U.S. patent application number 12/732634 was filed with the patent office on 2010-10-07 for hc-scr system for lean burn engines.
This patent application is currently assigned to BASF Catalysts LLC. Invention is credited to Patrick Burk, Chung-Zong Wan, Xiaolai Zheng.
Application Number | 20100251700 12/732634 |
Document ID | / |
Family ID | 42825039 |
Filed Date | 2010-10-07 |
United States Patent
Application |
20100251700 |
Kind Code |
A1 |
Wan; Chung-Zong ; et
al. |
October 7, 2010 |
HC-SCR System for Lean Burn Engines
Abstract
Systems and methods for abating NOx emission in an exhaust
stream are provided. Systems comprising hydrocarbon conversion over
a partial oxidation catalyst in a slip stream and a hydrocarbon
selective catalytic reduction catalyst are described. The emissions
treatment system is advantageously used for the treatment of
exhaust streams from lean burn engines including diesel engines,
lean burn gasoline engines and locomotive engines.
Inventors: |
Wan; Chung-Zong; (Somerset,
NJ) ; Burk; Patrick; (Freehold, NJ) ; Zheng;
Xiaolai; (Princeton Junction, NJ) |
Correspondence
Address: |
BASF CATALYSTS LLC
100 CAMPUS DRIVE
FLORHAM PARK
NJ
07932
US
|
Assignee: |
BASF Catalysts LLC
Florham Park
NJ
|
Family ID: |
42825039 |
Appl. No.: |
12/732634 |
Filed: |
March 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
61166047 |
Apr 2, 2009 |
|
|
|
61166603 |
Apr 3, 2009 |
|
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61169932 |
Apr 16, 2009 |
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Current U.S.
Class: |
60/287 ; 60/297;
60/301; 60/303 |
Current CPC
Class: |
F01N 3/0231 20130101;
F01N 2240/30 20130101; F01N 2410/00 20130101; F01N 2610/03
20130101; Y02T 10/24 20130101; F01N 3/103 20130101; F01N 13/009
20140601; Y02T 10/12 20130101; F01N 3/2066 20130101 |
Class at
Publication: |
60/287 ; 60/301;
60/303; 60/297 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/10 20060101 F01N003/10; F01N 3/035 20060101
F01N003/035 |
Claims
1. An emissions treatment system for NOx abatement in an exhaust
stream from a lean burn engine, comprising: a main exhaust conduit
in flow communication with the engine exhaust stream, and a
hydrocarbon selective catalytic reduction catalyst (HC-SCR) in flow
communication with the main exhaust conduit; a slip exhaust stream
conduit branched off of the main exhaust conduit connected to the
main exhaust conduit by a first junction to divert a portion of the
exhaust stream from the main exhaust conduit into the slip exhaust
stream conduit to provide a slip exhaust stream to flow through a
catalytic partial oxidation (CPO) catalyst in flow communication
with the slip exhaust stream conduit, and second junction
downstream from the first junction to reintroduce the slip exhaust
stream to the main exhaust conduit upstream of the HC-SCR; and a
hydrocarbon injector upstream of the CPO, wherein the CPO is
effective to convert a portion of the hydrocarbons to carbon
monoxide and hydrogen.
2. The emissions treatment system of claim 1, wherein the CPO is
designed to provide sufficient hydrogen and adequate amount of
hydrocarbons for the downstream HC-SCR catalyst.
3. The emissions treatment system of claim 1, wherein the
hydrocarbon injector device includes a metering device adapted to
control the amount of hydrocarbon injected into the slip exhaust
stream conduit.
4. The emissions treatment system of claim 1, wherein the
hydrocarbon is fuel.
5. The emissions treatment system of claim 1, wherein the portion
of the exhaust gas diverted into the slip exhaust stream conduit is
up to about 10% of the total exhaust flow.
6. The emissions treatment system of claim 1, further comprising a
metering device at the first junction of the slip exhaust stream
conduit to regulate the percentage of exhaust gas diverted to the
slip exhaust stream conduit.
7. The emissions treatment system of claim 6, wherein the
percentage of exhaust gas diverted to the slip exhaust stream
conduit is regulated up to about 10% of the total exhaust flow.
8. The emissions treatment system of claim 1, wherein the CPO
catalyst contains platinum group metals.
9. The emissions treatment system of claim 8, where the platinum
group metals of CPO catalyst are selected from of platinum,
palladium, rhodium and mixtures thereof.
10. The emissions treatment system of claim 1, wherein one or both
of the CPO catalyst and the HC-SCR are disposed on a flow through
monolith.
11. The emissions treatment system of claim 1, further comprising a
diesel particulate filter (DPF) downstream of the HC-SCR
catalyst.
12. The emissions treatment system of claim 1, further comprising a
diesel particulate filter (DPF) upstream of the HC-SCR
catalyst.
13. The emissions treatment system of claim 12, wherein the diesel
particulate filter (DPF) is located between the first junction and
the second junction in flow communication of the main exhaust
conduit.
14. The emissions treatment system of claim 11, further comprising
a diesel oxidation catalyst (DOC) upstream of the DPF and HC-SCR
catalyst.
15. The emissions treatment system of claim 12, further comprising
a diesel oxidation catalyst (DOC) upstream of the DPF and HC-SCR
catalyst.
16. The emissions treatment system of claim 13, further comprising
a diesel oxidation catalyst (DOC) upstream of the DPF and HC-SCR
catalyst.
17. The emissions treatment system of claim 14, wherein the diesel
oxidation catalyst (DOC) is located downstream of the first
junction in flow communication with the main exhaust conduit.
18. The emissions treatment system of claim 15, wherein the diesel
oxidation catalyst (DOC) is located downstream of the first
junction in flow communication with the main exhaust conduit.
19. The emissions treatment system of claim 16, wherein the diesel
oxidation catalyst (DOC) is located downstream of the first
junction in flow communication with the main exhaust conduit.
20. The emissions treatment system of claim 13, wherein the diesel
oxidation catalyst (DOC) is located downstream of the first
junction and upstream of the DPF and in flow communication with the
main exhaust conduit.
21. The emissions treatment system of claim 20, wherein the DOC and
DPF are integrated into a single component.
22. The emissions treatment system of claim 1, further comprising a
NH3-SCR catalyst downstream of the HC-SCR catalyst.
23. The emissions treatment system of claim 1, further comprising
an oxidation catalyst downstream of the HC-SCR catalyst.
24. A method of treating an exhaust stream comprising: passing the
exhaust stream through a main exhaust conduit and a portion of the
exhaust stream through a slip exhaust stream conduit, the main
exhaust conduit comprising a hydrocarbon selective catalytic
reduction catalyst (HC-SCR), the slip exhaust stream conduit
comprising a catalytic partial oxidation (CPO) catalyst, the slip
exhaust stream conduit branching-off of the main exhaust conduit at
a first junction and in flow communication with the CPO, the slip
exhaust stream conduit rejoining the main exhaust conduit at a
second junction upstream of the HC-SCR, a hydrocarbon injector
upstream of the CPO, where the CPO is adapted to convert a portion
of the hydrocarbons to carbon monoxide and hydrogen.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35 USC
.sctn.119(e) to U.S. Provisional Application Ser. No. 61/166,047,
filed Apr. 2, 2009, U.S. Provisional Application Ser. No.
61/166,603 filed Apr. 2, 2009 and U.S. Provisional Application Ser.
No. 61/169,932 filed Apr. 16, 2009 which are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates to emissions treatment systems
and methods useful for reducing contaminants in exhaust gas
streams. Specifically, embodiments of the invention are directed to
emissions treatment systems, and methods of use, for reducing NOx,
the systems including hydrocarbon conversion over a partial
oxidation catalyst to generate hydrogen and exhaust gas stream
partition.
