U.S. patent application number 12/331089 was filed with the patent office on 2009-06-18 for exhaust gas purification device.
Invention is credited to Yoshifumi Kato.
Application Number | 20090151340 12/331089 |
Document ID | / |
Family ID | 40276190 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090151340 |
Kind Code |
A1 |
Kato; Yoshifumi |
June 18, 2009 |
EXHAUST GAS PURIFICATION DEVICE
Abstract
An exhaust gas purification device has an injector and a
wall-flow type honeycomb support. The injector supplies fuel to the
honeycomb support. The honeycomb support has a plurality of porous
walls separating a plurality of inlet cells and a plurality of
outlet cells. The exhaust gas flowing into the inlet cells flows
through the porous walls into the outlet cells. Each porous wall
has an upstream surface facing the inlet cell and a downstream
surface facing the outlet cell. An upstream catalytic layer is
formed on the upstream surface and a downstream catalytic layer is
formed on the downstream surface. One of the upstream catalytic
layer and the downstream catalytic layer is composed of a fuel
reforming catalyst whose function is to reform the fuel to generate
a reducing agent, and the other of the upstream catalytic layer and
the downstream catalytic layer has a catalytic function which is
different from the function of the fuel reforming catalyst.
Inventors: |
Kato; Yoshifumi;
(Kariya-shi, JP) |
Correspondence
Address: |
Locke Lord Bissell & Liddell LLP;Attn: IP Docketing
Three World Financial Center
New York
NY
10281-2101
US
|
Family ID: |
40276190 |
Appl. No.: |
12/331089 |
Filed: |
December 9, 2008 |
Current U.S.
Class: |
60/299 |
Current CPC
Class: |
B01D 53/90 20130101;
B01D 2255/2042 20130101; B01D 2255/1021 20130101; F01N 2510/06
20130101; F01N 2610/03 20130101; B01D 2255/1025 20130101; F01N
3/106 20130101; F01N 3/2073 20130101; B01D 2255/1023 20130101; F01N
3/0842 20130101; B01D 53/9472 20130101; Y02T 10/24 20130101; F01N
2240/30 20130101; F01N 3/2066 20130101; B01D 2255/9022 20130101;
B01D 2255/91 20130101; F01N 3/2828 20130101; B01D 53/9468 20130101;
F01N 3/0814 20130101; F01N 13/009 20140601; B01D 2251/208 20130101;
B01D 2255/9032 20130101; Y02T 10/12 20130101; B01D 53/9477
20130101 |
Class at
Publication: |
60/299 |
International
Class: |
F01N 3/10 20060101
F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 13, 2007 |
JP |
2007-322241 |
Claims
1. An exhaust gas purification device comprising: an injector; and
a wall-flow type honeycomb support to which the injector supplies
fuel, the honeycomb support having a plurality of porous walls
separating a plurality of inlet cells and a plurality of outlet
cells, wherein the exhaust gas flowing into the inlet cells flows
through the porous walls into the outlet cells, wherein each porous
wall has an upstream surface facing the inlet cell and a downstream
surface facing the outlet cell, wherein an upstream catalytic layer
is formed on the upstream surface and a downstream catalytic layer
is formed on the downstream surface, wherein one of the upstream
catalytic layer and the downstream catalytic layer is composed of a
fuel reforming catalyst whose function is to reform the fuel to
generate a reducing agent, and the other of the upstream catalytic
layer and the downstream catalytic layer has a catalytic function
which is different from the function of the fuel reforming
catalyst.
2. The exhaust gas purification device according to claim 1,
wherein the upstream catalytic layer is composed of the fuel
reforming catalyst, wherein the downstream catalytic layer is
composed of a nitrogen oxide storage-reduction catalyst reducing
nitrogen oxide contained in the exhaust gas to generate nitrogen by
the reducing agent which is generated by the fuel reforming
catalyst.
3. The exhaust gas purification device according to claim 2,
further comprising a selective catalytic reduction catalyst at
downstream of the honeycomb support.
