Theoretical Study On CO₂ Reduction And N₂ Reduction Catalyzed By Transition Metal Complexes With Conjugated Ligand

Global warming and energy shortage are two major challenges of the world.Electrocatalytic CO₂ reduction reaction?CO₂RR?and nitrogen reduction reaction?NRR?are considered to be effective methods to solve the above two problems.The transition metal Re complexes containing conjugated ligands are considered as one of the most effective CO₂ electrocatalysts due to their high selectivity and low overpotential.However,the capture of reaction intermediates in experiment is still difficult,which limits the understanding of reaction mechanism at the atomic level and thus hinders the development of catalyst.Compared with the reaction condition of Haber-Bosch under high pressure and high temperature,the reaction condition of electrocatalytic NRR for ammonia is moderate?ambient temperature and normal pressure?.Therefore,electrocatalytic NRR is an ideal strategy to replace traditional Haber-Bosch process.The transition metal Fe complex containing conjugated ligands can effectively catalyze NRR,but the NRR catalyzed by Fe complex has the disadvantages of low faraday efficiency and high overpotential.Therefore,it is urgent to find more efficient NRR catalysts.In the past few decades,the design of functional materials has been an important chemical research.Molecular materials are versatile by appropriate adjustment of their properties,such as electron conduction,magnetism and self-activity.Among the molecular materials,?-conjugated molecular materials have received extensive attention.The?electrons distributed through the?conjugated network can reduce band gap,promote electron diffraction deviation,thermodynamic stability,and enhance intermolecular electronic interactions.?-conjugated compounds have become indispensable in molecular materials.The coordination of ligand and metal can form a more stable rigid molecular structure.In addition,the center metal ion and ligand have a good carrier transport type,which promotes the multi-electrons reaction of CO₂RR and NRR.With the increasing maturity of computer technology,quantum chemical calculation has received more attention as an important experimental supplement.Predicting the unknown compounds properties by theoretical calculations will greatly reduce the cost of design and synthesis new catalysts.In this thesis,the transition metal complexes containing conjugated ligands as catalysts in CO₂RR and NRR weer studied by using density functional theory?DFT?methods.The thesis consists four chapters:the first chapter is the introduction,which mainly contains the development of CO₂RR and NRR catalysts,and applications of?-conjugated molecular materials.The second chapter is the theoretical basis and calculation method.The third to fourth chapters are the main contents of the thesis:1.Based on DFT method,the processes of Re?py-bpy??CO?₃Cl?gpy=pyrazine coordination nanometer graphene;bpy=2,2'-bipyridine?as electrocatalyst for CO₂RR was studied.The results showed that the catalytic activity of Re?pyy-bpy??CO?₃Cl is determined by the strong redox ability of gpy-bpy conjugate ligand.The lower energy level between the highest occupied molecular orbital?HOMO?of CO₂ and the lowest unoccupied molecular orbital?LUMO?of the active catalyst than that between the HOMO of the proton source CH₃OH and the LUMO of the active catalyst is considered to be the intrinsic reason for the selectivity of CO₂ reduction.The present study would provide reliable theoretical guidance for improving the performance of catalysts.2.The performance of?–conjugated metal tetraphenyl porphyrins M–TPP?M=Ti,V,Cr,Mn,Co,Ni,Cu,Zn,Ru,Pd,Os,Pt,Mo,Rh,Ir,Re,V,W;TPP=tetraphenylporphyrin?as NRR electrocatalysts was investigated by DFT calculation.The results revealed that W–TPP is an efficient NRR electrocatalyst among all studied candidates.The W–TPP preferentially catalyzes NRR via the distal mechanism.The second NH₃ desorption is the rate-determining step,the corresponding Gibbs free energy change value and the overpotential of NRR on W–TPP catalyst are 0.65 eV and 0.49 V,respectively.This provides reliable theoretical guidance for the design of catalysts.