Mechanism of the Mn Promoter via CoMn Spinel for Morphology

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The Mechanism of Mn Promoter via CoMn Spinel for Morphology Control: Formation of Co2C Nanoprisms for Fischer-Tropsch to Olefins Reaction Zhengjia Li, Tiejun Lin, Fei Yu, Yunlei An, Yuanyuan Dai, Shenggang Li, Liangshu Zhong, Hui Wang, Peng Gao, Yuhan Sun, and Mingyuan He ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02144 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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The Mechanism of Mn Promoter via CoMn Spinel for Morphology Control: Formation of Co2C Nanoprisms for Fischer-Tropsch to Olefins Reaction Zhengjia Li a,b, Tiejun Lin b, Fei Yu b,c, Yunlei An b,d, Yuanyuan Dai b,c, Shenggang Li b,e, Liangshu Zhong b,*, Hui Wang b, Peng Gao b, Yuhan Sun b,e,*, Mingyuan He a a

School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, PR

China. b

CAS Key Laboratory of Low-carbon Conversion Science & Engineering, Shanghai Advanced Re-

search Institute, Chinese Academy of Sciences, Shanghai 201210, PR China c

University of the Chinese Academy of Sciences, Beijing 100049, PR China

d

School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, PR Chi-

na e

School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, PR China

ABSTRACT: The Fischer-Tropsch to olefins (FTO) reaction over the Co2C catalysts is structuresensitive, as the catalytic performance is strongly influenced by the surface structure of the active phase. The exposed facets determine the surface structure, and it remains a great challenge to precisely control the particle morphology of the FTO active phase. In this study, the controlling effect of the Mn promoter on the final morphology of the Co2C nanoparticles for the FTO reaction was investigated. The unpromoted catalyst and several promoted catalysts with Ce, La, and Al were also studied for comparison. For the Mn-promoted catalysts, the combination method of the Co and Mn components plays a crucial role on the final morphology of Co2C and thus the catalytic performance. For the CoMn catalyst prepared by co-precipitation, Co2C nanoprisms with specifically exposed facets of (101) and (020) can be obtained, which exhibit a promising FTO catalytic performance with high C2-4= selectivity, low methane selectivity, and high activity under mild reaction conditions. However, for the Mn/Co catalyst prepared via impregnation, Co2C nanospheres are formed, which exhibit high methane selectivity, low C2-4= selectivity and low activity. For the unpromoted catalyst and the catalysts promoted by Ce and La, Co2C nanospheres are also obtained, with similar catalytic performance as the Mn/Co catalyst prepared via impregnation. Due to the high stability of the Co2AlOx composite oxide, no Co2C phase can be formed for the catalyst promoted by Al.

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KEYWORDS: Fischer-Tropsch, Fischer-Tropsch to olefins, Syngas, Cobalt carbide, Facet effect, Manganese, Promoter.


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1. INTRODUCTION Lower olefins (C2-4=) are key building blocks of the chemical industry, and are widely used to synthesize a broad range of products such as polymers, solvents, drugs, cosmetics, detergents etc1-5. Traditionally, lower olefins are produced from naphtha cracking or fluid catalytic cracking (FCC)6. With increasing concerns on the availability of the feedstock and the impact on the environment, alternative routes starting with syngas (a mixture of carbon-monoxide and hydrogen) derived from natural gas, coal or renewable biomass for the production of lower olefins are more desirable7. Fischer-Tropsch synthesis (FTS) is a heterogeneously catalyzed polymerization reaction for the production of a wide variety of hydrocarbons from syngas8-9. The direct conversion of syngas into lower olefins via FTS without the intermediate steps is the so-called Fischer-Tropsch to Olefins (FTO) process1, 6, 10-12. Among the traditional FT catalysts (iron, cobalt and ruthenium), iron-based catalysts are attractive for the production of olefins13-14. Torres Galvis et al. found that Na and S-promoted iron catalysts using an inert support with weak interaction exhibited excellent selectivity to lower olefins (61 C%)1. Zhai et al. developed a Zn and Na-modulated Fe catalyst, where Zn served as the structural promoter and Na altered the electronic structure. The modification of the electronic structure of the catalyst surface suppressed the hydrogenation of double bonds and promoted the desorption of the olefin products, which rendered the catalyst unexpectedly selective toward olefins, especially C5+ olefins3. The FTO reaction is recognized as a structure-sensitive reaction in that the catalytic performance is strongly affected by the detailed surface structure, which is controlled by particle size and exposed facets of the active phase2, 15. The effect of particle size of the active phase has been extensively studied for iron-based2, 16-17 and cobalt-based18-20 catalysts. Torres Galvis et al. studied the size effect of iron carbide nanoparticles, and found that smaller iron carbide particles display higher surface specific activities mainly due to the higher methane production2. The size effect of metallic cobalt nanoparticles was also studied by Bezemer et al., and it was found that the catalytic performance was independent of the particle size for cobalt catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), but both activity and selectivity were strongly affected for catalysts with smaller cobalt particles19. Too small cobalt nanoparticles would lead to lower activity and higher methane selectivity. In addition, a few theoretical studies were also reported on the effect of the phase and exposed facets of the FT catalysts. Liu et al. reported that hcp Co catalysts prefer the direct dissociation route while fcc Co catalysts prefer the H-assisted route, due to the great influence of crystallographic structure and corresponding morphology effect on the formation of the various active sites with higher intrinsic activity and density21. Pham et al. predicted the equilibrium shape of χ-Fe5C2 crystallite by Wulff construction based on the surface energies of eleven facets observed by XRD patterns22. The thermodynamically most stable (510) facet was found to have the largest contribution to the total surface area of χ-Fe5C2 at 34.9 %. It was found that this surface ACS Paragon Plus Environment

