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Metal-catalysed steam reforming of ethanol in the production of hydrogen for fuel cell applications
Applied Catalysis B: Environmental 39 (2002) 65–74
Metal-catalysed steam reforming of ethanol in the production of hydrogen for fuel cell applications J.P. Breen, R. Burch∗ , H.M. Coleman School of Chemistry, Queen’s University Belfast, David Keir Building, Stranmillis Road, Belfast, N. Ireland BT9 5AG, UK Received 2 December 2001; received in revised form 10 March 2002; accepted 31 March 2002
Abstract A range of oxide-supported metal catalysts have been investigated for the steam reforming of ethanol/water mixtures for the production of hydrogen. Alumina-supported catalysts are very active at lower temperatures for the dehydration of ethanol to ethene which, at higher temperatures, is converted into H2 , CO, and CO2 as the major products and CH4 as a minor product. The order of activity of the metals is Rh > Pd > Ni = Pt. With ceria/zirconia-supported catalysts, the formation of ethene is not observed and the order of activity at higher temperatures is Pt ≥ Rh > Pd. By using combinations of a ceria/zirconia-supported metal catalyst with the alumina support it is shown that the formation of ethene does not inhibit the steam reforming reaction at higher temperatures. It is concluded that the support plays a significant role in the steam reforming of ethanol. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Fuel cell; Steam reforming; Metal catalyst
1. Introduction Fuel cells are considered to have the potential to provide a clean energy source for automotive applications as an alternative to gasoline or diesel engines. Both the fuel efficiency and the level of emissions of fuel cell-based systems offer improvements over conventional power sources. Currently, the most advanced system for mobile applications is based upon the polymer electrolyte membrane (PEM) cell which uses hydrogen as the fuel. However, since hydrogen is difficult to store and transport, it has to be produced from an easily transported liquid feedstock, with gasoline being the most convenient because of the extensive delivery infrastructure and the quality of the fuel available. On the other hand, gasoline is not ∗ Corresponding author. E-mail address: [email protected] (R. Burch).
a renewable resource and its consumption releases carbon dioxide. Therefore, there is also interest in biomaterials of which bioethanol is particularly important since it is readily produced from renewable resources and has a reasonably high hydrogen content. Further factors of importance in the on-board production of hydrogen are the size and weight of any system. Ideally, the fuel processor and the fuel cell should be no larger than a current engine and its associated accessories. Consequently, there is great interest in discovering very active catalysts for the conversion of organic compounds into hydrogen. The first step in such conversion may involve oxidation to generate the heat required for the second step which is steam reforming. Often these two steps will be combined since most catalysts that are active for steam reforming are also active for combustion. Subsequent steps are required to remove excess carbon monoxide since this is a poison of the fuel cell
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electrodes. This is achieved using water gas shift reactions (possibly both high temperature and low temperature shift catalysts will be required) and followed by a selective oxidation process to reduce the carbon monoxide to the ppm level. This paper is concerned with the conversion of bioethanol to hydrogen by using the steam reforming reaction. Although steam reforming has been known for a very long time, and is used extensively to produce hydrogen for chemical applications, there has been very little research in relation to mobile applications where the size and weight constraints are much more severe. In the case of ethanol, some preliminary work has been published recently on the use of Rh/Al2 O3 [1,2], Ni/La2 O3 [3], ZnO [4], Co/Al2 O3 [5], Cu/SiO2 combined with Ni/MgO [6], Cu, Ni, Pt, or Rh on various supports [7], potassium promoted Ni/Cu [8,9], and dual bed Pd/C Ni/alumina systems [10]. In contrast, there appear to have been no specific attempts in the context of ethanol reforming to investigate some of the very active catalysts that derive from automotive three-way catalyst (TWC) systems. Thus, Yee et al. in investigating the fundamental reactions of ethanol over M/CeO2 TWC catalysts draw attention to the fact that these catalysts are effective in water gas shift and related reactions. Similarly, in developing NOx storage materials, Matsumoto report that zirconia-supported Rh or Pt are much more active than the corresponding alumina-supported catalysts in the production of hydrogen by steam reforming [11]. In this paper, we present some preliminary results in which we compare the activity of various metals on alumina and on a ceria–zirconia support for the steam reforming of ethanol under experimental conditions which are relevant to the on-board production of hydrogen for PEM fuel cell applications using bioethanol as a renewable source of hydrogen.
