Simulation Of Key Reaction Processes In Polysilicon Production By Improved Sie-mens Method

To achieve the carbon peaking and carbon neutrality targets,the photovoltaic industry has become a strategic emerging industry in China.Polysilicon is the basic raw material of photovoltaic industry.At present,most enterprises in China mostly use the improved Siemens method to produce polysilicon,that is,the high-temperature hydrogenation reduction of SiHCl₃ above 1000℃.There are a large number of by-products SiCl₄ produced in the reduction process;at the same time,the power consumption of high-temperature reduction increases the cost of polysilicon production by improved Siemens method,which directly affects the market competi-tiveness of polysilicon products.The SiHCl₃ reduction power consumption generally accounts for more than 70%of the comprehensive power consumption.Therefore,improving the catalytic performance of Cu based catalyst for SiCl₄ hydrogenation to SiHCl₃ and clarifying the reduction mechanism of SiHCl₃ are the key to optimize the improved Siemens method.Based on the reaction mechanism of SiCl₄ hydrogenation to SiHCl₃ on Cu based catalyst,the effects of different content of Cl vacancy,Si doping and different crystal surface structure on the catalytic performance（activity and selectivity）of Cu based catalyst were studied by using density functional theory;for SiHCl₃ reduction reaction,the mechanism of SiHCl₃ reduction reaction was clarified by adjusting the crystal surface structure,Si vacancy and interface of silicon rod;the crystal surface structure and active sites of polysilicon deposition were clarified,so as to improve the silicon deposition rate,inhibit the production of SiCl₄ and lower the reduction temperature of SiHCl₃.The main conclusions were as follows:（1）The double Cl vacancy CuCl₂（001）has the best catalytic activity and selec-tivity for SiCl₄ hydrogenation to SiHCl₃.The optimal reaction path is SiCl₄→SiCl₃+Cl+（H）→SiHCl₃;Cl vacancy changes the micro geometry of CuCl₂（001）surface,and optimizes the coordination environment of Cu active site,and finally improves the catalytic activity and selectivity of CuCl₂ catalyst for SiCl₄hydrogenation to SiHCl₃.The Cl vacancy leads to the charge loss of Cu atom and makes the d-band center of Cu atom close to Fermi level,which is conducive to the adsorption and activation of surface species,and shows excellent catalytic activity and selectivity for SiCl₄ hydrogenation to SiHCl₃.（2）Cu-Si bond and Cl vacancy on CuCl₂（100）surface containing doped Si atom and single Cl vacancy synergistically catalyze SiCl₄ hydrogenation to SiHCl₃;Cu-Si bond provides sufficient active sites for the dissociation of SiCl₄,and Cl vacancy producing the low coordination Cu active sites gently promotes SiHCl₃ formation.Cl vacancy and Cu-Si bond synergistically activate the Si-Cl bond of SiCl₄,promoting SiCl₃ intermediate and improving the production rate of SiHCl₃.In industry,CuCl₂catalyst can be pretreated with silicon powder and hydrogen to obtain a new CuCl₂catalyst containing Cl vacancy and Cu-Si bond,so as to improve the activity and se-lectivity of SiCl₄hydrogenation to SiHCl₃.（3）（10-10）-Cu and（10-10）-Cu-Si with the highest proportion of hollow sites are conducive to H₂ dissociation.The catalytic activity of（10-10）-Cu and（10-10）-Cu-Si for SiCl₄ hydrogenation is higher than that of（10-10）-Si,but the selectivity for the formation of SiHCl₃ is weaker.The active center on（10-10）-Cu surface with square structure is composed of four Cu atoms and obtains electron;the active center pro-motes the excessive dissociation of SiCl₄,resulting in the production of by-products SiH₂Cl₂ and SiH₃Cl.The active center on（10-10）-Cu-Si surface with square structure is composed of two Cu atoms and two Si atoms;Si atoms cause the active center to lose electrons and inhibit the activity of Cu atoms,so that there is a competitive rela-tionship between SiHCl₃ and SiH₂Cl₂.The active center of（10-10）-Si surface is occu-pied by Si atom,inhibiting the activity of Cu atom and hindering the excessive disso-ciation of SiCl₄,which leads to the highest selectivity for SiHCl₃ formation.（4）Si atoms are easy to form and deposit on Si（111）surface,and the most fa-vorable path is SiHCl₃→SiCl₃+H→SiCl₂+Cl→SiCl+Cl→Si+Cl.Si（111）surface hin-ders the migration and interaction of SiCl₃ and Cl,and finally inhibits the formation of SiCl₄.Compared with Si（110）and Si（100）,Si（111）surface shows the highest atomic density and provides more active centers to promote the formation of Si.（5）Si（111）-V containing silicon vacancy is easy to promote SiHCl₃ reduction to Si and inhibit the by-products.The optimal reaction path for the formation of Si is SiHCl₃→SiCl₃+H→SiCl₂+Cl→SiCl+Cl→Si+Cl.The silicon vacancy on Si（111）-V surface changes the charge distribution of the surface,and more charges gather on the low coordination Si atoms around the silicon vacancy,which forms a state of electron loss in silicon vacancy;silicon vacancy also leads to charge transfer between surface Si and subsurface Si atoms.The change of electronic structure directly affects the ad-sorption and activation of active sites to species,so that Si（111）-V has the highest ac-tivity and selectivity for SiHCl₃ reduction to Si.In the polysilicon production by im-proved Siemens method,SiHCl₃ reduction to Si can be promoted by mainly exposing Si（111）surface containing Si vacancy of silicon rod,and optimizing the distribution of flow field and temperature field in the reactor.The research results provide a basis for polysilicon manufacturers to improve the activity and selectivity of SiCl₄ hydrogenation to SiHCl₃ by modifying Cu based cat-alyst,and improves the reduction speed and deposition efficiency of SiHCl₃ by ad-justing the crystal surface structure of silicon core,silicon vacancy and controlling polysilicon deposition.It provides theoretical support for reducing the energy con-sumption of polysilicon industry and meeting the low-carbon demand of photovoltaic products.