The Influence of Sulfurization and Carbonization on Mo-Based Catalysts for CH₃SH Synthesis

Abstract

Sulfur-resistant Mo-based catalysts have become promising for the one-step synthesis of methanethiol (CH₃SH) from CO/H₂/H₂S, but the low reactant conversion and poor product selectivity have constrained its development. Herein, we synthesized K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ catalysts via the sulfurization and carbonization of K-Mo-based catalysts in the oxidized state, respectively. During the synthesis of CH₃SH, both K-Mo₂C/Al₂O₃ and K-MoS₂/Al₂O₃ showed excellent catalytic performance, and the activity of the former is superior to that of the latter. The effect of different treatments on the catalytic performance of Mo-based catalysts was investigated by XRD, BET, Raman spectroscopy, H₂-TPR, and reactants-TPD characterization. The results showed that the sulfide-treated sample showed stronger metal-support interactions and contributed to the formation of K₂S, which exposed more active sites and stabilized the formation of C-S bonds. Carbonized samples enhanced the activation of H₂, which promoted the hydrogenation of the intermediate species of carbonyl sulfide (COS) and thus improved the selectivity of CH₃SH.

1. Introduction

The extensive use of coal, particularly coal gasification, has caused serious air pollution, such as CO, and reduced sulfur species (e.g., H₂S, COS, and CS₂), of which H₂S dominates [1]. Excessive emissions of these pollutants may worsen harmful environmental pollution (haze and acid rain) [2,3]. Due to the significant differences in the physicochemical properties of H₂S and CO, they typically require different removal strategies. Common methods for removing H₂S include catalytic hydrogenation [4], hydrolysis [5], and the Claus process [6]. However, most of the final products are converted to low-value solid sulfur or secondary pollutants, and the sulfur-containing species often poison the catalysts. CO is mainly treated using catalytic oxidation technologies [7,8], which wastes resources and consumes excessive energy. Therefore, a meaningful strategy would be the synergistic removal and resource utilization of CO and H₂S.

CH₃SH is an important precursor of methionine, an essential amino acid for poultry and animal feed [9]. Methionine also has applications in the fields of medicine, food, biology, and cosmetics and can be used as a moderator for free-radical polymerization to produce pesticides [10]. The thiolation of methanol is the main method for synthesizing methyl mercaptan in the industry [11,12], in which methanol can be synthesized by syngas (CO + H₂). To reduce the costs of synthesizing CH₃SH, H₂S produced during coal gasification and syngas (CO + H₂) can be directly utilized as a feedstock for the one-step synthesis of CH₃SH [13]. Theoretically, the product of the one-step synthesis of CH₃SH from CO/H₂/H₂S contains only CH₃SH and H₂O (Equation (1)) and thus provides a clean and economical synthetic route. However, the conversion of CO and the selectivity of CH₃SH are usually unsatisfactory in practice. Therefore, the development of high-performance catalysts for the synthesis of CH₃SH has received increasing attention.

Molybdenum disulfide (MoS₂) is widely used during hydrodesulfurization, low-carbon mixed-alcohol synthesis, mercaptan synthesis, etc. [10,14], due to its unique two-dimensional layered structure and semiconducting properties. MoS₂ is bonded by strong S-Mo-S bonds within layers and weak van der Waals forces between layers, and the addition of alkali metals can modulate the electronic effect on the Mo surface [15,16]. Therefore, molybdenum sulfide-based catalysts have become the main catalysts used in methanethiol synthesis due to their high sulfur resistance and stability [17,18,19]. In the past decades, the catalytic synthesis of CH₃SH in a mixed CO/H₂/H₂S gas system over K-Mo-based catalysts has been investigated in terms of gas components, support properties, additive type, calcination temperature and atmosphere, and preparation method. The nature of the support greatly influences the metal-support interactions as well as the dispersion of the active metal [20]. Stronger metal-support interactions can promote interfacial charge transfer, change the metal structure, and modulate molecular adsorption [21,22]. In addition, the dispersion of active metals can expose more active sites and accelerate the reaction [2]. However, there are still some acute problems limiting the practical applications of MoS₂ during the synthesis of CH₃SH, such as its low selectivity and poor stability.

