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Hydrogen assisted urea-SCR and NH3-SCR with silver–alumina as highly active and SO2-tolerant de-NOx catalysis
Applied Catalysis B: Environmental 77 (2007) 202–205 www.elsevier.com/locate/apcatb
Hydrogen assisted urea-SCR and NH3-SCR with silver–alumina as highly active and SO2-tolerant de-NOx catalysis Ken-ichi Shimizu *, Atsushi Satsuma Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Received 5 October 2006; received in revised form 17 July 2007; accepted 23 July 2007 Available online 27 July 2007
Abstract Addition of 0.5% H2 in the selective catalytic reduction of NO by urea (urea-SCR) caused a drastic improvement of NO reduction activity of Ag/Al2O3. Among various silver-based catalysts Ag/Al2O3 showed highest activity for H2 assisted urea-SCR, and high NO conversion (above 84%) and no formation of N2O were achieved over wide temperature range (200–500 8C) at GHSV of 75,000 h 1. After H2 assisted urea-SCR at 250 8C with 10% H2O and 50 ppm SO2 under GHSV of 380,000 h 1 for 24 h, NO conversion over Ag/Al2O3 decreased from 48 to 30%, though original activity was recovered when the deactivated catalyst was heated at 500 8C in the urea-SCR condition for 1 h. The catalyst also showed high SO2-tolerance in H2 assisted NH3-SCR at 200 8C. # 2007 Elsevier B.V. All rights reserved. Keywords: Nitrogen oxides; Silver alumina; Urea; Sulfur oxide
1. Introduction There is a possibility that fuel optimized engines combined with a urea-SCR system could reduce fuel consumption by 7%, resulting in less CO2 emission in a global perspective [1]. Great efforts have been made to develop SCR catalysts for automotive application. Although the established SCR process for stationary plants is based on the reduction of NOx by ammonia, this technology is not applicable for automotive use because of difficulties in the storing and handling of ammonia. Urea as ammonia source may be the best choice for automotive applications because urea is not toxic and can be easily transported on board as a high concentration aqueous solution, and hence urea-SCR has been studied for the NOx removal from diesel engine exhausts [1–7]. Vanadia-based catalysts are most effective for SCR by urea. However, for automotive applications, use of toxic vanadia-based catalysts pose some serious problems, such as high vapor pressure of the oxide leading to toxic emissions and the potential for water solubility and leaching of oxide from cold catalyst beds by condensing water vapor [7]. Another important demands for automotive use include the
enhancement of the activity at low temperature and reduction of the catalyst volume as well as the size of a urea solution tank. Hence, new urea-SCR catalysts having high NO conversion under high GHSV condition over wide temperature range and high stability must be developed using non-toxic components. Ag/Al2O3 is among the most active catalysts for SCR by alcohol [8] or higher hydrocarbons [9] under lean-burn exhaust conditions. Recent report by Satokawa [10] and our research group [11] and following studies [12,13] demonstrated that the activity of Ag/Al2O3 dramatically increased by hydrogen addition (H2–HC-SCR condition). Richter et al. discovered an unusual activity enhancement of Ag/Al2O3 for NH3-SCR [14,15]. Knowing the well-known fact that urea hydrolysis to yield NH3 and CO2 is catalyzed by acid catalysts such as alumina, we hypothesized that Ag/Al2O3 can be effective catalyst for H2 assisted urea-SCR. In this study we show a dramatic effect of H2 addition on the urea-SCR performance of Ag/Al2O3. The high activity under high GHSV condition at relatively low reaction temperature and high SO2-tolerance will make this catalytic system attractive for industrial application. 2. Experimental
