Performance Exploration of Ni-Doped MoS₂ in CO₂ Hydrogenation to Methanol

Abstract

The preparation of methanol chemicals through CO₂ and H₂ gas is a positive measure to achieve carbon neutrality. However, developing catalysts with high selectivity remains a challenge due to the irreversible side reaction of reverse water gas shift (RWGS), and the low-temperature characteristics of CO₂ hydrogenation to methanol. In-plane sulfur vacancies of MoS₂ can be the catalytic active sites for CH₃OH formation, but the edge vacancies are more inclined to the occurrence of methane. Therefore, MoS₂ and a series of MoS₂/Ni_x and MoS₂/Co_x catalysts doped with different amounts are prepared by a hydrothermal method. A variety of microscopic characterizations indicate that Ni and Co doping can form NiS₂ and CoS₂, the existence of these substances can prevent CO₂ and H₂ from contacting the edge S vacancies of MoS₂, and the selectivity of the main product is improved. DFT calculation illustrates that the larger range of orbital hybridization between Ni and MoS₂ leads to CO₂ activation and the active hydrogen is more prone to surface migration. Under optimized preparation conditions, MoS₂/Ni_0.2 exhibits relatively good methanol selectivity. Therefore, this strategy of improving methanol selectivity through metal doping has reference significance for the subsequent research and development of such catalysts.

1. Introduction

In the process of rapid human development, the excessive emission of carbon dioxide has led to a series of natural disasters such as global warming, continuous rise in sea levels, agricultural production reduction, and growing ocean acidification [1,2,3]. It is noteworthy that CO₂ can be described as a cheap and abundant C1 material if carbon dioxide is captured by a reasonable means and formed into chemical products with high industrial application value (methanol, aromatics, etc.) [4,5,6]. Among the abundant chemicals mentioned above, methanol is the preferred choice for the direct catalytic reaction of CO₂ gas to oxygenates because it can be considered an excellent precursor for some chemicals and a fuel substitute [7,8]. The efficient catalysis of CO₂ to CH₃OH is exothermic, and low temperatures favor equilibrium shifts in the product direction. However, the activation of the C–O inert bond in CO₂ requires a high temperature [9]. Therefore, considering the thermodynamic and kinetic contradictions of this reaction, it is extremely important and urgent to prepare a reasonable catalyst for the conversion of CO₂ and hydrogen into methanol under low-temperature conditions.

The catalyst systems mainly include Cu-based metal catalysts [10,11,12], precious metal catalysts [13,14], and metal oxide catalysts [15,16,17]. As is well known, with the birth of the industrialized Cu/ZnO/Al₂O₃ catalyst, copper-based catalysts are the most mature and deeply explored in the research field of CO₂ to methanol. However, the active phase is poisoned and inactivated due to the H₂O generated by the competitive side reaction of RWGS as well as the sulfur contained in the feed gas, which ultimately leads to poor stability. Precious metal catalysts are not suitable for large-scale applications due to their high price. Therefore, it is urgent to explore relevant catalysts with high conversion rates and high stability suitable for CO₂ to methanol under a low-temperature reaction environment.

As a two-dimensional layered material similar to graphene, the surface atoms of MoS₂ are almost completely exposed, so the atom utilization rate is greatly improved. The modifiability is strong, and the physical compatibility is high. There is a strong covalent bond between sulfur and molybdenum atoms, and the structure is stable. The H₂ adsorption on the MoS₂ surface is irreversible at low temperatures, indicating that MoS₂ has a certain hydrogen storage capacity. Because of its structural advantages, MoS₂ shows good chemical properties, so it has been widely applied in various catalytic areas such as electrochemistry, photochemistry, and thermochemistry. Combined with the structural similarity and strong conductivity of graphene, MoS₂ supported on graphene shows better catalytic properties for CO₂ to methane, and the selectivity of the main product is 95% [18,19]. The catalytic mechanism and performance of the Mo₆S₈ cluster for the chemical reaction between CO₂ and H₂ to obtain CH₃OH are explored by density functional theory (DFT). Calculation results prove that the C–O cleavage contained in H_xCO intermediates can be effectively promoted by MoS₂ [20]. Thus, catalyzed by MoS₂, the high selectivity of the methanol is explained. It is found that the CO₂ conversion is 12.5% at a low temperature of 180 °C catalyzed by the novel MoS₂. The selectivity to methanol is high (94.3%), and the stability is maintained for 3000 h without inactivation [21]. The study shows that the sulfur vacancies contained in the basal plane are conducive to methanol formation, while the edge sulfur vacancies are the active centers for the excessive hydrogenation of methane. From this perspective, edge S vacancies are meaningful for describing the CH₃OH selectivity, and minimizing the generation of sulfur vacancies at edge locations will be a promising strategy for improving the methanol selectivity catalyzed by MoS₂.

