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Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer–Tropsch synthesis performance
Accepted Manuscript
Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer-Tropsch synthesis performance
http://www.journals.elsevier.com/ journal-of-energy-chemistry/
Ting kuang , Shuai Lyu , Sixu Liu , Yuhua Zhang , Jinlin Li , Guanghui Wang , Li Wang PII: DOI: Reference:
S2095-4956(18)30507-2 https://doi.org/10.1016/j.jechem.2018.08.012 JECHEM 660
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
7 June 2018 25 July 2018 23 August 2018
Please cite this article as: Ting kuang , Shuai Lyu , Sixu Liu , Yuhua Zhang , Jinlin Li , Guanghui Wang , Li Wang , Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer-Tropsch synthesis performance, Journal of Energy Chemistry (2018), doi: https://doi.org/10.1016/j.jechem.2018.08.012
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Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer-Tropsch synthesis performance Ting kuanga, Shuai Lyua, b, Sixu Liua, Yuhua Zhanga, Jinlin Lia,*, Guanghui Wangb, Li Wanga,* a
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission &
Ministry of Education, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, Hubei, China Hubei Key Laboratory of Coal Conversion and New Carbon Material, School of Chemical
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b
Engineering and Technology, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China
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Abstract: Non-porous carbon sphere was used as support to synthesize supported cobalt Fischer-Tropsch catalysts with high activity and durability. Strong metal-support interaction was avoided and intrinsic activity of pristine cobalt nano-particles was studied. Thermal decomposition synthesis method was applied to
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obtain cobalt catalysts with high dispersion and narrow particle size distribution. Furthermore the cobalt size can be controlled by the molar ratio of o-dichlorobenzene/
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benzylamine. Compared with supported cobalt catalysts prepared by incipient wetness impregnation method and ultrasonic impregnation method, the catalyst prepared by
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thermal decomposition method showed higher catalytic activity, higher long chain
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hydrocarbons selectivity and lower methane selectivity.
Keywords: Fischer-Tropsch synthesis; Cobalt catalysts; Particle size; Thermal
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decomposition
*
Corresponding author. Tel: +86-27-67842922; Fax: +86-27-67842752 E-mail address: [email protected] (L. Wang); [email protected] (J. L. Li).
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1. Introduction Fischer-Tropsch synthesis (FTS), a heterogeneous catalytic process, can be used to produce transportation fuels and chemicals from syngas (CO + H2) which can be derived from biomass, coal and natural gas. Iron (Fe), cobalt (Co), nickel (Ni) and ruthenium (Ru) are effective in Fischer-Tropsch (FT) synthesis. Among them, Co and
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Fe catalysts are usually used in industry [1]. Compared with supported iron catalysts, Co catalysts are more expensive but exhibit higher catalytic activity, longer lifetime, higher selectivity for heavy hydrocarbons and low water-gas shift activity [2–4]. Therefore, it is a remaining challenge to design optimized Co FT catalysts.
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Increasing the exposed surface area per unit mass of Co metal by decreasing the Co particle size is one effective method. However, it has been reported that Co particle size smaller than 10 nm showed a sharp decrease in activity as a function of particle size [5,6]. The pioneering study of cobalt particle size effect showed that the
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specific activity decreased significantly with increasing Co dispersion [7]. On the other hand, the catalytic performance of supported cobalt catalyst can vary with
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different supports, dispersions and preparation methods [8]. In order to increase the dispersion of cobalt, some oxide materials, such as SiO2,
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Al2O3, TiO2 [9], are often used as catalyst supports for FTS. The main drawback of these supports is the formation of irreducible mixed compounds such as Co2SiO4,
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CoAl2O4, CoTiO4 due to strong metal-support interaction [10–12]. Therefore, the research concentrated on intrinsic cobalt particle size effect is profitably performed
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here using inert support material, carbon spheres. Moreover, it is documented that the catalyst preparation methods have a major
impact on catalyst activity and deactivation [13]. For example, aqueous impregnation of a support with cobalt precursor solution, either by co-impregnation or multiple impregnation is the most common preparation method. However, the obtained catalysts usually have wide particle size distributions, and the particles are easily to agglomerate under harsh conditions. To prevent such agglomeration, some supports with stronger metal-support interactions, such as titanium oxide and alumina oxide are
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usually needed [14]. Another interesting method is ultrasonic impregnation. Liu et al. prepared uniform cobalt oxide particles, and then loaded them on alumina oxide by ultrasonic dispersion, which successfully decreased metal-support interaction and increased the reduction degree of the catalyst [15]. Protection of surfactant is always needed to
easily to agglomerate during ultrasonic dispersion.
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achieve uniform nanoparticle size distribution. Otherwise such nanoparticles are
Thermal decomposition method is one of the most sophisticated methods to obtain high quality nanoparticles [16–18]. Herein, we applied thermal decomposition method to design and synthesis of FT cobalt catalysts. A series of supported cobalt
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catalysts were prepared using non-porous carbon spheres (CS) as support. In this contribution, we demonstrate that the supported cobalt catalyst prepared by thermal decomposition exhibited uniform cobalt nanoparticles, high catalytic activity and stability.