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exhibited high effective barriers of CH4 formation, suggesting the unfavorable formation of CH4 under FTS conditions23. Recently, we discovered Co2C nanoprisms with specifically exposed facets of (101) and (020) arising from the CoMn catalyst as a new FTO active phase exhibiting high C2-4= selectivity (60.8 C%), low methane selectivity (5.0 C%) and high activity (31.8 C%) under mild reaction conditions (250 oC and 1 bar)24. The addition of Na, which acted as an electronic donator to cobalt, resulted in stronger CO adsorption and enhanced CO dissociation, benefiting for the formation and stabilization of Co2C.25 However, little information was available on the effect of the Mn promoter on the catalyst structure and the catalytic performance. It was found that Co2C nanostructures possessed strong facet effects during syngas conversion. Co2C nanoprisms exhibited promising catalytic performance for the FTO reaction. In contrast, the formation of Co2C nanoparticles with sphere-like morphology has been considered as one of the main reasons for the deactivation of the Co-based FT catalysts, leading to very low FT activity with methane as the main product26. Nevertheless, it remains a great challenge to control the morphology of Co2C nanostructures with specific exposed facets. Product selectivity can be improved by the addition of various electronic promoters. Many different elements have been investigated as possible promoters to improve the C2-4= selectivity such as sodium27, manganese16-17, 28-29, cerium17, 30 and lanthanum31 during the FTS reaction process. The Mn promoter has been described as a typical electronic promoter, which could enhance both the activity and olefins selectivity. Bell et al. found that the Mn promoter led to higher CO surface coverage and facilitated CO dissociation, which resulted in a heavier hydrocarbon product distribution owing to less H availability for methanation and paraffin chain termination as well as more rapid C1 monomer generation28-29. A significant promoting effect of Mn was observed on TiO2-supported catalysts prepared by Morales32. The addition of small amounts of Mn to the Co/TiO2 catalyst affects the catalytic performance in the FTS by increasing the activity and suppressing the CH4 yield. They also found that the location of MnOx in the Co/TiO2 FTS catalysts was strongly affected by the preparation procedure and as a consequence determined the effectiveness of Mn as a promoter. The effects of the Mn promoter were more pronounced in the CoMn composite oxide than in the catalysts prepared by impregnation33. Bezemer et al. studied the promoting effects of Mn on carbon nanofiber-supported (CNF) cobalt catalysts for FT synthesis. Chain growth probability increased, and simultaneously, the product distribution shifted toward olefins at increasing MnO loadings34. Xiang et al. reported that the product distribution of aldehydes/alcohols and olefins/paraffins could be tuned by reaction conditions with the K-promoted CoMn catalyst. A Kstabilized Co2C along with a mixed valence Mn2(II)Mn3(IV)O8 phase was found.31 The two phases appeared to be in intimate contact during the reaction, and were anticipated to act synergistically to produce oxygenates and olefins. In the past decades, many experimental and theoretical studies have focused on the electronic effects of the Mn promoter, which improved the olefins selectivity by altering the electronic structure of the 4 ACS Paragon Plus Environment

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active phase. However, few studies were concerned with the effect of Mn on the morphology of the active phase. Here, we reported a morphology control effect of the Mn promoter on Co2C-based FTO catalysts. For the Mn-promoted catalyst, the effect of the combination method of Co and Mn on the final morphology of Co2C and the catalytic performance was investigated. In order to elucidate the effects of the Mn promoter on the Co2C nanoprisms formation, the unpromoted catalyst and several promoted catalysts (CoCe, CoLa and CoAl) were also studied. Connections were established between the combination method of Co and Mn, the morphology of Co2C and the catalytic performance in the FTO reaction to demonstrate the morphology control effect of the Mn promoter via CoMn spinel on Co2C-based catalysts. These findings suggested that the addition of the promoter greatly influenced the Co2C morphology, and the Mn promoter in the form of CoMn composite oxide promoted the formation of the Co2C nanoprisms with promising FTO performance.

2. EXPERIMENTAL SECTION

2.1 Catalyst Preparation

The CoMn composite oxide catalyst denoted as CoMn was prepared by the co-precipitation method. Typically, appropriate amount of cobalt nitrate (Co(NO3)2·6H2O, Sinopharm Chemical Reagent Co., Ltd.) and manganese nitrate (50 wt. % Mn(NO3)2, aqueous, Sinopharm Chemical Reagent Co., Ltd.) were dissolved in deionized water to form a 2 mol/L mixed salt solution (Co/Mn (atomic ratio) 2/1). Meanwhile, anhydrous sodium carbonate (Na2CO3, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in deionized water to form a 2 mol/L alkali solution as the precipitant. The aforementioned salt and alkali solution were simultaneously added dropwise into a beaker with 100 mL of deionized water with mechanical stirring. A constant pH was maintained at 8.0±0.1, and temperature was kept at 30±1 o

C during the precipitation process. After aging for 2 h at 30 oC, the obtained suspension was centri-

fuged, washed eight times with deionized water, and dried at 80 oC for 12 h. Then the samples were calcined in a furnace at 330 oC for 3 h under static air. The unpromoted catalyst and several other metal oxide-promoted catalysts were prepared with similar procedure and termed as Co3O4, CoCe, CoLa and CoAl, respectively as shown in Supporting Information. The atomic ratio of Co to other metals was also maintained as 2. Another Mn-promoted Co3O4 catalyst termed as Mn/Co (Co/Mn (atomic ratio) =2) was prepared by the impregnation method. Typically, appropriate amount of manganese nitrate was dissolved in 20 mL of deionized water. Then, the solution was mixed with 10 g of Co3O4 catalyst with magnetic stirring at


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room temperature for 1 h. After that, the mixture was dried for 12 h and then calcined in a furnace at 200 oC for 3 h under static air.

