Underpotential deposition, and

Kokkinidis, K., Underpotential deposition and electrocatalysis, J. Electroanal. Chem., 201, 217 (1986). 

Silver deposition on polycrystalline Pt electrodes at potentials positive to the equilibrium potential gave 2.5 atomic layers. Two binding types of Ag layers were found by anodic stripping the first Ag layer deposited on Pt, which seems to form an alloy of Ag-Pt, on which the second Ag deposition takes place in the Ag underpotential deposition region. STM images from the underpotential to the overpotential deposition region were observed for Cu underpotential deposition on Au(l 11) in sulfuric acid solution, where Cu underpotential deposition does not affect overpotential deposition, although the latter always takes place on the surface with Cu underpotential deposition and a metal. ... 

Cadmium Underpotential Deposition and Electrodeposition on Solid Electrodes... 

Willis, underpotential deposition. 1313 Wojtowicz, 1381 Work function of the metal, 887 and chemical potential 835 definition, 835 in electrochemistry, 835 and surface potential, 835 underpotential deposition and, 1316 Working electrode, potential, 1061 Wright, 1495... 

Unlike anions that specifically adsorb at electrodes, cations normally do not lose their solvation shell due to their smaller size and are electrostatically adsorbed at electrodes at potentials negative to the pzc. However, depending on the affinity with the foreign substrate, cations can be reduced to a lower oxidation state or even discharged completely to the corresponding metal atom at the sub-monolayer or monolayer level at potentials positive to the equilibrium Nernst potential for bulk deposition. This deposition of metal atoms on foreign metal electrodes at potential positive to that predicted by the Nernst equation for bulk deposition has been called underpotential deposition and has been extensively investigated in recent years. Detailed discussion of the... 

More than at mercury, it makes a difference whether the electrode is inert or not. In the first case, the electrode reaction is of the type Fe3+/ Fe2+ etc. and the modelling of processes is the same as with mercury. However, if the electrode reaction is of the type Zn2+/Zn, e.g. at a gold electrode, at least the electrode surface will be modified by the deposited zinc, Frequently, it is observed that the first monolayer of the foreign metal is deposited at a potential substantially positive to its standard potential. This phenomenon is named underpotential deposition and bears some resemblance to an electrode reaction that involves adsorption of the reacting species (see Sect. 6). 

Electrodes may also be modified by deposition of metal adatoms at potentials several hundred millivolts positive to the reversible potential for metal deposition. A submonolayer of adatoms may lower overpotentials for electron transfer processes or improve the selectivity of an electrocatalytic surface. Underpotential deposition and electrocatalysis have been discussed in a review [182]. 

CHANGES IN THE SURFACE STRUCTURE OF PLATINUM-TYPE METAL INDUCED BY METAL UNDERPOTENTIAL DEPOSITION AND ANION ADSORBATES... 

The deposition begins at potentials more positive than values where deposition of R occurs on bulk R. Consider, for example, the deposition of Ag on a 1-cm Pt electrode from a 0.01-L solution containing 10 M Ag". Let A = 1.6 X 10 cm and yo = yR. The potential for deposition of one-half of the silver (which forms about 0.05 monolayer) is = 0.35 V, compared to E = 0.43 V required for the same amount of deposition on a silver electrode. Deposition at potentials before that predicted by the Nernst equation with R = 1 is called underpotential deposition. The situation is much more complicated than the above treatment suggests, since the deposition potential depends on the nature of the substrate (material and pretreatment) and on adsorption of O. Also, the treatment assumes that formation of a second layer does not start until the first is complete. However, this is frequently not the case atoms of metal will often aggregate, rather than deposit on a foreign surface, and dendrites will form. Reviews on the nature of underpotential deposition and the deposition of solids in general are available (6-10). 

Te and Cu monolayers on gold, as well as Ag and Bi monolayers on platinum were obtained by cathodic underpotential deposition and investigated in situ by the potentiodynamic electrochemical impedance spectroseopy (PDEIS). PDEIS gives the graphical representation of the real and imaginary interfacial impedance dependencies on ac frequency and electrode potential in real-time in the potential scan. The built-in analyzer of the virtual spectrometer decomposes the total electrochemical response into the responses of the constituents of the equivalent electric circuits (EEC). Dependencies of EEC parameters on potential, especially the variation of capacitance and pseudocapacitance of the double layer, appeared to be very sensitive indicators of the interfacial dynamics. 

A typical set of results as observed during the underpotential deposition and dissolution of lead onto a silver electrode is shown in Fig. 5.146. The increased absorptivity of the silver electrode covered with lead results in an increased photoacoustic signal. As the electrode potential is scanned only in the underpotential deposition region, the deposited amount of lead should correspond to a monolayer of atoms or a fraction thereof. Measurements of the photoacoustic signal as a function of time at various electrode potentials illustrate this point. At electrode potentials in the underpotential region the signal rises rapidly to a constant value. 

Oscillatory kinetics with a surface reaction had been observed as early as in 1828 by Fechner [4] with an electrochemical system. As an example for these t) es of reactions. Fig. 7.1 shows the variation of the potential at a Pt electrode with time for the electrochemical oxidation of H2 in the presence of copper ions [5]. While the potential at low-current density j is constant (a), at higher j kinetic oscillations occur because of periodic poisoning and activation transitions of the electrode by underpotential deposition and dissolution of a passivating Cu overlayer. With further increase of , at first period doubling and then transition to an irregular situation (chaos) take place. 

Kokkinidis G (1986) Underpotential deposition and electrocatalysis. J Electroanal Chem Interfacial Electrochem 201(2) 217-236... 

Sanchez, P.L. and Elliott, J.M. (2005) Underpotential deposition and anodic stripping voltammetry at mesoporous microelectrodes. The Analyst, 130, 715-720. 

Combination of Underpotential Deposition and the Galvanic Replacement Reaction... 

Similar measurements for the cases of supported nanosized catalysts require preparation of the electrode in a specific way. Usually, catalyst is dispersed in suitably chosen media and desired amount is transfer to solid electrode serving as electrical contact [11]. This issue will be elaborated in more details later on. When stable thin catalyst layer is prepared cyclic voltammetry may be used to investigate surface electrochemical processes (Figure 4). In contrast to single-crystal and polycrystalline surface additional factor arises - namely, these processes depend also on the particle size. For example, in the case of supported Pt nanocatalysts it was observed that both hydrogen underpotential deposition and adsorption of oxygen species are clearly dependent on the particle size [12]. In specific, it was observed that smaller particles are more oxophilic and that surface oxidation is more irreversible for smaller particles. 

Herzog G, Arrigan DWM (2005) Underpotential deposition and stripping of lead at disorganized monolayer-modified gold electrodes. Electroanalysis 17 1816-1821... 

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