2. Experimental The catalysts (1% Rh/CeO2 /ZrO2 , 1% Rh/Al2 O3 , 1% Pt/Al2 O3 , 0.5% Pd/Al2 O3 , 1% Pt/CeO2 /ZrO2 , 1% Pd/CeO2 /ZrO2 , 5% Ni/Al2 O3 , 5% Ni/CeO2 /ZrO2 ) were prepared using conventional impregnation techniques of commercially available oxides (alumina or
ceria–zirconia). The quartz tube reactor had an internal diameter of 15 mm and the catalyst was held in place using glass wool plugs. A thermocouple was located in the catalyst bed. The reactor was heated by an electric furnace and connected to a gas chromatograph (Perkin-Elmer). The GC was equipped with an asymmetric thermal conductivity detector (TCD) which simultaneously measures H2 , CO2 , N2 , CO, and a flame ionisation detector (FID) which records CH4 , C2 H4 , CH3 CH2 OH and any other hydrocarbons produced. Acetaldehyde was not detected as a product although at low ethanol conversions we could not exclude the formation of small amounts of this partial oxidation product. The gas lines were stainless steel tubing wrapped in heating tape. The 100 mg of catalyst (250–425 m) and 900 mg of inert filler (250–425 m) were used unless otherwise stated. A mixture of ethanol and water was introduced using a syringe pump (RAZEL), with nitrogen as the carrier gas. A molar ratio of H2 O:EtOH 3:1 was used (gaseous flow rates of 39.3 cm3 min−1 H2 O and 13.1 cm3 min−1 EtOH) with 100 cm3 min−1 nitrogen, giving a total flow of 152.4 cm3 min−1 . The furnace was heated from 400 to 750 ◦ C at 50 ◦ C increments, with a sample for GC analysis being taken, and the catalyst bed temperature being recorded every 15 min. Several measurements were made at each temperature and the analyses were averaged. The observation (see Section 3) that ethene was a major product at lower temperatures on some catalysts led us to examine the steam reforming of ethene. 123 mg of catalyst and 900 mg of filler (250–425 m) were used with a molar ratio of H2 O:ethene of 3:1 (gaseous flow rates of 48 cm3 min−1 H2 O and 16 cm3 min−1 ethene) with 100 cm3 min−1 nitrogen, giving a total flow of 164 cm3 min−1 . 2.1. Analysis of results For convenience, we define a “useful conversion of ethanol” and a “total conversion of ethanol”. The term “useful conversion of ethanol” (CCOX ) is introduced because this is an indication of the amount of ethanol converted to hydrogen. This is defined as: CCOX (%) =
0.5 × [CO + CO2 ]out × 100 [ethanol]in
The total conversion (CTOT ) is defined as:
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Fig. 1. (a) Percentage useful conversion of ethanol and ethene over M/Al2 O3 catalysts. (b) Percentage total conversion of ethanol and ethene over M/Al2 O3 catalysts.
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Fig. 2. Product distribution for steam reforming of ethanol over 5% Ni/Al2 O3 (reduced).
0.5 × [CO + CO2 + CH4 ]out +[C2 H4 + CH3 CHO]out × 100 CTOT (%) = [ethanol]in where all amounts are based on molar quantities.
In the steam reforming of ethene, the conversions are defined as before with ethanol replaced by ethene. For the steam reforming of ethene and ethanol, the product distributions are given as percentages of the dry product stream.
Fig. 3. Product distribution for steam reforming of ethanol over 0.5% Pd/Al2 O3 .
