U.S. patent application number 11/746313 was filed with the patent office on 2007-09-06 for exhaust gas purification apparatus of internal combustion engine and catalyst for purifying exhaust gas of internal combustion engine.
This patent application is currently assigned to HITACHI, LTD.. Invention is credited to Shigeru Azuhata, Ryouta Doi, Hiroshi HANAOKA, Toshifumi Hiratsuka, Hidehiro Iizuka, Yuichi Kitahara, Osamu Kuroda, Toshio Manaka, Toshio Ogawa, Kojiro Okude, Norihiro Shinotsuka, Hisao Yamashita.
Application Number | 20070204595 11/746313 |
Document ID | / |
Family ID | 27456052 |
Filed Date | 2007-09-06 |
United States Patent
Application |
20070204595 |
Kind Code |
A1 |
HANAOKA; Hiroshi ; et
al. |
September 6, 2007 |
Exhaust Gas Purification Apparatus of Internal Combustion Engine
and Catalyst for Purifying Exhaust Gas of Internal Combustion
Engine
Abstract
An exhaust gas purification apparatus for use in an internal
combustion engine comprises an exhaust gas duct connected to the
engine through which exhaust gas containing NO.sub.x gas passes,
and a catalyst disposed in the exhaust gas duct such that it
contacts the exhaust gas. The catalyst chemically adsorbs NO.sub.x
when a stoichiometric amount of a gaseous oxidizing agent present
in the exhaust gas is larger than that of a gaseous reducing agent
present in the exhaust gas for reducing NO.sub.x, adsorbed NO.sub.x
is catalytically reduced in the presence of a reducing agent when
the stoichiometric amount of the oxidizing agent is not larger that
of the reducing agent.
Inventors: |
HANAOKA; Hiroshi;
(Kodaira-shi, JP) ; Kuroda; Osamu; (Hitachi-shi,
JP) ; Doi; Ryouta; (Naka-gun, JP) ; Iizuka;
Hidehiro; (Hitachinaka-shi, JP) ; Ogawa; Toshio;
(Takahagi-shi, JP) ; Yamashita; Hisao;
(Hitachi-shi, JP) ; Azuhata; Shigeru;
(Hitachi-shi, JP) ; Kitahara; Yuichi;
(Hitachinaka-shi, JP) ; Hiratsuka; Toshifumi;
(Hitachinaka-shi, JP) ; Okude; Kojiro;
(Hitachi-shi, JP) ; Shinotsuka; Norihiro;
(Hitachinaka-shi, JP) ; Manaka; Toshio;
(Hitachinaka-shi, JP) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
HITACHI, LTD.
Tokyo
JP
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
27456052 |
Appl. No.: |
11/746313 |
Filed: |
May 9, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10900320 |
Jul 28, 2004 |
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11746313 |
May 9, 2007 |
|
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|
10119075 |
Apr 10, 2002 |
7093432 |
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11746313 |
May 9, 2007 |
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09620650 |
Jul 20, 2000 |
6397582 |
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10119075 |
Apr 10, 2002 |
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09202243 |
Dec 10, 1998 |
6161378 |
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PCT/JP97/01955 |
Jun 9, 1997 |
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09620650 |
Jul 20, 2000 |
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Current U.S.
Class: |
60/274 |
Current CPC
Class: |
F01N 3/0871 20130101;
F02D 2200/0814 20130101; B01D 2255/2047 20130101; F01N 2570/04
20130101; Y02T 10/12 20130101; F01N 2610/03 20130101; F02D
2200/0806 20130101; B01D 2255/2065 20130101; F01N 3/0842 20130101;
F01N 2510/06 20130101; F02D 41/1465 20130101; F01N 2430/06
20130101; B01D 53/9495 20130101; B01D 2255/2061 20130101; F02D
41/1462 20130101; B01D 2255/2027 20130101; F01N 3/0878 20130101;
F01N 11/002 20130101; F01N 2250/12 20130101; B01D 2255/1021
20130101; B01D 2255/1023 20130101; B01D 2255/1025 20130101; F01N
3/0814 20130101; Y02A 50/20 20180101; Y02T 10/40 20130101; F01N
13/009 20140601; B01D 53/9422 20130101; F02D 41/146 20130101; F02D
41/027 20130101; F02D 41/1463 20130101; B01D 2255/2022 20130101;
F02D 41/0275 20130101; B01D 2255/2063 20130101; F02D 41/1455
20130101; B01D 2255/91 20130101 |
Class at
Publication: |
060/274 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 10, 1996 |
JP |
8-146981 |
Jun 14, 1996 |
JP |
8-153718 |
Aug 8, 1996 |
JP |
8-209587 |
Jan 28, 1997 |
JP |
9-13655 |
Claims
1. An exhaust gas purification apparatus using a catalyst, wherein
the catalyst comprising alumina and as catalytic components at
least two of Rh, Pt and Pd, at least one of Ce, La and Y, at least
one of Na, K and Sr.
2. The apparatus according to claim 1, which further contains
Mg.
3. The apparatus according to claim 2, wherein the catalyst is
supported on a honeycomb base body, an amount of catalytic
components being 18 to 36 g/L of the honeycomb structure.
4. The apparatus according to claim 3, wherein an amount of at
least one of the Na, Sr and K is larger than that of at least one
of Mg, Ti and Si.
5. The apparatus according to claim 1, which further contains at
least one of Ti and Si and Mg.
6. The apparatus according to claim 5, wherein the catalyst is
supported on a honeycomb base body, an amount of Mg being 2 to 4
g/L of the honeycomb structure.
7. The apparatus according to claim 1, wherein the catalyst
contains Ce, Rh, Pt, Mg, Na and Ti.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/900,320, filed Jul. 28, 2004, which is a
continuation of U.S. patent application Ser. No. 10/119,075, filed
Apr. 10, 2002, which is a continuation of U.S. patent application
Ser. No. 09/620,650, filed Jul. 20, 2000 (now U.S. Pat. No.
6,397,582), which is a continuation of U.S. patent application Ser.
No. 09/202,243, filed Dec. 10, 1998 (now U.S. Pat. No.
6,161,378).
BACKGROUND OF THE INVENTION
[0002] This application claims the priority of Japanese patent
documents No. 8-146981, filed Jun. 10, 1996; No. 8-153718, filed
Jun. 14, 1996; No. 8-209587, filed Aug. 8, 1996, and 9-13655, filed
Jan. 28, 1997, (PCT International Patent Application
PCT/JP97/01955, filed Jun. 8, 1997) the disclosures of which are
expressly incorporated by reference herein.
[0003] The present invention relates to a purification apparatus
for an exhaust gas which is discharged or emitted from an internal
combustion engine such as an automobile, and particularly to an
apparatus which includes a catalyst for purifying an exhaust gas
from an internal combustion engine that is operated under a lean
air-fuel ratio (a lean burn), and from an automobile which has such
a lean burn internal combustion engine.
[0004] Exhaust gas discharged from an internal combustion engine
such an automobile includes carbon monoxide (CO), hydrocarbon (HC)
and nitrogen oxide (NO.sub.x) etc. which pollute the environment,
adversely affect the human body, and disturb the growth and the
development of plants.
[0005] Accordingly, up to now a great deal of effort has gone into
reducing the amount of such pollutants by improving a combustion in
the internal combustion engine, and developing a method for
purifying the discharged or the emitted exhaust gas using a
catalyst to obtain a steady result.
[0006] Gasoline engine vehicles frequently utilize a three
component catalyst in which platinum (Pt) and rhodium (Rh) are main
active components. The oxidation of HC and CO and the reduction of
NO.sub.x are carried out at the same time to convert the above air
pollution materials to harmless materials.
[0007] It is characteristic of a three component catalyst, that it
operates effectively only for exhaust gases which are generated
within a range ("window") in the vicinity of a stoichiometric
air-fuel ratio.
[0008] In the conventional technique, the air-fuel ratio fluctuates
in accordance with an operation condition of the automobile. A
fluctuation region is principally controlled to the vicinity of the
stoichiometric air-fuel ratio, which is a ratio between A (weight
of air) and F (weight of fuel), being about 14.7 in case of the
gasoline. Hereinafter, in the present specification, the
stoichiometric air-fuel ratio is represented by A/F=14.7, but this
value varies in accordance with kinds of the fuels.
[0009] However, when the engine is operated under a lean air-fuel
ratio in comparison with the stoichiometric air-fuel ratio
atmosphere, the fuel consumption can be improved. Therefore, the
development of a lean burn combustion technique is promoted; and
recently automobiles have been developed in which the engine is
combusted under the lean area having the air-fuel ratio of more
than 18.
[0010] However, when a conventional three component catalyst is
adopted for purification of a lean burn exhaust gas, although the
oxidation purification with respect to HC and CO is performed
effectively, the reduction of NO.sub.x is not.
[0011] Accordingly, to promote the application of the lean burn
system for large size vehicles and to enlarge the lean burn
combustion time (that is, enlarge the operation area of the lean
burn system), it is necessary to develop an exhaust gas
purification technique which is suitable to the lean burn system.
Thus, the development of a technique for purifying HC, CO and
particularly NO.sub.x where a large quantity of oxygen (0.sub.2) is
included in the exhaust gas, has been promoted vigorously.
[0012] Japanese patent laid-open publication No. 61,706/1988
discloses a technique in which HC is supplied upstream of a lean
burn exhaust gas. The operation of a catalyst is facilitated by
lowering the oxygen (0.sub.2) concentration in the exhaust gas to a
concentration area for effective functioning of the catalyst.
[0013] Japanese patent laid-open publication No. 97,630/1987,
Japanese patent laid-open publication No. 106,826/1987 and Japanese
patent laid-open publication No. 117,620/1987, propose a technique
in which N included in the exhaust gas (after the conversion of an
easily absorbable NO.sub.2 by oxidizing NO) is absorbed and removed
by contact with a catalyst having NO.sub.x absorbing ability. When
the absorption efficiency decreases, by stopping a passing-through
of the exhaust gas accumulated NO.sub.x is reduction-removed using
H.sub.2, HC included in a methane gas and a gasoline etc., and so
that NO.sub.x absorbing ability of the catalyst is regenerated.
[0014] Further, WO 93/07363 and WO 93/08383 discloses an exhaust
gas purification apparatus in which an NO.sub.x absorbent material
arranged at an exhaust gas flow passage absorbs NO.sub.x from a
lean exhaust gas, and when an oxygen concentration in the exhaust
gas is lowered the NO.sub.x absorbent material discharges the
absorbed NO.sub.x. The exhaust gas absorbs NO.sub.x during the lean
atmosphere and the absorbed NO.sub.x is discharged by lowering
0.sub.2, concentration in the exhaust gas which flows into a
NO.sub.x absorbent.
[0015] However, in Japanese patent laid-open publication No.
61,708/1988, to attain a composition of the exhaust gas which
corresponds to the air-fuel ratio of A/F=14.7 where the catalyst
can function (0.sub.2 concentration having about 0.5%), it needs a
very large quantity of HC. A use of a blow-by gas in this patent
document is effective, but the blow-by gas does not have an amount
which is sufficient for efficient to treat an exhaust gas during an
operation of an internal combustion engine. It is possible
technically to throw the fuel but it eliminates the fuel
consumption gains achieved by the lean burn system.
[0016] In Japanese patent laid-open publication No. 97,630/1987,
Japanese patent laid-open publication No. 106,826/1987 and Japanese
patent laid-open publication No. 117,620/1987, to regenerate a NOx
absorbent material a flow of exhaust gas is stopped and the gaseous
reducing agent of HC etc. is contacted to NOx absorbent. Further,
two NOx absorbent materials are provided and the exhaust gas flows
alternately to these two NOx absorbent materials. It is therefore
necessary to provide an exhaust gas change-over mechanism, which
complicates the structure of the exhaust gas treatment
apparatus.
[0017] In WO 93/07363 and WO 93/08383, the exhaust gas is flowed
continuously to an NOx absorbent material, and the NOx in the
exhaust gas is absorbed during the lean atmosphere. By lowering
O.sub.2 concentration in the exhaust gas, the absorbed NOx is
discharged and the NOx absorbent material is regenerated.
Accordingly, since the change-over of the exhaust gas flow is
unnecessary, the problem in the above stated system can dissolved.
However, these systems require a material which can absorb NOx
during the lean condition and can discharge NOx when 0.sub.2
concentration in the exhaust gas is lowered. Since the repeated NOx
absorption and discharge inevitably causes a periodic change of a
crystal structure of the absorbent, it is necessary to take a
careful consideration about the durability of the absorbent.
Further, it is necessary to treat the discharged NOx; in the case
of a large quantity of the discharged NOx it may be necessary to
provide a post-treatment using a three component catalyst.
SUMMARY OF THE INVENTION
[0018] In the light of the problems in the above stated prior arts,
the object of the present invention is to provide an internal
combustion engine exhaust gas purification apparatus which has a
simple structure, consumes a small amount of gaseous reducing
agents, has superior endurance and which effectively removes
harmful components such as NOx from a lean burn exhaust gas
converting them to a harmless component.
