U.S. patent application number 12/095609 was filed with the patent office on 2010-11-04 for device for cleaning exhaust gas of internal combustion engine.
Invention is credited to Hirohito Hirata, Masaya Ibe, Masaru Kakinohana.
Application Number | 20100275586 12/095609 |
Document ID | / |
Family ID | 38092336 |
Filed Date | 2010-11-04 |
United States Patent
Application |
20100275586 |
Kind Code |
A1 |
Hirata; Hirohito ; et
al. |
November 4, 2010 |
DEVICE FOR CLEANING EXHAUST GAS OF INTERNAL COMBUSTION ENGINE
Abstract
The device for cleaning exhaust gas of the internal combustion
engine includes means provided in an exhaust gas passage (15) of
the internal combustion engine (10), such as a NOx catalyst (20), a
SOx trap catalyst (30) provided at a position upstream from the
exhaust gas cleaning means, and means (40, 41) for feeding ozone to
a position upstream from the SOx trap catalyst. SOx in the exhaust
gas is absorbed by the SOx trap catalyst before reaching the NOx
catalyst. Particularly, it is possible to oxidize SOx in the
exhaust gas by ozone as oxidizing gas to be an easily decomposable
state, whereby SOx is absorbable even at a low temperature.
Preferably, the SOx trap catalyst has no active points consisting
of precious metal. In such a case, the SOx trap catalyst is
prevented from being sulfur-poisoned whereby the deterioration of
the SOx absorbing ability is avoidable.
Inventors: |
Hirata; Hirohito;
(Shizuoka-ken, JP) ; Kakinohana; Masaru;
(Shizuoka-ken, JP) ; Ibe; Masaya; (Shizuoka-ken,
JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
38092336 |
Appl. No.: |
12/095609 |
Filed: |
November 28, 2006 |
PCT Filed: |
November 28, 2006 |
PCT NO: |
PCT/JP2006/324133 |
371 Date: |
July 19, 2010 |
Current U.S.
Class: |
60/299 |
Current CPC
Class: |
B01D 2255/1021 20130101;
B01D 53/9477 20130101; B01D 53/9422 20130101; F01N 3/0871 20130101;
Y02T 10/26 20130101; B01D 2258/012 20130101; F01N 2240/38 20130101;
B01D 53/9459 20130101; F01N 3/0842 20130101; Y02T 10/12 20130101;
F01N 3/085 20130101; F01N 3/2013 20130101; B01D 2257/302 20130101;
B01D 53/9409 20130101; B01D 2251/104 20130101; F01N 13/0097
20140603; B01D 2255/2042 20130101; F01N 13/009 20140601 |
Class at
Publication: |
60/299 |
International
Class: |
F01N 3/10 20060101
F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 1, 2005 |
JP |
2005-348292 |
Claims
1. A device for cleaning exhaust gas of an internal combustion
engine, comprising means provided in an exhaust gas passage of the
internal combustion engine, for cleaning exhaust gas discharged
from a combustion chamber, a SOx trap catalyst provided in said
exhaust gas passage at a position upstream from said exhaust gas
cleaning means, for absorbing SOx in the exhaust gas, and means for
feeding ozone to said exhaust gas passage at a position upstream
from said SOx trap catalyst, wherein said exhaust gas cleaning
means cleans at least one of HC, CO and NOx components in the
exhaust gas at a cleaning rate higher than that of said SOx trap
catalyst.
2. A device for cleaning exhaust gas of an internal combustion
engine as defined by claim 1, wherein said SOx trap catalyst
contains alkali metal element, alkaline earth metal element or rare
earth element.
3. A device for cleaning exhaust gas of an internal combustion
engine as defined by claim 1, wherein said exhaust gas cleaning
device comprises a NOx catalyst of a storage reduction type, and
said NOx catalyst of a storage reduction type carries more precious
metal than in said SOx trap catalyst.
4. A device for cleaning exhaust gas of an internal combustion
engine as defined by claim 1, wherein said SOx trap catalyst has no
active points consisting of precious metal.
Description
TECHNICAL FIELD
[0001] The present invention relates to a device for cleaning
exhaust gas of an internal combustion engine, particularly to a
device for cleaning exhaust gas of the internal combustion engine
provided with means for cleaning exhaust gas such as a
catalyst.
BACKGROUND ART
[0002] Recently, a NOx catalyst was put into practice for cleaning
nitrogen oxide (NOx) contained in exhaust gas of an internal
combustion engine of a lean combustion type. In the NOx catalyst,
for example, of a storage reduction type, alkaline earth such as
barium (Ba) and precious metal such as platinum (Pt) are carried on
a carrier such as alumina, and NOx in the exhaust gas is absorbed
and stored in the NOx catalyst in a form of nitrate
(Ba(NO.sub.3).sub.2). The NOx catalyst has a function for absorbing
and storing NOx contained in the exhaust gas if the internal
combustion engine is driven at a lean air-fuel ratio, while if
driven at a air-fuel ratio of lower than a theoretical air-fuel
ratio, the NOx catalyst releases the stored NOx and reduces the
same.
