JPH05277370A - Catalyst for purification of exhaust gas from engine - Google Patents

Abstract

PURPOSE:To stabilize activity at a low temp. by successively laminating first, second and third catalyst layers on a catalyst carrier, forming the first and second catalyst layers from alumina layers contg. different noble metal catalyst components, respectively, and incorporating Pd and CeO2 into the third catalyst layer. CONSTITUTION:First, second and third catalyst layers 3, 4, 5 are successively laminated on a catalyst carrier 2 made of cordierite, etc. The first and second catalyst layers 3, 4 are formed from alumina layers contg. respectively different noble metal catalyst components such as Pt and Rh, and Pd and CeO2 are incorporated into the third catalyst layer. By the oxygen storage ability of CeO2, Pd as a noble metal catalyst component in the third catalyst layer can be effectively inhibited from sintering at a low temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to CO, H in exhaust gas.
The present invention relates to an engine exhaust gas purifying catalyst used for reducing C and NOx.

[0002]

2. Description of the Related Art An exhaust gas purifying catalyst for purifying CO, HC and NOx in exhaust gas is disclosed in Japanese Patent Laid-Open No. 62-5767.
As shown in Japanese Patent Publication No. 1, first, second, and third catalyst layers are provided on a catalyst carrier in this order from the catalyst carrier toward the outside.
There is one in which different noble metal catalyst components are separately disposed in the first, second and third catalyst layers. In this case, the heat deterioration due to the alloying of the noble metal catalyst components is suppressed, and the purification performance is improved as compared with the conventional one having two catalyst layers.

[0003]

However, in the above exhaust gas purifying catalyst, even if it is said that each noble metal catalyst component is separately disposed in each catalyst layer and thermal deterioration is suppressed, it becomes the outermost surface. It is no different from the conventional method that the temperature of the 3rd catalyst layer comes into direct contact with the exhaust gas, and therefore the noble metal catalyst component of the 3rd catalyst layer causes sintering (coagulation coarsening) and heat deterioration. It tends to occur. As a result, the low temperature activity of the catalyst cannot be sufficiently stabilized. The present invention has been made in view of the above circumstances, and an object thereof is to provide a catalyst for purifying exhaust gas of an engine, which can sufficiently stabilize low temperature activity of the catalyst.

[0004]

In order to achieve the above object, the present invention (first invention) comprises a catalyst carrier,
In an exhaust gas purifying catalyst for an engine in which first, second and third catalyst layers are provided in order from the catalyst carrier toward the outside, the first and second catalyst layers are different from each other. It is composed of an alumina layer containing one noble metal catalyst component, and the third catalyst layer contains palladium and cerium oxide.

With the above-described structure of the present invention, it is possible to dispose one kind of noble metal catalyst component separately in the first, second and third catalyst layers to prevent the noble metal catalyst components from alloying with each other. However, since palladium having high heat resistance is arranged as a noble metal catalyst component in the third catalyst layer, which is the outermost surface, and cerium oxide is also contained in the third catalyst layer, palladium of the noble metal catalyst component not only the effect of the characteristic itself is obtained, the O ₂ adsorbed at high oxygen storage capacity effect (oxygen O ₂ concentration of cerium oxide, by releasing O ₂ when the O ₂ concentration is lower contribute to catalysis Based on the above effect), the temperature at which the palladium oxide of the third catalyst layer (palladium exists as an oxide) is dissociated into palladium is increased, and palladium as a noble metal catalyst component in the third catalyst layer is increased. So that it is possible to suppress the sintering in the stomach temperature effectively. Therefore, the thermal deterioration of the catalyst is effectively suppressed, and the low temperature activity of the catalyst can be sufficiently stabilized.

[0006]

EXAMPLES Examples of the present invention will be described below. Figure 1
FIG. 1 is a schematic enlarged view showing the catalyst according to the first embodiment. As shown in FIG. 1, the catalyst 1 is formed on the catalyst carrier 2 by the catalyst carrier 2
To the outer side (upper side in FIG. 1), the first, second, and third catalyst layers 3, 4, and 5 are provided in this order. As the material of the catalyst carrier 2, a known material such as cordierite is used, and the shape thereof is a known shape such as a honeycomb structure or a pellet type. The first catalyst layer 3 is
In the present embodiment, the alumina layer contains rhodium (hereinafter, Rh) as a noble metal catalyst component, and the noble metal catalyst component is only Rh in the first catalyst layer 3. In the present embodiment, the second catalyst layer 4 is an alumina layer containing platinum (hereinafter, Pt) as a noble metal catalyst component, and in the second catalyst layer 4, the noble metal catalyst component is only Pt. It is said that. In the present embodiment, the third catalyst layer 5 contains cerium oxide (hereinafter, CeO ₂ ) and alumina as main components, and palladium (hereinafter, Pd) is dispersedly contained therein.

