U.S. patent application number 12/521380 was filed with the patent office on 2010-12-16 for exhaust emission control apparatus for internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Hirohito Hirata, Masaya Ibe, Masaya Kamada, Mayuko Osaki.
Application Number | 20100313552 12/521380 |
Document ID | / |
Family ID | 39588421 |
Filed Date | 2010-12-16 |
United States Patent
Application |
20100313552 |
Kind Code |
A1 |
Hirata; Hirohito ; et
al. |
December 16, 2010 |
EXHAUST EMISSION CONTROL APPARATUS FOR INTERNAL COMBUSTION
ENGINE
Abstract
An exhaust emission control apparatus that is used with an
internal combustion engine to use both a NOx retention member and a
catalyst while avoiding a decrease in the exhaust gas purification
capacity. A NOx occlusion reduction type catalyst is placed in an
exhaust path of an internal combustion engine. The NOx occlusion
reduction type catalyst includes a base material. The base material
includes an exhaust inflow cell, which is closed at its downstream
side; and an exhaust outflow cell, which is closed at its upstream
side and is adjacent to the first cell with the partition wall
positioned in between. The exhaust inflow cell is configured so
that a NOx retention layer is formed on the inner surface thereof,
and the exhaust outflow cell is configured so that a catalyst layer
is formed on the inner surface thereof, respectively.
Inventors: |
Hirata; Hirohito;
(Shizuoka-ken, JP) ; Ibe; Masaya; (Shizuoka-ken,
JP) ; Osaki; Mayuko; (Shizuoka-ken, JP) ;
Kamada; Masaya; (Aichi-ken, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
39588421 |
Appl. No.: |
12/521380 |
Filed: |
December 20, 2007 |
PCT Filed: |
December 20, 2007 |
PCT NO: |
PCT/JP2007/074538 |
371 Date: |
June 26, 2009 |
Current U.S.
Class: |
60/287 ; 60/297;
60/301; 60/304 |
Current CPC
Class: |
F01N 3/0842 20130101;
B01D 2251/104 20130101; F01N 3/0222 20130101; Y02A 50/20 20180101;
B01D 2255/202 20130101; B01D 2255/2042 20130101; B01D 53/9431
20130101; B01D 2255/204 20130101; B01D 2255/91 20130101; F01N
2240/38 20130101; F01N 3/033 20130101; Y02A 50/2344 20180101; B01J
23/58 20130101; B01D 2255/1021 20130101; F01N 2570/14 20130101;
B01D 2255/1025 20130101; B01J 37/0242 20130101; F01N 3/0814
20130101 |
Class at
Publication: |
60/287 ; 60/301;
60/304; 60/297 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/10 20060101 F01N003/10; F01N 3/035 20060101
F01N003/035 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2006 |
JP |
2006-355739 |
Claims
1. An exhaust emission control apparatus for an internal combustion
engine, comprising: a NOx occlusion reduction type catalyst, which
is positioned in an exhaust path of the internal combustion engine;
and ozone supply means, which supplies ozone so that the ozone
mixes with an exhaust gas flowing into the NOx occlusion reduction
type catalyst; wherein the NOx occlusion reduction type catalyst
includes two or more cells, which are partitioned by a partition
wall that permits the passage of the exhaust gas, the two or more
cells including a first cell configured so that a downstream side
of the first cell is covered and a NOx retention layer containing a
NOx retention member is formed on an inner surface of the first
cell; and a second cell configured so that the second cell is
adjacent to the first cell with the partition wall positioned in
between, an upstream side of the second cell is covered, a catalyst
layer including a noble metal is formed on an inner surface of the
second cell, and an amount of the NOx retention member contained in
the catalyst layer is smaller than that in the NOx retention
layer.
2. The exhaust emission control apparatus according to claim 1,
wherein the partition wall, which permits the passage of the
exhaust gas, is a particulate filter for capturing particulates
contained in the exhaust gas.
3. The exhaust emission control apparatus according to claim 1,
wherein the catalyst layer formed on the inner surface of the
second cell is configured so that the amount of the NOx retention
member contained in the catalyst layer is substantially zero.
4. The exhaust emission control apparatus according to claim 1,
further comprising: ozone supply amount adjustment means for
adjusting the amount of ozone supply so that the mole ratio of
ozone to nitrogen monoxide (NO) in a gas mixture flowing into the
NOx occlusion reduction type catalyst is greater than 1.
5. The exhaust emission control apparatus according to claim 4,
wherein the ozone supply amount adjustment means adjusts the amount
of ozone supply so that the mole ratio of ozone (O.sub.3) to
nitrogen monoxide (NO) in the gas mixture flowing into the NOx
occlusion reduction type catalyst is not smaller than 2.
6. An exhaust emission control apparatus for an internal combustion
engine, comprising: a NOx occlusion reduction type catalyst, which
is positioned in an exhaust path of the internal combustion engine;
and an ozone supply unit, which supplies ozone so that the ozone
mixes with an exhaust gas flowing into the NOx occlusion reduction
type catalyst; wherein the NOx occlusion reduction type catalyst
includes two or more cells, which are partitioned by a partition
wall that permits the passage of the exhaust gas, the two or more
cells including a first cell configured so that a downstream side
of the first cell is covered and a NOx retention layer containing a
NOx retention member is formed on an inner surface of the first
cell; and a second cell configured so that the second cell is
adjacent to the first cell with the partition wall positioned in
between, an upstream side of the second cell is covered, a catalyst
layer including a noble metal is formed on an inner surface of the
second cell, and an amount of the NOx retention member contained in
the catalyst layer is smaller than that in the NOx retention layer.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust emission control
apparatus for an internal combustion engine.
BACKGROUND ART
[0002] It is known that a conventional exhaust emission control
apparatus disclosed, for instance, in JP-A-2002-89246 includes a
NOx occlusion reduction type catalyst. In the above conventional
art, an exhaust path of an internal combustion engine is provided
with a catalyst and a substance capable of occluding NOx
(hereinafter also referred to as the "NOx retention member"). Such
a configuration is formed so that NOx in an exhaust gas is occluded
by the NOx occlusion catalyst in a lean atmosphere, and that the
occluded NOx is released, reduced, and decomposed in a rich
atmosphere.
