Nickel Catalyst Driven by Direct Light Irradiation for Solar CO2

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Energy & Fuels 2002, 16, 1016-1023

Nickel Catalyst Driven by Direct Light Irradiation for Solar CO2-Reforming of Methane T. Kodama,* H. Ohtake, K-I. Shimizu, and Y. Kitayama 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 October 12, 2000

Catalytic activity and selectivity of Ni catalysts for solar CO2-reforming of methane were examined under direct irradiation of the catalyst by solar-simulated, concentrated Xe-arc lamp light, using a small-scaled transparent (quartz) packed-bed reactor. The Ni catalysts (17 wt %-Ni), supported with R-Al2O3, SiO2, ZrO2, and TiO2, were tested, and the Ni/R-Al2O3 showed the best activity and selectivity under the solar simulation, as well as under direct irradiation by infrared light. With the Ni/R-Al2O3 catalyst at the energy flux density of the incident light ) 810 kW m-2 and at the GHSV ) 18700 h-1, more than 90% of methane was chemically converted to syngas, in which 16% of the incident light energy was stored as chemical enthalpy. The activity data were compared to those for the catalyst irradiated by infrared light or UV-cut Xe light at various reaction temperatures or incident energy flux densities. This comparison indicated that the solarsimulated reforming with the Ni/R-Al2O3 was scarcely enhanced by UV-photochemical effect and that the activity was mainly determined by the thermochemical effect or the catalyst-bed temperature.

Introduction The efficient conversion of high-temperature heat from concentrated solar radiation to chemical fuels enables solar energy storage and transportation from the sun belt to remote population centers.1-3 Solar reforming of natural gas has been extensively examined as a solar thermochemical conversion process by many research groups such as the German Aerospace Research Center (DLR) in Germany, Sandia National Laboratories in the United States, and the Weizmann Institute of Science (WIS) in Israel.4-21 The reforming * Corresponding author. 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. J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 1999, 9, Pr3-9-Pr3-15. (3) Tamaura, Y. Solar Thermal 2000, Proceedings of the 10th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 2000, 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. (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.

process is a catalytic endothermic reaction between lowmolecular-weight hydrocarbons, like methane, with steam or carbon dioxide, which produces synthesis gas or syngas containing primarily CO and H2:

CH4 + H2O f CO + 3H2 ∆H°298K ) 206 kJ

(1)

CH4 + CO2 f 2CO + 2H2 ∆H°298K ) 247 kJ

(2)

These endothermic reactions are the basis for upgrading the calorific value of the hydrocarbons by roughly 25%, using solar energy. Solar reforming of methane was first developed for the closed-loop Solar Chemical Heat Pipe (SCHP) working on the cycle of methane reforming and reverse synthesis gas methanation.4,5,7,11-14 However, in recent years, the open loop thermochemical heat pipe based on steam or CO2 reforming of methane has rather (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: Netherlands, 1998; pp 213-218. (21) 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, 1920.

10.1021/ef000226n CCC: $22.00 © 2002 American Chemical Society Published on Web 08/07/2002