* Corresponding author. Fax: +81 52 789 3193. E-mail address: [email protected] (K.-i. Shimizu). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.07.021
Ag/Al2O3 catalysts (Ag = 1–10 wt.%) were prepared by impregnating g-AlOOH with an aqueous solution of silver
K.-i. Shimizu, A. Satsuma / Applied Catalysis B: Environmental 77 (2007) 202–205
nitrate followed by evaporation to dryness at 120 8C and by calcination in air at 600 8C for 4 h. Ag/Al2O3 catalyst with Ag loading of 2 wt.% (surface area = 251 m2 g 1) was used as a standard catalyst for catalytic reactions. Ag/SiO2 and Ag/SiO2– Al2O3 catalysts (Ag = 3 wt.%) were prepared by impregnating SiO2 (JRC-SIO-8, a reference catalyst of the Catalysis Society of Japan, SBET = 303 m2 g 1) and SiO2–Al2O3 (JRC-SAL-2, SiO2/Al2O3 = 5.6, SBET = 560 m2 g 1) with an aqueous solution of silver nitrate followed by evaporation to dryness at 120 8C and by calcination in air at 600 8C for 4 h. Ag+ ionexchanged zeolite (Ag-MFI, Ag = 3 wt.%) was prepared from H-MFI (Si/Al = 20, supplied from Tosoh Co. Japan) by ionexchange with an aqueous AgNO3 solution at room temperature, followed by filtering and drying at 120 8C, and by calcining in air at 550 8C for 2 h. V2O5–WO3/TiO2 catalyst (V– W/TiO2, V = 4 wt.%, W = 10 wt.%) was prepared by impregnating TiO2 (JRC-TIO-4, SBET = 50 10 m2 g 1) with an aqueous solution of ammonium metavanadate and ammonium metatungstate, followed by evaporation to dryness at 120 8C and by calcination in air at 500 8C for 4 h. Catalytic tests were performed using catalyst powders (particle size in a range 0.1–0.2 mm) in a fixed-bed flow reactor (inner diameter = 4 mm) at a flow rate of 100 cm3 min 1 under GHSV of 75,000 h 1 (catalyst weight = 50 mg) or 380,000 h 1 (catalyst weight = 10 mg). For the most representative condition using 10 mg of the Ag/Al2O3 catalyst (Ag = 2 wt.%), the volume of the catalyst bed was 0.016 cm3 and the catalyst bed length was 1.3 mm. Compositions of feed gas for urea-SCR in the absence and in the presence of H2 were NO/urea/H2/O2/ H2O/He = 0.1/0.05/0.5/10/10% per balance. Aqueous urea solution was used to get the desired water and urea concentration in the feed. Urea and water were introduced by passing part of the carrier gas to the saturator of aqueous urea solution. The reactant gas mixture was heated at ca. 130 8C before entering the catalyst bed. Urea concentration was determined by GC measurement of CO2 formed by complete combustion of urea by Pt/Al2O3 at 500 8C (GHSV = 3000 h 1). Product analysis at each temperature was performed after reaching a steady state condition (1 h). NO conversion into N2 or N2O was evaluated from GC analysis of N2 and N2O combined with NOx analyzer. Compositions of feed gas for NH3-SCR in the presence of H2 were NO/NH3/H2/O2/H2O/ He = 0.1/0.1/0.5/10/2% per balance. Compositions of feed gas for H2 assisted C3H8-SCR (H2–C3H8-SCR) reaction was NO/ C3H8/H2/O2/H2O/He = 0.1/0.1/0.5/10/2% per balance.
203
Table 1 Effect of H2 addition on NO conversion for various catalysts Catalyst
H2 = 0.5%
H2 = 0%
Ag/Al2O3 Ag-MFI Ag/SiO2–Al2O3 Ag/SiO2 Al2O3
90 61 3 4 0
0 18 0 0 0
T = 350 8C, GHSV = 75,000 h 1.