[0003] To ensure that the above reaction smoothly takes place, it
is preferred that the NOx occlusion reduction type catalyst reach
its activation temperature and fully exercise its activation
function. When an internal combustion engine starts up, however,
the catalyst temperature is low. Thus, the conventional exhaust
emission control apparatus addresses the above problem by adding
ozone (O.sub.3) to the exhaust gas at internal combustion engine
startup. Adding ozone to the exhaust gas oxidizes NOx in the
exhaust gas to accelerate a NOx occlusion reaction. Consequently,
even when the NOx occlusion reduction type catalyst is not fully
active at the time, for instance, of internal combustion engine
startup, the use of the above-described conventional technology
makes it possible to accelerate NOx occlusion and purify the
exhaust gas.
Patent Document 1: JP-A-2002-89246
Patent Document 2: JP-A-1993-192535
Patent Document 3: JP-A (PCT) No. 538295/2005
Patent Document 4: JP-A-1994-185343
Patent Document 5: JP-A-1998-169434
Patent Document 6: Japanese Patent No. 3551346
DISCLOSURE OF INVENTION
Problem to be Solved by the Invention
[0004] Meanwhile, the above-described NOx occlusion reduction type
catalyst is formed so that a layer containing the catalyst and NOx
retention member is coated onto a base material (which may also be
referred to as a support). This type of catalyst tends to have a
smaller exhaust gas purification capacity (a smaller capacity of
purifying NOx, HC, and CO) than a conventional three-way catalyst
without a NOx retention member. It means that the exhaust gas
purification function of the above-described NOx occlusion
reduction type catalyst is blocked.
[0005] The present invention has been made to solve the above
problem. An object of the present invention is to provide an
exhaust emission control apparatus that is used with an internal
combustion engine to use both a NOx retention member and a catalyst
while avoiding a decrease in the exhaust gas purification
capacity.
Means for Solving the Problem
[0006] To achieve the above-mentioned purpose, the first aspect of
the present invention is an exhaust emission control apparatus for
an internal combustion engine, comprising:
[0007] a NOx occlusion reduction type catalyst, which is positioned
in an exhaust path of the internal combustion engine; and
[0008] ozone supply means, which supplies ozone so that the ozone
mixes with an exhaust gas flowing into the NOx occlusion reduction
type catalyst; wherein
[0009] the NOx occlusion reduction type catalyst includes two or
more cells, which are partitioned by a partition wall that permits
the passage of the exhaust gas,
[0010] the two or more cells including a first cell configured so
that a downstream side of the first cell is covered and a NOx
retention layer containing a NOx retention member is formed on an
inner surface of the first cell; and a second cell configured so
that the second cell is adjacent to the first cell with the
partition wall positioned in between, an upstream side of the
second cell is covered, a catalyst layer including a noble metal is
formed on an inner surface of the second cell, and an amount of the
NOx retention member contained in the catalyst layer is smaller
than that in the NOx retention layer.
[0011] The second aspect of the present invention is the exhaust
emission control apparatus according to the first aspect, wherein
the partition wall, which permits the passage of the exhaust gas,
is a particulate filter for capturing particulates contained in the
exhaust gas.
[0012] The third aspect of the present invention is the exhaust
emission control apparatus according to the first or the second
aspect, wherein the catalyst layer formed on the inner surface of
the second cell is configured so that the amount of the NOx
retention member contained in the catalyst layer is substantially
zero.
[0013] The fourth aspect of the present invention is the exhaust
emission control apparatus according to any one of the first to
third aspects, further comprising:
[0014] ozone supply amount adjustment means for adjusting the
amount of ozone supply so that the mole ratio of ozone to nitrogen
monoxide (NO) in a gas mixture flowing into the NOx occlusion
reduction type catalyst is greater than 1.
[0015] The fifth aspect of the present invention is the exhaust
emission control apparatus according to the fourth aspect, wherein
the ozone supply amount adjustment means adjusts the amount of
ozone supply so that the mole ratio of ozone (O.sub.3) to nitrogen
monoxide (NO) in the gas mixture flowing into the NOx occlusion
reduction type catalyst is not smaller than 2.
ADVANTAGES OF THE INVENTION
[0016] The first aspect of the present invention is configured so
that the first cell includes a NOx retention layer whereas the
second cell includes a catalyst layer. Therefore, the catalyst can
properly exercise its exhaust gas purification function. It is
thought that the NOx retention member is a catalyst poison for a
noble metal element and a factor of decreasing the exhaust gas
purification capacity of the catalyst. According to the first
aspect of the present invention, the NOx retention layer and the
catalyst layer are provided in the first cell and in the second
cell, respectively. Further, the ozone supply means accelerates a
NOx occlusion reaction without resort to the catalyst layer.
Therefore, the first aspect of the present invention makes it
possible to occlude and reduce NOx while inhibiting the NOx
retention member from acting as a catalyst poison to keep the
catalyst's exhaust gas purification function intact.
[0017] The second aspect of the present invention enables the
catalyst to properly exercise its exhaust gas purification function
and allows the partition wall to capture particulates contained in
the exhaust gas.
[0018] The third aspect of the present invention makes it possible
to suppress the influence of the catalyst poison with higher
effectiveness than in the first aspect.
[0019] According to the fourth aspect of the present invention, NO
in the exhaust gas can be oxidized to generate NO.sub.3,
N.sub.2O.sub.5, and other nitrogen oxides of higher order than
NO.sub.2 (generate HNO.sub.3 as well if water exists). This makes
it possible to increase the amounts of NO.sub.3, N.sub.2O.sub.5,
and other nitrogen oxides of higher order than NO.sub.2, which are
contained in the exhaust gas that flows into a NOx retention
member. As a result, a NOx occlusion reaction can be accelerated to
increase the exhaust gas purification capacity.
[0020] According to the fifth aspect of the present invention, a
sufficient amount of ozone can be supplied as needed to generate
NO.sub.3, N.sub.2O.sub.5, and other nitrogen oxides of higher order
than NO.sub.2 (generate HNO.sub.3 as well if water exists) by
oxidizing NO. As a result, the NOx occlusion reaction can be
effectively accelerated to increase the exhaust gas purification
capacity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a diagram illustrating configuration of an exhaust
emission control apparatus according to a first embodiment of the
present invention.
[0022] FIGS. 2A and 2B are diagrams illustrating configuration of
the apparatus according to a first embodiment.
[0023] FIG. 3 is a flowchart illustrating a routine that the ECU 50
executes in the first embodiment.
[0024] FIG. 4 is a diagram to describe result of experiment for the
first embodiment.
[0025] FIG. 5 is a diagram to describe result of experiment for the
first embodiment.
[0026] FIGS. 6A and 6B are diagrams to describe result of
experiment for the first embodiment.
[0027] FIGS. 7A to 7C are diagrams to describe result of experiment
for the first embodiment.
