U.S. patent application number 11/053930 was filed with the patent office on 2005-09-01 for apparatus and method for purifying exhaust gas of internal combustion engine.
This patent application is currently assigned to Hitachi, Ltd.. Invention is credited to Hiratsuka, Toshiufumi, Iizuka, Hidehiro, Inoue, Takeshi, Kitahara, Yuichi, Kuroda, Osamu, Shinotsuka, Norihiro, Watanabe, Hiroko.
Application Number | 20050188684 11/053930 |
Document ID | / |
Family ID | 34697901 |
Filed Date | 2005-09-01 |
United States Patent
Application |
20050188684 |
Kind Code |
A1 |
Shinotsuka, Norihiro ; et
al. |
September 1, 2005 |
Apparatus and method for purifying exhaust gas of internal
combustion engine
Abstract
In an exhaust gas purifying apparatus provided with a lean NOx
catalyst supporting a catalyst layer, which contains a NOx trapping
component, on a honeycomb substrate formed so as not to cause an
alkali attack, the invention prevents trapped NOx from being
dissociated and exhausted during the time of a rich spike. A NOx
reducing catalyst with the function of reducing NOx by a reductant
in a rich or stoichiometric condition, e.g., a three-way catalyst,
is disposed downstream of the lean NOx catalyst. In the case of
increasing the amount of the NOx trapping component in the lean NOx
catalyst to enhance a NOx trapping capability, even if a part of
trapped NOx is dissociated during the time of the rich spike, the
dissociated NOx can be reduced by the NOx reducing catalyst
disposed on the downstream side.
Inventors: |
Shinotsuka, Norihiro;
(Hitachinaka, JP) ; Kuroda, Osamu; (Hitachi,
JP) ; Kitahara, Yuichi; (Hitachinaka, JP) ;
Inoue, Takeshi; (Hitachinaka, JP) ; Hiratsuka,
Toshiufumi; (Hitachinaka, JP) ; Watanabe, Hiroko;
(Oharai, JP) ; Iizuka, Hidehiro; (Mito,
JP) |
Correspondence
Address: |
CROWELL & MORING LLP
INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
Hitachi, Ltd.
Chiyoda-ku
JP
|
Family ID: |
34697901 |
Appl. No.: |
11/053930 |
Filed: |
February 10, 2005 |
Current U.S.
Class: |
60/286 ;
60/285 |
Current CPC
Class: |
F01N 2330/02 20130101;
Y02A 50/2344 20180101; Y02T 10/24 20130101; F01N 13/0093 20140601;
F01N 2330/12 20130101; F01N 3/101 20130101; F01N 3/0814 20130101;
F01N 3/0842 20130101; B01D 2255/91 20130101; Y02T 10/22 20130101;
F01N 2250/02 20130101; Y02A 50/20 20180101; B01D 53/9477 20130101;
F01N 13/009 20140601; F01N 2570/14 20130101; F01N 2330/06 20130101;
B01D 53/9422 20130101; B01D 53/944 20130101; F01N 2330/10 20130101;
Y02T 10/12 20130101; B01D 53/9431 20130101 |
Class at
Publication: |
060/286 ;
060/285 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 12, 2004 |
JP |
2004-34752 |
Claims
What is claimed is:
1. An exhaust gas purifying apparatus for an internal combustion
engine, comprising: a lean NOx catalyst disposed in an exhaust
passage of said internal combustion engine, having the function of
trapping NOx in a lean atmosphere under coexistence of oxygen, and
including a catalyst layer that has the function of reducing the
trapped NOx by a reductant in a rich or stoichiometric atmosphere
and is supported on a honeycomb substrate having a low affinity
with components of said catalyst layer; and a NOx reducing catalyst
disposed downstream of said lean NOx catalyst and having the
function of reducing NOx by a reductant in the rich or
stoichiometric atmosphere, wherein said NOx reducing catalyst
reduces NOx that is dissociated from said lean NOx catalyst when a
condition of exhaust gas in said exhaust passage is changed over
from a lean condition to a rich or stoichiometric condition.
2. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 1, wherein said lean NOx catalyst
contains a NOx trapping component and a NOx reducing component in
said catalyst layer.
3. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 1, wherein said lean NOx catalyst is a
three-way catalyst.
4. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 1, wherein said lean NOx catalyst
contains, as said NOx trapping component, at least one element
selected from among alkali metals and alkaline-earth metals, and
said honeycomb substrate contains no Si as a component thereof.
5. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 4, wherein the NOx trapping component in
said lean NOx catalyst contains at least one element selected from
among Li, Na, K, Cs, Sr and Ba, and an amount of the NOx trapping
component supported on said honeycomb substrate is not smaller than
20 g per bull volume of said honeycomb substrate.
6. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 4, wherein said lean NOx catalyst
contains a noble metal as the NOx reducing component.
7. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 1, wherein said lean NOx catalyst and
said NOx reducing catalyst are set to have a relative relationship
therebetween such that a linear speed of the exhaust gas flowing
through said NOx reducing catalyst is comparable to or higher than
a linear speed of the exhaust gas flowing through said lean NOx
catalyst.
8. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 7, wherein said NOx reducing catalyst is
constituted by a honeycomb substrate having a catalyst layer
supported thereon, and said honeycomb substrate of said NOx
reducing catalyst has a smaller porosity than said honeycomb
substrate of said lean NOx catalyst.
9. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 1, further comprising a three-way
catalyst disposed in said exhaust passage upstream of said lean NOx
catalyst.
10. An exhaust gas purifying apparatus for an internal combustion
engine, comprising an exhaust gas purifying catalyst and a filter
trapping and removing particulates which are disposed in an exhaust
passage of said internal combustion engine, wherein said exhaust
gas purifying catalyst comprises a lean NOx catalyst having the
function of trapping NOx in a lean atmosphere under coexistence of
oxygen and including a catalyst layer that has the function of
reducing the trapped NOx by a reductant in a rich or stoichiometric
atmosphere and is supported on a honeycomb substrate having a low
affinity with components of said catalyst layer, and a NOx reducing
catalyst having the function of reducing NOx by a reductant in the
rich or stoichiometric atmosphere; said lean NOx catalyst is
disposed upstream or downstream of said filter, and said NOx
reducing catalyst is disposed downstream of said lean NOx catalyst
and said filter; and said NOx reducing catalyst reduces NOx that is
dissociated from said lean NOx catalyst when a condition of exhaust
gas in said exhaust passage is changed over from the lean
atmosphere to the rich or stoichiometric atmosphere.
11. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 10, wherein said lean NOx catalyst is
disposed downstream of said filter, and said NOx reducing catalyst
is disposed downstream of said lean NOx catalyst.
12. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 10, wherein said lean NOx catalyst is
disposed upstream of said filter, and said NOx reducing catalyst is
disposed downstream of said filter.
13. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 10, wherein said lean NOx catalyst
contains, as said NOx trapping component, at least one element
selected from among alkali metals and alkaline-earth metals, and
said honeycomb substrate contains no Si as a component thereof.
14. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 13, wherein said lean NOx catalyst
contains a noble metal as the NOx reducing component.
15. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 10, wherein said NOx reducing catalyst is
a three-way.
16. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 10, wherein said lean NOx catalyst and
said NOx reducing catalyst are set to have a relative relationship
therebetween such that a linear speed of the exhaust gas flowing
through said NOx reducing catalyst is comparable to or higher than
a linear speed of the exhaust gas flowing through said lean NOx
catalyst.
17. An exhaust gas purifying apparatus for an internal combustion
engine according to claim 16, wherein said NOx reducing catalyst is
constituted by a honeycomb substrate having a catalyst layer
supported thereon, and said honeycomb substrate of said NOx
reducing catalyst has a smaller porosity than said honeycomb
substrate of said lean NOx catalyst.
18. An exhaust gas purifying method for use in an internal
combustion engine to purify exhaust gas exhausted from said
internal combustion engine by an exhaust gas purifying catalyst
disposed in an exhaust passage, the method comprising the steps of:
trapping NOx contained in the exhaust gas from said internal
combustion engine, which is operated in a lean atmosphere under
coexistence of oxygen, and removing the NOx from the exhaust gas by
a lean NOx catalyst having the function of trapping the NOx in a
lean atmosphere and including a catalyst layer that has the
function of reducing the trapped NOx by a reductant in a rich or
stoichiometric atmosphere and is supported on a honeycomb substrate
having a low affinity with components of said catalyst layer; and
temporarily changing over an operation mode of said internal
combustion engine to operation in the rich or stoichiometric
atmosphere, and reducing NOx dissociated from said lean NOx
catalyst with the changeover of the operation mode by a NOx
reducing catalyst having the function of reducing NOx by a
reductant in the rich or stoichiometric atmosphere.
19. An exhaust gas purifying method for an internal combustion
engine according to claim 18, wherein a linear speed of the exhaust
gas flowing through said NOx reducing catalyst is comparable to or
higher than a linear speed of the exhaust gas flowing through said
lean NOx catalyst.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an apparatus and a method
for purifying exhaust gas exhausted from an internal combustion
engine that is operated in a lean atmosphere.
