U.S. patent application number 12/675100 was filed with the patent office on 2011-05-12 for exhaust gas purification device.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Takeshi Matsumoto, Yasuyuki Tamane.
Application Number | 20110107746 12/675100 |
Document ID | / |
Family ID | 40387036 |
Filed Date | 2011-05-12 |
United States Patent
Application |
20110107746 |
Kind Code |
A1 |
Matsumoto; Takeshi ; et
al. |
May 12, 2011 |
EXHAUST GAS PURIFICATION DEVICE
Abstract
An exhaust gas purification device in which catalysts are
efficiently activated and in which pressure loss caused by
deposition of particulates exhausted from an internal combustion
engine is suppressed. The exhaust gas purification device has a
first purification section and a second purification section. The
first purification section has a metal honeycomb structure where a
first oxidation catalyst is carried. The second purification
section is placed on the downstream side of the first purification
section and has a particulate filter that carries different amounts
of second oxidation catalysts on the upstream side and on the
downstream side of an exhaust gas route.
Inventors: |
Matsumoto; Takeshi;
(Saitama, JP) ; Tamane; Yasuyuki; (Saitama,
JP) |
Assignee: |
HONDA MOTOR CO., LTD.
Minato-ku ,Tokyo
JP
|
Family ID: |
40387036 |
Appl. No.: |
12/675100 |
Filed: |
August 5, 2008 |
PCT Filed: |
August 5, 2008 |
PCT NO: |
PCT/JP2008/064034 |
371 Date: |
February 24, 2010 |
Current U.S.
Class: |
60/297 |
Current CPC
Class: |
B01J 29/7007 20130101;
B01D 2255/1021 20130101; B01J 37/0244 20130101; B01J 37/0246
20130101; B01D 2255/206 20130101; F01N 3/106 20130101; B01J 35/04
20130101; F01N 13/009 20140601; B01J 29/7415 20130101; B01J 37/0225
20130101; F01N 13/0093 20140601; Y02T 10/12 20130101; F01N 3/035
20130101; F01N 3/023 20130101; F01N 2510/0682 20130101; B01J 29/80
20130101; B01J 35/0006 20130101; F01N 13/0097 20140603; B01D
2255/9022 20130101; F01N 3/2842 20130101; F01N 2250/02 20130101;
B01D 53/945 20130101; F01N 3/0211 20130101; F01N 3/0222 20130101;
Y02T 10/22 20130101; B01J 29/44 20130101; B01J 29/40 20130101; B01D
2255/50 20130101 |
Class at
Publication: |
60/297 |
International
Class: |
F01N 3/035 20060101
F01N003/035 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2007 |
JP |
2007-226980 |
Claims
1. An exhaust gas purification device that is disposed in an
exhaust passage of an internal combustion engine, and purifies
exhaust gas emitted from the internal combustion engine, the device
comprising: a first purification unit including a honeycomb
structure of metal on which a first oxidation catalyst is
supported; and a second purification unit that is disposed
downstream of the first purification unit and includes a
particulate filter on which a second oxidation catalyst is
supported so that loaded amounts at an upstream side thereof and a
downstream side thereof are different.
2. The exhaust gas purification device according to claim 1,
wherein the honeycomb structure is retained at a retaining member,
the retaining member is formed in two pieces including an inner
tube and an outer tube, and an air space layer is formed between
the inner tube and the outer tube.
3. The exhaust gas purification device according to claim 1,
wherein the particulate filter contains a larger loaded amount of
the second oxidation catalyst at an upstream side thereof than at a
downstream side thereof.
4. The exhaust gas purification device according to claim 1,
wherein a region having a relatively larger loaded amount of the
second oxidation catalyst is formed from the upstream side of the
particulate filter in a region in the range of 20 to 800 of a
length of the particulate filter in an axial direction.
5. The exhaust gas purification device according to claim 1,
wherein the first oxidation catalyst at least contains Pt, alumina,
ceria and a zeolite.
6. The exhaust gas purification device according to claim 1,
wherein the first purification unit includes one or more of the
honeycomb structure.
7. The exhaust gas purification device according to claim 1,
wherein the second oxidation catalyst at least contains Pt, Pd and
alumina.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust gas purification
device, and in particular relates to an exhaust gas purification
device that purifies exhaust gas emitted from an internal
combustion engine such as a diesel engine with good efficiency.