[0003] Since sulfur (S) is contained in fuel and lubricant for the
engine, it is also contained in the exhaust gas. Thus, the NOx
catalyst absorbs sulfuric components in the exhaust gas, such as
sulfate, for example, BaSO.sub.4 and poisoned thereby. Since
sulfate absorbed and stored in the NOx catalyst has the stability
higher than that of nitrate, it is not released from the NOx
catalyst but gradually stored in the NOx catalyst even if the
air-fuel ratio in the exhaust gas is fuel rich. If an amount of
sulfate in the NOx catalyst increases, an amount of NOx capable of
being absorbed by the NOx catalyst gradually decreases, resulting
in a problem in that an amount of sulfate in the NOx catalyst
increases to lower the NOx absorbing ability of the NOx
catalyst.
[0004] To solve such a problem in that the NOx catalyst is poisoned
by sulfur, Japanese Patent Laid-Open No. 2000-145436 discloses a
device for absorbing SOx in the exhaust gas by a SOx absorbent
disposed upstream from the NOx absorbent. According to this device,
SOx in the exhaust gas is absorbed by the SOx absorbent before SOx
in the exhaust gas reaches the NOx absorbent, whereby the poisoning
of the NOx absorbent due to sulfur is prevented.
[0005] However, the device disclosed in Japanese Patent Laid-Open
No. 2000-145436 has insufficient for absorbing SOx when the SOx
absorbent is at a low temperature, whereby there is a problem in
that SOx passes through the SOx absorbent when the temperature is
low and is absorbed by the NOx absorbent. Also, there is another
problem in that the drop of the ability for absorbing SOx in the
SOx absorbent is relatively significantly lowered as time passes,
whereby it is difficult to maintain a sufficient SOx absorbing
ability for a long period.
DISCLOSURE OF THE INVENTION
[0006] The present invention has been made to solve the
above-mentioned problems, and an object thereof is to provide a
device for cleaning exhaust gas of an internal combustion engine
capable of exhibiting the sufficient SOx absorbing ability even at
a low temperature and of restricting the deterioration of the SOx
absorbing ability.
[0007] To achieve the above-mentioned object, according to one
aspect of the present invention, a device for cleaning exhaust gas
of an internal combustion engine is provided, comprising means for
cleaning exhaust gas discharged from a combustion chamber, provided
in an exhaust gas passage of the internal combustion engine, a SOx
trap catalyst provided in said exhaust gas passage at a position
upstream from said exhaust gas cleaning means, for absorbing SOx in
the exhaust gas, and means for feeding ozone to said exhaust gas
passage at a position upstream from said SOx trap catalyst, wherein
said exhaust gas cleaning means cleans at least one of HC, CO and
NOx components in the exhaust gas at a cleaning rate higher than
that of said SOx trap catalyst.
[0008] According to one aspect of the present invention, it is
possible to absorb SOx contained in the exhaust gas by the SOx trap
catalyst and prevent the exhaust gas cleaning means from being
poisoned by sulfur. Particularly, by ozone fed as strong oxidizing
gas at a position upstream from the SOx trap catalyst, it is
possible to oxidize SOx in the exhaust gas to be easily absorbable.
Accordingly, the absorption of SOx by the SOx trap catalyst is
possible even at a low temperature.
[0009] In this regard, the cleaning of exhaust gas includes the
absorption, adsorption or storage of special components in the
exhaust gas.
[0010] Here, the SOx trap catalyst preferably contains alkali metal
elements, alkaline earth metal elements or rare earth elements.
[0011] Also, the exhaust gas cleaning means preferably includes the
NOx catalyst of a storage reduction type, and the NOx catalyst of a
storage reduction type preferably carries more precious metal than
the SOx trap catalyst.
[0012] In this case, while an amount of precious metal carried on
the SOx trap catalyst from the upstream end to the downstream end
is maintained at a constant value, a total amount carried on the
storage reduction type Ox catalyst can be more than that carried on
the SOx trap catalyst. Or, the amount of precious metal may be
gradually increased from the upstream end of the SOx trap catalyst
to the downstream end of the storage reduction type NOx
catalyst.
[0013] Preferably, the SOx trap catalyst has no active points
consisting of precious metal.
[0014] In this case, the problem of the poisoning due to sulfur in
that as SOx has been stored in the SOx trap catalyst, the active
points are covered with sulfate is solved, and the deterioration of
the SOx absorbing ability as the time has passed is avoidable
[0015] According to the present invention, when SOx in the exhaust
gas is absorbed upstream from the exhaust gas cleaning means such
as the catalysts, an excellent effect is significant in that the
SOx absorbing ability is not lowered but sufficiently exhibited
even at a low temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 diagrammatically illustrates a system of a device for
cleaning exhaust gas of an internal combustion engine according to
one embodiment of the present invention;
[0017] FIG. 2 illustrates cells of a SOx trap catalyst in an
enlarged manner;
[0018] FIG. 3 illustrates carriers in an enlarged manner;
[0019] FIG. 4 illustrates a whole structure of test equipment for
carrying out tests in relation to the embodiment;
[0020] FIG. 5 illustrates an area V of FIG. 4 in more detail;
and
[0021] FIG. 6 is a graph illustrating the comparison of ratios of
sulfur trapped by the SOx trap catalysts used in the respective
examples.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] Embodiments of the present invention will be described below
with reference to the attached drawings.
