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Perovskite-type oxides as catalysts for selective reduction of nitric oxide by ethylene
ili~iii!i!iii!ii~!i~!iiii~i!~ii!i!iiiiiii~ surface science ELSEVIER
Applied Surface Science 121 / 122 (1997) 505-508
Perovskite-type oxides as catalysts for selective reduction of nitric oxide by ethylene T. Harada b, y. Teraoka a, S. Kagawa a,* Department of Applied Chemistry, Faculty of Engineering, Nagasaki UniversiO', Nagasaki 852, Japan b Fukuoka Industrial Technology Center, 332-1 Kamikoga, Chikushi, Fukuoka 818, Japan Received 1 November 1996; accepted 12 February 1997
Abstract The selective catalytic reduction of NO with C2H 4 (C2H4-SCR) was investigated over perovskites containing Co, Mn, Fe, Cr, A1, Sn and Ti as host B-site cations. The C2H4-SCR activity was observed over AI-, Sn- and Ti-based oxides which were free from redox-active metal cations such as Co and Cu and innately poor in oxidation activity. Over perovskites containing redox-active metal cations, the undesirable consumption of C2H 4 by O 2 proceeded preferentially. © 1997 Elsevier Science B.V. Keywords: Perovskite-type oxide; Selective reduction of NO; Ethylene
1. Introduction In 1990, Iwamoto et al. [1] and Held et al. [2] reported independently the selective catalytic reduction of NOx with hydrocarbons (HC-SCR). Thereafter, many reports have been published in which zeolitic materials and simple metal oxides like A1203 with and without transition metal promoters were almost exclusively used [3-5]. In contrast, little has been reported on the HC-SCR activity of crystalline mixed metal oxides. This paper reports the catalytic activity for the selective reduction of NO with C2H 4 (C 2H4-SCR) of perovskite-type oxides (ABO 3) containing Co, Mn, Fe, Cr, A1, Sn and Ti as host B-site cations. These perovskites are wide-ranging in terms
Corresponding author. Tel./fax: +81-958-489652; e-mail: [email protected].
of the oxidation activity, and are classified into active (Co, Mn), moderate (Cr, Fe) and less active (A1, Sn, Ti) catalysts [6]. In addition, perovskites of Co, Mn and Fe catalyzed the direct decomposition of NO [7,8]. Accordingly, the measurement of the C z H 4 - S C R activity of these perovskites must give information about the relation of the C2H4-SCR activity to the oxidation and NO decomposition activity.
2. Experimental Perovskite-type oxides were prepared by calcining appropriate mixtures of nitrates (Mg, Fe, AI), chlorides (Sn, Ti) and acetates (others) mostly at 850°C for 5 - 1 0 h. The catalytic activity was measured by feeding He-balanced reaction gases containing 0.44% NO and varying amounts of C 2 H 4 and 0 2 at a rate
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 3 5 4 - 1
T. Harada et al./Applied Surface Science 121 / 122 (1997) 505-508
506
of 15 cm 3 rain ~ over 0.25 g of a catalyst (W/F = 1.0 g s cm-3). After one hour on stream at each temperature at which the reaction reached the steady state, the gaseous components were analyzed by gas chromatography. N20 was never detected in all the cases and the NO reduction activity was evaluated by the conversion of NO into N 2 (X[N2]).
100
80 ..... 60. ~ ~°c ~
g 4o • og
2o
3. Results and discussion 2
Fig. 1 shows the temperature dependence of X[N2] in v a r i o u s reaction gases over Lao.4Sro.6Mno.sNio.203 which was reported to catalyze the direct decomposition of NO [8]. The oxide showed the activity for the NO decomposition in the N O - H e atmosphere above 500°C, and the NO decomposition activity decreased by the coexisting 02 because of the competition between NO and O 2 for the active sites [8]. In the presence of C2H 4, the reduction of NO by C2H 4 in the O2-free atmosphere ( N O - C 2 H 4) started around 400°C and increased monotonically with increasing temperature, while the NO reduction activity tremendously decreased when the excess oxygen (P(O2)/P(C2H4)=9.6) was added to the reaction gas. Fig. 2 shows the NO reduction activity of Lao.4Sro.6Mno.sNio.203 in N O - C 2 H 4 - O 2 atmosphere as a function of the partial pressure ratio of 02 to C2H 4. The drastic change of the NO reduction activity was observed at a P(O 2 ) / P ( C 2 H 4) ratio of 3 which corresponds to the stoichiometry of the complete oxidation of C 2 H 4 ( C 2 H 4 4- 3 0 2 ---->2 C O 2 100
z7
8O
-~- 60
g_ g 20
b
(D
0 3OO
400
500 600 700 Temperature / °C
800
Fig. 1. Conversion of NO into N 2 over Lao.4Sr0.6Mno.sNio.203 in (a) NO, (b) N O - O > (c) NO-C2H 4 and (d) N O - C 2 H 4 - O 2 reaction gases. NO, 0.44%; C2H4, 0.073%; 02, 0.7%.
