Oxidant specific adsorption

Effect of ligands and metal ions on surface protonation of a hydrous oxide. Specific Adsorption of cations and anions is accompanied by a displacement of alkalimetric and acidimetric titration curve (see Figs. 2.10 and 3.5). This reflects a change in surface protonation as a consequence of adsorption. This is illustrated by two examples ... 

Kleijn, J.M. and Lyklema, J., The electrical double layer on oxides Specific adsorption of chloride and methylviologen on ruthenium dioxide, J. Colloid Interf. Sci., 120, 511, 1987. 

A diffuse-layer minimum in C,E curves has not been found with electrodes kept 3 min at E = -0.74 V, i.e., at a potential close to the rest potential of Fe.728 Complete cathodic reduction at <<-0.74 V (SCE) is not achieved since a diffuse-layer minimum is not found for cathodically reduced electrodes. This effect has been explained by the oxidation of Fe. According to impedance data, strong specific adsorption of Cl anions at renewed Fe electrodes occurs since a very large shift of Eosq takes place going from KF to KC1 solutions. 

While the pzc of Hg in F solution has not changed by more than 1 mV for over 70 years, marginal variations are visible for Ga, Tl, In, Cd, Bi, Sn, and Sb that are related to electrolyte effects (weak specific adsorption or disturbance of the adsorbed water layer, as for Ga).847 Important variations can be seen, on the other hand, for polycrystalline Ag, Zn, Ni, Fe, and Cu. For all these metals a drop of the pzc to much more negative values has been recorded this is evidently related to an improvement in the preparation of the surface with more effective elimination of surface oxides. All these metals, with the exception of Ag, are naturally sensitive to atmospheric oxygen. Values of pzc for single-crystal faces first appeared in a 1974 compilation,23 in particular for the three main faces of Ag and for Au (110). Values for a number of other metals were reported in 1986.25 However, for sd-metals, an exhaustive, specific compilation of available experimental data was given by Hamelin etal. in 1983.24... 

A further effect which has been known for many years is that of anions, which are specifically adsorbed at high anodic potentials on platinum, on the products of the oxidation of carboxylate ions. For example, carbonium ion-derived products can be obtained in the presence of such specific adsorption and this demands a complete change in reaction route (Fioshin and Avrutskaya, 1967 Glasstone and Hickling, 1934). 

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Hohl, H., L. Sigg, and W. Stumm (1980), "Characterization of Surface Chemical Properties of Oxides in Natural Waters The Role of Specific Adsorption Determining the Specific Charge," in M. C. Kavanaugh and J. O. Leckie, Eds., Particulates in Water, Advances in Chemistry Series, ACS 189, 1-31. 

Schematic illustration on the specific adsorption on Fe-minerals of oxidants and reductants directly or through ligand bridges. The formation of these surface complexes (which is usually fast) facilitates the subsequent electron transfer.

The oxidation of Fe(II), V02+, Mn2+, Cu+ by oxygen is favored thermodynamically and kinetically by hydrolysis and by specific adsorption to hydrous oxide surfaces. As suggested in formula (IV) of Fig. 9.1 Fe(II) and the other transition elements Mn(II), VO(II), Cu(I) may more readily associate (probably outer-spherically) with... 

The large specific surface areas of the Fe solid phases (Fe(II,III)(hydr)oxides, FeS2, FeS, Fe-silicates) and their surface chemical reactivities facilitate specific adsorption of various solutes. This is one of the causes for the interdependence of the iron cycle with that of many other elements, above all with heavy metals, some metalloids, and oxyanions such as phosphate. 

The cyclic voitammogram for Pt (111) in 5 M sulfuric acid is shown in Fig. 2-21. Compared with that in 0.5 M sulfuric acid (Fig. 2-15), the anodic part of the two split hydrogen adsorption-desorption areas was compressed in the cathodic direction and became two sharp peaks while the cathodic part did not change its shape very much. The asymmetric peak at 700 mV shifted cathodicly and became more symmetric and sharp. The oxidation of platinum shifted about 100 mV in the anodic direction. All these changes could be attributed to the increase in specific adsorption of anions or the decrease of the activity of water as well as the pH change. 

One complication which may be present, when the Helmholtz model is in other respects appropriate, is that of specific adsorption. If one of the mobile species is to some extent chemically bound rather than being simply electrostatically bound to the metal electrode, Cji may show a dependence on the dc bias potential. Indeed this is the normal method of inferring specific adsorption. Another possibility in this case is that dl exhibits different high frequency and low frequency limits because at high frequencies the specific adsorption being an activated process is too slow to follow changes in interface potential. A further complication which is often present in real systems is the presence of an oxide layer on the surface of the metal electrode. Such an oxide layer can generate a potential... 

Adsorption of cations on iron oxides (Table 11.3) may be specific or non specific. With non specific adsorption, there is at least one water molecule between the adsorbing species and the surface functional group. Specific adsorption involves interaction with deprotonated surface hydroxyl groups to form mono- and bi-nuclear, inner sphere complexes, i.e. 

Specific adsorption of zinc and cadmium by iron and aluminum hydrous oxides. In Proc. 15 Hanford Life Sci. Symp. on Biol. Implications of Metals in the Environment, Hanford,Washington, USERDA-NTIS, 231-239... 

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