JP6287687B2 - Exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a composite oxide containing cerium (Ce), lanthanum (La), and zirconium (Zr), and an exhaust gas purifying catalyst using the composite oxide.

Exhaust gas discharged from internal combustion engines such as automobiles contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO _x ). After being purified by the catalyst for use, it is released into the atmosphere. Conventionally, a three-way catalyst that simultaneously performs oxidation of CO and HC and reduction of NO _x is used as the exhaust gas purification catalyst. As the three-way catalyst, alumina (Al ₂ O ₃ ), silica (SiO ₂ ) ₂ ), porous oxide carriers such as zirconia (ZrO ₂ ), titania (TiO ₂ ), etc., on which a noble metal such as platinum (Pt), palladium (Pd), rhodium (Rh) is supported are widely used. .

  In order to efficiently purify the harmful components in the exhaust gas using such a three-way catalyst, the air-fuel ratio (A / F), which is the ratio of air to fuel, of the air-fuel mixture supplied to the internal combustion engine is set. Must be near stoichiometric air-fuel ratio (stoichiometric). However, the actual air-fuel ratio becomes rich (excess fuel: A / F <14.7) or lean (excess oxygen: A / F> 14.7), mainly due to stoichiometry, depending on the driving conditions of the automobile, Correspondingly, the exhaust gas becomes rich or lean.

  In recent years, an OSC material, which is an inorganic material having an oxygen storage capacity (OSC), has been used as a carrier for an exhaust gas purification catalyst in order to enhance the exhaust gas purification performance of a three-way catalyst against fluctuations in oxygen concentration in the exhaust gas. It is used as. Cerium oxide is known to have an excellent oxygen storage capacity and has been widely used as an OSC material.

  As an oxide of cerium used as an OSC material for an exhaust gas purification catalyst, Patent Document 1 discloses a mixture or coprecipitate of a cerium (III) compound and a zirconium (IV) compound in an inert or non-oxidizing atmosphere. Ceria for exhaust gas purification co-catalyst characterized by heating and pyrolysis to form a cerium (III) -zirconium (IV) composite oxide and then heating it in an oxidizing atmosphere A method for producing a zirconia composite oxide is described.

Patent Document 2 discloses a composite oxide containing Ce and Zr, and when the valence of Ce is trivalent, the following composition formula: (Ce _1-xy , RE _x , AE _y ) ₂ (Zr _1-y , M _y ) ₂ O ₇ (RE is at least one rare earth ion other than Ce, AE is at least one of Ca, Sr, and Ba, M is at least one of Nb and Ta, 0 ≦ A composite oxide having a pyrochlore structure represented by x ≦ 0.9, 0 ≦ y ≦ 0.9, 0.4 ≦ x + y ≦ 0.9) is described. Patent Document 2 describes that the oxygen storage capacity per 1 mol of Ce of the composite oxide was excellent.

  In recent years, regarding exhaust gas purification catalysts for automobiles, it has been required to reduce the amount of noble metal used in the catalyst and the pressure loss of the catalyst in order to save resources, reduce costs, improve engine performance, and the like. However, if the amount of noble metal used is reduced, the oxygen storage capacity of the catalyst is lowered, so that it is desired to further improve the performance of the OSC material. In addition, various technologies have been developed in order to reduce the pressure loss of the catalyst that adversely affects the engine performance, and one example is to reduce the catalyst size. By reducing the catalyst size, it is possible to reduce the amount of OSC material used, leading to a reduction in the weight of the automobile, and thus the cost can be reduced. Therefore, it is required to increase the oxygen storage capacity per unit weight of the OSC material used as a carrier for the exhaust gas purification catalyst.

Japanese Patent Laid-Open No. 11-165067 JP 2005-231951 A

  As described above, the composite oxide used as the OSC material for the exhaust gas purification catalyst carrier is required to reduce the amount of precious metal used in the catalyst and reduce the catalyst pressure loss in order to save resources, reduce costs and improve engine performance. However, conventional exhaust gas purifying catalysts have room for improvement in oxygen storage capacity per unit weight of catalyst support.

  Accordingly, an object of the present invention is to provide a composite oxide containing Ce having an improved oxygen storage capacity per unit weight.

