JPH1119514A - Catalyst for cleaning lean exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for cleaning lean exhaust gas by which ceria/ zirconia solid solution in lean exhaust gas is inhibited from being poisoned by sulfur and durability is enhanced. SOLUTION: The catalyst for cleaning lean exhaust gas is constituted of oxide solid solution particles, in which solid solubility of zirconium oxide for cerium oxide in ceria/zirconia solid solution is >=50% and average diameter of crystallite is <=10 nm and furthermore ratio of zirconium is a range of 0.55<=Zr/(Ce+Zr)<=0.90 as mole ratio, of catalytic noble metal and a fire resistant porous body. Poisoning caused by sulfur is prevented because the amount of cerium easily forming sulfate is relatively little and solid solution phase is thermodynamically stabilized and sulfate is made difficult to be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lean exhaust gas capable of efficiently removing harmful substances contained in a lean exhaust gas, such as an exhaust gas from a diesel engine, which contains oxygen in excess of an amount necessary for oxidizing oxidizable components. It relates to a purification catalyst.

[0002]

2. Description of the Related Art Regarding gasoline engines, harmful substances in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of coping with it. However, in the case of diesel engines, regulations and development of technology have been delayed compared to gasoline engines due to the unique circumstances that harmful components are mainly emitted as particulates, and the development of exhaust gas purification catalysts that can reliably purify harmful substances has been developed. Is desired.

[0003] The diesel engine exhaust gas purifying catalysts which have been developed so far are roughly classified into trap catalysts and open type SOF (Soluble Organic Fractio).
n) Decomposition catalysts are known. Of these, the trap catalyst captures diesel particulates and regulates their emission, and is particularly effective for exhaust gas with a high dry suit ratio. However, trapping catalysts require regeneration equipment to incinerate trapped diesel particulates, leaving many practical problems, such as cracking of the catalyst structure during regeneration, clogging by ash, or complicated systems. ing.

On the other hand, an open-type SOF cracking catalyst, as disclosed in, for example, JP-A-3-38255, uses a catalyst in which a catalytic noble metal such as a platinum group metal is supported on a supporting layer of activated alumina or the like as in a gasoline engine. And CO
SOF is purified by oxidative decomposition together with (carbon monoxide) and HC (hydrocarbon). This open-type SOF cracking catalyst has a drawback that the removal rate of dry suit is low, but the amount of dry suit can be reduced by improving the diesel engine and the fuel itself, and there is no need for a regeneration treatment device. Because of the merits, further improvement in technology is expected in the future.

[0005] However, the open type SOF decomposition catalyst can efficiently decompose SOF at a high temperature, but has a disadvantage that the catalytic performance of a catalytic noble metal is low at a low temperature condition, so that the SOF purification performance is low. Further, since the catalytic noble metal is covered with the unburned SOF, the SOF poisoning in which the activity of the catalytic noble metal is reduced often occurs. Therefore, when the engine is started or the engine is idling, the temperature of the exhaust gas is low and undecomposed SOF is discharged. Also, a phenomenon occurs in which it becomes soot and accumulates in the honeycomb passage even if it is not discharged. Then, there is a problem that the catalyst is clogged by the deposited soot and the catalyst performance is reduced.

Therefore, conventionally, cerium oxide has been used in combination as a co-catalyst. Cerium oxide has an oxygen storage capacity, and oxygen released from cerium oxide has high low-temperature activity. Therefore, even when the engine is started or idling, the cerium oxide gives active oxygen to the catalytic noble metal, so that the SOF deactivating the catalytic noble metal easily burns and the catalytic activity is restored.

Since the exhaust gas purifying catalyst is used at a high temperature, it is necessary that the catalyst has a high purifying activity at a high temperature. Therefore, it is required that cerium oxide does not cause a decrease in specific surface area when used at a high temperature, that is, has excellent heat resistance. Therefore, it has been conventionally performed to stabilize cerium oxide by dissolving an oxide of a rare earth element other than zirconium oxide and cerium.

For example, Japanese Patent Laid-Open No. 4-55315 discloses that
Cerium oxide and zirconium oxide are coprecipitated from a mixed aqueous solution of a water-soluble salt of cerium and a water-soluble salt of zirconium, and a method for producing cerium oxide fine powder is disclosed, which is subjected to heat treatment. According to this manufacturing method, cerium and zirconium become a composite oxide by heat-treating the coprecipitate, and a ceria-zirconia solid solution forming a solid solution with each other is generated.

