Influence of Carbonization Conditions on Structural and Surface Properties of K-Doped Mo2C Catalysts for the Synthesis of Methyl Mercaptan from CO/H2/H2S

The cooperative transition of sulfur-containing pollutants of H2S/CO/H2 to the high-value chemical methyl mercaptan (CH3SH) is catalyzed by Mo-based catalysts and has good application prospects. Herein, a series of Al2O3-supported molybdenum carbide catalysts with K doping (denoted herein as K-Mo2C/Al2O3) are fabricated by the impregnation method, with the carbonization process occurring under different atmospheres and different temperatures between 400 and 600 °C. The CH4-K-Mo2C/Al2O3 catalyst carbonized by CH4/H2 at 500 °C displays unprecedented performance in the synthesis of CH3SH from CO/H2S/H2, with 66.1% selectivity and a 0.2990 g·gcat−1·h−1 formation rate of CH3SH at 325 °C. H2 temperature-programmed reduction, temperature-programmed desorption, X-ray diffraction and Raman and BET analyses reveal that the CH4-K-Mo2C/Al2O3 catalyst contains more Mo coordinatively unsaturated surface sites that are responsible for promoting the adsorption of reactants and the desorption of intermediate products, thereby improving the selectivity towards and production of CH3SH. This study systematically investigates the effects of catalyst carbonization and passivation conditions on catalyst activity, conclusively demonstrating that Mo2C-based catalyst systems can be highly selective for producing CH3SH from CO/H2S/H2.


Introduction
Coal is the main energy source in China. This produces large quantities of sulfurcontaining pollutants emitted by coal combustion, which has resulted in serious atmospheric environmental pollution [1][2][3]. Therefore, the removal and utilization of hydrogen sulfide resources have become research hotspots in the coal chemical industry [4,5]. In this industry, the Claus method and the improved Claus method are generally used to recover H 2 S to produce sulfur or sulfuric acid, and the added value of the products is low. The preparation of the high-value chemical methyl mercaptan (CH 3 SH) from H 2 S, along with CO and H 2 in the process of coal gasification, has become a valuable development direction [6][7][8][9]. This technology can not only effectively remove H 2 S but also provide a new route for downstream product development. The key to realizing the efficient utilization of raw materials and the high yield of products is to build a high-performance catalyst system. Until now, molybdenum sulfide based catalysts have been widely preferred because of their excellent sulfur resistance, but the reactant conversion and product selectivity are poor [10,11], which greatly restricts the development and application of molybdenum sulfide based catalysts.
In the industry, CH 3 SH is mainly synthesized from the thiolation of methanol over alkali-promoted transition metal catalysts [12,13], but methanol needs to be prepared with at a K:Mo molar ratio of 2:1. Next, 2 g support was added to the mixed solution and stirred. The mixture was allowed to stand overnight for aging and was then placed in an oven and dried at 110 • C for 6 h. Finally, the samples were transferred to a muffle furnace and heated from room temperature to 550 • C at 5 • C/min, with calcination in air for 5 h. The resulting catalyst was labelled as K-Mo/Al 2 O 3 .

Catalyst Characterization
Powder X-ray diffraction (XRD) of catalysts was performed on a Bruker Germany D8 ADVANCE instrument with Cu Kα-radiation (λ = 0.15418 nm) operating at 40 kV and 30 Ma. Raman spectroscopy (Jobin-Yvon France LabRam HR 800) was performed under 532 nm laser excitation, and the collection range was 200-1200 cm −1 . BET surface areas and pore structures of catalysts were determined by N 2 physisorption at 77 K using an America Quantachrome NOVA4200e analyzer. The temperature-programmed reduction of the sulfided sample (0.05 g) by hydrogen (H 2 -TPR) was performed using a thermal conductivity detector (TCD) at a heating rate of 10 • C/min until reaching 800 • C. The reduction atmosphere comprised a 10% H 2 /Ar (v/v) gas at a flow rate of 30 mL/min. The temperature-programmed desorption of carbon monoxide/hydrogen/hydrogen sulfide (CO/H 2 /H 2 S-TPD) was conducted in a fixed-bed flow reactor equipped with a thermal conductivity detector (TCD) as the detector. Taking CO-TPD as an example, prior to the analysis, the sample (0.05 g, 40-60 mesh) was first heated to 300 • C at a rate of 5 • C/min and kept at 300 • C for 0.5 h to remove adsorbed matter from the catalyst surface. After the sample was cooled down to room temperature, the gas stream was switched to 10% CO/He (30 mL/min). The adsorption was carried out at room temperature for 1 h, and then the sample was purged with Ar to remove the physically adsorbed CO. The reactor was heated from room temperature to 900 • C at a rate of 10 • C/min for temperature-programmed desorption. With other conditions remaining unchanged, H 2 -TPD and H 2 S-TPD were performed by replacing 10% CO/He with 10% H 2 /Ar and 10% H 2 S/He, respectively.

