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SiO2 catalyst activate in syngas mixture
Catalysis Communications 5 (2004) 635–638 www.elsevier.com/locate/catcom
Strong enhancement of the Fischer–Tropsch synthesis on a Co/SiO2 catalyst activate in syngas mixture V.A. de la Pen˜a OShea, J.M. Campos-Martı´n, J.L.G. Fierro
*
Instituto de Cata´lisis y Petroleoquı´mica, CSIC, c/Marie Curie, s/n, Cantoblanco, 28049 Madrid, Spain Received 26 January 2004; accepted 4 August 2004 Available online 11 September 2004
Abstract The pre-treatment of Co/SiO2 catalyst in a H2 + CO mixture produces a strong enhancement in CO conversion up to 90% which is five times higher than with the standard activation in H2. In addition, the H2 + CO activation almost depressed the appearance of unsaturated product. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Fischer–Tropsch; Cobalt; Pre-treatment; Syngas; Slurry reactor
1. Introduction Fischer–Tropsch (FT) technology for the conversion of natural gas-to-liquids (GTL) is nowadays an attractive process for the production of clean fuels, chemicals and added-value products [1–3]. The production of clean liquid hydrocarbons from syngas (CO + H2), via FT synthesis, is a promising, developing alternative for the production of alkanes, alkenes and oxygenates from natural gas. On scrutinizing the catalytic systems employed for the catalytic synthesis of hydrocarbons, it is clear that cobalt and iron are those most commonly used [4–6]. Cobalt catalysts are suited for producing high yields of long-chain alkanes in FT synthesis. The active species of the catalysts used in the FT synthesis is the metallic phase, which is produced by pretreatment of the precursors in a reducing environment prior to use in the reaction [7–11]. Although there is a tremendous body of work on the active catalysts, the mechanism of the reaction and engineering aspects to
*
Corresponding author. Tel.: +34 915854769; fax: +34 915854760. E-mail address: jlgfi[email protected] (J.L.G. Fierro). URL: http://www.icp.csic.es/eac/index.htm.
1566-7367/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.08.005
maximize the production of hydrocarbons, olefins and oxygenates, no so much attention has been paid to determine how the activating agent (H2, CO or CO + H2) influences on the catalyst performance [12]. Accordingly, this work was undertaken with the aim to investigate how the reducing agent employed in the activation of a cobalt–silica catalyst affect the reaction rate and product distributions during the FT synthesis.
2. Experimental methods For this purpose, a silica-supported catalyst was prepared by wet impregnation of a Grace Davison silica sample (SP9-10220, BET specific area 319 m2 g1 and pore volume 1.22 cm3 g1) with an aqueous solution of Co(NO3)2 Æ 6H2O of appropriate concentration to yield a 10.0 wt% cobalt on a dry basis. The impregnate was dried in air atmosphere at 373 K for 14 h, and finally it was calcined in air by heating at a rate of 10 K min1 to reach 723 K and then maintained for 2 h. This sample is labelled Co10-c. Specific surface areas of the catalysts were calculated by applying the BET method to the N2 adsorption
636
V.A. de la Pen˜a OShea et al. / Catalysis Communications 5 (2004) 635–638
isotherms, measured at liquid nitrogen temperature on Micrometrics TRISTAR equipment. Elemental analyses were performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) Perkin–Elmer optima 3300 DV. X-ray diffraction patterns were recorded from calcined, reduced and used catalysts using a Seifert 3000P vertical diffractometer and nickel-filtered Cu Ka radiation (k = 0.1538 nm), under constant instrument parameter. For each sample, Bragg angles between 5° and 80° were scanned a rate of 5 s per step (step size: 0.04° 2h). Temperature-programmed reduction (TPR) was carried out in a Micrometrics 3000 apparatus by passing a 10% H2/Ar flow (50 ml min1) through the sample. The temperature was increased from 323 to 1073 K at a rate of 10 K min1, and amount of H2 consumed was determined with a TCD, the effluent gas was passed through a cold trap placed before the TCD in order to remove water from the exit stream. Transmission electron micrographs were taken with a Fei Tecnai G30 type electron microscope. The acceleration voltage was set at 200 kV. The powdered sample was first suspended in acetone, after which a drop of the suspension was deposited on a copper grid covered by a fine carbon membrane evaporated under vacuum. Activity tests were carried out using an autoclave high pressure (Autoclave Engineers, Ltd.) catalytic reactor. The catalysts (0.5 g) were activated in a mixture of N2/H2 = 8:2 at 773 K for 10 h under atmospheric pressure. The reaction was conducted at 503 K and a pressure of 40 bar. The composition of the feed stream— CO (30%), H2 (60%) and N2 (10%)—was adjusted by electronic mass flow controllers (Unit) to a total flow of 100 mL(STP) min1. The reactor was suitably designed in order to allow on-line analysis of the gas fraction through a multiple GC column system. A Hewlett–Packard 6890 gas chromatograph equipped with an HP-1 capillary column and a Haysep Q packed column were used for product separation on-line. Product analysis was carried out with thermal conductivity (TCD) and flame ionisation (FID) detectors, which allows the analysis of H2, CO, CO2, C1–C6 hydrocarbons. C7–C30 hydrocarbons were analysed in a Hewlett–Packard 5890 gas chromatograph equipped with a HP-1 column.
