Thermal Decomposition and Photochemical Methods

The syntheses of palladium and platinum organosols [82-85] by the thermolyds of such precursors as palladium acetate, palladium acetylacetonate, and platinum acetylacetonate in hi boiling organic solvents like methyl- o-butylketone have been reported. Likewise, bimetallic colloids of copper and palladium have been prepared from the thermolysis of mixtures of their acetates in similar solvents. [86] These preparations were performed in the absence of stabilizing polymers, and as a residt, relatively broad size distributions and large partides were observed. 

In view of the extensive literature covering the syntheses and structures of bimetallic metal carbonyl clusters, it is even more surprising that few attempts to use the fadle decomposition of these compounds in the preparation of bimetallic colloids have been reported. Although this would be an unnecessarily complicated route to colloids of misdble metals if the salts of these metals could be coreduced to the same end, there remains the possibility of preparing bimetallic partides of immisdble metals starting from well defined molecular bimetallic clusters of immisdble metals. Such dusters are known to exist desjnte the immis-dbility of their constituent metals in the bulk, and there is reason to believe that bulk immisdbility might not apply to very small partides due to the importance of surface effects. 

Photochemical colloid syntheses fall into two categories the reduction of metal salts by radiolytically produced reducing agents such as solvated electrons and free radicals, and the photolysis of photolabile metal complexes. Again, the essence of these preparative procedures lies in the generation of zerovalent metal under conditions which prevent, or at least retard, the formation of bulk metal predpitates. 

The photolytic method has, of course, a long and important history in the formation of photographic images from silver halide emulsions. Over the past twenty years, predominantly from the work of Henglein and of Belloni, a wide variety of colloidal metals has been prepared by this method encompassing both main group metals such as cadmium [87, 88], thallium [89, 90] and lead, [91] as well as other noble [92-98] and non-noble transition metals [99-101]. Radiolytic methods differ in the type of redudng spedes which is formed under irradiation, which is a function of solvent and any added solute. The radiolysis of aqueous solutions of metal ions produces solvated electrons which may either react with the dissolved 

Similarly irradiation of Pd(NH3)4Q2 in aqueous uo-propanol results in the reduction of the palladium complex to colloidal palladium by both solvated electrons and 1-hydroxy-1-methylethyl radicals (CH3)2COH (formed by reaction of iyo-propanol with the initial radiolysis products H and OH). [102] Since mixtures of acetone and iso-propanol also yield (CH3)2COH under both radiolytic and photolytic conditions, they have been used in many studies on the radical chemistry of metal colloids. In many of these studies, the techniques of flash photolysis and pulse radiolysis were used to detect short lived intermediates, measure the kinetics, and determine the mechanisms of metal cation reduction, nucleation, and colloidal cluster formation. 

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