4. The exhaust gas purification device according to claim 1,
wherein the upstream catalytic layer is composed of an oxidizing
catalyst burning particulate matter contained in the exhaust gas,
wherein the downstream catalytic layer is composed of the fuel
reforming catalyst.
5. The exhaust gas purification device according to claim 4,
further comprising a selective catalytic reduction catalyst at
downstream of the honeycomb support.
6. The exhaust gas purification device according to claim 1,
wherein the fuel reforming catalyst is to reform diesel fuel to
generate at least one of carbon monoxide, hydrogen or hydrocarbon
as a reducing agent.
7. A honeycomb support comprising: a plurality of porous walls
separating a plurality of inlet cells and a plurality of outlet
cells, wherein exhaust gas flowing into the inlet cells flows
through the porous walls into the outlet cells, wherein each porous
wall has an upstream surface facing the inlet cell and a downstream
surface facing the outlet cell, wherein an upstream catalytic layer
is formed on the upstream surface and a downstream catalytic layer
is formed on the downstream surface, wherein one of the upstream
catalytic layer and the downstream catalytic layer is composed of a
fuel reforming catalyst whose function is to reform fuel to
generate a reducing agent, and the other of the upstream catalytic
layer and the downstream catalytic layer has a catalytic function
which is different from the function of the fuel reforming
catalyst.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an exhaust gas purification
device.
[0002] Japanese Unexamined Patent Publication No. 2006-291847
discloses an exhaust gas purification device for a diesel engine
utilizing a diesel fuel reforming catalyst and a nitrogen oxide
storage-reduction catalyst (hereinafter referred to as NSR
catalyst). The diesel fuel reforming catalyst reforms light oil, or
diesel fuel, to generate a reducing agent. The NSR catalyst reduces
nitrogen oxides (hereinafter referred to as NO.sub.x) contained in
exhaust gas into nitrogen (N.sub.2). According to the reference,
the diesel fuel reforming catalyst is connected to an exhaust
passage, and the NSR catalyst is connected downstream of the diesel
fuel reforming catalyst in the flow direction of the exhaust gas.
An injector for injecting the diesel fuel is provided upstream of
the diesel fuel reforming catalyst in the exhaust passage. The
diesel fuel reforming catalyst reforms diesel fuel in such a manner
that diesel fuel reacts with oxygen and water vapor in the exhaust
gas over the diesel fuel reforming catalyst, thereby generating
hydrogen. When the injector does not inject diesel fuel and the
air-fuel ratio of the exhaust gas is lean, the NSR catalyst absorbs
NO.sub.x in the exhaust gas. When the injector injects diesel fuel
and the air-fuel ratio of the exhaust gas becomes rich, the NSR
catalyst reduces the absorbed NO.sub.x into nitrogen, utilizing
hydrogen as a reducing agent generated by the diesel fuel reforming
catalyst.
[0003] However, the exhaust gas purification device in the above
reference has a problem of increasing the size of the whole device,
since the two catalysts, that is, the diesel fuel reforming
catalyst and the NSR catalyst having respective functions are
connected to the exhaust passage independently. The diesel fuel
injected by the injector into the exhaust passage is delivered to
the diesel fuel reforming catalyst in a mixed gaseous state of the
exhaust gas and the diesel fuel. In the mixed gaseous state, the
diesel fuel is distributed disuniformly, and is delivered
disuniformly to the diesel fuel reforming catalyst, thereby causing
another problem, such as deterioration of reaction efficiency of
the diesel fuel reforming catalyst.
[0004] The present invention is directed to provide a downsized
exhaust gas purification device in which reaction efficiency of
fuel reforming catalyst is improved.
SUMMARY OF THE INVENTION
[0005] In accordance with the present invention, an exhaust gas
purification device has an injector and a wall-flow type honeycomb
support. The injector supplies fuel to the honeycomb support. The
honeycomb support has a plurality of porous walls separating a
plurality of inlet cells and a plurality of outlet cells. The
exhaust gas flowing into the inlet cells flows through the porous
walls into the outlet cells. Each porous wall has an upstream
surface facing the inlet cell and a downstream surface facing the
outlet cell. An upstream catalytic layer is formed on the upstream
surface and a downstream catalytic layer is formed on the
downstream surface. One of the upstream catalytic layer and the
downstream catalytic layer is composed of a fuel reforming catalyst
whose function is to reform the fuel to generate a reducing agent,
and the other of the upstream catalytic layer and the downstream
catalytic layer has a catalytic function which is different from
the function of the fuel reforming catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The features of the present invention that are believed to
be novel are set forth with particularity in the appended claims.
