JP2009160556A - Catalyst for purifying exhaust gas, and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for purifying exhaust gas capable of preventing a catalytic activity reduction caused by palladium particle growth, and maintaining excellent catalytic activity for a long time under high temperature or oxidation reduction fluctuation, or further in case of long period usage. <P>SOLUTION: The catalyst for purifying exhaust gas is prepared by mixing a first heat resistant oxide having Pd dissolved therein, and containing cerium, zirconium, and a rare earth element (except for cerium) with a second heat resistant oxide having Pd supported thereon, and containing cerium, zirconium, and a rare earth element (except for cerium) by a weight mixing ratio of 1/9-9/1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst and a method for producing the same.

Exhaust gas discharged from internal combustion engines such as automobiles contains hydrocarbons (HC), nitrogen oxides (NO _x ), carbon monoxide (CO), etc., and exhaust gas purification to purify them. Catalysts for use are known.
As such an exhaust gas purification catalyst, a noble metal element as an active component is supported or dissolved in a composite oxide such as a cerium-based composite oxide, a zirconium-based composite oxide, or a perovskite-based composite oxide There are various known.

For example, cerium-based composite oxide particles in which palladium is supported on a cerium-based composite oxide represented by the general formula: Ce _0.5 Zr _0.375 Y _0.125 O _xide , activated alumina, barium sulfate, and alumina sol are mixed with a ball mill. An exhaust gas purifying catalyst produced by attaching a slurry obtained by pulverization to a monolith carrier, drying and firing is known (see Patent Document 1).
JP-A-11-207183

However, in the exhaust gas purifying catalyst described in Patent Document 1, the palladium particles move on the surface of the composite oxide and coalesce at high temperatures or under oxidation-reduction fluctuations, and also during long-term use. There is a disadvantage that growth occurs and the effective surface area of the palladium particles decreases, resulting in a decrease in catalytic activity.
An object of the present invention is to provide an exhaust gas purifying catalyst capable of preventing a decrease in catalytic activity due to palladium grain growth and achieving excellent catalytic activity over a long period of time under high temperature or oxidation-reduction fluctuations, and also during long-term use. And providing a manufacturing method thereof.

In order to achieve the above-mentioned object, the exhaust gas purifying catalyst of the present invention comprises a first heat-resistant oxide containing Pd in solid solution, containing cerium, zirconium and a rare earth element (excluding cerium), and Pd supported thereon, And a second heat-resistant oxide containing cerium, zirconium and rare earth elements (excluding cerium).
In the exhaust gas purifying catalyst of the present invention, it is preferable that the atomic ratio of cerium is 0.2 to 0.8 in the first heat-resistant oxide.

In the exhaust gas purifying catalyst of the present invention, it is preferable that the second heat-resistant oxide has a cerium atomic ratio of 0.1 to 0.6.
In the exhaust gas purifying catalyst of the present invention, the atomic ratio of cerium is 0.2 to 0.8 in the first heat-resistant oxide, and the atomic ratio of cerium is 0 in the second heat-resistant oxide. .1 to 0.6 is preferred.

In the exhaust gas purifying catalyst of the present invention, it is preferable that an alkaline earth metal is supported on the first heat-resistant oxide.
Furthermore, the method for producing an exhaust gas purifying catalyst of the present invention includes a step of producing a first heat-resistant oxide containing Pd and containing cerium, zirconium and a rare earth element (excluding cerium), and Pd. A step of producing a second heat-resistant oxide containing cerium, zirconium and a rare earth element (excluding cerium), and the first heat-resistant oxide and the second heat-resistant oxide, And the step of producing the first heat-resistant oxide comprises mixing a cerium salt, a zirconium salt and a rare earth element salt to prepare a mixed salt aqueous solution. A step of obtaining a coprecipitate by adding a neutralizing agent to the mixed salt aqueous solution, a step of obtaining a precursor oxide by subjecting the coprecipitate to primary firing at 400 to 1000 ° C., and the precursor oxide And Pd salt are mixed, 300 to 800 The first heat-resistant oxide is obtained by mixing the second firing step with the precursor oxide and the alkaline earth metal salt, followed by final firing at a temperature exceeding 600 ° C. And the step of producing the second heat-resistant oxide comprises mixing a cerium salt, a zirconium salt and a rare earth element salt to prepare a mixed salt aqueous solution, and neutralizing the mixed salt aqueous solution. A step of obtaining a coprecipitate by adding an agent, a step of obtaining a precursor oxide by subjecting the coprecipitate to primary baking at 400 to 1000 ° C., a mixture of the precursor oxide and a Pd salt, And a step of obtaining the second heat-resistant oxide by final baking at 600 ° C. or lower.

According to the method for producing an exhaust gas purifying catalyst of the present invention, Pd is dissolved, and the first heat-resistant oxide containing cerium, zirconium and rare earth elements (excluding cerium) and Pd are supported, and cerium, zirconium In addition, the exhaust gas-purifying catalyst of the present invention can be produced by mixing the second heat-resistant oxide containing rare earth elements (excluding cerium).
According to the exhaust gas purifying catalyst of the present invention, it is possible to prevent a decrease in catalytic activity due to palladium grain growth over a long period of time, and to maintain a high catalytic activity.

  As a result, if the exhaust gas purification catalyst of the present invention is used, excellent exhaust gas purification performance can be exhibited over a long period of time under high temperature or oxidation-reduction fluctuations.

  In the exhaust gas purifying catalyst of the present invention, Pd (palladium) is dissolved, and a first heat-resistant oxide containing cerium, zirconium, and a rare earth element (excluding cerium), Pd (palladium) is supported, cerium, A second heat-resistant oxide containing zirconium and a rare earth element (excluding cerium). For example, one particle (first particle) made of the first heat-resistant oxide and the second heat-resistant oxidation It is formed as an aggregate of particles including the other particle (second particle) made of a substance.

  In the first heat-resistant oxide, examples of rare earth elements (excluding cerium) include Sc (scandium), Y (yttrium), La (lanthanum), Pr (praseodymium), Nd (neodymium), and Pm (promethium). , Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), Lu (lutetium) Etc. Preferred rare earth elements include Y, La, Pr, and Nd. These rare earth elements may be used alone or in combination of two or more.

In the first heat-resistant oxide, the atomic ratio of cerium is preferably 0.2 to 0.8, and more preferably 0.3 to 0.7. If the atomic ratio of cerium is in the above range, the gas purification performance of the exhaust gas purification catalyst can be further improved.
In the exhaust gas purifying catalyst of the present invention, it is preferable that an alkaline earth metal is supported on the first heat-resistant oxide.

