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Development of active and stable supported noble metal catalysts for hydrogenation of carbon dioxide to methanol
Energy Convers. Mgmt
0196-8904(95)00085-2
Vol. 36, No. 6-9, pp. 633-636,
199.5
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0196-8904/95 $9.50 + 0.00
DEVELOPMENT OF ACTIVE AND STABLE SUPPORTED NOBLE METAL CATALYSTS FOR HYDROGENATION OF CARBON DIOXIDE TO METHANOL LI FAN
and KAORU FUJIMOTO
Department of Applied Chemistry, Faculty of Engineering The University of Tokyo, Tokyo, 113, Japan Abstract
- Strong metal support interaction (SMSI) appearing in
supported palladium catalysts improves greatly the selectivity and lifetime of the catalysts for methanol synthesis from CO, hydrogenation. Its mechanism is also discussed. 1. INTRODUCTION Catalytic hydrogenation of carbon dioxide into valuable chemicals
mid
fuels such as methanol
has been recently recognized as one of the promising recycling technologies for emitted CO,. Most of the resewch was focused on CuO-ZnO catalysts [ I-6J. Supported Pd catalysts are regarded as demanding catalysts for methanol synthesis from synthesis gas 171.On the other hand, under atmospheric pressure,
hydrogenation
of CO, on supported Pd catalysts produces only
methane [8]. Solymosi et al. studied Pd supported
on
AI,O,, SiO,, TiO, and MgO for CO, hy-
drogenation under pressurized condition but methanol selectivity was insufficient [9,10].
Here
we found that supported Pd catalyst which was reduced with hydrogen at 500°C exhibited high activity
and
long lifetime for methanol synthesis from CO, and Hz. 2. EXPERIMENTAL
Commercially available CeO, powder (99.99%) was moulded ancl crushed into 20-60 mesh, then it was impregnated with PtlCl, from its hot acidic aqueous solution kept at 90°C. Palladium loading was 4% by weight.
AlIer
tlegasificotion
and
evaporation, the calalysr precursor was dried
in air at 120°C for 12 h and then reduced by flowing hydrogen at 400°C Chlorine
WHS
not detected if the ciIti\lyst ;It this step
solution was analyzed by ion chro~natoglapli.
wx
for
4 h
to remove chlorine.
wushetl with hot water and the washing
Before reaction the catalyst was reduced in situ
with flowing hydrogen in the reactor at the desired temperature between 550°C and 200°C for 1 h. The reactor was a fixed-bed flow type, operated under high pressure conditions. Products were analyzed by a gas chromarograph. 3. RESULTS AND DISCUSSION In Table 1, reaction performances of Prt/CeO, reduced by tiyclrogen at different temperatures are compared, Reduction time for each catalyst \vaS 1 h. When the reduction temperature was between 200°C
and
4OO”C,no obvious change happened to the activity and selectivity of the
catalysts. The selcctivirics to methane
and
CO were high ;und the durability of the catalyst was
poor, because the methnnol selectivity decreased from 18% to 6% after 14 h from the start of the 633
FANand FUJIMOTO: HYDROGENATION OF CO, TO METHANOL
634
Table 1. Hydrogen Reduction Temperature Reduced
co2
Temp./OC
Couv./%
550 500 400 200
3.1 3.1 3.2 3.2 .
