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Article Cite This: ACS Omega 2018, 3, 10773−10780
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Efficient Catalysts for Cyclohexane Dehydrogenation Synthesized by Mo-Promoted Growth of 3D Block Carbon Coupled with Mo2C Hui Wang,†,∇ Na Zhang,†,∇ Rui Liu,† Ruihua Zhao,†,‡ Tianyu Guo,§,∥ Jinping Li,*,†,∥ Tewodros Asefa,*,⊥,# and Jianping Du*,†,∥ †
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, P. R. China Shanxi Kunming Tobacco Limited Liability Company, Taiyuan 030012, Shanxi, P. R. China § College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030600, Shanxi, P. R. China ∥ Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan 030024, Shanxi, P. R. China ⊥ Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States # Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854, United States
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‡
S Supporting Information *
ABSTRACT: Cyclohexane can serve as a good dihydrogen (H2) carrier and a safer medium to store and transport H2, as it is liquid under ambient conditions and it has a relatively high hydrogen density per unit volume (0.056 g(H2)/ cm3(Cy)liq.). However, cyclohexane can release H2 only with efficient cyclohexane dehydrogenation catalysts. Here, we report the synthesis of three-dimensional micron-sized block carbon−molybdenum carbide (BCMC) composite materials that can serve as noble-metal-free catalysts for cyclohexane dehydrogenation. The materials are synthesized by a facile hydrothermal synthetic route, and their structures and morphologies are characterized by various analytical techniques. The results show that the BCMCs, along with specific morphologies, form when a source of Mo is included in the precursor and that the sizes of the microparticles in them can be tailored by changing the relative amount of Mo used for their synthesis. The as-synthesized BCMC materials exhibit high catalytic activity for cyclohexane dehydrogenation while remaining stable and maintaining their catalytic activities during the reaction. The materials’ catalytic activity increases as the amount of Mo used to make the materials is increased. This is further found to be because the BCMC materials containing higher amounts of ammonium molybdate or higher densities of Mo2C provide substantially lower activation energies for the reaction. These materials can be expected to find industrial applications for catalytic production of H2 from hydrocarbons.
1. INTRODUCTION As the global fossil fuel reserves continue to deplete and the negative environmental impacts caused by fossil fuels intensify, the development and use of clean and environment-friendly energy sources have gained vital importance.1 At present, much research effort is dedicated to the production of dihydrogen (H2) for use as energy carrier because it has a high gravimetric energy density and hydrogen is an abundant element. However, H2 does not exist naturally and its production from any available source requires various catalytic dehydrogenation processes or catalysts. Transition-metal carbides, such as molybdenum and tungsten carbides, have recently attracted great attention not only for fundamental quantum mechanical studies2 but also for their potential applications in areas such as catalysis,3 energy storage,4 humidity sensors,5 and power metallurgy.6 More importantly, here, due to their Pt-like properties, metal carbides have particularly been increasingly pursued for their © 2018 American Chemical Society
catalytic applications in dehydrogenation as well as hydrogenation reactions,7,8 reforming reactions,9 hydrogen evolution reaction,10 and ammonia synthesis.11 However, the inherently low specific surface areas of many metal carbides synthesized by various methods7−11 limit their catalytic performances in various reactions, including dehydrogenation of alkanes to produce H2. One of the most commonly used approaches to overcome this issue has involved the synthesis of metal carbides in the form of small particles dispersed onto the surfaces of high-surface-area materials, such as activated carbon,12 porous silica,13 zeolite,14 and other carbon materials.15 While some of the resulting metal carbide materials, especially those supported on conductive materials such as carbon, show good catalytic activities for various Received: June 21, 2018 Accepted: August 23, 2018 Published: September 7, 2018 10773
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namely, β-cyclodextrin, phthalic acid, and ammonium molybdate (0.38 g), were mixed with deionized water, then the solution was hydrothermally treated at 140 °C, and finally, the solid product was carbonized in N2 atmosphere at 850 °C. The types of reagents as well as the relative amount of ammonium molybdate in the precursors were varied to investigate their effects on the compositions as well as the structures of the final materials. Accordingly, three different materials, namely, BCMC-5 (using 5 wt % Mo), BCMC-15 (using 15 wt % Mo), and BCMC-25 (using 25 wt % Mo), were synthesized. For comparative studies, a reference material, activated carbon-supported Mo2C (Mo2C/AC), was also synthesized. Figure 1 shows the X-ray diffraction (XRD) patterns of the different carbon−molybdenum carbide composite materials
