JP3238316B2 - Exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst using particles containing a solid solution of cerium oxide (hereinafter referred to as ceria) and zirconia oxide (hereinafter referred to as zirconia), such as an automobile. The present invention relates to a catalyst for efficiently purifying harmful components such as carbon monoxide, hydrocarbons, and nitrogen oxides contained in exhaust gas. The exhaust gas purifying catalyst of the present invention is excellent in exhaust gas purifying activity, has little reduction in purification performance even after high-temperature durability, and is excellent in durability.

[0002]

2. Description of the Related Art Since ceria has an oxygen storage capability of storing and releasing oxygen, it is widely used as a promoter of an exhaust gas purifying catalyst for purifying exhaust gas from an internal combustion engine. For example, a three-way catalyst that oxidizes and purifies carbon monoxide and hydrocarbons contained in exhaust gas and reduces and purifies nitrogen oxides controls the air / fuel (A / F) ratio to approximately the stoichiometric air-fuel ratio. However, the air-fuel ratio may deviate from the stoichiometric air-fuel ratio in the transition region. In such a case, if a catalyst supporting ceria is used, the air-fuel ratio can be controlled close to the stoichiometric air-fuel ratio by the oxygen storage / release action of ceria, and the width (window) of the air-fuel ratio expands, thereby improving the purification performance. . Further, since the oxygen partial pressure is adjusted to be substantially constant, there is also an effect that sintering of a noble metal as a catalyst component is prevented. Ceria is used as a powder because it is desirable to increase the specific surface area in order to enhance the oxygen storage capacity.

However, the exhaust gas purifying catalyst must be used at a high temperature and have a high purifying activity after being exposed to the high temperature. Therefore, even when ceria is used with a high specific surface area as a powder,
It is required that the specific surface area be less reduced at the time of use at a high temperature, that is, the heat resistance is excellent. Therefore, it has been conventionally proposed to make a solid solution of a rare earth element oxide other than zirconia and cerium in ceria.

For example, Japanese Patent Application Laid-Open No. 3-131343 discloses an exhaust gas purifying catalyst in which a mixed oxide of ceria and zirconia is formed by utilizing a precipitation reaction from an aqueous solution, and a slurry containing the mixed oxide is applied to a monolithic carrier. A manufacturing method is disclosed. In this exhaust gas purifying catalyst, since zirconia is dissolved in ceria, it shows excellent oxygen storage capacity as compared with the case where ceria is used alone or the case where ceria and zirconia are mixed in a powder state, Excellent heat resistance due to the stabilizing action of zirconia.

[0005] For example, Japanese Patent Application Laid-Open No. 4-55315 discloses a method for producing cerium oxide fine powder by co-precipitating ceria and zirconia from a mixed aqueous solution of a water-soluble salt of cerium and a water-soluble salt of zirconium, and heat treating the coprecipitated ceria and zirconia. A method is disclosed. According to this production method, cerium and zirconium are converted into oxides by heat-treating the coprecipitate, and particles containing an oxide solid solution dissolved in each other are generated.

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

[0007]

SUMMARY OF THE INVENTION However, Japanese Patent Laid-Open No. 3-1
Even the exhaust gas purifying catalyst manufactured by the method disclosed in Japanese Patent No. 31343 is still not sufficient for the strict exhaust gas regulations in recent years. That is, according to the study of the present inventors, it is clear that the solid solution of ceria and zirconia produced by the method disclosed in the above publication has excellent heat resistance but insufficient oxygen storage capacity. Was.

That is, since the pH of the aqueous solution to be precipitated differs between cerium and zirconium, the composition of the coprecipitate is not uniform in the method of coprecipitation from the mixed aqueous solution, and almost no solid solution occurs at the coprecipitate stage. In addition, in the impregnation method, there are a method of impregnating the ceria powder with an aqueous solution containing a zirconium salt and a method of impregnating the zirconia powder with an aqueous solution of a cerium salt.In each case, the primary particles of the oxide powder are large, The composition becomes non-uniform as a whole, and solid solution is hardly promoted.

