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Pre-reforming of liquefied petroleum gas on supported ruthenium catalyst
International Journal of Hydrogen Energy 26 (2001) 935–940
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Pre-reforming of lique)ed petroleum gas on supported ruthenium catalyst Takashi Suzuki ∗ , Hiko-ichi Iwanami, Osamu Iwamoto, Tamio Kitahara Research and Development Center, Cosmo Research Institute (CRI), 1134-2, Gongendo, Satte, Saitama 340-0193, Japan
Abstract Cylindrical shaped CeO2 –Al2 O3 support was prepared by calcining the pelletized powder mixture of cerium carbonate, aluminum hydroxide and poly-vinylalcohol. The speci)c surface area and porosity of the support were improved relative to those obtained by a conventional method. Ru was loaded on the porous carrier by using ruthenium trichloride and aqueous ammonia. The dispersion of Ru was improved 2.2 fold as much as that in the case using a conventional catalyst. The improved catalyst showed high activity for pre-reforming of lique)ed petroleum gas (LPG) with low proportion of S=C as 0.8 at ◦ 450 C. Finally, 7000 h of sustained run of the pre-reforming was successfully achieved on that catalyst. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Recently, production of deeply desulfurized fuels, fuel cell systems, gas to liquid (GTL) process, and others were considered to be important technologies developed to obtain clean energy sources. Rationalized reforming would be the key technology in the above mentioned categories. Moreover, it might be considered that the demand of hydrogen will be further increased in the near future. A steam reforming of hydrocarbons is one of the least expensive hydrogen production methods at the present time [1]. It is considered that the way to improve the energy e?ciency in the steam reforming process would be important. Some researchers have pointed out that low temperature reforming, namely, the pre-reforming is one of the e@ective techniques to enhance the thermal e?ciency [2– 4] and that the fuel saving in the process could be increased by 9.2% relative to the process without the pre-reformer [4]. The major role of the pre-reformer is to convert lighter hydrocarbons such as LPG into methane. In order to obtain methane preferentially, the pre-reforming of LPG should
∗ Corresponding author. Tel.: +81-480-42-2211; fax: +81480-42-3790. E-mail address: takashi [email protected] (T. Suzuki).
◦
be carried out at lower temperatures such as 400 –500 C from the equilibrium viewpoints. In addition, the carbon deposition caused by Boudouard reaction (2CO → CO2 + C ↓) [5] dominantly occurs in the range of lower temperatures. Therefore, a catalyst having higher activity and higher resistance to the carbon deposition is desired for the pre-reforming facility. It is suggested that ruthenium-based catalyst is very effective in preventing carbon deposition during the steam reforming, that the catalyst system is readily poisoned by sulfur compounds and that the carbon deposition takes place on the poisoned ruthenium based catalyst [6 –8]. Alkali metal is doped in reforming catalyst as a means of resisting carbon formation [9]. Recently, it has been reported on the Ru=Al2 O3 for high temperature steam reforming that cerium oxide has a role to prevent poisoning by sulfur and as a result carbon deposition can be reduced [10]. However, the cerium oxide doped Ru=Al2 O3 for high temperature reforming does not show su?cient activities for the pre-reforming. In order to improve the catalyst activity for the pre-reforming, an increase in the number of active sites by improving dispersion would be e@ective. In this work, when decomposable organic compounds such as poly-vinylalcohol were added to the starting material of a support, the surface area and porosity were improved, and as a result, the dispersion of ruthenium was increased 2.2 fold
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 3 6 - 2
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T. Suzuki et al. / International Journal of Hydrogen Energy 26 (2001) 935–940
in comparison with the support prepared by the conventional method. Finally, the catalyst was subjected to preliminary sustained run of the pre-reforming; the catalyst performance was maintained for 7000 h.
