arm. Next, plenty of O2 was added to the mixture gas, forming the mixture designated as the group 2 gas (dry gas; 2000 ppm NO, 2% O2, balance He), and the operation was repeated at the same flow rate of 60 mL/min. Finally, H2O vapor was added to the group 2 gas to form the group 3 gas (wet gas; 2000 ppm NO, 2% O2, 5% H2O, balance He). The results of NO removal with these three feed mixtures are all presented in Table 1. For group 1, all NO was converted, and the conversion of NO to N2 was 88.03%. For group 2, the conversion of NO to N2 decreased because of the addition of plenty of O2, and the total conversion of NO also decreased, whereas the conversion to NO2 increased. For group 3, although H2O vapor was injected, the conversion of NO to N2 decreased only slightly, whereas the conversion to NO2 increased markedly. Even under such conditions, about 60% of the NO was still converted to N2, and the total conversion of NO was higher than 84%. In summary, whenever excess O2 or excess O2 and a definite amount of H2O were added to the feed gas, NO was mainly converted to N2 in CMD. In particular, it was worth noting that, after the addition of H2O, the conversion of NO to N2 did not decrease too much. Meanwhile, the N mass balance was calculated by eq 5, which is also presented in Table 1. The results suggest that NO is essentially completely converted to N2 and NO2 only when the CMD method is used. Regardless of whether a pulsed corona discharge,1,19,20,23 a dielectric barrier discharge,22 or a surface discharge is used, if there is excess O2 in the reactants, NO is mainly converted to NO2. However, in the present work, all of the experiments were conducted

in the presence of excess O2. The results showed that NO was mainly converted to N2. Therefore, it can be concluded that the results of NO removal by CMD are different from those of the other discharge methods.1,20-24 It is known that the lifetime of N2O is about 120 years.25 Because the hazard of this compound can be not neglected, its formation might offset the advantage of the discharge method. Fortunately, in our experiments, BN calculated by eq 5 was nearly 1, which indicates that N2O was almost absent in the products when NO was removed by the CMD method. All of these results imply that CMD is a preferable and direct route to the elimination of NO. In CMD, microwaves induce a nonequilibrium discharge at atmospheric pressure. In this kind of discharge, the electrons have a very high temperature, but the ions and molecules are nearly at room temperature because only electrons are constantly accelerated in the electromagnetic field by the high-frequency power supply.21 In CMD, the high-energy electrons can have energies ranging from 5 to 10 eV.20 The high-energy electrons lose energy through collisions with gas molecules, which leads to the formation of a variety of species, including ions, metastable species, atoms, and free radicals. These products are chemically active and react easily with other gas molecules to form stable compounds.1 Subsequently, the most stable compounds are the main products. The bond dissociation energy of N2 is 9.8 eV, and those of O2, H2O, and NO are 5.1, 5.2, and 6.4 eV, respectively. In addition, the dissociation rate constant of N2 is much smaller than that of O2.26 Therefore, O2, H2O, and NO can be dissociated by CMD, but N2 can hardly be dissociated. Consequently, it is reasonable that N2 becomes the dominant component in the products. 3.1.2. Effect of Microwave Input Power. To study the effects of the microwave input power on the removal of NO, another experiment was conducted with the group 2 gas, and the results are shown in Figure 2. The total conversion of NO increased slightly with increasing input power, whereas the conversion of NO to N2 increased greatly and that to NO2 decreased markedly. When the input power was increased further, all conversions were leveled to constant values. This suggests that increasing the input power of the microwaves is mainly beneficial for the conversion of NO to N2. 3.1.3. Effect of O2 Concentration. From the effects of the input power, it can be concluded that variations of the input power in the range higher than 30 W have very little effect on the conversion of NO. Therefore, we selected an input power of 40-45 W to perform the following experiments. The gas flow rate was 60 mL/ min. The effect of the O2 concentration on the conversion of NO is shown in Figure 3, where NO in the feed gas was 2000 ppm and the balance was He. With increasing concentration of O2, the total conversion of NO de-

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Figure 2. Effect of microwave power on NO conversion. NO conversion to N2 (2), NO conversion to NO2 (b), total NO conversion (9). Gas mixture: 2000 ppm NO, 2% O2, balance He.