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Fig. 4. Product distribution for steam reforming of ethanol over 1% Rh/Al2 O3 .
3. Results and discussion Fig. 1a shows the useful conversion of ethanol and Fig. 1b the total conversion of ethanol as a function of temperature for the various alumina-supported catalysts that have been investigated. With regard to the useful conversion (to produce H2 ), alumina has a very low activity. Moreover, the Pd, Pt and reduced Ni
catalysts show very poor activity and the Rh being by far the most active of all these catalysts. Fig. 1b shows that the alumina has a high activity for converting ethanol to products other than CO and CO2 (see later). The alumina-supported Pt, Pd and Rh catalysts all show comparable total conversions while the Ni/Al2 O3 shows a more complex behaviour. Curiously, at lower temperatures this catalyst is less active
Fig. 5. Product distribution for steam reforming of ethanol over 1% Pt/Al2 O3 .
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Fig. 6. (a) Percentage useful conversion of ethanol and ethene over M/CeO2 /ZrO2 catalysts. (b) Percentage total conversion of ethanol and ethene over M/CeO2 /ZrO2 catalysts.
than the alumina support alone. At higher temperatures, the activity drops sharply and then rises again. This reflects a reversible self-poisoning deactivation at temperatures around 600–650 ◦ C. The corresponding detailed product distributions are shown in Figs. 2–5. Fig. 2 shows for the reduced Ni/Al2 O3 catalyst the formation of a significant amount of ethene that is gradually reformed into CO/CO2 and H2 as the temperature is raised above 550 ◦ C. This catalyst also produces a substantial amount of methane at the highest temperatures.
The Pd/Al2 O3 catalyst, which has comparable activity to the Ni catalyst, is seen in Fig. 3 to produce mainly ethene at low temperatures and this is gradually converted to H2 , CO and CO2 as the temperature is increased. This catalyst produces very little methane which, considering that it is not very active for ethanol steam reforming may indicate that the methane produced from the Ni catalyst arises as a secondary product from methanation of CO (or CO2 ) rather than from cracking of the ethanol or ethene. Fig. 4 shows that the Rh/Al2 O3 catalyst also produces a lot of ethene at low temperatures but this
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Fig. 7. Product distribution for steam reforming of ethanol over 1% Pd/CeO2 /ZrO2 .
has disappeared completely by 650 ◦ C, reflecting the higher steam reforming activity of this catalyst. Methane production is intermediate between that found for the Ni and Pd catalysts. The Pt/Al2 O3 catalyst, as shown in Fig. 5, produces ethene almost exclusively at temperatures below
550 ◦ C and even at 700 ◦ C there is still a substantial amount of ethene unconverted, reflecting the very low activity of this catalyst in the ethanol steam reforming reaction (see Fig. 1). The corresponding activity results for the ceria/ zirconia-supported catalysts are shown in Fig. 6a for
Fig. 8. Product distribution for steam reforming of ethanol over 1% Rh/CeO2 /ZrO2 .
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Fig. 9. Product distribution for steam reforming of ethanol over 1% Pt/CeO2 /ZrO2 .