Catalytic Performance Tests
Before the performance test, 0.8 g K-Mo/Al 2 O 3 (40-60 mesh) was placed in a quartz tube and a pre-treatment gas (carbonization gas: CH 4 /H 2 , C 2 H 6 /H 2 , C 3 H 8 /H 2 ) was introduced. Taking CH 4 /H 2 as an example, the ratio of CH 4 :H 2 was 1:9 (v/v, 40 mL/min); the sample was first heated from room temperature to 300 • C under a carbonization atmosphere at a rate of 1 • C/min and was maintained for 2 h; the sample was then further heated to the carbonization temperature stage (400 • C, 500 • C, 600 • C) at a rate of 1 • C/min and held for 2 h. K-Mo/Al 2 O 3 samples were carbonized in CH 4 /H 2 , C 2 H 6 /H 2 or C 3 H 8 /H 2 atmospheres and named as CH 4 -K-Mo 2 C/Al 2 O 3 or K-Mo 2 C/Al 2 O 3 , C 2 H 6 -K-Mo 2 C/Al 2 O 3 and C 3 H 8 -K-Mo 2 C/Al 2 O 3 , respectively.
The activity evaluation of the catalysts (0.4 g) was carried out on a fixed-bed reactor. A gas mixture of CO/H 2 /H 2 S (gas volume concentration: CO = 10%, H 2 = 40%, H 2 S = 50%) was used. N 2 was used as the equilibrium component and was introduced to the quartz tube with a pressure of 0.2 MPa and a flow rate of 40 mL/min. The catalytic activity was measured in the range of 275-400 • C. After reaching a steady state at each temperature, the products were analyzed online using three gas chromatographs (GC9790, China FULI INSTRUMENTS) equipped with FPD, TCD and FID.

Analysis of the Products
The CO conversion rate and product selectivity were calculated using the following equation: where C CO,in denotes the concentration of CO in the feed gas and C CO,out denotes the concentration of CO in the product; the product selectivity is calculated based on carbon balance, where i is the target product (CH 3 SH, COS, CO 2 , CH 4 , CS 2 ). C i is the product concentration.
The CO consumption rate and CH 3 SH formation rate were calculated using the following equation: where M CO is CO molar mass (g/mol); M CH3SH is the molar mass of CH 3 SH (g/mol). Q is the flow rate (L/h), V m is the molar volume of gas under standard condition (L/mol) and m cat is the catalyst amount (g). Figure 1 shows the effect of different carbonization temperatures on the catalytic performance of the K-Mo/Al 2 O 3 catalyst for the synthesis of CH 3 SH from a CO/H 2 /H 2 S mixture. From Figure 1A,B, it can be seen that the CO consumption rate first increased and then decreased with increasing temperature over 325 • C. Our former studies showed that the equilibrium constant of the CO consumption reaction (CO + H 2 S → COS + H 2 ) decreases with increasing temperature, which indicates that rising temperature is adverse to the occurrence of the CO consumption reaction. Therefore, the decrease in CO conversion at high temperatures is due to equilibrium limitations [7]. The CH 3 SH formation rate also showed a similar tendency to that of CO consumption. It can be obviously seen from the tendency that when the oxidation state material was carbonized at 500 • C, the catalyst was found to have the optimum catalytic activity, and the CO conversion (35.5%) and selectivity towards CH 3  From Figure 1C, it can be seen that there was no significant difference in the main products formed over K-Mo/Al 2 O 3 after being carbonized at different temperatures for the preparation of CH 3 SH from CO/H 2 /H 2 S; CH 3 SH, COS, CO 2 , CH 4 and CS 2 were the main products in this reaction system.

