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Hydrogen spillover within carbon-supported palladium catalyst prepared under ultrasound
Spillover and Migration of Surface Species on Catalysts Can Li and Qin Xin, editors 9 1997 Elsevier Science B.V. All rights reserved.
261
H y d r o g e n spillover within carbon-supported palladium catalyst p r e p a r e d under ultrasound Z.X.Cheng, S.B.Yuan a, J.W.Fan b, Q.M.Zhu and M.S.Zhen b State Key Laboratory of C 1 Chemistry and Technology, Department of Chemistry, Tsinghua University, Beijing 100084 aCollege of Chemistry and Molecular Engineer, Center of Physicochemistry, Peking University bInstitute of Chemical Defence, Beijing 1048 P.O.Box
The Pd/active-carbon catalyst was prepared in solution through a reduction of Pd Hwith formaldehyde, and the obtained metallic particles were around 10-30 A with a mean size about 16 A either applying ultrasound in catalyst preparation or varying palladium loading (0.8-5 wt.%). H2 spillover from Pd onto the carbon surface could occur in Pd/C at 200 ~ and H/Pd decreased with the increase of Pd loading; the ultrasound in Pd/C preparation led to a greatly increased amount of spillover H2, and H/Pd could reach a high value of 6.8 for 0.8 wt.% sonicated Pd/C catalysts. The Pd/C activity (mole Hjmin.g.Pd) in nitrobenzene hydrogenation at 30 ~ increased with the palladium loading.
1. INTRODUCTION Catalysts of carbon supported Nobel metals are widely used in industrial processes, including hydrogenation and organic synthesis. They could be prepared by general methods, such as impregnation or ionic exchange following by the drying and reduction in hydrogen. Meanwhile, there exists another appropriate method for their preparation, and it mainly consists of a reduction of metal precursors using a reductant (formaldehyde, hydrazine, or others) in aqueous solution, in which a carbon support is suspended (1). The latter one was adopted in this work. It was said that ultrasound might be beneficial in catalyst preparation, catalyst activation, and catalytic reaction involving solid catalyst/reagents. It is normally considered that the improvement of reactivity of solid catalysts was caused by the creation of surface defects, the reduction of particle size and the acceleration of mass transport upon ultrasound action (2). Therefore, we have investigated the effects of ultrasound in Pd/active-carbon catalyst preparation, and catalytic hydrogenation of nitrobenzene as well, as one would expect. In particular, hydrogen chemisorption was used to characterize the Pd/C catalyst, and the
262 ultrasound in catalyst preparation was found to play an important role in H2 spillover from the palladium surface onto the carbon surface. This is going to be reported here and discussed together with other experimental results including the catalytic activity in nitrobenzene hydrogenation.
2. E X P E R I M E N T A L
The active carbon used is a granular activated carbon made from coconut shell (BGW JX-101 as carrier) and bought from Tangshan Jianxin Activated Carbon Co. LTD. Its physical measurements are as follows: SB~-I,= 1300 m2/g;, total porosity = 1 cm3/g; microporosity (pore size < 15-16 A) =0.55 cm3/g and mesoporosity (pore size between 15-16 A and 1000-2000 A) =0.2 cm3/g. The carbon support was treated with 10 wt.% nitric acid at 80 ~ overnight and washed in distilled water. The Pd/C catalyst was prepared in a thermostated glass-reactor (200 ml) together with a magnetic stirring and a self-designed horn, which can generate ultrasound (20 KHZ, 30 W), was inserted in the solution during catalyst preparation. A temperature programmed flow reactor system was employed to measure the amount of H2 chemisorption on the catalyst. After a sample was treated in 10% H2 diluted with N2 at 200 ~ for 2h and cooled down to 25 ~ in 0.5h, 10% H2/N2 was then replaced with pure N2 to remove physically adsorbed H2 for 0.5h. Finally temperature programmed desorption (TPD) was carried out from 25 ~ to 600 ~ to quantitatively evaluate the amount of H2 adsorbed in Pd/C from the desorption peak area. Catalytic activity was evaluated for liquid-phase hydrogenation of nitrobenzene in a 300 ml glass reactor. Reaction rate (mole H2/mim.g.Pd) was calculated from the consumption of H2 in a reservoir. The standard reaction conditions were as follows: 30 ~ l atm H2, 100ml ethanol, 3ml nitrobenzene, 200mg Pd/C and 1500 rpm stirring.