useful conversion and in Fig. 6b for total conversion. The order of useful conversion at high temperatures is now Pd < Rh < Pt, and at intermediate temperatures Pd < Pt < Rh. At the lowest temperatures, Rh, Pd and Pt have comparable activities (within a factor of 2) but the decrease in activity initially with increasing temperature causes the activity of the Pt and the Pd to drop below that of Rh. The total conversion curves show that Pd is the least active at converting ethanol into any product, followed by Pt and Rh. Comparison between Figs. 1b and 6b show that the support has a significant effect on the reaction of ethanol with the alumina-support itself, and the alumina-supported catalysts, all showing much higher conversions. This difference can be traced to the fact that alumina is a much better support for dehydration of ethanol to ethene. Figs. 7–9 show the product selectivities for the various ceria/zirconia-supported catalysts, all of which have broadly similar properties. The main differences are that Pd/CeO2 /ZrO2 produces methane (ca. 10%) and some ethene (a maximum of ca. 4%). Rh/CeO2 /ZrO2 produces about 10% methane at low temperatures but this declines to only about 2% at the highest temperatures. This catalyst also produces virtually no ethene, possibly because Rh is active enough
to steam reform any ethene that might be formed as a primary product. Finally, the Pt/CeO2 /ZrO2 catalyst produces only small amounts of methane and no ethene above 550 ◦ C. In all cases, the trend in the selectivities to CO and CO2 reflects equilibrium behaviour that favours CO at higher temperatures. The results shown above suggest that the alumina support can play an important role in these catalysts by dehydrating ethanol to ethene even at low temperatures. Therefore, it is pertinent to consider whether the formation of ethene is beneficial or deleterious to the steam reforming reaction. In general, it might be anticipated that an alkene would adsorb very strongly on the metal component of the catalyst and that this could result in the deposition of a self-poisoning carbonaceous layer on the metal. Therefore, we have investigated the steam reforming of ethene on three representative catalysts and the results are also included in Figs. 1a and 6a. The Rh/Al2 O3 catalyst (see Fig. 1a) has a very high activity for the steam reforming of ethene even when this is introduced in high concentrations. The Pt/Al2 O3 catalyst is seen to be even more active for the conversion of ethene than for the conversion of ethanol. Finally, in the case of Rh/CeO2 /ZrO2 , Fig. 6a shows that the conversion of ethene corresponds closely to the conversion of ethanol. We conclude that
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Fig. 10. (a) Percentage conversion of ethanol over 100 mg 1% Rh/CeO2 /ZrO2 + 900 mg Al2 O3 . (b) Product distribution for steam reforming of ethanol over 100 mg 1% Rh/CeO2 /ZrO2 + 900 mg Al2 O3 .
dehydration of ethanol to ethene is unlikely to create a problem in the overall conversion of ethanol to hydrogen. This is confirmed by the results shown in Fig. 10a and b for the experiment in which we combined 100 mg Rh/CeO2 /ZrO2 and 900 mg alumina. At the lower temperatures, the effect of the alumina in de-
hydrating ethanol is clearly seen in the production of a large amount of ethene. However (compare the relevant curves in Figs. 6a and 10a), it is also clear that there is no significant change in the level of useful conversion, indicating that the Rh/CeO2 /ZrO2 catalyst is active irrespective of whether the feedstock is ethanol or ethene.
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In conclusion, the results of the present work show that the choice of support is very important in the steam reforming of ethanol. Alumina is very active in the dehydration reaction but the ceria/zirconia-supported catalysts are generally more active. Pt and Rh are much more active than Pd or Ni under our experimental conditions with both Pt and Rh showing 100% conversion at high space velocity at temperatures around 650 ◦ C. The formation of ethene as a primary product does not affect the overall production of hydrogen on the more active catalysts. The role of the support, the mechanism of the ethanol steam reforming reaction, and the effect of changing the metal particle size will be the subject of further research. Acknowledgements We are pleased to acknowledge the financial support of the European Commission through contract number ERK6-CT-1999-00012.
