Radii, ionic tetravalent metals

Another example of promotion by an added metal oxide is Cr/silica incorporating Sn(IV) ions [548,594], Like TiC>2, SnC>2 contains a tetravalent metal ion that can exist in tetrahedral coordination, and has a similar ionic radius. Indeed, SnC>2 and T1O2 are isomorphous. Mixed oxides of SnC>2 and SiC>2 are known to exhibit acidity [595-597], Figure 131 shows the result of adding SnC>2 to the Phillips catalyst. Silica was dried at 200 °C and then treated with an excess of SnCLi vapor. The support was then calcined at 500 °C to remove chloride. It was impregnated anhydrously with chromium and then activated at 500 °C in air. It was quite active in polymerization tests at 105 °C, and the MW distribution of tire polymer is shown in Figure 131. 

TABLE 1. Electronegativity "x", partial charge "5", ionic radius "r" and maximum coordination number "N" of some tetravalent metals (Z=4). 

Aluminium sulphate decomposes on heating into A1203 and S03, while the rare-earth sulphates are much more stable. Tetravalent sulphates of the type Z(S04)2 occur very rarely, and of the metals of the fourth column, only thorium, because of its large ionic radius, forms a sulphate of this type. 

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... 

The coordination chemistry of tetravalent cerium is in many aspects very similar to the coordination chemistry of tetravalent plutonium. The ionic radius of Ce" " (0.94 A) is within the experimental error identical to the ionic radius of Pu + (Shannon and Prewitt, 1969). Due to the similarity in the charge-to-ionic size ratio, the complex formation constants of tetravalent cerium are essentially the same as those of tetravalent plutonium. Complex formation causes for the two metal systems the same shift of the redox potential. 

Cerium(IV) is a strong oxidising agent and, as such, it has a number of chemical applications as an oxidant. In studying the hydrolysis of cerium(IV), a number of studies have utilised oxidation-reduction reactions in determining the relevant stability constants. As a tetravalent cation with a relatively small ionic radius, the hydrolysis of cerium(IV) occurs at very low pH. Consequently, data have been provided that show that the metal ion is extensively hydrolysed even at a pH of around 0 (i.e. Imoll" H" ) (Baes and Mesmer, 1976). Cerium(IV) is used as an analogue for plutonium(IV) in nuclear fuel manufacturing studies. 

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