3. RESULTS AND DISCUSSION 3.1. C A T A L Y S T P R E P A R A T I O N and TEM C H A R A C T E R I Z A T I O N
Place 2g HNO3-treated active carbon (200 mesh) and 50ml H20 in the reactor, as described above. Treat the mixture under ultrasound at room temperature for 0.5h. Add a given amount of PdCl2 solution in the reactor, in order to produce catalysts with palladium loadings varying from 0.8 wt.% to 5 wt.%. Then, treat again the mixture under ultrasound at 40 ~ for 2h, adjust pH=8 using 5 wt.% NaOH solution, and add a given amount of 36 wt.% formaldehyde (less than I ml) to reduce PdCI2 under sonication within l h. Finally, the solid sample was filtered off, washed in distilled water till pH=7, and dried at 110 ~ in air. The obtained sample will be stored in a desicator and ready for use. It is worthnoting that the active carbon used here is an excellent adsorbent for Pd H in solution under the above conditions and the amount of Pd n adsorbed in the carbon was over 90% of the totally added Pd H. This fact is independent of the ultrasound action. Transmission electron microscopy (TEM) was employed to characterize the prepared Pd/C catalysts. The observed palladium particles on electron micrographs were all nearly around 10-30 ,~, with a mean size of 16 ~, either varying Pd loading or applying ultrasound in catalyst
263 preparation. Histogram of particle size is presented in Figure 1 for a sonicated sample of 2 wt.% Pd/C, and unsonicated 5 wt.% Pd/C.
a Ii
60 o~ 50 (D
E 4O ~
30
o .,.-~
~
2o
lO 10
15
"
20
"
30
parti cl e size
60 S
50
E 4O = 30 0
~
20 lO 10
15 particle size
20
30
Figure 1. Distribution of particle size (A) in the Pd/C catalysts (a) 2 wt.% sonicated (b) 5 wt.% unsonicated. 3.2.
HYDROGEN
CHEMISORPTION
In general, TPD experiments indicated that the desorption of H2 adsorbed in Pd/C occurred between 80 and 350~ It needs to note that active carbon without Pd did not give any desorption peaks. The amount of desorbed H2 as a function of Pd loading is presented in Figure 2. The possible quantity of H2 adsorbed by Pd alone could be H/Pd=0.7, which was calculated from a mean size (16 A) of palladium particles for the catalysts Pd/C presented in Figure 2 according to the literature (1, 3). However, Figure 2b showed that only non-sonicated 5 wt.% Pd/C has a H/Pd-0.6 (less than 1), the others all have H/Pd larger than 1. A maximum
264 H/Pd=6.8 was obtained for 0.8 wt.% sonicated Pd/C. Furthermore, one can observe in Figure 2a that the non-sonicated catalysts adsorbed H2 in an amount which increased with the augmentation of Pd loading. It possibly indicates that the adsorbed H2 would mainly be related to the Pd surface; while the sonicated catalysts have an amount of adsorbed H2 larger than the non-sonicated catalysts, but varied a little bit with the Pd loading. It is very likely that the adsorbed Hz would mainly be on the carbon surface instead of the Pd surface. All these results strongly demonstrate that H2 spillover from the Pd surface onto the carbon surface occurred when catalyst was treated in H2 at 200 ~ This is in accordance with the literature (4).
300 ............. 9 o
o
250
........ 9 ......
S
"~ 200 ,.lZ
150
..-~ .............................................................A
O
o
A"
100 A"
50 '
0
NS
j S
!
'
!
1
,
i
,
/
,
!
2 3 4 Pd loading (wt.%)
5
Figure 2a. Variation of the amount of desorbed H2 (lamole/g.catalyst) with Pd loading in the Pd/C catalysts prepared under ultrasound (S) and without ultrasound (NS).
7
S 0
6 5 4
O
3
"O---
2 1
.
N S Lx........ zx.....zx..... !
1
9
!
............ ,
!
,
3 3, 2 Pd loading (wt.%)
,
O !
5
Figure 2b. Desorbed H2 per palladium (H/Pd) as a function of Pd loading in the Pd/C catalysts prepared under ultrasound (S) and without ultrasound (NS).
