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Factors controlling catalytic activity of H-form zeolites for the selective reduction of NO with CH4
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
1533
Factors Controlling Catalytic Activity of H - f o r m Zeolites for the Selective Reduction of N O with C H 4 A. Satsuma, M. Iwase, A. Shichi, T. Hattori and Y. Murakami Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
The selective reduction of NO with CH 4 in the presence of excess 0 2 over H-ZSM-5 and H-mordenites was investigated, and the factors controlling catalytic activity were discussed. The activity for the reduction of NO into N 2 was independent of crystal size and pellet size, indicating that the diffusion in zeolite channel and macro-pore has negligible effect on the catalytic activity. The catalytic activity proportionally increased with the acid amount, which strongly indicates that the acid amount is the controlling factor for this reaction. The activity was also dependent on the type of zeolites, which may be due to the difference in the acid strength, but not in the pore structure.
1. INTRODUCTION The reduction of NOx emissions from automotive and stationary sources is one of the serious problems to be solved. It has been reported that the selective catalytic reduction of NOx with hydrocarbons (HC-SCR) successfully proceeds even in the presence of excess oxygen over ion-exchanged zeolites, e.g., Cu/ZSM-5, Ga/ZSM-5, Co/ZSM-5 and so on [1-5]. It was clarified by several reports that the isolated cations are more active species than aggregated oxide clusters for HC-SCR [2, 6-8]. Thus, the dispersion of ions in zeolites is thought to be the most important factor for HC-SCR. However, a role of zeolite itself has not been well examined, i. e., crystal voidage and channels, pore structure, acid sites and so on.
Most practical zeolite catalysis takes place inside the framework of zeolites, and the internal capacity of zeolites provides the appropriate surfaces at which catalytic transformations can take place [9]. Consequently, the diffusion rate of reactants and pore structure often play an important role in the reaction using zeolites. As for HC-SCR, however, the effects of these properties of zeolites have not been well examined. In this study, in order to examine a role of zeolites on the catalytic activity for HC-SCR, the effects of crystal size, pellet size, structure of zeolites, acid amount and acid strength were investigated. As a first step, we examined the most simplest system in which the effect of diffusion is expected to be the smallest, i.e., the reduction of NO with CH 4, which is the smallest and least active hydrocarbon, over H-ZSM-5 and H-mordenites having various crystal sizes, pellet sizes and SiO2/A120 3 ratios.
1534 2. EXPERIMENTAL SECTION H-ZSM-5 and H-mordenites used in this study were listed in Table 1. It was confirmed that the aluminum content in surface layer measured by XPS agrees well with that in zeolite bulk measured by ICP. These catalysts were pressed and sieved in the range of 22-75 ktrn. Z-3 and M-2 sieved in the range of 300-600 lam were also prepared. For the estimation of crystal size, scanning electron micrographs (HITACHI S-800S), the adsorption isotherm of N 2 (OMNISORP SERIOUS 100CX), benzene filled pored method [10] and laser particle sizer (MALVERN 3600Ec) were used. The NO-CH4-O 2 reaction was carried out in a conventional continuous flow apparatus at atmospheric pressure. The reaction gas containing 1000ppm NO, 1000ppm CH 4 and 6.67% 0 2 diluted with He was fed to 0.5 g of catalyst bed at a flow rate of 42.0 cm3min -1 (SV=3200h -1). A steady state activity was measured after the exposure of catalysts to the feed gas for 1-2 h after the confirmation of a good carbon balance. After the NO-O 2 reaction, the mixture gas containing 1000ppm NO and 6.67% 0 2 diluted with He was fed, and the catalytic activity for NO oxidation was measured. The products were analyzed with gas chromatograph and NOx analyzer (Best BCL-100uH). The acid property of zeolites was measured with NH3-TPD (Temperature Programmed Desorption) in the same manner described previously [11 ].
Table 1 Source, composition and acid amount of H-ZSM-5 (Z) and H-mordenites (M) used in this study. Catalyst SiO~JA1203 a AI content a Acid amount / mmol g~ b No. Source ratio / mmol g-1 L c Hc Total Z- 1 Tosoh-H-ZSM-5 23.3 1.33 0.24 0.74 0.98 Z-2 JRC-Z5-25H d 24.6 1.27 0.45 0.84 1.29 Z-3 prepared 43.7 0.73 0.13 0.44 0.57 Z-4 prepared 44.1 0.73 0.13 0.42 0.55 Z-5 prepared 46.1 0.70 0.16 0.54 0.70 Z-6 prepared 79.4 0.41 0.06 0.39 0.45 Z-7 JRC-Z5-1000H d 1246.0 0.03 0.01 0.02 0.03 M-1 JRC-Z-HM10(3) d 10.2 2.80 0.35 0.40 0.75 M-2 JRC-Z-HM 15(1 ) d 14.9 2.01 0.17 1.50 1.67 M-3 JRC-Z-HM15(2) d 15.0 1.99 0.20 0.92 1.12 M-4 prepared 16.3 1.85 0.13 1.11 1.24 M-5 JRC-Z-HM20(3) d 19.5 1.57 0.17 1.10 1.27 M-6 JRC-Z-HM20(1 ) d 19.9 1.54 0.00 1.01 1.01 M-7 dealuminated HM15(1) 41.9 0.76 0.09 0.31 0.40 adetermined from chemical analysis using Inductive Coupled Plasma. bdetermined from NH3-TPD, CTPD profiles having two peak maxima were divided into those at lower temperature ( L ) and higher temperature (H), dsupplied from the Committee on Reference Catalyst of Catalysis Society of Japan.