2. Experimental 2.1. Porous CeO2 –Al2 O3 support Powder mixture of Ce2 (CO3 )3 · 8H2 O (Kanto Chemical Co.), Al(OH)3 (Kanto Chemical Co.) and poly-vinylalcohol (PVA; Nihon Gosei Co.) was well ground and sieved under 100 mesh. PVA was added to contain 10 wt% to the starting material. The powder mixture was pressed by a pelletizer (FK-1, Systems Engineering Co.) with 16 tons=cm2 . Finally, the resulting material was calcined overnight in an electric ◦ oven maintained at 650 C to obtain CeO2 –Al2 O3 support. Then ca. 3 mm o.d.×ca. 3 mm (H) of pellet comprising CeO2 (7.6 wt%) and Al2 O3 (92.4 wt%) was obtained. The speci)c surface area of the cylindrical support was around 250 m2 =g. 2.2. Conventional CeO2 –Al2 O3 support Powder mixture of CeO2 , and -Al2 O3 was used for preparation. The handling procedure is the same as in Section 2.1. except for adding PVA. The speci)c surface area of the carrier was around 140 m2 =g. The porous support and conventional support are denoted as support-A and -B, respectively. The chemical composition of support-B was almost analogous to that of support-A.
2.3. Ru loading Ru was loaded by means of an impregnating technique. The support was immersed into water soluble RuCl3 · H2 O (Mitsuwa Chemical Co.) for 3 h at room temperature and it ◦ was dried in air at 105 C for several hours. The resulting material was soaked in 7 M of aqueous ammonia for 2 h at ◦ 30 C. The solution was colored slightly rose pink which may be caused by the formation of ammonium complexes of Ru. The specimen was rinsed with de-ionized water to remove excess ammonia and ammonium chloride. It is deduced that the ruthenium chloride on the support would be changed into ruthenium hydroxide during the treatment in the alkaline solutions. 2.4. Pre-reforming reaction Pre-reforming reaction was performed in the conventional )xed bed reactor described in Scheme 1. Ten milliliter (ca. 9.5 g) of the catalyst was set in the )xed bed reactor ◦ and the catalyst was reduced by hydrogen for 3 h at 450 C prior to the pre-reforming. The commercially available LPG purchased from Cosmo Oil Co. (C4 H10 : 96 vol%, C3 H8 : 4 vol%) was used without further puri)cation. Molar proportions of steam=carbon in LPG (S=C) and GHSV (gas hourly space velocity) were 0.8 and ca. 1500 vol /vol=h1 ◦ at 450 C, respectively. The product distributions were determined with TCD-GC and FID-GC (GC390, GL Science Co. Ltd.). Active carbon (Unibeads-A, 60 –80 mesh, GL Science Co. Ltd.) was used for the separation of H2 , CO, CO2 , and CH4 , and Porapak-QS (60 –80 mesh, Waters Co. Ltd.) was used for the separation of hydrocarbons.
Scheme. 1. Diagram of the apparatus used for pre-reforming. PR: pressure regulating valve, FR: Kow regulating valve, PM: pressure maintaining valve, S: gas–liquid separator, GF: gas Kow meter, and SV: stop valve.
T. Suzuki et al. / International Journal of Hydrogen Energy 26 (2001) 935–940
937
3. Results and discussion 3.1. Ruthenium distributions in Support-A and -B
Scheme. 2. Flow diagram of CO adsorption instrument. SV: stop valve, PG: pressure gauge, PS: pressure sensor, 6DV: 6 directions valve, RM: rotameter, A: adsorption vessel, and TCD: thermal conductivity detector.
2.5. Catalyst characterization Speci)c surface area and pore volume were measured by means of a Belsorp 28 instrument (Bel Japan Co.) using nitrogen as an adsorbable molecule. The dispersion of Ru can be calculated appropriately on the assumption that CO is adsorbed dominantly with a stoichiometric adsorption rate of CO : Ru = 1 : 1, which is reported in the Refs. [11,12]. The dispersion is represented as [CO(a)]=[Ru]×100 (%), where [CO(a)] is the adsorbed amount of CO and [Ru] is the concentration of Ru in catalyst [11]. [CO(a)] is determined precisely by pulse adsorption method [13–15] and the concentration of [Ru] is obtained by a JXA-8600MX (JEOL Co.) ICP (induced coupled plasma) instrument. Prior to the pulse adsorption, 0.5 g of specimen was set in the ◦ quartz U-tube and it was reduced at 450 C for 3 h. Then ◦ ◦ the specimen was cooled from 450 C to 25 C under Kowing helium. After the pretreatment, CO pulse (0.2 ml) was dosed to the catalyst. The CO pulse was repeated until the adsorption was saturated. The amount of adsorbed CO was obtained by accumulating peak areas of TCD. The CO used was of 99.95% purity (research grade, Takachiho Chemical Co.), and the pulse experiment was carried out by using an Ohkura 6850 adsorption instrument equipped with TCD (Ohkura Riken Co.) . The connection diagram of the instrument is described in Scheme 2. The chemical compositions of Ru, CeO2 , and Al2 O3 in these catalysts were 1.6, 7.5, and 90.9 wt%, respectively. Distribution of ruthenium species in the catalyst was observed by an IRIS Advantage-RP (Jarrell-Ash Co.) EPMA (electron probe micro analysis) instrument.