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2. Experimental
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2.1. Carbon spheres (CSs) preparation
Carbon spheres were prepared by hydrothermal method. 60 g of glucose was
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dissolved into H2O (250 mL) and ultrasonicated for 30 min. The obtained solution was transferred into a stainless-steel autoclave of 500 mL capacity, and 100 mL H2O
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was added into the autoclave. The mixture was maintained at 180 ℃ for 10 h; afterward the brown precipitate was collected and washed with ethanol and deionized
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water until the filter liquor was clear. Then the precipitate was dried at 100 ℃ for 12 h followed by calcination at 800 ℃ in N2 for 2 h (heating rate: 2 ℃/min) in tube furnace. The obtained black power was denoted as carbon spheres (CSs). 2.2. Catalyst preparation CoO/C catalyst with cobalt content of 6.1 wt% was prepared by thermal decomposition method. In a typically synthesis, Co(acac)2 (0.84 mmol) and CSs (0.50 g) were dissolved in 0.14 mol of benzylamine. Subsequently, the obtained solution in
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the three-necked flask was heated to 190 ℃ with stirring in oil bath and refluxed for 2 h. After cooling down to 50–80 ℃, the sample was centrifuged and washed with ethanol for three times, followed by drying at 100 ℃ for 6 h. The sample was denoted as CoO/C-TD. For comparison, CoO/C catalysts with cobalt content of 5.2 wt% and 5.7 wt% were prepared by incipient wetness impregnation and ultrasonic impregnation,
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respectively. The cobalt nitrate hexahydrate was dissolve in appropriate amount of ethanol and deposited dropwise onto the carbon spheres, followed by evaporation in a rotary evaporator at 70 ℃ for 1 h (50 ℃ to 70 ℃ with heating rate of 5 ℃/ 30 min) under vacuum. The obtained powder was then dried at 100 ℃ for 10 h and calcined at
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350 ℃ for 6 h under N2 flow. The sample was denoted as CoO/C-IWI.
For the catalyst prepared by ultrasonic impregnation, CoO particles were first prepared by thermal decomposition method. The appropriate amount of CoO and CSs was ground with less ethanol for 30 min. Then the wet solid was dispersed in ethanol,
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transferred into a flask and ultrasonicated (generated at ultrasonic frequency of 40 kHz, ultrasonic power of 220 kV and 200 W) for 1 h. The solution was evaporated in
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a rotary evaporator at 70 ℃ for 1 h (50 ℃ to 70 ℃ with heating rate of 5 ℃/30 min) under vacuum. The obtained sample was dried at 100 ℃ for 6 h. The sample was
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denoted as CoO/C-UI.
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2.3. Characterization
The crystalline structure of the as-prepared catalysts was recorded by X-ray
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powder diffraction (XRD) (Cu Kα = 1.5404 Å) (Bruker D8 Advance, Germany). Scanning electron microscopy (SEM) (SU8010, Hitachi) was conducted to illustrate the morphology and size of catalysts. H2 temperature-programmed reduction (H2-TPR) experiment was performed on a Zeton Altamira AMI-200 unit to study the reduction behavior of the samples. Hydrogen chemisorption was carried out on a Micromeritics AutoChem II 2920 unit to give the number of active surface metal atoms. X-ray photoelectron spectroscopy (XPS) measurement was performed on a Thermal Electron VG multilab 2000 with Al Kα X-ray source under vacuum at 2×10-6 Pa to
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analyze the ratio of Co/C (all the binding energies were corrected by the C 1s peak with reference to 284.6 eV of the surface adventitious carbon). Inductively coupled plasma mass spectrometry (ICP-MS) was conducted on NexlON 300X to measure the content of cobalt at the RF power 1600 W, argon gas flow rates for the plasma, auxiliary and nebulizer flow were 18 L/min, 1.2 L/min and 0.98 L/min, respectively.
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2.4. Catalytic performance The Fischer-Tropsch synthesis was performed in a laboratory fixed bed reactor. Catalyst (0.1 g) mixed with carborundum (0.2 g) was added into the reactor and reduced in-situ using pure hydrogen (3 SL h-1 g-1) at 625 K for 3 h at atmospheric
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pressure. After reduction, the reactor temperature was decreased to 373 K, in flowing H2. Then, synthesis gas (H2/CO/N2 = 60/30/10 vol%, purity: 99.99%) was introduced. The space velocity of syngas was 2 SL h-1 g-1. The pressure was increased to 1 MPa, and the reaction temperature was gradually increased to 505 K. The effluent gas from the fixed bed reactor was analyzed online using gas chromatograph (GC) (Agilent
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7890B), equipped with one thermal conductivity detector (TCD) and two flame
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ionization detectors (FID), in which FID1 A and FID2 B were employed to analyze hydrocarbons, and TCD3 C was employed for the analysis of H2, N2, CO and CO2.
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3. Results and discussion
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3.1. Size-tunable CoO nanocrystals on the carbon spheres Large-scale production of uniform carbon spheres has been synthesized by a
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hydrothermal method. The SEM image of carbon spheres is shown in Fig. S1 (supporting information). All the SEM observations demonstrate that the product of hydrothermal process led to 100% carbon spheres with smooth surfaces. By using the Nano Measurer software (observations on more than 300 particles), the size distributions of carbon spheres have been analyzed (see inset in Fig. S1), and the average particle size is 339 nm. Fig. 1 shows typical XRD patterns of the CoO nanocrystals synthesized by using different o-dichlorobenzene/benzylamine (o-DCB/BN) molar ratios. The three peaks
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at 36.5°, 42.4° and 61.5° can be assigned to (111), (200) and (220) planes of CoO, respectively. It is worth noting that the three peaks are obviously broadened with increasing o-DCB/BN molar ratio. The results indicate that the particle size of CoO nanocrystals on the carbon sphere can be precisely tuned by changing o-DCB/BN molar ratio. The preparation and characterizations of these materials are described in
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detail in Tables S1 and S2 in supporting information.
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Fig. 1. XRD patterns of different particle sizes of CoO supported on carbon spheres.
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o-DCB/BN molar ratio: (1) 2.6, (2) 1.9, (3) 1.4, (4) 0.
Fig. 2. SEM images of different particle sizes of CoO supported on carbon spheres. o-DCB/BN molar ratio: (a) 2.6, (b) 1.9, (c) 1.4, (d) 0.