265 The oxidation of carbon in HNO3 could create active sites, probably being carbon surface oxygen groups, onto which H2 migrates from the Pd surface, as proposed in the literature (1). Interestingly, one can find that the sonication in Pd/C preparation resulted in an augmentation of H2 spillover amount in Pd/C, being several times high than the non-sonicated Pd/C catalysts, as seen in Figure 2. This is possibly due to the effects of ultrasound in Pd/C preparation, i.e. the sonolytic decomposition of water into several highly reactive species, including H202 (2), to oxidize the carbon surface and create carbon surface defects during the catalyst preparation in solution. The application of ultrasound in Pd/C preparation did not reduce the metal particle size even with a loading 5 wt.% of palladium, and only enhanced the amount of spillover H2. This is possibly related to the previous HNO3 oxidation of carbon, which has already created enough sites for Pd nucleation to produce very small particles. In fact, it was reported that the oxidation of carbon by HNO3 resulted in an increase of palladium dispersion about 4 times (1). TEM indicated that Pd particle size nearly did not grow up when Pd loading was raised from 0.8 wt.% to 5 wt.%. However, H/Pd lowered down very much for the sonicated Pd/C catalysts and the unsonicated Pd/C catalysts as well. It seems to imply that the nucleation sites of Pd particles could be the same as the sites for H2 spillover, i.e. defective surface sites, created in HNO3 oxidizing and ultrasound treating9 3.3. H Y D R O G E N A T I O N
OF NITROBENZENE
The catalytic activity of Pd/C (mole H2/min.g.Pd) in the hydrogenation of nitrobenzene was presented in Figure 3. One can find that the difference of activity between two different series (sonicated and non-sonicated) Pd/C was not too much. Most interestingly, the augmentation of Pd loading led to a large increase of catalytic activity.
= O
5.0 45 40 35
.
A ..'"
O
..~.. .. . . ..~.-
30-
..:.::::
2 5" o 20~;~ 1 5 1.00.5
,,:.'
~l)
.O-..
...
S o NS zx '
0
,,'"
..:~' ,~-"
'
i
i
,
,
2 3 4 Pd Loading (wt.%)
,
!
5
Figure 3. Variation of catalytic activity (mole HJmin g.pd) in nitrobenzene hydrogenation with the loading of Pd in Pd/C catalysts9 It has been pointed out that this hydrogenation reaction is sensitive to palladium particle size (5). Experimental results showed that neither sonication in catalyst preparation nor increase of palladium loading changed the palladium particle size in the Pd/C catalysts. It is therefore
266 possible to conclude that the variation of activity with Pd loading, as seen in Figure 3, could not be related to the particle size effect. The amount of spillover H2 in sonicated Pd/C was much larger than that in non-sonicated Pd/C, but there did not exist much large difference between their catalytic activities. When increasing Pd loading in Pd/C, the amount of spillover H2 reduced (Figure 2b), while the catalytic activity increased (Figure 3). It is thus likely that H2 spillover might not influence much the reaction rate of nitrobenzene hydrogenation. Perhaps, the variation of catalytic activity with Pd loading could be associated with other factors. For example, small metal particles locating in micropores lead to a decrease of metal particle accessibility for nitrobenzene molecules; or even sulfur and sodium residue in carbon, as detected by surface composition X-ray analysis, possibly contaminate the Pd surface.
4. CONCLUSION Either applying ultrasound in catalyst preparation or varying palladium loading (0.8-5 wt.%), the observed palladium particles from TEM were all nearly around 10-30 A, with a mean size of 16 A for carbon supported palladium catalysts. H2 spillover from Pd onto carbon surface could occur in Pd/C at a temperature 200 ~ and decreased with the augmentation of Pd loading. The ultrasound in Pd/C preparation led to a greatly increased amount of spillover H2. H/Pd could reach a high value of 6.8 for 0.8 wt.% sonicated Pd/C catalysts. The catalytic activity of Pd/C (mole Hjmin.g.Pd) in nitrobenzene hydrogenation at 30 ~ increased with palladium loading and is not much influenced by the application of ultrasound in the catalyst preparation.
ACKNOWLEDGMENT It is grateful for us to thank Nature Science Foundation of China for a financial support NO.29476264.
REFERENCES
1. J.S.Dong and T.J.Park, Carbon, 31, (1993) 427. 2. S.Ley and C.Low (eds.), Ultrasonics in Synthesis, Springer-Verlag Berlin Heidelberg, 1989. 3. J.E.Benson, H.S.Hwang and M.Boudart, J. Catal., 30 (1973) 146. 4. G.Chen, W.T.Chou and C.T.Yeh, Appl. Catal., 8 (1983) 389. 5. G.Carturan, G.Facchin, G.Cocco, G.Navazio and G.Gubitosa, J. Catal., 82 (1983) 56.
