Constrained Growth of MoS2 Nanosheets within a Mesoporous Silica

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Constrained Growth of MoS2 Nanosheets within a Mesoporous Silica Shell and Its Effects on Defect Sites and Catalyst Stability for H2S Decomposition Kelvin Mingyao Kwok, Sze Wei Daniel Ong, Luwei Chen, and Hua Chun Zeng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03123 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Constrained Growth of MoS2 Nanosheets within a Mesoporous Silica Shell and Its Effects on Defect Sites and Catalyst Stability for H2S Decomposition Kelvin Mingyao Kwok1,2, Sze Wei Daniel Ong2, Luwei Chen2*, Hua Chun Zeng1* 1 NUS

Graduate School for Integrative Sciences and Engineering and Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 2 Department

of Heterogeneous Catalysis, Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology and Research), 1 Pesek Road, Jurong Island, 627833, Singapore *Emails: [email protected], [email protected]

KEYWORDS: molybdenum disulfide, hydrogen sulfide, transition metal dichalcogenide, nanosheets, hydrogen production, core-shell, mesoporous silica

ABSTRACT: Molybdenum disulfide (MoS2) is a two-dimensional transition-metal-dichalcogenide that can form layered nanosheets with catalytically active sites present at edge or defect sites. The density of such active sites can be further tuned by modifying the length, layer number, strain, and surface defects of the sheets. Herein, a synthetic approach has been developed to encapsulate nanoscale MoS2 nanosheets inside a mesoporous silica shell. Small molybdenum (IV) oxide (MoO2) cores were synthesized and coated with a mesoporous silica phase, followed by a conversion to MoS2@SiO2. The space constraint on the inner cores resulted in short, few-layered, highly curved MoS2 nanosheets with circular or flowerlike morphology. The MoS2@SiO2 was evaluated as a catalyst for decomposition of hydrogen sulfide (H2S), which shows high catalytic turn-over frequency and superior thermal stability compared to unconstrained MoS2 catalysts.

1. Introduction Molybdenum disulfide (MoS2) is a graphene-like two-dimensional (2D) material, belonging to the class of transition metal dichalcogenides (TMDC) which include WS2, MoSe2, WSe2 and MoTe2. MoS2 predominantly forms layers in a 2H hexagonal arrangement with a sandwich structure (S-Mo-S), with different layers separated by a spacing of 0.61 nm (as compared to the 0.34 nm spacing in MoO2).1 Similar to graphite, “molecular sheets” of MoS2 are bound to each other through van-der-Waals forces with a Young’s modulus of 0.33.2 The number of layers present in a MoS2 nanomaterial can be elucidated by photoluminescence or Raman spectroscopy, with differences in the spectra being more obvious below six layers.3-4 Additionally, 2H-MoS2 has a bandgap in the semiconductor region that ranges from an indirect bandgap of 1.29 eV in bulk MoS2 to a direct bandgap of 1.9 eV in monolayer MoS2.4 Because of the possibility in bandgap engineering, MoS2 is being actively explored for nano-electronics and optoelectronics as an alternative to graphene. Single-layer MoS2 has been used as a n-type transistor with high electron mobility of up to 200 cm2/(Vs) and as a phototransistor with low switching times of 50 ms.5-6 The layered sheet-like structure of MoS2 also provides catalytic properties in addition to the electronic properties mentioned earlier. Recently, MoS2 has been studied for the hydrogen evolution reaction (the splitting of water to produce hydrogen), given its appeal as a non-precious metal

(unlike current Pt electrodes).1, 7-9 In addition, MoS2 has been traditionally used in the petroleum industry as a hydrodesulfurization (HDS) catalyst.10 Newer studies have focused on the application of MoS2 catalyst in similar reactions that use H2 such as hydrodeoxygenation, hydrodenitrogenation and synthesis gas conversion.11 The high catalytic activity of MoS2 has been found to be highly correlated to Mo edge sites along the nanosheets, sulfur vacancies in the basal plane, and grain boundaries between MoS2 films.12-14 In the case of the HDS process, the mechanism on MoS2 edge sites has been proposed as the adsorption of sulfur-containing molecules on the Mo site at the S vacancy, followed by adsorption of hydrogen, then step-wise elimination of the resultant molecule (i.e., after removal of sulfur) and H2S.15 These catalytically active Mo sites tend to form at the edges of the nanosheets where Mo atoms are exposed or poorly sandwiched by the two S layers, while the other parts of the basal plane are mostly inert except where sulfur vacancies or surface defects are present.16 In this regard, increasing the number of Mo active sites and hence improving the catalytic performance can be achieved by introducing strains in the MoS2 nanosheets to produce S vacancies. For instance, MoS2 nanosheets with kinked angles or thinner sheet slabs have been proposed to increase catalytic activity.17 The preferred synthesis method of MoS2 depends on the application that it is being used for. In general, semiconductor applications for MoS2 such as gas/chemical sensing, lithium-ion battery, or transistors, would require uniform

