JP3265534B2 - 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 used for an exhaust system of an automobile, and more particularly to an exhaust gas purifying catalyst containing a ceria-zirconia composite oxide and having improved heat resistance.

[0002]

2. Description of the Related Art As an exhaust gas purifying catalyst which is used in an exhaust system of an automobile, a three-way catalyst for purifying HC in exhaust gas, the CO and NO _x simultaneously has been widely used. The three-way catalyst has a structure in which a porous oxide such as alumina (Al ₂ O ₃ ) or silica (SiO ₂ ) is used as a carrier, and a noble metal such as platinum (Pt) or rhodium (Rh) is supported on the carrier. , together with purifying and oxidizing the adsorbed HC and CO in the catalyst, and purifying by reducing NO _x.

[0003] This three-way catalyst is designed to have the highest purification rate when it comes into contact with exhaust gas in a stoichiometric atmosphere having an air-fuel ratio of a stoichiometric air-fuel mixture. However, the actual air-fuel ratio fluctuates to a rich side or a lean side centering on stoichiometry depending on the driving conditions of the vehicle, so that the exhaust gas atmosphere also fluctuates. Therefore, high purification performance is not always ensured only by the three-way catalyst having the above configuration.

Therefore, conventionally, ceria (CeO ₂ )
Is known. Since ceria has a property of storing and releasing oxygen, oxygen is stored in ceria in a rich atmosphere and released in a lean atmosphere. Therefore, by containing ceria, the fluctuation of the atmosphere of the exhaust gas can be reduced, and the purification performance is improved. It is also preferable to include a ceria-zirconia composite oxide in which zirconia (ZrO ₂ ) is composited with ceria in the carrier. By combining zirconia, the thermal stability of ceria can be significantly improved.

[0005] For example, JP-A-10-182155 discloses an exhaust gas purifying catalyst in which a noble metal is supported on a composite oxide carrier containing at least two of cerium, zirconium and aluminum.

[0006]

However, due to the recent increase in high-speed running and the like, the exhaust gas temperature of automobiles has become extremely high. Therefore, the conditions for the durability test of the exhaust gas purifying catalyst also need to be stricter.

However, in the above-mentioned conventional exhaust gas purifying catalyst, no problem occurs in a durability test in which heating is performed at, for example, 1000 ° C. for several hours. However, when a severe durability test in which heating is performed at 1,000 ° C. for approximately 20 hours is performed, It became clear that phase separation may occur in the solid solution of ceria and zirconia depending on the composition of the carrier.

A general catalyst for purifying exhaust gas comprises forming a coat layer made of a carrier powder such as alumina or a composite oxide on a honeycomb-shaped carrier substrate made of cordierite or metal foil and having excellent heat resistance. The coat layer is configured to carry a noble metal. However, the conventional Japanese Patent Laid-Open No. 10-18
When a severe durability test is performed on a catalyst having a coat layer formed of a composite oxide carrier described in No. 2155, a crack occurs in the coat layer due to heat shrinkage of the carrier, or a problem such that the coat layer peels off. Was found to occur.

The present invention further improves the heat resistance of an exhaust gas purifying catalyst in which a noble metal is supported on a carrier containing a ceria-zirconia solid solution, exhibits a high purifying activity even after a severe durability test, and shows cracks in a coating layer and cracks. An object of the present invention is to provide an exhaust gas purifying catalyst in which cracks are prevented from being generated.

[0010]

According to a first aspect of the present invention, there is provided an exhaust gas purifying catalyst comprising a porous oxide powder and a composite oxide powder represented by the formula (1). An exhaust gas purifying catalyst comprising a carrier comprising a mixed powder and a noble metal carried on the carrier, wherein (Al ₂ O ₃ ) _a (CeO ₂ ) _b (ZrO ₂ ) _{1 -b} (1) In (1), a and b indicate a molar ratio, and a is 0.4
-2.5, b is in the range of 0.2-0.7) The mixed powder contains 30% by volume or more of powder having a particle size of 5 μm or more.

According to a second aspect of the present invention, there is provided an exhaust gas purifying catalyst comprising: a carrier comprising a mixed powder of a porous oxide powder and a composite oxide powder represented by the formula (2); An exhaust gas purifying catalyst comprising: (Al ₂ O ₃ ) _a (CeO ₂ ) _b (ZrO ₂ ) _{1 -b} (Y ₂ O ₃ ) _c (2) (in the formula (2), , B and c indicate the molar ratio, and a is
(0.4 to 2.5, b is in the range of 0.2 to 0.7, and c is in the range of 0.01 to 0.2) The mixed powder contains 30% by volume or more of powder having a particle size of 5 μm or more.

According to a third aspect of the present invention, there is provided a catalyst for purifying exhaust gas, comprising: a carrier comprising a mixed powder of a powder of a porous oxide and a powder of a complex oxide represented by the formula (3); An exhaust gas purifying catalyst comprising: (Al ₂ O ₃ ) _a (CeO ₂ ) _b (ZrO ₂ ) _{1 -b} (La ₂ O ₃ ) _d (3) (a in formula (3) , B and d indicate the molar ratio, and a is
(0.4 to 2.5, b is in the range of 0.2 to 0.7, and d is in the range of 0.005 to 0.1) The mixed powder contains 30% by volume or more of powder having a particle size of 5 µm or more.