1535 3. RESULTS AND DISCUSSION
3.1. Catalytic activity and acid property Fig. 1 shows a typical result of the NO-CH4-O 2 reaction as a function of the reaction temperature. The conversion of NO and CH 4 increased with the increase in the reaction temperature. The maximum activity of the reduction of NO into N 2 was observed at 773 K, and then decreased at 823 K. On the other hand, CH 4 conversion increased with increasing temperature up to 823 K. The decrease in NO conversion at higher temperature may be due to the increase in the competitive reaction of the oxidation of CH 4 with 0 2 [11]. The similar temperature dependencies were observed for all the catalysts tested. Fig. 2 shows the conversions of NO and CH 4 with the time-on-stream. The catalytic activity for the NO-CH4-O 2 reaction was constant for more than 350 min over both H-ZSM5 and H-mordenite. It should be noted that noticeable color change was not observed after the NO-CH4-O 2 reaction, though, after the NO-C3H6-O 2 reaction, H-mordenites were darkened due to the deposition of carbonaceous materials [ 11 ]. Fig. 3 shows NH3-TPD profiles before and after the NO-CH4-O 2 reaction. Solid lines represent the profiles before the reaction. The profiles were composed of two desorption peaks at lower temperature (around 480 K) and higher temperature (500-900 K). Since the amount of the peak at lower temperature varied with time of evacuation after NH 3 adsorption, this peak should arise from weakly adsorbed NH 3. The peak at higher temperature, which was not varied after the evacuation for 5 h, can be regarded as NH 3 strongly chemisorbed on acid sites of zeolites [ 11 ]. The high temperature peak was observed at around 630 K for HZSM-5 and at around 760 K for H-mordenites, indicating that the acid sites on H-mordenites are stronger than those on H-ZSM-5. This agrees well with the reported result that the
100
NO _,,,. A
tO
A
A
A
A
A
A
A
~ 6C
A
20
CH4
tO
o~ 30
10
L,-
tO
o
> cO
4c
..__NO ~
~
20
O
~
. . . .
v
v
0
-,~--CH I
I
700
800
900
Temperature / K
Fig. 1. Temperature dependence of conversion of ( O ) NO and ( G ) CH4 over H-ZSM-5 (Z-2).
30
10 0
I
0
9 0
0
0
4 I
i
I
100 200 300 400 Time / min
500
Fig. 2. Conversion of NO and CH 4 with time-on-stream over (A, A ) H-ZSM-5 (Z-4) and (O, O ) H-mordenites (M-3). Reaction temperature was 723 K.
1536
"r z
0 cD D I
400
I
I
600 800 Temperature / K
1000
Fig. 3. NH3-TPD profiles of (a) H-ZSM-5 (Z-4) and (b) H-mordenite (M-3). Solid lines and broken lines represent the profiles measured before and after NO-CH4-O 2 reaction, respectively.
enthalpy change in desorption (AH) is 130 kJ mo1-1 for Z-2 and 150 kJ mo1-1 for M-2, respectively [12]. Broken lines in the figure represent the profiles after the NO-CH4-O 2 reaction. The broken lines agreed well with the solid lines within an experimental error, indicating there was no change in the acid amount during the reaction. Table 1 summarizes the acid amount of all the catalysts measured by NH3-TPD before the reaction. Since there were no noticeable changes in the catalytic activity, the color of catalysts and the acid amount, it can be mentioned that the blocking of acid sites by carbonaceous materials did not occur under the NO-CH4-O 2 reaction.
