Oxygen storage material, catalyst for purifying exhaust gas, and method for producing oxygen storage material

Abstract

This oxygen occlusion material: has a chemical composition represented by Ce1-xZrxO2-δ (0.45≤x≤0.65, 0≤δ); in the XRD pattern, has, in the vicinity of 14.5°, a peak attributed to a cubic pyrochlore li

Description

Title of Invention: OXYGEN STORAGE MATERIAL, CATALYST FOR PURIFYING EXHAUST GAS, AND METHOD FOR PRODUCING OXYGEN STORAGE MATERIAL Technical Field

[0001] The present invention relates to an oxygen storage material, a catalyst for purifying exhaust gas, and a method for producing an oxygen storage material. The present application is based on PCT / JP2021 / 034566 filed on September 21, 2021, and the contents thereof are incorporated herein. Background Art

[0002] A three-way catalyst is used for purifying exhaust gas from an automatic vehicle or the like. The three-way catalyst is a catalyst for simultaneously removing three types of gas including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOX) in the exhaust gas. It is known that a purification rate depends on an air-fuel ratio, and CO, HC, and NOX can be simultaneously removed with high efficiency when an atmosphere of the exhaust gas is near a theoretical air-fuel ratio of 14.6. An OSC material that stores and releases oxygen is used for controlling the atmosphere near the theoretical air-fuel ratio. An oxygen storage capacity (OSC) indicates an amount of oxygen that can be stored and released by a material, which can store oxygen when an oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration in the exhaust gas is low according to the atmosphere, and is defined by an oxygen storage and release amount [pmol-02-g 4] per 1 g of the catalyst. Therefore, a material having a high OSC is required for the three-way catalyst.

[0003] CeO2 attracts attention due to having a relatively high OSC. CeO2 stores and releases oxygen according to the following reaction formula with a change in valence of Ce ions . Oxygen release: CeO2 CeO2-5 + 5 / 202 Oxygen storage: CeO2 - 6 + 5 / 202 — CeO2 Ce ions of CeO2 are Ce4+, and when all Ce4+ are reduced to Ce3+, 5 becomes the largest. At this time, 6 = 0.5.

[0004] However, since an ion radius is increased by about 1.2 times during the reduction from Ce4+ to Ce3+, a strain of a crystal lattice occurs, and the lattice becomes unstable. Therefore, the strain is relaxed by introducing Zr4+ having an ion radius smaller than that of the Ce ions. This relaxation effect eliminates the instability of the crystal lattice due to the change in valence of the Ce ions, resulting in an increase in OSC. The OSC of Cei-xZrx02 varies depending on a solid solution amount of Zr and becomes maximum in a composition near x = 0.5. A CeCh-ZrCk-based oxide has a high OSC due to the change in valence of the Ce ions, and is widely used as a co-catalyst of an exhaust gas purification catalyst.

[0005] In relation to this Ceo.5Zro.5O2, there are a tetragonal fluorite type structure (t ’ phase) in which Ce and Zr ions are randomly arranged and a cubic pyrochlore-like structure (k phase) in which Ce and Zr ions are arranged in order in a

[110] direction.

[0006] It is known that this k phase exhibits the highest OSC in a pseudo-binary system of CeO2-ZrO2. The t' phase has a reduction rate of about 50%, whereas the k phase releases oxygen of about 90% of a theoretical value. As shown in FIG. 1, a synthesis process of the k phase includes two steps (a first step (reduction step) and a second step (oxidation step) thereafter). In the first step (reduction step) , a precursor of CeO2-ZrO2 is subjected to a heat treatment at a high temperature of, for example, 1200°C or higher in a reduction atmosphere to synthesize pyrochlore phase Ce2Zr2O? in which Ce and Zr are arranged in order. Next, in the second step (oxidation step), a heat treatment is performed at, for example, about 600°C, and oxygen is introduced, thereby synthesizing K-Ce2Zr20g. Here, the pyrochlore phase Ce2Zr2O? and the k phase Ce2Zr20s have the same metal ion arrangement and both have a structure in which metal ions are arranged in order (hereinafter, sometimes referred to as a "cation-ordered structure"), and a difference is only the amount of oxygen. The pyrochlore type Ce2Zr2O? having a metal-to-oxygen ratio of 4:7 is further oxidized, and the k phase Ce2Zr20s has a metal-to-oxygen ratio of 4: (7 + 1) = 4:8 = 1:2.

[0007] NPL 3 discloses that the first step (reduction step) is performed at 1500°C for 4 hours in an atmosphere of 10% H2-Ar, and the second step (oxidation step) is performed at 600°C for 4 hours in the air. In addition, NPL 3 discloses that it has been examined that a reduction temperature and a reduction time in the first step (reduction step) has a relationship with a formation ratio of the k phase in a sample, when the reduction step is performed at 1300°C for 4 hours, 1400°C for 4 hours, and 1500°C for 4 hours, the formation ratio of the k phase is 91.2%, 97.1%, and 100%, respectively, and when the reduction step is performed at 1500°C for 30 minutes, 1 hour, and 2 hours, the formation ratio of the k phase is 72.5%, 85.0%, and 95.0%, respectively. Here, the formation ratio of the k phase in the sample is obtained based on an intensity ratio [1(14 / 29) value] of an intensity of a diffraction line at 20 = 14.5° and an intensity of a diffraction line at 20 = 29° in an X-ray diffraction pattern. NPL 4 discloses that the first step (reduction step) is performed at 1200°C and the second step (oxidation step) is performed at 500°C.

[0008] On the other hand, PTL 2 discloses that "a composite oxide comprising CeO2 and ZrO2, having one or more phases of a pyrochlore phase, a k phase, and an intermediate phase of both phases, and having a specific surface area of 20 m2 / g or more" (Claim 1) . PTL 2 discloses that, in any of synthesis processes of Examples and Comparative Examples, the first step (reduction step) is performed, but does not disclose that the second step (oxidation step) is performed. Specifically, it is disclosed in Example 1 that "the obtained powder was subjected to a reduction treatment at 1000°C for 2 hours in an N2 air flow containing 4% of H2 to obtain a composite oxide powder of the invention" (paragraph 0041), and it is disclosed in Example 2 that "the powder was subjected to the reduction treatment at 900°C for 2 hours in the N2 air flow containing 4% of H2 to obtain a composite oxide powder of the invention" (paragraph 0043). Since the second step (oxidation step) is not performed, it is considered that the k phase is not obtained. Further, in both Example 1 and Example 2, since the temperature in the first step (reduction step) is lower than 1200°C as described above, it is unknown whether the cation-ordered structure is obtained on the premise of the k phase. As described later, a peak near 29 = 15° in an XRD pattern is known as a piece of evidence supporting the presence of the cation-ordered structure, but an XRD pattern shown in FIG. 2 of PTL 2 shows only a low angle up to 25°, which does not support the presence of the cation-ordered structure.

