JP2025027415A - Oxygen absorbing and releasing materials and exhaust gas purification catalysts - Google Patents

Abstract

To provide an oxygen absorbing/desorbing material having low temperature actuation property and improved oxygen absorbing/desorbing ability.SOLUTION: The invention relates to oxygen absorbing/desorbing material including a Ruddlesden-Popper type laminar perovskite-type composite oxide represented by a formula (0):RE(Fe1-aMa)3Sr3O10-δ(in the formula, RE is at least one or more kinds selected from a group consisting of Pr, Nd, Sm, Eu, Gd, and La; M is at least one or more kinds selected from a group consisting of Mn, Ni, and Co; a is a real number between 0 or more and less than 1; δ is a real number between 0 or more and less than 1) and an exhaust gas purification catalyst using the oxygen absorbing/desorbing material.SELECTED DRAWING: Figure 1A

Description

The present invention relates to an oxygen absorbing/releasing material and an exhaust gas purification catalyst that uses the oxygen absorbing/releasing material.

With the progress of motorization on a global scale, the number of automobiles sold is on the rise, and from the viewpoint of global environmental conservation, high performance is required for catalysts for purifying exhaust gas from automobiles. Three-way catalysts, which are the mainstream catalysts for gasoline engines, purify these substances just enough at an air-fuel ratio that generates chemically equivalent amounts of oxidizing agents such as nitrogen oxides ( _NOx ) and reducing agents such as carbon monoxide (CO) and unburned hydrocarbons (HCs), using a feedback control system consisting of an air-fuel ratio sensor and an electronic fuel injection device. However, in an environment where the exhaust gas composition fluctuates drastically depending on the driving conditions, the purification ability cannot be fully demonstrated.

The oxygen absorbing/releasing material contained in the three-way catalyst absorbs and releases oxygen through a redox reaction of cations in the solid when the air-fuel ratio fluctuates, and adjusts the oxidizing agent and reducing agent to a stoichiometric ratio at which the catalyst can function. Ceria (CeO ₂ ) has traditionally been used as a material having such oxygen absorbing/releasing capacity (also called oxygen storage capacity or oxygen storage capacity), and in recent years, various types of composite oxides containing ceria have been researched, with ceria-zirconia (CeO ₂ -ZrO ₂ )-based composite oxides being particularly widely used. However, the amount of oxygen absorbed and released by ceria-zirconia-based composite oxides is approaching its theoretical limit.

As a cationic species that can exceed the limit of the cerium ion in the ceria-zirconia composite oxide, there are first transition metal ions with smaller atomic weights than the cerium ion, and various transition metal composite oxides have been developed. For example, Patent Document 1 discloses an oxygen storage material made of a composite oxide containing ferric oxide (Fe ₂ O ₃ ), zirconia (ZrO ₂ ), and a rare earth element oxide, but the oxygen absorption and release capacity of these iron-based oxygen absorbing and releasing materials is not necessarily sufficient.

This difference in performance between the oxygen absorbing/releasing material made of ceria-zirconia composite oxide and the iron-based oxygen absorbing/releasing material is believed to be due to the difference in the chemical properties of iron ions and cerium ions. Specifically, the standard electrode potential of the trivalent iron ions contained in ferric oxide and the like in a one-electron reaction (Fe3 ⁺ + ^e- = Fe2 ⁺ ) in an aqueous solution at 25°C is 0.771 V, which is lower than the equivalent potential of cerium ions (Ce4 ⁺ ), which is 1.71 V, and therefore shows a property of being difficult to chemically reduce. Furthermore, the equipotential in the reaction in which divalent iron ions contained in ferrous oxide etc. become zero-valent metal (Fe ²⁺ +2e ^- =Fe ⁰ ) is -0.44 V, whereas the equipotential in the reaction in which trivalent cerium ions (Ce ³⁺ ) become zero-valent metal (Ce ³⁺ +3e ^- =Ce ⁰ ) is -2.34 V, suggests that Fe ²⁺ has chemical properties that make it more easily reduced than Ce ³⁺ . Due to these differences in chemical properties, ferric oxide generally has a higher operating temperature than cerium oxide in terms of oxygen absorption/release capacity, and in a reducing atmosphere, the oxide structure collapses due to reduction to zero-valent metal, and improving these properties was one of the major challenges in investigating iron-based oxygen absorption/release materials.

In view of this problem, iron-based composite oxides capable of redox reactions (topotactic transition) without structural collapse in a high-temperature reducing atmosphere have been developed. For example, Non-Patent Document 1 discloses a strontium-iron-based composite oxide Sr ₃ Fe ₂ O _7-δ that exhibits excellent oxygen absorption/release capacity and high reduction resistance. However, Sr ₃ Fe ₂ O _7-δ has a high operating temperature for oxygen absorption/release capacity, and there is room for further improvement in the oxygen absorption/release capacity.

