JPH1043588A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalyst for purification of exhaust gas excellent in HC removing performance especially at a time immediately after starting by coating the top of a coating layer of a catalyst compsn. contg. Pt or Pd with an HC desorption inhibiting layer contg. a refractory inorg. material and allowing hydrocarbons to diffuse in various directions. SOLUTION: A coating layer of a catalyst compsn. contg. Pt or Pd and a refractory inorg. oxide based on activated alumina is formed on a catalyst carrier and the top of the coating layer is coated with an HC desorption inhibiting layer contg. a refractory inorg. material and allowing hydrocarbons to diffuse in various directions to obtain the objective catalyst for purification of exhaust gas. The HC desorption inhibiting layer is, e.g. a layer contg. two or more kinds of inorg. materials different from each other in the peaks of the pore diameter distributions, e.g. α-alumina, activated alumina, silica, a rare earth metal oxide, zirconium oxide, a metallic silicate and an arbitrary mixture of them.

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 for purifying exhaust gas discharged from an internal combustion engine such as an automobile engine (gasoline or diesel), a boiler or the like. The present invention relates to an exhaust gas purifying catalyst capable of reducing the amount of hydrocarbons (HC) to be discharged.

[0002]

2. Description of the Related Art In recent years, due to the problem of air pollution and global warming, exhaust gas regulations have become more and more strict, and therefore, development of exhaust gas purifying means has been receiving more and more attention. For example, JP-A-63-185451 discloses a refractory inorganic oxide having 5 to 30% by weight of platinum and 1 to 20% by weight of rhodium and having an average particle size of 0.5 to 20 μm. There has been proposed an exhaust gas purifying catalyst in which a catalyst composition containing particles is supported on a honeycomb carrier.

[0003]

However, in such an exhaust gas purifying catalyst, the average particle size of the refractory inorganic oxide is set to 0.5 to 20 μm to control the dispersibility of the noble metal and to improve the durability. Although the performance was improved, no attention was paid to the pore diameter peak of the inorganic oxide. For this reason, in the low temperature range of the exhaust gas immediately after the start of the internal combustion engine, HC is adsorbed by the noble metal as a catalyst component, and the adsorbed HC is exhausted at a stretch as the temperature of the exhaust gas rises. Therefore, there has been a problem that the catalyst performance cannot be sufficiently exhibited and the purification of HC becomes insufficient. The present invention has been made in view of such problems of the related art, and the purpose thereof is to:
In particular, it is an object of the present invention to provide an exhaust gas purifying catalyst having excellent HC purifying performance immediately after starting.

[0004]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and as a result, have found that the above problems can be solved by providing a specific HC desorption suppressing layer. It was completed. That is, the exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst obtained by coating a catalyst support with a catalyst composition containing a refractory inorganic oxide mainly composed of activated alumina containing platinum and / or palladium. The catalyst is characterized in that the catalyst composition coating layer is coated with a hydrocarbon desorption suppressing layer containing a refractory inorganic substance and in which hydrocarbons diffuse in various directions.

[0005]

In the present invention, a HC desorption-suppressing layer mainly composed of at least two or more refractory inorganic substances having different pore diameter peaks is coated on a carrier coated with the catalyst composition. Therefore, it is possible to effectively purify HC components discharged at a stretch when the temperature of the exhaust gas rises.

For example, a catalyst coated with activated alumina containing a noble metal component is widely used as a three-way catalyst for an internal combustion engine. Since the temperature of the surface of the noble metal component does not rise sufficiently to reach the activation temperature of the catalyst, no catalytic reaction occurs, and the HC component is adsorbed on the surface of the noble metal. This adsorbed HC component, with the rise of the exhaust gas temperature,
Since the noble metal component surface desorbs before reaching the catalytic reaction temperature and is discharged at once without being purified, such H
Sufficient purification ability cannot be obtained for the C component.

On the other hand, according to the present invention, as described above, two or more kinds of refractory inorganic substances having different peaks in the pore size distribution are coated on a catalyst layer such as an ordinary three-way catalyst. The desorbed HC component can be gas-diffused in the HC desorption suppressing layer at the time of desorption in various directions and can be prevented from desorbing at once, so that the original HC purification function of the noble metal component can be sufficiently exhibited. Will be possible.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the exhaust gas purifying catalyst of the present invention will be described in detail. As described above, the exhaust gas purifying catalyst of the present invention includes, on a catalyst carrier, a catalyst composition coat layer containing a refractory inorganic oxide whose main component is activated alumina containing platinum and / or palladium. here,
The catalyst carrier is not particularly limited, and various conventionally known carriers can be used. However, it is preferable to use a honeycomb-type carrier as an exhaust gas purifying catalyst for an automobile. The reason why platinum or palladium is included is that the present invention particularly intends to improve the HC purification ability, and as long as the HC purification ability can be exerted, a three-way catalyst to which rhodium or the like is added in addition to these. It is also possible to adopt a configuration of

