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Performance and durability of Pt-MFI zeolite catalyst for selective reduction of nitrogen monoxide in actual diesel engine exhaust
ELSEVIER
Applied Catalysis B: Environmental 5 ( 1994) Ll-L5
Performance and durability of Pt-MFI zeolite catalyst for selective reduction of nitrogen monoxide in actual diesel engine exhaust Masakazu Iwamoto a~*, Hidenori Yahiro a, Hyun Khil Shin a, Masami Watanabe b, Jianwei Guo ‘, Mitsuru Konno ‘, Takemi Chikahisa ‘, Tadashi Murayama ’ aCatalysis Research Center, Hokkaido Universiry, Sapporo 060, Japan b Ishikawajima-Harima Heavy Industries Co. Ltd., Koto-kn, Tokyo 135, Japan ’ Faculty of Engineering, Hokkaido
University, Sapporo 060, Japan
Received 15 September 1994; accepted 19 September 1994
Abstract Selective catalytic reduction of nitrogen monoxide (NO) by hydrocarbon in an oxidizing atmosphere has been studied over platinum-MFI zeolite (Pt-MFI) in synthesized or actual diesel engine exhaust gases. The activity of Pt-MFI in the synthesized gas, containing 10% water, changed in the early stage of the use, leveled off after 150-200 h, and remained constant for more than 800 h. The Pt-MFI catalyst also showed stable activity at 423-773 K and 10 000-150 000 h- I (gas hourly space velocity) in actual engine exhaust with light oil as a fuel. The degree of nitrogen monoxide reduction increased linearly upon addition of ethylene into the exhaust gas. Keywords: Diesel engine exhaust; MFI zeolite; Nitrogen monoxide; Platinum; Selective reduction; Zeolites
1. Introduction Selective catalytic reduction (SCR) of nitrogen monoxide by hydrocarbon in an oxidizing atmosphere (SCR-HC) has been extensively studied as a potential process for nitrogen monoxide removal from the emissions of diesel and lean-burn gasoline engines [ 11. Although many catalysts have been reported to be active, supported platinum catalysts, Pt/B203-Si02-A&O3 [2], Pt-zeolite [ 3,4], Pt/ AlTO3 [ 5,6], and Pt/SiO, [ 71 have widely been recognized to show the highest * Corresponding
author. Fax. ( +81-11) 7578126.
0926-3373/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSO10926-3373 (94)00047-6
L2
M. lwatnoto et al. /Applied Catalysis B: Etn~irwtttettta/ 5 (199-f) ,!.I-65
activity for the SCR-HC at temperatures as low as 453-573 K, An additionally interesting and significant point of platinum catalysts is the better performance in the presence of water and/or SO:! than those of the other catalysts. Obuchi et al. [5], however, have recently demonstrated that the catalytic activity of Pt/AI,O, in real diesel engine exhaust has gradually decreased with the time OFuse. In contrast, the performance of Pt-ZSM-5 zeolite has been reported not to change during the use in 8.6% water vapor except during the first few hours [ 31, Here the durability and the performance of Pt-ZSM-5 have been examined in actual diesel engine exhaust in more detail. 2. Experimental The MFI (ZSM-5, silica/alumina= 23.3) zeolite was supplied from Tosoh Corporation in the Na’ form. Pt-MFI catalysts were prepared by the ion exchange method described previously [ 31. The exchange level of platinum ion was 7 and 97%, which was determined by inductively coupled plasma spectroscopy (ICP) (SPS-1100, Seiko). Hereafter the sample was abbreviated as Pt-MFI-97 (cationzeolite structure-degree of exchange). The pellet size of the catalyst tested ranged between 173 and 290 mm. The catalytic activity was evaluated with a fixed-bed flow reactor [ 31. In the case of experiments using synthesized gas, the reactants were 500 ppm of NO, 500 ppm of C2H4, 10% of 02, 1% of CO1, and 10% of Hz0 (helium balance) and the gas hourly space velocity (GHSV) was 72 000 h-‘, unless otherwise stated. Two types of diesel engines were employed, direct injection, 4-cycle, watercooled diesel engines with displacement values of 825 and 1500 cm3 and compression ratios of 16.6 and 18.5, respectively. Details of the experimental system have been reported elsewhere [ 81. The engines were operated at 1700-1800 rpm. JIS No. 2 light oil (sulfur content, 0.33 wt.-%) was used as a fuel. When the air-tofuel ratio (A) was 1.8, the exhaust contained approximately 350-1000 ppm of NO,V, 80-330 ppmC of unburned hydrocarbon, 300-1200 ppm of CO, IO-15% of 02, and 1.4-5.5% of H20. The concentration of SO* in the emission was not measured. In the experiment using the synthesized gas, a gas chromatograph was employed to measure the respective concentrations of nitrogen and nitrous oxide formed. With the actual exhaust gas, a NO,r analyzer was used to determine the degree of reduction of NO, since it was very difficult to use the gas chromatograph because of particulates and SOz in the gas. 3. Results and discussion 3.1. Durability of Pt-ZSM-5 The catalytic activity of Pt-MFI catalyst was first measured as a function of the time of use with the synthesized gas. The catalytic run was continued for 1000 h
