In situ electron microscopy of LaNiO3 transformation during reduction and methane dry reforming.

By in situ electron microscopy we show the intermediate phase formation during decomposition of LaNiO3, a model methane dry reforming (DRM) catalyst, under different gas atmospheres. By combining dark- and bright-field imaging with secondary-electron contrast and operando electronic structure characterization, we localize the coupled formation of Ni(O) particles and La2NiO4 as key intermediates under DRM operation preceding full decomposition into metallic Ni and La2O3. Reduced and bar-level reactant partial pressures enable detailed observation of early decomposition stages. Ni particles formed in vacuum or DRM mixtures undergo transient surface oxidation by lattice oxygen, while highly dynamic Ni particles result from hydrogen reduction. At high temperatures, concurrent Ni formation, LaNiO3-to-La2NiO4 transitions, Ostwald ripening, and particle fragmentation/agglomeration govern the dynamics. DRM exposure of reduction-formed Ni leads to rapid oxidation to sintering-resistant NiO with size-dependent dissolution/agglomeration characteristics. As CO2 cannot directly oxidize Ni here, oxygen migration from the perovskite bulk drives this transient, DRM-detrimental NiO formation.