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Fig. 2 shows SEM images and the corresponding particle size analyses of CoO nanoparticles obtained by dissolving 0.84 mmol Co(acac)2 and 0.5 g carbon spheres in a mixture of o-dichlorobenzene and benzylamine (o-DCB/BN molar ratio: 2.6, 1.9, 1.4, 0). The average diameter of CoO nanoparticles decreased from 21.0 nm to 8.2 nm when the o-DCB/BN molar ratio increased, shown in the insets of Fig. 2. Consequently, increasing the o-DCB/BN molar ratio led to smaller CoO particle size
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on the carbon spheres, which was original from the decomposition of cobalt(II) acetylacetonate [19,20]. More cobalt crystal nucleus formation led to smaller CoO nanoparticles.
In addition, the CoO particle sizes can also be controlled by changing the
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concentration of precursor without o-dichlorobenzene. The synthesis process and characterization results of CoO particle sizes controlled by regulating the concentration of precursor are shown in the supporting information.
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3.2. Characterizations of various CoO/C catalysts prepared by different methods
Fig. 3. SEM images of cobalt catalysts prepared by different methods. (a) CoO/C-TD,
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(b) CoO/C-IWI, (c) CoO/C-UI.
As is well known, the catalyst preparation method is important to its catalytic
performance. For comparison with the CoO/C catalyst obtained by thermal decomposition method, we synthesized CoO/C catalysts by incipient wetness impregnation method and ultrasonic impregnation method, respectively. Fig. 3(a-c) depicts the SEM images of the CoO/C catalysts with different synthesis methods,
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denoted as CoO/C-TD, CoO/C-IWI and CoO/C-UI respectively. For CoO/C-TD catalyst obtained by thermal decomposition method, Fig. 3(a) shows that CoO particles are uniformly dispersed on carbon spheres. For CoO/C-IWI and CoO/C-UI catalysts, the particles agglomerated into micro-particles on the carbon spheres, as
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shown in Fig. 3(b-c).
Fig. 4. XRD patterns of cobalt catalysts prepared by different methods. (1)
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CoO/C-TD, (2) CoO/C-IWI, (3) CoO/C-UI.
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The crystalline structures of the CoO/C catalysts are examined by XRD and the results are shown in Fig. 4. All the peaks of the CoO/C catalysts are indexed to the
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cubic CoO phase. Additionally, the intensity of all the peaks for CoO/C-TD catalyst is weaker than that for CoO/C-IWI and CoO/C-UI catalysts, indicating higher dispersion
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of CoO particles under the thermal decomposition process. The H2 temperature-programmed reduction (H2-TPR) profiles of the CoO/C
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catalysts are displayed in Fig. 5. The main reduction peak at about 320–450 ℃ is attributed to the reduction of CoO to metal Co [21]. The small reduction peak at about 220–300 ℃ is assigned to the reduction of Co3O4 to CoO due to the oxidation of a small amount of CoO on the surface. The reduction peak at 536 ℃ is ascribed to the catalytic decomposition of the support [22]. It can be seen that the main reduction peak of CoO/C-IWI catalyst locates at lower temperature region relative to other catalysts, which is due to smaller CoO on CoO/C-IWI. Smaller CoO is easier to be reduced when cobalt species have lower interaction with support. And the reduction
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of CoO to Co occurs at similar temperature for CoO/C-TD and CoO/C-UI catalysts.
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Fig. 5. H2-TPR profiles of cobalt catalysts prepared by different methods. (1) CoO/C-TD, (2) CoO/C-IWI, (3) CoO/C-UI.
XPS measurements were carried out to elucidate the nature of surface species of the CoO/C catalysts. As shown in Fig. 6, for CoO/C-TD and CoO/C-UI catalysts, the
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peaks corresponding to Co 2p3/2 and 2p1/2 of Co3+ could be observed at 779.9/779.6 eV and 794.9/794.8 eV, respectively, while the peaks corresponding to Co 2p3/2 and
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2p1/2 of Co2+ are observed at 780.8/781.1 eV and 796.5/796.5 eV, respectively [23]. The presence of Co3+ demonstrates that small quantity Co3O4 over the surface may
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exist due to air exposure [24]. The peak intensity ratios of Co3+/(Co2++Co3+) are 0.42
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and 0.38 for CoO/C-TD and CoO/C-UI catalysts, indicating that over the surface the phase of cobalt species is mainly CoO. For CoO/C-IWI catalyst, only the presence of
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Co2+ and less intense satellite peak were detected. The presence of satellite peaks at about 785.8 eV and 802.4 eV indicates the presence of abundant Co2+ [25]. Thus the cobalt species of CoO/C-IWI is CoO phase. This is consistent with the result of TPR. The peak intensity for CoO/C-TD catalyst is stronger than that of CoO/C-IWI catalyst and CoO/C-UI catalyst, indicating that CoO/C-TD catalyst has higher dispersion. XPS is also used to monitor the surface content of Co species of catalysts (Table 1). It can be seen that the Co/C of the surface for CoO/C-TD catalyst is 0.17 (atomic ratio), which is much higher than that of CoO/C-IWI and CoO/C-UI catalysts, 0.06 (atomic
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ratio) and 0.01 (atomic ratio) respectively. It is shown that the dispersion of cobalt for CoO/C-TD catalyst is much better than that of CoO/C-IWI and CoO/C-UI catalysts.
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This is consistent with the results of the SEM and H2 chemisorption characterizations.
Fig. 6. XPS spectra of cobalt catalysts prepared by different methods. (1) CoO/C-TD, (2) CoO/C-IWI, (3) CoO/C-UI.
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Table1. XPS characterization results of different cobalt-based catalysts. Binding energy (eV) 2p3/2 3+
Co
CoO/C-IWI
3+
2+
Atomic ratio
(Co2++Co3+)a
of Co/C
Co
Co
Co
780.8
794.9
796.5
0.42
0.17
780.6
--
796.1
0
0.06
779.6
781.1
794.8
796.5
0.38
0.01
Peak intensity ratio of Co3+/(Co2++Co3+).
