JPH0418915A - Catalytic converter - Google Patents

Abstract

PURPOSE:To enhance the efficiency in removing NOX by forming a first wash coat layer of Cu/zeolite structure on a carrier on its front side and the first and second wash coat layers on the carrier on its rear side. CONSTITUTION:The NOX exhausted from a lean burn engine in the steady running are decomposed at the active site of the Cu/zeolite structure of the first wash coat layer 22, and the HC, CO and NOX exhausted from the engine are simultaneously reduced in the transient running carried out with the theoretical air-to-fuel ratio at the active site of the noble-metal elements such as Pt, Rh and Pd deposited on the second wash coat layer 23. Since the Cu/ zeolite structure is formed in this way on the front side of a carrier 15, the reducing agents such (H2, HC, etc.,) are secured close to the active site of the Cu/zeolite structure, and the efficiency in removing NOX is improved.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention aims to improve the purification efficiency of NOx emitted from an engine in which a lean mixture or a mixture at a stoichiometric air-fuel ratio is burned. Regarding catalytic converters that can

(Prior Art) It is known that one method for improving the fuel consumption rate of an engine is to burn a lean air-fuel mixture in the engine. This kind of engine is called a lean burn engine. Lean NOx having a Cu/zeolite structure having a purification efficiency characteristic curve shown in Fig. 5 is used as a catalyst for decomposing NOx emitted from such a lean burn engine.
x catalyst was used. Note that the Cu/zeolite structure refers to a structure in which Cu is supported on a crystalline structure made of AI and Si, called a zeolite structure.

(Problems to be Solved by the Invention) Incidentally, in the lean burn engine, the engine burns a lean air-fuel mixture during steady operation, and burns an air-fuel mixture at a stoichiometric air-fuel ratio during transient operation such as during acceleration.

However, when a conventional lean NOx catalyst is used to purify the exhaust gas of a lean pan engine, the engine burns a mixture at the stoichiometric air-fuel ratio during transient operations such as during acceleration, so a large amount of NOx is emitted near the stoichiometric air-fuel ratio. It was not possible to completely purify NOx. This is because, as shown in FIG. 5, the NOx purification efficiency of the conventional NOx catalyst is about 50% near the stoichiometric air-fuel ratio.

The present invention has been made in view of the above points, and its purpose is to provide a catalytic converter that can improve the purification efficiency of NOx emitted from an engine that burns a lean mixture or a mixture at a stoichiometric air-fuel ratio. It is about providing.

[Structure of the Invention] (Means for Solving the Problems) A first washfoot layer having a Cu/zeolite structure is formed on the front carrier, and first and second washcoat layers are formed on the rear carrier. , the first washcoat layer has a Cu/zeolite structure, and the second washcoat layer supports one or more noble metal elements such as Pt, Rh, and Pd. This is a catalytic converter.

(Function) NO emitted during steady operation of a lean burn engine
x is the first one: (u, /' of the 1-nosh coat layer) The active r gas of the theolite structure accelerates decomposition, and during transient operation at the stoichiometric air-fuel ratio, H emitted from the engine
C, Co, No,: as active points of noble metal elements such as Pt, Rh, and Pd supported on the second washer coat layer,
We are trying to reduce it at the same time.

(Example) A catalytic converter according to an example of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing how the catalyst is supported on the rear carrier, FIG. 2 is a sectional view showing how it is supported on the front carrier, and FIG. 3 is a sectional view taken along the axial direction of the catalytic converter. FIG. 4 is a radial cross-sectional view of the catalytic converter.

In FIG. 3, 11 is a catalytic converter.

This catalytic converter 11 has a cylindrical cross-section, and its outer periphery is composed of an intake section 12 into which exhaust gas flows, a container section 13 where the catalyst is housed, and a discharge section 14 from which purified exhaust gas is discharged. be done.

Furthermore, 15 is a carrier made of ceramic or metal and having a honeycomb shape. The structure of the front side (one part) of the carrier 15 is shown in FIG.
The structure of part (2) will be described later with reference to FIG.

A countless number of cells 16 surrounded by the carrier 5 in four directions are formed in the axial direction.

The structure on the carrier 15 is shown in FIGS. 1 and 2.
This will be explained with reference to the figures. First, on the front side (one portion) of the carrier 15, as shown in FIG. 2, a first wash coat layer 22 having a Cu/zeolite structure is formed on the carrier 15. Further, as shown in FIG. 1, on the rear side (■ part) of the carrier 15, a first wash coat layer 22 made of Cu/zeolite structure is formed, and on this first wash coat layer 22, a layer made of AlzOs is formed. A second washcoat layer 23 is formed. A plurality of noble metals such as Pt, Rh, and Pd are supported on the second wash coat layer 23.

By using the catalytic converter configured as described above, the NOy discharged during steady operation of the lean barge engine is absorbed by the first wash coat layer 22 (: u
/Due to the quality of the zeolite structure, decomposition is promoted, and during transient operation when operating at a stoichiometric air-fuel ratio, HC, Co, No, etc. discharged from the engine are transferred to the second washcoat layer 2.
At the same time, the active points of noble metal elements such as PtRh and Pc1, which are held in II4 and PtRh, are reduced at the same time. In other words,
The No.lambda. purification efficiency of the catalytic converter according to an embodiment of the present invention is shown by the solid line in FIG.

In this way, by forming the Cu y' ceolite striped structure on the front side of the carrier] 5, reducing agents (I (2, HC, etc.) are secured near the Cu / ceolite active sites, and the NOx purification efficiency is improved. I can do it.

In the above embodiment, the second wonyu coat layer 23 supports a plurality of noble metal elements such as Pt, Rh, Pd, etc., but it is also possible to support a single noble metal element among them.

Furthermore, the first washcoat layer 22 and the second washcoat layer 23 formed on the rear carrier 15 may be reverse layers.

[Effects of the Invention] As described in detail above, the present invention provides a catalytic converter that can improve the purification efficiency of NOx emitted from an engine combusted with a lean mixture or a mixture with a stoichiometric air-fuel ratio. Can you provide?

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing how a catalyst converter according to an embodiment of the present invention is supported on the rear carrier, FIG. 2 is a cross-sectional view showing how it is supported on the front carrier, and FIG. FIG. 4 is a cross-sectional view along the axial direction of the catalytic converter, FIG. 4 is a cross-sectional view along the radial direction of the catalytic converter, and FIG. 5 is a characteristic diagram showing the relationship between NOx purification efficiency and air-fuel ratio. 11...Catalytic converter, 15...Carrier, 22...
1st washcoat layer, 23... 2nd washcoat layer. Applicant's agent Patent attorney Suzue Low degree diagram

Claims (1)

[Claims] A first washcoat layer made of Cu/zeolite structure is formed on the front carrier, first and second washcoat layers are formed on the rear carrier, and the first washcoat layer is made of Cu/zeolite structure. A catalytic converter having a zeolite structure and having one or more noble metal elements such as Pt, Rh, and Pd supported on the second washcoat layer.