Release Mechanism of Double-Perovskite Type

Article pubs.acs.org/JPCC

Oxygen Intake/Release Mechanism of Double-Perovskite Type BaYMn2O5+δ (0 ≤ δ ≤ 1) Teruki Motohashi,* Taku Ueda, Yuji Masubuchi, and Shinichi Kikkawa Faculty of Engineering, Hokkaido University, N13, W8, Kita-ku, Sapporo 060-8628, Japan S Supporting Information *

ABSTRACT: The oxygen intake/release processes of double-perovskite type BaYMn2O5+δ (0 ≤ δ ≤ 1) were investigated from the viewpoints of crystal structure and reaction kinetics. The X-ray diffraction study on the partially oxidized/reduced BaYMn2O5+δ products has revealed a clear indication of successive phase changes among three distinct forms with δ ≈ 0, 0.5, and 1: the oxygen intake/release processes could proceed with nucleation followed by a growth of the generated phase. It has also appeared that the oxygen intake reaction is controlled by surface reaction at 348 ≤ T ≤ 403 °C, while the oxygen release process is surface-reactioncontrolled at 425 ≤ T ≤ 550 °C and nucleation-process-controlled at 600 ≤ T ≤ 750 °C. The activation energy for the oxygen intake process at 348 ≤ T ≤ 403 °C is Ea = 0.27 eV and is much smaller than those reported for any other perovskite-type oxides indicative of great affinity to oxygen molecules.

1. INTRODUCTION Nonstoichiometric oxides with reversible oxygen intake/release capability are called oxygen-storage materials (OSMs).1−11 OSMs have attracted increased interest for their technological importance which derives from a huge variety of oxygen-related chemical reactions in energy generations and environmental protection. The best-known OSM is CeO2−ZrO2 solid solution, the so-called CZ that has been widely used in modern three-way catalysts for the effective removal of NOx, CO, and hydrocarbons from automobile exhausts.1,12 Meanwhile, a new class of OSMs with excellent characteristics is highly desirable to realize oxygen-related future applications. We recently reported the remarkable oxygen intake/release characteristics of the manganese oxide BaYMn2O5+δ.7 This oxide rapidly stores/releases a large amount of oxygen at temperatures below 500 °C. The oxygen storage takes place topotactically in a perfect reversible manner even after 100 cycles making this oxide a potential candidate for oxygen storage applications. BaYMn2O5+δ is categorized as an A-site ordered double-perovskite which contains an arrangement of smaller yttrium and larger barium ions in separate layers at the perovskite A-site.13,14 The oxygen site within the yttrium plane is readily filled/unfilled depending on temperature or the surrounding atmosphere. The resultant oxygen nonstoichiometry ranges from δ = 0 to 1 corresponding to 3.85 wt % of oxygen-storage capacity (OSC; the amount of oxygen stored in the crystal lattice), which is much larger than the theoretically expected value for CZ, 2.8 wt %. BaYMn2O5+δ is found to exhibit a significant catalytic activity for flameless combustions of hydrocarbons.7 Also, this oxide may be applicable to oxygen separation and inert gas © XXXX American Chemical Society

purifications owing to its great affinity to oxygen molecules. To contribute to such practical applications, the enhancement of oxygen intake/release kinetics is indispensable. Our previous thermogravimetric study evidenced a crucial role of particle size in the oxygen intake/release characteristics.8 While the oxygen intake process at 500 °C was always fast and was completed only within 20 s, the time required for complete oxygen release was effectively shortened for fine powders prepared by a wetchemical route. This result emphasizes the importance of particle size control of BaYMn2O5+δ; nevertheless, the origin of the enhanced oxygen intake/release kinetics is still unclear and needs to be elucidated. Whereas the oxygen intake/release reactions are simply expressed as BaYMn2O5 (δ = 0) + 1/2O2 ↔ BaYMn2O6 (δ = 1), the mechanism underlying the actual processes may be more complicated. In fact, it was reported that BaYMn2O5+δ tends to crystallize in three stable forms with distinct oxygen contents, δ = 0, 0.5, and 1 (O5, O5.5, and O6 forms, respectively).14,15 These three forms are differentiated by the atomic arrangement within the yttrium plane as schematically drawn in Figure 1. Taking into account this characteristic feature, the oxygen intake/release processes are likely to proceed involving phase separation into the three forms rather than a continuous phase with the spatial gradient of oxygen content. Nevertheless, it has remained unclear which model is appropriate in reality. Received: February 25, 2013 Revised: May 24, 2013

