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SiO2 catalysts: Effect of Co-loading
Applied Catalysis A: General 373 (2010) 71–75
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Fischer Tropsch synthesis from a simulated biosyngas feed over Co(x)/SiO2 catalysts: Effect of Co-loading C. Medina a, R. Garcı´a a, P. Reyes a, J.L.G. Fierro b, N. Escalona a,* a b
Universidad de Concepcio´n, Facultad de Ciencias Quı´micas, Casilla 160C, Concepcio´n, Chile Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Cantoblanco, 28049 Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 August 2009 Received in revised form 20 October 2009 Accepted 25 October 2009 Available online 30 October 2009
A series of Co/SiO2 catalysts containing 10–30 wt.% Co were prepared by wet impregnation and tested in the Fischer Tropsch reaction using a simulated syngas (CO/H2) mixture similar to that obtained in biomass gasification. The catalysts characterization included N2 adsorption, temperature programmed reduction (TPR), X-ray photoelectron spectra (XPS) and transmission electron microscopy (TEM) techniques. The reaction was carried out in a stainless steel fixed bed reactor at 300 8C and 1 MPa. In general, activity increased almost linearly with increasing Co-loading, reaching maximum at about 20 wt.% Co and then levelled off. These results correlate with that derived from XPS. The Co/Si surface atomic ratios increased almost linearly up to about 20 wt.% Co due to gradual surface coverage by Co species. At higher loadings, cobalt form large crystal aggregates. The selectivity to C8–C9 hydrocarbons increases whereas the C14+ hydrocarbon follows an opposite trend upon increasing Co-loading. It was also observed that the selectivity to liquid fraction is a function of the Co crystallite size, that is longer hydrocarbons chains are formed on the smaller cobalt crystallites. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Fischer Tropsch synthesis Biosyngas Co/SiO2 catalysts Biomass
1. Introduction Nowadays there is great interest in the search of environmentally friendly fuels, as they contribute much less to the Greenhouse effect. Biomass appears as an attractive alternative as a clean, renewable and sustainable energy source. Different transformation routes have been used to convert biomass in liquid or gaseous fuels [1–3]. Following biomass gasification, methanol as well as liquid hydrocarbons can be obtained [3]. The first step of the overall process (biomass-to-liquid fuels, BTL) includes biomass gasification to yield a gas mixture containing mainly H2, CO, CO2 and CH4 which can then be adjusted to be desired H2/CO(CO2) ratio prior to being converted in the Fischer Tropsch synthesis (FTS) [4,5]. Currently the FTS is being used to synthesise hydrocarbons with different chain lengths by starting with H2/CO molar ratio of 2 in the feed, which is much higher than the H2/CO molar ratio close to 1 derived from biomass gasification [6,7]. Several papers have shown that is technical an economical feasible to obtain liquid hydrocarbons from biomass using integrated systems of gasification and catalytic FTS reaction [2,4,5]. Among the catalysts employed for the FT synthesis Co, Fe and Ru are the main active phases [8,9]. The Co-based catalysts exhibit in general high yield to
* Corresponding author. E-mail address: [email protected] (N. Escalona). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.10.039
long-chain hydrocarbons, alkenes and wax [10]. This behaviour has been attributed to the fact that these catalysts have the ability to readsorb the formed olefins on the metallic centres, thus, increasing the chain length which is accomplished with a decrease in the hydrogenation of the C5 5C bond and also in the isomerisation reactions [11,12]. It has been suggested that the activity of Cobased catalysts may be improved by following different approaches, such as the modification of the support, the increase in Co dispersion, addition of a second metal and improving the preparation procedure [13–16]. Furthermore, systematic studies of catalytic systems in the FTS using feedstock the biosyngas mixture is limited. Jun et al. [17] reported the synthesis of liquid hydrocarbons from biomass using Fe/Cu/Al/K catalysts. They found that these catalysts display a high yield to hydrocarbons and selectivity to alkenes. On the other hand, Lapidus et al. [18] reported high yield to liquid fuels on Co catalysts in the FTS from biosyngas. Additionally, they found that the presence of high concentration of CO2 and N2 in the reaction mixture affects significantly the catalytic behaviour because CO2 reduces the formation of long-chain hydrocarbons whereas N2 promotes the formation of diesel. Recently, Escalona et al. [19] have studied Co/SiO2 catalysts promoted by Re, Ru, Zn and Cu in the FT synthesis using a feed gas whose composition simulates biosyngas streams. Re and Ru additives led to a significant increase of both the activity and the selectivity to long-chain hydrocarbons, whereas in those modified with Cu and Zn, the effect was found to depend strongly on the amount of promoter. The product
