Fabrication of 3D Porous Hierarchical NiMoS Flowerlike Architectures

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Fabrication of 3D Porous Hierarchical NiMoS Flowerlike Architectures for Hydrodesulfurization Applications Wenjing Song, Weikun Lai, Zhou Chen, Jingyuan Cao, Haifeng Wang, Yixin Lian, Weimin Yang, and Xingmao Jiang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00299 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Nano Materials

Fabrication of 3D Porous Hierarchical NiMoS Flower-like Architectures for Hydrodesulfurization Applications Wenjing Song†, Weikun Lai*‡, Zhou Chen‡, Jingyuan Cao†, Haifeng Wang†, Yixin Lian‡, Weimin Yang§, Xingmao Jiang*†

† School of Chemical Engineering & Pharmacy, Wuhan Institute of Technology, Hubei 430073, P. R. China ‡ National Engineering Laboratory for Green Chemical Productions of Alcohols-ethers-esters, College of Chemistry and Chemical Engineering, Xiamen University, Fujian 361005, P. R. China § SINOPEC Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, PR China ABSTRACT. Layered transition-metal sulfides such as MoS2 often show a range of intriguing electronic, catalytic, and optical properties. Due to their high surface energy, layered materials generally tend to stack and prevent the exposure of additional edge sites. Here, we demonstrate a facile approach for the preparation of hierarchical NiMoS nanoflowers via SiO2-assisted hydrothermal synthesis. The structure and morphology of the nanomaterials are characterized by SEM, TEM, XRD, Raman and XPS analyses, revealing that different sizes of NiMoS nanoflowers assembled from various nanosheet thicknesses can be tuned by modifying the Si/Mo molar ratio. The key aspect of this strategy is to construct the 3D nanostructures around the SiO2

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nanospheres while maintaining a flower-like feature. The correlation between the nanosheet thickness, surface area and total dispersion of the Mo atoms indicated that the large quantity and efficient accessibility of NiMoS active sites originating from the synergistically multiscale structure and atom-scale modulations between MoS2 and Ni atoms are the determining factors for the observed impressive hydrodesulfurization (HDS) performance and the stable recyclability of these materials. KEYWORDS:

3D

nanoflowers,

Ultrathin

nanosheets,

Multiscale

structure,

NiMoS,

Hydrodesulfurization Transition-metal sulfides such as MoS2 have attracted great interest due to their unique electronic features that give rise to numerous applications in Li-ion batteries,1 hydrogen storage,2,3 and heterogeneous catalysis.4 Catalytic hydrotreatment is the most mature technology used by petroleum refineries to reduce the sulfur and nitrogen levels in fossil fuels.5 In industrial hydrodesulfurization (HDS) processes, alumina-supported molybdenum sulfide catalysts have been widely used during the last few decades.6-8 The HDS activity of the supported catalyst is restricted to some extent due not only to the strong interaction between the metal and support9 but also due to the low density of active sites and poor active structure.10 To overcome these disadvantages, unsupported sulfide catalysts have been designed.11,12 The edges of molybdenum disulfide (MoS2) are usually considered as the active sites, while the in-plane structure is not active in catalysis,13 leading to the unsatisfactory performance of bulk sulfide catalysts. Therefore, tremendous efforts have been devoted to developing various sulfide nanomaterials with greater number of exposed edge sites.14 Accompanied by the emergence of graphene, transition metal dichalcogenides such as twodimensional (2D) MoS2 have attracted increasing interest and a wide variety of nanostructured


