OXYGEN STORAGE MATERIAL, CATALYST FOR PURIFYING EXHAUST GAS, AND METHODS FOR MANUFACTURING OXYGEN STORAGE MATERIAL

Abstract

This oxygen storage material has a chemical composition represented by Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0), has a peak attributed to a cubic pyrochlore-like structure ( phase) near 14.5 in an XRD pattern, and has a specific surface area of 3 [m.sup.2/g] or more.

Claims

1. An oxygen storage material having a chemical composition represented by Ce1-xZrxO2- (0.45x0.65, 0), having a peak attributed to a cubic pyrochlore-like structure ( 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 claim 1, 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 claim 1.

6. A method for producing an oxygen storage material, which is a method for producing the oxygen storage material according to claim 1, 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 Ce1-xZrxO2- (0.45x0.65, 0); 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.

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 Ce1-xZrxO2- (0.45x0.65, 0).

8. The oxygen storage material according to claim 2, which has a peak attributed to Fe in the XRD pattern.

9. The oxygen storage material according to claim 3, which has a peak attributed to Fe in the XRD pattern.

10. A catalyst for purifying exhaust gas comprising: the oxygen storage material according to claim 2.

11. A catalyst for purifying exhaust gas comprising: the oxygen storage material according to claim 3.

12. A catalyst for purifying exhaust gas comprising: the oxygen storage material according to claim 4.

13. A method for producing an oxygen storage material, which is a method for producing the oxygen storage material according to claim 2, 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 Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0); 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.

14. A method for producing an oxygen storage material, which is a method for producing the oxygen storage material according to claim 3, 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 Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0); 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.

15. A method for producing an oxygen storage material, which is a method for producing the oxygen storage material according to claim 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 Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0); 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.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0038] FIG. 1 is a diagram schematically showing a phase synthesis process.

[0039] 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.

[0040] FIG. 3 shows XRD patterns of a calcined powder of Ce.sub.0.5Zr.sub.0.5O.sub.2 prepared by a Pechini method and Ce.sub.0.5Zr.sub.0.5O.sub.2 after addition of Fe.sub.3O.sub.4, -Fe.sub.2O.sub.3, and -Fe.sub.2O.sub.3.

[0041] (a) in FIG. 4 shows XRD patterns after reduction of additive-free Ce.sub.0.5Zr.sub.0.5O.sub.2 and samples added with Fe.sub.3O.sub.4, -Fe.sub.2O.sub.3, and -Fe.sub.2O.sub.3, and (b) in FIG. 4 is an enlarged diagram near 2=14.5.

[0042] FIG. 5 shows Raman spectra obtained by reducing additive-free Ce.sub.0.5Zr.sub.0.5O.sub.2 and a sample added with -Fe.sub.2O.sub.3 or oxidizing additive-free Ce.sub.0.5Zr.sub.0.5O.sub.2 and the sample after reduction.

[0043] (a) in FIG. 6 is a graph showing reduction time dependence of an XRD pattern of Ce.sub.0.5Zr.sub.0.5O.sub.2-5 vol % -Fe.sub.2O.sub.3, and (b) in FIG. 6 is an enlarged diagram near 2=14.5.

[0044] (a) in FIG. 7 is a graph showing reduction time dependence of an XRD pattern of Ce.sub.0.5Zr.sub.0.5O.sub.2-5 vol % Fe.sub.3O.sub.4, and (b) in FIG. 7 is an enlarged diagram near 2=14.5.

[0045] FIG. 8 is an XRD pattern of a sample added with -Fe.sub.2O.sub.3 and Ce.sub.0.5Zr.sub.0.5O.sub.2 prepared by a solid-state reaction method.

[0046] FIG. 9 shows an XRD pattern obtained after a reduction heat treatment in an atmosphere of 5% H.sub.2Ar at 800 C. for 3 hours.

[0047] FIG. 10 is a graph comparing specific surface areas of a t phase prepared by the Pechini method and a phase prepared by adding 5 vol % -Fe.sub.2O.sub.3 to the t phase.

