- Email: [email protected]
ACF catalysts for NO reduction with NH3 at low temperature
Catalysis Communications 10 (2009) 1538–1541
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Effect of SO2 on V2O5/ACF catalysts for NO reduction with NH3 at low temperature Yaqin Hou a,b, Zhanggen Huang a,*, Shijie Guo a a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Graduate University of Chinese Academy of Science, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2008 Received in revised form 13 February 2009 Accepted 2 April 2009 Available online 11 April 2009 Keywords: V2O5/ACF catalyst NO reduction Selective catalytic reduction SO2
a b s t r a c t Effect of SO2 on an activated carbon fiber-supported V2O5 (V2O5/ACF) for the low temperature (120–200 °C) selective catalytic reduction of NO with NH3 is studied using a transient method, FT-IR and MS. Results show that SO2 can significantly promote V2O5/ACF’s SCR activity when the V2O5 loading is lower than 5 wt%. The promoting of SO2 may be attributed to the sulfate species formed on the surface of V2O5/ACF catalyst, which can increase the surface acidity and NH3 adsorption. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx) in flue gas are major air pollutants which must be removed before emitting. Selective catalytic reduction (SCR) of NO with NH3 has been proven to be the most effective technology for NOx removal from flue gases [1]. The commercial catalyst is V2O5/TiO2, which has to be used at temperature higher than 350 °C to avoid catalyst deactivation by SO2 [2]. Low temperature SCR catalysts have been studied because of energy savings and easy retrofitting to existing boiler systems. Recently carbon materials are becoming excellent support for NO reduction at low temperature [3–5]. However, most researchers concluded that SO2 has a serious poisoning effect on SCR catalytic activity at low temperature [6–9]. Recently, Carja et al. [10] synthesized Mn–Ce/ZSM-5 catalyst exhibited 75–100% NO conversion within a broad temperature window (240–500 °C). Interestingly, SO2 had only slight effects on its activity. And it was found that the resistance to SO2 could be greatly enhanced for Ce modified Mn/TiO2 catalysts [11]. Activated carbon fibers (ACFs) can be used widely in water and air purification. Because there are many characteristics of catalyst and catalyst supports on ACF, such as microporous structure, relatively larger specific surface area and a variety of surface functional groups [12]. In addition, the fiber size being small (lm), the particle mass transfer resistance is almost negligible. Mochida et al. [13,14] proved the feasibility of using PAN and pitch-based ACF for low temperature de-NOx. Muniz et al. [15] showed that the high activity of NOx reduction at temperatures below 250 °C could be * Corresponding author. Tel./fax: +86 351 4043727. E-mail addresses: [email protected], [email protected] (Z. Huang). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.04.003
obtained with chemically and thermally treated ACF. Park et al. [16,17] reported the loading of Ag, Cu, Ni on ACF increased the catalytic activity of NO reduction. However, SO2 exists inevitably in the fuel gas, and the effect of which on the SCR reaction of ACF has still relatively unexplored. In this work, a novel V2O5/ACF was explored for NOx removal in the presence of SO2, and then the effect mechanism of SO2 on SCR activity of the V2O5/ACF catalyst was studied through a transient method, FT-IR and MS analyses. 2. Experimental 2.1. Catalyst preparation The support, cellulose activated carbon fiber (ACF), was obtained from Nantong Senyou Carbon Fiber Co. Its surface area was measured by BET method to 1032 m2/g. The V2O5/ACF catalysts were prepared by pore volume impregnation of ammonium metavanadate in oxalic acid. The V2O5 loading on the catalysts were determined by the concentration of ammonium metavanadate and confirmed by ICP analysis. After the impregnation, the samples were dried at 50 °C for 12 h and then at 110 °C for 6 h, followed by calcination in an Ar stream for 8 h at 500 °C and by preoxidation in air for 5 h. The catalyst used in this work contains 2 wt% V2O5 is termed V2/ACF for the sake of clarity. 2.2. Catalytic tests SCR activity of the catalysts for NO reduction was carried out in a fixed-bed glass reactor at 120–200 °C, with 1.5 g catalyst and 400 ml/min of gas flow rate corresponding to a GHSV of
1539
Y. Hou et al. / Catalysis Communications 10 (2009) 1538–1541