Performance of Catalysts Prepared at Different Carbonization Temperatures
The H 2 -TPR curves of K-Mo/Al 2 O 3 catalysts treated at different carbonization temperatures are shown in Figure 1D. The three materials with different carbonization temperatures exhibited similar reduction peak characteristics, with a low temperature reduction peak at 200-350 • C and a moderate temperature reduction peak at 450-600 • C. Moreover, the catalyst carbonized at 500 • C exhibited a reduction peak at a high temperature (above 700 • C), which was attributed to the reduction of Mo 4+ to Mo 0 , and the hydrogen consumption peaks were obviously lower than those of the other two materials at the same reduction temperature. These results indicated that K-Mo/Al 2 O 3 carbonized at 500 • C was more easily reduced under the same reduction conditions, resulting in more Mo with coordinatively unsaturated surface sites that were conducive to the adsorption of reactants and the desorption of intermediate products such as COS and CH 3 SH. Thus, the catalyst obtained by carbonization at 500 • C delivered the best catalytic performance, and the catalysts were carbonized at 500 • C in subsequent experiments. From Figure 1C, it can be seen that there was no significant difference in the main products formed over K-Mo/Al2O3 after being carbonized at different temperatures for the preparation of CH3SH from CO/H2/H2S; CH3SH, COS, CO2, CH4 and CS2 were the main products in this reaction system.
The H2-TPR curves of K-Mo/Al2O3 catalysts treated at different carbonization temperatures are shown in Figure 1D. The three materials with different carbonization temperatures exhibited similar reduction peak characteristics, with a low temperature reduction peak at 200-350 °C and a moderate temperature reduction peak at 450-600 °C. Moreover, the catalyst carbonized at 500 °C exhibited a reduction peak at a high temperature (above 700 °C), which was attributed to the reduction of Mo 4+ to Mo 0 , and the hydrogen consumption peaks were obviously lower than those of the other two materials at the same reduction temperature. These results indicated that K-Mo/Al2O3 carbonized at 500 °C was more easily reduced under the same reduction conditions, resulting in more Mo with coordinatively unsaturated surface sites that were conducive to the adsorption of reactants and the desorption of intermediate products such as COS and CH3SH. Thus, the catalyst obtained by carbonization at 500 °C delivered the best catalytic performance, and the catalysts were carbonized at 500 °C in subsequent experiments.

The Role of Passivation
According to the literature, fresh molybdenum carbide materials are prone to violent reactions with air to form molybdenum oxide [38][39][40]. Therefore, fresh molybdenum carbide materials need to be passivated. After passivation, a passivation layer is formed on the surface of the material, which makes it more stable (passivation treatment with

The Role of Passivation
According to the literature, fresh molybdenum carbide materials are prone to violent reactions with air to form molybdenum oxide [38][39][40]. Therefore, fresh molybdenum carbide materials need to be passivated. After passivation, a passivation layer is formed on the surface of the material, which makes it more stable (passivation treatment with O 2 /inert gas). To reveal the role of passivation in the catalytic performance of K-Mo/Al 2 O 3 for the synthesis of CH 3 SH from CO/H 2 /H 2 S, the catalytic activity of the catalysts before/after passivation was conducted. As shown in Figure 2, there was a significant change in the CO consumption rate within the reaction temperature range of 275-400 • C after passivation that decreased with increasing temperature. However, the formation rate trend of CH 3 SH remained almost unchanged, first increasing and then decreasing with increasing temperature. After comparing with the product analysis diagram, it was found that the formation of main products before/after passivation did not change (CH 3 SH, COS, CO 2 , CH 4 , and CS 2 ), but the selectivity towards CH 3 SH was significantly reduced and the selectivity of by-product COS was increased after passivation. These changes were likely due to the passivation layer on the passivated catalyst surface, which covered the active site for direct hydrogenation of some COS to CH 3 SH, resulting in the reduction of selectivity towards CH 3 SH. The result revealed that passivation treatment was not effective for the synthesis of CH 3 SH from CO/H 2 /H 2 S, as the existing of the passivation layer would inhibit the formation of CH 3 SH. O2/inert gas). To reveal the role of passivation in the catalytic performance of K-Mo/Al2O3 for the synthesis of CH3SH from CO/H2/H2S, the catalytic activity of the catalysts before/after passivation was conducted. As shown in Figure 2, there was a significant change in the CO consumption rate within the reaction temperature range of 275-400 °C after passivation that decreased with increasing temperature. However, the formation rate trend of CH3SH remained almost unchanged, first increasing and then decreasing with increasing temperature. After comparing with the product analysis diagram, it was found that the formation of main products before/after passivation did not change (CH3SH, COS, CO2, CH4, and CS2), but the selectivity towards CH3SH was significantly reduced and the selectivity of by-product COS was increased after passivation. These changes were likely due to the passivation layer on the passivated catalyst surface, which covered the active site for direct hydrogenation of some COS to CH3SH, resulting in the reduction of selectivity towards CH3SH. The result revealed that passivation treatment was not effective for the synthesis of CH3SH from CO/H2/H2S, as the existing of the passivation layer would inhibit the formation of CH3SH.