  As a result of various studies on means for solving the above problems, the present inventors have identified the composition ratio of Ce of the composite oxide having a pyrochlore structure containing Ce, La, and Zr. It has been found that the oxygen storage capacity per unit weight can be improved, and the present invention has been completed.

That is, the gist of the present invention is as follows.
(1) A composite oxide containing Ce, La and Zr and having a pyrochlore structure represented by the formula A ₂ B ₂ O ₇ , wherein the A site consists of Ce and La, the B site contains Zr, and A A composite oxide having a Ce composition ratio of 0.6 to 0.8.
(2) The composite oxide according to (1), wherein the composition ratio of Ce at the A site is 0.65 to 0.75 and the composition ratio of La is 1.25 to 1.35.
(3) An exhaust gas purifying catalyst in which a noble metal is supported on the composite oxide of (1) or (2).

  According to the present invention, it is possible to provide a composite oxide containing Ce with improved oxygen storage capacity per unit weight.

FIG. 1 is a diagram showing I values of initial and durable composite oxides of Examples 1 and 2 and Comparative Example 1-4. FIG. 2 is a diagram showing the OSC per unit weight of the composite oxides of Examples 1 and 2 and Comparative Example 1-3. FIG. 3 is a graph showing the relationship between the I value after durability of the composite oxides of Examples 1 and 2 and Comparative Example 1-3 and the Ce utilization rate. FIG. 4 is a graph showing the relationship between the Ce amount of the composite oxides of Examples 1 and 2 and Comparative Example 1-3 and the I value after durability. FIG. 5 is a graph showing the relationship between the Ce content of the composite oxides of Examples 1 and 2 and Comparative Example 1-3 and the OSC theoretical value. FIG. 6 is a graph showing the relationship between the amount of Ce of the composite oxide obtained from the general solution and the OSC per unit weight after durability in the examples.

  Hereinafter, preferred embodiments of the present invention will be described in detail.

The present invention relates to a composite oxide having a pyrochlore structure represented by the formula A ₂ B ₂ O ₇ containing cerium (Ce), lanthanum (La), and zirconium (Zr). In the composite oxide of the present invention, the A site of the formula A ₂ B ₂ O ₇ is composed of Ce and La and does not contain other elements. In the composite oxide of the present invention, the B site of the formula A ₂ B ₂ O ₇ contains Zr.

In the composite oxide of the present invention, having a pyrochlore structure means a crystal phase (pyrochlore phase) having a structure represented by the formula A ₂ B ₂ O ₇ and having a pyrochlore type ordered arrangement structure of Ce, La and Zr. ) Is configured. The pyrochlore phase has an oxygen defect site, and oxygen atoms enter the site, whereby the pyrochlore phase is expressed by the formula A ₂ B ₂ O _8-α (α is the atomic composition of La / 2). Phase change. On the other hand, the κ phase can change into a pyrochlore phase by releasing oxygen atoms. The oxygen storage capacity of the composite oxide containing Ce, La and Zr of the present invention is due to the absorption and release of oxygen by mutually changing the phase between the pyrochlore phase and the κ phase. When the composite oxide of the pyrochlore phase is used as the OSC material of the exhaust gas purification catalyst, it changes to the pyrochlore phase when rich and changes to the κ phase when lean.

  The crystal phase of the composite oxide of the present invention can be determined by X-ray diffraction (XRD) measurement. In the XRD pattern, diffraction lines near 2θ = 14 ° are diffraction lines belonging to the (111) plane of the regular phase (κ phase), and diffraction lines near 2θ = 29 ° belong to the (222) plane of the regular phase. Diffraction lines. In the present invention, the ratio of the intensity of the diffraction line belonging to the (111) plane to the diffraction line belonging to the (222) plane (the intensity of the (111) diffraction line / the intensity of the (222) diffraction line) is defined as I value. The value is used as an index indicating the durability (maintenance rate) of the ordered phase.

  In the composite oxide of the present invention, the composition ratio of Ce at the A site is 0.6 to 0.8, preferably 0.65 to 0.75. In the composite oxide of the present invention, the oxygen storage capacity per unit weight of the composite oxide is improved by setting the composition ratio of Ce at the A site to 0.6 to 0.8. Reduction in pressure loss is achieved, and furthermore, the amount of OSC material used can be reduced, which is excellent in terms of cost.

