Chinese Journal of Chemical Physics  2019, Vol. 32 Issue (6): 721-726

The article information

Jie Wang, Ying Xiang, Yi-yuan Ding, Yan-fei Xu, Xiang-hui Kong, Guang-yuan Ma, Chanatip Samart, Ming-yue Ding
王捷, 向颖, 丁毅远, 徐艳飞, 孔祥辉, 马光远, Chanatip Samart, 定明月
Bio-syngas Converting to Liquid Fuels over Co Modified Fe$_{3}$O$_{4}$-MnO$_{2}$ Catalysts
Chinese Journal of Chemical Physics, 2019, 32(6): 721-726
化学物理学报, 2019, 32(6): 721-726

Article history

Received on: April 29, 2019
Accepted on: June 24, 2019
Bio-syngas Converting to Liquid Fuels over Co Modified Fe$_{3}$O$_{4}$-MnO$_{2}$ Catalysts
Jie Wanga , Ying Xianga , Yi-yuan Dinga , Yan-fei Xua , Xiang-hui Konga , Guang-yuan Maa , Chanatip Samartb , Ming-yue Dinga,c     
Dated: Received on April 29, 2019; Accepted on June 24, 2019
a. School of Power and Mechanical Engineering, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Province Key Laboratory of Accountrement Technique in Fluid Machinery & Power Engineering, Wuhan University, Wuhan 430072, China;
b. Department of Chemistry, Faculty of Science and Technology, Thammasat University, Rangsit Campus, Klongluang, Pathumtani 12120, Thailand;
c. Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China
Abstract: A bifunctional Co modified Fe$_3$O$_4$-Mn catalyst was prepared for Fischer-Tropsch synthesis (FTS). The influence of Co loading on the synergistic effect of Fe-Co as well as FTS performance over Fe$_1$Co$_x$Mn$_1$ catalysts was studied. Incorporation of Co species into the Fe$_3$O$_4$-Mn catalyst promoted the reduction of iron oxides, increasing iron active sites during FTS. Moreover, the adding of Co species enhanced the electron transfer from Fe to Co metal, which strengthened the synergistic effect of Fe-Co, improving the catalytic performance. The Fe$_1$Co$_x$Mn$_1$ catalyst with higher Co loading promoted further the hydrogenation ability, favoring the shifting of the product distribution towards shorter hydrocarbons. Under optimized conditions of 280 ℃, 2.0 MPa and 3000 h$^{-1}$, the highest yield of liquid fuels was obtained for the Fe$_1$Co$_1$Mn$_1$ catalyst.
Key words: Fischer-Tropsch    Fe$_3$O$_4$-Mn    Co loading    Synergistic effect    

Fischer-Tropsch synthesis (FTS) process, converting syngas derived from coal, biomass to various alcohol, oil and high value-added hydrocarbon products, has received an increasing concern with the rapid development of economy and gradual depletion of crude oil in recent years [1-5]. The hydrocarbons product of FTS generally follows the Anderson-Schulz-Flory (ASF) model, with a distribution ranging from C$_1$ to C$_{50}$, which results in the difficulty for controlling selectively special hydrocarbons [6]. Designing of catalysts with high selectivity for special hydrocarbons is very important for industrial applications of FTS.

Fe, Co, Ni, and Ru are the conventional active metals for FTS [7], in which only Fe and Co are used for the production of liquid fuels in industry. Generally, less activity and producing long chain hydrocarbons are found over iron-based catalysts, whereas high activity and more expensiveness are displayed for cobalt-based catalysts. Thus, novel FTS catalyst is designed, combining with high activity of iron metal and heavy hydrocarbons selectivity of cobalt species. Actually, numerous attentions have been paid to preparing Fe-Co bifunctional catalysts [8-10]. Bragana et al. [11] found that Co-Fe bimetal supported on the HMS mesoporous material exhibited the highest activity and C$_5$$^+$ hydrocarbon selectivity. The results of Tavasoli et al. [12] showed that the single Co-based catalyst facilitated the formation of C$_5$$^+$ liquid hydrocarbons, whereas incorporation of iron into the Fe-Co catalyst did not change significantly the product distribution. In addition, Constant et al. [13] found that the formation of mixed Co-Fe bimetal species promoted the selectivity to light olefins while decreased catalytic activity compared to the Co-based catalyst. In addition, Lögdberg et al. [14] found that the combination of cobalt and iron improved the FTS catalytic activity in comparison to the monometallic Fe catalyst. Recently, Rothenberg et al. [3] considered that the interaction between Fe and Co species was of vital importance to the enhancement of activity and stability in FTS. Though abundant studies have been conducted on the Co-Fe bimetallic FTS catalyst, there exists the controversy on the FTS performance due to different conditions and/or different catalyst systems. Therefore, the intrinsic relationship between Co and Fe species is necessary to be further illustrated.