TOF is-1
14.3 14.3 0.95 0.93
Effect ou PdKe02
Catalyst
MeOH Sel./%
CH4 Sel./%
CO Sel./%
87.8 91.7 18.1 17.8
1.9 1.1 72.5 73.1
10.3 7.1 9.4 9.1
a
a.) total pressure: 30 bar* Temp. 23O’C;
Cat.weight OSg; i W/F=lOg-cat.hr.molb.) TOF was obtained by using Pd dispersion data partly listed in Table 2. reaction . For the catalyst reduced ut 500°C for 1 h, although the CO, conversion was nearly the same, the selectivity to methanol increased drastically from 18.1% to 91.7%, which was accompanied by a marked decrease in the formation OFCO and Cl-i,. For a Pd/CeO, catalyst treated by H, at higher temperature such as 55O”C, further improvement was not attained. Both the conversion and the methanol selectivity for the catalyst reduced at 500°C were kept constant over 100 h [ 111. From turnover frequency data listed in Table 1, it is found that TOP increased from 0.95 se’ to 14.3 s-‘with the change of reduction temperature from 400°C to 500°C. Similar results were obtained for Pd/Laz03 as well as Pd/TiO, catalysts 112,131. Although the reasons are not very clear now, the most probable explanation for the deactivation of the catalyst reduced at temperatures lower than 400°C should be the oxidation of Pd metal, resulting from water formed through CH, or CO formation, or CO,, as the PdO was confirmed on the deactivated catalyst by X-ray diffraction (XRD). From XRD patterns of fresh Pd/CeO, catalysts reduced at 400°C or 5OO”C, it was found that Pd metal and CeO, were discerned in both catalysts, while a new Ce,O, phase was formed in the 500°C-reduced Pd/CeO,. Also it should be noted that the particle size of Pd determined by the Sherrer method from the XRD peaks was the same for both catalysts. Considering the Pd particle size determined from CO chemisorption (Table 2), one is able to conclude that the partially-reduced support of the SOO”C-reduced catalyst, Ce,O,, migrated onto the Pd surface and covered most of the exposed Pd surface, which is a kind of SMSI phenomenon. As a result, CO uptake on the SOO’C-reducedcatalyst was so small that the calculated Pd particle size from CO chemisorption was too big. It is noteworthy also that Pd loadings measured by ICP listed in Table 2 were nearly the same, although they were higher than the originally-designed loading of 4% in catalyst preparation, which should be attributed to the loss of hydroxyl groups on the support or the reduction of the support in the preparation process of the catalyst.
As an evidence, from temperature pro-
grammed reduction (TPR) experiment, it was observed that much water formed between 300°C Table 2. CO Chemisorptiou Reductiou Temp. (oC) r
aud XRD Results of PdKe02 400 500 0.25 0.016 3720 b 58200
Pd Size from CO Adsorptiou (A) Pd Size by XRD (A) 3260 Pd Loading by ICP (%) 4.89 a.) CO uptake heie coutaius uo pl~ysically~ailsorbed CO. b.) Calculated value only.
3260 4.74
FAN and FUJIMOTO:
HYDROGENATION
635
OF CO, TO METHANOL
and 500°C. SMSI was suggested by Tauster et al.[ 14) to interpret the diminished H, and CO chemisorption on titania-supported platinum group metals reduced above 700K. SMSI behavior has been observed with a lot of reducible oxides including
Pd 011rare earth oxides used for CO hydrogenation.
But SMSI model is still a subject of controversy. One opinion is that metal particles are hexagonal in outline, of uniforul thickness and very thin, indicating a pill-box morphology, accompanied by partially-reduced oxide formation of the reducible support 1.15.1.The other theory is the migration and dispersion of p~utially-reduced oxide support 011the metal 1lb]. The SMSI effect here should belong to the latter category. From Temperature I’rogrammccl Desorption (TPD) data in Fig. 1, it was found that the ad-
, 0
100
200
300
400
Temperature/@ Figure I .CO2-‘IT’D Profiles of llie 5OOW-Reduced Helium flow, Rate: 4OOW/l1
sorbed CO, was decomposed to CO in large amount CO, was observed
OII
on
I’dlCeQ
Calalysl
SOO”C-reducedPd/CeO, catalyst, while little
this high-temperature-reduced catalyst. But
on
the low-temperature-reduced
Pd/CeO, catalyst, only small amount of CO was detected and most of the desorbed species was CO,.
Compared to the CO,-TPD result of pure CeO,, we found that carbon dioxide adsorbed
selectively
on
CeO, support and hardly adsorbed onto Pd metal [ 171.
Dumesic et al. proposed that an electronic effect was involved in SMSI behavior [18]. It was suggested that new active sites were created at the interface between the metal and the support. In fact, it is reported that l’:trtiullp-retl~lcetl TiO, can aid the dissociation of CO for CH, synthesis from syngas on Pd/TiO, 1191. Trimm et
~11. also
recognized that Ce,O, is a very strong reducing
agent which can reduce CO, to form CO or evetl carbon [20].
