First-principles Study On Cold Hydrogenation Of Silicon Tetrachloride

Currently,the modified Siemens method is the mainstream process for polysilicon production in the world,which produces a large amount of silicon tetrachloride（Si Cl₄,STC）while producing polysilicon.It is not only conducive to environmental protection but also waste of resources.The STC is hydrogenated to produce trichlorosilane（Si HCl₃,TCS）and further used to produce polysilicon,which can realize closed-loop production and clean utilization.STC hydrogenation mainly includes three processes:cold hydrogenation,thermal hydrogenation and plasma hydrogenation.Cold hydrogenation has attracted widespread attention due to its low reaction temperature and lower energy consumption.In recent years,the catalytic reaction of STC cold hydrogenation has progressed rapidly.Catalysts with high catalytic activity and selectivity have been obtained,but the reaction mechanism is still unclear due to the involvement of many by-products in the reaction,which is the key to developing more efficient catalysts and improving the reaction process.In this thesis,density functional theory calculation method was used to study the reaction path of STC and hydrogen and explore the active components.Considering the energy change and activation energy barrier of the reaction involved in the hydrogenation of STC,the effect of Cu vacancy number in the Cu-based catalyst on the reaction was systematically studied.The reaction paths of HCl and Si atoms on different surfaces to produce products.The main conclusions are as follows:（1）The energy calculation results show that the hydrogenation of STC to synthesize trichlorosilane,dichlorosilane,monochlorosilane and silane is endothermic,which requires external energy.The Si-Cl bond energy of silicon tetrachloride is lower than H-H bond energy of hydrogen,and the chain reaction is initiated by breaking the Si-Cl bond.The energy barrier to generate trichlorosilane is lower than the subsequent generation of dichlorosilane,monochlorosilane and silane.Trichlorosilane will be the main product when the raw materials are sufficient.（2）For the perfect Cu₃Si（001）surface,the direct reaction path of STC with H₂is the most kinetically favorable.For the Cu₃Si（001）surface with Cu vacancies,when a single Cu vacancy exists on the surface,DFT calculations indicate that there is a strong interaction between the catalyst and STC at this time,which directly promotes the direct dissociation path of STC.When two Cu vacancies exist on the surface,the interaction between the catalyst and STC is more intense,which further reduces the energy barrier of the direct dissociation path of STC.Because the direct dissociation pathway of STC is a rate-determining step,the increase of the number of Cu vacancies in Cu₃Si catalyst significantly improves the catalytic activity of Cu₃Si active materials.（3）We mainly explored the reaction path of generating TCS by the reaction of HCl with different silicon atoms.After calculation,it is found that it is difficult for Si atoms to desorb directly from the surface,and the path energy barrier of HCl reacting with Si from catalyst to generate products is even lower.On the surface of Si（100）-c（4×2）,the rate-determining step is the formation of Si H₂Cl₂,and the reaction energy barrier is 340.13 kmol^-1.For Cu₃Si catalyst,the rate-determining step is also the formation of Si H₂Cl₂with a reaction energy barrier of 221.60 k J mol^-1.For the single Cu vacancy Cu₃Si catalyst,the adsorption energy of the second HCl increases,which significantly reduces the energy barrier of the Si H₂Cl₂generation step.the rate-determining step is the Si HCl₃ generation reaction in the gas phase,and the energy barrier is 160.10 k J mol^-1.The results show that the Cu₃Si catalyst containing Cu vacancies exhibits great catalytic performance in this reaction.