  Examples of the alkaline earth metal include Be (beryllium), Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium), Ra (radium), and the like. As the alkaline earth metal, Ba, Sr, Ca and Mg are preferable, and Ba is more preferable. These alkaline earth metals may be used alone or in combination of two or more.

  In the first heat-resistant oxide, the alkaline earth metal preferably has a ratio of the atomic ratio of the alkaline earth metal to the sum of the atomic ratios of cerium, zirconium and rare earth elements of 2:95 to 30:70. Is more preferably supported in an amount of 5:95 to 20:80. If the alkaline earth metal is supported in the above range, the Pd grain growth can be effectively suppressed.

The alkaline earth metal may be supported on the first refractory oxide as a salt of an alkaline earth metal, and the constituent elements of the first refractory oxide, that is, cerium, zirconium and / or rare earth elements, Furthermore, Pd which will be described later may be formed as a composite oxide, and the composite oxide may be supported on the first heat-resistant oxide.
Pd is dissolved in the first heat-resistant oxide. The fact that Pd is dissolved in the first heat-resistant oxide means that Pd is coordinated in the crystal lattice of the first heat-resistant oxide, so that the first heat-resistant oxide and Pd form a solid solution. It is that. Note that all of Pd may not form a solid solution with the first heat-resistant oxide. For example, a part of Pd may be supported on the first heat-resistant oxide.

For example, when the first heat-resistant oxide is represented by the general formula: (Ce, Zr, R) O ₃ (R is the above-mentioned rare earth element), when Pd is dissolved, the first heat-resistant oxide is , Represented by the general formula: (Ce, Zr, R, Pd) O ₃ (R is the above-mentioned rare earth element). In addition, for example, when the first heat-resistant oxide carries the composite oxide containing the alkaline earth metal described above, the composite oxide containing the alkaline earth metal has the general formula: Ba (Ce, Pd ) O ₃ , Ba (Ce, Zr, Pd) O ₃ , Ba (Zr, Y, Pd) O ₃ , Ba (Ce, Zr, Y, Pd) O _{3 and the} like.

The solid solution ratio of Pd with respect to the composite oxide containing the first heat-resistant oxide and the alkaline earth metal is, for example, 20 to 100%, and preferably 60 to 100.
In addition, the solid solution rate (unit:%) of Pd can be measured by, for example, ICP emission spectroscopy (high frequency inductively coupled plasma emission spectroscopy). Specifically, for example, particles made of the first heat-resistant oxide are immersed in a solution in which Pd is insoluble and the first heat-resistant oxide is soluble, this solution is filtered, and the filtrate is subjected to ICP light emission. Measure by spectroscopic method. Thereby, the content of Pd contained in the filtrate is calculated, and the solid solution rate of Pd is calculated from the obtained calculated value.

The first heat-resistant oxide is not particularly limited and can be produced by an appropriate method for preparing the oxide, for example, a coprecipitation method, a citric acid complex method, an alkoxide method, or the like.
In the coprecipitation method, for example, a mixed salt aqueous solution containing the above-described salt of each element (excluding palladium salt and alkaline earth metal salt) in the above stoichiometric ratio is prepared, and a neutralizing agent is added to the mixed salt aqueous solution. In addition, after coprecipitation, the obtained coprecipitate is dried and then heat-treated.

Examples of the salt of each element include inorganic salts such as sulfate, nitrate, chloride, and phosphate, and organic acid salts such as acetate and oxalate. Further, the mixed salt aqueous solution can be prepared, for example, by adding the salt of each element to water at a ratio such as the above stoichiometric ratio and stirring and mixing.
Thereafter, a neutralizing agent is added to the mixed salt aqueous solution to cause coprecipitation. Examples of the neutralizing agent include ammonia, for example, organic bases such as amines such as triethylamine and pyridine, and inorganic bases such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, ammonium carbonate, and ammonium hydroxide. Is mentioned. As a neutralizing agent, Preferably, ammonium hydroxide salt is mentioned, More preferably, ammonium hydroxide aqueous solution is mentioned. In addition, a neutralizing agent is added so that the pH of the solution after adding the neutralizing agent may become about 6-10.

The obtained coprecipitate is washed with water if necessary, and dried by, for example, vacuum drying or ventilation drying, and then heat-treated (primary firing) at 400 to 1000 ° C., for example, to thereby obtain a precursor oxide. Get.
Next, the obtained precursor oxide is dispersed in an aqueous palladium salt solution, and this dispersion is filtered. Then, after the obtained filter cake is dried by, for example, vacuum drying or ventilation drying, the precursor is subjected to heat treatment (secondary firing) at, for example, 300 to 800 ° C., preferably 400 to 600 ° C. Pd is supported on the oxide (supported amount: 0.05 to 5% by weight).

Examples of the palladium salt include the same salts as described above, and can be prepared in the same manner as described above. Practically, nitrate aqueous solution, dinitrodiammine nitric acid solution, chloride aqueous solution and the like can be mentioned. Specific examples include an aqueous palladium nitrate solution, a dinitrodiammine palladium nitric acid solution, and a tetravalent palladium ammine nitric acid solution.
Next, an alkaline earth metal is supported on the precursor oxide supporting Pd.

The alkaline earth metal is not particularly limited, and the precursor oxidation is performed by an appropriate method for supporting the alkaline earth metal on the oxide, such as an adsorption supporting method, an impregnation method, a coprecipitation method, and an alkoxide method. It can be carried on an object.
In the adsorption carrying method, for example, a Pd carrying precursor oxide is added to an alkaline earth metal salt aqueous solution to prepare a slurry, and this slurry is filtered.

Alkaline earth metal salts include the same salts as described above, and can be prepared in the same manner as described above. Practically, an aqueous acetate solution, an aqueous nitrate solution, and the like can be given. Specifically, examples of the barium salt aqueous solution include a barium acetate aqueous solution and a barium nitrate aqueous solution.
Then, after the obtained filter cake is dried, for example, by vacuum drying or ventilation drying, depending on the amount of the alkaline earth metal supported, for example, 400 ° C. or more, preferably a temperature exceeding 600 ° C., More preferably, the alkaline earth metal is supported on the precursor oxide supporting Pd by heat treatment (final firing) at a temperature exceeding 600 ° C. and not exceeding 1100 ° C. In this way, the 1st particle | grains which consist of a 1st heat resistant oxide by which Pd was made into solid solution and the alkaline earth metal was carry | supported are obtained.

