Thermal stabilisation of alumina by lanthanide ions

Abstract

The effect of additions of lanthanide ions (La and Ce) and of the cerium ion concentration on the thermal stability of a γ alumina was studied. Ln//Al₂O₃ compositions were treated at high temperature (1100°C) for different calcination times (1, 5, and 24 h) and the resultant materials characterised by X-ray 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 more effective stabiliser than Ce.