M. Iwarnoto et al. /Applied
0’
I
I
200
0
Catalysis B: Environmental 5 (1994) LI-L5
r
I
400
600
800
L3
I
loo0
Time on stream I h Fig. 1.Change of the conversions to nitrogen (0) and nitrous oxide (0) with the reaction time over Pt-MFI-7 catalyst. NO, 500 ppm; C2H4,500 ppm; 02, 10%; CO*, 1%; H20, 10%. Total flow-rate, 150 cm3 min-‘; GHSV, 72 000 h-‘; Reaction temperature, 485 K.
mainly at 485 K and the change in the conversions to nitrogen and nitrous oxide is shown in Fig. 1. The conversion to nitrogen gradually decreased from 16% to 10% during the initial period of approximately 150 h, while the conversion to nitrous oxide increased from 11% to 20%. Both amounts then reached the respective constant values and did not change within the present experiment. It follows that Pt-MFI has a high durability for the SCR by ethene even in the presence of water. The effect of the concentration of ethylene and oxygen on the catalytic activity was investigated on the above stabilized catalyst and is summarized in Table 1. The nitrogen conversion increased with increasing concentration of ethylene. On the other hand the conversion to nitrous oxide also increased with the concentration of ethylene but leveled off at 20%. The results indicate that the ratio of the formation of nitrogen to nitrous oxide can be improved by increasing the concentration of ethylene. It should be noted that a decrease in the oxygen concentration resulted in a decrease in the conversions to nitrogen and nitrous oxide as well as in a reduction of the ethylene conversion. The activity was restored when the reaction conditions Table 1 Conversions to nitrogen and nitrous oxide of nitrogen dioxide on the stabilized Pt-MFI-7 catalyst ( COZ, 1%; H20, 10%; Temperature, 485 K; GHSV, 72 000 h-‘) Concentration
Conversion
NOippm
C,HJppm
021%
to N2 /%
to N,O/%
of C,H,/%
500 500 500 500
250 500 1000 500
10 10 10 5
4 10 15 6
12 20 20 7
100 100 60 40
L4
M. hamoto
et al. /Applied
Catalysis B: Envirormental5
(1993) Ll-W
were returned to the original values. As was previously pointed out [ 1J, it has again been confirmed that oxygen is the essential reactant to reduce nitrogen monoxide. 3.2. Catalytic activity of PT-ZSM-5 in actual exhaust gas The catalytic performance of Pt-MFI zeolite was first examined for the exhaust gas from the 825 cm3 diesel engine. The results on Pt-MFI-97 were illustrated as a function of reaction temperature in Fig. 2. The activity of Cu-MFI was also plotted for comparison. When the diesel engine was run with light oil the maximum conversion was observed at 523 K and the efficiency was about 30%. It is evident that the Pt-MFI catalyst has high efficiency for nitrogen monoxide reduction in the actual exhaust at a temperature as low as 500-600 IS. This is quite different from the decrease of the catalytic activity of A&O3or II-zeolites in the presence of water [ 91. Although the reason for the stability of Pt-MFI remains to be solved in future, the observation is very important from a practical point of view. The effect of ethylene addition into the actual diesel engine emission was examined on the 1500 cm3 diesel engine which emitted lower concentrations of NO, than the other engine. The durability and the effect of ethylene addition have been depicted in Fig. 3. The C/N value stands for the ratio of the number of carbon atoms in the exhaust (unburned hydrocarbon and added ethylene) to the number of NO, molecules included. The catalytic activity of Pt-MFI for the actual emission slightly decreased during the initial period of use but was entirely stable at 200500 h. In addition, the figure indicates the effectiveness of the addition of ethylene; the conversion to nitrogen increased monotonically with increasing the amount of 50
273
373
473
573
673
773
873
Reaction Temperature / K Fig. 2. Catalytic activity of Pt-MFI-97 for the exhaust from the 825 cm3 diesel engine. NO, loo0 ppm; unburned hydrocarbon, 250 ppmC; Ot, 10%; CO, 3 10 ppm; H,O, 1.4%;GHSV, 50 000 h- I.
