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C: Energy Conversion and Storage; Energy and Charge Transport 3

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Striking Oxygen-Release/Storage Properties of Fe-Site-Substituted SrFeO Kosuke Beppu, Saburo Hosokawa, Akito Demizu, Yudai Oshino, Kazuki Tamai, Kazuo Kato, Kenji Wada, Hiroyuki Asakura, Kentaro Teramura, and Tsunehiro Tanaka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12754 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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The Journal of Physical Chemistry

Striking Oxygen-Release/Storage Properties of Fe-site-substituted Sr3Fe2O7-δ Kosuke Beppu,† Saburo Hosokawa,*,†,‡ Akito Demizu,† Yudai Oshino,† Kazuki Tamai,† Kazuo Kato,§ Kenji Wada,¶ Hiroyuki Asakura,†,‡ Kentaro Teramura,†,‡ Tsunehiro Tanaka*,†,‡ †

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡ Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8245, Japan § Japan Synchrotron Radiation Research Institute, 1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ¶ Department of Chemistry for Medicine, Faculty of Medicine, Kagawa University, 1750-1 Ikenobe, Miki-cho, Kita-gun, Kagawa, 761-0793, Japan ABSTRACT: The present study explores the oxygen storage capacity of Sr3Fe2O7-δ doped by other transition metals. Sr3Fe2O7-δ doped with Mn, Co, or Ni exhibited higher oxygen storage amount and higher oxygen-release/storage rates than Sr3Fe2O7-δ; doping with Mn led to the highest oxygen release/storage rates. Although reduced Sr3(Fe0.8Co0.2)2O7-δ and Sr3(Fe0.8Ni0.2)2O7-δ had disordered coordination states around Fe and the transition-metal dopant, the local structure around the transition metals in the as-synthesized Sr3(Fe0.8Mn0.2)2O7-δ was almost kept intact even in the reduced state. These observations reveal that subtle differences in the ordering around transition metal species before and after lattice-oxygen release significantly affect the oxide ion mobilities in the perovskite structures, and also provide guidelines for designing novel oxygen storage materials.

1.

INTRODUCTION Oxygen storage materials (OSMs) have the abilities to store/release oxygen species in response to the surrounding atmosphere and can control the oxygen partial pressure. 1 Consequently, OSMs have been used in a variety of applications, 2-11 including gas separators,3,4 oxygen-ion conductors,5-7 and cocatalysts for automotive catalysts.8-11 Attention has been paid to the high oxygen storage abilities of perovskite-type oxides and perovskite-derived oxides,12-16 such as Ca0.8Sr0.2MnO312 and BaYMn2O5+δ14. These materials are known to store and release lattice oxygen while maintaining the original cation ordering. Such crystallographic transformations are referred to as “topotactic transitions”. 17,18 OSMs which transform via topotactic transitions hardly change their volumes during oxygen release/storage cycles, resulting in high mechanical strengths. Therefore, development of novel OSMs which utilize topotactic transitions is desired for practical application in the above-mentioned fields. Recently, we have reported that Sr3Fe2O7-δ, with a layered perovskite-type structure, exhibits topotactic oxygen storage/release behaviors, and has higher structural stability under severe reductive condition than SrFeO3-δ.19,20 The space group (I4/mmm) of Sr3Fe2O6.75 is identical with that of Sr3Fe2O6 obtained through the release of lattice oxygen. Only oxygen ions at the apical position between bilayer of FeO6 in Sr3Fe2O6.75 are topotactically eliminated by the reduction with H2 (Figure 1),

because the oxygen vacancy site at the apical position is more stable than that at the equatorial position in Sr3Fe2O6.75.21 It is uncommon for the symmetry of a crystal structure to be maintained by topotactic oxygen release; therefore, unit cell volume variations before and after the release of lattice oxygen from Sr3Fe2O7-δ are very small among OSMs. Furthermore, Sr3Fe2O7δ has a larger oxygen storage amount and faster oxygen release/storage rates than those of Pt/CeO2-ZrO2 solid solution which is a representative OSM, regardless of the absence of Pt loading. In other words, because of its excellent oxygen storage

Figure 1 Structure change from Sr3Fe2O6.75 to Sr3Fe2O6.