[0009] As an approach for improving catalyst activity of a ceria-zirconia (CeCt-ZrO?) composite oxide, it is known that iron (Fe) is formed as a solid solution in the CeO2-ZrO2 composite oxide (for example, PTL 3). Since the ion radius of Fe3+ is smaller than the ion radius of Zru, it is considered that Fe ions are selectively substituted with Zr sites, oxygen defects are generated in the ceria-zirconia-based composite oxide in which iron is formed as a solid solution due to such substitution with Fe ions, and the oxygen defects improve activity of oxygen in the composite oxide to exhibit an excellent oxygen storage capacity (OSC) (see paragraph 0021 of PTL 3).

[0010] PTL 3 discloses an oxygen storage material (Claim 1) "comprising a pyrochlore type ceria-zirconia-based composite oxide and iron added to the ceria-zirconia-based composite oxide, wherein a content ratio of iron to a total amount of cerium (Ce) and zirconium (Zr), i.e., (Fe / (Ce+Zr) x 100), is 0.5 at% to 9 at%, and a molar fraction of zirconium to a total number of moles of cerium (Ce) and zirconium (Zr), i.e., (X = Zr / (Ce+Zr) * 100), is X = 40% to 50%". In addition, it describes a reason why it can be determined that iron is sufficiently formed as a solid solution in the iron-containing pyrochlore type ceriazirconia-based composite oxide in the oxygen storage material based on a lattice constant and the intensity ratio of the diffraction line of 20 = 14.5° to the diffraction line of 20 = 29°, which are obtained from an X-ray diffraction pattern obtained by an X-ray diffraction measurement using CuKa before heating at 1100°C and after heating for 5 hours in the air (see paragraphs 0017 to 0021) . In order to sufficiently cause iron to be formed as a solid solution in this manner, it is disclosed that an iron-containing ceria-zirconium solid solution obtained by a coprecipitation method is pulverized to obtain an iron-containing ceria-zirconium solid solution powder, and the powder is pressure-molded under a pressure of 30 MPa to 350 MPa, then subjected to a reduction treatment under a temperature condition of 1400°C to 2000°C, and further subjected to an oxidation treatment to produce an oxygen storage material (see Claim 3). In Example 1, the reduction treatment is performed at 1700°C for 4 hours. Citation List Patent Literature

[0011] PTL 1: WO 2017 / 213038 PTL 2: JP2005-170554A PTL 3: JP2022-59284A Non Patent Literature

[0012] NPL 1: Y. Goto et al., Chern. Commun, 54 (2018) 3528-3531. NPL 2: Noriya Izu et al., J. Alloys Compd., 270 (1998) 107-114 . NPL 3: Y. Ding et al., Catalyst Today, 327 (2019), 262-270. NPL 4: Y. Nagai et al., Catalyst Today, 74 (2002), Summary of Invention Technical Problem

[0013] It has been reported that the high-temperature heat treatment (first step (reduction step)) at 1200°C or higher in the k phase synthesis process in the related art causes a great decrease in specific surface area, and specifically, it has been reported that the specific surface area of the t' phase is 17.08 [m2 / g] while the specific surface area of the k phase is 0.35 [m2 / g], which is about 1 / 50 of the specific surface area of the t' phase (see NPL 1). NPL 3 discloses that when a precursor of CeO2-ZrO2 is synthesized by the coprecipitation method, the specific surface area of the precursor is 64 [m2 / g], but the specific surface area after a reduction treatment is 1 [m2 / g] or less. In addition, it is also disclosed that when the precursor of CeCh^ZrOa is synthesized by a solvent-thermal method, the specific surface area of the precursor is 35 [m2 / g], but the specific surface area after a reduction treatment is 2 [m2 / g] to 3 [m2 / g]. As described above, although the k phase has a reduction rate ("89%" (see NPL 4)) higher than that of the t' phase, oxygen is stored and released via a surface, and thus, there is a problem that it is difficult to put the k phase into practical use since the specific surface area is greatly decreased.

[0014] Here, the composite oxide of PTL 2 is described to have a specific surface area with a very high value of 20 [m2 / g] or more (Claim 1), and the specific surface areas of Example 1 and Example 2 after the reduction treatment (corresponding to the first step) are 25 [m2 / g] and 52 [m2 / g] , respectively, while the specific surface area of Comparative Example 2 (paragraph 0048) in which the reduction treatment is not performed is 105 [m2 / g] (see Table 1) . It is considered that the high specific surface areas of Example 1 and Example 2 are results of the formation of the pyrochlore phase not proceeding in the composite oxide and the remaining of a large amount of the t' phase. In Example 1 and Example 2, the formation of the pyrochlore phase, which is the premise of the k phase, does not proceed, and the oxidation treatment (corresponding to the second step) for forming the k phase by introducing oxygen into vacant sites of the pyrochlore phase is not performed. Therefore, it is considered that Examples having high specific surface areas of 20 [m2 / g] or more described in PTL 2 do not contain the k phase, or even if the k phase is contained, the amount thereof is very small.

[0015] As a result of intensive studies, the present inventors have found a method of obtaining the k phase by a reduction heat treatment at a temperature lower than 1200°C, and made it possible to prepare a CeO2-ZrO2 oxide containing a k phase having a specific surface area larger than that in the related art, thereby completing the invention. In this method, the reduction heat treatment is performed by adding a Fe oxide during a reduction treatment of the CeO2^ZrO2 oxide, whereby a cation^ordered structure can be obtained by the reduction treatment at a low temperature as compared with the related art, thereby preventing growth of crystal grains, and as a result, a decrease in specific surface area is prevented.

