U.S. patent application number 15/014430 was filed with the patent office on 2016-07-21 for catalytic article with segregated washcoat and methods of making same.
This patent application is currently assigned to BASF Corporation. The applicant listed for this patent is BASF Corporation. Invention is credited to Keshavaraja Alive, Michael P. Galligan, Xinsheng Liu, Ye Liu, Pascaline Harrison Tran.
Application Number | 20160207029 15/014430 |
Document ID | / |
Family ID | 51529762 |
Filed Date | 2016-07-21 |
United States Patent
Application |
20160207029 |
Kind Code |
A1 |
Alive; Keshavaraja ; et
al. |
July 21, 2016 |
CATALYTIC ARTICLE WITH SEGREGATED WASHCOAT AND METHODS OF MAKING
SAME
Abstract
Provided herein are catalytic articles and methods of making
same using a single coat process. The catalytic article comprises
an elongated substrate monolith having a plurality of
longitudinally extending passages, each passage having at least a
first surface and a second surface opposite the first surface, the
first and second surfaces coated with at least a first coating and
a second coating, wherein the first coating comprises a first
catalyst composition and overlies the second coating on the first
surface, the second coating comprises a second catalyst composition
and overlies the first coating on the second surface, and wherein
the first catalyst composition and second catalyst composition have
a difference in surface charge. The washcoat may be applied as one
slurry, which then self-segregates into two coatings.
Inventors: |
Alive; Keshavaraja;
(Macungie, PA) ; Liu; Xinsheng; (Edison, NJ)
; Liu; Ye; (Holmdel, NJ) ; Galligan; Michael
P.; (Cranford, NJ) ; Tran; Pascaline Harrison;
(Holmdel, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BASF Corporation |
Florham Park |
NJ |
US |
|
|
Assignee: |
BASF Corporation
|
Family ID: |
51529762 |
Appl. No.: |
15/014430 |
Filed: |
February 3, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14208018 |
Mar 13, 2014 |
9283547 |
|
|
15014430 |
|
|
|
|
61783031 |
Mar 14, 2013 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 2255/1023 20130101;
Y02T 10/22 20130101; B01J 37/348 20130101; Y10T 428/24149 20150115;
B01J 35/0033 20130101; B01J 23/464 20130101; B01J 37/0215 20130101;
Y02T 10/12 20130101; B01J 35/0006 20130101; B01J 37/0248 20130101;
B01D 2255/1021 20130101; B01J 37/0203 20130101; B01D 2255/20715
20130101; B01J 23/63 20130101; F01N 2510/06 20130101; B01J 2523/00
20130101; B01J 23/40 20130101; B01J 35/04 20130101; B01D 53/945
20130101; B01D 2255/1025 20130101; B01D 2255/9022 20130101; F01N
3/2828 20130101; B01D 2258/014 20130101; B01J 23/44 20130101; B01J
37/0244 20130101; B01D 2255/2065 20130101; B01J 2523/00 20130101;
B01J 2523/3712 20130101; B01J 2523/824 20130101; B01J 2523/00
20130101; B01J 2523/31 20130101; B01J 2523/3712 20130101; B01J
2523/3718 20130101; B01J 2523/48 20130101; B01J 2523/822 20130101;
B01J 2523/824 20130101; B01J 2523/00 20130101; B01J 2523/3718
20130101; B01J 2523/48 20130101; B01J 2523/822 20130101; B01J
2523/00 20130101; B01J 2523/3712 20130101; B01J 2523/822 20130101;
B01J 2523/00 20130101; B01J 2523/31 20130101; B01J 2523/3712
20130101; B01J 2523/822 20130101; B01J 2523/824 20130101; B01J
2523/00 20130101; B01J 2523/31 20130101; B01J 2523/824
20130101 |
International
Class: |
B01J 23/63 20060101
B01J023/63; B01J 35/00 20060101 B01J035/00; B01J 37/02 20060101
B01J037/02 |
Claims
1-10. (canceled)
11. A method of making a catalytic article, the method comprising
providing a first support having a first surface charge and a first
platinum group metal thermally or chemically fixed to the first
support; providing a second support having a second surface charge
less than the first surface charge and a second platinum group
metal thermally or chemically fixed to the second support;
suspending the first and second supports in a single slurry
comprising a liquid medium and the first and second supports; and
applying the single slurry to a substrate.
