Effects of starting materials on the preparation of spherical pd powders by ultrasonic spray pyrolysis

Palladium powders were prepared from aqueous solutions of Pd(NO3)(2), PdCl2 and Pd(NH3)(4)Cl-2 by ultrasonic spray pyrolysis in nitrogen flow and the effects of starting materials and preparation conditions on the properties of prepared powders were investigated. Submicron powders were obtained from PdCl2 because it melts at 680 degrees C and disintegrates on evolving gas. Powders prepared from Pd(NH3)(4)Cl-2 often had a bimodal particle size distribution and irregular shapes because the porous particles of Pd formed at about 300 degrees C underwent either sintering or breaking up to pieces at higher temperatures, resulting in a mixture of large and small particles, Spherical solid powders were prepared from Pd(NH3)(4)Cl-2 only when the synthesis condition was carefully controlled within a narrow range, Dense spherical powders were obtained in a wide range of preparation condition by using Pd(NO3)(2). Pd(NO3)(2) hydrolyzes in heated droplets and then dissociates to PdO. At higher temperatures Pd particles formed by the dissociation of PdO sinter and densify successively to form single-crystal-like solid particles. Pd powders prepared from Pd(NO3)(2) and Pd(NH3)(4)Cl-2 oxidize less when they are heated in air.