[0023] FIG. 1 diagrammatically illustrates a system of a device for
cleaning exhaust gas of an internal combustion engine according to
one embodiment of the present invention. In this drawing, reference
numeral 10 denotes an internal combustion engine; a spark ignition
type internal combustion engine in this embodiment, more concretely
a direct injection type gasoline engine. The engine, however, may
be of a compressive ignition type, i.e., a diesel engine. The
present invention may be applicable to any types or kinds of
engines provided the exhaust gas thereof contains sulfuric
components. Reference numeral 11 denotes an intake manifold
communicated with an intake port, 12 denotes an exhaust manifold
communicated with an exhaust port, and 13 denotes a combustion
chamber. In this embodiment, fuel fed from a fuel tank not shown to
a high pressure pump 17 is compressively delivered to delivery pipe
18 by a high pressure pump 17 and stored there in a highly
compressed state. This high pressure fuel in the delivery pipe 18
is directly injected from a fuel injection valve 14 into the
combustion chamber 13. Exhaust gas from the engine 10 flows, after
passing through a turbocharger 19 via the exhaust gas manifold 12,
into the exhaust gas passage 15 downstream therefrom, and is
discharged to an outer air after being cleaned as stated later.
[0024] In the exhaust gas passage 15, a NOx catalyst 20 for
cleaning NOx in the exhaust gas is provided as means for cleaning
exhaust gas discharged from the combustion chamber 13. In this
connection, the exhaust gas cleaning means to which is applicable
the present invention is not limited to the NOx catalyst 20 but may
be any means that may be poisoned with sulfuric components
contained in the exhaust gas to lose the exhaust gas cleaning
ability inherent thereto. Such exhaust gas cleaning means includes
a three way catalyst, an HC absorbing agent, a NOx absorbing agent,
a particulate matter oxidation catalyst or others. Also, the
exhaust gas cleaning means may include the combinations of two or
more of them.
[0025] The NOx catalyst 20 according to this embodiment is a
storage reduction type NOx catalyst (NSR). In this case, the NOx
catalyst 20 is structured by carrying precious metal, for example,
platinum Pt as active points and NOx absorbing components on a
surface of a substrate consisting of oxide such as alumina
Al.sub.2O.sub.3. The NOx absorbing agent consists of at least one
selected from a group of alkali metal such as potassium K, sodium
Na, lithium Li or cesium Cs, alkaline earth such as barium Ba or
calcium Ca and rare earth such as lanthanum La or yttrium Y. The
storage reduction type NOx catalyst 20 absorbs NOx if an air fuel
ratio of the exhaust gas flowing thereinto is leaner than a
predetermined value (typically a theoretical air fuel ratio), and
contrarily, releases the absorbed NOx if the concentration of
oxygen in the exhaust gas becomes lower. According to this
embodiment, since a direct injection type gasoline engine capable
of executing a lean burn drive is used, the air fuel ratio of the
exhaust gas becomes lean during the lean burn drive, and the NOx
catalyst 20 absorbs NOx in the exhaust gas. Also, if the air fuel
ratio becomes rich by feeding the reducing agent upstream from the
NOx catalyst 20, the NOx catalyst 20 releases the absorbed NOx.
This released NOx reacts with the reducing agent and, is reduced
and cleaned.
[0026] The reducing agent may be those generating reduction
components such as hydrocarbon HC or carbon monoxide CO in the
exhaust gas, and the followings are usable, including gas such as
hydrogen or carbon monoxide, liquid or gaseous hydrocarbon such as
propane, propylene or butane and liquid fuel such as gasoline, gas
oil or kerosene. In this embodiment, gasoline used as fuel is
employed as the reducing agent for the purpose of avoiding the
complexity during the storage and/or supplementation. To supply the
reducing agent, fuel may be injected from an injection valve (not
shown) separately provided in the exhaust gas passage 15 upstream
from the NOx catalyst 20, or a larger amount of fuel than usual may
be injected from the fuel injection valve 14, or fuel is
post-injected from the fuel injection nozzle 14 at a final stage of
the expansion cycle or in the exhaust cycle. Such the supply of the
reducing agent for the purpose of releasing and reducing NOx in the
NOx catalyst 20 is called as a rich spike.