[0028] FIG. 8 is a diagram to describe result of experiment for the
first embodiment.
DESCRIPTION OF NOTATIONS
[0029] 10 an internal combustion engine [0030] 12 an exhaust path
[0031] 20 a catalytic device [0032] 30 an ozone supply device
[0033] 32 an ozone injection orifice [0034] 34 an air inlet [0035]
50 ECU [0036] 80 a NOx occlusion reduction type catalyst [0037] 82
a base material [0038] 86 a partition wall section [0039] 90 an
exhaust inflow cell [0040] 92 a NOx retention layer [0041] 94 a
catalyst layer [0042] 96 an exhaust outflow cell
BEST MODE FOR CARRYING OUT THE INVENTION
First Embodiment
Configuration of First Embodiment
[0043] FIG. 1 is a diagram illustrating an exhaust emission control
apparatus according to a first embodiment of the present invention.
As shown in FIG. 1, the exhaust emission control apparatus
according to the first embodiment includes a catalytic device 20,
which is placed in an exhaust path 12 of an internal combustion
engine 10. A NOx occlusion reduction type catalyst 80 is placed in
the catalytic device 20. As far as the exhaust emission control
apparatus is configured as described above, an exhaust gas passing
through the exhaust path 12 flows into the catalytic device 20 and
then into the NOx occlusion reduction type catalyst 80.
[0044] FIGS. 2A and 2B are cross-sectional views illustrating the
configuration of the NOx occlusion reduction type catalyst 80. The
cross-sectional views of the NOx occlusion reduction type catalyst
80 are taken along the direction of exhaust gas distribution. The
left-hand side of FIG. 2 corresponds to the upstream side into
which the exhaust gas flows, whereas the right-hand side of FIG. 2
corresponds to the downstream side from which the exhaust gas that
is purified during its passage through the NOx occlusion reduction
type catalyst 80 flows.
[0045] FIG. 2A schematically shows the overall configuration of the
NOx occlusion reduction type catalyst 80. The NOx occlusion
reduction type catalyst 80 is formed by coating a base material 82
shown in FIG. 2A with a NOx retention layer and a catalyst layer,
which will be described later. The base material 82 is a
honeycombed ceramic base material. The interior of the base
material 82 is partitioned by a partition wall to form a plurality
of cells.
[0046] As shown in FIG. 2A, the base material 82 includes an
exhaust inflow cell 90, which is open at its upstream side (the
left-hand side of the figure) and closed at its downstream side
(the right-hand side of the figure); and an exhaust outflow cell
96, which is closed at its upstream side and open at its downstream
side. These cells are extended in the direction of exhaust gas flow
(in the left-right direction in FIG. 2A).
[0047] FIG. 2B is an enlarged partial view of the NOx occlusion
reduction type catalyst 80. It illustrates the configuration of
cells included in the NOx occlusion reduction type catalyst 80. As
described above, the exhaust inflow cell 90 is configured so that
its upstream side is open to permit the inflow of the exhaust gas.
The downstream side of the exhaust inflow cell 90 is closed to
block the flow of the exhaust gas.
[0048] The inner surface of the exhaust inflow cell 90 is provided
with a NOx retention layer 92. The NOx retention layer 92 is formed
by coating the inner surface of the exhaust inflow cell 90 with a
NOx retention material containing BaCO.sub.3. BaCO.sub.3 functions
as a NOx retention member (which may also be referred to as a NOx
occlusion agent) that occludes NOx in the exhaust gas as nitrate
(or more specifically, Ba(NO.sub.3).sub.2). The occluded
Ba(NO.sub.3).sub.2 is actively released when mainly the exhaust gas
is rich or when the NOx retention member temperature is high.
Further, the NOx retention layer 92 is gas permeable to permit the
passage of the exhaust gas.
[0049] The exhaust outflow cell 96, on the other hand, is
configured so that its downstream side is open while its upstream
side is closed. This ensures that the gas existing in the exhaust
outflow cell 96 flows downstream and out of the NOx occlusion
reduction type catalyst 80.
[0050] The inner surface of the exhaust outflow cell 96 is provided
with a catalyst layer 94. The catalyst layer 94 is formed by
coating the inner surface of the exhaust outflow cell 96 with a
catalytic material containing Pt or other noble metal. Pt or other
noble metal functions as an active site that simultaneously
activates the oxidation reaction of CO and HC and the reduction
reaction of NOx. Thus, the catalyst layer 94 functions as a
three-way catalyst that simultaneously purifies NOx, CO, and HC.
Further, the catalyst layer 94 is gas permeable to permit the
passage of the exhaust gas.
[0051] As shown in FIG. 2B, the exhaust inflow cell 90 is adjacent
to the exhaust outflow cell 96 with a partition wall section 86 of
the base material 82 positioned in between. The partition wall
section 86, which is gas permeable to permit the passage of the
exhaust gas, functions as a filter that captures various
particulates (which may be abbreviated to PM) contained in the
exhaust gas when the exhaust gas passes through the partition wall
section 86.
[0052] When the above-described configuration is employed, the
exhaust gas flowing into the NOx occlusion reduction type catalyst
80 first flows into the exhaust inflow cell 90, then sequentially
passes through the NOx retention layer 92, partition wall section
86, and catalyst layer 94, reaches the exhaust outflow cell 94, and
flows downstream out of the exhaust outflow cell 94 (note the
arrows in FIG. 2B). This ensures that the exhaust gas passing
through the NOx occlusion reduction type catalyst 80 is purified as
needed by means of NOx occlusion, particulate removal, and
three-way activation during its distribution process.
[0053] The base material 82, which is honeycombed, is configured so
as to alternately close the upstream side opening and downstream
side opening of individual cells. This base material is similar to
a diesel particulate filter (DPF or simply referred to as the
"particulate filter"), which has been conventionally used to
capture particulates in the exhaust gas. Therefore, the
above-mentioned DPF, which is publicly known, can be used as needed
as the base material 82 for the first embodiment.
[0054] As shown in FIG. 1, the apparatus according to the first
embodiment also includes an ozone supply device 30. The ozone
supply device 30 is in communication with an air inlet 34. The
ozone supply device 30 can acquire air from the air inlet 34,
generate ozone (O.sub.3), and supply the ozone downstream. The
configuration, function, and other characteristics of an ozone
generator, which generates ozone from air, will not be described in
detail because a variety of related technologies are publicly
known.