2.2 Catalyst Characterization

The concentrations of the different elements were obtained with an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8000, PerkinElmer). X-ray diffraction (XRD) patterns of catalysts were recorded on a Rigaku Ultima IV X-ray powder diffractometer using Cu Kα radiation with the wavelength of 1.54056 Å at 40 kV and 40 mA. For phase identification, the samples were scanned from 10 to 90° at a rate of 4°/min in the mode of continuous scanning. For semi-quantitative analysis, the samples were scanned from 40° to 48° in a step scanning mode with a rate of 0.24°/min (5 s per 0.02°). The content of the different phases was calculated by reference intensity ratios (RIR). Generally, the integrated intensity of the most intense peak of each phase was obtained from fitting and deconvolution of the XRD patterns. The mass fraction of x phase (wx) was calculated from the following equation: =

∑

(1)

where I is the integrated intensity of the most intense peak, and the RIR values are available from the PDF cards. TEM micrographs and particle size distributions of the catalysts were obtained with a JEOL JEM 2000FX microscope operating at 200 kV. For the TEM measurements, the solids were dispersed in ethanol and dropped on a copper microgrid covered by a holey-carbon film. The nanoparticle size distribution for each sample was determined using samples of ~300 nanoparticles. The surface mean diameter of the nanoparticles in a catalyst sample was calculated from the equation

∑ ̅ = ∑

(2)

where ni is the number of particles with diameter di . For TEM characterization, the catalysts were reduced at 300 oC with a mixture of H2 and Ar (10 % H2 v/v, 12000 mL/(h gcat)) for 5 h at atmospheric pressure with a heating ramp of 1 oC/min. Temperature-programmed reduction (TPR) experiments were performed with a Micromeritics Autochem-II 2920 instrument equipped with a thermal conductivity detector (TCD) and an MKS Cirrus 2 mass spectrometer. The samples were pretreated under a helium flow at 200 oC for 120 min. Once the TCD signal was stable, the gas stream was switched to 5 % (v/v) H2/Ar, and the temperature was then increased from 50 to 800 oC at a rate of 5 oC/min. ACS Paragon Plus Environment

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Nitrogen adsorption measurements were performed on a Micromeritics 2420 instrument at -196 oC. The sample was degassed at 300 oC under vacuum for 6 h. The specific area was calculated by the Brunauer-Emmett-Teller (BET) equation. The total pore volume and the average pore size were determined by the Barrett–Joyner–Halenda (BJH) method.

2.3 Catalytic Evaluation

The FTO reaction was performed at 1 bar and 250 oC in a tubular quartz fixed-bed reactor (diameter 8 mm). A molding process was necessary to form large particles with sizes of 40-60 mesh (0.3-0.45 mm) prior to the reaction in order to avoid blockage and improve diffusion. The catalyst was diluted with 0.4 g of SiC powder to remove any temperature gradient within the catalyst bed. The catalyst was reduced prior to the reaction at 300 oC with a mixture of H2 and Ar (10 % H2 v/v, 12000 mL/(h gcat)) for 5 h at atmosphere pressure, and the heating ramp was 1 oC/min. The temperature was then dropped to 250 oC under a He (99.999 %) flow for 30 min to purge the residual reduction gas. Subsequently, the feed flow was switched to a mixture of N2, H2, and CO (N2/H2/CO=3/64.6/32.3 v/v/v, 3000 mL/(h gcat)). During the reaction, the reactor effluent was analyzed online at periodic intervals by an Agilent 7890A gas chromatograph equipped with two columns and two detectors. Analysis of N2 (reference), H2, CO, CH4, and CO2 was performed using a packed column (J&W Q&5A), and a thermal conductivity detector (TCD) using helium as the carrier gas. Hydrocarbons from C1 up to C7 were analyzed using a capillary column (Agilent 19095P-K25) and a flame ionization detector (FID). The selectivity was calculated as the percentage of equivalent carbons present in the hydrocarbon product (C%). CO conversion was calculated on a carbon atom basis according to the equation =

!"# $%&#!"# !"#

× 100%

(3)

Where COinlet and COoutlet represent moles of CO at the inlet and outlet, respectively. CO2 selectivity was calculated according to the equation + ,-.//0 =

%&#!"# !"# $%&#!"#

× 100%

(4)

Where CO2 outlet denotes moles of CO2 at the outlet. The selectivity of the individual product CnHm based was obtained from the equation 12 ,-.//0 =

34 %&#!"# !"# $%&#!"# $ %&#!"#

× 100%

(5)

Where CnHm outlet represents moles of C of the product at the outlet. As the selectivities of heavier hydrocarbon products were very low, it was very difficult to perform a full analysis of all the products at 1 bar. In this case, the method by difference was used to calculate the ACS Paragon Plus Environment

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C5+ + Oxy selectivity (CO converted to products other than CO2 and the C1-C4 products), where C5+ + Oxy represents hydrocarbons with 5 or more than 5 carbons and Oxy denotes oxygenate products.

3. RESULTS

3.1 ICP

The ratios of cobalt to the different promoters and the concentrations of the residual sodium for the studied samples are presented in Table 1. The Co/Metal ratios determined with ICP were approximately 2, similar to the nominal value. For all catalysts, the Na concentration ranged from 0.3 to 1.4 wt.%, which ensured the Co2C formation25. Table 1. Elemental analysis of the different catalysts. Sample Co/Metala Na concentration (wt.%)a Co3O4 1.4 Mn/Co 2.10 0.9 CoCe 2.01 1.0 CoLa 1.93 0.9 CoMn 2.06 0.6 CoAl 2.22 0.3 a Atomic ratio measured with ICP.

3.2 H2-TPR

Temperature-programmed reduction (TPR) profiles for all catalysts are shown in Figure 1 and the peak areas of different samples are presented in Table S1 in the Supporting Information. For the Co3O4 sample, two peaks centered around 227 and 328 oC were observed, corresponding to a two-step reduction process from Co3+ to Co2+ and Co2+ to metallic Co0 35. For the Mn/Co sample, analysis of the peak areas indicated that the reduction of Mn4+ to Mn2+ was overlapped with the reduction of Co3+ to Co2+ at 278 oC. Previous studies revealed that manganese hampered the reduction of Co2+ to Co0, as indicated by the third peak centered around 406 oC36. For the CoCe sample, analysis of the peak areas indicated that the second peak around 304 oC was assigned to the reduction of Ce4+ to Ce3+. For the CoLa sample, the first peak (297 oC) originated from the reduction of Co3+ to Co2+, and the second peak (386 oC) stemmed from the subsequent reduction to metallic Co0. For the CoMn sample, three peaks centered around 168, 266 and 389 oC were observed. The first peak around 168 oC was due to the partial reduc8 ACS Paragon Plus Environment

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tion of the CoMn composite oxide. The second peak around 266 oC was assigned to further reduction of the CoMn composite oxide. The third peak around 389 oC was originated from the reduction of the CoMn composite oxide to Co0 and MnO.25 Similar to the CoMn sample, cobalt oxide and alumina also can be combined as a composite oxide and the CoAl sample had similar reduction process as shown in Figure 1. Three peaks centered around 217, 303 and 597 oC were observed and all of the peaks shifted to higher temperature. It indicated that the CoAl sample was more difficult to reduce.

