Facile Fabrication of BCN Nanosheet-Encapsulated Nano-Iron as

Apr 10, 2017 - Notably, no obvious deactivation was observed after 1000 h running. The enhanced stability of the catalyst can be ascribed to the speci...
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Facile Fabrication of BCN Nanosheet-Encapsulated Nano-Iron as Highly Stable Fischer−Tropsch Synthesis Catalyst Jianghong Wu,†,‡ Liancheng Wang,*,† Baoliang Lv,† and Jiangang Chen*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

‡

S Supporting Information *

ABSTRACT: The few layered boron carbon nitride nanosheets (BCNNSs) have attracted widespread attention in the field of heterogeneous catalysis. Herein, we report an innovative one-pot route to prepare the catalyst of BCNNSs-encapsulated sub-10 nm highly dispersed nanoiron particles. Then the novel catalyst was used in Fischer−Tropsch synthesis for the first time and it exhibited high activity and superior stability. At a high temperature of 320 °C, CO conversion could reach 88.9%, corresponding catalytic activity per gram of iron (iron time yield, FTY) of 0.9 × 10−4 molCO gFe−1 s−1, more than 200 times higher than that of pure iron. Notably, no obvious deactivation was observed after 1000 h running. The enhanced stability of the catalyst can be ascribed to the special encapsulated structure. Furthermore, the formation mechanism of highly dispersed iron nanoparticle also was elaborated. This approach opens the way to designing metal nanoparticles with both high stability and reactivity for nanocatalysts in hydrogenation application. KEYWORDS: BCN nanosheets, Fe nanoparticles, encapsulate, Fischer−Tropsch synthesis, catalyst

1. INTRODUCTION Fischer−Tropsch synthesis (FTS) is a heterogeneous catalytic process, which can convert syngas (CO and H2) into a wide variety of hydrocarbons. In the process, the FTS catalyst conversion efficiency completely depends on catalyst. Therefore, the current research is mainly to develop high activity, selectivity, and stability of the catalyst. Nanocatalyst has been popular because of the high exposure of the active sites of catalyst and higher specific surface area. Because of aggregation, carbon deposition and active sites blocked, the outstanding performance of the so-called nanocatalyst is limited. To conquer this problem, several strategies have been adopted. Nowadays, a fashionable method is to design an encapsulated structure catalyst (core−shell structure) to preserve the active component and improve the activity effectively. Moreover, the confinement effect of shells has been proved to improve reaction activity and catalyst stability. Meanwhile, reactions happen on metal surfaces through intercalation under the 2D material shells.1−4 Up to now, porous silica,5,6 zeolites, 7,8 and carbon materials9,10 have usually been used as coating materials. However, silica pores can be easily subject to collapse in the basic environment, preventing molecules from contacting with © 2017 American Chemical Society

the active component. Moreover, for oxide coating (support), the strong interaction has a negative effect on the catalytic performance. The outstanding performances of these materials are limited due to imperfect features. The carbon-based coating are still considered to be advantageous over previously reported materials.11−13 Various functional groups (−COOH, −OH) may enhance the dispersibility of the nanoparticles. Furthermore, because of the stability of carbon materials at harsh application situations such as acids and alkalis, they can effectively protect the core materials and hinder aggregation of neighboring particles. However, carbon supports alone easily suffer from loss at harsh environment (high temperature) and catalyst recycling for reuse is a challenge for removal of carbon deposition. Generally, the regeneration of catalyst can be conveniently realized by calcination at higher temperature in ambient atmosphere. However, the method is limited as traditional carbon-based support for FTS reaction. Boron nitride (BN), a material closely related to graphite structure, is a quite versatile inorganic material that receives Received: January 12, 2017 Accepted: April 10, 2017 Published: April 10, 2017 14319

