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Particle Size Effects of Cobalt Carbide for Fischer-Tropsch to Olefins Yuanyuan Dai, Yonghui Zhao, Tiejun Lin, Shenggang Li, Fei Yu, Yunlei An, Xinxing Wang, Kang Xiao, Fanfei Sun, Zheng Jiang, Yongwu Lu, Hui Wang, Liangshu Zhong, and Yuhan Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03631 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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Particle Size Effects of Cobalt Carbide for Fischer-Tropsch to Olefins Yuanyuan Daia,b, Yonghui Zhaoa, Tiejun Lina, Shenggang Lia,c, Fei Yua,b, Yunlei Ana,b, Xinxing Wanga,b, Kang Xiaod, Fanfei Sunb,e, Zheng Jiange, Yongwu Lua, Hui Wanga, Liangshu Zhonga,c*, Yuhan Suna,c* a

CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced

Research Institute, Chinese Academy of Sciences, Shanghai 201203, PR China b

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

c

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

d

School of Materials Science & Engineering, Nanjing University of Posts and Telecommunications,

Nanjing, PR China e

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy

of Sciences, Shanghai 201204, PR China ABSTRACT: Particle size effects of the cobalt carbide (Co2C) catalyst on its catalytic performance for Fischer-Tropsch to olefins were investigated. When the Co2C nanoparticles were smaller than 7 nm, increasing the particle size led to enhanced intrinsic activity based on the turnover frequency (TOF), higher selectivity to lower olefins, higher ratio of olefin to paraffin, and lower methane selectivity. However, when the Co2C nanoparticles were larger than 7 nm, both intrinsic activity and product selectivity did not depend on the particle size. Further kinetic studies showed that both the apparent activation energy and the reaction order of H2 decreased, while the reaction order of CO increased with decreasing Co2C particle size when the size was smaller than 7 nm. In contrast, these kinetic parameters were nearly constant when the Co2C particle size was larger than 7 nm. Theoretical analysis revealed a strong correlation between the exposed facets and Co2C particle sizes, leading to the observed dependence of catalytic performance on the catalyst particle size. 1


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Keywords: Fischer-Tropsch, Fischer-Tropsch to Olefins, Syngas, Cobalt carbide, Nanoeffects, Size effects

INTRODUCTION Lower olefins (C2-4=), as basic building-block chemicals, are mainly produced through thermal cracking of naphtha or dehydrogenation of light alkanes.1-4 With the increasing concerns on the availability of the petroleum reserves, the production of lower olefins via conversion of syngas (a mixture of CO and H2) derived from non-petroleum resources attracts more and more interests from both academia and industry.5-11 Compared with other processes such as cracking of FT liquids, the methanol to olefins (MTO) or dimethyl ether to olefins (DMTO), the Fischer-Tropsch to olefins (FTO) process provides a competitive way to produce lower olefins from syngas owing to the shorter process with lower energy consumption.2, 12 Similar to the typical FTS, the catalytic mechanism of the FTO reaction is viewed as a polymerization process of surface CHx species.13 Modified FT catalysts were always used for the FTO reaction.14-16 A high selectivity towards lower olefins (60 wt%) was reported by Torres Galvis et al. over sulfur and sodium promoted iron catalysts.6, 17 Moreover, Zhai et al. found that the olefins selectivity reached 79 % for the zinc and sodium-modulated iron catalysts under 340 o

C and 20 bar.9 Recently, Co2C nanoprisms with exposed facets of (101) and (020) were reported as a

newly FTO active site with promising catalytic performance for olefins production under mild reaction conditions,7 although Co2C has traditionally been regarded as a possible cause of deactivation for Cobased FT catalysts,18-20 or viewed as the active site for CO insertion for alcohol formation.21-25 The residual Na acted as an electronic donator to enhance CO adsorption and facilitate the Co2C formation 26-27

, while the addition of Mn favored the morphology control for the formation of Co2C nanoprisms.28 Syngas conversion via FT reaction is structure-sensitive and the FT catalysts should possess 2


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strong nanoeffects. The phase, particle size and exposed facets of the active components greatly affect the catalytic performance.29-31 For Co-based catalysts, metallic Co, serving as the active phase for CO hydrogenation,32 may exist in both hexagonal close-packed (HCP) phase and the face-centered cubic (FCC) phase. It was suggested that the HCP phase was favorable for FT synthesis to the FCC phase.3335

