Crystal-Plane-Dependent Fischer–Tropsch Performance of Cobalt

Research Article Cite This: ACS Catal. 2018, 8, 9447−9455

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Crystal-Plane-Dependent Fischer−Tropsch Performance of Cobalt Catalysts Chuan Qin,†,‡ Bo Hou,† Jungang Wang,*,† Qiang Wang,† Gang Wang,§ Mengting Yu,†,‡ Congbiao Chen,†,‡ Litao Jia,† and Debao Li*,† †

ACS Catal. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/14/18. For personal use only.

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: Identification of the crystal plane effect of the Co derived from Co3O4 nanocrystals (NCs) on Fischer− Tropsch synthesis (FTS) is important for developing highperformance FTS solid catalysts. However, the achievement of this goal is hindered by the complexity of the FTS and the absence of sufficient crystallographic structure data. In this study, we report that the experimental FT performance of the Co catalysts depends on the exposed crystal facets of the Co3O4 NCs. The exposed Co3O4 NC {112} facets have the highest catalytic activity and the lowest methane selectivity (6.2%) in comparison to those of the {111} and {001} planes. The evolution of the crystal planes during the reduction was investigated further, and the preferred orientation relationship induced by the Co3O4 → Co transformation was {112} → {10− 11}, {111} → {0001}, and {001} → {11−20}. CO temperature-programmed surface reaction experiments and density functional theory calculations further verified that the high FT performance of Co3O4{112} can be attributed to the specific surface topology of its active phase (i.e., Co{10−11}). Our findings clarify that the activity and selectivity of the FTS reaction can be enhanced by the selective exposure of a specific crystal plane from Co3O4 and could open an avenue for the rational design of high-performance FTS catalysts. KEYWORDS: crystal plane, cobalt, reduction, Fischer−Tropsch synthesis, density functional theory

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INTRODUCTION Fischer−Tropsch synthesis (FTS) is a promising route to produce high-quality clean fuels and chemicals from feedstocks other than crude oil, such as natural gas, coal, and biomass.1 Co is the most widely used active phase in the low-temperature FTS process due to its low water-gas-shift activity, high activity, and selectivity toward long-chain paraffins.2 An increasing number of density functional theory (DFT) studies have been conducted to understand the effect of the crystal plane on the FTS activity and selectivity performance of the Co catalysts.3 Using DFT calculations, Liu et al. found that hexagonal close-packed (HCP) Co(11−21) and Co(10−11) facets have a relatively higher reaction rate for CO activation among all of the exposed HCP Co facets.4 Zhao et al. used DFT calculations to reveal that the insertion of HCO was an efficient alternative for chain growth on the Co(0001) surface in syngas conversion.5 However, because of the structural sensitivity of the cobalt catalysts and the notably easy oxidation of Co that makes it difficult for Co metal surfaces to exist,4,6 experimental identification of the crystal plane effect for a single-crystal Co catalyst remains a key challenge in the studies of FTS. © XXXX American Chemical Society

Recently, Zhong et al. reported that Co2C nanoprisms with exposed {101} and {020} facets exhibited high selectivity toward lower olefins and inhibited methane production in the Fischer−Tropsch to olefins (FTO) process.7 Studies of bulk Co single crystals found that CO dissociation on the zigzag grooved (11−20) facet proceeded more quickly than that on the close-packed (0001) surface.8 These studies prompted the speculation that the crystal facet of a cobalt nanoparticle may play a vital role in determining the FTS performance. However, these model systems differ significantly from real catalytic conditions, where the active cobalt species are not formed directly but rather are produced by the reduction of the Co3O4 precursor.1d,2b,9 Therefore, investigating Co3O4 single crystals with well-defined surfaces and exploring the evolution of the crystal plane during the FTS process is more useful for the design of better catalysts with maximum mass-specific reactivity in the actual reaction process. Received: April 6, 2018 Revised: August 22, 2018

