JPH08333116A - Powder - Google Patents

Abstract

PURPOSE: To obtain powder containing ceria, having high specific surface area and hardly causing lowering of specific surface area at high temperature. CONSTITUTION: This powder comprises particles in which ceria forms a solid solution with an oxide of rare earth element excluding cerium or zirconia, and >=50wt.% of the particles are composed of particles having particle diameter in a range of <=100nm particle. The powder can be produced by rubbing ceria powder and powder of a compound including rare earth element excluding cerium or zirconium in a pulverizing apparatus in which pulverizing media exist in a state in which pulverizing media or pulverizing media and a member of the pulverizing apparatus rub together in the pulverizing apparatus.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder which can be used as a catalyst and which comprises particles in which ceria and an oxide of a rare earth element or zirconia are in solid solution with each other.

[0002]

2. Description of the Related Art Ceria is used as a co-catalyst or the like in a catalyst for purifying exhaust gas from an internal combustion engine or the like because it has a function of storing and releasing oxygen (oxygen storage capacity) depending on a change in atmosphere. Further, in order to increase the specific surface area, ceria is used in a powder state and used as a catalyst or the like.

For example, when ceria powder is used as a co-catalyst in an exhaust gas purifying catalyst, the catalyst is used at a high temperature, and it is necessary to enhance the purifying activity at a high temperature. Therefore, it is required that the ceria powder has a high specific surface area and that the specific surface area does not decrease even when used at high temperature (high heat resistance).

As described above, in order to increase the specific surface area of the ceria powder and improve the heat resistance, it has been proposed to dissolve zirconia or an oxide of a rare earth element (excluding cerium) in ceria.

Conventionally, as a method of forming a solid solution of zirconia or an oxide of a rare earth element in ceria, a method of impregnating ceria powder with a solution containing zirnium or a rare earth element, followed by heat treatment (impregnation method), or a cerium compound And a compound of zirconium or a rare earth element are mixed in a solution state, and both are coprecipitated, followed by heat treatment (coprecipitation method).

[0006]

[Patent Document 1] Japanese Patent Application Laid-Open No. 4-55315
The publication describes the above coprecipitation method, and discloses a method in which a water-soluble zirconium salt is mixed with a water-soluble cerium salt to cause coprecipitation, and then the coprecipitate is heat-treated. In this method, by heat treating the coprecipitate,
Cerium and zirconium in the coprecipitate become oxides,
They form a solid solution with each other.

[0007] However, it is difficult to obtain independent fine-grained powder due to grain growth caused by heat treatment and strong particle adhesion or sintering. Further, the solid solution of ceria and zirconia is not sufficient, and a considerable amount of the components which are not solid solution remain. Further, if heat treatment is performed at a higher temperature in order to promote solid solution, the specific surface area of the powder decreases.

Further, Japanese Unexamined Patent Publication No. 4-284847 discloses that a powder in which ceria and zirconia or an oxide of a rare earth element are formed as a solid solution is produced by an impregnation method or a coprecipitation method.

However, in the coprecipitation method, the pH of the aqueous solution in which elements such as cerium are decomposed and deposited is different, so that the coprecipitate has a non-uniform composition and the solid solution is not sufficient before the heat treatment. In the impregnation method, there are cases of impregnating the ceria powder with a solution containing zirconium and the like, and cases of impregnating the powder containing zirconia with a solution containing a cerium compound. Since the primary particles of (the base material to be impregnated) are large, the composition of the entire particles becomes non-uniform, and it is difficult to promote solid solution. Therefore, solid solution is caused by heat treatment in both the coprecipitation method and the impregnation method.
Coprecipitation method 500 ~ 900 ℃, impregnation method 700-12
Although it is heat-treated at a temperature of 00 ° C, solid solution is not sufficient. Further, in both the coprecipitation method and the impregnation method, a neck is formed between the particles by heat-treating the powder, and a solid solution is promoted from this neck, but the grain growth and the sintering of the particles are promoted accordingly, and the powder The specific surface area of is reduced. Also, once the particles grow large, they cannot be easily crushed.

