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Oxygen Storage Property and Chemical Stability of SrFe TiO with Robust Perovskite Structure 1-x

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Akito Demizu, Kosuke Beppu, Saburo Hosokawa, Kazuo Kato, Hiroyuki Asakura, Kentaro Teramura, and Tsunehiro Tanaka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06078 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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The Journal of Physical Chemistry

Oxygen Storage Property and Chemical Stability of SrFe1-x Tix O 3-δ with Robust Perovskite Structure Akito Demizu, † Kosuke Beppu,† Saburo Hosokawa,*,†,‡ Kazuo Kato,§ 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

ABSTRACT: The compounds of SrFe 1-x Ti x O 3-δ with robust perovskite structure are synthesized by Ti substitution for Fe sites in SrFeO 3-δ , and they display improved oxygen storage properties and chemical stability in CO 2 atmosphere. The oxygen storage rate of SrFe 0.8 Ti 0.2 O 3-δ is over five times faster than that of SrFeO 3-δ . Further, in the presence of CO 2 , SrFe 1-x Ti x O 3-δ with x ≥ 0.2 maintains its oxygen storage property, while for x ≤ 0.1 the oxygen release rate drastically deteriorates due to the formation of SrCO 3 . The perovskite structures (space group I4/mmm) of SrFe 1-x TixO 3-δ (x ≥ 0.2) are preserved even after reduction treatment under H 2 atmosphere at 773 K. In contrast, the material with x ≤ 0.1 undergoes a phase transformation from perovskite (I4/mmm) to brownmillerite structures (Ima2), and the latter easily reacts with CO 2 to form a large amount of SrCO 3 on the surface. Thus, the robust perovskite structure maintains its original framework despite the reduction treatment, resulting in improved oxygen storage rate as well as the CO 2 resistance.

1.

INTRODUCTION Oxygen storage materials (OSMs) are among the key components in the catalytic system used for purifying automobile exhaust gases that contain hydrocarbons, CO and NO x .1-2 The hydrocarbons and CO are easily oxidized to CO 2 by using oxygen in the exhaust gas, while NO x must be reduced to N 2 regardless of the coexistence of oxygen. In other words, the purification efficiency of the system is strongly affected by the oxygen concentration in the exhaust gas.3-4 To suppress the variation of oxygen concentration on the surface of the automobile catalyst, OSMs are widely used in the purification systems.3-9 Therefore, OSMs with high oxygen storage amount as well as fast oxygen release/storage rates are highly desirable. Currently, the CeO 2 -ZrO 2 solid solution has been applied as an OSM in automobile catalytic systems, and it can store/release oxygen into/from the crystal structure with maintained Ce and Zr sites.10-11 Such crystallographic transformation with maintenance of the original framework is called a topotactic transition. Based on the topotactic transition of Sr-Fe mixed oxides,12 we have recently found that Sr 3 Fe 2 O 7-δ with a layered perovskite structure has better oxygen storage properties than the CeO 2 -ZrO 2 solid solution, and that Pd loading on Sr 3 Fe 2 O 713-14 δ dramatically enhances the oxygen release properties. SrFeO 3 -based materials have also been reported to be an effective oxide ion conductor.15 Unfortunately, the material reacts with CO 2 to form a dense SrCO 3 layer on the surface, leading to degraded performance in the presence of CO 2 . Therefore, it is difficult to apply simple Sr-Fe mixed oxides as OSMs in practical automobile catalytic systems, through which a large amount of CO 2 passes.4 In other words, the CO 2 resistance is