[0048] 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.

[0049] (a) in FIG. 12 is a photograph on which a model interface is formed by depositing a Fe.sub.2O.sub.3 thin film on a t phase CZ55 powder layer having a thickness of 3 m by PLD, and a mapping image obtained by performing line scanning on a boundary between a region (region 1) where the Fe.sub.2O.sub.3 thin film is deposited and a region (region 2) where the Fe.sub.2O.sub.3 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

[0050] 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.

(Oxygen Storage Material)

[0051] 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.

[0052] An oxygen storage material according to the present embodiment has a chemical composition represented by Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0), has a peak attributed to a cubic pyrochlore-like structure ( phase) near 14.5 in an XRD pattern, and has a specific surface area of 3 [m.sup.2/g] or more.

<Chemical Composition>

[0053] The oxygen storage material according to the present embodiment has the chemical composition represented by Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0). The oxygen storage material according to the present embodiment preferably has a chemical composition represented by Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.60, 0), and more preferably has a chemical composition represented by Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.55, 0).

[0054] As shown in Examples, the chemical composition is represented by Ce.sub.0.5Zr.sub.0.5O.sub.2, but it is known that a phase is obtained when x is in the range of 0.45x0.65 (see NPL 2).

[0055] In a simple substance of CeO.sub.2, oxygen on a crystal surface mainly contributes to OSC characteristics, and in CeO.sub.2ZrO.sub.2, oxygen inside a crystal also contributes to an OSC. Therefore, a CeO.sub.2ZrO.sub.2-based oxide has a high OSC. In Ce.sub.1-xZr.sub.xO.sub.2, a highest OSC characteristic is obtained near x=0.5. When oxygen in Ce.sub.0.5Zr.sub.0.5O.sub.2 is released, Ce.sub.0.5Zr.sub.0.5O.sub.2- is obtained. When a valence of Zr ions does not change, the largest 5 is 0.25. Ce.sub.1-xZr.sub.xO.sub.2 has a plurality of phases such as a t phase and a phase. As the oxygen storage material, the t phase or the phase is mainly used. The t phase is a tetragonal phase. The phase is a phase having a pyrochlore-like structure.

[0056] In addition, it is sufficient that 0. Theoretically, in the case of Ce.sub.0.5Zr.sub.0.5O.sub.2-, 00.25, and when x is close to 0, 00.5.

<XRD Pattern>

[0057] The oxygen storage material according to the present embodiment has a peak attributed to a cubic pyrochlore-like structure ( phase) near 14.5 in the XRD pattern.

[0058] In the present description, having a peak attributed to a cubic pyrochlore-like structure ( 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 ( phase). When an intensity of the peak attributed to the phase is weak, the peak can be checked by expanding a range of 2 including 14.5. As will be described later, since formation of the phase can also be checked by Raman spectroscopy, the presence of the peak attributed to the phase can be determined by using Raman spectroscopy in combination.

[0059] In addition, in the present description, near 14.5 means that it is sufficient to specify the peak attributed to the pyrochlore-like structure ( 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.

[0060] The oxygen storage material according to the present embodiment may have a peak attributed to Fe in the XRD pattern.

[0061] 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.

<Specific Surface Area>

[0062] The oxygen storage material according to the present embodiment has a specific surface area of 3 [m.sup.2/g] or more. The specific surface area is preferably 3.5 [m.sup.2/g] or more, more preferably 4 [m.sup.2/g] or more, and still more preferably 5 [m.sup.2/g] or more.

[0063] 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.)).

[0064] 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 phase, whereas the phase can be obtained even at 800 C. according to the invention. In the phase of the invention, since the 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.

[0065] The oxygen storage material according to the present embodiment can be used together with a three-way catalyst as a main catalyst.

[0066] The oxygen storage material according to the present embodiment can be used by being fixed to a carrier. Examples of the carrier include alumina (Al.sub.2O.sub.3), zirconia (ZrO.sub.2), magnesia (MgO), and silica (SiO.sub.2).