4 6 8 P(02) / P (C2H4)
10
Fig. 2. NO reduction activity of Lao4Sr06Mno.sNi~203 as a function of the partial pressure ratio of O, to C2H 4. NO(0.44~) C2H4(variable)-O2(0 or 0.7%).
+ 2H20). It can thus be concluded that in the oxygen-rich region, P ( O 2 ) / P ( C 2 H 4) > 3, the 0 2 C2 H4 reaction proceeds preferentially over the N O C2H 4 reaction and that in the oxygen-lean region, P(O2)/P(CeH 4) < 3, part of C2H 4 which is left after reacting with 02 reacts with NO to form N 2. The C2H4-SCR activity was not observed over Lao.sSro.2CoO 3 and La l_XSrXcoo.4Feo.603 (x = 0.4, 0.8) as well, which are reported to be active for the NO decomposition [7,8] and the complete oxidation of hydrocarbons [6,9]. These results demonstrate that perovskite catalysts active for NO decomposition do not necessarily show HC-SCR activity, as already pointed out by Iwamoto and Mizuno [3]. The reason why these catalysts are incapable to reduce NO in the presence of excess oxygen is the preferential consumption of C2H 4 by 02. The catalytic activities of perovskites which are innately less active than the Mn- and Co-based perovskites were investigated. The C2H4-SCR activity was not observed over moderately active Lao.2Sro.sFeO 3 and LaCrO 3 but over perovskites containing AI, Sn and Ti (Table 1). As shown in Fig. 3, the NO reduction over LaA103 in N O - C 2 H 4 - O 2 atmosphere occurred in a lower temperature range than that in N O - C 2 H 4 atmosphere, indicating the promotion effect of the coexisting oxygen on the NO reduction, that is, the occurrence of the C2H4-SCR reaction over LaAIO 3. It is to be noted that a significant amount of CO is formed in the temperature range of the N 2 formation. As can be seen from Table 1, the C2Ha-SCR
T. Harada et al./Applied Surface Science 121 / 122 (1997) 505-508 Table 1 Selective reduction of NO by C 2 H 4 over AI-, Sn- and Ti-based perovskite-type oxides Catalyst
Conversion of NO into N 2 (%) 300°C
400°C
500°C
600°C
700°C
11.3 87.6
1.0 0 0.2 0 0 0 0 0 0 19.5 62.3
20.0 15.2 19.5 0 0 1.3 1.8 1.7 4.3 29.2 44.9
14.2 21.3 14.5 2.6 2.4 1.5 2.6 8.0 11.1 56.8 18.4
5.5 7.6 6.0 6.6 5.0 4.2 4.0 3.7 6.3 18.5 13.0
LaA1Oa Lao sSr02 AIO3 LaAlo 9Mgo iO~ LaAl0 95Cu0.050 3 LaAl0 95Ni0 osOa LaAI 0 95 C00.0503
LaAlo.99Coo 0103 SrSnO 3 CaTiO 3 AI203 ~ Cu(ll7)-ZSM-5 b
Reaction conditions: NO (0.44%), C2H 4 (0.44%), 0 2 (4.4%), W / F = l . O g s c m 3. JRC-ALO-4. Catalysis Society of Japan. b Cu ion-exchange level: 117%.