In recent years, molybdenum carbide (Mo₂C) has received attention as a catalyst due to its high melting point, high hardness, high tensile strength, and high electrical and thermal conductivity, which are similar to ceramics [23,24,25]. Mo₂C is an intercalation compound formed by the entry of carbon atoms into the interstices of molybdenum atoms. The structure of molybdenum carbide is determined by a combination of geometrical and electronic factors. Carbon atoms that enter between molybdenum atoms occupy metal sites in the precursor, which changes the structure. Typically, crystals have face-centered cubic (fcc), hexagonal closed-packed (hcp), and simple hexagonal (hex) structures. Scholars believe that three types of bonding between atoms exist in Mo₂C: (i) metallic bonding (rearrangement of metal–metal bonds); (ii) covalent bonding (formation of bonds between metals and carbon); (iii) ionic bonding (charge transfer between metals and carbon) [26]. Mo₂C also possesses strong sulfur resistance and hydrogen evolution capacity, allowing it to be applied during CH₃SH synthesis. However, there have been no reports on the utilization of Mo₂C-based catalysts for synthesizing CH₃SH in CO/H₂/H₂S mixtures. Therefore, there is a need to compare the catalytic performance and physicochemical properties of K-Mo-based catalysts treated via sulfidation and carbonization to help guide the design of efficient Mo-based catalysts.

In this work, we first modulated the support and reaction pressure to determine the optimal reaction conditions to synthesize CH₃SH from CO/H₂/H₂S. Then, K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ catalysts were synthesized by sulfidation and carbonization treatments, respectively. This was performed to understand the effects of different treatments on their physicochemical properties and catalytic behaviors. Both catalysts showed excellent catalytic activity. BET, XRD, Raman spectroscopy, H₂-TPR, and reactants-TPD were used to characterize K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ catalysts. The results revealed that the key reason for the high performance of K-MoS₂/Al₂O₃ was its strong metal-support interactions, while the performance of K-Mo₂C/Al₂O₃ was attributed to its ability to activate H₂. This work provides new ideas for the modification of Mo-based catalysts applied to CH₃SH synthesis.

2. Results and Discussion

2.1. Effect of Reaction Pressure and Support

Figure 1 shows the CO conversion and product selectivity over K-MoS₂/SiO₂ and K-MoS₂/Al₂O₃ catalysts during the synthesis of CH₃SH from CO/H₂/H₂S. The effect of reaction pressure on the synthesis of CH₃SH was investigated in terms of the catalysts’ activity. As shown in Figure 1a, the CO conversion remained at about 5% and hardly changed upon increasing the temperature at a reaction pressure of 0 MPa. When the reaction pressure was increased to 0.2 MPa, the CO conversion significantly increased and further increased with the temperature, indicating that pressure directly affected the CO conversion. Figure 1b,c show the selectivity of the products under different pressures. When the reaction system was unpressurized, the main products were COS, CS₂, and CH₃SH, among which COS was the main product and showed the highest selectivity (~60%). The selectivity of CH₃SH was unsatisfactory. When the reaction system was pressurized to 0.2 MPa, CH₃SH was the main product, which had a maximum selectivity of 62.5% at 325 °C, accompanied by the generation of some COS and CO₂. When the pressure in the system increased, the hydrogenation of the intermediate product COS to generate CH₃SH was accelerated, while the hydrolysis of COS to CO₂ was promoted, and the decomposition of CH₃SH into CS₂ was inhibited. It can be seen that the synthesis of CH₃SH from CO/H₂/H₂S was greatly promoted by pressurizing the reaction system, and side reactions were also inhibited, which promoted the generation of CH₃SH.

The sulfided K-Mo catalysts with different supports were compared during the synthesis of CH₃SH from CO/H₂/H₂S at 0.2MPa. As shown in Figure 1d, the CO consumption rate was higher over the K-MoS₂/Al₂O₃ catalyst, which was almost three times that of K-MoS₂/SiO₂. K-MoS₂/Al₂O₃ showed the highest consumption rate of CO at 350 °C, exceeding 0.23 g(CO)·g⁻¹(cat)·h⁻¹. In Figure 1e,f, the products obtained over K-MoS₂/SiO₂ and K-MoS₂/Al₂O₃ catalysts both mainly consisted of CH₃SH, COS, CO₂, CH₄, and CS₂, among which there was no significant difference in the selectivity of CH₃SH. However, K-MoS₂/Al₂O₃ possessed a higher CH₄ selectivity at high temperatures due to the over-hydrogenation of CH₃SH, while K-MoS₂/SiO₂ had a weaker hydrogenation ability, resulting in a higher COS selectivity. In summary, K-MoS₂/Al₂O₃ using Al₂O₃ as the support under pressurized conditions was more favorable for the synthesis of CH₃SH from CO/H₂/H₂S.

