U.S. patent application number 15/197818 was filed with the patent office on 2017-01-05 for passive nox adsorber.
The applicant listed for this patent is Johnson Matthey Public Limited Company. Invention is credited to Jillian Elaine COLLIER, Sanyuan YANG.
Application Number | 20170001169 15/197818 |
Document ID | / |
Family ID | 56411815 |
Filed Date | 2017-01-05 |
United States Patent
Application |
20170001169 |
Kind Code |
A1 |
COLLIER; Jillian Elaine ; et
al. |
January 5, 2017 |
PASSIVE NOx ADSORBER
Abstract
A passive NO.sub.x adsorber is disclosed. The passive NO.sub.x
adsorber is effective to adsorb NO.sub.x at or below a low
temperature and release the adsorbed NO.sub.x at temperatures above
the low temperature. The passive NO.sub.x adsorber comprises a
noble metal and a molecular sieve having an LTL Framework Type. The
invention also includes an exhaust system comprising the passive
NO.sub.x adsorber, and a method for treating exhaust gas from an
internal combustion engine utilizing the passive NO.sub.x
adsorber.
Inventors: |
COLLIER; Jillian Elaine;
(Reading, GB) ; YANG; Sanyuan; (Savannah,
GA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Johnson Matthey Public Limited Company |
London |
|
GB |
|
|
Family ID: |
56411815 |
Appl. No.: |
15/197818 |
Filed: |
June 30, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62187866 |
Jul 2, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 53/9477 20130101;
B01J 29/80 20130101; B01D 2255/91 20130101; B01J 20/18 20130101;
B01D 2255/104 20130101; F01N 3/0814 20130101; B01D 53/944 20130101;
B01D 2255/1028 20130101; F01N 3/101 20130101; F01N 3/2066 20130101;
B01J 29/87 20130101; B01J 20/02 20130101; F01N 2370/02 20130101;
B01D 2255/9155 20130101; Y02T 10/24 20130101; F01N 13/009 20140601;
B01J 2229/36 20130101; F01N 3/0842 20130101; B01D 53/9445 20130101;
B01D 2255/1026 20130101; B01J 29/62 20130101; Y02T 10/12 20130101;
Y02T 10/22 20130101; B01J 29/7415 20130101; B01D 53/9431 20130101;
B01D 2255/106 20130101; B01D 2255/50 20130101; B01J 29/83 20130101;
B01D 2253/108 20130101; B01D 2255/1021 20130101; B01J 29/74
20130101; B01D 53/0462 20130101; B01D 53/9418 20130101; B01D
53/9422 20130101; B01J 29/85 20130101; B01J 35/0006 20130101; B01J
29/743 20130101; B01J 37/0246 20130101; F01N 2370/04 20130101; B01D
2255/9032 20130101; B01D 2253/25 20130101; B01D 2257/404 20130101;
B01J 29/44 20130101; B01J 37/0244 20130101; B01J 37/04 20130101;
B01D 53/9481 20130101; B01D 2255/1025 20130101; B01J 2229/186
20130101; B01J 37/0009 20130101; B01J 35/04 20130101; B01D 2255/10
20130101; B01D 2255/9022 20130101; B01D 2255/1023 20130101 |
International
Class: |
B01J 20/18 20060101
B01J020/18; B01J 29/74 20060101 B01J029/74; B01D 53/94 20060101
B01D053/94; B01J 20/02 20060101 B01J020/02 |
Claims
1. A passive NO.sub.x adsorber effective to adsorb NO.sub.x at or
below a low temperature and release the adsorbed NO.sub.x at
temperatures above the low temperature, said passive NO.sub.x
adsorber comprising a first noble metal and a molecular sieve
having an LTL Framework Type.
2. The passive NO.sub.x adsorber of claim 1 wherein the first noble
metal is selected from the group consisting of platinum, palladium,
rhodium, gold, silver, iridium, ruthenium, osmium, and mixtures
thereof.
3. The passive NO.sub.x adsorber of claim 1 wherein the first noble
metal is palladium.
4. The passive NO.sub.x adsorber of claim 1 wherein the molecular
sieve having an LTL Framework Type is selected from the group
consisting of aluminosilicate zeolite, an aluminophosphate zeolite,
a silicoaluminophosphate (SAPO) zeolite, and a metal-substituted
aluminosilicate or aluminophosphate zeolite.
5. The passive NO.sub.x adsorber of claim 1 wherein the molecular
sieve having an LTL Framework Type is selected from the group
consisting of zeolite L, Linde Type L, gallosilicate L, LZ-212,
LTL-type SAPO, and perlialite zeolite.
6. The passive NO.sub.x adsorber of claim 1 wherein the passive
NO.sub.x adsorber is coated onto a flow-through or filter
substrate.
7. The passive NO.sub.x adsorber of claim 1 wherein the passive
NO.sub.x adsorber is extruded to form a flow-through or filter
substrate.
