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Carbon-supported dehydrogenation catalysts composed of platinum and ruthenium metals
T. Inui et al. (Editors), New Aspects of Spillover Effect in Catalysis 0 1993 Elsevier Science Publishers B.V. All rights reserved.
313
Carbon-supported dehydrogenation catalysts composed of platinum and rutheniummetals Yuii ANDO, Xiaoniei LI, Eri ITO, Masaru YAMASHITA, a n d Yasukazu SAITO Department of Industrial Chemistry, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113, JAPAN
Abstract Catalytic dehydrogenation of alcohol o r alkane under boiling and refluxing conditions yielded molecular hydrogen and corresponding dehydrogenated compounds, taking the role of low-temperature endothermic reactions i n a newly-proposed system for chemical conversion of low-quality heats. The adsorbed hydrogen species on the surface of carbon-supported noble metals were capable to migrate from the site of C-H bond dissociation to the site of H a evolution, as demonstrated by the Ru-Pt composite catalyst. Hydrogen spillover to the carbon support proceeded over the Pd catalyst, for which little H~ evolution and facile acetone formation were observed from 2-propanol. 1. INTRODUCTION
Dehydrogenation reactions under mild conditions have attracted considerable attention in catalytic chemistry 111, since thermal or electric energies can be generated through the reverse processes, i. e., hydrogenation 121.
Among active dehydrogenation catalysts, carbon-supported noble metals are significantly important, because (1) both the C - H bond activation and the H2 formation proceed easily and ( 2 ) intimate and stable suspension is obtained in alcohol or alkane. Moreover, various kinds of composite catalysts a r e prepared from noble metals. In the present paper, spillover phenomena are investigated for the carbonsupported metal catalysts in the liquidphase.
2. CARBON-SUPPORTEDNOBLE METAL CATALYSIS FOR 2-PROPANOL DEHYDROGENATION The activity order of carbon-supported noble metal catalysts was
Ru>Rh>Pt>Pd(=O), a s far as the hydrogen evolution rates were concerned.
-
(CH3)ZCO + H2 (CH3)2CHOH The reaction was accompanied by the product retardation (Eq.(2)),
(1)
314
v = k / ( 1 + I([acetonel) where k and K are the rate Table 1 Characteristics of suspended constant and the retardation catalystsdehydrogenation of 2-propanol constant due to acetone Initial Retardation adsorption, respectively. Metal rate constnt Ref. As shown in Table 1, the mmol h-' 6.' mniol . dm.' high reaction rate on the R ~ - P ~+ / cPt(acac), 6290 0.009 a ruthenium catalyst and the Ru-Pt/C + Pd(acac);! 5660 0.009 a small acetone retardation Pt/C + H4R~4(C0)12 1700 0.008 a Ru-Pt/carbon 5870 0.014 b on the platinum catalyst 4630 0.017 C are noteworthy in contrast Ru/carbon Rli/carbon 1560 0.0 I4 C to those of the nickel fine390 0.004 C Pt/Ni fine particle 268 0.052 d particle catalysts with and without platinum deposition. Ni fine particle 96.9 0.066 d High reaction rates of Nickel bride 60 0.01 1 e 51.5 0.015 l 2-propanol dehydrogenation Raney nickel a) Present Work. b) M.Yamashila et al., Hydrogen Energy Progress 9. (Ed., T.N. per volume of the catalyst Vezieoglu. CDerive, J. Poltier). vol. I . Manif. Commun. Intern.. Paris (1992). solution are important for pp.197-205. c) Y.Sailo e l al.. Hydrogen Energy Progress R. (Ed.. T.N.Veriroglu. P.K.Takahashi), vol.1, Pergamon Press, New York. (1991). pp.339-344. d) M. designing the newlyNoda el 81.. Bull. Chem. SOC.Jpn.. 61,2541(19RR). e) D.E. Mears e l 81.. A. 1. Ch. proposed chemical heat E. Journal. 