U.S. patent application number 10/263011 was filed with the patent office on 2003-06-05 for method and device for the catalytic conversion of gaseous pollutants in the exhaust gas of combustion engines.
Invention is credited to Bohnke, Harald, Gieshoff, Jurgen, Kreuzer, Thomas, Losche, Bernd, Lox, Egbert, Pfeifer, Marcus, Staab, Roger, van Setten, Barry.
Application Number | 20030101718 10/263011 |
Document ID | / |
Family ID | 8178874 |
Filed Date | 2003-06-05 |
United States Patent
Application |
20030101718 |
Kind Code |
A1 |
Pfeifer, Marcus ; et
al. |
June 5, 2003 |
Method and device for the catalytic conversion of gaseous
pollutants in the exhaust gas of combustion engines
Abstract
The present invention is directed to a method and a device for
the catalytic conversion of harmful substances contained in the
exhaust gas of combustion engines, wherein the exhaust gas is
forced to pass through a catalyst-carrying porous support. The
support may be comprised of a catalytic material support itself,
have a catalytic material coating its pores and/or have a catalytic
layer on one or both of the surfaces through which the exhaust gas
will travel.
Inventors: |
Pfeifer, Marcus; (Solingen,
DE) ; Bohnke, Harald; (Darmstadt, DE) ; van
Setten, Barry; (Rodenbach, NL) ; Losche, Bernd;
(Gelnhausen, DE) ; Staab, Roger; (Freigericht,
DE) ; Gieshoff, Jurgen; (Biebergemund, DE) ;
Lox, Egbert; (Hanau, DE) ; Kreuzer, Thomas;
(Karben, DE) |
Correspondence
Address: |
KALOW & SPRINGUT LLP
488 MADISON AVENUE
19TH FLOOR
NEW YORK
NY
10022
US
|
Family ID: |
8178874 |
Appl. No.: |
10/263011 |
Filed: |
October 2, 2002 |
Current U.S.
Class: |
60/299 ;
60/301 |
Current CPC
Class: |
Y02T 10/22 20130101;
B01D 53/9431 20130101; F01N 3/2828 20130101; F01N 3/0222 20130101;
B01D 53/944 20130101; B01J 35/04 20130101; F01N 3/0842 20130101;
F01N 3/206 20130101; Y02T 10/12 20130101; F01N 3/035 20130101; B01J
23/42 20130101; B01D 53/9454 20130101; B01J 37/0215 20130101 |
Class at
Publication: |
60/299 ;
60/301 |
International
Class: |
F01N 003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 6, 2001 |
EP |
01123935.7 |
Claims
1. A method for the catalytic conversion of pollutants contained in
an exhaust gas of an internal combustion engine, said method
comprising contacting an exhaust gas with a catalyst, wherein the
catalyst comprises a catalyst-carrying porous support with an open
pore structure, wherein said support has a first surface and a
second surface, and wherein the exhaust gas enters the support
through said first surface, passes through the support, and leaves
the catalyst through the said second surface.
2. The method according to claim 1, wherein at least one catalytic
layer is present on said first surface and/or said second
surface.
3. The method according to claim 2, wherein the support comprises a
wall-flow-filter manufactured from an inert ceramic material.
4. The method according to claim 1, wherein the support comprises a
wall-flow filter manufactured by extrusion of a catalytic
material.
5. The method according to claim 3, wherein the internal combustion
engine is a diesel engine or a lean bum direct injection engine and
the catalyst is a NO.sub.x-storage catalyst, an SCR catalyst or a
HC-DeNO.sub.x catalyst for the reduction of nitrogen oxides.
6. The method according to claim 4, wherein the internal combustion
engine is a diesel engine or a lean burn direct injection engine
and the catalyst is a NO.sub.x-storage catalyst, an SCR catalyst or
a HC-DeNO.sub.x catalyst for the reduction of nitrogen oxides.
7. The method according to claim 3, wherein the internal combustion
engine is a diesel engine and the catalyst is a diesel oxidation
catalyst for the oxidation of carbon monoxide and hydrocarbons, as
well as soluble organic compounds adhered to soot particles.
8. The method according to claim 4, wherein the internal combustion
engine is a diesel engine and the catalyst is a diesel oxidation
catalyst for the oxidation of carbon monoxide and hydrocarbons, as
well as soluble organic compounds adhered to soot particles.
