In three-electrode cells without separation of the working and counter electrodes, the application of a sufficiently negative potential at the working electrode is accompanied by the development of a rather positive potential at the counter-electrode. If a platinum-flag counter-electrode is employed in acidic aqueous electrolyte, it undergoes anodic corrosion, to produce dissolved metal, perhaps as a Pt(IV) oxospecies. It serves in turn, as a source of platinum for cathodic electrodeposition at the negative working electrode. Scanning electron micrographs showed that controlled potential electrolysis at -1.2 V vs. SCE for 6 h (in stirred 1 M H2SO4) consistently produced dispersed platinum particles on a carbon working electrode. They were approximately spherical, had diameters of 20-40 nm, and were separated by distances of less than 30-50 nm. The platinum was not deposited when a carbon counter-electrode was employed. Simultaneous codeposition of platinum and polyoxometallates is feasible. Auger electron spectrometry, energy-dispersive X-ray fluorescence and secondary ion mass spectrometry were used to verify the presence of platinum. Shorter electrolysis times (e.g. 30-60 min) produced very low platinum loadings, which were difficult to determine with common surface analytical techniques. The As(III) electrocatalytic oxidation provided a good probe for the detection of traces of platinum, and permitted estimation of the platinum coverage on a carbon substrate.