According to a fourth aspect of the present invention, there is provided an exhaust gas purifying catalyst comprising: a carrier comprising a mixed powder of a porous oxide powder and a composite oxide powder represented by the formula (4); An exhaust gas purifying catalyst comprising: (Al ₂ O ₃ ) _a (CeO ₂ ) _b (ZrO ₂ ) _1-b (Y ₂ O ₃ ) _c (La ₂ O ₃ ) _d (4) (In the formula (4), a, b, c and d indicate a molar ratio,
a is 0.4 to 2.5, b is 0.2 to 0.7, c is 0.01 to 0.2, d
Is in the range of 0.005 to 0.1) The mixed powder contains 30% by volume or more of powder having a particle size of 5 μm or more.

The exhaust gas purifying catalyst according to claim 5 is characterized in that, in the exhaust gas purifying catalyst according to any one of claims 1 to 4, CeO ₂ and ZrO ₂ in the composite oxide form a solid solution. The solid solubility of ZrO ₂ in the composite oxide is 50% or more, more preferably 70% or more, and further preferably 80% or more.

According to a sixth aspect of the present invention, there is provided an exhaust gas purifying catalyst according to any one of the first to fifth aspects, wherein the carrier is calcined at 1000 ° C. for 5 hours in the composite oxide. The average crystallite diameter of the zirconia solid solution is 10 nm or less as measured by X-ray diffraction.

A feature of the exhaust gas purifying catalyst according to claim 7 is that, in the exhaust gas purifying catalyst according to any one of claims 1 to 6, the weight ratio between the porous oxide and the composite oxide in the carrier is reduced. 0.30 ≦ composite oxide / (composite oxide + porous oxide)
≤ 0.7.

The exhaust gas purifying catalyst according to claim 8 is characterized in that, in the exhaust gas purifying catalyst according to any one of claims 1 to 7, Pd is supported on the composite oxide, and the carrier further comprises Pd.
That is, at least one of Pt and Rh is supported.

The exhaust gas purifying catalyst according to the ninth aspect is characterized in that, in the exhaust gas purifying catalyst according to any one of the first to eighth aspects, the mixed powder contains powder having a particle size of 5 μm or more.
That is, it is contained at a ratio of less than volume%.

[0019]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, a carrier comprising a mixed powder of a porous oxide powder and a composite oxide powder represented by the formulas (1) to (4) is used. Particle size 5μm
The above powder is 30% by volume or more, preferably 40% by volume or more,
More preferably, the content is 50% by volume or more. As described above, the mixed powder contains powder having a particle size of 5 μm or more in an amount of 30% by volume or more, preferably 40% by volume or more, and more preferably 50% by volume or more. Even when a durability test is performed, problems such as cracking of the coat layer and peeling of the coat layer are prevented. If the powder having a particle diameter of 5 μm or more in the mixed powder is less than 30% by volume, the heat shrinkage during the durability test is considered to be large, and cracks and peeling occur in the coat layer as shown in Examples described later. The particle size refers to the particle size of the carrier made of the mixed powder.

In the composition of the composite oxide, the formula (1)
When the value of a in the formula (4) is smaller than 0.4, it is difficult to achieve a high dispersion of the ceria-zirconia composite oxide, and the specific surface area of the ceria-zirconia composite oxide is reduced after a severe durability test to purify the oxide. Performance decreases. If the value of a is 2.5
If it exceeds 3, the ceria-zirconia composite oxide is reduced in quantity, so that sufficient oxygen storage / release capability (hereinafter referred to as OSC) cannot be secured, and good purification performance cannot be obtained. The value of a is preferably 0.4 or more and 2.0 or less, more preferably 0.4 or more and 1.0 or less, more preferably 0.45 or more and 0.7 or less.
The following is good.

The value of b in equations (1) to (4) is 0.2
If it is smaller, the cerium content is small and sufficient OSC cannot be secured, so that good purification performance cannot be obtained.
If it exceeds 0.7, the thermal stability of the ceria-zirconia composite oxide decreases, and the purification performance after a severe durability test decreases. b
Is preferably in the range of 0.3 or more and 0.6 or less, more preferably 0.4 or more and 0.6 or less.

As shown in the formulas (2) and (4), by further adding Y ₂ O ₃ to the composite oxide, the durability of the ceria-zirconia composite oxide in an oxidizing atmosphere at 1000 ° C. or higher is further improved. As a result, phase separation of the ceria-zirconia composite oxide hardly occurs, and the activity of the noble metal is promoted. When the value of c in the formulas (2) and (4) is less than 0.01, the effect of adding Y ₂ O ₃ cannot be obtained, and when the value of c exceeds 0.2, the heat resistance of the ceria-zirconia composite oxide is reduced. As the activity decreases, the activity of the noble metal decreases.

As shown in equations (3) and (4),
By further adding La ₂ O ₃ to the composite oxide, the durability of the ceria-zirconia composite oxide in an oxidizing atmosphere at 1000 ° C. or more is further improved, and the ratio of the ceria-zirconia composite oxide is maintained even after a severe durability test. The surface area is kept high. When the value of d in the formulas (3) and (4) is less than 0.005, the effect of adding La ₂ O ₃ is not obtained, and when the value of d exceeds 0.1, the heat resistance of the ceria-zirconia composite oxide is reduced. descend. Note that at least one of Y ₂ O ₃ and La ₂ O _{3 in} the formulas (2) to (4) is preferably dissolved in a ceria-zirconia composite oxide and alumina.

