Energy & Fuels 2003, 17, 13-17
13
Catalytically Activated Metal Foam Absorber for Light-to-Chemical Energy Conversion via Solar Reforming of Methane T. Kodama,* A. Kiyama, and K.-I. Shimizu Department of Chemistry & Chemical Engineering, Faculty of Engineering, and Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan Received February 28, 2002
CO2 reforming of methane was examined under direct irradiation of the catalytically activated metal foam absorber by solar-simulated, concentrated visible light in the small receiver-reactor system with a transparent window. Rh, Ru, or Ni was applied as the active metal on the aluminacoated or noncoated Ni-Cr-Al metal foam disk for the preparation of the absorber. Rh showed the best activity on the noncoated metal foam while Ru was the best on the alumina-coated one. The most active and stable Ru/Al2O3/Ni-Cr-Al-foam absorber yielded a maximum methane conversion of 73% at a GHSV of 8500 h-1 and at ambient pressure, in which about 50% of the incident light energy reaching the absorber was stored as chemical enthalpy at a relatively low value of the average input-power density of radiation, F ) 180 kW m-2 (the peak flux density of 274 kW m-2 at the center of the irradiated surface of the absorber). This catalytically activate metal foam absorber will be used in the receiver-reactor system for solar methane reforming at relatively low-energy fluxes of concentrated solar radiation.
1. Introduction The conversion of concentrated solar heat to chemical fuels has the advantage of producing energy carriers for storing and transporting solar energy.1-3 The direct themochemical conversion of solar radiation energy is characterized by ideal high efficiency; its thermodynamic limit for the enthalpy storing is close to 100%. From this point of view of the chemical pathway for this process, solar reforming of natural gas has been investigated as the most promising “solar thermochemical process”.4-22 The following endothermic steam/CO2 reforming of methane is the basis for upgrading the * Author to whom correspondence should be addressed. Tel: +81-25-262-7335. Fax: +81-25-262-7010. E-mail: tkodama@ eng.niigata-u.ac.jp. (1) Fletcher, E. A. J. Minn. Acad. Sci. 1983/84, 49 (2), 30-34. (2) Grasse, W.; Tyner, C. E.; Steinfeld, A. International R & D collaboration in developing solar thermal technologies for electric power and solar chemistry: The solarPACES program of the International Energy Agency (IEA). J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies, 1999; 9: Pr3-17-Pr3-22. (3) Tamaura, Y. Solar hybrid methanol production from coal and natural gas by solar thermochemical process: CO2 reduction and cost evaluation. In Solar Thermal 2000, Proceedings of the 10th SolarPACES International Symposium on Solar Thermochemical Concentrating Technologies, Sydney, 2000; p 189-192. (4) Levitan, R.; Rosin, H.; Levy, M. Solar Energy 1989, 42 (3), 267272. (5) Bo¨hmer, M.; Langnickel, U.; Sanchez, M. Solar Energy Mater. 1991, 24, 441-448. (6) Buck, R.; Muir, J. F.; Hogan, R. E.; Skocypec, R. D. Solar Energy Mater. 1991, 24, 449-463. (7) Levy, M. Proceedings of the 6th International Symposium on Solar Thermal Concentrating Technology 1993, 2, 1003-1011. (8) Levy, M.; Levitan, R.; Rosion, H.; Rubin, R. Solar Energy 1993, 50, 179-189. (9) Muir, J. F.; Hogan, R. E., Jr.; Skocypec, R. D.; Buck, R. Solar Energy 1994, 52 (6), 467-477.
