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Energy & Fuels 2006, 20, 748-753
Production of COx-Free Hydrogen from Biomass and NaOH Mixture: Effect of Catalysts Minoru Ishida, Sakae Takenaka, Ichiro Yamanaka, and Kiyoshi Otsuka* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed September 1, 2005. ReVised Manuscript ReceiVed January 20, 2006
Hydrogen without CO and CO2 was produced through the reactions of cellulose with NaOH and water at 473-773 K according to the following overall stoichiometric reaction: (C6H10O5) + 12NaOH + H2O ) 6Na2CO3 + 12H2. Cellulose reacted with NaOH and water to produce hydrogen, Na2CO3, and a small amount of methane as the byproduct. The total yield of hydrogen obtained through the reaction at a temperature range 473-773 K was estimated to be 62% on the basis of the stoichiometric equation. The formation of hydrogen was specifically enhanced by the addition of Ni, Co, Rh, or Ru catalyst supported on Al2O3 to the mixture of cellulose and NaOH, and the total yields of hydrogen were dramatically improved to almost 100%. Among the many catalysts tested, Ni/TiO2 and Ni/Cr2O3 were promising catalysts which maintained catalytic activity during repeated reactions, keeping the hydrogen yield almost 100%.
Introduction Hydrogen energy is expected to be utilized in a H2/O2 fuel cell, because the cell can convert chemical energy of hydrogen and oxygen directly into electricity with high conversion efficiency without emission of any pollutant gases such as COx, NOx, or SOx. Among various types of fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) is most suitable for portable, automobile, and on-site applications due to low operational temperature, high power density, quick start-up, and rapid response to local changes. Industrial production of hydrogen has been performed through the steam reforming of natural gas and petroleum or by gasification of coal at temperatures >1000 K, followed by water gas shift reaction of CO.1,2 However, a huge quantity of CO2, the main greenhouse effect gas, is emitted into the atmosphere in these processes. In addition, the hydrogen formed by these processes cannot be supplied directly into the PEMFCs, because the hydrogen contains 2-3% of CO that strongly poisons Pt electrodes in PEMFCs. Thus, the concentration of CO has to be lowered to less than 20 ppm by the pressure-swing-absorption technique or by the selective oxidation of CO. These processes for CO removal make the reformer bulky and expensive. The second set of problems for practical use of the fuel cell systems consists of how to transport and store hydrogen. The methods for the storage of hydrogen such as high-pressure cylinder and hydrogen storage alloy3,4 are proposed. However, these methods need the consideration of issues concerning energy density, size of the system, the cost for the storage, and so on. As described above, the current technologies for the * Corresponding author. Tel: & Fax: +81-3-5734-2144. E-mail:
[email protected]. (1) Pena, M. A.; Gomez, J. P.; Fierro, J. L. G. Appl. Catal., A 1996, 144, 7. (2) Armor, J. N. Appl. Catal., A 1999, 176, 159. (3) Aoki, K. Denki Seiko 2001, 72 (4), 247. (4) Suzuki, R.; Tatemoto, K.; Kitagawa, H. J. Alloys Compd. 2004, 385, 173.
