Nonthermal Plasma Electrocatalytic Reduction of Nitrogen Oxide

Nov 12, 2003 - ... is about 15% higher compared to that by the SCR alone when 1000−3000 ppm of methane or ethylene is added to the inlet gas stream...
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Ind. Eng. Chem. Res. 2003, 42, 6682-6687

KINETICS, CATALYSIS, AND REACTION ENGINEERING Nonthermal Plasma Electrocatalytic Reduction of Nitrogen Oxide Z. Chen and V. K. Mathur* Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824

The high cost of a currently available selective catalytic reduction (SCR) process is driving R&D efforts toward finding alternative technologies to remediate NO emissions from utility boilers. A hybrid plasma-catalyst (P-C) system is investigated for NOx removal. The plasma (dielectric barrier discharge) and SCR techniques are each studied for the reduction of NOx for comparative purposes. γ-Al2O3 is used as a catalyst in the hybrid P-C system. The hybrid P-C experiments show effective NOx removal compared to the results obtained from either plasma or SCR experiments. NOx removal by the P-C system is about 15% higher compared to that by the SCR alone when 1000-3000 ppm of methane or ethylene is added to the inlet gas stream. The hybrid P-C system has the potential to be successfully used for the removal of NOx from utility boiler flue gases and diesel exhausts. Introduction NOx emissions from utility boilers and diesel engines are leading contributors to acid rain as well as to the photochemical smog through the formation of ozone. Current state-of-the-art processes for controlling NOx usually combine combustion improvements for lowering the NOx formation and postcombustion treatment for removing NOx from flue gases. Among the postcombustion technologies, selective catalytic reduction (SCR) of NOx has become dominant in controlling emissions from utility boilers and industrial furnace stacks.1 Although many improvements aimed at lowering the cost of SCR systems have been achieved, both fixed investment and operating cost are still high. Commercial SCR catalysts are generally oxides of titanium, vanadium, and, in some cases, tungsten. Many workers in this area believe that SCR technology can be modified and applied to the control of NO from diesel exhaust under lean-burn conditions. The development of inexpensive catalysts with high performance by using γ-Al2O3,2,3 ZSM-5,4,5 or TiO2 6,7 as the catalyst support has also been intensively conducted. Among the emerging technologies for postcombustion treatment, nonthermal plasma is one of the most promising technologies. Dielectric barrier discharges (DBDs) are one of the electrical breakdowns that can be classified as nonequilibrium discharges, also called nonthermal plasmas. An electrical discharge is a phenomenon wherein free electrons are produced and accelerated under the influence of an electric field. Through collisions with molecules in the gas, electrons cause excitation, ionization, electron multiplication, and formation of atoms and metastable compounds. It is the formation of atoms and compounds that gives an electrical discharge its unique chemical environment and makes it useful for chemical processing. * To whom correspondence should be addressed. Tel.: 603862-1917. Fax: 603-862-3747. E-mail: [email protected].

Based on this concept, a pilot plant is at the stage of field testing at a coal-fired power plant facility.8 As discussed in an earlier publication,9 nonthermal plasma in the DBD form is able to chemically reduce NOx into N2 under controlled conditions. However, the reactions in the DBD reactor significantly change as other components such as O2 or H2O are included in the feed gas. In the presence of O2 and/or H2O, NO oxidation into NO2 becomes dominant, partly forming HNO3. Therefore, plasma alone cannot complete the task of NOx removal. Literature review reveals that considerable efforts have been made to remove NOx via catalytic processes. The development of De-NOx catalysts has been carried out with a focus on the percent conversion, temperature range, life (resistance to poisons such as steam, sulfur, etc.), and manufacturing cost (replacing noble metals with inexpensive metals). Recently, coupling the plasma with a catalytic reactor for a better NOx removal has been receiving more and more attention. The plasma-catalyst (P-C) approach is aimed at obtaining a higher NOx conversion to N2 more effectively and economically than when each of the techniques is used individually. The plasma helps convert NO into NO2, which is then more easily reduced to N2 by a conventional catalytic process.10-12 The conversion of NOx to N2 is important for NOx removal from diesel truck exhausts, where NO2 from a plasma reactor alone is not easy to remove because of the difficulties in installing wet scrubbers. In this study, to gain a better understanding and knowledge of a P-C technique, a combined system is investigated to study the electrocatalytic effect on NOx conversion. The emphasis is on the study of gaseous chemicals formed during NOx passage through plasma, catalyst, and P-C systems rather than on the development of a new catalyst. N2O formation is also investigated. Readers interested in those relevant plasma reactions may refer to one of our previous papers.9

10.1021/ie030096g CCC: $25.00 © 2003 American Chemical Society Published on Web 11/12/2003

Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6683

Figure 1. Schematic diagram of a benchtop system.

