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. 2023 Oct 30;8(44):41182–41193. doi: 10.1021/acsomega.3c04059

Study on the Performance of Ni–MoS2 Catalysts with Different MoS2 Structures for Dibenzothiophene Hydrodesulfurization

Chuangchuang Yang 1, Anpeng Hu 1, Qiaoling Dai 1, Qinghe Yang 1,*, Ranran Hou 1, Zhiwei Liu 1
PMCID: PMC10634193  PMID: 37970013

Abstract

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Hydrodesulfurization (HDS) is an important process for the production of clean fuel oil, and the development of a new environmentally friendly, low-cost sulfided catalyst is key research in hydrogenation technology. Herein, commercial bulk MoS2 and NiCO3·2NiOH2·4H2O were first hydrothermally treated and then calcined in a H2 or N2 atmosphere to obtain Ni–MoS2 HDS catalysts with different structures. Mechanisms of hydrothermal treatment and calcination on Ni–MoS2 catalyst structures were investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). The catalytic performance of Ni–MoS2 catalysts was evaluated by the HDS reaction of dibenzothiophene (DBT) on a fixed bed reactor, and the structure–activity relationship between the structures of the Ni–MoS2 catalyst and the HDS of DBT was discussed. The results showed that the lateral size, the number of stacked layers, and the S/Mo atomic ratio of MoS2 in the catalyst decreased and then increased with the increase of the hydrothermal treatment temperature, reaching the minimum at the hydrothermal treatment temperature of 150 °C, i.e., the lateral size of MoS2 in the catalyst was 20–36 nm, the number of stacked layers of MoS2 was 5.4, and the S/Mo ratio in the catalyst was 1.80. In addition, the effects of different calcination temperatures and calcination atmospheres on the catalyst structures were investigated at the optimum hydrothermal treatment temperature. The Ni–Mo–S and NixSy ratios of the catalysts increased and then decreased with the increasing calcination temperature under a H2 atmosphere, reaching a maximum at a calcination temperature of 400 °C. Therefore, DBT exhibited the best HDS activity over the H-NiMo-150–400 catalyst, and the desulfurization rate of DBT reached 94.7% at a reaction temperature of 320 °C.

1. Introduction

In recent years, the production of clean fuels has faced serious challenges due to the substandard quality of crude oil, as well as the emphasis on environmental protection and increasingly stringent environmental regulations.1,2 Hydrodesulfurization (HDS) is the main method to remove the heteroatoms in petroleum fractions and to produce clean fuels.3 Because the catalyst is the core of HDS technology, the research on the HDS catalyst has attracted much attention.4,5

The most general and important hydrotreating catalysts, alumina or modified alumina-supported MoS2 usually promoted by Co or Ni atoms, have been used in the refining industry for more than half a century.6,7 In the industrial preparation of Mo-based hydrotreatment catalysts, solutions containing Ni2+ and oxidized molybdenum-containing anions, such as Mo7O246– and P2Mo5O236–, are usually first impregnated onto an Al2O3 support and then calcined to obtain an oxidized catalyst.8,9 In refinery hydrogenation reactors, the oxidized catalysts are reduced to sulfides using a mixture of a feed of H2 and of sulfur-containing compounds, such as H2S, CS2, dimethylsulfide (DMS), and dimethyldisulfide (DMDS).10−12 In general, the strong interaction between the active metal and the Al2O3 support during calcination of the supported catalyst promotes the formation of the Mo–O–Al phase.13 This strong interaction is conducive to the dispersion of Mo species to ensure high accessibility of the active sites to the reactants, but resulting in the subsequent sulfurization process needs to be carried out in a more stringent condition.1,14 Furthermore, the formation of spinel phases between Co/Ni and Al2O3 adversely affects MoS2 edge site catalytic performance.15 In order to avoid the negative impact of the support on the catalyst, researchers16,17 have proposed the preparation of unsupported HDS catalysts, which have been widely studied and become a new research direction due to their outstanding advantages, such as high utilization of active metals, high activity, and good resistance to carbon deposition. Liu et al.17 first synthesized NiMo oxides by the poly(vinylpyrrolidone) (PVP)-assisted chemical precipitation method and then prepared highly active unsupported HDS catalysts by in situ sulfidation of CS2 and H2 mixture. Chowdari et al.18 investigated the preparation of Ni–MoS2 nanotubes from bulk NiMo oxides by different sulfidation methods. The results showed that the activity of the unsupported HDS catalyst prepared by in situ sulfidation was 4 times higher than that of the catalyst prepared by non-in-situ sulfidation. Afanasiev19 prepared the unsupported Ni–MoS2 catalysts by first decomposing ammonium tetrathiomolybdate (ATM) to MoS2 at different temperatures and gas compositions and then introducing a promoter into the synthesized MoS2 by dissolving the organic complex of nickel or cobalt in an organic solution for reflux.

Therefore, most supported and unsupported HDS catalysts are first oxidized and then sulfided with sulfur-containing compounds. The use of sulfur-containing compounds, which are usually highly toxic, not only poses serious environmental problems but also causes severe corrosion of equipment and pipelines. In addition, the sulfidation process prolongs the refinery’s startup time, which is detrimental to the economics of the refinery.20−22 The preparation of sulfurized catalysts by simultaneous decomposition of precursors containing S and Mo, such as ATM, has the disadvantages of a complex precursor preparation process and a high price and is not suitable for industrial applications. To solve these problems, researchers are looking for an environmentally friendly and inexpensive precursor containing S and Mo. Researchers23,24 proposed to prepare HDS catalysts using MoS2, which is the active component of HDS catalysts and can be obtained by direct purification from molybdenum pyroxene concentrates.25,26 Based on the rim-edge model for unloaded HDS catalysts,27,28 the active sites responsible for the HDS reaction are situated at the rim sites located at the top and bottom of the layer-stacked MoS2, as well as the edge sites positioned between these upper and lower regions. Conversely, the basal surface of MoS2 remains inert. However, when MoS2 is obtained from molybdenum pyroxene concentrates, it exhibits a substantial transverse size of lamellar crystals (>150 nm) and a significant number of stacked layers (>30 layers). Consequently, this configuration exposes fewer rim and edge sites, thereby restricting the HDS activity. Wang et al.2 obtained MoS2 with a lateral size of about 30 nm and a stacking number of 6–8 layers by ball milling multilayers of commercial MoS2 with large lateral sizes. They found that ball milling exposed more Rim active sites of MoS2, thus improving the hydrogenation activity of the catalyst. Mahmoudabadi et al.23 prepared MoS2/GO catalysts by ultrasonic coexfoliation of commercial bulk MoS2 and graphene (GO) assisted by poly(vinylpyrrolidone) (PVP). The results showed that the HDS activity of MoS2/GO was higher than that of the conventional loaded catalysts. The promoter Ni/Co can exponentially increase the activity of the catalyst by modifying the MoS2 edge and increasing the electron cloud density of the adjacent Mo atoms. However, to the best of our knowledge, there are fewer studies focusing on the effects of Ni/Co atoms on the HDS catalysts prepared by modifying commercial MoS2, and furthermore, the different structures of MoS2 and the performance of HDS catalysts need to be further investigated.

Here, commercial bulk MoS2 and NiCO3·2NiOH2·4H2O were first hydrothermally treated and then calcined in a H2 or N2 atmosphere to obtain Ni–MoS2 hydrodesulfurization catalysts with different structures. The mechanisms of hydrothermal treatment and calcination on the structure of the Ni–MoS2 catalyst were investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). The HDS performance of the Ni–MoS2 catalyst was evaluated in a fixed bed reactor using DBT as a model compound, and the structure–activity relationship between the structure of the Ni–MoS2 catalyst and the HDS of DBT was discussed.

2. Experimental Section

2.1. Materials

No further purification was performed on all of the materials once they were received. Commercial bulk molybdenum disulfide (MoS2) was purchased from Aladdin Industrial Co. Basic nickel carbonate (NiCO3·2Ni(OH)2·4H2O) was supplied by the SINOPEC Changling Catalyst Company. Dibenzothiophene (DBT) and decalin were supplied by J&K Chemical, and n-decane was purchased from Tianjin Damao Chemical Reagent Factory.

2.2. Preparation of Sulfided NiMo Catalysts

A series of sulfided NiMo HDS catalysts were prepared by hydrothermal treatment of commercial bulk molybdenum disulfide (b-MoS2) and basic nickel carbonate (NiCO3·2Ni(OH)2·4H2O), according to the Ni/Mo atomic ratio of 0.5 at a certain temperature, and then calcined under a nonoxidizing atmosphere. Typically, 5 g of commercial MoS2 powder, 1.96 g of NiCO3·2Ni(OH)2·4H2O powder, and 70 mL of deionized water were added to a 100 mL Teflon-lined stainless steel rotary autoclave and hydrothermally treated at 90, 120, 150, 180, and 200 °C for 10 h, respectively. The black precipitate acquired through filtration is subjected to washing with deionized water, subsequent air drying at a temperature of 120 °C for a duration of 6 h, and subsequently undergoing calcination in a H2 atmosphere at temperatures of 350, 400, 500, and 600 °C for a duration of 3 h. Additionally, the mixture is calcined in a N2 atmosphere at a temperature of 400 °C for a duration of 3 h. The prepared catalysts were named H-NiMo-x-y and N-NiMo-x-y, where x represents the hydrothermal treatment temperature and y represents the calcination temperature.

Furthermore, to examine the impact of hydrothermal treatment and calcination in a hydrogen atmosphere on the structure of the catalyst, two separate catalysts were prepared: the NiMo-150 catalyst solely subjected to hydrothermal treatment without calcination and the H-NiMo-400 catalyst solely subjected to calcination in a hydrogen atmosphere without hydrothermal treatment. In this experiment, a 100 mL Teflon-lined stainless steel rotary autoclave was utilized to combine 5 g of commercial MoS2 powder, 1.96 g of NiCO3·2Ni(OH)2·4H2O powder, and 70 mL of deionized water. The resulting mixture was subjected to hydrothermal treatment at a temperature of 150 °C for a duration of 10 h. The resulting black precipitate was then filtered and washed with deionized water before being dried at 120 °C for 6 h, resulting in the formation of NiMo-150. In a separate procedure, the same quantities of MoS2, NiCO3·2Ni(OH)2·4H2O, and deionized water were combined in a beaker, and the mixture was stirred for 10 h at room temperature. The resulting black precipitate was filtered, washed with deionized water, and subsequently dried at 120 °C for 6 h. This dried precipitate was then subjected to calcination in a H2 atmosphere at a temperature of 400 °C for a duration of 3 h, resulting in the formation of H-NiMo-400.