Figure 1. H2-TPR profiles of the different catalysts.

3.3 N2 adsorption

Textural properties were acquired for all catalysts after the reduction and the reaction as presented in Table 2. Compared with the Co3O4 sample, the addition of metal oxide promoters increased the specific surface area and the pore volume for both the reduced and the spent samples. In contrast to the Co3O4 sample, the pore diameter of most of the promoted catalysts decreased, while those of the Mn/Co, CoCe and CoMn samples increased slightly after reaction. The SBET, Vtotal and Dpore of the reduced promoted catalysts ranged from 22.3 to 197.6 m2/g, 0.11 to 0.46 cm3/g and 8.5 to 16.9 nm, respectively. In addition, the SBET, Vtotal and Dpore of the spent promoted catalysts ranged from 11.3 to 185.2 m2/g, 0.06 to 0.50 cm3/g and 7.7 to 20.2 nm, respectively. It seemed that the specific surface area and the pore volume decreased, and the pore diameter increased for the promoted catalysts after the reaction.

Table 2. Textural properties of the different catalysts. Sample

BET Surface Area (m2/g)

Pore Volume (cm3/g)


Pore Diameter (nm) 9

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Co3O4 Mn/Co CoCe CoLa CoMn CoAl

Reduced 6.6 22.3 52.6 49.6 49.8 197.6

Spent 10.4 11.3 42.0 32.1 16.9 185.2

Reduced 0.04 0.11 0.20 0.22 0.13 0.46

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Spent 0.04 0.06 0.20 0.11 0.12 0.50

Reduced 25.6 16.9 13.4 15.1 9.5 8.5

Spent 15.0 18.4 16.4 7.7 20.2 9.3

3.4 XRD

X-ray powder diffraction was carried out to study the structure of the fresh, calcined, reduced and spent samples as shown in Figure 2. The XRD results suggested that only the CoMn sample was composite carbonate, while the other samples were amorphous. The diffraction peaks of the calcined CoMn and CoAl samples were attributed to the composite oxides, while those of the calcined Mn/Co, CoCe and CoLa samples were due to mixtures of Co3O4 and MnO2 (CeO2 or La2O3). The XRD result for the reduced Co3O4 sample indicated that the diffraction peaks corresponded to metallic Co (fcc and hcp). For the promoted catalysts, only CoMn and CoAl catalysts prepared by co-precipitation were composite oxides, while others were in the form of mixed oxides. In the case of the Mn/Co sample, a mixture of fcc Co and CoO was identified, while only CoO phase was observed in the case of the CoCe and CoLa samples. According to the H2-TPR results, both CeOx and MnOx underwent reduction during catalyst pretreatment, but only the Mn/Co sample showed MnOx with lower valency. The ceria nanoparticles were oxidized during the passivation. The broad peaks in the CoLa sample indicated that the CoO and La2O3 were well dispersed. According to our previous study24, a mixture of the CoMn composite oxide with lower valency and fcc Co0 was identified. Similar to the peaks of CoO, the diffraction peaks in the CoAl sample corresponded to the sole phase of the CoAl composite oxide with lower valency. The XRD measurement of the spent samples for Co3O4, Mn/Co, CoCe and CoLa revealed that all Co species were converted into the Co2C phase as a result of the residual Na promoting effects, which benefited the formation and stabilization of Co2C. We note that some diffraction peaks of metallic cobalt, CoO, and Co2C were overlapped in the range of 40~50°. However, the main differences in the peaks of metallic Co (fcc and hcp) from Co2C were around 44.23° (fcc Co0) and 44.26° (hcp Co0) as shown in our previous study25, and the absence of metallic cobalt phase in all spent samples in this study was inferred from the missing diffraction peaks of metallic cobalt around 44.23° and 44.26°. The main differences of the peaks of CoO from Co2C phase were around 61.51° and 73.69° (CoO), and these diffraction peaks of CoO phase were also not observed in all spent samples. Therefore, we concluded that except for the CoMn and CoAl samples, all Co species were converted into the Co2C phase. In the case of the CoMn ACS Paragon Plus Environment

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sample, the diffraction peaks corresponded to CoxMn1-xO, MnO and Co2C, indicating that CoxMn1-xO separated into Co2C and MnO. However, the CoAl composite oxide was very stable under the reaction conditions and no Co2C phase can be formed for the CoAl sample. To further study the formation and stability of Co2C nanoprisms, in-situ XRD characterization of the CoMn sample was performed as shown in Figure S1 in the Supporting Information. The diffraction peaks of the reduced CoMn sample corresponded to a mixture of the CoMn composite oxide with lower valency and fcc Co0. Once the feed flow was switched to syngas, the intensity of fcc Co0 decreased, and the peaks of Co2C were identified. With the increase of reaction time, the intensity of Co2C increased. The in-situ XRD results indicated that Co2C nanoprisms was formed during the initial stage of the reaction and was stable under the reaction condition.

Figure 2. XRD patterns of the different catalysts: (a) fresh; (b) calcination; (c) reduction; (d) spent (TOS = 85 h for the CoCe and CoLa samples, and TOS = 35 h for the Co3O4, Mn/Co, CoMn and CoAl samples).


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3.5 TEM

The TEM images of the fresh and calcined samples were shown in Figure S2 and S3 in the Supporting Information. Corresponding to the XRD results, only the CoMn sample was composite carbonate, while the other samples were amorphous. All fresh samples had irregular morphology. After calcination, all samples were spherical, and no nanoprisms were observed. The surface mean diameters were obtained by examining about 300 nanoparticles from the different regions in the TEM images of the reduced samples (Figure S4 in the Supporting Information) as shown in Figure 3. For all the reduced samples, only spherical morphology was observed. All samples displayed narrow particle size distributions. The surface mean diameter of the Co3O4 sample was 97.1 nm, and the addition of the metal oxide promoters resulted in a significant decrease in the mean diameters. For the Mn/Co sample, the particle size was 31.0 nm, while with the CoMn composite oxide sample the particle size further decreased to 16.3 nm. Similar to the CoMn sample, the surface mean diameter of the CoAl sample was 11.4 nm. The addition of Ce or La resulted in better dispersion. The CoCe and CoLa samples exhibited very small surface mean diameters, 5.0 and 4.2 nm, respectively.