Figure 3. Effect of O2 concentration on NO conversion. NO conversion to N2 (2), NO conversion to NO2 (b), total NO conversion (9). Gas mixture: 2000 ppm NO, balance He.

Figure 4. Effect of flow rate on NO conversion. NO conversion to N2 (2), NO conversion to NO2 (b), total NO conversion (9). Gas mixture: 2000 ppm NO, 2% O2, balance He.

creased. When the concentration of O2 was about 4%, the conversion of NO was almost the lowest at approximately 80%. Subsequently, the conversion of NO increased slowly with further increases in the O2 concentration. Then, the conversion of NO to N2 decreased continually and that to NO2 increased. When the concentration of O2 was 10%, the conversion of NO to N2 was about 4% higher than that to NO2. 3.1.4. Effect of Flow Rate. The effect of the flow rate on the conversion of NO is shown in Figure 4, in which the input power was also 40-45 W and the feeding gas comprised 2000 ppm NO, 2% O2, and the balance He. The gas flow rate ranged from 20 to 180 mL/min.

Figure 5. Dependence of reaction rate on flow rate. Formation rate of N2 (2), formation rate of NO2 (b), removal rate of NO (9). Gas mixture: 2000 ppm NO, 2% O2, balance He.

It can be seen that the effect of the flow rate on the total conversion of NO was insignificant. However, with increasing flow rate, the conversion of NO to N2 decreased greatly and that to NO2 increased markedly. When the flow rate was higher than 90 mL/min, the two conversions began to vary slowly. During the variation of the flow rate, even when the flow rate was 180 mL/min (i.e., 10.8 L/h), the conversion of NO to N2 was also above 45%, and that to NO2 was below 35%. The rates of NO removal and N2 and NO2 formation were calculated by using eqs 8-10, respectively. The dependence of these reaction rates on the flow rates is shown in Figure 5. All reaction rates increased with increasing flow rate. The rate of N2 formation was close to that of NO2 formation, and the rate of NO removal increased linearly with increasing total gas flow rate. All reaction rates were the highest when the flow rate was 10.8 L/h. When the flow rate increases, the gas residence time in the discharge region is reduced; this is equivalent to a reduction of the average input power for each mole of the gas. That is, increasing the flow rate is equivalent to indirectly decreasing average input power, which, in turn, gives rise to a decrease in the conversion of NO to N2 and an increase in the conversion of NO to NO2. The results were in accordance with the effect of the microwave input power. 3.1.5. Effect of NO Concentration. In CMD with a 40-45-W input power and 60 mL/min flow rate, using a feed gas containing 2% O2 and the balance He, the effect of the concentration of NO was investigated, and the results are shown in Figure 6. It was observed that the total conversion of NO decreased very slightly, whereas the increase in the conversion of NO to N2 as well as the decrease in the conversion of NO to NO2 were marked with increasing NO concentration. Thus, within a certain range, the higher the concentration of NO, the more effective the environmentally friendly removal of NO by the CMD method. However, it can also be seen that the conversion of NO to N2 will decrease if the concentration of NO is much greater. This issue was not investigated in the present work because of limitations on the experimental conditions. 3.2. Reduction of NO with CH4 under the CMD. 3.2.1. Reaction of CH4 with Selectively Excited Gas Mixtures. A gas mixture containing 2000 ppm NO, 2% O2, and the balance He was introduced into the D arm of the Y-type reactor, and by the cooperation between

Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 5997

Figure 6. Effect of NO concentration on NO conversion. NO conversion to N2 (2), NO conversion to NO2 (b), total NO conversion (9). The O2 concentration in the gas mixture is 2%; the balance is He.