The effects of particle size of metallic Co during the FT reaction have been widely studied. Iglesia

et al.36 investigated the structural sensitivity of supported cobalt catalysts and found that the turnover frequency (TOF) was independent of the cobalt particle size with a range of 9~200 nm. However, for the catalysts supported on carbon nanofibers (CNF) with cobalt particle sizes in a range of 2.6~27 nm, the TOF was found to be independent of cobalt particle size with sizes larger than 6~8 nm, but the activity and product selectivity varied greatly when the cobalt particle sizes ranged from 3 to 8 nm.37 Similar phenomena were also observed for other cobalt supported catalysts.38-42 Iron carbides as the active phase of Fe-based FT catalysts are easily formed under FT reaction conditions.16, 32, 43-44 The effect of particle size of iron carbide on the catalytic behavior for syngas conversion was also studied in recent years.1, 15, 45 Galvis et al.1 found that the TOF increased with decreasing iron carbide size from 7 to 2 nm for Na-promoted catalysts, whereas methane selectivity increased and lower olefins selectivity decreased. Due to the discrepancy of physicochemical properties for Co2C with metallic Co and iron carbide, the size effect of Co2C under realistic FTO reaction should be thoroughly investigated. According to our previous study of the sodium effect on the structure-performance relationship of Co-based catalyst during CO hydrogenation process26-27, sodium was specially introduced into the Co/SiO2 catalyst systems to accelerate the formation of Co2C. In this study, we focused on the particle size effect of Co2C derived from Na-Co/SiO2 on catalytic performance in FTO. The loading of cobalt on the SiO2 support varied from 0.5 wt% to 20 wt%, resulting in Co2C nanoparticles with sizes ranging from 5.3 3


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to 14 nm. Kinetic study and theoretical analysis were also performed to elucidate the intrinsic cobalt carbide particle size effects on the FTO performance.

EXPERIMENTAL SECTION Catalyst preparation. The SiO2 with specific surface area of 480 m2g-1 and average pore size of 7 nm purchased from Aladdin Industrial Corporation were used as support (SEM image, isotherms of N2 adsorption-desorption plots and pore distributions of SiO2 were shown in Figure S1). The Napromoted catalysts with different cobalt loadings (0.5, 1, 2, 3, 5, 10, 20 wt%) were prepared using incipient wetness impregnation. The Co/Na mass ratio of 10 was used and kept constant for all studied catalysts. Taking 10 wt% Co loading Na-promoted catalyst for example, 10 g SiO2 was impregnated with an aqueous solution including 5.72 g Co(NO3)2•6H2O and 0.93 g NaNO3. After impregnation, the sample was dried at 100 oC for 5 h, then calcined at 330 oC for 3 h under nitrogen flow (100 mL/min). The obtained samples are named as xIM, where x indicates the nominal Co loading and IM means the prepared method of the sample. Catalyst characterization. The Co and Na contents were determined by a PE Optima 2100DV inductive coupled plasma emission spectrometer (ICP). The structure phase and nanoparticles size were determined by powder X–ray diffraction (XRD) performed on a Rigatku Ultima IV4 X–ray diffractometer (40 kV, 40 mA) with Cu Ka radiation (λ = 1.54056 Å) in the range of 5o–90o at a scanning step length of 0.02o. The different phases were identified by JCPDS standard cards. The specific surface area and pore structure of catalysts were obtained on a TriStar II 3020 instrument using a multipoint Brunauer-Emmett-Teller (BET) method. Prior to the measurements, the samples were degassed at 200 oC in vacuum for 6 h, and then shifted to the analysis model for N2 4


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physical adsorption-desorption at –196 oC. The H2–TPR was performed on a Micromeritics ChemiSorb 2920 with a thermal conductivity detector (TCD). About 50 mg sample was pretreated under flowing helium of 40 mL min–1 at 200 oC for 60 min and then cooled down to 60 oC. After that, the gas was shifted to 5% H2/Ar and the temperature raised from 60 to 800 oC with a rate of 10 oC min–1. The particle size distribution and lattice fringe of phase were analyzed with transmission electron microscopy (TEM, JEOL JEM 2011) with a 200 kV accelerating voltage. Before measurement, the samples were dispersed in ethanol using ultrasonication and then deposited on a Cu microgrid. The STEM images of the spent samples were conducted on a FEI-Tecnai G2 microscope with 300 kV accelerating voltage. High-angle annular dark field (HAADF) images and elemental analysis (mapping) were obtained under STEM mode. The surface valence of catalysts was measured with XPS using Al Kα radiation (12 kV, 4 mA, hν = 1486.6 eV) on a Quantum 2000 Scanning ESCA Microprobe instrument. All binding energies were calibrated using the C 1s adventitious carbon peak with EB fixed at 284.6 eV. The data of EXAFS were collected at BL14W1 operated at 3.5 GeV in Shanghai Synchrotron Radiation Facility (SSRF) in transmission mode. The EXAFS spectra were measured at the Co Kedge (7712 eV) under ambient conditions. The energy was calibrated accordingly to the absorption edge of pure Co foil. The extraction of the data and the fitting of the profiles were performed on Athena and Artemis. Catalyst testing. The catalytic evaluation was performed in a fixed-bed reactor. Typically, 1.5 g of catalyst with 40-60 mesh was mixed with 3 g of the same size quartz sand. The catalysts were firstly in-situ reduced by 10 % H2/Ar (v/v, 200 mL/min) at 300 oC for 5 h at atmospheric pressure before reaction. After reduction, the temperature was then dropped to 250 oC in He (99.999%). A mixture of 5


