Anion adsorption, hydroxylated surface

Knowing the charge accumulated in IHP, one may calculate the concentration of [= SOH An-] = oj ,An/Bff and [= SO Ct+] = ap,ct/Bn respectively. The density of hydroxyl groups on the surface of the oxide, according to Sprycha, is proposed to be calculated, for high concentrations, from the difference between the total amount of hydroxyl group on the surface of the oxide Ns and cation and anion adsorption density in IHP... 

Anion Adsorption. The third paper of the series (6) proposed chemical reactions analogous to equations 5a and 6a for the adsorption of an anion (L/ ) by a hydroxylated surface ... 

Most studies of metal ion complexation rely on the two-pKH model. Schindler and Stumm [59, 69, 78, 82, 87] combine the two-pKH constant capacitance model with stoichiometric reactions of the metal ions with surface hydroxyls. Huang et al. [62] and Dzombak and Morel [63] tabulate ion affinity constants on the basis of the two-pKH GC model. Leckie and co-workers [88-90] combine the model cation and anion adsorption with the two-pKH TL model. Hayes makes a distinction between strongly and weakly adsorbing ions [89, 90]. A series of reviews on s.a. using the two-pKH model can be found in [91]. 

As discussed in the previous Sections, electrochemical oxidation of polyerystalline Ru involves about one order-of-magnitude larger currents than that of Ru(OOOl), starting as early as 0.2 V. Polyerystalline Ru is covered with hydroxyl ions from water very early in the potential scale even in acidic solutions, thus blocking the surface from anion adsorption by the supporting electrolyte. This conclusion is supported by the in situ IR spectra presented below. 

Because the only variable changed in this dissolution study was the type of alkali metal hydroxide, differences in dissolution rate must be attributed to differences in adsorption behavior of the alkali metal cations. The affinity for alkali metal cations to adsorb on silica is reported (8) to increase in a continuous way from Cs+ to Li+, so the discontinuous behavior of dissolution rate cannot simply be related to the adsorption behavior of the alkali metal cations. We ascribe the differences in dissolution rate to a promoting effect of the cations in the transport of hydroxyl anions toward the surface of the silica gel. Because differences in hydration properties of the cations contribute to differences in water bonding to the alkali metal cations, differences in local transport phenomena and water structure can be expected, especially when the silica surface is largely covered by cations. Lithium and sodium cations are known as water structure formers and thus have a large tendency to construct a coherent network of water molecules in which water molecules closest to the central cation are very strongly bonded slow exchange (compared to normal water diffusion) will... 

As the complexation of cations causes a release of H" and anions compete with surface bound hydroxyl groups (Tigand exchange ) the adsorption is strongly pH-dependant (see above). As a result, anions are preferably adsorbed at lower pH-values whereas cations are primarily adsorbed at higher pH values (Fig. 7.12). 

ANION ADSORPTION. The Constant capacitance model postulates that anions react with surface hydroxyl groups through the ligand exchange mechanism embodied in Eq. 5.37b.In typical applications, the value of b in Eq. 5.37b is either 1 or 2 and the corresponding conditional equilibrium constants have the form... 

ANION ADSORPTION. The triple layer model postulates that anions react with surface hydroxyl groups according to the general equation ... 

The surface of a metal oxide consists of exposed cations, oxide ions, and hydroxyl groups. It is clear that these cations and anions in the surface of an oxide cannot be coordinatively saturated and, hence, that they must develop characteristic properties. The degree of unsaturation of individual surface atoms will be determined by the requirement for retaining stoichiometry in the crystal, and the type of crystal lattice will determine the local symmetry of surface vacancies. A detailed knowledge of the properties, structural and electronic, at an atomic level would be required for an in-depth understanding of the surface chemistry of oxides at a molecular level. This information, however, is almost impossible to obtain experimentally for high-surface-area materials of practical importance in adsorption and catalysis. The descriptions of these surfaces at an atomic level are based almost exclusively on model surfaces and the assumption that certain well-defined crystal planes (preferentially those providing the lowest... 

Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

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