BACKGROUND ART
[0002] It has been known that particulate matter called
particulates (hereinafter referred to as PM) is contained in
exhaust gas emitted from an internal combustion engine in which
lean combustion is performed as in a diesel engine. In recent
years, the effects on the human body of these particulates has been
a concern, and thus an exhaust gas purification device that
efficiently collects particulates has been mounted on diesel
vehicles, which emit exhaust gas containing such particulates.
[0003] Generally, a diesel particulate collection filter (DPF), a
catalyzed soot filter (CSF) and the like have been used as an
exhaust gas purification device to collect particulates. Each of
these are devices that remove particulates by way of a filter
housed inside; however, since particulates collect inside the
filter with use, it is necessary to periodically or continuously
remove the particulates from the filter and clean the filter. If
the particulates deposit when exhaust gas passes through the
filter, resistance is imparted by the particulates thus deposited,
which is due to a pressure differential arising between the
upstream side and the downstream side of the filter, and thus a
so-called a pressure loss arising.
[0004] Here, as a general removal method of particulates, there is
a method in which fuel is periodically injected into the exhaust
system (exhaust path), the fuel thus injected is caused to combust
by an oxidation catalyst, and particulates are combustively removed
by the combustion heat generated at this time. However, in this
method, fuel is necessary to combust the particulates, whereby
there is a problem in that the fuel economy deteriorates.
[0005] On the other hand, as an alternate method, a method has been
suggested in which a catalyst carrier to which a noble metal has
been applied is arranged upstream of the filter, thereby improving
the ignition properties of the particulates deposited on the
filter, and the particulates are combusted at a relatively low
temperature. However, in this method, in a case where the distance
between the catalyst carrier and the filter is short, there has
been a problem in that it becomes easy for the particulates to
deposit on the filter.
[0006] Contrary to this, an exhaust gas purification device has
been disclosed that is provided with a casing in an exhaust side of
a diesel engine, houses inside this a filter that removes
particulates in the exhaust gas, and houses upstream of the filter
a catalyst carrier that causes harmful components of the exhaust
gas to oxidize, and in which the catalyst carrier and the filter
are disposed at a predetermined distance (for example, refer to
Patent Document 1). [0007] Patent Document 1: Japanese Unexamined
Patent Application, Publication No. 2001-98936
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0008] However, in the exhaust gas purification device disclosed in
Patent Document 1, an improvement had been implemented to the
arrangement and configuration of a carrier to which a catalyst is
loaded and a catalyst loaded to a carrier by only defining a
distance between the catalyst carrier and the filter. As a result,
a rise in pressure loss due to particulates depositing could be
suppressed; however, raising the efficiency of catalyst activity
could not be achieved simultaneously.
[0009] The present invention was made by taking the above such
issues into account, and an object thereof is to provide an exhaust
gas purification device that can suppress a rise in pressure loss
due to deposition of particulates emitted from an internal
combustion engine, and raise the efficiency of catalyst
activity.
Means for Solving the Problems
[0010] The present inventors have diligently researched to solve
the above-mentioned problems, whereby they discovered that
efficiency of catalyst activity could be raised and a rise in
pressure loss in the particulate filter could be suppressed by
disposing a honeycomb structure of metal to which a catalyst is
loaded at an upstream side, and disposing a particulate filter to
which an oxidation catalyst is loaded so that a loaded amount is
different at a downstream side of the honeycomb structure, thereby
arriving at completion of the present invention. More specifically,
the present invention provides an exhaust gas purification device
as described below.
[0011] According to a first aspect, an exhaust gas purification
device, which is disposed in an exhaust passage of an internal
combustion engine and purifies exhaust gas emitted from the
internal combustion engine, includes: a first purification unit
including a honeycomb structure of metal on which a first oxidation
catalyst is supported; and a second purification unit that is
disposed downstream of the first purification unit and includes a
particulate filter on which a second oxidation catalyst is
supported so that loaded amounts at an upstream side thereof and a
downstream side thereof are different.
[0012] In the exhaust gas purification device according to the
first aspect, the honeycomb structure of metal to which the first
oxidation catalyst is loaded is disposed at an upstream side. By
making the carrier to which the catalyst is loaded a honeycomb
structure of metal in this way, it is possible to raise the heat
conductivity. From this, it becomes possible to cause the catalyst
loaded to the honeycomb structure to be suitably activated.
[0013] In addition, since a honeycomb filter (honeycomb structure)
of metal is used, for example, since it is not necessary to provide
a heat insulating material layer in a matt shape between the filter
and the retaining member that retains the honeycomb filter, which
is necessary when using a ceramic filter or the like, it is
possible to employ a high capacity filter.
