Underpotential shift

The difference between the potential of the current peak for the desorption and the bulk deposition potential is known as the underpotential shift simple systems the value of Gibbs energies of adsorption and deposition shift both according to the Nernst equation. Deviations from this behavior may indicate coadsorption of other ions. 

Sanchez and Leiva [396] have considered density functional in the investigations of Cu UPD on Au(lll) and Au(lOO). The analysis of different energetic contributions to the underpotential shift has shown that deposition of Cu on these surfaces should occur at overpotentials. The authors have arrived at the conclusion that other factors, such as anion coadsorption, should be considered in order to explain their experimental observations. 

Fig. 7.145. Underpotential shift AEp, in different S(poly)/Mez+ systems as a function of the difference of electron work functions of polycrystalline S and Me, AO = Os - OMe. (Reprinted from E. Budevski, G. Staikov, and W. J. Lorenz, Electrochemical Phase Formation and Growth, p. 50, copyright 1996 John Wiley Sons. Reproduced by permission of John Wiley Sons, Ltd.)...

Table 2. Underpotential shifts (A p) of metal ions deposited on platinum.

To describe these UPDs quantitatively, Kolb [32] proposed values of AEp for a series of substrate-adatom systems. Table 2 gives estimated values of underpotential shifts (A p) for various metal ions deposited on platinum. Based on these estimated values, A p varies significantly, with the nature of the adatom. 

An attempt was made by Gerischer, Kolb, and Przasnyski [3.122, 3.229, 3.230] to empirically correlate the difference of work functions of 3D S and 3D Me phases, A0= 0s- 0Me, with a characteristic underpotential shift, AEp, corresponding to the most positive Meads desorption peak in an anodic stripping curve (cf. Figs. 3.2-3.6). A linear relation... 

Figure 3.8 Underpotential shift, A p, in different systems as a function of the difference...

Kolb et al. (23) observed that the potential of stripping of the bulk deposit is shifted towards lower po. n ials with respect to the potential of desorption of the UPD layer this difference was defined as the underpotential shift, AEp. The underpotential shift was correlated to the difference in work functions, O, between the substrate, S, and the UPD species, M, and expressed by the following equation ... 

Underpotential deposition of heavy metals on H2 evolving electrodes is a well known problem [133], The existence of a direct correlation between H2 evolution activity and metal work function, makes UPD very likely on high work function electrodes like Pt or Ni. Cathode poisoning for H2 evolution is aggravated by UPD for two reasons. First, deposition potentials of UPD metals are shifted to more anodic values (by definition), and second, UPD favors a monolayer by monolayer growth causing a complete coverage of the cathode [100]. Thus H2 evolution may be poisoned by one monolayer of cadmium for example, the reversible bulk deposition potential of which is cathodic to the H2 evolution potential. 

Surface limited reactions are well known in electrochemistry, and are generally referred to as underpotential deposits (UPD) [83-88], That is, in the deposition of one element on a second, frequently the first element will form an atomic layer at a potential under, or prior to, that needed to deposit the element on itself. One way of looking at UPD is that a surface compound, or alloy, is formed, and the shift in potential results from the free energy of formation of the surface compound. 

Alternatively, it might be that the underpotentials needed to form atomic layers of the elements were decreasing, shifting closer to the formal potentials for deposition of the bulk elements. This scenario may be a factor, but it is frequently observed that the steady state potentials are more negative then the formal potentials for the elements, where bulk deposits of the elements would be expected to form. 

The electrodeposition of alloys at potentials positive of the reversible potential of the less noble species has been observed in several binary alloy systems. This shift in the deposition potential of the less noble species has been attributed to the decrease in free energy accompanying the formation of solid solutions and/or intermetallic compounds [61, 62], Co-deposition of this type is often called underpotential alloy deposition to distinguish it from the classical phenomenon of underpotential deposition (UPD) of monolayers onto metal surfaces [63],... 

The occurrence of underpotential deposition of various metals on different metal substrates is an interesting subject Why is it possible The relation of the underpotential deposition shift potential was plotted... 

Underpotential deposition produces a change in the potential distribution at the interface, affects the organization of the solvent molecules at the interface, and shifts the potential of zero charge in the opposite direction to the effect observed with specific anion adsorption. 

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