On the crystal structure and cation valence of Mn in Mn-substituted Ba-beta-Al2O3

BaMnxAl12¡xO19 combustion catalysts with xD0.5, 1.0, 2.0, 3.0 have been investigated. The crystal structure and, specifically, the partitioning and the dominant oxidation state of Mn in the different crystallographic sites has been clarified by means of X-ray absorption spectroscopy and X-ray powder diffraction structure refinements of multiple diffraction powder data sets collected with both synchrotron and Cu-K® radiation at different wave lengths in proximity and far from the Mn K adsorption edge. The results show that at low loading (up to xD1) Mn preferentially enters tetrahedral Al(2) sites of Ba-¯I-Al2O3 as divalent cation. The occupancy of Ba sites in the mirror planes acts as a charge compensation mechanism to balance substitution of Al3C with Mn2C. At high Mn loading (x¸1) the occupation of Ba sites reaches unity and Mn preferentially enters octahedral Al(1) sites as Mn3C. Surface area measurements and catalytic activity tests in CH4 combustion have also been performed. The results indicate that the incorporation of Mn in the octahedral Al(1) sites causes reduction of surface area and has no beneficial effect on catalytic activity.