SiO2 As an Efficient Catalyst

Subscriber access provided by READING UNIV

Article

Direct synthesis of the reduced Co-C/SiO2 as an efficient catalyst for Fischer-Tropsch synthesis Yong-Hua Zhao, Chang Liu, Yong-Hong Song, Qi-Jian Zhang, Min-Li Zhu, Zhao-Tie Liu, and Zhong-Wen Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04149 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Direct synthesis of the reduced Co-C/SiO2 as an efficient catalyst for Fischer-Tropsch synthesis Yong-Hua Zhaoa,b, Chang Liua, Yong-Hong Songa, Qi-Jian Zhangb, Min-Li Zhua, Zhao-Tie Liua, Zhong-Wen Liua,* a

Key Laboratory of Applied Surface and Colloid Chemistry (MOE), School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China;

b

School of Chemistry & Environmental Engineering, Liaoning University of Technology, Jinzhou 121001, China.

Abstract The co-impregnation of the glucose and cobalt nitrate aqueous solution into SiO2 followed by the carbonization in an inert gas was demonstrated for the preparation of the directly reduced Co-xC/SiO2 (x: glucose to SiO2 mass ratio) Fischer-Tropsch (FT) catalysts. The as-prepared Co-xC/SiO2 (x=0.1, 0.5, 1.0, 1.5) without any pre-treatment was evaluated for FT synthesis under typical conditions. Significantly, the completely reduced Co-xC/SiO2 were obtained when the x was at or above 1.0, and the maximum normalized FT activity (cobalt-time-yield, CTY) of 3.5×10-4 molCO·g-1Co·s-1 over Co-1C/SiO2 was doubled the activity of the high-temperature reduced Co/SiO2. Moreover, light hydrocarbons (C1~C4) were favorably formed over Co-xC/SiO2, and the product distribution can be regulated via simply changing the ratio of the glucose to SiO2. The simple procedure and the cheap glucose make the directly reduced Co-xC/SiO2 catalysts very promising for the development of a more efficient FT process. Key words: Fischer-Tropsch, cobalt, in-situ reduction, carbon, glucose

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

*: Corresponding author (Z.-W. Liu) School of Chemistry & Chemical Engineering Shaanxi Normal University Xi’an 710119, China Tel: +86-29-8153-0801

Fax: +86-29-8153-0727

E-mail: [email protected]

2


Page 2 of 38

Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 Introduction Fischer-Tropsch (FT) synthesis is a promising process to produce high-value-added chemicals and clean liquid fuels from syngas (CO and H2) derived from carbon-containing resources such as natural gas, coal, and/or biomass. Recently, the FT process has aroused increasingly worldwide attention due to the limited petroleum reserve, the high demand for a decreased dependence on the petroleum-based fuels and chemicals, and the implementation of more stringent environmental legislations on liquid fuels.1,2 Generally, FT synthesis can be catalyzed by transition metals of Co, Fe, and Ru. Among them, the cobalt-based catalyst shows the most promising industrial prospect as a result of its high activity, high selectivity to long-chain hydrocarbons, high resistance to deactivation, low water gas-shift (WGS) activity, and a reasonable price and reserve.2-4 It is commonly known that the metallic cobalt (Co0) is the active site for FT reaction, and its activity is proportional to the number of the exposed Co0. Thus, the loading of a cobalt precursor such as cobalt nitrate over a porous oxide with a high surface area such as silica is a commonly practiced strategy to prepare Co-based catalysts5-10. To obtain the active Co0, the high-temperature reduction of the catalyst, e.g., 400 °C, is an indispensible step for activating the catalyst before the FT reaction. In this case, the specific activity of the catalyst is significantly affected by the extent of reduction and the dispersion of cobalt. Due to the simple procedure, the impregnation of the aqueous solution of cobalt salts into porous oxide supports such as SiO2 or Al2O3 is widely investigated as a convenient and practical method for preparing cobalt-based FT catalysts, and cobalt oxides are generally formed after the drying and calcination.3,5-9 However, the extent of reduction of cobalt over these catalysts is usually low due to strong cobalt-support interactions.5,7-9 3



Thus, increasing the extent of reduction and the dispersion of cobalt has long been a themed trend for a higher specific activity of cobalt for FT synthesis, and different strategies have been proposed and implemented. As indicated in many works,6,9-11 the introduction of noble metals such as Pt can facilitate the reduction of cobalt oxide. Moreover, the pretreatment of the precursor of the Co-based catalyst with the glow-discharge plasma of nitrogen and hydrogen can also improve the reducibility of the cobalt oxide.12 Alternatively, the co-impregnation of the sucrose as a chelating agent with precursors of cobalt and promoter into silica leads to Re- or Ru-promoted Co-based FT catalysts with an enhanced reducibility of cobalt oxides.6 By using polyvinyl pyrrolidone as a precursor, the core-shell catalyst based on the carbon modified mesoporous silica (Co3O4@C-m-SiO2) shows clearly higher reducibility of Co3O4 than that of the core-shell Co3O4@m-SiO2 without the carbon modification, and the reducibility is increased with increasing the carbon content over the catalyst due to weak interactions between cobalt and carbon.13 However, these methods suffer from either using the expensive noble metals or a rather complex procedure. Thus, less expensive and more facile methods are still desirable. In fact, regardless of the thus mentioned methods, cobalt oxides are exclusively formed, and the sufficient reduction of the catalyst at a temperature much higher than the FT reaction (~ 200 °C difference) is still required before the FT reaction.4,6-10,12,13 Thus, if the Co0 is directly formed during the preparation of the catalyst, the limitation originated from the high-temperature reduction step can be eliminated, and the catalyst with a highly dispersed Co0 can be reasonably expected. In this aspect, a surface impregnation combustion method by using citric acid as both a chelating agent 4


Page 4 of 38

Page 5 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and a reductant has recently been developed by Tsubaki and coworkers for the preparation of Co/SiO2 FT catalyst without further reduction.14-16 The reduction extent of the as-prepared Co/SiO2 is increased from 32 to 95% when the molar ratio of citric acid to the Co precursor is increased from 0.5 to 1.5, and the CO conversion over the optimal as-prepared catalyst (47.2%) is much higher than that over the reduced Co/SiO2 (16.7%) at 350 °C for 10 h.14 However, to prevent the oxidation of the catalyst after exposing in air, the as-prepared catalyst after combustion in Ar must be passivated with the diluted oxygen.14-16 Alternatively, by loading cobalt oxide on the nitrogen-doped carbon spheres (N-CSs), it is found that the cobalt oxide can be readily reduced by the N-CSs when it is treated in Ar at or above 480 oC.17 Under the same FT conditions, the Co/N-CSs obtained via high-temperature treatment in Ar shows much higher CO conversion than that treated in H2. The passivation of the as-prepared catalyst is not necessary, which may be due to the auto-reduction phenomenon in the presence of amorphous carbon over the catalyst.17,18 Based on these analyses, in this work, a simple impregnation-carbonation method by using glucose as both a reductant and carbon precursor was developed to prepare a directly reduced and highly dispersed metallic Co-C/SiO2 catalyst. After carbonization in an inert atmosphere, carbon was introduced into the catalyst, and the cobalt species were simultaneously reduced into the metallic Co. The as-prepared catalyst without further reduction and passivation was sufficiently active for FT synthesis. Under the same FT reaction conditions, the optimal catalyst showed almost doubled CO conversion than the impregnated Co/SiO2 catalyst after reduced at 400 oC. The use of the directly reduced Co-based catalyst without further high-temperature reduction is promising for the development of a more efficient FT process. 5