Performance of Catalysts Carbonized under Different Atmospheres
Different carbonization atmospheres can lead to the formation of different crystal forms of molybdenum carbide. For example, MoO3 will be converted into cubic molybdenum carbide with H2/toluene as the reaction gas at 673 K [41]; using H2/butane as the reaction gas, MoO3 is initially converted into face-centered cubic (fcc) molybdenum carbide, then the face-centered cubic molybdenum carbide gradually transforms into a

Performance of Catalysts Carbonized under Different Atmospheres
Different carbonization atmospheres can lead to the formation of different crystal forms of molybdenum carbide. For example, MoO 3 will be converted into cubic molybdenum carbide with H 2 /toluene as the reaction gas at 673 K [41]; using H 2 /butane as the reaction gas, MoO 3 is initially converted into face-centered cubic (fcc) molybdenum carbide, then the face-centered cubic molybdenum carbide gradually transforms into a hexagonal closely Nanomaterials 2023, 13, 2602 7 of 13 packed structure (fcp) with an increase in carbonization temperature [42]. Using alkanes with different carbon atomic numbers as a carbonization gas will generally lead to different carbonation degrees and then generate different numbers of active sites [27]. K-Mo/Al 2 O 3 was carbonized by different atmospheres (CH 4 /H 2 , C 2 H 6 /H 2 , C 3 H 8 /H 2 ), and the catalytic performance of different catalysts in the synthesis of CH 3 SH from CO/H 2 /H 2 S is shown in Figure 3. Remarkably, K-Mo/Al 2 O 3 samples treated with different atmospheres all showed similar tendencies of catalytic performance, whereby the CO consumption rate first increased and then decreased. Catalysts treated with C 3 H 8 /H 2 had the highest CO consumption rate, followed by catalysts treated with C 2 H 6 /H 2 , and catalysts treated with CH 4 /H 2 had the worst CO consumption rate. From Figure 3B, it can be seen that the CH 3 SH generation rate of the catalyst treated with C 3 H 8 /H 2 , which had the highest CO consumption rate, was not optimum. Combined with the product analysis ( Figure 3E), the selectivity towards by-product CO 2 of this catalyst was high, which might have been caused by the hydrolysis of COS or the water-gas shift reaction of CO. Compared with that of the K-Mo 2 C/Al 2 O 3 catalyst treated with C 3 H 8 /H 2 , the CO consumption rate of the K-Mo 2 C/Al 2 O 3 catalyst treated with C 2 H 6 /H 2 was slightly lower. It can be seen intuitively in Figure 3B that the CH 3 SH generation rate decreased sharply at high temperatures. And it was found that the selectivity of by-products of the catalyst was higher at high temperatures (COS, CH 4 ) and the CH 3 SH selectivity was lower, which might be due to easy carbon deposition on the catalyst surface at high temperatures, further leading to a decrease in COS hydrogenation to generate CH 3 SH and excessive hydrogenation of CH 3 SH to generate CH 4 ; thus, CH 3 SH selectivity decreased. Although the K-Mo 2 C/Al 2 O 3 catalyst treated with CH 4 /H 2 exhibited a poor CO consumption rate, it had a relatively stable CH 3 SH generation rate and high CH 3 SH selectivity. To reveal the role of carbonization atmospheres in catalytic performance, the characterization of catalysts was