U.S. patent application number 11/470186 was filed with the patent office on 2008-03-06 for system and method for reducing nox emissions.
Invention is credited to Karen Adams, Lifeng Xu.
Application Number | 20080053071 11/470186 |
Document ID | / |
Family ID | 38566521 |
Filed Date | 2008-03-06 |
United States Patent
Application |
20080053071 |
Kind Code |
A1 |
Adams; Karen ; et
al. |
March 6, 2008 |
System and Method for Reducing NOx Emissions
Abstract
An exhaust system, comprising of a first emission control system
in the exhaust system, said emission control system having a at
least a first region and a second region, said second region
physically segregated from the first region and at least partially
downstream of the first region, said first region including a
precious metal component dispersed on a metal oxide support that
stores NOx and said second region including a precious metal
component and a NOx sorbent component dispersed on a metal oxide
support, where said second region includes more of said NOx sorbent
component than said first region; and a second emission control
system including an SCR catalyst coupled downstream of said first
emission control system.
Inventors: |
Adams; Karen; (Ann Arbor,
MI) ; Xu; Lifeng; (Farmington Hills, MI) |
Correspondence
Address: |
ALLEMAN HALL MCCOY RUSSELL & TUTTLE, LLP
806 S.W. BROADWAY, SUITE 600
PORTLAND
OR
97205
US
|
Family ID: |
38566521 |
Appl. No.: |
11/470186 |
Filed: |
September 5, 2006 |
Current U.S.
Class: |
60/286 ; 60/295;
60/301 |
Current CPC
Class: |
F01N 13/009 20140601;
F01N 3/0814 20130101; F01N 3/206 20130101; F01N 2240/25 20130101;
Y02T 10/24 20130101; F01N 13/0093 20140601; Y02T 10/12
20130101 |
Class at
Publication: |
60/286 ; 60/295;
60/301 |
International
Class: |
F01N 3/00 20060101
F01N003/00; F01N 3/10 20060101 F01N003/10 |
Claims
1. An exhaust system, comprising: a first emission control system
in the exhaust system, said emission control system having a at
least a first region and a second region, said second region
physically segregated from the first region and at least partially
downstream of the first region, said first region including a
precious metal component dispersed on a metal oxide support that
stores NOx and said second region including a precious metal
component and a NOx sorbent component dispersed on a metal oxide
support, where said second region includes more of said NOx sorbent
component than said first region; and a second emission control
system including an SCR catalyst coupled downstream of said first
emission control system.
2. The exhaust system of claim 1, wherein the precious metal
component is selected from at least one of platinum, palladium,
rhodium, iridium, ruthenium, osmium, rhenium, silver, gold and/or
mixtures thereof.
3. The exhaust system of claim 1, wherein the precious metal
component includes platinum.
4. The exhaust system of claim 1, wherein the NOx sorbent component
is selected from at least one of alkali metal (lithium, sodium,
potassium, rubidium, cesium) and alkaline earth metal (beryllium,
magnesium, calcium, strontium, barium) oxides and carbonates of
said groups and/or mixtures thereof.
5. The exhaust system of claim 1, wherein the NOx sorbent component
includes barium oxide, and wherein the metal oxide support that
stores NOx is selected from at least one of alumina, zeolite,
silica dispersed on alumina, titania dispersed on alumina, zirconia
dispersed on alumina, and/or mixtures thereof.
6. The exhaust system of claim 1, wherein the metal oxide support
that stores NOx includes alumina.
7. The exhaust system of claim 1, wherein the metal oxide support
is selected from at least one of alumina, zeolite, silica, titania,
zirconia, and/or mixtures thereof.
8. The exhaust system of claim 1, wherein a washcoat in the first
catalyst region consists essentially of a precious metal component
dispersed on a metal oxide support that stores NOx.
9. The exhaust system of claim 1, wherein the first catalyst region
further comprises a NOx sorbent component.
10. The exhaust system of claim 1, wherein at least the second
region further comprises a ceria component.
11. The exhaust system of claim 1, where the first region and the
second region further comprise a ceria component where said second
region includes more of said ceria component than said first
region.
12. The exhaust system of claim 1, where the first region and the
second region further comprise a ceria component where said first
region includes more of said ceria component than said second
region.
13. The exhaust system of claim 1, wherein the first and second
regions are disposed on a monolithic flow-through substrate.
14. The exhaust system of claim 1, wherein the first and second
regions are disposed on a wall flow filter substrate.
15. The exhaust system of claim 1, wherein the first region and the
second region are disposed on a single substrate.
16. The exhaust system of claim 15, wherein the first region and
the second region are segregated by a segment of the substrate.
17. The exhaust system of claim 1 wherein the first region and the
second region are separated by an area of separation.
18. The exhaust system of claim 1 further comprising a urea
reductant injection system, wherein urea from the urea reductant
injection system is introduced between the first emission control
system and the second emission control system.
