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Palladium ion-exchanged SAPO-5 for a low temperature combustion of 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.
1647
Palladium ion-exchanged SAPO-5 for a low temperature combustion of CH 4 Yusaku Takita, Tatsumi Ishihara, Hiroyasu Nishiguchi, and Hideaki Sumi Department of Applied Chemistry, Faculty of Engineering Oita University, Dannoharu 700, Oita 870-11, Japan
The oxidation activity of molecular sieves ion-exchanged by Pd were investigated as a catalyst for the low temperature combustion of CH4. Among the molecular sieves examined, SAPO-5 ion-exchanged by Pd exhibited the highest activity for CH 4 combustion as well as the high thermal stability. Since the high dispersion of Pd particles was attained with ion-exchanged method, plateau in the temperature dependence of CH 4 conversion was hardly appeared on Pd-HSAPO-5 catalysts in the high space velocity of CH 4. Furthermore, the high activity of Pd-HSAPO-5 was sustained after the precalcination up to 1000 *C. Consequently, Pd-HSAPO-5 is a promising combustion catalyst in low temperature range. l.lntroduetion
Catalytic combustion, a new combustion technique without flames, is expected to be applicable to gas turbines. The advantages of oxidation catalysts for the application to combustion are the controlling combustion of fuels, less emission of pollutant such as NOx, and high efficiency of energy recovery. For the combustion catalyst, high oxidation activity as well as the thermal stability is required. At present, barium hexaaluminate is promising for the combustion catalysts at high temperature. In current catalytic combustors, fuel gases have to be preheated up to the temperature at which the combustion catalyst becomes active using open flame preheater. Thermal NOx is mainly formed at this preheater. Consequently, there is a possibility that a combustor with no NOx emission can be developed by the development of highly active catalyst which the preheater lights off at low temperature. Furthermore, methane emissions themselves are a potential environmental problem since the methane significantly contributes to the greenhouse effect. Removal of CH4 by combustion is also another important subject from an environmental prospective. Palladium catalysts supported on A1203 or A1203 based oxide generally use for a combustion catalysts in low temperature range [ 1]. However, the combustion activity of these conventional catalysts is not satisfactorily high and the development of the catalysts with high activity and high thermal stability is the important subject. Ion-exchanged method is effective for the preparation of highly dispersed metal catalysts and activity of metal catalysts is generally increased with improving the dispersion of metal. Therefore, it is expected that the molecular sieve ion-exchanged by Pd exhibits high activity to CH 4 combustion. Methane combustion of Pd 2§ ion-exchanged zeolite was reported by some researchers [2], however, the publications concerning the application of ion-exchanged catalysts for the combustion catalysts are limited up to now. Since the thermal stability of zeolite is not high, the catalytic activity of methane over Pd ion-exchanged ZSM-5 decreased significantly with time course in the presence of steam. In the present study, the activity of silicoaluminophosphate (SAPO) ion-
1648 Table 1 CH 4 oxidation activity of various molecular sieve ion-exchanged by Pd Surface Area a)
Pd Loading
m2g-1 Pd/AI203 Pd-H-Y Pd-H-USY e) Pd-H-Pentasil Pd-H-Ferrierite Pd-H-Mordenite Pd-H-SAPO-5 Pd-H-SAPO- 11 Pd-H-SAPO-34
wt%
109 411 597 280 315 408 107 90 380
1.00 1.00 0.69 0.73 0.88 0.70 0.96 0.69 0.53
CH 4 oxidation activity/~ b) Tlo
T30
Tso
T90
330 310 415 395 400 370 375 355 385
400 380 455 505 455 410 415 400 355
435 395 480 570 490 430 430 430 505
665 475 605 780 790 495 480 545 645
a) BET surface area b) Temperature at the CH 4 conversion attained at 10, 30, 50, and 90 %. c) Ultra-stable Y
exchanged by Pd for CH 4 combustion were investigated in detail, since it is reported that the framework of SAPO was sustained up to 1000 ~ [3].
2.Experimental Three types SAPO-5, -11, and -34 were synthesized according to U.S. Patent and 5 types of commercial zeolites were used. Molecular sieves were ion-exchanged to NH4§ with NH4NO3 and then calcined at 500 ~ for 6 h in air to obtain the molecular sieves ion-exchanged with H § H-type SAPO or zeolite thus prepared was ion-exchanged with Pd 2§ in a 0.01M[Pd(NH3)4]CI~ aqueous solution. The Pd ion-exchanged zeolite or SAPO were calcined in 02 stream at 500 ~ for 8h, H 2 stream 500 *C for 5h, and air 800 ~ 4h. The combustion activity of Pd ion-exchanged molecular sieves for CH 4 was measured with conventional fixed bed micro-flow reactor. A mixed gas consisting of CH 4 (1 vol%) and air was fed to the catalysts bed at S.V.=I(K)0(X) h -1. 90 The thermal stability of SAPO-5 was investigated with XRD and solid state M A S S - N M R . XRD 70 r.measurement was performed with o Cu K s line and NMR spectra for 5o 27A1 and 3ap were recorded with ID C)Pd-HSAI~-5 Bruker APR-300 spectrometer op30 o QPtV 2% erated at a field of 7 T. Spinning speeds of 4.5 kHz were used and the 10 chemical shifts of AI and P were referred to the external standard of 300 400 500 600 700 800 1.5M AI(H20)] § in AI(NO3)3 aqueTemperature / ~ ous solution a n d H2PO4(85%), respectively. The thermal stability of Fig. 1 Temperature dependence of the C H 4 conversion Pd ion-exchanged into SAPO-5 over Pd-HSAPO-5 and Pd/AI203 catalyst. 9
1649 was measured with TEM observation, adsorption of CO and ESR measurement. TEM observation was performed with JEM 2000FX (JEOL) and uptakes of CO was measured with the conventional adsorption apparatus. ESR measurement was performed on the reduced sample at room temperature with JEXFE1X(JEOL) and DPPH was used as an external standard for the calibration of g value. SAPO5 ion-exchanged by Pd was calcined at the various temperature in oxygen atmosphere (100 torr) for 6 h and then evacuated at 600 ~ for 3h.
