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Steam Reforming of n-Dodecane over Ru-Ni-Based Catalysts Vidya Sagar Guggilla,*,† Jale Akyurtlu,† Ates Akyurtlu,† and Isaiah Blankson‡ Chemical Engineering Department, Hampton UniVersity, Hampton, Virginia 23668, and NASA Glenn Research Center, CleVeland, Ohio 44135
Steam reforming of n-dodecane has been carried out with the goal of development of new and highly active catalysts for hydrogen production. Newly designed RuO-NiO-CeO2-Al2O3 catalysts have been successfully prepared with various loadings of Ni by sol-gel method. X-ray diffraction (XRD), H2 TPD, BET surface area, temperature-programmed reduction (TPR), and temperature-programmed desorption of CO2 (TPD of CO2) were used to characterize the prepared catalysts. The coke formation was studied by temperature programmed oxidation (TPO). The reforming of n-dodecane was carried out in a microreactor and investigated at different reaction temperatures, space velocity, steam/carbon ratio, and time-on-stream. Characterization results reveal that the presence of Ru and CeO2 enhances the catalyst reducibility. H2 TPD and CO2 TPD data indicate that 1 wt % Ru/2.5 wt % Ni/3 wt % CeO2/Al2O3 (1R2.5N3CA) catalyst exhibits larger nickel surface area and higher basicity compared to all the other catalysts. The activity and hydrogen yield of the 1R2.5N3CA catalyst are significantly higher than those of the other nickel catalysts under the same experimental conditions. Catalytic stability is also enhanced by the presence of ruthenium in nickel catalysts. Such improvement indicates that ruthenium plays an important role in the catalytic action of nickel. Overall, the bimetallic 1R2.5N3CA may be an effective catalyst for the production of hydrogen from n-dodecane. 1. Introduction Today’s rising concerns of global warming and environmental pollution has spurred an interest in alternative fuels. Among all alternatives, hydrogen has a potential to be an excellent energy carrier. It can be used directly as a fuel in internal combustion engines or indirectly to supply electricity using fuel cells. But, currently the use of hydrogen gas as an energy carrier is fraught with several technical problems. Low density makes it difficult and expensive to store hydrogen at high-energy density levels. This problem can be overcome by the production of hydrogen on site and on demand by reforming high-energy density fuels, such as methanol, ethanol, gasoline, jet fuel, or diesel.1 Reforming of jet fuel has military and general aviation applications. Jet fuel JP-8 is used by the U.S. Air Force, by the Navy, and by most of the diesel and gas turbine engines of the Army. Therefore, on-site or on-board reforming of jet fuel can provide hydrogen for fuel cells powering vehicles and portable devices. On-board generation of hydrogen from the reforming of jet fuel can also be used for fuel cells to provide the auxiliary power needs of civilian jet planes, thus, reducing the air pollution generated for this purpose. The conversion of hydrocarbon fuels to hydrogen can be carried out by several different reforming approaches including steam reforming, partial oxidation, and autothermal reforming.2-6 Catalytic steam reforming is an efficient process that yields high H2 concentrations. However, the presence of sulfur and heavy hydrocarbons in liquid fuels could cause deactivation of reforming catalysts;3,7 therefore, developing a highly sulfurtolerant and carbon-resistant catalyst for H2 production at high temperatures is critical for steam reforming of liquid hydrocarbon fuels (for example, of gasoline, jet fuel, or diesel). Industrially, hydrogen production is generally conducted by steam reforming of natural gas over nickel-based catalysts,8-10 * To whom correspondence should be addressed. Tel: (304)2939375. Fax: (304) 293-4139. E-mail:
[email protected]. Presently at the West Virginia University. † Hampton University. ‡ NASA Glenn Research Center.