production and storage of hydrogen are still far from satisfactory from the point of view of energy density, safe handling, size of the system, and the cost for the purification of hydrogen. Therefore, an innovative method for the production of hydrogen without CO or CO2 is desired to be established. Biomass is an important source of energy and the most important fuel worldwide after exhaustion of coal, petroleum, and natural gas, because biomass is a renewable energy resource synthesized from solar energy, carbon dioxide, and water. In addition, biomass consists of numerous and cheap sources widely distributed on the earth. It is derived from wood, cotton, paper, agricultural crops, major parts of household waste, and so on. Moreover, biomass does not add carbon dioxide to the atmosphere as it absorbs the same amount of carbon in growing as it releases when consumed as a fuel. For the reasons mentioned above, it is necessary to utilize biomass as an energy resource. Tomishige et al. proposed a method for the production of synthesis gas (H2 + CO) by the catalytic gasification of biomass.5-7 They demonstrated the gasification of various biomasses (cellulose, cedar wood, rice straw, jute stick, and bagasse) using a highly efficient Rh/CeO2/SiO2 catalyst at low temperatures (823-973 K), and they succeeded in that 98-99% of the carbon in the biomass was converted to the gas products at 873 K. The synthesis gas produced by the gasification of biomass can be used in the gas turbine for power generation or can be catalytically converted to many chemicals (methanol, dimethyl ether, Fischer-Tropsch oils, and so on). Recently, several methods for one-step production of pure hydrogen from carbon or biomass were proposed. Saxena proposed the formation of hydrogen through the reactions of (5) Asadullah, M.; Ito, S.; Kunimori, K.; Tomishige, K. Ind. Eng. Chem. Res. 2002, 41, 4567. (6) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Tomishige, K. Appl. Catal., A 2003, 246, 103. (7) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. Appl. Catal., A 2004, 267, 95.
10.1021/ef050282u CCC: $33.50 © 2006 American Chemical Society Published on Web 02/17/2006
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Energy & Fuels, Vol. 20, No. 2, 2006 749
carbon materials with NaOH in the presence of water vapor.8
C + 2NaOH + H2O ) 2H2 + Na2CO3 In this study, she demonstrated thermodynamics of hydrogen formation through the reactions of carbon with NaOH and water. Lin et al. examined a method for production of hydrogen through the reactions of organic materials (coal, wood) with water and CaO at 873-1023 K under high-pressure steam (4.2 MPa).9-11 In this method, organic materials are gasified to form H2 and CO2. Ca(OH)2 absorbs CO2 to form pure hydrogen.
C + H2O ) CO + H2 CO + H2O ) CO2 + H2
of eqs 1-3 were 40-60%. In addition, a small amount of methane was coproduced during the reactions of all the biomasses used in the previous studies. The formation of methane should be suppressed because formation of methane decreased the yield of hydrogen. In this study, we examined the effects of addition of several metal catalysts supported on various supports into the mixtures of cellulose and NaOH in order to increase the yield and the formation rate of hydrogen through the reactions of cellulose with water vapor and alkali metal hydroxides. The goals in this work are (1) to find effective catalysts for the production of hydrogen and (2) to find stable catalysts which have strong durability in repeated uses for the reactions. We will discuss the difference of the catalytic activity and durability for the formation of hydrogen during the reactions.
CaO + H2O ) Ca(OH)2 Experimental Section
Ca(OH)2 + CO2 ) CaCO3 + H2O However, the content percentages of CH4, CO2, and CO in the produced hydrogen were still far from satisfactory for the direct supply of the hydrogen to PEMFCs. Recently, we proposed a new method for the synthesis of hydrogen without CO or CO2 for PEMFCs through the reactions of biomasses (cellulose, sucrose, glucose, starch, cotton, or Japanese paper) with alkali metal hydroxides (NaOH, KOH, or RbOH) and water vapor at relatively low temperatures (473623 K) under atmospheric pressure.12 Biomass can be one of the energy sources substituted for coal, petroleum, and natural gas. In our method, biomass consisting of numerous and cheap sources widely distributed on the earth can be utilized as raw materials for the production of hydrogen. For example, cellulose[(C6H10O5)n], one of the main components of trees, grasses, and cotton, reacts with NaOH in the presence of water vapor to produce COx-free hydrogen and Na2CO3 according to eq 1:
C6H10O5 + 12NaOH + H2O ) 6Na2CO3 + 12H2 (1) The reactions of D-glucose[(C6H12O6)] and sucrose[(C12H22O11)] with NaOH and water vapor may be written as follows:
C6H12O6 + 12NaOH ) 6Na2CO3 + 12H2 C12H22O11 + 24NaOH + H2O ) 12Na2CO3 + 24H2
(2) (3)
The Gibbs free energy changes for the reactions of eqs 2 and 3 at 573 K are about -1220 kJ/mol-glucose (-101 kJ/mol-H2) and -2450 kJ/mol-sucrose (-102 kJ/ mol-H2), respectively.13 Therefore, the reactions of eqs 1-3 are thermodynamically allowed under the experimental conditions in this work. Na2CO3, the byproduct in eq 1, is one of the most important chemicals for the production of glass, soap, medicine, and so on. In the previous report, we reported the hydrogen formation through the reactions of cellulose, D-glucose, sucrose, and starch with NaOH and water at 473-623 K. These reactions produced hydrogen without COx, but the yields of hydrogen on the basis (8) Saxena, S. K. Int. J. Hydrogen Energy 2003, 28, 49. (9) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Energy ConVers. Manage. 2002, 43, 1283. (10) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Energy Fuels 2004, 18, 1014. (11) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2004, 83, 869. (12) Ishida, M.; Otsuka, K.; Takenaka, S.; Yamanaka, I. J. Chem. Technol. Biotechnol. 2005, 80, 281. (13) HSC Chemistry 4.1, Chemical Reaction and Equilibrium Software with Extensive Thermochemical Database, Outokumpu Research Oy, Finland, 1999.
The supported Ni, Co, Fe, Cu, Rh, Ru, Pd, and Pt catalysts used in this study were prepared by a conventional impregnation method. Ni(NO3)2‚6H2O, Co(NO3)2‚6H2O, Fe(NO3)3‚9H2O, Cu(NO3)2‚ 3H2O, RhCl3, RuCl3, H2PdCl4, and H2PtCl6 were utilized as the catalyst metal sources. Al2O3 (JRC-ALO8, specific surface area ) 148 m2 g-1), SiO2 (Cab-O-Sil supplied from Cabot Co., specific surface area ) 190 m2 g-1), TiO2 (JRC-TIO4, specific surface area ) 40 m2 g-1), ZrO2 (Wako Pure Chem. Ind., Ltd.; specific surface area ) 10 m2 g-1), Cr2O3 (Wako Pure Chem. Ind., Ltd.; specific surface area ) 1 m2 g-1) or CeO2 (Wako Pure Chem. Ind., Ltd.; specific surface area ) 3 m2 g-1) were utilized as catalytic supports. JRC-ALO8 and JRC-TIO4 were supplied from the Catalytic Society of Japan as reference catalysts. The catalytic supports were impregnated with an aqueous solution containing metal cations at about 353 K. The solvent was evaporated to dryness at about 353 K. The dried samples were calcined at 873 K for 5 h in air, followed by reduction with hydrogen at 773 K for 1 h except for Pt/Al2O3 which was reduced with hydrogen at 573 K for 1 h. The loading of metal on the supports was adjusted to 20 wt %. The reactions of cellulose with NaOH and water vapor were carried out with a conventional mass-controlled gas flow system under atmospheric pressure. Prior to the experiments, cellulose (Aldrich, high purity, 0.45 g) was mixed uniformly with an aqueous solution of NaOH (Wako Pure Chem. Ind., Ltd.; special grade) of 50 wt %. The amounts of cellulose (based on (C6H10O5) unit) and NaOH were adjusted to 2.78 and 33.3 mmol, respectively. In the case of the reactions of cellulose with NaOH in the presence of catalysts, the catalysts (0.18 g) were physically mixed with cellulose (0.45 g) and an aqueous solution of NaOH. The mixtures were mounted on an alumina boat which was placed at the center in a cylindrical stainless steel reactor (length 450 mm, inner diameter 21 mm). Air in the reactor was flushed out with Ar flow before the reaction. The temperature of the reactor was raised to 373 K with an electric furnace, and then the samples were dried at 373 K for 20 min under Ar flow. Then, water vapor (19.2 kPa) carried with Ar flow was passed through the samples. The flow rates of water vapor and Ar were 9.4 and 40.0 mL(STP) min-1, respectively, under 101 kPa total pressure. The temperature at the sample was monitored with a thermocouple and controlled within an error of (1 K. The temperature was raised linearly with reaction time from 373 to 773 K at a rate of 1.9 K min-1. During the reaction, a part of effluent gases from the reactor was sampled out and analyzed by a gas chromatograph. The detection limit of CO and CO2 in the effluent gas from the reactor was about 30 ppm in the present studies.
