JP2001062298A - Exhaust gas cleaning catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide lasting ability to remove HC, CO and NOx to an exhaust gas cleaning catalyst containing Pd as a catalyst component. SOLUTION: The objective exhaust gas cleaning catalyst has a substrate, an alumina-base carrier layer supported on the surface of the substrate and Pd and Cu supported on the carrier layer. Since Cu is present, the sintering of Pd supported alumina particles is prevented.

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 emitted from an internal combustion engine of an automobile or the like, and more particularly, to carbon monoxide (CO) and carbonized carbon contained in exhaust gas. Hydrogen (HC), nitrogen oxides (N
O _X) or the like can be efficiently reduced and purified to a high exhaust gas purifying catalyst durability.

[0002]

2. Description of the Related Art Conventionally, exhaust from an engine such as an automobile.
, CO, NO in exhaust gas _XPurifying exhaust
Many gas purification catalysts have been proposed, and at present,
HC, CO, NO under engine stoichiometric air-fuel ratio_XThe same
Three-way catalysts, which sometimes purify, have become mainstream.

[0003] The above three-way catalysts include alumina, alumina-
It is prepared by dispersing and supporting a catalyst component such as Pt, Pd, Rh or the like singly or in combination on a carrier layer made of a refractory inorganic oxide powder such as silica or silica.

[0004] Specifically, after a honeycomb-shaped substrate made of cordierite or metal is coated with a refractory inorganic oxide such as alumina, alumina-silica or silica as a carrier layer, the carrier layer is coated. Pt, P
In general, these catalyst components are supported on the surface of a carrier layer by immersing the catalyst components such as d and Rh alone or in combination in an aqueous solution.

[0005]

However, among the above-mentioned conventional catalysts, the three-way catalyst containing Pd as a catalyst component shows good catalytic activity at an early stage, but under the conditions of use of the three-way catalyst. Has a problem that the durability is not sufficient.

Accordingly, an object of the present invention to be achieved by a higher durability with respect to purification performance of exhaust gas purifying catalyst HC, CO, and NO _X containing Pd as a catalyst component.

[0007]

The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, the reason why the exhaust gas purifying catalyst containing Pd has low durability is that the exhaust gas purifying catalyst is poor. Was found to be due to sintering of particles carrying Pd during use, and further supporting Cu on a carrier carrying Pd containing alumina as a main component,
It has been discovered that sintering of particles carrying d is suppressed. The present invention has been made based on the above findings.

That is, an exhaust gas purifying catalyst for solving the above-mentioned problems comprises a substrate, a carrier layer mainly composed of alumina carried on the surface of the substrate, and a P layer carried on the carrier layer.
It is characterized by having d and Cu.

It is considered that this is because the presence of Cu prevents sintering between Pd-supported alumina particles.

Preferably, the carrier layer further contains a compound comprising at least one of a rare earth element, an alkaline earth metal and zirconium. Although the reason is not clear, the purification performance is improved by the synergistic effect with Cu and Pd.

The amount of copper carried is preferably 0.0001 mol or more per liter of catalyst volume.
It is more preferable that the content be from 0.01 to 0.1 mol. If the amount is less than this, the effect of improving the durability of the catalyst cannot be exhibited, and if the amount is more than this, the catalytic action of Pd is inhibited.

[0012] More preferably, copper is loaded after Pd is already loaded on the carrier layer. This is because covering the surface of Pd with Cu further prevents sintering of Pd. Also, if Pd and Cu are mixed at the molecular level even if the Pd surface is not covered by Cu,
This is a preferable state in which the durability can be improved as compared with the case where they are not mixed at the molecular level.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an embodiment of an exhaust gas purifying catalyst according to the present invention will be described in detail. Needless to say, the present invention is not limited by the following embodiments.

An exhaust gas purifying catalyst of the present invention comprises:
And a carrier layer.

The substrate forms a catalyst for purifying exhaust gas by carrying a carrier layer on the surface. The base material is a pellet,
A known base material having a large specific surface area such as a honeycomb shape can be used. Among them, a honeycomb shape is preferable because the specific surface area is large and the permeation resistance of the exhaust gas can be suppressed low.