generally supported on alumina and magnesium-aluminate because of their stabilities at high temperature.11 Numerous studies have been conducted to determine processing conditions and catalyst formulations that minimize carbon formation during steam reforming.12-16 However, the use of higher hydrocarbons that contain aromatics can lead to carbon formation during steam reforming both on the catalyst and before the catalyst bed.17 One approach to minimizing carbon formation in reforming is by changing the process conditions, such as increasing the steam to carbon ratio. Another approach is to use carbon-resistant catalysts. The first option will result in an increase in the energy and processing costs. One approach to develop carbon-resistant catalysts is to add a second catalytic species. Many researchers have tried an alkaline earth oxide, such as MgO and CaO, as a promoter to NiO/Al2O3,18-23 but the presence of such alkaline oxides in relatively small amounts is known to inhibit the reduction of NiO. Horiuchi et al.23 reported that the reforming activity of Ni in the CO2-reforming of CH4 is somewhat depressed by the presence of alkaline earth metals, while the carbon deposition was markedly suppressed. Introducing small amounts of molybdenum or tungsten into Ni catalysts has been shown to increase coking resistance with no decrease in catalytic activity.24-26 In literature, alkaline earth and rare earth oxides are usually added to the nickel-based catalysts as promoters to optimize the activity of the catalyst for methane reforming.27,28 Ceria is one of the best promoters among all rare earth oxides. It possesses high oxygen mobility29 and high oxygen storage capacity,30 promotes strong metal-support interaction,31,32 and has the ability to modify crystalline phase transformations.33 Its presence has a beneficial effect on the catalyst activity and inhibits the rate of carbon formation. Wang and Lu34 reported that adding CeO2 into Ni/γ-Al2O3 catalysts enhanced the nickel dispersion and reactivity of carbon deposits, leading to improved catalytic activity and stability in methane reforming with CO2. Noble metals of high reforming activity and low coking tendency have, also, been tried for the Ni-based catalysts as promoters.18,35-38 Hou and Yashima39 reported that Ni/R-Al2O3
10.1021/ie100811g 2010 American Chemical Society Published on Web 07/23/2010
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doped with only a small amount of Rh showed higher activity than Ni/R-Al2O3 catalyst in the CO2-reforming of methane. According to the studies for the CO2-reforming of methane over Ni-Ru and Ni-Pd bimetallic catalysts, the strong improvement in the activity and the stability observed on silica-supported Ni-Ru catalysts was attributed to the formation of Ni-Ru bimetallic clusters which led to an increase in metallic dispersion of Ni and favored the formation of a more reactive intermediate carbonaceous species.40,41 Another approach to making carbon-resistant catalysts is to prepare the catalyst with a higher surface area than conventional catalysts. Preparation of metal oxides by the sol-gel method results in the retention of hydroxyl-rich surfaces, which exhibit unique textural and chemical properties compared with those prepared by other conventional methods. In particular, alumina materials prepared by a sol-gel method have high surface areas and controllable textural and chemical properties. It was reported that an alumina aerogel with a well-developed pore structure inhibited carbon deposition in the CO2-reforming of methane, and this resulted in an enhancement of both methane conversion and coke resistance.42,43 The role and effect of xerogel catalysts on the catalytic performance in the steam reforming of higher hydrocarbons have not yet been clearly investigated. Therefore, developing a sol-gel-derived Ru-Ni-CeO2-Al2O3 catalyst for the steam reforming of n-dodecane is of great interest. In the present study, the activity and selectivity toward the production of hydrogen on ceria-alumina-supported Ni and Ru catalysts were evaluated for the steam reforming of n-dodecane, as a surrogate for jet fuels. The influence of Ni loading, and promoting effect of Ru on the hydrogen yield and coking resistance were investigated. These effects are discussed in the light of the catalyst characterization results obtained from H2TPR, XRD, H2 TPD, TPO, and surface area measurements. 2. Experimental Section 2.1. Catalyst Synthesis. Quaternary xerogel catalysts were prepared by the sol-gel method. Nickel and cerium precursors used were in acetate and nitrate form, respectively. Ruthenium precursor used was in chloride form. For the aluminum precursor, aluminum trisec-butoxide (ATB) was used. The following sol-gel parameters were used during the synthesis: ruthenium content ) 1 wt.%, nickel content ) 2.5, 10, and 20 wt.%, and ceria content ) 3 wt.