JP2002004844A - Exhaust emission control device for diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve regeneration efficiency of an NOx adsorbent and a particulate filter, by avoiding the decrease in heat transfer efficiency of respective exhaust post treatment devices, while simplifying the structure of an exhaust system by concentrating and integrating NOx-adsorbing function and an oxidation catalyst to the particulate filter, in an exhaust emission control device, in which the particulate filter interposed in the exhaust system of a diesel engine is provided with passage walls made of porous members and an upstream exhaust passage and a downstream exhaust passage, which are mutually adjacent to each other across the passage walls, and particulates in exhaust gas are collected by the wall, when the exhaust gas passes through the passage walls. SOLUTION: NOx adsorbents M are carried and held on first wall surfaces 5u, 6u which face upstream exhaust passages (u) of passage walls 5, 6, and oxidation catalysts M' are carried on second wall surfaces 5d, 6d which face downstream exhaust passage (d) of the passage walls.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a particulate filter provided in an exhaust system of a diesel engine.
A passage wall made of a porous member, and an upstream exhaust passage and a downstream exhaust passage adjacent to each other with the passage wall interposed therebetween;
The present invention relates to an exhaust gas purification device for a diesel engine in which particulates in exhaust gas are trapped in the passage wall when the exhaust gas passes through the passage wall from the upstream side to the downstream side.

[0002] In this specification, "particulate" means fine particulate matter (PM) contained in exhaust gas.
Which means that soot particles, various hydrocarbon particles, and other insoluble organic substances (SOOT) oxidize unburned fuel, unburned oil components, other soluble organic substances (SOF), and sulfur in fuel. It is formed by the adhesion of sulfur oxides generated by the process.

[0003]

2. Description of the Related Art Conventionally, the exhaust system of a diesel engine has been
Three exhaust aftertreatment devices (ie, NOx
An absorption catalyst device, a particulate filter, and an oxidation catalyst device) are interposed in series, and in this device, the transfer efficiency of the exhaust heat of the engine decreases as it goes to the downstream processing device. The purification efficiency and the regeneration efficiency of the processing apparatus also become lower toward the downstream side, and the structure of the exhaust system becomes complicated as a whole.

Therefore, NOx is added to the particulate filter.
In order to have an absorption function, for example, a NOx absorbent is supported on first and second wall surfaces on the upstream and downstream sides of a passage wall made of a porous member, for example, of a wall flow type particulate filter. Has already been proposed (see, for example, JP-A-6-272541).

[0005]

However, even with the above proposed structure, the oxidation catalyst device must be interposed in the exhaust system independently of the particulate filter, and especially in the downstream processing device. However, there still remain problems such as a reduction in purification efficiency and regeneration efficiency due to a reduction in the transfer efficiency of exhaust heat to the exhaust gas.

[0006] The present invention has been made in view of the above circumstances, and NOx is added to a particulate filter.
An object of the present invention is to provide a diesel engine exhaust purification device having a simple structure that can solve all the above-mentioned problems of the conventional structure by integrating not only the absorption function but also the oxidation catalyst function and forming an integrated structure.

[0007]

In order to achieve the above object, according to the first aspect of the present invention, a particulate filter provided in an exhaust system of a diesel engine comprises a passage wall made of a porous member, An upstream exhaust passage and a downstream exhaust passage that are adjacent to each other with a wall interposed therebetween, and when exhaust gas passes through the passage wall from the upstream side to the downstream side, the particulates in the exhaust at the passage wall are reduced. In the exhaust gas purifying device for a diesel engine, the NOx absorbent capable of absorbing NOx in the exhaust gas is carried on the first wall surface facing the exhaust passage upstream of the passage wall. An oxidation catalyst is supported on a second wall surface facing the downstream side exhaust passage.

