FORMATION OF SOLID PARTICLES BY HYDROLYSIS OF CERIUM(IV) SULFATE .1. TIME EVOLUTION OF THE HYDROLYZED SOLUTIONS

Uniform particles of controlled morphology of CeOSO4-H2O may be prepared by forced hydrolysis, at 90-degrees-C, of solutions of cerium (IV) sulphate. A structural description is given here of the different steps of the forced hydrolysis before precipitation takes place. It uses essentially the complementary techniques of extended X-ray absorption fine structure spectroscopy and small-angle X-ray scattering to characterize, at different length scales, the structural evolution of the solution. The first step, which occurs as temperature is raised above 60-degrees-C, is an inorganic polymerization that transforms molecular dimeric precursors Ce2(OH)2O12 into colloidal particles. In the second step, at the ageing temperature of 90-degrees-C, no chemical and structural changes are revealed; the solution has reached an equilibrium state characterized by the presence of 3 nm large monodisperse colloids which use 85% of the initial cerium ions and smaller particles (1 5%). The detailed local structure around the cerium atoms in the colloids is compatible with the formation of a chain-like structure of Ce(IV) ions via hydroxo bridges (Ce(OH)2)n2n+. A mechanism to explain the transformation of precursors into colloids is proposed.