In our previous work, a Fe$_3$O$_4$-Mn catalyst was synthesized by a hydrothermal method, which exhibited high olefins selectivity of about 80% [15]. It is expected that incorporation of cobalt species into Fe$_3$O$_4$-Mn might further optimize catalytic activity and product distribution, whereas the synergy between Fe$_3$O$_4$ and Co is not understood clearly. Herein, a Co modified Fe$_3$O$_4$-Mn catalyst was prepared and applied in the FTS field. The synergistic effect of Fe$_3$O$_4$-Co was investigated in detail by the combination of several techniques including N$_2$ adsorption-desorption analysis, powder X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H$_2$-TPR), X-ray photoelectron spectroscopy (XPS), and laser Raman spectroscopy (LRS).


The N$_2$ adsorption-desorption isotherms of all the prepared catalysts are shown in FIG. 1. Obvious hysteresis loops can be found in isotherms of all catalysts, which can be identified as Ⅳ type isotherm [16]. The type H1 hysteresis loop can be observed while the ratio of $P$/$P_0$ is over 0.9, ascribed to a wide macroporous size distribution [17], revealing the shaggy structure of prepared Fe$_1$Co$_x$Mn$_1$ catalysts. Similar macroporous structures are observed for all the Fe$_1$Co$_x$Mn$_1$ catalysts, indicating that addition of Co to the Fe$_3$O$_4$-Mn catalyst does not destroy the intrinsic structures. The BET surface area of Fe$_3$O$_4$-Mn is 90 m$^2$/g (Table S1 in supplementary materials), which decreases slightly with the adding of Co species, probably due to partial incorporation of Co particles into the macroporous structures of Fe$_3$O$_4$-Mn, decreasing BET surface area. Moreover, the average pore volume increases with the increasing of cobalt concentration. The average pore volume of Fe$_1$Co$_0$Mn$_1$ is 0.80 cm$^3$/g, which increases gradually to 1.40 cm$^3$/g as the ratio of Co/Fe raises to 1:1, demonstrating that Co species incorporated into Fe$_3$O$_4$-Mn are well dispersed inside the macroporous structures. The morphological and structural characteristics of Fe$_1$Co$_x$Mn$_1$ catalysts are demonstrated in SEM images in FIG. 2. All catalysts have a structure of nanoplates and the addition of Co has no effect on the structural characteristics.

FIG. 1 N$_2$ adsorption-desorption isotherms of the fresh Fe$_1$Co$_x$Mn$_1$ catalysts. (a) Fe$_1$Co$_0$Mn$_1$, (b) Fe$_1$Co$_{0.2}$Mn$_1$, (c) Fe$_1$Co$_{0.5}$Mn$_1$, (d) Fe$_1$Co$_{0.8}$Mn$_1$, (e) Fe$_1$Co$_1$Mn$_1$.
FIG. 2 SEM image of fresh catalysts. (a) Fe$_1$Co$_0$Mn$_1$, (b) Fe$_1$Co$_{0.2}$Mn$_1$, (c) Fe$_1$Co$_{0.5}$Mn$_1$, (d) Fe$_1$Co$_{0.8}$Mn$_1$, (e) Fe$_1$Co$_1$Mn$_1$.
B. H$_{2}$-TPR and XRD results