3. Results and discussion The nitrogen adsorption–desorption isotherms at 77 K revealed that the textural properties of the samples do not change substantially with the calcination treatment. This finding agrees with the data reported by scanning electron microscopy. X-ray diffraction (XRD) patterns were recorded for calcined, reduced and used catalysts. Only diffraction peaks belonging to a Co3O4 phase could be revealed in the Co10-c
sample. Transmission electron microscopy (TEM) study of the Co10-c sample showed a good contrast between the particles of cobalt oxide and silica. Cobalt oxide particles exhibited a nearly spherical shape with a size distribution depending on the silica particles on which they are deposited, so that cobalt oxide particles display uniform sizes within each silica particle with sizes ranging from 0.1 to 1.0 lm. Riva et al. [13] reported TEM analysis of silica-supported cobalt oxide catalysts and found that cobalt is not homogeneously distributed on the silica surface and that develops spherical aggregates in both the inner and the outer surface of the silica particles, with a mean particle size ranging from 0.3 to 0.5 lm for a catalysts containing 13% cobalt. Reduction profiles of the catalyst showed two reduction peaks, which are similar to those observed in bulk Co3O4 oxide. These profiles point to a two-step reduction process: the first one of low intensity starts at approximately 475 K and overlaps with the more intense second one whose maximum is placed at about 586 K. Thus, the reduction process of Co3O4 can be described by the reduction of Co3+ ions present in the spinel structure into Co2+ (Eq. (1)), with the subsequent structural change to CoO, followed by the reduction of CoO to metallic cobalt (Eq. (2)) [14–17] Co3 O4 þ H2 ! 3CoO þ H2 O
ð1Þ
3CoO þ 3H2 ! 3Co0 þ 3H2 O
ð2Þ
The shape of the reduction profiles of the bulk Co3O4 was essentially the same that in the Co10-c sample, though it appeared shifted to around 50 K higher temperatures. As the mean crystal size of bulk Co3O4 is around an order of magnitude higher than that of the Co10-c sample the shift in the reduction profile of bulk Co3O4 toward higher temperature is related to particle size effects. The catalyst pre-reduced at 773 K for 2 h under different reduction environments: H2 (20%), CO (20%) and synthesis gas diluted in N2 (H2/CO/N2 = 10:10:80 molar) was characterized by XRD and TEM. XRD patterns showed diffraction lines of an fcc crystalline structure. Besides, very weak peaks belonging to a CoO phase were also observed. This observation provides also support for the assignment of the reduction steps described by Eqs. (1) and (2). Notwithstanding, the reduction with CO resulted in the formation of several crystalline phases, few of the lines overlapping with each other. A metallic cobalt phase (hcp) was indexed together with crystalline graphitic carbon. The appearance of these graphitic carbon structures is due to the disproportionation reaction of carbon monoxide (Boudouard reaction) on the surface of the metallic cobalt particles 2CO ! CO2 þ C
ð3Þ
V.A. de la Pen˜a OShea et al. / Catalysis Communications 5 (2004) 635–638
637
% CO Conversion
100 H2+CO
80
60
40 H2
20
0
CO
0
5
10
15
20
25
Time/h Fig. 1. TEM micrographs of the Co10 sample activated at 773 K for 2 h in H2 (a) and in H2 + CO mixture (b).