300
,~ 400
O
O
= 500
OT,o}
"-.....
E 6 0 0 - A1"so Pd/AI203 700
"OTto} /XT~a Pd-HSAPO-5 0
10
20
30
50
40
Space velocity / 104 h "1 Fig.2 C H 4 conversion activity of Pd/AI203 and PdHSAPO-5 as a function of space velocity.
3. Results and Discussion 3.1. Catalytic activity of Pd ion-exchanged molecular sieve
u
u
I
400
Table 1 summarized the C H 4 oxidation activity of various molecular sieves ion-ex500 changed by Pd. Although the temperature at 10 % CH4 conversion became higher, the conversion at 90 % was attained at the lower tem600 perature upon a large part of Pd ion-exchanged molecular sieves compared with that I700 of Pd/Al203. In particular, Pd ion-exchanged HY and HSAPO-5 exhibited the high activity for CH 4 oxidation. Although the complete 800 oxidation of CH 4 attained at the lowest temI i I perature on Pd-HY among the catalysts exam800 900 1000 ined in Table 1, the framework was destructed Precalcination temperature/~ after CH 4 combustion at 800 ~ due to low thermal stability. In contrast, XRD analysis suggested that the framework of SAPO-5 was Fig.3 Effects of precalcination o n CH 4 comhardly changed after CH 4 combustion at 800 bustion activity. ~ Since the CH 4 conversion increased with increasing temperature without plateau, 90 % CH4 conversion attained on Pd-SAPO-5 was 170 ~ lower than that of Pd/AI203. Figure 1 shows the temperature dependence of C H 4 conversion on Pd-HSAPO-5 and Pd/ AI203. The activity for CH 4 combustion is higher on Pd/AIzO3 than Pd-HSAPO-5 in the temperature range lower than 450 ~ However, the CH 4 conversion increased drastically with increasing the reaction temperature on Pd-HSAPO-5. Contrarily, CH 4 conversion on Pd/AI203 increased through the plateau, resulting in the high temperature required to attain the complete oxidation of CH 4. Since it is reported that the plateau in the temperature dependence of CH 4
1650 combustion is appeared by the control of mass transfer process, no plateau in temperature dependence of CH 4 conversion on Pd-HSAPO-5 suggests that the effective surface area for CH 4 combustion is large on Pd-HSAPO-5. Figure 2 shows the dependence of the combustion activity of Pd-HSAPO-5 and Pd/Al203 on the space velocity of supplied CH 4. Since ion-exchanged Pd exists in the narrow pores of S A P O - 5 , it is anticipated that the mass transfer process greatly affects the CH 4 combustion activity in case of Pd-HSAPO-5. These effects of mass transfer process on CH 4 combustion becomes more significant in the high space velocity of supplied CH4. Temperatures at the 10 % and 50 % conversion level were elevated by increasing the space velocity of supplied CH 4 over both Pd-HSAPO-5 and Pd/Al203. However, Pd-HSAPO-5 exhibited the higher CH 4 conversion than Pd/Al203 at all examined space velocity. Since high dispersion of I'd attains on Pd-HSAPO-5 discussed later, it appears that the mass transfer process hardly affects on the CH 4 combustion on Pd-HSAPO-5 even though the active sites of Pd is dispersed in the narrow pore of SAPO-5. The effects of precalcination temperature on the C H 4 combustion activity were further studied (Fig.3). Although the temperature needs for 50 % C H 4 conversion were slightly increased with increasing the precalcination temperature, Pd-SAPO-5 exhibited the high activity for CH 4 oxidation even after precalcination at 1000 ~ This suggests that the activity for CH 4 combustion on Pd-SAPO-5 is sustained for a long period. Indeed, no degradation in CH 4 combustion activity of Pd-SAPO-5 was observed over 20 h at 600 ~ C H 4 conversion attained 90 % at 650 *C, which is 150 ~ lower than that of Pd/AI203 catalyst. Consequently, SAPO-5 ion-exchanged by Pd is the Table 2 Dependence of combustion rate of CH 4 upon promising catalysts for the low temthe partial presure of CH 4 and 0 2 at 400~ perature combustion of CH 4. Catalysts CH 4 dependence 0 2 dependence m n 3.2. R e a c t i o n k i n e t i c s of C H 4 c o m bustion
