Reforming of Isooctane over Ni−Al2O3 Catalysts for Hydrogen

Nov 13, 2006 - Production: Effects of Catalyst Preparation Method and Nickel. Loading .... (1) The reaction (eq 1) is exothermic, which makes it less ...
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Energy & Fuels 2007, 21, 570-580

Reforming of Isooctane over Ni-Al2O3 Catalysts for Hydrogen Production: Effects of Catalyst Preparation Method and Nickel Loading Hussam H. Ibrahim, Prashant Kumar, and Raphael O. Idem* Hydrogen Production Research Group, Process Systems Engineering, Faculty of Engineering, UniVersity of Regina, Regina, SK, Canada S4S 0A2 ReceiVed NoVember 13, 2006. ReVised Manuscript ReceiVed January 5, 2007

The production of hydrogen via the partial oxidation of isooctane was evaluated using a variety of NiAl2O3 catalysts. The effects of catalyst preparation method, nickel loading, and calcination temperature, as well as the reaction operating conditions (such as reaction temperature and space velocity), on the performance of the Ni-Al2O3 catalysts prepared were evaluated. These catalysts were thoroughly characterized to establish the relationship between catalyst characteristics and catalyst preparation conditions. Results showed that a high BET surface area enhances nickel dispersion, which, together with high catalyst reducibility, help to enhance the catalyst performance, in terms of isooctane conversion, H2 selectivity, and turnover number (TON). In each preparation method, it was not possible to obtain high catalyst reducibility together with high nickel dispersion. Thus, the trends observed were manifestations of the opposing tendencies of these characteristics. Also, an increase in calcination temperature was determined to have a detrimental effect on catalyst performance and resulted in an increase in the amount of carbon deposited during the reaction.

1. Introduction The present transportation power-generating internal combustion (IC) engines are well-known to have low energy-conversion efficiency1 and high emissions of gases such as oxides of nitrogen, sulfur dioxide (SO2), carbon monoxide (CO), and carbon dioxide (CO2), because of the use of fossil fuels. Although catalytic converters are designed to overcome emission problems,2 they are not completely capable of addressing SO2 emissions. Also, it does not eliminate CO2 emissions from automobile exhaust. In this context, fuel cells seem to be the driver that can address both the energy conversion efficiency (because of their inherent high energy-conversion efficiency) and emissions problems, if hydrogen is used as the direct fuel.3,4 Fuel cells are undergoing rapid development for both stationary and transportation applications.5,6 It seems, from the literature, that the low-temperature proton exchange membrane (PEM) fuel cell is well-suited as a source of power for portable devices, as well as for use in the transportation sector,7 provided the hydrogen production scheme is established. Typically, hydrogen can be produced from hydrocarbons or oxygenated hydrocar* Author to whom correspondence should be addressed. Tel.: (306) 5854470. Fax: (306) 585-4855. E-mail address: [email protected]. (1) Wang, L.; Murata, K.; Inaba, M. J. Power Sources 2005, 145 (2), 707-711. (2) Heck, R. M.; Farrauto, R. J. Catalytic Air Pollution Technology: Commercial Technology, 2nd Edition; Wiley: New York, 2002. (3) Pattersson, L. J.; Westerholm, R. Int. J. Hydrogen Energy 2001, 26, 243-264. (4) Carpenter, I. W.; Hayes, J. W. U.S. Patent Application No. US 2002/ 0127445, September 12, 2002. (5) Zhang, J.; Wang, Y.; Ma, R.; Wu, D. Appl. Catal., A 2003, 243 (2), 251-259. (6) Ahmed, S.; Krumpelt, M. Int. J. Hydrogen Energy 2001, 26 (4), 291301. (7) Ghenciu, A. F. Curr. Opin. Solid State Mater. Sci. 2002, 6 (5), 389399.

bons.8 Because of its abundance and reliability, fossil fuels can provide the primary source of hydrogen for fuel cells.9 For example, because, in the near-term, conventional fuels such as gasoline will continue to have a dominant role in ensuring the world’s transportation needs, gasoline or diesel therefore represents a very attractive source of hydrogen, because each already has a well-established production, transportation, storage, and dispensing infrastructure for vehicle applications.10 Moreover, gasoline has a higher energy density (heat value) and larger hydrogen content, compared to oxygenated hydrocarbons, such as methanol and ethanol,9-11 and, thus, would be a costbeneficial source of hydrogen. Hydrogen can be produced from gasoline via reforming technology.12 Other gases that are associated with the production of hydrogen from gasoline by reforming, such as CO2 and CO, can be removed via existing technologies such as water gas shift (WGS) reaction, preferential oxidation (PrOx), membrane separation, and amine-based CO2 capture. The reforming of any fuel to produce hydrogen onboard a vehicle is presently considered to be impractical, because the reformer adds to the size and weight of the vehicle. It can also affect other design considerations.13,14 Off-board reforming is (8) Roychoudhury, S.; Castadi, M.; Lyubovsky, M.; LaPierre, R.; Ahmed, S. J. Power Sources 2005, 152 (1), 75-86. (9) Zhu, W.; Xiong, G.; Han, W.; Yang, W. Catal. Today 2004, 93-95, 257-261. (10) Ran, R.; Xiong, G.; Sheng, S.; Yang, W.; Stroh, N.; Brunner, H. Catal. Lett. 2003, 88 (1-2), 55-59. (11) Moon, D. J.; Ryu, J. W. Catal. Lett. 2003, 89 (3-4), 207-212. (12) EG&G Services Parsons, Inc. Fuel Cell Handbook, 5th Edition; U.S. Department of Energy (DOE), Office of Fossil Energy, National Energy Technology Laboratory: Morgantown, WV, 2000. (13) Peppley, B. A. Fuel Processing for Fuel Cells: Can Catalysis Save the Day? Presented at the 18th Canadian Symposium on Catalysis, May 16-19, 2004, Montreal, Canada. (14) Anumakonda, A.; Yamanis, J.; Ferreall, J. U.S. Patent No. 6,221,280, April 24, 2001.