Figure 3. Particle size distribution of the different reduced catalysts: (a) Co3O4; (b) Mn/Co; (c) CoCe; (d) CoLa; (e) CoMn; (f) CoAl.


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Figure 4. TEM images of the different spent catalysts (TOS = 85 h for the CoCe and CoLa samples, and TOS = 35 h for the Co3O4, Mn/Co, CoMn and CoAl samples): (a, d) Co3O4; (b, e) Mn/Co; (c, f) CoCe; (g, j) CoLa; (h, k) CoMn; (i, l) CoAl. The nanoparticles marked by red rectangles were in the shape of nanoprism.


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TEM images of the spent samples are shown in Figure 4. For all samples, the Co species were converted to the Co2C phase as a result of the residual Na promoter effect, and metallic Co0 was not detected. In the case of the Co3O4 sample, only the Co2C phase with a spherical morphology was observed. The MnO and Ce2O3 were identified for the Mn/Co and CoCe samples, respectively. However, La2O3 was not detected for the CoLa sample because of the small particle size corresponding to the XRD results. In the case of the CoMn sample, Co2C nanoprisms with specifically exposed facets of (101) and (020) marked by red rectangle were observed as shown in Figure 4h and 4k. According to our previous work, the spherical particles near the Co2C nanoprisms, were CoMn composite oxide and MnO24. However, the CoAl composite oxide was not changed due to its high stability as shown in Figure 4i and 4l.

3.6 Catalytic Performance

All catalysts were tested to determine the influence of the promoters on the catalytic activity and product selectivity. The catalytic activities and product selectivities in the present study were obtained at around 10 h of time on stream (TOS) after the stabilization of the catalysts. The TOS for the stabilization of the CoCe and CoLa samples was about 70 h, and for the other catalysts it was about 20 h. According to the XRD and TEM results of the CoAl sample, the CoAl composite oxide was not changed and neither Co2C nor Co0 were observed after the reaction. At the meantime, the CO conversion was lower than 2.5 C% even at high reaction temperature (270 oC) as shown in Table S2 in the Supporting Information. Due to the very low activity, the CoAl sample is not correlated with other Co2C-based catalysts in this study. For other catalysts, it is important to compare the catalytic activity on the basis of the turnover frequencies. However, it is difficult to measure the number of active sites by H2 or CO chemisorption due to the reduction of Co2C by H2 and the adsorption of CO by metal oxide promoters. The XRD and TEM results of the spent-reduced CoMn sample, which was further reduced by 10 % H2/Ar at 250 oC after the reaction, were shown in Figure S5. The XRD results suggested that Co2C were formed under the reaction conditions. However, it could be converted into hcp Co under 10 % H2/Ar flows at 250 oC. In the TEM images of the spent-reduced CoMn sample, hcp Co was found but no Co2C nanoprisms were detected. In the present study, the activity was estimated by CO consumption and normalized by the weight of Co2C as shown in Figure 5 and Table S3 in the Supporting Information, and the activities were also calculated in terms of hydrocarbons formation from CO as shown in Figure S6 and Table S3. Usually, Fe-based catalysts with high olefin selectivity were applied to the FTO reaction, and the reaction conditions were severe (high reaction temperature and pressure). The activities of Fe-based FTO catalysts were always below 10×10-5 molCO/(gFe s)1-2, 9, 12, while similar activity was reached by the CoMn sample under milder reaction conditions (250 oC, 1 bar). The weight of Co2C in the CoMn samACS Paragon Plus Environment

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ple was calculated by the semi-quantitative analysis of the XRD pattern of the spent CoMn sample as shown in Figure S7 in the Supporting Information. For all samples, cobalt carbide time yield (CCTY) increased with reaction temperature as shown in Figure 5a. The addition of promoters to the Co3O4 sample led to a significant enhancement of the activity. The CCTY of the Mn/Co sample was almost 8 times that of the Co3O4 sample, while the CCTY of the CoMn sample was almost 5 times that of the Mn/Co sample. The CCTY of the CoLa and CoCe samples were 10~30 times that of the Co3O4 sample at different reaction temperatures. The activity of all samples based on the CCTY increased as follows: Co3O4 ＜ Mn/Co ＜ CoCe ＜ CoLa ＜ CoMn. As shown in Figure 5b, the activity of the CoMn sample was also increased with the increase of the H2/CO ratio, while it had little influence on the other samples.

Figure 5. Activity in terms of CO consumption of the different catalysts: (a) activity versus temperature at 1 bar, 3000 mL/(h gcat) and H2/CO = 2 (v/v); (b) activity versus the H2/CO ratio at 250 oC, 1 bar and 3000 mL/(h gcat).

The product selectivities of all samples at different reaction temperatures were presented in Figure 6. The active phase of metallic cobalt was inactive for the water-gas-shift reaction (WGS) for traditional Co-based FT catalysts, while the active phase (iron carbide) of Fe-based FT catalysts was highly active for the WGS reaction and led to high CO2 selectivity. The Co2C-based catalysts were similar to Febased catalysts and exhibited highly active for the WGS reaction with high CO2 selectivity as shown in Figure 6a. Higher reaction temperature resulted in higher CO2 selectivity. As shown in Figure 6b, 6c and 6d, the CH4 and C2-4= selectivity increased with the reaction temperature, and C5+ + Oxy selectivity decreased. The product distributions for all samples shifted toward lower molecular weight hydrocarbons with the increase of reaction temperature, as a consequence of the improvement in hydrogenation, which resulted in a decrease in chain growth probability. The addition of metal oxide promoters resulted in difACS Paragon Plus Environment