[0014] In addition, the second oxidation catalyst is loaded so that
the loaded amount of the upstream side and the downstream side of
the particulate filter is different. It thus becomes possible to
suppress a rise in pressure loss of the exhaust gas due to catalyst
loading by applying a so-called zone coat to the particulate
filter. It should be noted that zone coat implies coating a
catalyst at predetermined positions on a carrier so the loaded
amount of catalyst is different.
[0015] According to a second aspect, in the exhaust gas
purification device as described in the first aspect, the honeycomb
structure is retained at a retaining member, in which the retaining
member is formed in two pieces including an inner tube and an outer
tube, and an air space layer is formed between the inner tube and
the outer tube.
[0016] In the second aspect of the exhaust gas purification device,
the honeycomb structure is retained in a two-piece retaining member
formed with an air space layer between the inner tube and the outer
tube. As a result, for example, even in a case where heat
conductivity is high, and a honeycomb structure of metal that is
easily influenced by surrounding air and the like, it is possible
to minimize these influences.
[0017] According to a third aspect, in the exhaust gas purification
device as described in the first or second aspect, the particulate
filter contains a larger loaded amount of the second oxidation
catalyst at an upstream side thereof than at a downstream side
thereof.
[0018] In the exhaust gas purification device according to the
third aspect, the second oxidation catalyst is loaded to be
relatively abundant at an upstream side of the particulate filter
more than at a downstream side thereof. It thus becomes possible to
effectively activate the catalyst by loading the second oxidation
catalyst to be abundant at an upstream side of the particulate
filter, which is easily influenced by the heat of exhaust due to
contacting the exhaust gas first. Thus, catalyst activity is
improved, whereby it becomes possible to efficiency purify the
exhaust gas.
[0019] In addition, it becomes possible to suppress the influence
of deterioration of the second oxidation catalyst due to
temperature rising, even in a case where a heater and the like used
in regeneration of the particulate filter is disposed at a
downstream side, for example, by making the loaded amount of the
second oxidation catalyst on a downstream side of the particulate
filter less than that on an upstream side. From this it becomes
possible to decrease manufacturing cost and maintenance cost.
[0020] According to a fourth aspect, in the exhaust gas
purification device as described in any one of the first to third
aspects, a region having a relatively larger loaded amount of the
second oxidation catalyst is formed from the upstream side of the
particulate filter in a region in the range of 20 to 80% of a
length of the particulate filter in an axial direction.
[0021] In the exhaust gas purification device according to the
fourth aspect, a region in which the second oxidation catalyst is
loaded to be relatively abundant is formed from an upstream side of
the particulate filter at a region in the range of 20 to 80% of a
length in an axial direction of the particulate filter. In this
way, it becomes possible to suppress the influence on purification
performance due to exhaust gas accessibility, exhaust gas pressure
loss, catalyst deterioration and the like, even in a case where the
second oxidation catalyst is loaded to be dispersed so as to be
uniform over the entire particulate filter, by providing a region
in which the second oxidation catalyst is loaded to be relatively
abundant at a region in a range of 20 to 80% of a length of the
particulate filter from an upstream side of the particulate
filter.
[0022] According to a fifth aspect, in the exhaust gas purification
device as described in any one of the first to fourth aspects, the
first oxidation catalyst at least contains Pt, alumina, ceria and a
zeolite.
[0023] In the exhaust gas purification device according to the
fifth aspect, the first oxidation catalyst that is used in the
first purification unit contains at least Pt, alumina, ceria, and a
zeolite. By employing Pt, which has high oxidative power, in the
first oxidation catalyst, it becomes possible to combustively
remove HC and CO contained in the exhaust gas from a low
temperature. In addition, it becomes possible to adsorb NOx and
hydrocarbons up to an active temperature by having ceria, which is
a NOx adsorbent, and a zeolite, which is a hydrocarbon (HC)
adsorbent, coexisting. Moreover, since hydrocarbons oxidized with
Pt or Pd reduce NOx, it becomes possible to improve NOx
purification performance even in a case where NOx and hydrocarbons
desorb along with a rise in temperature. Furthermore, since ceria
and a zeolite coexist in the same catalyst layer, i.e. each exist
in the vicinity of one another, it becomes possible to promote
reaction.
[0024] According to a sixth aspect, in the exhaust gas purification
device as described in any one of the first to fifth aspects, the
first purification unit includes one or more of the honeycomb
structure.
[0025] The exhaust gas purification device according to the sixth
aspect comprises at least one honeycomb structure in the first
purification unit. In the case of honeycomb structures being
multiply installed, the purification efficiency can be
improved.
