NOx Oxidation and Storage Properties of a Ruddlesden–Popper-Type

Jul 1, 2019 - The development of NOx-trapping catalysts for automobiles is highly desired to meet the current strict exhaust emission regulations. Thi...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

NOx Oxidation and Storage Properties of a Ruddlesden−PopperType Sr3Fe2O7−δ-Layered Perovskite Catalyst Kazuki Tamai,† Saburo Hosokawa,*,†,‡ Hiroshi Okamoto,§ Hiroyuki Asakura,†,‡ Kentaro Teramura,†,‡ and Tsunehiro Tanaka*,†,‡

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†

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 § Advanced Research and Innovation Center, DENSO Corporation, Komenokicho-Minamiyama 500-1, Nisshin, Aichi 470-0111, Japan S Supporting Information *

ABSTRACT: The development of NOx-trapping catalysts for automobiles is highly desired to meet the current strict exhaust emission regulations. This study demonstrates that NOx oxidation and storage reactions proceed over Pt-free Sr3Fe2O7−δ with a Ruddlesden−Popper-type layered perovskite structure. Two types of Sr−Fe perovskite with oxygen storage capacity, namely, SrFeO3−δ and Sr3Fe2O7−δ, are studied as NOx-trapping catalysts. Sr3Fe2O7−δ shows higher NOx storage capacity than SrFeO3−δ; its activity is comparable to that of Pt/ Ba/Al2O3 calcined at 1273 K. NOx temperature-programmed desorption and diffuse reflectance infrared Fourier transform experiments confirm the superior NOx-trapping ability of Sr3Fe2O7−δ over SrFeO3−δ. In addition, NO temperature-programmed reactions and O2 temperature-programmed desorption experiments reveal that these catalysts operate through a novel NO oxidation mechanism involving the consumption of their lattice oxygens and topotactic structural changes at a temperature of around 350−400 K. The reduction performance of trapped NOx on Pd-modified Sr−Fe perovskites is investigated by lean−rich cycle experiments using H2 as the reductant. Pd/Sr3Fe2O7−δ shows significantly high NOx removal efficiency over the entirety of each lean−rich period. Modifying Sr3Fe2O7−δ with Pd is also effective for NOx storage in the presence of H2O and CO2 and the regeneration of the catalyst following SOx sorption. Sr3Fe2O7−δ, with both NOx adsorption and NO oxidation capabilities, acts as a Pt-free NOx-trapping catalyst, exhibiting both high NOx storage capacity and high thermal tolerance. KEYWORDS: NO oxidation, NOx-trapping catalyst, Pt-free, layered perovskite, Ruddlesden−Popper-type layered perovskite, oxygen storage material

1. INTRODUCTION Nitrogen oxides (NOx, NO + NO2) are serious air pollutants that cause a variety of environmental problems, such as acid rain and photochemical smog.1,2 Automotive exhaust gases are the main mobile NOx emission sources, and various techniques have been developed to date to remove NOx from exhaust gases.3,4 The use of a three-way catalyst (TWC) that contains Pt, Pd, and Rh as the active species is the most reliable technique for simultaneously converting NOx and other harmful materials, including hydrocarbons (HCs) and carbon monoxide (CO), into harmless products in stoichiometric airto-fuel (A/F) ratios. However, there is a global desire for reduced CO2 emissions and more fuel-efficient engines that operate at high A/F ratios, in other words, under fuel-lean conditions. Under such operating conditions, conventional TWCs cannot efficiently remove NOx from oxygen-rich exhaust gases. Selective catalytic reduction (SCR) catalysts © XXXX American Chemical Society

that use NH3 as the reductant, such as Cu-zeolites and Fezeolites, are used as alternatives, especially for diesel-engine exhaust gases.5,6 SCR catalysts efficiently reduce NOx to N2, even in the presence of excess oxygen; however, NH3-SCR systems for automotive applications require urea solution tanks and downstream NH3 oxidation catalysts that prevent NH3 slipping. The use of a NOx-trapping catalyst is another technique that removes NOx from automobile exhaust gases at high A/F ratios, under which the catalytic reduction of NOx to N2 is difficult. Pt/Ba/Al2O3 is a typical NOx-trapping catalyst known as a NOx storage and reduction catalyst.7−10 The catalyst removes NOx by oxidizing NO to NO2 at Pt sites and by Received: May 9, 2019 Accepted: July 2, 2019 Published: July 2, 2019 A

DOI: 10.1021/acsami.9b08139 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces trapping NOx on BaO as nitrite (NO2−) or nitrate (NO3−) under lean-fuel conditions. Occasionally, the trapped NOx on BaO is released under rich-spike conditions and reduced to N2 on Pt. However, NOx storage performance is significantly inferior at temperatures below 473 K because NO oxidation is kinetically hindered at low temperatures.11,12 In addition, Ptbased catalysts have poor thermal durabilities, with the NOx storage activity decreasing due to the sintering of Pt species after aging at high temperatures. Perovskite catalysts have recently received attention as Ptfree NOx-trapping catalysts.13−16 In particular, Sr-doped lanthanum mixed oxides have been extensively investigated because they exhibit high activities for NO oxidation, which is a key reaction for efficient NOx trapping.17−20 Kim et al. reported that La0.9Sr0.1CoO3 and La0.9Sr0.1MnO3 exhibit high NO oxidation activities comparable to that of the Pt/Al2O3 catalyst and that La0.9Sr0.1MnO3 is a Pt-free lean NOx trap (LNT) catalyst in simulated diesel exhaust gases when combined with Pd-Rh/BaO/CeO2-ZrO2.18,19 Shen et al. analyzed the kinetics of NO oxidation on La0.8Sr0.2MnO3 and found that the rate-determining step (RDS) was the desorption of NO2 in the 373−473 K temperature range, while the RDS on Pt/Al2O3 was determined to be the oxidation of gaseous NO by dissociative oxygen on the catalyst at 423−573 K.20 These Sr-doped lanthanum mixed oxides exhibit high NOx oxidation activities in the absence of Pt. For a NOx-trapping catalyst to be efficient, both its NO oxidation and NOx adsorption capabilities are important factors. However, the above-mentioned catalysts require additional modifications or compositions with NOx storage components such as BaO21,22 because their NOx adsorption abilities are not sufficiently high; hence, catalysts that exhibit both NO oxidation and NOx adsorption abilities are required to achieve high NOx storage performance. In this study, we focused on Sr3Fe2O7−δ with a Ruddlesden− Popper-type (RP-type) layered perovskite structure of the general formula: An+1BnO3n+1 (n = 1, 2, etc.). Sr3Fe2O7−δ (n = 2) consists of an alternating arrangement of two SrFeO3−δ perovskite layers and one SrO rock-salt layer (Figure 1) and