2000 h1. The simulated flue gas was a mixture of 600 ppm NO, 600 ppm NH3, 5% O2, 300 ppm SO2 (when used) and Ar to balance. The concentrations of NO, SO2 and O2 in the inlet and outlet of the reactor were simultaneously measured on-line by a flue gas analyzer (KM9106 Quintox, Kane International Limited). The SCR activity of the catalysts was expressed by the equation
X NO ¼
C in C out 100% C in
where Cin is the inlet concentration of NO, Cout is the outlet concentration of NO. 2.3. Catalyst characterization Temperature programmed desorption (TPD) of the sulfate formed on the surface of the V2O5/ACF catalyst was carried out in an Ar stream of 400 ml/min at a heating rate of 10 °C/min from 50 to 500 °C. The effluent SO2 concentration was continuously monitored using the flue gas analyzer during the whole process. FT-IR spectra were recorded on a Bio-Rad FTS 165 FT-IR spectrometer. The catalyst samples were mixed with potassium bromide (weight ratio of 1:400). Thirty-two scans were made and averaged to yield a spectrum with resolution of 4 cm1 over the spectral range of 400–4000 cm1. To clarify the effect of SO2 on the number and strength of acid sites over V2O5/ACF catalyst, adsorption of NH3 was performed in a fixed-bed glass reactor. About 0.6 g catalyst was preheated to 300 °C in a flow rate of 100 ml/min Ar for 2 h to eliminate water. The sample was then cooled to 50 °C in Ar and then exposed to a stream containing 1500 ppm NH3 and balance Ar. The outlet gas was monitored on-line by a mass spectrometer (Balzers QMS422) during the whole process. The release of NH3 was monitored by tracing the signal of m/e = 17.
3. Results and discussion 3.1. Effect of SO2 on different V2O5 loading Effect of SO2 on SCR activity of the V2O5/ACF catalyst is illustrated in Fig. 1 and compared with that of ACF. Under the employed reaction conditions without SO2, ACF shows relatively low activity for SCR reaction, with NO conversion of about 7%. By an introduction of very small amount of vanadium (0.5 wt%) on the
X NO / %
80
60
3.2. Promoting mechanism of SO2 To study the reason for the promoting role of SO2 on the lower V2O5 loadings catalyst, a transient response reaction of SO2 was carried out. The experimental procedure and the conditions are similar to the Fig. 1, and the results are shown in Fig. 2. The NO conversion increases from 50% to a steady state of 90% upon introduction of 300 ppm SO2 in 50 min. After a period of a steady state reaction, we remove the SO2 from the feed gas. However, more interestingly, the removal of SO2 does not cause any decline in NO conversion but exhibits a slightly increase. These results suggest the promoting effect of SO2 is not from the gas phase SO2 but from some sulfur-containing species formed and deposited on the catalyst surface. In the above study, the dependence on the V2O5 loading (see Fig. 1) suggests that the effect of SO2 is somehow with the ACF surface uncovered by the vanadium species. However, the support ACF itself shows very low and unstable activity in the presence of SO2. This phenomenon indicates that the promoting effect of
5wt%V2 O5 /ACF
100
2wt%V2 O5 /ACF
90 80
0.5wt%V2 O5 /ACF
XNO/ %
100
ACF, NO conversion can be significantly enhanced to 50% and further increased with increased V2O5 loading. It is necessary to point out that some NO2 and N2O can be formed in the studied operating conditions. But, when we measure the outlet fuel gas by the combustion analyzer KM9106 Quintox, only a small concentration of NO2 can be observed (<10 ppm), so in order to describe conveniently, this manuscript focuses on concentration of NO mainly. A higher NO conversion can be obtained up to 96% when V2O5 loading is up to 5 wt%. And the high NO conversion was mainly attributed to the chemical reaction, because the breakthrough time of NO adsorption is only 7 min. Upon addition of SO2 into the feed gas, the catalytic activity is significantly promoted by SO2 when the loading of V2O5 is lower than 5 wt%. Unlike the other vanadium catalysts supported on Al2O3, SiO2, which are easily poisoned by SO2 [18]. The activity increases sharply and reaches a steady-state NO conversion of 90% when a low V2O5 loading of 0.5 wt%. More interestingly, although the 5 wt% V2O5/ACF catalyst shows high activity without SO2, the addition of SO2 in the feed gas does not increase its activity, but NO conversion decreases quickly to a steady-state about 92%. These results clearly indicate that V2O5 loading is crucial for the SCR activity of the V2O5/ACF catalysts in the presence of SO2. At lower V2O5 loadings, SO2 plays a promoting role, but at higher V2O5 loadings, it acts as a poison.