Therefore, in both methods, a solid solution is caused by heat treatment, and heat treatment is performed at a temperature of 500 to 900 ° C. in the coprecipitation method and 700 to 1200 ° C. in the impregnation method, but the solid solution is not sufficient. In both methods, a heat treatment of the powder forms a neck between the particles, and the neck promotes solid solution.Accordingly, grain growth and sintering are promoted, so that the specific surface area of the powder decreases, The diameter of the child also increases. Also, once solid solution particles have grown large, they cannot be easily crushed.

For example, the solid solution obtained by the production method disclosed in JP-A-4-55315 has a solid solubility of at most 40.
%. Further, according to the method disclosed in JP-A-4-284847, only a solid solubility of 20% or less can be obtained. In such a conventional ceria-zirconia-based oxide solid solution having a low solid solubility, the oxygen storage capacity (hereinafter referred to as OSC) due to ceria is 100 to 150 μmol O at most.
_If the temperature is as low as about ₂ and the temperature is not higher than 500 ° C., the ability to absorb and release oxygen is not sufficiently exhibited, and there is a problem that the oxygen storage ability is low.

If the heat treatment is sufficiently performed at about 1600 ° C., the solid solution of the solid solution of ceria and zirconia becomes almost 100%.
% Is known. However, in this case, the OS
Although C is high, the average particle size of the crystallites becomes 1000 nm or more and the specific surface area becomes small, so that the
The release rate is insufficient, making it impractical as a cocatalyst.

The present invention has been made in view of such circumstances, and by using a ceria-zirconia-based solid solution having a large OSC and a sufficiently high oxygen storage / release rate, it is possible to further improve the performance in recent years and in the future. It is an object of the present invention to provide an exhaust gas purifying catalyst having excellent purifying performance that can meet strict exhaust gas regulations.

[0013]

According to the first aspect of the present invention, there is provided an exhaust gas purifying catalyst comprising a carrier comprising particles containing an oxide solid solution in which ceria and zirconia are dissolved in each other. An exhaust gas purifying catalyst comprising a catalytic noble metal supported on a carrier, wherein the solid solubility S of zirconia in ceria in the particles is 50% or more, and the average diameter of crystallites constituting the particles is 10 nm. Below ,
The specific surface area of the particles is at least 45 m ² / g .
Here, the solid solubility S (%) is a value calculated from the following equation (1): S = 100 × (x / C) × [(100−C) / (100−x)] (1) C is the mole% of zirconium , x is the concentration% of zirconia dissolved in ceria calculated from the lattice constant obtained from X-ray diffraction by equation (2), and a in equation (2) is the lattice constant ( Angstrom). x = (5.423-a) /0.003 (2)

Further features of the exhaust gas purifying catalyst of the present invention described in claim 2 for solving the above problems includes a co-catalyst, the exhaust gas purifying catalyst comprising a noble metal supported on a carrier, the cocatalyst and ceria Zirconia is a particle containing an oxide solid solution in which the zirconia and the zirconia are dissolved in each other, and the solid solubility S of zirconia to ceria in the particle is 50% or more, and the average diameter of crystallites constituting the particle is 10 nm or less . Oh, grain
The specific surface area of the element is set to 45 m ² / g or more . Here, the solid solubility S (%) is a value calculated from the following equation (1): S = 100 × (x / C) × [(100−C) / (100−x)] (1) C is the mole% of zirconium , x is the concentration% of zirconia dissolved in ceria calculated from the lattice constant obtained from X-ray diffraction by equation (2), and a in equation (2) is the lattice constant ( Angstrom). x = (5.423-a) /0.003 (2)

Further, a carrier of heat-resistant inorganic particles (eg, alumina, silica, titania, zirconia, etc.) can be added and mixed. With this form, the ceria-zirconia solid solution particles are in a more highly dispersed state.

[0016]

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the invention of claim 1, the carrier is formed from particles in which ceria and zirconia are dissolved.
A carrier substrate in the form of a honeycomb or pellets may be formed from the ceria-zirconia solid solution particles themselves, or a heat-resistant inorganic or metal substrate such as cordierite is coated with the particles to form a carrier substrate. You can also.