261
H y d r o g e n spillover within carbon-supported palladium catalyst p r e p a r e d under ultrasound Z.X.Cheng, S.B.Yuan a, J.W.Fan b, Q.M.Zhu and M.S.Zhen b State Key Laboratory of C 1 Chemistry and Technology, Department of Chemistry, Tsinghua University, Beijing 100084 aCollege of Chemistry and Molecular Engineer, Center of Physicochemistry, Peking University bInstitute of Chemical Defence, Beijing 1048 P.O.Box
The Pd/active-carbon catalyst was prepared in solution through a reduction of Pd Hwith formaldehyde, and the obtained metallic particles were around 10-30 A with a mean size about 16 A either applying ultrasound in catalyst preparation or varying palladium loading (0.8-5 wt.%). H2 spillover from Pd onto the carbon surface could occur in Pd/C at 200 ~ and H/Pd decreased with the increase of Pd loading; the ultrasound in Pd/C preparation led to a greatly increased amount of spillover H2, and H/Pd could reach a high value of 6.8 for 0.8 wt.% sonicated Pd/C catalysts. The Pd/C activity (mole Hjmin.g.Pd) in nitrobenzene hydrogenation at 30 ~ increased with the palladium loading.
1. INTRODUCTION Catalysts of carbon supported Nobel metals are widely used in industrial processes, including hydrogenation and organic synthesis. They could be prepared by general methods, such as impregnation or ionic exchange following by the drying and reduction in hydrogen. Meanwhile, there exists another appropriate method for their preparation, and it mainly consists of a reduction of metal precursors using a reductant (formaldehyde, hydrazine, or others) in aqueous solution, in which a carbon support is suspended (1). The latter one was adopted in this work. It was said that ultrasound might be beneficial in catalyst preparation, catalyst activation, and catalytic reaction involving solid catalyst/reagents. It is normally considered that the improvement of reactivity of solid catalysts was caused by the creation of surface defects, the reduction of particle size and the acceleration of mass transport upon ultrasound action (2). Therefore, we have investigated the effects of ultrasound in Pd/active-carbon catalyst preparation, and catalytic hydrogenation of nitrobenzene as well, as one would expect. In particular, hydrogen chemisorption was used to characterize the Pd/C catalyst, and the
262 ultrasound in catalyst preparation was found to play an important role in H2 spillover from the palladium surface onto the carbon surface. This is going to be reported here and discussed together with other experimental results including the catalytic activity in nitrobenzene hydrogenation.
2. E X P E R I M E N T A L
The active carbon used is a granular activated carbon made from coconut shell (BGW JX-101 as carrier) and bought from Tangshan Jianxin Activated Carbon Co. LTD. Its physical measurements are as follows: SB~-I,= 1300 m2/g;, total porosity = 1 cm3/g; microporosity (pore size < 15-16 A) =0.55 cm3/g and mesoporosity (pore size between 15-16 A and 1000-2000 A) =0.2 cm3/g. The carbon support was treated with 10 wt.% nitric acid at 80 ~ overnight and washed in distilled water. The Pd/C catalyst was prepared in a thermostated glass-reactor (200 ml) together with a magnetic stirring and a self-designed horn, which can generate ultrasound (20 KHZ, 30 W), was inserted in the solution during catalyst preparation. A temperature programmed flow reactor system was employed to measure the amount of H2 chemisorption on the catalyst. After a sample was treated in 10% H2 diluted with N2 at 200 ~ for 2h and cooled down to 25 ~ in 0.5h, 10% H2/N2 was then replaced with pure N2 to remove physically adsorbed H2 for 0.5h. Finally temperature programmed desorption (TPD) was carried out from 25 ~ to 600 ~ to quantitatively evaluate the amount of H2 adsorbed in Pd/C from the desorption peak area. Catalytic activity was evaluated for liquid-phase hydrogenation of nitrobenzene in a 300 ml glass reactor. Reaction rate (mole H2/mim.g.Pd) was calculated from the consumption of H2 in a reservoir. The standard reaction conditions were as follows: 30 ~ l atm H2, 100ml ethanol, 3ml nitrobenzene, 200mg Pd/C and 1500 rpm stirring.
