U.S. patent application number 10/253706 was filed with the patent office on 2003-06-26 for exhaust gas purifying catalyst and exhaust gas purifying system.
This patent application is currently assigned to MAZDA MOTOR CORPORATION. Invention is credited to Miyoshi, Seiji, Okamoto, Kenji, Takami, Akihide, Yamada, Hiroshi.
Application Number | 20030115855 10/253706 |
Document ID | / |
Family ID | 19114852 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030115855 |
Kind Code |
A1 |
Miyoshi, Seiji ; et
al. |
June 26, 2003 |
Exhaust gas purifying catalyst and exhaust gas purifying system
Abstract
An exhaust gas purifying catalyst comprises a substrate, an
inner catalytic layer coated on the substrate, an intermediate
catalytic layer containing Pd laid over said inner catalytic layer,
and an outer catalytic layer containing a NOx adsorption material
laid over said intermediate catalytic layer. The Pd isolated from
the NOx adsorption material is prevented from lowering its low
temperature activity.
Inventors: |
Miyoshi, Seiji; (Aki-gun,
JP) ; Takami, Akihide; (Aki-gun, JP) ; Yamada,
Hiroshi; (Aki-gun, JP) ; Okamoto, Kenji;
(Aki-gun, JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
8180 GREENSBORO DRIVE
SUITE 800
MCLEAN
VA
22102
US
|
Assignee: |
MAZDA MOTOR CORPORATION
Aki-gun
JP
|
Family ID: |
19114852 |
Appl. No.: |
10/253706 |
Filed: |
September 25, 2002 |
Current U.S.
Class: |
60/284 ;
60/301 |
Current CPC
Class: |
B01D 2255/902 20130101;
B01D 53/86 20130101; Y02T 10/22 20130101; B01J 37/0246 20130101;
B01D 2255/104 20130101; B01D 2255/20 20130101; B01D 2255/102
20130101; B01D 2255/91 20130101; B01D 2257/404 20130101; Y02T 10/12
20130101; B01D 53/945 20130101; B01D 2257/502 20130101; B01D
2257/702 20130101; B01J 37/0244 20130101; B01J 23/58 20130101 |
Class at
Publication: |
60/284 ;
60/301 |
International
Class: |
F01N 003/00; F01N
003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2001 |
JP |
2001-292979 |
Claims
What is claimed is:
1. An exhaust gas purifying catalyst comprising at least a metal
selected from a group of alkaline metals and alkaline earth metals;
and Pd, wherein said Pd is partly isolated from said metal so as to
be neither electrically nor chemically affected by said metal.
2. An exhaust gas purifying catalyst as defined in claim 1,
comprising two different catalytic layers formed on a substrate so
as to be laid over each other, said two different catalytic layers
containing said Pd and said metal separately.
3. An exhaust gas purifying catalyst as defined in claim 2, and
further comprising a layer of zeolite laid under said two different
catalytic layers, said layer of zeolite absorbing HC in an exhaust
gas while said exhaust gas purifying catalyst remains low in
temperature and releasing HC into said exhaust gas when said
exhaust gas purifying catalyst falls in temperature.
4. An exhaust gas purifying catalyst as defined in claim 3, wherein
outer one of said two different catalytic layer contains said metal
and inner one of said two different catalytic layer contains said
Pd.
5. An exhaust gas purifying catalyst as defined in claim 1, wherein
exhaust gas purifying catalyst contains said metal more than 15 g
per one liter of said substrate.
6. An exhaust gas purifying system for purifying exhaust gases
discharged into an exhaust line from an engine, said exhaust gas
purifying system comprising: a catalyst disposed in said exhaust
line, said catalyst comprising at least a metal selected from a
group of alkaline metals and alkaline earth metals, a NOx
adsorption material operative to adsorb NOx in said exhaust gas
while said exhaust gas has a comparatively high oxygen
concentration and release said NOx adsorbed therein and adsorbs SOx
as said exhaust gas lowers its oxygen concentration, and Pd; and
temperature control means for raising a temperature of said
catalyst so as thereby to cause said NOx adsorption material to
release said SOx adsorbed therein; wherein said Pd is partly
isolated from said NOx adsorption material so as to be neither
electrically nor chemically affected by said NOx adsorption
material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an exhaust gas purifying
catalyst and an exhaust gas purifying system using the exhaust gas
purifying catalyst.
[0003] 2. Description of Related Art
[0004] There have been widely known three-way catalysts that
performs oxidization of HC and CO and reduction of NOx
simultaneously. Such a three-way catalyst often comprises an active
metal such a noble metal as Pd, Pt and Rh, a support material such
as alumina that functions to stabilize the active metal and, as a
result, to increase a surface area of the active metal coming into
contact with exhaust gases so as thereby to improve the catalytic
conversion efficiency thereof, and an oxygen storage material such
as ceria (a supplemental catalyst). The three-way catalyst shows
poor catalytic performance at low exhaust gas temperature and
causes significant aggravation of NOx conversion efficiency at lean
air-fuel ratios.
