Formation of Hydrogen through the Decomposition of Kerosene over

Sakae Takenaka*, Yoji Kobayashi, and Kiyoshi Otsuka. Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of ...
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Energy & Fuels 2004, 18, 1775-1783

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Formation of Hydrogen through the Decomposition of Kerosene over Nickel-Based Catalysts Sakae Takenaka,* Yoji Kobayashi, and Kiyoshi Otsuka* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan Received April 23, 2004. Revised Manuscript Received August 4, 2004

The formation of hydrogen, without CO or CO2, through the decomposition of kerosene at 773873 K was performed. Ni/TiO2 catalyst showed the highest activity and the longest life among all the catalysts tested in the present study for the decomposition of a mixture of tertbutylcyclohexane, n-dodecane, and diethylbenzene (denoted as kerosene). Benzothiophene, as an impurity added in kerosene, did not affect the catalytic performance of Ni/TiO2 significantly for the hydrogen formation through the decomposition of kerosene. Therefore, the production of hydrogen without CO or CO2 from commercially available kerosene, which contains impurity sulfur, could be realized. Supported nickel catalysts were deactivated rapidly during the decomposition of diethylbenzene, whereas hydrogen was formed efficiently through the decomposition of tert-butylcyclohexane and n-dodecane. The catalytic performance of supported nickel catalysts for the decomposition of diethylbenzene was improved by the addition of Zn species. The addition of Zn species decreased the average crystallite size of nickel metal, which improved the catalytic performance for the decomposition of diethylbenzene.

1. Introduction Hydrogen is a clean fuel in the sense that no CO2 is emitted when it is used in H2-O2 fuel cells. Currently, hydrogen is produced mainly through the steam reforming of methane, which is the main component of natural gas, followed by a water gas shift reaction of CO. The steam reforming of methane is expected to be one of the promising methods for supplying hydrogen to the stationary fuel cells. The facility for hydrogen production should be constructed near pipelines of natural gas, because the storage and transportation of liquefied natural gas requires large energy inputs, because of the low liquefaction temperature of methane. Infrastructures for the distribution of natural gas as a utility gas have been already accomplished in urban areas of Japan. However, areas far removed from natural gas pipelines or less-populated areas in Japan, where infrastructures for the distribution of natural gas are lacking, must look for an alternative energy source for the hydrogen production. We believe that kerosene is one of the alternatives to natural gas. Kerosene is an inexpensive fuel and is easily available in Japan, because the distribution infrastructure has been accomplished throughout the country. Therefore, the steam reforming of kerosene has been recently expected to be a promising method of supplying hydrogen to fuel cells, especially in Japan.1,2 However, CO is contained inevitably in the hydrogen gas produced by the steam reforming of hydrocarbons such as natural gas and * Author to whom correspondence should be addressed. Telephone: 81-35734-2626. FAX: 81-35734-2879. E-mail: [email protected]. (1) Fukunaga, T.; Katsuno, H.; Matsumoto, H.; Takahashi, O.; Akai, Y. Catal. Today 2003, 84, 197-200. (2) Suzuki, T.; Iwanami, H.; Yoshinari, T. Int. J. Hydrogen Energy 2000, 25, 119-126.

kerosene, followed by the water gas shift reaction of CO. Polymer electrolyte fuel cells (PEFCs) require a thorough elimination of carbon monoxide (CO) from the fuel (H2), because CO strongly poisons the electrocatalysts in the cells. Therefore, the removal of CO through selective CO oxidation is required. This purification of CO inevitably adds extra cost and volume for the reformer. The decomposition of hydrocarbons to hydrogen and carbon is of current interest, from a viewpoint of an alternative route of hydrogen production.3-9 Because no CO was contained in the products from the hydrocarbon decomposition, the hydrogen can be supplied directly to PEFC. In the present study, the hydrogen production through the decomposition of kerosene was examined. Concerning the catalysts for the decomposition of methane, we have reported that the use of supported nickel catalysts is one of the most effective methods.10 The catalytic activity and life of supported nickel catalysts for the decomposition of methane were strongly dependent on the type of catalytic supports, as well as the type of metal additives. Ni/SiO2 showed higher catalytic activity and longer life for the decomposition of methane than other supported nickel catalysts such as Ni/TiO2, (3) Muradov, N. Z. Int. J. Hydrogen Energy 1993, 18, 211-215. (4) Shah, N.; Panjala, D.; Huffman, G. P. Energy Fuels 2001, 15, 1528-1534. (5) Zhang, T.; Amiridis, M. D. Appl. Catal., A 1998, 167, 161-172. (6) Li, Y.; Chen, J.; Chang, L.; Qin, Y. J. Catal. 1998, 178, 76-83. (7) Ermakova, M. A.; Ermakov, D. Y.; Kuvshinov, G. G. Appl. Catal., A 2000, 201, 61-70. (8) Choudhary, T. V.; Sivadinarayana, C.; Chusuei, C. C.; Klinghoffer, A.; Goodmann, D. W. J. Catal. 2001, 199, 9-18. (9) Otsuka, K.; Seino, T.; Kobayashi, S.; Takenaka, S. Chem. Lett. 1999, 1179-1180. (10) Otsuka, K.; Kobayashi, K.; Takenaka, S. Appl. Catal., A 2000, 190, 261-268.

