X-ray diffraction, FTIR, and NMR characterization of sol-gel alumina doped with lanthanum and cerium

Alumina doped with La and Ce was prepared by using the sol-gel technique. The doping concentrations were alternatively 2 and 5 wt% for each element. Samples were characterized as a function of temperature by using X-ray powder diffraction and FTIR spectroscopy. MAS-NMR studies showed that lanthanum and cerium interacted with alumina and produced a compound with aluminum ions in tetrahedral symmetry. These interactions stabilized alumina structure and shifted its transformations to higher temperatures. When cerium-doped samples were heated at temperatures higher than 1000 degrees C, CeO2 was segregated, For La doping such segregation was not observed. Bronsted acid sites were generated for the low doping concentrations of both elements, At the high doping concentration, however, only Lewis acid sites existed in a higher concentration than in undoped alumina. (C) 1997 Academic Press.