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properties and mechanical strength, Sr3Fe2O6.75-based materials have high potential as novel OSMs for use in applications. In the field of solid oxide fuel cells, heterogeneous elemental doping is applied to perovskite materials.22-29 For example, Ishihara et al. have reported that Sr and Mg doping enhance the oxygen ion conductivity of LaGaO3.23 Inspired by these examples of heterogeneous elemental doping, we very recently have revealed that Ti doping enhances the oxygen storage rate of SrFeO3-δ; unfortunately, however, no improvements in release rate and oxygen storage amount were observed by Ti doping into SrFeO3-δ.29 In this context, we investigated the effect of doping with other transition metals (Mn, Co, Ni) into the Fe sites of Sr3Fe2O7-δ having high structural stability, particularly with respect to the oxygen release/storage rate of the material. 2. EXPERIMENTAL SECTION Preparation: All reagents were purchased from Wako Pure Chemical Industries Ltd., Osaka, Japan. Sr3Fe2O7-δ, in which Fe was partially replaced by other transition metal (Sr3(Fe0.8M0.2)2O7-δ, where M = Mn, Co, or Ni), was synthesized by a polymerized-complex method. Citric acid (400 mmol, 98.0%) was dissolved in 180 mL of water at 353 K. The calculated amounts of SrCO3 (99.9%), Fe(NO3)3•9H2O (99.9%), Mn(NO3)2•6H2O (98%), Co(NO3)2•6H2O (99.0%), or Ni(NO3)2•6H2O (99.9%), were added and the mixture was stirred for 2 h to obtain a solution of the desired metal oxide complex, with a total cation content of 16.67 mmol, after which 400 mmol of ethylene glycol (99.5%) was added. The mixture was stirred at 403 K for 4 h during which time a gelatinous solution formed. After the gel was heated in a mantle heater at 623 K for 3 h, the resulting brown powder was calcined at 1273 K for 2 h to give Sr3(Fe0.8M0.2)2O7-δ. The amount of transition metal substitution to Fe site was set to 20 mol% to prevent the formation of by-products such as NiO, Co3O4 or Mn2O3. Note that 20 mol% of Ti addition into Fe site in SrFeO3-δ having perovskite-type structure has been found to improve its chemical stability and oxygen storage performance in our previous report.29 Temperature programmed reduction with H2 (H2-TPR): H2TPR was carried out using a flow-type reactor, as described in our previous reports.19,20,29 H2 (2 vol% in Ar at 30 mL min-1) was passed through the reactor charged with a 50-mg sample at atmospheric pressure. The reactor was heated to 1223 K with an electric furnace at a rate of 5 K min-1, and the amount of H2 consumed was monitored using the thermal conductivity (TC) detector of a Shimadzu GC8A gas chromatograph. Oxygen storage capacity (OSC) measurements: OSC measurements were performed with a Rigaku Thermoplus thermogravimeter (TG), as described in our previous reports. 19,20,29 Each sample (100 mg) was heated to 773 K in 5-vol% O2/Ar, and this temperature was maintained until constant weight was observed. The weight change of the samples was measured every 20 min, when the atmosphere was switched between 5 vol% O2/Ar and 5 vol% H2/Ar. Temperature-programmed oxygen desorption (O2-TPD): O2TPD was carried out in the same flow-type reactor as described above for the H2-TPR experiments. Each sample (50 mg) was heated to 1073 K for 1 h and cooled to room temperature under an atmosphere of pure O2 (30 mL min-1). The samples were then heated to 1223 K at a constant heating rate (5 K min -1) under a flow of He (30 mL min-1). The desorbed species were monitored using the TC detector of a Shimadzu GC8A gas chromatograph.