DOI: 10.1021/acsami.7b00561 ACS Appl. Mater. Interfaces 2017, 9, 14319−14327

Research Article

ACS Applied Materials & Interfaces

spectra were recorded on a Horiba LabRAM HR 800 spectrometer with an Ar laser with 514 nm wavelength. The Fourier-transformed infrared (FT-IR) spectra were carried out on a Nicolet IS50 spectrometer with the resolution of 4 cm−1 in the transmittance mode. X-ray photoelectron spectroscopy (XPS) was acquired on an Axis Ultra DLD imaging photoelectron spectrometer, Kratos Analytical Ltd. The source of X-ray was Al Ka, 1486.6 eV with a quartz monochrome. The calibration of the binding energy was referenced to the peak of graphitic carbon at 284.6 eV. Transmission electron microscopy (TEM) was performed with a Hitachi H-7650, operated at an acceleration voltage of 100 kV, to characterize the morphology of the prepared samples. High-resolution TEM (HRTEM, JEOL 2100) was used to analyze the structure of the prepared samples. The N2 adsorption and desorption isotherms were performed on a Tristar II (3020) instrument. The multipoint BET surface was estimated in a relative pressure range from 0.05 to 0.3 based on the Brunauer−Emmet−Teller (BET) model and pore size distribution was calculated from the adsorption branch of the isotherm according to the Barrett−Joyner−Halenda (BJH) method. The thermogravimetric analysis (TG) was carried out in Setsys Evolution TGA (16/18) instrument. The temperature-programmed mass spectra were obtained on a quadrupole mass spectrometer (OMNIstar GSD 301 O3; Pfeiffer Vacuum D-35614 Asslar) coupled to a thermogravimetric analysis. The Fe loading of the catalyst was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on an Atom 16 spectrometer (TJA, USA). 2.3. Catalytic Evaluation. The FTS reactions were carried out in a fixed bed reactor. About 3 mL catalyst (0.6 g) was diluted with 6 mL quartz sands of 40−80 mesh quartz sand. The Fe@BCNNSs catalyst need not be pretreated while the Fe/BNNSs catalyst was reduced by pure H2 at 400 °C for 6 h. The synthesis gas (molar ratio H2/CO = 2:1) was fed into the catalyst bed. The gas hourly space velocity (GHSV) is defined as the ratio of the volumes of syngas to the total volumes of catalyst. The flow rates of CO and H2 were controlled by two mass flow controllers (MFC, Brooks, model 5850E). The GHSV was regulated to 1000 h−1. Then the temperature was heated up with 2 °C min−1 to the designed temperature. During reaction process, the pressure of synthesis gas was 2.0 MPa all along. The description of the reactor and product analysis systems in detail is provided elsewhere.21,23

