Lower-valent cation

Fig. 18. Schematic diagram for a binary alloy with a passivating oxide film in contact to electrolyte with the reactions of (1) oxide formation, (2) electron transfer, and (3) corrosion, including (4) oxidation of lower-valent cations and the indication of ionic and atomic fractions X as variables for the composition of the layer and the metals surface.

Let us briefly consider in this context two important classes of materials. The first are the oxides of fluorite type such as Zr02 or Ce02. They can accommodate high concentrations of lower valent cations. Y203 doping of Zr0210 leads to the very important YSZ electrolyte. 

Figure 13 The ideal fluorite, AO2 oxide structure oxides with this structure have high-ionic conductivity when the host cations (such as A = Zr + or Ce +) are replaced by lower-valent cations such as Y3+ for Zr + in YSZ and Gd3+ for Ce + in Cei Gd t02 t/2. (Ref. 167. Reproduced by permission of Nature Publishing Group (www.nature.com))...

Such a mechanism is postulated to operate in the activation of methane at high temperatures in the process of the oxidative coupling (65). Catalysts which are both active and selective for the oxidative coupling of methane may be classified as strongly basic metal oxides. Substitution of lower-valent cations in their lattice generates oxygen vacancies, which constitute electron acceptor levels and are responsible for the appearance of electron holes in the valence band. These holes diffuse to the surface, because the lone pair orbitals of surface oxide ions are the HOMOs of the oxide and their energy levels form the top of the valence band. Localization of a hole on such lone pair orbital is equivalent to the formation of a surface O- species. 

Y. Mizutani, R. Yamane and T. Sata, Electrodialysis method to permeate lower valent cations selectively, Jpn. Pat., JP 46-23607 (examined application), US Pat. 3,510,417, 3,510,418, Br. Pat, 1,238,656 F. Hanada, N. Ohmura Y. Kagiyama and Y. Mizutani, Monovalent cation permselective membrane, Nippon Kaisui Gakkaishi Bull. Soc. Sea Water Sci), 1990, 44, 116. 

Tejuca et al. (1989) provided a brief review about this subject. Before 1989 the most prominent published contributions were those of Voorhoeve et al. (1975, 1976). The main B cations studied were Co and Mn while several cations were inserted in the A position partially substituting lanthanum. The rationale for this approach is found in the role assigned to oxygen vacancies in the dissociation of NO. The substitution of La by lower-valent cations induces the formation of oxygen vacancies. 

In addition, it should be noted that, although the generic formula of some minerals listed in Table XII-1 does not mention thorium, this element is often present in such minerals by snbstitution for quadrivalent or tervalent species, in percentages that may be in excess of 10%. This is, for instance, the case for Sm-monazites. Of course, in such substitutions charge balance must be maintained and the replacement of a tervalent ion by thorinm mnst be accompanied by an equivalent increased amount of a lower-valent cation (often Ca ) in the stmctnre. 

It may be shown that one may only hope to achieve this situation when the material is already dominated by electrons (and some positive point defects, such as oxygen vacancies) under hydrogen-free reducing conditions. By increasing pjj, the native positive defects are replaced by the protonic ones. Protons compensated by electrons are, however, hardly known in systems other than ZnO, and instead one tends to use acceptor-doped oxides in which the concentrations of protons and aU other positive defects are enhanced by the doping. The acceptors are most often substitutionaUy dissolved lower-valent cations, while, in principle, interstitial anions or substitutional higher-valent anions (e.g., N - substituting O ) can also be used. We shall simply refer to the acceptors as A, and, in the ionized state. A/. 

Cerium oxide, ceria, has a fluorite structure and shows oxide anion conducting behavior differ from other rare earth oxides. However, the O ionic conductivity of pure ceria is low because of a lack of oxide anion vacancies. For ion conduction, especially for anion, it is important to have such an enough vacancy in the crystal lattice for ion conduction. Therefore, the substitution of tetravalent Ce" by a lower valent cation is applied in order to introduce the anion vacancies. For the dopant cation, divalent alkaline earth metal ions and some rare earth ions which stably hold trivalent state are usually selected. Figure 9-28 shows the dopant ionic radius dependencies of the oxide ionic conductivity for the doped ceria at 800°C. In the case of rare earth doped Ce02, the highest O ion conductivity was obtained for... 

Also lower valent cations share this property of bringii about reversal of charge, but the required salt concentrations are then increasingly higher the lower the valency of the cation (see p. 276 Chapter IX 2). 

In gum arabic the frequency is much greater and it allows of coacervation with salts of the types 6—1, 5—1, 4—1, yet not with lower valent cations. 

Of these two pieces of information the former (a) does not yet concern us here, it will however play a great role in considering reversal of charge phenomena with lower valent cations or of positively charged colloids with lower valent anions in the 2-6 of this Chapter. 

The figure shows the general effect in choosing a lower valent cation, which consists in a nearly parallel displacement of the straight line towards higher con-centrations. " The nearly equal slope of both lines means that the reciprocal La number will indeed be practically the same as the reciprocal hexol number. The main effect consists thus in an enormous increase of the real reversal of charge concentration, the latter being 7 X 10 N for hexol nitrate and 3.6.10 for La(NOg)3. 

For this number must now be obtained from relatively not very different gross reversal of charge concentrations, which latter should for this purpose be known with much greater accuracy than in the case of hexol nitrate. Now in general choosing a lower valent cation, the slope of the electrophoretic velocity-concentration curve decreases, so that in fact the gross reversal of charge concentration with La(N03)s can be determined with less accuracy than with hexol nitrate. 

With decreasing equivalent weight of the colloid flocculability increases, in so far as flocculation or coacervation is already realisable with lower valent cations the lower the equivalent weight (see however also p. 295, 2 m),... 

Fig. 36. Reversal of charge, beginning and end of the coacervation or flocculation range as a function of the colloid concentration Ca. a in the complex coacervation in the narrow sense, (see p. 338, 2). b in the coacervation of gum arabic with hexol nitrate (see also p. 388). c in the coacervation or flocculation of gum arabic or an other acidoid with lower valent cations. (see p. 392).

Oxygen-deficient oxides doped with lower valent cations... 

---