Further, as described above, the exhaust gas purifying catalyst of the present invention is obtained by coating the above-mentioned catalyst composition coating layer with an HC desorption suppressing layer containing a refractory inorganic substance, and comprising platinum and / or palladium. HC adsorbed on the gas is diffused in various directions in the HC desorption suppressing layer. As the HC desorption suppressing layer, any layer capable of realizing the above gas diffusion is sufficient. Specifically, a layer containing two or more types of refractory inorganic substances having different peaks in the pore diameter distribution is used. Can be mentioned. It is not preferable to use the same refractory inorganic substance having the same peak of the pore diameter distribution or a single kind of the refractory inorganic substance, because gas diffusion occurs promptly and the effect of suppressing HC desorption is hardly generated.

Here, examples of the at least two or more kinds of refractory inorganic substances having different pore diameter peaks include α-alumina, activated alumina, silica, rare earth metal oxide, zirconium oxide, metal silicate and an arbitrary mixture thereof. can do. It is sufficient for these refractory inorganic substances to have different pore diameter peaks. For example, even if it is a single kind, activated alumina before firing and firing at a temperature of 600 ° C. or more in an air atmosphere, The peaks may be used in combination with alumina whose peak has been changed from that before firing.

[0011]

The present invention will be described below in more detail with reference to examples and comparative examples. (Example 1) 940 g of activated alumina powder mainly composed of γ-alumina carrying 4% by weight of Pd, 860 g of activated alumina powder mainly composed of γ-alumina, 200 g of cerium powder, and 2000 g of 2% nitric acid solution Was charged into a magnetic pot, mixed and pulverized with a vibration mill device to produce a first washcoat slurry. The slurry was applied to a cordierite monolithic carrier (1.3 L, 400 cells), dried, and calcined at 400 ° C. for 1 hour in an air atmosphere. This coating operation is performed after the applied amount is about 1
Repeat until the amount of Pd becomes 80 g / L.
The catalyst layer was formed so as to be cf. Next, α-
1000 g of alumina powder, 1000 g of activated alumina powder mainly composed of γ-alumina, 200 g of 2% nitric acid solution
Was charged into a magnetic pot and mixed and pulverized with a vibration mill to obtain a second washcoat slurry. The slurry was applied to the catalyst layer, dried, and calcined at 400 ° C. for 1 hour in an air atmosphere. The coating operation was repeated until the coating amount at this time became about 50 g / L after firing, and the exhaust gas purification catalyst No. 1 was used. 1 was obtained. Table 1 shows the catalyst coat layer structure.

(Example 2) ZSM instead of α-alumina
-5 zeolite (SiO ₂ / 2Al ₂ O ₃ = 30, H type)
Except that the exhaust gas purifying catalyst No. was used in the same manner as in Example 1 except that 2 was obtained.

(Example 3) In place of α-alumina, 95
Exhaust gas purification catalyst No. 1 was prepared in the same manner as in Example 1, except that activated alumina powder calcined in an air atmosphere at 0 ° C. for 4 hours was used. 3 was obtained.

Example 4 Except that cerium oxide was used in place of α-alumina, an exhaust gas purifying catalyst No. 1 was used in the same manner as in Example 1. 4 was obtained.

Example 5 Exhaust gas purification catalyst No. 1 was prepared in the same manner as in Example 1 except that zirconium oxide was used instead of α-alumina. 5 was obtained.

(Example 6) Exhaust gas purification catalyst No. 5 was prepared in the same manner as in Example 5 except that ZSM-5 zeolite was used instead of γ-alumina used in the second washcoat slurry. 6 was obtained.

(Example 7) Pd 8 wt% was used instead of Pd 4 wt% alumina, and the amount of Pd carried was 160 g / cf in the first wash coat slurry coating operation. Exhaust gas purification catalyst No. 7 was obtained.

Example 8 4% by weight Pd supported alumina 9
Instead of 40 g, 860 g of Pd 4 wt% supported alumina and 156 g of Rh 2 wt% supported alumina were used, and 714 g of activated alumina powder containing γ-alumina as a main component and 270 g of cerium powder were used to coat the first washcoat slurry. 160g / cf Pd loading
In the same manner as in Example 1, except that the exhaust gas purifying catalyst No. was used. 8 was obtained.

(Example 9) 4% by weight of Pd supported alumina 9
Instead of 40 g, 781 g of Pd 4 wt% supported alumina and 156 g of Rh 2 wt% supported alumina were used, 631 g of activated alumina powder mainly composed of γ-alumina and 350 g of cerium powder were used to coat the first washcoat slurry. 160g / cf Pd loading
In the same manner as in Example 1, except that the exhaust gas purifying catalyst No. was used. 9 was obtained.

