Theoretical Study On The Electrocatalytic Performance Of Transition Metal/Boron-Doped Two-Dimensional Materials For Carbon Dioxide And Nitrogen Reduction

Although rapid industrial development has brought great convenience to human life,the excessive use of fossil fuels has caused the shortage of resources and environmental problems,especially the greenhouse effect has caused a serious impact on the ecological environment.It has attracted great attention to recycle and utilize carbon dioxide（CO₂）.Among several feasible strategies（such as electrocatalysis,photocatalysis,and biocatalysis,etc.）,electrocatalytic CO₂reduction（CO₂RR）into low-carbon fuels and valuable chemicals is an effective way to realize sustainable utilization of carbon resources,which is performed under mild conditions and controlled environment.Except to improve the utilization rate of carbon through carbon cycle,developing clean energy and reducing carbon emissions are also effective ways to solve resource shortages and environmental problems.As a carbon-free energy,ammonia（NH₃）is an important hydrogen carrier that has large hydrogen capacity,high energy density and easy transportation.Compared with the traditional Haber-Bosch process with the harsh reaction conditions（300～500°C,150～200 atm）and the huge energy consumption（about 1%of the world’s total energy consumption）,the electrocatalytic nitrogen reduction reaction（NRR）has mild reaction conditions and zero CO₂ emissions,which can achieve green synthesis of ammonia.However,CO₂RR and NRR have disadvantages such as high overpotential,low reaction rate and low selectivity,which limit the large-scale application.Therefore,designing novel high-efficiency electrocatalysts,combined with elucidating their structure-property relationships and reaction mechanisms,is of great significance for improving the activity of catalysts and achieving high selectivity for specific products.In this thesis,the electrocatalytic performance of CO₂RR and NRR on transition metal/boron doped 2D materials was investigated by means of density functional theory（DFT）calculations.Thesis consists of six chapters.The first chapter is the introduction,which mainly introduces the reaction mechanism and electrocatalysts of CO₂RR/NRR.The second chapter is the theoretical basis and calculation methods;the third to eighth chapters are the main contents of the thesis:1.The effect of the coordination environment on the electrocatalytic CO₂RR performance of FeN_x-doped graphene（FeN_x-gra,x=0-4）was investigated by DFT method.The results reveal that adjusting the concentration of coordinating N atoms can tune the d-band center position of Fe atom and enhance the interaction between key intermediates and catalyst,among which FeN₃-gra exhibits the superior CO₂RR activity with a small limiting potential of-0.78V,in which CO*→HCO*is the potential-determining step.By adjusting the types of coordinating N atoms,the limiting potential of CO₂RR can be reduced from-0.78 V（pyridine N）to-0.57 V（pyrrolic N）.Our study provides theoretical guidance for deeper understanding the relationship between the coordination environment and the CO₂RR activity.2.Herein,the potential of two-dimensional covalent organic frameworks composed of pyrazine and metal-phthalocyanine linkages（M-COFs）as bifunctional electrocatalysts was explored by using the DFT methods.The results show that the Fe,Co,Ni,Cu,Zn,Ru,Rh,Pd and Ag can be firmly anchored on COFs as single atoms for forming stable single-atom catalysts.The metal atom in phthalocyanine has excellent catalytic activity towards CO₂RR,while the pyridine N in pyrazine is the active site of the hydrogen evolution reaction（HER）.Among studied candidates,Co-COF and Rh-COF are predicted to have limiting potential of-0.66/-0.11 V and-0.49/-0.49 V for CO₂RR/HER,respectively.The present study may provide a new strategy for designing novel bifunctional catalysts.3.The CO₂RR paths on bimetallic conjugated metal-organic frameworks（MOFs）with metal phthalocyanines（MN₄）as ligand and metal-bis（dihydroxy）complex（M’O₄）as linkage（MPc-O8-M’,M=Cu and Zn,M’=Fe,Co and Ni）were investigated by means of DFT calculations.According to the calculated free energy changes,Zn Pc-O8-Co exhibits outstanding catalytic performance for CO₂RR,which can reduce CO₂ to CH₃OH with the limiting potential of-0.63 V,and the first hydrogenation to form COOH*is the potential-determining step.The present work might provide reliable theoretical information for the design of novel bimetallic MOFs catalysts.4.The catalytic performance of heteroatom（B,P,Si,and S）doped C₃N for CO₂RR was investigated by DFT methods.The results show that the CO₂RR catalytic activity of heteroatom doped C₃N is affected by the type and doping site of heteroatom,among which the B-doped N vacancies of C₃N（B_N-C₃N）exhibits excellent CO₂RR activity with the low overpotential（0.14V）for CO₂ reduction to HCOOH.The low overpotential can effectively suppress the HER,thereby improving the product selectivity.This study provides the theoretical guidance for optimal design of efficient and selective carbon-based metal-free catalysts.5.NRR catalytic performance of M-COFs（M=Fe,Co,Ni,Cu,Zn,Mo,Ru,Rh,Pd,and Ag）formed by the pyrazine linkage and metal-phthalocyanine were explored using DFT methods.By establishing the linear relationship between the key hydrogenation steps of*N₂+H⁺+e^-→*NNH and*NH₂+H⁺+e^-→*NH₃,Mo-COF was screened to have the best NRR catalytic performance.The detailed reaction mechanism further verifies that Mo-COF efficiently catalyzes NRR through an enzymatic mechanism with the limiting potential of-0.19V.This theoretical study will help promote the development of 2D COFs in electrochemical NRR.6.By the means of DFT calculations,the potential of B-doped C₅N₂ as a metal-free electrocatalyst for NRR was investigated.The results show that B atom can be firmly doped into the interstitial site of C₅N₂(B_int-C₅N₂),which can effectively capture and activate N₂molecule.B_int-C₅N₂ exhibits high catalytic activity towards NRR via the alternating mechanism with an overpotential of 0.38 V.Our study provides the reference for elucidating the microscopic mechanism of NRR on B_int-C₅N₂,and has important theoretical significance for design of novel NRR catalysts.