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Plasma-assisted selective catalytic reduction of NOx by C2H2 over Co-HZSM-5 catalyst
Catalysis Communications 7 (2006) 297–301 www.elsevier.com/locate/catcom
Plasma-assisted selective catalytic reduction of NOx by C2H2 over Co-HZSM-5 catalyst Jinhai Niu a, Xuefeng Yang a,b, Aimin Zhu a,b, Lingling Shi a, Qi Sun a, Yong Xu a,b, Chuan Shi a,* b
a Laboratory of Plasma Physical Chemistry, Dalian University of Technology, Dalian 116024, China State Key Laboratory for Material Modification by Laser, Ion and Electron, Dalian University of Technology, Dalian 116024, China
Received 5 August 2005; accepted 31 October 2005 Available online 10 January 2006
Abstract Investigations on the combination of the dielectric barrier discharge plasma with Co-HZSM-5 catalyst for the selective catalytic reduction (SCR) of NOx by lower hydrocarbons were presented. Compared with the reductant of CH4 and C2H4, C2H2 showed a broader low temperature activity in the SCR of NOx with significant O2 tolerance, as well as enhanced CO2 selectivity under the combination of plasma with the catalyst. With a reactant gas mixture of 500 ppm NOx, 500 ppm C2H2, 15% oxygen in N2 and space velocity of 12 000 h1. The NOx conversion is higher than 50% in the temperature range of 150–450 °C in this plasma-assisted process. Larger than 90% of NOx conversion was obtained at 300 °C and Ein = 138 J l1. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Selective catalytic reduction (SCR) of NOx with hydrocarbons has been studied intensively as a potential method to remove NOx from the exhaust gases [1–3]. Various hydrocarbons have been tested in this process, including methane, ethylene, propene, and propane, as well as higher hydrocarbons. One challenge for the process is that the effective temperature window is relatively narrow, and should be broadened to lower temperatures for practical applications. Specially designed catalysts on the basis of promoted zeolites, supported metals or bifunctional catalyst mixtures are active also at lower temperatures (200– 400 °C) [4,5]. While for a technical application, they need further development. Recently, non-thermal plasma (NTP) catalytic processes for the abatement of noxious emissions in automotive exhaust gases have attracted much attention. In NOx *
Corresponding author. Tel./fax: +86 41182701243. E-mail address: [email protected] (C. Shi).
1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.10.016
removal, it has been well accepted that the oxidative potential of a NTP with the excess oxygen results in an effective conversion of NO to NO2, the presence of hydrocarbons in the gas, such as C2H4, C3H6 and so on, promotes the oxidation reactions. Therefore, the NTP itself cannot remove NOx from the exhaust. Thus, it is essential for the combination of the NTP with the SCR catalysis, since NO2, acting as the essential intermediate, could be effectively reduced by hydrocarbons over an appropriate catalyst in the SCR process. Recent papers reported on an efficient combination of a NTP with appropriate catalysts in the HC-SCR of NOx reaction [6–8]. However, it should be noted that NOx would be synthesized in N2/O2 system in some NTP processes. As reported by our group, significant amounts of NOx formed from N2 and O2 have been observed in a one-stage plasma-over-catalyst (POC) reactor at temperatures above 350 °C, indicating that the special necessity of using low-temperature performance in plasma-catalytic processes for the removal of NOx [9]. C2H2, with unsaturated triple bond and its sp hybridized C–H bond, should be easily activated at lower
298
J. Niu et al. / Catalysis Communications 7 (2006) 297–301
temperatures. Therefore, in the present paper, by comparing the reductants of C2H2 with CH4 and C2H4, the catalytic properties of a Co-HZSM-5 catalyst in a POC reactor for the SCR of NOx were investigated with and without the assistance of a dielectric barrier discharge plasma. Broadened effective temperature window, better oxygen tolerance and enhanced CO2 selectivity were clearly observed in plasma-assisted C2H2-SCR of NOx over Co-HZSM-5 catalyst.
3. Results and discussion 3.1. HC-SCR of NOx with and without discharge Fig. 1a–c shows the conversions of NOx as functions of reaction temperature over Co-HZSM-5 catalyst with and without discharge and over quartz pellets with discharge using CH4, C2H4 and C2H2 as reductant. Without discharge, the Co-HZSM-5 catalyst shows different catalytic behaviors in the CH4-SCR, C2H4-SCR, C2H2-SCR of 100
2. Experimental
NOx conversion ð%Þ ¼
NOx;in NOx;out N2 Oout 100 NOx;in ð1Þ
where NOx,in, NOx,out and N2Oout stand for NOx concentration in the inlet and outlet gas, and the amount of N2O in the outlet, respectively.
