Influence of pH of the Impregnation Solution on the Catalytic

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Energy & Fuels 2008, 22, 2885–2891

2885

Influence of pH of the Impregnation Solution on the Catalytic Properties of Co/γ-Alumina for Fischer-Tropsch Synthesis Jong Wook Bae, Yun-Jo Lee, Jo-Yong Park, and Ki-Won Jun* AlternatiVe Chemicals/Fuel Research Center, Korea Research Institute of Chemical Technology (KRICT), Post Office Box 107, Yuseong, Daejeon 305-600, Korea ReceiVed March 2, 2008. ReVised Manuscript ReceiVed June 25, 2008

The Co/γ-Al2O3 catalysts were prepared by the slurry impregnation of an aqueous solution of cobalt(II) nitrate precursor. Nitric acid or ammonium hydroxide was added to the cobalt nitrate solution, during impregnation, to give an acidic or basic environment. The changes in the particle size of cobalt species were estimated by X-ray diffraction (XRD) and hydrogen chemisorption. The reduction degree of cobalt oxides was measured by temperature-programmed reduction (TPR). The catalysts prepared under acidic conditions showed a higher reduction degree compared to those prepared at higher pH because of the reduced salt-support interaction. During the Fischer-Tropsch synthesis at 220 °C, employing the catalysts prepared at a different pH (0.80, 4.94, 9.96, and 11.12), a considerable difference in the initial activity was observed, depending upon the cobalt metal surface area. However, after stabilization, all of the catalysts attained a similar level of conversion, possibly because of the active-site rearrangement, deactivation, and wax formation on the catalyst surface. At a higher reaction temperature of 240 °C, the catalysts prepared at lower solution pH exhibited higher conversion than those prepared at higher solution pH. The cobalt species on the catalysts prepared under acidic conditions had a heterogeneous particle size distribution, showing higher steady-state activity, because of the reduced interaction with the support. The product distribution revealed a higher selectivity to C1 and C8+ on the catalyst prepared with a higher solution pH.

1. Introduction Fischer-Tropsch synthesis (FTS) is a promising route to synthesize clean and environmentally benign fuels in the near future.1–10 Cobalt is considered as the most favorable catalyst for the formation of high-molecular-weight and long-chain hydrocarbons from synthesis gas (syngas), which can further produce lubricants and diesel fuel by hydrocracking. Supported cobalt catalysts possess high resistance toward deactivation, and they show low activity for the water-gas shift reaction. The selection of a suitable support is an important aspect for the active cobalt-based catalyst in FTS, because the pore structure of the support has a significant effect on the Co3O4 crystallite size in supported cobalt catalysts. After examining several catalysts prepared with supports of the same chemical identity, the same method of preparation, and the same amount of active component, Borg et al.11 have recently reported the effects of physical parameters and chemical purity on the activity and selectivity of γ-Al2O3-supported cobalt catalysts. Xiong et al.12 have reported that the large pore size of γ-Al2O3 enhances * To whom correspondence should be addressed. Telephone: 82-42-8607671. Fax: 82-42-860-7388. E-mail: [email protected]. (1) Khodakov, A. Y.; Chu, W.; Fongarland, P. Chem. ReV. 2007, 107, 1692. (2) Madon, R. J.; Iglesisa, E. J. Catal. 1993, 139, 576. (3) Davis, B. H. Top. Catal. 2005, 32, 143. (4) Iglesia, E. Appl. Catal., A 1997, 161, 59. (5) Dry, M. E. Catal. Today 2002, 71, 227. (6) Jager, B.; Espinoza, R. Catal. Today 1995, 23, 17. (7) Wakamura, O. Nippon Steel Technical Report, 2005, 92. (8) Oukaci, R.; Singleton, A. H.; Goodwin, J. G., Jr. Appl. Catal., A 1999, 186, 129. (9) Rohde, M. P.; Unruh, D.; Schaub, G. Catal. Today 2005, 106, 143. (10) Schulz, H.; Nie, Z.; Ousmanov, F. Catal. Today 2002, 71, 351.