calorific value of the hydrocarbons, which produce syngas:
CH4 + H2O(l) f CO + 3H2 ∆H298K ) 250 kJ (1) CH4 + CO2 f 2CO + 2H2 ∆H298K ) 247 kJ (2) Because of the above endothermic reactions, the product syngas has greater heating value than the initial methane by roughly 25%. The product of syngas can be stored and transported to be combusted in a (10) Skocypec, R. D.; Hogan, R. E., Jr.; Muir, J. F. Solar Energy 1994, 52 (6), 479-490. (11) Buck, R.; Abele, M.; Bauer, H.; Seitz, A.; Tamme, R. ASME: J. Solar Energy Eng. 1994, 116, 73-78. (12) Epstein, M.; Spiewak, I.; Segal. A.; Levy, I.; Liebermann, D.; Meri, M.; Lerner, V. Proceedings of 8th International Symposium on Solar Thermal Concentrating Technology 1997, 3, 1209-1229. (13) Berman, A.; Levitan, R.; Epstein, M.; Levy, M. ASME: J. Solar Energy Eng. 1996, 118, 61-69. (14) Abele, M.; Bauer, H.; Buck, R.; Tamme, R.; Wo¨rner, A. ASME: J. Solar Energy Eng. 1996, 118, 339-346. (15) Edwards, J. H.; Do, K. T.; Maitra, A. M.; Schuck, S.; Fok, W.; Stein, W. Energy Convers. Manage. 1996, 37 (6-8), 1339-1344. (16) Tanashev, Y. Y.; Fedoseev, V. I.; Aristov, Y. I. Catal. Today 1997, 39, 251-260. (17) Aristov, Y. I.; Fedoseev, V. I.; Parmon, V. N. Int. J. Hydrogen Energy 1997, 22 (9), 869-874. (18) Anikeev, V. I.; Bobrin, A. S.; Ortner, J.; Schmidt, S.; Funken, K.-H.; Kuzin, N. A. Solar Energy 1998, 63 (2), 97-104. (19) Wo¨rner, A.; Tamme, R. Catal. Today 1998, 46, 165-174. (20) Berman, A.; Epstein, M. Hydrogen Power: Theoretical and Engineering Solutions; Kluwer Academic Publishers: The Netherlands, 1998; pp 213-218. (21) Kirillov, V. A. Solar Energy 1999, 66 (2), 143-149. (22) Fisher, U.; Sugarmen, C.; Tamme, R.; Buck, R.; Epstein, M. Proceedings of 10th SolarPACES International Symposium on Solar Thermal Concentrating Technology, Solar Thermal 2000; 2000; pp 1920.
10.1021/ef0200525 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/10/2002
14
Energy & Fuels, Vol. 17, No. 1, 2003
conventional gas turbine (GT) or a combined cycle (CC), to generate electricity at high conversion efficiency (up to 55% in a modern, large CC). Another future potential utilization of the product syngas or further processed methanol is to use them for fuel cells with high conversion efficiency, higher than GT and CC. In Japan, a project was recently proposed to develop a solar methanol production system in the sun belt.3 In this project, methanol or DME is to be produced from natural gas (methane) and coal via methane reforming and coal gasification using solar heat as the process heat in sun belt regions. Then, the produced liquid fuel is to be transported overseas to Japan by a modified oil tanker.3 Solar reforming of methane with CO2 has been investigated for more than 20 years.4-22 Due to the specific properties of a concentrated solar radiation energy source, modifications of the conventionally used reactor system are necessary for the solar reforming process. As a solar-specific reactor for reforming, a direct absorption concept was proposed in which the receiver and reformer are the same unit, which was realized in some solar reforming systems such as directly irradiated “volumetric” reactors developed by the German Aerospace Research Center (DLR) in Germany, Sandia National Laboratories in the United States, and the Weizmann Institute of Science (WIS) in Israel.6,9-11,14,18,19 In this concept, the concentrated solar radiation passes through a window and is absorbed by an absorber of catalytically active, reticulated ceramic foams which is mounted behind the window. In these volumetric reactors, reticulated ceramic foams, combining high gas permeability and turbulence of flow with a geometry suitable for effective and uniform absorption of solar radiation, are preferably used instead of conventional honeycomb structures. Reforming of methane with CO2 using the volumetric reactor system was first demonstrated in DLR and Sandia National Laboratories in 1990.6 A 200-300 kW volumetric reactor was demonstrated also at the WIS for CO2 reforming of methane.19 In the regions of the so-called “sun-belt” where the maximum insolation reaches about 1 kW m-2, solar volumetric reactors are operated under high-energy incident solar fluxes (the peak flux densities ) 8002500 kW m-2) and, hence, at high temperatures (above 1273 K), high conversions can be reached. If this solar reforming realizes an efficient conversion under lowerenergy incident solar fluxes, it may be applied for worldwide solar concentrating facilities where direct insolation is weaker or the area for collecting solar radiation is very restricted. Under lower-energy incident solar fluxes and, hence, at relatively low temperatures (below 1173 K), thermally resistant metallic foams having better heat-transfer behaviors could be used as a solar absorption receiver or absorber. In the present work, the reticulated metallic Ni-Cr-Al foam with a high-temperature resistance is selected as a base material for the solar absorber in the receiver-reactor system. The catalytically active metal foam (CAMF) absorber is prepared for solar CO2 reforming of methane. In a small-scale receiver-reactor system, the chemical and the light-to-chemical energy conversions are investigated under direct irradiation of the CAMF absorber at relatively low-energy incident flux densities of the solar-simulated, concentrated visible light.
