U.S. patent application number 11/564494 was filed with the patent office on 2008-05-29 for nox storage materials and traps resistant to thermal aging.
Invention is credited to Marcus Hilgendorff, Stanley Roth, Susanne Stiebels.
Application Number | 20080120970 11/564494 |
Document ID | / |
Family ID | 39145176 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080120970 |
Kind Code |
A1 |
Hilgendorff; Marcus ; et
al. |
May 29, 2008 |
NOx Storage Materials and Traps Resistant to Thermal Aging
Abstract
Nitrogen oxide storage materials and methods of manufacturing
nitrogen oxide storage materials are disclosed. The nitrogen oxide
storage materials can be used to manufacture catalytic trap
disposed in an exhaust passage of an internal combustion engine
which is operated periodically between lean and stoichiometric or
rich conditions, for abatement of NO.sub.x in an exhaust gas stream
which is generated by the engine. In one embodiment, the nitrogen
oxide storage material comprises alkaline earth material supported
on ceria particles having a crystallite size of between about 10
and 20 nm and the alkaline earth oxide having a crystallite size of
between about 20-40 nm.
Inventors: |
Hilgendorff; Marcus;
(Hannover, DE) ; Roth; Stanley; (Yardley, PA)
; Stiebels; Susanne; (Adenbuttel, DE) |
Correspondence
Address: |
BASF CATALYSTS LLC
100 CAMPUS DRIVE
FLORHAM PARK
NJ
07932
US
|
Family ID: |
39145176 |
Appl. No.: |
11/564494 |
Filed: |
November 29, 2006 |
Current U.S.
Class: |
60/299 ;
502/304 |
Current CPC
Class: |
B01J 35/006 20130101;
B01D 2255/20715 20130101; B01J 35/1038 20130101; B01D 2255/2065
20130101; B01D 2255/204 20130101; B01D 2255/9202 20130101; B01J
35/1019 20130101; B01D 2255/1025 20130101; B01D 53/9422 20130101;
B01D 2255/2092 20130101; B01J 37/0045 20130101; B01J 35/1061
20130101; B01D 2255/91 20130101; B01J 35/1014 20130101; B01J 23/63
20130101; B01D 2255/1021 20130101; B01J 23/10 20130101; B01J 35/023
20130101 |
Class at
Publication: |
60/299 ;
502/304 |
International
Class: |
F01N 3/10 20060101
F01N003/10; B01J 23/10 20060101 B01J023/10 |
Claims
1. A nitrogen oxide storage catalyst comprising: a coating on a
substrate, the coating comprising a nitrogen oxide storage material
comprising ceria particles having an alkaline earth oxide supported
on the particles, the ceria having a crystallite size of between
about 10 and 20 nm and the alkaline earth oxide having a
crystallite size of between about 20-40 nm.
2. The nitrogen oxide storage catalyst of claim 1, the coating
further comprising at least one member of platinum group metals
selected from the group consisting of platinum, palladium, rhodium,
iridium and mixtures thereof supported on refractory oxide
particles.
3. The nitrogen oxide catalyst of claim 2, wherein the refractory
oxide particles are selected from the group consisting of aluminum
oxide, mixed aluminum oxide and zirconium oxide, mixed aluminum
oxide and lanthanum oxide, mixed aluminum oxide and cerium oxide,
mixed aluminum oxide and magnesium oxide, and alumina oxide mixed
with one or more of zirconia and lanthana.
4. The nitrogen oxide catalyst of claim 2, wherein the ceria
particles have a particle size of between about 5 microns and about
50 microns and a BET surface area of between about 30 and 80
m.sup.2/g.
5. The nitrogen oxide catalyst of claim 4, wherein the ceria
particles have an average pore volume of about 0.3 to about 0.5
ml/g.
6. The nitrogen oxide catalyst of claim 5, wherein the pores in the
ceria particles have an average pore diameter of between about 3 nm
and about 30 nm.
7. The nitrogen oxide catalyst of claim 2, wherein the catalyst
exhibits improved nitrogen oxide storage capacity after aging at
850.degree. C. for 50 hours at a stoichiometric air fuel ratio
compared with a catalyst having non-spray-dried ceria particles
with baria supported on the ceria particles.
8. A catalytic trap disposed in an exhaust passage of an internal
combustion engine which operates periodically between lean and
stoichiometric or rich conditions, for abatement of NO.sub.x in an
exhaust gas stream which is generated by the engine, comprising a
catalytic trap material including a catalytic component effective
for promoting the reduction of NO.sub.x under stoichiometric or
rich conditions supported on a refractory metal oxide and a
NO.sub.x storage material effective for adsorbing the NO.sub.x
under lean conditions and desorbing and reducing the NO.sub.x to
nitrogen under stoichiometric or rich conditions, the NO.sub.x
storage material comprising particles of ceria having an alkaline
earth material supported on the ceria particles, the ceria having a
crystallite size of between about 10 and 20 nm and the alkaline
earth oxide having a crystallite size of between about 20-40 nm,
and the catalytic trap material being disposed on a refractory
carrier member.
9. The catalytic trap of claim 8, the catalytic component
comprising at least one member of platinum group metals selected
from the group consisting of platinum, palladium, rhodium, iridium
and mixtures thereof.
10. The catalytic trap of claim 9, wherein the refractory oxide
particles are selected from the group consisting of aluminum oxide,
mixed aluminum oxide and zirconium oxide, mixed aluminum oxide and
lanthanum oxide, mixed aluminum oxide and cerium oxide, mixed
aluminum oxide and magnesium oxide, and alumina oxide mixed with
one or more of zirconia and lanthana.
11. The catalytic trap of claim 9, wherein the ceria particles have
a particle size of between about 5 microns and about 20
microns.
12. The catalytic trap of claim 11, wherein the ceria particles
have an average pore volume of about 0.3 to about 0.5 ml/g.
13. The catalytic trap of claim 12, wherein the pores in the ceria
particles have an average pore diameter of between about 3 nm and
about 30 nm.
14. The catalytic trap of claim 8, wherein ceria particles are
spray-dried particles.
15. A method of making a nitrogen oxide storage material comprising
mixing a solution of barium with ceria particles, spray drying the
particles, heating the spray-dried particles, and coating the
particles on a substrate.
16. The method of claim 15, wherein the ceria particles have a
surface area of between about 50 and about 150 m.sup.2/g prior to
spray drying.
17. The method of claim 16, wherein the ceria particles has a
particle size of between about 5 microns and about 20 microns.
18. The method of claim 17, wherein the ceria particles have an
average pore volume of about 0.3 to about 0.5 ml/g.
19. The method of claim 15, wherein the pores in the ceria
particles have an average pore diameter of between about 3 nm and
about 30 nm.
20. The method of claim 15, wherein the nitrogen storage material
exhibits an improved nitrogen oxide storage capacity after aging at
850.degree. C. for 50 hours at a stoichiometric air fuel ratio
compared with a catalyst having non-spray-dried ceria particles
with baria supported on the ceria particles.
Description
TECHNICAL FIELD
[0001] Embodiments of the invention relate nitrogen oxide storage
materials and methods for their manufacture. More particularly,
embodiments of the invention pertain to NO.sub.x storage materials
that are resistant to thermal aging and methods of making such
materials. The nitrogen oxide storage materials may be part of a
catalytic trap used to treat exhaust gas streams, especially those
emanating from lean-burn gasoline or diesel engines.
BACKGROUND ART
[0002] Emission of nitrogen oxides ("NO.sub.x") from lean-burn
engines (described below) must be reduced in order to meet emission
regulation standards. Conventional three-way conversion ("TWC")
automotive catalysts are suitable for abating NO.sub.x, carbon
monoxide a ("CO") and hydrocarbon ("HC") pollutants in the exhaust
of engines operated at or near stoichiometric air/fuel conditions.
The precise proportion of air to fuel which results in
stoichiometric conditions varies with the relative proportions of
carbon and hydrogen in the fuel. An air-to-fuel ("A/F") ratio of
14.65:1 (weight of air to weight of fuel) is the stoichiometric
ratio corresponding to the combustion of a hydrocarbon fuel, such
as gasoline, with an average formula CH.sub.1.88. The symbol
.lamda. is thus used to represent the result of dividing a
particular A/F ratio by the stoichiometric A/F ratio for a given
fuel, so that; .lamda.=1 is a stoichiometric mixture, .lamda.>1
is a fuel-lean mixture and .lamda.<1 is a fuel-rich mixture.
[0003] Engines, especially gasoline-fueled engines to be used for
passenger automobiles and the like, are being designed to operate
under lean conditions as a fuel economy measure. Such future
engines are referred to as "lean-burn engines". That is, the ratio
of air to fuel in the combustion mixtures supplied to such engines
is maintained considerably above the stoichiometric ratio (e.g., at
an air-to-fuel weight ratio of 18:1) so that the resulting exhaust
gases are "lean", i.e., the exhaust gases are relatively high in
oxygen content. Although lean-burn engines provide enhanced fuel
economy, they have the disadvantage that conventional TWC catalysts
are not effective for reducing NO.sub.x emissions from such engines
because of excessive oxygen in the exhaust. Attempts to overcome
this problem have included operating lean-burn engines with brief
periods of fuel-rich operation (engines which operate in this
fashion are sometimes referred to as "partial lean-burn engines").
The exhaust of such engines is treated with a catalyst/NO.sub.x
sorbent which stores NO.sub.x during periods of lean (oxygen-rich)
operation, and releases the stored NO.sub.x during the rich
(fuel-rich) periods of operation. During periods of rich (or
stoichiometric) operation, the catalyst component of the
catalyst/NO.sub.x sorbent promotes the reduction of NO.sub.x to
nitrogen by reaction of NO.sub.x (including NO.sub.x released from
the NO.sub.x sorbent) with HC, CO and/or hydrogen present in the
exhaust.
[0004] Diesel engines provide better fuel economy than gasoline
engines and normally operate 100% of the time under lean
conditions, where the reduction of NOx is difficult due to the
presence of excess oxygen. In this case, the catalyst/NOx sorbent
is effective for storing NOx. As in the case of the gasoline
partial lean burn application, after the NOx storage mode, a
transient rich condition must be utilized to release/reduce the
stored NOx to nitrogen. In the case of the diesel engine, this
transient reducing condition will require unique engine calibration
or injection of a diesel fuel into the exhaust to create the next
reducing environment.
[0005] NO.sub.x storage (sorbent) components including alkaline
earth metal oxides, such as oxides of Mg, Ca, Sr and Ba, alkali
metal oxides such as oxides of Li, Na, K, Rb and Cs, and rare earth
metal oxides such as oxides of Ce, La, Pr and Nd in combination
with precious metal catalysts such as platinum dispersed on an
alumina support have been used in the purification of exhaust gas
from an internal combustion engine. For NO.sub.x storage, baria is
usually preferred because it forms nitrates at lean engine
operation and releases the nitrates relatively easily under rich
conditions. However, catalysts that use baria for NO.sub.x storage
exhibit a problem in practical application, particularly when the
catalysts are aged by exposure to high temperatures and lean
operating conditions. After such exposure, such catalysts show a
marked decrease in catalytic activity for NO.sub.x reduction,
particularly at low temperature (200 to 350.degree. C.) and high
temperature (450.degree. C. to 600.degree. C.) operating
conditions. In addition, NO.sub.x absorbents that include baria
suffer from the disadvantage that when exposed to temperatures
above 450.degree. C. in the presence of CO.sub.2, barium carbonate
forms, which becomes more stable than barium nitrate. Furthermore,
barium tends to sinter and to form composite compounds with support
materials, which leads to the loss of NO.sub.x storage
capacity.