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97 vol% syngas with different H2/CO ratios and 3 vol% N2 as inner standard was introduced to the reactor. The space velocity was varied to gain CO conversion of about 2%. The outlet gas was analyzed on-line with an Agilent 7890B gas chromatograph after passing through a hot trap (120 oC) and a cold trap (0 oC). The Agilent gas chromatograph was equipped with two columns and two detectors. H2, N2, CO, CH4 and CO2 were analyzed by a carbon molecular sieves column (TDX-1) with a thermal conductivity detector (TCD) using helium as the carrier gas. Hydrocarbons of C1-C7 were analyzed using a KCl-modified alumina capillary column (19095P-K25) and a hydrogen flame ionization detector (FID). The activation energy was obtained by adjusting the reaction temperature, and the reaction orders of H2 and CO were obtained by changing the partial pressures of H2 and CO. CO conversion was calculated on carbon atom basis according to eq 1:

CO Conversion 

COinlet  COoutlet

 100%

(1)

COinlet

where COinlet and COoutlet represent moles of CO at the inlet and the outlet, respectively. CO2 selectivity was calculated according to eq 2： CO2 Selectivity 

CO2outlet 100% COinlet  COoutlet

(2)

where CO2 outlet denotes moles of CO2 at the outlet. The selectivity to various individual product of CnHm was obtained from eq 3： Cn H m Selectivity 

COinlet

nCn H m outlet 100%  COoutlet  CO2outlet

(3)

where CnHm outlet represent moles of C of the product at the outlet. CTY (Co2C time yield) was calculated from the mass of the active sites according to eq 4:

CTY 

nCO

Conversion

mactive

phase

t

 100%

(4)

where nCO Conversion, mactive phase and t represent moles of CO converted, the mass of Co2C and time, 6


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respectively. TOF (turnover frequency) was calculated using simple geometric arguments for spherical particle shape and a surface cobalt carbide density of 7.6 g mL-1 corresponding to 16 Co atoms nm-2 and expressed by eq 5:

TOF 

nCO mactive

phase

conversion

 NA

 Sactive

phase

 N t

 100%

(5)

where NA, Sactive phase and N are the A F Gageiro constant, the surface area on per gram Co2C, and the atoms number of Co on per square nanoscale, respectively. For Co2C nanosphere: 𝑆𝑎𝑐𝑡𝑖𝑣𝑒

𝑝ℎ𝑎𝑠𝑒

=

π𝑑 2 𝜌π/6𝑑 3

6

= , where ρ and d are the density of Co2C and the 𝜌𝑑

diameter of Co2C nanospheres, respectively. RESULTS The BET surface area and average pore diameter of the calcined and spent samples were listed in Table S1. In comparison with the pure SiO2 support, the loading of cobalt (oxide and/or carbide) caused a decrease in the surface area, especially for those samples with high loadings, although the loading does not significantly affect the pore diameter. The loading of Co, the particle size for cobalt oxide and Co2C were measured and displayed in Table 1. TEM analysis of the fresh samples after calcination demonstrated that different cobalt oxide particle sizes in the range of 3.8~11.4 nm were obtained by controlling the cobalt loading (Figure S2). Analysis of XRD diffraction patterns (Figure S3) showed that Co3O4 crystallite sizes calculated by the Scherrer Formula were in good agreement with the particle sizes measured with TEM. However, it was difficult to detect cobalt oxide for those catalysts with low Co loading and only TEM was available to determine the particle size. Na was introduced to the samples specifically to enhance the formation of Co2C, and the Co/Na mass ratio was approximately 10 and kept constant for all samples. After 7


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treatment with syngas, Co2C nanoparticles with different sizes were synthesized. XRD patterns of Co/SiO2 with different cobalt loadings at different stages are displayed in Figure S3. The Co3O4 phase was converted into CoO and trace of Co0 during the reduction, and the Co2C phase was gradually evolved with time on-stream after exposure to syngas. The Co2C phase was detected exclusively after 24 h of treatment with syngas, and no diffraction peaks characteristic of Co0 was found, which was further proved by analyzing the XPS spectra of the treated samples (Figure S4). Moreover, the brightfield TEM of spent samples showed in Figure 1 again demonstrated that the Co species existed in the form of Co2C, and these cobalt species well distributed over the support. From the TEM results, it also can be found that the particle sizes of Co3O4 and Co2C increased with increasing Co loading, as plotted in Figure S5. Table 1 The Structure Properties of the Co/SiO2 Catalyst Samples. sample 0.50IM 1IM 2IM 3IM 5IM 10IM 20IM a

Co3O4 crystallite size

Co3O4 crystallite size

Co2C crystallite size

for the calcined

for the calcined sample

for the spent sample

sample(nm, XRD)

(nm, TEM)

(nm, TEM)

0.7

-

1.3 2.5 3.9 5.5 10.8 22.1

-

3.8 4.5 4.4 6.7 8.5 11.2 11.4

5.3 6.1 6.9 7.4 9.4 12.1 13.9

Co (wt%)

a

-

9.7 12.1 11.2

Measured by ICP.

8


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Figure 1. TEM images and particle size distributions of Co2C particles for the spent Co/SiO2 samples (a: 0.5IM, b: 1IM, c: 2IM, d: 3IM, e: 5IM, f: 10IM, g: 20IM).