Metal doping is an effective and feasible method for improving the performance of catalysts for various catalytic reactions. In addition to being directly used as a reasonable catalyst for producing methanol from CO₂ gas, MoS₂ can also be used as a single-atom support. The Pt monomer supported by MoS₂ that facilitates the methanol selectivity is 95.4% because of the synergistic impact between molybdenum disulfide and adjacent Pt monomers [22]. DFT analysis shows that the Co adatom-induced interstitial states play a major role in the breakage of CO₂ into the product methanol. Therefore, it is theoretically inferred that a single Co atom supported on a monolayer of molybdenum disulfide is an excellent catalyst for catalyzing carbon dioxide into methanol. This work proves, from the perspective of theoretical knowledge, that single-atom catalysts are supported on MoS₂ [23]. Encouraged by these reported results, herein, a kind of nanoscale MoS₂/Ni and MoS₂/Co catalysts with different proportions are prepared by hydrothermal method. Experimental exploration shows that the atomic doping of Ni and Co causes the structures of MoS₂ monolayers to be confined with NiS₂ and CoS₂ that are formed by Ni, Co, and S, respectively. The embedded NiS₂ and CoS₂ can prevent the CO₂ and H₂ from approaching the edge sulfur vacancy, so the CH₃OH selectivity is improved. As an experimental result, the optimal catalysis MoS₂/Ni_0.2 achieved a good methanol selectivity of 83.7%. Therefore, the methanol selectivity can be improved by the metal doping strategy.

2. Results and Discussions

2.1. Exploration of Various Characterization Results

As depicted in Figure 1, the crystalline phase structure of pure MoS₂ and Ni-doped samples is confirmed by XRD patterns. The pure MoS₂ catalyst exhibits a stable hexagonal 2H-type crystal structure when the metal Ni is undoped, and the individual diffraction peaks corresponding to the (002) (100) (103) (110) crystal planes of PDF#89-1495 are visible. All illustrations are presented in the form of broad reflections of lower intensity, indicating the poor crystallinity of the coherent scattering in the nanometer range. Ni doping makes the (002) crystal plane of MoS₂ broaden obviously, and the diffraction peak related to Ni is not observed, indicating that the excellent dispersibility of Ni doping is conducive to the reduction of catalyst grain size. The reduction of layers number in the MoS₂ stack after metal doping can be explained by this important result [24]. In other words, the growth of MoS₂ grains can be effectively suppressed by metal doping. As the doping amount of Ni increases to 0.2 mmol, the diffraction peak of MoS₂ is enhanced, and a peak of NiS₂ appears, which is because of the rapid binding of Ni ions with S²⁻ that is produced by the decomposition of thiourea. With the further increase in Ni doping amount, the sharp diffraction peaks and smaller half widths of NiS₂ and MoS₂ indicate that their crystallinities become stronger and the grain sizes are increased. Different from Ni doping, because the electronegativity of the Co element is weaker than Ni in the same period, the chemical forces between Co and MoS₂ are relatively weak, resulting in a worse dispersibility of Co doping than Ni, so the (002) crystal plane of MoS₂ is higher than Ni doping (Figure S1).

To further demonstrate the apparent structures of pure MoS₂ and the new catalysts formed by metal doping, scanning electronic microscopy (SEM) is analyzed. In MoS₂ structure, six S atoms are tightly packed up and down to form a sixfold triangular prism, and the triangular prism voids are formed. Mo atoms are filled into these voids, resulting in a two-dimensional sheet. Monolayers of 2D flakes can stack through van der Waals forces and weak coordination bonds and form different types of interlayer voids. Therefore, as shown in Figure 2a,b, MoS₂ is composed of abundant lamellar nanoflowers, and there are certain gaps between the layers, which is beneficial for deep adsorption of carbon dioxide gas and hydrogen with the catalyst. As depicted in Figure 2c, compared to pure MoS₂, a small amount of doped Ni ions is quickly combined with the S²⁻ that is generated by the decomposition of thiourea to form larger NiS₂ particle cubes, with a trend of concave surfaces along the centerline. As the reaction progresses, the raw materials continue to react with these large cubes, forming newly exposed surfaces from the centerline of the cube surface and gradually extending towards the interior of the cube, so the smaller NiS₂ volume cube particles are formed and separated.

When the Ni doping amount increases to 0.2 mmol, only irregular small particles are observed and are just filled at the outer edge of the MoS₂ layer, resulting in an increasing in interlayer spacing and a reduction of layers number, so the particles are reduced (Figure 2d), which are consistent with the XRD results. On the one hand, the stacking height and voids are increased, and the multilayer stacking structure is more stable [25]. On the other hand, the benign increase in interlayer spacing is conducive to the deep adsorption of carbon dioxide and hydrogen molecules between the layers, the location of NiS₂ leads to the exposure of less edge S vacancies, and, finally, the CH₃OH selectivity is effectively promoted. With further doping of Ni, the sample exhibits unseparated cubic blocks or irregular particles, a large amount of NiS₂ is accumulated and covers the surface of MoS₂, and the CH₃OH selectivity is reduced (Figure 2e,f). Therefore, the formation of NiS₂ particles is an anti-Ostwald ripening evolution model of large particle splitting and small particle growth separation [26]. An appropriate amount of metal doping can effectively improve the CH₃OH selectivity. On the contrary, excessive metal addition will cause a large area of NiS₂ accumulation in the material surface layer, which will affect the catalytic performance. Similar situations have also occurred with Co doping (Figure S2).