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MoS2 with low defect density and controllable layer numbers. Layer-by-layer exfoliation of bulk MoS2 coupled with lithium intercalation produces a single-layer MoS2, but mechanical exfoliation has a low yield and breaks the thin film into micron-length flakes that are undesirable for largescale electronic devices.18 Chemical exfoliation in liquid phase N-methyl-2-pyrrolidone (NMP) solvent has been shown to be more efficient and scalable for production of MoS2.19 Another common synthesis method is chemical vapor deposition (CVD), whereby an as-prepared Mo oxide thin layer (e.g., produced by e-beam evaporation of Mo on SiO2) is placed downstream from solid sulfur with the flow of hydrogen gas. At high temperatures (above the boiling point of sulfur (444°C)), elemental sulfur vaporizes and reacts with the reduced Mo layer to form MoS2 thin films.20 Other sulfide sources such as thioacetamide have also been employed to prepare MoS2 from MoO2 under hydrothermal conditions.1 Direct synthesis of MoS2 from thermal decomposition of ammonium tetrathiomolybdate, (NH4)2MoS4, is also a simple method of producing MoS2.21 However, such synthesis methods often lead to large (micrometer-length) MoS2 nanosheets or crystals which have a low density of edge sites. While they are suitable for microelectronic applications, such MoS2 nanosheets are not ideal for catalytic applications because of a lower population of active sites for reactions. Therefore, renewed interest in MoS2 for catalytic applications requires a paradigm shift away from making large uniform defect-free nanosheets of MoS2, and toward synthesizing well-dispersed nanoscale MoS2 sheets with a large population of defective Mo edge sites. Hydrothermal or solvothermal processes are suitable for the synthesis of nanomaterials and nanostructures, and have been investigated extensively in recent years. Similarly, these methods could be applied to transform Mo oxides to MoS2 or to cause chemical and structural modifications of MoS2 nanosheets in solution. Two recent studies, more specifically, have shown that controlled synthesis of MoS2 can result in highly defective MoS2 nanosheets and hence a high density of Mo active sites that lead to improved catalytic performance. As an example, disordered MoS2 with tunable lengths of 8–18 nm and thickness of 1.1– 5.1 nm (3–10 layers) had been synthesized through a hydrothermal reaction between MoO3 and Na2S at temperatures of up to 350°C under acidic conditions.17 Local defects in the MoS2 sheets were observed as curvatures with sharp angles of as low as 95°, and it was concluded that the MoS2 was probably formed by combination of many short nuclei to produce such angles, instead of sideway growth of the nuclei to form longer sheets (which would likely not produce curved or angled nanosheets). It was found that catalytic activity for hydrotreatment of light cycle oil is enhanced with shorter slab lengths and larger curvatures of MoS2, which is consistent with the theory that Mo active sites are located at edges and curves of MoS2 sheets. Separately, MoS2 sheets were embedded in carbon black through deposition-precipitation of MoS2 using a precursor compound (NH4)2MoS4.22 It was found that MoS2 was dispersed on or within the carbon black, with average slab

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lengths of 5.4 nm and an average of 4.6 MoS2 layers per slab. The MoS2 catalyst could be further doped with transition metals such as Fe, Co, and Ni, and the doped catalysts were also suitable for hydrogen evolution reaction. Hence, MoS2 nanosheets with short slab lengths and curved/kinked morphology were found to have improved catalytic activity. In general, it is quite difficult for nanoparticles to maintain their morphologies or catalytic activities under harsh reaction conditions. Both studies of catalytic MoS2 nanosheets did not test their long-term stability, although the crystal structure of MoS2 was changed after HDS process.17 At high temperatures, nanoparticles often agglomerate and lose their catalytic activity, because the edge sites or specific crystal facets are no longer accessible after agglomeration. In addition, at high temperatures, the MoS2 nanosheets often straighten, resulting in a reduction in defects and edge sites similar to thermal annealing in metal materials. Lastly, coking is a problem commonly associated with chemical reactions involving hydrocarbons, such as hydrodesulfurization, and will result in deteriorating catalytic activity as carbon deposits on the MoS2 nanosheets and blocks the Mo active sites. As such, the MoS2 nanosheets must be further supported or designed to overcome such problems. In this study, as depicted in Scheme 1, we have developed a synthetic route for MoS2 nanosheets encapsulated in mesoporous silica. Because the MoS2 is physically constrained in the central cavity of the spherical mesoporous silica, it is highly curved and correspondingly has a high density of Mo active sites. The mesoporous silica shell enhances thermal stability of the MoS2 as well as prevents its agglomeration. Hence, it can combine both the catalytic potential of MoS2 edge active sites as well as the long-term stability of traditional catalysts. Additionally, the mesoporous silica shell is resistant to acidic gases such as H2S. Such a catalyst would then have potential in the decomposition of H2S. Industrially, H2S is produced from hydrodesulfurization of hydrocarbons, and sulfur is then recovered from H2S through the Claus process, first patented by Carl Fredrich Claus in 1883. The overall chemical equation is H2S + ½O2 → S + H2O, requiring the inlet gas to be heated up to about 850°C. The thermal step (2H2S + 3O2  2SO2 + H2O, –518 kJ/mol) and the catalytic step (2H2S + SO2  3S + 2H2O, –1165.6 kJ/mol) are both highly exothermic, and the heat given off is typically recovered as low or medium pressure steam. Instead of oxidative production of water, in comparison, catalytic decomposition of H2S to H2 and S appears to be more atomically economical as it produces not only elemental sulfur but also the more valuable hydrogen gas. Although MoS2 has been used to study the catalytic decomposition of H2S from as early as 1974, information on improving the performance of this reaction has been scarce since then.23-24 By rational design of a nanostructured MoS2 catalyst, herein, we hope that it will enable the H2S decomposition to become a potential alternative to the Claus process in the future with a fringe benefit in support of a hydrogen or clean energy economy in the context of a greater research scope.

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Scheme 1. Synthetic route for the preparation of MoS2@SiO2 nanocatalyst in this work (hydrogen atoms in white and sulfur atoms in yellow color).