JP 2013-241328 A

Journal of Materials Chemistry A, 2015, 3, 13540-13545

As mentioned above, conventional iron-based oxygen absorbing/releasing materials have required further improvements in low-temperature operability and oxygen absorbing/releasing capacity. Therefore, the present invention aims to provide an oxygen absorbing/releasing material with improved low-temperature operability and oxygen absorbing/releasing capacity.

The inventors discovered that a Rudolfsden-Popper type layered perovskite complex oxide having a specific composition has excellent low-temperature operation and high oxygen absorption/release capacity, and thus completed the present invention.

That is, the gist of the present invention is as follows.
(1) An oxygen absorbing/releasing material containing a Rudolfsden-Popper type layered perovskite complex oxide represented by the following formula (0):
RE(Fe _1-a M _a ) ₃ Sr ₃ O _10-δ (0)
(In the formula, RE is one or more selected from the group consisting of Pr, Nd, Sm, Eu, Gd, and La, M is one or more selected from the group consisting of Mn, Ni, and Co, a is a real number equal to or greater than 0 and less than 1, and δ is a real number equal to or greater than 0 and less than 1.)
(2) An oxygen absorbing/releasing material containing a Rudolf Denn-Popper type layered perovskite complex oxide represented by the following formula (1):
La(Fe _1-a M _a ) ₃ Sr ₃ O _10-δ (1)
(In the formula, M is one or more selected from the group consisting of Mn, Ni, and Co, a is a real number equal to or greater than 0 and less than 1, and δ is a real number equal to or greater than 0 and less than 1.)
(3) The oxygen storage/release material according to (1) or (2) above, wherein in the formulas (0) and (1), a is 0.
(4) The oxygen storage/release material according to (1) or (2) above, wherein in the formulas (0) and (1), a is a real number greater than 0 and less than 1.
(5) The oxygen storage/release material according to (1), (2) or (4) above, wherein in the formulas (0) and (1), a is a real number of 0.1 or more and 0.3 or less.
(6) The oxygen storage/release material according to any one of (1), (3) to (5), wherein in the formula (0), RE is one or more selected from the group consisting of Pr, Nd, Eu and Gd.
(7) An exhaust gas purifying catalyst comprising the oxygen absorbing material according to any one of (1) to (6) above and a precious metal supported on the oxygen absorbing material.

The present invention makes it possible to provide an oxygen absorbing and releasing material with improved low-temperature operability and oxygen absorbing and releasing capacity.

1 shows a schematic diagram of a composite oxide (RE(Fe _1-a M _a ) ₃ Sr ₃ O _10-δ ) of the present invention having a Rudolfsden-Popper type layered perovskite crystal structure. 1 shows XRD patterns of Example 1, Comparative Example 1, and Comparative Example 2. The results of H ₂ -TPR analysis of Example 1, Comparative Example 1 and Comparative Example 2 are shown. _Figure 2A shows the XRD patterns of the new sample, the _{H2- treated} sample, and the reoxidized sample of Example 1. Figure 2B shows a schematic diagram of the transfer reaction between _{LaSr3Fe3O10 -δ} and _{LaSr3Fe3O9 .} Figure 2C shows the results of X-ray absorption spectrum analysis by XAFS of the new sample, the _H2 -treated sample, and the reoxidized sample of Example 1. 1 shows XRD patterns of Examples 1 to 4. The results of H ₂ -TPR analysis of Example 5, Comparative Example 3 and Comparative Example 4 are shown. The results of H ₂ -TPR analysis of Examples 5 to 8 are shown below. The XRD patterns of Examples 1, and 9 to 13 are shown. 1 shows XRD patterns of Comparative Examples 7 to 13. The results of H ₂ -TPR analysis of Examples 1 and 9 to 13 are shown below.

A preferred embodiment of the present invention is described in detail below.

The oxygen absorbing/releasing material of the present invention contains a Rudolfsden-Popper type layered perovskite complex oxide (hereinafter also referred to as the complex oxide of the present invention) represented by the following formula (0), and preferably consists of this complex oxide.
RE(Fe _1-a M _a ) ₃ Sr ₃ O _10-δ (0)
(In the formula, RE is one or more selected from the group consisting of praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), and lanthanum (La); M is one or more selected from the group consisting of manganese (Mn), nickel (Ni), and cobalt (Co); a is a real number equal to or greater than 0 and less than 1; and δ is a real number equal to or greater than 0 and less than 1.)

