Co Heterostructured Nanoparticles

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Construction of synergistic FeC/Co heterostructured nanoparticles as enhanced low temperature Fischer-Tropsch synthesis (FTS) catalyst Ce Yang, Bo Zhao, Rui Gao, Siyu Yao, Peng Zhai, Siwei Li, Jing Yu, Yanglong Hou, and Ding Ma ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01142 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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Figure 1. a-d) XRD patterns of Fe5C2/Co nanostructured NPs with different Fe/Co molar ratio (JCPDS No. 361248); XRD pattern of e) Fe5C2 and f) Co NPs. The solid dots indicate the peaks of ɛ-Co.28-30 241x203mm (300 x 300 DPI)

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Figure 2. a) TEM image and b) STEM image of 12Fe5C2/Co; c) HRTEM image of the area enclosed by the red frame in b). d) Co, e) Fe and f) total element distribution of the corresponding area enclosed by the red frame in b). 82x47mm (300 x 300 DPI)


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Figure 3. a) CO conversion of Fe5C2 and Fe5C2/Co catalysts with various Fe/Co ratio. b) Time on stream plot of CO conversion of the 12Fe5C2/Co, Fe5C2, Co and 7wt% Co catalysts. c) Selectivity of 12Fe5C2/Co catalyst. d) Selectivity of 7wt% Co catalyst. (measured in a fixed-bed reactor, P=3MPa, T=220oC and GHSV=15,000 cm3 h-1gcat-1 75x58mm (300 x 300 DPI)


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Figure 4. Pulse reactions of CO in He flow over 12Fe5C2/Co catalyst at a) 200oC and b) 250oC, and pulse reactions of CO in He flow over Fe5C2 catalyst at c) 250oC and 300oC. The peaks of CO2 (m/z=44) and CO (m/z=28) are shown in the profiles. 75x52mm (300 x 300 DPI)


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Figure 5. Normalized peak areas of a) CO2, b) H2O, c) CH4 and d) C2H4 produced on Fe5C2/Co , Fe5C2 and Co catalysts in the pulse reaction of syngas at different temperature.

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Construction of synergistic Fe5C2/Co heterostructured nanoparticles as enhanced low temperature Fischer-Tropsch synthesis (FTS) catalyst Ce Yang†,‡,ǁ,╢ , Bo Zhao‡,╢, Rui Gao₸,╧, Siyu Yao‡, Peng Zhai‡, Siwei Li‡, Jing Yu†,├, Yanglong Hou*,† and Ding Ma*,‡ †

BKL-MEMD, BIC-EAST, Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China

‡

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

₸ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, PO Box 165, Taiyuan, Shanxi 030001, China ╧ Synfuels China Co. Ltd, Beijing 100195, China ǁ

Chemical Science and Engineering division, Argonne National Laboratory, 9700 S Cass Ave, Lemont, IL 60439, United States

ABSTRACT. Iron and cobalt catalysts are two major categories for the commercial FischerTropsch synthesis (FTS) catalysts. The two types of catalysts have distinct merits and shortcomings while they are largely supplementary to each other. However, till now, there lacks


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an efficient way to properly combine those two catalysts into a synergistic one which possesses the benefits of both catalysts. Herein, the Fe5C2/Co heterostructured nanoparticles (NPs) were constructed by a secondary growth strategy, where the Fe/Co molar ratio can be tuned from 3.3 to 25. Based on the FTS reaction evaluation, we observed that only with 0.6 wt% Co (Fe/Co=12) incorporated, the Fe5C2/Co catalyst exhibits an activity four times higher than that of pure Fe5C2 catalyst at low temperature. In this catalyst, Co was responsible for the CO dissociation while Fe5C2 was for the chain growth at 220oC. The synergistic effect of the both sites may lead to enhanced performance in FTS reaction. This result provides a perspective for the construction of Fe-Co bimetallic FTS catalysts.