40
without SO2
with 300ppm SO2
removal SO 2
70 60
20
ACF 0
50
additional SO2
0
20
40
60
80
100
120
140
160
Time on stream/ min
40 0
50
100
150
200
250
Time on stream / min Fig. 1. Effect of SO2 on the activity of V2O5/ACF and ACF catalysts. Reaction conditions: 600 ppm NO, 600 ppm NH3, 5% O2, 300 ppm SO2 (when used), Ar balance; GHSV of 2000 h1; Reaction temperature of 180 °C.
Fig. 2. The SO2 transient response of the SCR reaction over 0.5 wt% V2O5/ACF. Reaction conditions are the same in Fig. 1.
Y. Hou et al. / Catalysis Communications 10 (2009) 1538–1541
V0.5/ACF+SO2 +SCR 3435
Transmitance ( a.u. )
SO2 on the V2O5/ACF catalyst may result from a coordinated mechanism involving both ACF and V2O5. So a TPD experiment of SO2 + O2 pre-treatment for ACF, V2O5, V2O5/ACF was carried out and the results are plotted in Fig. 3. It shows that the V2O5/ACF catalyst can adsorb a considerable amount of SO2, while the ACF or V2O5 does not. To further explore which form of the adsorbed SO2 over V2O5/ ACF catalyst, the catalyst samples before and after the SCR reaction in the presence of SO2 are analyzed by FT-IR, as shown in Fig. 4. As a reference, the spectra of ACF were also shown in the Fig. 4, and there was not any signal for it. But for the fresh catalyst 0.5 wt% V2O5/ACF catalyst before SCR reaction, a broad band in the range of 400–1300 cm1 was found, and the adsorption peak was at 650 and 1079 cm1. Frederickson [19] has reported the bulk V2O5 showing two IR adsorption peaks of 1020 and 825 cm1, respectively, which are corresponding to the stretching vibration of V5+@O and the V–O–V. So the peak at 1079 cm1 may be attributed to the stretching vibration of V5+@O, which is similar to the V2O5/ AC catalyst [20]. And it is presumed that the shift of the adsorption peaks of 1020–1079 cm1 may be ascribed to the cooperation of the active phase V2O5 and the support ACF. After the SCR reaction in the presence of SO2, the IR spectra has a great variety mainly in the range of 400–1300 cm1, they split into some new spectra, such as 612 and 1110 cm1. And the peak at 612 and 1110 cm1 can be attributed to the adsorption peak of SO2 4 . Nakamoto [21] and Chen and Yang [22] had reported that SO2 4 possessed a Td symmetric characteristic, which showed two IR adsorption peak at 1104 and 613 cm1, respectively. The catalyst after SCR reaction in the presence of SO2 was proved to contain SO2 4 by FT-IR, which indicated that the adsorbed SO2 over V2O5/ ACF catalyst was in the form of SO2 4 . It is true that SO2 can be oxidized to SO3 catalyzed by vanadium oxides, because we can not measure the sulfur trioxide concentration through the combustion analyzer KM9106 Quintox, so we did not analyze SO3 in the outlet of flue gas, but from the result of FT-IR, it is sure that some adsorption of SO3 or SO2 4 was formed on the catalyst surface.
3137
1395 1110
V0.5/ACF
612
1300
3435
1551 1079
650
ACF
4000
3000
2000
1000 -1
Wavenumber ( cm ) Fig. 4. FT-IR spectra of the ACF and the 0.5 wt% V2O5/ACF catalysts before and after SCR reaction in the presence of SO2 at 180 °C.
ACF
MS Intensity of NH3( a.u.)
1540
0
1000
2000
3000
4000
5000
6000
7000
8000
3000
4000
5000
6000
7000
8000
5000
6000
7000
8000
V0.5/ACF
0
1000
2000
(SO2+O2)-pretreated V0.5/ACF
3.3. Effect of SO2 on the NH3 adsorption It was reported that NH3 adsorption was a key step in the SCR reaction and the role of NO adsorption is negligible [23], so in order to further realize the promoting role of SO2, it is necessary to study the effect of SO2 to the NH3 adsorption. Amount of acidic sites on a catalyst surface can be characterized by the amount of NH3 adsorp-
SO2 concentration / ppm
4000 3200
V0.5/ACF ACF
2400
V2 O5
1600
0
1000
2000
3000
4000
Time (second) Fig. 5. Effluent NH3 profile during isothermal adsorption of NH3 at 50 °C on ACF, V0.5/ACF and SO2 + O2 pretreated V0.5/ACF.
tion. Fig. 5 shows effluent NH3 profiles measured by the MS during NH3 adsorption on ACF, V0.5/ACF, and V0.5/ACF pre-treated with SO2 + O2. The ACF has the lowest NH3 adsorption capacity with a breakthrough time of only 800 s. V0.5/ACF has a better NH3 adsorption capacity with a breakthrough time of 3100 s, indicating that ACF loaded by V2O5 can significantly enhance the adsorption ability of NH3. Compared with V0.5/ACF, V0.5/ACF pre-treated with SO2 + O2 shows the highest NH3 adsorption capacity with a breakthrough time of 6188 s, corresponding to the two times of V0.5/ ACF itself. Combined to the results in Figs. 1 and 5, it is possibly presumed that the promoting of SO2 can be ascribed to the formation of SO2 4 , which has the ability to enhance NH3 adsorption.