Japanese Patent Application Laid-Open No. 4-284847 discloses that
It is disclosed that a powder in which an oxide of zirconium oxide or a rare earth element and cerium oxide are dissolved in a solid solution is produced by an impregnation method or a coprecipitation method. By dissolving zirconium oxide in cerium oxide in this manner, a decrease in specific surface area due to high heat in the ceria-zirconia solid solution can be suppressed, and a decrease in oxygen storage capacity of cerium oxide can be prevented.

[0010]

According to the study of the present inventors, however, the oxygen storage capacity of the solid solution of ceria-zirconia is about 1 after the endurance test, although the decrease in specific surface area is suppressed. It has been clarified to decrease to / 10. This is because sulfur poisoning occurs in that the sulfur oxide contained in the exhaust gas reacts with the ceria-zirconia solid solution to form a sulfate, thereby deactivating the oxygen storage capacity. In particular, in the case of an exhaust gas in a lean atmosphere, it is difficult to reduce sulfate and it is difficult to restore oxygen storage capacity.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an exhaust gas purifying catalyst which suppresses sulfur poisoning of a solid solution of ceria-zirconia in lean exhaust gas and has excellent durability. I do.

[0012]

According to a first aspect of the present invention, there is provided a lean exhaust gas purifying catalyst comprising a ceria-zirconia solid solution in which zirconium oxide is dissolved in cerium oxide. The solid solubility of zirconium oxide in cerium oxide is 50% or more, the average diameter of crystallites is 10 nm or less, and the ratio of zirconium in the ceria-zirconia solid solution is 0.55 ≦ Zr / (Ce + Zr) in molar ratio. ) ≦ 0.90, comprising oxide solid solution particles, a catalytic noble metal, and a refractory porous material.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The solid solubility refers to a value defined by the following equation (1). Solid solubility (%) = 100 × (amount of zirconium oxide dissolved in the total amount of cerium oxide) / total amount of zirconium oxide (1) where, ceria-zirconia solid solution (solid solubility 100%)
Equation (2) exists between the zirconia concentration x (mol%) and the lattice constant a (angstrom).

X = (5.423-a) /0.003 (2) This was derived as follows. When the same preparation as in Example 1 described later is performed and the lattice constant is measured while increasing the amount of the surfactant in the mixing ratio of each cerium oxide and zirconium oxide, the value of the lattice constant asymptotically approaches a certain value. FIG. 2 shows an example of a composition of cerium oxide / zirconium oxide = 5/5. This is performed for each composition, and the lattice constant obtained in the region with a large amount of surfactant is plotted against the zirconium oxide concentration, as shown in FIG. Equation (2) is obtained by rearranging the values by the least square method.

In FIG. 3, the zirconium oxide concentrations of 0% and 1
The plot at 00% is the value shown on the JCPDS card. Each plot in FIG. 3 is Vegard
It can be determined that equation (2) shows the relationship between the zirconium oxide concentration and the lattice constant when the solid solubility is 100%. Based on equations (1) and (2), ceria
The solid solubility S (%) of the zirconia solid solution is represented by equation (3).

S = 100 × (x / C) × [(100−C) / (100−x)] (3) where x is obtained by equation (2). Oxidation in the sample
The zirconium content C is determined by the distribution of cerium and zirconium.
It is determined from the ratio. The exhaust gas of the present invention according to claim 1
For purification catalysts, cerium oxide in ceria-zirconia solid solution
Solid solubility of zirconium oxide in lithium is 50% or more
It is. If the solid solubility is 50% or more, oxygen storage capacity
(OSC) is 250-800 μmolO _Two/ G or more
It is extremely excellent in oxygen storage capacity.

In the oxide solid solution particles according to the present invention,
Crystallite of ceria-zirconia solid solution has an average diameter of 10 nm
It is as follows. 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) It is.

If the average diameter of the crystallite is 10 nm or less,
The crystallites are not densely packed, and are solid solution particles having pores between the crystallites. Average particle size is 10nm
If it exceeds, the pore volume and the specific surface area decrease, and the heat resistance also decreases. Note the specific surface area of the oxide solid solution particles in the present invention is 1 m ² / g or more, further 20 m ² / g or more, and more preferably at 50 m ² / g or more.

In the ceria-zirconia solid solution according to the present invention, a part of the cerium position is replaced by zirconium while maintaining the fluorite structure of cerium oxide to form a solid solution,
Zirconia is sufficiently dissolved. In the solid solution, zirconium ions are about 15% smaller than cerium ions. Therefore, the solid solution emits a lot of oxygen,
Accordingly, even if cerium ions change from tetravalent to trivalent and cause volume expansion of about 15%, the presence of zirconium ions can alleviate the distortion of the crystal lattice, and thus it is considered that oxygen is easily desorbed. . In addition, cubic crystals are easier to move oxygen ions than tetragonal crystals and monoclinic crystals.
Indicates a higher OSC.