another important factor to consider when developing OSMs. Various perovskite-related materials (BaYMn 2 O 5+δ 16 or LnBaCo 2 O 5+δ 17 with Ln = lanthanide elements) and oxysulfides (Pr 2 O 2 SO 4 20) have been reported to perform well as OSMs.1620 Still, little consideration has been paid toward their CO 2 resistance, even though alkaline earth or lanthanide elements contained in these OSMs are well-known to easily form carbonate species in CO 2 atmosphere because of their high basicity. 21-22 To the best of our knowledge, the CO 2 resistance of OSMs that are meant to be alternatives to the CeO 2 -ZrO 2 solid-solution in automobiles has not been clarified. In the field of solid-oxide fuel cells, some types of heterogeneous elemental doping into the perovskite structures have recently been reported to improve the structural stability of materials in the presence of CO 2 .23-27 For example, Yi et al. have found that Fe and Nb doping into BaCoO 3-δ enhances the performance of BaCoO 3-δ itself as an oxygen permeable membrane in CO 2 -containing atmosphere. The present study focuses on Ti substitution for Fe sites in SrFeO 3-δ with perovskite structure as an OSM for the following reasons. (1) The tolerance factor (t) defined by Goldchmidt is widely used as an indicator of stability and distortion for various perovskite materials, with t close to 1 regarded as stable. 28 In this sense, SrTiO 3 (t = 1.002) is a material with less distorted structure. (2) The Ti4+ ion is more stable under reducing conditions than the Fe4+ ion, and has higher chemical stability. Therefore, we expect that Fe site substitution with Ti ions can improve the structural rigidity of SrFeO 3-δ . We prepared a series of SrFe 1-x Ti x O 3-δ compounds (x = 0, 0.2, 0.4, …) to investigate their oxygen storage properties and structural stability under CO 2 -containing atmosphere. This is

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the first demonstration that the robust perovskite structure of SrFe 1-x Ti x O 3-δ (x = 0.2), which is stable even under reducing conditions, displays enhanced oxygen storage rate and CO 2 resistance compared to the unsubstituted SrFeO 3-δ . 2.

EXPERIMENTAL SECTION Preparation of SrFe 1-x Ti x O 3-δ : SrFe 1-x Ti x O 3-δ was synthesized by a polymerized complex method. Citric acid (400 mmol) was dissolved into deionized water (180 mL) at room temperature. Then, strontium carbonate (10 mmol), iron nitrate nonahydrate (10(1-x) mmol) and titanium (IV) tetraisopropoxide (10x mol) were added to the solution and stirred at 353 K for 2 h. The value of x is the concentration of titanium in SrFe 1x Ti x O 3-δ (x = 0–1). Ethylene glycol (400 mmol) was added to this solution, and the mixture was stirred at 403 K for 4 h to form a gelatinous solution. Afterwards, the gel was heated in a mantle heater at 623 K for 3 h. The obtained powder was calcined at 1273 K for 2 h. Characterization: The X-ray diffraction (XRD) patterns were obtained by Cu-Kα radiation on Rigaku Ultima IV or Rigaku MultiFlex systems. Rietveld analysis was carried out on the spectra recorded by the Rigaku MultiFlex system with the RIETAN-FP program.29 The three-dimensional visualization models of crystal structures were constructed using VESTA 3 software.30 X-ray photoelectron spectroscopy (XPS) measurement was performed on pelletized samples (0.05 g) by Mg-Kα radiation with a Shimadzu ESCA-3400 system. X-ray absorption fine structure (XAFS) spectra at Sr K-edge were measured at the beam line BL01B1 of SPring-8. The XAFS spectra were recorded in transmission mode at room temperature by using a Si(311) double-crystal monochromator. SEM images were obtained from a field emission scanning electron microscopy (FESEM, SU-8220, Hitachi High-Technologies). Temperature-