3. RESULTS AND DISCUSSION 3.1. C A T A L Y S T P R E P A R A T I O N and TEM C H A R A C T E R I Z A T I O N
Place 2g HNO3-treated active carbon (200 mesh) and 50ml H20 in the reactor, as described above. Treat the mixture under ultrasound at room temperature for 0.5h. Add a given amount of PdCl2 solution in the reactor, in order to produce catalysts with palladium loadings varying from 0.8 wt.% to 5 wt.%. Then, treat again the mixture under ultrasound at 40 ~ for 2h, adjust pH=8 using 5 wt.% NaOH solution, and add a given amount of 36 wt.% formaldehyde (less than I ml) to reduce PdCI2 under sonication within l h. Finally, the solid sample was filtered off, washed in distilled water till pH=7, and dried at 110 ~ in air. The obtained sample will be stored in a desicator and ready for use. It is worthnoting that the active carbon used here is an excellent adsorbent for Pd H in solution under the above conditions and the amount of Pd n adsorbed in the carbon was over 90% of the totally added Pd H. This fact is independent of the ultrasound action. Transmission electron microscopy (TEM) was employed to characterize the prepared Pd/C catalysts. The observed palladium particles on electron micrographs were all nearly around 10-30 ,~, with a mean size of 16 ~, either varying Pd loading or applying ultrasound in catalyst
263 preparation. Histogram of particle size is presented in Figure 1 for a sonicated sample of 2 wt.% Pd/C, and unsonicated 5 wt.% Pd/C.
a Ii
60 o~ 50 (D
E 4O ~
30
o .,.-~
~
2o
lO 10
15
"
20
"
30
parti cl e size
60 S
50
E 4O = 30 0
~
20 lO 10
15 particle size
20
30
Figure 1. Distribution of particle size (A) in the Pd/C catalysts (a) 2 wt.% sonicated (b) 5 wt.% unsonicated. 3.2.
HYDROGEN
CHEMISORPTION
In general, TPD experiments indicated that the desorption of H2 adsorbed in Pd/C occurred between 80 and 350~ It needs to note that active carbon without Pd did not give any desorption peaks. The amount of desorbed H2 as a function of Pd loading is presented in Figure 2. The possible quantity of H2 adsorbed by Pd alone could be H/Pd=0.7, which was calculated from a mean size (16 A) of palladium particles for the catalysts Pd/C presented in Figure 2 according to the literature (1, 3). However, Figure 2b showed that only non-sonicated 5 wt.% Pd/C has a H/Pd-0.6 (less than 1), the others all have H/Pd larger than 1. A maximum
264 H/Pd=6.8 was obtained for 0.8 wt.% sonicated Pd/C. Furthermore, one can observe in Figure 2a that the non-sonicated catalysts adsorbed H2 in an amount which increased with the augmentation of Pd loading. It possibly indicates that the adsorbed H2 would mainly be related to the Pd surface; while the sonicated catalysts have an amount of adsorbed H2 larger than the non-sonicated catalysts, but varied a little bit with the Pd loading. It is very likely that the adsorbed Hz would mainly be on the carbon surface instead of the Pd surface. All these results strongly demonstrate that H2 spillover from the Pd surface onto the carbon surface occurred when catalyst was treated in H2 at 200 ~ This is in accordance with the literature (4).
300 ............. 9 o
o
250
........ 9 ......
S
"~ 200 ,.lZ
150
..-~ .............................................................A
O
o
A"
100 A"
50 '
0
NS
j S
!
'
!
1
,
i
,
/
,
!
2 3 4 Pd loading (wt.%)
5
Figure 2a. Variation of the amount of desorbed H2 (lamole/g.catalyst) with Pd loading in the Pd/C catalysts prepared under ultrasound (S) and without ultrasound (NS).
7
S 0
6 5 4
O
3
"O---
2 1
.
N S Lx........ zx.....zx..... !
1
9
!
............ ,
!
,
3 3, 2 Pd loading (wt.%)
,
O !
5
Figure 2b. Desorbed H2 per palladium (H/Pd) as a function of Pd loading in the Pd/C catalysts prepared under ultrasound (S) and without ultrasound (NS).