reaction was catalyzed by LaA103, Lao.~Sro2AlO 3, LaAlo.gMgo.lO3, SrSnO 3 and CaTiO 3, and the A1based oxides were more active than the others. It should be stated, however, that the C2H4-SCR activity of perovskite catalysts is lower than that of representative HC-SCR catalysts of Cu-ZSM-5 and AI203 (Table 1). The partial substitution of redoxinactive Sr and Mg in LaA1Q caused a little change of the activity, while the C2H4-SCR activity disappeared by the substitution of redox-active Cu, Co and Ni for only 5 or 1 mol% of AI. This is in a marked contrast to the A1203 system in which the
100
•
80 60
b
~> 40
d......... a
20 0
,-"
o.___..o....... --e~
400
'
507
addition of a small amount of Cu and Co enhances the HC-SCR activity [3,4]. CaSno.sCo0.203 and CaTi0sFeo.203 also showed no activity for the C 2H4-SCR reaction. The substitution of such redox-active metal cations brought about the disappearance of the formation of CO irrespective of the substitution levels as well as an increase in the conversion of C 2 H 4 at the substitution levels of 5% or more. The conversion of C 2 H 4 at 500°C was comparable between the C 2 H 4 - S C R active LaA10.9Mg0.10 3 (57.5%) and inactive LaA10.99Co00~O 3 (64.4%), but the selectivity to CO formation was quite different between 24.7% for the former and 0% for the latter. Over the perovskite-type oxides showing the C2H4-SCR activity, the formation of CO was always observed and its temperature dependence was quite similar to that of the N 2 formation, as shown in Fig. 3 for LaA103. These results clearly indicate the implication of the formation of N 2 and CO over perovskite catalysts, and actually the parallelism of their formation was reported over rare earth metal oxides [10]. it might be speculated that the C2H4-SCR reaction takes place over perovskite catalysts which is too poor in the activity to c o n v e r t C 2 H 4 completely into CO 2 and/or the reaction proceeds through the mechanism involving the generation of CO. In conclusion, perovskite-type oxides containing redox-active metals cations as either host or substituent metal cations showed no C 2H4-SCR activity because of the preferential proceeding of an undesirable C 2 H 4 oxidation by 02. The C2H4-SCR activity was observed over AI-, Sn- and Ti-based perovskites which are innately poor in terms of oxidation activity. It has been reported that the oxidation activity of perovskites has a close implication to that of B-site cations [6]. Since simple oxides of their B-site cations (A1203, SnO2, TiO:) show the activity for the HCSCR reaction [4], it can be said that the nature of B-site cations is also important for the HC-SCR activity of perovskites.
0 ..........: " ' ~ A
500 600 Temperature/ "C
700
Fig. 3. Reduction of NO with C2H 4 over LaAIO 3. The conversion of NO into N 2 (a) and that of C 2 H 4 into CO 2 (b) and CO (c) in NO(0.44%)-C2H4(0.44%)-O2(4.4%). The conversion of NO into N z (d) in NO(0.44%)-C2H4(0.44%).
Acknowledgements This study was partly supported by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan.
508
T. Harada et al./ Applied Surface Science 121 / 122 (1997) 505-508
References [1] M. Iwamoto, H. Yahiro, Y. Yu-u, S. Shundo, N. Mizuno, Shokubai (Catalyst) 32 (1990) 430. [2] W. Held, A. K/3nig, T. Richter, L. Puppe, SAE Paper (1990) 900496. [3] M. lwamoto, N. Mizuno, J. Automobile Eng. 207 (1993) 23. [4] H. Hamada, Catal. Today 22 (1994) 21.
[5] [6] [7] [8]
M. Shelef, Chem. Rev. 95 (1995) 209. N. Yamazoe. Y. Teraoka, Catal. Today 8 (1990) 175. Y. Teraoka, H. Fukuda, S. Kagawa, Chem. Lett. (1990) 1. Y. Teraoka, T. Harada, H. Furukawa, S. Kagawa, Stud. Surf. Sci. Catal. 75 (1993) 2649. [9] H.M. Zhang, Y. Shimizu, Y. Teraoka, N. Miura, N. Yamazoe, J. Catal. 121 (1990) 432. [10] H. Hamada, Y. Kintaichi, M. Tabata, M. Sasaki, T. Ito, Sekiyu Gakkaishi 36 (1993) 149.
