Improvement of oxygen storage properties of hexagonal YMnO_3+δ by microstructural modifications

Hexagonal YMnO_3+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 YMnO_3+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 HoMnO_3+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 LnMnO_3+δ phases.