8. The passive NO.sub.x adsorber of claim 1 further comprising a
second molecular sieve catalyst, wherein the second molecular sieve
catalyst comprises a second noble metal and a second molecular
sieve, wherein the second molecular sieve does not have an LTL
Framework Type.
9. The passive NO.sub.x adsorber of claim 8 wherein the first noble
metal and the second noble metal are independently selected from
the group consisting of platinum, palladium, rhodium, gold, silver,
iridium, ruthenium, osmium, and mixtures thereof.
10. The passive NO.sub.x adsorber of claim 8 wherein the first
noble metal and the second noble metal are both palladium.
11. The passive NO.sub.x adsorber of claim 8 wherein the second
molecular sieve is a small, medium or large pore molecular sieve
selected from the group of Framework Type consisting of ACO, AEI,
AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB,
EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MAZ, MER, MON,
NSI, OFF, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SFW, SIV, THO, TSC,
UEI, UFI, VNI, YUG, ZON, BEA, and MFI, and intergrowths of two or
more.
12. The passive NO.sub.x adsorber of claim 11 wherein the small
pore molecular sieve is selected from the group Framework Type
consisting of AEI and CHA.
13. The passive NO.sub.x adsorber of claim 8 wherein the passive
NO.sub.x adsorber is coated onto a flow-through or filter
substrate.
14. The passive NO.sub.x adsorber of claim 8 having a first layer
and a second layer wherein the first layer comprises the first
noble metal and the molecular sieve having an LTL Framework Type
and the second layer comprises the second molecular sieve
catalyst.
15. The passive NO.sub.x adsorber of claim 8 having a first zone
and a second zone wherein the first zone comprises the first noble
metal and the molecular sieve having an LTL Framework Type and the
second zone comprises the second molecular sieve catalyst.
16. The passive NO.sub.x adsorber of claim 1 wherein the low
temperature is 250.degree. C.
17. An exhaust system for internal combustion engines comprising
the passive NO.sub.x adsorber of claim 1 and a catalyst component
selected from the group consisting a selective catalytic reduction
(SCR) catalyst, a particulate filter, a SCR filter, a NO.sub.x
adsorber catalyst, a three-way catalyst, an oxidation catalyst, and
combinations thereof.
18. A method for reducing NO.sub.x in an exhaust gas, said method
comprising adsorbing NO.sub.x onto the passive NO.sub.x adsorber of
claim 1 at or below a low temperature, thermally desorbing NO.sub.x
from the passive NO.sub.x adsorber at a temperature above the low
temperature, and catalytically removing the desorbed NO.sub.x on a
catalyst component downstream of the passive NO.sub.x adsorber.
19. The method of claim 18 wherein the catalyst component is
selected from the group consisting a selective catalytic reduction
(SCR) catalyst, a particulate filter, a SCR filter, a NO.sub.x
adsorber catalyst, a three-way catalyst, an oxidation catalyst, and
combinations thereof.
20. The method of claim 18 wherein the low temperature is
250.degree. C.
21. A catalyst comprising a substrate, a diesel oxidation catalyst,
and the passive NO.sub.x adsorber of claim 1, wherein the passive
NO.sub.x adsorber of claim 1 is located on a first zone or a first
layer on the substrate and the diesel oxidation catalyst is located
on a second zone or a second layer on the substrate.
22. The catalyst of claim 21 wherein the first zone is located
upstream of the second zone.
23. The catalyst of claim 21 wherein the first zone is located
downstream of the second zone.
24. The catalyst of claim 21 wherein the first layer is disposed on
the substrate and the second layer is disposed on the first
layer.
25. The catalyst of claim 21 wherein the second layer is disposed
on the substrate and the first layer is disposed on the second
layer.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a passive NON adsorber and its use
in an exhaust system for internal combustion engines.
BACKGROUND OF THE INVENTION
[0002] Internal combustion engines produce exhaust gases containing
a variety of pollutants, including nitrogen oxides ("NO.sub.x"),
carbon monoxide, and uncombusted hydrocarbons. These emissions are
the subject of governmental legislation. Emission control systems
are widely utilized to reduce the amount of these pollutants
emitted to atmosphere, and typically achieve very high efficiencies
once they reach their operating temperature (typically, 200.degree.
C. and higher). However, these systems are relatively inefficient
below their operating temperature (the "cold start" period).
[0003] For instance, current urea based selective catalytic
reduction (SCR) applications implemented for meeting Euro 6b
emissions require that the temperature at the urea dosing position
be above about 180.degree. C. before urea can be dosed and used to
convert NO.sub.x. NO.sub.x conversion below 180.degree. C. is
difficult to address using the current systems, and future European
and US legislation will stress the low temperature NO.sub.x storage
and conversion. Currently this is achieved by heating strategies
but this has a detrimental effect of CO.sub.2 emissions.
[0004] As even more stringent national and regional legislation
lowers the amount of pollutants that can be emitted from diesel or
gasoline engines, reducing emissions during the cold start period
is becoming a major challenge. Thus, methods for reducing the level
of NO.sub.x emitted during cold start condition continue to be
explored.