The carrier according to claim 2 includes alumina (Al ₂ O ₃ ), silica (SiO ₂ ), silica-alumina, titania (TiO ₂ ), zirconia (ZrO ₂ ),
Examples thereof include zeolite, potassium titanate oxide, and composite oxides thereof. A carrier substrate may be formed from these carriers, or a heat-resistant inorganic material such as cordierite or a metal substrate may be coated with these powders to form a carrier substrate. The shape of the carrier substrate can be the same as the conventional one such as a honeycomb type or a pellet type.

In the invention of claim 2 , the mixture may be a mixture of a co-catalyst and a carrier supporting a noble metal (in this case, the noble metal may be supported on the co-catalyst), or The co-catalyst and the noble metal may be supported (in this case, the co-catalyst and the noble metal may be supported on the same carrier, or the co-catalyst and the noble metal may be supported on separate carriers). .

As a noble metal as a catalyst component, platinum (P
t), palladium (Pd), rhodium (Rh), silver (A
g), gold (Au), etc., and one or more of these can be used. Also, iron (Fe), molybdenum (Mo), tungsten (W), nickel (Ni),
Chromium (Cr), manganese (Mn), cobalt (C
o) and an oxide of a base metal such as copper (Cu) may be used in combination. An appropriate amount of the noble metal carried is about 0.1 to 10 g per liter of the catalyst volume. If the supported amount is smaller than this, the purification performance becomes insufficient, and if the supported amount is larger than this, the effect is saturated and a problem in terms of cost may occur.

The carrier according to the first aspect and the co-catalyst according to the second aspect, which are the features of the present invention, are particles containing an oxide solid solution in which ceria and zirconia are dissolved in each other. The solid solubility is 50% or more, and the average diameter of crystallites constituting the particles is 10 nm or less. The solid solubility refers to a value defined by the following equation from the peak shift of X-ray diffraction.

Solid solubility (%) = 100 × (amount of zirconia dissolved in ceria) / total amount of zirconia The solid solubility S (%) is calculated by equation (1). S = 100 × (x / C) × [(100−C) / (100−x)] (1) where C is mol% of zirconium, and x is a formula (2) from a lattice constant obtained from X-ray diffraction. Is the concentration% of zirconia dissolved in ceria calculated by the following formula.

Note that a in the following equation is a lattice constant (Angstrom
). x = (5.423-a) /0.003 (2) In the exhaust gas purifying catalyst of the present invention, the solid solubility in the solid solution particles
Is 50% or more. If the solid solubility is less than 50%,
OSC of about 150μmolO_Two/ G or less. I
However, if the solid solubility is 50% or more, the OSC is 250 to 8
00 μmol O _Two/ G or more, oxygen storage capacity
It is extremely excellent.

In the exhaust gas purifying catalyst of the present invention, the average diameter of crystallites in the solid solution particles is 10 nm or less. The size of the crystallite is calculated from the half-width of the X-ray diffraction peak using the following Scherrer equation. D = kλ / (βcos θ) where k: constant 0.9, λ: X-ray wavelength (Å), β: diffraction line width of sample−diffraction line width of standard sample (radian), θ: diffraction angle (degree) If the average diameter of the crystallites is 10 nm or less, the crystallites are not densely packed and are solid solution particles having pores between the crystallites. When the average diameter exceeds 10 nm, the pore volume and the specific surface area decrease, and the heat resistance also decreases. For the same reason, the specific surface area is desirably 45 m ² / g or more. In the case of a solid solution of ceria and zirconia, part of the cerium position is replaced by zirconium while maintaining the fluorite structure of ceric oxide to form a zirconia skeleton, and the zirconia forms a solid solution. I have.
Therefore, a cubic crystal structure is obtained, and the cubic crystal is maintained even if the solid solution discharges a large amount of oxygen. Although the mechanism is not clear, the transfer of oxygen is considered to be easy in the case of the cubic system, and the OSC is higher than that of other tetragonal systems or monoclinic systems.