[0019] Another object of the present invention is to provide a
catalyst for use in an exhaust gas purification apparatus of an
internal combustion engine.
[0020] According to the present invention, an exhaust gas
purification apparatus for use in an internal combustion engine
comprises an exhaust gas duct connected to the engine, through
which the exhaust gas containing NOx gas passes, and a catalyst
disposed in the exhaust gas duct so that it contacts the exhaust
gas. The catalyst chemically adsorbs NOx when a stoichiometric
amount of a gaseous oxidizing agent present in the exhaust gas is
larger than the amount of a gaseous reducing agent present in the
exhaust gas for reducing NOx, while adsorbed NOx is catalytically
reduced in the presence of the reducing agent when the
stoichiometric amount of the oxidizing agent is not larger that of
the reducing agent.
[0021] According to the present invention, an apparatus for
purifying an exhaust gas from an internal combustion engine
comprises an exhaust gas duct connected to the engine, through
which the exhaust gas containing NOx gas passes, and a catalyst
disposed in the exhaust gas duct so that it contacts the exhaust
gas. The catalyst adsorbs NOx under when the amount of a gaseous
oxidizing agent present in the exhaust gas is larger than that of a
gaseous reducing agent for NOx added to the exhaust gas in a
stoichiometric relation, while adsorbed NOx is catalytically
reduced in the presence of the reducing agent when the amount of
the oxidizing agent is not larger that of the reducing agent in the
stoichiometric relation.
[0022] According to the present invention, an apparatus for
purifying an exhaust gas from an internal combustion engine
comprises an exhaust gas duct connected to the engine, through
which the exhaust gas containing NOx gas passes, and a catalyst
disposed in the exhaust gas duct so that it contacts with the
exhaust gas. The catalyst adsorbs NOx when a stoichiometric amount
of a gaseous oxidizing agent present in a lean combustion exhaust
gas is larger than that of a gaseous reducing agent present in the
lean combustion exhaust gas for reducing NOx, while the adsorbed
NOx is catalytically reduced in the presence of the reducing agent
when a stoichiometric amount of the oxidizing agent is not larger
that of the reducing agent in a stoichiometric or fuel rich
combustion exhaust gas.
[0023] According to the present invention, an apparatus for
purifying an exhaust gas from an internal combustion engine
comprises an exhaust gas duct connected to the engine, through
which the exhaust gas containing NOx gas passes, a device for
controlling the air-fuel ratio of the exhaust gas, and a catalyst
disposed in the exhaust gas duct so that it contacts with the
exhaust gas. The air-fuel ratio is switched from a range in which
the catalyst chemically adsorbs NOx in the lean combustion exhaust
gas to a range in which adsorbed NOx is catalytically reduced in
the presence of the reducing agent in a stoichiometric or fuel rich
combustion exhaust gas.
[0024] The catalyst according to the present invention comprises a
heat resistant carrier body and catalytic compounds supported
thereon. The catalytic compounds comprise at least one of sodium
and potassium, at least one of magnesium, strontium and calcium,
and at least one of platinum, palladium and rhodium. In at least
some embodiments, a base member supports the heat resistant carrier
body.
[0025] According to the present invention, the reducing agent to be
added to the lean combustion exhaust gas is at least one of
gasoline, light oil, kerosene, natural gas, their reformed
substances, hydrogen, alcohol, ammonia gas, engine blow-by gas and
canister purging gas. The reducing agent is supplied to the lean
combustion exhaust gas in response to signals from the
stoichiometric establishing unit.
[0026] The exhaust gas purification apparatus according to the
present invention further comprises a manifold catalyst which is
disposed in the exhaust gas duct immediately after the engine,
upstream of the catalyst, and functions as a three component
catalyst and a combustion catalyst.
[0027] According to another feature of the present invention, in
the exhaust gas purification apparatus, a second catalyst is
disposed in the exhaust gas duct of a direct fuel injection
engine.
[0028] According to yet another feature of the present invention,
the exhaust gas purification apparatus further comprises a three
component catalyst or a combustion catalyst is disposed in the
exhaust gas duct upstream of the catalyst.
[0029] According to the present invention, an apparatus for
purifying an exhaust gas from an internal combustion engine
comprises an exhaust gas duct connected to the engine, through
which the exhaust gas containing NOx, SOx and oxygen passes, and a
catalyst which chemically adsorbs NOx when the exhaust gas is
emitted from lean combustion, and in which adsorbed NOx is
catalytically reduced when a gaseous reducing agent is added to the
lean combustion exhaust gas in such an amount that a stoichiometric
amount of oxygen is not larger than that of the reducing agent.
[0030] The catalyst adsorbs or absorbs SO.sub.x in the lean
condition, and releases SOX in the stoichiometric or rich
condition.
[0031] According to the present invention, a catalyst for purifying
an exhaust gas from an internal combustion engine comprises a base
member, a heat resistant carrier body supported on the base member,
and catalyst components supported on the carrier body. The carrier
body has a number of small hollows extending in the direction of
gas flow of the exhaust gas.
[0032] According to the present invention, the catalyst compounds
comprise at least one alkali metal, at least one alkali earth metal
(other than barium), at least one noble metal and at least one rare
earth metal.
[0033] According to the present invention, an exhaust gas
purification apparatus for use in an internal combustion engine
comprises a catalyst which chemically adsorbs NOx when the amount
of a gaseous oxidizing agent is greater than that of a gaseous
reducing agent in a stoichiometric relation between the gaseous
oxidizing agent and the gaseous reducing agent, and catalytically
reduces adsorbed NOx when the gaseous reducing agent is equal to or
exceeds the gaseous oxidizing agent. The catalyst is provided in an
exhaust gas flow passage for a flow of an exhaust gas generated at
a lean air-fuel ratio and at a rich air-fuel ratio or a
stoichiometric air-fuel ratio.
[0034] According to the present invention, an exhaust gas
purification apparatus for use in an internal combustion engine
comprising a catalyst for chemically adsorbing NOx under a
condition where an gaseous oxidizing agent is more than a gaseous
reducing agent in a stoichiometric relation between the gaseous
oxidizing agent and the gaseous reducing agent and a device which
controls an air-fuel ratio to switch conditions between NOx
chemical adsorption to the catalyst and catalytic reduction
chemically of NOx to the catalyst.
[0035] According to the present invention, an exhaust gas
purification apparatus comprises a catalyst which chemically
adsorbs NOx when, in a stoichiometric relation between gaseous
oxidizing agent and gaseous reducing agent in an exhaust gas
flowing an exhaust gas flow passage in the internal combustion
engine, the amount of gaseous oxidizing agent is more than the
amount of gaseous reducing agent, and catalytic-reduces adsorbed
NOx when the amount of gaseous oxidizing agent equals or exceeds
the amount of gaseous reducing agent. The catalyst is provided in
the exhaust gas flow passage where an exhaust gas burned at a lean
air-fuel ratio and an exhaust gas burned at a rich or
stoichiometric air-fuel ratio flow into alternately.
[0036] The exhaust gas purification apparatus according to the
present invention provides a stoichiometric relation between
oxidation and reduction of gaseous oxidizing agent and gaseous
reducing agent. A control means for controlling the stoichiometric
relation between gaseous oxidizing agents and gaseous reducing
agents comprises a timing control means for controlling the time at
which the stoichiometric relation between gaseous oxidizing agent
and gaseous reducing agent changes over from a first condition in
which the amount of gaseous oxidizing agent is more than that of a
gaseous reducing agent, to another condition in which the amount of
gaseous reducing agent is equal to or exceeds the amount of gaseous
oxidizing agent. It also comprises a gaseous reducing agent excess
time control means for controlling a time when, in the
stoichiometric relation between oxidation and reduction, the
gaseous reducing agent is held to an amount equal to or greater
than the gaseous oxidizing agent.
[0037] According to the present invention, the catalyst has the
ability to chemical adsorb NOx, and to catalytic-reduce NOx. Even
when the oxygen concentration decreases, the catalyst does not
discharge NOx. These features can be obtained by a catalyst which
comprises at least one element selected from among the alkali
metals and alkali earth metals in the element periodic table (but
not including barium (Ba)), and at least one selected from noble
metals comprising platinum (Pt), palladium (Pd) and rhodium (Rh).
Further, the catalyst has an ability for catalytic oxidizing HC or
CO etc.
[0038] The catalyst has a high NOx absorption ability in a lean
atmosphere and further under the catalyst temperature of
250-500.degree. C. Further, to recover its NOx adsorbing ability by
reduction of the absorbed NOx, the catalyst must maintain a
stoichiometric atmosphere or a rich atmosphere for a time of about
30 seconds or less.
[0039] Accordingly, to perform effectively NOx adsorption and to
recover NOx adsorbing ability, it is desirable to provide the
catalyst at a position of an exhaust gas duct where an inlet port
gas temperature of the catalyst is 250-500.degree. C. The above
temperature range is one which can obtain normally under the
car-body floor.
[0040] NOx adsorbing ability of the catalyst is lowered due to
poisoning by SOx originated fuel (gasoline). However, when the
catalyst is maintained several (for example, ten) minutes at
400-800.degree. C. in a stoichiometric or rich atmosphere, SOx is
removed and then NOx adsorbing ability is recovered.
[0041] Accordingly, when the gasoline quality is bad (high sulfur
content) and the catalyst suffers from poisoning by SOx, it is
desirable to position the catalyst in an exhaust gas duct where an
inlet port gas temperature of the catalyst is 400-800.degree. C.
The above temperature range is one which can obtain under the
car-body floor.
[0042] When the catalyst is used for an automobile, it is desirable
to form it as a honeycomb having NOx adsorption ability of more
than 0.01 mol per an apparent honeycomb volume one (1) liter.
[0043] Further, it is desirable to set the specific surface area of
the catalyst layer on the honeycomb substrate (the honeycomb base
body), measured by absorbing nitrogen according to BET method, at
more than 50 m2/g.
[0044] In the exhaust gas, the gaseous oxidizing agents are O2, NO,
NO2, etc. and are mainly oxygen. The gaseous reducing agents are HC
supplied in an internal combustion engine, HC (including oxygen
containing hydrocarbon) generated in a combusting process as a
derivative from fuel, CO, H2 etc. Furthermore, a reducing material
such as HC can be added in the exhaust gas as a reducing
component.
[0045] When the lean exhaust gas contacts the three component
catalyst, HC, CO, H2 etc. as the gaseous reducing agents for
reducing NOx to nitrogen (N2), cause a combustion reaction with
oxygen (02) as the gaseous oxidizing agent in the exhaust gas. NOx
(NO and N02) reacts with these gaseous reducing agents and is
reduced to nitrogen (N2). Normally, since both reactions proceed in
parallel, a utilization rate of the gaseous reducing agents for
reducing NOx is low.
[0046] Particularly at a high reaction temperature of more than
500.degree. C. (depending on the catalyst material), an occupation
rate of the latter reaction becomes large. Hence, by separating NOx
from the exhaust gas (at least from O.sub.2) using the catalyst,
and then carrying out the catalytic-reaction with the gaseous
reducing agents, it is possible to achieve an effective reduction
of NOx to N2. According to the present invention, the catalyst is
used to adsorb and remove NOx in the lean exhaust gas, whereby NOx
in the exhaust gas is separated from 02.
[0047] Next, according to the present invention, with regard to the
oxidation reduction relation, namely the stoichiometric relation
between oxidation and reduction, which is constituted by the
gaseous oxidizing agents (02 and NOx etc.) and the gaseous reducing
agents (HC, CO, H2 etc.) in the exhaust gas, the gaseous reducing
agent is made equal to or larger than the gaseous oxidizing agent.
In this manner NOx adsorbed on the catalyst is reduced to N2
according to the catalytic-reaction with the gaseous reducing agent
such as HC.
[0048] NOx in the exhaust gas is substantially constituted of NO
and N02. The reaction property of N02 is rich in comparison with
that of NO. Accordingly, when NO is oxidized to NO2, the
adsorption-removal and the reduction of NOx in the exhaust gas are
performed easily.
[0049] The present invention includes a method of oxidizing and
removing NO.sub.x in the exhaust gas to N0.sub.2 by the coexistent
0.sub.2, and an oxidation means for attaining the above method
(such as means having NO oxidation function and the means for
providing an oxidation catalyst at a pre-stage of the
catalyst).
[0050] The reduction reaction of a chemically adsorbed NO.sub.2
according to the present invention will be described generally with
following reaction formulas: MO - NO 2 + HC .fwdarw. MO + N 2 + C
.times. .times. 0 2 + H 2 .times. 0 .fwdarw. MC .times. .times. 0 3
+ N 2 + H 2 .times. O ##EQU1## where M indicates a metal element
and MO--NO.sub.2 indicates a combination State Of N0.sub.2 of a
metal oxide surface. A reason for employing MC0.sub.3 as the
reduction generation substance will be explained later.