2.3. Characterization of Catalysts

The X-ray diffraction measurements were carried out on a Bruker D5005 X′ Pert diffractometer with Cu Kα radiation (λ = 1.5406 Å). Patterns were collected from 5 to 70° at a scanning speed of 2° min–1. The Bragg equation (d = nλ/2 sin θ) was used to calculate the interlayer spacing of the (002) crystal plane of MoS2. High-resolution transmission electron microscopy (HRTEM) was performed on a Philips Tecnai G2 F20 instrument with an electron energy of 200 kV, a conductive concentration of 0.5 mm, a resolution of 1.9 Å in TEM mode, and a sample tilt of ±40°. Images were captured with a TVIPS 1k × 1k CCD camera. In order to obtain an objective assessment of the lateral dimensions and stacking layers of MoS2 samples, we conducted a random count of 50 MoS2 nanosheets from the HRTEM images for each MoS2 sample. Subsequently, we computed the lateral dimension and average number of stacking layers. The X-ray photoelectron spectroscopy (XPS) studies were performed using an ESCALab250 spectrometer from VG Scientific. Element surface area was quantified using the sensitivity factors provided by the VG program. All binding energies (BEs) were calibrated to C 1s, a form of adventitious carbon (284.80 eV). In addition, Avantage software (version 5.967) can be used to fit the XPS spectrum to identify the chemical states of the Ni species (namely, Ni–Mo–S, Ni2+, NixSy, Ni shakeup). The Bruker A330 X-band continuous wave EPR spectrometer was utilized to measure the electron paramagnetic resonance (EPR) spectra. The paramagnetic samples were positioned within a rectangular resonator operating at a frequency of 9.853301 GHz. The surface morphology and the composition of the samples were characterized using scanning electron microscopy (SEM, JEOL JSM-6700F) and energy dispersive spectroscopy (EDS, Hitachi S-4800-I).

2.4. Catalytic Performance of Catalysts

The catalytic activities of sulfided NiMo catalysts were evaluated in a 500 mm long, 10.0 mm diameter fixed microreactor. The catalyst was ground to 40–60 mesh, and 0.45 g of catalyst was diluted with 1 g of quartz sand; then, the diluted sample was added to the constant temperature zone of the reactor, and the remainder was filled with 40–60 mesh quartz sand. A 1 wt % dibenzothiophene (DBT)/n-decane solution was chosen as the model reactant. HDS reactions of DBT were conducted at 260, 280, 300, and 320 °C, total pressure 4 MPa, WHSV 24.67 h–1, and H2/feed volume ratio of 500. The hydrodesulfurization products were analyzed on an Agilent 7890A series gas chromatograph, and decalin was used as an internal standard to calculate the content of each component in the product by normalization. According to the equation below, the conversion (denoted as x) of the reaction can be calculated.

2.4.

The HDS reaction of DBT occurs via two primary pathways, specifically hydrogenation (HYD) and direct desulfurization (DDS), as depicted in Scheme 1.29−31 In the DDS pathway, DBT completes the reaction by directly breaking the C–S bond at the S vacancy of the catalyst through the S atom adsorbed in the σ mode to form biphenyl (BP); in the HYD pathway, DBT is adsorbed onto the catalyst in the π-mode via the aromatic system and is first hydrogenated to form the intermediate tetrahydrodibenzothiophene (THDBT), which is then desulfided to form cyclohexylbenzene (CHB) and bicyclohexane (BCH).29,32 As a result of gas chromatography-mass spectrometry (GC-MS) analysis, several products were detected in the HDS of DBT. As hydrogenated intermediates in the hydrogenation (HYD) pathway, tetrahydrodibenzothiophene (THDBT) is further desulfurized to form cyclohexylbenzene (CHB) and bicyclohexane (BCH), and direct desulfurization (DDS) produces biphenyl (BP). The following equations were used to calculate the selectivity

2.4.
2.4.

Scheme 1. HDS Reaction Pathways of DBT.