[0026] According to a seventh aspect, in the exhaust gas
purification device as described in any one of the first to sixth
aspects, the second oxidation catalyst at least contains Pt, Pd and
alumina.
[0027] In the exhaust gas purification device according to the
seventh aspect, the second oxidation catalyst used in the second
purification unit contains at least Pt, Pd, and alumina. By
dispersing an active metal such as Pt and Pd upon coating an
inorganic oxide having fire resistance such as alumina, it becomes
possible to improve low temperature oxidation activity to HC and CO
contained in the exhaust gas.
Effects of the Invention
[0028] According to the present invention, an exhaust gas
purification device can be provided that is capable of raising the
efficiency of catalyst activity and suppress a rise in pressure
loss due to deposition of particulates emitted from an internal
combustion engine.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is an outline configuration of an internal combustion
engine 10 provided with an exhaust gas purification device 1
related to an embodiment of the present invention;
[0030] FIG. 2 is a view showing an overall configuration of an
exhaust gas purification device 1 related to the embodiment of the
present invention;
[0031] FIG. 3 is an outline configuration of a state in which a
portion of a casing 5 of the first purification unit 2 related to
the embodiment of the present invention has been cut away;
[0032] FIG. 4 is a longitudinal section of the first purification
unit 2 related to the embodiment of the present invention;
[0033] FIG. 5 is an outline configuration of a particulate filter
of a second purification unit 3 related to the embodiment of the
present invention;
[0034] FIG. 6 is an outline configuration of catalyst loaded to the
particulate filter related to the embodiment of the present
invention;
[0035] FIG. 7 is a schematic view showing adsorption/purification
of NOx and hydrocarbons (HC);
[0036] FIG. 8 is a schematic showing a reaction state of NOx and
hydrocarbons (HC); and
[0037] FIG. 9 is a graph showing a relationship between a
proportion (%) of zone coat and CO light-off temperature (.degree.
C.) obtained from the Examples and Comparative Examples.
EXPLANATION OF REFERENCE NUMERALS
[0038] 1 Exhaust gas purification device [0039] 2 first
purification unit [0040] 3 second purification unit [0041] 4
honeycomb filter [0042] 5, 7 casing [0043] 6 particulate filter
[0044] 51 inner tube [0045] 52 outer tube [0046] 53 air gap
PREFERRED MODE FOR CARRYING OUT THE INVENTION
[0047] Hereinafter, embodiments of the present invention are
explained with reference to the drawings. It should be noted that
"upstream" and "downstream" in the present embodiment are set to
indicate "upstream" and "downstream" relative to the flow of
exhaust gas emitted from the internal combustion engine.
Overall Configuration of Internal Combustion Engine
[0048] An outline configuration of an internal combustion engine
(hereinafter referred to as engine) 100 provided with an exhaust
gas purification device 1 related to an embodiment of the present
invention is shown in FIG. 1. As shown in FIG. 1, the engine 100
provided with the exhaust gas purification device 1 related to the
embodiment of the present invention includes an engine main body
101 in which combustion chambers are formed, and an exhaust system
102 connected to the engine main body 101, and forming an exhaust
gas path that guides exhaust gas, which is combustion gas exhausted
from the combustion chamber, to outside of the engine 100.
[0049] The exhaust system 102 is provided with an exhaust manifold
102a that is connected to a cylinder head 101a of the engine main
body, and an exhaust pipe 102b that is connected to the exhaust
manifold 102a and forms an exhaust path. Then, the exhaust
purification device 1 related to the embodiment of the present
invention is disposed through the exhaust pipe 102b directly after
the exhaust manifold 102a, which is connected to the engine main
body 101, adsorbs/purifies NOx and hydrocarbons (HC) emitted from
the engine 100, and purifies particulates (PM).
Overall Configuration of Exhaust Gas Purification Device
[0050] The exhaust gas purification device 1 related to the
embodiment of the present invention is shown in FIG. 2. As shown in
FIG. 2, the exhaust gas purification device 1 is provided with a
first purification unit 2 that is disposed at an upstream side and
includes a honeycomb filter 4, which is a honeycomb structure, and
a second purification unit 3 that is disposed at an downstream side
of the first purification unit 2 and has a particulate filter (a
high porosity material to which a second oxidation catalyst has
been applied) 6.
[0051] The second purification unit 3 is disposed on a downstream
side of the first purification unit 2 so as to be continuous with
the first purification unit 2. Then, the first purification unit 2
and the second purification unit 3 have inner regions that are
mutually communicative, and the inside thereof is made in a
configuration that allows exhaust gas to flow.
