Towards efficient oxygen separation from air: Influence of the mean rare-earth radius on thermodynamics and kinetics of reactivity with oxygen in hexagonal Y_1-xR_xMnO_3+δ

It is documented that the mean radius r¯ of rare-earth cations occupying Y_1-xR_x sublattice in Y_1-xR_xMnO_3+δ hexagonal oxides plays a decisive role in terms of thermodynamics and kinetics of reactivity of the materials with oxygen, and consequently, influences strongly the oxygen storage performance in thermal swing processes conducted in oxygen and air. Y_1-xR_xMnO_3+δ samples with designed r¯ being close to the critical one, at the border of stability between hexagonal- and perovskite-type phases, can reversibly incorporate/release significant amounts of oxygen in pure O₂ or air atmospheres, at the moderate temperatures on the order of 200–300 °C. Characteristic temperatures of oxidation and reduction are dependent on r¯, therefore, it is possible to adjust conditions of the temperature swing operation by the chemical doping in Y_1-xR_xMnO_3+δ with larger rare-earth elements. Crucial from a practical point of view, an increase of the oxidation temperature in such compounds greatly enhances the speed of the oxidation process (20 °C increase can reduce half-time of oxidation twice), which is found to be the limiting factor concerning the performance. Based on the comprehensive studies of the physicochemical properties of Y_1-xR_xMnO_3+δ, the optimized Y_0.95Pr_0.05MnO_3+δ composition is proposed, doped only with a small amount of more expensive praseodymium. The material exhibits excellent oxygen storage-related properties and is able for the effective production of oxygen in air by the thermal swing process, utilizing medium-/low-temperature industrial waste heat.