3.2. Effect of crystal size and pellet size Table 2 shows crystal size of zeolites estimated with various methods. An average crystal size was measured directly from laser particle sizer and scanning electron microgrphs (SEM), and was calculated from the external surface area measured by benzene filled pore method and adsorption isotherm of N 2. The crystal size estimated from laser particle sizer was the largest, while those estimated from the adsorption isotherm of N 2 and SEM were the smallest and equivalent to each other. Since it was confirmed from scanning electron micrographs that the crystals of zeolites were aggregated to larger particles, the crystal size estimated from the laser particle sizer and benzene pore filled method reflect the size of aggregated particles. Consequently, the values estimated from scanning electron microgaph were adopted as the crystal size of zeolites. Fig. 4 shows the conversion of NO into N 2 at 723 K as a function of the crystal size. In the case of H-mordenites, irrespective of crystal size, the conversion of NO into N 2 was around 18%. In the case of H-ZSM-5, the conversion of NO was also independent of the crystal size. The conversion of CH 4 was also independent of the crystal size of both HZSM5 and H-mordenites. The similar results were obtained at 673 K and 773 K. It should be
1537 noted that H-ZSM-5 having high aluminum content exhibited the higher activity, and that Hmordenites having higher aluminum content exhibited rather lower activity. These results seem to suggest that the acid amount and the type of zeolites are effective factors for the NOCH4-O 2 reaction. Table 3 shows the effect of pellet size of HZSM-5 and H-mordenite on the activity of the NO-CH4-O 2 reaction. Both in the cases of HZSM-5 and H-mordenite, the zeolites having the pellet size of 22-75 l.tm and 300-600 ILtmshowed the same activity for both NO and CH 4 conversion at 723 and 773 K.
Table 2
Estimation of crystal size of H-ZSM-5 and H-mordenites. Catalyst SiO2/A1203 External surface/m 2 g-1 Crystal size/l.tm No. ratio a b c a b SEM Z-1 23.3 14.3 37.9 1.7 0.23 0.09 0.07 Z-2 24.6 2.9 17.0 2.0 1.16 0.20 0.22 Z-3 43.7 6.7 47.4 2.8 0.50 0.07 0.08 Z-4 44.1 8.7 38.0 1.9 0.39 0.09 0.07 Z-5 46.1 6.9 34.2 2.1 0.49 0.10 0.12 M-2 14.9 9.9 22.0 1.5 0.36 0.16 0.08 M-3 15.0 12.0 19.8 1.1 0.29 0.18 0.08 M-4 16.3 8.8 8.5 1.8 0.39 0.41 0.61 aestimated with benzene filled pore method, bestimated from meso- and macro-pore volume, of which the diameter is larger than lnm, analyzed by t-plot of the adsorption isotherm of N 2, Cmeasured with laser particle sizer.
50 40 0 C
" - :3(] C 0
0
~ 2(] _ #
~~
C 0
0 z
0
I
0.2
014
I
0.6
Crystal size //~ m Fig. 4. Effect of crystal size of H-ZSM-5 ( O SIO2/A1203 = 23-25, 9 SIO2/A1203 = 43-47) and H-mordenites(A SiO2/AI20 3 = 14-16) on NO-CH4-O 2 reaction at 723K.
1538 Table 3 Effect of pellet size of H-ZSM-5 and H-mordenites on activity for NO-CH4-O 2 reaction. Catalyst Pellet size .. Conversion of NO / % Conversion of CH4 [ % /ktm 723 K 773 K 723 K 773 K Z-3 22-75 22 34 21 43 300-600 22 31 22 41 M-2 22-75 18 37 11 25 300-600 20 38 13 26
Increased crystal size or pellet size will result in an increase in pore length and thus in the Thiele modulus [13, 14]. When the effect of the rate of diffusion is not negligible, the increase in the crystal size or pellet size will result in a reduced effectiveness factor, vis. a reduced actual rate of reaction. However, in this case, there was no effect of the crystal size and the pellet size on the actual rate of reaction, as shown in Fig. 4 and Table 3. Therefore, it can be concluded that the effectiveness factor is unity, i.e., the diffusion rate of reactants in zeolite channel and macro-pore is faster than the rate of the reduction of NO with CH 4.
3.3. Effect of acid sites Fig. 5 shows NO conversion into N 2 at 723 K as a function of the acid amount measured by NH3-TPD. The activity for the reduction of NO increased proportionally with the acid amount. This figure strongly indicates that the acid amount is one of the factors controlling catalytic activity in the selective reduction of NO with CH 4. However, the slopes of the lines were different from each other, i.e., the activity of H-ZSM-5 was higher than that of Hmordenites at the same acid amount, indicating that the type of zeolites is another controlling factor. Fig. 6 shows NO conversion into NO 2 in the oxidation of NO as a function of the acid amount. Because the equilibrium conversion is limited at higher temperature, the activity for NO oxidation was compared at 573 K. The figure was very similar to Fig. 5, i.e., the activity proportionally increased with the increase in the acid amount and H-ZSM-5 exhibited higher activity than H-mordenites at the same acid amount. In the separate experiments, both H-ZSM-5 and H-mordenites exhibited no or little activity for the NO-CH 4 reaction and the CH4-O 2 reaction. These results indicate that both the acid amount and the zeolite type control the catalytic activity for the oxidation of NO into NO 2, which may control the activity for the reduction of NO with CH 4 under excess 0 2. This is in accordance with the literature which suggests an important role of the oxidation of NO into NO 2 over acid sites in the reduction of NO [15, 16]. As we reported previously [11], the catalytic activity proportionally increased with acid amount in the reduction of NO with C3H 6 over various zeolites including H-ZSM-5 and Hmordenites. Thus, the acid amount has the same effect on both the NO-CH4-O 2 reaction and the NO-C3H6-O 2 reaction. The effect of zeolite type, however, is different from reaction to reaction. In the NO-C3H6-O 2 reaction, H-ZSM-5 and H-mordenites showed the same activity per acid sites, while in the NO-CH4-O 2 reaction, H-ZSM-5 showed higher activity than H-mordenites.