Nickel Catalyst for Solar CO2-Reforming of Methane

received much attention.3,15,18,19,21 The calorifically upgraded product of syngas can be stored and transported for a conventional gas turbine (GC) or a combined cycle (CC), to generate electricity with a high conversion efficiency (up to 55% in a modern, large CC). The product syngas can be also readily be converted to easily transportable liquid fuels such as methanol.3 Another future potential utilization of the product syngas or the methanol is to use them for fuel cells with high conversion efficiency, higher than GC and CC. Catalysts based on Rh metal were first used for both the reforming and methanation reactions in the closedloop SCHP.4,6-11 In recent years, the price of Rh increased sharply and its availability became scarce. It was therefore necessary to develop alternatives to Rh catalyst for use in the SCHP. It was shown by Tucci and Streeter22 that supported low Ru loading catalysts containing 1-2% Ru are active and stable at 723-823 K for methanation. The Ru catalysts are active also for CO2 reforming of methane.23,24 Thus, Ru catalysts have been extensively investigated for the SCHP, especially at theWeizmann Institute.12-14,19 The kinetics of the CO2 reforming of methane on the 1%Ru/Al2O3 catalysts was studied using a nontransparent tubular reactor made from inconel and the appropriate kinetic models were developed. The results of long-term testing show that a 1%Ru/γ-Al2O3 (Engelhard) catalyst was the most suitable for a pilot 350 kW tubular reactor. 13 In the last 10 years, volumetric reactors have been developed to intensify the heat supply via the direct irradiation of a catalyst by concentrated sunlight in the reactor with light-transparent window.6,9-11,14,16-19 CO2 reforming of methane using this reactor system was first demonstrated in the “catalytically enhanced solar absorption receiver” experiment conducted by DLR and Sandia National Laboratories USA in 1990.6 Later, 100-300 kW volumetric reactors were demonstrated for CO2 reforming of methane.9-11,14,19 Rh and Ru catalysts, supported on γ-Al2O3, were mainly used in these volumetric reactors. Solar volumetric reactors can be operated at high temperatures (above 1273 K), and higher conversion can be reached. However, in the vicinity of 1273 K and in the presence of water, the specific area of the γ form of alumina is strongly decreased. This phenomena is associated with the transformation of γ-alumina into R-phase. Therefore, Berman and Epstein20 examined the promoted Ru-Ce catalyst to improve the activity and thermal stability of Ru/Al2O3 catalyst for solar CO2reforming of methane. The Ni-based catalyst is usually used for the industrial reforming processes, which is the best one for the industrial use from an economical point of view in comparison to active noble metal catalysts. The Ni catalysts had not been extensively examined for the solar reforming of methane in the closed-loop SCHP project because specific problems arise due to reforming with low CO2/CH4 or H2O/CH4 ratios under the non (22) Tucci, E. R.; Streeter, R. C. Hydrocarbon Process. 1980, April, 107-112. (23) Solymosi, F.; Kutsa´n, Gy.; Erdo¨helyi, A. Catal. Lett. 1991, 11, 149-156. (24) Edwards, J. H.; Maitra, A. M. Fuel Process. Technol. 1995, 42, 269-289.