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MoS2 have been reported.1,15,16 However, the reaction activities in any practical applications have always been restrained by the exposure of fewer active sites, caused by the restacking and aggregation of the 2D nanosheets caused by the interlayer van der Waals forces and high surface energy.17,18 One effective strategy for addressing this challenge is to assemble the 2D MoS2 nanosheets into a 3D hierarchical architecture, allowing the nanosheets to grow separately rather than to aggregate or restack in order to increase the number of exposed active surface as more as possible.3,19,20 As reported by Wang et al., the large surface area and high number of large pores of the as-prepared bimodal mesoporous MoS2 nanosheets is expected to lead to high hydrodeoxygenation activities.21 Porous, nanostructured MoS2 prepared by ultrasonic spray pyrolysis (USP) method using colloidal silica as a sacrificial template show a higher surface area and enhanced thiophene HDS activity than those of the conventional MoS2.4 Recently, Bao’s group reported a template-assisted strategy for the synthesis of multiscale mesoporous MoS2 foam and demonstrated high hydrogen evolution activity and stability.22 Although MoS2 catalysts with high surface areas have been prepared by the aforementioned methods, the addition of a promoter also affected the textural properties of the amorphous Mo sulfide by decreasing the surface area and changing the pore characteristics.23 The inhibited growth of the crystallized MoS2 particles when Ni or Co was incorporated restricts the construction of Ni(Co)MoS nanostructure with a high surface area (Table S1).24,25 On the other hand, the HDS activity is closely related to the proportion of the NiMoS sites, the structure and morphology of the Ni-Mo-S active phase and the dispersion of active metal.10 Ni or Co promoter atoms substitute Mo atoms at the edges of MoS2 slabs in the form of so-called Ni(Co)-Mo-S structure.26-29 The addition of the promoter also affected the construction of the uniform hierarchy NiMoS architecture due to the uncontrollable fast precipitation between the metal ions


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and S2- ion.27 The hierarchical MoS2−Ni3S2 nanostructures prepared by Gao et al. are composed of inner 1D Ni3S2 nanorod surface decorated by two-dimensional (2D) MoS2 nanosheets.30 Therefore, it is highly desirable to develop a better and uniform 3D structure of the NiMoS composite with a high number of specific exposed NiMoS sites and excellent practical applications. Inspired by the synthesis of porous MoS2 by the above hydrothermal method assisted with SiO2 as the sacrificial template, we integrated these design principles to synthesize porous hierarchical 3D architectures of NiMoS and successfully prepared uniform NiMoS nanoflowers with high surface area in this study. Construction the 3D NiMoS nanostructures around the SiO2 nanospheres had been found to be a key feature in the synthesis processes. This synthesis route for porous hierarchical NiMoS flower-like architectures may open new opportunities for rational design of transition-metal composite sulfides through a multiscale structural control to enhance the catalytic activity and other energy related process. RESULTS AND DISCUSSION The uniform porous NiMoS nanoflowers were prepared using the synthetic procedure illustrated in Scheme 1. First, the SiO2-supported Ni and Mo powders were prepared via the wet impregnation method with the size of ~100 nm. Second, the hydrothermal synthesis of NiMoS material consisting of nanosheets was proposed, using hydrazine monohydrate as the reductant and elemental sulfur as the sulfurizing-agent, as in the previous study.26 In the hydrothermal process, Ni-Mo precursors would react with elemental sulfur and the formed sulfide nanosheets were further self-assembled into nanoflowers confined within the domains around the SiO2 nanospheres. Finally, the SiO2 particles were dissolved by HF acid treatment, thus generating the porous hierarchical NiMoS nanoflowers.


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Scheme 1 Illustration of the fabrication of 3D porous hierarchical NiMoS flower-like architectures and the hydrothermal-sensitive morphology evolutions. To reveal the formation mechanism, we monitored the growth process of the NiMoS nanoflowers at different time intervals. As displayed in Scheme 1, at the initial stage of the hydrothermal process, the Ni-Mo precursors are re-dissolved at a relatively slow rate and assemble into spherical agglomerates driven by the minimization of the surface energy. The weak interaction between NiMoS and SiO2 makes the NiMoS sample be confined in the interconnecting regions of the SiO2 spheres, the NiMoS nanoflowers appeared with a diameter of 100-200 nm and located nearby SiO2 nanosphere. Other smaller crystallites that remain out of the nanospheres are easily dissolved and diffused outwards accompanied by crystallization.31 As the hydrothermal time was increased, many embedded nanosheets and small particles can be observed on the surfaces and these nanospheres demonstrate a curved stripe-like feature on the surface (e.g., 24 h). A similar process was reported in the synthesis of the CoSx nanostructures 32,33

and porous NiCo2O4 core-shell microspheres.31 When the hydrothermal duration is increased