1539
40
o~
6O
cY 5o
z 30
z o
O t-
40
.c_ '- 30 ._o
o.-
._o 20
_
00 ~0
O0
0
t,-
a~ 2(; > tO
oolc
o1(; O
Oz
z
0
9
9
Acid amount / mmol gl Fig. 5. Effect of acid amount on catalytic activity for selective reduction of NO with CH 4 over ( O ) H Z S M - 5 and (Q))Hmordenites at 723 K.
0
015 Acid a m o u n t / m m o l g-1
Fig. 6. Effect of acid amount on catalytic activity for oxidation of NO into NO 2 over ( O ) HZSM-5 and (Q)H- mordenites at 573 K.
The most significant difference between H-ZSM-5 and H-mordenite is the difference in the structure of channel, i.e., the framework of ZSM-5 consists of 3-dimensional channel systems with 10 oxygen membered tings, while the framework of mordenite consists of intersecting channel systems with 12 and 8 oxygen membered tings [9]. Thus, one might suspect that the diffusion of reactants may cause the difference in the activity of H-ZSM-5 and H-mordenites. However, since it was clarified in the previous section that the diffusion is not the rate determining, the structure of channel is not a factor controlling catalytic activity. Another possibility is the difference in the acid strength. In the case of the NO-C3H6-O 2 reaction, the strong acid sites on H-mordenites were blocked by carbonaceous materials. It should be added that above-mentioned correlation between the catalytic activity and the acid amount was obtained on the basis of the residual acid amount after the reaction. In the case of the NO-CH4-O 2 reaction, however, the strong acid sites on H-mordenite remained unblocked, as shown in Fig. 3. Thus, the type of zeolite had an effect on the catalytic activity, only when the strong acid sites remained unblocked. This strongly suggests that the acid strength may be another factor controlling catalytic activity, i.e., the strong acid sites of H-mordenites may be less active than the moderate acid sites. Since NO 2 is said to adsorb on Lewis acid sites of alumina, ion-exchanged zeolites, and so on [17], the low activity of strong acid sites may be rationalized by assuming excess stabilization of NO 2 on the strong acid sites.
1540 4. CONCLUSION In order to clarify the controlling factors of zeolites for the selective reduction of NO with CH 4, the dependence of activity on crystal size, pellet size and acid amount were examined. It was found that the activity for the selective reduction of NO is independent of the crystal size and the pellet size of zeolites, i.e., the diffusion rate of reactants is faster than the rate of the reduction of NO with CH 4. The activity for the reduction of NO depends on the acid amount and, possibly, acid strength.
5. ACKNOWLEDGMENT Authors appreciate Dr. A. Isogai (Toyota Central R&D Labs.) for the measurement of XPS and Mr. Y. Fujita and Mr. S. Komai (Nagoya University) for the measurement of SEM. Authors also appreciate Prof. Y. Izumi, Prof. M. Onaka and Dr. T. Shinoda (Nagoya University) for the use of OMNISORP SERIOUS 100CX. This work was partly supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan (No. 07242105 and 06555242).