800
4. Conclusions 0 200
250
300
350
400
450
500
O
Temperature / C Fig. 3. TPD patterns of SO2 on different catalysts after SO2 + O2 pre-treatment.
The effect of SO2 on the catalysis activity of V2O5/ACF catalysts for low temperature SCR reaction depends strongly on the loading of V2O5. When the loading of V2O5 is lower than 5 wt%, SO2 plays a positive effect. Otherwise, it plays a negative effect. And the promoting effect of SO2 is not from the gas phase SO2 but from some sulfur-containing species formed and deposited on the catalyst
Y. Hou et al. / Catalysis Communications 10 (2009) 1538–1541
surface. The kinds of SO2 adsorbed on the catalyst surface can be recognized as SO2 4 , which can increase the surface acidity and NH3 adsorption on the V2O5/ACF catalyst, and thus enhance the SCR activity. The disappearance of NO with the reaction time was mainly attributed to the chemical reaction not the adsorption. Acknowledgments The authors gratefully acknowledge financial support from the National High-Tech Research and Development Program (2007AA05Z310), the National Nature Science Funds (20877078) and the Institute of Coal Chemistry Open Funds of the State Key Laboratory of Coal Conversion (No. 08-309). References [1] X.L. Zhang, Z.G. Huang, Z.Y. Liu, Catal. Commun. 9 (2008) 842. [2] H. Bosch, F. Janssen, Catal. Today 2 (1998) 369. [3] A. Boyano, N. Lombardo, M.E. Galvez, M.J. Lazaro, R. Moliner, Chem. Eng. J. 144 (2008) 347.
1541
[4] M.E. Galvez, A. Boyano, M.J. Lazaro, R. Moliner, Chem. Eng. J. 144 (2008) 12. [5] A. Boyano, M.C. Iritia, I. Malpartida, M.A. Larrubia, L.J. Alemany, R. Moliner, M.J. Lazaro, Catal. Today 137 (2008) 226. [6] W.S. Kijlstra, D.S. Brands, E.K. Poels, A. Bliek, Catal. Today 50 (1999) 133. [7] W. Sjoerd, M. Biervliet, K. Poels, Appl. Catal. B: Environ. 16 (1998) 327. [8] Z.P. Zhu, Z.Y. Liu, S.J. Liu, H.X. Niu, T.D. Hu, T. Liu, Y.N. Xie, Appl. Catal. B: Environ. 26 (2000) 25. [9] Y. Huang, Z.Q. Tong, B. Wu, J.F. Zhang, J. Fuel Chem. Technol. 36 (2008) 616. [10] G. Carja, Y. Kameshima, K. Okada, C.D. Madhusoodana, Appl. Catal. B 73 (2007) 60. [11] Z.B. Wu, R.B. Jin, H.Q. Wang, Y. Liu, Catal. Commun. 10 (2009) 935. [12] G. Vivekanand, A. Ritesh, V. Nishith, Carbon 44 (2006) 60. [13] I. Mochida, M. Ogaki, H. Fujitsu, Fuel 64 (1985) 1054. [14] I. Mochida, S. Kawana, S. Kisamori, H. Fujitsu, Carbon 32 (1994) 1750. [15] J. Muniz, G. Marban, A.B. Fuertes, Appl. Catal. B 27 (2000) 36. [16] S.-J. Park, B.-J. Kim, J. Colloid Interface Sci. 282 (2005) 124. [17] S.-J. Park, J.-S. Shin, J. Colloid Interface Sci. 264 (2003) 39. [18] F. Forzatti, Catal Today 62 (2000) 51. [19] L. Frederickson, D.M. Hausen, Anal. Chem. 35 (1963) 818. [20] Z.P. Zhu, H.X. Niu, Z. Y Liu, S.J. Liu, J. Catal. 197 (2000) 6. [21] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, fourth ed., Wiley, New York, 1986. [22] J.P. Chen, R.T. Yang, J. Catal. 139 (1993) 277. [23] Z.G. Huang, Z.P. Zhu, Z.Y. Liu, Q.Y. Liu, J. Catal. 214 (2003) 213.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Effect of SO2 on V2O5/ACF catalysts for NO reduction with NH3 at low temperature Yaqin Hou a,b, Zhanggen Huang a,*, Shijie Guo a a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Graduate University of Chinese Academy of Science, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2008 Received in revised form 13 February 2009 Accepted 2 April 2009 Available online 11 April 2009 Keywords: V2O5/ACF catalyst NO reduction Selective catalytic reduction SO2
a b s t r a c t Effect of SO2 on an activated carbon fiber-supported V2O5 (V2O5/ACF) for the low temperature (120–200 °C) selective catalytic reduction of NO with NH3 is studied using a transient method, FT-IR and MS. Results show that SO2 can significantly promote V2O5/ACF’s SCR activity when the V2O5 loading is lower than 5 wt%. The promoting of SO2 may be attributed to the sulfate species formed on the surface of V2O5/ACF catalyst, which can increase the surface acidity and NH3 adsorption. