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Synergistic Roles of NO and NO2 in Selective Catalytic Reduction of NOx by NH3
159 Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) All rights reserved reserved © 2006 Elsevier B.V. All
441 441
Synergistic Roles of NO and NO2 in Selective Catalytic Reduction ofNO x byNH 3 Joon Hyun Baika, Jeong Hyun Ron", Sung Dae Yimb, In-Sik Nam*'", Jong-Hwan Leec, Byong K. Choc and Se H. Ohc department of Chemical Engineering/School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Pohang 790-784, Korea b
Fuel Cell Research Center, Korea Institute of Energy Research (KIER), 71-2 Jang-dong, Daejoen 305-343, Korea "General Motors R&D Center, Warren, Michigan 48090-9055, USA 1. Introduction It has been reported that NOx reduction activity of SCR catalysts can be improved by pre-oxidation of NO to NO2 before the SCR reaction in the catalytic reactor [1], which can be done by installing an oxidation catalyst or a nonthermal plasma device upstream of the SCR catalysts [2]. However, the role of NO2 during the catalytic reduction of NOx has not been clearly understood yet. According to the literature, the rate of reaction between NO2 and NH3 is much faster than that between NO and NH3 over Fe-exchanged TiO2-pillared clay [3] and HZSM5 catalysts [4]. Kiovsky et al. [5] also observed that NO2 was more reactive than NO toward NH3 over HM, and an increase of the NO2/NO ratio from 2 to 12 in the feed gas stream significantly enhanced NOx reduction activity. However, Koebel et al. [6] reported that the reaction with equal amounts of NO and NO2 by NH3 was much faster than that with NO (or NO2) only over V2O5-WO3/TiO2 catalyst. Long and Yang [7] reported that the reactivity of FeZSM5 with NO and NO2 increases in the following order: NO+NO2 > NO2 ^ NO. These literature reports suggest that there may indeed be a synergistic role of NO2 with NO in the feed gas stream for catalytic reduction of NOx by NH3, even though there is no comprehensive quantitative analysis on the role of NO2 for the SCR reaction. In this study, the synergistic effect of NO and NO2 in the feed gas stream on the catalytic NOx reduction activity has been systematically examined to better understand
442
the roles of NO and NO2 in the SCR process. 2. Experimental Three representative SCR catalysts - V2O5/T1O2, FeZSM5 and CuZSM5 - were used in this study. The "V^CVTiC^ catalyst was prepared by the wet impregnation method with an aqueous solution of NH4VO3 (Aldrich) on TiO2 support (Hombikat UV-100). The CuZSM5 and FeZSM5 catalysts were prepared by the wet ion exchange method using (CH3COO)2Cu-H2O (0.01 M, Aldrich) and FeCl2 (0.05 M, Aldrich) solutions with 15 g of ZSM5 (Tosoh, HSZ-830NHA) at room temperature. The contents of V, Cu and Fe are 2.83, 3.06 and 2.67 wt.%, respectively. The synergistic effects of NO and NO2 on the NOx reduction activity of the SCR catalysts were examined using a packed-bed flow reactor containing 20/30 mesh size of the catalyst by varying the NO/NO2 feed ratio. In initial tests of CuZSM5 catalysts, the effect of NO2 was not detectable at the reactor space velocity of 100,000 h'1. When the reactor space velocity was increased to 500,000 h"1, the synergistic effects of NO and NO2 became clearly detectable over the entire temperature range from 200 to 450 °C. The concentrations of NO, NH3 and NO2 were determined by on-line chemiluminescence NO-NOx analyzer (Thermo Environmental Instrument, Model 42H), NH3 analyzer with NDIR (Rosemount Analytical, Model 880A) and NO2 analyzer with electrochemical cell (Testo, Model 350M), respectively. The details of the experimental system and procedures have been already described elsewhere [8]. 3. Results and Discussion Fig. 1 shows the conversions of NO, NO2 and NOx as a function of the NO2/NOX feed ratio to the SCR reactor. For all three catalysts (Fig. lb, Id and If), the synergistic effects of NO and NO2 on NOx conversion are most pronounced at low temperatures, gradually disappearing with the rising catalyst temperatures. Interestingly, this synergistic effect completely disappears above 400 °C for both V2Os/TiO2 and FeZSM5 catalysts, while vanishing above 300 °C for the CuZSM5 catalyst. It is of practical importance to note for the CuZSM5 catalyst that the effect of NO2 is significant only at low temperatures below 300 °C, above which the kinetic behaviors of NO and NO2 are essentially identical. The optimum NO2/NOX feed ratio for the best synergistic effect is 0.75 for both FeZSM5 and CuZSM5 catalysts, while it varies from 0.5 to 0.75 for the V2O5/TiO2 catalyst.
