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Energy & Fuels 2004, 18, 820-829
Storage and Supply of Pure Hydrogen from Methane Mediated by Modified Iron Oxides Sakae Takenaka,* Van Tho Dinh Son, 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 December 10, 2003
Storage and supply of pure hydrogen through the reduction of Fe3O4 with methane and the subsequent oxidation of the formed iron metal with water vapor were demonstrated. The modification of iron oxides with Cr cations prevented the sintering of Fe3O4/Fe particles during the redox, whereas the mediator without Cr cations was deactivated quickly, because of the sintering. The addition of Cu species into the iron oxides modified with Cr cations enhanced both the reduction with methane and the oxidation with water vapor at lower temperatures. The iron oxides modified with Cu and Cr (denoted as Cu-Cr-FeOx) can store and supply pure hydrogen repeatedly through the reduction with methane and the subsequent oxidation with water vapor. During the redox of Cu-Cr-FeOx, the Cu species were always present as copper metal and Cr cations were always stabilized on the B-sites in ferrite CrxFe3-xO4. Copper metal in the Cu-Cr-FeOx must activate methane in the reduction and water in the oxidation. The CrxFe3-xO4 ferrite in the Cu-Cr-FeOx might prevent the sintering of iron metal particles by blocking their contacts with each other.
1. Introduction Hydrogen is expected to become an important energy carrier for sustainable energy consumption with a reduced impact on the environment. A hydrogen-based energy system is regarded as a viable and advantageous option for delivering high-quality energy services. In particular, hydrogen should be used as a fuel for H2O2 fuel cells due to high conversion efficiency of chemical energy of hydrogen into electricity without emission of any pollutant gases. However, one of the major obstacles to using hydrogen as an energy carrier is the lack of safe, efficient, and low-cost storage systems that are suitable for various stationary and mobile applications. With regard to this problem, we have proposed a simple, safe, and environmentally benign technology for the storage and supply of hydrogen to fuel cells.1,2 Our technology is based on a redox reaction of magnetite, as given in reaction 1:
Fe3O4 + 4H2 T 3Fe + 4H2O
(1)
In this reaction, magnetite (Fe3O4) is reduced by hydrogen to iron metal. The recovery and the supply of hydrogen can be accomplished via the reverse reaction, i.e., the reoxidation of iron metal with water vapor. Using this method, one mole of iron can store and regenerate 1.33 moles of H2, which corresponds to 4.8 wt % of iron metal. Because the iron metal is stable at * Author to whom correspondence should be addressed. Telephone: 81-3-5734-2626. FAX: 81-3-5734-2879. E-mail: stakenak@ o.cc.titech.ac.jp. (1) Otsuka, K.; Mito, A.; Takenaka, S.; Yamanaka, I. Int. J. Hydrogen Energy 2001, 26, 191-194. (2) Otsuka, K.; Mito, A.; Takenaka, S.; Yamanaka, I. Stud. Surf. Sci. Catal. 2001, 136, 215-220.
room temperature, the method is very safe for the storage of hydrogen. In addition, iron metal or iron oxide is easily available and a very inexpensive material. Our technology for the storage and the supply of hydrogen via the redox reaction between Fe3O4 and iron metal would stand on the condition that we could get inexpensive and pure hydrogen to reduce Fe3O4 to iron metal. At present, hydrogen is produced through the steam reforming of methane, which is a main component of natural gas, followed by a water gas-shift reaction of CO. The steam reforming of methane requires temperatures of >1000 K, as well as a large amount of energy input, because it is a highly endothermic reaction. Moreover, the hydrogen from the processes contains ca. 20 vol % of CO2 and an intolerable impurity of CO (1-2 vol %); thus, further purification is usually indispensable for the supply of the hydrogen to fuel cells. If Fe3O4 can be reduced directly with methane instead of hydrogen, our technology could be more attractive from the standpoint of the storage and the supply of pure hydrogen from natural gas as the primary energy source. The principle of our technology is analogous to the old steam iron process that was extensively studied by the group at the Institute of Gas Technology for the production of hydrogen-rich fuel gas at high temperatures (>1073 K) and pressures (>70 bar), using a gas mixture of CO, CO2, and H2 from coal and biomass for the reduction of iron ores.3,4 In addition, Steinfeld proposed the sequential production of synthesis gas and (3) Tarman, P. B.; Punwani, D. V. Proc. Intersoc. Energy Convers. Eng. Conf. 1976, 11, 286-293. (4) Tarman, P. B.; Biljetina, R. Coal Process. Technol. 1979, 5, 114116.
