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Alumina aerogel for support of a methane combustion catalyst
Applied Catalysis A: General, 88 (1992) 137-148
137
Elsevier Science Publishers B.V., Amsterdam APCAT A2309
Alumina aerogel for support of a methane combustion catalyst Yasuyuki Mizushima and Makoto Hori Colloid Research Institute, 350-10gura, Yahatahigashi-ku, Kitakyushu 805 (Japan) (Received 23 September 1991, revised manuscript received 24 March 1992 )
Abstract Catalytic metal (platinum or palladium 1 wt.-%)-supported alumina aerogels were prepared. The platinum (or palladium ) -supported alumina aerogel catalyst was tested for methane combustion. The activities of platinum-supported alumina aerogel and commercial 7-alumina prepared by the conventional dipping method were similar, palladium-supported alumina aerogel showed higher activity than palladium-supported commercial y-alumina. This difference in activity became larger after treatment at 1200 °C for 100 h. However, the specific surface area and pore-size distribution of the alumina aerogel was similar to commercial alumina after treatment. Palladium (1 wt.-% )-supported silica ( 10 mol-% )alumina aerogel, which retains a higher specific surface area than alumina aerogel after treatment, was also prepared and its catalytic activity was measured. It showed better activity at low temperatures than the palladium-supported alumina aerogel. Completion of the reaction was not as good as with alumina aerogel. This was due to the difference in the electron state of palladium on the surface of each support.
Keywords: alumina aerogel, combustion catalyst, palladium, platinum, methane combustion.
INTRODUCTION
Aerogel was first studied by Kistler [ 1 ] and since then a number of investigations on aerogels have been reported although only a few reports on the use of aerogel as a catalyst support were published before 1970 [ 2-4 ]. With regards to alumina aerogel supports for catalysts, Gardes et al. have reported the use of NiO-AleO3 aerogel for the partial oxidation of alkene and the nitroxidation of hydrocarbons into nitriles [5 ]. Sayari et al. have reported the use of CreO3A1203 aerogel for the nitroxidation of hydrocarbons into nitriles [6] and Abouarnadasse et al. have reported the use of PbO-AIeO3 aerogel for the same nitroxidation [7]. Bianchi et al. have used FeeQ-A1203 aerogel for FischerCorrespondence to: Dr. Y. Mizushima, at his present address: Research and Development Department, N G K Spark Plug Co., Ltd., Komaki Factory, 2808, Iwasaki, Komaki, Aichi 485, Japan. Tel. ( 4-81-568)761225, fax. (+81-568)761295.
0926-3373/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
138
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
Tropsch synthesis [8]. Copper-, nickel-, and palladium-alumina aerogels have been studied for the same kind of organic reactions [9-11]. Effective and clean combustion is required nowadays from the viewpoint of saving energy and protecting the environment. Catalytic combustion has been studied to fulfill these requirements [ 12 ]. Catalysts for the combustion of natural gas (main gas is methane) have been studied from the viewpoint of fuel supply in Japan [13]. Combustion catalysts require the following properties: (1) they should ignite gas at a low temperature; (2) should produce gas for complete combustion without any poisonous emission gas (that is, high activity); (3) show no deterioration even when exposed at 1000-1300 ° C; and (4) have a long life [14]. Moreover, sintering of the support of the catalyst is a serious problem causing increased catalyst deactivation. Authors have studied alumina aerogels and have found that alumina aerogels show high heat resistance [15]. Platinum and palladium were selected as catalysts and platinum (or palladium ) -supported alumina aerogels were prepared. Catalytic activities were measured for methane combustion. EXPERIMENTAL Aluminium s e c . - t r i b u t o x i d e [AI (OBu sec )3] (10.0 g) and ethyl acetoacetate (etac) (4.403 g) were mixed and reacted to make a AI(OBu se~ )3-etac chelate complex. Ethanol (10 ml) was added. Water (2.192 g) diluted with 20 ml of ethanol was gradually added and hydrolyzed. Platinum chloric acid (0.110 g) [or palladium chloride (0.069 g) ] was added to the hydrolyzed alumina sol and pyridine (1.4 ml) was also added to make a platinum chloric acid and pyridine complex as in the reaction: H2PtC16 ÷ NC~Hs-~ CsH~NH2PtCI~. This complex prevents reduction and precipitation of platinum, so the alumina sol containing platinum can gellate and be aged without any precipitation. This is the same in the case of palladium chloride. Moreover, pyridine causes alumina sol's gellation to accelerate and the flame of the gel to strengthen. This alumina gel was supercritically dried in an autoclave at 270 ° C, 26.5 MPa under the supercritical state of ethanol. Thus, a catalyst-supported alumina aerogel was prepared. The amount of platinum in the ethanol taken out of the autoclave was measured by an inductivity-coupled argon plasma emission spectrophotometer (ICPV-1000, Shimadzu). No platinum was detected. The metal-supported alumina aerogel was calcined and crushed into 1-3 mm diameter grains. 1 g of these grains were packed into a quartz tube and set in the fixed-bed flow reactor and the activity of the catalyst for methane combustion was measured. 1 vol.-% of methane and 99 vol.-% of air were mixed and the flow-speed was regulated at 1 1/min. The combusted gas was analyzed with a gas chromatograph using a thermal conductivity detector (GC-A8, Shimadzu).