M. Iwarnoto et al. /Applied
Catalysis B: Environmental 5 (1994) Ll-LS
L5
80
0
C/N Ratio / Fig. 3. Effect of ethylene addition on the catalytic activity of Pt-MFI-97. The conversions were weasured at the initial stage of use (0) and after the stabilization of activity (200-500 h, 0). The 1500 cm3 diesel engine; NO,, 350 ppm; unburned hydrocarbon, 83 ppmC; 02, 15.0%; CO 500 ppm; H,O, 5.5%; Reaction temperature 483 K; GHSV, 70 000 h-‘.
ethylene added. This strongly suggests the present selective reduction-system to be a feasible method for diesel engines. The dependence of the catalytic activity on the GHSV is shown in Fig. 4. The conversion of NO, at C/N = 3 was approximately 49% at 50 000 h- ’ and 39% at 150 000 h-‘. Clearly Pt-MFI catalyst has excellent activity for the SCR-HC even at such high GHSV. 80
0 2
I
I
I
6
IO
14
18
GHSV / 1 O4 h-’ Fig. 4. GHSV dependence of the catalytic activity of Pt-MFI-97. NO,, 350 ppm; unburned hydrocarbon, 83 ppmC; added ethylene, 480 ppm; OS, 9.6%; CO, 500 ppm; H20, 5.5%; Reaction temperature, 483 K.
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M. hoamoto et al. /Applied Catalysis& EnhnmentalS
(1993) Ll-L.5
The high catalytic activity and the stability of the activity of Pt-MFI were here confirmed in actual diesel engine exhaust. The formation of nitrous oxide, however, remains an important problem of the Pt-MFI catalyst, since nitrous oxide is a strong greenhouse gas.
Acknowledgements This work was supported in part by a grant-in-Aid for Scientific Research from the ministry of Education, Science, and Culture of Japan, and Development of an Environment Friendly Co-generation System of PEC.
References [ 11 M. Iwamoto and N. Mizuno, J. Automobile Eng., 207 (1993) 23. [2] G. Zhang, T. Yamaguchi, H. Kawakami and T. Suzuki, Appl. Catal, B, 1 ( 1992) L15, [3] H. Hirabayashi, H. Yahiro, N. Mizuno and M, Iwamoto, Chcm. Lett., ( 1992) 2235. [4] H.K. Shin, H. Hirabayashi, H. Yahiro, N. Mizuno and M. Iwamoto, Shokubai, 35 ( 1993) 402. [5] A. Obuchi, A. Ohi, M. Nakamura, A. Ogata, K. Mizuno and H. Ohuchi, Appl. Catal. B, 2 (1993) 71. [6] M. Truex, PlatinumMetals Rev., 36 (1992) 2. [7] H. Hamada, Y. Kintaichi, T. Yoshinari, M. Sasaki and T. Ito, Proc. National Meeting of the Catalysis Sot. of Japan, Niigata, Oct. 1992, p. 103. [S] M. Konno, T. Chikahisa, T. Murayama and h?. Iwamoto, SAB Transactions, 920091 ( 1992) , [9] H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito and M. Tabata, Appl. Catal., 64 ( 1990) Ll.