BACKGROUND
[0003] Operation of lean burn engines, e.g., diesel engines, lean
burn gasoline engines and locomotive engines, provide excellent
fuel economy, and have very low emissions of gas phase hydrocarbons
and carbon monoxide due to their operation at high air/fuel ratios
under fuel lean conditions. Diesel engines, in particular, also
offer significant advantages over gasoline engines in terms of
their durability, and their ability to generate high torque at low
speed. Effective abatement of NOx from lean burn engines is
difficult to achieve because NOx conversion rates under fuel lean
conditions is very low. Thus, conversion of the NOx component of
exhaust streams to innocuous components generally requires
specialized NOx abatement strategies for operation under fuel lean
conditions.
[0004] One such strategy for the abatement of NOx in the exhaust
stream from lean burn engines uses NOx storage reduction (NSR)
catalysts, which are also known in the art as "lean NOx traps
(LNT)." LNT catalysts contain NOx sorbent materials capable of
adsorbing or "trapping" oxides of nitrogen under lean conditions
and platinum group metal components to provide the catalyst with
oxidation and reduction functions. In operation, the LNT catalyst
promotes a series of elementary steps which are depicted below in
Equations 1-5. In an oxidizing environment, NO is oxidized to
NO.sub.2 (Equation 1), which is an important step for NOx storage.
At low temperatures, this reaction is typically catalyzed by a
platinum group metal component, e.g., a platinum component. The
oxidation process does not stop here. Further oxidation of NO.sub.2
to nitrate, with incorporation of atomic oxygen, is also a
catalyzed reaction (Equation 2). There is little nitrate formation
in the absence of the platinum group metal component even when
NO.sub.2 is used as the NOx source. The platinum group metal
component has the dual functions of oxidation and reduction. For
its reduction role, the platinum group metal component first
catalyzes the release of NOx upon introduction of a reductant,
e.g., CO (carbon monoxide), H.sub.2 (hydrogen) or HC (hydrocarbon)
to the exhaust (Equation 3). This step may recover some NOx storage
sites but contribute to limited reduction of NOx species. The
released NOx is then further reduced to gaseous N.sub.2 in a rich
environment (Equations 4 and 5). NOx release can be induced by fuel
injection even in a net oxidizing environment. However, the
efficient reduction of released NOx by H.sub.2, CO or HC requires
overall net rich conditions. A temperature surge can also trigger
NOx release because metal nitrate is less stable at higher
temperatures. NOx trap catalysis is a cyclic operation. Metal
compounds are believed to undergo a carbonate/nitrate conversion,
as a dominant path, during lean/rich operations.
[0005] Oxidation of NO to NO.sub.2
NO+1/2O.sub.2.fwdarw.NO.sub.2 (1)
[0006] NOx Storage as Nitrate
2NO.sub.2+MCO.sub.3+1/2O.sub.2.fwdarw.M(NO.sub.3).sub.2+CO.sub.2
(2)
[0007] NOx Release
M(NO.sub.3).sub.2+2CO.fwdarw.MCO.sub.3+NO.sub.2+NO+CO.sub.2 (3)
[0008] NOx Reduction to N.sub.2
NO.sub.2+CO.fwdarw.NO+CO.sub.2 (4)
2NO+2CO.fwdarw.N.sub.2+2CO.sub.2 (5)
[0009] In Equations 2 and 3, M represents a divalent metal cation.
M can also be a monovalent or trivalent metal compound in which
case the equations need to be rebalanced.
[0010] While the reduction of NO and NO.sub.2 to N.sub.2 occurs in
the presence of the NSR catalyst during the rich period, it has
been observed that ammonia (NH.sub.3) can also form as a by-product
of a rich pulse regeneration of the NSR catalyst. For example, the
reduction of NO may proceed with Equations 6 and 7.
[0011] Reduction of NO to NH.sub.3
CO+H.sub.2O.fwdarw.H.sub.2+CO.sub.2 (6)
2NO+5H.sub.2.fwdarw.2NH.sub.3+2H.sub.2O (7)
[0012] This property of the NSR catalyst mandates that NH.sub.3,
which is itself a noxious component, must also now be converted to
an innocuous species before the exhaust is vented to the
atmosphere.
[0013] An alternative strategy for the abatement of NO.sub.x under
development of mobile applications (including treating exhaust from
lean burn engines) uses selective catalytic reduction (SCR)
catalyst technology. The strategy has been proven effective as
applied to stationary sources, e.g., treatment of flue gases. In
this strategy, NO.sub.x is reduced with a reductant, e.g.,
NH.sub.3, to nitrogen (N.sub.2) over an SCR catalyst that is
typically composed of base metals. This technology is capable of
NO.sub.x reduction greater than 90%, thus it represents one of the
best approaches for achieving aggressive NO.sub.x reduction
goals.
[0014] Ammonia is one of the most effective reductants for NO.sub.x
at lean condition using SCR technologies. One of the approaches
being investigated for abating NO.sub.x in diesel engines (mostly
heavy duty diesel vehicles) utilizes urea as a reductant. Urea,
which upon hydrolysis produces ammonia, is injected into the
exhaust in front of an SCR catalyst in the temperature range
200-600.degree. C. One of the major disadvantages for this
technology is the need for an extra large reservoir to house the
urea on board the vehicle. Another significant concern is the
commitment of operators of these vehicles to replenish the
reservoirs with urea as needed, and the requirement of an
infrastructure for supplying urea to the operators. Therefore, less
burdensome and alternative technology utilizing on board fuel as
reductant for the NOx treatment of exhaust gases are desirable.
[0015] Selective catalytic reduction of NO.sub.x using hydrocarbons
(HC-SCR) has been studied extensively as a potential alternative
method for the removal of NOx under oxygen-rich conditions.
Ion-exchanged base metal zeolite catalysts (e.g., Cu-ZSM5) have
typically not been sufficiently active under typical vehicle
operating conditions, and are susceptible to degradation by sulfur
dioxide and water exposure. Catalysts employing platinum-group
metals (e.g., Pt/Al.sub.2O.sub.3) operate effectively over a narrow
temperature window between 180.degree. C. and 220.degree. C. and
are highly selective towards N.sub.2O production.
[0016] Catalytic devices using alumina-supported silver
(Ag/Al.sub.2O.sub.3) have received attention because of their
ability to selectively reduce NO.sub.x under lean exhaust
conditions with a wide variety of hydrocarbon species. The use of
hydrocarbons and alcohols, aldehydes and functionalized organic
compounds over Ag/Al.sub.2O.sub.3 allows reduction of NO.sub.x at
temperatures below 450.degree. C. In addition to the molecules
listed above, diesel fuel could also be used as a reductant. Diesel
fuel does not require additional tanks for diesel-powered vehicles.
The diesel fuel can be supplied to the emissions system by changing
engine management or by supplying an additional injector of diesel
fuel to the exhaust train. However, current HC-SCR catalyst systems
exhibit insufficient durability. Catalyst coking caused by fuel
decomposition and deposition on catalyst and sulfur poisoning
derived from fuel and oil cause catalyst performance deterioration
in relatively short periods of operation time. The catalyst has to
undergo frequent and costly regeneration to sustain desirable
performance.
[0017] Despite these various alternatives, there is no commercially
available practical hydrocarbon SCR catalyst using diesel fuel as
reductant. Therefore, there is a need in the art for systems and
methods for providing durable NOx reduction activity with HC-SCR
technology.
SUMMARY
[0018] One or more embodiments of the invention are directed to
emissions treatment systems for NOx abatement in an exhaust stream
from a lean burn engine. One embodiment of a system comprises a
main exhaust conduit in flow communication with the engine exhaust
stream, and a hydrocarbon selective catalytic reduction catalyst
(HC-SCR) in flow communication with the main exhaust conduit. A
slip exhaust stream conduit branches off of the main exhaust
conduit and is connected to the main exhaust conduit by a first
junction to divert a portion of the exhaust stream from the main
exhaust conduit into the slip exhaust stream conduit to provide a
slip exhaust stream to flow through a catalytic partial oxidation
(CPO) catalyst in flow communication with the slip exhaust stream
conduit. A hydrocarbon injector is located upstream of the CPO
catalyst in the slip stream. A second junction downstream from the
first junction reintroduces the slip exhaust stream to the main
exhaust conduit upstream of the HC-SCR. The CPO is effective to
convert a portion of the hydrocarbons to carbon monoxide and
hydrogen.
