Techniques for preparation of managanese-substituted lanthanum hexaaluminates

Abstract

Manganese substituted lanthanum hexaaluminates (LaMnAl11O19) have been prepared by two different co-precipitation techniques, i.e. precipitation as hydroxides and carbonates, and by a sol-gel technique. Pore size distribution after calcination up to 600 °C could be controlled with the use of a modifier, acetyl-acetone (Acac), that was added to the alkoxide solution during the sol-gel synthesis. Two different drying techniques were used in the test, oven drying at low pressure yielding xerogels and supercritical drying yielding aerogels. The crystal-phase, surface area and pore size were compared after consecutive calcinations at 600 °C, 1000 °C and 1200 °C. The samples prepared with the carbonate and sol-gel methods showed pure hexaaluminate- phases at 1200 °C. The highest thermal stability, 28 m2/g at 1200 °C, was found for the aerogel samples. It was shown that a high initial surface area did not correspond to a high surface area after calcination to high temperatures. To retain a high surface area at high temperatures, an initial broad pore size distribution was needed. Small pores, below 50 Å, seemed to collapse more readily than larger pores and should be avoided if high thermal stability is desired. The sol-gel technique combined with supercritical drying gave the most promising material, although the carbonate method also showed promising results.