Theoretical Study On The Structure And Catalytic Properties Of Ligand-Protected Gold Clusters And Nanoporous Gold

Bulk gold is chemically inert and well known to have a face-centered-cubic(fcc)structure.However,when a bulk material is reduced in size to nanoscale,the resulting atomically precise nanoparticles have unique structures which differ significantly from the bulk.Ligand protected gold nanoclusters is one of most intensively studied gold nanomaterials owning to their distinctive physical chemistry properties which are vastly different from the bulk and potential usage in biomedicines and catalysis,and chemical sensing.In this paper,we focus on investigating the structural evolution of the thiolate-protected gold clusters and the gold-based catalysts basing on the density functional theory(DFT).In this work,we theoretically predict an unprecedented?14 kDa core mass AuNP species,denoted as Au68(SR)36,which is composed a symmetric,face-centered-cubic(fcc)68-gold atom framework.The fcc gold kernel in the Au68(SR)36 are made of eight 13-atom Au-cubotahedrons sharing twelve square faces showing a standard 2×2×2 magic cube formula.The Au68(SR)36 is thought to be a key intermediate NP bridges the evolution of Au44(SR)28 and Au92(SR)44.The DFT calculations indicate the Au68(SR)36 has a sizable HOMO-LUMO gap of 0.98 eV and relative high thermodynamic stability.The fcc 68-atom gold framework in the Au68(SR)36 also presents a new candidate to address the atomic structure of recently reported water soluble mercaptobenzoic acid protected Au68NPs.By employing a crystal facet cleavage(CFC)method,we theoretically predict five fcc-structured isomer structures of Au20(SR)16 cluster.We explore many different kinds of large ligands to investigate the stability of the isomers.The dispersion correction density functional theory(DFT-D)calculations indicate the nanocrystal like Au20(SR)-6 processes comparable and even lower energies than that of the non-fcc structure reported by previous experimental studies.The cubic nanocrystal like Au20-Isol isomer structure is suggested as a better candidate to understand the atomic structure of Au20(SC2H4Ph)16.In this work,we theoretical study the structure evolution of a series of large size Au145-3N(SR)60-2N(N=1-8)clusters based on an "[Au2@Au(SR)2]fragmentation"pathway starting from a model Au145(SR)60 cluster.Through comprehensively searching the atomic structure of various clusters and evaluating their stabilities by meanings of first principle calculations,the stabilization mechanism of experimentally reported Au130(SR)50 and Au133(SR)52 clusters is first rationalized.Our studies indicate that the Au130(SR)50 and Au133(SR)52 are two critical size clusters at which the gold cores underwent configuration transitions between decahedral and icosahedral ones.The energy comparisons of various cluster isomer structures indicate that the Au130(SR)50,Au127(SR)48,Au124(SR)46 and Au121(SR)44 clusters favor a decahedral core,while the Au133(SR)52,Au136(SR)54,Au139(SR)56,and Au142(SR)58 prefer icosahedral gold cores.Furthermore,we also find that the cuboctahedral gold core is less stable in the cluster size region between?120 to?140 gold atoms.The optical absorption properties and the relative thermodynamics stabilities of the Au145-3N(SR)60-2N(N=1-8)clusters are also surveyed by density functional theory(DFT)and time-dependent DFT calculations.We report a systematically study of reaction mechanism of CO oxidation over nanoporous(NPG)based on the recent progress of structural characterization of NPG.The Agx@Au-(111)and Agx@Au-(100)slabs(x=1-3)are built to mimic the local structure of NPG.The CO oxidation pathways over two model catalysts are investigated via the density functional theory(DFT)calculations.Totally 50 reaction pathways are examined at different active site of two model catalysts.A simplified microkinetics model termed the Sabatier analysis which is built on the adsorption energies and activation barriers is used to evaluate the reaction rate of different reaction pathways.The role of the residual Ag atom and effect of the co-adsorbed CO molecule are discussed.Our theoretical results propose that the presence of residual Ag content in NPG may promote the adsorption of O2 species.However,the barrier height of OCOO*intermediate dissociation is affected strongly by the local surface structure and the presence of nearby co-adsorbed CO molecule.The co-adsorbed CO molecule nearby the reaction active site can promote the dissociation of OCOO*reaction intermediate via an electrophilic attack process,which is denoted as a tri-molecular CO self-promoting oxidation mechanism.Such tri-molecular route has reduced reaction steps and higher reaction rate in comparison to the conventional bi-molecular reaction mechanism.The presence of residual Ag atom can facilitate the attack of co-adsorbed CO to OCOO*intermediate and lead to increased reaction rate.