[0005] One of catalysts of the type containing an HC absorption
material and a NOx adsorption material is disclosed in Japanese
Unexamined Patent Publication No. 2001-113173. This catalyst stores
HC and NOx in an exhaust gas during cold engine operation
immediately after an engine start and releases and converts the
adsorbed HC and NOx after activation of a catalytic metal. The
catalyst comprises an inner catalytic layer of HC absorption
material that contains zeolite on a surface of a substrate and an
outer catalytic layer of catalytic metal that contains a noble
metal such as Pd and a NOx adsorption material such as Ba. The NOx
adsorption material is selected from a group of alkaline metals and
alkaline earth metals. The catalyst contains the NOx adsorption
material between 60/40 and 99/1 in weight ratio.
[0006] Another catalyst of the type containing an HC absorption
material and a NOx adsorption material is disclosed in Japanese
Unexamined Patent Publication No. 11-13462. This catalyst comprises
an inner catalytic layer of HC absorption material that contains
zeolite on a surface of a substrate and one or two outer catalytic
layers of catalytic metal each of which contains a noble metal such
as Pd and is impregnated with a solution of barium nitrate.
[0007] As disclosed, for example, in Japanese Unexamined Patent
Publication No. 9-79026, it has been known to dispose a NOx
adsorption material and an HC absorption material coated on a
single substrate together with a three-way catalyst in an exhaust
line.
[0008] The investigation of peculiarities of this type of catalysts
that was conducted by the inventors of the present application
showed that, although Pd in the catalyst inherently had catalytic
activity on HC at comparatively low temperatures, there were cases
where the low temperature catalytic performance of Pd was lowered
depending on the catalytic composition when the catalyst was
exposed to a high temperature exhaust gas. It was found that these
cases appeared in the catalyst that contained an alkaline metal or
an alkaline earth metal as the NOx adsorption material. It was also
found that the more the low temperature activity of Pd was
deteriorated more as the amount of NOx adsorption material
increased.
[0009] FIG. 8 shows light off temperatures (T50) regarding HC, CO
and NOx conversion for various comparative catalysts containing
different amounts of NOx adsorption material after aging. The
comparative catalyst comprises two catalytic layers, namely an
inner catalytic layer coated on a substrate and an outer catalytic
layer coated over the inner catalytic layer. The inner catalytic
layer has a catalytic component consisting of Ag and Bi supported
on .beta.-type zeolite. The outer catalytic layer has a catalytic
component consisting of Pt, Rh and Pd supported on alumina and
ceria. The measurement of light off temperature was made on four
comparative catalysts having outer catalytic layers that contain no
NOx adsorption material, 16 g/L of NOx adsorption material (10 g/L
of Ba; 3 g/L of Sr and 3 g/L of Mg), 32 g/L of NOx adsorption
material (20 g/L of Ba; 6 g/L of Sr and 6 g/L of Mg) and 50 g/L of
NOx adsorption material (30 g/L of Ba; 10 g/L of Sr and 10 g/L of
Mg), respectively. Each comparative catalyst was aged at 80.degree.
C. for 24 hours in the atmosphere.
[0010] As demonstrated in FIG. 8, each of HC, CO and NOx shows a
rise in light off temperature that becomes greater as the amount of
NOx adsorption material increases. In particular, each of HC and CO
shows a prominent tendency to rise the light off temperature.
SUMMARY OF THE INVENTION
[0011] It is therefore an object of the invention to provide an
exhaust gas purifying catalyst containing both Pd and NOx
adsorption material that prevents Pd from causing aggravation in
low temperature HC purification performance when the exhaust gas
purifying catalyst is exposed to a high temperature exhaust
gas.
[0012] It is another object of the invention to provide an exhaust
gas purifying catalyst containing both Pd and NOx adsorption
material that prevents Pd from causing aggravation of its low
temperature activity more as the NOx adsorption material is
increased in amount.
[0013] The present invention was accomplished on the basis of the
revelation that the functional aggravation of an exhaust gas
purifying catalyst containing both Pd and a NOx adsorption material
as an alkaline metal or an alkaline earth metal is prevented by
isolating Pd from the NOx adsorption material in the exhaust gas
purifying catalyst.
[0014] According to an aspect of the present invention, the exhaust
gas purifying catalyst contains at least a metal, selected from a
group of alkaline metals and alkaline earth metals; and Pd, such
that the Pd is partly isolated from the metal so as to be not
affected by said metal electrically nor chemically. Specifically,
the exhaust gas purifying catalyst comprises an outer catalytic
layer containing a metal and an inner catalytic layer containing
Pd. These Pd and metal are prevented from electrical and chemical
interaction with each other, so that the isolated Pd is prevented
from causing aggravation in low temperature activity.
[0015] The exhaust gas purifying catalyst may further comprise an
innermost layer of zeolite that is operative to adsorb HC in an
exhaust gas while the exhaust gas purifying catalyst remains low in
temperature and to release the adsorbed HC into the exhaust gas
when the exhaust gas purifying catalyst falls in temperature. The
zeolite adsorbs HC in an exhaust gas while the exhaust gas
purifying catalyst is at a low temperature, so that HC is prevented
from emitting into the atmosphere is prevented. Since the metal, an
alkaline metal or an alkaline earth metal, does not affect the low
temperature activity of Pd, HC released from the zeolite catalytic
layer is reliably oxidized, and so purified, by Pd.
