Microgravimetric and voltammetric study of Zn underpotential deposition on platinum in alkaline medium

The microgravimetric and voltammetric responses of a polycrystalline Pt electrode in 0.1 mol L−1 NaOH solution in the presence and the absence of 1.6 × 10−4 Zn2+, at 0.1 V s−1 were analyzed. During the positive potential sweep, the water molecules are progressively substituted by OH− ions, prior to PtO formation. The voltammetric charges obtained under the Znads dissolution peaks suggest that 0.7 monolayers are deposited, with each Zn ad-atom occupying one active site and transferring two electrons. The total loss of mass due to the dissolution of the Zn ad-layer was 136.6 ng cm−2 and the mass increase due to PtO formation was found to be only 12 ng cm−2 less than the theoretical ones, 157.5 and 37.4 ng cm−2, respectively, indicating that both processes are overlapped. In this way it is proposed that an adsorbed by-layer of Zn and OH− ions is formed.