The chemical states of elements of the best-performing MoS₂/Ni_0.2 catalyst are measured through X-ray photoelectron spectroscopy (XPS) for further study. The XPS measured spectra are shown in Figure 3, and all surface composition values are obtained from the spectral results. As displayed in Figure 3a, the Mo, S ratio in MoS₂ conforms to the chemical formula, while the ratio of these two elements in MoS₂/Ni_0.2 is lower than the chemical formula. The spectrum of Ni 2p is composed of spin-orbit peaks at 853.2 eV and 870.8 eV of Ni 2p_3/2 and Ni 2p_1/2, respectively. An oscillatory satellite peak occurs from the Ni element, displaying the presence of Ni²⁺ (Figure 3b) [27]. The XPS spectrum related to Mo 3d region is displayed in Figure 3c. The binding energies at 232.0 eV and 229.1 eV are attributed to Mo 3d_3/2 and Mo 3d_5/2, respectively, proving that Mo appears with Mo⁴⁺ valence state in MoS₂/Ni_0.2. The characteristic peak at 226.1 eV corresponds to S 2s. This phenomenon is in agreement with the spectral phenomena of the Mo 3d region of molybdenum disulfide without Ni doping, which proves that the doping of Ni does not result in a fundamental change in the valence state of molybdenum and the successful formation of MoS₂ in MoS₂/Ni_0.2 [28]. It is worth mentioning that the appearance of the Ni–Mo chemical bond confirms that there is a strong chemical force between MoS₂ surfaces and Ni in the MoS₂/Ni_0.2 catalyst, which may weaken the Mo–S bond. XPS shows two strong S 2p peaks corresponding to binding energies at 163.2 eV as well as 161.8 eV for the S 2p_1/2 and S 2p_3/2 binding energies of S²⁻ in the MoS₂/Ni_0.2 sample [29,30,31], which is attributed to the low surface coordination of sulfide ions and the lattice S of metal–sulfur bonds in NiS₂ (Figure 3d) [32]. Therefore, the doping of Ni leads to the charge-density redistribution, the density of edge S vacancies is decreased, and the CH₃OH selectivity is improved [33].

The chemical adsorption and effective catalysis of carbon dioxide are related to the alkalinity of the MoS₂ surface [34,35]. To further explore the role of doped metals in the CO₂ adsorption characteristics, the CO₂–TPD results of MoS₂/Ni_x (x = 0.1, 0.2, 0.3, 0.5) are shown in Figure 4. By comparing the desorption peak areas and peak positions of the four different Ni-doped catalysts, these materials have a certain adsorption capacity for acidic CO₂ molecules. The CO₂–TPD curve of MoS₂/Ni_x (x = 0.3, 0.5) catalysts mainly contain medium-strength alkaline sites. The MoS₂/Ni_x (x = 0.1, 0.2) catalyst exhibits three CO₂ desorption regions, and the MoS₂/Ni_0.2 has the strongest alkaline site. The strong alkaline sites are closely related to methanol selectivity, which may lead to the MoS₂/Ni_0.2 catalyst having the best methanol selectivity [36]. As the amount of Ni added further increases, the desorption peak slowly becomes flat and wider, indicating that the bonding ability between CO₂ and the catalyst is gradually weakened, and the catalytic performance decreased accordingly. This result indicates that an appropriate amount of Ni is doped on MoS₂, with the effective acid–base interaction between the basic S–Mo and S–Ni functional groups of the catalyst and the acidic CO₂ molecules. In addition, the dipole–dipole chemical interaction between carbon dioxide and the polar sites related to S functional groups also plays a key role in the adsorption level of CO₂ gas, which has a positive effect on improving product selectivity [37].

However, excessive Ni doping results in ineffective contact between CO₂ and catalyst sites. Interestingly, the peak shapes and peak areas of materials are also different with different Ni doping amounts, indicating that the CO₂ adsorption is regulated by both the active component content and the MoS₂ support. Similarly, the CO₂–TPD curves of MoS₂/Co catalysts prepared under the same conditions with different Co doping levels are shown in Figure S3. From the comparison chart of the adsorption level of the CO₂ molecule, it can be seen that the four samples all have the corresponding desorption peaks in the intermediate and high-temperature regions. Although the strong alkaline site of MoS₂/Co_0.2 moves towards the high-temperature zone, the medium alkaline site moves towards the low-temperature zone. Thus, through CO₂–TPD analysis, it can be deduced that MoS₂/Ni_0.2 is an ideal catalyst for carbon dioxide hydrogenation to methanol, which must be confirmed by activity evaluation.