In formula (0), RE is at least one selected from the group consisting of Pr, Nd, Sm, Eu, Gd, and La, and preferably at least one selected from these. From the viewpoint of excellent oxygen absorption/release capacity at lower temperatures, RE is preferably at least one selected from the group consisting of Pr, Nd, Eu, and Gd, and more preferably Pr, Nd, Eu, or Gd.

In one embodiment, the oxygen storage/release material of the present invention is one in which RE is La in formula (0), and is represented by the following formula (1).
La(Fe _1-a M _a ) ₃ Sr ₃ O _10-δ (1)
(In the formula, M is one or more selected from the group consisting of manganese (Mn), nickel (Ni) and cobalt (Co), a is a real number equal to or greater than 0 and less than 1, and δ is a real number equal to or greater than 0 and less than 1.)

The composite oxide of the present invention can exhibit excellent oxygen absorption/release capacity even at low temperatures without structural collapse even when exposed to a high-temperature reducing atmosphere. Therefore, the oxygen absorption/release material containing the composite oxide of the present invention has high oxygen absorption/release capacity and excellent low-temperature operability.

The complex oxide of the present invention has a Rudolfsden-Popper type layered perovskite crystal structure. Because the complex oxide of the present invention has this crystal structure, it can exhibit excellent oxygen absorption and release capacity without structural collapse even when exposed to a high-temperature reducing atmosphere. In the present invention, the crystal structure of the complex oxide can be determined, for example, by powder X-ray diffraction (XRD) measurement.

FIG. 1A shows a schematic diagram of the composite oxide of the present invention (RE(Fe1 _{-aM a} ) _{3Sr3O10 -δ )} having a Rudlesden-Popper type layered perovskite crystal structure. Generally, it is known that the oxidation number of Fe in iron compounds is 3 or 2, with 3 being the most stable oxidation number. In formula (0), δ is a real number equal to or greater than 0 and less than 1, and therefore, for example, when a is 0, the oxidation number of Fe is higher than 3. It is believed that the composite oxide of the present invention is able to exhibit high oxygen absorption/release capacity due to the abnormally high valence of Fe.

In one embodiment, in formulas (0) and (1), a is 0, i.e., the composite oxide does not contain the metal element M.

In another embodiment, in formulas (0) and (1), a is a real number greater than 0 and less than 1, i.e., the composite oxide contains a metal element M. The composite oxide of the present invention further improves the oxygen absorption/release capacity by containing one or more metal elements M selected from the group consisting of Mn, Ni, and Co. In formulas (0) and (1), from the viewpoint of resource chemistry and cost, a is preferably a real number of 0.1 or more and 0.3 or less.

In formulas (0) and (1), δ is a real number between 0 and 1, preferably between 0 and 0.5, and more preferably 0.25. As described above, it is believed that the complex oxide of the present invention can exhibit high oxygen absorption and release capacity because the oxidation number of Fe is abnormally high, that is, the oxidation number of Fe is higher than 3.

The composite oxide of the present invention can be prepared, for example, by a complex polymerization method. In a preferred embodiment, the composite oxide of the present invention can be prepared, for example, by dissolving in water salts of each metal constituting the composite oxide, a carboxylic acid (preferably citric acid) and a glycol (preferably ethylene glycol) in a predetermined amount relative to the total amount of the metal salts to obtain a mixture of these, heating and stirring this mixture at, for example, 70°C to 200°C to produce a gel, which is then pre-baked and baked. In this embodiment, the temperature for the bake is usually 800°C to 1600°C, preferably 1200°C to 1600°C.

The composite oxide of the present invention exhibits high oxygen absorption and release capacity, making it suitable for use as an oxygen absorption and release material.

The present invention also includes an exhaust gas purification catalyst using the oxygen absorbing/releasing material. The exhaust gas purification catalyst of the present invention contains the oxygen absorbing/releasing material and a precious metal (preferably a precious metal supported on the oxygen absorbing/releasing material). In another embodiment, the exhaust gas purification catalyst of the present invention does not need to contain a precious metal.

Noble metals that can be used in the exhaust gas purification catalyst of the present invention are not particularly limited and include, for example, gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru) and osmium (Os), preferably Pt, Pd and Rh, and more preferably Pd.

In the exhaust gas purification catalyst of the present invention, the content of the precious metal is usually 0.001% to 10% by weight, preferably 0.01% to 5% by weight, relative to the oxygen absorbing/releasing material.

The present invention will be described in more detail below using examples. However, the technical scope of the present invention is not limited to these examples.

