Fabrication Of Ternary Sulfur Based Photocatalysts For Photocatalytic Reduction Of N₂ And Their Mechanism

Ammonia（NH₃）is an essential chemical and has been widely applied in industry and agriculture.Haber-Bosch process has been described as one of the most influential inventions in the 20th century,which has several disadvantages:high cost,harsh reaction conditions,low reaction yield and huge energy consumption.Highly effective utilization of solar energy for N₂ reduction and green energy conversion are currently facing critical challenges.However,most of traditional catalysts could be excited by ultraviolet light,and photo-generated electron-hole pairs are easily to recombine,leading to ultimately poor quantum efficiency.Additionally,it has been commonly recognized that the rate-determining step of N₂ fixation process is the dissociation of the N≡N bond,due to the extremely large bond energy of 940.95 k J mol^-1.Hence,how to further enhance the reaction efficiency and reveal the mechanism of N₂ activation on the surface of material are key scientific issues.To solve these problems,the ternary sulfur base nanomaterials,Ag In S₂/MXene and Mo_1-xW_xS₂,were synthesized to explore the catalytic properties of photoreduction N₂.With the aid of in situ Fourier Transform Infrared（FTIR）and the quantum chemical calculation method,the reaction process of photocatalytic synthesis of NH₃ were analyzed.In addition,the separation and migration of charges at the interface,its dynamic behaviors,and reaction mechanism had also been discussed thoroughly.The main achievements are as follows:In order to enhance the transport efficiency of photogenic carriers,0D Ag In S₂nanoparticles with different proportions were in situ grown on the surface of MXene nanosheets to synthesize 0D/2D Ag In S₂/MXene Z-scheme heterojunction nanosheets.It is found that 0D Ag In S₂ nanoparticles are employed as electron donors,and the MXene（Ti₃C₂）ultrathin nanosheets serve as the sites for N₂ chemisorption and electron transfer.This hybrid exhibits a short charge transport distance and excellent interfacial charge transfer abilities.The DFT calculations clearly indicated that the activation of N₂ in a dinuclear end-on bound structure could induce larger adsorption energy of N₂(E_ad=-5.20 e V)in the Ti₃C₂（001）surface with a high NH₃ synthesis rate of 38.8μmol g^-1_cat h^-1 under visible light.To improve activation efficiency and explore the absorption and activation mechanism,the mixed 2H/1T alloyed Mo_1-xW_xS₂ nanosheets were prepared.The results show that the efficiency of photoreduction N₂ can reach about 111μmol g^-1_cat h^-1 under visible light over Mo_1-xW_xS₂ nanosheets with the ratio of 2H/1T=1:1.With the aid of density functional theory（DFT）calculations and in situ N₂ adsorption X-ray absorption near-edge fine structure（XANES）techniques,the coordination chemistry and adsorption behavior of N₂ over the crystal interface were investigated during the N₂fixation process.The results show that the interface distortion with W doping leads to the largest adsorption energy（-2.05 e V）and higher electron density state in W 5d orbitals,which can not only polarize the adsorbed N₂ molecules for better activation but also promote the electron transfer to them for greatly enhancing photocatalytic efficiency.In situ infrared spectroscopy analysis showed that during the reaction process,the synthesis of NH₃ was accompanied by the formation of N₂H_y species.