Since the average diameter of the crystallites is as small as 10 nm or less, the grain boundaries between the crystallites become large, and the oxygen ions moving through the grain boundaries are easily moved. Thus, the oxidation-reduction reaction of CO, HC and NOx can be rapidly performed, and high purification performance can be obtained. When the specific surface area of the particles is 45 m ² / g or more, oxygen is absorbed and released on the surface of the particles. Therefore, if the specific surface area is large, the above action is further promoted.

In the solid solution particles of ceria and zirconia, the molar ratio of cerium to zirconium is 0.2%.
The range of 5 <Zr / (Ce + Zr) <0.75 is preferable, and the range of 0.4 <Zr / (Ce + Zr) <0.6 is particularly preferable. When the content of zirconium is 25 mol% or less, the action of forming a skeleton of zirconium in the solid solution crystal is weakened, and it becomes difficult to maintain a cubic crystal having a fluorite structure, and the OSC is reduced. In addition, since the ability to occlude and release oxygen depends on the valence of trivalent and tetravalent cerium, when the zirconium content is 75 mol% or more, the OSC is reduced due to a shortage of the absolute amount of cerium.

Furthermore, if solid solution particles of ceria and zirconia are used as a co-catalyst by mixing them with a carrier such as alumina, the solid solution particles are in a highly dispersed state, so that a high contact area with the exhaust gas is ensured and mutual interaction is ensured. Since the activity decrease due to sintering is less likely to occur, the solid solution particles can be stably held up to a high temperature. As a method for producing the solid solution particles of ceria and zirconia, a first method of obtaining a precipitate by adding a surfactant and an alkaline substance to an aqueous solution in which a cerium element and a zirconium element are dissolved is described.
There is a method of performing a step and a second step of heating the precipitate to obtain a powder containing particles of a solid solution in which ceria and zirconia are dissolved in each other.

In the first step, a precipitate is obtained by adding a surfactant and an alkaline substance to an aqueous solution in which a cerium compound and a zirconium compound are dissolved. Here, the action of the surfactant is not clear, but is presumed as follows. In other words, in the state just neutralized with the alkaline substance, the cerium element and the zirconium element precipitate in a state of a very fine hydroxide or oxide having a particle size of several nm or less. Conventionally, it is dried as it is, but in the present invention, plural kinds of precipitated particles are uniformly taken into micelles of the surfactant by adding the surfactant. As the neutralization proceeds in the micelle, the generation of particles proceeds in a small space in which a plurality of components are uniformly contained. Furthermore, the surfactant improves the dispersibility of the precipitated fine particles, reduces segregation, and increases the degree of contact. Thereby, the solid solubility can be increased and the average particle size of the crystallite can be reduced.

The surfactant may be added before the alkaline substance, at the same time as the alkaline substance, or after the alkaline substance. However, if the surfactant is added too late, segregation will occur. Therefore, it is desirable to add the surfactant at the same time as or before the addition of the alkali substance. Compounds of the elements that become oxides in aqueous solution include cerium (III) nitrate, ammonium cerium (IV) nitrate, cerium (III) chloride, cerium (III) sulfate, cerium (IV) sulfate, zirconium oxynitrate, Zirconium oxychloride and the like are exemplified. In addition, as the alkaline substance, any substance that exhibits alkalinity as an aqueous solution can be used. Ammonia, which can be easily separated on heating, is particularly desirable. However, other alkaline substances such as alkali hydroxide can be used because they can be easily removed by washing with water.