It is preferable that the complex oxide represented by the formulas (1) to (4) further contain a trace amount of an additional element selected from alkali metals, alkaline earth metals, rare earth elements and transition metal elements. By including such an additive element, the heat resistance of the ceria-zirconia composite oxide is further improved. It is desirable that the amount of the additional element be 0.05 or less in terms of the molar ratio of the oxide to the formulas (1) to (4). That is, when the additive element M is added, for example, the formula (2)
The _{(Al 2 O 3) a (} CeO 2) b (ZrO 2) 1-b (Y 2 O 3) c (MO)
is expressed as _e, the molar ratio e is in the range of 0 to 0.05.

As the alkali metal, Na, K, Li, and Cs are preferable, as the alkaline earth metal, Mg, Ca, Sr, and Ba are preferable. As the rare earth element, La, Pr, Nd, Pm, Sm, Eu, and G are used.
It may be selected from d, Tb, Dy, Ho, Er, Tm, Yb, and Lu. As transition metal elements, Fe, Cu, Ni, Co, Mn
And the like. One of these may be used, or two or more of them may be used in combination. When two or more kinds are used in combination, they are added so that the total amount thereof is in the range of 0.05 or less in atomic ratio.

At least CeO ₂ and ZrO ₂ in the above composite oxide
Forms a solid solution, and the solid solubility of ZrO _{2 in} the solid solution is preferably 50% or more, more preferably 70% or more, and further preferably 80% or more. As a result, the durability of the ceria-zirconia composite oxide in an oxidizing atmosphere at 1000 ° C. or higher is further improved, and phase separation of the ceria-zirconia composite oxide is less likely to occur.

The solid solubility refers to a value defined by the following equation (5).

Solid solubility (%) = 100 × (amount of oxide B dissolved in the total amount of oxide A) / total amount of oxide B (5) Here, solid solution in the total amount of oxide A Oxide B is converted to Oxide A
Is assumed to be uniformly dissolved with respect to the total amount of

For example, in the case of a solid solution of ceria and zirconia, ceria corresponds to oxide A and zirconia corresponds to oxide B. The solid solubility (%) is 100 × (the amount of zirconia dissolved in the total amount of ceria). ) / Total amount of zirconia. Here, between the zirconia concentration x (mol%) of the ceria-zirconia solid solution (solid solubility 100%) and the lattice constant f (Å), JC
According to the value shown in the PDS card, the following equation (6) holds.

X = (5.411−f) /0.00253 (6) In the oxide solid solution particles according to the present invention, the average diameter of crystallites in the solid solution particles is 10 nm or less. The size of this crystallite is calculated from the half value width of the diffraction peak at the X-point using the following Scherrer equation.

D = kλ / (β COSθ) where k: constant 0.9, λ: wavelength of X-ray (Å), β: diffraction line width of sample−diffraction line width of standard sample (radian), θ: diffraction angle (Degree) When 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 crystallites. When the average crystallite diameter exceeds 10 nm, the pore volume and the specific surface area decrease, and the heat resistance also decreases. The specific surface area 10 m ² / g or more, further 20 m ² / g or more, and more preferably at 50 m ² / g or more.

In the composite oxide according to the present invention, the solid solution of ceria and zirconia becomes a solid solution by substituting a part of the cerium position with zirconium while maintaining the fluorite structure of cerium oxide. Solid solution. A zirconia skeleton is formed in the solid solution. Therefore, the crystal structure of the cubic crystal becomes stable, 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.

Further, since the average diameter of the crystallites is as small as 10 nm or less, the number of grain boundaries between the crystallites increases, and oxygen ions moving through the grain boundaries easily move. The oxygen storage capacity is further improved.
If the specific surface area is as large as 10 m ² / g or more, since oxygen is stored and released on the surface, the rate of storing and releasing oxygen is sufficiently high, and excellent oxygen storage capability is exhibited in combination with high OSC.

The composite oxide maintains a substantially uniform dispersion state in which each constituent element is in a solid solution or the like. The crystallite diameter of ceria-zirconia constituting the composite oxide is preferably 5 nm or less. Further, the composite oxide carrier has a ceria-zirconia crystallite size of 10 nm or less even after being treated at 1000 ° C. for 5 hours in air, and 60% or more of cerium and zirconium contained in the composite oxide. 30 nm or less,
More preferably 20 nm or less, more preferably 80% or more is dispersed as 30 nm or less, more preferably 20 nm or less, still more preferably 90% or more is 30 nm or less, more preferably 20 nm or less, most preferably 10 nm or less. It is desirable. It is confirmed from the results of SEM observation, TEM observation, and elemental analysis that 60% or more of the composite oxide in Examples of the present invention is dispersed as particles having a size of 30 nm or less.

The definition of the crystallite diameter and the particle diameter in this case means the ceria-zirconia crystallite and the distance between alumina particles or aluminum atoms interposed between the particles. In addition, ceria is added to alumina particles and alumina crystallites.
The spacing may be such that zirconia crystals or particles or atoms are dispersed as inclusions. When the crystallite size or particle size in the composite oxide is larger than 30 nm, the ratio of ceria and / or zirconia effectively acting as a composite oxide carrier catalyst decreases, the activity of noble metals decreases, and the OSC as a catalyst decreases. NO _x, H of the three-way catalyst in a variation atmosphere but decreases
It is not preferable because the purification performance for C and CO decreases.
Further, it is not preferable that the composite (solid solution) of cerium oxide and zirconium oxide is not sufficient because the OSC and purification performance are lowered.

