U.S. patent application number 12/706558 was filed with the patent office on 2010-08-19 for catalyst system for the reduction of nox and nh3 emissions.
This patent application is currently assigned to Ford Global Technologies, LLC. Invention is credited to John Vito Cavataio, Yisun Cheng, Haren S. Gandhi, Robert Henry Hammerle.
Application Number | 20100209321 12/706558 |
Document ID | / |
Family ID | 32092201 |
Filed Date | 2010-08-19 |
United States Patent
Application |
20100209321 |
Kind Code |
A1 |
Gandhi; Haren S. ; et
al. |
August 19, 2010 |
Catalyst System for the Reduction of NOx and NH3 Emissions
Abstract
This catalyst system simultaneously removes ammonia and enhances
net NOx conversion by placing an NH.sub.3--SCR catalyst formulation
downstream of a lean NOx trap. By doing so, the NH.sub.3--SCR
catalyst adsorbs the ammonia from the upstream lean NOx trap
generated during the rich pulses. The stored ammonia then reacts
with the NOx emitted from the upstream lean NOx trap-enhancing the
net NOx conversion rate significantly, while depleting the stored
ammonia. By combining the lean NOx trap with the NH.sub.3--SCR
catalyst, the system allows for the reduction or elimination of
NH.sub.3 and NOx slip, reduction in NOx spikes and thus an improved
net NOx conversion during lean and rich operation.
Inventors: |
Gandhi; Haren S.; (West
Bloomfield, MI) ; Cavataio; John Vito; (Dearborn,
MI) ; Hammerle; Robert Henry; (Franklin, MI) ;
Cheng; Yisun; (Ann Arbor, MI) |
Correspondence
Address: |
ROTHWELL, FIGG, ERNST & MANBECK, P.C.
1425 K STREET, N.W., SUITE 800
WASHINGTON
DC
20005
US
|
Assignee: |
Ford Global Technologies,
LLC
Dearborn
MI
|
Family ID: |
32092201 |
Appl. No.: |
12/706558 |
Filed: |
February 16, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12325787 |
Dec 1, 2008 |
7674743 |
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12706558 |
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11684064 |
Mar 9, 2007 |
7485273 |
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12325787 |
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10065470 |
Oct 22, 2002 |
7332135 |
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11684064 |
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Current U.S.
Class: |
423/213.2 |
Current CPC
Class: |
B01D 53/8631 20130101;
F01N 13/0097 20140603; F01N 2240/25 20130101; Y02T 10/22 20130101;
Y02C 20/10 20130101; F01N 3/0842 20130101; F01N 3/0814 20130101;
Y02T 10/24 20130101; F01N 3/108 20130101; F01N 2510/06 20130101;
Y02T 10/12 20130101 |
Class at
Publication: |
423/213.2 |
International
Class: |
B01D 53/94 20060101
B01D053/94 |
Claims
1-10. (canceled)
11. A method for the reduction of nitrogen oxides and ammonia from
diesel engine exhaust gas, said method comprising: storing nitrogen
oxides present in the exhaust gas in a nitrogen oxide adsorber
during lean cycles with a lean exhaust gas air ratio; reducing
stored nitrogen oxides from the nitrogen oxide adsorber during rich
pulses, which contain a rich exhaust gas air ratio, and thereby
generating ammonia; adsorbing and storing ammonia in a
NH.sub.3--SCR catalyst arranged downstream of the nitrogen oxide
adsorber; reacting the stored ammonia with nitrogen oxides in the
NH.sub.3--SCR catalyst; operating the engine under lean conditions
during lean cycles; during lean cycles, oxidizing carbon monoxide
and hydrocarbons contained in the exhaust gas in an oxidizing
catalytic converter arranged upstream of the nitrogen oxide
adsorber and NH.sub.3--SCR catalyst; operating the engine under
rich conditions during rich cycles.
12. The method according to claim 1, further comprising filtering
particulate matter in the exhaust gas.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/325,787 filed Dec. 1, 2008, which is a continuation of U.S.
application Ser. No. 11/684,064 filed Mar. 9, 2007, now U.S. Pat.
No. 7,485,273 which is a continuation of U.S. application Ser. No.
10/065,470, filed Oct. 22, 2002, now U.S. Pat. No. 7,332,135.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is directed to a catalyst system to
facilitate the reduction of nitrogen oxides (NOx) and ammonia from
an exhaust gas. More particularly, the catalyst system of this
invention includes a lean NOx trap in combination with an ammonia
selective catalytic reduction (NH.sub.3--SCR) catalyst, which
stores the ammonia formed in the lean NOx trap during rich air/fuel
operation and then reacts the stored ammonia with nitrogen oxides
to improve NOx conversion to nitrogen when the engine is operated
under lean air/fuel ratios. In an alternate embodiment, a three-way
catalyst is designed to produce desirable NH.sub.3 emissions at
stoichiometric conditions and thus reduce NOx and NH.sub.3
emissions.
[0004] 2. Background Art
[0005] Catalysts have long been used in the exhaust systems of
automotive vehicles to convert carbon monoxide, hydrocarbons, and
nitrogen oxides (NOx) produced during engine operation into
non-polluting gases such as carbon dioxide, water and nitrogen. As
a result of increasingly stringent fuel economy and emissions
standards for car and truck applications, it is preferable to
operate an engine under lean conditions to improve vehicle fuel
efficiency and lower CO.sub.2 emissions. Lean conditions have
air/fuel ratios greater than the stoichiometric ratio (an air/fuel
ratio of 14.6), typically air/fuel ratios greater than 15. While
lean operation improves fuel economy, operating under lean
conditions increases the difficulty in treating some polluting
gases, especially NOx.