Experimental Apparatus and Procedure A schematic diagram of the benchtop reactor system for NO removal is shown in Figure 1. It includes the barrier discharge reactor, catalytic reactor, high-voltage (HV) power supply and measuring instruments, gas supply, and gas analytical instrumentation. The piping system allows the two reactors to be operated either individually or in series with the catalytic reactor downstream of the DBD reactor. This provides three experimental modes including DBD reactions only, catalytic reactions only, and hybrid electrocatalytic reactions. Details can be found elsewhere.13 DBD Reactor. The reactor geometry used to initiate a barrier discharge in a gas space consists of two concentric stainless steel tubes. Each of these cylinders serves as an electrode, while the outer cylinder also serves as the pressure boundary. A HV is applied between the electrodes, with the outer electrode (tube) at ground potential for safety reasons. The distance between the two electrodes is 10 mm. Two quartz tubes are installed adjacent to the electrodes for producing barrier discharge, and the gas spacing between the two quartz tubes is 6 mm. The effective length of the reaction space is where both electrodes function and generate microdischarges. It is 305 mm. The gas mixture enters the inlet plenum of the reactor and exits through the outlet plenum. The outer stainless steel electrode for the benchtop reactor is wrapped with copper tubing fed by a constant-temperature bath, cooling or heating the reactor as necessary for temperature control. Catalytic Reactor. The catalytic reactor consists of a Vycor tube packed with a catalyst (γ-Al2O3 as a 1/16in. or 1.6-mm sphere, surface area 350 m2/g, bulk density 0.662 g/cm3; Alcoa Industrial Chemicals) and placed in a tubular heater. A thermocouple probe is embedded in the center of the catalyst bed. It is shown in the center section of Figure 1. The i.d. and o.d. of the Vycor tube are 21 and 25 mm, respectively. The reactor tube is packed with a catalyst that is held in place with ceramic wool containing Al2O3, CaO, and Si3N4. The length of the packed bed is 30 mm, corresponding to

6.875 g of γ-Al2O3 as the catalyst. The tubular electric heater is type 55035 from Lindberg with a maximum attainable temperature of 1100 °C. HV Power Measurement. The HV supplied to the DBD reactor is monitored and measured by a Tektronix 2211 digital storage oscilloscope sampling at 20 MHz. The HV signal is first reduced with a Tektronix P6015 HV probe (1000×) in series with a Tektronix P6109 probe (10×) before being fed to channel 1 of the oscilloscope. The current in the secondary circuit of the HV transformer is indirectly measured using a resistor of 50 Ω and a Tektronix P6027 probe (1×). This signal is fed to channel 2 of the oscilloscope. The power consumption in the DBD reactor is calculated using the voltage and current traces recorded and stored by the oscilloscope. When the corresponding current and voltage values are multiplied at the same phase angle, a curve of point power is obtained and it is averaged to give the discharge power. An online computer reads the values recorded by the oscilloscope and calculates the discharge power. The voltage and current signals are taken from the second circuit of the transformer where there are some inefficiencies including transformers, inductors, and resistances. However, these inefficiencies are negligible compared to discharge power consumption. Gas Analytical Instrumentation. Several commercial gas analyzers are installed online. A ThermoElectron model 10A/R chemiluminescent NO-NO2NOx analyzer is used to measure NO and NOx (NO and NO2 are collectively referred to as NOx). A Nova model 375 portable combustion analyzer is used to measure the O2, CO2, and CO concentrations in the gas stream. A Hewlett-Packard 5890 series II gas chromatograph (GC) with a Supelco column packed with 60/80 Carboxen-1000 is used to analyze CH4, C2H4, and C2H2 in the gas stream when a reducing agent such as CH4 or C2H4 is added to the simulated gas mixture. A thermal conductivity detector installed in the GC is used with helium as the carrier and reference gas. In addition, a Nicolet 520 Fourier transform infrared (FTIR) spectrometer is used to analyze the gaseous products,