extensive attention because of many advantages such as high thermal stability and oxidation resistance, large thermal conductivity, nontoxicity, and environmental friendliness. Its high stability and resistance to oxidation can endow the catalyst ability of remove the deposited carbon by a simple calcination around 600 °C, enabling it an ideal candidate as an encapsulating material. Apart from the well-studied usage as excellent planelike support for dispersing and stabilizing active component.14,15 It has also garnered notice in harsh conditions as an encapsulated shell.16 Recently, Van Bokhoven et al.17 found that boron nitride coated rhodium black displayed the performance of stable production of syngas. However, microsized Ru particles exposing low active sites influenced the catalytic performances. Fu et al. employed porous BN confined metal Ni nanoparticles to study methanation reaction.2 The usage of the silica support and sequent thermal treatment by ammonia at 850 °C could affect its catalytic activity. In addition, as we know, FTS is involved with adsorption and desorption for CO and H2 molecule. In contrast to the pristine BN/C nanosheets, which have the reversible covalently adsorbed H2 and CO molecule adsorption,18,19 the h-BN/C heterostructures benefit the improved H2 adsorption.20 Inspired and encouraged by the properties of h-BN/C heterostructures, it is highly desirable to realize assembly of BN and C into BCN nanostructures. In the present work, we choose Fe-catalyzed syngas (CO +H2) conversion as a probe reaction. Generally, the widely used supported Fe2O3 catalysts showed no activity and needed to be reduced at relative higher temperature before it was used as FTS catalyst. In the process, these active components were subject to migrate and grew into bigger particles lead to low activity. In previous work, although we in situ synthesized Fe0/ BNNSs catalyst and used in FTS for the first time but bigger iron particles sizes (about 30 nm) and poor dispersion had a negative effect on the catalytic activity.21 Here, we report a moderate and facile synthetic route for BCN nanosheets (BCNNSs)-encapsulated highly dispersed nanoiron (nZVI) catalyst by a one-pot thermal decomposition with boric acid, urea and ferric nitrate, with the aim of design and fabrication of highly stable FTS and other hydrogenation catalyst.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. 3.1.1. Structure Characterization of Fe@BCNNSs Composite. The Fe@BCNNSs materials were prepared by a simple thermal decomposition of ferric nitrate, boric acid and urea in 5% H2/N2 atmosphere. Figure 1a shows the XRD pattern of the fresh prepared catalysts. There were three phases defected in the samples. Because the (002) planes of BN and C are overlapping, the peak observed at 2θ = 26.2° could be assigned to (002) plane of hexagonal boron nitride (h-BN) (JCPDS card No. 34−0421) or carbon (JCPDS card No. 41−1387). In addition, this twophase case was also detected in iron (α-Fe and γ-Fe) of the [email protected] In Figure 1a, the strongest peak at 2θ = 44.6° was assigned to (110) of α-Fe (JCPDS card No. 06−0696), and the next strongest peak was assigned to (111) of γ-Fe.24 As we known, the wide (002) plane of BCN peaks around 26.2° indicated the thin thickness of BCNNSs. After diluted hydrochloric acid (2 mol/L) treatment at 80 °C for 24 h, the sharp (002) plane was observed (Figure S1). In addition, the plane of iron at 44.6° was still observed. The (002) peak position of BCNNSs (26.2°) was close to that of pristine h-BN powder (2θ = 26.7°). Compared with XRD patterns of BNNSs (Figure S2) and BCNNSs (Figure S1), BCNNS crystallinity is obviously promoted. The results also demonstrate the metal Fe actually aid the formation of BCN with enhanced crystallinity, which may be conducive to catalytic performance. It is worth

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. In a typical procedure, boric acid (0.62 g) and urea (3.6 g) was dissolved in the deionized water and stirred for 15 min. A certain amount of iron nitrate hydrate (0.5 g) was added to the solution to form the yellow solution. After vigorously stirring overnight at 65 °C, a condensed solid precursor that showed light green did not formed until the solvent was removed completely. Subsequently, the precursor was subjected to a tube furnace and heated up with a heating rate of 5 °C min−1 to 900 °C and maintained for 3 h under 5% H2/N2 atmosphere. In the experiment, H2 is beneficial for formation of boron carbon nitride nanosheets. The final product was marked Fe@BCNNSs and corresponding support was indicated as BCNNSs. As a reference, the precursor without iron nitrate hydrate was annealed in accordance with the above steps, which was a typical synthetic route for BNNSs.22 The corresponding Fe/ BNNSs catalyst was prepared by incipient wetness impregnation method with aqueous solution of iron nitrate (2 mol/L). The asprepared samples were stored in air at room temperature. In the experiment, BNNSs was a white solid powder, but the BCNNSs composite was black. 2.2. Catalyst Characterization. The powder XRD experiment was performed to analyze the phase with a Scintag XDS 2000 X-ray diffractometer equipped with a Cu Kα X-ray source. For analysis, the scan was done from 10° to 90° with a step size of 2° min−1. Raman 14320