The speci)c surface area of Support-A is ca. 250 m2 =g, whereas that of Support-B is 0.56 fold in comparison with the case of support-A. In order to obtain further information, the pore volume on the catalysts was con)rmed. The pore volume of Support-B was 0.42 ml=g, whereas that of Support-A was 0.64 ml=g. These observations implied that the texture of the support was improved by adding PVA. When Support-A and Support-B were used for catalyst preparations, dispersion of Ru on Support-A was 64.8% ((CO(a)= 2:4 ml=g-cat (stp)), whereas that of Ru on Support-B was 29.7% ((CO(a)= 1:1 ml=g-cat(stp)) and as a result, the dispersion of Ru on Support-A was increased 2.2 fold in comparison with the case of Support-B. Fig. 1 shows the EPMA results on Ru=Support-A and Ru=Support-B. In the case of Ru=Support-B, ruthenium species preferentially existed in the external zone, while in the case of Ru=Support-A, ruthenium species existed not only in the external zone but also in the internal zone. It is, therefore, considered that the dispersion of ruthenium might be improved on Support-A and that the di@used LPG and steam in the internal zone may react, and as a result an increase in the whole reactivity of the cylindrical shape of the catalyst could be expected. In addition, it was deduced that the porosity of the support might be improved by adding PVA as an estimated model described in Fig. 2. In this section, it has been suggested that the addition of a decomposable component such as PVA can increase the speci)c surface area which in turn could inKuence the dispersion of active metal as well. 3.2. Con4rmation of catalytic performance on Ru=Support-A and -B Comparing the catalytic performance of pre-reforming on Ru=Support-A and Ru=Support-B, these catalysts were ◦ subjected to the reaction at 450 C with S=C = 0:8 under 2 8 kg=cm and GHSV was set at 1500 vol=vol=h1 . When the Ru=Support-B was used for the pre-reforming, the conversion of LPG was calculated by using Eq. (1) and it was 85.2%, whereas when using the Ru=Support-A, LPG reacted completely as shown in Table 1. Conversion of LPG (%) = 100 × [C] in CO + CO2 + CH4 =[C] in LPG:
(1)
Here, one can suspect that the lower conversion observed on Ru=Support-B would be caused by carbon formation. It is anticipated that the carbon deposition might take place readily in the reaction conditions as stated in the beginning. In order to clarify the di@erence of the amount in the deposited carbon on these catalysts, they were subjected to
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T. Suzuki et al. / International Journal of Hydrogen Energy 26 (2001) 935–940
Fig. 1. The EPMA image of Ru dispersed on Support-A and -B.
Fig. 2. Estimated model to form pores during the calcining. (•) oxides, (◦) decomposable component (PVA).