[0055] The ozone supply device 30 has an ozone injection orifice
32, which injects a gas within the catalytic device 20. The ozone
injection orifice 32 is positioned upstream of the NOx occlusion
reduction type catalyst 80 in the catalytic device 20. When this
configuration is employed to inject ozone from the ozone injection
orifice 32, the ozone or air can be added to the exhaust gas
passing through the exhaust path 12. The added ozone or air then
mixes with the exhaust gas so that the resulting gas mixture flows
into the NOx occlusion reduction type catalyst 80.
[0056] The exhaust emission control apparatus according to the
first embodiment includes an ECU (Electronic Control Unit) 50. The
ECU 50 is connected to the ozone supply device 30. The ECU 50
transmits a control signal to the ozone supply device 30 for the
purpose of controlling the timing and amount of ozone injection.
The use of the above-described configuration makes it possible to
supply ozone at desired timing.
[0057] To efficiently induce a NOx occlusion reaction where the NOx
retention layer 92 occludes NOx in the exhaust gas, it is preferred
that NOx in the exhaust gas is oxidized to a great extent. In the
first embodiment, the ozone supply device 30 can add ozone to the
exhaust gas as needed. This makes it possible to effectively purify
the exhaust gas by oxidizing NOx in the exhaust gas during a gas
phase reaction.
[0058] The ECU 50 is also connected, for instance, to various
sensors, which are provided for the internal combustion engine 10.
Therefore, the ECU 50 can acquire information, for instance, about
the temperature, engine speed Ne, air-fuel ratio A/F, load, and
intake air amount of the internal combustion engine 10.
Features of First Embodiment
Features of Configuration
[0059] As described above, the NOx retention member (BaCO.sub.3 in
the first embodiment) contained in the NOx retention member is
capable of occluding NOx in the exhaust gas. The noble metal
contained in the catalyst (Pt, Rh, Pd, etc. in the first
embodiment) functions as an active site during exhaust gas
purification. To achieve NOx occlusion reduction and exhaust gas
purification with high efficiency, it is important that the above
functions be exercised effectively in a coordinated manner.
[0060] Various conventional catalysts formed by integrating the
abovementioned NOx retention member and catalyst are known. These
catalysts are disclosed, for instance, in Japanese Patent No.
3551346 and also referred to as a "NOx occlusion reduction type
catalyst" or "NSR catalyst." The NOx occlusion reduction type
catalyst can accelerate the NOx occlusion reaction by promoting the
oxidation of NOx. Further, when NOx is to be released, the catalyst
can purify the exhaust gas.
[0061] However, if the NOx retention member is combined with the
catalyst as described above, the exhaust gas purification capacity
of the catalyst (the capacity for purifying NOx, HC, and CO)
becomes smaller than that of a conventional three-way catalyst that
does not include a NOx retention member. The reason would be that
the NOx retention member acts as a catalyst poison for the catalyst
(noble metal element) and impairs the catalyst's activation
function. To achieve exhaust gas purification with high efficiency,
it is preferred that such an adverse effect be avoided to fully
exercise the catalyst's function.
[0062] In view of the above circumstances, the exhaust emission
control apparatus according to the first embodiment configures the
NOx occlusion reduction type catalyst 80 by providing the exhaust
inflow cell 90 with the NOx retention layer 92 and the exhaust
outflow cell 96 with the catalyst layer 94, thereby making the NOx
retention layer 92 and catalyst layer 94 independent of each other.
As mentioned earlier, the catalyst's exhaust gas purification
capacity decreases when the NOx retention member acts as a catalyst
poison. The first embodiment prevents the NOx retention member from
acting as a catalyst poison for the catalyst layer 94 because the
NOx retention layer 92 and catalyst layer 94 are made independent
of each other with the partition wall section 86 positioned in
between. The following describes operations that are performed for
NOx occlusion and NOx release when the configuration according to
the first embodiment is employed.
(Operation Performed for Nox Occlusion)
[0063] As described above, the NOx occlusion reduction type
catalyst 80 according to the first embodiment is configured so that
the catalyst layer 94 is formed on the inner surface of the exhaust
outflow cell 96. The catalyst layer 94 includes Pt or other noble
metal and can simultaneously purify NOx, CO, and HC (this function
may be hereinafter referred to as the "exhaust gas purification
function"). However, to enable the catalyst to exercise its exhaust
gas purification function, it is necessary that the catalyst be
heated to an adequate activation temperature. Therefore, when the
internal combustion engine 12 starts up, particularly at a cold
temperature, it is difficult to purify NOx contained in the exhaust
gas because the temperature of the NOx occlusion reduction type
catalyst 80 is low.
[0064] In the above situation, therefore, the present embodiment
causes the NOx retention layer 92 to occlude NOx. Further, to
accelerate such a NOx occlusion, the present embodiment uses the
ozone supply device 30 to supply ozone so that the ozone mixes with
the exhaust gas flowing into the NOx occlusion reduction type
catalyst 80. When ozone is added to the exhaust gas in the above
manner, NOx in the exhaust gas is oxidized to facilitate NOx
occlusion.
[0065] The NOx oxidized by ozone flows into the exhaust inflow cell
90 and reaches the NOx retention layer 92. An occlusion reaction
then occurs in the NOx retention layer 92 so that NOx is occluded
as nitrate. As the above-described operation is performed, it is
possible to prevent the NOx in the exhaust gas from flowing
downstream of the catalytic device 20 even in a situation where the
catalyst layer 94 has not reached its activation temperature at
startup of the internal combustion engine 12.
(Operation Performed for Nox Release)
[0066] As the aforementioned NOx occlusion takes place after
startup of the internal combustion engine 12, the temperature of
the NOx occlusion reduction type catalyst 80 rises. Therefore, when
an adequate period of time elapses after startup of the internal
combustion engine 12, the temperature of the catalyst layer 94 in
the NOx occlusion reduction type catalyst 80 reaches an activation
temperature. Consequently, when the catalyst layer 94 reaches its
activation temperature and is ready to fully exercise its exhaust
gas purification function, the first embodiment shuts off the
supply of ozone and exercises control to slightly enrich the fuel
injection amount of the internal combustion engine 12.
[0067] When the supply of ozone shuts off, the NOx occlusion
reaction stops being accelerated. Further, when the temperature of
the NOx occlusion reduction type catalyst 80 is high, the
temperature of the NOx retention layer 92 is also high. As the
temperature rises and the atmosphere becomes enriched, the NOx
retention layer 92 actively releases the occluded NOx. Therefore,
the NOx release reaction actively occurs due to the above-described
control.