Iodometric titration: The sample (20 mg) and KI (700 mg) were dissolved in 1 M HCl solution to form the iodine by the reduction of tetra- or tri-valent transition metal cations. The formation amount of iodine was evaluated by a titration of 0.015 M Na2S2O3 solution. During the titration, starch as an indicator of the end-point was added to the solution. The end-point was determined from disappearance of a blue color in the solution. Characterization: Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku UltimaIV instrument using CuKα radiation. SEM images were obtained from a field emission scanning electron microscope (FE-SEM, SU-8220, Hitachi High-Technologies). The three-dimensional visualization models of crystal structures were constructed using VESTA 3 software.30 X-ray absorption fine-structure (XAFS) spectra at the Fe, Mn, Co, and Ni K-edges were measured at the BL11S2 beamline of the Aichi Synchrotron Radiation Center (AichiSR); these spectra were recorded in transmission mode at room temperature using a Si(111) double-crystal monochromator. Timeresolved in-situ energy-dispersive XAFS (DXAFS) spectra at the Sr K-edge were measured with a polychromator in transmission mode at the BL28B2 beam line of the SPring-8, as described in our previous reports.20,29 Each sample (30 mg) was pressed into a pellet and placed in a batch-type in-situ cell. The sample was pretreated with pure H2 at 873 K for 15 min, and then and with pure O2 under the same conditions. The in-situ cell was evacuated after which pure H2 or O2 (1 atm) was injected. A time-resolved DXAFS spectrum was recorded every 72 ms under pure H2 or O2 at 873 K. 3. RESULTS AND DISCUSSION Oxygen release/storage behavior of Sr3(Fe0.8M0.2)2O7-δ: Figure 2 displays the OSC profiles obtained by TG analyses of Sr3(Fe0.8M0.2)2O7-δ at 773 K. The weight changes in these profiles correspond to the amounts of oxygen species stored/released from the materials. Weight changes were observed in all samples in response to atmospheric switching, and the amounts of oxygen storage by Sr3Fe2O7-δ were improved by transitionmetal doping. As mentioned in our previous reports,19,20,29 the oxygen release/storage response rates for Sr3Fe2O7-δ are very fast. Consequently, its fundamental response rates cannot be evaluated by OSC measurements using TG due to the poor time resolution (of order of seconds) of the apparatus; hence, we measured the storage and release rates of Sr3(Fe0.8M0.2)2O7-δ using the in-situ DXAFS technique, which has a high time resolution (of the order of milliseconds). Figure S1 shows the Sr K-edge Xray absorption near edge structure (XANES) spectra of

Figure 2 (A) OSC profiles obtained by TG analyses at 773 K of Sr3(Fe0.8M0.2)2O7-δ (M = Fe, Mn, Co and Ni). Black line, Fe (undoped); purple line, Mn; green line, Co and red line, Ni. (B) Oxygen storage amount of Sr3(Fe0.8M0.2)2O7-δ.


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Figure 3 (A) Oxygen release and (B) storage profiles at 873 K of Sr3(Fe0.8M0.2)2O7-δ (M = Fe, Mn, Co and Ni) obtained by in-situ DXAFS. Black line, Fe (undoped); purple line, Mn; green line, Co and red line, Ni. (C) Initial oxygen release and storage rates of Sr3(Fe0.8M0.2)2O7-δ.

Sr3(Fe0.8M0.2)2O7-δ. Changes in the XANES spectra were observed at 16117 eV following H2 or O2 injection. These spectral changes correspond to local structural changes around the Sr ions with oxygen storage/release. To determine the oxygen storage and release rates for Sr3(Fe0.8M0.2)2O7-δ, a linear combination fitting was carried out using the spectra obtained from DXAFS. The XANES spectrum was fitted by two spectra. During the oxygen release process, one XANES spectrum corresponds to that of the sample before H2 injection, and the other is the XANES spectrum with the lowest white line intensity among the examined spectra. On the other hand, during the oxygen storage process, one XANES spectrum is that of the sample before O2 injection, and the other is the XANES spectrum with the highest white line intensity among the examined spectra. The y-axis is allocated for the amount of oxygen released/stored, and the amounts of oxygen stored by Sr3(Fe0.8M0.2)2O7-δ are estimated from the OSC profiles displayed in Figure 2. The oxygen storage/release behaviors of Sr3Fe2O7-δ changed upon transition-metal doping, as shown in Figures 3 (A) and (B) (for detailed analytical procedure, see Figure S1). The oxygen release/storage rates estimated from changes in the DXAFS spectra just after the H2 or O2 injection in the first cycle are shown in Figure 3(C) (for detailed calculation procedure, see Figure S2). The oxygen-release response rates were significantly enhanced by transition-metal doping (Figure 3(C)). The oxygen-release rates followed the order: Mn >> Co >> Ni > bare. The response rate of the oxygen storage was much higher than that of the oxygen release, and it is difficult to obtain high-precision storage rates even by the DXAFS technique. However, the order of the storage rates for