The invention together with objects and advantages thereof, may
best be understood by reference to the following description of the
presently preferred embodiments together with the accompanying
drawings in which:
[0007] FIG. 1 is a schematic view showing an exhaust gas
purification device according to a first preferred embodiment of
the present invention;
[0008] FIG. 2 is a perspective view of the exhaust gas purification
device according to the first preferred embodiment;
[0009] FIG. 3 is a cross-sectional side view of a honeycomb support
according the first preferred embodiment;
[0010] FIG. 4 is a fragmentally enlarged cross-sectional side view
of the honeycomb support according to the first preferred
embodiment;
[0011] FIG. 5 is a fragmentally enlarged cross-sectional side view
of the honeycomb support according to the first preferred
embodiment;
[0012] FIG. 6 is a schematic view of an exhaust gas purification
device according to a second preferred embodiment;
[0013] FIG. 7 is a fragmentally enlarged cross-sectional side view
of a honeycomb support according to the second preferred
embodiment; and
[0014] FIG. 8 is a fragmentally enlarged cross-sectional side view
of the honeycomb support according to the second preferred
embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] FIG. 1 shows an exhaust gas purification device of a first
preferred embodiment according to the present invention. A diesel
engine 1 has a cylinder head 1A to which an intake manifold 1B and
an exhaust manifold 1C are connected. The intake manifold 1B
introduces air into the diesel engine 1. The exhaust manifold 1C
conducts exhaust gas outside the diesel engine 1. The exhaust
manifold 1C is connected to an exhaust passage 2 so that exhaust
gas from the diesel engine 1 flows in a direction indicated by an
arrow A in FIG. 1. In the midway of the exhaust passage 2, an
injector 3 is provided so as to bore through the wall of the
exhaust passage 2 from the outside to the inside. The injector 3 is
connected through a fuel passage 4 to a fuel tank which is not
shown in the drawings. Diesel fuel as a fuel is stored in the fuel
tank and is injected into the exhaust passage 2.
[0016] A honeycomb support 5 is connected downstream of the
injector 3 in the direction indicated by the arrow A in FIG. 1. As
shown in FIG. 2, the honeycomb support 5 is formed in a cylindrical
shape and made of a porous ceramic material such as cordierite. The
honeycomb support 5 has a plurality of rectangular cross-sectional
cells. The cells extend through the honeycomb support 5 in the flow
direction of the exhaust gas. One of each two adjacent cells is
closed by a plug 6A (as shown in FIG. 3) inserted in the downstream
end so as to form an inlet cell 5A. The other of the two adjacent
cells, which is adjacent to the inlet cell 5A, is closed by a plug
6B inserted in the upstream end so as to form an outlet cell 5B.
Porous walls 5C are formed to separate the inlet cells 5A and
outlet cells 5B. In flowing through the honeycomb support 5, the
exhaust gas flows into the inlet cells 5A, as indicated by arrows
B1 in FIG. 3, and then into the outlet cells 5B through the porous
walls 5C as indicated by arrows B2, to the outside of the honeycomb
support 5.
[0017] As shown in FIG. 4, each of the porous walls 5C separating
the inlet cells 5A and the outlet cells 5B has an upstream surface
5D and a downstream surface 5E at the opposite side of the upstream
surface 5D. The upstream surface 5D faces the inlet cell 5A. The
downstream surface 5E faces the outlet cell 5B. An upstream
catalytic layer 7 is formed on the upstream surface 5D. The
upstream catalytic layer 7 is composed of a thin-layered fuel
reforming catalyst. A downstream catalytic layer 8 is formed on the
downstream surface 5E. The downstream catalytic layer 8 is composed
of a thin-layered NSR catalyst. When exhaust gas passes through the
honeycomb support 5, the exhaust gas flows through the upstream
catalytic layer 7 on the upstream surface 5D, the porous wall 5C,
and the downstream catalytic layer 8 on the downstream surface 5E,
sequentially in this order, as indicated by an arrow E in FIG.