REFERENCES W. Held, A. K6nig and T. Richter, L. Pupper, SAE Paper 900496 (1990). S. Sato, Y. Tu-u, H. Yahiro, N. Mizuno and M. Iwamoto, Appl. Catal., 70 (1991) L 1. M. Misono and K. Kondo, Chem. Lett., (1991) 1001. K. Yogo, S. Tanaka, M. Ihara and E. Kikuchi, Chem. Lett., (1992) 1025. Y. Li and J. N. Armor, Appl. Catal. B, 1 (1992) L31. Y. Li and J. N. Armor, Appl. Catal. B, 2 (1993) 239. Z. Chajar, M. Primet, H. Praliaud, M. Chevrier, C. Gauthier and F. Mathis, Appl. Catal. B, 4 (1994) 199. 8. B. Coq, D. Tachon, F. Figuras, G. Mabilon and M. Prigent, Appl. Catal. B, 6 (1995) 271. 9. A. Dyer, "An Introduction to Zeolite Molecular Sieves", p. 118, John Wiley and Sons, New York (1988). 10. M. Inomata, M. Yamada, S. Okada, M. Niwa and Y. Murakami, J. Catal., 100 (1986) 264. 11. A. Satsuma, K. Yamada, T. Moil, M. Niwa, T. Hattori and Y. Murakami, Catal. Lett., 31(1995) 367. 12. M. Sawa, M. Niwa and Y. Murakami, Zeolites, 10 (1990) 307. 13. E.W. Thiele, Ind. Eng. Chem., 31 (1939) 916. 14. N.Y. Chen, T. F. Degnan, Jr. and C. M. Smith, "Molecular Transport and Reaction in Zeolites - Design and Application of Shape Selective Catalysts", p.55, VCH, New Yoak (1994). 15. M. Sasaki, H. Hamada, Y. Kintaichi and T. Ito, Catal. Lett., 15 (1992) 297. 16. E. Kikuchi and K. Yogo, Stud. Surf. Sci. Catal., 84 (1994) 1547. 17. H. Kn6zinger, Adv. Catal., 25 (1976) 184. 1. 2. 3. 4. 5. 6. 7.
1533
Factors Controlling Catalytic Activity of H - f o r m Zeolites for the Selective Reduction of N O with C H 4 A. Satsuma, M. Iwase, A. Shichi, T. Hattori and Y. Murakami Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
The selective reduction of NO with CH 4 in the presence of excess 0 2 over H-ZSM-5 and H-mordenites was investigated, and the factors controlling catalytic activity were discussed. The activity for the reduction of NO into N 2 was independent of crystal size and pellet size, indicating that the diffusion in zeolite channel and macro-pore has negligible effect on the catalytic activity. The catalytic activity proportionally increased with the acid amount, which strongly indicates that the acid amount is the controlling factor for this reaction. The activity was also dependent on the type of zeolites, which may be due to the difference in the acid strength, but not in the pore structure.
1. INTRODUCTION The reduction of NOx emissions from automotive and stationary sources is one of the serious problems to be solved. It has been reported that the selective catalytic reduction of NOx with hydrocarbons (HC-SCR) successfully proceeds even in the presence of excess oxygen over ion-exchanged zeolites, e.g., Cu/ZSM-5, Ga/ZSM-5, Co/ZSM-5 and so on [1-5]. It was clarified by several reports that the isolated cations are more active species than aggregated oxide clusters for HC-SCR [2, 6-8]. Thus, the dispersion of ions in zeolites is thought to be the most important factor for HC-SCR. However, a role of zeolite itself has not been well examined, i. e., crystal voidage and channels, pore structure, acid sites and so on.
Most practical zeolite catalysis takes place inside the framework of zeolites, and the internal capacity of zeolites provides the appropriate surfaces at which catalytic transformations can take place [9]. Consequently, the diffusion rate of reactants and pore structure often play an important role in the reaction using zeolites. As for HC-SCR, however, the effects of these properties of zeolites have not been well examined. In this study, in order to examine a role of zeolites on the catalytic activity for HC-SCR, the effects of crystal size, pellet size, structure of zeolites, acid amount and acid strength were investigated. As a first step, we examined the most simplest system in which the effect of diffusion is expected to be the smallest, i.e., the reduction of NO with CH 4, which is the smallest and least active hydrocarbon, over H-ZSM-5 and H-mordenites having various crystal sizes, pellet sizes and SiO2/A120 3 ratios.
1534 2. EXPERIMENTAL SECTION H-ZSM-5 and H-mordenites used in this study were listed in Table 1. It was confirmed that the aluminum content in surface layer measured by XPS agrees well with that in zeolite bulk measured by ICP. These catalysts were pressed and sieved in the range of 22-75 ktrn. Z-3 and M-2 sieved in the range of 300-600 lam were also prepared. For the estimation of crystal size, scanning electron micrographs (HITACHI S-800S), the adsorption isotherm of N 2 (OMNISORP SERIOUS 100CX), benzene filled pored method [10] and laser particle sizer (MALVERN 3600Ec) were used. The NO-CH4-O 2 reaction was carried out in a conventional continuous flow apparatus at atmospheric pressure. The reaction gas containing 1000ppm NO, 1000ppm CH 4 and 6.67% 0 2 diluted with He was fed to 0.5 g of catalyst bed at a flow rate of 42.0 cm3min -1 (SV=3200h -1). A steady state activity was measured after the exposure of catalysts to the feed gas for 1-2 h after the confirmation of a good carbon balance. After the NO-O 2 reaction, the mixture gas containing 1000ppm NO and 6.67% 0 2 diluted with He was fed, and the catalytic activity for NO oxidation was measured. The products were analyzed with gas chromatograph and NOx analyzer (Best BCL-100uH). The acid property of zeolites was measured with NH3-TPD (Temperature Programmed Desorption) in the same manner described previously [11 ].
