Mechanism Research On Co-transformation And Utilization Of Sulfur-carbon Species Over MoS₂ Materials

Sustained large-scale hazy weather has a significant negative impact on the human health and transportation.Sulfate aerosol is the behind-the-scenes and the main driving precursor of hazy.Coal,the main energy source in China,is one of the important sources of sulfur compounds.The removal of various types of reduced sulfur species generated by the clean utilization of coal(coal gasification)has become a research hotspot in the field of environmental pollution control and coal chemical industry.Synergistic catalytic conversion of sulfur-carbon species(CO/H₂S/H₂)produced by coal gasification process into a high value-added product-methyl mercaptan(CH₃SH,an irreplaceable raw material for the synthesis of essential amino acids(methionine))is a new way to alleviate hazy pollution.High-efficiency catalysts are the key factor for synthesizing CH₃SH from CO/H₂S/H₂ mixtures.Potassium-molybdenum(K-Mo)-based catalysts are potential catalysts due to their excellent sulfur resistance and excellent catalytic performance.However,the reaction system faced by K-Mo-based catalysts is relatively complicated,and the reaction pathway and the structure of catalytically active phase are still contradictory.Meanwhile,the structure-activity correlation and reaction mechanisms between K-Mo-based catalysts and CH₃SH synthesis are not clear.These have restricted the further development of the research system for the utilization of sulfur-carbon pollutants.In view of the above problems,a series of K-Mo-based catalysts was prepared and the catalytic performance for the synthesis of CH₃SH was investigated by exploring different types of supports,different vulcanization temperatures and vulcanization times,and K/Mo molar ratios.The modern advanced characterization technique demonstrates the active species types and the properties on K-Mo based catalysts as well as the reaction pathways for synthesis of CH₃SH from CO/H₂S/H₂.Moreover,the reaction mechanisms for the synthesis of CH₃SH were clarified,and the structure-activity relationship between K-doped MoS₂ phases and the catalytic performance was established.The main research contents and conclusions are as follows:(1)Study on the types and properties of K-doped MoS₂ active phase on K-Mo-based catalysts.We tackle this issue through investigating the effect of supports on the nature of MoS₂ catalysts via the characterizations of XRD,TPS,XPS,HR-TEM,as well as HAADF-STEM combined with energy-dispersive spectroscopy(EDS),etc.The results suggested that K species could not only lead to formation of different oxidized precursors but also affected the microstructure,surface morphology and electronic properties of active MoS₂ phases.Three sulfided species,i.e.,pure MoS₂,K-decorated MoS₂ and K-intercalated MoS₂ nanoslabs,were simultaneously detected.These sulfide species were demonstrated to be derived from the reduction and sulfidation of oxidized precursors via O-S exchange mechanism.Furthermore,the activity results showed that pure MoS₂ species worked only for the reaction of CO with H₂S to COS,while K-decorated MoS₂ and K-intercalated MoS₂nanoslabs exhibited unique ability for the hydrogenation of COS to produce CH₃SH.(2)Structure-property correlations between different types of K-doped MoS₂ phases and catalytic activity.By controlling the vulcanization way(vulcanization temperature and vulcanization time),the phase behavior for transforming K-Mo oxide precursors into sulfides during vulcanization was investigated.The structure-property relationships between K-decorated MoS₂ phase(and K-intercalated MoS₂ phase)and the catalytic performance of CH₃SH synthesis were established.Resluts showed that K-Mo oxide has complex phase transition,phase separation and phase recombination behavior during vulcanization.K-intercalated MoS₂ phase has higher catalytic performance than K-decorated MoS₂ phase,the former has positively linearly correlation with the catalytic performance of CH₃SH synthesis,while the latter exhibited the negatively relationship with the catalytic performance.(3)The reaction pathway for synthesizing methyl mercaptan(CH₃SH)using H₂S-containing syngas(CO/H₂S/H₂)as reactant gas over SBA-15 supported K-Mo-based catalysts.In light of designed kinetic experiments together with the thermodynamics analyses,the corresponding reaction pathways synthesizing CH₃SH over K-Mo/SBA-15were proposed.COS was demonstrated to be generated firstly via the reaction between CO and H₂S,and then CH₃SH was formed via two reaction pathways.The first one is the direct hydrogenation of COS,and the second one is the hydrogenation of CS₂ originated from the disproportionation of COS.(4)Study the effect of K/Mo molar ratio on the modulation behavior and reaction mechanism of K-Mo catalysts.Supported few-layers and ordered unpromoted and K promoted MoS₂ nanosheets with different K/Mo molar ratio were rapidly synthesized via the ultrasound induced impregnation method assisted by microwave thermal treatment.The as-prepared samples were characterized by TEM,XPS,N₂ absorption-desorption,XRD,Raman,H₂-TPRS,and COS-TPD as well as Density functional theory calculations(DFT).The results showed that supported K promoted MoS₂ nanosheets exhibited the higher conversion of CO and the selectivity of CH₃SH than that of conventional impregnated catalysts and the catalysts reported in the literature.The catalytic activity was demonstrated to be interpreted,for the first time,by the bond energy model in the spirit of Sabatier principle.The doping of K additive over MoS₂ species in a certain range was considered to increase the strength of Mo-S bond to the moderate intensity via the electronic effect,as demonstrated by H₂-TPRS and DFT calculation,which facilitated the stabilization of C-S bond while activating COS molecule.The size effect originated from the different preparation methods was also found to pose a significant influence on the chemical bonding environment of Mo atom,thus weakening the Mo-S bond strength to moderate degree.(5)Study on the treatment of malodorous CH₃SH tail gas.The reaction kinetics for CH₃SH catalytic decomposition was established by preparing La modified ZSM-5 catalyst.The preparation of SBA-15 supported Mo O₃ material achieved high low-temperature activity and long-term stability,and the reaction mechanisms and inactivation behavior for catalytic decomposition of CH₃SH ware clarified.Based on the above results,the K-Mo-based catalyst for synthesis of CH₃SH was successfully applied for the catalytic decomposition of CH₃SH tail gas to achieve dual-function application of the catalyst.