conducted. hexagonal closely packed structure (fcp) with an increase in carbonization temperature [42]. Using alkanes with different carbon atomic numbers as a carbonization gas will generally lead to different carbonation degrees and then generate different numbers of active sites [27]. K-Mo/Al2O3 was carbonized by different atmospheres (CH4/H2, C2H6/H2, C3H8/H2), and the catalytic performance of different catalysts in the synthesis of CH3SH from CO/H2/H2S is shown in Figure 3. Remarkably, K-Mo/Al2O3 samples treated with different atmospheres all showed similar tendencies of catalytic performance, whereby the CO consumption rate first increased and then decreased. Catalysts treated with C3H8/H2 had the highest CO consumption rate, followed by catalysts treated with C2H6/H2, and catalysts treated with CH4/H2 had the worst CO consumption rate. From Figure 3B, it can be seen that the CH3SH generation rate of the catalyst treated with C3H8/H2, which had the highest CO consumption rate, was not optimum. Combined with the product analysis ( Figure 3E), the selectivity towards by-product CO2 of this catalyst was high, which might have been caused by the hydrolysis of COS or the water-gas shift reaction of CO. Compared with that of the K-Mo2C/Al2O3 catalyst treated with C3H8/H2, the CO consumption rate of the K-Mo2C/Al2O3 catalyst treated with C2H6/H2 was slightly lower. It can be seen intuitively in Figure 3B that the CH3SH generation rate decreased sharply at high temperatures. And it was found that the selectivity of by-products of the catalyst was higher at high temperatures (COS, CH4) and the CH3SH selectivity was lower, which might be due to easy carbon deposition on the catalyst surface at high temperatures, further leading to a decrease in COS hydrogenation to generate CH3SH and excessive hydrogenation of CH3SH to generate CH4; thus, CH3SH selectivity decreased. Although the K-Mo2C/Al2O3 catalyst treated with CH4/H2 exhibited a poor CO consumption rate, it had a relatively stable CH3SH generation rate and high CH3SH selectivity. To reveal the role of carbonization atmospheres in catalytic performance, the characterization of catalysts was conducted.

Characterization of Catalysts Carbonized under Different Atmospheres
The N 2 adsorption-desorption isotherms and physical properties of the catalysts assessed using N 2 physisorption are displayed in Figure 4 and Table 1. As shown in Figure 4, the oxidation state sample was a typical mesoporous structure with an IV curve and an H1 hysteresis ring, and the surface area and pore volume were 116.1 m 2 /g and 0.269 cc/g, respectively. After carbonization, the specific surface area of all samples decreased slightly, with CH 4 -K-Mo 2 C/Al 2 O 3 decreasing to 103.9 m 2 /g, C 2 H 6 -K-Mo 2 C/Al 2 O 3 decreasing to 101.3 m 2 /g and C 3 H 8 -K-Mo 2 C/Al 2 O 3 decreasing to 113.4 m 2 /g. However, the average pore volume of CH 4 -K-Mo 2 C/Al 2 O 3 , C 2 H 6 -K-Mo 2 C/Al 2 O 3 and C 3 H 8 -K-Mo 2 C/Al 2 O 3 increased to 0.321 cc/g, 0.290 cc/g and 0.390 cc/g, respectively. It can be therefore be concluded that the small decrease in specific surface area and the increase in pore volume after carbonization were caused by the collapse of some small pores during the carbonization process.

