Colloidal metal particle dispersions

It is important to state the difference between particles and particulate films at the onset of this section. Particles are separate nanometer- to micron-sized colloids dispersed in solution. Physically interconnected colloidal metal particles constitute a particulate film which may be supported by a monolayer floating on an aqueous subphase or be deposited on a solid substrate. 

The linear CO stretching frequency for the carbonylated platinum colloid while lower than that found for surface bound CO, is in the range reported for the platinum carbonyl clusters [Pt 3 (CO) 6 ] n / sind we find that the carbonylated colloid is easily transformed into the molecular cluster [Pt 12 (CO) 24 ] (10) reaction with water. The cluster was isolated in 50 yield based on platinum content of the precipitate by extraction with tetraethylammonium bromide in methanol from the aluminum hydroxide precipitated when water is added to the aluminoxane solution. The isolation of the platinum carbonyl cluster reveals nothing about the size or structure of the colloidal platinum particles, but merely emphasizes the high reactivity of metals in this highly dispersed state. The cluster isolated is presumably more a reflection of the stability of the [Pt3(CO)6]n family of clusters than a clue to the nuclearity of the colloidal metal particles - in a similar series of experiments with colloidal cobalt with a mean particle size of 20A carbonylation results in the direct formation of Co2(CO)8. 

It is interesting to include in this eategory of interphase systems, the use of highly water-soluble proteetive agents, like polymers, surfactants, or ionic species, to stabilize colloidal metallic particles finely dispersed in water. Larpent and Patin have developed this original approach [72]. Ruthenium eolloids ean efficiently catalyze the oxidation, under room eonditions, of cyclo-octane by t-butylhydroperoxide [73]. Recent results show that... 

Highly effective catalysts have been prepared by using the protective properties of some hydrophilic polymers, such as polyvinylpyrrolidone (PVPD), polyvinylmethyl ester (PVME), dextrin and PVA. The protective role of these synthetic polymers is to prevent the aggregation of colloidal metal particles and to stabilize the homogeneous dispersion of small particles. These catalysts possess high activity and selectivity. They are, moreover, easily separated from reaction products and can be repeatedly reused [36, 37]. 

It was shown in [5 3,54] that there are two methods to fabricate the homogeneous polymer-immobilized dispersions of colloidal metal particles (Fe, Co, Cr, Mo, W, Mn, Re, Ni, Pd, Pt, Ru, Rh, Os, Ir) using the precursors thermal decomposition. In the former case an active polymer solution (containing amino-, amido-, imino-, nitrilo-, hydroxy-, and other functional groups) is used. In an inert solvent a labile metal compound is gradually added to the solution (this operation creates the favorable conditions for the chemisorption interaction) followed by the suspension thermal decomposition at 370-440 K or by radiation. 

Boutonnet, M., Kizling, L, Stenius, R, and Maire, G. 1982. The preparation of mono-disperse colloidal metal particles from microemulsions. Colloids Surf., 5, 209-225. 

Recently, various sol-gel based recipes incorporating metal colloids have been shown to result in successful SERS substrates (49, 50, 52-57). Large Raman enhancements have been found for nano-sized metal particles dispersed in the resulting gel structure, in part, due to the large stabilized metal surface areas available to molecular-sized scatterers. In contrast, the sol-gel derived SiOa SERS substrate produced by the procedure described here is covered by immobilized clustered aggregates of monodispersed sized gold nano-particles that have been grown in-situ. 

PVP, a water soluble amine-based pol5mer, was found to be an optimum protective agent because the reduction of noble metal salts by polyols in the presence of other surfactants often resulted in non-homogenous colloidal dispersions. PVP was the first material to be used for generating silver and silver-palladium stabilized particles by the polyol method [231-233]. By reducing the precur-sor/PVP ratio, it is even possible to reduce the size of the metal particles to few nanometers. These colloidal particles are isolable but surface contaminations are easily recognized because samples washed with the solvent and dried in the air are subsquently not any more pyrophoric [231,234 236]. 

For trace analysis in fluids, some Raman sensors (try to) make use of the SERS effect to increase their sensitivity. While the basic sensor layout for SERS sensors is similar to non-enhanced Raman sensors, somehow the metal particles have to be added. Other than in the laboratory, where the necessary metal particles can be added as colloidal solution to the sample, for sensor applications the particles must be suitably immobilised. In most cases, this is achieved by depositing the metal particles onto the surfaces of the excitation waveguide or the interface window and covering them with a suitable protection layer. The additional layer is required as otherwise washout effects or chemical reactions between e.g. sulphur-compounds and the particles reduce the enhancement effect. Alternatively, it is also possible to disperse the metal particles in the layer material before coating and apply them in one step with the coating. Suitable protection or matrix materials for SERS substrates could be e.g. sol-gel layers or polymer coatings. In either... 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

It is operationally difficult to distinguish between dissolved and colloidally dispersed substances. For example, colloidal metal-ion precipitates occasionally have particle sizes smaller than 100 A, sufficiently small to pass through a membrane filter, and organic substances can exist as a stable colloidal suspension. Information on the types of species encountered under different chemical conditions (type of complexes, their stabilities, rate of formation) is a prerequisite to better understanding of the transformation in properties of toxic chemicals in a water body. 

Despite the fact that the hydrolysis of the ferric ion is exceedingly sensitive to various experimental parameters (temperature, pH, etc.), hematite (a-Fe203) and akageneite ((3-FeOOH) were apparently the first reasonably uniform colloidal metal (hydrous) oxides dispersions reported in the literature, as already indicated in the introduction. Since then, this family of compounds has been the most extensively investigated, with specific emphases on particle uniformity, composition, and morphology. 

Colloidal dispersions of fine metal particles can usually be prepared by reduction of metal ions. The first scientific report to synthesize colloidal dispersion of metals was presented by Faraday, who prepared metal colloids without stabilizers (5). In his case counteranions may have played the role of the stabilizer. In most recent cases, however, stabilizers are usually added to the system to stabilize the colloidal dispersions. 

In this section, the general concept and practical procedures to synthesize the colloidal dispersions of fine metal particles in homogeneous solution are described. 

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