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a
2p1/2
--
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CoO/C-UI
779.9
2+
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CoO/C-TD
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Catalysts
Co3+/
3.3. The effects of preparation methods on FTS performance The catalytic activity of cobalt-based catalysts prepared by different synthesis methods was tested in a fixed-bed reactor under FTS reaction conditions: T = 230 ℃, P = 1 MPa, H2/CO = 2, GHSV = 2 SL h-1 g-1. The catalytic results are summarized in Table 2. The CoO/C-TD catalyst with higher dispersion shows a considerable improvement of the catalytic performance compared to CoO/C-IWI and CoO/C-UI
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catalysts. The temperature evolution of CO conversion for three catalysts is presented in Fig. 7(a). The CoO/C-TD catalyst has a catalytic activity at 180 ℃ and the CO conversion is 2%. However, under the same condition, CoO/C-IWI and CoO/C-UI do not have catalytic activity until the temperature reaches at 210 ℃ and 220 ℃, respectively. The activity of the catalysts increases with the increase of temperature. Moreover, the CO conversion of CoO/C-TD catalyst is higher than that of CoO/C-IWI
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and CoO/C-UI catalysts at 180–230 ℃. The time-on-stream evolution of CO conversion for cobalt-based is presented in Fig. 7(b). It can be seen that the CoO/C-TD catalyst shows higher CO conversion (21.0%) than CoO/C-IWI and CoO/C-UI catalysts (4.8% and 3.0%, respectively) at similar Co content. Such a
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higher activity can be related to higher dispersion of CoO/C-TD catalyst [26]. The FTS product selectivity for CoO/C-TD, CoO/C-IWI and CoO/C-UI catalysts is presented in Fig. 7(c). It can be seen that CoO/C-TD catalyst shows higher C5+
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selectivity (81.9%) and lower methane selectivity (11.2%).
Table 2. Dispersion and catalytic activity of cobalt-based Fischer-Tropsch synthesis
CO2
Dispersion a
TOF
CO conversion
(%)
(10-3 s-1)
(%)
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Catalysts
Product selectivity (%)
selectivity CH4
C2–C4
C5+
(%)
24.1
4.2
21.0
1.3
11.2
6.9
81.9
CoO/C-IWI
10.1
1.1
4.8
4.4
16.3
24.2
59.5
CoO/C-UI
4.9
1.4
3.0
5.4
13.9
26.6
59.5
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CoO/C-TD
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catalysts obtained by different methods.
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Obtained from H2 chemisorption.
In order to examine the durability of the CoO/C-TD catalyst under reaction conditions, long-term tests were performed. The CO conversion and hydrocarbon selectivity vs. time on stream are shown in Fig. 8, indicating no observed deactivation in the 120 h reaction. In addition, the product distributions over the CoO/C-TD
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catalyst are shown in Fig. S5 in the supporting information. The model of Anderson-Schulz-Flory (ASF) distribution was used to calculate the chain growth probability (α-value), with the results shown in the inset of Fig. S5. It is shown that the CoO/C-TD catalyst has a α-value of 0.85, referring to high probability towards
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long-chain hydrocarbon formation.
Fig. 7. Fischer-Tropsch synthesis performance of cobalt catalysts prepared by
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different methods (Reaction condition: CO/H2 = 1/2, 230 ℃, 1 MPa, 2 SL h-1 g-1).
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Fig. 8. CO conversion and hydrocarbon selectivity vs. time on stream for the CoO/C-TD catalyst (Reaction condition: CO/H2 = 1/2, 230 ℃, 1 MPa, 2 SL h-1 g-1).
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Fig. 9. TEM images of (a) fresh CoO/C-TD catalyst; (b) spent CoO/C-TD catalyst,
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HRTEM images of (c) fresh CoO/C-TD catalyst, (d) spent CoO/C-TD catalyst.
Furthermore, subsequent characterization of TEM on the fresh and spent
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CoO/C-TD catalysts shows the morphology and the average diameter of cobalt nanocrystals on the carbon spheres are essentially unchanged during the 120 h FTS reaction, as shown in Fig. 9. The average particle sizes of the fresh and spent CoO/C-TD catalysts are 20.0 and 19.9 nm, respectively. In addition, visible lattice fringes with d spacings of ~2.49 and ~2.04 Ǻ were observed by HRTEM and shown in Fig. 8(c,d), corresponding to the (111) planes of CoO and Co, respectively. It can be concluded that the CoO/C-TD catalyst was stable during catalytic reaction.
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4. Conclusions In summary, the particle size of carbon sphere supported cobalt catalysts can be precisely controlled by thermal decomposition method. SEM images and H2 chemisorption results indicate that the dispersion of CoO/C-TD catalyst prepared by thermal decomposition method was much higher compared with CoO/C-IWI and
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CoO/C-UI catalysts. For the Fischer-Tropsch synthesis, the CoO/C-TD catalyst showed higher catalytic activity, higher long chain hydrocarbon selectivity and lower methane selectivity. Furthermore, the ability to control the cobalt size provides a
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novel approach to explore excellent model catalyst.
Acknowledgments
This work was supported by the Key Program project of the NSFC and China Petrochemical Corporation Joint Fund (Grant No. U1463210), the Natural Science
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Foundation of Hubei Province of China (2013CFA089) and the Fundamental Research Funds for the Central Universities, South-Central University for
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Graphical abstract Thermal decomposition method has been successfully applied to prepare cobalt
catalysts with high dispersion. The procedure does not require calcination and avoids nanoparticles agglomeration. The CoO/C-TD catalyst shows high catalytic activity and C5+ selectivity.