oxygen diffusion within the structure was very small (1.09 eV) due to the presence of oxygen migration pathways between the equatorial oxygen sites of FeO6 and the apical oxygen sites between FeO6 bilayers.26 We proposed that such high oxygen mobility could be used to oxidize NO in kinetically hindered temperature regions, with the SrO rock-salt interlayers expected to act as NOx-trapping sites. Therefore, we investigated the NOx storage performance of SrFeO3−δ and Sr3Fe2O7−δ from the viewpoints of their NO oxidation and NOx adsorption abilities, focusing on the effect of the perovskite structure and the roles of lattice oxygens. In addition, the reusabilities of the catalysts following reductive treatment of the trapped NOx with H2 and the effect of coexisting gases (CO2, H2O, and SO2) on NOx storage performance were also investigated for the Pd-modified Sr−Fe perovskites.

2. EXPERIMENTAL SECTION 2.1. Preparation of SrFeO3−δ and Sr3Fe2O7−δ. SrFeO3−δ and Sr3Fe2O7−δ were synthesized by the polymerized complex method. Iron nitrate nonahydrate (Fe(NO3)3·9H2O; 20 mmol for SrFeO3−δ and 13.3 mmol for Sr3Fe2O7−δ) and strontium carbonate (SrCO3; 20 mmol) were added to an aqueous solution of citric acid (180 mL, 400 mmol for SrFeO3−δ or 333 mmol for Sr3Fe2O7−δ), and the resulting solution was stirred for 2 h at 353 K to produce a solution of the metal oxide complex. Ethylene glycol (400 mmol for SrFeO3−δ or 333 mmol for Sr3Fe2O7−δ) was added to the solution, after which it was stirred at 403 K for 4 h to produce a gelatinous solution. The gel was pyrolyzed in a mantle heater at 623 K for 3 h, and the brown powder obtained in this manner was finally calcined at 1273 K for 2 h (SrFeO3−δ) or 5 h (Sr3Fe2O7−δ) in an electronic furnace under a flow of air (1 L min−1). In some experiments, 1 wt % Pd/SrFeO3−δ and 1 wt % Pd/Sr3Fe2O7−δ were also used. The Pd-modified catalysts were synthesized by the following simple impregnation method. Calcined Sr−Fe perovskite (0.4 g) was added to an evaporating dish with an ethyl acetate solution containing Pd acetate (9 mL). The suspension was evaporated on a hot plate, and the obtained powder was calcined at 1073 K for 5 h under a flow of air. To compare NOx storage activities, the conventional Pt/Ba/Al2O3 (1 wt % Pt, 7 wt % Ba) Pt-based NOx-trapping catalyst was synthesized by an impregnation method. Al2O3 (JRC-ALO-7) powder was added to the aqueous solution containing Pt(NH3)2(NO2)2 and Ba(OOCCH3)2, and the suspension was dried at 353 K overnight. The powder was calcined at 773 or 1273 K for 2 h (referred to as “Pt/ Ba/Al2O3_773” or “Pt/Ba/Al2O3_1273”, respectively). 2.2. NOx Storage Experiments. NOx storage experiments were carried out using a conventional fixed-bed flow system. Catalyst granules (0.3−0.6 mm) were introduced into a quartz reactor, which was then pretreated at 773 K under a flow of 10% O2 in He (50 mL min−1) for 1 h. After the temperature was decreased to 573 K, the reaction gas (200 ppm NO, 3% O2, and He balance, at total flow rate of 100 mL min−1) was introduced at a gas hourly space velocity (GHSV) of 50,000 h−1. The outlet concentration of NOx was measured using a chemiluminescence NOx analyzer (PG-335, Horiba Ltd., Japan). Desorption profiles for NOx stored on the Sr−Fe perovskites were obtained by a NOx temperature-programmed desorption technique (NOx-TPD). Following NOx storage for 1 h, the catalyst was heated from room temperature to 973 K at a rate of 20 K min−1 in 10% O2/He, and NOx desorption was determined by the NOx analyzer. To investigate the effects of poisoning gases on NOx storage performance, NOx storage experiments in a reaction gas containing CO2 and H2O (200 ppm NO, 3% O2, 5% CO2, 10% H2O, and He balance, 100 mL min−1) were also carried out. H2O was supplied to the reactor by a plunger pump (LP-3000, Lab-Quatec, Japan) through a vaporizer and heated line. 2.3. Lean−Rich Cycle Experiments. NOx storage performance after reductive treatment with H2 was examined in lean−rich cycle

Figure 1. Schematic crystal structures of a cubic SrFeO3 and a Ruddlesden−Popper-type Sr3Fe2O7. The circles correspond to Sr cations (green) and oxygen anions (red), which are located at the corners of a FeO6 octahedron.

exhibits oxygen storage capacity (OSC) due to the oxygen nonstoichiometries of the perovskite layers.23,24 We reported the use of Sr3Fe2O7−δ as a three-way catalyst support.25 Pd/ Sr3Fe2O7−δ demonstrated TWC activity over wider A/F ranges than Pd/Al2O3 by maintaining the reactive metallic state of the modified Pd through the use of its oxygen storage/release functionality. Ota et al. calculated the oxygen vacancy and ionic transport in Sr3Fe2O7−δ and found that the barrier for B