(Method for Producing Oxygen Storage Material)

[0067] 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 Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0), 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.

[0068] A method for preparing an oxide having a chemical composition represented by Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0) 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.

[0069] A method for preparing a composite in which a Fe compound is added to the oxide having a chemical composition represented by Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0) is also not particularly limited, and for example, it can be prepared by mixing in a planetary ball mill.

[0070] 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.

<Fe Compound>

[0071] Examples of the Fe compound to be added include Fe oxides such as Fe.sub.2O.sub.3 and Fe.sub.3O.sub.4, and Fe-containing oxides containing metals other than Fe such as CoFe.sub.2O.sub.4.

[0072] 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 Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0).

[0073] 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 Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0).

[0074] 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 Ce.sub.1-xZr.sub.xO.sub.2- (0.45x0.65, 0).

[0075] 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.

(Catalyst for Purifying Exhaust Gas)

[0076] A catalyst for purifying exhaust gas according to the present embodiment contains the above-described oxygen storage material according to the invention.

[0077] 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.

[0078] 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.

[0079] 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 (Al.sub.2O.sub.3), zirconia (ZrO.sub.2), magnesia (MgO), and silica (SiO.sub.2).

EXAMPLES

1. Preparation of Sample Powder

(1) Preparation by Pechini Method

[0080] Hereinafter, a step of preparing a CeO.sub.2ZrO.sub.2 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 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.

TABLE-US-00001 TABLE 1 Element Composition Purity (%) Mixing mole ratio*.sup.1 Ce Ce(NO.sub.3).sub.36H.sub.2O 99.9 1:3:3 Zr ZrO(NO.sub.3).sub.22H.sub.2O 99 1:9:9 *.sup.1Mixing mole ratio = cation:citric acid:propylene glycol

(2) Preparation by Solid-State Reaction Method

[0081] Hereinafter, a step of preparing the CeO.sub.2ZrO.sub.2 sample using the solid-state reaction method will be described.

[0082] The CeO.sub.2ZrO.sub.2 sample was prepared by the general solid-state reaction method as the method for synthesizing ceramics, different from the Pechini method. In the solid-state 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 CeO.sub.2 (manufactured by Anan Kasei Co., Ltd.) and ZrO.sub.2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) were weighed so as to satisfy Ce.sub.0.5Zr.sub.0.5O.sub.2, 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.

(3) Preparation Ce.sub.0.5Zr.sub.0.5O.sub.2Fe Oxide Composite

[0083] A composite of a Fe oxide and Ce.sub.0.5Zr.sub.0.5O.sub.2 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, Fe.sub.3O.sub.4 (manufactured by FUJIFILM Wako Pure Chemical Corporation), -Fe.sub.2O.sub.3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and -Fe.sub.2O.sub.3 (manufactured by Kojundo Chemical Lab. Co., Ltd.) were used, and an addition amount thereof was about 5 vol % with respect to the amount of Ce.sub.0.5Zr.sub.0.5O.sub.2.

[0084] The crystal structures of -Fe.sub.2O.sub.3 and -Fe.sub.2O.sub.3 are different, -Fe.sub.2O.sub.3 is a corundum type, and -Fe.sub.2O.sub.3 is a spinel type.

2. Evaluation of Materials

(1) Phase Identification Method

[0085] 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. CuK 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 HeNe laser (=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 m, and a slit value was 100 m. In the measurement, a subtractive filter having an optical density of 0.3 to 1 was used.

[0086] 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.