9. The method according to claim 3, wherein the internal combustion
engine is a stoichiometrically operated gasoline engine and the
catalyst is a three-way catalyst for simultaneous conversion of
nitrogen oxides, hydrocarbons and carbon monoxide.
10. The method according to claim 4, wherein the internal
combustion engine is a stoichiometrically operated gasoline engine
and the catalyst is a three-way catalyst for simultaneous
conversion of nitrogen oxides, hydrocarbons and carbon
monoxide.
11. A device for the catalytic conversion of harmful substances in
an exhaust gas of an internal combustion engine, wherein the device
comprises a catalyst-carrying porous support with an open pore
structure, and wherein said open pore structure has a first surface
that permits the entry of the exhaust gas and a second surface that
permits the exit of the exhaust gas.
12. The device according to claim 11, wherein said
catalyst-carrying porous support comprising a catalytic
material.
13. The device according to claim 12, wherein the catalytic
material is a HC-absorber, a NO.sub.x-storage catalyst, a diesel
oxidation catalyst, an SCR catalyst, a HC-DeNO.sub.x catalyst or a
three-way catalyst.
14. The device according to claim 13, wherein the porous support is
in the form of a wall-flow filter.
15. The device according to claim 11, wherein a catalyst layer is
attached to said first surface and/or said second surface.
16. The device of claim 11, wherein said support comprises an inert
ceramic material.
17. An exhaust system comprising the device of claim 11, wherein
the device is located within an exhaust gas pipe and said first
surface is oriented upstream of said second surface.
18. The exhaust system of claim 17, wherein said device spans a
cross-section of said exhaust gas pipe.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of catalytic
conversion of gaseous pollutants.
BACKGROUND OF THE INVENTION
[0002] Combustion engines can be, on the one hand, part of
stationary combustion plants or, on the other hand, used in mobile
vehicles. Unfortunately, both types of engines produce gases that
contain harmful substances that pollute the environment
considerably. These harmful substances or pollutants typically
comprise carbon monoxide (CO), hydrocarbons (HC) and nitrogen
oxides (NO.sub.x).
[0003] Over the years, governments around the world have reduced
the allowable limits for emission of these pollutants. In order to
comply with these requirements, persons skilled in the art have
tried to optimize the combustion processes in combustion engines,
especially in diesel and gasoline engines. These optimization
techniques have reduced the emission of harmful substances
considerably. However, known optimization processes are not
sufficient to comply with the current legal limits.
[0004] Consequently, in order to lower further the amount of
pollutants emitted with exhaust gases, catalysts that convert the
pollutants into harmless compounds such as water, carbon dioxide
and nitrogen have been employed.
[0005] By way of example, it is known to use three-way catalysts.
These types of catalysts simultaneously convert nitrogen oxides,
hydrocarbons and carbon monoxide into less noxious substances.
[0006] It is also known to use SCR catalysts, which are especially
used for the treatment of the exhaust gas of combustion engines in
stationary combustion plants. The acronym SCR stands for "selective
catalytic reduction," which means that nitrogen oxides contained in
the exhaust gas are chemically reduced to nitrogen and water even
in the presence of oxygen, by adding ammonia to the exhaust gas and
contacting the mixture of nitrogen oxides, ammonia and the other
exhaust gas components with the SCR-catalyst.
[0007] These types of catalysts are typically applied as layers,
referred to as "wash coat layers," onto the surfaces of solid
supports. The solid supports may, for example, be in the form of
pellets or tablets consisting of, for example, alumina.
[0008] Known catalyst support systems may, for example, be designed
in the form of flow-through substrates. Flow-through substrates
typically have honeycomb structures with a plurality of flow
channels through which the exhaust gas may flow, extending from an
inlet to an outlet and are made of metal or ceramic. In these
structures, a catalytic coating may be applied to the inner
surfaces of the flow channels.
[0009] Alternatively, flow-through substrates can be made entirely
from one or more catalytically active materials. For this purpose,
the one ore more catalytic materials are mixed with additional
components such as pore-formers, plasticising materials, etc. and
then formed into the desired honeycomb shape by extrusion.