NOx Conversion (%)
80 60 40 20 0 100
150
200
250
300 o T/ C
350
400
450
150
200
250
300 o T/ C
350
400
450
150
200
250
300 o T/ C
350
400
450
a 100
NOx Conversion (%)
80 60 40 20 0 100
b 100 80
NOx Conversion (%)
The POC reactor used in this work consisted of an outer quartz tube (i.d. 10 mm), a stainless steel tube (o.d. 3 mm) placed along the axis of the outer tube as the high voltage electrode and 2.8 ml of catalyst pellets (20–40 mesh) filled between the two tubes. A stainless steel wire mesh wound on the outside surface of the quartz tube was used as the ground electrode. The DBD power supply source was capable of supplying a bipolar sine wave output with 0–60 kV peak-to-peak voltage (Up) at an a.c. frequency of 50 Hz. The input electric discharge power (Pin) at a certain experimental condition was measured via the area of the voltage-charge Lissajous figures [10]. The input discharge energy density, Ein (J l1), was defined as Pin(W) divided by the inlet gas flow rate (l s1). The temperature of the plasma reactor was controlled by a temperature-programmed controller (Xiamen Yuguang Co., China), the average of Tc (the center temperature measured from the central stainless tube) and Tout (outside temperature of the outer quartz tube) was taken as the reaction temperature, Treac. Co-HZSM5 catalyst was prepared via a three-times repeated ionexchange method at 80 °C by using H-ZSM-5 (SiO2/ Al2O3 = 25, Nankai University, China) and aqueous cobalt acetate solutions, followed by drying at 120 °C for 8 h and calcination at 500 °C for 3 h after each ion exchange. A reacting gas mixture was prepared by a mass flow controller (MFC) (SevenStar Co., China) system. And a reacting gas flow rate corresponding to GHSV = 12 000 h1 was always used. Individual NO and NO2 concentration was monitored by a chemiluminescence analyzer (Monitor ML9841AS), the concentrations of CO2, CO, and N2O in the outflow gas were determined by an infrared absorption spectrometer (SICK-MAIHAK-S710). Before reaction, the catalyst was pretreated in a N2 stream at 500 °C for 2 h. NOx conversion was defined as follows:
60 40 20 0 100
c
Fig. 1. NOx conversion as functions of temperature over: (j) Co-HZSM5 catalyst alone, (d) discharge over Co-HZSM-5 catalyst and (m) discharge over quartz pellets (500 ppm NOx, 15% O2, N2 as balance gas, flow rate 520 ml min1, GHSV=12 000 h1, (a) 500 ppm C2H2, (b) 500 ppm C2H4 and (c) 1000 ppm CH4, Ein (150–250 °C) = 144 J l1, Ein (300–350 °C) = 138 J l1).
J. Niu et al. / Catalysis Communications 7 (2006) 297–301
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NOx reactions. In CH4-SCR, only 15% conversion of NOx was obtained at 350 °C, and the maximum NOx conversion of 35% was achieved at 400 °C. By using C2H4 and C2H2 as reductants, the SCR reactions were initiated at much lower temperatures. At 150 °C, NOx conversion was 16% in the C2H4-SCR reaction and 24% in the C2H2-SCR reaction. The maximum conversion efficiency of NOx was 82% at 300 °C for C2H4 and 89% for C2H2 at 350 °C. When DBD plasma was operated over Co-HZSM-5 catalyst, the removal efficiency of NOx was greatly enhanced especially in lower temperature regions (150–300 °C). A synergistic effect was clearly observed in the C2H2/NO/ O2/N2 system. At 150 °C and Ein = 144 J l1, more than 50% of NOx was converted in plasma-catalytic process (discharge over Co-HZSM-5), which was larger than the sum of the pure catalytic efficiency of 23.7% and pure plasmainduced (discharge over quartz pellets) conversion percentage of 6.4%. The negative value came from the formation of a little amount of N2O. Conversion of NOx increased with the increase of temperature from 150 to 300 °C. At 300 °C and Ein = 138 J l1, pure plasma-induced and plasma-catalytic NOx conversion percentages are 2.9% and 93%. NOx conversion in the plasma-catalytic process is larger than the sum of it in pure catalytic and in pure plasma-induced process and which is also larger than the maximum value obtained in pure catalytic process at 350 °C. When temperature was increased to 350 °C, NOx conversion decreased to 89%. (The discharge temperature controlled was no higher than 350 °C, since the discharge above 350 °C was unstable.) But in the C2H4/NO/O2/N2 system, synergistic effects can be achieved only below 250 °C. This could be partly interpreted by the increase of the formation rate of NOx from N2 and O2 over the catalyst at higher temperatures [9]. However, by using CH4 as reductant, the pure catalytic, pure plasma-induced and plasmacatalytic NOx conversion percentages were 1.2%, 23% and 25%, respectively, at 150 °C. When the reaction temperature increased to 350 °C, only 5% of NOx was eliminated by discharge over quartz pellets and a little promotion effect on NOx conversion was observed in plasma-catalytic process. This indicated that the NOx conversion mainly came from the contribution of plasma decomposition at low temperatures and catalytic conversion might play a major role at high temperatures. It also could be seen that even in the presence of plasma and catalyst, NOx cannot be effectively reduced by methane at lower temperatures. Comparing the reductants CH4 and C2H4, as well as C2H2, it was clear that by using C2H2 as reductant, the catalyst showed higher activities even in the low temperature region of 150–350 °C, and obviously a synergistic effect between dielectric barrier discharge plasma and catalyst was also observed below 300 °C.