Characterization of Catalysts Carbonized under Different Atmospheres
The N2 adsorption-desorption isotherms and physical properties of the catalysts assessed using N2 physisorption are displayed in Figure 4 and Table 1. As shown in Figure  4, the oxidation state sample was a typical mesoporous structure with an IV curve and an H1 hysteresis ring, and the surface area and pore volume were 116.1 m 2 /g and 0.269 cc/g, respectively. After carbonization, the specific surface area of all samples decreased slightly, with CH4-K-Mo2C/Al2O3 decreasing to 103.9 m 2 /g, C2H6-K-Mo2C/Al2O3 decreasing to 101.3 m 2 /g and C3H8-K-Mo2C/Al2O3 decreasing to 113.4 m 2 /g. However, the average pore volume of CH4-K-Mo2C/Al2O3, C2H6-K-Mo2C/Al2O3 and C3H8-K-Mo2C/Al2O3 increased to 0.321 cc/g, 0.290 cc/g and 0.390 cc/g, respectively. It can be therefore be concluded that the small decrease in specific surface area and the increase in pore volume after carbonization were caused by the collapse of some small pores during the carbonization process.   The phase compositions of the catalysts of different carbonization atmospheres were characterized by XRD and Raman spectroscopy. As shown in Figure 5A, the diffraction peaks at 2θ = 38.1°, 39.4°, 61.7° and 75.7° were attributed to β-Mo2C (JCPDS card No.65-8766) and correspond to the (002), (101), (110) and (201) crystal planes, respectively. Small diffraction peaks of K2MoO4 species (JCPDS card No.29-1021) were also detected. In the Raman spectrum ( Figure 5B), diffraction peaks were detected at 325 cm −1 , 896 cm −1 and 917 cm −1 , which were attributed to monomolybdate MoO4 2− species and K2MoO4 on the sample surface [43,44], indicating an interaction between the precursor potassium and molybdenum. The diffraction peak was detected at 215 cm −1 , assigned to well-dispersed AlMo6O24H6 3+ anderson heteropolymetalate anions (AlMo6) formed by an alumina carrier and molybdenum-based aqueous solution [45][46][47]. The diffraction peaks at 661 cm −1 , 818  The phase compositions of the catalysts of different carbonization atmospheres were characterized by XRD and Raman spectroscopy. As shown in Figure 5A, the diffraction peaks at 2θ = 38.1 • , 39.4 • , 61.7 • and 75.7 • were attributed to β-Mo 2 C (JCPDS card No.65-8766) and correspond to the (002), (101), (110) and (201) crystal planes, respectively. Small diffraction peaks of K 2 MoO 4 species (JCPDS card No.29-1021) were also detected. In the Raman spectrum ( Figure 5B), diffraction peaks were detected at 325 cm −1 , 896 cm −1 and 917 cm −1 , which were attributed to monomolybdate MoO 4 2− species and K 2 MoO 4 on the sample surface [43,44], indicating an interaction between the precursor potassium and molybdenum. The diffraction peak was detected at 215 cm −1 , assigned to well-dispersed AlMo 6 O 24 H 6 3+ anderson heteropolymetalate anions (AlMo 6 ) formed by an alumina carrier and molybdenum-based aqueous solution [45][46][47]. The diffraction peaks at 661 cm −1 , 818 cm −1 and 990 cm −1 were also assigned to β-Mo 2 C [48,49]; among these, the peaks at 818 cm −1 and 990 cm −1 correspond to the stretching vibrations of Mo-C-Mo and Mo-C, respectively. The results of XRD and Raman spectroscopy showed that there was no significant difference in the phase structure of the catalysts under different carbonization atmospheres.
cm −1 and 990 cm −1 were also assigned to β-Mo2C [48,49]; among these, the peaks at 818 cm −1 and 990 cm −1 correspond to the stretching vibrations of Mo-C-Mo and Mo-C, respectively. The results of XRD and Raman spectroscopy showed that there was no significant difference in the phase structure of the catalysts under different carbonization atmospheres. H2-TPR was carried out on the catalysts under different carbonization atmospheres to investigate redox properties; the results for the catalysts are displayed in Figure 6. It can be seen that all K-Mo2C/Al2O3 catalysts had three obvious hydrogen consumption peaks, two of which appeared at 200-400 °C and 400-600 °C, with larger hydrogen consumption peaks at >600 °C. Generally, the hydrogen consumption peak at low temperatures (200-400 °C) was the reduction of the passivation layer on the surface of the molybdenum carbide [50]. The moderate temperature reduction peak (400-600 °C) can be attributed to the reduction of Mo 6+ to Mo 4+ or other high-valent Mo species [51]. The hightemperature reduction peak (>600 °C) can be attributed to the reduction of Mo 4+ or