conversion over Ag/Al2O3 catalyst in urea-SCR or H2–urea-SCR with water vapor. The H2 addition dramatically increased NO conversions, and high NO conversions (above 80%) were observed over a wide temperature range (200–500 8C) at GHSV of 75,000 h 1. It should be noted that no appreciable formation of N2O was confirmed. NO conversions above 84% were achieved even under high GHSV condition (GHSV = 380,000 h 1) in a temperature range of 300–500 8C. Fig. 2 shows the effect of Ag loading of Ag/Al2O3 on the NO conversion to N2 in H2–ureaSCR at 200 8C (GHSV = 75,000 h 1). The NO conversion increased with the loading to 2 wt.% and slightly decreased with further increase in the loading. Thus, Ag/Al2O3 with the optimal loading (2 wt.%) was used as a standard catalyst. The activity of Ag/Al2O3 was compared with that of V–W/ TiO2, which is the most effective catalyst for urea-SCR. Fig. 3 shows the activity of these catalysts for H2–urea-SCR (GHSV = 380,000 h 1). Note that H2 addition resulted in no change in NO conversion over V–W/TiO2 (results not shown). Ag/Al2O3 showed higher NO conversion than V–W/TiO2 at low temperature region (below 300 8C). The turnover frequencies (TOF) of Ag/Al2O3, expressed as mol NO converted to N2 per mol Ag per hour were 17 h 1 at 150 8C and 110 h 1 at 200 8C. The former value is higher than TOF for one of the most effective vanadia-based catalyst reported by Popa et al. (TOF = 2.2 h 1 at 150 8C) for NH3-SCR [16]. The NO conversions over Ag/Al2O3 in H2–urea-SCR condition were lower than those in the H2 assisted NH3-SCR (H2–NH3-SCR) condition, and were higher
3. Results and discussion Table 1 shows the effect of H2 addition on the NO conversion to N2 in urea-SCR at 400 8C. In the absence of H2, all the silver catalysts except for Ag-MFI showed no activity for urea-SCR. Addition of 0.5% H2 increased NO conversions over Ag/Al2O3 and Ag-MFI catalysts. The promoting effect was most drastic for Ag/Al2O3; the NO conversion increased from 0 to 90%. In contrast, NO conversions were below 4% even in H2–urea-SCR condition on Ag/SiO2 and Ag/SiO2–Al2O3 catalysts. Al2O3 showed no activity in H2–urea-SCR reaction. Fig. 1 shows NO
Fig. 1. NO conversion to N2 on Ag/Al2O3 (Ag = 2 wt.%) in H2–urea-SCR under different conditions.
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Fig. 2. Effect of silver loading of Ag/Al2O3 on NO conversion in H2–urea-SCR at 250 8C (GHSV = 75,000 h 1).
than those in H2 assisted C3H8-SCR (H2–C3H8-SCR) condition below 450 8C. Effect of SO2 addition on the catalytic activity of Ag/Al2O3 for H2–urea-SCR at 250 8C (GHSV = 380,000 h 1) is shown in Fig. 4. After the measurements of steady state conversion in SO2 free condition (NO conversion = 48% at t = 0 h), 50 ppm SO2 was added to the reaction gas mixture and NO conversion was measured as a function of time. Upon SO2 addition, the NO conversion gradually decreased after 10 h of SO2 co-feed, and after 24 h a stable NO conversion (30%) was obtained. Then, H2–urea-SCR in the absence of SO2 was subsequently carried out at 500 8C for 1 h. After this treatment, the NO conversion at 250 8C was completely recovered to the initial value. This result suggests that the occupation of the active sites by certain adspecies, such as ammonium sulfate caused activity decrease at 250 8C, but the active site is regenerated by a thermal decomposition of them. Mobile diesel engines work under temperature-changing conditions, and hence, we believe that
Fig. 4. NO conversion to N2 vs. time of 50 ppm SO2 addition in H2–urea-SCR at 250 8C (GHSV = 380,000 h 1).
Fig. 5. NO conversion to N2 (*) and N2O (*) vs. time of 50 ppm SO2 addition in H2–NH3-SCR at 200 8C (GHSV = 75,000 h 1).
the catalyst deactivation by ammonium sulfate is not essential and the catalyst activity could be recovered during diesel exhaust conditions. Effect of SO2 addition on the catalytic activity of Ag/Al2O3 for H2–NH3-SCR at 200 8C (GHSV = 75,000 h 1) is shown in Fig. 5. The catalyst showed NO conversion above 90% for 120 h with no indication of catalyst deactivation, demonstrating high SO2-tolerance of this catalytic system. 4. Conclusion
Fig. 3. NO conversion to N2 on Ag/Al2O3 (Ag = 2 wt.%) in H2–urea-SCR, H2– NH3-SCR and H2–C3H8-SCR reactions (GHSV = 380,000 h 1). NO conversion to N2 on V–W/TiO2 in H2–urea-SCR (!) is also included.