[0016] It has been known in the art that, although Pd is
advantageous to the low temperature HC conversion, it is easy to
cause thermal deterioration and to be poisoned with lead and
sulfur. Contradistinctively, the exhaust gas purifying catalyst of
the present invention in which the inner catalytic layer containing
Pd is covered by the outer catalytic layer containing the metal, so
that the outer catalytic layer functions as a functional barrier,
preventing the Pd from thermal deterioration and lead and sulfur
poisoning. Therefore, On this account, the low temperature activity
of the exhaust gas purifying catalyst is ensured.
[0017] The exhaust gas purifying catalyst may preferably contain
the metal more than 15 g per one liter of a substrate on which the
exhaust gas purifying catalyst is formed. Even though the exhaust
gas purifying catalyst contains a large amount of the metal, an
alkaline metal or an alkaline earth metal, the exhaust gas
purifying catalyst prevents the Pd from lowering its lower
temperature activity. In the case where an alkaline metal or an
alkaline earth metal is employed as a NOx adsorption material, it
is preferred for the exhaust gas purifying catalyst to contain the
metal 30 g, but less than 59 g, per one liter of the substrate.
[0018] According to another aspect of the present invention, an
exhaust gas purifying system comprises a catalyst that is disposed
in the exhaust line and comprises at least a metal selected from a
group of alkaline metals and alkaline earth metals, a NOx
adsorption material operative to adsorb NOx in the exhaust gas
while the exhaust gas has a comparatively high oxygen
concentration, to release the adsorbed NOx into the exhaust gag,
and to adsorb SOx in the exhaust gas as said exhaust gas lowers its
oxygen concentration, and Pd, and temperature control means for
raising a temperature of the catalyst so as thereby to cause the
NOx adsorption material to release the adsorbed SOx into the
exhaust gas. In the catalyst, the Pd is partly isolated from the
NOx adsorption material so as to be not affected by said NOx
adsorption material electrically nor chemically.
[0019] The catalyst containing a NOx adsorption material encounters
the problem of sulfur poisoning that refers to a loss of the NOx
adsorption function due to a salt formed in the form of an oxide of
sulfur resulting from adsorption of sulfur in an exhaust gas. In
the event of an occurrence of this problem, although the oxide of
sulfur can be released from the NOx adsorption material by raising
the catalytic temperature, for example to 400.degree. C., the NOx
adsorption material encourages the Pd in deteriorating its low
temperature activity at a so high catalytic temperature, so as to
cause the Pd to aggravate its low temperature activity
significantly.
[0020] Contradistinctively, since the catalyst used in the exhaust
gas purifying system of the present invention in which the Pd is at
least partly isolated from the NOx adsorption material so as to be
not affected by the NOx adsorption material electrically nor
chemically, the Pd is prevented from being encouraged in
deteriorating its low temperature activity by the NOx adsorption
material when the catalytic temperature is raised. The raise in
exhaust gas temperature may be performed by making an air-fuel
ratio rich, by retarding an ignition timing or a fuel injection
timing, or by supplying a secondary gas into an exhaust gas stream
so as to assist oxidative reaction of HC and CO with a result of
providing an increase in reaction heat.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The above and other objects and features of the present
invention will be understood from the following description of a
specific embodiment thereof when considering in conjunction with
the accompanying drawings, in which:
[0022] FIG. 1 is a schematic view of an engine equipped with an
exhaust gas purifying system of the present invention;
[0023] FIG. 2 is a cross-sectional view of an exhaust gas purifying
catalyst according to an embodiment of the present invention;
[0024] FIG. 3 is graphic representation showing HC absorption
ratios, HC oxidation ratios, total HC conversion ratio and NOx
conversion ratio of the exhaust gas purifying catalyst shown in
FIG. 2 before and after aging;
[0025] FIG. 4 is a cross-sectional view of a comparative exhaust
gas purifying catalyst;
[0026] FIG. 5 is graphic representation showing HC absorption
ratios, HC oxidation ratios, total HC conversion ratio and NOx
conversion ratio of the exhaust gas purifying catalysts shown in
FIGS. 2 and 4 after aging;
[0027] FIG. 6 is graphic representation showing the relationship
between inlet temperature and HC conversion ratio of the exhaust
gas purifying catalysts shown in FIGS. 2 and 4 after aging;
[0028] FIG. 7 is a flowchart illustrating a sequence routine of
fuel injection control; and
[0029] FIG. 8 is graphic representation showing the relationship
between light off temperature and amount of NOx adsorption material
of an exhaust gas purifying catalyst after aging for HC, CO and
NOx.
DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT
[0030] The term "light off temperature (T50)" as used herein shall
mean and refer to an inlet gas temperature at which the conversion
efficiency of a catalyst amounts to 50%. Further, the term "exhaust
gas of an air-fuel ratio (A/F) of X" as used herein shall mean and
refer to an exhaust gas produced resulting from combustion of an
air-fuel mixture of an air-fuel ratio (A/F) of X.
