1 Introduction

MoS2 (molybdenum disulfide) is a typical transition metal dichalcogenide with a layered structure, in which a molybdenum atom layer is sandwiched between two layers of sulfur atoms [1, 2] and the MoS2 layers hold each other by weak van der Waals interactions [3]. MoS2 has attracted growing attention due to its structural properties and potential application as a lubricant [4], in lithium ions batteries [5], and as a catalyst in hydrogen evolution (HER) [6], hydrodeoxygenation (HDO) [7], hydrodesulfurization (HDS) [8,9,10,11,12], hydrodenitrogenation (HDN) [13], and hydrogenation (HYD) [14,15,16,17] reactions.

The catalytic active sites of MoS2 have been investigated in various reactions. It has been suggested that the sulfur vacancies generated at the edge planes of a layered MoS2 structure could be associated with the active sites for HDS [18, 19] and methanation [20]. It was also reported that the lower valence state of the MoS2 and the number of anion vacancies could contribute to the catalytic activity of MoS2 for a thiophene hydrogenolysis reaction [21]. In the rim-edge model proposed by Dagge and Chianelli [22], the rim sites are mainly responsible for a HYD reaction and the edge ones for sulfur removal. On the other hand, it was observed that the “inflection” on the basal plane of MoS2 could be highly active in HYD [23].

Unsupported MoS2 catalysts can be prepared by several methods such as chemical vapor deposition (CVD) [24], sonochemical synthesis [25], thermal decomposition [26], exfoliation [27, 28], solvothermal synthesis [29], and hydrothermal synthesis [30, 31]. In particular, hydrothermal preparation is much superior to the others in the control of nucleation, particle shape, dispersion, and reaction rate by adjusting temperature and pressure at a low synthesis temperature [32]. A lot of research has examined the effect of temperature on catalyst preparation, whereas that of pressure has been less studied. When MoS2 was prepared by thermal decomposition at 300 or 350 °C in H2, the variation of pressure between 30 and 50 bar in the preparation did not significantly affect the catalytic performance in the HDS reaction [33]. In the preparation of MoS2 using ammonium tetrathiomolybdate (ATTM, or (NH4)2MoS4), with the open-flow isostatic pressing method, it was found that the increase in the synthesis pressure from 6.9 to 55.2 bar could improve the catalytic activity in HDS [34]. An in-situ decomposition by heating ATTM to 350 °C at a pressure of 31 bar in an H2 environment could produce highly disordered MoS2 and a large surface area, which resulted in enhanced activity in the HDS reaction of dibenzothiophene (DBT) [35]. Furthermore, the stacks of the single MoS2 layer could be obtained by varying the pressure of hydrothermal synthesis in an autoclave between 170 and 200 °C [36].

Although it has been known that the increase in the pressure in the preparation of MoS2 can improve the catalytic activity, especially for the HDS reaction, there has not been any systematic study of the effect of preparation conditions on the morphology and the methanation reaction of MoS2. In this study, unsupported MoS2 catalysts are hydrothermally prepared at various pressures and temperatures in a continuous flow of H2 and are then examined for physical and chemical characteristics, and catalytic performance, in the methanation reaction.

2 Experimental

2.1 Catalyst Preparation

The schematic diagram of the hydrothermal reaction system for the preparation of MoS2 is shown in Fig. 1. ATTM 99.99% from Alfa Aesar was used as the precursor of MoS2. 5 g of ATTM and 250 mL of doubly distilled water were stirred in a 1 L Inconel 600 cylindrical vessel at 200 rpm for 30 min in 80 mL/min of Ar gas flow, followed by a further 1 h in 30 mL/min of H2 gas flow. The pressure inside the vessel was increased up to a predetermined value between 5 and 40 bar. The temperature was also controlled to a desired value between 300 and 400 °C. After the hydrothermal reaction, the temperature inside the vessel was cooled down to room temperature for 12 h in 200 mL/min of Ar flow.

