The fate of Fe3+ ions in the system {AlO(OH)-xerogel/Fe-compounds} after mechanical activation and different thermal treatments studied by Mössbauer, ESR spectroscopy and thermal analysis

Co-milling of AlO(OH) xerogels with various iron(III) compounds such as α-Fe2O3 or K3[Fe(CN)6] has been used for mechanochemical activation leading to Fe/Al oxide phases in coexistence with the activated main component. The results obtained allow a deeper insight into the chemical processes occurring during activation, doping, and thermal treatment in atmospheres of different chemical activity (H2O; N2; O2/N2; H2/N2). Evidence will be given for Fe3 + species being able to form suitable crystallization germs and to lower the crystallization temperature. One can distinguish between the incorporation of Fe3 + ions into the alumina matrix at the crystallite growth (e.g. on Fe2O3-like seeds) and the incorporation of Fe3 + ions during the last steps of crystallization. These last-mentioned Fe3 + ions are provided by coexisting Fe/Al oxide phases. In contrast to alumina matrices equilibrated at high temperatures (e.g. at 1,200–1,500°C), the matrices formed in situ are able to incorporate Fe3 + ions by diffusion at temperatures of ∼900–1,100°C in a short time regime.