Design And Mechanism Study For Doped Carbon-based Catalysts Based On Density Functional Theory

As an important scientific field,catalysis plays an indispensable role in energy,environment,chemical industry,health,and pharmaceutical,etc.The development of better catalytic systems is essential to meet the needs of contemporary society,and such catalysts are the main factor in promoting sustainable development and advances in chemical synthesis.Common oxygen reduction reaction（ORR）,oxygen evolution reaction（OER）and CO oxidation reaction（COOR）still mostly rely on precious metal platinum（Pt）and precious metal oxides such as ruthenium oxide（RuO₂）and iridium oxide（IrO₂）as catalysts.These catalysts still have many deficiencies in catalytic activity,stability,selectivity.Among them,the high cost and scarcity of catalysts are important factors restricting their large-scale commercial application.Therefore,developing low-cost and high-performance catalysts to replace them and meet the needs of contemporary society is the focus of catalytic research.Due to the characteristics of abundant sources,high conductivity and tunable molecular structure,carbon-based materials have made great progress in different fields of catalysis.In this paper,we studied the activity and selectivity of four types for doped carbon-based catalysts in ORR,OER,and COOR by density functional theory（DFT）calculations,which provided reference for experimental research.The main research results are as follows:（1）The feasibility of heteroatom-doped fullerene C₇₀((X（Cn）,with B,N,O or Si as dopant X,replacing five different types of carbon atoms（Cn,n=1-5）on C₇₀))catalyzing ORR and OER is verified by DFT calculations.Formation energy results show that X（Cn）is a thermodynamically stable complex.ΔG_*OH shows a good linear relationship withΔG_*OOH andΔG_*O.It is worth noting that the ORR overpotential values（η）of N（C1）,N（C2）,N（C3）and N（C4）are 0.87 V,0.75 V,0.67 V and 0.73 V,respectively,which are all greater than Pt(η_ORR=0.45 V),but it shows that ORR can still be catalyzed.Compared with the pristine C₇₀,both B and N doping can reduce OER overpotential value and improve OER performance.In particular,N（C4）(η_OER=0.55 V)has the closest overpotential to traditional noble metal OER catalysts such as Ru O₂(η_OER=0.42 V),indicating that it can be used as a potential candidate for OER catalysts.According to the volcano plots,the best ORR and OER activities of X（Cn）appear atΔG_*OH=0.56 e V andΔG_*O-ΔG_*OH=1.78 e V,respectively.This work can provide some useful information for the design and discovery of new non-metal doped carbon-based catalysts.（2）The catalytic activities of Ru-N₄ doped fullerenes(Ru-N₄-C₅₄ and Ru-N₄-C₆₄)for ORR and COOR are systematically investigated by DFT from the aspects of thermodynamics and kinetics.For ORR,DFT results display that Ru-N₄-C₅₄ and Ru-N₄-C₆₄ have satisfactory catalytic behavior both thermodynamically and kinetically.The relative energy curves show that the entire four-electron process of the most favorable ORR path on Ru-N₄-C₅₄ and Ru-N₄-C₆₄ are exothermic,and the activation energies of the rate-limiting step are 0.60 and 0.40?e V,respectively,which are lower than the calculated value 0.79?e V for Pt（111）.For COOR,DFT results reveal that Ru-N₄-C₆₄ has better catalytic property both thermodynamically and kinetically.The relative energy curve shows that the entire COOR process of the preferred Langmuir–Hinshelwood mechanism on Ru-N₄-C₆₄ is exothermic,and the activation energy of the rate-limiting step is 0.93?e V.Therefore,two hopeful ORR catalysts(Ru-N₄-C₅₄ and Ru-N₄-C₆₄)with performance comparable to Pt（111）and one promising COOR catalyst(Ru-N₄-C₆₄)are identified.This work provides a new possibility for the experimental design of metal-nonmetal co-doped carbon-based catalysts.（3）A novel ORR catalyst consisting of transition metal M and non-metal atoms N₄co-doped to vacancy fullerenes(M-N₄-C₆₄,M=Fe,Co,or Ni)is explored using DFT.Mulliken charge analysis shows that the metal center is the reaction site of ORR.PDOS analysis indicates that the interaction between Fe-N₄-C₆₄ and the adsorbate is the strongest,followed by Co-N₄-C₆₄ and Ni-N₄-C₆₄.This is consistent with the calculated adsorption energies.By analyzing and comparing the adsorption energies of ORR intermediates and activation energies and reaction energies of all elemental reactions in M-N₄-C₆₄（M=Fe,Co,and Ni）,two favorable ORR electrocatalysts,Fe-N₄-C₆₄ and Co-N₄-C₆₄,are selected.Both exhibited conduction through the more efficient 4e^-reduction pathway.Moreover,the relative energy curves indicate that the whole reaction energy variation in the favorable ORR pathways of Fe-N₄-C₆₄ and Co-N₄-C₆₄is degressive,which is conducive to positive-going reactions.This work provides some valuable information for the design and discovery of novel metal-nonmetal co-doped carbon-based catalysts.（4）In recent years,one of the research directions of proton exchange membrane fuel cells（PEMFCs）is to develop efficient ORR catalysts to replace precious metals.In this study,based on DFT calculations,we design a new type single-atom ORR catalyst by doping single iron atoms into the nitrogen coordination cavity of graphite carbonitride（Fe/g-C₃N₄）.Combined with Mulliken charge analysis,the adsorption sites and adsorption energies of ORR intermediates are analyzed,and the activation energy barriers and reaction energies of all elementary reactions are discussed.The possible ORR reaction pathways are explored and the most favorable ORR mechanism is identified.Our computational results indicate that Fe/g-C₃N₄ is a potential ORR catalyst.This work provides some theoretical guidance for the experimental study of metal single-atom-doped carbon-based catalysts for PEMFCs.