Scheme 1

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3. Results and Discussion

3.1. Effect of Hydrothermal Treatment Temperature on the Structure of Sulfided NiMo Catalysts

3.1.1. XRD Analysis

Figure 1 shows the XRD patterns of b-MoS2 and sulfided NiMo catalysts prepared at different hydrothermal treatment temperatures. It can be seen from Figure 1 that the diffraction peaks are at 14.4, 32.7, 33.5, 58.4°, etc., in the XRD pattern of b-MoS2 corresponding to planes (002), (100), (101), (110), etc., in hexagonal 2H-MoS2 (JCPDS No. 37–1492). Compared with that of b-MoS2, the characteristic peak intensity of each crystal plane of sulfided NiMo prepared by different hydrothermal treatment temperatures is obviously reduced. Especially, the intensity of the (002), (100), and (110) characteristic peaks is reduced for the (002) crystal plane, representing the number of MoS2 stacking layers, and the (100) crystal plane and (110) crystal plane, representing the lateral size of MoS2, which means the reduction of the stacking layers and lateral size of the prepared catalysts.2,33 In the high-temperature and high-pressure hydrothermal environment, a large amount of high-pressure airflow accelerates the MoS2 extension and distortion, thus leading to a certain degree of disruption of the van der Waals forces between MoS2 layers and Mo–S bonds within the layers.34,35 At the same time, due to the adsorption of Ni salt on the surface of MoS2,36−38 the reaggregation of exfoliated MoS2 is prevented. When the hydrothermal temperature was increased from 90 to 150 °C, the intensity of the diffraction peaks, especially the (002) peak in the XRD patterns of the obtained catalyst samples, first gradually decreased and then showed a gradual increase as the hydrothermal temperature continued to increase from 150 to 200 °C. This may be due to the partial desorption of NiCO3·2Ni (OH)2·4H2O adsorbed on the MoS2 surface, resulting in slight aggregation of MoS2 as the hydrothermal treatment temperature continues to increase.37,39

Figure 1.

Figure 1

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XRD patterns of sulfided NiMo catalysts obtained at different hydrothermal treatment temperatures.

Interestingly, as shown in the amplified spectrum in Figure 1, the characteristic peak of the (002) crystal plane of the resulted catalyst is slightly shifted to a lower angle than that of b-MoS2, indicating a slight expansion of the MoS2 interlayer spacing caused by the lattice distortion of the catalyst.40 The results of calculations based on the Bragg equation (Table 1) show that the interlayer spacing of MoS2 increases with an increase in the hydrothermal treatment temperature. Increasing the MoS2 layer spacing could potentially facilitate the adsorption and desorption of reactant molecules (DBT) on the active sites, thereby contributing to the enhancement of the HDS activity.41,42

Table 1. Interlayer Spacing, Lateral Dimensions, and Number of Stacked Layers of Commercially Available Molybdenum Disulfide and Sulfided NiMo Catalysts with Different Hydrothermal Treatments.
catalysts 2θ (deg) interlayer spacing (Å) lateral dimension (nm) average number of layers
b-MoS2 14.41 6.14 150–170 32.1
H-NiMo-90–400 14.27 6.20 45–55 6.6
H-NiMo-120–400 14.23 6.22 35–47 6.2
H-NiMo-150–400 14.19 6.24 20–36 5.4
H-NiMo-180–400 14.16 6.25 37–50 6.8
H-NiMo-200–400 14.13 6.26 47–60 7.1
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Furthermore, the diffraction peaks at 2θ = 44.3 and 50.1° in the XRD spectra of sulfided NiMo catalysts can be indexed to the (202) and (211) crystal planes of Ni3S2, which is in good agreement with the standard Ni3S2 (JCPDS card number 44–1418) and previous studies.43−45

3.1.2. HRTEM Analysis

The morphology and microstructure of sulfided NiMo catalysts obtained from different hydrothermal treatment temperatures were studied with high-resolution TEM (HRTEM). As shown in Figure 2, the stripes with a lattice spacing of about 0.62 nm correspond to the (002) plane of hexagonal MoS2. The commercial bulk molybdenum disulfide (b-MoS2) sample is composed of regular sheets with lateral sizes of several hundred nanometers and stacked layers of more than 30 (Figure 2a), while the lateral size and stacked layers of MoS2 in the catalyst samples prepared by hydrothermal treatment are significantly reduced, which is consistent with the XRD results. In addition, the large number of discontinuous lattice stripes found in the hydrothermally treated catalysts indicated the presence of abundant structural defects46,47 (Figure 2b,c,d,e,f). The reduction of MoS2 lateral size and stacking layer number as well as the presence of defects would favor the catalyst to expose more active sites and thus increase the HDS activity.2,48 The statistics of lateral dimensions and the average number of stacked layers of MoS2 for HRTEM images are shown in Table 1. As can be seen in Table 1, when the hydrothermal treatment temperature was increased from 90 to 150 °C, the lateral size and the number of stacked layers of MoS2 in the obtained catalysts first gradually decreased and then rather slightly increased with further increasing the hydrothermal treatment temperature, which is consistent with the XRD results.

Figure 2.

Figure 2

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HRTEM images of sulfided NiMo catalysts obtained at different hydrothermal treatment temperatures: (a) b-MoS2, (b) H-NiMo-90–400, (c) H-NiMo-120–400, (d) H-NiMo-150–400, (e) H-NiMo-180–400, and (f) H-NiMo-200–400.