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Fig. 1. Effect of NO2 on the reduction of NOx by NH3. Feed gas composition: 500 ppm NOx (NO + NO2), 500 ppm NH3) 5 % O2, 10 % H2O and N2 balance; Reactor SV: 500,000 h"1. The NO and NO2 conversion data (Fig. la, l c and le) indicate that the presence of NO improves the NO2 conversion while NO2 improves the NO conversion, with some minor exceptions in the case of V2O5/T1O2 catalyst.
It is particularly noteworthy in Fig. l e that a
small amount of NO (i.e., NO2/NOx=0.75) can make a big improvement in the NO2
444
conversion over the CuZSM5 catalyst at 200 °C, resulting in a large enhancement in NOx conversion in Fig. If. When a 1:1 mixture of NO and NO2 (i.e., NO2/NOx=0.5) is fed to the SCR reactor at low temperature (200 °C) where the thermodynamic equilibrium between NO and NO2 is severely constrained by kinetics, the NO2 conversion is much greater than (or nearly twice) the NO conversion for all three catalysts. This observation is consistent with the following parallel reactions of the SCR process [6]: Reaction (2) is the dominant reaction due to its reaction rate much faster than the others, resulting in an equal conversion of NO and NO2. On the other hand, Reaction (3) is more favorable than Reaction (1), which leads to a greater additional NO2 conversion by Reaction (3) compared with the NO conversion by Reaction (1). 4NH3 + 4NO + O2 -> 4N2 + 6H2O 4NH3 + 2NO + 2NO2 -» 4N2 + 6H2O 4NH3 + 3NO2 -> 3.5N2 + 6H2O
(1) (2) (3)
4. Conclusion •
• •
In general, NO and NO2 are mutually beneficial for NOx reduction over the SCR catalysts tested. That is, the presence of NO enhances the NO2 conversion, and vice versa. This results in the synergistic effects of NO and NO2 in the catalytic reduction of NOx with NH3 over CuZSM5, FeZSM5 and V2O5/TiO2 catalysts. The synergistic effect is most pronounced at low temperatures - below 300 °C for CuZSM5 and below 400 °C for FeZSM5 and V2O5/TiO2. The optimum NO/NO2 feed ratio for the best synergistic effect depends on the catalyst and its temperature.
References [1] S. Broer and T. Hammer, Appl. Catal. B: Environ. 28 (2000) 101. [2] S.J. Schmieg, B.K. Cho and S.H. Oh, Appl. Catal. B: Environ. 49 (2004) 113. [3] R.Q. Long and R.T. Yang, J. Catal. 190 (2000) 22. [4] S.A. Stevenson and J.C. Vartuli, J. Catal. 208 (2002) 100. [5] J.R. Kiovsky, P.B. Koradia and C.T. Lim, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 218. [6] M. Koebel, G Mania and M. Elsener, Catal. Today 73 (2002) 239. [7] R.Q. Long and R.T. yang, J. Catal. 207 (2002) 224. [8] J.H. Baik, S.D. Yim, I.-S. Nam, J.-H. Lee, B.K. Cho and S.H. Oh, Top. Catal. 30-31 (2004) 37.
