Oxidation Kinetics of Nanocrystalline Hexagonal RMn_1-TiO₃ (R = Ho, Dy)

Hexagonal manganites, RMnO₃ (R = Sc, Y, Ho-Lu), are potential oxygen storage materials for air separation due to their reversible oxygen storage and release properties. Their outstanding ability to absorb and release oxygen at relatively low temperatures of 250-400 °C holds promise of saving energy compared to current industrial methods. Unfortunately, the low temperature of operation also implies slow kinetics of oxygen exchange in these materials, which would make them inefficient in applications such as chemical looping air separation. Here, we show that the oxidation kinetics of RMnO₃ can be improved through Ti⁴⁺-doping as well as by increasing the rare earth cation size. The rate of oxygen absorption of nanocrystalline RMn_1-TiO₃ (R = Ho, Dy; x = 0, 0.15) was investigated by thermogravimetric analysis, X-ray absorption near-edge structure, and high-temperature X-ray diffraction (HT-XRD) with in situ switching of atmosphere from N₂ to O₂. The kinetics of oxidation increases for larger R and even more with Ti⁴⁺ donor doping, as both induce expansion of the ab-plane, which reduces the electrostatic repulsion between oxygen in the lattice upon oxygen ion migration. Surface exchange rates and activation energies of oxidation were determined from changes in lattice parameters observed through HT-XRD upon in situ switching of atmosphere.