Theoretical Study On The Reduction Of Carbon Dioxide By Nitrogen-coordinated Scandium And Uranium Complexes

As industrial production consumes a great amount of fossil energy,the carbon dioxide content in the air exceeds the standard,which not only causes serious environmental problems,but also trigers worldwide energy crisis.As a safe and renewable C1 energy source,CO₂ reduction can lower the amount of greenhouse gas on the one hand,and achieve the goal of recycling energy.At present,precious metals have been the main catalysts for carbon dioxide.Limited by their high cost and scarcity,there is an urgent need to develop alternative catalysts.Due to special electronic properties,the early group transition metals can effect efficient CO₂ reduction by carefully selecting ligands.In recent studies,low-valent actinides have shown certain advantages in the field of energy.Therefore,in this thesis,relativistic density functional theory(DFT)was used to study structures of group 3 metal and uranium complexes with various N-dornor ligands and their catalytic CO₂ reduction reactions(CO₂RR).Firstly,silylamide(L^Me and L)complexes[M^m(L^Me)₃]ⁿ and[M^mL₃]ⁿ(M=Sc,Y,La and Ac;M=III and II;n=0 and-1)were designed and calculated.It is shown that the M-N bond is of weak dative single bond.The topological analysis of chemical bonds finds that the metal and carbon dioxide are also chemically coupled by dative bonds.The small reaction free energy?_rG(0.03?0.13 e V)is needed for the CO₂ adsorption by catalyst.The?_rG of subsequent hydrogenation reactions are all negative,and the de-sorption of newly functionalized CO is an exothermic process.Thermodynamic and kinetic calculations show that the catalytic capacity of the metal complexes increases gradually from Sc?Y?La?Ac.On the basis of these reactions,the activation of low-valent uranium complexes on a series of small molecules has been explored.The order of chemical bond strength between uranium and small molecules is NO>CO>N₂>CO₂.Secondly,complexes[MPz]ⁿ(M=Sc,Y,La,Ac and U;n=0 and 1)have been constructed by using azaporphyrin ligand(Pz).It is found that the II valence Y,La,Ac and U complexes are able to adsorb CO₂ and form two coordination configurations.One is that an oxo of CO₂ oxygen coordinates the metal,and the linear CO₂ displays a tilt angle with respect to the plane of complex.Second,O and C of CO₂ are simultaneously coordinating the metal.In the former,there is almost no electron density distribution around the CO₂ part,while in the latter,0.533?0.641 electrons flow to moieties of ligand and carbon dioxide.The spin density of the ligands varies from 0.000to 1.949,indicating that the ligands play a major role in the reaction process.In the reduction process of adding hydrogen atoms,it is found that the bond order of C and O atoms in CO₂ changes from 2.16 to 1.30 as the reaction goes on.Carbon dioxide tends to break and generate CO and H₂O,which reveals that these complexes have a reducing effect on carbon dioxide.Finally,the complexes[M^m(CN)](M=Sc,Y,La,Ac and U;m=II and III),where CN is a molecular unit of g-C₃N₄.It is found that the?_rG of adsorbing CO₂ by the trivalent complex ranges from-0.48 to-0.66 e V.The relatively small?_rG in the range of-0.21?-0.47 e V was calculated for the bivalent complexes.All the calculated reaction energies are negative,which are thermodynamically feasible.Moreover,trivalent complexes are more favorable for the CO₂ adsorption.