Results and Discussion Reactions over Various Metal Catalysts Supported on Al2O3. Figure 1 shows the changes of the formation rates of hydrogen and methane with reaction temperature which was
750 Energy & Fuels, Vol. 20, No. 2, 2006
Figure 1. Changes of the formation rates of H2 and CH4 during the reactions of cellulose with NaOH and H2O.
Figure 2. Changes of the formation rates of H2 and CH4 during the reactions of cellulose with NaOH and H2O in the presence of transition metal catalysts supported on Al2O3.
increased linearly with time on stream during the reactions of cellulose with NaOH in the presence of water vapor [P(H2O) ) 19.2 kPa]. Hydrogen was formed at above 473 K, and the formation rates of hydrogen showed several peaks as shown in Figure 1. Methane was formed at above 623 K, but neither CO nor CO2 was observed at all during the reaction. The formation of hydrogen and that of methane terminated at about 750 K. After the reaction in Figure 1, residues remaining in the alumina boat were analyzed by X-ray diffraction (XRD) measurements. The XRD spectra showed that most of the NaOH in the alumina boat was converted into Na2CO3. Therefore, the reactions of cellulose with NaOH and water vapor may proceed according to eq 1. The total amount of hydrogen formed was estimated by integration of the formation rates of hydrogen against time in Figure 1. The yield of hydrogen was estimated to be 62% of the theoretical value expected from eq 1. The low yield of hydrogen can be caused partly by the formation of methane. In addition, some products (trace amounts of ethane, propane, benzene, cyclohexane, 2-pentanone, and 3-pentanone) from cellulose would not react with NaOH and water to form hydrogen. To improve the hydrogen yield, we examined the effect of catalysts added into the mixtures of cellulose and NaOH. Figure 2 shows the changes of the formation rates of hydrogen and methane with time on stream during the reactions of cellulose containing transition metals (Ni, Co, Fe, or Cu) catalysts
Ishida et al.
Figure 3. Changes of the formation rates of H2 and CH4 during the reactions of cellulose with NaOH and H2O in the presence of precious metal catalysts supported on Al2O3.
supported on Al2O3 and NaOH in the presence of water vapor [P(H2O) ) 19.2 kPa] from 373 to 773 K. The transition metal catalysts supported on Al2O3 (0.18 g) were physically mixed with cellulose (0.45 g). The loading amount of these metals was adjusted to 20 wt %. The result in the absence of catalyst was also shown in Figure 2 for comparison. The formation rates of hydrogen in the presence of Ni/Al2O3 and Co/Al2O3 increased from 500 K and reached the maximum at around 523 K. In the case of the reaction without catalyst, the formation rates of hydrogen also increased from 500 K and reached the maximum at around 523 K. However, the formation rates of hydrogen in the presence of Ni/Al2O3 and Co/Al2O3 were significantly higher than those in the absence of catalyst. In addition, the second peak of the formation rates of hydrogen appeared at around 550 K during the reactions in the presence of Ni/Al2O3 and Co/ Al2O3, while the second peak was observed at around 600 K during the reactions in the absence of catalyst. These results suggested that Ni/Al2O3 and Co/Al2O3 promoted hydrogen formation at low temperatures. As described earlier, methane was coproduced during the reactions in the absence of catalyst. It should be noted that a trace of methane was produced during the reactions in the presence of Ni/ Al2O3 and Co/Al2O3. Thus, the addition of Ni/Al2O3 and Co/Al2O3 into cellulose was quite effective for the production of pure hydrogen. The addition of Cu/Al2O3 and Fe/Al2O3 into cellulose also increased the formation rates of hydrogen at around 500 K. However, the increment of the formation rates of hydrogen by the addition of Cu/Al2O3 and Fe/Al2O3 was not so large as that by the addition of Ni/ Al2O3 and Co/Al2O3. In addition, the formation of methane was not