In the case of a solid solution of ceria and zirconia, the molar ratio of cerium to zirconium is 0.3 ≦ Zr /
(Ce + Zr) ≦ 0.8 is preferable, and 0.4 ≦ Zr / (Ce +
The range of Zr) ≦ 0.6 is particularly preferred. When the content of zirconium is 30 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 of the fluorite structure due to desorption of oxygen. It cannot be desorbed and the OSC decreases. In addition, since the ability to insert and extract oxygen depends on the change in the valence of trivalent and tetravalent cerium, when the zirconium content exceeds 80 mol%, the OSC is reduced due to the lack of the absolute amount of cerium.

The weight ratio between the porous oxide and the composite oxide in the carrier is preferably 0.30 ≦ composite oxide / (composite oxide + porous oxide) ≦ 0.7, and is in the range of 0.4 to 0.6. Is particularly desirable. If the value of {composite oxide / (composite oxide + porous oxide)} is less than 0.30, the amount of ceria-zirconia composite oxide is reduced in quantity and sufficient O
SC cannot be secured, and good purification performance cannot be obtained. Also, the value of {composite oxide / (composite oxide + porous oxide)} is
If it exceeds 0.7, the noble metal carrying property is reduced and the noble metal is mainly carried on the porous oxide, so that the cocatalyst function represented by the OSC of the ceria-zirconia composite oxide does not work effectively.

By configuring the support as described above, the average crystallite size of the ceria-zirconia solid solution in the composite oxide after firing the support at 1000 ° C. for 5 hours becomes 10 nm or less as measured by X-ray diffraction. Even after a severe durability test, the specific surface area of the ceria-zirconia composite oxide is maintained high, and high purification performance can be maintained.

The porous oxide constituting a part of the carrier includes alumina, zeolite, mordenite, FSM, alumina-zirconia composite oxide, silica, zirconia,
A porous oxide such as titania and silica-alumina, which has been used as a carrier of a conventional catalyst for purifying exhaust gas, can be used. The particle size of the porous oxide is not particularly limited, but the specific surface area is optimally in the range of 5 to 300 m ² / g,
If the ratio is out of this range, the loading ratio of the noble metal may be biased, and the activity may be reduced or it may be difficult to achieve a high dispersion of the composite oxide powder. The carrier may contain components added by the impregnation method or components added as a sol in addition to the mixed powder.

As the noble metal supported on the carrier, P
Precious metals such as t, Rh, Pd, and Ir used in conventional exhaust gas purifying catalysts can be used. The amount of the noble metal supported varies depending on the type of the noble metal.
Preferably, it is in the range of 20% by weight. If the supported amount of the noble metal is less than this, sufficient purification activity cannot be obtained, and if it is more than this, the purification activity is saturated and the cost rises.

In order to produce the composite oxide according to the present invention,
For example, the manufacturing method disclosed in JP-A-10-182155 can be used. That is, at least Al, Ce and Zr
A composite oxide can be produced by preparing an oxide precursor in a short time from a salt solution of the above by a coprecipitation method and calcining it in the air. Alternatively, Al, Ce and Zr may be supplied as alkoxides, and the oxide precursor may be produced by a sol-gel method.

To produce the exhaust gas purifying catalyst of the present invention, the mixed oxide powder and the porous oxide powder are mixed to form a mixed powder, which is then mixed with a binder such as alumina sol, boehmite, aluminum nitrate and the like. Mix with water to form a slurry. This slurry is used to wash coat a honeycomb-shaped carrier substrate formed of cordierite or metal foil, and then dried and fired to form a coat layer. Then, the catalyst for exhaust gas purification of the present invention can be manufactured by adsorbing and supporting a noble metal on the coat layer using a solution of a noble metal salt or a noble metal complex. As the binder, silica sol, zirconia sol, titania sol or a salt solution of these metal elements can be used.

A mixed powder of the composite oxide powder and the porous oxide powder is impregnated with a noble metal salt solution or the like, and is used as it is or is filtered, dried or evaporated and dried to obtain a noble metal-supported powder. This noble metal-carrying powder may be coated on a carrier substrate. Also, by preparing a noble metal salt or the like in the solution at the time of preparing the oxide precursor of the composite oxide, the noble metal is supported on the composite oxide, and then mixed with the porous oxide powder to form a noble metal-supported powder. You can also. Further, in order to make the catalyst layer porous, an organic substance or carbon powder may be added to the slurry for forming the coat layer.

[0044]

The present invention will be specifically described below with reference to examples and comparative examples.

