Effect of large cations (La and Ba) on the catalytic performance of Mn-substituted hexaaluminates for N2O decomposition

Mn-substituted La-hexaaluminate (LaMn_xAl_(12−x)O₁₉) and Ba-hexaaluminate (BaMn_xAl_(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 °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.