[0003] 2. Description of the Related Art
[0004] A lean NOx catalyst is known as a catalyst for effectively
purifying nitrogen oxides (hereinafter referred to as "NOx")
contained in exhaust gas in which oxygen is also contained. One
known example of the lean NOx catalyst is a NOx trapping type
catalyst that traps and reduces NOx contained in exhaust gas with
absorption, adsorption and other reactions.
[0005] The lean NOx catalyst of the NOx trapping type contains, in
a catalyst layer, a NOx trapping component and a NOx reducing
component. Examples generally used as the NOx trapping component
are alkali metals, such as lithium (Li), sodium (Na), potassium
(K), and cesium (Cs), which are highly basic and belong to group IA
of the Periodic Table. Also, examples generally used as the NOx
reducing component are noble metals, such as platinum (Pt), rhodium
(Rh), and palladium (Pd).
[0006] However, the lean NOx catalyst containing the alkali metal
element has a problem of causing an alkali attack phenomenon. The
term "alkali attack phenomenon" means a phenomenon that the alkali
metal in the catalyst layer drifts into a substrate. The lean NOx
catalyst is usually of a structure in which the catalyst layer is
supported on a honeycomb substrate made of cordierite. The
cordierite contains, as one component, silicon (Si) that tends to
very easily bind with the alkali metal. Accordingly, when a heat
load is applied to the lean NOx catalyst, the alkali metal drifts
into the cordierite and the alkali attack phenomenon occurs.
[0007] The occurrence of the alkali attack phenomenon reduces the
amount of the alkali metal present in the catalyst layer and hence
deteriorates the NOx trapping capability of the catalyst. Further,
the strength of the honeycomb substrate is reduced due to a
resulting chemical change in composition of the cordierite.
[0008] In order to suppress the alkali attack phenomenon, it has
been proposed to form the honeycomb substrate by using a material
that contains no Si (see, e.g., JP-A-10-165817 (abstract and
claims)).
SUMMARY OF THE INVENTION
[0009] The alkali attack phenomenon can be suppressed by using the
honeycomb substrate that has a low affinity with the alkali metal.
If the alkali attack phenomenon does not occur, a reduction in the
strength of the honeycomb substrate resulting from the alkali
attack phenomenon can be avoided. It is therefore possible to
increase the amount of the alkali metal supported on the lean NOx
catalyst, and to enhance the NOx trapping capability of the
catalyst.
[0010] Meanwhile, the inventors have found that the following
problem arises in the case of increasing the amount of the alkali
metal supported on the lean NOx catalyst and enhancing the NOx
trapping capability of the catalyst. More specifically, when the
trapped NOx is reduced with the operation atmosphere changed over
to a stoichiometric or rich state, this process relatively easily
causes a phenomenon that a part of the trapped NOx is dissociated
from the catalyst due to heat generated from the reduction
reaction, etc. while it remains not reduced. Such a NOx
dissociation phenomenon is apt to occur particularly in a state
that a large mount of NOx is trapped by the lean NOx catalyst, or
in a state that the temperature of exhaust gas is high and the NOx
trapping capability of the catalyst is reduced.
[0011] The term "dissociation" used herein means that the NOx
trapped by the catalyst with adsorption, absorption, occlusion and
other reactions is detached from the catalyst when the NOx is
reduced in the stoichiometric or rich state. With regards to a NOx
absorbing type catalyst, the word "release" is often used as
representing the meaning in contrast with "absorption", and the
"release" is also included in the category of the
"dissociation".
[0012] In an exhaust gas purifying apparatus provided with a lean
NOx catalyst having a honeycomb substrate which totally does not
cause or hardly causes the alkali attack phenomenon, it is an
object of the present invention to overcome the problem of
dissociation of NOx occurred when an operation atmosphere is
changed over to reduce trapped NOx.
[0013] To achieve the above object, an exhaust gas purifying
apparatus according to the present invention includes, in an
exhaust passage of an internal combustion engine, a lean NOx
catalyst having the function of trapping NOx in a lean atmosphere
under coexistence of oxygen and including a catalyst layer that has
the function of reducing the trapped NOx by a reductant in a rich
or stoichiometric atmosphere and is supported on a honeycomb
substrate having a low affinity with components of the catalyst
layer, and a NOx reducing catalyst disposed downstream of the lean
NOx catalyst and having the function of reducing NOx by a reductant
in the rich or stoichiometric atmosphere.
[0014] In the present invention, the term "catalyst layer" means a
state in which catalyst active components including a NOx trapping
component and a NOx reducing component are supported on a support
made of, e.g., alumina.
[0015] The exhaust gas purifying apparatus of the present invention
may further comprise a pre-catalyst, which is constituted as a
three-way catalyst, upstream of the lean NOx catalyst.
Alternatively, the exhaust gas purifying apparatus may further
comprise a filter for trapping and removing particulates upstream
or downstream of the lean NOx catalyst.
