Alumina-supported Pt catalysts, the sizes of which were between one and ten nanometer, were synthesized by water-in-oil microemulsion method. The effect of the Pt particle size on the electrocatalytic oxidation of CO was studied in detail. Transmission electron microscopy (TEM), scanning electron micrograph (SEM) and cyclic voltammetry (CV) were used for the characterization of the catalysts. It has been found that the catalytic activity obviously depended on the size of Pt nanoparticle, and the range of the optimum particle size was 5～6 nm, which may result from the number of adsorption sites and the relative coverages of CO_ad and OH_ad species.

Key words: microemulsion, Pt/Al₂O₃, nanoparticle, size effect