INCORPORATION OF LANTHANIDES IN ALUMINA MATRICES BY A SOL-GEL PROCESSEMPLOYING HETEROMETALLIC ALKOXIDES, M[AL(OPRI)(4)](3), AS PRECURSORS

Spectroscopic studies of the hydrolysis of heterometallic alkoxides, M [Al(OPri)(4)](3), M = La, Ce, suggest that they do not dissociate dur ing the early stages of hydrolysis. Gels from these heterometallic alk oxides can be prepared by two methods (i) direct hydrolysis in (PrOH)- O-i, and (ii) peptization with acetic acid. Changes that occur in the structure and surface properties of gels during heat treatment are mon itored by BET surface area measurements and X-ray powder diffraction ( XRD) studies. No crystalline phase is observed below 500 degrees C in xerogels derived from La [Al(OPri)(4)](3) and Ce [Al(OPri)(4)](3) or t heir mixtures. The XRD of the gel derived from La [Al(OPri)(4)](3) sho ws diffraction peaks at 900 degrees C due to LaAlO3. Cerium oxide star ts to separate out at 600 degrees C in xerogels prepared from Ce [Al(O Pri)(4)](3), and at 700 degrees C in xerogels prepared from mixtures o f La [Al(OPri)(4)](3) and Ce [Al(OPri)(4)](3). Raman spectra of the ge ls prepared from the mixtures of La [Al(OPri)(4)](3) and Ce [Al(OPri)( 4)](3) show that the CeO2 crystallite sizes are > 100 Angstrom, and Ce O2, in fact, contains lanthanum in a mixed fluorite structure. High-re solution electron microscopy confirms that the lanthanum is part of th e CeO, fluorite structure even in lanthanum rich areas. X-Ray photoele ctron spectroscopic (XPS) analysis of the lanthanum containing gels, h eated to 500 and 1000 degrees C, shows a peak at 836.5 eV for La 3d(5/ 2) which could be due to the presence of M-O-Al type bridges in the sa mples. The contribution of mu''' in the Ce 3d peaks suggests that both oxides, CeO2 and Ce2O3, are present in cerium containing gels. The pr esence of lanthanum does not affect the distribution of Ce-III and Ce- IV in our samples.