Theoretical Calculation Studies On Two-dimensional Carbon-based Single-atom Catalysts

Ammonia（NH₃）and hydrogen peroxide（H₂O₂）are important chemical raw materials in modern industrial production,which have a great influence on human life and global economic development.Electrocatalytic reduction of NO₃^-in industrial sewage to NH₃ is a feasible and environmentally-friendly method for ammonia production.Meanwhile,electrocatalytic reduction of O₂ to H₂O₂ is also a green and efficient way for H₂O₂ production.However,the application of efficient and stable catalysts is the key link to realizing the above processes.This dissertation aims to seek efficient catalysts for the electrocatalytic reduction of NO₃^-and O₂ through theoretical calculations,and the work content is as follows:A total of 24 TM@GDY single-atom models were constructed by embedding transition metal atoms（TM=Ti-Cu,Zr-Ag,Hf-Au）on the graphdiyne（GDY）monolayer,with the catalytic performance for NO₃RR investigated as well.The binding energy between TM and GDY was calculated to demonstrate that there were thirteen relatively stable TM@GDY models（TM=Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zr,Pd,Ag,Hf,Pt）.Then,the reaction mechanism of the NO₃RR process was discussed,and for the above thirteen TM@GDY models,the Gibbs free energy changes of each elementary reaction in the NO₃RR process were calculated.Notably,for the five TM@GDY models（TM=Ti,Zr,Pd,Hf,Pt）,the NO₃H and NO₂H adsorbate could not be stably adsorbed.Therefore,the energy barrier of the potential determination step（PDS）for the eight TM@GDY models（TM=V,Cr,Mn,Fe,Co,Ni,Cu,Ag）was selectively compared.The results displayed that the V@GDY and Ag@GDY models possessed lower energy barriers for the PDS.It was also found that the energy barriers for the release of NO₂,NO,N₂O,and N₂ were higher than that of NH₃.Therefore,the two models showed better catalytic performance and product selectivity for the NO₃RR process.Finally,the reasons for the great catalytic performance are discussed from the density of state,energy band,charge analysis,and project Crystal Orbital Hamilton Populations.In this part,the novel TM@GDY single-atom models have innovatively combined with NO₃RR electrocatalytic reaction for systematic discussion,which greatly enriched the research field of NO₃RR electrocatalytic reaction.Using the Co-N₄ coordinate structure on the graphene support as the prototype,carbon vacancy defects were introduced near the Co-N₄ coordinate structure.As a result,39 types of Co-N₄ coordinate structure models with carbon vacancy defects were constructed,and the catalytic performance of the two-electron oxygen reduction reaction（2e^-ORR）for all the models was investigated.The stability of the models was evaluated by calculating the binding energy between the Co site and the defective supports.The results displayed six models with poor stability.For the rest of the 33models,there was a volcanic relationship between the Gibbs free energy of the*OOH intermediate(ΔG_*OOH)and the 2e^-ORR performance,with 13 models displaying great activity and 7 models displaying terrible activity for the 2e^-ORR.Then,factors of 2e^-ORR performance were discussed from adsorption energy,adsorption morphology,and electronic structure.In the end,it was concluded that the overlap degree between Co 3d orbitals and O 2p orbitals was the key factor affecting the 2e^-ORR performance.In this part,it is the first time from the perspective of unique defective supports to systematically study the effects of carbon vacancy defects on the Co-N₄ coordinate structures to the 2e^-ORR process.