and without discharge by using C2H2, C2H4 and CH4 as reductants were shown in Fig. 2a–c. Without discharge, NOx conversion strongly depended on the oxygen content in the C2H2-SCR and C2H4-SCR reactions. The removal percentage of NOx increased from 13.8% to 60% in the C2H2-SCR reaction and 6.5% to 49% in the C2H4-SCR reaction with the increase of the oxygen content from 0 to 12%. Further increase of O2 concentration made the NOx conversion decrease. When the discharge was on, NOx removal was promoted, especially in the absence of oxygen, 73% and 68% of NOx conversions were achieved in NO/C2H2/N2 and NO/C2H4/N2 mixtures at 250 °C
3.2. Oxygen content effect on NOx removal
Fig. 2. Dependence of NOx conversion on oxygen content over: (j) CoHZSM-5 only and (d) Co-HZSM-5 plus ac DBD plasma (500 ppm NOx, (a) 500 ppm C2H2, (b) 500 ppm C2H4 and (c) 1000 ppm CH4, N2 as balance gas, flow rate 520 ml min1, GHSV = 12 000 h1, Ein (250 °C) = 144 J l1).
NOx conversion as a function of the oxygen content at 250 °C and Ein = 144 J l1 over Co-HZSM-5 catalyst with
NOx conversion (%)
100 80 60 40 20 0 0
2
4
6
8
10
12
14
16
14
16
Oxygen content (%)
a
NOx conversion (%)
100 80 60 40 20 0 0
2
4
6
8
10
12
Oxygen content (%)
b
NOx conversion (%)
100 80 60 40 20 0 0
c
2
4
6
8
10
12
Oxygen content (%)
J. Niu et al. / Catalysis Communications 7 (2006) 297–301
C2H2 to COx conversion(%)
100
60 40 20
a
150 200 250 300 350 400 450 500 550
T/
o
C
100 80 60 40 20 0 100
b
150 200 250 300 350 400 450 500 550
T/
o
C
100
3.3. Hydrocarbon’s conversion to COx (x = 1,2) and CO2/CO ratio in HC-SCR of NOx with and without discharge Hydrocarbon’s conversion to COx (x = 1,2), defined by the ratio of the total amount of COx and the inlet carbon atom concentration of hydrocarbons, as a function of temperature were shown in Fig. 3a–c. With Co-HZSM-5 catalyst only, 12% of C2H2 and 8% of C2H4 were converted to COx at 150 °C, while only 2.5% of methane was oxidized to COx at 250 °C. 100% of C2H4 converted to COx (x = 1,2) occurred above 300 °C, and that of C2H2 did shift to 350 °C. As high as 500 °C was needed to fully convert methane over Co-HZSM-5 catalyst. With the plasma assistance, the hydrocarbon’s conversion to COx (x = 1,2) for C2H2 and C2H4 were significantly enhanced in the range of 150–300 °C (e.g. at 150 °C, they are 62% and 74% for C2H2 and C2H4, respectively). After correlation of these data with those in Fig. 1, it was obvious that the selectivity of C2H2 was higher than that of C2H4 for the SCR of NOx. For example, at 200 °C, the selectivities of the plasma catalytic processes for C2H2 and C2H4 were 80% and 52%, respectively. (Since the N2 production cannot be measured in our experiments, the selectivity here is defined by the
80
0 100
C2H4 to COx conversion(%)
and Ein = 144 J l1. The improved NOx conversion by discharge might come from the direct decomposition of NOx, since it was found that the NOx conversion could reach 62% in NO/N2 mixtures under the same conditions. NOx conversions decreased slightly when 2% of oxygen was added. With further increase in O2 content, the NOx conversions increased to get to the maximum value at 12% and 8% oxygen content in C2H2-SCR and C2H4-SCR reactions, respectively, and then decreased. The difference between pure catalytic and plasma-induced catalytic SCR reactions illustrated that NOx conversion might go through different pathways in the plasma catalytic process with the increase of oxygen content. In the oxygen content of 0–2%, the NOx removal mainly came from the discharge induced decomposition; with the oxygen content increase from 2% to 15%, due to the inhibition of plasma decomposition by the excess oxygen, the plasma-enhanced reduction by the reductants contributed the most percentage of NOx conversion. When CH4 was used as reductant, catalysis over CoHZSM-5 catalyst was neglective at 250 °C as shown in Fig. 2c, while NOx conversions were obviously enhanced when DBD plasma was on. NOx conversion was higher than 50% in the absence of O2, and decreased with the increasing O2 content, the conversions could be maintained at ca. 30% in the O2 content of about 12%. This suggested that NOx conversion mainly came from the direct decomposition of NOx by DBD plasma and higher O2 content might inhibit the decomposition reactions. From another point of view, it was shown that the plasma catalytic process could be operated over a wide oxygen concentration range from 0 to 15% both in the C2H2-SCR and C2H4-SCR systems; and C2H2 contained system was more tolerant to oxygen than C2H4.