As described above, since two or more phases of ceria and zirconia or an oxide of a rare earth element coexist in the particles and a large amount of undissolved components remain, ceria and an oxide of zirconia or a rare earth element are sufficient. It never dissolves in. Moreover, a powder composed of particles having a large specific surface area has not been obtained. Among the finest powders of ceria in which zirconia or an oxide of a rare earth element is solid-dissolved, only those having 80% by weight or more of particles having a particle diameter of 1000 nm or more are obtained. Therefore, with a high specific surface area in ceria powder,
Moreover, the requirement that the specific surface area is not reduced at high temperature is not satisfied.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a powder containing ceria, which has a high specific surface area and a small decrease in specific surface area at high temperature.

[0012]

The powder of the present invention is composed of particles in which at least one oxide of rare earth elements other than cerium and zirconium and ceria are solid-soluted with each other, and the content of the powder is 50% by weight or more. Particle size is 100 nm
It is characterized by being in the following range.

[0013]

In the powder of the present invention, 50% by weight or more of the particles have a particle size in the range of 100 nm or less, and the particle size is very small, so that the original specific surface area of the powder is high. Moreover, since the oxide of at least one of rare earth elements other than cerium and zirconium is sufficiently dissolved in ceria, even if it is used at high temperature, the decrease in the specific surface area of the powder is small.
Therefore, for example, when it is used as a catalyst, it can maintain a high activity even when used at high temperature.

[0014]

EFFECTS OF THE INVENTION The powder of the present invention has a high specific surface area, and even if it is used at a high temperature, the specific surface area is not significantly reduced.
Therefore, for example, when it is used as a catalyst, it has high activity such as oxygen storage capacity even at high temperatures.

[0015]

EXAMPLES The invention (other inventions) which makes the present invention more specific will be described in detail below.

(Description of Other Inventions) The powder of the present invention comprises:
An oxide of at least one kind of rare earth element other than cerium and zirconium and ceria are solid-solved with each other, and the particle size of 50% by weight or more is 10
It is in the range of 0 nm or less.

In the present invention, the rare earth elements are Sc,
Y, lanthanoids (La, Pr, Nd, etc.) and actinides (Ac, Th, Pa, etc.) are elements belonging to Group IIIa of the periodic table, and exclude cerium.

At least one oxide of rare earth elements other than cerium and zirconium (hereinafter referred to as an oxide of a rare earth element or the like) and ceria are in solid solution with each other. But it's okay.

In the powder of the present invention, 50% by weight or more of the particles have a particle size of 100 nm or less. When the number of particles having a particle size larger than the above range increases, the specific surface area decreases.
Further, it is desirable that the particle diameter of 80% by weight or more is 50 nm or less. Within this range, the specific surface area of the powder is further increased.

The particle size defined in the present invention is measured by an electron microscope, a particle size distribution measuring instrument or the like. As a principle for obtaining a particle size distribution suitable for measuring the particle size, for example, the photon correlation method can be mentioned.

Next, a method for producing the powder of the present invention will be described.

A compound containing at least one of rare earth elements other than cerium and zirconium (hereinafter referred to as a compound such as a rare earth element) and ceria powder are crushed in a crushing apparatus in which crushing media exist. By crushing one another or the crushing media and a member of the crushing device in friction with each other, particles of at least one oxide of rare earth elements other than cerium and zirconium and ceria are solid-solved with each other. There is a method for producing the powder.

In this manufacturing method, the ceria powder and the compound such as the rare earth element are crushed by utilizing the frictional force, so that the oxide such as the rare earth element can be sufficiently dissolved in the ceria.

The reason for this is not clear, but it is presumed as follows.

