Theoretical Study On The Structure, Optical Spectra And Catalytic Properties Of Gold Clusters

Due to the unique physicochemical properties and potential in several practicalapplications such as catalysis, nanotechnology and magnetic materials etc., nanosizedgold clusters have attract intensive research interests during the past decades.Tremendous experimental and theoretical efforts have been devoted to this field. Thestudies on gold cluster can be roughly classified into two categories: one is about thestructure and catalytic properties of bare gold cluster, which includes thedetermination of precise atomic structure of clusters and another extensively studiedsystem is the thiolate protected gold clusters.In this thesis, we have employed density functional theory （DFT） to investigatethe structural evolution and structure-catalytic properties of a series of nanosized goldclusters and thiolate protected gold clusters. The thesis contains two parts. The firstpart focuses on the relationship between the structure and the catalytic activity of thebare gold clusters in the CO oxidation reactions. In the second part of the thesis, weintroduce our recently developed force-field based ‘divide-and-protect’ method andapplications to the quick and reasonable prediction of structure of thiolate protectedgold clusters, e.g. Au₂₁（SR）₁₄^-and Au₂₄（SR）₂₀clusters. On basis of the predictedcluster structures, we have theoretically studied the UV-vis adsorption spectrum andpowder X-ray diffraction curves and compared them with experimental data. Goodagreements between theoretical and experimental curves were obtained. Finally, theUV-vis absorption spectrum of newly synthesized Au₃₆（SR）₂₄was studied.In the first part of thesis, we have comprehensively studied the catalytic reactionmechanism of a variety of nanosized gold clusters （with size ranges from0.3-0.8nm）towards CO oxidation using DFT calculations. The structure–catalytic activityrelationship is obtained. We find that the triangular Au3site on the cluster surface isthe major active site, which can stabilize both OCOO*intermediate and transitionstate structure of O-O bond scission. On basis of this discovery, we further observedthe effects of co-adsorbed CO molecules nearby the active site on the whole catalyticreaction. Our calculation results indicate that the co-adsorbed CO molecule at the Au3active site can not only serve as the reactant, but also act as promoter to the oxidationprocess. There are two major creativities in this mechanism. First, over past twodecades, researchers generally consider the CO oxidation on nanosized gold clusters is a bi-molecular process. Few studies examine the role of co-adsorbed CO moleculeon the reaction process. Our proposed new mechanism suggests the role of theco-adsorbed CO molecule is not negligible. It can also participate into the reactions.Second, our extensive theoretical calculations revealed for the first time that theco-adsorbed CO molecule can attack the formed OCOO*intermediate throughelectrophilic attack, which significantly lower the energy barrier of O-O bond brokenand hence accelerate the reactions with the direct formation of two product CO₂molecules. As a result, the total elementary reaction step in the reaction is reduced.We therefore call this new reaction mechanism as CO self-promoting oxidationreaction mechanism. In order to further verify this mechanism, we also carry out someother theoretical studies such as Born–Oppenheimer molecular dynamics simulation,CO oxidation reaction on metal oxide supported gold nanostructure, and Hirshfeldatomic charge analysis. The additional calculation results confirm again our proposednew mechanism and revealed the electronic nature of CO-self-promoting oxidation.The results provide a new insight into the nature CO oxidation reaction catalyzed bygold cluster, therefore it is highlighted by the famous international chemical andchemical engineering review journal Chemical&Engineering News.In the second part, the structure and UV-vis adsorption spectrum of three thiolateprotected gold clusters are investigated using DFT calculations and our recentdeveloped force-field based ‘divide and protect’ method. The structure and opticalspectrum of Au₂₁（SR）₁₄^-is a major research subject in this part. The magic stableAu₂₁（SR）₁₄^-cluster has been observed in several mass spectrometry （MS） experimentof many thiolated Au clusters feature by a high peak in spectrum. On basis of the‘divide and protect’ method, we predict the Au₂₁（SR）₁₄^-contains a high symmetryAu13core. The Au-core is protected by two dimer and four monomer staple motifs.Based on the predicted cluster structure, we also calculate its UV-vis absorptionspectrum using TDDFT for the further experimental verification. The formationmechanism and origin of its magic stability observed in several MS experiments isdiscussed via the first principle molecular dynamic simulation, which indicate theformation of Au₂₁（SR）₁₄^-is probably a thermodynamics-dominated process. Besides,we apply the ‘divide and protect’ method in the structural predicting of Au₂₄（SR）₂₀, inwhich the most stable isomer turns out to be a D2dsymmetry Au8core covered by twosets of catenane-like [Au₃（SR）₄][Au₅（SR）₆] staple motifs. The UV-vis and XRD curveof this lowest-energy isomer re also calculated, which are in good agreement with the experimental curve reported before. In last part, we have studied the optical propertiesof Au36（SR）24in collaboration with Prof. Rongchao Jin professor in Carnegie MellonUniversity. The electron transition pattern of each feature absorption peak observed inthe experimental curve is explained by TDDFT calculations.Through the theoretical investigation on two distinctive gold cluster systems, weoffer reasonable explaining and analysis to the structure, catalytic and opticalproperties of a series of different nanosized gold clusters. We also theoreticallypredict the structure of some thiolate-protected Au clusters that are still explored inexperiment. We hope the work in this paper will promote and deepen ourunderstanding on the structure and property of Au nanocluster, at the same timemotivate more relevant researches in this field.