19. The exhaust system of claim 1 further comprising a particulate
filter system, wherein said filter system is located upstream of
the first emission control system.
20. The exhaust system of claim 1 further comprising a particulate
filter system, wherein said filter system is located downstream of
the second emission control system.
21. The exhaust system of claim 1 further comprising a sensor
coupled in the exhaust system and a controller for adjusting engine
operation to vary an exhaust air-fuel ratio in response to said
sensor.
22. An exhaust system, comprising: a first emission control device
with a washcoat having a first amount of precious metal physically
segregated from NOx storage material and a second amount of
precious metal physically mixed with NOx storage material, where
first amount is upstream of said second amount; and a second
emission control device downstream of said first device, said
second device including an SCR catalyst configured to store
NH3.
23. An exhaust system, comprising: a first emission control system
in the exhaust system, said emission control system having a at
least a first region and a second region, said second region
physically segregated from the first region and at least partially
downstream of the first region, said first region including a
precious metal component and said second region including a
precious metal component and a NOx sorbent component, where said
second region includes more of said NOx sorbent component than said
first region, and said first and second regions are substantially
free of ceria; and a second emission control system including an
SCR catalyst coupled downstream of said first emission control
system.
Description
TECHNICAL FIELD
[0001] The present application relates to the field of automotive
emission control systems and methods.
BACKGROUND
[0002] Emission treatment systems may optimize the abatement of
nitrogen oxides (NOx) by using a downstream a selective catalytic
reduction (SCR) catalytic converter to adsorb ammonia (NH3)
produced by a NOx trap during rich excursions to reduce any NOx
that slips past the NOx trap during lean operation. In this regard,
a NOx trap capable of high NH3 generation may be advantageous
combined with a SCR catalytic converter to reduce NOx emissions.
Further, the development of a low temperature NOx abatement system
may be attractive for emission control, particularly for light duty
diesel vehicles. Sufficient NH3 generation from the NOx trap may be
beneficial to delete a reservoir needed to supply a reductant to
the downstream SCR catalytic converter.
[0003] One approach to convert NOx to NH3 in a NOx trap upstream of
a SCR catalytic converter is described in U.S. Application No.
2005/0129601. In the '601 reference, the NOx trap combines a NOx
sorbent and a platinum group metal (PGM) dispersed on a substrate.
The NOx sorbent is an alkali or alkaline earth metal oxygenated
compound. PGMs can selectively convert stored NOx to NH3 during
rich excursions, while the NOx sorbent component effectively traps
NOx during lean operation. Specifically, in one example, a
two-layered substrate is used with less NOx sorbent material in the
top layer than in the bottom layer. Optionally, the NOx trap
contains oxygen storage components (OSCs), such as ceria, which
have benefits such as improving desulfation, however OSCs limit NH3
production.
[0004] However, the inventors herein have recognized that NOx
sorbents may deactivate PGMs, thereby reducing NOx conversion at
low temperatures. Further, the inventors herein have recognized NOx
traps that contain OSCs may provide low temperature NOx abatement,
however OSCs may limit NH3 production over the broad temperature
range of operation on vehicles. Thus, the approach of '601 may
improve low temperature NOx conversion if OSCs are used, but may
result in insufficient NH3 generation over the broad range of
operating temperatures for optimum NOx conversion.
[0005] Another approach to achieve low temperature NOx abatement is
described in U.S. Pat. No. 6,182,443. Therein, a catalyst composed
of PGM dispersed on an aluminum oxide containing support is located
upstream of an SCR catalyst, and this catalyst system operates in
lean exhaust without any rich excursions. At low temperatures, PGM
may facilitate oxidation and storage of NOx as aluminum nitrate. As
temperatures rise, this nitrate decomposes back to NOx, which may
be reduced to N2 over the downstream SCR with added NH3 or
hydrocarbon reductant.
[0006] However, the inventors herein have recognized that, for the
'443 catalyst system, since the NOx storage catalyst upstream of
the SCR catalyst does not provide NOx reduction, NOx conversion is
less efficient than if both catalysts provide NOx reduction.
Further, the inventors herein have recognized that if the upstream
PGM/aluminum oxide catalyst were exposed to rich excursions, a
different operation than described in '443, its stored NOx may be
reduced to NH3, but limited to low temperature operation. Thus, the
approach and the catalyst configuration of '443 may be insufficient
to provide optimum overall NOx conversion.
SUMMARY
[0007] In one approach, the above issues may be addressed by an
exhaust system, comprising: a first emission control system in the
exhaust system, said emission control system having at least a
first region and a second region, said second region physically
segregated from the first region and at least partially downstream
of the first region, said first region including a precious metal
component dispersed on a metal oxide support that stores NOx and
said second region including a precious metal component and a NOx
sorbent component dispersed on a metal oxide support, where said
second region includes more of said NOx sorbent component than said
first region; and a second emission control system including an SCR
catalyst coupled downstream of said first emission control system.
