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γ-Al2O3
Fuel xxx (2014) xxx–xxx
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The characterization of micro-structure of cobalt on c-Al2O3 for FTS: Effects of pretreatment on Ru–Co/c-Al2O3 Jae-Sun Jung a,b, Jae-Suk Lee a, Garam Choi a, S. Ramesh a, Dong Ju Moon a,b,⇑ a b
Clean Energy Research Center, KIST, Seoul, Republic of Korea Clean Energy & Chemical Engineering, UST, Daejeon, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nano particles of Co3O4 are highly
dispersed over hydrogen calcined Ru– Co/c-Al2O3 catalyst due to direct reduction. The hydrogen calcined Ru–Co/c-Al2O3 preferentially formed the metallic cobalt of HCP structure. The activity for FTS is affected by the microstructure of cobalt and addition of ruthenium.
a r t i c l e
i n f o
Article history: Received 17 March 2014 Received in revised form 3 July 2014 Accepted 28 August 2014 Available online xxxx Keywords: Fischer-Tropsch Synthesis Calcination atmosphere GTL-FPSO Direct reduction
a b s t r a c t Ruthenium-promoted c-Al2O3-supported cobalt catalysts were prepared by different slurry impregnation methods and calcined in a hydrogen/air atmosphere. The prepared catalysts were well characterized by N2 physisorption, TPR, XRD, TEM and XPS techniques. The addition of Ru caused the reduction peaks to shift towards lower temperatures due to the spillover of hydrogen from the Ru species. The rutheniumpromoted catalyst calcined under a hydrogen atmosphere leads to metallic cobalt with an HCP structure and shows good catalytic activity for Fischer Tropsch Synthesis under the reaction conditions studied. The catalyst calcined under an air atmosphere leads to metallic cobalt with FCC structures and showed lower catalytic activity. It was also found that the calcination atmosphere and structure of the catalyst have a profound effect on the activity and selectivity for FT synthesis. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Production of synthetic fuel by FTS has received considerable interest due to higher octane numbers and lower sulfur and aromatic contents. It has been suggested that FTS is a key process in gas-to-liquids technology (GTL). Limited petroleum reservoirs, high oil prices and global warming makes GTL an attractive alternative technology for sustainable development [1]. Under a marine environment, it was expected that structured catalysts with good
⇑ Corresponding author at: Clean Energy Research Center, KIST, Seoul, Republic of Korea. Tel.: +82 2 958 5867; fax: +82 2 958 5809. E-mail address: [email protected] (D.J. Moon).
mechanical strength are required for avoiding the pressure drop in a fixed bed reactor [2,3]. Many researchers have reported heterogeneous catalysts for FT synthesis, and among the reported results, cobalt-based catalysts are more effective for CO hydrogenation. Reported results clearly demonstrated that the cobaltbased catalysts had many advantages over iron catalysts, such as high conversion, long lifetime and selectivity towards higher hydrocarbons. The Fischer-Tropsch Synthesis (FTS) activity of the cobalt-based catalyst depends on the number of active sites located on the surface of the crystalline metal after the reduction. The activity and selectivity of the supported cobalt catalysts depend on several factors, such as particle size, the nature of the
http://dx.doi.org/10.1016/j.fuel.2014.09.001 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
support, the presence of the promoters, and preparation methods including pretreatment conditions [4,5]. The highly dispersed Co catalyst requires the initial formation of small sizes of CoO or Co3O4 crystallites. The formation of highly dispersed catalysts requires a strong interaction between the supports and the cobalt precursors. This may be the reason that alumina is one of the most employed supports for cobalt catalysts with its favorable mechanical and surface properties. However, alumina-supported cobalt catalysts have limited reducibility as irreducible species form on the surface of the support. The formation of irreducible species between cobalt and the support may occur during pretreatment and/or during the reaction. Catalysts forming such compounds lose some part of the active cobalt metal phase, resulting in lower activities in FT synthesis. The reducibility of Co/Al2O3 was affected in two possible ways: (1) by increasing the cobalt-support interaction and (2) by facilitating the migration of cobalt ions into the tetrahedral sites of Al2O3 to form nonreducible cobalt aluminates. It is well known that in CoAl2O4, Co2p ions sit on a spinel structure. The term ‘‘non-reducible’’ is applied because these cobalt aluminates can only be reduced above 800–900 °C [6,7]. To enhance the reducibility, noble metal promoters are introduced. Many researchers focus on the role and the actual structure of the alloy and bimetallic catalysts created by introducing the promoter, although the role of the surface composition is still debated with uncertainty. The group VIII noble metals have been suggested to influence catalyst performance in a number of different ways. They may act to promote hydrogen spillover or to increase the reducibility of Co, and metals such as ruthenium and platinum reduce at a lower temperature than cobalt oxides and catalyze reduction by spillover from the promoter surface [8]. The activity, by preventing the buildup of carbonaceous deposits, may exhibit cluster and ligand effects and may provide a combination of enhanced Co reducibility and dispersion [9]. It has also been further reported that the addition of a second metal component has no detrimental effect on Co hydrogenation activity [10]. Tsubaki et al. [11] concluded that the Ru was enriched on cobalt, but Pt or Pd dispersed well in the form of Pt–Co or Pd–Co alloys. The Ru-promoted catalyst has the highest performance with a weight ratio of 1/50, but the alloy catalysts and bimetallic catalysts were mingled in the discussion. It is also known that several parameters of catalysts and calcinations atmospheres can influence the performance of cobalt FT catalysts [12]. The benefits of direct reduction of cobalt nitrate on cobalt dispersion have been reported by Iglesia [13]. De Jong et al. [14,15] have shown that changing the gas atmosphere from air to 1% NO/He during thermal treatment of supported Ni-nitrate or Co-nitrate delivers an improved metal oxide dispersion. Enache et al. [16] found that the direct reduction of cobalt nitrate produced a more active Co/Al2O3 catalyst under conditions favoring methanation, but their studies showed that direct reduction of cobalt nitrate was not able to make hexagonal close packed (HCP) on alumina due to strong metal-support interactions (SMSI). Zirconia was found to promote poorly crystalline hexagonal metallic cobalt, and it is known that modification of the preparation parameters strongly affects the microstructure of cobalt supported catalysts [17]. The microstructure of the metallic phase has been the subject of several studies by X-ray diffraction (XRD), as cobalt can exist in both hexagonal close packed (HCP) and face-centered cubic (FCC) forms. However, it is difficult to extract from the literature a clear image of the specific effect of cobalt particle microstructure on Fischer-Tropsch activity because of the number of parameters. It was proposed that amorphous cobalt or HCP cobalt with crystallographic defects are active phases in the Fischer-Tropsch reactions, while FCC crystallized cobalt has lower catalytic activity. On nanometer scale, both FCC and HCP forms may coexist,
possibly leading to an influence on the resulting stacking faults and the catalytic activity [18]. Karaca et al. [19] also found that the cobalt HCP phase was more favorable for FT synthesis than cobalt FCC. There have been reports that cobalt preferentially forms the FCC phase over SiO2, as determined by in situ XRD investigation, while the HCP structure is detected over alumina-supported cobalt catalysts, as reported by Elbashir [20]. O’Shea et al. [21] found that the FT activity of a silica-supported cobalt catalyst was strongly enhanced when activation was carried out under H2 + CO (1/1) compared to a typical H2 reduction. The authors attributed this effect to the formation of Co (FCC) particles under H2 atmosphere, while highly dispersed Co (HCP) nano-sized particles are obtained for syngas (H2 + CO) activation. This method is too complex to apply for GTL-FPSO processes due to the usage of carbon. The objective of this work is to study the effect of the calcination atmosphere with usage of direct reduction on the cobalt over the structural catalyst such as granule-type c-Al2O3. This work also investigates the impact on the microstructure by introducing ruthenium by the slurry impregnation method. To neglect the consideration of the term alloy and bimetallic catalyst, the relatively higher weight ratio of Ru to total Co was fixed to 2/10. The confirmation of those investigations was studied with the help of temperature-programmed reduction (TPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis.
2. Experimental 2.1. Catalyst preparation 2.1.1. Preparation of Co/Al2O3 Commercial samples of c-Al2O3with a 1.8-mm diameter were procured from SASOL (South Africa) and calcined at 500 °C for 5 h before being used in the catalyst preparation. The supported cobalt catalysts were prepared as reported in the literature [22]. Then, 15 wt% cobalt-loaded catalysts were prepared by the slurry impregnation method with multiple steps using aqueous cobalt nitrate solution (Co(NO3)26H2O 98.0%, Junsei Chem., Japan). The catalysts were dried and calcined in different atmospheres (air, hydrogen) at 400 °C for 4 h and are designated as Co(H2) and Co(Air). Catalyst calcination was performed under a hydrogen atmosphere, and the hydrogen flow was maintained at GHSV = 1500 h1. 2.1.2. Ru–Co/Al2O3 (Co-slurry impregnation) Supported catalysts containing 15 wt% cobalt nitrate and a specified amount of ruthenium (III) chloride hydrate (Cl3RuxH2O) were prepared by co-impregnation. The metal solution was prepared following fiducial points. The weight ratio of Ru to total Co was 2/10. The molar ratio of Ru to total Co was 6.16%. The prepared catalysts were denoted in terms of the different atmospheres and impregnation steps as Ru–Co (1st H2) and Ru–Co (1st Air). 2.1.3. Ru–Co/Al2O3 (subsequent-impregnation) The 15 wt% Co/c-Al2O3 was first calcined in an air atmosphere at 400 °C. The amount of ruthenium (III) chloride hydrate (Cl3RuxH2O) prepared in a slurry solution was the same as the amount prepared by the co-impregnation method, and the sample was impregnated on previously calcined 15 wt% Co/c-Al2O3. The prepared catalysts were dried at 100 °C overnight to remove excess water and finally calcined in the different atmospheres at 400 °C (hydrogen and air atmosphere). The prepared catalysts were denoted in terms of the different atmospheres and impregnation steps as Ru–Co (2nd H2) and Ru–Co (2nd Air).
