Outer-sphere surface reaction

In the outer-sphere electrode reaction, the reactant and the product do not interact strongly with the electrode surface, and usually they are at a distance of at least a solvent layer from the electrode the original configuration of the reactant is nearly main-... 

Given that the concentration deviation for species E, H20, will be negligible, the second term on the right side of Eq. 4.32b will reduce to -k bAcD, making the corresponding term in Eq. 4.34b equal to - (k b + k f)AcD, and a equal to (k b + k f) in Eq. 4.36e. From Eq. 4.39 it then follows that the second term on the right in Eq. 4.41b will be simply k b, without the equilibrium concentrations of species D and E, in this case. Thus, if outer-sphere surface complexation is much faster than inner-sphere surface complexation and if the effect of any perturbation of the reactions in Eq. 4.40 on the concentration of water is negligible, the linear relationships... 

This subsequence is useful to consider if the time scale for proton adsorption-desorption reactions is comparable to or longer than that for outer-sphere surface complexation. It is a special case of the abstract scenario listed third in Table 4.3. Under the conditions given there, the protonation-proton dissociation reaction (A = SOH, B = H C = SOH2) is assumed to be much faster than outer-sphere surface complexation-dissociation, such that (kf ka, kb kd, k f kf, k b kb here)... 

Because of the millisecond time scale for these reactions, pressure-pulse perturbation (Fig. 4.1) with conductivity detection of the response can be used, as in the molybdate adsorption example. Evidently, the inner-sphere surface complexion step for sulfate occurs on time scales very much longer than those for its outer-sphere surface complexation, and therefore it was not observed experimentally with the method used. 

For anions that complex weakly with surface hydroxyl groups (e.g., perchlorate), it is possible that the outer-sphere surface complexation step in Eq. 4.43 will be faster than both protonation and proton dissociation. This condition is just the opposite of that given for the third reaction sequence in Table 4.3. Its effect on the time constants for the sequence can be derived by applying the approach in Eqs. 4.30-4.38.20 In place of Eqs. 4.36c-e, one finds the factors in multiplying Aca (= A[SOH] = A[ET]) and AcD(= A[L ]) in Eq. 4.34 to be... 

Upon reaction with an adsorptive in aqueous solution (which then becomes an adsorbate), surface functional groups can engage in adsorption complexes, which are immobilized molecular entities comprising the adsorbate and the surface functional group to which it is bound closely [18]. A further classification of adsorption complexes can be made into inner-sphere and outer-sphere surface complexes [19]. An inner-sphere surface complex has no water molecule interposed between the surface functional group and the small ion or molecule it binds, whereas an outer-sphere surface complex has at least one such interposed water molecule. Outer-sphere surface complexes always contain solvated adsorbate ions or molecules. Ions adsorbed in surface complexes are to be distinguished from those adsorbed in the diffuse layer [18] because the former species remain immobilized on a clay mineral surface over time scales that are long when compared, e.g., with the 4-10 ps required for a diffusive step by a solvated free ion in aqueous solution [20]. Outer-sphere surface complexes formed in the interlayers of montmorillonite by Ca2+ or Mg2+ are immobile on the molecular time scale... 

Electron transfer rate constants of outer sphere redox reactions can be measured relatively easily at n-type semiconductor electrodes. This is because electrons are withdrawn from the surface under depletion conditions, so that their concentration is lower than in the bulk. Under ideal... 

These theoretical considerations also gave a basis for the consideration of the optimal distance of discharge, which is a result of competition between the activation energy AG and the overlap of electronic wave functions of the initial and final states. The reaction site for outer-sphere electrochemical reactions is presumed to be separated from the electrode surface by a layer of solvent molecules (see, for instance, [129]). In consequence, the influence of imaging interactions on AGJ predicted by the Marcus equation is small, which explains why such interactions are neglected in many calculations. However, considerations of metal field penetration show that the reaction sites close to the electrode are not favored [128], though contributions to ks from more distant reaction sites will be diminished by a smaller transmission coefficient. If the reaction is strongly nonadiabatic, then the closest approach to the electrode is favorable. 

Such an energy transfer is taken into account in the encounter pre-equilibrium model [131, 133], which considers the outer-sphere electrode reaction to be a two-step process. In the first step the reactant diffuses to the reaction zone with a thickness dr at the electrode surface, where the probability of the charge transfer process between reactant and electrode is significant [133]. Here the electrode and reactant in the reaction zone are similar to a pair of reactants which exchange the electrons in a homogeneous reaction. 

Ion pairing reactions form outer-sphere surface complexes with the background electrolyte ... 

ELECTRON SPIN RESONANCE SPECTROSCOPY Electron spin resonance (ESR) is a technique that can also be used on aqueous samples and has been used to study the adsorption of copper, manganese, and chromium on aluminum oxides and hydroxides. Copper(II) was found to adsorb specifically on amorphous alumina and microcrystalline gibbsite forming at least one Cu-O-Al bond (McBride, 1982 McBride et al., 1984). Manganese(II) adsorbed on amorphous aluminum hydroxide was present as a hydrated outer-sphere surface complex (Micera et al., 1986). Electron spin resonance combined with electron spin-echo experiments revealed that chromium(III) was adsorbed as an outer-sphere surface complex on hydrous alumina that gradually converted to an inner-sphere surface complex over 14 days of reaction time (Karthein et al., 1991). 

Reductive dissolution reactions can be described by a three-step reaction sequence (Stone, 1986). The steps are (i) adsorption of the reductant forming either an inner- or outer-sphere surface complex, (ii) electron transfer from a reducing agent to a surface metal ion, and (iii) release of the reduced ion. For Mn(III/IV) oxides the reductive step is complicated by the necessity for Mn(IV) to be reduced first to Mn(III) then to Mn(II). All natural Mn oxides, however, contain both Mn(IV) and Mn(III) (McKenzie, 1989). Even laboratory MnO2 preparations contain some Mn(III). 

As mentioned earlier, complex formation reactions at hydrous metal oxide surfaces can be treated as an extension of classic coordination chemistry metal centers on mineral surfaces participate in inner-sphere and outer-sphere coordination reactions with molecules adsorbed from overlying solution, including H2O, OH , O, and solute molecules (Schindler, 1981 Schindler and Stumm, 1987). A variety of protonation/deprotonation and complex-formation reactions determine the speciation of surface sites. A few... 

Inner- or outer-sphere surface complex formation is a. necessary prerequisite for most surface chemical redox reactions. (ESR may provide important information regarding the nature of the precursor complex.) When electron transfer is fast k2 ArJArOH]), overall rates of reaction are influenced by rates of organic reductant adsorption. When electron transfer is slow kl < ArJArOH]), Eq. [18] can be modeled as a pseudoequilibrium reaction, using the equilibrium constant... 

In triple layer approximations the location of each adsorbate with respect to the surface must be specified. Protons and all ions assumed bound as inner-sphere complexes (specifically or chemically adsorbed species) are assumed to lose part of their hydration sheaths, bonding directly to sites in the surface itself. Adsorbates assumed to remain hydrated, forming outer-sphere surface complexes, are assigned to the OHP. In the intrinsic equilibrium constants for adsorption reactions, K ", the activities of ions transferred from solution to the surface are corrected for the electrical potential they experience, % or (27). 

METAL CATION ADSORPTION. The formation of outer-sphere surface complexes involving metal cations has been described typically in the triple layer model by the reactions ... 

---