the formation of large Co3O4 crystallite. Similar studies have also been reported on silica-supported cobalt catalysts.13–17 Saib et al.13 and Khodakov et al.14,15 have shown higher reducibility of cobalt in wide-pore silica-supported catalysts than cobalt deposited on narrow-pore structured supports. Khodakov et al.18 have also shown that the ease of reduction decreases from large pores to small pores on silica-supported cobalt catalysts. Another important factor that influences the FT catalyst development is the strong cobalt-support interaction, which leaves a fraction of cobalt chemically inactive after reduction, affecting the catalytic activity and selectivity during FTS. Promoters, such as Ru, Re, and Pt, in combination with cobalt, have shown a remarkable enhancement of the catalyst performance.19,20 However, because of the high price of the precious metals, the design of highly dispersed cobalt-based catalysts on porous supports,

(11) Borg, O.; Eri, S.; Blekkan, E. A.; Storsaeter, S.; Wigum, H.; Rytter, E.; Holmen, A. J. Catal. 2007, 248, 89. (12) Xiong, H.; Zhang, Y.; Wang, W.; Li, J. Catal. Commun. 2005, 6, 512. (13) Saib, A. M.; Claeys, M.; van Steen, E. Catal. Today 2002, 71, 395. (14) Khodakov, A. Y.; Griboval-Constant, A.; Bechara, R.; Villain, F. J. Phys. Chem. B 2001, 105, 9805. (15) Khodakov, A. Y.; Griboval-Constant, A.; Bechara, R.; Zholobenko, V. L. J. Catal. 2002, 206, 230. (16) Khodakov, A. Y.; Bechara, R.; Griboval-Constant, A. Appl. Catal., A 2003, 254, 273. (17) Song, D.; Li, J. J. Mol. Catal. A: Chem. 2006, 247, 206. (18) Khodakov, A. Y.; Lynch, J.; Bazin, D.; Rebours, B.; Zanier, N.; Moisson, B.; Chaumette, P. J. Catal. 1997, 168, 16. (19) Tsubaki, N.; Sun, S.; Fujimoto, K. J. Catal. 2001, 199, 236. (20) Hilmen, A. M.; Schanke, D.; Holmen, A. Catal. Lett. 1996, 38, 143.

10.1021/ef800155v CCC: $40.75  2008 American Chemical Society Published on Web 07/24/2008

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such as Al2O3,21–29 TiO2,22 SiO2,30–35 and ZrO2,36 has been the subject of investigation in the recent past. The reactivity of cobalt catalysts during FTS also depends upon the particle size, reducibility, and dispersion of cobalt. The cobalt dispersion is largely affected by the concentration of hydroxyl groups on the support. Xiong et al.12 reported a negative correlation between the Co3O4 particle size and the degree of reduction for γ-Al2O3-supported cobalt catalysts. The pH of impregnation solution and the point of zero charge (PZC) of the support37,38 are the critical parameters during catalyst preparation that decide whether a homo- or heterogeneous particle size distribution can be obtained on the surface of the support. In the case of γ-Al2O3, which is a commercially important support, it is advantageous to enhance the cobalt oxide reducibility to the maximum possible extent to achieve high FTS activity. Thus, the change in pH of the solution during catalyst preparation can be an effective way to influence the cobalt-support interaction and attain the desired reducibility of cobalt oxide. Although the effect of solution pH on catalytic activity has been numerously investigated on Al2O3, SiO2, and TiO21 and the influence of pore size discussed, the variation in particle size with solution pH needs to be examined thoroughly. In the present investigation, we have closely examined the change in the salt-support interaction with the change in the pH of the solution during preparation of the Co/γ-Al2O3 catalyst. The size and morphology of the cobalt particles thus obtained have been correlated with the FTS activity. The catalysts have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), hydrogen chemisorption, surface area measurement, and pore size distribution. The aim of the present investigation is to