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx) in flue gas are major air pollutants which must be removed before emitting. Selective catalytic reduction (SCR) of NO with NH3 has been proven to be the most effective technology for NOx removal from flue gases [1]. The commercial catalyst is V2O5/TiO2, which has to be used at temperature higher than 350 °C to avoid catalyst deactivation by SO2 [2]. Low temperature SCR catalysts have been studied because of energy savings and easy retrofitting to existing boiler systems. Recently carbon materials are becoming excellent support for NO reduction at low temperature [3–5]. However, most researchers concluded that SO2 has a serious poisoning effect on SCR catalytic activity at low temperature [6–9]. Recently, Carja et al. [10] synthesized Mn–Ce/ZSM-5 catalyst exhibited 75–100% NO conversion within a broad temperature window (240–500 °C). Interestingly, SO2 had only slight effects on its activity. And it was found that the resistance to SO2 could be greatly enhanced for Ce modified Mn/TiO2 catalysts [11]. Activated carbon fibers (ACFs) can be used widely in water and air purification. Because there are many characteristics of catalyst and catalyst supports on ACF, such as microporous structure, relatively larger specific surface area and a variety of surface functional groups [12]. In addition, the fiber size being small (lm), the particle mass transfer resistance is almost negligible. Mochida et al. [13,14] proved the feasibility of using PAN and pitch-based ACF for low temperature de-NOx. Muniz et al. [15] showed that the high activity of NOx reduction at temperatures below 250 °C could be * Corresponding author. Tel./fax: +86 351 4043727. E-mail addresses: [email protected], [email protected] (Z. Huang). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.04.003
obtained with chemically and thermally treated ACF. Park et al. [16,17] reported the loading of Ag, Cu, Ni on ACF increased the catalytic activity of NO reduction. However, SO2 exists inevitably in the fuel gas, and the effect of which on the SCR reaction of ACF has still relatively unexplored. In this work, a novel V2O5/ACF was explored for NOx removal in the presence of SO2, and then the effect mechanism of SO2 on SCR activity of the V2O5/ACF catalyst was studied through a transient method, FT-IR and MS analyses. 2. Experimental 2.1. Catalyst preparation The support, cellulose activated carbon fiber (ACF), was obtained from Nantong Senyou Carbon Fiber Co. Its surface area was measured by BET method to 1032 m2/g. The V2O5/ACF catalysts were prepared by pore volume impregnation of ammonium metavanadate in oxalic acid. The V2O5 loading on the catalysts were determined by the concentration of ammonium metavanadate and confirmed by ICP analysis. After the impregnation, the samples were dried at 50 °C for 12 h and then at 110 °C for 6 h, followed by calcination in an Ar stream for 8 h at 500 °C and by preoxidation in air for 5 h. The catalyst used in this work contains 2 wt% V2O5 is termed V2/ACF for the sake of clarity. 2.2. Catalytic tests SCR activity of the catalysts for NO reduction was carried out in a fixed-bed glass reactor at 120–200 °C, with 1.5 g catalyst and 400 ml/min of gas flow rate corresponding to a GHSV of
1539
Y. Hou et al. / Catalysis Communications 10 (2009) 1538–1541
2000 h1. The simulated flue gas was a mixture of 600 ppm NO, 600 ppm NH3, 5% O2, 300 ppm SO2 (when used) and Ar to balance. The concentrations of NO, SO2 and O2 in the inlet and outlet of the reactor were simultaneously measured on-line by a flue gas analyzer (KM9106 Quintox, Kane International Limited). The SCR activity of the catalysts was expressed by the equation
X NO ¼
C in C out 100% C in
where Cin is the inlet concentration of NO, Cout is the outlet concentration of NO. 2.3. Catalyst characterization Temperature programmed desorption (TPD) of the sulfate formed on the surface of the V2O5/ACF catalyst was carried out in an Ar stream of 400 ml/min at a heating rate of 10 °C/min from 50 to 500 °C. The effluent SO2 concentration was continuously monitored using the flue gas analyzer during the whole process. FT-IR spectra were recorded on a Bio-Rad FTS 165 FT-IR spectrometer. The catalyst samples were mixed with potassium bromide (weight ratio of 1:400). Thirty-two scans were made and averaged to yield a spectrum with resolution of 4 cm1 over the spectral range of 400–4000 cm1. To clarify the effect of SO2 on the number and strength of acid sites over V2O5/ACF catalyst, adsorption of NH3 was performed in a fixed-bed glass reactor. About 0.6 g catalyst was preheated to 300 °C in a flow rate of 100 ml/min Ar for 2 h to eliminate water. The sample was then cooled to 50 °C in Ar and then exposed to a stream containing 1500 ppm NH3 and balance Ar. The outlet gas was monitored on-line by a mass spectrometer (Balzers QMS422) during the whole process. The release of NH3 was monitored by tracing the signal of m/e = 17.