6684 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003

particularly N2O. A gas cell is used to transport samples. The path length of the gas cell is 10 cm. Results and Discussion This study identifies the important variables in the barrier discharge process and their effects on the removal of NOx. NOx is usually referred to as the total of NO and NO2. Destruction of NOx is measured in terms of inlet and outlet concentrations as follows:

NOx conversion (%) )

Cinlet - Coutlet × 100 Cinlet

(1)

Accordingly, NO conversion can be referred to as the percentage of inlet NO converted to NO2, HNO3, N2O, and/or N2. NOx conversion is also calculated from eq 1. Cinlet is CNO plus CNO2 in the inlet stream, while Coutlet is CNO plus CNO2 in the outlet stream. The term NOx conversion is the measurement of the removal of both NO and NO2. NOx conversion can be used to estimate inlet NO converted to N2 when HNO3 and N2O produced are considered to be negligible. The energy density is the power deposited into 1 L of a gas mixture at standard conditions (J/L), i.e., the discharge power divided by the total gas flow rate. All of the experimental results are obtained using a simulated mixture of a pollutant and other gases. The system is operated at atmospheric pressure and room temperature, except it is maintained at 65 °C when highly moist gases are used. It is found that the ratio of discharge power to total energy consumption reached the maximum at a frequency of 150 Hz for this barrier discharge system. Therefore, the present experiments are conducted at a frequency of 150 Hz.9 The plasma startup usually needs 5-10 min to reach a steady state. A concentration reading is taken after it remains within the acceptable range for 5 min. The discharge power is calculated with the help of a computer, and usually three to five readings of the discharge power are taken to get one averaged data point. Repeated experiments are conducted, and the data error is found to be within (3%. Effect of the Catalytic Reactor Temperature on NOx Reduction in a P-C System. Experiments are first conducted with a feed gas of 250 ppm NO, 230 ppm CO, 3% O2, 7.6% CO2, and balance N2 using the plasma reactor alone. The conversions of NO/NOx are found to be 44% and 9%, respectively. A concentration of 700 ppm CO is measured in the outlet stream. Next, experiments are conducted using a catalytic reactor alone and a P-C system. Sampling points are at the outlets of the plasma and catalytic reactors. The same feed gas is bypassed around the plasma reactor and fed directly to the catalytic reactor to obtain the effect on NO/NOx conversions by catalysis alone. Presented in Figure 2 are the NO/NOx conversions as a function of the catalytic reactor temperature. The catalytic reactor alone provides a maximum of NO/NOx conversions of 66% and 9%, respectively, for the abovestated experimental parameters at 495 °C. When the plasma and catalytic reactors are combined, the P-C system provides a maximum of NO/NOx conversions of 98% and 42%, respectively, at 450 °C. The plasma helps to increase the NO/NOx conversions for the catalytic reactor even at a slightly lower catalyst temperature. Table 1 presents the NO/NOx conversions for three systems for comparison.

Figure 2. Effects of catalysis alone and plasma catalysis on NO/ NOx conversions as a function of temperature: 0, NO catalysis; 9, NOx catalysis; O, NO with a P-C system; b, NOx with a P-C system. Total flow rate 1000 SCCM, 250 ppm NO, 230 ppm CO, 3% O2, 7.6% CO2, N2 balance. Plasma: 320 J/L, room temperature. Catalyst: 6.875 g of γ-Al2O3, space velocity 5780/h. Table 1. Comparison of Maximum Conversions of NO/ NOx for Three Experimental Modes conversion

plasma alone

catalyst alone

P-C system

% NO % NOx

44 9

66 9

98 42

Table 2. Gas Composition Comparison between the Initial and Plasma-Conditioned Streams feed stream initial conditioned