[0031] Referring to the drawings in detail and, in particular, to
FIG. 1 schematically showing an engine 1 equipped with an exhaust
gas purifying system of the present invention, the engine 1, that
is of a spark-ignition type, comprises a plurality of cylinders 2
(only one of which is shown) in each of which a combustion chamber
4 is formed, a fuel injector 3 operative to spray fuel directly
into the combustion chamber 4, a spark plug 5. Air is introduced
into the engine through an intake passage 6 and an exhaust gas is
discharged from the engine 1 through an exhaust passage 7 equipped
with an exhaust gas purifying catalyst 8 therein.
[0032] FIG. 2 shows a cross-section of the exhaust gas purifying
catalyst 8. As shown, the exhaust gas purifying catalyst 8
comprises a substrate 11 such as a cordierite honeycomb bed, an
inner catalytic layer 12 coated on the substrate 11, an
intermediate catalytic layer 13 laid over the inner catalytic layer
12 and an outer catalytic layer 14 laid over the intermediate
catalytic layer 13. The inner catalytic layer 12 contains Ag and Bi
in addition to zeolite, and a binder. The intermediate catalytic
layer 13 contains a catalytic component that comprises Pd supported
on alumina and ceria, and a binder. The outer catalytic layer 14
comprises a catalytic component that contains Pt, Rh, Ba, Sr and Mg
supported on alumina and ceria, and a binder. The zeolite of the
exhaust gas purifying catalyst 8, that functions as an HC
absorption material, is of a .beta.-type having a salic ratio of
300.
[0033] The catalytic component comprising Pt and Rh supported on
alumina such as .gamma.-alumina powder and ceria functions as a
three-way catalyst. Ba, Sr and Mg in the exhaust gas purifying
catalyst 8 adsorb NOx in an exhaust gas when the exhaust gas has an
oxygen concentration higher than, for example, 4% that is
represented by an air-fuel ratio greater than 16 and release the
adsorbed NOx into the exhaust gas when the exhaust gas lowers the
oxygen concentration below, for example, 2%. The alumina is used in
the powder form. As the ceria, that indicates oxides containing a
ceria component and functions as an oxygen storage material, one of
Ce.Pr composite oxides (Ce.sub.0 9Pr.sub.0 1O.sub.2) is employed in
this embodiment. In order to put a restraint on thermal
deterioration of the alumina, a small amount, for example 5%, of La
is added into the alumina powder. As the binder of each of the
inner catalytic layer 12 and the intermediate catalytic layer 13,
an alumina binder is employed a. On the other hand, as the binder
of the outer catalytic layer 14, a basic binder such as zirconia
binder. Seen in that sense, the exhaust gas purifying catalyst 8
can be called a lean NOx adsorption catalyst having an HC trapping
function.
[0034] A sample exhaust gas purifying catalyst 8 was produced in
the following process. First of all, catalytic powder A and B were
prepared. The catalytic powder A was made up by mixing active
alumina powder, ceria, palladium nitrate and water all together and
calcinating the mixture at 500.degree. C. after drying. The
catalytic powder B was made up by mixing active alumina powder,
ceria, barium acetate, strontium acetate, magnesium acetate,
dinitro-diamine platinum nitrate, rhodium nitrate and water all
together, and calcinating the mixture at 500.degree. C. after
drying.
[0035] The inner catalytic layer 12 was formed by coating a given
amount of a slurry of a mixture of zeolite, hydrated alumina
(alumina binder). The mixture slurry was prepared by mixing zeolite
and hydrated alumina with water and stirring the mixture. The
honeycomb substrate 11 was dipped into and drawn out from the
mixture slurry so as to form a slurry layer, and then the slurry
layer was exposed to air blows so as to remove an excessive part of
the mixture slurry. This process was repeated until a layer
consisting of the given amount of the mixture slurry is formed. The
eventual layer was finished by calcinations at 500.degree. C. after
drying. The intermediate catalytic layer 13 was formed over the
inner catalytic layer 12 in the same process as the inner catalytic
layer 12 using the catalytic powder A in place of zeolite. The
inner and intermediate catalytic layers 12 and 13 were impregnated
with a mixed solution of silver nitrate and bismuth acetate and
then calcinated at 500.degree. C. after drying. Thereafter, the
outer catalytic layer 14 was formed over the intermediate catalytic
layer 13 in the same process as the inner catalytic layer 12 using
the catalytic powder B and a zirconia binder in place of zeolite
and an alumina binder, respectively.
[0036] The exhaust gas purifying catalyst 8 was adjusted so as to
have the following quantitative formula:
1 Inner catalytic layer 12 .beta.-type zeolite: 100 g/L
Intermediate catalytic layer 13 Pd: 2.0 g/L; Alumina: 30 g/L;
Ceria: 10 g/L Outer catalytic layer 14 Pt: 3.5 g/L; Rh: 0.3 g/L;
Ba: 30 g/L; Sr: 10 g/L; Mg: 10 g/L; Alumina: 100 g/L; Ceria: 100
g/L Impregnated constituent Ag: 10 g/L Bi: 0.5 g/L
[0037] The inner and outer catalytic layers 12 and 14 were adjusted
so that the inner catalytic layer 12 contains a total amount of Ba,
Sr and Mg less than 1% of the total amount of those of the outer
catalytic layer 14. Each of the respective catalytic layers 12, 13
and 14 contained impurities less than 1%.