which is one of the essential steps in the Fischer–Tropsch synthesis. The presence of a major crystalline metallic Co phase (fcc) and a minor CoO phase in the X-ray diffraction profiles of the Co10-c sample reduced in a H2/CO mixture does not differ substantially from the results derived from the samples reduced in hydrogen. On the other hand, no graphitic carbon produced by the Boudouard reaction was observed since the simultaneous presence of hydrogen minimizes carbon formation. An attempt was made to reveal the structure of the reduced catalyst by TEM. Fig. 1 displays TEM micrographs of the Co10-c sample reduced at 773 K for 2 h in H2 (A) and H2/CO (B). The sample activated in H2 does not show substantial structural differences with respect the unreduced one. However, the cobalt particles appear much more dispersed on the silica substrate when the Co10-c sample was activated in the H2 + CO mixture. The catalysts were tested in the Fischer–Tropsch reaction. The reaction was conducted in a batch reactor by keeping the catalyst in suspension in n-hexadecane. CO conversion was strongly affected by the pre-treatment. As shown in Fig. 2, the conventional activation of the catalyst in H2 allows reaching stationary CO conversion levels around 28%, but these levels decreased to about 10% very likely as a consequence of the formation of carbon deposits by the Boudouard reaction on the metallic Co crystallites. Surprisingly, a tremendous enhancement in CO conversion, reaching levels in the order of 90%, was achieved upon catalyst pre-treatment under the H2 + CO mixture. The excellent performance of this catalyst appears to be related to the increase of the number of Co active sites produced by activating in H2 + CO mixture which produces an increase in the active area of the catalyst,
Fig. 2. Evolution of CO conversion with the time on-stream of the Co10 catalyst activated in CO, H2 and H2 + CO.
Table 1 Influence of activation treatment on products selectivity Activation
H2 CO H2/CO a b
% Selectivity toa
C2–C4 olefinityb
C1
C2
C3
C4
C5+
24.5 25.3 31.8
2.8 4.4 2.2
4.8 5.3 3.6
3.6 3.2 1.2
62.9 61.7 58.4
0.57 0.40 0.02
Selectivity = mol CO to product/mol CO total consumed. Olefinity = mol alkenes formed/mol alkanes formed.
and hence an increase in CO conversion. The smaller crystal size of Co particles of these catalyst determined by TEM provides also support for this interpretation. In addition, the H2 + CO pre-treatment led to a change in product distribution (Table 1). Though no substantial changes were found in selectivity to a given product, selectivity to C1 increased in detriment of all other hydrocarbons. An examination of the alkenes/ alkanes ratios indicates that H2 + CO activation basically produces alkanes and only a very amount of alkenes. Since alkenes are expected to be primary products in the Fischer–Tropsch reaction mechanism [18], the small Co particles of the Co10-c catalysts activated in H2 + CO mixture have stronger hydrogenation ability.
4. Conclusion In short, this preliminary work demonstrates that pre-treatment of the Co10 catalyst in a H2 + CO mixture produces a strong enhancement in CO conversion up to 90% which is five times higher than with the standard activation in H2. In addition, the H2 + CO activation almost depressed the appearance of unsaturated products.
638
V.A. de la Pen˜a OShea et al. / Catalysis Communications 5 (2004) 635–638
Acknowledgements A fellowship (V.P.O.) granted by the Repsol-YPF Foundation is acknowledged. J.M.C.M. acknowledges a fellowship granted by Repsol-YPF. This work was partly supported by MCYT (Spain) (Project MAT2001-2215-C03-01).