Fig. 1
figure 1

Schematic of the hydrothermal reaction system for MoS2 preparation

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2.2 Characterization

The prepared MoS2 catalysts were subjected to characterization. The content of elemental sulfur was analyzed using a SC-432DR Sulfur Analyzer (LECO Co., USA). The SEM images were obtained using a Zeiss Supra 50VP scanning electron microscope equipped with EDS. The shape of the nano-sized particles was examined using a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2010). The structure of the catalysts was analyzed by Rigaku D/Max-2500 X-ray diffractometer (40 kV, 100 mA) with Cu Kα (λ = 1.5414 Å) as the radiation source at 2°/min in a range of 2θ between 5° and 90°, and identified using the JCPDS (Joint Committee on Powder Diffraction Standards) library. An X-ray photoelectron spectroscopy (XPS) study was carried out to distinguish the chemical states of Mo and S on the surface of MoS2 using an Axis Nova spectrometer (Kratos) with a monochromatic Al Kα X-rays source (1486.6 eV), operated at 15 kV and 10 mA under a chamber pressure of 10−8 Torr. All binding energies were referenced to that of O 1s (531.0 eV). The baseline corrections for the peak fitting were carried out using the Shirley method. The BET surface area of the sample was determined by measuring N2 adsorption–desorption isotherms at 77 K with the ASAP 2020, Micromeritics Instrument. CO-TPD (temperature programmed desorption), and H2-TPR (temperature programmed reduction) measurements were performed using Micromeritics AutoChem 2920. In CO-TPD measurements, 100 mg of a sample was pretreated at 250 °C for 1 h in 50 mL/min of He gas flow. After CO adsorption at 400 °C for 3 h in 30 mL/min of 10% CO/He gas flow, a sample was cooled down to room temperature in 40 mL/min of He flow. The TCD signal was collected during CO desorption by heating up to 900 °C at 10 °C/min. For H2-TPR analysis, 100 mg of sample was pretreated at 120 °C for 1 h in 30 mL/min of Ar gas flow, followed by heating from room temperature to 800 °C at 10 °C/min in 30 mL/min of 10% H2/Ar gas flow.

2.3 Catalytic Reaction

The prepared MoS2 catalysts were evaluated in catalytic activity for methanation by using an Inconel 600 cylindrical fixed-bed reactor with an internal diameter of 8 mm and a length of 450 mm, as shown in Fig. 2. 0.5 g of catalyst was placed between two layers of silica wool in the middle of the cylindrical reactor. Two K-type thermocouples were installed above and below the catalyst bed to monitor and control the temperature. The difference in temperature between the two thermocouples was within ± 2 °C. The catalysts were subjected to methanation at 400 °C at 30 bar. The composition of the reactant gas mixture was 50% H2/50% CO (i.e., a H2/CO ratio of 1.0). The flow rate of the reactant gas mixture was 80 mL/min, corresponding to the gas hourly space velocity (GHSV) of 4800 h−1. An HP 6890 Series II gas chromatograph using a packed column filled with Carbosphere® was employed for the analysis of the composition of reactant and product gases. The catalytic activity of MoS2 in the methanation reaction for Eq. (1) was assessed in terms of CO conversion as Eq. (2) [37] under the assumption that the mass balance is satisfied by 100%.

Fig. 2
figure 2

Schematic of the methanation reaction system for evaluation of the catalytic activity of MoS2

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$${{\text{H}}_{\text{2}}}+{\text{ CO}} \to \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} {\text{C}}{{\text{H}}_{\text{4}}}+{\text{ }}\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} {\text{C}}{{\text{O}}_{\text{2}}}$$
(1)
$${\text{CO conversion }}\left( \% \right){\text{ }}={\text{ }}\left( {\sum {{\text{n}}_{\text{i}}}{{\text{V}}_{\text{i}}}/{{\text{V}}_{{\text{co}}}}} \right) \times {\text{1}}00$$
(2)

where n i is the number of carbon atoms in product i, V i the volume fraction of product i detected, and V CO the volume fraction of carbon monoxide in the reactant gas.