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ferent changes in products selectivities. In comparison with the Co3O4 sample, the CH4 selectivity and C5+ + Oxy selectivity of the Mn/Co sample decreased and increased significantly, respectively. At a high reaction temperature (260 oC), the addition of the Mn promoter led to a decrease of CH4 selectivity from 21.8 C% to 9.6 C% and an increase of C5+ + Oxy selectivity from 36.4 C% to 55.2 C%. This indicated that the Mn promoter enhanced the heavier hydrocarbons selectivity and led to an increase in the chain growth probability. Except for the Mn/Co sample, all metal oxide promoters increased the C2-4= selectivity and decreased the C5++Oxy selectivity to some extent. The CoMn sample exhibited high C2-4= selectivity more than 45 C% at all temperatures and the highest C2-4= selectivity (54.1 C%) of all samples. At the meantime, the CoMn and CoCe samples inhibited methane formation. The CoMn sample also exhibited lowest CH4 selectivity less than 8 C% at all temperatures and the lowest CH4 selectivity (3.1 C%) of all samples. The C2-4= selectivity of all samples decreased with the promoter element as follows: CoMn ＞ CoCe ＞ CoLa ＞ Co3O4, Mn/Co, and the CH4 selectivity of all samples increased with the promoter element as follows: CoMn ＜ Mn/Co ＜ CoCe ＜ CoLa, Co3O4. Obviously, the CoMn sample exhibited simultaneously the highest C2-4= selectivity and lowest CH4 selectivity under mild reaction temperature. The effect of the H2/CO ratio on product selectivity was also studied as shown in Figure S8 in the Supporting Information. For all the samples, the CO2 selectivities were higher than 25 C% and increased with the H2/CO ratio. As shown in Figure S8b, S8c and S8d in the Supporting Information, the CH4 and C2-4= selectivities increased with H2/CO ratio, and the C5++Oxy selectivity decreased. This indicated that the product distributions for all samples shifted toward lower molecular weight hydrocarbons, as a consequence of higher H2 concentrations, which resulted in a decrease in the chain growth probability. Even at the low H2/CO ratio such as 0.5, the C2-4= selectivity of the CoMn sample (29.3 C%) was much higher than those of the others (less than 15 C%) with a low CH4 selectivity (1.4 C%). This indicated that the CoMn catalysts were more tolerant to syngas with low H2/CO ratio derived from coal and had promising industrial potential. The Olefin/Paraffin ratios (O/P ratios) of lower olefins at different H2/CO ratios were shown in Figure S9 in the Supporting Information. All O/P ratios decreased with the increase of H2/CO ratios as a result of the further hydrogenation. However, the CoMn sample still exhibited higher O/P ratios (C2-4 = 24.4) even at a H2/CO ratio of 2. Higher H2/CO ratio is disadvantageous to the formation light olefins, and may result in higher selectivity of CH4 in FTS. The H2/CO ratios of the effluent for the CoMn sample at different H2/CO ratios of the feed gas were also analyzed as shown in Table S4. These results suggested that the H2/CO ratios of the effluent were increased slightly with inlet H2/CO ratios of 1 and 2 (v/v), while it decreased with inlet H2/CO ratio of 0.5 (v/v). For CoMn FTO catalysts, the overall reaction can be written as the following: 2x CO + x H2 ↔ CxH2x +x CO2 Thus, the theoretical H2/CO ratio of the feed gas for olefins production with high CO2 selectivity is 0.5. The lower H2/CO ratio results in higher O/P ratios and lower CH4 selectivity, and is beneficial to olefins 16 ACS Paragon Plus Environment

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production. Therefore, the CoMn FTO catalysts were more suitable for the biomass and coal-based syngas with low H2/CO ratios. We also carried out the FTO reaction using the studied CoMn catalyst under industry-relevant reaction conditions with higher reaction pressures and longer TOS as presented in Table S5, Figures S10, S11 and S12. With the increase of reaction pressure from 1 to 10 bar, the CO conversion and CH4 selectivity increased significantly, while the C2-4= selectivity and O/P ratios decreased due to the extensive secondary hydrogenation reaction. The diffraction peaks of the spent CoMn sample under 10 bar reaction pressure were attributed to a mixture of Co2C and MnCO3, and the Co2C nanoprisms were observed in the spent sample. In our previous study24, we found no obvious deactivation over 600 hours of reaction at 250 oC, 3 bar, H2/CO=1 (v/v) and 6000 mL/(h gcat). As shown in Figure S11, significant improvement in the stability to about 1200 h of TOS without obvious deactivation was achieved using SiC as the diluent for the enhancement of heat transferring. A large amount of Co2C nanoprisms were observed in the spent sample as shown in Figure S12b. These results revealed that the Co2C nanoprisms were stable under industry-relevant reaction conditions, suggesting promising potential for industrial applications.


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Figure 6. Product selectivity of the different catalysts at different temperatures, 1 bar, 3000 mL/(h gcat), and H2/CO = 2 (v/v): (a) CO2 selectivity; (b) CH4 selectivity; (c) C2-4= selectivity; (d) C5++Oxy selectivity.

The product distributions of all samples at different reaction temperatures and H2/CO ratios were presented in Figures 7 and 8, respectively. Product distributions of the Co3O4 and CoLa samples fitted the traditional Anderson-Schultz-Flory (ASF) model well and the deviation of methane was not observed. A slight deviation of CH4 from the traditional ASF model was observed for the product distributions for the Mn/Co and CoCe samples. Great deviation from the traditional ASF model was found for the CoMn sample due to the strong facet effects of Co2C nanoprisms. At low reaction temperature or low H2/CO ratio, an even greater deviation from the traditional ASF model was observed.


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Figure 7. Product distributions of the different catalysts at different temperatures, 1 bar, 3000 mL/(h gcat), and H2/CO = 2 (v/v): (a) 230 oC; (b) 240 oC; (c) 250 oC; (d) 260 oC.

Figure 8. Product distributions of the different catalysts at different H2/CO ratios, 250 oC, 1 bar and 3000 mL/(h gcat): (a) H2/CO = 0.5; (b) H2/CO = 1; (c) H2/CO = 2.