KEYWORDS: Fe5C2, Cobalt, Synergistic, Low temperature, Fischer Tropsch Synthesis

INTRODUCTION Fischer-Tropsch synthesis (FTS) is to convert syngas (CO+H2) to liquid fuels. Among all the catalysts for industrial FTS process, iron- and cobalt-based catalysts are two major categories whose active phases are correspondingly iron carbides and metallic cobalt.1-5 Traditionally, ironbased catalysts are considered as less active, less selective towards long-chain hydrocarbons but inexpensive.1,

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Cobalt-based catalysts, on the contrary, are found as highly active, highly

selective towards long-chain hydrocarbons but much more expensive compared with the ironbased catalysts.3, 8-14 Nevertheless, recently, iron carbides were suggested by Hensen et al. to have higher intrinsic selectivity towards long-chain hydrocarbons theoretically compared with cobalt at relatively low


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reaction temperature, and the lower selectivity towards long-chain hydrocarbons observed experimentally is actually a result of high reaction temperature required for CO dissociation.15-16 Yet, the high selectivity of cobalt towards long-chain hydrocarbons is a result of low reaction temperature which is rooted in its high CO dissociative activity at that temperature.15-20 Therefore, it is highly desired to construct a new FTS catalyst based on the high activity of Co as well as the cheap price and high long-chain hydrocarbons selectivity of iron carbide. Indeed, many efforts have been attempted to obtain favorable Fe-Co-based catalysts. Early reports noticed an incremental activity and C5+ product selectivity with increased corporation of Fe for certain range of Fe/Co molar ratio.21-22 Whereas, according to the more systematic studies from Coville group

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and Gevert group

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, the activity and C5+ product selectivity of the

catalysts have an apparently negative correlation with the amount of Fe component due to the alloying between Fe and Co parts. Recently, Rothenberg and co-workers produced Fe-Co-based catalysts from core-shell Fe3O4/Co3O4 precursor and proposed that the interaction between cobalt phase and iron phase was the main contribution for the enhanced activity and stability.29 Nevertheless, in all the current works, the catalysts was composed by large portion of Co and it is almost impossible to study the synergistic mechanism of the situation where amount of Co is small and Co is used as promoter. In our previous work, pure-phase Fe5C2 NPs was synthesized by a solution chemical method and was identified as an active phase for iron-based FTS process.30 It is expected that Fe5C2 NPs are ideal platforms to construct Fe-Co-based catalysts, examining the synergistic effects between Fe and Co parts in the catalysts. Herein, Fe5C2 NPs are employed as a starting material, which was then decorated with small amount of metallic Co particles through a secondary crystal growth process (Fe/Co=12) to construct a Co/Fe5C2 interfacial catalyst. The bimetallic Fe5C2/Co


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shows outstanding catalytic activity at low temperature while maintains the characteristics of iron carbide components in terms of chain propagation and termination behavior. The synergy of the two components were studied and discussed in detail. EXPERIMENTAL SECTION Synthesis of Fe5C2/Co heterostructures Fe5C2/Co heterostructured NPs with Fe/Co molar ratio of 12 (denoted as 12Fe5C2/Co) were prepared via a secondary growth strategy. In a typical process, octadecylamine was used as both the solvent and surfactant, cetyltrimethyl ammonium bromide (CTAB) was used as the inducing agent. Co2(CO)8 and Fe(CO)5 were utilized as the precursors. In a four-neck flask, a mixture of octadecylamine (14.5 g) and CTAB (0.113 g) was stirred sufficiently, degassed under N2 flow and heated to 120 oC. Then, under inert atmosphere, Fe(CO)5 (0.5 mL, 3.6 mmol) was injected into the solvent under a N2 blanket. The mixture was heated to 180 oC at 10 oC/min and kept at this temperature for 20 min. After that, the mixture was further heated to 350 oC at 10 oC/min and kept for 10 min before it was cooled down to 180 o

C again. Following that, Co2(CO)8 (0.1g, 0.3 mmol) were dissolved in 5 ml hexane, and

subsequently injected into the system. The reaction was kept at 180 oC for 30 min for the growth of Co NPs onto Fe5C2 NPs. The product was washed with ethanol and hexane, and collected for further characterization. The synthetic procedures of Fe5C2/Co with Fe/Co ratio of 25, 6.5 and 3.3 are similar with the above case except that 0.05 g, 0.2 g and 0.3 g Co2(CO)8 were introduced, respectively. Those samples were denoted as 25Fe5C2/Co, 12Fe5C2/Co, 6.5Fe5C2/Co and 3.3Fe5C2/Co. Synthesis of Co NPs