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DOI: 10.1021/acscatal.8b01333 ACS Catal. 2018, 8, 9447−9455

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ACS Catalysis

at 10 kV and a transmission electron microscope (JEOL JEM· 2010FEF) at an accelerating voltage of 200 kV. The composition and phase of the product were identified by powder X-ray diffraction (XRD) on a PANalytical Empyrean X’pert power diffractometer with monochromated Cu Kα radiation. The scan ranged from 10 to 80° at a rate of 8° min−1. With the same device, the in situ XRD profiles were recorded in the 2θ range of 5 to 40° at a rate of 1.8° min−1 after reduction at 400 °C for 6 h with monochromated Mo Kα radiation. Fourier transform infrared (FT-IR) spectra were obtained using a Thermo Scientific Nicolet 6700 spectrometer with a resolution of 4 cm−1 and an accumulation of 32 times. N2 adsorption−desorption isotherms were collected using a MicroActive ASAP 2460 2.01 instrument at −196 °C in order to determine the textural properties. The specific surface areas of the three types of Co3O4 were measured by the Brunauer− Emmett−Teller (BET) method. Hydrogen chemisorption experiments were conducted using a Micromeritics Autochem 2920 instrument. The samples were reduced in pure H2 at 400 °C for 6 h (1 °C min−1) and evacuated at this temperature. The isotherms were measured at 150 °C. Temperature-programmed reduction in hydrogen (H2-TPR) was carried out to assess the reducibility of the catalysts using a TP-5080 multipurpose automatic adsorption instrument. The sample (30 mg) was initially filled with argon at 120 °C for 2 h and cooled to 50 °C. Next, a 5 vol % H2/N2 mixture was flowed in (30 mL min−1), and the temperature was increased to 960 °C at a heating ramp of 10 °C min−1. The consumption of H2 was detected by a TCD detector calibrated previously using the reduction of CuO as a standard. The degree of reduction (DOR) of cobalt was measured with the same equipment after reducing the samples by a procedure applied in previous catalysis experiments. Typically, the catalyst sample (100 mg) was reduced in situ at 400 °C for 6 h at a heating ramp of 1 °C min−1 in a flow (30 mL min−1) of a 5 vol % H2/ N2 mixture. Next, the temperature was further increased to 900 °C at a rate of 10 °C min−1. The amount of hydrogen consumption was calculated from 50 to 400 °C (after holding at 400 °C for 6 h), and the DOR of the cobalt was obtained using a method reported in a previous study,1a where it was assumed that complete reduction of Co3O4 to CoO and partial reduction of CoO to Co0 occurred during the reduction at 400 °C. The CO temperature-programmed surface reaction (COTPSR) was used to evaluate the reactivity of the surface Co0 sites. The experiments were initiated by in situ reduction of the catalysts (100 mg, 0.18−0.25 mm) at 400 °C for 6 h with a stream of hydrogen under atmospheric pressure, using a heating ramp of 1 °C min−1. Next, the system was purged by an argon stream at 30 mL min−1 for 30 min at 50 °C. Subsequently, the CO probe gas was introduced into the equipment for 30 min. After the physically absorbed CO was removed using an argon stream, the gas was changed to a flow of H2, and the temperature was increased to 700 °C at a heating ramp of 10 °C min−1, at which point the spectra were recorded.

In this study, three Co3O4 model catalysts (>15 nm) with well-defined shapes and different exposed crystal planes were synthesized in the absence of the promoter and the support in order to exclude the additional effects due to the phasesensitive promoter and support that may change the route of the plane transition and affect the catalytic performance.10 Thus, we have been able to correlate the intrinsic catalytic activity and product selectivity with the corresponding crystal facet of the active site. We found that the Co3O4 nanoplates with {112} facets show the highest FT activity and the lowest selectivity for methane (6.2%), while Co3O4{001} surfaces did not produce any olefins. The evolution of the crystal plane during the reduction was further investigated by an extra pretreatment. By a combination of the experimental results with the results obtained by DFT calculations, the mechanisms of the facet-dependent catalytic performance of the cobalt catalysts in the FTS reaction were explored and revealed.