We think that decomposition of
CO, to CO and oxygen atom shoulcl be the key step in this reaction.
The active site for this
decomposition st~oulcl be the reduced ceria (Ce,O,) because it was observed that CO, adsorbed only onto the support.
As high temperature reduction led to the electron-rich state of Ce cation
(maybe Pd was also at the electron-rich state), these electrons could be denoted to the anti-bonding orbitals of the adsorbed CO, to accelerate its decomposition. Formed CO could be hydrogenated to methanol on Pd surface and surface oxygen atom might react with hydrogen to form water. As the oxygen atom reacted with hydrogen instead of palladium atom in the high-temperature-reduced catalyst, palladium could be protected from oxidation. As mentioned above, we already observed that PdO formed on the deactivated low-temperature-reduced catalyst. In other word, a redox EM 36:6/9-S
FAN and FUJIMOTO: HYDROGENATION OF CO, TO METHANOL
636
cycle on Ce, where Ce,O, was not oxidized by the oxygen atom, should be the prerequisite to keep the life time of Ce,O, as well as the catalyst. 4. CONCLUSION Change of reduction temperature showed a remarkable effect onto the catalytic performances of the supported palladium catalyst during the hydrogenation of carbon dioxide to methanol. A reduction in flowing hydrogen at 500°C of Pcl/CeO, catalyst resulted in a high activity for methanol synthesis, especially in its selectivity and durability. However, reduction at 400°C or lower exhibited a low activity and a poor durability. This major difference should be attributed structurally to the partially-reduced support formed
on
the high-temperature reduced catalyst, where SMSI effect
behaved. This partially-reduced state of the support could improve markedly the decomposition of CO, to CO, which was the key step here. REFERENCES 1. Inui, T., Takeguchi, T., Kohama, A., Kitagawa, K., Proc. 10th Int. Congr. Catal., Budapest, 1992. 2. Fujitani, T., Saito, M., Kanai, Y., Watanabe, T., Card. Lett., 25, 271( 1994). 3. Dubois, J. L., Sayarnn, K., and Arakawa, H., Cherrr. Lat., 1115 (1992). 4. Fujiwara, M., Anrlo,II., Tanaka, M., and Souma, Y., Bull. Chern.
&JC.
Jp.,
67, 546( 1994).
5. Nitta, Y., Suwata, O., Okamoto, Y., and ImiuXtka,T., Cat&. Lert., 26, 345( 1994). 6. Fujimoto, K. and Sbikndn, ‘I‘.,A/I/J/. Catd. 31, 13 (1987). 7. Poutsma, M., Elek, L. F., Ibarbia, P. A., Risch, A. P. and Rabo, J. A., J. Cntd., 52, 157 (1978). 8. Vannice, M. A., J. Catal. 40, 129 (1975). 9. Erdohelyi, A., Pasztor, M. and Solymosi, F., J. Cataf. 98, 166 (1986). 10. Solymosi, F., Erdohelyi, A. and Lancz, M., J. Catal. 95,567 (1985). 11. Fan, L. and Fujimoto, K., Appl. Cutal. A. General., 106, Ll (1993). 12. Fan, L. and Fujimoto, K., C/tern. Left., 105 (1994). 13. Fan, L. and Fujimoto, K., Bu11.Che/n. Sot.
Jpn.,
67, 1773 (1994).
14. ‘l’auster, S. J., Fung, S. C. and Garten, R. L., J. Am. Chm. Sot. 100, 170 (1978). 15. Baker, R. T. K., Prestridge, E. 13.:t;ld Garten, R. L., J. Cad.
56,390 (1979).
16. Sadeghi, 1-I.R. and Henrich, V. E., J. Coral. 87, 279 (1984). 17. Fan, L. and Fujimoto, K., to be published 18. Raupp, G. B. and Dulnesic, J. A., ./. Plrys. Chcnz. 88, 660 (1984). 19. Rieck, J. S. and Bell, A. T., ./. Ctrtol. 99, 262 (1986). 20. Padeste, C., Cant, N., ‘I’rinlnl, D. L., Ccml. Lett., 18, 305(1993).