[0068] When NOx is released from the NOx retention layer 92, the
released NOx passes through the partition wall section 86 and
reaches the catalyst layer 94. The NOx in the catalyst layer 94 is
then reduced to N.sub.2, H.sub.2O, CO.sub.2, etc. by HC and other
reductants contained in the exhaust gas. As described earlier, the
present embodiment is configured so that the NOx retention layer 92
and catalyst layer 94 are formed independently of each other. This
configuration prevents the NOx retention member from acting as a
catalyst poison for the catalyst layer 94. Consequently, the
present embodiment makes it possible to purify the exhaust gas
effectively without blocking the exhaust gas purification function
of the catalyst layer 94.
[0069] As described above, the present embodiment certainly
prevents the NOx retention member from acting as a catalyst poison
because the NOx retention layer 92 and catalyst layer 94 are formed
independently of each other with the partition wall section 86
positioned in between. This makes it possible to unfailingly
prevent the exhaust gas purification capability of the catalyst
layer 94 from being hindered. Further, the present embodiment
causes the ozone supply device 30 to supply ozone and accelerates
the NOx occlusion reaction without resort to the catalyst.
Therefore, NOx can be occluded and reduced while fully exercising
the exhaust gas purification function of the catalyst.
[0070] In addition, when a NOx oxidation method based on ozone is
used, NOx can be oxidized with increased certainty during a gas
phase reaction without resort to the catalyst even when the
temperature is low at the time, for instance, of internal
combustion engine startup. Moreover, when water vapor exists,
nitric acid arises and easily reacts with the NOx retention member.
This makes it possible to occlude NOx with high efficiency.
Details of Process Performed by First Embodiment
[0071] A process performed by the exhaust emission control
apparatus according to the first embodiment will now be described
in detail with reference to FIG. 3. FIG. 3 is a flowchart
illustrating a routine that the ECU 50 executes in the first
embodiment. The routine is executed when the internal combustion
engine 10 starts at a low temperature (e.g., at a cold start).
[0072] First of all, the routine shown in FIG. 3 performs step S100
to supply ozone. More specifically, the ECU 50 transmits a control
signal to the ozone supply device 30 so that ozone is supplied at a
predetermined flow rate. Ozone injection then occurs in accordance
with the control signal. As a result, NO in the exhaust gas is
oxidized to NO.sub.3 so that an occlusion reaction occurs
efficiently within the NOx retention layer 92.
[0073] Next, the routine performs step S110 to judge whether an
O.sub.3 supply shutoff condition is established. More specifically,
step S110 is performed to judge whether a certain period of time,
which is required for the catalyst layer 94 to reach its activation
temperature has elapsed. The certain priod can be predetermined on
the basis of, for instance, an experiment. If the obtained judgment
result does not indicate that the O.sub.3 supply shutoff condition
is established, the routine concludes that the catalyst layer 94
has not reached its activation temperature, and repeats steps S100
and beyond.
[0074] If, on the other hand, the obtained judgment result
indicates that the O.sub.3 supply shutoff condition is established,
the routine proceeds to step S130, shuts off the supply of O.sub.3,
and controls the operating status of the internal combustion engine
10 so that the air-fuel ratio changes from stoichiometric to
slightly rich. As a result, the NOx occluded in the NOx retention
layer 92 is released. The released NOx then passes through the
partition wall section 86, reaches the catalyst layer 94, and
becomes reduced and purified. Subsequently, the routine comes to an
end.
[0075] When the above process is performed, it is possible to
unfailingly prevent the NOx retention member from acting as a
catalyst poison and achieve NOx occlusion and reduction while fully
exercising the exhaust gas purification function of the catalyst
layer 94. Further, when a NOx oxidation method based on ozone is
used, NOx can be surely oxidized without resort to the catalyst
even when the temperature is low at the time, for instance, of
internal combustion engine startup. This makes it possible to
obtain excellent emission characteristics.
[0076] As described earlier, the partition wall section 86
according to the first embodiment is made of a material that is
capable of capturing particulates contained in the exhaust gas.
Therefore, the particulates can be captured when the exhaust gas
passes through the partition wall section 86. Trace amounts of
particulates may be generated not only from a diesel engine but
also from a gasoline engine. It is therefore important that
particulates be effectively removed no matter what type of internal
combustion engine is used. The present embodiment can not only
achieve NOx occlusion and reduction and exhaust gas purification,
but also effectively dispose of generated particulates.
[0077] Further, the first embodiment is configured so that the
exhaust inflow cell 90 is opened in alignment with one surface of
the base material 82 (the left-hand side surface of FIG. 2) whereas
the exhaust outflow cell 96 is opened in alignment with the other
surface of the base material 82 (the right-hand side surface of
FIG. 2). As far as the above configuration is employed, it is easy
to provide the exhaust inflow cell 90 with the NOx retention layer
92 and the exhaust outflow cell 96 with the catalyst layer 94.
Therefore, the configuration according to the first embodiment is
excellent in that it makes it possible to form the NOx retention
layer 92 and catalyst layer 94 in isolation from each other and
makes it easy to form them on an individual basis.
[0078] In the first embodiment, which has been described above, the
NOx occlusion reduction type catalyst 80 corresponds to the "NOx
occlusion reduction type catalyst" according to the first aspect of
the present invention; and the ozone supply device 30 corresponds
to the "ozone supply means" according to the first aspect. Further,
in the first embodiment, which has been described above, the
partition wall section 86 of the base material 82 corresponds to
the "partition wall" according to the first aspect of the present
invention; the exhaust inflow cell 90 corresponds to the "first
cell" according to the first aspect; the exhaust outflow cell 96
corresponds to the "second cell" according to the first aspect; the
NOx retention layer 92 corresponds to the "NOx retention layer"
according to the first aspect; and the catalyst layer 94
corresponds to the "catalyst layer" according to the first
aspect.
[0079] Furthermore, in the first embodiment, which has been
described above, the partition wall section 86 corresponds to the
"particulate filter" according to the second aspect of the present
invention.
Results of Experiment for First Embodiment
[0080] Results of experiment for the first embodiment of the
present invention will now be described with reference to FIGS. 4
to 7.