The physical adsorption of hydrogen molecules on the catalyst surface is replaced by effective chemical adsorption at a certain temperature. Therefore, as shown in Figure 5, the reduction performance of the catalyst is studied through H₂–TPR experiments. The active hydrogen rapidly migrates between adjacent S atoms to form H–S–H and the Mo or S edge is gradually reduced through “dissociation diffusion” [38,39]. Because of different particle shapes and dispersion levels in MoS₂, different Ni doping differs in adsorption temperature and intensity. For MoS₂/Ni_0.1, one main peak appears at 324 °C [40]. With the suitable increase in metal doping, the double active sites are gradually enhanced, and the reduction energy consumption decreases, so that the reduction peaks of H₂–TPR shift to the low-temperature direction. After the Ni doping amount is increased to 0.2 mmol, two well-separated peaks appear at 282 °C and 362 °C; the first peak significantly moves to lower reduction temperatures, and MoS₂/Ni_0.2 has the lowest reduction temperature, indicating that the metal sulfur bonds energy is reduced.

Combining XRD and SEM analysis, it is found that the strong interaction between doped metals and MoS₂ is conducive to the dispersion of the catalyst and the reduction of catalyst particle size. The increased interlayer spacing of MoS₂ results in an increase in Ni/Mo edge site. Hydrogen participates in the surface reaction, and the sulfur atom is reductively removed, resulting in an irreversible change of MoS₂ structure. Thus, the Mo–S bond strength is weakened, the reducibility of MoS₂ is enhanced, and a good reaction basis for the carbon dioxide hydrogen to methanol is provided [40]. The doping amount of Ni increased to 0.3 and 0.5 mmol, and the excessive and irregular accumulation of NiS₂ is covered on the MoS₂ surface. The reduction temperature is increased, and the reduction ability of the catalyst is weakened. Compared with MoS₂/Co_x catalysts, the MoS₂/Co_0.2 catalyst also has a good downward shift of the first peak position, but the reduction temperature is still higher than MoS₂/Ni_0.2 (Figure S4). Thus, the catalytic performance of Co-doped MoS₂ may be lower than that of MoS₂/Ni_X.

The specific surface area and pore size distribution of the material play an important role in the performance of the catalyst. The MoS₂/Ni catalyst with Ni doping amount of 0.2 mmol and MoS₂ catalyst are analyzed by N₂ physical adsorption and desorption technology. The N₂ adsorption and desorption trend and pore size distribution characteristics are listed in Figure 6 and

Table 1. Measurement results of pore structure parameters and specific surface area.

Sample	BET Specific Surface Area (m2·g−1)	Surface Pore Diameter (nm)	Pore Volume (cm3·g−1)
MoS2	41.70	20.65	0.214
MoS2/Ni0.2	40.73	10.13	0.090

. The adsorption isotherms of the MoS₂ catalyst and MoS₂/Ni_0.2 catalyst belong to class VI isotherms. According to the standard classification of IUPAC, the characteristics of such isotherms reflect the characteristics of mesoporous materials (Figure 6a,b). When the relative pressure of P/P₀ of MoS₂ is 0.45~1.0, and the relative pressure of P/P₀ of MoS₂/Ni_0.2 is 0.8~1.0, the adsorption capacity is improved. The pore size distribution ranges of the involved materials are shown in Figure 6c,d, which are obtained from the analysis of the pore size distribution curve. The highest point corresponding to the pore radius is greater than 2 nm, which further proves that both MoS₂ and MoS₂/Ni_0.2 catalysts are mesoporous materials. Compared with MoS₂, the MoS₂/Ni_0.2 catalyst exhibits a relatively smaller specific pore diameter as well as pore volume. It can be seen that the excellent CH₃OH selectivity of MoS₂/Ni_0.2 may be due to its relatively small pore volume (0.090 cm³·g⁻¹) and pore size (10.13 nm).