Since the raw material powder is pulverized in a state where the pulverizing media are in friction with each other or the pulverizing media and the member of the pulverizing apparatus, the pulverizing energy received by the raw material powder is large, and microscopically the temperature of the pulverized particles is pulverized. The temperature becomes higher than the average temperature of the whole product (raw material powder). Further, the stress applied to the particles during the pulverization promotes the diffusion of the element into the ceria particles. Further, the crushed raw material particles are strongly rubbed with each other. Due to these effects, an amorphous phase is formed on the surface of the ceria particle, and each element (rare earth element except cerium and at least one element of zirconium and oxygen) is effectively contained in the internal crystal lattice through the amorphous phase. It is considered that they are taken in and solid solution is promoted. It should be noted that although the temperature of the crushed particles increases, the average temperature of the entire crushed product does not increase rapidly, so grain growth and particle sintering are suppressed.

When a compound such as a rare earth element is an oxide as a raw material, it is solid-solved in ceria in an oxide state, and when it is other than an oxide, it is present in the atmosphere during pulverization. Oxygen (When pulverizing with a dispersion medium present,
Oxygen present in the dispersion medium also participates) and a compound such as a rare earth element react to change to an oxide such as a rare earth element,
Dissolves in ceria.

During the pulverization, the raw material ceria powder and the compound such as the rare earth element become fine particles due to friction. When an oxide of a rare earth element or the like forms a solid solution in the particles of ceria powder, the particles themselves become large, but the particles become fine particles by pulverization. Therefore, by continuing the pulverization, particles in which ceria and an oxide such as a rare earth element form a solid solution with each other,
50% by weight or more of the particles have a particle size of 100 nm or less and are fine.

Further, since no heat treatment is performed, there is no grain growth or sintering of the grains, and it is possible to obtain fine grains having a large specific surface area and a small grain size.

The shape of the raw material compound such as a rare earth element is not particularly limited, and may be in the form of powder, lump, or aqueous solution. Further, the crushing media or the member of the crushing device may be made of a compound such as a rare earth element, and the wear pieces generated by friction due to the crushing may be used as a raw material of the compound such as the rare earth element.

Examples of the compound such as a rare earth element as a raw material include oxides, hydroxides, salts and the like. In the case of a form other than the oxide, oxygen existing in the atmosphere reacts with a compound such as a rare earth element during pulverization to be converted into an oxide such as a rare earth element, which is solid-dissolved in ceria. When the dispersion medium is present when the raw material is pulverized, oxygen in the dispersion medium also contributes to the formation of the oxide.

The ceria powder as a raw material is not particularly limited. However, in order to improve the pulverization efficiency, fine porous particles having a small primary particle diameter and a weak degree of secondary aggregation are desirable.

The crushing device having the crushing media is for crushing the raw material powder by frictional force and crushing it. Examples of the crushing device include an attrition type mill (horizontal medium stirring mill).

The crushing media refer to balls, rods, rollers (which come into contact with the members of the crushing device), etc., and the members of the crushing device include the container of the crushing device, the stirring propeller, the stirring rod,
A stirring disk, a roller (one that does not come into contact with the members of the crushing device), etc.

Using a crushing device having crushing media, crushing the raw materials by frictional force by crushing the raw materials in a state where the crushing media or the crushing media and the members of the crushing device rub each other. You can

That is, the crushing media or members of the crushing device rotate or vibrate, and the crushing media or the crushing media and the members of the crushing device rub against each other, so that the raw material particles existing in the crushing device are impacted. Force and shear force can be applied.

It is desirable that the impact force and the shearing force applied to the raw material particles are strong. By applying strong impact force or shearing force, independent ultrafine particles with less adhesion can be obtained. A centrifugal acceleration of about 10 to 100 times the gravitational acceleration is made more effectively by using the rotational force of a motor, for example, more effectively than a method in which the main acceleration applied to the crushing media is due to the natural fall due to gravity like a normal ball mill. A method utilizing a crusher of the type utilizing In particular, in the method using this crusher, if the centrifugal acceleration is 100 times or more the gravitational acceleration, the crushing media can crush the raw material particles more finely and more frequently,
desirable.