First Purification Unit
[0052] An outline configuration of a state in which a portion of
the casing 5, which is a retaining member of the first purification
unit 2 related to the embodiment of the present invention has been
cut away is shown in FIG. 3. In addition, FIG. 4 shows a
longitudinal section of the first purification unit 2 related to
the embodiment of the present invention. As shown in FIG. 3, the
first purification unit 2 includes a honeycomb filter 4 that has a
honeycomb structure in which multiple cells are formed of metal
(not illustrated), and a casing 5 that is formed in a cylindrical
shape and retains the honeycomb filter 4.
[0053] The honeycomb filter is formed by way of loading the first
oxidation catalyst by wash coating or the like onto a honeycomb
carrier of cylindrical shape. As the honeycomb carrier, for
example, one formed by superimposing each of a corrugated board and
a flat board composed of metallic foil, and winding these into a
roll shape may be used. In addition, the material of the honeycomb
carrier may be any metal so long as having electrical conductivity
and, for example, stainless steel, Ni--Cr alloy or the like can be
exemplified.
[0054] Pt, Pd, alumina, silica alumina, a NOx adsorbent, and a HC
adsorbent can be exemplified as the catalyst components configuring
the first oxidation catalyst loaded to the honeycomb filter. As the
NOx adsorbent, for example, a ceria-zirconia composite oxide that
has been stabilized by ceria and zirconia can be exemplified. In
addition, a beta zeolite and MFI zeolite can be exemplified as the
HC adsorbent, for example.
[0055] It should be noted that the catalyst layer with the first
oxidation catalyst of the present embodiment includes two layers of
catalyst layers: a catalyst layer composed of an MFI zeolite and
ceria-zirconia composite oxide, and a catalyst layer composed of a
beta zeolite and ceria-zirconia composite oxide.
[0056] As a method of coating the first oxidation layer, a general
coating method is used in which a slurry is prepared with the first
oxidation catalyst material, the honeycomb filter is dipped into
this slurry, and then dried/fired after removing excess slurry.
[0057] As shown in FIG. 4, the casing 5 retains the honeycomb
filter 4 so that the axial direction of the honeycomb filter 4,
which is formed in a cylindrical shape, is the direction in which
exhaust gas flows. In addition, the casing 5 includes an inner tube
51 that retains the honeycomb filter 4, and an outer tube 52 that
is disposed on an outer side of the inner tube 51 so as to cover
the inner tube 51. In other words, the casing 5 is formed so as to
make a two layer structure of the inner tube 51 and the outer tube
52.
[0058] The inner tube 51 is formed in a cylindrical form that can
retain the honeycomb filter 4, which is formed in a cylindrical
shape. A projecting portion 54, which is formed by an outer
circumferential surface of the outer tube 52 protruding in an
outward direction of the casing 5, is provided in the outer tube
52. The projecting portion 54 is formed so that a diameter of the
outer tube 52 is larger than a diameter of the outer tube 52 at
both end portions in the axial direction.
[0059] When the inner tube 51 and the outer tube 52 formed as such
are overlapped so that the outer tube 52 is an outer layer, an air
gap 53, which is a air space layer, is formed between the
projecting portion 54 and the inner tube 51. Stainless steel,
Ni--Cr alloy and the like can be exemplified as the material
configuring the casing 5, for example.
Second Purification Unit
[0060] An outline configuration of the particulate filter 6 of the
second purification unit 3 related to the embodiment of the present
invention is shown in FIG. 5. In addition, FIG. 6 shows an outline
configuration of the catalyst loaded to be supported on the
particulate filter 6 related to the embodiment of the present
invention. The second purification unit 3 has the particulate
filter 6 that purifies particulates, and a casing 7 that is formed
in a cylindrical shape and retains the particulate filter 6.
[0061] As shown in FIG. 5, the particulate filter 6 uses a wall
flow-type filter, and has porous walls and a catalyst coating
layer. The catalyst coating layer is configured with the second
oxidation catalyst, and is formed on a surface of the porous
walls.
[0062] The structure of the porous wall may be of wall flow-type as
in the present embodiment and, for example, may be a structure in
which both end faces are alternately sealed by a three-dimensional
mesh or honeycomb structure. Furthermore, it may be a structure in
which a fibrous material has been laminated and molded into a felt
shape.
[0063] Cordierite, silicon carbide, mullite, alumina and the like
can be exemplified as the material of the particulate filter 6.