12.313(1966). 0 F.Claes e l al.. Bull. SOC.Chim. France, 25, I167 pump (1958). .system as compact as possible, as it reduces the size of the endothermic low80 temperature reactor directly. The small extent of acetone retardation is desirable for this system as well, 6O since large acetone concentrations E in the catalyst solution make it E possible to perform the distillation = .Z 40 for separating acetone from 2-propanol a t high thermal 9 efficiencies. i From the viewpoint of the spillover phenomena on catalysts, carbon-supported palladium metal was interesting, because hydrogen 0 5 10 15 20 25 evolution was quice poor, wnereas (0)Homogeneous Ru,(OAc), solution in 2-propanol acetone yielded in the solution ( 0 ) Pd/C suspension surface-modified with Rul(OAc), amounted to gg times of metallic Reaction conditions : Boiling and refluxing(82.4 c ) palladium in molar after the Figure 1. Activity enhancement of ruthenium complex reaction for 5 h. we have such for 2-propanol dehydrogenation by modifying the a view that the product hydrogen surface of catalyst palladium sneaked out from palladium to carbon. In fact, the activity of hydrogen evolution over the carbon-supported palladium catalyst was generated by modifying the palladium surface with [Ru30(OAc)$ H20 131+. The
.
315
homogeneous activity of this complex was exceeded by the complex-modified palladium catalyst(Fig. 1). Deuterium kinetic isotope effects concerning (CH3)2CDOH and (CH3)2CHOD revealed that dissociation of the methine C-H bond was rate-determining for the carbon-supported platinum or rhodium catalyst, whereas the step of hydrogen evolution was slow on the rutheniudcarbon catalyst. Provided t h a t the surface hydrogen species migrate rapidly, the facile steps of C-H splitting on Ru and of H a formation on Pt or Rh would be combined. Actually, the activities of 2propanol dehydrogenation were enhanced over the composite catalysts of Pt, Rh and Pd with Ru(Tab1e 2). H-D exchange between the substrate (CD3)$DOD and the support carbon containing H was unneglible over the Ru-Pt composite catalyst, a s demonstrated by the products in both the gas and liquid phases(Fig. 2 ) . According to the high-resolution TEM images, the carbon-supported R u - P t ( l : l ) composite catalyst was prepared a s fine particles, some of which were single crystals. From the lattice constants of the f.c.c.cl11)plane, dilution of platinum with ruthenium was ascertained. Table 2 Composite metal catalysts for 2-propanol dehydrogenation
Metal
Facile step
Rh
H2 elimin.
C-H dissoc.
Ru
C-H dissoc.
H2 elimin.
Pt
H~)elimin
C-H dissoc.
a) Molar ratio = 9:l
-8 a
s
Slow step Compositea) Initial rateb) Ru-Rh Ru-Pt
500 682 550 629 226
b) Unit: mmol.g-'.h-'
1.0
0.0 0.0
1.0
2.0
3.0
4.0
H transfer ar from to mdrogen carbon solution evolution Reaction time : 5 h
5.0
Catalyst solution: Ru-Pt( 1 :1.5 wt%) on carbon 200 mg / 5 ml 2-propanol-d8 Reaction conditions: Boiling and refluxing(82.4
c)
Figure 2. Spillover of H from carbon during 2-propanol-d8dehydrogenation
316
3. CARBON-SUPPORTEDNOBLE METAL CATALYSTS FOR CYCLOHEXANE DEHYDROGENATION
The reaction of cyclohexane dehydrogenation yielding benzene and hydrogen under mild conditions(Eq. (3)) is important [31, because a fuel cell is constructed using its reverse reaction, i. e., hydrogenation of benzene 141.