3 Results

3.1 Physical and Chemical Characteristics

3.1.1 Elemental Composition

Table 1 lists the surface and bulk S/Mo mole ratios. EDS determined the surface S/Mo mole ratio. Total sulfur analysis determined the bulk S/Mo mole ratio, assuming that only S and Mo constituted the MoS2. This table shows that the bulk and surface S/Mo mole ratios are close to 2, which is the stoichiometric S/Mo mole ratio of MoS2, and that the S/Mo ratio is not significantly dependent on the preparation conditions.

Table 1 Bulk and surface S/Mo mole ratios of the MoS2 catalysts with the preparation conditions
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3.1.2 X-ray Powder Diffraction

Figure 3 shows the XRD patterns of MoS2 prepared at various elevated pressures (5–40 bar) at a temperature of 350 °C. As broad XRD peaks generally result from the presence of relatively less crystalline materials, the XRD patterns in Fig. 3 show that only the MoS2 phase exists, and particles of the MoS2 catalyst are present in a relatively less crystalline form, which is not clearly affected by the preparation pressure. A similar MoS2 structure was observed in those prepared by the thermal decomposition of ATTM [38]. It was also reported that the relatively less crystalline MoS2 has a disordered structure, in which the (0 0 2) planes are stacked to build a few layers [34]. Furthermore, the relatively less crystalline MoS2 is known to be highly active for catalytic reactions [17, 20].

Fig. 3
figure 3

X-ray diffraction patterns of the MoS2 catalysts: a S_05_350, b S_10_350, c S_20_350, d S_40_300, e S_40_350, and f S_40_400

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As shown in Table 2, the slab height calculated by Scherrer’s equation [22, 23, 38] using FWHM (full width at half maximum) of the peak corresponding to the (0 0 2) plane and the number of the layers stacked in MoS2, decreases with increasing the preparation pressure. The average height of the crystallites of crystalline MoS2 can be estimated from the diffraction peak for the (0 0 2) plane in the low 2θ range, while the length of the basal plane can be estimated from the ones for the (1 0 0), (1 0 1), or (1 1 0) [8, 22]. However, the length of the basal plane of relatively less crystalline MoS2 cannot be easily estimated from XRD patterns due to random defects, an overlap of peaks, and the folding of basal planes [8, 22, 39]. Moreover, it is well known that an increase in the FWHM indicates a decrease in the stacking and crystallite size (H), where the number of the stacked layer (n) is calculated by n =\(\hspace{0.17em}\text{H}\)/6.17 (\(\text{H}\) in Å) [22]. In general, the height of the MoS2 peaks increases, and the width becomes narrower by increasing the preparation temperature at a preparation pressure of 40 bar. This phenomenon is in agreement with the results of Afanasiev [9], Daage and Chianelli [22], and Iwata et al. [23], where the particles of MoS2 can grow in the height of MoS2 slab by increasing preparation temperature.

Table 2 Physical and chemical characteristics of the prepared MoS2 catalysts
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3.1.3 Scanning Electron Microscopy (SEM)

All MoS2 catalysts prepared in this study are present in agglomerates of irregular nano-sized particles (see Fig. 4). The particles of the MoS2 catalyst prepared at a preparation pressure of 5 bar are fused together to form the agglomerates. As the preparation pressure is increased from 5 to 40 bar, the fused portion gradually disappears, indicating that a high preparation pressure could reduce the extent of agglomeration. In studies of MoS2 prepared by using a mechanically pressurized precursor in the aqueous phase, the catalyst consisted of irregularly rounded particles, and the degree of agglomeration was significantly reduced by the pressurization [35, 40]. Thus, it is expected that high pressure of H2 gas in the preparation facilitates the segregation of MoS2 particles from each other.

Fig. 4
figure 4

SEM photomicrographs of the MoS2 catalysts: a S_05_350, b S_10_350, c S_20_350, d S_40_300, e S_40_350, and f S_40_400

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On the other hand, an increase in preparation temperature from 300 to 400 °C at 40 bar intensifies the fusion of the particles to result in compact agglomerates (see Fig. 4). Calais et al. [39] also reported that an increase in decomposition temperature from 400 to 700 °C rapidly sintered the particles of MoS2 during thermal decomposition.