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
139
RESULTS AND DISCUSSION
Platinum-supported alumina aerogel Fig. i shows transmission electron microscope (TEM, JEM 2000EX 200kV, JEOL) micrographs of platinum particles prepared by two different methods. One was prepared by mixing platinum chloric acid in the alumina sol as previously described (method A). The other was prepared as follows: the alumina aerogel was calcined at 1000°C for 2 h and dipped into ethanol containing platinum chloric acid (method B) (Fig. lb). These catalyst-supported alumina aerogels were treated at 500°C for 2 h in air [2] and investigated by
Fig. I. T E M micrographs of platinum-supported alumina aerogel treated at 500°C for 2 h. (a) Platinum particlesprepared b F method A. (b) Platinum particlesprepared by method B after calcinationof the alumina aerogel.
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Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
TEM. The platinum particles prepared by method A were smaller than those prepared by method B, after calcination. Their particle size distributions were plotted from T E M micrographs (Fig. 2 ). The particle size distribution of platinum prepared by method A was narrow and the average diameter of the platinum particles was 6 nm. Platinum supported by method B after calcination was broad and the average diameter was 15 nm. Platinum (1 wt.-% )-supported commercial 7-alumina (Catalyst and Chemicals) was prepared by method B in the same way as described above. The
t
/
I
\ 0
10 Diameter
2'0 (nm)
30
Fig. 2. Platinum particle diameter according to the preparation method. ( ) method B, after calcination.
) Method A; ( ....
1 O0
//,'/ > o
so
C
~
300
(2~
y 400
/ ,/"/'/i
I
500 600 Temp. ('C)
f
700
800
Fig. 3. Catalytic activity for methane combustion of the platinum (1 wt.-% )-supported alumina aerogel and commercial alumina treated at 500 ° C for 5 h. ( • ) Alumina aerogel (method A ); ( O ) alumina aerogel (method B ); ( • ) commercial alumina (method B ).
141
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
o--
1 O0
v
01
o 50 ~O
300
400
500 600 Temp. ('C)
"700
800
Fig. 4. Catalytic activity for methane combustion of the platinum (1 wt.-% )-supported alumina aerogel and commercial alumina treated at 1200°C for 5 h. (O) Alumina aerogel (method A); (O) alumina aerogel (method B ); ( A ) commercial alumina (method B).
platinum-supported alumina aerogel and commercial alumina were calcined at 500 °C for 2 h in air and for 2 h in hydrogen. Fig. 3 shows the activities of the platinum-supported aluminas for methane combustion. The platinum-supported commercial alumina showed better activity than the others, and no large difference existed in the alumina aerogels. In spite of the fact that the platinum-supported alumina aerogel prepared by method A showed a good dispersibility of platinum particles its activity was not so good compared with the other aluminas. Fig. 4 shows the catalytic activity for methane combustion of platinum-supported aluminas after firing at 1200 °C for 5 h. As the activity of each catalyst deteriorated, the platinum-supported alumina aerogel, produced by method A showed a slightly better activity than the others.
Palladium-supported alumina aerogel The palladium (1 wt.-% )-supported aluminas were also prepared in the same way as described above. Fig. 5 shows the catalytic activities for methane combustion of palladium-supported aluminas. The palladium-supported alumina aerogels showed better activity than the palladium-supported commercial yalumina. The dispersibility of the palladium~particles prepared by method A were better than those prepared by method B (see the result of Figs. 1 and 2 ). However, the palladium-supported alumina aerogels prepared by method A showed less activity than those prepared by method B. This result shows that only a limited number of palladium particles were able to react with methane
142
Y. Mizushima and M. Hori/Appl. Catal, A 88 (1992) 137-148
I oo
,p,-. ~ o,
/ ~ . ~ "-
500
600
.~
g
~
v
300
400
700
800
Temp. ('(3)
Fig. 5. Catalytic activity for methane combustion of the palladium (1 wt.-% )-supported alumina aerogel and the commercial alumina treated at 500 ° C for 5 h. ( • ) Alumina aerogel (method A); (O) alumina aerogel (method B); ( • ) commercial alumina (method B). I oo
,,.i.
./ /
'~ 5O
/
e-
,
t
Z
•
jl m 300
400
500 600 Temp. ('C)
700
800
Fig. 6. Catalytic activity for methane combustion of the palladium (1 wt.-% )-supported alumina aerogel and commercial alumina treated at 1200°C for 5 h. ( • ) Alumina aerogel (method A); (O) alumina aerogel (method B); ( • ) commercial alumina (method B ).