Please cite this article in press as: Jung J-S et al. The characterization of micro-structure of cobalt on c-Al2O3 for FTS: Effects of pretreatment on Ru–Co/cAl2O3. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.09.001
J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
2.2. Catalyst characterization 2.2.1. Nitrogen physisorption The surface area, pore volume, and average pore diameter of the catalysts were measured by N2 physisorption. The isotherms were obtained using a MoonsorpIIsystem (KIST, Korea) at 196 °C. Before the adsorption–desorption measurements, the samples were degassed at 200 °C for 3 h to remove impurities. 2.2.2. X-ray diffraction (XRD) The fresh and reduced samples were characterized by XRD techniques. XRD measurements were performed on XRD-6000 (Shimadzu Co., Japan) with monochromatized Cu Ka radiations. The average crystallite thickness of cobalt oxide was calculated from the Scherrer equation using the Co3O4 (3 1 1) peak located at 2h = 36.9°. The particle size of Co3O4 was converted to the corresponding cobalt metal particle size according to the relative molar volumes of metallic cobalt and Co3O4. The resulting conversion factor for the diameter of a given Co3O4 particle being reduced to metallic cobalt is:
dðCo0 Þ ðnmÞ ¼ 0:75 dðCo3 O4 Þ
ð1Þ
Cobalt metal dispersion (D) can be calculated by assuming spherical uniform cobalt metal particles with a site density of 14.6 atom/nm2 using the following formula:
Dð%Þ ¼ 96=d
ð2Þ
where D is the dispersion of cobalt on the c-Al2O3, and d is the particle diameter of cobalt [23]. 2.2.3. Temperature programmed reduction (TPR) The reduction behaviors of the prepared catalysts were analyzed by the temperature programmed reduction (TPR). TPR experiment was carried out with AutoChem II 2920 (Micromeritics Co., USA) under a flow of 5% H2 in Ar (40 ml/min). The sample in a quartz reactor was heated at 10 °C min1 by an electronic furnace. The TPR profiles were measured under the temperature ranges of 100–900 °C. The degree of reduction (DOR) was calculated using Eq. (3), which is defined as the ratio of hydrogen consumed for the complete reduction of metal oxides from ambient temperature to 900 °C to the amount of hydrogen calculated for this complete reduction [24].
DORð%Þ ¼
peakÞ H2 consumption ðmmolÞðfirst gcat Total H2 consumption mmol gcat
ð3Þ
2.2.4. Transmission electron microscopy (TEM) The metal particles and support structure were analyzed by field-emission transmission electron microscopy [FETEM, Tecnai G2] and high angle annular dark field (HAADF)-STEM with energy dispersive X-ray spectroscopy [EDX, Philips CM-30]. The sample specimens for TEM analysis were prepared by ultrasonic dispersion of the catalysts in alcohol. 2.2.5. X-ray photoelectron spectroscopy (XPS) XPS measurements were carried out in an ULVAC-PHI (PHI-5800) electron spectrometer fitted with an Al Ka source. The anode was operated at a power of 150 W (4 kV, 25 mA), and the analyzer was operated at constant pass energy of 50 eV. All of the spectra were recorded at similar spectrometer parameters. 2.3. Catalytic activity studies A pressurized flow type reaction apparatus with a fixed-bed reactor was used for the FT synthesis studies. The apparatus was
3
equipped with an electronic temperature controller for a furnace, thermal mass flow controllers for gas flows and a back-pressure regulator. A thermocouple was set at the axial center of the tubular reactor. The stainless steel tube (OD = 14.6 mm and ID = 12.7 mm) was used as a fixed-bed reactor. A 1.0-g aliquot of catalyst was placed in the reactor with inert quartz wool above and below the catalyst. All of the catalysts were reduced in the flow of 5% H2 in nitrogen at 450 °C for 12 h. After the reduction, the reactor was cooled to 230 °C, and the reaction mixture was admitted, increasing the pressure to 20 bar. The mole ratio of reactants was maintained at 2 throughout the studies and a space velocity (GHSV) of 2300 h1. The effluent gas and higher hydrocarbon products were also analyzed by two consecutive in-lines GC (HP7890, TCD GC equipped with Carbosphere 80/100 packed column and FID GC equipped with GS-GasPro capillary column). The liquid products along with water were condensed in the traps and water was removed by decantation at the end of the test. The products were analyzed using an Agilent 5975C GC/MS system (Gas chromatography–mass spectrometry) equipped with a HP-5 column. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Physical properties of the catalysts To investigate the structural modification of c-alumina during the calcination conditions in the case of the air atmosphere series (A) and hydrogen atmosphere series (B), the nitrogen adsorption and BHJ plot (insertion) were obtained on both catalysts, as shown in Fig. 1. The BET surface area, pore volume and average pore radius for the support and catalysts are listed in Table 1. According to the IUPAC nomenclature, the isotherms of c-alumina-supported cobalt catalysts are identified as type IV and H1 hysteresis loop. Based on the BJH method, c-alumina has a narrow pore-size distribution. After the impregnation of the cobalt on c-Al2O3, the isotherm and pore-size distribution were not significantly changed compared to c-Al2O3. Both catalyst series exhibit a reduction in the BET surface area, average pore diameter and total pore volume with 15 wt% Co loading(or Ru-promoted catalyst) after different atmosphere calcination. This indicates that the induced cobalt fills the pores of c-Al2O3, and there may be slightly less blocking of narrow pores by smaller crystallites in the case of series (A). BET analysis clearly demonstrated that calcinations atmosphere has no effect on structural properties of the catalysts. Structural properties depend only on the amount of metal loading [25]. 3.1.2. Temperature programmed reduction (TPR) Reduction behaviors of the catalysts calcined in different atmospheres were recorded and are represented in Fig. 2. The reduction behaviors were assumed and divided into three areas in comparison to Co3O4 as the reference. The estimated information of the degree of reduction (DOR) and the amount of hydrogen consumption are listed in Table 2. Cobalt oxide gives two reduction steps, and CoO could be considered as an intermediate species. The reduction process includes the following steps (4) and (5):
Co3 O4 þ H2 ! 3CoO þ H2 O
ð4Þ
CoO þ H2 ! Co0 þ H2 O
ð5Þ
The TPR profiles did not follow the above-mentioned two steps in the hydrogen-calcined catalyst. These trends could be attributed to two major reasons. (i) These trends were affected by the homogeneity of the cobalt cluster distribution by a slurry
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Fig. 1. The nitrogen adsorption–desorption isotherms and pore size distribution curve of (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd).
Table 1 Physical and chemical properties of the prepared catalysts. Catalysts
Calcination atmosphere
Cobalt contents (%)
Physisorption a
c-Al2O3 Co Ru–Co (1st) Ru–Co (2nd) Co Ru–Co (1st) Ru–Co (2nd) a b c d
– Hydrogen Hydrogen Hydrogen Air Air Air
– 15 15 15 15 15 15
2
XRD a
3
a
BET S.A. (m /g)
Total P.V (cm /g)
Ave. P.D (nm)
d(Co3O4)b (nm)
d(Co0)xc (nm)
D %d (nm)
190 153 123 148 160 135 142
0.50 0.38 0.33 0.34 0.38 0.35 0.36
11.0 11.0 10.8 9.3 10.9 10.8 10.1
4.3 5.6 5.3 10.3 10.6 14.1
3.2 4.2 4.0 7.7 8.0 10.6
29.8 22.8 24.2 12.4 12.1 9.1
Measured by N2 physisorption [Moonsorp-II, KIST]. d(Co3O4) was calculated by Scherrer equation from XRD data. Mean Co0 particle size was estimated by d(Co3O4), the corresponding the equation: d(Co0) = 3/4 d(Co3O4). Dispersion was estimated by d(Co0), the corresponding the equation : D% = 96/d(Co0).
impregnation method and also matched with previous reports [25–27]. (ii) The amount of Co3O4 (or CoO) remaining was less than that of the air-calcined catalysts as evidenced in the XRD pattern.
