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

BaMnxAl12-xO19-alpha combustion catalysts with x = 0.5, 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 alpha radiation at different wave lengths in proximity and far from the Mn K adsorption edge. The results show that at low loading (up to x=1) Mn preferentially enters tetrahedral Al(2) sites of Ba-beta I-Al2O3 as divalent cation- The occupancy of Ba sites in the mirror planes acts as a charge compensation mechanism to balance substitution of Al3+ With Mn2+. At high Mn loading (x greater than or equal to 1) the occupation of Ba sites reaches unity and Mn preferentially enters octahedral Al(1) sites as Mn3+. 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. (C) 1998 Academic Press.