atoms on the alumina surface and some particles were probably surrounded by alumina and as such were unable to react with methane. This must be the reason why both platinum-supported alumina aerogels showed almost the same activity in Fig. 3. Fig. 6 shows the catalytic activities for methane combustion of palladium (1 wt.-% )-supported alumina aerogels after treatment at 1200°C for 5 h. The
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activity of the palladium-supported alumina aerogel produced by method A was better than that treated at 500 ° C for 5 h, while the activity of the palladium-supported alumina aerogel produced by method B did not change much after heat treatment and the activity of the palladium-supported commercial alumina deteriorated. It is supposed that the palladium particles prepared by method A within the alumina aerogel appeared on the surface of the alumina and activity increased, while the palladium particles prepared by method B had a weak attachment with the support and were easily sintered making the activity worse. We also found that the heat resistance of palladium was much better than platinum with respect to methane combustion [16]. Silica (10 mol-% )-alumina aerogel has a higher specific surface area than the aerogel prepared at 1200°C for a 5-h period [15]. So palladium (1 wt.-% )supported silica (10 mol-% )-alumina aerogel was prepared and the catalytic activity was measured. Fig. 7 shows the activity of the palladium-supported silica (10 mol-% )-alumina aerogel treated at 1200°C for 5 h. The palladiumsupported silica (10 tool-% )-alumina aerogel showed a higher activity at low temperatures than the alumina aerogel and the alumina aerogel showed a higher activity on completion of the reaction. The specific surface area of the palladium-supported alumina aerogel and the silica (10 mol-%)-alumina aerogel after treatment at 1200°C for 5 h were 57.0 and 93.4 m2/g, respectively. The better activity of the silica ( 10 mol-% )-alumina aerogel at lower temperatures may be attributed to its higher specific surface area. While, the palladiumsupported silica (10 mol-% )-alumina aerogel showed poorer activity on completion of the reaction than alumina aerogel, it is supposed that the addition o
1O0
g
E 50 o
,g
-0
300
400
500
600
700
800
TemD. (°C)
Fig. 7. Catalytic activity for methane combustion of the palladium (1 wt.-% )-supported alumina aerogel and silica (10 mol-% )-alumina aerogel treated at 1200 °C for 5 h. ( • ) Alumina aerogel; ( O ) silica (10 mol-% )-alumina aerogel.
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of silica made the alumina show a higher acidity than alumina itself and this effect altered the activity for methane combustion [17,18]. The activity of the palladium-supported alumina aerogel prepared by method A increased by heat treatment at 1200°C for 5 h (Figs. 5, 6). So the same kind of test was applied regardless of whether the palladium-supported silica (10 mol-% )-alumina aerogel showed similar results or not. The activities of the palladium-supported silica (10 mol-% )-alumina aerogel were investigated after heat treatment at 500, 800, 1000 and 1200°C for 5 h (Fig. 8). Higher heat treatment caused higher activity, however, the completion of the reaction did not change or only temperature slightly deteriorated. The activities of these aerogels were measured in the same way after treatment at 1200 °C for 100 h (Fig. 9), because catalysts need to have a long life. The palladium-supported alumina aerogel and the palladium-supported silica (10 mol-% )-alumina aerogel deteriorated slightly and showed the same tendency as those treated at 1200°C for 5 h. The commercial alumina lost its activity almost completely. The specific surface areas of the alumina aerogel, silica (10 mol-% )-alumina aerogel and commercial alumina are shown in Table 1. The palladium-supported alumina aerogel showed a much higher activity than the palladium-supported commercial alumina in spite of having almost the same specific surface area. However, the activity of the palladium-supported silica (10 mol-% )-alumina aerogel on completion of methane combustion worsened. Fig. 10 shows the pore-size distribution of the alumina supports after treatment at 1200°C for 100 h. The alumina aerogel and commercial alumina support showed almost the same pore-size distribution and specific surface area,
1O0
.-'-
,
300
400
•
:
500 600 Temp. ('C)
700
800
Fig. 8. Catalytic activity for methane combustion of the palladium (1 wt.-% ) -supported silica (10 mol-% )-alumina aerogel. ( O ) 500°C; ( • ) 800°C; (Z~) 1000°C; ( • ) 1200°C.
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
145
(%) 1 oo
{ 300
400
500
600
700
8 0 0 ('C)
Temp.
Fig. 9. Catalytic activity for methane combustion of the palladium (1 wt.-% )-supported alumina aerogel, silica (10 mol-% )-alumina aerogel and commercial alumina treated at 1200 ° C for 100 h. ( • ) Alumina aerogel; ( [] ) silica ( 10 mol-% )-alumina aerogel; ( • ) commercial alumina.
TABLE 1 Specific surface area (SSA) of supports treated at 1200°C for 100 h Support
SSA (m2/g)
Alumina aerogel Silica (10 mol-% )-alumina aerogel Commercial alumina
8.5 66.7 5.5
£3
10 -3
10 -2
10 "1
1
101
Pore d,ameter (/~m)
Fig. 10. Pore-size distributions of alumina aerogel, silica (10 mol-% )-alumina aerogel and commercial alumina treated at 1200°C for 100 h. ( ) Alumina aerogel; ( .... ) silica (10 mol-% )alumina aerogel; (---) commercial alumina.