[0051] The above reaction is exothermic. Alkali metals and alkali
earth metals are used as the metal M, and the reaction heat is
estimated by representing Na and Ba respectively as follows, under
a standard condition (1 atmosphere, 25.degree. C.): 2 .times. NaNO
3 .function. ( s ) + 5 / 9 .times. C 3 .times. .times. H 6
.function. ( g ) .fwdarw. 2 .times. Na 2 .times. NO 3 + N 2
.function. ( g ) + 2 / 3 .times. CO 2 .function. ( g ) + 5 / 3
.times. CO 2 .function. ( g ) .function. [ - .DELTA. .times.
.times. H = 873 .times. .times. k .times. .times. joule ] ##EQU2##
Ba .function. ( NO 3 ) 2 .times. ( s ) + 5 / 9 .times. C 3 .times.
H 6 .function. ( g ) .fwdarw. BaCO 3 .function. ( s ) + N 2
.function. ( g ) + 2 / 3 .times. CO 2 .function. ( g ) + 5 / 3
.times. H 2 .times. O .function. ( g ) .function. [ - .DELTA.
.times. .times. H = 751 .times. .times. k .times. .times. joule ]
##EQU2.2## wherein, s indicates a solid state and g indicates a
gaseous state. Here, a thermodynamic value of the solid state is
used, as that of an adsorbed state.
[0052] It should be noted that the combustion heat of 5/9 mole
C.sub.3H.sub.6 is 1,070 k joule, and each of the above reactions is
exothermic. The heat, which matches that for the combustion beat of
HC, heat is transferred to the exhaust gas, and thus a local rise
in temperature of a surface of the catalyst can be restrained.
[0053] Where the catching agent for NO.sub.x is an NO.sub.x
absorbent, NO.sub.x which is taken into the mass of the absorbent
is reduced. Heat transfer to the exhaust gas is limited, causing a
rise in temperature of the absorbent. This exothermically generated
heat shifts the balance of the absorption reaction to that of
NO.sub.x discharging or NO.sub.x emission. absorption .fwdarw. MC
.times. .times. 0 3 .times. ( s ) + 2 .times. NO 2 + 1 / 20 2
.times. .0..0. .times. .times. M .function. ( NO 3 ) 2 + CO 2
.rarw. discharging ##EQU3##
[0054] Even though the concentration of the gaseous reducing agents
is increased to reduce rapidly NO.sub.x concentration in the
exhaust gas which is discharged to the outside of the absorbent,
the reaction between NO.sub.2 and HC in the gaseous phase does not
proceed.
[0055] Accordingly, the amount of discharged NO.sub.x cannot be
reduced fully by an increment of the gaseous reducing agents.
Further, at a stage where the adsorption amount of NO.sub.x is
small, a reduction operation may occur; however, since the
regeneration frequency of NO.sub.x absorbent increases, it is not
put to practical use.
[0056] The catalyst according to the present invention generates a
small absolute amount of exothermic heat so as to catch NO.sub.x
near its surface in accordance with chemical adsorption. Also the
rise in temperature of the catalyst is small, so as to transfer
heat rapidly to the exhaust gas. Accordingly, it is possible to
prevent the discharge of NO.sub.x after it is captured.
[0057] The catalyst according to the present invention utilizes a
material which catches NOx at or near its surface by chemical
adsorption, and does not cause NOx discharging or NO.sub.x emission
in accordance with the exothermic reaction during the reduction of
NO.sub.x.
[0058] Further, the catalyst according to the present invention
adsorbs NO.sub.x contained in lean exhaust gas at its surface, and
during the reduction of NO.sub.x it does not cause NO.sub.x
discharge in accordance with the lowering of the oxygen
concentration.
[0059] The inventors of the present invention have determined that
the above stated features can be realized using a catalyst which is
selected from at least one of alkali metals and alkali earth metals
(classified in an element periodic table) and at least one of noble
metals selected from platinum (Pt), rhodium (Rh) and palladium
(Pd), but does not contain barium (Ba). Preferably, the catalyst
according to the present invention includes at least one element
selected from potassium (K), sodium (Na), and strontium (Sr), and
noble metal elements.
[0060] In the exhaust gas purification apparatus according to the
present invention, a catalyst arranged at an exhaust gas flow
passage includes at least one element selected from potassium (K),
sodium (Na), magnesium (Mg), strontium (Sr) and calcium (Ca), as
well as noble metal elements.
[0061] In the exhaust gas purification apparatus, in the case of a
stoichiometric relation between oxidation and reduction of each of
the components included in the exhaust gas, the gaseous oxidizing
agent is equal to or greater than the gaseous reducing agent, and
NO.sub.x is chemically adsorbed on the catalyst. On the other hand,
when the gaseous reducing agent is equal to or greater than the
gaseous oxidizing agent, NO.sub.x which has been absorbed on the
catalyst is reduced, according to the catalytic-reaction with the
gaseous reducing agent, to harmless N.sub.2
[0062] The catalyst in the present invention can be applied
suitably in particular by following substances.
[0063] The composition is constituted of a metal and a metal oxide
substance (or a complex oxide substance) which contains at least
one element selected from potassium (K), sodium (Na), magnesium
(Mg), strontium (Sr) and calcium (Ca), at least one selected from
rare earth metals, and at least one element selected from the noble
metals including platinum (Pt), rhodium, (Rh), and palladium (Pd).
This composition, which is supported on a porous heat-withstanding
metal oxide substance, has a superior NO.sub.x adsorbing
ability.
[0064] As the earth metal element, cerium (Ce) or lanthanum (La),
particularly Ce, is preferable. The earth metal element has a
function for exhibiting the three component function to the
catalyst under the stoichiometric atmosphere or the rich
atmosphere.
[0065] At least one of titanium (Ti) and silicon (Si) can be added
to the catalyst according to the present invention, improving the
heat resistant property and SO.sub.x endurance property of the
catalyst. Ti or Si has a function for adsorbing or absorbing
SO.sub.x under the lean atmosphere, or for discharging the adsorbed
or absorbed SO.sub.x in a stoichiometric atmosphere or a rich
atmosphere.
[0066] In the catalyst of the present invention, alkali metals,
alkali earth metals, noble metals, rare earth elements, titanium
(Ti) and silicon (Si) are held on the porous support or porous
carrier member, which is supported or carried on a substance body.
For the purpose of heat resistance, .quadrature.-A1.sub.20.sub.1 is
preferably employed as the porous support. As the substance body, a
cordierite, mullite, a metal, for example, a stainless steel is
preferable.
[0067] As the crystal structure of Ti which is held on the porous
support, an amorphous oxide state is preferable. Further, in case
where the catalyst includes Si and alkali earth metals at the same
time, as both the crystal structures Si and alkali earth metals, an
amorphous oxide state is preferable.
[0068] In the catalyst of the present invention, it is preferable
to include in the porous support (porous carrier member), alkali
metals of 5-20 wt %, and alkali earth metals of 3-40 wt %. Further,
it is also preferable to include Pt of 0.5-3 wt %, Rh of 0.05-0.3
wt %, and Pd of 0.5-15 wt %, respectively. Mg prevents the cohesion
or condensation of the active components which are held on the
porous support, such as the noble metal.
[0069] It is also preferable to include the rare earth metals of
5-30 wt %, Ti of 0.1-30 wt %, and Si of 0.6-5 wt % as silica in the
porous support.
[0070] The present invention provides a catalyst which comprises,
on the porous support, sodium (Na), magnesium (Mg), and at least
one element selected from platinum (Pt), palladium (Pd), and
rhodium (Rh), as well as at least one selected from cerium (Ce) and
lanthanum (La). Further, the porous support also preferably
includes Na of 5-20 wt %, Mg of 1-40 wt % under a weight ratio
Mg/(Na+Mg), Pt of 0.5-3 wt %, Rh of 0.05-0.3 wt %, and Pd of 0.5-15
wt % are included.
[0071] In the exhaust gas purification apparatus according to the
present invention, to chemically adsorb NO.sub.x to the catalyst,
or to catalytically reduce the chemically adsorbed NO.sub.x, means
must be provided for controlling the stoichiometric relation
between oxidation and reduction by the gaseous oxidizing agents and
the gaseous reducing agents in the exhaust gas. By providing a
means for controlling the stoichiometric relation between oxidation
and reduction, it is possible to assure that the gaseous reducing
agent equals or exceeds the gaseous oxidizing agent. For example,
the combustion condition in the internal combustion engine can be
adjusted to a stoichiometric or a rich air-fuel ratio, or a gaseous
reducing agent can be added to the lean burn exhaust gas.
[0072] One method of achieving the former is to control the fuel
injection amount in accordance with the output of the oxygen
concentration sensor and the output of the intake air flow amount
sensor provided in the exhaust gas duct. In this method, some of
the cylinders are operated with a rich mixture and the remainder
are operated in a lean mixture. In the mixed components of the
exhaust gas all of the cylinders the gaseous reducing agent is
equal to or is greater than the gaseous oxidizing agent in the
stoichiometric relation between the oxidation and the
reduction.
[0073] The latter can be attained by each of following methods.
[0074] One method is to add a gaseous reducing agent to the exhaust
gas flow upstream of the catalyst. As the gaseous reducing agent,
gasoline, light gas oil, natural gas, reforming material thoseof,
hydrogen, alcohol materials and ammonium materials can be applied.
It is effective to introduce the blow-by gas and the canister
purging gas at the upstream of the catalyst and to add the gaseous
reducing agent such as hydrocarbon (HC) contained in the above
materials. In an internal combustion engine with direct fuel
injection, it is effective to inject the fuel during the exhausting
process and to add the fuel as the gaseous reducing agent.
[0075] As the catalyst in the present invention, various shapes can
be applied. In addition to a honeycomb shape which is obtained by
coating the catalyst components onto a honeycomb shaped member
comprised of cordierite or metal materials such as a stainless
steel, a pellet shape, a plate shape, a particle shape and a powder
shape can be applied.
[0076] In the present invention, the apparatus can provide means
for establishing the time when the gaseous reducing agent is equal
to or is greater than the gaseous oxidizing agent. The above timing
is obtained by each of following methods.
[0077] In one technique, in accordance with the air-fuel ratio
setting signal which is determined by ECU (engine control unit),
the engine rotation number signal, the intake air amount signal,
the intake air pipe pressure signal, the speed signal, the throttle
valve opening degree signal, the exhaust temperature etc., the
NO.sub.x discharging amount during lean operation is estimated and
the integration value thereof is exceeds over a predetermined
setting value.
[0078] In another method, in accordance with the signal of the
oxygen sensor (or A/F sensor) arranged upstream or downstream of
the catalyst in the exhaust gas flow passage, the accumulated
oxygen amount is detected, and the accumulated oxygen amount
exceeds over a predetermined amount. As a modified embodiment, the
accumulated oxygen amount during the lean operation time exceeds a
predetermined amount.
[0079] In another approach, in accordance with the signal of
NO.sub.x sensor arranged at the upstream of the catalyst in the
exhaust gas flow passage, the accumulated NO.sub.x amount is
detected, and the accumulated NO.sub.x amount during the lean
operation time exceeds over a predetermined amount.
[0080] In still another method, in accordance with the signal of
NO.sub.x sensor arranged at the downstream of the catalyst in the
exhaust gas flow passage, NO.sub.x concentration is detected, and
NO.sub.x concentration exceeds over a predetermined
concentration.
[0081] According to the present invention, further the apparatus
provides the means for establishing the maintenance time where the
gaseous reducing agent is equal to or exceeds the gaseous oxidizing
agent. The time during which the gaseous reducing agent excess
condition and the throw-in gaseous reducing agent amount is
maintained can be determined taking into consideration the
specifications and characteristics of the adsorbent and the
internal combustion engine. The above methods can be realized by
adjusting the stroke, the injection time and the injection interval
of the fuel injector.
