Complementary and Depth‐Dependent Structural Insight: A Coupled Operando Raman‐X‐Ray Diffraction Study on Iron Molybdate Catalysts

Coupling Raman spectroscopy with X‐ray diffraction (XRD) provides complementary structural information on solid materials such as heterogeneous catalysts. A laboratory‐based operando 532 nm Raman‐XRD setup combining structural analysis is presented with simultaneous quantitative gas‐phase analysis using infrared (IR) spectroscopy. To avoid the effect of sample heating by laser irradiation and for better statistics in the XRD patterns, the sample is constantly rotated during experiments. The potential of the setup is illustrated for the depth‐dependent structural analysis of iron molybdate (Fe 2 (MoO 4 ) 3 ) catalyst during the oxidative dehydrogenation of ethanol, the latter monitored by IR spectroscopic analysis of the gas phase. When applying reductive conditions to Fe 2 (MoO 4 ) 3 , structural dynamics are observed, accompanied by changes in the catalytic performance. The reduction of Fe 2 (MoO 4 ) 3 to β‐FeMoO 4 is quantified by Rietveld analysis (XRD) and the (sub)surface structure of Fe 2 (MoO 4 ) 3 is probed via molybdate stretching vibrations (Raman). Temporal analysis reveals different timescales of catalyst reduction attributed to a faster (sub)surface contribution (Raman) and a slower bulk response (XRD), enabling depth‐dependent analysis. The presented coupled Raman‐XRD setup offers high versatility for detailed laboratory‐based structural analysis under in situ and operando conditions, providing complementary information on the catalyst's structure and structural dynamics.