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The used solvent was collected and purified by centrifuging at 12,000 rpm for 10min. Subsequently, a NbFeB magnet was used to separate any residual Fe5C2 NPs using the magnetic property of Fe5C2 NPs.31 Then the used solvent was utilized as solvent to fabricate Co NPs. Note that the reason we utilize the used solvent is that the solvent changed dramatically after the synthesis of Fe5C2 NPs, and we tried to make the synthesis of separate Co NPs parallel to that of the Co NPs in 12Fe5C2/Co catalyst.30,

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Following that, Co2(CO)8 (0.1g, 0.3 mmol) were

dissolved in 5 ml hexane, and subsequently injected into the system. The reaction was kept at 180 oC for 30 min for the growth of Co NPs. The product was washed with ethanol and hexane, and collected for further characterization. Catalyst preparation To test the catalytic performance of Fe5C2/Co, Fe5C2 and Co NPs, 80 mg of as-prepared Fe5C2/Co, Fe5C2 and Co NPs were dispersed in ethanol. Those NPs was impregnated with SiO2 support and dried at room temperature. A supported Co catalyst (7wt%) was prepared by impregnation method: cobalt nitrate ethanol solution was added to SiO2 support, then dried and reduced at 400 oC in H2, which is termed as 7%Co. For Fe5C2/Co catalysts, the loading of metal is around 7 wt%. Depends on the ratio of Fe5C2 and Co, it is termed as xFe5C2/Co, where x is the ratio. For Fe5C2 catalysts, the loading of metal is around 7 wt%, which is termed as (pure) Fe5C2. For Co NPs catalysts, the loading of metal is around 0.6 wt%, which is termed as Co. Characterization Transmission electron microscopy (TEM) was carried out on an FEI Tecnai T20 microscopy. High-resolution TEM (HRTEM) was carried out on an FEI Tecnai F30 microscopy. Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive Spectrometer (EDS) line scan were also carried out on an FEI Tecnai F30 microscopy. X-ray diffraction (XRD) patterns


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were obtained using a Rigaku DMAX-2400 X-ray diffractometer equipped with Cu Kα radiation. The accelerating voltage and current were 40 kV and 100 mA. Extended X-ray absorption fine structure (EXAFS) were characterized on BL14W1-XAFS at the Shanghai Synchrotron Radiation Facility (SSRF), the storage ring operated at 3.5 GeV and 300 mA. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an Axis Ultra (Kratos Analytical Ltd.) imaging photoelectron spectrometer using a monochromatized aluminum Kα anode, and the C1s peak at 284.8 eV was taken as an internal standard. A Gas Chromatography machine (Agilent 7890A GC) was used to study the catalytic performance. Pulse reactions were performed on a Mass Spectrometry machine (PFEIFFER MC-D200M PRO MS). FTS reaction condition The FTS reaction was carried on a fixed-bed flow reactor with mixed gas containing 32% CO, 63% H2 and 5% Ar at a temperature of 220 oC. 100 mg of supported catalyst was loaded in a stainless steel tube lined with a quartz layer. The gas hourly space velocity (GHSV) of the reactions was set at 15,000 cm3 h-1gcat-1. The pressure of the reactor was set at 3 MPa and the temperature was ramped from 30 to 220 oC at 3 oC /min in the syngas atmosphere. The product and reactant in the gas phase were detected online by an Agilent 7890 GC. C1-C4 ranged alkanes were analyzed by a Plot Al2O3 capillary column with FID; CO, CO2, CH4 and Ar were analyzed by a Porapark Q and 5A molecular sieve packed column with TCD. The 5% Ar in the syngas was used as the internal standard for the calculation of CO conversion. The product with large molecular weight was collected in a cold trap. Hydrocarbon was analyzed by a 7820 GC with HP-5 capillary column and FID, oxygenates in water was analyzed by a 7820 GC with HP-innowax capillary column and FID. The selectivity of the products was all on a carbon basis. TPH experiment of 12Fe5C2/Co