DOI: 10.1021/acsami.7b00561 ACS Appl. Mater. Interfaces 2017, 9, 14319−14327

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The Raman shift suggested the incorporation of carbon in the BN layers smeared out Raman signals of BN due to the distortion of the layer symmetry.26 At the same time, it can be found that carbon D and G band still existed when the fresh catalyst was calcined under 600 °C about 3 h in the air atmosphere, which also shown the doped carbon in BCNNSs displayed high thermal stability (Figure S4). Figure 1c shows FT-IR spectrum of the synthesized Fe@ BCNNSs catalyst. The absorption bands located at 803 cm−1 can be attributed to out-of-plane B−N−B bending vibrations while 1383 cm−1 absorption peak can be assigned to in-plane B−N stretching vibration of h-BN, respectively. However, outof-plane B−N−B bending vibration of h-BN was about 803 cm−1, the peak shift to low wavenumber of which also indicated that BCNNSs had some defects.27 Except for BN characteristic peaks, some new peaks were also found. The peaks at 1600 and 1023 cm−1 could be assigned to CN and B−O absorption peaks. The broad absorption band near 3400 cm−1 could be assigned to the O−H and NH bonds due to the surface B−OH, N−H groups and absorbed water. The formation of CN bond suggested that C doped into the h-BN could form the B− C−N hybrid the compounds.28 This result corresponded well to that of Raman spectrum confirming BCN feature of the final product. At the same time, the peaks 580 cm−1 in Fe@ BCNNSs catalyst was attributed to the Fe−O characteristic absorption peak. Figure S5 gives some photographs of the samples in the experiment. The precursor solution was yellow, whereas the dried precursor powder showed lightly green. From yellow solution to green solid, the color change was mainly caused by reducibility of the urea solution, through the Fe3+ to Fe2+ ions via electron transfer.29 3.1.2. Morphology of the As-Prepared Fe@BCNNSs Sample. The morphology and structure of the as-prepared Fe@BCNNSs catalyst were characterized by TEM. Figure 2a gives low-magnification TEM image of the Fe@BCNNSs. The black nanoparticles were distributed uniformly within thin BCNNSs. The size distributions of the iron particles was also displayed (Figure S6). More specifically, the particle size of black nanoparticles was mainly in the range of 2−15 nm, with average size of 8 nm, based on the statistical measurement results of 100 particles from TEM result. Figure 2b shows high magnification TEM image of the Fe@BCNNSs. Obviously, black nanoparticles were surrounded by BCNNSs, which also indicated the formation of BCNNSs, as shown in Figure 2b. The coating may prevent active component leaching under FTS conditions. To clarify phases of the black particles and nanosheets, the enlarged TEM images were carried out. Figure 2c shows an HRTEM image of Fe nanoparticle covered by BCNNSs. The lattice spacing of dark particle is 0.21 nm, whereas the encapsulate layer has a lattice spacing of 0.34 nm, as shown in Figure 2d. The thickness of BCN layer covering the iron particles was about 3−8 layers, as displayed in Figure 2d. It is noted that our Fe@BCNNSs was synthesized at 900 °C. The BCNNSs encapsulation might retard the aggregation or sintering of the small core particles. In addition, it was found that the iron nanoparticles were not completely removed by diluted HCl (2 mol/L) treatment over 24 h in 80 °C autoclave, as shown in Figure 2e. The residual Fe was measured to be 3.9 wt % by ICP analysis. Figure 2f gives TEM image of Fe/ BNNSs. It was found that the iron particle size was too small to be detected, which also indicated good dispersion of the active phase. The particle size was smaller than 3 nm.

Figure 1. (a) XRD pattern, (b) Raman spectrum, and (c) FT-IR spectrum of the as-prepared Fe@BCNNSs catalyst.