Example 1 A high-concentration aqueous solution (aqueous solution) in which aluminum nitrate, cerium nitrate, zirconyl oxynitrate and lanthanum nitrate were dissolved at a ratio of a = 0.5, b = 0.5 and d = 0.007 in the formula (3) The solid content after firing contained therein was about 10% by weight). To this aqueous solution was added a hydrogen peroxide solution containing hydrogen peroxide in an amount equivalent to 1.2 times the amount of cerium, followed by stirring and mixing.Ammonia water was added in a short time to neutralize the solution, and cerium hydroxide, zirconium hydroxide, and water were added. A suspension containing a precipitate of lanthanum oxide and aluminum hydroxide was obtained. This was calcined at 300 ° C. for 5 hours and calcined at 700 ° C. for 5 hours. The obtained powder is put into a ball mill together with water, and
The powder was crushed until the particle size of the powder of not less than 15% by volume became 15 μm or less to prepare a composite oxide powder. In this case, the firing is performed at 700 ° C., but the firing may be performed at about 600 to 1100 ° C. at which an oxide can be formed.

Next, with respect to 100 parts by weight of the composite oxide powder, 100 parts by weight of a commercially available heat-resistant activated alumina powder (specific surface area: 180 m ² / g) as a porous oxide powder, aluminum nitrate nonahydrate 20 parts Parts by weight, 5 parts by weight of boehmite (AlO (OH)) and water in such an amount that the solid content after firing is 50% by weight,
A slurry was prepared by mixing with an attritor for about 3 minutes.
The average particle size after pulverizing the slurry was 7 μm.

A 1.3 L cordierite honeycomb-shaped carrier base material was prepared, dipped in the slurry, pulled up, blown off excess slurry, dried and fired at 650 ° C. for 1 hour to form a coat layer. The coating layer is made of the carrier substrate 1
200 g formed per liter.

Then, the carrier substrate having the coating layer was immersed in an aqueous solution of platinum nitrate having a predetermined concentration, lifted up, blown off extra droplets, dried and baked at 250 ° C. for 1 hour to carry Pt.
Next, it was immersed in an aqueous solution of rhodium nitrate having a predetermined concentration, and Rh was supported in the same manner. Pt 1.5 per liter of carrier substrate
g and 0.3 g of Rh.

The obtained exhaust gas purifying catalyst of Example 1 was attached to the exhaust pipe of a 2-liter gasoline engine,
An accelerated endurance running test was performed for 100 hours in a running pattern simulating European driving. The maximum catalyst bed temperature at this time is about
It will be 1000 ° C.

Thereafter, the exhaust gas during traveling simulating European traveling was measured at the upstream and downstream of the catalyst at the same time.
HC, CO, and averages were calculated purification ratio by measuring the purification rate of NO _x. Table 1 shows the results.

Example 2 The same amount of zirconia powder having a specific surface area of 50 m ² / g was used in place of activated alumina powder as a porous oxide, and zirconia sol (ZrO ₂ : 10 wt.) Was used in place of aluminum nitrate and boehmite. %) An exhaust gas purifying catalyst of Example 2 was prepared in the same manner as in Example 1 except that 100 parts by weight was used. Then, the average purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1.

(Example 3) Aluminum nitrate, cerium nitrate and zirconyl oxynitrate were prepared by substituting a = 0.
5. Exhaust gas purification catalyst of Example 3 was prepared in the same manner as in Example 1 except that a high concentration aqueous solution dissolved at a ratio of b = 0.33 was used. Then, the average purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1.

Example 4 A high-concentration aqueous solution in which aluminum nitrate, cerium nitrate, zirconyl oxynitrate and yttrium nitrate were dissolved at a ratio of a = 0.5, b = 0.6 and c = 0.05 in the formula (2) was used. Except for this, the exhaust gas-purifying catalyst of Example 4 was prepared in the same manner as in Example 1. Then, the average purification rate was measured in the same manner as in Example 1, and the results were shown in Table 1.
Shown in

(Example 5) Aluminum nitrate, cerium nitrate, zirconyl oxynitrate, yttrium nitrate and lanthanum nitrate were prepared by a = 0.5, b = 0.5 and c = 0 in formula (4).
05, a high concentration aqueous solution dissolved at a ratio of d = 0.015 was prepared. An exhaust gas purifying catalyst of Example 5 was prepared in the same manner as in Example 1 except that coprecipitation was performed using this aqueous solution. Then, the average purification rate was measured in the same manner as in Example 1,
Table 1 shows the results.

Example 6 A composite oxide powder prepared in the same manner as in Example 1 was impregnated with a predetermined amount of an aqueous solution of palladium nitrate having a predetermined concentration, filtered, dried and calcined at 250 ° C.
Pd was loaded. With respect to 100 parts by weight of the Pd-supported composite oxide powder, 100 parts by weight of the same activated alumina powder as in Example 1 as the porous oxide powder, and aluminum nitrate 9 hydrate 20
Parts by weight and 5 parts by weight of boehmite were added and mixed with an attritor for about 3 minutes to prepare a slurry.

Using this slurry, a coat layer was formed in the same manner as in Example 1, and then immersed in an aqueous solution of rhodium nitrate having a predetermined concentration, pulled up, blown off excess droplets, and dried to carry Rh. An exhaust gas purifying catalyst of Example 6 was prepared. 1.5 g of Pd is supported per liter of carrier substrate, and Rh
Is supported by 0.3 g. Then, the average purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1.

Example 7 A high-concentration aqueous solution was prepared by dissolving aluminum nitrate, cerium nitrate, zirconyl oxynitrate and lanthanum nitrate in the ratio of a = 0.5, b = 0.5 and d = 0.015 in the formula (3). did. An exhaust gas purifying catalyst of Example 7 was prepared in the same manner as in Example 6, except that the aqueous solution was coprecipitated and mordenite was mixed instead of the activated alumina powder. 1.5 g of Pd and 0.3 g of Rh are supported per liter of the carrier substrate. Then, the average purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1.