The surfactant is desirably a surfactant capable of forming a micelle to form a narrow space therein, for example, a spherical micelle. Further, those having a critical micelle concentration (cmc) of 0.1 mol / L or less are desirable. More desirably, a surfactant of 0.01 mol / L or less is desirable. Examples of these surfactants include alkylbenzene sulfonic acid and its salts, α-olefin sulfonic acid and its salts, alkyl sulfate acid, alkyl ether sulfate, phenyl ether sulfate, methyl taurate, and sulfosuccinate. , Ether sulfates, alkyl sulfates, ether sulfonates, saturated fatty acids, and salts thereof, unsaturated fatty acids such as oleic acid, and salts thereof, other carboxylic acids, sulfonic acids,
Anionic surfactants such as sulfuric acid, phosphoric acid, and phenol derivatives, polyoxyethylene polypropylene alkyl ether, polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene polystyryl phenyl ether, polyoxyethylene polyoxy Polypropylene alkyl ether, polyoxyethylene polyoxypropylene glycol, polyhydric alcohol; glycol; glycerin; sorbitol; mannitol; pentaethritol; sucrose; polyhydric alcohol, fatty acid partial ester, polyhydric alcohol; glycol; glycerin; Mannitol; pentaethritol; sucrose; etc. polyoxyethylene polyhydric alcohol fatty acid partial ester, polyoxyethylene fatty acid ester, poly Non-ionic surfactants such as xyethylenated castor oil, polyglycerin fatty acid ester, fatty acid diethanolamide, polyoxyethylene alkylamine, triethanolamine fatty acid partial ester, trialkylamine oxide, primary fatty amine salt, secondary fatty amine Salt, tertiary fatty amine salt, tetraalkylammonium salt; trialkylbenzylammonium salt; alkylpyridinium salt; 2-alkyl-1-alkyl-1
-Hydroxyethyl imidazolinium salt; N, N-dialkylmorpholinium salt; quaternary ammonium salt such as polyethylene polyamine fatty acid amide salt; etc .; cationic surfactant such as betaine compound; zwitterionic surfactant such as betaine compound It is at least one selected from agents.

The critical micelle concentration (cmc) is the lowest concentration at which a certain surfactant forms micelles. The addition amount of the surfactant is desirably in the range of 1 to 50 parts by weight based on 100 parts by weight of the oxide solid solution particles to be produced. When the content is 1 part by weight or more, the solid solubility is further improved. If it exceeds 50 parts by weight, the surfactant may not be able to effectively form micelles.

In the above-mentioned production method, attention must be paid to the valence of cerium. In the case of tetravalent cerium, ceria easily forms a solid solution with zirconia, and thus the solid solution particles according to the present invention can be produced by the above production method. However, in the case of trivalent cerium, since ceria hardly forms a solid solution with zirconia, it is desirable to employ another means.

Therefore, it is proposed that hydrogen peroxide coexist. Thereby, cerium (III) forms a complex with hydrogen peroxide and is oxidized to cerium (IV), so that ceria can be easily dissolved in zirconia. The timing of adding hydrogen peroxide is not particularly limited, and it may be before the addition of the alkaline substance and the surfactant, or may be added simultaneously with or after these. Hydrogen peroxide is a particularly desirable oxidizing agent since post-treatment is not required. However, depending on the case, other oxidizing agents such as oxygen gas, ozone and peroxide can be used.

Further, an aqueous solution in which the cerium element and the zirconium element are dissolved is added with an alkaline substance while stirring at a high shear rate of 10 ³ sec ^-1 or more, more desirably 10 ⁴ sec ^-1 or more, to precipitate the precipitate. It is also preferred to obtain. Although some degree of segregation of the components in the precipitated fine particles, which are the neutralized product, cannot be avoided, the segregation is uniform and the dispersibility is improved by vigorous stirring, so that the degree of contact between the cerium source and the zirconium source is further improved. improves.

For example, when coprecipitating from an aqueous solution of a cerium salt and a zirconium salt, the precipitated particles of the same type tend to be aggregated because of the different precipitation pH. Thus, by high-speed stirring at a high shear rate, a group of the same type of precipitated fine particles is destroyed, and the precipitated particles are well mixed. Also,
Even when trivalent cerium is used, since solid solution is promoted, it is not necessary to oxidize to tetravalent with hydrogen peroxide.

Therefore, according to the above manufacturing method, the solid solubility can be improved and the average particle size of crystallites can be further reduced. If the shear rate is less than 10 ³ sec ⁻¹ ,
The effect of promoting solid solution is not sufficient. Note that the shear rate V is V
= V / D. Here, v is the speed difference (m / sec) between the rotor and the stator of the stirrer, and D is the gap (m) between the rotor and the stator.