%. Initially a known amount of aluminum precursor (aluminum sec-butoxide, Sigma-Aldrich) was dissolved in ethanol at 353 K with vigorous stirring. For the partial hydrolysis of the aluminum precursor, small amounts of nitric acid and distilled water (40% nitric acid) solution, which had been mixed with ethanol, were slowly added to the solution containing the aluminum precursor, and stirred for 10 min at room temperature. Then the desired amounts of ceria, nickel, and ruthenium salts were added sequentially into the solution containing the aluminum precursor and stirred for 30 min at room temperature. Subsequently, the temperature was raised to 353 K and kept there for 2 h under vigorous stirring. The resulting clear solution was then cooled to room temperature with vigorous stirring. A transparent gel was formed within a few minutes by adding an appropriate amount of water diluted with ethanol to the solution. After aging the gel for 3 days, it was dried in an oven at 313 K for 48 h. The resulting xerogel was finally calcined at 773 K for 5 h in air. The catalysts prepared in this work were 3 wt % CeO2-Al2O3, 1 wt % Ru/3 wt %CeO2-Al2O3, 10 wt % Ni/3 wt %CeO2-Al2O3, 1 wt % Ru/ 2.5 wt % Ni/3 wt % CeO2-Al2O3, 1 wt % Ru/10 wt % Ni/3 wt % CeO2-Al2O3, and 1 wt % Ru/20 wt % Ni/3 wt %CeO2-Al2O3,
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which are denoted as 3CA, 1R3CA, 10N3CA, 1R2.5N3CA, 1R10N3CA, and 1R20N3CA, respectively. 2.2. Catalyst Characterization. X-ray powder diffraction patterns were obtained with a XPERT diffractometer, using Cu KR radiation (1.5406 Å) at 40 kV and 30 mA and a secondary graphite monochromator. The measurements were recorded in steps of 0.045° with a count time of 0.5 s in the 2θ range of 10-65°. The specific surface areas of the catalyst samples were calculated from N2 adsorption-desorption data acquired on a single point Pulse Chemisorb 2705 instrument (Micromeritics, U.S.A.) at liquid N2 temperature. The powders were first outgassed at 423 K to ensure a clean surface prior to construction of the adsorption isotherm. A cross-sectional area of 0.164 nm2 of the N2 molecule was assumed in the calculations of the specific surface areas using the method of Brunauer, Emmett, and Teller (BET). Temperature-programmed reduction studies were carried out on a Pulse Chemisorb 2705 instrument (Micromeritics, U.S.A.) to study the metal dispersion and reducibility. In a typical experiment, ca. 100 mg of oven-dried sample (dried at 383 K overnight) was mounted on a quartz wool plug and placed in a U-shaped quartz sample tube. Prior to TPR studies, argon gas was passed with a flow of 50 mL/min at 393 K for 2 h to pretreat the catalyst sample. After pretreatment, the sample was cooled to ambient temperature and TPR analysis was carried out in a flow of 10% H2/Ar mixture (50 mL/min) from ambient temperature to 1273 K at a heating rate of 10 K/min. Temperature-programmed desorption of H2 and CO2 were carried out in the same system as described for TPR. After reduction at 873 K for 2 h, the sample was heated up to 1073 K and then cooled down to room temperature in argon flow. Then, the adsorption of H2 or CO2 took place for 45 min. After adsorption, the flow was switched from H2 or CO2 to argon or helium and the temperature was elevated at a rate of 10 K/min. The desorbed H2 (hydrogen uptake) or CO2 amount were detected using a TCD. The nickel/ruthenium surface area was calculated assuming a stoichiometry of one hydrogen molecule per two surface nickel/ruthenium atoms and the atomic crosssectional areas were taken as 6.49 × 10-20 m2/Ni atom and 6.1 × 10-20 m2/Ru atom.44 2.3. Reforming Experiments. The steam reforming experiments were carried out in a quartz vertical flow microreactor system. The schematic diagram of the experimental setup is illustrated in Figure 1. The reactor consisted of a 43 cm long quartz tube with 1.2 cm o.d. and 1.0 cm inner diameter. Powder catalyst samples were dispersed on quartz wool and packed between two plugs of quartz wool inside the reactor tube. The typical catalyst weight was 25-125 mg. Inconel-sheathed K-type thermocouples were placed at upstream and downstream sides of the catalyst bed in quartz thermowells. The reactor was then placed inside a thermostat-controlled vertical tube furnace whose temperature was maintained constant within (5 K. The flows of nitrogen and argon were metered into the system by AALBORG AFC 3600 mass flow controllers. Flows of fuel and water were both metered into the system by KD Scientific 200 series syringe pumps. All three components (feed gases, water, fuel) were mixed in a heated cross which exited into a 2 m long section of heat-traced 6 mm o.d. stainless steel tubing that served as a vaporizer with its surface at a temperature of approximately 483 K. To determine whether the fuel was fully vaporized, the mass balance on the fuel through a blank reactor tube held at 523 K was performed. The blank run showed complete vaporization of the fuel with better than 99% closure
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Figure 1. Schematic diagram of the experimental setup for the steam-reforming reaction.