[0019] In one or more embodiments, the CPO is designed to provide
sufficient hydrogen and adequate amount of hydrocarbons for the
downstream HC-SCR catalyst. In one or more embodiments, the
hydrocarbon injector device includes a metering device adapted to
control the amount of hydrocarbon injected into the slip exhaust
stream conduit. The hydrocarbon is fuel according to one or more
embodiments.
[0020] In one or more embodiments, the portion of the exhaust gas
diverted into the slip exhaust stream conduit is up to about 10% of
the total exhaust flow. One or more embodiments further comprise a
metering device at the first junction of the slip exhaust stream
conduit to regulate the percentage of exhaust gas diverted to the
slip exhaust stream conduit. In one or more embodiments the CPO
catalyst contains platinum group metals. Examples of the platinum
group metals of CPO catalyst include platinum, palladium, rhodium
and mixtures thereof.
[0021] In one or more embodiments, one or both of the CPO catalyst
and the HC-SCR are disposed on a flow through monolith. In one or
more embodiments, the system includes a diesel particulate filter
(DPF) downstream of the HC-SCR catalyst. In one or more
embodiments, the system includes a diesel particulate filter (DPF)
upstream of the HC-SCR catalyst. In one or more embodiments, the
diesel particulate filter (DPF) is located between the first
junction and the second junction in flow communication of the main
exhaust conduit. In one or more embodiments, the system includes a
diesel oxidation catalyst (DOC) upstream of the DPF and HC-SCR
catalyst. In one or more embodiments, the system includes a diesel
oxidation catalyst (DOC) upstream of the DPF and HC-SCR catalyst.
In one or more embodiments, the system includes a diesel oxidation
catalyst (DOC) upstream of the DPF and HC-SCR catalyst. In one or
more embodiments, the system includes the diesel oxidation catalyst
(DOC) is located downstream of the first junction in flow
communication with the main exhaust conduit. In one or more
embodiments, the system includes the diesel oxidation catalyst
(DOC) is located downstream of the first junction in flow
communication with the main exhaust conduit. In one or more
embodiments, the system includes the diesel oxidation catalyst
(DOC) is located downstream of the first junction in flow
communication with the main exhaust conduit.
[0022] In one or more embodiments, the system includes the diesel
oxidation catalyst (DOC) is located downstream of the first
junction and upstream of the DPF and in flow communication with the
main exhaust conduit. In one or more embodiments, the system
includes the DOC and DPF are integrated into a single component. In
one or more embodiments, the system includes a NH3-SCR catalyst
downstream of the HC-SCR catalyst. In one or more embodiments, the
system includes an oxidation catalyst downstream of the HC-SCR
catalyst.
[0023] Another aspect of the invention pertains to methods of
treating an exhaust stream. In one method embodiment, the exhaust
stream is passed through a main exhaust conduit and a portion of
the exhaust stream through a slip exhaust stream conduit. The main
exhaust conduit comprises a hydrocarbon selective catalytic
reduction catalyst (HC-SCR), and the slip exhaust stream conduit
comprises a catalytic partial oxidation (CPO) catalyst in flow
communication with the slip stream conduit and a hydrocarbon
injector upstream of the CPO. The slip exhaust stream conduit
branches-off of the main exhaust conduit at a first junction and is
in flow communication with the CPO. The slip exhaust stream conduit
rejoins the main exhaust conduit at a second junction upstream of
the HC-SCR. The CPO is adapted to convert a portion of the
hydrocarbons in the slip exhaust stream conduit to carbon monoxide
and hydrogen.
[0024] In one or more method embodiments, the amount of hydrocarbon
injector into the slip exhaust stream conduit is controlled by a
metering device. The hydrocarbon is onboard fuel in one or more
embodiments. In one or more embodiments, a first percentage of the
exhaust stream passes through the main exhaust conduit and a second
percentage of the exhaust stream passes through the slip exhaust
stream conduit, where the first percentage is greater than the
second percentage. In one or more embodiments, the second
percentage of the exhaust stream is controlled by a metering device
located within the slip exhaust stream conduit near the first
junction. One or more embodiments of the method further comprise
passing the exhaust stream in the main exhaust conduit through a
diesel particulate filter located upstream of the HC-SCR
catalyst.
[0025] One or more method embodiments may include passing the
exhaust stream in the main exhaust conduit through a diesel
particulate filter located downstream of the HC-SCR catalyst. One
or more method embodiments may include passing the exhaust stream
in the main exhaust conduit through a diesel particulate filter
located downstream of the first junction and upstream of the second
junction. One or more method embodiments may include passing the
exhaust stream in the main exhaust conduit through a diesel
oxidation catalyst located upstream of the diesel particulate
filter. One or more method embodiments may include passing the
exhaust stream in the main exhaust conduit through a diesel
oxidation catalyst located upstream of the diesel particulate
filter. One or more method embodiments may include passing the
exhaust stream in the main exhaust conduit through a diesel
oxidation catalyst located upstream of the diesel particulate
filter. One or more method embodiments may include passing the
exhaust stream in the main exhaust conduit through a diesel
oxidation catalyst located downstream of the first junction. One or
more method embodiments may include passing the exhaust stream in
the main exhaust conduit through a diesel oxidation catalyst
located downstream of the first junction.
[0026] One or more method embodiments may include passing the
exhaust stream in the main exhaust conduit through a diesel
oxidation catalyst located downstream of the first junction. One or
more method embodiments may include passing the exhaust stream in
the main exhaust conduit through a diesel oxidation catalyst
located downstream of the first junction and upstream of the diesel
particulate filter. One or more method embodiments may include
passing the exhaust stream in the main exhaust conduit through a
diesel oxidation catalyst located downstream of the first junction
and upstream of the diesel particulate filter. One or more method
embodiments may include passing the exhaust stream in the main
exhaust conduit through a diesel oxidation catalyst located
downstream of the first junction and upstream of the diesel
particulate filter.
[0027] In one or more method embodiments, the diesel oxidation
catalyst and the diesel particulate filter are integrated. In one
or more method embodiments, the diesel oxidation catalyst and the
diesel particulate filter are integrated. The method can comprise
according to one or more embodiments passing the exhaust stream in
the main exhaust conduit through a NH3-SCR catalyst located
downstream of the HC-SCR catalyst. One or more method embodiments
include passing the exhaust stream in the main exhaust conduit
through an oxidation catalyst located downstream of the HC-SCR
catalyst.
[0028] The various embodiments of the invention may include, in a
multitude of configurations, various components including, but not
limited to, diesel oxidation catalysts, catalyzed soot filters,
HC-selective catalytic reduction catalysts, NH.sub.3-- selective
catalytic reduction catalyst and oxidation catalysts. The exhaust
gas may be passed through these optional components in a variety of
sequences.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic view showing an engine emission
treatment system according to a detailed embodiment;
[0030] FIG. 2 is a schematic view showing an engine emission
treatment system according to another embodiment;
[0031] FIG. 3 is a schematic view showing an integrated engine
emission treatment system according to an embodiment;
[0032] FIG. 4 is an alternative emission treatment system according
to one or more embodiments of the invention;
[0033] FIG. 5 is an alternative emission treatment system according
to one or more embodiments of the invention;
[0034] FIG. 6 is an alternative emission treatment system according
to one or more embodiments of the invention;
[0035] FIG. 7 is an alternative emission treatment system according
to one or more embodiments of the invention;
[0036] FIG. 8 is an alternative emission treatment system according
to one or more embodiments of the invention;
[0037] FIG. 9 is an alternative emission treatment system according
to one or more embodiments of the invention;
[0038] FIG. 10 is an alternative emission treatment system
according to one or more embodiments of the invention;
[0039] FIG. 11 is an alternative emission treatment system
according to one or more embodiments of the invention;
[0040] FIG. 12 is an alternative emission treatment system
according to one or more embodiments of the invention;
[0041] FIG. 13 is a perspective view of a wall flow filter
substrate;
[0042] FIG. 14 is a cut-away view of a section of a wall flow
filter substrate; and
[0043] FIG. 15 shows a graph of the percent NOx conversion as a
function of catalyst inlet temperature for an HC-SCR under various
operating conditions.