According to this feature, when the exhaust gas passes through the passage wall of the particulate filter, the particulates in the exhaust gas are mainly collected and deposited on the first wall surface of the passage wall, and the first wall surface is collected. Above, it has both the action of removing the trapped particulates by oxidative combustion and the action of absorbing NOx in the exhaust gas with the NOx absorbent. That is, during normal operation of a diesel engine in which the exhaust air-fuel ratio is lean, in addition to the oxidizing action of CO and HC, the oxidation reaction of carbon component C in particulates (2NO ₂ + C → 2NO + CO ₂ ) and the NOx absorption reaction By the occurrence of (2NO + O ₂ → 2NO ₂ → M · 2NO ₂ ), it becomes possible to simultaneously obtain the NOx absorbing action of the absorbent (M) and the action of purifying the carbon component in the particulates. In particular, when the NOx absorbent (M) is in the first on the walls, NO ₂ is produced in large quantities by oxidizing NO, the the a part is absorbed in the NOx absorbent, can not be absorbed NO ₂ is particulate Since it also easily reacts with the carbon component C therein, the oxidizing and burning reaction of the particulates can be promoted in a lower exhaust gas temperature atmosphere than when the absorbent is not on the first wall surface, thereby reducing the regeneration efficiency of the particulate filter. improves.

The NOx absorbed by the NOx absorbent
For the release regeneration of the fuel, a release regeneration technique based on a fuel-rich state (that is, an excess air ratio λ ≦ 1) is generally known. However, when the regeneration temperature is relatively low, the regeneration efficiency of the NOx absorbent (NOx Release efficiency). However, the soluble organic substance (SOF) contained in the particulate matter is in a fuel vapor state even at a low temperature of about 250 ° C., which can contribute to fuel richness. NO collected particulates
When it is near the x absorbent (on the first wall surface), the NOx regeneration efficiency can be sufficiently ensured even if the fuel-rich amount is reduced as compared with the conventional case. That is, in the state where particulates (particularly SOF) coexist with the NOx absorbent,
It becomes possible to increase the release amount of NOx at a lower temperature than in a state without coexistence.

It is known that the NOx absorbing / releasing function of the NOx absorbent decreases with an increase in the SOx absorption amount of the NOx absorbent. The invention of claim 1 is effective.
That is, during the lean operation, the sulfur component {(M (SO ₄ ) _n } absorbed {SO ₂ + M + O ₂ → (M (SO ₄ ) _n )} as sulfuric acid in the NOx absorbent (M) becomes stoichiometric or rich operation. Occasionally, the combustion reaction with the carbon component C in the particulates existing in the same atmosphere ｛(M (SO ₄ ) _n + C →
Since M + CO ₂ + SO ₂易 becomes easy, the effect of removing the sulfur component is achieved, and the release regeneration temperature is reduced.

The exhaust gas that has passed from the first wall surface through the pores inside the passage wall to the second wall surface is subjected to the action of the oxidation catalyst on the second wall surface. HC and CO generated at the same time can be effectively oxidized (particularly at cold start).

According to a second aspect of the present invention, there is provided a particulate filter interposed in an exhaust system of a diesel engine.
A passage wall made of a porous member, and an upstream exhaust passage and a downstream exhaust passage adjacent to each other with the passage wall interposed therebetween;
In an exhaust gas purification apparatus for a diesel engine, particulates in exhaust gas are captured in the passage wall when the exhaust gas passes from the upstream side to the downstream side of the passage wall. An oxidation catalyst is supported on a first wall surface facing the exhaust passage, and a NOx absorbent capable of absorbing NOx in the exhaust is supported on a second wall surface facing the exhaust passage downstream of the passage wall. .

According to this feature, when the exhaust gas passes through the passage wall of the particulate filter, the particulates in the exhaust gas are mainly collected and deposited on the first wall surface of the passage wall. An oxidation catalyst exists on one wall surface, and during lean operation, a large amount of N generated by an oxidation reaction of NO (2NO + O ₂ → 2NO ₂ ) on the first wall surface.
O ₂ facilitates the combustion reaction of the carbon component C in the particulates (2NO ₂ + C → 2NO + CO ₂ ), so that the effect of removing and burning the particulates is further enhanced (compared with the first aspect of the invention). Further, since a large amount of NO ₂ is also contained in the exhaust gas that has passed through the passage wall and reached the second wall surface side, it is easily absorbed by the NOx absorbent on the second wall surface, and as a whole NOx Can be further improved.

According to any of the above features, the particulate filter has not only a NOx absorption function but also an oxidation catalyst function and has an integrated structure, so that the heat transfer efficiency of each exhaust after-treatment device is reduced as much as possible. Therefore, the chemical reaction at the time of the NOx absorption function and the particulate oxidizing combustion function is promoted, and the regeneration efficiency of the NOx absorbent and the particulate filter is improved.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below based on embodiments of the present invention illustrated in the accompanying drawings.