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The TPH experiment was carried out as follows: The TPH experiment was carried out as follows: 50 mg of 12Fe5C2/Co catalyst was used, and the gas hourly space velocity (GHSV) was 15,000 cm3 h-1gcat-1. The whole reactor was purge with 10% H2 for 30 min. Then, the catalyst was heated to 300oC at rate of 1.5 oC/min and keep at that temperature for 1h. The Mass Spectrometry started to record as soon as the system started to heat up, and the m/e value detected as follows: 18 for water and 15 for methane. Pulse reactions of CO 50 mg Fe5C2/Co and Fe5C2 catalysts which described in the ‘catalyst preparation’ section were used. The catalysts were reduced in 10% H2 at 300oC for 30 min before the pulse reactions. The pulse reactions of CO were carried out under 0.1MPa, with the carried gas of He, and the gas hourly space velocity (GHSV) of 7,500 cm3 h-1gcat-1. Around 250 µL CO was pulsed into the reactor through a sample loop at each pulse. The pulse reactions on Fe5C2/Co were carried out under 200 oC and 250 oC, and pulse reactions on Fe5C2 were carried out under 250 oC and 300 oC, respectively. Pulse reactions of syngas 50 mg Fe5C2/Co, Fe5C2 and Co NPs catalysts which described in the ‘catalyst preparation’ section were used. The catalysts were reduced in 10% H2 at 300oC for 30 min before the pulse reactions. The pulse reactions of syngas were carried out under 0.1MPa, with the carried gas of He, and the gas hourly space velocity (GHSV) of 7,500 cm3 h-1gcat-1. Around 250 µL syngas was pulsed into the reactor through a sample loop at each pulse. The experiments were carried out under 25 oC, 120 oC, 150 oC, 180 oC, 200 oC, 220 oC and 250 oC, respectively. The peak areas of CO2 (m/z=44), H2O (m/z=17), CH4 (m/z=15) and C2 (m/z=26) were calculated for each pulse.


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The normalized peak areas (S) were obtained by the following formula:

S=

ܵ௧ − ܵ௕ ܵ௦

Where St is the peak area integrated directly from the test, Sb is the peak area of blank test, and Ss is the standard peak area of one pulse. RESULTS AND DISCUSSION Structural characterization of Fe5C2/Co. The composition of Fe5C2/Co NPs was studied by ICP and XRD. Based on ICP results, the as-prepared samples have Fe/Co molar ratios of 25, 12, 6.5 and 3.3, respectively, with corresponding XRD patterns shown in Figure 1. All the XRD patterns are in accordance with the standard Fe5C2 pattern, suggesting the Fe5C2 phase was kept after secondary growth. With the injection of Co2(CO)8 precursor, Co NPs were formed as ε-Co phase.33-35 However, due to the peak overlap between XRD peaks of Fe5C2 and ε-Co, the existence of Co NPs can be hardly resolved by XRD except for a small peak of (301) arose in Figure 1f (as indicated by the black dot). Therefore, to examine whether there is the existence of metallic cobalt, we characterized as-synthesized heterostructured 12Fe5C2/Co NPs by X-ray Absorption Fine Structure (XAFS) measurement (Figure S1). The Fourier Transformation of Cobalt k-edge EXAFS spectrum shows that the back-scattering of Co was influenced by two CoO and Co-Co bond by comparing with the Co foil and standard pattern of Co3O4. Moreover, analyzing the XANES spectrum by fitting it with Co foil and standard pattern of Co3O4, it was found that there is 19% cobalt oxide and 81% metallic cobalt. As Co NPs are sensitive to air, the existence of small amount of cobalt oxide is due to the slight surface oxidation resulted from the


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exposure to air. Thus, both XANES and EXAFS spectra prove the formation of metallic cobalt in Fe5C2/Co heterostructures.

Figure 1. a-d) XRD patterns of Fe5C2/Co nanostructured NPs with different Fe/Co molar ratio (JCPDS No. 36-1248); XRD pattern of e) Fe5C2 and f) Co NPs. The solid dots indicate the peaks of ɛ-Co.33-35 To further look into the structural information of Fe5C2/Co NPs, TEM, STEM and EDSmapping were employed to characterize the samples. For the precursor Fe5C2 sample, identical to what we have reported earlier, the dominant products under TEM are large NPs with an average size of 40-50 nm and 1-2 nm amorphous carbon shell (Figure S2a). From the HRTEM, it can be also observed that the Fe5C2 NPs are multi-crystalline with grain size of 15 nm (Figure S2b). However, as shown in Figure 2a, small NPs with the size of 5-8 nm were observed to be grown