3.1.4 Transmission Electron Microscopy (TEM)

As all samples show similar images, a representative of the low resolution TEM images is exhibited for the sample prepared at 350 °C and 5 bar in Fig. 5a. The shape of the particles in the low-resolution image of Fig. 5a can be termed “rag-structure”, based on the observation by Chianelli et al. [1]. On the other hand, the high-resolution images of Fig. 5b–d reveal that the MoS2 particles involve a lattice fringe with a spacing of 0.65 nm, which is quite close to the 0.62 nm one for crystalline MoS2 (2H–MoS2) [1, 20, 39]. Furthermore, bent fringes with multi-layered structures become severe with an increase in the preparation pressure. It was reported that several MoS2 layers were folded and disordered in the rag-structure [1]. However, variation of the preparation temperature between 300 and 400 °C at 40 bar does not significantly affect the shape of the fringes. Even though it was reported that a bent lattice fringe becomes straight at a temperature above 700 °C [9], it is supposed that the preparation temperatures in this study are too low to straighten the bent fringe.

Fig. 5
figure 5

TEM images of the MoS2 catalysts: a low-resolution image of S_40_300, and high -resolution images of b S_05_350, c S_10_350, d S_20_350, e S_40_300, f S_40_350, and g S_40_400

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3.1.5 N2 Adsorption–Desorption Isotherms

The N2 adsorption–desorption isotherms of the MoS2 catalysts in Fig. 6 show that an increase in preparation pressure from 5 to 40 bar can sharply raise the amounts of N2 adsorbed on MoS2. Furthermore, it is noteworthy that the type of the N2 adsorption–desorption isotherms in Fig. 6 is dependent on the preparation pressure. According to the IUPAC classification standard [41], the MoS2 prepared at 20 bar has a Type IV N2 adsorption isotherm with a Type H2 hysteresis loop, whereas that prepared at 40 bar has a Type II N2 adsorption isotherm with a Type H3 hysteresis loop. It has been well known that a Type IV N2 isotherm is the typical characteristic of mesoporous solids such as industrial adsorbents [41, 42], and a Type II N2 isotherm is normally obtained from macroporous adsorbents [41]. In addition, a Type H2 hysteresis loop has been obtained in aggregates or agglomerates of spheroidal particles [42] having pores with a nonuniform size or shape [42]. A Type H3 hysteresis loop has been normally observed in aggregates or agglomerates of plate-like or edged particles giving rise to slit-shaped pores with nonuniform size or shape [42]. Therefore, it is found that the MoS2 prepared at 20 bar exists as aggregates with mesopores of nonuniform size or shape, whereas the one prepared at 40 bar has some macropores as well as mesopores.

Fig. 6
figure 6

Nitrogen adsorption–desorption isotherms of the MoS2 catalysts: The open and closed symbols denote adsorption and desorption, respectively; a S_05_350, b S_10_350, c S_20_350, and d S_40_350

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The average pore size and BET surface areas summarized in Table 2 are acquired from the N2 adsorption–desorption isotherms of the MoS2 samples. The average pore size and BET surface area significantly increase as the preparation pressure is increased. However, in the variation of preparation temperature at 40 bar, the average pore size and BET surface area are maximized at a temperature of 350 °C.

3.1.6 X-ray Photoelectron Spectroscopy (XPS)

The oxidation states of Mo and S on the surface of the prepared MoS2 are found from the Mo 3d and S 2p XPS spectra. Since the shapes of XPS spectra are similar for all catalysts, the representing spectra of the one prepared at 350 °C and 40 bar are shown in Fig. 7, including the deconvolution of the peaks for Mo 3d and S 2p. The Mo 3d XPS spectrum is deconvoluted using an intensity ratio of 2/3 and a splitting of 3.1 eV between two Mo 3d peaks (3d5/2 and 3d3/2) [43], employing a combination of 20% Gaussian and 80% Lorentzian distributions. On the other hand, the S 2p XPS spectrum is deconvoluted using an intensity ratio of 2/1 and a splitting of 1.2 eV between two S 2p peaks (2p3/2 and 2p1/2) [43], employing a combination of 35% Gaussian and 65% Lorentzian distributions.