<Synthesis of Complex Oxides>
The composite oxides of the present invention were synthesized by a complex polymerization method. Specifically, each metal salt, citric acid and ethylene glycol (8.57 equivalents relative to the total amount of the metal salts) were dissolved in ion-exchanged water, and then the mixture was heated and stirred to produce a gel, which was then pre-baked and then baked to synthesize each composite oxide powder. The specific procedure is described below. Note that the number of moles of hydrates is the number of moles of anhydrides.

Example 1: _{LaSr3Fe3O10 - δ}
Citric acid (400 mmol) was added to a beaker and dissolved in ion-exchanged water (180 mL). Fe(NO ₃ ) ₃ ·9H ₂ O (20 mmol), SrCO ₃ (20 mmol), La(NO ₃ ) ₃ ·9H ₂ O (6.67 mmol) were added thereto, and then ethylene glycol (400 mmol) was added and stirred at room temperature for 30 minutes. After stirring, the mixture was aged at 130°C for 30 minutes to 1 hour to create a gel, and then pre-baked at 350°C for 1 to 2 hours to obtain a precursor of LaSr ₃ Fe ₃ O _10-δ . The precursor powder was then baked at 1400°C for 2 hours to synthesize the desired composite oxide.

Example 2: _LaSr3 ( _Fe0.8Mn0.2 ) _{3O10 - δ}
The target composite oxide was synthesized in the same manner as in Example 1, except _that Fe(NO ₃ ) 3.9H ₂ O (20 mmol) was changed to Fe(NO ₃ ) _{3.9H 2 O} (16 mmol) and Mn(NO ₃ ) 2.6H ₂ O (4 mmol).

Example 3: _LaSr3 ( _Fe0.8Co0.2 ) _{3O10 - δ}
The target composite oxide was synthesized in the same manner as in Example 1, except _that Fe(NO ₃ ) 3.9H ₂ O (20 mmol) was changed to Fe(NO ₃ ) _{3.9H 2 O} (16 mmol) and Co(NO ₃ ) 2.6H ₂ O (4 mmol).

Example 4: _LaSr3 ( _Fe0.8Ni0.2 ) _{3O10 - δ}
The target composite oxide was synthesized in the same manner as in Example 1, except _that Fe(NO ₃ ) 3.9H ₂ O (20 mmol) was changed to Fe(NO ₃ ) _{3.9H 2 O} (16 mmol) and Ni(NO ₃ ) 2.6H ₂ O (4 mmol).

Comparative example 1: Sr ₃ Fe ₂ O _7-δ
Citric acid (400 mmol) was added to a beaker and dissolved in ion-exchanged water (180 mL). Fe(NO ₃ ) _{3.9H 2} O (13.3 mmol) and SrCO ₃ (20 mmol) were added thereto, followed by ethylene glycol (400 mmol) and stirring at room temperature for 30 minutes. After stirring, the mixture was aged at 130°C for 30 minutes to 1 hour to create a gel, which was then pre-baked at 350°C for 1 to 2 hours to obtain a precursor of Sr ₃ Fe ₂ O _7-δ . The precursor powder was then baked at 1000°C for 5 hours to synthesize the desired composite oxide.

Comparative example 2: SrFeO _3-δ
Citric acid (400 mmol) was added to a beaker and dissolved in ion-exchanged water (180 mL). Fe(NO ₃ ) _{3.9H 2} O (20 mmol) and SrCO ₃ (20 mmol) were added thereto, followed by ethylene glycol (400 mmol) and stirring at room temperature for 30 minutes. After stirring, the mixture was aged at 130°C for 30 minutes to 1 hour to create a gel, which was then pre-baked at 350°C for 1 to 2 hours in a mantle heater to obtain a precursor of SrFeO _3-δ . The precursor powder was then baked at 1000°C for 2 hours to synthesize the desired composite oxide.

<Analysis and performance evaluation>
Powder X-ray diffraction (XRD)
The analysis was performed using a horizontal sample type multipurpose X-ray diffraction apparatus (Miniflex and SmartLab Studio II, Rigaku Corp.) with Cu Kα [40 kV, 15 mA (Miniflex), 50 mA (SmartLab Studio II)] as the X-ray source.

Hydrogen Temperature Programmed Reduction (H ₂ -TPR)
Belcat A manufactured by Microtrackbell was used. After weighing out about 50 mg of sample powder and introducing it into a sample tube, the temperature was raised to 500° C. while 20% O ₂ /He was circulated at 30 mL/min, and the sample was pretreated for 10 minutes and then cooled. After replacing with Ar gas, 5% H ₂ /Ar was circulated at 30 mL/min, and the sample was heated at 10° C./min while being heated, and the hydrogen (H ₂ ) consumption was analyzed. The analysis was performed with a thermal conductivity detector (TCD), and a desiccant was placed in front of the TCD to trap the water produced.