Also, in a crusher that does not use centrifugal force, for example, a crusher that uses vibration of balls, it is preferable to generate the same crushing force as described above to crush the raw material.

It is to be noted that it is better to prepare the grinding media or the member of the grinding apparatus from a compound such as a rare earth element and use the wear pieces produced by friction during grinding as a raw material for the compound such as a rare earth element. It is desirable because more oxide forms a solid solution.

50% by weight or more of particles have a particle size of 100 nm
In order to produce a powder consisting of particles in the following range, it can be achieved by pulverizing for a long time or by reducing the proportion of the pulverized raw material.

When using a normal powder, when crushing using a ball mill, crushing for 100 hours or more, and when crushing using a Dyno mill, crushing for 10 hours or more, 50% by weight or more is obtained. Particle size is 100 nm
A powder consisting of particles in the following ranges can be produced.

In the case of wet pulverization using a dyno mill, if the raw material concentration is 17 g / l or less, the pulverization is performed for 3 to 4 hours or longer. Except for the wet pulverization, the proportion of the raw material in the pulverizer is 150 g / l or less. Then, by pulverizing for 10 hours or more, it is possible to produce a powder composed of particles in which 50% by weight or more of the particles have a particle diameter of 100 nm or less.

Before the raw material is pulverized by using the above pulverizer, the raw material ceria powder is heat-treated in a reducing atmosphere, or when the ceria powder and a compound such as a rare earth element are pulverized by the pulverizer, the ceria powder is removed. It is better to heat-treat in a reducing atmosphere. By this process, at least a part of ceria becomes trivalent (CeO _2-x (0 <X <0.5)). If a compound such as a rare earth element is present in this state, an oxide such as a rare earth element is taken into the ceria crystal and solid solution is promoted.

In particular, when ceria is heat-treated in a reducing atmosphere, oxygen is released from the crystal particles of ceria,
Generate oxygen defects. It is considered that the presence of this defect further promotes solid solution of an oxide such as a rare earth element in ceria.

The heat treatment may cause grain growth and sintering of the raw material particles. However, since the grain growth and sintering of the raw material particles can be crushed by the crushing apparatus, the production can be performed. The specific surface area of the powder is small.

The heat treatment of the ceria powder in the above reducing atmosphere may be performed, for example, by the following method.

A furnace in which graphite is present, eg a heater,
The ceria powder is placed in a furnace in which the heat insulating material is made of graphite, or a furnace in which graphite powder is placed, and heat treatment is performed at a vacuum degree of not less than that obtained by an oil rotary pump (10 ^-2 torr). In the case of this method, it is desirable to obtain an even higher vacuum degree with an oil diffusion pump, a turbo molecular pump, or the like.

In a normal furnace, a reducing gas,
For example, CO gas, H ₂ gas, hydrocarbon gas such as CH ₄ , C ₂ H ₄ or the like is circulated or sealed, and the ceria powder is arranged and heat-treated.

The heat treatment temperature is 300 ° C. to 1200 ° C.
It is preferably in the range of ℃, more preferably 500-80
A range of 0 ° C is preferable. By preventing the grain growth and sintering of the raw material particles, it is possible to improve the efficiency of crushing using the crushing device. Therefore, the heat treatment temperature should be low.

When the ceria powder is heat-treated in a reducing atmosphere before being crushed, the ceria powder may be subjected to a reduction treatment in a reducing atmosphere, but the heat treatment is preferably performed in the following state.

Ceria powder and a compound such as a rare earth element are mixed by using an ordinary mixing device (ball mill or the like) so that the ceria powder and a compound such as a rare earth element are allowed to coexist, or a cerium salt and a rare earth element are mixed. It is preferable to heat-treat the ceria powder in a reducing atmosphere in the state where the ceria powder and the compound such as the rare earth element coexist by coprecipitating the compound such as. When heat treatment is performed under this condition, solid solution progresses during the heat treatment process. When heat-treating the ceria powder in a reducing atmosphere in the state where the ceria powder and the compound such as the rare earth element coexist by coprecipitating the cerium salt and the compound such as the rare earth element, first, the cerium salt in the process of coprecipitation Changes to cerium hydroxide and changes to ceria powder during the subsequent heat treatment.