Figure 7. Results of reaction between unexcited CH4 and excited gas mixture containing NO. NO conversion to N2 (2), CH4 conversion (9). Reactants: 2000 ppm NO, 2% O2, 1600 ppm CH4, balance He.

the quartz wool placed in the reactor arm and the inner wall of the arm, discharge was produced in the D arm. At the same time, no discharge was found in the junction of the Y-type reactor. CH4 (1600 ppm) was then introduced directly downstream of the discharge region (namely, at the junction) along the G arm. The total flow rate was kept 60 mL/min. The results of the reaction of CH4 with excited NO are present in Figure 7. Although CH4 did not pass through the discharge region, CH4 and NO were still both converted. This indicates that NO or O2 excited by CMD interacted with the unexcited CH4. Moreover, with increasing input power, the conversions of CH4 and NO both increased markedly. The conversion of NO to N2 upon excitation of the gas mixture in the presence of CH4 was compared to the conversion obtained in the absence of CH4, and the results are shown in Figure 8. It can be seen that, regardless of whether CH4 was used, the conversion of NO to N2 increased with increasing input power. However, with a similar input power, the addition of CH4 clearly enhanced the conversion of NO to N2. When NO is excited, N2 can be produced first by direct NO decomposition in the CMD process. At the same time, long-lived O species from NO conversion can induce CH4 to form CH3 and H active species in the junction.27 Subsequently, these active species can react with NO to form N2.28 Therefore, the conversion of NO to N2 is much high when NO is excited in the presence of CH4.

Figure 8. Effect of CH4 addition on NO conversion to N2 in an excited gas mixture containing NO. With CH4 addition (2), without CH4 addition (9). Reactants: 2000 ppm NO, 2% O2, 1600 ppm CH4, balance He.

Figure 9. Results of reaction between excited CH4 and an unexcited gas mixture containing NO. NO conversion to N2 (2), CH4 conversion (9). Reactants: 2000 ppm NO, 2% O2, 1600 ppm CH4, balance He.

3.2.2. Reaction of NO with Selectively Excited CH4. Except for interchanging the passages of the gas mixture and CH4, the other conditions were the same for the investigation described here as for that described in section 3.2.1. Under such conditions, CH4 was excited, but the mixture gas was not. The results of the reaction of the excited CH4 with the unexcited gas mixture are shown in Figure 9. It can be seen that the conversions of both NO and CH4 increased with increasing input power. Compared to the results presented in section 3.2.1, after the discharge mode had been changed, the conversions of NO to N2 were affected markedly. These results are displayed in Figure 10. It can be noted that the N2 yield with excited NO was far greater than that obtained with excited CH4. In CH4 discharge, excited species such as H and CHx have been observed.29 Of these species, CH3 and H radical have relatively long lifetimes. Because the reactions of these active species with NO took place in the junction of the reactor, only the active species with long enough lifetimes were able to reach that region and react. Therefore, CH3 and H radicals from CH4 were probably the key species for NO removal under these conditions. 3.3. Energy Efficiencies. The energy efficiency for NO removal, ENO (mass flow rate of removal of NO divided by input power), was calculated using eq 6, and the results are shown as a function of input power in Figure 11. This plot indicates that the energy efficiency of NO removal decreases with increasing discharge

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Figure 10. Results of NO conversion to N2 in different discharge modes. Excited gas mixture (2), excited CH4 (9). Reactants: 2000 ppm NO, 2% O2, 1600 ppm CH4, balance He.

Figure 13. Effects of different discharge modes on energy efficiencies. Excited gas mixture with injected CH4 (9), excited gas mixture without injected CH4 (2), excited CH4 with injected gas mixture (b).

powers. The energy efficiency of excited CH4 is lowest. From these results, it is apparent that our subsequent investigations should be directed toward increasing the energy efficiency of the conversion of NO to N2 by selecting proper additives and optimizing the reactor used in CMD. 4. Conclusions

Figure 11. Dependence of energy efficiency on input power. Gas mixture: 2000 ppm NO, 2% O2, balance He.