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dx.doi.org/10.1021/jp401965b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Schematic illustration of the crystal structures of BaYMn2O5+δ with oxygen contents δ = 0, 0.5, and 1 (O5, O5.5, and O6 phases, respectively). The illustration was drawn with VESTA software16 on the basis of the structural models reported in the literature. The unit cell of each structure is represented by a black frame.

2.2. Preparation and Characterization of Partially Oxidized/Reduced Products. To study the structural changes upon oxygen intake/release processes, partially oxidized/reduced BaYMn2O5+δ products were prepared and were characterized by the following procedures. Approximately 40 mg of the δ = 0 product was heated with a heating rate of 10 °C/min and was kept at 350 °C in flowing 5% H2/95% Ar gas mixture until the sample reached its equilibrium state. Then, the gas flow was switched to 1% O2/99% Ar gas mixture for gradual oxygenation. This heat treatment was carried out in a commercial thermobalance (TG; Rigaku TG8120GH) with a gas flow rate of 100 mL/min. The weight gain due to oxygen intake was monitored with TG until the sample weight reached a target value, and then the sample was rapidly cooled to room temperature (>50 °C/min) in flowing N2 gas. Four partially oxidized products with δ = 0.21, 0.38, 0.59, and 0.81 were successfully obtained. Also, products with δ ≈ 1/3 were prepared through oxidation under highly O2-diluted atmospheres (0.01% and 0.1% O2/N2 balanced) to study the effect of oxygen partial pressure on the resultant phases. A similar procedure was carried out for the δ = 1 product heated at 450 °C in flowing 5% H2/95% Ar gas mixture to prepare five partially reduced products with δ = 0.78, 0.74, 0.59, 0.40, and 0.19. The δ values estimated from the weight changes were in good agreement with those determined by iodometric titration (see Supporting Information). The phase identification was done by means of XRD (Rigaku Ultima IV) for the resultant partially oxidized/reduced products as well as for the fully reduced (δ = 0) and fully oxidized (δ = 1) products. XRD patterns were collected with monochromatized Cu Kα radiation over the 2θ range of 10−70° with a step size of 0.02° and a scanning rate of 2°/min. The data were quantitatively analyzed by means of a biphasic Rietveld method using RIETAN-2000 software.17 2.3. Isothermal TG Experiments for Reaction Kinetics Analysis. TG experiments were performed on the δ = 0 and 1 products at various fixed temperatures. Approximately 20 mg of the δ = 0 product was placed in a shallow platinum pan (5 mmϕ × 2.5 mm) and was heated at a target temperature in flowing 5% H2/95% Ar gas mixture. Then, the flowing gas was switched to 1% O2/99% Ar gas mixture to gradually oxidize the sample, while the weight was measured with the time subjected.

Having been motivated by these issues, we carried out the following two experiments in the present work. First, the structural changes upon oxygen intake/release were studied by X-ray powder diffraction (XRD) on partially oxidized/reduced products. The three stable forms are easily distinguishable by their XRD profiles.14 Second, time variation of the sample weight was measured by means of thermogravimetry (TG) to identify the rate-controlling step on the basis of reaction kinetics analyses. Our XRD study on the partially oxidized/ reduced products has revealed a clear indication of successive phase changes among three distinct forms with δ ≈ 0, 0.5, and 1: the oxygen intake/release processes could proceed with nucleation of the generated phase at the grain surface and a subsequent growth of the phase. It has also appeared that the oxygen intake/release processes are either surface-reactioncontrolled or nucleation-process-controlled rather than diffusion-controlled in a wide temperature range.