2.2. Performance of CO₂ Feed Gas to CH₃OH Conversion Catalyzed by MoS₂, MoS₂/Ni_x, and MoS₂/Co_x

Under the condition settings of 260 °C, 5 MPa, and 12,000 mL h⁻¹ g_cat⁻¹ of gas hourly space velocity (GHSV), all catalytic evaluations are performed by a fixed-bed reactor, and the experimental results after testing are listed in Figure 7. As a comparative catalyst, the property of 2H-type MoS₂ is studied first. The result shows that the CO₂ conversion rate is 3.36% (Figure S5) and the methanol selectivity is 11.34%, and the main byproducts are methane and carbon monoxide. The catalytic characteristic of MoS₂ for CO₂ to product is well demonstrated by the experimental results, but the conversion and selectivity are expected to be improved. With the increase in Ni and Co doping amount, the CH₃OH selectivity is also increased. When the Ni and Co doping amount is 0.2 nmol, the CH₃OH of the catalyst MoS₂/Ni_0.2 and MoS₂/Co_0.2 reach the highest 83.73% and 73.82% (Figure 7). However, when the doping amount is further increased from 0.2 mmol to 0.3 mmol, the methanol selectivity shows a downward trend, because the appropriate proportion of Ni and Co addition leads to the blocking effect of edge S vacancies. On the contrary, excessive Ni and Co metals are doped, a large amount of NiS₂ and CoS₂ are stacked and covered on the reaction surface of the catalyst, the exposed sulfur vacancy active sites are blocked, and the catalytic performance is reduced. By comparing the CH₃OH selectivity of Ni and Co-doped catalysts, it is determined that the catalytic property of the Ni-doped MoS₂ catalyst slightly surpasses the Co-doped MoS₂ catalyst.

2.3. Intermediates and Mechanisms Properties Involved in CO₂ Hydrogenation to Methanol

2.3.1. Determination of Intermediates through In Situ Infrared Spectroscopy

The key adsorbents and intermediates included in the formation of methanol on the MoS₂/Ni_0.2 catalyst with the best performance are shown in Figure 8. The characteristic peak of CO₂ gas appears at approximately 2350 cm⁻¹ [41]. The peaks at 720 cm⁻¹ and 907 cm⁻¹ are attributed to O* characteristic and the peak intensity is obvious [21]. The vibration peak at 1080 cm⁻¹ is attributed to the methoxy group (CH₃O*) [42]. In situ DRIFTS experimental results show that the chemical bond of CO₂ adsorbed on the MoS₂/Ni_0.2 surface is broken and forms CO* and O*, then CO* and dissociated H* react to form HCO* species. The generation of methoxy and methanol is due to subsequent gradual hydrogenation.

2.3.2. Determination of Mechanisms Properties through DFT Calculation

To provide a deeper explanation of metal doping in improving methanol selectivity, the reaction mechanisms of MoS₂ and Ni/MoS₂ are investigated by DFT theoretical calculations. The chemical adsorption of H₂ and CO₂ on the catalyst surface is determined as a prerequisite for subsequent reactions. Figure 9a shows the optimal adsorption structure for CO₂ on the catalyst. The CO₂ angle is from 180° to 120.20° and the O–C bond is elongated from 1.25 Å to 1.38 Å. The corresponding differential charge density (Figure 9b) confirms that Mo electrons are transferred to the adsorbed CO₂. The corresponding PDOS analysis shows that the CO₂ molecular orbital is not only shifted to a lower energy level after being adsorbed on the catalyst surface (Figure 9c), but also overlaps with the Mo 4d state after the CO₂ gas becomes an adsorbed state on the MoS₂ surface, proving the CO₂ activation. As shown in Figure 9e, Ni doping leads to more unsaturated exposure of Mo, and the formed active hydrogen is more prone to surface migration. The interaction between Ni and the charge-rich center S in NiS₂ gives it an appropriate affinity and bonding range. The local action of negative charge on the S atom and H₂ occurs at the highest spin density position. The activation process involves the transfer of negative charge from the S atom to the H₂ antibonding orbital σ*, the electron cloud distribution and orbital energy of the hydrogen molecule are changed, the four-center transition state is formed, the S–S and H–H bonds are weakened and heterocleaved, and the S–H bond is strengthened, which eventually leads to the breaking of the former and the formation of the latter. A larger range of orbital hybridization near the Fermi level (Figure 9f) makes the H₂ bond break more thoroughly (Figure 9d). Combined with in situ infrared spectroscopy, the effective dissociation of hydrogen gas is conducive to the gradual hydrogenation of CO*, so the methanol selectivity is more highly catalyzed by MoS₂/Ni_0.2.

The chemically adsorbed CO₂ is considered to have two reduction pathways: the formate pathway and CO hydrogenation. According to in situ DRIFTS spectra, CO₂ hydrogenation catalyzed by MoS₂/Ni tends to an oxidation–reduction pathway, and the optimal adsorption structures of different intermediates as well as barrier energy are shown in Figure 10. CO₂ and H₂ are directly dissociated into CO*, O*, and 2H* through the energy barrier of 0.63 eV. The large adsorption energy indicates that the interaction between CO* and MoS₂ is very strong (E_ads = −1.27 eV), and is more likely to undergo further reaction on the support without being released and forming CO gas. The calculation results indicate that CO* can be hydrogenated to CHO* by crossing only a potential barrier of 0.69 eV. It is evident that the carbon involved in CO is more susceptible to H proton attack than the oxygen atom, and forms CHO anchoring at the active site for subsequent hydrogenation reaction. The second hydrogenation step of CHO to CH₂O needs a reaction barrier of 1.22 eV. The CH₂O is further hydrogenated to CH₃O, which just needs a barrier energy of 0.79 eV, and this hydrogenation process is exothermic, indicating that the CH₃O is a beneficial intermediate. In the fourth hydrogenation reaction, CH₃O can be reduced to CH₃OH through an energy barrier of 1.33 eV (the decisive step of the reaction system) and easily desorbed from the catalyst surface. Therefore, based on the above mechanism exploration, methanol is the final product catalyzed by MoS₂/Ni. Finally, the reaction mechanism is summarized as CO₂* → CO* → CHO* → CH₂O* → CH₃O* → CH₃OH.