In the promoted catalyst, the addition of Ru caused the peaks to shift to lower temperatures, presumably due to the spillover of hydrogen from the reduced promoter to reduce the cobalt oxide species, so that the DOR (%) was relatively higher than that of
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Fig. 2. TPR profiles of the fresh catalysts and the bulk of Co3O4 as reference: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd). Table 2 Total H2 consumption and degree of reduction of the catalysts. Catalysts
Co Ru–Co (1st) Ru–Co (2nd)
Calcination atmosphere
Air Air Air
H2 consumption (mmol/g catalyst)
Degree of reduction (%)
DOR (%)
Total
1st Peak
2nd Peak
3rd Peak
1st Peak
2nd Peak
3rd Peak
4.52 4.48 5.22
– 0.55 1.10
2.01 3.93 4.12
2.52 – –
– 12.27 21.07
44.46 87.73 78.93
55.54 – –
the unpromoted catalysts in the area of realistic reduction temperatures. These trends might be explained by some of the Co strongly interacting with alumina (CoXOY–Al2O3), which could be reduced at a lower temperature [8,25]. The first intense peak at 150– 200 °C assigned to the reduction of Ru2O3 to Ru0 and the second peak at 230–450 °C assigned to the reduction of Co3O4 to Co0 were segregated, and these peaks were shifted to higher positions compared to those of Ru–Co (1st). These results indicate that following different impregnation methods may allow the easy formation of mobile RuOx, which migrates to c-Al2O3 during oxidation in the calcination step [13,28]. 3.1.3. XRD patterns of cobalt based catalysts The XRD patterns of the calcined unpromoted and Ru-promoted catalysts are shown in Fig. 3. The peaks at 46.1 and 66.5° correspond to the c-Al2O3 support, while other peaks correspond to the different crystal planes of Co3O4, CoO and CoAl2O4. The Co3O4 phase was indexed at 2h = 31.3°, 36.9°, 65.4°; the CoO phase was indexed at 2h = 36.4°, 62.7°; and the CoAl2O4 spinel phase was indexed at 2h = 55.8° and 59.1° in both calcinations methods. The catalyst calcined in a hydrogen atmosphere has less intensity due to Co3O4 and CoAl2O4 than the air calcined catalysts, whereas the intensity of CoO followed the reverse trend. It was apparent that the hexagonal metallic phase (HCP) of cobalt and the face-centered cubic metallic phase of cobalt (FCC) with structural imperfections were observed, which were indexed at 2h = 41.5° (1 0 0), 44.2° (0 0 2) and 44.4° (1 1 1), respectively. These trends were attributed to the direct reduction. It was suggested that the hydrogen calcination of nitrate precursors leads to weaker interactions, which was proven by the reduction of the CoAl2O4 and Co3O4 spinel phases, and these weaker interactions help to increase the quantity of amorphous or poorly crystalline hexagonal metallic phase and face-centered cubic metallic phase. The coexistence of hexagonal metallic and face-centered cubic metallic phase was explained by previous reports. Ducreux et al. [17] have shown that using simulated X-ray patterns that increase in peak intensity corresponds to the (0 0 2) HCP plane of Co with an increase of stacking faults in the
45 100 100
FCC metallic clusters. This is because the (1 1 1) plane of Co FCC overlaps exactly with the (0 0 2) plane of Co HCP. In promoted catalysts, the promoter peaks appear at 28.1° for Ru deposited on Co/c-Al2O3, which could be attributed to (1 1 0) of RuO2 [29]. The intensity of RuO2 was different because of the degree of segregation created by following the impregnation method, but the Co3O4 spinel phases were increased marginally. Interestingly, the peaks related to RuO2 did not remain, but the HCP (or FCC) phase becomes more crystallized, and the HCP (1 0 1) at 47.4° of metallic cobalt appeared in the hydrogen-calcined catalysts. Of particular interest to Ru–Co (2nd), the highest-intensity diffraction patterns of HCP (or FCC) were present, although the amount of mobile RuOX was higher due to the formation of Ru0. The hexagonal metallic phase of metallic ruthenium was located at 44.0° (1 0 1), which is relatively close to 44.2° (0 0 2) and 44.4° (1 1 1) of the cobalt phases, so it is difficult to discriminate in XRD studies. The HCP (1 0 1) was not related to metallic ruthenium, as shown in Fig. 4. The XRD patterns of the reduced catalysts are represented in Fig. 4. The intensity of the peak corresponding to Co3O4 was decreased, and the intensity of other peaks is almost same. In (a) Co, the metallic cobalt of HCP and FCC exhibited a narrow and intense peak for the catalyst, and the reduced catalyst becomes an ordered microstructure compared to the fresh catalyst. However, only the (1 1 1) plane of Co FCC phase, located at 2h = 44.4°, was observed for the air-calcined catalyst. The hydrogen calcined catalyst has the coexistence of a plane of Co HCP and FCC phases at 2h = 44.2° and 44.4°, respectively. The reduction behavior of the promoted catalyst was changed marginally as explained by the TPR results. It was found that the calcination conditions and the deposition of ruthenium influence the reduction of the cobalt species and affect the crystalline structure. These trends may be possible in two ways. First, the microstructure was affected by the metalsupport interaction which was retarded by the formation of mixed surface compounds between Co3O4 and c-Al2O3 during the oxidation. The direct reduction of nitrate precursors leads to weaker
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Fig. 3. XRD patterns of the calcined Co/c-Al2O3 catalysts (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) c-Al2O3, (b) Co, (c) Ru–Co (1st), and (d) Ru–Co (2nd).
interactions, which was caused by the reduction of mixed-surface compounds and might induce the formation of a poorly crystalline hexagonal metallic phase. Second, the cooperation of ruthenium might assist the direct reduction of nitrate precursors using hydrogen because the ruthenium is known to have a higher ability for spillover. However, it is difficult to define which surface composition shows higher synergy effectively from only the XRD studies. Of course, it was shown that the Ru–Co (2nd) catalyst led to a more crystallized metallic phase from the XRD studies. 3.1.4. TEM images of Ru–Co/c-Al2O3 catalysts The morphologies of the fresh catalysts are shown in Fig. 5. The overview of the microstructure shows the darker Co surface species and Ru species dispersed on the alumina grains. The aggregation of cobalt surface species and Ru species on the larger support grains show bright contrast relative to the substrate, which were confirmed by the EDS. There are some differences
between 5(A) and 5(B). The air-calcined catalysts showed aggregated Co3O4 and CoO crystallites more than the hydrogen-calcined catalysts did. It was suggested that the hydrogen calcined catalyst has more uniformly distributed macula on the surface with a diameter of approximately 10–30 compared to the air calcined catalyst. This phenomenon occurred because the phase of Co3O4 was converted to the CoO and Co phase [30,31]. In promoted catalysts, (c) Ru–Co (2nd) is more segregated than (b) Ru–Co (1st) due to the presence of more aggregation of the Co surface species and Ru species, which changed dramatically upon hydrogen calcination. These results are attributed to the increased formation of metallic state species, as determined by TPR and XRD. These trends may be explained in detail following the results. Fig. 6 indicates that lattice spacing’s were measure directly from the HRTEM images on a 2-nm scale. The lattice spacing’s of 2.43 Å and 2.03 Å correspond to the (3 1 1) and (1 1 1) planes of Co3O4 and the FCC structure, respectively, in the hydrogen-calcined catalyst.
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
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Fig. 4. XRD patterns of the reduced Co/c-Al2O3 catalysts (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) c-Al2O3, (b) Co, (c) Ru–Co (1st), and (d) Ru–Co (2nd).
Fig. 5. HRTEM/HAADF images of catalysts for (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd).
The lattice spacing of 1.91 Å, which corresponds to the (1 0 1) plane of HCP, appeared in the reduced hydrogen-calcined catalyst. However, only the lattice spacing of 2.03 Å, corresponding to the
(1 1 1) plane of FCC, exists in the reduced air-calcined catalyst. The trends matched the XRD results except for the existence of the (1 1 1) plane and the (0 0 2) plane. It is difficult to distinguish
Please cite this article in press as: Jung J-S et al. The characterization of micro-structure of cobalt on c-Al2O3 for FTS: Effects of pretreatment on Ru–Co/cAl2O3. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.09.001
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
between the two phases on the basis of morphology and spatial distribution because of the overlapping of the (1 1 1) plane of FCC and the (0 0 2) plane of HCP as stacking faults [17,32]. Individual metallic particles in the bimetallic Ru–Co were analyzed by EDS, as shown in Fig. 6. The Co was very rich in (a) Ru–Co (1st) during the calcination-reduction step, but the Ru was very rich in Ru–Co (2nd). These trends were maintained in the reduced air-calcination catalyst. The studies of the TEM images are sensitive to atomic number and density and show the changes in morphology clearly. From the TEM studies, the results could be explained by the following two suggestions. First, the Ru-rich spots on the Co surfaces gave a higher degree of reduction, as well as large particles of Ru–Co and metallic structures of cobalt formed during the reduction process. Second, the Ru of Ru–Co (2nd) might be more segregated than Ru–Co (1st) due to different mobility caused by the different impregnation methods. These results are also proved by Table 4. The values of Ru/Co for Ru–Co (2nd) were maintained and were higher than those of Ru–Co (1st) during the oxidationreduction steps. 3.1.5. XPS spectra of Ru–Co/c-Al2O3 catalysts To investigate the interaction, oxidation and reduced state in ruthenium-promoted catalysts, XPS was employed, and the results
are shown in Fig. 7. The calcined catalysts and reduced catalysts were studied via Co2p XPS spectra obtained under different calcination conditions, and the corresponding spectral parameters are represented in Table 3. A Co2p3/2 component of 779.7 eV and a low intensity of the shake-up satellite peak at approximately 787 eV were observed, which is typical for Co2+ and Co3+ ions in the Co3O4 spinel phase [30]. In the case of the air-calcined Ru–Co (1st) and Ru–Co (2nd), the binding energies (BEs) of Co2p3/2 were higher (780.4) compared to that of Co3O4 (779.7), which served as the reference. It was clear that the electronic structure of the cobalt species present in the calcined catalyst from cobalt nitrate was very different from Co3O4 due to the interaction of cobalt and alumina. However, the hydrogen-calcined catalysts shifted to lower energy binding energies. After the reduction, binding energy values shifted towards lower value when compared to original catalyst. The binding energy featured the Co2p3/2 component peak at 780.05 eV and a satellite peak at approximately 787 eV, which is typical for ligand-to-metal charge transfer transitions in the CoO intermediate phase and the Co metallic phase from Co3O4 [30,32,33]. The reduced hydrogen-calcined catalyst values were close to those of metallic cobalt. In other words, the fact that the hydrogen-calcined catalysts have the metallic cobalt species was proven, and it
Fig. 6. The lattic images obtained from HRTEM and Energy dispersive spectra obtained for (A) Hydrogen calcination catalyst, (B) Reduced hydrogen calcination catalyst and (C) Reduced air calcination catalyst: (a) Ru–Co (1st) (b) Ru–Co (2nd).