12. The method of claim 11, wherein the single slurry provides two
compositionally distinct coatings, a first coating and a second
coating.
13. The method of claim 11, wherein the substrate comprises an
elongated substrate monolith having a plurality of longitudinally
extending passages, each passage having at least a first surface
and a second surface opposite the first surface, and wherein the
first coating overlies the second coating on the first surface, and
the second coating overlies the first coating on the second
surface.
14. The method of claim 11, wherein the difference in surface
charge is equal to or greater than about 10 Coulombs/m.sup.2.
15. The method of claim 11, wherein the first support comprises
high density ceria and the second support comprises low density
zirconia doped with Pr.
16. The method of claim 11, wherein thermally fixing the platinum
group metal to the first support and second support comprises
impregnating the platinum group metal onto the first and second
support to provide an impregnated support and calcining impregnated
support.
17. The method of claim 11, wherein the pH of the slurry is
maintained in the range of about 3 to about 9.
18. The method of claim 11, wherein the platinum group metal
thermally or chemically fixed to the first support comprises Rh in
an amount of about 0.5-10% by weight of the total catalyst.
19. The method of claim 11, wherein the platinum group metal
thermally fixed to the second support comprises Pd or Pt in an
amount of about 0.05-5% by weight of the total catalyst.
20. The method of claim 11, wherein the first support comprises
alumina or zirconia, and the second support comprises ceria.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 14/208,018, filed Mar. 13, 2014, which claims priority under 35
U.S.C. .sctn.119(e) to U.S. Provisional Application No. 61/783,031,
filed Mar. 14, 2013, the entire contents of each of which are
herein incorporated by reference.
TECHNICAL FIELD
[0002] This invention pertains generally to catalysts for the
treatment of exhaust gases. More specifically, this invention is
directed to catalytic articles having a segregated washcoat.
BACKGROUND
[0003] The exhaust gases of internal combustion engines contain
pollutants such as hydrocarbons, carbon monoxide and nitrogen
oxides (NO.sub.x) that foul the air. Emission standards for
unburned hydrocarbons, carbon monoxide and nitrogen oxide
contaminants have been set by various governments and must be met
by older as well as new vehicles. In order to meet such standards,
catalytic converters containing a three way catalyst (TWC) may be
located in the exhaust gas line of internal combustion engines. The
use of exhaust gas catalysts have contributed to a significant
improvement in air quality. The TWC is the most commonly used
catalyst and it provides the three functions of oxidation of CO,
oxidation of unburned hydrocarbons (HC's) and reduction of NOx to
N2. TWCs typically utilize one or more platinum group metals (PGM)
to simultaneously oxidize CO and HC and reduce NOx compounds. The
most common catalytic components of a TWC are platinum (Pt),
rhodium (Rh) and palladium (Pd).
[0004] The platinum group metals (PGM) in the TWC catalysts (e.g.,
platinum, palladium, rhodium, ruthenium and iridium) are typically
dispersed on a high surface area, refractory metal oxide support,
e.g., a high surface area alumina coating, or on an oxygen storage
component (OSC), or their mixtures. The support is carried on a
suitable substrate such as a monolithic substrate comprising a
refractory ceramic or metal honeycomb structure, or refractory
particles such as spheres or short, extruded segments of a suitable
refractory material. The TWC catalyst substrate may also be a wire
mesh, typically a metal wire mesh, which is particularly useful in
small engines.
[0005] Many TWC catalysts are manufactured with at least two
separate catalyst coating compositions (washcoats) that are applied
in the form of aqueous dispersions as successive layers on a
substrate (for example, a honeycomb body composed of ceramic or
metal) in order to separate noble metals, such as, palladium and
rhodium which represent the main catalytically active species.