noting that no Fe−B, Fe−C, Fe−O, and other crystalline matter were detected by XRD pattern. This also suggests that the BCNNS-encapsulated Fe nanoparticles remain stable in air at room temperature without notable oxidation. To get further information on physics and chemistry of Fe@ BCNNSs, we collected Raman spectra. It can identify the nature of defects and disorder that has certain advantages in the catalytic reaction. Figure 1b shows the Raman spectrum of the fresh prepared catalysts. As we know, the single peak at 1368 cm−1 for pure commercial BN assigned to the typical B−N stretching vibration mode (E2g) (Figure S3). However, the Raman spectrum recorded with the 514 nm wavelength excitation revealed that the products showed two distinct characteristic peaks due to D- and G- bands at 1344.6 and 1589.0 cm−1, respectively, which indicated that sp2 hybrid carbon element existed in the samples. By deconvoluted D peak,25 a small peak (E2g of BN) was observed at 1368 cm−1. 14321

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products contained all of C−C, B−C, B−N, and C−N bonds, suggesting the hybrid network in chemical compositions of the products. Thus, XPS analysis very clearly supported doping carbon into the h-BN, which was in line with the result of Raman spectrum. The B: C: N: O: Fe atomic ratio calculated from the XPS spectra was 1:0.21:0.85:0.70:0.02. Compared with 20.9 wt % iron loading, the detected low Fe content also indicated that iron particles was encapsulated by BCNNSs. The presence of the heteroatom-containing groups and the defects on BCNNSs disclosed by FT-IR, Raman, and XPS was also beneficial for anchoring the iron nanoparticles.34 3.1.4. Pore Structure of Composites. Figure 4 shows the N2 adsorption and desorption isotherms of the Fe@ BCNNSs composite. The composite had a specific surface area of 171.9 m2 g−1 calculated using the BET model. The composite exhibited a type IV isotherm and H2 hysteresis loop, which was a typical of mesoporous structure. In addition, the isotherm rising at high relative pressure (>0.9) indicated the presence of macro-porous structure. The corresponding pore size distribution is presented in embedded in Figure 4. The pore size distribution also showed presence of mesopores and macrospores, which was consistent with the result of isotherms. This hierarchical pore structure is beneficial for adsorption of CO and H2 and diffusion of the hydrocarbon product.1 Though even higher surface area of the bare BNNSs (953 m2 g−1), the surface area of Fe/BNNSs catalyst sharply decreased to 49 m2 g−1. It can be speculated that the iron particles filled up the microporous on the surface of the support, which resulted in sharp decrease of BET. The N2 adsorption and desorption isotherms of the BNNSs and Fe/BNNSs are also displayed (Figure S7). 3.1.5. Formation Mechanism of Fe@BCNNSs. To study the formation mechanism of Fe@BCNNSs composites, we examined the pyrolytic details of precursor by TG-MS measurement, as shown in Figure 5. The thermogravimetric curves may be described into four stages. A first TG step, about 2% weight loss of the precursor was observed. The obvious MS signal of 17 (NH3), 18 (H2O), and 44 (CO2) below 100 °C showed that the partial dehydration of hydrated ferric nitrate, and the urea partial decomposition with the assistance of released water. The second step ranging from 100 to 250 °C with about 60% weight loss occurred. The second signal of 17, 18, and 44 originated from free water of hydrated ferric nitrate, H2O decomposed of boric acid and urea.35 It was found that the MS signal of NO (m/z = 30) and NO2 (m/z = 46, not shown) occurred due to iron nitrate decomposition above 270 °C, accompanied by formation of oxide iron nanoparticles. However, the decomposition temperature of pure iron nitrate generally occurred about 150 °C.36,37 The higher decomposition temperature above 270 °C indicated that urea would delay the decomposition of iron nitrate. As the temperature increased successively up to 500 °C, residue urea would polymerized and carbonized with corresponding signal of CO2, H2O and NH3.38 With the increase in heat treatment temperature, iron oxide was transformed into zero iron nanoparticles under 5% H2/N2 atmosphere. In the literature,37,39 author explained onion-like C or BN formed in carbon or B, N related species dissolved into iron nanoparticles and separated out as coated layers. In our case, it can be speculated that the formation of onion-like BCNNSs coating structures was finished before the iron nanoparticles came into contact with each other and sinter, as was shown in Figure 2b, d. Therefore, during their formation

Figure 2. (a) Low- and (b) high-magnification TEM images of the catalyst, (c, d) HRTEM images of catalyst, (e) TEM image of catalyst with treatment with 80 °C autoclave with acid leaching, (f) TEM image of Fe/BNNSs.

