Operando XAS/XRD and Raman Spectroscopic Study of Structural Changes of the Iron Molybdate Catalyst during Selective Oxidation of Methanol

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 alpha-MoO3 and Fe-2(MoO4)(3) towards only Fe-2(MoO4)(3). XRD and Raman spectroscopy also showed disappearance of the alpha-MoO3 phase with time on stream. The results evidenced that the alpha-MoO3 component evaporated completely, while the Fe-2(MoO4)(3) component remained stable. This was linked to a decrease in catalytic activity. Further studies unraveled that the rate of alpha-MoO3 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 alpha-MoO3 evaporation needs to be prevented to optimize MoO3-based catalysts for selective oxidation of methanol.