[0016] A exhaust gas purifying method of the present invention
comprises the steps of trapping NOx contained in the exhaust gas in
the lean atmosphere and removing the NOx from the exhaust gas by
the lean NOx catalyst having the above-described structure, and
then changing over an operation atmosphere to the rich or
stoichiometric atmosphere, thereby reducing NOx dissociated from
the lean NOx catalyst with the changeover of the operation
atmosphere into nitrogen (N.sub.2) by the NOx reducing
catalyst.
[0017] The present invention is applicable to any kind of lean NOx
catalyst that causes a phenomenon similar to the alkali attack
phenomenon. For example, when a catalyst contains, as the NOx
trapping component, an alkaline-earth metal element such as
strontium (Sr) and barium (Ba), the alkaline-earth metal element
causes an attack phenomenon against the honeycomb substrate in a
similar way although the attack phenomenon is not so strong as that
caused by alkali metal. The present invention can be applied to
such a catalyst as well.
[0018] According to the present invention, by employing, as the
honeycomb substrate, a material that has a low affinity with the
NOx trapping component, when a lean NOx catalyst supporting the NOx
trapping component thereon in an increased amount is used, it is
possible to overcome the problem that a part of trapped NOx is
dissociated when the trapped NOx is reduced in the rich or
stoichiometric atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a graph showing a change of a NOx purification
rate when the operation mode is changed over from the lean
operation to the stoichiometric operation;
[0020] FIG. 2 is a schematic view showing a basic construction of
an exhaust gas purifying apparatus according to the present
invention;
[0021] FIG. 3 is a graph showing results of Test Example 1;
[0022] FIG. 4 is a graph showing results of Test Example 2;
[0023] FIG. 5 is a graph showing results of Test Example 3;
[0024] FIG. 6 shows another embodiment of the present invention and
is a schematic view of an exhaust gas purifying apparatus including
a pre-catalyst;
[0025] FIG. 7 is a graph showing results of Test Example 4
conducted for the exhaust gas purifying apparatus including the
pre-catalyst;
[0026] FIG. 8 shows still another embodiment of the present
invention and is a schematic view of an exhaust gas purifying
apparatus including a DPF (Diesel Particulate Filter);
[0027] FIG. 9 is a sectional view of the DPF as viewed in the
direction of flow of exhaust gas; and
[0028] FIG. 10 is a schematic view of an exhaust gas purifying
apparatus according to still another embodiment of the present
invention, in which layout of the DPF is changed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] The present invention will be described in detail.
[0030] In the following description, an attack phenomenon caused by
drift of a NOx trapping component is called an alkali attack for
the sake of convenience. Also, a step of temporarily changing over
the operation mode to the stoichiometric or rich operation during
the lean operation is called a rich spike.
[0031] FIG. 1 is a graph showing a time-dependent change of a NOx
purification rate, particularly at the time when the operation mode
is changed over from the lean operation to the stoichiometric
operation, for a lean NOx catalyst in which the occurrence of the
alkali attack is suppressed by employing, as a honeycomb substrate,
a material that has a low affinity with components of a catalyst
layer.
[0032] The lean NOx catalyst traps NOx contained in exhaust gas
during the lean operation of an internal combustion engine. Because
the NOx trapping capability is finite, the NOx purification rate
lowers as the amount of NOx trapped by the lean NOx catalyst
increases.
[0033] When the amount of the trapped NOx reaches a certain value,
the operation mode of the internal combustion engine is temporarily
changed over to the stoichiometric or rich combustion mode so that
the trapped NOx is reduced for purification. In other words, the
rich spike is performed.
[0034] The rich spike causes, on the lean NOx catalyst, a reaction
between the trapped NOx and a reducing gas component in the exhaust
gas, whereby the NOx is reduced to N.sub.2.
[0035] Because the NOx reduction reaction is exothermic, a part of
the trapped NOx may dissociate from the lean NOx catalyst while it
remains not reduced, and a negative purification rate may occur
transiently.
[0036] The phenomenon of dissociation of the trapped NOx becomes
more noticeable when the exhaust gas is at a high temperature level
at which the NOx trapping capability lowers. Also, that phenomenon
becomes more noticeable when the amount of the NOx trapping
component in the catalyst layer is increased and the lean operation
time is prolonged to increase the amount of the trapped NOx.
[0037] Thus, even in the case using the honeycomb substrate that
does not cause the alkali attack, there is naturally a limit in the
amount of an alkali metal, i.e., the NOx trapping component,
supported on the catalyst from the viewpoint of avoiding such a
transient phenomenon, as described above.