(21) Jacobs, G.; Patterson, P. M.; Zhang, Y.; Das, T. K.; Li, J.; Davis, B. H. Appl. Catal., A 2002, 233, 215. (22) Jacobs, G.; Das, T. K.; Zhang, Y.; Li, J.; Racoillet, G.; Davis, B. H. Appl. Catal., A 2002, 233, 263. (23) Rohr, F.; Lindvag, O. A.; Holmen, A.; Blekkan, E. A. Catal. Today 2000, 58, 247. (24) van de Loosdrecht, J.; Balzhinimaev, B.; Dalmon, J. A.; Niemantsverdriet, J.; Tsybulya, S. V.; Saib, A. M.; van Berge, P. J.; Visagie, J. L. Catal. Today 2007, 123, 293. (25) Borg, O.; Eri, S.; Blekkan, E. A.; Storsaeter, S.; Wigum, H.; Rytter, E.; Holmen, A. J. Catal. 2007, 248, 89. (26) van Berge, P. J.; van de Loosdrecht, J.; Barradas, S.; van der Kraan, A. M. Catal. Today 2000, 58, 321. (27) Xiong, H.; Zhang, Y.; Liew, K.; Li, J. J. Mol. Catal. A 2005, 231, 145. (28) Xiong, H.; Zhang, Y.; Wang, W.; Li, J. Catal Commun. 2005, 6, 512. (29) Bae, J. W.; Kim, I. G.; Lee, J. S.; Lee, K. H.; Jang, E. J. Appl. Catal., A 2003, 240, 129. (30) Zhang, J.; Chen, J.; Ren, J.; Sun, Y. Appl. Catal., A 2003, 243, 121. (31) Martinez, A.; Rollan, J.; Arribas, M. A.; Cerqueira, H. S.; Costa, A. F.; Aguiar, E. F. S. J. Catal. 2007, 249, 162. (32) Zhang, Y.; Shinoda, M.; Tsubaki, N. Catal. Today 2004, 93-95, 55. (33) Shinoda, M.; Zhang, Y.; Yoneyama, Y.; Hasegawa, K.; Tsubaki, N. Fuel Process. Technol. 2004, 86, 73. (34) Ernst, B.; Libs, S.; Chaumette, P.; Kinnemann, P. Appl. Catal., A 1999, 186, 145. (35) Reubroycharoen, P.; Vitidsant, T.; Liu, Y.; Yang, G.; Tsubaki, N. Catal. Commun. 2007, 8, 375. (36) Liu, Y.; Chen, J.; Fang, K.; Wang, Y.; Sun, Y. Catal. Commun. 2007, 8, 945. (37) Schwarz, J. A. Catal. Today 1992, 15, 395. (38) Park, G. A. Chem. ReV. 1965, 65, 177.

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elucidate the importance of solution pH during catalyst preparation on the structure and FTS activity of Al2O3-supported Co catalysts. 2. Experimental Section 2.1. Catalytic Preparation. The 15 wt % Co supported on γ-Al2O3 (laboratory-made, surface area: ∼350 m2/g) catalysts were prepared by the conventional wet impregnation method by changing the pH of the slurry with the addition of NH4OH or HNO3 solution in an excess of deionized water. Cobalt nitrate [Co(NO3)2 · 6H2O] was used as a precursor. The alumina used was synthesized from an aluminum isopropoxide-adopting sol-gel method.40 The slurry solution was kept for 12 h in a rotary evaporator at room temperature, and the excess solution was evacuated finally at 70 °C and vacuum condition. The catalysts were successively dried at ∼100 °C for 12 h and subsequently calcined at 500 °C for 5 h. Alumina-supported 20 and 30 wt % cobalt catalysts were also prepared by the same procedure for the sake of comparison. The catalyst are represented by the notation CoAl(X), where Co and Al represents cobalt and γ-Al2O3 and X stands for the pH of cobalt nitrate solution during impregnation. 2.2. Catalytic Activity Test. The catalysts were initially activated at 400 °C in a fixed-bed reactor (id ) 12.7 mm) for 12 h with 5% H2/N2. The activity tests were conducted for more than 80 h under the following reaction conditions: reaction T ) 220-240 °C; P ) 2 MPa; SV (L/kgcat/h) ) 2000; feed composition, (H2/ CO/CO2/Ar; mol %) ) 57.3:28.4:9.3:5.0. The effluent gas from the reactor was analyzed by an online gas chromatograph (YoungLin Acme 6000 GC) employing a GS-GASPRO capillary column connected with a flame ionization detector (FID) for the analysis of hydrocarbons and a Porapak Q/Molecular Sieve (5A) packed column connected with a thermal conductivity detector (TCD) for the analysis of carbon oxides, using Ar as an internal standard. 2.3. Catalyst Characterization. The Brunauer-Emmett-Teller (BET) surface areas, pore volumes, and pore size distributions were estimated from nitrogen adsorption and desorption isotherm data obtained at -196 °C using a constant volume adsorption apparatus (Micromeritics, ASAP-2400). The pore volumes were determined at a relative pressure (P/Po) of 0.99. The calcined samples were degassed at 300 °C with a He flow for 4 h before the measurements. The pore size distributions of the samples were calculated using the Barett-Joyner-Halenda (BJH) model. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku diffractometer using Cu KR radiation to identify the phases of Co/γ-Al2O3 catalysts and their crystallinity. The calcined samples as well as the samples that were reduced at 400 °C for 12 h in 5% H2/N2 flow followed by passivation with 0.1% O2/He for 0.5 h at room temperature (RT), were characterized separately to identify the phases of Co3O4 and cobalt metal. The morphology of Co/γAl2O3 catalysts was also characterized using TEM (TECNAI G2 instrument). The TPR experiments were performed to determine the reducibility of the surface Co3O4. Prior to the TPR experiments, the samples were pretreated in a He flow up to 350 °C and kept for 2 h to remove the adsorbed water and other contaminants followed by cooling to 50 °C. The reducing gas containing a 5% H2/Ar mixture was passed over the samples at a flow rate of 30 mL/min with a heating rate of 10 °C/min up to 750 °C and kept at that temperature for 30 min. The effluent gas was passed over a molecular sieve trap to remove the generated water and analyzed by a GC equipped with a TCD. The surface area and particle size of cobalt metal were measured by hydrogen chemisorption at 100 °C under static conditions using a Micromeritics ASAP 2000 instrument equipped with a pump system for providing a high vacuum of 10-6 torr. Prior to adsorption measurements, the sample (∼0.5 g) was reduced in situ at 400 °C (39) Bezemer, G. L.; Radstake, P. B.; Koot, V.; van Dillen, A. J.; Geus, J. W.; de Jong, K. P. J. Catal. 2006, 237, 291. (40) Bae, J. W.; Potdar, H. S.; Kang, S. H.; Jun, K. W. Energy Fuels 2008, 22, 223.