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(0.05 g) were heated to 1073 K for 1 h under 100% O 2 (30 ml min-1), and cooled to room temperature. Then, the pretreated sample was heated to 1223 K at a constant heating rate (10 K min-1) under He flow (30 ml min-1). The desorption species were monitored using a thermal conductivity detector (Shimadzu GC8A gas chromatograph). OSC measurement: The oxygen storage capacity (OSC) was measured by using a thermogravimeter (Rigaku Thermo plus). The sample (0.1 g) was heated up to 773 K in O 2 /Ar (5 vol%, 100 mL min-1) and held until a constant weight was achieved. Then, the weight changes while switching the atmosphere every 20 min between 5% O 2 /Ar and 5% H 2 /Ar were recorded. OSC measurement in the presence of CO 2 was carried out by adding CO 2 (5 mL min-1) into 5% O 2 /Ar or 5% H 2 /Ar (with the total flow rate being 105 mL min-1). Time-resolved in-situ energy-dispersive XAFS (DXAFS) data at the Sr K-edge was measured with a polychromator in a transmission mode at the beam line BL28B2 of SPring-8. The sample (0.03 g) was pressed into a pellet and placed in a batchtype in-situ cell, and pretreated under pure H 2 and O 2 at 823 K for 15 min each. Then, the in-situ cell was evacuated. The timeresolved DXAFS spectra were recorded every 70 ms under pure H 2 or O 2 at 823 K. 3. RESULTS AND DISCUSSION Oxygen Storage Properties in the Absence of CO 2 : XRD patterns (Figure 1(A)) of SrFe 1-x Ti x O 3-δ (x = 0−1) synthesized by a polymerized complex method showed pure perovskite phase without any byproducts such as TiO 2 . The morphology of SrFeO 3-δ was slightly different from that of Ti-containing material (Figure S1): Ti doping to SrFeO 3-δ pulverized the particle of SrFeO 3-δ with a particle size of about 2 µm. The peak position gradually shifted to lower angle with increasing Ti concen-

Figure 1. (A and B) XRD patterns and (C) H2-TPR profiles of SrFe1-xTixO3-δ (x = 0–1).

programmed reduction with hydrogen (H 2 -TPR) was carried out by flowing hydrogen (2 vol% in Ar, 30 mL min-1) with samples (0.05 g) bedded in a tube reactor. The sample was heated with an electric furnace with a heating rate of 5 K min-1 up to 1223 K. Then, the amount of consumed H 2 was monitored with a thermal conductivity detector (Shimadzu GC8A gas chromatograph). Oxygen temperature-programmed desorption (O 2 TPD) was carried out with a flow-type reactor. The samples

tration (Figure 1(B)), indicating that Fe4+ ions (0.585 Å) in the perovskite structure are substituted by Ti4+ ions (0.605 Å).31 H 2 -TPR profiles of SrFe 1-x Ti x O 3-δ are shown in Figure 1(C). No reduction peak was observed for x = 1 (SrTiO 3-δ ), while two reduction peaks at around 700 K and above 900 K were clearly detected for Fe–containing materials (x = 0–0.6). Since Fe3+ ion can easily take the tetrahedral coordination, the reducibility of octahedral Fe4+ ions in the perovskite structure must be much


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Figure 2. (A) Weight change of SrFe1-xTixO3-δ in the absence of CO2. (B) Oxygen release and (C) storage profiles of SrFeO3-δ (black line) and SrFe0.8Ti0.2O3-δ (red line) obtained by in-situ DXAFS analysis. (D) Initial oxygen release and storage rates of SrFeO3-δ (black line) and SrFe0.8Ti0.2O3-δ (red line) calculated from DXAFS analysis.

higher than that of Ti4+ ion which is preferential to the octahedral coordination. These results suggest that Ti4+ ion is more stable under H 2 atmosphere than Fe4+ ion in the perovskite structure. The reduction peaks were assigned based on the XRD patterns of SrFeO 3-δ samples after reduction at 773 and 1273 K. The reduction product at 773 K was Sr 2 Fe 2 O 5 with a brownmillerite structure (Figure S2). Hence, the reduction peak at around 700 K is due to the reduction from Fe4+ to Fe3+ species in SrFeO 3-δ . The theoretical amount of H 2 consumption can be calculated by the following reaction equation, assuming that all the Fe4+ in SrFeO 3-δ is reduced to Fe3+:

higher time resolution (in the order of milliseconds).32 Figure S3 shows the Sr K-edge X-ray absorption near edge structure (XANES) spectra of the as-prepared SrFeO 3-δ and Sr 2 Fe 2 O 5 . The difference between the spectra at 16115 eV reflects the change of local structure around the Sr ion during the oxygen release. To estimate the oxygen storage and release rates of SrFeO 3-δ (black line) and SrFe 0.8 Ti 0.2 O 3-δ (red line), a linear combination fitting was performed by using the XANES spectra before/after oxygen release obtained from DXAFS.33 In the results shown in Figure 2(B, C), for SrFeO 2.81 the unit of y axis is the amount of oxygen storage/release, in which the spectral change from SrFeO 2.81 to Sr 2 Fe 2 O 5 is allocated for the oxygen storage amount estimated from OSC measurement. The initial rates of oxygen release from SrFeO 3-δ and SrFe 0.8 Ti 0.2 O 3-δ were essentially identical. Surprisingly, the initial rate of oxygen storage in SrFe 0.8 Ti 0.2 O 3-δ was definitely enhanced by Ti substitution (Figure 2(D)), being more than 5 times faster than that of SrFeO 2.81 . O 2 -TPD under He flow reveals that the O 2 desorption behavior in SrFeO 3-δ is almost the same with that in SrFe 0.8 Ti 0.2 O 3-δ (Figure S4). However, the desorption peak around 940 K in SrFeO 3-δ is hardly detected for SrFe 0.8 Ti 0.2 O 3-δ . The peak observed at high temperature range above 800 K is reported to correspond to the phase transition from SrFeO 2.75 to Sr 2 Fe 2 O 5

2SrFeO 3 + H 2 → Sr 2 Fe 2 O 5 + H 2 O Since the H 2 consumption (1642 μmol-H 2 g-1) corresponds to the reduction from SrFeO 3-δ to Sr 2 Fe 2 O 5 , the composition of the former was determined to be SrFeO 2.81 . The area of the reduction peak at around 700 K decreased with increasing amount of Ti substitution, suggesting that the amount of reducible Fe4+ species is lowered by Ti substitution, as expected. The XRD pattern of the sample reduced at 1223 K indicated Fe metal and Sr 3 Fe 2 O 6 , but not Sr 2 Fe 2 O 5 (Figure S2). Hence, the reduction from Fe3+ to Fe metal at above 900 K proceeds with the structural collapse of Sr 2 Fe 2 O 5 . Remarkably, the peak observed above 900 K due to the structural collapse shifted to higher temperature with increasing Ti substitution, indicating that Ti substitution in Fe sites in SrFeO 3-δ enhances the structural stability in severely reducing conditions. OSC profiles of SrFe 1-x Ti x O 3-δ (x = 0, 0.2, 0.8, and 1) in the absence of CO 2 were measured by using thermal gravimetric (TG) analysis (Figure 2(A)). The weight change mainly corresponds to the storage and release of oxygen in the crystal structure, when the gas flow is switched between O 2 and H 2 at 773 K. The oxygen storage amount (751 μmol-O 2 g-1) of SrFeO 3-δ is almost the same as that estimated by the reduction from SrFeO 2.81 to Sr 2 Fe 2 O 5 (821 μmol-O 2 g-1). This amount decreased with increasing amount of Ti substitution. This tendency agrees well with the behavior of H 2 consumption observed at around 700 K in the H 2 -TPR profiles of SrFe 1-x Ti x O 3δ . Therefore, the oxygen storage/release processes occur by the Fe4+/Fe3+redox reaction. Although the TG analysis can roughly evaluate the oxygen storage and release rates at a low time resolution (in the order of seconds), it is unfortunately inadequate for measuring the faster rates of SrFe 1-x Ti x O 3-δ . Instead, we measured these rates of SrFe 1-x Ti x O 3-δ by using in-situ DXAFS which has a much

(A)

(B)

(C)

(D)

Sr O Fe Ti b

c

Figure 3. Crystal structures of as-prepared (A) SrFeO3-δ and (C) SrFe0.8Ti0.2O3-δ. (B) and (D) are the respective crystal structures obtained after reduction at 773 K.