3. Results and discussion 3.1. Effect of SO2 on different V2O5 loading Effect of SO2 on SCR activity of the V2O5/ACF catalyst is illustrated in Fig. 1 and compared with that of ACF. Under the employed reaction conditions without SO2, ACF shows relatively low activity for SCR reaction, with NO conversion of about 7%. By an introduction of very small amount of vanadium (0.5 wt%) on the
X NO / %
80
60
3.2. Promoting mechanism of SO2 To study the reason for the promoting role of SO2 on the lower V2O5 loadings catalyst, a transient response reaction of SO2 was carried out. The experimental procedure and the conditions are similar to the Fig. 1, and the results are shown in Fig. 2. The NO conversion increases from 50% to a steady state of 90% upon introduction of 300 ppm SO2 in 50 min. After a period of a steady state reaction, we remove the SO2 from the feed gas. However, more interestingly, the removal of SO2 does not cause any decline in NO conversion but exhibits a slightly increase. These results suggest the promoting effect of SO2 is not from the gas phase SO2 but from some sulfur-containing species formed and deposited on the catalyst surface. In the above study, the dependence on the V2O5 loading (see Fig. 1) suggests that the effect of SO2 is somehow with the ACF surface uncovered by the vanadium species. However, the support ACF itself shows very low and unstable activity in the presence of SO2. This phenomenon indicates that the promoting effect of
5wt%V2 O5 /ACF
100
2wt%V2 O5 /ACF
90 80
0.5wt%V2 O5 /ACF
XNO/ %
100
ACF, NO conversion can be significantly enhanced to 50% and further increased with increased V2O5 loading. It is necessary to point out that some NO2 and N2O can be formed in the studied operating conditions. But, when we measure the outlet fuel gas by the combustion analyzer KM9106 Quintox, only a small concentration of NO2 can be observed (<10 ppm), so in order to describe conveniently, this manuscript focuses on concentration of NO mainly. A higher NO conversion can be obtained up to 96% when V2O5 loading is up to 5 wt%. And the high NO conversion was mainly attributed to the chemical reaction, because the breakthrough time of NO adsorption is only 7 min. Upon addition of SO2 into the feed gas, the catalytic activity is significantly promoted by SO2 when the loading of V2O5 is lower than 5 wt%. Unlike the other vanadium catalysts supported on Al2O3, SiO2, which are easily poisoned by SO2 [18]. The activity increases sharply and reaches a steady-state NO conversion of 90% when a low V2O5 loading of 0.5 wt%. More interestingly, although the 5 wt% V2O5/ACF catalyst shows high activity without SO2, the addition of SO2 in the feed gas does not increase its activity, but NO conversion decreases quickly to a steady-state about 92%. These results clearly indicate that V2O5 loading is crucial for the SCR activity of the V2O5/ACF catalysts in the presence of SO2. At lower V2O5 loadings, SO2 plays a promoting role, but at higher V2O5 loadings, it acts as a poison.
40
without SO2
with 300ppm SO2
removal SO 2
70 60
20
ACF 0
50
additional SO2
0
20
40
60
80
100
120
140
160
Time on stream/ min
40 0
50
100
150
200
250
Time on stream / min Fig. 1. Effect of SO2 on the activity of V2O5/ACF and ACF catalysts. Reaction conditions: 600 ppm NO, 600 ppm NH3, 5% O2, 300 ppm SO2 (when used), Ar balance; GHSV of 2000 h1; Reaction temperature of 180 °C.