C. Medina et al. / Applied Catalysis A: General 373 (2010) 71–75
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Table 1 Cobalt content (wt.%), specific area (SBET), total pore volume and cobalt particle size estimated from TEM of Co/SiO2 catalysts. Catalysts
Co (%)
Vp (cm3 g1)
SBET (m2 g1)
TEM (nm)
Co(10) Co(15) Co(20) Co(25) Co(30)
10 15 20 25 30
0.26 0.22 0.20 0.20 0.20
123 119 113 100 104
37 38 41 47 52
distribution of the liquid fraction revealed the presence of C6–C14+ hydrocarbons in most catalysts. In line with the above, the present work was undertaken with the aim to study the effect of Coloading in Co(x)/SiO2 catalysts in the FT synthesis by feeding a biosyngas gas mixture. 1.1. Catalyst preparation Cobalt-based catalysts containing metal loading of 10–25 wt.% Co were prepared by wet impregnation of silica commercial support (BASF D11-11, SBET = 136 m2 g1) with appropriate amounts of aqueous solution of Co(NO3)2 (Aldrich, p.a.), in a rotary evaporator. After impregnation, the samples were dried at 110 8C for 12 h and then calcined at 500 8C for 4 h. Prior to characterization a testing the catalysts were activated in flowing hydrogen at 500 8C. The catalysts are listed in Table 1. 1.2. Catalyst characterization The BET specific surface area (SBET) was evaluated from nitrogen adsorption at 196 8C in an automatic Micromeritics system Model ASAP 2010. Temperature programmed reduction (TPR) studies were carried out in a conventional system having a thermal conductivity detector and a quartz cell. In each experiment 100 mg of the sample was used, with a heating rate of 10 8C min1 and the temperature range from 25 to 1050 8C. The mixture used in the reduction experiments was a 5%H2/Ar with a flow of 50 cm3/min. TEM micrographs were obtained in a transmission electron microscopy (TEM) micrographs, taken with a Jeol Model JEM1200 EXII System. The X-ray diffraction (XRD) studies was carried out on a Rigaku diffractometer using a Ni-filter and Cu Ka1 radiation and scanning 2u angles in the range from 10 to 708. X-ray photoelectron spectra (XPS) were acquired with a Fisons Escalab 200R spectrometer equipped with a hemispherical electron analyzer and an Al Ka (hn = 1486.6 eV, 1 eV = 1.602 1019 J) 120 W X-ray source. Prior to the analysis, the samples were reduced in situ in H2 flow at 500 8C for 10 h. All binding energies (BEs) were referenced to the Si 2p line at 103.4 eV. This reference gave BE values within an accuracy of 0.2 eV. Atomic ratios were calculated from the intensity ratios normalized by atomic sensitivity factors.
2. Results and discussion 2.1. Catalyst characterization The SBET and total pore volume (obtained at 0.95 of P/P8) of calcined Co/SiO2 catalysts are shown in Table 1. As shown, the SBET decreases slightly with the Co-loading. This result suggests that Co species were highly dispersed into the pores of the silica substrate and that pore blockage was almost absent. In Table 1, the cobalt particle average size estimated from TEM of Co/SiO2 catalysts are summarized. In general, the Co/SiO2 catalysts present a broadening in the metal particles size distribution upon increasing Co-loading and the average particle was found to increase gradually with Co-loading form 37 to 52 nm (Table 1). Fig. 1 shows the XRD lines of Co(x)/SiO2 catalysts as function of Co content. It shows that all catalysts present only a diffraction plane centred at 44.58, which was assigned to Co0 species [20]. The intensity of the diffraction line increases and become slightly broader with the Co content. The particle size obtained by Scherrer equation is in line with the increases in the particle size detected by TEM. TPR profiles of the oxide precursors are given in Fig. 2. It can be observed that the reduction process of the catalysts occurs in two distinct stages. The first peak centred at 350 8C is ascribed to the transformation of Co3O4 to CoO, whereas the second stage centred to 406 8C represents the transformation of CoO to Co [21,22]. The relative intensity and width of the second reduction peak increases with Co-loading to a higher extent than the first peak, suggesting a higher reduction degree of CoO to metallic Co with an increase of the average diameter of Co3O4 particles, in agreement with results reported