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stationary conditions.25 The main problem is that the deactivation rapidly occurs due to the deposition of carbon on an Ni catalyst surface under these conditions. In the open loop thermochemical heat pipe, however, reforming with high CO2/CH4 or H2O/CH4 ratios can be conducted to prevent carbon depositions. Moreover, the activity and stability of the catalyst may be improved by rare earth-based promoters or alkaline-earth metal ions.26-33 Yamazaki et al.28 found that Ni-on-MgO-CaO containing catalyst showed excellent activity and stability for CO2 reforming of methane. Especially, higher CaO-containing catalyst showed more excellent stability. Tomishige et al.33 reported that nicke-magnesia solid solution Ni0.03Mg0.97O has high and stable activity without carbon deposition for a long time (100 days) in the CO2 reforming of methane. In the solar reforming under the direct irradiation of the Ni catalyst by sunlight, an additional enhancement of the catalytic activity or stability may be achieved due to a direct photochemical action of intense light flux. In this paper, we examined Ni catalysts for the solar CO2-reforming of methane under direct irradiation of the catalyst by the solar-simulated Xe light. On the Ni/ R-Al2O3, which showed the greatest activity among the Ni catalysts tested here, the reaction enhancement by thermochemical and UV-photochemical effects was studied using the catalysts irradiated by infrared light or UV-cut Xe light, as well as by the solar-simulated Xe light. Experimental Section Preparation of Catalysts. Ni catalyst, supported with R-Al2O3, SiO2, ZrO2, or TiO2, was prepared by a conventional impregnation method using nickel nitrate. The nickel content of the catalyst was set to 17 wt %. The R-Al2O3, SiO2, ZrO2, or TiO2 was suspended in the nickel nitrate solution. The suspended solution was evaporated to dryness. After grinding the dried powder in a mortar, it was calcined at 1023 K for 3 h in air and then reduced by H2 at 723 K for 0.5 h. The Ni catalysts thus prepared were characterized by X-ray diffractometry (XRD) with Cu KR radiation (Rigaku, RAD-γA diffractometer). In the XRD pattern, only the peaks due to metallic Ni appeared along with those due to R-Al2O3, SiO2, ZrO2, or TiO support. A 3 wt % Ru/R-Al2O3 catalyst was also prepared by the following procedure, and the activity was tested for comparison. Ruthenium acetylacetonate was dissolved in acetone. The R-Al2O3 was suspended in the solution. The suspended solution was evaporated to dryness. After grinding the dried powder in a mortar, it was calcined at 873 K for 3 h in air and then reduced by H2 at 723 K for 0.5 h. In the XRD pattern, the small peaks due to metallic Ru were (25) Richardson, J. T.; Paripatyador, S. A. Appl. Catal. 1990, 61, 293-309. (26) Gadalla, A. M.; Sommer, M. E. J. Am. Ceram. Soc. 1989, 72 (4), 683-687. (27) Gadalla, A. M.; Sommer, M. E. Chem. Eng. Sci. 1989, 44 (12), 2825-2829. (28) Yamazaki, O.; Nozaki, T.; Omata, K.; Fujimoto, K. Catal. Lett. 1992, 1953-1954. (29) Takayasu, O.; Soman, C.; Takegahara, Y.; Matsuura, I. Stud. Surf. Sci. Catal. 1994, 88, 281-288. (30) Choudhary, V. R.; Uphade, B. S.; Mamman, A. S. Catal. Lett. 1995, 32, 387-390. (31) Gronchi, P.; Fumagalli, D.; Del Rosso, R.; Centola, P. J. Thermal Anal. 1996, 47, 227-234. (32) Horiuchi, T.; Sakuma, K.; Fukui, T.; Kubo, Y.; Osaki, T.; Mori, T. Appl. Catal. A: General 1996, 114, 111-120. (33) Tomishige, K.; Yamazaki, O.; Chen, Y.; Yokoyama, K.; Li, X.; Fujimoto, K. Catal. Today 1998, 45, 35-39.

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Table 1. Catalysts Used for CO2 Reforming of Methane catalyst

metal loading/ wt %

BET surface area/ m2-g-1

Ni/R-Al2O3 Ni/SiO2 Ni/ZrO2 Ni/TiO2 Ru/R-Al2O3

17 17 17 17 3

7.5 1.3 8.0 2.4 1.1

observed along with those of R-Al2O3 support. The BET surface areas of the Ni and Ru catalysts were determined by nitrogen adsorption (Shimadzu, Micromeritics Flow Sorb II 2300) and are listed in Table 1. These catalysts had relatively small surface areas of 1.1-8.0 m2 g-1. Activity Test under Direct Irradiation of Catalyst by Solar-Simulated Xe Light. The experimental setup is illustrated in Figure 1a. The catalyst (0.33-1.0 g) was packed in the reactor of a transparent quartz tube with a inner diameter of 7 mm. The length of the catalyst bed in the reactor tube was set to 10 or 25 mm. A CH4-CO2 mixture (pCH4 ) 0.5, and pCO2 ) 0.5) was fed to the reactor at a flow rate of 40-200 N cm3 min-1 and then the catalyst in the reactor was irradiated by the solar-simulated Xe light in order to carry out the methane reforming. The concentrated Xe-arc lamp (Ushio U-Tech, 3kW XEBEX HI-BEAM IIIR, Tokyo, Japan) light was used to simulate concentrated solar radiation. In the original Xe-arc lamp light, there exist the strong line spectra in the wavelength range of 800-1000 nm, which are characteristic of Xe-arc emission. Therefore, for solar simulation, about 90% of the concentrated 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 catalyst bed in the reactor. The circular diameter of the focal area was fitted to the catalyst-bed length (10 or 25 mm) to irradiate the bed throughout. The energy flux density (FD; kW m-2) of the incident solar-simulated light at the center of the circular focal area was previously measured by a heat flux transducer with sapphire window attachment (Medtherm, 64-100-20/SW-1C-150) placed in the center position of the focal area. The circular diameter of the sapphire window of the heat flux transducer was about 10 mm. The temperature of the catalyst bed was measured using a K-type thermocouple placed at the center of the catalyst bed in contact with it packed inside the reactor. The energy flux density FD of the incident solar-simulated light depends on the power supply to the Xe-arc lamp and the diameter of the circular focal area. Figure 2 shows the change in the FD of the solar-simulated light at the center of the focal area as a function of the power supply, for the different focal diameters of 10 and 25 mm. The FD increased linearly with an increase in the power supply in the both cases. Activity Test under Direct Irradiation of Catalyst by UV-Cut Xe Light. The same reactor system of Figure 1a was used for the activity test but the UV-cut Xe light was used for irradiation of the catalyst. The solar-simulated Xe light was transmitted by the UV-cut filter (Kenko, Sharp Cut Filter Colorless L-42) to cut UV light with the wavelengths shorter than 420 nm, prior to direct irradiation of the catalyst in the transparent reactor. The Ni/R-Al2O3 catalyst (0.35 g) was packed in the reactor and the catalyst-bed length was 10 mm. The circular diameter of the focal area was set to 10 mm to irradiate the catalyst bed throughout. A CH4-CO2 mixture