2. Experimental Section 2.1 Chemicals. Ammonium heptamolybdate tetrahydrate (>99%) was from Merck; ethanol (analytical grade) and acetone (analytical grade) were from VWR; polyvinylpyrrolidone (PVP, average molecular weight 40000), 25% cetyltrimethylammonium chloride (CTAC) solution in H2O, tetraethylorthosilicate (TEOS, >99%), thioacetamide (98%) and molybdenum(IV) disulfide nanopowder (99%, 97%) was from Nacalai Tesque; nickel(II) acetate tetrahydrate (>99%) was from Fluka; and deionized water was collected through the Elga MicroMeg purified water system. 2.2 Synthesis of MoO2 nanocores. MoO2 core synthesis was adapted from a previous study.25 To synthesize MoO2 cores, 1200 mg of ammonium heptamolybdate was first dissolved in 176 mL of deionized water under magnetic stirring. Then, 80 mL of ethanol and 4000 mg of polyvinylpyrrolidone (PVP) were added, and the mixture was stirred at room temperature for 30 min. The resulting solution was transferred equally into eight 40 mL Teflon-lined stainless steel autoclaves and treated hydrothermally at 180°C for 16 h in an electric oven. The autoclave was cooled in ambient air and the resultant black precipitate was separated by four repeated cycles of centrifugation-washing with acetone-ethanol cosolvent, and finally dried at 80°C overnight in an electric oven. 2.3 Synthesis of MoO2@SiO2 core-shell spheres. 120 mg of the MoO2 cores was dispersed in a mixed solvent of 132 mL of water and 80 mL of ethanol with 60 min of sonication. Then, 2.2 mL of 25% cetyltrimethylammonium chloride solution (CTAC) was added, followed by 0.8 mL of diethanolamine. The mixture was stirred for 30 min, and then 1.6 mL of tetraethyl orthosilicate (TEOS) was added dropwise to the solution while mixing. The solution was stirred for 16 h at room temperature, and the resultant grey solid was separated by centrifugation, washed with acetone-ethanol cosolvent and dried at 80°C overnight in an electric oven to yield MoO2@SiO2. Pure mesoporous silica spheres (mSiO2) could also be synthesized using the same method except without the addition of MoO2 cores.

2.4 Syntheses of MoS2@SiO2 and its doped core-shell spheres. The above-obtained MoO2@SiO2 sample was then converted to MoS2@SiO2 or its doped derivatives with either Co (to form MoS2-Co@SiO2) or Ni (to form MoS2Ni@SiO2). 100 mg of MoO2@SiO2 was dispersed in 40 mL of water with 20 min of sonication, followed by addition of 150 mg of thioacetamide. In a separate experiment, 150 mg of Na2S was used instead of thioacetamide as the sulfide source. To produce Co or Ni doped samples, 20 mg of cobalt(II) acetate tetrahydrate or nickel(II) acetate tetrahydrate was added to the solution together with MoO2@SiO2. The solution was transferred to a 40 mL Teflon lined stainless steel autoclave container and hydrothermally treated at 200°C for 24 h. The autoclave was then cooled in ambient air and the resultant black precipitate was collected by centrifugation, washed with ethanol, and dried at 80°C overnight in an electric oven to obtain MoS2@SiO2, MoS2Co@SiO2, and MoS2-Ni@SiO2. 2.5 Syntheses of MoS2-NP and MoS2-NP@SiO2. MoS2 nanoparticles (MoS2-NP) were also obtained from conversion of MoO2 cores. Briefly, 50 mg of MoO2 cores were dispersed in 40 mL of water with 20 min of sonication, followed by addition of 150 mg of thioacetamide. The solution was transferred to a 40 mL of Teflon lined stainless steel autoclave container and hydrothermally treated at 200°C for 24 h. Subsequently, the autoclave was cooled in ambient air and the resulting black precipitate was collected by centrifugation, washed with ethanol, and dried at 80°C overnight in an electric oven. Mesoporous silica was coated on the MoS2-NP with the scaled down version of the same procedure that formed MoO2@SiO2 from MoO2 cores (Section 2.3). For example, 30 mg of MoS2-NP was re-dispersed in a mixed solvent of 33 mL of water and 20 mL of ethanol with 30 min of sonication. Then, 0.55 mL of 25% CTAC solution was added followed by 0.2 mL of diethanolamine. The mixture was stirred for 30 min, then 0.4 mL of TEOS was added dropwise to the solution while mixing. The solution was stirred for 16 h at room temperature, separated by centrifugation, washed with acetone-ethanol cosolvent and dried at 80°C overnight in an electric oven to yield MoS2-NP@SiO2. 2.6 Synthesis of MoS2@Stöber-SiO2. Solid SiO2 phase was coated on the MoO2 cores using a modified Stöber process. Briefly, 450 mg of MoO2 cores (Section 2.2) was dissolved in a mixture of 12 mL of deionized water and 80 mL of ethanol. Then, 6 mL of TEOS was added, followed by adding 2.56 mL of 25% aqueous NH3, and the solution was stirred at ambient temperature for 6 h. The resultant gray precipitate was separated by centrifugation, washed with a 1:1 water-ethanol cosolvent, and dried at 80°C overnight in an electric oven. The product MoO2@Stöber-SiO2 then underwent the same type of treatment to convert MoO2 to MoS2 as described in Section 2.4. 2.7 Catalytic decomposition of H2S. Decomposition of gaseous H2S was carried out in a packed bed flow reactor with a 1/8” quartz tube at atmospheric pressure. In a typical experimental run, 50 mg of catalyst was loaded between quartz wool in the center of the quartz tube. A stream of