Catalytic Properties of Co/γ-Al2O3 for FTS

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Table 1. Physical Properties of Co/Al2O3 Catalysts

samplea

pH/additive during impregnation

surface area (m2/g)

pore volume (cm3/g)

average pore size (nm)

γ-Al2O3 CoAl(1) CoAl(5) CoAl(10) CoAl(11)

0.80/HNO3 4.94/none 9.96/NH4OH 11.12/NH4OH

351 206 227 225 243

1.437 0.272 0.678 0.698 0.786

12.5 4.1 9.8 9.9 10.1

a Catalyst notation: CoAl(X) represents the cobalt impregnated on γ-Al2O3, where X stands for the pH of cobalt nitrate solution during impregnation.

Figure 1. Pore size distribution (PSD) of CoAl catalysts prepared at a different pH of solution and bare γ-Al2O3.

for 12 h. Hydrogen uptakes were calculated from the difference of the two successive isotherms measured at 100 °C. The cobalt metal dispersion and surface area were calculated with the assumption of H/Co stoichiometry of 1.

3. Results and Discussion 3.1. Physicochemical Properties of Co/γ-Al2O3. The results of surface area measurements by nitrogen physisorption at -196 °C, for the 15 wt % Co/γ-Al2O3 catalysts prepared at different pH levels of impregnation solution, are given in Table 1, and the BJH pore size distributions are shown in Figure 1. The support γ-Al2O3 shows broad pore diameter distribution with an average size of 12.5 nm and a BET surface area of 351 m2/ g. After impregnation of the cobalt species, the surface areas of the samples are obtained in the range of 206-243 m2/g. The surface areas of the CoAl catalysts are much lower when the pH of the preparation solution is lower. Similarly, the pore volumes have also decreased considerably. From the pore size distribution patterns shown in Figure 1, the decrease in the average pore size (from 12.5 nm of the support to 4.1 nm) for the catalysts prepared with low pH of 0.80 suggests that some pore blockage has occurred because of the collapse of larger pores of γ-Al2O3 at the acidic solution condition. All of the samples have shown unimodal distribution. Besides, in the case of CoAl(1), the peak maximum shifts to the lower pore diameter region because of the structural changes in γ-Al2O3. These changes in pore size distribution could alter the cobalt particle size distribution and salt-support interaction and eventually the catalytic performance in FTS. 3.2. H2 Chemisorption, XRD and TPR Measurements on Co/γ-Al2O3 Catalysts. Hydrogen chemisorption over cobaltbased catalysts provides valuable information about the cobalt metal surface area and particle size. It has been reported that hydrogen chemisorption on cobalt catalysts is more reliable than