In the above method, after the preparation of the Pd-supported precursor oxide, the alkaline earth metal is supported on the Pd-supported precursor oxide. For example, the alkaline earth metal is supported on the first heat-resistant oxide. If not, final firing may be performed after the preparation of the Pd-supported precursor oxide.
Further, in the above method, an aqueous solution of all constituent elements (including palladium and alkaline earth metal) is prepared, and a co-precipitate is obtained after adding a neutralizing agent to the aqueous solution. After drying, heat treatment (final firing) can also be performed.

  Further, in the citric acid complex method, for example, citric acid and salts of each of the above elements (excluding palladium salts and alkaline earth metal salts) are used, and each of the above elements (excluding palladium salts and alkaline earth metal salts) are used. A citric acid mixed salt aqueous solution is prepared by adding a slightly excess citric acid aqueous solution to the stoichiometric ratio, and the citric acid mixed salt aqueous solution is dried to solidify each element (palladium salt and alkaline earth metal salt). And the obtained citric acid complex is pre-baked and then heat-treated.

Examples of the salt of each element include the same salts as described above. For the citric acid mixed salt aqueous solution, for example, a mixed salt aqueous solution is prepared in the same manner as described above, and an aqueous citric acid solution is added to the mixed salt aqueous solution. It can be prepared by adding.
Thereafter, the citric acid mixed salt aqueous solution is dried to form a citric acid complex of each element described above. Drying removes moisture at a temperature at which the formed citric acid complex does not decompose, for example, from room temperature to 150 ° C. Thereby, a citric acid complex of each element described above (excluding palladium salts and alkaline earth metal salts) can be formed. Thereafter, the formed citric acid complex is temporarily calcined. In the preliminary firing, for example, heating is performed at 250 to 350 ° C. in a vacuum or an inert atmosphere.

And a precursor oxide is obtained by heat-processing (primary baking) at 400-1000 degreeC, for example.
Next, the obtained precursor oxide is dispersed in an aqueous palladium salt solution in the same manner as in the coprecipitation method, and this dispersion is filtered. Then, after the obtained filter cake is dried by, for example, vacuum drying or ventilation drying, the precursor is subjected to heat treatment (secondary firing) at, for example, 300 to 800 ° C., preferably 400 to 600 ° C. Pd is supported on the oxide (supported amount: 0.05 to 5% by weight).

  Then, as in the coprecipitation method, an alkaline earth metal is supported on the precursor oxide on which Pd is supported, and then, depending on the amount of the alkaline earth metal supported, for example, 400 ° C. or higher, preferably The first heat-resistant oxide in which Pd is solid-solved and further supported by an alkaline earth metal by heat treatment (final firing) at a temperature exceeding 600 ° C., more preferably, exceeding 600 ° C. and not exceeding 1100 ° C. First particles consisting of are obtained.

In the citric acid complex method, when the alkaline earth metal is not supported on the first heat-resistant oxide, Pd is supported on the precursor oxide, followed by oxidation of the Pd-supported precursor as in the coprecipitation method. What is necessary is just to finally bake a thing.
In the alkoxide method, for example, a mixed alkoxide solution containing the alkoxide of each of the above elements (excluding palladium salts and alkaline earth metal salts) in the above stoichiometric ratio is prepared, and water is added to the mixed alkoxide solution. To obtain a precipitate.

Examples of the alkoxide of each element include (mono, di, tri) alcoholate formed from each element and alkoxy such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, and the following general formula (1). Examples include (mono, di, tri) alkoxy alcoholates of each element.
_{E [OCH (R 1) -} (CH 2) i -OR 2] j (1)
(In the formula, E represents each element, R ₁ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, R ₂ represents an alkyl group having 1 to 4 carbon atoms, and i represents 1 to 1) (An integer of 3 and j represents an integer of 2 to 4.)
More specifically, examples of the alkoxy alcoholate include methoxyethylate, methoxypropylate, methoxybutyrate, ethoxyethylate, ethoxypropylate, propoxyethylate, butoxyethylate, and the like.

And a mixed alkoxide solution can be prepared by, for example, adding the alkoxide of each element to an organic solvent so that it may become said stoichiometric ratio, and stirring and mixing.
The organic solvent is not particularly limited as long as the alkoxide of each element can be dissolved, and examples thereof include aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, ketones, and esters. Preferably, aromatic hydrocarbons such as benzene, toluene and xylene are used.

Then, the obtained precipitate is evaporated to dryness, and then dried by, for example, vacuum drying or ventilation drying, followed by heat treatment (primary firing) at 400 to 1000 ° C., for example, to oxidize the precursor. Get things.
Next, the obtained precursor oxide is dispersed in an aqueous palladium salt solution in the same manner as in the coprecipitation method, and this dispersion is filtered. Then, after the obtained filter cake is dried by, for example, vacuum drying or ventilation drying, the precursor is subjected to heat treatment (secondary firing) at 400 to 800 ° C., preferably 400 to 600 ° C., for example. Pd is supported on the oxide.

  Then, as in the coprecipitation method, an alkaline earth metal is supported on the precursor oxide on which Pd is supported, and then, depending on the amount of the alkaline earth metal supported, for example, 400 ° C. or higher, preferably The first heat-resistant oxide in which Pd is solid-solved and further supported by an alkaline earth metal by heat treatment (final firing) at a temperature exceeding 600 ° C., more preferably, exceeding 600 ° C. and not exceeding 1100 ° C. First particles consisting of are obtained.

In the alkoxide method, when the alkaline earth metal is not supported on the first heat-resistant oxide, Pd is supported on the precursor oxide and then the Pd-supported precursor oxide is added as in the coprecipitation method. What is necessary is just to carry out final baking.
In the alkoxide method, for example, after preparing a homogeneous mixed solution containing a mixed alkoxide solution and an organometallic salt of palladium so as to have the above stoichiometric ratio, and adding water thereto to precipitate it, The obtained precipitate can be dried and subjected to heat treatment (secondary firing described above) to obtain a precursor oxide carrying Pd.

  As the organometallic salt of palladium, for example, a palladium carboxylate formed from acetate, propionate, etc., for example, β-diketone compound or β-ketoester compound represented by the following general formula (2), and / or Or the metal chelate complex etc. of palladium formed from the (beta) -dicarboxylic acid ester compound shown by following General formula (3) are mentioned.