electrochemical reactions,16 others show catalytic activity in conventional thermocatalytic reactions. For example, nanocomposite materials comprising Mo2C and onion-like carbon were found to effectively catalyze dehydrogenation reactions17,18 and nanocarbon-supported molybdenum carbides were successfully shown to catalyze dehydrogenation of various alkanes.19 Notably also, various hybrid materials prepared by dispersing or embedding Mo2C on different shaped nanocarbon materials, including nano-octahedron-shaped carbons,16 carbon nanofibers,20 hollow carbon microspheres,21 pomegranate-shaped carbons,22 and hierarchically ordered microflowers,23 were found to exhibit good electrocatalytic activity toward the hydrogen evolution reaction. These previous works suggest that hybrid materials possessing molybdenum carbides supported on other types of uniquely shaped micro/nanocarbons may have good catalytic properties for various reactions, especially those involving dehydrogenation of alkanes to produce H2. It is widely known that the storage, safe handling, and transportation of H2 are challenging due to its high vapor pressure. It is also known that many liquid organic compounds (LOCs) can serve as hydrogen carriers and deliver H2 through dehydrogenation reactions.24 Among many LOCs that can carry and deliver H2, cyclohexane is a great candidate because it is liquid under ambient condition and it has a relatively high hydrogen density per unit volume (0.056 g(H2)/cm3(Cy)liq.). Thus, cyclohexane can potentially be used to safely store H2 and transport it over a desirable distance. In addition, cyclohexane entails relatively low cost during H2 storage and transportation. However, like many other LOCs and hydrogen carriers, cyclohexane requires dehydrogenation catalysts to release the hydrogen in it in the form of H2. Furthermore, the catalysts must be efficient to be viable candidates for large-scale commercial applications, i.e., they should make the dehydrogenation process occur over the minimal activation energy barrier or at low energy expense. For dehydrogenation of cyclohexane and other related compounds, although noble metals such as platinum are efficient catalysts,25 their high cost and scarcity limit their extensive use. Herein, we report the synthesis of noble-metal-free composite materials composed of three-dimensional (3D) block carbon and molybdenum carbide (BCMC) particles that show efficient catalytic activity for dehydrogenation of cyclohexane. The materials are synthesized by hydrothermal carbonization and characterized using various analytical methods. Their catalytic properties toward cyclohexane dehydrogenation are evaluated using a fixed-bed reactor, and the results indicate that BCMCs effectively catalyze the reaction. Their catalytic activities for the reaction are improved by tailoring the sizes of the BCMC particles via optimization of the amount of Mo precursor used for the synthesis of the materials. Additionally, the reason behind the materials’ high catalytic activity for cyclohexane dehydrogenation reaction is also studied. Catalytic reaction data obtained at different reaction temperatures suggest that the BCMC materials, especially those made using higher amounts of Mo precursor, render low activation energy (Ea) to the reaction.
Figure 1. XRD patterns of different BCMC materials: (a) BCMC-5, (b) BCMC-15, and (c) BCMC-25.
resulted from the synthesis. The peaks observed at 34.4, 37.9, 39.4, 52.1, 61.5, 69.6, 74.6, and 75.5° were attributed to the (100), (002), (101), (102), (110), (103), (112), and (201) planes of hexagonal molybdenum carbide (Mo2C), respectively. A peak at 26°, which can be assigned to the crystalline phase of carbon, is observed in the XRD pattern of BCMC-15, which was synthesized with moderate amount of Mo. This indicates that a moderate amount of Mo favors the graphitization of some of the carbon formed in the material during carbonization. On the other hand, a broad peak indicating the presence of amorphous carbon was observed in the XRD pattern of BCMC-5, which was synthesized with the smallest amount of Mo. This must have been due to the fact that the amount of residual carbon left behind during the carbonization step was high since only a small amount of carbon went to form molybdenum carbide with the small amount of Mo used. In the case of the XRD pattern of BCMC25, which was prepared with a high amount of Mo, no diffraction peak associated with amorphous or graphitic carbon was observed; here, the carbon must have almost exclusively gone to form Mo carbides, leaving behind no residual carbon. No peaks other than those of carbon and Mo2C were observed in the XRD patterns, suggesting that the materials were carbon−molybdenum carbide composite materials. Interestingly, even if the amount of ammonium molybdate used for the synthesis of the materials was increased, from 5 wt % Mo to 25 wt % Mo, the intensity of the characteristic peak associated with Mo2C did not increase, implying that the Mo2C nanoparticles forming in the materials via the synthesis
2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. The synthesis of BCMC composite materials was achieved via a simple procedure comprising three steps (see Experimental Section for details). First, three commercially available precursors, 10774
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Figure 2. SEM images of BCMC composite materials: (a, b) BCMC-5 and (c, d) BCMC-15.