In the accompanying drawings, FIG. 1 shows a first embodiment of the present invention.
FIG. 2 is a longitudinal sectional view of the particulate filter according to the embodiment, FIG. 2 is a sectional view taken along line 2-2 of FIG. 1, FIG. 3 is an enlarged sectional view of a main part of the particulate filter according to the first embodiment, and FIG.
FIG. 5 is a reaction explanatory view of the embodiment, FIG. 5 is an enlarged sectional view of a main part of the particulate filter according to the second embodiment, and FIG. 6 is a reaction explanatory view of the second embodiment.

First, a first embodiment will be described. Figure 1
In the exhaust system EX of the diesel engine, a particulate filter F for capturing particulates in exhaust gas is interposed. The particulate filter F has a honeycomb structure called a wall flow type in the illustrated example. The particulate filter F has a metal casing 1 formed in a pipe shape with both ends open, and is fitted inside the casing 1. , Fixed filter body 2
It is composed of

The filter body 2 has a plurality of vertical passage walls 5 and a plurality of horizontal passage walls 6 as passage walls for partitioning the inside thereof into a number of exhaust passages parallel to each other. Each of the vertical passage wall 5 and the horizontal passage wall 6 is formed in a flat plate shape from a porous member such as a porous ceramic. The plurality of vertical passage walls 5 extend in the up-down direction, and are arranged in parallel at equal intervals, and the plurality of lateral passage walls 6 extend in the left-right direction, and are parallel at equal intervals. Lined up.

The vertical passage wall 5 and the horizontal passage wall 6 are arranged orthogonally. The vertical passage wall 5 and the horizontal passage wall 6 extend in the flow direction of exhaust gas (right direction in FIG. 1) and are aligned vertically and horizontally. A large number of upstream exhaust passages u and downstream exhaust passages d having a rectangular cross section are defined. Moreover, the upstream exhaust passage u and the downstream exhaust passage d are arranged so as to alternately line up, down, left and right. Thus, the upstream exhaust passage u and the downstream exhaust passage d are arranged adjacent to each other with the passage walls 5 and 6 interposed therebetween.

Each upstream exhaust passage u is hermetically closed by a fixed plug 7u at the downstream end (right end in FIG. 1) of the casing 1 and upstream at the upstream end (left end in FIG. 1) of the casing 1. It has an exhaust port ui that is always open to the side. Each downstream exhaust passage d is airtightly closed by a fixed plug 7d at the upstream end (left end in FIG. 1) of the casing 1, and is always downstream at the downstream end (right end in FIG. 1) of the casing 1. It has an open exhaust outlet do.

As a material of the passage walls 5 and 6, for example, porous ceramics such as cordierite and Si--C are generally used. In the present invention, at least the upstream side of the passage walls 5 and 6 is provided. First wall surfaces 5u, 6u facing exhaust passage u
And the second wall surfaces 5d and 6d facing the downstream side exhaust passage d,
Any material that can support a catalytic noble metal component serving as a NOx absorbent or a noble metal component serving as an oxidation catalyst, and has innumerable pores large enough to capture particulates on the first wall surfaces 5u and 6u. I just need.

In particular, in the first embodiment, the NOx absorbent M is carried on the first wall surfaces 5u and 6u on the upstream side of the passage walls 5 and 6, and the second wall surfaces 5d and 5d on the downstream side of the passage walls 5 and 6 are provided. The oxidation catalyst M 'is supported on 6d. In carrying out the above-mentioned loading, a conventionally known method for loading the catalyst (for example, a method of applying an absorbent M or the like to the wall surface of the carrier and firing the same) can be appropriately adopted.

The catalyst or the like serving as the NOx absorbent M is selected from, for example, alkali metals such as potassium, sodium, lithium and cesium, alkaline earth metals such as barium and calcium, and rare earth metals such as lanthanum and yttrium. And at least one and a noble metal such as platinum. The NOx absorbent M has a function of absorbing NOx in exhaust gas during an operation with a lean exhaust air-fuel ratio, and releasing the absorbed NOx into the exhaust gas during a stoichiometric or rich operation.