2 Experimental 2.1 Catalyst preparation SiO2 (Q15, Fujisilica) was degassed at 70 °C for 4 h on a rotary evaporator. After this, the temperature was cooled down to 30 °C. Subsequently, the aqueous solution of cobalt nitrate (the mass ratio of metallic cobalt to Q15 was 0.1) and the desired amount of the glucose (D-glucose, AR grade, Tianjin Kermel Chemical Regent Co. Ltd.) was slowly added under vacuum. After drying at 120 oC, the carbonization of the samples was carried out in a tubular furnace at 650 °C for 4 h with a heating rate of 2 °C·min-1 under Ar flow. After cooling to the ambient temperature, the Co-supported catalysts were obtained, and labeled as Co-xC/Q15, where x is the mass ratio of glucose to Q15 (x=0, 0.1, 0.5, 1.0, and 1.5, respectively). 2.2 Characterization techniques To confirm the formation of Co-C/Q-15 catalyst, the Co-1.5C/Q15 before the carbonization (ca. 100 mg) was loaded in the tube on a Micromeritics Autochem 2920 equipment. After purging in a flow of an Ar at 50 oC for 30 min, it was heated from 50 to 900 oC, and the tail gas was monitored by a thermal conductivity detector (TCD) and mass spectrometer (MS), respectively. The textural properties of the samples were determined by N2 physisorption at -196 °C using a BelSorp-Max (Bel Japan Inc.). Before the physisorption, the samples (ca. 1000 mg) were degassed at 350 °C for 5 h. The pore size distribution was established from the adsorption branch of the isotherm using the Barret-Joyner-Halender (BJH) method.8 X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance X-ray diffractometer with the monochromatised Cu/Kα radiation (40 kV, 40 mA). The samples were scanned from 5 to 6


Page 6 of 38

Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70o (2θ) with a step size of 0.02o and a counting time of 0.2 s per step. The hydrogen temperature-programmed reduction (H2-TPR) was conducted on a Micromeritics Autochem 2920 device. Prior to each test, the catalyst (ca. 50 mg) was purged in a flow of argon at room temperature for 30 min. Then 10 vol.% hydrogen in argon stream was introduced through the catalyst bed with a flow rate of 30 cm3·min-1 while the temperature was increased to 900 °C at a heating ramp of 10 °C·min-1. The hydrogen consumption and the released gases were monitored and analyzed by a TCD and MS, respectively. The hydrogen temperature-programmed desorption (H2-TPD) was performed also using the Micromeritics Autochem 2920 device to estimate the metallic Co dispersion. Firstly, the Co/Q15 catalyst (ca. 100 mg) was reduced in a flow of high-purity hydrogen (30 cm3·min-1) at 400 °C for 4 h. Then, it was cooled to 70 °C in the same gas flow. Secondly, the hydrogen was switched to argon at the same temperature, and the flow was kept for about 1 h to remove the physically adsorbed hydrogen. Finally, the H2-TPD was performed from 70 to 400 oC under Ar flow (30 cm3·min-1), and the desorbed hydrogen was monitored by a TCD. In the cases of the Co-C/Q15 catalysts, the sample (ca. 100 mg) was directly heated to 70 oC in a high-purity hydrogen (30 cm3·min-1), and was purged for 1 h. After that, the same procedure as that of Co/Q15 was applied for the H2-TPD experiments. To calculate the dispersion (D %) of cobalt over the catalyst, the atomic ratio of the adsorbed H to Co is assumed to be 1. Based on the dispersion and a spherical uniform model of the metallic cobalt, the surface average crystallite size of the metallic Co (d(Co0)) was calculated as explained in detail in the referenced work.8 The content of carbon over all of the catalysts was measured with a Vario EL III automatic 7



elemental analyzer. TEM observations were performed on a JEM 2100 electron microscope (JEOL, Japan) operated at 200 kV. The powdered sample was ultrasonically dispersed in ethanol and deposited on a copper grid prior to the measurements. 2.3 FT reaction The catalytic experiments were carried out in a stainless steel tubular reactor (i.d. = 10 mm) under the conditions of 235 oC, 1.0 MPa, and the ratio of the amount of the catalyst (g) to the flow rate of the syngas (W/F) = 5.0 g·h·mol-1. The flowchart of the reaction setup is shown in Figure 1. In the reaction system, the line tube between the outlet of the reactor and ice-water trap was heated at 180 °C to avoid the potential condensation of heavy FT hydrocarbons. For each test, 0.5 g of the catalyst (40-60 mesh) was diluted with the same quantity and size of quartz beads. In the case of Co/Q15 catalyst, a pre-reduction was performed at 400 °C for 4 h in a pure H2 flow of 50 cm3·min-1. In the cases of Co-C/Q15, the catalyst without any pre-treatments was directly applied for FT synthesis. The syngas (H2/CO molar ratio of 2) containing 4% Ar as an internal standard was used for all of the tests. The effluents after cooling in an ice-water trap were analyzed on an online GC (GC-9560, Shanghai Huaai Chromatographic Analysis Co., Ltd., China). An alumina capillary column (0.20 mm × 50 m, 0.5 µm) was used for separating the hydrocarbons, and determined by a flame ionization detector (FID). The CO, CH4, Ar, and CO2 were analyzed on a TCD with a packed activated-carbon column (3 mm × 2 m). The selectivity of different hydrocarbons was calculated on the basis of the carbon number. The cobalt weight normalized activity, i.e., cobalt-time-yield (CTY, molCO·g-1Co·s-1), was calculated as the converted CO molecule per second divided by the total amount of the cobalt over the catalyst loaded in the reactor.11 8