Kodama and Kiyama
2. Experimental Section 2.1. Preparation of Catalytically Active Foam Absorbers. An Ni-Cr-Al metal foam disk (CELMET, supplied by Sumitomo Electric Industries, Ltd.) with a high-temperature resistance (∼1173 K in air) was used for preparation of the catalytically active metal foam (CAMF) absorber, which is composed of a Ni-based alloy containing 20-30% Cr and 4-7% Al. The disk was a 30-mm-diameter and 10-mm-long foam with 27-33 cells per linear inch. The cell size was about 0.8 mm and the surface area of the foam disk was 2500 m2/m3. The ceramic (alumina) foam disk (supplied by Krosakiharima Co.) was also used for preparation of the catalytically active ceramic foam (CACF) absorber for comparison. The ceramic foam disk had the same dimensions (30-mm diameter and 10-mm long) with a similar cell size and surface area (about 30 cells per linear inch and 2500 m2/m3) as the Ni-Cr-Al metal foam disk used. Two types of the metal foam absorber were prepared: one was an alumina-support-coated type and another was a noncoated type. For the noncoated type, active metal of Ru, Rh, or Ni was applied directly on the Ni-Cr-Al metal foam disk noncoated with any support material such as alumina. A metal salt ethanol solution of RuCl3, RhCl3, or Ni(CH3COO)2 was added dropwise to the noncoated metal foam disk, allowed to dry at room temperature overnight, and calcined. For Rhor Ni-applied foam, it was calcined at 1023 K in air for 3 h and reduced at 723 K in 100% H2 for 1 h. For the Ru-applied one, it was calcined at 823 K in air for 3 h and reduced at 873 K in 10% H2-90% N2 for 1 h. Metal loadings were set to 7.3 µmol-metal/cm2 of the surface of the metal foam, assuming that all of the metal added to the foam disk was loaded. In the preparation of the alumina-support-coated type, the metal or ceramic foam disk was first coated with a catalyst support of alumina. Afterward, the catalytically active metal of Ru, Rh, or Ni was applied. There exist two variations for coating the foam disk with an alumina support. In the first case the disk was soaked in an Al(NO3)3 solution and dried at room temperature overnight. Then, it was calcined at 1273 K for 1 h (designated as Procedure A). In another case, fine γ-Al2O3 support particles were used. The metal or ceramic foam was soaked into an aqueous slurry of the γ-Al2O3 support, dried at room temperature overnight, and calcined at 1273 K for 1 h (designated as Procedure B). The density of this solution has to be kept low enough to prevent a clogging up of the pores of the foam structure. These alumina coating processes were repeated until the alumina coating reached up to 10 wt %: the alumina loadings were calculated from the masses of coated and uncoated foams. Then, Ru, Rh, or Ni was applied in the same procedures as used in the preparation of the noncoated type of the absorber. Metal loadings were set to 7.3 or 14.6 µmol-metal/cm2 of the surface of the metal or ceramic foam. 2.2. Activity Tests of Absorbers. A double-walled quartz reactor was used for the catalytic activity tests of the absorbers. The experimental setup is illustrated in Figure 1. The inner diameter of the outer quartz reactor was 39 mm. The inner quartz tube has the inner diameter of 31 mm. The thickness of the quartz tubes was about 2 mm. The absorber disk was placed on a porous quartz plate in the inner tube to be fixed. The reactor was insulated with refractory bricks. The reaction feed gas was fed to the outer annulus and allowed to flow through the foam absorber disk and out from the inner tube of the reactor. A 50% CH4-50% CO2 gas mixture was fed to the reactor at a flow rate of 1.0 N dm3 min-1 and at ambient pressure. The GHSV was set to 8500 h-1. Then, the absorber was irradiated with the solar-simulated Xe light in order to carry out the CO2 reforming of methane. The Xe-arc lamp house set (Ushio U-Tech, 3 kW XEBEX HIBEAM IIIR, Tokyo, Japan, or CINEMECCANICA, 5 kW ZX8000H, Milano, Italy) was used to simulate concentrated solar radiation. In the original Xe-arc lamp light, there exist the strong line