10.1021/ef0498972 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/10/2004

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Ni/Al2O3, and Ni/MgO.11,12 The addition of palladium or copper into supported nickel catalysts improved the catalytic life significantly for the decomposition of methane.13-15 However, the catalytic performance of these nickel-based catalysts could be different among the types of hydrocarbons that are decomposed. Therefore, the catalysts that are active for the decomposition of kerosene (a mixture of linear alkanes, cycloalkanes, and aromatics) are worth exploring. In addition, kerosene contains aromatic sulfur compounds such as benzothiophene derivatives as an impurity. For example, kerosene of a Japanese Industrial Standard grade (JIS1) contained ca. 30-55 ppm sulfur impurities. It is generally accepted that sulfur impurities would strongly deactivate catalysts for many catalytic reactions. Therefore, the effect of sulfur impurity on the decomposition of kerosene should be also investigated. In the present study, we perform the decomposition of kerosene to form hydrogen, without CO or CO2, over different catalysts and demonstrate the superior catalytic performance of Ni/TiO2 for the reaction. In addition, the catalytic performance of Ni/TiO2 catalysts for the decomposition of aromatics was improved by the addition of Zn species. 2. Experimental Section All the catalysts used in the present study were prepared by a conventional impregnation method. Vapor-grown carbon fiber (VGCF, specific surface area of 10 m2/g, from Showa Denko), Al2O3 (JRC-ALO4, specific surface area of 140 m2/g), SiO2 (Cab-O-Sil, specific surface area of 200 m2/g, from Cabot Co.), and TiO2 (JRC-TIO4, specific surface area of 40 m2/g) were utilized as catalytic supports. JRC-ALO4 and JRC-TIO4 were supplied by the Catalysis Society of Japan as reference catalysts. Fe(NO3)3, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2, and PdCl2 were utilized as metal sources. Catalytic supports were impregnated with aqueous solutions that contained metal cations at 363 K, and the solvents were evaporated to dryness at 363 K. The dried samples except for the sample supported on VGCF were calcined in air at 873 K for 5 h. The loading of iron, cobalt, and nickel metal supported on the catalytic supports was adjusted to 20 wt %. Cu or Pd species were added to the Ni (20 wt %)/SiO2 catalysts, with a molar ratio of Cu/ (Cu + Ni) ) 0.1 or Pd/(Pd + Ni) ) 0.5, respectively. Zn species were added to the Ni (20 wt %)/TiO2 catalyst, with a molar ratio of Zn/Ni ) 0.2. The decomposition of kerosene was performed, using a conventional gas flow system with a fixed catalyst bed. Catalyst samples were packed in a tubular reactor that was made from quartz (inner diameter of 2 cm and length of 60 cm). Prior to the reaction, catalyst samples were treated with hydrogen at 573 K and were heated at the reaction temperatures (773 or 873 K) for 30 min under argon. The decomposition of kerosene was initiated by contacting kerosene diluted with argon (partial pressure of kerosene of 4 kPa, total flow rate of 60 mL/min) with the catalysts. After effluent gases through the catalyst bed were introduced into a trap that had been cooled at 273 K, a portion of the gases was sampled and analyzed via gas chromatography (GC). (11) Takenaka, S.; Ogihara, H.; Yamanaka, I.; Otsuka, K. Appl. Catal., A 2001, 217, 101-110. (12) Wang, P.; Tanabe, E.; Ito, K.; Jia, J.; Morioka, H.; Shishido, T.; Takehira, K. Appl. Catal., A 2002, 231, 35-44. (13) Reshetenko, T. V.; Avdeeva, L. B.; Ismagilov, Z. R.; Chuvilin, A. L.; Ushakov, V. A. Appl. Catal., A 2003, 247, 51-63. (14) Takenaka, S.; Shigeta, Y.; Tanabe, E.; Otsuka, K. J. Catal. 2003, 220, 468-477. (15) Takenaka, S.; Shigeta, Y.; Otsuka, K. Chem. Lett. 2003, 32, 2627.

