JPH0522261Y2 - - Google Patents

Description

[Detailed explanation of the idea]

[Industrial Field of Application] This invention relates to a monolithic exhaust gas purifying catalyst used for purifying exhaust gas in internal combustion engines such as automobiles. [Prior art] Conventional catalysts for exhaust gas purification are made by depositing activated alumina powder containing cerium on the surface of a monolithic carrier base material, and then uniformly applying Pt, Rh, Pd, etc. individually or in combination to the carrier. The catalyst supported on
−159391 etc. [Problems with the prior art] However, with such conventional catalysts for purifying exhaust gas, due to the reaction heat generated during purification of the HC components contained in the exhaust gas, a considerable amount of HC components are removed, especially during acceleration. Since it is contained in the exhaust gas, the reaction heat generated when purifying the exhaust gas causes the area near the active site of the catalyst to be locally exposed to quite high temperatures. There was a problem in that this localized high temperature caused deterioration of the catalyst. [Means for solving the problem] This idea was made by focusing on such conventional problems.The catalyst carrier is heated at least in air on the upstream side to increase the particle size and stabilize it thermally. The above problem can be solved by providing a part that supports oxidized platinum, reacting HC such as propane selectively at a certain temperature, reducing the amount of HC flowing into the subsequent part, and reducing the exothermic components in the succeeding part. This is an attempt to resolve the issue. In addition, at least the part coated with platinum on the upstream side and the catalyst on the downstream side may be an integrated catalyst or may be separate from each other. In the latter case, the upstream and downstream catalysts may be in close contact with each other, or may be separated by a slight gap. Also good. In addition, at least the portion coated with platinum on the upstream side preferably has a length of 1/10 to 1/2 of the total length of the catalyst carrier. [Operation] The internal combustion engine of a car produces a considerable amount of energy during acceleration.
HC is discharged, and the HC contains a large amount of paraffinic HC such as propane. Among the engine exhaust gas purification reactions, oxidation of HC is the largest exothermic reaction, and it is the one that leads to a rise in the temperature of the catalyst bed.
This is oxidation of HC. Therefore, when gas containing a large amount of HC comes into contact with the catalyst during exhaust gas purification, the catalyst is locally exposed to high temperatures, causing sintering or the like in that area, resulting in deterioration of the catalyst. However, when a platinum catalyst supported on alumina is exposed to high temperatures, the degree of dispersion decreases and the particle size increases, but the oxidation performance of HC such as propane does not depend on the degree of dispersion.
It shows constant activity. On the other hand, in general, the larger the particle size, the worse the performance in purifying reactions such as CO. In other words, by increasing the particle size, HC such as propane can be oxidized selectively to some extent. Therefore, as in the present invention, in the platinum/alumina catalyst section installed upstream of the catalyst, the average particle diameter of platinum is 85 Å (dispersity 0.1) to 170 Å (dispersity 0.05), and propane is produced with a large amount of heat. By reacting paraffin-based HC such as to some extent and sending the remaining gas component with less HC into the subsequent part, preferably with an average platinum particle size of 30 Å (dispersity 0.3), local heat generation is generated in the subsequent part. This suppresses the thermal deterioration of platinum/rhodium that follows, and maintains a highly dispersed state. Therefore, the durability of the catalyst as a whole is improved. The present invention will be explained below with reference to Examples, Comparative Examples, and Test Examples. Example 1 FIG. 1 is a diagram showing an example of this invention. The catalytic converter 1 houses therein a carrier catalyst 2 made of inorganic material, and the carrier catalyst is composed of an upstream catalyst section 3 and a downstream catalyst section 4. Exhaust gas enters from the upstream inlet 5 of the catalytic converter 1, causes HC in the exhaust gas to react to some extent in the upstream catalyst section 3 containing at least Pt, which is located upstream of the catalytic converter, and then passes through the subsequent downstream catalyst section 4. The remaining gas enters, is purified, and is discharged from the outlet 6 on the downstream side of the catalytic converter. A cordierite support with a density of 400 cells per square inch and a capacity of 1.7 was prepared, and activated alumina to which cerium was pre-attached was uniformly coated over the entire substrate, dried, and then fired at 650°C for 2 hours. . Next, the carrier is immersed in a Pt solution from one end to 1/3 of its total length to support only Pt.
℃, 2 hours baking, 750℃, 4 hours heat treatment Pt 0.9g
A catalyst part (a) containing the following was obtained. Subsequently, the remaining end-supported portion was immersed in a mixed solution containing Pt and Rh, dried, and heated to 650°C.
After calcination at ℃ for 2 hours, a catalyst part B supporting 1.0 g of Pt and 0.19 g of Rh was obtained. In this case, the part that was first impregnated only with Pt should not be impregnated with noble metal again. Through this process, the catalyst part A is made of noble metal that is thermally stabilized by heat treatment and has a large particle size.
A catalyst A was obtained in which only Pt was supported, and Pt and Rh were supported in a uniform and highly dispersed manner in the catalyst part (b). In addition, in the catalyst A obtained in this example, the average particle diameter of Pt in catalyst part (a) was 130 Å, and the average particle diameter of Pt in catalyst part (b) was 20 Å. Example 2 Catalyst B was obtained in the same manner as in Example 1, except that the amount of Pt supported in catalyst section A was 0.5 g, and the amounts of Pt and Rh supported in catalyst section B were 1.5 g and 0.19 g, respectively. Furthermore, catalyst B
The average particle diameter of Pt in catalyst part (a) was 130 Å, and the average particle diameter of Pt in catalyst part (b) was 20 Å. Example 3 Pt and Rh from one end of the carrier to 1/5 of the total length
The same procedure as in Example 1 was carried out, except that the amount of Pt supported in catalyst part A was 0.38 g and the amount of Rh supported was 0.04 g, and the amount of Pt and Rh supported in catalyst part B was 1.5 g and 0.15 g, respectively. Catalyst C was obtained. In addition, in catalyst C, the average particle diameter of Pt in catalyst part A is 130 Å, and the Pt in catalyst part B is 130 Å.
The average particle diameter was 20 Å. Comparative Example 1 Capacity with density of 400 cells per square inch
Activated alumina pre-deposited with cerium is uniformly coated on the cordierite support No. 1.7 over the entire substrate, dried, and then fired at 650°C for 2 hours.
Next, the entire carrier is immersed in a mixed solution containing Pt and Rh.
After drying and firing at 650℃ for 2 hours, Pt 1.9g/piece,
A catalyst X supporting 0.19 g/piece of Rh was obtained. Note that the average particle diameter of Pt in Catalyst X was 20 Å. Test Example 1 An actual vehicle durability test (engine durability) was conducted on catalysts A, B, and C obtained in Examples 1, 2, and 3 and catalyst X obtained in Comparative Example 1 under the following conditions, and the purification rate of 10 mode emission was Measure the purification rate by √×
are shown in Table 1. Engine durability conditions Catalyst inlet temperature 750℃ Durability time 100hr Space velocity Approx. 70000hr ^-1 Evaluation conditions Vehicle 1960 Skyline Engine Displacement 2000c.c.ECCS