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to 72 h, the growth process of the nanoflowers proceeded, resulting in the complete formation of multi-layered flower-like structures, with similar size to the initial spherical agglomerates. In addition, it is important to note that a higher temperature is beneficial for the evolution of the NiMoS sample with a more obvious morphology. Detailed XRD patterns of the NiMoS samples under different hydrothermal conditions are displayed in Figure S1 (ESI). Both samples show similar diffraction patterns, proving the existence of the same crystal phase in both samples. During the hydrothermal procedure, Ni atoms are favorable to substitute for the Mo atoms in the MoS2 matrix and induce the formation of sulfur vacancies and defect sites.34,35 To further gain insight into the effect of the SiO2 spheres on the construction of the 3D hierarchical NiMoS flower-like architectures, we carried out the synthesis of NiMoS under different Si/Mo molar ratios. For comparison, the NiMoS sample synthesized without the SiO2 nanospheres was also obtained using the hydrothermal approach. SEM and TEM images of these samples are displayed in Figure 1. The flower-like feature could be observed in all these samples, and each flower structure was a cluster composed of different amounts of the nanosheets. The TEM images clearly reveal a morphological change accompanied by the changes in the Si/Mo ratio. The NiMoS samples obtained without the SiO2 nanospheres consist of nanoflowers with the sizes of 400-600 nm, and the NiMoS nanostructure growing in the domain of larger SiO2 spheres (~200 nm) resulted in a larger nanoflower size (Figure S2). Besides, the NiMoS-NF2 sample synthesized at a low Si/Mo molar ratio also shows a blurry flowerlike structure. The introduction of greater amounts of SiO2 nanospheres with Si/Mo ratios greater than 9 clearly results in the appearance of a uniform flowerlike structure, and the size of the nanoflowers significantly decrease. Hence, during the entire fabrication process, the Si/Mo molar ratio plays an important role in determining the size of the flower-like nanostructure.


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Figure 1 SEM and TEM images of the (a, b) NiMoS-NF0, (c, d) NiMoS-NF2, (e, f) NiMoS-NF9 and (g, h) NiMoS-NF22 samples (enlarged-view inset).


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Figure 2 Electron microscopic images of the NiMoS-NF9 sample: Low magnification (a) SEM and (b) TEM image (close-up view inset), (c, d) high-resolution TEM image (side-view inset), (e) HAADF-STEM image of the clusters, (f) corresponding bright field and STEM-EDX mapping images reflect Ni L, Mo L and S K intensities. The morphology and structure of the hierarchical nanosheet-based NiMoS were further characterized by electron microscopy. As shown in Figures 2(a, b), the SEM and TEM images of the NiMoS-NF9 sample display a uniform distribution of the nanoflowers. Figure 2(b) inset


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provides a close-up view of the NiMoS flower-like architectures, showing that the hierarchical nanoflower consists of ultrathin nanosheets with the thickness of 3-6 nm, while the nanosheets of the sample without the SiO2 addition are nearly twice as thick. This indicates that during the synthesis of the hierarchical NiMoS composites, SiO2 strongly inhibit the stacking of the MoS2 layers. Figure 2(c) demonstrates the well-stacked layered structure of MoS2 on a nanosheet consisting of 2-5 stacking layers with the longitudinal length of 3-10 nm, similar to the results of a previous work.26 Actually, the MoS2 slabs lengths for the series of the NiMoS-NFx samples are mainly 5-8 nm, far smaller than that for the bulk NiMoS sample. Lattice distances identified from the magnified TEM image in Figure 2(d) are approximately 0.64 and 0.27 nm, in good agreement with the lattice spacings of the (002) and (100) planes. In addition, the d-spacing of the (002) plane is slightly broader than that of bulk MoS2 (0.61 nm), verifying the expanded interlayer spacing. This can be attributed to the substrate effect and the ultrathin nature of the MoS2 nanosheets with an undulated surface36 and the presence of defects involved in the MoS2 layers.37 Furthermore, many slight rotations and crosses from the individual (002) and (100) planes on the basal surface can be observed, suggesting a disordered atomic arrangement and resulting in larger dislocations and distortions dispersed on the nanosheets.38 Hence, the cracking of basal planes occurred, leading to an additional edge and a defect-rich structure. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 2(e)) of the nanoflower clusters confirms its nanosheets assembled flower-like feature. EDX elemental mapping images in Figure 2(f) confirm an even distribution of Ni, Mo and S elements throughout the entire nanostructure. A trace of the Si element was observed on the NiMoS samples by HF-etched in the EDS analysis (Figure S3). Furthermore, the HAADF-STEM image and corresponding EDX maps of NiMoS-SiO2 prior to the HF etching prove that the NiMoS