(2) Crystal Structure Evaluation on Ce.sub.0.5Zr.sub.0.5O.sub.2Fe Oxide Composite

[0087] FIG. 3 shows XRD patterns of a calcined powder of Ce.sub.0.5Zr.sub.0.5O.sub.2 prepared by the Pechini method and Ce.sub.0.5Zr.sub.0.5O.sub.2 after addition of Fe.sub.3O.sub.4, -Fe.sub.2O.sub.3, and -Fe.sub.2O.sub.3. The four XRD patterns in FIG. 3 are, in order from the top, an XRD pattern of the calcined powder of Ce.sub.0.5Zr.sub.0.5O.sub.2, an XRD pattern of a sample after addition of Fe.sub.3O.sub.4, an XRD pattern of a sample after addition of -Fe.sub.2O.sub.3, and an XRD pattern of a sample after addition of -Fe.sub.2O.sub.3. Regarding Ce.sub.0.5Zr.sub.0.5O.sub.2, all of the observed peaks were attributed to a tetragonal fluorite type structure of Ce.sub.0.5Zr.sub.0.5O.sub.2 having a space group P4.sub.2/nmc. In each sample to which the Fe oxide was added, the same peak as that of the raw material was observed.

[0088] In the XRD pattern, a peak indicated by attributes to the tetragonal fluorite type structure, a peak indicated by .diamond-solid. is a peak attributed to Fe.sub.3O.sub.4, a peak indicated by .Math. is a peak attributed to -Fe.sub.2O.sub.3, and a peak indicated by .box-tangle-solidup. is a peak attributed to -Fe.sub.2O.sub.3.

(3) Crystal Structure Evaluation on Sample after Reduction Heat Treatment

[0089] (a) in FIG. 4 shows XRD patterns after reduction of additive-free Ce.sub.0.5Zr.sub.0.5O.sub.2 and samples added with Fe.sub.3O.sub.4, -Fe.sub.2O.sub.3, and -Fe.sub.2O.sub.3. (b) in FIG. 4 is an enlarged diagram near 2=14.5. The XRD pattern is obtained for a sample after a reduction heat treatment and before an oxidation treatment.

[0090] The pyrochlore type Ce.sub.2Zr.sub.2O.sub.7 obtained by the reduction heat treatment is subjected to an oxidation treatment at an appropriate temperature to introduce oxygen to obtain K phase Ce.sub.2Zr.sub.2O.sub.8. As described above, both the pyrochlore type Ce.sub.2Zr.sub.2O.sub.7 and the phase Ce.sub.2Zr.sub.2O.sub.8 have a structure in which cations are arranged in order.

[0091] Although the additive-free Ce.sub.0.5Zr.sub.0.5O.sub.2 has the tetragonal fluorite type structure, when the phase is changed to pyrochlore type Ce.sub.2Zr.sub.2O.sub.7 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.

[0092] A main difference between the crystal structure of the fluorite type Ce.sub.0.5Zr.sub.0.5O.sub.2 and the crystal structure of the pyrochlore type Ce.sub.2Zr.sub.2O.sub.7 (crystal structure of phase Ce.sub.2Zr.sub.2O.sub.8) 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 Ce.sub.2Zr.sub.2O.sub.7 from the fluorite type Ce.sub.0.5Zr.sub.0.5O.sub.2.

[0093] The reduction heat treatment was performed under a condition of an atmosphere of 5% H.sub.2Ar at 800 C. for 3 hours. In the case of the additive-free Ce.sub.0.5Zr.sub.0.5O.sub.2, a peak of the tetragonal fluorite type structure was observed even after the reduction. On the other hand, in the samples added with Fe.sub.2O.sub.3 and Fe.sub.3O.sub.4, a peak derived from the ordering of cations was observed near 14.5, and the phase change to the pyrochlore type Ce.sub.2Zr.sub.2O.sub.7 was confirmed. In particular, in the sample added with Fe.sub.2O.sub.3, as compared with Fe.sub.3O.sub.4, a peak near 14.5 is remarkably observed, and it is considered that pyrochlore type Ce.sub.2Zr.sub.2O.sub.7 having a higher degree of ordering is obtained.