The material of the base material has a strength that can be practically used in the state of use of the exhaust gas purifying catalyst, and preferably has a high affinity for the carrier layer and is chemically and physically compatible with the exhaust gas. It is better to be stable. For example, known materials such as cordierite and metal can be used.

Alumina is preferably in a form having a large specific surface area, for example, powdered alumina. The carrier layer is formed by coating the surface of a substrate with alumina by a known method. The carrier layer is preferably made porous for the purpose of increasing the specific surface area. Also, alumina itself is preferably porous to increase the specific surface area.

Further, the carrier layer may be further formed of various elements in addition to the alumina layer. Among them, it is preferable to add at least one compound among rare earth elements, alkaline earth metals, and zirconium.

These compounds are preferably oxides, inorganic salts or organic salts of the respective elements. It is to be noted that the compound includes a state of an element simple substance.

The method of adding these compounds to the carrier layer is as follows:
Alumina powder may be mixed at the powder level and coated on the surface of the substrate, or a compound obtained by adding these compounds at the molecular level at the time of forming alumina may be coated on the surface of the substrate. After forming the carrier layer by the above, a layer containing these elements may be formed on the surface thereof. Here, the rare earth elements are a total of 17 elements of Sc, Y and lanthanoid, and the alkaline earth metals are Ca, Sr, Ba, R
a. Among them, Ce, La and Ba are preferable from the viewpoint of the effect of increasing the durability of the exhaust gas purifying catalyst.

Further, other refractory inorganic oxides may be added to the carrier layer. For example, known carrier materials such as zeolite, silica and titania can be added. These refractory inorganic oxides are preferably porous in order to increase the specific surface area.

Pd is supported on the carrier layer as a catalyst component. The carrier layer further supports Cu. This is because Cu can prevent sintering of Pd. As a method for supporting Pd on the carrier layer, a known supporting method can be used. For example, a method of supporting by a water supply method, an impregnation method, an ion exchange method, a spray method, a slurry mixing method and the like can be mentioned. The method of supporting Cu on the carrier can be performed by a method similar to the method of supporting Pd.

For supporting Pd and Cu on alumina or the like, Pd or the like may be supported on the surface of the carrier layer after forming the carrier layer, or the powder surface of alumina or the like may be previously formed before forming the carrier layer. May be loaded with Pd or the like, or may be mixed when generating the carrier powder.

[0024] In addition to Pd, other catalyst components may be supported on the carrier layer. For example, a known catalyst component such as Rh or Pt can be supported.

The supported amount of Cu is 0.1 to 1 liter of the catalyst volume.
0001 mol or more, preferably 0.001
It is more preferable that the amount be 0.1 mol. If the amount is less than this, the effect of improving the durability of the catalyst cannot be exhibited, and if the amount is more than this, the catalytic action of Pd is inhibited.

The amount of Pd carried is 0.1 to 1 L of catalyst volume.
1 to 50 g, more preferably,
It is preferably 0.5 to 20 g. If it is less than this, the exhaust gas purifying catalyst cannot exert its exhaust gas purifying ability,
Further, even if the amount is more than this, the exhaust gas purifying ability is not improved because it is saturated, but merely increases the cost.

The order in which Pd and Cu are supported is not particularly limited, but the order in which Pd and Cu are
It is more preferable to support Cu after supporting Pd so as to form a layer.

[0028]

EXAMPLES The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples.

<Preparation of Catalyst> Catalysts of Examples 1 and 2 (Formation of First Layer) γ-alumina was impregnated with an aqueous solution of palladium nitrate, dried, and palladium was supported on the surface. Further, it was impregnated with an aqueous solution of copper nitrate and dried to obtain palladium-copper-supported alumina.

Cerium-zirconium mixed oxide powder, barium sulfate powder, palladium-copper-supported alumina, a mixture of alumina hydrate and water were mixed to form a slurry. This slurry was coated on a substrate made of cordierite honeycomb. After drying, firing was performed at 350 ° C. or higher in the air for 1 hour.

