Theoretical Study Of The Alloying Mechanism And Catalytic Properties Of Ligand-Protected Metal Nanoclusters

Metal nanoclusters（NCs）have a high degree of dispersion,good stability and precise structure.The applications of metal nanoclusters depend on their properties determined by their structures.Precise control of the alloying site of NCs is a challenging study.NCs research aims to establish a comprehensive guide for the design and synthesis of NCs for specific applications.NCs can be used to construct nanocomposites with excellent catalytic properties,and well-controlled synthesis methods and advanced characterization tools allow one to design and synthesize catalysts with good activity and selectivity at the atomic level.In this thesis,density functional theory（DFT）computational studies and theoretical design were carried out for the alloying mechanism of ligand-protected gold and silver NCs and cluster-loaded single-atom catalysts based on the recent experimental research progress.Based on the spontaneous alloying reaction between[Au₂₅（PET）₁₈]^-（PET=SCH₂CH₂Ph）and[Ag₂₅（DMBT）₁₈]^-（DMBT=2,4-dimethylbenzenethiol）clusters observed by experimental mass spectrometry measurements,this thesis investigates the reaction mechanism of inter-cluster exchange by means of DFT.It was found that the inter-cluster alloying reaction of[Au₂₅（PET）₁₈]^-and[Ag₂₅（DMBT）₁₈]^-clusters was proceeded through formation of dianionic adduct[Au₂₅Ag₂₅（PET）₁₈（DMBT）₁₈]^2-.Once the dianionic adduct was formed,its structure evolved gradually by breaking and recombining the ligand shells.During this process,the ligand shell metal exchange took place first.Thereafter,the heterometal atom in ligand-shell was swapped with the metal atom in the icosahedral M₁₃-kernel.The mechanisms of two kinds of metal exchange processes were determined.Through comprehensively studying the metal exchange pathways,our studies further revealed that the[Au₂₅（PET）₁₈]^-cluster had a higher activity to form the kernel-doped alloy cluster than the[Ag₂₅（DMBT）₁₈]^-cluster.Based on the study of metal atom exchange and ligand exchange reactions between the atomically precise[Ag₄₄（p-MBA）₃₀]^4-cluster with[Au₂（p-NTP）₂Cl]^-,a three-stage alloying reaction mechanism of[Ag₄₄（p-MBA）₃₀]^4-cluster was proposed.During the first stage,an exchange of ligand-shell metal atoms took place.During the second stage,the motif exchanged on the[Au Ag₄₃（p-MBA）₃₀]^4-cluster.During the third stage,the Au（I）atom in the ligand-shell was swapped with a Ag（0）atom of the icosahedral Ag₁₂-core.The DFT calculation results demonstrated that the metal exchange proceeded via different mechanism at the different reaction stages.In the reaction Stage I and II,the metal exchange proceeded via formation of a dianionic[Ag₄₄（p-MBA）₃₀]^4--[Au₂（p-NTP）₂Cl]^-intermediate and then broke and recombined with the ligand-shell.In the Stage III,the diffusion of the Au（I）to icosahedral Ag₁₂-core is proceeded via a motif catalyzed heterometal atom diffusion mechanism.We hope that this work will provide a new perspective for the precise control of alloy position in alloyed nanomaterials.The method of grafted metal atoms by ligands can construct’grafted single-atom catalysts’with good catalytic activity without destroying the cluster structure.The catalytic performance of Fe²⁺,Co²⁺and Ni²⁺grafted by Cys ligand-protected Au₂₅ via L-cysteine as a bridging ligand in OER or ORR was investigated by DFT.Among various candidates,Ni²⁺grafted single atom catalyst with one cyclic bridging ligand(Au₂₅-1Cys-Ni²⁺)is an excellent bi-functional OER and ORR catalyst,with the overpotential of OER and ORR as low as 0.48 and 0.27 V.The volcano plot between the overpotential of OER and ORR and the adsorption energy of oxygenated species on Au₂₅-n Cys-M showed that the adsorption strength of oxygenated intermediates can be regulated by the grafted method and the number of bridging ligands,thus affecting its catalytic performance.Simultaneously,spin magnetic moments of metal cations can also be used as descriptors of the adsorption energies of intermediates on the grafted single atom catalyst,the excellent catalytic performance of Au₂₅-n Cys-Ni²⁺is originated from the smaller spin magnetic moment of Ni²⁺.Single atom catalysts have gained wide attention due to their own combination of isolated active sites of homogeneous catalysts and easy recycling of multiphase catalysts.Based on the constructed catalysts with different atomic defect types of Mg O（100）substrates loaded with single gold atoms,the selectivity of different types of catalysts for CO oxidation pathways was investigated,and finally the activity of the catalysts for CO oxidation was verified by a differential kinetic simulation.The computational results show that the Mg-defective surface of Mg O（100）loaded Au atom catalyst（Au₁/V-center）transforms into a unique carbonyl-modified single Au-atom active site（2CO-Au₁/V-center）during CO oxidation.It can efficiently catalyze CO oxidation via a trimolecular ER or bimolecular LH reaction mechanism.The catalytic activity of single Au atoms adsorbed on different Mg O surfaces was evaluated by microkinetic modeling at different temperatures and CO/O₂ partial pressures to analyze the conversion of CO₂ for different types of catalysts.The results of microkinetic simulations show that 2CO-Au₁/V-center can efficiently carry out the CO conversion reaction at ambient temperature.