X-ray absorption fine structure (XAFS)
Measurements were performed using a fluorescence method at beamline BL01B1 of the SPring-8 synchrotron radiation facility of the Japan Synchrotron Radiation Research Institute, a public interest incorporated foundation.

Oxygen storage test by thermogravimetric analysis Oxygen absorption and release capacity was measured using a thermogravimetric analyzer (TG STA 2500 Regulus, NETZSCH). 100 mg of sample (sized to 0.3 mm to 0.7 mm) was placed on a balance and held at 500°C for 1 hour under 2% O ₂ /Ar flow, and then 2% H ₂ /Ar (100 mL/min) and 2% O ₂ /Ar (100 mL/min) were flowed at 500°C while switching between them every 20 minutes, and the weight change was analyzed to determine the amount of oxygen absorbed and released.

Oxygen storage test by CO/ _O2 alternating pulse method 2g of sample pellets were placed in a fixed-bed flow reactor, and the conversion behavior of each gas was examined using an FT-IR analyzer (Best Instruments SESAM-HL) and an oxygen meter (BEX). The sample was heated to 500°C while flowing 10% _O2 / _N2 gas at a flow rate of 10 L/min. 1% _O2 / _N2 and 2% CO/ _N2 were introduced sequentially at the same flow rate and temperature for 6 cycles, each for 120 seconds, and the average value of the 2nd to 5th cycles of the _CO2 generation amount during the CO introduction period was calculated as the oxygen absorption and release amount.

<Analysis and performance evaluation of complex oxides>
Figure 1B shows the XRD patterns of LaSr ₃ Fe ₃ O _10-δ (Example 1), Sr ₃ Fe ₂ O _7-δ (Comparative Example 1), and SrFeO _3-δ (Comparative Example 2). As shown in Figure 1B, the diffraction lines of Comparative Example 2 matched the diffraction pattern characteristic of cubic perovskite. On the other hand, the diffraction lines of Example 1 and Comparative Example 1 each had the strongest line at about 32° split into two, matching the diffraction pattern characteristic of Rdolphden-Popper type layered perovskite.

The reduction behavior of Fe ions was evaluated by hydrogen temperature-programmed reduction (H ₂ -TPR) for LaSr ₃ Fe ₃ O _10-δ (Example 1), Sr ₃ Fe ₂ O _7-δ (Comparative Example 1), and SrFeO ₃ -δ (Comparative Example 2). Figure 1C shows the H 2 -TPR analysis results for LaSr ₃ Fe ₃ O _10-δ (Example 1), Sr ₃ Fe ₂ O _7-δ (Comparative Example 1), and SrFeO _3-δ (Comparative Example 2). In Figure _1C , the hydrogen consumption peak at 500°C for Comparative Example 2 is due to a topotactic transition reaction without collapse of the oxide structure, as shown by the formula 1: SrFeO ₃ + 1/2H ₂ → SrFeO _2.5 + 1/2H ₂ O. On the other hand, the hydrogen consumption observed at 700°C or higher in Comparative Example 2 is due to an irreversible reaction accompanied by the collapse of the oxide structure, as shown by the formula ₂ : _3SrFeO3 + ₁ / _2H2 → _Sr3Fe2O6 + Fe + 1/ _2H2O . On the other hand, in Example 1 and Comparative Example 1, hydrogen consumption with main peaks at 490°C and 510°C, respectively, was detected, and both were attributed to hydrogen consumption due to topotactic transition. Unlike Comparative Example 2, in Example 1 and Comparative Example 1, hydrogen consumption due to the collapse of the oxide structure at 700°C or higher was hardly observed, indicating that the structural stability was increased compared to Comparative Example 2 due to the unique structure of the layered perovskite. In addition, the hydrogen consumption peak temperature was 490°C in Example 1, which was observed at a lower temperature than 510°C in Comparative Example 1, and it was shown that Example 1 exhibited oxygen desorption from a lower temperature than Comparative Example 1, and was superior in low-temperature operation.