4. DISCUSSION


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Mn is widely used as a promoter for FTS catalysts, particularly for improving the activity and selectivity17, 28-29. It has also been reported that Mn acts not only as an electronic promoter to alter the chemisorption of the reactants on the catalyst but also as a structural promoter to enhance dispersion of the active phase and to stabilize the catalyst during the FTS process25. For the reduced unpromoted Co3O4 sample, the surface mean particle size was 97.1 nm, while that of the reduced promoted samples decreased sharply. The particle size of the reduced Mn/Co sample was 31.0 nm. Furthermore, the particle size of the reduced CoMn sample decreased to 16.3 nm. Therefore, it is reasonable to speculate that Mn also acted as a structural promoter in the CoMn FTO catalysts and the formation of the CoMn composite oxide precursor benefited the cobalt dispersion. In addition, the particle sizes of the reduced CoAl, CoCe and CoLa samples were 11.4, 5.0 and 4.2 nm, respectively. As a structural promoter, the promoting effect is likely related to the strength of the interaction between the promoter and cobalt oxide. For the typical FT reaction, the products always obey the ASF distribution. According to the ASF model, product selectivity depends on the chain growth probability, and it is impossible to selectively synthesize parts of the hydrocarbons as FTS proceeds via polymerization of the CHx species37. Hence, a major challenge for the FTO process is to break the restriction of the ASF model in order to achieve high olefins selectivity with low methane selectivity. Mn has been proved to be an effective promoter of Fe-based catalysts to increase the selectivity of lower olefins and suppress the formation of undesirable methane as a result of the enhancement of CO dissociation5, 38. However, the reported methane selectivity is still rather high, especially at high reaction temperature. In this study, the product distribution of the unpromoted Co3O4 sample totally conformed to the ASF model under different reaction temperatures and H2/CO ratios. The product distribution of the CoLa sample was the same as that of the Co3O4 sample without deviation of methane selectivity from the ASF model. For the Mn/Co and CoCe samples, certain suppression effect for CH4 formation was observed. Strong suppression effect for CH4 formation was observed for the CoMn sample with the formation of Co2C nanoprisms. Combined with our previous study24, it is reasonable to speculate that the deviation of the CH4 selectivity from the ASF distribution for the CoMn sample mainly originated from the strong facet effect of Co2C nanoprisms with specifically exposed facets of (101) and (020). For the CoMn sample, the precursor was the CoMn composite oxide promoted with the residual Na from the precipitant Na2CO3. The Na promoter acted as an electronic donator to cobalt resulted in stronger CO adsorption and enhanced CO dissociation, benefiting Co2C formation and stabilization. To date, several studies have reported the Co-based FTS catalysts, especially the CoMn composite oxide, with or without alkali promoters, but the Co2C nanostructures with specifically exposed facets have not been observed28, 31, 35, 39-40. The conditions for the formation of Co2C nanoprisms are still ambiguous. In the present study, we compared the CoMn composite oxide precursor with other precursors (Co3O4, Mn/Co, CoCe, CoLa and CoAl) in order to explore the formation mechanism of the Co2C nanoprisms. 20 ACS Paragon Plus Environment

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Except for the CoAl sample, the cobalt species were converted into Co2C and no metallic Co was detected as shown in Figure 2. According to the TEM images of the spent samples, Co2C nanospheres were observed for the samples of Co3O4, Mn/Co (impregnation), CoCe and CoLa, while the CoMn composite oxide precursor successfully resulted in the formation of Co2C nanoprisms. All the samples after reduction had spherical morphology, and no Co2C nanoparticles were detected. According to our previous work, Co2C nanoprisms were formed during the first 20 hours of the reaction, and were separated from the CoMn precursor24. Based on the results of DFT calculations, which had reported in our previous work24, it was concluded that on the Co2C(101) surface, CH2CH2 was the most stable species from both thermodynamics and kinetics points of view. Meanwhile, methane formation was inhibited on both Co2C(101) and Co2C(020). All exposed facets of Co2C nanoprisms belonged to these two specific facets, and the percentage of the exposed facets (101) and (020) for Co2C nanoprisms was 100 %. It revealed that the Mn promoter had a controlling effect on the final morphology of Co2C nanoparticles and promoted the formation of Co2C nanoprisms. However, the combination method between the Mn promoters and cobalt species also had a significant influence on the final morphology of Co2C nanoparticles. The Mn/Co sample was synthesized by precipitation of Co3O4 using Na2CO3 as the precipitant followed by impregnation of Mn from a nitrate solution, leading to a similar elemental composition as the CoMn sample (~2.1 Co/Mn and 0.6 to 0.9 wt.% Na). The two samples showed different Co2C morphologies and distinct catalytic performance. The Co2C nanospheres in the Mn/Co sample exhibited low C24

=

selectivity and low activity. Only the CoMn sample with the combination method of composite ox-

ides formed the Co2C nanoprisms. The Co2C nanoprisms in the CoMn sample exhibited simultaneously highest C2-4= selectivity, lowest methane selectivity and extra high activity. The Na promoter acted as an electronic donator to cobalt resulted in stronger CO adsorption and enhanced CO dissociation, benefiting Co2C formation and stabilization. Based on the previous studies19, 24, the method to incorporate the Na promoter into the catalysts is not relevant. However, the intimate contact between cobalt species and the Mn promoter achieved by co-precipitation is the key to achieve the nanoprism morphology. The addition of the Na or Mn promoter individually would not facilitate the formation of Co2C nanoprisms during the reaction, and the desired morphology was only achieved when the cobalt species and the Mn promoter were in close contact and under the presence of the Na promoter. The synergy effect between the Na and Mn promoters benefited for the formation of the Co2C nanoprisms. The XRD and TEM results of the reduced CoAl sample indicated that the cobalt species and alumina were also combined as composite oxides. However, the Co2C nanoprisms were undetected for the spent CoAl sample. Concerning with the high reduction temperature of the TPR results in Figure 1, it is reasonable to speculate that the CoAl composite oxide remained during the reaction process due to its high stability. We inferred that the CoMn composite oxide may work as sources for the formation of small crystal nucleus of Co2C and then continuously grew to form nanoprisms. However, for the CoAl composite oxide, it was hard to ACS Paragon Plus Environment

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form crystal nucleus of Co2C due to the high stability of CoAl composite oxide. Therefore, the Mn promoter combined with cobalt species as a composite oxide with suitable stability had a morphology controlling effect on final morphology of Co2C nanoparticles and led to the formation of the Co2C nanoprisms with specifically exposed facets of (101) and (020) and excellent FTO catalytic performance.