carbon analysis by using carbon analyzer (Model EMIA-110, Hitachi-Horiba Co.). The amount of deposited carbon was determined on the basis of the amount of carbon dioxide generated in a micro electric furnace with pure oxygen. The deposited carbon at 100 h was ca. 0.1 g per g-catalyst on Ru=Support-A and -B. Thus the deposited carbon is not so much and the signi)cant di@erence with regard to carbon deposition was not observed on those catalysts. In addition, product distributions were almost analogous in the two catalysts (Table 1). It is inferred from these results that the function of active sites on Ru=Support-A and -B were the same but the unreacted LPG in Ru=Support-B might be due to an insu?ciency of active sites on the catalyst. Thus,
the feed was converted completely on the texture improved catalyst (Ru=Support-A) which may have su?cient active sites. As stated above, the pre-reforming reaction was successfully performed on the Ru=Support-A with lower S=C such ◦ as 0.8 around 450 C. These results led us to the preliminary sustained run to con)rm the catalyst performance. 3.3. Preliminary sustained run In order to con)rm the pre-reforming performance of Ru=Support-A, the catalyst was subjected to the sustained ◦ run for 7000 h at 450 C under 8 kg=cm2 in the micro reactor
T. Suzuki et al. / International Journal of Hydrogen Energy 26 (2001) 935–940
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Table 1 Conversion of butane and product distributions in low temperature steam reforminga Time (h)
Catalyst
S=C
Conv. of butane (%)
Product distributions (%) H2 CO CH4
CO2
100 100
Ru=Support-A Ru=Support-B
0.8 0.8
100 85.2
9.6 9.3
16.8 17.8
a Catalyst:
0.4 0.3
73.2 72.6
◦
7 ml (6.5 g), GHSV : 1500 v=v=h1 , Press.: 8 kg=cm2 , Temp.: 450 C.
Fig. 3. Catalyst life test of Ru=Support-A for pre-reforming. (?) conversion of LPG, (•) methane, (4) carbon dioxide, () hydrogen, and ◦ (5) carbon monoxide. S=C = 0:8 (at initial stage S=C = 1:0), GHSV = 1500, reaction temperature= 450 C, and pressure= 8 kg=cm2 .
as seen in Scheme 1. Fig. 3 shows the conversion of LPG, and the composition of products during the pre-reforming. Evaluating the composition change in the products versus the S=C value, the S=C was set at unity in the initial stage. The compositions of H2 , CO, CH4 , and CO2 in that condition were 13.5, 0.3, 71.9, and 14.3 vol%, respectively. After 1100 h, feeding rate of steam was decreased to maintain S=C = 0:8. The compositions of H2 , CO, CH4 , and CO2 were 10.9, 0.3, 74.9, and 13.9 vol%, respectively. Thus, methane was formed preferentially at lower S=C proportion. Unreacted LPG was not detected at all by means of FID-GC until 2500 h; after this period, an in)nitesimal amount of butane was observed at outlet. Finally, unreacted LPG (ca. 0.3 vol% of butane) was observed after the elapse of 7000 h. However, it was observed that such a small amount of LPG was converted completely by increasing the temperature ◦ within 10 C. So far, the S=C was maintained at a higher ratio in the conventional Ni based catalyst from the viewpoint of preventing the carbon formations. The developed catalyst shows a stable performance even in lower S=C such as 0.8 for a long term. These results indicate that the developed Ru=CeO2 – Alumina catalyst may have potential for commercial applications.
References [1] Saito M. Methanol synthesis by catalytic hydrogenation of carbon dioxide. Shokubai (Catalysis) 1993;35:485–91. [2] Vali VK, Singh AK, Kumar A. Catalyst selection for optimum performance. Fert News 1998;43:33–8. [3] Hansen JHB, Storgaard L, Pedersen PS. Aspects of modern reforming technology and catalysts. Ammonia Plant Saf 1992;32:52– 62. [4] Foreman JM. Pre-reformer aids syngas units; adding another step to steam reforming saves fuel and boosts output. Hydrocarbon Process 1990;69:34B–34D. [5] Rideal EK. Concept in catalysis. London and New York: Academic Press, 1968. [6] Okada O, Tabata T, Masuda M, Matsui H. Catalyst deactivation by sulfur poisoning. Shokubai (Catalysis) 1993;35:224 –30. [7] Okada O, Ipponmatsu M, Masuda M, Takami S. A study of sulfur poisoning on Ru=Al2 O3 methanation catalyst. Nenryokyokaishi (J Jpn Fuel Sci Technol) 1989;68: 39– 44. [8] Masuda M, Tabata T, Okada O, Wood BJ, Tong TG, MaCarty JG. The role of sulfur in the deposition of carbon on ruthenium and nickel steam reforming catalyst. Stud Surf Sci Catal 1991;68:185–93.