DOI: 10.1021/acsami.9b08139 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces experiments. The rich gas (3% H2 diluted with He, 100 mL min−1) was repeatedly flowed for 5 min following the NOx storage reaction in the lean gas (200 ppm NO, 3% O2, and He balance, 100 mL min−1) for 15 min at 573 K. The Pd/SrFeO3−δ and Pd/Sr3Fe2O7−δ catalysts were preconditioned at 573 K in the rich gas for 30 min after pretreatment at 773 K under a flow of 10% O2/He. 2.4. Temperature-Programmed Reactions with NO. The reactivities of the lattice oxygens in the perovskite catalysts toward NO molecules were evaluated by a NO temperature-programmed reaction (NO-TPR). Prior to NO-TPR, the catalyst (50 mg) was preoxidized in flowing pure O2 at 973 K for 1 h, after which it was gradually cooled to 323 K. The He gas was allowed to flow for at least for 2 h to remove residual oxygen in the reactor. The catalyst was heated at a rate of 5 K min−1 under a flow of 1.00% NO in He, and the consumption of NO, or the total consumption of NO and NO2, was determined by the NOx analyzer. 2.5. Characterization. X-ray powder diffraction (XRD) patterns were obtained using a MultiFlex diffractometer (Rigaku Corp., Japan) (2θ range, 10°−120°; step size, 0.02°; duration time, 3.2 s) with Cu Kα radiation (λ = 1.5405 Å). Scanning electron microscopy (SEM) images were acquired on a field-emission scanning electron microscope (SU-8220, Hitachi High Technologies). N2 adsorption isotherms were obtained on a BELSORP-mini II system (MicrotracBEL Corp., Japan) at 77 K, and the specific surface area (SBET) of each catalyst was calculated using the Brunauer−Emmett−Teller (BET) method. Temperature-programmed oxygen desorption (O2TPD) experiments were carried out in a similar process to that used for NO-TPR. The sample (50 mg) was pre-oxidized in flowing pure O2 at 1073 K for 1 h, after which it was gradually cooled to room temperature (r.t.). A He gas was flowed until the signal from the thermal conductivity detector (TCD) stabilized. The catalyst was heated at 5 K min−1 under a flow of He, and the desorption of oxygen was detected by the TCD fitted to a GC-8A unit devoid of columns (Shimadzu Corp., Japan). In situ diffuse reflectance infrared Fourier transform (DRIFT) spectra following NO2 adsorption were acquired using an ISDR-600 spectrometer (JASCO Corp., Japan) equipped with a mercury−cadmium−tellurium (MCT) detector with a resolution of 4 cm−1 and cooled with liquid nitrogen. The sample powder (30 mg) was placed in a diffuse reflectance cell fitted with a CaF2 window. The background spectrum of the sample was recorded at r.t. following oxidizing pretreatment at 973 K in 10% O2/He. The IR spectrum of the NOx-adsorbed sample was obtained at r.t. after the sample was saturated with 0.1% NO2 in He at 573 K.

found to belong to the I4/mmm space group, were obtained as pure phases in Sr/Fe ratios of 1 and 3/2, respectively. These materials have oxygen nonstoichiometries from the ideal SrFeO3 and Sr3Fe2O7 formulas due to the presence of oxygen vacancies in the perovskite units. The amount of oxygen vacancy is represented by the symbol “δ”. The chemical formulas of these samples were determined by thermogravimetry to be SrFeO2.86 and Sr3Fe2O6.67, respectively (see Figure S1a,b, Supporting Information). The SEM images (Figure 3) of SrFeO3−δ and Sr3Fe2O7−δ show well-defined crystallized surface structures, with particles

Figure 3. SEM images of (a, b) SrFeO3−δ and (c, d) Sr3Fe2O7−δ calcined at 1273 K.

approximately 1−3 μm and 0.5−1 μm in size, respectively; these sizes are consistent with the specific surface areas determined by N2 adsorption isotherm experiments using the BET method (0.8 and 1.7 m2 g−1, respectively). Sr3Fe2O7−δ is composed of particles that are smaller than those of SrFeO3−δ, which indicates that Sr3Fe2O7−δ was less sintered than SrFeO3−δ during high-temperature calcination. The high thermal durability of Sr3Fe2O7−δ is attributable to its stacked structure composed of SrO rock-salt layers between SrFeO3−δ perovskite layers. Similar enhancements in thermal and chemical stabilities due to SrO rock-salt interlayers have been previously reported for Srn+1MnO3n+1 (M = Fe, Ti), La2−xSrxMO4−δ (M = Mn, Fe, Co, Ni), and other RP-type materials.27−30 3.2. NOx Storage Capacities of Sr−Fe Perovskites. Figure 4 shows the time courses of the NOx storage reactions at 573 K on Sr−Fe perovskites and Pt/Ba/Al2O3 catalysts. In the absence of a catalyst, the outlet NOx concentration was 200 ppm following the introduction of NO. On the other

3. RESULTS AND DISCUSSION 3.1. Structures of the Sr−Fe Perovskites. Figure 2a,c displays the XRD patterns of the prepared Sr−Fe perovskites calcined at 1273 K. SrFeO3−δ and Sr3Fe2O7−δ, which were

Figure 2. XRD patterns of (a) SrFeO3−δ and (b) SrFeO3−δ following reactions with NO at 673 K and (c) Sr3Fe2O7−δ and (d) Sr3Fe2O7−δ following reactions with NO at 673 K. XRD patterns of SrFeO3−δ and Sr3Fe2O7−δ are produced from the standard cards of 91063-ICSD and 74436-ICSD, respectively.

Figure 4. Time courses of NOx storage reactions at 573 K: (a) without catalyst and on (b) SrFeO3−δ, (c) Sr3Fe2O7−δ, (d) Pt/Ba/ Al2O3_773, and (e) Pt/Ba/Al2O3_1273. C

DOI: 10.1021/acsami.9b08139 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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nm−2, respectively. These values are much greater than those of Pt/Ba/Al2O3_773 and Pt/Ba/Al2O3_1273, namely, 1.4 and 0.5 nm−2, respectively. When the (001) facets of SrFeO3 and Sr3Fe2O7 are considered,31,32 the densities of surface Sr atoms are both 6.7 nm−2. The surface NOx densities of the Sr−Fe perovskites are comparable or higher than the densities of their surface atoms; that is, Sr−Fe perovskites exhibit high NSCs despite their small surface areas because these catalysts can densely store NOx on their surfaces. Previous studies on Pt/Ba/Al2O3 concluded that the distance between Pt particles (NOx oxidation sites) and Ba species (NOx storage sites) is important for efficient NOx trapping.8,33,34 The XRD pattern of Pt/Ba/Al2O3_1273 (not shown) reveals that calcination at a high temperature results in the aggregation of Pt metal particles on the catalyst, which reduces the interface around the Pt particles and decreases the NOx-trapping efficiency. In other words, the segregation of NOx oxidation sites and NOx storage sites decreases the NOxtrapping efficiency of the Pt-based catalyst. On the other hand, on Pt-free Sr−Fe perovskites, both NOx oxidation and storage processes proceed on the same catalyst surface, and segregation does not occur. Consequently, Sr−Fe perovskites can store NOx at high densities even if high-temperature calcination promotes sintering of the catalyst surface. Figure 5 shows NOx desorption profiles of the Sr−Fe perovskites following NOx storage reactions. The amounts of

