The reduction of Mg-Fe-O and Mg-Fe-Al-O complex oxides studied by temperature-programmed reduction combined with in situ Mossbauer spectroscopy

In a H-2/N-2 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 Mossbauer 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 degreesC, respectively, indicating that the reduction of the oxide catalyst proceeded via two stages. Mossbauer spectroscopy and XRD showed that the phases in the sample were MgFe2O4 and MgO before TPR. At the first TPR peak (446 degreesC), MgFe2O4 was completely reduced to Mg1-xFexO. The Mossbauer spectrum of Mg1-xFexO exhibited a doublet with IS = 0.83 mm/s and QS = 0.87 mm/s. The second TPR peak (696 degreesC) corresponded to the reduction of Mg1-xFexO to Fe-0. For the Mg-Fe-Al-O sample, two peaks were also observed in the TPR profile with temperatures at 506 and 936 degreesC, respectively. Mossbauer spectroscopy and XRD showed that the phases in the sample were alpha -Fe2O2, MgFeAlO4, and MgO before TPR. The Mossbauer spectrum of MgFeAlO4, was a doublet with IS = 0.33 mm/s and QS = 0.76 mm/s. At the first TPR peak (506 degreesC), alpha -Fe2O3 was reduced to Fe2+ and Fe-0, while MgFeAlO4 was transformed into Mg1-xFexO. The second TPR peak (936 degreesC) corresponded to the reduction of Mg1-xFexO to metallic Fe-0. Effects of the presence of magnesium and aluminum in the solid solutions on the reduction of iron species were discussed. (C) 2001 Academic Press.