of the mass balance. From the vaporizer, the reactant mixture flowed into a 21 cm long preheating section of the quartz tube reactor, in which any nonvaporized liquid was vaporized. The temperature in the reactor was raised to 873 K with a ramp of 10 K per minute for the reduction of the catalyst in 10% H2/Ar for 2 h. The system was, then, flushed with nitrogen for 30 min to remove the physisorbed hydrogen on catalyst. The steam, prepared on the bypass line, was subsequently introduced into the reactor, followed by the fuel after 10 min. The temperature of the feed stream was 873 K prior to reaching the catalyst bed. The reactor effluent passed through a condenser to knock out any condensable liquids before entering the gas analysis train. The gaseous reactor effluent was analyzed by two online Varian 3800 gas chromatographs: one equipped with a TCD, which was used for permanent gas analysis, and an FID for heavier hydrocarbon analysis and the second with a TCD for H2 and Ar analysis. The concentrations of gaseous product compounds were determined from calibration curves. The fractional conversion of n-dodecane resulting from steam reforming (Xref) and the product yields of H2, CO, CO2, and CH4 were calculated as follows: YH2 )
YCO2 )
YCO )
XREF )
FH2,out 13.FC12H26,in FCO2,out 12.FC12H26,in FCO,out 12.FC12H26,in
FCO,out + FCO2,out 12.FC12H26,in
(1)
(2)
(3)
(4)
In all the equations, F is the molar flow rate of the subscripted species in mol s-1. The molar flow rate of n-dodecane was calculated from the pump volumetric flow rate. The flow rate of the individual product species was determined from the
Table 1. Specific Surface Areas of Original and Spent Catalysts (Catalysts Calcined at 773 K for 5 h) catalyst
BET specific surface area of original catalysts (m2/g)
BET specific surface area of spenta catalysts (m2/g)
Al2O3 3CA 1R3CA 10N3CA 1R2.5N3CA 1R10N3CA 1R20N3CA
350 370 380 428 377 360 317
54 176 142 200 173
a
Catalyst weight 125 mg, steam reforming for 260 min.
species effluent concentration and the total effluent flow rate on the dry basis. 3. Results and Discussion 3.1. Physical Properties of Catalysts. The BET specific surface areas determined by nitrogen physisorption for all the catalysts are presented in Table 1. The specific surface areas of pure alumina and 3CA support were found to be 350 and 370 m2/g respectively. The BET surface area increased by the addition of ruthenium or nickel to the 3CA substrate. This might be due to the homogeneous distribution of metal in the support matrix due to the use of the sol-gel method in catalyst preparation. It is also probable that the active metal precursors may have caused surface roughening on the support matrix. In the ruthenium-promoted catalysts/bimetallic catalysts, the BET surface area decreased as a function of the nickel content; this might be due to surface hydroxyl groups of the support being consumed by the reaction with the active phase precursor. Such a surface reaction may have caused the decrease of available surface area of the substrate, probably by closure of the pores. 3.2.1. X-ray Diffraction (XRD). The phases on the prepared catalysts were characterized by means of XRD. The powder X-ray diffraction patterns of supports and various catalysts calcined at 773 K are shown in Figure 2. The broad peaks that correspond to γ-Al2O3 in Figure 2-a indicate that bare alumina and ceria-alumina supports calcined at 773 K are relatively amorphous or poorly crystalline; it is known that alumina is transformed from boehmite (AlOOH) to γ-Al2O3 at temperatures above 773 K.45 In bare CeO2-Al2O3 catalyst, no crystalline phases from cerium species were detected; this may be the result
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Figure 2. X-ray diffraction patterns of calcined support and various Ru-Ni catalysts: (a) Al2O3 pure support, (b) 3CA, (c) 1R3CA, (d) 10N3CA, (e)1R2.5N3CA, (f)1R10N3CA, and (g) 1R20N3CA (9 from γ-Al2O3; b from CeO2).