Figure 4 (A) H2-TPR profiles of Sr3(Fe0.8M0.2)2O7-δ (M = Fe, Mn, Co and Ni). (B) Accumulated oxygen release amount calculated from H2-TPR profiles of Sr3(Fe0.8M0.2)2O7-δ. Black line, Fe (undoped); purple line, Mn; green line, Co and red line, Ni.

Sr3(Fe0.8M0.2)2O7-δ, which was roughly estimated by the change of XANES spectra, was essentially identical with that of the release rate. Thus, the doping of Sr3Fe2O7-δ with Mn was most effective for rapid oxygen release and storage. Figure S3 shows SEM images of the Sr3(Fe0.8M0.2)2O7δ. Co- and Ni-doped materials contained irregularly-shaped particles with a size of around 1 µm, and these morphologies were similar with that of Sr3Fe2O7−δ itself. On the other hand, Mndoped material consisted of smaller particles less than 500 nm. Therefore, the small particle size of Mn-doped material might affect the high oxygen release and storage rates, while only the particle size effect could not explain the improvement of oxygen release/storage performances of the Ni- and Co-doped materials. Figure 4 shows the H2-TPR profiles for Sr3(Fe0.8M0.2)2O7-δ. Sr3Fe2O7-δ and Sr3(Fe0.8Mn0.2)2O7-δ showed reduction peaks only at below 800 K. The profiles for Sr3(Fe0.8Co0.2)2O7-δ and Sr3(Fe0.8Ni0.2)2O7-δ exhibited similar reduction peaks at below 800 K, as well as reduction peaks at above 1000 K. We reported that the reduction peak observed at above 1000 K corresponds to the structural collapse of the perovskite unit,19 which, by analogy, means that Co or Ni doping unfortunately promotes the structural collapse of Sr3Fe2O7-δ. On the other hand, the reduction peak observed at below 800 K in Sr3Fe2O7-δ has been previously assigned to the topotactic release of oxygen. The amounts of released oxygen for Sr3(Fe0.8M0.2)2O7-δ determined from this reduction peak (Figure 4(B)) correspond well to the oxygen storage amounts calculated from the OSC experiments, suggesting that the oxygen storage amount estimated by OSC is related to the topotactic release of oxygen from Sr3(Fe0.8M0.2)2O7-δ. H2-TPR experiments are generally used to evaluate sample reducibility. The reduction temperature due to topotactic oxygen release was lowered by doping. An example of a reduction scheme is shown: Sr3(Fe0.8M0.2)2O7-δ + (1-δ) H2 → Sr3(Fe0.8M0.2)2O6 + (1-δ) H2O. Sr3(Fe0.8Co0.2)2O7-δ began to release the lattice oxygen at the lowest temperature among the examined samples, although Sr3(Fe0.8Mn0.2)2O7-δ showed the fastest oxygen-release rate. In other words, molecular H2 easily reacts with the lattice oxygens in Sr3(Fe0.8Co0.2)2O7-δ. We recently reported that Pd-loaded Sr3Fe2O7-δ releases lattice oxygen at extremely low temperatures under a flow of H2;20 therefore, Pd dramatically enhances the oxygen-release rate of Sr3Fe2O7-δ. The Pd/Sr3Fe2O7-δ results imply that improvements in the reactivity of the OSC material



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Table 1 Chemical formulations and lattice parameters of Sr3(Fe0.8M0.2)2O7-δ Lattice parameter (Å) Chemical formulation b)