441 441
Synergistic Roles of NO and NO2 in Selective Catalytic Reduction ofNO x byNH 3 Joon Hyun Baika, Jeong Hyun Ron", Sung Dae Yimb, In-Sik Nam*'", Jong-Hwan Leec, Byong K. Choc and Se H. Ohc department of Chemical Engineering/School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Pohang 790-784, Korea b
Fuel Cell Research Center, Korea Institute of Energy Research (KIER), 71-2 Jang-dong, Daejoen 305-343, Korea "General Motors R&D Center, Warren, Michigan 48090-9055, USA 1. Introduction It has been reported that NOx reduction activity of SCR catalysts can be improved by pre-oxidation of NO to NO2 before the SCR reaction in the catalytic reactor [1], which can be done by installing an oxidation catalyst or a nonthermal plasma device upstream of the SCR catalysts [2]. However, the role of NO2 during the catalytic reduction of NOx has not been clearly understood yet. According to the literature, the rate of reaction between NO2 and NH3 is much faster than that between NO and NH3 over Fe-exchanged TiO2-pillared clay [3] and HZSM5 catalysts [4]. Kiovsky et al. [5] also observed that NO2 was more reactive than NO toward NH3 over HM, and an increase of the NO2/NO ratio from 2 to 12 in the feed gas stream significantly enhanced NOx reduction activity. However, Koebel et al. [6] reported that the reaction with equal amounts of NO and NO2 by NH3 was much faster than that with NO (or NO2) only over V2O5-WO3/TiO2 catalyst. Long and Yang [7] reported that the reactivity of FeZSM5 with NO and NO2 increases in the following order: NO+NO2 > NO2 ^ NO. These literature reports suggest that there may indeed be a synergistic role of NO2 with NO in the feed gas stream for catalytic reduction of NOx by NH3, even though there is no comprehensive quantitative analysis on the role of NO2 for the SCR reaction. In this study, the synergistic effect of NO and NO2 in the feed gas stream on the catalytic NOx reduction activity has been systematically examined to better understand
442
the roles of NO and NO2 in the SCR process. 2. Experimental Three representative SCR catalysts - V2O5/T1O2, FeZSM5 and CuZSM5 - were used in this study. The "V^CVTiC^ catalyst was prepared by the wet impregnation method with an aqueous solution of NH4VO3 (Aldrich) on TiO2 support (Hombikat UV-100). The CuZSM5 and FeZSM5 catalysts were prepared by the wet ion exchange method using (CH3COO)2Cu-H2O (0.01 M, Aldrich) and FeCl2 (0.05 M, Aldrich) solutions with 15 g of ZSM5 (Tosoh, HSZ-830NHA) at room temperature. The contents of V, Cu and Fe are 2.83, 3.06 and 2.67 wt.%, respectively. The synergistic effects of NO and NO2 on the NOx reduction activity of the SCR catalysts were examined using a packed-bed flow reactor containing 20/30 mesh size of the catalyst by varying the NO/NO2 feed ratio. In initial tests of CuZSM5 catalysts, the effect of NO2 was not detectable at the reactor space velocity of 100,000 h'1. When the reactor space velocity was increased to 500,000 h"1, the synergistic effects of NO and NO2 became clearly detectable over the entire temperature range from 200 to 450 °C. The concentrations of NO, NH3 and NO2 were determined by on-line chemiluminescence NO-NOx analyzer (Thermo Environmental Instrument, Model 42H), NH3 analyzer with NDIR (Rosemount Analytical, Model 880A) and NO2 analyzer with electrochemical cell (Testo, Model 350M), respectively. The details of the experimental system and procedures have been already described elsewhere [8]. 3. Results and Discussion Fig. 1 shows the conversions of NO, NO2 and NOx as a function of the NO2/NOX feed ratio to the SCR reactor. For all three catalysts (Fig. lb, Id and If), the synergistic effects of NO and NO2 on NOx conversion are most pronounced at low temperatures, gradually disappearing with the rising catalyst temperatures. Interestingly, this synergistic effect completely disappears above 400 °C for both V2Os/TiO2 and FeZSM5 catalysts, while vanishing above 300 °C for the CuZSM5 catalyst. It is of practical importance to note for the CuZSM5 catalyst that the effect of NO2 is significant only at low temperatures below 300 °C, above which the kinetic behaviors of NO and NO2 are essentially identical. The optimum NO2/NOX feed ratio for the best synergistic effect is 0.75 for both FeZSM5 and CuZSM5 catalysts, while it varies from 0.5 to 0.75 for the V2O5/TiO2 catalyst.