Addition of 0.5% H2 in the selective catalytic reduction of NO by urea (urea-SCR) caused a drastic improvement of NO reduction activity of Ag/Al2O3. The high NO conversion under high GHSV condition in a wide temperature range and high SO2-tolerance make this catalytic system attractive for industrial application. Additional advantage of this catalytic system is its high activity for H2 assisted HC-SCR reaction at high temperature (500 8C), indicating a possibility of urea-free de-NOx catalysis at high temperature region. These properties are suitable for diesel de-NOx catalysis, which should be
K.-i. Shimizu, A. Satsuma / Applied Catalysis B: Environmental 77 (2007) 202–205
operated under transient conditions in a wide temperature window. Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research (KAKENHI) from the Ministry of Education, Science and Culture, Japan. Authors would like to appreciate the financial support by a feasibility study of Japan Science and Technology Agency. References [1] Pa¨r L.T. Gabrielsson, Top. Catal. 28 (2004) 177. [2] W. Held, A. Konig, T. Richter, L. Puppe, SAE Technical Paper, Series no. 900496. [3] M. Koebel, M. Elsener, T. Marti, Combust. Sci. Tech. 121 (1996) 85. [4] M. Koebel, M. Elsener, O. Krocher, C. Scha¨r, R. Ro¨thlisberger, F. Jaussi, M. Mangold, Top. Catal. 30–31 (2004) 43.
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[5] M.E. Seker, N. Yasyerli, E. Gulari, C. Lambert, R.H. Hammerle, Appl. Catal. B 37 (2002) 27. [6] J. Park, H.J. Park, J.H. Baik, I.-S. Nam, C.-H. Shin, J.-H. Lee, B.K. Cho, S.H. Oh, J. Catal. 240 (2006) 47. [7] E. Seker, E. Gulari, R.H. Hammerle, C. Lambert, J. Leerat, S. Osuwan, Appl. Catal. A 226 (2002) 183. [8] T. Miyadera, K. Yoshida, Chem. Lett. (1993) 1483. [9] K. Shimizu, A. Satsuma, T. Hattori, Appl. Catal. B 25 (2000) 239. [10] S. Satokawa, Chem. Lett. (2000) 294. [11] S. Satokawa, J. Shibata, K. Shimizu, A. Satsuma, T. Hattori, Appl. Catal. B 42 (2003) 179. [12] R. Burch, J.P. Breen, C.J. Hill, B. Krutzsch, K. Konrad, E. Jobson, L. Cider, K. Era¨nen, F. Klingstedt, L.-E. Lindfors, Top. Catal. 30–31 (2004) 19. [13] M. Richter, U. Bentrup, E. Eckelt, M. Schneider, M.-M. Pohl, R. Fricke, Appl. Catal. B 51 (2004) 261. [14] M. Richter, R. Fricke, R. Eckelt, Catal. Lett. 94 (2004) 115. [15] E. Kondratenko, V.A. Kondratenko, M. Richter, R. Fricke, J. Catal. 239 (2006) 23. [16] A.F. Popa, P.H. Mutin, A. Vioux, G. Delahay, B. Coq, Chem. Commun. (2004) 2214.