2.2. Performance of the Sulfided and Carbonized Catalysts

2.2.1. Characterization of the Phase Structure

The N₂ adsorption–desorption of K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ catalysts was characterized to further elucidate the differences in their physical properties. The corresponding isotherms are shown in Figure 2a, and the detailed information is shown in

Table 1. Physical properties of K-Mo₂C/Al₂O₃ and K-MoS₂/Al₂O₃.

Sample	Surface Area (m2/g)	Pore Volume (cc/g)	Pore Diameter Dv (d) (nm)
K-Mo2C/Al2O3	103.9	0.321	9.618
K-MoS2/Al2O3	82.2	0.326	8.615

. The isotherms of K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ were assigned as type-IV with H-1type hysteresis loops, indicating mesoporous structures [27,28,29]. In contrast, the BET surface area of K-Mo₂C/Al₂O₃ was larger than that of K-MoS₂/Al₂O₃, possibly due to the accumulation of sulfur on the surface of K-MoS₂/Al₂O₃ due to vulcanization, whereas the carbonization treatment avoided this.

X-ray diffraction (XRD) was used to reveal the crystalline phases of K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ catalysts, and the corresponding XRD patterns are shown in Figure 2b. Diffraction peaks (2θ = 38.1, 39.4, 61.7, and 75.7°) corresponding to the (002), (101), (110), and (201) planes of β-Mo₂C (JCPDS#65-8766) were observed in the pattern of K-Mo₂C/Al₂O₃. Moreover, K-Mo₂C/Al₂O₃ also displayed weak characteristic diffraction peaks attributed to MoC (JCPDS#08-0384). For K-MoS₂/Al₂O₃, the diffraction peaks (2θ = 14, 33.2, and 58.7°) corresponding to the (002), (100), and (110) planes of MoS₂ (JCPDS#75-1539) were observed. In addition, the same diffraction peaks (30.7, 32.6, and 50.2°) in the patterns of both K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ were assigned to K₂MoO₄ (JCPDS- ICDD#29-1021), indicating interactions between K and Mo.

Raman spectroscopy was used to determine the molecular structure of the samples by analyzing the vibrational and rotational modes of their chemical bonds. Figure 2c shows the Raman spectra of the K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ to provide further evidence of the effects of vulcanization and carbonization treatments. K-Mo₂C/Al₂O₃ displayed an intense band near 215 cm⁻¹, which corresponded to the well-dispersed AlMo₆O₂₄H₆³⁺ heteropolyacid anion (AlMo₆) formed by mixing an alumina support with a molybdenum-based aqueous solution [30]. The bands at 325 cm⁻¹, 896 cm⁻¹, and 917 cm⁻¹ were attributed to K₂MoO₄ and K₂Mo₂O₇, which is consistent with the XRD patterns [31,32]. K-Mo₂C/Al₂O₃ catalyst also showed distinct characteristic peaks at 661 cm⁻¹, 818 cm⁻¹, and 990 cm⁻¹, which corresponded to the stretching vibrations of Mo-C-Mo and Mo-C, respectively, indicating the presence of Mo₂C [33]. For K-MoS₂/Al₂O₃, the main bands at 378 cm⁻¹ and 404 cm⁻¹ were assigned to the in-plane E¹_2g mode resulting from the opposite vibrations of two S atoms with respect to the Mo atom. The A_1g mode was related to the out-of-plane vibrations of S atoms in opposite directions, respectively, further demonstrating the existence of MoS₂.

