Molybdenum loss, phase transformations and catalytic performance of an iron molybdate catalyst.

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Combination of in situ multi-edge X-ray absorption spectroscopy at the Mo K- and Fe K-edges in combination with X-ray diffraction successfully uncovered structural dynamics and phase transformations of an iron molybdate catalyst during redox cycling.

The structural changes of an iron molybdate/molybdenum oxide (Mo/Fe = 2.0) catalyst for the selective oxidation of methanol to formaldehyde were studied using combined operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) as well as operando Raman spectroscopy. Under operating conditions, the Mo K-edge XANES spectra showed a transition from a mixture of α-MoO 3 and Fe 2 (MoO 4 ) 3 towards only Fe 2 (MoO 4 ) 3 . XRD and Raman spectroscopy also showed disappearance of the α-MoO 3 phase with time on stream. The results evidenced that the α-MoO 3 component evaporated completely, while the Fe 2 (MoO 4 ) 3 component remained stable. This was linked to a decrease in catalytic activity. Further studies unraveled that the rate of α-MoO 3 evaporation increased with increasing MeOH concentration, decreasing O 2 concentration and increasing temperature. The simultaneous measurements of catalytic activity and spectroscopy allowed to derive a structure-activity relationship showing that α-MoO 3 evaporation needs to be prevented to optimize MoO 3 -based catalysts for selective oxidation of methanol.

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