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2500 ppm H2S in N2 was introduced to the reactor at 40 mL/min, and the reaction was conducted at 500–800°C, with 2 h of runtime at each 100°C interval, and 6 h of runtime at 800°C. Solid elemental sulfur was collected at the outlet of the quartz tube at room temperature. The outlet gas stream was first sent through a gas bubbler containing saturated aqueous Cu(II) acetate solution to remove unconverted H2S, before entering a Shimadzu GC-2010 gas chromatography (GC) machine equipped with a GSGASPRO column (60 m, 0.32 mm) and analyzed by a thermal conductivity detector (TCD). The H2 composition was measured by the GC every 7 min using N2 as an internal standard to calculate H2S conversion. 2.8 Characterization methods. The morphology and chemical composition of the above samples were examined using field emission scanning electron microscopy (FESEM, JSM-6700F), transmission electron microscopy (TEM, JEM-2010, 200 kV), and high-resolution transmission electron microscopy with energy dispersive X-ray spectroscopy (HRTEM/EDX, JEM-2100F, 200 kV). The crystal structures of the samples were investigated using powder X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα radiation, λ = 1.5406 Å) with a scanning rate of 1.6°/min. The pore structures of the samples were studied by nitrogen physisorption (Micromeritics ASAP-2420), with a degassing pretreatment of N2 at 200°C for 24 h, and the specific surface areas were determined using Brunauer-Emmett-Teller (BET) method. Surface compositions of the samples were investigated with X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical) using a monochromatic Al Kα exciting radiation (1486.71 eV). Binding energies (BEs) were referenced to the adventitious C 1s peak (BE = 284.8 eV). Peak areas for the individual Mo (MoIV, MoV, and MoVI) and S (S2–, and S22–) species from the deconvoluted XPS spectra were used to determine the normalized fraction of defect sites present in each catalyst sample. Raman spectra (Bruker, Senterra) were recorded using a 532 nm laser at 2 mW with an aperture of 50 μm, resolution of 3–5 cm–1, microscope objective of 20×, spectral range of 60–1555 cm–1, acquisition time of 10 seconds and 10 co-additions, with the sample powder pressed flat on a glass slide and measurements taken at room temperature in ambient air in a darkfield setting in enclosed chamber of equipment. Mo loadings in the samples were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo Scientific iCAP 6000). Enhanced X-ray absorption fine structure (EXAFS) spectroscopy was carried out at the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF) under transmission mode, with the ring of the BSRF operating at 250 mA electron beam current under top-up mode.

3. Results and Discussion 3.1 Synthesis and morphology of MoS2@SiO2. The synthetic route from MoO2 core to MoS2@SiO2 core-shell is shown in Scheme 1 and Figure 1. By using PVP as a capping agent under the hydrothermal condition, the product particles of MoO2 have a size distribution of around 40 nm

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(Figure 1a). The MoO2 cores maintained their size distribution even after excess PVP capping agent was removed by washing with acetone-ethanol, and they could be easily redispersed in the water-ethanol cosolvent. Using the above MoO2 cores, a uniform silica shell could be further deposited, resulting in the product, MoO2@SiO2 core-shell spheres, where the CTAC surfactant created one-dimensional mesoporous channels perpendicular to the shell surface. The concentration of diethanolamine and the ratio of water to ethanol in the cosolvent had been optimized to control the hydrolysis rate of TEOS and to prevent aggregation of silica at lower water to ethanol ratios or coreless silica spheres at higher water to ethanol ratios. It is noted that most of the core-shell spheres contain only one MoO2 core (Figure 1b), with the occasional spheres containing two or more MoO2 particles (Figure 1c).

Figure 1. TEM images of (a) MoO2 cores, (b) MoO2@SiO2, (c, e-f) MoS2@SiO2 and (d) SEM image of MoS2@SiO2.

To convert the MoO2 core in MoO2@SiO2 to MoS2, thioacetamide was employed as a sulfide source. Under hydrothermal conditions, thioacetamide reacts with MoO2 to form MoS2 with acetamide as a by-product. The conversion of MoO2@SiO2 to MoS2@SiO2 requires at least 200°C for 24 h, below which (for example, 170°C for 24 h or 200°C for 16 h) only amorphous or single-sheet MoS2 is observed inside the silica shell (no lattice fringes can be observed, Figure


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S1). The FESEM image (Figure 1d) shows that the core-shell spheres maintain their original size and smooth outer shell with little leakage of MoS2 onto the surface, while the TEM images (Figure 1e) show that the interior of the silica sphere becomes hollow. TEM-EDX mapping (Figure 2) confirms that the MoS2 is confined within the silica shell. Therefore, the resultant MoS2 sheets are constrained to the hollow interior of the silica sphere. Hence, they either form a randomly stacked assemblage (Figure 1e) in the interior space, or grow closely on the inner wall (Figure 1f). The bending curvature of these MoS2 sheets could be very high (Figure 1e), with angles as low as 90° (Figure 1f).

Figure 2. FETEM-EDX maps of MoS2@SiO2 (scale bar in insert TEM image: 100 nm).

3c-d), similar to how MoO2 cores were transformed to MoO2@SiO2 (Figure 1b), the MoS2-NP gets covered by mesoporous silica. However, it loses its curved nanosheet morphology and lacks the hollow interior seen in the MoS2@SiO2 sample. Therefore, it is necessary to encapsulate the MoO2 cores with silica before converting it to MoS2 but not vice versa, in order to create the curved MoS2 nanosheet structure inside the hollow interior of mesoporous silica.

Figure 3. TEM images of (a-b) MoS2-NP and (c-d) MoS2NP@SiO2.

Similarly, MoS2-Co@SiO2 and MoS2-Ni@SiO2 samples were synthesized by doping with Co and Ni acetates respectively during the transformation from MoO2@SiO2 to MoS2@SiO2. A TEM-EDX mapping study shows that the Co and Ni atoms are generally well-dispersed within the spherical particles of MoS2@SiO2 (Figures S2 and S3). ICPOES analysis indicates about 7.5 wt% loading of Mo in the MoS2@SiO2 sample (Table S1). Doping with Co or Ni has also been found to further increase catalytic performance as Co can be preferentially located at S vacancy sites along edges of the MoS system, while Ni leads to truncated morphologies.26 MoO2 nanoparticle cores can also be directly converted to MoS2 nanoparticles (MoS2-NP, i.e., particulate aggregates of MoS2) without the intermediate step of encapsulating silica (Figure 3a-b). The MoS2 forms nanosheets outward from the original MoO2 core, but because the nanosheets are not constrained by a silica shell, they tend to become less curved and must have less defects in comparison to those formed inside the MoS2@SiO2 sample. In addition, the nanosheets in MoS2-NP agglomerate easily to form larger particles of 100–150 nm, which are about double the size of the MoS2 cores in MoS2@SiO2 (MoS2 cores at 60 nm). If the as-formed MoS2 NP is used as a core to be encapsulated inside mesoporous silica (MoS2-NP@SiO2, Figure

Figure 4. TEM Images of MoO2@Stöber-SiO2 before (a) and after (b) thioacetamide treatment, and MoO2@SiO2 treated with Na2S after (c) 6 h and (d) 12 h.