Pd-HSAPO-5 0.32 0.33 Effects of partial pressure of oxygen and methane on the combustion Pd/AI203 1.62 0.55 activity were examined for the reaction kinetics of CH 4 combustion on Pd-HSAPO-5 and Pd/AI203. The resuits were expressed by the empirical rate equation r=kPcn4mPo2 (1) where r is the combustion rate of methane, and m and n are the reaction orders for methane and oxygen partial pressure, respectively. In this experiment, CH 4 combustion was performed at 400 ~ to keep the CH 4 conversion level below 10 %. Table 2 summarized the reaction order of 02 and CH 4 on Pd-HSAPO-5 and Pd/Al203. Reaction order of oxygen on Pd-HSAPO-5 was almost the same value as that on Pd-HSAPO-5. On the other hand, the reaction order of CH 4 was far larger on Pd/AI203 than that of Pd-HSAP-5. The following two reasons were considered for the small reaction order of CH 4 on Pd-HSAPO-5 compared with Pd/Al203. One is the difference of facility in CH 4 activation and the other is the changes in the reaction mechanism. For studying the reaction mechanism on the both catalysts, the reaction rate was analyzed based on the Langmuir-Hinshelwood mechanism as listed in the following reaction equations. CH 4 + s ~ CH4s (2) 20 2+4s ~ 4Os (3) CH4s + 4Os ---" CO2s+2H2Os+2s (4)
1651
'
i
'
I
''
I
"
I
'
0
' ' I '"'
I
'
I
' 7
''
I
'
1
'
'
'
0
E
E ~
t~ o
o
I
~
10
"'1
8
I
,
E
o 6 -~
I--I Pd/AI203 !
!
I
I
I
0.02
I
L
0.04
40
'
'
0 0. . 2" ' ' ' '
'4'
(Po2/atm) 1/2
PcH4/atm
Fig.4 Plots of (pcm/r) 'Is vs. Pca4 and (Po22/r)'/Svs. (P02) 1/2 in CH 4 combustion on Pd-HSAPO-5 and Vd/Al203 (400~ CO2s ~ 2H2Os ~
CO 2 + s 2H20 + 2s
(5) (6)
Here s means the surface adsorption sites. The oxidation of hydrocarbon on noble metal catalysts generally proceeds by the surface reaction between adsorbed oxygen and hydrocarbon. [4] For analyzing the reaction kinetics above, we assumed that the rate determining steps is the surface reaction between adsorbed methane and dissociatively adsorbed oxygen. The combustion rate under the above assumption was expressed in the following equation, r =
k,n~4Ko,P~,Po,[S0] *
(7)
( 1+Kcri4Pcri4+Ko21/4po2u2)s where Kci.x4 and Ko2 are adsorption equilibrium constants of oxygen and methane, respectively and [S o] represents the total surface adsorption site. 1%is a rate constant in equation (3). Equation (6) rewritten to test the fit of experimental data in the following equation;
(Pci.Jr) us =
I+Ko21/4po21/2
Kcrx4Pcti4 +
(kKcH4Ko2P02) 1/5180]
(Po22/r)1Is =
1 + KcH4PcH4 (kKcH4Ko2PcH4)'/S[So]
(8) (kKcH4Ko2P02) 1/5[Sj
+
ko2Po21/2
(9)
(kKcH4Ko2PcH4) x/S[So]
Experimentally obtained (Po,/r)t/s and (Pcri4/r)t/s values are plotted against P02~/2 and PCH4' respectively as shown in Fig. 4. Since the plots in Fig. 4 are quite linear on both catalysts, the
1652 reaction kinetics on both PdHSAPO-5 and Pd/AI203 catalysts are LangmuirHinschelwood type and the ai rate determining steps is the surface reaction between adsorbed CH 4 and "~ After CII4 combustion dissociatively adsorbed oxygen. Consequently, the small values in the reaction order of CH 4 in Table 2 is not caused by a different reaction lo 2b do mechanism but bythe high 20/degree ability for CH 4 activation 9 Namely, it appears that the Fig.5 XRD pattems of Pd-HSAPO-5 before (a) and methane is easily activated after (b) CH 4 combustion. on Pd-HSAPO-5 compared with Pd/AI20 v Therefore, Pd-HSAPO-5 exhibits the high activity for CH 4 combustion. I
,
,,I
,
3.3. Thermal stability of Pd-HSAPO-5 Thermal stability is the another important factors for the combustion catalysts. Figure 5 shows the XRD patterns of Pd-SAPO-5 before and after CH4 combustion at 800 ~ All diffraction peaks were assigned to those for SAPO-5, and there were no diffraction peaks from a second phase, and there were no changes in the angle as well as the relative strength of the diffraction peaks from SAPO-5 after CH 4combustion at 800 ~ It is considered that the temperature of a catalyst surface reaches to 1100 ~ under CH4 combustion. The framework of 2"/Al
.
40
.