[0006] Regarding NOx reduction for diesel and lean burn gasoline
engines in particular, lean NOx adsorber (trap) technologies have
been widely used to reduce exhaust gas NOx emissions. Lean NOx
adsorbers operate in a cyclic fashion of lean and rich durations.
The lean NOx trap functions by adsorbing NOx when the engine is
running under lean conditions-until the NOx trap reaches the
effective storage limit-followed by NOx reduction when the engine
is running under rich conditions. Alternatively, NOx reduction can
proceed by simply injecting into the exhaust a sufficient amount of
reductant that is independent of the engine operation. During this
rich cycle, a short rich pulse of reductants, carbon monoxide,
hydrogen and hydrocarbons reduces the NOx adsorbed by the trap
during the lean cycle. The reduction caused during the rich cycle
purges the lean NOx adsorber, and the lean NOx adsorber is then
immediately available for the next lean NOx storage/rich NOx
reduction cycle. In general, poor NOx reduction is observed if the
air excess ratio .lamda. is above 1. NOx reduction generally
increases over lean NOx adsorbers as the .lamda. ratio is decreased
lower than 1. This air excess or lambda ratio is defined as the
actual air/fuel ratio divided by the stoichiometric air/fuel ratio
of the fuel used. The use of lean NOx adsorber (trap) technology,
and in particular the rich pulse of reductants, can cause the
.lamda. ratio to reach well below 1.
[0007] Lean NOx traps, however, often have the problem of low NOx
conversion; that is, a high percentage of the NOx slips through the
trap as NOx. NOx slip can occur either during the lean portion of
the cycle or during the rich portion. The lean NOx slip is often
called "NOx breakthrough." It occurs during extended lean operation
and is related to saturation of the NOx trap capacity. The rich NOx
slip is often called a "NOx spike." It occurs during the short
period in which the NOx trap transitions from lean to rich and is
related to the release of stored NOx without reduction. Test
results depicted in FIG. 1a have shown that during this lean-rich
transition, NOx spikes, the large peaks of unreacted NOx accounts
for approximately 73% of the total NOx emitted during the operation
of a lean NOx trap. NOx breakthrough accounts for the remaining 27%
of the NOx emitted.
[0008] An additional problem with lean NOx traps arises as a result
of the generation of ammonia by the lean NOx trap. As depicted in
FIG. 1b, ammonia is emitted into the atmosphere during rich pulses
of the lean NOx adsorber. In laboratory reactor experiments,
ammonia spikes as high as 600 ppm have been observed under typical
lean NOx adsorber operation (see FIG. 1b). While ammonia is
currently not regulated, ammonia emissions are being closely
monitored by the U.S. Environmental Protection Agency; and,
therefore, reduction efforts must be underway. Ammonia is created
when hydrogen or hydrogen bound to hydrocarbons reacts with NOx
over a precious metal, such as platinum. The potential for ammonia
generation increases for a precious metal catalyst (such as a lean
NOx trap) as the .lamda. ratio is decreased, as the duration of the
rich pulse increases, and the temperature is decreased. There is
thus an optimum .lamda. and rich pulse duration where the maximum
NOx reduction is observed without producing ammonia. Attempts to
enhance conversion of NOx by decreasing the .lamda. ratio of the
rich pulse duration leads to significant production of ammonia and
thus results in high gross NOx conversion
(NOx.fwdarw.N.sub.2.fwdarw.NH.sub.3), but much lower net NOx
conversion (NOx.fwdarw.N.sub.2).
[0009] In addition to nitrogen, a desirable non-polluting gas, and
the undesirable NH.sub.3 described above, N.sub.2O is another NOx
reduction products. Like NH.sub.3, N.sub.2O is generated over NOx
adsorbers and emitted into the atmosphere during rich pulses. The
gross NOx conversion is the percent of NOx that is reduced to
N.sub.2, N.sub.2O and N.sub.3. The net NOx conversion is the
percent of NOx that is reduced to nitrogen, N.sub.2, only.
Accordingly, the gross NOx conversion is equal to the net NOx
conversion if nitrogen is the only reaction product. However, the
net NOx conversion is almost always lower than the gross NOx
conversion. Accordingly, a high gross NOx conversion does not
completely correlate with the high portion of NOx that is reduced
to nitrogen.
[0010] The NOx conversion problem is magnified for diesel vehicles,
which require more than a 90% NOx conversion rate under the 2007
U.S. Tier II BIN 5 emissions standards at temperatures as low as
200.degree. C. While high NOx activity is possible at 200.degree.
C., it requires extreme measures such as shortening the lean time,
lengthening the rich purge time, and invoking very rich air/fuel
ratios. All three of these measures, however, result in the
increased formation of NOx or ammonia. Accordingly, while it may be
possible to achieve 90+% gross NOx conversion at 200.degree. C., to
date there has not been a viable solution to achieve 90+% net NOx
conversion.