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nanoflowers grow near the SiO2 nanospheres and reveal the weak interaction between them (Figure S4). These results further demonstrate that the hierarchical nanosheet-based NiMoS can be synthesized successfully through SiO2-assisted hydrothermal method.

Figure 3 (a) Nanoflower size and (b) Nanosheet thickness distribution, (c) nitrogen adsorptiondesorption isotherms and (d) pore size distribution of the series of NiMoS samples. For a quantitative comparison, the well-defined flower-like morphology was further investigated, and the nanoflower size and nanosheet thickness distributions were obtained through statistical analyses based on at least 300 individual nanoflowers or nanosheets obtained from the different regions of each sample. As depicted in Figure 3, the Si/Mo molar ratios modulated the lamellar thickness, which in turn determined the porous structure and nanoflower


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size. Compared to NiMoS-NF22 (125 nm), the NiMoS-NF9 sample has the mean nanoflower size of approximately 153 nm, and lower Si/Mo ratios give larger nanoflowers with the diameter of approximately 264 nm, while the nanoflowers with the mean size of approximately 525 nm were obtained in the absence of SiO2. This result further reveals that SiO2 plays a significant role in the fabrication of the smaller nanostructure. The varied size of flower morphology will result in the discriminating thickness of nanosheets and the discrepancies in the density values of the hierarchical structures. The nanosheets are interconnected, forming porous networks. As displayed in Figure 3(b), the apparent nanosheets thickness strikingly different between the clusters assembled by the nanosheets with different Si/Mo ratio. The average thickness of the nanosheets without SiO2 was estimated to be 6.9 nm. The increase in the Si/Mo ratio leads to a thinner nanosheet, which may change the defect density, the surface area and pore structure. The distinct specific surface area and porosity are analyzed in detail and the results are summarized in Table 1. Without SiO2 addition, a low surface area for NiMoS nanoflowers is obtained (36.3 m2/g) that is comparable to that of the NiMoS-bulk (27.2 m2/g); however, with the help of SiO2, the surface area can be controlled and substantially enhanced. In fact, a higher Si/Mo ratio gives a higher surface area, the NiMoS-NF22 sample showed the highest specific surface area (117.2 m2/g) which can be ascribed to the smaller thickness of the sheet structure. Figures 3(c, d) display the N2 adsorption-desorption isotherms and pore size distributions. It is clear that all NiMoS samples exhibit a type IV isotherm and no plateau region in the isotherms which are the characteristics of a typical mesoporous material. The mesoporous volume increases with the increasing of Si/Mo ratio, indicating the crucial role of SiO2 in forming the mesoscale pores. The mesopores are related to the thickness of the NiMoS nanosheets. These nanosheets are ultrathin and wrinkled, providing sufficient void space between the neighboring


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nanosheets. Pore size distributions reveal that the relatively narrow pore-size distribution has a peak at approximately 3 nm, and another larger pore distribution broader than 5 nm may originate from the gap between the nanostructures. Since the larger mesopores are much more important for the mass transfer resistance for reacting molecules, such a combination on the hierarchical structures is potentially ideal for the catalytic reaction. Table 1 Chemical properties of NiMoS samples. Sample

NiMoSNF22 NiMoSNF9 NiMoSNF2 NiMoSNF0 NiMoSBulk a

SBET (m2·g-1)

VPore (cm3·g-1)

dPore (nm)

dNFa (nm)

TNSb (nm)

fsurfb

nab (nm-2)

ftotc

MoSd (%)