- 0 ppm
100
0
-
ppm,
Fig.6 27A1and 31p-MASS NMR sepctra of SAPO-5 before and after calcination at 1000 ~ (*);Spining side band
1653 SAPO-5 is extremely stable and sustained up to 1200 ~ There' 0 ' ' ' fore, the thermal stability of 2 < SAPO-5 is satisfactory as a support "~ 1.10 [ _ ~1 for the CH4cOmbustiOn catalyst in ~ , O-'-----: 11 the low-temperature region. The ~ 1.00 ~ W thermal stability of Pd-HSAPO-5 ~ ~ t was further studied in more detail i~ in the following part of present ~ ",1,2 "o study. < " 0.90~ , . . . . . , I, , I . .~ | Figure 6 shows the solid state 800 900 1000 NMR spectra of 27A1 and 31p in Preealcination temperature/*C SAPO-5 before and after calcined at 1000 ~ In accordance with the results of XRD, changes in the Fig.7 Average Pd particle size in SAPO-5 estimated NMR spectra of 27A1 and 31p in from CO adsorption amount as a function of calcination SAPO-5 are quit small. Consetemperature. quently, local structure as well as the long range order of framework 600"C precalcination of SAPO-5 are hardly changed up to 1000 ~ x2.5 Since the thermal stability of SAPO-5 support is satisfactory, the thermal stability of Pd ion-exgii=2.916 f N "9'-- gu=2"657 changed in HSAPO-5 was further x 2.5 studied with CO adsorption as shown in Fig.7. The average particle size of Pd estimated from the 1000"(2 precalcination linear type adsorption of CO was 1.1 nm and hardly dependent on the precalcination temperature up to 1000 'C. TEM observation g_t_=2.130 shows that the some part of Pd ionexchanged aggregated and exuded from the pores of SAPO-5. However, large part of Pd is stably exFig.8 ESR spectra of Pd-HSAPO-5 after pre-calcinaisted in the pores of SAPO-5, since tion at the marked temperature followed by evacuation the average particle size of Pd estiat 600~ mated with TEM and the uptakes of CO are 5 nm and 1.1 nm, respectively. Consequently, high dispersion of Pd is stably sustained up to the high temperature range. Since the smaller particle is generally more easily sintered to form larger particle, it is interesting that the high dispersion of Pd particle was attained in the temperature range up to 1000 ~ On the other hand, TEM observation and CO adsorption measurement suggests that the Pd particle with different thermal stability exists in HSAPO-5, namely, easily aggregated one and highly stable one. Figure 8 shows the ESR spectra of Pd-HSAPO-5 after the evacuation at 600 ~ for 6 h. Since no ESR signal was observed on the specimens which was calcined in the air, the most common oxidation state of Pd is divalent, Pd(II), in the as-synthesized specimens. On the other ~
I'
'
'
'
~
|
- -
M
-
1654 hand, partially reduced sample gives an ESR signal of which parameters is g~ =2.130. The similar ESR spectra were reported for various palladium-exchanged zeolite and SAPO-5[5]. Considering the results reported by Yu et al.[5], the large part of Pd in SAPO-5 1 was reduced to monovalent, Pd(I), afe " ter the evacuation at 600 ~ for 3 h. The hyperfine signal from Pd(I) also confirmed the results that a large part of Pd is in a state of h~gh dispersion in atomic level. It is clearly shown in Fig.8 that the two signals in ESR spectra are obFig.9 SAPO-5 framwork and ion-exchanging served at ga--2.657 and 2.916 which are sites [5]. assigned to two kind of Pd at two different environments. These two kinds of Pd may assign to Pd ion existed at the center of a double 6-ring ( site I in Fig.9 ) which forms a 6-ring channel and the site dispersed into a 12-ring channel (site II or II* in Fig.9). The ESR spectra of Pd(I) in SAPO-5 was hardly changed after the precalcination at 1000 ~ as shown in Fig.9. Consequently, framework of SAPO-5 as well as Pd particle ion-exchanged in SAPO-5 are highly stable and consequently, it is expected that the high activity of Pd-HSAPO-5 for CH 4 combustion is stably exhibited for a long period. ,I-
- -
-.
4. Conclusion In the conventional studies on the combustion catalysts, supported metal catalysts have been generally investigated. However, metal ion-exchanged molecular sieve catalysts are highly active for the combustion of hydrocarbOn, since the high dispersion of metal can be attained by ion-exchanging method. In particular, framework of SAPO-5 is extremely stable and consequently, highly dispersed state of Pd in SAPO-5 is also stable sustained under the combustion. The large effective area of I'd improved the ability for CH 4 conversion. Consequently, PdHSAPO-5 exhibits the high activity for CH 4 combustion in the low temperature range. Acknowledgment The authors are grateful for the financial support from Saneyoshi Scholarship Foundation. The TEM experiments in this article were carried out in the High Voltage Electron Microscopy Laboratory of Kyushu University. References 1) D.L. Trimm, Appl. Catal., 7, 249 (1983). 2) Y. Li and J.N. Armor, Appl. Catal. B, 3, 275 (1994). 3) R. Vomscheid, M. Briend, M.J. Peltre, P. Massiani, P.P. Man, D. Barthomeuf, J. Chem. Soc., Chem. Comm. 1993, 544 (1993). 4) H. Arai, T. Yamada, K. Eguchi, and T. Seiyama, Appl. Catal., 26, 265 (1986). 5) J. Yu, J. Comets, L. Kevan, J. Phys. Chem., 97, 10433 (1993).