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on the surface of larger NPs after the secondary growth process (12Fe5C2/Co), which is also confirmed by STEM observation (Figure 2b). We believe that the secondary growth indeed successfully introduced the 2nd metal over initial Fe5C2 NPs, forming metallic Co decorated Fe5C2 heterostructure. To identify the composition of those heterostructures, EDS mapping was used to check the elemental distribution in the area enclosed by the red frame in Figure 2b. Clearly, the small NPs are Co NPs which is attached with the large Fe5C2 NPs (Figure 2d-f). HRTEM image of the same area (Figure 2c) indicates that the lattice spacing values are 0.201 nm in the smaller NPs and 0.204 nm in the larger NPs, which are in line with the interplane distance of (221) planes in ε-Co and (510) planes in Fe5C2, respectively.

Figure 2. a) TEM image and b) STEM image of 12Fe5C2/Co; c) HRTEM image of the area enclosed by the red frame in b). d) Co, e) Fe and f) total element distribution of the corresponding area enclosed by the red frame in b).


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However, if less amount of Co2(CO)8 were introduced, Co NPs cannot be observed clearly on the surface of Fe5C2 NPs (25Fe5C2/Co; Figure S3a). In addition, if further increasing the amount of Co2(CO)8 precursor in the secondary growth process, Co NPs will no longer homogeneously attached to the surface of Fe5C2 NPs to form the heterostuctures, but instead tend to aggregate together themselves (6.5Fe5C2/Co; Figure S3b). This demonstrates that the Fe5C2/Co ratio of 12 benefits the formation of interfacial Fe5C2-Co heterostructures while excessively high concentration of Co in the secondary growth process favors the phase segregation and independent assembly of Co NPs and Fe5C2 NPs. When the concentration of Co2(CO)8 is even higher, the size of Co NPs will continuously grow to 30 nm and lost close contact with Fe5C2 surface as shown in Figure S3c and d. Catalytic properties. FTS reaction over Fe5C2/Co catalysts with different Fe/Co molar ratio was evaluated at 220oC. As shown in Figure 3a, pure Fe5C2 has relatively low activity at low reaction temperautre of 220oC, with the conversion of CO around 5%. Although Fe5C2 has been identified as one of the most active phase of Fe-based FTS process, normally the working temperature of Fe-based catalysts is around 270-350 oC.36-38 Obviously, 220oC is too low a temperature for Fe5C2 catalyst, and therefore a low CO conversion is observed at this temperature. Instead, by adding small amount of Co in the synthesis process to form the Co decorated Fe5C2 heterostructures (25Fe5C2/Co), CO conversion increased to 7%. Further increasing the amount of Co to Fe/Co=12 (12Fe5C2/Co), CO conversion dramatically increased to 22%, although for this catalyst, the weight percentage concentration of Co is just 0.6%. It is obvious that this small amount of Co has major impact on catalytic reactivity. In order to find the underlying reasons of the enhanced FTS performance of 12Fe5C2/Co catalyst, we compared the catalytic performance of 12Fe5C2/Co with two control catalysts, i.e. 7%Co/SiO2 catalyst


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prepared by impregnation method and pure Co NPs supported over SiO2 (with the Co concentration of 0.6%, termed as Co). The former has identical total metal loading with that of 12Fe5C2/Co catalyst (7%), while the latter has same amount of Co as that in 12Fe5C2/Co catalyst but without Fe5C2. Then, we plotted the on-stream CO conversion of the four catalysts as shown in Figure 3b. For 12Fe5C2/Co catalyst, we found that the catalyst experienced an induction period in the first 10 hours. As metallic Co was believed to be the active phase of Co-based FTS process, we attribute the induction period to the reduction of Co species (see Figure S4 for the Co 2p XPS of fresh catalyst, which shows the partial oxidation of Co when exposing to air) and removal of surface oxygen species on Fe5C2. Note that, according to the TPH of the 12Fe5C2/Co catalyst, there was not any CoCx formed during the synthesis, which means the activation period of the catalyst cannot be attributed to decarburization of CoCx (Figure S7). After FTS for 25 hours, the CO conversion of 12Fe5C2/Co catalyst stables at 22%, which is almost the same as conventional impregnated Co catalyst with ten times higher Co loading (7wt%) as shown by the green line. If we decreased the loading of Co to 0.6% (Co catalyst), which is same with that in 12Fe5C2/Co catalyst, CO conversion is very small, namely around 3%. Obviously, the individual components in 12Fe5C2/Co catalyst have extremely low FTS reactivity, but integrating them into a heterostructure with Fe5C2-Co interface makes it a very active catalyst in FTS reaction, even comparable with that of pure Co catalyst with same metal loading. This is to say, now we are able to use a Fe-based catalyst but with the decoration of tiny amount of Co to realize the reactivity of pure Co catalyst at low temperature. CO is activated over the Co sites in the heterostructured catalyst, while it is highly possible that the successive chain propagation was happened on the adjacent Fe5C2 surface.