Of course, the mesoporous channels in the silica shell are also required for the thioacetamide to enter and reach the MoO2 cores. When the MoO2 cores were coated with nonporous Stöber silica instead, the subsequent treatment


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with the same thioacetamide not only could not convert the MoO2 core to MoS2 sheets, but also caused some dissolution-precipitation of the silica shell into adjacent smaller 50–100 nm coreless silica spheres (Figure 4a-b). When Na2S was used as a sulfide source instead of thioacetamide (see Section 2.5) for the transformation of MoO2@SiO2, the higher alkalinity of the solution caused the SiO2 shell to be gradually dissolved, eventually leaving only the original MoO2 cores after 12 h (Figure 4c-d). The dissolution of SiO2 by the alkaline solution probably hindered the formation of MoS2 nanosheets. The commercial MoS2 nanopowder (see Section 2.1; called as “bulk MoS2” hereafter) consists of aggregates of MoS2 crystals ranging from 100 to 400 nm (Figure S4) with only small amounts of stacked nanosheet morphology visible. 3.2 Catalyst characterization. The crystallography structures of MoO2 nanoparticles, MoO2@SiO2 core-shell spheres, MoS2 nanoparticles, MoS2@SiO2 core-shell spheres and the spent catalysts from the latter two samples were characterized by powder XRD (Figure 5). The XRD peaks of MoO2 NP at 36.5°, 53.7° and 65.7° correspond to MoO2 (JCPDS 50-0739). Those peaks become weaker in the MoO2@SiO2 sample due to the lower loading of MoO2 cores within the silica shell, while the 21.7° peak indicates the presence of amorphous SiO2. In the fresh MoS2-NP and fresh MoS2@SiO2 samples, two broad peaks at 32.2° and 56.9° correspond to the (100) and (106) peaks of MoS2 in the 2H phase (JCPDS 37-1492), while the same peak for SiO2 is present in both fresh and spent MoS2@SiO2 catalyst samples. Notably, the (002) peak typically at 14.4°, which corresponds to periodicity in the c-axis (normal to the MoS2 basal plane), is not visible. The (002) peak indicates the presence of multilayer MoS2 nanosheets, and has been shown to be undetected on monolayer or few layer MoS2.21, 27 Hence, this shows that the MoS nanosheets formed in 2 the MoS2-NP and MoS2@SiO2 samples are few-layered. In contrast, the XRD pattern of bulk MoS2 (Figure S5) shows that the (002) reflection is many times more intense than the others, indicating that bulk MoS2 is highly stacked. A significant divergence in XRD peak patterns emerged after the two different MoS2 samples (i.e., MoS2-NP and MoS2@SiO2) were used for the H2S decomposition reaction. The flow of H2S gas at temperatures of up to 800°C is expected to increase the crystallinity of the MoS2 nanosheets. In the case of the MoS2-NP, the (100) and (106) peaks become more intense, additional MoS2 peaks appear at 39.4° (103), 49.4° (105), 58.3° (110) and 68.9° (201), and the (002) peak at 14.4° now appears with high intensity, suggesting that the MoS2-NP nanosheets have undergone significant crystallization and sintering to form multi-layered bulk-like MoS2. In contrast, the spent MoS2@SiO2 catalyst after reaction has a (100) peak that is sharper but with a similar intensity, while a much smaller (002) peak is formed compared to the spent MoS2-NP catalyst. Even after taking into account the lower loading of Mo in the

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MoS2@SiO2 sample (7.5%) compared to the MoS2-NP sample (59.9%), the areas of the peaks in MoS2@SiO2 would still be smaller than MoS2-NP. Furthermore, (103) and (105) peaks that correspond to long-range order are not observed in MoS2@SiO2 compared to MoS2-NP, suggesting that such long-range order is not significant for the nanosheets inside MoS2@SiO2. This comparative observation suggests that the silica shell is able to effectively constrain the movement of the MoS2 nanosheets, preventing excessive sintering or combination into multi-layered MoS2 that can lead to deactivation due to the reduction in the total number of exposed active sites. The nature of the silica shell goes beyond a physical protective role, and importantly, it provides additional thermal stability and catalytic activity of the MoS2@SiO2, as will be seen later in the catalytic performance results.

Figure 5. XRD patterns of MoO2-NP, MoO2@SiO2, MoS2-NP, spent MoS2-NP, MoS2@SiO2, and spent MoS2@SiO2 catalysts. Indexed planes are shown for the MoS2 phase. Note that XRD peak heights are not scaled according to metal content.

Our N2 physisorption analysis (Figure 6) further indicates that the MoS2@SiO2 sample has a type IV physisorption isotherm with a type H4 hysteresis loop, which is typical for mesoporous silica. The sample has a BET surface area of 145 m2/g, pore volume of 0.19 cm3/g and pore size of 5.44 nm, confirming the presence of mesopores. The layered structure of bulk MoS2-NP, MoS2-NP and MoS2@SiO2 were further studied by Raman spectroscopy (Figure 7). The laser power of 2 mW adopted in this Raman measurement was sufficiently low that local heating or oxidation of the MoS2 sample was not significant and we could observe characteristic Raman peaks. Strain-induced Raman shifts are widely observed and studied in graphene and graphene-like layered materials. Type E modes of symmetry are in-plane displacements while type A symmetry are out-of-plane displacements, both of which red-shift to lower wavenumbers as the lattice expands and interatomic or interlayer interactions weaken.28, 29 MoS2 has two significantly different atomic plane distances, 0.27 nm for (100) and (010) planes and 0.61 nm for the (001) plane, a result of


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the different interatomic forces along the different planes. The in-plane lattice is held together by MoS covalent bonds, while adjacent nanosheets in the [001] direction are held by weak van der Waals’ forces. MoS2 nanosheets are typically characterized by two peaks, E12g (383 cm–1) corresponding to in-plane vibration and A1g (408 cm–1) corresponding to out-of-plane vibration.30 The MoS2-NP and MoS2@SiO2 samples both exhibit these two peaks, but their peaks are broader than that of bulk MoS2. The E12g and A1g Raman peaks have been shown to blue-shift and red-shift respectively as the number of MoS2 layers decrease from bulk to monolayer, with the effect becoming obvious at below 4 layers, and so the larger full width at half maxima for the MoS2-NP and MoS2@SiO2 suggests that there is a mixture of MoS2 nanosheets with varying layer number. This is also consistent with TEM imagery of both samples (Figure 1e and Figure 3b), where a different number of MoS2 layers ranging from one to more than five can be observed.