previously by Martinez et al. [25]. This is also consistent with those obtained by TEM and XRD results. Fig. 3 shows the XPS of the Co 2p region of reduced Co(x)/SiO2 catalysts. XP spectra of all catalysts in the Co 2p region showed the characteristic Co 2p3/2–Co 2p1/2 doublet. The width of the corresponding 2p3/2 and 2p1/2 peaks reveals presence of two species of cobalt, which are summarized in Table 2 for the peak with higher intensity (2p3/2). The most intense Co 2p3/2 peak was fitted to two components: one at 778.0 eV belonging to metallic cobalt [23] and at 780.0 eV originated from cobalt oxide [20]. The relative intensity of the two Co 2p peaks was calculated and the values obtained are also shown in Table 2. It can be seen, that the proportion of Co metallic phase increases gradually with Co-loading whereas the cobalt oxide phase follows an opposite
1.3. Catalytic reaction Activity tests were carried out in a fixed bed stainless steel reactor. The reaction conditions were: space velocity (GHSV) of 1800 ml g1 h1, pressure of 1 MPa and reaction temperature of 300 8C. The feed was a representative mixture obtained from biomass gasification having H2/CO/CO2/CH4/N2 in the molar proportion 32%/32%/12%/18%/6%, respectively [7]. Prior to the reaction, the catalysts were reduced in situ at 500 8C during 10 h under hydrogen flow. Activity data were taken at steady-state conditions, approximately after 80 h on-stream. The analyses of products were performed by gas chromatography using a PerkinElmer 3920B system provided with a thermal conductivity detector, using a n-octane Porasil-C column for liquids and a Carbosieve II column for the gaseous products.
Fig. 1. X-ray diffraction patterns of reduced Co(x)/SiO2 catalysts.
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Table 2 Binding energies (eV) and atomic surface ratios of Co/SiO2 reduced catalysts. Si 2p (eV)
Co 2p3/2 (eV)
Co/Si (at/at)
Co(10)
103.5
0.020
Co(15)
103.4
Co(20)
103.5
Co(25)
103.4
Co(30)
103.4
778.0 780.6 778.0 780.6 778.0 780.6 778.0 780.6 778.0 780.6
Catalysts
(36) (64) (41) (59) (62) (38) (71) (29) (80) (20)
0.033 0.043 0.063 0.092
trend. This behaviour suggests that at low Co content Co2+ interacts strongly with the SiO2 support and is not completely reduced to Co metallic. On the contrary, at higher Co-loading cobalt is present in the form of larger particles which are easier to reduced. In order to examine the extent of dispersion of the active phase over the silica surface, the Co/Si atomic ratio was calculated (see Table 2). The variation of the Co/Al atomic surface ratio as a function of Co-loading of the Co(x)/SiO2 catalysts is shown in Fig. 4. Clearly, the Co phase appears to be as rather small crystals (around of 38 nm) dispersed up to 20% of Co. The observed deviation from linearity above 20% of Co suggests the formation of large crystalline aggregate (over 38 nm). The higher Co/Si ratios observed at higher Co-loading, especially for Co(25)/SiO2 and Co(30)/SiO2 catalysts, results likely from the presence of a high density of cobalt oxide particles and therefore keep less exposed silica surface. This is in a good agreement with TPR and TEM results. 2.2. Catalytic activity The conversion of CO over Co(x)/SiO2 catalysts as a function of Co-loading at 300 8C is shown in Fig. 5. The activity, expressed as Fig. 3. X-ray photoelectron spectra of the Co 2p region of reduced Co(x)/SiO2 catalysts.
percent of CO conversion, increases linearly with increasing Coloading up to about 20%, and then slightly decreases. Similar behaviour has been reported by other authors; the maximum of CO conversion depends mainly of cobalt precursor salt, pretreatment conditions and type of support [24–26]. The initial linear increase of CO conversion is a consequence of an increase in the amount of well-dispersed Co species. This is consistent with the observation of Co species are deposited as small crystals up to 20%, as was observed by XPS. Above this Co-loading, the formation of Co large crystalline aggregates takes place and the Co dispersion decreases, leading therefore to a slight decrease in the CO conversion. On the other hand, taking into account that the catalyst activity should be proportional to the number of surface metal cobalt (Co0), the CO conversion initially increase with the Co content due to an increase in the proportion of Co metallic species, observed by XPS. However, above 20% of metallic Co species increase and the CO conversion decreases. This result clearly shows that the activity drop is due to the lost of dispersion and the formation of the large metallic Co clusters. 2.3. Selectivity
Fig. 2. Temperature programmed reduction profiles of Co/SiO2 catalysts.