10.1021/ef060566u CCC: $37.00 © 2007 American Chemical Society Published on Web 02/20/2007

Reforming of Isooctane oVer Ni-Al2O3 Catalysts

generating considerable interest, because hydrogen can be produced, stored, and dispensed in a hydrogen refuelling station and, also, the CO2 byproduct can be captured and sequestered. Typical reforming technologies include (a) partial oxidation (POX) reforming,

C8H18 + 4O2 / 8CO + 9H2

(∆H298° ) -659.9 kJ/mol) (1)

(b) steam reforming (SR) and (c) autothermal reforming (ATR) (eqs 2 and 3, respectively),

C8H18 + 8H2O / 8CO + 17H2 (∆H298° ) 1274.8 kJ/mol) (2) C8H18 + 2O2 + 12H2O / 8CO2 + 21H2 (∆H298° ) -22.0 kJ/mol) (3) and (d) carbon dioxide reforming (DR),

C8H18 + 8CO2 / 16CO + 9H2 (∆H298° ) 1604.0 kJ/mol) (4) The reaction

CO + H2O / CO2 + H2

(∆H298° ) -41.2 kJ/mol) (5)

is known as the water gas shift reaction, which is complementary to almost every reforming technique that produces CO as a byproduct. Here, we use the partial oxidation approach, because of the following advantages:15 (1) The reaction (eq 1) is exothermic, which makes it less energy-intensive than steam reforming (eq 2); (2) A smaller reformer can be used to achieve both high conversion of the hydrocarbon and high selectivity toward the production of H2 at short contact times; and (3) The partial oxidation setup is more compact and mechanically simpler than steam reforming, because no additional heating is required. However, the challenge lies in developing a suitable catalyst for reforming gasoline to hydrogen in a compact, yet efficient, fuel processor.16,17 The catalysts investigated thus far contain either noble metals (such as platinum (Pt) and palladium (Pd)) or non-noble transition metals (such as nickel (Ni) and copper (Cu)) supported on an oxide substrate (such as alumina (Al2O3) and ceria (CeO2)). Although the noble-metal-containing catalysts are proven to have good resistance to coke formation and sulfur poisoning,4,13,18 their high cost limits their application.19,20 On the other hand, base-metal-containing catalysts represent a very appealing alternative, provided a high-surface-area nanocomposite with a high dispersion of metals are prepared. In particular, the use of nickel-based catalysts in heavy liquid hydrocarbon reforming has attracted widespread attention3,10,11,21-26 of late. However, Ni-based catalysts normally (15) Li, C.; Yu, C.; Shen, S. Catal. Lett. 2000, 67, 139-145. (16) Krumpelt, M.; Ahmed, S.; Kumar, R. U.S. Patent No. 6,110,861, August 29, 2000. (17) Lemonidou, A. A.; Goula, M. A.; Vasalos, L.A. Catal. Today 1998, 46 (2-3), 175-183. (18) Becerra, A. M.; Iriarte, M. E.; Castro-Luna, A. D. React. Kinet. Catal. Lett. 2003, 79 (1), 119-125. (19) Xu, S.; Zhao, R.; Wang, X. Fuel Process. Technol. 2004, 86 (2), 123-133. (20) Wang, S.; Lu, G. Q. M. Appl. Catal., B 1998, 16 (3), 269-277. (21) Avci, A. K.; Trimm, D. L.; Akoylu, A. E.; Onsan, Z. I. Appl. Catal., A 2004, 258 (2), 235-240.