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EXPERIMENTAL SECTION All chemicals used for the catalyst preparation were of analytical grade and were used as received without further purification. Synthesis of Co3O4 Nanoplates. Hexagonal Co3O4 nanoplates (denoted as p-Co) were synthesized by following the procedure reported by Hu et al. with slight modifications.11 In a typical procedure, Co(NO3)2·6H2O (4.8 g) and poly(vinylpyrrolidone) (PVP, 4 g) were added to a mixture of ethanol and deionized water (40 mL) in a 1:1 volume ratio to form a pink solution. Next, a 0.4 M NaOH aqueous solution (100 mL) was slowly added with vigorous stirring for 30 min. Next, the obtained solution was transferred into a 200 mL autoclave and heated for 10 h at 120 °C. After the solution was cooled to room temperature, the products were obtained by centrifugation and washed three times with deionized water and one time with ethanol, after which they were dried at 60 °C for 12 h and calcined in air at 500 °C for 5 h. Synthesis of Co3O4 Nanosheets. The Co3O4 nanosheets (denoted as s-Co) were prepared by a hydrothermal method as follows: Co(CH3COO)2·4H2O (0.75 g) was dissolved in H2O (114 mL), and PVP (2.4 g) was added and dissolved with stirring. Next, a 1 M NaOH aqueous solution (6 mL) was added with stirring for 30 min. The mixture was stirred for another 20 min and later transferred into a 200 mL Teflon autoclave that was then sealed and heated to 180 °C for 3 h. After the mixture was cooled to room temperature, the solid precipitate was collected by centrifugation, washed several times with deionized water and ethanol, dried at 60 °C, and calcined at 500 °C for 5 h in air. Synthesis of Co3O4 Nanocubes. The Co3O4 nanocubes (denoted as c-Co) were prepared according to the reported route with slight modifications.12 The synthesis procedure was as follows: Co(CH3COO)2·4H2O (2 g) was added to a mixture of ethanol and deionized water (100 mL) in a 3:2 volume ratio to form a red solution, and 25% ammonia (10 mL) was added dropwise with vigorous stirring. The obtained solution was stirred for an additional 20 min and transferred into a 200 mL Teflon-lined autoclave and heated for 3 h at 170 °C. Next, the final product was separated by centrifugation, washed several times with deionized water and ethanol, and dried at 60 °C for 12 h. Characterization Techniques. The morphological and size analyses of the obtained nanoparticles were performed using a scanning electron microscope (JSM-7001F) operated

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RESULTS AND DISCUSSION Characterization of Co3O4 NCs with Different Shapes. Uniform Co3O4 nanoplates (denoted as p-Co), nanosheets (denoted as s-Co), and nanocubes (denoted as c-Co) were prepared by hydrothermal synthesis. The p-Co are uniform hexagons (Figure 1a1 and Figure S1a) with average thickness 9448

DOI: 10.1021/acscatal.8b01333 ACS Catal. 2018, 8, 9447−9455

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Figure 1. Structural analysis of Co3O4 NCs. HRTEM, ED patterns, and FFT images of Co3O4 NCs: (a1−a4) p-Co; (b1−b4) s-Co; (c1−c4) c-Co. Insets in a3−c3 are the FFTs of the high-resolution images. a4−c4 give the surface structures of the Co3O4 (112), (111), and (001) surfaces, respectively, exposed on p-Co, s-Co, and c-Co. Blue and red spheres represent Co and O atoms, respectively. Insets in a4−c4 are the corresponding schematic diagrams.