Table 1 Source, composition and acid amount of H-ZSM-5 (Z) and H-mordenites (M) used in this study. Catalyst SiO~JA1203 a AI content a Acid amount / mmol g~ b No. Source ratio / mmol g-1 L c Hc Total Z- 1 Tosoh-H-ZSM-5 23.3 1.33 0.24 0.74 0.98 Z-2 JRC-Z5-25H d 24.6 1.27 0.45 0.84 1.29 Z-3 prepared 43.7 0.73 0.13 0.44 0.57 Z-4 prepared 44.1 0.73 0.13 0.42 0.55 Z-5 prepared 46.1 0.70 0.16 0.54 0.70 Z-6 prepared 79.4 0.41 0.06 0.39 0.45 Z-7 JRC-Z5-1000H d 1246.0 0.03 0.01 0.02 0.03 M-1 JRC-Z-HM10(3) d 10.2 2.80 0.35 0.40 0.75 M-2 JRC-Z-HM 15(1 ) d 14.9 2.01 0.17 1.50 1.67 M-3 JRC-Z-HM15(2) d 15.0 1.99 0.20 0.92 1.12 M-4 prepared 16.3 1.85 0.13 1.11 1.24 M-5 JRC-Z-HM20(3) d 19.5 1.57 0.17 1.10 1.27 M-6 JRC-Z-HM20(1 ) d 19.9 1.54 0.00 1.01 1.01 M-7 dealuminated HM15(1) 41.9 0.76 0.09 0.31 0.40 adetermined from chemical analysis using Inductive Coupled Plasma. bdetermined from NH3-TPD, CTPD profiles having two peak maxima were divided into those at lower temperature ( L ) and higher temperature (H), dsupplied from the Committee on Reference Catalyst of Catalysis Society of Japan.
1535 3. RESULTS AND DISCUSSION
3.1. Catalytic activity and acid property Fig. 1 shows a typical result of the NO-CH4-O 2 reaction as a function of the reaction temperature. The conversion of NO and CH 4 increased with the increase in the reaction temperature. The maximum activity of the reduction of NO into N 2 was observed at 773 K, and then decreased at 823 K. On the other hand, CH 4 conversion increased with increasing temperature up to 823 K. The decrease in NO conversion at higher temperature may be due to the increase in the competitive reaction of the oxidation of CH 4 with 0 2 [11]. The similar temperature dependencies were observed for all the catalysts tested. Fig. 2 shows the conversions of NO and CH 4 with the time-on-stream. The catalytic activity for the NO-CH4-O 2 reaction was constant for more than 350 min over both H-ZSM5 and H-mordenite. It should be noted that noticeable color change was not observed after the NO-CH4-O 2 reaction, though, after the NO-C3H6-O 2 reaction, H-mordenites were darkened due to the deposition of carbonaceous materials [ 11 ]. Fig. 3 shows NH3-TPD profiles before and after the NO-CH4-O 2 reaction. Solid lines represent the profiles before the reaction. The profiles were composed of two desorption peaks at lower temperature (around 480 K) and higher temperature (500-900 K). Since the amount of the peak at lower temperature varied with time of evacuation after NH 3 adsorption, this peak should arise from weakly adsorbed NH 3. The peak at higher temperature, which was not varied after the evacuation for 5 h, can be regarded as NH 3 strongly chemisorbed on acid sites of zeolites [ 11 ]. The high temperature peak was observed at around 630 K for HZSM-5 and at around 760 K for H-mordenites, indicating that the acid sites on H-mordenites are stronger than those on H-ZSM-5. This agrees well with the reported result that the
100
NO _,,,. A
tO
A
A
A
A
A
A
A
~ 6C
A
20
CH4
tO
o~ 30
10
L,-
tO
o
> cO
4c
..__NO ~
~
20
O
~
. . . .
v
v
0
-,~--CH I
I
700
800
900
Temperature / K
Fig. 1. Temperature dependence of conversion of ( O ) NO and ( G ) CH4 over H-ZSM-5 (Z-2).
30
10 0
I
0
9 0
0
0
4 I
i
I
100 200 300 400 Time / min
500
Fig. 2. Conversion of NO and CH 4 with time-on-stream over (A, A ) H-ZSM-5 (Z-4) and (O, O ) H-mordenites (M-3). Reaction temperature was 723 K.