CH 4 to COx conversion(%)
300
80 60
40 20 0 100
c
150 200 250 300 350 400 450 500 550
T/
o
C
Fig. 3. Hydrocarbon’s conversion to COx (x = 1,2) as functions of temperature over: (j) Co-HZSM-5 only and (d) Co-HZSM-5 plus ac DBD plasma. (500 ppm NOx, 15% O2, N2 as balance gas, flow rate 520 ml min1, GHSV=12 000 h1, (a) 500 ppm C2H2, (b) 500 ppm C2H4 and (c) 1000 ppm CH4, Ein (150–250 °C) = 144 J l1, Ein (300–350 °C) = 138 J l1).
ratio of the NOx conversion and the hydrocarbon’s conversion to COx.) For CH4, only above 300 °C, its conversion to COx (x = 1,2), could be enhanced. This indicated that CH4 was more difficult to be activated than C2H4 and C2H2 even in the combination of DBD plasma with the Co-HZSM-5 catalyst. Another aspect which should be noted is the CO2/CO ratio. As listed in Table 1, the CO2/CO ratio was increased with the increase of temperature and the amount of CO produced at high temperatures can be neglected with C2H2, C2H4 and CH4 as reductants. With the assistance
J. Niu et al. / Catalysis Communications 7 (2006) 297–301
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Table 1 CO2/CO ratio in HC-SCR of NOx at different temperatures over Co-HZSM-5 catalyst with and without plasma (reaction conditions are the same as in Fig. 1) Temperature/°C
150 200 250 300 350
C2H2-SCR
C2H4-SCR
CH4-SCR
Catalyst
Catalyst + plasma
Catalyst
Catalyst + plasma
Catalyst
Catalyst + plasma
1.1 1.2 1.3 1.9 94.9
1.2 1.2 1.4 2.9 216.4
1.0 1.4 2.1 6.6 96.3
0.9 0.9 1.6 11.7 98.6
– – – 11.8 73.7
– – – 14.3 103.7
of plasma, at higher temperatures (300–350 °C), the CO2 selectivities were somehow enhanced in C2H4-SCR, C2H2-SCR, as well as in CH4-SCR reactions.
20203002), the National High Technology Research and Development Program (‘‘863 Programm’’) of China (Grant No. 2002AA649140) and the Provincial Grants of Science and Technology of Liaoning, China (No. 20022112).
4. Conclusions A synergistic effect between dielectric barrier discharge plasma and catalyst was observed in C2H2-SCR of NOx at 150–300 °C in the presence of excess oxygen. Over one-stage POC reactor, more than 50% conversion of NOx can be obtained at a broad temperature window (150–450 °C): at low temperatures (150–300 °C), it can be realized by the combination of plasma with catalyst; at higher temperatures (above 300 °C), catalyst alone can put it into practice (the plasma is turned off). Acknowledgements This work is supported by National Natural Science Foundation of China (Grant Nos. 20077005 and
References [1] L. Ren, T. Zhang, D. Liang, C. Xu, J. Tang, L. Lin, Appl. Catal. B 35 (2002) 317. [2] B. Cho, J. Catal. 42 (1993) 418. [3] A. Shichi, A. Satsuma, T. Hattori, Appl. Catal. A 207 (2001) 315. [4] M. Neylow, M. Castagnola, N. Castagnola, C. Marshall, Catal. Today 96 (2004) 53. [5] M. Misono, Y. Hirao, C. Yokoama, Catal. Today 38 (1997) 157. [6] H. Miessner, K.-P. Francke, R. Rudolph, Appl. Catal. B 36 (2002) 53. [7] S. Yoon, A.G. Panov, R.G. Tonkyn, A.C. Ebeling, S.E. Barlow, M.L. Balmer, Catal. Today 72 (2002) 243. [8] Z. Chen, V. Mathur, Ind. Eng. Chem. Res. 42 (2003) 6682. [9] Q. Sun, A.-M. Zhu, X.-F. Yang, J.-H. Niu, Y. Xu, Chem. Commun. 12 (2003) 1418. [10] L. Rosenthal, D. Davis, IEEE Trans. Ind. Appl. 11 (1975) 328.