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Y. M izushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
so any difference in activities did not depend on the specific surface area or pore-size distributions of the supports. Auger electron spectroscopy (JAMP30, JEOL) of the palladium-supported aerogels and commercial alumina at an accelerating voltage of 3.0 kV showed that only the commercial alumina contained calcium. This kind of impurity affects the activity of palladium as well as the preparation method. The palladium peak of the alumina aerogel and commercial alumina were at 325.5 eV and that of the silica (10 mol-% )-alumina aerogel was 328 eV. The XRD pattern of palladium oxide and palladium metal were observed (RAD-IIC, Rigaku) in both alumina aerogel and silica (10 mol-%)-alumina aerogel. The ratio of peak square between palladium oxide (101) and palladium (111) was similar between alumina aerogel and silica (10 mol-% )-alumina aerogel [ 19 ]. So only the electron state on the surface of palladium in each aerogel was different and this affected the activity for methane combustion [ 20 ]. Fig. 11 shows the catalytic activity of palladium (1 wt.-% )-supported alumina aerogel for methane combustion after treatment at 1300 °C for 5 h. The catalytic activity decreased between 750-800 ° C. Activity decreased especially with decreasing temperature. This p h e n o m e n o n was also observed in the palladium-supported silica (10 mol-% )-alumina aerogel treated at 1200°C for 100 h (Fig. 9), while it was not observed in the alumina aerogel at all. The palladium particles on the alumina sintered at 1300°C and the interaction became weaker between the palladium particles and the alumina support. Then, the nature of the palladium metal became dominant and the following reaction occurred between 750-900 °C [21,22]: P d O - ~ P d + 1/202. This result shows that palladium in the alumina aerogel also sintered during the heat treatment at 1300 ° C for 5 h, although the palladium-supported alumina aerogel prepared by method A showed a good dispersibility of palladium particles and high ac1oo J
I-~j}
='-
300
I
I
I
I
I
400
500
600
700
800
Temp. (°C)
Fig. 11. Catalytic activity for methane combustion of palladium (1 wt.-%)-supported alumina aerogel at 1300°C for 5 h: ( O ) on increasing temperature; ( O ) on decreasing temperature.
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
147
tivity, a n d p a l l a d i u m oxide m o r e easily b e c a m e p a l l a d i u m m e t a l in the silica (10 mol-% ) - a l u m i n a aerogel. T h i s was also due to t h e difference in the elect r o n s t a t e of p a l l a d i u m as has b e e n explained. CONCLUSIONS (1) P l a t i n u m - a n d p a l l a d i u m - s u p p o r t e d a l u m i n a aerogel were prepared. T h e p a l l a d i u m - s u p p o r t e d a l u m i n a aerogel s h o w e d high activity for m e t h a n e comb u s t i o n a n d v e r y good h e a t resistance, while t h e p l a t i n u m - s u p p o r t e d one does not. (2) H i g h activity of t h e p a l l a d i u m - s u p p o r t e d a l u m i n a aerogel as a m e t h a n e c o m b u s t i o n c a t a l y s t is d e p e n d e n t o n t h e dispersibility of active p a l l a d i u m particles a n d on a s t r o n g c o n t a c t w i t h t h e a l u m i n a aerogel, n o t on the specific surface area or pore-size d i s t r i b u t i o n o f t h e a l u m i n a support. (3) A silica a d d i t i o n to t h e a l u m i n a aerogel increases catalytic activity at low t e m p e r a t u r e s . H o w e v e r , this decreases on c o m p l e t i o n o f the reaction. (4) H i g h h e a t t r e a t m e n t (1300 ° C) causes t h e p a l l a d i u m in the a l u m i n a aerogel to s i n t e r with a c t i v i t y d e c r e a s i n g b e t w e e n 750-800 ° C. ACKNOWLEDGEMENTS T h e a u t h o r s sincerely t h a n k Prof. Arai a n d Dr. M a c h i d a for t h e i r advice on m a k i n g t h e fixed-bed flow r e a c t o r a n d on e x p e r i m e n t a l procedures.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
S.S. Kistler, J. Phys. Chem., 36 (1932) 52. S.S. Kistler, S. Swann and E.G. Appel, Ind. Eng. Chem., 26 (1934) 388. S. Swann, E.G. Appel and S.S. Kistler, Ind. Eng. Chem., 26 (1934) 1014. H.D. Foster and D.B. Keyes, Ind. Eng. Chem., 29 (1937) 1254. G.G.E. Gardes, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1976) 1321. A. Sayari, A. Ghorbel, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1981) 220. S. Abouarnadasse, G.M. Pajonk and S.J. Teichner, in Actas Xe Simposia Iberoamericano de Catalysis, Merida, Venezuela, 1986, Vol. II, p. 615. D. Bianchi, H. Batis-Landoulsi, C.O. Bennett, G.M. Pajonk, P. Vergnon and S.J. Teichner, Bull. Soc. Chem. Fr., (1981) 345. G. Pajonk, M.B. Taghavi and S.J. Teichner, Bull. Soc. Ft., (1975) 983. G.E.E. Gardes, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1976) 1327. J.N. Armor, E.J. Carlson and P.M. Zambri, Appl. Catal., 19 (1985) 339. L.D. Pfefferle and W.C. Pfefferle, Catal. Rev.-Sci, Eng., 29 (1987) 219. T. Ono and S. Ichihara, Jpn. Catalyst, 29 (1987) 305. S. Ichihara and K. Tuchitani, Jpn. Ceramics, 24 (1989) 1065. Y. Mizushima and M. Hori, J. Non-Crystal. Solids, (submitted). R. Prasad, L.A. Kennedy and E. Ruckenstein, Catal. Rev.-Sci. Eng., 26 (1984) 1. R.C. Hansford, Ind. Eng. Chem., 39 (1947) 849.