[0094] In the XRD patterns in FIG. 4, a peak indicated by attributes to a tetragonal fluorite type structure of fluorite type Ce.sub.0.5Zr.sub.0.5O.sub.2, a peak indicated by is a peak attributed to pyrochlore type Ce.sub.2Zr.sub.2O.sub.7, and a peak indicated by is a peak attributed to Fe.

[0095] FIG. 5 shows a Raman spectrum of additive-free Ce.sub.0.5Zr.sub.0.5O.sub.2, a Raman spectrum when an -Fe.sub.2O.sub.3 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% H.sub.2Ar 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 -Fe.sub.2O.sub.3 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.sup.1, 440 cm.sup.1, and 600 cm.sup.1 after reduction. After oxidation, a plurality of peaks attributed to the phase at 400 cm.sup.1 to 600 cm.sup.1 can be observed in addition to the strong peaks near 280 cm.sup.1 and 440 cm.sup.1. Therefore, formation of the phase can be confirmed by using the X-ray diffraction and the Raman spectroscopy in combination.

(4) Influence 1 of Reduction Time on Formation of Pyrochlore Type Ce.sub.2Zr.sub.2O.sub.7

[0096] With (3), regarding the samples added with Fe.sub.2O.sub.3 and Fe.sub.3O.sub.4, an influence of a time required for the formation of pyrochlore type Ce.sub.2Zr.sub.2O.sub.7 was examined. The reduction heat treatment was performed under a condition of an atmosphere of 5% H.sub.2Ar 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 Ce.sub.0.5Zr.sub.0.5O.sub.2- vol % -Fe.sub.2O.sub.3. (b) in FIG. 6 is an enlarged diagram near 2=14.5. It was found that, due to reduction for 30 minutes, in Ce.sub.0.5Zr.sub.0.5O.sub.2- vol % -Fe.sub.2O.sub.3, the peak near 14.5 was observed and the phase changed to pyrochlore type Ce.sub.2Zr.sub.2O.sub.7.

[0097] In the XRD pattern of (a) in FIG. 6, a peak indicated by is the peak attributed to pyrochlore type Ce.sub.2Zr.sub.2O.sub.7, a peak indicated by x is a peak attributed to FeO, and a peak indicated by is a peak attributed to Fe.

(5) Influence 2 of Reduction Time on Formation of Phase

[0098] (a) in FIG. 7 shows reduction time dependence of an XRD pattern of Ce.sub.0.5Zr.sub.0.5O.sub.2- vol % Fe.sub.3O.sub.4. (b) in FIG. 7 is an enlarged diagram near 2=14.5. The reduction heat treatment was performed under a condition of an atmosphere of 5% H.sub.2Ar 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 Ce.sub.0.5Zr.sub.0.5O.sub.2- vol % Fe.sub.3O.sub.4, a reduction heat treatment for about 3 hours was required for the phase change to pyrochlore type Ce.sub.2Zr.sub.2O.sub.7.

[0099] In the XRD pattern of (a) in FIG. 7, a peak indicated by attributes to fluorite type Ce.sub.0.5Zr.sub.0.5O.sub.2, a peak indicated by is a peak attributed to pyrochlore type Ce.sub.2Zr.sub.2O.sub.7, and a peak indicated by is a peak attributed to Fe.

(6) Crystal Structure Evaluation on Sample after Reduction Heat Treatment of Ce.sub.0.5Zr.sub.0.5O.sub.2Fe Oxide Composite Using Ce.sub.0.5Zr.sub.0.5O.sub.2 Prepared by Solid-State Reaction Method

[0100] The CeO.sub.2ZrO.sub.2 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 Ce.sub.2Zr.sub.2O.sub.7 was confirmed by adding a Fe oxide.

[0101] FIG. 8 shows an XRD pattern of a sample added with -Fe.sub.2O.sub.3 and Ce.sub.0.5Zr.sub.0.5O.sub.2 prepared by the solid-state reaction method. All the observed peaks of Ce.sub.0.5Zr.sub.0.5O.sub.2 prepared by the solid-state reaction were attributed to the tetragonal fluorite type structure of Ce.sub.0.5Zr.sub.0.5O.sub.2 having a space group P4.sub.2/nmc, and peaks derived from -Fe.sub.2O.sub.3 were observed in the composite.