[0006] NO.sub.x storage materials comprising barium fixed to ceria
particles have been reported, and these NO.sub.x materials have
exhibited improved thermal aging properties compared to the
catalyst materials described above. Despite these improvements,
there is an ongoing need to improve the performance of NO.sub.x
storage materials, particularly the ability of these materials to
operate over a wide temperature range and to operate effectively
after exposure to high temperature. It is also desirable to improve
the kinetics of NO.sub.x oxidation (required in advance of NOx
storage) and the kinetics of NOx reduction (required following NOx
release). Thus, there is a need to provide improved NO.sub.x
storage materials and methods for their manufacture.
SUMMARY
[0007] Aspects of the invention include nitrogen oxide storage
materials, catalytic traps for the abatement of nitrogen oxide,
methods for manufacturing both the nitrogen oxide storage materials
and the catalytic traps for the abatement of nitrogen oxides, and
methods of abating nitrogen oxide in an exhaust gas stream.
[0008] According to one embodiment, the nitrogen oxide storage
materials comprise ceria particles having alkaline earth oxides,
for example, baria, supported on the particles, the ceria having a
crystallite size of between about 10 and 20 nm and the alkaline
earth oxides having a crystallite size of between about 20 and 40
nm. Other suitable alkaline earth oxides include oxides of Mg, Sr,
and Ca. In certain embodiments, the composite particles have a BET
surface area of between about 30 and 80 m.sup.2/g. In another
embodiment, a nitrogen oxide storage catalyst is provided
comprising a coating on a substrate, the coating comprising a
nitrogen oxide storage material comprising spray-dried ceria
particles having baria supported on the particles.
[0009] In certain embodiments, the coating of the nitrogen oxide
storage catalyst further comprises at least one member of platinum
group metals selected from the group consisting of platinum,
palladium, rhodium, iridium and mixtures thereof supported on
refractory oxide particles. The refractory oxide particles may be
selected from the group consisting of aluminum oxide, mixed
aluminum oxide and zirconium oxide, mixed aluminum oxide and
lanthanum oxide, mixed aluminum oxide and cerium oxide, mixed
aluminum oxide and magnesium oxide, and alumina oxide mixed with
one or more of zirconia and lanthana.
[0010] Another embodiment relates to a catalytic trap disposed in
an exhaust passage of an internal combustion engine which operates
periodically between lean and stoichiometric or rich conditions,
for abatement of NO.sub.x in an exhaust gas stream which is
generated by the engine. The catalytic trap comprises a catalytic
trap material including a precious metal catalytic component
effective for oxidizing NO to NO.sub.2 under lean conditions and
promoting the reduction of released NO.sub.x to nitrogen under
stoichiometric or rich conditions supported on a refractory metal
oxide, and a NO.sub.x storage material effective for adsorbing the
NO.sub.x under lean conditions and desorbing the NO.sub.x under
stoichiometric or rich conditions, the NOx storage material
comprising particles of ceria having alkaline earth carbonate
supported on the ceria particles, having a crystallite size of
between about 10 and 20 nm and the alkaline earth oxide having a
crystallite size of between about 20 and 40 nm, and the catalytic
trap material being disposed on a refractory carrier member. Still
another embodiment relates to a method of making a nitrogen oxide
storage material comprising mixing a solution of barium with ceria
particles, spray drying the particles, heating the spray-dried
particles, mixing the composite particles with a precious metal
supported catalyst and coating the slurry mixture of particles on a
substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a graph showing the nitrogen oxide conversion
efficiency of a catalyst in accordance with an embodiment of the
invention and a comparative reference catalyst;
[0012] FIG. 2 is a graph comparing the nitrogen oxide storage
capacity of various catalysts;
[0013] FIG. 3 is a graph comparing the nitrogen oxide storage
capacity of catalysts;
[0014] FIG. 4 is a graph comparing the nitrogen oxide storage
capacity of two catalysts;
[0015] FIG. 5 is a graph comparing the nitrogen oxide storage
capacity of two catalysts; and
[0016] FIG. 6 is a SEM image of the spray dried and calcined
BaCO.sub.3/CeO.sub.2 composite material.
DETAILED DESCRIPTION
[0017] Before describing several exemplary embodiments of the
invention, it is to be understood that the invention is not limited
to the details of construction or process steps set forth in the
following description. The invention is capable of other
embodiments and of being practiced or being carried out in various
ways.
[0018] In one embodiment of the invention, a spray-dried NO.sub.x
storage material comprising alkaline earth carbonate or mixtures of
carbonates, for example, BaCO.sub.3 or mixtures of BaCO.sub.3 and
MgCO.sub.3 supported on CeO.sub.2 particles is provided. According
to one or more embodiments of the invention, Ba sintering and Ba
composite compound formation is reduced under the conditions of
thermal stress in an exhaust gas of a lean burn engine. The
NO.sub.x storage material according to embodiments of the present
invention demonstrates improved NO.sub.x storage capacity after
thermal aging when used in a catalytic trap.
