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Energy & Fuels 1999, 13, 1074-1081
Isolation of Oxygen Formed during Catalytic Reduction of Carbon Dioxide Using a Solid Electrolyte Membrane Shigeki Furukawa,* Masaki Okada, and Yohichi Suzuki Department of Industrial Chemistry, College of Industrial Technology, Nihon University, 1-2-1, Izumi-cho, Narashino-shi, Chiba, Japan Received March 10, 1999
We studied the separative recovery of oxygen formed during the hydrogenation of carbon dioxide using a composite system consisting of yttria-stabilized zirconia (YSZ) membrane-equipped Ag electrodes as a pump and a nickel/zeolite catalyst to initiate the hydrogenation of carbon dioxide. The methanation of carbon dioxide proceeded efficiently in the presence of the nickel/zeolite catalyst even at 873 K, the lowest functional temperature at which YSZ has ionic conductivity. Carbon dioxide conversion and methane yield reached 100% and 80%, respectively, at H2/CO2 ) 10, space velocity < 6200 h-1, and 873 K. Carbon dioxide was adsorbed by the crystal structure of the zeolite even at 873 K, and methanation of the adsorbed carbon dioxide (CO2 ad) proceeded by the following one-step reaction: CO2 ad + 3H2 f CH4 + H2O. The rate constant of methanation was estimated to be 1.6 × 10-2 cm3 g-cat-1 s-1 for the pseudo-first-order reaction. Electrochemical isolation of oxygen formed during hydrogenation of carbon dioxide was carried out under galvanostatic conditions. It was assumed that the oxygen was converted to water by the nickel/ zeolite catalyst and then transported through the YSZ electrolyte after the water was ionized. We assumed that the oxygen was not formed by the dissociation of carbon dioxide directly on the cathode electrode. It appeared that the charge-transfer process was the rate-determining step for isolation of oxygen on the cathode electrode in the presence of the nickel/zeolite catalyst.
1. Introduction Global warming due to an increase of carbon dioxide in the atmosphere is a matter of great concern. Emission control of carbon dioxide and other measures are being considered. To alleviate this problem, attempts have been made to reduce the production of carbon dioxide and to reuse carbon dioxide waste. The majority of carbon dioxide is waste produced when fossil fuels are burned to obtain energy. It is meaningless, however, to consume large amounts of energy to convert waste (carbon dioxide) to resource (hydrocarbons). Methods for reusing carbon dioxide should consume as little artificial energy as possible. Methods investigated and recently reported are photo1-4 or electrochemical5,6 reduction, low-temperature catalytic reduction,7-12 and biological reduction.13-15 * Author to whom correspondence should be addressed. (1) Lehn, J. M.; Ziessel, R. Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 701. (2) Hawecker, J.; Lehn, J. M.; Ziessel, R. J. Chem. Soc., Chem. Commun. 1985, 56. (3) Kitamura, N.; Tazuke, S. Chem. Lett. 1983, 1109. (4) Lehn, J. M.; Ziessel, R. J. Organomet. Chem. 1990, 382, 157. (5) Halmann, M. M. Chemical Fixation of Carbon Dioxide; CRC Press: Boca Raton, FL, 1993. (6) Electrochemical and Electrocatalytic Reactions of Carbon Dioxide; Sullivan, B. P., Krist, K., Guard, H. E., Ed.; Elsevier: New York, 1993. (7) Sexton, B. A.; Somorjai, G. A. J. Catal. 46, 167. (8) Luengo, C. A.; Carbera, A. L.; Mackay, H. B.; Maple, M. B. J. Catal. 1997, 47, 1. (9) Inui, T.; Funabiki, M.; Suehiro, M.; Sezume, T. J. Chem. Soc., Faraday Trans. I 1979, 75, 787. (10) Inui, T.; Takeguchi, T.; Kohama, A.; Tanida, K. Energy Convers. Mgmt. 1992, 33, 513. (11) Inui, T.; Takeguchi, T.; Kohama, A.; Kitagawa, K. Proc. 10th Int. Congr. Catal., 1992 (Budapest) 1993, 245. (12) Saito, M. Syokubai 1993, 35, 485. (13) Igarashi, Y.; Kodama, T. Biseibutsu 1989, 5, 525.
Many of these investigations succeeded only in reducing the production of carbon dioxide but not in recovering oxygen from carbon dioxide. The “natural” world has formed a very complicated and well-balanced cyclical system of materials and energy in which there is no terminal waste, whereas there is no cyclical system built into human activities, with the flow typically from resource to waste. These two systems are incompatible in terms of material, quantity, and time. The origin of the environmental problems today is based on the incompatibility between the two systems as described above. To live in harmony with nature, we must create an artificial-materials recycling system whereby waste is converted to resource. Accordingly, carbon dioxide should be recognized as a source of carbon and oxygen. We have created a system to recover the oxygen formed during hydrogenation of carbon dioxide. The system consists of a solidstate electrolyte, which acts as an oxygen-isolating membrane, and a metal-supported zeolite, which acts as the hydrogenating catalyst of carbon dioxide. Yttriastabilized zirconia, which is an oxygen-ion conducting, solid-state electrolyte, has been proposed to have applications as a fuel cell, oxygen sensor, or oxygen pump.16-20 The oxygen pump can be used in various other chemical reactions by applying an electric poten(14) Igarashi, Y. Kagaku to Seibutsu 1992, 30, 394. (15) Nishio, N. Kagaku to Seibutsu 1992, 30, 537. (16) Iwahara, H. PETROTECH 1989, 12 (3), 60. (17) Tagawa, H. J. JSME 1991, 94 (1), 81. (18) Fukutome, A. PETEROTECH 1991, 14 (4), 25. (19) Ouki, M.; Sano, H. Ceramics 1986, 21 (3), 215. (20) Yoshida, F. Ceramics 1985, 21 (3), 183.