Takenaka et al. Two types of substrates were utilized for the kerosene decomposition in the present study. One is a mixture of n-dodecane (n-C12H26, 52 wt %), tert-butylcyclohexane (C6H11C(CH3)3, 21 wt %), and diethylbenzene (C6H4(C2H5)2, 27 wt %), which, hereafter, is denoted as the model kerosene. The composition of the model kerosene corresponds to 55 vol % n-dodecane, 20 vol % tert-butylcyclohexane, and 25 vol % diethylbenzene. The other is the commercially available JIS-1 grade of kerosene (denoted hereafter as the commercial kerosene). The composition of the commercial kerosene is 82 vol % saturated hydrocarbons and 18 vol % aromatics. In addition, the commercial kerosene contains 45 ppm of sulfur impurity. To examine the effect of sulfur impurity on the kerosene decomposition, 16 and 125 ppm of benzothiophene were added into the model kerosene. X-ray diffraction (XRD) patterns of the catalyst samples were measured by a Rigaku model RINT 2500V diffractometer, using Cu KR radiation, at room temperature. Scanning electron microscopy (SEM) images of carbon that was deposited on the catalysts by the decomposition of hydrocarbons were obtained using a Hitachi model FE-SEM S-800 field-emission-gun scanning electron microscope. X-ray absorption spectra (XANES and EXAFS) were measured on the beamline BL-9A facility at the Photon Factory in the Institute of Materials Structure Science for High Energy Accelerator Research Organization at Tsukuba in Japan (Proposal No. 2002G255). Ni and Zn K-edge XANES and EXAFS of the catalyst samples were measured in fluorescence mode with a Si(111) two-crystal monochromator at room temperature. The nickel loading in the catalysts for the measurements of Ni K-edge XANES/EXAFS was adjusted to 10 wt %. Prior to the measurements of XANES and EXAFS, the catalyst samples, which had been reduced with hydrogen at 573 K, were contacted with the model kerosene at 773 K. After the reaction, the catalyst samples were cooled to room temperature under an argon stream. Each catalyst with deposited carbon was then packed in a polyethylene bag under an argon atmosphere. The analysis of EXAFS data was performed using an EXAFS analysis program (REX, Rigaku Co.). For EXAFS analysis, the oscillation was extracted from the EXAFS data by a spline smoothing method. The oscillation was normalized by the edge height of ∼70-100 eV above the threshold. The Fourier transformation of k3-weighted EXAFS oscillation was performed over a k-range of 3.5-14.5 Å-1. Inversely Fourier transformed data for each Fourier peak were analyzed by a curve-fitting method, using theoretical phaseshift and amplitude functions that were derived by the FEFF8 program.16

