Adsorbed anions

There is, or may be, an iimer layer of specifically adsorbed anions on the surface these anions have displaced one or more solvent molecules and have lost part of their iimer solvation sheath. An imaginary plane can be drawn tlirough the centres of these anions to fomi the inner Helmholtz plane (IHP). 

Taguchi S and Aramata A 1995 Voltammetric study of underpotentiai deposition (upd) of Zn " ions on Pt(111) effect of adsorbed anion J. Electroanal. Chem. 396 131 -7... 

Fig. 1. The structure of the electrical double layer where Q represents the solvent CD, specifically adsorbed anions 0, anions and (D, cations. The inner Helmholtz plane (IHP) is the center of specifically adsorbed ions. The outer Helmholtz plane (OHP) is the closest point of approach for solvated cations or molecules. O, the corresponding electric potential across the double layer, is also shown.

Early studies on oxide films stripped from iron showed the presence of chromium after inhibition in chromate solutionand of crystals of ferric phosphate after inhibition in phosphate solutions. More recently, radio-tracer studies using labelled anions have provided more detailed information on the uptake of anions. These measurements of irreversible uptake have shown that some inhibitive anions, e.g. chromateand phosphate are taken up to a considerable extent on the oxide film. However, other equally effective inhibitive anions, e.g. benzoate" pertechnetate and azelate , are taken up to a comparatively small extent. Anions may be adsorbed on the oxide surface by interactions similar to those described above in connection with adsorption on oxide-free metal surfaces. On the oxide surface there is the additional possibility that the adsorbed anions may undergo a process of ion exchange whereby... 

Electrode Electrolyte Adsorbed anion Elg/Vvs. SHE vs. SHE Adsorption Atomic state (terrace) density/cm-2 References... 

Interestingly the electrochemical promotional effect was found only in the case of perchloric acid supporting electrolyte. No promotion effect was found in presence of strongly adsorbed anions (HS04 Cl ). 

Solid-surface fluorescence and phosphorescence quantum yield values were obtained from +23° to -180°C for the anion of p-aminobenzoic acid adsorbed on sodium acetate (11). Fhosphorescence lifetime values were also obtained for the adsorbed anion from +23° to -196°C. Table 1 gives the fluorescence and phosphorescence quantum yield values acquired. The fluorescence quantum yield values remained practically constant as a function of temperature. However, the phosphorescence quantum yield values changed substantially with temperature. The phosphorescence lifetime experiments indicated two decaying components. Each component showed a gradual increase in phosphorescence lifetime with cooler temperatures, but then the increase appeared to level off at the coldest temperatures. 

The enhanced adsorption of anions and other substances that occurs at increasingly positive potentials causes a gradual displacement of water (or other solvent) molecules from the electrolyte layer next to the electrode. This leads to a markedly slower increase in the rate of oxygen evolution from water molecules and facilitates a further change of potential in the positive direction. As a result, conditions arise that are favorable for reactions involving the adsorbed species themselves (Fig. 15.9). In particular, adsorbed anions are discharged forming adsorbed radicals ... 

Further STM and SXS smdies [Wu et al., 1998] concerning this phenomenon indicated that the presence of specifically and nonspecifically adsorbing anions as well as organic molecules (e.g., pyridine, bipyridine, and uracil) may also lift the reconstructed surface by exhibiting a structural transition, and it has been extensively studied and reviewed in [Kolb, 1996]. 

Increasing the temperature seems to facilitate the oxidation of CO, since lower amounts are accumulated on the surface [Kardash and Korzeniewski, 2000]. The presence of strongly adsorbed anions in the elecfrolytic solution affects the distribution of products. It has been shown that adsorbed (bi)sulfate diminishes the production of CO,... 

OHads formation has a clear voltammetric signature on a number of surfaces, including the (lll)-oriented surfaces of platinum group metals, Pt(lll) in alkaline and acid electrolytes of non-adsorbing anions [Markovic and Ross, 2002], and Au(lll), Au(lOO), and Ag(lll) in neutral and alkaline electrolytes [Savinova et al., 2002]. On these surfaces, the reaction has a reversible character. Anderson and co-workers calculated the reversible potential of Reaction (9.1) on Pt to be 0.62 V with respect to a reversible hydrogen electrode (RHE) [Anderson, 2002]. The Pt(lll)-OH bond energy has been estimated to be about 1.4 eV in an alkaline electrolyte [Markovic and Ross, 2002]. 

Corrigan and Weaver, 1998 Kunimatsu et al., 1985a, b Korzeniewski et al., 1986 Tian et al., 1997]. At the onset of chemisorbed CO oxidation, the CO frequency evidences the anomalous Stark tuning behavior noted by Ross and Markovic and coworkers (Fig. 12.8), who associated it with compression of CO islands by adsorbed anions [Stamenkovic et al., 2005]. 

The introduction of the concept of the micropotential permits derivation of various expressions for the potential difference produced by the adsorbed anions, i.e. for the potential difference between the electrode and the solution during specific adsorption of ions. It has been found that, with small coverage of the surface by adsorbed species, the micropotential depends almost linearly on the distance from the surface. The distance between the inner and outer Helmholtz planes is denoted as xx 2 and the distance between the surface of the metal and the outer Helmholtz plane as jc2. The micropotential, i.e. the potential difference between the inner and... 

Fig. 4.6 The adsorbed anion A induces a positive charge excess at the outer Helmholtz plane, which can be represented as its image A. The plane of imaging is identical with the outer Helmholtz plane. (According to B. B. Damaskin, O. A. Petrii and V. V. Batrakov)...

The adsorption of ions is determined by the potential of the inner Helmholtz plane 0n while the shift of Epzc to more negative values with increasing concentration of adsorbed anions is identical with the shift in 0(m). Thus, the electrocapillary maximum is shifted to more negative values on an increase in the anion concentration more rapidly than would follow from earlier theories based on concepts of a continuously distributed charge of adsorbed anions over the electrode surface (Stern, 1925). Under Stern s assumption, it would hold that 0(m) = 0X (where, of course, 0X no longer has the significance of the potential at the inner Helmholtz plane). 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

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