[0011] Accordingly, a need exists for a catalyst system that
eliminates NOx breakthrough during the lean operation as well has
the NOx spikes during the lean-rich transition period. There is
also a need for a catalyst system that is capable of improving net
NOx conversion. Finally, there is a need for a catalyst system
capable of reducing ammonia emissions.
SUMMARY OF THE INVENTION
[0012] This invention provides a solution for all of the above
problems and, in particular, reduces or eliminates ammonia
emissions and improves the net NOx conversion of the catalyst
system. These problems are solved by simultaneously removing
ammonia and enhancing NOx conversion with the use of an
NH.sub.3--SCR catalyst placed downstream of the lean NOx adsorber
catalyst, as shown in FIG. 2. The NH.sub.3--SCR catalyst system
serves to adsorb the ammonia emissions from the upstream lean NOx
adsorber catalyst generated during the rich pulses. Accordingly, as
shown in FIG. 2, the ammonia emissions produced by the lean NOx
adsorber is stored and effectively controlled by the NH.sub.3--SCR
catalyst rather than being emitted. This reservoir of adsorbed
ammonia then reacts directly with the NOx emitted from the upstream
lean NOx adsorber. As a result, as shown in FIG. 3, the overall net
conversion is enhanced from 55% to 80%, while depleting the stored
ammonia, as a function of the SCR reaction:
NH.sub.3+NOx.fwdarw.N.sub.2. The NH.sub.3--SCR catalyst is then
replenished with ammonia by subsequent rich pulses over the lean
NOx adsorber.
[0013] During the lean cycle for this lean NOx
adsorber+NH.sub.3--SCR system, the NOx breakthrough from the
upstream lean NOx adsorber is reduced continuously as it passes
over the NH.sub.3--SCR until the reservoir of ammonia is depleted.
In addition, during the rich cycle, large spikes of unreacted NOx,
are created. The downstream NH.sub.3--SCR catalyst thus serves to
dampen these large NOx, spikes by reacting the unreacted NOx, with
the reservoir of stored ammonia emitted from the lean NOx adsorber.
In general, the combination of the lean NOx, adsorber+NH.sub.3--SCR
catalyst system allows for the reduction, or elimination, of
ammonia emissions and NOx slip, i.e., reduction of NOx breakthrough
and NOx spikes and, therefore, improved net NOx conversion during
lean and rich operation.
[0014] Additionally, under this invention, urea and/or ammonia does
not need to be injected into the exhaust system to effectuate the
reaction between NOx and ammonia. Rather, the ammonia is
automatically generated from the NOx present in the exhaust gas as
it passes over the precious metal lean NOx adsorber during the rich
pulses. The generated ammonia is then stored on the downstream
NH.sub.3--SCR catalyst, to react with the unreacted NOx and thereby
convert the unreacted NOx to nitrogen.
[0015] The NH.sub.3--SCR catalyst thus serves to adsorb the ammonia
from the upstream lean NOx adsorber catalyst generated during the
rich pulses. Under this system, the ammonia is stored and
effectively controlled rather than being emitted. This reservoir of
adsorbed ammonia then reacts directly with any NOx emitted from the
upstream lean NOx adsorber. As a result, the overall net NOx
conversion is enhanced from 55% to 80%, while the overall gross NOx
conversion is enhanced from 68% to 82%, as shown in FIG. 3.
[0016] In one alternative embodiment of this invention, the
catalyst system can be optimized and NOx reduction increased by
vertically slicing the lean NOx trap and NH.sub.3--SCR catalyst
substrates to create separate catalyst zones, such that the
catalytic converter shell or can would have alternating sections of
lean NOx trap and NH.sub.3--SCR catalysts, as shown in FIGS. 4a, 4b
and 4c. Under this embodiment, both technologies, the lean NOx trap
formulation and the NH.sub.3--SCR formulation, can be incorporated
into a single substrate and/or a single converter can rather than
placing the NH.sub.3--SCR catalyst downstream of the lean NOx
adsorber as two separate and distinct catalyst substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1a is a graph illustrating the NOx spikes that occur
during the NOx trap lean-rich transition;
[0018] FIG. 1b is a graph illustrating NOx and NH.sub.3 emissions
from a typical prior art lean NO.sub.x adsorber system;
[0019] FIG. 2 depicts the lean NOx and NH.sub.3--SCR catalyst
system of the present invention;
[0020] FIG. 3 depicts reduced NOx emissions and NH.sub.3 emissions
as a result of the use of the lean NOx and NH.sub.3--SCR catalyst
system of the present invention, as shown in FIG. 2;
[0021] FIGS. 4a, 4b, and 4c depict three different zoned catalyst
embodiments of the lean NOx and NH.sub.3--SCR catalyst system;
[0022] FIGS. 5a, 5b, and 5c provide graphs illustrating the reduced
levels of NOx and NH.sub.3 emissions resulting from each of the
three zoned catalyst embodiments depicted in FIGS. 4a, 4b, and 4c
at a 250.degree. C. inlet gas temperature and operating at a 50
second lean cycle and 5 second rich cycle;
[0023] FIGS. 6a, 6b and 6c provide graphs illustrating the reduced
levels of NOx and NH.sub.3 emissions resulting from each of the
three zoned catalyst embodiments depicted in FIGS. 4a, 4b and 4c at
a 200.degree. C. inlet gas temperature and operating at a 25 second
lean cycle and a 5 second rich cycle;
[0024] FIGS. 7a, 7b and 7c show three proposed examples of washcoat
configurations incorporating the lean NOx trap and NH.sub.3--SCR
formulations into the same substrate;
[0025] FIG. 8 is a graph illustrating the impact of NOx conversion
after hydrothermal aging; and
[0026] FIG. 9 depicts a modified three-way catalyst and
NH.sub.3--SCR catalyst system of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] In this invention, net NOx conversion is improved and
ammonia emissions reduced through the use of a lean NOx trap and
NH.sub.3--SCR catalyst system which operate together to produce and
store ammonia and reduce NOx to nitrogen. In so doing, the catalyst
system of the present invention solves three problems of lean NOx
traps; namely, reducing NOx breakthrough, NOx spikes and ammonia
emissions.