Figure 6. (a) N2 adsorption-desorption isotherms and (b) volumetric pore size distribution (using BJH method based on the desorption data) of MoS2@SiO2.

Figure 7. Comparison of Raman spectra among the bulk MoS2, MoS2-NP and MoS2@SiO2 samples. Solid lines indicate fresh catalysts and dashed lines indicate spent catalysts.

Furthermore, there is a red-shift in the Raman peaks for both MoS2-NP and MoS2@SiO2 samples, with the red-shift more significant in the MoS2@SiO2. The E12g peak is shifted from 382.6 cm–1 in bulk MoS2 to 377.0 cm–1 in MoS2-NP and 371.4 cm–1 in MoS2@SiO2, while the A1g peak is shifted from 407.8 cm–1 in bulk MoS2 to 402.6 cm–1 in MoS2-NP and 397.0 cm–1 in MoS2@SiO2. First-principle studies supported by experimental Raman measurement on monolayer MoS2 subjected to uniaxial strain have been demonstrated by Rice et al.28 They showed a red shift of -2.1 cm–1/%-strain for E12g and -0.4 cm–1/%-strain for A1g, which would correspond to about 5.3% strain for our MoS2@SiO2 catalyst based on the E12g shifts (11.2 cm–1). While the A1g peaks for our MoS2 catalysts shift more significantly (10.8 cm–1) than suggested by them, the difference in nanomaterial synthesis probably account for such a difference, as they use a different synthesis method, a top-down synthesis of large 20 μm monolayer MoS2 prepared using mechanical cleavage of bulk MoS2. As the E12g and A1g correspond to different lattice displacements, it is possible that the two peaks can red-shift at different rates in different MoS2 nanomaterials, especially for submicron-sized nanosheets where the aspect ratio is significantly smaller. On the other hand, Yang et al uses a bottom-up hydrothermal synthesis of MoS2 from colloidal solutions, and we can see from their MoS2 nanosheets exhibit Raman red-shifts of similar magnitude for both E12g and A1g peaks (383.1 cm–1 to 379.3 cm–1 for E12g, and 408.2 to 403.1 cm–1 for A1g).29 A similar effect is also observed in recent MoS2-Au composite nanoscrolls synthesized by Hwang et al, where both the E12g and A1g peaks are shifted by similar magnitudes.31 The red-shifts in the Raman peaks were shown by Yang et al to be associated with out-of-plane uniaxial tensile strain along the [001] direction and a corresponding in-plane biaxial compressive strain in the basal plane as a result of their MoS2 nanosheets forming a 3D network with highly curved walls.29 Furthermore, they showed that the strain, and hence the amount of red-shift, can be tuned by pH, with a higher pH producing a more curved morphology, more


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strain and more red-shift of the Raman peaks. Our MoS2NP and MoS2@SiO2 samples were synthesized in alkaline solutions containing thioacetamide as the sulfiding agent, and TEM images show the highly curved morphology of the MoS2 nanosheets formed. The Raman peaks of our freestanding MoS2-NP are red-shifted to a similar extent as the MoS2 nanosheets synthesized by Yang et al, while the Raman peaks of our MoS2@SiO2 are red-shifted even further, which is consistent with a trend that the MoS2 nanosheets grown in higher pH solution have higher curvature and hence higher red-shift of Raman peaks.29 In the case of MoS2@SiO2, given that the MoS2 nanosheets expand outward from the center MoO2 core due to its larger lattice constant, while it is constrained by the outer mesoporous silica shell, it is likely that the MoS2 nanosheets formed inside the silica shell are under more strain than the freefloating MoS2-NP particles, resulting in a greater red-shift of its Raman peak. The Raman spectra for the spent catalysts shows little change in the bulk MoS2 catalyst; only a slight blue-shift in the Raman peaks for MoS2-NP (E12g of 403.8 cm–1 and A1g of 379.9 cm–1) is observed, indicating less strain in the nanosheets as they aggregated and became more bulk-like. However, an even smaller blue-shift is seen in the Raman peaks for our MoS2@SiO2 (E12g of 370.6 cm–1 and A1g of 397.8 cm–1) indicating a better preservation of the nanosheets in the confined space of catalyst (Figure 7).

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responsible for the significantly higher hydrogen evolution reaction (HER) activity of MoS2 nanostructures containing these defect sites, and have also been suggested as active sites in H2S decomposition.32-35 After these catalysts have been used at high temperatures for the H2S decomposition reaction, the peak intensities of the various Mo and S species change significantly. After reaction, the XPS spectrum of the spent MoS2-NP sample more closely resembled bulk MoS2, where the intensity of the main MoIV peak increased significantly, and the MoV and disulfide S22– peaks all but disappeared, suggesting that most of the defect sites have been lost due to rearrangement of Mo and S atoms as the crystallinity of the MoS2 nanosheets increased. This effect is less pronounced in the spent MoS2@SiO2 sample, where small amounts of MoV and a substantial amount of S22– are still present in its XPS sample. This suggests that the defects sites that contribute to increased catalytic activity are more stable in the MoS2@SiO2 compared to the bare MoS2 nanoparticles.

