This journal is © the Owner Societies 2017 Phys. Chem. Chem. Phys., 2017, 19, 19243--19251 | 19243 Cite this: Phys. Chem. Chem. Phys., 2017, 19, 19243 Oxygen storage properties of hexagonal HoMnO 3+d † Konrad S ´wierczek, ab Alicja Klimkowicz, * c Kengo Nishihara, c Shuntaro Kobayashi, c Akito Takasaki, c Maleeha Alanizy, d Stanislaw Kolesnik, d Bogdan Dabrowski, d Seungho Seong e and Jeongsoo Kang e Structural and oxygen content changes of hexagonal HoMnO 3+d manganite at the stability boundary in the perovskite phase have been studied by X-ray diffraction and thermogravimetry using in situ oxidation and reduction processes at elevated temperatures in oxygen and air. The oxygen storage properties during structural transformation between stoichiometric Hex0 and oxygen-loaded Hex1 phases, transition temperatures and kinetics of the oxygen incorporation and release are reported for materials prepared by the solid-state synthesis and high-impact mechanical milling. Long-term annealing experiments have shown that the Hex0 (d = 0) - Hex1 (d E 0.28) phase transition is limited by the surface reaction and nucleation of the new phase for HoMnO 3+d 15MM. The temperatures of Hex0 2 Hex1 transitions have been established at 290 1C and 250 1C upon heating and cooling, respectively, at a rate of 0.11 min À1 , also indicating that the temperature hysteresis of the transition could possibly be as small as 10 1C in the equilibrium. Ball-milling of HoMnO 3+d has only a small effect on improving the speed of the reduction/oxidation processes in oxygen, but importantly, allowed for considerable oxygen incorporation in air at a temperature range of 220–255 1C after prolonged heating. The Mn 2p XAS results of the Mn valence in oxygen loaded samples support the oxygen content determined by the TG method. The magnetic susceptibility data of the effective Mn valence gave inconclusive results due to dominating magnetism of the Ho 3+ ions. Comparison of HoMnO 3+d with previously studied DyMnO 3+d indicates that a tiny increase in the ionic size of lanthanide has a huge effect on the redox properties of hexagonal manganites and that practical properties could be significantly improved by synthesizing the larger average size (Y,Ln)MnO 3+d manganites. 1. Introduction Usage of pure or highly enriched oxygen gas is nowadays ubiquitous in various industrial technologies because of increased efficiency and/or the uniqueness of chemical processes requiring it. Enriched oxygen gas is essential, among others, in the production processes of steel and non-ferrous metals, chemicals, petro- chemicals, glass, ceramics, and paper, and also it is utilized in healthcare and medical treatments. 1 At a large scale, the oxygen is produced by cryogenic methods, which involve large capital investment, require specific conditions of pressure and temperature, and consequently, are considered highly energy-consuming and costly. 2 Alternative methods rely on the separation of air components via pressure- and/or temperature-driven adsorption processes 3,4 and the emerging membrane-related technologies. 5,6 Interestingly, with the recent progress in the field of so-called oxygen storage materials (OSMs), several novel compounds appear to be capable of economical production of oxygen by using thermal or pressure swing-type reactions. 7–11 Moreover, depending on their intrinsic properties, these OSMs are also considered for implementation in many important existing and emerging technological processes: for example, inert gas purification; solar water splitting; non-aerobic oxidation including flameless combustion (e.g., synthesis gas production); high-temperature production of steel, glass or plastic production that requires high-purity oxygen; oxy-fuel and chemical looping combustion processes; solid oxide fuel cell technology and the three-way catalytic converters for auto- motive exhaust systems. 12–21 a AGH University of Science and Technology, Faculty of Energy and Fuels, Department of Hydrogen Energy, al. A. Mickiewicza 30, 30-059 Krakow, Poland b AGH Centre of Energy, AGH University of Science and Technology, ul. Czarnowiejska 36, 30-054 Krakow, Poland c Shibaura Institute of Technology, Department of Engineering Science and Mechanics, 3-7-5 Toyosu, Koto-ku, 135-8548 Tokyo, Japan. E-mail: klimkowicz.alicja.ewa.j2@shibaura-it.ac.jp d Department of Physics, Northern Illinois University, DeKalb, IL 60115, USA e Department of Physics, The Catholic University of Korea (CUK), Bucheon 14662, Korea † Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cp03556j Received 26th May 2017, Accepted 26th June 2017 DOI: 10.1039/c7cp03556j rsc.li/pccp PCCP PAPER Open Access Article. Published on 12 July 2017. Downloaded on 10/14/2021 1:34:55 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue