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 thermodynamic 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 of this system 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 electrochemical potential of electrons at the working electrode, it may be appealing to use the position of the peaks found for this system as a measure for the stability of the different upd ad-phases being formed. In this spirit, Kolb et al. [2][3][4] introduced in the 1970s the concept of underpotential shift, ΔE upd . This quantity was defined as the difference in the potential of the desorption peak for a layer of a metal M adsorbed on a foreign substrate S and the potential of the peak corresponding to the dissolution of the pure metal M. In the present case, ΔE upd ¼ E 3 À E 1 , as marked in the red segment in Fig. 3.1. At the time Kolb et al. developed their modeling of upd, single crystal surfaces were not available for performing electrochemical experiments, so that all the data employed by these authors were restricted to polycrystalline surfaces. On the basis

Underpotential Deposition of Silver on Gold for Surface Catalysis of Plasmon-Enhanced Reduction of 4-Nitrothiophenol

“…Underpotential deposition (UPD) involves electrodeposition of a species at potentials less negative than Nernst potentials based on favorable interactions of the species to be deposited with the substrate. , UPD enables conformal deposition of foreign metals at a monolayer level on metal substrates, and the resulting heterostructures have been studied in comparison with pure metals. − Electrostatic and chemical interactions of surfaces of UPD metals with molecules or ions affect adsorption structures , and interfacial dynamics. , Accordingly, the heterostructures formed by UPD have been studied for electrocatalysis. − However, the role of UPD metals has not been studied in more complicated reactions, such as photocatalysis with surface plasmons. , …”