Example 8 Aluminum nitrate, cerium nitrate, zirconyl oxynitrate and lanthanum nitrate were dissolved at a ratio of a = 0.5, b = 0.5 and d = 0.015 in the formula (3), and palladium nitrate was further dissolved. A highly concentrated aqueous solution was prepared. A Pd-supported composite oxide powder was prepared in the same manner as in Example 6, except that coprecipitation was performed using this aqueous solution.

Based on 100 parts by weight of the Pd-supported composite oxide powder, 100 parts by weight of activated alumina powder as in Example 1 as a porous oxide powder, 20 parts by weight of aluminum nitrate nonahydrate, and 5 parts by weight of boehmite And add about 3
Mix for minutes to prepare a slurry.

Using this slurry, a coat layer was formed in the same manner as in Example 1, and Pt and Rh were further supported.
Was prepared. 1.5 g of Pd, 1.5 g of Pt and 0.3 g of Rh are supported per liter of the carrier substrate. Then, the average purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1.

Example 9 Aluminum nitrate, cerium nitrate, zirconyl oxynitrate and lanthanum nitrate were dissolved in a ratio of a = 0.5, b = 0.5 and d = 0.015 in the formula (3), and palladium nitrate was further dissolved. A highly concentrated aqueous solution was prepared. A Pd-supported composite oxide powder was prepared in the same manner as in Example 6, except that coprecipitation was performed using this aqueous solution.

With respect to 100 parts by weight of the Pd-supported composite oxide powder, 50 parts by weight of the same activated alumina powder as in Example 1 and an alumina-zirconia composite oxide having a specific surface area of 70 m ² / g as a porous oxide powder 50 parts by weight of the powder, 20 parts by weight of aluminum nitrate 9-hydrate and 5 parts by weight of boehmite were added, and mixed with an attritor for about 3 minutes to prepare a slurry.

Using this slurry, a coat layer was formed in the same manner as in Example 1, and Pt and Rh were further supported.
Was prepared. 1.5 g of Pd, 1.5 g of Pt and 0.3 g of Rh are supported per liter of the carrier substrate. Then, the average purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1.

(Comparative Examples 1 to 5) Aluminum nitrate, cerium nitrate and zirconyl oxynitrate were replaced by a =
An exhaust gas purifying catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that a high-concentration aqueous solution dissolved at a ratio of 2.6, b = 0.5 and d = 0 was used. Then, the average purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 2 prepared in the same manner as in Examples 1 to 4 except that a to d were outside the scope of the present invention.
Table 1 shows the compositions of Nos. To 5 and the results.

<Evaluation>

[0067]

[Table 1]

As can be seen from Table 1, the exhaust gas purifying catalysts of the examples exhibited extremely high average purifying rates of 95 to 99% even after a severe durability test at 1000 ° C. for 100 hours. It can be seen that the durability is extremely superior to that of the catalyst.

As a result of X-ray diffraction, cerium and zirconium formed almost uniform solid solutions in the catalysts of the respective examples.
Its average crystallite diameter is about 7 nm, and the solid solubility of ZrO ₂ is 85%.
That was all.

As a result of polishing the cross section of the coat layer and observing it with an electron microscope, it was found that a solid solution of cerium and zirconium was highly dispersed in alumina, and that this composite oxide powder was alumina powder or alumina-zirconia composite oxide. It was confirmed that the particles were dispersed in a state surrounded by the powder.

Further, when the specific surface area of the powder from which the coat layer was scraped off was measured, the catalyst of Example 2 had a large specific surface area of 18 m ² / g, and the catalysts of the other examples had a large specific surface area of about 70 m ² / g or more. It was confirmed that. As a result of measuring the OSC of the powder, it was confirmed that the OSC per mol of cerium at 300 ° C. showed a high value of 0.09 (mol-O ₂ / mol-Ce) or more.

As a method for measuring the OSC, the catalyst was filled in a quartz reaction tube, and the measurement was performed at 300 ° C. using an infrared image furnace. Specifically, 1% H ₂ + is added to 0.1 to 1 g of the catalyst.
He gas 100cc / min and 1% O ₂ + He gas 50cc / min for several tens of seconds
The mixture was alternately circulated at intervals of several minutes (mainly 2.5 minutes), and the outgas of the catalyst was analyzed by a quadrupole mass spectrometer, and measured at each temperature for 13 minutes. _{2 From} + He gas
From the time delay when the gas was switched to H ₂ + He gas, the amount of oxygen stored in the catalyst was calculated.

From the above results, since the ceria-zirconia solid solution is highly dispersed and has a large specific surface area even after the endurance test, the exhaust gas purifying catalysts of the respective examples exhibit high purification performance due to their high OSC. It is clear that it was maintained. This is considered to be due to the fact that the values of the molar ratios a to d of the composite oxide were set in the composition range of the present invention.

(Examples 10 to 28 and Comparative Examples 6 to 10) Except for different compositions, 25 kinds of powders shown in Table 2 were synthesized in the same manner as in Examples 1 to 4, and 950 shown in 2
The firing was performed under the conditions of 11100 ° C.