[0016] The invention has been made in view of the above circumstances, and provides an oxygen storage material including a CeO2^ZrO2 oxide containing a k phase having a specific surface area larger than that in the related art, a catalyst for purifying exhaust gas, and a method for producing an oxygen storage material. Solution to Problem

[0017] In order to solve the above problems, the invention provides the following means.

[0018] An oxygen storage material according to a first aspect of the invention has a chemical composition represented by Cei^xZrxO2^6 (0.45 <x 0.65, 0^5), has a peak attributed to a cubic pyrochlore-like structure (k phase) near 14.5° in an XRD pattern, and has a specific surface area of 3 [m2 / g] or more.

[0019] The oxygen storage material according to the above aspect may have a specific surface area of 3.5 [m2 / g] or more .

[0020] The oxygen storage material according to the above aspect may have a specific surface area of 5 [m2 / g] or more.

[0021] The oxygen storage material according to the above aspect may have a peak attributed to Fe in the XRD pattern.

[0022] A catalyst for purifying exhaust gas according to a second aspect of the invention contains the oxygen storage material according to the above aspect.

[0023] A method for producing an oxygen storage material according to a third aspect of the invention includes a composite preparation stage of preparing a composite in which a Fe compound is added to an oxide having a chemical composition represented by Cei-xZrxC>2-5 (0.45 x 0.65, 0 6), and a reduction heat treatment stage of subjecting the composite to a reduction heat treatment at a temperature of 700°C or higher and 850°C or lower.

[0024] In the method for producing an oxygen storage material according to the above aspect, in the composite preparation stage, the Fe compound may be added in an amount of 1 vol% or more and 10 vol% or less with respect to the oxide having the chemical composition represented by Cei-xZrxO2-s (0.45 x <0.65, 0^6). Advantageous Effects of Invention

[0025] According to the invention, it is possible to provide an oxygen storage material including a CeO2^ZrO2 oxide containing a k phase having a larger specific surface area than that in the related art. Brief Description of Drawings

[0026] [FIG. 1] FIG. 1 is a diagram schematically showing a k phase synthesis process. [FIG. 2] FIG. 2 is a diagram conceptually showing a state in which exhaust gas is purified using a catalyst for purifying according to the present embodiment fixed to a carrier . [FIG. 3] FIG. 3 shows XRD patterns of a calcined powder of Ceo.5Zro.5O2 prepared by a Pechini method and Ceo.5Zro.5O2 after addition of FeaO4, y-Fe2O3, and «-Fe2O3. [FIG. 4] (a) in FIG. 4 shows XRD patterns after reduction of additive-free Ceo.5Zro.5O2 and samples added with FesOo y-FeoOs, and a-FeoOs, and (b) in FIG. 4 is an enlarged diagram near 29 = 14.5°. [FIG. 5] FIG. 5 shows Raman spectra obtained by reducing additive-free Ceo.5Zro.5O2 and a sample added with a-FeaOs or oxidizing additive-free Ceo.5Zro.5O2 and the sample after reduction. [FIG. 6] (a) in FIG. 6 is a graph showing reduction time dependence of an XRD pattern of Ceo.sZro.502-5 vol% y-FeoOs, and (b) in FIG. 6 is an enlarged diagram near 29 = 14.5°. [FIG. 7] (a) in FIG. 7 is a graph showing reduction time dependence of an XRD pattern of Ceo.sZro.502-5 vol% Fe.3O4z and (b) in FIG. 7 is an enlarged diagram near 29 = 14.5°. [FIG. 8] FIG. 8 is an XRD pattern of a sample added with y-Fe2O3 and Ceo.5Zro.5O2 prepared by a solid-state reaction method. [FIG. 9] FIG. 9 shows an XRD pattern obtained after a reduction heat treatment in an atmosphere of 5% H2-Ar at 800°C for 3 hours. [FIG. 10] FIG. 10 is a graph comparing specific surface areas of a t' phase prepared by the Pechini method and a k phase prepared by adding 5 vol% a-Fe2O3 to the t’ phase . [FIG. 11] FIG. 11 is a conceptual diagram showing a mechanism related to cation ordering of a ceria^zirconia composite oxide related to the presence of Fe ions. [FIG. 12] (a) in FIG. 12 is a photograph on which a model interface is formed by depositing a Fe2Oa thin film on a t' phase CZ55 powder layer having a thickness of 3 pm by PLD, and a mapping image obtained by performing line scanning on a boundary between a region (region 1) where the Fe2Oa thin film is deposited and a region (region 2) where the Fe2Oa thin film is not deposited by a Raman spectrum method after a reduction heat treatment, and (b) in FIG. 12 shows a Raman spectrum of each original mapping image shown in (a) in FIG. 12. Description of Embodiments

[0027] Hereinafter, an oxygen storage material, a catalyst for purifying exhaust gas, and a method for producing an oxygen storage material according to an embodiment to which the invention is applied will be described in detail.

[0028] (Oxygen Storage Material) An oxygen storage material can release oxygen when oxygen is insufficient and can store oxygen when oxygen is excessive in a three-way catalyst. Accordingly, even when an air-fuel ratio deviates from an ideal range, three types of gas including carbon monoxide, hydrocarbons, and nitrogen oxides in an exhaust gas can be simultaneously removed.

[0029] An oxygen storage material according to the present embodiment has a chemical composition represented by Cei^ xZrxO2^s (0.45 x 0.65, 0^5), has a peak attributed to a cubic pyrochlore-like structure (k phase) near 14.5° in an XRD pattern, and has a specific surface area of 3 [m2 / g] or more .

[0030] <Chemical Composition> The oxygen storage material according to the present embodiment has the chemical composition represented by Cei-xZrxO2^s (0.45 x 0.65, 0^6). The oxygen storage material according to the present embodiment preferably has a chemical composition represented by Cei-xZrxO2-6 (0.45 x 0.60, 0^6), and more preferably has a chemical composition represented by Cei^xZrxO2^ (0.45 <x <0.55, 0 <6) . As shown in Examples, the chemical composition is represented by Ceo.5Zro.5O2, but it is known that a k phase is obtained when x is in the range of 0.45 <x <0.65 (see NPL 2).

