Mechanism, of underpotential deposition

One of the most important processes in metallurgy is copper deposition. Galvanoplastic conditions imply a copper-acid bath over a pre-multilayer of copper from a copper-alkaline cyanide bath. The study of copper underpotential deposition on platinum is of special interest in electrochemistry in spite of its catalytic properties being rather poor with respect to fuel cell applications [101-104]. The mechanism of copper deposition and the presence of copper(I) ions as an intermediate have been... 

The major areas of application of reflectance spectroscopy have been the elucidation of reaction mechanisms, double layer studies, investigations of underpotential deposition (UPD), and studies of the electroreflectance effect (ER). This range is too large for an in depth discussion to be given here. Instead, two examples of the type of information that can be obtained will be described (a third system, hydrogen adsorption on platinum, has been discussed in Chapter 7). Those readers interested in more details are referred to a recent review [1], and the literature cited therein. 

When the potential step method is employed to elucidate the mechanism of the underpotential deposition process, the current j against time t is readily found to be in the form of... 

One of the subjects that is still quite intensively developed (using electrochemical methods frequently combined with nonelectrochemical techniques) concerns reduction of Hg compounds at various surfaces (e.g. Pt or Au), with the emphasis laid on underpotential deposition (UPD) of mercury. Deposition of mercury on other metals is generally important for better understanding of the mechanism of the formation of amalgams. Moreover, underpotential Hg deposition characteristics constitute a significant source of information on Hg-metal interactions. In turn, mercury film electrodes obtained by such deposition have a significant appKcation in electrochemical analysis ofvarious species. 

Unlikely ion implantation, adatoms can be deposited in situ on the cathode surface at underpotential. The mechanism of adsorption, its potential dependence and the state of the adsorbed atoms has been widely discussed in the literature [66, 193, 194,... 

Fig. 14. Activity of Pt for hydrogen evolution in 1 M HC104 as a function of the coverage with underpotential deposited Pb. A change in mechanism with Pb coverage is indicated. After ref. 303, by permission of Pergamon Press.

The importance of electronic interaction as the basic mechanism leading to synergetic effects is also suggested by results showing that the simultaneous underpotential deposition of Cd and Tl on Ag(lll) results in an enhancement of the hydrogen evolution rate while inhibitory effects are shown by the two kinds of adatoms separately [317]. 

Oxide electrodes have been observed to be almost immune from poisoning effects due to traces of metallic impurities in solution [99]. This is undoubtedly due primarily to the extended surface area. It can be anticipated that the calcination temperature must have a sizable effect. But in addition, a different mechanism of electrodeposition must be operative. Chemisorption on wet oxides is usually weak because metal cations are covered by OH groups [479]. As a consequence, underpotential deposition of metals is not observed on Ru02, although metal electrodeposition does takes place. However, electrodeposited metals give rise to clusters or islands and not to a monomolecular layer like on Pt. Therefore, the oxide active surface remains largely uncovered even if metallic impurities are deposited [168]. Thus, the weak tendency of oxides to adsorb ions, and its dependence on the pH of the solution is linked to their favorable behavior observed as cathodes in the presence of metallic impurities. 

Mechanisms of electrochemical and photoelectrochemical deposition of metal selenide clusters (Me = Pb, Cd, Zn, Bi, In) onto the surface as well as into the selenium films have been studied. These clusters are formed as a result of underpotential and overpotential deposition of the metals onto Se. Photoinduced underpotential deposition of Bi onto Se was used to cover selenium colloidal particles with BiiSes clusters. The PbSe and Bi2Sc3 clusters modify the Se surface and form electronic surface states in the Se bandgap, thus promoting electron exchange processes between the valence band and redox species in solution and the rise of the subbandgap photocurrent. 

The electrochemical monolayer deposition in cathodic reactions above the Nemst potential, the underpotential deposition (UPD), is one of the most promising instruments for assembling of nanostructures from atoms and atomic monolayers [1], Though the UPD phenomenon has been known for decades [2], its technological application is still hindered, because of insufficient knowledge of the dynamics and mechanisms of reactions in the monolayers. 

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