Figure 1. Experimental setups for CO2 reforming of methane under direct irradiation of catalyst (a) by the solar-simulated Xe light and (b) by the infrared light.



Figure 2. Energy flux density (FD) of the incident light at the center of the focal area as a function of power supply to the Xe-arc lamp. Symbols: circles for the irradiation by the (non UV-cut) solar-simulated Xe light with a 10-mm diameter (b) of the circular focal area and with a 25 mm diameter (O). Solid triangles for the irradiation by the UV-cut Xe light with a 10-mm diameter of the circular focal area (2). (pCH4 ) 0.5, and pCO2 ) 0.5) was fed to the reactor at a flow rate of 40 N cm3 min-1 and then the irradiation of catalyst by the UV-cut Xe light was commenced for the methane reforming. The FD of the UV-cut Xe light at the center of the circular focal area with the diameter of 10 mm is also given in Figure 2. The FD of the UV-cut light was reduced by about 25% in comparison to the original solar-simulated Xe light. Activity Test under Direct Irradiation of Catalyst by Infrared Light. The experimental setup is illustrated in Figure 1b. The Ni catalyst (1.0 g) was packed in the reactor of a transparent quartz tube with a inner diameter of 7 mm. The length of the catalyst bed was set to 25 mm. The catalyst in the reactor was heated in an infrared furnace (ULVAC, E45) to 773-1173 K while passing a stream of N2 gas at a flow rate of 10 N cm3 min-1 through the reactor. The temperature was controlled using a K-type thermocouple placed at the center of the catalyst bed in contact with it packed inside the reactor. After reaching the desired temperature within 5 min, a CH4CO2 mixture (pCH4 ) 0.5 ,and pCO2 ) 0.5) was fed to the reactor at a flow rate of 40 N cm3 min-1 in order to commence the methane reforming. Methane Conversion and Energy Conversion. The steam in the effluent gases from the reactor was condensed in a cooling trap connected to the outlet of the reactor. The dry effluent gases were analyzed by gas chromatography equipment (Shimadzu, GC-4C) with a TCD detector to determine the gas compositions. The conversion of methane, X, during the CO2 reforming 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 MALT234 software program was used to compute the equilibrium composition of the system CH4 + CO2 at 1 atm and at temperatures of interest. Then, the equilibrium conversion of methane was estimated using eq 3, based on the computed equilibrium compositions. (34) Yamauchi, S. Netsu Sokutei 1985, 12 (3), 142-144.