3.1.3. Chemical Structure of Fe@BCNNSs Composites. To further investigate the structure of the BCNNSs, we performed XPS measurement. The XPS spectra of full range scan (Figure 3a) revealed that B, C, and N, O, Fe elements were the primary surface species in the products. Figure 3b shows B 1s XPS spectrum, which were deconvoluted into three peaks that corresponded to boron atoms in different chemical environments. The main peak of B 1s (190.4 eV) was assigned to B atoms surrounded by N atoms.30 The subpeaks at 189.9 and 192.3 eV were assigned to the B−C and B−O dangling bond, the latter potentially being surface defect.31 The C 1s signals can be deconvoluted into four peaks. The main peak position was located at 284.6 eV, which was same to the value observed in graphite. This suggested graphite domains existed in material. The small shoulder peak seen at the higher binding energy (285.7 eV) could be caused by the presence of C−N bond, whereas the peak at lower binding energy (283.9 eV) was attributed to a C−B bond structure. The small peak at 289.5 eV was assigned to the C−O bonding structure.32 Figure 3d showed the high-resolution and deconvoluted N 1s XPS spectrum. The peaks at binding energy near 398.0 and 399.1 eV may be related to N−B and N−C bonds, which were in agreement with the reported data.33 Figure 3e displays the XPS spectrum of Fe 2p. The peaks at 706 eV of zerovalent iron was not detected in the spectrum. Because of the partial oxidation of Fe0, the two small peaks at 724.4 (Fe 2p1/2) and 711.2 (Fe 2p3/2) eV attributed to the surface iron oxides could be observed. The XPS spectra revealed that the as-prepared 14322

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Figure 3. (a) XPS spectra of scan survey, (b) B 1s, (c) N 1s, and (d) C 1s, (e) Fe 2p, (f) a schematic illustration of BCNNSs.

of the catalyst.9 The activity of Fe@BCNNSs and Fe/BNNSs was both evaluated, as shown in Table 1 and Figure 6. It was apparent that reaction temperature has a significant effect on the catalytic activity and selectivity for two catalysts. As shown in Figure 6, CO conversion improved remarkably with the rising reaction temperature at 320 °C and reached 95% at 340 °C over Fe@BCNNSs catalyst, while the activity of catalyst is less than 10% below 300 °C. We speculated that the phenomenon was the result of the variation of the metal phase. The active component was zerovalent iron and was carbonized to need to go through induction period in the experiment. When temperature increased unceasingly to 320 °C, CO conversion can reach 88.9%, corresponding to catalytic activity per gram of iron (iron time yield, FTY) of 0.9 × 10−4 molCO g−1 s−1. The BCNNSs coating could prevent highly dispersed nanoiron immigrating (average size of ∼8 nm) so that the Fe@ BCNNSs catalyst could display a considerable catalytic activity due to the exposed a large number of active sites. In addition, from 320 to 360 °C, there was no obvious deactivation for Fe@ BCNNSs catalyst, the support of which also was expected to use in high-temperature FTS conditions. In contrast, CO conversion was merely 10.9% for impregnated Fe/BNNSs catalyst even at 340 °C. The huge difference of CO conversion could be related to their structure difference. A strong interaction between iron oxide and BNNSs was revealed by H2-TPR for Fe/BNNSs and higher reduction temperature (522 °C) showed it was not easy

Figure 4. N2 adsorption and desorption isotherm isotherms and size distribution (inset).