In this way, it is possible to advantageously achieve NOx storage
and ammonia generation across a larger temperature range, which
includes low temperatures, so that NOx abatement improves.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic depiction of an exemplary embodiment
of an engine.
[0009] FIG. 2 is a block diagram illustrating an exemplary
combination NOx trap/NH3-SCR emission treatment system.
[0010] FIG. 3 is an exemplary graph of data acquired from a NOx
trap illustrating the effect of a ceria component on the selective
conversion of NOx to NH3 at a range of temperatures.
[0011] FIG. 4 is an exemplary graph of data acquired from a NOx
trap illustrating the effect of a ceria component on the total
conversion over a range of temperatures.
[0012] FIG. 5 is an exemplary graph of data acquired from a NOx
trap illustrating the effect of a NOx sorbent component on the
total conversion of at a range of temperatures.
[0013] FIG. 6A-B is a flowchart of an exemplary method for the
treatment of exhaust from a combustion engine by the emission
control system described respecting FIG. 2 at rich operation (FIG.
6A) and at lean operation (FIG. 6B).
DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS
[0014] FIG. 1 is a schematic diagram showing one cylinder of
multi-cylinder engine 10, which may be included in a propulsion
system for a vehicle. Engine 10 may be controlled at least
partially by a control system including controller 12 and by input
from a vehicle operator 132 via an input device 130. In this
example, input device 130 includes an accelerator pedal and a pedal
position sensor 134 for generating a proportional pedal position
signal PP. Combustion chamber (e.g., cylinder) 30 of engine 10 may
include combustion chamber walls 32 with piston 36 positioned
therein. Piston 36 may be coupled to crankshaft 40 so that
reciprocating motion of the piston is translated into rotational
motion of the crankshaft. Crankshaft 40 may be coupled to at least
one drive wheel of the passenger vehicle via a transmission system.
Further, a starter motor may be coupled to crankshaft 40 via a
flywheel to enable a starting operation of engine 10.
[0015] Combustion chamber 30 may receive intake air from intake
passage 44 via intake manifold 42 and may exhaust combustion gases
via exhaust passage 48. Intake passage 44 and exhaust passage 48
can selectively communicate with combustion chamber 30 via
respective intake valve 52 and exhaust valve 54. In some
embodiments, combustion chamber 30 may include two or more intake
valves and/or two or more exhaust valves.
[0016] Fuel injector 66 is shown coupled directly to combustion
chamber 30 for injecting gasoline or diesel fuel directly therein
in proportion to the pulse width of signal FPW received from
controller 12 via electronic driver 68. In this manner, fuel
injector 66 provides what is known as direct injection of fuel into
combustion chamber 30. The fuel injector may be mounted in the side
of the combustion chamber or in the top of the combustion chamber,
for example. Fuel may be delivered to fuel injector 66 by a fuel
system (not shown) including a fuel tank, a fuel pump, and a fuel
rail. In some embodiments, combustion chamber 30 may alternatively
or additionally include a fuel injector arranged in intake passage
44 in a configuration that provides what is known as port injection
of fuel into the intake port upstream of combustion chamber 30.
[0017] Intake manifold 42 may include a throttle 62 having a
throttle plate 64. In this particular example, the position of
throttle plate 64 may be varied by controller 12 via a signal
provided to an electric motor or actuator included with throttle
62, a configuration that is commonly referred to as electronic
throttle control (ETC). In this manner, throttle 62 may be operated
to vary the intake air provided to combustion chamber 30 among
other engine cylinders. The position of throttle plate 64 may be
provided to controller 12 by throttle position signal TP. Intake
manifold 42 may include a mass air flow sensor 120 and a manifold
air pressure sensor 122 for providing respective signals MAF and
MAP to controller 12.
[0018] Ignition system 88 can provide an ignition spark to
combustion chamber 30 via spark plug 92 in response to spark
advance signal SA from controller 12, under select operating modes.
Though spark ignition is shown in FIG. 1, in some embodiments,
combustion chamber 30 or one or more other combustion chambers of
engine 10 may be operated in a compression ignition mode, with or
without an ignition spark. For example, engine 10 may be a diesel
engine without a spark plug.
[0019] Exhaust gas sensor 126 is shown coupled to exhaust passage
48 upstream of emission control device 70. Sensor 126 may be any
suitable sensor for providing an indication of exhaust gas air/fuel
ratio such as a linear oxygen sensor or UEGO (universal or
wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO, a
HEGO (heated EGO), a NOx, HC, or CO sensor.