10.1021/ef030188i CCC: $27.50 © 2004 American Chemical Society Published on Web 04/10/2004
Pure Hydrogen from Methane Mediated by Iron Oxides
hydrogen via the reduction of Fe3O4 with methane (4CH4 + Fe3O4 f 3Fe + 8H2 + 4CO) and the subsequent oxidation of iron metal with water vapor (4H2O + 3Fe f 4H2 + Fe3O4).5 They assumed that the energy needed for the reduction and oxidation of iron samples should be supplied from concentrated solar infrared radiation. Tamaura et al.6 and Kodama et al.7 proposed a similar process that utilized the redox reaction of metal oxides under an irradiation of concentrated solar energy. However, these processes must be performed at temperatures higher than1000 K, because the formation of synthesis gas from Fe3O4 and methane is favorable at temperatures higher than1000 K, according to the thermodynamic equilibrium. Generally speaking, the sintering of iron species during the redox reaction occurred more easily as the reaction temperatures increased. The redox between Fe3O4 and iron metal for the storage and supply of hydrogen based on methane should be performed at temperatures as low as possible, from an economical viewpoint. We intended to convert methane to CO2 and H2O, instead of synthesis gas, during the reduction of Fe3O4 with methane. In the present study, we demonstrate the possibility of a new technology for the storage and supply of hydrogen from methane as the primary energy source, i.e., magnetite is reduced with methane to iron metal and subsequently iron metal is oxidized with water vapor to form hydrogen repeatedly. The favorable effects of metal additives to magnetite on the redox reactions will be investigated and discussed. 2. Experimental Section 2.1. Preparation of Iron Oxides. Iron oxide (Fe2O3) was prepared via the precipitation of an aqueous solution of Fe(NO3)3‚9H2O by the hydrolysis of urea at 363 K. The iron oxide samples modified with metal additives (M) were prepared by coprecipitation methods of a mixed aqueous solution of Fe(NO3)3‚9H2O and the added metal cations by the hydrolysis of urea at 363 K. The precipitates were dried at 373 K for 10 h and calcined at 773 K for 10 h. The amount of metal additives (M) in iron oxides was adjusted to be 5 mol % of total metal cations (M/(M + Fe) ) 0.05). When two types of metal additives (M1 and M2) were added into the iron oxides, the amount of each additive was adjusted to be 5 mol % of total metal cations (M1/(M1 + M2 + Fe) ) M2/(M1 + M2 + Fe) ) 0.05). Hereafter, iron oxide samples without and with a foreign metal (M) were denoted as FeOx and M-FeOx, respectively. 2.2. Redox of Iron Oxides. Reduction of iron oxides with methane and the subsequent oxidation of the reduced iron oxides with water vapor were performed with a conventional gas-flow system with a fixed bed. An iron oxide sample (0.2 g) was packed in a tubular quartz reactor (inner diameter of 1.8 cm and lengths of 40 cm). For the reduction, methane (at a pressure (P) of 101.3 kPa and flow rate (F) of 30 mL/min) was introduced into a tubular reactor at 473 K and the temperature at the reactor increased linearly with time to 1023 K at a rate of 3 K/min. The temperature was maintained at 1023 K until the formation of CO and CO2 could not be observed. After argon was used to purge the methane remaining in the reactor, water vapor that was balanced with argon (P(H2O) ) 18.5 kPa and total flow rate of 70 mL/min) was contacted with the (5) Steinfeld, A.; Kuhn, P.; Karni, J. Energy 1993, 18, 239-249. (6) Tamaura, Y.; Wada, Y.; Yoshida, T.; Tsuji, M. Energy 1997, 22, 337-342. (7) Kodama, T.; Aoki, A.; Shimizu, T.; Kitayama, Y. Energy Fuels 1998, 12, 775-781.
Energy & Fuels, Vol. 18, No. 3, 2004 821 reduced iron oxide samples at 473 K. The temperature of the reactor was increased linearly over time to 823 K at a rate of 4 K/min. The oxidation with water vapor was continued at 823 K until the hydrogen formation could not be observed. Unless otherwise noted, the reduction with methane and the subsequent oxidation with water vapor for the iron oxide samples were conducted repeatedly under the same conditions. During the reactions, some of the effluent gases from the catalyst bed were sampled out and analyzed by gas chromatography (GC). The detection limit of CO and CO2 was ca. 0.1 vol % of methane in the reduction of Fe3O4 and 0.3 vol % of H2 in the reoxidation of iron metal. 2.3. Characterization of Iron Oxides. X-ray absorption spectra (X-ray absorption near-edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS) analyses) were measured on the beam line BL-9A facility at the Photon Factory in the Institute of Materials Structure Science for High Energy Accelerator Research Organization (Tsukuba, Japan) (under Proposal No. 2002G108). Cu or Cr K-edge XANES and EXAFS of Cu-Cr-FeOx were measured in fluorescence mode with a Si(111) two-crystal monochromator at room temperature. The spectra of reference samples (copper foil, Cu2O, CuO, and Cr2O3) were measured in transmission mode at room temperature. Prior to the measurements, Cu-Cr-FeOx samples were reduced with methane at 923 K and subsequently oxidized with water vapor at 773 K. Each sample then was packed in a polyethylene bag under an argon stream. X-ray diffraction (XRD) patterns of iron oxide samples were recorded with a Rigaku RINT 2500V diffractometer, using Cu KR radiation at room temperature.
3. Results and Discussion 3.1. Modification of Iron Oxide with Different Metal Cations. We have reported that, in the redox of the reduction with hydrogen and the subsequent oxidation with water vapor, Al-FeOx and Cr-FeOx produced hydrogen repeatedly with a high reproducibility.8-10 The addition of Al and Cr cations prevented the sintering of Fe species during the repeated redox, whereas FeOx without the additives was deactivated quickly for the redox. In the present study, we examined the effects of these additives to FeOx on the redox of the reduction with methane and the subsequent oxidation with water vapor. The results for the reduction with methane and the subsequent oxidation with water vapor for FeOx, Al-FeOx, and Cr-FeOx are shown in Figures 1 and 2. For the reduction of all the iron oxide samples with methane (Figure 1), the formation of CO and CO2 was observed due to the reduction of iron oxide samples with methane. Although hydrogen could be observed during the reduction with methane, the formation of hydrogen at a time-on-stream value of