0- 0
3H2
+
(3)
The activity order of Pt > Pd > Rh > Ru(= 0)and the Langmiur-type product retardation were obtained for cyclohexanes as well as for 2-propanol. The dehydrogenation activity over the platinudcarbon catalyst was substantially improved by alloying with ruthenium, which itself exhibited no catalytic activity solely. This synergetic effect would also be ascribed to the surface migration of adsorbed hydrogen species. The almost equal activity of the PdWcarbon catalyst with the sum of the Pdcarbon and Wcarbon 6 1 catalysts might be derived from 5 the characteristics of Pd and Pt metals similar with each other. i - 4 Fig. 3 demonstrates not i2 only the high extent of activity \ E l enhancement but also a large effect of retardation due to added benzene with respect to the Pt-Ru composite catalyst. 1 Adsorption constant of benzene toward the noble metal catalyst 0must be large for the efficient Pt Pt+Ru t-Ru Pt-Pd Pt+Pd Pd electrode reaction (0.2 mmol) (0.2 / 0.2 mmol) (0.2 mmol) Of C 6 H 6 + 6 H + + 6 e - ' C g H u , 0 : H, evolution without C6Headdentl. which constitutes the counter I : H, evolution with CH , , (0.5 ~01%). I: H transfer from EtC,H, I to C,H, part in the proposed thermo-regenerative Catalyst solution : Metal on 0.78 g carbon / 100 ml erhylcyclohexane Reaction conditions : Boiling and refluxing (132 'C). fuel cell system 121.
-
aQ)
Y
4. REFERENCES
I
Figure 3. Rates of hydrogen evolution and hydrogen transfer from ethylcyclohexane with suspended metal catalysts
M. Yamashita, T. Kawamura, M. Suzuki, Y. Saito., Bull. Chem. SOC. Jpn., 64,(1991)272;T. Fujii, Y. Saito, J. Chem. SOC., Chem. Commun., (1990) 757. 2 Y.Saito, M. Yamashita, K. Yukawa, H. Itagaki, Hydrogen Energy Progress 9 (Ed., T. N. Vezieoglu, C. Derive, J.Pottier-1, vol. 1, Munif. Commun. Intern., Paris(19921, pp. 113-121. 3 K. Yukawa, T. Fujii, Y. Saito, J. Chem SOC.,Chem. Commun., (1991) 1548. 4 S. H. Langer, S. Yurchak, J. Electrochem. SOC.,116,(1968) 1228. 1
313
Carbon-supported dehydrogenation catalysts composed of platinum and rutheniummetals Yuii ANDO, Xiaoniei LI, Eri ITO, Masaru YAMASHITA, a n d Yasukazu SAITO Department of Industrial Chemistry, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113, JAPAN
Abstract Catalytic dehydrogenation of alcohol o r alkane under boiling and refluxing conditions yielded molecular hydrogen and corresponding dehydrogenated compounds, taking the role of low-temperature endothermic reactions i n a newly-proposed system for chemical conversion of low-quality heats. The adsorbed hydrogen species on the surface of carbon-supported noble metals were capable to migrate from the site of C-H bond dissociation to the site of H a evolution, as demonstrated by the Ru-Pt composite catalyst. Hydrogen spillover to the carbon support proceeded over the Pd catalyst, for which little H~ evolution and facile acetone formation were observed from 2-propanol. 1. INTRODUCTION
Dehydrogenation reactions under mild conditions have attracted considerable attention in catalytic chemistry 111, since thermal or electric energies can be generated through the reverse processes, i. e., hydrogenation 121.
Among active dehydrogenation catalysts, carbon-supported noble metals are significantly important, because (1) both the C - H bond activation and the H2 formation proceed easily and ( 2 ) intimate and stable suspension is obtained in alcohol or alkane. Moreover, various kinds of composite catalysts a r e prepared from noble metals. In the present paper, spillover phenomena are investigated for the carbonsupported metal catalysts in the liquidphase.
2. CARBON-SUPPORTEDNOBLE METAL CATALYSIS FOR 2-PROPANOL DEHYDROGENATION The activity order of carbon-supported noble metal catalysts was
Ru>Rh>Pt>Pd(=O), a s far as the hydrogen evolution rates were concerned.