3.1.3. EPR and SEM-EDS Analyses

To conduct a more thorough investigation into the S vacancies of the catalysts synthesized at various hydrothermal temperatures, the catalysts underwent EPR characterization. The EPR spectra of the catalysts prepared with b-MoS2 at different hydrothermal temperatures are depicted in Figure 3. The spectra reveal that all samples exhibit the lowest EPR signal at g = 0.2003. The intensity of the EPR signal directly correlates with the concentration of S vacancies,49−51 and the peak intensity of the prepared catalysts surpasses that of b-MoS2. This observation implies that the catalysts possess a higher abundance of sulfur vacancies. The sulfur vacancy concentration of the catalysts prepared at different hydrothermal temperatures decreases in the following order: H-NiMo-150–400 > H-NiMo-120–400 > H-NiMo-180–400 > H-NiMo-90–400 > H-NiMo-200–400. Figure S6 displays the typical SEM micrographs and EDS spectra of H-NiMo-150–400, revealing a uniform dispersion of Ni in the catalyst. Table 2 presents the elemental contents and S/Mo atomic ratios of b-MoS2 and the catalysts prepared at various hydrothermal treatment temperatures, as determined by EDS analysis. The catalysts exhibit varying concentrations of S vacancies, with a decreasing trend observed in the following sequence: H-NiMo-150–400 > H-NiMo-120–400 > H-NiMo-180–400 > H-NiMo-90–400 > H-NiMo-200–400.

Figure 3.

Figure 3

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EPR patterns of sulfided NiMo catalysts obtained at different hydrothermal treatment temperatures.

Table 2. EDS Results of Catalysts Prepared at Different Hydrothermal Treatment Temperatures.
catalysts Ni, % Mo, % S, % O, % Ni/Mo S/Mo
b-MoS2 0 32.12 63.62 4.26 0 1.98
H-NiMo-90–400 12.02 28.35 54.19 5.44 0.42 1.91
H-NiMo-120–400 12.95 27.42 51.66 7.97 0.47 1.88
H-NiMo-150–400 14.96 26.79 49.22 9.03 0.56 1.84
H-NiMo-180–400 11.22 28.29 53.48 7.01 0.4 1.89
H-NiMo-200–400 14.27 27.63 53.17 4.93 0.52 1.92
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3.1.4. XPS Analysis

The chemical composition and valence states in the sulfided NiMo catalysts obtained at different hydrothermal treatment temperatures were further investigated by XPS, and the Mo 3d and S 2p spectra of the six samples are shown in Figure 4. Spectra were fitted by Gaussian–Lorentzian curves, and the peak shifts were corrected by using the standard C 1s peak at 284.6 eV.2,40 As shown in Figure 4a, the binding energies are located at about 229.0 and 232.1 eV attributed to the Mo 3d5/2 and Mo 3d3/2 characteristic peaks of Mo4+, while the binding energies of Mo6+ appear at about 232.5 and 235.7 eV, and the binding energy of the S 2s peak appears at about 226.2 eV.1,52 In the S 2p fitting spectrum illustrated in Figure 4b, the observed peaks at 162.3 and 163.5 eV in the b-MoS2 sample can be ascribed to the S 2p3/2 and S 2p1/2 orbitals of S2–, respectively. The peak at 169.6 eV is a characteristic peak due to the partial oxidation of S on the MoS2 surface to form SO42–.53,54 Interestingly, the Mo 3d peaks of the catalyst samples prepared by hydrothermal treatment were shifted by about 0.3 eV toward lower binding energies compared to commercial bulk molybdenum disulfide (b-MoS2), while the same shift occurred for S 2p peaks. The lower binding energy of the Mo 3d signal and the S 2p signal of the catalysts (Ni3S2–MoS2) prepared by hydrothermal treatment indicates not only a strong electronic interaction between Ni3S2 and MoS2 in the catalysts55,56 but also a reduction in the number of S atoms and the creation of S vacancies,57,58 which contributes to the HDS reactivity of the catalysts.59,60Table 3 shows the S/Mo atomic ratios of the six samples based on the XPS peak area ratio between S 2p and Mo 3d. It can be seen from Table 3 that the S/Mo atomic ratio in the sulfided NiMo catalyst is lower compared to that of b-MoS2, further confirming the presence of a large number of S vacancies in the catalyst samples prepared by hydrothermal treatment. The number of S vacancies in the catalyst decreases in the following order: H-NiMo-150–400 > H-NiMo-120–400 > H-NiMo-180–400 > H-NiMo-90–400 > H-NiMo-200–400, which is consistent with the results of EPR and SEM-EDS.

Figure 4.

Figure 4

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Fitted curves corresponding to Mo 3d and S 2p for sulfided NiMo catalysts obtained at different hydrothermal treatment temperatures: (a) Mo 3d fitted spectrum and (b) S 2p fitted spectrum.

Table 3. S/Mo Atomic Ratio and Different Ni Species Proportions of Commercial Molybdenum Disulfide and Sulfided NiMo Catalysts with Different Hydrothermal Treatments.
catalysts S/Mo Ni2+, % Ni–Mo–S, % NixSy, % Ni–Mo–S + NixSy, %
b-MoS2 2.02 0 0 0 0
H-NiMo-90–400 1.88 66.3 18.9 14.8 33.7
H-NiMo-120–400 1.83 62.7 21.0 16.3 37.3
H-NiMo-150–400 1.80 57.7 23.4 18.9 42.3
H-NiMo-180–400 1.84 63.7 20.8 15.5 36.3
H-NiMo-200–400 1.89 68.1 18.0 13.9 31.9
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The Ni 2p3/2 spectra and corresponding deconvolution curves of sulfided NiMo catalysts at different hydrothermal treatment temperatures are shown in Figure 5. There is about 854.1 eV binding energy for the Ni 2p3/2 peak for the Ni–Mo–S active phase, 853.2 eV for NixSy, and 856.7 eV for oxidized Ni2+ species.1,61 The peaks observed at an energy level of 862.2 eV are attributed to the shakeup satellite structures originating from the Ni 2p3/2 orbital.1 Many studies concluded that NixSy in the catalyst is less active in the HDS reaction,62−64 but the presence of a certain amount of NixSy can take a synergistic effect with MoS2, making the hydrogen spillover effect more effective and thus improving the HYD selectivity of DBT.59,65−68 The proportion of active species at different hydrothermal treatment temperatures is shown in Table 3. The relative proportions of active species (Ni–Mo–S + NixSy) decrease in the following order: H-NiMo-150–400 > H-NiMo-120–400 > H-NiMo-180–400 > H-NiMo-90–400 > H-NiMo-200–400.

