Synthesis Of MoS₂ And Its Applications As Hydrogenation Catalysts

MoS₂ as a typical layered inorganic analogue of graphene,has been considered as ideal catalysts for the hydro-treating process of petroleum.The catalytic performance of MoS₂ is associated with its structures,sizes and morphologies due to its anistropic structures.New methods for preparing MoS₂ with controllable nanostructures are demanded in order to prepare highly active MoS₂ catalysts.Herein,this dissertation is focusing on exploring novel approaches to synthesize nano-MoS₂ with controllable structures,sizes and morphologies,discussing the formation mechanism of the nano-MoS₂ samples,and investigating their catalytic activities for the hydrogenation of polycyclic aromatic hydrocarbons,taking anthracene as the model compound.The main results are summarized as follows:MoS₂ nanosheets were synthesized using ammonium thiomolybdate and thiourea as the precursors with hydrothermal method.The structures,sizes,compositions,and morphologies of MoS₂ nanosheets were characterized and its catalytic activities over MoS₂ nanosheets for anthracene hydrogenation were tested.Intercalated MoS₂5 intermediate MoS₂ and crystallized 2H-MoS₂ were prepared by adjusting crystallization time.Intercalated MoS₂ possesses enlarged interlayer distance,rich defects and dislocations.Intermediate MoS₂ is the intermediate state between intercalated MoS₂ and crystallized 2H-MoS₂,which contains enlarged interlayer distance,rich defects,dislocations and desired crystallinity.Crystallized 2H-MoS₂ has high crystallinity.The growth mechanism of MoS₂ nanosheets may undergo three steps:the nucleation,the combination,and the Ostwald ripening.The results of catalytic reactions revealed that intermediate MoS₂ prepared at 200 ℃ for 24 h with pH value of 5 exhibits optimized catalytic activities of anthracene hydrogenation.The selectivity to deep hydrogenation product octahydroanthraquinone（8HN）and the hydrogenation ratio of anthracene are 47.5%and 41.2%,respectively.MoS₂ catalysts were synthesized using ammonium thiomolybdate and thiourea as the precursors with surfactants-assisted hydrothermal method.The adopted surfactants included PEG,PVP,P123,AOT,SDS,and CTAB.The structures,sizes and arrangements of MoS₂ nano sheets could be controlled by adjusting surfactants,resulting in MoS₂ catalysts with various morphologies.Spherical MoS₂ catalysts were prepared using PEG and PVP as surfactants.Bulk-like MoS₂ catalysts were prepared using P123 and AOT as surfactants.Flower-like MoS₂ catalysts were prepared using SDS and CTAB as surfactants.It has been found,highly dispersed spherical MoS₂ catalysts can be synthesized with the assistance of PEG,in which MoS₂ nano-sheets with slab length of shorter than 15 nm and stacking layers of less than 6.During the growth of MoS₂ catalysts,surfactants act as the templates.MoS₂ catalyst synthesized with the assistance of PEG exhibited the highest catalytic activities for anthracene hydrogenation.The selectivity to deep hydrogenation product 8HN and the hydrogenation ratio of anthracene are 54.0%and 44.4%,respectively.MOS2 catalysts were synthesized using ammonium thiomolybdate and thiourea as the precursors,CTAB as surfactant,the mixed solution of ethanol and water as the solvent with solvothermal method.The sizes and morphologies of MoS₂ catalysts could be controlled by adjusting the volume ratio of ethanol and water.Hollow spherical MoS₂ catalysts were prepared from the mixed solutions with volume ratio V（C2H5OH）/V（H2O）of 1/39～11/29.Hollow spherical MoS₂ particles prepared from the mixed solvents with the volume ratio V（C2H5OH）/V（H2O）of 1/19 have the diameters of 1.8～2.0 μm and the uniform wall thickness about 150-200 nm,in which MoS₂ nano-sheets have few stacking layers（about 4～6 layers）.The hollow spherical MoS₂ catalyst exhibited the highest catalytic activities for anthracene hydrogenation.The selectivity to deep hydrogenation product 8HN and the hydrogenation ratio of anthracene are 55.4%and 45.3%,respectively.MoS₂ particles were prepared using ATTM as the single precursor with surfactant-assisted hydrothermal method.The morphologies of MoS₂ particles could be controlled by adjusting surfactants.Poorly spherical MoS₂ particles with serious aggregation were prepared with CTAB,AOT and PEG as surfactants.Highly dispersed ellipsoidal MoS₂ particles were synthesized with PVP surfactant.The long axis and short axis of ellipsoidal MoS₂ particles were 800 nm and 400 nm,respectively and exhibited the highest catalytic activities for anthracene hydrogenation.The selectivity to deep hydrogenation product 8HN and the hydrogenation ratio of anthracene are 65.1%and 46.8%,respectively.Colloidal MoS₂ can be synthesized with PVP-assisted solvothermal method.The size of MoS₂ colloidal particles could be controlled by adjusting the solvothermal temperature.MoS₂ colloidal particles synthesized at 140 ℃ were about 50 nm and exhibited the highest catalytic activities for anthracene hydrogenation.The selectivity to deep hydrogenation product 8HN and the hydrogenation ratio of anthracene are 51.1%and 41.1%,respectively.Above all,the catalytic activities of MoS₂ catalysts with various structures,sizes and morphologies decreased as the following order:ellipsoidal MoS₂ particles>MoS₂ hollow spheres>MoS₂ solid spheres>MoS₂ nano-sheets.The catalytic activities of MoS₂ catalysts are closely associated with the exposure of active edges.Shorter slab and fewer stacking layer of MoS₂ nano-sheets benefit to expose much more active edges.In addition,the enlarged interlayer distance,rich defects and dislocations of MoS₂ nano-sheets facilitate the exposure of active edges.Spheres,hollow spheres and ellipsoidal MoS₂ with smaller size and higher dispersion could provide much more exposed active edges.