suppressed by the addition of Cu/Al2O3 and Fe/Al2O3. Figure 3 shows the changes of the formation rates of hydrogen with time on stream during the reactions of cellulose added with precious metal catalysts (Rh, Ru, Pt, or Pd supported on Al2O3) and NaOH in the presence of water vapor from 373 to 773 K. The precious metal catalysts supported on Al2O3 (0.18 g) were physically mixed with cellulose (0.45 g). The loading amount of these metals was adjusted to 20 wt %. The addition of these metals supported on Al2O3 dramatically enhanced the formation rates of hydrogen, especially by the addition of Rh/Al2O3, Pd/ Al2O3, or Ru/Al2O3. The formation rates of hydrogen at above 523 K increased approximately twice or more by the addition of these catalysts. The formation rates of hydrogen at around 500 K in the presence of Rh/Al2O3, Pd/Al2O3, and Ru/Al2O3
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Energy & Fuels, Vol. 20, No. 2, 2006 751
Figure 4. Hydrogen yields through the reactions of cellulose with NaOH and H2O in the presence of various metal catalysts supported on Al2O3.
were higher than those in the presence of Ni/Al2O3 and Co/ Al2O3. In addition, the formation rates of methane during the reactions in the presence of Rh/Al2O3 and Ru/Al2O3 were very low, while methane was formed at 650-700 K in the absence of catalyst and in the presence of Pt/Al2O3 and Pd/Al2O3. These results suggest that Rh/Al2O3 and Ru/Al2O3 are also quite effective catalysts for hydrogen formation through the reactions of cellulose with NaOH and water vapor. Figure 4 shows the total yields of hydrogen through the reactions of cellulose with NaOH and water vapor in the addition of various metal catalysts supported on Al2O3. The yields were calculated from the results in Figures 1-3 by assuming eq 1. In the case of the reactions of cellulose and NaOH without catalyst, the yield of hydrogen was 62%. The addition of Rh/ Al2O3, Ru/Al2O3, Ni/Al2O3, or Co/Al2O3 to the mixtures of cellulose and NaOH improved the yields of hydrogen dramatically. The hydrogen yields for these catalysts were estimated to be 100% within an experimental error of (7%. These results strongly support that the overall reaction of cellulose with NaOH and water vapor occurs according to eq 1 if methane was not formed during the reaction. The yields of hydrogen were estimated to be 85% for Pd/Al2O3, 78% for Pt/Al2O3, 74% for Cu/Al2O3, and 70% for Fe/Al2O3, respectively. The total yields of hydrogen became higher in the order of Rh/Al2O3, Ni/Al2O3, Ru/Al2O3, Co/Al2O3 > Pd/Al2O3 > Pt/Al2O3 > Cu/Al2O3 > Fe/Al2O3. The addition of Rh, Ni, Ru, or Co catalysts decreased the formation of methane and increased the formation of hydrogen. We speculate that Ni, Co, Rh, and Ru catalysts promote cleavage of C-H bonds of cellulose derivatives, reaction intermediates, and desorption of H species as H2 to the gas phase. Therefore, the methane formation was significantly suppressed and the hydrogen formation was accelerated at low temperatures. Reactions over Ni Catalysts Supported on Various Supports. Figure 5 shows the changes of the formation rates of hydrogen with time on stream during the reactions of cellulose with NaOH and water vapor in the presence of Ni catalysts supported on various supports (Al2O3, TiO2, ZrO2, SiO2, Cr2O3, or CeO2). The supported Ni catalysts (0.18 g) were physically mixed with cellulose (0.45 g). The loading amount of Ni was adjusted to 20 wt %. The addition of Ni/Al2O3, Ni/TiO2, and Ni/ZrO2 enhanced the formation rates of hydrogen, especially in the temperature range 523-573 K. Besides the catalysts shown in Figure 5, Ni/Cr2O3 and Ni/CeO2 enhanced the hydrogen formation with an extent similar to that of Ni/ ZrO2. On the other hand, the addition of Ni/SiO2 did not affect the formation rates of hydrogen in the temperature range 523-550
Figure 5. Changes of the formation rates of H2 and CH4 during the reactions of cellulose with NaOH and H2O in the presence of various Ni catalysts.