Figure 3. Results of CO2 reforming of methane at 973 K and at a low GHSV of 2500 h-1 with various Ni catalysts under irradiation by the infrared light: time variations of (a) CH4 conversion (X) and (b) H2/CO ratio in the effluent. An amount of 1.0 g of the Ni catalyst was used and the catalyst-bed length was set to 25 mm. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 40 N cm3 min-1. Symbols: Ni/R-Al2O3(0), Ni/TiO2(4), Ni/ZrO2((), Ni/SiO2(O). The energy conversion from incident light to chemical energy 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 ∆H°298K ) 41 kJ

(4)

and the H2/CO ratio in the product gas becomes lower than the stoichiometric ratio of one for eq 2. 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 ∆H°298K(overall) ) 247 + 41x kJ

(5) (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 effluent. Thus, the energy stored as chemical enthalpy by the overall reaction, Qchem, can be experimentally estimated by

Qchem ) FCH4,in × X × ∆H°298K(overall)

(7)

where FCH4,in represents the molar flow rate of CH4 to the inlet of the reactor. The energy of the incident Xe light to the surface of the catalyst bed, Qincident is written by

Qincident ) AFD × IS

(8)

where AFD is the average energy flux density of the incident Xe light in the focal area, and IS is the irradiated surface area of the catalyst bed. For the case of the circular focal diameter ) 10 mm, we assumed that the radiation beam is parallel (Thus, IS ) 7 mm i.d. of the reactor tube × 10 mm of the catalyst-bed length) and the energy flux distribution is homo-

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Figure 4. Results of CO2 reforming of methane at temperature around 800-900 K and at a low GHSV of 2500 h-1 with various Ni catalysts under irradiation by the solar-simulated Xe light: time variations of (a) catalyst-bed temperature, (b) CH4 conversion (X), and (c) H2/CO ratio in the effluent. An amount of 1.0 g of the Ni catalyst was used and the catalystbed length was set to 25 mm. The circular diameter of the focal area was 25 mm and the FD at the center position was 630 kW m-2. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 40 N cm3 min-1. Symbols: Ni/R-Al2O3(0), Ni/TiO2(4), Ni/ZrO2((), Ni/SiO2(O). geneous within a circular diameter of 10 mm. Since the size of the window of the heat flux transducer for measurement (the circular diameter of about 10 mm) almost fitted that of the catalyst-bed surface area irradiated (IS), the FD value measured at the center of the focal area (Figure 2) approximately indicates the average energy flux density in the focal area. Therefore, in the case of using the focal diameter of 10 mm, we used for calculation the FD values in Figure 2 as the AFD for eq 8. The energy conversion, ηchem, of incident Xe light to chemical energy or enthalpy is defined by

ηchem ) Qchem/Qincident

(9)

Results and Discussion Support Effect on Activity of Ni Catalyst. The activity tests of the Ni catalysts, supported on various oxide supports, were first carried out at 973 K under direct irradiation of the catalyst by the infrared light. A low GHSV of 2500 h-1 was used here. The methane conversion X and the H2/CO ratio in the effluent as a function of reaction time are shown in Figure 3. The Ni/TiO2 showed the highest X value but the H2/CO ratio largely exceeded one, indicating the significant occurrence of methane decomposition to bulk carbon and hydrogen:

Figure 5. Results of CO2 reforming of methane at higher temperatures around 1000-1100 K and at a higher GHSV of 18700 h-1 with various Ni catalysts under irradiation by the solar-simulated Xe light: time variations of (a) catalyst-bed temperature, (b) CH4 conversion (X), and (c) H2/CO ratio in the effluent. The catalyst-bed length was set to 10 mm. The circular diameter of the focal area was 10 mm and the FD at the center position was 810 kW m-2. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 120 N cm3 min-1. Mass of the catalyst used (symbols) were Ni/R-Al2O3: 0.35 g (0); Ni/TiO2: 0.33 g (4); Ni/ZrO2: 0.35 g ((); and Ni/SiO2: 0.41 g (O).