Table 3 shows the selectivity to CH4, CO2 and C5+ hydrocarbons for the Co(x)/SiO2 catalysts as a function of Co-loading. High selectivity to CH4 was observed for all catalysts, similar to that observed for silica supported Co-promoted catalysts [19]. The Co(20) catalysts display the lowest CH4 formation, whereas the other catalysts show a CH4 formation close to 96%. The formation
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Table 3 Selectivity under steady-state conditions in the Fischer Tropsch reaction over Co(x)/ SiO2 catalysts at 300 8C and 1 MPa. Catalyst
Co(10) Co(15) Co(20) Co(25) Co(30)
Selectivity (C mol%) CH4
CO2
C5+
96.5 96.4 93.5 95.4 96.4
0.5 0.4 0.4 0.5 0.4
3.0 3.2 6.1 4.1 3.1
of CO2 is constant and close to 0.5% in all the catalysts. Coke is also formed on the surface of Co/SiO2 catalysts and this can be explained by the disproportion reaction of carbon monoxide on the surface of Co crystallites according to the Boudouard reaction [27,28]. However, the CO2 formation might be formed through recombination of atomic oxygen produced in the course of CO dissociation with other CO molecules as proposed by Krishnamoorthy et al. [29]. In this model CO is adsorbed and dissociated on the carbon surface, where the hydrogen does not play any role, as shown in the following reactions:
Fig. 5. CO conversion over Co/SiO2 catalysts as function of Co-loading.
CO ! Ca þ Oa Oa þ CO ! CO2 It can be seen in Table 3 that all catalysts show C5+ hydrocarbon selectivity lower than 8%. It is known that supported-cobalt catalysts show high yields to long-chain alkanes in the traditional FTS (H2/CO = 2) [8,9,27]. This is no the case for the Co/SiO2 catalyst used in this work which exhibit low selectivity C5+ hydrocarbons and high selectivity to CH4 when using biosyngas in the feed. Although the reason for this behaviour is not clear, it may be considered that the selectivity in the FTS reactions is very sensitive to feed composition, metals dispersion, promoter, support and particle size effects. Fig. 6 shows the distribution of condensable products over Co(x) catalysts as function of Co-loading. An increase in Co-loading produces a shift in the distribution of condensable products. Thus, the C8–C9 production of hydrocarbon chain increases with increasing Co-loading. Conversely, the production of C14+ hydrocarbons decreases with Co content. Bezemer et al. [30] and Girardon et al. [31] found that metal particles size has a strong impact on the cobalt selectivity, i.e. a decrease in cobalt particle size to 6–8 nm results in a higher selectivity to methane and higher yield to olefinic product. C5+
Fig. 6. Product distribution in the liquid extract produced in the Fischer Tropsch reaction over Co/SiO2 catalysts as function of Co-loading.
selectivity was also smaller with cobalt particles smaller than 6– 8 nm. However, Khodakov et al. [32] have shown that the catalytic properties of small cobalt particles in FT synthesis could be different from the larger ones. Therefore, the variation in the yield to hydrocarbon chain may be related to metal particle size. The increase of cobalt particle size favours the formation of hydrocarbons centred in C8–C9 chain length and decreases production of heavier hydrocarbons. This variation in the hydrocarbon selectivity on larger Co particles can be attributed to the rate of secondary reactions such as olefin readsorption, as has been proposed earlier [33]. 3. Conclusions The Co/SiO2 catalysts show high activity in the FT synthesis using simulated biosyngas feed. The liquid hydrocarbons fraction has a length chain from C6 to larger than C14+. The CO conversion over Co/SiO2 catalysts is mainly determined by the dispersion of Co species. The distribution of liquid hydrocarbons mainly depends on the Co particles size. Higher metal particles decrease the production of C14+ hydrocarbon and favour the synthesis of smaller hydrocarbon chain lengths (C8–C9). Acknowledgment
Fig. 4. Relationship between the Co/Si atomic surface ratio and the nominal surface density of Co for Co/SiO2 catalysts.
The authors thank to CONICYT for the financial support (FONDECYT 1070548) grant.
