Noble metal clusters

When a supported metal on an oxide is prepared from an adsorbed precursor incorporating a noble metal bonded to an oxophilic metal, the result may be small noble metal clusters, each more-or-less nested in a cluster of atoms of the oxophilic metal, which is cationic and anchored to the support through metal-oxygen bonds [44,45]. The simplest such structure is modeled on the basis of EXAFS data as Re4Pt2, made from Re2Pt(CO)i2 (Fig. 6) [45]. 

As was mentioned previously, photoemission has proved to be a valuable tool for measurement of the electronic structure of metal cluster particles. The information measured includes mapping the cluster DOS, ionization threshold, core-level positions, and adsorbate structure. These studies have been directed mainly toward elucidation of the convergence of these electronic properties towards their bulk analogues. Although we will explore several studies in detail, we can say that studies from different laboratories support the view that particles of 150 atoms or more are required to attain nearly bulk-like photoemission properties of transition and noble metal clusters. This result is probably one of the most firmly established findings in the area of small particles. 

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

Figure 6. Absorption spectra of spherical non-interacting nanoclusters embedded in no absorbing matrices (a) effect of the size for Ag nanoclusters in silica (b) effect of the matrix for R = 2.5 nm Au clusters (the refractive index n = and the position of the plasma resonance are reported for each considered matrix) (c) effect of the cluster composition for i = 5 nm noble-metal clusters (Ag, Au, Cu) in silica. (Reprinted from Ref [1], 2005, with permission from Italian Physical Society.)...

Dendrimer interior functional groups and cavities can retain guest molecules selectively, depending on the nature of the guest and the dendritic endoreceptors, the cavity size, the structure, and the chemical composition of the peripheric groups. Two main methods are known for the synthesis of metal nanoparticles inside dendrimers. The first method consists of the direct reduction of dendrimer-encapsulated metal ions (Scheme 9.4) the second method corresponds to the displacement of less-noble metal clusters with more noble elements [54]. 

In addition, the rate of Oz reduction, forming 02 by electron, is of importance in preventing carrier recombination during photocatalytic processes utilizing semiconductor particles. 02 formation may be the slowest step in the reaction sequence for the oxidation of organic molecules by OH radicals or directly by positive holes. Cluster deposition of noble metals such as Pt, Pd, and Ag on semiconductor surfaces has been demonstrated to accelerate their formation because the noble metal clusters of appropriate loading or size can effectively trap the photoinduced electrons [200]. Therefore, the addition of a noble metal to a semiconductor is considered as an effective method of semiconductor surface modification to improve the separation efficiency of photoinduced electron and hole pairs. 

Z. C. Zhang, J. Hare, and B. Beard, Basicity of nanosized noble metal clusters in catalysis, 13th Int. Congr. Catalysis, Paris Abstract, 2-017 (2004). 

In the spill-over or catalytic model, the noble-metal clusters on the surface act as catalytic reaction sites (Fig. 2.4a). Reacting species such as oxygen can be dissociated more easily at these sites. When they move from the metal cluster to the grain... 

In this section, two methods used to prepare dendrimer-encapsulated metal nanoclusters are discussed direct reduction of dendrimer-encapsulated metal ions and displacement of less noble metal clusters with more noble elements. 

Nonnoble metal clusters are considerably more fragile to corrosion by the solvent [11] compared to noble metal clusters. Therefore the production of stable small particles results from a compromise between the smallest size and the longest stability [104]. 

DENSITY FUNCTIONAL STUDIES OF NOBLE METAL CLUSTERS. ADSORPTION OF O2 AND CO ON GOLD AND SIIVER CLUSTERS... 

Structure of neutral and charged noble metal clusters... 

Figure 1. Equilibrium geometries of the two lowest energy isomers, Xy and Xy, of anionic, neutral, and cationic noble metal clusters. The roman numerals identify each clusters in Table 1.

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