[0005] For instance, PCT Intl. Appl. WO 2008/047170 discloses a
system wherein NO.sub.x from a lean exhaust gas is adsorbed at
temperatures below 200.degree. C. and is subsequently thermally
desorbed above 200.degree. C. The NO.sub.x adsorbent is taught to
consist of palladium and a cerium oxide or a mixed oxide or
composite oxide containing cerium and at least one other transition
metal.
[0006] U.S. Appl. Pub. No. 2011/0005200 teaches a catalyst system
that simultaneously removes ammonia and enhances net NO.sub.x
conversion by placing an ammonia-selective catalytic reduction
("NH.sub.3-SCR") catalyst formulation downstream of a lean NO.sub.x
trap. The NH.sub.3-SCR catalyst adsorbs the ammonia that is
generated during the rich pulses in the lean NO.sub.x trap. The
stored ammonia then reacts with the NO.sub.x emitted from the
upstream lean NO.sub.x trap, which increases NO.sub.x conversion
rate while depleting the stored ammonia.
[0007] PCT Intl. Appl. WO 2004/076829 discloses an exhaust-gas
purification system which includes a NO.sub.x storage catalyst
arranged upstream of an SCR catalyst. The NO.sub.x storage catalyst
includes at least one alkali, alkaline earth, or rare earth metal
which is coated or activated with at least one platinum group metal
(Pt, Pd, Rh, or Ir). A particularly preferred NO.sub.x storage
catalyst is taught to include cerium oxide coated with platinum and
additionally platinum as an oxidizing catalyst on a support based
on aluminum oxide. EP 1027919 discloses a NO.sub.x adsorbent
material that comprises a porous support material, such as alumina,
zeolite, zirconia, titania, and/or lanthana, and at least 0.1 wt %
precious metal (Pt, Pd, and/or Rh). Platinum carried on alumina is
exemplified. U.S. Appl. Pub. No. 2012/0308439 A1 teaches a cold
start catalyst that comprises (1) a zeolite catalyst comprising a
base metal, a noble metal, and a zeolite, and (2) a supported
platinum group metal catalyst comprising one or more platinum group
metals and one or more inorganic oxide carriers.
[0008] U.S. Appl. Pub. No. 2015/0158019 discloses a passive
NO.sub.x adsorber (PNA) that comprises a noble metal and a small
pore molecular sieve such as chabazite (CHA). Although noble
metal/zeolite PNA catalysts such as Pd/CHA and Pd/Beta show good
NO.sub.x storage performance and improved sulfur tolerance compared
to non-zeolite PNAs, the temperature at which the NO.sub.x is
released is too low for the downstream SCR component to convert all
of the NO.sub.x to N.sub.2.
[0009] As with any automotive system and process, it is desirable
to attain still further improvements in exhaust gas treatment
systems, particularly under cold start conditions. We have
discovered a new passive NO.sub.x adsorber that provides enhanced
cleaning of the exhaust gases from internal combustion engines. The
new passive NO.sub.x adsorber not only increases the NO.sub.x
release temperature but also increases the total NO.sub.x storage
capacity.
SUMMARY OF THE INVENTION
[0010] The invention is a passive NO.sub.x adsorber that is
effective to adsorb NO.sub.x at or below a low temperature and
release the adsorbed NO.sub.x at temperatures above the low
temperature. The passive NO.sub.x adsorber comprises a first noble
metal and a molecular sieve having an LTL framework. The invention
also includes an exhaust system comprising the passive NO.sub.x
adsorber, and a method for treating exhaust gas from an internal
combustion engine utilizing the passive NO.sub.x adsorber.
BRIEF DESCRIPTION OF THE DRAWING
[0011] FIG. 1 shows the NO.sub.x storage and release vs. time
profiles for a PNA of the invention and a comparative PNA in the
fresh state and after hydrothermal aging.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The passive NO.sub.x adsorber of the invention is effective
to adsorb NO.sub.x at or below a low temperature and release the
adsorbed NO.sub.x at temperatures above the low temperature.
Preferably, the low temperature is about 250.degree. C. The passive
NO.sub.x adsorber comprises a first noble metal and a molecular
sieve having an LTL Framework Type. The first noble metal is
preferably palladium, platinum, rhodium, gold, silver, iridium,
ruthenium, osmium, or mixtures thereof; more preferably, palladium,
platinum, rhodium, or mixtures thereof. Palladium is particularly
preferred.
[0013] The molecular sieve has an LTL Framework Type and may be any
natural or a synthetic molecular sieve, including zeolites, and is
preferably composed of aluminum, silicon, and/or heteroatoms (e.g.,
Ga) and has an LTL framework. The LTL molecular sieve typically has
a three-dimensional arrangement of TO.sub.4 (T=Si, Al, Ga) units or
tetrahedra that are joined by the sharing of oxygen atoms, and are
characterized by a channel system comprising 1-D 12-ring channels.