In the above production method, the element in the precipitate is turned into an oxide by heating the precipitate. This heating atmosphere may be any of an oxidizing atmosphere, a reducing atmosphere, and a neutral atmosphere. The reason why the element in the precipitate becomes an oxide is that oxygen contained in water or the like in the aqueous solution used as a raw material participates and oxidizes the element in the precipitate during heating. Therefore, an oxide can be obtained even when heated in a reducing atmosphere.

Further, as another method for producing solid solution particles of ceria and zirconia, a first step of obtaining a precipitate by adding an alkaline substance to an aqueous solution in which cerium element and zirconium element are dissolved, And heating the product in a reducing atmosphere to obtain a powder containing particles composed of an oxide solid solution in which a plurality of oxides are dissolved in each other.

In this method, the solid solubility of the oxide in the particles is high by heating the precipitate in a reducing atmosphere in the second step without adding a surfactant to the aqueous solution of the raw material.
In addition, oxide solid solution particles having a small average diameter of crystallites in the particles can be obtained. This is considered to be due to the following reasons. Since the heating atmosphere is in a reduced state, the cerium atom becomes trivalent in the oxide, oxygen vacancies are formed, and the interdiffusion of cerium and zirconium becomes easy, so that the solid solution of ceria and zirconia is easily promoted. Become. further,
The zirconium is regularly arranged, and the zirconia skeleton can be reliably formed in ceria. Further, in the heat treatment in the air, a temperature of 1600 ° C. or more is required to obtain a sufficient solid solution state.
Solid solution proceeds sufficiently at a heat treatment temperature of 300 ° C. or lower. Therefore, the size of the formed crystallite is reduced. But 90
When the reduction heat treatment at 0 to 1300 ° C. is performed, the crystallite size becomes larger than the heat treatment at a lower temperature.
In some cases, it is necessary to perform heat treatment at 250 ° C. or lower.

As the reducing atmosphere, carbon monoxide, hydrogen,
It is preferable that the gas contains a gas such as a hydrocarbon. As described above, even in a reducing atmosphere, an oxide contained in water or the like in an aqueous solution used as a raw material oxidizes elements in a precipitate during heating, so that an oxide is obtained.

As the reducing atmosphere, for example, when the reducing atmosphere is made of CO, the CO concentration is preferably 0.1 to 30%. Within this range, the solid solubility is further improved. In addition, as a method for collecting the precipitate, a method of drying the filter cake after squeezing most of the water from the precipitate with a filter press or the like, a method of spraying the precipitate and water with a spray drier or the like, and drying are exemplified. You. Drying of the precipitate can be performed promptly at 100 ° C. or higher,
Drying at 200 ° C. or more, and more preferably at 300 ° C. or more, is preferable because ammonium nitrate generated by the neutralization is easily decomposed and removed.

The method of heating the precipitate may be any method. The heating temperature is 150
The range of -600 ° C is desirable. If the heating temperature is lower than 150 ° C., it is difficult to obtain an oxide, and if the heating temperature is higher than 600 ° C., the obtained oxide may be sintered, and the particles may aggregate. When the drying temperature reaches the above-mentioned heating temperature (150 to 600 ° C.) when drying at the time of collecting the precipitate, the drying step and the heating step of the precipitate can be used in combination.

It is desirable that the obtained oxide solid solution particles be subjected to the following post-treatment. That is, it is preferable to heat-treat the oxide solid solution particles at 800 to 1300 ° C. in a reducing atmosphere (for example, in a state containing a gas such as carbon monoxide, hydrogen, and hydrocarbon) because the solid solution is promoted. For example, zirconia skeletons can be more reliably formed in ceria to increase OSC, which is preferable.

As described above, during the heat treatment in the reducing atmosphere, a part of the oxygen in the obtained solid solution is lost and a part of the cation (cerium) is reduced to a low valence (trivalent). It has become. But then in air about 300 ° C
If heated as described above, it easily returns to the original state. Therefore, in the cooling process after the heat treatment, a process of bringing the material into contact with air at a temperature of about 600 ° C. or lower to restore the original valence may be performed.