Compositiond

117.2

0.33

10.4

125

3.7

0.133

1.36

0.060

88.5

Ni0.27MoS2.3

88.0

0.31

13.0

153

4.4

0.134

1.60

0.056

83.0

Ni0.29MoS2.4

54.3

0.15

10.6

264

5.3

0.141

1.90

0.040

85.1

Ni0.31MoS2.3

36.3

0.11

11.2

525

6.9

0.138

2.43

0.034

86.7

Ni0.25MoS2.1

27.2

0.09

13.3

--

--

0.068

2.75

0.029

84.9

Ni0.32MoS2.2

Average NiMoS nanoflowers size determined by statistical analysis of SEM images.

b

Average NiMoS nanosheets thickness (TNS), dispersion of the surface MoS2 crystals (fsurf) and density of edge Mo atoms (na), determined by statistical analysis of HRTEM images.

c

Total Mo dispersion, obtained by HRTEM, BET and XPS results.

d

Sulfidation of Mo and atomic composition determined by XPS spectra.

In light of the differences in the nanosheet thickness and nanoflower size, the dispersion of MoS2 slabs was evaluated by TEM analysis. As indicated in Table 1, it is clear that the MoS2 crystallites with short slab length are highly dispersed for all NiMoS nanoflowers, with the exception of the bulk NiMoS. Here, the conventional dispersion indicator of the active surface of the crystals is calculated according to39,40: t

t

f surf = (∑6ni − 6) / (∑3ni 2 − 3ni + 1) i =1

i =1

(1)


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In particular, the fraction of the Mo atoms on the edge surface of the MoS2 crystals (fsurf) is determined by assuming that all MoS2 slabs are present on the catalyst surface as perfect hexagons. ni denotes the number of the Mo atoms along one edge of a MoS2 slab determined from its length (L = 0.32(2ni-1) nm) and t is the total number of the slabs. As shown in the HRTEM image, we could obtain the slab length (L) and slabs number (t) of the MoS2 crystals. This dispersion calculation is related to a supported catalyst and MoS2 crystals distribute on the support surface. Hence, for the unsupported NiMoS samples, MoS2 crystals will possibly be present in the bulk phase. Considering this point, we define a parameter ftot for denoting the total proportion of the Mo atoms at the edges over the unsupported NiMoS catalyst. The total dispersion of active Mo atoms can be statistically evaluated by dividing the total number of the Mo atoms at the edge surface by the total number of the Mo atoms on the unsupported catalyst, using the number of the edge Mo atoms calculated by the TEM analysis and the surface area determined from the nitrogen adsorption-desorption measurements. This can be expressed by the following equations, t

k

na = (∑6ni − 6) / (∑ Aj ) i =1

j =1

f tot = na ⋅ S BET / ( N A ⋅ mMo ⋅ MoS )

(2) (3)

where na is the number of the Mo atoms at the edges per unit surface area, and k and Aj denote the number and the area of the sample observed in the TEM images. mMo and MoS mean the total number of moles of Mo and the Mo degree of sulfidation, respectively. The detailed calculation is provided in Figure S5. Quantitative comparison shows that although the number of the Mo atoms at the edges per unit area (na) in NiMoS-NF0 or NiMoS-bulk is larger than that of the sample prepared with SiO2 addition, the total proportion of the Mo edge atoms in the catalyst


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(ftot) improved significantly with increasing of Si/Mo ratio. As shown in Table 1, the NiMoSNF0 delivers a larger na (2.43 nm-1) and a lower ftot (0.034). Higher total dispersion of active sites in the NiMoS samples is mainly attributed to thinner nanosheets and higher surface area of the highly oriented flower-like architecture.