2.2.2. Redox Properties of K-Mo Catalysts

H₂-temperature programmed reduction (H₂-TPR) experiments are commonly used to investigate the properties and amounts of active sulfur species over MoS₂-based catalysts. The H₂-TPR curves of K-MoS₂/SiO₂ and K-MoS₂/Al₂O₃ catalysts are shown in Figure S1. Two reduction peaks were observed over K-MoS₂/SiO₂ and K-MoS₂/Al₂O₃ catalysts at 300–400 °C and 500–600 °C. Typically, the reduction peaks located in the low-temperature region (300–400 °C) are attributed to surface-active sulfur species on the exposed coordinatively unsaturated sites (CUS) of MoS₂ [34]. The reduction peaks in the mid-temperature region (500–600 °C) are assigned to the consumption of hydrogen during K-S bond breakage in K-containing crystalline phases [34]. As reported in the literature, the consumption of hydrogen in the low-temperature region of H₂-TPR determines the strength of the Mo-S bonds, which reflects the number of surface-active sulfur species. The H₂ consumption over K-MoS₂/Al₂O₃ was much larger than that over K-MoS₂/SiO₂, indicating that more reactive sulfur species were present on the surface of K-Mo/Al₂O₃. Furthermore, the reduction temperature over K-MoS₂/Al₂O₃ was higher than that over K-MoS₂/SiO₂, indicating stronger metal-support interactions over the K-MoS₂/Al₂O₃ catalyst [35], which promoted the dispersion of active species on the support [36]. Active sulfur species exhibit high performance during the synthesis of CH₃SH from CO/H₂/H₂S; therefore, K-MoS₂/Al₂O₃ is a more suitable choice, which is consistent with the activity tests in Figure 1. Notably, three distinct hydrogen consumption peaks were observed over the K-Mo₂C/Al₂O₃ catalyst at 200–350 °C, 400–600 °C, and >600 °C, respectively (Figure 3). Typically, the hydrogen consumption peaks in the low-temperature region are assigned to the reduction of the passivation layer on the surface of molybdenum carbide [37]. The reduction peak in the mid-temperature region is attributed to the reduction of Mo⁶⁺ to Mo⁴⁺ [38], and the reduction peak in the high-temperature region is attributed to the reduction of Mo⁴⁺ to Mo [39]. However, the sample was not passivated, so it can be assumed that there was no passivation layer on its surface. According to the previous literature, the hydrogen consumption peak at low temperatures (200–400 °C) was also attributed to the reduction of high-valent molybdenum oxides on the surface. Combined with the H₂-TPR results for K-containing molybdenum sulfide catalysts, the three hydrogen consumption peaks (200–350 °C, 400–600 °C, and >600 °C) were attributed to the reduction of Mo⁴⁺, the breakage of K-C bonds in the K crystalline phase, and the reduction of Mo²⁺ (Mo₂C) to elemental Mo, respectively. Compared with the H₂-TPR curves of K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃, the K-MoS₂/Al₂O₃ catalyst exhibited a higher reduction temperature and greater hydrogen consumption. This indicates that K-MoS₂/Al₂O₃ displayed stronger metal-support interactions and more coordinatively unsaturated molybdenum sites (Mo-CUS) than K-Mo₂C/Al₂O₃.