(Formation of Second Layer) A slurry was prepared by mixing cerium-zirconium composite oxide powder, lanthanum carbonate, γ-alumina, alumina hydrate and water. This slurry was coated on the first layer, dried, and fired at 350 ° C. or higher in the air for 1 hour.

(Support of Pt and Rh) The catalyst was impregnated with an aqueous solution of Pt nitrate and dried. Further, it was impregnated with an aqueous Rh chloride solution and dried.

Catalyst of Comparative Example (Formation of First Layer) γ-alumina was impregnated with an aqueous solution of palladium nitrate, dried, and palladium was supported on the surface to obtain palladium-supported alumina.

A mixed slurry of cerium-zirconium mixed oxide powder, barium sulfate powder, alumina supported on palladium-copper, alumina hydrate, and water was mixed. This slurry was coated on a substrate made of cordierite honeycomb. After drying, firing was performed at 350 ° C. or higher in the air for 1 hour.

(Formation of Second Layer) A slurry was prepared by mixing cerium-zirconium composite oxide powder, lanthanum carbonate, γ-alumina, alumina hydrate and water. This slurry was coated on the first layer, dried, and fired at 350 ° C. or higher in the air for 1 hour.

(Support of Pt and Rh) The catalyst was impregnated with an aqueous solution of Pt nitrate and dried. Further, it was impregnated with an aqueous Rh chloride solution and dried.

<Compositions of Catalysts of Examples and Comparative Examples> The compositions of the prepared catalysts of Examples 1 and 2 and Comparative Examples are shown in Table 1.

[0038]

[Table 1]

<Durability test> Put the catalyst in the housing,
Displacement 4 for operating with an air-fuel ratio of 14.6 stoichiometric air-fuel ratio
It was connected to the exhaust of a 000cc engine. The engine was operated at a catalyst-containing gas temperature of 900 ° C. for 100 hours to perform an endurance test on the exhaust gas purifying ability of the catalyst.

The measured HC of <exhaust gas purification rate measurement> catalyst containing gas by connecting the catalyst to the exhaust port to 2000cc engine before and after the durability test and the catalyst outlet gas, CO, the concentration of the NO _X respectively, in accordance with the following formula Each purification rate was determined. (Purification rate) = 100− (concentration of gas exiting catalyst) / (concentration of gas entering catalyst) × 100 The operating conditions of the engine were a stoichiometric air-fuel ratio of 14.6. The temperature of the gas containing the catalyst was 460 ° C.

<Results> Table 2 shows the exhaust gas purification rates before the durability test, and Table 3 shows the exhaust gas purification rates after the durability test.

[0042]

[Table 2]

[0043]

[Table 3]

From the test results, when the carrier layer of the exhaust gas purifying catalyst containing Pd in the carrier layer further contains copper, deterioration of the exhaust gas purifying ability of the catalyst is suppressed as compared with the case where copper is not contained. It became clear that.

[0045]

The exhaust gas purifying catalyst according to the present invention comprises HC,
CO, and has the effect of high durability with respect to cleaning ability NO _X.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G069 AA03 BA01A BA01B BA13A BA13B BA17 BB06B BB16B BC08A BC13B BC31A BC31B BC38A BC42A BC43A BC51A BC51B BC71B BC72A BC72B BC75B CA03 CA09 EC22Y

Claims (4)

[Claims] 1. An exhaust gas purifying catalyst comprising: a substrate; a carrier layer mainly composed of alumina supported on the surface of the substrate; and Pd and Cu supported on the carrier layer. . 2. The exhaust gas purifying catalyst according to claim 1, wherein the carrier layer further contains a compound comprising at least one of a rare earth element, an alkaline earth metal and zirconium. 3. The exhaust gas purifying catalyst according to claim 1, wherein the carried amount of Cu is 0.0001 mol or more per 1 L of the present exhaust gas purifying catalyst volume. 4. The exhaust gas purifying catalyst according to claim 1, wherein the carried amount of Cu is 0.001 to 0.1 mol per 1 L of the present exhaust gas purifying catalyst volume.