When the ceria powder and the compound such as the rare earth element are coexisted in the reduced state of the ceria powder, the heat treatment may be performed under the same condition as the heat treatment of the ceria powder alone.

By heat-treating the ceria powder in a reducing atmosphere in this way, solid solution of ceria and oxides such as rare earth elements is promoted.

Further, it is preferable to add a reducing agent when the raw material powder is pulverized by the pulverizing apparatus. This reducing agent also reduces ceria and promotes solid solution of oxides such as rare earth elements in ceria. It is considered that the promotion of this solid solution is due to a mechanochemical phenomenon.

As the above-mentioned reducing agent, a reducing agent capable of at least partially reducing ceria to CeO _2−x (0 <X <0.5) is preferable. Specifically, methanol,
Examples thereof include alcohols such as ethanol and isopropanol, glycols such as ethylene glycol, glycerin, and aldehydes such as acetaldehyde. In addition,
When the powder obtained in the present invention is used as a catalyst,
A substance containing a considerable amount of a harmful substance, or a substance that produces a harmful substance by post-treatment such as drying or heat treatment at 1000 ° C. or higher is not suitable as a reducing agent.

As a method for forming a solid solution with another element in ceria, for example, when the other element is a metal or a salt softer than the metal, a method called mechanical alloying is used. In this method, a large mechanical force is applied to the raw material powder to mechanically change the composition of the particles constituting the powder, and the different elements are physically mixed.

However, in the method of adding a reducing agent as described above, the solid solution phenomenon of other elements in ceria is slightly different from the method of mechanical alloying. A substance that does not easily form a solid solution with simple force is promoted by utilizing the action of a reducing agent (mechanochemical phenomenon).

Report on coexisting system of ceria and zirconia (Solid State Ionics 3/4 (1981) p477
.About.481), at 1200.degree. C., about 20 mol% zirconia is solid-dissolved in the ceria while the tetragonal zirconia is about 20 mol% ceria. Zirconia 2
In the range of 0 to 80 mol%, cubic ceria solid solution and tetragonal zirconia solid solution coexist in two phases. 1200
At temperatures below ℃, the solid solution range of both gradually narrows.
At 1000 ° C. or lower, zirconia transforms into monoclinic crystal and maintains two-phase coexistence with cubic ceria.

However, in the present invention, if the ceria powder is heat-treated in a reducing atmosphere as described above, or if a reducing agent is present when the raw material is pulverized, part of the ceria is CeO 2.
The state becomes _2-X (0 <X <0.5). In this state, when ceria and an oxide such as a rare earth element form a solid solution with each other, the oxide of a rare earth element such as a tetragonal crystal forms a solid solution while the ceria keeps a cubic crystal, and takes the form of a solid solution having an arbitrary composition. be able to. Therefore, if the amount of solid solution increases and part of the ceria is in the state of CeO _2−x (0 <X <0.5), oxides of rare earth elements and the like will form solid solution at almost all rates.

When the ceria is reduced as described above, the ceria powder is preferably fine and porous particles in order to improve the reduction efficiency.

As a method for pulverizing the raw material powder, wet pulverization is preferable.

For example, using a dispersion medium such as ethanol, ceria powder and a compound such as a rare earth element are dispersed (made into a slurry state) in this dispersion medium and pulverized. A liquid reducing agent such as ethanol is preferable as the dispersion medium because it promotes solid solution of an oxide such as a rare earth element in ceria.

When milling by the wet method, the state of dispersing the raw materials is not particularly limited, but it is preferable that the compound such as rare earth element is finely dispersed than the ceria powder. In this state, the area in which the particles of the compound such as a rare earth element come into contact with the ceria particles is high, and moreover, the oxide of the rare earth element or the like is easily dissolved in the ceria.

The crushing time depends on the efficiency of crushing,
The crushing method with high crushing energy shortens the crushing time.