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Figure 4. XRD patterns of SrFe1-xTixO3-δ reduced at 773 K.

with brownmillerite structure,34 implying that Ti-substituted SrFe 1-x Ti x O 3-δ affect the structural change accompanied with O 2 desorption. To investigate the possible structural change under H 2 flow at 773 K, we performed Rietveld analysis for SrFeO 2.81 (x = 0) and SrFe 0.8 Ti 0.2 O 3-δ (x = 0.2) with/without the reduction treatment at 773 K (Figure 3, Figure S5, S6, S7 and S8). SrFeO 2.81 displays a phase transition from the perovskite (I4/mmm) to brownmillerite structures (Ima2) after the reduction treatment. On the other hand, both as-synthesized and reduced SrFe 0.8 Ti 0.2 O 3-δ belong to the same space group (I4/mmm). Since Fe4+ ions in SrFe 1-x Ti x O 3-δ samples are mainly substituted with Ti4+ as mentioned above, the composition of SrFe 0.8 Ti 0.2 O 3-δ before oxygen release is determined to be SrFe 0.8 Ti 0.2 O 2.81 based on the identification of SrFeO 2.81 for unsubstituted SrFeO 3-δ . Considering the H 2 consumption (1043 μmol-H 2 g-1) obtained from H 2 -TPR data of SrFe 0.8 Ti 0.2 O 2.81 , the SrFe 0.8 Ti 0.2 O 3-δ sample after oxygen release is considered to be SrFe 0.8 Ti 0.2 O 2.62 . From the Rietveld analysis of SrFe 0.8 Ti 0.2 O 2.62 , oxygen ions in 2b, 8h and 4c sites in SrFe 0.8 Ti 0.2 O 3-δ with space group I4/mmm seem to be eliminated by the reduction. These results indicate that the perovskite framework (I4/mmm) of the as-prepared SrFe 0.8 Ti 0.2 O 3-δ is preserved under the reducing conditions. The oxygen storage process includes two steps: the dissociation of O 2 molecule followed by the diffusion of oxide ions in the crystal lattice. It has been suggested that the oxygen storage

