Preparation of Fine Particles of Noble Metals by Spray-pyrolysis Technique

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Spray-pyrolysis technique was applied for the preparation of ultrafine and spherical particles of Ag, Pd and Ag-Rd alloy. Ag particle - Solution of Ag <NO_3> gave Ag particles at the pyrolysis temperature above 650゜C Ag particles became solid and spherical at pyrolysis temperatures above melting point of Ag. The particle sizes (0.2-0.5 <micro> m) were determined by the degree of the splitting of atomized droplets during the pyrolysis. When solution of Ag <NO_3> was atomized into <H_2> - <O_2> flame, Ag particles were formed by two routes depending on flame temperature : Ag <NO_3> Ag(1)->Ag(s)(route C) and Ag <NO_3> ->Ag vapor->Ag(s)(route D). Ag particles by route D were finer (0.02-0.2 <micro> m) than those by route C. In route D, the particle sizes were determined by the feed rate of Ag <NO_3> . It was important to prevent the co-occurence of the two routes in order to produce uniform-sized Ag particles.Pd particles - Pd <(NO_3)_2> solution was atomized into <H_2> - <O_2> flame. Spherical and uniform-sized Pd particles (0.5-0.8 <micron> m) were obtained at around 1900゜C. Pd powders produced at higher temperatures contained ultrafine particles by partial evaporation of Pd. Ag-Pd alloy particles - Spray pyrolysis of Ag <NO_3> - Pd <(NO_3)_2> solution in <H_2> - <O_2> flame gave submicron alloy particles. In order to prepare the alloy particles with uniform composition, the flame temperature must be controlled in the range where alloy particles melt without evaporation of both components.