Colloidal Dispersions of Platinum and Palladium Clusters Embedded in the Micelles.Preparation and Application to the Catalysis for Hydrogenation of Olefins

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Abstract
Colloidal dispersions of platinum and palladium clusters were prepared by the reduction of the corresponding metal salts in the presence of various surfactants. Hydrogen- and photo-reduction methods were examined for the micellar solution of hexachloroplatinic acid. The photo-reduction was proved to be more suitable than the hydrogen-reduction from the viewpoint of small size and good dispersion in the colloidal clusters. The higher surfactant concentration than the critical micelle concentration (cmc) is favorable for the production of the homogeneous colloidal dispersions. The photo-reduction method is also available for the case of palladium. The colloidal platinum and palladium clusters, thus obtained, act as highly active catalysts for the hydrogenation of olefin in an aqueous solution. The colloidal clusters prepared by the photo-reduction method showed higher activity than those prepared by other methods. The roles of the surfactant micelle for the hydrogenation are not only protecting hydrophobic colloidal particles, but also both solubilizing the hydrophobic substrates in an aqueous solution and assisting their approach toward the active site on the surface of the cluster particles. In the hydrogenation of unsaturated fatty acids, the surfactant micelle surrounding the clusters can control the orientation of the substrates, resulting in novel regioselective hydrogenation.