Design Of Supported Single-atom Electrocatalysts And Theoretical Studies On The Catalytic Mechanism

The development of clean energy technology is the main strategy to solve the two major problems,depletion of fossil energy and environmental pollution,which are currently faced by mankind.The use of electrochemical conversion technology can provide clean and sustainable energy for our future.For example,we can use oxygen reduction reaction(ORR)and hydrogen oxidation(HOR)to prepare fuel cells.We can use carbon monoxide reduction reaction(CORR)and carbon dioxide reduction reaction(CO2RR)to convert carbon monoxide and carbon dioxide into high value-added chemicals(alkanes,alkenes and alcohols).We can use nitric oxide reduction(NORR)and nitrogen reduction(NRR)to convert nitric oxide and nitrogen into ammonia.The core of realizing efficient electrocatalytic conversion technology is to find suitable electrocatalysts.Single-atom catalysts have become a research hotspot in the field of catalysis due to their maximum atom utilization,high catalytic activity and selectivity.However,due to the high specific surface free energy of single atoms,single atoms are easy to agglomerate and couple into nanoparticles,which will cause the catalysts to lose activity.Therefore,finding a suitable support for anchoring single atoms is the key to achieving a stable single-atom catalyst structure.This paper discussed the design of single-atom catalysts based on two novel non-metallic materials and the catalytic mechanism of related reactions.First,among many materials,the electron-deficient nature of boron materials makes it a potential substrate for stabilizing single metal atoms with electron-rich properties.The electron-deficient characteristics of nano-boron clusters and their intrinsic vacancies make the electron-rich transition metal atoms loaded on them prone to charge transfer,thereby enhancing the metal-carrier coordination bond.Therefore,we choose B36,a typical representative of nano-boron clusters,as the carrier for anchoring single atoms.Secondly,as an isoelectronic material of graphene,the introduction of point defects(boron and nitrogen defects)for hexagonal boron nitride(h-BN)can improve its chemical activity and provide a richer metal coordination environment.Therefore,it can catalyze more complicated electrochemical processes and become an excellent catalyst carrier to anchor single metal atom.This work includes four parts.The first part mainly summarizes the research background,research significance and research status and determines the importance and necessity of the research content.The second part introduces the theoretical basis,related methods and software.The third part studies the typical representative of nano-boron cluster,B36,supported single-atom to catalyze fuel cell cathode reaction-oxygen reduction reaction.We judge the catalytic activity and selectivity by calculating the Gibbs free energy change in the reaction,and use Bader charge,charge density difference,and local density of states to analyze the charge transfer and metal-carrier interaction.The fourth part studies the catalytic process of transition metal single atoms anchored on the boron-defected h-BN for nitric oxide reduction.We calculate the Gibbs free energy changes of different systems and analyze their electronic structure to find the source of catalytic activity.The main conclusions are summarized as follows:(1)We study the binding energy and charge transfer of a series of transition metal atoms with B36 and the calculation results show that single metal atoms can be stably loaded in the central hexagonal hole of B36.On the basis of determining the structural stability of the single-atom catalysts,we further studied the mechanism of the oxygen reduction reaction on M@B36.By calculating the Gibbs free energy change in the reaction,it is determined that Ni@B36 can reduce oxygen to water with lower overpotential and higher selectivity.(2)Hexagonal boron nitride(h-BN),as an analog of graphene,can be used as a carrier to anchor single atoms through defect engineering.We systematically studied the application of h-BN loaded transition metal single atom with boron defect in the electroreduction of NO to NH3.The results show that the single metal Ni and Cu loaded on h-BN can reduce NO to NH3 with high activity and Faraday efficiency.Among them,the limiting potential of Cu@h-BN in catalyzing this reaction is only 0.23V,which is lower than that of noble metal-based catalysts.The density of states analysis shows that in the vicinity of the Fermi level,the d orbital of metallic Cu hybridizes with the p orbital of the nitrogen atom in the NO molecule,which will activate the NO molecule,so it has a relatively good catalytic activity.