The iron oxidation state in Mg-Al-Fe mixed oxides derived from layered double hydroxides: An XPS study

Mixed oxides with large surface area and high thermal stability can be obtained by thermal treatment of the layered double hydroxides (LDH). Mg-Al-Fe mixed oxide samples with varying Mg/Al ratio and 5 mol.% of Fe were prepared in this way and the iron oxidation state (Feox) in these compounds was studied by X-ray photoelectron spectroscopy (XPS), using a calibration based on the relation of Feox to the splitting between the O 1s and Fe 2p3/2 centroids. The XPS results confirm Fe3+ as a dominant oxidation state in the studied mixed oxides. A vacuum-induced reduction of iron in the Fe2O3 and Mg-Al-Fe oxide samples has been observed and an influence of the Mg:Al ratio on this effect in mixed oxides has been detected. The role of the local variations of the electron density distribution in the close neighbourhood of the surface oxygen atoms in the mixed oxides in the reduction processes is discussed.