2.2.3. Temperature-Programmed Desorption of Reactants (CO/H₂/H₂S-TPD)

Adsorption is an important stage during any catalytic reaction; thus, it is necessary to further consider the adsorption properties over catalysts for the reactants. As shown in Figure 4, CO/H₂/H₂S-TPD was carried out to test the chemisorption and activation of CO/H₂/H₂S over K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ catalysts. In Figure 4a, both K-MoS₂/Al₂O₃ and K-Mo₂C/Al₂O₃ displayed three desorption peaks of CO at 100–200 °C, 400–500 °C, and 500–700 °C. In the low-temperature region, the desorption peak of CO over K-Mo₂C/Al₂O₃ was due to the physical adsorption of CO on Mo₂C, whereas the desorption peak of CO over K-MoS₂/Al₂O₃ was attributed to the non-dissociative adsorption of CO over Mo-CUS. The peak area of K-Mo₂C/Al₂O₃ was much higher than that of K-MoS₂/Al₂O₃, indicating the larger surface area of the former. In the mid-temperature regions, the CO desorption peaks for K-Mo₂C/Al₂O₃ and K-MoS₂/Al₂O₃ were attributed to the weak chemisorption on Mo₂C and the desorption of CO from –SH groups [40], respectively. In the high-temperature region, the desorption peak of CO for K-Mo₂C/Al₂O₃ was attributed to strong chemisorption on Mo₂C, while the desorption peaks of CO for K-MoS₂/Al₂O₃ were related to the dissociative adsorption of CO with kinetic control or sulfur-containing compounds. Generally, dissociatively adsorbed CO tends to form hydrocarbon products. During the hydrogenation reaction, the adsorption and activation of H₂ molecules are important. H₂ molecules that are readily activated to adsorbed H* facilitate the formation of CH₃S* species with C-S intermediates, while the presence of gaseous H₂ facilitates the hydrolysis of C-S intermediates to form CH_x* and SH*. The latter of these is detrimental to the synthesis of CH₃SH. The H₂-TPD curves in Figure 4b show that K-Mo₂C/Al₂O₃ produced a large amount of desorbed H in the mid-temperature region, with a clear H₂ inversion peak at 700 °C, indicating that hydrogen may have dissociated into adsorbed H*. However, there was no obvious H₂ desorption peak over the K-Mo₂S/Al₂O₃ catalyst. In summary, the K-Mo₂C/Al₂O₃ catalyst showed better adsorption and activation ability for CO and H₂, and the formed H* stabilized the C-S bond and hydrogenated CH₃SH. As for H₂S-TPD in Figure 4c, almost no difference was observed between K-Mo₂C/Al₂O₃ and K-MoS₂/Al₂O₃ in the low- and high-temperature regions. A weak desorption peak of H₂S appeared over the K-Mo₂C/Al₂O₃ catalyst near 400 °C, while almost no desorption peak appeared for K-MoS₂/Al₂O₃, indicating that Mo₂C adsorbed H₂S at moderate temperatures. According to Figure 1, the optimal reaction temperature for the synthesis of CH₃SH was in the range of about 300–400 °C, so the appropriate desorption temperature of H₂S for the K-Mo₂C/Al₂O₃ catalyst promoted the synthesis of CH₃SH.

2.2.4. Catalytic Performance of Sulfided and Carbonized Catalysts

The consumption rate of CO, the formation rate of CH₃SH, and the selectivity of products over K-Mo₂C/Al₂O₃ and K-MoS₂/Al₂O₃ catalysts during the synthesis of CH₃SH from COS/H₂/H₂S are presented in Figure 5. K-Mo₂C/Al₂O₃ exhibited better excellent catalytic activity in Figure 5a,b, and the rates of CO consumption and CH₃SH formation with temperature followed similar trends over both K-Mo₂C/Al₂O₃ and K-MoS₂/Al₂O₃ catalysts. The highest catalytic activity over the K-Mo₂C/Al₂O₃ catalyst was achieved at 325 °C when the consumption rate of CO reached 0.2660 g·g_cat⁻¹·h⁻¹, and the formation rate of CH₃SH reached 0.2990 g·g_cat⁻¹·h⁻¹. The consumption rate of CO over the K-MoS₂/Al₂O₃ catalyst reached a maximum of only 0.2257 g·g_cat⁻¹·h⁻¹ at 350 °C, and the formation rate of CH₃SH reached a maximum of only 0.1795 g·g_cat⁻¹·h⁻¹ at 325 °C. As shown in Figure 5c,d, there was no significant difference in the variety of products (CH₃SH, COS, CO₂, CH₄, and CS₂), and CH₃SH was the main product. K-Mo₂C/Al₂O₃ exhibited higher CH₃SH selectivity and lower COS selectivity, and no significant difference was found in the selectivity of the remaining products. These results indicate that the carbonized catalyst possessed a higher COS hydrogenation capacity.

2.3. Discussion

First, the reaction conditions for CH₃SH synthesis were optimized by regulating the pressure during the reaction and trying different supports. The pressurized conditions improved the CO conversion efficiency and CH₃SH selectivity, which was attributed to the presence of more active sulfur species on the surface of K-MoS₂/Al₂O₃ catalysts and the favorable dispersion of the active species when using Al₂O₃ as the support. In addition, the ex situ characterization of sulfided K-MoS₂/Al₂O₃ catalyst and carbonized K-Mo₂C/Al₂O₃ catalyst were combined with their activity tests during CH₃SH synthesis from COS/H₂/H₂S. The carbonized K-Mo₂C/Al₂O₃ catalyst exhibited higher catalytic performance than the sulfided K-MoS₂/Al₂O₃ catalyst. The XRD and Raman spectra show that the molybdenum sulfide and molybdenum carbide phases were formed in the oxidized K-Mo/Al₂O₃ samples, which differed depending on sulfurization and carbonization treatment, respectively. K-Mo oxides remained in the catalysts after pretreatment, indicating interactions between the molybdate and K. H₂-TPR showed that the sulfided K-MoS₂/Al₂O₃ catalyst exhibited higher hydrogen reduction peaks, but this was not directly related to the catalytic activity. According to CO/H₂/H₂S-TPD, K-Mo₂C/Al₂O₃ showed a greater activation capacity for CO and H₂ than the sulfided K-MoS₂/Al₂O₃ catalyst. The activity tests showed that the K-Mo₂C/Al₂O₃ catalysts exhibited a higher CO consumption rate. Analysis of the product selectivity showed that the K-Mo₂C/Al₂O₃ catalyst showed superior hydrogenation capacity for COS and CS₂, which may be related to its excellent hydrogen activation capacity.