[0082] Other objects, advantages and novel features of the present
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0083] FIG. 1 is a schematic illustration of a representative
embodiment of an exhaust gas purification apparatus for use in an
internal combustion engine according to the present invention;
[0084] FIG. 2 is a time lapse characteristic of NO.sub.x
purification rate where a rich operation and a lean operation are
repeated alternately in accordance with a method for purifying an
exhaust gas of an internal combustion engine according to the
present invention;
[0085] FIG. 3 shows the relationship between NO.sub.x concentration
and NO.sub.x purification rate in a lean exhaust gas;
[0086] FIG. 4 is NO.sub.x purification rate in a stoichiometric
exhaust gas;
[0087] FIGS. 5A and 5B show the relationships between an inlet port
NO.sub.x purification rate and an outlet port NO.sub.x purification
rate of a catalyst when a rich (stoichiometric) operation is
changed over to a lean operation;
[0088] FIGS. 6A and 6B shows the relationships between an inlet
port NO.sub.x purification rate and an outlet port NO.sub.x
purification rate of a catalyst when a rich (stoichiometric)
operation is changed over to a lean operation;
[0089] FIG. 7 a block diagram showing a method for controlling an
air-fuel ratio;
[0090] FIG. 8 is a flow chart showing a method for controlling an
air-fuel ratio;
[0091] FIG. 9 is a flow chart showing a method for accumulating
NO.sub.x discharging amount during a lean operation;
[0092] FIG. 10 is a flow chart showing NO.sub.x amount assumption
in the flow chart shown in FIG. 8;
[0093] FIG. 11 is a flow chart showing NO.sub.x amount assumption
in the flow chart shown in FIG. 8;
[0094] FIG. 12 is a flow chart showing NO.sub.x amount assumption
in the flow chart shown in FIG. 8;
[0095] FIG. 13 is a flow chart Showing NO.sub.x amount assumption
in the flow chart shown in FIG. 8;
[0096] FIG. 14 is a schematic depiction of an embodiment of an
exhaust gas purification apparatus in which a manifold catalyst is
provided;
[0097] FIG. 15 shows an embodiment of an exhaust gas purification
apparatus in a fuel direct injection in-cylinder engine;
[0098] FIG. 16 shows an embodiment of an exhaust gas purification
apparatus in which a post-catalyst is provided;
[0099] FIG. 17 shows an embodiment of an exhaust gas purification
apparatus in which a gaseous reducing agent is added to an upstream
of a catalyst;
[0100] FIGS. 18A, 18B and 18C are views showing NO.sub.x
purification characteristic where a mode operation is carried
out;
[0101] FIG. 19 is a graph showing an NO.sub.x purification
characteristic where an oxygen concentration is varied using a
model gas;
[0102] FIG. 20 is a graph showing an NO.sub.x purification
characteristic where an oxygen concentration is varied using a
model gas;
[0103] FIG. 21 shows an NO.sub.x purification rate for optimizing
Na supported amount;
[0104] FIG. 22 shows an NO.sub.x purification rate for optimizing
Mg supported amount;
[0105] FIG. 23 shows an NO.sub.x purification rate for optimizing
Ce supported amount;
[0106] FIG. 24 shows an NO.sub.x purification rate for optimizing
Rh and Pt supported amount;
[0107] FIG. 25 shows an NO.sub.x purification rate for optimizing
Pd and Pt supported amount; and
[0108] FIG. 26 shows an NO.sub.x purification rate for optimizing
Sr supported amount.
BEST MODE FOR CARRYING OUT THE INVENTION
[0109] Hereinafter, one embodiment of an exhaust gas purification
apparatus and an exhaust gas purification catalyst for use in an
internal combustion engine according to the present invention will
be explained. However, the present invention will not be limited
within the following embodiments.
Characteristics and Representative Embodiments of Catalyst
[0110] The characteristics of a catalyst for use in an internal
combustion engine in accordance with the present invention will now
be explained.
(Catalyst Preparation Method)
[0111] A catalyst N--N9 was obtained according to a following
method.
[0112] In a first embodiment, nitric acid alumina slurry was
obtained by mixing alumina powders with alumina-sol obtained by
peptizing boehmite by nitric acid. Then a cordierite honeycomb was
immersed in the slurry and the cordierite honeycomb was pulled up
rapidly. The slurry enclosed in a cell was removed by performing an
air-blow process, and after drying of the honeycomb the honeycomb
was calcined at 450.degree. C.
[0113] The above processes was carried out repeatedly and alumina
of 150 g per liter of an apparent volume of the honeycomb was
coated.
[0114] On this alumina coated honeycomb catalyst active components
were held and then a honeycomb shape catalyst was obtained.
[0115] In another example, the alumina coated honeycomb was
immersed in a cerium nitrate (Ce(NO.sub.3).sub.2) solution and the
honeycomb was dried at 200.degree. C. After drying, the honeycomb
was calcined for 1 (one) hour at 600.degree. C. In succession, it
is immersed in a mixture liquid comprised of sodium nitrate
(NaNO.sub.3) solution, titania-sol solution and magnesium nitrate
(Mg(NO.sub.3).sub.2) solution. The honeycomb was then dried at
200.degree. C. and calcined for 1 (one) hour at 600.degree. C.
[0116] In still another example, the honeycomb was immersed in a
solution comprised of dinitrodiamime Pt nitrate solution and
rhodium nitrate (Rh(NO.sub.3).sub.2) solution. The honeycomb was
then dried at 200.degree. C. the honeycomb, and calcined for 1
(one) hour at 450.degree. C. Finally, it was immersed in a
magnesium nitrate (Mg(NO.sub.3).sub.2) solution, dried at
200.degree. C., and calcined for 1 (one) hour at 450.degree. C.
[0117] With the above process, a honeycomb shaped catalyst was
obtained, in which Ce, Mg, Na, Ti, Rh and Pt were held on alumina
(Al.sub.20.sub.3) and is 2Mg-(0.2Rh, 2.7Pt)-(18Na, 4Ti,
2Mg)-27ce/Al.sub.20.sub.3. Herein, Al.sub.20.sub.3 shows that the
active component was held on Al.sub.20.sub.3; the numerical value
preceding the element is the weight (g) of the indicated metal
component which was supported per 1 (one) liter of the apparent
volume of the honeycomb.
[0118] An expressed order shows a support order or a carrier order
and the elements were held in accordance with an order of the
component for separating from the expressed component near to
Al.sub.2O.sub.3, namely, on Al.sub.2O.sub.3, 27Ce, (18Na, 4Ti,
2Mg), (0.2Rh, 2.7Pt), and 2Mg are held in sequence, and the
component bound up with parenthesis were supported at the same
time. The amount of each of the respective active components which
was deposited onto the carrier can be varied by varying the active
component concentration in the immersed solution.
[0119] A catalyst N--K9 was prepared according to the following
method.
[0120] In place of sodium nitrate (NaNO.sub.3) solution used in the
preparation for the catalyst N--N9, potassium nitrate (KNO.sub.3)
solution was used; otherwise a method was used which was similar to
that for preparing the catalyst N--N9. As a result, a catalyst
N--K9 comprised of 2Mg-(0.2Rh, 2.7Pt)-(18K, 4Ti,
2Mg)-27Ce/Al.sub.2O.sub.3, was obtained.
[0121] Further, with a similar method, a comparison catalyst N--R2
comprised of 2Mg-(0.2Rh, 2.7Pt)-27Ce/Al.sub.2O.sub.3 was
obtained.
(Performance Evaluation Method)
[0122] After the catalysts obtained by the above stated methods
were thermally treated at 700.degree. C. under an oxidizing
atmosphere for 5 (five) hours, the characteristics were evaluated
using the following methods.
[0123] On an automobile in which a lean burn gasoline engine was
mounted having the displacement volume of 1.8 liters, the honeycomb
shape catalysts having a volume of 1.7 liters prepared using the
above stated methods were mounted, and the NO.sub.x purification
characteristics were evaluated.
(Characteristics of Catalysts)
[0124] When the catalyst N--N9 was mounted, and a 30 second period
of rich operation (having A/F=13.3) and an about 20 minutes period
of a lean operation (having A/F=22) were repeated alternately, the
NO.sub.x purification characteristics shown in FIG. 2 were
obtained.
[0125] When the fuel is gasoline, a mass composition of the
gasoline comprises carbon (C) of about 85.7% and hydrogen (H) of
about 14.3%. Herein, when the gasoline of F (g) is burned, carbon
(C) of 0.857.times.F (g) reacts with oxygen (0.sub.2) of 0.857 F/12
mol and hydrogen (H) of 0.143.times.F (g) reacts with oxygen
(0.sub.2) of 0.143 F/4 mol. A necessary air amount A (g) is
expressed as following:
A=((0.857/12+0.143/4)F.times.28.8)/0.21=14.7F wherein, 28.8 is air
molecular amount (g/mol) and 0.21 is oxygen (O.sub.2) amount rate
in air. A/F=14.7 indicates a stoichiometric air-fuel ratio.
[0126] Accordingly, when A/F=13.3, to the fuel (a gaseous reducing
agent) of F (g), it shows an insufficient amount (a reduction
atmosphere) of the air (the gaseous oxidizing agent) of 1.39 F (g).
On the other hand in case of A/F=22, to the fuel of F (g), it shows
an excess (an oxidization atmosphere) of the air (the gaseous
oxidizing agent) of 7.31 F (g).
[0127] As shown in FIG. 2, NO.sub.x during the lean operation time
was purified in accordance with this catalyst.
[0128] The NO.sub.x purification rate during lean operation
decreased gradually; that is, the initial purification rate of 100%
decreased to about 40% after 20 minutes. However, the reduced
purification rate recovered to 100% after 30 seconds of rich
operation. When lean operation was carried out again, NO.sub.x
purification performance was recovered and the above stated change
in time lapse was repeated.
[0129] Even when the lean operation and the rich operation were
repeated in plural times, the velocity of the lowering in time
lapse of NO.sub.x purification rate during the lean operation was
unchanging. Thus, the NO.sub.x absorption performance was fully
regenerated according to the rich operation.
[0130] The vehicle speed was constant at about 40 km/h (the exhaust
gas space velocity (SV) was constant at about 20,000/h) and
NO.sub.x concentration in the exhaust gas was varied by varying the
ignition periods. The relationship between NO.sub.x concentration
and NO.sub.x purification rate in the lean exhaust gas was
determined, and is shown in FIG. 3.
[0131] NO.sub.x purification rate decreased with time; however, the
more NO.sub.x concentration lowered, the lower the rate of
decrease. The amount of NO.sub.x that was removed prior to NOx
purification rates of 50% and 30% was estimated through FIG. 3, and
is shown in Table 1. TABLE-US-00001 TABLE 1 NO.sub.x concentration
NO.sub.x amount purified NO.sub.x amount purified in inlet port
until purification until purification exhaust gas (ppm) rate 50%
(mol) rate 30% (mol about 50 0.030 0.041 about 120 0.031 0.047
about 230 0.030 0.045 about 450 0.030 0.042 about 550 0.026
0.038
[0132] NO.sub.x amounts which were caught were substantially
constant, regardless of NO.sub.x concentration. The absorption
amount did not depend to the concentration (pressure) of the
adsorbent, which is a feature of the chemical adsorption.
[0133] In the test catalysts, first Pt particles were used as an
NO.sub.x absorption medium. A CO adsorption amount evaluation which
is frequently used as means for evaluating the exposed Pt amount
was carried out; and CO adsorption amount (at 100.degree. C.) was
found to be 4.5.times.10.sup.-4 mol. This value was about 1/100 of
the above stated NO.sub.x adsorption amount; and therefore it was
clear that Pt does not work as a main role for NO.sub.x
adsorbent.
[0134] On the other hand, BET surface area (measured by nitrogen
adsorption) of this catalyst (including cordierite as a substrate)
was measured about 25 m.sup.2/g and was 28,050 m.sup.2 per the
honeycomb 1.7 liters. Further, the chemical structure of the Na
contained in the catalyst according to the present invention was
studied. From the facts that C0.sub.2 gas was generated and
dissolved in an inorganic acid, and from a value of a point of
inflection in a pH neutralization titration curve line, it was
determined that Na existed mainly as Na.sub.2CO.sub.3.
[0135] Supposing all of the surface was occupied by
Na.sub.2CO.sub.3, Na.sub.2CO.sub.3 of 0.275 mol would be exposed on
the surface. (As the specific gravity of Na.sub.2CO.sub.3 is 2.533
g/mol, the volume of one molecular of Na.sub.2CO.sub.3 can be
estimated. Supposing Na.sub.2CO.sub.3 is a cube, an area of one
face of Na.sub.2CO.sub.3 was estimated and the estimated area was
used as the occupation area of the surface Na.sub.2CO.sub.3).
[0136] In accordance with the above stated reaction formula,
Na.sub.2CO.sub.3 of 2.75 mol has an ability to adsorb N0.sub.2 of
0.55 mol. However, the amount of NO.sub.x which had been removed by
the catalyst according to the present invention is on the order of
0.04 mol, which is less than 1/10 of the ability.
[0137] The above difference attributed the cause that BET method
evaluated the physical surface area and the surface area of
Al.sub.20.sub.3 with the surface area of Na.sub.2CO.sub.3 was
evaluated.
[0138] The above stated evaluations show the adsorbed NO.sub.x
amount was far greater than the NO.sub.x catching power of
Na.sub.2CO.sub.3 bulk, and at least NO.sub.x was caught at a
limited area of the surface or the vicinity of the surface of
Na.sub.2CO.sub.3.
[0139] Further, in FIG. 3, the rate of decrease of the purification
rate lowered at about NO.sub.x purification rate of 20%, however
this shows that the reducing reaction caused according to the
catalyst function.
[0140] FIG. 4 shows the NO.sub.x purification rate immediately
after the lean operation has changed over to stoichiometric
operation.
[0141] FIGS. 5A and 6A show the NO.sub.x purification
characteristic when the lean operation has changed over to the
stoichiometric operation; and FIGS. 5B and 6B show the NO.sub.x
purification characteristic when the lean operation has changed
over to rich operation.
[0142] FIGS. 5A and 5B shows NO.sub.x concentrations of an inlet
port and an outlet port of the catalyst N--N9. FIG. 5A shows the
case where the air-fuel ratio is changed from lean operation at
A/F=22 to stoichiometric operation at A/F=14.2. At A/F=14.2, to the
fuel of F (g), it shows a shortage of air (the gaseous oxidizing
agent) of 0.5 F (g); That is, a reduction atmosphere.