[0038] A rig test was carried out in order to evaluate catalytic
performance--HC purification efficiency (HC absorption ratio, HC
oxidation ratio and HC conversion efficiency) and lean NOx
purification efficiency--of the sample exhaust gas purifying
catalyst 8 before and after aging. The aging of the sample exhaust
gas purifying catalyst 8 was made in the atmosphere at 800.degree.
C. for 24 hours. The evaluation of HC purification efficiency was
performed in the test mode consisting of raising the inlet gas
temperature of an N.sub.2 gas streaming at a spatial velocity of
25000h.sup.-1 in which the sample exhaust gas purifying catalyst 8
was disposed to 80.degree. C. (first step); supplying HC, NO and
O.sub.2 gases into the stream of N.sub.2 gas for two minutes
keeping the inlet gas temperature so that the HC, NO and O.sub.2
contents amount to 1500 ppmC, 100 ppmC and 1.0%, respectively,
(second step); and subsequently raising the inlet gas temperature
from 80.degree. C. to 400.degree. C. at a rate of 30.degree.
C./min. after interrupting the supply of HC gas (third step). The
N.sub.2 gas was streamed at a spatial velocity of
25000h.sup.-1.
[0039] HC absorption ratio was determined on the basis of inlet and
outlet HC concentrations of the N.sub.2 gas stream for the period
of two minutes during the second step. The HC oxidation ratio, that
is the ratio of oxidized HC relative to absorbed HC, was determined
on the basis of an absorbed amount of HC in the sample exhaust gas
purifying catalyst 8 for the period of two minutes during the
second step and the outlet HC concentration during the third step.
The HC conversion efficiency was defined by the product of HC
absorption ratio and HC oxidation ratio. The lean NOx purification
efficiency was defined as a NOx conversion efficiency for a period
of 60 seconds from a point of time at which a simulated exhaust gas
is changed over in gas composition to a specific gas composition A
after repeating exposure of the sample exhaust gas purifying
catalyst 8 to a simulated exhaust gas having the gas composition A
for a period of 60 seconds and subsequently to a simulated exhaust
gas having a specific gas composition B for a period of 60 seconds
five times. The simulated exhaust gas was maintained at 35.degree.
C. and streamed at a spatial velocity of 25000h.sup.-1. The table I
shows the gas composition A for a simulated exhaust gas of a lean
air-fuel ratio (A/F) of 22 and the gas composition B for a
simulated exhaust gas of a rich air-fuel ratio (A/F) of 14.5.
2 TABLE I Gas composition A Gas composition B Lean (A/F = 22) Rich
(A/F = 14.5) HC(C.sub.3H.sub.6) 1333 ppm 1333 ppm NO 260 ppm 260
ppm CO 0.16% 0.16% CO.sub.2 9.75% 9.75% H.sub.2 650 ppm 650 ppm
O.sub.2 7% 0.5% N.sub.2 Reminder Remainder
[0040] The evaluation result is shown in FIG. 3. As demonstrated in
FIG. 3, the HC oxidation ratio shows a deterioration due to aging
but not so large. This event will be described in detail later in
connection with a rig test of a comparative exhaust gas purifying
catalyst. On the other hand, the HC absorption ratio shows
deterioration a little due to aging. The lean NOx purification
efficiency shows deterioration a little due to aging. The fact that
deterioration of the HC absorption ratio is quite a little
indicates that the .beta.-type zeolite in the inner catalytic layer
12 causes almost no functional defect due to aging. This event
proves that the .beta.-type zeolite causes almost no decrease in
specific surface area.
[0041] Bi impregnated in the intermediate catalytic layer 13, that
exists an accessible atom to a Pd atom, prevents Pd from reacting
on Ag and thereby producing a Pd--Ag alloy, in other words,
prevents Pd from causing a deterioration in low temperature
activity on HC conversion or prevents a decrease in the amount of
Ag that effectively functions in the improvement of HC absorption
performance.
[0042] FIG. 4 shows a comparative exhaust gas purifying catalyst 8C
that comprises an inner catalytic layer 12C coated on a substrate
11 such as a cordierite honeycomb bed and an outer catalytic layer
14C directly laid over the inner catalytic layer 12C. The inner
catalytic layer 12C is identical in constituent with and formed in
the same process as the inner catalytic layer 12 of the sample
exhaust gas purifying catalyst 8. The outer catalytic layer 14C
comprises a catalytic component that contains Pt, Rh, Pd, Ba, Sr
and Mg supported on alumina and ceria, and a binder. That is, the
comparative exhaust gas purifying catalyst 8C is different from the
sample exhaust gas purifying catalyst 8 in that no intermediate
catalytic layer is formed between the inner and outer catalytic
layers 12C and 14C, but is there Pd additionally contained in the
outer catalytic layer 14C.
