Stabilisation of aluminas by rare earth and alkaline earth ions

Abstract

The effect of added rare earth and alkaline earth oxides on the resistance of two aluminas to loss of surface area at high temperature has been investigated. In tests of short duration ( ⩽ 4 h) at 1100 ° C, La³⁺ and Ce⁴⁺ are equally effective in reducing loss of area for a commercial γ-alumina. However, in extended tests (24 h) La³⁺is much more effective than Ce⁴⁺. X-ray diffraction shows that the latter has segregated as CeO₂ and the alumina is largely α-phase. No separate rare earth oxide phase can be observed with La₂O₃ and θ-alumina is dominant. The same effects are discernable at higher temperature and with a washcoat alumina. Tests with other rare earth oxides and alkaline earth oxides show that an ion size effect is operative. Under severe conditions Ca²⁺, Yb³⁺ and Sm³⁺ are ineffective, Pr³⁺ and Sr²⁺are moderately effective while La³⁺ and Ba²⁺ are the most effective. Samples containing the latter both exhibit the presence of a hexaaluminate-type phase after tests at 1200° C. Diffuse reflectance infrared spectroscopy has been used to investigate surface hydroxyl groups formed after reequilibration of samples with ambient air. The groups present on sintered alumina differ only slightly from those on unsintered material. There is a good correlation between hydroxyl group band area, in Kubelka-Munk units, and surface area. Samples with added La₂O₃ and CeO₂ show only AlOH groups which also closely resemble those present on alumina alone. However the band area for oxide stabilised samples appears to be slightly below that for pure alumina of equivalent area. At most 30% of the surface is made up of a capping layer of the added oxide. X-ray photoelectron spectroscopy (XPS ) binding energies, and La/ Al and Ce/Al atom ratios determined by XPS, are consistent with the above picture.