Plasma-assisted selective catalytic reduction of NOx by C2H2 over Co-HZSM-5 catalyst Jinhai Niu a, Xuefeng Yang a,b, Aimin Zhu a,b, Lingling Shi a, Qi Sun a, Yong Xu a,b, Chuan Shi a,* b
a Laboratory of Plasma Physical Chemistry, Dalian University of Technology, Dalian 116024, China State Key Laboratory for Material Modification by Laser, Ion and Electron, Dalian University of Technology, Dalian 116024, China
Received 5 August 2005; accepted 31 October 2005 Available online 10 January 2006
Abstract Investigations on the combination of the dielectric barrier discharge plasma with Co-HZSM-5 catalyst for the selective catalytic reduction (SCR) of NOx by lower hydrocarbons were presented. Compared with the reductant of CH4 and C2H4, C2H2 showed a broader low temperature activity in the SCR of NOx with significant O2 tolerance, as well as enhanced CO2 selectivity under the combination of plasma with the catalyst. With a reactant gas mixture of 500 ppm NOx, 500 ppm C2H2, 15% oxygen in N2 and space velocity of 12 000 h1. The NOx conversion is higher than 50% in the temperature range of 150–450 °C in this plasma-assisted process. Larger than 90% of NOx conversion was obtained at 300 °C and Ein = 138 J l1. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Selective catalytic reduction (SCR) of NOx with hydrocarbons has been studied intensively as a potential method to remove NOx from the exhaust gases [1–3]. Various hydrocarbons have been tested in this process, including methane, ethylene, propene, and propane, as well as higher hydrocarbons. One challenge for the process is that the effective temperature window is relatively narrow, and should be broadened to lower temperatures for practical applications. Specially designed catalysts on the basis of promoted zeolites, supported metals or bifunctional catalyst mixtures are active also at lower temperatures (200– 400 °C) [4,5]. While for a technical application, they need further development. Recently, non-thermal plasma (NTP) catalytic processes for the abatement of noxious emissions in automotive exhaust gases have attracted much attention. In NOx *
Corresponding author. Tel./fax: +86 41182701243. E-mail address: [email protected] (C. Shi).
1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.10.016
removal, it has been well accepted that the oxidative potential of a NTP with the excess oxygen results in an effective conversion of NO to NO2, the presence of hydrocarbons in the gas, such as C2H4, C3H6 and so on, promotes the oxidation reactions. Therefore, the NTP itself cannot remove NOx from the exhaust. Thus, it is essential for the combination of the NTP with the SCR catalysis, since NO2, acting as the essential intermediate, could be effectively reduced by hydrocarbons over an appropriate catalyst in the SCR process. Recent papers reported on an efficient combination of a NTP with appropriate catalysts in the HC-SCR of NOx reaction [6–8]. However, it should be noted that NOx would be synthesized in N2/O2 system in some NTP processes. As reported by our group, significant amounts of NOx formed from N2 and O2 have been observed in a one-stage plasma-over-catalyst (POC) reactor at temperatures above 350 °C, indicating that the special necessity of using low-temperature performance in plasma-catalytic processes for the removal of NOx [9]. C2H2, with unsaturated triple bond and its sp hybridized C–H bond, should be easily activated at lower
298
J. Niu et al. / Catalysis Communications 7 (2006) 297–301
temperatures. Therefore, in the present paper, by comparing the reductants of C2H2 with CH4 and C2H4, the catalytic properties of a Co-HZSM-5 catalyst in a POC reactor for the SCR of NOx were investigated with and without the assistance of a dielectric barrier discharge plasma. Broadened effective temperature window, better oxygen tolerance and enhanced CO2 selectivity were clearly observed in plasma-assisted C2H2-SCR of NOx over Co-HZSM-5 catalyst.