[0019] According other embodiments of the invention, methods of
manufacturing NO.sub.x storage materials and catalytic traps
including these storage materials are provided. Other embodiments
of the invention pertain to a catalytic trap for abatement of
NO.sub.x in an exhaust gas stream which is generated by an internal
combustion engine which is operated periodically between lean and
stoichiometric or rich conditions. According to one or more
embodiments, the catalytic trap comprises a catalytic trap material
including a catalytic component effective for promoting the
reduction of NO.sub.x under stoichiometric or rich conditions
supported on a refractory metal oxide and a NO.sub.x storage
material effective for adsorbing the NO.sub.x under lean conditions
and desorbing and reducing the NO.sub.x to nitrogen under
stoichiometric or rich conditions, the NOx storage material
comprising spray-dried particles of ceria having alkaline earth
carbonate, for example, barium carbonate, supported on the ceria
particles, the catalytic trap material being disposed on a
refractory carrier member.
[0020] Embodiments of this invention pertain to a process for
abatement of NO.sub.x in an exhaust gas stream generated by an
internal combustion engine which periodically operates alternately
between lean and stoichiometric or rich conditions, comprising
locating the above-described catalytic trap in an exhaust passage
of the engine and treating the exhaust gas stream with a catalytic
trap whereby at least some of the NO.sub.x in the exhaust gas
stream is adsorbed by the catalytic trap during the periods of lean
conditions and is desorbed from the catalytic trap and reduced to
nitrogen during the periods of stoichiometric or rich
conditions.
[0021] The refractory metal oxide support of the catalytic trap may
be porous in nature and has a high surface area such as alumina,
for example, gamma-alumina. Other suitable support materials
include titania, titania-alumina, zirconia, zirconia-alumina,
baria-alumina, lanthana-alumina, lanthana-zirconia-alumina
titania-zirconia, and mixtures thereof. Desirably, the refractory
metal oxide support will have a surface area of between about 5 and
about 350 m.sup.2/g, and more particularly between about 100 and
200 m.sup.2/g. Typically, the support will be present on the coated
substrate in the amount of about 1.5 to about 7.0 g/in.sup.3, for
example between about 3 and 6 g/in.sup.3. A suitable support
material for the precious metal is alumina, which may be doped with
one or more other materials. Alumina having a BET surface area of
about 200 m.sup.2/g and doped with 10%-30% ZrO.sub.2 and 1%-4% LaO
provided good results.
[0022] In one or more embodiments of the present invention the
catalytic component preferably comprises a precious metal
component, i.e., a platinum group metal component. Suitable
precious metal components include platinum, palladium, rhodium and
mixtures thereof. The catalytic component will typically be present
in an amount of about 20 to about 200 g/ft.sup.3, more
specifically, about 60 to 120 g/ft.sup.3.
[0023] The NO.sub.x storage material employed in the catalytic trap
according to embodiments of the present invention comprises a
spray-dried NO.sub.x storage material comprising BaCO.sub.3
supported on CeO.sub.2 particles.
[0024] In one or more embodiments, the catalytic trap is disposed
on a refractory carrier member. Examples of such substrates
include, for example, stainless steel, titanium, aluminum
zirconate, aluminum titanate, aluminum phosphate, cordierite,
mullite and corundum. The carrier member may be employed as a
monolithic honeycomb structure, spun fibers, corrugated foils,
layered materials, etc.
[0025] In a gasoline vehicle application, a catalytic device
employing a three-way conversion ("TWC") catalyst may be used in
conjunction with the catalytic trap of the invention. Such a device
will be located in an exhaust passage of the internal combustion
engine and will be disposed upstream and/or downstream of the
catalytic trap. The TWC catalyst would typically include platinum,
palladium and rhodium catalytic components dispersed on a high
surface area refractory support and may also contain one or more
base metal oxide catalytic components such as oxides of iron,
manganese or nickel. Such catalysts can be stabilized against
thermal degradation by expedients such as impregnating an activated
alumina support with one or more rare earth metal oxides, e.g.,
ceria. Such stabilized catalysts can sustain very high operating
temperatures. For example, if a fuel cut technique is utilized,
temperatures as high as 1050.degree. C. may be sustained in the
catalytic device.
[0026] If the catalytic device is employed and is located upstream
of the catalytic trap of the invention, the catalytic device would
be mounted close to the exhaust manifold of the engine. In such an
arrangement, the TWC catalyst would warm up quickly and provide for
efficient cold start emission control. Once the engine is warmed
up, the TWC catalyst will remove HC, CO and NO.sub.x from the
exhaust gas stream during stoichiometric or rich operation and HC
and CO during lean operation. The catalytic trap of the invention
would be positioned downstream of the catalytic device where the
exhaust gas temperature enables maximum NO.sub.x trap efficiency.
During periods of lean engine operation, when NO.sub.x passes
through the TWC catalyst, NO.sub.x is stored on the catalytic trap.
The catalytic trap is periodically desorbed and the NO.sub.x is
reduced to nitrogen under periods of stoichiometric or rich engine
operation. If desired, a catalytic device containing a TWC catalyst
may be employed downstream of the catalytic trap of the invention.
Such catalytic device will serve to remove further amounts of HC
and CO from the exhaust gas stream and, in particular, will provide
for efficient reduction of the NO.sub.x to nitrogen under periods
of stoichiometric or rich engine operation.
[0027] In a diesel vehicle application, the catalytic NOx-trap
according to embodiments of the invention may be used in
conjunction with a diesel oxidation catalyst (DOC), and a catalyzed
soot filter (CSF); where the DOC and CSF are placed either before
or after the catalytic device of this invention. In another
embodiment of the invention, it is possible to place the NOx-trap
catalyst directly onto the filter media.
[0028] The several components of the catalytic trap material may be
applied to the refractory carrier member, i.e., the substrate, as a
mixture of two or more components or as individual components in
sequential steps in a manner which will be readily apparent to
those skilled in the art of catalyst manufacture. A typical method
of manufacturing the catalytic trap of the present invention is to
provide the catalytic trap material as a coating or layer of
washcoat on the walls of the gas-flow passages of a suitable
carrier member. This may be accomplished, by impregnating a fine
particulate refractory metal oxide support material, e.g., gamma
alumina, with one or more catalytic metal components such as a
precious metal, i.e., platinum group, compound or other noble
metals or base metals, drying and calcining the impregnated support
particles and forming an aqueous slurry of these particles.