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx Table 3 Binding energy (BE) of Co2p3/2 (ev) from XPS characterization techniques. Catalysts
Calcination atmosphere
Cobalt contents (%)
BE of Co2p3/2 (ev) Calcined
Reduced
Co3O4 Ru–Co Ru–Co Ru–Co Ru–Co
Reference Hydrogen Hydrogen Air Air
– 15 15 15 15
779.7 780.2 779.9 780.4 780.4
– 779.5 779.4 780.2 780.2
(1st) (2nd) (1st) (2nd)
Table 4 Effect of the reduction and calcination atmosphere treatments on the XPS peak intensity ratio of noble metal and cobalt in the mixture of noble metal and cobalt. Catalysts
Ru–Co Ru–Co Ru–Co Ru–Co
(1st) (2nd) (1st) (2nd)
Calcination atmosphere
Cobalt contents (%)
XPS atomic ratio (Ru/Co) Oxide state
Reduced state
Hydrogen Hydrogen Air Air
15 15 15 15
0.016 0.122 0.031 0.125
0.016 0.102 0.016 0.233
suggested that the metallic cobalt phase is maintained during the reduction process. Although the other intense peak was normalized to compare the positions of the binding energy, it was interesting that for Ru–Co (1st), the peaks for Co2p3/2 peaks were sharp. This result was attributed to the fact that the Co existed in the metallic state. These trends agree with a previous report [11].
3.2. FT synthesis with cobalt based catalysts To investigate different microstructures by surface composition effects on the catalytic activity, catalytic tests were performed under the reaction conditions of 230 °C, 20 bar and H2/CO = 2 in a fixed bed reactor system with a space velocity (GHSV) of 2300 h1 for 100 h, it was observed that steady state activity was reached after 30 h and the performance results are represented in Fig. 8 and Table 5. The promoted catalyst has good performance with higher CO conversion and almost similar C5+ hydrocarbon selectivity compared to the unpromoted catalyst. The C5+ hydrocarbon could be
9
affected by the pore size because the reaction carried out on active sites was controlled by mass diffusion through the inner and outer pores of the catalyst [27,34,35]. A maximum conversion of 82.7% and C5+ selectivity of 97.2 C-mol% was reached over Ru–Co (1st) with good stabilization, as shown in Fig. 8 and Table 5. The observed performance could be explained based on the characterization results. First, the enhancement of spillover is due to the presence of ruthenium. The relative amount of reduction was increased at a realistic temperature of 450 °C. Second, ruthenium was enriched on cobalt. Finally, the Ru-rich spot on the Co surface determined a high degree of reduction and a large particle size of cobalt oxide species on the Ru-promoted catalysts. These trends agree with literature reports, which claimed that the surface of larger metallic cobalt particles is enriched in electrons. It also seems that the adsorption of CO molecules is influenced by the metallic cluster size. [11,17,31,36]. The results also suggested that the direct reduction of the catalyst by hydrogen certainly leads to the formation of the HCP phase, which has marginal effects on the CO conversion and the range of carbon number distribution under the tested FTS conditions. It is interesting that different values of CO conversion are obtained through the comparison of Co (Air) and Co (H2), but the difference decreases due to the presence of Ru, which slightly enhanced the cluster sizes and reducibility, as determined by XRD and TPR. When compared to Ru–Co (1st, H2) and Ru–Co (2nd, H2), as shown by TEM and XPS, the degree of segregation was too severe to negatively affect methane selectivity, although the combination of hydrogen spillover and the direct reduction of nitrate precursors led to a sharp intensity of the metallic cobalt. It was also interesting that Ru–Co (1st, H2) generates more products in the carbon range of C9–C12 than higher hydrocarbons although the catalyst has reached a carbon number of C34 as shown in Fig. 9. In other words, these results were proposed as evidence that the ruthenium might have assisted informing the HCP structure but did not make porous HCP for efficient mass diffusion [37]. The air-calcined Ru-promoted catalyst has predominantly the FCC phase after the reduction, as determined by XRD and TEM. It was found that the air-calcined Ru-promoted catalyst has a good performance with lower olefin-to-paraffin ratio, which effects are chain-length dependent by an olefin read sorption mechanism under the tested conditions [38–40].
Fig. 7. Co2p core-level spectra of the prepared catalyst of (A) calcined catalyst and (B) reduced catalyst: the air calcined catalyst of (a) Ru–Co (1st) and (b) Ru–Co (2nd) and the hydrogen catalyst of (c) Ru–Co (1st) and (d) Ru–Co (2nd).
Please cite this article in press as: Jung J-S et al. The characterization of micro-structure of cobalt on c-Al2O3 for FTS: Effects of pretreatment on Ru–Co/cAl2O3. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.09.001
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
Fig. 8. Time on stream of activity test for (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd).
Table 5 Catalytic performance of the prepared catalysts for F–T synthesis. Catalysts
Co Ru–Co Ru–Co Co Ru–Co Ru–Co
(1st) (2nd) (1st) (2nd)
Calcination atmosphere
Hydrogen Hydrogen Hydrogen Air Air Air
CO conversion (%)
58.9 82.7 81.2 50 79.3 76.0
CO2 conversion (%)
6.6 5.6 13.0 3.5 4.5 5.0
Product distribution (C-mol%) CH4
C2-C4
C5+
2.3 3.2 11.2 2.6 3.6 3.6
0.5 0.6 0.6 0.4 0.6 0.6
97.2 96.2 88.2 97.0 95.7 96.0
Olefin/paraffin in C2–C4 (%)
0.76 0.56 0.66 0.45 0.22 0.33
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
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Fig. 9. Carbonnumer distribution obtained from GC/MS system (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd).
4. Conclusions Ruthenium-promoted alumina-supported cobalt catalysts were prepared by different slurry impregnation methods and were
calcined under different atmospheres. The catalysts were well characterized and probed by FTS. Air-calcined catalysts have strong interactions with the support, which induced metallic phases related to HCP and FCC phases. In contrast, the catalysts calcined
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
in a hydrogen atmosphere had weak interactions with HCP structures that showed uniform distribution of the active species on the support. These phenomena are clearer in the Ru promoted catalyst, which was affected by spillover and retained the cobalt cluster sizes. It was also observed that the hydrogen-calcined catalyst shows a marginal effect on CO conversion and definitely makes the shift to the middle distillation range by the presence of the metal microstructure.
References [1] Kang JS, Awate SV, Lee YJ, Kim SJ, Park MJ, Lee SD, et al. J Nanosci Nanotechnol 2010;10:3700. [2] Moon DJ, Lee YJ, Kang JS, Park MJ, Kim SJ, Lee SD, et al. Korea Patent 1,152,666 2012. [3] Moon DJ, Lee SD, Lee BG, Lee HJ, Lee YJ, Kang JS, et al. Korea Patent 1,157,205 2012. [4] Davis BH. Ind Eng Chem Res 2007;46:8938. [5] Prieto G, Martínez A, Concepción P, Moreno-Tost R. J Catal 2009;266:129. [6] Chernavskii PA, Khodakov AY, Pankina GV, Girardon J-S, Quinet E. Appl Catal A 2006;306:108. [7] Chu W, Chernavskii PA, Gengembre L, Pankina GA, Fongarland P, Khodakov AY. J Catal 2007;252:215. [8] Morales F, Weckhuysen BM. Catalysis 2006;19:1. [9] Khodakov AY, Chu W, Fongarland P. Chem Rev 2007;107:1692. [10] Coqa B, Figueras F. J Mol Catal A: Chem 2001;173:117. [11] Tsubaki N, Sun S, Fujumoto K. J Catal 2001;199:236. [12] van de Loosdrecht J, Barradas S, Caricato EA. Top Catal 2003;26:121. [13] Iglesia E. Appl Catal A 1997;161:59. [14] Sietsma JRA, Meeldijk JD, Den Breejen JP, Versluijs-Helder M, Van Dillen AJ, De Jongh PE, et al. Angew Chem Int Ed 2007;46:4547.