Hydrogen assisted urea-SCR and NH3-SCR with silver–alumina as highly active and SO2-tolerant de-NOx catalysis Ken-ichi Shimizu *, Atsushi Satsuma Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Received 5 October 2006; received in revised form 17 July 2007; accepted 23 July 2007 Available online 27 July 2007
Abstract Addition of 0.5% H2 in the selective catalytic reduction of NO by urea (urea-SCR) caused a drastic improvement of NO reduction activity of Ag/Al2O3. Among various silver-based catalysts Ag/Al2O3 showed highest activity for H2 assisted urea-SCR, and high NO conversion (above 84%) and no formation of N2O were achieved over wide temperature range (200–500 8C) at GHSV of 75,000 h 1. After H2 assisted urea-SCR at 250 8C with 10% H2O and 50 ppm SO2 under GHSV of 380,000 h 1 for 24 h, NO conversion over Ag/Al2O3 decreased from 48 to 30%, though original activity was recovered when the deactivated catalyst was heated at 500 8C in the urea-SCR condition for 1 h. The catalyst also showed high SO2-tolerance in H2 assisted NH3-SCR at 200 8C. # 2007 Elsevier B.V. All rights reserved. Keywords: Nitrogen oxides; Silver alumina; Urea; Sulfur oxide
1. Introduction There is a possibility that fuel optimized engines combined with a urea-SCR system could reduce fuel consumption by 7%, resulting in less CO2 emission in a global perspective [1]. Great efforts have been made to develop SCR catalysts for automotive application. Although the established SCR process for stationary plants is based on the reduction of NOx by ammonia, this technology is not applicable for automotive use because of difficulties in the storing and handling of ammonia. Urea as ammonia source may be the best choice for automotive applications because urea is not toxic and can be easily transported on board as a high concentration aqueous solution, and hence urea-SCR has been studied for the NOx removal from diesel engine exhausts [1–7]. Vanadia-based catalysts are most effective for SCR by urea. However, for automotive applications, use of toxic vanadia-based catalysts pose some serious problems, such as high vapor pressure of the oxide leading to toxic emissions and the potential for water solubility and leaching of oxide from cold catalyst beds by condensing water vapor [7]. Another important demands for automotive use include the
enhancement of the activity at low temperature and reduction of the catalyst volume as well as the size of a urea solution tank. Hence, new urea-SCR catalysts having high NO conversion under high GHSV condition over wide temperature range and high stability must be developed using non-toxic components. Ag/Al2O3 is among the most active catalysts for SCR by alcohol [8] or higher hydrocarbons [9] under lean-burn exhaust conditions. Recent report by Satokawa [10] and our research group [11] and following studies [12,13] demonstrated that the activity of Ag/Al2O3 dramatically increased by hydrogen addition (H2–HC-SCR condition). Richter et al. discovered an unusual activity enhancement of Ag/Al2O3 for NH3-SCR [14,15]. Knowing the well-known fact that urea hydrolysis to yield NH3 and CO2 is catalyzed by acid catalysts such as alumina, we hypothesized that Ag/Al2O3 can be effective catalyst for H2 assisted urea-SCR. In this study we show a dramatic effect of H2 addition on the urea-SCR performance of Ag/Al2O3. The high activity under high GHSV condition at relatively low reaction temperature and high SO2-tolerance will make this catalytic system attractive for industrial application. 2. Experimental
* Corresponding author. Fax: +81 52 789 3193. E-mail address: [email protected] (K.-i. Shimizu). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.07.021
Ag/Al2O3 catalysts (Ag = 1–10 wt.%) were prepared by impregnating g-AlOOH with an aqueous solution of silver
K.-i. Shimizu, A. Satsuma / Applied Catalysis B: Environmental 77 (2007) 202–205