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Fig. 1. Effect of NO2 on the reduction of NOx by NH3. Feed gas composition: 500 ppm NOx (NO + NO2), 500 ppm NH3) 5 % O2, 10 % H2O and N2 balance; Reactor SV: 500,000 h"1. The NO and NO2 conversion data (Fig. la, l c and le) indicate that the presence of NO improves the NO2 conversion while NO2 improves the NO conversion, with some minor exceptions in the case of V2O5/T1O2 catalyst.
It is particularly noteworthy in Fig. l e that a
small amount of NO (i.e., NO2/NOx=0.75) can make a big improvement in the NO2
444
conversion over the CuZSM5 catalyst at 200 °C, resulting in a large enhancement in NOx conversion in Fig. If. When a 1:1 mixture of NO and NO2 (i.e., NO2/NOx=0.5) is fed to the SCR reactor at low temperature (200 °C) where the thermodynamic equilibrium between NO and NO2 is severely constrained by kinetics, the NO2 conversion is much greater than (or nearly twice) the NO conversion for all three catalysts. This observation is consistent with the following parallel reactions of the SCR process [6]: Reaction (2) is the dominant reaction due to its reaction rate much faster than the others, resulting in an equal conversion of NO and NO2. On the other hand, Reaction (3) is more favorable than Reaction (1), which leads to a greater additional NO2 conversion by Reaction (3) compared with the NO conversion by Reaction (1). 4NH3 + 4NO + O2 -> 4N2 + 6H2O 4NH3 + 2NO + 2NO2 -» 4N2 + 6H2O 4NH3 + 3NO2 -> 3.5N2 + 6H2O
(1) (2) (3)
4. Conclusion •
• •
In general, NO and NO2 are mutually beneficial for NOx reduction over the SCR catalysts tested. That is, the presence of NO enhances the NO2 conversion, and vice versa. This results in the synergistic effects of NO and NO2 in the catalytic reduction of NOx with NH3 over CuZSM5, FeZSM5 and V2O5/TiO2 catalysts. The synergistic effect is most pronounced at low temperatures - below 300 °C for CuZSM5 and below 400 °C for FeZSM5 and V2O5/TiO2. The optimum NO/NO2 feed ratio for the best synergistic effect depends on the catalyst and its temperature.
References [1] S. Broer and T. Hammer, Appl. Catal. B: Environ. 28 (2000) 101. [2] S.J. Schmieg, B.K. Cho and S.H. Oh, Appl. Catal. B: Environ. 49 (2004) 113. [3] R.Q. Long and R.T. Yang, J. Catal. 190 (2000) 22. [4] S.A. Stevenson and J.C. Vartuli, J. Catal. 208 (2002) 100. [5] J.R. Kiovsky, P.B. Koradia and C.T. Lim, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 218. [6] M. Koebel, G Mania and M. Elsener, Catal. Today 73 (2002) 239. [7] R.Q. Long and R.T. yang, J. Catal. 207 (2002) 224. [8] J.H. Baik, S.D. Yim, I.-S. Nam, J.-H. Lee, B.K. Cho and S.H. Oh, Top. Catal. 30-31 (2004) 37.