2. EXPERIMENTAL SECTION 2.1. Sample Synthesis. Samples of BaYMn2O5+δ were synthesized via a solid-state reaction route utilizing the oxygenpressure-controlled encapsulation technique.7,14 A stoichiometric mixture of BaCO3 (Kanto Chemical, 99%), Y2O3 (Wako Pure Chemical, 99.9%), and Mn2O3 (Soekawa Chemical, 99.9%) was preheated in flowing N2 gas (99.99%) at 1000 °C to obtain a precursor powder. The precursor was pressed into pellets and then was placed in an evacuated silica ampule together with FeO powder, which acts as a getter for excess oxygen. The silica ampule was heated at 1100 °C for 24 h followed by rapid cooling to room temperature. The XRD analysis (Rigaku Ultima IV) indicated that the resultant product was essentially a single phase of oxygen-deficient BaYMn2O5 (δ = 0). The oxygen content was indeed close to 5.0 according to the result of iodometric titration. The product contained large particles of several micrometers with a smaller value for the Brunauer−Emmett−Teller (BET) specific surface area (s = 0.50 m2/g). Such a coarse microstructure was favorable to decelerate the oxygen intake/release and facilitated our reaction kinetics analysis. A part of the BaYMn2O5 powder was postannealed at 600 °C in flowing O2 gas to prepare fully oxidized BaYMn2O6 (δ = 1). B



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The flowing gas was effectively replaced within a short period (∼10 s) using an optional pumping system designed for our thermobalance. The isothermal measurements at various temperatures were carried out using a single sample batch. From the TG data, nominal oxygen content δ was determined assuming the initial and saturated values as δ = 0 and 1, respectively. Similar isothermal TG experiments were also performed on the δ = 1 product in flowing 5% H2/95% Ar gas mixture for oxygen release. The composition of flowing gas was important to obtain time versus δ plots with sufficient accuracy. In fact, a preliminary oxygen intake experiment at 400 °C in pure O2 gas flow resulted in instantaneous oxygenation only within 20 s accompanied by a sharp rise in the sample temperature as high as 500 °C. Because the oxygen intake process is exothermic, a fast reaction rate induces significant selfheating;18 thereby, the condition for isothermal reaction kinetics does not hold. We found that the use of diluted O2, that is, the 1% O2 /99% Ar gas mixture, considerably decreased the oxygen intake rate and avoided the significant self-heating. Even with such a moderate condition, however, accurate measurements were not possible above 410 °C because of the accelerated oxygen intake kinetics at elevated temperatures. Meanwhile, the oxygen release process in the 5% H2/95% Ar gas mixture led to lower reaction rates, which enabled us to perform reliable TG experiments up to 750 °C.

Figure 2. X-ray powder diffraction (XRD) patterns for the partially oxidized products with δ = 0.21, 0.38, 0.59, and 0.81 together with the fully reduced (δ = 0) and fully oxidized (δ = 1) products. These products were prepared through partial oxygenation of the δ = 0 powder in 1% O2 /99% Ar gas mixture. The patterns for the δ = 0 and 1 are indexed on the basis of tetragonal P4/nmm and triclinic P-1 space groups, respectively. Note that hkl indices for strong diffraction lines are only shown. The inset shows magnified patterns at 47.0 ≤ 2θ ≤ 48.5°.

