Theoretical Study On The Performance Of Hydrogen Evolution Reaction On Keggin Type Polyoxometalates And Chirality Of Dawson Type Polyoxometalates

Polyoxometalates?POMs?represent a class of multi-metal oxyanions usually formed by the bonding of early transition metals(such as V^V,Mo^V,Nb^V,Ta^V,Mo^VI and W^VI)with oxygen.POMs have certain structures and unique redox performance.POMs can accommodate many electrons without changing their structures.POMs are widely used in electrocatalysis,asymmetric catalysis,magnetism,optics,biology and other fields.Electrocatalysts derived from POMs,such as POMOF,metal sulphides and metal oxides,can be used in hydrogen evolution reduction?HER?,oxygen evolution reduction?OER?,and carbon dioxide electrocatalytic reduction?CO₂RR?.Among them,HER is an important way to obtain clean energy.Considering the high overpotential of HER,the reaction must be carried out by means of catalysts.In recent years,a large number of POMs-based derivatives have been synthesized to drive the HER.Meanwhile,chiral POMs have been widely used in asymmetric catalysis,chiral medicine,nonlinear materials and other fields.Due to rapid racemization and partial hydrolysis,most chiral POMs exist in solution as racemic mixtures.However,POMs are more important in asymmetric catalysis and biological activity.Therefore,the resolution of racemic mixture becomes an urgent problem.Due to the complex structures of POMs,neither the electrocatalytic reaction mechanism of HER involved nor the chiral resolution process are very clear.The theoretical calculations are very useful to solve these questions.In this thesis,density functional theory?DFT?methods were used to simulate the reaction intermediates,and explore the mechanism of POMs participating in HER and the chiral resolution process.It can provide reliable theoretical guidance for the catalysis applications.The research work of this theis consists of two aspects:1.The redox and acidity properties of?-Keggin H_3+nPM₁₂O₄₀(PM₁₂2 ne^-,M=W,Mo;n=0 to 5)with different oxidation states and hydrogen atoms adsorption were calculated by means of DFT.In the process of adsorbing hydrogen atoms,a single electron of the hydrogen atom is transferred to a d-orbital of the metal in POMs,and the oxygen site of the POMs acts as a base to receive a proton.Therefore,the accepting electron ability of POMs and the proton adsorption ability of oxygen atoms will affect their hydrogen adsorption performance.The results show that the accepting electron ability of POMs decrease with the introduction of reduced electrons,and there is a linear dependence between the single-electron reduction energy?RE?and LUMO orbital energy level of POMs.For PM₁₂2 ne^-,of oxidation state and multi-electron reduced states,the negative charges of different oxygen sites is:O_d<O_c<O_b.Correspondingly,the ability for adsorbing protons also follows the above trend.The Gibbs free energy results show that PMo₁₂ ne^-,possesses better performance fo hydrogen evolution by introducing electrons,and PW₁₂ ne^-has ideal hydrogen evolution perfromance in oxidation state or low reduced states.This work aims to provide a new idea for designing better HER electrocatalysts.2.The chiral induction and turnover of chiral[P₂Mo₁₈O₆₂]^6-and[PMo₉O₃₁?OH₂?₃]^3-under the action of histidine were studied by DFT.Compared with complete oxidation state[P₂Mo₁₈O₆₂]^6-/[PMo₉O₃₁?OH₂?₃]^3-,single-electron reduced state[P₂Mo₁₈O₆₂]^7-/[PMo₉O₃₁?OH₂?₃]^4-Mo-O_b bond length tends to be average,which indicates that electron transfer will affect the chirality of POM to a large extent.Therefore,it is speculated that chiral ligand histidine can be used to transfer electrons to POM molecules during chiral separation,so as to complete chiral induction.The results show that,compared with _L-histidine,the interaction between _D-histidine and _L-isomer is smaller,so _L,D-[P₂Mo₁₈O₆₂]^6-can be separated under the induction of chiral histidine.At the same time,ECD spectra of the POMs-histidine ligand complex showed charge transfer from ligand to POM,which further demonstrated the rationality of chiral histidine induced molecular separation of chiral POMs.The energy difference between chiral L-[P₂Mo₁₈O₆₂]^6-with D₃ symmetry and idealized[P₂Mo₁₈O₆₂]^6-with D_3h symmetry is 5.88 kcal mol^-1,suggesting that chirality inversion may occur from L-isomer to D-isomer through idealized[P₂Mo₁₈O₆₂]^6-by crossing small energy barrier.The present work not only explains the experimental phenomenon successfully,but also helps to deeply understand the process of resolution and provide ideas for the experiment.