References [1] H. Schulz, Appl. Catal. A: General 186 (1999) 3. [2] M.E Dry., Catal. Today 71 (2002) 227. [3] J.M. Ogden, R.H. Williams, E.D. Larson, Energy Policy 32 (2004) 7. [4] B.W. Wojciechowski, Catal. Rev. Sci. Eng. 30 (1998) 629. [5] R.B. Anderson, The Fischer–Tropsch Synthesis, Academic Press, New York, 1984. [6] R.J. Madon, E. Iglesia, S.C. Reyes, in: S.L. Suib, M.E. Davis (Eds.), Selectivity in Catalysis, American Chemical Society, Washington, DC, 1993, p. 382.
[7] D.B. Bukur, L. Nowicki, R.K. Manne, X. Lang, J. Catal. 155 (1995) 366. [8] K. Sudsakorn, J.G. Goodwin Jr., A.A. Adeyiga, J. Catal. 213 (2003) 204. [9] B. Jongsomjit, J.G. Goodwin Jr., Catal. Today 77 (2002) 191. [10] M.D. Shroff, D.S. Kalakkad, K.E. Coulter, S.D. Ko¨hler, M.S. Harrington, N.B. Jackson, A.G. Sault, A.K. Datye, J. Catal. 156 (1995) 185. [11] B. Ernst, A. Bensaddik, L. Hilarie, P. Chaumette, A. Kienneman, Catal. Today 39 (1998) 329. [12] D.B. Bukur, X. Lang, Y. Ding, Appl. Catal. A: Gen. 186 (1999) 255. [13] R. Riva, H. Miessner, R. Vitali, G. Del Piero, Appl. Catal.: A General 196 (2000) 111. [14] A. Lapidus, A. Krylova, V. Kazanskii, V. Borovkov, A. Zaitsev, J. Rathousky, A. Zukal, M. Janca´lcova´, Appl. Catal. 73 (1991) 65. [15] Y. Okamoto, T. Nagata, T. Adachi, T. Imanaka, K. Inamura, T. Takyu, J. Chem. Phys. 95 (1991) 310. [16] G.J. Haddad, J.G. Goodwin, J. Catal. 157 (1995) 56. [17] E. van Steen, G.S. Sewell, R.A. Makhothe, H. Micklethwaite, H. Manstein, M. de Lange, C.T. OConnor, J. Catal. 162 (1996) 220. [18] See, e.g., H. Schulz, Erdo¨l Kohle 30 (1977) 123.
Strong enhancement of the Fischer–Tropsch synthesis on a Co/SiO2 catalyst activate in syngas mixture V.A. de la Pen˜a OShea, J.M. Campos-Martı´n, J.L.G. Fierro
*
Instituto de Cata´lisis y Petroleoquı´mica, CSIC, c/Marie Curie, s/n, Cantoblanco, 28049 Madrid, Spain Received 26 January 2004; accepted 4 August 2004 Available online 11 September 2004
Abstract The pre-treatment of Co/SiO2 catalyst in a H2 + CO mixture produces a strong enhancement in CO conversion up to 90% which is five times higher than with the standard activation in H2. In addition, the H2 + CO activation almost depressed the appearance of unsaturated product. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Fischer–Tropsch; Cobalt; Pre-treatment; Syngas; Slurry reactor
1. Introduction Fischer–Tropsch (FT) technology for the conversion of natural gas-to-liquids (GTL) is nowadays an attractive process for the production of clean fuels, chemicals and added-value products [1–3]. The production of clean liquid hydrocarbons from syngas (CO + H2), via FT synthesis, is a promising, developing alternative for the production of alkanes, alkenes and oxygenates from natural gas. On scrutinizing the catalytic systems employed for the catalytic synthesis of hydrocarbons, it is clear that cobalt and iron are those most commonly used [4–6]. Cobalt catalysts are suited for producing high yields of long-chain alkanes in FT synthesis. The active species of the catalysts used in the FT synthesis is the metallic phase, which is produced by pretreatment of the precursors in a reducing environment prior to use in the reaction [7–11]. Although there is a tremendous body of work on the active catalysts, the mechanism of the reaction and engineering aspects to
*
Corresponding author. Tel.: +34 915854769; fax: +34 915854760. E-mail address: jlgfi[email protected] (J.L.G. Fierro). URL: http://www.icp.csic.es/eac/index.htm.