Separation has been necessary historically because some platinum
group metals, such as palladium and rhodium, can form an alloy
which is known to be less catalytically active.
[0006] There is a need to provide single washcoat compositions
containing precious metals (i.e., palladium and rhodium) while
maintaining and/or improving catalytic performance as compared to
compositions that provide these metals individually for separate
layers. There is also a need for applying the single washcoat
composition in one coating step. There is also continuing need to
provide a TWC catalyst composites that utilize precious metals
efficiently and remain effective to meet regulated HC, NOx, and CO
conversions.
SUMMARY
[0007] A first aspect of the invention pertains to a catalytic
article comprising an elongated substrate monolith having a
plurality of longitudinally extending passages, each passage having
at least a first surface and a second surface opposite the first
surface, the first and second surfaces coated with at least a first
coating and a second coating, wherein the first coating comprises a
first catalyst composition and overlies the second coating on the
first surface, the second coating comprises a second catalyst
composition and overlies the first coating on the second surface,
and wherein the first catalyst composition and second catalyst
composition have a difference in surface charge. In one or more
embodiments, the difference in surface charge is equal to or
greater than about 10 Coulombs/m.sup.2.
[0008] In one or more embodiments, the first composition comprises
a first platinum group metal. In some embodiments, the first
platinum group metal is thermally or chemically fixed to a first
support material. In one or more embodiments, the first platinum
group metal comprises palladium and/or platinum. In some
embodiments, the palladium is present in an amount of about
0.05-10% by weight of the total catalyst. In one or more
embodiments, the second composition comprises a second platinum
group metal thermally or chemically fixed to a second support
material. In some embodiments, the second platinum group metal is
thermally or chemically fixed to a second support material. In one
or more embodiments, the second platinum group metal comprises
rhodium. In some embodiments, the rhodium is present in an amount
of about 0.005-5% by weight of the total catalyst.
[0009] In one or more embodiments, the second support comprises
ceria. In some embodiments, the first platinum group metal
comprises palladium and the second platinum group metal comprises
rhodium, and the palladium and rhodium are present in a ratio of
about 1:10 to about 100:1. In one or more embodiments, the
palladium and rhodium are present in an amount of about 5 to about
300 grams per cubic feet. In some embodiments, the first support
comprises alumina, and the second support comprises ceria. In one
or more embodiments, the first support comprises zirconia, and the
second support comprises ceria. In some embodiments, the zirconia
is doped with ceria.
[0010] Another aspect of the invention relates to a method of
making a catalyst article. The method comprises providing a first
support having a first surface charge and a first platinum group
metal thermally or chemically fixed to the first support particle;
providing a second support having a second surface charge less than
the first surface charge and a second platinum group metal
thermally or chemically fixed to the second support particle;
suspending the first and second supports in a single slurry
comprising a liquid medium and the first and second supports;
applying the single washcoat to a substrate.
[0011] In one or more embodiments, the single washcoat provides two
compositionally distinct coatings, a first coating and a second
coating. In some embodiments, the substrate comprises an elongated
substrate monolith having a plurality of longitudinally extending
passages, each passage having at least a first surface and a second
surface opposite the first surface, and wherein the first coating
overlies the second coating on the first surface, and the second
coating overlies the first coating on the second surface. In one or
more embodiments, the difference in surface charge is equal to or
greater than about 10 Coulombs/m.sup.2. In some embodiments, the
first support comprises ceria and the second support comprises
zirconia. In one or more embodiments, the first support comprises
high density ceria and the second support comprises low density
zirconia doped with Pr.
[0012] In some embodiments, thermally fixing the platinum group
metal to the first support and second support comprises
impregnating the platinum group metal onto the first and second
support to provide an impregnated support and calcining impregnated
support. In one or more embodiments, the impregnated supports are
calcined for at least about 0.25 to about 4 hours at a temperature
ranging from about 100 to about 800.degree. C. In some embodiments,
wherein the pH of the slurry is maintained in the range of about 3
to about 9. In one or more embodiments, wherein the slurry pH is
controlled by adding an organic acid. In some embodiments, wherein
the organic acid comprises tartaric acid, formic acid or nitric
acid. In one or more embodiments, wherein the platinum group metal
thermally or chemically fixed to the first support particle
comprises Rh in an amount of about 0.5-10% by weight of the total
catalyst. In some embodiments, wherein the platinum group metal
thermally fixed to the second support particle comprises Pd or Pt
in an amount of about 0.05-5% by weight of the total catalyst.