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require a high temperature to be activated,27 and coke deposition is much faster than on noble metal supported catalysts.28 In the recent literature, there has been progress in regard to the partial oxidation of light hydrocarbons (those with 5 C atoms) to hydrogen-rich gas is still in its early stages.3,9-11,14,16,24-26,35-39 Of the studies on the catalytic partial oxidation of heavy hydrocarbons to hydrogen, only a few attempted the conversion of gasoline or a gasoline surrogate with a nickel-based catalyst system.3,9-11,24-26 Also, most of the catalysts used for the gasoline or simulated gasoline are based on noble metals. The limited studies on the use of base-metal catalysts (typically nickel-based catalysts) could not alleviate the problem of coke formation and catalyst deactivation. On the other hand, a NiAl2O3 catalyst system could be used for this process, because (i) the γ-Al2O3 acid support facilitates hydrocarbon cracking and the reforming of heavy hydrocarbons;40 (ii) γ-Al2O3 provides a high surface area and thermal stability;41 (iii) nickelbased catalysts have been proven to have good activity for the partial oxidation of natural gas, ethane, propane, and n-heptane;42 (iv) nickel is highly active for synthesis gas production with high hydrogen selectivity during the partial oxidation process;43 (v) the activity of nickel is comparable to that of noble-metal catalysts;44 and (vi) both nickel and γ-Al2O3 are relatively low in cost and are readily available. The use of Ni-Al2O3 catalysts in reforming reactions is not new. However, the effects of the catalyst preparation method (22) Ming, Q.; Healey, T.; Allen, L.; Irving, P. Catal. Today 2002, 77, 51-64. (23) Praharso; Adesina, A. A.; Trimm, D. L.; Cant, N. W. Chem. Eng. J. 2004, 99, 131-136. (24) Trimm, D. L.; Adesina, A. A.; Praharso; Cant, N. W. Catal. Today 2004, 93-95, 17-22. (25) Yanhui, W.; Diyong, W. Int. J. Hydrogen Energy 2001, 26 (7), 795800. (26) Moon, D. J.; Ryu, J. W.; Lee, S. D.; Lee, B. G.; Ahn, B. S. Appl. Catal., A 2004, 272, 53-60. (27) Choudhary, V. R.; Prabhakar, B.; Rajput, A. M. J. Catal. 1995, 157 (2), 752-754. (28) Hou, Z.; Yokota, O.; Tanaka, T.; Yashima, T. Catal. Lett. 2003, 89 (1-2), 121-127. (29) Hohn, K. L.; Schmidt, L. D; Reyes, S. C.; Feeley, J. S. U.S. Patent Application No. US 2001/0027258, October 4, 2001. (30) Reyniers, M. F.; de Smet, C. R.; Menon, P. G.; Marin, G. B. CATTECH 2002, 6 (4), 140-149. (31) Avci, A. K.; Onsan, Z. I.; Trimm, D. L. Top. Catal. 2003, 22 (34), 359-367. (32) Joensen, F.; Rostrup-Nielsen, J. J. Power Sources 2002, 105, 195201. (33) Armor, J. N. Appl. Catal., A 1999, 176 (2), 159-176. (34) Liu, X.; Wang, J.; Liu, C.; He, F.; Eliasson, B. React. Kinet. Catal. Lett. 2003, 79 (1), 69-76. (35) Bakker, G. M.; Cracknell, R. F.; Kramer, G. J.; Morley, C.; Vos, E. J. U.S. Patent Application No. US 2004/0136901, July 15, 2004. (36) Schmidt, L. D.; Krummenacher, J. J.; West, K. N. U.S. Patent Application No. US 2004/01990038, October 7, 2004. (37) Villegas, L.; Guilhaume, N.; Provendier, H.; Masset, F.; Mirodatos, C. Presented at the 1st European Hydrogen Energy Conference 2003, September 2-5, 2003, Grenoble, France. (38) Moon, D. J.; Sreekumar K.; Lee, S. D. Appl. Catal., A 2001, 215, 1-9. (39) Praharso, Adesian, A. A.; Trimm, D. L.; Cant, N. W. Korean J. Chem. Eng. 2003, 20 (3), 468-470. (40) Gai, P. L.; Boyes, E. D. Electron Microscopy in Heterogeneous Catalysis; DuPont Experimental Station: Wilmington, DE, 2003. (41) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd Edition; McGraw-Hill: New York, 1991. (42) Ran, R.; Xiong, G.; Sheng, S.; Yang, W.; Stroh, N.; Brunner, H. Catal. Lett. 2003, 88 (1-2), 55-59. (43) York, A. P. E.; Xiao, T.; Green, M. L. H. Top. Catal. 2003, 22 (3-4), 345-358. (44) Wang, H.; Li, Z.; Tian, S. React. Kinet. Catal. Lett. 2003, 79 (1), 69-76.

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Table 1. Catalysts Chemical Composition, Surface Area, Pore Size, and Pore Volume Composition (wt %) γ-Al2O3 PRE PRE PRE PRE PRE IMP IMP IMP a

Ni ( nominal value)

Ni

Al

Na

BET surface area (m2/g)

average pore diameter (Å)

total pore volume (cm3/g)

0 1 2 3 6 9 3 6 9

n/aa 1.0 2.0 3.0 5.6 8.7 2.9 6.2 9.5

n/aa 45.3 45.7 46.2 41.0 42.8 45.1 45.7 42.6

n/aa 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0

260.0 ( 0.2 196.9 ( 0.9 177.1 ( 0.8 153.9 ( 0.2 147.0 ( 0.2 152.8 ( 0.2 148.7 ( 0.2 142.3 ( 0.2 136.1 ( 0.2

37.6 93.1 92.3 90.4 87.6 84.2 92.8 91.2 89.1

0.3 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4

Not applicable.

and nickel loading on the performance of Ni-Al2O3 catalysts for the partial oxidation of gasoline to produce hydrogen have not been reported before in the literature. Thus, the present work investigates the impact of using two simple catalyst preparation methods (precipitation and impregnation) and the effect of nickel loading on the performance of Ni-Al2O3 catalysts in the partial oxidation of isooctane, which is used here as a surrogate for gasoline. This work also evaluates the relationships between nickel loading and the catalyst preparation methods with the characteristics of the catalysts, and their inter-relationships with catalyst performance. These results are presented in this paper. 2. Experimental Section 2.1. Catalyst Preparation. Two methods of synthesissnamely, precipitation (PRE) and impregnation (IMP)swere used to prepare Ni-Al2O3 catalysts with different nickel loadings. Overall, eight catalysts were prepared for performance evaluation. The supported nickel catalysts were prepared using Ni(NO3)2‚6H2O (Aldrich) as the metal precursor. The material for the support was γ-Al2O3 (EM Science). Sodium carbonate (EM Science) was used to induce precipitation. 2.1.1. Precipitation (PRE). Predetermined concentrations of aqueous solutions of Ni(NO3)2‚6H2O (depending on the desired nickel loading) was introduced from a burette in a dropwise fashion into a container that contained a specified amount of γ-Al2O3 suspended in a fixed amount of sodium carbonate solution. The final pH of the resulting slurry was maintained at 9 with vigorous mixing throughout the aging period of 24 h. The precipitate was filtered, washed thoroughly with warm water, dried at 90 °C for 5 h, and further dried at 110 °C overnight before calcination. 2.1.2. Wetness Impregnation (IMP). Five catalysts with different nickel concentrations, while keeping the amount of water and γ-Al2O3 fixed, were prepared. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis for these five samples was then utilized to generate a calibration curve to assist in subsequent precise nickel loading on supports. This method is similar to the PRE method, but without the addition of a precipitation agent (i.e., sodium carbonate) and the absence of the washing step. The results obtained were then used to generate a calibration curve, which was used to prepare the catalyst samples with the required amount of nickel loading. 2.2. Catalyst Activation. The activation of the catalysts was performed in two consecutive steps: a calcination step, followed by an in situ reduction step. The calcination was performed at 650 °C in a muffle furnace in flowing air. The furnace was ramped from room temperature to the calcination temperature at a rate of 10 °C/min and then held at the setpoint temperature for 3 h. At the end of the calcination cycle, the furnace was shut off and the calcined samples were allowed to cool overnight. The nominal compositions and designations of catalysts prepared are given in Table 1, which also shows the actual compositions obtained from ICP-AES. The reduction step was performed in situ in the reactor prior to the actual performance evaluation of the catalyst. All the catalyst samples were reduced at 640 °C for 2 h with 5% H2/N2 flowing at a rate of 100 mL/min.