R ₃ COCHR ₅ COR ₄ (2)
(In the formula, R ₃ represents an alkyl group having 1 to 6 carbon atoms, a fluoroalkyl group having 1 to 6 carbon atoms, or an aryl group, and R ₄ represents an alkyl group having 1 to 6 carbon atoms, or 1 to 6 carbon atoms. A fluoroalkyl group, an aryl group or an alkoxy group having 1 to 4 carbon atoms, and R ₅ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.)
R ₇ CH (COR ₆ ) ₂ (3)
(In the formula, R ₆ represents an alkyl group having 1 to 6 carbon atoms, and R ₇ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.)
In the general formula (2) and the general formula (3), examples of the alkyl group having 1 to ₆ carbon atoms of R ₃ , R ₄ and R ₆ include methyl, ethyl, propyl, isopropyl, n-butyl, s -Butyl, t-butyl, t-amyl, t-hexyl and the like. The alkyl group having 1 to 4 carbon atoms of R ₅ and R _7, for example, methyl, ethyl, propyl, isopropyl, n- butyl, s- butyl, t- butyl.

In the general formula (2), examples of the fluoroalkyl group having 1 to 6 carbon atoms of R ₃ and R ₄ include trifluoromethyl. The aryl group of R ₃ and R _4, for example, phenyl. Examples of the alkoxy group having 1 to 4 carbon atoms of R ₃ include methoxy, ethoxy, propoxy, isopropoxy, n-butoxy, s-butoxy, t-butoxy and the like.

  More specifically, β-diketone compounds are, for example, 2,4-pentanedione, 2,4-hexanedione, 2,2-dimethyl-3,5-hexanedione, 1-phenyl-1,3-butanedione. 1-trifluoromethyl-1,3-butanedione, hexafluoroacetylacetone, 1,3-diphenyl-1,3-propanedione, dipivaloylmethane and the like.

More specific examples of the β-keto ester compound include methyl acetoacetate, ethyl acetoacetate, t-butyl acetoacetate, and the like.
Specific examples of the β-dicarboxylic acid ester compound include dimethyl malonate and diethyl malonate.
In the second heat-resistant oxide, examples of the rare earth element (excluding cerium) include the rare earth elements described above, and preferably Y, La, Pr, and Nd. The above rare earth elements may be used alone or in combination of two or more.

In the second heat-resistant oxide, the atomic ratio of cerium is preferably 0.1 to 0.6, and more preferably 0.2 to 0.5.
Further, Pd is supported on the second heat-resistant oxide. The phrase “Pd is supported on the second heat-resistant oxide” means that Pd is held on the surface without being dissolved in the second heat-resistant oxide.

In the second heat-resistant oxide, the amount of Pd supported is preferably 0.05 to 5% by weight.
The second heat-resistant oxide is not particularly limited as in the case of the first heat-resistant oxide, and may be any suitable method for preparing the oxide, such as a coprecipitation method, a citric acid complex method, an alkoxide. It can be manufactured by the law.

In the coprecipitation method, for example, a mixed salt aqueous solution containing the above-described salt of each element (excluding the palladium salt) at the above stoichiometric ratio is prepared, and a neutralizing agent is added to the mixed salt aqueous solution and coprecipitated. Then, the obtained coprecipitate is dried and then heat-treated.
Examples of the salt of each element include the organic acid salts described above. Further, the mixed salt aqueous solution can be prepared, for example, by adding the salt of each element to water at a ratio such that the above stoichiometric ratio is obtained and stirring and mixing.

Thereafter, a neutralizing agent is added to the mixed salt aqueous solution to cause coprecipitation. Examples of the neutralizing agent include the neutralizing agent described above, and preferably an aqueous ammonia solution. In addition, a neutralizing agent is added so that the pH of the solution after adding the neutralizing agent may become about 6-10.
Then, the obtained coprecipitate is washed with water if necessary, and dried by, for example, vacuum drying or ventilation drying, followed by heat treatment (primary firing) at 400 to 1000 ° C., for example, to thereby obtain a precursor oxide. obtain.

Next, the obtained precursor oxide is dispersed in an aqueous palladium salt solution, and this dispersion is filtered. Then, after the obtained filter cake is dried by, for example, vacuum drying or ventilation drying, heat treatment (final firing) is performed at, for example, 600 ° C. or less, preferably 300 to 600 ° C., thereby supporting Pd. Second particles made of the second heat-resistant oxide are obtained.
In the above method, an aqueous solution of all the constituent elements (including palladium) is prepared, and a neutralizing agent is added thereto for coprecipitation, and then the obtained coprecipitate is dried and heat-treated. (Final firing) can also be performed.

  In addition, in the citric acid complex method, for example, citric acid and salts of each of the above elements (excluding palladium salts) are slightly excess from the stoichiometric ratio with respect to each of the above elements (excluding palladium salts). An aqueous solution is added to prepare a citric acid mixed salt aqueous solution, and the citric acid mixed salt aqueous solution is dried to form a citric acid complex of each of the above elements (excluding the palladium salt). The complex is calcined and then heat treated.

Examples of the salt of each element include the same salts as described above. For the citric acid mixed salt aqueous solution, for example, a mixed salt aqueous solution is prepared in the same manner as described above, and an aqueous citric acid solution is added to the mixed salt aqueous solution. It can be prepared by adding.
Thereafter, the citric acid mixed salt aqueous solution is dried to form a citric acid complex of each element described above. Drying removes moisture at a temperature at which the formed citric acid complex does not decompose, for example, from room temperature to 150 ° C. Thereby, a citric acid complex of each element described above (excluding palladium salts and alkaline earth metal salts) can be formed. Thereafter, the formed citric acid complex is temporarily calcined. In the preliminary firing, for example, heating is performed at 250 to 350 ° C. in a vacuum or an inert atmosphere.

And a precursor oxide is obtained by heat-processing (primary baking) at 400-1000 degreeC, for example.
Next, the obtained precursor oxide is dispersed in an aqueous palladium salt solution in the same manner as in the coprecipitation method, and this dispersion is filtered. Then, after the obtained filter cake is dried by, for example, vacuum drying or ventilation drying, heat treatment (final firing) is performed at, for example, 600 ° C. or less, preferably 300 to 600 ° C., thereby supporting Pd. Second particles made of the second heat-resistant oxide are obtained.

