This work reviews recent studies of underpotential deposition (UPD) of several metals on Pt and Au substrates performed in the Grupo de Materiais Eletroquímicos e Métodos Eletroanalíticos (IQSC -- USP, São Carlos). The UPD Cu, Cd and Pb on Pt were analysed in terms of their influence in the oxygen evolution reaction. Partial blockage of surface active sites, promoted by Pb ad-atoms, resulted in a change from water to hydrogen peroxide as the final product. The Ag UPD on Pt and Au substrates was also discussed in this work. A detailed model of charge calculation for Ag monolayer was developed and confirmed by the rotating ring-disk data. The partial charge transfer in UPD studies was analysed in the Cd/Pt and Cd/Au systems and a value of 0.5 was found for the adsorption electrovalence of Cd ad-ions. The Sn/Pt UPD systems were studied from the point of view of the valences of metallic ions in solution. The deposition from Sn(IV) generates a full monolayer with a maximum occupation of approximately 40% of the surface active sites (340 µC cm-2) plus 105 µC cm-2 of Hads (half monolayer). Changing the metallic ion for Sn(II), it was possible to deposit a full monolayer (210 µC cm-2) without any detectable Hads. Finally, the effect of anions was discussed in the Zn/Pt and Zn/Au systems. Here...
The underpotential deposition of Zn on polycrystalline Pt was studied in three different acid solutions for different Zn2+ concentrations. The voltammetric charges and the inhibition promoted by the maximum coverage of Zn ads in the hydrogen adsorption and evolution reactions were used to postulate adsorption models. It was shown that the co-adsorption of anions HSO4-, ClO4- and F-, either on the substrate or on the ad-atoms, exerts a marked influence in the ad-layer nature. On Pt, the maximum values found for the redissolution charge were around 210 muC cm-2, except for 10-3 mol L-1 Zn2+ in fluoride medium where a value of 350 muC cm-2 was obtained. A negligible inhibition of the hydrogen evolution reaction was recorded in sulfuric and perchloric acid solutions while a strong inhibition was found for fluoride medium. These results were related to the interaction of the large oxy-anions either with the substrate or the ad-atoms. Collection experiments were performed with the rotating ring-disk electrode system (RRDE) and the results confirmed the large charge value obtained in the HF electrolyte. The experiments performed with the RDDE also demonstrated that the voltammetric peaks analyzed here are associated with UPD Zn.
The theory of statistical-mechanical lattice-gas modeling of adsorption is
reviewed and shown to be applicable to a range of electrochemical problems
dominated by effective, lateral adsorbate--adsorbate interactions. A general
strategy for applying the method to specific systems is outlined, which
includes microscopic model formulation, calculation of zero-temperature phase
diagrams, numerical calculation of thermodynamical and structural quantities at
nonzero temperatures, and estimation of effective, lateral interaction energies
that cannot be obtained by first-principles methods. Phenomena that are
discussed include poisoning and enhanced adsorption, and illustrative
applications to specific systems are reviewed. Particular problems considered
are: the poisoning by sulfur of hydrogen adsorption on platinum (111), the
electrochemical adsorption of naphthalene on polycrystalline copper and of urea
on single-crystal platinum (100), and the underpotential deposition of copper
on single-crystal gold (111).; Comment: 24 pages LaTex, 8 uuencoded Postscript figs in separate file