The catalyst 1 as described above is formed, for example, as follows. That is, first, γ-alumina (A
1 ₂ O ₃ ) powder, an aqueous rhodium nitrate solution is added, mixed and stirred, dried and fired, and then pulverized with a ball mill to obtain a fine powder. Then, 480 g of γ-Al ₂ O _{3 having} Rh immobilized thereon, 120 g of beehmite (hydrated alumina), 1000 cc of water, and 10 cc of nitric acid were mixed and stirred by a homomixer to obtain an alumina slurry, which was then added to the alumina slurry. After immersing the catalyst carrier 2 having a honeycomb structure and pulling it up, excess slurry is removed by a high pressure air blower.
And dried at 250 ° C. for 2 hours, then 60
The first catalyst layer 3 is formed on the catalyst carrier 2 by firing at 0 ° C. for 2 hours. Next, similarly to the case of forming the first catalyst layer 3,
An aqueous solution of gintrodiamine platinum is added to γ-Al ₂ O ₃ , mixed and stirred, dried and baked, and then pulverized by a ball mill to obtain fine powder. Then, 480 g of γ-Al ₂ O ₃ having Pt fixed thereon, 120 g of boehmite,
1000 cc of water and 10 cc of nitric acid were mixed and stirred with a homomixer to obtain an alumina slurry.
After the catalyst carrier formed up to the catalyst layer 3 is dipped and pulled up to remove excess slurry, it is dried at 200 ° C. for 2 hours and then calcined at 600 ° C. for 2 hours to deposit on the first catalyst layer 3. The second catalyst layer 4 is formed. Next, an aqueous solution of gintrodiamine palladium is added to the CeO ₂ powder, mixed and stirred, dried and fired, and then pulverized with a ball mill to obtain a fine powder. 540 g of this powder, 60 g of boehmite, 1000 cc of water, and 10 cc of nitric acid were mixed and stirred,
Get the slurry. Then, in the slurry, the first and second
The catalyst carrier 2 formed up to the catalyst layers 3 and 4 is immersed and pulled up to remove excess slurry, then dried at 200 ° C. for 2 hours and then calcined at 600 ° C. for 2 hours to obtain a second catalyst layer. The third catalyst layer 5 is formed on the surface 4. Incidentally, each of the layers 3,
The washcoat amounts of 4 and 5 were adjusted so that each was 14 wt% with respect to the weight of the catalyst carrier 2, and the amount of Pt, Rh, and Pd supported as the noble metal catalyst component (based on the total volume of the catalyst layer in the product) was adjusted. Precious metal weight) is 1.33 for Pt
g / l, 0.27 g / l for Rh, and 1.0 g / l for Pd.

In such a catalyst, the noble metal catalyst component is Pt in the first catalyst layer 3 and Rh in the second catalyst layer 4.
Since only the Pd is provided solely in the third catalyst layer 5, alloying can be suppressed, and while Pd having high heat resistance is contained in the third catalyst layer 5 together with CeO ₂ , Based on the oxygen storage capacity effect of CeO ₂ ,
The dissociation of Pd (Pd oxide → Pd) can be increased to suppress sintering between Pd. As a result, it is possible to effectively suppress the thermal deterioration of the catalyst 1 and sufficiently stabilize the low temperature activity (heat resistance) of the catalyst.

Next, in order to support the heat resistance performance of the catalyst 1, a heat resistance test was conducted under the following test conditions while comparing with Comparative Example 1 (see FIG. 2) and Comparative Example 2 (see FIG. 3) below. went.