3. Results and discussion 3.1. HC-SCR of NOx with and without discharge Fig. 1a–c shows the conversions of NOx as functions of reaction temperature over Co-HZSM-5 catalyst with and without discharge and over quartz pellets with discharge using CH4, C2H4 and C2H2 as reductant. Without discharge, the Co-HZSM-5 catalyst shows different catalytic behaviors in the CH4-SCR, C2H4-SCR, C2H2-SCR of 100
2. Experimental
NOx conversion ð%Þ ¼
NOx;in NOx;out N2 Oout 100 NOx;in ð1Þ
where NOx,in, NOx,out and N2Oout stand for NOx concentration in the inlet and outlet gas, and the amount of N2O in the outlet, respectively.
NOx Conversion (%)
80 60 40 20 0 100
150
200
250
300 o T/ C
350
400
450
150
200
250
300 o T/ C
350
400
450
150
200
250
300 o T/ C
350
400
450
a 100
NOx Conversion (%)
80 60 40 20 0 100
b 100 80
NOx Conversion (%)
The POC reactor used in this work consisted of an outer quartz tube (i.d. 10 mm), a stainless steel tube (o.d. 3 mm) placed along the axis of the outer tube as the high voltage electrode and 2.8 ml of catalyst pellets (20–40 mesh) filled between the two tubes. A stainless steel wire mesh wound on the outside surface of the quartz tube was used as the ground electrode. The DBD power supply source was capable of supplying a bipolar sine wave output with 0–60 kV peak-to-peak voltage (Up) at an a.c. frequency of 50 Hz. The input electric discharge power (Pin) at a certain experimental condition was measured via the area of the voltage-charge Lissajous figures [10]. The input discharge energy density, Ein (J l1), was defined as Pin(W) divided by the inlet gas flow rate (l s1). The temperature of the plasma reactor was controlled by a temperature-programmed controller (Xiamen Yuguang Co., China), the average of Tc (the center temperature measured from the central stainless tube) and Tout (outside temperature of the outer quartz tube) was taken as the reaction temperature, Treac. Co-HZSM5 catalyst was prepared via a three-times repeated ionexchange method at 80 °C by using H-ZSM-5 (SiO2/ Al2O3 = 25, Nankai University, China) and aqueous cobalt acetate solutions, followed by drying at 120 °C for 8 h and calcination at 500 °C for 3 h after each ion exchange. A reacting gas mixture was prepared by a mass flow controller (MFC) (SevenStar Co., China) system. And a reacting gas flow rate corresponding to GHSV = 12 000 h1 was always used. Individual NO and NO2 concentration was monitored by a chemiluminescence analyzer (Monitor ML9841AS), the concentrations of CO2, CO, and N2O in the outflow gas were determined by an infrared absorption spectrometer (SICK-MAIHAK-S710). Before reaction, the catalyst was pretreated in a N2 stream at 500 °C for 2 h. NOx conversion was defined as follows:
60 40 20 0 100
c
Fig. 1. NOx conversion as functions of temperature over: (j) Co-HZSM5 catalyst alone, (d) discharge over Co-HZSM-5 catalyst and (m) discharge over quartz pellets (500 ppm NOx, 15% O2, N2 as balance gas, flow rate 520 ml min1, GHSV=12 000 h1, (a) 500 ppm C2H2, (b) 500 ppm C2H4 and (c) 1000 ppm CH4, Ein (150–250 °C) = 144 J l1, Ein (300–350 °C) = 138 J l1).