carbon monoxide chemisorption, 1,41 because the latter is almost irreversible under FT conditions because of its dissociation. Therefore, hydrogen chemisorption is adopted for measuring the number of cobalt active sites on the catalyst surface. The values of dispersion of cobalt and metal surface area derived from hydrogen chemisorption are reported in Table 2. It can be observed that CoAl(1) and CoAl(5) show higher cobalt metal surface areas than CoAl(10) and CoAl(11). At a pH of the impregnation solution lower than the PZC of γ-Al2O3, the repulsive interaction between the surface of support and cobalt ions (Co2+) occurs and induces a nonhomogeneous, possibly a bimodal, cobalt particle size distribution with a combination of particles of smaller (4 nm) sizes. The discrepancy in the metal surface areas of CoAl(1) and CoAl(5) could be a result of the structure collapse of γ-Al2O3 under more acidic conditions. On the other hand, at pH 10, the cobalt precursor could be impregnated by deposition and partial precipitation. Similar results were reported by Bezemer et al.39 during the preparation of cobalt catalysts supported on carbon nanofibers (CNFs) and silica. Deposition-precipitation induces stronger metal-support interaction, leading to a considerable decrease in the size of the cobalt species. Thus, the difference in the formation mechanism could be a possible reason for the difference in the metal surface area exhibited by different catalysts. Similarly, the decrease in metal surface area in the case of CoAl(11) compared to that on CoAl(10) can be attributed to the very low reduction degree because of the strong cobalt metal-support interaction, which originates from the formation of an ammonium complex, such as (NH4)2Co8(CO3)6(OH)6(H2O)4.42 The changes in the phase composition of the Co/γ-Al2O3 catalysts were determined by taking the powder X-ray diffraction patterns of the calcined 15 wt % catalysts prepared at a different pH of impregnation solution. The patterns are shown in Figure 2a. All of the samples have shown intense reflections because of the diffraction peaks from Co3O4 with the characteristic γ-Al2O3 peaks at around 2θ ) 45.9° and 66.9°. The average crystallite size of Co3O4 was calculated using Scherrer’s equation11 from the most intense Co3O4 peak (2θ ) 36.8°). The values of particle size thus calculated are given in Table 2. The results suggest that the particle size of Co3O4 decreases from 20.7 to 13.8 nm with an increasing pH of the impregnating solution. The TPR profiles of Co/γ-Al2O3 samples, prepared by varying the pH of solution, are shown in Figure 3. The data reveal that the change in pH during catalyst preparation influences the reduction behavior of the finished catalysts. All of the samples, except CoAl(1), have shown two major peaks with different intensities. These two peaks can be attributed to the reduction of Co3O4 to CoO at a temperature of ∼400 °C and CoO to metallic cobalt at a much higher temperature. The broad peaks at higher temperature arise because of the interaction of cobalt particles with γ-Al2O3 support. Under basic preparation conditions, the two reduction peaks are distinct, indicating a relatively homogeneous size distribution (unimodal) of cobalt particles. As the pH of solution decreases, the Tmax of the first peak shifts toward a lower temperature region, except for CoAl(1). This is due to the weaker interaction of cobalt species with γ-Al2O3 by the formation of larger Co3O4 particles under the precipitation conditions. As revealed from Table 1 and Figure 1, the pore size distribution for CoAl(1) catalyst is obviously narrower and (41) Schanke, D.; Vada, S.; Blekkan, E. A.; Hilmen, A. M.; Hoff, A.; Holmen, A. J. Catal. 1995, 156, 85. (42) Petrov, K.; Deleva, E.; Garcia-Martinez, O. Solid State Ionics 1996, 92, 303.

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Table 2. Summary of Characterization Results from Chemisorption, TPR, and XRD H2 chemisorption

H2 consumption from TPR (mmol of H2/g)

sample

before pre-reduction

after pre-reduction

reduction degree (%)a

Co surface area (m2/g Co metal)

CoAl(1) CoAl(5) CoAl(10) CoAl(11)

2.079 2.263 1.488 1.465

0.676 0.647 0.909 1.251

41.5 47.8 17.2 6.3

11.7 13.6 8.7 4.6

XRD

uncorrected particle size (nm)

corrected particle size (nm)b

Co3O4 particle size (nm)

57.8 49.5 77.3 147.6

24.0 23.7 13.3 9.4

20.7 19.7 16.7 13.8

a The reduction degree was calculated from the integrated TPR peak areas of the samples with and without pre-reduction at 400 °C for 12 h as shown in Figure 3. The difference between theoretical H2 consumption and H2 consumption before pre-reduction is responsible for the unreducible cobalt particles, such as cobalt aluminate. Reduction degree (%) ) [(H2 consumption before pre-reduction - H2 consumption after pre-reduction)/theoretical H2 consumption for 15 wt % Co/Al2O3 samples (3.379 mmol of H2/g) × 100]. b The particle size of Co was calculated from the results of H2 chemisorption and corrected by the reduction degree.

