Understanding the structure of catalysts under realistic reaction conditions remains a major challenge in heterogeneous catalysis, particularly in the presence of the strong spatial gradients found in industrial reactors. In this work, the authors introduce iso-potential operando X-ray diffraction (IPO-XRD), a novel methodology that decouples the operation of the reactor from the diffraction measurements while preserving the local reaction conditions.
A compact profile reactor is combined with a standard XRD transmission capillary through iso-potential coupling. In this way, IPO-XRD enables the direct correlation of gas composition, temperature and the evolution of the crystalline phases along catalytic fixed beds.
The approach is demonstrated for the oxidative dehydrogenation of ethane over a catalyst of 30 wt% MoO₃ on γ-Al₂O₃, a system with pronounced and reversible structural dynamics. IPO-XRD captures the spatial transition from oxidised molybdenum phases, Al₂(MoO₄)₃ and molybdenum suboxides, to reduced MoO₂, which is directly linked to the depletion of oxygen in the gas phase. Excellent agreement between the iso-potential measurements and direct reactor measurements at a synchrotron confirms the validity of the concept, while additional experiments on a laboratory diffractometer demonstrate its broader applicability.
By providing spatially resolved structural information under true reaction conditions, IPO-XRD represents a powerful and versatile tool for investigating catalyst dynamics. The method enables an efficient use of synchrotron beamtime and opens new avenues for catalyst characterisation in the laboratory and in industrially relevant environments.
The study is a collaboration between the Institute of Chemical Reaction Engineering at Hamburg University of Technology, the Deutsches Elektronen-Synchrotron DESY and REACNOSTICS GmbH, carried out within the Collaborative Research Centre CRC 1615 (SMART Reactors).
Gökhan Gizer, Hafiz Muhammad Zeeshan, Hemanth Appala, Kavya Chandana Bharatula, Birte Wollak, Oliver Korup, Fernando Igoa Saldaña, Philipp Glaevecke, Ann-Christin Dippel, Michael Schmidt, Raimund Horn (2026). Iso-Potential Operando X-Ray Diffraction (IPO-XRD) - A Novel Approach for Monitoring Catalyst Dynamics in Reactors at Synchrotron and Laboratory Sources. ChemCatChem, 18 (21), e71148.