3. Results and Discussion 3.1. Decomposition of the Model Kerosene. Figure 1 shows changes in the formation rates of hydrogen (Figure 1a) and methane (Figure 1b), as a function of the time-on-stream during the decomposition of the model kerosene at 773 K over different nickel-based catalysts. Ni/VGCF, Ni/SiO2, Ni/TiO2, and Ni-Cu/SiO2 were used as catalysts. For all the reactions, the formation of hydrogen and methane was observed as gaseous products. All the catalysts showed high activity for the hydrogen formation at early stages of the decomposition of the model kerosene; however, the formation rate of hydrogen declined gradually or quickly as the time-on-stream value increased. The formation rate of hydrogen at early stages of the reactions ( Ni-Cu/SiO2 > Ni/VGCF > Ni/SiO2. The life of the catalysts became longer in the following order: Ni/TiO2 ≈ Ni-Cu/SiO2 > Ni/VGCF ≈ Ni/SiO2. When the formation of hydrogen was terminated, methane could not be observed at the same time in the effluent gases from the catalyst bed. In addition, any hydrocarbons such as ethane and ethene, except for methane, were not observed in the effluent gases through a trap cooled at 273 K. We have reported the decomposition of various alkanes and alkenes of C2-C8 over Ni/SiO2.17,18 These reactions produced hydrogen and methane as the primary gaseous products, regardless of the type of hydrocarbons that were decomposed. Therefore, we concluded that the formation of methane resulted from the hydrogenation of carbon that had been deposited on the catalysts with gaseous hydrogen. Note that hydrogen is consumed during the kerosene decomposition through the hydrogenation of carbon to methane. The hydrogen yield in the kerosene decomposition would be improved if the hydrogenation of carbon to methane does not proceed. The yields of hydrogen and methane in the kerosene decomposition shown in Figure 1 were evaluated by integrating the formation rates of these products against time-on-stream. The results are shown in Figure 2. The yields of hydrogen and methane for the catalysts that are not shown in Figure 1 are also listed in Figure 2. It is generally accepted that iron, cobalt, and nickel metals are effective catalysts for the decomposition of hydrocarbons; in particular, nickel catalysts showed the highest yield of hydrogen through the methane decomposition in the temperature range of 773-873 K among the three catalysts.10,19-23 However, in the case of the decomposition of kerosene, the yields of hydrogen for iron, cobalt, and nickel supported on Al2O3 were similar to each other. In contrast, the hydrogen yields for (17) Otsuka, K.; Kobayashi, S.; Takenaka, S. Appl. Catal., A 2001, 210, 371-379. (18) Otsuka, K.; Shigeta, Y.; Takenaka, S. Int. J. Hydrogen Energy 2002, 27, 11-18. (19) Takenaka, S.; Serizawa, M.; Otsuka, K. J. Catal. 2004, 222, 520-531. (20) Baker, R. T. K. Carbon 1989, 27, 315-323.

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Figure 2. Yields of hydrogen and methane in the decomposition of the model kerosene at 773 K. Catalysts ) 0.090 g, partial pressure of the model kerosene ) 4 kPa, total flow rate ) 60 mL/min.

supported nickel catalysts were strongly dependent on the type of catalytic supports. The hydrogen yield for the Ni/TiO2 catalyst was the highest among those for all the supported nickel catalysts. We have examined the effects of catalytic supports for nickel catalysts on the catalytic activity and life for the decomposition of methane.11 In the case of methane decomposition at 773 K, the Ni/SiO2 catalyst was more active, in comparison to Ni/VGCF, Ni/Al2O3, and Ni/TiO2. The difference in the catalytic performance of the supported nickel catalysts for the decomposition of methane, according to the type of the catalytic support, resulted from the difference of size distribution of nickel metal particles on the catalysts, i.e., nickel metal particles with diameters of 40-100 nm showed higher activity and longer life for the methane decomposition.21,24 The size distribution of nickel metal particles was changed, according to the type of catalytic supports.11,25 An average size of the nickel metal particles on Ni/TiO2 was relatively smaller than those on Ni/SiO2 and Ni/VGCF. Therefore, it is likely that relatively smaller nickel metal particles were more effective for the kerosene decomposition. On the other hand, Ni-Cu or Ni-Pd alloys showed a high catalytic activity and long catalytic life for the decomposition of methane at 773-1073 K.13,14 For the decomposition of the model kerosene at 773 K, the hydrogen yield for Ni-Cu/SiO2 was higher than that for Ni/SiO2 or Ni-Pd/SiO2. However, the yield of hydrogen for NiCu/SiO2 was appreciably lower than that for Ni/TiO2 and the yield of methane, relative to that of hydrogen, for the former catalyst was higher than that for the latter catalyst. Figure 3 shows the change in the formation rates of hydrogen (Figure 3a) and methane (Figure 3b), as a function of the time-on-stream in the decomposition of the model kerosene over different nickel-based catalysts at 873 K. As for all the catalysts, the formation rates of hydrogen and methane at 873 K were higher than those (21) Takenaka, S.; Kobayashi, S.; Ogihara, H.; Otsuka, K. J. Catal. 2003, 217, 79-87. (22) Lee, C. J.; Park, J.; Yu, J. A. Chem. Phys. Lett. 2002, 360, 250255. (23) Avdeeva, L. B.; Kochubey, D. I.; Shaikhutdinov, S. K. Appl. Catal., A 1999, 177, 43-51. (24) Otsuka, K.; Ogihara, H.; Takenaka, S. Carbon 2003, 41, 223233. (25) Takenaka, S.; Kato, E.; Tomikubo, Y.; Otsuka, K. J. Catal. 2003, 219, 176-185.