Applied Catalysis B: Environmental 5 ( 1994) Ll-L5
Performance and durability of Pt-MFI zeolite catalyst for selective reduction of nitrogen monoxide in actual diesel engine exhaust Masakazu Iwamoto a~*, Hidenori Yahiro a, Hyun Khil Shin a, Masami Watanabe b, Jianwei Guo ‘, Mitsuru Konno ‘, Takemi Chikahisa ‘, Tadashi Murayama ’ aCatalysis Research Center, Hokkaido Universiry, Sapporo 060, Japan b Ishikawajima-Harima Heavy Industries Co. Ltd., Koto-kn, Tokyo 135, Japan ’ Faculty of Engineering, Hokkaido
University, Sapporo 060, Japan
Received 15 September 1994; accepted 19 September 1994
Abstract Selective catalytic reduction of nitrogen monoxide (NO) by hydrocarbon in an oxidizing atmosphere has been studied over platinum-MFI zeolite (Pt-MFI) in synthesized or actual diesel engine exhaust gases. The activity of Pt-MFI in the synthesized gas, containing 10% water, changed in the early stage of the use, leveled off after 150-200 h, and remained constant for more than 800 h. The Pt-MFI catalyst also showed stable activity at 423-773 K and 10 000-150 000 h- I (gas hourly space velocity) in actual engine exhaust with light oil as a fuel. The degree of nitrogen monoxide reduction increased linearly upon addition of ethylene into the exhaust gas. Keywords: Diesel engine exhaust; MFI zeolite; Nitrogen monoxide; Platinum; Selective reduction; Zeolites
1. Introduction Selective catalytic reduction (SCR) of nitrogen monoxide by hydrocarbon in an oxidizing atmosphere (SCR-HC) has been extensively studied as a potential process for nitrogen monoxide removal from the emissions of diesel and lean-burn gasoline engines [ 11. Although many catalysts have been reported to be active, supported platinum catalysts, Pt/B203-Si02-A&O3 [2], Pt-zeolite [ 3,4], Pt/ AlTO3 [ 5,6], and Pt/SiO, [ 71 have widely been recognized to show the highest * Corresponding
author. Fax. ( +81-11) 7578126.
0926-3373/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSO10926-3373 (94)00047-6
L2
M. lwatnoto et al. /Applied Catalysis B: Etn~irwtttettta/ 5 (199-f) ,!.I-65
activity for the SCR-HC at temperatures as low as 453-573 K, An additionally interesting and significant point of platinum catalysts is the better performance in the presence of water and/or SO:! than those of the other catalysts. Obuchi et al. [5], however, have recently demonstrated that the catalytic activity of Pt/AI,O, in real diesel engine exhaust has gradually decreased with the time OFuse. In contrast, the performance of Pt-ZSM-5 zeolite has been reported not to change during the use in 8.6% water vapor except during the first few hours [ 31, Here the durability and the performance of Pt-ZSM-5 have been examined in actual diesel engine exhaust in more detail. 2. Experimental The MFI (ZSM-5, silica/alumina= 23.3) zeolite was supplied from Tosoh Corporation in the Na’ form. Pt-MFI catalysts were prepared by the ion exchange method described previously [ 31. The exchange level of platinum ion was 7 and 97%, which was determined by inductively coupled plasma spectroscopy (ICP) (SPS-1100, Seiko). Hereafter the sample was abbreviated as Pt-MFI-97 (cationzeolite structure-degree of exchange). The pellet size of the catalyst tested ranged between 173 and 290 mm. The catalytic activity was evaluated with a fixed-bed flow reactor [ 31. In the case of experiments using synthesized gas, the reactants were 500 ppm of NO, 500 ppm of C2H4, 10% of 02, 1% of CO1, and 10% of Hz0 (helium balance) and the gas hourly space velocity (GHSV) was 72 000 h-‘, unless otherwise stated. Two types of diesel engines were employed, direct injection, 4-cycle, watercooled diesel engines with displacement values of 825 and 1500 cm3 and compression ratios of 16.6 and 18.5, respectively. Details of the experimental system have been reported elsewhere [ 81. The engines were operated at 1700-1800 rpm. JIS No. 2 light oil (sulfur content, 0.33 wt.-%) was used as a fuel. When the air-tofuel ratio (A) was 1.8, the exhaust contained approximately 350-1000 ppm of NO,V, 80-330 ppmC of unburned hydrocarbon, 300-1200 ppm of CO, IO-15% of 02, and 1.4-5.5% of H20. The concentration of SO* in the emission was not measured. In the experiment using the synthesized gas, a gas chromatograph was employed to measure the respective concentrations of nitrogen and nitrous oxide formed. With the actual exhaust gas, a NO,r analyzer was used to determine the degree of reduction of NO, since it was very difficult to use the gas chromatograph because of particulates and SOz in the gas. 3. Results and discussion 3.1. Durability of Pt-ZSM-5 The catalytic activity of Pt-MFI catalyst was first measured as a function of the time of use with the synthesized gas. The catalytic run was continued for 1000 h