[0020] Aftertreatment device 70 is shown arranged along exhaust
passage 48 downstream of exhaust gas sensor 126 where in
aftertreatment device 70 may be a three way catalyst (TWC), NOx
trap, various other emission control devices, or combinations
thereof as to be detailed further in FIG. 2. Aftertreatment device
70 may be configured to adsorb NOx when engine 10 is operating with
a lean air to fuel ratio. Controller 12 may be configured to
periodically provide a rich exhaust stream (for example, by
performing an additional injection of fuel after top dead center of
the compression stroke, or by operating with rich combustion) to at
least react some adsorbed NOx with HC and CO to purge
aftertreatment device 70 of stored NOx. Selective catalytic
reduction (SCR) catalytic converter 74 is configured to adsorb NH3
that exits aftertreatment device 70. SCR catalytic converter 74 is
shown arranged along exhaust passage 48 downstream of
aftertreatment device.
[0021] Controller 12 is shown in FIG. 1 as a microcomputer,
including microprocessor region 102, input/output ports 104, an
electronic storage medium for executable programs and calibration
values shown as read only memory chip 106 in this particular
example, random access memory 108, keep alive memory 110, and a
data bus. Controller 12 may receive various signals from sensors
coupled to engine 10, in addition to those signals previously
discussed, including measurement of inducted mass air flow (MAF)
from mass air flow sensor 120; engine coolant temperature (ECT)
from temperature sensor 112 coupled to cooling sleeve 114; a
profile ignition pickup signal (PIP) from Hall effect sensor 118
(or other type) coupled to crankshaft 40; throttle position (TP)
from a throttle position sensor; and absolute manifold pressure
signal, MAP, from sensor 122. Engine speed signal, RPM, may be
generated by controller 12 from signal PIP. Manifold pressure
signal MAP from a manifold pressure sensor may be used to provide
an indication of vacuum, or pressure, in the intake manifold. Note
that various combinations of the above sensors may be used, such as
a MAF sensor without a MAP sensor, or vice versa.
[0022] Controller 12 may determine various conditions of
aftertreatment device 70 and SCR catalytic converter 74 in any
suitable manner. For example, the temperature Tatd of
aftertreatment device 70 may be provided by a temperature sensor 72
and the temperature Tcat of SCR catalytic converter 74 may be
provided by a temperature sensor 76. In addition, sensor 110
provides an indication of both the oxygen concentration in the
exhaust gas as well as NOx concentration. Signal 112 provides
controller 12 a voltage indicative of the O.sub.2 concentration
while signal 114 provides a voltage indicative of NOx
concentration. Under some conditions, controller 12 may be
configured to provide a rich exhaust stream based on various
conditions, such as NOx concentration downstream of SCR catalytic
converter 74.
[0023] As described above, FIG. 1 shows only one cylinder of a
multi-cylinder engine, and that each cylinder may similarly include
its own set of intake/exhaust valves, fuel injector, etc.
[0024] FIG. 2 is an exemplary embodiment of an emission treatment
system 200 that may include an aftertreatment device 70 and a SCR
catalytic converter 74. In general, the aftertreatment device may
store NOx and reduce stored NOx to nitrogen (N2) or NH3 or some
combination thereof, although some NOx that may not be stored or
converted may pass through the aftertreatment device. The SCR
catalytic converter generally may adsorb NH3 on the catalyst
wherein the adsorbed NH3 may selectively and catalytically reduce
NOx exiting the aftertreatment device. While not shown in FIG. 2, a
particular filter may be included upstream and/or downstream of
device 70 and/or device 74.
[0025] The aftertreatment device and the SCR catalytic converter
may be arranged in various configurations. In the embodiment
described herein, the SCR catalytic converter may be arranged
downstream from the aftertreatment device. By doing this, NH3 that
may be generated by the aftertreatment device under some conditions
may be used as the reductant in the SCR catalytic converter for the
selective and catalytic reduction of NOx leaving the aftertreatment
device.
[0026] Referring back to FIG. 2, the SCR catalytic converter may
facilitate the adsorption of NH3 and the reduction of NOx by the
adsorbed NH3. Various catalysts may be suitable to adsorb NH3 and
reduce NOx accordingly. For example, base metal (e.g., copper,
iron) exchanged zeolite compositions or various vanadia-based
compositions may be used to form the SCR catalyst. The SCR catalyst
may be in the form of self supporting catalyst particles or as a
honeycomb monolith formed of the SCR catalyst. Further, the SCR
catalyst may be disposed on a substrate, such as a ceramic or metal
honeycomb structure, for example. Various other catalyst
compositions and forms may be disposed on substrates suitable for
the application described herein.
[0027] The aftertreatment device may facilitate the storage of NOx
and the reduction of stored NOx to N2 or NH3 or some combination
thereof. In the embodiment described herein, it may be desirable to
substantially convert stored NOx to NH3 such that NH3 leaving the
aftertreatment device may be adsorbed in the SCR catalytic
converter wherein the adsorbed NH3 may reduce NOx exiting the
aftertreatment device.
[0028] The aftertreatment device may include a first catalyst
region 210 and a second catalyst region 220 wherein the first
catalyst region may be upstream of the second catalyst region. The
catalyst regions may include one or more components dispersed on a
support wherein a catalyst region may be disposed on a substrate,
such as a ceramic or metal honeycomb monolith.