CH4 f C + 2H2

(10)

A large amount of carbon was deposited on the catalyst, and visually the volume of the catalyst bed increased due to the carbon deposition. Because of the carbon deposition, the reactor was plugged at 120 min of the reaction. The order of the activity, except Ni/TiO2, was that Ni/R-Al2O3 > Ni/SiO2 > Ni/ZrO2. The H2/CO ratio with the Ni/R-Al2O3 almost agreed with the stoichiometric ratio of one. The activity of these Ni catalysts was also tested under direct irradiation of the catalyst by the solarsimulated Xe light (Figure 4): the reaction conditions of the mass of the catalyst used (1.0 g), the catalystbed length (25 mm), and the GHSV (2500 h-1) were the same as those used for the experiments by the infraredlight irradiation of Figure 3. The circular diameter of the focal area was set to 25 mm, being fitted to the catalyst-bed length. The FD at the center position of the focal area was 630 kW m-2. The catalyst-bed temperatures rapidly increased over 800 K within 10 min of the irradiation and then gradually increased to around 900 K (Figure 4a). As shown by Figure 4b, c, the best active and selective catalyst was the Ni/R-Al2O3 in the solar simulation, as well as in the infrared-light irradiation. Except for the Ni/TiO2, the order of the activity of


Figure 6. Variations of (a) catalyst-bed temperature, (b) CH4 conversion (X), H2/CO ratio in the effluent, and (c) energy conversion (ηchem) as a function of the energy flux density (FD) of the incident light in CO2 reforming of methane with the Ni/R-Al2O3 catalyst under irradiation by the solar-simulated Xe light. The data were taken from the values at 120 min of the reaction. An amount of 0.35 g of the catalyst was used and the catalyst-bed length was set to 10 mm. The circular diameter of the focal area was 10 mm. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 120 N cm3 min-1. The GHSV was 18700 h-1.

the Ni catalysts corresponded to that in the infraredlight irradiation. To conduct the activity test at higher temperatures under irradiation by the solar-simulated light, the catalyst-bed length was reduced to 10 mm (the mass of the catalyst ) 0.33-0.41 g) and the diameter of the focal area was set to 10 mm, in which the FD of 810 kW m-2 could be obtained at the maximum power supply to the Xe lamp (Figure 2). A higher GHSV of 18700 h-1 was used here. The catalyst-bed temperature reached around 1000-1100 K under these conditions (Figure 5a). At the higher temperatures and the higher GHSV (Figure 5b,c), the Ni/R-Al2O3 showed a much higher activity with a good selectivity than those with the other Ni catalysts. Reaction Performance and Energy Conversion Efficiency with Ni/r-Al2O3 Catalyst under Solar Simulation. The solar-simulated CO2 reforming of methane with the Ni/R-Al2O3 was performed using various FD of the solar-simulated light (Figure 6): the catalyst-bed length (0.35 g of the Ni/R-Al2O3) was set to 10 mm and the GHSV was 18700 h-1. The catalystbed temperature linearly increased from 800 to 1050 K with an increase in the FD from 410 to 810 kW m-2. With increasing FD or catalyst-bed temperature, the X increased to 90% with the H2/CO ratio approaching the


Figure 7. Variations of (a) catalyst-bed temperature, (b) CH4 conversion (X), H2/CO ratio in the effluent, and (c) energy conversion (ηchem) as a function of the space velocity (GHSV) of the reactant gas in CO2 reforming of methane with the Ni/ R-Al2O3 catalyst under irradiation by the solar-simulated Xe light. The data were taken from the values at 120 min of the reaction. An amount of 0.35 g of the catalyst was used and the catalyst-bed length was set to 10 mm. The circular diameter of the focal area was 10 mm and the FD at the center position was 810 kW m-2. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 40-200 N cm3 min-1.

stoichiometric value of one. The energy conversion, ηchem, significantly increased with increasing FD, attaining to 16% at FD ) 810 kW m-2 and at 1050 K. The experiments on a GHSV dependence were carried out by changing the flow rate of the reactant gas mixture (Figure 7): the catalyst-bed length (0.35 g of the Ni/R-Al2O3) was 10 mm and the FD was fixed to 810 kW m-2. With increasing GHSV, the catalyst-bed temperature slightly decreased in the range of 950-1073 K. High methane conversions over 90% with the H2/ CO ratios close to one were obtained at the GHSV 18700 h-1 and then the X rapidly decreased with a higher GHSV. On the other hand, the energy conversion showed the maximum (ηchem ) 16%) at the GHSV ) 18700-24900 h-1. The low Ru loading (3 wt %-Ru) catalyst, supported on R-Al2O3, was compared to this relatively high Ni loading (17 wt %) catalyst of the Ni/R-Al2O3 under the irradiation by the solar-simulated Xe light with the FD ) 810 kW m-2 (Figure 8). The catalyst-bed temperatures around 900-950 K were obtained in the both cases. Our high Ni loading catalyst obviously showed a much higher activity than the low Ru loading catalyst, on the R-Al2O3 support, under solar simulation.