M. Iwarnoto et al. /Applied
0’
I
I
200
0
Catalysis B: Environmental 5 (1994) LI-L5
r
I
400
600
800
L3
I
loo0
Time on stream I h Fig. 1.Change of the conversions to nitrogen (0) and nitrous oxide (0) with the reaction time over Pt-MFI-7 catalyst. NO, 500 ppm; C2H4,500 ppm; 02, 10%; CO*, 1%; H20, 10%. Total flow-rate, 150 cm3 min-‘; GHSV, 72 000 h-‘; Reaction temperature, 485 K.
mainly at 485 K and the change in the conversions to nitrogen and nitrous oxide is shown in Fig. 1. The conversion to nitrogen gradually decreased from 16% to 10% during the initial period of approximately 150 h, while the conversion to nitrous oxide increased from 11% to 20%. Both amounts then reached the respective constant values and did not change within the present experiment. It follows that Pt-MFI has a high durability for the SCR by ethene even in the presence of water. The effect of the concentration of ethylene and oxygen on the catalytic activity was investigated on the above stabilized catalyst and is summarized in Table 1. The nitrogen conversion increased with increasing concentration of ethylene. On the other hand the conversion to nitrous oxide also increased with the concentration of ethylene but leveled off at 20%. The results indicate that the ratio of the formation of nitrogen to nitrous oxide can be improved by increasing the concentration of ethylene. It should be noted that a decrease in the oxygen concentration resulted in a decrease in the conversions to nitrogen and nitrous oxide as well as in a reduction of the ethylene conversion. The activity was restored when the reaction conditions Table 1 Conversions to nitrogen and nitrous oxide of nitrogen dioxide on the stabilized Pt-MFI-7 catalyst ( COZ, 1%; H20, 10%; Temperature, 485 K; GHSV, 72 000 h-‘) Concentration
Conversion
NOippm
C,HJppm
021%
to N2 /%
to N,O/%
of C,H,/%
500 500 500 500
250 500 1000 500
10 10 10 5
4 10 15 6
12 20 20 7
100 100 60 40
L4
M. hamoto
et al. /Applied
Catalysis B: Envirormental5
(1993) Ll-W
were returned to the original values. As was previously pointed out [ 1J, it has again been confirmed that oxygen is the essential reactant to reduce nitrogen monoxide. 3.2. Catalytic activity of PT-ZSM-5 in actual exhaust gas The catalytic performance of Pt-MFI zeolite was first examined for the exhaust gas from the 825 cm3 diesel engine. The results on Pt-MFI-97 were illustrated as a function of reaction temperature in Fig. 2. The activity of Cu-MFI was also plotted for comparison. When the diesel engine was run with light oil the maximum conversion was observed at 523 K and the efficiency was about 30%. It is evident that the Pt-MFI catalyst has high efficiency for nitrogen monoxide reduction in the actual exhaust at a temperature as low as 500-600 IS. This is quite different from the decrease of the catalytic activity of A&O3or II-zeolites in the presence of water [ 91. Although the reason for the stability of Pt-MFI remains to be solved in future, the observation is very important from a practical point of view. The effect of ethylene addition into the actual diesel engine emission was examined on the 1500 cm3 diesel engine which emitted lower concentrations of NO, than the other engine. The durability and the effect of ethylene addition have been depicted in Fig. 3. The C/N value stands for the ratio of the number of carbon atoms in the exhaust (unburned hydrocarbon and added ethylene) to the number of NO, molecules included. The catalytic activity of Pt-MFI for the actual emission slightly decreased during the initial period of use but was entirely stable at 200500 h. In addition, the figure indicates the effectiveness of the addition of ethylene; the conversion to nitrogen increased monotonically with increasing the amount of 50
273
373
473
573
673
773
873
Reaction Temperature / K Fig. 2. Catalytic activity of Pt-MFI-97 for the exhaust from the 825 cm3 diesel engine. NO, loo0 ppm; unburned hydrocarbon, 250 ppmC; Ot, 10%; CO, 3 10 ppm; H,O, 1.4%;GHSV, 50 000 h- I.
