JPH0422706A - catalytic converter - Google Patents

Abstract

PURPOSE:To enhance the purifying efficiency of NOx in the exhaust gas of an engine, in which lean mixture gas or a mixture gas in theoretical air fuel ratio is combusted, by allowing a wash coat layer of Cu/zeolite structure, which is formed on a bearer, to bear noble metal element. CONSTITUTION:A catalyst converter 11 has a cylindrical section and comprises a suction part 12 for forming-in of the exhaust gas, a vessel part 13 for accommodation of catalyst, and a discharge part 14 for exhaustion of the purified exhaust gas. The catalyst has a honeycomb-shaped bearer 15 made of ceramic or metal, and a number of cells 16 with four sides surrounded by this bearer 15 are formed in the axial direction. A wash coat layer 21 of Cu/zeolite structure is formed on this bearer 15, and a plurality of noble metals such as Pt, Rh, Pd are borne by this wash coat layer 21. This provides a high purifying efficiency for NOx exhausted from an engine, in which a lean mixture gas or a mixture gas in the theoretical air fuel ratio is combusted.

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 combust a lean air-fuel mixture in the engine. This kind of engine is called a lean burn engine. As a catalyst for decomposing NOx emitted from such a lean burn engine, lean NOx having a Cu/zeolite structure having a purification efficiency characteristic curve shown in Fig. 4 is used.
X catalyst was used. Note that the Cu/zeolite structure refers to a structure in which Cu is supported on a crystal structure consisting of AI and Si, called a zeolite structure.

(Question 8 to be solved by the invention) By the way, in the lean burn engine described above, during steady operation, the engine burns a lean air-fuel mixture, and during transient operation, such as during acceleration, the engine burns a mixture at a stoichiometric air-fuel ratio. .

However, when a conventional lean NOx catalyst is used to purify the exhaust gas of a lean-burn 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 as shown in Figure 4.
This is because the NOx purification efficiency of the conventional lean 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 NO, which is emitted from an engine in which a lean mixture or a mixture at a stoichiometric air-fuel ratio is combusted. Our goal is to provide the following.

[Structure of the invention] (Means for solving the problem) A single or multiple noble metal elements such as Pt, Rh, and Pd are supported on a wash coat layer such as a Cu/zeolite structure formed on a carrier. This is a catalytic converter characterized by the following.

(Function) NOx emitted during steady operation of a lean burn engine
is decomposed at active sites in the Cu/zeolite structure, and during transient operation at the stoichiometric air-fuel ratio, HC, Co, and N are discharged from the engine.

8 at the active sites of noble metal elements such as Pt, Rh, and Pd.

(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 the state of support on a carrier, FIG. 2 is a sectional view taken along the axial direction of the catalytic converter, and FIG. 3 is a sectional view taken along the radial direction of the catalytic converter.

First, in FIG. 2, 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 a 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. Then, by the carrier 15, 4
A countless number of cells 16 are formed in the axial direction.

The structure on the carrier 15 will be explained with reference to FIG. 1. A washcoat layer 21 having a Cu/zeolite structure is formed on the carrier 15, and a plurality of noble metals such as Pt, Rh, and Pd are supported on this washcoat layer 21.

By using the catalytic converter configured as described above, NOx emitted during steady operation of the lean burn engine is accelerated in decomposition at the active sites of the Cu/zeolite structure of the washcoat layer 21, and the engine is operated at the stoichiometric air-fuel ratio. During transient operation, HC, Co, and N discharged from the engine
Ox is simultaneously reduced at the active sites of noble metal elements such as Pt, Rh, and Pd. In other words, the NOx purification efficiency of the catalytic converter according to one embodiment of the present invention is shown by the solid line in FIG.

In addition, in the above embodiment, the wash coat layer 21 includes Pt,
Although a plurality of noble metal elements such as Rh and Pd are supported, a single noble metal element may be supported.

[Effects of the Invention] As detailed above, the present invention provides a catalytic converter that can improve the purification efficiency of NOx emitted from an engine burned with a lean mixture or a mixture with a stoichiometric air-fuel ratio. can be provided.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing how a catalytic converter is supported on a carrier according to an embodiment of the present invention, FIG. 2 is a cross-sectional view along the axial direction of the catalytic converter, and FIG. 3 is a radial direction of the catalytic converter. 4 is a characteristic diagram showing the relationship between NOx purification efficiency and air-fuel ratio. 11...Catalytic converter, 15...Carrier, 21...
Washcoat layer.

Claims (1)

[Claims] A catalytic converter characterized in that a single or multiple noble metal elements such as Pt, Rh, and Pd are supported on a washcoat layer such as a Cu/zeolite structure formed on a carrier.