[0031] In a simple substance of CeO2, oxygen on a crystal surface mainly contributes to OSC characteristics, and in CeO2-ZrO2, oxygen inside a crystal also contributes to an OSC. Therefore, a CeCh-ZrCh-based oxide has a high OSC. In Cei-xZrxO2, a highest OSC characteristic is obtained near x = 0.5. When oxygen in Ceo.5Zro.5O2 is released, Ceo.5Zro.502-6 is obtained. When a valence of Zr ions does not change, the largest 6 is 0.25. Cei^xZrxO2 has a plurality of phases such as a t' phase and a k phase. As the oxygen storage material, the t' phase or the k phase is mainly used. The t' phase is a tetragonal phase. The k phase is a phase having a pyrochlore-like structure. In addition, it is sufficient that 0 5. Theoretically, in the case of Ceo.sZro.sOo-s, 0^5^ 0.25, and when x is close to 0, 0 6 0.5.

[0032] <XRD Pattern> The oxygen storage material according to the present embodiment has a peak attributed to a cubic pyrochlore-like structure (k phase) near 14.5° in the XRD pattern.

[0033] In the present description, "having a peak attributed to a cubic pyrochlore-like structure (k phase)" means that a peak appears near 14.5° in the XRD pattern to the extent that the peak can be specified to attribute to the cubic pyrochlore-like structure (k phase). When an intensity of the peak attributed to the k phase is weak, the peak can be checked by expanding a range of 26 including 14.5°. As will be described later, since formation of the k phase can also be checked by Raman spectroscopy, the presence of the peak attributed to the k phase can be determined by using Raman spectroscopy in combination. In addition, in the present description, "near 14.5°" means that it is sufficient to specify the peak attributed to the pyrochlore-like structure (k phase), and it is intended not to be strictly limited to 14.5° in consideration of a deviation depending on measurement conditions, an apparatus, and the like in an actual measurement.

[0034] The oxygen storage material according to the present embodiment may have a peak attributed to Fe in the XRD pattern . Since the oxygen storage material according to the present embodiment is subjected to a reduction heat treatment by adding a Fe compound, Fe remains in the oxygen storage material at a stage of preparation. Since Fe itself is considered to have no influence or have a small influence on the OSC, the oxygen storage material can be used while Fe remains. When Fe remains, a Fe compound may be formed in the oxygen storage material. In addition, when Fe flows due to an acid or the like, the oxygen storage material does not contain or hardly contains Fe.

[0035] <Specific Surface Area> The oxygen storage material according to the present embodiment has a specific surface area of 3 [m2 / g] or more. The specific surface area is preferably 3.5 [m2 / g] or more, more preferably 4 [m2 / g] or more, and still more preferably 5 [m2 / g] or more. In the present description, a value of "specific surface area" is a value obtained by adsorbing nitrogen molecules at a liquid nitrogen temperature of 77K and calculating the specific surface area based on an adsorption isotherm using a BET theory. The "specific surface area" can be measured by using, for example, a gas adsorption amount measuring apparatus BELSORP 18 Plus (manufactured by BEL JAPAN, Inc. (current MicrotracBEL Corp.)).

[0036] When a crystal grain becomes larger, a surface with respect to the entire crystal grain becomes smaller, and thus the specific surface area becomes smaller. The crystal grain grows as a temperature increases during the heat treatment. In the related art, it is necessary to perform a reduction heat treatment at 1200 °C or higher during the preparation of the k phase, whereas the k phase can be obtained even at 800°C according to the invention. In the k phase of the invention, since the k phase can be prepared at a temperature lower than that in the related art, growth of the crystal grain is prevented as compared with the related art, and the specific surface area is increased.

[0037] The oxygen storage material according to the present embodiment can be used together with a three-way catalyst as a main catalyst . The oxygen storage material according to the present embodiment can be used by being fixed to a carrier. Examples of the carrier include alumina (AI2O3) , zirconia (ZrO2), magnesia (MgO), and silica (SiOz) •

[0038] (Method for Producing Oxygen Storage Material) A method for producing an oxygen storage material according to the present embodiment includes a composite preparation stage of preparing a composite in which a Fe compound is added to an oxide having a chemical composition represented by Cei^xZrxO2^ (0.45 x 0.65, 0^6), and a reduction heat treatment stage of subjecting the composite to a reduction heat treatment at a temperature of 700°C or higher and 850°C or lower.

[0039] A method for preparing an oxide having a chemical composition represented by Cei-xZrxC>2-5 (0.45 x 0.65, 0 6) is not particularly limited, and for example, a Pechini method or a general solid-state reaction method as a method for synthesizing ceramics can be used. A method for preparing a composite in which a Fe compound is added to the oxide having a chemical composition represented by Cei^xZrxO2^s (0.45 x 0.65, 0^6) is also not particularly limited, and for example, it can be prepared by mixing in a planetary ball mill.

[0040] The temperature in the reduction heat treatment in the reduction heat treatment stage is preferably 750°C or higher and 850°C or lower. The temperature in the reduction heat treatment is preferably 800°C ± 30°C.

[0041] <Fe Compound> Examples of the Fe compound to be added include Fe oxides such as Fe2O3 and FeaCh, and Fe-containing oxides containing metals other than Fe such as CoFe204 .

[0042] An addition amount of the Fe compound in the composite preparation stage is preferably 1 vol% or more and 10 vol% or less with respect to the oxide having the chemical composition represented by Cei-xZrxO2-6 (0.45 x 0.65, 0 5) • The addition amount of the Fe compound is preferably 2 vol% or more, and more preferably 3 vol% or more, with respect to the oxide having the chemical composition represented by Cei-xZrxO2-s (0.45 <x <0.65, 0^6). The addition amount of the Fe compound is preferably 8 vol% or less, and more preferably 7 vol% or less, with respect to the oxide having the chemical composition represented by Cei-xZrxO2-s (0.45 <x <0.65, 0^6).

[0043] After the reduction heat treatment stage, an oxidation treatment is performed at a temperature of 300°C to 800°C. For example, the oxidation treatment can be performed at 600°C.

[0044] (Catalyst for Purifying Exhaust Gas) A catalyst for purifying exhaust gas according to the present embodiment contains the above-described oxygen storage material according to the invention. The oxygen storage material according to the invention may be contained together with the main catalyst as a co catalyst, or may be used alone as the catalyst for purifying exhaust gas .

