JP2003313011A - Method for producing metal oxide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a metal oxide suitable for use as a catalyst carrier of a catalyst for cleaning of exhaust gas of an internal combustion engine and maintaining a large specific surface area with age and to provide a method for producing a multiple metal oxide having a widely selective composition and a large specific surface area. <P>SOLUTION: In the method for producing a metal oxide, a solution prepared by dissolving at least one metal compound is impregnated into a porous material having pores and reducible to ashes, and after firing in a non-oxidizing atmosphere the porous material is burned and removed. Preferably the pores have 2-40 nm average pore diameter. When the multiple metal oxide is produced, preferably a solution prepared by dissolving at least two metal compounds is impregnated into the above porous material and the porous material is exposed to gaseous ammonia and fired in a non-oxidizing atmosphere. In the case where the metal compound is an aluminum compound, α-alumina is produced. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a metal oxide, and more particularly to a method for producing a fine metal oxide suitable for a catalyst carrier such as an exhaust gas purifying catalyst, and an exhaust gas purifying catalyst. .

[0002]

2. Description of the Related Art A catalyst for accelerating the reaction of reaction components contained in a gas is generally constructed by supporting a catalyst component and the like on a catalyst carrier. For example, exhaust gas emitted from an internal combustion engine such as an automobile engine decomposes nitrogen oxides (NO _x ) which are reaction components contained in the exhaust gas, and decomposes hydrocarbons (HC) and carbon monoxide (CO). It is purified by a burning three-way catalyst, and is also purified by a storage-reduction type NO _x purification catalyst having a function of absorbing NO _x in a lean atmosphere added to a conventional three-way catalyst and having an enhanced NO _x purification capability. These exhaust gas purifying catalysts generally include a catalyst carrier such as γ-alumina, a catalyst component of a noble metal such as platinum, and, in some cases, NO.
_x It is configured to carry a co-catalyst component such as an occlusion agent.

[0003] Here, in order for the catalyst to efficiently promote the purification of the reaction components, it is necessary for the catalyst to provide a high surface area to the gas so that the gas contacts the catalyst efficiently. The high surface area is generally formed by supporting a catalyst component or the like on a catalyst carrier having a high specific surface area. And this high surface area needs to be maintained over time under the conditions in which the catalyst is used.

For example, in the case of a catalyst for purifying exhaust gas of an automobile engine, the temperature repeatedly fluctuates between room temperature and about 1000 ° C., and the concentration of HC and CO is relatively high, and O ₂
This high surface area must be maintained under conditions where a reducing atmosphere with a low concentration and an oxidizing atmosphere with a relatively low HC and CO concentration and a high O ₂ concentration are repeated.

Therefore, for the catalyst carrier of the exhaust gas purifying catalyst, a material having a high specific surface area which is durable even under such severe conditions is selected. Normally, γ-alumina having a specific surface area of about 180 m ² / g is selected. Is used. At present, this γ-alumina is regarded as a catalyst carrier material of an exhaust gas purifying catalyst having a high specific surface area and excellent durability in the best balance.

[0006]

However, even in such γ-alumina, the stability of the specific surface area at high temperature is not sufficient, and when it is exposed to high temperature for a long time, the crystal structure changes and the specific surface area also changes. Is reduced. Here, the high temperature type crystal structure of alumina is α type, and if it is α-alumina, further change in crystal structure does not occur and it is predicted that the specific surface area will be stable even at high temperature.

Therefore, it is desired to provide α-alumina having a high specific surface area such as γ-alumina. However, it is extremely difficult to produce α-alumina having such a high specific surface area by usual grinding or the like. Is.

On the other hand, it has been studied to improve durability by including a plurality of elements in an oxide. For example, JP-A-10-182155 includes a step of forming an oxide precursor from a salt solution. The composite metal oxides produced are described.

On the other hand, the specific composite metal oxide has not only durability but also functionality such as oxygen storage performance, oxygen ion conductivity, acid-base point, and adsorptivity. For this reason,
If a catalyst carrier having these functionalities is combined with a catalyst component such as platinum, there is a possibility that an exhaust gas purification catalyst with significantly improved exhaust gas purification performance can be provided. Therefore, the present applicants have proposed in Japanese Patent Application No. 2001-165418 a catalyst carrier having a high specific surface area, which is manufactured by using a supercritical fluid.

