Cerium oxides acid-base properties

The main usefulness of Eh-pH diagrams consists in the immediacy of qualitative information about the effects of redox and acid-base properties of the system on elemental solubility. Concerning, for instance, cerium, figure 8.20 immediately shows that, within the stability field of water, delimited upward by oxidation boundary curve o and downward by reduction boundary curve r, the element (in the absence of other anionic ligands besides OH groups) is present in solution mainly as trivalent cerium Ce and as soluble tetravalent hydroxide Ce(OH)2. It is also evident that, with increasing pH, cerium precipitates as trivalent hydroxide Ce(OH)3. 

In another study, information on the acid-base properties of lanthanum and cerium oxides was obtained by adsorption microcalorimetry of ammonia and carbon dioxide and correlated to reaction selectivities in the dehydration of 4-methylpentan-2-ol. Ceria is more acidic than La203, whereas the interaction with CO2 is more pronounced for La203 than for Ce02, in terms of both number and strength of the adsorption sites [99]. 

CutrufeUo, M.G., Ferino, L, Monad, R., Rombi, E., and Solinas, V. Acid-base properties of zirconium cerium and lanthanum oxides by calorimetric and catalytic investigation. Top. Catal. 2002,19, 225-240. 

Ceric oxide, Ce02, is prepared by igniting cerium or any cerous or ceric salt of an oxy-acid. Its density depends upon the method of preparation. The color is the subject of much discussion.) We would expect a white oxide like zirconia and thoria. White oxides have been prepared, but Brauner has shown that such ceria contains impurities. As usually obtained ceria has a pale yellow color. It volatilizes without melting or decomposition at about 1900°. Besides being a weak base it shows slight evidence of faintly acidic properties. While pure ceria is insoluble in nitric or hydrochloric acids, a mixture of rare earth oxides dissolves readily... 

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