1566-7367/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.08.005
maximize the production of hydrocarbons, olefins and oxygenates, no so much attention has been paid to determine how the activating agent (H2, CO or CO + H2) influences on the catalyst performance [12]. Accordingly, this work was undertaken with the aim to investigate how the reducing agent employed in the activation of a cobalt–silica catalyst affect the reaction rate and product distributions during the FT synthesis.
2. Experimental methods For this purpose, a silica-supported catalyst was prepared by wet impregnation of a Grace Davison silica sample (SP9-10220, BET specific area 319 m2 g1 and pore volume 1.22 cm3 g1) with an aqueous solution of Co(NO3)2 Æ 6H2O of appropriate concentration to yield a 10.0 wt% cobalt on a dry basis. The impregnate was dried in air atmosphere at 373 K for 14 h, and finally it was calcined in air by heating at a rate of 10 K min1 to reach 723 K and then maintained for 2 h. This sample is labelled Co10-c. Specific surface areas of the catalysts were calculated by applying the BET method to the N2 adsorption
636
V.A. de la Pen˜a OShea et al. / Catalysis Communications 5 (2004) 635–638
isotherms, measured at liquid nitrogen temperature on Micrometrics TRISTAR equipment. Elemental analyses were performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) Perkin–Elmer optima 3300 DV. X-ray diffraction patterns were recorded from calcined, reduced and used catalysts using a Seifert 3000P vertical diffractometer and nickel-filtered Cu Ka radiation (k = 0.1538 nm), under constant instrument parameter. For each sample, Bragg angles between 5° and 80° were scanned a rate of 5 s per step (step size: 0.04° 2h). Temperature-programmed reduction (TPR) was carried out in a Micrometrics 3000 apparatus by passing a 10% H2/Ar flow (50 ml min1) through the sample. The temperature was increased from 323 to 1073 K at a rate of 10 K min1, and amount of H2 consumed was determined with a TCD, the effluent gas was passed through a cold trap placed before the TCD in order to remove water from the exit stream. Transmission electron micrographs were taken with a Fei Tecnai G30 type electron microscope. The acceleration voltage was set at 200 kV. The powdered sample was first suspended in acetone, after which a drop of the suspension was deposited on a copper grid covered by a fine carbon membrane evaporated under vacuum. Activity tests were carried out using an autoclave high pressure (Autoclave Engineers, Ltd.) catalytic reactor. The catalysts (0.5 g) were activated in a mixture of N2/H2 = 8:2 at 773 K for 10 h under atmospheric pressure. The reaction was conducted at 503 K and a pressure of 40 bar. The composition of the feed stream— CO (30%), H2 (60%) and N2 (10%)—was adjusted by electronic mass flow controllers (Unit) to a total flow of 100 mL(STP) min1. The reactor was suitably designed in order to allow on-line analysis of the gas fraction through a multiple GC column system. A Hewlett–Packard 6890 gas chromatograph equipped with an HP-1 capillary column and a Haysep Q packed column were used for product separation on-line. Product analysis was carried out with thermal conductivity (TCD) and flame ionisation (FID) detectors, which allows the analysis of H2, CO, CO2, C1–C6 hydrocarbons. C7–C30 hydrocarbons were analysed in a Hewlett–Packard 5890 gas chromatograph equipped with a HP-1 column.