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[9] Raymond AH. The design of catalysts and catalytic process for the steam reforming of hydrocarbon feedstocks. Catal Sci Technol 1990;1:317–20. [10] Suzuki T, Iwanami H, Yoshizawa T. Steam reforming catalyst for manufacture of hydrogen from liquid hydrocarbons. JP09010586, Jpn Kokai Tokkyo Koho (1997); Chem Abstr 1997;126:201531. [11] Tsumaki H, Shinjyo H. Measurement of metal dispersion in Cl free supported reference noble metal catalysts by CO-pulse method. Nippon Kagaku Kaishi (J Chem Soc Jpn) 1989;12:1990 –8.
[12] Narita T, Miura H, Ohira M, Hondou H, Sugiyama K, Mastuda T, Gonzarez RD. The e@ect of reduction temperature on the chemisorptive properties of Ru=Al2 O3 . Appl Catal 1987;32:185–90. [13] Freel J. Chemisorption on supported platinum, I. Evaluation of a pulse method. J Catal 1972;25:139. [14] Gruber HL. An adsorption Kow method for speci)c metal surface area determination. Anal Chem 1962;34:1828. [15] Nojiri N. Measurement of speci)c metal surface area by CO pulse method. Shokubai (Catalysis) 1981;23:488.
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Pre-reforming of lique)ed petroleum gas on supported ruthenium catalyst Takashi Suzuki ∗ , Hiko-ichi Iwanami, Osamu Iwamoto, Tamio Kitahara Research and Development Center, Cosmo Research Institute (CRI), 1134-2, Gongendo, Satte, Saitama 340-0193, Japan
Abstract Cylindrical shaped CeO2 –Al2 O3 support was prepared by calcining the pelletized powder mixture of cerium carbonate, aluminum hydroxide and poly-vinylalcohol. The speci)c surface area and porosity of the support were improved relative to those obtained by a conventional method. Ru was loaded on the porous carrier by using ruthenium trichloride and aqueous ammonia. The dispersion of Ru was improved 2.2 fold as much as that in the case using a conventional catalyst. The improved catalyst showed high activity for pre-reforming of lique)ed petroleum gas (LPG) with low proportion of S=C as 0.8 at ◦ 450 C. Finally, 7000 h of sustained run of the pre-reforming was successfully achieved on that catalyst. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Recently, production of deeply desulfurized fuels, fuel cell systems, gas to liquid (GTL) process, and others were considered to be important technologies developed to obtain clean energy sources. Rationalized reforming would be the key technology in the above mentioned categories. Moreover, it might be considered that the demand of hydrogen will be further increased in the near future. A steam reforming of hydrocarbons is one of the least expensive hydrogen production methods at the present time [1]. It is considered that the way to improve the energy e?ciency in the steam reforming process would be important. Some researchers have pointed out that low temperature reforming, namely, the pre-reforming is one of the e@ective techniques to enhance the thermal e?ciency [2– 4] and that the fuel saving in the process could be increased by 9.2% relative to the process without the pre-reformer [4]. The major role of the pre-reformer is to convert lighter hydrocarbons such as LPG into methane. In order to obtain methane preferentially, the pre-reforming of LPG should
∗ Corresponding author. Tel.: +81-480-42-2211; fax: +81480-42-3790. E-mail address: takashi [email protected] (T. Suzuki).