NO

NO2

O2

CO2

CO

N2

250 ppm 0 ppm 3% 7.6% 230 ppm balance 140 ppm 87 ppm 3% 7.6% 700 ppm balance

The NOx conversion of 42% by the P-C system is not a simple addition of the individual conversions by plasma and catalyst (9% each). There is a great synergy achieved with this P-C system. NOx reduction in the P-C system can be considered to be due to conditioning of the gas stream by plasma, which significantly changes the gas mixture components and compositions (Table 2). A considerable increase in the CO concentration from 230 to 700 ppm after plasma conditioning probably accounts for the remarkable increase in NOx conversion to N2. Formation of CO is due to electro-impact dissociation of CO2 in the plasma as follows:

e + CO2 f e + O + CO

(2)

The De-NOx pathway can be proposed as follows:

2{CO} + 2{NO} f {CNO-NCO} + 2{O}

(3)

where {} indicates active species adsorbed on the γ-Al2O3 catalyst surface. Schmal et al.14 reported that the above reaction over Pd-xMo/Al2O3 formed isocyanate (CNO-NCO). CNO-NCO was thought to be an intermediate in the SCR of NO by C3H6 over Pt/Al2O3.15 The following reactions are also proposed for the present study:

NO + {O} f NO2

(4)

{CNO-NCO} f N2 + 2CO

(5)

CO + NO2 f CO2 + NO

(6)

Table 2 shows that the NO2 concentration increases from 0 to 87 ppm after conditioning. It may also contribute to the increase in NOx conversion by the

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stream accounts for the higher NOx conversion by the P-C system compared to that by the catalytic reactor alone, as shown in Table 3. In the case of the conditioned feed, there are CO and other CH4-oxidized products as a result of part oxidation of CH4 by the plasma process. A high NO2 concentration of 141 ppm (Table 4) is found in the conditioned feed. These compounds may play an important role in the SCR of NOx. One of the NOx removal pathways can be considered to be the same as that proposed earlier; i.e., CO and NO form isocyanate, an intermediate. CH4 and CH4-oxidized products probably combine with NO2, forming CH3NO2, an intermediate indicating another pathway to NOx reduction: Figure 3. NO/NOx conversions vs concentrations of CH4 added: 0, NO plasma; 9, NOx plasma; ], NO catalysis; [, NOx catalysis; O, NO with a P-C system; b, NOx with a P-C system. Total flow rate 1000 SCCM, 250 ppm NO, 3% O2, and N2 balance. Plasma: 150 Hz, 260 J/L, glass wool packing. Catalyst: 6.875 g of γ-Al2O3, 500 °C, space velocity 5780 /h. Table 3. Comparison of NO/NOx Conversions with 3000 ppm CH4 Addition conversion

plasma alone

catalyst alone

P-C system

% NO % NOx

65 8

85 40

94 55

Table 4. Feed Composition Comparison between the Initial and Plasma-Conditioned Streams for 3000 ppm CH4 Addition feed stream

NO

NO2

O2

CH4

N2

initial conditioned

250 ppm 89 ppm

0 ppm 141 ppm

3% 3%

3000 ppm 2850 ppma

balance balance

a

5% of the initial CH4 is oxidized into CO and HC-oxidized products.

catalytic reactor if NO2 is an intermediate, indicating another pathway to NOx reduction. CH4 Effect on NOx Reduction with a P-C System. Being chemically active, propylene is often used as a reducing agent in conducting studies of SCR of NO.16,17 Methane, however, is attractive because of its low cost and easy availability. Especially for a natural gas combustion system, the exhaust probably contains some unburned methane that could be used in a downstream catalytic process. To identify the CH4 effect, experiments are conducted in the absence of CO2 because plasma would dissociate CO2 into CO, which would enhance NOx conversion. The experiments are conducted with CH4 additions of 0, 1000, 2000, and 3000 ppm. For comparison, the same gas feed is bypassed around the plasma reactor and is directly fed to the catalytic reactor to obtain the effect of the catalytic reactor alone on NO/NOx conversions. Figure 3 shows the NO/NOx conversions as a function of concentrations of CH4 added to the inlet feed stream to the P-C system. When the NOx conversions between a P-C system and catalyst alone are compared, the improvement in NOx conversion is found to remain constant at about 15% in the CH4 concentration range of 1000-3000 ppm. Presented in Table 3 is the comparison of conversions for CH4 addition of 3000 ppm. The NO/NOx conversions for the P-C system are 94% and 55%, respectively, compared with 65% and 8% for the plasma alone and 85% and 40% for the catalyst alone. Table 4 compares the compositions of initial and conditioned feeds. Plasma conditioning of the feed

