Metal deposition, electrode surface

Figure 9.3-3 illustrates the superposition of several metal electrode reactions for various metal ion activities. It is apparent that there are three methods for the reduction of metal ions to metal. By applying an external potential more negative than the half-cell potential, metal reduction occurs, resulting in the deposition of surface layers at the metal-metal ion electrode surface by electrolysis. A second method results when a metal ion in solution, is contacted by another metal. M2, whose potential is more negative. This results in the deposition of M, on M2 and is known as contact reduction or cementation. In... 

Much attention, particularly among the Russian workers, has been given to the applicability of the Heyrovsky-Ilkovic versus the Kolthoff-Lingane equation for electrode reactions involving metal deposition. In practice, one plots the electrode potential versus log[(/i — /)//] or log(/ — i) and examines the linearity of the plot and also the value of the slope (theoretically 2.3RTjnF). It is expected that the Heyrovsky-Ilkovic equation should be applicable when alloy formation with the electrode material takes place and the metal formed diffuses away from the surface so that its surface activity is a function of the current density. Alloying and diffusion in the electrode will be functions of the metal deposited, electrode material, temperature, and the rate of deposition (current density) therefore, comparison is difficult or meaningless if several of these variables are varied simultaneously. 

The major types of interferences in ASV procedures are overlapping stripping peaks caused by a similarity in the oxidation potentials (e.g., of the Pb, Tl, Cd, Sn or Bi, Cu, Sb groups), the presence of surface-active organic compounds that adsorb on tlie mercury electrode and inhibit the metal deposition, and the formation of intermetallic compounds (e.g., Cu-Zn) which affects the peak size and position. Knowledge of these interferences can allow prevention through adequate attention to key operations. 

Consider the solid electrolyte cell shown in Figure 5.20. For simplicity we consider only a working (W) and reference (R) electrode deposited on a solid electrolyte, such as YSZ or p"-Al203. The two electrodes are made of the same metal or of two different metals, M and M. The partial pressures of 02 on the two sides of the cell are p02 and po2 Oxygen may chemisorb on the metal surfaces so that the workfunctions w and R(p 02). 

VI. Incomplete calcination of the metal catalyst electrode, resulting to coke deposition on the catalyst-electrode surface. 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

In metal deposition, the primary products form adsorbates on the electrode surface rather than a supersaturated solution. Their excess chemical potential is directly related to polarization and given by nFAE. The total excess surface energy = 2 S,o,. Otherwise, all the results described above remain valid. 

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