DETAILED DESCRIPTION
[0044] Provided are emissions treatment systems that can be used
for treating exhaust gas from lean burn engines, and methods of
using these systems to treat engine exhaust. Lean burn engines
include, but are not limited to, diesel engines, lean burn gasoline
engines and locomotive engines.
[0045] Small amount of hydrogen (typically <2000 ppm) present in
the lean exhaust can significantly enhance the performance of the
HC-SCR catalyst, reversing damage done to the catalyst through
normal operation. Embodiments of the invention are directed to
systems and methods that can provide the fuel reductant and
hydrogen to the HC-SCR catalyst. The system comprises a slip stream
from the main exhaust to generate hydrogen with on-board fuel and a
catalytic partial oxidation catalyst (CPO) in a net reducing
condition. The slip stream containing unconverted fuel species and
hydrogen is combined with the main exhaust to provide the favorable
HC-SCR reaction conditions. Hydrogen can be generated from a
hydrocarbon feed by a partial oxidation process in which a portion
of the feed reacts with the oxygen in the slip stream under a fuel
rich condition.
[0046] FIG. 1 shows a schematic description of one aspect of the
present invention and is described in greater detail below.
Briefly, a slip stream (typically 1-10% of the total exhaust flow
emanating from the engine) is bypassed upstream of a HC-SCR
catalyst located downstream from the engine. A sufficient amount of
the fuel is introduced into the slip stream to produce a fuel rich
condition. A catalytic partial oxidation catalyst, as described
further below, is placed in the slipstream downstream of the point
of fuel introduction. The slip stream containing unconverted fuel
species and hydrogen is combined with the main exhaust and
introduced to the HC-SCR catalyst. In one embodiment of this
invention, the HC-SCR may be placed downstream of a diesel
oxidation catalyst and diesel particulate filter devices so that
the HC-SCR can be regenerated at the same time as the regeneration
of the DOC and DPF devices. The system can be optionally optimized
with a bypass flow control valve and a fuel control delivery device
to minimize the fuel penalty of the system.
[0047] The following terms shall have, for the purposes of this
application, the respective meanings set forth below.
[0048] "Lean NOx catalyst", "LNC", "hydrocarbon selective catalytic
reduction catalyst" and "HC-SCR" may be used interchangeably within
the specification. These are different than a lean NOx trap (LNT)
which has a NOx storage and release function.
[0049] "Lean gaseous streams" including lean exhaust streams mean
gas streams that have a .lamda.>1.0.
[0050] "Lean periods" refer to periods of exhaust treatment where
the exhaust gas composition is lean, i.e., has a
.lamda.>1.0.
[0051] "Platinum group metal components" refer to platinum group
metals or one of their oxides.
[0052] "Rare earth metal components" refer to one or more oxides of
the lanthanum series defined in the Periodic Table of Elements,
including lanthanum, cerium, praseodymium and neodymium.
[0053] "Rich gaseous streams" including rich exhaust streams mean
gas streams that have a .lamda.<1.0.
[0054] "Rich periods" refer to periods of exhaust treatment where
the exhaust gas composition is rich, i.e., has a
.lamda.<1.0.
[0055] "Washcoat" has its usual meaning in the art of a thin,
adherent coating of a catalytic or other material applied to a
refractory substrate, such as a honeycomb flow through monolith
substrate or a filter substrate, which is sufficiently porous to
permit the passage there through of the gas stream being
treated.
[0056] "Flow communication" means that the components and/or
conduits are adjoined such that exhaust gases or other fluids can
flow between the components and/or conduits.
[0057] "Downstream" refers to a position of a component in an
exhaust gas stream in a path further away from the engine than the
component preceding component. For example, when a diesel
particulate filter is referred to as downstream from a diesel
oxidation catalyst, exhaust gas emanating from the engine in an
exhaust conduit flows through the diesel oxidation catalyst before
flowing through the diesel particulate filter. Thus, "upstream"
refers to a component that is located closer to the engine relate
to another component.
[0058] Reference to an "ammonia-generating component" means a part
of the exhaust system that supplies ammonia (NH.sub.3) as a result
of its design and configuration driven by engine-out emissions and
dosing of reductant (H.sub.2, CO and/or HC) via engine management
or via injection into exhaust. Such a component excludes gas dosing
or other externally supplied sources of NH.sub.3. Examples of
ammonia-generating components include NOx storage reduction (NSR)
catalysts and lean NOx traps (LNT).
[0059] FIG. 1 shows an emissions treatment system 2 for NOx
abatement in an exhaust stream from a lean burn engine 4 according
to one embodiment. An exhaust gas stream containing gaseous
pollutants (e.g., unburned hydrocarbons, carbon monoxide, nitrogen
oxides) and particulate matter is conveyed via a main exhaust
conduit 6 in flow communication with a lean burn engine 4. The
exhaust gas stream in the main exhaust conduit 6 is passed through
a hydrocarbon selective catalytic reduction catalyst (HC-SCR) 8 in
flow communication with the conduit 6. A slip exhaust stream
conduit 10 branches off of the main exhaust conduit 6. The slip
exhaust stream conduit 10 is connected to the main exhaust conduit
6 by a first junction 12 to divert a portion of the exhaust stream
from the main exhaust conduit 6 into the slip exhaust stream
conduit 10 to provide a slip exhaust stream. The slip exhaust
stream flows through a catalytic partial oxidation (CPO) catalyst
14 in flow communication with the slip exhaust stream conduit 10. A
second junction 16 downstream from the first junction 12
reintroduces the slip exhaust stream from the slip exhaust stream
conduit 10 to the main exhaust conduit 6 upstream of the HC-SCR 8.
Line 20 leads to the tail pipe and out of the system.
[0060] Hydrocarbon feed can be introduced through the conduit 18 in
the slip stream upstream of a CPO catalyst 14. The CPO 14 is
effective to convert a portion of the hydrocarbons to carbon
monoxide and hydrogen. The CPO 14 according to one or more
embodiments is designed to provide sufficient hydrogen to enhance
the performance of the HC-SCR 8.
[0061] In some embodiments, the main exhaust conduit 6 includes an
optional additional exhaust system component 22 downstream of the
first junction 12. The additional exhaust system component 22 can
be, for example, one or more of a diesel oxidation catalyst, a
diesel particulate filter, a reductant injector and an air injector
in flow communication with the main exhaust conduit. In one
specific embodiment, the optional additional exhaust system
component 22, for example, one or more of a diesel oxidation
catalyst and a diesel particulate filter, can be placed upstream of
the first junction 12 of the main exhaust conduit 6.
[0062] As shown in the embodiment of FIG. 2, the hydrocarbon
injector 18 can include a metering device 24 adapted to control the
amount of hydrocarbon injected into the slip exhaust stream conduit
10. In specific embodiments, the injected hydrocarbon is on-board
fuel.
[0063] Detailed embodiments of the invention include a metering
device 26 at the first junction 12 of the slip exhaust stream
conduit 10 to regulate the percentage of exhaust gas diverted to
the slip exhaust stream conduit 10. In some embodiments of the
invention, the portion of the exhaust gas diverted from the main
exhaust conduit 6 into the into the slip exhaust stream conduit 10
is up to about 10% of the total exhaust flow. In other detailed
embodiments, the portion of the exhaust gas diverted from the main
exhaust conduit 6 is in the range of about 0.5% to about 15%, or in
the range of about 1% to about 10%. In other detailed embodiments
the portion of the exhaust gas diverted from the main exhaust
conduit 6 is up to about 15%, 14%, 13%, 12% 11%, 10%, 9%, 8%, 7%,
6%, 5%, 4%, 3% or 2% of the total exhaust flow.
[0064] In specific embodiments one or more of the CPO catalyst and
the HC-SCR are disposed on a flow through monolith.
[0065] The emissions treatment system of various embodiments, as
shown in FIG. 2, further comprises a diesel particulate filter
(DPF) 28 located between the first junction 12 and the second
junction 16 in flow communication with the main exhaust conduit 6.