Figure 3. (a, b) SEM images, (c) energy-dispersive X-ray spectrum, and (d, e) elemental mapping images of BCMC-25.
remained similar in size. The result also suggested that the Mo2C nanoparticles formed with the carbon materials were present in a well-dispersed form and they did not tend to grow, even when the amount of ammonium molybdate included in the precursors was increased.
The scanning electron microscopy (SEM) images of BCMC5 and BCMC-15, displayed in Figure 2, clearly showed blockshaped particles. In the images, especially those of BCMC-15, some irregularly shaped blocks of carbon particles were also evident (Figure 2c). Magnified images (Figure 2b,d) showed 10775
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Figure 4. (a, b) TEM and (c, d) HRTEM images; (e) X-ray photoelectron spectrum and deconvoluted peaks, and (f) N2 adsorption/desorption isotherm (inset, pore-size distribution) of BCMC-25.
a type IV-type isotherm with H3 hysteresis loop in the relative pressure (P/P0) range of 0.6−1.0 (Figures 4f and S1a), suggesting the presence of a mesoporous structure in all of the materials. The pore sizes were distributed over wide ranges (inset in Figures 4f and S1b), with average pore diameters of ca. 24, 16, and 14 nm for BCMC-5, BCMC-15, and BCMC-25, respectively. The presence of significant structural defects in the materials was confirmed by Raman spectroscopy (Figure S2). To indirectly assess the nature of growth and the role of Mo in the growth process of the polyhedron-shaped 3D block carbon coupled with Mo2C, several other control materials were synthesized without including β-cyclodextrin, phthalic acid, or ammonium molybdate in the precursor. In the case of the precursor containing only ammonium molybdate and βcyclodextrin (i.e., no phthalic acid), instead of molybdenum carbide, MoO2 was formed after the carbonization step (Figure S3). This may have been due to the fact that not enough carbon was provided by the precursor during the carbonization step due to the absence of phthalic acid in it. Meanwhile, when ammonium molybdate and phthalic acid were used in the precursor, no solid product was obtained. However, when βcyclodextrin and phthalic acid were used in the precursor without ammonium molybdate, the resulting material contained only spherical carbon nanoparticles (Figure S4). These results indicate that molybdate ions play a key role in the growth of 3D block carbon−molybdenum carbide composite materials, besides slowly turning into molybdenum carbide particles during carbonization. Furthermore, as the molybdenum carbide grows, it starts to restrain the growth of spherical carbon particles, favoring instead the formation of the blockshaped BCMC particles. In other words, during the carbonization step, ammonium molybdate indirectly promotes the anisotropic growth of the 3D Mo-polymer precursor that finally turns into 3D block carbon decorated with Mo2C
that some of the BCMC nanoparticles had irregular shapes but clear edges. In the case of BCMC-5, the thicknesses of the larger blocks were slightly greater than 200 nm, whereas in the case of BCMC-15, the thicknesses of the particles were all ca. 200 nm (marked in the inset) and those of the smaller ones were ca. 400 nm. Additionally, well-defined particles, mostly pentagon- and hexagon-shaped ones, were prevalent in the latter, suggesting that the amount of ammonium molybdate precursor dictated the nature of growth of the 3D block carbon particles, besides their sizes. Figure 3 shows the SEM and elemental distribution images of a sample of BCMC-25. BCMC-25 could be seen to have small particles, most of which have similar sizes (Figure 3a). The magnified SEM image (Figure 3b) showed that the sizes of most of the polyhedron-shaped particles were smaller than 400 nm and the thicknesses of the major particles were slightly greater than 200 nm. The energy-dispersive X-ray spectrum of BCMC-25 (Figure 3c) indicated that the particles contained only carbon and molybdenum. Furthermore, elemental mapping showed that carbon and molybdenum elements were well dispersed in