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In addition to that, the heterostructured 12Fe5C2/Co catalyst also shows improved selectivity compared with Fe5C2 catalyst. As can be seen in Figure 3c, at 220oC, the 12Fe5C2/Co catalyst shows a lower selectivity towards undesired product CO2 (6.4%), and a higher selectivity towards C5+ about 46% at CO conversion of 22%. Instead, Fe5C2 catalyst exhibits CO2 selectivity of 10.8% and a C5+ selectivity of 37% at CO conversion of 5% (Figure S5, surely at very low conversion, the determination of selectivity is not so accurate). Although the (510) sruface of Fe5C2, which also observed in the as-synthesized Fe5C2 NPs, has been considered to favor the chain growth,17-18 the low C5+ selectivity of Fe5C2 rooted in the insufficient supply of C1 species under 220oC which resulted from a lower C1 species coverage at that temperature. Moreoevr, 12Fe5C2/Co catalyst produces less C5+ than the Co catalyst does, and the chain growth probability of 12Fe5C2/Co differs from that of Co catalyst (0.70 vs 0.77) (Figure 3d and S6), but comparing with Fe5C2, we did observe a trend of the increase of C5+ selectivity from 37% to 46% (which is close to that of Co catalyst) and decrease of C1-4 selectivity from 52.5% to 45% in 12Fe5C2/Co catalyst. Therefore, the heterostructures of Fe5C2 and Co not only improve the reactivity but also played a key role in the change of FTS selectivity. By further increasing the concentration of Co in the Fe5C2/Co catalysts, the CO conversion drops gradually (Figure 4a), and eventually reaches a level around 5%. This indicates that Fe5C2/Co=12 is the optimal bimetallic catalyst that has the highest CO conversion. As shown in Figure S3b-d, too high the concentraion of Co in Fe5C2/Co structure will lead to the seperation of Co NPs from the surface of Fe5C2, which indicates that the interface of Co-Fe5C2 no longer exists. This may accounts for the low reactivity of those catalysts.


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Figure 3. a) CO conversion of Fe5C2 and Fe5C2/Co catalysts with various Fe/Co ratio. b) Time on stream plot of CO conversion of the 12Fe5C2/Co, Fe5C2, Co and 7wt% Co catalysts. c) Selectivity of 12Fe5C2/Co catalyst. d) Selectivity of 7wt% Co catalyst. (measured in a fixed-bed reactor, P=3MPa, T=220oC and GHSV=15,000 cm3 h-1gcat-1) To understand the mechanism of the modulated catalytic performance of 12Fe5C2/Co catalyst in more detail, we performed pulse reactions of CO in He over 12Fe5C2/Co and Fe5C2 catalysts at different temperature. The catalysts were pre-reduced in hydrogen at 300 oC for a half hr to reduce partially oxidized Co to metallic Co. Note that the reduction treatment will not hydrogenate the carbidic carbon in Fe5C2 NPs (Figure S7). As can be seen in Figure 4 a and 4b,


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in the CO pulse reaction, CO2 started to be observed on 12Fe5C2/Co catalyst at 250oC. Since the catalysts were pretreated under H2 before the pulse reaction, this indicates the extra oxygen atom can only be generated from dissociation of CO, which suggests the activation of CO takes place under 250 oC on 12Fe5C2/Co catalyst in the pulse reaction (at 0.1 MPa). Nevertheless, as can be seen in Figure 4c and d, the peaks for CO2 did not show up under either 250 oC or 300 oC on Fe5C2 catalyst, which indicate the activation of CO is more difficult on the surface of pure Fe5C2 even at 300 oC under the pulse reaction condition. Therefore, the pulse reactions of CO illustrate that the incorporation of cobalt in 12Fe5C2/Co catalyst benefits the activation of CO.