When the powder obtained by the present invention is used as a catalyst or the like by mixing it with a powder having a particle size on the order of micron such as normal gamma-alumina, it is preferable to carry out normal heat drying.

However, when used in combination with powders of other components in the form of ultrafine particles, as in the method for producing a catalyst developed by some of the present inventors (Japanese Patent Application No. 6-81951), after mixing, In order to avoid dry agglomeration, the powder is preferably combined (mixed) with the powder of other component in the form of ultrafine particles without being dried as it is. However, if it is unavoidable to dry the powder obtained in the present invention before combining it with the powder of the other components in the form of ultrafine particles, freeze drying, supercritical drying, or the like is performed, or the dry powder is made into a slurry. It is better to reconstitute, ultrasonically disperse or lightly regrind.

Production method of this catalyst (Japanese Patent Application No. 6-8195)
No. 1), a catalyst component, finely divided alumina powder,
It is a method for producing a catalyst by mixing a substance that prevents sintering of amylna particles such as finely divided silica powder, and a substance that does not form a solid solution or a compound by reacting with alumina that is added as necessary. The powder of the present invention can be used as a substance that does not react with the catalyst component or alumina to form a solid solution or a compound.

As described above, in the present invention, by adding the reducing agent, a larger amount of oxide such as a rare earth element can be dissolved in ceria.

Since the powder of the present invention has a sufficient solid solution of ceria and an oxide such as a rare earth element, it can be used as a catalyst, particularly a catalyst used at high temperature, an exhaust gas catalyst for automobiles and the like.

For example, when the powder of the present invention is used for an application (catalyst or the like) which requires a function of occluding and releasing oxygen, the following functions are exhibited.

The function of ceria in the powder of the present invention is
The oxygen partial pressure is adjusted by storing and releasing oxygen. It has a function as a co-catalyst that enhances catalytic activity by storing oxygen in an oxidizing atmosphere and releasing oxygen in a reducing atmosphere. In the catalyst, it is necessary to rapidly store and release the above oxygen. The particles constituting the powder of the present invention have a high specific surface area because ceria has a sufficient solid solution of an oxide of zirconia or a rare earth element and has a small particle size. Therefore, a high specific surface area can be maintained even when used at high temperatures, and oxygen can be occluded and released rapidly.

For example, when used as a catalyst, the catalyst can be prepared by mixing with alumina or the like as in the conventional ceria powder, and the obtained catalyst contains a rare earth element oxide or zirconia sufficiently in ceria. It has a high heat resistance because it is a solid solution, and it has high activity due to its large specific surface area. For example, the powder of the present invention can be used in a method for producing a catalyst developed by a part of the present inventors (Japanese Patent Application No. 6-81951).

The powder of the present invention has a large specific surface area. To measure the specific surface area, the BET method using a gas such as nitrogen or the mercury injection method is used to measure up to about 30Å. There is a method of calculating from the pore diameter and the pore volume.

Examples of the present invention will be described below.

The particle diameters of the particles shown in the following examples are those measured by a particle size distribution measuring instrument using the photon correlation method, and are the particle diameters of the particles having the highest existence frequency by weight ratio.

Example 1 1500 cc of ethanol was used as a dispersion medium (also serving as a reducing agent), and this ethanol was used together with primary particles having a particle size of 6 nm and secondary particles having a particle size of 10 nm.
μm and 100 g of ceria powder having a specific surface area of 100 m ² / g and a horizontal medium stirring mill (Dy manufactured by Willey A. Bachofen AG)
No mill) and crushed. During milling, the inner wall of the mill, balls and stirring blades should be made of zirconia.
The stirring blade was rotated at 000 rpm. Grinding was performed for 40 hours to produce a powder.

In this embodiment, the ceria powder and the zirconia are mixed by the fragments mixed into the ceria powder due to abrasion from the inner wall of the zirconia mill, the balls and the stirring blades.