For each sample after firing, the solid solubility of zirconia in the contained ceria-zirconia solid solution,
The average crystallite diameter of the zirconia solid solution, the specific surface area of the composite oxide of alumina, ceria and zirconia were measured, respectively.
Table 2 shows the results. In addition, about Example 23, Example 1
Although the synthesis was carried out using a sol-gel method which is usually performed in addition to the same method as in the above, it should be noted that almost the same measured values were obtained by either of the synthesis methods.

<Evaluation>

[0077]

[Table 2]

As can be seen from Table 2, the powders of Examples 10 to 28 have high ZrO ₂ solid solubility and high specific surface area even after firing at a high temperature in the air, and have an average crystallite diameter of ceria-zirconia solid solution. small.

On the other hand, in the powders of Example 28 and Comparative Examples 6 to 10,
As evident in Example 28, the solid solubility of ZrO ₂ after firing at 1100 ° C. was not sufficient, or the average crystallite diameter of the ceria-zirconia solid solution was large or the specific surface area was notable in Comparative Examples 6 to 9. There is a problem that it becomes smaller. In the case of Comparative Example 10 having a large amount of alumina, the amount of ceria contained was relatively small, so that the OSC was low. As a result,
High catalytic performance cannot be obtained in a fluctuating atmosphere.

In Table 2, ZrO _{2 has} a solid solubility exceeding 100. This is because it is simply obtained from the ceria-zirconia solid solution (cubic) 311 diffraction line using the above-mentioned equation (6). Because Y and La
Is because the solid solution forms in both alumina and the ceria-zirconia solid solution, so that the lattice constant of the ceria-zirconia solid solution changes depending on the difference in ionic radius between Y and La. However, since the effect is less than 10%, this effect is ignored for the sake of convenience.

The method of measuring the OSC is described below.

Examples 10 to 28 after endurance test at 1000 ° C. for 20 hours
In addition, 1 g of platinum was supported by impregnation per 100 g of the composite oxide of Comparative Examples 6 to 10. After compacting the catalyst powder, a pellet catalyst having a diameter of 0.5 to 1 mm was prepared. Using 0.3 g of this catalyst, O was prepared in the same manner as the catalyst of Example 2.
SC was measured. OSC per unit weight from measurement results
Was calculated, and the OSC of the catalyst using the composite oxide of Example 11 was calculated.
Are shown in Table 2 as relative values with respect to 100.

From Table 2, it is clear that the catalyst having a high solid solubility and within the composition range of the example has a high OSC.

<Effect of Particle Size> The effect of the particle size of the mixed powder was examined.

An average particle diameter of 2 μm, 5 μm,
Four types of mixed powders of μm, 7.7 μm and 8.8 μm were prepared. Then, four types of exhaust gas purifying catalysts were prepared in the same manner as in Example 1 using the respective mixed powders.

These exhaust gas purifying catalysts were attached to the exhaust pipe in the same manner as in Example 1, and the same accelerated durability running test was performed for 10 hours. Thereafter, the cross section of the catalyst was observed with an electron microscope. As a result, as shown in FIG. 1, in the case of using the mixed powder having an average particle size of 2 μm, large cracks were observed in the coat layer, and peeling occurred.

Next, air was blown from the end face of the catalyst at a pressure of 0.1 MPa, and the cross section of the catalyst was observed with an electron microscope. As a result, as shown in FIG. 2, when the mixed powder having an average particle size of 5 μm was used, some cracks were observed in the coat layer, but peeling was very small.

Further, in the case of using the mixed powder having the average particle diameters of 7.7 μm and 8.8 μm, as shown in FIGS. 3 and 4, no cracking or peeling was observed in the coat layer even after air blowing, and The strength was excellent.

The above four kinds of mixed powders and the other four
For each type of mixed powder, the particle size distribution was measured using a laser diffraction / scattering type particle size distribution measuring device, and the degree of peeling was relatively evaluated. The results are shown in Table 3.

[0090]

[Table 3]

From Table 3, it can be seen that when 30% or more of powder having a particle size of 5 μm or more is contained, the degree of peeling and cracking is reduced. It is also found that the content is preferably 40% or more, and more preferably 50% or more. However, it is not good that the particle size is large, and peeling increases when the particle size is 5 μm or more when it is 85% or more. Further, the average particle size is 3.5 to 20 μm, preferably 4.5 to 14 μm, more preferably 5.5 to 11 μm, and still more preferably 7 to 9 μm.
It turns out that m is good.

(Examples 29 to 32, Comparative Examples 11 to 13)
The results of an experiment focusing on the weight ratio between the porous oxide and the composite oxide in the carrier are shown. Using a composite oxide having the same composition ratio as in Example 28 as an example, and using a composite oxide and γ
Example 1 except that the weight ratio to -Al ₂ O ₃ was changed respectively.
An exhaust gas purification catalyst similar to that described above was prototyped. This catalyst was subjected to the same accelerated durability running test as in Example 1 for 100 hours.

The peel rate of the coat layer was calculated from the difference in weight of the catalyst before and after the durability test. The results are shown in Table 4. Also,
The catalyst after the durability test was placed in an evaluation device, and the OSC at 300 ° C. was measured. The results are also shown in Table 4.

[0094] Further, each fitted with a respective catalysts after the durability test in an exhaust system of a 2 liter engine, gradually increased incoming gas temperature from room temperature to measure the purification rate of HC and NO _x. Then, the respective 50% purification temperatures were calculated, and the results are shown in Table 4.

