Theoretical Study On Photo- And Electrocatalytic Reduction Of CO₂ With Manganese Complexes

With the rapid development of human society,the global demand for energy is growing,and carbon dioxide emissions are increasing sharply,causing many environmental problems.To reduce the amount of CO₂ in the atmosphere is of great urgency.The catalytic reduction of CO₂into fuel molecules such as CH₃OH,HCOOH and CO by transition metal complexes is one of the important CO₂ mitigation strategies.Hence,it is a challenge to design high efficiency molecular catalysts to reduce CO₂.In recent years,manganese carbonyl complexes have been widely developed as catalysts for photo-and electroreduction of CO₂.To improve the selectivity and efficiency of catalysts,rational design of redox active ligands is crucial.In this thesis,the mechanisms of photo-and electrocatalytic reduction of CO₂ mediated by manganese carbonyl complexes containing bipyridine（bpy）,terpyridine（tpy）and pyridine-oxazoline（pyrox）ligands were studied using density functional theory.The first system is a theoretical study on the electrocatalytic reduction of CO₂ to CO by Mn-bpy and Mn-tpy carbonyl complexes which is assisted by CH₃OH as external proton source.The results indicate that Mn-bpy and Mn-tpy catalysts have similar ECE two-electron reduction processes to generate active catalysts.The first one-electron reduction is located on the bpy/tpy ligand generating[Mn^I(R-bpy^·-)（CO）₃Br]^-/[Mn^I(R-tpy^·-)（CO）₃Br]^-.Then,Br^-is dissociated immediately leading neutral radical[Mn⁰（R-bpy）（CO）₃]/[Mn⁰（R-tpy）（CO）₃].The second one-electron reduction generates the active catalysts[Mn(R-bpy（CO）₃]^-/[Mn（R-tpy）（CO）₂]^-.However,due to the larger delocalization of tpy,the reduction potential of Mn-tpy is slightly smaller.[Mn（R-bpy）（CO）₃]^-is more nucleophilic towards CO₂ attributing to stronger back-donation of tpy,which reduces the electron density of Mn in HOMO.Sequentially,[Mn-CO₂]^-extract one hydrongen from CH₃OH giving[Mn-COOH].Two paths are followed from[Mn-COOH]named reduction-first and protonation-first.The protonation-first path can be ruled out for the high barriers.For the reduction-first path,[Mn-COOH]^-is resulted by getting one electron which determines catalytic reduction potential.Further protonation of[Mn-COOH]^-can break C-OH bond which is the rate-determing step of the catalytic cycle.Then,CO is produced and catalyst is regenerated after decarbonylation and reduction.When the reaction is conducted at the equilibrium potential of[Mn-COOH]to[Mn-COOH]^-,Mn-tpy has lower C-OH cleavage barrier for two reasons:First,more electrons are located in COOH group in[tpy-Mn-COOH]^-.Thus,C-OH bond in[tpy-Mn-COOH]^-is more activated and fewer electrons are required to break it.Second,during the process of C-OH disconnection,the electrons are supplied by tpy ligand in[tpy-Mn-COOH]^-while they are provided by CO ligands in[bpy-Mn-COOH]^-.Moreover,by studying the effect of CH₃OH concentration,we found that the C-OH cleavage barrier can be diminished by forming hydrogen bonds.The second system mainly studied the reaction mechanism of photocatalytic and electrocatalytic reduction of CO₂ by pyridine-oxazoline-based manganese complex Cat1-Cat3.For photocatalytic reaction,proton transfer between BIH⁺and TEOA is thermodynamically and kinetically unfavorable based on current calculation results,which limits the photocatalytic efficiency.For electrocatalytic reaction,the product selectivity of electrocatalysis is mainly determined by the abilities of the active catalyst to attack CO₂ and extract hydrogen from Br（?）nsted acid.When TFE is used,Mn is more likely to attack CO₂ to selectively generate CO.The studies of orbital interactions demonstrate that theπ*of CO₂ is attacked by the dz² of Mn while theσ*of O-H is attacked when abstracting hydrogen.In the next step,we will continue to improve the photocatalytic mechanism,and expand the study of orbital interactions to explore the regulations affecting product selectivity.