[0038] In the present invention, the alkali attack is suppressed by
using the honeycomb substrate that has a low affinity with the
components of the catalyst layer, and a NOx reducing catalyst is
disposed downstream of the lean NOx catalyst to reduce the NOx
having dissociated from the lean NOx catalyst, thereby eventually
increasing the amount of the NOx trapping component.
[0039] FIG. 2 shows a basic construction of an exhaust gas
purifying apparatus according to the present invention.
[0040] A lean NOx catalyst 3 and a NOx reducing catalyst 4 are
disposed in an exhaust passage 2 of an engine 1, i.e., an internal
combustion engine, in this order from the upstream side.
[0041] The lean NOx catalyst 3 is constituted such that a catalyst
layer containing an alkali metal and a noble metal is supported on
the honeycomb substrate made of a ceramic that contains, e.g.,
alumina and calcium oxide as components. The catalyst layer can
further contain any of alkaline-earth metals such as magnesium
(Mg), calcium (Ca), strontium (Sr) and barium (Ba), and rare-earth
metals such as cerium (Ce).
[0042] The catalyst layer is supported on the honeycomb substrate
by coating or any other suitable method. The catalyst layer can be
prepared by causing the above-mentioned catalyst active components
to be supported on a support, which is made of at least one of
metal oxides selected from among alumina, titanium oxide and
zirconium oxide, by impregnation or any other suitable method.
[0043] In addition to the above-mentioned ceramic honeycomb, the
honeycomb substrate can also be constituted as a metal honeycomb
that is manufactured by forming metal foils made of primarily
stainless steel into a desired shape or by joining the metal foils
to each other and then forming them into the desired shape. When
the catalyst layer contains the alkali metal as the NOx trapping
component, the alkali attack can be suppressed by using, as the
honeycomb substrate, a material that contains no Si.
[0044] In the case using the honeycomb substrate which totally does
not cause or hardly causes the alkali attack, the amount of the NOx
trapping component in the lean NOx catalyst can be increased. On
the other hand, such a case raises a problem that the NOx trapping
component may drift out of the lean NOx catalyst during the time of
the rich spike. The drifted-out NOx trapping component may adhere
to the NOx reducing catalyst and may deteriorate the NOx purifying
performance of the NOx reducing catalyst.
[0045] To cope with the problem that the NOx trapping component
drifts out of the lean NOx catalyst, the present invention proposes
it to set the linear speed of the exhaust gas flowing through the
NOx reducing catalyst comparable to or preferably higher than the
linear speed of the exhaust gas flowing through the lean NOx
catalyst. By so setting the linear speed of the exhaust gas flowing
through the NOx reducing catalyst located on the downstream side,
the flow speed of the exhaust gas passing through the NOx reducing
catalyst is increased, and therefore the NOx trapping component
becomes hard to adhere to the NOx reducing catalyst
correspondingly.
[0046] The linear speed of the exhaust gas varies with a change in
the cross-sectional area of a flow path in the catalyst. For
example, the linear speed of the exhaust gas flowing through the
catalyst on the downstream side can be increased by changing a
catalyst outer diameter such that the cross-sectional area of the
catalyst on the downstream side is smaller than the cross-sectional
area of the catalyst on the upstream side.
[0047] As an alternative, when the catalyst on the upstream side
and the catalyst on the downstream side have the same outer
diameter, the linear speed of the exhaust gas can be varied by
changing a porosity of the honeycomb. When the number of cells per
unit area is the same, the porosity reduces as the thickness of a
cell wall is increased. The smaller the porosity, the higher is the
linear speed of the exhaust gas.
[0048] When the amount of the alkali metal or the alkaline-rare
metal supported on the lean NOx catalyst exceeds 20 g per bulk
volume of the honeycomb, the trapped NOx cannot be often completely
purified because of the large amount of the trapped NOx, even if
the air-fuel ratio during the time of the rich spike is set to a
lower value to increase the NOx reducing capability. The not
completely purified NOx is more apt to dissociate. The present
invention is, however, free from such a problem because the NOx
reducing catalyst is disposed downstream of the lean NOx catalyst
and the dissociated NOx is reduced by the NOx reducing catalyst.
Thus, the present invention can be said as providing a method that
is effective for the case of increasing the amount of the alkali
metal or the alkaline-rare metal supported on the lean NOx catalyst
over 20 g per bulk volume of the honeycomb, to thereby increase the
amount of NOx to be trapped.
[0049] Note that the NOx reducing catalyst 4 is preferably made of
a three-way catalyst, but it may be a lean NOx catalyst
instead.
[0050] FIG. 6 shows an exhaust gas purifying apparatus according to
another embodiment of the present invention in which a pre-catalyst
5 made of a three-way catalyst is disposed in the exhaust passage 2
upstream of the lean NOx catalyst 3. The provision of the
pre-catalyst 5 increases the capability of purifying hydrocarbons
(HC).