Table 1. Synthesis Performance of Co3O4 NCs Catalysts in the FTS Reaction product selectivity (wt %) catalyst p-Co s-Co c-Co

T (°C) a

220 250b 250a 250a

CO conversn (%)

CH4

C2−C4

C5+

C3−C4 (o/p)

36.5 43.9 37.2 44.2

6.2 10.6 14.8 32.9

17.8 26.8 27.7 55.3

75.9 62.6 57.5 11.9

0.8 0.7 0.8 0

TOFCOc (10−2 s−1)

TOFCH4c (10−3 s−1)

8.5

9.0

6.4 3.7

9.5 12.2

a Reaction conditions: H2/CO = 2, P = 2 MPa, GHSV = 1.7 Lsyngas h−1 gCo−1, and TOS = 48 h. bData were obtained by changing the GHSV to 3.1 Lsyngas h−1 gCo−1. cTurnover frequency (TOF) was calculated at 250 °C on the basis of H2 chemisorption.

and s-Co are 16.0 and 11.2 m2 g−1, respectively. In this instance, these two catalysts were selected because they had the same morphology and similar sizes, enabling us to exclude the influence of the shape and the specific surface area. Another synthesized catalyst (i.e., c-Co) is monodisperse with a uniform edge length of approximately 20 nm (Figure 1c1 and Figure S1c) and is enclosed by six {001} planes that are the only planes normal to the set of the (220) planes with a lattice distance of 0.28 nm. The specific surface area of c-Co is 40.5 m2 g−1. We note that the particle sizes of all catalysts exceed 15 nm, so that the effect of the size on the catalytic properties can be excluded; such an effect would become noticeable for particles with sizes less than 8−10 nm.6,13

and edge length of 15−25 and 60−100 nm, respectively, whereas the corresponding values for s-Co are approximately 30 and 100 nm. The electron diffraction (ED) patterns (Figure 1a2−c2) demonstrate the single-crystalline structure of the Co3O4 NCs without the presence of twins or stacking faults. As observed from the high-resolution transmission electron microscope (HRTEM) images (Figure 1a2,b2), the dominant exposed planes of p-Co are {112}, which are the only planes perpendicular to both the set of the (220) planes with a lattice distance of 0.28 nm and the set of the (111) planes with a square crossing lattice space of 0.46 nm. The major exposed planes of s-Co are {111}, which are the only planes normalized by (220) and (022) planes with a lattice space of 0.28 nm and an interfacial angle of 60°. The specific surface areas of p-Co 9449

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Figure 2. (a) Long-chain hydrocarbon composition, (b) ASF plots, and (c) CO-TPSR profiles obtained for the p-Co, s-Co, and c-Co catalysts.

and 32.9%. The order of CH4 selectivity is established as {112} < {111} < {001}, and C5+ selectivity displayed the opposite trend. The product differences among the investigated catalysts for long-chain hydrocarbon selectivity have been analyzed (Figure 2a), and the distribution of the hydrocarbon chain follows the Anderson−Schulz−Flory (ASF) model for all three catalysts. The calculated chain growth probability α (Figure S4) is largely consistent with the results of previous studies.16 Another interesting finding is that hardly any olefins were found over the {001} facets, while the {112} and {111} surfaces displayed similarly higher o/p ratios. This facet effect is discussed below on the basis of the results of DFT calculations. Reactivity of Surface Co0 Sites Studied by CO-TPSR. To elucidate the surface topology of the exposed active sites in the selected catalysts, the CO temperature-programmed surface reaction (CO-TPSR) was carried out on the reduced surfaces, and it was found that all of the obtained curves displayed two separate peaks (Figure 2c). The intense signals observed in the temperature range of 150−280 °C are assigned to the hydrogenation of the carbon species resulting from CO dissociation. A high-temperature characteristic peak in the range of 450−700 °C originated from the hydrogenation of the deposited resilient carbon,17 which is assumed to be due to the aging of the carbon species during the temperature ramp. The intense high-temperature peak, which is observed at a temperature that was higher than the temperature of a typical FT reaction, exhibits large differences for Co3O4 NC catalysts. The CH4 evolution feature peak for c-Co is considerably larger than those of the other two catalysts, indicating that the {001} facets form carbon species more easily in comparison to the {111} and {112} surfaces. This may be one of the reasons for the high CH4 selectivity on the {001} facets that is expected to generate methane by the hydrogenation of the carbon species. Meanwhile, the catalytic activity will also be affected by the carbon species, which may poison the active species or physically decrease the accessibility of the Co surface.17b In the low-temperature regime, the CO-TPSR profile can be interpreted as a signature of the intrinsic catalytic activity.18 It is observed that the temperature of the maximum CH4 formation is in the order p-Co < s-Co ≈ c-Co, indicating the higher activity of the Co0 sites for p-Co. This is in good agreement with the trend observed for the FTS reaction. Hence, the activity of the catalysts is indeed related to the distinct intrinsic activities of the Co0 sites toward CO hydrogenation. Depending on the different initial Co3O4 planes, a gradual evolution toward cobalt metal that exposes preferentially dissimilar surfaces of Co0 atoms is hypothesized.