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
1647
Palladium ion-exchanged SAPO-5 for a low temperature combustion of CH 4 Yusaku Takita, Tatsumi Ishihara, Hiroyasu Nishiguchi, and Hideaki Sumi Department of Applied Chemistry, Faculty of Engineering Oita University, Dannoharu 700, Oita 870-11, Japan
The oxidation activity of molecular sieves ion-exchanged by Pd were investigated as a catalyst for the low temperature combustion of CH4. Among the molecular sieves examined, SAPO-5 ion-exchanged by Pd exhibited the highest activity for CH 4 combustion as well as the high thermal stability. Since the high dispersion of Pd particles was attained with ion-exchanged method, plateau in the temperature dependence of CH 4 conversion was hardly appeared on Pd-HSAPO-5 catalysts in the high space velocity of CH 4. Furthermore, the high activity of Pd-HSAPO-5 was sustained after the precalcination up to 1000 *C. Consequently, Pd-HSAPO-5 is a promising combustion catalyst in low temperature range. l.lntroduetion
Catalytic combustion, a new combustion technique without flames, is expected to be applicable to gas turbines. The advantages of oxidation catalysts for the application to combustion are the controlling combustion of fuels, less emission of pollutant such as NOx, and high efficiency of energy recovery. For the combustion catalyst, high oxidation activity as well as the thermal stability is required. At present, barium hexaaluminate is promising for the combustion catalysts at high temperature. In current catalytic combustors, fuel gases have to be preheated up to the temperature at which the combustion catalyst becomes active using open flame preheater. Thermal NOx is mainly formed at this preheater. Consequently, there is a possibility that a combustor with no NOx emission can be developed by the development of highly active catalyst which the preheater lights off at low temperature. Furthermore, methane emissions themselves are a potential environmental problem since the methane significantly contributes to the greenhouse effect. Removal of CH4 by combustion is also another important subject from an environmental prospective. Palladium catalysts supported on A1203 or A1203 based oxide generally use for a combustion catalysts in low temperature range [ 1]. However, the combustion activity of these conventional catalysts is not satisfactorily high and the development of the catalysts with high activity and high thermal stability is the important subject. Ion-exchanged method is effective for the preparation of highly dispersed metal catalysts and activity of metal catalysts is generally increased with improving the dispersion of metal. Therefore, it is expected that the molecular sieve ion-exchanged by Pd exhibits high activity to CH 4 combustion. Methane combustion of Pd 2§ ion-exchanged zeolite was reported by some researchers [2], however, the publications concerning the application of ion-exchanged catalysts for the combustion catalysts are limited up to now. Since the thermal stability of zeolite is not high, the catalytic activity of methane over Pd ion-exchanged ZSM-5 decreased significantly with time course in the presence of steam. In the present study, the activity of silicoaluminophosphate (SAPO) ion-
1648 Table 1 CH 4 oxidation activity of various molecular sieve ion-exchanged by Pd Surface Area a)
Pd Loading
m2g-1 Pd/AI203 Pd-H-Y Pd-H-USY e) Pd-H-Pentasil Pd-H-Ferrierite Pd-H-Mordenite Pd-H-SAPO-5 Pd-H-SAPO- 11 Pd-H-SAPO-34
wt%
109 411 597 280 315 408 107 90 380
1.00 1.00 0.69 0.73 0.88 0.70 0.96 0.69 0.53
CH 4 oxidation activity/~ b) Tlo
T30
Tso
T90
330 310 415 395 400 370 375 355 385
400 380 455 505 455 410 415 400 355
435 395 480 570 490 430 430 430 505
665 475 605 780 790 495 480 545 645
a) BET surface area b) Temperature at the CH 4 conversion attained at 10, 30, 50, and 90 %. c) Ultra-stable Y
exchanged by Pd for CH 4 combustion were investigated in detail, since it is reported that the framework of SAPO was sustained up to 1000 ~ [3].