3. RESULTS 3.1. XRD Analysis on the Partially Oxidized/Reduced Products. Figure 2 presents XRD patterns for the partially oxidized products with δ = 0.21, 0.38, 0.59, and 0.81 together with the fully reduced (δ = 0) and fully oxidized (δ = 1) products. The patterns for δ = 0 and 1 are consistent with the tetragonal O5 and triclinic O6 models, respectively, as reported in the literature.19,20 While all of the XRD profiles are similar to each other, a careful look at the 004 diffraction peaks around 2θ = 47.0−48.5° highlights a structural feature in the course of the oxygen intake process because the change in the lattice parameter c is reported to be much larger than those in the a and b values. The profiles of the partially oxidized products are found to consist of two strong Kα1−Kα2 doublets, whose positions coincide with the 004 lines of the O5 (004O5, lower angle side) and the O6 (004O6, higher angle side) phases as shown in the inset of Figure 2. The relative intensity of the 004O6 lines increases with increasing the nominal δ value. This result clearly indicates that the oxygen intake process involves phase separation into two distinct forms with the lowest (O5) and highest (O6) oxygen contents. XRD patterns for the partially reduced products are summarized in Figure 3. The pattern for the δ = 0.59 product (Figure 3d) contains diffraction lines which are assigned to the intermediate O5.5 phase (orthorhombic, a ≈ 0.816 nm, b ≈ 0.755 nm, and c ≈ 1.528 nm)15 in addition to the original O6 phase. Interestingly, the diffraction lines of O6 disappear for δ = 0.19 and 0.40 (Figure 3b and c), and the profiles are explained as a mixture of the two other stable forms, that is, O5.5 and O5. Meanwhile, the profiles of δ = 0.74 and 0.78 are somewhat complicated (Figure 3e and f). These patterns consist of two sets of diffraction lines: one is for the original O6 phase, and the other is indexed on the basis of an orthorhombic unit cell, most likely a perovskite-related structure, but its lattice parameters, a ≈ 0.800 nm, b ≈ 0.761 nm, and c ≈ 1.523 nm, are largely different from the reported values for the O5.5 phase.15 We

assume that the novel orthorhombic phase would be an O5.5type phase with oxygen content slightly larger than 5.5. More detailed information on this phase is given in the Supporting Information. In any case, the oxygen release process appears to involve phase separation into two distinct forms as in the case of oxygen intake. Because most of the diffraction peaks of the O5 phase overlap with those of the O6 phase, our biphasic Rietveld refinements for the partially oxidized products were unsuccessful. Meanwhile, the partially reduced products, which contain either O6/ O5.5 or O5.5/O5 phases, could be analyzed quantitatively to give molar ratios of the constituent phases. This is because large differences in the unit cell volume between O5/O6 and O5.5 phases lead to fairly nice separation of the diffraction peaks. The resultant nominal δ values calculated from the phase composition are essentially in agreement with those estimated from the weight variations of the TG data indicating the correctness of our interpretation (see Supporting Information for details). The XRD data were measured at ambient temperature whereas the syntheses were carried out at 350 and 450 °C for the oxidized and reduced products, respectively, implying that the XRD data do not necessarily reflect the situation under the synthesis conditions. In fact, all of the O5, O5.5, and O6 phases possess highly ordered atomic arrangements, which might involve order/disorder transitions associated with the oxygen vacancies. We suggest that the oxygen-vacancy orderings of the three phases are rather robust at moderate temperatures as the distortions of MnO5/O6 polyhedra are cooperatively coupled with the oxygen-vacancy ordering, which could effectively C



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Figure 4. XRD patterns for the partially oxidized products with δ ≈ 1/ 3 under P(O2) = 0.01, 0.1, and 1%. Diffraction peaks for O5, O5.5, and O6 are marked with blue diamonds, green triangles, and red circles, respectively. Figure 3. XRD patterns for the partially reduced products with δ = 0.19, 0.40, 0.59, 0.74, and 0.78 together with the fully reduced (δ = 0) and fully oxidized (δ = 1) products. These products were prepared through partial deoxygenation of the δ = 1 powder in 5% H2 /95% Ar gas mixture. Diffraction peaks for O5, O5.5, O6, and the novel orthorhombic phase (see main text) are marked with blue diamonds, green triangles, red circles, and orange crosses, respectively. For δ = 0.59, diffraction lines of the O5.5 phase are indexed (indices of strong lines are only shown).