Finally, we elucidated the corresponding catalytic reaction mechanisms for the conversion of CO/H₂/H₂S to CH₃SH over carbonization-treated and sulfidation-treated K-Mo-based catalysts. The similarity was that COS was first generated by the reaction of CO and H₂S, and then CH₃CH was synthesized by direct or indirect hydrogenation of COS. Over the K-MoS₂/Al₂O₃ catalyst, the stronger metal-support interactions and the greater number of exposed unsaturated molybdenum sites favored the adsorption and activation of CO and H₂S. The production of K₂S was key to the synthesis of CH₃SH and also stabilized the C-S bonds, thus avoiding the conversion of COS to side products in the gas phase. For the K-Mo₂C/Al₂O₃ catalyst, Mo_xC facilitated the adsorption and activation of H₂, and active H atoms accelerated the hydrogenation of COS to SH₃CH, which explains why K-Mo₂C/Al₂O₃ exhibited a higher SH₃CH selectivity.

3. Experimental Section

3.1. Catalyst Preparation

γ-Al₂O₃ (purity > 99.98%) and SiO₂ (purity > 99.99%) were purchased from Chongqing Chuandong Chemical Co., Ltd., Chongqing, China; (NH₄)₆Mo₇O₂₄⋅4H₂O (purity > 98%) was purchased from Tianjin No. 4 Chemical Reagent Factory, China; K₂CO₃ (purity > 99.99%) was purchased from Lianyungang Xinfu Rare Earth Co., Ltd., Lianyungang, China. All medicines were not further purified.

3.1.1. Preparation of K-MoO₃/Al₂O₃ and K-MoO₃/SiO₂

K-MoO₃/Al₂O₃ and K-MoO₃/SiO₂ catalysts were prepared by using the incipient-wetness co-impregnation method with γ-Al₂O₃ and SiO₂ as supports, respectively, K₂CO₃ as the K precursor, and (NH₄)₆Mo₇O₂₄·4H₂O as the molybdenum precursor. Before γ-Al₂O₃ (noted as “Al₂O₃” hereafter) was used, it was calcined in a muffle furnace at 550 °C for 6 h. The loading of Mo (based on MoO₃) and the molar ratios of K:Mo were 10 wt% and 2:1, respectively. After impregnation, the samples were dried at 110 °C for 12 h and then calcined at 550 °C for 5 h in muffle furnace under still air. After cooling, the tablets were pressed and sieved through 40–60 mesh to obtain oxidized samples, noted as K-MoO₃/Al₂O₃ and K-MoO₃/SiO₂.

3.1.2. Preparation of K-MoS₂/Al₂O₃ and K-MoS₂/SiO₂

K-MoO₃/Al₂O₃ or K-MoO₃/SiO₂ samples (0.8 g; 40–60 mesh) were fixed at the center of the tube furnace reactor, and then a mixture of H₂/H₂S with a total flow rate of 40 mL/min was introduced, and the temperature of the reactor was increased to 400 °C and held for 4 h. The volume ratio of H₂: H₂S was 9:1, and the heating rate was 5 °C/min. The sulfated samples were noted as K-MoS₂/Al₂O₃ and K-MoS₂/SiO₂, respectively.