[0013] In one or more embodiments, wherein the first support
comprises alumina, and the second support comprises ceria. In some
embodiments, wherein the first support comprises zirconia, and the
second support comprises ceria. In one or more embodiments, wherein
the zirconia is doped with ceria.
BRIEF DESCRIPTION OF DRAWINGS
[0014] FIGS. 1A and B are representations of two supports in a
slurry before and after coating a substrate in accordance with one
or more embodiments of the invention;
[0015] FIGS. 2A-D are transmission electron microscope images of a
catalytic article in accordance with one or more embodiments of the
invention; and
[0016] FIGS. 3A-D are transmission electron microscope images of a
catalytic article in accordance with one or more embodiments of the
invention.
DETAILED DESCRIPTION
[0017] Provided are catalytic articles and methods of producing the
same. The catalytic articles are advantageously made from a single
washcoat process, whereby the washcoat segregates into two or more
coatings based on the surface charge of the catalytic particles
contained therein. In some embodiments, the catalytic articles
produced by these processes feature an ordering of the coatings
which have the opposite ordering on the opposite of a given channel
in a substrate. That is, where one coat overlies a second, the
second will overlie the first on the opposing wall. In other
embodiments, the catalytic articles produced by these processes are
conventional; that is, the same layer is always closer to the
substrate.
Catalytic Article
[0018] Accordingly, one aspect of the invention relates to a
catalytic article comprising an elongated substrate monolith having
a plurality of longitudinally extending passages, each passage
having at least a first surface and a second surface opposite the
first surface, the first and second surfaces coated with at least a
first coating and a second coating, wherein the first coating
comprises a first catalyst composition and overlies the second
coating on the first surface, the second coating comprises a second
catalyst composition and overlies the first coating on the second
surface, and wherein the first catalyst composition and second
catalyst composition have a difference in surface charge.
[0019] Segregation of the two first and second coating is achieved
by creating or selecting materials with a difference in surface
charge of the first and second catalyst compositions. Accordingly,
in one or more embodiments, the difference in surface charge is
equal to or greater than about 10 Coulombs/m.sup.2, or greater than
about 12 Coulombs/m.sup.2. Surface charge can be measured using a
Zeta potential probe analyzer. Therefore, surface charge
modification can play a key role in achieving precious metal
segregation in a single layer catalyst. For example, zirconia is
less basic than ceria; thus, ceria would show relatively less
affinity for anions compared to zirconia. So zirconia covered with
nitrate ions tends to form a separate layer in a slurry and when
applied as a washcoat. FIGS. 1A and B illustrate this concept. FIG.
1A shows a container with a slurry containing zirconia-based
particles and ceria-based particles. FIG. 1B. shows the slurry
after it was been washcoated onto a substrate. Once the slurry is
coated onto the substrate, it will separate, resulting in discrete
zirconia and ceria layers. In some embodiments, the surface can
further be modified by modification of the surface using adsorption
of species (e.g., organic ions and/or organic surfactants).
[0020] In one or more embodiments, the catalyst compositions
comprise precious or platinum group metals. In some embodiments,
the first composition comprises a first platinum group metal. In
one or more embodiments, the second composition comprises a second
platinum group metal thermally or chemically fixed to a second
support material. In some embodiments, the first platinum group
metal is thermally or chemically fixed to a first support material.
In one or more embodiments, the first platinum group metal
comprises palladium or platinum. In one or more embodiments, the
palladium is present in an amount of about 0.2, 0.5, 1, 2, 3, 4, 5,
6, 7, 8, 9 or 10% by weight of the total catalyst. In some
embodiments, the palladium is present in an amount of about 1-3% by
weight of the total catalyst. In further embodiments, the palladium
is present in an amount of about 1.4% by weight of the total
catalyst. In one or more embodiments, the first support comprises
zirconia.