H$_2$-TPR is used to characterize the reduction behavior of Fe$_1$Co$_x$Mn$_1$. Four peaks appear when temperature is around 200-500 ℃ or 600-800 ℃ for the single Fe$_3$O$_4$-Mn catalyst (FIG. 3). According to our previous study, the peaks at around 400 ℃ and 750 ℃ are due to continual reduction of Fe$_3$O$_4$$\rightarrow$FeO$\rightarrow$Fe, respectively [15]. And the shoulder peak at around 200-500 ℃ corresponds to the reduction of MnO$_2$ [18]. For the Fe$_1$Co$_x$Mn$_1$ catalysts, new main peaks appear at around 200-320 ℃ and 400-500 ℃, which is due to reduction of Co$_3$O$_4$$\rightarrow$CoO$\rightarrow$Co, respectively [19]. Different from monometallic catalysts, reduction pattern of multi-metallic catalysts is more difficult to analyze because metallic oxide reduction steps are easily overlapped [20, 21]. It is apparent that the adding of cobalt species into Fe$_3$O$_4$-Mn results in the reduction peaks of iron oxides shifting towards lower temperature to some extent, indicating that the addition of Co promotes the reduction of metallic oxide via weakening the Fe-Mn interaction. This is in agreement with Díaz et al. [22], who demonstrated that Co species promoted the reduction of iron oxides.

FIG. 3 H$_2$-TPR profiles of the fresh Fe$_1$Co$_x$Mn$_1$ catalysts. (a) Fe$_1$Co$_0$Mn$_1$, (b) Fe$_1$Co$_{0.2}$Mn$_1$, (c) Fe$_1$Co$_{0.5}$Mn$_1$, (d) Fe$_1$Co$_{0.8}$Mn$_1$, (e) Fe$_1$Co$_1$Mn$_1$.

XRD measurements are carried out to investigate phase structures of our catalysts. From FIG. 4 it can be found that all catalysts exhibit the diffraction peaks at 2$\theta$ of 30.0$^{\circ}$, 35.4$^{\circ}$, 53.4$^{\circ}$, 62.5$^{\circ}$, which are ascribed to the typical Fe$_3$O$_4$ (JCPDS: 00-019-0629) [23]. In addition, an amorphous peak is observed at 2$\theta$=40.5$^{\circ}$, corresponding to the amorphous structure of MnO$_2$. No diffraction peak of Co species appears for the Co-modified Fe-Mn catalysts, implying that Co species is well dispersed in the Fe$_1$Co$_x$Mn$_1$ catalysts. Notably, with the gradual adding of Co species into the catalysts, the diffraction peaks of Fe$_3$O$_4$ shifted slightly towards higher angles, which may be due to the combination of Fe and Co species. The results of Lögdberg et al. [14] demonstrated that addition of Co into the Fe$_2$O$_3$ resulted in diffraction peaks of Fe$_2$O$_3$ shifting towards higher 2$\theta$. Therefore, it may be the Fe$_3$O$_4$-Co solid solution in the Fe$_1$Co$_x$Mn$_1$ catalysts contracts the crystal lattice of Fe$^{2+}$(0.78 Å) and Fe$^{3+}$(0.61 Å) via the incorporation of Co$^{2+}$(0.75 Å) and Co$^{3+}$(0.61 Å) [24].

FIG. 4 XRD patterns of the fresh Fe$_1$Co$_x$Mn$_1$ catalysts with different ratio.
C. XPS and Raman results

Phase compositions and electronic state on the surface layers of Fe$_1$Co$_x$Mn$_1$ with different Co contents are characterized by XPS. In FIG. 5(A), all the catalysts show two obvious Fe 2p peaks at around 723.5 and 710.8 eV that can be ascribed to Fe$_3$O$_4$, which is in agreement with the XRD results in FIG. 4. Particularly, with the addition of Co content, Fe 2p peaks marginally shift to a higher binding energy. Moreover, at around 781.2 and 795.6 eV, Co 2p$_{3/2}$ and Co 2p$_{1/2}$ peaks gradually appear, demonstrating the existence of Co$_3$O$_4$ (FIG. 5(B)). In addition, the peak intensity of Co has an increasing tendency when Co content increases, probably due to the accumulation of added cobalt in both the bulky and surface layers. Different from the Fe 2p peaks, Co 2p peaks shift towards a lower binding energy while Co content increases. It is generally accepted that the electron transfers from a low electronegative element to a high electronegative element [18]. The electronegativities of Fe and Co are 1.8 and 1.9, respectively, indicating that the electron transfer occurs from the low electronegative Fe element to the high electronegative Co element. Both the XPS and XRD results suggest that incorporation of Co species into the Fe$_3$O$_4$-Mn catalyst adjusts effectively electron structures of iron species. In addition, a typical Mn 2p$_{3/2}$ peak at 642.2 eV and a characteristic Mn 2p$_{1/2}$ peak at 654.0 eV can be observed in all Fe$_1$Co$_x$Mn$_1$ catalysts, confirming the existence of MnO$_2$ [25, 26].