[0028] In order to meet increasingly stringent fuel economy
standards, it is preferable to operate an automotive engine under
lean conditions. However, while there is improvement in fuel
economy, operating under lean conditions has increased the
difficulty in reducing NOx emissions. As an example, for a
traditional three-way catalyst, if the air/fuel ratio is lean even
by a small amount, NOx conversion drops to low levels. With
traditional three-way catalysts, the air/fuel ratio must be
controlled carefully at stoichiometric conditions to maximize
reduction of hydrocarbons, carbon monoxide and NOx.
[0029] Throughout this specification, NOx refers to nitrogen
oxides, which include nitrogen monoxide NO and nitrogen dioxide
NO.sub.2. Further, lean NOx adsorber and lean NOx trap are used
interchangeably throughout this specification.
[0030] To achieve NOx reduction, under lean operating conditions,
one option is the inclusion of a lean NOx trap. While the lean NOx
trap is generally effective in NOx reduction, lean NOx traps are
known to have the problems referred to as "NOx slip" which includes
breakthrough of NOx during the extended lean operation of the NOx
trap and also NOx spikes generated during the transition from the
lean to the rich cycle.
[0031] NOx spikes, or NOx emissions during the lean-rich
transition, are believed to occur due to the exothermic heat
generated from the oxidation of reductants, carbon monoxide,
hydrocarbons and hydrogen, by the oxygen released from the oxygen
storage material--the temperature rise can be as high as
80-100.degree. C.
[0032] The problem of NOx spikes is illustrated in FIG. 1a, and the
problem of insufficient net NOx conversion is illustrated in FIG.
1b. FIG. 1b depicts laboratory reactor data of a lean NOx adsorber
system operating in an 85 second lean and 5 second rich cyclic
pattern. The plot in FIG. 1b shows the nitrogen species
concentration as a function of time. The laboratory reactor data
depicted in FIG. 1b resulted from a catalyst having an engine swept
volume (ESV) of 100%. Additionally, the reactor used to obtain the
results in FIG. 1b was at a temperature of 300.degree. C. To begin
the cycle, 500 ppm of nitrogen oxide was fed into the reactor where
much of it was stored during the 85 second lean duration. During
the 5 second rich duration, nitrogen oxide was reduced; however, a
significant amount of ammonia was formed. As illustrated in FIG.
1b, the data shows ammonia spikes as high as 600 ppm under typical
lean NOx adsorber operation. Conversion, however, is generally
improved as the .lamda. ratio is decreased during the rich pulse.
Decreasing the .lamda. ratio also leads to significant production
of ammonia and thus results in high gross NOx conversion
(NO.sub.x.fwdarw.N.sub.2+NH.sub.3), but much lower net NOx
conversion (NOx.fwdarw.N.sub.2). As illustrated in FIG. 1b, the net
NOx conversion to nitrogen for this lean NOx adsorber system was
only 55%.
[0033] Under the catalyst system of this invention, ammonia is
reduced and the net NOx conversion improved simultaneously by
placing an NH.sub.3--SCR catalyst formulation downstream of the
lean NOx adsorber catalyst, as shown in FIG. 2.
[0034] FIG. 2 is an illustration of the catalyst system of this
invention, which is capable of simultaneously eliminating ammonia
emissions and improving net NOx conversion. As illustrated in FIG.
2, NOx produced during engine operation is stored by the lean NOx
adsorber during the lean cycle. Following the lean cycle, during
the rich cycle of the lean NOx adsorber NOx is reduced and ammonia
generated. The lean NOx adsorber stores much of the NOx during the
lean operation and then reduces NOx during rich pulses of the
reductants. During the same rich pulses, significant amounts of
ammonia are generated, as further illustrated in FIG. 1. As
illustrated in FIG. 2, the lean NOx adsorber emits NO, NO.sub.2,
NH.sub.3, and N.sub.2O. These same gases then pass through the
NH.sub.3--SCR, where NH.sub.3 is stored. Accordingly, the addition
of the NH.sub.3--SCR catalyst downstream allows for the adsorption
of NH.sub.3 and subsequent reaction with any NOx that slips through
the upstream lean NOx adsorber, which thus improves the overall net
NOx conversion (NH.sub.3+NO.fwdarw.N.sub.2). As can be seen in FIG.
2, the catalyst system of this invention results in a significant
net NOx conversion improvement, the elimination of ammonia
emissions, and the production of non-polluting gases nitrogen and
N.sub.2O.