In the alkoxide method, for example, a mixed alkoxide solution containing the alkoxide of each of the above elements (excluding the palladium salt) in the above stoichiometric ratio is prepared, and the mixed alkoxide solution is hydrolyzed by adding water. To obtain a precipitate.
Examples of the alkoxide of each element include the alkoxide described above.
And a mixed alkoxide solution can be prepared by, for example, adding the alkoxide of each element to an organic solvent so that it may become said stoichiometric ratio, and stirring and mixing.

The organic solvent is not particularly limited as long as the alkoxide of each element can be dissolved, and examples thereof include the organic solvents described above. Preferably, aromatic hydrocarbons such as benzene, toluene and xylene are used.
Then, the obtained precipitate is evaporated to dryness, and then dried by, for example, vacuum drying or ventilation drying, followed by heat treatment (primary firing) at 400 to 1000 ° C., for example, to oxidize the precursor. Get things.

  Next, the obtained precursor oxide is dispersed in an aqueous palladium salt solution in the same manner as in the coprecipitation method, and this dispersion is filtered. Then, after the obtained filter cake is dried by, for example, vacuum drying or ventilation drying, heat treatment (final firing) is performed at, for example, 600 ° C. or less, preferably 300 to 600 ° C., thereby supporting Pd. Second particles made of the second heat-resistant oxide are obtained.

In the alkoxide method, for example, after preparing a homogeneous mixed solution containing a mixed alkoxide solution and an organometallic salt of palladium so as to have the above-described stoichiometric ratio, and adding water to this to cause precipitation, The obtained precipitate is dried and subjected to a heat treatment (the above-described final firing), whereby a precursor oxide carrying Pd can also be obtained.
Examples of the organometallic salt of palladium include the above-described organometallic salts.

The exhaust gas purifying catalyst of the present invention is an appropriate method for mixing particles (powder) with the first particles made of the first heat-resistant oxide and the second particles made of the second heat-resistant oxide. For example, it can be manufactured by mixing by a mixing method such as dry mixing or wet mixing. Preferably, it can be produced by dry mixing.
In the exhaust gas purification catalyst, the weight mixing ratio of the first particles made of the first heat-resistant oxide and the second particles made of the second heat-resistant oxide is, for example, 1/9 to 9/1, preferably 3/7 to 8/2. If these weight mixing ratios are in the above-described range, palladium grain growth can be effectively suppressed.

The mixed particles (mixed powder) of the first particles made of the first heat-resistant oxide and the second particles made of the second heat-resistant oxide thus obtained can be used as they are as an exhaust gas purification catalyst. However, it is usually prepared as an exhaust gas purifying catalyst by a known method such as loading on a catalyst carrier.
Examples of the catalyst carrier include known catalyst carriers such as a honeycomb monolith carrier made of cordierite. In order to carry on the catalyst carrier, for example, first, water is added to the mixed particles (mixed powder) obtained above to form a slurry, which is then coated on the catalyst carrier, dried, and then 300 It heat-processes at -800 degreeC, Preferably, it is 300-600 degreeC.

In the exhaust gas purifying catalyst of the present invention thus obtained, palladium grain growth is effectively suppressed even during long-term use, and the dispersion state of the palladium composite oxide is well maintained.
As a result, if the exhaust gas purification catalyst of the present invention is used, excellent exhaust gas purification performance can be exhibited over a long period of time under high temperature or oxidation-reduction fluctuations.

  The exhaust gas purification catalyst of the present invention can achieve excellent exhaust gas purification performance over a long period of time. For example, exhaust gas for purifying exhaust gas discharged from an internal combustion engine such as a gasoline engine or a diesel engine or a boiler. It can be used effectively as a gas purification catalyst.

Next, although this invention is demonstrated based on an Example and a comparative example, this invention is not limited by the following Example.
(Production of particles made of Pd solid solution / Ba-supported composite oxide)
Production Example 1 (Production of Pd solid solution / Ba / Ce _0.50 Zr _0.45 Y _0.05 O ₃ particles A)
Cerium nitrate 0.050 mol in terms of Ce Zirconium oxynitrate 0.045 mol in terms of Zr Yttrium nitrate 0.005 mol in terms of Y By adding the above components to a round bottom flask, adding 500 mL of deionized water and stirring to dissolve. A mixed salt aqueous solution was prepared. Next, a 10 wt% ammonium hydroxide aqueous solution was gradually added dropwise to the mixed salt aqueous solution at room temperature to form a coprecipitate in the mixed salt aqueous solution.

  Next, the mixed salt aqueous solution in which the coprecipitate was produced was stirred for 60 minutes, and then this aqueous solution was filtered to obtain a coprecipitate. Subsequently, the coprecipitate was sufficiently washed with deionized water, vacuum dried at 110 ° C., and further calcined at 500 ° C. for 3 hours in an air atmosphere. Then, the coprecipitate is pulverized into particles, and these particles are fired at 800 ° C. for 5 hours (primary firing) in the atmosphere to thereby obtain composite oxide particles composed of cerium, zirconium and yttrium (precursor oxidation). Product).

Next, 50 g of the particles were added to a round bottom flask, and 500 mL of deionized water was added and stirred for 10 minutes to prepare a slurry by dispersing in deionized water.
Next, this slurry was added and dispersed in an aqueous palladium nitrate solution in an amount such that the amount of palladium supported on the particles (precursor oxide) was 0.8% by weight. Next, the dispersion was subjected to suction filtration and then vacuum-dried at 110 ° C. for 12 hours. Subsequently, palladium was supported on the precursor oxide particles by firing at 500 ° C. for 1 hour in the air atmosphere (secondary firing).

In addition, when the element contained in the filtrate when the dispersion liquid was suction filtered was measured by ICP emission spectroscopic analysis, palladium was not contained in the filtrate. Thereby, it was found that most of palladium was contained in the filter cake when the dispersion was suction filtered.
Thereafter, barium acetate was added to the round bottom flask and 100 mL of deionized water was added and dissolved. Next, 50 g of precursor oxide particles carrying palladium were added to an aqueous barium acetate solution to prepare a slurry. The concentration of the barium acetate aqueous solution was adjusted so that the ratio of the sum of the atomic ratios of cerium, zirconium and yttrium to the atomic ratio of barium was 100: 10 in particles A described later.

Next, this slurry was vacuum-dried to remove moisture, thereby further supporting barium on the precursor oxide supporting palladium.
Then, by firing (final firing) at 1000 ° C. for 3 hours in an air atmosphere, Pd is solid-solved and Ba is supported, and a composite oxide containing Ce, Zr and Y (Pd solid-solution Ba / Ce) Particles A consisting of _0.50 Zr _0.45 Y _0.05 O ₃ ) were obtained.