[0143] Regeneration starts immediately after the stoichiometric
change-over. Since the exhaust gas NO.sub.x concentration in the
exhaust gas of A/F=14.2 is high at that time, the inlet port
NO.sub.x concentration in the stoichiometric operation increases
substantially, and at the same time the outlet NO.sub.x
concentration increases in a transient manner. However, the outlet
port NO.sub.x concentration is always substantially lower than the
inlet port NO.sub.x concentration. The regeneration proceeds
rapidly, and the outlet port NO.sub.x concentration approaches zero
(0) within a short time.
[0144] FIG. 5B shows case where the air-fuel ratio is changed over
from lean operation at A/F=22 to rich operation at A/F=13.2.
Similarly to FIG. 5A, the outlet port NOx concentration is always
much lower than the inlet port NOx concentration. Further, the
outlet port NOx concentration reaches the vicinity of zero (0)
within a shorter time.
[0145] As clearly understood from the above, the A/F value during
regeneration affects the time required for regeneration. A/F value,
the time and gaseous reducing agent amount suitable for the
regeneration are influenced by the composition (such as the shape,
temperature and SV value) of the catalyst, the kind of the gaseous
reducing agent, and the shape and the length of the exhaust gas
flow passage. Accordingly, the regeneration condition is determined
overall by all of the above stated items.
[0146] FIGS. 6A and 6B show NOx concentrations of an inlet port and
an outlet port of the catalyst N--K9. FIG. 6A shows case where the
air-fuel ratio is changed from lean operation (A/F=22) to
stoichiometric operation (A/F=14.2), and FIG. 6B shows case where
the air-fuel ratio is changed over from lean operation at A/F 22 to
rich operation at A/F=13.2.
[0147] Similarly to case of the above catalyst N--N9, the outlet
port NOx concentration is always much less than the inlet port NOx
concentration, further the regeneration of the catalyst proceeds
within a short time.
(Basic Characteristics of Catalyst)
[0148] Using a model gas, the basic characteristics (in particular,
the effect of oxygen concentration on the NO.sub.x purification
rate) were evaluated.
[0149] The cordierite honeycomb of the catalyst N--N9 having 6 ml
was filled up in a silica reaction tube having an inner diameter of
28 mm and the model gas was passed through the tube. The oxygen
concentration in the model gas was varied, and the effect on the
NOx purification rate was studied. The reaction temperature was
300.degree. C. at the inlet port gas temperature of the
catalyst.
[0150] Initially, the gas composition comprises of 0.sub.2 of 5%
(volume ratio: same hereinafter), NO of 600 ppm, C.sub.3H.sub.4 of
500 ppm (1,500 ppm as Cl), CO of 1,000 ppm, CO2 of 10%, H2O of 10%,
and the balance of N2. After ten (10) minutes, when NOx
purification rate has been stable, the oxygen concentration was
lowered to a predetermined value and maintained for twenty (20)
minutes. Finally, the gas mixture was then returned again to its
initial composition. NOx concentration change for the above time
interval was varied with six different 0.sub.2 concentrations, (0%,
0.5%, 0.7%, 1%, 2% and 3%), and the graphs shown in FIG. 19 was
obtained.
[0151] FIG. 19 shows that when the oxygen concentration in the lean
gas was decreased (that is, the oxidization atmosphere became weak
and the reduction atmosphere became strong), it was admitted that
NOx purification rate tended to increase. This suggests that this
catalyst purifies NOx by way of reduction.
[0152] Further, in FIG. 19, the NOx purification rate was positive
at all times, and accordingly, the catalyst NOx concentration
passed through the catalyst did not increase, regardless of the
oxygen concentration.
[0153] Next, a gas comprising 0.sub.2 of 5%, NO of 600 ppm, and the
balance of N.sub.2 was used, and the oxygen concentration was
similarly varied. FIG. 20 shows the results. In this examination,
the gaseous reducing agent was not included in the gas. In FIG. 20,
by lowering the oxygen concentration and even the oxidization
atmosphere was weakened, then NO.sub.x purification rate did not
improve. This suggests that this catalyst purifies NO.sub.x by
reduction.
[0154] In FIG. 20, the NO.sub.x purification rate did not have a
positive value. Further in this catalyst according to the
oxidization atmosphere caught NO.sub.x is not discharged or
emitted.
An Exhaust Gas Purification Apparatus
[0155] FIG. 1 shows one embodiment of an exhaust gas purification
apparatus according to the present invention, which comprises an
air intake system having an engine 99 which is suitable for lean
burn operation, an air flow sensor 2, a throttle valve 3 etc. The
exhaust system has an oxygen concentration sensor 19 (or A/F
sensor), an exhaust gas temperature sensor 17 and a catalyst 18
etc., and an engine control unit (ECU) etc.
[0156] ECU comprises an I/O LSI as an input/output interface, an
execution processing unit MPU, memory units (RAM and ROM) for
storing many control programs, and a timer counter, etc.
[0157] The above stated exhaust gas purification apparatus
functions as follows. After the intake air to the engine 99 is
filtered through an air cleaner 1, it is metered by the air flow
sensor 2. Further, the air passes through the throttle valve 3, is
received for fuel injection through an injector 5, and supplied to
the engine 99 as an air-fuel mixture. The signals of the air flow
sensor 2 and others are inputted into ECU (engine control
unit).
[0158] In the ECU, by the latter stated method the operation
conditions of the internal combustion engine and of the catalyst
are evaluated, and an air-fuel ratio is determined. By controlling
the injection period etc. of the injector 5, the fuel concentration
of the air-fuel mixture is established at a predetermined
value.
[0159] The air-fuel mixture sucked into the cylinders is ignited
and burned by means of an ignition plug 6, which is controlled
according to signals from ECU. The combustion exhaust gas is led to
an exhaust gas purification system, which includes a catalyst 18.
During stoichiometric operation NO.sub.x, HC and CO are purified
according to the three component catalyst function. During lean
operation NO.sub.x is purified by adsorption and at the same time
HC and CO are purified by combustion.
[0160] Further, in accordance with the judgment and the control
signal of the ECU, the NO.sub.x purification ability of the
catalyst 18 is monitored continuously during lean operation, and
when its NO.sub.x purification capacity lowers, the air-fuel ratio
etc. is shifted to the rich side, so that the NO.sub.x purifying
ability is recovered. With the above stated operation, in this
apparatus, the exhaust gas is purified effectively under all engine
combustion conditions, including lean operation and stoichiometric
operation (including rich operation).
[0161] The fuel concentration (the air-fuel ratio) of the air-fuel
mixture supplied to the engine is controlled as shown in the block
diagram of FIG. 7.
[0162] Based on signals from an accelerator pedal depression
sensor, an intake air amount metered by the air flow sensor 2, an
engine rotational speed detected by a crank angle sensor, a
throttle sensor signal for detecting the throttle valve opening, an
engine water coolant temperature signal, a starter signal etc., ECU
25 determines the air-fuel ratio (A/F), and this signal is
compensated according to a signal which is fed-back from the oxygen
sensor.
[0163] In cases of low temperature, an idling time and a high load
time etc., the above feed-back control according to the signal of
the respective sensor and the respective switching means is
stopped. Further, the apparatus has an air-fuel ratio compensation
leaning function which enables the apparatus to adapt accurately to
both delicate and abrupt changes of the air-fuel ratio.
[0164] When the determined air-fuel ratio is stoichiometric
(A/F=14.7) or rich (A/F<14.7), the injection conditions for the
injector 5 are determined by the ECU, and thereby the
stoichiometric operation and the rich operation are carried
out.
[0165] On the other hand, when lean operation (A/F>14.7) is
detected, the NO.sub.x adsorbing ability of the catalyst 18 is
evaluated. When it is judged that the apparatus has adsorbing
ability, the fuel injection amount for carrying out lean operation
is determined. When, however, it is judged that the apparatus has
no adsorbing ability, the air-fuel ratio is shifted to the rich
side for a predetermined period, and the catalyst 18 is
regenerated.
[0166] FIG. 8 shows a flow chart of the air-fuel ratio control. In
step 1002, signals indicating various operation conditions are read
in. Based on these signals, in step 1003, the air-fuel ratio is
determined, and in step 1004 the determined air-fuel ration is
detected. In step 1005, the determined air-fuel ratio is compared
to the stoichiometric air-fuel ratio.
[0167] The stoichiometric air-fuel ratio used for this comparison,
to put it more precisely, is the air-fuel ratio in which the
velocity of the catalytic reduction reaction of NO.sub.x in the
catalyst exceeds the NO.sub.x pick-up velocity in accordance with
the adsorption. This stoichiometric air-fuel ratio is determined by
evaluating the characteristics of the catalyst in advance, and the
air-fuel ratio of the vicinity of the stoichiometric air-fuel ratio
is selected.
[0168] Herein, when the established air-fuel ratio is equal to or
less than the stoichiometric air-fuel ratio, processing advances to
step 1006, and without regeneration operation of the catalyst the
air-fuel ratio operation followed by the indication is carried
out.
[0169] If the established air-fuel ratio is greater than the
stoichiometric air-fuel ratio, the NO.sub.x adsorption amount is
estimated in step 1007, and in step 1008, the estimated amount is
compared with a predetermined limitation amount.
[0170] The limitation adsorption amount is set to a value which
permits NO.sub.x in the exhaust gas to be fully purified, based on
experimentally determined NO.sub.x removal characteristics of the
catalyst, taking into account the exhaust gas temperature and the
catalyst temperature etc.
[0171] If the apparatus retains NO.sub.x adsorbing ability,
processing advances to step 1006, without the regeneration of the
catalyst the air-fuel ratio operation followed by the indication is
carried out. If, however, the apparatus has no NO.sub.x adsorbing
ability, processing goes to step 1009, and the air-fuel ratio is
shifted to the rich side. In step 1010, the rich shift time is
counted, and when the elapsed time Tr exceeds a predetermined time
(Tr).sub.c, the rich shift finishes.
[0172] The evaluation of NO.sub.x adsorbing ability of the catalyst
will be carried out as following.
[0173] FIG. 9 shows a method for judging and accumulating the
NO.sub.x discharge amount according to the various kinds operation
conditions during the lean operation time.
[0174] In step 1007-EO1, the signals relating to the working
conditions of the catalyst such as the exhaust gas temperature and
various kinds of engine operation conditions which affect to
NO.sub.x concentration in the exhaust gas are read in, and NO.sub.x
amount E.sub.n that is adsorbed in a unit time is estimated.
[0175] In step 1007-EO2, NO.sub.x amount E.sub.n is accumulated,
and in step 1008-EO1 the accumulated value .SIGMA.E.sub.n is
compared with the upper limitation value (E.sub.n).
[0176] When the accumulated value .SIGMA.E.sub.n is equal to or is
less than the upper limitation value (E.sub.n).sub.c, the
accumulation is continued; but if the accumulated value
.SIGMA.E.sub.n exceeds the upper limitation value (E.sub.n).sub.c
the accumulation is released in step 1008-EO2 and the process
advances to step 1009.
[0177] FIG. 10 shows a method for evaluating the NO.sub.x adsorbing
ability of the catalyst according to the accumulated lean operation
time.
[0178] In step 1007-HO1, the lean operation time is accumulated,
and in step 1008-HOl the value .SIGMA.H.sub.L is compared with an
upper limitation value (H.sub.L).sub.c.
[0179] When the accumulated value .SIGMA.H.sub.L is equal to or is
less than the upper limitation value (H.sub.L).sub.c accumulation
is continued; however, when the accumulation value .SIGMA.H.sub.L
exceeds the upper limitation value (H.sub.L).sub.c the accumulation
is released in step 1008-HO2, and processing goes to step 1009.
[0180] FIG. 11 shows a method for evaluating the NO.sub.x adsorbing
ability of the catalyst according to the oxygen sensor signal
during the lean operation time.
[0181] In step 1007-001, the oxygen amount Qo is accumulated, and
in step 1008-001 the accumulated value .SIGMA.Qo is compared with
an upper limitation value (Qo)c.
[0182] If the accumulated value .SIGMA.QN is equal to or is less
than the upper limitation value (QO)c, the accumulation is
continued, but when the accumulation value .SIGMA.QO exceeds the
upper limitation value (QO)c the accumulation is released in step
1008-002, and processing advances to step 1009.
[0183] FIG. 12 shows a method for evaluating the NOx adsorbing
ability of the catalyst according to the NOx concentration sensor
signal which is detected in the inlet port of the catalyst during
lean operation.
[0184] In step 1007-NO1, the NOx amount QN in the inlet port of the
catalyst is accumulated based on NOx concentration sensor signal.
In step 1008-NO1 the accumulated value .SIGMA.QN is compared with
the upper limitation value (QN)c. If the accumulated value
.SIGMA.QN is equal to or is less than the upper limitation value
(QN)c, the accumulation is continued; but when the accumulation
value .SIGMA.QN exceeds the upper limitation value (QN)c the
accumulation is released in step 1008-002, and processing goes to
step 1009.
[0185] FIG. 13 shows a method for judging NOx adsorbing ability
according to NOx concentration sensor signal which is detected in
the outlet port of the catalyst during the lean operation time.
