Polymer-capped Bimetallic Nanoclusters as Active and Selective Catalysts

Catalysts play a very important role not only in chemical processes in industry but also in chemical reactions (enzymatic reactions) in the human body. 

In contrast, colloid chemistry has provided the colloidal dispersion of metal fine particles in water.As early as the 1950 s, colloidal dispersion of fine particles of precious metals was conducted and applied to catalyses. Although they contain fine metal particles, the size of the particles was not so uniform. In addition they were not very stable when used in solution. Thus, there still remained many problems in the reproducibility of the preparation and catalysis using metal nanoparticles. In 1976, the author s group prepared colloidal dispersions of rhodium particles protected by water-soluble polymers by reduction of rhodium(III) ions under mild conditions, i.e., reduction with refluxing alcohol in the presence of water-soluble polymers. They were applied to the catalyst for hydrogenation of olefins. In 1989, we developed colloidal dispersions of Pd/Pt bimetallic nanoclusters by the simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP). 

Colloidal dispersions of metal nanoclusters usually work at rather low temperatures as homogeneous catalysts. On this point, metal nanoclusters are similar to enzymes and are often regarded as artificial enzymes. From the viewpoint of green chemistry (less energy, fewer by-products, more efficiency, more selectivity, etc.), enzymes are model industrial catalysts. Thus, metal nanoclusters may provide the means for industrial catalysts to step up from the present practical heterogeneous catalysts to more ideal and enzyme-like ones. 

In this section the preparation and catalytic properties of the colloidal dispersion of polymer-capped bimetallic nanoclusters will be discussed, emphasizing the importance of polymers for controlling the structure and stabilizing the dispersions of bimetallic nanoclusters in preparation processes as well as for the improvement of the activity and selectivity in catalyses. Although there are many reports on the preparation of industrial catalysts composed of bimetallic nanoclusters and inorganic supports, they are not included in this section. 

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N. Toshima, Polymer-capped bimetallic nanoclusters as active and selective catalysts, in N. Ueyama, A. Harada (eds.) Macromolecular Nanostructured Materials, Kodansha/ Springer, Tokyo/Berlin, 2004, 182. 

Polymer-capped Bimetallic Nanoclusters as Active and Selective Catalysts... 

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