Catalytic Absorber for Light-to-Chemical Energy Conversion
Energy & Fuels, Vol. 17, No. 1, 2003 15
Figure 1. Schematic of experimental setup for solar-simulated methane reforming using a foam absorber. spectra in the wavelength range of 800-1000 nm, which are characteristic of Xe-arc emission. Therefore, for solar simulation, about 90% of the Xe-arc lamp light in the wavelength range of 750-1200 nm was cut by a heat absorbing filter. The solar-simulated Xe light thus prepared was used to directly irradiate the absorber in the reactor. The energy flux distribution of the incident solar-simulated light on the surface of the absorber was previously measured using a heat flux transducer with a sapphire window attachment (Medherm, 64100-20/SW-1C-150) placed in the position of the irradiated surface of the absorber. The maximum temperature (Tm) of the absorber was measured using a K-type thermocouple placed at the center of the irradiated surface of the absorber. The steam in the effluent gases from the reactor was condensed in a cooling trap connected to the exit of the reactor. The dry effluent gases were analyzed by gas chromatography equipment (Simadzu, GC-4C) with a TCD detector to determine the gas composition. The methane conversion (X) was estimated by the following equation:
yCO + yH2 X) 4yCH4 + yCO + yH2
(3)
where yCH4, yCO, and yH2 are the dry mole fractions for CH4, CO, and H2, respectively, in the effluent. The light-to-chemical energy conversion (ηchem) was estimated as follows. First, the overall reaction occurring in the gas phase was determined from the experimental data. CO2 reforming of methane of eq 2 is frequently associated with the reverse water-gas shift reaction:
H2 + CO2 f CO + H2O
∆H298K ) 41 kJ
(4)
and the H2/CO ratio in the product gas becomes lower than the stoichiometric ratio of one. In this case, the overall reaction in the gas phase is written by
CH4 + (1 + x)CO2 f (2 + x)CO + (2 - x)H2 + xH2O (5) ∆H298K(overall) ) 247 + 41x kJ
(6)
where x indicates the contribution of the reverse water-gas shift reaction. The x value was determined from the experimental H2/CO ratio in the product gas. Thus, the energy stored as chemical enthalpy by the overall reaction (Wchem) can be
experimentally estimated by
Wchem ) FCH4‚X‚∆H298K(overall)
(7)
where FCH4 represents the molar flow rate of CH4 to the reactor. The energy conversion, ηchem, of the incident light to chemical energy or enthalpy is defined by
ηchem ) Wchem/Winc
(8)
where Winc is the total energy of the light incident onto the absorber. Since the Winc value was measured by the heat flux transducer as described above, the light-to-chemical energy conversion (ηchem) is the power absorbed in chemical reaction divided by the input of thermal energy.
3. Results and Discussion Table 1 shows the absorber samples investigated. Figure 2a shows the energy flux distribution of the incident solar-simulated light on the irradiated surface of the absorber (designated as Flux distribution type I). The central peak flux density reached 357 kW m-2. The average energy density of radiation (F) was 192 kW m-2, which is defined by F ) Winc/S where S is the directly irradiated absorber surface (3 cm-diameter circle). Effects of the applied metals, Ru, Rh, and Ni, on the catalytic activity of the absorber under the light irradiation were compared for the noncoated type. Figure 3 shows the time variations of the methane conversion when using the Ni7.3/metal, Rh7.3/metal, and Ru7.3/ metal foam absorbers listed in Table 1. For these absorbers, the metal loadings were set to 7.3 µmolmetal/cm2 of the surface (2500 m2/m3) of the metal foam. The Ru- and Ni-applied absorbers were rapidly deactivated. The Rh was the most active and stable on the noncoated metal foam absorber. Coating of the alumina-support material drastically changed the catalytic activities of the metal-applied NiCr-Al foam absorbers. The time variations of the methane conversion by the alumina-coated types of Ni7.3-A/metal, Rh7.3-A/metal, and Ru7.3-A/metal foam