A sample (H2-treated sample) was prepared by treating LaSr ₃ Fe ₃ O _10-δ (Example 1: new) with 2% H ₂ / _Ar at 500°C to reduce it, and a sample (reoxidized sample) was prepared by reoxidizing it with 2% O ₂ /Ar, and these were analyzed. Figure 2A shows the XRD patterns of the new sample of Example 1, the sample after H ₂ treatment, and the sample after reoxidation. In Figure 2A, the XRD pattern of the sample after H ₂ treatment is consistent with the XRD pattern of LaSr ₃ Fe ₃ O ₉ , which indicates that oxygen was inserted and removed from the intermediate layer of LaSr ₃ Fe ₃ O _10-δ without the oxide structure being destroyed by the H ₂ treatment, and Fe was transformed from 6-fold to 4-fold coordination. Figure 2B shows a schematic diagram of the transition reaction between LaSr ₃ Fe ₃ O _10-δ and LaSr ₃ Fe ₃ O _9. On the other hand, the XRD pattern of the reoxidized sample was completely consistent with that of the new sample, indicating that the topotactic adsorption and desorption of oxygen proceeds reversibly in this composite oxide.

FIG. 2C shows the results of the analysis of the X-ray absorption spectrum by XAFS of the new sample of Example 1, the sample after H ₂ treatment, and the sample after reoxidation. As shown in FIG. 2C, in the structure near the X-ray absorption edge, the Fe K-absorption edge energy of the new sample of Example 1 was roughly equivalent to 3.5 valence. The absorption edge energy of the sample after H ₂ treatment shifted to the low energy side and was reduced to trivalent Fe. By reoxidation, the oxidation number of Fe was restored to 3.5 valence, the same as that of the new sample. These results, similar to the XRD analysis results shown in FIG. 2A, showed that the adsorption and desorption of oxygen in this composite oxide proceeded reversibly. Thus, the oxidation number of Fe in LaSr ₃ Fe ₃ O _10-δ (Example 1) is an abnormally high valence higher than 3, which is thought to contribute to the development of excellent oxygen adsorption and desorption capacity.

Fig. 3 shows the XRD patterns of LaSr ₃ Fe ₃ O _10-δ (Example 1) and Examples 2 to 4 in which Fe in LaSr ₃ Fe ₃ O _10-δ was partially substituted with Mn, Co, or Ni, respectively. As shown in Fig. 3, in Examples 2 to 4 as well, as in Example 1, the diffraction lines each had the strongest line at about 32° split into two, which matched the diffraction pattern characteristic of the Rdolphin-Popper type layered perovskite.

The results of the oxygen storage test by thermogravimetric analysis for the composite oxides of Examples 1 to 4 and Comparative Example 1 are shown in Table 1. As shown in Table 1, LaSr ₃ Fe ₃ O _10-δ (Example 1) showed a larger oxygen absorption/release amount than Sr ₃ Fe ₂ O _7-δ (Comparative Example 1), which is capable of reversible topotactic transition. Both the composite oxides of Example 1 and Comparative Example 1 have an abnormally high valence of Fe, the oxidation number of which is higher than 3, and are therefore considered to be capable of exhibiting high oxygen absorption/release capacity. The composite oxide of Example 1 has a higher Fe content per weight than the composite oxide of Comparative Example 1, which is presumably one of the reasons for the higher oxygen absorption/release capacity compared to the composite oxide of Comparative Example 1. In addition, the composite oxides of Examples 2 to 4, in which 20% of the Fe in LaSr ₃ Fe ₃ O _10-δ (Example 1) was replaced with Mn, Co, or Ni, respectively, showed even larger oxygen absorption/release amounts compared to the composite oxide of Example 1.

<Preparation of catalyst>
Catalysts were prepared by supporting palladium on each of the composite oxides of Examples 1 to 4 and Comparative Examples 1 and 2 so that the amount of palladium was 1% by weight relative to the composite oxide. In addition, catalysts were prepared using ceria-zirconia composite oxide. The specific procedure is described below.

Catalysts of Examples 5 to 8 and Comparative Examples 3 and 4 Acetone was added to a beaker to dissolve palladium acetate. Each of the composite oxides of Examples 1 to 4 and Comparative Examples 1 and 2 was added thereto, and the mixture was heated and stirred to distill off the solvent. After drying overnight at 120°C, the resulting solid was crushed in a mortar and calcined at 500°C for 2 hours to prepare a catalyst. The catalysts prepared using each of the composite oxides of Examples 1 to 4 and Comparative Examples 1 and 2 were designated as Examples 5 to 8 and Comparative Examples 3 and 4, respectively.

Catalysts of Comparative Examples 5 and 6 Distilled water was added to a beaker, and palladium nitrate was dissolved therein. A commercially available ceria-zirconia composite oxide was added thereto, and the mixture was heated and stirred to remove the solvent. After drying overnight at 120°C, the resulting solid was crushed in a mortar and fired at 500°C for 2 hours to prepare a catalyst. As the ceria-zirconia composite oxide, composite oxides of CeO ₂ /ZrO ₂ = 67/33 and CeO ₂ /ZrO ₂ = 33/67 were used, and the catalysts prepared using these were designated Comparative Examples 5 and 6, respectively.