nitrate followed by evaporation to dryness at 120 8C and by calcination in air at 600 8C for 4 h. Ag/Al2O3 catalyst with Ag loading of 2 wt.% (surface area = 251 m2 g 1) was used as a standard catalyst for catalytic reactions. Ag/SiO2 and Ag/SiO2– Al2O3 catalysts (Ag = 3 wt.%) were prepared by impregnating SiO2 (JRC-SIO-8, a reference catalyst of the Catalysis Society of Japan, SBET = 303 m2 g 1) and SiO2–Al2O3 (JRC-SAL-2, SiO2/Al2O3 = 5.6, SBET = 560 m2 g 1) with an aqueous solution of silver nitrate followed by evaporation to dryness at 120 8C and by calcination in air at 600 8C for 4 h. Ag+ ionexchanged zeolite (Ag-MFI, Ag = 3 wt.%) was prepared from H-MFI (Si/Al = 20, supplied from Tosoh Co. Japan) by ionexchange with an aqueous AgNO3 solution at room temperature, followed by filtering and drying at 120 8C, and by calcining in air at 550 8C for 2 h. V2O5–WO3/TiO2 catalyst (V– W/TiO2, V = 4 wt.%, W = 10 wt.%) was prepared by impregnating TiO2 (JRC-TIO-4, SBET = 50 10 m2 g 1) with an aqueous solution of ammonium metavanadate and ammonium metatungstate, followed by evaporation to dryness at 120 8C and by calcination in air at 500 8C for 4 h. Catalytic tests were performed using catalyst powders (particle size in a range 0.1–0.2 mm) in a fixed-bed flow reactor (inner diameter = 4 mm) at a flow rate of 100 cm3 min 1 under GHSV of 75,000 h 1 (catalyst weight = 50 mg) or 380,000 h 1 (catalyst weight = 10 mg). For the most representative condition using 10 mg of the Ag/Al2O3 catalyst (Ag = 2 wt.%), the volume of the catalyst bed was 0.016 cm3 and the catalyst bed length was 1.3 mm. Compositions of feed gas for urea-SCR in the absence and in the presence of H2 were NO/urea/H2/O2/ H2O/He = 0.1/0.05/0.5/10/10% per balance. Aqueous urea solution was used to get the desired water and urea concentration in the feed. Urea and water were introduced by passing part of the carrier gas to the saturator of aqueous urea solution. The reactant gas mixture was heated at ca. 130 8C before entering the catalyst bed. Urea concentration was determined by GC measurement of CO2 formed by complete combustion of urea by Pt/Al2O3 at 500 8C (GHSV = 3000 h 1). Product analysis at each temperature was performed after reaching a steady state condition (1 h). NO conversion into N2 or N2O was evaluated from GC analysis of N2 and N2O combined with NOx analyzer. Compositions of feed gas for NH3-SCR in the presence of H2 were NO/NH3/H2/O2/H2O/ He = 0.1/0.1/0.5/10/2% per balance. Compositions of feed gas for H2 assisted C3H8-SCR (H2–C3H8-SCR) reaction was NO/ C3H8/H2/O2/H2O/He = 0.1/0.1/0.5/10/2% per balance.
203
Table 1 Effect of H2 addition on NO conversion for various catalysts Catalyst
H2 = 0.5%
H2 = 0%
Ag/Al2O3 Ag-MFI Ag/SiO2–Al2O3 Ag/SiO2 Al2O3
90 61 3 4 0
0 18 0 0 0
T = 350 8C, GHSV = 75,000 h 1.
conversion over Ag/Al2O3 catalyst in urea-SCR or H2–urea-SCR with water vapor. The H2 addition dramatically increased NO conversions, and high NO conversions (above 80%) were observed over a wide temperature range (200–500 8C) at GHSV of 75,000 h 1. It should be noted that no appreciable formation of N2O was confirmed. NO conversions above 84% were achieved even under high GHSV condition (GHSV = 380,000 h 1) in a temperature range of 300–500 8C. Fig. 2 shows the effect of Ag loading of Ag/Al2O3 on the NO conversion to N2 in H2–ureaSCR at 200 8C (GHSV = 75,000 h 1). The NO conversion increased with the loading to 2 wt.% and slightly decreased with further increase in the loading. Thus, Ag/Al2O3 with the optimal loading (2 wt.%) was used as a standard catalyst. The activity of Ag/Al2O3 was compared with that of V–W/ TiO2, which is the most effective catalyst for urea-SCR. Fig. 3 shows the activity of these catalysts for H2–urea-SCR (GHSV = 380,000 h 1). Note that H2 addition resulted in no change in NO conversion over V–W/TiO2 (results not shown). Ag/Al2O3 showed higher NO conversion than V–W/TiO2 at low temperature region (below 300 8C). The turnover frequencies (TOF) of Ag/Al2O3, expressed as mol NO converted to N2 per mol Ag per hour were 17 h 1 at 150 8C and 110 h 1 at 200 8C. The former value is higher than TOF for one of the most effective vanadia-based catalyst reported by Popa et al. (TOF = 2.2 h 1 at 150 8C) for NH3-SCR [16]. The NO conversions over Ag/Al2O3 in H2–urea-SCR condition were lower than those in the H2 assisted NH3-SCR (H2–NH3-SCR) condition, and were higher
3. Results and discussion Table 1 shows the effect of H2 addition on the NO conversion to N2 in urea-SCR at 400 8C. In the absence of H2, all the silver catalysts except for Ag-MFI showed no activity for urea-SCR. Addition of 0.5% H2 increased NO conversions over Ag/Al2O3 and Ag-MFI catalysts. The promoting effect was most drastic for Ag/Al2O3; the NO conversion increased from 0 to 90%. In contrast, NO conversions were below 4% even in H2–urea-SCR condition on Ag/SiO2 and Ag/SiO2–Al2O3 catalysts. Al2O3 showed no activity in H2–urea-SCR reaction. Fig. 1 shows NO
Fig. 1. NO conversion to N2 on Ag/Al2O3 (Ag = 2 wt.%) in H2–urea-SCR under different conditions.