10.1021/ef990039t CCC: $18.00 © 1999 American Chemical Society Published on Web 08/14/1999
Isolation of Oxygen Formed during Catalytic Reduction of CO2
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tial across the membrane,21-27 examples of which are oxidative dehydrogenation of ethyl benzene21 and electrolytic reduction of nitrogen oxides (NOx)22 and carbon dioxide.23,24,27 In particular, electrolytic reduction of carbon dioxide is difficult because it must apply a high reduction potential in order to form anionic radicals as intermediates. In this study, we have given attention to constructing a material recycling system which is economical and have attempted to accelerate hydrogenation of carbon dioxide by loading the nickel/zeolite catalyst onto an electrode in an oxygen-pump-type, solid-electrolyte reactor. 2. Experimental Section Preparation of Hydrogenation Catalyst. Hydrous X-type zeolite (Merck Co.) (20 g), 2 mm in diameter, was added to 100 mL of an aqueous solution of 1 M sodium nitrate. The ion-exchange reaction was carried out for 24 h. The zeolite particles were then rinsed with distilled water, dried, and put into 100 mL of an aqueous solution of 0.2-1.0 M nickel nitrate for 24 h. The nickel/ zeolite catalyst thus prepared was rinsed in water, dried at 353 K for 1 day, calcined at 573 K in air for 4 h, and further calcined at 773 K in a hydrogen atmosphere for 2.5 h. For determination of nickel content, we used the dimethylglyoxime assay method (JIS-K-0102). Preparation of the Reactor Tube. An 8 mol % yttria-stabilized zirconia tube (YSZ) (C. C. Co.) with a 10 mm outside diameter, 7 mm inside diameter, and 500 mm length was used as a reactor. The inner and outer surface of YSZ was polished with No. 80 waterproof sandpaper, washed with distilled water and acetone, and dried. The electrode was prepared by applying silver paste (Niraco Co.; Silver paste Product 597) to the outer and inner surfaces of the middle portion of the tube over a length of 100 mm. A platinum wire of 0.5 mm diameter to be used as the lead wire was affixed to the electrode tube. After drying for 24 h at about 353 K, the electrode was calcined for 10 min at 973 K. A schematic diagram of the reactor tube is shown in Figure 1(a). The reactor tube was then inserted into a branched quartz tube with an outer diameter of 20 mm and length of 400 mm. Thus, two chambers were created, allowing gaseous material to flow in two separate currents (Figure 1b). Loading the Catalyst. The nickel/zeolite catalyst was loaded into the reaction chamber located between the YSZ tube and the quartz tubular casing. The catalyst was fixed at both ends by quartz wool. To evaluate the influence of the catalyst on the electrode reaction, three methods of catalyst loading were used as shown in Figure 2. The three methods were tested to elucidate the following effects: the mode of contact (21) Michaels, J. N.; Vayenas, C. G. J. Electrochem. Soc. 1984, 131 (11), 2544. (22) Gu¨r, T. M.; Huggins, R. A. J. Electrochem. Soc. 1979, 126 (6), 1067. (23) Erstfeld, T. E.; Mullins, O. Pap. Am. Inst. Aeronaut. Astronaut. 1979, 79-1375. (24) Gu¨r, T. M.; Huggins, R. A. J. Catal. 1986, 2, 443. (25) Hibino, T.; Hamakawa, S.; Iwahara, H. Nikkashi 1993, 3, 238. (26) Tsiakaras, P. T.; Vayenas, C. G. Mater. Sci. Forum 1991, 76, 179. (27) Gu¨r, T. M.; Wise, H.; Huggins, R. A. J. Catal. 1991, 129, 216.
Figure 1. Illustration of YSZ tube (a) and reactor (b).
Figure 2. Location of filled-up catalyst.
between the catalyst and the cathode electrode and the variation of gas phase composition by the catalytic reaction. We varied (1) the amount of contact between the catalyst and the cathode electrode and (2) the gasphase composition by the catalytic reaction. Construction of Apparatus. Reaction apparatus and the electric circuit used in our study are shown in Figure 3. After being washed and dried, the carbon dioxide (99.9 vol %) and hydrogen (99.99 vol %) in cylinders were supplied to the reactor tube in the chamber between the YSZ and quartz tubes at a controlled flow rate through a micro-adjusting valve and a manometer. In this procedure, the inner part of the YSZ was pre-filled with helium gas (99.99%) for the purpose of adjusting the initial partial pressure of oxygen to a level lower than that of air. Water vapor formed during the reaction was analyzed by Gas Chromatograph GC-4C (GC1, see Figure 3)(Shimazu Co., detector: TCD; carrier gas: Ar; flow rate: 40 mL/min;
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Figure 3. Reaction apparatus.
injection temperature: 333 K; Gaskuropak54 column: 80/100 mesh, 2 m × 3 mm). Other species in the gaseous product were dehydrated, then analyzed using the Gas Chromatograph GC-4BIT (GC2, see Figure 3)(Shimazu Co., detector: TCD; carrier gas: He; flow rate: 40 mL/ min; injection temperature: 333 K; silica gel column: 60/80 mesh, 3 m × 3 mm). The oxygen-permeated YSZ was analyzed by the same method as above (molecular sieve 5A column: 60/80 mesh, 4 m × 3 mm). Basic conditions applied in this experiment are summarized as follows: reaction temperature: 773-873 K; flow rate of mixed gas H2/CO2: 9.2 × 10-4 dm3/s; mixing ratio (by volume) of H2/CO2:1-15; catalyst quantity: 1-10 g; space velocity (SV):