Figure 5.

Figure 5

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Ni 2p3/2 spectra and corresponding deconvoluted curves of sulfided NiMo catalysts obtained at different hydrothermal treatment temperatures.

3.2. Effects of Different Calcination Conditions (Temperature and Atmosphere) on the Structure of Sulfided NiMo Catalysts

3.2.1. XRD Analysis

Calcination is a key process for NiCO3·2Ni (OH)2·4H2O to decompose and interact with MoS2 to form Ni–Mo–S and NixSy. The optimum hydrothermal treatment temperature was selected to study the effects of different calcination temperatures and calcination atmospheres on the catalyst structure. The XRD patterns of the sulfided NiMo catalysts with different calcination temperatures and calcination atmospheres are shown in Figure 6. The XRD spectra of the H-NiMo-150–350 sample calcinated at 350 °C in a H2 atmosphere showed only the characteristic peaks of MoS2, but the characteristic peaks of Ni3S2 appeared at 2θ = 44.4 and 50.2° for H-NiMo-150–400. The presence of Ni3S2 in the H-NiMo-150–400 sample was able to prevent the aggregation of MoS2, and then, the intensity of the MoS2 characteristic peak was reduced. The spectra of H-NiMo-150–500 and H-NiMo-150–600 exhibit a greater number of Ni3S2 peaks at 2θ = 21.8, 31.1, 37.8, 54.6, and 55.3°, along with a continuous increase in the intensities of these characteristic peaks from a calcination temperature of 400 to 600 °C. This phenomenon can be attributed to the reduction in edge sites caused by the sintering of MoS2 at higher calcination temperatures. Consequently, the availability of Ni for accommodation at the MoS2 edge to form the Ni–Mo–S phase decreases, leading to the generation of thermodynamically stabilized Ni3S2 in a calcined environment. This finding aligns with previous research.69−71 Moreover, the diffraction peak at 2θ = 51.9° in the spectrum of the H-NiMo-150–600 sample was attributed to Ni (JCPDS No. 87–0712), which can be ascribed to the reduction of Ni species (NiO or NixSy) in the catalyst when the calcination temperature was increased to 600 °C in a atmosphere. Unlike the catalyst samples prepared by calcination in a H2 atmosphere, the XRD spectra of the catalyst N-NiMo-150–400 samples prepared by calcination of the catalyst precursor at 400 °C in a N2 atmosphere do not exhibit the diffraction peak of Ni3S2 but exhibit the characteristic peaks of NiO at 2θ = 37.2 and 62.8°. When the catalyst precursor was calcined in a N2 atmosphere, NiCO3·2Ni(OH)2·4H2O adsorbed on the surface of MoS2 decomposed into NiO.72,73 At the same time, the decomposition of NiCO3·2Ni(OH)2·4H2O into NiO still exists during calcination in a H2 atmosphere, but the process was also accompanied by the activation on some S in MoS2 by H274 and the activated S combined with Ni species to produce Ni3S2.

Figure 6.

Figure 6

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XRD patterns of sulfided NiMo catalysts obtained at different calcination temperatures and calcination atmospheres.

3.2.2. XPS Analysis

XPS was used to further characterize the chemical state of Ni species on the surface of the catalysts prepared at different calcination temperatures and calcination atmospheres, and the XPS fitted spectra of Ni species are shown in Figure 7. The active components Ni–Mo–S and NixSy of the catalysts calcined in a H2 atmosphere were at 854.1 and 853.2 eV,1,61 respectively, while the Ni 2p3/2 fitted spectra of the catalyst N-NiMo-150–400 calcined in a N2 atmosphere contained only a small fraction of the Ni–Mo–S peaks. The proportions of different Ni species for the catalysts with different calcination temperatures and calcination atmospheres are shown in Table 4. As can be seen in Table 4, the percentages of active catalyst components (Ni–Mo–S and NixSy) obtained by calcination in a H2 atmosphere showed a trend of increasing and then decreasing with the increased calcination temperature, reaching a maximum at a calcination temperature of 400 °C. According to the widely accepted Ni–Mo–S model,75,76 Ni replaces some of Mo at the edges of MoS2. When the calcination temperature is 350 °C, the incomplete decomposition of Ni salts and the limited activation reaction of H2 on S in MoS2 limits the Ni substitution of Mo at the edges of MoS2 so that the Ni–Mo–S and NixSy percentages in the H-NiMo-150–350 sample are smaller than those in the H-NiMo-150–400 sample. With the progressive increase in calcination temperature under a H2 atmosphere, the sintering of MoS2 intensifies, resulting in a reduction of its edge sites that can accommodate Ni and facilitate the formation of Ni–Mo–S. Additionally, the elevated temperature leads to the segregation of Ni species from MoS2, further generating Ni3S2.69−71 Consequently, the NixSy/Ni–Mo–S ratios increase as the calcination temperature increases. When the catalyst precursor was calcined in a N2 atmosphere, the Ni salt adsorbed on the MoS2 surface interacted with MoS2 during decomposition, causing Ni to partially replace Mo at the edge of MoS2 to form a Ni–Mo–S structure. The irreducible N2 could not activate the S in MoS2 to react with the decomposed Ni salt to form NixSy, so only a small amount of Ni–Mo–S was formed in the N-NiMo-150–400 sample.