If 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. The oxygen storage capacity is further improved. And because oxygen is absorbed and released on the surface,
When the specific surface area is as large as 20 m ² / g or more, the oxygen storage / release speed becomes sufficiently high, and excellent oxygen storage capability is exhibited in combination with high OSC. The specific surface area is 50m ² /
g is more preferable.

Cerium and zirconium in the ceria-zirconia solid solution have a molar ratio of 0.55 ≦ Zr / (Ce + Z
r) ≦ 0.90. When the content of zirconium is 55 mol% or less, the action of forming the skeleton of zirconium in the solid solution crystal is weakened, and it becomes difficult to maintain a cubic crystal having a fluorite structure due to desorption of oxygen. It cannot be desorbed and the OSC decreases. In addition, since the amount of cerium that easily forms a sulfate is relatively large, or the solid solution phase is thermodynamically unstable and the sulfate is easily formed, sulfur poisoning is likely to occur, and oxygen after durability has been increased.
SC decreases.

Further, since the ability to occlude and release oxygen depends on the trivalent and tetravalent changes in cerium, when the zirconium content exceeds 90 mol%, the absolute amount of cerium becomes insufficient and the OSC decreases. . That is, in the exhaust gas purifying catalyst of the present invention, the ceria-zirconia solid solution contained in the oxide solid solution particles has a molar ratio of 0.55 ≦ Zr
By setting /(Ce+Zr)≦0.90, zirconium, which hardly forms a sulfate, is relatively more than cerium, which easily forms a sulfate, and zirconium, which has a large amount of zirconium, makes the solid solution phase thermodynamically stable. As a result, it becomes difficult to form sulfate, so that sulfur poisoning is prevented and durability is improved.

The oxide solid solution particles according to the present invention may be composed of only ceria-zirconia solid solution particles, or may be composed of a mixture of alkaline earth metal oxides, or at least partially composed of ceria-zirconia, as described later. A configuration in which the solid solution is dissolved in a zirconia solid solution may be employed. Further, oxides of other metals may be mixed or dissolved as long as the properties are not adversely affected.

In order to produce the oxide solid solution particles, for example, an alkali solution is prepared by adding hydrogen peroxide, a surfactant and an alkaline substance to an aqueous solution in which a compound containing trivalent cerium and a compound containing zirconium are dissolved. The first step of forming a precipitate in the solution, the second step of evaporating the alkaline solution containing the precipitate to dryness to obtain a dried product, and heating the dried product, the zirconium oxide dissolved in the cerium oxide. Including ceria-zirconia solid solution, ceria
The solid solubility of zirconium oxide to cerium oxide in the zirconia solid solution is 50% or more and the average diameter of crystallites is 10%.
nm or less, and the ratio of zirconium in the ceria-zirconia solid solution is 0.55 ≦ Zr / (Ce
+ Zr) a third step of obtaining oxide solid solution particles comprising ceria-zirconia solid solution particles in the range of 0.90;
Can be adopted.

In the above-mentioned production method, first, in the first step, hydrogen peroxide, a surfactant and an alkaline substance are added to an aqueous solution in which a compound containing trivalent cerium and a compound containing zirconium are dissolved, thereby forming a precipitate. Get. In the first step, first, trivalent cerium forms a complex with hydrogen peroxide and is oxidized to tetravalent cerium.
Cerium oxide is easily dissolved with zirconium oxide.

The amount of hydrogen peroxide to be added is preferably at least 1/4 of cerium ion. If the added amount of hydrogen peroxide is less than 1/4 of the cerium ion, the solid solution of cerium oxide and zirconium oxide becomes insufficient. Excessive addition of hydrogen peroxide has no particular adverse effect, but is only disadvantageous in terms of economy and has no merit, and is more preferably in the range of 1/2 to 2 times the cerium ion.

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 because no post treatment is required. The addition of the alkaline substance makes the system alkaline, and cerium and zirconium precipitate as insoluble hydroxides or oxides.