When the precipitate is heated by spray drying during heating, an oxide solid solution is obtained as a powder (dry matter). Further, in the case of using other heating methods, since the obtained oxide solid solution is a solid, the oxide solid solution particles can be obtained by dry grinding with a hammer mill, a ball mill, a vibration mill or the like. In addition, if necessary, a catalyst to which a rare earth oxide, an alkaline earth metal, an alkali metal, or the like is added can similarly improve the purification performance after durability.

[0046]

The present invention will be described below in detail with reference to examples and comparative examples. Note that the present invention is not limited to the following embodiments. (Example 1) An aqueous solution was prepared by mixing cerium (III) nitrate and zirconyl nitrate in a molar ratio of Ce / Zr = 5/5, and ammonia water was added dropwise with stirring to neutralize the precipitate. Generated. Subsequently, an aqueous solution containing hydrogen peroxide containing hydrogen peroxide of half the number of moles of cerium ions contained in the mixed aqueous solution and an aqueous solution containing alkylbenzenesulfonic acid at 10% by weight of the obtained oxide were added and mixed. Stirred.

The obtained slurry was sprayed in an atmosphere of an incoming gas of 400 ° C. and an outgoing gas of 250 ° C., dried by a spray drying method, and coexisted ammonium nitrate was evaporated and decomposed to prepare an oxide solid solution powder. When the solid solubility of this oxide solid solution was calculated from the equation (1) using the lattice constant by X-ray diffraction, the solid solubility was 100%. When the average diameter of the crystallite was calculated from the 311 peak of the X-ray diffraction pattern by the Schuler's formula, the average diameter of the crystallite was 10 nm. Further, the specific surface area of the oxide solid solution powder measured by the BET method was 45 m ² / g.

500 g of the obtained oxide solid solution powder and L
5 g of a was added and 1000 g of heat-stabilized γ-alumina powder (BET specific surface area 140 g / m 2), 100 g of boehmite, and 1000 g of a 10% nitric acid aqueous solution were stirred and mixed in an alumina ball mill for 1 hour to form a slurry. A cordierite honeycomb carrier base material (1.7 liters) is immersed in the slurry, pulled up to blow off excess slurry, and then heat-treated at 600 ° C. for 2 hours to form a supporting layer containing a promoter. A honeycomb carrier was formed. The amount of the slurry attached was adjusted to 250 g by dry weight.

Next, the honeycomb carrier having the carrier layer was immersed in an aqueous solution of dinitrodiammine platinum, pulled up and wiped off excess water droplets, dried at 250 ° C. for 2 hours, then immersed in an aqueous solution of rhodium nitrate and pulled up. After wiping off excess water droplets, drying at 250 ° C. for 2 hours, Pt and R
h was carried to prepare an exhaust gas purifying catalyst of this example.
The supported amount of the catalytic noble metal is 1 g of Pt and 0.2 g of Rh per liter of the honeycomb carrier. Comparative Example 1 Cerium (III) nitrate and zirconyl nitrate were mixed at a molar ratio of Ce / Zr = 8/2,
An exhaust gas purifying catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that no surfactant was used. In addition,
The solid solubility of the obtained oxide solid solution particles was 38%, the average diameter of crystallites was 7 nm, and the specific surface area of the oxide solid solution powder measured by the BET method was 70 m ^2.
/ G. (Test / Evaluation) The oxide solid solution particles obtained during the production of the exhaust gas purifying catalysts of Example 1 and Comparative Example 1 were each measured for OSC. The measurement of OSC was obtained by repeating the oxidation-reduction of the sample by flowing hydrogen and oxygen alternately using a thermogravimetric analyzer and measuring the weight change at that time. As a result, the OSC of the oxide solid solution of Example 1 was 4
While the OSC of the oxide solid solution of Comparative Example 1 was 50 μmol O ₂ / g, it was a considerably low value of 150 μmol O ₂ / g. This is apparently due to the difference in solid solubility.

Each of the above two types of exhaust gas purifying catalysts was attached to the exhaust system of the engine, and the incoming gas temperature was 850 ° C.
Endurance test was conducted for 50 hours under the following conditions. And
For each catalyst after the durability test, the same engine as in the durability test was used, A / F = 14.6, and the input gas temperature was 40.
The HC, CO, and NO _x purification rates were measured at 0 ° C.
Table 1 shows the results.

[0051]

[Table 1] Table 1 shows that the exhaust gas purifying catalyst of Example 1 has a higher purification rate than Comparative Example 1 and is excellent in purification performance after durability. This is apparently due to the difference between the solid solubility of the promoter and the particle size of the crystallite. (Example 2) Exhaust gas purification of Example 2 was carried out in the same manner as in Example 1 except that only the oxide solid solution powder formed in Example 1 was slurried and adhered to a honeycomb carrier to form a support layer. A catalyst was prepared.

[0052] The similarly for the exhaust gas purifying catalyst of Example 1 HC, CO, was measured _the NO _x purification rate, showed comparable purification performance as in Example 1.

[0053]

According to the exhaust gas purifying catalyst of the present invention, excellent purification performance is exhibited by high OSC, and excellent purification performance which can meet recent and future stricter exhaust gas regulations. ing.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Tadashi Suzuki 41 Toyoda Central Research Institute, Inc., Nagakute-cho, Aichi-gun 41, Yokomichi, Toyota Central Research Institute, Inc. (72) Inventor Yoshio Ukyo 41, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Yokomichi 41, Toyoda Central Research Institute, Inc. (1) Inside the Toyota Central R & D Laboratories Co., Ltd. (72) Inventor Norio Kimura No. 41 at the Toyota Central R & D Laboratories, Nagakute-cho Yo 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Yasuo Ikeda Toyo, Toyota City, Aichi Prefecture No. 1 in Tamachi Inside Toyota Motor Corporation (56) References JP-A-9-40425 (JP, A) JP-A-8-215569 (JP, A) JP-A-8-333116 (JP, A) Carla de Leitenburg, et al. "A Novel and Simple Route to Catalysts with a High Oxgen Storage Capacity: the Direct Room-Temperature Nature of Ceremony of the Year of the Union of Cereals, O2-ZrO2Sol. CH EM. SOC. , CHEM. COMMU N. , No. 21, p. 2181-2182 (58) Fields investigated (Int.Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/86 B01D 53/94 C01F 17/00

Claims (2)

(57) [Claims] 1. A catalyst for purifying exhaust gas, comprising: a carrier comprising particles containing an oxide solid solution in which cerium oxide and zirconium oxide are dissolved in each other; and a noble metal carried on the carrier. The solid solubility S of the zirconium oxide with respect to the cerium oxide is 50% or more, and the average diameter of crystallites constituting the particles is 10 nm or less , and a ratio table of the particles
An exhaust gas purifying catalyst having an area of 45 m ² / g or more . Here, the solid solubility S (%) is a value calculated from the following equation (1): S = 100 × (x / C) × [(100−C) / (100−x)] (1) C is the mole% of zirconium , x is the concentration% of zirconia dissolved in ceria calculated from the lattice constant obtained from X-ray diffraction by equation (2), and a in equation (2) is the lattice constant ( Angstrom). x = (5.423-a) /0.003 (2) 2. An exhaust gas purifying catalyst comprising a co-catalyst and a noble metal supported on a carrier, wherein the co-catalyst is a particle containing an oxide solid solution in which cerium oxide and zirconium oxide are dissolved in each other. The solid solubility S of the zirconium oxide with respect to the cerium oxide in the particles is 50% or more, the average diameter of crystallites constituting the particles is 10 nm or less , and the specific surface area of the particles is 4 % or less.
An exhaust gas purifying catalyst characterized by being at least 5 m ² / g . Here, the solid solubility S (%) is a value calculated from the following equation (1): S = 100 × (x / C) × [(100−C) / (100−x)] (1) C is the mole% of zirconium , x is the concentration% of zirconia dissolved in ceria calculated from the lattice constant obtained from X-ray diffraction by equation (2), and a in equation (2) is the lattice constant ( Angstrom). x = (5.423-a) /0.003 (2)