Inner-sphere surface reaction

In heterogeneous redox reactions similar reaction sequences are observed usually an encounter (outer-sphere or inner-sphere) surface complex is formed to facilitate the subsequent electron transfer. 

By following the reaction scheme proposed by dos Santos Afonso and Stumm (22) for the reductive dissolution of hematite surface sites (Scheme 1), we were able to explain perfectly the observed pH pattern of the oxidation rate of H2S. The rate is proportional to the concentration of inner-sphere surface complexes of HS" formed with either the neutral (>FeOH) or the protonated (>FeOH2+) ferric oxide surface sites. 

An example of a speciation calculation involving metals and ligands that adsorb to form only inner-sphere surface complexes is shown in Table 9.10 for a soil solution at pH 7.5. The adsorption reactions for these metals and ligands are exemplified by the first and eighth rows in Table 9.7 ... 

A reaction sequence somewhat in parallel with the Eigen-Wilkins-Werner mechanism can also be expressed for the inner-sphere surface complexation of bivalent metal cations by an ionised surface hydroxyl group (cf equation (9.9a) ... 

Equation 4.3 is formally similar to a complexation reaction between SR(s) and the aqueous solution species on the left side. Indeed, the solid-phase product on the right side can be interpreted on the molecular level as either an outer-sphere or an inner-sphere surface complex. The latter type of adsorbed species was invoked in connection with the generic adsorption-desorption reactions in Eqs. 3.46 and 3.61, which were applied to interpret mineral dissolution processes. In general, adsorbed species can be either diffuse-layer ions or surface complexes,7 and both species are likely to be included in macroscopic composition measurements based on Eq. 4.2. Equation 4.3, being an overall reaction, does not imply any particular adsorbed species product, aside from its stoichiometry and the electroneutrality condition in Eq. 4.4. 

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... 

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. 

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... 

The interfacial aqueous coordination chemistry of natural particles, in particular their surface complexation reactions, owes much of its development to the research of Werner Stumm. Beginning with the tentative interpretation of specific adsorption processes in terms of chemical reactions to form inner-sphere surface complexes, his seminal questions spawned a generation of research on the detection and quantitation of these surface species. The application of noninvasive spectroscopy in this research is exemplified by electron spin resonance and extended X-ray absorption fine structure studies. These studies, in turn, indicate the existence of a rich variety of surface species that transcend the isolated surface complex in both structure and reactivity, thereby stimulating future research in molecular conceptualizations of the particle-water interface. 

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). 

A reduction in system pH enhances the solubility of PR, making the precipitation of pyromorphite minerals possible. However, the sorption of Pb decreases sharply as the system pH decreased, producing a sigmoidal function, usually referred to as an adsorption edge, which reflects the affinity of a metal species for a mineral surface (Sposito, 1984). The ability of Pb to form inner-sphere surface complexes is related to the ability of a species in solution to form hydroxides. In fact, it has been shown that surface affinity of metal cations for Fe-oxide and Fe-hydroxide surfaces agrees with their hydrolysis values (Hayes and Katz, 1996). An analogy between solution complexation and surface complexation is represented in the following reactions (Hayes and Katz, 1996) ... 

These corrected values correspond to an oxygenation of surface complexes extrapolated to a bimolecular reaction in solution. The close agreement in the oxygenation kinetics of dissolved VO(OH)+ and with that of the surface complexes (=MO)2VO supports the evidence for inner-sphere surface coordination from spectroscopic and thermodynamic experiments. 

METAL CATION ADSORPTION. The formation of inner-sphere surface complexes involving metal cations is typically described in the constant capacitance model with the chemical reactions ... 

We now turn to the inner-sphere redox reactions in polar solvents in which the coupling of the electron with both the inner and outher solvation shells is to be taken into account. For this purpose a two-frequency oscillator model may the simplest to use, provided the frequency shift resulting from the change of the ion charges is neglected. The "adiabatic electronic surfaces of the solvent before and after the electron transfer are then represented by two similar elliptic paraboloids described by equations (199.11), where x and y denote the coordinates of the solvent vibrations in the outer and inner spheres, respectively. The corresponding vibration frequencies and... 

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