Electrosorption zero charge, potential

The particular enhancement of a radical dimerisation reaction, Eqs. (45a) and (45b), is observed on hydrophobic metals like Hg, Pb, and Sn at potentials close to their respective point of zero charge where electrosorption is strongest and diminishes at more negative and positive potential. If the reduction potential of the oxo-compound, which in accordance with the equilibrium reaction (45b) is pH dependent (60 mV per pH unit), is shifted by more than 100 mV away from the point of zero charge, then the ketyl radicals are no longer adsorbed strongly enough and they are displaced from the... 

The electrosorption valency of several neutral aliphatic molecules on mercury was examined by Koppitz at al.54 As usual, the experimental l values were referred to the potential of zero charge in the absence of specific adsorption, Ez, and to low surface coverages. With the remarkable exception of thiourea, all these molecules do not undergo pet (A = 0). Since they are also neutral (z = 0), their electrosorption valency is exclusively determined by the two dipole terms ... 

Electrochemistry shares many concepts with surface science, and for the last two decades there has been an exchange of methods and ideas between these two neighboring disciplines. However, the electrosorption valency has no equivalent in surface science, since experiments at the solid/gas or solid/vacuum interface cannot be performed at constant potential. However, for low coverages, and near the potential of zero charge, the electrosorption valency can be related to the dipole moment of the adsorbate, which can be measured both in surface science and, though with greater difficulty, also in electrochemistry. In the following, we point out the relation between these two quantities. 

Table 1 summarizes a few values of the dipole moment fx and of the electrosorption valency l of halide and alkali metal ions adsorbed on mercury. The experimental values of l are relative to low coverages near the potential of zero charge and are taken from Schultze and Koppitz,50 while the corresponding // values were calculated from Eq. (75). The theoretical values in the last column are from a hard sphere electrolyte model. Further data can be found in the article by Schmickler49 Note that, in the electrochemical environment, the dipole moments are much smaller than in vacuo, where they can reach values of the order of 7 D for the alkali metal ions. No doubt, this difference is caused by the screening of the adsorbate dipole by the solvent molecules. 

The electrosorption of pyridine on mercury is shown in Fig. 2J. One should note that the dependence of 0 on is roughly bell-shaped for most compounds, with the maximum of adsorption occurring at a potential that is slightly negative with respect to the potential of zero charge. 

The experimental methods for determination of the potential of zero charge have been reviewed by Swinkels, Conway, and by Argade and Gileadi. Methods of measurement of electrosorption of organic compounds on solid electrodes have been discussed recently by Gileadi. ... 

The electrosorption ofhisulfate on polyerystalline copper [145, 146] shows that it binds much more strongly than to gold. We may assume that the electrosorption potential of the bisulfate varies linearly with the amount of copper in the surface. Thus, guided by the work ofTrasatti on the influence of the anion on the potential of zero charge [147], we take... 

The calculations were performed near the potential of zero charge, when the outer potential of the metal and solution are equal. The charge transfer coefficients were calculated for energy widths of A = 0.5eV and A = — leV, which is a reasonable range for weak adsorption. The degrees of adatom adsorption were set at 0 = 1/3 and o = 2/3. o = 0, which corresponds to the vacuum is also shown. Experimental values of the electrosorption valenciesare shown in the last column of the table. 

The electrosorption valency expresses the charge flow during the electrosorption process at a constant potential. It takes values ranging between zero and the ionic charge z. Values very close to z show that the adsorbed species are almost completely discharged, while values of i lower thanz indicate adsorption with partial charge transfer. The adsorbed species are recognized as ad-atoms when... 

Kim et al. ° has recently studied the electrosorption of Co(II) ions from aqueous solutions on a cellulose-based activated carbon fiber KF-1500 after modifying the ACF surface by dipping in IM HNO3 and IM NaOH solutions at room temperature. The zeta potential changes of different ACF samples with pH of the solution (Figure 6.33) showed that the zero point charge (pH pc) of the untreated sample was... 

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