[0021] In one or more embodiments, the second platinum group metal
is thermally or chemically fixed to a second support material. In
some embodiments, the second platinum group metal comprises
rhodium. In one or more embodiments, the rhodium is present in an
amount of about 0.2, 0.5, 1, 2, 3, 4, or 5% by weight of the total
catalyst. In some embodiments, the rhodium is present in an amount
of about 1-5, or 1-3% by weight of the total catalyst. In one or
more embodiments, the rhodium is present in an amount of about
0.25% by weight of the total catalyst. In some embodiments, the
second support comprises ceria. In one or more embodiments, the
first platinum group metal comprises palladium and the second
platinum group metal comprises rhodium, and the palladium and
rhodium are present in a ratio of about 1:1 to about 50:1 or 5:1 to
about 20:1 or about 10 to about 1. In some embodiments, the
palladium and rhodium are present in an amount of about 5 to about
300, or about 10 to about 100 grams per cubic feet. If there is
more than one platinum group metal on a given support, the platinum
group metals may be in the form of alloy, particle assemblies, core
shells, etc.
[0022] The layers are formed from a single washcoat layer that
contains two or more precious metals, each of which is on its own
support, resulting in a homogeneous mixture of the metals in the
same layer on a substrate. One or more of the precious metals are
fixed to their individual support, which means that the precious
component is not soluble in the washcoat dispersion. Fixing of
precious metals can occur by chemical or thermal fixation. For
thermal fixing, to produce a "thermally-fixed" precious metal, it
is meant that the impregnated supports are treated with heat such
that the precious metals are converted to their oxide forms and
that upon use of the thermally-fixed precious metals on supports in
an aqueous slurry, the precious metals are not soluble and do not
alloy/agglomerate. For chemical fixation, the pH or some other
parameter of the dispersion of the precious metal salt with support
is changed to render the precious metal component insoluble in the
washcoat dispersion. The thickness of the washcoat will depend on
the washcoat loading. In some embodiments, the thickness is from
about 100 to about 200 microns. The porosity of a washcoat depends
on the particle itself and between particles. The latter depends on
the particle size of the support materials. Pore size for the
particle itself is within about 10 to about 200 nanometers, and
between particles is normally within about 100 nanometers to about
30 microns.
[0023] Reference to a "support" in a catalyst washcoat layer refers
to a material that receives precious metals, stabilizers,
promoters, binders, and the like through association, dispersion,
impregnation, or other suitable methods. Useful high-surface area
supports include one or more refractory oxides. These oxides
include, for example, silica, alumina, titania, zirconia, and mixed
oxide forms thereof such as silica-alumina, aluminosilicates (which
may be amorphous or crystalline), alumina-zirconia, alumina-ceria,
titanium-alumina, and zirconium-silicate. In one embodiment, the
support is comprised of alumina. Alumina includes the members of
the gamma, delta, theta or transitional aluminas, such as gamma and
beta aluminas, and, if present, a minor amount of other refractory
oxide, e.g., about up to 20 weight percent. High surface area
refractory metal oxide supports refer to support particles having
high external surface area, pores larger than 20 A, and a wide pore
distribution. High surface area refractory metal oxide supports,
e.g., "gamma alumina" or "activated alumina," used with oxidation
catalysts typically exhibit a BET surface area in excess of 60
square meters per gram ("m.sup.2/g"), often up to about 200
m.sup.2/g or higher. "BET surface area" refers to the Brunauer,
Emmett, Teller method for determining surface area by N2
adsorption. As used herein, the term "activated alumina" refers to
a high surface area phase of alumina, such as, but not limited to,
gamma-alumina. Such activated alumina is usually a mixture of the
gamma and delta phases of alumina, but may also contain substantial
amounts of eta, kappa, and theta alumina phases. Refractory metal
oxides other than activated alumina may be utilized as a carrier
for at least some of the catalytic components in a given catalyst.