(Example 10) Except that the coating amount of the surface layer of Example 1 was changed to 100 g / L instead of 50 g / L, the same procedure as in Example 1 was carried out.
10 was obtained.

Comparative Example 1 γ-supporting 4% by weight of Pd
940 g of activated alumina powder containing alumina as a main component;
860 g of activated alumina powder mainly composed of γ-alumina
And 200 g of cerium powder and 2000 g of a 2% nitric acid solution
And into a magnetic pot, mixed and crushed with a vibration mill device,
A washcoat slurry was produced. The slurry was applied to a cordierite monolithic carrier (1.3 L, 400 cells), dried, and calcined at 400 ° C. for 1 hour in an air atmosphere. This coating operation is repeated until the applied amount becomes about 150 g / L after firing, and the Pd carrying amount becomes 80 g / cf so that the exhaust gas purifying catalyst N
o. 11 was obtained.

(Comparative Example 2) γ-supporting 4% by weight of Pd
940 g of activated alumina powder containing alumina as a main component;
860 g of activated alumina powder mainly composed of γ-alumina
And 200 g of cerium powder and 2000 g of a 2% nitric acid solution
And into a magnetic pot, mixed and crushed with a vibration mill device,
A first washcoat slurry was produced. This slurry was used as a cordierite monolith carrier (1.3 L 40
0 cells), dried and calcined at 400 ° C. for 1 hour in an air atmosphere. This coating operation was repeated until the applied amount became about 150 g / L after firing, and the catalyst layer was formed such that the Pd carrying amount became 80 g / cf. Next, 2000 g of α-alumina powder and 2000 g of a 2% nitric acid solution were charged into a magnetic pot, and mixed and pulverized by a vibration mill to obtain a second washcoat slurry. The slurry was applied to the catalyst layer, dried, and calcined at 400 ° C. for 1 hour in an air atmosphere. The coating operation was repeated until the coating amount at this time became about 50 g / L after firing, and the catalyst for exhaust gas purification No. 1 was used. 12 was obtained.

The exhaust gas purifying catalyst obtained in each example was evaluated for HC purification characteristics (FTP 75) using a vehicle (displacement 3 L) manufactured by Nissan Motor Co., Ltd. under the following evaluation conditions.
A-bag), and the obtained results are shown in Table 1. Performance evaluation conditions Catalyst capacity 1.7L Evaluation vehicle Nissan Motor Co., Ltd. V-type 6-cylinder 3000cc engine Evaluation mode LA4-CH (A-bag) Hydrocarbon (in gas at catalyst inlet) discharged at engine start Carbon number C _{2 ~C 3} 21.2% (excluding C1 _{component) C 4 ~C 6 33.0% C} 7 ~C 9 45.8%

[0024]

[Table 1]

Although the present invention has been described in detail with reference to examples and comparative examples, the present invention is not limited to these examples, and various modifications can be made within the scope of the present invention. For example, in each of Examples and Comparative Examples, a cordierite honeycomb carrier was used, but a metal honeycomb carrier can also be used.

[0026]

As described above, according to the present invention, since the specific HC desorption suppressing layer is coated, the exhaust gas which can exhaustively purify the HC components discharged immediately after the start can be effectively purified. A gas purification catalyst can be provided.

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication location B01J 29/44 F01N 3/10 ZABA 35/10 301 3/20 ZABD F01N 3/10 ZAB B01D 53 / 36 104Z 3/20 ZAB B01J 23/56 301A

Claims (6)

[Claims] 1. An exhaust gas purifying catalyst comprising a catalyst support and a catalyst composition containing a refractory inorganic oxide mainly composed of activated alumina containing platinum and / or palladium, the catalyst composition comprising: On the material coat layer, contains a refractory inorganic material,
An exhaust gas purifying catalyst comprising a hydrocarbon desorption suppressing layer in which hydrocarbons diffuse in various directions. 2. The exhaust gas according to claim 1, wherein the refractory inorganic substance contained in the hydrocarbon desorption suppressing layer comprises at least two or more kinds of refractory inorganic substances having different pore diameter distribution peaks. Purification catalyst. 3. The at least one refractory inorganic substance having different pore diameter distribution peaks selected from the group consisting of α-alumina, activated alumina, silica, rare earth metal oxide, zirconium oxide and metal silicate. The exhaust gas purifying catalyst according to claim 2, characterized in that: 4. The two or more refractory inorganic substances having different peaks in the pore size distribution are activated alumina, and the activated alumina is mixed with activated alumina before firing at a temperature of 600 ° C. or higher in air. The exhaust gas purifying catalyst according to claim 3, comprising alumina. 5. The exhaust gas purifying catalyst according to claim 1, wherein the catalyst carrier is a honeycomb type carrier. 6. The exhaust gas purifying catalyst according to claim 1, wherein the hydrocarbon reduction rate is 35% or more.