Sr3Fe2O7-δ

Sr3(Fe0.8Mn0.2)2O7-δ

Sr3(Fe0.8Co0.2)2O7-δ

Sr3(Fe0.8Ni0.2)2O7-δ

as-synthesized

Sr3Fe2O6.73

reduced a)

Sr3Fe2O5.98

as-synthesized

Sr3(Fe0.8Mn0.2)2O6.60

reduced

Sr3(Fe0.8Mn0.2)2O5.92

as-synthesized

Sr3(Fe0.8Co0.2)2O6.56

reduced

Sr3(Fe0.8Co0.2)2O5.84

as-synthesized

Sr3(Fe0.8Ni0.2)2O6.71

reduced

Sr3(Fe0.8Ni0.2)2O5.80

Volume (Å3) a

c

3.866 (3.866) c) 3.893 3.862 (3.863) 3.877 3.859 (3.862) 3.896 3.854 (3.856) 3.879

20.153 (20.153) 20.052 20.145 (20.153) 20.136 20.136 (20.145) 20.128 20.119 (20.127) 20.196

301.21 (301.21) 303.90 300.46 (300.79) 302.67 299.86 (300.42) 305.52 298.83 (299.21) 303.88

The variation of the unit cell volume (%) d)

0.89

0.74

1.89

1.69

a) Reduced samples were obtained by the reduction of the as-synthesized samples under 5% H2/Ar flow (100 mL min-1) at 773 K for 1 h. b) Chemical formulation of each sample was determined by the iodometric titration. c) The lattice parameters inside the brackets show those of reoxidized samples which were obtained by the oxidation of the reduced samples under 5% O2/Ar flow (100 mL min-1) at 773 K for 1 h. d) The variation of unit cell volume was calculated from as-synthesized and reduced samples.

toward molecular H2 enhance the oxygen-release rate. While reactivity toward molecular H2 correlated with the oxygen-release rate in the previous study, the oxygen-release-rate order determined by the DXAFS analysis in the present work cannot be interpreted solely on the basis of the reactivity of Sr3(Fe0.8M0.2)2O7-δ toward molecular H2. Consequently, we investigated in detail the crystal structures of these materials before and after oxygen release. Characterization of Sr3(Fe0.8M0.2)2O7-δ before/after oxygen release: The XRD patterns and extended X-ray absorption fine structure (EXAFS) oscillations of as-synthesized Sr3(Fe0.8M0.2)2O7-δ (M = Mn, Co or Ni) are shown in Figures 5, 6, and S4 (black lines). XRD peaks due to the Ruddlesden-Popper-type layered perovskite structure were observed for all samples. The EXAFS oscillations of all doped transition-metal species are essentially identical to those of the undoped (Fe) species. These results indicate that all transition-metal species are

Figure 5 (A) Over all view and (B) magnified view of the XRD patterns of (a, b) Sr3(Fe0.8Mn0.2)2O7-δ, (c, d) Sr3(Fe0.8Co0.2)2O7-δ and (e, f) Sr3(Fe0.8Ni0.2)2O7-δ. (b, d, f and black line), as-synthesized sample; (a, c, e and red line), the sample reduced under 5% H2/Ar flow (100 mL min-1) at 773 K for 1 h.

