The production of nanoparticulate ceria using reverse micelle sol gel techniques

Abstract

Mesoporous ceria nanoparticles have, for the first time, been synthesized from cerium isopropoxide as precursor in a sol gel type process by controlled hydrolysis using reverse micelles. Water-filled reverse micelles have been facilitated by adding a non-ionic surfactant (TX-100) to an hydrophobic solvent (cyclohexane) above the critical micelle concentration. The detailed thermal chemistry of the gels has been studied at temperatures between 423 and 1123 K using XRD, SEM, TGA/DSC, elemental analysis and physical adsorption measurements. Calcination of gel in air produces porous nanoparticles of cerium dioxide with the fluorite structure at 823 K. Particle size and crystallinity increases with temperature along with loss of porosity indicating surface sintering and recrystallisation are taking place. Thermogravimetric analysis of ceria gel under air shows surfactant decomposition starts at ca. 500 K and is complete by ca. 700 K. Parallel calcination studies under nitrogen or argon show similar data to each other emphasising the importance of oxygen in the removal of carbonaceous material from the gels. Thus, in the absence of atmospheric O₂, oxygen is instead removed from the fluorite ceria phase which is believed to result in the formation of non-stoichiometric CeO_(2−x). This causes loss of crystallisation of the fluorite CeO₂ phase producing glassy material rather than nanoparticles and prevents porosity developing. This also increases the diffusion limitations on carbon loss which results in C being retained in the gel until higher temperatures.