(Example 2) In Example 1, zirconia powder 2 having a specific surface area of 50 m ² / g when crushing ceria powder 2
Pulverization was performed in the same manner as in Example 1 except that 0 g was added and the pulverization time was 25 hours to produce a powder.

Example 3 100 g of ceria powder having a primary particle diameter of 6 nm, a secondary particle diameter of 10 μm and a specific surface area of 100 m ² / g was dispersed in water, and this dispersed water and zirconyl nitrate dihydrate 90 g Of water. Next, ammonia was added to the mixed aqueous solution to neutralize it and dried. After that, the ceria powder in Example 1 was replaced with this dried product,
Pulverization was carried out in the same manner as in Example 1 except that the pulverization time was 16 hours to produce a powder.

Example 4 The inner wall, balls and stirring blades of the horizontal medium stirring mill in Example 3 were made of Sialon, the addition amount of zirconyl nitrate dihydrate was 155 g, and the grinding time was 50 hours, except that Pulverization was performed in the same manner as in Example 3 to produce a powder.

Example 5 100 g of ceria powder having a primary particle size of 6 nm, secondary particle size of 10 μm and a specific surface area of 100 m ² / g was placed in a furnace having a heater and an inner wall made of graphite.
It heat-processed at 700 degreeC for 2 hours, maintaining a vacuum using an oil diffusion pump. Then, the ceria powder in Example 1 was replaced with this heat-treated product, ethanol was replaced with 1500 cc of water, and crushing was performed in the same manner as in Example 1 except that the crushing time was 20 hours, to produce a powder.

(Example 6) In the heat treatment before crushing of Example 5, the dried product of Example 3 before crushing was used as a raw material.
The mixture was heat-treated in the same manner as in Example 5 except that the heat treatment temperature was 1100 ° C. Then, the raw material in Example 4 was replaced with the heat-treated product, and the pulverization was performed in the same manner as in Example 4 except that the pulverization time was 10 hours, to produce a powder.

Example 7 An aqueous cerium nitrate solution and an aqueous zirconyl nitrate dihydrate solution were mixed, and ammonia was added dropwise to neutralize the mixture to obtain a coprecipitate, which was further dried. Then, crushing was performed in the same manner as in Example 3 except that the dried material was used instead of the ceria powder in Example 3, to produce a powder.

Comparative Example 100 g of ceria powder having a primary particle diameter of 6 nm, a secondary particle diameter of 10 μm and a specific surface area of 100 m ² / g was impregnated with an aqueous zirconyl nitrate dihydrate solution, and then 700 atmosphere. Heat treatment was performed at 0 ° C. to produce a powder having a zirconia content of 20 mol%.

Powders having ratios of Ce and Zr other than those shown above in Examples 1 to 7 and Comparative Example were also manufactured.

(Evaluation Test of Powder) With respect to the powders obtained in Examples 1 to 7, the solid solution state of zirconium in ceria was observed by calculating the lattice constant from the shift of each diffraction peak by X-ray diffraction. Each of the powders was a solid solution of zirconia.

Further, the particle size of the powder before grinding and the obtained powder (the powder after grinding) of Example 3 were analyzed by the photon correlation method. The results are shown in FIG. 1 (ceria powder before crushing) and FIG. 2 (powder after crushing). 1 and 2
The frequency on the vertical axis of is the existence frequency according to the weight ratio measured by the particle size distribution analyzer using the photon correlation method. Further, powders other than that of Example 3 (powder of Examples 1, 2, 4 to 7) are also shown in FIG.
The particle size distribution was almost the same as.

It can be seen from FIGS. 1 and 2 that most of the particles are crushed by the crushing treatment, and most of them have a particle size of about 20 nm. As a result, all of the powders of Examples 1 to 7 had a particle size of 70% by weight or more and a particle size of 100 nm.
It was below.

As described in Examples 1 to 3, when the inner wall of the horizontal medium stirring mill, the balls and the stirring blades are made of zirconia, the zirconia powder generated by the abrasion of the inner wall, the balls and the stirring blades is Dissolved in ceria powder. Moreover, the amount of solid solution increased as the pulverization time increased.