completely substituted with Fe ion in Sr3Fe2O7-δ having the layered perovskite structure. Sr3(Fe0.8M0.2)2O7-δ, obtained by reduction at 773 K under 5% H2/Ar flow at 773 K for 1 h, was characterized (Figures 5, 6, and S4 (red lines)); the XRD patterns of the reduced samples were similar to those of the as-synthesized samples devoid of impurity phases. The peaks due to the 200 facet of the reduced Sr3(Fe0.8M0.2)2O7-δ appeared at lower angles when compared to those of the as-synthesized samples. These results indicate that Sr3(Fe0.8M0.2)2O7-δ topotactically releases lattice oxygen, by analogy with the topotactic transition from Sr3Fe2O6.75 to Sr3Fe2O6.19 Comparing the lattice parameters of the a and c axes of the various samples, the changes observed for these parameters upon release of lattice oxygen depend on the transition-metal dopant (Table 1). The variation in the unit-cell volume was the smallest for Sr3(Fe0.8Mn0.2)2O7-δ, which exhibited the highest-oxygen release rate. The lattice parameters of all reduced samples almost restituted to those of as-synthesized samples by the reoxidation under 5% O2/Ar flow at 773 K for 1 h. We performed the iodometric titration to determine chemical formulations of as-synthesized and reduced samples. The result of the iodometric titration is summarized in Table 1. The chemical formulations of the as-synthesized and reduced Sr3Fe2O7-δ were Sr3Fe2O6.73 and Sr3Fe2O5.98, respectively. These values agreed well with those estimated from Rietveld analysis and H2-TPR profile in our previous report.19 The oxygen content in as-synthesized Sr3Fe2O7-δ decreased by Mn- and Co-doping, and was maintained in the Ni-doped material. In the reduced Sr3(Fe0.8M0.2)2O7-δ (M = Mn, Co or Ni), the oxygen contents were apparently below 6.0. These results imply that the valence state of transition metal species in Sr3(Fe0.8M0.2)2O7-δ partly converts to divalent under the reduction condition, while Fe4+ species in Sr3Fe2O7-δ itself were not reduced to Fe2+.19 The edge position of XANES spectrum represents the valence state of the target element. However, since the edge position in XANES spectrum is known to depend on not only the valence state but also the coordination state, it is difficult to determine the valence state from only XANES spectrum.31,32 We have previously reported that the valence state of Fe in


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The Journal of Physical Chemistry Sr3Fe2O7-δ vary between Fe4+ and Fe3+ during the oxygen release/storage behavior. The edge positions of Fe K-edge XANES spectra in the as-synthesized and reduced Sr3(Fe0.8M0.2)2O7-δ (M = Mn, Co or Ni) were almost identical to those of Sr3Fe2O7-δ, and these shapes were essentially identical with those of as-synthesized and reduced Sr3Fe2O7-δ (Figures S5, S6 and S7). Then, these results indicate that Fe4+ species in Sr3(Fe0.8M0.2)2O7-δ (M = Mn, Co or Ni) might be reduced to Fe3+, but not to Fe2+. The edge positions of Mn and Co K-edge XANES spectra shifted to low energy by the reduction treatment, but did not agree with those of tri- or di-valent references. However, considering the results of the iodometric titration, Mn or Co species could be partially reduced to divalent. On the other hand, the edge position of Ni species in reduced Sr3(Fe0.8Ni0.2)2O7-δ corresponded with that of NiO, indicating that the Ni species were reduced to a divalent state. Summarizing the results, Fe species in Sr3(Fe0.8M0.2)2O7-δ (M = Mn, Co or Ni) changed between Fe4+ and Fe3+, and the valence of the transition metal dopant partially varied between tetra- or tri-valent and divalent during the oxygen release/storage procedures at 773 K. The estimated changes of the valence states for the transition metal species in Sr3(Fe0.8M0.2)2O7-δ were summarized in Table S1. EXAFS oscillations give information about the local structure around the target element. The amplitude of the EXAFS oscillations shows the information about the coordination number of the atom existing around the target element. The phase of the oscillations represents distance and species of the atom existing around the target element.33 Then, local structures in the reduced samples were investigated by EXAFS oscillations in order to observe clear differences among the reduced samples. The Fe K-edge EXAFS oscillations of reduced Sr3(Fe0.8Co0.2)2O7-δ and Sr3(Fe0.8Ni0.2)2O7-δ were obscure in the 8–12 Å-1 range, compared with those of Sr3Fe2O7-δ or the assynthesized sample. Similarly, EXAFS oscillations corresponding to the Co and Ni K-edges were hardly observed in this range. The Fourier transformed EXAFS spectra of the reduced Sr3(Fe0.8Co0.2)2O7-δ and Sr3(Fe0.8Ni0.2)2O7-δ samples revealed that the amplitudes of the peaks in the 2–4 Å range were much smaller than those of the as-synthesized samples. In addition, the Co and Ni K-edge XANES spectral features of these samples were altered by the reduction treatment (Figures S5 and S6). No impurity phases due to independent transition-metal species, such as FeO, CoO, or NiO, were observed in the XRD patterns of the reduced samples, indicating that local structures around the transition metals are largely disordered by the reduction treatment, while maintaining the perovskite framework. Indeed, variations in the unit-cell volumes before/after reduction in the Co- and Ni-doped materials are much larger than that of the Mndoped material or Sr3Fe2O7-δ. It is noteworthy that the XANES spectra and EXAFS oscillation of the reduced Sr3(Fe0.8Co0.2)2O7-δ and Sr3(Fe0.8Ni0.2)2O7-δ samples completely recover to those of the as-synthesized samples by reoxidation under 5% O2/Ar flow at 773 K for 1 h (Figures S5 and S6). On the other hand, the reduced Sr3Fe2O7-δ clearly showed EXAFS oscillation at 9–12 Å-1, but the oscillation pattern of this sample was clearly different to that of the as-synthesized sample. However, it should be noted that the phase of the EXAFS oscillations in as-synthesized Sr3(Fe0.8Mn0.2)2O7-δ did not change even after the reduction, while the amplitude of the oscillations was decreased. The result indicates that the nearest coordination number around Fe and Mn changed by the release of oxygen ions, with the maintenance of the distance between