Fig. 2. The SO2 transient response of the SCR reaction over 0.5 wt% V2O5/ACF. Reaction conditions are the same in Fig. 1.
Y. Hou et al. / Catalysis Communications 10 (2009) 1538–1541
V0.5/ACF+SO2 +SCR 3435
Transmitance ( a.u. )
SO2 on the V2O5/ACF catalyst may result from a coordinated mechanism involving both ACF and V2O5. So a TPD experiment of SO2 + O2 pre-treatment for ACF, V2O5, V2O5/ACF was carried out and the results are plotted in Fig. 3. It shows that the V2O5/ACF catalyst can adsorb a considerable amount of SO2, while the ACF or V2O5 does not. To further explore which form of the adsorbed SO2 over V2O5/ ACF catalyst, the catalyst samples before and after the SCR reaction in the presence of SO2 are analyzed by FT-IR, as shown in Fig. 4. As a reference, the spectra of ACF were also shown in the Fig. 4, and there was not any signal for it. But for the fresh catalyst 0.5 wt% V2O5/ACF catalyst before SCR reaction, a broad band in the range of 400–1300 cm1 was found, and the adsorption peak was at 650 and 1079 cm1. Frederickson [19] has reported the bulk V2O5 showing two IR adsorption peaks of 1020 and 825 cm1, respectively, which are corresponding to the stretching vibration of V5+@O and the V–O–V. So the peak at 1079 cm1 may be attributed to the stretching vibration of V5+@O, which is similar to the V2O5/ AC catalyst [20]. And it is presumed that the shift of the adsorption peaks of 1020–1079 cm1 may be ascribed to the cooperation of the active phase V2O5 and the support ACF. After the SCR reaction in the presence of SO2, the IR spectra has a great variety mainly in the range of 400–1300 cm1, they split into some new spectra, such as 612 and 1110 cm1. And the peak at 612 and 1110 cm1 can be attributed to the adsorption peak of SO2 4 . Nakamoto [21] and Chen and Yang [22] had reported that SO2 4 possessed a Td symmetric characteristic, which showed two IR adsorption peak at 1104 and 613 cm1, respectively. The catalyst after SCR reaction in the presence of SO2 was proved to contain SO2 4 by FT-IR, which indicated that the adsorbed SO2 over V2O5/ ACF catalyst was in the form of SO2 4 . It is true that SO2 can be oxidized to SO3 catalyzed by vanadium oxides, because we can not measure the sulfur trioxide concentration through the combustion analyzer KM9106 Quintox, so we did not analyze SO3 in the outlet of flue gas, but from the result of FT-IR, it is sure that some adsorption of SO3 or SO2 4 was formed on the catalyst surface.
3137
1395 1110
V0.5/ACF
612
1300
3435
1551 1079
650
ACF
4000
3000
2000
1000 -1
Wavenumber ( cm ) Fig. 4. FT-IR spectra of the ACF and the 0.5 wt% V2O5/ACF catalysts before and after SCR reaction in the presence of SO2 at 180 °C.
ACF
MS Intensity of NH3( a.u.)
1540
0
1000
2000
3000
4000
5000
6000
7000
8000
3000
4000
5000
6000
7000
8000
5000
6000
7000
8000
V0.5/ACF
0
1000
2000
(SO2+O2)-pretreated V0.5/ACF
3.3. Effect of SO2 on the NH3 adsorption It was reported that NH3 adsorption was a key step in the SCR reaction and the role of NO adsorption is negligible [23], so in order to further realize the promoting role of SO2, it is necessary to study the effect of SO2 to the NH3 adsorption. Amount of acidic sites on a catalyst surface can be characterized by the amount of NH3 adsorp-
SO2 concentration / ppm
4000 3200
V0.5/ACF ACF
2400
V2 O5
1600
0
1000
2000
3000
4000
Time (second) Fig. 5. Effluent NH3 profile during isothermal adsorption of NH3 at 50 °C on ACF, V0.5/ACF and SO2 + O2 pretreated V0.5/ACF.
tion. Fig. 5 shows effluent NH3 profiles measured by the MS during NH3 adsorption on ACF, V0.5/ACF, and V0.5/ACF pre-treated with SO2 + O2. The ACF has the lowest NH3 adsorption capacity with a breakthrough time of only 800 s. V0.5/ACF has a better NH3 adsorption capacity with a breakthrough time of 3100 s, indicating that ACF loaded by V2O5 can significantly enhance the adsorption ability of NH3. Compared with V0.5/ACF, V0.5/ACF pre-treated with SO2 + O2 shows the highest NH3 adsorption capacity with a breakthrough time of 6188 s, corresponding to the two times of V0.5/ ACF itself. Combined to the results in Figs. 1 and 5, it is possibly presumed that the promoting of SO2 can be ascribed to the formation of SO2 4 , which has the ability to enhance NH3 adsorption.