process, this special mechanism led to the final formation of uniformly iron nanoparticles encapsulated by BCNNSs. Coincided with the dehydrated of hydrated ferric nitrate and decomposition of urea and boric acid at the thermo-hydrolysis process, a large number of gaseous species were generated (∼80 wt %) and released (NH3, NO2, O2, H2O and CO2), and porous structure was formed, which was also confirmed by N2 adsorption and desorption isotherms. 3.1.6. Catalyst Activation and Stability. Syngas conversion was chosen as a probe reaction because its activity of FTS reaction is sensitive to the structural and electronic properties 14323

DOI: 10.1021/acsami.7b00561 ACS Appl. Mater. Interfaces 2017, 9, 14319−14327

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ACS Applied Materials & Interfaces

Figure 5. (a) Thermogravimetric curves of precursor, (b) the mass spectra of the released gas for the precursor, (c) the schematic of all major reactions for the pyrolysis of urea, boric acid and hydrated ferric nitrate.

to be reduced (Figure S8). Ma et al.40 show that the residual oxygen of graphene can bind parts of the active sites and suppressed the activity and chain growth probability factor. Here, the small size of iron particles of Fe/BNNSs may be caused by the nature of the rich B−O defects, and the lower CO conversion may be caused by the enrichment of oxidized sites, low surface CHx coverage and the irreversible adsorption of CO for the small size iron-based catalyst.41 Besides, the CO conversion of pure Fe is about 30.8% at 320 °C, corresponding FTY about 0.45 × 10−6 molCO g−1Fe s−1, less than two hundredth of Fe@BCNNSs catalyst.

The olefin/paraffin selectivity is an important parameter. As shown in Table 1, the olefin/paraffin selectivity of their catalysts is below 1.6, not good, which is probably related to the BCN nature. According to Nash et. al’s report, the defect-laden BN can act as metal free hydrogenation catalyst for olefin.42 The as-formed olefin in FTS could be further converted to the paraffin, and yield a low olefin/paraffin selectivity. Nonetheless, it is possible to tune the density of surface defect to adjust the hydrogenating ability. Simultaneously, the selectivity for CH4 and C2−4 increased at the expense of C5+ as the reaction temperature increased. It is expected that a weak CO adsorption of the small catalyst 14324

DOI: 10.1021/acsami.7b00561 ACS Appl. Mater. Interfaces 2017, 9, 14319−14327

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ACS Applied Materials & Interfaces Table 1. Catalytic Performance of Fe@BCNNSs and Fe/BNNSs Catalysts at Different Temperaturesa hydrocarbon selectivity (%) samples Fe@BCNNSs catalyst

Fe/BNNSs catalyst

T (°C)

CO conversion (%)

CH4

C2−4

C5+

CO2 yield (%)

olefin/paraffin in C2−C4

300 320 340 360 320 340 360

7.6 88.9 95 94.9 8.7 10.9 12.3

34.6 22.7 24.1 27.7 54.8 56.9 57.9

44.8 49.2 51.6 53.5 33.3 36.4 38.3

20.6 28.1 24.3 18.8 11.9 6.7 3.8

8.6 15.6 33.6 29.5 9.1 9.6 11.2

1.13 0.47 0.10 0.08 1.60 1.03 0.96

Reaction condition: P = 2 MPa, GHSV = 1000 h−1, H2/CO = 2. The loading amount of iron of Fe/BCNNSs and Fe/BNNSs catalysts were 20.9 wt %, 18.9 wt %, respectiviely. a

(40%) of the leached catalyst is obviously multiplied in comparison with pristine Fe@BCNNSs. Besides the activity, stability is also a crucial factor for catalyst. Figure 7 shows CO conversion and selectivity of the

Figure 6. CO conversion of Fe@BCNNSs and Fe/BNNSs at different temperatures.