Here, the action of the surfactant is not clear, but is presumed as follows. That is, in the state just neutralized with an alkaline substance, cerium and zirconium precipitate in the state of very fine hydroxide or oxide having a particle size of several nm or less. Then, by adding the surfactant, plural kinds of precipitated particles are uniformly taken into the micelles of the surfactant, and neutralization, aggregation and aging progress in the micelles, whereby the plural components are uniformly contained and concentrated. The formation of solid solution particles proceeds in the small space. Furthermore, the dispersing effect of the surfactant improves the dispersibility of the precipitated fine particles, reduces segregation, and increases the degree of contact. As a result, the solid solubility is increased, and the average diameter of crystallites 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. In the second step, the alkaline solution containing the precipitate is evaporated to dryness, and the dried substance is heated in the third step to generate oxide solid solution particles composed of ceria-zirconia solid solution particles.

The heating temperature in the third step is 300
It is desirable to be in the range of -1000 ° C. When the temperature is lower than 300 ° C., the generation of ceria-zirconia solid solution particles becomes insufficient, and the heat resistance decreases. If the temperature is higher than 1000 ° C., the OSC decreases due to a decrease in specific surface area. Compounds containing trivalent cerium include cerium nitrate (III
), Cerium chloride (III), cerium sulfate (III) and the like. Examples of the compound containing zirconium include zirconium oxynitrate and zirconium oxychloride. As the alkaline earth metal, calcium, strontium, barium or the like is used.

Further, as the alkaline substance, any substance which 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 metal hydroxides can be used because they can be easily removed by washing with water. As a surfactant,
Any of anionic, cationic and nonionic types can be used, and among them, a surfactant capable of easily forming a micelle to form a narrow space therein, for example, a spherical micelle is desirable. In addition, the critical micelle concentration (cmc) is 0.1 mol under the condition of forming a precipitate by alkali.
/ Liter or less, more preferably 0.01 mo
Less than 1 / liter of surfactant is desirable. The micelle formed by the surfactant preferably has a narrow space inside such as a sphere.

Examples of these surfactants include alkyl benzene sulfonic acid and its salts, α-olefin sulfonic acid and its salts, alkyl sulfates, alkyl ether sulfates, phenyl ether sulfates, and methyl taurate. , Sulfosuccinate, ether sulfate, alkyl sulfate, ether sulfonate,
Saturated fatty acids and their salts, unsaturated fatty acids such as oleic acid and its salts, other anionic surfactants such as carboxylic acid, sulfonic acid, sulfuric acid, phosphoric acid and phenol derivatives, polyoxyethylene polyprolene 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; pentaerythritol; sucrose; fatty acid partial esters of polyhydric alcohols, such as polyhydric alcohol; glycol; glycerin; sorbitol; mannitol; Sucrose; polyoxyethylene fatty acid partial esters, such as polyhydric alcohols, polyoxyethylene fatty acid esters, polyoxyethylene castor oil,
Polyglycene fatty acid ester, fatty acid diethanolamide, polyoxyethylene alkylamine, triethanolamine fatty acid partial ester, nonionic surfactant such as trialkylamine oxide, primary fatty amine salt,
A second fatty amine salt, a tertiary fatty amine salt, a tetraalkylammonium salt; a trialkylbenzylammonium salt; an alkylpyridinium salt; a 2-alkyl-1-alkyl-1-hydroxyethylimidazolinium salt;
It is at least one selected from cationic surfactants such as quaternary ammonium salts such as N-dialkylmorpholinium salts; polyethylene polyamine fatty acid amide salts; and zwitterionic surfactants such as betaine compounds.

The critical micelle concentration (cmc) is the lowest concentration at which a certain surfactant forms micelles. The addition amount of the surfactant is 1 to 5 parts by weight based on 100 parts by weight of the ceria-zirconia solid solution particles to be produced.
A range of 0 parts by weight is desirable. 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, it is desirable to subject the obtained ceria-zirconia solid solution particles to the following post-treatment. That is, a reducing atmosphere (for example,
Contains gas such as carbon monoxide, hydrogen and hydrocarbons)
In this case, heat treatment at 800 to 1300 ° C. is preferable because solid solution is promoted and a zirconia skeleton can be surely formed in ceria to increase OSC. Thus, 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). ing. However, if it is heated to about 300 ° C. or more in the air thereafter, 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, a composite oxide is obtained as a powder (dry matter). In the case of using other heating methods, since the obtained composite oxide is in a lump, a powder is obtained by dry pulverization using a hammer mill, a ball mill, a vibration mill or the like. By the way, the ceria-zirconia solid solution is not a thermodynamically stable phase, so that its solid solubility may decrease when used in an oxidizing atmosphere. Therefore, it is desirable that at least a part of the rare earth oxide other than the alkaline earth metal or cerium be dissolved in the ceria-zirconia solid solution. Thereby, the solid solution phase can be further stabilized. Suitable for this purpose are calcium, yttrium, and the like, whose ionic radius is close to that of cerium and zirconium.