5.
[0018] The fuel reforming catalyst of the upstream catalytic layers
7 includes, for example, rhodium (Rh), alumina (Al.sub.2O.sub.3)
and the like. The fuel reforming catalyst reforms diesel fuel by
the reaction with oxygen (O.sub.2) and water vapor (H.sub.2O) in
the exhaust gas. As a result, carbon monoxide (CO), hydrogen
(H.sub.2), and hydrocarbon (HC) are generated as reducing agents
for the NSR catalyst of the downstream catalytic layers 8. The
porous ceramic material for the porous walls 5C has greater flow
resistance than the fuel reforming catalyst for the upstream
catalytic layers 7. Therefore, the diesel fuel flowing in the inlet
cells 5A to pass through the upstream catalytic layers 7 remains in
the vicinity of the upstream catalytic layers 7, or, between the
porous walls 5C and the upstream catalytic layers 7 for a long
time. Part of diesel fuel does not react when flowing through the
upstream catalytic layers 7. Such unburned diesel fuel and other
components such as carbon monoxide, hydrogen, and hydrocarbon,
which are generated by the upstream catalytic layers 7, pass
through the porous walls 5C and reach the downstream catalytic
layers 8 in an uniformly-distributed state in the exhaust gas.
Further, the reducing agents such as carbon monoxide generated by
the upstream catalytic layers 7 reach the downstream catalytic
layers 8 immediately after flowing through the porous walls 5C.
Therefore, the reaction between the reducing agents and components
in the exhaust gas is suppressed, and the elimination of the
reducing agents is suppressed to the minimum, accordingly.
[0019] The NSR catalyst of the downstream catalytic layers 8
includes alkaline-earth metal such as barium (Ba) and the like, as
an absorbent. When the air-fuel ratio of the exhaust gas is lean,
that is, when the exhaust gas is in an oxidizing atmosphere in
which the air concentration is high, the NSR catalyst temporarily
absorbs NO.sub.x contained in exhaust gas. When carbon monoxide,
hydrogen, and hydrocarbon are supplied while the exhaust gas is in
a rich state, that is, in the reducing atmosphere in which the air
concentration is low, the NSR catalyst reduces absorbed NO.sub.x to
nitrogen so as to generate ammonia (NH.sub.3).
[0020] Referring back to FIG. 1, a selective catalytic reduction
catalyst (hereinafter referred to as SCR catalyst) 9 is connected
downstream of the honeycomb support 5. The SCR catalyst 9 reduces
NO.sub.x contained in exhaust gas to nitrogen in such a manner that
NO.sub.x contained in the exhaust gas reacts with ammonia generated
by the NSR catalyst of the downstream catalytic layers 8.
[0021] Next, the operation of the exhaust gas purification device
of the first preferred embodiment according to the present
invention will be described. The following will describe the
operation when the injector 3 does not inject diesel fuel yet, and
the air-fuel ratio of the exhaust gas flowing into the honeycomb
support 5 is in a lean state. As shown in FIG. 1, the exhaust gas
discharged from the diesel engine 1 flows into the honeycomb
support 5 through the exhaust manifold 1C and the exhaust passage
2. The exhaust gas in the honeycomb support 5 flows through the
inlet cell 5A (as shown in FIG. 4), the upstream catalytic layer 7,
the porous wall 5C, and the downstream catalytic layer 8,
sequentially in this order, into the outlet cell 5B. Since the
injector 3 does not inject diesel fuel, the fuel reforming catalyst
of the upstream catalytic layer 7 does not react on the upstream
surface 5D of the porous wall 5C.
