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

Abstract

BaMnxAl12−xO19−α 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. The crystal structure of the final material and the dominant oxidation state of Mn in the different crystallographic sites, have been investigated by means of XANES spectroscopy and Rietveld refinements of diffraction powder data sets collected in proximity and far from the MnK absorption edge. It was 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-βI-Al2O3 structure is obtained. A formation mechanism involving Ba ion diffusion within the γ-Al2O3 spinel blocks, similar to that observed for Mn-free samples, is active also in this case. However, the presence of Mn ions favours the formation of the Ba-βI-Al2O3 that occurs at lower temperatures. At low Mn loading (up to x = 1), Mn preferentially enters the tetrahedral Al(2) sites of the Ba-βI-Al2O3 as Mn2+. At higher loading, Mn preferentially enters the octahedral Al(1) sites as Mn3+. A charge compensation mechanism, involving the occupancy of Ba sites in the mirror planes, operates to balance the substitution of Al3+ with Mn2+. The presence of Mn ions also affects the morphological properties of the final material.