(Configuration of Measurement System)
[0081] FIG. 4 shows a measurement system that was used for the
experiment. The measurement system includes a model gas generator
230 and a plurality of gas cylinders 232 in order to generate a
model gas, which represents the exhaust gas of an internal
combustion engine. The model gas generator 230 can mix the gases in
the gas cylinders 232 to create the following simulant gas:
Simulant gas composition
C.sub.3H.sub.6 1000 ppm
CO 7000 ppm
NO 1500 ppm
O.sub.2 7000 ppm
CO.sub.2 10%
H.sub.20.sub.3%
Remainder N.sub.2
[0082] The model gas generator 230 is in communication with an
electric furnace in which a test piece 222 is placed. FIG. 5 is an
enlarged view of the test piece 222 and its vicinity. As shown in
FIG. 5, the test piece 222 is configured so that an embodiment
sample 224 is housed in a quartz tube. The experiment involves the
use of a comparative example for which the same experiment is to be
conducted as with the embodiment sample 224 with a later-described
comparative sample substituted for the embodiment sample 224.
[0083] The measurement system shown in FIG. 4 includes an oxygen
cylinder 240. The downstream end of the oxygen cylinder 240 is in
communication with flow rate control units 242, 244. The flow rate
control unit 242 is in communication with the ozone generator 246.
The ozone generator 246 receives oxygen, which is supplied from the
oxygen cylinder 240, and generates ozone. The ozone generator 246
communicates with the downstream end of the model gas generator 230
and the upstream end of the test piece 222 through an ozone
analyzer 248 and a flow rate control unit 250.
[0084] Meanwhile, the downstream end of the flow rate control unit
244 directly communicates with the ozone analyzer. In a situation
where the above-described configuration is employed, turning ON the
ozone generator 246 supplies a gas mixture of O.sub.3 and O.sub.2
to the upstream end of the test piece 222 whereas turning OFF the
ozone generator 246 supplies only O.sub.2 to the upstream end of
the test piece 222.
[0085] When the flow rate control units 242, 244 and the ozone
generator 246 are used as appropriate, the measurement system shown
in FIG. 4 makes it possible to create the following two types of
gases, which differ in composition. Each of these gases is to be
injected into the test piece 222 and will be hereinafter simply
referred to as an "injection gas."
Injection gas composition (1) O.sub.3, 30,000 ppm; remainder,
O.sub.2 (2) O.sub.2 only
[0086] The flow rate control unit 250 can supply the injection gas
at a desired flow rate.
[0087] Exhaust gas analyzers 260, 262 and an ozone analyzer 264 are
positioned downstream of the test piece 222. These analyzers can
measure gas components that flow out of the test piece 222.
[0088] The following measuring instruments were used during the
experiment:
Ozone generator 246; Iwasaki Electric, OP100W Ozone analyzer 248
(upstream); Ebara Jitsugyo, EG600 Ozone analyzer 264 (downstream);
Ebara Jitsugyo, EG2001B Exhaust gas analyzers 260, 262; Horiba,
MEXA9100D (HC/CO/NOx measurement); Horiba, VAI-510 (CO.sub.2
measurement)
(Sample Preparation Method)
[0089] FIGS. 6A and 6B illustrate an embodiment sample and a
comparative sample that were used during the experiment. FIG. 6A
shows the embodiment sample 224, which is also shown in FIG. 5. The
embodiment sample 224 has the same configuration as the NOx
occlusion reduction type catalyst 80 according to the first
embodiment. FIG. 6B shows a comparative sample 324. The comparative
sample 324 uses the same honeycombed base material as the
embodiment sample 224, but is coated in a manner different from
that for the embodiment sample 224.
[0090] The embodiment sample 224 shown in FIG. 6A was prepared by
performing the procedure described below. First of all,
.gamma.-Al.sub.2O.sub.3 was dispersed in ion exchange water. An
aqueous solution of barium acetate was then added. The resulting
mixture was heated to remove water from it, dried at 120.degree.
C., and pulverized to powder. The powder was then burned for two
hours at 500.degree. C. The burnt powder was immersed in a solution
containing ammonium hydrogen carbonate, and then dried at
250.degree. C. to obtain barium that was supported on
Al.sub.2O.sub.3 (hereinafter also referred to as the
"barium-supported catalyst"). The support quantity of barium was
0.2 mole per 120 g of .gamma.-Al.sub.2O.sub.3.
[0091] Next, .gamma.-Al.sub.2O.sub.3 was dispersed in ion exchange
water. An aqueous solution containing dinitro-diamine platinum was
then added to support Pt. The resulting mixture was dried,
pulverized, and burned for one hour at 450.degree. C. to obtain
platinum that was supported on Al.sub.2O.sub.3 (hereinafter also
referred to as the "platinum-supported catalyst"). The support
quantity of platinum was 4 g per 120 g of
.gamma.-Al.sub.2O.sub.3.
[0092] Next, a 30 mm diameter, 50 mm long, 12 mil/300 cpsi
cordierite DPF (hereinafter also referred to as the base material
282) was prepared. As described earlier, the DPF has the same
configuration as the base material 82 according to the first
embodiment. In the experiment, therefore, the DPF was used as the
base material 282. One surface of the base material 282 (the
left-hand side surface of FIG. 6A) was coated with the
barium-supported catalyst and burned for one hour at 450.degree. C.
to obtain a NOx retention layer. The coating amount was such that
Al.sub.2O.sub.3 was coated at a rate of approximately 60 g/L.
[0093] Next, the other surface of the base material 282 (the
right-hand side surface of FIG. 6A), which was coated as described
above, was coated with the platinum-supported catalyst. The coated
base material 282 was burned for one hour at 450.degree. C. to
obtain a catalyst layer. The coating amount was such that
Al.sub.2O.sub.3 was coated at a rate of 60 g/L. As a result of the
above process, the embodiment sample 224, which corresponds to the
NOx occlusion reduction type catalyst 80 according to the first
embodiment, was obtained.
[0094] Consequently, the obtained embodiment sample 224 was such
that the overall Pt support quantity was 2 g, and that the Ba
support quantity was 0.1 mole/Al.sub.2O.sub.3 120 g, and further
that the coating amount was 120 g/L (Al.sub.2O.sub.3).
[0095] Meanwhile, the comparative sample 324, which is shown in
FIG. 6B, was prepared by performing the procedure described below.
First of all, .gamma.-Al.sub.2O.sub.3 was dispersed in ion exchange
water. An aqueous solution of barium acetate was then added. The
resulting mixture was heated to remove water from it, dried at
120.degree. C., and pulverized to powder. The powder was then
burned for two hours at 500.degree. C. The burnt powder was
immersed in a solution containing ammonium hydrogen carbonate, and
then dried at 250.degree. C. to obtain the barium-supported
catalyst.
[0096] The obtained barium-supported catalyst was dispersed in ion
exchange water. An aqueous solution containing dinitro-diammine
platinum was then added to support Pt. The resulting mixture was
dried, pulverized, and burned for one hour at 450.degree. C. In
this manner, a comparative coating catalyst was obtained. The
obtained catalyst was such that the barium support quantity was 0.1
mole per 120 g of .gamma.-Al.sub.2O.sub.3, and that the platinum
support quantity was 2 g per 120 g of .gamma.-Al.sub.2O.sub.3.