[0027] The NOx catalyst 20 may be of a selective catalytic
reduction type (SCR). Examples of the selective reduction catalytic
type NOx catalyst may be that carrying precious metal such as
platinum on the surface of substrate made, for example, of zeolite
or alumina, that carrying transition metal such as Cu on the
substrate surface by the ion exchange, or that carrying
titania/vanadium catalyst (V.sub.2O.sub.3/WO.sub.3/TiO.sub.2) on
the substrate surface. In the selective catalytic reduction type
NOx catalyst, HC and NO in the exhaust gas are constantly and
simultaneously are reacted to be N.sub.2, O.sub.2 or H.sub.2O and
cleaned under the condition wherein the air fuel ratio in the
flowing-in exhaust gas is lean. In this regard, the existence of HC
is indispensable when NOx is cleaned. Since unburned HC is surely
contained in the exhaust gas even if the air fuel ratio is lean, it
is possible to reduce and clean NOx by using the same. As described
above, the reducing agent may be fed by carrying out the rich spike
as in the case of the storage reduction type NOx catalyst. In this
case, ammonia or urea may be used as the reducing agent other than
described above.
[0028] Now, the description will be made on other means for
cleaning exhaust gas. The three way catalyst carries precious metal
such as Pt, Pd or Rh on porous oxide such as alumina or ceria, and
is capable of simultaneously cleaning HC, CO and NOx in the exhaust
gas in an atmosphere in the vicinity of the stoichiometric air fuel
ratio. The HC absorbing agent is made, for example, of porous
absorbing agent mainly composed of silica (such as that carries
SiO.sub.2 between layered crystals of SiO.sub.4) or porous material
such as zeolite, formed to be in a cylindrical shape having a
number of thin axial flow passages (cells), capable of absorbing a
HC component in the incoming exhaust gas within porous thin holes
when the absorbing agent temperature is low and releasing the
absorbed HC component therefrom when the absorbing agent
temperature is high. This is particularly effective for decreasing
so-called cold HC during the cold start of the engine. The NOx
absorbing agent is formed of porous zeolite or others and maintains
NO or NO.sub.2 in the exhaust gas as it is without converting to
nitrate. The particulate matter oxidation catalyst is carried on a
surface of a particulate filter for trapping particulate matter
(PM) mainly discharged from a diesel engine and oxidizing (burning)
the trapped particulate matter at a relatively low temperature.
This catalyst is at least one selected from a group consisting of
precious metal such as platinum Pt, palladium Pd or Rhodium Rh,
alkali metal such as potassium K, sodium Na, lithium Li or cesium
Cs, alkaline rare earth such as barium Ba, calcium Ca or strontium
Sr, rare earth such as lanthanum La, yttrium Y or cerium Ce and
transition metal such as iron Fe.
[0029] Now referring again to FIG. 1, according to this embodiment,
a SOx trap catalyst 30 for absorbing SOx in the exhaust gas is
provided in the exhaust gas passage 15 at a position upstream from
the NOx catalyst 20. According thereto, it is possible to absorb
(or adsorb or trap) SOx contained in the exhaust gas by the SOx
trap catalyst 30 before SOx reaches the NOx catalyst 20, whereby it
is possible to prevent the NOx catalyst from being poisoned with
sulfur. Also, there may be a possibility of eliminating the
regeneration control for recovering the NOx catalyst from the
sulfuric poisoning as being generally carried out. This
regeneration control is carried out by temporarily setting the air
fuel ratio to be stoichiometric or rich when the exhaust gas
temperature is relatively high (for example, 400.degree. C.). Thus,
sulfate is decomposed to sulfur oxide (SOx) and removed from the
NOx catalyst.
[0030] Complementarily, while the exhaust gas temperature at which
sulfate is removable from the NOx catalyst 20, for example,
400.degree. C., is relatively easily reached by the gasoline engine
as in this embodiment, it is relatively difficult to be reached by
the diesel engine wherein the temperature of exhaust gas is
inherently low. Contrary thereto, the temperature at which the
stored NOx is releasable from the NOx catalyst and reducible is
lower than the temperature at which sulfate is removable, for
example, 200 to 300.degree. C. Sulfate is more stable than nitrate
and, therefore impossible to be removed unless the atmospheric
temperature is higher than in a case of nitrate. The exhaust gas
temperature at which sulfate is removable is various in accordance
with materials or structures of exhaust gas cleaning means; for
example, there may be a case of 500.degree. C. or higher.
[0031] As illustrated in FIG. 1, ozone feeding means capable of
feeding ozone (O.sub.3) is provided in the exhaust gas passage 15
at a position upstream from the SOx trap catalyst 30. The ozone
feeding means includes n ozone feeding member 40 inserting into the
exhaust gas 15 at a position upstream from the SOx trap catalyst
30, and an ozone generator 41 connected via an ozone feeding
passage 42 to the ozone feeding member 40. Ozone generated in the
ozone generator 41 reaches the ozone feeding member 40 via the
ozone feeding passage 42, and is injected into the exhaust gas
passage 15 in the downstream direction from feeding ports 43
provided in the ozone feeding member 40. According to this
embodiment, while a plurality (two) of the feeding ports 43 are
provided, it may be only one. The ozone feeding member 40 extends
in the diametrical direction of the exhaust gas passage 15, and the
feeding ports 43 thereof are arranged at a predetermined interval
in the longitudinal direction of the ozone feeding member 40 so
that ozone is dispersed evenly in the exhaust gas passage 15.