M. Iwarnoto et al. /Applied
Catalysis B: Environmental 5 (1994) Ll-LS
L5
80
0
C/N Ratio / Fig. 3. Effect of ethylene addition on the catalytic activity of Pt-MFI-97. The conversions were weasured at the initial stage of use (0) and after the stabilization of activity (200-500 h, 0). The 1500 cm3 diesel engine; NO,, 350 ppm; unburned hydrocarbon, 83 ppmC; 02, 15.0%; CO 500 ppm; H,O, 5.5%; Reaction temperature 483 K; GHSV, 70 000 h-‘.
ethylene added. This strongly suggests the present selective reduction-system to be a feasible method for diesel engines. The dependence of the catalytic activity on the GHSV is shown in Fig. 4. The conversion of NO, at C/N = 3 was approximately 49% at 50 000 h- ’ and 39% at 150 000 h-‘. Clearly Pt-MFI catalyst has excellent activity for the SCR-HC even at such high GHSV. 80
0 2
I
I
I
6
IO
14
18
GHSV / 1 O4 h-’ Fig. 4. GHSV dependence of the catalytic activity of Pt-MFI-97. NO,, 350 ppm; unburned hydrocarbon, 83 ppmC; added ethylene, 480 ppm; OS, 9.6%; CO, 500 ppm; H20, 5.5%; Reaction temperature, 483 K.
L6
M. hoamoto et al. /Applied Catalysis& EnhnmentalS
(1993) Ll-L.5
The high catalytic activity and the stability of the activity of Pt-MFI were here confirmed in actual diesel engine exhaust. The formation of nitrous oxide, however, remains an important problem of the Pt-MFI catalyst, since nitrous oxide is a strong greenhouse gas.
Acknowledgements This work was supported in part by a grant-in-Aid for Scientific Research from the ministry of Education, Science, and Culture of Japan, and Development of an Environment Friendly Co-generation System of PEC.
References [ 11 M. Iwamoto and N. Mizuno, J. Automobile Eng., 207 (1993) 23. [2] G. Zhang, T. Yamaguchi, H. Kawakami and T. Suzuki, Appl. Catal, B, 1 ( 1992) L15, [3] H. Hirabayashi, H. Yahiro, N. Mizuno and M, Iwamoto, Chcm. Lett., ( 1992) 2235. [4] H.K. Shin, H. Hirabayashi, H. Yahiro, N. Mizuno and M. Iwamoto, Shokubai, 35 ( 1993) 402. [5] A. Obuchi, A. Ohi, M. Nakamura, A. Ogata, K. Mizuno and H. Ohuchi, Appl. Catal. B, 2 (1993) 71. [6] M. Truex, PlatinumMetals Rev., 36 (1992) 2. [7] H. Hamada, Y. Kintaichi, T. Yoshinari, M. Sasaki and T. Ito, Proc. National Meeting of the Catalysis Sot. of Japan, Niigata, Oct. 1992, p. 103. [S] M. Konno, T. Chikahisa, T. Murayama and h?. Iwamoto, SAB Transactions, 920091 ( 1992) , [9] H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito and M. Tabata, Appl. Catal., 64 ( 1990) Ll.