In a diesel engine, lean operation is performed during normal operation, so the NOx absorbent M absorbs NOx in exhaust gas. In addition, if the exhaust gas is made rich by injecting (ie, post-injecting) a reducing agent or fuel into the exhaust system from an injection device (not shown) disposed in the exhaust system on the upstream side of the particulate filter F, etc. , NOx absorbent M emits the absorbed NOx to exhaust gas.

Next, the operation of the first embodiment will be described. In the particulate filter F, the exhaust gas flowing through the exhaust system Ex while the diesel engine is operating first enters the respective upstream exhaust passages u, and passes therefrom across the passage walls 5 and 6. , And reach each adjacent downstream exhaust passage d (see arrows in FIGS. 1 and 3), and further flow through the downstream exhaust pipe. When the exhaust gas passes through the passage walls 5 and 6, the particulates in the exhaust gas are mainly collected on the first wall surfaces 5u and 6u of the passage walls 5 and 6, and are deposited there.

The first wall surfaces 5u and 6u have both the function of removing the collected and deposited particulates by oxidative combustion and the function of absorbing NOx in the exhaust gas to the NOx absorbent M. . That is, at the time of the lean operation, in addition to the oxidizing action of CO and HC, the oxidizing reaction of the carbon component C in the particulate (2NO ₂ + C → 2NO +
CO ₂ ) and the NOx absorption reaction (2NO + O ₂ → 2NO ₂ → M · 2NO ₂ ) by the NOx absorbent M occur simultaneously, so that the NOx absorption effect and the carbon component purification effect in the particulates are simultaneously exhibited. Is done. Especially NO
x absorbent M first wall 5u, to be on 6u, NO ₂ is produced in large quantities by oxidizing NO, the the a part is absorbed in the NOx absorbent M, NO ₂ is particulate which can not be absorbed Since the oxidation reaction with the carbon component C in the curate becomes easy,
The oxidizing and burning reaction of the particulates can be promoted in an exhaust gas temperature atmosphere lower than the case where the NOx absorbent M is not present on the first wall surfaces 5u and 6u, whereby the particulate filter F can be efficiently regenerated. .

The release regeneration of the NOx absorbed by the NOx absorbent M is performed by temporarily injecting and supplying (post-injection) a reducing agent or fuel into the exhaust gas upstream of the particulate filter F, for example. Is performed in a fuel-rich state (ie, excess air ratio λ ≦ 1). However, in this method, when the regeneration temperature is relatively low, the NOx release efficiency of the NOx absorbent M tends to decrease. .

However, the soluble organic substance (SOF) contained in the particulate matter is in a fuel vapor state even at a low temperature of about 250 ° C., which can contribute to fuel richness. When the collected particulates are collected in the vicinity of the NOx absorbent (on the first wall surfaces 5u, 6u), even if the fuel-rich amount is smaller than in the conventional case, NO
x regeneration efficiency can be sufficiently ensured. That is, as is clear from Table 1 below, in the state where particulates (particularly, SOF content) coexist with NOx absorbent M,
It becomes possible to increase the release amount of NOx at a lower temperature than in a state without coexistence.

[0029]

[Table 1]

During lean operation, the NOx absorbent M
, The sulfur component in the exhaust gas absorbed in the form of sulfate {SO ₂
+ M + O ₂ → (M (SO ₄ ) _n }, and the absorbed sulfur component {(M (SO ₄ ) _n } is different from the carbon component C in particulates existing in the same atmosphere during stoichiometric or rich operation. Oxidation reaction ｛(M (SO ₄ ) _n + C → M +
Since CO ₂ + SO ₂ is easily absorbed, the removal removal of sulfur components is promoted by using trapping particulates.
Therefore, NOx of NOx absorbent M caused by sulfur component absorption
A decrease in absorption / release function (sulfur poisoning) can be effectively suppressed.

The exhaust gas that has passed through the pores of the passage walls 5 and 6 from the first wall surfaces 5u and 6u and reached the second wall surfaces 5u and 6u has an oxidation catalytic action on the second wall surfaces 5d and 6d. Therefore, excess HC and CO generated during the regeneration of the particulate filter F and cannot be absorbed by the NOx absorbent M can be effectively oxidized.