800
4. Conclusions 0 200
250
300
350
400
450
500
O
Temperature / C Fig. 3. TPD patterns of SO2 on different catalysts after SO2 + O2 pre-treatment.
The effect of SO2 on the catalysis activity of V2O5/ACF catalysts for low temperature SCR reaction depends strongly on the loading of V2O5. When the loading of V2O5 is lower than 5 wt%, SO2 plays a positive effect. Otherwise, it plays a negative effect. And the promoting effect of SO2 is not from the gas phase SO2 but from some sulfur-containing species formed and deposited on the catalyst
Y. Hou et al. / Catalysis Communications 10 (2009) 1538–1541
surface. The kinds of SO2 adsorbed on the catalyst surface can be recognized as SO2 4 , which can increase the surface acidity and NH3 adsorption on the V2O5/ACF catalyst, and thus enhance the SCR activity. The disappearance of NO with the reaction time was mainly attributed to the chemical reaction not the adsorption. Acknowledgments The authors gratefully acknowledge financial support from the National High-Tech Research and Development Program (2007AA05Z310), the National Nature Science Funds (20877078) and the Institute of Coal Chemistry Open Funds of the State Key Laboratory of Coal Conversion (No. 08-309). References [1] X.L. Zhang, Z.G. Huang, Z.Y. Liu, Catal. Commun. 9 (2008) 842. [2] H. Bosch, F. Janssen, Catal. Today 2 (1998) 369. [3] A. Boyano, N. Lombardo, M.E. Galvez, M.J. Lazaro, R. Moliner, Chem. Eng. J. 144 (2008) 347.
1541
[4] M.E. Galvez, A. Boyano, M.J. Lazaro, R. Moliner, Chem. Eng. J. 144 (2008) 12. [5] A. Boyano, M.C. Iritia, I. Malpartida, M.A. Larrubia, L.J. Alemany, R. Moliner, M.J. Lazaro, Catal. Today 137 (2008) 226. [6] W.S. Kijlstra, D.S. Brands, E.K. Poels, A. Bliek, Catal. Today 50 (1999) 133. [7] W. Sjoerd, M. Biervliet, K. Poels, Appl. Catal. B: Environ. 16 (1998) 327. [8] Z.P. Zhu, Z.Y. Liu, S.J. Liu, H.X. Niu, T.D. Hu, T. Liu, Y.N. Xie, Appl. Catal. B: Environ. 26 (2000) 25. [9] Y. Huang, Z.Q. Tong, B. Wu, J.F. Zhang, J. Fuel Chem. Technol. 36 (2008) 616. [10] G. Carja, Y. Kameshima, K. Okada, C.D. Madhusoodana, Appl. Catal. B 73 (2007) 60. [11] Z.B. Wu, R.B. Jin, H.Q. Wang, Y. Liu, Catal. Commun. 10 (2009) 935. [12] G. Vivekanand, A. Ritesh, V. Nishith, Carbon 44 (2006) 60. [13] I. Mochida, M. Ogaki, H. Fujitsu, Fuel 64 (1985) 1054. [14] I. Mochida, S. Kawana, S. Kisamori, H. Fujitsu, Carbon 32 (1994) 1750. [15] J. Muniz, G. Marban, A.B. Fuertes, Appl. Catal. B 27 (2000) 36. [16] S.-J. Park, B.-J. Kim, J. Colloid Interface Sci. 282 (2005) 124. [17] S.-J. Park, J.-S. Shin, J. Colloid Interface Sci. 264 (2003) 39. [18] F. Forzatti, Catal Today 62 (2000) 51. [19] L. Frederickson, D.M. Hausen, Anal. Chem. 35 (1963) 818. [20] Z.P. Zhu, H.X. Niu, Z. Y Liu, S.J. Liu, J. Catal. 197 (2000) 6. [21] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, fourth ed., Wiley, New York, 1986. [22] J.P. Chen, R.T. Yang, J. Catal. 139 (1993) 277. [23] Z.G. Huang, Z.P. Zhu, Z.Y. Liu, Q.Y. Liu, J. Catal. 214 (2003) 213.