Various ligand-stabilized metal nanoclusters as homogeneous and heterogeneous catalysts in the liquid phase

Ligand-stabilized noble metal nanoclusters, prepared by various chemical methods by different research groups in Japan and Germany, were characterized and examined by a common method for application to the catalysis for hydrogenation of olefins in homogeneous and heterogeneous systems in the liquid phase. The mean diameters of palladium, platinum, rhodium and Pd/Pt nanoclusters stabilized by various ligands range from 1.3 to 3.2 nm if prepared by a single reaction, and from 2.2 to 4.0 nm if prepared by a stepwise growth method. The Stokes radii of metal nanoclusters stabilized by surfactants range from 1.7 to 2.1 nm, suggesting a thickness of the protective layer from 1.1 to 1.4 nm, whereas those stabilized by polymers give much larger values, suggesting the formation of aggregates. The catalytic activities of the metal nanoclusters, evaluated by hydrogenation of 1,3-cyclooctadiene and methyl acrylate, depend mainly upon the particle size, i.e. the smaller the size, the higher the activity, However, a strongly interacting ligand like tetraoctylammonium halide and 1,10-phenanthroline can disturb the hydrogenation, In contrast, the activities of heterogeneous catalysts supported on charcoal depend strongly on the covering strength of the stabilizer. Copyright (C) 2001 John Wiley & Sons, Ltd.