[0064] Pt, Pd, alumina, silica alumina, and a HC adsorbent can be
exemplified as the catalyst components of the second oxidation
catalyst supported on the particulate filter 6. A beta zeolite and
an MFI zeolite can be exemplified, for example, as the HC
adsorbent.
[0065] As shown in FIG. 6, the particulate filter 6 has the second
oxidation catalyst coated so that the loaded amounts of catalyst at
the upstream side thereof and downstream side thereof are
different. For example, zone coating to make a loaded amount of Pt
of the second oxidation catalyst be relatively small at a
downstream side is conducted by further coating the second
oxidation catalyst, which has a relatively large loaded amount of
Pt, at an upstream side. A region in the range of 20 to 80% of the
length in the axial direction of the particulate filter 6 is
preferable as a region on the upstream side at which the loaded
amount of Pt applied is relatively increased.
[0066] As a method of coating the second oxidation catalyst, a
method similar to that of the first oxidation catalyst can be used.
In addition, in the so-called zone coat, in which second oxidation
catalysts having different loaded amounts of catalyst are coated to
the upstream side and the downstream side, first the second
oxidation catalyst that has a relatively small loaded amount of Pt
is coated over the entire body, and dried, after which the second
oxidation catalyst, in which the catalyst is controlled to a
predetermined concentration to have a relatively large loaded
amount of Pt, is further coated at predetermined areas.
Operation of Exhaust Gas Purification Device of Present
Invention
[0067] Next, operation of the exhaust gas purification device of
the present invention will be explained using FIGS. 7 and 8. FIG. 7
is a schematic view showing adsorption/purification of NOx and
hydrocarbons (HC). FIG. 8 is a schematic view showing a reaction
state of NOx and hydrocarbons.
[0068] Upon the engine 100 starting, exhaust gas from the engine
100 is emitted. Hydrocarbons, PM, NOx, etc. are contained in the
exhaust gas, and these flow to the exhaust gas purification device
1 via the exhaust pipe 102b. As shown in FIG. 7, in the honeycomb
filter 4 of the first purification unit 2 disposed at an upstream
side of the exhaust gas purification device 1, a hydrocarbon
adsorbent (e.g., MFI zeolite or beta zeolite) and a NOx adsorbent
(e.g., CeZrOx) are respectively loaded in the first catalyst layer
41 and the second catalyst layer 42, and a portion of the
hydrocarbons and NOx contained in the exhaust gas are temporarily
adsorbed to the hydrocarbon adsorbent and NOx adsorbent here.
[0069] When the exhaust gas emitted from the engine 100 raises in
temperature with the elapse of time, the hydrocarbon adsorbent
becomes such that hydrocarbons are desorbed more than hydrocarbons
are adsorbed, and thus the amount of hydrocarbons desorbed from the
hydrocarbon adsorbent becomes gradually larger that the amount of
hydrocarbons adsorbed to the hydrocarbon adsorbent. Similarly, the
NOx adsorbent becomes such that NOx is desorbed more than NOx is
adsorbed, and thus the amount of NOx desorbed from the NOx
adsorbent gradually becomes larger than the amount of NOx adsorbed
to the NOx adsorbent.
[0070] As shown in FIG. 8, the hydrocarbons thus desorbed from the
hydrocarbon adsorbent contact with the first oxidation catalyst
(e.g., Pt and Al.sub.2O.sub.3), contact with NOx thus desorbed from
the NOx adsorbent, and reduce the NOx. Then, the hydrocarbons are
combusted by contacting with the first oxidation catalyst. At this
time, since the hydrocarbon adsorbent and NOx adsorbent are loaded
on the same catalyst layer and exist in close proximity, the
reaction between NOx and hydrocarbons is promoted, thereby
improving NOx purification performance.
[0071] The combustion heat generated from combustion at the first
oxidation catalyst spreads and conducts to the surroundings due to
the honeycomb filter 4 of metal, which excels in heat conductance.
Furthermore, the combustion heat thus generated conducts to the
second purification unit 3 provided at a downstream side of the
first purification unit 2 as well as from the upstream side of the
second purification unit 3 also, and the second oxidation catalyst
raises in temperature. It should be noted that the honeycomb filter
4 is not easily affect by outside air due to being retained by the
casing 5, which has a two layer structure, and thus the combustion
heat generated is efficiently conducted.
[0072] In addition, hydrocarbons and NOx that has not been adsorbed
to the honeycomb filter 4 of the first purification unit 2 flows to
the second purification unit 3. Here, the hydrocarbons contact with
the second oxidation catalyst (e.g., Pt, Pd and Al.sub.2O.sub.3)
loaded in the catalyst layer of the particulate filter 6 of the
second purification unit 3, contact with NOx, and reduce the NOx.
