Anions adsorption potentials

The influence of the surface structure on the anion adsorption. Potential dependence of the adsorption of sulfate ions at various crystal faces of copper as studied by radiotracer technique [101] is shown in Fig. 1. 

From the first reaction, assuming that the anion adsorption process is rate determining, and neglecting the potential difference as shown in Eq. (75a), the average dissolution current density is written as a function of the surface concentration of the aggressive ions, (CJx, y, 0, /)), i.e.,... 

Two types of EDL are distinguished superficial and interfacial. Superficial EDLs are located wholly within the surface layer of a single phase (e.g., an EDL caused by a nonuniform distribution of electrons in the metal, an EDL caused by orientation of the bipolar solvent molecules in the electrolyte solution, an EDL caused by specific adsorption of ions). Tfie potential drops developing in tfiese cases (the potential inside the phase relative to a point just outside) is called the surface potential of the given phase k. Interfacial EDLs have their two parts in dilferent phases the inner layer with the charge density in the metal (because of an excess or deficit of electrons in the surface layer), and the outer layer of counterions with the charge density = -Qs m in the solution (an excess of cations or anions) the potential drop caused by this double layer is called the interfacial potential... 

In situation (b) the anion adsorption is compensated by the negative overall potential of the dme. In situation (c), with a further increase in the negative potential, an electric double layer will now be formed with cations from the solution, so that the apparent <7Hg is lowered again. Hence crHg as a function of the negative dme potential, yielding the so-called electrocapillary curve, shows a maximum at about -0.52 V (see Fig. 3.18). 

Figure 24. Schematic representation of the effect of anion adsorption on the potential profile at the O/S interface, showing the potential profile before (dashed line) and after adsorption (solid line).

These experimental results demonstrate that the specific adsorption of anions leads to a significant decrease in junction stability, as illustrated in particular in the potential regions of disordered anion adsorption, as well as in the fluctuations in the plateau currents at potentials of the 2D-ordered anion adlayers. The instability of the conductance plateaus were statistically analyzed, by determining the standard... 

A closer inspection of the predominant peak in the conductance histogram at G0 (=77.5 pS) reveals that its position and magnitude depend on the applied electrode potential, as well as on the strength of anion adsorption (Fig. 11). The peak position shifts in the presence of weakly specifically adsorbed ions (e.g., C104-, SO)2 ) to value smaller than G0. 

Two other small peaks appear at E=0.45 and 0.7 V on the Pt(lll) surface. They are shifted to more negative potentials with increasing H SO concentration, suggesting their link with anion adsorption. A peak at 0.35 V for the Pt(100) surface, where one would expect the so-called double-layer region for Pt, is also intriguing. It remains to be seen whether or not it is due to sulphate adsorption on the (111)-oriented imperfections. 

For simplicity, we will consider the case in which surface charge and potential are positive, and that only anions adsorb. Furthermore, the potential drop in the Gouy-Chapman layer will be assumed to be small enough that its charge/potential relation can be linearized. The V o/oo/pH relationship can then be derived parametrically, with the charge in the Gouy-Chapman layer cr4 as the parameter. The potential at the plane of anion adsorption can then be calculated and substituted in Equation 28 to give ... 

Fig. 6-29. Change in potential profile across a compact layer due to anionic contact adsorption at constant potential on a metal electrode solid line = without contact anion adsorption broken line = with contact anion adsorption 4m - inner potential of metal electrode, 4s = inner potential of solution 4ihp = inner potential at IHP.

Such anion adsorption can be prevented by chemisorbing a mono-layer of a strongly adherent thiol molecule to the Au surfaces [97,98]. 1-Propanethiol (PT) was used here because the gold nanotubules can still be wetted with water after chemisorption of the PT monolayer [97].t The Em versus applied potential curves for an untreated and PT-treated gold nanotubule membrane, with KBr solutions present on either side of the membrane, are shown in Fig. 13. The untreated membrane shows only cation permselectivity, but the permselectivity of the PT-treated membrane can be switched, exactly as was the case with the nonadsorbing electrolyte (Fig. 12). 

In addition to anion adsorption, there exists the possibility of adsorption of cations at negative potentials along with coadsorption phenomena. For example, mixed layers of alkali cations with iodine on Au(llO) [291] or cyanide on Pt(lll) [292] have been reported. In fact, coadsorption has proven to be quite common among numerous underpotential metal deposition reactions as described below. 

Practical considerations and implementation. Most investigations involve the use of distilled/deionised water with KNO3 as the nitrate ion source thereby avoiding any potential impact of water hardness and dissolved salts on the catalytic removal of nitrates. It has been pointed out that in the presence of anions such as S04 and bicarbonates, which may be present in tap-water at concentrations of above 90 ppm, reduced nitrate reduction rates are to be expected as a result of competitive anion adsorption. Pintar and co-workers have indicated that nitrate removal rates are reduced when using drinking water as opposed to distilled water. Chloride ion is known to reduce the rate of nitrate removal while the choice of cation as counter ion influences the rate in the order, < Na < Ca < Mg + <... 

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