Characterization of Oxygen Storage and Structural Properties of Oxygen-Loaded Hexagonal RMnO_3+δ (R = Ho, Er, and Y)

Abstract

Single-phase polycrystalline samples of stoichiometric RMnO 3+δ (R = Er, Y, and Ho) were achieved in the hexagonal P6 3 cm structure through solid state reaction at ∼1300 °C. Thermogravimetric measurements in oxygen atmospheres demonstrated that samples with the larger Ho and Y show rapid and reversible incorporation of large amounts of excess oxygen (0.3 > δ > 0) at an unusually low temperature range of ∼190− 325 °C, indicating the industrial usefulness of RMnO 3+δ materials for lower cost thermal swing adsorption processes for oxygen separation from air. Further increase of the excess oxygen intake to δ ∼ 0.38 was achieved for all the investigated materials when annealed under high pressures of oxygen. The formation of three oxygen stable phases with δ = 0, 0.28, and 0.38 was confirmed by thermogravimetric measurements, synchrotron X-rays, and neutron diffraction. In situ synchrotron diffraction proved the thermal stability of these single phases and the regions of their creation and coexistence, and demonstrated that the stability of the δ = 0.28 phase increases with the ionic size of the R ion. Structural modeling using neutron powder diffraction for oxygen excess phases describes the formation and details of a large R3c superstructure observed for HoMnO 3.28 by tripling the c-axis of the original parent unit cell. Modeling of the RMnO 3.38 (R = Y and Er) oxygen-loaded phase converged on a structural model consistent with the symmetry of Pca2 1 .

Oxygen Incorporation and Release in Metastable Bixbyite V2O3 Nanocrystals

“…For comparison, HoMnO 3 , a hexagonal manganite material demonstrating oxidation at exceptionally low temperatures, requires an oxidation temperature of 190°C, while heating in pure O 2 gas, and a reduction temperature of 325°C. 25 Bixbyite V 2 O 3 demonstrates nearly the same oxygen storage capacity at transformation (~1.8%) while operating at a lower temperature and ppO 2 . In addition to aiding in the stabilization of the phase, the high surface to volume ratio in bixbyite NCs is expected to result in faster diffusion, and therefore faster switching kinetics, than in the bulk.…”

Oxygen storage properties of hexagonal HoMnO3+δ

“…25 Alternative to materials using the oxygen partial pressure swing process, the hexagonal-type manganites LnMnO 3+d (Ln = smaller lanthanides, e.g. Dy, Ho, Er and Y) were found to operate in the much more promising temperature swing process in pure oxygen or air, as recently documented by Remsen et al 7 and Abughayada et al 8,26 The advantage of such compounds stems from a low and narrow temperature range at which the oxygen exchanges occur, and most importantly, there is no need to use special gas mixtures for the reduction process. However, the kinetics of the redox processes reported to date have been relatively slow, and the achievable OSC has been somewhat lower than that for the layer-ordered perovskites.…”

“…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][8][9][10][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 automotive exhaust systems. [12][13][14][15][16][17][18][19][20][21] The underlying basis of oxygen incorporation/release into/ from OSMs is associated with a large change of the oxygen stoichiometry in the bulk of the material, which is essentially a redox-type process.…”

“…For smaller ones (Ho-Lu, In and Sc), the hexagonal structure (Hex0, d = 0) is formed with layers of 8-fold coordinated Ln 3+ cations and layers of the corner-shared MnO 5 trigonal-bipyramids. 8,26 This five-fold coordination of Mn 3+ causes splitting of the 3d 4 electronic orbitals into three sets: empty a 0 (d z 2) and filled up e 0 (d x 2 Ày 2, d xy ) and e 00 (d xz , d yz ) with high spin states e 02 and e 002 . As a result of orbital filling, the elongation of basal Mn-O bonds and the compression of the apical Mn-O bond are observed.…”

“…Furthermore, similar behavior was also observed for the hexagonal HoMnO 3+d , ErMnO 3+d , and YMnO 3+d , obtained by high oxygen pressure annealing (Hex2, d E 0.41) for which OSC, as well as the temperatures of oxidation and reduction, were reported depending on the ionic radius of R 3+ . 8 However, because the observed kinetics of the oxidation/reduction processes were very slow, the determination of the respective phase diagrams of stability of the stoichiometric and oxygen-loaded phases was not complete. Moreover, such a slow kinetics was responsible for the reduced OSC, which could hinder applicability of hexagonal LnMnO 3+d manganites for use at elevated temperatures where waste heat is frequently available from several industrial processes.…”