[0051] FIGS. 8 to 10 show exhaust gas purifying apparatuses
according to other embodiments of the present invention in which a
filter capable of trapping and removing particulates, i.e., a
diesel particulate filter 6 (hereinafter referred to as a "DPF"),
is additionally disposed. In the exhaust gas purifying apparatus of
FIG. 8, the DPF 6 is disposed upstream of the lean NOx catalyst 3.
In the exhaust gas purifying apparatus of FIG. 10, the DPF 6 is
disposed between the lean NOx catalyst 3 and the NOx reducing
catalyst 4. FIG. 9 shows a cross-section of the DPF 6 in FIG. 8.
These embodiments are effective in a diesel engine. Stated another
way, since the DPF 6 serves to remove soot contained in diesel
exhaust gas, it is possible to avoid a reduction in the NOx
purifying performance of the catalyst disposed on the downstream
side.
[0052] The DPF 6 can be constituted of, e.g., a monolithic
honeycomb filter that is formed by alternately sealing off inlets
and outlets of a porous cordierite honeycomb. Another practicable
example of the DPF 6 is a ceramic fiber laminated filter prepared
by wrapping ceramic fibers around a porous tube. Still other
practicable examples of the DPF 6 are a filter prepared by forming
a metal wire mesh into a hollow cylindrical shape, or a filter
prepared by laminating sintered metal sheets one above another.
[0053] The arrangement of FIG. 8 can provide an additional
advantage that since the particulates are less accumulated in the
lean NOx catalyst and the NOx reducing catalyst, both the lean NOx
catalyst and the NOx reducing catalyst can be prolonged in service
life. The arrangement of FIG. 10 can provide an additional
advantage that since the concentration of the exhaust gas is not
averaged by the DPF 6 during the time of the rich spike, the rich
level is less apt to become shallow and hence the air-fuel ratio is
less apt to become high, whereby the trapped NOx can be more easily
purged.
[EXAMPLE]
[0054] The present invention will be described in more detail below
in connection with an example. In the following example, the lean
NOx catalyst had a bulk volume of 0.71 L, and the honeycomb had an
outer diameter of 105.7 mm.phi. and a length of 81.2 mm. Also, the
lean NOx catalyst, the NOx reducing catalyst, and the pre-catalyst
were mounted in a test vehicle after being subjected to thermal
endurance treatment at 850.degree. C. for 50 hours in advance.
[0055] (Preparation of Lean NOx Catalyst A)
[0056] The honeycomb substrate used in the example was a ceramic
honeycomb made of alumina and calcium oxide and having cells in
nominal number of 400 cells/inch.sup.2 (about 62 cells/cm.sup.2).
An alumina slurry made of alumina and nitrate acidic alumina was
coated on the ceramic honeycomb such that the amount of alumina was
190 g per bulk volume. Then, an alumina coated honeycomb was
obtained through steps of drying and firing.
[0057] The alumina coated honeycomb was impregnated with a solution
of cerium (Ce) nitrate and was subjected to drying at about
100.degree. C. and then to firing for 1 hour at about 600.degree.
C.
[0058] Subsequently, the alumina coated honeycomb was impregnated
with a mixed dipping liquid, i.e., a mixed solution of sodium (Na)
nitrate, magnesium (Mg) nitrate, titania sol, and
dinitrodiammine-platinum (Pt) nitrate, followed by drying at about
100.degree. C. and then firing for 1 hour at about 600.degree.
C.
[0059] With the process described above, a lean NOx catalyst A
according to the example of the present invention was obtained in
which catalyst active components were supported in amounts of Ce:
27 g, Na: 50 g, Mg: 1.8 g, Ti: 4 g, and Pt: 3 g per bulk volume of
the honeycomb.
[0060] (Preparation of Lean NOx Catalyst B)
[0061] A lean NOx catalyst B according to the example of the
present invention was prepared exactly in the same manner as the
lean NOx catalyst A except for using, as the honeycomb substrate, a
metal honeycomb made of primarily stainless steel and having cells
in nominal number of 400 cells/inch.sup.2 (about 62
cells/cm.sup.2). The amounts of the catalyst active components per
bulk volume of the honeycomb were the same as those in the lean NOx
catalyst A.
[0062] (Preparation of Lean NOx Catalyst C)
[0063] A lean NOx catalyst C according to a comparative example was
prepared exactly in the same manner as the lean NOx catalyst A
except for using, as the honeycomb substrate, a honeycomb made of
cordierite and having cells in nominal number of 400
cells/inch.sup.2 (about 62 cells/cm.sup.2). The amounts of the
catalyst active components per bulk volume of the honeycomb were
the same as those in the lean NOx catalyst A.
