Adsorption of chloride ions

Macdonald et al.25 28 maintained that the adsorption of chloride ions enhances the formation of cation vacancies of metal ions and their transfer... 

MacDonald on the adsorption of chloride ions in passivation, 237 of CO on electrochemically facetted platinum, 135 of diols on mercury, 188 of neutral compounds on electrodes, 185 of perchlorate ions, copper and, 94 specific adsorption, anodic dissolution and, 256... 

It is interesting to compare these results with the electrophoretic measurements made under identical electrolyte concentrations. Figure 8 shows that the variation of electrophoretic mobility with sodium chloride concentration is different for the bare and the PVA-covered particles. For the bare particles, the mobility remains constant up to a certain salt concentration, then increases to a maximum and decreases sharply, finally approaching zero. The maximum in electrophoretic mobility-electrolyte concentration curve with bare particles has been explained earlier (21) by postulating the adsorption of chloride ions on hydrophobic polystyrene particles. In contrast, for the PVA-covered particles, the mobility decreases with increasing electrolyte concentration until it approaches zero at high salt concentration. 

For the adsorption of chloride ions on the interface of metallic electrode in aqueous potassium chloride solution, the Gibbs adsorption equation is written as in Eqn. 5-18 ... 

The shift of the potential of zero charge toward the negative direction induced by the contact adsorption of chloride ions has been found not only with liquid mercury electrodes but also with solid metal electrodes such as gold [Jiang-Seo-Sato, 1990]. 

Fig. 5-30. Potential profile across a compact layer estimated by calculations at various electrode potentials for a mercury electrode in a 03 M sodium chloride solution electrode potential changes fivm No. 1 (a cathodic potential) to No. 6 (an anodic potential), and contact adsorption of chloride ions takes place at anodic potentials. E = electrode potential = zero charge potential x = distance fix>m the interface. [From (3raham, 1947.]...

Also, Cuesta and Kolb [52] have employed STM to investigate adsorption of chloride ions on Au(lOO) surfaces. 

Fig. 6.89. Plot of the Gibbs energy of adsorption vs. electrode charge density for the adsorption of chloride ions at an Au(111) electrode by the chronocou-lometry technique. Adapted from Z. Shi and J. Upkowski, J. Electroanal. Chem. 403 225, copyright 1996, Fig. 7b, with permission of Elsevier Science.)...

Figure P6.4 shows the logarithmic dependence of the adsorption of chloride ions, log(T - r/r, on time, /(min). The adsorbing metal is aluminum, the concentration of chloride ions in solution is 1(T3 mol dm-3 at pH 12, and the potential in the hydrogen scale is -0.9 V. On the assumption that the adsorption is diffusion controlled, determine the standard free energy of adsorption of chloride ions on the surface concerned. 

Multicomponent vC—C bands were also observed in the SER spectra of phenylacetylene adsorbed on copper, silver (114), and gold (83) electrodes. The principal components characterizing the species on Ag and Au are at ca. 2017 and 1985 cm"1 (An —93 and —125 cm"1), respectively. The higher wavenumber band is displaced by the adsorption of chloride ions when the potential was changed from —0.6 to 0.0 V. Four nC—C components were observed for the species on the copper electrode, centered... 

It Is caused by adsorption of chloride Ions on the platinum and the attachment of the quaternary Ion by Ion pairing. One can cause alternating Increases and decreases In the rate of electrooxl-datlon and catalytic esterification by the presence of monolayers, bllayers, etc. 

The same general approach can be used to describe adsorption of chloride ion, which may be written... 

Fig. 39), a systematic change of the C(E) curves was observed for the first time the more negative capacity peak increases with the number of atomic steps at the gold surface in parallel, the middle peak decreases and the more positive peak shifts negatively. The most negative capacity peak seems connected to adsorption of chloride ion on monoatomic steps at the surface and the middle peak to adsorption on the terraces (see TLK model. Section III.4). 

For aluminum, the outer surface of the oxide layer in humid environments is considered to be a mixture of aluminum oxide and aluminum hydroxide. After the adsorption of chloride ions, an ion exchange can occur leading to the substitution of hydroxyl ions by chloride ions [179, 180]. After the chemical attack of the oxide, aluminum is electrochemicaUy dissolved. The chloride ions are regenerated after the dissolution of the transitory hydrox-ychloride compounds. Thus, a relatively small amount of chloride ions can result in a progressive attack of the protective layer. Within the head of the filiform filament, the anodic dissolution of aluminum leads to a local acidification of the anolyte due to the hydration of aluminum ions. It has been observed that a secondary cathodic reaction, the reduction of hydrogen ions, can occur. Hydrogen evolution has been observed within the head [166]. 