Comparative Example 1 Neodymium (Nd), Lanthanum (La), Cerium (C
Each nitrate of e) is absorbed in activated alumina powder, dried and then calcined at 700 ° C. for 2 hours to prepare alumina powder containing 0.1 mol / l of Nd, La or Ce, respectively. Alumina powder 100 containing Nd
Parts by weight and alumina sol 7 with an alumina content of 10 wt%
0 parts by weight and 20 parts by weight of water are mixed and stirred to produce an alumina slurry. A catalyst carrier was dipped in this slurry, pulled out from the slurry, blown off the slurry in the cell with an air stream, and dried at 250 ° C. for 2 hours.
Firing was carried out for 2 hours at a temperature of ℃ to form a first alumina coat layer. This carrier is immersed in an aqueous solution of palladium chloride (PdCl ₂ ) to support Pd on the alumina coat layer. Next, an activated alumina powder containing La was used to prepare an alumina slurry in the same manner, and the first slurry was prepared in this slurry.
The carrier on which the layer is formed is immersed, and the second layer is formed in the same manner as above. After that, it is immersed in a rhodium chloride (RhCl ₃ ) aqueous solution to support Rh on the second layer. Furthermore, using activated alumina containing Ce, the third layer was formed in the same manner as above, and the carrier on which the first layer and the second layer were formed was treated with gintrodiamine platinum (Pt (NH ₃ ) ₂ (NO ₂ ). ₂ ) Immersed in an aqueous solution, Pt was supported on this third layer to obtain a monolith catalyst as shown in FIG. At this time, the amount of noble metal catalyst component supported (weight of noble metal to the total volume of the catalyst layer in the product)
Is adjusted to 1.33 g / l for Pt, 0.27 g / l for Rh, and 1 g / l for Pd.

Comparative Example 2 On a catalyst carrier, first and second catalyst layers were provided in order from the catalyst carrier to the outside, and Pt and Rh were dispersed in alumina in the first catalyst layer. In the second catalyst layer, Pd is dispersedly contained in the main component CeO ₂ and alumina. In this case, the washcoat layer of each layer is adjusted to 14 wt% with respect to the weight of the catalyst carrier, and the catalyst loading amount in the first catalyst layer is 1.6 g.
/ L (Pt / Rh = 5/1), Pd in the second catalyst layer
The supported amount is 1.0 g / l.

Test conditions (1) Catalyst capacity: 24 ml (2) Aging conditions: The catalyst was heated at a temperature of 1000 ° C. for 5 hours.
Heat for 0 hours. (3) Space velocity of 600,000H under an air-fuel ratio of 14.5
^-1 .

As a result of the heat resistance test, the contents shown in FIG. 4 were obtained. According to this FIG. 4, the catalyst 1 according to Example 1 exhibits a higher HC purification rate from a lower temperature than Comparative Examples 1 and 2, and the catalyst 1 exhibits higher heat resistance than Comparative Examples 1 and 2. It was

FIG. 5 shows a second embodiment. The catalyst 1 according to the second embodiment is configured such that the catalyst carrier 2 is provided with the first, second, and third catalyst layers 3, 4, and 5 as in the first embodiment. The noble metal catalyst component of the catalyst layers 3 and 4 is changed from that of the first embodiment. That is, the first catalyst layer 3 contains Pt as a noble metal catalyst component.
In the present embodiment, the alumina layer further contains Ba. The second catalyst layer 4 is an alumina layer containing Rh as a noble metal catalyst component, and the alumina layer further contains Zr in this example.

The formation (manufacture) of the catalyst 1 according to the second embodiment is basically the same as that of the first embodiment, but the noble metal catalyst components in the first and second catalyst layers 3 and 4 are changed. Accordingly, only the formation of the first and second catalyst layers 3 and 4 is different. That is, in the formation of the first catalyst layer 3, an aqueous solution of gintroamine platinum is added to the γ-Al ₂ O ₃ powder, mixed and stirred, dried and fired, and then ground with a ball mill to obtain a fine particle state. Use as powder. Then, an aqueous barium nitrate solution is added to this powder, mixed and stirred, and then dried and fired. Thereby, γ-Al ₂ O
₃ , Pt and Ba are fixed in the alumina particles. Then, using the above-mentioned Pt and Ba-immobilized γ-Al ₂ O ₃ , alumina slurry was obtained as in the formation of the first catalyst layer of the first embodiment, and the catalyst carrier 2 was dipped in it and pulled up. After that, it is dried and calcined to form the first catalyst layer 3 on the catalyst carrier 2. In forming the second catalyst layer 4, γ-Al ₂ O ₃
An aqueous rhodium nitrate solution is added to and mixed with stirring, dried and fired, and then pulverized with a ball mill to obtain fine powder. Then, an aqueous zirconium nitrate solution is added to this powder, mixed and stirred, and dried and fired. This allows
In γ-Al ₂ O ₃ , Rh and Zr are fixed in the alumina particles. And Rh and Z above
Using γ-Al ₂ O _{3 in} which r and r are immobilized,
As in the case of forming the second catalyst layer in the example, an alumina slurry was obtained,
The catalyst carrier formed up to the first catalyst layer is dipped therein and pulled up, and then dried and fired to form the second catalyst layer 4 on the first catalyst layer 3. In addition, each layer 3, 4,
The wash coat amount of 5 and the amount of the precious metal catalyst component supported are
As in the first embodiment, the amounts of Ba and Zr added are 5 to 8 (wt% / wash coat amount) in this embodiment.