-
(CH3)ZCO + H2 (CH3)2CHOH The reaction was accompanied by the product retardation (Eq.(2)),
(1)
314
v = k / ( 1 + I([acetonel) where k and K are the rate Table 1 Characteristics of suspended constant and the retardation catalystsdehydrogenation of 2-propanol constant due to acetone Initial Retardation adsorption, respectively. Metal rate constnt Ref. As shown in Table 1, the mmol h-' 6.' mniol . dm.' high reaction rate on the R ~ - P ~+ / cPt(acac), 6290 0.009 a ruthenium catalyst and the Ru-Pt/C + Pd(acac);! 5660 0.009 a small acetone retardation Pt/C + H4R~4(C0)12 1700 0.008 a Ru-Pt/carbon 5870 0.014 b on the platinum catalyst 4630 0.017 C are noteworthy in contrast Ru/carbon Rli/carbon 1560 0.0 I4 C to those of the nickel fine390 0.004 C Pt/Ni fine particle 268 0.052 d particle catalysts with and without platinum deposition. Ni fine particle 96.9 0.066 d High reaction rates of Nickel bride 60 0.01 1 e 51.5 0.015 l 2-propanol dehydrogenation Raney nickel a) Present Work. b) M.Yamashila et al., Hydrogen Energy Progress 9. (Ed., T.N. per volume of the catalyst Vezieoglu. CDerive, J. Poltier). vol. I . Manif. Commun. Intern.. Paris (1992). solution are important for pp.197-205. c) Y.Sailo e l al.. Hydrogen Energy Progress R. (Ed.. T.N.Veriroglu. P.K.Takahashi), vol.1, Pergamon Press, New York. (1991). pp.339-344. d) M. designing the newlyNoda el 81.. Bull. Chem. SOC.Jpn.. 61,2541(19RR). e) D.E. Mears e l 81.. A. 1. Ch. proposed chemical heat E. Journal. 12.313(1966). 0 F.Claes e l al.. Bull. SOC.Chim. France, 25, I167 pump (1958). .system as compact as possible, as it reduces the size of the endothermic low80 temperature reactor directly. The small extent of acetone retardation is desirable for this system as well, 6O since large acetone concentrations E in the catalyst solution make it E possible to perform the distillation = .Z 40 for separating acetone from 2-propanol a t high thermal 9 efficiencies. i From the viewpoint of the spillover phenomena on catalysts, carbon-supported palladium metal was interesting, because hydrogen 0 5 10 15 20 25 evolution was quice poor, wnereas (0)Homogeneous Ru,(OAc), solution in 2-propanol acetone yielded in the solution ( 0 ) Pd/C suspension surface-modified with Rul(OAc), amounted to gg times of metallic Reaction conditions : Boiling and refluxing(82.4 c ) palladium in molar after the Figure 1. Activity enhancement of ruthenium complex reaction for 5 h. we have such for 2-propanol dehydrogenation by modifying the a view that the product hydrogen surface of catalyst palladium sneaked out from palladium to carbon. In fact, the activity of hydrogen evolution over the carbon-supported palladium catalyst was generated by modifying the palladium surface with [Ru30(OAc)$ H20 131+. The
.
315
homogeneous activity of this complex was exceeded by the complex-modified palladium catalyst(Fig. 1). Deuterium kinetic isotope effects concerning (CH3)2CDOH and (CH3)2CHOD revealed that dissociation of the methine C-H bond was rate-determining for the carbon-supported platinum or rhodium catalyst, whereas the step of hydrogen evolution was slow on the rutheniudcarbon catalyst. Provided t h a t the surface hydrogen species migrate rapidly, the facile steps of C-H splitting on Ru and of H a formation on Pt or Rh would be combined. Actually, the activities of 2propanol dehydrogenation were enhanced over the composite catalysts of Pt, Rh and Pd with Ru(Tab1e 2). H-D exchange between the substrate (CD3)$DOD and the support carbon containing H was unneglible over the Ru-Pt composite catalyst, a s demonstrated by the products in both the gas and liquid phases(Fig. 2 ) . According to the high-resolution TEM images, the carbon-supported R u - P t ( l : l ) composite catalyst was prepared a s fine particles, some of which were single crystals. From the lattice constants of the f.c.c.cl11)plane, dilution of platinum with ruthenium was ascertained. Table 2 Composite metal catalysts for 2-propanol dehydrogenation
Metal
Facile step
Rh
H2 elimin.