3. Material and Methods

3.1. Materials

All the materials contained in the experiments are shown in

Table 2. Experimental reagents and gas raw materials.

Chemical Reagent Name	Chemical Formula	Specification	Manufacturer
Cobalt nitrate hexahydrate	Co(NO3)2·6H2O	Analytical pure AR	Sinopharm group chemical reagent Co., Ltd. (Shanghai, China)
Nickel nitrate hexahydrate	Ni(NO3)2·6H2O	Analytical pure AR	Sinopharm group chemical reagent Co., Ltd.
Ammonium molybdate tetrahydrate	(NH4)6Mo7O24·4H2O	Analytical pure AR	Sinopharm group chemical reagent Co., Ltd.
Thiourea	CH4N2S	Analytical pure AR	Tianjin damao chemical reagent factory (Tianjin, China)
Quartz sand	SiO2	Analytical pure AR	Shanghai mclean biochemical technology Co., Ltd. (Shanghai, China)
Raw gas	H2/CO2/N2	72/24/4 (molar ratio)	Ningxia guangli comprehensive trading Co., Ltd. (Yinchuan, China)
Reducing gas	H2/N2	H2/N2 = 30/70	Beijing yanan weiye gas Co., Ltd. (Beijing, China)
High purity nitrogen	N2	99.999%	Ningxia guangli comprehensive Trading Co., Ltd.
High purity helium	He	99.999%	Ningxia guangli comprehensive trading Co., Ltd.
Air	Air	100%	air generator (Yinchuan, China)

.

3.2. Experimental Apparatus

All the instruments and types of equipment for experiments are shown in

Table 3. Instruments and types of equipment for experiments.

Equipment Name	Instrument Model	Manufacturer
Carbon dioxide catalytic conversion evaluation device	Φ240 × Φ22 × Φ510	Beijing Kunlun yongtai Technology Co., Ltd. (Beijing, China)
Gas chromatograph	Agilent GC8890	Agilent Technologies (China) Co., Ltd. (Beijing, China)
Electric heating constant temperature blast drying oven	GZX-9240MBE	Shanghai boxun industrial Co., Ltd. medical equipment factory (Shanghai, China)
Tube furnace	TL1200	Nanjing boyuntong instrument technology Co., Ltd. (Nanjing, China)
Desktop high-speed centrifuge	H/T16MM	Hunan hercy instrument equipment Co., Ltd. (Changsha, China)
Magnetic stirrer	DF-101S	Shandong juancheng hualu electric heating instrument Co., Ltd. (Heze, China)
Powder tablet press	35 mm	Hefei kejing material technology Co., Ltd. (Hefei, China)
Electronic balance	PL602	Mettler toledo Instruments Ltd. (Zurich, Switzerland)
Hydrogen generator	SPH-300	Beijing China HP analytical technology research institute (Beijing, China)
Air generator	SPB-3	Beijing China HP analytical technology research institute
Column (TCD)	HP-PLOT Q	Agilent Technologies (China) Co., Ltd. (Beijing, China)
Column (FID)	MolSieve 5A	Agilent Technologies (China) Co., Ltd.

.

3.3. Preparation of MoS₂, MoS₂/Ni, and MoS₂/Co Materials

MoS₂/Ni and MoS₂/Co are prepared by hydrothermal method. Firstly, 1 mmol ammonium molybdate tetrahydrate and thiourea (14 mmol) are transferred to distilled water (35 mL). To completely dissolve the reactants and form a homogeneous solution, the beaker containing the mixed reaction liquid should be placed on a magnetic suspension stirrer and stirred for 30 min. Different contents of nickel nitrate hexahydrate or cobalt nitrate hexahydrate are weighed and placed into the above solution to be completely dissolved. The thoroughly dissolved solution is poured into a hydrothermal kettle with Teflon-lined stainless steel, and the Teflon lid is tightly closed. The hydrothermal kettle is placed into a vacuum drying box, then reacted at 210 °C for 24 h. After the completion of catalyst preparation, the hydrothermal kettle is taken out and the temperature drops to cool down. The reacted substance is washed with deionized water 3 times and then washed again with anhydrous ethanol for the same process. Finally, the washed and centrifuged material is effectively dried by a drying oven, the drying temperature is 80 °C and the time is 12 h, and the final product sample is obtained. For experimental comparison, MoS₂ is prepared by dissolving ammonium molybdate tetrahydrate (1 mmol) and thiourea (14 mmol) using the same preparation process as MoS₂/Ni and MoS₂/Co.