2.3. Catalyst Characterization. The surface area of the catalysts was determined by the BET method, whereas the average pore sizes and pore volumes were measured by adsorption/desorption of nitrogen at 77 K. A Micromeritics instrument ASAP-2010 (Micromeritics Instruments Inc.) was used for the analysis. Prior to analysis, the fresh calcined catalyst was first pretreated in a vacuum at 90 °C for 2 h and then at 250 °C for at least 10 h. Nickel dispersion was evaluated using the H2 chemisorption technique. The experiment was performed at 35 °C in the chemisorption arm of the same ASAP-2010 (Micromeritics) equipment described previously. The sample (0.7 g) was first pretreated thermally with oxygen (Praxair) and evacuated at 350 °C, and then was reduced with hydrogen (Praxair) at the actual reduction temperature (640 °C). The analysis was then performed at 35 °C. The chemisorption results were then used to calculate the nickel dispersion, following the procedure described by Idem and Bakhshi.45 The metal dispersion was calculated by assuming the adsorption stoichiometry of one H atom per Ni surface atom. Thermogravimetric experiments were conducted in a model TGA-DSC-1100 thermogravimetric analysis system (Setaram Scientific and Industrial Equipment) using flowing ultrahigh-purity (UHP) argon (Praxair) at a flow rate of 30 mL/min. The measurements were obtained in terms of weight loss, due to thermal treatment (TGA). Analysis results were used to determine the temperature after which the catalyst remains stable without a further loss in weight. This facilitates the selection of the appropriate calcination temperature for every catalyst. Simultaneously, differential scanning calorimetry (DSC) analysis was also performed, to provide information about the heat associated with decomposition/dehydration steps. Typically, a dry sample (50 mg) was loaded into the thermobalance and heated from room temperature to 850 °C at a heating rate of 10 K/min. X-ray diffraction (XRD) measurements were obtained on the dried, calcined, and reduced catalyst samples, using a Bruker model D8/GADDS diffractometer, using Cu KR radiation (λ ) 1.5418 Å). The voltage was 40 kV, and the electric current was 40 mA. The scanning rate was 5°/min, whereas the 2θ scanning range was 20°-80°. Typically, a powder sample was mounted on the sample holder and then scanned to cover the indicated 2θ range. The measurements were used to identify the crystalline phases and possibly measure the nickel crystallite sizes. Temperature-programmed reduction (TPR) studies were performed in a fixed-bed quartz tube (ChemBET-3000, Quantachrome Corporation), using 0.1 g of calcined catalyst. Before the analysis, the samples were treated with helium at 250 °C for 1 h and then cooled to ambient temperature in a nitrogen flow. The reactor then was heated from room temperature to 1000 °C with a 10 °C/min increasing temperature ramp in a 3% H2/N2 mixture at a flow rate of 35 mL/min. TPR analysis was used to provide information on the reducibility of the catalyst and aids in the selection of the optimum reduction temperature. Also, TPR could be used in conjunction with XRD analysis to determine the species present in the catalysts. (45) Idem, R. O.; Bakhshi, N. N. Ind. Eng. Chem. Res. 1994, 33, 20472055.

Reforming of Isooctane oVer Ni-Al2O3 Catalysts Temperature-programmed oxidation (TPO) was performed in the TGA-DSC-1100 system described previously. The analysis gas in this case was a 3% O2/Ar (Praxair) at a flow rate of 30 mL/min. Spent sample (0.1 g) was loaded into the thermobalance and pretreated with argon at 110 °C for 1 h to dry the sample prior to analysis. TPO analysis is particularly important to quantify the carbon deposited on the catalyst after the reaction. 2.4. Catalytic Activity Tests. An Inconel fixed-bed reactor (inner diameter (ID) of 12.7 mm) housed in a furnace with a single heating zone was used for the evaluation of catalyst performance. Liquid isooctane was introduced by a syringe pump (kd Scientific-200), while the gas flows were metered and regulated by an Aalborg digital flow controller (DFC-26). The air-to-isooctane molar ratio was 3.0, which translates to an oxygen-to-isooctane molar ratio of 0.63. The catalyst bed temperature was measured by means of a sliding thermocouple that was dipped inside the catalyst bed. The diluent used in the catalyst bed was alumina in the R-phase that had the same particle size as that of the catalyst (150 µm). Pure R-Al2O3 (150 µm) was used in the preheating zone and the section after the catalyst bed. The reaction product was first passed through a water-cooled condenser and then through a knockout ice bath, to separate the permanent gases from the condensate. The gas product was analyzed with an on-line gas chromatograph (HP-6890, Agilent Technologies) that was equipped with a TCD, using Haysep and Molsieve columns (Alltech Associates) for complete gas product separation. The unreacted isooctane/liquid product was analyzed with a gas chromatography-mass spectrometry (GC-MS) (HP-6890/5073, HP) using 30 m GS-GasPro column (J&W Scientific) equipment. The catalysts were evaluated for isooctane conversion, hydrogen selectivity, hydrogen yield, and performance stability. Typically, 0.5 g of catalyst powder (150 µm) diluted with inert R-Al2O3 at a ratio of 1:20 was reduced in situ in flowing 5% H2/N2 (100 mL/ min) at 640 °C for 2 h prior to reaction. Once the selected operation conditions were obtained at 630 °C, samples were taken at intervals of 30 min. The reactant molar feed ratio was kept within stoichiometric limits. Further, an additional run was performed using the 9% Ni-Al2O3 catalyst prepared via the precipitation method at a reaction temperature of 750 °C, commonly used in POX reactions, while keeping all the other parameters the same. The purpose of the run was to verify whether or not an elevated temperature would have any beneficial effect on the catalyst performance during isooctane conversion to hydrogen under the given conditions. The basis is that, thermodynamically, exothermic reactions favor lower temperatures. This would imply that an increase in temperature would drive an exothermic reaction backward, thereby reducing conversion.