[0186] In step 1007-CO1, NOx amount CN in the outlet port of the
catalyst is accumulated according to NOx concentration sensor
signal. In step 1008-CO1 the size between the accumulation value
.SIGMA.CN and the upper limitation value (CN)c of the accumulated
NOx amount is compared.
[0187] When the accumulated value .SIGMA.CN is equal to or is less
than the upper limitation value (CN)c the accumulation is
continued; but if the accumulated value .SIGMA.CN is more than the
upper limitation value (CN)c, the process advances to step
1009.
[0188] FIG. 14 shows another embodiment of an exhaust gas
purification apparatus according to the present invention. The
structural difference between the embodiment shown in FIG. 1 and
this embodiment is the provision of a manifold catalyst 17 which is
arranged on the exhaust air duct in the vicinity of the engine
99.
[0189] Reinforcement of the discharging regulation of the exhaust
gas in the automobile is necessary to purify the harmful materials
such as HC which are discharged immediately after the engine
starting time. In the prior technique, however, harmful materials
are discharged without treatment until the catalyst reaches to the
working temperature. It is thus necessary to reduce substantially
the non-treated amount of the harmful materials. A method for
abruptly heating the catalyst to the working temperature is
effective for this purpose.
[0190] FIG. 14 shows apparatus which diminishes the amount of HC
and CO which is discharged during engine starting, and also
provides exhaust gas purification during lean and stoichiometric
operation (including rich operation time).
[0191] In the construction shown in FIG. 14, the manifold catalyst
17, may be a three component catalyst comprised mainly of Pt, Rh,
and Ce02, a material in which Pd is added to the three component
catalyst, and a combustion catalyst which is comprised of
combustion active components such as Pd as a main component. In
this construction, during the engine starting the temperature of
the manifold catalyst 17 rises rapidly and the purification of HC
and CO is carried out immediately after engine starts.
[0192] During stoichiometric operation both the manifold catalyst
17 and the catalyst 18 function, and purification for HC, CO and
NO.sub.x is carried out. During lean operation the catalyst 18
adsorption-purifies NO.sub.x.
[0193] For regeneration of the catalyst 18, the air-fuel ratio is
shifted to the rich side. Because HC and CO (gaseous reducing
agents) are largely chemically changed by the manifold catalyst 17,
they can reach the catalyst 18 and regenerates it. The above stated
construction, which includes the catalyst 18 is thus an important
advantageous feature.
[0194] FIG. 15 shows a further embodiment of an exhaust gas
purification apparatus according to the present invention. The
difference between the construction shown in FIG. 1 and that of
FIG. 15 is that the engine 99 is a direct fuel injection system. As
this figure demonstrates, the apparatus according to the present
invention, can suitably apply to the direct fuel inject system
engine.
[0195] FIG. 16 shows a further embodiment of an exhaust gas
purification apparatus according to the present invention. The
difference between this construction and the ones shown in FIG. 1
and FIG. 15 is that a post-catalyst element 24 is provided
downstream of the catalyst 18. For example, by arranging the
combustion catalyst on the post-catalyst element 24, HC
purification ability can be improved. Further, arrangement of the
three way catalyst on the post-catalyst 24 can reinforce the three
way function during the stoichiometric operation.
[0196] FIG. 17 shows a further embodiment of an exhaust gas
purification apparatus according to the present invention. The
difference between this construction and the ones shown in FIG. 1
and FIGS. 14-16 resides is that upon indication of a rich-shift,
the fuel is added upstream of the catalyst 18 through a gaseous
reducing agent injector 23. In this system, the desired operation
conditions of the engine can be established regardless of the
conditions of the catalyst 18.
[0197] Hereinafter, the effects achieved by the present invention
will be explained by reference to the graphic depictions in FIGS.
18-26.
[0198] The exhaust gas purification performance of the catalyst and
of the exhaust gas purification apparatus according to the present
invention were evaluated as follows: A first test catalyst and a
comparison catalyst were mounted on a lean burn specification
automobile having a displacement volume of 1.8 liters; and the
automobile was run on a chassis dynamometer. Both the test
catalysts were of a honeycomb shape, having 400 cell/in.sup.2 and a
volume of 1.7 liters, they were heat-treated at 700.degree. C.
under an oxidization atmosphere and were put below the floor.
[0199] The running speed was kept constant at 10-15 modes running,
based on the Japanese exhaust gas regulation measurement method.
The exhaust gas was analyzed using two methods. In a first method,
NO.sub.x, HC, and CO concentrations in the exhaust gas were
measured and analyzed directly using the automobile exhaust gas
measurement apparatus. The second method provided for measuring CVS
(constant volume sampling) value, using the automobile constant
volume dilution sampling apparatus.
[0200] Further, in 10-15 mode running, the A/F ratio was maintained
in the lean range (A/F=22-23) during constant speed running time,
during acceleration from 20 km/h to 40 km/h at 10 mode, and
acceleration time from 50 km/h to 70 km/h at 15 mode. The rest of
the running was performed at the stoichiometric A/F ratio.
[0201] FIGS. 18A-18C show NO.sub.x concentrations at the inlet port
and the outlet port of the catalyst at the last of the 10 modes
(which were repeated three times) and 15 mode which succeeded the
last 10 mode, using the catalyst N--N9 according to the present
invention (FIG. 18A), the catalyst N--K9 according to the present
invention (FIG. 18B), and the catalyst N--R2 according to the
comparison example (FIG. 18C). The comparison catalyst had the
composition shown in Table 2 (below).
[0202] In FIGS. 18A and 18B, it can be seen that for the catalysts
N--N9 and N--K9, the outlet port NO.sub.x concentration was lower
than the inlet port NO.sub.x concentration at all operation areas.
Since lean operation and the stoichiometric operation were carried
out repeatedly, the catalyst was regenerated effectively and
NO.sub.x purification ability was held and continued. On the other
hand, in the comparison catalyst N--R2, it can be seen that during
a portion of the time the outlet port NO.sub.x concentration
exceeded the inlet port NO.sub.x concentration.
[0203] Tables 2 and 3 show the CVS value obtained by the various
kinds of the catalyst and the comparison catalyst, together with
the catalyst compositions. The preparations for the catalyst and
the comparison catalyst were performed by the above stated methods.
However, as the preparation raw materials, barium nitrate was used
as barium (Ba) and silica-sol was used as silicon (Si). It is
assumed that Si exists a silica (SiO.sub.2) or a complex oxide
thereof. TABLE-US-00002 TABLE 2 CVS value (g/km) marks composition
NO.sub.x HC CO comparison N-R1 (0.2Rh,2.7Pt)--27Ce/Al.sub.2O.sub.3
0.15 0.02 0.07 catalyst N-R2
2Mg--(0.2Rh,2.7Pt)--27Ce/Al.sub.2O.sub.3 0.15 0.02 0.04 adsorption
N-S1 2Mg--(0.2Rh,2.7Pt)--30Sr--27Ce/Al.sub.2O.sub.3 0.08 0.10 0.08
catalyst N-S2 2Mg--(0.2Rh,2.7Pt)--(30Sr,2Mg)--27Ce/Al.sub.2O.sub.3
0.09 0.05 0.08 N-S3 (0.2Rh,2.7Pt)--(30Sr,4Ti)--27Ce/Al.sub.2O.sub.3
0.11 0.08 0.11 N-S4
2Mg--(0.2Rh,2.7Pt)--(30Sr,4Ti)--27Ce/Al.sub.2O.sub.3 0.10 0.07 0.09
N-S5 2Mg--(0.2Rh,2.7Pt)--(30Sr,4Si)--27Ce/Al.sub.2O.sub.3 0.10 0.08
0.09 N-N1 2Mg--(0.2Rh,2.7Pt)--18Na--27Ce/Al.sub.2O.sub.3 0.06 0.08
0.03 N-N2 2Mg--(0.2Rh,2.7Pt)--(18Na,2Mg)--27Ce/Al.sub.2O.sub.3 0.06
0.12 0.04 N-N3 (0.2Rh,2.7Pt)--(18Na,4Ti)--27Ce/Al.sub.2O.sub.3 0.07
0.16 0.10 N-N4 2Mg--(0.2Rh,2.7Pt)--(18Na,4Ti)--27Ce/Al.sub.2O.sub.3
0.06 0.15 0.08 N-N5 (0.2Rh,2.7Pt)--(18Na,4Si)--27Ce/Al.sub.2O.sub.3
0.05 0.10 0.12 N-N6
2Mg--(0.2Rh,2.7Pt)--(18Na,4Si)--27Ce/Al.sub.2O.sub.3 0.06 0.08 0.06
N-N7 2Mg--(0.2Rh,2.7Pt)--(10Na,10Sr)--27Ce/Al.sub.2O.sub.3 0.05
0.10 0.05 N-N8 (0.2Rh,2.7Pt)--(18Na,4Ti,2Mg)--27Ce/Al.sub.2O.sub.3
0.07 0.12 0.07 N-N9
2Mg--(0.2Rh,2.7Pt)--(18Na,4Ti,2Mg)--27Ce/Al.sub.2O.sub.3 0.04 0.11
0.04 N-N10 2Mg--(0.2Rh,2.7Pt)--(10
Na,4Ti,2Mg)--27Ce/Al.sub.2O.sub.3 0.04 0.06 0.04
[0204] TABLE-US-00003 TABLE 3 CVS value (g/km) marks composition
NO.sub.x HC CO adsorbtion N-K1
2Mg--(0.2Rh,2.7Pt)--18K--27Ce/Al.sub.2O.sub.3 0.06 0.08 0.03
catalyst N-K2 2Mg--(0.2Rh,2.7Pt)--(18K,2Mg)--27Ce/Al.sub.2O.sub.3
0.05 0.10 0.05 N-K3 (0.2Rh,2.7Pt)--(18K,4Si)--27Ce/Al.sub.2O.sub.3
0.08 0.11 0.06 N-K4
2Mg--(0.2Rh,2.7Pt)--(18K,4Si)--27Ce/Al.sub.2O.sub.3 0.05 0.08 0.05
N-K5 2Mg--(0.2Rh,2.7Pt)--(18K,4Si)--27Ce/Al.sub.2O.sub.3 0.06 0.08
0.05 N-K6 (0.2Rh,2.7Pt)--(18K,10Sr)--27Ce/Al.sub.2O.sub.3 0.07 0.12
0.08 N-K7 2Mg--(0.2Rh,2.7Pt)--(18K,4Ti,2Mg)--27Ce/Al.sub.2O.sub.3
0.05 0.10 0.05 N-K8
(0.2Rh,2.7Pt)--(18K,4Ti,2Mg)--27Ce/Al.sub.2O.sub.3 0.05 0.10 0.07
N-K9 2Mg--(0.2Rh,2.7Pt)--(18K,4Ti,2Mg)--27Ce/Al.sub.2O.sub.3 0.05
0.07 0.04 N-K10
2Mg--(0.2Rh,2.7Pt)--(10K,10Sr,2Mg)--27Ce/Al.sub.2O.sub.3 0.04 0.07
0.08 N-M1 2Mg--(0.2Rh,2.7Pt)--(10Na,10K,4Ti)--27Ce/Al.sub.2O.sub.3
0.05 0.05 0.08 N-M2
2Mg--(0.2Rh,2.7Pt)--(10Na,10K,10Si)--27Ce/Al.sub.2O.sub.3 0.06 0.05
0.08 N-M3 2Mg--(0.2Rh,2.7Pt)--(10Na,10K,10Sr)--27Ce/Al.sub.2O.sub.3
0.06 0.10 0.05 N-M4
2Mg--(0.2Rh,2.7Pt)--(10Na,10K,4Ti,2Mg)--27Ce/Al.sub.2O.sub.3 0.05
0.11 0.05
[0205] For the various kinds of the catalyst, the exhaust gas
purification performances were measured using a model exhaust
gas.
Modified Catalysts
Experiment 1:
(Preparation Method)
[0206] The alumina powders and alumina nitrate slurry as a
precursor thereof were coated on a cordierite honeycomb (400
cell/in.sup.2), and the honeycomb had deposited thereon an alumina
coating of 150 g per liter of apparent volume of the honeycomb. The
alumina coated honeycomb was then impregnated with a cerium nitrate
solution, dried at 200.degree. C., and calcined for 1 (one) hour at
600.degree. C. Thereafter, the honeycomb was impregnated with a
mixture liquid comprised of sodium nitrate (NaNO.sub.3) solution
and magnesium nitrate (MgNO.sub.3) solution was similarly dried and
calcined.
[0207] Next, the honeycomb was impregnated by a mixture solution
comprised of dinitrodiamime platinum nitrate solution and rhodium
nitrate (Rh(NO.sub.3).sub.2) solution, dried at 200.degree. C. and
calcined for 1 (one) hour at 450.degree. C. Finally, a magnesium
nitrate (Mg(N0.sub.3).sub.2) solution was impregnated, and the
honeycomb was dried at 200.degree. C. and calcined for 1 (one) hour
at 450.degree. C.