[15] Sietsma JRA, Den Breejen JP, De Jongh PE, van Dillen AJ, Bitter JH, De Jong KP. Stud Surf Sci Catal 2007;167:55. [16] Enache DI, Rebours B, Roy-Auberger M, Revel R. J Catal 2002;205:346. [17] Ducreux O, Rebours B, Lynch J, Bazin D. Oil Gas Sci Technol 2009;64:49. [18] Bezemer GL, Radstake PB, Koot V, van Dillen AJ, Geus JW, de Jong KP. J Catal 2006;237:291. [19] Karaca H, Safonova OV, Chambrey S, Fongarland P, Roussel P, GribovalConstant A, et al. J Catal 2011;277:14. [20] Elbashir NO, Dutta P, Manivannan A, Seehra MS, Roberts CB. Appl Catal A 2005;285:169. [21] de la Pena O Shea VA, Homs N, Fierro JLG, Ramirez de la Piscina P. Catal Today 2006;114:422. [22] Espinoza RL, Visagie JL, Bolder FH. US Patent 5,733,839 1998. [23] Soled SL, Iglesia E, Fiato RA, Baumgartner JE, Vroman H, Miseo S. Top Catal 2003;26:101. [24] Abbaslou RMM, Tavasoli A, Dalai AK. Appl Catal A: Gen 2009;355:33. [25] Lee JS, Jung JS, Moon DJ. J Nanosci Nanotechnol; in press. [26] Jacobs G, Das TK, Zhang Y, Li J, Racoillet G, Davis BH. Appl Catal A: Gen 2002;233:263. [27] Jacobs G, Das TK, Patterson PM, Li J, Sanchez L, Davis BH. Appl Catal A 2003;247:335. [28] Jongsomjit B, Panpranot J, Goodwin Jr JG. J Catal 2001;204:98. [29] Lee DJ, Kang SW, Rhee SW. Thin Solid Films 2002;413:237. [30] Potoczna-Petru D, Kepinski L. Catal Lett 2001;73:41. [31] Xiong H, Zhang Y, Wang S, Li J. Catal Commun 2005;6:512. [32] Bidaux J-E, Schaller R, Benoit W. Acta Metall 1989;37:803. [33] van Steen E, Claeys M, Dry ME, van de Loosdrecht J, Viljoen EL, Visagie JL. J Phys Chem B 2005;109:3575. [34] Song JR, Wen LX, Xia ZM, Chen J-F. Fuel Process Technol 2007;88:443. [35] Jung JS, Kim SW, Moon DJ. Catal Today 2012;185:168. [36] Martínez A, López C, Márquez F, Díaz I. J Catal 2003;220:486. [37] Sadeqzadeh M, Karaca H, Roussel P, Khodakov AY. Catal Today 2011;164:62. [38] Cheng J, Song T, Hua P, Lok CM, Ellis P, French S. J Catal 2008;255:20–8. [39] Kapteijn F, de Deugd RM, Moulijn JA. Catal Today 2005;105:350–6. [40] Shi B, Davis BH. Catal Today 2005;106:129–31.
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
The characterization of micro-structure of cobalt on c-Al2O3 for FTS: Effects of pretreatment on Ru–Co/c-Al2O3 Jae-Sun Jung a,b, Jae-Suk Lee a, Garam Choi a, S. Ramesh a, Dong Ju Moon a,b,⇑ a b
Clean Energy Research Center, KIST, Seoul, Republic of Korea Clean Energy & Chemical Engineering, UST, Daejeon, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nano particles of Co3O4 are highly
dispersed over hydrogen calcined Ru– Co/c-Al2O3 catalyst due to direct reduction. The hydrogen calcined Ru–Co/c-Al2O3 preferentially formed the metallic cobalt of HCP structure. The activity for FTS is affected by the microstructure of cobalt and addition of ruthenium.
a r t i c l e
i n f o
Article history: Received 17 March 2014 Received in revised form 3 July 2014 Accepted 28 August 2014 Available online xxxx Keywords: Fischer-Tropsch Synthesis Calcination atmosphere GTL-FPSO Direct reduction
a b s t r a c t Ruthenium-promoted c-Al2O3-supported cobalt catalysts were prepared by different slurry impregnation methods and calcined in a hydrogen/air atmosphere. The prepared catalysts were well characterized by N2 physisorption, TPR, XRD, TEM and XPS techniques. The addition of Ru caused the reduction peaks to shift towards lower temperatures due to the spillover of hydrogen from the Ru species. The rutheniumpromoted catalyst calcined under a hydrogen atmosphere leads to metallic cobalt with an HCP structure and shows good catalytic activity for Fischer Tropsch Synthesis under the reaction conditions studied. The catalyst calcined under an air atmosphere leads to metallic cobalt with FCC structures and showed lower catalytic activity. It was also found that the calcination atmosphere and structure of the catalyst have a profound effect on the activity and selectivity for FT synthesis. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Production of synthetic fuel by FTS has received considerable interest due to higher octane numbers and lower sulfur and aromatic contents. It has been suggested that FTS is a key process in gas-to-liquids technology (GTL). Limited petroleum reservoirs, high oil prices and global warming makes GTL an attractive alternative technology for sustainable development [1]. Under a marine environment, it was expected that structured catalysts with good
⇑ Corresponding author at: Clean Energy Research Center, KIST, Seoul, Republic of Korea. Tel.: +82 2 958 5867; fax: +82 2 958 5809. E-mail address: [email protected] (D.J. Moon).
mechanical strength are required for avoiding the pressure drop in a fixed bed reactor [2,3]. Many researchers have reported heterogeneous catalysts for FT synthesis, and among the reported results, cobalt-based catalysts are more effective for CO hydrogenation. Reported results clearly demonstrated that the cobaltbased catalysts had many advantages over iron catalysts, such as high conversion, long lifetime and selectivity towards higher hydrocarbons. The Fischer-Tropsch Synthesis (FTS) activity of the cobalt-based catalyst depends on the number of active sites located on the surface of the crystalline metal after the reduction. The activity and selectivity of the supported cobalt catalysts depend on several factors, such as particle size, the nature of the
http://dx.doi.org/10.1016/j.fuel.2014.09.001 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
support, the presence of the promoters, and preparation methods including pretreatment conditions [4,5]. The highly dispersed Co catalyst requires the initial formation of small sizes of CoO or Co3O4 crystallites. The formation of highly dispersed catalysts requires a strong interaction between the supports and the cobalt precursors. This may be the reason that alumina is one of the most employed supports for cobalt catalysts with its favorable mechanical and surface properties. However, alumina-supported cobalt catalysts have limited reducibility as irreducible species form on the surface of the support. The formation of irreducible species between cobalt and the support may occur during pretreatment and/or during the reaction. Catalysts forming such compounds lose some part of the active cobalt metal phase, resulting in lower activities in FT synthesis. The reducibility of Co/Al2O3 was affected in two possible ways: (1) by increasing the cobalt-support interaction and (2) by facilitating the migration of cobalt ions into the tetrahedral sites of Al2O3 to form nonreducible cobalt aluminates. It is well known that in CoAl2O4, Co2p ions sit on a spinel structure. The term ‘‘non-reducible’’ is applied because these cobalt aluminates can only be reduced above 800–900 °C [6,7]. To enhance the reducibility, noble metal promoters are introduced. Many researchers focus on the role and the actual structure of the alloy and bimetallic catalysts created by introducing the promoter, although the role of the surface composition is still debated with uncertainty. The group VIII noble metals have been suggested to influence catalyst performance in a number of different ways. They may act to promote hydrogen spillover or to increase the reducibility of Co, and metals such as ruthenium and platinum reduce at a lower temperature than cobalt oxides and catalyze reduction by spillover from the promoter surface [8]. The activity, by preventing the buildup of carbonaceous deposits, may exhibit cluster and ligand effects and may provide a combination of enhanced Co reducibility and dispersion [9]. It has also been further reported that the addition of a second metal component has no detrimental effect on Co hydrogenation activity [10]. Tsubaki et al. [11] concluded that the Ru was enriched on cobalt, but Pt or Pd dispersed well in the form of Pt–Co or Pd–Co alloys. The Ru-promoted catalyst has the highest performance with a weight ratio of 1/50, but the alloy catalysts and bimetallic catalysts were mingled in the discussion. It is also known that several parameters of catalysts and calcinations atmospheres can influence the performance of cobalt FT catalysts [12]. The benefits of direct reduction of cobalt nitrate on cobalt dispersion have been reported by Iglesia [13]. De Jong et al. [14,15] have shown that changing the gas atmosphere from air to 1% NO/He during thermal treatment of supported Ni-nitrate or Co-nitrate delivers an improved metal oxide dispersion. Enache et al. [16] found that the direct reduction of cobalt nitrate produced a more active Co/Al2O3 catalyst under conditions favoring methanation, but their studies showed that direct reduction of cobalt nitrate was not able to make hexagonal close packed (HCP) on alumina due to strong metal-support interactions (SMSI). Zirconia was found to promote poorly crystalline hexagonal metallic cobalt, and it is known that modification of the preparation parameters strongly affects the microstructure of cobalt supported catalysts [17]. The microstructure of the metallic phase has been the subject of several studies by X-ray diffraction (XRD), as cobalt can exist in both hexagonal close packed (HCP) and face-centered cubic (FCC) forms. However, it is difficult to extract from the literature a clear image of the specific effect of cobalt particle microstructure on Fischer-Tropsch activity because of the number of parameters. It was proposed that amorphous cobalt or HCP cobalt with crystallographic defects are active phases in the Fischer-Tropsch reactions, while FCC crystallized cobalt has lower catalytic activity. On nanometer scale, both FCC and HCP forms may coexist,
possibly leading to an influence on the resulting stacking faults and the catalytic activity [18]. Karaca et al. [19] also found that the cobalt HCP phase was more favorable for FT synthesis than cobalt FCC. There have been reports that cobalt preferentially forms the FCC phase over SiO2, as determined by in situ XRD investigation, while the HCP structure is detected over alumina-supported cobalt catalysts, as reported by Elbashir [20]. O’Shea et al. [21] found that the FT activity of a silica-supported cobalt catalyst was strongly enhanced when activation was carried out under H2 + CO (1/1) compared to a typical H2 reduction. The authors attributed this effect to the formation of Co (FCC) particles under H2 atmosphere, while highly dispersed Co (HCP) nano-sized particles are obtained for syngas (H2 + CO) activation. This method is too complex to apply for GTL-FPSO processes due to the usage of carbon. The objective of this work is to study the effect of the calcination atmosphere with usage of direct reduction on the cobalt over the structural catalyst such as granule-type c-Al2O3. This work also investigates the impact on the microstructure by introducing ruthenium by the slurry impregnation method. To neglect the consideration of the term alloy and bimetallic catalyst, the relatively higher weight ratio of Ru to total Co was fixed to 2/10. The confirmation of those investigations was studied with the help of temperature-programmed reduction (TPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis.
