Synthesis and characterization of nanosized ceria powders and high-concentration ceria sols

Nanoscale, high surface area cerium oxide (ceria) powders and stable, high-concentration (> 1 M) ceria sols were prepared by a new method based on homogeneous precipitation in an acidic environment using cerium(IV) nitrate as the precursor. The results are compared with ceria powders and sols prepared by a hydrothermal method in a basic environment with cerium(III) nitrate as the precursor. Hydrolysis and condensation of the cerium(IV) and cerium(III) nitrates yield the ceria precursor precipitates with molecular formula of CeO₂·2H₂O [or Ce(OH)₄] and CeO ₂-H₂O [or Ce(OH)₂O], respectively. The dried ceria precursor powders from Ce(IV) and Ce-(III) are well dispersed in the form of primary particles of about 4 nm in size. Calcination at 450 °C causes phase transformation of the amorphous portion accompanying growth of the Ce(IV)-derived ceria crystallite or aggregation of the Ce(III)-derived ceria particles. The Ce(III)-derived ceria crystallites have a smaller lattice parameter than the Ce(IV)-derived sample whose lattice parameter decreases with increasing calcination temperature. Stable ceria sols at a solid concentration up to 300 g of CeO₂/L were obtained from a Ce(IV)-derived ceria precursor powder, as compared to a maximum stable solid concentration of 20 g of CeO₂/L for the Ce(III)-derived ceria sols. These results are discussed in terms of the shape and aggregation tendency of the primary ceria particles prepared by the two different methods.