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Figure 8. Comparison of the activity between the Ni/R-Al2O3(17 wt %-Ni) and the Ru/R-Al2O3(3 wt %-Ru) for CO2 reforming of methane under irradiation by the solar-simulated Xe light: time variations of (a) catalyst-bed temperature and (b) CH4 conversion (X). An amount of 0.35 g of the catalyst was used and the catalyst-bed length was set to 10 mm. The circular diameter of the focal area was 10 mm and the FD at the center position was 810 kW m-2. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 160 N cm3 min-1. The GHSV was 25000 h-1. Symbols: Ni/R-Al2O3(9), Ru/R-Al2O3(O).

Thermochemical and UV-Photochemical Effects on Ni/r-Al2O3 Catalyst. On the Ni/R-Al2O3, we compared the temperature dependence of the activity between the two cases under irradiation by the solarsimulated Xe light and by the infrared light. The X and H2/CO ratio were plotted against the catalyst-bed temperature (Figure 9): the catalyst-bed length was 25 mm (1.0 g of the Ni/R-Al2O3) and the GHSV was 2500 h-1 here. We could not see a significant difference in the temperature dependence of the X and H2/CO ratio between the two cases. Equilibrium conversion is also given in Figure 9. At a higher temperature, the X value became closer to the equilibrium conversion and almost reached the equilibrium value at 1173 K. To examine the UV-photochemical effect on the reaction enhancement of the solar-simulated reforming, the CO2 reforming with the Ni/R-Al2O3 was carried out under direct irradiation of the catalyst by the UV-cut Xe light for comparison. The activity data were compared to those under irradiation by the (non UV-cut) solar-simulated light. The catalyst-bed length of 10 mm (0.35 g of the Ni/R-Al2O3) and the GHSV of 6200 h-1 were used here. Figure 10 shows the catalyst-bed temperature as a function of the FD of the incident UVcut or non UV-cut solar-simulated light. In the both cases, the bed temperature linearly increased with increasing FD. However, higher bed temperatures were observed for the UV-cut Xe light than those for the non UV-cut light although the FD values were the same. This would be due to the fact that the fraction of the infrared-light component was relatively increased in the UV-cut light in comparison to that in the non UV-cut light. The energy due to the infrared-light component

Kodama et al.

Figure 9. Comparison of the activity of the Ni/R-Al2O3 for CO2 reforming of methane under irradiation by the solarsimulated Xe light (O) and by the infrared light (b): variations of (a) CH4 conversion (X) and (b) H2/CO ratio in the effluent as a function of the catalyst-bed temperature. The data were taken from the values at 120 min of the reaction. An amount of 1.0 g of the catalyst was used and the catalyst-bed length was set to 25 mm. The circular diameter of the focal area for the irradiation by the solar-simulated Xe light was 25 mm and the FD at the center position was varied up to 630 kW m-2. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 40 N cm3 min-1. The GHSV was 2500 h-1.

Figure 10. Relation between the catalyst-bed temperature and the energy flux density of the incident light in CO2 reforming of methane with the Ni/R-Al2O3 under irradiation by the (non UV-cut) solar-simulated Xe light (9) and by the UV-cut Xe light (0). The data were taken from the values at 120 min of the reaction. An amount of 0.35 g of the catalyst was used and the catalyst-bed length was set to 10 mm. The circular diameter of the focal area was 10 mm. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 40 N cm3 min-1. The GHSV was 6200 h-1.