In this embodiment, not only the NOx absorption function but also the oxidation catalyst function are integrated in the particulate filter F, and an integrated structure is provided, so that the efficiency of heat transfer to the respective exhaust after-treatment means M and M 'is reduced. Since the reduction can be avoided as much as possible, the NOx absorption reaction and the particulate combustion removal reaction are promoted, and the regeneration efficiency of the NOx absorbent M and the particulate filter F is improved.

FIGS. 5 and 6 show a second embodiment of the present invention. In this embodiment, contrary to the first embodiment, the oxidation catalyst M 'is carried on the first wall surfaces 5u, 6u on the upstream side of the passage walls 5, 6, and the first wall 5u, 6u on the downstream side of the passage walls 5, 6 is provided. 2 wall 5
The NOx absorbent M is supported on d and 6d.

According to this configuration, when the exhaust gas passes through each of the passage walls 5 and 6 of the particulate filter F, the particulates in the exhaust gas mainly pass through the first of the passage walls 5 and 6.
It is collected and deposited on the wall surfaces 5u, 6u. On the first wall surfaces 5u, 6u, an oxidation catalyst M 'is present.
During the lean operation, a large amount of NO ₂ generated by the oxidation reaction of NO (2NO + O ₂ → 2NO ₂ ) on the first wall surfaces 5u and 6u causes the carbon component C in the particulates to increase.
Is more easily oxidized (2NO ₂ + C → 2NO + CO ₂ ), so that the effect of removing and burning particulates is further enhanced (compared to the first embodiment). In addition, passage wall 5,
Since a large amount of NO ₂ is also contained in the exhaust gas that has passed through the second wall 5d and reached the second wall 5d, the second wall 5d
It becomes easy to be absorbed also by the NOx absorbent M on d and 6d, and the NOx absorption efficiency can be further improved as a whole.

Also in the second embodiment, the particulate filter F integrates not only the NOx absorbing function but also the oxidizing function and has an integrated structure, so that the efficiency of heat transfer to the respective exhaust after-treatment means M and M 'is improved. Therefore, the NOx absorption reaction and the particulate combustion removal reaction can be promoted, and the regeneration efficiency of the NOx absorbent M and the particulate filter F can be improved.

[0036]

As described above, according to the invention of each claim,
A particulate filter provided in an exhaust system of a diesel engine includes a passage wall made of a porous member, and an upstream exhaust passage and a downstream exhaust passage adjacent to each other with the passage wall interposed therebetween. In an exhaust gas purification apparatus in which particulates in exhaust gas are captured by the passage wall when passing through the passage wall, a first wall surface facing the upstream exhaust passage and a second wall facing the downstream exhaust passage of the passage wall.
Since the NOx absorbent was supported on one of the wall surfaces and the oxidation catalyst was supported on the other, the N filter was applied to the particulate filter.
By integrating both the Ox absorption function and the oxidation catalyst function and forming an integrated structure, the structure of the exhaust system can be simplified as a whole, and furthermore, the reduction of the heat transfer efficiency of each exhaust aftertreatment device can be avoided as much as possible. The reaction in each processing device is promoted, and the efficiency of regeneration of the NOx absorbent and the particulate filter is improved.

According to the first aspect of the present invention, the NOx absorbent is supported on the upstream first wall surface where particulates in the exhaust gas are collected and deposited, while the NOx absorbent is supported on the downstream second wall surface. since supporting the oxidation catalyst, by using the nO ₂ which can not be absorbed in the NOx absorbent of the nO ₂ generated by oxidation of nO in the first on the walls, the particulates deposited on the first on the walls of the combustion reaction Therefore, the oxidative combustion reaction of the particulates proceeds even in a relatively low exhaust gas temperature atmosphere, and the regeneration efficiency of the particulate filter is improved. In addition, since the soluble organic substance (SOF) contained in the particulates can contribute to the fuel richness, the release regeneration efficiency can be sufficiently ensured even when the fuel rich amount is made smaller than before when releasing the NOx of the NOx absorbent. Furthermore, since the sulfur component absorbed by the NOx absorbent during the lean operation easily reacts with the carbon component in the particulate matter near the NOx absorbent (on the first wall surface) during the rich operation, the sulfur component is released and removed. It is performed effectively and is effective in suppressing sulfur poisoning of NOx absorbents. In addition, the exhaust gas that has passed through the pores of the passage wall from the first wall surface and reached the second wall surface is subjected to an oxidation catalytic action on the second wall surface, and is generated during regeneration of the particulate filter and the like. Excess HC and the like contained in the exhaust gas can be effectively oxidized and removed.