Note that when the particle A was subjected to X-ray diffraction analysis, the particle A contained a complex oxide represented by the general formula: (Ce, Zr, Y, Pd) O ₃ as a main component, It was confirmed that a composite oxide represented by the formula: Ba (Ce, Pd) O ₃ was included. As a result, in the particle A, it was confirmed that Ba (Ce, Pd) O ₃ was supported on (Ce, Zr, Y, Pd) O ₃ .

The particles A were immersed in an aqueous hydrogen fluoride solution (HF / H ₂ O = 1/10) maintained at room temperature for 12 hours, this liquid was filtered, and the filtrate was measured by ICP emission spectroscopic analysis. As a result, it was confirmed that the solid solution rate of palladium was 80%.
Production Example 2 (Production of Pd solid solution / Ba / Ce _0.55 Zr _0.40 Y _0.05 O ₃ particles B)
Cerium nitrate 0.055 mol in terms of Ce Zirconium oxynitrate 0.040 mol in terms of Zr Yttrium nitrate 0.005 mol in terms of Y Except using the above components, Pd was dissolved in the same manner as in Production Example 1, and Ba Was obtained, and particles B made of a composite oxide containing Ce, Zr and Y (Pd solid solution, Ba / Ce _0.55 Zr _0.40 Y _0.05 O ₃ ) supported thereon.

Production Example 3 (Production of Pd solid solution / Ba / Ce _1.00 O ₃ particles C)
Cerium nitrate 0.100 mol in terms of Ce Except for using the above components, a complex oxide containing Ce (Pd solid solution / Ba /) in which Pd is solid-solved and Ba is supported by the same method as in Production Example 1. Particles C consisting of Ce _1.00 O ₃ ) were obtained.

Production Example 4 (Production of Pd solid solution / Ba / Ce _0.80 Zr _0.15 Y _0.05 O ₃ particles D)
Cerium nitrate 0.080 mol in terms of Ce Zirconium oxynitrate 0.015 mol in terms of Zr Yttrium nitrate 0.005 mol in terms of Y Except for using the above components, Pd was dissolved in the same manner as in Production Example 1, and Ba Was obtained, and particles D made of a composite oxide containing Ce, Zr and Y (Pd solid solution, Ba / Ce _0.80 Zr _0.15 Y _0.05 O ₃ ) on which was supported.

Production Example 5 (Production of Pd solid solution / Ba / Ce _0.40 Zr _0.50 Y _0.05 La _0.05 O ₃ particles E)
Cerium nitrate 0.040 mol in terms of Ce, zirconium oxynitrate 0.050 mol in terms of Zr Yttrium nitrate 0.005 mol in lanthanum nitrate Y in terms of 0.005 mol in terms of La The same method as in Production Example 1 except that the above components are used As a result, a particle E composed of a complex oxide containing Pd, Ba, and supported Ce, Zr, Y, and La (Pd solid solution, Ba / Ce _0.40 Zr _0.50 Y _0.05 La _0.05 O ₃ ) is obtained. It was.

Production Example 6 (Production of Pd solid solution / Ba / Ce _0.22 Zr _0.68 Y _0.03 La _0.02 Nd _0.05 O ₃ particles F)
Cerium nitrate 0.022 mol in terms of Ce zirconium oxynitrate 0.068 mol in terms of Zr Yttrium nitrate 0.003 mol in lanthanum nitrate Y in terms of 0.002 mol in neodymium nitrate Neodymium nitrate 0.005 mol in terms of Nd Is a composite oxide containing Ce, Zr, Y, La and Nd in which Pd is solid-solved and Ba is supported by the same method as in Production Example 1 (Pd solid-solution Ba / Ce _0.22 Zr _0.68 Y _0.03 Particles F made of La _0.02 Nd _0.05 O ₃ ) were obtained.
(Production of particles made of Pd-supported composite oxide)
Production Example 7 (Production of Pd / Ce _0.50 Zr _0.45 Y _0.05 O ₂ Particle G)
Cerium nitrate 0.050 mol in terms of Ce Zirconium oxynitrate 0.045 mol in terms of Zr Yttrium nitrate 0.005 mol in terms of Y By adding the above components to a round bottom flask, adding 500 mL of deionized water and stirring to dissolve. A mixed salt aqueous solution was prepared. Next, an aqueous ammonia solution prepared by dissolving 51 g of ammonia in 1 L of deionized water was gradually added dropwise to the mixed salt aqueous solution at room temperature to form a coprecipitate in the mixed salt aqueous solution.

  Subsequently, the mixed salt aqueous solution in which the coprecipitate was generated was stirred for 60 minutes, and then this aqueous solution was filtered to obtain a coprecipitate. Subsequently, the coprecipitate was sufficiently washed with deionized water, vacuum-dried at 110 ° C., and further calcined at 500 ° C. in an air atmosphere for 3 hours. Then, the coprecipitate is pulverized into particles, and the particles are fired at 800 ° C. for 5 hours (primary firing) in an air atmosphere to obtain composite oxide particles containing cerium, zirconium and yttrium (precursor oxidation). Product).

Next, 50 g of the particles were added to a round bottom flask, and 500 mL of deionized water was added and stirred for 10 minutes to prepare a slurry by dispersing in deionized water.
Next, this slurry was added and dispersed in an aqueous palladium nitrate solution in an amount such that the amount of palladium supported on the particles (precursor oxide) was 0.8% by weight. Next, the dispersion was subjected to suction filtration and then vacuum-dried at 110 ° C. for 12 hours. Subsequently, by firing at 500 ° C. for 1 hour in an air atmosphere, particles G made of a composite oxide containing Pd and containing Ce, Zr and Y (Pd / Ce _0.50 Zr _0.45 Y _0.05 O ₂ ) are obtained. It was.

Further, the particles G were immersed in an aqueous hydrogen fluoride solution (HF / H ₂ O = 1/10) maintained at room temperature for 12 hours, this liquid was filtered, and the filtrate was measured by ICP emission spectroscopic analysis. As a result, it was confirmed that palladium did not form a solid solution. Thus, it was confirmed that Pd was supported on the composite oxide containing Ce, Zr and Y.
Production Example 8 (Production of Pd / Ce _0.80 Zr _0.15 Y _0.05 O ₂ Particle H)
Cerium nitrate 0.080 mol in terms of Ce Zirconium oxynitrate 0.015 mol in terms of Zr Yttrium nitrate 0.005 mol in terms of Y Except for the above components, Ce was supported by Pd in the same manner as in Production Example 7. , Zr and Y-containing composite oxide (Pd / Ce _0.80 Zr _0.15 Y _0.05 O ₂ ) was obtained.