2.Experimental Three types SAPO-5, -11, and -34 were synthesized according to U.S. Patent and 5 types of commercial zeolites were used. Molecular sieves were ion-exchanged to NH4§ with NH4NO3 and then calcined at 500 ~ for 6 h in air to obtain the molecular sieves ion-exchanged with H § H-type SAPO or zeolite thus prepared was ion-exchanged with Pd 2§ in a 0.01M[Pd(NH3)4]CI~ aqueous solution. The Pd ion-exchanged zeolite or SAPO were calcined in 02 stream at 500 ~ for 8h, H 2 stream 500 *C for 5h, and air 800 ~ 4h. The combustion activity of Pd ion-exchanged molecular sieves for CH 4 was measured with conventional fixed bed micro-flow reactor. A mixed gas consisting of CH 4 (1 vol%) and air was fed to the catalysts bed at S.V.=I(K)0(X) h -1. 90 The thermal stability of SAPO-5 was investigated with XRD and solid state M A S S - N M R . XRD 70 r.measurement was performed with o Cu K s line and NMR spectra for 5o 27A1 and 3ap were recorded with ID C)Pd-HSAI~-5 Bruker APR-300 spectrometer op30 o QPtV 2% erated at a field of 7 T. Spinning speeds of 4.5 kHz were used and the 10 chemical shifts of AI and P were referred to the external standard of 300 400 500 600 700 800 1.5M AI(H20)] § in AI(NO3)3 aqueTemperature / ~ ous solution a n d H2PO4(85%), respectively. The thermal stability of Fig. 1 Temperature dependence of the C H 4 conversion Pd ion-exchanged into SAPO-5 over Pd-HSAPO-5 and Pd/AI203 catalyst. 9
1649 was measured with TEM observation, adsorption of CO and ESR measurement. TEM observation was performed with JEM 2000FX (JEOL) and uptakes of CO was measured with the conventional adsorption apparatus. ESR measurement was performed on the reduced sample at room temperature with JEXFE1X(JEOL) and DPPH was used as an external standard for the calibration of g value. SAPO5 ion-exchanged by Pd was calcined at the various temperature in oxygen atmosphere (100 torr) for 6 h and then evacuated at 600 ~ for 3h.
300
,~ 400
O
O
= 500
OT,o}
"-.....
E 6 0 0 - A1"so Pd/AI203 700
"OTto} /XT~a Pd-HSAPO-5 0
10
20
30
50
40
Space velocity / 104 h "1 Fig.2 C H 4 conversion activity of Pd/AI203 and PdHSAPO-5 as a function of space velocity.
3. Results and Discussion 3.1. Catalytic activity of Pd ion-exchanged molecular sieve
u
u
I
400
Table 1 summarized the C H 4 oxidation activity of various molecular sieves ion-ex500 changed by Pd. Although the temperature at 10 % CH4 conversion became higher, the conversion at 90 % was attained at the lower tem600 perature upon a large part of Pd ion-exchanged molecular sieves compared with that I700 of Pd/Al203. In particular, Pd ion-exchanged HY and HSAPO-5 exhibited the high activity for CH 4 oxidation. Although the complete 800 oxidation of CH 4 attained at the lowest temI i I perature on Pd-HY among the catalysts exam800 900 1000 ined in Table 1, the framework was destructed Precalcination temperature/~ after CH 4 combustion at 800 ~ due to low thermal stability. In contrast, XRD analysis suggested that the framework of SAPO-5 was Fig.3 Effects of precalcination o n CH 4 comhardly changed after CH 4 combustion at 800 bustion activity. ~ Since the CH 4 conversion increased with increasing temperature without plateau, 90 % CH4 conversion attained on Pd-SAPO-5 was 170 ~ lower than that of Pd/AI203. Figure 1 shows the temperature dependence of C H 4 conversion on Pd-HSAPO-5 and Pd/ AI203. The activity for CH 4 combustion is higher on Pd/AIzO3 than Pd-HSAPO-5 in the temperature range lower than 450 ~ However, the CH 4 conversion increased drastically with increasing the reaction temperature on Pd-HSAPO-5. Contrarily, CH 4 conversion on Pd/AI203 increased through the plateau, resulting in the high temperature required to attain the complete oxidation of CH 4. Since it is reported that the plateau in the temperature dependence of CH 4
1650 combustion is appeared by the control of mass transfer process, no plateau in temperature dependence of CH 4 conversion on Pd-HSAPO-5 suggests that the effective surface area for CH 4 combustion is large on Pd-HSAPO-5. Figure 2 shows the dependence of the combustion activity of Pd-HSAPO-5 and Pd/Al203 on the space velocity of supplied CH 4. Since ion-exchanged Pd exists in the narrow pores of S A P O - 5 , it is anticipated that the mass transfer process greatly affects the CH 4 combustion activity in case of Pd-HSAPO-5. These effects of mass transfer process on CH 4 combustion becomes more significant in the high space velocity of supplied CH4. Temperatures at the 10 % and 50 % conversion level were elevated by increasing the space velocity of supplied CH 4 over both Pd-HSAPO-5 and Pd/Al203. However, Pd-HSAPO-5 exhibited the higher CH 4 conversion than Pd/Al203 at all examined space velocity. Since high dispersion of I'd attains on Pd-HSAPO-5 discussed later, it appears that the mass transfer process hardly affects on the CH 4 combustion on Pd-HSAPO-5 even though the active sites of