Figure 3. TPR profiles of the CoAl catalysts.

Figure 2. (a) XRD patterns of the calcined CoAl catalysts. (b) XRD patterns of the reduced and subsequently passivated CoAl catalysts.

the average pore diameter is much smaller. For the cobalt species located in the smaller pores, the reduction is obviously impeded. This is clearly reflected from the TPR patterns. To calculate the degree of reduction, a second TPR experiment was performed after pre-reduction of the catalysts in H2 at 400 °C for 12 h. In Figure 3, the dashed lines correspond to the pre-reduced catalysts. Hydrogen consumption values were calculated for the two TPR runs. The reduction degree (%) was defined as [(H2 consumption before pre-reduction - H2 consumption after pre-reduction)/theoretical H2 consumption for 15 wt % Co/Al2O3 samples (∼3.379 mmol of H2/g) × 100]. The difference between theoretical H2 consumption and H2 consumption before pre-reduction is responsible for the unreducible cobalt particle formation, such as cobalt aluminate. The particle size of Co was recalculated from the results of hydrogen chemisorption, taking the degree of reduction into account. The trend of corrected particle size of cobalt metal is consistent with

the XRD results representing the Co3O4 particle size. Although the cobalt particle size calculated from H2 chemisorption was larger on the CoAl(1) and CoAl(5) than that of the XRD measurement, the reduction degree is higher on CoAl(5), showing a high cobalt metal surface area as shown in Table 2. These discrepancies in particle size could be induced from the sintering phenomena during the reduction step or different stoichiometric factor for H/Co because of the molecular adsorption of H2. The total amounts of hydrogen consumption are larger on the catalysts prepared from acidic conditions than that from basic conditions. Furthermore, the reduction degrees calculated from the two different TPR experiments showed lowest value for the CoAl(11). The CoAl(5) showed the highest reducibility at low temperature (Figure 3). As previously reported, these results could be correlated with the initial catalytic activities and the steady-state activity may change because of the reoxidation or rearrangement of cobalt metals.4,10,21,24,26 However, the initial states of cobalt particles could alter the product distribution at the steady state because of the intrinsically different morphologies of cobalt particles and their electronic states.43 The difference in the values of particle size estimated by the two different techniques (Table 2) could be due to the nonuniform cobalt particle size distribution, particularly in CoAl(1), the inherent limitation of the XRD technique to identify particles of size less than 4 nm, the change in morphology of Co3O4 during the reduction step, and also the correction given to the particle size based on the degree of reduction. These compounding effects are further explained in the following sections to elucidate the effects of solution pH on catalytic activity. (43) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; Jos van Dillen, A.; de Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956.

Catalytic Properties of Co/γ-Al2O3 for FTS

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Figure 4. TEM analysis and cobalt particle size distribution of CoAl catalysts prepared at a different pH of solution: (a) CoAl(1), (b) CoAl(5), (c) CoAl(10), and (d) CoAl(11).

3.3. XRD and TEM Analysis of Co/γ-Al2O3. The ease of reducibility of the cobalt species on the catalysts was further characterized by the XRD analysis of the reduced catalysts. All samples were pre-reduced at 400 °C for 12 h and passivated with 0.1% O2/He gas at room temperature for 30 min prior to the measurements (Figure 2b). In all of the samples, cobalt metal and CoO diffraction peaks were noticed except in the case of CoAl(1). The intensity of the line because of metallic cobalt is much lower in CoAl(11) compared to that of CoAl(10). This could be due to the usage of an excess amount of NH4OH solution during the precipitation. In the case of CoAl(1), sharp diffraction peaks because of Co3O4 are detected. This confirms that the nature of the cobalt-support interaction is considerably different in the case of CoAl(1). The existence of cobalt particles, which are easily reoxidized during passivation, becomes evident. Such easy reducibility is possible in the case of small particles, non-interacting with the support. Therefore, the assumption of the existence of small particles along with the bigger ones seems plausible, reflecting the bimodal particle size distribution. This is an important observation, which needs to be understood. Alumina can dissolve in the strong acid solutions when the pH is less than 1. The dissolved aluminum ions interact with the Co ions and form a hydrotalcite-like structure, e.g., Co6Al2Co3(OH)16.4H2O.44 This amorphous hydrotalcite-like structure is physically and loosely bonded to the original alumina surface, and eventually, these species could modify the alumina surface. Because of the weak interaction of impregnated Co species on the modified alumina with hydrotalcite-like structure, the Co species on that modified alumina are easily reducible. The high reduction degree observed in the case of catalysts prepared at low solution pH also supports this proposition. Thus, it can be understood that the catalysts prepared at low pH have their cobalt species easily reducible and the distribution (44) van Berge, P. J.; van de Loosdrecht, J.; Caricato, E. A.; Barrados, S. Patent W099/42214.

