First-principles And Synchrotron Radiation Studies Of Single-atom Electrocatalytic Reaction Mechanisms

With the consumption of fossil fuels and the emission of carbon dioxide gas,energy shortages and climate change issues have received more and more attention.Electrochemical technology,which uses sustainable green energy sources(such as solar,wind and tidal energy)to directly convert CO2 into fuel or high value-added chemicals,is currently one of the most effective solutions.The choice of catalyst is crucial in CO2 electrochemical reduction reaction due to the disadvantages of high overpotential,the low Faraday efficiency and poor product selectivity.Single-atom catalysts of the M-Nx-C type,which has the advantages of abundant raw materials,good conductivity,high stability,and high metal atom utilization,are the main research interests at present.The combination of synchrotron radiation X-ray absorption fine structure(XAFS)spectroscopy and theoretical calculations is an important technical tool in single-atom electrocatalysis studies.Because of the easy aggregation of single-atom catalysts during preparation,synchrotron radiation XAFS is required to provide important electronic structure information for single-atom catalysts.Especially,the combination with DFT can accurately identify the coordination structure of M-Nx.In addition,the different coordination structures of metal atoms can effectively tune the catalytic activity of single-atom catalysts and the reactivity of electrocatalytic reactions.Therefore,the DFT study of CO2 electrocatalytic reactions on M-Nx-C can provide guidance for experimental research based on synchrotron radiation XAFS.The combination of in situ synchrotron radiation XAFS and DFT can be used to study the self-structural changes of the catalyst in the service state and reveal the reaction mechanism of electrocatalytic reactions.In Chapter 3,Fe/g-C3N4,Co/g-C3N4 and Ni/g-C3N4 catalyst models are constructed based on the DFT method for the studying reaction mechanism of CO2 electrocatalytic reduction reaction.Fe-,Co-and Ni-doped g-C3N4 form the coordination structures of Fe-N4,Co-N3 and Ni-N3,respectively,and structural information around the metal atoms is obtained by the combination of synchrotron radiation XAFS and DFT.Because of the weak chemical bonding between the metal atom and the pyridine-like N atom on the cavities of g-C3N4,Fe/g-C3N4,Co/g-C3N4 and Ni/g-C3N4 catalysts have strong interactions for both CO2 and intermediates of the CO2 electrocatalytic reaction.The self-structural changes of the catalysts during the electrochemical reaction provide key intermediate information for in situ synchrotron radiation XAFS-based reaction mechanism studies.Owing to the strong interaction between*CO and catalyst,it is difficult for*CO to desorb from the surfaces of Fe/g-C3N4,Co/g-C3N4 and Ni/g-C3N4,which leads to further reduction reaction to yield products such as methane and methanol.Limiting potentials for methane and methanol forming on Co/g-C3N4 are both 0.81 V,which is the largest among three catalysts,since the strongest interaction between catalyst and product,making it difficult to desorb.The limiting potentials for the generation of methane and methanol are 0.67 V and 0.69 V,respectively,on Fe/g-C3N4 and both are 0.72 V on Ni/g-C3N4.The results after employing implicit solvent model indicate that Co/g-C3N4 is more sensitive to the solvation effect and the limiting potentials for forming methane and methanol change to 1.11 V and 1.01 V.However,solvent effect has little impact on Fe/g-C3N4 and Ni/g-C3N4.In Chapter 4,Fe-N4/CNT,Co-N4/CNT and Ni-N4/CNT catalyst models are prepared by DFT method,namely,Fe,Co,and Ni single metal atoms are supported on nitrogen-doped carbon nanotube materials.The reaction mechanism of CO2 electrocatalytic reduction on Fe-N4/CNT,Co-N4/CNT and Ni-N4/CNT is investigated using computational hydrogen electrode model.It is found that the weak interaction between CO and Ni-N4/CNT makes CO easily desorb from surface and tends to directly generate CO,while*CO on Fe-N4/CNT and Co-N4/CNT prefer to undergo further reduction reactions to produce methane and methanol due to the stronger interaction.The limiting potential of forming CO,methane and methanol on Fe-N4/CNT are 1.08 V,0.68 V and 0.68 V,respectively and on Co-N4/CNT are 0.43 V,0.56 V and 0.56 V,respectively.The combination of synchrotron XAFS and DFT is used in the study of the electrocatalytic reaction mechanism of unsaturated M-Nx coordination structures,and it is found that unsaturated M-Nx has a significant effect on the reaction activity.Hence,Ni-Nx with different coordination structures embedded on the edges of carbon nanotubes and Ni-N4 structures supported on zigzag or chiral carbon nanotubes are used as the catalysts for CO2 electrochemical reaction.It is found that different Ni-Nx coordination structures have a significant effect on the catalytic activity.Furthermore,the catalytic activity of multi-walled carbon nanotubes is stronger than that of single-walled carbon nanotubes due to the interaction with inner C atoms.Then,the catalytic activity of single-atom catalysts based on g-C3N4 and carbon nanotubes is compared for CO2 electrocatalytic reaction.It is observed that the catalytic activity of Co/g-C3N4 and Ni/g-C3N4 is stronger than that of Co-N4/CNT and Ni-N4/CNT.However,the limiting potential of the corresponding product on Co/g-C3N4 is higher than that of Co-N4/CNT,which implies that the external voltage of CO2 electrocatalytic reduction is not linearly related to the catalytic activity of the catalyst.And Ni/g-C3N4 prefers to generate methanol and methane,while Ni-N4/CNT has high product selectivity to CO.Significantly,because of the same coordination structure of Fe-N4/CNT and Fe/g-C3N4,their catalytic activities are similar and limiting potentials of the corresponding products are also similar.In Chapter 5,six explicit water molecules are placed on Cu-N4-C catalyst for simulating explicit water model to study the solvation effect on CO2 electrocatalytic reduction.It is found that the binding energies of explicit water model and implicit solvent model on*CHO,*OCH3,*O,and*OH differ greatly,and the implicit solvent model cannot accurately describe the solvation effect on CO2 electrocatalytic 1 reaction.DFT combined with in situ synchrotron radiation XAFS reveals that the catalyst undergoes structural reorganization with applied voltage,thus,the research of the solvation effect provides help for the mechanism study of Operando synchrotron radiation XAFS in electrocatalytic reactions.This thesis systematically studies the reaction mechanism of CO2 electrocatalytic reduction on M-Nx-C materials and finds that the different M-Nx coordinated structures have a great influence on catalytic activity and product selectivity.This work provides a solid basis for subsequent experiments on the CO2 electrocatalytic reaction based on synchrotron radiation.