Page 8 of 38

Page 9 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 Results and discussion 3.1 Formation of the metallic Co-C/SiO2 To understand the mechanism for the direct formation of the metallic Co-C/Q15 catalysts, the heating of the Co-1.5C/Q15 catalyst precursor, i.e., the Co-1.5C/Q15 before the carbonization, in an Ar flow from 50 to 900 oC was monitored by a TCD and MS, and the results are given in Figure 2. From Figure 2A, three TCD peaks at about 83, 264 and 452 oC were clearly seen, indicating that gaseous products are released during the carbonization process. The gaseous products determined by MS (Figure 2B) were hydrogen, methane, water, and carbon dioxide with a mass charge ratio (m/e) of 2, 16, 18, and 44, respectively. Moreover, the small peak (m/e = 18) at the peak maximum of about 86 oC can reasonably be ascribed to the physically adsorbed water over the sample. Following the MS results, the TCD peak at about 83 oC was due to the physically adsorbed H2O, and the TCD peak at about 264 oC was contributed mainly from the produced H2O, CO2, and CH4. Moreover, the TCD peak at the highest temperature was mainly originated from H2 besides the possible contribution of the H2O, CO2, and CH4. Thus, the gaseous products generated during heating the catalyst precursor were clearly determined. To shed further lights on the origin of released gases and the formation of carbon, the results in Figure 2 were examined in detail. As indicated in Figure 2, the starting temperatures for the MS peak assigned to methane, water, and carbon dioxide were much similar (~190 oC), indicating that the water, carbon dioxide, and methane are evolved almost simultaneously. However, at a much higher temperature of about 380 oC, hydrogen was started to be released, indicating the much delayed formation of hydrogen. By comparing the integrated peak area shown in Figure 2B, the 9



Page 10 of 38

total amount of water or carbon dioxide released was much larger than that of hydrogen or methane. Thus,

both

the

dehydration

(C6H12O6→6C+6H2O)

and

the

reforming

(C6H12O6+6H2O→12H2+6CO2) reactions of the glucose occurred, which is consistent with the results for the pyrolysis or reforming of sugars.19,20 Besides these reactions, the formation of methane indicates that the decomposition of the glucose or the partially dehydrated glucose also occurred, which has been observed for the pyrolysis of sucrose.19 These observations were also supported by the carbon content over the carbonized catalysts (Table 1), which is measured by elemental analyses. Taking the Co-1.5C/Q15 catalyst as an example, the calculated carbon content was 33 wt.% based on the stoichiometric dehydration of the glucose into C and H2O while a much lower carbon content of 18.9 wt.% was measured. When the reduction of cobalt oxides was concerned, the amount of the hydrogen produced can be roughly determined if only the dehydration and reforming reactions of the glucose are taken into account. In this case, considering the smallest amount of the added glucose, Co-0.1C/Q15 was used as an example to estimate the possible reduction of Co3O4 into Co0. Based on the carbon content over the catalyst given in Table 1, about 0.4 mole of hydrogen was produced by carbonizing 100 g of the catalyst, which is doubled the stoichiometric amount of hydrogen for the reduction of the Co3O4 over the catalyst to Co0. Thus, in our case, the large amount of reducing gases such as hydrogen produced during the carbonization process is sufficient for the reduction of cobalt oxides into Co0 provided a sufficiently high kinetics. In the following, the characteristics of the carbonized Co-C/Q15 catalysts with different mass ratios of glucose to silica will be discussed. 3.2 Crystal characteristics 10


Page 11 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The XRD patterns of the carbonized catalysts are presented in Figure 3. In the case of Co/Q15, intense diffractions indexed to the cubic Co3O4 were clearly observed besides the broad peak due to the amorphous silica at about 22o (2θ). Moreover, big particles of Co3O4 were expected from the sharp diffractions. In contrast, diffractions of the cubic Co3O4 over all of the Co-C/Q15 catalysts were disappeared although the diffraction of the amorphous silica was still kept almost the same. Moreover, no diffractions due to the carbon derived from the glucose were found even for the Co-1.5C/Q15 having the highest carbon content of 18.9 wt.% (Table 1). This indicates that the carbon formed during the carbonization process should be amorphous. Instead, broad diffractions due to CoO or metallic Co were observed over all of the Co-C/Q15 catalysts, indicating the reduction of the cubic Co3O4 to either CoO or metallic Co. This is well agreeable with the results discussed in Section 3.1. Thus, as a result of the released hydrogen and methane during the carbonization process, the cobalt species were reduced into CoO or metallic Co, which is expectedly dependent on the amount of the glucose added, i.e., the glucose to Q15 ratio, since the amount of the cobalt precursor was kept the same for all of the catalysts. Indeed, in the case of the Co-0.1C/Q15 with the smallest amount of the glucose used, broad diffractions assigned to CoO were clearly observed while those due to the Co3O4 phase were completely disappeared. With increasing the amount of the glucose added during the preparation of catalysts, the peak intensity for CoO gradually decreased while broad and weak diffractions of the metallic Co became more pronounced. When the ratio of the glucose to Q15 increased to be 1.5, only broad and weak diffractions of the metallic Co were observed. Considering the gradual releasing of the reducing gases and the simultaneous formation of the carbon, the migration and sintering of cobalt species 11



can be hindered. Thus, during the carbonization, the catalyst was completely reduced if the released reducing gases were sufficient, and the well dispersed small Co0 particles were expected as revealed from broad diffractions of the metallic cobalt. 3.3 Textural properties The N2 adsorption-desorption isotherms of the Q15 and Co-supported catalysts are given in Figure 4. In the case of Q15, a typical type IV (a) isotherm together with a steep H1 hysteresis loop at relative pressures of 0.8-0.9 was observed according to the IUPAC classification.21 After loading 10 wt.% cobalt (Co/Q15), a very similar isotherm to that of the Q15 was obtained. However, the amount of the adsorbed N2 was significantly decreased, indicating a decreased pore volume. In comparison with the Co/Q15, Co-C/Q15 catalysts showed very similar adsorption isotherms and almost the same amount of the adsorbed N2. However, with increasing the carbon content over the catalyst, i.e., from Co-0.1C/Q15 to Co-1.5C/Q15, the lower relative pressure for the hysteresis loop was continuously shifted from about 0.65 to 0.44, leading to the gradual transition of the hysteresis loop from H1 to H4. This indicates the pore blocking in a wide range of pore necks and the filling of micropores as discussed in the reference,21 which can further be revealed from the pore size distribution shown in Figure 5. In the case of Q15, a broad pore size distribution centered at about 24 nm was observed. After loading 10 wt.% Co, a clear decrease in the peak pore size (~14 nm) was obtained. Moreover, the obvious decrease in dv/dw indicated a decreased pore volume, which is clearly reflected from the data given in Table 1. For the catalyst with the smallest amount of carbon, i.e., Co-0.1C/Q15, a very similar pattern of the pore size distribution to that of Co/Q15 was observed, indicating that the changing in the porous property of Co/Q15 is very limited after 12