The designation 12-ring refers to the number of tetrahedral atoms
(e.g., Si, Al) or oxygen atoms that make up a ring system.
Molecular sieve frameworks are typically anionic, which are
counterbalanced by charge compensating cations, typically alkali
and alkaline earth elements (e.g., Na, K, Mg, Ca, Sr, and Ba),
ammonium ions, and also protons. Other metals (e.g., Fe, Ti) may be
incorporated into the framework of the LTL molecular sieve to
produce a metal-incorporated molecular sieve.
[0014] Preferably, the LTL-framework molecular sieve is an
aluminosilicate zeolite, an aluminophosphate zeolite, a
silicoaluminophosphate (SAPO) zeolite, or other metal-substituted
aluminosilicate or aluminophosphate zeolite. More preferably, the
LTL-framework molecular sieve is zeolite L, Linde Type L,
gallosilicate L, LZ-212, LTL-type SAPO, or perlialite zeolite.
[0015] The passive NO.sub.x adsorber may be prepared by any known
means. For instance, the first noble metal may be added to the
LTL-framework molecular sieve to form the passive NO.sub.x adsorber
by any known means, the manner of addition is not considered to be
particularly critical. For example, a noble metal compound (such as
palladium nitrate) may be supported on the molecular sieve by
impregnation, adsorption, ion-exchange, incipient wetness,
precipitation, spray drying, or the like. Alternatively, the noble
metal may be added during molecular sieve synthesis. Other metals
may also be added to the passive NO.sub.x adsorber.
[0016] Preferably, the passive NO.sub.x adsorber further comprises
a flow-through substrate or filter substrate. The flow-through or
filter substrate is a substrate that is capable of containing
catalyst components. The substrate is preferably a ceramic
substrate or a metallic substrate. The ceramic substrate may be
made of any suitable refractory material, e.g., alumina, silica,
titania, ceria, zirconia, magnesia, zeolites, silicon nitride,
silicon carbide, zirconium silicates, magnesium silicates,
aluminosilicates, metallo aluminosilicates (such as cordierite and
spudomene), or a mixture or mixed oxide of any two or more thereof.
Cordierite, a magnesium aluminosilicate, and silicon carbide are
particularly preferred.
[0017] The metallic substrates may be made of any suitable metal,
and in particular heat-resistant metals and metal alloys such as
titanium and stainless steel as well as ferritic alloys containing
iron, nickel, chromium, and/or aluminum in addition to other trace
metals.
[0018] The flow-through substrate is preferably a flow-through
monolith having a honeycomb structure with many small, parallel
thin-walled channels running axially through the substrate and
extending throughout from an inlet or an outlet of the substrate.
The channel cross-section of the substrate may be any shape, but is
preferably square, sinusoidal, triangular, rectangular, hexagonal,
trapezoidal, circular, or oval.
[0019] The filter substrate is preferably a wall-flow monolith
filter. The channels of a wall-flow filter are alternately blocked,
which allow the exhaust gas stream to enter a channel from the
inlet, then flow through the channel walls, and exit the filter
from a different channel leading to the outlet. Particulates in the
exhaust gas stream are thus trapped in the filter.
[0020] The passive NO.sub.x adsorber may be added to the
flow-through or filter substrate by any known means. A
representative process for preparing the passive NO.sub.x adsorber
using a washcoat procedure is set forth below. It will be
understood that the process below can be varied according to
different embodiments of the invention.
[0021] The pre-formed passive NO.sub.x adsorber may be added to the
flow-through or filter substrate by a washcoating step.
Alternatively, the passive NO.sub.x adsorber may be formed on the
flow-through or filter substrate by first washcoating unmodified
molecular sieve onto the substrate to produce a molecular
sieve-coated substrate. Noble metal may then be added to the
molecular sieve-coated substrate, which may be accomplished by an
impregnation procedure, or the like.
[0022] The washcoating procedure is preferably performed by first
slurrying finely divided particles of the passive NO.sub.x adsorber
(or unmodified LTL-framework molecular sieve) in an appropriate
solvent, preferably water, to form the slurry. Additional
components, such as transition metal oxides, binders, stabilizers,
or promoters may also be incorporated in the slurry as a mixture of
water soluble or water-dispersible compounds. If unmodified
LTL-framework molecular sieve is utilized in the slurry, a noble
metal compound (such as palladium nitrate) may be added into the
slurry in order to form the noble metal/LTL-framework molecular
sieve during the washcoating process.
[0023] The slurry preferably contains between 10 to 70 weight
percent solids, more preferably between 20 to 50 weight percent.
Prior to forming the slurry, the passive NO.sub.x adsorber (or
unmodified LTL-framework molecular sieve) particles are preferably
subject to a size reduction treatment (e.g., milling) such that the
average particle size of the solid particles is less than 20
microns in diameter.