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Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer-Tropsch synthesis performance
http://www.journals.elsevier.com/ journal-of-energy-chemistry/
Ting kuang , Shuai Lyu , Sixu Liu , Yuhua Zhang , Jinlin Li , Guanghui Wang , Li Wang PII: DOI: Reference:
S2095-4956(18)30507-2 https://doi.org/10.1016/j.jechem.2018.08.012 JECHEM 660
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
7 June 2018 25 July 2018 23 August 2018
Please cite this article as: Ting kuang , Shuai Lyu , Sixu Liu , Yuhua Zhang , Jinlin Li , Guanghui Wang , Li Wang , Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer-Tropsch synthesis performance, Journal of Energy Chemistry (2018), doi: https://doi.org/10.1016/j.jechem.2018.08.012
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Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer-Tropsch synthesis performance Ting kuanga, Shuai Lyua, b, Sixu Liua, Yuhua Zhanga, Jinlin Lia,*, Guanghui Wangb, Li Wanga,* a
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission &
Ministry of Education, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, Hubei, China Hubei Key Laboratory of Coal Conversion and New Carbon Material, School of Chemical
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b
Engineering and Technology, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China
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Abstract: Non-porous carbon sphere was used as support to synthesize supported cobalt Fischer-Tropsch catalysts with high activity and durability. Strong metal-support interaction was avoided and intrinsic activity of pristine cobalt nano-particles was studied. Thermal decomposition synthesis method was applied to
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obtain cobalt catalysts with high dispersion and narrow particle size distribution. Furthermore the cobalt size can be controlled by the molar ratio of o-dichlorobenzene/
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benzylamine. Compared with supported cobalt catalysts prepared by incipient wetness impregnation method and ultrasonic impregnation method, the catalyst prepared by
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thermal decomposition method showed higher catalytic activity, higher long chain
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hydrocarbons selectivity and lower methane selectivity.
Keywords: Fischer-Tropsch synthesis; Cobalt catalysts; Particle size; Thermal
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decomposition
*
Corresponding author. Tel: +86-27-67842922; Fax: +86-27-67842752 E-mail address: [email protected] (L. Wang); [email protected] (J. L. Li).
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1. Introduction Fischer-Tropsch synthesis (FTS), a heterogeneous catalytic process, can be used to produce transportation fuels and chemicals from syngas (CO + H2) which can be derived from biomass, coal and natural gas. Iron (Fe), cobalt (Co), nickel (Ni) and ruthenium (Ru) are effective in Fischer-Tropsch (FT) synthesis. Among them, Co and
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Fe catalysts are usually used in industry [1]. Compared with supported iron catalysts, Co catalysts are more expensive but exhibit higher catalytic activity, longer lifetime, higher selectivity for heavy hydrocarbons and low water-gas shift activity [2–4]. Therefore, it is a remaining challenge to design optimized Co FT catalysts.
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Increasing the exposed surface area per unit mass of Co metal by decreasing the Co particle size is one effective method. However, it has been reported that Co particle size smaller than 10 nm showed a sharp decrease in activity as a function of particle size [5,6]. The pioneering study of cobalt particle size effect showed that the
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specific activity decreased significantly with increasing Co dispersion [7]. On the other hand, the catalytic performance of supported cobalt catalyst can vary with
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different supports, dispersions and preparation methods [8]. In order to increase the dispersion of cobalt, some oxide materials, such as SiO2,
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Al2O3, TiO2 [9], are often used as catalyst supports for FTS. The main drawback of these supports is the formation of irreducible mixed compounds such as Co2SiO4,
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CoAl2O4, CoTiO4 due to strong metal-support interaction [10–12]. Therefore, the research concentrated on intrinsic cobalt particle size effect is profitably performed
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here using inert support material, carbon spheres. Moreover, it is documented that the catalyst preparation methods have a major
impact on catalyst activity and deactivation [13]. For example, aqueous impregnation of a support with cobalt precursor solution, either by co-impregnation or multiple impregnation is the most common preparation method. However, the obtained catalysts usually have wide particle size distributions, and the particles are easily to agglomerate under harsh conditions. To prevent such agglomeration, some supports with stronger metal-support interactions, such as titanium oxide and alumina oxide are
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usually needed [14]. Another interesting method is ultrasonic impregnation. Liu et al. prepared uniform cobalt oxide particles, and then loaded them on alumina oxide by ultrasonic dispersion, which successfully decreased metal-support interaction and increased the reduction degree of the catalyst [15]. Protection of surfactant is always needed to
easily to agglomerate during ultrasonic dispersion.
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achieve uniform nanoparticle size distribution. Otherwise such nanoparticles are
Thermal decomposition method is one of the most sophisticated methods to obtain high quality nanoparticles [16–18]. Herein, we applied thermal decomposition method to design and synthesis of FT cobalt catalysts. A series of supported cobalt
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catalysts were prepared using non-porous carbon spheres (CS) as support. In this contribution, we demonstrate that the supported cobalt catalyst prepared by thermal decomposition exhibited uniform cobalt nanoparticles, high catalytic activity and stability.