Spray-dried particles of the bulk NO.sub.x sorbent may be included
in the slurry. Alternatively, the NO.sub.x storage material or
sorbent may be dispersed into the support, preferably in an
impregnation operation, as described below. Activated alumina may
be thermally stabilized before the catalytic components are
dispersed thereon, as is well known in the art, by impregnating it
with, e.g., a solution of a soluble salt of barium, lanthanum,
zirconium, rare earth metal or other suitable stabilizer precursor,
and thereafter drying (e.g., at 110.degree. C. for one hour) and
calcining (e.g., at 550.degree. C. for one hour) the impregnated
activated alumina to form a stabilizing metal oxide dispersed onto
the alumina. Base metal catalysts may optionally also have been
impregnated into the activated alumina, for example, by
impregnating a solution of a base metal nitrate into the alumina
particles and calcining to provide a base metal oxide dispersed in
the alumina particles.
[0029] The carrier may then be immersed into the slurry of
impregnated activated alumina and excess slurry removed to provide
a thin coating of the slurry on the walls of the gas-flow passages
of the carrier. The coated carrier is then dried and calcined to
provide an adherent coating of the catalytic component and,
optionally, the catalytic trap material, to the walls of the
passages thereof. The carrier may then be immersed into a slurry of
fine particles of component of the NO.sub.x storage material as a
second or overlayer coating deposited over the layer of catalytic
component. A magnesium component, e.g., a solution of a magnesium
salt such as magnesium nitrate, acetate, sulfate, hydroxide, etc.,
may be combined with the slurry of component of the NO.sub.x
storage material or it may be applied as a third or overlayer
coating deposited over the second layer of the NO.sub.x storage
material. The carrier is then dried and calcined to provide a
finished catalyst trap member in accordance with one embodiment of
the present invention.
[0030] Alternatively, the alumina or other support particles
impregnated with the catalytic component may be mixed with bulk or
supported particles of the NO.sub.x storage material in an aqueous
slurry, and this mixed slurry of catalytic component particles and
NO.sub.x storage material particles may be applied as a coating to
the walls of the gas-flow passages of the carrier. Preferably,
however, for improved dispersion of the NO.sub.x storage material,
the washcoat of catalytic component material, after being dried and
calcined, is immersed (post-dipped) into a solution of a component
(NO.sub.x storage material precursor compound (or complex) and a
magnesium precursor compound (or complex) to impregnate the
washcoat with the NO.sub.x storage material precursor. The
impregnated washcoat is then dried and calcined to provide the
NO.sub.x storage material dispersed throughout the washcoat.
[0031] Separate discrete layers of washcoat may be applied in
successive impregnating/drying/calcining operations, e.g., to
provide a bottom washcoat layer containing a platinum catalytic
component in a bottom washcoat layer and a palladium and/or rhodium
catalytic component in a top washcoat layer. The NO.sub.x storage
material may be dispersed by impregnation into both the top and
bottom layers.
[0032] In use, the exhaust gas stream which is contacted with the
catalytic trap of the present invention is alternately adjusted
between lean and stoichiometric/rich operating conditions so as to
provide alternating lean operating periods and stoichiometric/rich
operating periods. It will be understood that the exhaust gas
stream being treated may be selectively rendered lean or
stoichiometric/rich either by adjusting the air-to-fuel ratio fed
to the engine generating the exhaust or by periodically injecting a
reductant into the gas stream upstream of the catalytic trap. For
example, the composition of the present invention is well suited to
treat the exhaust of engines, including diesel engines, which
continuously run lean. In such case, in order to establish a
stoichiometric/rich operating period, a suitable reductant, such as
fuel, may be periodically sprayed into the exhaust immediately
upstream of the catalytic trap of the present invention to provide
at least local (at the catalytic trap) stoichiometric/rich
conditions at selected intervals. Partial lean-burn engines, such
as partial lean-burn gasoline engines, are designed with controls
which cause them to operate lean with brief, intermittent rich or
stoichiometric conditions.
[0033] Without intending to limit the invention in any manner,
embodiments of the present invention will be more fully described
by the following examples.
EXAMPLE 1
Preparation of NO.sub.x Storage Material
[0034] BaCO.sub.3 and CeO.sub.2 were intimately mixed and finely
dispersed in a weight ratio of between about 1:3 and about 1:5.
Cerium oxide having a BET surface area of between about 50-150
m.sup.2/g was mixed with a solution of barium acetate such that the
BaCO.sub.3/CeO.sub.2 composite had a BaCO.sub.3 content of about
10-30 wt %. After mixing, the suspension of soluble barium acetate
and CeO.sub.2 was then spray-dried at a temperature of between
about 90.degree. C. and 120.degree. C. to obtain a solid mixture of
barium acetate and ceria.
[0035] After spray-drying, the mixture was then heated at about
550.degree. C. to 800.degree. C. for about 2 hours to form
particles of ceria having barium carbonate supported on the ceria
particles. The resulting BaCO.sub.3 had a crystallite size of
between about 20 and 40 nm. The BaCO.sub.3 and CeO.sub.2
crystallites formed particles with a size of between about 5 and 50
microns. The BET surface area of the particulate mixture is between
about 30 and 80 m.sup.2/g.
Preparation of Catalytic Component
[0036] To provide a fully formulated NO.sub.x storage catalyst or
catalytic trap as described above, in addition to the manufacture
of barium carbonate supported on ceria, a precious metal can be
supported on a refractory oxide according to the following
description. Pt and Rh are impregnated onto Al.sub.2O.sub.3 by an
incipient wetness procedure to yield 1.8 weight percent Pt and 0.1
weight percent Rh. Pd is impregnated separately onto alumina to a
Pd loading of 1.4 weight percent.