[0029] The components and support may further include one or more
elements or compounds or some combination thereof. For example, a
precious metal component may include the element Pt, a NOx sorbent
component may include the compound barium oxide (BaO), and a
support may include the compound alumina (Al2O3).
[0030] Further, a component as well as the support may exhibit
various properties under some conditions. For example, a property
of a precious metal component, such as Pt, may be to catalyze the
oxidation and storage of NOx. A property of a NOx sorbent
component, such as BaO, may be NOx storage when catalyzed at high
temperature ranges. A property of a refractory metal oxide support,
such as Al2O3, may also be NOx storage but when catalyzed at low
temperature ranges. Another property of a precious metal component,
such as Pt, may be to catalyze the preferential conversion of
stored NOx to NH3 during rich excursions.
[0031] Accordingly, one exemplary embodiment of the catalyst
regions for the components and support described herein (e.g., Pt,
BaO, and Al2O3) may include a precious metal component (e.g., Pt)
and a support (e.g., Al2O3) for the first catalyst region 210 and a
precious metal component (e.g., Pt), a NOx sorbent component (e.g.,
BaO), and a support (e.g., Al2O3) for the second catalyst region
220, wherein the precious metal component may include at least one
precious metal element (e.g., Pt), the NOx sorbent component may
include at least one suitable alkali or alkaline earth compound
(e.g., BaO), and the support may include at least one suitable
refractory metal oxide (e.g., Al2O3).
[0032] The properties of the catalyst regions may relate to the
properties of the components and the support included therein.
However, the components and the support in the catalyst region may
interact such that the aggregate properties of the components and
the support included therein may be distinct for the properties of
the catalyst region. In particular, the properties of the
components and the support included in the catalyst region may be
diluted by various other components included therein.
[0033] For example, the first catalyst region having a precious
metal component may have no constituents of a NOx sorbent component
(e.g., it may be substantially pure of NOx sorbent components, for
example the precious metal component may be Pt), and may exhibit
properties that may include catalyzing the oxidation and storage of
NOx on the Al2O3 support at low temperatures. However, the precious
metal component may be at least partially deactivated by the
presence, or an increased amount, of a NOx sorbent component, such
as BaO. As such, the contaminated precious metal component may less
able to abate NOx at low temperatures.
[0034] For the aftertreatment device described herein, the catalyst
regions may include a precious metal component and a NOx sorbent
component wherein the ratio of the precious metal component to the
NOx sorbent component may be greater in one catalyst region than
the ratio in another catalyst region from which it is physically
segregated, where the region with a higher ratio may be located
upstream of the region with a lower ratio. Likewise, the catalyst
regions may include a precious metal component and a NOx sorbent
component wherein the amount or weight of the NOx sorbent component
may be greater in one catalyst region than in another catalyst
region from which it is physically segregated, where the region
with a higher amount of NOx sorbent may be located downstream of
the region with a lower amount (or substantially no) NOx
sorbent.
[0035] In one particular embodiment, it may be advantageous to
distribute the components such that the components may be disposed
in the aftertreatment device such that the ratio of the precious
metal component to the NOx sorbent component may be greater in the
first catalyst region than the ratio in the downstream second
catalyst region from which it is physically segregated. By doing
this, the first catalyst region including a relatively higher
percent of the precious metal may be exposed to the exhaust when
the concentration of NOx may be the greatest (e.g. as the exhaust
from the engine enters the emission treatment system). As such, the
amount of NOx stored and subsequently converted to NH3 for the SCR
catalytic converter may be increased. Conversely, the components
may be disposed in the aftertreatment device such that the ratio of
the precious metal component to the NOx sorbent component may be
less in the first catalyst region than said ratio in the upstream
second catalyst region for various other reasons.
[0036] Referring back to the embodiment illustrated in FIG. 2, the
first catalyst region may be physically segregated from the second
catalyst region. In one approach, the first catalyst region may be
housed in a first NOx abatement device and the second catalyst
region may be housed in a second separate downstream NOx abatement
device. Further, the first catalyst region may be segregated to
various degrees from the second catalyst region by an area of
separation 230 within the aftertreatment device wherein the
catalyst regions may be segregated to at least partially reduce or
prevent the deactivation of a component in one catalyst region by a
component in another catalyst region. For example, a precious metal
component in one catalyst region may be deactivated by a NOx
sorbent component, such as BaO, in another catalyst region, and
thus by providing an appropriate degree of segregation, improved
performance may be achieved.
[0037] The area of separation may employ various approaches or
structures to segregate the catalyst regions. For example, the area
of separation may include a divider wherein the divider may include
suitable materials capable of at least partially segregating the
catalyst regions while facilitating suitable exhaust flow. In
another example, the area of separation may be a void wherein a
void of some distance segregates the catalyst regions. In another
embodiment of the aftertreatment device segregated washcoat may be
used to configure the catalyst regions. Further, the area of
separation may be configured perpendicular or parallel or to
various other degrees relative to the exhaust flow.