Then, the hydrocarbons combust by contacting with the second
oxidation catalyst.
[0073] Here, in the particulate filter 6, it becomes possible to
effectively activate the catalyst, and possible to suppress a rise
in pressure loss by applying zone coating in which the oxidation
catalyst such as Pt is loaded to be abundant at an upstream side
which is easily affected by the heat of exhaust gas due to
contacting the exhaust gas first. From this it becomes possible for
the exhaust gas to be more efficiently purified.
EXAMPLES
[0074] Next, the present invention is further explained based on
Examples; however, the present invention is not to be limited
thereto.
Honeycomb Filter
[0075] A honeycomb carrier of stainless steel was prepared as the
honeycomb filter. Pt, alumina, silica-alumina, a ceria-zirconia
composite oxide (CeZrOx), a beta zeolite and a ZSM-5 zeolite were
prepared as the first oxidation catalyst to be loaded to the
honeycomb carrier. The honeycomb filter was configured with
catalyst layers in two layers that contained beta zeolite and Pt,
alumina, silica-alumina, ceria-zirconia composite oxide (CeZrOx);
and ZSM-5 zeolite and Pt, alumina, silica-alumina, and
ceria-zirconia composite oxide (CeZrOx).
Particulate Filter
[0076] A wall-flow type DPF of silicon carbide with a length of
152.4 mm (L) and diameter of 143.8 mm (.phi.) was prepared as the
particulate filter. Pt, Pd, alumina, silica-alumina, beta zeolite
and ZSM-5 zeolite were prepared as the second oxidation catalyst
supported to the particulate filter.
Example 1
[0077] The particulate filter was immersed in a slurry composed of
the second oxidation catalyst, dried after excess slurry was
removed therefrom, and fired, after which the second oxidation
catalyst was coated from the upstream side of the particulate
filter up to a position of 80% thereof, dried, and then fired. As a
result, a particulate filter having a zone coating proportion of
80% was prepared. In the particulate filter, a Pt amount in the
zone coated portion (portion in which loaded amount of the second
oxidation catalyst is relative high) was 2.25 g/L, a Pt amount in
the non-zone coated portion (portion in which loaded amount of the
second oxidation catalyst is relative low) was 1.00 g/L, and an
overall Pt amount was 2.00 g/L. An exhaust gas purification device
was prepared in which the above-mentioned honeycomb filter and this
particulate filter were respectively disposed at an upstream side
and a downstream side thereof.
[0078] In the exhaust gas purification device, at constant
conditions of an engine revolution speed of 2000 rpm, the exhaust
gas temperature was changed by changing the load on the diesel
engine, and the HC light-off T50 temperature was measured. It
should be noted that the HC light-off T50 temperature is the
exhaust gas temperature when HC purification efficiency shows
50%.
Example 2
[0079] A particulate filter was prepared having a zone coating
proportion of 66.7% by the same method as Example 1. In the
particulate filter, a Pt amount in the zone coated portion was 2.50
g/L, a Pt amount in the non-zone coated portion was 1.00 g/L, and
an overall Pt amount was 2.00 g/L. An exhaust gas purification
device was prepared in which the above-mentioned honeycomb filter
and this particulate filter were respectively disposed at an
upstream side and a downstream side thereof. This exhaust gas
purification device was operated similarly to Example 1.
Example 3
[0080] A particulate filter was prepared having a zone coating
proportion of 50% by the same method as Example 1. In the
particulate filter, a Pt amount in the zone coated portion was 3.00
g/L, a Pt amount in the non-zone coated portion was 1.00 g/L, and
an overall Pt amount was 2.00 g/L. An exhaust gas purification
device was prepared in which the above-mentioned honeycomb filter
and this particulate filter were respectively disposed at an
upstream side and a downstream side thereof. This exhaust gas
purification device was operated similarly to Example 1.
Example 4
[0081] A particulate filter was prepared having a zone coating
proportion of 33.3% by the same method as Example 1. In the
particulate filter, a Pt amount in the zone coated portion was 4.10
g/L, a Pt amount in the non-zone coated portion was 1.00 g/L, and
an overall Pt amount was 2.03 g/L. An exhaust gas purification
device was prepared in which the above-mentioned honeycomb filter
and this particulate filter were respectively disposed at an
upstream side and a downstream side thereof. This exhaust gas
purification device was operated similarly to Example 1.