[0043] The comparative exhaust gas purifying catalyst 8C was
produced in the following process. In the first place, catalytic
powder C was prepared. The catalytic powder C was made up by mixing
active alumina powder, ceria, barium acetate, strontium acetate,
magnesium acetate, dinitro-diamine platinum nitrate, rhodium
nitrate, palladium nitrate and water all together and calcinating
the mixture at 500.degree. C. after drying.
[0044] The inner catalytic layer 12C was formed in the same process
as that of the sample exhaust gas purifying catalyst 8. The inner
catalytic layer 12C was subsequently impregnated with a mixed
solution of silver nitrate and bismuth acetate and then calcinated
at 500.degree. C. after drying. Thereafter, the outer catalytic
layer 14C was formed over the inner catalytic layer 12C in the same
process as that of the sample exhaust gas purifying catalyst 8.
[0045] The exhaust gas purifying catalyst 8C was adjusted so as to
have the following quantitative formula:
3 Inner catalytic layer 12C .beta.-type zeolite: 100 g/L Outer
catalytic layer 14C Pt: 3.5 g/L; Rh: 0.3 g/L; Pd: 2.0 g/L; Ba: 30
g/L; Sr: 10 g/L; Mg: 10 g/L; Mg: 10 g/L; Alumina: 100 g/L; Ceria:
100 g/L Impregnated constituent Ag: 10 g/L Bi: 0.5 g/L
[0046] The inner and outer catalytic layers 12C and 14C were
adjusted so that the inner catalytic layer 12C contains a total
amount of Ba, Sr and Mg less than 1% of the total amount of those
of the outer catalytic layer 14. Each of the respective catalytic
layers 12C and 14C contained impurities less than 1%.
[0047] A rig test was carried out in order to make comparative
evaluations of the catalytic performance--HC purification
efficiency and lean NOx purification efficiency--between the sample
and comparative exhaust gas purifying catalysts 8 and 8C after
aging in the same test mode as described in connection with the
evaluation rig test of the sample exhaust gas purifying catalyst 8.
The evaluation result is shown in FIG. 5.
[0048] As demonstrated in FIG. 5, there is almost no difference in
the lean NOx conversion efficiency between the sample and
comparative exhaust gas purifying catalysts 8 and 8C. The sample
exhaust gas purifying catalyst 8 shows an HC absorption ratio lower
than the comparative exhaust gas purifying catalyst 8C and,
however, an HC oxidation ratio higher than the comparative exhaust
gas purifying catalyst 8C. As a whole, the sample exhaust gas
purifying catalyst 8 has an HC conversion ratio higher than the
comparative exhaust gas purifying catalyst 8C.
[0049] It is conceivable that the high HC oxidation ratio of the
sample exhaust gas purifying catalyst 8 is due to the conformation
of Pd in the intermediate catalytic layer 13 isolated from Ba, Sr
and Mg functioning as NOx adsorption elements in the outer
catalytic layer 14. That is, because Pd is higher in low
temperature activity on HC conversion due to oxidation, the HC
oxidation ratio is greatly influenced by how the Pd is active. As
for the sample exhaust gas purifying catalyst 8, the Pd in the
intermediate catalytic layer 13 is hardly affected electrically and
chemically by the NOx adsorption elements, so that the Pd does not
cause a significant deterioration in the low temperature activity
due to aging. That is, it is conceived that the NOx adsorption
elements do not encourage the Pd in deteriorating its low
temperature activity.
[0050] Rig tests were carried out to measure HC conversion ratios
of the sample and comparative exhaust gas purifying catalysts 8 and
8C in order to evaluate light off temperatures (T50) regarding HC
conversion. Measurements of HC conversion ratio were made on each
catalyst after aging as the inlet gas temperature was gradually
raised. Aging of the catalyst was performed in the same condition
as previously described. A simulated exhaust gas that was used in
the rig test was of an air-fuel ratio (A/F) of 14.7.+-.0.9. That
is, while a main simulated exhaust gas of an air-fuel ratio (A/F)
of 14.7 was stationarily streamed at a spatial velocity of
25000h.sup.-1, a specified amount of modifying gas was spouted into
the stationary main exhaust gas stream on a cycle of 1 Hz so as to
force the air-fuel ratio to pulsate between 14.7.+-.0.9.
[0051] The table II shows the gas composition of the main simulated
exhaust gas of an air-fuel ratio (A/F) of 14.7.
4TABLE II CO.sub.2 O.sub.2 CO H.sub.2 C.sub.3H.sub.6(HC) NO N.sub.2
13.9% 0.6% 0.6% 0.2% 0.056% 0.1% Remainder
[0052] A gas of O.sub.2 was employed for the modifying gas in order
to force the air-fuel ratio to vary to a lean side to 15.6. On the
other hand, a mixture gas of H.sub.2 and CO was employed for the
modifying gas in order to force the air-fuel ratio to vary to a
rich side to 13.8.