Page 12 of 38

Page 13 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


introducing a small amount of carbon. This observation was directly seen from the results of the BET surface area, pore volume, and average pore size as given in Table 1. A further increase of the carbon content over the catalyst, i.e., Co-0.5C/Q15, led to a slight decrease in the peak pore size (~12 nm) and a broader pore size distribution (curve d in Figure 5). When the carbon content was further increased (Co-1C/Q15), the peak was disappeared, and a very broad pore size distribution from 2 to 40 nm was observed with the increased proportion of the smaller pores (curve e in Figure 5). In the case of the Co-1.5C/Q15 with the highest amount of carbon, very similar pattern to that of Co-1C/Q15 was obtained. Thus, after introducing cobalt and the increased amount of carbon, the pore size distribution generally became broader with an increased proportion of the smaller pores. These results suggest that the introduced cobalt and carbon may mainly be located in the inner pores of the Q15. This was further supported by the increased volume of the adsorbed N2 with increasing the content of carbon, which can be clearly seen by comparing the isotherms of different samples in the range of p/p0 < 0.1. As discussed in Section 3.1, reducing gases were released accompanied with the formation of the carbon and the reduction of cobalt oxides, leading to the creation of micropores. As a result, the average pore size of the sample was decreased, which is directly reflected from the data in Table 1. Moreover, with increasing the carbon content from Co-0.1C/Q15 to Co-1.5C/Q15, the pore volume was only slightly decreased while the average pore size was obviously decreased, leading to the increased BET surface area. 3.4. Particle size and dispersion of cobalt As discussed in Section 3.2, well dispersed small Co particles can be suggested from the weak and broad XRD diffractions of Co-C/Q15 catalysts (Figure 3). To confirm this, typical samples 13



were applied for TEM observations, and the results are given in Figure 6. In the case of Co/Q15, big Co3O4 particles were clearly seen, and the size distribution in the range of 40 to over 100 nm was obtained, which are well agreeable with the intense XRD diffractions (Figure 3). In contrast, much smaller particles of cobalt species (less than 15 nm) with a narrowed size distribution was observed for Co-1C/Q15 and Co-1.5C/Q15. Moreover, Co-1C/Q15 showed a slightly smaller peak size but a marginally wider size distribution. Thus, the introduction of the glucose with the glucose to silica ratios of 1 and 1.5 can unambiguously decrease the particle size and its distribution of cobalt over silica. As discussed in Section 3.3, the introduced carbon and cobalt species were mainly deposited in the inner pore wall of Q15, led to a decreased pore size with increasing the content of carbon (Figure 5 and Table 1). As a result of the limitation of the pores, the cobalt species over Co-C/Q15 were confined to smaller sizes, which is agreeable with the results reported by Khodakov et al.22 It is commonly known that the interaction between Co and silica is much stronger than that between Co and carbon. Thus, considering the high-temperature carbonization process, slightly bigger particles of cobalt species were formed if the amount of the introduced carbon was high. This was exactly the case for Co-1.5C/Q15, which gives slightly bigger cobalt particles than Co-1C/Q15. To further insight into the impact of the introduced carbon on the dispersion of cobalt, H2-TPD was performed for all of the catalysts. In the cases of Co-C/Q15 catalysts, the H2-TPD experiment was directly carried out. For Co/Q15, prior to the H2 chemisorption, it was reduced at 400 oC for 4 h in a flow of a pure H2. From the results given in Table 2, the total H2 uptake was decreased in the order of Co-0.5C/Q15 > Co-1C/Q15 > Co-1.5C/Q15 >> Co/Q15, leading to the same changing pattern for the dispersion of Co over different catalysts. Thus, the introduction of a small amount of 14


Page 14 of 38

Page 15 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


carbon can sharply improve the dispersion of Co over silica. When the Co particles were concerned, the surface average crystallite size of the metallic Co was calculated from the dispersion and a uniform spherical model. The reduced Co/Q15 had Co0 crystals of about 36 nm. In contrast, the Co0 crystals over Co-C/Q15 were only about 3 to 5 nm, which is slightly dependent on the carbon content over the catalyst (Table 2). Considering experimental errors, these results were well consistent to the TEM observations (Figure 6). 3.5 Reduction behavior As discussed in Section 3.1, the simultaneous introduction of glucose and cobalt nitrate into silica followed by the carbonization in an inert gas led to the formation of the reduced Co-C/Q15. Moreover, the cobalt species over the catalysts, i.e., Co3O4, CoO, and Co0 as revealed from XRD (Figure 3), were strongly dependent on the amount the glucose used during the preparation of the catalysts. Thus, to further investigate the chemical state of cobalt species over different catalysts, H2-TPR was carried out, and the results monitored by TCD and MS are given in Figures 7 and 8, respectively. In the case of Co/Q15, a big peak centered at about 350 oC together with a shoulder at about 325 oC was obtained (Figure 7). This can be reasonably attributed to the two-step reduction of the big particles of Co3O4 to Co0 via the CoO intermediate as discussed in our previous work.23 This was further confirmed from the MS signal, which only H2O at an m/e of 18 was detected (Figure 8A) and no methane was formed (Figure 8B). Moreover, the tiny reverse peak at about 90 oC for all of the samples (Figure 7) was revealed to be water from the MS profiles (Figure 8), which can be attributed to the physically adsorbed water. In the cases of Co-C/Q15 catalysts, the most obvious observation was the significantly 15



decreased peak intensity in comparison with Co/Q15 (Figure 7), which indicates the decreased H2 consumption or the less reducible species over the catalysts. In comparison with Co/SiO2, however, a new broad peak centered at about 550 oC was observed (Figure 7). As revealed from Figure 8B, this was mainly contributed from methane at an m/e of 16. By comparing the results in Figures 7 and 8, it can be concluded that the amount of cobalt oxides was significantly decreased with increasing the carbon content over Co-C/Q15. Moreover, as shown in Figure 8A, the peak tail at a high temperature also became less significant with increasing the carbon content over the catalysts, indicating the decreased amount of the difficultly reducible cobalt oxides. These observations were well consistent with the XRD results. Noteworthy, the small peak centered at about 300 oC for Co-1.5C/Q15 (Figure 8A) may be due to the reduction of the surface cobalt oxides based on (1) only metallic Co is revealed from XRD; (2) the easy potential oxidation of the highly dispersed Co0 under atmospheric conditions. In addition, a small tail peak at about 450 oC for Co-0.5C/Q15 (Figure 8A) may be due to the small particles of cobalt oxides as revealed from Figure 6, which is believed to be reduced at a higher temperature.22,24 Thus, consistent with the XRD results, the introduction of glucose led directly to the reduced Co-C/Q15, and the extent of reduction of cobalt oxides depends on the amount of the glucose introduced. As indicated in Figure 8B, there was no methane detected over the Co/Q15 during the H2-TPR process. In contrast, methane was clearly formed over all of the Co-C/Q15 catalysts under the same conditions. Moreover, the area by integrating the peak assigned to the methane in Figure 8B was increased with increasing the carbon content over the catalyst, i.e., 1.16, 1.93, and 2.32 for Co-0.5C/Q15, Co-1C/Q15, and Co-1.5C/Q15, respectively, indicating a positive correlation 16