0196-8904(95)00085-2
Vol. 36, No. 6-9, pp. 633-636,
199.5
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0196-8904/95 $9.50 + 0.00
DEVELOPMENT OF ACTIVE AND STABLE SUPPORTED NOBLE METAL CATALYSTS FOR HYDROGENATION OF CARBON DIOXIDE TO METHANOL LI FAN
and KAORU FUJIMOTO
Department of Applied Chemistry, Faculty of Engineering The University of Tokyo, Tokyo, 113, Japan Abstract
- Strong metal support interaction (SMSI) appearing in
supported palladium catalysts improves greatly the selectivity and lifetime of the catalysts for methanol synthesis from CO, hydrogenation. Its mechanism is also discussed. 1. INTRODUCTION Catalytic hydrogenation of carbon dioxide into valuable chemicals
mid
fuels such as methanol
has been recently recognized as one of the promising recycling technologies for emitted CO,. Most of the resewch was focused on CuO-ZnO catalysts [ I-6J. Supported Pd catalysts are regarded as demanding catalysts for methanol synthesis from synthesis gas 171.On the other hand, under atmospheric pressure,
hydrogenation
of CO, on supported Pd catalysts produces only
methane [8]. Solymosi et al. studied Pd supported
on
AI,O,, SiO,, TiO, and MgO for CO, hy-
drogenation under pressurized condition but methanol selectivity was insufficient [9,10].
Here
we found that supported Pd catalyst which was reduced with hydrogen at 500°C exhibited high activity
and
long lifetime for methanol synthesis from CO, and Hz. 2. EXPERIMENTAL
Commercially available CeO, powder (99.99%) was moulded ancl crushed into 20-60 mesh, then it was impregnated with PtlCl, from its hot acidic aqueous solution kept at 90°C. Palladium loading was 4% by weight.
AlIer
tlegasificotion
and
evaporation, the calalysr precursor was dried
in air at 120°C for 12 h and then reduced by flowing hydrogen at 400°C Chlorine
WHS
not detected if the ciIti\lyst ;It this step
solution was analyzed by ion chro~natoglapli.
wx
for
4 h
to remove chlorine.
wushetl with hot water and the washing
Before reaction the catalyst was reduced in situ
with flowing hydrogen in the reactor at the desired temperature between 550°C and 200°C for 1 h. The reactor was a fixed-bed flow type, operated under high pressure conditions. Products were analyzed by a gas chromarograph. 3. RESULTS AND DISCUSSION In Table 1, reaction performances of Prt/CeO, reduced by tiyclrogen at different temperatures are compared, Reduction time for each catalyst \vaS 1 h. When the reduction temperature was between 200°C
and
4OO”C,no obvious change happened to the activity and selectivity of the
catalysts. The selcctivirics to methane
and
CO were high ;und the durability of the catalyst was
poor, because the methnnol selectivity decreased from 18% to 6% after 14 h from the start of the 633
FANand FUJIMOTO: HYDROGENATION OF CO, TO METHANOL
634
Table 1. Hydrogen Reduction Temperature Reduced
co2
Temp./OC
Couv./%
550 500 400 200
3.1 3.1 3.2 3.2 .
