Phenomenology and Thermodynamics of Underpotential Deposition

Abstract

As can be found from the other chapters of this book, underpotential deposition shows a wide variety of behaviors, which involve the occurrence of several surface phases, formation of submonolayers, monolayers (ML) and eventually the formation of bilayers. Adsorption may be commensurate or incommensurate, where the ML may undergo compression, and metal adatoms may coadsorb with anions to generate new phases. To start the discussion and briefly go into the history of the development of thermodynamics models for upd, we consider a relatively “simple” system, as shown in Fig. 3.1, which corresponds to Ag deposition on Pt(111) [1]. The voltammogram shown there presents three cathodic and three anodic peaks, which correspond to ML formation/desorption(3), bilayer formation/desorption(2) and bulk deposition/oxidation(1) of Ag. The peak potentials of the complementary processes do not coincide, denoting that at the present sweep rate a quasi equilibrium state has still not been reached. For the discussion below, we choose the anodic peaks, that we will denote with E ₁, E ₂ and E ₃ (see Fig. 3.1). At the sight of the features of this voltammogram, and although the abscissa axis gives a measure for the of electrons at the working electrode, it may be appealing to use the position of the peaks found there as a measure for the stability of the different upd ad-phases being formed. In this spirit, Kolb et al. [2–4] introduced in the 1970s the concept of underpotential shift, ΔE ^upd.