[0035] It should be noted that for diesel applications, lean NO,
NOx adsorbers must operate at lower temperatures compared to
gasoline lean NOx adsorbers since the exhaust temperatures of
diesel engines are significantly lower. More ammonia is generated
at 200.degree. C. than at 300.degree. C. over lean NOx adsorbers
and thus the catalyst system of this invention has an even greater
potential for diesel applications. Likewise, the problem of NOx
spikes is more critical at higher temperatures, the temperatures
used for gasoline applications; and thus the catalyst system of
this invention is beneficial to control the unreacted NOx spikes
that result from the operation of a lean NOx adsorber at operating
temperatures typical for gasoline lean NOx adsorber
applications.
[0036] The NH.sub.3--SCR catalyst thus serves to adsorb the ammonia
produced naturally from the upstream lean NOx adsorber catalyst
generated during the rich pulses. As a result, the NH.sub.3--SCR
catalyst stores the ammonia, controlling it rather than allowing it
to be emitted into the atmosphere. This reservoir of adsorbed
NH.sub.3 in the NH.sub.3--SCR catalyst reacts directly with the NOx
emitted from the upstream lean NOx adsorber (trap).
[0037] In general, this invention works to clean NOx emissions--and
thus has applicability for stationary sources as well as for moving
vehicles. This invention may be used to reduce NOx emissions for
nitric acid plants, or any other stationary source that requires
the reduction of NOx emissions. This invention is nonetheless
particularly directed for use with gasoline and diesel vehicles
which, unlike stationary sources, have a wide range of operating
parameters, especially temperature parameters-which cannot be
precisely controlled. The present invention has the ability to
store large quantities of ammonia across a broad temperature range
to effectuate the reaction between ammonia and nitrogen oxides and
thereby convert NOx to nitrogen.
[0038] As illustrated in FIG. 3, laboratory experiments have
demonstrated that the use of a lean NOx adsorber plus NH.sub.3--SCR
catalyst system improves net NOx conversion from 55%, as
illustrated in FIG. 1, to 80%. FIG. 3 is a graph displaying
laboratory data obtained using the catalyst system of this
invention, wherein NOx ppm are charted as a function of time. As
illustrated in FIG. 3, the catalyst system of this invention
completely eliminated the ammonia spikes created during the rich
pulses of the lean NOx adsorber. In this system, ammonia is stored
on the NH.sub.3--SCR catalyst where it reacts with NOx during the
85 second lean duration, which thus improves the net NOx conversion
from 55% to 80% with no additional fuel economy penalty. As shown
in FIG. 3, the improved net NOx conversion can be observed by the
much narrower profile-zero ppm NOx is emitted for a significant
amount of time as compared to the graph shown in FIG. 1 of a system
lacking the NH.sub.3--SCR+lean NOx adsorber combination.
[0039] The reaction between the stored ammonia and NOx increases
the overall net NOx conversion, which is enhanced from 55%--the
amount of NOx converted in prior art lean NOx trap systems--to
80%--as a result of the combination of a lean NOx trap and
NH.sub.3--SCR catalyst system. Moreover, in addition to improving
net NOx conversion, the ammonia stored in the NH.sub.3--SCR
catalyst is depleted during the SCR reaction wherein ammonia and
nitrogen oxide are reacted to produce nitrogen. The NH.sub.3--SCR
catalyst is replenished with ammonia by subsequent rich pulses over
the lean NOx adsorber that causes a portion of the NOx to react
with hydrogen to form ammonia.
[0040] It should be noted that no urea or ammonia needs to be
injected into the exhaust system to effectuate the reaction between
ammonia and NOx. Rather, the ammonia is naturally generated from
the NOx present in the exhaust gas as it passes over the lean NOx
trap during rich pulses. More specifically, ammonia is naturally
created during the fuel rich cycle of the lean NOx trap. Ammonia is
naturally produced as it passes over the precious metal active
component of the lean NOx trap. Similarly, the ammonia could also
be generated in a conventional precious metal based TWC located
upstream of a LNT/NH.sub.3--SCR system.
[0041] For this invention, the lean NOx trap is optimized for
ammonia generation by removing oxygen storage capacity (OSC) and
thereby enhancing the rich cycle and thus creating a greater
quantity of ammonia for reaction with the NOx in the downstream
NH.sub.3--SCR catalyst. In a preferred embodiment, the lean NOx
trap includes platinum as the precious metal. Platinum is the
preferred precious metal because it is believed that a greater
quantity of NH.sub.3 is produced over platinum than rhodium,
palladium and/or a combination of the precious metals. Nonetheless,
other precious metals such as palladium and rhodium, and the
combination of one or more of the precious metals platinum,
palladium and rhodium may also be used to generate NH.sub.3.
[0042] Additionally, the lean NOx trap of this invention preferably
includes a "VNOx adsorbing material" or NOx storage
component/material, which can be alkali and alkali earth metals
such as barium, cesium, and/or rare earth metals such as cerium
and/or a composite of cerium and zirconium. Although an alternative
catalyst formulation that does not contain a NOx storage component
but generates ammonia from NOx, may also be utilized, in the most
preferred embodiment, the NOx storage material should have the
ability to store NOx at low temperature ranges, specifically in the
range of 150.degree. C.-300.degree. C. The NH.sub.3 thermodynamic
equilibrium under rich conditions is maximized during the
temperature range of 150.degree. C.-300.degree. C.