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Figure 4. Pulse reactions of CO in He flow over 12Fe5C2/Co catalyst at a) 200oC and b) 250oC, and pulse reactions of CO in He flow over Fe5C2 catalyst at c) 250oC and d) 300oC. The peaks of CO2 (m/z=44) and CO (m/z=28) are shown in the profiles.

The unique catalytic performance of Fe5C2/Co heterostructures demonstrated in the pulse reactions of CO indicates that the “dual center model” may indeed hold, i.e. one site is responsible for the CO dissociation and the other site is responsible for chain propagation. 20, 39 To check the validity of this concept, we conduct the pulse reactions of syngas on the 12Fe5C2/Co, Fe5C2 and Co catalysts respectively at 0.1 MPa. The peak of CO2 (m/z=44), H2O (m/z=17), CH4 (m/z=15) and C2 (m/z=26) of each catalyst at different temperatures were followed and integrated/normalized with the formula described in the experimental section. The MS profiles of the pulse reactions of syngas were shown in Figure S8-S11, and the results were summarized as shown in Figure 5. Clearly, CO2 started to appear on 12Fe5C2/Co and Co catalysts at only 120oC, which suggests the dissociation of CO on both catalyst at such temperature. Meanwhile, we did not detect CO2, H2O or CH4 products on the Fe5C2 catalyst at 120oC, indicating its incapability to dissociate CO at such low temperature. This is to say with the integration of Co NPs over Fe5C2 surface, we introduced the CO dissociation center on 12Fe5C2/Co catalyst. Indeed, we investigated the diffusion barriers of the CH species on Co and from Co to Fe5C2 through a DFT calculation, and found that the CH diffusion from Co to iron carbide sites is a thermodynamic favored process, which is in agreement with our experiment conclusion (Figure S12-S13). Furthermore, when elevating the reaction temperature from 120 to 220oC, the activity for CO activity on 12Fe5C2/Co catalysts gradually increased. The activity of Fe5C2 begin at around 150-


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180 oC and increased slowly because of its poor ability to dissociate CO in an atmosphere of low concentration of syngas. The activity of Co catalyst changes very slowly from 150-220 oC. On one hand, it is because that the extremely low loading of Co catalyst (0.6wt%) provided insufficient active sites. On the other hand, the low methanation rate limited the removal of C1 species, and hence blocking the sites for further CO dissociation.15-16, 40 On the contrary, the fast growing amount of product generated from 12Fe5C2/Co catalyst with the elevating temperature demonstrates that the activity of 12Fe5C2/Co catalyst increased rapidly along with the temperature. This is because that beside hydrogenation on Co sites, the C1 species could also migrate onto Fe5C2 surface to finish the hydrogenation reaction.15 In addition to that, as reported earlier by the theoretical modeling, the moderate metal-C interaction strength of Fe5C2, neither too strong nor too weak for C-C coupling, can also facilitate the formation of longer chains.15 Indeed, at 250oC, the formation of C2 hydrocarbon was observed on 12Fe5C2/Co catalyst, demonstrating that the C1 species generated mostly on Co sites can migrate and undergo chain propagation on Fe5C2 sites (Figure 5 d). We therefore conclude that in the 12Fe5C2/Co catalyst, since Co has much lower energy barrier for CO dissociation compared with that of FeCx, CO can be easily dissociated on Co site and provide enough C1 building blocks. At a same time, chain growth reaction can take place on the FeCx site under relatively low temperature. This consumes the produced C1 intermediates and prevent the Co site from being poisoned by the growing chains. Thus, by properly combining the two parts into a single catalyst, the FTS performance of 12Fe5C2/Co catalyst was boosted drastically under low temperature.


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Figure 5. Normalized peak areas of a) CO2, b) H2O, c) CH4 and d) C2H4 produced on Fe5C2/Co , Fe5C2 and Co catalysts in the pulse reaction of syngas at different temperature. CONCLUSION In a summary, to combine the merits of each iron- and cobalt-based FTS catalysts, we successfully construct heterostructured Fe5C2/Co dual center catalysts. By varying the Fe/Co molar ratio, with the optimal incorporation of only 0.6

wt%

Co, the heterostrucuted catalyst

exhibited activity comparable to conventional impregnated Co one. The pulse reactions of syngas along with DFT calculation demonstrated that Co is responsible for CO dissociation while Fe5C2 is for most of hydrogenation and chain growth. This work not only provides a new perspective for building the Fe-Co bimetallic FTS catalysts, but also investigates the working mechanism for such Fe-Co-based catalysts for the


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first time. Moreover, our result also proves that Fe5C2 NPs can work as an ideal platform for the catalysts of syngas conversion processes. Their intrinsic activity facilitates the tuning of the catalytic properties is crucial for the further investigation of the catalytic mechanism.

AUTHOR INFORMATION Corresponding Author * Email for Y. H. [email protected] * Email for D. M. [email protected] Author Contributions ╢These authors contributed equally. Present Addresses ├

Present address of Jing Yu: College of Materials Science and Engineering, Zhejiang University

of Technology, Zhejiang province, 311122, China, Funding Sources This work was financially supported by the Natural Science Foundation of China (51125001, 51590882, 91645115, 21473003, 81421004, 51602285, 51672010), The State key Project of Research and Development of China (2016YFA0200102, 2017YFA0206301, 2017YFB0602200), 973 Project (2013CB933100) and China Postdoctoral Science Foundation (Grant No. 2015M580011) Notes


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The authors declare no competing ﬁnancial interest.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. XAFS and XPS of Fe5C2/Co nanostructure, TEM image of Fe5C2/Co with different Fe/Co ratio, selectivity of Fe5C2 catalyst, chain growth probability of 12Fe5C2/Co and Co catalysts, TPH of 12Fe5C2/Co catalyst, original profiles for the pulse reactions of syngas and computational detail.

ACKNOWLEDGMENT We thank the Shanghai Synchrotron Radiation Facility for providing of the beamline. REFERENCES 1. de Smit, E.; Weckhuysen, B. M. Chem. Soc. Rev. 2008, 37, 2758-2781. 2. Zhai, P.; Xu, C.; Gao, R.; Liu, X.; Li, M. Z.; Li, W. Z.; Fu, X. P.; Jia, C. J.; Xie, J. L.; Zhao, M.; Wang, X. P.; Li, Y. W.; Zhang, Q. W.; Wen, X. D.; Ma, D. Angew. Chem. Int. Ed. 2016, 55, 9902-9907. 3. Wang, H.; Zhou, W.; Liu, J. X.; Si, R.; Sun, G.; Zhong, M. Q.; Su, H. Y.; Zhao, H. B.; Rodriguez, J. A.; Pennycook, S. J.; Idrobo, J. C.; Li, W. X.; Kou, Y.; Ma, D. J. Am. Chem. Soc. 2013, 135, 4149-4158. 4. Weststrate, C. J.; van de Loosdrecht, J.; Niemantsverdriet, J. W. J. Catal. 2016, 342, 1-16. 5. Yu, J. W.; Li, W. Z.; Zhang, T.; Ma, D.; Zhang, Y. W. Catal. Sci. Tech. 2016, 6, 8355-8363. 6. Galvis, H. M. T.; Bitter, J. H.; Davidian, T.; Ruitenbeek, M.; Dugulan, A. I.; de Jong, K. P. J. Am. Chem. Soc. 2012, 134, 16207-16215. 7. de Smit, E.; Cinquini, F.; Beale, A. M.; Safonova, O. V.; van Beek, W.; Sautet, P.; Weckhuysen, B. M. J. Am. Chem. Soc. 2010, 132, 14928-14941. 8. Borg, O.; Dietzel, P. D. C.; Spjelkavik, A. I.; Tveten, E. Z.; Walmsley, J. C.; Diplas, S.; Eri, S.; Holmen, A.; Ryttera, E. J. Catal. 2008, 259, 161-164. 9. den Breejen, J. P.; Sietsma, J. R. A.; Friedrich, H.; Bitter, J. H.; de Jong, K. P. J. Catal. 2010, 270, 146-152.


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TOC


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Co Heterostructured Nanoparticles

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