Figure 12. Dependence of energy efficiency on flow rate. Gas mixture: 2000 ppm NO, 2% O2, balance He.

input power, following a curve similar to those observed with the electron beam and pulse corona NO reduction techniques. The dependence of the energy efficiency, ENO, on the gas flow rate is shown in Figure 12. The energy efficiency increases linearly with increasing flow rate, which is in accordance with the dependence of the reaction rate on the flow rate. For an approximate evaluation of the effects of different discharge modes on energy efficiencies, the energy efficiencies of NO conversion to N2, i.e., EN2, were calculated by eq 7. The results of EN2 for all discharge modes are shown in Figure 13. These efficiencies are lower than the energy efficiency of NO removal. The value of EN2 upon addition of CH4 is clearly higher than that without CH4 addition, especially for lower input

A novel technique for the removal of NO was developed, and the effect of CH4 injection on NO removal was investigated experimentally using a Y-type reactor. The results show that the process is effective for the environmentally friendly removal of NO. NO is mainly converted to N2 both in dry gas and in wet gas. Increasing the flow rate of the feed gas is helpful in enhancing the reaction rate and energy efficiency. The input power, as well as the O2 and NO concentrations, also clearly affects the conversion of NO to N2. However, regardless of the O2 concentration up to 10% or the flow rate up to 10.8 L/h, N2 still is the main product. No N2O formation was observed in these experiments. The interactions between the reactants from the G arm and the D arm were also observed when only the reactants from the D arm were excited. This indicates that some radicals or excited species from the D arm have long enough lifetimes to reach the junction and react. At the same time, the addition of CH4 greatly affects the removal of NO and energy efficiency. To improve the energy efficiency and further clarify the reaction mechanism, additional studies must be performed. Literature Cited (1) Sathiamoorthy, G.; Kalyana, S.; Finney, W. C.; Clark, R. J.; Locke, B. R. Chemical reaction kinetics and reactor modeling of NOx removal in a pulsed streamer corona discharge reactor. Ind. Eng. Chem. Res. 1999, 38, 1844. (2) Lawrence, M. G.; Crutzen, P. J. Influence of NOx emissions from ships on tropospheric photochemistry and climate. Nature 1999, 402, 167. (3) Madia, G.; Koebel, M.; Elsener, M.; Wokaun, A. The effect of an oxidation precatalyst on the NOx reduction by ammonia SCR. Ind. Eng. Chem. Res. 2002, 41, 3512. (4) Popp, P. J.; Bishop, G. A.; Stedman, D. H. Method for commercial aircraft nitric oxide emission measurements. Environ. Sci. Technol. 1999, 33, 1542. (5) Parvulescu, V. I.; Centeno, M. A.; Grange, P.; Delmon, B. NO decomposition over Cu-Sm-ZSM-5 zeolites containing lowexchanged copper. J. Catal. 2000, 191, 445.

Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 5999 (6) Iwamoto, M.; Yahiro, H.; Mine, Y.; Kagawa, S. Excessively copper ion-exchanged ZSM-5 zeolites as highly active catalysts for direct decomposition of nitrogen monoxide. Chem. Lett. 1989, 213. (7) Iwamoto, M.; Yahiro, H.; Torikai, Y.; Yoshioka, T.; Mizuno, N. Novel preparation method of highly copper ion-exchanged ZSM-5 zeolites and their catalytic activities for NO decomposition. Chem. Lett. 1990, 1967. (8) Parvulescu, V. I.; Grange, P.; Delmon, B. Catalytic removal of NO. Catal. Today 1998, 46, 233. (9) Kikuchi, E.; Yogo, K.; Tanaka, S.; Abe, M. Selective reduction of NO with propylene on Fe-silicate catalysts. Chem. Lett. 1991, 1063. (10) Huang, J.; Suib, S. L. Dimerization of methane through microwave plasmas. J. Phys. Chem. 1993, 97, 9403. (11) Marun, C.; Suib, S. L.; Dery, M.; Harrison, J. B.; Kablaoui, M. J. Effect of dielectric constant, cavities in series, and cavities in parallel on the product distribution of the oligomerization of methane via microwave plasmas. J. Phys. Chem. 1996, 100, 17866. (12) Eliasson, B.; Liu, C. J.; Kogelschatz, U. Direct conversion of methane and carbon dioxide to higher hydrocarbons using catalytic dielectric-barrier discharges with zeolites. Ind. Eng. Chem. Res. 2000, 39, 1221. (13) Fridman, A. A.; Rusanov, V. D. Theoretical basis of nonequilibrium near atmospheric-pressure plasma chemistry. Pure Appl. Chem. 1994, 66, 1267. (14) Roh, H. S.; Park, Y. K.; Park, S. E. Superior decomposition of NO over plasma-assisted catalytic system induced by microwave. Chem. Lett. 2000, 578. (15) Tang, J.; Zhang, T.; Liang, D.; Sun, X.; Lin, L. Conversion of NO to N2 in continuous microwave discharge. Chem. Lett. 2000, 916. (16) Tang, J.; Zhang, T.; Wang, A.; Ren, L.; Yang, H.; Ma, L.; Lin, L. Removal of NO by microwave discharge with the addition of CH4. Chem. Lett. 2001, 140. (17) Reid, D. W. Chemical catalysis with RF power mechanisms, systems and costs. Res. Chem. Intermed. 1994, 97. (18) Suib, S. L.; Brock, S. L.; Marquez, M.; Luo, J.; Matsumoto, H.; Hayashi, Y. Efficient catalytic plasma activation of CO2, NO, and H2O. J. Phys. Chem. B 1998, 102, 9661. (19) Masuda, S. Pulse corona induced plasma chemical processs A horizon of new plasma chemical technologies. Pure Appl. Chem. 1988, 60, 727.

(20) Mok, Y. S.; Ham, S. W. Conversion of NO to NO2 in air by a pulsed corona discharge process. Chem. Eng. Sci. 1998, 53 (9), 1667. (21) Mok, Y. S.; Ham, I. S. Role of organic chemical additives in pulsed corona discharge process for conversion of NO. J. Chem. Eng. Jpn. 1998, 31 (3), 391. (22) Takaki, K.; Jani, M. A.; Fujiwara, T. Removal of nitric oxide in flue gases by multipoint to plane dielectric barrier discharge. IEEE Trans. Plasma Sci. 1999, 27 (4), 1137. (23) Kim, H. H.; Takashima, K.; Katasura, S.; Mizuno, A. Lowtemperature NOx reduction process using combined systems of pulsed corona discharge and catalysis. J. Phys. D: Appl. Phys. 2001, 34, 604. (24) Broer, S.; Hammer, T. Selective catalytic reduction of nitrogen oxides by combining a nonthermal plasma and a V2O5WO3/TiO2 catalyst. Appl. Catal. B 2000, 28 (2), 101. (25) Fluckiger, J.; Dauenbach, A.; Blunier, T.; Stauffer, B.; Stocker, T. F.; Raynaud, D.; Barnola, J. M. Variations in atmospheric N2O concentration during abrupt climatic changes. Science 1999, 285, 227. (26) Amirov, R. H.; Asinovsky, E. I.; Samoilov, I. S.; Shepelin, A. V. Non-Thermal Plasma Techniques for Pollution Control: Part B; Springer-Verlag: Berlin, 1993. (27) Kilpinen, P.; Glarborg, P.; Hupa, M. Reburning chemistrys A kinetic modeling study. Ind. Eng. Chem. Res. 1992, 31, 1477. (28) Krylov, O. V. Catalytic reactions of partial methane oxidation. Catal. Today 1993, 18, 209. (29) De la Cal, E.; Tafalla, D.; Tabares, F. L. Characterization of He/CH4 DC glow-discharge plasmas by optical-emission spectroscopy, mass-spectrometry, and actinometry. J. Appl. Phys. 1993, 73, 948.

Received for review May 14, 2003 Revised manuscript received August 15, 2003 Accepted September 29, 2003 IE0304208