References [1] S. Cavallaro, Energy and Fuels 14 (2000) 1195. [2] S. Freni, J. Power Sources 94 (2001) 14. [3] A.N. Fatsikostas, D.I. Kondarides, X.E. Verykios, Chem. Commun. (2001) 851. [4] J. Llorca, P.R. de la Piscina, J. Sales, N. Homs, Chem. Commun. (2001) 641. [5] F. Haga, T. Nakijima, K. Yamashita, S. Mishima, React. Kinet. Catal. Lett. 63 (1998) 253. [6] S. Freni, N. Mondello, S. Cavallaro, G. Cacciola, V.N. Parmon, V.A. Sobyanin, React. Kinet. Catal. Lett. 71 (2000) 143. [7] S. Cavallaro, S. Freni, Int. J. Hydrogen Energy 21 (1996) 465. [8] F.J. Marino, E.G. Cerrella, S. Duhalde, M. Jobbady, M.A. Laborde, Int. J. Hydrogen Energy 23 (1998) 1095. [9] F. Marion, M. Jobbagy, G. Baronetti, M. Laborde, Stud. Surf. Sci. Catal. 130 (2000) 2147. [10] V.V. Galvita, G.L. Semin, V.D. Belyaev, V.A. Semikolenov, P. Tsiakaras, V.A. Sobyanin, Appl. Catal. A: Gen. 220 (2001) 123. [11] S. Matsumoto, Y. Ikeda, H. Suzuki, M. Ogai, N. Miyoshi, Appl. Catal. B: Environ. 25 (2000) 115.
Metal-catalysed steam reforming of ethanol in the production of hydrogen for fuel cell applications J.P. Breen, R. Burch∗ , H.M. Coleman School of Chemistry, Queen’s University Belfast, David Keir Building, Stranmillis Road, Belfast, N. Ireland BT9 5AG, UK Received 2 December 2001; received in revised form 10 March 2002; accepted 31 March 2002
Abstract A range of oxide-supported metal catalysts have been investigated for the steam reforming of ethanol/water mixtures for the production of hydrogen. Alumina-supported catalysts are very active at lower temperatures for the dehydration of ethanol to ethene which, at higher temperatures, is converted into H2 , CO, and CO2 as the major products and CH4 as a minor product. The order of activity of the metals is Rh > Pd > Ni = Pt. With ceria/zirconia-supported catalysts, the formation of ethene is not observed and the order of activity at higher temperatures is Pt ≥ Rh > Pd. By using combinations of a ceria/zirconia-supported metal catalyst with the alumina support it is shown that the formation of ethene does not inhibit the steam reforming reaction at higher temperatures. It is concluded that the support plays a significant role in the steam reforming of ethanol. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Fuel cell; Steam reforming; Metal catalyst
1. Introduction Fuel cells are considered to have the potential to provide a clean energy source for automotive applications as an alternative to gasoline or diesel engines. Both the fuel efficiency and the level of emissions of fuel cell-based systems offer improvements over conventional power sources. Currently, the most advanced system for mobile applications is based upon the polymer electrolyte membrane (PEM) cell which uses hydrogen as the fuel. However, since hydrogen is difficult to store and transport, it has to be produced from an easily transported liquid feedstock, with gasoline being the most convenient because of the extensive delivery infrastructure and the quality of the fuel available. On the other hand, gasoline is not ∗ Corresponding author. E-mail address: [email protected] (R. Burch).
a renewable resource and its consumption releases carbon dioxide. Therefore, there is also interest in biomaterials of which bioethanol is particularly important since it is readily produced from renewable resources and has a reasonably high hydrogen content. Further factors of importance in the on-board production of hydrogen are the size and weight of any system. Ideally, the fuel processor and the fuel cell should be no larger than a current engine and its associated accessories. Consequently, there is great interest in discovering very active catalysts for the conversion of organic compounds into hydrogen. The first step in such conversion may involve oxidation to generate the heat required for the second step which is steam reforming. Often these two steps will be combined since most catalysts that are active for steam reforming are also active for combustion. Subsequent steps are required to remove excess carbon monoxide since this is a poison of the fuel cell
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electrodes. This is achieved using water gas shift reactions (possibly both high temperature and low temperature shift catalysts will be required) and followed by a selective oxidation process to reduce the carbon monoxide to the ppm level. This paper is concerned with the conversion of bioethanol to hydrogen by using the steam reforming reaction. Although steam reforming has been known for a very long time, and is used extensively to produce hydrogen for chemical applications, there has been very little research in relation to mobile applications where the size and weight constraints are much more severe. In the case of ethanol, some preliminary work has been published recently on the use of Rh/Al2 O3 [1,2], Ni/La2 O3 [3], ZnO [4], Co/Al2 O3 [5], Cu/SiO2 combined with Ni/MgO [6], Cu, Ni, Pt, or Rh on various supports [7], potassium promoted Ni/Cu [8,9], and dual bed Pd/C Ni/alumina systems [10]. In contrast, there appear to have been no specific attempts in the context of ethanol reforming to investigate some of the very active catalysts that derive from automotive three-way catalyst (TWC) systems. Thus, Yee et al. in investigating the fundamental reactions of ethanol over M/CeO2 TWC catalysts draw attention to the fact that these catalysts are effective in water gas shift and related reactions. Similarly, in developing NOx storage materials, Matsumoto report that zirconia-supported Rh or Pt are much more active than the corresponding alumina-supported catalysts in the production of hydrogen by steam reforming [11]. In this paper, we present some preliminary results in which we compare the activity of various metals on alumina and on a ceria–zirconia support for the steam reforming of ethanol under experimental conditions which are relevant to the on-board production of hydrogen for PEM fuel cell applications using bioethanol as a renewable source of hydrogen.
