Phase Relations in the System Fe2O3–Cr2O3–TiO2 between 1000 and 1300°C and the Stability of (Cr,Fe)2Tin-2O2n-1 Crystallographic Shear Structure Compounds

In a H2/N2 atmosphere, the reduction processes of Mg–Fe–O and Mg–Fe–Al–O complex oxide catalysts, derived from their hydrotalcite precursors, were studied by means of temperature-programmed reduction (TPR). In particular, the reduction process of the Mg–Fe–O samples with a Mg/Fe ratio of 1/1 and Mg–Fe–Al–O with a Mg/Fe/Al ratio of 2/1/1 were investigated in detail in combination with in situ M?ssbauer spectroscopy and X-ray diffraction (XRD). For the Mg–Fe–O sample, two peaks were observed in the TPR profile with peak temperatures at 446 and 696°C, respectively, indicating that the reduction of the oxide catalyst proceeded via two stages. M?ssbauer spectroscopy and XRD showed that the phases in the sample were MgFe2O4 and MgO before TPR. At the first TPR peak (446°C), MgFe2O4 was completely reduced to Mg1-xFexO. The M?ssbauer spectrum of Mg1-xFexO exhibited a doublet with IS=0.83 mm/s and QS=0.87 mm/s. The second TPR peak (696°C) corresponded to the reduction of Mg1-xFexO to Fe0. For the Mg–Fe–Al–O sample, two peaks were also observed in the TPR profile with temperatures at 506 and 936°C, respectively. M?ssbauer spectroscopy and XRD showed that the phases in the sample were -Fe2O3, MgFeAlO4, and MgO before TPR. The M?ssbauer spectrum of MgFeAlO4 was a doublet with IS=0.33 mm/s and QS=0.76 mm/s. At the first TPR peak (506°C), -Fe2O3 was reduced to Fe2+ and Fe0, while MgFeAlO4 was transformed into Mg1-xFexO. The second TPR peak (936°C) corresponded to the reduction of Mg1-xFexO to metallic Fe0. Effects of the presence of magnesium and aluminum in the solid solutions on the reduction of iron species were discussed.