[0037] A slurry mixture containing about 34 wt % of alumina
previously mixed with Pt/Rh, about 9 wt % Pd on alumina, a solution
of zirconium acetate with a content of about 3 wt % ZrO.sub.2,
magnesium acetate to yield 9 wt % MgO, and 45 wt %
BaCO.sub.3/CeO.sub.2 spray-dried powder is milled at pH 6-8 until a
particle size of 11 micron (d.sub.90) is obtained.
Coating of a Substrate
[0038] Ceramic or metallic honeycomb substrates are coated with the
slurry in a dip coating manner and then dried in a dryer and
subsequently calcined in a furnace under air at about 450.degree.
C.-550.degree. C. The coating procedure is then repeated until a
loading of about 4-6.5 g/in.sup.3 is achieved. The coating on the
honeycomb catalyst comprises about 3-30 micron BaCO.sub.3/CeO.sub.2
particles and about 1-20 micron alumina particles. BaCO.sub.3 is
fixed within the pores of the ceria particles in such a way that it
does not migrate to the alumina particles. It is believed that the
contact of BaCO.sub.3 and alumina would lead to the formation of
inactive Ba/Al.sub.2O.sub.3 composite compound formation upon
aging, which has a reduced NO.sub.x storage capacity compared to
BaCO.sub.3.
COMPARATIVE EXAMPLE 2
[0039] Samples were prepared in accordance with Example 1 above,
except that the barium acetate/ceria solution was not spray
dried.
EXAMPLE 3
NO.sub.x Storage Capacity Testing
[0040] Two catalytic traps were prepared, a first catalytic trap
was prepared in accordance with Example 1 and a comparative
catalytic trap was prepared in accordance with Comparative Example
2. Both catalytic traps A were evaluated after aging for 8 hours at
850.degree. C.
[0041] Both catalytic traps were evaluated as follows. An engine
was set to an air/fuel ratio of 11.6 for 2 minutes at the desired
temperature to remove all stored NO.sub.x and oxygen from the
catalyst. This mode represents rich engine operation. Subsequently,
the engine was adjusted to an air/fuel ratio of 29.6 under constant
NO.sub.x mass flow. This mode represents lean engine operation.
During the whole test, the NO.sub.x concentration was measured
before and after the NO.sub.x trap using a NO.sub.x analyzer.
[0042] After the 2 minute rich operation followed by a 60 second
lean operation, the engine was set to a 3 second rich operation to
remove stored NO.sub.x without having hydrocarbon and carbon
monoxide tailpipe emissions. This 60 sec lean/3 sec rich cycle is
repeated 10 times to establish constant catalyst conditions. For
the time period of the 10 lean/rich cycles the NO.sub.x efficiency
(U) is calculated from the NO.sub.x inlet and NO.sub.x outlet
concentrations via equation (1): NOx storage mass in g is
calculated via equation (2): [0043] NOx=NO.sub.x concentration
(ppm) [0044] V=volume flow (m.sup.3/h) [0045] V.sub.ideal=ideal
molar volume (l/mol) at STP [0046] M.sub.s=Molar weight of NO.sub.2
(g/mol) [0047] dt=time interval (s)
[0048] After the 10 lean/rich cycles, the engine is operated for 1
min rich to remove the stored NO.sub.x completely. Subsequently,
the engine is operated under lean condition until no more NO.sub.x
is stored in the trap. Under these conditions, the overall NO.sub.x
storage capacity is evaluated. However, to achieve a NO.sub.x
conversion of greater than 80%, the NO.sub.x storage capacity at
high NO.sub.x efficiency is decisive. FIG. 1 demonstrates that the
NO.sub.x storage capacity of catalytic trap prepared in accordance
with Example 1 utilizing a spray-drying process exhibited superior
capacity compared to the Comparative reference Example.
EXAMPLE 4
Barium Concentration and Calcination Temperature
[0049] Different amounts of Ba were impregnated into ceria of
different surface area, using the procedures described in Example
1. Ceria powders with different BET surface areas were used to
determine the effect of the resulting Ba/Ceria composite
powder.
[0050] Characterization of the impregnated powder included BET
surface area measurement. In addition fully formulated NOx trap
catalysts were prepared using the procedures described in Example 1
that contain the particular Ba/Ceria composite material as NO.sub.x
storage component. The NOx storage properties of the catalysts have
been evaluated after aging for 8 hours at 850.degree. C. under air
with 10% H.sub.2O in a laboratory reactor. The results are shown in
Table I and Table II below.
[0051] Table I shows the result of a variation of the BaCO.sub.3
and CeO.sub.2 concentration together with a variation of the ceria
used. After impregnation, all samples were calcined at 550.degree.
C. in air to decompose the impregnated Ba precursor into
BaCO.sub.3.
TABLE-US-00001 TABLE I CeO2 Crystallite size of BET BET BaCO3 BaCO3
Ceria Surface Ba/Ceria Crystallite Crystallite in Nox Nox area
calcined size size Ba/Ceria Storage Storage of CeO.sub.2 4 h As
Aged, aged at at BaCO3 CeO2 Ceria Crystallite 800.degree. C.
Prepared 4 h 800.degree. C. 4 h 800.degree. C. 300.degree. C.