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Figure 3. Change in the formation rates of hydrogen (a) and of methane (b) as a function of time on stream during the decomposition of the model kerosene over Ni-based catalysts at 873 K. Catalysts ) 0.090 g, partial pressure of the model kerosene ) 4 kPa, total flow rate ) 60 mL/min.

Figure 4. Yields of hydrogen and methane in the decomposition of the model kerosene at 873 K. Catalysts ) 0.090 g, partial pressure of the model kerosene ) 4 kPa, total flow rate ) 60 mL/min.

at 773 K (shown in Figure 1). The catalytic life did not change significantly with increasing reaction temperatures. The formation of methane was enhanced, compared to that of hydrogen, in the reaction of the Ni-Cu/ SiO2 catalyst when the reaction temperature increased from 773 K to 873 K. The increase of the formation rate of methane is not preferable, from the viewpoint of hydrogen formation, because the methane formation decreases the yield of hydrogen. Figure 4 shows the yields of hydrogen and methane in the decomposition of the model kerosene at 873 K. Hydrogen yields for all the catalysts became higher by increasing the reaction temperatures from 773 K to 873 K. The hydrogen yields at 873 K were higher in the order of Ni/TiO2 > Pd-Ni/SiO2 > Fe/Al2O3 > Ni-Cu/SiO2, whereas the order at 773 K was Ni/TiO2 > Ni-Cu/SiO2 > Ni-Pd/SiO2 > Ni/VGCF. From the results described previously, Ni/TiO2 is the most effective catalyst for the hydrogen formation through the decomposition of the model kerosene at 773-873 K. Therefore, the catalytic performance of Ni/ TiO2 for the kerosene decomposition was investigated in detail.

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Figure 5. Change in the formation rates of (a) hydrogen and (b) methane as a function of time-on-stream in the decomposition of the model kerosene with different levels of benzothiophene and the commercial kerosene. Mass of Ni/TiO2 catalysts ) 0.090 g, partial pressure of kerosene ) 4 kPa, total flow rate ) 60 mL/min ((O) the model kerosene without benzothiophene, (b) the model kerosene with benzothiophene of 16 ppm, (2) the model kerosene with benzothiophene of 125 ppm, and (4) the commercial kerosene).

3.2. Effect of Benzothiophene Impurity on the Decomposition of Kerosene. Generally, commercially available kerosene contains benzothiophene derivatives. Therefore, the effects of benzothiophene impurity on the decomposition of the model kerosene were examined. Model kerosenes with different levels of benzothiophene (0, 16, and 125 ppm) were decomposed over Ni/TiO2 catalysts at 873 K. The results were shown in Figure 5. The decomposition of the model kerosenes over a Ni/ TiO2 catalyst proceeded to form hydrogen and methane as gaseous products, regardless of the level of benzothiophene impurity. Note that the formation rates of hydrogen at early stages of the reactions ( diethylbenzene. Note that Ni/ SiO2 was deactivated immediately after contact with diethylbenzene. We have examined the reactivities of different types of C6 hydrocarbons (n-hexane, cyclohexane, cyclohexene, and benzene) on Ni/SiO2.18,28 The catalytic life of Ni/SiO2 was significantly shorter during the decomposition of benzene, compared to that for the other hydrocarbons. These results indicate that supported nickel catalysts are deactivated quickly during the decomposition of aromatics. As will be described below in detail, carbon nanofibers were formed by the decomposition of any hydrocarbons used in the present study. A nickel metal particle was present at the tip of each carbon nanofiber.20 The exposed surfaces of nickel metal particles present at the tips of carbon nanofibers decomposed kerosene to carbon and hydrogen. The process was followed by the diffusion of C atoms in the bulk of a nickel metal particle from one side of the particle. The precipitation of C atoms at the other side of the nickel metal particles formed carbon nanofibers. Aromatics such as diethylbenzene would interact more strongly with the surface of nickel metal via π-electron in aromatics, compared to alkanes and alkenes. The strong interaction of aromatics with nickel metal would result in a rapid deposition of C atoms on the nickel metal surface. Therefore, the deposition rate of C atoms on the nickel metal surface from aromatics may overcome the diffusion rate of C atoms in the bulk of nickel metal, which must cause the formation of graphite layers on the active nickel metal surface. This could be one of reasons why supported nickel catalysts are deactivated quickly during the decomposition of aromatics. The rapid deactivation of supported nickel catalysts for the decomposition of diethylbenzene is dramatically improved by the addition of Zn species into the catalysts. Figure 8 shows the changes in the formation rates of (28) Otsuka, K.; Abe, Y.; Kanai, N.; Kobayashi, Y.; Takenaka, S.; Tanabe, E. Carbon 2004, 42, 727-736.