Figure 6 Fe K-edge and doped transition metal K-edge (A) EXAFS oscillations and (B) Fourier transformed EXAFS of Sr3(Fe0.8M0.2)2O7-δ. Black dot line, as-synthesized sample; red line, the sample reduced under 5% H2/Ar flow (100 mL min-1) at 773 K for 1 h. *The chemical formulations represent those of assynthesized samples determined by the iodometric titration. The states of reduced samples were shown in Table 1.

Fe(Mn) ions and/or between Fe(Mn) ion and Sr ion. In particular, peaks in the Fourier transformed EXAFS spectra belonging to the second coordination sphere (2–4 Å) were observed even in the reduced sample. Similarly, the XANES spectral features of Sr3(Fe0.8Mn0.2)2O7-δ were almost identical before and after reduction, but slight shifts in the edge positions were observed (Figure S7). Furthermore, the change in the unit-cell volume observed for Sr3(Fe0.8Mn0.2)2O7-δ is the smallest among the



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transition metal in Sr3(Fe0.8Co0.2)2O7-δ is largely distorted by reduction, Co doping dramatically enhances the oxygen release rate of Sr3Fe2O7-δ due to high reactivity toward molecular H2 as shown in H2-TPR (Figure 4). Therefore, the enhancement of the release rate for Sr3(Fe0.8Co0.2)2O7-δ is mainly attributed to improvement in reactivity toward molecular H2. On the other hand, the results obtained from Sr3(Fe0.8Mn0.2)2O7-δ reveal that the high response rate is achieved by the improvement of latticeoxygen diffusion rate rather than the reactivity of the surface oxygen species toward the reductant. The present observations strongly suggest that the design of robust crystal frameworks which exhibit extremely small structural changes before and after the release of lattice oxygen is indispensable for improving the lattice-oxygen diffusion rate. 4. Figure 7 O2-TPD profiles under He flow of (black line) Sr3Fe2O7-δ and (purple line) Sr3(Fe0.8Mn0.2)2O7-δ.