Applied Surface Science 121 / 122 (1997) 505-508
Perovskite-type oxides as catalysts for selective reduction of nitric oxide by ethylene T. Harada b, y. Teraoka a, S. Kagawa a,* Department of Applied Chemistry, Faculty of Engineering, Nagasaki UniversiO', Nagasaki 852, Japan b Fukuoka Industrial Technology Center, 332-1 Kamikoga, Chikushi, Fukuoka 818, Japan Received 1 November 1996; accepted 12 February 1997
Abstract The selective catalytic reduction of NO with C2H 4 (C2H4-SCR) was investigated over perovskites containing Co, Mn, Fe, Cr, A1, Sn and Ti as host B-site cations. The C2H4-SCR activity was observed over AI-, Sn- and Ti-based oxides which were free from redox-active metal cations such as Co and Cu and innately poor in oxidation activity. Over perovskites containing redox-active metal cations, the undesirable consumption of C2H 4 by O 2 proceeded preferentially. © 1997 Elsevier Science B.V. Keywords: Perovskite-type oxide; Selective reduction of NO; Ethylene
1. Introduction In 1990, Iwamoto et al. [1] and Held et al. [2] reported independently the selective catalytic reduction of NOx with hydrocarbons (HC-SCR). Thereafter, many reports have been published in which zeolitic materials and simple metal oxides like A1203 with and without transition metal promoters were almost exclusively used [3-5]. In contrast, little has been reported on the HC-SCR activity of crystalline mixed metal oxides. This paper reports the catalytic activity for the selective reduction of NO with C2H 4 (C 2H4-SCR) of perovskite-type oxides (ABO 3) containing Co, Mn, Fe, Cr, A1, Sn and Ti as host B-site cations. These perovskites are wide-ranging in terms
Corresponding author. Tel./fax: +81-958-489652; e-mail: [email protected].
of the oxidation activity, and are classified into active (Co, Mn), moderate (Cr, Fe) and less active (A1, Sn, Ti) catalysts [6]. In addition, perovskites of Co, Mn and Fe catalyzed the direct decomposition of NO [7,8]. Accordingly, the measurement of the C z H 4 - S C R activity of these perovskites must give information about the relation of the C2H4-SCR activity to the oxidation and NO decomposition activity.
2. Experimental Perovskite-type oxides were prepared by calcining appropriate mixtures of nitrates (Mg, Fe, AI), chlorides (Sn, Ti) and acetates (others) mostly at 850°C for 5 - 1 0 h. The catalytic activity was measured by feeding He-balanced reaction gases containing 0.44% NO and varying amounts of C 2 H 4 and 0 2 at a rate
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 3 5 4 - 1
T. Harada et al./Applied Surface Science 121 / 122 (1997) 505-508
506
of 15 cm 3 rain ~ over 0.25 g of a catalyst (W/F = 1.0 g s cm-3). After one hour on stream at each temperature at which the reaction reached the steady state, the gaseous components were analyzed by gas chromatography. N20 was never detected in all the cases and the NO reduction activity was evaluated by the conversion of NO into N 2 (X[N2]).
100
80 ..... 60. ~ ~°c ~
g 4o • og
2o
3. Results and discussion 2
Fig. 1 shows the temperature dependence of X[N2] in v a r i o u s reaction gases over Lao.4Sro.6Mno.sNio.203 which was reported to catalyze the direct decomposition of NO [8]. The oxide showed the activity for the NO decomposition in the N O - H e atmosphere above 500°C, and the NO decomposition activity decreased by the coexisting 02 because of the competition between NO and O 2 for the active sites [8]. In the presence of C2H 4, the reduction of NO by C2H 4 in the O2-free atmosphere ( N O - C 2 H 4) started around 400°C and increased monotonically with increasing temperature, while the NO reduction activity tremendously decreased when the excess oxygen (P(O2)/P(C2H4)=9.6) was added to the reaction gas. Fig. 2 shows the NO reduction activity of Lao.4Sro.6Mno.sNio.203 in N O - C 2 H 4 - O 2 atmosphere as a function of the partial pressure ratio of 02 to C2H 4. The drastic change of the NO reduction activity was observed at a P(O 2 ) / P ( C 2 H 4) ratio of 3 which corresponds to the stoichiometry of the complete oxidation of C 2 H 4 ( C 2 H 4 4- 3 0 2 ---->2 C O 2 100
z7
8O
-~- 60
g_ g 20
b
(D
0 3OO
400
500 600 700 Temperature / °C
800
Fig. 1. Conversion of NO into N 2 over Lao.4Sr0.6Mno.sNio.203 in (a) NO, (b) N O - O > (c) NO-C2H 4 and (d) N O - C 2 H 4 - O 2 reaction gases. NO, 0.44%; C2H4, 0.073%; 02, 0.7%.