Figure 7.

Figure 7

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Ni 2p3/2 spectra and corresponding deconvoluted curves of sulfided NiMo catalysts obtained at different calcination temperatures and calcination atmospheres.

Table 4. Percentage of Ni Species for Different Calcination Temperatures and Calcination Atmospheres.
catalysts Ni2+, % Ni–Mo–S, % NixSy, % Ni–Mo–S + NixSy, %
H-NiMo-150–350 77.3 14.7 8.0 22.7
H-NiMo-150–400 57.7 23.4 18.9 42.3
H-NiMo-150–500 78.0 8.4 13.6 22.0
H-NiMo-150–600 82.1 5.7 12.2 17.9
N-NiMo-150–400 90.9 9.1 0 9.1
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3.3. Catalytic Activity and Selectivity

DBT was used as a reactant to evaluate the HDS catalytic performance of samples. The relationship between the conversion and reaction temperature of DBT for the reaction on different catalysts is shown in Figure 8. The conversion of DBT increased with an increase of the reaction temperature. The conversion of DBT over sulfided NiMo catalysts was much higher than that of the commercial bulk molybdenum disulfide (b-MoS2) sample at the same reaction temperature, which was attributed to the addition of the promoter Ni that could change the electronic properties of the active center and form a new Ni–Mo–S active phase, thus improving the HDS catalytic activity.29,32 In addition, the conversion of DBT reacted on other catalysts except the catalyst N-NiMo-150–400 obtained by calcination in a N2 atmosphere was almost linear with increasing temperature. According to the results of XRD (Figure 6) and XPS (Figure 7), the existing state of Ni species in the catalyst N-NiMo-150–400 is mainly NiO, and DBT acts as a sulfiding agent to further sulfide NiO into NixSy and Ni–Mo–S active phases under reaction conditions, and the fitted spectrum of Ni 2p3/2 of the reacted N-NiMo-150–400 sample is shown in Figure 9. Therefore, the conversion of DBT in N-NiMo-150–400 was significantly increased at the reaction temperature of 300 °C.

Figure 8.

Figure 8

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Temperature dependence of the DBT conversion on different catalyst samples.

Figure 9.

Figure 9

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Ni 2p3/2 spectra and corresponding deconvoluted curves of N-NiMo-150–400 after the reaction.

The order of activity for the sulfided NiMo catalysts, which were subjected to calcination under a hydrogen atmosphere, can be observed in Figure 7. The ranking, from highest to lowest, is as follows: H-NiMo-150–400, H-NiMo-120–400, H-NiMo-180–400, H-NiMo-90–400, H-NiMo-200–400, H-NiMo-150–350, H-NiMo-150–500, and H-NiMo-150–600. The catalyst activity was found to be influenced by several key factors, including the quantity of active sites in MoS2, the presence of sulfur vacancies, and the concentration of active components (Ni–Mo–S and NixSy) within the catalyst.5,29,40 As the hydrothermal treatment temperature increases, the lateral size, the number of stacked layers, and S/Mo atomic ratio of MoS2 in the catalyst first decrease and then increase (Tables 1, 2, and 3), reaching a minimum when the hydrothermal treatment temperature is 150 °C, i.e., in the following order: H-NiMo-150–400 < H-NiMo-120–400 < H-NiMo-180–400 < H-NiMo-90–400 < H-NiMo-200–400. The smaller lateral size of MoS2 and the reduced number of stacked layers are more favorable for the exposed active sites.2,77 Additionally, these factors promote the formation of a higher proportion of Ni–Mo–S and NixSy during the calcination process, thereby contributing to the improved activity of the catalyst.78,79 At the same time, the smaller S/Mo ratio indicates the higher number of S vacancies in the catalyst,80−82 which helps to improve the activity of the DBT reaction on the catalyst. Therefore, the order of activity of the DBT reaction on the catalyst is consistent with the results of XRD (Figure 1 and Table 1), TEM (Figure 2 and Table 1), ERP (Figure 3), SEM-EDS (Table 2), and XPS (Figure 4 and Table 3) characterization. Due to the incomplete decomposition of Ni salts83,84 and the limited activation of H2 and S in MoS2,32 the content of active components in the H-NiMo-150–350 sample was lower than that in H-NiMo-150–400 (Table 4), which made the conversion rate of DBT in H-NiMo-150–350 lower than that in H-NiMo-150–400. As the calcination temperature continued to increase, the Ni–Mo–S structure began to decompose continuously to form Ni3S2 and to detach from MoS2,78,85 which not only made the aggregation of MoS2 and Ni3S2 increase (Figure 6) but also lead to the proportion of active components decrease (Table 4), resulting in lower conversion of DBT on the H-NiMo-150–500 and H-NiMo-150–600 samples than on the H-NiMo-150–400 sample.