[0064] (Preparation of NOx Reducing Catalyst X)
[0065] An alumina coated honeycomb having an alumina amount of 100
g per bulk volume of the honeycomb was prepared by coating an
alumina slurry on a honeycomb made of cordierite and having cells
in nominal number of 400 cells/inch.sup.2 (about 62
cells/cm.sup.2), and by subjecting the honeycomb to steps of drying
and firing. Additionally, the alumina slurry used herein was the
same as that used in preparing the lean NOx catalyst A.
[0066] The alumina coated honeycomb was impregnated with a solution
of cerium (Ce) nitrate, and was subjected to drying at about
100.degree. C. and then to firing for 1 hour at about 600.degree.
C. Thereafter, the alumina honeycomb was impregnated with a mixed
solution of dinitrodiammine-platinum (Pt) nitrate and rhodium (Rh)
nitrate, followed by steps of drying and firing.
[0067] In such a way, a NOx reducing catalyst X was obtained in
which catalyst active components were supported in amounts of Ce:
27 g, Pt: 2 g, and Rh: 0.2 g per bulk volume of the honeycomb.
[0068] (Preparation of NOx Reducing Catalyst Y)
[0069] A NOx reducing catalyst Y was prepared exactly in the same
manner as the NOx reducing catalyst X except for using, as the
honeycomb substrate, a honeycomb made of cordierite and having an
outer diameter of 86 mm.phi., a length of 122 mm, a bulk volume of
0.71 L, and cells in nominal number of 400 cells/inch.sup.2 (about
62 cells/cm.sup.2). This NOx reducing catalyst Y had the same
volume as the NOx reducing catalyst X, but the honeycomb
cross-sectional area of the NOx reducing catalyst Y was {fraction
(2/3)} time that of the NOx reducing catalyst X, namely the linear
speed of the exhaust gas in the catalyst Y is 1.5 times that in the
catalyst X.
[0070] (Preparation of NOx Reducing Catalyst Z)
[0071] A NOx reducing catalyst Z was prepared exactly in the same
manner as the NOx reducing catalyst X except for using, as the
honeycomb substrate, a honeycomb made of cordierite and having an
outer diameter of 86 mm.phi., a length of 122 mm, a bulk volume of
0.71 L, and cells in nominal number of 600 cells/inch.sup.2 (about
93 cells/cm.sup.2). This NOx reducing catalyst Z gave the same
linear speed of the exhaust gas as that in the NOx reducing
catalyst Y, but it had the number of cells 1.5 times that in the
latter.
[0072] (Preparation of Pre-Catalyst)
[0073] An alumina coated honeycomb having an alumina amount of 100
g per bulk volume of the honeycomb was prepared by coating the same
alumina slurry as that used in preparing the lean NOx catalyst A on
a honeycomb made of cordierite and having a bulk volume of 0.3 L,
and by subjecting the honeycomb to steps of drying and firing.
[0074] The alumina coated honeycomb was impregnated with a solution
of cerium (Ce) nitrate, and was subjected to drying at about
100.degree. C. and then to firing at about 600.degree. C.
Thereafter, the alumina honeycomb was impregnated with a mixed
solution of dinitrodiammine-platinum (Pt) nitrate, rhodium (Rh)
nitrate, and dinitrodiammine-palladium (Pd) nitrate, followed by
drying at about 100.degree. C. and then firing at about 600.degree.
C.
[0075] In such a way, a pre-catalyst (three-way catalyst) was
obtained in which catalyst active components were supported in
amounts of Ce: 27 g, Pt: 1.5 g, Rh: 0.15 g, and Pd: 5 g per bulk
volume of the honeycomb.
[TEST EXAMPLE 1]
[0076] Test Example 1 was conducted to examine respective NOx
purifying performances of the lean NOx catalysts A, B and C.
[0077] First, the lean NOx catalyst A was mounted in an exhaust
pipe of a lean burn vehicle (test vehicle) having a displacement of
2.0 L.
[0078] Then, the test vehicle was held fixed on a chassis
dynamometer and was operated to run at a constant speed on
condition that the air-fuel (A/F) ratio was set to 20 and the
vehicle speed was set to 40 km/h (with the temperature at the
catalyst inlet being about 300.degree. C).
[0079] During the run, the operation mode of the test vehicle was
changed over from the lean burn operation to the rich operation to
perform the rich spike. After 1 minute from the changeover to the
rich operation, the NOx concentration at an engine outlet (i.e.,
the NOx concentration at a catalyst inlet) and the NOx
concentration at an exhaust pipe outlet (i.e., the NOx
concentration at a catalyst outlet) were measured and a NOx
purification rate (%) was computed based on the following formula
(1): NOx purification rate (%)=(NOx concentration at engine
outlet-NOx concentration at exhaust pipe outlet)+NOx concentration
at engine outlet.times.100 (1)
[0080] A similar test was conducted for the case of running the
test vehicle at a constant vehicle speed of 70 km/h (with the
temperature at the catalyst inlet being about 400.degree. C).