[0043] In general, to increase the NOx storage function of the lean
NOx trap and effectuate the NOx conversion reaction, in the
preferred embodiment, the lean NOx trap has the following
characteristics: (1) the inclusion of platinum as the precious
metal; (2) the ability to store NOx between 150.degree. C. and
500.degree. C. during the lean portion of the cycle; (3) the
ability to maximize the duration of the lean NOx trap rich cycle;
(4) the ability to generate ammonia at the 150.degree.
C.-500.degree. C. temperature range; (5) minimize OSC to lessen
fuel penalty; and (6) lower to generate more ammonia. Ammonia
production is maximized at the preferred temperature range,
150.degree. C.-300.degree. C.--which also correlates with the
steady state equilibrium range for ammonia creation. It bears
emphasis that other NOx storage components may be utilized,
especially for stationary sources, where sulfur poisoning does not
pose a threat.
[0044] Most simply, the NH.sub.3--SCR catalyst may consist of any
material or combination of materials that can adsorb ammonia and
facilitate the NOx+NH.sub.3 to yield nitrogen. The NH.sub.3--SCR
catalyst should preferably be made of a base metal catalyst on a
high surface area support such as alumina, silica, titania, zeolite
or a combination of these. More preferably, the NH.sub.3--SCR
catalyst should be made of a base metal selected from the group
consisting of Cu, Fe, and Ce and/or a combination of these metals,
although other base metals may be used. Base metals generally are
able to effectuate NOx conversion using ammonia while both the base
metals and the high surface support material serves to store
NH.sub.3. The base metal and high surface area support such as
zeolite selected should preferably be one that can store NH.sub.3
over the widest possible temperature range. Likewise, the base
metal selected is preferably one that can convert NO and NO.sub.2
to N.sub.2 across the widest possible temperature range and the
widest range of NO/NO.sub.2 ratios.
[0045] The advantage of the catalyst system of this invention is
the use of a combination of a lean NOx trap and an NH.sub.3--SCR
catalyst. The use of a lean NOx trap in the present system allows
for much greater storage of NOx R, because the NOx breakthrough
that would otherwise happen can be controlled by the NH.sub.3--SCR
catalyst. Additionally, the use of a lean NOx trap as part of this
system allows for the operation of the engine at lean conditions
for a longer time and thus provides improved fuel economy. If, for
example, a three-way catalyst is used as the NOx storage mechanism,
NOx storage is significantly limited, as well as the production of
ammonia. To maximize the reduction of emissions, a three-way
catalyst must be operated at stoichiometric conditions.
Accordingly, unless the three-way catalyst is run on the rich side
100% of the time, ammonia production is significantly less than for
a typical lean NOx trap. As set forth above, the efficiency of a
three-way catalyst is compromised if it is operated at conditions
other than at stoichiometric conditions. Thus, the combination of a
lean NOx trap and NH.sub.3--SCR catalyst allows for significant NOx
storage and ammonia production and thus increases net NOx
conversion.
[0046] In a preferred embodiment, the lean NOx trap and
NH.sub.3--SCR catalyst constitute alternating zones in a single
substrate and/or a single catalytic converter can. This zoned
design, as shown in three different embodiments in FIGS. 4a-4c, is
believed to maximize the reaction between ammonia and NOx.
[0047] As illustrated in FIG. 4, three zoned catalyst system
embodiments were evaluated on a laboratory flow reactor. The total
catalyst system dimensions were held constant at a 1'' diameter and
2'' length. The first system, labeled "4a", had a 1'' long lean NOx
trap followed by a 1'' long NH.sub.3--SCR catalyst. In the second
system, labeled "4b", the catalyst samples were sliced in half to
yield alternating 1/2'' long sections. Finally, in the third
system, labeled "4c", the same catalyst samples were further cut in
half to yield 1/4'' long sections, again of the lean NOx trap and
NH.sub.3--SCR catalyst technologies. It should be noted that each
time the catalysts were sliced, as shown in "4b" and "4c", the
overall length of the catalyst system was reduced slightly,
approximately 3/16'' total. The alternating lean NOx trap and
NH.sub.3--SCR catalyst zones can be created in a single substrate
or the lean NOx trap and NH.sub.3--SCR catalyst prepared, cut as
desired and then placed adjacent one another in a single can. The
zones are preferably formed in a single substrate. However, cut
substrates placed in alternating fashion also exhibit improved net
NOx conversion.
[0048] Under the zoned catalyst designs shown in FIGS. 4a-4c, where
alternating lean NOx and NH.sub.3--SCR catalyst zones are provided,
the ammonia formed by the lean NOx trap is believed to be
immediately adsorbed by the NH.sub.3--SCR catalyst for use in the
NOx conversion reaction. It is further believed that the greater
the separation between the lean NOx trap and the NH.sub.3--SCR
catalyst, the greater chance there is for the ammonia to be
converted back into NOx. It is further believed that oxygen is more
abundant in the back of a catalyst substrate and thus the oxygen
may be available to effectuate the unwanted conversion of the
ammonia back to nitrogen oxide. Accordingly, if the catalyst
substrate is too long, there may be some undesired conversion that
takes place; and thus in a preferred embodiment, the substrate is
designed so that ammonia is available for immediate reaction with
NOx.
[0049] FIGS. 5a-5c illustrate laboratory reactor data of the three
different zoned catalyst system embodiments shown in FIGS. 4a-4c.