1536
"r z
0 cD D I
400
I
I
600 800 Temperature / K
1000
Fig. 3. NH3-TPD profiles of (a) H-ZSM-5 (Z-4) and (b) H-mordenite (M-3). Solid lines and broken lines represent the profiles measured before and after NO-CH4-O 2 reaction, respectively.
enthalpy change in desorption (AH) is 130 kJ mo1-1 for Z-2 and 150 kJ mo1-1 for M-2, respectively [12]. Broken lines in the figure represent the profiles after the NO-CH4-O 2 reaction. The broken lines agreed well with the solid lines within an experimental error, indicating there was no change in the acid amount during the reaction. Table 1 summarizes the acid amount of all the catalysts measured by NH3-TPD before the reaction. Since there were no noticeable changes in the catalytic activity, the color of catalysts and the acid amount, it can be mentioned that the blocking of acid sites by carbonaceous materials did not occur under the NO-CH4-O 2 reaction.
3.2. Effect of crystal size and pellet size Table 2 shows crystal size of zeolites estimated with various methods. An average crystal size was measured directly from laser particle sizer and scanning electron microgrphs (SEM), and was calculated from the external surface area measured by benzene filled pore method and adsorption isotherm of N 2. The crystal size estimated from laser particle sizer was the largest, while those estimated from the adsorption isotherm of N 2 and SEM were the smallest and equivalent to each other. Since it was confirmed from scanning electron micrographs that the crystals of zeolites were aggregated to larger particles, the crystal size estimated from the laser particle sizer and benzene pore filled method reflect the size of aggregated particles. Consequently, the values estimated from scanning electron microgaph were adopted as the crystal size of zeolites. Fig. 4 shows the conversion of NO into N 2 at 723 K as a function of the crystal size. In the case of H-mordenites, irrespective of crystal size, the conversion of NO into N 2 was around 18%. In the case of H-ZSM-5, the conversion of NO was also independent of the crystal size. The conversion of CH 4 was also independent of the crystal size of both HZSM5 and H-mordenites. The similar results were obtained at 673 K and 773 K. It should be
1537 noted that H-ZSM-5 having high aluminum content exhibited the higher activity, and that Hmordenites having higher aluminum content exhibited rather lower activity. These results seem to suggest that the acid amount and the type of zeolites are effective factors for the NOCH4-O 2 reaction. Table 3 shows the effect of pellet size of HZSM-5 and H-mordenite on the activity of the NO-CH4-O 2 reaction. Both in the cases of HZSM-5 and H-mordenite, the zeolites having the pellet size of 22-75 l.tm and 300-600 ILtmshowed the same activity for both NO and CH 4 conversion at 723 and 773 K.
Table 2
Estimation of crystal size of H-ZSM-5 and H-mordenites. Catalyst SiO2/A1203 External surface/m 2 g-1 Crystal size/l.tm No. ratio a b c a b SEM Z-1 23.3 14.3 37.9 1.7 0.23 0.09 0.07 Z-2 24.6 2.9 17.0 2.0 1.16 0.20 0.22 Z-3 43.7 6.7 47.4 2.8 0.50 0.07 0.08 Z-4 44.1 8.7 38.0 1.9 0.39 0.09 0.07 Z-5 46.1 6.9 34.2 2.1 0.49 0.10 0.12 M-2 14.9 9.9 22.0 1.5 0.36 0.16 0.08 M-3 15.0 12.0 19.8 1.1 0.29 0.18 0.08 M-4 16.3 8.8 8.5 1.8 0.39 0.41 0.61 aestimated with benzene filled pore method, bestimated from meso- and macro-pore volume, of which the diameter is larger than lnm, analyzed by t-plot of the adsorption isotherm of N 2, Cmeasured with laser particle sizer.
50 40 0 C
" - :3(] C 0
0
~ 2(] _ #
~~
C 0
0 z
0
I
0.2
014
I
0.6
Crystal size //~ m Fig. 4. Effect of crystal size of H-ZSM-5 ( O SIO2/A1203 = 23-25, 9 SIO2/A1203 = 43-47) and H-mordenites(A SiO2/AI20 3 = 14-16) on NO-CH4-O 2 reaction at 723K.
1538 Table 3 Effect of pellet size of H-ZSM-5 and H-mordenites on activity for NO-CH4-O 2 reaction. Catalyst Pellet size .. Conversion of NO / % Conversion of CH4 [ % /ktm 723 K 773 K 723 K 773 K Z-3 22-75 22 34 21 43 300-600 22 31 22 41 M-2 22-75 18 37 11 25 300-600 20 38 13 26
Increased crystal size or pellet size will result in an increase in pore length and thus in the Thiele modulus [13, 14]. When the effect of the rate of diffusion is not negligible, the increase in the crystal size or pellet size will result in a reduced effectiveness factor, vis. a reduced actual rate of reaction. However, in this case, there was no effect of the crystal size and the pellet size on the actual rate of reaction, as shown in Fig. 4 and Table 3. Therefore, it can be concluded that the effectiveness factor is unity, i.e., the diffusion rate of reactants in zeolite channel and macro-pore is faster than the rate of the reduction of NO with CH 4.