  In the composite oxide of the present invention, the composition ratio of La at the A site is 1.2 to 1.4, preferably 1.25 to 1.35, corresponding to the composition ratio of Ce. In the composite oxide of the present invention, when the A site contains La, durability of the ordered pyrochlore phase is improved, and Ce utilization is improved.

  In the composite oxide of the present invention, the composition ratio of the B site containing Zr is 2. The B site can also contain a metal element other than Zr. Examples of metal elements other than Zr include Ti, Fe, Y, Nb, and Hf.

The composite oxide of the present invention preferably has the formula Ce _x La _2-x Zr ₂ O ₇ (wherein x is 0.6 to 0.8, preferably 0.65 to 0.75). A composite oxide having a pyrochlore structure represented by

  By setting the composition ratio of Ce and La in the composite oxide of the present invention, the durability of the ordered pyrochlore phase is improved as the amount of substitution of La is increased, and the Ce utilization rate is improved. The balance between the effect of improving the storage capacity and the effect of decreasing the amount of Ce per unit weight and decreasing the oxygen storage capacity as the La substitution amount increases is optimized. As a result, the composite oxide of the present invention is superior in oxygen storage capacity per unit weight as compared with a composite oxide in which the composition ratio of Ce and La is not in this range.

Here, in the composite oxide of the present invention, the durability of the ordered pyrochlore phase is improved by increasing the substitution amount of La. (1) The trivalent La is substituted and dissolved, and κ Oxygen vacancies also occur in the phase, resulting in a difference in the number of oxygen coordination at the Ce and Zr sites. (2) Since trivalent La having a large ion size is substituted at the Ce site, (3) a stable phase A possible reason is that the composition is close to La ₂ Zr ₂ O ₇ .

  The composite oxide of the present invention can exhibit excellent oxygen storage capacity over a wide temperature range, but is preferably used in a low / medium temperature range of about 350 ° C. to 550 ° C.

  The composite oxide having a pyrochlore structure of the present invention can be produced by a usual method such as a solid phase method, a liquid phase method, or an alkoxide method. For example, in the composite oxide of the present invention, an aqueous solution of a cerium compound, a zirconium compound and a lanthanum compound and an aqueous solution of a complexing agent are mixed and dried to precipitate a product containing Ce, La and Zr. Manufactured by firing in a reducing atmosphere. A cerium compound, a zirconium compound, and a lanthanum compound can also be used as a solution of a non-aqueous solvent, for example, an alcohol or an organic carboxylic acid ester.

  Examples of the cerium compound include water-soluble compounds such as nitrates such as cerium nitrate and diammonium cerium nitrate, sulfates such as cerium sulfate, and chlorides such as cerium chloride. Further, as the zirconium compound, water-soluble compounds such as nitrates such as zirconium oxynitrate, sulfates such as zirconium sulfate, and chlorides such as zirconium oxychloride can be used. As the lanthanum compound, water-soluble compounds such as nitrates such as lanthanum nitrate, sulfates such as lanthanum sulfate, and chlorides such as lanthanum chloride can be used.

  The complexing agent is not particularly limited, and examples thereof include organic carboxylic acids, organic sulfonic acids, β-diketones, cyclopolyenes, alcohols and the like. Examples of the organic carboxylic acid include formic acid, acetic acid, propionic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, citric acid, tartaric acid, aminoacetic acid and the like. Is preferred. Examples of the organic sulfonic acid include benzenesulfonic acid and p-toluenesulfonic acid. Examples of β-diketone include acetylacetone. Examples of cyclopolyene include cyclooctatetraene and cyclopentadiene. Examples of the alcohol include alkyl alcohols such as butyl alcohol.

  When a precipitate is formed by mixing an aqueous solution of a cerium compound, a zirconium compound and a lanthanum compound and an aqueous solution of a complexing agent, the precipitate may be filtered and dried.

  Drying of a mixed solution of an aqueous solution of a cerium compound, a zirconium compound and a lanthanum compound and an aqueous solution of a complexing agent can be usually performed at 50 to 80 ° C. for 5 to 48 hours.