hand, when the above-mentioned catalysts were used, the NOx concentration was 0 ppm for a certain period of time, after which it gradually increased to 200 ppm. This behavior indicates that the catalysts remove NOx from the reaction gas by entrapment. Sr3Fe2O7−δ was able to store much more NOx than SrFeO3−δ despite their similar specific surface areas. The NOx concentration was maintained below 2 ppm (conversion of NOx > 99%) for 278 s with Sr3Fe2O7−δ, which is about eight times longer than the time observed for SrFeO3−δ (36 s). The amounts of trapped NOx species when NOx concentrations exceeded 180 ppm, in other words, when the NOx-trapping efficiencies were 10%, were calculated as NOx storage capacities (NSCs). Detailed calculational methods are provided in the Supporting Information. The NSCs of SrFeO3−δ and Sr3Fe2O7−δ were 13 and 69 μmol g−1, respectively (Table 1). The reusabilities of these catalysts Table 1. NOx Storage Capacity and BET-Specific Surface Area of Each Catalyst catalyst

SBET (m2 g−1)

trapped NOxa (μmol)

NSCb (μmol g−1)

surface NOx densityc (nm−2)

SrFeO3−δ Sr3Fe2O7−δ Pt/Ba/Al2O3_773 Pt/Ba/Al2O3_1273

0.8 1.7 156 108

2.7 14.8 24.7 5.2

13 69 357 85

9 25 1.4 0.5

a Amount of trapped NOx when the NOx concentration exceeded 180 ppm (90% of the introduced NO). bNOx storage capacity (NSC) was calculated by dividing the amount of trapped NOx by the catalyst weight. cSurface NOx density was calculated by dividing the NSC by SBET. See the Supporting Information for further details.

were tested by repeating the NOx storage reactions after regenerating the catalysts by the NOx desorption procedure at 973 K in 10% O2/He (Figure S2). Although the NSCs were lower in the second runs, the storage activities were almost identical during the second and third runs. These changes in NOx storage performance are explained by the reconstruction of the catalyst surface. During the catalyst calcination process, SrCO3 species decompose and react with iron oxide to form Sr−Fe perovskites at around 900 K, as shown in the thermogravimetric analysis profiles (Figure S3). However, tiny amounts of residual SrCO3 species should exist on the surface even after the calcination at 1273 K due to the chemical equilibrium with the gas-phase CO2 in the air. During the NOx storage reaction, residual carbonate species are substituted by nitrate species. Sr(NO3)2 is less stable than SrCO3 and decomposes during the NOx desorption procedure. This decomposition accompanies the surface reconstruction of the catalyst. As a consequence, the NOx storage performance was different during the second run but hardly changed during the third run. Pt/Ba/Al 2 O 3 exhibited a high NSC, with the NO x conversion maintained above 99% for 594 s when the catalyst was calcined at 773 K; however, the capacity was significantly lower when the catalyst was calcined at 1273 K. The NOx concentration on the Pt/Ba/Al2O3_1273 catalyst rapidly increased to 170 ppm over 300 s. Although the catalyst continued to trap NOx after 300 s, the NOx-trapping efficiency was only 15%. The NSC standardized by the catalyst surface area of each catalyst is summarized in Table 1 as the surface NOx density. The surface densities of trapped NOx on SrFeO3−δ and Sr3Fe2O7−δ were calculated to be 9 and 25

Figure 5. NOx desorption profiles of (a) SrFeO3−δ and (b) Sr3Fe2O7−δ following 1 h NOx storage reactions.

desorbed NOx are almost equal to those of stored NOx. Usually, NOx-TPD is carried out under a flow of He gas; however, in this study, it was carried out in 10% O2/He to prevent the formation of N2O and N2, which would be formed in the reaction of trapped NOx with oxygen vacancy sites on the perovskite catalysts during NOx-TPD. NOx desorption peaks for SrFeO3−δ and Sr3Fe2O7−δ were observed at 687 and 750 K, respectively. These profiles suggest that significantly more NOx is adsorbed by Sr3Fe2O7−δ than SrFeO3−δ and that this NOx is more strongly adsorbed; these strong NOx adsorption sites result in the superior NSC of Sr3Fe2O7−δ compared to SrFeO3−δ. To investigate the differences in the NOx adsorption properties of SrFeO3−δ and Sr3Fe2O7−δ, DRIFT spectra were acquired following NO2 adsorption (Figure 6). The IR absorptions observed at 1629 and 1602 cm−1 for both SrFeO3−δ and Sr3Fe2O7−δ are attributed to gaseous NO2.35 These IR bands quickly disappeared when the gas was switched from 0.1% NO2/He to He. Bidentate nitrate species also have absorptions in this region; however, such species were not observed in the spectra of the Sr−Fe perovskites, which may be due to the strong IR absorptions of the Sr−Fe perovskites themselves and their small specific D

DOI: 10.1021/acsami.9b08139 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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determined, and the corresponding NO-TPR profiles are shown in Figure S5. The NOx concentration hardly changed during the NO-TPR of SrFeO3−δ, which suggests that the observed consumption of NO by SrFeO3−δ is mainly due to the oxidation of NO to NO2 (Figure S5a). On the other hand, NOx adsorption (positive peak at 500−600 K) and desorption (negative peak at 600−750 K) behavior was observed for Sr3Fe2O7−δ (Figure S5b). On the basis of these two NO-TPR profiles, we conclude that the consumed NO in Figure 7 is mainly due to the oxidation of NO to NO2 as well as the adsorption of NOx by the catalysts, while negligible amounts of other products, such as N2 and N2O, were formed during the NO-TPR experiments. The formation of NO2 in the absence of oxygen in the feed gas indicates that NO molecules are oxidized by reactions with the lattice oxygens in the Sr−Fe perovskites. The temperatures of the NO consumption peaks reveal that the lattice oxygens in SrFeO3−δ react more readily with NO molecules than those in Sr3Fe2O7−δ. To confirm the source of the oxygen species responsible for the oxidation of NO during NO-TPR, O2-TPD experiments were also carried out (Figure 8). Sharp and stepwise oxygen