Fig. 7
figure 7

Representative XPS spectra of the MoS2 catalyst (S_10_350) with the deconvoluted peaks: a Mo 3d and b S 2p

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In Fig. 7a, two deconvoluted peaks at 229.1 and 232.2 eV correspond to Mo4+ in MoS2 [43, 44], and those at 231.4 and 234.6 eV to Mo6+ in MoS3 [43]. As the oxidation state of Mo at 230.3 eV of Mo 3d5/2 peak and 233.4 eV of Mo 3d3/2 peak is higher than that corresponding to Mo4+ and lower than that of Mo6+, it is estimated that the above peaks can be attributed to Mo5+ in Mo2S5 [43, 45, 46]. According to the study by Wang’s research group [45], Mo2S5 could exist as an intermediate product between MoS2 and MoS3 in MoS2 preparation by heating an ATTM solution. On the other hand, the peak at 226.3 eV is allocated for S 2s of MoS2, based on the research by Wang et al. [43], and Roy and Srivastava [44].

In Fig. 7b of S 2p XPS spectrum, two S 2p3/2 peaks at ca. 162.0 eV and ca. 168.6 eV can be attributed to S2− in MoS2 [43, 44, 47] and SO42− species [43, 47], respectively. According to Wang et al. [43], sulfate may remain as a residue when sulfuric acid is used in MoS2 preparation. Pathak et al. [46] announced that the existence of the SO42− species is related to the bisulfite (HSO3−) and bisulfate (H2SO4−) residues in the precipitates formed in the MoS2 preparation. In addition, the production of the sulfate has been accounted for through the oxidation of the adsorbed H2S, or low balance sulfur by oxidants such as H2O or CO2 [47]. Accordingly, it is postulated that SO42− species at about 168.6 eV may be attributed to the sulfate residue formed by the oxidation of a part of H2S produced from ATTM in an aqueous phase in H2 during the hydrothermal synthesis of MoS2.

The mole ratios of Mo4+ to total Mo and SO42− to the sum of SO42− and S2− are summarized in Table 2. Although the Mo4+ mole ratios are not much different between the catalysts, it is worth noting that the ratio is slightly increased by increasing the preparation pressure and temperature. Thus, the high pressure and temperature in the preparation might lead to favorably transform Mo6+ of ATTM as a precursor into Mo4+ of MoS2. Moreover, the sulfate residue seems to be the highest in the MoS2 catalyst prepared at 350 °C and 20 bar, which might imply that most H2S adsorbed at the preparation conditions could be likely oxidized.

3.1.7 CO-Temperature Programmed Desorption (TPD)

In the CO-TPD profiles of the MoS2 samples shown in Fig. 8, three CO desorption peaks are observed: the first (Peak 1) is located at approximately 100 °C, the second (Peak 2) between 600 and 700 °C, and the third (Peak 3) at approximately 780 °C. In general, the number of desorption peaks represents the number of adsorption sites, which have different adsorption enthalpy and desorption activation energy [48]. In a comparison between Peak 1 and Peak 2 of all catalysts, Peak 1 is relatively weaker than Peak 2 in intensity, which means that most of the CO is strongly chemisorbed in the MoS2 catalysts. Peak 3 is observed only in the CO-TPD of the catalyst prepared at 40 bar, which implies that the adsorption site corresponding to Peak 3 can more strongly adsorb CO than that of Peak 2.

Fig. 8
figure 8

CO-TPD profiles of the MoS2 catalysts: a S_05_350, b S_10_350, c S_20_350, and d S_40_350

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The amount of CO desorption corresponding to Peak 1 increases with an increase in the preparation pressure, which is indicative of the enhancement of weak adsorption of CO species [49]. In addition, the overall CO desorption of Peak 2 and Peak 3 rises in amount with increasing preparation pressure, inducing the improvement of the catalytic activity of the MoS2. It has been suggested that the strong CO adsorption site is associated with the sulfur vacancy as a catalytic active site [50].