9


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To further investigate the nature of Co species in the spent samples, extended X-ray absorption fine structure (EXAFS) and scanning transmission electron microscope (STEM) characterization of the spent samples were performed. Figure 2 shows the k3-weighted EXAFS Fourier transform magnitude spectra with different cobalt contents and the corresponding fitting curves with Co2C. The fitting data of Co K-edge EXAFS are also listed in Table 2. It was found that all the spectra of the spent samples were fitted well with Co2C. The fitting bond distance of Co-C and Co-Co (Co2C) were close to that of the Co2C standard sample, so as to the coordination shell. However, the R value of CoC shell decreased with increasing content of Co, indicating the distance of the first Co-C coordination shells increased when the cobalt content was low. As the distance of Co-O is slightly larger than that of Co-C, the increasing distance of Co-C indicated the content of Co-O increased. In-situ XRD of Co2C in different temperatures under 2% O2/Ar conditions (Figure S6) suggested that Co2C was quite stable in air, thus the Co-O band would not be generated from the oxidation of Co2C. As the support was SiO2, it was hard to avoid Co-O band between the support and Co species, and the percentage of Co-O band would increase with decreasing particle size of cobalt species. The increasing distance of Co-C with decreasing content of Co just suggested that the Co2C particle size was smaller under lower cobalt content, which was in line with the result of XRD and TEM. In addition, the coordination number for first shell of Co-C increased from 1.3 to 1.6 as the content of Co increased from 0.5 wt% to 20 wt%, which further demonstrated that the size of Co2C nanoparticles increased with the increase of Co content. As for the cobalt silicate species, we cannot obtain rational fitting result using αCo2SiO4 or β-Co2SiO4. That means that the content of cobalt silicate was quite low and possibly only existed at the interface of the SiO2 support and the supported cobalt species, which was hard to contact with the reactant CO and H2. STEM results of 0.5IM sample with 0.5wt% cobalt demonstrated a uniform element dispersion for the spent catalysts (Figure S7). The well dispersion of Na on the 10


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support surface played a vital role in promoting the formation of Co2C. The bright spots in Figure S7a under HAADF mode represented the Co2C nanoparticles with the average size of 5.2 nm, which was close to the value of 5.3 nm calculated with bright-field TEM. The Co2C particles size was found to vary from 5.3 nm to 13.9 nm depending on the Co3O4 size.

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a

Co-Co

0

2

Co-Co

4

6

0

8

Radius (Angstroms)

c

d

6

Co-Co

8

3IM

0

2

4

6

8

Radius (Angstroms)

f

10IM Fit

20IM

C-Co

Fit

FT Amplitude (a.u.)

Co-Co

C-Co

0

8

C-Co

Radius (Angstroms)

e

6

Co-Co

FT Amplitude (a.u.)

FT Amplitude (a.u.)

C-Co

4

4

Fit

Co-Co

2

2

Radius (Angstroms) 2IM Fit

0

1IM Fit

C-Co

FT Amplitude (a.u.)

FT Amplitude (a.u.)

b

0.5IM Fit

C-Co

FT Amplitude (a.u.)

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

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2

4

6

8

0

Radius (Angstroms)

2

4

6

8

Radius (Angstroms)

Figure 2. k3-weighted extended X-ray absorption fine structure (EXAFS) Fourier transform magnitude spectra of the spent samples with different Co content (a: 0.5IM, b: 1IM, c: 2IM, d: 3IM, e: 10IM, f: 20IM). The black solid lines are the K-edge EXAFS spectra and the red dotted lines are the fitting curves with Co2C.

12


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Table 2. Co K-edge EXAFS Fitting Results for the Co/SiO2 Catalyst Samples. sample

shell

N

R(Å)

∆𝐸0 (eV)

CoO

Co-O Co-Co Co–C Co–Co1 Co–Co2 Co–Co3 Co–Co4 Co–C Co–Co1 Co–Co2 Co–Co3 Co–Co4 Co–C Co–Co1 Co–Co2 Co–Co3 Co–Co4 Co–C Co–Co1 Co–Co2 Co–Co3 Co–Co4 Co–C Co–Co1 Co–Co2 Co–Co3 Co–Co4 Co–C Co–Co1 Co–Co2 Co–Co3 Co–Co4 Co–C Co–Co1 Co–Co2 Co–Co3 Co–Co4

6 12 1.8 0.7 3.4 2.6 1.2 1.3 0.7 3.3 3.1 1.2 1.3 0.7 3.2 3.0 1.4 1.4 0.7 2.9 2.7 1.3 1.5 0.8 3.0 2.8 1.4 1.5 0.7 3.1 2.9 1.5 1.6 0.8 3.4 3.0 1.5

2.13 3.01 1.9 2.51 2.63 2.79 3.00 2.03 2.46 2.64 2.82 2.99 2.07 2.49 2.67 2.85 3.01 2.09 2.53 2.72 2.90 3.06 2.02 2.46 2.65 2.84 3.00 1.99 2.50 2.62 2.78 2.98 1.97 2.54 2.73 2.85 2.97