hinder the disordered atomic arrangement. Nevertheless, the stability of the ordered atomic arrangements at elevated temperatures is open to dispute and merits further structural studies. It is worthwhile to point out the different features between oxygen intake/release processes in BaYMn2O5+δ. While the oxygen intake reaction is always accompanied by the coexistence of O5 and O6, the oxygen release reaction seems to proceed along two successive steps, that is, the first step at δ ≥ 0.5 involving phase separation into O6 and O5.5 (or slightly oxidized O5.5-type phase) and the second step at δ ≤ 0.5 as mixtures of O5.5 and O5. Upon oxygen release, the O5 phase has never appeared unless the whole part of O6 is transformed into O5.5. The absence of O5.5 upon oxygen intake suggests the kinetically caused instability of oxygen/vacancy arrangement in the rapid oxidation process. In fact, as shown in Figure 4, the partially oxidized product (δ ≈ 1/3) under highly O2-diluted atmospheres (0.01∼0.1% O2) resulted in phase separation into O5 and O5.5 in contrast to the products oxidized under 1% O2 atmosphere which are always mixtures of O5 and O6. This result suggests that the local exothermic heat upon oxygen intake tends to induce phase instability of the intermediate O5.5 and thereby the direct phase transformation to the fully oxidized O6 in O2-rich atmospheres. 3.2. Isothermal TG Experiments for Reaction Kinetics Analysis. Figure 5 presents isothermal TG data of the δ = 0 powder under the 1% O2/99% Ar gas mixture at 348 ≤ T ≤ 403 °C. It appears that the sample weight increases with time (t) almost linearly up to δ ≈ 0.5. The data are analyzed on the basis of the method of Hancock and Sharp21 to discuss the

Figure 5. Isothermal TG data of the δ = 0 powder under the 1% O2/ 99% Ar gas mixture at various temperatures. The inset shows the time variation of the fraction reacted α as a function of the reduced time t/ t0.5.

reaction mechanisms. As shown in the inset of Figure 5, the time variation of the fraction reacted α (=δ in this case) falls on a single curve in a range of 0 ≤ α ≤ 0.50 when plotted as a function of the reduced time t/t0.5, where t0.5 is the time for 50% reaction (δ = 0.50). Plots of ln [−ln(1 − α)] versus ln t are used to specify the rate-controlling process upon oxygen intake of BaYMn2O5+δ (see Supporting Information). The data points at each temperature are well fitted with a straight line at 0.15 ≤ α ≤ 0.50. The slope of the plots (m) is nearly unity suggesting surface-reaction-controlled according to the method of Hancock and Sharp. The following equations have been proposed if the surface reaction is limited by movement of an interface between the reactant and the product at a constant velocity:22 1 − (1 − α)1/2 = kt D

(1)



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and 1 − (1 − α)1/3 = kt

(2)

where the circular and spherical geometries are respectively assumed. As pointed out by Sharp et al., it is difficult to differentiate the two models solely on the basis of this analysis.21,22 We tentatively assume the circular geometry to calculate the reaction rate constant values taking into account the two-dimensional nature of the oxygen intake process. Both the circular and the spherical models have given similar values of the activation energies (Ea): the surface geometry in the reaction model does not affect our interpretation. The results of the oxygen release process shown in Figure 6 were also analyzed by the same method. Whereas the oxygen Figure 7. Temperature dependence of the reaction rate constants, k, obtained from the oxygen intake/release experiments. Data for the oxygen intake and release processes are shown with red circles and blue triangles, respectively.

the oxygen intake process is 0.27 eV for the surface-reaction rate law at 348 ≤ T ≤ 403 °C. Also, the Ea values for the oxygen release process are determined to be 0.68 eV for the surfacereaction rate law at 425 ≤ T ≤ 550 °C and 0.17 eV for the Avrami nucleation rate law at 600 ≤ T ≤ 750 °C.