The three catalysts have similar XRD patterns (Figure S2a) and can be indexed as face-centered-cubic structures of Co3O4 (space group Fd3m). They are in good agreement with the values in the literature (JCPDS# 00-001-1152) and do not show the presence of any other crystalline impurities or intermediates. Meanwhile, the FT-IR spectrum in Figure S3 demonstrates the stretching vibrations of the metal−oxygen bond of a Co3O4 spinel oxide with no evident impurities detected, illustrating the formation of pure Co3O4. The reduction behavior of the catalysts was investigated by H2TPR. The reduction profiles (Figure S2b) are similar in shape, and all of them displayed two separated peaks that are typically assigned to the stepwise reduction of Co3O4 to CoO and of CoO to Co0.14 The peak positions are in the sequence pCo3O4 > s-Co3O4 > c-Co3O4, and the corresponding reduction degrees are 80.1%, 80.3%, and 81.9%, respectively. These similar values could exclude the influence of the degree of reduction as a factor that gives rise to the different performances of the catalysts. Catalytic Performance of Different Co3O4 NCs. The evaluated FTS results (Table 1) demonstrate that different Co3O4 NCs exhibit facet-dependent catalytic activity in the FTS reaction that was compared to an identical GHSV and CO conversion (approximately 40%). The required reaction temperatures are 220 °C for p-Co and 250 °C for s-Co and cCo, indicating that the p-Co with exposed {112} facets exhibits higher activity in comparison to the other catalysts. p-Co and s-Co have similar surface areas and shapes, and the difference in degree of reduction between them is too trivial to interpret as the origin of the obvious changes in the catalytic performance. Therefore, the distinct catalytic activity difference of p-Co and s-Co can be assigned to the different crystal planes of the catalysts. For c-Co, although the degree of reduction and the specific surface area are higher than those of the other two catalysts, the FTS activity is lower than that of p-Co and is similar to that of s-Co. It is known that the catalytic activity mainly depends on the number of active metallic cobalt sites that is positively correlated with the degree of reduction and the specific surface area.15 Consideration of the effects of the coexisting surface planes suggests that the {001} planes have the lowest activity for FTS, which is confirmed by the TOFCO results shown in Table 1. In addition to the activity, the selectivity of the Co3O4 NCs catalysts in the FTS is also sensitive to the crystal plane. It is observed that, at the same reaction temperature (250 °C) and CO conversion, catalysts with different exposed crystal planes exhibit different CH4 selectivities following the order p-Co < sCo < c-Co. p-Co exhibits the lowest CH4 selectivity of 10.6%, while s-Co and c-Co, respectively, show selectivities of 14.8% 9450

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Figure 3. Structural analysis of the Co NCs. HRTEM, ED patterns, and FFT images of the Co NCs: (a1−a4) p-Co-R; (b1−b4) s-Co-R; (c1−c4) c-Co-R. Insets in a1−c1 show the Wulff constructions of the corresponding Co NCs. Different colors represent different crystal planes: yellow, {10−11}; green, {0001}; pink. {11−20}. Insets in a3−c3 are the FFTs of the high-resolution images. a4−c4 show the top views of different surfaces.