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18 C.L. Thomas, Ind. Eng. Chem., 41 (1949) 2564. 19 E. Kikuchi and T. Matsuda, Ninth Symposium for Catalytic Combustion (Proceedings), The Catalysis Society of Japan, 1990, p. 11. 20 L. Hilaire and F. Legate, Y. Holl and G. Maire, Solid State Commun., 32 (1979) 157, 21 T. Hayata, Electrochem., 10 (1989) 953. 22 T. Matsuda, E. Kikuchi, Proceedings of the 66th Catalyst Meeting, The Catalysis Society of Japan, 1990, p. 186.
137
Elsevier Science Publishers B.V., Amsterdam APCAT A2309
Alumina aerogel for support of a methane combustion catalyst Yasuyuki Mizushima and Makoto Hori Colloid Research Institute, 350-10gura, Yahatahigashi-ku, Kitakyushu 805 (Japan) (Received 23 September 1991, revised manuscript received 24 March 1992 )
Abstract Catalytic metal (platinum or palladium 1 wt.-%)-supported alumina aerogels were prepared. The platinum (or palladium ) -supported alumina aerogel catalyst was tested for methane combustion. The activities of platinum-supported alumina aerogel and commercial 7-alumina prepared by the conventional dipping method were similar, palladium-supported alumina aerogel showed higher activity than palladium-supported commercial y-alumina. This difference in activity became larger after treatment at 1200 °C for 100 h. However, the specific surface area and pore-size distribution of the alumina aerogel was similar to commercial alumina after treatment. Palladium (1 wt.-% )-supported silica ( 10 mol-% )alumina aerogel, which retains a higher specific surface area than alumina aerogel after treatment, was also prepared and its catalytic activity was measured. It showed better activity at low temperatures than the palladium-supported alumina aerogel. Completion of the reaction was not as good as with alumina aerogel. This was due to the difference in the electron state of palladium on the surface of each support.
Keywords: alumina aerogel, combustion catalyst, palladium, platinum, methane combustion.
INTRODUCTION
Aerogel was first studied by Kistler [ 1 ] and since then a number of investigations on aerogels have been reported although only a few reports on the use of aerogel as a catalyst support were published before 1970 [ 2-4 ]. With regards to alumina aerogel supports for catalysts, Gardes et al. have reported the use of NiO-AleO3 aerogel for the partial oxidation of alkene and the nitroxidation of hydrocarbons into nitriles [5 ]. Sayari et al. have reported the use of CreO3A1203 aerogel for the nitroxidation of hydrocarbons into nitriles [6] and Abouarnadasse et al. have reported the use of PbO-AIeO3 aerogel for the same nitroxidation [7]. Bianchi et al. have used FeeQ-A1203 aerogel for FischerCorrespondence to: Dr. Y. Mizushima, at his present address: Research and Development Department, N G K Spark Plug Co., Ltd., Komaki Factory, 2808, Iwasaki, Komaki, Aichi 485, Japan. Tel. ( 4-81-568)761225, fax. (+81-568)761295.
0926-3373/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
138
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
Tropsch synthesis [8]. Copper-, nickel-, and palladium-alumina aerogels have been studied for the same kind of organic reactions [9-11]. Effective and clean combustion is required nowadays from the viewpoint of saving energy and protecting the environment. Catalytic combustion has been studied to fulfill these requirements [ 12 ]. Catalysts for the combustion of natural gas (main gas is methane) have been studied from the viewpoint of fuel supply in Japan [13]. Combustion catalysts require the following properties: (1) they should ignite gas at a low temperature; (2) should produce gas for complete combustion without any poisonous emission gas (that is, high activity); (3) show no deterioration even when exposed at 1000-1300 ° C; and (4) have a long life [14]. Moreover, sintering of the support of the catalyst is a serious problem causing increased catalyst deactivation. Authors have studied alumina aerogels and have found that alumina aerogels show high heat resistance [15]. Platinum and palladium were selected as catalysts and platinum (or palladium ) -supported alumina aerogels were prepared. Catalytic activities were measured for methane combustion. EXPERIMENTAL Aluminium s e c . - t r i b u t o x i d e [AI (OBu sec )3] (10.0 g) and ethyl acetoacetate (etac) (4.403 g) were mixed and reacted to make a AI(OBu se~ )3-etac chelate complex. Ethanol (10 ml) was added. Water (2.192 g) diluted with 20 ml of ethanol was gradually added and hydrolyzed. Platinum chloric acid (0.110 g) [or palladium chloride (0.069 g) ] was added to the hydrolyzed alumina sol and pyridine (1.4 ml) was also added to make a platinum chloric acid and pyridine complex as in the reaction: H2PtC16 ÷ NC~Hs-~ CsH~NH2PtCI~. This complex prevents reduction and precipitation of platinum, so the alumina sol containing platinum can gellate and be aged without any precipitation. This is the same in the case of palladium chloride. Moreover, pyridine causes alumina sol's gellation to accelerate and the flame of the gel to strengthen. This alumina gel was supercritically dried in an autoclave at 270 ° C, 26.5 MPa under the supercritical state of ethanol. Thus, a catalyst-supported alumina aerogel was prepared. The amount of platinum in the ethanol taken out of the autoclave was measured by an inductivity-coupled argon plasma emission spectrophotometer (ICPV-1000, Shimadzu). No platinum was detected. The metal-supported alumina aerogel was calcined and crushed into 1-3 mm diameter grains. 1 g of these grains were packed into a quartz tube and set in the fixed-bed flow reactor and the activity of the catalyst for methane combustion was measured. 1 vol.-% of methane and 99 vol.-% of air were mixed and the flow-speed was regulated at 1 1/min. The combusted gas was analyzed with a gas chromatograph using a thermal conductivity detector (GC-A8, Shimadzu).