[0038] As noted herein, the aftertreatment device may store NOx and
reduce stored NOx into N2 or NH3. In the embodiment described
herein, it may be desirable to substantially convert NOx to NH3
such that NH3 leaving the aftertreatment device may be adsorbed in
the SCR catalytic converter wherein the adsorbed NH3 may reduce NOx
exiting the aftertreatment device. Conversely, in another approach,
a NOx abatement device may include catalyst regions wherein the
components of the catalyst may not form relatively high amounts of
NH3 or at least one component of the catalyst may hinder NH3
generation, although some NH3 may be generated. For example, some
NOx abatement devices may include an oxygen storage component (OSC)
such as ceria (CeO2) wherein the CeO2 component may limit the
formation of NH3. The CeO2 component may be used for various other
reasons, such as improving low temperature NOx conversion, and also
the intermittent storage of oxygen.
[0039] FIG. 3 is a graph 300 of an exemplary embodiment of data
that may demonstrate the effect of a CeO2 component on the
selective conversion of NOx to NH3 at a range of temperatures. For
example, graph 300 may show the percentage of NOx converted to NH3
at various temperatures for a Pt--BaO--CeO2 catalyst region as
indicated by 310 and for a Pt--BaO catalyst region as indicated by
320. It may be understood that the catalyst regions described
herein (e.g., Pt--BaO--CeO2 and Pt--BaO) may generally be disposed
on a suitable support, such as alumina (Al2O3), although the
support may be excluded from the description so as to focus
specifically on the behavior of the various catalyst
combinations.
[0040] Referring back to graph 300, the exclusion of the CeO2
component in the catalyst region may convert a higher percentage of
NOx to NH3 at a wider temperature range relative to the catalyst
that includes a CeO2 component. As such, it may be beneficial under
some conditions to exclude the CeO2 component from one or more
catalyst regions, such as region 210, 220, or both. By doing this,
the aftertreatment device may generate sufficient NH3 such that a
liquid urea tank or ammonia storage vessel may be excluded from the
emission treatment system described herein. However, the liquid
tank may also be included, if desired.
[0041] The inclusion of the CeO2 component in the Pt--BaO catalyst
composition may convert at least partially higher percentage of NOx
to NH3 at lower temperatures, although the percentage of NOx
converted to NH3 may still not be sufficient for low temperature
ranges. As such, it may be desirable to include at least a
component such as Pt--BaO substantially or at least partially free
of CeO2 so as to generate higher NH3 amounts over a wider
temperature range and at least another component that may
facilitate at least sufficient NH3 generation at a lower
temperature range.
[0042] The NOx that may not react to form NH3 may be converted by
various other mechanisms or may not be stored or converted and exit
the aftertreatment device. The percentage of the NOx that may react
to form NH3 or other products by various mechanisms or some
combination thereof may be referred to cumulatively as the
percentage of total NOx conversion. Specifically, the percentage of
total NOx conversion may include the intermittent conversion of NOx
in a catalyst region for storage or the reduction of NOx to N2 or
the formation of NH3 from NOx or a variety of other mechanisms of
NOx conversion or some combination thereof.
[0043] While it may be advantageous to convert a substantial
percentage of NOx to NH3 to use as a reductant in the SCR catalytic
converter, the emission system described herein may further benefit
by converting the residual NOx not used to generate NH3. By doing
this, a more modest amount of NH3 may be suitable to substantially
reduce the NOx in the SCR catalytic converter. Even further, the
NOx that may be stored in a catalyst region in the aftertreatment
device may be utilized to generate additional NH3 under some
conditions, such as the exhaust composition. As such, a high
percentage of total NOx conversion may be advantageous to
facilitate the optimization of NOx abatement, as will be described
further with regard to FIG. 4 and the corresponding structure
described herein
[0044] FIG. 4 is a graph 400 of another exemplary embodiment of
data that may demonstrate the effect of a CeO2 component on the
percentage of total NOx conversion at a range of temperatures. For
example, graph 400 may show the percentage of total NOx converted
at various temperatures for a Pt--BaO--CeO2 catalyst region as
indicated by 410 and for a Pt--BaO catalyst region as indicated by
420. In particular, graph 400 may show that the exclusion of CeO2
component in the catalyst region may convert lower levels of total
NOx.
[0045] While the inclusion of CeO2 in the catalyst compositions may
convert a higher percentage of total NOx, NH3 generation may still
be insufficient for the emission treatment system described herein.