Figure 6. DRIFT spectra of adsorbed NOx species on (a) SrFeO3−δ and (b) Sr3Fe2O7−δ.

surface areas, which make observing surface NOx species difficult. On the other hand, the spectrum of NO2-adsorbed Sr3Fe2O7−δ exhibited strong absorptions at 1451, 1405, and 1362 cm−1. According to the IR spectrum of NO2-adsorbed SrO (Figure S4) and the literature,36−38 the IR band at 1451 cm−1 is attributed to surface monodentate nitrate, and IR bands at 1405 and 1362 cm−1 are attributed to bulk ionic nitrate species. These additional adsorbed NOx species explain the superior NOx adsorption ability of Sr3Fe2O7−δ, as shown in NOx desorption profiles (Figure 5). Considering the structural differences between SrFeO3−δ and Sr3Fe2O7−δ, SrO rock-salt layers between the SrFeO3−δ perovskite layers may act to trap NOx and improve the NOx adsorption capacity of Sr3Fe2O7−δ, as shown in the following reaction scheme: SrO + 2NO2 + O* → Sr(NO3)2

In this scheme, O* refers to a reactive oxygen species derived from the catalyst or generated by the disproportionation of NO2 to NO. 3.3. Reactivities of Lattice Oxygens in Sr−Fe Perovskites with NO. The lattice oxygens of Sr−Fe perovskites are expected to be involved in the oxidation of NO. Therefore, NO temperature-programmed reaction (NO-TPR) experiments were carried out to evaluate the reactivities of the lattice oxygens in the Sr−Fe perovskites toward NO molecules in the absence of O2 (Figure 7). SrFeO3−δ and Sr3Fe2O7−δ

Figure 8. O2-TPD profiles of (a) SrFeO3−δ and (b) Sr3Fe2O7−δ.

desorptions were observed at around 500−700 K and 700− 940 K on SrFeO3−δ, while Sr3Fe2O7−δ exhibited a broad oxygen desorption at above 550 K. SrFeO3−δ began to desorb oxygen at a temperature that was about 50 K lower than that of Sr3Fe2O7−δ; this ordering is consistent with that of NO-TPR. The oxygen desorption profile of SrFeO3−δ can be explained by the presence of intermediate SrFeO3−δ phases with different oxygen deficiencies. SrFeO3 devoid of oxygen vacancy sites transforms with increasing oxygen deficiency as follows:39,40 SrFeO3 (cubic, Pm-3m) → SrFeO2.875 (tetragonal, I4/mmm) → SrFeO2.75 (orthorhombic, Cmmm) → SrFeO2.5 (orthorhombic, Icmm). The total amount of desorbed oxygen during O2-TPD was 2094 μmol-O g−1. Assuming that the stable chemical state after total oxygen desorption is SrFeO2.5, this amount corresponds to the change from SrFeO2.88 to SrFeO2.5, and the calculated composition at 700 K is SrFeO2.72. On the basis of these results, oxygen desorption at around 500−700 K is attributed to the transformation between the two intermediate phases, namely, from SrFeO2.875 to SrFeO2.75, and the desorption at around 700−940 K is attributed to the transformation of SrFeO2.75 to SrFeO2.5. The shoulders observed ahead of each oxygen desorption peak are due to changes in the rate-determining steps since oxygen migration in the bulk and transformations of the catalyst structure affect the rate of oxygen desorption as well as the oxygen desorption from the catalyst surface. Sr3Fe2O7−δ exhibits no apparent intermediate phase between Sr3Fe2O7 and Sr3Fe2O6, and the

Figure 7. NO-TPR profiles of (a) SrFeO3−δ and (b) Sr3Fe2O7−δ when detecting NO alone.

were heated in a flow of NO gas, and the consumption of NO by each catalyst was examined from a temperature of around 350−400 K. The main NO consumption peaks for SrFeO3−δ and Sr3Fe2O7−δ were observed at 525 and 537 K, respectively. In the case of SrFeO3−δ, an apparent shoulder was observed ahead of the main consumption peak, while Sr3Fe2O7−δ exhibited only a weak consumption peak in this temperature region. The total concentrations of NO + NO2 were E