[0053] The evaluation result is shown in FIG. 6. As demonstrated in
FIG. 6, the sample exhaust gas purifying catalyst 8 in which Pd is
isolated from Ba, Sr and Mg is superior in low temperature activity
to the comparative exhaust gas purifying catalyst 8C in which Pd
coexists with Ba, Sr and Mg. Further, the sample exhaust gas
purifying catalyst 8 is improved in light off temperature (T50) by
approximately 27.degree. C. as compared with the comparative
exhaust gas purifying catalyst 8C. This event proves that the
isolation of Pd from a NOx adsorption material is advantageous to
preventing an exhaust gas purifying catalyst from causing a
deterioration of low temperature activity.
[0054] The exhaust gas purifying catalyst according to the
embodiment of the present invention contains a NOx adsorption
material such as Ba and adsorbs NOx in exhaust gases while an
air-fuel ratio remains lean. Accordingly, when there is an increase
in the amount of adsorbed NOx, it is necessary to cause reduction
purification of NOx by means of making an air-fuel ratio rich, i.e.
by means of lowering an oxygen concentration of exhaust gas, so as
to release the adsorbed NOx. In addition, when the amount of NOx
adsorption material poisoned with sulfur becomes large, it is
necessary to revitalize the exhaust gas purifying catalyst by means
of raising a catalytic temperature.
[0055] The following description will be directed to fuel injection
control for NOx release and revitalization of sulfur poisoned
catalyst. In the fuel injection control, an air-fuel ratio is made
rich in order to cause a rise in catalytic temperature.
[0056] FIG. 7 shows a flowchart illustrating a sequence routine of
the fuel injection control. When the sequence logic commences and
control proceeds to a function block at step S1 where various data
are input. The data includes at least an amount of intake air, an
engine speed, an accelerator position or engine load and a
catalytic temperature. After determining a basic amount of fuel
injection Qpb that meets a target air-fuel ratio determined on the
basis of the data according to an engine operating condition at
step S2, an amount of NOx adsorption NOe and an amount of SOx
(oxides of sulfur) adsorption are estimated as integrated values at
steps S3 and S4, respectively. In this instance, when the air-fuel
ratio remains lean, i.e. an excess air ratio .lambda. is greater
than 1, this estimate is performed with respect to an engine
operating condition (engine speeds and engine loads) by reference
to a map that defines amounts of NOx or SOx adsorption according to
engine operating conditions by way of experiment. On the other
hand, when the air-fuel ratio remains rich, i.e. an excess air
ratio .lambda. is equal to or less than 1, the estimate is
performed by gradually reducing the integrated amount of NOx or SOx
adsorption by a diminution constant that is changed larger as the
excess air ratio .lambda. becomes lower and as an interval for
which the air-fuel ratio remains rich becomes longer.
[0057] Thereafter, a determination is made at step S5 as to whether
the estimated amount of NOx adsorption NOe is larger than a
predetermined threshold amount of NOx adsorption NOo. The threshold
amount of NOx adsorption NOo refers to an amount of NOx adsorption
that is conceived to be as large as the exhaust gas purifying
catalyst 8 involves NOx release control, in other words, the
catlyst lowers its NOx adsorptive power. When the estimated amount
of NOx adsorption NOe is larger than the predetermined threshold
amount of NOx adsorption NOo, this indicates that the sulfur
poisoning of NOx adsorption material is as serious as the exhaust
gas purifying catalyst 8 needs to be revitalized, then, another
determination is made at step S6 as to whether the last attribute
TN.sub.n-1 that a counter shows is zero. The attribute TN
represents a period of time for which the fuel injection control
keeps up control of increasing the amount of fuel injection. When
the last attribute TN.sub.n-1 is zero, after establishing a
threshold time TNo at step S7, after establishing a threshold time
TSo at step S7, the current attribute TN.sub.n is changed by an
increment of one at step S8. On the other hand, when the last
attribute TN.sub.n-1 is not zero, the current attribute TN.sub.n is
changed by an increment of one at step S8 without establishing a
threshold time TNo. In this instance, the threshold time TNo is set
to an appropriate value, for example between 0.5 and 5 seconds in
actual time, according to the current attribute TN.sub.n.
Specifically, the threshold time TNo is small while the catalytic
temperature is in a range, for example, between 200 and 400.degree.
C. where NOx is easily released and is, on the other hand, large
when the catalytic temperature is out of that temperature
range.
[0058] Subsequently to changing the current attribute TN.sub.n by
an increment of one at step S8, a determination is made at step S9
as to whether the current attribute TN.sub.n is larger than the
threshold time TNo. When the current attribute TN.sub.n is equal to
or smaller than the threshold time TNo, the basic amount of fuel
injection Qpb is replaced with an amount of fuel injection
Qp.lambda. for enrichment at step S10. Thereafter, after
substituting the basic amount of fuel injection Qpb (i.e.
Qp.lambda.) for an actual amount of fuel injection Qp at step S11,
fuel injection is controlled so as to spray the actual amount of
fuel injection Qp at step S12. On the other hand, when the current
attribute TN.sub.n is larger than the threshold time TNo at step
S9, after resetting the attribute TN.sub.n and the estimated amount
of NOx adsorption NOe at step S13, the basic amount of fuel
injection Qpb is substituted for an actual amount of fuel injection
Qp at step S11 and then, fuel injection is controlled so as to
spray the actual amount of fuel injection Qp at step S12.
