Microcrystalline metal particles

For the benefit of clarity, this Chapter has been restricted fundamentally to the discussion of the chemistry of molecular transition metal clusters no dinuclear compounds, which were analyzed to some extent in Chapter 1, nor microcrystalline metal particles are considered. For the same reason the main emphasis is given to homonuclear metal compounds. However, heteronuclear species with different transition metals or containing main group atoms are taken into account whenever they are useful for a better understanding of cluster chemistry. 

For small particles supported on thin films of amorphous or microcrystalline materials it is not easy to determine whether there is any consistent correlation between the particle orientation and the orientation of the adjacent locally ordered region of the substrate. For some samples of Pt and Pd on gamma-alumina, for example, nanodiffraction shows that the support films have regions of local ordering of extent 2 to 5 nm. Patterns from the metal particles often contain spots from the alumina which appear to be consistently related to the metal diffraction spots. 

If metal ions in solution are reduced to atoms by an appropriate redudng agent, they normally will combine into colloids or into microcrystalline particles which finely predpitate out. Such processes can occur within seconds, as is known for instance from the formation of silver mirrors on glass by the reduction of silver ions. If the growth of the metal particles in solution can be retarded and if appropriate ligands are additionally offered, it may happen that metal rich ligand stabilized transition metal clusters of uniform rize are formed. 

Any Mg(N03)g and excess acid present are separated by further decantation filtration is not recommended because of the fine particle size of the metal. The Cr residue is dried on a steam bath. Yield about 27 g. of light-gray, microcrystalline powder whose Cr content is 99.6%. 

At low temperature, theophrastite ( -Ni(OH)2(s)) appears to be less soluble than bunsenite (NiO(s)), which is a somewhat surprising result. However, theophrastite is the stable form of nickel(II) oxyhydroxide phases below a temperature of about 77 C (Palmer and Gamsjager, 2010) (this is a much lower temperature than the 160 °C indicated by Ziemniak and Goyette (2004)). Consequently, bunsenite is unlikely to control the solubility of nickel at temperatures near ambient. Gamsjager et al. (2005) noted that a number of studies had likely examined the solubility of a microcrystalline form of theophrastite. This form was found to be substantially more soluble than crystalline forms of the same phase, but this particle size dependence is much greater than that normally observed for metal oxides/hydroxides (Palmer and Gamsjager, 2010). Table 11.45 also lists solubility data for the micro crystalline and crystalline forms of theophrastite (this distinction is, in part, based around the reported solubility constants). 

---