FIG. 5 XPS spectra of the as-synthesized catalysts. (A) Fe 2p, (B) Co 2p, and (C) Mn 2p. (a) Fe$_1$Co$_0$Mn$_1$, (b) Fe$_1$Co$_{0.2}$Mn$_1$, (c) Fe$_1$Co$_{0.5}$Mn$_1$, (d) Fe$_1$Co$_{0.8}$Mn$_1$, (e) Fe$_1$Co$_1$Mn$_1$.

In order to have a better insight into surface structure and carbonaceous species of used Fe$_1$Co$_x$Mn$_1$ catalysts, Raman analysis is carried out to identify the carbon species on the surface of catalysts. As shown in FIG. 6, two main peaks observed at 1342 cm$^{-1}$ and 1607 cm$^{-1}$ are ascribed to the disordered and ordered carbonaceous species, respectively [27]. The intensity of surface carbonaceous species shows an apparent increase when Co content is higher, demonstrating that addition of more Co to Fe$_1$Co$_x$Mn$_1$ catalysts facilitates the formation of carbonaceous species on the surface layers. Moreover, the G/D ratio (the ratio of ordered and disordered carbon species) shows an apparent decrease from 0.94 to 0.21 as the Co content increases from 0 to 33%. The disordered carbon species is related to the structural disorder at the surface while the ordered carbon species is connected with the vibration of the sp$^2$ bonded carbon atoms [28, 29]. It is clear that incorporation of Co species can decrease the graphitization degree and increase the defect sites in graphene lattice, which can weaken the mobility of catalyst and suppress catalyst's tendency to agglomerate [26].

FIG. 6 Raman spectrum profiles of the spent Fe$_1$Co$_x$Mn$_1$ catalysts. (a) Fe$_1$Co$_0$Mn$_1$, (b) Fe$_1$Co$_{0.2}$Mn$_1$, (c) Fe$_1$Co$_{0.5}$Mn$_1$, (d) Fe$_1$Co$_{0.8}$Mn$_1$, (e) Fe$_1$Co$_1$Mn$_1$.
D. Catalytic evaluation

Fischer-Tropsch performance of the catalysts is carried out at the conditions of 280 ℃, 2.0 MPa, 3000 h$^{-1}$ and CO:H$_2$=1:1 for 30 h. CO conversion of the Fe$_3$O$_4$-Mn catalyst is 43.32%, which shows an increase as Co content increases. (Table Ⅰ). When the ratio of Co/Fe reaches 1:1, CO conversion increases to 58.90%, demonstrating that the addition of Co species to the Fe$_3$O$_4$-Mn promotes catalytic activity. The results of Yang et al. [30] showed that tuning Fe/Co molar ratio in the Fe$_5$C$_2$/Co catalyst changed obviously the FTS performance. Incorporation of 0.6 wt% Co into the Fe$_5$C$_2$/Co catalyst improved the catalytic activity about 4 times compared to pure Fe$_5$C$_2$ catalyst. Constant et al. [13] found that increasing cobalt content in Co-Fe bimetallic catalysts facilitated the FTS rate increase. In the present study, the adding of Co species into the Fe$_3$O$_4$-Mn catalyst promotes the reduction of iron oxides, as confirmed by H$_2$-TPR, which facilitates the formation of iron active sites. Furthermore, cobalt metal is used widely as the active metal to optimize the FTS performance [31-33]. The incorporation of Fe and Co in Fe$_1$Co$_x$Mn$_1$ catalysts improves the electron migration from Fe into Co, which may enhance the synergistic effect of two metals, further increasing catalytic performance.

Table Ⅰ The catalytic performance of various catalysts in the FTS reaction.