The range of the coexisting alkaline earth metal is such that at least one (M) of the oxide of the alkaline earth metal is 0 <M / (C) in a molar ratio with respect to the ceria-zirconia solid solution particles.
e + Zr + M) ≦ 0.15 is desirable. And
The average diameter of crystallites of the oxide particles is desirably 10 nm or less, and the specific surface area is desirably 20 m ² / g or more. More preferably, it is 50 m ² / g.

The coexistence of the alkaline earth metal oxide with the ceria-zirconia solid solution has the effect of increasing the stability of the ceria-zirconia solid solution phase in an oxidizing atmosphere at 900 ° C. or higher. That is, in an environment where ceria-zirconia solid solution has a solid solubility of 50% or more and cerium works ideally, the alkaline earth metal oxide is in equilibrium with the ceria-zirconia solid solution, and the alkaline earth metal oxide is ceria-zirconia. It is located between zirconia solid solution particles and hinders the progress of sintering. Therefore, aggregation of the ceria-zirconia solid solution particles is suppressed, and a decrease in specific surface area is prevented. For this purpose, an alkaline earth element such as barium which has a larger ionic radius than cerium and is hard to form a solid solution is suitable.

In terms of increasing the OSC of the ceria-zirconia solid solution, the effect of the coexistence of the alkaline earth metal oxide is hardly clear. However, when used for a long time in a high-temperature oxidizing atmosphere, the solid solution phase in the ceria-zirconia solid solution gradually separates into two phases, and the OSC tends to decrease. When used under such conditions, if an alkaline earth metal oxide is present, the solid solution phase becomes relatively stable, so that the OSC after use hardly decreases.

It should be noted that at least one of the alkaline earth metals (M) is present in a molar ratio of 0 with respect to the ceria-zirconia solid solution.
If it exceeds the range of <M / (Ce + Zr + M) ≦ 0.15, the OSC will be rather small. In addition, the number of components increases, which is disadvantageous in cost. If the average diameter of the crystallites of the alkaline earth metal oxide is greater than 10 nm or the specific surface area is less than 20 m ² / g, the rate of oxygen absorption and desorption is reduced, so that the atmosphere in which the oxygen partial pressure changes rapidly. , The OSC may be substantially reduced.

In order to produce the above-mentioned composite oxide, in the above-mentioned production method, for example, in the first step, a compound containing at least one kind of alkaline earth metal, a compound containing trivalent cerium and a compound containing zirconium are used. Use a dissolved aqueous solution. Then, when cerium and zirconium precipitate as insoluble hydroxides or oxides due to the addition of the alkaline substance, the compound containing the alkaline earth metal remains dissolved in the solution.

When the alkaline solution containing the precipitate is evaporated to dryness in the second step to obtain a dried product, a compound containing an alkaline earth metal is also precipitated in the dried product. Then, by heating the dried product in the third step, ceria-zirconia solid solution particles are generated, and oxide particles composed of a composite oxide of an alkaline earth metal oxide and zirconium oxide are generated. The reason that the composite oxide of the alkaline earth metal oxide and the zirconium oxide is formed is that the ionic radius of the alkaline earth metal is different from Ce and Zr, so that CeO ₂
And zirconium are difficult to form a solid solution in a solid solution of ZrO ₂ and zirconium easily forms a composite oxide with an alkaline earth metal.

As another method for producing the above-mentioned composite oxide, hydrogen peroxide, a surfactant and an alkaline substance are added to an aqueous solution in which a compound containing trivalent cerium and a compound containing zirconium are dissolved. A first step of forming a first solution containing an alkaline earth metal and adding an acid in which a salt with the alkaline earth metal is insoluble to an aqueous solution in which at least one compound of the alkaline earth metal is dissolved. A second step of forming a second solution, mixing the first solution and the second solution, evaporating to dryness, and then heating to include a ceria-zirconia solid solution in which zirconium oxide is dissolved in cerium oxide; The solid solubility of zirconium oxide in cerium oxide is not less than 50% and the average diameter of crystallites is not more than 10 nm; Ratio of Koniumu is in a molar ratio 0.5
Ceria-zirconia solid solution particles comprising ceria-zirconia solid solution particles in the range of 5 ≦ Zr / (Ce + Zr) ≦ 0.90 and oxide particles composed of a composite oxide of an alkaline earth metal oxide and zirconium oxide And a third step of obtaining a composite oxide in which the oxide particles and the oxide particles are mixed with a particle diameter of 50 nm or less, respectively.

