Structure of Au clusters

In subsection 3.1, we will present GGA and LDA calculations for Au clusters with 6first principles method outlined in section 2, which employs the same scalar-relativistic pseudo-potential for LDA and GGA (see Fig 1). These calculations show the crucial relevance of the level of density functional theory (DFT), namely the quality of the exchange-correlation functional, to predict the correct structures of Au clusters. Another, even more critical, example is presented in subsection 3.2, where we show that both approaches, LDA and GGA, predict the cage-like tetrahedral structure of Au2o as having lower energy than amorphous-like isomers, whereas for other Au clusters, namely Auig, Au ... 

In this work we recalculate the structures of Au clusters with 6scalar relativistic Troullier-Martins pseudo-potentials , respectively, and within the SIESTA code" . In Fig 2 we present our results for the structures and relative binding energies. We see that GGA leads to planar structures whereas LDA favors 3D structures for n>7 clusters. Thus, in addition to relativistic effects, the observed planarity of Au clusters is accounted for using only the GGA level of theory. 

Figure 4- Left Density of states (DOS) of cage-like and amorphous structures of Au clusters with 18 and 20 atoms, calculated at the LDA and GGA levels of theory. Right Total energy difference between cage-like and compact equilibrium structures of cationic, neutral, and anionic clusters with 18 (starts), 20 (crosses), and 32 (circles) atoms.

Let us now consider higher closed-shell clusters and show in this section that closed-shell structures of Au clusters... 

Hybrid density functional calculations have been carried out for AU-O2, Au-CO, Aui3, AU13-O2, Au -CO, AU13-H2, and AU55 clusters to discuss the catalytic behavior of Au clusters with different sizes and structures for CO oxidation [179]. From these calculations, it was found that O2 and CO could adsorb onto several Au model systems. Especially, icosahedral Aun cluster has a relatively weak interaction with O2 while both icosahedral and cubooctahedral Aui3 clusters have interactions with CO. These findings suggest that the surfaces of the Au clusters are the active sites for the catalytic reactions on the supported and unsupported Au catalysts. 

Molecular-dynamics simulations also showed that spherical gold clusters is stable in the form of FCC crystal structure in a size range of = 13-555 [191]. This is more likely a key factor in developing extremely high catalytic activity on reducible Ti02 as a support material. Thus, it controls the electronic structure of Au nanoparticles (e.g. band gap and BE shift of Au 4f7/2 band) and thereby the catalytic activity. 

Recent GGA first principles pseudopotential calculations conclude that Aun clusters adopt planar structures up to larger sizes than silver and copper, particularly the anionic species, due to relativistic effects. Specifically, Fernandez and coworkers obtain planar structures for the ground state of anionic v=l), neutral v=0), and cationic (z/=+l) species of Au(( clusters... 

Figure 2. Left equilibrium geometries of the two lowest energy isomeric states of Au clusters obtained using LDA or GGA scalar relativistic pseudo-potentials. The ground state is Au for GGA and Auj for LDA (except for n=6, which LDA structure is also Aue). Right difference in the binding energy per atom of the planar and 3D structures given in the left panel for neutral gold clusters with 6

Figure 5. Left panel lowest energy equilibrium structures of AunTM+ clusters with 2

While the stability of the monolayer Pt alloy catalyst concept was initially unclear and therefore threatened to make the monolayer catalyst concept a questionable longer term solution, a very recent discovery seems to lend support to the claim that Pt monolayer catalyst could be made into stable catalyst structures Zhang et al. [94] reported the stabilizing effect of Au clusters when deposited on top of Pt catalysts. The presence of Au clusters resulted in a stable ORR and surface area profile of the catalysts over the course of about 30,000 potential cycles. X-ray absorption studies provided evidence that the presence of the Au clusters modified the Pt oxidation potentials in such a way as to shift the Pt surface oxidation towards higher electrode potentials. 

An alternative porcupine imagery of Au clusters involves radial AuP—L quills attached to a central Auc atom (p, peripheral c, central) with angular fluxionality. This view is supported by additional structural features the radial Auc—Aup distances are always appreciably less than the tangential Aup—Aup distances, and Auc—Aup—L angles are close to 180° except where ligand chelation interferes. Linear P—Au—Au—P moieties are sometimes evident. 

Fig. 2. Structures of the clusters [AuOs3(/i-H)(CO)10(PR3)] (R = E 1 °r Et) and [Os3-(/x-H)2(CO)10], showing the isolobal relationship between the edge-br dging Au(PR3) unit and the hydrido ligand.

Fig. 6. Structures of the clusters [MRu4H3(CO)l2(PPh3)] (M = Cu, Ag, or Au), showing the site preferences of the M(PPh3) units.

Fig. 15. Structures of the clusters [h R HzfCOlufPPlb ] and [M2Ru4(m-CO)3-(CO)10(PPh3)2] (M = (Cu, Ag, or Au), showing the change in skeletal geometry when two hydrido ligands are formally replaced by a CO group. [Reprinted with permission of the Royal Society of Chemistry 142) and Elsevier Sequoia S.A. (44).]...

Fig. 23. Structure of the cluster cation [Au3Rh(/ -H)(CO)(PPh3)5]+, with the hydrido ligand, which is thought to bridge the Au—Rh vector trans to the CO ligand, omitted for clarity. [Reprinted with permission of Freund Publishing House Ltd. (22).]...

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