Positions of the feeding ports 43 becomes ozone feeding positions
in the exhaust gas passage 15.
[0032] The ozone generator 41 may be of a type wherein ozone is
generated while air or oxygen is flowing as raw material through a
discharge tube capable of applying a high voltage, or of any other
types. Air or oxygen used as the raw material is a gas taken from
outside of the exhaust gas passage 15, and not a gas contained
within the exhaust gas in the exhaust gas passage 15. In the ozone
generator 14, the efficiency for producing ozone is higher when
using a raw material having a higher temperature than when using
that having a lower temperature. Accordingly, by generating ozone
while using gas fed from outside of the exhaust gas passage 15 as
described above, it is possible to improve the efficiency for
producing ozone.
[0033] The ozone generator 41 is connected to an electronic control
unit (hereinafter referred to as ECU) 100 as control means, and
when it is switched ON by ECU 100, it generates ozone, and when it
is switched OFF by ECU 100, it stops the generation of ozone. Ozone
thus generated is fed from the feeding ports 43 of the ozone
feeding member 40 into the exhaust gas passage 15 as described
above, whereby the feeding of ozone is carried out. In this
embodiment, while ozone produced by switching the ozone generator
41 to ON is immediately fed, ozone may be preliminarily produced
and stored, and may be fed by switching a valve. Also, ozone may be
compressed by a pump or a compressor before it is fed.
[0034] ECU 100 executes the rich spike control for releasing NOx
from the NOx catalyst 20 in accordance with a predetermined program
preliminarily stored therein. That is, when the conditions for
executing the predetermined rich spike are satisfied, ECU 100
simultaneously executes the rich spike by injecting fuel a
separately provided injection valve for the rich spike or injecting
a more amount of fuel than usual from the fuel injection valve 14,
or executing the post injection from the fuel injection valve 14.
Thereby, the air fuel ratio of the exhaust gas flowing into the NOx
catalyst 20 becomes richer than the theoretical air fuel ratio, and
NOx stored in the NOx catalyst 20 is released and reacted with
unburned components (CO, HC) in the exhaust gas to be reduced and
cleaned. In such a manner, the rich spike control means is
constituted by ECU 100.
[0035] FIG. 2 illustrates an enlarged view of cells in the SOx trap
catalyst 30. The SOx trap catalyst 30 includes a substrate 32 made,
for example, of cordielite in a cylindrical shape as a whole, and
the substrate 32 is formed to define a number of mesh-like or
honeycomb-like cells 33 used as exhaust gas holes. The cell 33
extends in the axial direction (the front to backward direction of
FIG. 2) of the SOx trap catalyst 30, and is opened at opposite ends
thereof to define an inlet and an exit for the exhaust gas,
respectively. On the inner wall of the cell 33, a carrier 34 is
formed all over the surface thereof as a wash coat layer. The
carrier 34 is formed, for example, of alumina (Al.sub.2O.sub.3)
having a thickness of 20 to 50 .mu.m.
[0036] FIG. 3 illustrates an enlarged view of the carrier 34. The
carrier 34 is formed of a number of particles 35 microscopically
agglomerated together so that voids 36 are formed between the
particles 35 for allowing gas to diffuse therein. A diameter of the
particle 35 is, for example, approximately several ten nm. The
carrier 34 is formed by mixing and dispersing powder of material
forming the carrier 34 with solution such as water, in which
solution, the substrate 32 is immersed and after being dried,
sintered to be a final product. On the surface of the particle 35
forming the carrier 34, a number of components reacted with SOx to
produce sulfate; i.e., SOx reactive components are provided.
[0037] In a case of such a prior art SOx trap catalyst as disclosed
in Japanese Patent Laid-Open No. 2000-145436, a number of active
points 37 consisting of precious metal such as Pt or Pd are
provided on the surface of the particle 35, and the active points
are necessary. Contrary to this, according to this embodiment,
there are no such active points 37 on the surface of the particle
35, and thus the carrier 34 does not contain the active points 37.
In the device for cleaning the exhaust gas according to the present
invention, although the SOx trap catalyst 30 may contain the active
points 37, they are not indispensable but rather preferably
eliminated.
[0038] The SOx reactive components 38 are preferably alkali metal
element, alkaline earth metal element or rare earth element. The
alkali metal element is preferably Li, Na or K, the alkaline earth
element is preferably Ba, Ca or Sr, the rare earth element is
preferably Ba, Ca or Sr, and the rare earth element is preferably
La.
