Study On The Controllable Synthesis Of Ag-based Clusters And Their Electrocatalytic CO₂reduction Performance

Atomically precise coinage metal nanoclusters（NCs）have been the subject of intense study owing to their versatile applications in catalysis,assembly,optoelectronics,and biolabeling.The last decade has witnessed great advancements of molecular Au nanoclusters in structural elucidation,intensive property exploration and a manifold of applications.Such great success actually benefits from the effective synthetic approaches for Au NCs.However,for Ag NCs and Ag based alloy clusters（e.g.Ag Au clusters）,these methods might not be applicable,as Ag is much more sensitive to air oxidation.The current methods for synthesizing Ag and Ag-based alloy clusters are still quite limited,plus the tedious procedure,and extremely low yields,these factors have significantly restricted the further property study and application of Ag and Ag-alloyed nanoclusters.At the same time,silver-based nanomaterials have been receiving more and more attention in electrocatalytic carbon dioxide reduction（CO₂RR）in recent years,but the corresponding mechanism investigation lags far behind.The emergence of atomically precise silver clusters makes it possible to elucidate the catalytic mechanism.Therefore,to advance the development of this field,it is imperative to develop rapid and high yield synthetic methods of silver nanoclusters and to explore the catalytic mechanism of silver nanoclusters in the electrocatalytic reduction of carbon dioxide.In this thesis,through the construction of a reverse micelle system,p-mercaptobenzoic acid（p-MBA）protected Ag₄₄（p-MBA）₃₀ and the corresponding alloy cluster of Au₁₂Ag₃₂（p-MBA）₃₀ were successfully synthesized with high efficiency.The rapid adding method of reducing agent at high temperature was used to synthesize the all-alkynyl protected Ag₁₅（C≡C-^tBu）₁₂ for the first time and applied it to CO₂RR.The main contents of this paper are as follows:（1）Using silver nitrate as silver source,p-mercaptobenzoic acid（p-MBA）as ligand,sodium borohydride as reductant and cetyltrimethyl ammonium bromide（CTAB）as surfactant,Ag₄₄（p-MBA）₃₀ and Au₁₂Ag₃₂（p-MBA）₃₀ were synthesized rapidly（15 min）and in high yield（85%）by constructing CTAB reverse micelle system.Their characteristic absorbance peaks were observed in the UV-vis spectrum,and the composition of the product was determined by electrospray ionization mass spectrometry.The system before and after the reaction was characterized by Raman spectroscopy and nuclear magnetic resonance（NMR）.It was proved that the CTAB reverse micelles played the role of microreactor in this system,and the complex processes such as size focusing were avoided by encapsulating the short-chain Ag-p-MBA precursors in the reverse micelles,so this method can synthesize the target products with high efficiency.（2）Using silver trifluoroacetate as silver source,tert-butylacetylene（HC≡C-^tBu）as ligand and sodium cyanoborohydride as reductant,the smallest known silver cluster Ag₁₅（C≡C-^tBu）₁₂with homoleptic alkynyl protection was successfully synthesized for the first time by the rapid adding method of reducing agent at high temperature.Its structure was successfully characterized by single crystal X-ray diffraction.It was found that the silver nucleus of Ag₁₅（C≡C-^tBu）₁₂ contains a Ag@Ag₈@Ag₆ kernel and the whole structure possesses a BCC（body-centered cubic）configuration.When applied to CO₂RR,it is found that the dominat product is CO,and the Faradaic efficiency is as high as 95%.At the same time,the Faraday efficiency for CO is more than 70%in the range of-0.8V to-1.5V（vs RHE）,and the side hydrogen evolution reaction can be effectively suppressed.Ag₁₅（C≡C-^tBu）₁₂ as a catalyst maintained good stability before electrocatalytic reaction.