From Table Ⅰ it can be found that the Fe$_3$O$_4$-Mn catalyst exhibits a low CH$_4$ selectivity (3.58%) with a high C$_5$$^+$ selectivity (74.72%). The ratio of C$_{2-4}$$^{=}$/C$_{2-4}$$^\textrm{o}$ is 4.43. With the adding of Co species into the Fe$_3$O$_4$-Mn catalyst, the hydrocarbons product shifts towards lighter hydrocarbons, and the alkenes selectivity decreases gradually, demonstrating that the combination of Co and Fe facilitates the formation of shorter hydrocarbons. The results of Bezemer et al. [34] demonstrated that smaller cobalt metal particles in the Fe-Co bimetallic catalyst had higher hydrogenation ability in FTS, promoting the formation of lower C$_5$$^+$ hydrocarbons. Díaz et al. [22] suggested that the combination of Co and Fe bimetallic active sites enhanced the activation of reactants and desorption of intermediate hydrocarbons, favoring the production of lighter hydrocarbons. Therefore, incorporation of Co metal into the Fe$_3$O$_4$-Mn catalyst in this study strengthens the synergistic effect between Fe and Co active sites, which promotes the activation of reactants and desorption of intermediate hydrocarbons, changing the product distribution to lighter hydrocarbons. Furthermore, Co species added in the Fe$_1$Co$_x$Mn$_1$ catalysts provides higher hydrogenation ability for olefins hydrogenation reaction at relatively high reaction temperature (260 ℃), favoring the production of paraffins.

E. Reaction conditions

The reaction conditions (temperature, GHSV and pressure) have great influence on the performance of the catalyst. In this study, the optimal Fe$_1$Co$_1$Mn$_1$ catalyst is selected to carry out the optimization of reaction conditions. The effect of reaction temperature is shown in FIG. S1 (supplementary materials). It can be found that CO conversion increases from 58.90% to 96.03% when reaction temperature raises from 260 ℃ to 340 ℃. The selectivity of CH$_4$ and C$_2$-C$_4$ raises from 11.01% and 24.60% to 30.11% and 38.90%, respectively while that of C$_5$$^+$ hydrocarbons shows a sharp drop from 64.39% to 30.99%. It may be attributed to that higher reaction temperature suppresses the carbon chain growth of hydrocarbons, resulting in the formation of light hydrocarbons, which is consistent with the previous study [35].

The influence of reaction pressure on the catalytic performance of Fe$_1$Co$_1$Mn$_1$ catalyst at 280 ℃ and 3000 h$^{-1}$ is presented in FIG. S2 (supplementary materials). The CO conversion increases from 17.98% to 59.86% while the reaction pressure increase from 1.0 MPa to 4.0 MPa (FIG. S1), demonstrating that higher pressure promotes catalytic activity. Moreover, the selectivity of CH$_4$ and C$_2$-C$_4$ increases from 5.75% and 13.55% to 11.83% and 21.25%, respectively; while that of C$_5$$^+$ hydrocarbons has a slight decrease from 80.69% to 66.92% with the increasing pressure. This suggests that lower reaction pressure is propitious to the formation of C$_5$$^+$ hydrocarbons.

FIG. S3 in supplementary materials shows the impact of GHSV on catalytic performance of Fe$_1$Co$_1$Mn$_1$ catalyst at 280 ℃ and 2.0 MPa. CO conversion has an obvious decrease from 50.10% to 16.32% with the continual increasing of GHSV, which may be attributed to the shortened contact time in higher GHSV. Interestingly, the higher GHSV has slight influence on the hydrocarbons selectivity. According to these results above, the optimized performance of syngas to liquid fuels is achieved on the Fe$_1$Co$_1$Mn$_1$ catalyst, which displays the highest yield of liquid fuels of 36.65% at 280 ℃, 2.0 MPa and 3000 h$^{-1}$.


Briefly, the influence of cobalt on the synergy of Fe-Co and FTS performances over the Fe$_1$Co$_x$Mn$_1$ catalyst was studied. Introduction of Co into the Fe$_3$O$_4$-Mn catalyst did not damage the catalyst macroporous structures, and enlarged the average pore volume. The increasing addition of cobalt enhanced the reduction of Fe$_3$O$_4$, which facilitated the formation of iron carbide. In addition, the Fe$_1$Co$_x$Mn$_1$ catalyst with higher Co loading enhanced the electron transfer from Fe$_3$O$_4$ to Co$_3$O$_4$, probably strengthening the synergistic effect of Fe-Co and improving the catalytic performance. Furthermore, incorporation of Co species increased the hydrogenation ability of intermediate hydrocarbons, resulting in the hydrocarbon distribution shifting to shorter carbon number. The optimized Fe$_1$Co$_1$Mn$_1$ catalyst displayed the highest yield of liquid fuels of 36.65% at 280 ℃, 2.0 MPa and 3000 h$^{-1}$.

Supplementary materials: Experimental details, catalyst characterization equipment, textural properties of the fresh catalysts, effect of reaction conditions on conversion and hydrocarbon distribution are available.