DOI: 10.1021/acsami.9b08139 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces structure remains in the I4/mmm space group over the entire range due to its layered structure. The total amount of desorbed oxygen during O2-TPD was 1490 μmol-O g−1, which corresponds to the change from Sr3Fe2O6.72 to Sr3Fe2O6 using a similar assumption to that used for SrFeO3−δ. The amounts of lattice oxygen in SrFeO3−δ and Sr3Fe2O7−δ consumed by NO at 673 K during NO-TPR were calculated to be 941 and 893 μmol g−1, respectively; these values correspond to the following changes: SrFeO2.88 to SrFeO2.71 and Sr3Fe2O6.72 to Sr3Fe2O6.29 on the basis of the O2-TPD results. The amounts of consumed oxygen were about 45 and 60% of the desorbed oxygen during O2-TPD, respectively. Such large amounts of consumed oxygen are attributable to the participation of lattice oxygens in the bulk as well as adsorbed oxygens and lattice oxygens in the subsurface of the catalyst. In the case of SrFeO3−δ, the amount of oxygen consumed during NO-TPR at 673 K is consistent with the amount of desorbed lattice oxygen at around 500−700 K. These results reveal that the lattice oxygens in the bulk of a Sr−Fe perovskite are successively consumed by reactions with NO near the catalyst surface due to high oxygen mobility; the NO-TPR profiles reflect the properties of the lattice oxygens in Sr−Fe perovskites. During NO-TPR, the oxygen consumption started at a temperature about 150 K lower than that for oxygen desorption during O2-TPD because oxygen desorption is essentially an endothermic process that is promoted by the exothermic reaction, namely, oxidation of NO to NO2. NO consumption was also observed above 600 K, which is above the main NO consumption peak; however, this peak does not precisely reflect the characteristics of the lattice oxygens for the following two reasons. First, the chemical equilibrium between NO and NO2 is biased to NO at high temperatures; consequently, NO2 formed through the reaction with lattice oxygens will decompose to NO and O2 at these temperatures. Second, the desorption of stored NOx species cannot be ignored above 600 K, as evidenced by Figure 5. These two reasons make it difficult to observe the behavior of lattice oxygens during the oxidation of NO. The consumption of lattice oxygens in Sr−Fe perovskites is also supported by the XRD patterns of the catalysts acquired after stopping NO-TPR at 673 K (Figure 2). These XRD patterns revealed that, although slight peak shifts were observed, the perovskite framework of the as-synthesized sample was unaltered following NO-TPR. The observed peak shifts are the result of unit-cell expansion due to increasing numbers of oxygen vacancies.24 In contrast, NO oxidation involving lattice oxygens and such structural changes were not observed with Pt/Ba/Al2O3 in the absence of OSC (Figure S6a). The consumption of NO observed above 600 K on Pt/Ba/Al2O3_773 is ascribable to the direct decomposition of NO to N2O and N2.41,42 Interestingly, α-Fe2O3 (Figure S6b) did not consume NO during NO-TPR, although it can be reduced by H2 at lower temperatures than SrFeO3−δ and Sr3Fe2O7−δ, as shown in their H2-TPR profiles (Figure S7). α-Fe2O3 (Fe3+) is reduced to Fe3O4 (mixture of Fe3+ and Fe2+) at about 612 K, whereas SrFeO3−δ and Sr3Fe2O7−δ (mixture of Fe4+ and Fe3+) are reduced to SrFeO2.5 and Sr3Fe2O6 (Fe3+) at around 734 and 770 K, respectively. In addition, α-Fe2O3 did not release oxygen during O2-TPD (not shown). These results suggest that the oxygen-releasing properties of the Sr−Fe perovskites play important roles during the oxidation of NO to NO2 with lattice oxygens.

In our previous researches about OSC measurements of SrFeO3−δ and Sr3Fe2O7−δ using H2 as a reductant, these materials exhibited significantly faster oxygen-releasing rates than α-Fe2O3.24,43 This is attributed to the oxygen-releasing properties of SrFeO3−δ and Sr3Fe2O7−δ that can release oxygen while maintaining their perovskite frameworks, as shown in Figure 2; in other words, these materials undergo topotactic changes.24,39,44,45 On the other hand, when α-Fe2O3 (hematite with a corundum structure) releases oxygen, its structure is significantly altered as it transforms into Fe3O4 (magnetite with an inverse spinel structure) accompanied with the rearrangement of cations (Fe ions). Topotactic change is kinetically preferred for the release of oxygen from the catalyst. The thermodynamics of oxygen release from an ABO3 perovskite is described by the change in the Gibbs free energy (ΔG) using the change of enthalpy (ΔH) and entropy (ΔS) in accordance with 2 2 ABO3 → ABO3 − δ + O2 ; ΔG = ΔH − T ΔS δ δ

Oxygen desorption is essentially an endothermic (ΔH > 0) and endoentropic (ΔS > 0) reaction. Since the contribution of the entropy term increases with temperature, the Gibbs energy becomes negative, and oxygen release is energetically enabled at high temperatures. The magnitude of the enthalpy change corresponds to the oxygen bonding strength and the heat associated with structural changes. Strong oxygen bonding or significant structural change requires more heat to release oxygen. Comparing the enthalpy changes associated with the release of 1 mol-O2, the enthalpy changes for SrFeO3−δ, Sr3Fe2O7−δ, and α-Fe2O3 are reported to be about 200, 150, and 469 kJ mol−1, respectively.46−48 In contrast to the oxidation reaction of H2 with O2 to H2O (2H2 + O2 = 2H2O; ΔH298 ° = −484 kJ mol−1), the oxidation of NO to NO2 (2NO + O2 = 2NO2; ΔH°298 = −114 kJ mol−1) is less exothermic;48 lattice oxygens in α-Fe2O3 cannot be extracted by NO. The high oxygen-releasing abilities in SrFeO3−δ and Sr3Fe2O7−δ, which are accompanied with topotactic oxygen release, are essential for the oxidation of NO to proceed with the lattice oxygens. 3.4. NOx Storage Performance after Reducing Treatment with H2 and the Effect of Coexisting Gases. The NOx trapped on a NOx-trapping catalyst needs to be removed to restore the NOx storage capacity of the catalyst, for example, by reducing treatment. Figure 9 shows the results of lean−rich cycle experiments on Pd-modified Sr−Fe perovskites and Pt/ Ba/Al2O3 catalysts. Overall NOx removal efficiencies are listed in Table 2. Pd/Sr3Fe2O7−δ showed a significantly high NOx removal efficiency with an outlet NOx concentration of almost 0 ppm in each lean and rich period. This result suggests that trapped NOx species are successfully removed by H2 during rich periods and that NOx-trapping sites are regenerated. Pd modification was essential for regeneration of the NOxtrapping sites. NOx storage performance cannot be restored on bare Sr3Fe2O7−δ after the rich period, as shown in Figure S8a. When Pd/Sr3Fe2O7−δ was used without H2 pretreatment prior to the reaction, NOx removal efficiencies were not high in the first two cycles but gradually improved (Figure S8b). PdO species on Pd/Sr3Fe2O7−δ are reduced to metallic Pd species by H2 pretreatment, and these metallic Pd species are active for NOx reduction, as reported previously by us.25 In this study, H2 pretreatment of Pd/Sr3Fe2O7−δ also increased the NOx storage performance, as shown in Figure 9b. The enhanced F