[0024] The flow-through or filter substrate may then be dipped one
or more times into the slurry or the slurry may be coated on the
substrate such that there will be deposited on the substrate the
desired loading of catalytic materials. If the first noble metal is
not incorporated into the LTL-framework molecular sieve prior to,
or during, washcoating the flow-through or filter substrate, the
molecular sieve-coated substrate is typically dried and calcined
and then, the first noble metal may be added to the molecular
sieve-coated substrate by any known means, including impregnation,
adsorption, or ion-exchange, for example, with a noble metal
compound (such as palladium nitrate).
[0025] The passive NO.sub.x adsorber coating can cover the total
length of the substrate, or alternately can only cover a portion of
the total length of the substrate such that only an inlet zone or
outlet zone of passive NO.sub.x adsorber coating is formed.
Preferably, the entire length of the substrate is coated with the
passive NO.sub.x adsorber slurry so that a washcoat of the passive
NO.sub.x adsorber covers the entire surface of the substrate.
[0026] After the flow-through or filter substrate has been coated
with the passive NO.sub.x adsorber, and impregnated with noble
metal if necessary, the coated substrate is preferably dried and
then calcined by heating at an elevated temperature to form the
passive NO.sub.x adsorber-coated substrate. Preferably, the
calcination occurs at 400 to 600.degree. C. for approximately 1 to
8 hours.
[0027] In an alternative embodiment, the flow-through or filter
substrate is comprised of the passive NO.sub.x adsorber. In this
case, the passive NO.sub.x adsorber is extruded to form the
flow-through or filter substrate. The passive NO.sub.x adsorber
extruded substrate is preferably a honeycomb flow-through
monolith.
[0028] Extruded molecular sieve substrates and honeycomb bodies,
and processes for making them, are known in the art. See, for
example, U.S. Pat. Nos. 5,492,883, 5,565,394, and 5,633,217 and
U.S. Pat. No. Re. 34,804. Typically, the molecular sieve material
is mixed with a permanent binder such as silicone resin and a
temporary binder such as methylcellulose, and the mixture is
extruded to form a green honeycomb body, which is then calcined and
sintered to form the final small pore molecular sieve flow-through
monolith. The molecular sieve may contain the first noble metal
prior to extruding such that a passive NO.sub.x adsorber monolith
is produced by the extrusion procedure. Alternatively, the first
noble metal may be added to a pre-formed molecular sieve monolith
in order to produce the passive NO.sub.x adsorber monolith.
[0029] Additionally, the passive NO.sub.x adsorber may further
comprise a second molecular sieve catalyst. The second molecular
sieve catalyst comprises a second noble metal and a second
molecular sieve, wherein the second molecular sieve does not have
an LTL Framework Type. In this embodiment, the passive NO.sub.x
adsorber may comprise one or more additional molecular sieve
catalysts (e.g., a third molecular sieve catalyst and/or a fourth
molecular sieve catalyst), provided that the additional molecular
sieve(s) are different than the first and second molecular
sieves.
[0030] The first noble metal and the second noble metal are
independently selected from platinum, palladium, rhodium, gold,
silver, iridium, ruthenium, osmium, or mixtures thereof;
preferably, they are independently selected from palladium,
platinum, rhodium, or mixtures thereof. More preferably, the first
noble metal and the second noble metal are both palladium.
[0031] The second molecular sieve is preferably a small pore
molecular sieve having the Framework Type of ACO, AEI, AEN, AFN,
AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI,
ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU,
PHI, RHO, RTH, SAT, SAV, SFW, SIV, THO, TSC, UEI, UFI, VNI, YUG,
and ZON, a medium pore molecular sieve having the Framework Type of
MFI, FER, MWW, or EUO, a large pore molecular sieve having the
Framework Type of CON, BEA, FAU, MAZ, MOR, OFF, or EMT, as well as
mixtures or intergrowths of any two or more. More preferably, the
small pore zeolite is AEI or CHA, the medium pore zeolite is MFI,
and the large pore zeolite is BEA.
[0032] The passive NO.sub.x adsorber containing the second
molecular sieve catalyst may be prepared by processes well known in
the prior art. The noble metal/LTL-framework molecular sieve and
the second molecular sieve catalyst may be physically mixed to
produce the passive NO.sub.x adsorber. Preferably, the passive
NO.sub.x adsorber further comprises a flow-through substrate or
filter substrate. In one embodiment, the noble metal/LTL-framework
molecular sieve and the second molecular sieve catalyst are coated
onto the flow-through or filter substrate, and preferably deposited
on the flow-through or filter substrate using a washcoat procedure
to produce the passive NO.sub.x adsorber.