3.1.3. Preparation of K-Mo₂C/Al₂O₃

K-MoO₃/Al₂O₃ samples (0.8 g; 40–60 mesh) were fixed in the center of the tube furnace reactor. Then, a mixture of H₂/CH₄ with a total flow rate of 40 mL/min was introduced, and the temperature of the reactor was increased to 300 °C and maintained for 2 h. Then, it was increased to 500 °C for 2 h (1 °C/min), where the volume ratio of H₂:CH₄ was 9:1, and the heating rate was 5 °C/min. The carbonized sample was noted as K-Mo₂C/Al₂O₃.

3.2. Catalyst Characterization

The details of catalyst characterizations are listed in the Supplementary Materials.

3.3. Catalytic Performance Evaluation

The catalyst activity was evaluated over a 6 mm i.d. quartz fixed-bed using 0.4 g of catalyst (40–60 mesh). A gas mixture of CO/H₂/H₂S = 1/4/5 at a pressure of 0.2 MPa and a flow rate of 40 mL/min was used. WHSV used for tests was 6000 mL·g⁻¹·h⁻¹. The catalytic activity was measured in the range of 275–400 °C at 25 °C intervals. Samples were collected after the target temperature was reached and stabilized for 30 min, 60 min, and 90 min followed by taking the average value. The reaction products were detected online using three GC fitted with one thermal conductivity detector (TCD), one flame ionization detector (FID), and two flame photometric detectors (FPD). The CO conversion and CH₃SH selectivity were calculated by the following equations:

$$X_{\left(CO\right)}\left(\%\right)=\frac{C_{CO,in}-C_{CO,out}}{C_{CO,in}}\times 100\%$$

$${\mathrm{S}}_{{CH}_{3}SH}(\%)=\frac{C_{{CH}_{3}SH}}{C_{{CH}_{3}SH}+C_{COS}{+C}_{{CO}_2}{+C}_{{CH}_4}{+C}_{{CS}_2}}\times 100\%$$

where C_CO,in denotes the concentration of CO in the feed gas; C_CO,_out denotes the concentration of CO in the product; and C_CH3SH, C_COS, C_CO2, C_CH4, and C_CS2 represent the concentration of CH₃SH, COS, CO₂, CH₄, and CS₂ in the product, respectively.

The consumption rate of CO and the formation rate of CH₃SH were calculated by the following equation:

$$r_{\left(CO\right)}=\frac{M_{co}\times C_{co,in}\times X_{\left(CO\right)}\times Q}{V_m{\times m}_{cat}}=\mathrm{g}\times {{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}}^{-1}\times {\mathrm{s}}^{-1}$$

$$r_{\left({CH}_{3}SH\right)}=\frac{M_{CH3SH}\times Q}{V_m{\times M}_{cat}}=\mathrm{m}\mathrm{o}\mathrm{l}\times {{\mathrm{m}\mathrm{o}\mathrm{l}}_{\mathrm{c}\mathrm{a}\mathrm{t}}}^{-1}\times {\mathrm{s}}^{-1}$$

where M_CO is the CO molar mass (g/mol), M_CH3SH is the CH₃SH molar of the product (mol), Q is the flow rate, V_m is the molar volume of gas at standard conditions (L/mol), and M_cat is the molar of catalyst used (mol).

4. Conclusions

In this work, we compared the catalytic performance of K-Mo₂S catalysts with different supports. Based on the H₂-TPR results, the metal-support interactions over the K-MoS₂/Al₂O₃ catalyst were much stronger than those over the K-MoS₂/SiO₂ catalyst, which improved the dispersion of active species on the support surface. The exposure of more Mo-CUS over the K-MoS₂/Al₂O₃ catalyst accelerated the reaction. Moreover, the K-Mo₂C/Al₂O₃ catalyst obtained by the carbonization of oxidized K-Mo-based catalysts also showed excellent catalytic performance for the conversion of CO/H₂/H₂S to CH₃SH. K-Mo₂C/Al₂O₃ exhibited a higher CO consumption rate and CH₃SH selectivity than K-MoS₂/Al₂O₃. More importantly, the CO/H₂/H₂S-TPD experiments and characterization results confirmed that the strong metal-support interactions and the formation of K₂S over the K-MoS₂/Al₂O₃ catalyst favored the adsorption and activation of CO/H₂S. The K-Mo₂C/Al₂O₃ catalyst showed a superior ability to activate H₂. In contrast, the activation of hydrogen was more favorable for the hydrogenation of the intermediate species COS to CH₃SH, thus improving the selectivity of CH₃SH in the products.