[0022] The exhaust gas passing through the upstream catalytic layer
7 flows through the porous wall 5C to reach the downstream
catalytic layer 8. Since the air-fuel ratio of the exhaust gas
which reaches the downstream catalytic layer 8 is lean, the NSR
catalyst of the downstream catalytic layer 8 on the downstream
surface 5E of the porous wall 5C temporarily absorbs NO.sub.x
contained in the exhaust gas. The exhaust gas flows through the
downstream catalytic layer 8 into the outlet cell 5B, and is
discharged to the outside of the honeycomb support 5. Then, the
exhaust gas flows through the SCR catalyst 9. However, the SCR
catalyst 9 does not react with the exhaust gas, since the NSR
catalyst of the downstream catalytic layer 8 does not generate
ammonia when the exhaust gas is in a lean state.
[0023] The following will explain the operation when the injector 3
injects diesel fuel, and the air-fuel ratio of the exhaust gas
flowing into the honeycomb support 5 is in a rich state. When the
injector 3 injects diesel fuel into the exhaust passage 2, the
mixed gas of the exhaust gas and the diesel fuel flows through the
porous wall 5C of the honeycomb support 5 (as shown in FIG. 4).
Diesel fuel is supplied to the fuel reforming catalyst of the
upstream catalytic layer 7 on the upstream surface 5D of the porous
wall 5C. The supplied diesel fuel reacts with oxygen and water
vapor contained in the exhaust gas over the fuel reforming catalyst
to generate carbon monoxide, hydrogen, and hydrocarbon as reducing
agents for the NSR catalyst of the downstream catalytic layer 8. It
is noted that the porous ceramic material of the porous wall 5C has
greater flow resistance than the fuel reforming catalyst of the
upstream catalytic layer 7. Therefore, the diesel fuel flowing
through the upstream catalytic layer 7 remains for a long time
between the upstream surface 5D of the porous wall 5C and the
upstream catalytic layer 7, thereby improving the reaction
efficiency of the fuel reforming catalyst.
[0024] The unburned diesel fuel which does not react in flowing
through the upstream layer 7, carbon monoxide, hydrogen, and
hydrocarbon generated by the upstream layer 7, pass through the
porous wall 5C to reach the downstream layer 8. Such components are
distributed uniformly in the exhaust gas while flowing through the
porous wall 5C. Therefore, the components are supplied to all over
the NSR catalyst of the downstream catalytic layers 8 to improve
the reaction efficiency of the NSR catalyst. The reducing agents,
such as carbon monoxide generated by the fuel reforming catalyst of
the upstream catalytic layer 7, flow through the porous wall 5C to
reach the downstream catalytic layer 8 immediately. The
disappearance of the reducing agents by the reaction with other
components in the exhaust gas is suppressed to the minimum.
Therefore, the reaction efficiency in the NSR catalyst is further
improved.
[0025] When the reducing agents generated by the upstream catalytic
layer 7 and the exhaust gas in a rich state reach the downstream
catalytic layer 8, the NSR catalyst of the downstream catalytic
layer 8 reduces the absorbed NO.sub.x into nitrogen and generates
ammonia. Nitrogen, which is reduced by the downstream catalytic
layer 8, and ammonia, which is generated by the downstream
catalytic layer 8, flow from the outlet cell 5B to the outside of
the honeycomb support 5. Then nitrogen and ammonia flow through the
SCR catalyst 9 connected to the downstream side. When NO.sub.x
which is not reduced by the NSR catalyst remains in the exhaust
gas, the SCR catalyst 9 reduces the residual NO.sub.x into nitrogen
by reacting the residual NO.sub.x with ammonia generated by the NSR
catalyst.
[0026] Thus, each of the porous walls 5C of the honeycomb support 5
has the upstream catalytic layer 7 on the upstream surface 5D and
the downstream catalytic layer 8 on the downstream surface 5E. The
upstream catalytic layers 7 are composed of the fuel reforming
catalyst and the downstream catalytic layers 8 are composed of the
NSR catalyst, thereby the single honeycomb support 5 has two
different catalytic functions. Further, the honeycomb support 5 is
of a wall flow type in which exhaust gas passes through the porous
walls 5C. Therefore, diesel fuel remains between the upstream
catalytic layers 7 and the porous walls 5C so as to extend the
residence time of the diesel fuel in the vicinity of the fuel
reforming catalyst of the upstream catalytic layers 7. The reaction
efficiency of the fuel reforming catalyst is improved, accordingly.