This work was supported by International Cooperation and Exchange Program of the National Natural Science Foundation of China (No.51861145102), Science and Technology Program of Shenzhen (No.JCYJ20180302153928437), and Fundamental Research Fund for the Central Universities (No.2042019kf0221).

Supporting Information Contents

S1. Experimental

S2. Catalyst characterization

S3. Fischer Tropsch synthesis reaction

S4. Table and figures

Figure S1. Effect of reaction temperature on CO conversion and hydrocarbon distribution. Reaction conditions: P = 2.0 MPa, GHSV = 3000 h-1, TOS = 10 h.

Figure S2. Effect of reaction pressure on CO conversion and hydrocarbon distribution. Reaction conditions: T = 260 ℃, GHSV = 3000 h-1, TOS = 10 h.

Figure S3. Effect of the GHSV on CO conversion and hydrocarbon distribution. Reaction conditions: T = 260 ℃, P = 2.0 MPa, TOS = 10 h.

S1. Experimental S1.1. Preparation of Fe3O4-Mn catalyst

The Fe3O4-Mn catalyst was prepared by a one-pot co-precipitation method. All chemicals used in our experiment are analytical grade and are purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). FeSO4·7H2O (1.39 g) and PVP (1.00 g, K-30) were dissolved in deionized water (100 mL) to form a light green solution and kept it for one hour. An aqueous solution of sodium hydroxide was added into the solution to generate a green suspension. After 5 min, KMnO4 aqueous solutions were added dropwise and the green suspension became to turn dark brown. The newly produced brown precipitate was then aged with the mother liquid for 12 h under stirring. At last, the sample was washed by demineralized water and ethanol, followed by drying in a vacuum.

S1.2. Preparation of Fe1CoxMn1 catalysts

Fe1CoxMn1 (x = 0, 0.2, 0.5, 0.8, 1) catalysts were synthesized by the similar one-pot co-precipitation method, where the molar ratio of Fe: Co: Mn is 1: x: 1. Typically, FeSO4·7H2O, CoSO4·7H2O and PVP were dissolved in deionized water (100 mL) to form a light orange solution. After one hour, an aqueous solution of sodium hydroxide was added into the solution to generate a green suspension. After 5 min, KMnO4 aqueous solution was added dropwise and the precipitate was aged with the mother liquid for 12 h. Last, the precipitate was collected by centrifugation, followed by washing and drying.

S2. Catalyst characterization

N2 adsorption and desorption isotherms were measured by an automatic physical adsorption analyzer (Micromeritics ASAP2020) at -196 ℃. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method and pore size distribution was analyzed by Barrett-Joyner-Halenda (BJH) method. Before testing, all catalysts were degassed under vacuum at 120 ℃ for 3 h.

Powder X-ray diffraction (XRD) spectra were obtained on a Bruker AXS-D8 Advance (Germany) diffractometer equipped with Co Kα (λ = 1.78 Å) radiation (35 kV, 40 mA) with a scanning angle (2θ) range of 10 to 90°.

X-ray photoelectron spectroscopy (XPS) measurements were conducted on a VG system (MultiLab 2000) equipped with an Al Kα (1486.6 eV) quartz monochromator source. All peaks were corrected by setting the C 1s peak of 284.6 eV as the reference. Laser Raman spectroscopy (LRS) of our catalysts was performed on a RM-1000 confocal Raman microscope.

H2-temperature programmed reduction (H2-TPR) measurements were carried out in a quartz-tube fixed-bed microreactor. Fifty milligrams of sample were in situ pretreated with a high purity N2 stream (30 mL·min-1) at 150 ℃ to remove the residual water and other contaminants. After exposed to N2 for 1 h, the catalyst was switched to a 5%H2/95%N2 flow exposure and heated to 800 ℃ at a rate of 10 ℃·min-1. The H2 contents of the tail gas were continuously recorded by a thermal conductivity detector (TCD).

S3. Catalytic evaluation

The catalytic tests were conducted in a fixed-bed, stainless flow micro-reactor. Typically, the catalyst (0.5 g) and quartz sand (0.5 g) were mixed and loaded into the reactor. The catalyst was reduced in H2 atmosphere at 350 ℃, 0.1 MPa and 3000 h-1 for 10 h. After reduction described above and cooling to 200 ℃, syngas (H2: CO = 1:1) was introduced into the reactor, followed by a rise of reaction temperature with a rate of 1 ℃/min. The reaction was carried out at the desired temperature, gas space velocity (GHSV) and pressure. The operation conditions for the tandem catalysts were as follows: temperature of 280 - 360 ℃, GHSV of 3000 - 12000 h-1 and pressure of 1.0 - 4.0 MPa.