FTS performance, as was verified in the subsequent experiments. After the reduction, the XRD patterns completely change from those of the pristine Co3O4 into those of the HCP phase of the Co metal (Figure S7). SEM (Figure S6) and TEM images (Figure 3) show that the morphologies of the starting Co3O4 NCs were well maintained, with no obvious changes in size. The surfaces of the obtained Co NCs were covered with a passivated layer of CoO that was formed by the passivation process with 0.5% O2/N2 after the reduction. The existence of the CoO layer was further confirmed by the corresponding H2TPR profiles shown in Figure S9. Uniform Co nanoplates (denoted as p-Co-R), nanosheets (denoted as s-Co-R), and nanocubes (denoted as c-Co-R) were obtained from the corresponding Co3O4 NCs. Such a morphology-preserving reduction can be attributed to the large size of the Co3O4 NCs and the low reduction temperature.19 The ED patterns (Figure 3a2−c2) confirm that all acquired Co NCs are single crystals. As observed for p-Co-R, two lattice spacings of 0.19 and 0.15 nm were observed with an angle of 50° in the HRTEM image (Figure 3a3), indicating the presence of the (10−11) and (10−12) planes of the HCP Co, respectively. On the basis of these results and the corresponding FFT image, the two flat planes of the p-Co-R could be referenced to {10−11}. In addition, the HRTEM image of s-Co-R (Figure 3b3) shows the (10−10) and (01− 10) atomic planes with a lattice distance of 0.21 nm and an interfacial angle of 60°. Together with the corresponding FFT

To verify this hypothesis, a morphology-preserved reduction was applied to the selected Co3O4 NC catalysts. Further Investigation of the Change of the Surface Plane during the Reduction Process. In the cobalt-based FTS reaction, the active species is the cobalt metal rather than the cobalt oxide. The catalytic performance of cobalt-based systems depends directly on the number and availability of the cobalt metal atoms that provide the active sites for CO dissociation, hydrogenation to form the CHx monomer, and the subsequent carbon-chain growth.15b Generally, the asprepared cobalt-based FTS catalysts are activated under a reducing atmosphere to obtain a metallic cobalt phase (Co0). Hence, it is necessary to investigate how the Co3O4 crystal planes change and affect the reaction rate and the product distributions of the FTS reaction. To reveal the nature of the active sites and elucidate the structure−performance relationship, we designed an additional pretreatment to investigate the evolution of the morphology and the final crystal facet of Co. In detail, three different Co3O4 NCs were subjected to in situ reduction using the same procedure used in the FTS process, but with the reduction atmosphere diluted to 5% H2 with nitrogen instead of being pure H2. The passivation of the pretreated samples was carried out in a flow of 0.5% O2/N2 for 2 h at room temperature prior to the characterization. This strategy was adopted to obtain morphology-preserved uniform Co NCs from the corresponding Co3O4 NCs. The Co NCs that were reduced in 5% H2 can represent the corresponding Co3O4 NCs and have the same 9451

DOI: 10.1021/acscatal.8b01333 ACS Catal. 2018, 8, 9447−9455

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ACS Catalysis

Figure 4. (a) Detailed hydrocarbon product distribution and (b) o/p ratio obtained over Co NCs and Co3O4 NCs catalysts. (c) Reaction paths for CH hydrogenation into CH4 and (d) propene hydrogenation into propane on Co(10−11), Co(0001), and Co(11−20) surfaces.

