Hexagonal YMnO3+δ is shown to be an effective temperature-swing oxygen storage material working at low temperatures (150–300 °C) in pure oxygen if adequately processed or obtained having sub-micrometer primary particles with limited number of big agglomerates. A substantial increase of a practical oxygen storage capacity is observed for a sample synthesized by a solid-state method, which was subjected to a high impact mechanical milling. However, even better properties can be achieved for the sol-gel technique-produced YMnO3+δ. The reversible incorporation and release of the oxygen is associated with a structural transformation between stoichiometric YMnO3 (Hex0) phase and a mixture of oxygen-loaded Hex1 with δ ≈ 0.28 and Hex2 with δ ≈ 0.41 phases, as documented by in situ structural X-ray diffraction studies, supported by thermogravimetric experiments. Contrary to HoMnO3+δ, it was not possible to obtain single phase Hex1 material in oxygen, as well as to oxidize YMnO3 in air. Results confirm crucial role of the ionic size of rare earth element Ln on the oxygen storage-related properties and stability of the oxygen-loaded LnMnO3+δ phases.
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