3. Results and discussion The nitrogen adsorption–desorption isotherms at 77 K revealed that the textural properties of the samples do not change substantially with the calcination treatment. This finding agrees with the data reported by scanning electron microscopy. X-ray diffraction (XRD) patterns were recorded for calcined, reduced and used catalysts. Only diffraction peaks belonging to a Co3O4 phase could be revealed in the Co10-c
sample. Transmission electron microscopy (TEM) study of the Co10-c sample showed a good contrast between the particles of cobalt oxide and silica. Cobalt oxide particles exhibited a nearly spherical shape with a size distribution depending on the silica particles on which they are deposited, so that cobalt oxide particles display uniform sizes within each silica particle with sizes ranging from 0.1 to 1.0 lm. Riva et al. [13] reported TEM analysis of silica-supported cobalt oxide catalysts and found that cobalt is not homogeneously distributed on the silica surface and that develops spherical aggregates in both the inner and the outer surface of the silica particles, with a mean particle size ranging from 0.3 to 0.5 lm for a catalysts containing 13% cobalt. Reduction profiles of the catalyst showed two reduction peaks, which are similar to those observed in bulk Co3O4 oxide. These profiles point to a two-step reduction process: the first one of low intensity starts at approximately 475 K and overlaps with the more intense second one whose maximum is placed at about 586 K. Thus, the reduction process of Co3O4 can be described by the reduction of Co3+ ions present in the spinel structure into Co2+ (Eq. (1)), with the subsequent structural change to CoO, followed by the reduction of CoO to metallic cobalt (Eq. (2)) [14–17] Co3 O4 þ H2 ! 3CoO þ H2 O
ð1Þ
3CoO þ 3H2 ! 3Co0 þ 3H2 O
ð2Þ
The shape of the reduction profiles of the bulk Co3O4 was essentially the same that in the Co10-c sample, though it appeared shifted to around 50 K higher temperatures. As the mean crystal size of bulk Co3O4 is around an order of magnitude higher than that of the Co10-c sample the shift in the reduction profile of bulk Co3O4 toward higher temperature is related to particle size effects. The catalyst pre-reduced at 773 K for 2 h under different reduction environments: H2 (20%), CO (20%) and synthesis gas diluted in N2 (H2/CO/N2 = 10:10:80 molar) was characterized by XRD and TEM. XRD patterns showed diffraction lines of an fcc crystalline structure. Besides, very weak peaks belonging to a CoO phase were also observed. This observation provides also support for the assignment of the reduction steps described by Eqs. (1) and (2). Notwithstanding, the reduction with CO resulted in the formation of several crystalline phases, few of the lines overlapping with each other. A metallic cobalt phase (hcp) was indexed together with crystalline graphitic carbon. The appearance of these graphitic carbon structures is due to the disproportionation reaction of carbon monoxide (Boudouard reaction) on the surface of the metallic cobalt particles 2CO ! CO2 þ C
ð3Þ
V.A. de la Pen˜a OShea et al. / Catalysis Communications 5 (2004) 635–638
637
% CO Conversion
100 H2+CO
80
60
40 H2
20
0
CO
0
5
10
15
20
25
Time/h Fig. 1. TEM micrographs of the Co10 sample activated at 773 K for 2 h in H2 (a) and in H2 + CO mixture (b).
which is one of the essential steps in the Fischer–Tropsch synthesis. The presence of a major crystalline metallic Co phase (fcc) and a minor CoO phase in the X-ray diffraction profiles of the Co10-c sample reduced in a H2/CO mixture does not differ substantially from the results derived from the samples reduced in hydrogen. On the other hand, no graphitic carbon produced by the Boudouard reaction was observed since the simultaneous presence of hydrogen minimizes carbon formation. An attempt was made to reveal the structure of the reduced catalyst by TEM. Fig. 1 displays TEM micrographs of the Co10-c sample reduced at 773 K for 2 h in H2 (A) and H2/CO (B). The sample activated in H2 does not show substantial structural differences with respect the unreduced one. However, the cobalt particles appear much more dispersed on the silica substrate when the Co10-c sample was activated in the H2 + CO mixture. The catalysts were tested in the Fischer–Tropsch reaction. The reaction was conducted in a batch reactor by keeping the catalyst in suspension in n-hexadecane. CO conversion was strongly affected by the pre-treatment. As shown in Fig. 2, the conventional activation of the catalyst in H2 allows reaching stationary CO conversion levels around 28%, but these levels decreased to about 10% very likely as a consequence of the formation of carbon deposits by the Boudouard reaction on the metallic Co crystallites. Surprisingly, a tremendous enhancement in CO conversion, reaching levels in the order of 90%, was achieved upon catalyst pre-treatment under the H2 + CO mixture. The excellent performance of this catalyst appears to be related to the increase of the number of Co active sites produced by activating in H2 + CO mixture which produces an increase in the active area of the catalyst,
Fig. 2. Evolution of CO conversion with the time on-stream of the Co10 catalyst activated in CO, H2 and H2 + CO.