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Figure 8. Change in the formation rates of (a) hydrogen and (b) methane as a function of time-on-stream during the decomposition of diethylbenzene at 773 K over Ni/TiO2 and Ni-Zn/TiO2 catalysts. Mass of catalysts ) 0.02 g, partial pressure of diethylbenzene ) 4 kPa, and total flow rate ) 60 mL/min.

Figure 9. Change in the formation rates of (a) hydrogen and (b) methane as a function of time-on-stream in the decomposition of the model kerosene at 773 K over Ni/TiO2 and Ni-Zn/ TiO2 catalysts. Mass of catalysts ) 0.09 g, partial pressure of the model kerosene ) 4 kPa, and total flow rate ) 60 mL/ min.

hydrogen and methane, as a function of the time-onstream in the decomposition of diethylbenzene over the Ni/TiO2 catalyst and the catalyst with Zn species (denoted hereafter as Ni-Zn/TiO2 catalyst). Zinc was added into the Ni/TiO2 catalysts with a molar ratio of Zn/Ni ) 0.2. Catalytic activity of Ni/TiO2 without Zn species for the decomposition of diethylbenzene at 773 K was very poor, even at early stages of the reaction, and a trace of hydrogen was detected in the effluent gases after 10 min. The addition of zinc into Ni/TiO2 increased the formation rate of hydrogen significantly and improved the catalytic life. Nevertheless, the formation rate of methane was not increased by the addition of zinc into the catalyst. The decomposition of the model kerosene was performed at 773 K over the Ni-Zn/TiO2 catalyst. The results are shown in Figure 9. The formation rate of hydrogen at early stages of the reaction for Ni-Zn/TiO2 was less than that for Ni/TiO2, although the catalytic life was improved slightly by the addition of Zn species. Hydrogen yields for the Ni/TiO2 and Ni-Zn/TiO2 catalysts in the kerosene decomposition at 773 K were estimated to be 186 and 144 mmol g-cat-1, respectively. These results indicated that the addition of zinc into

H2 Formation through Kerosene Decomposition

Figure 10. Scanning electron microscopy (SEM) images of carbon formed by the decomposition of hydrocarbons: (a) carbon deposited on Ni/TiO2 by the decomposition of the model kerosene with 0 ppm, (b) carbon deposited on Ni/TiO2 by the decomposition of the model kerosene with 16 ppm, (c) carbon deposited on Ni/TiO2 by the decomposition of the model kerosene with 125 ppm of benzothiophene at 873 K, (d), carbon deposited on Ni/TiO2 by the decomposition of the commercial kerosene at 873 K, and (e) carbon deposited on a Ni-Zn/TiO2 catalyst by the decomposition of diethylbenzene at 773 K.

Ni/TiO2 is not effective for the decomposition of kerosene. It is likely that the addition of Zn species into Ni/ TiO2 improved the catalytic performance for the decomposition of aromatics only. 3.5. Carbons Formed by Kerosene Decomposition. Figure 10 shows SEM images of carbon deposited on Ni/TiO2 catalysts by the decomposition of the model kerosene without and with benzothiophene at 873 K, and by the decomposition of a commercial kerosene at 873 K. An SEM image of carbon deposited on the NiZn/TiO2 catalyst by the decomposition of diethylbenzene at 773 K is also shown in Figure 10e. The Ni/TiO2