<Catalyst analysis and performance evaluation>
FIG. 4 shows the results of H _{2 -TPR} analysis of Pd/LaSr ₃ Fe ₃ O _10-δ (Example 5), Pd/Sr ₃ Fe ₂ O _7-δ (Comparative Example 3), and Pd/SrFeO _3-δ (Comparative Example 4). In FIG. 4, in Comparative Example 4, hydrogen consumption attributable to the topotactic transition with a peak at 85° C. was observed, and hydrogen consumption due to the collapse of the oxide structure was detected at 700° C. or higher, similar to SrFeO _3-δ (Comparative Example 2) in FIG. 1C. On the other hand, in Example 5 and Comparative Example 3, hydrogen consumption attributable to the topotactic transition was observed at 83° C. and 98° C., respectively, but hydrogen consumption due to the collapse of the oxide structure at 700° C. or higher was hardly observed. These results showed that catalysts carrying precious metals such as palladium also have high structural stability of the Rdolphin-Popper type layered perovskite. Furthermore, the hydrogen consumption peak temperature (83°C) of Example 5 was observed at a lower temperature than that of Comparative Example 3 (98°C), indicating that Example 5 exhibited superior low-temperature operability, with oxygen desorption occurring at a lower temperature. Thus, both LaSr ₃ Fe ₃ O _10-δ (Example 1) and Pd/LaSr ₃ Fe ₃ O _10-δ (Example 5) had high structural stability in a reducing atmosphere, and exhibited superior low-temperature operability compared to conventional composite oxides and catalysts using the same (Comparative Examples 1 and 3), which are known to exhibit high oxygen storage and release capacity.

5 shows the results of H 2 -TPR analysis of Pd/LaSr ₃ Fe ₃ O _10-δ (Example 5) and catalysts (Examples 6 to 8) using composite oxides in which Fe in LaSr ₃ Fe ₃ O _{10 -δ} is partially substituted with Mn, Co, or Ni, respectively. As shown in FIG. 5, in all of Examples 5 to 8, hydrogen consumption attributed to topotactic transition was observed near 100°C, and hydrogen consumption due to the collapse of the oxide structure at 700°C or higher was hardly observed. These results showed that the catalysts using composite oxides in which part of Fe in LaSr ₃ Fe ₃ O _10-δ is substituted with Mn, Co, or Ni, respectively, also have high structural stability of the Rdolphin-Popper type layered perovskite.

The results of oxygen storage tests by the CO/ _O2 alternating pulse method for the catalysts of Examples 5 to 8 and Comparative Examples 3, ₅ , and 6 are shown in Table 2. As shown in Table 2, Pd/ _{LaSr3Fe3O10 -δ} (Example 5) showed a larger amount of oxygen absorption and release and an excellent oxygen absorption and release capacity than Pd/ _{Sr3Fe2O7 -δ} (Comparative Example 3), which is a catalyst in which Pd is supported on a conventional iron-based composite oxide capable of reversible topotactic transition, _and Pd/ _Ce0.7Zr0.3O2 (Comparative Example 5) and Pd/ _Ce0.3Zr0.7O2 (Comparative Example ₆ ), which are catalysts in which _Pd is supported on a conventional ceria-zirconia _{composite oxide} known to exhibit high oxygen absorption and release capacity. Furthermore, the catalysts of Examples 6 to 8, in which 20% of the Fe in the composite oxide of Pd/LaSr ₃ Fe ₃ O _10-δ (Example 5) was replaced with Mn, Co or Ni, respectively, had a larger oxygen absorption and release amount than the catalyst of Example 5 and showed even higher oxygen absorption and release capacity.

<Synthesis of Complex Oxide 2>
The composite oxides of Examples 9 to 13 and Comparative Examples 7 to 13 were prepared by changing La in the composite oxide (LaSr ₃ Fe ₃ O _10-δ ) of Example 1 to another metal. Note that the number of moles shown for hydrates is the number of moles for the anhydride.

Example 9: _{PrSr3Fe3O10 - δ}
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Pr(NO ₃ ) _{3.6H 2} O (6.67 mmol).

Example 10: _{NdSr3Fe3O10 - δ}
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Nd(NO ₃ ) _{3.6H 2} O (6.67 mmol).

Example 11: _{SmSr3Fe3O10 - δ}
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Sm(NO ₃ ) _{3.6H 2} O (6.67 mmol).

Example 12: _{EuSr3Fe3O10 - δ}
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Eu(NO ₃ ) _{3.6H 2} O (6.67 mmol).

Example 13: _{GdSr3Fe3O10 - δ}
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Gd(NO ₃ ) _{3.5H 2} O (6.67 mmol).