3.1.8 H2-Temperature Programmed Reduction (TPR)

In Fig. 9 for the H2-TPR profile of the MoS2, H2 consumption occurs mainly at a low temperature of approximately 230 °C. It can be seen that H2 consumption increases with an increase in the preparation pressure (see Fig. 9a), and it is maximized at 350 °C with varying preparation temperatures (see Fig. 9b). According to Jacobsen et al. [13] and Mangnus et al. [51], the appearance of H2-TPR peaks in the low temperature range is due to the sulfur weakly bonding to the surface of the catalyst, or extra sulfur present. In a H2-TPR study of transition metal sulfides, a peak in the low temperature domain was addressed due to the weakly bonded sulfur related to the surface reaction, whereas that in the high temperature one was due to a bulk reduction [52]. In the same study, the weakly bonded sulfur could produce coordinated unsaturated sites (CUS), which might be related to the catalytic activity of transition sulfides. Li et al. [53] proposed that sulfur vacancies could be formed by the formation of H2S through the reaction of S with H2 on the surface of MoS2. Hence, high pressure and a temperature of 350 °C in the preparation possibly develops the formation of sulfur vacancies for H2 consumption.

Fig. 9
figure 9

H2-TPR profiles of the MoS2 catalysts: a variation of preparation pressures (5–40 bar) at 350 °C, and b variation of preparation temperatures (300–400 °C) at 40 bar

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3.2 Catalytic Performance

Figure 10 shows CO conversion profiles against reaction time in the methanation reaction over the MoS2 catalysts. In general, maximum CO conversions of the catalysts are reached within 90 min of reaction time, and conversions after 90 min monotonically decrease with reaction time. In Fig. 10, for the catalysts prepared at 350 °C, the decreasing rate of CO conversion after 90 min is more significant in the catalyst prepared at 5 bar than those at 20 and 40 bar. On the other hand, in the catalysts prepared at 40 bar with varying temperatures, the decreasing rates of CO conversion after 90 min are much lowered, especially at 400 °C. To compare the catalysts in catalytic performance, the CO conversions between 3 and 7 h are averaged and summarized in Table 3. It is apparent that the catalytic performance of the MoS2 catalysts in CO conversion is improved by increasing the preparation pressure. Furthermore, the methanation reactivity of catalysts is high at preparation temperatures in the order of 350, 300, and 400 °C. Therefore, the results suggest that the optimum conditions for MoS2 preparation are 350 °C and 40 bar in terms of CO conversion.

Fig. 10
figure 10

CO conversions of the MoS2 catalysts in the methanation: a variation of preparation pressures (5–40 bar) at 350 °C, and b variation of preparation temperatures (300–400 °C) at 40 bar

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Table 3 Summary of the catalytic performance of the prepared MoS2 catalysts in methanation
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4 Discussion

The hydrothermally prepared MoS2 catalysts are formed in bent fringes with a nano-sized slab by stacking the (0 0 2) planes, which is confirmed by the results of TEM and XRD. The nano-sized particles are chemically composed of MoS2, with some Mo2S5 and MoS3, where a little of SO42− is also present on the surface. Thus, some Mo6+ of ATTM might be incompletely decomposed to Mo2S5 and MoS3 that are probably located on the curved edges of the bent fringes during the preparation. In addition, as the existence of sulfur vacancies in the catalysts is observed in CO-TPD and H2-TPR, the surface SO42− is postulated to be associated with the vacancies. The sulfur in Mo2S5 and MoS3 might be depleted by reacting with H2 to H2S, which leaves a sulfur vacancy on the surface. After the synthesized catalysts are exposed to the atmospheric environment, it is expected that the vacancy could get oxidized to SO42− on the surface. In general, it has been observed that the sulfur vacancy could improve the catalytic activity of MoS2 [20, 52, 53]. It was also suggested that the sulfur vacancies facilitating the methanation reaction are also involved in the sulfur species weakly bonded to the surface of MoS2 [20].