Further, unburned diesel fuel and reducing agents generated by the
fuel reforming catalyst of the upstream catalytic layers 7 are
distributed uniformly in flowing through the porous walls 5C, and
are supplied uniformly to the NSR catalyst of the downstream
catalytic layers 8. The reaction efficiency of the NSR catalyst is
improved, accordingly. Therefore, the exhaust gas purification
device is downsized, and the reaction efficiency of the fuel
reforming catalyst is improved.
[0027] The upstream catalytic layers 7 are composed of the fuel
reforming catalyst which reforms diesel fuel to generate reducing
agents. The downstream catalytic layers 8 are composed of the NSR
catalyst which reduces NO.sub.x contained in the exhaust gas into
nitrogen by the reducing agents generated by the fuel reforming
catalyst. The honeycomb support 5 has two different catalytic
functions, accordingly. One of the functions is to generate
reducing agents by reforming diesel fuel, and the other is to
reduce NO.sub.x in exhaust gas into nitrogen. The reducing agents
generated by the fuel reforming catalyst of the upstream catalytic
layers 7 immediately reach the NSR catalyst of the downstream
catalytic layers 8, without flowing through the exhaust passage 2.
Therefore, the elimination of reducing agents by oxygen and the
like in the exhaust gas is suppressed, and the reaction efficiency
of the NSR catalyst is improved.
[0028] The following will describe an exhaust gas purification
device according to a second preferred embodiment of the present
invention. The exhaust gas purification device of the second
embodiment differs from that of the first embodiment in that the
upstream and downstream catalytic layers are composed of an
oxidizing catalyst and a fuel reforming catalyst, respectively.
Like or same parts or elements will be referred to by the same
reference numerals as those in FIGS. 1 through 5, and the
description thereof will be omitted.
[0029] FIG. 6 shows the structure of the exhaust gas purification
device of the second preferred embodiment. The honeycomb support 5
is connected at the downstream side of the injector 3 in the
direction indicated by an arrow A of FIG. 6. At the downstream side
of the honeycomb support 5, an NSR catalyst 19 and a SCR catalyst 9
are connected serially in this order. The NSR catalyst 19 is
similar to that of the downstream catalytic layer 8 in the first
embodiment.
[0030] As shown in FIG. 7, upstream catalytic layers 17 composed of
an oxidizing catalyst are formed on the upstream surfaces 5D of the
porous walls 5C in the honeycomb support 5. Similarly, downstream
catalytic layers 18 composed of a fuel reforming catalyst are
formed on the downstream surfaces 5E. The oxidizing catalyst of the
upstream catalytic layers 17 includes a precious metal such as
platinum (Pt), palladium (Pd), and the like, and serves to oxidize
and burn particulate matter (hereinafter referred to as PM)
contained in exhaust gas. The honeycomb support 5 is of a wall flow
type in which exhaust gas flows through the porous walls 5C. As
shown in FIG. 8, PM contained in exhaust gas is collected by the
upstream catalytic layer 17 when exhaust gas passes through the
honeycomb support 5. The fuel reforming catalyst of the downstream
catalytic layers 18 is similar to that of the upstream catalytic
layers 7 in the first embodiment. When diesel fuel is supplied,
carbon monoxide, hydrogen, hydrocarbon are generated as reducing
agents for the NSR catalyst 19. Other structures are similar to
those of the first embodiment.
[0031] The following will describe the operation of the exhaust gas
purification device of the second preferred embodiment. Firstly,
the operation of the exhaust gas purification device when the
injector 3 does not inject diesel fuel yet will be described. As
shown in FIG. 7, exhaust gas flowing into the honeycomb support 5
flows through the inlet cell 5A, the upstream catalytic layer 17,
the porous wall 5C, and the downstream catalytic layer 18, serially
in this order, into the outlet cell 5B. PM contained in exhaust gas
is collected on the oxidizing catalyst of the upstream catalytic
layer 17, as shown in FIG. 8, and is burned to be removed by the
function of the oxidizing catalyst. The heat generated when PM is
burned is transferred to the downstream catalytic layer 18 through
the porous wall 5C so as to heat the fuel reforming catalyst of the
downstream catalytic layer 18.
