Anion adsorption and charge transfer

Anion Adsorption and Charge Transfer on Single-Crystal Electrodes... 

Anion(s), role in boundary layer at metal-electrolyte solution interface, 126-127 Anion adsorption and charge transfer on single-crystal electrodes advantages and disadvantages, 169 anion coverages, 161,162/ ball models for adlayer structures, 161,163/ charge correction, 167,169 defects, influence on adsorption, 165,166/... 

Anion adsorption and charge transfer on single-crystal electrodes—Continued experimental procedure, 157-159 polycrystalline electrode activated by electrochemical cycling, 165,167 Pt(531) electrode, 167,168/ state of electrode surface using cyclic voltammetry, 164,165/ voltammetric profile, 158/, 160 Antigen-antibodies, use in glass surfaces, 202-209... 

FIGURE 13.2 An electrostatic adsorption mechanism (a) surface charging, metal adsorption, and proton transfer, (b) monolayer coverage of Pt anions with hydration sheath. 

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. 

The adsorption strength of anions on Au electrodes follows the sequence of C104 < S042- < Cl < Br < I. The strong specific adsorption of halide ions leads to a partial charge transfer between the adsorbate and the metal electrode [234]. 

Wifckowski and coworkers [37] have reported adsorption of bisulfate anions on Au(lll), Pt(lll), and Rh(lll) electrodes in sulfuric acid solution using electrochemical and several nonelectrochemical techniques. It was concluded from the low-energy electron-diffraction data that the structure of bisulfate on gold is different from that on Pt(lll) and Rh(lll). Adsorption of bisulfate on Au(lll) is associated with a charge transfer from the electrode to the adsorbate. However, the formation of this particular bond does... 

The presence of adsorbed ions can alter the potential of the OHP and thereby influence the rate of charge transfer at the electrode. For example, a specifically adsorbed anion causes the potential at the OHP to be more negative than in the absence of adsorption. 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

Solution of these equations eventually gives an expression fornj (0), the concentration of electrons at the surface, in terms of the parameters already defined, together with kCT, the rate constant for charge transfer across the interface (see Fig. 10.21) and two recombination constants, one for the bulk and one for the surface. Recombination of hole-electron pairs is taken into account in the development, as is also the formation of surface states by a surface-dependent anion adsorption at a degree of coverage, 9. 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

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