Effect of large cations (La3+ and Ba2+) on the catalytic performance of Mn-substituted hexaaluminates for N2O decomposition

Mn-substituted La-hexaaluminate (LaMnxAl(12-x)O19) and Ba-hexaaluminate (BaMnxAl(12-x)O19) catalysts were prepared using the carbonates route and investigated for high-concentration of N2O decomposition. it was for the first time found that the Ba-hexaaluminate exhibited higher activity than the La-hexaaluminate at a given Mn content, both of which were much more active than Mn/Al2O3 after being subjected to high-temperature (1400 degrees C) treatment. The catalytic activity varied with the Mn content and attained the best one at x = 1. X-ray diffraction (XRD) characterizations showed that a small amount of Mn (up to x = 1) promoted greatly the formation of phase-pure hexaaluminate, while excess Mn caused formation of catalytically inactive impurity phases, such as LaAlO3, BaAl2O4, Mn3O4, and LaMnO3, which covered partially the active sites and then led to a loss of the activity. UV-visible spectra showed that Mn2+ preferentially enter tetrahedral Al sites at a low Mn content (x = 0.5) for the La-hexaaluminate, which is quite different from the case of Ba-hexaaluminate where Mn3+ can substitute octahedral Al sites even at x = 0.5. Such a difference in the number of catalytically active Mn3+ sites in the octahedral position should be responsible for the higher activity of the Mn-substituted Ba-hexaaluminate. (C) 2009 Elsevier B.V. All rights reserved.