C. Medina et al. / Applied Catalysis A: General 373 (2010) 71–75
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Fischer Tropsch synthesis from a simulated biosyngas feed over Co(x)/SiO2 catalysts: Effect of Co-loading C. Medina a, R. Garcı´a a, P. Reyes a, J.L.G. Fierro b, N. Escalona a,* a b
Universidad de Concepcio´n, Facultad de Ciencias Quı´micas, Casilla 160C, Concepcio´n, Chile Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Cantoblanco, 28049 Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 August 2009 Received in revised form 20 October 2009 Accepted 25 October 2009 Available online 30 October 2009
A series of Co/SiO2 catalysts containing 10–30 wt.% Co were prepared by wet impregnation and tested in the Fischer Tropsch reaction using a simulated syngas (CO/H2) mixture similar to that obtained in biomass gasification. The catalysts characterization included N2 adsorption, temperature programmed reduction (TPR), X-ray photoelectron spectra (XPS) and transmission electron microscopy (TEM) techniques. The reaction was carried out in a stainless steel fixed bed reactor at 300 8C and 1 MPa. In general, activity increased almost linearly with increasing Co-loading, reaching maximum at about 20 wt.% Co and then levelled off. These results correlate with that derived from XPS. The Co/Si surface atomic ratios increased almost linearly up to about 20 wt.% Co due to gradual surface coverage by Co species. At higher loadings, cobalt form large crystal aggregates. The selectivity to C8–C9 hydrocarbons increases whereas the C14+ hydrocarbon follows an opposite trend upon increasing Co-loading. It was also observed that the selectivity to liquid fraction is a function of the Co crystallite size, that is longer hydrocarbons chains are formed on the smaller cobalt crystallites. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Fischer Tropsch synthesis Biosyngas Co/SiO2 catalysts Biomass
1. Introduction Nowadays there is great interest in the search of environmentally friendly fuels, as they contribute much less to the Greenhouse effect. Biomass appears as an attractive alternative as a clean, renewable and sustainable energy source. Different transformation routes have been used to convert biomass in liquid or gaseous fuels [1–3]. Following biomass gasification, methanol as well as liquid hydrocarbons can be obtained [3]. The first step of the overall process (biomass-to-liquid fuels, BTL) includes biomass gasification to yield a gas mixture containing mainly H2, CO, CO2 and CH4 which can then be adjusted to be desired H2/CO(CO2) ratio prior to being converted in the Fischer Tropsch synthesis (FTS) [4,5]. Currently the FTS is being used to synthesise hydrocarbons with different chain lengths by starting with H2/CO molar ratio of 2 in the feed, which is much higher than the H2/CO molar ratio close to 1 derived from biomass gasification [6,7]. Several papers have shown that is technical an economical feasible to obtain liquid hydrocarbons from biomass using integrated systems of gasification and catalytic FTS reaction [2,4,5]. Among the catalysts employed for the FT synthesis Co, Fe and Ru are the main active phases [8,9]. The Co-based catalysts exhibit in general high yield to
* Corresponding author. E-mail address: [email protected] (N. Escalona). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.10.039
long-chain hydrocarbons, alkenes and wax [10]. This behaviour has been attributed to the fact that these catalysts have the ability to readsorb the formed olefins on the metallic centres, thus, increasing the chain length which is accomplished with a decrease in the hydrogenation of the C5 5C bond and also in the isomerisation reactions [11,12]. It has been suggested that the activity of Cobased catalysts may be improved by following different approaches, such as the modification of the support, the increase in Co dispersion, addition of a second metal and improving the preparation procedure [13–16]. Furthermore, systematic studies of catalytic systems in the FTS using feedstock the biosyngas mixture is limited. Jun et al. [17] reported the synthesis of liquid hydrocarbons from biomass using Fe/Cu/Al/K catalysts. They found that these catalysts display a high yield to hydrocarbons and selectivity to alkenes. On the other hand, Lapidus et al. [18] reported high yield to liquid fuels on Co catalysts in the FTS from biosyngas. Additionally, they found that the presence of high concentration of CO2 and N2 in the reaction mixture affects significantly the catalytic behaviour because CO2 reduces the formation of long-chain hydrocarbons whereas N2 promotes the formation of diesel. Recently, Escalona et al. [19] have studied Co/SiO2 catalysts promoted by Re, Ru, Zn and Cu in the FT synthesis using a feed gas whose composition simulates biosyngas streams. Re and Ru additives led to a significant increase of both the activity and the selectivity to long-chain hydrocarbons, whereas in those modified with Cu and Zn, the effect was found to depend strongly on the amount of promoter. The product