TOF is-1
14.3 14.3 0.95 0.93
Effect ou PdKe02
Catalyst
MeOH Sel./%
CH4 Sel./%
CO Sel./%
87.8 91.7 18.1 17.8
1.9 1.1 72.5 73.1
10.3 7.1 9.4 9.1
a
a.) total pressure: 30 bar* Temp. 23O’C;
Cat.weight OSg; i W/F=lOg-cat.hr.molb.) TOF was obtained by using Pd dispersion data partly listed in Table 2. reaction . For the catalyst reduced ut 500°C for 1 h, although the CO, conversion was nearly the same, the selectivity to methanol increased drastically from 18.1% to 91.7%, which was accompanied by a marked decrease in the formation OFCO and Cl-i,. For a Pd/CeO, catalyst treated by H, at higher temperature such as 55O”C, further improvement was not attained. Both the conversion and the methanol selectivity for the catalyst reduced at 500°C were kept constant over 100 h [ 111. From turnover frequency data listed in Table 1, it is found that TOP increased from 0.95 se’ to 14.3 s-‘with the change of reduction temperature from 400°C to 500°C. Similar results were obtained for Pd/Laz03 as well as Pd/TiO, catalysts 112,131. Although the reasons are not very clear now, the most probable explanation for the deactivation of the catalyst reduced at temperatures lower than 400°C should be the oxidation of Pd metal, resulting from water formed through CH, or CO formation, or CO,, as the PdO was confirmed on the deactivated catalyst by X-ray diffraction (XRD). From XRD patterns of fresh Pd/CeO, catalysts reduced at 400°C or 5OO”C, it was found that Pd metal and CeO, were discerned in both catalysts, while a new Ce,O, phase was formed in the 500°C-reduced Pd/CeO,. Also it should be noted that the particle size of Pd determined by the Sherrer method from the XRD peaks was the same for both catalysts. Considering the Pd particle size determined from CO chemisorption (Table 2), one is able to conclude that the partially-reduced support of the SOO”C-reduced catalyst, Ce,O,, migrated onto the Pd surface and covered most of the exposed Pd surface, which is a kind of SMSI phenomenon. As a result, CO uptake on the SOO’C-reducedcatalyst was so small that the calculated Pd particle size from CO chemisorption was too big. It is noteworthy also that Pd loadings measured by ICP listed in Table 2 were nearly the same, although they were higher than the originally-designed loading of 4% in catalyst preparation, which should be attributed to the loss of hydroxyl groups on the support or the reduction of the support in the preparation process of the catalyst.
As an evidence, from temperature pro-
grammed reduction (TPR) experiment, it was observed that much water formed between 300°C Table 2. CO Chemisorptiou Reductiou Temp. (oC) r
aud XRD Results of PdKe02 400 500 0.25 0.016 3720 b 58200
Pd Size from CO Adsorptiou (A) Pd Size by XRD (A) 3260 Pd Loading by ICP (%) 4.89 a.) CO uptake heie coutaius uo pl~ysically~ailsorbed CO. b.) Calculated value only.
3260 4.74
FAN and FUJIMOTO:
HYDROGENATION
635
OF CO, TO METHANOL
and 500°C. SMSI was suggested by Tauster et al.[ 14) to interpret the diminished H, and CO chemisorption on titania-supported platinum group metals reduced above 700K. SMSI behavior has been observed with a lot of reducible oxides including
Pd 011rare earth oxides used for CO hydrogenation.
But SMSI model is still a subject of controversy. One opinion is that metal particles are hexagonal in outline, of uniforul thickness and very thin, indicating a pill-box morphology, accompanied by partially-reduced oxide formation of the reducible support 1.15.1.The other theory is the migration and dispersion of p~utially-reduced oxide support 011the metal 1lb]. The SMSI effect here should belong to the latter category. From Temperature I’rogrammccl Desorption (TPD) data in Fig. 1, it was found that the ad-
, 0
100
200
300
400
Temperature/@ Figure I .CO2-‘IT’D Profiles of llie 5OOW-Reduced Helium flow, Rate: 4OOW/l1
sorbed CO, was decomposed to CO in large amount CO, was observed
OII
on
I’dlCeQ
Calalysl
SOO”C-reducedPd/CeO, catalyst, while little
this high-temperature-reduced catalyst. But
on
the low-temperature-reduced
Pd/CeO, catalyst, only small amount of CO was detected and most of the desorbed species was CO,.
Compared to the CO,-TPD result of pure CeO,, we found that carbon dioxide adsorbed
selectively
on
CeO, support and hardly adsorbed onto Pd metal [ 171.
Dumesic et al. proposed that an electronic effect was involved in SMSI behavior [18]. It was suggested that new active sites were created at the interface between the metal and the support. In fact, it is reported that l’:trtiullp-retl~lcetl TiO, can aid the dissociation of CO for CH, synthesis from syngas on Pd/TiO, 1191. Trimm et
~11. also
recognized that Ce,O, is a very strong reducing
agent which can reduce CO, to form CO or evetl carbon [20].