The present invention uses a metal oxide which can be used as a catalyst carrier of an exhaust gas purifying catalyst for an internal combustion engine and can maintain a high specific surface area with time by a method completely different from those of the prior art. Another object of the present invention is to provide a method for producing a composite metal oxide having a high specific surface area and having a composition that can be selected over a wide range.

[0011]

The above object is to impregnate a burnable porous material having pores with a solution in which at least one metal compound is dissolved, and after firing in a non-oxidizing atmosphere, This is achieved by a method for producing a metal oxide, which comprises burning and removing the porous material.

That is, the method of the present invention comprises impregnating a solution of a metal compound to deposit the metal compound in each of the pores of the porous material, followed by firing in a non-oxidizing atmosphere. A target metal oxide is obtained by producing a metal oxide or a precursor thereof and burning and removing the porous material.

The precipitation of the metal compound in the pores can be carried out by impregnating and then drying the solution when the metal compound is one kind. In this case, each of the pores serves as a drying container, and the metal compound can be deposited as fine particles significantly smaller than the pore size.

When a plurality of kinds of metal compounds are used for the purpose of producing a composite metal oxide, it is preferable that the porous material be impregnated with an aqueous solution in which a plurality of metal compounds are dissolved and then dried. The porous material is exposed to ammonia gas and a plurality of metal compounds are coprecipitated from the aqueous solution in the pores. In this case, each of the pores serves as a reaction container, and a plurality of metal compounds can be simultaneously deposited as fine particles having a size significantly smaller than the pore size.

In the subsequent firing, in each case where one kind or a plurality of kinds of metal compounds are used, each of the pores serves as a firing container, and the metal oxide is deposited in a state where the precipitation of the metal compound is suppressed. Alternatively, a precursor thereof can be produced. The firing temperature is preferably set to a temperature not higher than a temperature at which a hydroxide that is a precursor of a metal oxide is generated and a solvent such as water is evaporated. Thus, by burning away the porous material and heating it at an appropriate temperature, a metal oxide having a high temperature type crystal structure can be obtained in a fine state.

As described above, according to the present invention, the fine pores of the porous material are used as a drying container or a reaction container, and further as a baking container to produce the desired fine metal oxide. . Preferably, by selecting a porous material having an average pore diameter of 2 to 40 nm, it is possible to obtain a metal oxide having a high specific surface area suitable as a catalyst carrier for an exhaust gas purification catalyst.

The method for producing a metal oxide of the present invention is
Although it differs from the above-mentioned method using the “supercritical fluid” in the prior art in that the “solution” in which the metal compound is dissolved is used, the method using the supercritical fluid is the pores of the porous material. While a metal oxide equivalent to the pore size obtained by transferring the shape of is obtained, the method utilizing the solution of the present invention is a method of precipitating a metal compound in the pores, and The obtained metal oxide is also different in that a remarkably fine metal oxide can be obtained.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION In the method of the present invention, first, a burnable porous material having pores is impregnated with a solution in which at least one metal compound is dissolved. This burnable porous material can be selected from various activated carbons, carbon nanotubes, carbon nanohorns, Ketjen black (hollow carbon black, Ketjen Black), etc., and preferably has an average pore diameter of 2 to 40 nm. Those that have are used.

The at least one metal compound can be a compound of various metal elements, including s-block metal elements, d-block metal elements, p-block metal elements, f-
It can be selected from a wide range of block metal elements, specifically, sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lanthanum (La), yttrium (Y). ), Cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (S
m), europium (Eu), gadolinium (Gd), titanium (Ti), tin (Sn), zirconium (Zr), manganese
(Mn), iron (Fe), cobalt (Co), nickel (Ni),
Chromium (Cr), Niobium (Nb), Copper (Cu), Vanadium
(V), molybdenum (Mo), tungsten (W), zinc (Z
n), aluminum (Al), silicon (Si), tantalum (Ta), or the like.