4 6 8 P(02) / P (C2H4)
10
Fig. 2. NO reduction activity of Lao4Sr06Mno.sNi~203 as a function of the partial pressure ratio of O, to C2H 4. NO(0.44~) C2H4(variable)-O2(0 or 0.7%).
+ 2H20). It can thus be concluded that in the oxygen-rich region, P ( O 2 ) / P ( C 2 H 4) > 3, the 0 2 C2 H4 reaction proceeds preferentially over the N O C2H 4 reaction and that in the oxygen-lean region, P(O2)/P(CeH 4) < 3, part of C2H 4 which is left after reacting with 02 reacts with NO to form N 2. The C2H4-SCR activity was not observed over Lao.sSro.2CoO 3 and La l_XSrXcoo.4Feo.603 (x = 0.4, 0.8) as well, which are reported to be active for the NO decomposition [7,8] and the complete oxidation of hydrocarbons [6,9]. These results demonstrate that perovskite catalysts active for NO decomposition do not necessarily show HC-SCR activity, as already pointed out by Iwamoto and Mizuno [3]. The reason why these catalysts are incapable to reduce NO in the presence of excess oxygen is the preferential consumption of C2H 4 by 02. The catalytic activities of perovskites which are innately less active than the Mn- and Co-based perovskites were investigated. The C2H4-SCR activity was not observed over moderately active Lao.2Sro.sFeO 3 and LaCrO 3 but over perovskites containing AI, Sn and Ti (Table 1). As shown in Fig. 3, the NO reduction over LaA103 in N O - C 2 H 4 - O 2 atmosphere occurred in a lower temperature range than that in N O - C 2 H 4 atmosphere, indicating the promotion effect of the coexisting oxygen on the NO reduction, that is, the occurrence of the C2H4-SCR reaction over LaAIO 3. It is to be noted that a significant amount of CO is formed in the temperature range of the N 2 formation. As can be seen from Table 1, the C2Ha-SCR
T. Harada et al./Applied Surface Science 121 / 122 (1997) 505-508 Table 1 Selective reduction of NO by C 2 H 4 over AI-, Sn- and Ti-based perovskite-type oxides Catalyst
Conversion of NO into N 2 (%) 300°C
400°C
500°C
600°C
700°C
11.3 87.6
1.0 0 0.2 0 0 0 0 0 0 19.5 62.3
20.0 15.2 19.5 0 0 1.3 1.8 1.7 4.3 29.2 44.9
14.2 21.3 14.5 2.6 2.4 1.5 2.6 8.0 11.1 56.8 18.4
5.5 7.6 6.0 6.6 5.0 4.2 4.0 3.7 6.3 18.5 13.0
LaA1Oa Lao sSr02 AIO3 LaAlo 9Mgo iO~ LaAl0 95Cu0.050 3 LaAl0 95Ni0 osOa LaAI 0 95 C00.0503
LaAlo.99Coo 0103 SrSnO 3 CaTiO 3 AI203 ~ Cu(ll7)-ZSM-5 b
Reaction conditions: NO (0.44%), C2H 4 (0.44%), 0 2 (4.4%), W / F = l . O g s c m 3. JRC-ALO-4. Catalysis Society of Japan. b Cu ion-exchange level: 117%.