— — -5.6 -5.6 -5.6 -5.6 -5.6 1.8 1.8 1.8 1.8 1.8 4.2 4.2 4.2 4.2 4.2 8.8 8.8 8.8 8.8 8.8 4.7 4.7 4.7 4.7 4.7 4.2 4.2 4.2 4.2 4.2 5.1 5.1 5.1 5.1 5.1

Co2Ca

0.5IM

1IM

2IM

3IM

10IM

20IM

∆𝜎 2 × 103 (Å2 ) — — 4.0 6.5 6.5 6.5 6.5 8.4 2.8 5.6 4.4 4.1 6.5 2.4 4.9 2.9 1.5 4.4 1.4 3.8 1.6 0.9 0.3 9.3 2.7 1.3 9.5 3.0 1.7 3.7 2.2 1.5 6.7 3.8 1.6 1.9 7.6

R-factor (%)

0.4

0.9

0.9

0.4

0.2

0.5

Note: N, coordination number; R, interatomic distance; 𝜎, disorder parameter; ∆𝐸0 , energy shift. All the fitting analysis were performed in the R space, ∆R = 1.0–3.0 and ∆𝐾= 3.0–12.0. 𝑆02 = 0.9 was obtained from fitting the Co-foil standard sample. a

The data was from reference.46 13


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The catalytic performance as a function of time-on-stream (TOS) for the reduced samples after exposure to syngas were investigated and presented in Figure S8. All of hydrocarbons selectivity was calculated without considering CO2. Over the first 24 h, both the CO conversion and product selectivity changed remarkably. In the beginning, high activity was observed when syngas was introduced to the reduced catalysts, and with TOS the activity dramatically decreased, while the CO2 selectivity increased, indicating the gradual evolution of Co2C, as reported in our previous studies7, 2627

. After 24 h, the catalytic activity gradually became stable. The methane selectivity and C2-4=

selectivity were rather constant during the reaction for the samples with low Co loading (less than 2 wt%). However, the product selectivity changed significantly for the samples with higher Co loading. Over the first 8 h, the methane selectivity decreased dramatically and then reached a minimum value. In the following stage (10-20 h), the methane selectivity increased gradually. The C2-4= selectivity presented a similar trend as methane, but the selectivity decreased rapidly at the initial stage of 1-2 h. All the catalysts displayed relatively high stability in terms of hydrocarbon selectivity after 20 h. In order to compare the performance of Co2C with different particle sizes, all reported data were collected after the catalytic performance reached the stability state. The catalytic performance results after reaching stability, including activity, product distribution and the ratio of olefin to paraffin under different H2/CO ratio, were summarized in Table 3. The cobalt time yield (molCO·gCo2C-1·s-1) for different Co2C particle size was also calculated. As shown in Figure 3a, an obvious relationship between the cobalt time yield (CTY) and Co2C particle size was observed. The CTY decreased when the Co2C particle size increased from 7 nm to 14 nm, and the rapidly decreasing CTY could also be observed when the Co2C particle size was smaller than 7 nm. The maximum CTY value was obtained for the sample with Co2C particle size of about 7 nm, regardless of the H2/CO ratio in the feed gas. This trend is very similar to the result of Co particle size in the FTS 14


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process.37 It was also observed that the CTY values were enhanced by increasing the H2/CO ratio. When the H2/CO ratio increased from 0.5 to 2, the CTY value for the sample with Co2C particle size of about 7 nm increased from 0.13 to 0.46 ×10-3 molCO·gCo2C-1·s-1. To characterize the intrinsic activity of the catalyst, the surface-specific activity or turnover frequency (TOF) was also calculated, as shown in Figure 3b. In the previous studies on Co-based FT catalysts, the TOF was calculated using the dispersion obtained from H2 chemisorption assuming complete reduction and an H/Cos adsorption stoichiometry of 1.37-38 For Fe particles, the dispersion is obtained from CO chemisorption assuming stoichiometric adsorption ratio of CO/Fe=1/2.47 However, as the adsorption stoichiometry of H or CO on iron carbide were unclear, the TOF value was calculated using simple geometric arguments for the spherical particle shape that the density of Fe5C2 (ρ = 7.57 g mL-1) corresponding to 14 Fe atoms nm-2.1 In our study, TOF values were also calculated using simple geometric arguments for the spherical nanoparticles and a surface cobalt carbide density of 7.6 g mL-1 corresponding to 16 Co atoms nm-2. Under this assumption, the as-obtained TOF values were found to increase rapidly from 0.53 to 1.52×10-3 s-1 with the increasing Co2C particle size from 5.3 to 7 nm for the H2/CO ratio of 2. However, it remained nearly constant with the value of about 1.5×10-3 s-1 when further increasing the particle size from 7 to 14 nm. Decreasing the H2/CO ratio from 2 to 0.5 resulted in reduced TOF from 1.5 to 0.45×10-3 s-1 at the stable region. In Figure 3c, the methane selectivity was plotted against the Co2C particle size. For these samples, methane selectivity decreased from 27 C% to 16 C% at the H2/CO ratio of 2 when the Co2C particle size increased from 5.3 to 7 nm, whereas the selectivity remained almost the same when further increasing the Co2C particle size. The relationship between the Co2C particle size and C5+ selectivity were also displayed in Table 3 and plotted in Figure 3d. In contrast to methane selectivity, C5+ selectivity increased gradually when the Co2C particle size increased from 5.3 to 7 nm and then kept 15


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ACS Catalysis

constant. Higher methane selectivity and lower C5+ selectivity for Co2C with small particle size indicated its lower chain-growth capability.

