Structural Modulation Of Single-Atom Catalysts For Efficient Electrocatalytic Reaction

Energy crisis and environmental pollution are urgent problems to be solved in society.Electrocatalytic processes are very important in the transition from fossil fuels to renewable energy,and it is necessary to use catalysts to speed up the reaction and improve the reaction selectivity.Combining the advantages of homogeneous and heterogeneous catalysts,single-atom catalysts(SACs)have attracted extensive research interest in various electrocatalytic reactions.Metal dispersion at the atomic scale yields maximum utilization efficiency,uniform active sites,unsaturated coordination environments,and strong metal-support interactions can significantly improve the activity,selectivity,and stability of metal atomic sites.It is found that the local coordination environment of single atoms has an important influence on its electronic structure and catalytic behavior.The structure of the active site is precisely designed at the atomic scale to control the electronic structure of the active center,thereby optimizing the adsorption energy of the intermediates in the catalytic reaction,and establishing the rational structure-activity relationship.However,it is difficult to design targeted tunable structures due to the limitations of synthetic methods and the immaturity of atomic-scale control techniques.In addition,it is difficult to accurately judge the structural environment and active sites,which is not conducive to the rationalization of the design of the structure-property relationship.In this paper,single-atom catalysts are used as materials to carry out research.Through precise design and controllable synthesis of catalyst atomic size structure,efficient electrocatalytic energy conversion technology is realized,and the structure-activity relationship between structure and activity is established.The main contents of this paper are as follows:(1)The coordination structure of W single-atom catalysts enables efficient nitrogen fixation.Precise design of coordination structure enables controllable customization of reactivity and selectivity.The coordination structure of the first shell of the W single-atom catalyst is regulated by the metal-oxygen bridge strategy,and the electronic structure of the active center W is coordinated by the N,O double atoms,thereby optimizing the catalytic reaction energy barrier and improving the activity and selectivity of nitrogen reduction reaction.Compared with the W-N4,the W-N2O2 has a higher NH3 yield(12.62 μg h-1 mg-1cat)and a higher Faradaic efficiency(8.35%),indicating that W sites of W-N2O2 are the source of activity.Isotope labeling experiments confirmed that the detected NH3 was produced by electrochemical reduction of nitrogen gas.Density functional theory calculations indicate that the unique electronic structure of W-N2O2 optimizes the binding energy of NRR intermediates.(2)Functional regulation of the main group Sb central site stimulates the oxygen reduction activity.Under the guidance of first-principles calculations,the oxygen reduction activity of the main group Sb-N4 structure can be stimulated by regulating the epoxy groups near the Sb center,and the interaction between the regulation of functionalized groups and the oxygen reduction activity can be established.Based on this principle,Sb single-atom catalysts with epoxy group-optimized Sb-N4 structure on N-doped graphene was synthesized.Theoretical calculations prove that the epoxy groups are thermodynamically more inclined to be close to the Sb center,and the electron-rich epoxy groups can adjust the electronic structure of the Sb center,thereby optimizing the adsorption energy barrier of the reaction intermediate on the Sb site.The Sb single-atom catalyst with epoxy group exhibits excellent oxygen reduction performance,which is close to that of commercial Pt/C.(3)Ni-base superstructure with dual active sites to accelerate alkaline HER kinetics.The efficient multi-step catalytic reaction process requires the construction of multiple active sites.Based on the Ni-based MOF structure,a three-dimensional porous sponge-like macrostructure was controllably synthesized by utilizing the structural anisotropy.Based on the surface free energy of metal atoms,the kinetic competition relationship between atomization and aggregation is regulated by pyrolysis temperature,and the high-temperature dynamic transformation of Ni atoms in graphite nanocages is realized,thereby constructing the superstructure with Ni nanoparticles and Ni single atoms(Ni SA/Ni NPs).This structure with multiple active sites synergistically enhances the kinetic rate of the alkaline hydrogen evolution reaction(HER).The catalytic sites were evaluated by acid washing to remove Ni NPs and using KSC to shield Ni atomic sites,which proved that the reaction process requires the synergistic catalysis of Ni nanoparticles and Ni single atoms.The synergistic catalytic mechanism was further proved by first-principles calculations.Ni single atom was the reaction site for the first step of water dissociation in the catalytic process,and Ni NPs were used as the reaction site for the second step of hydrogen evolution.