In other embodiments, a diesel oxidation catalyst (DOC) 30 is
located downstream of the first junction 12 and upstream of the DPF
28 and in flow communication with the main exhaust conduit 6.
[0066] In an alternative embodiment, shown in FIGS. 3 and 4, the
DOC 30 and DPF 28 are integrated into a single component or
substrate 32. For example, the DOC 30 and DPF 28 may be disposed in
separate zones of the same substrate 32, where the DOC 30 is
disposed on the upstream segment of the substrate 32, and the CSF
28 is disposed on the downstream segment of the substrate 32.
[0067] Additional embodiments of the invention are directed to
methods of treating an exhaust stream from a lean burn engine.
Referring to FIGS. 1, 2 and 4, the exhaust stream is passed through
a main exhaust conduit 6 and a portion of the exhaust stream
through a slip exhaust stream conduit 10. The main exhaust conduit
6 comprises a hydrocarbon selective catalytic reduction catalyst
(HC-SCR) 8. The slip exhaust stream conduit 10 comprises a
catalytic partial oxidation (CPO) catalyst 14 and hydrocarbons can
be injected into the slip exhaust stream conduit 10 upstream of the
CPO 14 using a hydrocarbon injector 18. The slip exhaust stream
conduit 10 branches-off of the main exhaust conduit 6 at a first
junction 12 and is in flow communication with the CPO 14. The slip
exhaust stream conduit 10 rejoins the main exhaust conduit 6 at a
second junction 16. The second junction 16 is located upstream of
the HC-SCR 8. The CPO 14 is adapted to convert hydrocarbons in the
slip exhaust stream conduit 10 to carbon monoxide and hydrogen.
[0068] The amount of hydrocarbon injected into the slip exhaust
stream conduit 10 may be controlled by a metering device 24. In
detailed embodiments, the hydrocarbon is on-broad fuel.
[0069] According to one or more embodiments of the invention, a
first percentage of the exhaust stream passes through the main
exhaust conduit 6 and a second percentage of the exhaust stream
passes through the slip exhaust stream conduit 10, where the first
percentage is greater than the second percentage. The second
percentage of the exhaust stream in some embodiments is controlled
by a metering device 26 located within the slip exhaust stream
conduit 10 near the first junction 12.
[0070] Various embodiments of the invention further comprise
passing the exhaust stream in the main exhaust conduit 6 through a
diesel particulate filter 28 located downstream of the first
junction 12 and upstream of the second junction 16. In other
embodiments, the exhaust stream in the main exhaust conduit 6 is
passed through a diesel oxidation catalyst 30 located downstream of
the first junction 12 and upstream of the diesel particulate filter
28. In detailed embodiments, as shown in FIG. 4, the diesel
oxidation catalyst 30 and the diesel particulate filter 28 are
integrated into a single component 32.
[0071] The CPO 14, DOC 30, DPF 28 as well as optional components 22
can be made of compositions well known in the art and may comprise
base metals (e.g., ceria) and/or platinum group metals as catalytic
agents. In the upstream position, the DOC and/or particulate filter
provides several advantageous functions. The catalyst serves to
oxidize unburned gaseous and non-volatile hydrocarbons (i.e., the
soluble organic fraction of the diesel particulate matter) and
carbon monoxide to carbon dioxide and water. Removal of substantial
portions of the SOF, in particular, assists in preventing too great
a deposition of particulate matter on the HC-SCR 8. In specific
embodiments, the platinum group metal is selected from the group
consisting of platinum, palladium, rhodium and combinations
thereof.
[0072] In certain embodiments of the invention, one or more of the
DOC 30, DPF 28 and optional components 22 are coated on a soot
filter, for example, a wall flow filter to assist in the removal of
the particulate material in the exhaust stream, and, especially the
soot fraction (or carbonaceous fraction) of the particulate
material. The DOC, in addition to the other oxidation function
mentioned above, lowers the temperature at which the soot fraction
is oxidized to CO.sub.2 and H.sub.2O. As soot accumulates on the
filter, the catalyst coating assists in the regeneration of the
filter. As shown in FIG. 5, a DPF 28 may be located downstream of
the HC-SCR 8 to convert the CO and unconverted fuel species. FIG. 6
shows another alternate embodiment where the DPF 28 is located
upstream of the first junction 12 to minimize or prevent fouling of
the downstream HC-SCR with particulate material. FIG. 7 shows an
alternate embodiment where a DOC 30 is located upstream of the
first junction 12 and the HC-SCR 8 and DPF 28 are located
downstream of the second junction. FIG. 8 shows an alternate
embodiment where a DOC 30 and DPF 28 are located upstream of the
first junction. In another alternate embodiment, as shown in FIG. 9
the system further comprises a NH.sub.3-SCR catalyst downstream of
the HC-SCR to convert any NH.sub.3 emissions generated in the
system. In another specific embodiment, shown in FIG. 10, the
system further comprises an oxidation catalyst downstream of the
HC-SCR to oxidize CO and any unconverted fuel species. FIG. 11
shows an alternative embodiment of an emission treatment system
comprising an ammonia oxidation (AMOX) catalyst 36 located
downstream of the HC-SCR 8 catalyst. The ammonia oxidation catalyst
36 may be useful for removing or abating residual ammonia, which
may be referred to as slipped ammonia through the system.
[0073] FIG. 12 shows an alternate embodiment where the hydrogen
source 38 is an off-line source or component other than a CPO
catalyst. The hydrogen source 38, as shown here, may include a
metering device 40 capable of controlling the amount of hydrogen
being injected into the main exhaust conduit 6. The off-line
H.sub.2 source (and HC reductant) can include the output from a CPO
reaction (a partial oxidation of fuel with oxygen) as previously
described.
[0074] Various optional components 22 can be included in the
exhaust conduit 6. These optional components 22 can be located
upstream of the hydrogen injector 38, downstream of the hydrogen
injector 38 or downstream of the HC-SCR catalyst 8. It is
conceivable that an optional component may be located within the
hydrogen injector 38 prior to the junction with the main exhaust
conduit 6. The alternate embodiments shown are merely indicative of
various ways the invention can be practiced and should not be taken
as limiting. The components can arranged in other configurations
and remain within the scope of the invention.
[0075] As will be understood by those skilled in the art, the
various metering devices may also be connected to a controller. The
controller can include, amongst other components, sensors and
processors. The sensors can be suitable for measuring the
components of the gaseous composition and can be placed at various
locations within the exhaust conduits. The processor can evaluate
data from the sensors and adjust the metering devices to optimize
the function of the various catalytic components.
[0076] Optional components 22 for use with various embodiments of
the invention can be, for example, one or more of a diesel
oxidation catalyst, a diesel particulate filter, a reductant
injector, an air injector, an ammonia oxidation catalyst, an
ammonia selective catalytic reduction catalyst in flow
communication with the main exhaust conduit. Additionally, the
optional components 22 may be a combination of integrated
components, including, but not limited to, those shown in FIG.
3.
Substrates
[0077] In detailed embodiments, any or all of the catalysts,
including the HC-SCR 8, CPO 14, and DOC 30, are disposed on a
substrate. The substrate may be any of those materials typically
used for preparing catalysts, and will typically comprise a ceramic
or metal honeycomb structure, for example, a flow through monolith.
Any suitable substrate may be employed, such as a monolithic
substrate of the type having fine, parallel gas flow passages
extending therethrough from an inlet or an outlet face of the
substrate, such that passages are open to fluid flow therethrough
(referred to as honeycomb flow through substrates). The passages,
which are essentially straight paths from their fluid inlet to
their fluid outlet, are defined by walls on which the catalytic
material is coated as a washcoat so that the gases flowing through
the passages contact the catalytic material. The flow passages of
the monolithic substrate are thin-walled channels, which can be of
any suitable cross-sectional shape and size such as trapezoidal,
rectangular, square, sinusoidal, hexagonal, oval, circular, etc.
Such structures may contain from about 60 to about 600 or more gas
inlet openings (i.e., cells) per square inch of cross section.