[0033] Suitable flow-through or filter substrates are described
above, as well as procedures for washcoating the substrates with
the noble metal/LTL-framework molecular sieve and the second
molecular sieve catalyst. The order of addition of the noble
metal/LTL-framework molecular sieve and the second molecular sieve
catalyst onto the flow-through or filter substrate is not
considered critical. Thus, the noble metal/LTL-framework molecular
sieve may be washcoated on the substrate prior to the second
molecular sieve catalyst or the second molecular sieve catalyst may
be washcoated on the substrate prior to the noble
metal/LTL-framework molecular sieve or both the noble
metal/LTL-framework molecular sieve and the second molecular sieve
catalyst can be washcoated on the substrate simultaneously.
[0034] In an alternative embodiment, the flow-through or filter
substrate is comprised of the noble metal/LTL-framework molecular
sieve, the second molecular sieve catalyst, or both the noble
metal/LTL-framework molecular sieve and second molecular sieve
catalyst. In this case, the noble metal/LTL-framework molecular
sieve, the second molecular sieve catalyst, or both are extruded to
form the flow-through or filter substrate. If not included in the
extruded substrate, the noble metal/LTL-framework molecular sieve
or second molecular sieve catalyst is coated onto the extruded
flow-through or filter substrate. The extruded substrate is
preferably a honeycomb flow-through monolith.
[0035] Preferably, the passive NO.sub.x adsorber comprises a first
layer comprising the noble metal/LTL-framework molecular sieve and
a second layer comprising the second molecular sieve catalyst.
Typically, the first layer may be disposed on a substrate and the
second layer is disposed on the first layer. Alternatively, the
second layer may be disposed on a substrate and the first layer
disposed on the second layer.
[0036] In a separate embodiment, the passive NO.sub.x adsorber
comprises a first zone comprising the noble metal/LTL-framework
molecular sieve and a second zone comprising the second molecular
sieve catalyst. The first zone may be upstream of the second zone
such that the first zone contacts the exhaust gas prior to the
second zone, or alternatively the second zone may be upstream of
the first zone such that the second zone contacts the exhaust gas
prior to the first zone. Preferably, the second zone is located
upstream of the first zone such that the exhaust gas contacts the
second molecular sieve catalyst prior to contacting the first
molecular sieve catalyst. The two zones may be on the same catalyst
component (or catalyst brick), or the first zone comprising the
noble metal/LTL-framework molecular sieve may be located on a
separate brick (or catalyst component) than the second zone
comprising the second molecular sieve catalyst.
[0037] The invention also includes an exhaust system for internal
combustion engines comprising the passive NO.sub.x adsorber. The
exhaust system preferably comprises one or more additional
after-treatment devices capable of removing pollutants from
internal combustion engine exhaust gases at normal operating
temperatures. Preferably, the exhaust system comprises the passive
NO.sub.x adsorber and one or more other catalyst components
selected from: (1) a selective catalytic reduction (SCR) catalyst,
(2) a particulate filter, (3) a SCR filter, (4) a NO.sub.x adsorber
catalyst, (5) a three-way catalyst, (6) an oxidation catalyst, or
any combination thereof. The passive NO.sub.x adsorber is
preferably a separate component from any of the above
after-treatment devices. Alternatively, the passive NO.sub.x
adsorber can be incorporated as a component into any of the above
after-treatment devices. That is, the passive NO.sub.x adsorber may
be incorporated as a zone on a substrate that contains the other
catalyst component; or may be incorporated as a layer on a
substrate that contains the other catalyst component. As an
illustrative example, the passive NO.sub.x adsorber may be a front
zone on a substrate that also contains a diesel oxidation catalyst
as the rear zone, (or the passive NO.sub.x adsorber may be a rear
zone on a substrate that also contains a diesel oxidation catalyst
as the front zone); or the passive NO.sub.x adsorber may be the
lower layer on a substrate with a diesel oxidation catalyst forming
an overlayer over the lower layer, or the passive NO.sub.x adsorber
may be an overlayer covering a lower layer comprising a diesel
oxidation catalyst.
[0038] So the invention also includes a catalyst comprising a
substrate, a diesel oxidation catalyst, and the passive NO.sub.x
adsorber. The passive NO.sub.x adsorber is located on a first zone
or a first layer on the substrate; and the diesel oxidation
catalyst is located on a second zone or a second layer on the
substrate. Preferably when the substrate contains a first zone of
the passive NO.sub.x adsorber and a second zone of the diesel
oxidation catalyst, the first zone is located upstream of the
second zone. Alternatively, the first zone is located downstream of
the second zone. When the substrate contains a first layer of the
passive NO.sub.x adsorber and a second layer of the diesel
oxidation catalyst, the first layer is preferably disposed on the
substrate and the second layer is disposed on the first layer.
Alternatively, the second layer is disposed on the substrate and
the first layer is disposed on the second layer.
[0039] These after-treatment devices are well known in the art.
Selective catalytic reduction (SCR) catalysts are catalysts that
reduce NO.sub.x to N.sub.2 by reaction with nitrogen compounds
(such as ammonia or urea) or hydrocarbons (lean NO.sub.x
reduction). A typical SCR catalyst is comprised of a
vanadia-titania catalyst, a vanadia-tungsta-titania catalyst, or a
metal/zeolite catalyst such as iron/beta zeolite, copper/beta
zeolite, copper/SSZ-13, copper/SAPO-34, Fe/ZSM-5, or
copper/ZSM-5.
