THE REDUCTION BEHAVIOR OF FE-MO-O CATALYSTS STUDIED BY TEMPERATURE-PROGRAMMED REDUCTION COMBINED WITH IN-SITU MOSSBAUER-SPECTROSCOPY AND X-RAY-DIFFRACTION

The reduction processes of Fe-Mo-O catalysts with different Fe/Mo atom ic ratios were studied by means of temperature-programmed reduction (T PR). In particular, the reduction process of a sample with ail Fe/Mo r atio of 0.29 was investigated in detail in combination with in situ Mo ssbauer spectroscopy (MBS) and X-ray diffraction (XRD). Five peaks are observed in the TPR profile of this sample with peak temperatures at 853, 910, 952, 1014, and 1233 K, respectively, indicating that the red uction of the catalyst proceeds via five stages. MBS and XRD showed th at the phases in the sample are MoO3 and Fe-2(MoO4)(3) before TPR. At the first TPR peak (853 K), Fe-2(MoO4)(3) was completely transformed i nto beta-FeMoO4 and Mo4O11, while MoO3 was not affected. The beta-FeMo O4 phase that formed can convert to alpha-FeMoO4 when the reduced samp le is exposed to air at room temperature. The second TPR peak (910 K) corresponds to the reduction of MoO3 to MoO2. At the third TPR peak (9 52 K), Mo4O11 was reduced to MoO2 and most of the beta-FeMoO4 was tran sformed to Fe2Mo3O8 and Fe3O4. The Mossbauer spectrum of FeMo3O8 can b e fitted with two doublets (IS = 0.90 mm/sec, QS = 0.58 mm/sec and IS = 1.03 mm/sec, QS = 1.05 mm/sec). It is interesting to note that at th is stage some Fe2+ ions were reoxidized to Fe3+, and Fe3O4 is formed, probably due to the reaction between Mo6+ and Fe2+ in beta-FeMoO4. At the fourth TPR peak (1014 K), beta-FeMoO4, was completely converted an d the Fe-Mo alloy (a singlet in the Mossbauer spectrum with IS = -0.21 mm/sec) was formed, resulting from the reduction of MoO2 and Fe3O4 or Fe2Mo3O8. At the last peak (1233 K), all the metal oxides (MoO2 and F e3O4 or Fe2Mo3O8) were reduced to metals. The products detected are Fe 3Mo alloy and metallic molybdenum. (C) 1995 Academic Press, Inc.