[0010] In flow-through substrates, the exhaust gas flows along the
flow channels. For catalytically converting the pollutants
contained in the exhaust gas, the exhaust gas components must leave
the main flow direction of the exhaust gas stream and diffuse into
the catalytic coating or into the bulk catalytic material and
contact the catalytically active centers of the catalyst
material.
[0011] Thus, before the gaseous pollutants can react at the
catalytic centers, they need to be transported by diffusion into
the catalyst material. Further, after catalytic conversion, the
reaction products need to diffuse out of the catalyst material and
to rejoin the exhaust gas stream in the flow channel.
[0012] Diffusion of the gaseous pollutants only takes place when
the energy of the pollutants is larger than a certain activation
energy. Thus, depending on the temperature of the exhaust gas and
the catalyst, the conversion of the gaseous pollutants may be
limited by the transport mechanisms of these substances, i.e., it
is not the reaction speed at the catalytic centers but the
transport of the pollutants to the catalytic centers that
determines the velocity of the catalytic conversion of the
pollutants. This means, in cases of unfavorable reaction
conditions, there are a considerable number of catalytic centers
that are not reached by the pollutants. Consequently, the highest
possible chemical conversion of harmful substances at the catalytic
centers is not achieved. Thus, there remains a need to improve the
flow of reactants to and the flow of chemical products away from
catalytic materials. The present invention provides one
solution.
SUMMARY OF THE INVENTION
[0013] The present invention is directed to the catalytic
conversion of gaseous pollutants in the exhaust gas of a combustion
engine. It provides both a device and a method for improving the
catalytic conversion of these substances.
[0014] According to one embodiment, the present invention provides
a device for the catalytic conversion of harmful substances
contained in the exhaust gas of an internal combustion engine. The
device comprises a catalyst-carrying porous support, wherein said
catalyst-carrying porous support has a first surface and a second
surface, and wherein said first surface permits the entry of
exhaust gas and said second surface permits the exit of said
exhaust gas. Preferably, the first surface and the second surface
are located on opposite sides of said substrate.
[0015] The catalyst-carrying porous support may be comprised of
and/or coated with a catalyst material. Alternatively, bound to the
first surface may be a first catalyst layer and/or bound to the
second surface may be a second catalyst layer. Wherever a catalyst
layer is applied, it is applied such that it does not prevent the
exhaust from flowing from the first surface to the second
surface.
[0016] According to a second embodiment, the present invention
provides a method for the catalytic conversion of pollutants
contained in an exhaust gas of an internal combustion engine, said
method comprising contacting an exhaust gas with a catalyst,
wherein the catalyst comprises a catalyst-carrying porous support
with an open pore structure, wherein said support has a first
surface and a second surface, and wherein the exhaust gas enters
the support through said first surface, passes through the support,
and leaves through said second surface.
BRIEF DESCRIPTION OF THE FIGURES
[0017] FIG. 1 is an enlarged cutaway illustration of the schematic
structure and operation of a prior art catalyst system.
[0018] FIG. 2 is an enlarged cutaway illustration of the principal
structure and operation of the device according to the present
invention.
[0019] FIG. 3 is a schematic drawing of a catalyst-carrying support
in the form of a wall-flow filter for the removal of gaseous
harmful substances from the exhaust gas of combustion engines.
[0020] FIG. 3b is a representation of a wall-flow filter according
to FIG. 3a with the downstream side plugs of the incident flow
channels cut away.
[0021] FIG. 4 is a representation of a diesel-light-off-test with a
diesel oxidation catalyst with a wall-flow-filter closed on the
downstream side according to FIG. 3a. (Light-off-test with 270 ppm
C.sub.3H.sub.6(C.sub.1))
[0022] FIG. 5 is a representation of a diesel-light-off-test with a
diesel oxidation catalyst with a wall-flow-filter open on the
downstream side according to FIG. 3b. (Light-off-test with 270 ppm
C.sub.3H.sub.6(C.sub.1))
DETAILED DESCRIPTION
[0023] The present invention provides a device and a method for the
catalytic conversion of pollutants contained in the exhaust gas of
an internal combustion engine. According to the present invention,
an exhaust gas is contacted with a catalyst, wherein the catalyst
comprises a catalyst-carrying porous support with an open pore
structure bounded by a first surface and a second surface and the
exhaust gas is forced to enter into the support by said first
surface, pass through the porous structure of the support and leave
the catalyst via said second surface.