To further substantiate the nature of the curved morphology of MoS2@SiO2, the Mo K-edge EXAFS spectrum of MoS2@SiO2 (Figure S6) also shows the intensity of Mo–Mo (compared to Mo–S) of both the fresh and spent catalysts are significantly lower than that of the bulk MoS 2 catalyst, which indicates that MoS2@SiO2 had less Mo–Mo coordination than bulk MoS2, confirming that the MoS2@SiO2 was more dispersed and had more edge sites. The shift of the Mo–Mo distance to a lower value in the fresh MoS2@SiO2 catalyst also indicates strain in the encapsulated catalyst and the presence of defect sites (possibly molybdenum oxysulfides or Mo–O–Si bonds) which distort the Mo–Mo coordination and hence the nanosheet morphology. In addition, the chemical states of Mo and S in bulk MoS2, MoS2-NP and MoS2@SiO2 samples were investigated by XPS technique (Figure 8). The strongest peaks in all samples correspond to 229.5 eV for MoIV 3d5/2 (with a spinorbit splitting of 3.15 eV between 3d5/2 and 3d3/2) and 162.4 eV for S2– 2p3/2 (with a spin-orbit splitting of 1.18 eV between 2p3/2 and for 2p1/2). These peaks are the only ones present for bulk MoS2. However, both Mo peaks of MoS2-NP and MoS2@SiO2 samples have additional shoulder peaks corresponding to MoV (230.5 eV) and MoVI (233.0 eV) species, and their S 2p peaks have an additional peak corresponding to disulfide (S22–, 163.6 eV). These additional peaks indicate the presence of edge defect sites, such as MoV=O sites that are comparable to molybdenum oxysulfides (MoOxSy), bridging disulfides (S–S)br2–, shared disulfides (S–S)sh2– and terminal disulfides (S–S)t2–.32-34 In particular, the disulfide ligands at these defect sites can be easily removed to expose unsaturated MoIV sites, and have been proposed as the active sites

Figure 8. XPS spectra of the bulk MoS2, MoS2-NP, and MoS2@SiO2 samples and their spent catalysts after H2S decomposition reaction, where "4" indicates that the intensity is multiplied by 4 in order to increase clarity of peaks.

3.3 H2S decomposition over MoS2 catalysts. To test its catalytic activity and chemical stability, the above studied samples of MoS2@SiO2, MoS2-Ni@SiO2, MoS2-Co@SiO2, MoS2@SiO2, MoS2-NP and 7.5%MoS2@SiO2 were tested as catalysts for H2S decomposition. For a better comparison, 7.5%MoS2/SiO2 (with 7.5 wt% Mo) was synthesized by dispersing MoS2-NP on mesoporous SiO2 spheres with physical mixing and drying. In addition, bulk MoS2 was used as a benchmark catalyst in this evaluation, and a blank test with no catalyst loaded was also run for a more complete comparison. The blank test shows less than 2% conversion from 500–700°C and only 9% conversion at thermal decomposition temperature of 800°C (Figure 9a). As an addi-


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ACS Catalysis

tional control experiment, we also synthesized pure mesoporous silica (mSiO2) using the same synthesis method in Section 2.3, except without adding any MoO2 cores, hence obtaining spherical mesoporous silica. The mSiO2 was then tested using the same procedure as the other catalysts, and H2S conversion was found to be similar to a blank run (without loading any catalyst), hence proving that mesoporous silica has no intrinsic catalytic activity for H2S decomposition (Figure 9a). The results for the catalyst testing were normalized against the number of Mo atoms (determined using ICP-OES analysis) to compare their turnover frequencies (TOF), as shown in Figure 9b.

will simply agglomerate at high temperatures to larger particle sizes, which will result in a lower exposed surface area of active sites. In addition, the number of active sites decrease for MoS2-NP because the nanosheets anneal at the high reaction temperatures and become more straight (less curved), resulting in less defect sites, which are typically located along edges or along curved kinks in the MoS2 nanosheet, an effect that is evident in the XPS results (Section 3.2 and Figure 8).

As sulfur is one of the products of H2S decomposition and it has a boiling point of 444°C, the reaction was carried out from 500–800°C. In the quartz tube immediately outside the furnace, the gaseous sulfur cooled and condensed into solid sulfur. The mass of the sulfur collected (Figure S7) was measured and found to be in good agreement with the calculated amount of solid sulfur produced based on the total H2S converted over time for each catalyst. The highest temperature of 800°C was run for 6 h to assess the thermal stability of the various MoS2 catalysts over time. Above its boiling point, sulfur can exist in various forms from S1 to S8, but equilibrium studies have established that S2 gas dominates the gas phase at 800°C and below.36 Hence, the overall H2S decomposition reaction can be written as:37 H2S  H2+ ½ S2 ΔĤ° = +20.4 kJ/mol Since the above reaction is endothermic, the conversion increases at higher temperatures, even for the blank run. Comparing the TOF of the catalysts, the bulk MoS2 catalyst has the lowest performance. The currently prepared MoS2NP underperformed the MoS2@SiO2 catalyst and appeared to deactivate significantly even after just 6 h of reaction at 800°C. As a result, the MoS2@SiO2 catalyst had a higher TOF than the MoS2-NP with simply the addition of the silica shell (Figure 9). Upon examining the spent catalyst samples under TEM imaging, we notice that while the size of the MoS2 cores in the MoS2@SiO2 catalyst remained the same, the MoS2-NP particles agglomerated to large clusters ranging from 200–500 nm in size (Figure S8). In addition, the SiO2-coated MoS2 catalyst was found to be more stable than the MoS2-NP, with its performance decreasing by only 2% after 6 h of reaction (55.8% to 54.7%), compared to a performance decrease of 19% for the MoS2NP sample without the SiO2 shell (62.6% to 50.6%). The MoS2@SiO2 catalyst was further tested to be stable for an additional 24 h with no further decrease in catalytic performance, for a total time-on-stream of 38 h (Figure S9). To account for the difference in Mo loading of MoS2-NP and MoS2@SiO2, we also dispersed MoS2-NP on mesoporous SiO2 spheres by physical mixing and drying, and tested the resultant 7.5MoS2/SiO2 mixture. It exhibits similar initial catalytic performance as the MoS2@SiO2 sample, but was not as stable as MoS2@SiO2, as the conversion of H2S with the 7.5MoS2/SiO2 decreased by 17% after 6 h of reaction, similar to the MoS2-NP sample (Figure 9a). Without a mesoporous silica shell constraining it in a core, the MoS2-NP

Figure 9. (a) Conversion and (b) turnover frequency (TOF) of the MoS2 catalysts. Reaction conditions: 50 mg catalyst, 40 mL/min of 2500 ppm H2S in N2, 2 h of reaction at 500°C, 600°C, 700°C, and 800°C, respectively, after which another 4 h of test was applied (i.e., 2 h + 4 h = 6 h at 800°C).