the nanocomposite materials (Figure 3d,e). The block-shaped carbon and dispersed Mo2C particles were also seen in the transmission electron microscopy (TEM) images (Figure 4a,b). The image of a single carbon particle showed block morphology, with Mo2C nanoparticles (the sizes of which are ca. 10 nm) anchoring on its surfaces (Figure 4b). The high-resolution transmission electron microscopy (HRTEM) images exhibited a clear crystal fringe and lattice distances of ca. 0.227 and 0.250 nm, corresponding to the crystal planes of (101) and (100) (Figure 4c,d). The X-ray photoelectron spectrum of Mo 3d and the sum of the peakfitted curves (Figure 4e) revealed peaks corresponding to Mo0, Mo3+, Mo4+, and Mo6+ species.26 The N2 adsorption/ desorption isotherms of the different BCMC materials showed 10776
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2.2. Catalytic Properties. The dehydrogenation of cyclohexane over the various BCMC composite materials and the control material (Mo2C/AC) was measured for 210 min at 315 °C (Figure 5a) as well as at other temperatures (Figures S7a−S9a). The catalytic activity of BCMC materials to dehydrogenate cyclohexane increased in the order of BCMC-5 < BCMC-15 < BCMC-25, with a selectivity of ca. 100% toward dehydrogenation (or H2 production) in all cases, regardless of the reaction temperature at which the catalytic tests were performed. This result implies that decreasing the size of 3D block carbon leads to high conversion in cyclohexane dehydrogenation. Notably also, BCMC-15 (which has a Brunauer−Emmett−Teller (BET) surface area of 19 m2/g) and BCMC-25 (which has a BET surface area of 28 m2/g) showed better catalytic activities than Mo2C/AC (whose BET surface area is 440 m2/g) at all of the temperatures. Furthermore, all of the BCMC materials (i.e., BCMC-5, BCMC-15, and BCMC-25) showed better catalytic activities than activated carbon-supported Mo2C (Mo2C/AC) above 310 °C, although Mo2C nanoparticles were well dispersed on AC surfaces (Figure S10). This means, compared to their activated carbon-supported Mo2C counterpart, 3D block carbon-coupled Mo2C particles are more efficient catalysts for cyclohexane dehydrogenation reaction, even despite that they have relatively lower surface area. The catalytic performance of BCMC for cyclohexane dehydrogenation (conversion % and selectivity) also fares well with some
nanoparticles (Scheme 1). When a higher amount of ammonium molybdate is used in the precursor, it creates a Scheme 1. Schematic Illustration of the Synthetic Route Leading to 3D Block Carbon Containing Mo2C Nanoparticle (BCMC) Materials
barrier between Mo-polymeric structures, restraining their further growth (Figure S5). Hence, an increase in the amount of ammonium molybdate (from 5 to 25 wt % Mo) leads to smaller BCMC particles. However, when the amount of Mo was increased further to 35 wt % and more, instead of Mo carbides, MoO2 was largely formed in the products (see Figure S6).
Figure 5. (a) Percentage of cyclohexane conversion through catalytic dehydrogenation reaction over different BCMC and control catalysts at 315 °C. (b) Initial and final percentages of cyclohexane conversion over different BCMC catalysts at different reaction temperatures. (c) Catalytic activity of the as-prepared BCMC catalysts toward cyclohexane dehydrogenation at 315 °C. (d) Activation energy of cyclohexane dehydrogenation over BCMC catalysts. 10777
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catalytic properties of the materials toward cyclohexane dehydrogenation could be tailored. The catalytic test results indicated that the BCMC materials were suitable for dehydrogenation of cyclohexane with those containing more well-dispersed Mo2C particles, in particular, exhibiting the highest catalytic activities. Besides their structures, the reason that BCMC materials effectively catalyzed the dehydrogenation reaction was because they could render low activation energy for the reaction. During the reactions at various temperatures, the materials have also remained stable and maintained their catalytic activities.