Mo 2+ to Mo atoms [52]. However, the sample was not subjected to passivation treatment, and the presence of Mo 6+ was not detected in the XRD spectrum. According to the literature [39,52], the hydrogen consumption peak at low temperatures (200-400 °C) is the reduction of surface high-valent molybdenum oxides, while the consumption peak at 650-800 °C is the reduction of Mo 4+ or Mo 2+ to Mo atoms. Therefore, in this work, the three hydrogen consumption peaks of 200-400 °C, 400-600 °C and >600 °C can attributed to the reduction of Mo 4+ , the reduction of potassium carbide species and the reduction of Mo 2+ to Mo atoms, respectively. K-Mo2C/Al2O3 catalysts synthesized under different carbonization atmospheres exhibited similar H2-TPR reduction peaks, but the CH4-K-Mo2C/Al2O3 catalyst showed larger hydrogen consumption peaks at low and medium temperatures, indicating that more Mo coordinatively unsaturated surface sites were formed on the catalyst, which is more conducive to the adsorption of reactants and the desorption of products, consistent with the above catalytic performances. H 2 -TPR was carried out on the catalysts under different carbonization atmospheres to investigate redox properties; the results for the catalysts are displayed in Figure 6. It can be seen that all K-Mo 2 C/Al 2 O 3 catalysts had three obvious hydrogen consumption peaks, two of which appeared at 200-400 • C and 400-600 • C, with larger hydrogen consumption peaks at >600 • C. Generally, the hydrogen consumption peak at low temperatures (200-400 • C) was the reduction of the passivation layer on the surface of the molybdenum carbide [50]. The moderate temperature reduction peak (400-600 • C) can be attributed to the reduction of Mo 6+ to Mo 4+ or other high-valent Mo species [51]. The high-temperature reduction peak (>600 • C) can be attributed to the reduction of Mo 4+ or Mo 2+ to Mo atoms [52]. However, the sample was not subjected to passivation treatment, and the presence of Mo 6+ was not detected in the XRD spectrum. According to the literature [39,52], the hydrogen consumption peak at low temperatures (200-400 • C) is the reduction of surface high-valent molybdenum oxides, while the consumption peak at 650-800 • C is the reduction of Mo 4+ or Mo 2+ to Mo atoms. Therefore, in this work, the three hydrogen consumption peaks of 200-400 • C, 400-600 • C and >600 • C can attributed to the reduction of Mo 4+ , the reduction of potassium carbide species and the reduction of Mo 2+ to Mo atoms, respectively. K-Mo 2 C/Al 2 O 3 catalysts synthesized under different carbonization atmospheres exhibited similar H 2 -TPR reduction peaks, but the CH 4 -K-Mo 2 C/Al 2 O 3 catalyst showed larger hydrogen consumption peaks at low and medium temperatures, indicating that more Mo coordinatively unsaturated surface sites were formed on the catalyst, which is more conducive to the adsorption of reactants and the desorption of products, consistent with the above catalytic performances. The TPD technique is commonly used to characterize the surface properties and chemical reactions of a material. As an important step in a catalytic reaction, the adsorption of reactant molecules by the catalyst plays a crucial role in the catalytic performance [53,54]. To investigate the adsorption and activation abilities of K-Mo2C/Al2O3 catalysts under different carbonization atmospheres for reactants, the catalysts were characterized The TPD technique is commonly used to characterize the surface properties and chemical reactions of a material. As an important step in a catalytic reaction, the adsorption of reactant molecules by the catalyst plays a crucial role in the catalytic performance [53,54]. To investigate the adsorption and activation abilities of K-Mo 2 C/Al 2 O 3 catalysts under different carbonization atmospheres for reactants, the catalysts were characterized by carbon monoxide, hydrogen and hydrogen sulfide adsorption and desorption; the results of CO-TPD, H 2 -TPD and H 2 S-TPD of the catalysts are displayed in Figure 7. As shown in Figure 7A, the CH 4 -K-Mo 2 C/Al 2 O 3 catalyst exhibited three CO desorption peaks, indicating three different CO adsorption sites and on the catalyst with different adsorption strengths. According to the literature, CH 4 -K-Mo 2 C/Al 2 O 3 has a low-temperature desorption peak at around 120 • C, which was assigned to the desorption of CO and physical adsorption of Mo 2 C. In the high-temperature region, the