According to the second aspect of the present invention, the oxidation catalyst is carried on the upstream first wall surface where particulates in the exhaust gas are collected and deposited, while NOx is deposited on the downstream second wall surface. Since the absorbent is loaded, during lean operation,
The oxidation reaction of the carbon component in the particulates can be promoted by the NO ₂ generated by the oxidation reaction on the first wall surface, whereby the effect of burning and removing the particulates deposited on the first wall surface can be further improved. Can be enhanced. Further, since a large amount of NO ₂ is contained in the exhaust gas that has reached the second wall surface after passing through the passage wall, it is easily absorbed by the NOx absorbent on the second wall surface, and as a whole, This can greatly contribute to the improvement of NOx absorption efficiency.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a particulate filter according to a first embodiment of the present invention.

FIG. 2 is a sectional view taken along line 2-2 of FIG.

FIG. 3 is an enlarged sectional view of a main part of the particulate filter according to the first embodiment.

FIG. 4 is a diagram illustrating the reaction of the first embodiment.

FIG. 5 is an enlarged sectional view of a main part of a particulate filter according to a second embodiment.

FIG. 6 is a diagram illustrating the reaction of the second embodiment.

[Explanation of symbols]

 Ex: Exhaust system F: Particulate filter M: NOx absorbent M ': Oxidation catalyst d: Downstream exhaust passage u: Upstream exhaust passage 5, 6: Vertical, horizontal Passage wall (passage wall) 5u, 5u ... first wall surface 5d, 6d ... second wall surface

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/18 F01N 3/28 301Q 3/28 301 B01D 53/34 129Z 53/36 103C (72) Inventor Kenichi Ogawara 1-4-1 Chuo, Wako-shi, Saitama F-term in Honda R & D Co., Ltd. (Reference) 3G090 AA02 AA03 BA01 3G091 AA18 AB00 AB02 AB06 AB13 BA03 BA04 BA11 BA14 BA15 BA19 CA18 CB02 FA02 FA04 FB02 FB10 FB11 FB12 FC02 FC07 GA06 GA18 GA21 GA24 GB01X GB02W GB03W GB04W GB05W GB06W GB10X GB17X HA14 HA47 4D002 AA08 AA12 AA40 AC10 BA03 BA05 CA13 DA56 DA70 EA03 EA07 HA01 4D048 AA14 AB01 AB03 BB02 CA01 CC38 CD05 EA02

Claims (2)

[Claims] A particulate filter (F) interposed in an exhaust system (Ex) of a diesel engine includes a passage wall (5, 6) made of a porous member, and a passage wall (5, 6).
An upstream exhaust passage (u) and a downstream exhaust passage (d) adjacent to each other with the exhaust gas interposed therebetween.
6), the particulates in the exhaust are trapped in the passage walls (5, 6) when passing from the upstream side to the downstream side. In the first wall surface (5u, 6u) facing the upstream exhaust passage (u) of (5, 6), NO
While carrying a NOx absorbent (M) capable of absorbing x,
A second exhaust passage (d) on the downstream side of the passage wall (5, 6);
An exhaust purification device for a diesel engine, characterized in that an oxidation catalyst (M ') is supported on wall surfaces (5d, 6d). 2. A particulate filter (F) interposed in an exhaust system (Ex) of a diesel engine includes a passage wall (5, 6) made of a porous member, and the passage wall (5, 6).
An upstream exhaust passage (u) and a downstream exhaust passage (d) adjacent to each other with the exhaust gas interposed therebetween.
6), the particulates in the exhaust are trapped in the passage walls (5, 6) when passing from the upstream side to the downstream side. The oxidation catalyst (M ') is supported on the first wall surfaces (5u, 6u) facing the upstream exhaust passage (u) of the (5, 6), while the downstream exhaust passage (d) of the passage walls (5, 6). A NOx absorbent (M) capable of absorbing NOx in exhaust gas is supported on second wall surfaces (5d, 6d) facing the exhaust gas purifier.