C-S-H gel is participating in chemisorption and adsorption of chloride ions too. According to Beaudoin [206] and Ramachandran [207], the three types of chloride ions can be distinguish the free ions easily extracted in an alcohol, the chemisorbed ones on the gel surface, and in the interlayers area—they cannot be extracted by alcohol and, finally—the strongly bound chlorides, presumably occurring as a solid solution in C-S-H. The latter cannot be leached by water. The C-S-H ability of chloride ions binding depends on the H/S and C/S ratio and increases with them [207]. 

The rest of this chapter is organized as follows. We start with the mobility of metal surfaces, which we investigated by the KMC technique. This requires the rates of all possible processes as input, which we obtain by a combination of DFT and a semiempirical potential. The first application is to a Ag(l(X)) electrode, which is quite mobile even at room temperature. Interestingly, the mobility increases when the electrode potential is raised, which we explain by field-dipole interactions. Explicitly, we consider island shapes and dynamics, step fluctuations, and Ostwald ripening for this surface. In contrast to silver, a clean Au(lOO) surface is not mobile at ambient temperatures however, the adsorption of chloride ions enhances the mobility. We explain the underlying mechanism and present results for Ostwald ripening. 

FIGURE 7.S Side view models for the adsorption and insertion of chloride ions on the hydroxylated NiO(l 11) surface showing the Cl-free hydroxylated surface, adsorption of chloride ions by substitution of hydroxide ions, and subsurface insertion by exchange between oxygen and chloride ions. A (2x2) supercell with a Cl coverage of 25% is shown. Reprinted from Bouzoubaa et al. [47], with permission from Elsevier. 

The adsorption of chloride ion on a Pt electrode depends on the ion concentration, Pt morphology, temperature, and electrode potential. The coverage of CP on Pt decreases when both temperature [147,148] and concentration [147] increase, and when electrode potential decreases [147,148]. At 20°C, the coverage of CP on polycrystalline Pt was found to be ca. 0.5 monolayers at 0.7 V and ca. 0.45 monolayers at 0.6 V, when CP concentration in the electrolyte was 10 M [147]. Stamenkovic et al. [149] found that Pt(lll) is more active than Pt(lOO) for both the ORR and the HOR when CP is present in the electrolyte. They attributed the differences in catalytic activities to a much stronger interaction of Cl j with the (100) sites than with the (111) sites, and they proposed that Cl d strongly adsorbed onto Pt(lOO) can simultaneously suppress both the adsorption of O2 and H2 molecules and the formation of pairs of Pt sites needed to break the 0-0 and H-H bonds. 

Molina, RV. and D. Posadas. 1988. Temperature dependence of the adsorption of chloride ion on platinum electrodes. Electrochim. Acta 33 661-665. 

CMoride ions in acpieous solutions represent a specific corrosive agent for pitting corrosion on passive materials. In seawater as well, the chloride ions are responsible for pitting corrosion. The starting point for pitting corrosion is a locally increased adsorption of chloride ions to damaged or weak points in the passivation layer. 

The reason for this is that oxygen access to the crevice is hindered and, the adsorption of chloride ions in the crevice and hindrance of liquid exchange. The result is a raised concentration of chloride ions in the electrolyte solution in the crevice compared to the rest of the electrolyte solution. The corrosion potential is exceeded in the crevice once the concentration of these ions is high enough. 

However, it is important to note that the Ret values are not directly related to the susceptibility to corrosion of the different alloys and composites. They are related to the rate of charge transfer reactions that give rise to the formation of a passive layer on the surface of the samples (the impedance measurements are at open circuit potential only). The protective characteristics of these passive films depend on the preparation conditions of the alloys, the distribution of elements in the alloy and the presence on the surface of active sites for adsorption of chloride ion. 

From an adsorption viewpoint, adsorption competition always prevails on the alloy surface between chloride ions and dissolved oxygen atoms. Notably, no oxide layer forms once chloride ions adsorb on the alloy surface, in which the metal ions readily dissolve. Therefore, the adsorption of chloride ions increases the reacting current density (as indicated by a comparison of Figs. 1 and 9), subsequently increasing the rate of metal dissolution. 

Horanyi G, Inzelt G (1978) Periodical changes in the adsorption of chloride ions accompanying the potential oscillations produced in the course of galvanostatic electrooxidation. J Electroanal Chem 87 423-427... 

As the concentration of chloride ions is increased, the passive film undergoes active dissolution due to adsorption of chloride ions, and... 

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