Studies of the electrodeposition of platinum metal from a hexachloroplatinic acid bath

The mechanism of platinum deposition onto carbon from a bath containing H₂PtCl₆ in 0.1 M HCl is discussed. Although voltammograms show multiple cathodic waves, it is shown that they all result from the reduction of Pt(IV) to Pt(0) and there is no evidence for Pt(II) as a stable intermediate at any potential; the multiple waves arise because the hexachloroplatinic acid exists in solution as a mixture of kinetically inert Cl⁻/H₂O complexes. The nucleation of the Pt centres on carbon is a very slow process even with a large overpotential and this leads to surfaces with well dispersed hemispherical nuclei. Moreover, each Pt centre appears to grow only to a limiting size before growth ceases.