CHARACTERIZATION OF HIGH-TEMPERATURE CALCINED LANTHANUM-MODIFIED ALUMINA BY X-RAY PHOTOELECTRON-SPECTROSCOPY AND X-RAY-DIFFRACTION

Previous studies in this laboratory have shown that for the La/gamma-Al2O3 system as the atomic ratio La/Al is increased from 0.05 to 2.0 the X-ray photoelectron spectroscopy (XPS) La 3d5/2 binding energy progressively decreases from 835.2 to 832.9 eV. The shifts in binding energy demonstrate a change in the coordination of lanthanum coinciding with a transformation from dispersed to particulate phases. It is of particular interest to understand more completely the chemical interaction between lanthana and alumina at lower lanthanum loadings, which is known to retard the sintering of gamma-alumina and promote catalysis. In this study lanthanum-modified alumina was calcined at high temperature (1500-degrees-C). Up to a La/Al ratio of 0.10, X-ray diffraction (XRD) identified the dominant lanthanum phase as lanthanum-beta-alumina (La2O3.11Al2O3). The XPS La binding energy for lanthanum-beta-alumina was 835.2 eV, which corresponds to that measured previously for dispersed phase lanthanum in gamma-alumina. These results suggest that at La/Al less-than-or-equal-to 0.10 the highly dispersed La/gamma-Al2O3 species is a precursor of crystalline lanthanum-beta-alumina formed at higher temperatures. At higher lanthanum concentrations XRD identified highly ordered crystalline LaAlO3 and La2O3, consistent with the phase diagram of the La/Al2O3 system. According to XPS binding energy shifts the precipitation of LaAlO3 requires a higher lanthanum concentration at 925-degrees-C (gamma-alumina phase) than at 1500-degrees-C (beta-alumina phase).