Calcination of the product containing Ce, La and Zr can be usually performed by heating and holding at 600 ° C. to 1500 ° C. for 2 hours to 10 hours in a reducing atmosphere. The reducing atmosphere can be an inert gas atmosphere or a non-oxidizing atmosphere, but is preferably an atmosphere containing a reducing gas such as H ₂ or CO.

  The composite oxide having a pyrochlore structure is a composite of κ phase in which the valence of Ce becomes tetravalent by holding at 500 ° C. to 1000 ° C. for 1 hour to 10 hours in an oxidizing atmosphere such as air or an oxygen atmosphere. It can be an oxide.

The composite oxide of the present invention can be used as an exhaust gas purifying catalyst for automobiles or the like by supporting a noble metal by itself as a carrier. Therefore, the present invention also relates to an exhaust gas purification catalyst in which a noble metal is supported on the composite oxide. The exhaust gas purifying catalyst of the present invention preferably contains a platinum group noble metal as a main catalyst. Examples of the platinum group noble metal include ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt), and it is particularly preferable to use Rh, Pt, and Pd. . The supported amount may be the same as that of the conventional exhaust gas purifying catalyst, but is preferably 0.01% by weight to 5% by weight with respect to the exhaust gas purifying catalyst. The exhaust gas purifying catalyst of the present invention may contain a support material other than the composite oxide of the present invention as a support. Examples of the carrier material other than the composite oxide of the present invention include metal oxides that are porous and have excellent heat resistance. For example, aluminum oxide (alumina: Al ₂ O ₃ ), zirconium oxide (zirconia: ZrO ₂ ), silicon oxide (silica: SiO ₂ ), or composite oxides mainly composed of these metal oxides can be used. In the exhaust gas purifying catalyst of the present invention, a conventional supporting method such as an adsorption supporting method or a water absorbing supporting method can be used as the supporting method.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the technical scope of the present invention is not limited to these examples.

Example 1
Preparation of Ce _0.7 La _1.3 Zr ₂ O _7.35 (κ phase) Diammonium cerium (IV) nitrate 7 mmol / pure water 5 ml, zirconium oxynitrate 20 mmol / pure water 10 ml and lanthanum nitrate 13 mmol / pure water 10 ml Each aqueous solution was prepared, and these aqueous solutions were mixed and stirred. An aqueous solution of citric acid 40 mmol / pure water 10 ml was prepared, and the mixed solution of the Ce salt, Zr salt and La salt was added to the prepared citric acid solution, and the mixture was stirred at 300 rpm for 30 minutes. While stirring, the solution was heated at about 60 ° C. to evaporate the water. After confirming that the viscosity of the solution became high and the rotational speed of the stirring bar was disturbed, the solution was transferred to a vacuum dryer and vacuum-dried at 60 ° C. overnight. The dried and precipitated product was pulverized and then transferred to a glass container and heated on a hot plate. Heating was started at 150 ° C., and the temperature was increased by 10 ° C. until the nitrate was decomposed (this sample decomposed at 190 ° C.). The obtained product containing Ce, Zr and La was baked in an electric furnace at 600 ° C. for 5 hours to obtain a composite oxide of Ce, Zr and La. The obtained composite oxide was transferred to a tubular channel, and 5% H ₂ / Ar balance gas was circulated at a flow rate of 1 L / min for 2 hours. Then, while flowing 5% H ₂ / Ar balance gas at a flow rate of 300 mL / min, the electric furnace was heated from room temperature to 1300 ° C. over 2 hours and 10 minutes and held at 1300 ° C. for 3 hours to sinter the composite oxide did. Thereafter, the heat source was turned off and naturally dissipated to obtain a composite oxide of a pyrochlore phase represented by the formula Ce _0.7 La _1.3 Zr ₂ O ₇ . A part of the pyrochlore phase composite oxide was subjected to XRD analysis, and the diffraction pattern of the initial pyrochlore phase composite oxide was measured.

