1. INTRODUCTION

Metal nanoparticles have attracted a great interest in scientific research and industrial applications, owing to their unique large surface-to-volume ratios and quantum-size effects. ^{1 3} Since industrial catalysts usually work on the surface of metals, the metal nanoparticles, which possess much larger surface area per unit volume or weight of metal than the bulk metal, have been considered as promising materials for catalysis.

Industrial catalysts are usually composed of inorganic supports and metals on the supports. They are often prepared by heat treatment of metal ions on the support at high temperature sometimes under hydrogen. They have very complex structures. For example, they are the mixtures of metal particles with various sizes and shapes. Metal particles often strongly interact with the inorganic supports, thus resulting in the structure of half balls, for example.

In contrast, colloid chemistry has provided the colloidal dispersion of metal fine particles in water.4 In the 1950's colloidal dispersions of metal fine particles were already prepared and applied to catalysis. Although they contained metal nanoparticles, the size of nanoparticles was not sufficiently monodispersed. In addition they were not stable enough when used in solution. Thus, there still remain many problems in the reproducibility of the preparation and catalysis of metal nanoparticles. In 1976, we prepared colloidal dispersions of rhodium nanoparticles 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.5 These nanoparticles were applied as catalysts for hydrogenation...