[0040] Particulate filters are devices that reduce particulates
from the exhaust of internal combustion engines. Particulate
filters include catalyzed particulate filters and bare
(non-catalyzed) particulate filters. Catalyzed particulate filters
(for diesel and gasoline applications) include metal and metal
oxide components (such as Pt, Pd, Fe, Mn, Cu, and ceria) to oxidize
hydrocarbons and carbon monoxide in addition to destroying soot
trapped by the filter.
[0041] Selective catalytic reduction filters (SCRF) are
single-substrate devices that combine the functionality of an SCR
and a particulate filter. They are used to reduce NO.sub.x and
particulate emissions from internal combustion engines. In addition
to the SCR catalyst coating, the particulate filter may also
include other metal and metal oxide components (such as Pt, Pd, Fe,
Mn, Cu, and ceria) to oxidize hydrocarbons and carbon monoxide in
addition to destroying soot trapped by the filter.
[0042] NO.sub.x adsorber catalysts (NACs) are designed to adsorb
NO.sub.x under lean exhaust conditions, release the adsorbed NO
under rich conditions, and reduce the released NO.sub.x to form
N.sub.2. NACs typically include a NO.sub.x-storage component (e.g.,
Ba, Ca, Sr, Mg, K, Na, Li, Cs, La, Y, Pr, and Nd), an oxidation
component (preferably Pt) and a reduction component (preferably
Rh). These components are contained on one or more supports.
[0043] Three-way catalysts (TWCs) are typically used in gasoline
engines under stoichiometric conditions in order to convert
NO.sub.x to N.sub.2, carbon monoxide to CO.sub.2, and hydrocarbons
to CO.sub.2 and H.sub.2O on a single device.
[0044] Oxidation catalysts, and in particular diesel oxidation
catalysts (DOCS), are well-known in the art. Oxidation catalysts
are designed to oxidize CO to CO.sub.2 and gas phase hydrocarbons
(HC) and an organic fraction of diesel particulates (soluble
organic fraction) to CO.sub.2 and H.sub.2O. Typical oxidation
catalysts include platinum and optionally also palladium on a high
surface area inorganic oxide support, such as alumina,
silica-alumina and a zeolite.
[0045] The passive NO.sub.x adsorber is preferably a separate
component from any of the above after-treatment devices.
Alternatively, the passive NO.sub.x adsorber can be incorporated as
a component into any of the above after-treatment devices. For
instance, a substrate may comprise and upstream zone of the passive
NO.sub.x adsorber and a downstream zone of SCR catalyst on the same
substrate.
[0046] The exhaust system can be configured so that the passive
NO.sub.x adsorber is located close to the engine and the additional
after-treatment device(s) are located downstream of the passive
NO.sub.x adsorber. Thus, under normal operating conditions, engine
exhaust gas first flows through the passive NO.sub.x adsorber prior
to contacting the after-treatment device(s). Alternatively, the
exhaust system may contain valves or other gas-directing means such
that during the low temperature period (below a temperature ranging
from about 150 to 220.degree. C., preferably 200.degree. C., about
as measured at the after-treatment device(s)), the exhaust gas is
directed to contact the after-treatment device(s) before flowing to
the passive NO.sub.x adsorber. Once the after-treatment device(s)
reaches the operating temperature (about 150 to 220.degree. C.,
preferably 200.degree. C., as measured at the after-treatment
device(s)), the exhaust gas flow is then redirected to contact the
passive NO.sub.x adsorber prior to contacting the after-treatment
device(s). This ensures that the temperature of the passive
NO.sub.x adsorber remains low for a longer period of time, and thus
improves efficiency of the passive NO.sub.x adsorber, while
simultaneously allowing the after-treatment device(s) to more
quickly reach operating temperature. U.S. Pat. No. 5,656,244, the
teachings of which are incorporated herein by reference, for
example, teaches means for controlling the flow of the exhaust gas
during cold-start and normal operating conditions.
[0047] The invention also includes a method for treating exhaust
gas from an internal combustion engine. The method comprises
adsorbing NO.sub.x onto the passive NO.sub.x adsorber at
temperatures at or below a low temperature, thermally desorbing
NO.sub.x from the passive NO.sub.x adsorber at a temperature above
the low temperature, and catalytically removing the desorbed
NO.sub.x on a catalyst component downstream of the passive NO.sub.x
adsorber. Preferably, the low temperature is about 250.degree.
C.
[0048] The catalyst component downstream of the passive NO.sub.x
adsorber is a SCR catalyst, a particulate filter, a SCR filter, a
NO.sub.x adsorber catalyst, a three-way catalyst, an oxidation
catalyst, or combinations thereof.
[0049] The following examples merely illustrate the invention.