[0081] Further, those tests were likewise repeated for each of the
lean NOx catalysts B and C in place of the lean NOx catalyst A.
[0082] Test results are shown in FIG. 3. When each of the lean NOx
catalysts A and B is solely used, high NOx purifying performance in
match with its high NOx trapping capability is not obtained. The
NOx purifying performance of the lean NOx catalyst C employing the
cordierite honeycomb is more inferior to those of the lean NOx
catalysts A and B. The reason why the NOx purifying performance of
the lean NOx catalyst C is particularly inferior is presumably in
that the amount of the alkali metal in the catalyst layer is
reduced due to the alkali attack. The honeycomb strength of the
lean NOx catalyst C is also reduced due to the alkali attack at an
excessive level.
[TEST EXAMPLE 2]
[0083] Tests similar to those in Test Example 1 were conducted by
disposing the lean NOx catalyst A in the exhaust pipe of the test
vehicle on the upstream side, and disposing the NOx reducing
catalyst X therein on the downstream side.
[0084] Further, similar tests were repeated for each of the lean
NOx catalysts B and C in place of the lean NOx catalyst A. The
tests were also conducted for the case in which the lean NOx
catalyst B was disposed in the exhaust pipe on the upstream side,
and the lean NOx catalyst C was disposed therein on the downstream
side.
[0085] FIG. 4 shows the NOx purifying rates computed based on the
above formula (1). Comparing FIGS. 3 and 4 with each other, the
advantage resulting from the provision of the NOx reducing catalyst
can be understood. More specifically, with the arrangement in which
the NOx reducing catalyst X or the lean NOx catalyst C is disposed
upstream of the lean NOx catalyst A or B, the NOx purifying
performance is increased. With the arrangement in which the lean
NOx catalyst C is disposed on the upstream side, however, the NOx
purifying performance is hardly increased even when the lean NOx
catalyst X is disposed on the downstream side. The reason why the
arrangements according to the embodiments of the present invention
exhibit relatively high NOx purifying performance is presumably in
that NOx having dissociated during the time of the rich spike are
purified by the NOx reducing catalyst X or the lean NOx catalyst C
having the NOx reducing function.
[TEST EXAMPLE 3]
[0086] Tests similar to those in Test Example 2 were conducted by
disposing the lean NOx catalyst A in the exhaust pipe of the test
vehicle on the upstream side, and disposing the NOx reducing
catalyst Y or Z therein on the downstream side in place of the NOx
reducing catalyst X.
[0087] Test results are shown in FIG. 5. FIG. 5 also shows the test
results obtained with the combination of the lean NOx catalyst A
and the NOx reducing catalyst X in Test Example 2.
[0088] There is no significant difference in the NOx purifying
performance among the NOx reducing catalysts X, Y and Z.
Accordingly, it is preferable to employ the NOx reducing catalyst Y
or Z because the linear speed of the exhaust gas is increased and
the alkali metal having drifted out of the lean NOx catalyst is
less apt to adhere to the NOx reducing catalyst as compared with
the case using the NOx reducing catalyst X.
[TEST EXAMPLE 4]
[0089] Tests similar to those in Test Example 1 were conducted by
disposing, in the exhaust pipe of the test vehicle, the
pre-catalyst 5, the lean NOx catalyst 3 and the NOx reducing
catalyst 4 in this order from the upstream side, as shown in FIG.
6.
[0090] FIG. 7 shows test results of Test Example 4-1 representing
an example of the present invention which employs the pre-catalyst,
the lean NOx catalyst A and the NOx reducing catalyst X, and Test
Example 4-2 representing a comparative example which employs the
pre-catalyst, the lean NOx catalyst C and the NOx reducing catalyst
X.
[0091] The exhaust gas temperature at the inlet of the lean NOx
catalyst was 350.degree. C. at the vehicle speed of 40 km/h, and
was 450.degree. C. at the vehicle speed of 70 km/h.
[0092] As seen from FIG. 7, Test Example 4-1 representing the
example of the present invention exhibits higher NOx purifying
performance than Test Example 4-2 representing the comparative
example.
[0093] Thus, the present invention is able to overcome the problem
that, when the amount of the NOx trapping component in the lean NOx
catalyst is increased to enhance the NOx trapping capability, a
part of trapped NOx is dissociated during the time of the rich
spike and the NOx purifying performance is deteriorates. As a
result, the lean operation time can be prolonged and the fuel
consumption can be improved.
* * * * *