[0097] Next, both surfaces of a base material 382 (the left- and
right-hand surfaces of FIG. 6B), which has the same structure as
the base material 282, were coated with the comparative coating
catalyst, which was prepared as described above, and burned for one
hour at 450.degree. C. One surface was coated so that
Al.sub.2O.sub.3 was coated at a rate of 60 g/L. The overall coating
amount, including both surfaces, was such that Al.sub.2O.sub.3 was
coated at a rate of 120 g/L.
[0098] Consequently, the prepared comparative sample 324 is similar
to the embodiment sample 224 in that the overall Pt support
quantity was 2 g, and that the Ba support quantity was 0.1
mole/Al.sub.2O.sub.3 120 g, and further that the coating amount was
120 g/L (Al.sub.2O.sub.3). As described above, the embodiment
sample 224 and comparative sample 324 were configured so that they
contained the same amounts of Pt and Ba.
(Description of Experiment)
[0099] In the measurement system described above, the
aforementioned simulant gas and injection gas were combined and
supplied to the test piece 222 under the following conditions. The
electric furnace was controlled so that the catalyst temperature
rose at the following rate. The amounts of components of the gas
flowing downstream were then determined.
Temperature: 30.degree. C. to 500.degree. C.
[0100] Temperature rise rate: 10.degree. C./min (constant) Simulant
gas flow rate: 30 L/min Injection gas flow rate: 6 L/min
[0101] The injection gas was supplied when the temperature was
between 30.degree. C. and 300.degree. C. When the temperature was
between 300.degree. C. and 500.degree. C., only the simulant gas
was distributed without supplying the injection gas.
(Purification Efficiency Calculation Method)
[0102] FIGS. 7A to 7C are images illustrating how the exhaust gas
purification efficiency was calculated in the experiment. FIG. 7A
is an image illustrating the amount of a component of the supplied
exhaust gas, which was determined by multiplying the simulant gas
concentration by the test time. In accordance with the image, the
amount of a component of the exhaust gas supplied within the
measurement time was calculated in the experiment by multiplying
the product of the concentration of the component in the simulant
gas and a simulant gas flow rate by the test time.
[0103] FIG. 7B is an image illustrating the amount of a component
of the exhaust gas flowing downstream, which was determined by
multiplying the concentration of the gas flowing downstream of the
test piece 222 by the test time. In accordance with the image, the
amount of the component flowing downstream was calculated by
multiplying the product of a component concentration, which was
detected by an exhaust gas analyzer, and a gas flow rate by the
test time.
[0104] The above calculated values were then used to determine the
exhaust gas purification efficiency as shown in FIG. 7C. More
specifically, the amount of a component flowing downstream (FIG.
7B) was subtracted from the amount of gas supplied within the
measurement time (FIG. 7A). Further, the obtained value was divided
by the amount of gas supplied within the measurement time (FIG. 7A)
to calculate the exhaust gas purification efficiency as a
percentage.
(Results of Experiment)
[0105] FIG. 8 is a graph illustrating a first portion of the
results of the experiment. The graph in FIG. 8 indicates that the
use of the embodiment sample 224 exhibited higher purification
efficiencies for NOx, HC, and CO than the use of the comparative
sample 324.
[0106] The results of experiment, which have been described above,
indicate that the first embodiment induces a NOx occlusion reaction
while averting the influence of a catalyst poison. It means that
the catalyst fully exercises its exhaust gas purification function
to obtain excellent emission characteristics. Further, as mentioned
above, the embodiment sample 224 contains the same amounts of
barium and platinum as the comparative sample 324. In other words,
the first embodiment makes it possible to use the NOx retention
member and noble metal with high efficiency.
Modifications of First Embodiment
First Modification
[0107] The first embodiment coats the base material 82 with the NOx
retention layer 92 that contains BaCO.sub.3. However, the material
for the NOx retention layer is not limited to the one described
above. For instance, an alkali metal, such as Na, K, Cs, or Rb, an
alkali earth metal, such as Ba, Ca, or Sr, or a rare earth element,
such as Y, Ce, La, or Pr can be used as needed, as described in
Japanese Patent No. 3551346.
[0108] Therefore, when the NOx retention member occludes NOx as
nitrate, the composition of the nitrate is not limited to
Ba(NO.sub.3).sub.2, which is mentioned in connection with the first
embodiment. It should be noted that Ba has a large occlusion
capacity (1 mole of Ba can occlude 3 moles of NO.sub.3), exhibits
higher thermal stability than the other materials, and is suitable
as a NOx retention member for use with an exhaust emission control
apparatus.
[0109] The material for the catalyst layer 94 is not limited to Pt,
Rh, Pb, or other materials described earlier. Various catalyst
materials known as noble metal materials constituting an exhaust
gas purification catalyst may be applied to the present invention.
Further, ceramic, alumina (Al.sub.2O.sub.3), and other appropriate
materials may be used as a support material for a noble metal or
NOx retention member.
Second Modification
[0110] The first embodiment uses the ozone supply device 30 to add
ozone to the exhaust gas. Preferably, however, such an ozone
addition may be made in the manner described below. It is known
that NOx in the exhaust gas oxidizes due to a gas phase reaction
when ozone (O.sub.3) is added to the exhaust gas. More
specifically, the NOx reacts with the ozone to induce the following
reactions:
NO+O.sub.3->NO.sub.2+O.sub.2 [1]
NO.sub.2+O.sub.3->NO.sub.3+O.sub.2 [2]
NO.sub.2+NO.sub.3->N.sub.2O.sub.5 [3]
(NO.sub.2+NO.sub.3<-N.sub.2O.sub.5)
[0111] In the subsequent explanation, reaction formula [1] may be
referred to as the "first formula;" reaction formula [2], the
"second formula;" and reaction formula [3], the "third formula."In
the third formula, only the arrow indicating a rightward reaction
is included; however, the parenthesized leftward reaction may also
occur.
[0112] NOx occlusion in the NOx retention member is achieved when a
high-order nitrogen oxide, which is generated when NOx is oxidized,
or HNO.sub.3, which is generated when such a nitrogen oxide reacts
with water, is occluded as Ba(NO.sub.3).sub.2 or other nitrate by
the NOx retention member. When, for instance, NO.sub.3 changes to
Ba(NO.sub.3).sub.2 or other nitrate, it is occluded by the NOx
occlusion member. To induce a NOx occlusion reaction with high
efficiency, therefore, it is preferred that an increased amount of
NOx in the exhaust gas change to NO.sub.3, N.sub.2O.sub.5, and
other nitrogen oxides of higher order than NO.sub.2.