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Fig. 2. Effect of silver loading of Ag/Al2O3 on NO conversion in H2–urea-SCR at 250 8C (GHSV = 75,000 h 1).
than those in H2 assisted C3H8-SCR (H2–C3H8-SCR) condition below 450 8C. Effect of SO2 addition on the catalytic activity of Ag/Al2O3 for H2–urea-SCR at 250 8C (GHSV = 380,000 h 1) is shown in Fig. 4. After the measurements of steady state conversion in SO2 free condition (NO conversion = 48% at t = 0 h), 50 ppm SO2 was added to the reaction gas mixture and NO conversion was measured as a function of time. Upon SO2 addition, the NO conversion gradually decreased after 10 h of SO2 co-feed, and after 24 h a stable NO conversion (30%) was obtained. Then, H2–urea-SCR in the absence of SO2 was subsequently carried out at 500 8C for 1 h. After this treatment, the NO conversion at 250 8C was completely recovered to the initial value. This result suggests that the occupation of the active sites by certain adspecies, such as ammonium sulfate caused activity decrease at 250 8C, but the active site is regenerated by a thermal decomposition of them. Mobile diesel engines work under temperature-changing conditions, and hence, we believe that
Fig. 4. NO conversion to N2 vs. time of 50 ppm SO2 addition in H2–urea-SCR at 250 8C (GHSV = 380,000 h 1).
Fig. 5. NO conversion to N2 (*) and N2O (*) vs. time of 50 ppm SO2 addition in H2–NH3-SCR at 200 8C (GHSV = 75,000 h 1).
the catalyst deactivation by ammonium sulfate is not essential and the catalyst activity could be recovered during diesel exhaust conditions. Effect of SO2 addition on the catalytic activity of Ag/Al2O3 for H2–NH3-SCR at 200 8C (GHSV = 75,000 h 1) is shown in Fig. 5. The catalyst showed NO conversion above 90% for 120 h with no indication of catalyst deactivation, demonstrating high SO2-tolerance of this catalytic system. 4. Conclusion
Fig. 3. NO conversion to N2 on Ag/Al2O3 (Ag = 2 wt.%) in H2–urea-SCR, H2– NH3-SCR and H2–C3H8-SCR reactions (GHSV = 380,000 h 1). NO conversion to N2 on V–W/TiO2 in H2–urea-SCR (!) is also included.
Addition of 0.5% H2 in the selective catalytic reduction of NO by urea (urea-SCR) caused a drastic improvement of NO reduction activity of Ag/Al2O3. The high NO conversion under high GHSV condition in a wide temperature range and high SO2-tolerance make this catalytic system attractive for industrial application. Additional advantage of this catalytic system is its high activity for H2 assisted HC-SCR reaction at high temperature (500 8C), indicating a possibility of urea-free de-NOx catalysis at high temperature region. These properties are suitable for diesel de-NOx catalysis, which should be
K.-i. Shimizu, A. Satsuma / Applied Catalysis B: Environmental 77 (2007) 202–205
operated under transient conditions in a wide temperature window. Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research (KAKENHI) from the Ministry of Education, Science and Culture, Japan. Authors would like to appreciate the financial support by a feasibility study of Japan Science and Technology Agency. References [1] Pa¨r L.T. Gabrielsson, Top. Catal. 28 (2004) 177. [2] W. Held, A. Konig, T. Richter, L. Puppe, SAE Technical Paper, Series no. 900496. [3] M. Koebel, M. Elsener, T. Marti, Combust. Sci. Tech. 121 (1996) 85. [4] M. Koebel, M. Elsener, O. Krocher, C. Scha¨r, R. Ro¨thlisberger, F. Jaussi, M. Mangold, Top. Catal. 30–31 (2004) 43.
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