is more readily transformed to heat on the surface of the catalyst. Figure 11a shows the relation between the X (and the H2/CO ratio) and FD. As can be seen here, the irradiation of the catalyst by the UV-cut light gave higher X values rather than by the non UV-cut light at the same FD value, indicating the minor UV-photo-


Figure 11. Comparison of the activity of the Ni/R-Al2O3 for CO2 reforming of methane under irradiation by the (non UVcut) solar-simulated Xe light (circles) and by the UV-cut light (triangles): variations of CH4 conversion (X) (solid symbols) and H2/CO ratio (open symbols) in the effluent (a) as a function of the energy flux density (FD) of the incident light, and (b) as a function of the catalyst-bed temperature. The data were taken from the values at 120 min of the reaction. An amount of 0.35 g of the catalyst was used and the catalyst-bed length was set to 10 mm. The circular diameter of the focal area was 10 mm. A CH4-CO2 mixture (CH4:CO2 ) 1:1) was fed at a flow rate of 40 N cm3 min-1. The GHSV was 6200 h-1.

chemical effect on the reaction enhancement. The relation between the X (and the H2/CO ratio) and the catalyst-bed temperature is shown in Figure 11b. Within experimental error, there was no significant difference in the temperature dependence of the X or H2/CO ratio between in the two cases of irradiation by the UV-cut light and by the non UV-cut light. Thus, the activity of the Ni/R-Al2O3 was determined by the catalyst-bed temperature or the thermochemical effect in the solar CO2 reforming of methane. Conclusions The Ni/R-Al2O3 catalyst was found to be active for solar CO2 reforming of methane. This relatively high Ni loading catalyst (17 wt %-Ni) showed a higher activity than the low Ru loading catalyst (3 wt %-Ru),


on the R-Al2O3 support, under the solar simulation. Direct photochemical action of intense light flux had no effect on the activity of the Ni/R-Al2O3 which could be completely explained by the thermochemical effect. Sixteen percent of the energy conversion from light to chemical energy with the methane conversion over 90% was obtained via the CO2 reforming of methane in a small-scaled transparent packed-bed reactor. This energy conversion efficiency was relatively low in comparison to those reported in the literatures concerning the solar reforming of methane. For example, Tanashev et al.16 reported about 25-74% of the light-to-chemical energy conversion via steam reforming of methane using a volumetric reactor and a 10-kW Xe lamp in which 2000 kW m-2 of the flux density of the lamp could be obtained. In the present work, the catalyst used had a relatively small surface area of 7.5 m2 g-1. In addition, the reactor was scarcely thermally isolated and its heat loss would be very large. The used energy flux density of the incident light to the reactor (810 kW m-2) was relatively low in comparison to those used in the existing solar facilities. The flux power density of the concentrated solar radiation can reach around 1000-4000 W m-2 for modern helioconcentrators.35 Therefore, by increased surface area of the catalyst, a higher energy flux density of the incident light, the thermal isolation of the reactor, and utilization of the heat of gas products, the energy conversion efficiency with the Ni/R-Al2O3 will be improved. From the economical viewpoint, the Ni/R-Al2O3 is one of the most coat-effective industrial catalysts for reforming. Moreover, the R-phase of alumina used for the catalyst support is stable even at high temperatures above 1273 K. The problems may arise due to carbon deposition on the surface of the Ni/R-Al2O3 catalyst, as many researchers reported on the nickel catalysts supported on alumina. However, the catalytic stability of Ni/Al2O3 catalysts may be improved, probably by alkali and alkaline-earth metal ions.26,27,29,30,32 For example, Horiuch et al.32 reported that the added oxides of Na, K, Mg, and Ca into Ni/Al2O3 catalyst markedly suppressed the carbon deposition during the CO2 reforming. Gadalla et al.26,27 reported that the serious carbon buildup on Ni/Al2O3 catalysts tends to disappear as magnesia is substituted for alumina, leading to enhanced catalyst stability. Further investigations to improve the activity and stability of Ni/R-Al2O3 catalyst will be needed. EF000226N (35) Segal, A.; Epstein, M. J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 1999, 9, Pr3-53-Pr3-58.

Nickel Catalyst Driven by Direct Light Irradiation for Solar CO2

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