C. Medina et al. / Applied Catalysis A: General 373 (2010) 71–75
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Table 1 Cobalt content (wt.%), specific area (SBET), total pore volume and cobalt particle size estimated from TEM of Co/SiO2 catalysts. Catalysts
Co (%)
Vp (cm3 g1)
SBET (m2 g1)
TEM (nm)
Co(10) Co(15) Co(20) Co(25) Co(30)
10 15 20 25 30
0.26 0.22 0.20 0.20 0.20
123 119 113 100 104
37 38 41 47 52
distribution of the liquid fraction revealed the presence of C6–C14+ hydrocarbons in most catalysts. In line with the above, the present work was undertaken with the aim to study the effect of Coloading in Co(x)/SiO2 catalysts in the FT synthesis by feeding a biosyngas gas mixture. 1.1. Catalyst preparation Cobalt-based catalysts containing metal loading of 10–25 wt.% Co were prepared by wet impregnation of silica commercial support (BASF D11-11, SBET = 136 m2 g1) with appropriate amounts of aqueous solution of Co(NO3)2 (Aldrich, p.a.), in a rotary evaporator. After impregnation, the samples were dried at 110 8C for 12 h and then calcined at 500 8C for 4 h. Prior to characterization a testing the catalysts were activated in flowing hydrogen at 500 8C. The catalysts are listed in Table 1. 1.2. Catalyst characterization The BET specific surface area (SBET) was evaluated from nitrogen adsorption at 196 8C in an automatic Micromeritics system Model ASAP 2010. Temperature programmed reduction (TPR) studies were carried out in a conventional system having a thermal conductivity detector and a quartz cell. In each experiment 100 mg of the sample was used, with a heating rate of 10 8C min1 and the temperature range from 25 to 1050 8C. The mixture used in the reduction experiments was a 5%H2/Ar with a flow of 50 cm3/min. TEM micrographs were obtained in a transmission electron microscopy (TEM) micrographs, taken with a Jeol Model JEM1200 EXII System. The X-ray diffraction (XRD) studies was carried out on a Rigaku diffractometer using a Ni-filter and Cu Ka1 radiation and scanning 2u angles in the range from 10 to 708. X-ray photoelectron spectra (XPS) were acquired with a Fisons Escalab 200R spectrometer equipped with a hemispherical electron analyzer and an Al Ka (hn = 1486.6 eV, 1 eV = 1.602 1019 J) 120 W X-ray source. Prior to the analysis, the samples were reduced in situ in H2 flow at 500 8C for 10 h. All binding energies (BEs) were referenced to the Si 2p line at 103.4 eV. This reference gave BE values within an accuracy of 0.2 eV. Atomic ratios were calculated from the intensity ratios normalized by atomic sensitivity factors.
2. Results and discussion 2.1. Catalyst characterization The SBET and total pore volume (obtained at 0.95 of P/P8) of calcined Co/SiO2 catalysts are shown in Table 1. As shown, the SBET decreases slightly with the Co-loading. This result suggests that Co species were highly dispersed into the pores of the silica substrate and that pore blockage was almost absent. In Table 1, the cobalt particle average size estimated from TEM of Co/SiO2 catalysts are summarized. In general, the Co/SiO2 catalysts present a broadening in the metal particles size distribution upon increasing Co-loading and the average particle was found to increase gradually with Co-loading form 37 to 52 nm (Table 1). Fig. 1 shows the XRD lines of Co(x)/SiO2 catalysts as function of Co content. It shows that all catalysts present only a diffraction plane centred at 44.58, which was assigned to Co0 species [20]. The intensity of the diffraction line increases and become slightly broader with the Co content. The particle size obtained by Scherrer equation is in line with the increases in the particle size detected by TEM. TPR profiles of the oxide precursors are given in Fig. 2. It can be observed that the reduction process of the catalysts occurs in two distinct stages. The first peak centred at 350 8C is ascribed to the transformation of Co3O4 to CoO, whereas the second stage centred to 406 8C represents the transformation of CoO to Co [21,22]. The relative intensity and width of the second reduction peak increases with Co-loading to a higher extent than the first peak, suggesting a higher reduction degree of CoO to metallic Co with an increase of the average diameter of Co3O4 particles, in agreement with results reported previously by Martinez