of the best non-noble metal catalysts and even noble-metal catalysts reported in the literature (Table S1). Besides, all of the BCMC materials showed no obvious decrease in their catalytic activity toward cyclohexane dehydrogenation for at least 210 min (Figures 5a and S7a− S9a), indicating their stability during the reaction. The initial and final rates of conversions of reactants over the materials, shown in Figure 5b, further corroborated that BCMC materials reasonably maintained their catalytic activities, although BCMC-25 deactivated at a relatively higher rate than the other catalysts due to its smaller Mo carbide particles, and thus their higher rates of sintering at high reaction temperature (Figure S10). Among all of the BCMC materials, BCMC-25 (i.e., the 3D block carbon−Mo2C materials made from 25 wt % Mo) exhibited the highest catalytic activity toward cyclohexane dehydrogenation (Figures 5c and S7b−S9b). After reaction, all of the catalysts maintained their structures and morphology (Figures S11 and S12) and the Mo in them remained in the same oxidation state as that in the original catalysts (Figure S13). The possible reasons for the high catalytic activity of BCMC materials toward dehydrogenation may be related to their morphology and the degree of defects in them. Threedimensional block carbon has rough surfaces. Moreover, increasing the amount of Mo makes the size of 3D block carbon decrease, and correspondingly the surface area of the carbon block increases. These can all lead to increased dispersion of Mo2C particles and a high density of catalytically active sites for cyclohexane dehydrogenation. Thus, the selfregulating effect of higher amount of Mo in decreasing the size of carbon support and increasing the dispersion of Mo2C indirectly plays a key role in the materials’ ability to catalyze the dehydrogenation reaction. However, further increase in the amount of Mo did not continue to result in the same effect because when the amount was increased (to form, e.g., BCMC35 and BCMC-45), MoO2, which exhibits no activity in dehydrogenation, was formed (Figure S6) rather than molybdenum carbides. To further explore the reasons behind BCMC materials’ high catalytic activity toward dehydrogenation, the activation energy (Ea) of the reaction over the materials was calculated by obtaining kinetics data at different reaction temperatures and then fitting the curves over the data. The results are displayed in Figure 5d. The slopes of the lines decreased as the amount of Mo used to make the BCMC materials was increased. The calculated Ea values were 225, 175, and 155 kJ/mol for BCMC5, BCMC-15, and BCMC-25, respectively. Interestingly, the value of Ea for BCMC-25 (155 kJ/mol) was less than that of Mo2C/AC (171 kJ/mol, whose fitting line is not shown), even though they both contained the same amount of Mo. Thus, the low activation energy is most likely another reason behind BCMC’s high catalytic activity toward cyclohexane dehydrogenation reaction.
4. EXPERIMENTAL SECTION 4.1. Synthesis of BCMC. In a typical synthesis of BCMC materials, in a round-bottom flask, β-cyclodextrin (1.84 g) was dissolved in deionized water (30 mL) and in a second flask phthalic acid (5.65 g) was dissolved in deionized water (15 mL). The two solutions were then mixed together by a magnetic stirrer. The resulting solution was added into another solution prepared in a flask by dissolving ammonium molybdate (0.38 g) and deionized water (15 mL). Subsequently, the solution was transferred into a stainless steel autoclave and treated at 140 °C for 12 h. The mixture was then centrifuged, and the solid product was washed and dried in an oven at 70 °C and finally carbonized in N2 atmosphere at 850 °C for 2 h. This gave block the carbon−molybdenum carbide (BCMC) composite material, which was denoted as BCMC-5 (in which “5” indicates the 5 wt % Mo used to make the material). By following a similar procedure but using 15, 25, 35, and 45 wt % Mo, BCMC-15, BCMC-25, BCMC-35, and BCMC-45 materials, respectively, were synthesized. To determine the effect of the type of precursor on the morphology of the products, control materials involving only two of the three precursors, i.e., ammonium molybdate and β-cyclodextrin, or ammonium molybdate and phthalic acid, through an otherwise similar synthetic procedure, were synthesized. Moreover, a reference material, activated carbon-supported Mo2C, was synthesized by the incipient impregnation method. In this case, 0.46 g of ammonium molybdate was dissolved in 10 mL of deionized water and the solution was added dropwise onto 1 g of pretreated activated carbon to impregnate molybdate