Controlled Synthesis And Electrocatalytic CO₂ Reduction Application Of Homoleptic Alkynyl-protected Coinage Metal Nanoclusters

Monolayer ligand protected coinage metal nanoclusters（NCs）have been gradually developing into ideal model molecules for basic research and have a wide range of applications in catalysis,optoelectronics,and biomedicine due to their defined composition,definitive structure and unique properties conferred by quantum confinement effects.Ligand engineering is one of the important means to tune the structure and properties of coinage metal NCs,and the commonly used ligands include thiolate,phosphine,halogen,etc.In recent years,ligand engineering research has gradually extended into alkynyl ligands,mainly due to the unique coordination mode of alkyne molecules,which have a smaller coordination number than that of thiolate-protected NCs and can yield novel NCs with unique structures and properties.Therefore,the development of homoleptic alkynyl-protected metal NCs has become a hot topic in the field of cluster research currently.However,the development of alkynyl-protected metal NCs is in its initial stage,and there are still many problems need to be solved,such as there are only few synthetic methods and low yields,little knowledge regarding the formation process,and lacking the development of cluster applications.To resolve the above problems,this thesis focuses on the four aspects of homoleptic alkynyl-protected coinage metal NCs,including controlled synthesis,reaction mechanism of cluster formation process,structure-property relationship and electrocatalytic CO₂ reduction reaction（CO₂RR）applications.The details are listed as follows:（1）The Au₁₄₄（PA）₆₀ NCs（PA=Ph C≡C-,phenylacetylene）were prepared by a“direct reduction（DR）”synthesis method,and the changes of the component content during their formation process were monitored in real time.On one hand,we investigated the influence of the precursor aggregation state toward the target NCs in the“direct reduction”synthesis strategy;On the other hand,the changes of the components during the formation of the target NCs were in situ monitored,and the ligand autocatalytic formation of a cumulene during the growth of the NCs was discovered for the first time,and their single-crystal structures were obtained for structural characterization.Finally,the three reaction stages during the formation process of Au₁₄₄（PA）₆₀ NCs was proposed.（2）A universal synthetic method with“synchronous nucleation and passivation（SNP）”was developed by us to fabricate homoleptic alkynyl-protected Au NCs such as Au₂₂（PA）₁₈（>70%）and Au₃₆（PA）₂₄（>17%）with high yields.Subsequently,Au₃₆（PA）₂₄ were used as an example to obtain the reaction intermediate Au₂₂（PA）₁₈ and the by-product Au₆（PA）₆ by manipulating their growth kinetics,and the“disproportionation reaction”between the three components was obtained by monitoring the changes of each component in the reaction process.Finally,the structural evolution model of the reaction intermediate Au₂₂（PA）₁₈ to the target NC of Au₃₆（PA）₂₄ was proposed based on the single crystal structure information.Finally,according to the quantitative analysis of the concentration change of each component during the reaction,the accuracy of the structural evolution model was verified,and the formation mechanism of Au₃₆（PA）₂₄ was revealed.（3）A novel homoleptic alkynyl-protected Ag Cu superatom of[Ag₉Cu₆（^tBu C≡C）₁₂]⁺was first time synthesized in high yield（>40%）by the“Two-in-One”method.In addition,Au/Ag superatoms[Au₇Ag₈（^tBu C≡C）₁₂]⁺with the same metal number and the alkynyl ligand were prepared in high yields（>86%）by an“anti-galvanic reaction（AGR）”method.Subsequently,they were characterized in detail in terms of structure,composition,and properties.Based on the crystal structure information,their optical absorption properties were explored in depth.The relationship between the metal composition on the M₁@M₈@M₆ metal structure and their optical absorption properties was also investigated with the aid of time-dependent density functional theory（TD-DFT）calculations.Finally,the effects of structural composition on the differences in physical/chemical stability were investigated separately.The results show that,compared with Au₇Ag₈ cluster,Ag₉Cu₆ exhibited higher thermal stability with better tolerance to Lewis base（such as CH₃ONa）,but is more sensitive to oxidant（such as H₂O₂）.（4）Three M₁₅（M,the metal core）series NCs with different compositions were applied to electrocatalytic CO₂RR,and they all exhibited excellent catalytic performance.First,we prepared the Au Ag Cu trimetallic M₁₅ series superatom of[Au₂Ag₈Cu₅（^tBu C≡C）₁₂]⁺in high yield by an“tailoring chemistry”synthetic strategy and characterized their structure,composition and properties detailedly.The crystal structure has both the inner part of Au₇Ag₈and the outermost part of Ag₉Cu₆.Therefore,the absorption properties of the three NCs are similar,which also experimentally verifies the results of DFT calculation in Chapter 5.Subsequently,we applied the three M₁₅ series NCs（Au₇Ag₈,Ag₉Cu₆ and Au₂Ag₈Cu₅）to CO₂RR for the first time,which showed excellent catalytic performance with the stability of maintaining the metal core structure(Au₇Ag₈:-0.5 V vs.RHE,FE_CO=94.42%（FE,Faraday efficiency）;Ag₉Cu₆:-1.2 V vs.FE_HCOOH=46.97%;Au₂Ag₈Cu₅:-1.0 V vs.RHE,FE_HCOOH=28.29%).Among them,Ag₉Cu₆ has a stronger suppression of the hydrogen evolution reaction（HER）(FE_H2≤7.00%)and its main product is HCOOH in a certain potential range.Theoretical simulations to elucidate the cluster structure/function relationship are still underway,and this part of the study is expected to elucidate the reaction mechanism of Cu-based coinage NCs catalyzing CO₂ into HCOOH at the atomic level.