Preferably, the at least one metal compound is a metal oxide suitable as a catalyst carrier for an exhaust gas purification catalyst, for example, a metal oxide such as alumina, silica or zirconia, and silica-alumina, zirconia-ceria, It is a compound of a metal forming a composite metal oxide such as alumina-ceria-zirconia, ceria-zirconia-yttria, and zirconia-calcia. By converting these metals into compounds such as acetates, nitrates, carbonates, chlorides, alkoxides and acetylacetonates, a solution dissolved in a suitable solvent can be formed.

In the method of the present invention, the above-mentioned burnable porous material is impregnated with a solution of these metal compounds. This impregnation is carried out, for example, by dispersing the porous material in a solution of the metal compound under normal pressure to form a slurry and, if necessary, depressurizing the porous material to degas the porous material, and then to the inside of the pores of the porous material. This can be carried out by immersing the solution, and then by drying, the metal compound can be precipitated in the pores of the porous material.

When the metal compound is one, the solvent of the solution is
A solvent suitable for such impregnation and drying is sufficient, and it can be substantially any solvent capable of dissolving the metal compound. Here, when a composite metal oxide is produced using a plurality of types of metal compounds, preferably, the solvent is water, and the porous material is impregnated with a solution in which at least two types of metal compounds are dissolved, and then the porous material is used. Are exposed to ammonia gas to coprecipitate a plurality of metal compounds in the pores of the porous material. That is, by exposing the solution to ammonia gas, the pH of the solution impregnated into the pores is adjusted to a pH at which co-precipitation of a plurality of metal compounds occurs in the pores, whereby a precipitate containing finely mixed metals is obtained. Can be obtained. Then it is dried and the solvent is evaporated.

The precipitate thus obtained is
In a non-oxidizing atmosphere, that is, an atmosphere in which the porous material is not burned out.
It is fired in the atmosphere. This atmosphere is N₂Gas, CO₂Moth
Su, H ₂It can be gas, vacuum or the like. Firing temperature
Is appropriately selected according to the intended metal oxide,
Preferably, the firing temperature is preferably a metal oxide.
Produces hydroxide, which is a precursor of, and evaporates water and other solvents
The temperature is set to the temperature or lower. Then burn off the porous material
By doing so, the high-temperature type crystal structure metal oxide can
Can be obtained in the state. Such high temperature type crystal structure metal
Oxides are generally exposed to exhaust gas for long periods of time.
However, since the crystal structure is stable, the specific surface area decreases.
There is no such thing.

The obtained metal oxide can be used as a catalyst carrier as it is or after subjecting it to mild crushing if necessary. Preferably, the metal oxide produced by such a method is platinum (Pt), palladium (Pd),
A catalytic metal such as rhodium (Rh) is carried and used as an exhaust gas purifying catalyst. As a general method, the catalyst metal is supported by platinum dinitrodiammine Pt (NH ₃ )
₂ (NO ₂ ) ₂ , palladium nitrate Pd (NO ₃ ) ₂ , rhodium nitrate Rh (NO ₃ ) _{3 and the} like are impregnated with a solution of a metal compound, which is then dried and fired. You can Hereinafter, the present invention will be described more specifically with reference to Examples.

[0025]

Example 1 100 g of aluminum nitrate anhydrous hydrate Al (NO ₃ ) ₃ was dissolved in 200 g of ion-exchanged water to obtain 100 g of activated carbon A.
(Specific surface area 718 m2 / g, pore volume 0.66 cc / g,
The peak pore size of 6.5 nm) was impregnated with 75 g of this solution. Next, the activated carbon A impregnated with aluminum nitrate was dried at 120 ° C. for 5 hours, and then baked at 400 ° C. for 2 hours in an N ₂ gas atmosphere containing ₂ % by volume of H ₂ gas. Then, the activated carbon was burned and removed by heating at 600 ° C. for 3 hours in the air atmosphere, and then the heating was performed at 1200 ° C. for 3 hours in the air atmosphere. The powder thus obtained was confirmed to be α-alumina by powder X-ray diffraction, and the specific surface area by BET method was 286 m ^2.
/ G. Further, this α-alumina powder had a particle size distribution shown in FIG. 1 as a result of measurement by image analysis.