In such a catalyst 1, the above-mentioned first catalyst is used.
In addition to producing the same effect as the catalyst according to the example, the second
Since Rh is contained in the catalyst layer 4, CeO in the third catalyst layer 5 is formed at the layer boundary between the second and third catalyst layers 4 and 5.
₂ Based on the oxygen storage capacity effect, the purification performance of NO _x can be significantly improved.

Next, in order to support the heat resistance performance (HC purification rate measurement) and NO _x purification performance of the catalyst according to the second embodiment, the catalyst according to the first embodiment, Comparative Example 1 and Comparative Example 2 are described. For comparison, a heat resistance test and a NO _x purification test were performed under the same test conditions as the heat resistance test described above.

Regarding the results of the heat resistance test, the characteristic line shown in FIG. 4 was obtained. According to this content, the catalyst according to the second example exhibited higher heat resistance than the catalyst according to the first example. As for the result of the NO _x purification test, the content of FIG. 6 was obtained. According to the contents of FIG. 6, the catalyst according to the second embodiment also showed high NO _X purification performance.

FIG. 7 shows a third embodiment. The catalyst according to the third embodiment is basically configured in the same manner as the catalyst according to the first embodiment described above, but in the present embodiment, the first, second and third catalyst layers 3 are formed. The specific surface areas of Nos. 4, 5 are sequentially increased from the first catalyst layer 3 toward the third catalyst layer 5, and the particles of each layer 3, 4, 5 become finer toward the third catalyst layer 5. Has been done.

In this embodiment, in order to adjust the specific surface area of each of the layers 3, 4 and 5, the material powder of each of the layers 3, 4 and 5 is heat-treated under different conditions. The catalyst is to be formed by the manufacturing method of the first embodiment described above. That is, in order to reduce the specific surface area of the first catalyst layer 3, the γ-Al ₂ O ₃ powder is heat-treated in the air at 1000 ° C. for 50 hours, so that the specific surface area of the second catalyst layer 4 is moderate. Γ-Al ₂ O ₃
The powder is heat-treated in air at 900 ° C. for 50 hours, and Ce is added in order to increase the specific surface area of the third catalyst layer 5.
The O ₂ powder is heat-treated in the air at 800 ° C. for 50 hours.

In such a catalyst 1, since the third catalyst layer 5 forming the outermost surface of the reaction has a large specific surface area, the reaction rate is promoted even at a low temperature and the low temperature activity is properly improved. Moreover, the denseness of the third catalyst layer 5 due to the large specific surface area prevents the invasion of phosphorus, lead, etc., and also improves the poisoning resistance. On the other hand, the second,
Since the specific surface area is made smaller toward the first catalyst layers 4 and 3, the ventilation resistance in the catalyst can be reduced.

Next, in order to support the (low temperature activity) performance of the above catalyst 1, the catalyst according to the first embodiment (heat treatment of CeO ₂ powder was performed at 800 ° C. for 50 hours in air). The HC purification test was conducted on the basis of comparison, and the first
~ HC50% by varying the specific surface area of the third catalyst layer
An HC 50% purification temperature test was conducted to measure the purification temperature for obtaining the purification rate. The test conditions for both tests are the same as the test conditions for the heat resistance test described above.

As a result of the above HC purifying test, FIG.
And the content of FIG. 9 for the above HC50% purification temperature test. According to the contents of FIG. 8, the third
The catalyst according to the example shows higher purification performance from a lower temperature than the catalyst according to the first example, and according to the content of FIG. 9, the combination of the specific surface areas of the layers in the present example is the specific surface area of each layer. Shows that the combination of specific surface areas of the layers in this example is appropriate in that it has a low temperature activity similar to that of all the above, and can also have the effect of reducing the ventilation resistance in the catalyst. It was

FIG. 10 shows a fourth embodiment.
The catalyst according to the fourth embodiment is basically the same as the above first catalyst.
Although it has the same structure as the catalyst according to the example, in this example, Pd of the third catalyst layer 5 is fixed at the time of fixing to CeO ₂ and at the time of firing the slurry coat layer. , Calcination in an oxidizing atmosphere in which O ₂ is added by 5% relative to the case of air (CeO before obtaining a slurry for immersing the catalyst carrier 2 etc.)
Calcination for fixation to ₂ and calcination of this slurry-coat layer: see the manufacturing method in the first embodiment), Pt of the second catalyst layer 4 and Rh of the first catalyst layer 3 are respectively γ-
When the slurry coat layer is fixed to Al ₂ O ₃ and each of the slurry coat layers is fired, it is fired in a reducing atmosphere in which H ₂ is added by 5% with respect to the case of air.

