Identification of the chemical state of Fe in barium hexaaluminate using Rietveld refinement and 57Fe Mössbauer spectroscopy

The catalytic activity of hexaaluminate is closely related to the chemical state of substituted active metal ions. In this paper, the mechanism of stabilization of Fe ions in βI-Al2O3 and magnetoplumbite-type Fe-substituted Ba hexaaluminates was proposed at the molecule level on the basis of X-ray diffraction, Rietveld refinement, and Mössbauer spectroscopy. Fe3+ ions originated from oxidic entities dispersed on Ba-modified γ-Al2O3 mainly entered into the sites in the loosely packed mirror plane of the hexaaluminates. In particular, Fe3+ ions at low concentration preferentially occupied the tetrahedral Al(5) sites of the βI-Al2O3 phase, while Fe3+ ions at high concentration mainly occupied the trigonal bipyramidal Al(5) and octahedral Al(3) sites in the magnetoplumbite phase. Meanwhile, tetrahedral Fe3+ ions in the intermediate spinel-type BaAl2O4 phase preferentially entered into the tetrahedral Al(2) sites in the spinel block of hexaaluminates. Fe ions in the Al(5) sites of βI-Al2O3 and the Al(3) sites of magnetoplumbite phase were highly active for N2O decomposition.