ions into the pores of the carbon material. The material was dried under ambient condition at room temperature and then in an oven at 70 °C. The product was subjected to carbonization at 850 °C for 2 h in N2 atmosphere. This finally gave activated carbon-supported Mo2C, which was denoted as Mo2C/AC. 4.2. Characterization. The crystallinity of the materials (catalysts) synthesized above was characterized by X-ray diffraction (XRD, Rigaku MiniFlex) operating with Cu Kα radiation (λ = 0.1542 nm). The diffraction patterns were collected in the 2θ range of 10−80°. The morphologies and structures of the materials were characterized by a scanning electron microscope (SEM, SU8010) operating at 200 kV and a scanning transmission electron microscope (S/TEM, FEI Tecnai G2 F20) operating also at 200 kV. In addition, using the SEM instrument, energy-dispersive X-ray spectroscopy (EDS) was performed to map the elemental distribution in the materials. The materials were analyzed by X-ray photoelectron spectroscopy (XPS) using ESCALAB 250 instrument equipped with a monochromatic X-ray source (Al Kα hν = 1486.6 eV). The surface area (BET) and pore-size distribution of the materials were determined by N2 adsorption/desorption
3. CONCLUSIONS Composite materials composed of 3D block carbon and molybdenum carbide (denoted BCMC) and containing different amounts of Mo (or Mo2C) have been successfully synthesized by the hydrothermal synthetic method. During the synthesis, the amount of Mo precursor has been found to play an important role in dictating the size and uniformity of BCMC. So, by controlling the relative amount of Mo in the precursor, the structures and morphologies as well as the 10778
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Chinese Scholar, Scientific Research Foundation for the Returnees, College Student Innovation Program of Shanxi (2017084), and the US National Science Foundation (NSF, CBET-1508611).
analysis, which was performed using a Micromeritics Tristar II 3020 instrument. To assess the relative defects in the materials, Raman scattering spectroscopy was performed with a Kratos, XSM 800 instrument. The excitation wavelength of the spectrometer was 633 nm (1.96 eV). 4.3. Catalysis. The catalytic properties of the materials were tested using a fixed-bed reactor under constant pressure. In each catalytic run, 150 mg of sample was placed at the center of a quartz tube (with internal diameter, ID = 5 mm), which was then tightly sealed. The temperature around the reactor was programmed to: (1) increase from room temperature to 473 K over a period of 30 min under N2 atmosphere, (2) then increase from 473 to 623 K at a rate of 5 °C/min and remain at 623 K for 2 h under H2 atmosphere, and (3) finally decrease at a rate of 5 °C/min from 623 K to one of the following reaction temperatures: 578, 583, 588, or 593 K. Once the temperature reached one of these reaction temperatures, cyclohexane was pumped into the reactor at a flow rate of 0.01 mL/min. The reaction products were analyzed on-line by a gas chromatography instrument equipped with a thermal conductivity detector and a dioctyl sebacate column, and the conversion % of cyclohexane was then calculated. To compare the catalytic activity of the materials to those of other conventional carbon-supported molybdenum carbide materials, the catalytic performance of activated carbon-supported Mo2C (Mo2C/AC) was also evaluated under a similar reaction condition. The turnover frequency of BCMC catalysts during the reaction was calculated based on the mole of cyclohexane converted to benzene per mole of Mo per second.
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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/acsomega.8b01411. Various characterization data and catalytic results for BCMC and control materials; electron microscopy images; and tables containing comparative catalytic data (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (J.L.). *E-mail:
[email protected] (T.A.). *E-mail:
[email protected] (J.D.). ORCID
Jinping Li: 0000-0002-2628-0376 Tewodros Asefa: 0000-0001-8634-5437 Author Contributions ∇
H.W. and N.Z. contributed equally to the work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support by National Natural Science Foundation of China (51572185) and Natural Science Foundation of Shanxi Province (2014011016-4). This study was also supported by Coal-Based Scientific and Technological Key Project of Shanxi Province (MQ2014-10), Shanxi Province Technology Foundation for Selected Overseas 10779
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DOI: 10.1021/acsomega.8b01411 ACS Omega 2018, 3, 10773−10780