[0032] The exhaust gas from which PM is removed by the upstream
catalytic layer 17 passes through the porous wall 5C and reaches
the downstream catalytic layer 18. Since the injector 3 does not
inject diesel fuel, the reaction over the fuel reforming catalyst
of the downstream catalytic layer 18 does not occur. The exhaust
gas passing through the downstream catalytic layer 18 is discharged
to the outside of the honeycomb support 5, and passes through the
NSR catalyst 19 connected downstream of the honeycomb support 5.
Since the air-fuel ratio of the exhaust gas passing through the NSR
catalyst 19 is in a lean state, the NSR catalyst 19 absorbs
NO.sub.x temporarily, similar to that of the downstream catalytic
layer 8 in the first embodiment. In case the air-fuel ratio of the
exhaust gas is in a lean state, the NSR catalyst 19 does not
generate ammonia, and the reaction over the SCR catalyst 9 does not
occur.
[0033] Next will describe the operation of the exhaust gas
purification device when the injector 3 injects diesel fuel. When
the injector 3 injects diesel fuel into the exhaust passage 2, the
mixed gas of exhaust gas and diesel fuel passes through the porous
walls 5C of the honeycomb support 5. Part of diesel fuel in the
mixed gas passes through the upstream catalytic layers 17 while not
being burned, and reaches the downstream catalytic layers 18. The
unburned diesel fuel reacts with oxygen and water vapor contained
in the exhaust gas over the fuel reforming catalyst of the
downstream catalytic layers 18 to generate carbon monoxide,
hydrogen, and hydrocarbon as reducing agents for the NSR catalyst
19. The reaction is similar to that in the upstream catalytic
layers 7 of the first embodiment.
[0034] In passing through the porous wall 5C, the unburned diesel
fuel which reaches the downstream catalytic layers 18 is
distributed uniformly in the whole exhaust gas. Therefore, the
unburned diesel fuel is supplied uniformly to the entire fuel
reforming catalyst of the downstream catalytic layers 18, thereby
improving the reaction efficiency of the fuel reforming catalyst.
Since the fuel reforming catalyst is heated by the heat generated
when the oxidizing catalyst of the upstream catalytic layers 17
burns PM, the reaction efficiency of the fuel reforming catalyst is
further improved. The exhaust gas in a rich state is discharged to
the outside of the honeycomb support 5, and reaches the NSR
catalyst 19. In this state, the exhaust gas includes diesel fuel
which does not react over the fuel reforming catalyst of the
downstream catalytic layers 18. The NSR catalyst 19 when the
exhaust gas is in a rich state reacts similar to the NSR catalyst
of the downstream catalytic layers 8 in the first embodiment.
[0035] Thus, in the porous walls 5C of the honeycomb support 5, the
upstream catalytic layers 17 are composed of the oxidizing catalyst
and the downstream catalytic layers 18 are composed of the fuel
reforming catalyst. Therefore, the single honeycomb support 5 has
two different catalytic functions, thereby downsizing the exhaust
gas purification device, similar to the first embodiment. Further,
the unburned diesel fuel which passes through the upstream
catalytic layers 17 is in an uniformly-distributed state in the
exhaust gas when passing through the porous walls 5C. The diesel
fuel is supplied uniformly to the entire downstream catalytic
layers 18, thereby improving the reaction efficiency in the fuel
reforming catalyst of the downstream catalytic layers 18.
[0036] Further, the upstream catalytic layers 17 are composed of
the oxidizing catalyst which burns PM. Therefore, the heat
generated when PM is burned is transferred to the downstream
catalytic layers 18 through the porous walls 5C so as to heat the
fuel reforming catalyst of the downstream catalytic layers 18.
Therefore, the reaction efficiency of the fuel reforming catalyst
is further improved.
[0037] Therefore, the present examples and embodiments are to be
considered as illustrative and not restrictive, and the invention
is not to be limited to the details given herein but may be
modified within the scope of the appended claims.
* * * * *