MoS2 catalysts are commonly doped with Co and Ni metals to increase its performance in reactions such as hydrogen evolution reaction and hydrodeoxygenation, as these metals can be incorporated into the MoS2 layered framework in close proximity to Mo atoms at edge sites and can enrich structural defects and stabilize hydrogen atoms.22, 38-40 Furthermore, doping the MoS @SiO catalyst with Co 2 2 or Ni increases the TOF of the catalyst. While the Ni-doped catalyst had the highest initial performance at 800°C, its TOF performance decreased over time to become similar to the Co-doped catalyst (Figure 9b).


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When using XPS analysis to quantify the percentage of Mo and S defect sites (such as MoV, MoVI, and S22–), we could observe a correlation between the catalytic performance (both initial and after 6 h) of the various MoS2 catalysts and the percentage of defect sites (Figure 10; XPS spectra of MoS2-Co@SiO2 and MoS2-Ni@SiO2 are shown in Figure S10). It has been proposed in the case of MoS2 catalysts for the hydrogen evolution reaction, that there are three main types of catalytic active sites: the wellknown Mo edge sites, sulfur vacancies, and grain boundaries.13, 41 The disulfide S22– species observed from XPS analysis can be eliminated during reaction to form unsaturated MoIV sites (i.e., sulfur vacancies)12-13, while the MoV and MoVI species are likely indicative of Mo defect sites similar to those observed at MoS2 thin film grain boundaries, amorphous MoS2 or MoS2 nanoclusters.32 When we plotted the initial and long-term H2S conversion at 800°C (initial conversion representing the fresh and long-term conversion representing the spent catalysts) versus the fraction of (MoV + MoVI) sites, we found an increase in conversion as the fraction of Mo defect sites increases. In the case of MoS2-Ni@SiO2, the fraction of Mo defect sites is initially very high, but drops significantly after the reaction, which could explain its high initial catalytic performance which decreases (along with the Mo defect site fraction) to be similar to MoS2-Co@SiO2 after 6 h of reaction.

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the sent catalysts: 50 mg catalyst, 40 mL/min of 2500 ppm H2S in N2, and reaction time of 2 h at 800°C.

A similar trend is not as obvious for the S22– defect sites. Though we are not able to independently vary the Mo and S defect sites with our synthesis method, we could nonetheless observe that for MoS2-Co@SiO2, their fresh and spent catalysts both have the similar fractions of Mo defect sites (0.44 vs. 0.43, Figure 10a), but a decrease in fraction of S22– sites (0.30 vs. 0.09, Figure 10b), yet the H2S conversion only decreased from 58% to 57%, suggesting that disulfide S22– species may play a less significant role in catalytic activity. Additionally, the bulk MoS2 still shows catalytic activity despite having the lowest fraction of defect sites, which suggests that MoIV in crystalline MoS2 also has some intrinsic active sites, probably from Mo edge sites.

4. Conclusion In summary, we have synthesized a mesoporous silicaencapsulated MoS2 nanocatalyst (MoS2@SiO2) with a high density of active sites through a solvothermal process. The nanosheets of MoS2 inside the mesoporous silica are short, few-layered with a highly curved morphology that results in lower crystallinity, higher strain and higher defect density. Such “molecular sheets” of MoS2 exhibit significantly higher catalytic performance for the H2S decomposition reaction compared to bulk MoS2, and the encapsulation of MoS2 nanosheets in mesoporous silica further increases the performance and chemical stability of the catalyst, with 54.7% conversion for up to 38 h. In comparison, although there is currently no industrial implementation of H2S decomposition, a recent study on H2S decomposition used perovskite catalysts reaching up to 37.7% conversion at 950°C.42 While the current treatment of H2S obtained from hydrodesulfurization (HDS) in the Claus process only yields water, the catalytic decomposition of H2S can produce hydrogen gas, which is even more valuable for many chemical processes, including HDS (For example, the hydrogen can be recycled back as a feedstock for HDS). In view of huge global presence and natural reserve of H2S, this decomposition approach must possess enormous economic value, since about 64 million tons of sulfur is produced annually from the Claus process, and about 4.6 billion tons of H2S is estimated to exist in the Black Sea.43 In addition, H2S removal from sour gas streams is also becoming more important as the H2S concentrations in natural gas streams from conventional and unconventional resources are higher than in crude oil.44

ASSOCIATED CONTENT Supporting Information Figure 10. H2S Conversion at 800°C plotted against the fraction of (a) MoV and MoVI, and (b) S22–. Reaction conditions for

FETEM-EDX mapping images of MoS2-Co@SiO2 and MoS2Ni@SiO2 samples and TEM images of bulk MoS2, MoS2Co@SiO2, MoS2-Ni@SiO2, MoS2@SiO2 samples synthesized


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ACS Catalysis

with varied conditions, and spent MoS2-NP catalyst. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Emails: [email protected], [email protected] ACKNOWLEDGMENT The authors gratefully acknowledge the financial supports provided by the Ministry of Education Singapore, the National University of Singapore, and the Institute of Chemical and Engineering Sciences, A*STAR. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program. The authors thank Dr. Yonghua Du for his insights in EXAFS analysis.

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Constrained Growth of MoS2 Nanosheets within a Mesoporous Silica

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