Here, we report the remarkable oxygen intake/release capability of a double perovskite BaYMn2O5+δ. This oxide rapidly stores/releases a large amount of oxygen (>3.7 wt %) at moderate temperatures in a perfectly reversible manner. The oxygen intake/release behaviors of BaYMn2O5+δ are clearly beyond those of any conventional oxides in terms of the magnitude and sharpness of the processes. It also appeared that this oxide exhibits a significant catalytic activity for flameless combustions of hydrocarbons, presumably owing to its oxygen-storage ability.

Remarkable Oxygen Intake/Release Capability of BaYMn2O5+δ: Applications to Oxygen Storage Technologies

Polycrystalline samples of hexagonal P63cm (space group No. 185) Dy1–xYxMnO3+δ were synthesized over complete solubility ranges under reducing conditions with oxygen partial pressures of 5 ppm to air at high temperatures (1200–1400 °C). All samples displayed unusually large, reversible oxygen content excess (0 ≤ δ ≤ 0.35) at low temperatures of ∼200–400 °C under air and oxygen atmospheres in thermogravimetric measurements. During heating, stoichiometric samples showed rapid uptakes of oxygen at 200–300 °C and equally fast releases of oxygen at 275–375 °C when materials transformed back to the stoichiometric P63cm phase. Upon slow cooling from the stoichiometric P63cm phase, all samples displayed rapid uptakes of oxygen at 200–350 °C. Similar changes were observed as a function of oxygen partial pressure at constant temperature. Increased, reversible changes in oxygen content were also achieved by high-pressure annealings in oxygen and hydrogen reduction at 400 °C. Thermogravimetric and X-ray diffraction m...

Synthesis and Oxygen Storage Capacities of Hexagonal Dy1–xYxMnO3+δ

Formation of Co3+ octahedra and tetrahedra in YBaCo4O8.1

The oxygen storage capability was investigated for Ca2(AlxMn1–x)2O5+δ (0.50 ≤ x ≤ 0.67) with a Brownmillerite-type structure. This oxide can store/release a large amount of excess oxygen (∼3.0 wt %) topotactically in response to variations in temperature and the surrounding atmosphere in a highly reversible manner. The capacity and response of oxygen storage are remarkable only in the vicinity of x = 0.50, that is, Ca2AlMnO5+δ, and rapidly deteriorated as the Al content increases. Owing to the high sensitivity in terms of oxygen nonstoichiometry, Ca2AlMnO5+δ exhibits oxygen intake/release ability when temperature swing between 500 and 700 °C is applied. With this characteristic feature of this oxide, a facile method for oxygen enrichment is demonstrated.

Oxygen Storage Capability of Brownmillerite-type Ca2AlMnO5+δ and Its Application to Oxygen Enrichment

Here we show that the low-temperature oxygen-storage capability of RBaCo4O7+δ (R = Y, Dy, Ho, Er, Tm, Yb, and Lu) depends on the size of the rare earth constituent R. For all these R constituents the RBaCo4O7+δ phase can be charged and discharged with large amounts of oxygen in a narrow temperature range below 400 °C. With decreasing ionic radius, r(RIII), the oxygen-release temperature (TR) decreases while the temperature where the phase decomposes (TD) increases such that the “safety window” between TR and TD widens. On the other hand, the maximum amount of excess oxygen taken by the RBaCo4O7+δ lattice in 1 atm O2 decreases from δ ≈ 1.4 (for R = Dy) to ∼1.0 (for R = Lu) with decreasing r(RIII). Hence the optimum oxygen-storage characteristics are found about R = Tm in terms of r(RIII). We also discuss the crystal structures of the oxygen-annealed RBaCo4O7+δ samples.

R-Site Substitution Effect on the Oxygen-Storage Capability of RBaCo4O7+δ

An uncommon oxygen absorption/desorption behavior is reported for the cation-stoichiometric cobalt oxide, YBaCo4O7, structurally composed of two kinds of layers of corner-sharing CoO4 tetrahedra. We have found that YBaCo4O7+δ absorbs and desorbs oxygen up to δ ≈ 1.5 in a narrow temperature range below 400 °C. The oxygen uptake/release process is highly reversible, being controlled by both temperature and oxygen partial pressure. Such a large low-temperature oxygen-content tunability is of great promise in regard to applications related to, for example, oxygen storage. Materials with similar characteristics are, to a large degree, lacking today.

S. Otani

Papers

Oxygen Nonstoichiometry in YBaCo4O7+δ: Large Low-Temperature Oxygen Absorption/Desorption Capability