Coupled Operando Mössbauer‐Raman Spectroscopic Analysis of Iron Molybdate Catalysts During Oxidative Dehydrogenation

Coupling operando synchrotron-based Mössbauer spectroscopy (SMS) with operando Raman spectroscopy provides complementary structural information during catalytic reactions. We demonstrate the potential of this approach for the oxidative dehydrogenation of ethanol to acetaldehyde over an iron molybdate catalyst. The coupled SMS-Raman approach enables the simultaneous detection of bulk ($^{57}$Fe Mössbauer) and (sub)surface (Raman) structural changes and their correlation with the catalytic conversion and selectivity recorded by online mass spectrometry. Based on the high photon flux at the synchrotron and absence of a non-resonant background, SM spectra could be recorded with high time resolution, enabling simultaneous analysis by SMS and Raman spectroscopy. Exposure to reducing conditions (absence of gaseous oxygen), while the reaction to acetaldehyde is maintained, revealed a significantly faster response of Raman signals from crystalline phases (MoO$_3$ , Fe$_2$ (MoO$_4$)$_3$) in comparison to SMS, which originates from the higher surface sensitivity of Raman spectroscopy. SMS as a bulk technique monitors the reduction of iron, leading to β-FeMoO$_4$ formation under reductive conditions. Our results show that the application of coupled (sub)surface and bulk operando methods strongly facilitates a detailed understanding of the structure-activity relationship of bulk metal oxide catalysts in selective oxidation reactions, including oxygen transport from the bulk to the catalyst surface.