Catalytic combustion of methane on substituted barium hexaaluminates

A sol-gel method using metallic barium, aluminum alkoxides and metal nitrates has been used to synthesize barium hexaaluminate partially substituted by either manganese, iron or both metal ions. The p-alumina structure was obtained by calcination under oxygen at 1200 degrees C. X-ray analysis revealed that formation of a pure single phase BaMxAl12-xO19 occurred up to x=4 for Fe, x=3 for Mn and for Fe1Mn1 in the case of mixed substituted hexaaluminates. Incorporation of Mn in excess leads to another phase formation (manganese oxide or spinel). As far as the valence state of transition metal ions is concerned, the introduced Fe ions were always trivalent, whereas the Mn ones were either divalent or trivalent. In the latter case, the first Mn ions were introduced in the matrix essentially as Mn2+ and only for BaMn3Al9O19 does manganese exist exclusively as Mn3+, the higher the Mn concentration, the higher the proportion of Mn3+. All solids were aged at 1200 degrees C under water and oxygen and showed a good thermal resistance. Activity for methane combustion has been measured for fresh and aged solids, light-off temperatures were observed in the 560-640 degrees C range. However, the highest activity was obtained for catalysts containing either 3 Mn, 2 Fe or 1 Fe+1 Mn ions per unit cell. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species. (C) 2000 Elsevier Science B.V. All rights reserved.