[0024] The present disclosure is not intended to be a treatise on
catalyst conversion of gaseous pollutants. Readers are referred to
appropriate available texts for background on this subject.
[0025] According to the present invention, high catalytic
conversion is achieved by forcing exhaust gas to pass through the
porous catalytically active material of the catalyst. This action
increases the contact probability of the pollutants with the
catalytic centers, which in turn exposes the catalytic centers of
the catalytic material directly to the exhaust gas stream. Contrary
to the prior art, in the method according to the present invention,
contact of the exhaust gas stream with the catalytic centers is
enhanced. As the exhaust gas passes through the catalyst, the
entire catalytic material is directly exposed to the exhaust gas
stream.
[0026] In one embodiment, the present invention provides a device
that may be used to facilitate the directional motion of an exhaust
gas to a catalyst. The device comprises a catalyst-carrying porous
support that has a first surface and a second surface. The first
surface and the second surface are on different sides of the
support and are preferably located on opposite sides of the
support.
[0027] The support is preferably located within an exhaust system
in the direct path of the exhaust gas and oriented such that the
first surface is upstream of the second surface. The phrase
"exhaust system" refers to any structure through which an exhaust
gas will flow during emission from an engine, for example, an
exhaust pipe. Because the device is located in the direct path of
the exhaust stream and is porous, the exhaust gas will enter
through the first surface and exit out of the second surface.
Preferably, the device spans most or all of a cross-section of an
exhaust pipe, thereby forcing a part or all of the exhaust gas to
pass through it. For example, it may be oriented perpendicularly to
the length of an exhaust pipe and span the entire cross-section of
the exhaust gas pipe.
[0028] The support may itself be made of a catalytic material.
Alternatively, the support may be coated with a catalytic material
such that the coating forms catalytic centers in the pores of the
support at which the exhaust gas can be converted into less noxious
compounds, while continuing to travel through the support.
[0029] In a preferred embodiment of the present invention, the
porous support is in the form of a wall-flow-filter. This filter
can be manufactured entirely from catalytic material or from an
inert material and coated thereafter with a catalytic coating.
[0030] Alternatively, or in addition to forming the support out of
a catalytic material or coating the support with a catalytic
material, one may attach a catalyst layer to the first surface
and/or the second surface. When a catalyst layer is attached to the
first surface and/or the second surface, it must be sufficiently
porous that the exhaust gas may travel through it.
[0031] The device according to the present invention is suitable
for combustion engines in stationary combustion plants, as well as
for combustion engines in mobile vehicles. Depending on the type of
the combustion engine, diesel or gasoline engine, the catalyst can,
for example, be a hydrogen-adsorber (HC-Adsorber), a
NO.sub.x-storage catalyst, a diesel oxidation catalyst, an SCR
catalyst, a HC-DeNO.sub.x catalyst or a three-way catalyst.
[0032] In a second embodiment, the present invention provides a
method for cleaning the exhaust gas of a diesel engine or a
gasoline lean burn direct injection engine. Preferably, the porous
support is in the form of a wall-flow filter with a catalytic
material consisting of a NO.sub.x-storage catalyst, an SCR catalyst
or a HC-DeNO.sub.x catalyst for the reduction of nitrogen oxides in
the exhaust gas of said engines.
[0033] According to this method, the pollutants are transported to
the catalytically active centers of the catalyst by a directional
motion due to the force of the exhaust gas as it leaves the engine
and travels through the exhaust pipes, and not by diffusion. Thus,
the limitation of the catalytic conversion of the harmful
substances due to transport effects of the substances is mostly
avoided.
[0034] In this embodiment, exhaust gas is forced to enter a porous
support through a first surface and to exit the material through a
second surface. The support is located in the flow stream;
consequently, the exhaust gas does not need to change direction
solely for the purpose of being exposed to the catalyst.
[0035] Either the support itself, or associated with the support,
is a catalytic material. This material may, as described above,
form a layer on the first surface and/or the second surface.
Additionally or alternatively, it may form a coating on the pores
of the substrate itself or be the substrate itself.