[0045] When the catalyst for purifying exhaust gas according to the present embodiment contains the oxygen storage material according to the invention as the co-catalyst, a known three-way catalyst precious metal can be used as the main catalyst. Specific examples of the known three-way catalyst precious metal include rhodium, platinum, and palladium.

[0046] As shown in FIG. 2, the catalyst for purifying exhaust gas according to the present embodiment can be used by being fixed to the carrier. In FIG. 2, reference numeral 1 denotes the oxygen storage material, reference numeral 2 denotes the three-way catalyst as the main catalyst, and reference numeral 3 denotes the carrier. Examples of the carrier according to the invention include alumina (AI2O3) , zirconia (ZrO2) , magnesia (MgO), and silica (SiOz) . Examples

[0047] 1 . Preparation of Sample Powder (1) Preparation by Pechini Method Hereinafter, a step of preparing a CeO2^ZrO2 sample using the Pechini method will be described. The Pechini method is a kind of liquid phase synthesis method, and has an advantage that metal ions can be mixed at an atomic level and a uniform and fine product can be obtained, although a generation amount that can be prepared at one time is smaller than that in a solid-state method. Metal nitrates, citric acid, and propylene glycol shown in Table 1, and distilled water in an amount of 1 / 2 of a weight of citric acid were added to obtain the desired composition, and the mixture was stirred for 24 hours. Thereafter, the temperature was increased stepwise from room temperature to 300°C by a hot stirrer to prepare a precursor. The completely solidified precursor was held in an electric furnace at 200°C, 300°C, and 400°C for 2 hours each to perform carbonization. Thereafter, the mixture was pulverized in a mortar for about 10 minutes, then further calcined in the air at 800°C for 2 hours, and pulverized in a planetary ball mill at 400 rpm for 2 hours to form fine particles. After ball milling, 2-propanol was sufficiently dried and mixed by hand for about 10 minutes in an agate mortar to obtain a powder sample. A heating rate was 5°C / min in the process of the carbonization and 10°C / min in the process of the calcination.

[0048] [Table 1] Element Composition Purity (%) Mixing mole ratio*1 Ce Ce (NO3) 3- 6H2O 99.9 1:3:3 Zr ZrO (NO3) 2-2H2O 99 1:9:9 x: Mixing mole ratio = cation: citric acid:propylene glycol

[0049] (2) Preparation by Solid-State Reaction Method Hereinafter, a step of preparing the CeO2-ZrO2 sample using the solid-state reaction method will be described. The CeO2-ZrO2 sample was prepared by the general solidstate reaction method as the method for synthesizing ceramics, different from the Pechini method. In the solidstate reaction method, raw material powders such as oxides and carbonates are mixed, then atoms are diffused by a heat treatment at a high temperature, and the reaction proceeds. Although uniformity of the sample is inferior to that of the liquid phase method, a large amount of sample can be prepared at one time. Raw materials CeO2 (manufactured by Anan Kasei Co., Ltd.) and ZrO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) were weighed so as to satisfy Ceo.5Zro.5O2, and then mixed in a planetary ball mill at 300 rpm for 2 hours. After ball milling, 2-propanol was sufficiently dried, and the mixed powder was pelletized using a hydraulic single-shaft hand press under a condition of 35 MPa for 1 minute. Thereafter, isostatic compression molding was performed at 250 MPa for 1 minute using a cold isostatic press machine (CPA-50s, NPa System, Co., Ltd.). The temperature was increased to 1600°C at 10°C / min and held for 10 hours in calcination. The calcined pellets were coarsely pulverized in an HD mortar and pulverized in a planetary ball mill at 300 rpm for 12 hours to obtain a calcined powder.

[0050] (3) Preparation Ceo.5Zro.502-Fe Oxide Composite A composite of a Fe oxide and Ceo.5Zro.5O2 prepared by the Pechini method and the solid-state reaction method was prepared by performing mixing in a planetary ball mill at 300 rpm for 1 hour. As the Fe oxide, FeaO4 (manufactured by FUJIFILM Wako Pure Chemical Corporation), a-FeoOs (manufactured by Kojundo Chemical Lab. Co., Ltd.), and FeoOs (manufactured by Kojundo Chemical Lab. Co., Ltd.) were used, and an addition amount thereof was about 5 vol% with respect to the amount of Ceo.5Zro.5O2. The crystal structures of a-FeoOs and y^Fe2O3 are different, a-FeoOs is a corundum type, and Y_Fe2O3 is a spinel type .

[0051] 2. Evaluation of Materials (1) Phase Identification Method For phase identification of the prepared sample, powder X-ray diffraction (XRD) (D8 Advance, Bruker) was used The powder sample was filled in a dedicated glass holder and was subjected to a measurement. All samples were measured by a concentration optical system. Cu-Ka rays were used as an X-ray source, a tube bulb voltage was 40 kV, and a filament current was 40 mA. In addition to XRD, a microscopic Raman spectroscopy apparatus (HR-800 manufactured by HORIBA, Ltd.) was used for the phase identification of the sample. A He-Ne laser (X = 632.84 nm) was used as an excitation source. The sample was expanded by an objective lens of 100 times using an optical microscope, and a grading of 600 lines / nm was used. A confocal hole value was 1000 pm, and a slit value was 100 pm. In the measurement, a subtractive filter having an optical density of 0.3 to 1 was used. For the measurement of the specific surface area, a high-precision fully automatic gas adsorption apparatus (BELSORP 18 PLUS, BEL JAPAN, Inc.) was used. About 1 g of the sample powder was weighed and charged into a tube. The powder in the sample tube was subjected to a heat treatment in a pretreatment system of the adsorption apparatus at 350°C for 2 hours under vacuum to remove adsorbed water and the like. The measurement was performed using nitrogen as adsorption species at a liquefied nitrogen temperature (77K) The measurement was performed in a range of an introduction pressure of 0.2 kPa to 0.95 relative pressure of an actually measured saturated vapor pressure. Based on the obtained adsorption isotherm, the specific surface area was determined based on the BET theory.