400.degree. C. Sample Wt % wt % (m2/g) (nm) (m2/g) (nm) (nm) (nm)
(g/l) (g/l) A 29 71 90 12 13 20 34 28 2.8 2.4 B 29 71 40 18 9 22 30
34 1.4 2.0 C 25 75 66 16 14 21 32 28 2.6 2.7 D 20 80 90 12 17 22 40
27 3.5 1.9 E 20 80 40 18 13 20 26 31 2.3 2.4
[0052] After 800.degree. C. aging, the highest NOx storage activity
at 400.degree. C. is obtained with sample C, having a medium Ba
concentration and a CeO.sub.2 material with a medium BET surface
area and crystallinity. A high BET surface area and relative low Ba
concentration is especially beneficial for NOx storage at
300.degree. C. It is particularly interesting that sample D having
the largest BaCO.sub.3 crystallite size after aging yields the best
NOx storage at low temperature. In addition, increased Ba
concentration resulted in decreased BET surface area and increase
in CeO.sub.2 crystal size.
TABLE-US-00002 TABLE II CeO.sub.2 crystallite size of CeO.sub.2 BET
BaCO.sub.3 Ceria BaCO.sub.3/ crystallite BaCO3 Ba/ crystallite in
Ba/ BET Ceria calcination BET Ba/ size crystallite Ceria size Ceria
Nox Nox surface area Temp .degree. C. Ceria of size aged Aged, aged
storage storage of (2 h after after Ceria in Ba/ after 4 h 4 h 4 h
at at BaCO.sub.3 CeO.sub.2 Ceria spray calcination Ceria
calcination 800.degree. C. 800.degree. C. 800.degree. C.
300.degree. C. 400.degree. C. Sample Wt % wt % (m2/g) drying)
(m.sup.2/g) (nm) (nm) (m2/g) (nm) (nm) (g/l) (g/l) F 29 71 200 550
66 9 18 17 37 29 2.0 1.7 G 29 71 200 650 54 10 28 16 40 26 3.5 1.8
H 29 71 200 750 21 24 40 16 45 28 2.5 2.7 I 29 71 200 850 14 33 37
12 40 32 1.1 1.3
[0053] In order to determine an optimum BaCO.sub.3/CeO.sub.2
composite, the Ba/CeO.sub.2 is calcined after Ba impregnation at
different temperatures. This is done to decompose the Ba precursor
to the carbonate and to conditioning the composite for optimum NOx
adsorption capacity. The data in Table II demonstrates that a
calcination temperature between 550 and 750.degree. C. after
impregnation of Ba onto CeO.sub.2 provided the best results for
NO.sub.x storage. The samples calcined within this temperature
range had higher surface area and exhibited higher NOx storage
after aging than a sample calcined at 850.degree. C. Furthermore, a
BaCO.sub.3 crystallite size of between about 20-50 nm, for example,
45 nm, and a CeO.sub.2 crystallite size of between about 25-30 nm
in combination with a sufficient BET surface area after aging
yielded the highest NOx storage at 400.degree. C. According to the
data in Tables I and II, an as-prepared BET surface area between
40-60 m.sup.2/g and a ceria crystal size between about 10-and 20 nm
and a BaCO.sub.3 crystallite size of between about 20-and 40 nm
yielded the best performance after aging.
[0054] An example of a desirable morphology of spray dried and
calcined BaCO.sub.3/CeO.sub.2 mixture is shown in the SEM image of
FIG. 6. FIG. 6 shows about 10-20 nm size CeO.sub.2 crystals
agglomerated to particles of about 5-50 microns in size. Adhering
to these about 5-50 micron size CeO.sub.2 particles are BaCO.sub.3
particles of about 20-150 nm size. The BaCO.sub.3 particles are
likely agglomerates of smaller crystallites.
EXAMPLE 5
Ceria Type and Doping
[0055] Various types of ceria and doping with different materials
were evaluated for effect on BET surface area and decomposition
temperature of the barium carbonate. The decomposition temperature
is the temperature at which Ba reacts with ceria to form
BaCeO.sub.3. The samples below were prepared by an incipient
wetness preparation instead of spray-drying prior to calcination.
The results are shown in Table III:
TABLE-US-00003 TABLE III BET surface BET after surface aging phases
observed Decomposition as at by XRD after Temperature prepared
950.degree. C. thermal treatment Material of BaCO3 (.degree. C.)
(m2/g) (m2/g) (950.degree. C.) A (90% CeO2, 10% 914 13 1.8 BaCeO3,
CeO2, La) + 15% Ba BaCO3 C (57% CeO2, 43% 950 44 6 BaCeO3, CeO2 Pr)
+ 15% Ba D (72% CeO2, 28% 770 31 6 BaCeO3, CeO2 La) + 15% Ba B (90%
CeO2, 10% 945 30 6.6 BaCeO3, CeO2, La) + 15% Ba BaCO3 (Example 5B)
E (95% CeO2, 5% 945 25 10 BaCeO3, CeO2, La) + 15% Ba BaCO3 F (90%
CeO2, 10% 945 30 10 BaCeO3, CeO2, La) + 15% Ba BaCO3 (Example 5F) G
(100% CeO2) + 15% 942 41 13 BaCeO3, CeO2, Ba BaCO3 H (91% CeO2, 9%
950 86 16 BaCeO3, CeO2 Pr) + 15% Ba
[0056] According to the data in table III, doping Ceria with La or
Pr to a level of 10% does not influence the decomposition
temperature of BaCO.sub.3. Only sample D with 28% La has a much
lower temperature of BaCO.sub.3 decomposition while sample C even
with 43% of Pr has a high resistance towards reaction with
BaCO.sub.3.
[0057] The preferred BaCO.sub.3/CeO.sub.2/dopand material should
have a BET surface area >10 m.sup.2/g after aging and a high
resistance towards reaction to BaCO.sub.3 as shown in table
III.