Figure 3. The CTY (a), TOF (b), CH4 selectivity (c) and C5+ selectivity (d) as a function of the Co2C particle size under different H2/CO ratio (250 oC, 1 bar).

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Table 3 Catalytic Activity and Product Distribution for Various Catalysts with Different Co2C Size (250 oC, 1 bar) Product Distribution

Olefin/Paraffin ratio

Sample

H2/CO ratio

CTY -5 (10 molCO·gCo2C-1·s-1)

TOF （10-3 s-1）

CH4

C2-4=

C5+

C2

C3

C4

C2-4

0.50IM 1IM 2IM 3IM 5IM 10IM 20IM 0.50IM 1IM 2IM 3IM 5IM 10IM 20IM 0.50IM 1IM 2IM 3IM 5IM 10IM 20IM

2 2 2 2 2 2 2 1 1 1 1 1 1 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.21 0.33 0.46 0.43 0.31 0.25 0.24 0.11 0.23 0.26 0.22 0.18 0.16 0.13 0.06 0.12 0.13 0.11 0.10 0.08 0.08

0.53 0.97 1.52 1.53 1.40 1.45 1.60 0.28 0.67 0.86 0.78 0.81 0.93 0.87 0.15 0.35 0.43 0.39 0.45 0.47 0.53

26.8 22.6 20.6 17.0 14.9 16.0 17.1 22.7 17.7 15.0 12.8 10.7 11.7 12.3 19.0 14.5 12.1 9.6 8.6 9.1 9.9

37.4 38.9 38.2 42.9 44.7 44.8 44.1 36.5 38.8 39.8 42.9 44.2 44.1 43.4 35.3 38.9 38.5 40.2 41.5 42.0 41.3

31.5 34.8 38.7 37.6 37.7 36.4 36.3 38.0 41.1 43.1 42.4 43.2 42.0 42.2 44.1 45.0 48.0 48.7 48.4 47.3 47.3

7.9 8.5 9.3 11.5 12.6 10.9 13.1 9.3 11.8 13.9 17.5 16.8 14.2 15.0 14.0 15.2 20.1 21.5 21.1 21.2 20.9

16.3 18.9 22.7 23.1 24.2 22.7 23.0 20.1 21.4 26.5 30.1 29.5 28.3 27.1 25.1 26.0 33.2 35.3 36.3 35.3 34.4

13.0 15.9 16.6 16.6 16.8 16.6 16.2 15.7 18.2 19.2 22.1 22.6 19.9 19.4 21.2 22.8 24.4 27.2 27.2 25.4 25.3

9.0 12.3 15.1 16.7 17.3 15.9 17.4 12.9 17.1 19.4 22.9 22.4 20.5 20.3 21.3 23.5 25.5 28.1 28.2 27.4 26.8

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ACS Catalysis

The C2-4= selectivity and the ratio of olefin to paraffin (O/P) as a function of the Co2C particle size were plotted in Figure 4 and Figure 5. Both C2-4= selectivity and O/P ratio of C2-4 increased with increasing Co2C particle size and then remained constant when the size was larger than 7 nm. For example, in the case of H2/CO = 2, C2-4= selectivity increased from 37 C% to 45 C%, while the O/P ratio of C2-4 increased from 8 to 16 with the size increasing from 5.3 to 7 nm, and then remained unchanged with further increasing the Co2C size. Reducing the H2/CO ratio from 2 to 0.5 resulted in a slightly decrease in C2-4= selectivity from 45 C% to 41 C%, whereas the O/P ratio for the C2-4 slate increased from 16 to 28.

Figure 4. C2-4= selectivity and C2-4=/C2-40 as a function of the Co2C particle size under different H2/CO ratio (250 oC, 1 bar).

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Figure 5. The O/P ratio as a function of the Co2C particle size under different H2/CO ratio (250 oC, 1 bar).