[0052] (2) Crystal Structure Evaluation on Ceo .sZro .sOo^Fe Oxide Composite FIG. 3 shows XRD patterns of a calcined powder of Ceo.5Zro.5O2 prepared by the Pechini method and Ceo.5Zro.5O2 after addition of Fe3O4, y^FeoOs, and a^FeoOs. The four XRD patterns in FIG. 3 are, in order from the top, an XRD pattern of the calcined powder of Ceo.5Zro.5O2, an XRD pattern of a sample after addition of FeaO4, an XRD pattern of a sample after addition of y-FeoOs, and an XRD pattern of a sample after addition of a^FeoOs. Regarding Ceo.5Zro.5O2, all of the observed peaks were attributed to a tetragonal fluorite type structure of Ceo.5Zro.5O2 having a space group P42 / nmc. In each sample to which the Fe oxide was added, the same peak as that of the raw material was observed. In the XRD pattern, a peak indicated by attributes to the tetragonal fluorite type structure, a peak indicated by is a peak attributed to FeaO4, a peak indicated by is a peak attributed to y^FeoOs, and a peak indicated by "A" is a peak attributed to a-FeoOs.

[0053] (3) Crystal Structure Evaluation on Sample after Reduction Heat Treatment (a) in FIG. 4 shows XRD patterns after reduction of additive-free Ceo.5Zro.5O2 and samples added with FegO4, y-FegOg, and a-FegOg . (b) in FIG. 4 is an enlarged diagram near 29 = 14.5°. The XRD pattern is obtained for a sample after a reduction heat treatment and before an oxidation treatment. The pyrochlore type CegZrgO? obtained by the reduction heat treatment is subjected to an oxidation treatment at an appropriate temperature to introduce oxygen to obtain k phase CegZrgOg. As described above, both the pyrochlore type CeoZroO? and the k phase CegZrgOg have a structure in which cations are arranged in order. Although the additive-free Ceo.5Zro.5O2 has the tetragonal fluorite type structure, when the phase is changed to pyrochlore type CegZrgO? in which cations are arranged in order by the reduction heat treatment, a peak derived from the ordering of cations appears near 14.5° in the XRD pattern. A main difference between the crystal structure of the fluorite type Ceo.5Zro.5O2 and the crystal structure of the pyrochlore type CegZrgOg (crystal structure of k phase Ce2Zr20g) is due to whether cations are arranged in order or randomly arranged. Therefore, a peak appears at a very close position in the XRD pattern. Among them, since the peak near 14.5° appears only when the cations are in order, the presence or absence of this peak serves as an index for distinguishing the pyrochlore type Ce2Zr2O? from the fluorite type Ceo.5Zro.5O2.

[0054] The reduction heat treatment was performed under a condition of an atmosphere of 5% H2-Ar at 800°C for 3 hours. In the case of the additive-free Ceo.5Zro.5O2, a peak of the tetragonal fluorite type structure was observed even after the reduction. On the other hand, in the samples added with FeoOa and FeaO^, a peak derived from the ordering of cations was observed near 14.5°, and the phase change to the pyrochlore type CeoZroO? was confirmed. In particular, in the sample added with Fe2O3, as compared with FeaO4, a peak near 14.5° is remarkably observed, and it is considered that pyrochlore type CeoZroO? having a higher degree of ordering is obtained.

[0055] In the XRD patterns in FIG. 4, a peak indicated by attributes to a tetragonal fluorite type structure of fluorite type Ceo.5Zro.5O2, a peak indicated by "o" is a peak attributed to pyrochlore type CeoZroO?, and a peak indicated by "V" is a peak attributed to Fe.

[0056] FIG. 5 shows a Raman spectrum of additive-free Ceo.5Zro.5O2, a Raman spectrum when an a-FeaOs added sample is subjected only to reduction, and a Raman spectrum in the case of reoxidation after reduction. The reduction heat treatment was performed under a condition of an atmosphere of 5% H2-Ar at 800°C for 3 to 5 hours, and the oxidation treatment was performed under a condition of the air at 800°C for 2 hours. In the Raman spectrum shown in FIG. 5, in the a-Fe2O3 added sample in which the peak is observed near 14.5° in the powder X-ray diffraction pattern (FIG. 4), peaks attributed to the pyrochlore type structure can be observed near 280 cm-1, 440 cm-1, and 600 cm-1 after reduction After oxidation, a plurality of peaks attributed to the k phase at 400 cm-1 to 600 cm-1 can be observed in addition to the strong peaks near 280 cm"1 and 440 cm”1. Therefore, formation of the k phase can be confirmed by using the X-ray diffraction and the Raman spectroscopy in combination.

[0057] (4) Influence 1 of Reduction Time on Formation of Pyrochlore Type Ce2Zr2O? With (3), regarding the samples added with Fe2Os and FeaO4, an influence of a time required for the formation of pyrochlore type Ce2Zr2O? was examined. The reduction heat treatment was performed under a condition of an atmosphere of 5% H2~Ar at 800°C, and the oxidation treatment was performed under a condition of the air at 800°C for 2 hours, (a) in FIG. 6 shows reduction time dependence of an XRD pattern of Ceo.sZro.sOz-S vol% y-Fe2O3. (b) in FIG. 6 is an enlarged diagram near 20 = 14.5°. It was found that, due to reduction for 30 minutes, in Ceo.5Zro.502-5 vol% Y_Fe2O3, the peak near 14.5° was observed and the phase changed to pyrochlore type Ce2Zr2O?.

[0058] In the XRD pattern of (a) in FIG. 6, a peak indicated by "o" is the peak attributed to pyrochlore type Ce2Zr2O7, a peak indicated by "x" is a peak attributed to FeO, and a peak indicated by "V" is a peak attributed to Fe.

[0059] (5) Influence 2 of Reduction Time on Formation of k Phase (a) in FIG. 7 shows reduction time dependence of an XRD pattern of Ceo.5Zro.502-5 vol% FeaCh. (b) in FIG. 7 is an enlarged diagram near 20 = 14.5°. The reduction heat treatment was performed under a condition of an atmosphere of 5% H2^Ar at 800°C. After the reduction heat treatment for 1 hour and 2 hours, the peak near 14.5° was not observed. Based on the above, it was found that in the case of Ceo.5Zro..502-5 vol% FesCg, a reduction heat treatment for about 3 hours was required for the phase change to pyrochlore type Ce2Zr2O7.