desorption peak was the dissociation adsorption peak formed by the strong chemical adsorption between the catalyst and CO [52]. C 2 H 6 -K-Mo 2 C/Al 2 O 3 and C 3 H 8 -K-Mo 2 C/Al 2 O 3 showed one and two CO desorption peaks, respectively, which were obviously lower than those of CH 4 -K-Mo 2 C/Al 2 O 3 . According to the related research, the Mo coordinatively unsaturated sites serve as the adsorption sites for the activation of CO [6,7,55]. Therefore, the CH 4 /H 2 atmosphere promotes the generation of more Mo coordinatively unsaturated sites on the catalyst and thus facilitates CO adsorption and activation, which also enhances the selectivity of CH 3 SH. The TPD technique is commonly used to characterize the surface properties and chemical reactions of a material. As an important step in a catalytic reaction, the adsorption of reactant molecules by the catalyst plays a crucial role in the catalytic performance [53,54]. To investigate the adsorption and activation abilities of K-Mo2C/Al2O3 catalysts under different carbonization atmospheres for reactants, the catalysts were characterized by carbon monoxide, hydrogen and hydrogen sulfide adsorption and desorption; the results of CO-TPD, H2-TPD and H2S-TPD of the catalysts are displayed in Figure 7. As shown in Figure 7A, the CH4-K-Mo2C/Al2O3 catalyst exhibited three CO desorption peaks, indicating three different CO adsorption sites and on the catalyst with different adsorption strengths. According to the literature, CH4-K-Mo2C/Al2O3 has a low-temperature desorption peak at around 120 °C, which was assigned to the desorption of CO and physical adsorption of Mo2C. In the high-temperature region, the desorption peak was the dissociation adsorption peak formed by the strong chemical adsorption between the catalyst and CO [52]. C2H6-K-Mo2C/Al2O3 and C3H8-K-Mo2C/Al2O3 showed one and two CO desorption peaks, respectively, which were obviously lower than those of CH4-K-Mo2C/Al2O3. According to the related research, the Mo coordinatively unsaturated sites serve as the adsorption sites for the activation of CO [6,7,55]. Therefore, the CH4/H2 atmosphere promotes the generation of more Mo coordinatively unsaturated sites on the catalyst and thus facilitates CO adsorption and activation, which also enhances the selectivity of CH3SH.  In H 2 -TPD, the CH 4 -K-Mo 2 C/Al 2 O 3 catalyst exhibited slightly strong hydrogen activation ability, producing a large amount of desorbed H at medium temperature, and there was an obvious H 2 negative peak, indicating that hydrogen might be dissociated into adsorbed H* at this temperature. Moreover, as shown in the H 2 S-TPD results ( Figure 7C), the CH 4 -K-Mo 2 C/Al 2 O 3 catalyst showed that it had strong H 2 S adsorption and dissociation abilities, very similar to the other two catalysts in the low temperature range but slightly stronger above 500 • C. In short, the use of CH 4 /H 2 as a carbonization atmosphere facilitates the adsorption and activation of reactants of K-Mo 2 C/Al 2 O 3 , thus delivering a high activity and selectivity towards CH 3 SH.

Conclusions
In conclusion, a series of K-Mo 2 C/Al 2 O 3 catalysts were successfully obtained through the impregnation method with a carbonization process at temperatures of 400-600 • C. Through careful control of the carbonization temperature, passivation process and carbonization atmosphere (500 • C, unpassivated and CH 4 /H 2 being optimal), a CH 4 -K-Mo 2 C/Al 2 O 3 catalyst with unusually high activity and selectivity towards CH 3 SH produced from CO/H 2 /H 2 S at 325 • C could be obtained (e.g., 66.1% selectivity and 0.2990 g·g cat −1 ·h −1 formation rate to CH 3 SH). Structural characterization studies revealed that passivation of the catalysts would lead to a layer formed on the surface of the material, covering the active sites of the reaction and reducing the activity of the catalyst. Under different carbonization temperatures and atmospheres (CH 4 /H 2 , C 2 H 6 /H 2 , C 3 H 8 /H 2 ), the catalyst carbonized by CH 4 /H 2 at 500 • C exhibited higher catalytic activity because it was more easily reduced under the same reduction conditions and resulted in more Mo coordinatively unsaturated sites, which was conducive to the adsorption of reactants and the desorption of intermediate products, thus showing the best overall performance. This work identifies K-Mo 2 C/Al 2 O 3 as a promising new Mo-based catalyst for the production of CH 3 SH from CO/H 2 /H 2 S.