◦
be carried out at lower temperatures such as 400 –500 C from the equilibrium viewpoints. In addition, the carbon deposition caused by Boudouard reaction (2CO → CO2 + C ↓) [5] dominantly occurs in the range of lower temperatures. Therefore, a catalyst having higher activity and higher resistance to the carbon deposition is desired for the pre-reforming facility. It is suggested that ruthenium-based catalyst is very effective in preventing carbon deposition during the steam reforming, that the catalyst system is readily poisoned by sulfur compounds and that the carbon deposition takes place on the poisoned ruthenium based catalyst [6 –8]. Alkali metal is doped in reforming catalyst as a means of resisting carbon formation [9]. Recently, it has been reported on the Ru=Al2 O3 for high temperature steam reforming that cerium oxide has a role to prevent poisoning by sulfur and as a result carbon deposition can be reduced [10]. However, the cerium oxide doped Ru=Al2 O3 for high temperature reforming does not show su?cient activities for the pre-reforming. In order to improve the catalyst activity for the pre-reforming, an increase in the number of active sites by improving dispersion would be e@ective. In this work, when decomposable organic compounds such as poly-vinylalcohol were added to the starting material of a support, the surface area and porosity were improved, and as a result, the dispersion of ruthenium was increased 2.2 fold
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 3 6 - 2
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T. Suzuki et al. / International Journal of Hydrogen Energy 26 (2001) 935–940
in comparison with the support prepared by the conventional method. Finally, the catalyst was subjected to preliminary sustained run of the pre-reforming; the catalyst performance was maintained for 7000 h.
2. Experimental 2.1. Porous CeO2 –Al2 O3 support Powder mixture of Ce2 (CO3 )3 · 8H2 O (Kanto Chemical Co.), Al(OH)3 (Kanto Chemical Co.) and poly-vinylalcohol (PVA; Nihon Gosei Co.) was well ground and sieved under 100 mesh. PVA was added to contain 10 wt% to the starting material. The powder mixture was pressed by a pelletizer (FK-1, Systems Engineering Co.) with 16 tons=cm2 . Finally, the resulting material was calcined overnight in an electric ◦ oven maintained at 650 C to obtain CeO2 –Al2 O3 support. Then ca. 3 mm o.d.×ca. 3 mm (H) of pellet comprising CeO2 (7.6 wt%) and Al2 O3 (92.4 wt%) was obtained. The speci)c surface area of the cylindrical support was around 250 m2 =g. 2.2. Conventional CeO2 –Al2 O3 support Powder mixture of CeO2 , and -Al2 O3 was used for preparation. The handling procedure is the same as in Section 2.1. except for adding PVA. The speci)c surface area of the carrier was around 140 m2 =g. The porous support and conventional support are denoted as support-A and -B, respectively. The chemical composition of support-B was almost analogous to that of support-A.
2.3. Ru loading Ru was loaded by means of an impregnating technique. The support was immersed into water soluble RuCl3 · H2 O (Mitsuwa Chemical Co.) for 3 h at room temperature and it ◦ was dried in air at 105 C for several hours. The resulting material was soaked in 7 M of aqueous ammonia for 2 h at ◦ 30 C. The solution was colored slightly rose pink which may be caused by the formation of ammonium complexes of Ru. The specimen was rinsed with de-ionized water to remove excess ammonia and ammonium chloride. It is deduced that the ruthenium chloride on the support would be changed into ruthenium hydroxide during the treatment in the alkaline solutions. 2.4. Pre-reforming reaction Pre-reforming reaction was performed in the conventional )xed bed reactor described in Scheme 1. Ten milliliter (ca. 9.5 g) of the catalyst was set in the )xed bed reactor ◦ and the catalyst was reduced by hydrogen for 3 h at 450 C prior to the pre-reforming. The commercially available LPG purchased from Cosmo Oil Co. (C4 H10 : 96 vol%, C3 H8 : 4 vol%) was used without further puri)cation. Molar proportions of steam=carbon in LPG (S=C) and GHSV (gas hourly space velocity) were 0.8 and ca. 1500 vol /vol=h1 ◦ at 450 C, respectively. The product distributions were determined with TCD-GC and FID-GC (GC390, GL Science Co. Ltd.). Active carbon (Unibeads-A, 60 –80 mesh, GL Science Co. Ltd.) was used for the separation of H2 , CO, CO2 , and CH4 , and Porapak-QS (60 –80 mesh, Waters Co. Ltd.) was used for the separation of hydrocarbons.
Scheme. 1. Diagram of the apparatus used for pre-reforming. PR: pressure regulating valve, FR: Kow regulating valve, PM: pressure maintaining valve, S: gas–liquid separator, GF: gas Kow meter, and SV: stop valve.