3.4. Characterization Methods

The crystal phase composition of the material is determined by a German BmkerD8 advanced X-ray diffractometer (XRD). The radiation source is Cu K_α rays, the tube voltage is 40 kV, and the tube current is 40 mA. The scanning range is 3–85°, and the scanning speed is 8°/min.

The specific surface area and the pore size distribution of the materials, are characterized by a Quanta Autosorb IQ automatic physical and chemical adsorption instrument. The adsorbate is N₂, and the physical adsorption test is carried out under the condition of vacuum liquid nitrogen (−196 °C). Before the adsorption test, the samples needed to be degassed at a temperature of 200 °C for 6 h. The specific surface area is obtained by linear regression of the multipoint BET (Brunauer–Emmett–Teller) model, and the pore size distribution is obtained by the DFT model.

The microscopic morphology of the material is characterized by scanning electron microscopy (SEM) (Carl Zeiss Company, Jena, Germany) NanoPorts QUANTA250/QUANTA430.

The H₂–TPR test is carried out on a chemisorption apparatus with automatic temperature (AutoChem II 2920) from Micromeritics Company, Georgia, USA. A 50–100 mg sample is weighed and placed in reaction tubes, and the test temperature rises from 25 °C to 300 °C with a rate of 10 °C/min for drying pretreatment, then cooled to 50 °C under He gas flow (30–50 mL/min) for 1 h. A 10% H₂/Ar mixture is passed into the reaction tube with a fixed gas flow rate (30–50 mL/min) for 1 h to be saturated, and then switched to Ar with the same gas flow to eliminate weak physical adsorption of H₂. Finally, the tested catalyst is heated to 500 °C in Ar atmosphere (heating rate is 10 °C/min) and desorbed, and the desorbed gas is detected by TCD.

The CO₂–TPD test is carried out on a chemical adsorber (AutoChem II 2920) from Micromeritics, USA, which is a fully automatic temperature program. A 50–100 mg sample is weighed and placed in reaction tubes, and the test temperature is increased from 25 °C to 300 °C (heating rate 10 °C/min) for drying pretreatment, then cooled to 50 °C under He gas flow (30–50 mL/min) for 1 h. A 10% CO₂/He mixture is passed into the reaction tube at a certain gas flow rate is 30–50 mL/min for 1 h to be saturated, and then switched to He at a certain gas flow rate of 30–50 mL/min for 1 h to eliminate weak physical adsorption of carbon dioxide on the surface. The last step involves heating the catalyst to 700 °C at a heating rate (10 °C/min) in He atmosphere and desorbed, and the desorbed gas is detected by TCD.

A Thermo Scientific K-Alpha spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) used for the XPS test, and the content and valence state of catalyst surface substances are analyzed under the excitation source of Al Kα rays (hv = 1486.6 eV). A suitable amount of the pressed catalyst is connected to the corresponding sample tray in a standardized manner and placed in the sample detection chamber of the Thermo Scientific K-Alpha XPS instrument. The sample is sent to the analysis chamber when the indoor pressure of the sample chamber reaches the standard value (less than 2.0 × 10⁻⁷ bar). The important test conditions include spot size (400 μm), work voltage (12 kV), and filament current (6 mA).

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis is performed by Thermo Nicolet iS20 (Thermo Fisher Scientific, Shanghai, China) which has a liquid-nitrogen-cooled mercury cadmium-telluride (MCT) detector. Firstly, the catalyst is reduced at room temperature for 30 min under a pure N₂ (20 mL/min) atmosphere. The background spectra are obtained under N₂ blowing. Six batches of DRIFTS characterization are performed on the pretreated catalyst. Mixed CO₂ and H₂ (CO₂:H₂ = 1:3) are introduced into the chamber at 150 °C, 180 °C, 200 °C, 220 °C, 240 °C, and 260 °C for 30 min.

3.5. Catalytic Performance Test

As shown in Figure 11, the catalytic characteristic for the reaction of CO₂ to CH₃OH is measured on fixed-bed equipment. A certain amount of catalyst (0.2 g) and 0.4 g of quartz sand (40–60 mesh) are mixed uniformly and loaded into the constant temperature zone in the middle of the reaction tube (8 mm internal diameter). Before the performance is evaluated, the catalyst is pretreated in situ with 10 mL/min H₂/N₂ for 3 h under test conditions of 0.1 MPa and 300 °C. After reduction, the feed gas (H₂:CO₂ = 3:1) is transmitted through the reactor and fully contacts the catalyst. The subsequent reactions are performed under 5 MPa, a temperature range from 180 °C to 260 °C, and GHSV is maintained at 12,000 mL h⁻¹ g_cat⁻¹. The gas phase product is detected by Agilent 8890 (Agilent Technologies (China) Co., Ltd, Beijing, China) online chromatography, and the composition and content of the product are determined according to the peak time of the product in the chromatographic column. The specific amounts of CO₂, CO, CH₄, and H₂ included in the reaction product are monitored online by MolSieve 5A (Agilent Technologies (China) Co., Ltd, Beijing, China) packed column (TCD). Organic compounds such as CH₄ and CH₃OH in the reaction product are detected online by HP-PLOT Q-packed column (FID).