Pd is dispersed in the narrow pore of SAPO-5. The effects of precalcination temperature on the C H 4 combustion activity were further studied (Fig.3). Although the temperature needs for 50 % C H 4 conversion were slightly increased with increasing the precalcination temperature, Pd-SAPO-5 exhibited the high activity for CH 4 oxidation even after precalcination at 1000 ~ This suggests that the activity for CH 4 combustion on Pd-SAPO-5 is sustained for a long period. Indeed, no degradation in CH 4 combustion activity of Pd-SAPO-5 was observed over 20 h at 600 ~ C H 4 conversion attained 90 % at 650 *C, which is 150 ~ lower than that of Pd/AI203 catalyst. Consequently, SAPO-5 ion-exchanged by Pd is the Table 2 Dependence of combustion rate of CH 4 upon promising catalysts for the low temthe partial presure of CH 4 and 0 2 at 400~ perature combustion of CH 4. Catalysts CH 4 dependence 0 2 dependence m n 3.2. R e a c t i o n k i n e t i c s of C H 4 c o m bustion
Pd-HSAPO-5 0.32 0.33 Effects of partial pressure of oxygen and methane on the combustion Pd/AI203 1.62 0.55 activity were examined for the reaction kinetics of CH 4 combustion on Pd-HSAPO-5 and Pd/AI203. The resuits were expressed by the empirical rate equation r=kPcn4mPo2 (1) where r is the combustion rate of methane, and m and n are the reaction orders for methane and oxygen partial pressure, respectively. In this experiment, CH 4 combustion was performed at 400 ~ to keep the CH 4 conversion level below 10 %. Table 2 summarized the reaction order of 02 and CH 4 on Pd-HSAPO-5 and Pd/Al203. Reaction order of oxygen on Pd-HSAPO-5 was almost the same value as that on Pd-HSAPO-5. On the other hand, the reaction order of CH 4 was far larger on Pd/AI203 than that of Pd-HSAP-5. The following two reasons were considered for the small reaction order of CH 4 on Pd-HSAPO-5 compared with Pd/Al203. One is the difference of facility in CH 4 activation and the other is the changes in the reaction mechanism. For studying the reaction mechanism on the both catalysts, the reaction rate was analyzed based on the Langmuir-Hinshelwood mechanism as listed in the following reaction equations. CH 4 + s ~ CH4s (2) 20 2+4s ~ 4Os (3) CH4s + 4Os ---" CO2s+2H2Os+2s (4)
1651
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Fig.4 Plots of (pcm/r) 'Is vs. Pca4 and (Po22/r)'/Svs. (P02) 1/2 in CH 4 combustion on Pd-HSAPO-5 and Vd/Al203 (400~ CO2s ~ 2H2Os ~
CO 2 + s 2H20 + 2s
(5) (6)
Here s means the surface adsorption sites. The oxidation of hydrocarbon on noble metal catalysts generally proceeds by the surface reaction between adsorbed oxygen and hydrocarbon. [4] For analyzing the reaction kinetics above, we assumed that the rate determining steps is the surface reaction between adsorbed methane and dissociatively adsorbed oxygen. The combustion rate under the above assumption was expressed in the following equation, r =
k,n~4Ko,P~,Po,[S0] *
(7)
( 1+Kcri4Pcri4+Ko21/4po2u2)s where Kci.x4 and Ko2 are adsorption equilibrium constants of oxygen and methane, respectively and [S o] represents the total surface adsorption site. 1%is a rate constant in equation (3). Equation (6) rewritten to test the fit of experimental data in the following equation;
(Pci.Jr) us =
I+Ko21/4po21/2
Kcrx4Pcti4 +
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(8) (kKcH4Ko2P02) 1/5[Sj
+
ko2Po21/2
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(kKcH4Ko2PcH4) x/S[So]
Experimentally obtained (Po,/r)t/s and (Pcri4/r)t/s values are plotted against P02~/2 and PCH4' respectively as shown in Fig. 4. Since the plots in Fig. 4 are quite linear on both catalysts, the
1652 reaction kinetics on both PdHSAPO-5 and Pd/AI203 catalysts are LangmuirHinschelwood type and the ai rate determining steps is the surface reaction between adsorbed CH 4 and "~ After CII4 combustion dissociatively adsorbed oxygen. Consequently, the small values in the reaction order of CH 4 in Table 2 is not caused by a different reaction lo 2b do mechanism but bythe high 20/degree ability for CH 4 activation 9 Namely, it appears that the Fig.5 XRD pattems of Pd-HSAPO-5 before (a) and methane is easily activated after (b) CH 4 combustion. on Pd-HSAPO-5 compared with Pd/AI20 v Therefore, Pd-HSAPO-5 exhibits the high activity for CH 4 combustion. I
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3.3. Thermal stability of Pd-HSAPO-5 Thermal stability is the another important factors for the combustion catalysts. Figure 5 shows the XRD patterns of Pd-SAPO-5 before and after CH4 combustion at 800 ~ All diffraction peaks were assigned to those for SAPO-5, and there were no diffraction peaks from a second phase, and there were no changes in the angle as well as the relative strength of the diffraction peaks from SAPO-5 after CH 4combustion at 800 ~ It is considered that the temperature of a catalyst surface reaches to 1100 ~ under CH4 combustion. The framework of 2"/Al
.