2CH3NO2 + O2 f N2 + CO + CO2 + 3H2O

(7)

Figure 3 shows that NO conversion decreases as the methane concentration is increased under plasma alone conditions. This can be explained by the following main reactions taking place in the plasma:

e + N2 f e + N + N

(8)

N + NO f N2 + O

(9)

e + O2 f e + O(3P) + O(3P)

(10)

e + O2 f e + O(3P) + O(1D)

(11)

O(3P) + NO + M f NO2 + M

(12)

O + O2 + M f O3 + M

(13)

O3 + NO f NO2 + O2

(14)

where O(3P) and O(1D) are ground-state and metastable excited-state oxygen atoms, respectively, and M is either N2 or O2. As methane is added, the following reactions are proposed to occur in the plamsa:

CH4 + e f CH3 + H + e

(15)

CH4 + O f CH3 + OH

(16)

Reaction 15 competes for energetic electrons under plasma condition, leading to fewer electrons available for the formation of radicals such as O and O3 to convert NO into NO2 via reactions 12 and 14. Reaction 16 also consumes O. As the methane concentration is increased from 1000 to 3000 ppm, NO conversion slowly decreases. C2H4 Effect on NOx Reduction with a P-C System. Ethylene is added to the inlet stream to study its effect on NO/NOx conversions. Ethylene (C2H4) is used rather than propylene (C3H6) because it is closer to CH4 with respect to the numbers of C and H atoms. Very little information is available in the literature for its use as a reducing chemical in the SCR reactions. To investigate the C2H4 effect on NO/NOx conversions, experiments are conducted in the absence of CO2. The experiments are conducted with C2H4 additions of 0, 1000, 2000, and 3000 ppm to the inlet stream. For comparison, the same gas feed is bypassed around the plasma reactor and is directly fed to the catalytic reactor to obtain the effect by catalyst alone on NO/NOx conversions. Figure 4 shows the NO/NOx conversions as a function of concentrations of C2H4 added to the various systems.

6686 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 7. Feed Composition Comparison between the Initial and Plasma-Conditioned Streams for 3000 ppm C2H4 Addition feed stream

NO

NO2

O2

C2H4

N2

initial conditioned

250 ppm 19 ppm

0 ppm 230 ppm

3% 3%

3000 ppm 2640 ppma

balance balance

a 12% of the initial C H is oxidized into CO and other HC2 4 oxidized products.

These compounds play important roles in the SCR of NOx. A high NO2 concentration of 230 ppm is found in the conditioned feed. It may be contributing to the increase in NOx conversion by the catalytic reactor if NO2 is also an intermediate and assists in NOx reduction. Figure 4 shows that C2H4 significantly enhances NO conversion to NO2. The following reactions are proposed to occur in the plasma process: Figure 4. NO/NOx concentrations vs concentrations of C2H4 added: 0, NO plasma; 9, NOx plasma; ], NO catalysis; [, NOx catalysis; O, NO with a P-C system; b, NOx with a P-C system. Total flow rate 1000 SCCM, 250 ppm NO, 3% O2, and N2 balance. Catalyst: 6.875 g of γ-Al2O3, 500 °C. Plasma: 150 Hz, 260 J/L, glass wool packing, space velocity 5780 /h. Table 5. Comparison of Conversions for 3000 ppm C2H4 Addition conversion

plasma alone

catalyst alone

P-C system

% NO % NOx

92 0.4

99 84

99 90

Table 6. Improvement in NOx Conversions by a P-C System for C2H4 Addition C2H4 addition, ppm catalyst alone (a), % NOx P-C system (b), % NOx improvement (b-a), % NOx