Page 16 of 38

Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


between the carbon content over the catalyst and the amount of the released methane during the H2-TPR. Thus, the formation of methane was reasonably attributed to the presence of carbon over the catalyst. In the case of the Co-0.5C/Q15, the peak temperatures for the formation of methane were about 360 and 590 oC, respectively. However, the same peak temperatures of 300 and 590 oC were obtained for Co-1C/Q15 and Co-1.5C/Q15. This may be due to the carbon existed in different manners over the catalysts. As discussed in Section 3.2, only amorphous carbon was revealed from XRD results (Figure 3), which may favor the formation of methane. However, the details need to be further studied. 3.6 FT performance Based on the characterization results, the Co-C/Q15 catalysts without any pre-reduction were directly applied for FT synthesis while the Co/Q15 catalyst was reduced at 400 oC for 4 h before the FT reaction. The calculated CTY as a function of time on stream (TOS) is shown in Figure 9, and the main results at a TOS of 10 h are summarized in Table 3. As shown in Figure 9, all of the catalysts showed stable FT performance, and the activity indexed by CTY at TOS of 10 h was increased in the order of Co-0.1C/Q15 < Co/Q15 < Co-1.5C/Q15 < Co-0.5C/Q15 < Co-1C/Q15. The lowest CTY of 1.3×10-5 molCO·g-1Co·s-1 was obtained over Co/0.1C-Q15, which is significantly lower than other catalysts. This can be easily explained as that the cobalt species over the as-prepared Co/0.1C-Q15 were mainly CoO (Figure 3), which is not active for the FT reaction. Thus, in comparison with the reduced Co/Q15, the as-prepared Co-C/Q15 without further reduction was more active for FT synthesis when the mass ratio of the glucose to the support was 0.5 or larger. Noteworthy, the Co-C/Q15 catalyst was not 17



passivated with the diluted oxygen after the carbonization, and its storage and transfer to the reactor were under atmospheric conditions. Thus, the re-oxidation of the metallic cobalt over the Co-C/Q15 by the air was negligible, which can be explained as the auto-reduction phenomenon by the amorphous carbon suggested in previous works.17,18 Moreover, the FT activity of the Co-C/Q15 was not proportional to the ratio of the glucose to the support, and the highest CTY of 3.5×10-4 molCO·g-1Co·s-1 was obtained over Co-1C/Q15 (Figure 9 and Table 3). This may be explained based on the dispersion and the size effect of Co0. As reported in many works, the CTY of the FT synthesis over Co-based catalysts was first increased and then decreased with decreasing the size of Co0, and the maximum CTY was found at a Co0 size of about 4-6 nm.25,26 Taking the size effect of cobalt into account, the clearly smaller CTY over Co/Q15 than those over Co-0.5C/Q15, Co-1C/Q15, and Co-1.5C/Q15 can be reasonably ascribed to its much larger Co0 particles (Table 2 and Figure 6). However, the Co-0.5C/Q15, which has comparable size of Co0 particles with Co-1C/Q15, exhibited a clearly lower CTY (2.93 vs. 3.50×10-4 molCO·g-1Co·s-1). This may be due to the incomplete reduction of Co-0.5C/Q15 during the carbonization as a result of a smaller amount of the glucose used. To confirm this, the as-prepared Co-0.5C/Q15 was reduced at 400 oC for 4h, and was labeled as Co-0.5C/Q15-R. As indicated in Figure 9 and Table 3, the TOS activity and the CTY over Co-0.5C/Q15-R was almost the same as that over Co-1C/Q15, which well supports our suspect. Moreover, this further proved that the cobalt species could not be completely reduced when the ratio of glucose to Q15 was 0.5, which is agreeable with the characterization results of the catalyst. In the case of the lower CTY over Co-1.5C/Q15 (2.23×10-4 molCO·g-1Co·s-1) than that over Co-1C/Q15, it can be reasonably ascribed to its clearly lower dispersion of Co0 (18.7 vs. 28.5%, 18


Page 18 of 38

Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2). The results of the product distribution over different catalysts are summarized in Table 3. Irrespective of the amount of the carbon introduced into Co/Q15, less than 1% selectivity of CO2 was obtained, which is consistent to the low activity of cobalt for WGS reaction.4 Moreover, the impact of the carbon on the activity of Co/Q15 for WGS reaction was negligible. In the case of the selectivity of hydrocarbons, a sharply increased selectivity of light hydrocarbons over Co-C/Q15 was generally observed in comparison with the Co/Q15. Specifically, the selectivity of methane over Co-C/Q15 (~32 - 43%) was much higher than that over Co/Q15 (~11%), and the highest methane selectivity of 43.08% was obtained over Co-1C/Q15 (Table 3). Moreover, in comparison with the selectivity of C2-C4 hydrocarbons over Co/Q15 (~6%), it was around 15% over Co-C/Q15 catalysts, the extent of which is almost independent on the carbon content over the catalyst. Correspondingly, the lowest selectivity of the C5+ hydrocarbons was obtained over the Co-1C/Q15 (39.97%). Thus, the one-step synthesized Co-C/Q15 showed a significantly varied product distribution in comparison with the high-temperature reduced Co/Q15, and the product distribution can be easily regulated via changing the ratio of the glucose to silica during the impregnation. Irrespective of the supports, smaller Co0 particles are generally favorable for the formation of light hydrocarbons while larger Co0 particles are advantageous for the production of C5+ heavier hydrocarbons25,27-29 although the exact critical size of Co0 particles is not unambiguously determined. By correlating the results in Figure 6 and Table 2 with the data in Table 3, the much larger size of Co0 particles over Co/Q15 than that over Co-C/Q15 can be explained as one of the reasons for the higher selectivity of light hydrocarbons over Co-C/Q15. 19



However, if the selectivity of methane over the Co-0.5C/Q15, Co-1C/Q15, and Co-1.5C/Q15 was compared, the results cannot be explained based only on the crystal size of Co0, in which the reason should be further explored. In fact, it is commonly known that the diffusivity of H2 in micropores is much higher than that of CO, and a higher H2/CO molar ratio over the catalyst pores always leads to a higher selectivity of light hydrocarbons, especially methane.15,30,31 When the textural properties of the catalysts (Figure 5 and Table 1) were examined, the average and peak pore size was decreased in the order of Co/Q15 > Co-0.5C/Q15 > Co-1C/Q15, which matches well with the changing pattern for the selectivity of methane, i.e., a higher selectivity of methane over the catalyst with a smaller pore size. Thus, the diffusional effect of H2 and CO in the micropores of the catalyst can be another reason for the higher selectivity of methane over Co-1C/Q15 than that over Co-0.5C/Q15 or Co/Q15. Following this explanation, an exception occurred for the Co-1.5C/Q15, which shows a lower selectivity of methane than Co-1C/Q15, although the average and peak pore size over the two catalysts was comparable (Figure 5 and Table 1). It has been reported that the formation of methane is enhanced with increasing the proportion of cobalt oxides over the reduced cobalt-based FT catalyst.22 In this case, if the contribution of cobalt oxide is considered, the higher selectivity of methane over Co-1C/Q15 than that over Co-1.5C/Q15 can be explained since more cobalt oxides are present over Co-1C/Q15 as revealed from Section 3.5. This is further supported from the much higher selectivity of methane over Co-0.5C/Q15 (32.78%) than that over the reduced Co-0.5C/Q15 (Co-0.5C/Q15-R, 24.64%). Thus, the selectivity of light hydrocarbons over Co-C/Q15 can be well explained based on the properties of the crystal size of Co0, the micropores, and the presence of 20