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2. Experimental
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2.1. Carbon spheres (CSs) preparation
Carbon spheres were prepared by hydrothermal method. 60 g of glucose was
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dissolved into H2O (250 mL) and ultrasonicated for 30 min. The obtained solution was transferred into a stainless-steel autoclave of 500 mL capacity, and 100 mL H2O
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was added into the autoclave. The mixture was maintained at 180 ℃ for 10 h; afterward the brown precipitate was collected and washed with ethanol and deionized
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water until the filter liquor was clear. Then the precipitate was dried at 100 ℃ for 12 h followed by calcination at 800 ℃ in N2 for 2 h (heating rate: 2 ℃/min) in tube furnace. The obtained black power was denoted as carbon spheres (CSs). 2.2. Catalyst preparation CoO/C catalyst with cobalt content of 6.1 wt% was prepared by thermal decomposition method. In a typically synthesis, Co(acac)2 (0.84 mmol) and CSs (0.50 g) were dissolved in 0.14 mol of benzylamine. Subsequently, the obtained solution in
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the three-necked flask was heated to 190 ℃ with stirring in oil bath and refluxed for 2 h. After cooling down to 50–80 ℃, the sample was centrifuged and washed with ethanol for three times, followed by drying at 100 ℃ for 6 h. The sample was denoted as CoO/C-TD. For comparison, CoO/C catalysts with cobalt content of 5.2 wt% and 5.7 wt% were prepared by incipient wetness impregnation and ultrasonic impregnation,
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respectively. The cobalt nitrate hexahydrate was dissolve in appropriate amount of ethanol and deposited dropwise onto the carbon spheres, followed by evaporation in a rotary evaporator at 70 ℃ for 1 h (50 ℃ to 70 ℃ with heating rate of 5 ℃/ 30 min) under vacuum. The obtained powder was then dried at 100 ℃ for 10 h and calcined at
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350 ℃ for 6 h under N2 flow. The sample was denoted as CoO/C-IWI.
For the catalyst prepared by ultrasonic impregnation, CoO particles were first prepared by thermal decomposition method. The appropriate amount of CoO and CSs was ground with less ethanol for 30 min. Then the wet solid was dispersed in ethanol,
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transferred into a flask and ultrasonicated (generated at ultrasonic frequency of 40 kHz, ultrasonic power of 220 kV and 200 W) for 1 h. The solution was evaporated in
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a rotary evaporator at 70 ℃ for 1 h (50 ℃ to 70 ℃ with heating rate of 5 ℃/30 min) under vacuum. The obtained sample was dried at 100 ℃ for 6 h. The sample was
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denoted as CoO/C-UI.
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2.3. Characterization
The crystalline structure of the as-prepared catalysts was recorded by X-ray
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powder diffraction (XRD) (Cu Kα = 1.5404 Å) (Bruker D8 Advance, Germany). Scanning electron microscopy (SEM) (SU8010, Hitachi) was conducted to illustrate the morphology and size of catalysts. H2 temperature-programmed reduction (H2-TPR) experiment was performed on a Zeton Altamira AMI-200 unit to study the reduction behavior of the samples. Hydrogen chemisorption was carried out on a Micromeritics AutoChem II 2920 unit to give the number of active surface metal atoms. X-ray photoelectron spectroscopy (XPS) measurement was performed on a Thermal Electron VG multilab 2000 with Al Kα X-ray source under vacuum at 2×10-6 Pa to
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analyze the ratio of Co/C (all the binding energies were corrected by the C 1s peak with reference to 284.6 eV of the surface adventitious carbon). Inductively coupled plasma mass spectrometry (ICP-MS) was conducted on NexlON 300X to measure the content of cobalt at the RF power 1600 W, argon gas flow rates for the plasma, auxiliary and nebulizer flow were 18 L/min, 1.2 L/min and 0.98 L/min, respectively.
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2.4. Catalytic performance The Fischer-Tropsch synthesis was performed in a laboratory fixed bed reactor. Catalyst (0.1 g) mixed with carborundum (0.2 g) was added into the reactor and reduced in-situ using pure hydrogen (3 SL h-1 g-1) at 625 K for 3 h at atmospheric
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pressure. After reduction, the reactor temperature was decreased to 373 K, in flowing H2. Then, synthesis gas (H2/CO/N2 = 60/30/10 vol%, purity: 99.99%) was introduced. The space velocity of syngas was 2 SL h-1 g-1. The pressure was increased to 1 MPa, and the reaction temperature was gradually increased to 505 K. The effluent gas from the fixed bed reactor was analyzed online using gas chromatograph (GC) (Agilent
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7890B), equipped with one thermal conductivity detector (TCD) and two flame
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ionization detectors (FID), in which FID1 A and FID2 B were employed to analyze hydrocarbons, and TCD3 C was employed for the analysis of H2, N2, CO and CO2.
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3. Results and discussion
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3.1. Size-tunable CoO nanocrystals on the carbon spheres Large-scale production of uniform carbon spheres has been synthesized by a
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hydrothermal method. The SEM image of carbon spheres is shown in Fig. S1 (supporting information). All the SEM observations demonstrate that the product of hydrothermal process led to 100% carbon spheres with smooth surfaces. By using the Nano Measurer software (observations on more than 300 particles), the size distributions of carbon spheres have been analyzed (see inset in Fig. S1), and the average particle size is 339 nm. Fig. 1 shows typical XRD patterns of the CoO nanocrystals synthesized by using different o-dichlorobenzene/benzylamine (o-DCB/BN) molar ratios. The three peaks
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at 36.5°, 42.4° and 61.5° can be assigned to (111), (200) and (220) planes of CoO, respectively. It is worth noting that the three peaks are obviously broadened with increasing o-DCB/BN molar ratio. The results indicate that the particle size of CoO nanocrystals on the carbon sphere can be precisely tuned by changing o-DCB/BN molar ratio. The preparation and characterizations of these materials are described in
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detail in Tables S1 and S2 in supporting information.
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Fig. 1. XRD patterns of different particle sizes of CoO supported on carbon spheres.
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o-DCB/BN molar ratio: (1) 2.6, (2) 1.9, (3) 1.4, (4) 0.
Fig. 2. SEM images of different particle sizes of CoO supported on carbon spheres. o-DCB/BN molar ratio: (a) 2.6, (b) 1.9, (c) 1.4, (d) 0.