The obtained pyrochlore phase composite oxide was baked in an electric furnace in the atmosphere at 550 ° C. for 2 hours to form a composite of κ phase represented by the formula Ce _0.7 La _1.3 Zr ₂ O _7.35. An oxide was obtained. A part of the κ-phase composite oxide was subjected to XRD analysis, and the diffraction pattern of the initial κ-phase composite oxide was measured. In addition, the composite oxide of κ phase was durable in air at 1000 ° C. for 5 hours, and XRD analysis and OSC measurement were performed on this durable product.

(Example 2)
Preparation of Ce _0.8 La _1.2 Zr ₂ O _7.4 (κ phase) The amount of metal salt used was changed from 7 mmol of diammonium cerium (IV) nitrate, 20 mmol of zirconium oxynitrate and 13 mmol of lanthanum nitrate to diammonium cerium nitrate. (IV) Pyrochlore phase complex oxide and formula represented by the formula Ce _0.8 La _1.2 Zr ₂ O ₇ in the same manner as in Example 1 except that 8 mmol, zirconium oxynitrate 20 mmol and lanthanum nitrate 12 mmol were used. A composite oxide of κ phase represented by Ce _0.8 La _1.2 Zr ₂ O _7.4 was obtained, each was subjected to XRD analysis, and the obtained composite oxide of κ phase was 1000 in the air. Durability was performed at 5 ° C. for 5 hours, and XRD analysis and OSC measurement were performed on the durable product.

(Comparative Example 1)
Preparation of Ce ₂ Zr ₂ O ₈ (κ phase) The amount of metal salt used was 7 mmol of diammonium cerium (IV) nitrate, 20 mmol of zirconium oxynitrate and 13 mmol of lanthanum nitrate, 20 mmol of diammonium cerium (IV) nitrate, zirconium oxynitrate A composite oxide of a pyrochlore phase represented by the formula Ce ₂ Zr ₂ O ₇ and a composite oxide of a κ phase represented by the formula Ce ₂ Zr ₂ O ₈ were obtained in the same manner as in Example 1 except that the amount was 20 mmol. Then, XRD analysis was performed, and the obtained composite oxide of κ phase was durable in air at 1000 ° C. for 5 hours, and XRD analysis and OSC measurement were performed on this durable product.

(Comparative Example 2)
Preparation of CeLaZr ₂ O _7.5 (κ phase) The amount of metal salt used was 7 mmol of diammonium cerium (IV) nitrate, 20 mmol of zirconium oxynitrate and 13 mmol of lanthanum nitrate, 10 mmol of diammonium cerium (IV) nitrate, zirconium oxynitrate A composite oxide of pyrochlore phase represented by the formula CeLaZr ₂ O ₇ and a composite oxide of κ phase represented by the formula CeLaZr ₂ O _7.5 are the same as in Example 1 except that 20 mmol and 10 mmol of lanthanum nitrate were used. Each was subjected to XRD analysis, and the obtained composite oxide of κ phase was durable in air at 1000 ° C. for 5 hours, and XRD analysis and OSC measurement were performed on this durable product.

(Comparative Example 3)
Preparation of Ce _0.5 La _1.5 Zr ₂ O _7.25 (κ phase) The amount of the metal salt was changed from 7 mmol of diammonium cerium (IV) nitrate, 20 mmol of zirconium oxynitrate and 13 mmol of lanthanum nitrate to diammonium cerium nitrate. (IV) Pyrochlore phase composite oxide and formula represented by the formula Ce _0.5 La _1.5 Zr ₂ O ₇ in the same manner as in Example 1 except that 5 mmol, zirconium oxynitrate 20 mmol and lanthanum nitrate 15 mmol were used. A composite oxide of κ phase represented by Ce _0.5 La _1.5 Zr ₂ O _7.25 was obtained, each was subjected to XRD analysis, and the obtained composite oxide of κ phase was 1000 in the air. Durability was performed at 5 ° C. for 5 hours, and XRD analysis and OSC measurement were performed on the durable product.

(Comparative Example 4)
Preparation of La ₂ Zr ₂ O ₇ (pyrochlore phase) The amount of metal salt used was changed from 7 mmol of diammonium cerium (IV) nitrate, 20 mmol of zirconium oxynitrate and 13 mmol of lanthanum nitrate to 20 mmol of zirconium oxynitrate and 20 mmol of lanthanum nitrate. In the same manner as in Example 1, a pyrochlore phase composite oxide represented by the formula La ₂ Zr ₂ O ₇ was obtained, and XRD analysis and OSC were performed on the initial product and the durable product that was durable in air at 1000 ° C. for 5 hours. Measurements were performed. The composite oxide of Comparative Example 4 did not have oxygen storage capacity, but was used as a comparative sample for durability of the pyrochlore phase.