of a homogeneous dispersion of ceria within the alumina matrix. The possible crystalline phases in the catalyst calcined at 773 K are RuO, NiO, NiAl2O4, CeO2 and γ-Al2O3. Due to peak broadening and superimposition of γ-Al2O3 and NiAl2O4 phases, it is difficult to clearly distinguish γ-Al2O3 and NiAl2O4 phases by means of XRD.46 Two crystalline phases, that is, γ-Al2O3 and CeO2, were detected in all the catalyst samples as can be observed from Figure 2. Interestingly, the CeO2 crystalline phase that was observed in Ru-containing catalysts was not detected in the 10N3CA catalyst. This phenomenon indicated that the cerium species could have been homogeneously dispersed in the 10N3CA catalyst compared to the other catalysts or they may be present within the NiO matrix, thus, making its detection by XRD impossible. In supported ruthenium or rutheniumpromoted nickel catalysts, cerium species were converted to ceria. This may be due to the fact that cerium ions show a higher tendency of hydroxylation than ruthenium ions. Therefore, due to the addition of ruthenium chloride precursor to the sol-gel solution, variation in the pH of the solution may occur leading to the dissolution of small crystals and the growth of ceria.47 XRD reflections due to crystalline CeO2 were observed at 2θ ) 28.5, and the intensities of these reflections were found to increase with the increase of metal (Ni) loading. The XRD patterns also suggested that there were no detectable diffraction peaks representing crystalline NiO and RuO present in the samples; this clearly indicates that the NiO and RuO species are present in a highly dispersed amorphous state in the support matrix. Additionally, the diffraction line at 43.3° is difficult to distinguish since (012) reflection of NiO may be overlapped with (113) reflection of R-Al2O3.48 However, the presence of a highly dispersed crystalline surface NiO and RuO phases cannot be ruled out, because such particles would be highly dispersed on the CeO2-Al2O3 surface as smaller crystallites of 1R2.5N3CA > 10N3CA > 1R20N3CA and 1R3CA. After 10 h of reaction, hydrogen yield decreased in the order 1R2.5N3CA < 1R10N3CA < 10N3CA < 1R20N3CA and 1R3CA, with 1R2.5N3CA exhibiting the lowest deactivation. This is almost in accordance with the order in Ni dispersion and total basic sites, which are measured by H2 TPD, and CO2 TPD, respectively. Note that the formation of carbon was very low on 1R2.5N3CA, while formation of more carbon was observed on the other catalysts. Compared to 10N3CA, 1R3CA catalyst deactivates more slowly but gives lower yields of H2, CO, and CO2, and higher yield of methane. Methane formation may be the result of the methanation reaction (CO + 3H2 f CH4 + H2O) or because of the cracking of n-dodecane. In rutheniumpromoted 1R2.5N3CA catalyst, the initial activity is lower compared to 1R10N3CA, but it is more stable; this may be due to the number of dispersed nickel sites and basic sites that are more available on the surface of the 1R2.5N3CA catalyst than the other catalysts, and also, there is less carbon formation on the 1R2.5N3CA catalyst when compared to the other catalysts. In ruthenium-promoted bimetallic catalysts, steam reforming activity decreases and carbon formation increases with an increase in nickel loading. These results are well correlated with the results from hydrogen chemisorption, total number of basic sites and carbon formation. 3.7. TPO Experiments. The temperature-programmed oxidation profile of carbon deposited on various catalysts after 5 h of steam reforming of n-dodecane at 1073 K, H2O/C ratio 2 with a space velocity of 103 000 h-1 are shown in Figure 9. In the graphs, several CO2 peaks were observed, indicating different types of carbonaceous species with different reactivities toward oxidation. All catalysts showed some CO2 evolution between 350 and 550 K. These peaks were the only peaks observed for 1R2.5N3CA. In the case of 10N3CA, the lowtemperature CO2 peaks extended down to 325 K. Some degree of CO2 evolution was also observed between 600 and 800 K for 3CA, 1R3CA, 10N3CA, and 1R10N3CA. These peaks were most pronounced for 1R3CA. The 3CA support had a large CO2 peak centered around 975 K. A small peak was also observed for 1R10N3CA at 900 K.12,32,34,60,61