2. Experimental The catalysts (1% Rh/CeO2 /ZrO2 , 1% Rh/Al2 O3 , 1% Pt/Al2 O3 , 0.5% Pd/Al2 O3 , 1% Pt/CeO2 /ZrO2 , 1% Pd/CeO2 /ZrO2 , 5% Ni/Al2 O3 , 5% Ni/CeO2 /ZrO2 ) were prepared using conventional impregnation techniques of commercially available oxides (alumina or
ceria–zirconia). The quartz tube reactor had an internal diameter of 15 mm and the catalyst was held in place using glass wool plugs. A thermocouple was located in the catalyst bed. The reactor was heated by an electric furnace and connected to a gas chromatograph (Perkin-Elmer). The GC was equipped with an asymmetric thermal conductivity detector (TCD) which simultaneously measures H2 , CO2 , N2 , CO, and a flame ionisation detector (FID) which records CH4 , C2 H4 , CH3 CH2 OH and any other hydrocarbons produced. Acetaldehyde was not detected as a product although at low ethanol conversions we could not exclude the formation of small amounts of this partial oxidation product. The gas lines were stainless steel tubing wrapped in heating tape. The 100 mg of catalyst (250–425 m) and 900 mg of inert filler (250–425 m) were used unless otherwise stated. A mixture of ethanol and water was introduced using a syringe pump (RAZEL), with nitrogen as the carrier gas. A molar ratio of H2 O:EtOH 3:1 was used (gaseous flow rates of 39.3 cm3 min−1 H2 O and 13.1 cm3 min−1 EtOH) with 100 cm3 min−1 nitrogen, giving a total flow of 152.4 cm3 min−1 . The furnace was heated from 400 to 750 ◦ C at 50 ◦ C increments, with a sample for GC analysis being taken, and the catalyst bed temperature being recorded every 15 min. Several measurements were made at each temperature and the analyses were averaged. The observation (see Section 3) that ethene was a major product at lower temperatures on some catalysts led us to examine the steam reforming of ethene. 123 mg of catalyst and 900 mg of filler (250–425 m) were used with a molar ratio of H2 O:ethene of 3:1 (gaseous flow rates of 48 cm3 min−1 H2 O and 16 cm3 min−1 ethene) with 100 cm3 min−1 nitrogen, giving a total flow of 164 cm3 min−1 . 2.1. Analysis of results For convenience, we define a “useful conversion of ethanol” and a “total conversion of ethanol”. The term “useful conversion of ethanol” (CCOX ) is introduced because this is an indication of the amount of ethanol converted to hydrogen. This is defined as: CCOX (%) =
0.5 × [CO + CO2 ]out × 100 [ethanol]in
The total conversion (CTOT ) is defined as:
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Fig. 1. (a) Percentage useful conversion of ethanol and ethene over M/Al2 O3 catalysts. (b) Percentage total conversion of ethanol and ethene over M/Al2 O3 catalysts.