Example 5
[0082] A particulate filter was prepared having a zone coating
proportion of 20.0% by the same method as Example 1. In the
particulate filter, a Pt amount in the zone coated portion was 6.00
g/L, a Pt amount in the non-zone coated portion was 1.00 g/L, and
an overall Pt amount was 2.00 g/L. An exhaust gas purification
device was prepared in which the above-mentioned honeycomb filter
and this particulate filter were respectively disposed at an
upstream side and a downstream side thereof. This exhaust gas
purification device was operated similarly to Example 1.
Example 6
[0083] A particulate filter was prepared having a zone coating
proportion of 10.0% by the same method as Example 1. In the
particulate filter, a Pt amount in the zone coated portion was
11.00 g/L, a Pt amount in the non-zone coated portion was 1.00 g/L,
and an overall Pt amount was 2.00 g/L. An exhaust gas purification
device was prepared in which the above-mentioned honeycomb filter
and this particulate filter were respectively disposed at an
upstream side and a downstream side thereof. This exhaust gas
purification device was operated similarly to Example 1.
Comparative Example 1
[0084] An exhaust gas purification device arranged with a
particulate filter having a zone coating proportion of 100% was
prepared. In the particulate filter, the overall Pt amount was set
to 2.00 g/L. The exhaust gas purification device was prepared in
which the above-mentioned honeycomb filter and this particulate
filter were respectively disposed at an upstream side and a
downstream side thereof. This exhaust gas purification device was
operated similarly to Example 1.
[0085] A relationship of Pt amounts in the zone coated portion and
non-zone coated portion for the Examples and Comparative Example is
shown in Table 1. In addition, a chart plotting the HC light-off
T50 temperature (.degree. C.) relative to a proportion of zone
coating (%) obtained from the Examples and Comparative Example is
shown in FIG. 9.
TABLE-US-00001 TABLE 1 Non-zone Zone coating Zone coated coated HC
light-off proportion portion, Pt portion, Pt Total Pt temperature
(%) amount (g/L) amount (g/L) amount (g/L) (.degree. C.) Example 1
80.0 2.25 1.00 2.00 207.2 Example 2 66.7 2.50 1.00 2.00 205.7
Example 3 50.0 3.00 1.00 2.00 203.2 Example 4 33.3 4.10 1.00 2.03
200.3 Example 5 20.0 6.00 1.00 2.00 198.4 Example 6 10.0 11.00 1.00
2.00 201.5 Comparative 100.0 2.00 -- 2.00 207.9 Example 1
[0086] As shown in FIG. 9, by setting a proportion of a zone coated
portion of catalyst loaded on the particulate filter to be no more
than 80% relative to a length of the particulate filter, it becomes
possible to dispose a catalyst abundantly containing active metal
at an upstream portion that exhaust gas easily contacts, increase
the purification efficiency of exhaust gas with the catalyst, and
improve low temperature HC purification performance.
[0087] In addition, when the zone coating proportion is greater
than 80%, the performance difference over that uniformly coated
becomes small, whereby it is understood that the effectiveness of
zone coating, when taking into consideration catalyst deterioration
due to a temperature rise at a tail end portion during DPF
regeneration, is small.
[0088] On the other hand, when the zone coating proportion is lower
than 20%, although an improvement in low temperature HC
purification performance can be observed compared to a case of
being uniformly coated, there is concern over performance
deteriorating since particle growth is promoted by sintering due to
the concentration of active metal being high in addition to the
metal intergranular spacing being small, and thus contact
probability with the exhaust gas becomes low. Furthermore,
manufacturing control of the catalyst during production of zone
coating becomes tight, and there is concern over performance
variation deterioration.
[0089] Next, the exhaust gas purification devices used by Example 5
and Comparative Example 1 was prepared. Air at a flow rate of 300
m.sup.3/h was flowed through each of the exhaust gas purification
devices. Pressure gauges were set at the upstream side and the
downstream side of the exhaust gas purification device, and the
pressure differential (pressure loss) between the upstream side and
down stream side of the exhaust gas purification device was
measured. In the exhaust gas purification device of Example 5,
which had a zone coating proportion of 20%, the pressure
differential was 2.06 kPa. In the exhaust gas purification device
of Comparative Example 1, which had a zone coating proportion of
100%, the pressure differential was 2.56 kPa. From this it can be
understood that the exhaust gas purification device according to
Example 5, which had a zone coating proportion of 20%, has a
smaller pressure loss than the exhaust gas purification device
according to Comparative Example 1, which had a zone coating
proportion of 100%.
* * * * *