3. Results and Discussion 3.1. Characterization Results. The chemical composition given in Table 1 shows that the actual values for catalysts prepared by precipitation and impregnation were very similar to the target values. The presence of a small amount of sodium was detected in the catalysts that were prepared via the precipitation method. Table 1 also shows the results for surface area, average pore size, and average pore volume. The results show that an increase in nickel loading results in a decrease in the surface area, pore size, and pore volume for both the PRE and IMP catalysts. As expected, the trend for BET surface area for catalysts prepared by the precipitation method was affected by the amount of sodium present after washing. For example, the largest amount of sodium was detected on the 6% Ni/Al prepared via the precipitation method,which had the lowest surface area for PRE catalysts. Figures 1a and 1b show typical results for differential thermal analysis (DTA) performed on the fresh dry catalyst samples prepared by the precipitation and impregnation methods,

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respectively. The plot of this differential provides information on the exothermic and endothermic reactions that are occurring as the dry catalyst sample is heated at a linear programmed rate. The results provide information on the temperature at which weight loss becomes insignificant, which, in turn, gives an indication of the optimum calcination temperature for the catalyst. Generally, two endothermic peaks, which correspond to two major weight losses, were observed. A temperature of ∼100 °C was sufficient for removing physisorbed water,46,47 whereas the desorption of structural water and/or adsorbed CO2, following decomposition from a carbonate, required a higher temperature (∼300 °C for catalysts prepared via the precipitation method and ∼400 °C for catalysts prepared via the impregnation method).46,48 For both preparation methods, there was a shift of the peaks toward higher temperatures as the nickel loading increased from 3% to 9%. This indicates that high nickel loading catalysts are more prone to having strongly adsorbed water and/ or adsorbed CO2. Another observation is the high intensity of peaks for the catalyst prepared via the precipitation method for the same nickel loading. This could be attributed to the presence of an excessive amount of water used for washing for the peak at 100 °C and the strong presence of carbonate that remained after washing for the peak at 300 °C. Also, the wider peak for the catalyst prepared via the impregnation method at 400 °C could be assigned to the collapse of the layered structure and the decomposition of entrapped Ni(NO3) during catalyst preparation.46 Bera et al.49 also reached the same conclusion, that, at high temperatures, the nickel hydroxide (Ni(OH)2) loses water and forms a reducible nickel oxide (NiO) phase. The figures also indicate that the samples prepared via PRE had the largest weight loss, which occurred as a result of the elimination of structural water. The results for XRD analyses (not shown) of catalysts obtained for different preparation methods with different nickel loadings at various stages of catalyst synthesis showed that none of the prepared calcined and reduced catalysts exhibited any diffraction patterns corresponding to any nickel species, such as metallic nickel (in reduced catalysts) and NiO and NiAl2O4 (in calcined catalysts). This indicates that the nickel loading on the catalysts is still within monolayer coverage. This phenomenon is consistent with low nickel loadings, as reported by Kim et al.47 and Guo et al.48 However, the absence of NiO peaks, even at the highest loading (9%), may also imply that this phase is amorphous or highly dispersed, considering that 9 wt % nickel is capable of exceeding monolayer coverage. There were no noticeable peaks corresponding to metallic nickel in the reduced sample, and the catalyst during all phases showed more or less the same pattern of diffractions. The only peaks observed on all spectra were those of γ-Al2O3. However, note that the position of the 2θ ) 46° diffraction peak shifted to smaller diffraction angles with increasing nickel content. This indicates that γ-Al2O3 contained an increasing concentration of Ni atoms in its crystalline structure51 as the nickel loading increased. This increased concentration of nickel in the γ-Al2O3 structure is expected to reduce the nickel dispersion as the nickel loading increases. This was verified using TPR and H2 chemisorption results, as follows. (46) Tsyganok, A. I.; Tsunoda, T.; Hamakawa, S.; Suzuki, K.; Takehira, O. K.; Hayakawa, T. J. Catal. 2003, 213 (2), 191-203. (47) Kim, P.; Kim, Y.; Kim, H.; Song, I. K.; Yi, J. Appl. Catal., A 2004, 272, 157-166. (48) Guo, J.; Lou, H.; Zhao, H.; Chai, D.; Zheng, X. Appl. Catal., A 2004, 273, 75-82. (49) Bera, P.; Rajamathi, M.; Hegde, M. S.; Kamath, P. V. Bull. Mater. Sci. 2000, 23 (1), 141-145.

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Figure 1. Rate of thermal weight loss for catalysts prepared (a) via the precipitation method and (b) via the impregnation method.

TPR results for the calcined catalysts are given in Figures 2a and 2b. The purpose of such experimentation is to identify the different reducible nickel species present in the calcined catalysts and their corresponding reduction temperatures. Figure 2a shows the effect of nickel concentration on the reducibility of PRE catalysts. Mainly, two reducible nickel species could be observed in Figure 2a. The first, at ∼580 °C, corresponds to nickel oxides that are not completely integrated in the spinel structure but do have a certain degree of interaction with the support.51 On the other hand, the significant reduction peak at ∼900 °C is an indication of a strong metal support interaction and is mainly attributed to the presence of NiAl2O4 spinel. Free nickel oxide (typically observed at ∼250 - 400 °C) was not evident for catalysts prepared via the precipitation method, which indicates good incorporation of the nickel species in the texture of the support. Also, the absence of free nickel species could be attributed to the high dispersion and small crystalline size. This result coincides with results obtained earlier from XRD analyses. Another set of TPR results was obtained for the catalysts prepared via the impregnation method (Figure 2b). In this case, however, peaks at ∼580 °C were almost absent, which made the catalysts less reducible, as compared to the catalysts prepared by the precipitation method. Nickel species