Further, as in Examples 2 and 3, by adjusting the amount of zirconia powder or zirconyl nitrate dihydrate to be added, the solid solution amount of zirconia could be increased.

When the inner wall of the horizontal medium agitating mill, the balls and the agitating blades are made of sialon having high abrasion resistance as in Example 4, since the sialon is less mixed in the ceria powder, The amount of zirconia solid solution is less affected by the grinding time. Therefore, it was easy to set the composition of the powder to be produced.

In Example 4, zirconia powder could be used instead of zirconyl nitrate dihydrate.

The zirconia content in the powder obtained by changing the ratio of Ce and Zr in Example 1 (the content of zirconia when the total amount of ceria and zirconia in the powder is 100 mol%) 3) and the lattice constant of the powder are shown in FIG.

From FIG. 3, the relationship between the zirconia content and the lattice constant had a clear relationship according to the Regard's law, as indicated by the broken line. This indicates that almost all zirconia contained in the powder is in solid solution in ceria.

With respect to the powder obtained in Example 2, the pore distribution and specific surface area before and after pulverization were measured by the mercury porosimetry method. The results are shown in FIG. 4 (ceria powder before crushing) and FIG. 5 (powder after crushing). 4 and 5, the solid line shows the specific surface area, and the broken line shows the pore volume.

From FIGS. 4 and 5, it can be seen that the specific surface area is increased by 2.5 times due to the pulverization.

Further, in the powder of the comparative example, about 20% of the added zirconia was dissolved in ceria, and the remaining 80% was added.
Existed as a simple substance of zirconia without solid solution. The particle size of 95% by weight or more of the particles was 5000 nm or more.

Further, the powders of Example 3 and the comparative example (both the weight ratio of Ce and Zr in the powder is Ce: Zr = 5: 5) are mixed with 2.5 times by weight of gamma alumina. Then, 1 g of platinum and 0.2 g of rhodium were added per 100 g of gamma alumina. This produced two types of catalysts.

These catalysts were added to automobile exhaust gas in an amount of 10
After holding at 00 ° C for 5 hours (durability test), CO, NO,
The purification rate of hydrocarbons (HC) was measured. FIG. 6 shows the temperature at which the purification rate of CO, NO, and hydrocarbons (HC) reaches 50% (50% purification temperature).

From FIG. 6, it can be seen that the catalyst using the powder of this example has a lower 50% purification temperature of each component after the endurance test and higher activity than the catalyst using the powder of the comparative example. .

[Brief description of drawings]

FIG. 1 is a diagram showing a particle size distribution of powder before pulverization in an example of the present invention.

FIG. 2 is a diagram showing a particle size distribution of powder after pulverization in Examples of the present invention.

FIG. 3 is a diagram showing the relationship between the zirconia content in the powder of the example of the present invention and the lattice constant.

FIG. 4 is a diagram showing the relationship between the pore diameter, the specific surface area and the pore volume of the powder before pulverization in Examples of the present invention.

FIG. 5 is a diagram showing the relationship between the pore size, the specific surface area and the pore volume of the powder after pulverization in the examples of the present invention.

FIG. 6 is a diagram showing a 50% purification temperature of exhaust gas components after a durability test of catalysts using the powders of Examples of the present invention and Comparative Examples.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideo Sofukawa, Nagakute-cho, Aichi-gun, Aichi Prefecture, Nagakage 1 1st side street, Toyota Central Research Institute Co., Ltd. (72) Inventor Masayuki Fukui, Nagakute-cho, Aichi-gun, Aichi Prefecture No. 41 Nagamichi Yokomichi 1 Toyota Central Research Institute Co., Ltd.

Claims (1)

[Claims] 1. A particle comprising at least one oxide of rare earth elements other than cerium and zirconium, and ceria, which are solid-solved with each other, and 50% by weight or more of the particles have a particle size of 100 nm or less. A powder characterized by being.