Table 1 Influence of activation treatment on products selectivity Activation
H2 CO H2/CO a b
% Selectivity toa
C2–C4 olefinityb
C1
C2
C3
C4
C5+
24.5 25.3 31.8
2.8 4.4 2.2
4.8 5.3 3.6
3.6 3.2 1.2
62.9 61.7 58.4
0.57 0.40 0.02
Selectivity = mol CO to product/mol CO total consumed. Olefinity = mol alkenes formed/mol alkanes formed.
and hence an increase in CO conversion. The smaller crystal size of Co particles of these catalyst determined by TEM provides also support for this interpretation. In addition, the H2 + CO pre-treatment led to a change in product distribution (Table 1). Though no substantial changes were found in selectivity to a given product, selectivity to C1 increased in detriment of all other hydrocarbons. An examination of the alkenes/ alkanes ratios indicates that H2 + CO activation basically produces alkanes and only a very amount of alkenes. Since alkenes are expected to be primary products in the Fischer–Tropsch reaction mechanism [18], the small Co particles of the Co10-c catalysts activated in H2 + CO mixture have stronger hydrogenation ability.
4. Conclusion In short, this preliminary work demonstrates that pre-treatment of the Co10 catalyst in a H2 + CO mixture produces a strong enhancement in CO conversion up to 90% which is five times higher than with the standard activation in H2. In addition, the H2 + CO activation almost depressed the appearance of unsaturated products.
638
V.A. de la Pen˜a OShea et al. / Catalysis Communications 5 (2004) 635–638
Acknowledgements A fellowship (V.P.O.) granted by the Repsol-YPF Foundation is acknowledged. J.M.C.M. acknowledges a fellowship granted by Repsol-YPF. This work was partly supported by MCYT (Spain) (Project MAT2001-2215-C03-01).
References [1] H. Schulz, Appl. Catal. A: General 186 (1999) 3. [2] M.E Dry., Catal. Today 71 (2002) 227. [3] J.M. Ogden, R.H. Williams, E.D. Larson, Energy Policy 32 (2004) 7. [4] B.W. Wojciechowski, Catal. Rev. Sci. Eng. 30 (1998) 629. [5] R.B. Anderson, The Fischer–Tropsch Synthesis, Academic Press, New York, 1984. [6] R.J. Madon, E. Iglesia, S.C. Reyes, in: S.L. Suib, M.E. Davis (Eds.), Selectivity in Catalysis, American Chemical Society, Washington, DC, 1993, p. 382.
[7] D.B. Bukur, L. Nowicki, R.K. Manne, X. Lang, J. Catal. 155 (1995) 366. [8] K. Sudsakorn, J.G. Goodwin Jr., A.A. Adeyiga, J. Catal. 213 (2003) 204. [9] B. Jongsomjit, J.G. Goodwin Jr., Catal. Today 77 (2002) 191. [10] M.D. Shroff, D.S. Kalakkad, K.E. Coulter, S.D. Ko¨hler, M.S. Harrington, N.B. Jackson, A.G. Sault, A.K. Datye, J. Catal. 156 (1995) 185. [11] B. Ernst, A. Bensaddik, L. Hilarie, P. Chaumette, A. Kienneman, Catal. Today 39 (1998) 329. [12] D.B. Bukur, X. Lang, Y. Ding, Appl. Catal. A: Gen. 186 (1999) 255. [13] R. Riva, H. Miessner, R. Vitali, G. Del Piero, Appl. Catal.: A General 196 (2000) 111. [14] A. Lapidus, A. Krylova, V. Kazanskii, V. Borovkov, A. Zaitsev, J. Rathousky, A. Zukal, M. Janca´lcova´, Appl. Catal. 73 (1991) 65. [15] Y. Okamoto, T. Nagata, T. Adachi, T. Imanaka, K. Inamura, T. Takyu, J. Chem. Phys. 95 (1991) 310. [16] G.J. Haddad, J.G. Goodwin, J. Catal. 157 (1995) 56. [17] E. van Steen, G.S. Sewell, R.A. Makhothe, H. Micklethwaite, H. Manstein, M. de Lange, C.T. OConnor, J. Catal. 162 (1996) 220. [18] See, e.g., H. Schulz, Erdo¨l Kohle 30 (1977) 123.