Thermal stabilisation of alumina by lanthanide ions

The effect of additions of lanthanide ions (La and Ce) and of the cerium ion concentration on the thermal stability of a gamma alumina was studied. Ln/Al2O3 compositions were treated at high temperature (1100 degrees C) for different calcination times (1, 5, and 24 h) and the resultant materials characterised by X-lay diffraction (XRD), BET surface area, X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction (TPR). It was observed that cerium ion stabilisation was dependent on concentration with 6 wt-%Ce being the most efficient. It is suggested that the stabilisation is due to occupation of vacant cation sites in the spinel structure by Ce3+, and that stabilisation breaks down on prolonged exposure at high temperatures because of oxidation of Ce3+ to CeO2. The same stabilisation mechanism is considered to operate for the lanthanum ion but, unlike Ce3+, it is not subject to oxidation, making La a mole effective stabiliser than Ce.