In this manufacturing method, first, in the first step,
By adding hydrogen peroxide, a surfactant, and an alkaline substance to an aqueous solution in which a compound containing trivalent cerium and a compound containing zirconium are dissolved, a first solution containing a precipitate is formed. In the first step, the cerium and zirconium compounds become insoluble and precipitate by the same operation as in the first step of the above-described manufacturing method.

In the second step, a second solution containing a precipitate is formed by adding an acid in which a salt with the alkaline earth metal is insoluble to an aqueous solution in which at least one compound of the alkaline earth metal is dissolved. Is done. In this second step,
Insoluble alkaline earth metal salts precipitate. The acid to be added may be an acid, or may be used in the form of a salt if it reacts with a compound of an alkaline earth metal.

In the third step, the first solution and the second solution are mixed, evaporated to dryness, and heated. As a result, ceria-zirconia solid solution particles are generated, and oxide particles composed of a composite oxide of an alkaline earth metal oxide and zirconium oxide are generated. The reason why the composite oxide of alkaline earth metal oxide and zirconium oxide is generated
The ionic radius of the alkaline earth metal is different from Ce and Zr, so that it is difficult to form a solid solution in the solid solution of CeO ₂ and ZrO ₂ ;
There are two possible reasons that zirconium easily forms a composite oxide with an alkaline earth metal. Furthermore, by separately preparing a ceria-zirconia solid solution and a composite oxide of an alkaline earth metal oxide and zirconium oxide,
It is thought that each will be easier to generate.

In the above two production methods, the obtained oxide solid solution particles contain a salt of an alkaline earth metal. Therefore, washing with an acid such as dilute sulfuric acid is also preferable. Pt, Rh,
Examples of Pd, Ir, and the like, and examples of the refractory porous body include alumina, titania, zirconia, silica, silica-alumina, and zeolite. The catalytic noble metal is generally supported on the porous body, but may be supported on the oxide solid solution particles.

The loading amount of the catalyst noble metal varies depending on the kind of the noble metal, but is generally 0.1 to 2 g per liter of the volume of the exhaust gas purifying catalyst. It is desirable that the oxide solid solution particles be contained in an amount of 0.1 to 1 mol (moles of cations) per 1 liter of the volume of the exhaust gas purifying catalyst. When the amount of the oxide solid solution particles is less than 0.1 mol / liter, the purification performance at low temperature is reduced. When the amount is more than 1 mol / liter, the porous particles are relatively small, and the purification performance is reduced due to a decrease in adsorption ability.

The exhaust gas purifying catalyst of the present invention may be used after being formed into a pellet shape or a honeycomb shape from a powder shape, or may be used after being coated on a honeycomb carrier substrate formed in advance.

[0049]

The present invention will be specifically described below with reference to examples and comparative examples. Note that the present invention is not limited to the following embodiments. Example 1 Cerium (III) nitrate and zirconium oxynitrate were mixed at a molar ratio of Ce / Zr = 45/55 @ Zr /
An aqueous solution was prepared by mixing so that (Ce + Zr) = 0.55 °, and ammonia water was added dropwise with stirring to neutralize to form a precipitate. Subsequently, a hydrogen peroxide solution containing hydrogen peroxide in an equimolar number to the cerium ion contained in the mixed aqueous solution and an aqueous solution containing 10% by weight of the obtained oxide of alkylbenzenesulfonic acid were added, and the mixture was stirred.

The obtained slurry was dried at 120 ° C. for 24 hours to remove water, and then heated at 300 ° C. for 5 hours to remove ammonium nitrate as a by-product.
Oxide solid solution particles of 0 nm or less were prepared. With respect to the oxide solid solution particles, the solid solubility of zirconium oxide calculated from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials according to the formula (3) was 90%. The average diameter of the crystallites was calculated from the (311) peak of the X-ray diffraction pattern using Scherrer's formula, and the average diameter was 7 nm.
Further, the specific surface area of the oxide solid solution particles measured by the BET method was 80 m ² / g.