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In the literature, three types of carbon were identified during hydrocarbon reforming: 1. Pyrolitic (coating) carbon formed from cracking of hydrocarbons at temperatures greater than 870 K at low H2O/C ratios over acidic catalysts. It reacts with oxygen around 665 K. It was hypothesized that one source of coating carbon in case of higher HC’s is oxidative dehydrogenation to isobutene, propylene, methane, CO and adsorbed carbon and the other source may be from the further cracking or decomposition of low-carbon alkanes/ alkenes. This amorphous carbon is reactive and may be removed by gasification. 2. Filamentous (whisker) carbon that formed above 724 K at low H2O/C ratios and with aromatic feeds. It is graphitic in the case of methane reforming and forms because of methane decomposition or the Boudouard reaction. It reacts with oxygen around 850 K. In case of higher hydrocarbons, the source of this carbon is the same as above but whisker forms at lower H2O/C ratios. 3. Encapsulating carbon is a film of nonreactive deposits that contain C-H bonds and is formed on Ni surfaces by polymerization below 770 K at low H2O/C ratios and with aromatic feeds. This type of carbon causes progressive catalyst deactivation. Since 3CA is basic, encapsulating carbon should not form on it. The low temperature peaks in Figure 9 in the range of 350-550 K may be due to the formation of nickel carbide (NiC3 form),32,34 which has a low thermal stability47 or a different type of carbon as observed by Wang and Lu,34 and a comparison between 10N3CA, 1R10N3CA, and 1R2.5N3CA indicate that the amount of this carbon increases with an increase in nickel loading. This peak is absent on the 3CA. Chen et al47 performed ATR of iso-octane on Ni-CZO catalysts. When they oxidized the spent catalysts in a TGA, they observed a peak at 665 K, which they attributed to coating carbon which forms from the cracking of isooctane. The peaks on Figure 9 in the range of 550-800 K may be caused by coating carbon, but because of the basicity of the catalysts used in this study, only a small amount of carbon oxidizing in this range was observed. Christensen et.al62 state that the growth of filamentous carbon requires both a critical Ni particle size and a certain concentration of Ni particles at a site. Wang and Lu34 reported that if the carbon migration is assisted by the structure of the catalyst, the growth of filamentous carbon is impeded. The preparation method used in this study may have resulted in a Ni-CeO2Al2O3 interaction which facilitated the migration of carbon. Baker63 found that the metal support interaction affected the type of the carbon formed; weak metal-support interaction caused the growth of whisker carbon, while strong metal support interaction resulted in extruded carbon filaments. Also, the extent of the reduction of Ni is an important factor in the whisker carbon formation, the reduced particles leading to carbon forming over all the Ni particles, thus, hindering the diffusion of reactants to the particle surface. This will cause the isolation of the Ni particle and will lead to its encapsulation by carbon. Therefore, in our case, it seems that the two types of carbon that formed are NiC3 form and extruded carbon, which did not grow into whisker-type of filamentous carbon or encapsulating carbon. This might be due to the possibility of formation of Ru-Ni clusters in our catalysts, with an ensemble size unsuitable for the growth of filaments, although we did not study the size of clusters.