16
Energy & Fuels, Vol. 17, No. 1, 2003
Kodama and Kiyama
Table 1. Absorbers of Catalytically Activated Foams Investigated absorber Ni7.3/metalc Rh7.3/metalc Ru7.3/metalc Ni7.3-A/metalc Rh7.3-A/metalc Ru7.3-A/metalc Ru14.6-A/metald Ru14.6-B/metald Ru14.6-B/ceramicd
foam disk Ni-Cr-Al Ni-Cr-Al Ni-Cr-Al Ni-Cr-Al Ni-Cr-Al Ni-Cr-Al Ni-Cr-Al Ni-Cr-Al alumina
alumina support coatinga
metal loadingb
Procedure A, 10 wt % Procedure A, 10 wt % Procedure A, 10 wt % Procedure A, 10 wt % Procedure B, 10 wt % Procedure B, 10 wt %
(2 wt %-Nia) (3 wt %-Rha) (2 wt %-Rua) 11 wt %-Ni (2 wt %-Nia) 17 wt %-Rh (3 wt %-Rha) 18 wt %-Ru (2 wt %-Rua) 30 wt %-Ru (5 wt %-Rua) 30 wt %-Ru (5 wt %-Rua) 23 wt %-Ru (4 wt %-Rua)
a With respect to the mass of the foam disk. b With respect to the mass of the support. c Metal loadings were set to 7.3 µmol-metal/cm2 of the surface of the metal foam. d Metal loadings were set to 14.6 µmol-metal/cm2 of the surface of the metal or ceramic foam.
Figure 3. Effects of applied metals of Ni, Rh, and Ru on the methane conversions for the alumina-support-coated metal foam absorbers [Symbol: (]) the Ni7.3-A/metal, (0) Rh7.3-A/ metal, and (4) Ru7.3-A/metal foam absorbers] and for the noncoated metal foam absorbers [Symbol: ([) Ni7.3/metal, (9) Rh7.3/metal, and (2) Ru7.3/metal foam absorbers]. The fluxdistribution Type I was used for irradiation of the absorber.
Figure 2. Energy-flux distribution of the incident visible light on the irradiated surface of the absorber. (a) Type I: Winc ) 136 W, (b) Type II: Winc ) 128 W.
absorbers are also shown in Figure 3. The catalytic activities were improved in all the cases of Ru, Rh, and Ni. The Ru was most improved by the alumina-support effect and the Ru-applied absorber in the aluminacoated type showed the greatest and most stable activity. The metal foam absorbers were coated with alumina support in two different methods (Procedures A and B), and then applied with Ru: Here the metal loadings were set to 14.6 µmol-metal/cm2 of the foam surface. These absorbers thus prepared (the Ru14.6-A/metal and Ru14.6B/metal absorbers in Table 1) were compared on the basis of their activity in Figure 4. The stability of the activity was improved in the absorber prepared by washcoating with the γ-Al2O3 particles (Procedure B).
Figure 4. Effects of alumina-support coating method on the methane conversion for the Ru-applied metal foam absorbers. The Ru14.6-A/metal (4) and Ru14.6-B/metal (b) foam absorbers were tested. The flux-distribution Type I was used for irradiation of the absorber.
To improve the reaction efficiency on the absorber, the flux energy distribution of the incident light on the irradiated surface of the absorber was modified to be more uniform (Figure 2b, designated as Flux distribu-
Catalytic Absorber for Light-to-Chemical Energy Conversion
Figure 5. Time variations of methane conversion with the Ru-applied absorbers prepared using metal and ceramic foams. The Ru14.6-B/metal (b) and Ru14.6-B/ceramic (]) foam absorbers were tested. The flux-distribution Type II was used for irradiation of the absorber. Table 2. Some Experimental Resultsa for Activity Tests of Absorbers absorber Ni7.3/metal Rh7.3/metal Ru7.3/metal Ni7.3-A/metal Rh7.3-A/metal Ru7.3-A/metal Ru14.6-A/metale Ru14.6-B/metal Ru14.6-B/metal Ru14.6-B/ceramic
flux-distribution type Tmb/K I I I I I I I I II II
1306 1248 1328 1324 1199 1163 1094 1098 1145 1206
Xc 0.04 0.27 0.10 0.19 0.36 0.38 0.45 0.48 0.73 0.56
H2/CO ηchemd 0.31 1.05 0.49 0.70 0.89 0.77 0.79 0.81 0.88 0.89
0.03 0.16 0.08 0.12 0.22 0.24 0.28 0.30 0.49 0.38
a Results were for the values obtained at 180 min of irradiation. Temperature at the center of the irradiated surface of the absorber. c Methane conversion. d Light-to-chemical energy conversion. e Results were for values obtained at 120 min of irradiation.