2. Experimental 2.1. Catalyst preparation 2.1.1. Preparation of Co/Al2O3 Commercial samples of c-Al2O3with a 1.8-mm diameter were procured from SASOL (South Africa) and calcined at 500 °C for 5 h before being used in the catalyst preparation. The supported cobalt catalysts were prepared as reported in the literature [22]. Then, 15 wt% cobalt-loaded catalysts were prepared by the slurry impregnation method with multiple steps using aqueous cobalt nitrate solution (Co(NO3)26H2O 98.0%, Junsei Chem., Japan). The catalysts were dried and calcined in different atmospheres (air, hydrogen) at 400 °C for 4 h and are designated as Co(H2) and Co(Air). Catalyst calcination was performed under a hydrogen atmosphere, and the hydrogen flow was maintained at GHSV = 1500 h1. 2.1.2. Ru–Co/Al2O3 (Co-slurry impregnation) Supported catalysts containing 15 wt% cobalt nitrate and a specified amount of ruthenium (III) chloride hydrate (Cl3RuxH2O) were prepared by co-impregnation. The metal solution was prepared following fiducial points. The weight ratio of Ru to total Co was 2/10. The molar ratio of Ru to total Co was 6.16%. The prepared catalysts were denoted in terms of the different atmospheres and impregnation steps as Ru–Co (1st H2) and Ru–Co (1st Air). 2.1.3. Ru–Co/Al2O3 (subsequent-impregnation) The 15 wt% Co/c-Al2O3 was first calcined in an air atmosphere at 400 °C. The amount of ruthenium (III) chloride hydrate (Cl3RuxH2O) prepared in a slurry solution was the same as the amount prepared by the co-impregnation method, and the sample was impregnated on previously calcined 15 wt% Co/c-Al2O3. The prepared catalysts were dried at 100 °C overnight to remove excess water and finally calcined in the different atmospheres at 400 °C (hydrogen and air atmosphere). The prepared catalysts were denoted in terms of the different atmospheres and impregnation steps as Ru–Co (2nd H2) and Ru–Co (2nd Air).
Please cite this article in press as: Jung J-S et al. The characterization of micro-structure of cobalt on c-Al2O3 for FTS: Effects of pretreatment on Ru–Co/cAl2O3. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.09.001
J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
2.2. Catalyst characterization 2.2.1. Nitrogen physisorption The surface area, pore volume, and average pore diameter of the catalysts were measured by N2 physisorption. The isotherms were obtained using a MoonsorpIIsystem (KIST, Korea) at 196 °C. Before the adsorption–desorption measurements, the samples were degassed at 200 °C for 3 h to remove impurities. 2.2.2. X-ray diffraction (XRD) The fresh and reduced samples were characterized by XRD techniques. XRD measurements were performed on XRD-6000 (Shimadzu Co., Japan) with monochromatized Cu Ka radiations. The average crystallite thickness of cobalt oxide was calculated from the Scherrer equation using the Co3O4 (3 1 1) peak located at 2h = 36.9°. The particle size of Co3O4 was converted to the corresponding cobalt metal particle size according to the relative molar volumes of metallic cobalt and Co3O4. The resulting conversion factor for the diameter of a given Co3O4 particle being reduced to metallic cobalt is:
dðCo0 Þ ðnmÞ ¼ 0:75 dðCo3 O4 Þ
ð1Þ
Cobalt metal dispersion (D) can be calculated by assuming spherical uniform cobalt metal particles with a site density of 14.6 atom/nm2 using the following formula:
Dð%Þ ¼ 96=d
ð2Þ
where D is the dispersion of cobalt on the c-Al2O3, and d is the particle diameter of cobalt [23]. 2.2.3. Temperature programmed reduction (TPR) The reduction behaviors of the prepared catalysts were analyzed by the temperature programmed reduction (TPR). TPR experiment was carried out with AutoChem II 2920 (Micromeritics Co., USA) under a flow of 5% H2 in Ar (40 ml/min). The sample in a quartz reactor was heated at 10 °C min1 by an electronic furnace. The TPR profiles were measured under the temperature ranges of 100–900 °C. The degree of reduction (DOR) was calculated using Eq. (3), which is defined as the ratio of hydrogen consumed for the complete reduction of metal oxides from ambient temperature to 900 °C to the amount of hydrogen calculated for this complete reduction [24].
DORð%Þ ¼
peakÞ H2 consumption ðmmolÞðfirst gcat Total H2 consumption mmol gcat
ð3Þ
2.2.4. Transmission electron microscopy (TEM) The metal particles and support structure were analyzed by field-emission transmission electron microscopy [FETEM, Tecnai G2] and high angle annular dark field (HAADF)-STEM with energy dispersive X-ray spectroscopy [EDX, Philips CM-30]. The sample specimens for TEM analysis were prepared by ultrasonic dispersion of the catalysts in alcohol. 2.2.5. X-ray photoelectron spectroscopy (XPS) XPS measurements were carried out in an ULVAC-PHI (PHI-5800) electron spectrometer fitted with an Al Ka source. The anode was operated at a power of 150 W (4 kV, 25 mA), and the analyzer was operated at constant pass energy of 50 eV. All of the spectra were recorded at similar spectrometer parameters. 2.3. Catalytic activity studies A pressurized flow type reaction apparatus with a fixed-bed reactor was used for the FT synthesis studies. The apparatus was
3
equipped with an electronic temperature controller for a furnace, thermal mass flow controllers for gas flows and a back-pressure regulator. A thermocouple was set at the axial center of the tubular reactor. The stainless steel tube (OD = 14.6 mm and ID = 12.7 mm) was used as a fixed-bed reactor. A 1.0-g aliquot of catalyst was placed in the reactor with inert quartz wool above and below the catalyst. All of the catalysts were reduced in the flow of 5% H2 in nitrogen at 450 °C for 12 h. After the reduction, the reactor was cooled to 230 °C, and the reaction mixture was admitted, increasing the pressure to 20 bar. The mole ratio of reactants was maintained at 2 throughout the studies and a space velocity (GHSV) of 2300 h1. The effluent gas and higher hydrocarbon products were also analyzed by two consecutive in-lines GC (HP7890, TCD GC equipped with Carbosphere 80/100 packed column and FID GC equipped with GS-GasPro capillary column). The liquid products along with water were condensed in the traps and water was removed by decantation at the end of the test. The products were analyzed using an Agilent 5975C GC/MS system (Gas chromatography–mass spectrometry) equipped with a HP-5 column. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Physical properties of the catalysts To investigate the structural modification of c-alumina during the calcination conditions in the case of the air atmosphere series (A) and hydrogen atmosphere series (B), the nitrogen adsorption and BHJ plot (insertion) were obtained on both catalysts, as shown in Fig. 1. The BET surface area, pore volume and average pore radius for the support and catalysts are listed in Table 1. According to the IUPAC nomenclature, the isotherms of c-alumina-supported cobalt catalysts are identified as type IV and H1 hysteresis loop. Based on the BJH method, c-alumina has a narrow pore-size distribution. After the impregnation of the cobalt on c-Al2O3, the isotherm and pore-size distribution were not significantly changed compared to c-Al2O3. Both catalyst series exhibit a reduction in the BET surface area, average pore diameter and total pore volume with 15 wt% Co loading(or Ru-promoted catalyst) after different atmosphere calcination. This indicates that the induced cobalt fills the pores of c-Al2O3, and there may be slightly less blocking of narrow pores by smaller crystallites in the case of series (A). BET analysis clearly demonstrated that calcinations atmosphere has no effect on structural properties of the catalysts. Structural properties depend only on the amount of metal loading [25]. 3.1.2. Temperature programmed reduction (TPR) Reduction behaviors of the catalysts calcined in different atmospheres were recorded and are represented in Fig. 2. The reduction behaviors were assumed and divided into three areas in comparison to Co3O4 as the reference. The estimated information of the degree of reduction (DOR) and the amount of hydrogen consumption are listed in Table 2. Cobalt oxide gives two reduction steps, and CoO could be considered as an intermediate species. The reduction process includes the following steps (4) and (5):
Co3 O4 þ H2 ! 3CoO þ H2 O
ð4Þ
CoO þ H2 ! Co0 þ H2 O
ð5Þ
The TPR profiles did not follow the above-mentioned two steps in the hydrogen-calcined catalyst. These trends could be attributed to two major reasons. (i) These trends were affected by the homogeneity of the cobalt cluster distribution by a slurry
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Fig. 1. The nitrogen adsorption–desorption isotherms and pore size distribution curve of (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd).
