Thermal evolution crystal structure and cation valence of Mn in substituted Ba-beta-Al2O3 prepared via coprecipitation in aqueous medium

BaMnxAl12-xO19-alpha combustion catalysts with x = 0.5, 1.0, 2.0, 3.0 have been prepared via coprecipitation in aqueous medium. Thermal evolution from 380 up to 1670 K has been followed by XRD and surface area measurements. T he crystal structure of the final material and the dominant oxidation state of Mn in the different crystallographic sites, have been investigated by m eans of XANES spectroscopy and Rietveld refinements of diffraction powder d ata sets collected in proximity and far from the MnK absorption edge. It wa s found that, except for the highest Mn content (x = 3), monophasic samples are obtained upon calcination at 1470 K. For the whole compositional range the Ba-beta(1)-Al2O3 structure is obtained. A formation mechanism involvin g Ba ion diffusion within the gamma-Al2O3 spinel blocks, similar to that ob served for Mn-free samples, is active also in this case. However, the prese nce of Mn ions favours the formation of the Ba-beta(1)-Al2O3 that occurs at lower temperatures. At low Mn loading (up to x = 1), Mn preferentially ent ers the tetrahedral Al(2) sites of the Ba-beta(1)-Al2O3 as Mn2+. At higher loading, Mn preferentially enters the octahedral Al(1) sites as Mn3+. A cha rge compensation mechanism, involving the occupancy of Ba sites in the mirr or planes, operates to balance the substitution of Al3+ with Mn2+. The pres ence of Mn ions also affects the morphological properties of the final mate rial. (C) 1999 Kluwer Academic Publishers.