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catalyst produced carbon nanofibers preferentially through the decomposition of the model kerosene without benzothiophene, as shown by Figure 10a. The diameters of the carbon nanofibers were in the range of 30-60 nm. A nickel metal particle was observed at the tip of each carbon nanofiber from the backscattering electron images (BEIs) measured at the same time as the SEM images (the BEIs are not shown). The diameter of the carbon nanofibers was controlled by the size of the nickel metal particles present at the tips of the fibers.21 The nickel metal particles at the tips of the carbon nanofibers decomposed kerosene to grow the fibers.20 As shown in Figure 10b and 10c, carbon nanofibers with diameters of 30-60 nm were produced by the decomposition of the model kerosene with 16 and 125 ppm of benzothiophene. The diameter ranges of the carbon nanofibers observed in Figures 10a, b, and c were similar to each other, irrespective of the different contents of benzothiophene impurity. In addition, carbon nanofibers similar to those produced from the model kerosene (images a-c) were formed by the decomposition of a commercial kerosene (image d). These results implied that the benzothiophene impurity in kerosene did not affect the growth mechanism of the carbon nanofibers in the decomposition of kerosene over the Ni/ TiO2 catalyst. On the other hand, the decomposition of diethylbenzene over the Ni-Zn/TiO2 catalyst produced carbon nanofibers of a shape similar to those formed from the model kerosene and the commercial kerosene. However, the diameters of the carbon nanofibers from diethylbenzene on Ni-Zn/TiO2 (image e) were smaller (20-40 nm) than those from the kerosenes (30-60 nm). It is likely that diethylbenzene is decomposed preferentially on smaller nickel metal particles to form carbon nanofibers with smaller diameters. The addition of Zn species into Ni/TiO2 may decrease the average size of the nickel metal particles. 3.6. Structures of Catalytically Active Components in the Catalysts. As described previously, the addition of Zn species into Ni/TiO2 improved the catalytic performance for the decomposition of diethylbenzene significantly. Therefore, the structures of Zn and Ni species in Ni-Zn/TiO2 were investigated by Zn and Ni K-edge XANES and EXAFS. The structure of Ni species in Ni-Zn/TiO2 after the decomposition of the model kerosene was compared to that in Ni/TiO2 to clarify the effect of zinc addition. The model kerosene was used as the substrate of the reaction, because Ni/ TiO2 did not exhibit the catalytic activity for the decomposition of diethylbenzene as shown in Figure 8. The loading of nickel in Ni/TiO2 and Ni-Zn/TiO2 for the measurement of XANES/EXAFS was adjusted to 10 wt %. Zn species were added into Ni/TiO2 with a molar ratio of Zn/Ni ) 0.1. Figure 11 shows Zn K-edge XANES spectra of Ni-Zn/TiO2 catalyst before and after the decomposition of the model kerosene at 773 K, as well as the spectra of ZnO and zinc foil as references. The threshold for XANES spectrum of the fresh Ni-Zn/TiO2 catalyst was observed at ∼9665 eV and the position of the threshold was similar to that for ZnO reference. The position of threshold for XANES spectra of metal species is sensitive to the oxidation state of the metal species.29 (29) Tanaka, T.; Yoshida, T.; Yoshida, S.; Baba, T.; Ono, Y. Physica B 1995, 208&209, 687-688.

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Figure 11. Zn K-edge XANES spectra of Ni-Zn/TiO2 before and after the decomposition of the model kerosene at 773 K, and the spectra of a zinc foil and ZnO as references.

XANES spectrum of the fresh catalyst did not change after the decomposition of the model kerosene. These results implied that Zn species in Ni-Zn/TiO2 were always present as Zn2+ during the reaction. Zn K-edge XANES spectra of Ni-Zn/TiO2 are not similar to that of ZnO, which is composed of a tetrahedral ZnO4 unit, but to the spectrum of ZnCO3, which is composed of an octahedral ZnO6 unit.30,31 Therefore, Zn species in the Ni-Zn/TiO2 catalyst were stabilized as octahedral ZnO6 mainly during the decomposition of kerosene. It was reported that zinc and TiO2 formed several titanates, such as ZnTiO3, Zn2Ti3O8, and Zn2TiO4.32-34 XRD patterns of Ni-Zn/TiO2 before and after the decomposition of kerosene did not show any peaks assignable to these titanates containing Zn species, because of a low loading of zinc. Therefore, crystallized titanates containing Zn species would not be formed on the Ni-Zn/TiO2 catalyst. Among these titanates (ZnTiO3, Zn2Ti3O8, and Zn2TiO4), ZnTiO3 is composed of an octahedral ZnO6 unit. The compound oxides such as ZnTiO3 may be formed on the Ni-Zn/TiO2 catalyst. Figure 12 shows Ni K-edge XANES spectra of Ni/TiO2 and Ni-Zn/TiO2 after the decomposition of the model kerosene at 773 K, as well as the spectrum of nickel foil. Ni K-edge XANES spectrum of Ni-Zn/TiO2 was consistent with that of Ni/TiO2. The XANES spectra of these catalysts were similar to that of nickel foil, although two peaks observed in the range of 8345-8360 eV were not as clear for these catalysts as those for nickel foil. These results suggested that Ni species in Ni-Zn/TiO2 and Ni/TiO2 were present as nickel metal crystallites during the decomposition of kerosene. Figure 13 shows Fourier transforms of Ni K-edge k3weighted EXAFS (radial structure functions, RSFs) of Ni/TiO2 and Ni-Zn/TiO2 after the decomposition of the model kerosene at 773 K, as well as a RSF of the nickel (30) Takahashi, M.; Tanida, H.; Kawauchi, S.; Harada, M.; Watanabe, I. J. Synchrotron Rad. 1999, 6, 278-280. (31) Waychunas, G. A.; Fuller, C. C.; Davis, J. A.; Rehr, J. J. Geochim. Cosmochim. Acta 2003, 67, 1031-1043. (32) Yamaguchi, O.; Morimi, M.; Kawabata, H.; Shimizu, K. J. Am. Ceram. Soc. 1987, 70, C-97-C-98. (33) Bartram, S. T.; Slepetys, R. A. J. Am. Ceram. Soc. 1961, 44, 493-499. (34) Kim, H. T.; Kim, Y.; Matjaz, V.; Suvorov, D. J. Am. Ceram. Soc. 2001, 84, 1081-1086.