[0113] In view of the above circumstances, the second modification
induces the reactions indicated by the second and third formulae by
adding ozone in such a manner that the mole ratio of ozone to NO in
the gas mixture is greater than 1. More specifically, ozone
addition is made so that the following relational expression is met
by the ratio between Mol(O.sub.3), which is a mole equivalent of
the amount of ozone in the gas mixture, and Mol(NO), which is a
mole equivalent of the amount of nitrogen monoxide in the gas
mixture:
Mol(O.sub.3)/Mol(NO)>1 [4]
[0114] In the subsequent explanation, formula [4] above may be
referred to as the "fourth formula."
[0115] When the mole ratio of ozone to NO in the gas mixture is not
greater than 1 (Mol(O.sub.3)/Mol(NO).ltoreq.1), NO.sub.3 and
N.sub.2O.sub.5 will not be generated due to the reactions indicated
in the second and third formulae although NO.sub.2 is generated due
to the reaction indicated in the first formula. As such being the
case, the second modification is configured so that the substance
quantity of ozone to be added is greater than the substance
quantity of NO in the exhaust gas. Therefore, an adequate amount of
ozone can be supplied to generate NO.sub.2 and N.sub.2O.sub.5 by
oxidizing NO (to induce the reactions indicated in the second and
third formulae). As a result, the amounts of high-order nitrogen
oxides in the exhaust gas can be certainly increased to achieve NOx
occlusion effectively.
[0116] The process described above is implemented when the ECU 50
performs a "process for adjusting an ozone supply amount so that
the mole ratio of ozone to nitrogen monoxide (NO) in the gas
mixture flowing into the NOx occlusion reduction type catalyst is
greater than 1" (ozone supply amount adjustment processing). This
process can be performed, for instance, before step S100 of the
routine shown in FIG. 3. The ozone supply amount for providing the
above mole ratio can be defined, for instance, by allowing the ECU
50 to estimate the molar quantity of NOx contained in the exhaust
gas in accordance with the operating status (engine speed Ne,
air-fuel ratio A/F, load, intake air amount, etc.) of the internal
combustion engine 10 and calculate the flow rate of ozone to be
supplied in accordance with the estimated molar quantity of
NOx.
Third Modification
[0117] Alternatively, the ozone supply amount may be further
increased so that the mole ratio of ozone to nitrogen monoxide in
the gas mixture is not smaller than 2
(Mol(O.sub.3)/Mol(NO).gtoreq.2). When the mole ratio of ozone
(O.sub.3) to nitrogen monoxide (NO) in the gas mixture is greater
than 1, the ozone still remains in the gas mixture even after NO is
oxidized to NO.sub.2 as indicated in the first formula. Therefore,
the reactions indicated in the second and third formulae occur to
generate NO.sub.3 and N.sub.2O.sub.5. However, if a trace amount of
ozone remains after the reaction indicated in the first formula,
the amounts of NO.sub.3 and N.sub.2O.sub.5 to be generated during
the reactions indicated in the second and third formulae are
decreased.
[0118] In view of the above circumstances, the third modification
adjusts the ozone supply amount so that the mole ratio between
ozone and NO in the gas mixture is not smaller than 2
(Mol(O.sub.3)/Mol(NO).gtoreq.2). This ensures that an adequate
amount of ozone remains after the reaction indicated in the first
formula and contributes to the reactions indicated in the second
and third formulae, thereby certainly increasing the amounts of
high-order nitrogen oxides. As described above, the third
modification makes it possible to supply an adequate amount of
ozone for generating NO.sub.3 and N.sub.2O.sub.5 by oxidizing NO
and effectively accelerate the NOx occlusion reaction.
[0119] The process described above is implemented when the ECU 50
performs a "process for adjusting the ozone supply amount so that
the mole ratio of ozone (O.sub.3) to nitrogen monoxide (NO) in the
gas mixture flowing into the NOx occlusion reduction type catalyst
is not smaller than 2." This process can be performed, for
instance, before step S100 of the routine shown in FIG. 3.
Fourth Modification
[0120] The first embodiment is configured so as to supply ozone
with the ozone supply device 30 installed outside the catalytic
device 20 and the ozone injection orifice 32 positioned inside the
catalytic device 20. However, the present invention is not limited
to the use of such a configuration. Ozone may be added to the
exhaust gas by using various publicly known ozone generation
devices methods. For example, a configuration for generating ozone
directly by plasma discharge may be formed within the exhaust path
12 or catalytic device 20.
[0121] As mentioned earlier, the base material for use with the
present invention may be substituted by a DPF or made of various
publicly known materials that have been used for a DPF. It means
that the structures and materials applicable to the base material
for the present invention include the structures and materials of a
conventionally used DPF.
[0122] However, it can also be said that the base material for the
present invention is not limited to a DPF, and that the structure
and material of the base material 82 are not limited to those of
the DPF. More specifically, the present invention may employ a base
material that is configured to include the exhaust inflow cell and
exhaust outflow cell, which are adjacent to each other with the
partition wall positioned in between, while the partition wall is
made of a material that permits the passage of the exhaust gas. The
NOx occlusion reduction type catalyst for the present invention may
be configured so that the NOx retention layer is formed on the
inner surface of the exhaust inflow cell with the catalyst layer
formed on the inner surface of the exhaust outflow cell.
[0123] In the first embodiment, therefore, the partition wall
section 86 corresponds to a particulate filter that captures
particulates. However, the "partition wall" for the present
invention is not limited to such a particulate filter. In other
words, the use of a particulate filter is not always essential as
far as the employed partition wall is gas permeable to permit the
passage of the exhaust gas.
[0124] The NOx retention member may not only occlude NOx but also
adsorb NOx. More specifically, the NOx occlusion reduction type
catalyst 80 may not only occlude NOx but also adsorb NOx.
Therefore, the "retention" operation performed by the NOx retention
member means not only the "occlusion" of NOx but also the
"adsorption" of NOx.
[0125] It is preferred that the amount of NOx retention substance
contained in the catalyst layer 94 be substantially zero. However,
the present invention is not limited to the use of such a catalyst
layer. The present invention may alternatively be configured so
that the catalyst layer 94 contains a smaller amount of NOx
retention substance than the NOx retention layer 92.
* * * * *