4. DISCUSSION Our XRD study on the partially oxidized/reduced products has revealed a clear indication of phase separation into three distinct forms with δ ≈ 0, 0.5, and 1. The oxygen intake/release processes are thus understood with nucleation followed by a growth of the generated phase. The oxygen intake process of BaYMn2O5 (δ = 0) can be divided into the following four elementary steps. Step 1 is physisorption of an O2 molecule on the oxide grain surface. This step seems sufficiently fast and is unlikely to be rate-controlling. Step 2 is a redox reaction between the oxide and the O2 molecule to form two O2− ions involving four-electron transfer (O2 + 4e− → 2O2−) from the oxide. This is one of the key reactions in solid oxide fuel cells and often causes large overpotential in perovskite oxide cathodes.24 As step 3, nucleation of the fully oxidized O6 phase takes place, which is triggered by the arrangement of the incorporated O2− ions within the BaYMn2O5 lattice. Then, step 4 is bulk diffusion of the O2− ions resulting in a growth of the O6 phase. These four elementary steps are only based on a sketched model to figure out the oxygen intake. Similar successive steps can also be applicable to the oxygen release process, where a H2 molecule reacts with the O6 (or O5.5) phase to form the O5.5 (or O5) phase and H2O molecule as a product. The fact that the oxygen intake/release processes are surfacereaction-controlled or nucleation-process-controlled in a wide temperature range implies that the diffusion step (i.e., step 4) tends to be faster than the redox reaction/nucleation steps (steps 2 and 3). This aspect should be emphasized because many solid-state reactions are subject to diffusion-controlled because of poor diffusivity of relevant ions, and particularly for perovskite-type oxides, the oxide-ion diffusion is usually taken into account to discuss the oxygen intake/release reaction kinetics.25,26 We point out that the cation-ordered structure of BaYMn2O5+δ potentially contains effective diffusion pathways, which could play a crucial role in the remarkable oxide-ion diffusivity. In fact, a recent quantum-chemistry calculation on

Figure 6. Isothermal TG data of the δ = 1 powder under the 5% H2/ 95% Ar gas mixture at various temperatures. The inset shows the time variation of the fraction reacted α as a function of the reduced time t/ t0.5.

release reaction was found to proceed along the two successive steps at δ ≥ 0.5 (O6/O5.5 mixtures) and δ ≤ 0.5 (O5.5/O5 mixtures), no singularity of the reaction rate was evidenced at δ = 0.5. We thus simply defined the fraction reacted α as 1 − δ. The α versus t/t0.5 plots clearly indicate the presence of two distinct regimes at 425 ≤ T ≤ 550 °C and 600 ≤ T ≤ 750 °C. The α versus t/t0.5 relation at 425 ≤ T ≤ 550 °C is essentially similar to that of oxygen intake at 348 ≤ T ≤ 403 °C, while Sshaped curves are observed at 600 ≤ T ≤ 750 °C. Plots of ln [−ln(1 − α)] versus ln t lead to different slopes, that is, m ≈ 1 and ≈2 for 425 ≤ T ≤ 550 °C and 600 ≤ T ≤ 750 °C, respectively (see Supporting Information), implying surfacereaction-controlled for the former and nucleation-processcontrolled (the so-called Avrami-Erofe’ev model) for the latter. The data points at 600 ≤ T ≤ 750 °C nicely obey the following two-dimensional Avrami-Erofe’ev equation:22 [−ln(1 − α)]1/2 = kt

(3)

Such a two-dimensional reaction model was also applicable to the reaction kinetics for pyridine intercalation of the layered oxychloride FeOCl.23 The reaction rate constants (k) upon oxygen intake/release follow the Arrhenius equation in both the lower and the higher temperature regimes as indicated in Figure 7. The Ea value of E



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Table 1. Activation Energy (Ea) Values of Chemical Oxygen Exchange for Various Perovskite-Type Oxidesa compound