[0039] Now, the exhaust gas discharged from the combustion chamber
13 of the engine 10 is supplied with ozone from the ozone feeding
member 40, and thereafter, sequentially passes the SOx trap
catalyst 30 and the NOx catalyst 20. SOx in the exhaust gas is
oxidized with ozone as a strong oxidizing gas to be SO.sub.3 which
is easily absorbable. SO.sub.3 reacts with the SOx reactive
components 38 in the SOx trap catalyst 30 without the assistance of
the active points 37 to generate sulfate. Tis sulfate is absorbed
by the carrier 34, and as a result, SOx in the exhaust gas is
absorbed by the SOx trap catalyst 30. Since the exhaust gas enters
the voids 36 between the particles 35, The generation and
absorption of sulfate are carried out in a wide area on the surface
of the particle.
[0040] Particularly, the absorption of SOx is possible even if the
temperature of the exhaust gas or the SOx trap catalyst 30 is low.
This is because SOx is oxidized by ozone even at the low
temperature to be an easily absorbable state. Contrary to this,
according to the conventional SOx trap catalyst, since the easily
absorbable state is not obtainable without using the active points
37, it is necessary to rise the catalyst temperature to the active
temperature or higher. Accordingly, as a result, SOx is not
absorbable at the low temperature and SOx freely passes the SOx
trap catalyst to adhere to the NOx catalyst or others, resulting in
the sulfuric poisoning. According to the inventive device for
cleaning exhaust gas, such a problem is eliminated, and it is
possible to prevent the NOx catalyst from being poisoned by sulfur
directly after the engine start or during the low temperature
drive.
[0041] Also, in this embodiment, there is an advantage in that the
SOx absorbing ability of the SOx trap catalyst 30 has been
sufficiently maintained for the long period without being degraded.
That is, if the absorption of SOx in the SOx trap catalyst
continues, sulfate is stored on the carrier 35. In the conventional
SOx catalyst, the active points 37 are gradually covered with
sulfate, whereby the sulfuric poisoning becomes worse to lower the
activity and the SOx absorbing ability of the catalyst. Contrarily,
according to the inventive device for cleaning exhaust gas, since
ozone is fed thereto, it is possible to absorb SOx without the
assistance of the active points 37, and, in fact, there are no
active points 37 in this embodiment. Accordingly, even if sulfate
is stored on the carriers 35, the activity and the SOx absorbing
ability of the catalyst do not so worsen as in the conventional SOx
trap catalyst. Thus, it is possible to maintain the sufficient SOx
absorbing ability for a long period.
[0042] In this embodiment, since the SOx trap catalyst 30 has no
active points 37, there are advantages described below. That is, if
sulfate absorbed by the SOx trap catalyst is decomposed and
released, sulfuric components thereof are absorbed by the
downstream NOx catalyst 20 to result in the sulfuric poisoning of
the NOx catalyst 20. In a case of the conventional SOx trap
catalyst having active points 37, in the same manner as the
regeneration of sulfuric poisoning in the NOx catalyst, by rising
the atmospheric temperature of the catalyst to a high value (for
example, 400.degree. C. or higher) at which sulfate is releasable
from the catalyst as well as causing the atmosphere of the catalyst
to the reducing (rich) atmosphere, sulfate is decomposed and
released from the SOx trap catalyst. On the other hand, when the
rich spike is executed for the purpose of releasing NOx from the
NOx catalyst, the SOx trap catalyst is similarly exposed to the
reducing atmosphere. At that time, if the exhaust gas temperature
is as high as allowing sulfate to be releasable, for example, due
to the high load drive, sulfate is decomposed and released from the
SOx trap catalyst to poison the downstream NOx catalyst with
sulfur.
[0043] The operation of the conventional SOx trap catalyst for
releasing sulfate is caused by the active points consisting of
precious metal. Namely, the reaction starts from the active points
to decompose and release sulfate. Since there are no active points
37 in the SOx trap catalyst 30 according to this embodiment,
however, it is difficult to release sulfate even if the atmosphere
is hot and reductive as described above. Thereby, it is possible to
prevent the downstream NOx catalyst 20 from being poisoned by
sulfate released from the SOx trap catalyst 30 when the rich spike
is executed.
[0044] According to this embodiment, the absorption of SOx in the
SOx trap catalyst 30 is executed with the assistance of the ozone
feeding. Therefore, it is desirable to always feed ozone even at a
small amount during the operation of the engine.
[0045] In this connection, as the SOx trap catalyst 30 absorbs SOx,
a total amount of the absorbed SOx reaches the maximum absorbable
amount of the SOx trap catalyst 30 in due course, and the SOx
absorbing ability is significantly lowered (that is, saturated).
This state is referred to as the saturation of the SOx trap
catalyst 30, and the SOx trap catalyst 30 may be exchanged to a
fresh one. To inform this timing to the user, alarm means such as a
lamp or a buzzer may be provided. Or, means for detecting the
saturation of the SOx trap catalyst 30 may be provided. For
example, ECU 100 integrates an amount of consumed fuel based on the
detected value of a fuel meter, and the saturation of the SOx trap
catalyst 30 may be determined thereby.