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Fig. 2. Product distribution for steam reforming of ethanol over 5% Ni/Al2 O3 (reduced).
0.5 × [CO + CO2 + CH4 ]out +[C2 H4 + CH3 CHO]out × 100 CTOT (%) = [ethanol]in where all amounts are based on molar quantities.
In the steam reforming of ethene, the conversions are defined as before with ethanol replaced by ethene. For the steam reforming of ethene and ethanol, the product distributions are given as percentages of the dry product stream.
Fig. 3. Product distribution for steam reforming of ethanol over 0.5% Pd/Al2 O3 .
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Fig. 4. Product distribution for steam reforming of ethanol over 1% Rh/Al2 O3 .
3. Results and discussion Fig. 1a shows the useful conversion of ethanol and Fig. 1b the total conversion of ethanol as a function of temperature for the various alumina-supported catalysts that have been investigated. With regard to the useful conversion (to produce H2 ), alumina has a very low activity. Moreover, the Pd, Pt and reduced Ni
catalysts show very poor activity and the Rh being by far the most active of all these catalysts. Fig. 1b shows that the alumina has a high activity for converting ethanol to products other than CO and CO2 (see later). The alumina-supported Pt, Pd and Rh catalysts all show comparable total conversions while the Ni/Al2 O3 shows a more complex behaviour. Curiously, at lower temperatures this catalyst is less active
Fig. 5. Product distribution for steam reforming of ethanol over 1% Pt/Al2 O3 .
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Fig. 6. (a) Percentage useful conversion of ethanol and ethene over M/CeO2 /ZrO2 catalysts. (b) Percentage total conversion of ethanol and ethene over M/CeO2 /ZrO2 catalysts.
than the alumina support alone. At higher temperatures, the activity drops sharply and then rises again. This reflects a reversible self-poisoning deactivation at temperatures around 600–650 ◦ C. The corresponding detailed product distributions are shown in Figs. 2–5. Fig. 2 shows for the reduced Ni/Al2 O3 catalyst the formation of a significant amount of ethene that is gradually reformed into CO/CO2 and H2 as the temperature is raised above 550 ◦ C. This catalyst also produces a substantial amount of methane at the highest temperatures.
The Pd/Al2 O3 catalyst, which has comparable activity to the Ni catalyst, is seen in Fig. 3 to produce mainly ethene at low temperatures and this is gradually converted to H2 , CO and CO2 as the temperature is increased. This catalyst produces very little methane which, considering that it is not very active for ethanol steam reforming may indicate that the methane produced from the Ni catalyst arises as a secondary product from methanation of CO (or CO2 ) rather than from cracking of the ethanol or ethene. Fig. 4 shows that the Rh/Al2 O3 catalyst also produces a lot of ethene at low temperatures but this
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Fig. 7. Product distribution for steam reforming of ethanol over 1% Pd/CeO2 /ZrO2 .
has disappeared completely by 650 ◦ C, reflecting the higher steam reforming activity of this catalyst. Methane production is intermediate between that found for the Ni and Pd catalysts. The Pt/Al2 O3 catalyst, as shown in Fig. 5, produces ethene almost exclusively at temperatures below
550 ◦ C and even at 700 ◦ C there is still a substantial amount of ethene unconverted, reflecting the very low activity of this catalyst in the ethanol steam reforming reaction (see Fig. 1). The corresponding activity results for the ceria/ zirconia-supported catalysts are shown in Fig. 6a for
Fig. 8. Product distribution for steam reforming of ethanol over 1% Rh/CeO2 /ZrO2 .