J. Niu et al. / Catalysis Communications 7 (2006) 297–301
299
NOx reactions. In CH4-SCR, only 15% conversion of NOx was obtained at 350 °C, and the maximum NOx conversion of 35% was achieved at 400 °C. By using C2H4 and C2H2 as reductants, the SCR reactions were initiated at much lower temperatures. At 150 °C, NOx conversion was 16% in the C2H4-SCR reaction and 24% in the C2H2-SCR reaction. The maximum conversion efficiency of NOx was 82% at 300 °C for C2H4 and 89% for C2H2 at 350 °C. When DBD plasma was operated over Co-HZSM-5 catalyst, the removal efficiency of NOx was greatly enhanced especially in lower temperature regions (150–300 °C). A synergistic effect was clearly observed in the C2H2/NO/ O2/N2 system. At 150 °C and Ein = 144 J l1, more than 50% of NOx was converted in plasma-catalytic process (discharge over Co-HZSM-5), which was larger than the sum of the pure catalytic efficiency of 23.7% and pure plasmainduced (discharge over quartz pellets) conversion percentage of 6.4%. The negative value came from the formation of a little amount of N2O. Conversion of NOx increased with the increase of temperature from 150 to 300 °C. At 300 °C and Ein = 138 J l1, pure plasma-induced and plasma-catalytic NOx conversion percentages are 2.9% and 93%. NOx conversion in the plasma-catalytic process is larger than the sum of it in pure catalytic and in pure plasma-induced process and which is also larger than the maximum value obtained in pure catalytic process at 350 °C. When temperature was increased to 350 °C, NOx conversion decreased to 89%. (The discharge temperature controlled was no higher than 350 °C, since the discharge above 350 °C was unstable.) But in the C2H4/NO/O2/N2 system, synergistic effects can be achieved only below 250 °C. This could be partly interpreted by the increase of the formation rate of NOx from N2 and O2 over the catalyst at higher temperatures [9]. However, by using CH4 as reductant, the pure catalytic, pure plasma-induced and plasmacatalytic NOx conversion percentages were 1.2%, 23% and 25%, respectively, at 150 °C. When the reaction temperature increased to 350 °C, only 5% of NOx was eliminated by discharge over quartz pellets and a little promotion effect on NOx conversion was observed in plasma-catalytic process. This indicated that the NOx conversion mainly came from the contribution of plasma decomposition at low temperatures and catalytic conversion might play a major role at high temperatures. It also could be seen that even in the presence of plasma and catalyst, NOx cannot be effectively reduced by methane at lower temperatures. Comparing the reductants CH4 and C2H4, as well as C2H2, it was clear that by using C2H2 as reductant, the catalyst showed higher activities even in the low temperature region of 150–350 °C, and obviously a synergistic effect between dielectric barrier discharge plasma and catalyst was also observed below 300 °C.
and without discharge by using C2H2, C2H4 and CH4 as reductants were shown in Fig. 2a–c. Without discharge, NOx conversion strongly depended on the oxygen content in the C2H2-SCR and C2H4-SCR reactions. The removal percentage of NOx increased from 13.8% to 60% in the C2H2-SCR reaction and 6.5% to 49% in the C2H4-SCR reaction with the increase of the oxygen content from 0 to 12%. Further increase of O2 concentration made the NOx conversion decrease. When the discharge was on, NOx removal was promoted, especially in the absence of oxygen, 73% and 68% of NOx conversions were achieved in NO/C2H2/N2 and NO/C2H4/N2 mixtures at 250 °C
3.2. Oxygen content effect on NOx removal
Fig. 2. Dependence of NOx conversion on oxygen content over: (j) CoHZSM-5 only and (d) Co-HZSM-5 plus ac DBD plasma (500 ppm NOx, (a) 500 ppm C2H2, (b) 500 ppm C2H4 and (c) 1000 ppm CH4, N2 as balance gas, flow rate 520 ml min1, GHSV = 12 000 h1, Ein (250 °C) = 144 J l1).
NOx conversion as a function of the oxygen content at 250 °C and Ein = 144 J l1 over Co-HZSM-5 catalyst with
NOx conversion (%)
100 80 60 40 20 0 0
2
4
6
8
10
12
14
16
14
16
Oxygen content (%)
a
NOx conversion (%)
100 80 60 40 20 0 0
2
4
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Oxygen content (%)
b
NOx conversion (%)
100 80 60 40 20 0 0
c
2
4
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Oxygen content (%)
J. Niu et al. / Catalysis Communications 7 (2006) 297–301
C2H2 to COx conversion(%)
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a
150 200 250 300 350 400 450 500 550
T/
o
C
100 80 60 40 20 0 100
b
150 200 250 300 350 400 450 500 550
T/
o
C
100