The obtained oxide solid solution particles (40 g) and γ-
60 g of alumina powder was mixed with 1 liter of water for 2 hours using a ball mill. A dinitrodiammineplatinum nitrate solution containing 1 g of Pt was added to the whole amount of the obtained slurry, and the mixture was stirred and evaporated to dryness to carry Pt, thereby preparing a catalyst powder of Example 1. Example 2 Cerium (III) nitrate and zirconium oxynitrate were mixed at a molar ratio of Ce / Zr = 40/60 ／ Zr /
Except that an aqueous solution was prepared by mixing and dissolving so that (Ce + Zr) = 0.6 °, in the same manner as in Example 1,
Oxide solid solution particles were prepared.

When the solid solubility of the oxide solid solution particles was calculated from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials according to the formula (3), the solid solubility of zirconium oxide was 90%. The average diameter of the crystallites was calculated from the (311) peak of the X-ray diffraction pattern using Scherrer's formula, and the average diameter was 7 nm. Further BET
Surface area of the composite oxide particles measured by the method is 75 m
² / g.

Then, in the same manner as in Example 1, the catalyst powder of Example 2 was prepared. Example 3 Cerium (III) nitrate and zirconium oxynitrate were mixed at a molar ratio of Ce / Zr = 30/70 @ Zr /
Except that an aqueous solution was prepared by mixing and dissolving (Ce + Zr) = 0.7 ° in the same manner as in Example 1,
Oxide solid solution particles were prepared.

The solid solubility of the oxide solid solution particles was calculated from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials according to the formula (3). As a result, the solid solubility of zirconium oxide was 95%. The average diameter of the crystallites was calculated from the (311) peak of the X-ray diffraction pattern using Scherrer's formula, and the average diameter was 6 nm. Further BET
Surface area of the composite oxide particles measured by the method is 83 m
² / g.

Then, in the same manner as in Example 1, the catalyst powder of Example 3 was prepared. Example 4 Cerium (III) nitrate and zirconium oxynitrate were mixed in a molar ratio of Ce / Zr = 20/80 ／ Zr /
Except that an aqueous solution was prepared by mixing and dissolving so that (Ce + Zr) = 0.8 °, in the same manner as in Example 1,
Oxide solid solution particles were prepared.

The solid solubility of the oxide solid solution particles was calculated from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials according to the formula (3). The solid solubility of zirconium oxide was 100%. The average diameter of the crystallites was calculated from the (311) peak of the X-ray diffraction pattern using Scherrer's formula, and was found to be 8 nm. Further BE
The specific surface area of the composite oxide particles measured by the T method is 68
m ² / g.

Then, in the same manner as in Example 1, the catalyst powder of Example 4 was prepared. Example 5 Cerium (III) nitrate and zirconium oxynitrate were mixed at a molar ratio of Ce / Zr = 10/90 ｛Zr /
Except that an aqueous solution was prepared by mixing and dissolving so that (Ce + Zr) = 0.9 °, in the same manner as in Example 1,
Oxide solid solution particles were prepared.

With respect to the oxide solid solution particles, the solid solubility of zirconium oxide calculated from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials by the formula (3) was 100%. In addition, the average diameter of the crystallites was calculated as (31)
1) Calculating from the peak using Scherrer's formula,
The average diameter was 6 nm. Further, the specific surface area of the composite oxide particles measured by the BET method was 72 m ² / g.

Then, in the same manner as in Example 1, the catalyst powder of Example 5 was prepared. Comparative Example 1 Ce / Zr = 70/30 @ Zr / Cerium (III) nitrate and zirconium oxynitrate in molar ratio
Except that an aqueous solution was prepared by mixing and dissolving so that (Ce + Zr) = 0.3 °, in the same manner as in Example 1,
Oxide solid solution particles were prepared.

The solid solubility of the oxide solid solution particles was calculated from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials according to equation (3). The solid solubility of zirconium oxide was 90%. The average diameter of the crystallites was calculated from the (311) peak of the X-ray diffraction pattern using Scherrer's formula, and the average diameter was 7 nm. Further BET
Surface area of the composite oxide particles measured by the method is 73 m
² / g.

Then, a catalyst powder of Comparative Example 1 was prepared in the same manner as in Example 1. Comparative Example 2 Ce / Zr = 60/40 ＝ Zr / Cerium (III) nitrate and zirconium oxynitrate in molar ratio
Except that an aqueous solution was prepared by mixing and dissolving so that (Ce + Zr) = 0.4 °, in the same manner as in Example 1,
Oxide solid solution particles were prepared.

The solid solubility of the oxide solid solution particles was calculated from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials according to the formula (3). The solid solubility of zirconium oxide was 90%. The average diameter of the crystallites was calculated from the (311) peak of the X-ray diffraction pattern using Scherrer's formula, and the average diameter was 7 nm. Further BET
Surface area of the composite oxide particles measured by the method is 90 m
² / g.