et al. [25]. This is also consistent with those obtained by TEM and XRD results. Fig. 3 shows the XPS of the Co 2p region of reduced Co(x)/SiO2 catalysts. XP spectra of all catalysts in the Co 2p region showed the characteristic Co 2p3/2–Co 2p1/2 doublet. The width of the corresponding 2p3/2 and 2p1/2 peaks reveals presence of two species of cobalt, which are summarized in Table 2 for the peak with higher intensity (2p3/2). The most intense Co 2p3/2 peak was fitted to two components: one at 778.0 eV belonging to metallic cobalt [23] and at 780.0 eV originated from cobalt oxide [20]. The relative intensity of the two Co 2p peaks was calculated and the values obtained are also shown in Table 2. It can be seen, that the proportion of Co metallic phase increases gradually with Co-loading whereas the cobalt oxide phase follows an opposite
1.3. Catalytic reaction Activity tests were carried out in a fixed bed stainless steel reactor. The reaction conditions were: space velocity (GHSV) of 1800 ml g1 h1, pressure of 1 MPa and reaction temperature of 300 8C. The feed was a representative mixture obtained from biomass gasification having H2/CO/CO2/CH4/N2 in the molar proportion 32%/32%/12%/18%/6%, respectively [7]. Prior to the reaction, the catalysts were reduced in situ at 500 8C during 10 h under hydrogen flow. Activity data were taken at steady-state conditions, approximately after 80 h on-stream. The analyses of products were performed by gas chromatography using a PerkinElmer 3920B system provided with a thermal conductivity detector, using a n-octane Porasil-C column for liquids and a Carbosieve II column for the gaseous products.
Fig. 1. X-ray diffraction patterns of reduced Co(x)/SiO2 catalysts.
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Table 2 Binding energies (eV) and atomic surface ratios of Co/SiO2 reduced catalysts. Si 2p (eV)
Co 2p3/2 (eV)
Co/Si (at/at)
Co(10)
103.5
0.020
Co(15)
103.4
Co(20)
103.5
Co(25)
103.4
Co(30)
103.4
778.0 780.6 778.0 780.6 778.0 780.6 778.0 780.6 778.0 780.6
Catalysts
(36) (64) (41) (59) (62) (38) (71) (29) (80) (20)
0.033 0.043 0.063 0.092
trend. This behaviour suggests that at low Co content Co2+ interacts strongly with the SiO2 support and is not completely reduced to Co metallic. On the contrary, at higher Co-loading cobalt is present in the form of larger particles which are easier to reduced. In order to examine the extent of dispersion of the active phase over the silica surface, the Co/Si atomic ratio was calculated (see Table 2). The variation of the Co/Al atomic surface ratio as a function of Co-loading of the Co(x)/SiO2 catalysts is shown in Fig. 4. Clearly, the Co phase appears to be as rather small crystals (around of 38 nm) dispersed up to 20% of Co. The observed deviation from linearity above 20% of Co suggests the formation of large crystalline aggregate (over 38 nm). The higher Co/Si ratios observed at higher Co-loading, especially for Co(25)/SiO2 and Co(30)/SiO2 catalysts, results likely from the presence of a high density of cobalt oxide particles and therefore keep less exposed silica surface. This is in a good agreement with TPR and TEM results. 2.2. Catalytic activity The conversion of CO over Co(x)/SiO2 catalysts as a function of Co-loading at 300 8C is shown in Fig. 5. The activity, expressed as Fig. 3. X-ray photoelectron spectra of the Co 2p region of reduced Co(x)/SiO2 catalysts.
percent of CO conversion, increases linearly with increasing Coloading up to about 20%, and then slightly decreases. Similar behaviour has been reported by other authors; the maximum of CO conversion depends mainly of cobalt precursor salt, pretreatment conditions and type of support [24–26]. The initial linear increase of CO conversion is a consequence of an increase in the amount of well-dispersed Co species. This is consistent with the observation of Co species are deposited as small crystals up to 20%, as was observed by XPS. Above this Co-loading, the formation of Co large crystalline aggregates takes place and the Co dispersion decreases, leading therefore to a slight decrease in the CO conversion. On the other hand, taking into account that the catalyst activity should be proportional to the number of surface metal cobalt (Co0), the CO conversion initially increase with the Co content due to an increase in the proportion of Co metallic species, observed by XPS. However, above 20% of metallic Co species increase and the CO conversion decreases. This result clearly shows that the activity drop is due to the lost of dispersion and the formation of the large metallic Co clusters. 2.3. Selectivity
Fig. 2. Temperature programmed reduction profiles of Co/SiO2 catalysts.