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Fig.6 27A1and 31p-MASS NMR sepctra of SAPO-5 before and after calcination at 1000 ~ (*);Spining side band
1653 SAPO-5 is extremely stable and sustained up to 1200 ~ There' 0 ' ' ' fore, the thermal stability of 2 < SAPO-5 is satisfactory as a support "~ 1.10 [ _ ~1 for the CH4cOmbustiOn catalyst in ~ , O-'-----: 11 the low-temperature region. The ~ 1.00 ~ W thermal stability of Pd-HSAPO-5 ~ ~ t was further studied in more detail i~ in the following part of present ~ ",1,2 "o study. < " 0.90~ , . . . . . , I, , I . .~ | Figure 6 shows the solid state 800 900 1000 NMR spectra of 27A1 and 31p in Preealcination temperature/*C SAPO-5 before and after calcined at 1000 ~ In accordance with the results of XRD, changes in the Fig.7 Average Pd particle size in SAPO-5 estimated NMR spectra of 27A1 and 31p in from CO adsorption amount as a function of calcination SAPO-5 are quit small. Consetemperature. quently, local structure as well as the long range order of framework 600"C precalcination of SAPO-5 are hardly changed up to 1000 ~ x2.5 Since the thermal stability of SAPO-5 support is satisfactory, the thermal stability of Pd ion-exgii=2.916 f N "9'-- gu=2"657 changed in HSAPO-5 was further x 2.5 studied with CO adsorption as shown in Fig.7. The average particle size of Pd estimated from the 1000"(2 precalcination linear type adsorption of CO was 1.1 nm and hardly dependent on the precalcination temperature up to 1000 'C. TEM observation g_t_=2.130 shows that the some part of Pd ionexchanged aggregated and exuded from the pores of SAPO-5. However, large part of Pd is stably exFig.8 ESR spectra of Pd-HSAPO-5 after pre-calcinaisted in the pores of SAPO-5, since tion at the marked temperature followed by evacuation the average particle size of Pd estiat 600~ mated with TEM and the uptakes of CO are 5 nm and 1.1 nm, respectively. Consequently, high dispersion of Pd is stably sustained up to the high temperature range. Since the smaller particle is generally more easily sintered to form larger particle, it is interesting that the high dispersion of Pd particle was attained in the temperature range up to 1000 ~ On the other hand, TEM observation and CO adsorption measurement suggests that the Pd particle with different thermal stability exists in HSAPO-5, namely, easily aggregated one and highly stable one. Figure 8 shows the ESR spectra of Pd-HSAPO-5 after the evacuation at 600 ~ for 6 h. Since no ESR signal was observed on the specimens which was calcined in the air, the most common oxidation state of Pd is divalent, Pd(II), in the as-synthesized specimens. On the other ~
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1654 hand, partially reduced sample gives an ESR signal of which parameters is g~ =2.130. The similar ESR spectra were reported for various palladium-exchanged zeolite and SAPO-5[5]. Considering the results reported by Yu et al.[5], the large part of Pd in SAPO-5 1 was reduced to monovalent, Pd(I), afe " ter the evacuation at 600 ~ for 3 h. The hyperfine signal from Pd(I) also confirmed the results that a large part of Pd is in a state of h~gh dispersion in atomic level. It is clearly shown in Fig.8 that the two signals in ESR spectra are obFig.9 SAPO-5 framwork and ion-exchanging served at ga--2.657 and 2.916 which are sites [5]. assigned to two kind of Pd at two different environments. These two kinds of Pd may assign to Pd ion existed at the center of a double 6-ring ( site I in Fig.9 ) which forms a 6-ring channel and the site dispersed into a 12-ring channel (site II or II* in Fig.9). The ESR spectra of Pd(I) in SAPO-5 was hardly changed after the precalcination at 1000 ~ as shown in Fig.9. Consequently, framework of SAPO-5 as well as Pd particle ion-exchanged in SAPO-5 are highly stable and consequently, it is expected that the high activity of Pd-HSAPO-5 for CH 4 combustion is stably exhibited for a long period. ,I-
- -
-.
4. Conclusion In the conventional studies on the combustion catalysts, supported metal catalysts have been generally investigated. However, metal ion-exchanged molecular sieve catalysts are highly active for the combustion of hydrocarbOn, since the high dispersion of metal can be attained by ion-exchanging method. In particular, framework of SAPO-5 is extremely stable and consequently, highly dispersed state of Pd in SAPO-5 is also stable sustained under the combustion. The large effective area of I'd improved the ability for CH 4 conversion. Consequently, PdHSAPO-5 exhibits the high activity for CH 4 combustion in the low temperature range. Acknowledgment The authors are grateful for the financial support from Saneyoshi Scholarship Foundation. The TEM experiments in this article were carried out in the High Voltage Electron Microscopy Laboratory of Kyushu University. References 1) D.L. Trimm, Appl. Catal., 7, 249 (1983). 2) Y. Li and J.N. Armor, Appl. Catal. B, 3, 275 (1994). 3) R. Vomscheid, M. Briend, M.J. Peltre, P. Massiani, P.P. Man, D. Barthomeuf, J. Chem. Soc., Chem. Comm. 1993, 544 (1993). 4) H. Arai, T. Yamada, K. Eguchi, and T. Seiyama, Appl. Catal., 26, 265 (1986). 5) J. Yu, J. Comets, L. Kevan, J. Phys. Chem., 97, 10433 (1993).