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Fig. 9. Product distribution for steam reforming of ethanol over 1% Pt/CeO2 /ZrO2 .
useful conversion and in Fig. 6b for total conversion. The order of useful conversion at high temperatures is now Pd < Rh < Pt, and at intermediate temperatures Pd < Pt < Rh. At the lowest temperatures, Rh, Pd and Pt have comparable activities (within a factor of 2) but the decrease in activity initially with increasing temperature causes the activity of the Pt and the Pd to drop below that of Rh. The total conversion curves show that Pd is the least active at converting ethanol into any product, followed by Pt and Rh. Comparison between Figs. 1b and 6b show that the support has a significant effect on the reaction of ethanol with the alumina-support itself, and the alumina-supported catalysts, all showing much higher conversions. This difference can be traced to the fact that alumina is a much better support for dehydration of ethanol to ethene. Figs. 7–9 show the product selectivities for the various ceria/zirconia-supported catalysts, all of which have broadly similar properties. The main differences are that Pd/CeO2 /ZrO2 produces methane (ca. 10%) and some ethene (a maximum of ca. 4%). Rh/CeO2 /ZrO2 produces about 10% methane at low temperatures but this declines to only about 2% at the highest temperatures. This catalyst also produces virtually no ethene, possibly because Rh is active enough
to steam reform any ethene that might be formed as a primary product. Finally, the Pt/CeO2 /ZrO2 catalyst produces only small amounts of methane and no ethene above 550 ◦ C. In all cases, the trend in the selectivities to CO and CO2 reflects equilibrium behaviour that favours CO at higher temperatures. The results shown above suggest that the alumina support can play an important role in these catalysts by dehydrating ethanol to ethene even at low temperatures. Therefore, it is pertinent to consider whether the formation of ethene is beneficial or deleterious to the steam reforming reaction. In general, it might be anticipated that an alkene would adsorb very strongly on the metal component of the catalyst and that this could result in the deposition of a self-poisoning carbonaceous layer on the metal. Therefore, we have investigated the steam reforming of ethene on three representative catalysts and the results are also included in Figs. 1a and 6a. The Rh/Al2 O3 catalyst (see Fig. 1a) has a very high activity for the steam reforming of ethene even when this is introduced in high concentrations. The Pt/Al2 O3 catalyst is seen to be even more active for the conversion of ethene than for the conversion of ethanol. Finally, in the case of Rh/CeO2 /ZrO2 , Fig. 6a shows that the conversion of ethene corresponds closely to the conversion of ethanol. We conclude that
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Fig. 10. (a) Percentage conversion of ethanol over 100 mg 1% Rh/CeO2 /ZrO2 + 900 mg Al2 O3 . (b) Product distribution for steam reforming of ethanol over 100 mg 1% Rh/CeO2 /ZrO2 + 900 mg Al2 O3 .
dehydration of ethanol to ethene is unlikely to create a problem in the overall conversion of ethanol to hydrogen. This is confirmed by the results shown in Fig. 10a and b for the experiment in which we combined 100 mg Rh/CeO2 /ZrO2 and 900 mg alumina. At the lower temperatures, the effect of the alumina in de-
hydrating ethanol is clearly seen in the production of a large amount of ethene. However (compare the relevant curves in Figs. 6a and 10a), it is also clear that there is no significant change in the level of useful conversion, indicating that the Rh/CeO2 /ZrO2 catalyst is active irrespective of whether the feedstock is ethanol or ethene.
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In conclusion, the results of the present work show that the choice of support is very important in the steam reforming of ethanol. Alumina is very active in the dehydration reaction but the ceria/zirconia-supported catalysts are generally more active. Pt and Rh are much more active than Pd or Ni under our experimental conditions with both Pt and Rh showing 100% conversion at high space velocity at temperatures around 650 ◦ C. The formation of ethene as a primary product does not affect the overall production of hydrogen on the more active catalysts. The role of the support, the mechanism of the ethanol steam reforming reaction, and the effect of changing the metal particle size will be the subject of further research. Acknowledgements We are pleased to acknowledge the financial support of the European Commission through contract number ERK6-CT-1999-00012.
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