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5% CO2. Spikes in the NOx concentration profiles are due to the oversupply of H2O when water droplets condensed in the line are occasionally vaporized in the reactor. The amount of trapped NOx on bare Sr3Fe2O7−δ for 1 h decreased to 10% (16 μmol → 1.6 μmol) in the presence of H2O and CO2, whereas the amount increased on Pd/Sr3Fe2O7−δ (17 μmol → 27 μmol). This amount is comparable to the amount observed for Pt/Ba/Al2O3_773 K (23 μmol). The main effect of the Pd on Sr3Fe2O7−δ was to improve the NO oxidation activity. The proportion of NO2 in the outlet NOx stream at 1 h from the start of the reaction was only 2% on bare Sr3Fe2O7−δ; this proportion increased to 52% on Pd/Sr3Fe2O7−δ. These results suggest that NO oxidation to NO2, which is a key step in the NOx storage reaction, is suppressed due to the suppression of NO adsorption by the competitive adsorption of H2O and CO2. Pd catalysts have recently been studied as cold NOx traps because the Pdn+−NO bond is stable in the presence of H2O and CO2.50 Modified Pd species may improve NO adsorption and promote the subsequent oxidation reaction with surface oxygen species on Sr3Fe2O7−δ. As a result, Pd modification increases the NOx storage performance of Sr3Fe2O7−δ in the presence of H2O and CO2. In addition, we also investigated the SOx tolerances of Pd/ Sr3Fe2O7−δ. Unfortunately, the NOx storage performance of Pd/Sr3Fe2O7−δ was dramatically lower after SOx sorption due to blockage of NOx adsorption sites by strongly adsorbed SOx (Figure S9). NOx storage performance was observed to partly recover after catalyst regeneration by H2 at 773 K for 30 min; however, the NSC decreased to 31% of the value prior to SOx sorption. Some simple ABO3-type perovskite catalysts, such as BaFeO3 and Ba0.9Sr0.1Ti0.8Cu0.2O3, have been reported as being tolerant to SOx.51,52 The tolerance is affected by the compositions of the A and B sites and can be finely tuned by substitutions with other elements. In addition, the A and B site elements in RP-type perovskites can be substituted by other elements. The substitutions of these sites in Sr3Fe2O7−δ would improve the SOx tolerance as well as the NOx oxidation and adsorption abilities.

Figure 9. Lean−rich cycle experiments at 573 K on (a) Pd/SrFeO3−δ, (b) Pd/Sr 3Fe2 O7−δ , (c) Pt/Ba/Al2 O3 _773, and (d) Pt/Ba/ Al2O3_1273. Pd/SrFeO3−δ and Pd/Sr3Fe2O7−δ were preconditioned for 30 min in rich phase gas before the first lean period. Lean gas: 200 ppm NO, 3% O2, He balance (100 mL min−1); rich gas: 3% H2, He balance (100 mL min−1).

Table 2. NOx Removal Efficiency of Each Catalyst in Lean− Rich Cycle Experiments catalyst

NOx removal efficiencya (%)

Pd/SrFeO3−δ Pd/Sr3Fe2O7−δ Pt/Ba/Al2O3_773 Pt/Ba/Al2O3_1273

63 100 96 44

NOx removal efficiency (%) = 100 × ∫ (CNOx−in − CNOx−out)dt/ ∫ CNOx−in dt a

NOx storage performance of the Pd-modified perovskite catalysts following H2 pretreatment has been reported by the Ozensoy et al.;49 they found that H2 pretreatment of Pd/ LaCoO3 and Pd/LaMnO3 increased their NOx adsorption abilities and concluded that the oxygen defects generated on these catalysts act as strong NOx adsorption sites and that metallic Pd assists in the oxidation of NOx. The same can be said about Pd/Sr3Fe2O7−δ after H2 pretreatment. Pd/SrFeO3−δ also recovered its NOx storage activity during rich periods; however, overall NOx removal efficiency was not particularly high because of an insufficient NSC. Although Pt/Ba/ Al2O3_773 K showed high NOx removal efficiency, the efficiency was significantly lower for Pt/Ba/Al2O3_1273 K, in which Pt particles were sintered by high-temperature calcination. Pd modification also improved the NOx storage performance in the presence of coexisting gases. Figure 10 shows the results of the NOx storage reaction in the presence of 10% H2O and

4. CONCLUSIONS SrFeO3−δ and Sr3Fe2O7−δ perovskites with OSCs exhibit high NSCs that are comparable to that of the Pt/Ba/Al2O3 and are very thermally stable. These Sr−Fe perovskites densely store NOx on the catalyst surfaces and exhibit high NSCs despite their small surface areas. Sr3Fe2O7−δ, with an RP-type layered perovskite structure, exhibited higher NSC than SrFeO3−δ. Pd/ Sr3Fe2O7−δ effectively stores NOx in the presence of H2O and CO2 and regenerates the catalyst following SOx sorption. Pd/ Sr3Fe2O7−δ shows a significantly high NOx removal efficiency in lean−rich cycling experiments, in which trapped NOx is successfully removed by the reductive treatment with H2. On the basis of NOx-TPD and DRIFT experiments, we conclude that Sr3Fe2O7−δ has superior NOx adsorption ability compared to SrFeO3−δ due to the presence of its SrO rock-salt interlayers that adsorb ionic NO3− species, which contributes to the high NSC. In addition, NO-TPR and O2-TPD experiments reveal that SrFeO3−δ and Sr3Fe2O7−δ exhibit unique NO oxidation capabilities that use the lattice oxygens in these catalysts. Such NO oxidation ability is not observed for Pt/Ba/Al2O3 in the absence of OSC or α-Fe2O3, which bonds strongly to oxygen and requires significant structural changes to release oxygen. Sr−Fe perovskites oxidize NO to NO2 because the oxygen bonding is relatively weak and these catalysts topotactically

Figure 10. Time courses of NOx storage reaction at 573 K with (solid line) and without (dashed line) 5% CO2 and 10% H2O on (a, b) Sr3Fe2O7−δ, (c, d) Pd/Sr3Fe2O7−δ, and (e, f) Pt/Ba/Al2O3_773. G