Figure 6. Effects of re-use of Ni/ZrO2 catalyst on the formation rates of H2 and CH4 during the reactions of cellulose with NaOH and H2O.
K, but the catalyst improved the hydrogen formation at above 550 K. The formation of methane was specifically suppressed by the addition of Ni/Al2O3, Ni/TiO2, and Ni/ZrO2. In the case of Ni/ SiO2, a small amount of methane was formed at above 570 K. The total yields of hydrogen were estimated to be almost 100% for all catalysts. To examine the durability of Ni catalysts, the reactions were performed two or three times. Ni catalysts supported on Al2O3, ZrO2, CeO2, TiO2, and Cr2O3 were utilized. The catalyst (0.18 g) which had been used in the first run was collected by filtration, and it was physically mixed with cellulose (0.45 g) again. The mixtures of cellulose with the catalyst were mixed uniformly with an aqueous solution of NaOH of 50 wt %. Figure 6 shows the effects of reuse of Ni/ZrO2 catalyst on the formation rates of hydrogen and methane during the reactions of cellulose with NaOH and water vapor. At the first run in the presence of Ni/ZrO2 catalyst, the maxima peaks for hydrogen formation were observed at 520, 560, and 630 K. At the second run for the same catalyst, three peaks for hydrogen formation were observed, but the positions of peaks were changed, that is, the second and third peaks were positioned at 570 and 670 K while the first peak was not shifted. At the third run, the third peak was observed at around 700 K. In addition, the first peak at around 520 K became smaller gradually with
752 Energy & Fuels, Vol. 20, No. 2, 2006
Ishida et al. Table 2. Change in the Average Size of Ni Crystallites on Various Ni Catalysts average Ni crystallite size/nm
a
Figure 7. Effects of re-use of Ni/TiO2 catalyst on the formation rates of H2 and CH4 during the reactions of cellulose with NaOH and H2O. Table 1. Effects of Re-use of Ni Catalysts on the Yields of H2 H2 yield in each run/% catalyst
first
second
third
Ni/ZrO2 Ni/Al2O3 Ni/CeO2 Ni/TiO2 Ni/Cr2O3
107 98 103 101 102
93 90 85 98 100
73
102
repeated runs. These results indicated that Ni/ZrO2 catalyst was deactivated gradually for hydrogen formation through the reactions of cellulose with NaOH and water vapor. Figure 7 shows the effects of reuse of Ni/TiO2 catalyst on the formation rates of hydrogen and methane during the reaction of cellulose with NaOH and water vapor. Three peaks for hydrogen formation were observed at 520, 550, and 620 K in the first run experiment with Ni/TiO2. After the first run, three peaks for hydrogen formation were always observed. The position of the first peak did not change from the first to the third run, although this peak became smaller gradually with repeated runs. On the other hand, the second and the third peaks were shifted to higher temperatures gradually with repeated runs. These results indicated that Ni/TiO2 catalyst was also deactivated for hydrogen formation. However, it should be noted that the deactivation of Ni/TiO2 with repeated runs was slower than that of Ni/ZrO2. Besides Ni/ZrO2 and Ni/TiO2, Ni/Al2O3, Ni/CeO2, and Ni/ Cr2O3 were utilized repeatedly for the reactions (Table 1). Changes in hydrogen yields with repeated runs depended on the types of catalytic supports. As shown in Table 1, the hydrogen yields for Ni/ZrO2, Ni/Al2O3, and Ni/CeO2 were decreased gradually with repeated runs. On the other hand, the hydrogen yields for Ni/TiO2 and Ni/Cr2O3 were almost 100% and did not change with repeated runs. These