STABILIZATION OF ALUMINA BY ADDITION OF LANTHANUM

Lanthanum was introduced on a commercial alumina using the La(NO3)3 precursor. Low and high loaded La-Al2O3 samples were prepared. Pure alumina and lanthanum modified aluminas were aged at 1050 and 1220-degrees-C under dry and wet air (20 vol.-% water). They were characterized by BET, X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) measurements. The introduction of low lanthanum contents led to a stabilization of alumina towards thermal sintering below 1050-degrees-C. The presence of water increased the rate of sintering. In contrast, an atomic ratio of lanthanum-to-aluminium close to 0.01 does not prevent the formation of alpha-Al2O3 associated with a drastic loss of specific area. By increasing the lanthanum-to-aluminium ratio until a value of 0.05, a stabilization was observed even at 1220-degrees-C. The positive effect of lanthanum was assigned to the formation of microdomains of lanthanum aluminate on the alumina surface. In the perovskite structure, aluminium ions are stabilized in octahedral coordination. Lanthanum ions must be present at a sufficient concentration to ensure the complete LaAlO3 formation from the more reactive surface aluminium ions. The formation of the La2O3, 11Al2O3 phase (lanthanum beta-Al2O3) observed at high temperature was associated with the loss of the stabilizing effect. This phase may be formed by a solid state reaction between Al2O3 and LaAlO3.