exposed on the HCP Co catalyst.4 Therefore, the high FTS activity on the Co{10−11} facets can be ascribed to the low activation energy barriers of this more open Co surface. In addition to the activity, the methane, C5+, and o/p selectivities are all in agreement with the trends depicted in Table 1. Therefore, the Co NC samples reduced by 5% H2 can represent the corresponding Co3O4 NCs and have the same FTS performance characteristics. DFT Calculations. To elucidate the origin of the facetdependent selectivity performance of Co NCs, detailed DFT calculations for the formation of methane and of lower olefins (C3H6 as an example) were performed on the Co(10−11), Co(0001), and Co(11−20) surfaces that simulate the p-Co-R, s-Co-R, and c-Co-R catalysts, respectively. As depicted in Figure 4c, the effective barriers for methane formation from CH and H on Co(10−11), Co(0001), and Co(11−20) surfaces are 1.28, 0.95, and 0.79 eV, respectively. Thus, these calculations readily explain the observation that the selectivity of CH4 is in the order Co(10−11) < Co(0001) < Co(11−20). Recently, Liu et al. also reported that both CH and CH2 are favored CHx (x = 1−3) monomers and are considerably easier to form than CH4 and CH3OH on the HCP Co(10−11) surfaces.22 In addition, for the Co(10−11), Co(0001), and Co(11−20) surfaces, the first hydrogenation (C3H6 + H → C3H7) barriers are 0.44, 0.45, and 0.26 eV (Figure 4d), and the calculated barriers for C3H7 + H → C3H8 are 0.96, 0.87, and 0.59 eV, respectively. In comparison to the other surfaces, the hydrogenation barrier of C3H6 is the lowest on the Co(11− 20) surface for the overall pathways, indicating that C3H6 is more easily hydrogenated, which is expected to improve the C3H8 selectivity in comparison to those for the Co(0001) and Co(10−11) surfaces. Thus, these theoretical calculations provide strong evidence to support the experimental

image, this result suggests that the dominant facets for s-Co-R are {0001}. For c-Co-R (Figure 3c3), the lattice fringes of 0.20 and 0.15 nm that are observed with an angle of 43° correspond to the (0002) and (10−12) planes, respectively. This confirmed that c-Co-R is predominantly enclosed by the {11−20} facets. Hence, the preferred orientation relationship induced by the Co3O4 → Co transformation is {112} → {10− 11}, {111} → {0001}, and {001} → {11−20}. Meanwhile, the HRTEM images of the corresponding Co NCs that were reduced in pure H2 (Figure S10) demonstrated the same plane transformation as the Co NCs that were reduced by 5% H2. However, because the Co structure may change by varying the temperature and pretreating the catalysts,4,18a,20 it is still necessary to investigate whether the Co NCs that were reduced in 5% H2 can represent the corresponding Co3O4 NCs and have the same FTS performance. Therefore, in situ XRD and FTS experiments were performed on the reduced catalysts. In situ XRD diffraction patterns (Figure S8) at 400 °C show the complete reduction of Co3O4 to HCP Co (JCPDS# 00001-1278) for all of the samples. The shape and intensity of the reduction peaks for Co NCs are identical with those of the corresponding Co3O4 NCs samples. In addition, the catalytic performance of the Co NCs was further evaluated under FTS conditions identical with those described above. The activity and selectivity performance characteristics of the Co NC catalysts (Figure 4a,b) are very similar to those of the corresponding Co3O4 NCs catalysts. p-Co-R with major exposed {10−11} facets exhibits the highest activity for CO conversion that is obtained from Co3O4{112}. According to the principle of Wulff construction, the Co{10−11} facets are rich in the B5-type sites,21 which are known to facilitate CO dissociation, CHx hydrogenation, and C−C bond formation. Recently, Liu et al. presented a comparative analysis of the CO dissociation rates on HCP Co and predicted that the Co(10− 11) facet is almost the most active facet among the other facets 9452