Table 1 Physical and chemical properties of the prepared catalysts. Catalysts
Calcination atmosphere
Cobalt contents (%)
Physisorption a
c-Al2O3 Co Ru–Co (1st) Ru–Co (2nd) Co Ru–Co (1st) Ru–Co (2nd) a b c d
– Hydrogen Hydrogen Hydrogen Air Air Air
– 15 15 15 15 15 15
2
XRD a
3
a
BET S.A. (m /g)
Total P.V (cm /g)
Ave. P.D (nm)
d(Co3O4)b (nm)
d(Co0)xc (nm)
D %d (nm)
190 153 123 148 160 135 142
0.50 0.38 0.33 0.34 0.38 0.35 0.36
11.0 11.0 10.8 9.3 10.9 10.8 10.1
4.3 5.6 5.3 10.3 10.6 14.1
3.2 4.2 4.0 7.7 8.0 10.6
29.8 22.8 24.2 12.4 12.1 9.1
Measured by N2 physisorption [Moonsorp-II, KIST]. d(Co3O4) was calculated by Scherrer equation from XRD data. Mean Co0 particle size was estimated by d(Co3O4), the corresponding the equation: d(Co0) = 3/4 d(Co3O4). Dispersion was estimated by d(Co0), the corresponding the equation : D% = 96/d(Co0).
impregnation method and also matched with previous reports [25–27]. (ii) The amount of Co3O4 (or CoO) remaining was less than that of the air-calcined catalysts as evidenced in the XRD pattern.
In the promoted catalyst, the addition of Ru caused the peaks to shift to lower temperatures, presumably due to the spillover of hydrogen from the reduced promoter to reduce the cobalt oxide species, so that the DOR (%) was relatively higher than that of
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Fig. 2. TPR profiles of the fresh catalysts and the bulk of Co3O4 as reference: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd). Table 2 Total H2 consumption and degree of reduction of the catalysts. Catalysts
Co Ru–Co (1st) Ru–Co (2nd)
Calcination atmosphere
Air Air Air
H2 consumption (mmol/g catalyst)
Degree of reduction (%)
DOR (%)
Total
1st Peak
2nd Peak
3rd Peak
1st Peak
2nd Peak
3rd Peak
4.52 4.48 5.22
– 0.55 1.10
2.01 3.93 4.12
2.52 – –
– 12.27 21.07
44.46 87.73 78.93
55.54 – –
the unpromoted catalysts in the area of realistic reduction temperatures. These trends might be explained by some of the Co strongly interacting with alumina (CoXOY–Al2O3), which could be reduced at a lower temperature [8,25]. The first intense peak at 150– 200 °C assigned to the reduction of Ru2O3 to Ru0 and the second peak at 230–450 °C assigned to the reduction of Co3O4 to Co0 were segregated, and these peaks were shifted to higher positions compared to those of Ru–Co (1st). These results indicate that following different impregnation methods may allow the easy formation of mobile RuOx, which migrates to c-Al2O3 during oxidation in the calcination step [13,28]. 3.1.3. XRD patterns of cobalt based catalysts The XRD patterns of the calcined unpromoted and Ru-promoted catalysts are shown in Fig. 3. The peaks at 46.1 and 66.5° correspond to the c-Al2O3 support, while other peaks correspond to the different crystal planes of Co3O4, CoO and CoAl2O4. The Co3O4 phase was indexed at 2h = 31.3°, 36.9°, 65.4°; the CoO phase was indexed at 2h = 36.4°, 62.7°; and the CoAl2O4 spinel phase was indexed at 2h = 55.8° and 59.1° in both calcinations methods. The catalyst calcined in a hydrogen atmosphere has less intensity due to Co3O4 and CoAl2O4 than the air calcined catalysts, whereas the intensity of CoO followed the reverse trend. It was apparent that the hexagonal metallic phase (HCP) of cobalt and the face-centered cubic metallic phase of cobalt (FCC) with structural imperfections were observed, which were indexed at 2h = 41.5° (1 0 0), 44.2° (0 0 2) and 44.4° (1 1 1), respectively. These trends were attributed to the direct reduction. It was suggested that the hydrogen calcination of nitrate precursors leads to weaker interactions, which was proven by the reduction of the CoAl2O4 and Co3O4 spinel phases, and these weaker interactions help to increase the quantity of amorphous or poorly crystalline hexagonal metallic phase and face-centered cubic metallic phase. The coexistence of hexagonal metallic and face-centered cubic metallic phase was explained by previous reports. Ducreux et al. [17] have shown that using simulated X-ray patterns that increase in peak intensity corresponds to the (0 0 2) HCP plane of Co with an increase of stacking faults in the
45 100 100
FCC metallic clusters. This is because the (1 1 1) plane of Co FCC overlaps exactly with the (0 0 2) plane of Co HCP. In promoted catalysts, the promoter peaks appear at 28.1° for Ru deposited on Co/c-Al2O3, which could be attributed to (1 1 0) of RuO2 [29]. The intensity of RuO2 was different because of the degree of segregation created by following the impregnation method, but the Co3O4 spinel phases were increased marginally. Interestingly, the peaks related to RuO2 did not remain, but the HCP (or FCC) phase becomes more crystallized, and the HCP (1 0 1) at 47.4° of metallic cobalt appeared in the hydrogen-calcined catalysts. Of particular interest to Ru–Co (2nd), the highest-intensity diffraction patterns of HCP (or FCC) were present, although the amount of mobile RuOX was higher due to the formation of Ru0. The hexagonal metallic phase of metallic ruthenium was located at 44.0° (1 0 1), which is relatively close to 44.2° (0 0 2) and 44.4° (1 1 1) of the cobalt phases, so it is difficult to discriminate in XRD studies. The HCP (1 0 1) was not related to metallic ruthenium, as shown in Fig. 4. The XRD patterns of the reduced catalysts are represented in Fig. 4. The intensity of the peak corresponding to Co3O4 was decreased, and the intensity of other peaks is almost same. In (a) Co, the metallic cobalt of HCP and FCC exhibited a narrow and intense peak for the catalyst, and the reduced catalyst becomes an ordered microstructure compared to the fresh catalyst. However, only the (1 1 1) plane of Co FCC phase, located at 2h = 44.4°, was observed for the air-calcined catalyst. The hydrogen calcined catalyst has the coexistence of a plane of Co HCP and FCC phases at 2h = 44.2° and 44.4°, respectively. The reduction behavior of the promoted catalyst was changed marginally as explained by the TPR results. It was found that the calcination conditions and the deposition of ruthenium influence the reduction of the cobalt species and affect the crystalline structure. These trends may be possible in two ways. First, the microstructure was affected by the metalsupport interaction which was retarded by the formation of mixed surface compounds between Co3O4 and c-Al2O3 during the oxidation. The direct reduction of nitrate precursors leads to weaker
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Fig. 3. XRD patterns of the calcined Co/c-Al2O3 catalysts (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) c-Al2O3, (b) Co, (c) Ru–Co (1st), and (d) Ru–Co (2nd).
interactions, which was caused by the reduction of mixed-surface compounds and might induce the formation of a poorly crystalline hexagonal metallic phase. Second, the cooperation of ruthenium might assist the direct reduction of nitrate precursors using hydrogen because the ruthenium is known to have a higher ability for spillover. However, it is difficult to define which surface composition shows higher synergy effectively from only the XRD studies. Of course, it was shown that the Ru–Co (2nd) catalyst led to a more crystallized metallic phase from the XRD studies. 3.1.4. TEM images of Ru–Co/c-Al2O3 catalysts The morphologies of the fresh catalysts are shown in Fig. 5. The overview of the microstructure shows the darker Co surface species and Ru species dispersed on the alumina grains. The aggregation of cobalt surface species and Ru species on the larger support grains show bright contrast relative to the substrate, which were confirmed by the EDS. There are some differences
between 5(A) and 5(B). The air-calcined catalysts showed aggregated Co3O4 and CoO crystallites more than the hydrogen-calcined catalysts did. It was suggested that the hydrogen calcined catalyst has more uniformly distributed macula on the surface with a diameter of approximately 10–30 compared to the air calcined catalyst. This phenomenon occurred because the phase of Co3O4 was converted to the CoO and Co phase [30,31]. In promoted catalysts, (c) Ru–Co (2nd) is more segregated than (b) Ru–Co (1st) due to the presence of more aggregation of the Co surface species and Ru species, which changed dramatically upon hydrogen calcination. These results are attributed to the increased formation of metallic state species, as determined by TPR and XRD. These trends may be explained in detail following the results. Fig. 6 indicates that lattice spacing’s were measure directly from the HRTEM images on a 2-nm scale. The lattice spacing’s of 2.43 Å and 2.03 Å correspond to the (3 1 1) and (1 1 1) planes of Co3O4 and the FCC structure, respectively, in the hydrogen-calcined catalyst.
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
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Fig. 4. XRD patterns of the reduced Co/c-Al2O3 catalysts (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) c-Al2O3, (b) Co, (c) Ru–Co (1st), and (d) Ru–Co (2nd).
Fig. 5. HRTEM/HAADF images of catalysts for (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd).
