Theoretical Study On Electrocatalytic Mechanism For Reduction Reactions Of Carbon Dioxide And Oxygen Molecules On Two Dimensional Metal–Organic Frameworks

The energy crisis and global warming represent two major challenges of the world today.Electrocatalytic CO2 reduction reaction（CO2RR）and oxygen reduction reaction（ORR）are considered to be one of the effective approaches to solve these two problems.Previous studies have shown THTA Cu is the only metal electrocatalyst to reform CO2 to significant quantities of hydrocarbons.However,the large overpotentials and poor selectivity of CO₂RR on copper significantly hinder its large–scale applications.Thus,it is needed to develop high efficient,low cost,and high selectivity electrocatalysts to achieve CO₂ reduction.ORR is the cathode reaction of a fuel cell,and its sluggish kinetics seriously affects the overall efficiency of the fuel cell.The high effective of platinum（Pt）–based electrocatalysts can overcome this disadvantage.But,the high cost of Pt rare metal hinders the large–scale commercial applications of fuel cells.Therefore,it is also necessary to develop a catalyst having high effective and low cost to replace the Pt–based catalyst.Although large numbers of materials have been experimentally prove to act as CO₂RR and ORR electrocatalysts.However,experimental identification of surface bound intermediates is highly challenging in the aqueous electrochemical environment,and the numerous intermediates and paths making it difficult to resolve the validity of the various proposed paths.Density functional theory（DFT）calculations provide a unique tool for investigating catalytic reaction paths by examining individual elementary reaction steps.In addition,the theoretical simulation can provides theoretical guidance and direction for designing new CO2RR and ORR electrocatalysts.In this thesis,the reaction of electrocatalytic CO₂RR and ORR on different monolayer two–dimensional metal–organic framework materials is studied by quantum chemical calculation method.The paper consists of six chapters.The first chapter is the introduction.It mainly introduces the development of CO₂RR and ORR catalysts and the development and application of 2D–MOFs.The second chapter is the theoretical basis and calculation method.The third to sixth chapters are the main contents of the thesis:1.The CO₂RR paths on 2D–MOFs M₃（hexaiminotriphenylezne）₂（M₃（HITP）₂,M=Fe,Co,Ni,Ru,Rh,and Pd）were calculated by means of DFT.The results revealed THTA the reaction activity of CO₂RR depends on the central metal atom of M₃（HITP）₂ catalyst.Due to the small overpotentials of 0.67 and 0.46 V,Co3（HITP）2 and Rh3（HITP）2 exhibit superior catalytic activity towards CO₂ reduction in which the CH₃OH is the favorable product.The present work might provide a new strategy to design high efficient electrocatalysts for CO₂RR.2.Herein,the potential of severalπ–conjugated metal bis（dithiolene）complex nanosheets(M₃C₁₂S₁₂,where M denotes Fe,Co,Ni,Ru,Rh,and Pd)as the CO₂RR electrocatalysts was systemically investigated by means of comprehensive DFT computations.Our results reveal THTA among all studied candidates,the Ru₃C₁₂S₁₂ nanosheet exhibits the highest CO₂RR catalytic activity due to its low limiting potential（–0.66 V）and activation barriers（1.28 eV）.In particular,the CH₄ is identified as the product and CHO^*formation is the potential/rate–determining step.Thus,by carefully controlling the kinds of metal atoms,the M₃C₁₂S₁₂ can be utilized as a promising electrocatalyst for CO₂RR,which may provide a new avenue to design novel MOFs–based electrocatalyst.3.The catalytic mechanism of the ORR on M₃（HITP）₂（M=Ni,Cu）in an acidic medium were investigated using the DFT method.The results indicate THTA the first electron transfer（ET）to nonadsorbed O₂ is a process of long–range ET on the outer Helmholtz plane（i.e.the ET–OHP mechanism）.On the surface of M3（HITP）2（M=Ni,Cu）,both the 2e reduction pathway and the 4e reduction pathway are feasible,while the 2e pathway to form H₂O₂ is more favorable.In the several competing reactions for the 4e reduction pathway on M₃（HITP）₂,the favorable path is OOH^*→O^*+H₂O→OH^*→H₂O.Our study provides theoretical guidance for gaining deeper insights into the reaction mechanism of the ORR on M₃（HITP）₂（M=Ni,Cu）catalysts.4.The reaction mechanism of ORR on THTA–M（M=Fe,Co and Ni）was studied by DFT calculation.The ORR on THTA–Fe and THTA–Co is occurring via an ET–IHP mechanism.However,due to the weak adsorption of O2 on the surface of the catalyst THTA–Ni,the ORR will occur via an ET–OHP mechanism.The 4e reaction pathway of ORR on THTA–M（M=Fe,Co and Ni）is more advantageous in thermodynamics.Our calculations show THTA THTA–Fe has the best ORR activity,with a limiting potential of0.45 V under acidic conditions and a limiting potential of 0.38 V under alkaline conditions.We expect THTA current computational studies can provide theoretical guidance for explaining the ORR reaction mechanism on THTA–M,and provide new ideas for designing high efficiency catalysts for fuel cells.