Operando Spatially and Time-Resolved Reduction of Hollow Polycrystalline NiO: How to Promote Ni Nanoparticle Redispersion

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Oxidation–reduction treatments are widely used to rejuvenate sintered metal catalysts, yet the specific role of the reduction step in driving nanocatalyst redispersion remains poorly understood. In this work, we present the first spatially and time-resolved operando study of Ni redispersion via in situ isothermal reduction of hollow, polycrystalline NiO at 400–600 °C in 1 bar 20%H 2 /N 2 . By combining in situ environmental transmission electron microscopy (ETEM) with tailored semantic segmentation, we extracted grain-level NiO reduction kinetics and found that a moderate 450 °C reduction slows and desynchronizes transformation across neighboring NiO grains─thereby promoting Ni fragmentation even from intermediate NiO morphologies not conducive to redispersion under high-temperature reduction. These new insights reveal the dual need of spatial separation (hollow oxide formation) and temporal offset (slower, asynchronous reduction) for effective metal redispersion, providing mechanistic guidance for the rational design of oxidation–reduction regeneration protocols.