In such a catalyst 1, since it can be considered that the proportion of PdO increases, the dissociation of Pd (PdO
→ Pd) Temperature can be raised and heat resistance (stabilization of low temperature activity) can be improved.

In order to support the heat resistance performance of such a catalyst, a heat resistance test was conducted under the same test conditions as the heat resistance test in the above-mentioned first embodiment while comparing with the catalyst according to the second embodiment. went. As a result of this heat resistance test, the contents of FIG. 11 were obtained. According to the contents of FIG. 11, the catalyst according to the fourth example exhibited a slightly higher heat resistance performance than the catalyst according to the first example. Similarly, when the NO _x purification performance was also tested, the content shown in FIG. 12 was obtained. According to the content of FIG. 12, the catalyst according to the fourth embodiment is significantly higher than the catalyst according to the first embodiment in the NO _x purification performance in the lean air-fuel ratio operation region of the engine.

[0027]

As described above, the present invention can sufficiently stabilize the low temperature activity of the catalyst.

[Brief description of drawings]

FIG. 1 is a diagram conceptually showing a catalyst according to a first embodiment.

FIG. 2 is a view conceptually showing a catalyst according to Comparative Example 1.

FIG. 3 is a diagram conceptually showing a catalyst according to Comparative Example 2.

FIG. 4 is a characteristic diagram showing the results of heat resistance tests of the catalysts according to the first and second examples and the catalysts according to comparative examples 1 and 2.

FIG. 5 is a view conceptually showing a catalyst according to a second embodiment.

FIG. 6 is a characteristic diagram showing the results of a NO _x purification test of the catalyst according to the second example.

FIG. 7 is a diagram conceptually showing a catalyst according to a third embodiment.

FIG. 8 is a characteristic diagram showing the results of a heat resistance test of the catalyst according to the third example.

FIG. 9 is a characteristic diagram showing the results of an HC 50% purification temperature test when various combinations of the specific surface areas of the first, second and third catalyst layers were changed.

FIG. 10 is a view conceptually showing a catalyst according to a fourth embodiment.

FIG. 11 is a characteristic diagram showing the results of the heat resistance test of the catalyst according to the fourth example.

FIG. 12 is a characteristic diagram showing the results of a NO _x purification performance test of the catalyst according to the fourth example.

[Explanation of symbols]

 1 catalyst 2 catalyst carrier 3 first catalyst layer 4 second catalyst layer 5 third catalyst layer

Claims (6)

[Claims] 1. An exhaust gas purifying catalyst for an engine, wherein a first catalyst layer, a second catalyst layer and a third catalyst layer are provided on a catalyst carrier in this order from the catalyst carrier toward the outside. The exhaust gas of an engine, wherein the second catalyst layer is composed of an alumina layer containing different one kind of noble metal catalyst component, and the third catalyst layer contains palladium and cerium oxide. Purification catalyst. 2. The first catalyst layer according to claim 1, wherein one of platinum and rhodium is contained as a noble metal catalyst component, and the other catalyst of platinum and rhodium is contained as a noble metal catalyst component in the second catalyst layer. A catalyst for purifying engine exhaust gas, which is characterized in that 3. The exhaust gas purifying catalyst for an engine according to claim 1, wherein the first catalyst layer contains rhodium and the second catalyst layer contains platinum. 4. The exhaust gas purifying catalyst for an engine according to claim 1, wherein the first catalyst layer contains platinum and the second catalyst layer contains rhodium. 5. The specific surface area according to claim 1, wherein the specific surface areas of the first, second and third catalyst layers are increased from the first catalyst layer toward the third catalyst layer. A catalyst for purifying engine exhaust gas, which is characterized in that 6. The exhaust gas of an engine according to claim 1, wherein as the palladium in the third catalyst layer, one that is fired in an oxidizing atmosphere when being immobilized on cerium oxide is used. Purification catalyst.