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observation that p-Co exhibits the lowest methane selectivity while c-Co shows a high selectivity to methane and alkanes. Origin of Facet-Dependent Chain Growth Performance of Co3O4 NCs. In addition to the selectivity for methane and lower olefins, the facet-dependent chain growth performance of Co3O4 NC catalysts may also attributed to the specific surface topology of its active phase. The chain growth mechanism and selectivity have been demonstrated to be dramatically influenced by the intrinsic structure in FT synthesis.23 An investigation by Su et al. revealed that the chain growth tends to proceed through the CO insertion mechanism on the close-packed Co(0001), whereas the carbide mechanism is more favorable on the more open Co(10−11), accompanied by higher selectivity to C2 hydrocarbons than to CH4.23a The origin of this phenomenon was assigned to the structure-sensitive adsorption of the key intermediates, in particular the C/CH species. As the C/CH bond strength increases, the total barrier for chain growth is decreased, resulting in higher selectivity toward chain growth rather than methanation. In addition, our previous work has investigated the formation mechanism of long-chain hydrocarbons on the Co(10−11)22 and found that the estimated C/ CHx binding was higher on Co(10−11) than on the other reported surfaces, such as Co(0001) and Co(11−20),24 indicating a lower total barrier and higher selectivity toward chain growth on the Co(10−11) surface. Furthermore, it is widely accepted that methanation is the main competing pathway for chain growth in the overall FTS process. A number of publications have reported a remarkable linear relationship between the selectivity to CH4 and C5+.25 In our study, a similar plot is shown in Figure S5. Since the trend of the methane selectivity has been established as Co3O4{112} < Co3O4{111} < Co3O4{001}, it is reasonable to assume that the C5+ selectivity displayed the opposite trend.

AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.W.: [email protected]. *E-mail for D.L.: [email protected]. ORCID

Chuan Qin: 0000-0002-4851-6474 Qiang Wang: 0000-0001-7301-9341 Congbiao Chen: 0000-0001-5833-0270 Litao Jia: 0000-0002-4733-2884 Debao Li: 0000-0002-6891-4787 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Nos. U1710104, 21703273, and 21706271), Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA 21020202) and the Natural Science Foundation of ShanXi Provincial (No. 201701D221056) for providing financial support. We thank the staff of the 610 research group at the State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Shanxi, People’s Republic of China, for assistance during data collection.

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ABBREVIATIONS FTS,Fischer−Tropsch synthesis; NCs,nanocrystals; DFT,density function theory; HCP,hexagonal close packed; FFT,fast Fourier transformed; CO-TPSR,CO temperature-programmed surface reaction

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REFERENCES

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CONCLUSIONS Our experimental and computational results are the first to clearly demonstrate a facet-dependent catalytic performance of cobalt catalysts under the realistic conditions of the FTS reaction, and it was found that the Co3O4{112} facets exhibit the best activity and selectivity. The evolution of the crystal facet and the preferred orientation relationship induced by the Co3O4 → Co transformation during reduction was further investigated. By a combination of the characterization of the structure of the reduced surface and DFT calculations, the mechanisms of the facet-dependent catalytic performance for the Co3O4 NCs in the FTS reaction were explored and confirmed. These findings show that the catalytic performance of the FTS reaction can be enhanced by the selective exposure of a specific crystal plane from Co3O4 and open a new avenue to the rational design of high-performance FTS catalysts. Our strategy can be further adapted for other complex chemical reactions for which the active phase is not formed directly but rather requires a reduction step.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01333. Experimental procedures and structural details and supplemental figures as described in the text (PDF) 9453

DOI: 10.1021/acscatal.8b01333 ACS Catal. 2018, 8, 9447−9455

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DOI: 10.1021/acscatal.8b01333 ACS Catal. 2018, 8, 9447−9455