Theoretical Study On Electrocatalytic Synthesis Of Ammonia By Transition Metal Modified Boron Cluster

Ammonia（NH₃）plays an important role as the raw material of agricultural fertilizer and the carrier of renewable energy.The traditional industrial synthetic ammonia method（Haber-Bosch）relies on fossil fuels,but the large-scale use of fossil fuels is at odds with the carbon neutrality policy advocated by modern society.It is a long-term way to find sustainable green energy.Electrochemical reduction of nitrogen（N₂）or nitric oxide（NO）to NH₃（Nitrogen Reduction Reaction,NRR or Nitric Oxide Reduction Reaction,NORR）is a method with broad application prospects.However,NRR and NORR catalysts exhibit low activity and selectivity in most studies due to the competition between high overpotential and hydrogen evolution reaction（HER）.Therefore,it is of great significance to explore cheap and high-performance NH₃catalytic materials.In this thesis,we explored the strategy of catalytic synthesis of ammonia based on TM₄B₁₈ cluster system and proposed the idea of reducing N₂ and NO to synthesize NH₃ with low energy consumption.The TM₄B₁₈ clusters doped with different metals were theoretically studied as highly active and stable materials for capturing N₂ and NO molecules.The geometric structure and electronic properties of TM₄B₁₈clusters,the adsorption of N₂ and NO,and the application potential of TM₄B₁₈ in two ammonia synthesis reactions were studied by density functional theory（DFT）calculations.The main results are as follows:（1）We systematically studied 18 transition metal boron clusters TM₄B₁₈（TM=3d,4d,5d）as promising catalysts for NRR.First-principles calculations show that these TM₄B₁₈ clusters have excellent water environment stability and high selectivity under HER.We studied the NRR mechanism of the remote,alternate and enzymatic reaction pathways.The TM₄B₁₈ material was screened by the“four-step method”,and three candidate catalysts(Zr₄B₁₈,V₄B₁₈ and Mn₄B₁₈)with excellent performance for ammonia synthesis were screened.Most notably,through the enzymatic mechanism,the Zr₄B₁₈ cluster performs best,with an onset potential as low as-0.50 e V,due to the occupation of the d orbital part of the active metal atom.（2）TM₄B₁₈（TM=3d）was also studied as a catalyst for electrocatalytic reduction of NO.After screening the catalyst by“three-step screening method”,it was found that Ni₄B₁₈ has good catalytic activity and ability to activate NO and can spontaneously carry out NORR reaction.Ni₄B₁₈ generates NH₃ by*NOH as a reactant,and the cluster has high selectivity.Our research predicts a new TM₄B₁₈ catalyst with high catalytic activity and stability,which provides some research ideas and explorations for electrocatalytic synthesis of ammonia and is also of great significance for denitrification and emission reduction.It is expected that the current results will provide useful physical insights for the development of new B cluster catalysts for the activation and conversion of N₂ and NO,also provide important theoretical perspectives for guiding the experimental synthesis of doped boron clusters and the development of functional materials.