The formation and physicochemical characterization of Al2O3-doped manganese ferrites

Mn/Fe mixed oxide solids doped with Al2O3 (0.32–1.27 wt.%) were prepared by impregnation of manganese nitrate with finely powdered ferric oxide, then treated with different amounts of aluminum nitrate. The obtained samples were calcined in air at 700–1000 °C for 6 h. The specific surface area (SBET) and the catalytic activity of pure and doped precalcined at 700–1000 °C have been measured by using N2 adsorption isotherms and CO oxidation by O2. The structure and the phase changes were characterized by DTA and XRD techniques. The obtained results revealed that Mn2O3 interacted readily with Fe2O3 to produce well-crystallized manganese ferrite (MnFe2O4) at temperatures of 800 °C and above. The degree of propagation of this reaction increased by Al2O3-doping and also by increasing the heating temperature. The treatment with 1.27 wt.% Al2O3 followed by heating at 1000 °C resulted in complete conversion of Mn/Fe oxides into the corresponding ferrite phase. The catalytic activity and SBET of pure and doped solids were found to decrease, by increasing both the calcination temperature and the amount of Al2O3 added, due to the enhanced formation of MnFe2O4 phase which is less reactive than the free oxides (Mn2O3 and Fe2O3). The activation energy of formation (ΔE) of MnFe2O4 was determined for pure and doped solids. The promotion effect of aluminum in formation of MnFe2O4 was attributed to an effective increase in the mobility of reacting cations.