[0059] On the other hand, when the estimated amount of NOx
adsorption NOe is equal to or smaller than the predetermined
threshold amount of NOx adsorption NOo, this indicates that the
sulfur poisoning of NOx adsorption material is not serious, then,
another determination is made at step S14 as to whether the counter
is still counting, i.e. whether it is still in the process of the
control of increasing control of increasing the amount of fuel
injection. When it is still in the process of the fuel increasing
control, then, the sequence logic jumps to the control of releasing
NOx through steps S8 to S12. On the other hand, when it is out of
the process of the fuel increasing control, then, a determination
is made at step S15 as to whether the estimated amount of SOx
adsorption SOe is larger than a predetermined threshold amount of
SOx adsorption SOo. The threshold amount of SOx adsorption SOo
refers to an amount of SOx adsorption that is conceived to be
poisoned with SOx as large as the exhaust gas purifying catalyst 8
involves SOx release control. When the estimated amount of SOx
adsorption SOe is still below the predetermined threshold amount of
SOx adsorption SOo, fuel injection is performed so as to splay the
basic amount of fuel Qpb through steps S11 and S12.
[0060] On the other hand, when the estimated amount of SOx
adsorption SOe is above the predetermined threshold amount of SOx
adsorption SOo at step S15, another determination is made at step
S16 as to whether the last attribute TS.sub.n-1 that a counter
shows is zero. The attribute TS represents a period of time for
which the fuel injection control keeps up control of increasing the
amount of fuel injection. When the last attribute TS.sub.n-1 is
zero, after establishing a threshold time TSo at step S17, the
current attribute TN.sub.n is changed by an increment of one at
step S18. On the other hand, when the last attribute TS.sub.n-1 is
not zero, the current attribute TS.sub.n is changed by an increment
of one at step S18 without establishing a threshold time TSo. In
this instance, the threshold time TSo is set to an appropriate
value in a range between, for example, 1 and 10 minutes in actual
time according to a current catalytic temperature while the
catalytic temperature is higher than a specific temperature of, for
example, 400.degree. C. Specifically, the threshold time TSo is
made as the catalytic temperature becomes higher.
[0061] Subsequently, a determination is made at step S19 as to
whether the current attribute TN.sub.n is larger than the threshold
time TSo. When the current attribute TS.sub.n is equal to or
smaller than the threshold time TSo, the sequence logic jumps to
the function at step S10 so as to replace the basic amount of fuel
injection Qpb with an amount of fuel injection Qp.lambda. for
enrichment. Thereafter, after substituting the basic amount of fuel
injection Qpb (i.e. Qp.lambda.) for an actual amount of fuel
injection Qp at step S11, fuel injection is controlled so as to
spray the actual amount of fuel injection Qp at step S12. On the
other hand, when the current attribute TS.sub.n is larger than the
threshold time TSo at step S19, after resetting the attribute
TS.sub.n the estimated amount of NOx adsorption NOe and the
estimated amount of SOx adsorption SOe at step S20, the basic
amount of fuel injection Qpb is substituted for an actual amount of
fuel injection Qp at step S11 and then, fuel injection is
controlled so as to spray the actual amount of fuel injection Qp at
step S12.
[0062] As described above, when the amounts of adsorbed NOx and
adsorbed SOx increase, the air-fuel ratio is made rich by
increasing the amount of fuel injection even while the engine is
operated with a lean air-fuel ratio, so as to lower the oxygen
concentration of exhaust gas. As a result, the exhaust gas
purifying catalyst 8 releases NOx from the NOx adsorption material
and then reduces the NOx with the catalytic novel metals in the
outer catalytic layer 14 thereof. Further, when the exhaust gas
purifying catalyst 8 is poisoned with sulfur to a significantly
increased degree, the air-fuel ratio is made rich by increasing the
amount of fuel injection even while the engine is operated with a
lean air-fuel ratio. As a result, since, while the exhaust gas
raises its temperature, the exhaust gas purifying catalyst 8 is
made active in oxidation reaction, the exhaust gas purifying
catalyst 8 raises its own temperature, resulting in releasing SOx
from the NOx adsorption material. This leads to revitalization of
the NOx adsorption material.
[0063] When the NOx adsorption material is seriously poisoned with
sulfur, the exhaust gas temperature may be raised by retarding an
ignition timing in addition to making an air-fuel ratio rich.
Further, the catalytic temperature may be further raised by
supplying secondary air into an exhaust gas stream upstream from
the exhaust gas purifying catalyst 8 so as to assist the exhaust
gas purifying catalyst 8 in causing oxidative reaction.
[0064] It is to be understood that although the present invention
has been described with regard to preferred embodiments thereof,
various other embodiments and variants may occur to those skilled
in the art, which are within the scope and spirit of the invention,
and such other embodiments and variants are intended to be covered
by the following claims.
* * * * *