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rate is affected by the structural change during oxygen storage or the distortion of crystal structure.13, 18, 35, 36 The reasons are that a major rearrangement of the metal cations during the redox process delays the redox kinetics, and crystal structures with a large number of disordered sites suppress the oxygen absorption. SrFe 0.8 Ti 0.2 O 3-δ can store oxide ions in the crystal lattice without rearranging the Sr and Fe ions, and its structural symmetry is higher than that of Sr 2 Fe 2 O 5 . Therefore, the fast oxygen storage rate of SrFe 0.8 Ti 0.2 O 3-δ must be attributed to the robust perovskite structure, which can maintain a highly symmetrical crystal structure despite the reducing treatment. During the oxygen storage process, the volume expansion (8.9 %) associated with the structural transformation from Sr 2 Fe 2 O 5 to SrFeO 2.81 is much larger than that in the case of SrFe 0.8 Ti 0.2 O 2.81 (4.6 %). Since a large volume construction/expansion under redox atmospheres tends to create structural defects such as crack or warp in the material, SrFe 0.8 Ti 0.2 O 2.81 is expected to have superior mechanical strength compared to SrFeO 2.81 . Figure 4 shows the XRD patterns of SrFe 1-x Ti x O 3-δ (x = 0– 1) after reduction at 773 K. SrFe 1-x Ti x O 3-δ (x ≥ 0.2) showed similar diffraction patterns to that derived from the perovskite structure with space group I4/mmm, while those with x ≤ 0.1 showed diffraction patterns of a brownmillerite structure (Ima2). Therefore, Ti substitution at Fe sites in SrFeO 3-δ (SrFe 1x Ti x O 3-δ , x ≥ 0.2) leads to improved rigidity in the as-prepared perovskite structure. These results allow us to conclude that the robust perovskite structure formed by Ti substitution contributes to the enhanced structural stability, which was observed in the reduction behavior of H 2 -TPR profiles at around 900 K. Oxygen Storage Properties in the Presence of CO 2 : The measured OSC profiles of SrFe 1-x Ti x O 3-δ (x = 0, 0.2, 0.8, and 1) in the presence of CO 2 are shown in Figure 5(A). The oxygen storage and release rates of the materials with x = 0 and 0.1 drastically deteriorated in the presence of CO 2 . However, SrFe 0.8 Ti 0.2 O 3-δ (x = 0.2) could quickly release and storage oxygen even in the presence of CO 2 . The oxygen storage performances of SrFe 1-x Ti x O 3-δ are summarized in Figure 5(B–D). These rates were almost the same for all SrFe 1-x Ti x O 3-δ (when x ≥ 0.2) with or without CO 2 , while the OSC performance of samples with x < 0.2 was clearly retarded by the presence of CO 2 . In other words, there is a threshold in the OSC performance of SrFe 1-x Ti x O 3-δ at around x = 0.2. Figure 6(A) shows the weight change of SrFeO 3 during two hours in an oxidation (100 ml min-1 5% O 2 /Ar + 5 ml min-1 CO 2 ) or reduction (100 ml min-1 5% H 2 /Ar + 5 ml min-1 CO 2 )

Figure 5. (A) Weight change of SrFe1-xTixO3-δ in the presence of CO2. (B) Oxygen storage amount, (C) oxygen release rate, and (D) oxygen storage rate of SrFe1-xTixO3-δ. Filled red circles: without CO2, open black circles: with CO2.


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Figure 6. (A) TG profiles of SrFeO3-δ in oxidation or reduction atmospheres with coexisting CO2 at 773 K. (B) Weight gain from initially reduced SrFe1-xTixO3-δ under the reducing atmosphere in the presence of CO2. (C) C 1s and Sr 3p XPS spectra of SrFe1-xTixO3-δ after reduction at 773 K with coexisting of CO2. (D) Surface ratio of carbonate species relative to Sr on SrFe1-xTixO3-δ treated in the reducing atmosphere.

atmosphere in the presence of CO 2 at 773 K. The weight remained constant during the oxidation treatment, but increased noticeably during the reduction treatment following an initial decrease. These results indicate that a side reaction occurred in the CO 2 atmosphere under the reducing conditions but not under the oxidizing conditions. This weight increase was drastically suppressed by doping Ti ions into SrFeO 3-δ , as shown in Figure 6(B). To evaluate the chemical species formed on the surface during the reduction treatment in the presence of CO 2 , XPS analysis was performed for SrFe 1-x Ti x O 3-δ . The C 1s peak derived from SrCO 3 appeared at 289.5 eV,37 and those due to Sr 3p 1/2 and 3p 3/2 were observed at 281 and 270 eV, respectively. Then, the ratio of carbonate species relative to Sr on the SrFe 1-x Ti x O 3δ surface was estimated from the C 1s peak of carbonate species and Sr 3d peaks (Figure 6(C)). As shown in Figure 6(D), the surface ratio of carbonates decreased with increasing amount of Ti substitution in SrFe 1-x Ti x O 3-δ , showing an inflection point at x = 0.2. These results suggest that the generation of SrCO 3 is drastically suppressed by Ti substitution for Fe sites in SrFeO 3δ . The weight increase under the reduction condition is attributed to the generation of SrCO 3 . Interestingly, the trend of SrCO 3 formation estimated from XPS analysis (Figure 6(D)) against the Ti substitution amount was different from that obtained from TG analysis. XPS analysis shows that the amount of SrCO 3 gradually decreases even when x > 0.2, while the weight increase due to the generation of SrCO 3 when x > 0.2 is less clear according to the TG analysis (Figure 6(B)). Since the XPS analysis is more sensitive to the surface structure than TG analysis, this discrepancy implies that SrCO 3 must be preferentially generated near the surface of SrFe 1-x Ti x O 3-δ . The threshold of OSC performance in the presence of CO 2 at x = 0.2 agrees well with the inflection points observed from XPS and TG analyses of the sample treated for a long time under the reduction condition in CO 2 -containing atmosphere. As mentioned above (Figure 4), the reduced samples of SrFe 1x Ti x O 3-δ with x ≤ 0.1 had the brownmillerite structure, while those with x ≥ 0.2 maintained the original perovskite structure. Considering these results, the brownmillerite structure formed during reduction treatment must react more easily with CO 2 to form a larger amount of SrCO 3 . Since the coordination number (C.N.) of 8 around the Sr ion in the brownmillerite structure is lower than that (C.N. ≈ 12) in