Takenaka et al.

Figure 12. Ni K-edge XANES spectra of Ni/TiO2 and Ni-Zn/ TiO2 after the decomposition of the model kerosene at 773 K and the spectrum of nickel foil.

Figure 13. Fourier transforms of Ni K-edge k3-weighted EXAFS of Ni/TiO2 and Ni-Zn/TiO2 after the decomposition of the model kerosene at 773 K, and EXAFS of nickel foil.

foil. The RSF features for all the samples shown in Figure 13 were similar to each other, although the intensity of the peaks was different among the three samples. Therefore, Ni species in Ni/TiO2 and Ni-Zn/ TiO2 were present as nickel metal crystallites. The peak at 2.1 Å in the RSF of Ni/TiO2 was more intense than that of Ni-Zn/TiO2, which suggests that an average crystallite size of nickel metal on Ni-Zn/TiO2 was smaller than that for Ni/TiO2. The structural parameters of nickel metal crystallites on these catalysts were estimated by a curve-fitting method for the EXAFS. Inversely Fourier transformed data of the Fourier peak in an R-range of 1.0-2.9 Å of the RSFs for Ni/TiO2 and Ni-Zn/TiO2 catalysts were analyzed. By the curve-fitting of EXAFS, the coordination number and the interatomic distance of Ni-Ni bonds were estimated to be 7.3 and 2.46 Å, respectively, for Ni/TiO2, and 5.8 and 2.46 Å, respectively, for Ni-Zn/TiO2. These results indicated that an average crystallite size of nickel metal on Ni/TiO2 became smaller by the addition of Zn species, which would improve the catalytic performance of Ni/TiO2 for the decomposition of diethylbenzene. We have reported that nickel metal particles on supported nickel catalysts were aggregated before the growth of carbon nanofibers, when the catalysts were contacted with hydrocarbons.35 As described previously, Zn species in Ni-Zn/TiO2 would (35) Takenaka, S.; Ogihara, H.; Otsuka, K. J. Catal. 2002, 208, 5463.

H2 Formation through Kerosene Decomposition

be present as the compound oxides such as ZnTiO3. Interaction between Ni species and ZnTiO3 may be stronger than that between Ni species and TiO2. Because the strong interaction of Ni species with the compound oxides such as ZnTiO3 would prevent the severe aggregation of nickel metal particles during the reaction, the sizes of the nickel metal particles on NiZn/TiO2 would be kept smaller during the reaction. 4. Conclusion On the basis of the results described previously, we have concluded the following:

Energy & Fuels, Vol. 18, No. 6, 2004 1783

(1) Ni/TiO2 was the most effective catalyst for the hydrogen formation through the decomposition of kerosene at 773-873 K among all the catalysts tested in the present study. (2) Benzothiophene impurities in kerosene did not appreciably affect the catalytic activity or life of Ni/TiO2 for the decomposition of kerosene. (3) The addition of Zn species into Ni/TiO2 catalysts improved the catalytic performance for the decomposition of diethylbenzene. EF0498972