Real-time tracking of iron oxide particle reduction with hydrogen in a fluidized bed reactor

Reduction of micron-sized Fe$_2$O$_3$ particles with hydrogen was investigated in a fluidized bed reactor (FBR), designed and developed as part of this study and equipped with X-ray transparent glassy carbon windows. Simultaneous in situ X-ray absorption spectroscopy (XAS) at a synchrotron source and quadrupole mass spectrometry (QMS) enabled correlated real-time monitoring of solid-phase transformations and gas-phase composition during isothermal reduction at 773 K in a 1:1 H$_2$/N$_2$ atmosphere. Time-resolved XAS revealed a sequential transformation from Fe$_2$O$_3$ to Fe$_3$O$_4$ and metallic Fe, while QMS detected a corresponding increase and subsequent decay of H$_2$O. A systematic temporal offset between solid- and gas-phase signals indicates that the diagnostics probe slightly different particle ensembles: XAS captures particles present in the freeboard, whereas QMS reflects the integrated H$_2$O formation from the entire reacting bed. Temporary trapping of H$_2$O within the evolving pore network, previously reported for reducing iron oxide particles, may further contribute to the delayed gas-phase response. No evidence for a persistent FeO phase was observed under the investigated conditions. ... mehrThe reduction proceeds rapidly during the initial stage and subsequently slows down, likely associated with the formation of passivating Fe/Fe$_3$O$_4$ layers that limit hydrogen access and hinder the transport of water from the particle core. The present work establishes an experimental framework for synchrotron operando studies of iron oxide reduction in fluidized bed reactors, enabling correlated XAS-QMS measurements and providing the basis for upcoming systematic investigations of particle properties, morphologies and operating parameters.
Novelty and significance statement: Braun et al. investigated quasi-single iron oxide reduction in inert-particle-diluted fixed bed reactor cells under low hydrogen concentrations and varying temperatures. While providing kinetic insights, these experiments were limited to the solid phase and did not capture the multi-particle interactions, improved temperature control, or enhanced mass transport characteristic of fluidized bed reactors (FBRs). This work presents real-time tracking of iron oxide reduction with hydrogen in an X-ray absorption spectroscopy (XAS)-compatible FBR designed in this study and operated under practically relevant isothermal conditions. The combination of in situ XAS and quadrupole mass spectrometry (QMS) enables, for the first time, simultaneous monitoring of solid- and gas-phase transformations and reveals their correlated evolution during reduction. The XAS-QMS approach addresses the scarcity of operando studies in FBRs and is transferable to different reaction conditions, particle properties, and iron ores, extending its relevance from energy storage to steelmaking, materials, and catalysis.