Production Example 9 (Production of Pd / Ce _0.55 Zr _0.40 Y _0.05 O ₂ Particle I)
Cerium nitrate 0.055 mol in terms of Ce Zirconium oxynitrate 0.040 mol in terms of Zr Yttrium nitrate 0.005 mol in terms of Y Except for using the above components, Ce was supported with Pd in the same manner as in Production Example 7. , Zr and Y-containing composite oxide (Pd / Ce _0.55 Zr _0.40 Y _0.05 O ₂ ) was obtained.

Production Example 10 (Production of Pd / Ce _0.30 Zr _0.60 Y _0.05 La _0.05 O ₂ Particle J)
Cerium nitrate 0.030 mol in terms of Ce Zirconium oxynitrate 0.060 mol in terms of Zr Yttrium nitrate 0.005 mol in terms of Y Lanthanum nitrate 0.005 mol in terms of La The same method as in Production Example 7 except that the above components are used As a result, particles J made of composite oxide containing Pd, Ce, Zr, Y and La (Pd / Ce _0.30 Zr _0.60 Y _0.05 La _0.05 O ₂ ) were obtained.

Production Example 11 (Production of Pd / Ce _0.22 Zr _0.68 Y _0.03 La _0.02 Nd _0.05 O ₂ Particle K)
Cerium nitrate 0.022 mol in terms of Ce zirconium oxynitrate 0.068 mol in terms of Zr Yttrium nitrate 0.003 mol in lanthanum nitrate Y in terms of 0.002 mol in neodymium nitrate Neodymium nitrate 0.005 mol in terms of Nd Is a particle composed of a composite oxide containing Pd and containing Ce, Zr, Y, La and Nd (Pd / Ce _0.22 Zr _0.68 Y _0.03 La _0.02 Nd _0.05 O ₂ ) by the same method as in Production Example 7. K was obtained.

Production Example 12 (Production of Pd / Ce _0.11 Zr _0.67 Y _0.15 La _0.02 Nd _0.05 O ₂ Particle L)
Cerium nitrate 0.011 mol in terms of Ce Zirconate 0.067 mol in terms of Zr Yttrium nitrate 0.01 in terms of Y Lanthanum nitrate in terms of Y 0.002 mol in terms of La neodymium nitrate 0.005 mol in terms of Nd Other than using the above components Is a particle made of a composite oxide (Pd / Ce _0.11 Zr _0.67 Y _0.15 La _0.02 Nd _0.05 O ₂ ) containing Ce, Zr, Y, La and Nd on which Pd is supported in the same manner as in Production Example 7. L was obtained.

Production Example 13 (Production of Pd / Zr _0.83 La _0.08 Nd _0.09 O ₂ Particle M)
Zirconium oxynitrate 0.083 mol lanthanum nitrate in terms of Zr 0.008 mol in terms of La neodymium nitrate 0.009 mol in terms of Nd Except for using the above components, Pd was supported in the same manner as in Production Example 7, Particles M made of a composite oxide containing Zr, La and Nd (Pd / Zr _0.83 La _0.08 Nd _0.09 O ₂ ) were obtained.

Examples 1-3 and Comparative Examples 1-2
Table 1 shows the particles (Pd solid solution, Ba / Ce _0.50 Zr _0.45 Y _0.05 O ₃ ) obtained in Production Example 1 and the particles obtained in Production Example 7 (Pd / Ce _0.50 Zr _0.45 Y _0.05 O ₂ ). The mixed particles (mixed powder) were prepared by mixing in a mortar according to the weight mixing ratio shown in FIG. That is, mixed particles (mixed powders) with different weight mixing ratios were prepared with the Ce atomic ratio in each composite oxide being constant.

実施例４〜８および比較例３
製造例２で得られた粒子（Ｐｄ固溶・Ｂａ／Ｃｅ_0.55Ｚｒ_0.40Ｙ_0.05Ｏ₃）に対して、表２に従ってＰｄ担持複合酸化物を、５０：５０の重量混合比で、乳鉢で混合することにより混合粒子（混合粉末）を調製した。すなわち、Ｐｄ固溶・Ｂａ担持複合酸化物におけるＣｅの原子割合を一定とし、この複合酸化物に対して、Ｃｅの原子割合がそれぞれ異なる、Ｐｄ担持複合酸化物を混合して混合粒子（混合粉末）を調製した。

Examples 4 to 8 and Comparative Example 3
A Pd-supported composite oxide was mixed in a mortar at a weight mixing ratio of 50:50 with respect to the particles (Pd solid solution / Ba / Ce _0.55 Zr _0.40 Y _0.05 O ₃ ) obtained in Production Example 2 according to Table 2. Thus, mixed particles (mixed powder) were prepared. That is, the atomic ratio of Ce in the Pd solid solution / Ba-supported composite oxide is made constant, and the mixed oxide is mixed with Pd-supported composite oxides having different Ce atomic ratios (mixed powder). ) Was prepared.

実施例９〜１２および比較例４
製造例９で得られた粒子（Ｐｄ／Ｃｅ_0.55Ｚｒ_0.40Ｙ_0.05Ｏ₂）に対して、表３に従ってＰｄ固溶・Ｂａ担持複合酸化物を、５０：５０の重量混合比で、乳鉢で混合することにより混合粒子（混合粉末）を調製した。すなわち、Ｐｄ担持複合酸化物におけるＣｅの原子割合を一定とし、この複合酸化物に対して、Ｃｅの原子割合がそれぞれ異なる、Ｐｄ固溶・Ｂａ担持複合酸化物を混合して混合粒子（混合粉末）を調製した。

Examples 9-12 and Comparative Example 4
A Pd solid solution / Ba-supported composite oxide was mixed in a mortar at a weight mixing ratio of 50:50 with respect to the particles (Pd / Ce _0.55 Zr _0.40 Y _0.05 O ₂ ) obtained in Production Example 9 according to Table 3. Thus, mixed particles (mixed powder) were prepared. That is, the atomic ratio of Ce in the Pd-supported composite oxide is made constant, and mixed particles (mixed powder) are mixed with Pd solid solution / Ba-supported composite oxide having different Ce atomic ratios. ) Was prepared.