[0036] In another embodiment of the present invention, the method
is used to clean the exhaust gas of a stoichiometrically operated
gasoline engine, and the porous support is in the form of a
wall-flow filter with a catalytic coating consisting of a three-way
catalyst for the simultaneous conversion of nitrogen oxides,
hydrocarbons and carbon monoxide contained in the exhaust gas of
said engine.
[0037] The present invention may be more fully understood by
reference to the figures. FIG. 1 is an enlarged cutaway
illustration of the schematic structure and operation of a known
catalyst system for the conversion of pollutants in the exhaust gas
of combustion engines. The porous support, 1, is formed, for
example, by the walls of the flow channels of a flow-through type
honeycomb support. On the surface of the catalyst support, 1, a
porous catalyst layer, 2, is applied. This catalyst layer, the
so-called wash coat, contains catalytic centers, 3, that are
located on the surface, as well as in the volume of the catalyst
layer, 2.
[0038] The catalyst layer, 2, is exposed to the exhaust gas that
flows parallel to the surface of the catalyst layer. The flow
direction of the exhaust gas stream is indicated with A.
[0039] In order to contact the catalytic centers, 3, the harmful
substances of the exhaust gas need to diffuse into the catalyst
layer and to the catalytic centers. The main direction of this
diffusion motion is directed transverse to the flow direction of
the exhaust gas in the flow channel. In FIG. 1, the directions of
the diffusion motion are shown schematically and identified by B
and C. FIG. 1 shows that the exhaust gas needs to enter the
catalyst layer, 2, in the direction indicated with B in order to
reach the catalyst centers, 3, in the interior of the catalyst
layer, 2. The reaction products of the catalytic conversion need to
diffuse back to the surface of the catalyst layer, 2, along the
direction indicated with C. Depending on the temperature of the
exhaust gas and catalyst and on the porosity of the catalyst layer,
the transport of the pollutants to the catalytic centers, 3, might
be slow and limit the achievable conversion rate for the
pollutants.
[0040] FIG. 2 shows an enlarged cutaway illustration of the
principal structure and operation of the device according to the
present invention for the catalytic conversion of gaseous harmful
substances in the exhaust gas of combustion engines. The device
comprises a porous support, 4. On each of the essentially flat
first (front) and second (back) surfaces of the porous support, 4,
a catalyst layer, 2, is applied. This catalyst layer is similar to
the catalyst layer of FIG. 1.
[0041] However, contrary to the assembly according to FIG. 1, the
exhaust gas does not flow parallel to the surface of the catalyst
layer but is forced to traverse the catalyst layers and the porous
support, 4. The exhaust gas enters the catalyst layer, 2 on the
(first surface) front side of the porous support, 4, passes the
support and exits this configuration via the catalyst layer on the
(second surface) back side of the porous support, 4.
[0042] The flow direction A of the exhaust gas as shown in FIG. 2
ensures that both catalyst layers, 2, and thus the catalytic
centers, 3 contained therein, are exposed directly to the full
exhaust gas stream. The directional flow A of the exhaust gas
through the catalyst layers replaces the undirected diffusion
motion of the exhaust gas in the prior art method according to FIG.
1 that always requires a certain activation energy to take place.
In the method according to the present invention, the chemical
conversion of the pollutants at the catalytic centers, 3, is no
longer transport limited.
[0043] Alternatively to the assembly according to FIG. 2, the
porous support can be manufactured entirely from catalytic
material. In that case, no distinct catalyst coatings on the
surfaces of the support, 4, are necessary.
[0044] Dependent on the type of combustion engine and on the
harmful substances in the exhaust gas of the combustion engine,
different catalytic materials can be used.
[0045] For the treatment of the exhaust gas of diesel engines and
of gasoline lean burn direct injection engines, a NO.sub.x-storage
catalyst can be used for converting the nitrogen oxides into
harmless substances. Alternatively, it is also possible to coat the
porous support with an SCR-catalyst and to convert the nitrogen
oxides by combining the present invention with the method of
selective catalytic reduction. Suitable SCR catalytic materials are
made, for example, of a mixture of V.sub.2O.sub.2, WO.sub.3 and
TiO.sub.3. To perform selective catalytic reduction ammonia or a
precursor component that can be converted into ammonia needs to be
added to the exhaust gas stream as a reducing agent.