Product distributions were shown in Figure S9 and the probabilities of chain growth (α) for hydrocarbons obtained by fitting the results of C3-7 using the ASF model were listed in Table S2. The probability was found to increase with increasing Co2C particle size from 5.3 to 7 nm and then remained almost unchanged. The samples with smaller Co2C particle sizes displayed lower α values. The α value also increased with decreasing H2/CO value. As shown in Figure S9d, the α value at H2/CO = 2 was about 0.65 for Co2C with particle sizes larger than 7 nm, while this value increased to 0.78 under H2/CO = 0.5. The effect of the reaction temperature on the FTO performance over various catalysts with different Co2C particle sizes was also investigated. As shown in Figure S10, with the increasing temperature from 250 to 270 oC, the CTY value increased to 0.85 from 0.46×10-3 molCO·gCo2C-1·s-1, and the TOF value increased from 1.52 to 2.82×10-3 s-1 under H2/CO = 2 and 1 bar for 2IM sample. In addition, the CH4 selectivity increased and C5+ selectivity decreased with the increasing temperature. At the reaction temperatures studied, similar relationships between the catalytic performance and Co2C particles sizes were observed, clearly demonstrating that the activity and product selectivity depended on the Co2C particle size. 19


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ACS Catalysis

To further determine the effects of the Co2C particle size on the apparent activation energy (Ea), Ea was measured by calculating the rate constants (k) at different temperatures and fitting the Arrhenius equation:

k   exp(

Ea ) RT

(4)

where A and Ea are the pre-exponential factor and the apparent activation energy, respectively. The logarithm of the k was correlated with the reciprocal temperature (1/T) as shown in Figure S11 and Table S3. A good linear relationship between lnk and 1/T was found for each catalyst. The values of Ea determined from the above procedure were further plotted against the Co2C particle size as shown in Figure 6. It was found that the Ea value decreased from 105 to 83 kJ/mol with increasing Co2C particle size from about 5.3 to 7 nm, but it again became stable when further increasing the Co2C particle size. Based on the CO consumption rate model, kinetic experiments were also carried out to study the intrinsic Co2C particle size effect on the FTO reaction and the results were shown in Figure S12 and Figure S13. It can be seen that the reaction order of CO increased from 0.7 to 1.2 with an increase of the Co2C particle size from 5.3 to 7 nm, but an opposite trend was found for H2, whose reaction order decreased from 1.7 to 0.4, suggesting that samples with small particles were more sensitive to the H2 partial pressure rather than CO, while samples with large particle sizes were more sensitive to CO. When the Co2C particle size was larger than 8 nm, both CO and H2 reaction orders seemed unchanged and remained at 1.2 and 0.4, respectively.

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Figure 6. The activation energy and the reaction order as a function of the cobalt carbide particle size (a: the activation energy; b-c: the reaction order of CO and H2).

Theoretical analysis was further performed to shed light on the relationship between the catalytic performance and Co2C particle size. Although different facets possess varying stabilities, more than one type of facet will appear in the resulting morphology. It was assumed that the ideal morphology of Co2C nanosphere-like particle was dominated by the (111) facet. The spherical nanoparticle models were built based on the polyhedral equation and the exposed surfaces were cut from the bulk structure of Co2C with preferentially terminating (111) facet. The types and numbers of the typical active sites were manually identified by inspection of the surfaces of the nanoparticles of different diameters. The structures of the active sites were identified based on those studied in our previous work.10 Co2C nanoparticles of 4, 5, 6, 7 and 8 nm were built containing 3693, 7133, 12381, 19677 and 29319 atoms, respectively. Due to the large number of atoms, it is impossible for us to perform density functional theory (DFT) study on them. Different side views of the 6 nm cluster are displayed in Figure 7 to show the available active sites, which were manually counted. Those for the other sizes are displayed in Figure S14 to Figure S17, whereas the possible active sites on these nanoparticles are collected in Table 4. There are four types of Co2C active sites as shown in Figure 7, which can be involved in CO activation and CHx hydrogenation. The four types of active sites are named as flat-Co2C(111), stepped21


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ACS Catalysis

Co2C(111), stepped-Co2C(101) and stepped-Co2C(020), respectively.7, 23, 27 We previously found the (101) and (020) facets to play key roles during syngas conversion, which favored olefin production and inhibited methane formation.7, 27 Furthermore, Co2C nanoprisms were found to mainly expose the (020) facet, while Co2C nanospheres were dominated by the (111) facet, resulting in the higher catalytic activity and lower methane selectivity of Co2C nanoprisms than Co2C nanospheres. As shown in Table 4, analysis of the structures of the Co2C nanoparticles shows that there is no Co2C(101) or Co2C (020) exposed facets when the Co2C particle size is less than 6 nm, which is consistent with the observed lower catalytic activity and higher methane selectivity for the catalysts with small Co2C particle sizes.

Figure 7. Possible active sites on a 6 nm Co2C nanoparticle with 12381 atoms. Co atoms at the active sites are displayed in yellow, whereas the rest of the Co atoms and the C atoms are shown in blue and in grey, respectively.