results showed that Ni/TiO2 and Ni/Cr2O3 were suitable catalysts for repeated reactions of cellulose with NaOH and water vapor. We considered a few reasons for the deactivation of the catalysts during repeated cycles, as below. First, we speculated that the deactivation of catalyst was due to the change of Ni metal particle size on various supports. Average sizes of Ni crystallites on various supports were estimated by XRD analysis with the Scherrer equation before and after the reaction in Table 2. Crystallite sizes of Ni of fresh catalysts were about 20 nm except for Ni/Al2O3 (11 nm) and Ni/CeO2 (39 nm). After
catalyst
fresha
after the reaction
Ni/ZrO2 Ni/Al2O3 Ni/CeO2 Ni/SiO2 Ni/TiO2 Ni/Cr2O3
20.6 10.7 39.2 23.3 20.4 22.7
37.3 31.1 37.3 35.8 35.7 31.7
Reduction with H2 at 773 K for 1 h.
the reaction, crystallite sizes of Ni were about 35 nm of all catalysts. We could not observe significant differences in the crystallite sizes after the reaction. Therefore, we could not explain the differences in the stability of the catalysts by Ni metal particle size. Second, we speculated that the reason for deactivation of the catalyst was carbon deposition over Ni during the reaction. If this model was true, catalytic activity would revive by oxidation with air. Therefore, we calcined used Ni/ZrO2 at 423 K for 3 h and 873 K for 5 h in air, and reduced with H2 at 773 K for 1 h. However, hydrogen yield of the reaction by the revived Ni/ ZrO2 catalyst was the same as that of the deactivated one. This experimental fact suggested that carbon deposition on Ni was not the reason for the deactivation of catalyst. We conjectured that the stability of the catalysts would be associated with the stability of Ni metal particles on the various supports. ZrO2, Al2O3, and SiO2 would react with NaOH, forming sodium zirconate, sodium aluminate, and sodium silicate, respectively. The catalytic activity of the supported Ni catalysts may decrease because Ni metal particles may be left out from these supports and/or be buried in the bulk of these supports. On the other hand, Cr2O3 is resistant to NaOH; therefore, the Ni metal supported on Cr2O3 would be stable. Because of the high stability of Ni/Cr2O3, the catalytic activities of Ni/Cr2O3 would be kept high through the 1-2 cycles. However, these assumptions do not agree with the results in the case of the TiO2 support. TiO2 is susceptible to reaction with NaOH, forming sodium titanate. If the assumptions which are described above are right, the catalytic activity and durability of Ni/TiO2 may decline such as ZrO2, Al2O3, or SiO2. However, the catalytic activities of Ni/TiO2 were kept high through the 1-3 cycles. We could not explain the stability of Ni/TiO2 at this moment. To make clear the role of catalysts and catalytic supports in the reactions of cellulose, NaOH, and water vapor, further studies are definitely needed. Conclusion We concluded as follows on the basis of the results described above. (1) One-step production of pure hydrogen without CO or CO2 was possible through the reactions of cellulose with NaOH and water vapor at 473-623 K. Other components of biomass such as sucrose, glucose, starch, cotton, or Japanese paper could be used as raw materials. The overall reaction of cellulose with NaOH and water vapor to produce hydrogen would be expressed as (C6H10O5) + 12NaOH + H2O ) 6Na2CO3 + 12H2. (2) The addition of Ni, Co, Rh, or Ru catalysts supported on Al2O3 specifically enhanced the formation of hydrogen at