of the Co particles are heterogeneous. In the contrary, the catalysts prepared at high solution pH exhibit strong metalsupport interaction and their Co particles could be more homogeneously distributed. These results are further supported by TEM investigations, whose micrograms are depicted in Figure 4. The TEM images clearly show the existence of both smaller (below 4 nm) and larger (>4 nm) Co3O4 particles in CoAl(1) and CoAl(5) catalysts (Figure 4a). The bimodal cobalt particle size distribution on CoAl(1) was observed with the size of below 3 nm and above 7 nm; however, CoAl(11) showed a unimodal particle size distribution with the maximum frequency at 5 nm in size. It can be understood that, owing to the existence of these smaller particles, the hydrogen consumption is found to be higher in the TPR analysis (Figure 3 and Table 2) and a facile reoxidation is observed after reduction and passivation as confirmed by XRD analysis (Figure 2b). In general, these small cobalt particles exhibit high initial acitivity in the FTS, and they are also easily deactivated by the well-known deactivation mechanisms, such as reoxidation by water or sintering. 4,10,26 To verify this phenomenon, the catalysts have been examined by conducting the reaction at two different temperatures and the results are discussed in the next section. 3.4. Catalytic Activities on Co/γ-Al2O3 Prepared at a Different pH of Solution. The particle size of cobalt is one of the important factors to obtain high catalytic activity and selectivity.4,29,30 A correlation between the FTS activity, along with product selectivity, can be seen from the data presented in Table 3 and Figure 5. The CoAl(1) and CoAl(5) catalysts prepared under acidic conditions show higher initial activity than that on CoAl(11), and the decrease in catalytic activity is also faster as can be seen from the TOS plots. The initial catalytic activity on the CoAl(11) catalyst, which shows a difficult reducibility and low metallic surface area, is low (∼16%) but increases up to ∼50% followed by an abrupt decrease and stabilization at ∼20%. The low initial activity could be due to

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Table 3. Summary of Reaction Results on Co/γ-Alumina Catalystsa selectivity (C mol %) notation CoAl(1) CoAl(5) CoAl(10) CoAl(11) 20CoAl(5)b 30CoAl(5)c

temperature (°C)

conversion of CO

C1

C2-C4

C5-C7

C8+

olefins in C2-C4

220 240 220 240 220 240 220 240 220 240 220 240

21.4 56.8 20.3 46.9 17.4 35.7 19.7 42.1 17.7 63.4 21.3 60.6

13.1 16.2 14.2 15.9 14.9 17.8 16.7 17.1 20.7 18.7 20.0 22.3

15.7 16.1 12.5 12.9 10.6 12.4 10.4 12.9 14.0 17.3 16.0 18.3

15.7 13.7 13.8 12.7 12.6 13.1 11.9 13.8 14.3 13.7 14.3 11.8

55.5 54.0 59.5 58.5 61.9 56.7 61.0 56.2 51.0 50.3 49.7 47.6

46.8 27.6 40.6 29.6 35.5 29.2 30.8 27.8 29.7 15.5 21.4 13.5

a The steady-state values correspond to a TOS of around 50 h at 220 °C and around 30 h at 240 °C. b 20CoAl(5) catalyst was prepared with 20 wt % of cobalt content at the solution pH of 4.67. c 30CoAl(5) catalyst was prepared with 30 wt % of cobalt content at the solution pH of 4.55.

