Five different compositions of mixed acidic PdCl2/SnCl2 catalysts for electroless plating were investigated mainly by electrochemical and x-ray photoelectron spectroscopy measurements, and also by ultraviolet and visible photo-spectroscopy, transmission electron microscopy, and Auger electron spectroscopy. It is shown that the potential sweep technique using a catalyzed gold electrode in 1M HC1 yields voltammograms which depend upon the catalyst activity, and that this technique is applicable for simple diagnosis of the activity of mixed PdCl2/SnCl2 catalyst systems. It was found by transmission electron microscopy that large colloidal particles are required for high catalytic activities. X-ray photoelectron spectroscopy showed that these colloidal particles consist of a system in which Pd and Sn atoms have an interaction such as in Pd-Sn intermediates or alloys. The ratio of Pd/Sn after acceleration is not constant but depends on the catalyst activity. The accelerator (1M NH4BF4 or 1:1 HC1) not only removes Sn(iV) ions from the substrate but also causes coagulation of small particles adsorbed on the substrate, and these actions result in a drastic increase in the activity of low activity catalysts.
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