Page 20 of 38

Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cobalt oxides. Noteworthy, the Co-C/Q15 catalysts showed much higher selectivity of methane than the Co/Q15 (Table 3). This observation is agreeable with the result over Co/carbon materials for FT synthesis, which has been explained as the more pronounced difference for the diffusivity of H2 and CO in the micropores of amorphous carbon.14,32 4 Conclusions In summary, the co-impregnation followed by the carbonization was successfully developed for the preparation of the directly reduced Co-xC/SiO2. Without any high-temperature reduction and passivation, the as-prepared Co-xC/SiO2 could be an efficient catalyst for FT synthesis, and the mass ratio of the glucose to SiO2 was revealed to be the critical factor in determining the activity and product distribution of the catalyst for FT synthesis. The reducing gases of H2 and CH4 released during the carbonization, the amount of which is dependent on the added glucose, were responsible for the formation of the reduced Co-xC/SiO2. Moreover, the proportion of Co0 and amorphous carbon over Co-xC/SiO2 was increased with increasing the mass ratio of the glucose to SiO2. When the mass ratio was increased from 0.1 to 1.5, the CTY of the Co-xC/SiO2 was first increased and then decreased. The Co-1C/SiO2 showed the highest CTY of 3.5×10-4 molCO·g-1Co·s-1, which is much higher than the high-temperature reduced Co/SiO2 (1.75×10-4 molCO·g-1Co·s-1). Moreover, the selectivity of light hydrocarbons (C1~C4) over Co-xC/SiO2 was clearly higher than than that over the high-temperature reduced Co/SiO2, and the product distribution can be regulated via simply changing the mass ratio of the glucose to SiO2 during the impregnation step. The highest activity of the Co-1C/SiO2 for FT reaction was reasonably explained as the high dispersion and the smaller 21



size of Co0 particles (less than 15 nm) over the catalyst. Moreover, the smaller Co particle size, the diffusional effect of CO and H2 in the carbon-containing micropores, and the presence of cobalt oxides over the catalysts were responsible for the higher selectivity of light hydrocarbons over the Co-xC/SiO2 catalysts. The simple procedure, the cheap glucose, and the avoidable high-temperature reduction make Co-xC/SiO2 a very promising catalyst for the development of a more efficient FT process. Acknowledgements The financial supports by the National Natural Science Foundation of China (21376146 and 21636006) and the Fundamental Research Funds for the Central Universities (GK201601005) are highly appreciated. Dr. Y.-H. Zhao thanks the Foundation for the Excellent Doctoral Dissertation of Shaanxi Normal University (X2011YB06). Literature Cited (1) Dry, M. E. The Fischer-Tropsch Process: 1950-2000. Catal. Today 2002, 71, 227. (2) Zhang, Q.; Kang, J.; Wang, Y. Development of Novel Catalysts for Fischer-Tropsch Synthesis: Tuning the Product Selectivity. ChemCatChem 2010, 2, 1030. (3) Khodakov, A. Y.; Chu, W.; Fongarland, P. Advances in the Development of Novel Cobalt Fischer-Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels. Chem. Rev. 2007, 107, 1692. (4) Wang, G. W.; Hao, Q. Q.; Liu, Z. T.; Liu, Z. W. Fischer-Tropsch Synthesis over Co/Montmorillonite-Insights into the Role of Interlayer Exchangeable Cations. Appl. Catal. A: Gen. 2011, 405, 45. 22


Page 22 of 38

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5) Sun, S.; Tsubaki, N.; Fujimoto, K. The Reaction Performances and Characterization of Fischer-Tropsch Synthesis Co/SiO2 Catalysts Prepared from Mixed Cobalt Salts. Appl. Catal. A: Gen. 2000, 202, 121. (6) Girardon, J. S.; Quinet, E.; Griboval-Constant, A.; Chernavskii, P. A.; Gengembre, L.; Khodakov, A. Y. Cobalt Dispersion, Reducibility, and Surface Sites in Promoted Silica-Supported Fischer-Tropsch Catalysts. J. Catal. 2007, 248, 143. (7) Ma, W.; Jacobs, G.; Sparks, D. E.; Gnanamani, M. K.; Pendyala, V. R. R.; Yen, C. H.; Klettlinger, J. L. S.; Tomsik, T. M.; Davis, B. H. Fischer-Tropsch Synthesis: Support and Cobalt Cluster Size Effects on Kinetics over Co/Al2O3 and Co/SiO2 Catalysts. Fuel 2011, 90, 756. (8) Hao, Q. Q.; Zhao, Y. H.; Yang, H. H.; Liu, Z. T.; Liu, Z. W. Alumina Grafted to SBA-15 in Supercritical CO2 as a Support of Cobalt for Fischer-Tropsch Synthesis. Energy Fuels 2012, 26, 6567. (9) Shimura, K.; Miyazawa, T.; Hanaoka, T.; Hirata, S. Fischer-Tropsch Synthesis over Alumina Supported Cobalt Catalyst: Effect of Promoter Addition. Appl. Catal. A: Gen. 2015, 494, 1. (10) Kogelbauer, A.; Goodwin, J. G.; Oukaci, J. R. Ruthenium Promotion of Co/Al2O3 Fischer-Tropsch Catalysts. J. Catal. 1996, 160, 125. (11) Tsubaki, N.; Sun, S.; Fujimoto, K. Different Functions of the Noble Metals Added to Cobalt Catalysts for Fischer-Tropsch Synthesis. J. Catal. 2001, 199, 236. (12) Chu, W.; Wang, L. N.; Chernavskii, P. A.; Khodakov, A. Y. Glow-Discharge Plasma-Assisted Design of Cobalt Catalysts for Fischer-Tropsch Synthesis. Angew. Chem. 2008, 120, 5130. (13) Xie, R. Y.; Wang, H.; Gao, P.; Xia, L.; Zhang, Z. Z.; Zhao, T. J.; Sun, Y. H. Core@Shell 23