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
139
RESULTS AND DISCUSSION
Platinum-supported alumina aerogel Fig. i shows transmission electron microscope (TEM, JEM 2000EX 200kV, JEOL) micrographs of platinum particles prepared by two different methods. One was prepared by mixing platinum chloric acid in the alumina sol as previously described (method A). The other was prepared as follows: the alumina aerogel was calcined at 1000°C for 2 h and dipped into ethanol containing platinum chloric acid (method B) (Fig. lb). These catalyst-supported alumina aerogels were treated at 500°C for 2 h in air [2] and investigated by
Fig. I. T E M micrographs of platinum-supported alumina aerogel treated at 500°C for 2 h. (a) Platinum particlesprepared b F method A. (b) Platinum particlesprepared by method B after calcinationof the alumina aerogel.
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Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
TEM. The platinum particles prepared by method A were smaller than those prepared by method B, after calcination. Their particle size distributions were plotted from T E M micrographs (Fig. 2 ). The particle size distribution of platinum prepared by method A was narrow and the average diameter of the platinum particles was 6 nm. Platinum supported by method B after calcination was broad and the average diameter was 15 nm. Platinum (1 wt.-% )-supported commercial 7-alumina (Catalyst and Chemicals) was prepared by method B in the same way as described above. The
t
/
I
\ 0
10 Diameter
2'0 (nm)
30
Fig. 2. Platinum particle diameter according to the preparation method. ( ) method B, after calcination.
) Method A; ( ....
1 O0
//,'/ > o
so
C
~
300
(2~
y 400
/ ,/"/'/i
I
500 600 Temp. ('C)
f
700
800
Fig. 3. Catalytic activity for methane combustion of the platinum (1 wt.-% )-supported alumina aerogel and commercial alumina treated at 500 ° C for 5 h. ( • ) Alumina aerogel (method A ); ( O ) alumina aerogel (method B ); ( • ) commercial alumina (method B ).
141
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o--
1 O0
v
01
o 50 ~O
300
400
500 600 Temp. ('C)
"700
800
Fig. 4. Catalytic activity for methane combustion of the platinum (1 wt.-% )-supported alumina aerogel and commercial alumina treated at 1200°C for 5 h. (O) Alumina aerogel (method A); (O) alumina aerogel (method B ); ( A ) commercial alumina (method B).
platinum-supported alumina aerogel and commercial alumina were calcined at 500 °C for 2 h in air and for 2 h in hydrogen. Fig. 3 shows the activities of the platinum-supported aluminas for methane combustion. The platinum-supported commercial alumina showed better activity than the others, and no large difference existed in the alumina aerogels. In spite of the fact that the platinum-supported alumina aerogel prepared by method A showed a good dispersibility of platinum particles its activity was not so good compared with the other aluminas. Fig. 4 shows the catalytic activity for methane combustion of platinum-supported aluminas after firing at 1200 °C for 5 h. As the activity of each catalyst deteriorated, the platinum-supported alumina aerogel, produced by method A showed a slightly better activity than the others.
Palladium-supported alumina aerogel The palladium (1 wt.-% )-supported aluminas were also prepared in the same way as described above. Fig. 5 shows the catalytic activities for methane combustion of palladium-supported aluminas. The palladium-supported alumina aerogels showed better activity than the palladium-supported commercial yalumina. The dispersibility of the palladium~particles prepared by method A were better than those prepared by method B (see the result of Figs. 1 and 2 ). However, the palladium-supported alumina aerogels prepared by method A showed less activity than those prepared by method B. This result shows that only a limited number of palladium particles were able to react with methane
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I oo
,p,-. ~ o,
/ ~ . ~ "-
500
600
.~
g
~
v
300
400
700
800
Temp. ('(3)
Fig. 5. Catalytic activity for methane combustion of the palladium (1 wt.-% )-supported alumina aerogel and the commercial alumina treated at 500 ° C for 5 h. ( • ) Alumina aerogel (method A); (O) alumina aerogel (method B); ( • ) commercial alumina (method B). I oo
,,.i.