5. CONCLUSIONS

The controlling effect of several promoters (Mn, Ce, La and Al) on the morphology of the Co2C phase for the FTO reaction was investigated in details. The combination method and the resulting interaction between the cobalt species and the studied promoters played an important role on the final morphology of the Co2C nanoparticles and the catalytic performance. For the CoMn catalyst prepared by coprecipitation, Co2C nanoprisms with specifically exposed facets of (101) and (020) can be obtained exhibiting a promising FTO catalytic performance with high C2-4= selectivity, low methane selectivity and high activity under mild reaction conditions. The formation of the CoMn spinel via co-precipitation method seemed to act as a precursor for the formation of the very selective Co2C nanoprisms. However, for the Mn/Co catalyst prepared via impregnation, Co2C nanospheres were formed exhibiting high methane selectivity, low C2-4= selectivity and low activity. CoCe, CoLa and the unpromoted catalyst also generated Co2C nanospheres with similar catalytic performance as that of the Mn/Co catalyst prepared via impregnation. Due to the very high stability of the Co2AlOx composite oxide, no Co2C phase can be obtained for the CoAl sample. Based on the above studies, Mn not only acted as a typical electronic or structural promoter, but also had a strong effect on the morphology resulting in the formation of Co2C nanoprisms with specifically exposed facets of (101) and (020).

ASSOCIATED CONTENT Supporting Information. H2-TPR peak areas, activity, catalytic performance, TEM images, XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail for L. S. Zhong: [email protected] *E-mail for Y. H. Sun: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work has been supported by the Natural Science Foundation of China (21403278, 21573271, 91545112), Shanghai Municipal Science and Technology Commission, China (15DZ1170500, 16DZ1206900),

the

Ministry

of

Science

and

Technology

of

China

(2017YFB0602202,

2016YFA0202802) and the Chinese Academy of Sciences (QYZDB-SSW-SLH035, Youth Innovation Promotion Association). REFERENCES (1) Torres Galvis, H. M.; Bitter, J. H.; Khare, C. B.; Ruitenbeek, M.; Dugulan, A. I.; De Jong, K. P. Science 2012, 335, 835-838. (2) Torres Galvis, H. M.; Bitter, J. H.; Davidian, T.; Ruitenbeek, M.; Dugulan, A. I.; De Jong, K. P. J. Am. Chem. Soc. 2012, 134, 16207-16215. (3) 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, 99029907. (4) Cheng, Y.; Lin, J.; Xu, K.; Wang, H.; Yao, X. Y.; Pei, Y.; Yan, S. R.; Qiao, M. H.; Zong, B. N. ACS Catal. 2016, 6, 389-399. (5) Wang, D.; Zhou, X. P.; Ji, J.; Duan, X. Z.; Qian, G.; Zhou, X. G.; Chen, D.; Yuan, W. K. J. Mater. Chem. A 2015, 3, 4560-4567. (6) Torres Galvis, H. M.; De Jong, K. P. ACS Catal. 2013, 3, 2130-2149. (7) Lu, J.; Yang, L.; Xu, B.; Wu, Q.; Zhang, D.; Yuan, S.; Zhai, Y.; Wang, X.; Fan, Y.; Hu, Z. ACS Catal. 2014, 4, 613-621. (8) Todic, B.; Ma, W.; Jacobs, G.; Davis, B. H.; Bukur, D. B. J. Catal. 2014, 311, 325-338. (9) Oschatz, M.; Lamme, W. S.; Xie, J.; Dugulan, A. I.; De Jong, K. P. ChemCatChem 2016, 8, 28462852. (10) Torres Galvis, H. M.; Koeken, A. C. J.; Bitter, J. H.; Davidian, T.; Ruitenbeek, M.; Dugulan, A. I.; De Jong, K. P. Catal. Today 2013, 215, 95-102. (11) Cheng, K.; Ordomsky, V. V.; Legras, B.; Virginie, M.; Paul, S.; Wang, Y.; Khodakov, A. Y. Appl. Catal., A 2015, 502, 204-214. (12) Torres Galvis, H. M.; Koeken, A. C. J.; Bitter, J. H.; Davidian, T.; Ruitenbeek, M.; Dugulan, A. I.; De Jong, K. P. J. Catal. 2013, 303, 22-30. (13) Setthapun, W.; Williams, W. D.; Kim, S. M.; Feng, H.; Elam, J. W.; Rabuffetti, F. A.; Poeppelmeier, K. R.; Stair, P. C.; Stach, E. A.; Ribeiro, F. H.; Miller, J. T.; Marshall, C. L. J. Phys. Chem. C 2010, 114, 9758-9771. (14) Rausch, A. K.; Schubert, L.; Henkel, R.; Van Steen, E.; Claeys, M.; Roessner, F. Catal. Today 2016, 275, 94-99. (15) Iglesia, E.; Soled, S. L.; Fiato, R. A. J. Catal. 1992, 137, 212-224. (16) Xie, J.; Torres Galvis, H. M.; Koeken, A. C.; Kirilin, A.; Dugulan, A. I.; Ruitenbeek, M.; De Jong, K. P. ACS Catal. 2016, 6, 4017-4024. (17) Xie, J.; Yang, J.; Dugulan, A. I.; Holmen, A.; Chen, D.; De Jong, K. P.; Louwerse, M. J. ACS Catal. 2016, 6, 3147-3157. (18) Munnik, P.; De Jongh, P. E.; De Jong, K. P. J. Am. Chem. Soc. 2014, 136, 7333-7340. (19) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X. D.; Kapteijn, F.; Van Dillen, A. J.; De Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956-3964. ACS Paragon Plus Environment

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Mechanism of the Mn Promoter via CoMn Spinel for Morphology

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