3.3. Hydrocarbon’s conversion to COx (x = 1,2) and CO2/CO ratio in HC-SCR of NOx with and without discharge Hydrocarbon’s conversion to COx (x = 1,2), defined by the ratio of the total amount of COx and the inlet carbon atom concentration of hydrocarbons, as a function of temperature were shown in Fig. 3a–c. With Co-HZSM-5 catalyst only, 12% of C2H2 and 8% of C2H4 were converted to COx at 150 °C, while only 2.5% of methane was oxidized to COx at 250 °C. 100% of C2H4 converted to COx (x = 1,2) occurred above 300 °C, and that of C2H2 did shift to 350 °C. As high as 500 °C was needed to fully convert methane over Co-HZSM-5 catalyst. With the plasma assistance, the hydrocarbon’s conversion to COx (x = 1,2) for C2H2 and C2H4 were significantly enhanced in the range of 150–300 °C (e.g. at 150 °C, they are 62% and 74% for C2H2 and C2H4, respectively). After correlation of these data with those in Fig. 1, it was obvious that the selectivity of C2H2 was higher than that of C2H4 for the SCR of NOx. For example, at 200 °C, the selectivities of the plasma catalytic processes for C2H2 and C2H4 were 80% and 52%, respectively. (Since the N2 production cannot be measured in our experiments, the selectivity here is defined by the
80
0 100
C2H4 to COx conversion(%)
and Ein = 144 J l1. The improved NOx conversion by discharge might come from the direct decomposition of NOx, since it was found that the NOx conversion could reach 62% in NO/N2 mixtures under the same conditions. NOx conversions decreased slightly when 2% of oxygen was added. With further increase in O2 content, the NOx conversions increased to get to the maximum value at 12% and 8% oxygen content in C2H2-SCR and C2H4-SCR reactions, respectively, and then decreased. The difference between pure catalytic and plasma-induced catalytic SCR reactions illustrated that NOx conversion might go through different pathways in the plasma catalytic process with the increase of oxygen content. In the oxygen content of 0–2%, the NOx removal mainly came from the discharge induced decomposition; with the oxygen content increase from 2% to 15%, due to the inhibition of plasma decomposition by the excess oxygen, the plasma-enhanced reduction by the reductants contributed the most percentage of NOx conversion. When CH4 was used as reductant, catalysis over CoHZSM-5 catalyst was neglective at 250 °C as shown in Fig. 2c, while NOx conversions were obviously enhanced when DBD plasma was on. NOx conversion was higher than 50% in the absence of O2, and decreased with the increasing O2 content, the conversions could be maintained at ca. 30% in the O2 content of about 12%. This suggested that NOx conversion mainly came from the direct decomposition of NOx by DBD plasma and higher O2 content might inhibit the decomposition reactions. From another point of view, it was shown that the plasma catalytic process could be operated over a wide oxygen concentration range from 0 to 15% both in the C2H2-SCR and C2H4-SCR systems; and C2H2 contained system was more tolerant to oxygen than C2H4.
CH 4 to COx conversion(%)
300
80 60
40 20 0 100
c
150 200 250 300 350 400 450 500 550
T/
o
C
Fig. 3. Hydrocarbon’s conversion to COx (x = 1,2) as functions of temperature over: (j) Co-HZSM-5 only and (d) Co-HZSM-5 plus ac DBD plasma. (500 ppm NOx, 15% O2, N2 as balance gas, flow rate 520 ml min1, GHSV=12 000 h1, (a) 500 ppm C2H2, (b) 500 ppm C2H4 and (c) 1000 ppm CH4, Ein (150–250 °C) = 144 J l1, Ein (300–350 °C) = 138 J l1).
ratio of the NOx conversion and the hydrocarbon’s conversion to COx.) For CH4, only above 300 °C, its conversion to COx (x = 1,2), could be enhanced. This indicated that CH4 was more difficult to be activated than C2H4 and C2H2 even in the combination of DBD plasma with the Co-HZSM-5 catalyst. Another aspect which should be noted is the CO2/CO ratio. As listed in Table 1, the CO2/CO ratio was increased with the increase of temperature and the amount of CO produced at high temperatures can be neglected with C2H2, C2H4 and CH4 as reductants. With the assistance
J. Niu et al. / Catalysis Communications 7 (2006) 297–301
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Table 1 CO2/CO ratio in HC-SCR of NOx at different temperatures over Co-HZSM-5 catalyst with and without plasma (reaction conditions are the same as in Fig. 1) Temperature/°C
150 200 250 300 350
C2H2-SCR
C2H4-SCR
CH4-SCR
Catalyst
Catalyst + plasma
Catalyst
Catalyst + plasma
Catalyst
Catalyst + plasma
1.1 1.2 1.3 1.9 94.9
1.2 1.2 1.4 2.9 216.4
1.0 1.4 2.1 6.6 96.3
0.9 0.9 1.6 11.7 98.6
– – – 11.8 73.7
– – – 14.3 103.7
of plasma, at higher temperatures (300–350 °C), the CO2 selectivities were somehow enhanced in C2H4-SCR, C2H2-SCR, as well as in CH4-SCR reactions.
20203002), the National High Technology Research and Development Program (‘‘863 Programm’’) of China (Grant No. 2002AA649140) and the Provincial Grants of Science and Technology of Liaoning, China (No. 20022112).
4. Conclusions A synergistic effect between dielectric barrier discharge plasma and catalyst was observed in C2H2-SCR of NOx at 150–300 °C in the presence of excess oxygen. Over one-stage POC reactor, more than 50% conversion of NOx can be obtained at a broad temperature window (150–450 °C): at low temperatures (150–300 °C), it can be realized by the combination of plasma with catalyst; at higher temperatures (above 300 °C), catalyst alone can put it into practice (the plasma is turned off). Acknowledgements This work is supported by National Natural Science Foundation of China (Grant Nos. 20077005 and
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