[0060] In the XRD pattern of (a) in FIG. 7, a peak indicated by attributes to fluorite type Ceo.5Zro.5O2, a peak indicated by "o" is a peak attributed to pyrochlore type CeoZroO?, and a peak indicated by "V" is a peak attributed to Fe .

[0061] (6) Crystal Structure Evaluation on Sample after Reduction Heat Treatment of Ceo.sZro.sOo-Fe Oxide Composite Using Ceo.5Zro.5O2 Prepared by Solid-State Reaction Method The CeOo-ZrOo sample was also prepared by the solid^ state reaction method which is the general method for synthesizing ceramics, and a low-temperature synthesis of pyrochlore type CeoZroO? was confirmed by adding a Fe oxide. FIG. 8 shows an XRD pattern of a sample added with y-FeoOs and Ceo.5Zro.5O2 prepared by the solid-state reaction method. All the observed peaks of Ceo.5Zro.5O2 prepared by the solid-state reaction were attributed to the tetragonal fluorite type structure of Ceo.5Zro.5O2 having a space group Pio / nmc, and peaks derived from y-FeoOs were observed in the composite .

[0062] In the XRD pattern in FIG. 8, a peak indicated by attributes to fluorite type Ceo.5Zro.5O2, and a peak indicated by is a peak attributed to y-FeoOa.

[0063] FIG. 9 shows an XRD pattern of Ceo. sZr o. 5O2-y-Fe2O3 obtained after a reduction heat treatment in an atmosphere of 5% H2-Ar at 800°C for 3 hours. After the reduction, the peak derived from the ordering of cations was observed near 14.5°, and it was found that the phase changed to the pyrochlore type Ce2Zr20v. Therefore, it was found that even in the case of Ceo.5Zro.5O2 prepared by the solid-state reaction method, the low-temperature synthesis of the pyrochlore type Ce2Zr20o by the addition of a Fe oxide was able to be performed.

[0064] In the XRD pattern in FIG. 9, a peak indicated by "o" is a peak attributed to pyrochlore type CeoZroO?, and a peak indicated by "V" is a peak attributed to Fe.

[0065] (7) Specific Surface Areas of t' Phase Prepared by Pechini Method and k Phase Prepared by Adding 5 vol% a-FeoOa to t’ Phase FIG. 10 is a graph comparing specific surface areas of the t’ phase prepared by the Pechini method and the k phase prepared by adding 5 vol% a-Fe2O3 to the t' phase. In FIG. 10, the t' phase was a phase prepared by the Pechini method and then not subjected to a reduction heat treatment, and the k phase was a phase subjected to a reduction heat treatment in an atmosphere of 5% H2-Ar at 800°C for 5 hours and a phase subjected to a reduction heat treatment in an atmosphere of 5% H2-Ar at 800°C for 1 hour. The k phase was subjected to an oxidation treatment in the air at 800°C for 2 hours. The specific surface area of the t’ phase is 22.3 [m2 / g] , and the specific surface areas of the k phase according to the invention are 3.8 [m2 / g] and 5.2 [m2 / g], respectively. The specific surface areas of the k phase according to the invention were respectively about 11 times and about 15 times the specific surface area of the k phase reported in NPL 1. Average particle diameters of the t' phase, the k phase (subjected to the reduction heat treatment at 800°C for 5 hours), and the k phase (subjected to the reduction heat treatment at 800°C for 1 hour) were 40.6 nm, 174 nm, and 324 nm, respectively. The average particle diameter was calculated on an assumption that particles were spherical using the measured specific surface area.

[0066] FIG. 11 is a conceptual diagram showing a mechanism related to cation ordering of a ceria-zirconia composite oxide due to the presence of Fe ions. First, CeFeOa is formed at an interface of a Fe oxide at 800°C, and Ce3+ and Ce vacancies formed in this process promote cation diffusion In order to maintain electrical neutrality, oxygen is released from a surface of the t’ phase Ceo.5Zro.5O2 (hereinafter, sometimes referred to as "CZ55") or via CeFeO.3 having the highest oxygen transport capacity. In general, an ordered phase is thermodynamically more stable at a low temperature than a disordered phase. In addition, thermal energy is dynamically required for the diffusion, and particularly, the thermal energy is required for the slow diffusion of cations. As described above, when a sufficiently high mobility for rearranging a cation arrangement is achieved at a low temperature, the formation of the ordered phase easily proceeds. This is a case of a ceria-zirconia composite oxide doped with FezOs . Further, it is presumed that when the ordered phase is formed as a seed, an ordered phase region spreads over the entire region by a strain caused by a difference in molar volume between the ordered phase and the disordered phase as a driving force .

[0067] In order to verify the mechanism shown in FIG. 11, as shown in (a) in FIG. 12, a Fe2O3 thin film was deposited by PLD on a t' phase CZ55 powder layer having a thickness of 3 pm formed on a quartz substrate to form a model interface. Next, a reduction heat treatment was performed at 900 °C for 5 hours, then a boundary between a region in which the FeaCh thin film was deposited (Fe2Oa; region 1) and a region in which the Fe2O3 thin film was not deposited (no Fe2Oa; region 2) in (a) in FIG. 12 was subjected to line scanning by a Raman spectrum method to obtain a Raman spectrum shown in (b) in FIG. 12. Near the region 1 where CZ55 was covered with the Fe2Oa thin film, peaks caused by a cat ion-ordered structure were clearly observed near 280 cm”1 and 440 cm”1. A region having the cation-ordered structure is further extended by 40 pm in length toward the region 2 not covered with the FeaOa thin film. This is a piece of direct evidence that cation ordering occurs in the presence of Fe2O3 and the ordered region can be rapidly expanded to the vicinity with a scale of several tens of pm. It should be noted that the cation ordering of CZ55 at a low temperature in the presence of FeaOa occurs only in the powder sample, and does not occur in a fired bulk sample. This suggests that a mechanical stress and a stress due to lattice mismatch around the interface in (b) in FIG. 11 may play an important role in the expansion of the cation ordering in the ceria-zirconia composite oxide. Fe is considered to have a role in promoting cation ordering of CZ55 without being formed as a solid solution in CZ55. Reference Signs List

[0068] 1: oxygen storage material 2: three-way catalyst 3: carrier

Claims

1. An oxygen storage material having a chemical composition represented by Cei-xZrxO2-s (0.45 f x 1 0.65, 0 b 5) , having a peak attributed to a cubic pyrochlore-like structure (k phase) near 14.5° in an XRD pattern, and having a specific surface area of 3 [m2 / g] or more.