present as NiAl2O4 spinel still represented the most dominant reducible nickel species. Generally, lower nickel loadings exhibited better dispersions, which resulted in broader peaks and a shift of these NiAl2O4 spinel peaks toward higher reduction temperatures. The relatively higher temperature needed to reduce the catalysts with lower nickel loading is attributed to the presence of a spinel NiAl2O4 structure, which resulted from strong metal-support interaction. Generally, the TPR profiles showed a strong presence of the nickel spinel phase, with a predominant single peak at temperatures of >850 °C. This typically occurs with Ni-Al2O3 systems calcined at temperatures of >550 °C.50 The high temperatures required to reduce the catalyst samples were also a strong indication of the well-dispersed nickel species and the absence of bulk oxides on the support.51 Furthermore, a predominant single peak is evidence of a single-step reduction from Ni2+ to Ni0 without going through intermediate oxides.51,52 At the calcination temperature used for this study, most of the nickel (50) Molina R.; Poncelet, G. J. Catal. 1998, 173, 257-267. (51) Scheffer, B.; Molhoek, P.; Moulijn, J. A. Appl. Catal. 1989, 46 (1), 11-30. (52) Hu, C.; Yao, J.; Yang, H.; Chen, Y.; Tian, A. J. Catal. 1996, 166, 1-7.

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Figure 2. TPR patterns of (a) calcined 3%, 6%, and 9% precipitated (PRE) catalysts and (b) calcined 3%, 6%, and 9% impregnated (IMP) catalysts.

in the support is in the tetrahedral coordination.53 This structure is also confirmed by the Ni-Al2O3 spinel predominance. Figure 3 shows the effect of nickel loading on nickel dispersion for PRE and IMP catalysts. As expected, the figure shows that the nickel dispersion increases as the nickel loading decreases. This result is consistent with XRD and TPR results. This is in agreement with the results of Ming-Tseh54 on a supported nickel catalyst, in which the author observed that, at low nickel contents, the dispersion was greater than that at high (53) Scheffer, B.; Heijeinga J. J.; Moulijn, J. A. J. Phys. Chem. 1987, 91 (1), 4752-5759 (54) Ming-Tseh, T.; Chang, F. Appl. Catal., A 2002, 203, 15-22. (55) Perego, C.; Villa, P. Catal. Today 1997, 34, (3-4), 281-305. (56) Hutchings, G. J. Catal. Lett. 75 2001, 75 (1-2), 1-12. (57) Pinna, F. Catal. Today 1998, 41 (1-3), 129-137.

nickel contents and decreased gradually with loading. This was attributed to the fact that, at low nickel content, the small nickel crystallites exhibit no agglomeration, whereas for high nickel contents, there was the existence of agglomeration, because of the presence of significant nickel density. 3.2. Catalyst Performance Evaluation Studies. The gas products, as analyzed by an on-line gas chromatograph, consisted mainly of hydrogen, methane (CH4), ethane (C2H6), ethylene (C2H4), CO, and CO2. Additional analysis of the gas product in a separate GC-MS analysis showed the presence of propene and 2-methyl-1-propene in the gas. On the other hand, analysis of the liquid product showed the presence of propene, 2-methyl-1-propene, 2-methyl-1,3-butadiene, and 2-methyl-1butene.

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Figure 3. Effect of nickel dispersion on isooctane conversion.

The catalysts were evaluated in terms of isooctane conversion, turnover number for conversion (TON), and hydrogen selectivity, given in eqs 6-8, respectively. The conversion of isooctane (XiC8) was calculated based on a carbon balance for C1 carboncontaining products. Because the higher hydrocarbons are regarded as unconverted and are subject to further conversion,

isooctane conversion, XiC8 )

outlet outlet Noutlet CH4 + NCO + NCO2

8Ninlet iC8

(6)

where Noutlet is the number of moles of species i in the outlet i is the number of moles of gas stream. In the same way, Ninlet i isooctane in the inlet stream.

turnover number for isooctane conversion (TON) ) XiC8 (7) weight of nickel on catalyst hydrogen selectivity, SH2 (%) ) actual moles of hydrogen produced × 100 theoretically expected moles of hydrogen × XiC8 (8) Material balance calculations based on elemental carbon were performed for a selected number of runs. The overall recovery was >93% in all cases. Figure 4a confirms our observation that partial oxidation reaction at a high-temperature such as 750 °C does not enhance the isooctane conversion. In fact, the conversion at the lower temperature of 630 °C started much higher and then stabilized after 3 h. The two curves eventually converged as a function of the time-on-stream, which indicates that similar conversion values could be obtained by reaction at both temperatures. Stability does seem to have been better for the high-temperature run. Also, hydrogen selectivity was better at 750 °C than at 630 °C, as shown in Figure 4a. This can be attributed to a better dehydrogenation phenomenon at a higher temperature. Other activity measurements for the prepared catalysts were performed at the reaction temperature of 630 °C and the weight hourly space velocity (WHSV) of isooctane is 14.1 kgiC8 kg-cat-1 h-1. Figures 4b and 4c show the isooctane conversions of catalysts obtained as a function of time-on-stream for catalysts prepared via the PRE method for nickel loadings of 1, 2, 3, 6, and 9 wt % and for IMP catalysts with nickel loadings of 3, 6, and 9 wt

Figure 4. (a) Effect of reaction temperature on the isooctane conversion and hydrogen selectivity for the 9% Ni-Al2O3 prepared via the precipitation method. (b) Isooctane conversion as a function of the time-on-stream for PRE catalysts with nickel loadings of 1, 2, 3, 6, and 9 wt %. (c) Isooctane conversion as a function of the time-onstream for IMP catalysts with nickel loadings of 3, 6, and 9 wt %.