The Ce, La, and Zr composition analyzes of the composite oxides of Examples 1 and 2 and Comparative Example 1-4 were performed by the following Na ₂ O ₂ melting-ICP analysis method.

[Ce, La, Zr composition analysis]
A 0.1 g sample was weighed and ₂ g of the melting agent Na ₂ O ₂ was added to the sample and placed in an electric furnace heated to 400 ° C. The electric furnace was heated and the sample was heated at 800 ° C. for 30 minutes to melt. The obtained melt was transferred to a beaker, 40 ml of HCl (1 + 1) was added, and the mixture was heated and dissolved on a heater. The volume was adjusted to 100 ml and ICP analysis was performed. ICP analysis was performed three times.

(Test Example 1)
The XRD analysis of the initial product and durable product (durability in air at 1000 ° C. for 5 hours) of the composite oxides of the κ phase of Examples 1 and 2 and Comparative Example 1-3 and the pyrochlore phase of Comparative Example 4 was performed. In the XRD pattern of the complex oxide (κ phase), the diffraction line belonging to the (111) plane near 2θ = 14 ° is one of the diffraction lines representing the double period of the regular phase. In the measured XRD pattern of each sample, the diffraction attributed to the (222) plane of the ordered phase having the intensity of the diffraction line near 2θ = 14 °, which is the diffraction line attributed to the (111) plane of the ordered phase (κ phase). A ratio ((111) diffraction line intensity / (222) diffraction line intensity) to the intensity of the diffraction line near 2θ = 29 °, which is a line, was calculated as an I value. The I value can be used as an index indicating the durability (maintenance rate) of the ordered phase. The results are shown in Table 1 and FIG.

  From Table 1 and FIG. 1, it was shown that the I value after durability of the composite oxides of Examples 1 and 2 was larger than that of Comparative Examples 1 and 2, but smaller than that of Comparative Examples 3 and 4. . Further, as the substitution amount of La increases, the post-endurance I value increases and the ordered phase is maintained, but it is shown that the La value is almost constant at La = 1.5 (Comparative Example 3) or more. It was.

  In addition, since the relationship between the amount of substitution of La and the d values of (111) and (222) of the XRD pattern is proportional, and the Vegard rule is established, La is substituted for the Ce site. It was confirmed.

(Test Example 2)
The oxygen storage capacity (OSC) was measured using the κ-phase composite oxides of Examples 1 and 2 and Comparative Example 1-3 as exhaust gas purifying catalysts. As a measurement sample, 1 g of the composite oxide of κ phase of Examples 1 and 2 and Comparative Example 1-3 and 1 wt% Pt / Al ₂ O ₃ (specific surface area (SSA) of Al ₂ O ₃ : 100 m) ² / g) ² g of 1 g physically mixed was used.

The measurement conditions are as follows:
Measurement gas flow rate: Carbon dioxide (CO) 2 volume% gas (balance gas is nitrogen) and oxygen (O ₂ ) 1 volume% gas (balance gas is nitrogen) at a rate of 10 L / min. The pulse was introduced twice.
Measurement temperature: 400 ° C, 500 ° C
OSC data: At the time of introducing the third CO gas pulse, the number of moles of O ₂ molecules occluded was calculated from the amount of CO ₂ produced for 2 minutes and used as OSC.

The OSC per gram of the composite oxide after durability is shown in Table 2 and FIG.

  From Table 2 and FIG. 2, the exhaust gas purifying catalyst using the composite oxides of Examples 1 and 2 is 2 minutes per gram of OSC material (composite oxide) at both 400 ° C. and 500 ° C. OSC was excellent.

Next, the relationship between the Ce content of the composite oxide represented by the formula Ce _x La _{2 -x} Zr ₂ O ₇ and the oxygen storage capacity per unit weight after durability was determined as follows.