the perovskite structure (I4/mmm), the vicinity of the Sr ions in SrFe 1-x Ti x O 3-δ is more closely packed with oxide ions compared to that in Sr 2 Fe 2 O 5 . Therefore, the Sr ions in the brownmillerite structure would be more accessible to the CO 2 molecules. Thus, similar to the reasons for the improved oxygen storage rate, the robust perovskite structure of SrFe 1-x Ti x O 3-δ (x ≥ 0.2) enhances the CO 2 resistance compared to SrFeO 3-δ . 4.

CONCLUSION

Ti substitution for Fe sites in SrFeO 3-δ is found to inhibit the structural collapse of the material in severely reducing conditions. In the absence of CO 2 , OSC measurements indicate that the oxygen storage rate is dramatically enhanced in the Ti-substituted SrFe 1-x Ti x O 3-δ . In the presence of CO 2 , SrFe 1-x Ti x O 3-δ with x ≥ 0.2 has faster oxygen storage and release rates, because the Ti substitution suppresses SrCO 3 formation under the reducing conditions. Fe site substitution by Ti ions in SrFeO 3-δ (SrFe 1-x Ti x O 3-δ , x ≥ 0.2) with perovskite structure (I4/mmm) maintains its structural framework even under strong reducing conditions, while SrFeO 3-δ displays a phase transition to brownmillerite structure (Ima2). The formation of a robust perovskite structure by Ti substitution could thus improve the oxygen release rate of SrFeO 3δ , as well as its OSC performance in the presence of CO 2 . These results provide valuable information for the development of not only functional OSMs but also oxide ion conductors in fuel cells.

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

SEM images of SrFeO 3-δ and SrFe 0.8 Ti 0.2 O 3-δ , XRD patterns of as-synthesized SrFeO 3-δ and reduced samples at various temperature, Sr K-edge XANES spectra of SrFeO 3δ before and after reduction, O 2 -TPD profiles of SrFeO 3-δ and SrFe 0.8 Ti 0.2 O 3-δ and results of Rietveld analysis of SrFeO 3-δ and SrFe 0.8 Ti 0.2 O 3-δ with/without the reduction treatment (PDF)

AUTHOR INFORMATION



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

ORCID Saburo Hosokawa: 0000-0003-1251-3543 Kosuke Beppu: 0000-0002-1913-2033 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.

ACKNOWLEDGMENT This study was supported by the Program for Elements Strategy Initiative for Catalysts & Batteries (ESICB). The DXAFS experiments were performed with the approval of SPring-8 (Proposal Nos. 2014B1371, 2015B1754, 2016A1379, and 2016B1743). The authors thank Prof. Kenji Wada (Kagawa University), Dr. Junya Ohyama (Nagoya University), and Mr. Takuya Shibano (Kyoto University) for assisting with the DXAFS experiments.

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