We think that decomposition of
CO, to CO and oxygen atom shoulcl be the key step in this reaction.
The active site for this
decomposition st~oulcl be the reduced ceria (Ce,O,) because it was observed that CO, adsorbed only onto the support.
As high temperature reduction led to the electron-rich state of Ce cation
(maybe Pd was also at the electron-rich state), these electrons could be denoted to the anti-bonding orbitals of the adsorbed CO, to accelerate its decomposition. Formed CO could be hydrogenated to methanol on Pd surface and surface oxygen atom might react with hydrogen to form water. As the oxygen atom reacted with hydrogen instead of palladium atom in the high-temperature-reduced catalyst, palladium could be protected from oxidation. As mentioned above, we already observed that PdO formed on the deactivated low-temperature-reduced catalyst. In other word, a redox EM 36:6/9-S
FAN and FUJIMOTO: HYDROGENATION OF CO, TO METHANOL
636
cycle on Ce, where Ce,O, was not oxidized by the oxygen atom, should be the prerequisite to keep the life time of Ce,O, as well as the catalyst. 4. CONCLUSION Change of reduction temperature showed a remarkable effect onto the catalytic performances of the supported palladium catalyst during the hydrogenation of carbon dioxide to methanol. A reduction in flowing hydrogen at 500°C of Pcl/CeO, catalyst resulted in a high activity for methanol synthesis, especially in its selectivity and durability. However, reduction at 400°C or lower exhibited a low activity and a poor durability. This major difference should be attributed structurally to the partially-reduced support formed
on
the high-temperature reduced catalyst, where SMSI effect
behaved. This partially-reduced state of the support could improve markedly the decomposition of CO, to CO, which was the key step here. REFERENCES 1. Inui, T., Takeguchi, T., Kohama, A., Kitagawa, K., Proc. 10th Int. Congr. Catal., Budapest, 1992. 2. Fujitani, T., Saito, M., Kanai, Y., Watanabe, T., Card. Lett., 25, 271( 1994). 3. Dubois, J. L., Sayarnn, K., and Arakawa, H., Cherrr. Lat., 1115 (1992). 4. Fujiwara, M., Anrlo,II., Tanaka, M., and Souma, Y., Bull. Chern.
&JC.
Jp.,
67, 546( 1994).
5. Nitta, Y., Suwata, O., Okamoto, Y., and ImiuXtka,T., Cat&. Lert., 26, 345( 1994). 6. Fujimoto, K. and Sbikndn, ‘I‘.,A/I/J/. Catd. 31, 13 (1987). 7. Poutsma, M., Elek, L. F., Ibarbia, P. A., Risch, A. P. and Rabo, J. A., J. Cntd., 52, 157 (1978). 8. Vannice, M. A., J. Catal. 40, 129 (1975). 9. Erdohelyi, A., Pasztor, M. and Solymosi, F., J. Cataf. 98, 166 (1986). 10. Solymosi, F., Erdohelyi, A. and Lancz, M., J. Catal. 95,567 (1985). 11. Fan, L. and Fujimoto, K., Appl. Cutal. A. General., 106, Ll (1993). 12. Fan, L. and Fujimoto, K., C/tern. Left., 105 (1994). 13. Fan, L. and Fujimoto, K., Bu11.Che/n. Sot.
Jpn.,
67, 1773 (1994).
14. ‘l’auster, S. J., Fung, S. C. and Garten, R. L., J. Am. Chm. Sot. 100, 170 (1978). 15. Baker, R. T. K., Prestridge, E. 13.:t;ld Garten, R. L., J. Cad.
56,390 (1979).
16. Sadeghi, 1-I.R. and Henrich, V. E., J. Coral. 87, 279 (1984). 17. Fan, L. and Fujimoto, K., to be published 18. Raupp, G. B. and Dulnesic, J. A., ./. Plrys. Chcnz. 88, 660 (1984). 19. Rieck, J. S. and Bell, A. T., ./. Ctrtol. 99, 262 (1986). 20. Padeste, C., Cant, N., ‘I’rinlnl, D. L., Ccml. Lett., 18, 305(1993).