T. Suzuki et al. / International Journal of Hydrogen Energy 26 (2001) 935–940
937
3. Results and discussion 3.1. Ruthenium distributions in Support-A and -B
Scheme. 2. Flow diagram of CO adsorption instrument. SV: stop valve, PG: pressure gauge, PS: pressure sensor, 6DV: 6 directions valve, RM: rotameter, A: adsorption vessel, and TCD: thermal conductivity detector.
2.5. Catalyst characterization Speci)c surface area and pore volume were measured by means of a Belsorp 28 instrument (Bel Japan Co.) using nitrogen as an adsorbable molecule. The dispersion of Ru can be calculated appropriately on the assumption that CO is adsorbed dominantly with a stoichiometric adsorption rate of CO : Ru = 1 : 1, which is reported in the Refs. [11,12]. The dispersion is represented as [CO(a)]=[Ru]×100 (%), where [CO(a)] is the adsorbed amount of CO and [Ru] is the concentration of Ru in catalyst [11]. [CO(a)] is determined precisely by pulse adsorption method [13–15] and the concentration of [Ru] is obtained by a JXA-8600MX (JEOL Co.) ICP (induced coupled plasma) instrument. Prior to the pulse adsorption, 0.5 g of specimen was set in the ◦ quartz U-tube and it was reduced at 450 C for 3 h. Then ◦ ◦ the specimen was cooled from 450 C to 25 C under Kowing helium. After the pretreatment, CO pulse (0.2 ml) was dosed to the catalyst. The CO pulse was repeated until the adsorption was saturated. The amount of adsorbed CO was obtained by accumulating peak areas of TCD. The CO used was of 99.95% purity (research grade, Takachiho Chemical Co.), and the pulse experiment was carried out by using an Ohkura 6850 adsorption instrument equipped with TCD (Ohkura Riken Co.) . The connection diagram of the instrument is described in Scheme 2. The chemical compositions of Ru, CeO2 , and Al2 O3 in these catalysts were 1.6, 7.5, and 90.9 wt%, respectively. Distribution of ruthenium species in the catalyst was observed by an IRIS Advantage-RP (Jarrell-Ash Co.) EPMA (electron probe micro analysis) instrument.
The speci)c surface area of Support-A is ca. 250 m2 =g, whereas that of Support-B is 0.56 fold in comparison with the case of support-A. In order to obtain further information, the pore volume on the catalysts was con)rmed. The pore volume of Support-B was 0.42 ml=g, whereas that of Support-A was 0.64 ml=g. These observations implied that the texture of the support was improved by adding PVA. When Support-A and Support-B were used for catalyst preparations, dispersion of Ru on Support-A was 64.8% ((CO(a)= 2:4 ml=g-cat (stp)), whereas that of Ru on Support-B was 29.7% ((CO(a)= 1:1 ml=g-cat(stp)) and as a result, the dispersion of Ru on Support-A was increased 2.2 fold in comparison with the case of Support-B. Fig. 1 shows the EPMA results on Ru=Support-A and Ru=Support-B. In the case of Ru=Support-B, ruthenium species preferentially existed in the external zone, while in the case of Ru=Support-A, ruthenium species existed not only in the external zone but also in the internal zone. It is, therefore, considered that the dispersion of ruthenium might be improved on Support-A and that the di@used LPG and steam in the internal zone may react, and as a result an increase in the whole reactivity of the cylindrical shape of the catalyst could be expected. In addition, it was deduced that the porosity of the support might be improved by adding PVA as an estimated model described in Fig. 2. In this section, it has been suggested that the addition of a decomposable component such as PVA can increase the speci)c surface area which in turn could inKuence the dispersion of active metal as well. 3.2. Con4rmation of catalytic performance on Ru=Support-A and -B Comparing the catalytic performance of pre-reforming on Ru=Support-A and Ru=Support-B, these catalysts were ◦ subjected to the reaction at 450 C with S=C = 0:8 under 2 8 kg=cm and GHSV was set at 1500 vol=vol=h1 . When the Ru=Support-B was used for the pre-reforming, the conversion of LPG was calculated by using Eq. (1) and it was 85.2%, whereas when using the Ru=Support-A, LPG reacted completely as shown in Table 1. Conversion of LPG (%) = 100 × [C] in CO + CO2 + CH4 =[C] in LPG:
(1)
Here, one can suspect that the lower conversion observed on Ru=Support-B would be caused by carbon formation. It is anticipated that the carbon deposition might take place readily in the reaction conditions as stated in the beginning. In order to clarify the di@erence of the amount in the deposited carbon on these catalysts, they were subjected to
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T. Suzuki et al. / International Journal of Hydrogen Energy 26 (2001) 935–940
Fig. 1. The EPMA image of Ru dispersed on Support-A and -B.