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4. Conclusions Various sol-gel Ni-CeO2-Al2O3 catalysts promoted with ruthenium were studied in the steam reforming of n-dodecane. The activity and hydrogen yield of CeO2-Al2O3-supported RuNi catalysts increased when the Ni loading is increased from 0 to 10 wt % and then decreased with further increase of Ni loading at low space velocities. At high space velocities, 1R2.5N3CA catalysts remained stable during experiments lasting about 10 h. When small amounts of Ru are added to the Ni-CeO2-Al2O3 catalysts, their activity, selectivity, and stability significantly improved. This indicates that ruthenium plays an important role in the catalytic action. An improvement in catalytic activity and selectivity with respect to hydrogen yield appears to be due to easier reduction of nickel species to a metallic state and larger nickel surface area. The remarkable catalytic performance of 1R2.5N3CA catalyst is attributed to the combination of several factors, that is, high dispersion, high Ni metal area, and high basicity. The catalyst developed in this work, 1% Ru/2.5% Ni supported on CeO2-modified Al2O3, shows excellent promise as a catalyst that can successfully reform n-dodecane for hydrogen production. Acknowledgment The authors gratefully acknowledge financial support from NASA grant (NCC3-1037). They thank Dr. V. Jagasivamani for his assistance with the XRD experiments. Literature Cited (1) Trimm, D. L.; Onsan, Z. I. Onboard fuel conversion for hydrogenfuel-cell-driven vehicles. Catal. ReV.sSci. Eng. 2001, 43, 31–84. (2) Brown, L. F. A comparative study of fuels for on-board hydrogen production for fuel-cell-powered automobiles. Int. J. Hydrogen Energy 2001, 26, 381–397. (3) Song, C. S. Fuel processing for low-temperature and high-temperature fuel cells: Challenges, and opportunities for sustainable development in the 21st century. Catal. Today 2002, 77, 17–49. (4) Rostrup-Nielsen, J. R. Conversion of hydrocarbons and alcohols for fuel cells. Phys. Chem. Chem. Phys. 2001, 3, 283–288. (5) Gould, B. D.; Tadd, A. R.; Schwank, J. W. Nickel-catalyzed autothermal reforming of jet fuel surrogates: n-Dodecane, tetralin, and their mixture. J. Power Sources 2007, 164, 344–350. (6) Li, Y.; Wang, X.; Xie, C.; Song, C. Influence of ceria and nickel addition to alumina-supported Rh catalyst for propane steam reforming at low temperatures. Appl. Catal., A 2009, 357, 213–222. (7) Acres, G. J. K. Recent advances in fuel cell technology and its applications. J. Power Sources 2001, 100, 60–66. (8) Armor, J. R. The multiple roles for catalysis in the production of H2. Appl. Catal., A 1999, 176, 159–176. (9) Rostrup-Nielsen, J. R. Industrial relevance of coking. Catal. Today 1997, 37, 225–232. (10) Rostrup-Nielsen, J. R. Production of synthesis gas. Catal. Today 1994, 18, 305–324. (11) Rider, D. E.; Twigg, M. V. In Catalysis, Science and Technology; Twigg, M. V., Ed.; Springer: Berlin, 1984; Vol. 5, p 1. (12) Trimm, D. L. Catalysis for the control of coking over steam reforming. Catal. Today 1999, 49, 3–10. (13) Borowiecki, T.; Machocki, A.; Ryczkowski, J. Induction period of coking in the steam reforming of hydrocarbons. Stud. Surf. Sci. Catal. 1994, 88, 537–542. (14) Trimm, D. L. The formation and removal of coke from nickel catalyst. Catal. ReV.sSci. Eng. 1977, 16, 155–189. (15) Rostrup-Nielsen, J. R. Criteria for carbon formation (steam reforming and methanation). NATO ASI Ser., Ser. E 1982, 54, 127–149. (16) Strohm, J. J.; Zheng, J.; Song, C. Low-temperature steam reforming of jet fuel in the absence and presence of sulfur over Rh and Rh-Ni catalysts for fuel cells. J. Catal. 2006, 238, 309–320. (17) Rostrup-Nielsen, J. R.; Christensen, T. S.; Dybkjaer, I. Characterization of Ca-Promoted Ni/R-Al2O3 catalyst for CH4 reforming with CO2. Stud. Surf. Sci. Catal. 1998, 113, 81–95.
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ReceiVed for reView April 3, 2010 ReVised manuscript receiVed June 17, 2010 Accepted July 1, 2010 IE100811G