For example, bulk ceria, zirconia, alpha-alumina and other
materials are known for such use. Although many of these materials
have a lower BET surface area than activated alumina, that
disadvantage tends to be offset by the greater durability of the
resulting catalyst or a beneficial interaction with precious metal
deposited on the support. Examples of supports include, but are not
limited to, high surface area refractory metal oxides and
composites containing oxygen storage components. Exemplary support
materials are high surface area aluminum oxide (>80, 90, 100,
125, or even 150 m.sup.2/g) (in various modifications), zirconium
oxide components that can be combined with stabilizers such as
lanthana (i.e., Zr--La composites), and oxygen storage components
(i.e. cerium-zirconium mixed oxides in various embodiments).
Exemplary high surface area refractory metal oxides can comprise an
activated alumina compound selected from the group consisting of
alumina, alumina-zirconia, alumina-ceria-zirconia,
lanthana-alumina, lanthana-zirconia-alumina, baria-alumina, baria
lanthana-alumina, baria lanthana-neodymia alumina, and
alumina-ceria. Other suitable supports may include silica, titania
zeolites, etc.
[0024] In one or more embodiments, the first support comprises
alumina and the second support comprises ceria. In other
embodiments, the first support comprises zirconia and the second
support comprises ceria. Supports may also be promoted with other
elements. For example, a support may comprise Pr-promoted zirconia,
or B a/La-promoted zlumina.
[0025] In one or more embodiments, one or more catalyst
compositions are disposed on a "substrate." The substrate may be
any of those materials typically used for preparing catalysts, and
will preferably comprise a ceramic or metal honeycomb structure.
Any suitable substrate may be employed, such as a monolithic
substrate of the type having fine, parallel gas flow passages
extending therethrough from an inlet or an outlet face of the
substrate, such that passages are open to fluid flow therethrough
(referred to as honeycomb flow through substrates). The passages,
which are essentially straight paths from their fluid inlet to
their fluid outlet, are defined by walls on which the catalytic
material is coated as a washcoat so that the gases flowing through
the passages contact the catalytic material. The flow passages of
the monolithic substrate are thin-walled channels, which can be of
any suitable cross-sectional shape and size such as trapezoidal,
rectangular, square, sinusoidal, hexagonal, oval, circular, etc.
Such structures may contain from about 60 to about 900 or more gas
inlet openings (i.e., cells) per square inch of cross section.
[0026] The substrate can also be a wall-flow filter substrate,
where the channels are alternately blocked, allowing a gaseous
stream entering the channels from one direction (inlet direction),
to flow through the channel walls and exit from the channels from
the other direction (outlet direction). A dual oxidation catalyst
composition can be coated on the wall-flow filter. If such a
substrate is utilized, the resulting system will be able to remove
particulate matters along with gaseous pollutants. The wall-flow
filter substrate can be made from materials commonly known in the
art, such as cordierite or silicon carbide.
[0027] The ceramic substrate may be made of any suitable refractory
material, e.g., cordierite, cordierite-alumina, silicon nitride,
zircon mullite, spodumene, alumina-silica magnesia, zircon
silicate, sillimanite, a magnesium silicate, zircon, petalite,
alumina, an aluminosilicate and the like.
[0028] In alternative embodiments, one or more catalyst
compositions may be deposited on an open cell foam substrate. Such
substrates are well known in the art, and are typically formed of
refractory ceramic or metallic materials.
[0029] In some embodiments, the catalytic article further comprises
a third coating, wherein the third coating comprises a third
catalyst composition. The third catalyst composition would have
also have a difference in surface charge from the first and second
to product a third coating. The ordering of the coatings is
opposite relative to the surface. That is, if the first coating
overlays the second coating, which overlies the third coating on
the first surface, the third coating will overlay the second
coating, which will overlay the first coating on the second
surface. In some embodiments, additional layers may be added.