The OSC per unit weight after durability of the composite oxide is represented by the following formula (I):
OSC per unit weight after endurance = Ce utilization factor x OSC theoretical value (I)
It is represented by

  First, the Ce utilization was determined as a relational expression for the I value after durability. The I value indicates the degree of regular phase maintenance rate.

  Regarding the Ce utilization ratio of the composite oxides of Examples 1 and 2 and Comparative Example 1-3, the theoretical value of OSC per gram of composite oxide of each sample (OSC when Ce was used for 100% OSC) was calculated. The ratio (actual value / theoretical value) of the OSC actual value (400 ° C., 500 ° C.) to the theoretical value was calculated. The results are shown in Table 3.

  The relationship between the I value after the durability of each sample and the Ce utilization rate is shown in FIG. FIG. 3 shows that the I value and the Ce utilization rate are in a proportional relationship, and that the Ce utilization rate increases when the pyrochlore structure is maintained. This is considered to be because oxygen defects are present in the pyrochlore phase, so that oxygen is readily occluded / released due to the oxidation / reduction of Ce, and the diffusion rate in the bulk is high.

From FIG. 3, the Ce utilization rate at 400 ° C. is represented by the following formula (II):
Ce utilization rate (y) = 9.3768 × I value (x) −0.0217 (II)
The Ce utilization at 500 ° C. is represented by the following formula (III):
Ce utilization rate (y) = 15.671 × I value (x) +0.0012 (III)
It is represented by

Next, the relationship between the I value after endurance and the Ce amount is shown in FIG. From FIG. 4, the I value after endurance is the following exponential function formula (IV) with Ce amount as a variable:
I value after endurance (y) = 0.0396exp (−1.162 × x) (IV)
Can be approximated by

FIG. 5 shows the relationship between the OSC theoretical value per unit weight of the composite oxide and the Ce amount. From FIG. 5, the OSC theoretical value is the following formula (V):
OSC theoretical value = 427.32 × x + 3.3947 (V)
It is represented by

The OSC per unit weight after endurance of the composite oxide of the formula (I) is calculated from the formulas (II) to (V):
OSC = [a (0.0396exp (−1.162 × x) + b] × [427.32 × x + 3.3947]
(A and b are coefficients in the respective equations for 400 ° C. and 500 ° C. shown in FIG. 3)
It is represented by FIG. 6 shows the relationship between the OSC per unit weight after endurance of the composite oxide calculated from this equation and the Ce amount at 400 ° C. and 500 ° C., respectively. From FIG. 6, the OSC per unit weight after endurance is excellent at a composition ratio of Ce = 0.6 to 0.8. Particularly, at 400 ° C., the OSC per unit weight is maximum at this composition ratio. It was shown that it became.

  As described above, the composite oxides of Examples 1 and 2 are inferior in the durability of the ordered phase as compared with those of Comparative Examples 3 and 4, but the amount of Ce in the composite oxide is large. In addition, the composite oxides of Examples 1 and 2 have less Ce content in the composite oxide than those of Comparative Examples 1 and 2, but the ordered phase has high durability and Ce utilization is high. .

  In the composite oxides of Examples 1 and 2, by replacing part of the Ce site with La, durability of the ordered pyrochlore phase is improved, Ce utilization is improved, and oxygen storage capacity is improved. And by replacing with La, the balance between the effect of reducing the amount of Ce per unit weight and reducing the oxygen storage capacity is optimized. It is considered that the OSC per unit weight became the maximum.

  By using the exhaust gas purifying catalyst of the present invention, it is possible to provide an exhaust gas purifying catalyst having improved OSC performance per unit weight.

Claims (3)

A composite oxide containing a pyrochlore structure represented by the formula A ₂ B ₂ O ₇ , comprising Ce, La and Zr, wherein the A site comprises Ce and La, the B site comprises Zr, and the Ce of the A site A composite oxide having a composition ratio of 0.6 to 0.8.   The composite oxide according to claim 1, wherein the composition ratio of Ce at the A site is 0.65 to 0.75, and the composition ratio of La is 1.25 to 1.35.   An exhaust gas purifying catalyst comprising a noble metal supported on the composite oxide according to claim 1 or 2.

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