Fig. 2. Estimated model to form pores during the calcining. (•) oxides, (◦) decomposable component (PVA).
carbon analysis by using carbon analyzer (Model EMIA-110, Hitachi-Horiba Co.). The amount of deposited carbon was determined on the basis of the amount of carbon dioxide generated in a micro electric furnace with pure oxygen. The deposited carbon at 100 h was ca. 0.1 g per g-catalyst on Ru=Support-A and -B. Thus the deposited carbon is not so much and the signi)cant di@erence with regard to carbon deposition was not observed on those catalysts. In addition, product distributions were almost analogous in the two catalysts (Table 1). It is inferred from these results that the function of active sites on Ru=Support-A and -B were the same but the unreacted LPG in Ru=Support-B might be due to an insu?ciency of active sites on the catalyst. Thus,
the feed was converted completely on the texture improved catalyst (Ru=Support-A) which may have su?cient active sites. As stated above, the pre-reforming reaction was successfully performed on the Ru=Support-A with lower S=C such ◦ as 0.8 around 450 C. These results led us to the preliminary sustained run to con)rm the catalyst performance. 3.3. Preliminary sustained run In order to con)rm the pre-reforming performance of Ru=Support-A, the catalyst was subjected to the sustained ◦ run for 7000 h at 450 C under 8 kg=cm2 in the micro reactor
T. Suzuki et al. / International Journal of Hydrogen Energy 26 (2001) 935–940
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Table 1 Conversion of butane and product distributions in low temperature steam reforminga Time (h)
Catalyst
S=C
Conv. of butane (%)
Product distributions (%) H2 CO CH4
CO2
100 100
Ru=Support-A Ru=Support-B
0.8 0.8
100 85.2
9.6 9.3
16.8 17.8
a Catalyst:
0.4 0.3
73.2 72.6
◦
7 ml (6.5 g), GHSV : 1500 v=v=h1 , Press.: 8 kg=cm2 , Temp.: 450 C.
Fig. 3. Catalyst life test of Ru=Support-A for pre-reforming. (?) conversion of LPG, (•) methane, (4) carbon dioxide, () hydrogen, and ◦ (5) carbon monoxide. S=C = 0:8 (at initial stage S=C = 1:0), GHSV = 1500, reaction temperature= 450 C, and pressure= 8 kg=cm2 .
as seen in Scheme 1. Fig. 3 shows the conversion of LPG, and the composition of products during the pre-reforming. Evaluating the composition change in the products versus the S=C value, the S=C was set at unity in the initial stage. The compositions of H2 , CO, CH4 , and CO2 in that condition were 13.5, 0.3, 71.9, and 14.3 vol%, respectively. After 1100 h, feeding rate of steam was decreased to maintain S=C = 0:8. The compositions of H2 , CO, CH4 , and CO2 were 10.9, 0.3, 74.9, and 13.9 vol%, respectively. Thus, methane was formed preferentially at lower S=C proportion. Unreacted LPG was not detected at all by means of FID-GC until 2500 h; after this period, an in)nitesimal amount of butane was observed at outlet. Finally, unreacted LPG (ca. 0.3 vol% of butane) was observed after the elapse of 7000 h. However, it was observed that such a small amount of LPG was converted completely by increasing the temperature ◦ within 10 C. So far, the S=C was maintained at a higher ratio in the conventional Ni based catalyst from the viewpoint of preventing the carbon formations. The developed catalyst shows a stable performance even in lower S=C such as 0.8 for a long term. These results indicate that the developed Ru=CeO2 – Alumina catalyst may have potential for commercial applications.
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