[0046] On the other hand, fuel having the significantly low
sulfuric concentration has recently been developed and partially
put into practice. In an automobile engine using such fuel, there
may be a possibility in that the SOx trap catalyst does not
saturate until the lifetime of the automobile has been reached, but
continues to exhibit the sufficient SOx absorbing ability.
Accordingly, in such a case, it is unnecessary to consider the
exchange of the SOx trap catalyst.
[0047] Here, of course, the NOx catalyst 20 has a function for
cleaning NOx in the exhaust gas at a cleaning rate higher than that
of the SOx trap catalyst 30. While the SOx trap catalyst 30 having
no precious metal forming the active points 37 is used in the
above-mentioned embodiment, it is also possible to use the SOx trap
catalyst 30 having precious metal. In either cases, an amount of
precious metal carried on the downstream NOx catalyst 20 is
preferably larger than that carried on the upstream SOx trap
catalyst 30. In this case, it is possible that while the amount of
precious metal carried on SOx trap catalyst 30 is made uniform from
the upstream end to the downstream end thereof and that of the
precious metal carried on the NOx catalyst 20 is made uniform from
the upstream end to the downstream end thereof, a total amount of
precious metal carried on the NOx catalyst 20 is made more than
that on the SOx trap catalyst 30. Or, an amount of precious metal
may be gradually increased from the upstream end of the SOx trap
catalyst 30 to the downstream end of the NOx catalyst 20.
[0048] Next, results of tests using model gas in relation to this
embodiment will be described below.
(1) Test Equipment
[0049] FIG. 4 illustrates a test equipment as a whole, and FIG. 5
illustrates an area V of FIG. 4 in detail. Reference numeral 61
denotes a plurality of gas bombs filled with raw material gas for
producing model gas similar to the composition of exhaust gas
discharged from a gasoline engine. The raw material gas referred to
herein may be N.sub.2, O.sub.2, CO or others. Reference numeral 62
denotes a model gas generator having a mass flow controller, for
mixing predetermined amounts of the respective raw material gases
to produce the model gas MG. As shown in FIG. 5 in detail, the
model gas MG passes through a three-way elbow 72, after which
sequentially passes a SOx trap catalyst 64 and a NOx catalyst 65 of
a storage reduction type, and finally is discharged from an exhaust
duct not shown into an outer air.
[0050] As shown in FIG. 4, gaseous oxygen O.sub.2 fed from an
oxygen bomb 67 is bifurcated, one of which is controlled in a flow
rate by a flow rate control unit 68 and fed to an ozone generator
69. In the ozone generator 69, oxygen is selectively and partially
converted to ozone O.sub.3, and these oxygen and ozone (or oxygen
alone) reaches an ozone analyzer 70. The other of the bifurcated
oxygen is controlled in a flow rate by another flow rate control
unit 71, and thereafter mixed with gas fed from the ozone generator
69 and reaches the ozone analyzer 70. In the ozone analyzer 70, the
concentration of ozone in the incoming gas, i.e., supplied gas, is
measured, and thereafter a flow rate of the supplied gas is
controlled by the flow rate control unit 71. Redundant gas is
discharged outside from an exhaust duct not shown, and the supplied
gas controlled in flow rate is mixed with the model gas MG in the
three-way elbow 72 as shown in FIG. 5. This mixed gas sequentially
passes through the SOx trap catalyst 64 and the storage reduction
type NOx catalyst 65, and thereafter, is processed by an exhaust
gas analyzer 78 for measuring the concentration of SOx, SO.sub.2,
H.sub.2S and an ozone analyzer 79 for measuring the concentration
of ozone, and finally discharged outside from an exhaust duct not
shown.
[0051] An electric heater 74 for controlling the temperature of the
SOx trap catalyst 64 is provided on the outer circumference of a
quartz tube 63. Also, a temperature sensor 75 is provided for
measuring the temperature of a catalyst floor of the SOx trap
catalyst 64.
[0052] The NOx catalyst 65 includes a honeycomb-shaped cordielite
substrate having a diameter of 30 mm, a length of 25 mm, a cell
wall thickness of 4 mil (milli inch length; approximately 0.1 mm)
and the number of cells of 400 cps (cells per square inch;
approximately 62 cells per 1 square centimeter) coated with
.gamma.-Al.sub.2O.sub.3 as a carrier. A coated amount is 120 g/L (a
denominator L means 1 liter of catalyst). Barium acetate is carried
thereon with water and calcined at 500.degree. C. for two hours. An
amount of barium acetate carried thereon is 0.2 mol/L. This
catalyst is immersed into a solution containing ammonium
bicarbonate and dried at 250.degree. C. Further, Pt is carried by
using aqueous solution containing dinitrodiamine platinum, and
after being dried, calcined at 450.degree. C. for one hour. An
amount of Pt carried thereon is 2 g/L.