b
tion type II). The central peak flux density was reduced from 357 to 274 kW m-2. The average energy density of radiation was F ) 181 kW m-2. This uniform distribution may suppress the reradiation energy loss from the central peak flux position of the absorber that is overheated. Furthermore, the insulation was improved using thicker refractory bricks. These improvements of the reaction conditions increased the methane conversion with the Ru14.6-B/metal absorber from 48 to 73%, as shown by Figures 4 and 5. As shown in Table 2, the H2/CO ratio in the product gas, obtained by the Ru14.6-B/metal absorber, was 0.88, being close to the stoichiometric ratio of one (eq 2). The ceramic or alumina foam disk with the same dimensions and the similar porosities (about 30 cells per linear inch and 2500 m2/m3 of the surface) was also used for the preparation of the absorber, and its activity was compared to the Ru14.6-B/metal foam absorber. The same procedure as that for the Ru14.6-B/metal absorber was used for the preparation of this ceramic absorber (Ru14.6-B/ceramic in Table 1). As shown by Figure 5, this ceramic foam absorber gave a lower methane conversion than the metal foam absorber under the same reaction conditions. The better heat transfer of the NiCr-Al metal foam may improve the reaction efficiency of the absorber, as expected. However, the activity of “catalytically active absorber” is determined by many
Energy & Fuels, Vol. 17, No. 1, 2003 17
factors such as the reflectivity, the surface area, the catalyst dispersion, as well as the heat transfer behavior. Here we could not clarify the main effective factor of this new metal foam absorber. In the next step the measurement or characterization of the optical and the thermal characteristics of this metal absorber are needed. Table 2 shows some experimental results for the activity tests. The energy conversions are also listed in Table 2. The Ru14.6-B/metal foam absorber under irradiation with Flux distribution II attained 49% of the energy conversion. The maximum temperature (Tm) of the absorber was 1145 K, which is less than the limit of the heat resistance of the Ni-Cr-Al foam (∼1173 K in air). The original Ni-Cr-Al metal foam has the thermal resistance at temperatures below about 1173 K in air because it is gradually oxidized at temperatures above 1173 K. Under the reducing atmosphere used here, the foam will have the thermal resistance at higher temperatures than 1173 K. In all the experiments we did here, visually no major damage to the absorber due to the thermal shock was observed after the reforming tests. In the most of the previous literature, the performances of the catalytically active ceramic absorbers in the volumetric receiver-reactors were demonstrated in much larger reactor scales and higher input power levels although light-to-chemical conversions approaching 66% were reported.6,14,19 Thus, their values could not be directly compared with our results in Table 2. However, Tanashev et al.16 studied the solar steam reforming of methane under direct irradiation of a commercial Rubased catalyst by concentrated Xe-arc-lamp or visible light in an experimental scale (the reactor scale and input power levels) similar to the one we used. They reported the light-to-chemical energy conversions of 3674% for the average energy density of radiation F ) 130-670 kW m-2 using a commercial Ru-based catalyst with a 20-mm diameter (the thickness of the catalyst bed was 4-20 mm). The energy conversion tended to increase with an increase in the F value. For low flux density levels of F ) 130-300 kW m-2, the energy conversion increased from 36 to only 46%. However, here we have attained about 50% of the energy conversion at a low value of F ) 180 W m-2. 4. Conclusion The active CAMF absorber was prepared for use in the receiver-reactor system for solar methane reforming at relatively low-energy fluxes of concentrated solar radiation. The Ru/Al2O3/Ni-Cr-Al-foam absorber yielded about 50% of the light-to-chemical energy conversion at a low-energy density of F ) 180 kW m-2 for the irradiation of the absorber. Acknowledgment. A part of the present work was financially supported by a Grant-in-Aid for Science Research No.13780404 from the Japan Society for the Promotion of Science. Note Added after ASAP Posting. Figure 2 was printed with the wrong units in its legends. Several instances occurred in which “pore(s)” was used instead of “cell(s)”. The incorrect version was published on the Web on 10/10/2002 (ASAP). The current version was posted on the Web on 10/30/2002. EF0200525