EXAMPLE 6
Precious Metal Support
[0058] Various alumina supports were evaluated for stability. It
was found that the support material for the precious metal tends to
react with BaCO.sub.3 at a certain temperature. If this temperature
for a specific material is reached most or all of the BaCO.sub.3
has formed a compound with the support material and this compound
has much diminished tendency to adsorb NO.sub.x compared to
BaCO.sub.3.
[0059] The table below shows a list of different support materials
derived from ZrO.sub.2 or Al.sub.2O.sub.3. Materials A and B show
higher BaCO.sub.3 decomposition temperature than pure or La, Ba or
ZrO.sub.2 doped aluminas. However the surface area of those
materials is relatively small compared to other doped or undoped
materials. Furthermore, it was found that the higher the surface
area of a material in presence of Ba the higher is the NO.sub.x
storage capacity of an aged catalyst containing this material.
[0060] In particular, it was found that ZrO.sub.2 doped aluminas
and also La and ZrO.sub.2 doped materials have very thermally
stable surface areas in presence of Ba. The preferred alumina
should have a BET surface area of 150-250 m.sup.2/g, a pore volume
of 0.3-0.8 ml/g an average pore size of 3-20 nm.
TABLE-US-00004 TABLE IV BET phases surface BET observed by
Decomposition as after XRD Al2O3/ZrO2 derived temperature prepared
4 h after thermal support + 15% BaO of BaCO3 (.degree. C.) (m2/g)
900.degree. C. treatment A (92% ZrO2, 8% 820 36 BaZrO3, BaCO3,
La2O3) ZrO2 B (31% MgO, 69% 830 64 39 MgAl2O4, Al2O3) BaAl2O4 C
(20% Ba, 80% 740 101.3 61 BaCO3, BaAl2O4 Al2O3) D (4% La, 20% ZrO2,
736 96 CeO2, Al2O3 76% Al2O3) BaAl2O4, E (100% Al2O3) 765 73.6 67.9
Al2O3, ZrO2 F (90% Al2O3 + 10% 730 81 73 CeO2, BaAl2O4 CeO2) G (30%
ZrO2, 70% 740 88 BaAl2O4, ZrO2 Al2O3) H (20% Ba, 80% 695 156 83
BaAl2O4 Al2O3) I (82% Al2O3, 11% 720 118 80 Al2O3, BaCO3 CeO2, 7%
ZrO2) J (100% Al2O3) 720 116 106 BaAl2O4 K (72% Al2O3, 28% 750 130
100 MgAl2O4, Mg) BaAl2O4, BaCO3 L (90% Al2O3, 10% 700 133.5
BaAl2O4, Al2O3 ZrO2) M (80% Al2O3, 20% 720 133 100 CeO2, Al2O3
CeO2) N (80% Al2O3, 20% 720 121.7 100.5 Al2O3, ZrO2 ZrO2) O (4%
La/15% 700 126 BaAl2O4, ZrO2) ZrO2, Al2O3 P (21% Mg, 10% Zr, 730
142 BaCO3, 69% Al2O3) MgAl2O4, ZrO2 Q (97% Al2O3, 3% 720 152 121
Al2O3 La) R (75% Al2O3 25% 700 135 ZrO2, Al2O3, ZrO2) BaCO3 S (90%
Al2O3 10% 700 154 124.1 Al2O3, BaCO3, ZrO2) BaAl2O4 T (85% Al2O3
15% 700 142 ZrO2, Al2O3, ZrO2) BaCO3 U (74.6% Al2O3, 0.4% 748 156
132 BaAl2O4, La/15% ZrO2) Al2O3, ZrO2, BaCO3
EXAMPLE 7
Optimization of Aged NO.sub.x Storage Capacity
[0061] Various samples were tested for aged NO.sub.x storage
capacity for samples aged at 850.degree. C. in an oven with 10%
steam in air. A sample prepared in accordance with comparative
Example 1 having the NO.sub.x sorbent samples with
BaCO.sub.3/CeO.sub.2 concentration of sample C in Table I but spray
dried and calcined to different surface areas as indicated (Example
7A=41 m.sup.2/g and Example 7B=52 m.sup.2/g). In addition, samples
and B and F from table III were tested after spray drying with
barium, preparing in accordance with Example 1 and aging in a
laboratory reactor. The results shown in FIG. 2 demonstrate the
spray dried and calcined material 7B, with a BET surface area of 52
m.sup.2/g exhibited the highest NOx storage capacity, while the
other samples had similar performance. According to these tests,
there is no benefit associated with doping ceria with 10% of
La.
[0062] Various samples were tested for aged NO.sub.x storage
capacity for samples aged at 850.degree. C. for 50 hours in an
engine at stoichiometric air fuel ratio. In this case, the
evaluation has been done at an engine. Samples prepared in
accordance with comparative Example 2 (NO.sub.x sorbent prepared by
impregnation), by spray drying and by spray drying with optimized
BET surface area were prepared. The results shown in FIG. 3
demonstrate the spray-dried sample exhibited superior results,
particularly the sample with optimized BET surface area.
EXAMPLE 8
[0063] The effect of the refractory oxide to support the precious
metal component of the catalyst was also tested. Samples N (Example
8A) and O (Example 8B) from Table IV above were prepared into fully
formulated catalysts and tested for NOx storage capacity after
aging for 50 hours at 850.degree. C. under stoichiometric
conditions. FIG. 4 shows that the lanthana and zirconia doped
sample (Example O) exhibited the best results between 250 and
400.degree. C. Similar results were observed for sample N aged 50
hours at 750.degree. C. (Example 8C) and sample O aged 50 hours at
750.degree. C. (Example 8D) at a lean air fuel ratio, and these
results are shown in FIG. 5.
[0064] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit or scope of the invention. Thus,
it is intended that the present invention cover modifications and
variations of this invention provided they come within the scope of
the appended claims and their equivalents.
* * * * *