Incorporation of lanthanides in alumina matrices by a sol-gel process employing 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 during the early stages of hydrolysis. Gels from these heterometallic alkoxides 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 monitored 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 their mixtures. The XRD of the gel derived from La [Al(OPri)(4)](3) shows diffraction peaks at 900 degrees C due to LaAlO3. Cerium oxide starts to separate out at 600 degrees C in xerogels prepared from Ce [Al(OPri)(4)](3), and at 700 degrees C in xerogels prepared from mixtures of La [Al(OPri)(4)](3) and Ce [Al(OPri)(4)](3). Raman spectra of the gels 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 CeO2, in fact, contains lanthanum in a mixed fluorite structure. High-resolution electron microscopy confirms that the lanthanum is part of the CeO, fluorite structure even in lanthanum rich areas. X-Ray photoelectron spectroscopic (XPS) analysis of the lanthanum containing gels, heated 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 samples. The contribution of mu''' in the Ce 3d peaks suggests that both oxides, CeO2 and Ce2O3, are present in cerium containing gels. The presence of lanthanum does not affect the distribution of Ce-III and Ce-IV in our samples.