As such, it may be desirable to include a precious metal component
and a NOx sorbent component such as Pt--BaO in one catalyst region
so as to facilitate NH3 generation and NOx conversion over a
relatively wider higher temperature range (FIGS. 3-4) and a
component or combination of components in another physically
segregated catalyst region that may facilitate NOx conversion at
lower temperatures in the aftertreatment device. Such an approach
may be applied to regions 210 and 220 of FIG. 2. For example,
region 220 may include more CeO2 than region 210, or region 210 may
be substantially free of CeO2.
[0046] FIG. 5 is a graph 500 of an exemplary embodiment of data
that may demonstrate the effect of a NOx sorbent component such as
BaO on the catalyst region. For example, graph 500 may show the
percentage of total NOx converted at various temperatures for
precious metal catalyst region such as the Pt--Rh region as
indicated by 510 and for a precious metal catalyst region such as
the Pt--Rh--BaO region as indicated by 520 wherein the precious
metal catalyst region may include the elements Pt and Rh. In
particular, graph 500 may show that a catalyst region that includes
a substantially pure precious metal component such as Pt--Rh may
convert high levels of total NOx at low temperatures. Said another
way, the exclusion of or reduction of the NOx sorbent component
from a catalyst region to a greater degree than another catalyst
region may facilitate low temperature NOx conversion.
[0047] While the exclusion or reduction of a NOx sorbent component
such as BaO in an upstream region, such as 210, may facilitate low
temperature NOx conversion, NOx conversion at higher temperature
ranges may be substantially reduced. As such, it may be desirable
to combine the properties of a catalyst region that that may
exhibit the NOx conversion properties a precious metal and NOx
sorbent catalyst region (e.g., NH3 generation and total NOx
conversion over a relatively wider higher temperature range) such
as Pt--BaO and the properties of a catalyst region that may exhibit
properties of a region with precious metals having less NOx sorbent
materials (e.g., low temperature NOx conversion) such as Pt--Rh as
demonstrated by FIGS. 3-5 and shown in FIG. 2.
[0048] The combination of a Pt--Rh catalyst region (substantially
free of CeO2) physically segregated and upstream of a Pt--BaO
catalyst region (also substantially free of CeO2) is one exemplary
embodiment of a combination that may be included in the
aftertreatment device. Various other suitable ceria and non-ceria
combinations of components may be used. In general, the catalyst
regions may include at least a first precious metal component
wherein a precious metal component may include one or more precious
metals (e.g. Pt, Pd, Rh, Ir, Ru, Os, Re, Ag, and Au) wherein
precious metals include PGMs (e.g., Pt, Pd, Rh) with a first amount
of a NOx sorbent (which may be zero) and at least a second precious
metal component with a second amount of the NOx sorbent component,
where the second amount is greater than the first amount and where
a NOx sorbent component may include one or more NOx sorbents, such
as an oxide or carbonate of an alkali metal (e.g. Li, Na, K, Rb,
Cs) or an alkaline earth metal (e.g. Be, Mg, Ca, Sr, Ba,), or some
combination thereof. The above components may be dispersed on a
high surface area metal oxide support (e.g., alumina, titania,
zirconia, zeolites, etc), such that the first support is a metal
oxide that may store NOx (e.g., alumina or zeolite). The components
and their support typically are disposed on a monolithic
flow-through substrate, wall flow filter substrate, honeycomb
structures, layered materials, or spun fibers, among other
configurations.
[0049] In one approach, the first precious metal component may be
blended with the second precious metal component and the NOx
sorbent component wherein the blend may be housed in at least one
catalyst region. However, the function of some components may be
reduced or deactivated in the presence of other components. For
example, the NOx sorbent component may reduce the activity of
precious metal components such that higher temperatures may be
required to achieve suitable levels of NOx conversion. Said another
way, the first precious metal component may convert substantially
less NOx at lower temperatures in the presence of the NOx sorbent
component.
[0050] In the embodiment described herein, the first precious metal
component may be physically segregated from the NOx sorbent
component to address the deactivation of the precious metal
component by the NOx sorbent component. In particular, the first
precious metal component may be disposed in one catalyst region and
the second precious metal component with the NOx sorbent may be
disposed in another catalyst region wherein the catalyst regions
may be segregated by various modes as described herein.
[0051] Generally, the physical segregation of the catalyst regions
within the aftertreatment device may facilitate various mechanisms
wherein at least one catalyst region may promote the formation of
NH3 from NOx (e.g., adsorbed NOx or NOx in the exhaust) or the
reduction of stored NOx to N2 or some combination thereof during
rich excursions and one catalyst region may promote the storage of
NOx during lean operation.
[0052] Referring back to FIG. 2, the aftertreatment device and the
SCR system described herein may interact to regulate the exhaust
constituents introduced to the system and various intermediary
byproducts that may be generated within the system. Accordingly,
the SCR catalytic converter responds to the exhaust exiting the
aftertreatment device so as to adsorb NH3 generated during rich
excursions or utilize adsorbed NH3 to reduce NOx. As such, the
aftertreatment device and the SCR catalytic converter engage to
facilitate the optimization of NOx abatement in the emission
treatment system.