Table 3 shows the selectivity to CH4, CO2 and C5+ hydrocarbons for the Co(x)/SiO2 catalysts as a function of Co-loading. High selectivity to CH4 was observed for all catalysts, similar to that observed for silica supported Co-promoted catalysts [19]. The Co(20) catalysts display the lowest CH4 formation, whereas the other catalysts show a CH4 formation close to 96%. The formation
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Table 3 Selectivity under steady-state conditions in the Fischer Tropsch reaction over Co(x)/ SiO2 catalysts at 300 8C and 1 MPa. Catalyst
Co(10) Co(15) Co(20) Co(25) Co(30)
Selectivity (C mol%) CH4
CO2
C5+
96.5 96.4 93.5 95.4 96.4
0.5 0.4 0.4 0.5 0.4
3.0 3.2 6.1 4.1 3.1
of CO2 is constant and close to 0.5% in all the catalysts. Coke is also formed on the surface of Co/SiO2 catalysts and this can be explained by the disproportion reaction of carbon monoxide on the surface of Co crystallites according to the Boudouard reaction [27,28]. However, the CO2 formation might be formed through recombination of atomic oxygen produced in the course of CO dissociation with other CO molecules as proposed by Krishnamoorthy et al. [29]. In this model CO is adsorbed and dissociated on the carbon surface, where the hydrogen does not play any role, as shown in the following reactions:
Fig. 5. CO conversion over Co/SiO2 catalysts as function of Co-loading.
CO ! Ca þ Oa Oa þ CO ! CO2 It can be seen in Table 3 that all catalysts show C5+ hydrocarbon selectivity lower than 8%. It is known that supported-cobalt catalysts show high yields to long-chain alkanes in the traditional FTS (H2/CO = 2) [8,9,27]. This is no the case for the Co/SiO2 catalyst used in this work which exhibit low selectivity C5+ hydrocarbons and high selectivity to CH4 when using biosyngas in the feed. Although the reason for this behaviour is not clear, it may be considered that the selectivity in the FTS reactions is very sensitive to feed composition, metals dispersion, promoter, support and particle size effects. Fig. 6 shows the distribution of condensable products over Co(x) catalysts as function of Co-loading. An increase in Co-loading produces a shift in the distribution of condensable products. Thus, the C8–C9 production of hydrocarbon chain increases with increasing Co-loading. Conversely, the production of C14+ hydrocarbons decreases with Co content. Bezemer et al. [30] and Girardon et al. [31] found that metal particles size has a strong impact on the cobalt selectivity, i.e. a decrease in cobalt particle size to 6–8 nm results in a higher selectivity to methane and higher yield to olefinic product. C5+
Fig. 6. Product distribution in the liquid extract produced in the Fischer Tropsch reaction over Co/SiO2 catalysts as function of Co-loading.
selectivity was also smaller with cobalt particles smaller than 6– 8 nm. However, Khodakov et al. [32] have shown that the catalytic properties of small cobalt particles in FT synthesis could be different from the larger ones. Therefore, the variation in the yield to hydrocarbon chain may be related to metal particle size. The increase of cobalt particle size favours the formation of hydrocarbons centred in C8–C9 chain length and decreases production of heavier hydrocarbons. This variation in the hydrocarbon selectivity on larger Co particles can be attributed to the rate of secondary reactions such as olefin readsorption, as has been proposed earlier [33]. 3. Conclusions The Co/SiO2 catalysts show high activity in the FT synthesis using simulated biosyngas feed. The liquid hydrocarbons fraction has a length chain from C6 to larger than C14+. The CO conversion over Co/SiO2 catalysts is mainly determined by the dispersion of Co species. The distribution of liquid hydrocarbons mainly depends on the Co particles size. Higher metal particles decrease the production of C14+ hydrocarbon and favour the synthesis of smaller hydrocarbon chain lengths (C8–C9). Acknowledgment
Fig. 4. Relationship between the Co/Si atomic surface ratio and the nominal surface density of Co for Co/SiO2 catalysts.
The authors thank to CONICYT for the financial support (FONDECYT 1070548) grant.
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