EFFECT OF MECHANICAL ACTIVATION ON THE REAL STRUCTURE AND REACTIVITY OF IRON(III) OXIDE WITH CORUNDUM-TYPE STRUCTURE

For highly dispersed alpha-Fe2O3 the effect of mechanical activation i n the high-powered ball mill under Ar atmosphere on its real structure and reactivity has been studied by using methods sensitive to the bul k (X-ray diffraction, TEM, IR spectroscopy of the lattice vibrations, static magnetic method) as well as to surface properties (SIMS, IR spe ctroscopy of adsorbed CO and surface hydroxyls, catalytic CO oxidation ), Before activation, initial sample was shown to have a rather high d ensity of surface and near-surface extended defects with local spinel structure probably stabilized by admired sodium ions, while the bulk i s free from such disturbances, Cooperative distortion of the lattice ( increase of the distance between cation layers and some equalizing of the O-O distances in the anion sublattice) was observed and explained by an effect of the admired sodium cations in the interstitial positio ns. Mechanical activation was found to generate anion vacancies in the bulk that was accompanied by a further increase of the distance betwe en cation layers, whereas a mode of relaxation within the anion layers approaches that typical for the perfect hematite, A fall of the densi ty of near-surface extended defects caused by mechanical activation wa s explained by the glide and/or climb mechanisms and was found to corr elate with the decrease of the hematite reactivity. (C) 1996 Academic Press, Inc.