Noble metal atoms, colloidal

Compared with normal Raman spectroscopy, the SERS technique gives a considerably lower limit of determination of the substance to be analyzed. This is achieved by bringing colloidal noble metal atoms into close proximity to the sample molecules after they have been separated by TLC. Further information can he found in [90, 91]. 

In materials chemistry, nanoparticles of noble metals are an original family of compounds. Well-defined in terms of their size, structure and composition, zero-valent transition-metal colloids provide considerable current interest in a variety of applications. Here, the main interest is their application in catalysis. Zerovalent nanocatalysts can be generated in various media (aqueous, organic, or mixture) from two strategic approaches according to the nature of the precursor, namely (i) mild chemical reduction of transition-metal salt solutions and (ii) metal atom... 

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

Unless the metal is introduced as such, e.g. as a colloid or by metal-atom-vapour deposition (see later), the final and critical step is inevitably a reduction, performed either ex situ or in situ (or both). Molecular hydrogen is most often used, although carbon monoxide has a thermodynamic advantage, which is useful for less easily reducible species because the carbon dioxide produced is less effective than water in reversing the process. Reduction of a base metal oxide can be effected by hydrogen atoms spilling over (see Section 3.34) from reduced noble metal particles." More exotic reductants (e.g. Cr ions," oxirane" and... 

It has been shown, however, that in some cases, catalysts working efficiently even if they are used in very small quantities (0.1 ppm) such as the Speier catalyst H2PtCl6 or the Karstedt catalyst [Pt(CH2=CHSiMe20SiMe2CH=CH2)2], are indeed functioning in a colloidal nanoparticle form. Such nanoparticles are giant metal clusters containing several hundred atoms that are readily produced for instance by reduction of noble metal salts or complexes (see Chap. 2.2.1 and 20.7.3). 

To describe the behavior of clusters the classical electrodynamics may be used for colloidal particles whereas for nano-crystals of less than 100 atoms the optical functions become size dependent. Simplified models can be set up for ideal clusters such as sodium or potassium not being disturbed by interband interference. Nevertheless, it should be clear that these calculation are of reduced value due to the fact that almost all clusters of practical applicability are made of noble metals or semiconductors with a fer more complex behavior. 

---