Preparation, characterisation and catalytic activity of pure and substituted La-hexaaluminate systems for high temperature catalytic combustion

La-Al-O, La-Mg-Al-O, La-Mn-Al-O and La-Mg-Mn-Al-O hexaaluminates have been prepared using the carbonates route previously developed for M-substituted and unsubstituted Ba-beta -Al2O3. Starting from amorphous precursors, the formation of a final magnetoplumbite (MP) phase is observed upon calcination at T greater than or equal to 1100 degreesC. In the case of LaMn1Al11O19, the NIP phase already forms at 900 degreesC evidencing a promotion effect of Mn ions. Upon calcination at 1300 degreesC, monophasic samples can be obtained only for Mg-substituted samples (LaMg1Al11O19 and LaMg0.5Mn0.5Al11O19), whereas in the other samples the presence of LaAlO3 is always detected. This behaviour is associated with the stabilisation, via a charge compensation mechanism, of the MP phase due to the introduction of Mg2+ ions in the structure. The co-presence of Mg and Mn in the final catalyst has resulted in a higher specific catalytic activity per Mn mol. Such a behaviour is likely associated with the stabilisation of Mn ions at high oxidation state due to the co-presence of Mg2+. (C) 2001 Elsevier Science B.V. All rights reserved.