Temperature Monitoring of Iron Molybdate Catalysts during ODH Reactions Using Coupled Operando Raman/XRD Analysis

ABSTRACT

Oxidative dehydrogenation (ODH) reactions represent highly exothermic reactions. Due to their exothermicity, temperature gradients and hot spots are formed within the catalyst bed that may cause overoxidation, leading to lower selectivity, and present challenges regarding heat management. For a detailed understanding of ODH reactions, including structure–property relations, new approaches enabling in situ monitoring of the catalyst temperature are needed. Using lab‐based coupled operando Raman/X‐ray powder diffraction (XRD) analysis, we investigated iron molybdate (Fe ₂ (MoO ₄ ) ₃ ) catalysts during ethanol ODH regarding temperature‐dependent structural dynamics. To identify heat‐induced effects, we added hexagonal boron nitride (h‐BN) as an internal standard and performed depth‐dependent temperature monitoring, revealing a correlation of (sub‐)surface and bulk temperatures with ethanol conversion rates, highlighting the relevance of catalyst heating at high conversions. Using this newly developed in situ thermometer, the temperature of the catalyst can be probed within the penetration depths of the two applied methods. For the ethanol ODH over iron molybdate, no temperature gradient was detected, showing that the surface and the bulk are thermally equilibrated. Our findings demonstrate the importance of considering heat effects during ODH reactions for technical applications but also for the analysis and interpretation of operando structural data in the context of mechanistic studies.