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Fig. 2 shows SEM images and the corresponding particle size analyses of CoO nanoparticles obtained by dissolving 0.84 mmol Co(acac)2 and 0.5 g carbon spheres in a mixture of o-dichlorobenzene and benzylamine (o-DCB/BN molar ratio: 2.6, 1.9, 1.4, 0). The average diameter of CoO nanoparticles decreased from 21.0 nm to 8.2 nm when the o-DCB/BN molar ratio increased, shown in the insets of Fig. 2. Consequently, increasing the o-DCB/BN molar ratio led to smaller CoO particle size
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on the carbon spheres, which was original from the decomposition of cobalt(II) acetylacetonate [19,20]. More cobalt crystal nucleus formation led to smaller CoO nanoparticles.
In addition, the CoO particle sizes can also be controlled by changing the
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concentration of precursor without o-dichlorobenzene. The synthesis process and characterization results of CoO particle sizes controlled by regulating the concentration of precursor are shown in the supporting information.
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3.2. Characterizations of various CoO/C catalysts prepared by different methods
Fig. 3. SEM images of cobalt catalysts prepared by different methods. (a) CoO/C-TD,
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(b) CoO/C-IWI, (c) CoO/C-UI.
As is well known, the catalyst preparation method is important to its catalytic
performance. For comparison with the CoO/C catalyst obtained by thermal decomposition method, we synthesized CoO/C catalysts by incipient wetness impregnation method and ultrasonic impregnation method, respectively. Fig. 3(a-c) depicts the SEM images of the CoO/C catalysts with different synthesis methods,
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denoted as CoO/C-TD, CoO/C-IWI and CoO/C-UI respectively. For CoO/C-TD catalyst obtained by thermal decomposition method, Fig. 3(a) shows that CoO particles are uniformly dispersed on carbon spheres. For CoO/C-IWI and CoO/C-UI catalysts, the particles agglomerated into micro-particles on the carbon spheres, as
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shown in Fig. 3(b-c).
Fig. 4. XRD patterns of cobalt catalysts prepared by different methods. (1)
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CoO/C-TD, (2) CoO/C-IWI, (3) CoO/C-UI.
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The crystalline structures of the CoO/C catalysts are examined by XRD and the results are shown in Fig. 4. All the peaks of the CoO/C catalysts are indexed to the
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cubic CoO phase. Additionally, the intensity of all the peaks for CoO/C-TD catalyst is weaker than that for CoO/C-IWI and CoO/C-UI catalysts, indicating higher dispersion
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of CoO particles under the thermal decomposition process. The H2 temperature-programmed reduction (H2-TPR) profiles of the CoO/C
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catalysts are displayed in Fig. 5. The main reduction peak at about 320–450 ℃ is attributed to the reduction of CoO to metal Co [21]. The small reduction peak at about 220–300 ℃ is assigned to the reduction of Co3O4 to CoO due to the oxidation of a small amount of CoO on the surface. The reduction peak at 536 ℃ is ascribed to the catalytic decomposition of the support [22]. It can be seen that the main reduction peak of CoO/C-IWI catalyst locates at lower temperature region relative to other catalysts, which is due to smaller CoO on CoO/C-IWI. Smaller CoO is easier to be reduced when cobalt species have lower interaction with support. And the reduction
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of CoO to Co occurs at similar temperature for CoO/C-TD and CoO/C-UI catalysts.
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Fig. 5. H2-TPR profiles of cobalt catalysts prepared by different methods. (1) CoO/C-TD, (2) CoO/C-IWI, (3) CoO/C-UI.
XPS measurements were carried out to elucidate the nature of surface species of the CoO/C catalysts. As shown in Fig. 6, for CoO/C-TD and CoO/C-UI catalysts, the
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peaks corresponding to Co 2p3/2 and 2p1/2 of Co3+ could be observed at 779.9/779.6 eV and 794.9/794.8 eV, respectively, while the peaks corresponding to Co 2p3/2 and
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2p1/2 of Co2+ are observed at 780.8/781.1 eV and 796.5/796.5 eV, respectively [23]. The presence of Co3+ demonstrates that small quantity Co3O4 over the surface may
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exist due to air exposure [24]. The peak intensity ratios of Co3+/(Co2++Co3+) are 0.42
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and 0.38 for CoO/C-TD and CoO/C-UI catalysts, indicating that over the surface the phase of cobalt species is mainly CoO. For CoO/C-IWI catalyst, only the presence of
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Co2+ and less intense satellite peak were detected. The presence of satellite peaks at about 785.8 eV and 802.4 eV indicates the presence of abundant Co2+ [25]. Thus the cobalt species of CoO/C-IWI is CoO phase. This is consistent with the result of TPR. The peak intensity for CoO/C-TD catalyst is stronger than that of CoO/C-IWI catalyst and CoO/C-UI catalyst, indicating that CoO/C-TD catalyst has higher dispersion. XPS is also used to monitor the surface content of Co species of catalysts (Table 1). It can be seen that the Co/C of the surface for CoO/C-TD catalyst is 0.17 (atomic ratio), which is much higher than that of CoO/C-IWI and CoO/C-UI catalysts, 0.06 (atomic
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ratio) and 0.01 (atomic ratio) respectively. It is shown that the dispersion of cobalt for CoO/C-TD catalyst is much better than that of CoO/C-IWI and CoO/C-UI catalysts.
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This is consistent with the results of the SEM and H2 chemisorption characterizations.
Fig. 6. XPS spectra of cobalt catalysts prepared by different methods. (1) CoO/C-TD, (2) CoO/C-IWI, (3) CoO/C-UI.
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Table1. XPS characterization results of different cobalt-based catalysts. Binding energy (eV) 2p3/2 3+
Co
CoO/C-IWI
3+
2+
Atomic ratio
(Co2++Co3+)a
of Co/C
Co
Co
Co
780.8
794.9
796.5
0.42
0.17
780.6
--
796.1
0
0.06
779.6
781.1
794.8
796.5
0.38
0.01
Peak intensity ratio of Co3+/(Co2++Co3+).