C-H dissoc.
Ru
C-H dissoc.
H2 elimin.
Pt
H~)elimin
C-H dissoc.
a) Molar ratio = 9:l
-8 a
s
Slow step Compositea) Initial rateb) Ru-Rh Ru-Pt
500 682 550 629 226
b) Unit: mmol.g-'.h-'
1.0
0.0 0.0
1.0
2.0
3.0
4.0
H transfer ar from to mdrogen carbon solution evolution Reaction time : 5 h
5.0
Catalyst solution: Ru-Pt( 1 :1.5 wt%) on carbon 200 mg / 5 ml 2-propanol-d8 Reaction conditions: Boiling and refluxing(82.4
c)
Figure 2. Spillover of H from carbon during 2-propanol-d8dehydrogenation
316
3. CARBON-SUPPORTEDNOBLE METAL CATALYSTS FOR CYCLOHEXANE DEHYDROGENATION
The reaction of cyclohexane dehydrogenation yielding benzene and hydrogen under mild conditions(Eq. (3)) is important [31, because a fuel cell is constructed using its reverse reaction, i. e., hydrogenation of benzene 141.
0- 0
3H2
+
(3)
The activity order of Pt > Pd > Rh > Ru(= 0)and the Langmiur-type product retardation were obtained for cyclohexanes as well as for 2-propanol. The dehydrogenation activity over the platinudcarbon catalyst was substantially improved by alloying with ruthenium, which itself exhibited no catalytic activity solely. This synergetic effect would also be ascribed to the surface migration of adsorbed hydrogen species. The almost equal activity of the PdWcarbon catalyst with the sum of the Pdcarbon and Wcarbon 6 1 catalysts might be derived from 5 the characteristics of Pd and Pt metals similar with each other. i - 4 Fig. 3 demonstrates not i2 only the high extent of activity \ E l enhancement but also a large effect of retardation due to added benzene with respect to the Pt-Ru composite catalyst. 1 Adsorption constant of benzene toward the noble metal catalyst 0must be large for the efficient Pt Pt+Ru t-Ru Pt-Pd Pt+Pd Pd electrode reaction (0.2 mmol) (0.2 / 0.2 mmol) (0.2 mmol) Of C 6 H 6 + 6 H + + 6 e - ' C g H u , 0 : H, evolution without C6Headdentl. which constitutes the counter I : H, evolution with CH , , (0.5 ~01%). I: H transfer from EtC,H, I to C,H, part in the proposed thermo-regenerative Catalyst solution : Metal on 0.78 g carbon / 100 ml erhylcyclohexane Reaction conditions : Boiling and refluxing (132 'C). fuel cell system 121.
-
aQ)
Y
4. REFERENCES
I
Figure 3. Rates of hydrogen evolution and hydrogen transfer from ethylcyclohexane with suspended metal catalysts
M. Yamashita, T. Kawamura, M. Suzuki, Y. Saito., Bull. Chem. SOC. Jpn., 64,(1991)272;T. Fujii, Y. Saito, J. Chem. SOC., Chem. Commun., (1990) 757. 2 Y.Saito, M. Yamashita, K. Yukawa, H. Itagaki, Hydrogen Energy Progress 9 (Ed., T. N. Vezieoglu, C. Derive, J.Pottier-1, vol. 1, Munif. Commun. Intern., Paris(19921, pp. 113-121. 3 K. Yukawa, T. Fujii, Y. Saito, J. Chem SOC.,Chem. Commun., (1991) 1548. 4 S. H. Langer, S. Yurchak, J. Electrochem. SOC.,116,(1968) 1228. 1