examined samples (Table 1). These results indicate that the local structures around Mn and Fe ions in the reduced Sr3(Fe0.8Mn0.2)2O7-δ nearly completely preserve their long period structure of as-synthesized perovskite framework even by the reduction treatment, compared with that of Sr3Fe2O7-δ. The XAFS spectra and XRD pattern of the reduced Sr3(Fe0.8Mn0.2)2O7-δ sample completely recovered to those of the as-synthesized sample upon the reoxidation. The local structures around the Fe and transitionmetal dopants in the reduced Sr3(Fe0.8M0.2)2O7-δ differ according to the transition metal involved, and the differences in the degrees of disorder between the as-synthesized and reduced samples appear to contribute to the oxygen-release rates of Sr3(Fe0.8M0.2)2O7-δ. As the evaluation of oxygen release rate by DXAFS analysis performed at 873 K, H2 dissociation proceeded in all samples judging from H2-TPR results. Therefore, the oxygen-release rate seems to reflect the diffusion rate of lattice oxygen within the particles rather than the reactivities of the lattice oxygens toward molecular H2. That is, Sr3(Fe0.8Mn0.2)2O7-δ, which has the most accurately ordered local structure around Fe and Mn among the examined samples, must have high diffusion rate of oxygen species within its particles. When the oxygen desorption behavior of Sr3(Fe0.8Mn0.2)2O7-δ was evaluated by O2-TPD under He flow (Figure 7), its oxygen desorption temperature was observed at lower temperature than that of Sr3Fe2O7-δ. Also regarding the storage rate in which the oxygen diffusion in the particle is directly involved (Figure 3(C)), Sr3(Fe0.8Mn0.2)2O7-δ showed higher storage rate than other samples. In a similar manner, Motohashi et al. have proposed that the disordered nature of the local structure around Mn and Al in Ca2AlMnO5-δ, which was generated by changing the Al/Mn ratio, signiﬁcantly deteriorates the oxygen storage rate.15 In other words, OSC materials which have accurately ordered local structures exhibit superb oxygen diffusion properties. The release/storage rates in OSM mainly depend on not only diffusion rate of lattice oxygen in particle inside, but also the reactivity between the surface oxygen and reductant such as H2. If the reactivity of oxygen species on the surface of OSM with H2 molecule is unusually high, the release rate of lattice oxygen generally becomes fast by generating a large concentration gradient for lattice oxygen between the surface and particle inside of OSM. Actually, although the ordering around

CONCLUSION The OSCs of Sr3Fe2O7-δ, in which Fe sites were doped by Mn, Co and Ni, were investigated. The lattice-oxygen release temperature of Sr3Fe2O7-δ is lowered by transition metal doping. In particular, Co doping releases lattice oxygen at the lowest temperature under a flow of H2, resulting in the enhancement of the oxygen release rate in Sr3Fe2O7-δ. Sr3(Fe0.8Mn0.2)2O7-δ exhibited the highest oxygen release/storage rates among the examined samples while maintaining a high oxygen storage amount. The reduced Sr3(Fe0.8Mn0.2)2O7-δ preserved the ordered local structures around the Fe and Mn ions of the as-synthesized sample, although the local structures of transition-metal species in the reduced Sr3(Fe0.8Co0.2)2O7-δ and Sr3(Fe0.8Ni0.2)2O7-δ were distorted by the reduction treatment. The extremely subtle structural and ordering changes around Fe and transition metal dopant before and after lattice-oxygen release are revealed to greatly contribute to the enhancement of oxygen release/storage rates. Our observations provide useful information for the design of novel oxygen storage materials with rapid redox properties.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

XANES spectra of Sr3(Fe0.8M0.2)2O7-δ (M = Mn, Co, or Ni) during the DXAFS experiment, SEM images of Sr3(Fe0.8M0.2)2O7-δ (M = Mn, Co, or Ni), XRD patterns of Sr3Fe2O7-δ before and after reduction, XAFS spectra of Sr3(Fe0.8M0.2)2O7-δ (M = Mn, Co, or Ni) during OSC measurement and estimated valence states of the transition metal species in the Sr3(Fe0.8M0.2)2O7-δ.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S.H) [email protected] (T.T)

ORCID Kosuke Beppu: 0000-0002-1913-2033 Saburo Hosokawa: 0000-0003-1251-3543 Hiroyuki Asakura: 0000-0001-6451-4738 Kentaro Teramura: 0000-0003-2916-4597 Tsunehiro Tanaka: 0000-0002-1371-5836 Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This study was supported by the Program for Element Strategy Initiative for Catalysts & Batteries (ESICB). DXAFS measurements were performed with the approval of SPring-8 (Proposal Nos. 2016B1457 and 2017A1419). XAFS measurements were performed with the approval of AichiSR (Proposal No. 201703079). The authors thanked Prof. Teruki Motohashi (Kanagawa University) for the support of iodometric titration.

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