surface could promote the generation of methane.43 Hence, the CH4 selectivity of Fe/BNNSs with smaller particle size was over twice that of Fe@BCNNSs (54.8% vs 22.7% at 320 °C). The active phase Fe can be converted into magnetic Fe2O3 in hydrothermal condition (Figure S9) and partial to Fe3O4 in FTS condition. Therefore, our Fe@BCNNSs catalyst also accompanied by a water−gas shift (WGS) reaction, and CO2 conversion increased with the increasing reaction temperature (Table 1). As shown in Figure 2e, the capsules, cracked holes in the coated layer can be observed in the acid treated Fe@BCNNSs catalyst, indicating the some cracked BCN shell and the core is partially wrapped, so the syngas can penetrate and contact with the active phases therein. In some previous reports, the carbon or BN encapsulated metal can hardly be totally removed in diluted acid at room temperature,44,45 and some well-wrapped one therefore remains. Could those well coated layers shield the syngas from active phase? Is the catalyst inactive in FTS? To examine this point, we first treated the Fe@BCNNSs catalyst with diluted HNO3 (3.5 M) over 2 h at room temperature (TEM image, Figure S11) and then evaluate its FTS activity. The HCl was avoided since the Cl contamination will suppresses the hydrogenation activity of the FTS reaction. Though the iron content decreased to 2.8 wt % (based on ICP), the leached catalyst still display an initial 43% CO conversion at 320 °C and 38% after 120 h running, the FTY was 3.0 × 10−4 molCO g−1 s−1, about triple that of untreated Fe@BCNNSs. Fu et al.’ reviewed that small molecules can penetrated into the BNNSs or graphene coated matrix through the inherent vacancies and defects,46 then the FTS reaction can proceed beneath the BCNNS layers in this study. The enhanced FTY of the leached Fe@BCNNSs might be related to the confinement effect.2,4,46 Besides, the CH4 selectivity

Figure 7. CO conversion and selectivity as a function of time on stream in the FTS process at 320 °C during 1000 h on stream.

composites as a function of time on stream. The initial CO conversion was 38.8% at 15 h and reached steady state at about 48 h, which also indicated that the composite needed to go through an induction period to form the active phase. Then, CO conversion decreased slightly and retained a value of 83.4% after 1000 h on stream. The superior stability is closely related with the special structure. On the one hand, it originated from the structure of iron encapsulated by the BCNNSs. Pan et al. point out that the confinement effect of support and active phase not only can improve the catalytic activity but also improve the stability.2 In this study, the mean particle sizes of a typical spent catalyst increase to 18 nm, and the particles did not agglomerate seriously at 320 °C (Figure S10). On the other hand, the excellent thermal conductivity of BCNNSs should also be accounted, which effectively impeded the sintering of the active sites at high temperature.

4. CONCLUSION In this work, we successfully developed a feasible strategy to synthesize highly dispersed Fe nanoparticles covered by BCN layers via a one-pot thermal decomposition. The formation mechanism of the Fe@BCNNSs was elaborated in detail. The encapsulated BCNNS shells stabilized the iron nanoparticle in air at room temperature. The iron nanoparticles (mean size below 10 nm) with a large number of active sites feature a considerable catalytic activity, and BCNNSs shell prevent the deactivation of the catalyst under high temperature conditions. 14325

DOI: 10.1021/acsami.7b00561 ACS Appl. Mater. Interfaces 2017, 9, 14319−14327

Research Article

ACS Applied Materials & Interfaces

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The catalyst displays excellent stability even after running for 1000 h. This study could provide an alternative choice for the design and fabrication of highly stable FTS and other hydrogenation BCN-based catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00561. Size distributions of the iron particles of fresh catalyst, XRD patterns, Raman spectrum of the treated catalyst, TEM image of spent catalyst, XRD pattern of BNNSs, and some photographs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected] (L.W.). *E-mail: [email protected] (J.C.). ORCID

Liancheng Wang: 0000-0003-1117-1326 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China ( 21503253 and 21373254) and Natural Science Foundation of Shan-Xi province of China (2015011010).

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REFERENCES

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DOI: 10.1021/acsami.7b00561 ACS Appl. Mater. Interfaces 2017, 9, 14319−14327

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DOI: 10.1021/acsami.7b00561 ACS Appl. Mater. Interfaces 2017, 9, 14319−14327