Method of Forming Catalytic Article
[0030] There is a substantial challenge of combining two individual
platinum group metals in one coating composition due to the
solubility of precious metal salts in water. In conventional TWC
catalysts, for example, the platinum group metals palladium and
rhodium are individually applied by impregnation to the support
materials and are then subsequently incorporated into an aqueous
washcoat dispersion. Specifically, prior art methods included:
[0031] a. Application of a first noble metal by impregnation with a
metal salt solution without regard to dilution to a first support
(aluminum oxide or OSC) to form a first impregnated support;
[0032] b. Production of a first aqueous washcoat dispersion using
the first impregnated support;
[0033] c. Application of a second noble metal by impregnation with
a metal salt solution without regard to dilution to a second
support (aluminum oxide or OSC) to form a second impregnated
support;
[0034] d. Production of a second aqueous washcoat dispersion using
the second impregnated support;
[0035] e. Application of a first layer onto substrate using the
first aqueous washcoat dispersion and calcination of the first
layer;
[0036] f. Application of a second layer onto substrate using the
second aqueous washcoat dispersion and calcinations of the second
layer.
[0037] As discussed above, generally, if both noble metals are
processed in a single aqueous washcoat dispersion utilizing
conventional methods, the probability of the two noble metals
forming an alloy within the washcoat layer as a result of the use
of water-soluble metal salts would be greatly increased. This would
lead to the performance of the TWC catalyst being poorer in this
case than in the case of separate palladium and rhodium layers.
[0038] Accordingly, one aspect of the invention relates to a method
of making a catalyst article. The method allows for a single
washcoat to be applied to a substrate, but the platinum group
metals are kept separate, thereby preventing alloying. Accordingly,
the method comprises providing a first support having a first
surface charge and a first platinum group metal thermally or
chemically fixed to the first support particle; providing a second
support having a second surface charge less than the first surface
charge and a second platinum group metal thermally or chemically
fixed to the second support particle; suspending the first and
second supports in a single slurry comprising a liquid medium and
the first and second supports; and applying the single washcoat to
a substrate. In one or more embodiments, the single washcoat
provides two compositionally distinct coatings, a first coating and
a second coating. In one or more embodiments, other chemical
aspects of the supports and/or slurry may aid in layer segregation.
For example, the supports may have different densities, which help
to form multi layers within the slurry.
[0039] In some embodiments, the substrate comprises an elongated
substrate monolith having a plurality of longitudinally extending
passages, each passage having at least a first surface and a second
surface opposite the first surface, and wherein the first coating
overlies the second coating on the first surface, and the second
coating overlies the first coating on the second surface. In one or
more embodiments, the difference in surface charge is equal to or
greater than about 10 Coulombs/m.sup.2.
[0040] A first support having a first surface charge and a first
platinum group metal thermally fixed to the first support particle
must first be provided. Any of the variants described above with
respect to the finished catalytic article may be applied here. For
example, rhodium may be supported onto a ceria and thermally fixed.
Palladium may be supported on a Pr-doped zirconia and thermally
fixed.
[0041] As discussed above, in some embodiments, the precious metal
may be thermally fixed to support. In one or more embodiments,
thermally fixing a precious metal onto a support may include
impregnating the precious metal onto a powder and then calcining
the impregnated support. Calcination affixes the metal onto the
support. Methods known in the art may be employed. A general
temperature range for calcination is about 100 to about 800.degree.
C., or more specifically about 300 to about 600.degree. C. However,
the specific temperature may depend on the specific salts used. For
example, where sulfates are used, higher temperatures (e.g., about
750.degree. C.) may be utilized. Amine acetates will generally
necessitate lower temperatures, less than about 400.degree. C.
Duration of the calcination can vary anywhere from about 0.25 to
about 4 hours, or about 0.5 hours to about 4 hours. Any combination
of time and temperature may be utilized, as fits for the specific
conditions. Exemplary calcination conditions are at 550.degree. C.
for two hours.
[0042] In some embodiments, the precious metal may be chemically
fixed onto the support using methods known in the art. For example,
palladium may be fixed with BaOH and Rh may be fixed with
mono-ethanolamine (MEA).
[0043] Once the platinum group metals have been affixed to the
support, a washcoat can be prepared with the supports. The first
and second supports may be suspended in a single slurry comprising
a liquid medium and the first and second supports.