[0208] With the above stated method, Embodiment Catalyst 1 was
obtained, in which to 100 wt % Al.sub.2O.sub.3, 18 wt % Ce, 12 wt %
Na, and 1.2 wt % Mg were held at the same time. In addition this
Embodiment Catalyst 1 also contained 1.6 wt % Pt, 0.15 wt % Rh, and
1.5 wt % Mg.
[0209] Using a similar method, Embodiment Catalysts 2-4 were
obtained.
[0210] As clearly shown in above, an NO.sub.x adsorption catalyst
is provided in the exhaust gas flow passage, and under the
oxidization atmosphere of a lean exhaust gas, NO.sub.x is caught
according to the chemical adsorption. Thus, a reduction atmosphere
is formed and the adsorption catalyst is regenerated. Accordingly,
NO.sub.x etc. in the lean burn exhaust gas can be purified with
high efficiency, without significantly affecting the fuel
consumption.
[0211] The compositions of the prepared catalysts are shown in
Table 4. The support order in this Table 4 indicates that after a
first component has been applied, a second component and then the
third and the fourth were applied, with the amount of each being
indicated preceding the symbol. TABLE-US-00004 TABLE 4 support
order first second third component component component fourth
Embodiment 18 wt % Ce 12 wt % Na 0.15 wt % Rh 1.5 wt % Mg Catalyst
1 1.2 wt % Mg 1.6 wt % Pt Embodiment 18 wt % Ce 12 wt % Na 0.15 wt
% Rh Catalyst 2 1.2 wt % Mg 1.6 wt % Pt 7 wt % Sr Embodiment 18 wt
% Ce 12 wt % Na 0.15 wt % Rh Catalyst 3 1.2 wt % Mg 1.6 wt % Pt
Embodiment 18 wt % Ce 1.5 wt % Na 1.5 wt % Pd Catalyst 4 1.5 wt %
Mg 1.5 wt % pt
(Experimentation Manner)
[0212] (1) honeycomb shape catalyst of 6 cc (17 mm square; 21 mm
length) was filled up in a Pyrex reaction tube.
[0213] (2) The reaction tube was put into a ring shaped electric
furnace and heated to the temperature of 300.degree. C. (As the
reaction temperature, the honeycomb inlet port gas temperature was
measured.) After the temperature stabilized at 300.degree. C., a
flow of a model exhaust gas of the stoichiometric ratio (a
"stoichiometric model exhaust gas") was started. After 3 (three)
minutes, the flow of the stoichiometric model exhaust gas was
stopped, and a flow of a lean ratio model exhaust gas (a "lean
model exhaust gas") was started.
[0214] NO.sub.x in the gas discharged from the reaction tube was
measured by the chemical luminescence detection method, and HC was
measured by the FID method.
[0215] NO.sub.x purification performance and HC purification
performance obtained by this method were made as an initial period
performance.
[0216] (3) The reaction tube where the honeycomb catalyst used in
(2) above was filled up, put into the ring shaped electric furnace
and raised to the temperature of 300.degree. C. (As the reaction
temperature, the honeycomb inlet port gas temperature was
measured.)
[0217] After the temperature stabilized at 300.degree. C., a flow
of the stoichiometric model exhaust gas which contains SO.sub.2 gas
as a catalyst poison was started. The SO.sub.2 poisoning operation
was finished by flowing the poisoning gas SO.sub.2 gas for five
hours.
[0218] After the above stated SO.sub.2 poisoning using the
honeycomb catalyst, a test similar to (2) above was carried out and
NO.sub.x purification performance and HC purification performance
of after SO.sub.2 poisoning were obtained.
[0219] (4) The honeycomb catalyst used in (2) above was put into a
baking furnace and under the air atmosphere the honeycomb catalyst
was calcined at 300.degree. C. for five hours. After the cooling of
the honeycomb catalyst, NO.sub.x purification performance and HC
purification performance similar to (2) above were measured.
[0220] As the stoichiometric model exhaust gas, the gas comprised
of 0.1 vol % NO, 0.05 vol % C.sub.3H.sub.6, 0.6 vol % CO, 0.6 vol %
O.sub.2, 0.2 vol % H.sub.2, 10 vol % water vapor and the balance of
N.sub.2.
[0221] Further, as the lean model exhaust gas, the gas comprised of
0.06 vol % NO, 0.04 vol % C.sub.3H.sub.6, 0.1 vol % CO, 10V01%
C0.sub.2, 5 vol % O.sub.2, 10 vol % water vapor and the balance of
N.sub.2.
[0222] In the above stoichiometric model exhaust gas,
C.sub.3H.sub.6, CO, H.sub.2 are the gaseous reducing agents and
O.sub.2 and NO are the gaseous oxidizing agents. The gaseous
oxidizing agent amount is 0.65% for converting into O.sub.2 and the
gaseous reducing agent amount is 0.575% for converting into O.sub.2
consumption ability. Accordingly, both were balanced.
[0223] In the lean mode exhaust gas, C.sub.3H.sub.6 and CO are the
gaseous reducing agents and O.sub.2 and NO are the gaseous
oxidizing agents. The gaseous oxidizing agent amount is 5.03% for
converting into O.sub.2 and the gaseous reducing agent amount is of
0.23% for converting into O.sub.2 consumption ability. Accordingly,
there is an excess amount of oxygen (O.sub.2) of 4.8%.
[0224] As the gas used for catalyst poisoning, the gas comprised of
0.1 vol % NO, 0.05 vol % C.sub.3H.sub.6, 0.6 vol % CO, 0.005 vol %
S0.sub.2, 10 vol % water vapor and the balance of N.sub.2.
[0225] The space velocities of the above three kinds gases were
30,000/h in the dry gas base (not containing the water vapor).
[0226] Table 5 shows the purification rate of the honeycomb
catalyst of the initial period and after the poisoning by SO.sub.2.
Here, NO.sub.x purification rate was obtained after 1 (one) minute
where the stoichiometric model exhaust gas was changed over to the
lean model exhaust gas. TABLE-US-00005 TABLE 5 NO.sub.x
purification NO.sub.x rate (%) purification initial NO.sub.x after
rate (%) purification poisoning by after 800.degree. C. rate (%)
SO.sub.2 calcination 300.degree. C. 400.degree. C. 300.degree. C.
400.degree. C. 300.degree. C. 400.degree. C. Embodiment 91 94 76 67
58 49 Catalyst 1 Embodiment 98 97 85 70 67 62 Catalyst 2 Embodiment
91 94 75 65 55 45 Catalyst 3 Embodiment 90 90 70 60 55 45 Catalyst
4
[0227] NO.sub.x purification rate and HC purification rate were
calculated according to following formula. NO.sub.x purification
rate=(NO.sub.x concentration in inlet port gas-NO.sub.x
concentration in outlet port gas).times.100/(NO.sub.x concentration
in inlet port gas) Equation (1) HC purification rate=(HC
concentration in inlet port gas-HC concentration in outlet port
gas).times.100/(HC concentration in inlet port gas) Equation
(2)
[0228] Embodiment Catalysts 1-4 had a high initial period
performance and a heat resistant ability and a SO.sub.x endurance
ability.
[0229] Embodiment Catalyst 5 was obtained by replacing the support
material of Embodiment Catalyst 1 with La and Al complex oxide
material (La-.beta.-A1.sub.2O.sub.3) where the composition ratio of
La--Al was 1-20 mol % La and 100 mol % sum of La and Al. The
performances of Embodiment Catalyst 5 are shown in Table 6.
TABLE-US-00006 TABLE 6 NO.sub.x NO.sub.x purification purification
rate (%) rate (%) initial NO.sub.x after after purification
poisoning by calcination rate (%) SO.sub.2 at 800.degree. C.
300.degree. C. 400.degree. C. 300.degree. C. 400.degree. C.
300.degree. C. 400.degree. C. Embodiment 90 88 75 65 67 65 Catalyst
5
[0230] As the support was constituted by La--Al complex oxide
material (La-.beta.-A1.sub.2O.sub.3) the heat resistant ability was
improved.
[0231] In Embodiment Catalyst 1, the initial period performance of
NO.sub.x purification rate at 400.degree. C. was measured, while
the support amount of Na of the second component was varied. The
catalyst preparation was similar to Embodiment Catalyst 1 and the
experimentation manner was similar to Experiment 1. The results are
shown in FIG. 21. To achieve a high NO.sub.x purification rate, it
was suitable to make the Na supported amount 5-20 wt % to the total
support.
[0232] In Embodiment Catalyst 1, the initial period performance of
NO.sub.x purification rate at 400.degree. C. was measured, while
the supported amount of Mg of the second component was varied. The
results are shown in FIG. 22. To achieve a high NO.sub.x
purification rate, it was suitable to make the weight ratio between
the Mg supported amount and (Na supported amount+Mg support amount)
at 1-40 wt %.
[0233] In Embodiment Catalyst 1, the initial period performance of
NO.sub.x purification rate at 400.degree. C. was measured, while
the supported amount of Ce of the first component was varied. The
results are shown in FIG. 23. To achieve a high NO.sub.x
purification rate, it was suitable to make Ce supported amount 5-30
wt %.
[0234] In Embodiment Catalyst 1, the initial period performance of
NO.sub.x purification rate at 400.degree. C. was measured, while
the supported amounts of Pt and Rh were varied. The results are
shown in FIG. 24. To achieve a high NO.sub.x purification rate, it
was suitable to make the supported amount of Pt 0.5-3 wt % and the
supported amount of Rh 0.05-0.3 wt %.
[0235] In Embodiment Catalyst 1, the initial period performance of
NO.sub.x purification rate at 400.degree. C. was measured, while
the supported amounts of Pt and Pd were varied. The results are
shown in FIG. 25. To achieve a high NO.sub.x purification rate, it
was suitable to make the supported amount of Pt 0.5-3 wt % and the
supported amount of Pd 0.5-15 wt %.
[0236] In Embodiment Catalyst 2, initial period NO.sub.x
purification rate at 400.degree. C., after calcination at
800.degree. C., was measured. The results are shown in FIG. 26.
Since the supported amount of Sr was 1-20 wt %, the high NO.sub.x
purification rate and the high heat resistant ability were
obtained.
Experiment 2:
(Preparation Method)
[0237] Water and dilute nitric acid were added to boehmite powers
to form a slurry, which wash-coated to a cordierite made honeycomb.
After drying, the honeycomb was calcined for 1 (one) hour at
600.degree. C., thereby obtaining an alumina coating of 150 g per
liter.
[0238] The alumina coated honeycomb was immersed in Ce nitrate
solution, dried and calcined for 1 (one) hour at 600.degree. C.
Next, the honeycomb was immersed in Sr nitrate solution, dried and
calcined for 1 (one) hour at 600.degree. C. Thereafter, the
honeycomb was immersed in titaniasol solution as a precursor for
titania, dried and calcined for 1 (one) hour at 600.degree. C.
[0239] The honeycomb was then immersed in a solution containing
dinitrodiamime Pt nitrate and Rh nitrate. After drying, it was
calcined for 1 (one) hour at 450.degree. C. Finally, the honeycomb
was immersed in Mg nitrate solution, dried and calcined for 2 (two)
hours at 450.degree. C., forming honeycomb A.
[0240] The catalyst composition of the honeycomb catalyst A was
comprised of alumina (Al.sub.2O.sub.3) of 100 Wt %' Mg of 1 wt %,
Rh of 0.15 wt %, Pt of 1.9 wt %, Ti of 5 wt %, Sr of 15 wt %, and
Ce of 18 wt %, which is the standard for other Embodiment
Catalysts.
[0241] Further, for manufacturing the honeycomb catalyst, as an
alternative to the above method of impregnating the catalyst
components to the alumina coated honeycomb, it is possible to
employ a method where the catalyst components are immersed into the
aluminum powders and after the catalyst powders are prepared they
are made into the slurry, which is coated onto the honeycomb
substrate (honeycomb base body). Further, as the precursor of
titania, in addition to the above titania-sol, an organo titanium
compound, the titanium sulfate and the titanium chloride etc. can
be used. As the alkali earth metals in place of Sr nitrate
(Sr(NO.sub.3).sub.2) using Ca nitrate (Ca(N0.sub.3).sub.2), a
honeycomb catalyst B was obtained.
[0242] Except for as the rare earth metals in place of cerium
nitrate (Ce(NO.sub.3).sub.2) using lanthanum nitrate
(La(NO.sub.3).sub.2), and a method similar to that used for
honeycomb catalyst A, a honeycomb catalyst C was obtained. Further,
using iridium nitrate, a honeycomb catalyst D was obtained.
[0243] By varying the concentration of the titania-sol used in the
preparation of the honeycomb catalyst A, three kinds of honeycomb
catalysts E, F and G having different Ti supported amounts were
obtained.
[0244] By varying the concentration of strontium nitrate
(Sr(NO.sub.3).sub.2) used in the preparation of the honeycomb
catalyst A, three kinds of honeycomb catalysts H, I and J having
different Sr supported amounts were obtained.
[0245] By varying the concentration of dinitrodiamime Pt solution
used in the preparation of the honeycomb catalyst A, three kinds of
honeycomb catalysts K, L and M having different Pt supported
amounts were obtained.