[0102] In the XRD pattern in FIG. 8, a peak indicated by Q attributes to fluorite type Ce.sub.0.5Zr.sub.0.5O.sub.2, and a peak indicated by T is a peak attributed to -Fe.sub.2O.sub.3.

[0103] FIG. 9 shows an XRD pattern of Ce.sub.0.5Zr.sub.0.5O.sub.2--Fe.sub.2O.sub.3 obtained after a reduction heat treatment in an atmosphere of 5% H.sub.2Ar 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 Ce.sub.2Zr.sub.2O.sub.7. Therefore, it was found that even in the case of Ce.sub.0.5Zr.sub.0.5O.sub.2 prepared by the solid-state reaction method, the low-temperature synthesis of the pyrochlore type Ce.sub.2Zr.sub.2O.sub.7 by the addition of a Fe oxide was able to be performed.

[0104] In the XRD pattern in FIG. 9, a peak indicated by is a peak attributed to pyrochlore type Ce.sub.2Zr.sub.2O.sub.7, and a peak indicated by is a peak attributed to Fe.

(7) Specific Surface Areas of t Phase Prepared by Pechini Method and Phase Prepared by Adding 5 vol % -Fe.sub.2O.sub.3 to t Phase

[0105] FIG. 10 is a graph comparing specific surface areas of the t phase prepared by the Pechini method and the phase prepared by adding 5 vol % -Fe.sub.2O.sub.3 to the t phase.

[0106] 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 phase was a phase subjected to a reduction heat treatment in an atmosphere of 5% H.sub.2Ar at 800 C. for 5 hours and a phase subjected to a reduction heat treatment in an atmosphere of 5% H.sub.2Ar at 800 C. for 1 hour. The phase was subjected to an oxidation treatment in the air at 800 C. for 2 hours.

[0107] The specific surface area of the t phase is 22.3 [m.sup.2/g], and the specific surface areas of the phase according to the invention are 3.8 [m.sup.2/g] and 5.2 [m.sup.2/g], respectively. The specific surface areas of the phase according to the invention were respectively about 11 times and about 15 times the specific surface area of the phase reported in NPL 1.

[0108] Average particle diameters of the t phase, the phase (subjected to the reduction heat treatment at 800 C. for 5 hours), and the 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.

[0109] 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, CeFeO.sub.3 is formed at an interface of a Fe oxide at 800 C., and Ce.sup.3+ 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 Ce.sub.0.5Zr.sub.0.5O.sub.2 (hereinafter, sometimes referred to as CZ55) or via CeFeO.sub.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 Fe.sub.2O.sub.3. 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.

[0110] In order to verify the mechanism shown in FIG. 11, as shown in (a) in FIG. 12, a Fe.sub.2O.sub.3 thin film was deposited by PLD on a t phase CZ55 powder layer having a thickness of 3 m 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 Fe.sub.2O.sub.3 thin film was deposited (Fe.sub.2O.sub.3; region 1) and a region in which the Fe.sub.2O.sub.3 thin film was not deposited (no Fe.sub.2O.sub.3; 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 Fe.sub.2O.sub.3 thin film, peaks caused by a cation-ordered structure were clearly observed near 280 cm.sup.1 and 440 cm.sup.1. A region having the cation-ordered structure is further extended by 40 m in length toward the region 2 not covered with the Fe.sub.2O.sub.3 thin film. This is a piece of direct evidence that cation ordering occurs in the presence of Fe.sub.2O.sub.3 and the ordered region can be rapidly expanded to the vicinity with a scale of several tens of m. It should be noted that the cation ordering of CZ55 at a low temperature in the presence of Fe.sub.2O.sub.3 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

[0111] 1: oxygen storage material [0112] 2: three-way catalyst [0113] 3: carrier