[Effect of idea]

As explained above, according to this idea,
In the monolithic exhaust gas purification catalyst, the upstream side of the catalyst supports at least platinum with an average particle size of 85 Å to 170 Å, and the subsequent downstream part contains rhodium and platinum with a smaller particle size than the platinum on the upstream side. Because of the structure in which it carries
In order to cause HC to react to some extent, the downstream catalyst
Local heat generation during HC purification can be suppressed,
The effect of suppressing thermal deterioration of platinum and rhodium and improving the durability of the catalyst can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the present invention,
FIG. 2 is a diagram showing the relationship between the oxidation activity and the degree of dispersion of a platinum catalyst. 1...Catalytic converter, 2...Carrier catalyst, 3...
...upstream catalyst section, 4...downstream catalyst section, 5...upstream inlet, 6...downstream outlet, 7...Pt(NO ₃ ) ₂
Reaction of propane-O ₂ system impregnated with (NH ₃ ) ₂ solution,
8... Reaction of propane-O ₂ system impregnated with H ₂ PtCl ₆ solution, 9... Impregnated with Pt(NO ₃ ) ₂ (NH ₃ ) ₂ solution
CO-O ₂ system reaction, 10... CO-O ₂ system reaction impregnated with H ₂ PtCl ₆ solution.

Claims (1)

[Scope of utility model registration request] In the monolithic exhaust gas purification catalyst, the catalyst supports platinum with an average particle size of at least 85 Å to 170 Å on the upstream side, and the subsequent downstream portion supports rhodium and platinum with a particle size smaller than the platinum on the upstream side. An exhaust gas purifying catalyst characterized by comprising a catalyst supporting platinum.