Ea/eV

T/°C

P(O2)/atm

methodb

remarks

BaYMn2O5+δ La0.7Sr0.3MnO3 SrFeO3−δ La0.5Sr0.5CoO3‑δ BaPrCo2O5+δ BaGdCo2O5+δ

0.27 0.82−1.16 1.51 0.69 0.67 0.85

348−403 600−900 790−1000 619−696 300−500 250−600

0.01 6.6 × 10−4 0.01 1.00 0.21 1.00

TG ECR ECR ECR ECR TG

present work thin film, ref 27 ref 25 ref 26 ref 28 single crystal, ref 29

a The Ea values for these oxides were determined on the basis of oxygen intake rates upon increasing oxygen partial pressure [P(O2)] by means of either thermogravimetry or electrical conductivity relaxation. bTG, thermogravimetry; ECR, electrical conductivity relaxation.

this oxide suggested18 that the activation energies for oxide-ion hopping between the adjacent Y and MnO2 layers are smaller (0.18 eV for Y layer → Mn layer and 0.70 eV for Mn layer → Y layer) than those for other pathways (0.78−1.37 eV). This means that the zigzag movement along the Y/Mn layers is most energetically favorable for oxide-ion migration. The present result also indicates that the enhanced oxygen intake/release kinetics of BaYMn2O5+δ fine powders is interpreted as a consequence of the large specific surface area rather than of the reduced diffusion length. The oxygen release rate at 500 °C is indeed proportional to the specific surface area of the powders,8 consistent with our interpretation. It is worthwhile to compare the Ea values for BaYMn2O5+δ with those for other perovskite-type oxides. Because the value of oxygen intake at 348 ≤ T ≤ 403 °C is related to the surfacereaction-controlled process, its comparison with literature data of the chemical oxygen exchange process (i.e., oxygen exchange in the presence of chemical potential gradient) may be informative. Table 1 summarizes the Ea values for various Mn/Fe/Co oxides with simple-perovskite type or doubleperovskite type structure.25−29 The value for our BaYMn2O5+δ is much smaller than those for any other perovskite-type oxides indicative of great affinity to oxygen molecules. The doubleperovskite type BaRECo2O5+δ (RE = Pr and Gd) shows somewhat large Ea values28,29 implying that the smaller Ea value for BaYMn2O5+δ is hard to be explained only on the basis of its layered atomic arrangement. We suggest that the small activation energy of oxygen exchange in BaYMn2O5+δ is attributed to the characteristic oxygen intake process involving phase separation into the particularly stable forms (δ = 0, 0.5, and 1.0) as revealed in the present work.

drastically enhanced with appropriate promoter metals for accelerating O2 molecule dissolution or H2O molecule formation at the oxide grain surface.

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ASSOCIATED CONTENT

S Supporting Information *

Results of oxygen content analysis by means of iodometric titration, biphasic Rietveld refinements for the partially reduced products, structural aspect of the novel orthorhombic phase in the partially reduced products with δ ≈ 0.8, and plots of ln [−ln(1−α)] versus ln t to specify the rate-controlling process. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81(0)11 706 6741. Fax: +81(0)11 706 6740. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by a Grant-in-Aid for Science Research (contract no. 22750181) from the Japan Society for the Promotion of Science. T.M. acknowledges financial support from the Inamori Foundation.

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REFERENCES

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5. CONCLUSIONS The oxygen intake/release mechanism of double-perovskite type BaYMn2O5+δ was discussed on the basis of the crystal structure and reaction kinetics studies. Our XRD study on the partially oxidized/reduced BaYMn2O5+δ products has revealed a clear indication of successive phase changes among three distinct forms with δ ≈ 0, 0.5, and 1: the oxygen intake/release processes are thus understood with nucleation followed by a growth of the generated phase. It has also appeared that the oxygen intake reaction is surface-reaction-controlled at 348 ≤ T ≤ 403 °C, while the oxygen release is a surface-reactioncontrolled process at 425 ≤ T ≤ 550 °C and a nucleationprocess-controlled process at 600 ≤ T ≤ 750 °C. The small activation energy for the oxygen intake process strongly indicates the great affinity to oxygen molecules. The present work has invoked an importance of surface reactivity as well as grain morphology control to achieve the enhanced performance of BaYMn2O5+δ. As a future prospect, the oxygen intake/release kinetics of BaYMn2O5+δ could be F



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Release Mechanism of Double-Perovskite Type

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