Then, in the same manner as in Example 1, a catalyst powder of Comparative Example 2 was prepared. Comparative Example 3 Cerium (III) nitrate and zirconium oxynitrate were mixed at a molar ratio of Ce / Zr = 50/50 ｛Zr /
Except that an aqueous solution was prepared by mixing and dissolving so that (Ce + Zr) = 0.5 °,
Oxide solid solution particles were prepared.

When the solid solubility of the oxide solid solution particles was calculated from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials according to the formula (3), the solid solubility of zirconium oxide was 90%. The average diameter of the crystallites was calculated from the (311) peak of the X-ray diffraction pattern using Scherrer's formula, and was found to be 8 nm. Further BET
Surface area of the composite oxide particles measured by the method is 84 m
² / g.

Then, a catalyst powder of Comparative Example 3 was prepared in the same manner as in Example 1. Comparative Example 4 Cerium (III) nitrate and zirconium oxynitrate were mixed at a molar ratio of Ce / Zr = 5/95 ｛Zr / (C
e + Zr) = Oxide solid solution particles were prepared in the same manner as in Example 1, except that an aqueous solution was prepared by mixing and dissolving so as to satisfy 0.95 °.

With respect to the oxide solid solution particles, the solid solubility of zirconium oxide calculated by the formula (3) from the lattice constant by X-ray diffraction and the mixing ratio of the starting materials was 100%. In addition, the average diameter of the crystallites was calculated as (31)
1) Calculating from the peak using Scherrer's formula,
The average diameter was 7 nm. Further, the specific surface area of the composite oxide particles measured by the BET method was 75 m ² / g.

Then, in the same manner as in Example 1, a catalyst powder of Comparative Example 4 was prepared. <Evaluation of heat resistance> 0.5 g of each of the catalyst powders of the examples and the comparative examples was packed in a quartz tube having an inner diameter of 10 mm.
A durability test was performed by flowing a gas containing SO ₂ at 2 ° C. for 2 hours. The composition of the gas was ₂ cc SO ₂ gas at 20 cc / mi.
n, oxygen 33 cc / min, nitrogen 280 cc / mi
n, water vapor is 10 cc / min. OSC of each catalyst powder before and after endurance was measured by a temperature-raised CO pulse test shown below.

In the heating CO pulse test, a catalyst bed filled in a reaction tube was previously heated to 600 ° C. using a fixed bed flow reactor.
After oxidizing sufficiently, the CO generated when the temperature is raised from room temperature to 600 ° C. in He gas under CO pulse injection.
₂ was measured. CO pulse size is 7.72
The measurement was performed under the conditions of μmol and a heating rate of 10 ° C./min. The OSC value is calculated from the room temperature to 300 ° C.
O ₂ amount was used.

The OSC value of Comparative Example 3 before endurance was 5 μmol in terms of the amount of CO _2.
FIG. 1 shows the post-durability OSC of each catalyst. From FIG. 1, it is apparent that the OSC of each comparative example decreases to about 1/10 or less after the durability, whereas the catalyst of each example maintains the high OSC after the durability.

[0070]

According to the lean exhaust gas purifying catalyst of the present invention, a decrease in OSC during durability is suppressed, so that the amount of sulfur contained in exhaust gas in a low temperature region such as at start-up or idling is reduced.
The high purification ability of OF is maintained for a long time.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the Zr / (Ce + Zr) ratio and the OSC after endurance (ratio to the value before endurance).

FIG. 2 is a graph showing the relationship between the addition ratio of alkylbenzenesulfonic acid and the lattice constant of crystals of the formed oxide solid solution particles.

FIG. 3 is a graph showing the relationship between the zirconia concentration in ceria-zirconia solid solution particles and the lattice constant.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshio Jintori 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Institute, Inc. 41 at Yokomichi, Toyota Central Research Laboratory, Inc.

Claims (1)

[Claims] 1. A solid solution of ceria-zirconia in which zirconium oxide is dissolved in cerium oxide, wherein the solid solubility of zirconium oxide in cerium oxide in the ceria-zirconia solid solution is 50% or more, and the average diameter of crystallites Is not more than 10 nm, and the molar ratio of zirconium in the ceria-zirconia solid solution is 0.55 ≦ Zr /
A lean exhaust gas purifying catalyst comprising: oxide solid solution particles in a range of (Ce + Zr) ≦ 0.90; a catalytic noble metal; and a refractory porous material.