Example 2 Activated carbon A in Example 1 was replaced with activated carbon B (specific surface area 794).
m2 / g, pore volume 1.38 cc / g, peak pore diameter 13
nm-) was used to obtain α-alumina powder in the same manner as in Example 1. This α-alumina has a specific surface area of 122
m ² / g, and as a result of measurement by image analysis, it had a particle size distribution shown in FIG. 1. 2 shows the measurement results of the pore size distribution based on the BJH method using a pore size distribution measuring device Asap 2000 (sold by Shimadzu Corporation) for activated carbons A and B.

Comparative Example 1 100 g of aluminum nitrate anhydrate was dissolved in 200 g of ion-exchanged water, 75 g of this solution was dried, and then heated in an air atmosphere at 250 ° C. for 2 hours, then in the air. Heating was performed at 1200 ° C. for 3 hours in the atmosphere. The powder thus obtained has an α value determined by powder X-ray diffraction.
-Alumina was confirmed, and the specific surface area by the BET method was 1.6 m ² / g. The α-alumina powder had an average particle size of more than 300 nm as a result of measurement by image analysis.

Example 3 108.6 g of cerium nitrate Ce (NO ₃ ) ₃ .6H ₂ O and 56.3 g of zirconium nitrate Zr (NO ₃ ) ₂ .2H ₂ O were dissolved in 200 g of ion-exchanged water to obtain 100 g. 75 g of this solution was impregnated into activated carbon A above. Then
Activated carbon A impregnated with this solution was exposed to N ₂ gas at 20 ° C. containing 10% by volume of NH ₃ gas for 2 hours, and then 120 ° C.
It was subjected to drying for 5 hours. Then, in the atmosphere, 60
After heating at 0 ° C for 3 hours to burn off the activated carbon,
The heating was performed at 800 ° C. for 3 hours.

The powder thus obtained was confirmed to be a cerium-zirconium composite oxide by powder X-ray diffraction, and the specific surface area by the BET method was 205.1 m ² / g.
Met. The cerium-zirconium composite oxide powder had a particle size distribution shown in FIG. 1 as a result of measurement by image analysis, and had an average particle size of 4.5 nm.

-Catalyst Performance Evaluation- The α-alumina powder of Example 1 was impregnated with an aqueous dinitrodiammine platinum solution, dried at 120 ° C for 2 hours, and then calcined at 500 ° C for 2 hours in the atmosphere, Catalyst A of the present invention was prepared by supporting 2 parts by mass of Pt on 98 parts by mass of α-alumina. Similarly, supporting Pt on α-alumina of zirconium and α-alumina of Comparative Example 1,
The catalyst B of the present invention and the catalyst a of the comparative example were prepared. Similarly, 98 parts by mass of γ-alumina was loaded with 2 parts by mass of Pt to prepare a catalyst b of a comparative example.

Each of these catalysts is compressed and crushed to have a diameter of 1 to
A 2 mm pellet was prepared, and 1 g of each catalyst sample was subjected to temperature characteristic evaluation by a fixed bed flow reactor. After placing each catalyst sample in a fixed bed flow reactor, the catalyst bed temperature was raised at a rate of 10 ° C./minute in an atmosphere in which the following lean gas and rich gas were switched at a flow rate of 7 liters / minute every 1 second. While doing so, the 50% purification temperature of HC (C ₃ H ₆ ) was measured. Also, each catalyst sample was placed in a fixed bed flow reactor, subjected to a durability treatment of heating at a catalyst bed temperature of 1000 ° C. for 5 hours in a model gas atmosphere corresponding to A / F = 14.6, and then similarly. The 50% purification temperature of HC (C ₃ H ₆ ) was measured. The results are summarized in Table 1 as the initial stage and after the endurance treatment.