reaction was catalyzed by LaA103, Lao.~Sro2AlO 3, LaAlo.gMgo.lO3, SrSnO 3 and CaTiO 3, and the A1based oxides were more active than the others. It should be stated, however, that the C2H4-SCR activity of perovskite catalysts is lower than that of representative HC-SCR catalysts of Cu-ZSM-5 and AI203 (Table 1). The partial substitution of redoxinactive Sr and Mg in LaA1Q caused a little change of the activity, while the C2H4-SCR activity disappeared by the substitution of redox-active Cu, Co and Ni for only 5 or 1 mol% of AI. This is in a marked contrast to the A1203 system in which the
100
•
80 60
b
~> 40
d......... a
20 0
,-"
o.___..o....... --e~
400
'
507
addition of a small amount of Cu and Co enhances the HC-SCR activity [3,4]. CaSno.sCo0.203 and CaTi0sFeo.203 also showed no activity for the C 2H4-SCR reaction. The substitution of such redox-active metal cations brought about the disappearance of the formation of CO irrespective of the substitution levels as well as an increase in the conversion of C 2 H 4 at the substitution levels of 5% or more. The conversion of C 2 H 4 at 500°C was comparable between the C 2 H 4 - S C R active LaA10.9Mg0.10 3 (57.5%) and inactive LaA10.99Co00~O 3 (64.4%), but the selectivity to CO formation was quite different between 24.7% for the former and 0% for the latter. Over the perovskite-type oxides showing the C2H4-SCR activity, the formation of CO was always observed and its temperature dependence was quite similar to that of the N 2 formation, as shown in Fig. 3 for LaA103. These results clearly indicate the implication of the formation of N 2 and CO over perovskite catalysts, and actually the parallelism of their formation was reported over rare earth metal oxides [10]. it might be speculated that the C2H4-SCR reaction takes place over perovskite catalysts which is too poor in the activity to c o n v e r t C 2 H 4 completely into CO 2 and/or the reaction proceeds through the mechanism involving the generation of CO. In conclusion, perovskite-type oxides containing redox-active metals cations as either host or substituent metal cations showed no C 2H4-SCR activity because of the preferential proceeding of an undesirable C 2 H 4 oxidation by 02. The C2H4-SCR activity was observed over AI-, Sn- and Ti-based perovskites which are innately poor in terms of oxidation activity. It has been reported that the oxidation activity of perovskites has a close implication to that of B-site cations [6]. Since simple oxides of their B-site cations (A1203, SnO2, TiO:) show the activity for the HCSCR reaction [4], it can be said that the nature of B-site cations is also important for the HC-SCR activity of perovskites.
0 ..........: " ' ~ A
500 600 Temperature/ "C
700
Fig. 3. Reduction of NO with C2H 4 over LaAIO 3. The conversion of NO into N 2 (a) and that of C 2 H 4 into CO 2 (b) and CO (c) in NO(0.44%)-C2H4(0.44%)-O2(4.4%). The conversion of NO into N z (d) in NO(0.44%)-C2H4(0.44%).
Acknowledgements This study was partly supported by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan.
508
T. Harada et al./ Applied Surface Science 121 / 122 (1997) 505-508
References [1] M. Iwamoto, H. Yahiro, Y. Yu-u, S. Shundo, N. Mizuno, Shokubai (Catalyst) 32 (1990) 430. [2] W. Held, A. K/3nig, T. Richter, L. Puppe, SAE Paper (1990) 900496. [3] M. lwamoto, N. Mizuno, J. Automobile Eng. 207 (1993) 23. [4] H. Hamada, Catal. Today 22 (1994) 21.
[5] [6] [7] [8]
M. Shelef, Chem. Rev. 95 (1995) 209. N. Yamazoe. Y. Teraoka, Catal. Today 8 (1990) 175. Y. Teraoka, H. Fukuda, S. Kagawa, Chem. Lett. (1990) 1. Y. Teraoka, T. Harada, H. Furukawa, S. Kagawa, Stud. Surf. Sci. Catal. 75 (1993) 2649. [9] H.M. Zhang, Y. Shimizu, Y. Teraoka, N. Miura, N. Yamazoe, J. Catal. 121 (1990) 432. [10] H. Hamada, Y. Kintaichi, M. Tabata, M. Sasaki, T. Ito, Sekiyu Gakkaishi 36 (1993) 149.