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Table 4. Possible Active Sites on Co2C Nanoparticles of Different Sizes from 4 nm to 8 nm. Active sites

flat-Co2C(111)

4 nm

5 nm

6 nm

7 nm

8 nm

8

16

12

12

28

8

12

12

12

20

0

0

4

12

28

0

0

12

18

8

steppedCo2C(111)

steppedCo2C(101)

steppedCo2C(020)

DISCUSSION FTS has been shown to be a complex and structure-sensitive process, and the catalytic performance strongly dependents on the active phase and particle size. For Co-based FT reaction, the formation of nanosphere-like Co2C was usually regarded as one of the main reasons for the deactivation. However, our previous studies demonstrated that Co2C nanoprisms exhibited high selectivity to olefins with high O/P ratios.7 In addition, methane selectivity was much lower than that predicted by the ASF model. Strong facet effect of Co2C nanostructures for syngas conversion has therefore been illustrated in detail. To further investigate the particle size effect of Co2C on the FTO reaction, it is necessary to controllably synthesize Co2C nanostructures with different sizes. Although Co2C nanoprisms are very promising for the FTO reaction, the synthesis of this kind of Co2C 23


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ACS Catalysis

morphology with different particle sizes is quite challenging. Thus, sphere-like Co2C nanoparticles, instead of Co2C nanoprisms, with sizes in the range of 5.3~14 nm were successfully synthesized by controlling the Co loadings. The studies on the size effect of sphere-like Co2C particles in the FTO process provides a crucial way to understand the fundamental surface process occurring on a working catalyst. According to our previous work,26 Co2C could be obtained through mild reduction and carbonization of Na-promoted Co/SiO2 catalysts. Both XRD and TEM analysis proved that the formed Co2C was very stable under reaction conditions for the Na-promoted catalysts, and there was also no deactivation by sintering or carbon deposition at the steady state. It can be assumed that the surface of Co2C is relatively clean, even for a long time-on-stream. It is very interesting to find that the particle size effect observed for sphere-like Co2C is very similar to the results of Co0 in the FTS, and both of them are in sharp contrast to the iron carbide. For Co2C, the TOF and C5+ selectivity decline dramatically below a certain critical particle size (~7 nm), while the CH4 selectivity displayed the opposite trend. The relationship of lower olefins selectivity or O/P ratio with the Co2C particle size also suggests that there exists a certain critical particle size of about 7 nm. Kinetic experiments and theoretical analysis were further carried out to study the size effect of Co2C on syngas conversion. As mentioned above, the apparent activation energy (Ea) of Co2C increased sharply with the size decreasing from 7 to 5.3 nm. The higher Ea for Co2C with smaller size leads to the lower activity of a working catalyst with small Co2C nanoparticles. The measured kinetic parameters suggested that Co2C with the particle size smaller than 7 nm was more sensitive to the H2 partial pressure than CO partial pressure, resulting in high methane selectivity and lower olefin selectivity, which can be further demonstrated by comparing the product distributions of Co2C with different particle sizes under different H2/CO ratios. Our previous DFT calculations showed that CH2CH2 was the most stable intermediate on the Co2C(020), Co2C(111) facets and especially the 24


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Co2C(101) facet, and methane formation was significantly suppressed on both Co2C(101) and Co2C(020).7, 48 The Co2C(111) facet was found to dominate the surface for nanoparticles smaller than 6 nm, and the absence of Co2C(101) and Co2C(020) suggesting methane formation to be favorable. Larger Co2C sizes were found to have a large number of Co2C(101) and Co2C(020) facets, which would be conducive to olefins production. Furthermore, similar to the case of metallic Co, it can be speculated that Co2C nanoparticles with smaller size are dominated of low coordination sites at corners and edges instead of active step edge sites at terraces, and the strongly bonded CO blocks the site and hampers CO dissociation,49 leading to low catalytic activity with high possibility for methane formation.

CONCLUSIONS A series of Na-promoted Co/SiO2 catalysts with different Co loadings were prepared by incipient wetness impregnation. The corresponding Co2C catalysts with particle sizes ranging from 5.3 to 14 nm were then obtained by in situ carbonization of reduced Na-promoted Co/SiO2 catalysts with syngas. The effect of Co2C particle size on its FTO performance was systematically investigated. The FTO performance was found to be independent of the Co2C particle size for samples larger than 7 nm in size, whereas both catalytic activity and product distribution were significantly influenced by the particle size for catalysts with Co2C particle size smaller than 7 nm. With these smaller Co2C particles, the TOF and olefin selectivity improved with the increasing particle size, while the larger particle also had lower methane selectivity. Increasing the H2/CO ratio in syngas resulted in higher TOF and selectivity toward methane and C2-4= but low C5+ selectivity and O/P ratio. Although raising the reaction temperature led to enhanced catalytic activity and shifted the products toward lower carbon numbers, the effect of the Co2C particle size on its catalytic performance under different reaction 25


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ACS Catalysis

conditions were similar. Thus, relatively large Co2C particles of 6~7 nm were needed for maximizing both the catalytic activity and selectivity of the targeted products. Our kinetic studies showed that the activation energy decreased with increasing Co2C particle size from 5.3 nm to 7 nm, and remained nearly constant with the further increase in the particle size. Co2C particles of smaller sizes had higher activation energies leading to lower TOF values. The reaction order of CO increased with increasing Co2C particle size, but the opposite was found for that of H2. This was consistent with the observation that the intermediate species were fully hydrogenated resulting in higher methane selectivity over smaller Co2C particles. Theoretical analysis showed that smaller spherical Co2C nanoparticles of

Particle Size Effects of Cobalt Carbide for Fischer ... - ACS Publications

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