Stability And Catalytic Performance Of Transition Metals On Nitrogen Doped Graphene:A First-principles Study

Ubiquitous chemical reactions are the foundation of human survival and social development.As a core part of high selectivity and fast reaction efficiency,finding and optimizing catalysts has always been a long-term goal of modern social development.Graphene is a single-atom-thick,hexagonal grid structure,carbon material composed of sp² bonds,which has unique geometric structure,electronic properties,and good chemical stability in harsh acidic or alkaline environments.Therefore,it has become an ideal carrier for understanding the basic principles of the physical chemistry of atomically dispersed supported catalysts.However,pure graphene is often insensitive to the adsorption of metal atoms,while nitrogen-doped graphene can change its Fermi level,thereby changing the energy band structure and enhancing the catalytic activity.At the same time,graphene containing defect vacancies can strengthen the binding of supported metal atoms,thereby immobilizing metal atoms.The concept of single-atom catalysts（SACs）was first proposed by Academician Zhang Tao and others in 2011.Since then,SACs have developed rapidly in the field of catalysis.So far,nitrogen-doped graphene containing defect vacancies has been used as a substrate for immobilizing transition metal single atoms in many chemical reactions for the synthesis of SACs catalysts.In recent years,dual-atom catalysts（DACs）have attracted more and more attention due to their higher metal loadings,more metal active sites,and unique reactivity.At the same time,the dual-atoms can also act synergistically to further adjust the geometry and electronic structure of the active site.Today,atomically dispersed supported catalysts have become a research hotspot due to their high catalytic activity,selectivity,and atomic utilization.In this paper,nitrogen-doped graphene containing defect vacancies was used as the substrate.Based on the calculation method of density functional theory,the geometric distribution of metal atoms on the substrate of 24 transition metal elements under different atomic concentrations was systematically studied.The stability was then analyzed,and a potential catalyst for hydrogen evolution reaction was finally screened out.Chapter One,briefly summarize the status of catalysts in today’s social development,the research progress of single-atom and dual-atom catalysts,the progress of hydrogen evolution reaction,and lead to the discussion of transition metal atoms on defect-vacancy nitrogen-doped graphene in this paper.Chapter two,introduce first principles based on density functional theory,the theoretical basis of electrocatalysis,as well as the functional selection,calculation parameters and model construction of our research system.Chapter three,the density functional theory（DFT）calculation is carried out according to the model we built,and the calculation result of the theoretical model is obtained.Our theoretical research is mainly analyzed from two aspects:（a）geometry;（b）stability;（c）HER reaction catalytic activity.（a）Geometric structure analysis:The stable configurations of 24 metal elements on three-nitrogen doped single-vacancy graphene（SVGN₃）substrate were qualitatively analyzed.（1）When single atoms are on pure graphene substrate,13 elements（Ti,V,Fe,Co,Ni,Zr,Nb,Tc,Ru,Rh,Hf,Ta,Os）are optimally adsorbed on the hole sites,7elements（Cr,Mo,Pd,Ag,W,Ir,Pt）are optimally adsorbed at the bridge site,and the rest elements（Mn,Cu,Re,Au）are optimally adsorbed at the top site.（2）When the single atoms are on the SVGN₃ substrate,all single atoms are stably adsorbed on the single-defect vacancies.（3）When the dual-atoms（M1 and M2）are on the SVGN₃substrate,the optimal configuration of one element（W）is:M1 is in the bridge position between the two nitrogen atoms,and M2 is at top position of the third nitrogen atom.Among the remaining 23 elements,M1 is a single-defect vacancy located in the SVGN₃substrate:3 elements（Ti,Zr,Hf）,M2 is located in a single-defect vacancy on the other side of the substrate;3 elements（Nb,In V,Ta）,M2 is at the top of nitrogen atom;2elements（Rh,Pd）,M2 is at the top of carbon atom;15 elements（Cr,Mn,Fe,Co,Ni,Cu,Mo,Tc,Ru,Ag,Re,Os,Ir,Pt,Au）,M2 is located in the single-defect vacancy on the same side of the substrate and perpendicular to the substrate.（b）Stability analysis:The binding energy of atoms on the substrate is counted,and the stability of the stable configuration is further analyzed.（1）Comparing single atoms on pure graphene and SVGN₃ substrates,for elements of each period,the changing trends of the binding energy of elements in the same main group are roughly similar,and all single atoms tend to be adsorbed in single-defect vacancy on the SVGN₃substrate.（2）When the number of metal atoms is underloaded,7 elements（Mo,Tc,Re,Os,Ir,Pt,Au）may form dimers on the SVGN₃ substrate.（3）When the number of metal atoms is overloaded,M2 will be more stably attached to M1 among the 24 elements,thus forming dimers at the single-defect vacancy.（c）Analysis of the catalytic activity of the HER reaction:Potential single-atom and dual-atom hydrogen evolution reaction catalysts were screened by H overpotential,and Bader charge analysis was performed on the hydrogen evolution reaction catalysts.（1）In the SACs,Co,Ni,Pd SACs can be used as potential HER reaction catalysts.（2）In the DACs,Re DAC can be used as potential HER reaction catalysts.（3）The catalytic activity of the HER reaction can be attributed to the additional atoms regulating the charge transfer on the metal atoms.Chapter four,this paper not only deepens the understanding of the stability and activity of SACs on SVGN₃ substrate,but also provides a new strategy for designing efficient DACs based on SACs.It also lays a solid foundation for subsequent research.