Theoretical Study On The Structure And Catalytic Properties Of Gold Nanoclusters

Due to the unique physicochemical properties and potential usages in practicalapplications such as catalysis, nanotechnology and magnetic materials etc., nanosizedgold clusters have attracted widespread research interests during the past few decades.Tremendous experimental and theoretical efforts have been devoted to the studies ofstructure and properties of nano-sized gold clusters. To date, the studies on goldclusters can be roughly classified into two categories: the structure and properties ofgold clusters with or without ligand-protection.In this thesis, we carried out systematical theoretical studies on two types of goldclusters using the density functional theory (DFT) calculations. First, the theoreticalstudies on the structure prediction of thiolate-protected gold nanocluster Au44(SR)28and its optical properties are performed. Second, the catalytic activity and reactionpathways of styrene oxidation on subnanometer gold clusters in presence of O2as thesole oxidant or the H2/O2mixture are investigated.In the first part of thesis, we have summarized a general structural rule for thiolategold clusters by analyzing the ligand-layer structure of known thiolate-protected goldnanoclusters. On basis of the "divide-and-protect" scheme, we are able to quicklysearch and predict the structure of Au-kernel of the object gold clusters and hencedetermine the accurate structure of ligand protected gold cluster. In brief, the"divide-and-protect" scheme is an efficient structural search algorithm, includingbasin-hopping potential energy surface sampling combined with the Sutton-Chenempirical parameters. The structure of a thiolate-protected Au44clusters,[Au44(SR)28],is theoretically predicted via density functional theory calculations combined with the"divide-and-protect" method. The Au44(SR)28is predicted to contain a “two-shell”face-centered-cubic type of Au kernel and possess chirality. The predicted clusterstructure is validated by comparison of optical absorption properties between theoryand previous experiments, as well as energy evaluations. Our theoretical resultsindicate the dianionic Au44(SR)28has a quite small HOMO/LUMO gap (~0.3eV).The properties of neutral Au44(SR)28show much better agreement with previousexperimental results than the dianionic one, such as the optical gap and shape ofoptical curve. Based on the predicted cluster structure, the magic stability ofAu44(SR)28is understood from the superatom electronic confguration and formation of a unique double-helix superatom network inside.In the second part of thesis, we performed a comprehensive study of mechanismof styrene selective oxidation to benzaldehye and styrene epoxide on subnanometergold clusters with the cluster size ranges from around0.4to1.0nm via the densityfunctional theory (DFT) calculation. The major focuses of studies are the intrinsiccatalytic selectivity and size-dependent activities of gold clusters towards the styreneoxidations. The selectivity of the catalysis reactions, e.g. selective formation tobenzaldehyde or styrene epoxide, with the presence of dioxygen as the sole oxidant orthe H2/O2mixture as the reactant is discussed. A new reaction channel involving theformation of a metastable four-membered ring CCOO*intermediate starting from theoxametallacycle intermediate (OMME) is proposed for the first time. Thefour-membered ring CCOO*intermediate can readily break into benzaldehyde andaldehyde to finish the catalytic cycle. The calculated energy curves indicate that theproposed new reaction channel leading to selective formation of benzaldehyde isfavorable in energy than the known pathway that leaded to the styrene epoxide,explaining the recent experimental observations of high yield of benzaldehyderesulted from the styrene oxidation on~1.4nm gold clusters. The mechanism ofdramatic improvement of product selectivity to epoxide with the H2as the additive isexplored as well. We find that the major role of H2additive is facilitating thedissociation of O2into active O-atom on subnanometer gold clusters, which leads tohigh selectivity to epoxide product. This systematic study enables a quantitativeassessment of the size-dependent activity and selectivity of subnanometer goldclusters towards styrene selective oxidation.Through the studies of structure and catalytic properties of a series of goldnanocluster systems, we obtained a deeper understanding of the structure, catalyticand optical properties of gold nanoclusters. We hope that the current results maypromote our understanding of unique properties of nanogold clusers and lay somefoundations for future experimental and theoretical studies in this field.