(2) Test Conditions
[0053] Tests were executed under the following conditions on the
respective SOx trap catalysts 64 in Examples 1 to 4 described
later. First, the electric heater 74 is controlled so that the
temperature detected by the temperature sensor 75 is maintained
constant (200.degree. C.). When the temperature is stable, the
model gas of the following composition is made to flow and
simultaneously therewith supplied gas is mixed with the model gas
at the three-way elbow 72. When ozone is fed, the ozone generator
69 is made ON. Thereby, the supplied gas becomes the mixed gas of
ozone and oxygen. Contrarily, if ozone is not fed, the ozone
generator 69 is made OFF. Thereby, the supplied gas is oxygen
alone. The composition of the model gas is SO.sub.2 of 50 ppm,
H.sub.2O of 3% and N.sub.2 of residue. The composition of the
supplied gas containing ozone is ozone O.sub.3 of 50000 ppm and
O.sub.2 of residue. The flow rate of the supplied gas is 1
L/min.
[0054] An amount of sulfur trapped by the SOx trap catalyst 64 and
the NOx catalyst 65 for two hours after feeding the model gas was
obtained by the inductively coupled plasma emission spectrochemical
analysis (ICP analysis).
(3) Examples (SOx Trap Catalyst)
Example 1
[0055] The SOx trap catalyst was formed of a cordielite honeycombe
substrate having a diameter of 30 mm, a length of 25 mm, a cell
wall thickness of 4 mil (milli inch length; 1/1000 inch) and the
number of cells of 400 cpsi (cells per square inch; approximately
62 per 1 square centimeter), coated with .gamma.-Al.sub.2O.sub.3 as
a carrier. An amount of coaing is 120 g/L. Barium acetate was
carried thereon with water and calcined at 500.degree. C. for two
hours. An amount of barium acetate is 0.2 mol/L. This catalyst was
immersed into a solution containing ammonium bicarbonate and dried
at 250.degree. C. Further, Pt is carried by using an aqueous
solution containing dinitrodiamine platinum, and after being dried,
calcined at 450.degree. C. for one hour. An amount of carried Pt is
2 g/L.
Example 2
[0056] Example 2 is the same as Example 1 except that Pt is not
carried.
Example 3
[0057] The SOx trap catalyst was formed of a cordielite honeycombe
substrate having a diameter of 30 mm, a length of 25 mm, a cell
wall thickness of 4 mil (milli inch length; 1/1000 inch) and the
number of cells of 400 cpsi (cells per square inch; approximately
62 per 1 square centimeter), coated with .gamma.-Al.sub.2O.sub.3 as
a carrier. An amount of coaing is 120 g/L. Further, Pt is carried
by using an aqueous solution containing dinitrodiamine platinum,
and after being dried, calcined at 450.degree. C. for one hour. An
amount of carried Pt is 2 g/L. Further, potassium acetate was
carried thereon with water and calcined at 500.degree. C. for two
hours. A carried amount of potassium acetate is 0.2 mo/L.
Example 4
[0058] Example 4 is the same as Example 3 except that Pt is not
carried.
(4) Test Results
[0059] FIG. 6 illustrate the comparison of ratios of sulfuric
components trapped by the SOx trap catalysts in Examples 1 to 4. As
apparent from the drawing, when the ozone generator 69 was switched
ON to feed ozone, it was possible to trap approximately 100% of the
sulfuric components in either of Examples. From this result, it is
possible to confirm the effect of the present invention for
preventing SOx from flowing to the downstream side of the SOx trap
catalyst as well as avoiding the sulfuric poisoning of the NOx
catalyst.
[0060] On the other hand, when the ozone generator 69 is switched
OFF not to feed ozone, a ratio of sulfuric components trapped by
the SOx trap catalyst becomes less in comparison with a case
wherein ozone is fed. The reason therefor is that SO.sub.2 in the
model gas is not so sufficiently oxidized as in a case of ozone.
However, in Examples 1 and 3 having Pt, more amounts of sulfuric
components could be trapped than in Examples 2 and 4 having no Pt.
The reason therefor is that Pt forming the active points can
oxidize or activate SO.sub.2 in the model gas although not so
effective as ozone. In this regard, since this test was carried out
for a short period as two hours after being exchanged to a fresh
one, it is expected that if the trapping of sulfuric components is
conducted for a longer period, Pt in Examples 1 and 3 are gradually
covered with sulfate to lower the SOx absorbing ability and finally
a level of Examples 2 and 4 is reached.
[0061] While the present invention has been described above with
reference to the embodiment, the present invention should not be
limited to this embodiment. For example, although the NOx catalyst
20 and the SOx trap catalyst 30 are separately provided from each
other in the above embodiment, these may be integrated together by
using a common substrate or a casing.
[0062] The present invention includes all variations, modifications
or equivalents contained in a spirit of the present invention as
defined by attached claims. Accordingly, the present invention
should not be limitative but applicable to any other techniques
included within a spirit of the present invention.
INDUSTRIAL APPLICABILITY
[0063] The present invention is applicable to a device for cleaning
exhaust gas of an internal combustion engine provided with exhaust
gas cleaning means such as catalysts.
* * * * *