./ /
'~ 5O
/
e-
,
t
Z
•
jl m 300
400
500 600 Temp. ('C)
700
800
Fig. 6. Catalytic activity for methane combustion of the palladium (1 wt.-% )-supported alumina aerogel and commercial alumina treated at 1200°C for 5 h. ( • ) Alumina aerogel (method A); (O) alumina aerogel (method B); ( • ) commercial alumina (method B ).
atoms on the alumina surface and some particles were probably surrounded by alumina and as such were unable to react with methane. This must be the reason why both platinum-supported alumina aerogels showed almost the same activity in Fig. 3. Fig. 6 shows the catalytic activities for methane combustion of palladium (1 wt.-% )-supported alumina aerogels after treatment at 1200°C for 5 h. The
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activity of the palladium-supported alumina aerogel produced by method A was better than that treated at 500 ° C for 5 h, while the activity of the palladium-supported alumina aerogel produced by method B did not change much after heat treatment and the activity of the palladium-supported commercial alumina deteriorated. It is supposed that the palladium particles prepared by method A within the alumina aerogel appeared on the surface of the alumina and activity increased, while the palladium particles prepared by method B had a weak attachment with the support and were easily sintered making the activity worse. We also found that the heat resistance of palladium was much better than platinum with respect to methane combustion [16]. Silica (10 mol-% )-alumina aerogel has a higher specific surface area than the aerogel prepared at 1200°C for a 5-h period [15]. So palladium (1 wt.-% )supported silica (10 mol-% )-alumina aerogel was prepared and the catalytic activity was measured. Fig. 7 shows the activity of the palladium-supported silica (10 mol-% )-alumina aerogel treated at 1200°C for 5 h. The palladiumsupported silica (10 tool-% )-alumina aerogel showed a higher activity at low temperatures than the alumina aerogel and the alumina aerogel showed a higher activity on completion of the reaction. The specific surface area of the palladium-supported alumina aerogel and the silica (10 mol-%)-alumina aerogel after treatment at 1200°C for 5 h were 57.0 and 93.4 m2/g, respectively. The better activity of the silica ( 10 mol-% )-alumina aerogel at lower temperatures may be attributed to its higher specific surface area. While, the palladiumsupported silica (10 mol-% )-alumina aerogel showed poorer activity on completion of the reaction than alumina aerogel, it is supposed that the addition o
1O0
g
E 50 o
,g
-0
300
400
500
600
700
800
TemD. (°C)
Fig. 7. Catalytic activity for methane combustion of the palladium (1 wt.-% )-supported alumina aerogel and silica (10 mol-% )-alumina aerogel treated at 1200 °C for 5 h. ( • ) Alumina aerogel; ( O ) silica (10 mol-% )-alumina aerogel.
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of silica made the alumina show a higher acidity than alumina itself and this effect altered the activity for methane combustion [17,18]. The activity of the palladium-supported alumina aerogel prepared by method A increased by heat treatment at 1200°C for 5 h (Figs. 5, 6). So the same kind of test was applied regardless of whether the palladium-supported silica (10 mol-% )-alumina aerogel showed similar results or not. The activities of the palladium-supported silica (10 mol-% )-alumina aerogel were investigated after heat treatment at 500, 800, 1000 and 1200°C for 5 h (Fig. 8). Higher heat treatment caused higher activity, however, the completion of the reaction did not change or only temperature slightly deteriorated. The activities of these aerogels were measured in the same way after treatment at 1200 °C for 100 h (Fig. 9), because catalysts need to have a long life. The palladium-supported alumina aerogel and the palladium-supported silica (10 mol-% )-alumina aerogel deteriorated slightly and showed the same tendency as those treated at 1200°C for 5 h. The commercial alumina lost its activity almost completely. The specific surface areas of the alumina aerogel, silica (10 mol-% )-alumina aerogel and commercial alumina are shown in Table 1. The palladium-supported alumina aerogel showed a much higher activity than the palladium-supported commercial alumina in spite of having almost the same specific surface area. However, the activity of the palladium-supported silica (10 mol-% )-alumina aerogel on completion of methane combustion worsened. Fig. 10 shows the pore-size distribution of the alumina supports after treatment at 1200°C for 100 h. The alumina aerogel and commercial alumina support showed almost the same pore-size distribution and specific surface area,
1O0
.-'-
,
300
400
•
:
500 600 Temp. ('C)
700
800
Fig. 8. Catalytic activity for methane combustion of the palladium (1 wt.-% ) -supported silica (10 mol-% )-alumina aerogel. ( O ) 500°C; ( • ) 800°C; (Z~) 1000°C; ( • ) 1200°C.
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
145
(%) 1 oo
{ 300
400
500
600
700
8 0 0 ('C)
Temp.
Fig. 9. Catalytic activity for methane combustion of the palladium (1 wt.-% )-supported alumina aerogel, silica (10 mol-% )-alumina aerogel and commercial alumina treated at 1200 ° C for 100 h. ( • ) Alumina aerogel; ( [] ) silica ( 10 mol-% )-alumina aerogel; ( • ) commercial alumina.