試験例１（活性評価）
１）耐久試験
不活性雰囲気３分、酸化雰囲気３分、不活性雰囲気３分および還元雰囲気３分の計１２分を１サイクルとし、このサイクルを２５０サイクル、合計５０時間繰り返して、各実施例および各比較例で得られた混合粒子（混合粉末）を、酸化雰囲気と還元雰囲気とに交互に暴露した後、還元雰囲気のまま室温まで冷却した。

Test example 1 (activity evaluation)
1) Durability test 3 cycles of inert atmosphere, 3 minutes of oxidizing atmosphere, 3 minutes of inert atmosphere, and 3 minutes of reducing atmosphere are defined as 1 cycle, and this cycle is repeated for 250 cycles for a total of 50 hours. The mixed particles (mixed powder) obtained in each comparative example were alternately exposed to an oxidizing atmosphere and a reducing atmosphere, and then cooled to room temperature in the reducing atmosphere.

The inert atmosphere, the oxidizing atmosphere, and the reducing atmosphere correspond to exhaust gas atmospheres that are exhausted when the stoichiometric, lean, and rich air-fuel mixtures are burned, respectively.
Each atmosphere was prepared by supplying a gas having a composition shown in Table 4 containing high-temperature steam at a flow rate of 300 × 10 ⁻³ m ³ / hr. The ambient temperature was maintained at about 1050 ° C.
2) 50% purification temperature (Examples 1-3 and Comparative Examples 1-2)
The test pieces were prepared by molding the particles (powder) of Examples 1 to 3 and Comparative Examples 1 and 2 after endurance into pellets having a size of 0.5 mm to 1.0 mm. Using the model gas composition shown in Table 5, while increasing the temperature of the exhaust gas exhausted by combustion of this model gas from room temperature to 450 ° C. at a rate of 20 ° C./min, the model gas is applied to each test piece. The temperature at which 50% of HC, NO _x, and CO in the exhaust gas was purified (50% purification temperature: ° C.) was measured. The results are shown in FIG. That is, the graph shown in FIG. 1 shows the change in the 50% purification temperature of each of HC, NO _x, and CO when the weight mixing ratio of the Pd solid solution / Ba supported composite oxide and the Pd supported composite oxide is changed. Represents.
3) 450 ° C. purification rate (Examples 4 to 8 and Comparative Example 3)
The particles (powder) of Examples 4 to 8 and Comparative Example 3 after endurance were molded into pellets having a size of 0.5 mm to 1.0 mm to prepare test pieces. Using the model gas composition shown in Table 5, the temperature of the exhaust gas exhausted by the combustion of the model gas is increased from room temperature to 450 ° C. at a rate of 20 ° C./min. Then, the respective purification rates of HC, NO _x and CO at 450 ° C. were measured. In the measurement, the sample weight was 0.5 g. The flow rate was 2.5 L / min. The results are shown in FIG. That is, the graph shown in FIG. 2 represents the change in the purification rate at 450 ° C. for each of HC, NO _x, and CO when the Ce atomic ratio of the Pd-supported composite oxide is changed.
4) 450 ° C. purification rate (Examples 9 to 12 and Comparative Example 4)
The purification rates of HC, NOx and CO at 450 ° C. were measured by the same method as 3). The results are shown in FIG. That is, the graph shown in FIG. 3 represents the change in the purification rate at 450 ° C. for HC, NO _x, and CO when the Ce atomic ratio of the Pd solid solution / Ba-supported composite oxide is changed.

When changing the mixing weight ratio of Pd solid solution · Ba supporting composite oxide and Pd-supporting composite oxide, HC, is a graph showing the change of the NO _x and CO, respectively 50% purification temperature. When changing the atomic ratio of Ce and Pd-supporting composite oxide, HC, is a graph showing a change in purification rate of NO _x and CO respectively 450 ° C.. When changing the atomic ratio of Ce and Pd solid solution · Ba supporting composite oxide, HC, is a graph showing a change in purification rate of NO _x and CO respectively 450 ° C..

Claims (6)

A first heat-resistant oxide in which Pd is dissolved and contains cerium, zirconium and a rare earth element (excluding cerium);
A catalyst for purifying exhaust gas, comprising Pd, and a second heat-resistant oxide containing cerium, zirconium, and a rare earth element (excluding cerium).   2. The exhaust gas purifying catalyst according to claim 1, wherein the first heat-resistant oxide has an atomic ratio of cerium of 0.2 to 0.8.   2. The exhaust gas purifying catalyst according to claim 1, wherein the second heat-resistant oxide has an atomic ratio of cerium of 0.1 to 0.6.   In the first heat-resistant oxide, the atomic ratio of cerium is 0.2 to 0.8, and in the second heat-resistant oxide, the atomic ratio of cerium is 0.1 to 0.6. The exhaust gas-purifying catalyst according to claim 1.   The catalyst for exhaust gas purification according to any one of claims 1 to 4, wherein an alkaline earth metal is supported on the first heat-resistant oxide. Producing a first heat-resistant oxide having Pd and containing cerium, zirconium and a rare earth element (excluding cerium);
Producing a second heat-resistant oxide having Pd and containing cerium, zirconium and a rare earth element (excluding cerium);
A step of mixing the first heat-resistant oxide and the second heat-resistant oxide at a weight mixing ratio of 1/9 to 9/1,
The step of producing the first heat-resistant oxide includes
Mixing a cerium salt, a zirconium salt and a rare earth element salt to prepare a mixed salt aqueous solution;
Adding a neutralizing agent to the mixed salt aqueous solution to obtain a coprecipitate;
A step of subjecting the coprecipitate to primary firing at 400 to 1000 ° C. to obtain a precursor oxide;
Mixing the precursor oxide and the Pd salt and performing secondary firing at 300 to 800 ° C .;
A step of obtaining the first heat-resistant oxide by mixing the precursor oxide that has been subjected to secondary firing and the alkaline earth metal salt and performing final firing at a temperature exceeding 600 ° C .;
The step of producing the second heat-resistant oxide comprises:
Mixing a cerium salt, a zirconium salt and a rare earth element salt to prepare a mixed salt aqueous solution;
Adding a neutralizing agent to the mixed salt aqueous solution to obtain a coprecipitate;
A step of subjecting the coprecipitate to primary firing at 400 to 1000 ° C. to obtain a precursor oxide;
And a step of obtaining the second heat-resistant oxide by mixing the precursor oxide and the Pd salt and final firing at 600 ° C. or lower. Method.

Images