Reaction products were separated by a hot trap (kept at 50 ℃) and a cold trap (kept at 1 ℃). The tail gases were detected online by gas chromatographs (FULI GC 97) equipped with a TCD and a FID. The liquid products were collected and analyzed off-line using a gas chromatograph (FULI GC 97) with a FID. Catalytic activity and product selectivity were calculated on a carbon-atom basis.

The CO conversion is obtained by:

$ CO\;conversion\; = \frac{{C{O_{in}} - C{O_{out{\rm{ }}}}}}{{C{O_{in}}}} \times 100\% $

COin: the volume of CO at the inflow; COout: the volume of CO at the outflow. The CO conversion observed in the blank tests (in the absence of catalyst, only by effect of the temperature) is zero.

The CO2 selectivity is obtained by:

$ C{O_2}\ selectivity = \frac{{C{O_{2\;out}}{\rm{ }}}}{{C{O_{in}} - C{O_{out}}}} \times 100\% $

CO2 out: the volume of CO2 at the outflow.

The hydrocarbon selectivity (product distribution) is calculated by:

${C_{\rm{i}}}{H_m}\;selectivity = \frac{{i \times {C_{\rm{i}}}{H_{m\;out}}{\rm{ }}}}{{\sum\nolimits_{i = 1}^n i \times {C_{\rm{i}}}{H_{m\;out}}{\rm{ }}}} \times 100\% $

The selectivity to a hydrocarbon does not take into account the formation of CO2.

CiHm out: the mole number of the hydrocarbon with i carbons at the outflow.

FTY, which is the converted CO moles per gram in unit time, can be obtained by:

$ FTY\left( {mol \cdot g_{Fe}^{ - 1} \cdot {s^{ - 1}}} \right) = \frac{{{Q_{in}}{v_{CO}}{x_{CO}}}}{{{V_m}{m_{Fe}}}} $

Qin: the inlet total volume flow rate; vCO: the molar percentage of CO in syngas; xCO: the CO conversion; Vm: the gas molar volume under standard conditions, Vm = 22, 400 mL/mol; mFe: the mass of Fe in the catalyst.

S4. Table and figures
Table S1 Textural properties of the fresh catalysts.
FIG. S1 Effect of reaction temperature on CO conversion and hydrocarbon distribution. Reaction conditions: P = 2.0 MPa, GHSV = 3000 h-1, TOS = 10 h.
FIG. S2 Effect of reaction pressure on CO conversion and hydrocarbon distribution. Reaction conditions: T = 260 ℃, GHSV = 3000 h-1, TOS = 10 h.
FIG. S3 Effect of the GHSV on CO conversion and hydrocarbon distribution. Reaction conditions: T = 260 ℃, P = 2.0 MPa, TOS = 10 h.
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王捷a , 向颖a , 丁毅远a , 徐艳飞a , 孔祥辉a , 马光远a , Chanatip Samartb , 定明月a,c     
a. 武汉大学动力与机械学院,武汉 430072;
b. 泰国国立法政大学化学学院,巴吞他尼府 12120;
c. 南开大学先进能源材料化学教育部重点实验室,天津 300071
摘要: 本文制备了用于费托合成反应的钴改性Fe$_3$O$_4$-MnO$_2$双功能催化剂,并探究了钴负载量对Fe-Co协同效应的影响以及Fe$_1$Co$_x$Mn$_1$催化剂的费托合成反应性能.实验发现,在Fe$_3$O$_4$-Mn催化剂中加入Co可促进铁氧化物的还原、增加反应过程中铁位点的活性.此外,Co的加入可增强Fe-Co金属间的电子转移,加强两者的协同作用,提高催化性能. Co负载较高的Fe$_1$Co$_x$Mn$_1$催化剂可进一步促进加氢反应能力,使产品分布向短链烃方向转移.在280 ℃、2.0 MPa和3000 h$^{-1}$的最佳工况条件下,Fe$_1$Co$_1$Mn$_1$催化剂的液体燃料收率最高.
关键词: 费托合成    Fe$_3$O$_4$-Mn    钴负载    协同效应