Periodicity, noble metal clusters

Bonding and Structure in Solids Ligand Field Concept Metal Cluster Chemistry Nanosized Inorganic Clusters Noble Metals Periodic Table Sintering... 

A second feature of metal halide cluster chemistry is that the early transition metals are more prone to form metal -metal bonds than are the later noble metals and coinage metals. Again the polynuclear metal carbonyls differ in this facet of metal-metal bond behavior, and, in fact, metal carbonyl clusters become more common on going from the left to the right of the Periodic Table. 

The metals supported on a 2-D surface are investigated by periodic models, with the aim of characterising the properties of the supported metal particles as function of their cluster size from isolated supported metal atoms to regular overlayers. We will discuss two different noble metals Pd and Pt. The properties of these metals are similar, but when supported on an oxide surface they show in many cases different behaviour. One important distinction between them concerns the shape of the supported clusters, which are known to depend on the strength of the metal-support interaction, and therefore on parameters such as the choice of the substrate, working temperature and loading [11]. Pt is reported to form 2-dimensional particles followed by transformation into 3-dimensional particles [12], while Pd tends to form 3-dimensional but flat (raft-like) particles when supported on a Z1O2 support [13]. 

We start by giving a short report of the computational methods employed in our calculations, including a geometrical description of the systems investigated. We describe first the cluster calculations on silica based materials, followed by periodic calculations on noble metals supported on oxides. The results for both systems yield detailed information on structure and bonding on these complexes. 

C. Clusters of noble metals also give periodicities... 

Access to clusters of principally all elements in the periodic table has opened new possibilities for test of calculations and comparison with available experimental data and what is known from surface science. The geometry of the free clusters will, as discussed above, be determined by the electronic properties of the constituent atoms. This means that clusters of alkali and coinage metals will in a first approximation be determined by the free electrons while clusters of transition elements will be determined by the balance between the nd and (n -k l)s valence electrons. Noble gas atoms will behave as hard spheres, which under certain thermodynamic conditions can form larger clusters of icosahe-dral symmetry [134,135]. The geometry of these free clusters are quite different what one obtains if the cluster is constructed as a piece of the lattice known for the corresponding solid. 

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