%. Based on these results (Figures 4b and 4c), the catalysts with high conversion that maintained satisfactory stability for the entire duration of the experimental run were 3% Ni-PRE, 3% Ni-IMP, and 6% Ni-IMP. After 100 min of time-onstream, the conversions were 43 mol % for the 9 wt % catalyst, 39 mol % for the 6% catalyst, and 28 mol % for the 3 wt % catalyst. In contrast, the activity of IMP9 continued to decline with time-on-stream. 3.3. Effect of Catalyst Characteristics on Catalyst Performance. We are ultimately attempting to correlate the catalyst performance to catalyst characteristics such as nickel loading, catalyst reducibility from TPR analysis, metal dispersion from H2 chemisorption, and BET surface area from N2 adsorption.

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Figure 5. Effect of nickel loading on the isooctane conversion.

Figure 6. Effect of nickel loading on H2 selectivity.

Catalyst performance was evaluated using isooctane conversion, H2 selectivity, and TON for isooctane conversion. Figure 5 shows the effect of catalyst nickel loading for IMP and PRE catalysts on the stable isooctane conversion (or conversion obtained after 100 min of time-on-stream). Figure 5 shows that the isooctane conversion increases with catalyst nickel loading for IMP catalysts. The isooctane conversions increased with nickel loading from 28 mol % for IMP3 to 47 mol % for IMP9. In contrast, for PRE catalysts, the conversion increased with nickel loading from 35 mol % for PRE1 to a maximum of 44 mol % for PRE3. Beyond PRE3, the conversions declined to 28 wt.% for PRE9. Based on the result, the optimum nickel loading to achieve the highest conversion for the PRE catalysts was 3%. Figure 6 shows the effect of catalyst nickel loading for both PRE and IMP catalysts on hydrogen selectivity. Similar to Figure 5, the hydrogen selectivity for IMP catalysts increased as the nickel loading increased, whereas it peaked at 3% nickel loading for those prepared via the precipitation method. The turnover number for isooctane conversion (TON) was defined in our work as the mole percentage of isooctane converted per gram of nickel on the catalyst. Figures 7a and 7b illustrate the typical effect of the nickel content on the TON. Generally, the TON increased as the nickel loading decreased, as expected. This is attributed to the increased nickel dispersion with decreasing nickel loading, as was shown in the results from H2 chemisorption, TPR, and XRD analyses. We have evaluated the effects of additional characteristics of the catalysts such as catalyst reducibility, nickel dispersion, and BET surface area

Figure 7. Turnover number (TON) as a function of the time-onstream for (a) PRE catalysts and (b) IMP catalysts.

Figure 8. Effect of catalyst reducibility on isooctane conversion.

on catalyst performance to provide an explanation for the trends obtained from nickel loading. Catalyst reducibility was measured in terms of the minimum temperature required for the complete reduction of the catalyst, as provided by the TPR results. The effect of catalyst reducibility on isooctane conversion is presented in Figure 8. The figure shows that isooctane conversion increased with reducibility for samples prepared via the impregnation method. The 9% NiAl2O3 catalyst, which was the most reducible catalyst (830 °C), had the highest conversion (43 mol %; see Figure 5) and hydrogen selectivity (78 mol %; see Figure 6). In the case of

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Figure 9. Effect of nickel dispersion on the turnover number for isooctane.

Figure 10. Hydrogen selectivity as function of catalyst metal dispersion.

PRE catalysts, there was a maximum conversion for PRE3, even though it was not the most reducible. This implies that factors other than reducibility alone contributed toward catalyst performance. One of these factors is nickel dispersion. The effects of nickel dispersion on isooctane conversion, TON for isooctane conversion, and hydrogen selectivity are shown in Figures 3, 9, and 10, respectively. Figure 3 shows that isooctane conversion decreased as the nickel dispersion for the IMP catalysts increased. When viewed against the background of the reducibility results, this result shows that the beneficial effects on conversion of increased nickel dispersion with a decrease in nickel loading cannot overcome the detrimental effect of increased reducibility with nickel loading for the IMP catalysts. This reasoning becomes very clear in the PRE catalysts, where there is a maximum conversion with PRE3 having an intermediate nickel dispersion. As shown in Figure 3, going from PRE9 toward PRE3, the beneficial effect of nickel dispersion surpassed the detrimental effect of catalyst reducibility. From PRE3 to PRE1, the detrimental effects of reducibility surpassed the beneficial effects of nickel dispersion, with the result that a maximum occurred for PRE3 for the PRE catalysts. The beneficial effects of nickel dispersion are shown in Figure 9, in terms of the TON. The figure shows the TON increased with metal dispersion for both PRE and IMP catalysts. On the other hand, Figure 10 is an illustration of the effects of nickel dispersion on H2 selectivity. This figure shows the trends for

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Figure 11. Effect of catalyst surface area on the isooctane conversion.