The selectivity of DBT over different catalysts at a reaction temperature of 280 °C is shown in Table 5, which indicates that the HYD/DDS ratios of DBT are similar over catalysts prepared at different hydrothermal treatment temperatures. Interestingly, the HYD pathway selectivity of DBT on the catalysts increased with increasing calcination temperature in a H2 atmosphere at the same hydrothermal treatment temperature and even the selectivity of the HYD pathway of DBT over that of the DDS pathway appeared in the H-NiMo-150–600 catalyst samples. This result is due to the decomposition of part of the Ni–Mo–S structure into Ni3S2 when H-NiMo-150–500 and H-NiMo-150–600 are calcined in a H2 atmosphere, which leads to an increase in the NixSy/Ni–Mo–S ratio. The mutual contact between NixSy and MoS2 plates partially limits the accessibility of S vacancies in MoS2 plates, making the DBT DDS reaction pathway limited.65,86 Meanwhile, the hydrogen spillover phenomenon between NixSy and MoS2 makes the catalyst more selective for the HYD pathway.65 In contrast to other catalyst samples, DBT reacted with the highest HYD/DDS of 1.34 on the N-NiMo-150–400 sample prepared by calcination in a N2 atmosphere. On the one hand, due to the limited amount of S activation of MoS2 edge and corner sites in a N2 atmosphere, which means that a very small amount of sulfur vacancies is generated, this is a prerequisite for DBT to carry out the DDS reaction pathway. On the other hand, the MoS2 edge and corner active sites are covered by NiO obtained by decomposition, and under the reaction conditions, DBT acts as a vulcanizing agent to convert more NiO into NixSy (Figure 9), and the generation of NixSy facilitates the selectivity of the HYD pathway.

Table 5. Reaction Rate Constants and Selectivities of Different Catalysts.

catalysts SaHYD (%) SaDDS (%) HYD/DDS
b-MoS2 23.67 76.33 0.31
H-NiMo-90–400 26.46 73.54 0.36
H-NiMo-120–400 26.93 74.07 0.35
H-NiMo-150–400 25.38 74.62 0.34
H-NiMo-180–400 26.93 74.07 0.35
H-NiMo-200–400 26.46 73.54 0.36
H-NiMo-150–350 23.31 76.69 0.32
H-NiMo-150–500 46.23 53.77 0.86
H-NiMo-150–600 52.82 47.18 1.12
N-NiMo-150–400 57.23 42.77 1.34
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a

Calculated at a reaction temperature of 280 °C.

4. Conclusions

In summary, the hydrothermal treatment of commercial bulk MoS2 and alkaline nickel carbonate, followed by calcination in either a H2 or a N2 atmosphere, yielded sulfided HDS catalysts with notable activity. The findings from characterization and HDS activity assessment indicated that the lateral size, number of stacked layers, and S/Mo atomic ratio of MoS2 within the catalysts exhibited a decreasing trend, followed by an increasing trend as the hydrothermal treatment temperature increased. The optimal values for these parameters were achieved at a hydrothermal treatment temperature of 150 °C. Furthermore, the impact of various calcination temperatures on the catalyst structure was examined at the optimal hydrothermal treatment temperature. When subjected to a H2 atmosphere, the MoS2 plates underwent initial destruction and subsequent aggregation as the calcination temperature increased. Notably, the catalyst prepared at a calcination temperature of 400 °C exhibited the most favorable structure. Consequently, DBT demonstrated superior hydrodesulfurization (HDS) activity over H-NiMo-150–400. Additionally, it was further substantiated that the inclusion of NixSy contributed to an enhancement in the hydrogenation (HYD) selectivity of the catalysts, as evidenced by the HYD/DDS ratios of DBT in H-NiMo-150–500 and H-NiMo-150–600. This study not only enhances our comprehension of the formation mechanism of the Ni–Mo–S active phase in the HDS reaction and the impact of catalyst preparation conditions on its structure but also offers valuable insights for the advancement and implementation of novel, ecofriendly, and cost-effective catalysts for sulfide hydrogenation.

Acknowledgments

The authorsare delighted to express our gratitude for the invaluable support extended to this research by the National Key Research and Development Project (Grant No. 2021YFA1501204) and the China Petrochemical Corporation (Sinopec Group 121043-2).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c04059.

  • Effects of hydrothermal treatment and H2 atmosphere calcination on the structure of the catalyst: XRD analysis results, HRTEM analysis results, XPS analysis results and activity evaluation results of hydrothermal treatment only, calcination only and hydrothermal treatment before calcination; SEM-EDS and STEM-EDS analysis results of H-NiMo-150–400; Mo 3d spectra for different calcination conditions; and correlation between Ni–Mo–S content and activity (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c04059_si_001.pdf (1.1MB, pdf)

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