0

1000

2000

3000

2 34 32

54 70 16

76 88 12

84 90 6

Presented in Table 5 is the comparison of NO/NOx conversions with C2H4 addition of 3000 ppm in the inlet feed streams. The NO/NOx conversions for the P-C system are 99% and 90%, respectively, compared with 92% and 0.4% for the plasma alone and 99% and 84% for the catalyst alone. When the NOx conversions achieved between the P-C system and catalyst alone are compared, the improvement in NOx conversion is found to be about 6% for C2H4 addition of 3000 ppm. For both the P-C system and catalyst alone, NOx conversion to N2 is of importance. Therefore, only NOx conversions for different C2H4 concentrations are listed in Table 6 for comparison. The highest improvement of 32% for the P-C system without C2H4 addition is of little importance because the total NOx conversion is only 34%. When 1000 ppm C2H4 is added, the NOx conversion is 70% for the P-C system, 16% higher than 54% for the catalyst alone. The improvement decreases from 16% to 12% as the C2H4 concentration is increased from 1000 to 2000 ppm. Only 6% advantage in NOx conversion is achieved by the P-C system when the C2H4 concentration is increased from 2000 to 3000 ppm. Table 7 compares the chemical compositions between the initial and plasma-conditioned feeds. Plasma conditioning of the feed stream accounts for the higher NOx conversion by the P-C system compared to the conversion by the catalytic reactor alone. In the conditioned feed, there are CO and other C2H4-oxidized products as a result of part oxidation of C2H4 by the plasma process.

C2H4 + O + O f HCO + HCO + H + H

(17)

HCO + O2 f CO + HO2

(18)

HO2 + NO f NO2 + OH

(19)

Reactions 17-19 are dominant, resulting in up to 90% of NO conversion under the plasma condition only. Investigation of N2O Formation. N2O is formed as a side product of the plasma reaction under controlled conditions such as a dry gas mixture feed. Its removal before emission is vital to the P-C system under investigation. When the same feed mixture of NO/O2/ CH4/N2 is passed through the P-C system, 17 ppm N2O is found in the outlet gas stream. However, this N2O concentration is reduced to less than 5 ppm in the presence of 2% H2O. Addition of 3000 ppm C2H4 to the mixture of NO/O2/N2 successfully suppresses N2O formation in the P-C system even in the absence of water vapors in the inlet stream. Flue gas or exhaust typically contains 5-10% H2O vapors. Therefore, there is not likely to be any N2O formation problem using a P-C system for NOx removal. Our previous paper9 reported the mechanism of N2O formation as

NO2 + N f N2O + O

(20)

The presence of H2O or C2H4 probably suppresses radical N formation, leading to less or no N2O formation. Conclusions The present study on reduction of NOx using a P-C system has led to the following conclusions: 1. For the inlet gas mixture of 250 ppm NO/3% O2/ 3000 ppm CH4/N2, the NOx conversion for the P-C system (plasma energy density of 260 J/L) is 55% compared to 40% for the catalyst alone, achieving an improvement of 15%. 2. An addition of 2000 ppm C2H4 to the gas mixture of 250 ppm NO/3% O2/N2 enhances the NOx conversion to 88% for the P-C system (plasma energy density of 260 J/L) compared to 76% for the catalyst alone, achieving an improvement of 12%. 3. SCR of NO by C2H4 is much more effective than that by CH4. γ-Al2O3 is an effective and less expensive SCR catalyst. 4. The synergy achieved with the P-C system can be accounted for by the plasma conditioning of the gas

Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6687

mixture and the production of HC-oxidized compounds that promote the downstream catalytic reduction of NOx into N2. 5. Addition of 2% H2O to the dry mixture of NO/O2/ N2 successfully suppresses N2O formation, showing less than 5 ppm N2O in the outlet gas stream of the P-C system. The present work shows that a hybrid P-C system using inexpensive catalysts and reducing agents has the potential to be successfully used for the removal of NOx from flue gases and exhausts of diesel engine trucks, a major problem faced by the power and transportation industries. Acknowledgment Z.C. is thankful to the Department of Chemical Engineering, University of New Hampshire, for financial support. Thanks are due to Dr. Sterling Tomellini, Department of Chemistry, University of New Hampshire, for allowing us the use of his FTIR laboratory for N2O measurements. Literature Cited (1) Control of Nitrogen Oxide Emissions: Selective Catalytic Reduction (SCR); Technical Report 9; DOE and Southern Company Services, Inc.: July 1997. (2) Meunier, F.; Breen, J.; Zuzaniuk, V.; Olsson, M.; Ross, J. Mechanistic Aspects of the Selective Reduction of NO by Propene over Alumina and Silver-Alumina Catalysts. J. Catal. 1999, 187, 497. (3) Sirdeshpande, A.; Lighty, J. Kinetics of the Selective Catalytic Reduction of NO with NH3 over CuO/γ-Al2O3. Ind. Eng. Chem. Res. 2000, 39, 1781. (4) Chen, H.; Voskoboinikov, T.; Sachtler, W. Reaction Intermediates in the Selective Catalytic Reduction of NOx over Fe/ZSM5. J. Catal. 1999, 186, 91. (5) Parvulescu, V.; Centeno, M.; Grange, P.; Delmon, B. NO Decomposition over Cu-Sm-ZSM-5 Zeolites Containing LowExchanged Copper. J. Catal. 2000, 191, 445. (6) Kondarides, D.; Chafik, T.; Verykios, X. Catalytic Reduction of NO by CO over Rhodium Catalysts. J. Catal. 2000, 191, 147.

(7) Long, R.; Yang, R. FTIR and Kinetic Studies of the Mechanism of Fe3+-Exchanged TiO2-Pillared Clay Catalyst for Selective Catalytic Reduction of NO with Ammonia. J. Catal. 2000, 190, 22. (8) McLarnon, C. Update on the ECO Process for MultiPollutant Control of Coal-Fired Utility Boilers. Electric Power 2002 Conference, Mar 21, 2002. (9) Chen, Z.; Mathur, V. Non-Thermal Plasma for Gaseous Pollution Control. Ind. Eng. Chem. Res. 2002, 41 (9), 2082. (10) Penetrante, B.; Brusasco, R.; Merritt, B.; Pitz, W.; Vogtlin, G.; Kung, M.; Kung, H.; Wan, C.; Voss, K. NOx Conversion Chemistry In Plasma-Assisted Catalysis. Proceedings of the 1998 Diesel Engine Emissions Reduction Workshop, Castine, ME, Jul 6-9, 1998; pp 177-186. (11) Oda, T.; Kato, T.; Takahashi, T.; Shimizu, K. Nitric Oxide Decomposition in Air by Using Nonthermal Plasma Processing with Additives and Catalyst. IEEE Trans. Ind. Appl. 1998, 34, 268. (12) Hoard, J.; Balmer, M. L. Plasma-Catalysis for Diesel NOx Remediation. Proceedings of the 1998 Diesel Engine Emissions Reduction Workshop. Castine, ME, Jul 6-9, 1998. (13) Chen, Z. Nitric Oxide Reduction by Non-Thermal Plasma and Catalysis. Ph.D. Dissertation, University of New Hampshire, Durham, NH, Dec 2002. (14) Schmal, M.; Baldanza, M.; Vannice, M. Pd-xMo/Al2O3 Catalysts for NO Reduction by CO. J. Catal. 1999, 185, 138. (15) Captain, D.; Amiridis M. In Situ FTIR Studies of the Selective Catalytic Reduction of NO by C3H6 over Pt/Al2O3. J. Catal. 1999, 184, 377. (16) Denton, P.; Giroir-Fendler, A.; Praliaud, H.; Primet, M. Role of the Nature of the Support (Alumina or Silica), of the Support Porosity, and of the Pt Dispersion in the Selective Reduction of NO by C3H6 under Lean-Burn Conditions. J. Catal. 2000, 189, 410. (17) Kudla, R.; Subramanian, S.; Chattha, M.; Hoost, T. Effect of Tungsten Oxide Addition on the Catalytic Activity of γ-Al2O3 for NOx Reduction from Fuel-Lean Gas Mixtures. Ind. Eng. Chem. Res. 1996, 35, 4394.

Received for review February 3, 2003 Revised manuscript received September 30, 2003 Accepted September 30, 2003 IE030096G