Rich gas composition: 0.15% C ₃ H ₆ + 1.05% CO + 0.33% O ₂
+ 0.3% NO + 0.35% H ₂ + 14.19% CO ₂
+ 10.0% H ₂ O (remainder: nitrogen) Lean gas composition: 0.05% C ₃ H ₆ + 0.14% CO + 0.94% O ₂
+ 0.34% NO + 14.27% CO ₂ + 10.0% H
₂ O (remainder: nitrogen)

[0033]

[Table 1]

From the results shown in Table 1, the catalysts A and B of the examples are
It can be seen that the 50% purification temperature of HC is considerably lower both in the initial stage and after the endurance treatment than the catalysts a and b of the comparative example. Also,
Comparing the BET specific surface areas of the catalyst after the initial treatment and after the durability treatment, the catalyst of the present invention shows substantially no change, whereas the catalyst b of the comparative example shows a remarkable decrease, and the catalyst a of the comparative example shows that It can be seen that the BET specific surface area is extremely lower than in the initial stage.

From these results, by using α-alumina having a high specific surface area obtained by the method of the present invention as the catalyst carrier, the high specific surface area of the catalyst carrier is maintained even after the durability treatment, whereby the durability of the catalyst carrier after the durability treatment is maintained. It is judged that the high catalyst performance is maintained.

[0036]

The present invention provides a method for producing a metal oxide suitable for a catalyst carrier of an exhaust gas purifying catalyst for an internal combustion engine, which maintains a high specific surface area over time, and has a specific surface area having a composition that can be selected over a wide range. It is possible to provide a method for producing a high composite metal oxide.

[Brief description of drawings]

FIG. 1 is a graph showing a particle size distribution of a metal oxide obtained by the method of the present invention.

FIG. 2 is a graph showing the pore size distribution of activated carbon.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C01F 7/30 B01D 53/36 104A C01G 25/00 B01J 23/56 301A (72) Inventor Yusuke Ito Aichi 1 Toyota Town, Toyota City, Toyota Motor Co., Ltd. (72) Inventor, Akihiko Suda, Aichi District, Nagakute Town, Aichi District, Aichi Prefecture, No. 41, Yokoshiro, Toyota Central Research Institute Co., Ltd. (72) Inventor, Hideo Sofugawa, Aichi Prefecture 1 in 41 Yokomichi, Nagakute-machi, Aichi-gun 1 in Toyota Central Research Laboratory Co., Ltd. (72) Inventor Naoki Takahashi 1 in 41, Nagakute-machi, Nagakute-machi, Aichi-gun 1 in Toyota Central Research Laboratory F-term ( Reference) 4D048 AA06 AA13 AA18 AB05 BA01X BA08X BA19X BA30X BB01 BB17 4G042 DA01 DA02 DB10 DB26 DB31 DC03 DD06 DE06 DE12 4G048 AA03 AB02 AB06 AC08 AD04 AD06 AE07 4G069 AA01 AA03 AA08 BA01A BA01B BA08C BB02A BB02B BB06A BB06B BC43A BC43B BC51A BC51B BC75A BC75B CA03 CA09 EA01Y EB01 BA01 FA02 BA01 BA02 BA02 FA02 AB02 FC02 AB02 A02A02 A02A02 A02

Claims (6)

[Claims] 1. A method of impregnating a burnable porous material having pores with a solution in which at least one metal compound is dissolved, firing the material in a non-oxidizing atmosphere, and then burning and removing the porous material. A method for producing a metal oxide, comprising: 2. The method for producing a metal oxide according to claim 1, wherein the pores have an average pore diameter of 2 to 40 nm. 3. The porous material is impregnated with a solution in which at least two metal compounds are dissolved, the porous material is exposed to ammonia gas, and then fired in a non-oxidizing atmosphere. The method for producing a metal oxide according to 1. 4. The method for producing a metal oxide according to claim 1, wherein the metal compound is an aluminum compound and the metal oxide is α-alumina. 5. The porous material is activated carbon.
5. The method for producing a metal oxide according to any one of 4 to 4. 6. An exhaust gas purifying catalyst comprising a metal oxide produced by the method according to any one of claims 1 to 5 and a catalytic metal supported on the metal oxide.