Comparative Example 7: YSr ₃ Fe ₃ O _10-δ
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) ₃ .9H ₂ O (6.67 mmol) was changed to Y(NO ₃ ) ₃ .6H ₂ O (6.67 mmol).

Comparative example 8: CeSr ₃ Fe ₃ O _10-δ
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Ce(NO ₃ ) _{3.6H 2} O (6.67 mmol).

Comparative Example 9: TbSr ₃ Fe ₃ O _10-δ
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Tb(NO ₃ ) _{3.5H 2} O (6.67 mmol).

Comparative example 10: DySr ₃ Fe ₃ O _10-δ
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Dy(NO ₃ ) _{3.5H 2} O (6.67 mmol).

Comparative Example 11: HoSr ₃ Fe ₃ O _10-δ
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Ho(NO ₃ ) _{3.5H 2} O (6.67 mmol).

Comparative Example 12: ErSr ₃ Fe ₃ O _10-δ
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Er(NO ₃ ) _{3.5H 2} O (6.67 mmol).

Comparative example 13: YbSr ₃ Fe ₃ O _10-δ
The target composite oxide was synthesized in the same manner as in Example 1, except that La(NO ₃ ) _{3.9H 2} O (6.67 mmol) was changed to Yb(NO ₃ ) _{3.5H 2} O (6.67 mmol).

FIG. 6 shows the XRD patterns of Examples 1 and 9 to 13. FIG. 7 shows the XRD patterns of Comparative Examples 7 to 13. As shown in FIG. 6, the diffraction lines of Examples 9 to 13, like the diffraction lines of Example 1, have the strongest line around 32° split into two, and match the diffraction pattern characteristic of the Rudlesden-Popper type layered perovskite. On the other hand, as shown in FIG. 7, in Comparative Examples 7 to 13, the diffraction lines of SrFeO ₃ and (RE) ₂ O ₃ (RE is Y, Ce, Tb, Dy, Ho, Er, or Yb, respectively) were detected, and it was recognized that the Rudlesden-Popper type layered perovskite was not generated.

FIG. 8 shows the results of H ₂ -TPR analysis of Examples 1 and 9 to 13. As described above with respect to FIG. 1C, in Example 1, a hydrogen consumption peak was detected at 490° C., which was attributed to hydrogen consumption due to topotactic transition. As described above, in Example 1, hydrogen consumption due to the collapse of the oxide structure at 700° C. or higher was hardly observed, structural stability was enhanced, and oxygen desorption occurred from a low temperature of 490° C., resulting in excellent low-temperature operability. As shown in FIG. 8, the hydrogen consumption peaks of Examples 9 to 13 were observed at temperatures equal to or lower than those of Example 1, indicating that oxygen desorption occurs from a low temperature equal to or lower than those of Example 1. Furthermore, in Examples 9 to 13, almost no hydrogen consumption was observed at 600° C. or higher, which indicated that almost no structural collapse occurred due to reduction to zero-valent metal.

Claims (7)

An oxygen absorbing/releasing material comprising a Rudolfsden-Popper type layered perovskite complex oxide represented by the following formula (0):
RE(Fe _1-a M _a ) ₃ Sr ₃ O _10-δ (0)
(In the formula, RE is one or more selected from the group consisting of Pr, Nd, Sm, Eu, Gd, and La, M is one or more selected from the group consisting of Mn, Ni, and Co, a is a real number equal to or greater than 0 and less than 1, and δ is a real number equal to or greater than 0 and less than 1.) An oxygen absorbing/releasing material comprising a Rudolfsden-Popper type layered perovskite complex oxide represented by the following formula (1):
La(Fe _1-a M _a ) ₃ Sr ₃ O _10-δ (1)
(In the formula, M is one or more selected from the group consisting of Mn, Ni, and Co, a is a real number equal to or greater than 0 and less than 1, and δ is a real number equal to or greater than 0 and less than 1.) The oxygen absorbing and releasing material according to claim 1 or 2, wherein a is 0 in the formulas (0) and (1). The oxygen absorbing and releasing material according to claim 1 or 2, wherein in the formulas (0) and (1), a is a real number greater than 0 and less than 1. The oxygen absorbing and releasing material according to claim 4, wherein in the formulas (0) and (1), a is a real number of 0.1 or more and 0.3 or less. The oxygen absorbing and releasing material according to claim 1, wherein in formula (0), RE is one or more selected from the group consisting of Pr, Nd, Eu, and Gd. 3. An exhaust gas purifying catalyst comprising the oxygen absorbing material according to claim 1 or 2 and a precious metal supported on the oxygen absorbing material.