This laboratory data was obtained with the three catalyst systems
operating at a 250.degree. C. inlet gas temperature and operating
with 50 second lean and 5 second rich cycles. Additionally, the
inlet concentration of the NOx feed gas was 500 ppm and the overall
space velocity was 15,000 per hour. As illustrated in FIGS. 5a-5c,
with the use of a two-zoned catalyst system as depicted in FIG. 5a,
approximately 50 ppm of NO is emitted. This two-zone catalyst
system resulted in a gross NOx conversion of 95% and a net NOx
conversion of 66%. The four-zone catalyst embodiment, depicted as
FIG. 5b, significantly reduced NOx emissions, well below the 15 ppm
range, to result in gross NOx conversion of 99% and a net NOx
conversion of 86%. Finally, as illustrated by the eight zoned
catalyst embodiment, FIG. 5c, gross NOx conversion is 100% and net
NOx conversion is 97.5%. The improvement comes from the reduction
of N.sub.2O elimination of the NH.sub.3 breakthrough and reduction
of NOx. Accordingly, as the catalyst system is zoned down from 1''
sections to 1/4'' sections, the test results revealed an associated
improvement in net NOx conversion.
[0050] As shown in FIGS. 5a-5c, a zoned catalyst, with alternating
lean NOx and NH.sub.3--SCR catalysts in 1'' to 1/4'' sections
significantly improves the net NOx conversion from 66% to 97.5%. In
addition, the gross NOx conversion is improved from 95% to 100%. In
general, the improvement in the net NOx conversion is the function
of the elimination of the ammonia slip, reduction in N.sub.2O, and
extra NOx reduction related to the NH.sub.3+NOx reaction on the
NH.sub.3--SCR catalyst. It is further believed that the drop in
N.sub.2O emissions is likely due to a higher fraction of the NOx
reduction reaction proceeding on the NH.sub.3--SCR catalyst rather
than the lean NOx trap. NOx reduction over a
platinum-containing-lean NOx trap results in high levels of
N.sub.2O generation, whereas the NH.sub.3--SCR catalyst has a high
selectivity to nitrogen.
[0051] FIGS. 6a-6c depicts laboratory data obtained using the
three-zoned catalyst embodiments originally shown in FIGS. 4a-4c at
a 200.degree. C. inlet gas temperature operating with a 25 second
lean cycle and a 5 second rich cycle. As compared to FIGS. 5a-5c,
it should be noted that shortening the lean time from 50 seconds,
as used in FIGS. 5a-5c, to 25 seconds, resulted in a substantial
higher steady emission of ammonia-a fact which results in reduced
net NOx conversion rates, as compared to the data charted in FIGS.
5a-5c. As can be seen in FIGS. 6a-6c, the use of smaller zoned
sections from two zones to eight zones and thus 1'' sections down
to 1/4'' sections, as illustrated in FIGS. 6a and 6c, improves the
net NOx conversion from 50% to 81%. Again, this improvement is
believed to come mainly from the reduction of ammonia breakthrough
and a small reduction in N.sub.2O emissions. This lab data was
obtained with an inlet concentration of the NOx feed gas at 500 ppm
and an overall space velocity at 15,000 per hour.
[0052] As set forth above, in the preferred embodiment, the lean
NOx trap washcoat and NH.sub.3--SCR washcoat are combined in a
single substrate rather than placing the NH.sub.3--SCR formulation
downstream of the lean NOx adsorber as two separate catalyst
substrates. Under this embodiment, the catalyst formulations can be
incorporated together by mixing or layering the washcoats on a
substrate.
[0053] FIGS. 7a-7c show three proposed washcoat configurations
incorporating the lean NOx trap and NH.sub.3--SCR formulations into
the same substrate. As shown in FIGS. 7a and 7b, the first and
second proposed configurations have the lean NOx trap and
NH.sub.3--SCR washcoat formulations on the bottom and top layer,
respectively. It is believed that the top layer could be a highly
porous structure that allows better and faster contact between the
chemicals and gas phase and the active sites in the second layer.
The third configuration, as shown in FIG. 7c, involves the use of a
one layer washcoat containing both lean NOx trap and NH.sub.3--SCR
washcoat formulations. Under this third configuration, shown in
FIG. 7c, the washcoat composition of the lean NOx trap and
NH.sub.3--SCR catalyst could be homogeneously or heterogeneously
mixed. For a heterogeneously mixed composition, the formulation of
the lean NOx trap and NH.sub.3--SCR catalyst are separated.
However, they contact each other in varying degrees by controlling
the size of the grain structures. The homogeneously mixed
composition allows for a more intimate contact between the two
formulations and is thus preferred.
[0054] The invention also contemplates engineering such
combinations within the pores of the monolithic substrate. An
example of this is incorporating washcoat into porous substrates
used for filtering diesel particulate matter. Thus, this lean NOx
trap/NH.sub.3--SCR catalyst concept can be integrated into diesel
particulate matter devices.
[0055] This very active SCR reaction of NOx and ammonia proceeds
with or without oxygen present. Koebel et al. reports that the
fastest SCR reaction involves equal molar amounts of NO and
NO.sub.2 NO and NO.sub.2 then react with two NH.sub.3 to yield
N.sub.2 in the absence of oxygen. In contrast, the lean NOx
adsorber reaction of NOx plus CO is highly reactive only in an
oxygen-free environment. In a lean NOx adsorber system, NOx is
adsorbed during the lean cycle duration, NOx is not reduced.