Further, Table 4 also shows the results of scraping off the coat layer from a part of the catalyst after the measurement and measuring the specific surface area.

[0096]

[Table 4]

As apparent from Table 4, the composite oxide and γ
-Al ₂ O ₃ weight ratio 0.3 ≦ composite oxide / (composite oxide +
By setting γ-Al ₂ O ₃ ) ≦ 0.7, preferably 0.4 ≦ composite oxide / (composite oxide + γ-Al ₂ O ₃ ) ≦ 0.6, the peeling rate of the coat layer is suppressed to be small, and high OSC and 50% The rise in purification temperature could be kept low.

[0098]

According to the exhaust gas purifying catalyst of the present invention, cracks and cracks in the coating layer are prevented even after a severe durability test, and the specific surface area of the ceria-zirconia composite oxide is high and the dispersion is high. Since the state is maintained, the durability is extremely excellent.

[Brief description of the drawings]

FIG. 1 is a photomicrograph showing the particle structure of a cross section of a catalyst using a mixed powder having an average particle size of 2 μm after a durability test.

FIG. 2 is a photomicrograph showing the particle structure of a cross section after blowing air after a durability test of a catalyst using a mixed powder having an average particle size of 5 μm.

FIG. 3 is a photomicrograph showing the particle structure of a cross section after blowing air after a durability test of a catalyst using a mixed powder having an average particle size of 7.7 μm.

FIG. 4 is a photomicrograph showing the particle structure of a cross section after blowing air after a durability test of a catalyst using a mixed powder having an average particle size of 8.8 μm.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-10-277389 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53 / 86 B01D 53/94

Claims (9)

(57) [Claims] An exhaust gas purifying catalyst comprising: a carrier comprising a mixed powder of a porous oxide powder and a composite oxide powder represented by the formula (1); and a noble metal carried on the carrier. (Al ₂ O ₃ ) _a (CeO ₂ ) _b (ZrO ₂ ) _{1 -b} (1) (In the formula (1), a and b indicate a molar ratio, and a is 0.4
-2.5, b is in the range of 0.2-0.7) The exhaust gas purifying catalyst characterized in that the mixed powder contains 30% by volume or more of powder having a particle size of 5 μm or more. 2. An exhaust gas purifying catalyst comprising a carrier comprising a mixed powder of a porous oxide powder and a composite oxide powder represented by the formula (2), and a noble metal supported on the carrier. (Al ₂ O ₃ ) _a (CeO ₂ ) _b (ZrO ₂ ) _{1 -b} (Y ₂ O ₃ ) _c (2) (in the formula (2), a, b and c indicate a molar ratio, and a Is
0.4 to 2.5, b is in the range of 0.2 to 0.7 and c is in the range of 0.01 to 0.2) The mixed powder contains 30% by volume or more of powder having a particle size of 5 μm or more. 3. An exhaust gas purifying catalyst comprising a carrier comprising a mixed powder of a powder of a porous oxide and a powder of a complex oxide represented by the formula (3), and a noble metal carried on the carrier. Then, (Al ₂ O ₃ ) _a (CeO ₂ ) _b (ZrO ₂ ) _{1 -b} (La ₂ O ₃ ) _d (3) (in the formula (3), a, b and d indicate a molar ratio, and a Is
0.4 to 2.5, b is in the range of 0.2 to 0.7, and d is in the range of 0.005 to 0.1) The mixed powder contains 30% by volume or more of powder having a particle size of 5 μm or more. 4. An exhaust gas purifying catalyst comprising a carrier comprising a mixed powder of a porous oxide powder and a composite oxide powder represented by the formula (4), and a noble metal carried on the carrier. Then, (Al ₂ O ₃ ) _a (CeO ₂ ) _b (ZrO ₂ ) _{1 -b} (Y ₂ O ₃ ) _c (La ₂ O ₃ ) _d (4) (in the formula (4), a, b, c And d represent a molar ratio,
a is 0.4 to 2.5, b is 0.2 to 0.7, c is 0.01 to 0.2, d
Is in the range of 0.005 to 0.1) An exhaust gas purifying catalyst characterized in that the mixed powder contains 30% by volume or more of powder having a particle size of 5 µm or more. 5. The solid oxide of CeO ₂ and ZrO ₂ in the composite oxide, wherein the solid solubility of ZrO ₂ in the composite oxide is 50% or more. The exhaust gas purifying catalyst according to any one of the above. 6. An average crystallite size of the ceria-zirconia solid solution in the composite oxide after calcining the carrier at 1000 ° C. for 5 hours is 10 nm or less as measured by X-ray diffraction. 6. The exhaust gas purifying catalyst according to any one of claims 1 to 5. 7. The weight ratio of the porous oxide and the complex oxide in the carrier is 0.30 ≦ complex oxide / (complex oxide + porous oxide) ≦ 0.7. Item 7. The exhaust gas purifying catalyst according to any one of Items 1 to 6. 8. The exhaust gas purifying apparatus according to claim 1, wherein Pd is carried on the composite oxide, and at least one of Pt and Rh is further carried on the carrier. catalyst. 9. The exhaust gas purifying catalyst according to claim 1, wherein the mixed powder contains a powder having a particle size of 5 μm or more in a proportion of less than 85% by volume.