[0078] FIGS. 13 and 14 illustrate a wall flow filter substrate 50
which has a plurality of alternately blocked channels 52 and can
serve as a particulate filter. The passages are tubularly enclosed
by the internal walls 53 of the filter substrate. The substrate has
an inlet end 54 and an outlet end 56. Alternate passages are
plugged at the inlet end 54 with inlet plugs 58 and at the outlet
end 56 with outlet plugs to form opposing checkerboard patterns at
the inlet 54 and outlet 56. A gas stream enters through the
unplugged channel inlet 60, is stopped by outlet plug and diffuses
through channel walls 53 (which are porous) to the outlet side. The
gas cannot pass back to the inlet side of walls because of inlet
plugs 58. If such substrate is utilized, the resulting system will
be able to remove particulate matters along with gaseous
pollutants.
[0079] Wall flow filter substrates can be composed of ceramic-like
materials such as cordierite, .alpha.-alumina, silicon carbide,
aluminum titanate, silicon nitride, zirconia, mullite, spodumene,
alumina-silica-magnesia or zirconium silicate, or of porous,
refractory metal. Wall flow substrates may also be formed of
ceramic fiber composite materials. Specific wall flow substrates
are formed from cordierite, silicon carbide, and aluminum titanate.
Such materials are able to withstand the environment, particularly
high temperatures, encountered in treating the exhaust streams.
[0080] Wall flow substrates for use in the inventive system can
include thin porous walled honeycombs (monoliths) through which the
fluid stream passes without causing too great an increase in back
pressure or pressure across the article. Ceramic wall flow
substrates used in the system can be formed of a material having a
porosity of at least 40% (e.g., from 40 to 75%) having a mean pore
size of at least 10 microns (e.g., from 10 to 30 microns).
[0081] In specific embodiments where extra functionality is applied
to the filter (DOC 30, DPF 28 and optional components 22), the
substrates can have a porosity of at least 59% and have a mean pore
size of between 10 and 20 microns. When substrates with these
porosities and these mean pore sizes are coated with the techniques
described below, adequate levels of desired catalyst compositions
can be loaded onto the substrates. These substrates are still able
retain adequate exhaust flow characteristics, i.e., acceptable back
pressures, despite the catalyst loading. U.S. Pat. No. 4,329,162 is
herein incorporated by reference with respect to the disclosure of
suitable wall flow substrates.
[0082] Typical wall flow filters in commercial use are typically
formed with lower wall porosities, e.g., from about 42% to 50%. In
general, the pore size distribution of commercial wall flow filters
is typically very broad with a mean pore size smaller than 25
microns.
[0083] The porous wall flow filter can be catalyzed in that the
wall of the element has thereon or contained therein one or more
catalytic materials. Catalytic materials may be present on the
inlet side of the element wall alone, the outlet side alone, both
the inlet and outlet sides, or the wall itself may consist all, or
in part, of the catalytic material. This invention includes the use
of one or more washcoats of catalytic materials and combinations of
one or more washcoats of catalytic materials on the inlet and/or
outlet walls of the element. The filter may be coated by any of a
variety of means well known to the art.
[0084] The substrates useful for the catalysts of the present
invention may also be metallic in nature and be composed of one or
more metals or metal alloys. The metallic substrates may be
employed in various shapes such as corrugated sheet or monolithic
form. Suitable metallic supports include the heat resistant metals
and metal alloys such as titanium and stainless steel as well as
other alloys in which iron is a substantial or major component.
Such alloys may contain one or more of nickel, chromium and/or
aluminum, and the total amount of these metals may advantageously
comprise at least 15 wt. % of the alloy, e.g., 10-25 wt. % of
chromium, 3-8 wt. % of aluminum and up to 20 wt. % of nickel. The
alloys may also contain small or trace amounts of one or more other
metals such as manganese, copper, vanadium, titanium and the like.
The surface or the metal substrates may be oxidized at high
temperatures, e.g., 1000.degree. C. and higher, to improve the
resistance to corrosion of the alloys by forming an oxide layer on
the surfaces the substrates. Such high temperature-induced
oxidation may enhance the adherence of the refractory metal oxide
support and catalytically promoting metal components to the
substrate.
[0085] In alternative embodiments, one or all of the HC-SCR 8, CPO
14, DOC 30, DPF 28 and optional components 22 may be deposited on
an open cell foam substrate. Such substrates are well known in the
art, and are typically formed of refractory ceramic or metallic
materials.
CPO Catalyst
[0086] The principle of the CPO catalyst is the reaction of fuel
with oxygen to yield carbon monoxide and hydrogen according to
Equation 8.
C.sub.nH.sub.m+(n/2)O.sub.2.fwdarw.nCO+(m/2)H.sub.2 (8)
[0087] The catalytic partial oxidation reaction prevails when
sufficiently high temperature and limited contact time of reactive
gas with the catalyst (high space velocity) are provided. In
detailed embodiments, the CPO 14 catalyst contains platinum and
palladium. In specific embodiments, the platinum group metal
loading is in the range of about 20 g/ft.sup.3 to about 200
g/ft.sup.3. In more specific embodiments, the platinum to palladium
metal ratio in the CPO is in the range of about 1:9 to about 9:1.
In one or more embodiments, the CPO operates between 600.degree. C.
and 700.degree. C. in excess of 100,000 hr.sup.-1 space velocity
(sometimes >250,000 hr.sup.-1). The platinum to palladium metal
ratio of detailed embodiments can be in the range of about 1:5 to
about 5:1, or about 1:4 to about 4:1, or about 1:3 to about 3:1, or
about 1:2 to about 2:1, or about 1:1.
[0088] In a specific embodiment the CPO comprises a suitable high
surface area refractory metal oxide support layer is deposited on a
substrate as described above to serve as a support upon which
finely dispersed catalytic metal may be distended. In particular
embodiments, high surface area refractory metal oxide supports can
be utilized, e.g., alumina support materials, also referred to as
"gamma alumina" or "activated alumina," typically exhibit a BET
surface area in excess of 60 square meters per gram ("m.sup.2/g"),
often up to about 200 m.sup.2/g or higher. Such activated alumina
is usually a mixture of the gamma and delta phases of alumina, but
may also contain substantial amounts of eta, kappa and theta
alumina phases. Refractory metal oxides other than activated
alumina can be used as a support for at least some of the catalytic
components in a given catalyst. For example, bulk ceria, zirconia,
alpha alumina and other materials are known for such use. Although
many of these materials suffer from the disadvantage of having a
considerably lower BET surface area than activated alumina, that
disadvantage tends to be offset by a greater durability or
performance enhancement of the resulting catalyst. "BET surface
area" has its usual meaning of referring to the Brunauer, Emmett,
Teller method for determining surface area by N.sub.2 adsorption.
Pore diameter and pore volume can also be determined using BET-type
N.sub.2 adsorption or desorption experiments.
[0089] A specific support coating is alumina, for example, a
stabilized, high-surface area transition alumina. As used herein
and in the claims, "transition alumina" includes gamma, chi, eta,
kappa, theta and delta forms and mixtures thereof. It is known that
certain additives such as, e.g., one or more rare earth metal
oxides and/or alkaline earth metal oxides may be included in the
transition alumina (usually in amounts comprising from 2 to 10
weight percent of the stabilized coating) to stabilize it against
the generally undesirable high temperature phase transition to
alpha alumina, which is a relatively low surface area. For example,
oxides of one or more of lanthanum, cerium, praseodymium, calcium,
barium, strontium and magnesium may be used as a stabilizer.
[0090] The platinum group metal catalytic component of the
catalytic partial oxidation catalyst comprises palladium and
platinum and, optionally, one or more other platinum group metals.
As used herein and in the claims, "platinum group metals" means
platinum, palladium, rhodium, iridium, osmium and ruthenium.