The lattice spacing of 1.91 Å, which corresponds to the (1 0 1) plane of HCP, appeared in the reduced hydrogen-calcined catalyst. However, only the lattice spacing of 2.03 Å, corresponding to the
(1 1 1) plane of FCC, exists in the reduced air-calcined catalyst. The trends matched the XRD results except for the existence of the (1 1 1) plane and the (0 0 2) plane. It is difficult to distinguish
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between the two phases on the basis of morphology and spatial distribution because of the overlapping of the (1 1 1) plane of FCC and the (0 0 2) plane of HCP as stacking faults [17,32]. Individual metallic particles in the bimetallic Ru–Co were analyzed by EDS, as shown in Fig. 6. The Co was very rich in (a) Ru–Co (1st) during the calcination-reduction step, but the Ru was very rich in Ru–Co (2nd). These trends were maintained in the reduced air-calcination catalyst. The studies of the TEM images are sensitive to atomic number and density and show the changes in morphology clearly. From the TEM studies, the results could be explained by the following two suggestions. First, the Ru-rich spots on the Co surfaces gave a higher degree of reduction, as well as large particles of Ru–Co and metallic structures of cobalt formed during the reduction process. Second, the Ru of Ru–Co (2nd) might be more segregated than Ru–Co (1st) due to different mobility caused by the different impregnation methods. These results are also proved by Table 4. The values of Ru/Co for Ru–Co (2nd) were maintained and were higher than those of Ru–Co (1st) during the oxidationreduction steps. 3.1.5. XPS spectra of Ru–Co/c-Al2O3 catalysts To investigate the interaction, oxidation and reduced state in ruthenium-promoted catalysts, XPS was employed, and the results
are shown in Fig. 7. The calcined catalysts and reduced catalysts were studied via Co2p XPS spectra obtained under different calcination conditions, and the corresponding spectral parameters are represented in Table 3. A Co2p3/2 component of 779.7 eV and a low intensity of the shake-up satellite peak at approximately 787 eV were observed, which is typical for Co2+ and Co3+ ions in the Co3O4 spinel phase [30]. In the case of the air-calcined Ru–Co (1st) and Ru–Co (2nd), the binding energies (BEs) of Co2p3/2 were higher (780.4) compared to that of Co3O4 (779.7), which served as the reference. It was clear that the electronic structure of the cobalt species present in the calcined catalyst from cobalt nitrate was very different from Co3O4 due to the interaction of cobalt and alumina. However, the hydrogen-calcined catalysts shifted to lower energy binding energies. After the reduction, binding energy values shifted towards lower value when compared to original catalyst. The binding energy featured the Co2p3/2 component peak at 780.05 eV and a satellite peak at approximately 787 eV, which is typical for ligand-to-metal charge transfer transitions in the CoO intermediate phase and the Co metallic phase from Co3O4 [30,32,33]. The reduced hydrogen-calcined catalyst values were close to those of metallic cobalt. In other words, the fact that the hydrogen-calcined catalysts have the metallic cobalt species was proven, and it
Fig. 6. The lattic images obtained from HRTEM and Energy dispersive spectra obtained for (A) Hydrogen calcination catalyst, (B) Reduced hydrogen calcination catalyst and (C) Reduced air calcination catalyst: (a) Ru–Co (1st) (b) Ru–Co (2nd).
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx Table 3 Binding energy (BE) of Co2p3/2 (ev) from XPS characterization techniques. Catalysts
Calcination atmosphere
Cobalt contents (%)
BE of Co2p3/2 (ev) Calcined
Reduced
Co3O4 Ru–Co Ru–Co Ru–Co Ru–Co
Reference Hydrogen Hydrogen Air Air
– 15 15 15 15
779.7 780.2 779.9 780.4 780.4
– 779.5 779.4 780.2 780.2
(1st) (2nd) (1st) (2nd)
Table 4 Effect of the reduction and calcination atmosphere treatments on the XPS peak intensity ratio of noble metal and cobalt in the mixture of noble metal and cobalt. Catalysts
Ru–Co Ru–Co Ru–Co Ru–Co
(1st) (2nd) (1st) (2nd)
Calcination atmosphere
Cobalt contents (%)
XPS atomic ratio (Ru/Co) Oxide state
Reduced state
Hydrogen Hydrogen Air Air
15 15 15 15
0.016 0.122 0.031 0.125
0.016 0.102 0.016 0.233
suggested that the metallic cobalt phase is maintained during the reduction process. Although the other intense peak was normalized to compare the positions of the binding energy, it was interesting that for Ru–Co (1st), the peaks for Co2p3/2 peaks were sharp. This result was attributed to the fact that the Co existed in the metallic state. These trends agree with a previous report [11].
3.2. FT synthesis with cobalt based catalysts To investigate different microstructures by surface composition effects on the catalytic activity, catalytic tests were performed under the reaction conditions of 230 °C, 20 bar and H2/CO = 2 in a fixed bed reactor system with a space velocity (GHSV) of 2300 h1 for 100 h, it was observed that steady state activity was reached after 30 h and the performance results are represented in Fig. 8 and Table 5. The promoted catalyst has good performance with higher CO conversion and almost similar C5+ hydrocarbon selectivity compared to the unpromoted catalyst. The C5+ hydrocarbon could be
9
affected by the pore size because the reaction carried out on active sites was controlled by mass diffusion through the inner and outer pores of the catalyst [27,34,35]. A maximum conversion of 82.7% and C5+ selectivity of 97.2 C-mol% was reached over Ru–Co (1st) with good stabilization, as shown in Fig. 8 and Table 5. The observed performance could be explained based on the characterization results. First, the enhancement of spillover is due to the presence of ruthenium. The relative amount of reduction was increased at a realistic temperature of 450 °C. Second, ruthenium was enriched on cobalt. Finally, the Ru-rich spot on the Co surface determined a high degree of reduction and a large particle size of cobalt oxide species on the Ru-promoted catalysts. These trends agree with literature reports, which claimed that the surface of larger metallic cobalt particles is enriched in electrons. It also seems that the adsorption of CO molecules is influenced by the metallic cluster size. [11,17,31,36]. The results also suggested that the direct reduction of the catalyst by hydrogen certainly leads to the formation of the HCP phase, which has marginal effects on the CO conversion and the range of carbon number distribution under the tested FTS conditions. It is interesting that different values of CO conversion are obtained through the comparison of Co (Air) and Co (H2), but the difference decreases due to the presence of Ru, which slightly enhanced the cluster sizes and reducibility, as determined by XRD and TPR. When compared to Ru–Co (1st, H2) and Ru–Co (2nd, H2), as shown by TEM and XPS, the degree of segregation was too severe to negatively affect methane selectivity, although the combination of hydrogen spillover and the direct reduction of nitrate precursors led to a sharp intensity of the metallic cobalt. It was also interesting that Ru–Co (1st, H2) generates more products in the carbon range of C9–C12 than higher hydrocarbons although the catalyst has reached a carbon number of C34 as shown in Fig. 9. In other words, these results were proposed as evidence that the ruthenium might have assisted informing the HCP structure but did not make porous HCP for efficient mass diffusion [37]. The air-calcined Ru-promoted catalyst has predominantly the FCC phase after the reduction, as determined by XRD and TEM. It was found that the air-calcined Ru-promoted catalyst has a good performance with lower olefin-to-paraffin ratio, which effects are chain-length dependent by an olefin read sorption mechanism under the tested conditions [38–40].
Fig. 7. Co2p core-level spectra of the prepared catalyst of (A) calcined catalyst and (B) reduced catalyst: the air calcined catalyst of (a) Ru–Co (1st) and (b) Ru–Co (2nd) and the hydrogen catalyst of (c) Ru–Co (1st) and (d) Ru–Co (2nd).
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Fig. 8. Time on stream of activity test for (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd).
Table 5 Catalytic performance of the prepared catalysts for F–T synthesis. Catalysts
Co Ru–Co Ru–Co Co Ru–Co Ru–Co
(1st) (2nd) (1st) (2nd)
Calcination atmosphere
Hydrogen Hydrogen Hydrogen Air Air Air
CO conversion (%)
58.9 82.7 81.2 50 79.3 76.0
CO2 conversion (%)
6.6 5.6 13.0 3.5 4.5 5.0
Product distribution (C-mol%) CH4
C2-C4
C5+
2.3 3.2 11.2 2.6 3.6 3.6
0.5 0.6 0.6 0.4 0.6 0.6
97.2 96.2 88.2 97.0 95.7 96.0
Olefin/paraffin in C2–C4 (%)
0.76 0.56 0.66 0.45 0.22 0.33
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J.-S. Jung et al. / Fuel xxx (2014) xxx–xxx
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Fig. 9. Carbonnumer distribution obtained from GC/MS system (A) air calcination catalyst and (B) hydrogen calcination catalyst: (a) Co, (b) Ru–Co (1st), and (c) Ru–Co (2nd).
4. Conclusions Ruthenium-promoted alumina-supported cobalt catalysts were prepared by different slurry impregnation methods and were
calcined under different atmospheres. The catalysts were well characterized and probed by FTS. Air-calcined catalysts have strong interactions with the support, which induced metallic phases related to HCP and FCC phases. In contrast, the catalysts calcined
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in a hydrogen atmosphere had weak interactions with HCP structures that showed uniform distribution of the active species on the support. These phenomena are clearer in the Ru promoted catalyst, which was affected by spillover and retained the cobalt cluster sizes. It was also observed that the hydrogen-calcined catalyst shows a marginal effect on CO conversion and definitely makes the shift to the middle distillation range by the presence of the metal microstructure.
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Please cite this article in press as: Jung J-S et al. The characterization of micro-structure of cobalt on c-Al2O3 for FTS: Effects of pretreatment on Ru–Co/cAl2O3. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.09.001