Figure 4 (a) X-ray diffraction patterns and (b) Raman spectra of the as-prepared NiMoS catalysts after hydrogen treated, (c) XPS spectra of Ni 2p, Mo 3d and S 2p of the NiMoS samples. Furthermore, the composition, crystallinity and chemical state of NiMoS with various Si/Mo ratios are also characterized. As revealed by the XRD patterns, all samples give weak XRD peaks, consistent with the relatively poor crystalline structures of MoS2 (JCPDS-ICDD 371492). The (002) plane peak at 2θ ≈ 14° is due to the slabs stacking along the c-axis, while the


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(110) plane peak at 2θ ≈ 59° is due to a slab layer. It is noteworthy that the MoS2 (002) diffraction peak is located at 13.8° (d002 = 0.637 nm), indicating the expansion of the interlayer distance, as shown in Figure 2(d). Interestingly, although a sharp NiS2 peak was observed in the as-prepared NiMoS samples (Figure S6a), the hydrogen treatment could redistribute the nickel atoms and the Ni3S2 species partially formed in the sample, consistent with the XPS results for the reduced NiMoS sample. The Ni atoms substitute Mo atoms in the Ni-Mo-S crystal phase have not yet been distinguished from the pure MoS2 in the micrographs or XRD patterns. Normally, the slab structures of MoS2 observed in the HRTEM images correspond mainly to the NiMoS phase because of the much higher intrinsic activity of the NiMo catalysts.41,42 From the EDX elemental mapping images in Figure 2(f), an even distribution of Ni, Mo and S elements was displayed throughout the entire nanostructure. Although the Ni content in these substances is not specified, as characterized by XPS, the proportion of the NiMoS phase is approximately 70% for all NiMoS samples (Table S2 in ESI). Aside from the MoS2 reflections, a few of NiSx species (99%), and thiophene (C4H4S, AR, >98%) were purchased from Aladdin Industrial Corporation (Shanghai, China). 40 wt% colloidal silica suspension in H2O (~100 nm) were purchased from Shandong Peak-tech New Material Co., Ltd. Hydrofluoric acid (HF, 40% aqueous solution), sulfur, nickel nitrate hexahydrate (Ni(NO3)2·6H2O, AR, >98%) were obtain from Sinopharm Chemical Reagent Co., Ltd. Deionized water (DI) was prepared using an OPK apparatus (Shanghai Lakecore, One instrument type short for “OPK” in that company)

Synthesis of 3D hierarchical NiMoS nanoflowers: two steps were involved in synthesis of 3D hierarchical NiMoS nanoflowers: precursor preparation and precursor reduction and sulfidation.


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In the first step, Ni(NO3)2·6H2O was dissolved in 20 mL deionized water containing (NH4)6Mo7O24·4H2O, and sonicated for 10 min, followed by the introduction of silica sol, with the Si/Mo/Ni molar ratio of x/1/0.3, where x varies between 0, 2, 9 and 22. Then, the mixture was stirred for 24 h at room temperature to evaporate most of the solvent and was then further dried at 80 °C overnight. In the second step, the obtained dry powder and elemental sulfur were dispersed in the 18 mL aqueous solution by ultra-sonication for 30 min. The S/Mo molar ratio was 2.5 for all cases. After adding 10 mL hydrazine monohydrate, the mixture was transferred into a 35 mL Teflon-lined autoclave and maintained at 160-200 °C for 24-72 h. After cooling to room temperature naturally, the black precipitate was collected and treated with 30 mL HF (~ 40%) for 5 h under room temperature, followed by washing two times with 100 mL water and washing one time with 100 mL absolute ethanol and finally drying at 80 °C in a vacuum oven overnight. The obtained samples were denoted as ‘NiMoS-NFx’ for NiMoS nanoflowers, where x represents the various Si/Mo molar ratios (0, 2, 9 and 22). For comparison, pure MoS2 was prepared following the same procedure for NiMoS-NF9 without the addition of Ni(NO3)2·6H2O. The bulk NiMoS sample was prepared via the vaporization of aqueous solutions of 2.46 g Ni(NO3)2·6H2O and 5 g (NH4)6Mo7O24·4H2O followed by drying at 110 °C for 24 h and calcination in air at 500 °C for 4 h. Prior to the measurements, the oxide sample was sulfurized in situ at 400 °C for 2 h in a stream of 15 vol.% H2S/H2 under atmospheric pressure. The NiMoS/γ-Al2O3 catalyst containing 3.0 wt% NiO and 12.0 wt% MoO3 was prepared by an incipient-wetness impregnation method, and then sulfurized under the same conditions as mentioned above.