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a
2p1/2
--
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CoO/C-UI
779.9
2+
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CoO/C-TD
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Catalysts
Co3+/
3.3. The effects of preparation methods on FTS performance The catalytic activity of cobalt-based catalysts prepared by different synthesis methods was tested in a fixed-bed reactor under FTS reaction conditions: T = 230 ℃, P = 1 MPa, H2/CO = 2, GHSV = 2 SL h-1 g-1. The catalytic results are summarized in Table 2. The CoO/C-TD catalyst with higher dispersion shows a considerable improvement of the catalytic performance compared to CoO/C-IWI and CoO/C-UI
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catalysts. The temperature evolution of CO conversion for three catalysts is presented in Fig. 7(a). The CoO/C-TD catalyst has a catalytic activity at 180 ℃ and the CO conversion is 2%. However, under the same condition, CoO/C-IWI and CoO/C-UI do not have catalytic activity until the temperature reaches at 210 ℃ and 220 ℃, respectively. The activity of the catalysts increases with the increase of temperature. Moreover, the CO conversion of CoO/C-TD catalyst is higher than that of CoO/C-IWI
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and CoO/C-UI catalysts at 180–230 ℃. The time-on-stream evolution of CO conversion for cobalt-based is presented in Fig. 7(b). It can be seen that the CoO/C-TD catalyst shows higher CO conversion (21.0%) than CoO/C-IWI and CoO/C-UI catalysts (4.8% and 3.0%, respectively) at similar Co content. Such a
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higher activity can be related to higher dispersion of CoO/C-TD catalyst [26]. The FTS product selectivity for CoO/C-TD, CoO/C-IWI and CoO/C-UI catalysts is presented in Fig. 7(c). It can be seen that CoO/C-TD catalyst shows higher C5+
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selectivity (81.9%) and lower methane selectivity (11.2%).
Table 2. Dispersion and catalytic activity of cobalt-based Fischer-Tropsch synthesis
CO2
Dispersion a
TOF
CO conversion
(%)
(10-3 s-1)
(%)
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Catalysts
Product selectivity (%)
selectivity CH4
C2–C4
C5+
(%)
24.1
4.2
21.0
1.3
11.2
6.9
81.9
CoO/C-IWI
10.1
1.1
4.8
4.4
16.3
24.2
59.5
CoO/C-UI
4.9
1.4
3.0
5.4
13.9
26.6
59.5
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CoO/C-TD
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catalysts obtained by different methods.
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Obtained from H2 chemisorption.
In order to examine the durability of the CoO/C-TD catalyst under reaction conditions, long-term tests were performed. The CO conversion and hydrocarbon selectivity vs. time on stream are shown in Fig. 8, indicating no observed deactivation in the 120 h reaction. In addition, the product distributions over the CoO/C-TD
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catalyst are shown in Fig. S5 in the supporting information. The model of Anderson-Schulz-Flory (ASF) distribution was used to calculate the chain growth probability (α-value), with the results shown in the inset of Fig. S5. It is shown that the CoO/C-TD catalyst has a α-value of 0.85, referring to high probability towards
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long-chain hydrocarbon formation.
Fig. 7. Fischer-Tropsch synthesis performance of cobalt catalysts prepared by
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different methods (Reaction condition: CO/H2 = 1/2, 230 ℃, 1 MPa, 2 SL h-1 g-1).
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Fig. 8. CO conversion and hydrocarbon selectivity vs. time on stream for the CoO/C-TD catalyst (Reaction condition: CO/H2 = 1/2, 230 ℃, 1 MPa, 2 SL h-1 g-1).
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Fig. 9. TEM images of (a) fresh CoO/C-TD catalyst; (b) spent CoO/C-TD catalyst,
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HRTEM images of (c) fresh CoO/C-TD catalyst, (d) spent CoO/C-TD catalyst.
Furthermore, subsequent characterization of TEM on the fresh and spent
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CoO/C-TD catalysts shows the morphology and the average diameter of cobalt nanocrystals on the carbon spheres are essentially unchanged during the 120 h FTS reaction, as shown in Fig. 9. The average particle sizes of the fresh and spent CoO/C-TD catalysts are 20.0 and 19.9 nm, respectively. In addition, visible lattice fringes with d spacings of ~2.49 and ~2.04 Ǻ were observed by HRTEM and shown in Fig. 8(c,d), corresponding to the (111) planes of CoO and Co, respectively. It can be concluded that the CoO/C-TD catalyst was stable during catalytic reaction.
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4. Conclusions In summary, the particle size of carbon sphere supported cobalt catalysts can be precisely controlled by thermal decomposition method. SEM images and H2 chemisorption results indicate that the dispersion of CoO/C-TD catalyst prepared by thermal decomposition method was much higher compared with CoO/C-IWI and
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CoO/C-UI catalysts. For the Fischer-Tropsch synthesis, the CoO/C-TD catalyst showed higher catalytic activity, higher long chain hydrocarbon selectivity and lower methane selectivity. Furthermore, the ability to control the cobalt size provides a
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novel approach to explore excellent model catalyst.
Acknowledgments
This work was supported by the Key Program project of the NSFC and China Petrochemical Corporation Joint Fund (Grant No. U1463210), the Natural Science
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Foundation of Hubei Province of China (2013CFA089) and the Fundamental Research Funds for the Central Universities, South-Central University for
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Graphical abstract Thermal decomposition method has been successfully applied to prepare cobalt
catalysts with high dispersion. The procedure does not require calcination and avoids nanoparticles agglomeration. The CoO/C-TD catalyst shows high catalytic activity and C5+ selectivity.
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