Co3O4@C-m-SiO2 Catalysts with Inert C Modified Mesoporous Channel for Desired Middle Distillate. Appl. Catal. A: Gen. 2015, 492, 93. (14) Shi, L.; Tao, K.; Kawabata, T.; Shimamura, T.; Zhang, X. J.; Tsubaki, N. Surface Impregnation Combustion Method to Prepare Nanostructured Metallic Catalysts without Further Reduction: As-Burnt Co/SiO2 Catalysts for Fischer-Tropsch Synthesis. ACS Catal. 2011, 1, 1225. (15) Shi, L.; Zeng, C.; Lin, Q.; Lu, P.; Niu, W.; Tsubaki, N. Citric Acid Assisted One-Step Synthesis of Highly Dispersed Metallic Co/SiO2 without Further Reduction: As-Prepared Co/SiO2 Catalysts for Fischer-Tropsch Synthesis. Catal. Today 2014, 228, 206. (16) Shi, L.; Jin, Y.; Xing, C.; Zeng, C.; Kawabata, T.; Imai, K.; Matsuda, K.; Tan, Y.; Tsubaki, N. Studies on Surface Impregnation Combustion Method to Prepare Supported Co/SiO2 Catalysts and Its Application for Fischer-Tropsch Synthesis. Appl. Catal. A: Gen. 2012, 435-436, 217. (17) Xiong, H.; Moyo, M.; Rayner, M. K.; Jewell, L. L.; Billing, D. G.; Coville, N. J. Autoreduction and Catalytic Performance of a Cobalt Fischer-Tropsch Synthesis Catalyst Supported on Nitrogen-Doped Carbon Spheres. ChemCatChem 2010, 2, 514. (18) Cheng, K.; Subramanian, V.; Carvalho, A.; Ordomsky, V. V.; Wang, Y.; Khodakov, A. Y. The Role of Carbon Pre-Coating for the Synthesis of Highly Efficient Cobalt Catalysts for Fischer-Tropsch Synthesis. J. Catal. 2016, 337, 260.

(19) Chen, H.; Dou, B.; Song, Y.; Xu, Y.; Zhang, Y.; Wang, C.; Zhang, X.; Tan, C. Pyrolysis Characteristics of Sucrose Biomass in a Tubular Reactor and a Thermogravimetric Analysis. Fuel 2012, 95, 425. (20) Tanksale, A.; Wong, Y.; Beltramini, J. N.; Lu, G. Q. Hydrogen Generation from Liquid Phase 24


Page 24 of 38

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Catalytic Reforming of Sugar Solutions Using Metal-Supported Catalysts. Int. J. Hydrogen Energ. 2007, 32, 717. (21) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez-Reinoso, F.; Rouquerol, J. Sing, K. S. W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1051. (22) Khodakov, A. Y.; Griboval-Constant, A.; Bechara, R.; Zholobenko, V. L. Pore Size Effects in Fischer Tropsch Synthesis over Cobalt-Supported Mesoporous Silicas. J. Catal. 2002, 206, 230. (23) Hao, Q. Q.; Wang, G. W.; Liu, Z. T.; Lu, J.; Liu, Z. W. Co/Pillared Clay Bifunctional Catalyst for Controlling the Product Distribution of Fischer-Tropsch Synthesis. Ind. Eng. Chem. Res. 2010, 49, 9004. (24) Borg, Ø.; Eri, S.; Blekkan, E. A.; Storsæter, S.; Wigum, H.; Rytter, E.; Holmen, A. Fischer-Tropsch Synthesis over γ-Alumina-Supported Cobalt Catalysts: Effect of Support Variables. J. Catal. 2007, 248, 89. (25) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; Van Dillen, A. J.; De Jong, K. P. Cobalt Particle Size Effects in the Fischer-Tropsch Reaction Studied with Carbon Nanofiber Supported Catalysts. J. Am. Chem. Soc. 2006, 128, 3956. (26) Den Breejen, J. P.; Sietsma, J. R. A.; Friedrich, H.; Bitter, J. H.; De Jong, K. P. Design of Supported Cobalt Catalysts with Maximum Activity for the Fischer-Tropsch Synthesis. J. Catal. 2010, 270, 146. 25



(27) Tavasoli, A.; Sadagiani, K.; Khorashe, F.; Seifkordi, A. A.; Rohani, A. A.; Nakhaeipour, A. Cobalt Supported on Carbon Nanotubes—A Promising Novel Fischer-Tropsch Synthesis Catalyst. Fuel Process. Technol. 2008, 89, 491. (28) Fu, T.; lv, J.; Li, Z. Effect of Carbon Porosity and Cobalt Particle Size on the Catalytic Performance of Carbon Supported Cobalt Fischer-Tropsch Catalysts. Ind. Eng. Chem. Res. 2014, 53, 1342. (29) Den Breejen, J. P.; Radstake, P. B.; Bezemer, G. L.; Bitter, J. H.; Frøseth, V.; Holmen, A.; De Jong, K. P. On the Origin of the Cobalt Particle Size Effects in Fischer-Tropsch Catalysis. J. Am. Chem. Soc. 2009, 131, 7197. (30) Liu, Y.; Fang, K.; Chen, J.; Sun, Y. Effect of Pore Size on the Performance of Mesoporous Zirconia-Supported Cobalt Fischer-Tropsch Catalysts. Green Chem. 2007, 9, 611. (31) Tsubaki, N.; Zhang, Y.; Sun, S.; Mori, H.; Yoneyama, Y.; Li, X.; Fujimoto, K. A New Method of Bimodal Support Preparation and Its Application in Fischer-Tropsch Synthesis. Catal. Commun. 2001, 2, 311. (32) Zaman, M.; Khodadi, A.; Mortazavi, Y. Fischer-Tropsch Synthesis over Cobalt Dispersed on Carbon Nanotubes-Based Supports and Activated Carbon. Fuel Process. Technol. 2009, 90, 1214. .

26


Page 26 of 38

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Summary of the textural properties of the samples Samples

BET surface area (m2·g-1)

Pore volume (cm3·g-1)

Average pore size (nm)

C (wt.%)*

Co (wt.%)

Q15

229

1.30

23.0

-

-

Co/Q15 Co-0.1C/Q15 Co-0.5C/Q15 Co-1C/Q15 Co-1.5C/Q15

199 203 225 293 353

0.68 0.67 0.61 0.59 0.58

13.6 13.1 10.9 8.0 6.6

0.9 4.5 12.4 18.9

9.1 9.0 8.7 8.0 7.4

*: Determined by elemental analyses. Table 2 H2-TPD and peak area of MS signals during H2-TPR for different catalysts H2-TPD

H2-TPR 0

Catalysts

H2 uptake (µmol·g-1)

Dispersion (%)

d (Co ) (nm)

Peak area of H2O (a.u.)*

Peak area of CH4 (a.u.)**

Co/Q15 Co-0.5C/Q15 Co-1C/Q15 Co-1.5C/Q15

20.8 216.9 193.4 117.4

2.7 29.4 28.5 18.7

35.7 3.2 3.4 5.1

2.49 1.97 1.42 0.99

1.16 1.93 2.32

*: Obtained by integrating the MS peak of H2O (Figure 8A). **: Obtained by integrating the MS peak of CH4 (Figure 8B).