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and Expected Health Implications. Atmos. Environ. 2008, 42, 454− 465. (3) Matsumoto, S. Recent Advances in Automobile Exhaust Catalysts. Catal. Today 2004, 90, 183−190. (4) Roy, S.; Hegde, M. S.; Madras, G. Catalysis for NOx Abatement. Appl. Energy 2009, 86, 2283−2297. (5) Koebel, M.; Elsener, M.; Kleemann, M. Urea-SCR: A Promising Technique to Reduce NOx Emissions from Automotive Diesel Engines. Catal. Today 2000, 59, 335−345. (6) Li, J.; Chang, H.; Ma, L.; Hao, J.; Yang, R. T. Low-temperature Selective Catalytic Reduction of NOx with NH3 over Metal Oxide and Zeolite CatalystsA Review. Catal. Today 2011, 175, 147−156. (7) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.-i.; Tanizawa, T.; Tanaka, T.; Tateishi, S.-s.; Kasahara, K. The New Concept 3-way Catalyst for Automotive Lean-burn Engine: NOx Storage and Reduction Catalyst. Catal. Today 1996, 27, 63−69. (8) Roy, S.; Baiker, A. NOx Storage−Reduction Catalysis: From Mechanism and Materials Properties to Storage−Reduction Performance. Chem. Rev. 2009, 109, 4054−4091. (9) Liu, G.; Gao, P.-X. A Review of NOx Storage/Reduction Catalysts: Mechanism, Materials and Degradation Studies. Catal. Sci. Technol. 2011, 1, 552−568. (10) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E., II Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts. Catal. Rev. 2004, 46, 163−245. (11) Svedberg, P.; Jobson, E.; Erkfeldt, S.; Andersson, B.; Larsson, M.; Skoglundh, M. Influence of the storage material on the storage of NOx at low temperatures. Top. Catal. 2004, 30, 199−206. (12) Olsson, L.; Jozsa, P.; Nilsson, M.; Jobson, E. Fundamental Studies of NOx Storage at Low Temperatures. Top. Catal. 2007, 42, 95−98. (13) Constantinou, C.; Li, W.; Qi, G.; Epling, W. S. NOx Storage and Reduction over a Perovskite-based Lean NOx Trap Catalyst. Appl. Catal., B 2013, 134-135, 66−74. (14) He, X.; Meng, M.; He, J.; Zou, Z.; Li, X.; Li, Z.; Jiang, Z. A Potential Substitution of Noble Metal Pt by Perovskite LaCoO3 in ZrTiO4 Supported Lean-burn NOx Trap Catalysts. Catal. Commun. 2010, 12, 165−168. (15) Xian, H.; Zhang, X.; Li, X.; Zou, H.; Meng, M.; Zou, Z.; Guo, L.; Tsubaki, N. Effect of the Calcination Conditions on the NOx Storage Behavior of the Perovskite BaFeO3−x Catalysts. Catal. Today 2010, 158, 215−219. (16) López-Suárez, F. E.; Illán-Gómez, M. J.; Bueno-López, A.; Anderson, J. A. NOx Storage and Reduction on a SrTiCuO3 Perovskite Catalyst Studied by Operando DRIFTS. Appl. Catal., B 2011, 104, 261−267. (17) Dong, Y.-H.; Xian, H.; Lv, J.-L.; Liu, C.; Guo, L.; Meng, M.; Tan, Y.-S.; Tsubaki, N.; Li, X.-G. Influence of Synthesis Conditions on NO Oxidation and NOx Storage Performances of La0.7Sr0.3MnO3 Perovskite-type Catalyst in Lean-burn Atmospheres. Mater. Chem. Phys. 2014, 143, 578−586. (18) Kim, C. H.; Qi, G.; Dahlberg, K.; Li, W. Strontium-Doped Perovskites Rival Platinum Catalysts for Treating NOx in Simulated Diesel Exhaust. Science 2010, 327, 1624−1627. (19) Choi, S. O.; Penninger, M.; Kim, C. H.; Schneider, W. F.; Thompson, L. T. Experimental and Computational Investigation of Effect of Sr on NO Oxidation and Oxygen Exchange for La1−xSrxCoO3 Perovskite Catalysts. ACS Catal. 2013, 3, 2719−2728. (20) Shen, B.; Lin, X.; Zhao, Y. Catalytic Oxidation of NO with O2 over Pt/γ-Al2O3 and La0.8Sr0.2MnO3. Chem. Eng. J. 2013, 222, 9−15. (21) Qi, G.; Li, W. NOx Adsorption and Reduction over LaMnO3 Based Lean NOx Trap Catalysts. Catal. Lett. 2014, 144, 639−647. (22) Wen, W.; Wang, X.; Jin, S.; Wang, R. LaCoO3 Perovskite in Pt/ LaCoO3/K/Al2O3 for the Improvement of NOx Storage and Reduction Performances. RSC Adv. 2016, 6, 74046−74052. (23) Raveau, B.; Hervieu, M.; Pelloquin, D.; Michel, C.; Retoux, R. A Large Family of Iron Ruddlesden-Popper Relatives: from Oxides to

release lattice oxygen, maintaining their basic structures. This NO oxidation mechanism, which uses stored lattice oxygen, is a novel NO oxidation process under low-temperature conditions in which the catalytic oxidation of NO to NO2 on a Pt-based catalyst is kinetically suppressed. SrFeO3−δ has superior NOx oxidation ability; however, it has poor NOx adsorption ability and exhibits a low NSC. On the other hand, Sr3Fe2O7−δ has NOx oxidation and adsorption abilities, resulting in the realization of a high NSC. RP-type layered perovskites with OSCs are good candidates as Pt-free NOxtrapping catalysts since they display both NOx oxidation and adsorption capabilities. Their fine-tuning by A and/or B site substitution with other elements should be required to improve the NSC at low temperatures and the tolerance to the poisoning gases.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b08139. Supplementary results for determining oxygen deficiency by thermogravimetry; details of the NSC calculational methods; recycle experiments of NOx storage reaction on SrFeO3−δ and Sr3Fe2O7−δ; thermogravimetric analysis profiles of the catalyst precursors; a DRIFT spectrum of NO2-adsorbed SrO; NO-TPR profiles on SrFeO3−δ and Sr3Fe2O7−δ when detecting NO + NO2; NO-TPR profiles on Pt/Ba/Al2O3_773 and α-Fe2O3, H2-TPR profiles on SrFeO3−δ, Sr3Fe2O7−δ, and α-Fe2O3; lean− rich cycling experiments on bare Sr3Fe2O7−δ and Pd/ Sr3Fe2O7−δ without preconditioning; and NOx storage reactions after SOx sorption and regeneration treatment on Pd/Sr3Fe2O7−δ (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

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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ACKNOWLEDGMENTS This work was performed under the management of the “Elements Strategy Initiative for Catalysts & Batteries (ESICB)” project supported by the Ministry of Education Culture, Sports, Science and Technology (MEXT), Japan, under the “Elements Strategy Initiative to Form Core Research Center” program. K.T. is thankful for the JSPS Research Fellowship for Young Scientists. The crystal structures in this paper were produced by the VESTA program.53

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REFERENCES

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