3.3. Effect of acid sites Fig. 5 shows NO conversion into N 2 at 723 K as a function of the acid amount measured by NH3-TPD. The activity for the reduction of NO increased proportionally with the acid amount. This figure strongly indicates that the acid amount is one of the factors controlling catalytic activity in the selective reduction of NO with CH 4. However, the slopes of the lines were different from each other, i.e., the activity of H-ZSM-5 was higher than that of Hmordenites at the same acid amount, indicating that the type of zeolites is another controlling factor. Fig. 6 shows NO conversion into NO 2 in the oxidation of NO as a function of the acid amount. Because the equilibrium conversion is limited at higher temperature, the activity for NO oxidation was compared at 573 K. The figure was very similar to Fig. 5, i.e., the activity proportionally increased with the increase in the acid amount and H-ZSM-5 exhibited higher activity than H-mordenites at the same acid amount. In the separate experiments, both H-ZSM-5 and H-mordenites exhibited no or little activity for the NO-CH 4 reaction and the CH4-O 2 reaction. These results indicate that both the acid amount and the zeolite type control the catalytic activity for the oxidation of NO into NO 2, which may control the activity for the reduction of NO with CH 4 under excess 0 2. This is in accordance with the literature which suggests an important role of the oxidation of NO into NO 2 over acid sites in the reduction of NO [15, 16]. As we reported previously [11], the catalytic activity proportionally increased with acid amount in the reduction of NO with C3H 6 over various zeolites including H-ZSM-5 and Hmordenites. Thus, the acid amount has the same effect on both the NO-CH4-O 2 reaction and the NO-C3H6-O 2 reaction. The effect of zeolite type, however, is different from reaction to reaction. In the NO-C3H6-O 2 reaction, H-ZSM-5 and H-mordenites showed the same activity per acid sites, while in the NO-CH4-O 2 reaction, H-ZSM-5 showed higher activity than H-mordenites.
1539
40
o~
6O
cY 5o
z 30
z o
O t-
40
.c_ '- 30 ._o
o.-
._o 20
_
00 ~0
O0
0
t,-
a~ 2(; > tO
oolc
o1(; O
Oz
z
0
9
9
Acid amount / mmol gl Fig. 5. Effect of acid amount on catalytic activity for selective reduction of NO with CH 4 over ( O ) H Z S M - 5 and (Q))Hmordenites at 723 K.
0
015 Acid a m o u n t / m m o l g-1
Fig. 6. Effect of acid amount on catalytic activity for oxidation of NO into NO 2 over ( O ) HZSM-5 and (Q)H- mordenites at 573 K.
The most significant difference between H-ZSM-5 and H-mordenite is the difference in the structure of channel, i.e., the framework of ZSM-5 consists of 3-dimensional channel systems with 10 oxygen membered tings, while the framework of mordenite consists of intersecting channel systems with 12 and 8 oxygen membered tings [9]. Thus, one might suspect that the diffusion of reactants may cause the difference in the activity of H-ZSM-5 and H-mordenites. However, since it was clarified in the previous section that the diffusion is not the rate determining, the structure of channel is not a factor controlling catalytic activity. Another possibility is the difference in the acid strength. In the case of the NO-C3H6-O 2 reaction, the strong acid sites on H-mordenites were blocked by carbonaceous materials. It should be added that above-mentioned correlation between the catalytic activity and the acid amount was obtained on the basis of the residual acid amount after the reaction. In the case of the NO-CH4-O 2 reaction, however, the strong acid sites on H-mordenite remained unblocked, as shown in Fig. 3. Thus, the type of zeolite had an effect on the catalytic activity, only when the strong acid sites remained unblocked. This strongly suggests that the acid strength may be another factor controlling catalytic activity, i.e., the strong acid sites of H-mordenites may be less active than the moderate acid sites. Since NO 2 is said to adsorb on Lewis acid sites of alumina, ion-exchanged zeolites, and so on [17], the low activity of strong acid sites may be rationalized by assuming excess stabilization of NO 2 on the strong acid sites.
1540 4. CONCLUSION In order to clarify the controlling factors of zeolites for the selective reduction of NO with CH 4, the dependence of activity on crystal size, pellet size and acid amount were examined. It was found that the activity for the selective reduction of NO is independent of the crystal size and the pellet size of zeolites, i.e., the diffusion rate of reactants is faster than the rate of the reduction of NO with CH 4. The activity for the reduction of NO depends on the acid amount and, possibly, acid strength.
5. ACKNOWLEDGMENT Authors appreciate Dr. A. Isogai (Toyota Central R&D Labs.) for the measurement of XPS and Mr. Y. Fujita and Mr. S. Komai (Nagoya University) for the measurement of SEM. Authors also appreciate Prof. Y. Izumi, Prof. M. Onaka and Dr. T. Shinoda (Nagoya University) for the use of OMNISORP SERIOUS 100CX. This work was partly supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan (No. 07242105 and 06555242).
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