Suitable platinum group metal components are palladium and platinum
and, optionally, rhodium. Desirable catalysts for partial oxidation
should have at least one or more of the following properties. They
should be able to operate effectively under conditions varying from
oxidizing at the inlet to reducing at the exit; they should operate
effectively and without significant temperature degradation over a
temperature range of about 427.degree. C. to 1315.degree. C.); they
should operate effectively in the presence of carbon monoxide,
olefins and sulfur compounds; they should provide for low levels of
coking such as by preferentially catalyzing the reaction of carbon
with H.sub.2O to form carbon monoxide and hydrogen thereby
permitting only a low level of carbon on the catalyst surface; they
should resist poisoning from such common poisons as sulfur and
halogen compounds. For example, in some otherwise suitable
catalysts, carbon monoxide may be retained by the catalyst metal at
low temperatures thereby decreasing or modifying its activity. The
combination of platinum and palladium is a highly efficient
oxidation catalyst for the purposes of the present invention.
Generally, the catalytic activity of platinum-palladium combination
catalysts is not simply an arithmetic combination of their
respective catalytic activities; the disclosed range of proportions
of platinum and palladium have been found to provide efficient and
effective catalytic activity in treating a rather wide range of
hydrocarbon feeds with good resistance to high temperature
operation and catalyst poisons.
[0091] Rhodium may optionally be included with the platinum and
palladium. Under certain conditions, rhodium is an effective
oxidation as well as a steam reforming catalyst, particularly for
light olefins. The combined platinum group metal catalysts can
catalyze the autothermal reactions at quite low ratios of H.sub.2O
to carbon (atoms of carbon in the feed) and oxygen to carbon,
without significant carbon deposition on the catalyst. This feature
provides flexibility in selecting H.sub.2O to C and O.sub.2 to C
ratios in the inlet streams to be processed.
[0092] The platinum group metals employed in the catalysts of
embodiments of the present invention may be present in the catalyst
composition in any suitable form, such as the elemental metals, as
alloys or intermetallic compounds with the other platinum group
metal or metals present, or as compounds such as an oxide of the
platinum group metal. As used in the claims, the terms palladium,
platinum and/or rhodium "catalytic component" or "catalytic
components" is intended to embrace the specified platinum group
metal or metals present in any suitable form. Generally, reference
in the claims or herein to platinum group metal or metals catalytic
component or components embraces one or more platinum group metals
in any suitable catalytic form. Suitable CPO catalysts are
described in U.S. Pat. No. 4,522,894, the entire content of which
is incorporated herein by reference.
HC-SCR Catalyst
[0093] A HC-SCR catalyst comprising silver on an alumina support is
generally useful for the emissions treatment system of this
invention. In a detailed embodiment, the catalyst contains "well
dispersed" silver species on the surface of an alumina. In a
specific embodiment, the catalyst substantially free of silver
metal and/or silver aluminate.
[0094] The catalyst may be prepared by impregnation of an alumina
support with ionic silver. The alumina support can be any suitable
alumina, including but not limited to, boehmite pseudoboehmite,
diaspore, norstrandite, bayerite, gibbsite, hydroxylated alumina,
calcined alumina, and mixtures thereof. An exemplary silver-alumina
catalyst comprises about 3 to 4 weight percent (wt. %) silver on an
Ag.sub.2O basis supported on alumina. In one embodiment, the
catalyst is prepared by depositing ionic silver on highly
hydroxylated alumina. As used herein, the term "hydroxylated" means
that the surface of the alumina has a high concentration of surface
hydroxyl groups in the alumina as it is obtained, for example
boehmite, pseudoboehmite or gelatinous boehmite, diaspore,
norstrandite, bayerite, gibbsite, alumina having hydroxyl groups
added to the surface, and mixtures thereof. Pseudoboehmite and
gelatinous boehmite are generally classified as non-crystalline or
gelatinous materials, whereas diaspore, norstrandite, bayerite,
gibbsite, and boehmite are generally classified as crystalline.
According to one or more embodiments of the invention, the
hydroxylated alumina is represented by the formula
Al(OH).sub.xO.sub.y where x=3-2y and y=0 to 1 or fractions thereof.
In their preparation, such aluminas are not subject to high
temperature calcination, which would drive off many or most of the
surface hydroxyl groups. In alternative embodiments, the alumina
may be of a type subject to higher temperature calcinations to
provide gamma, delta, theta and alpha-alumina and combinations
thereof.
[0095] Impregnating the alumina with a water soluble, ionic form of
silver such as silver acetate, silver nitrate, etc., and then
drying and calcining the ionic silver-impregnated alumina at a
temperature low enough to fix the silver and decompose the anion
(if possible). Typically for the nitrate salt this would be about
450-550 degrees centigrade to provide an alumina that has
substantially no silver particles greater than about 20 nm in
diameter. In certain embodiments, the diameter of the silver is
less than 10 nm, and in other embodiments, the silver is less than
about 2 nm in diameter. In one or more embodiments, the processing
is performed so that the silver is present in substantially ionic
form and there is substantially no silver metal present as
determined by UV spectroscopy. In one or more embodiments there is
substantially no silver aluminate present. The absence of silver
metal and silver aluminate can be confirmed by x-ray diffraction
analysis. In the presence of small amount of hydrogen and adequate
hydrocarbon fuel species, the catalyst can still perform well when
it has been deposited with carbonaceous and sulfur species.
DOC Catalyst
[0096] The oxidation catalyst can be formed from any composition
that provides effective combustion of unburned gaseous and
non-volatile hydrocarbons (i.e., the VOF) and carbon monoxide. In
addition, the oxidation catalyst should be effective to convert a
substantial proportion of the NO of the NOx component to NO.sub.2.
As used herein, the term "substantial conversion of NO of the NOx
component to NO.sub.2" means at least 20%, and specifically between
30 and 60%. Catalyst compositions having these properties are known
in the art, and include platinum group metal- and base metal-based
compositions. The catalyst compositions can be coated onto
honeycomb flow-through monolith substrates formed of refractory
metallic or ceramic (e.g., cordierite) materials. Alternatively,
oxidation catalysts may be formed on to metallic or ceramic foam
substrates which are well-known in the art. These oxidation
catalysts, by virtue of the substrate on which they are coated
(e.g., open cell ceramic foam), and/or by virtue of their intrinsic
oxidation catalytic activity provide some level of particulate
removal. The oxidation catalyst may remove some of the particulate
matter from the exhaust stream upstream of the wall flow filter,
since the reduction in the particulate mass on the filter
potentially extends the time before forced regenerations.
[0097] One specific oxidation catalyst composition that may be used
in the emission treatment system contains a platinum group
component (e.g., platinum, palladium or rhodium components)
dispersed on a high surface area, refractory oxide support (e.g.,
.gamma.-alumina) which is combined with a zeolite component (e.g.,
a beta zeolite). A specific platinum group metal component
comprises platinum and palladium. When the composition is disposed
on a refractory oxide substrate, e.g., a flow through honeycomb
substrate, the concentration of platinum group metal is typically
from about 10 to 150 g/ft.sup.3. In specific embodiments, the
platinum group metal is typically in the range of about 20
g/ft.sup.3 to about 130 g/ft.sup.3, or about 30 g/ft.sup.3 to about
120 g/ft.sup.3, or about 40 g/ft.sup.3 to about 110 g/ft.sup.3 or
about 50 g/ft.sup.3 to about 100 g/ft.sup.3. In other detailed
embodiments the platinum group metal is present in a concentration
greater than about 10 g/ft.sup.3, about 20 g/ft.sup.3, about 30
g/ft.sup.3, about 40 g/ft.sup.3, about 50 g/ft.sup.3, about 60
g/ft.sup.3, about 70 g/ft.sup.3, about 80 g/ft.sup.3, about 90
g/ft.sup.3, about 100 g/ft.sup.3, about 110 g/ft.sup.3 or about 120
g/ft.sup.3. In still other detailed embodiments, the platinum group
metal is present in a concentration less than about 120 g/ft.sup.3,
about 110 g/ft.sup.3, about 100 g/ft.sup.3, about 90 g/ft.sup.3,
about 80 g/ft.sup.3, about 70 g/ft.sup.3, about 60 g/ft.sup.3,
about 50 g/ft.sup.3, about 40 g/ft.sup.3, or about 30 g/ft.sup.3.