Materials Characterization. X-ray powder diffraction (XRD) patterns were recorded with a D/MAX 2500/PC powder diffractometer (Rigaku) using a CuKα radiation source (λ = 0.15406


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nm) operated at 40 kV and 100 mA. Nitrogen adsorption–desorption measurements were performed using a Micromeritics Tristar 3020 adsorption automatic instrument at the liquid nitrogen temperature. BET surface area was determined using the adsorption data in the relative pressure (P/P0) range of 0.05-0.25. SEM images were obtained using a field emission SEM (FESEM) instrument (Zeiss SIGMA, Germany). TEM experiments were performed using an FEI Tecnai 30 high-resolution transmission electron microscope (Philips Analytical) operated at the accelerating voltage of 300 kV. Raman spectra were recorded using a Renishaw Invia Raman spectrometer over five random spots on the powers. The excitation source was a Spectra-Physics Excelsior CW solid state laser operated at λ = 532 nm. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientic Escalab 250 instrument under ultrahigh vacuum with the pressure close to 2 × 10-9 mbar. The C 1s peak at 284.6 eV was used as the internal standard to compensate for the sample charging, and the corresponding spectra were fitted by GaussianLorentzian curves using the XPSPEAK software.

Catalyst activity measurements. Thiophene HDS activity of NiMoS catalysts was measured in a 100 mL stainless steel autoclave. In a typical procedure, 0.1 g catalyst was dispersed in a 40 mL tetradecane solution containing 3 wt% thiophene. After purging in He several times, the temperature was raised to the reaction temperature under constant stirring, and then the sealed autoclave was charged with H2 at the pressure of 3.0 MPa. The reaction was conducted at 220300 °C with stirring for 0-5 h. Approximately 0.1 mL of the reaction mixture collected from the reaction equipment (Figure S9) was analyzed by a gas chromatograph equipped with an FID detector and an HP-5 capillary column. Thiophene conversion was calculated using the mole ratio of the thiophene in the product and feedstock, and the reproducibility was better than ±3%.


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The evaluation of the 4,6-DMDBT HDS activity was carried out in the same setup, using 0.20 wt% 4,6-DMDBT in tetradecane (300 ppm S) as the model reactant. The 4,6-DMDBT HDS reaction was performed at 300 °C with the total H2 pressure of 5.0 MPa. The sulfur content in the sample was analyzed by a sulfur Fluorescence analyzer (KDS-300). The desulfurization rate was calculated using the mass fractions of the total sulfur in the feedstock and product. In the catalyst recycling experiments, we simply isolated the catalyst from the reaction system by centrifugation and then thoroughly washed it with ethanol and dried at 80 °C in a vacuum oven overnight. Then, the dried sample was reused for subsequent HDS runs and could be activated immediately under identical reaction conditions at 300 °C with the H2 pressure of 5.0 MPa.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD pattern of the NiMoS-NF9 samples obtained at different hydrothermal time and temperature; SEM images for NiMoS sample prepared by SiO2-100nm and SiO2-200nm; XRD of the NiMoS catalysts before and after HF etching; EDS spectrum of the samples after HF etching; HAADF-STEM image and corresponding EDX maps with red rectangle in HAADFSTEM image of NiMoS-SiO2 before HF etching; TEM images for NiMoS-NF9 to calculate the length of MoS2 slab on the sheet and the area of the nanosheet; XRD and XPS survey spectrum of the as-prepared NiMoS samples; XPS spectra of Ni 2p, Mo 3d and S 2p of the NiMoS-NF0 before and after hydrogen treated; XRD profiles, SEM images and desulfurization rate of 4,6DMDBT HDS on NiMoS-NF0 sample before and after HF etching; Schematic diagram of reaction equipment; A table for comparison of the properties of MoS2 or Ni(Co)MoS materials


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with other reported catalysts; A table of XPS experiments: binding energies (BE in eV) for Mo 3d, Ni 2p and S 2p, at.% of species and ratios for NiMoS-NFx samples.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The research is supported by National Natural Science Foundation of China (21703179, 21373034, U1463210) and the Fundamental Research Funds for the Central Universities of China (No. 20720170103). Financial support of the postdoctoral fellowship of Wuhan Institute of Technology is recognized.

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