Table 3 Results of FT synthesis over different catalysts* Catalysts

Co/Q15 Co-0.1C/Q15 Co-0.5C/Q15 Co-1C/Q15 Co-1.5C/Q15 Co-0.5C/Q15-R

CO conversion (%) 28.97 2.19 46.24 50.89 30.00 56.56

CTY -4

-1

10 molCO·g

-1

Co·s

1.75 0.13 2.93 3.50 2.23 3.58

CO2 Selectivity (%) 0.14 0.29 0.53 0.22 0.51

Hydrocarbon distribution (%) CH4

C2~C4

C5+

11.22 32.78 43.08 38.05 24.64

6.07 16.64 16.95 15.44 14.87

82.71 50.58 39.97 46.51 60.49

*: Operating conditions: W/F=5.0 g·h·mol-1, P=1.0 MPa, T=235 oC, TOS=10 h.

27



Figure captions Figure 1 Flowchart of the reaction system for FT synthesis Figure 2 The TCD (A) and MS (B) profiles recorded during heating the Co-1.5C/Q15 catalyst precursor in an Ar atmosphere. Figure 3 XRD patterns of Co/Q15 (a), Co-0.1C/Q15 (b), Co-0.5C/Q15 (c), Co-1C/Q15 (d), and Co-1.5C/Q15 (e). Figure 4 N2 adsorption-desorption isotherms of Q15 (a), Co/Q15 (b), Co-0.1C/Q15 (c), Co-0.5C/Q15 (d), Co-1C/Q15 (e), and Co-1.5C/Q15 (f). Figure 5 Pore size distributions calculated by the BJH method using the adsorption branch for Q15 (a), Co/Q15 (b), Co-0.1C/Q15 (c), Co-0.5C/Q15 (d), Co-1C/Q15 (e), and Co-1.5C/Q15 (f). Figure 6 TEM images and the particle size distributions (Insets) of cobalt species over Co/Q15 (A), Co-1C/Q15 (B), and Co-1.5C/Q15 (C). Figure 7 H2-TPR profiles of Co/Q15 (a), Co-0.5C/Q15 (b), Co-1C/Q15 (c), and Co-1.5C/Q15 (d). Figure 8 The MS signals of H2O (A) and CH4 (B) during the H2-TPR for Co/Q15 (a), Co-0.5C/Q15 (b), Co-1C/Q15 (c), and Co-1.5C/Q15 (d). Figure 9 The TOS activity indexed by CTY for the FT synthesis over different catalysts.

28


Page 28 of 38

Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

3

5

4

p

1 6

H2

N2

CO + H2

p

7 9

TCD FID

GC

8

1. Pressure regulator 2. Stop valve 3. Mass flow controller 4. Check valve 5. Pressure gauge 6. Reactor 7. Electric heating belt 8. Ice-water condenser 9. Back pressure regulator

Figure 1 Flowchart of the reaction system for FT synthesis.

29



A

TCD Signal / a.u.

83

264

452

100

200

300

400

500

600

700

800

900

o

Temperature / C

B 266

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a, m/e=2 b, m/e=18 c, m/e=16 d, m/e=44

310

457

a 86

b c d 100

240

200

420

300

400

500

600

700

800

900

o

Temperature / C Figure 2 The TCD (A) and MS (B) profiles recorded during heating the Co-1.5C/Q15 catalyst precursor in an Ar atmosphere.

30


Page 30 of 38

Page 31 of 38

Co

e

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


d c CoO

b a Co3O4

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75

2θ /

ο

Figure 3 XRD patterns of Co/Q15 (a), Co-0.1C/Q15 (b), Co-0.5C/Q15 (c), Co-1C/Q15 (d), and Co-1.5C/Q15 (e).

31



3

-1

-1

200 cm . g

3

Va / cm · g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a b c d e f 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure P/P0

Figure 4 N2 adsorption-desorption isotherms of Q15 (a), Co/Q15 (b), Co-0.1C/Q15 (c), Co-0.5C/Q15 (d), Co-1C/Q15 (e), and Co-1.5C/Q15 (f).

32


Page 32 of 38

Page 33 of 38

-1

0.08

a b c d e f

0.06

3

-1

dv/dw / cm ·g ·nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.04

0.02

0.00 2

4

6

8

20

40

Pore width / nm Figure 5 Pore size distributions calculated by the BJH method using the adsorption branch for Q15 (a), Co/Q15 (b), Co-0.1C/Q15 (c), Co-0.5C/Q15 (d), Co-1C/Q15 (e), and Co-1.5C/Q15 (f).

33



15 10 5

200 nm

40

20 15 10 5 0

60 80 100 Particle size (nm)

30

C

25

Frequency (%)

20

0

30

B

25

25 Frequency (%)

30

A Frequency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

5

15 10 5 0

10 15 Particle size (nm)

100 nm

20

5

10 Particle size (nm)

15

100 nm

Figure 6 TEM images and the particle size distributions (Insets) of cobalt species over Co/Q15 (A), Co-1C/Q15 (B), and Co-1.5C/Q15 (C).

34


Page 35 of 38

350 325

TCD Signal / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a b c d

100

200

300

400

500

600

700

800

900

o

Temperature / C

Figure 7 H2-TPR profiles of Co/Q15 (a), Co-0.5C/Q15 (b), Co-1C/Q15 (c), and Co-1.5C/Q15 (d).

35



A 355

Intensity / a.u.

285

90

a b c d 100

200

300

400

500

600

700

800

900

o

Temperature / C

B

300

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

364

d 590

c b a 100

200

300

400

500

600

700

800

900

o

Temperature / C Figure 8 The MS signals of H2O (A) and CH4 (B) during the H2-TPR for Co/Q15 (a),

Co-0.5C/Q15 (b), Co-1C/Q15 (c), and Co-1.5C/Q15 (d).

36


Page 36 of 38

Page 37 of 38

4

CTY / 10-4 molCO . g-1Co . s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2 Co/Q15 Co-0.1C/Q15 Co-0.5C/Q15

1

Co-1C/Q15 Co-1.5C/Q15 Co-0.5C/Q15-R

0 0

2

4

6

8

10

Time on stream / h Figure 9 The TOS activity indexed by CTY for the FT synthesis over different

catalysts.

37



For Table of Contents Only 3.5

SiO2

Co-C/SiO2 Impregnation Co(NO3)2 Glucose

Carbonization 650 oC for4 h

Procedure for direct synthesis of metallic Co-C/SiO2

CTY (10-4 molCO g-1 s-1) Co

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.0

Reduced at 4000C

2.5 2.0 1.5 1.0 0.5 0.0 x=0.5

1.0 1.5 Co-xC/SiO2

38


0

Page 38 of 38

SiO2 As an Efficient Catalyst

Recommend Documents