3.6. Calculation Methods for Reactant Conversion as Well as Product Selectivity

The reactant conversion is calculated using the normalization method, and the formulas for CO₂ conversion and selectivity of all products are shown in Equations (1) and (2).

$$X\left({CO}_2\right)=\frac{f_{CO}{A}_{CO}+i\left(f_{{CH}_4}{A}_{{CH}_4}+f_{{CH}_{3}OH}{A}_{{CH}_{3}OH}\right)}{f_{{CO}_2}{A}_{{CO}_2}+f_{CO}{A}_{CO}+i\left(f_{{CH}_4}{A}_{{CH}_4}+f_{{CH}_{3}OH}{A}_{{CH}_{3}OH}\right)}$$

(1)

$$i=\frac{f_{{CH}_4-TCD}{A}_{{CH}_4-TCD}}{f_{{\mathrm{CH}}_4-\mathrm{FID}}{A}_{{\mathrm{CH}}_4-\mathrm{FID}}}$$

(2)

$$S\left({CH}_{3}OH\right)=\frac{f_{{CH}_{3}OH}{A}_{{CH}_{3}OH}}{f_{CO}{A}_{CO}+i\left(f_{{CH}_4}{A}_{{CH}_4}+f_{{CH}_{3}OH}{A}_{{CH}_{3}OH}\right)}$$

(3)

X(CO₂): CO₂ conversion, Sel (CH₃OH): CH₃OH selectivity, A: peak area, f: correction factor, i: conversion factor.

The correction factor values for each component in tail gas are shown in

Table 4. Correction factor value of components in tail gas.

Component i	CO2	CO	CH3OH	CH4-TCD	CH4-FID
Correction factor fi	1.06	1.24	2.13	1.66	1

.

3.7. Calculation Methods

The Vienna Ab Initio Simulation Package (VASP) is used to calculate the relevant properties of the MoS₂/Ni system [21]. Exchange correlation is measured through Perdew–Burke–Ernzerhof (PBE) method [43]. The plane-wave cut-off energy is 400 eV and the Monkhorst–Pack k-point sampling is 1 × 1 × 1 for the structural optimization involved in this system, while the electronic performance is calculated with the larger k-point of 3 × 3 × 1 [44]. A reasonable initial model is constructed as a 5 × 5 supercell with a vacuum thickness of 15 Å, and the size of the calculation model is 15.83 × 15.83 Å. Structural convergences are completed with Hellmann–Feynman residual force convergences between each atom below 0.02 eV/Å. A CI-NEB method is used to search the transition state involved in each elementary reaction [45]. The adsorption energy of the reactant on the catalyst surface (E_ads) is represented by Equation (4):

E_ads = E_AB − E_A − E_SM

(4)

where E_AB is the energy value of the reaction system where small reactant molecules are adsorbed on the material surface, E_A is the energy of material structure before adsorption, and E_SM is the energy possessed by reactant molecules adsorbed on the catalyst surface.

4. Conclusions

In conclusion, the MoS₂ catalyst is prepared by hydrothermal synthesis and doped with different amounts of Ni and Co metal to obtain MoS₂/Ni and MoS₂/Co composite catalyst materials. The catalytic properties of these materials in CO₂ hydrogenation to CH₃OH are investigated.

Compared with 2H-MoS₂, under the optimal reaction conditions, the MoS₂/Ni_0.2 composite catalyst shows excellent product selectivity for methanol. Thus, the high methanol selectivity from CO₂ and H₂ gas is achieved.

Through the analysis of XRD, XPS, BET, H₂–TPR SEM, and CO₂–TPD characterization results of MoS₂, Ni/MoS₂, and Co/MoS₂ composite catalysts with different metal loading ratios, it is confirmed that the addition of Ni, Co metals are beneficial for blocking more edge S vacancies, and, thus, the CH₃OH selectivity is higher. Through metal doping, the H₂ consumption and CO₂ adsorption are improved, indicating that the ability to activate H₂ and CO₂ is improved.

In situ DRIFTS spectra and DFT calculation confirmed that the hydrogenation of CO₂ to methanol on MoS₂/Ni catalyst follows an oxidation–reduction pathway. CO₂ is activated into CO* and gradually hydrogenated to form methoxy groups; ultimately, methanol is generated. The improvement of CH₃OH selectivity by metal doping involved in this paper has insight significance for subsequent related research.