The physical properties, such as the shape of the fringes, the BET surface area, and the height of the slab, are greatly dependent on the preparation pressure. Increasing the preparation pressure at 350 °C, it is seen that the curved basal planes of MoS2 is increased in number, and the slab height is reduced. In addition, high preparation pressure can develop the pore structure, which raises the BET surface area of the MoS2 catalyst. According to the results of XPS, CO-TPD, and H2-TPR, more surface Mo4+ and sulfur vacancies appear at higher preparation pressures, which probably leads to the improvement of the catalytic performance of MoS2 catalysts in methanation. Analogously, it was reported that the catalytic activity of MoS2 for hydrodesulfurization is associated with BET surface area [12, 34]. On the other hand, Iwata’s research group proposed that active sites such as sulfur vacancies could exist on the curvature of the basal planes of MoS2, as well as on the edge planes [17, 23]. Thus, it is suggested that the development of the bent fringes and the pore structure at high preparation pressure could induce the appearance of surface Mo4+ and sulfur vacancies in the catalysts, which can provide highly active sites for CO adsorption, H2 reduction, and the methanation.

Even though the fringe shape does not change much with preparation temperature at a fixed pressure of 40 bar, the fringes slightly grow in slab height with an increase in the preparation temperature. Thus, it is expected that the BET surface area will decrease with an increase in the slab height by the preparation temperature. However, the surface area seems to be maximized at a preparation temperature of 350 °C in this study. In general, it has been reported that an increase in preparation temperature from 350 to 700 °C could decrease the BET surface area of MoS2 prepared by thermal decomposition of ATTM [9, 23]. Afanasiev [9], in particular, claimed that a preparation temperature between 400 and 700 °C could decrease BET surface area of MoS2 due to the plugging of the micropores in the catalyst, as well as the growth of the MoS2 slab. However, as the mole fraction of surface Mo4+ in the catalyst increases with an increase in the preparation temperature (see Table 2), it should be noted that the preparation temperature can affect not only the slab height but also the decomposition of ATTM precursor into MoS2. It has been known that the thiosalt precursor could be incompletely decomposed into MoS2 at a temperature below 350 °C [34, 45]. Thus, with preparation temperatures at or below 350 °C, the decomposition of the ATTM precursor into MoS2 could be dominant to enhance the BET surface area by producing the pore structure. On the other hand, for a preparation temperature above 350 °C, the increase in the MoS2 slab height could primarily cause plugging of pores and lower the BET surface area. Furthermore, such a phenomenon could severely affect the amount of the active sites related to the surface Mo4+ species and the sulfur vacancies. As the surface Mo4+ is observed to be increasingly present with an increase in the preparation temperature, it is obvious that a high preparation temperature must be beneficial to prompt the decomposition and create the active sites. Nevertheless, the catalytic activity in the methanation drops sharply in the catalyst prepared at 400 °C (see Table 3). Thus, according to the H2-TPR result showing the maximum incidence of sulfur vacancies in the catalyst prepared at 350 °C, it is considered that the entrances of the pores generated at 350 °C of the preparation temperature could be closed at 400 °C, preventing reactants from getting adsorbed on the active sites.

5 Conclusions

Unsupported MoS2 catalysts prepared by hydrothermal reaction using ammonium tetrathiomolybdate (ATTM, (NH4)2MoS4) show relatively broad XRD peaks, which are the characteristics of less crystalline materials. The particles of the catalysts are formed in the bent fringes by stacking the (0 0 2) planes. Mo6+ of the ATTM precursor are mainly transformed into MoS2, with some Mo2S5 and MoS3 produced during the synthesis. Also, a small amount of the surface sulfur species is present in sulfate form. The increase in preparation pressure results in the increase in the curvature on the basal planes of MoS2, and a decrease in the degree of stacking of slab related to the crystallite size of MoS2, which could enhance the BET surface area. It is suggested that the increase in surface Mo4+ and sulfur vacancies observed at high preparation pressures could improve the catalytic activity of MoS2 for methanation. Although the increase in the preparation temperature from 300 to 400 °C could likely decompose the ATTM precursor into MoS2, a high preparation temperature above 350 °C decreases the BET surface area and the amount of the catalytic active sites, such as sulfur vacancies, available for the methanation reaction. Accordingly, it is suggested that an optimum preparation temperature of 350 °C is required to maximize the catalytic performance for methanation by completing the decomposition of the precursor and securing the accessible active sites.