2. The oxygen storage material according to claim 1, wherein the specific surface area is 3.5 [m2 / g] or more.

3. The oxygen storage material according to claim 2, wherein the specific surface area is 5 [m2 / g] or more.

4. The oxygen storage material according to any one of claims 1 to 3, which has a peak attributed to Fe in the XRD pattern .

5. A catalyst for purifying exhaust gas comprising: the oxygen storage material according to any one ofclaims 1 to 4.

6. A method for producing an oxygen storage material, which is a method for producing the oxygen storage material according to any one of claims 1 to 4, comprising:a composite preparation stage of preparing a composite in which a Fe compound is added to an oxide having a chemical composition represented by Cei-xZrxO2-s (0.45 d x t 0.65, 0 d 5); anda reduction heat treatment stage of subjecting the composite to a reduction heat treatment at a temperature of 700°C or higher and 850°C or lower.

7. The method for producing an oxygen storage material according to claim 6, wherein in the composite preparation stage, the Fe compound is added in an amount of 1 vol% or more and 10 vol% or less with respect to the oxide having the chemical composition represented by Cei^xZrxO2^6 (0.45 x <0.65, 0 <6).INTERNATIONAL SEARCH REPORT International application No. PCT / JP2022 / 035I81 A. CLASSIFICATION OF SUBJECT MATTER B01J 35 / / 0(2006.0 l)i: B01J 2J / «3(2006.01)i; B01J 37 / 15(2006.01)i FI: B01J35 / 10 301J; B01J23 / 83 A; B01J37 / 18 According to International Patent Classification (IPC) or to both national classification ar id IPC B. FIELDS SEARCHEDMinimum documentation searched (classification system followed by classification symbols)B01J35 / 10; B01J23 / 83; B01J37 / 18Documentation searched other than minimum documentation to the extent that such documents are included in the fields searchedPublished examined utility model applications of Japan 1922-1996Published unexamined utility model applications of Japan 1971-2022Registered utility model specifications of Japan 1996-2022Published registered utility model applications of Japan 1994-2022Electronic data base consulted during the international search (name of data base and, where practicable, search terms used)JSTPlus / JMEDPlus / JST7580 (JDreamlll); JSTChina (JDreamlll)DOCUMENTS CONSIDERED TO BE RELEVANTCategory* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. X DING. Y et. al., A novel method for the synthesis of CexZrl-xO2 solid solution with high purity of Kappa phase and excellent reactive activity, Catalysis Today, 2019, vol. 327, pp. 262-270, doi: 10.1016 / j.cattod.2018.04.040, abstract, 1., 2.1., 3.1., 3.

2. 1,5 Y 4,6-7 A 2-3 X JP 2005-170774 A (TOSOH CORP.) 30 June 2005 (2005-06-30) claims 1-3, 6, paragraphs [0001], [0026], [0033] 1-3,5 Y 4,6-7 Y GU, Zhenhua et. al., Structure and catalytic property of CeO2-ZrO2-Fe2O3 mixed oxide catalysts for diesel soot combustion: Effect of preparation method, Journal of Rare Earths, 2014, vol. 32 no. 9, pp. 817-823 4,6-7 abstract, pp. 817, 1.1.3, 1.1.4 Y JP 2015-080736 A (TOYOTA MOTOR CORP.) 27 April 2015 (2015-04-27) claims 1, 3, 6, paragraph [0020] 4,6-7| | Further documents are listed in the continuation of Box C. | / | See patent family annex.* Special categories of cited documents:“A” document defining the general state of the art which is not considered“T” later document published after the international filing date or priority“O”“P”to be of particular relevanceearlier application or patent but published on or after the international filing datedocument which may throw doubts on priority claim(s) or which is cited to establish the publication date of another citation or other special reason (as specified)document referring to an oral disclosure, use, exhibition or other meansdocument published prior to the international filing date but later than the priority date claimed‘Y’date and not in conflict with the application but cited to understand the principle or theory underlying the inventiondocument of particular relevance; the claimed invention cannot be considered novel or cannot be considered to involve an inventive step when the document is taken alonedocument of particular- relevance; the claimed invention cannot be considered to involve an inventive step when the document is combined with one or more other such documents, such combination being obvious to a person skilled in the artdocument member of the same patent familyDate of the actual completion of the international searchDate of mailing of the international search report28 October 202208 November 2022Name and mailing address of the ISA / JPJapan Patent Office (ISA / JP)3-4-3 Kasumigaseki, Chiyoda-ku, Tokyo 100-8915JapanAuthorized officerTelephone No.INTERNATIONAL SEARCH REPORT International application No. PCT / JP2022 / 035I81C. DOCUMENTS CONSIDERED TO BE RELEVANT Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. A ANEGGI, Eleonora et. al., Promotional effect of rare earths and transition metals in the combustion of diesel soot over CeO2 and CeO2-ZrO2, Catalysis Today, 30 April 2006, vol. 114, no. 1, pp. 40-47, doi: 10.1016 / j.cattod.2006.02.008 entire text 1-7 A JP 2013-241328 A (TOYOTA CENTRAL R&D LABS., INC.) 05 December 2013 (2013-12-05) entire text 1-7 A JP 2005-231951 A (TOSOH CORP.) 02 September 2005 (2005-09-02) entire text 1-7INTERNATIONAL SEARCH REPORT Information on patent family membersInternational application No.PCT / JP2022 / 035I81Patent document cited in search report Publication date (day / month / year) Patent family member)s) Publication date (day / month / year) JP 2005-170774 A 30 June 2005 (Family: none) JP 2015-080736 A 27 April 2015 (Family: none) JP 2013-241328 A 05 December 2013 US 2015 / 0080211 Al entire text WO 2013 / 162029 Al EP 2805919 Al CN 104220379 A JP 2005-231951 A 02 September 2005 (Family: none)

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