PRE and IMP catalysts, which are similar to those for conversion versus nickel dispersion (Figure 3). The reasons of the interplay between the effects of catalyst reducibility and nickel dispersion given for conversion are also applicable here for H2 selectivity. Figure 11 shows the effect of BET surface area on the isooctane conversion. For catalysts prepared via the impregnation method, an increase in the catalyst nickel loading resulted in a decrease in the catalyst surface area, as shown in our earlier results. On the other hand, the PRE catalysts seem to mirror the trend for nickel dispersion in Figure 3 with a slight discrepancy for PRE6, because of the presence of a larger-thanexpected amount of sodium (see Table 1). Thus, nickel dispersion seems to be related to BET surface area, in that a high BET surface area provides a good environment for high nickel dispersion. 3.4. Summary. Earlier, we had attributed the major differences in performance of the catalysts prepared by the two methods to reducibility and metal dispersion. Also, we had demonstrated that the synthesis methods also affected the BET surface area and whether or not sodium is trapped in the catalyst. The interplay and ultimate effect of these variables on isooctane conversion has been illustrated in our earlier results. The summary is that a high BET surface area enhances nickel dispersion, which, together with high catalyst reducibility, helps to enhance the catalyst performance. In each preparation method, it was not possible to obtain high catalyst reducibility with high nickel dispersion. In this situation, the trends observed are manifestations of these opposing tendencies. 3.5. Effect of Reaction Temperature. The effect of the isooctane partial oxidation reaction temperature on the isooctane conversion and H2 selectivity was studied at 590, 610, 630, and 640 °C for the PRE 3% Ni-Al2O3 catalyst, because it had the best combination of conversion and H2 selectivity. The weight hourly space velocity was fixed at WHSV ) 22.2 h-1 for all of the runs. The results are given in Figure 12. This figure shows that both isooctane conversion and hydrogen selectivity increase with the reaction temperature. This is expected for the range of reaction temperatures used for this study, because the reaction is exothermic. Any reaction at >630 °C did not show any increase in conversion over that obtained at 630 °C, as evidenced by the results obtained earlier at 750 °C. On the other hand, hydrogen selectivity showed a steady increase with temperature. This was also observed for results obtained at 750 °C, as shown in Figure 4a. For the range of reaction temperatures up to 630 °C, the isooctane conversion increased from 14.9 mol % to 29.9

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Figure 12. Isooctane conversion and H2 selectivity as a function of the reaction temperature for PRE 3% Ni-Al2O3 catalyst at WHSV ) 22.22 h-1 and Tcalcination ) 650 °C.

Figure 14. Isooctane conversion, hydrogen selectivity, and yield, as a function of WHSV for PRE 3% Ni-Al2O3 catalyst at Treaction ) 610 °C and Tcalcination ) 650 °C.

Figure 13. Effect of calcination temperature on isooctane conversion, hydrogen selectivity, and yield for the PRE 3% catalyst at Treaction ) 630 °C, WHSV ) 0.071 h-1.

Figure 15. Effect of calcination temperature on the amount of carbon deposition for the spent 3% precipitated catalyst. Curves that show a monotonic decrease are weight loss profiles; curves with two maximums are heat-flow profiles.

mol % and the hydrogen selectivity increased from 74.5 mol % to 96.3 mol %. 3.6. Effect of Calcination Temperature. Figure 13 gives the effect of catalyst calcination temperature on the isooctane conversion, hydrogen selectivity, and hydrogen yield. The three performance parameters exhibited a detrimental inverse monotonic relationship. The profiles for the hydrogen selectivity showed a slight decrease (2%) when going from 500 °C to 600 °C and an appreciable decrease (16%) after the 600 °C calcination temperature. This could be attributed to sintering when the catalyst is subjected to high temperatures, resulting in a rapid increase in the size of the nickel crystallite and catalyst structural collapse. 3.7. Effect of Space Velocity. Another parameter studied was the WHSV. Experiments were conducted on the 3% Ni-Al2O3 prepared by the PRE method in the space velocity range of WHSV ) 14.1-26.3 h-1 at a reaction temperature of 610 °C. Figure 14 illustrates the effect of the space velocity on the isooctane conversion, hydrogen selectivity, and hydrogen yield. The general trends are slight decreases in the conversion, selectivity, and yield for an increase in the space velocity. For example, a decrease of only ∼4% in the isooctane conversion and H2 selectivity was observed for the entire range of WHSV values. Figure 14 shows that the isooctane conversion declined from 21.4 mol % for WHSV ) 14.1 h-1 to 17.41 mol % for WHSV ) 26.32 h-1. 3.8. Coke Formation. It was decided to determine whether or not there was carbon deposition during the reaction for the

PRE3 catalyst. TPO analyses were performed on the spent catalysts to quantify any carbon that had deposited on the catalyst during reaction. Typical results are shown in Figure 15 for the spent 3% PRE Ni-Al2O3 catalysts for various calcinations temperatures. Two profilessnamely, weight loss (curves with monotonic decrease) and heat flow (curves with two maximums)sare given in the figure. The results show two major peaks, corresponding to two weight reductions: the first peak (at ∼100 °C) is an endothermic peak at 100 °C that could be assigned to the thermal desorption of water, and the second peak (at ∼400-600 °C) shows the largest reduction in weight and corresponds to the main part of the coke deposited on the catalyst. This was an exothermic peak related to the oxidation of coke to CO and CO2. The TG profile in Figure 15 indicates that the total loss in sample weight was 3.3 mg, from which ∼1.3 mg was due to carbon oxidation. Figure 15 also shows the effect of calcination temperature on the amount and rate of carbon deposition. The profiles for TG and DTA show that the amount and rate of carbon deposition on a given catalyst increases as catalyst calcination temperature decreases. This suggests that the phase transformations and structural collapse during calcination have an effect on the final amount of carbon deposited on the catalyst. This is also related to the NiO crystallite size, which is dictated by the calcination temperature. This suggests that larger crystallite sizes for NiO produced by higher calcination temperatures are more prone to carbon deposition. The overall trend for all the other catalyst samples was more or less identical to those shown in Figure 15 for TPO

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results and weight loss observations. The overall observation was the increase in the amount of carbon deposited on the surface of the catalysts as the nickel content increased from 3 wt % to 9 wt %. 4. Conclusions A high BET surface area enhances nickel dispersion, which, together with high catalyst reducibility, helps to enhance the catalyst performance, in terms of isooctane conversion, H2 selectivity, and turnover number (TON). In each preparation method, it was not possible to obtain high catalyst reducibility

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with high nickel dispersion. Thus, the trends observed were manifestations of these opposing tendencies. Also, an increase in calcination temperature was determined to have a detrimental effect on catalyst performance and resulted in an increase in the amount of carbon deposited during the reaction. Acknowledgment. The authors would like to thank the Auto 21 Networks of Centers of Excellence (NCE), Canada for their financial support. EF060566U