[0053] In the embodiment described herein, rich excursions may
include a condition wherein a reducing agent (e.g. hydrocarbons
from fuel not combusted in the engine, injected fuel, etc.) may be
present in the exhaust and relatively low concentrations of NOx may
be detected, while lean operation may include a condition wherein
NOx and excess oxygen may be abundant in the exhaust.
[0054] FIGS. 6A-B may generally describe the method for the
treatment of exhaust from a combustion engine by the emission
control system described herein. In particular, method 600 is a
generalized sequence of actions responsive to the engine operating
conditions wherein the sequence may be schematically illustrated by
the flowchart of 6A during rich excursions and by flowchart of 6B
during normal lean operation.
[0055] The flowcharts are a conceptual representation that may be
simplified for clarity.
[0056] Referring specifically to FIG. 6A, the exhaust from a rich
excursion enters the aftertreatment device at the inlet of the
emission treatment system where. In particular, hydrocarbons (HC)
and relatively low amounts of NOx may enter the aftertreatment
device as constituents of the exhaust wherein HC may reduce NOx
under some conditions.
[0057] At 630, NOx may be reduced to NH3 or reduced to N2 or some
combination thereof in the aftertreatment device. The
aftertreatment device as described herein may promote the
conversion of a relatively high percentage of NOx to NH3, although
some NOx may be reduced to N2 wherein the reducing agent may
include HC in the exhaust. The NOx may be adsorbed in the
aftertreatment device during lean operation or may enter with the
exhaust or some combination thereof. The NH3 generated at the
aftertreatment device may proceed to a downstream SCR catalytic
converter accordingly. Continuing with the figure of 6A, the NH3
entering the SCR catalytic converter may substantially adsorb the
NH3 at 640.
[0058] Referring now to FIG. 6B, the method begins again at the
inlet of the emission treatment system where the exhaust from lean
operation of the engine enters the aftertreatment device. In
particular, relatively high amounts of NOx may enter the
aftertreatment device as constituents of the exhaust. At 650, the
NOx may be adsorbed in the aftertreatment device, although some NOx
may escape adsorption. As such, residual NOx exiting the
aftertreatment device may proceed to the SCR catalytic converter
accordingly. Continuing with flowchart 620, NH3 adsorbed in the SCR
catalytic converter during a rich excursion may reduce the residual
NOx from the aftertreatment device. Accordingly, the exhaust from
the SCR catalytic converter may exit the emission treatment system
substantially free of NOx.
[0059] The emission control system may be regulated by the
controller to follow the above approaches. In particular, the
controller may facilitate the release of NOx and/or NH3 from the
aftertreatment device to the SCR catalytic device during various
conditions by adjusting fuel injection, engine airflow, or
combinations thereof. For example, it may be advantageous to end a
rich excursion and initiate lean operation when the SCR catalytic
converter may be approaching the threshold capacity of NH3
adsorption. At this condition, the controller may prompt lean
operation in response to a signal from a sensor (e.g., oxygen
sensor, NOx sensor, NH3 sensor, etc.), for example, at the SCR
catalytic converter. By doing this, NOx abundant in the exhaust
during lean operation may be reduced by the NH3 saturated in the
SCR catalytic converter.
[0060] Note that the example control and estimation routines
included herein can be used with various engine and/or vehicle
system configurations. The specific routines described herein may
represent one or more of any number of methoding strategies such as
event-driven, interrupt-driven, multi-tasking, multi-threading, and
the like. As such, various steps, operations, or functions
illustrated may be performed in the sequence illustrated, in
parallel, or in some cases omitted. Likewise, the order of
methoding is not necessarily required to achieve the features and
advantages of the example embodiments described herein, but are
provided for ease of illustration and description. One or more of
the illustrated steps or functions may be repeatedly performed
depending on the particular strategy being used. Further, the
described steps may graphically represent code to be programmed
into the computer readable storage medium in the engine control
system.
[0061] It will be appreciated that the configurations and routines
disclosed herein are exemplary in nature, and that these specific
embodiments are not to be considered in a limiting sense, because
numerous variations are possible. For example, the above technology
can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine
types. The subject matter of the present disclosure includes all
novel and nonobvious combinations and subcombinations of the
various systems and configurations, and other features, functions,
and/or properties disclosed herein.
[0062] The following claims particularly point out certain
combinations and subcombinations regarded as novel and nonobvious.
These claims may refer to "an" element or "a first" element or the
equivalent thereof. Such claims should be understood to include
incorporation of one or more such elements, neither requiring nor
excluding two or more such elements. Other combinations and
subcombinations of the disclosed features, functions, elements,
and/or properties may be claimed through amendment of the present
claims or through presentation of new claims in this or a related
application. Such claims, whether broader, narrower, equal, or
different in scope to the original claims, also are regarded as
included within the subject matter of the present disclosure.
* * * * *