Figure 5. Conversion of CO with time on stream (h). Reaction conditions: T ) 220-240 °C; P ) 2 MPa; SV (L/kgcat/h) ) 2000; feed compositions (H2/CO/CO2/Ar; mol %) ) 57.3:28.4:9.3:5.0.

low metallic surface area (TPR and H2 chemisorption). The increase to maximum point could be explained by the in situ rearrangement of cobalt on the surface during the course of the FTS. The difference in catalytic activity at steady state in CoAl catalysts could not be established because they all show a stable activity around 20% at 220 °C, possibly because of the influence of wax formation on the catalyst surface. A clear indication about the difference in activity could be established at a higher reaction temperature of 240 °C. The catalytic activity is better reflected in terms of cobalt metallic surface area and its reducibility. The higher the metallic surface area, the higher the activity obtained. Thus, it has been confirmed that the catalysts prepared at low solution pH exhibit better activity because of the presence of a heterogeneous distribution of particles, which are easily reducible. The bimodal distribution of the Co particle helps attain stable activity. 3.5. Product Distribution on Co/γ-Al2O3 Prepared at a Different pH of Solution. The CO2 formation on CoAl catalysts by the water-gas shift reaction is marginal and below 1.0%. With the increase in solution pH, the catalysts exhibit higher selectivity to C8+ products with a concomitant decrease in the selectivities of C2-C7 hydrocarbons, as shown in Table 3 and Figure 6. On the smaller cobalt metal particles, the probability of olefin re-adsorption and secondary hydrogenation of olefins formed during the FTS reaction could be low and results in showing a high selectivity to C2-C7 and olefins. A higher selectivity to C2-C7 and a lower selectivity to C8+ is observed on CoAl(1), which is due to the limited secondary reaction on small cobalt particles [CoAl(1)]. Therefore, the small cobalt particle size on CoAl(1) and the limitation of olefin re-adsorption for further propagation to higher molecular-weight hydrocarbon

Figure 6. Conversion of CO and product distribution on the CoAl catalysts. Reaction conditions: T ) 220 °C; P ) 2 MPa; SV (L/kgcat/ h) ) 2000; feed compositions (H2/CO/CO2/Ar; mol %) ) 57.3:28.4: 9.3:5.0.

formation restricts larger chain length formation. It is interesting to note that the ratio of O/(O + P) [stands for the mole ratio of olefin to (olefin plus paraffin) in the range of C2-C4] decreases with the increase of the pH of the impregnating solution. 3.6. Effect of Co Loading on Catalysts Prepared with the Similar pH of Solution. Furthermore, the activities of Co/ γ-Al2O3 with 20 and 30 wt % loading (prepared at almost the same pH) during FTS are also shown in Table 3. These catalysts also showed an initial high conversion (results not shown) at the reaction temperature of 220 °C, which decreases with time on stream until 50 h of operation. The steady-state conversion did not change appreciably with the increase of cobalt loading at the reaction temperature of 220 °C. However, at higher temperature (∼240 °C), the conversion increases substantially to 63.4% for 20 wt % Co/γ-Al2O3 (20CoAl) catalyst. A further increase of cobalt has little effect on the CO conversion. These results also substantiate the observations made on catalysts prepared with low solution pH. 4. Conclusions The pH of the impregnating solution during the catalyst preparation strongly influences the physicochemical and catalytic properties of Co/γ-Al2O3 in the FTS reaction. The control of cobalt particle size and their distribution on γ-Al2O3 during catalyst preparation plays an important role to determine the CO conversion and product distribution. The catalysts prepared at the basic conditions showed low cobalt surface areas because of a very low reduction degree. Although the initial catalytic

Catalytic Properties of Co/γ-Al2O3 for FTS

activities of catalysts prepared at low pH of solution are high, the steady-state activities are dependent upon cobalt metal surface areas. Catalysts prepared under acidic conditions show heterogeneous particle size distribution with a high degree of reducibility because of the reduced salt-support interaction. These catalysts exhibit higher CO conversion activity with lower selectivity to C8+ compounds compared to their high pH analogues. The mole ratio of olefin to (olefin plus paraffin) in the range of C2-C4 decreases with the increase of the pH of the impregnating solution.

Energy & Fuels, Vol. 22, No. 5, 2008 2891 Acknowledgment. The authors acknowledge the financial support of KEMCO and GTL Technology Development Consortium (Korea National Oil Corp., Daelim Industrial Co., Ltd., Doosan Mecatec Co., Ltd., Hyundai Engineering Co., Ltd., and SK Energy Co., Ltd.) under “Energy and Resources Technology Development Programs” of the Ministry of Knowledge Economy, Republic of Korea. We thank Dr. P. S. Sai Prasad, visiting scientist under the KOFST Brain-Pool fellowship for helpful discussions. EF800155V