The Reduction of Mg-Fe-O and Mg-Fe-Al-O Complex Oxides Studied by Temperature-Programmed Reduction Combined withIN sITU mOSSBAUER sPECTROSCOPY

摘要

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 an X-ray diffraction (XRD). For the Mg-Fe-O sample, two peaks were observed in the TPR profile with peak temperatures at 446 and 196 deg C, respectively, indicating that the reduction of e oxide catalyst proceeded via two stages. Mossbauer spectroscopy and XRD showed that the phases in the sample were MgFe_2O_4 and MgO before TPR. At the first TPR peak (446 deg C), MgFe_2O_4 and MgO before TPR. At the first TPR peak (446 deg C), MgFe_2O_4 was completely reduced to Mg_(1-x)FeO exhibited a doublet with IS = 0.83 mm/s and QS = 0.87 mm/s. The second TPR peak (696 deg C) corresponded to the reduction of Mg_(1x)Fe_xO 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 deg C, respecitively. Mossbauer spectroscopy and XRD showed that the phases in the sample were #alpha#-Fe_2O_3, MgFeAlO_4, and MgO before TPR. The Mossbauer spectrum of MgFeAlO_4 was a doublet with IS = 0/33 mm/s and QS = 0.76 mm/s. At the first TPR peak (506 deg C), #alpha#-Fe_2O_3 was reduced to Fe~(2+) and Fe~0, while MgFeAlO_4 was transformed into Mg_(1x)FeO. The second TPR peak (936 deg C) corresponded to the reduction of Mg_(1x)Fe_xO to metallic Fe~0. Effects of the presence of magnesium and aluminum in the solid solutions on the reduction of iron species were discussed.