Theoretical Study On Electrocatalytic Reduction Of CO₂ With Tricarbonyl Manganese Complex

Electrochemical reduction of CO₂ to generate fuels or fuel precursors and value-added chemicals is a promising method for CO₂ conversion and utilization.Transition metal complexes as catalysts for electrochemical reduction of CO₂,especially the use of cheap and abundant manganese complexes as catalysts,has become a hot topic in recent years.The main challenges in this field are to improve the catalytic activity of molecular catalysts and reduce overpotential.Regulating the ligand structure or changing the ligand framework can affect the electronic and steric structures of the electrocatalyst,and then adjust the catalytic reduction potential and catalytic activity.In addition,the introduction of functional groups in the second coordination sphere can enhance intramolecular interactions and thus improve the catalytic reaction rate and selectivity.In this dissertation,the mechanisms of the following two works are studied by the density functional theory?DFT?method.In the first system,the reaction mechanism of catalytic reduction of CO₂ by pyridine-oxazoline-based manganese complex with the assist of weak Br?nsted acid was first studied by DFT calculation.Based on this,we investigated the effects of a series of electron-donating and electron-withdrawing substituents on the reduction potential and catalytic activity of pyridine-oxazoline manganese complexes.The calculation results show that the reduction potential of the complexes with electron-withdrawing substituents is reduced,which is beneficial to decrease the overpotential of the reaction.Electron-donating substituted complexes can significantly improve catalytic activity.Furthermore,the activity of bisoxazoline manganese complexes for electrocatalytic reduction of CO₂ were studied demonstrating enhanced catalytic activity with larger overpotentials.By extending the?framework and introducing electron-withdrawing substituents for ligand modification,the overpotential can be reduced,but the catalyst activity also decreases.In the second system,the detailed mechanism of catalytic reduction of CO₂ to CO by the manganese complex{Mn[bpyMe?ImMe?]?CO?₃Br}PF₆?1PF₆,bpyMe?ImMe?=6-?methyl-imidazoium?-6-methyl-2,2'-bipyridine?with imidazole group in the second coordination sphere with the aid of weak Br?nsted acid H₂O was firstly studied in detail by DFT calculation.The calculation results show that the strong acidic C2-H in the imidazole group plays an important role in the steps of CO₂ coordination,protonation and C-OH bond cleavage.There are strong intramolecular interactions between imidazole groups and carboxyl and proton sources,which enhance the catalytic activity of the catalyst.By changing the heterocyclic substituents in the second coordination layer,and calculating the potential energy surface of the catalytic reduction of CO₂,we further explored the effect of different types of heterocyclic substituents on the catalytic activity,looking for catalysts with better catalytic performance.Studies on different heterocyclic substituents show that the benzimidazole manganese complex{Mn[bpyMe?BenImMe?]?CO?₃Br}PF₆?L3?has a significantly enhanced catalytic activity at comparable reduction potentials.Therefore,L3 might be a more efficient electrocatalys theoretically.