[0046] For the removal of harmful substances from the exhaust gas
of stoichiometrically operated gasoline engines,
three-way-catalysts can be used. By means of these catalysts, the
simultaneous reduction of nitrogen oxides, as well as the oxidation
of hydrocarbons and carbon monoxide is achieved.
[0047] For the removal of harmful substances from the exhaust gas
of diesel engines, diesel oxidation catalysts can be used. By means
of these catalysts, the oxidation of CO and HC, as well as of
soluble, organic components adhered to soot particles in the
exhaust gas of diesel engines is achieved.
[0048] Finally, HC-DeNO.sub.x catalysts for the reduction of
nitrogen oxides can be used for the treatment of the exhaust gas of
diesel engines.
[0049] FIG. 3a shows a preferred embodiment of the device according
to the present invention for the conversion of gaseous pollutants
in the exhaust gas of combustion engines. The device is essentially
made of a wall-flow-filter, 5.
[0050] The wall-flow-filter, 5, has an alternating arrangement of
entrance flow channels, 6, and exit flow channels, 7. The entrance
flow channels, 6, and the exit flow channels, 7 are separated by
wall elements that form the porous support, 4.
[0051] The inner surfaces of the entrance flow channels, 6, are
coated with catalyst layers, 2. The entrance flow channels, 6, are
closed by plugs, 8, on the downstream side. The exit flow channels,
7, are closed by similar plugs, 8, on the upstream side of the
wall-flow-filter.
[0052] Therefore, the exhaust gas enters the wall-flow-filter, 5,
via the entrance flow channels, 6, but cannot exit on the channels'
downstream side. Thus, the exhaust gas is forced to traverse the
separating walls and catalyst layers, 2, between two adjacent
entrance and exit flow channels and then leaves the
wall-flow-filter, 5, via the open downstream ends of the exit flow
channels.
[0053] FIG. 3b shows a wall-flow-filter, 5, that was used for
comparison purposes. Contrary to the filter according to FIG. 3a,
the plugs, 8, on the downstream side are removed in the
wall-flow-filter according to FIG. 3b. Thus, the exhaust gas can
flow through the entrance flow channels, 6, of the
wall-flow-filter, 5, without being forced to pass the separating
walls and enter the exit flow channels.
[0054] The following example is intended to explain further the
method according to the present invention. The catalyst
arrangements of FIGS. 3a and 3b were used to compare the method of
the present invention with the prior art method of cleaning the
exhaust gas.
EXAMPLE 1
[0055] A wall flow filter according to FIG. 3a made from silicon
carbide (SiC) with a cell density of 31 cm.sup.-2 (200 cpsi; cells
per square inch) and a length of 15.24 cm (6 inches) and 2.54 cm (1
inch) in diameter was coated with a conventional diesel oxidation
catalyst on the basis of platinum on alumina.
[0056] Coating was done from the entrance side of the wall flow
filter. The achieved loading of the filter with the catalyst
material was 44 g per liter of filter volume and the noble metal
concentration was 0.46 g/l (13 g/ft.sup.3).
[0057] The light-off behavior for HC and CO of this catalyst was
determined by means of a model gas test bench at a space velocity
of 25,000 h.sup.-1. The results are shown in FIG. 4. For CO and for
HC light-off temperatures of 152.degree. C. and 200.degree. C. were
measured.
[0058] After these measurements, 3 mm in length of the
wall-flow-filter were cut off on the downstream side so that the
plugs of the exit flow channels 7 of the wall-flow-filter were
removed and the exhaust gas could now pass freely through the
entrance flow channels without being forced to pass the walls
between adjacent entrance and exit channels. This situation is
shown in FIG. 3b.
[0059] Again the CO and HC light-off temperatures of the catalyst
were determined. The resulting temperature for CO was 185.degree.
C. and for HC was 209.degree. C. The results are shown in FIG.
5.
[0060] These latter light-off temperatures are clearly higher than
those obtained with the arrangement according to FIG. 3a. These
results illustrate the improvements in exhaust gas cleaning in the
method according to the invention.
[0061] Having thus described and exemplified the invention with a
certain degree of particularity, it should be appreciated that the
claims that follow are not to be so limited but are to be afforded
a scope commensurate with the wording of each element of the claims
and equivalents thereof.
* * * * *