In further detailed embodiments, the range of platinum group metal
concentrations is between any combination of the previously listed
minimum and maximum concentrations.
[0098] Platinum group metal-based compositions suitable for use in
forming the oxidation catalyst are also described in U.S. Pat. No.
5,100,632 (the '632 patent) hereby incorporated by reference. The
'632 patent describes compositions that have a mixture of platinum,
palladium, rhodium, and ruthenium and an alkaline earth metal oxide
such as magnesium oxide, calcium oxide, strontium oxide, or barium
oxide with an atomic ratio between the platinum group metal and the
alkaline earth metal of about 1:250 to about 1:1, and specifically
about 1:60 to about 1:6.
[0099] Catalyst compositions suitable for the oxidation catalyst
may also be formed using base metals as catalytic agents. For
example, U.S. Pat. No. 5,491,120 (the disclosure of which is hereby
incorporated by reference) discloses oxidation catalyst
compositions that include a catalytic material having a BET surface
area of at least about 10 m.sup.2/g and consist essentially of a
bulk second metal oxide which may be one or more of titania,
zirconia, ceria-zirconia, silica, alumina-silica, and
.alpha.-alumina.
[0100] Also useful are the catalyst compositions disclosed in U.S.
Pat. No. 5,462,907 (the '907 patent, the disclosure of which is
hereby incorporated by reference). The '907 patent teaches
compositions that include a catalytic material containing ceria and
alumina each having a surface area of at least about 10 m.sup.2/g,
for example, ceria and activated alumina in a weight ratio of from
about 1.5:1 to 1:1.5. Optionally, platinum may be included in the
compositions described in the '907 patent in amounts effective to
promote gas phase oxidation of CO and unburned hydrocarbons but
which are limited to preclude excessive oxidation of SO.sub.2 to
SO.sub.3. Alternatively, palladium in any desired amount may be
included in the catalytic material.
NH.sub.3-SCR Catalyst
[0101] In one specific embodiment of the present invention, the
system may further comprise a NH.sub.3-SCR catalyst downstream of
the HC-SCR catalyst. The NH.sub.3-SCR catalyst can prevent any
NH.sub.3 generated in the system from releasing to the environment.
Suitable NH.sub.3-SCR catalysts can be any of the known SCR
catalysts useful in the Urea-SCR application. It is advantageous
that NH.sub.3-SCR catalyst comprising molecular sieves with CHA
X-ray crystal structures (e.g. Cu-CHA, Cu-SAPO) are employed. These
molecular sieves with CHA structure exhibiting superior
hydrothermal stability and durability are particularly useful in
this invention.
Gasoline Lean Burn Engines
[0102] While the embodiments described above are with respect to a
diesel engine having a DOC and a DPF downstream of a diesel engine,
it will be appreciated that systems in according with one or more
embodiments of the invention can be used in gasoline lean burn
engines. Accordingly, an exemplary system would include a system of
the type shown in FIG. 1, wherein exhaust from a lean burn engine
is in flow communication with component 22, which may be a suitable
catalyst for oxidizing carbon monoxides and hydrocarbons. An
example of a suitable catalyst for a gasoline engine is a three-way
catalyst (TWC). TWC catalysts which exhibit good activity and long
life comprise one or more platinum group metals (e.g., platinum or
palladium, rhodium, ruthenium and iridium) located upon a high
surface area, refractory oxide support, e.g., a high surface area
alumina coating.
Hydrogen Sources
[0103] In some alternative embodiments, the hydrogen may be
generated by an external source or hydrogen generator. Suitable
hydrogen sources include, but are not limited to, electrolyzers,
plasma reformers, thermal decomposition devices, steam reformers,
compressed gases container and liquefied gases containers.
[0104] Electrolyzers, such as proton exchange membranes (PEM), can
be used to produce hydrogen on board the a vehicle. The PEM splits
water into hydrogen and oxygen molecules which can then be
compressed and injected into the exhaust. PEM systems require only
small amounts of water maintained in the system.
[0105] Plasma reformers convert gaseous hydrocarbons, like
gasoline, diesel fuel, methane, ethane, etc., to hydrogen. A
reaction chamber is charged with sufficient fuel and air and a
plasma is ignited. The plasma based reaction results in hydrogen
evolution. The hydrogen evolution can be optimized with catalytic
components.
[0106] Thermal decomposition devices can crack, or pyrolyze, fuel
to yield hydrogen and carbon oxide species. Thermal decomposition
generally requires high temperatures for efficient conversion.
[0107] Steam reformers can generate hydrogen by reacting fuel with
water. The reaction is exothermic, resulting in an increased
reaction rate. Like electrolysis, a small onboard water source is
required for this type of hydrogen injector.
Example
[0108] To illustrate the effect of hydrogen present in the exhaust,
separate experiments were conducted with an engine aged HC-SCR
catalyst sample in a laboratory reactor. A simulated diesel exhaust
containing 450 ppm carbon C1 (as diesel), 150 ppm NO, 5% CO.sub.2,
5% H.sub.2O, 10% O.sub.2 balance N.sub.2 was introduced to a core
sample taken from the inlet section of an engine aged HC-SCR
catalyst sample at 30,000 hr.sup.-1. The evaluations were conducted
with a down ramp of the catalyst inlet temperature from 500.degree.
C. to 260.degree. C. The HC-SCR catalyst sample was silver
containing monolithic catalyst. A 1 M solution of silver nitrate
was prepared using deionized water. The resulting solution was
stored in a dark bottle away from light sources. The available pore
volume of the various supports was determined by titrating the bare
support with water while mixing until incipient wetness was
achieved. This resulted in a liquid volume per gram of support.
Using the final target Ag.sub.2O level and the available volume per
gram of support, the amount of 1M AgNO.sub.3 solution needed was
calculated. DI water was added to the silver solution, if needed,
so that the total volume of liquid was equal to amount needed to
impregnate the support sample to incipient wetness. If the amount
of AgNO.sub.3 solution needed exceeds the pore volume of the
support, then multiple impregnations were done.
[0109] The appropriate AgNO.sub.3 solution was added slowly to the
support with mixing. After incipient wetness is achieved, the
resulting solid was dried at 90.degree. C. for 16 h, then calcined
at 540.degree. C. for 2 hours. The catalyst was also optionally
subjected to a flowing stream of about 10% steam in air for at
least about, typically about 16 hours at 650.degree. C.
[0110] Catalysts were prepared as described above using
commercially available pseudoboehmite (Catapal.RTM. C1, 270
m.sup.2/g, 0.41 cc/g pore volume, 6.1 nm average pore diameter,
produced by Sasol, North America) and boehmite (P200 (from Sasol),
100 m.sup.2/g, 0.47 cc/g pore volume, 17.9 nm average pore
diameter) alumina supports. Each alumina was processed until the
silver content of the finished catalyst was about 3% by weight on
an Ag.sub.2O basis. A monolith having about 300 cells per square
inch was washcoated with the alumina, resulting in a loading of
about 2 g/in.sup.3. The HC-SCR catalyst was placed in front of a
DOC/DPF in a configuration in an engine exhaust system and aged on
an engine for 50 hours. During the aging, a number of fuel burning
cycles to simulate the regeneration of DOC/DPF were employed.
[0111] The NOx conversion results are shown in FIG. 13. The engine
aged sample as received without any treatment shows about 20% NOx
conversion in the entire temperature range evaluated. The catalyst
exhibits 10% better NOx conversion at 500.degree. C., 20% better
NOx conversion at 400.degree. C., and 50% better NOx conversion at
300.degree. C. when 1000 ppm H.sub.2 is introduced into the
simulated exhaust. The presence of hydrogen in the exhaust
drastically enhances the NOx performance of the severely
deactivated catalyst.
[0112] While this invention has been described with an emphasis
upon preferred embodiments, it will be obvious to those of ordinary
skill in the art that variations in the preferred devices and
methods may be used and that it is intended that the invention may
be practiced otherwise than as specifically described herein.
Accordingly, this invention includes all modifications encompassed
within the spirit and scope of the invention as defined by the
claims that follow.
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