[0246] By varying the concentration of rhodium nitrate solution
used in the preparation of the honeycomb catalyst A, three kinds of
honeycomb catalysts N, 0 and P having different Rh supported
amounts were obtained.
[0247] By varying the concentration of cerium nitrate solution used
in the preparation of the honeycomb catalyst A, three kinds of
honeycomb catalysts Q, R and S having different Ce supported
amounts were obtained.
[0248] A honeycomb catalyst T was also obtained, which did not
contain titanium (Ti) such as in the honeycomb catalyst A.
[0249] The catalyst compositions of the honeycomb catalysts A-T are
shown in Table 7.
Test Manner 1:
[0250] As to the honeycomb catalysts A-T, under following
conditions, NO.sub.x purification reaction activity was
evaluated.
[0251] A honeycomb catalyst having 6 cc was filled up in a quartz
reaction tube having an inner diameter of 25 mm and was arranged in
an electric furnace.
[0252] The reaction tube was heated by the electric furnace, the
inlet port gas temperature of the reaction tube was set at
300.degree. C. constant and the following model gas was let
flow.
[0253] As the exhaust gas for a condition where an internal
combustion engine was operated with the stoichiometric air-fuel
ratio, the model gas comprised 0.1% (volume ratio) NO, 0.05%
C.sub.3H.sub.6, 0.6% CO, 0.5% O.sub.2, 0.2% H.sub.2, 10% H.sub.2O,
and the balance of N.sub.2, and flowed at a space velocity of
30,000/h.
[0254] As the exhaust gas for a condition where an internal
combustion engine was operated with the lean air-fuel ratio, the
model gas comprised 0.06% (volume ratio) NO, 0.04% C.sub.3H.sub.6,
0.1% CO, 5% O.sub.2, 10% H.sub.20, and the balance of N.sub.2, and
flowed at a space velocity of 30,000/h. The above model gases for
the stoichiometric and lean air-fuel ratios were flowed alternately
every three (3) minutes each.
[0255] In these model exhaust gases, the gaseous oxidizing agent
amount is 0.55 vol % for converting into O.sub.2 and the gaseous
reducing agent amount of 0.625 for converting into O.sub.2
consumption ability. Accordingly, both were substantially
balanced.
[0256] The model gas for the stoichiometric air-fuel ratio and the
model gas for the lean air-fuel ratio were flowed alternately every
three (3) minutes each, and the inlet port NO.sub.x concentration
and the outlet port NO.sub.x concentration of the catalyst at this
time were measured according to the chemical luminescence detection
NO.sub.x analyzer. And NO.sub.x purification rate one minute after
the stoichiometric air-fuel ratio model exhaust gas was changed to
the lean air-fuel ratio model exhaust gas was calculated according
to the formula shown in Experiment 1.
Test Manner 2:
[0257] Except that the inlet port gas temperature was heated by the
electric furnace at 400.degree. C. constant, for each of the
honeycomb catalysts A-T, NO.sub.x purification reaction activity
was evaluated in a manner similar to Test Manner 1.
Test Manner 3:
[0258] Similar to Test Manner 1, the inlet port gas temperature was
heated by the electric furnace at 300.degree. C. constant. The gas,
in which to the model gas used for the lean air-fuel ratio 0.005%
S0.sub.2 gas was added, was flowed at a space velocity of 30,000/h
for 3 (three) hours. After that, for each of the honeycomb
catalysts A-T, NO.sub.x purification reaction activity under the
inlet port gas temperature of 300.degree. C. was evaluated in a
manner similar to Test Manner 1.
Test Manner 4:
[0259] Similar to Test Manner 3, the gas, in which to the model for
the lean air-fuel ratio 0.005% S0.sub.2 gas was added, was flowed
at 300.degree. C. for 3 (three) hours. After that, for each of the
honeycomb catalysts A-T, NO.sub.x purification reaction activity
under the inlet port gas temperature of 400.degree. C. was
evaluated in a manner similar to Test Manner 2.
[0260] With respect to the honeycomb catalysts A-T, the results of
the evaluations according to Test Manners 1 and 3 are shown in
Table 7 and the results of the evaluations according to Test
Manners 2 and 4 are shown in Table 8. TABLE-US-00007 TABLE 7
NO.sub.x purification catalyst composition (wt %) rate (%) rare
alkali initial after SO.sub.2 earth earth Ti Pt Rh Mg period
endurance A Ce 18 Sr 15 5 1.9 0.15 1 83 77 B Ce 18 Ca 15 5 1.9 0.15
1 82 72 C La 18 Sr 15 5 1.9 0.15 1 80 73 D Y 18 Sr 15 5 1.9 0.15 1
81 72 E Ce 18 Sr 15 0.1 1.9 0.15 1 80 71 F Ce 18 Sr 15 1 1.9 0.15 1
81 73 G Ce 18 Sr 15 30 1.9 0.15 1 80 72 H Ce 18 Sr 3 5 1.9 0.15 1
80 70 I Ce 18 Sr 7.5 5 1.9 0.15 1 82 75 J Ce 18 Sr 40 5 1.9 0.15 1
81 72 K Ce 18 Sr 15 5 0.2 0.15 1 80 69 L Ce 18 Sr 15 5 1 0.15 1 81
73 M Ce 18 Sr 15 5 4 0.15 1 85 76 N Ce 18 Sr 15 5 1.9 0.15 1 80 73
O Ce 18 Sr 15 5 1.9 0.5 1 82 74 P Ce 18 Sr 15 5 1.9 1 1 81 72 Q Ce
5 Sr 15 5 1.9 0.15 1 80 70 R Ce 10 Sr 15 5 1.9 0.15 1 82 73 S Ce 40
Sr 15 5 1.9 0.15 1 84 72 T Ce 18 Sr 15 0 1.9 0.15 1 80 69
[0261] TABLE-US-00008 TABLE 8 NO.sub.x purification catalyst
composition (wt %) rate (%) rare alkali initial after SO.sub.2
earth earth Ti Pt Rh Mg period endurance A Ce 18 Sr 15 5 1.9 0.15 1
76 55 B Ce 18 Ca 15 5 1.9 0.15 1 73 54 C La 18 Sr 15 5 1.9 0.15 1
74 51 D Y 18 Sr 15 5 1.9 0.15 1 70 48 E Ce 18 Sr 15 0.1 1.9 0.15 1
67 45 F Ce 18 Sr 15 1 1.9 0.15 1 72 50 G Ce 18 Sr 15 30 1.9 0.15 1
74 53 H Ce 18 Sr 3 5 1.9 0.15 1 67 46 I Ce 18 Sr 7.5 5 1.9 0.15 1
72 49 J Ce 18 Sr 40 5 1.9 0.15 1 77 54 K Ce 18 Sr 15 5 0.2 0.15 1
65 44 L Ce 18 Sr 15 5 1 0.15 1 70 51 M Ce 18 Sr 15 5 4 0.15 1 80 54
N Ce 18 Sr 15 5 1.9 0.15 1 66 45 O Ce 18 Sr 15 5 1.9 0.5 1 76 52 P
Ce 18 Sr 15 5 1.9 1 1 72 48 Q Ce 5 Sr 15 5 1.9 0.15 1 68 44 R Ce 10
Sr 15 5 1.9 0.15 1 72 50 S Ce 40 Sr 15 5 1.9 0.15 1 75 52 T Ce 18
Sr 15 0 1.9 0.15 1 65 43
[0262] Each of Embodiment catalysts A-S has a high NO.sub.x
purification rate after S0.sub.2 endurance and a strong S0.sub.2
resistivity in comparison with the catalyst T which does not
contain titanium (Ti).
[0263] As to the honeycomb catalyst A, an X-ray analyzing spectrum
was measured and the crystallization structure was identified. In
the X-ray analyzing spectrum of the honeycomb catalyst A, there was
no peak caused by titania (TiO.sub.2) and it was considered that
titania (TiO.sub.2) had maintained a non-crystalline structure. It
was understood that strontium (Sr) as alkali earth metals was held
as carbonate.
Experiment 3:
[0264] Cerium nitrate (Ce(N0.sub.3).sub.2) solution was impregnated
in alumina (Al.sub.20.sub.3) and after drying at 200.degree. C. the
alumina was calcined at 600.degree. C. for 1 hour. In succession, a
mixture solution obtained by mixing strontium nitrate
(Sr(NO.sub.3).sub.2) with silica-sol was impregnated, similarly in
the cerium (Ce) coated alumina was dried and calcined. With the
above process, the catalyst powders were obtained, such catalyst
powders were comprised of, to 100 wt % alumina (Al.sub.2O.sub.3),
18 wt % Ce, 15 wt % Sr, 4 wt % SiO.sub.2, 1.6 wt % Pt, 0.15 wt %
Rh, and 1.5 wt % Mg. Alumina-sol and aluminum nitrate
(Al(NO.sub.3).sub.2) were added to the catalyst powders and the
slurry was obtained by agitating and mixed with them and the
obtained slurry was coated to a cordierite made honeycomb (400
cell/in.sup.2). The calcination temperature was about 450.degree.
C. and Embodiment Catalyst 100 having a final coating amount of 200
g/l.
[0265] According to a method similar to the above, Embodiment
Catalysts 100-105 were obtained.
[0266] The composition of the prepared catalysts are show in Table
9. The support order in this Table 9 indicates that after a first
component has applied, a second component is applied and next a
third component and a fourth component are applied successively.
Further, the supported amount is indicated before the supported
metal symbol. TABLE-US-00009 TABLE 9 Embodi- support order ment
first second third Catalyst component component component fourth
100 18 wt % Ce 15 wt % Sr 0.15 wt % Rh 1.5 wt % Mg 4 wt % SiO.sub.2
1.6 wt % Pt 101 18 wt % Ce 15 wt % Sr 0.15 wt % Rh 4 wt % SiO.sub.2
1.6 wt % Pt 102 18 wt % Ce 7 wt % Sr 0.15 wt % Rh 7 wt % Ca 1.6 wt
% Pt 4 wt % SiO.sub.2 103 18 wt % Ce 15 wt % Sr 0.15 wt % Rh 1.6 wt
% pt 104 18 wt % Ce 15 wt % Sr 0.15 wt % Rh 1.6 wt % Pt 105 18 wt %
Ce 7 wt % Sr 0.15 wt % Rh 7 wt % Ca 1.6 wt % Pt
[0267] The testing manner was similarly to (1), (2) and (3) shown
in Experiment 1 and further the composition of the model gas was
similarly to that of shown in Experiment 1.
[0268] Table 10 shows the NO.sub.x gas purification rate at one
minute after the start of flow of the stoichiometric model exhaust
gas, and the purification rate one minute after the start of the
lean model exhaust gas, obtained by the honeycomb catalyst during
the initial period and after SO.sub.2 poisoning. The NO.sub.x gas
purification rate was calculated according to the formula shown in
Experiment 1. TABLE-US-00010 TABLE 10 NO.sub.x purification initial
rate (%) poisoning by SO2 period NO.sub.x lowering purification
rate (%) rate (%) of lean Embodiment stoichio stoichic- to Catalyst
metric lean metric lean initial 100 100 80 100 75 6 101 100 50 100
43 14 102 100 65 100 59 9 103 100 65 100 50 23 104 100 47 100 32 32
105 100 58 100 40 31
[0269] Further, the decrease of the lean NO.sub.x gas purification
rate by SO.sub.2 poisoning was calculated by the following formula.
Lowering rate of NO.sub.x gas purification rate=(initial period
NO.sub.x gas purification rate-NO.sub.x gas purification rate after
SO.sub.2 poisoning)/(initial period NO.sub.x gas purification rate
formula (2)
[0270] The decrease of the lean NO.sub.x gas purification rate by
SO.sub.2 poisoning according to the support of Si0.sub.2 is
improved at -5%-15%.
[0271] In Embodiment Catalyst 100, the NO.sub.x gas purification
rate was measured while the Si0.sub.2 supported amount was varied.
The catalyst preparation manner and the experimental manner were
similarly to those of Embodiment Catalyst 100. The results are
shown in Table 11. By supporting SiO.sub.2, the initial period
NO.sub.x gas purification rate is improved. Further, the support
amount of SiO.sub.2, was 0.6 wt %-5 wt %, NO.sub.x gas purification
rate after SO.sub.2 poisoning can obtain 60%. TABLE-US-00011 TABLE
11 SiO.sub.2 initial period NO.sub.x NO.sub.x purification
supported purification rate rate (%) after SO.sub.2 amount (%)
poisoning (wt %) stoichiometric lean stoichiometric lean 0 100 55
100 50 0.5 100 55 100 50 0.8 100 70 100 63 1 100 75 100 68 2 100 80
100 75 3 100 80 100 75 4 100 78 100 72 5 100 65 100 60 8 100 50 100
53 9 100 55 100 49 10 100 50 100 46
[0272] The foregoing disclosure has been set forth merely to
illustrate the invention and is not intended to be limiting. Since
modifications of the disclosed embodiments incorporating the spirit
and substance of the invention may occur to persons skilled in the
art, the invention should be construed to include everything within
the scope of the appended claims and equivalents thereof.
* * * * *