Accordingly, NOx reduction is limited to only the rich pulse time
duration. On the other hand, the lean NOx adsorber+NH.sub.3--SCR
catalyst system allows for NOx reduction reaction to proceed during
both the lean and rich time durations. Accordingly, ammonia as a
reductant can be considered as a much more robust reductant than
carbon monoxide.
[0056] As set forth above, the fastest SCR reaction involves equal
molar amounts of NO and NO.sub.2 Accordingly, FIG. 8 illustrates
the impact of varying NO:NO.sub.2 ratios after hydrothermal aging.
FIG. 8 is a graph of three NH.sub.3--SCR catalyst formulations over
a wide NO:NO.sub.2 range. In the laboratory, it was possible to
control the NO:NO.sub.2 ratio entering the downstream NH.sub.3--SCR
catalyst. Accordingly, the NO:NO.sub.2 ratio entering the
NH.sub.3--SCR catalyst was solely dependent on the upstream lean
NOx adsorber. In some cases, the majority of the feed NOx
(especially NOx spikes) are made up of mostly NO rather than
NO.sub.2 Accordingly, it is believed that the catalyst formulations
of this invention will enhance reported net NOx efficiency--and
thus the preferred catalyst is one that is capable of operating
across the broadest range of NO:NO.sub.2 ratios, and at a full
spectrum of temperature ranges.
[0057] In general, since NH.sub.3--SCR catalysts do not contain
precious metals, they are significantly less costly than a typical
lean NOx trap. Accordingly, it is more cost effective to have an
overall catalyst system containing a lean NOx trap adsorber and an
NH.sub.3--SCR catalyst system, rather than one that uses two lean
NOx trap adsorbers. Additionally, the incorporation of both a lean
NOx trap and NH.sub.3--SCR washcoat into a single substrate will
significantly reduce substrate costs.
[0058] In another embodiment of this invention, NH.sub.3 and NOx in
an exhaust stream are reduced using a stoichiometric three-way
catalyst system. This three-way catalyst system has particular
application for high speed/high flow rate conditions (i.e., US06
conditions). Currently, three three-way catalysts are used for such
high speed condition applications, wherein the third three-way
catalyst is primarily directed to NOx removal for high speed/high
flow rate conditions. Under this alternate embodiment, the third
three-way catalyst can be substituted with an NH.sub.3--SCR
catalyst to store NH.sub.3 for reaction with NOx to improve net NOx
conversion, eliminate NH.sub.3 emissions and reduce catalyst
costs.
[0059] To improve net NOx and NH.sub.3 reduction, the second
three-way catalyst can be modified to enhance the three-way
catalyst's ability to generate NH.sub.3 emissions. To this end, in
a preferred embodiment, the three-way catalyst is designed to
generate desirable NH.sub.3 creation by using platinum as the
precious metal of the three-way catalyst, by placing platinum on
the outer layer of the three-way catalyst to maximize the
NO+H.sub.2--NH.sub.3 reaction. Likewise, the oxygen storage
capacity (OSC) of the three-way catalyst can be removed to further
promote the creation of "desirable" NH.sub.3. By doing so, the
NH.sub.3 purposely generated during rich operation can then be
stored by the NH.sub.3--SCR catalyst for subsequent reaction with
NOx emissions, and thereby control both NOx and NH.sub.3 emissions
under all operating conditions.
[0060] When a car is operated under rich conditions, the air/fuel
ratio is less than 14.6, hydrogen is produced in the exhaust via
the water-gas shift reaction: CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2.
The hydrogen that is produced then reacts with NOx, as it passes
over the precious metal surface to create "desirable" ammonia. The
ammonia produced is then stored on an NH.sub.3--SCR catalyst to
help reduce net NOx conversion. The reaction of
NOx+NH.sub.3-->N.sub.2+H.sub.2O can then take place on a
separate NH.sub.3 selective catalyst, capable of converting
NO.sub.2 and NO to N.sub.2.
[0061] As shown in FIG. 9, a stoichiometric three-way
catalyst/NH.sub.3--SCR catalyst system 10 is depicted, including a
first three-way catalyst 14 positioned in close proximity to the
engine 12 to reduce cold start emissions. The second three-way
catalyst 16 is modified as described above to enhance the ability
of the second three-way catalyst 16 to generate NH.sub.3 emissions.
Downstream of the second three-way catalyst 16 is an NH.sub.3--SCR
catalyst 18 that functions to store NH.sub.3 produced by the
modified second three-way catalyst 16 for reaction with NOx
emissions, to reduce both NOx and NH.sub.3 emissions.
[0062] By substituting the third three-way catalyst as currently
used with an NH.sub.3--SCR catalyst and thereby eliminating the
need for a third precious metal containing catalyst, significant
cost savings can be achieved.
[0063] It should further be noted that this invention also
contemplates the use of a three-way catalyst, in combination with a
lean NOx trap and an NH.sub.3--SCR catalyst.
[0064] While the best mode for carrying out the invention has been
described in detail, those familiar with the art to which this
invention relates will recognize various alternative designs and
embodiments for practicing the invention as defined by the
following claims.
* * * * *