265 The oxidation of carbon in HNO3 could create active sites, probably being carbon surface oxygen groups, onto which H2 migrates from the Pd surface, as proposed in the literature (1). Interestingly, one can find that the sonication in Pd/C preparation resulted in an augmentation of H2 spillover amount in Pd/C, being several times high than the non-sonicated Pd/C catalysts, as seen in Figure 2. This is possibly due to the effects of ultrasound in Pd/C preparation, i.e. the sonolytic decomposition of water into several highly reactive species, including H202 (2), to oxidize the carbon surface and create carbon surface defects during the catalyst preparation in solution. The application of ultrasound in Pd/C preparation did not reduce the metal particle size even with a loading 5 wt.% of palladium, and only enhanced the amount of spillover H2. This is possibly related to the previous HNO3 oxidation of carbon, which has already created enough sites for Pd nucleation to produce very small particles. In fact, it was reported that the oxidation of carbon by HNO3 resulted in an increase of palladium dispersion about 4 times (1). TEM indicated that Pd particle size nearly did not grow up when Pd loading was raised from 0.8 wt.% to 5 wt.%. However, H/Pd lowered down very much for the sonicated Pd/C catalysts and the unsonicated Pd/C catalysts as well. It seems to imply that the nucleation sites of Pd particles could be the same as the sites for H2 spillover, i.e. defective surface sites, created in HNO3 oxidizing and ultrasound treating9 3.3. H Y D R O G E N A T I O N
OF NITROBENZENE
The catalytic activity of Pd/C (mole H2/min.g.Pd) in the hydrogenation of nitrobenzene was presented in Figure 3. One can find that the difference of activity between two different series (sonicated and non-sonicated) Pd/C was not too much. Most interestingly, the augmentation of Pd loading led to a large increase of catalytic activity.
= O
5.0 45 40 35
.
A ..'"
O
..~.. .. . . ..~.-
30-
..:.::::
2 5" o 20~;~ 1 5 1.00.5
,,:.'
~l)
.O-..
...
S o NS zx '
0
,,'"
..:~' ,~-"
'
i
i
,
,
2 3 4 Pd Loading (wt.%)
,
!
5
Figure 3. Variation of catalytic activity (mole HJmin g.pd) in nitrobenzene hydrogenation with the loading of Pd in Pd/C catalysts9 It has been pointed out that this hydrogenation reaction is sensitive to palladium particle size (5). Experimental results showed that neither sonication in catalyst preparation nor increase of palladium loading changed the palladium particle size in the Pd/C catalysts. It is therefore
266 possible to conclude that the variation of activity with Pd loading, as seen in Figure 3, could not be related to the particle size effect. The amount of spillover H2 in sonicated Pd/C was much larger than that in non-sonicated Pd/C, but there did not exist much large difference between their catalytic activities. When increasing Pd loading in Pd/C, the amount of spillover H2 reduced (Figure 2b), while the catalytic activity increased (Figure 3). It is thus likely that H2 spillover might not influence much the reaction rate of nitrobenzene hydrogenation. Perhaps, the variation of catalytic activity with Pd loading could be associated with other factors. For example, small metal particles locating in micropores lead to a decrease of metal particle accessibility for nitrobenzene molecules; or even sulfur and sodium residue in carbon, as detected by surface composition X-ray analysis, possibly contaminate the Pd surface.
4. CONCLUSION Either applying ultrasound in catalyst preparation or varying palladium loading (0.8-5 wt.%), the observed palladium particles from TEM were all nearly around 10-30 A, with a mean size of 16 A for carbon supported palladium catalysts. H2 spillover from Pd onto carbon surface could occur in Pd/C at a temperature 200 ~ and decreased with the augmentation of Pd loading. The ultrasound in Pd/C preparation led to a greatly increased amount of spillover H2. H/Pd could reach a high value of 6.8 for 0.8 wt.% sonicated Pd/C catalysts. The catalytic activity of Pd/C (mole Hjmin.g.Pd) in nitrobenzene hydrogenation at 30 ~ increased with palladium loading and is not much influenced by the application of ultrasound in the catalyst preparation.
ACKNOWLEDGMENT It is grateful for us to thank Nature Science Foundation of China for a financial support NO.29476264.
REFERENCES
1. J.S.Dong and T.J.Park, Carbon, 31, (1993) 427. 2. S.Ley and C.Low (eds.), Ultrasonics in Synthesis, Springer-Verlag Berlin Heidelberg, 1989. 3. J.E.Benson, H.S.Hwang and M.Boudart, J. Catal., 30 (1973) 146. 4. G.Chen, W.T.Chou and C.T.Yeh, Appl. Catal., 8 (1983) 389. 5. G.Carturan, G.Facchin, G.Cocco, G.Navazio and G.Gubitosa, J. Catal., 82 (1983) 56.