TABLE 1 Specific surface area (SSA) of supports treated at 1200°C for 100 h Support
SSA (m2/g)
Alumina aerogel Silica (10 mol-% )-alumina aerogel Commercial alumina
8.5 66.7 5.5
£3
10 -3
10 -2
10 "1
1
101
Pore d,ameter (/~m)
Fig. 10. Pore-size distributions of alumina aerogel, silica (10 mol-% )-alumina aerogel and commercial alumina treated at 1200°C for 100 h. ( ) Alumina aerogel; ( .... ) silica (10 mol-% )alumina aerogel; (---) commercial alumina.
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so any difference in activities did not depend on the specific surface area or pore-size distributions of the supports. Auger electron spectroscopy (JAMP30, JEOL) of the palladium-supported aerogels and commercial alumina at an accelerating voltage of 3.0 kV showed that only the commercial alumina contained calcium. This kind of impurity affects the activity of palladium as well as the preparation method. The palladium peak of the alumina aerogel and commercial alumina were at 325.5 eV and that of the silica (10 mol-% )-alumina aerogel was 328 eV. The XRD pattern of palladium oxide and palladium metal were observed (RAD-IIC, Rigaku) in both alumina aerogel and silica (10 mol-%)-alumina aerogel. The ratio of peak square between palladium oxide (101) and palladium (111) was similar between alumina aerogel and silica (10 mol-% )-alumina aerogel [ 19 ]. So only the electron state on the surface of palladium in each aerogel was different and this affected the activity for methane combustion [ 20 ]. Fig. 11 shows the catalytic activity of palladium (1 wt.-% )-supported alumina aerogel for methane combustion after treatment at 1300 °C for 5 h. The catalytic activity decreased between 750-800 ° C. Activity decreased especially with decreasing temperature. This p h e n o m e n o n was also observed in the palladium-supported silica (10 mol-% )-alumina aerogel treated at 1200°C for 100 h (Fig. 9), while it was not observed in the alumina aerogel at all. The palladium particles on the alumina sintered at 1300°C and the interaction became weaker between the palladium particles and the alumina support. Then, the nature of the palladium metal became dominant and the following reaction occurred between 750-900 °C [21,22]: P d O - ~ P d + 1/202. This result shows that palladium in the alumina aerogel also sintered during the heat treatment at 1300 ° C for 5 h, although the palladium-supported alumina aerogel prepared by method A showed a good dispersibility of palladium particles and high ac1oo J
I-~j}
='-
300
I
I
I
I
I
400
500
600
700
800
Temp. (°C)
Fig. 11. Catalytic activity for methane combustion of palladium (1 wt.-%)-supported alumina aerogel at 1300°C for 5 h: ( O ) on increasing temperature; ( O ) on decreasing temperature.
Y. Mizushima and M. Hori/Appl. Catal. A 88 (1992) 137-148
147
tivity, a n d p a l l a d i u m oxide m o r e easily b e c a m e p a l l a d i u m m e t a l in the silica (10 mol-% ) - a l u m i n a aerogel. T h i s was also due to t h e difference in the elect r o n s t a t e of p a l l a d i u m as has b e e n explained. CONCLUSIONS (1) P l a t i n u m - a n d p a l l a d i u m - s u p p o r t e d a l u m i n a aerogel were prepared. T h e p a l l a d i u m - s u p p o r t e d a l u m i n a aerogel s h o w e d high activity for m e t h a n e comb u s t i o n a n d v e r y good h e a t resistance, while t h e p l a t i n u m - s u p p o r t e d one does not. (2) H i g h activity of t h e p a l l a d i u m - s u p p o r t e d a l u m i n a aerogel as a m e t h a n e c o m b u s t i o n c a t a l y s t is d e p e n d e n t o n t h e dispersibility of active p a l l a d i u m particles a n d on a s t r o n g c o n t a c t w i t h t h e a l u m i n a aerogel, n o t on the specific surface area or pore-size d i s t r i b u t i o n o f t h e a l u m i n a support. (3) A silica a d d i t i o n to t h e a l u m i n a aerogel increases catalytic activity at low t e m p e r a t u r e s . H o w e v e r , this decreases on c o m p l e t i o n o f the reaction. (4) H i g h h e a t t r e a t m e n t (1300 ° C) causes t h e p a l l a d i u m in the a l u m i n a aerogel to s i n t e r with a c t i v i t y d e c r e a s i n g b e t w e e n 750-800 ° C. ACKNOWLEDGEMENTS T h e a u t h o r s sincerely t h a n k Prof. Arai a n d Dr. M a c h i d a for t h e i r advice on m a k i n g t h e fixed-bed flow r e a c t o r a n d on e x p e r i m e n t a l procedures.
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18 C.L. Thomas, Ind. Eng. Chem., 41 (1949) 2564. 19 E. Kikuchi and T. Matsuda, Ninth Symposium for Catalytic Combustion (Proceedings), The Catalysis Society of Japan, 1990, p. 11. 20 L. Hilaire and F. Legate, Y. Holl and G. Maire, Solid State Commun., 32 (1979) 157, 21 T. Hayata, Electrochem., 10 (1989) 953. 22 T. Matsuda, E. Kikuchi, Proceedings of the 66th Catalyst Meeting, The Catalysis Society of Japan, 1990, p. 186.