Those skilled in the art will recognize many variations that are
within the spirit of the invention and scope of the claims.
EXAMPLE 1
Preparation of Passive NO.sub.x Adsorbers (PNAs)
[0050] PNA 1A: 1 wt. % Pd/LTL
[0051] Palladium is added to LTL zeolite (crystalline structure;
silica-to-alumina ratio (SAR) of about 6; NH.sub.4.sup.+ exchanged)
according to the following procedure to produce PNA 1A: The powder
catalyst is prepared by wet impregnation of the zeolite using a
soluble palladium compound as the precursor. After drying at
105.degree. C., the sample is calcined at 500.degree. C. to provide
the fresh catalyst, and a portion of the fresh catalyst is then
hydrothermally aged at 750.degree. C. in an air atmosphere
containing 10% H.sub.2O. The Pd loading of PNA 1A is 1 wt. %.
[0052] PNA 1 B: 3 wt. % Pd/LTL
[0053] PNA 1B is produced using the same procedure as PNA 1A with
the exception that a higher amount of palladium is loaded onto the
LTL zeolite. The Pd loading of PNA 1B is 3 wt. %.
[0054] Comparative PNA 2A: 1 wt. % Pd/CHA
[0055] Comparative PNA 2A is produced using the same procedure as
PNA 1A with the exception that a small pore chabazite (CHA) zeolite
with a silica-to-alumina ratio (SAR) of 25 is used in place of LTL.
The Pd loading of Comparative PNA 2A is 1 wt. %.
[0056] Comparative PNA 2B: 3 wt. % Pd/CHA
[0057] Comparative PNA 2B is produced using the same procedure as
PNA 1B with the exception that a small pore chabazite (CHA) zeolite
with a silica-to-alumina ratio (SAR) of 25 is used in place of LTL.
The Pd loading of Comparative PNA 2B is 3 wt. %.
EXAMPLE 2
NO.sub.x Storage Capacity Testing Procedures
[0058] The catalyst (0.4 g) is held at the adsorption temperature
of about 100.degree. C. for 5 minutes in an NO-containing gas
mixture flowing at 2 liters per minute at a MHSV of 300
L*hr.sup.-1*g.sup.-1. This adsorption stage is followed by
Temperature Programmed Desorption (TPD) at a ramping rate of
17.degree. C./minute in the presence of the NO-containing gas until
the bed temperature reaches about 450.degree. C. in order to purge
the catalyst of all stored NO.sub.x for further testing.
[0059] The NO-containing gas mixture during both the adsorption and
desorption comprises 10 vol. % O.sub.2, 60 ppm NO, 5 vol. %
CO.sub.2, 1500 ppm CO, 130 ppm C.sub.3H.sub.6, and 5 vol. %
H.sub.2O in N.sub.2.
[0060] The NO.sub.x storage is calculated as the amount of NO.sub.2
stored per liter of catalyst with reference to a monolith
containing a catalyst loading of about 3 g/in.sup.3. The results
are shown in Table 1 and the NO.sub.x uptake and release profiles
are shown in FIG. 1 for fresh and aged catalysts (hydrothermally
aged at 750.degree. C. as described in Example 1).
[0061] The results at Table 1 show that the PNAs of the invention
(PNA 1A and PNA 1B) store a greater amount of NO.sub.x compared to
the Comparative PNAs over the entire testing period in the fresh
state and clearly demonstrate the higher NO.sub.x release
temperatures for Pd/LTL, both fresh and aged compared to the
corresponding Pd/CHA comparative catalysts. Although at the initial
100.degree. C. storage period the NO.sub.x storage of the Pd/CHA
samples was higher than that of the Pd/LTL samples, during the
temperature ramping period above 100.degree. C. the Pd/LTL samples
store more NO.sub.x than the comparative Pd/CHA samples as shown in
FIG. 1. FIG. 1 also shows the NO.sub.x thermally releasing at
higher temperatures than the comparative Pd/CHA examples
(.about.370.degree. C. vs .about.260.degree. C.) and that the
NO.sub.x release characteristics are maintained upon hydrothermal
aging.
TABLE-US-00001 TABLE 1 NO.sub.x Storage Comparison Results NO.sub.x
release NO.sub.x storage NO.sub.x release peak capacity until onset
temper- Pd breakthrough.sup.1 temperature ature Catalyst Zeolite
loading (g NO.sub.2/L) (.degree. C.) (.degree. C.) 1A Fresh LTL 1
wt % 0.55 285 370 2A Fresh* CHA 1 wt % 0.51 230 260 1B Fresh LTL 3
wt % 1.48 295 360 2B Fresh* CHA 3 wt % 0.7 260 330 1A Aged LTL 1 wt
% 0.39 300 365 2A Aged* CHA 1 wt % 0.64 220 255
.sup.1"Breakthrough" is defined as the temperature at which the
concentration of NO.sub.x in the gas stream rises above the initial
inlet value of 60 ppm.
* * * * *