[0044] Generally, any method that modifies the surface charge may
be used to obtain the necessary charge differential. For example,
modifications of the surface may be carried out using adsorption of
species such as inorganic ions or organic surfactants, as well as
thermal treatment. In some embodiments, the pH of the slurry can be
controlled by using one or more acids. In some embodiments, the
acid comprises an organic acid. In further embodiments, the acid
comprises tartaric acid, formic acid or nitric acid. While not
wishing to be bound to any particular theory, it is thought that
the slurry can be visualized precious metal oxide particles
dispersed in aqueous medium with a minimum amount of ions (e.g.,
tartarates and H+). In some embodiments, the pH of the slurry is
maintained at about 3 to about 9, or 5.5 to about 6 The resulting
slurry will separate into two visible layers when left
undisturbed.
[0045] In embodiments where the precious metal is chemically fixed
to the support, the slurry can be visualized as precious metal
hydroxide particles dispersed well in aqueous medium with lots of
negatively and positively charged ions (e.g., nitrates, acetates,
tartarates, H+, Ba++, etc).
[0046] Once the slurry has been prepared, it may be applied to a
substrate using methods known in the art. As described above, only
one washcoat need be applied to achieve more than one layer. This
is advantageous, as less process steps are required for production
of the catalytic article, increasing throughput and saving
cost.
[0047] TWC catalysts that exhibit good activity and long life
comprise one or more platinum group metals (e.g., platinum,
palladium, rhodium, rhenium and iridium) disposed on a high surface
area, refractory metal oxide support, e.g., a high surface area
alumina coating. The support is carried on a suitable substrate
such as a monolithic substrate comprising a refractory ceramic or
metal honeycomb structure, or refractory particles such as spheres
or short, extruded segments of a suitable refractory material. The
refractory metal oxide supports may be stabilized against thermal
degradation by materials such as zirconia, titania, alkaline earth
metal oxides such as baria, calcia or strontia or, most usually,
rare earth metal oxides, for example, ceria, lanthana and mixtures
of two or more rare earth metal oxides. For example, see U.S. Pat.
No. 4,171,288 (Keith). TWC catalysts can be formulated to include
an oxygen storage component (OSC) including, for example, ceria and
praseodymia.
[0048] High surface refractory metal oxide supports refer to
support particles having pores larger than 20 A and a wide pore
distribution. High surface area refractory metal oxide supports,
e.g., alumina support materials, also referred to as "gamma
alumina" or "activated alumina," typically exhibit a BET surface
area in excess of 60 square meters per gram ("m.sup.2/g"), often up
to about 200 m.sup.2/g or higher. Such activated alumina is usually
a mixture of the gamma and delta phases of alumina, but may also
contain substantial amounts of eta, kappa and theta alumina phases.
Refractory metal oxides other than activated alumina can be used as
a support for at least some of the catalytic components in a given
catalyst. For example, bulk ceria, zirconia, alpha alumina and
other materials are known for such use. Although many of these
materials suffer from the disadvantage of having a considerably
lower BET surface area than activated alumina, that disadvantage
tends to be offset by a greater durability of the resulting
catalyst. "BET surface area" has its usual meaning of referring to
the Brunauer, Emmett, Teller method for determining surface area by
N2 adsorption.
[0049] The catalytic layer may also contain stabilizers and
promoters, as desired. Suitable stabilizers include one or more
non-reducible metal oxides wherein the metal is selected from the
group consisting of barium, calcium, magnesium, strontium and
mixtures thereof. Preferably, the stabilizer comprises one or more
oxides of barium and/or strontium. Suitable promoters include one
or more non-reducible oxides of one or more rare earth metals
selected from the group consisting of lanthanum, praseodymium,
yttrium, zirconium and mixtures thereof.
[0050] It should be noted that in some embodiments, the above
processes will result in a conventional coating pattern. That is, a
layer will always be the layer closer to the substrate, and the
other layer will always be the layer above the first layer.
[0051] Another aspect of the invention pertains to having a
substrate with a charge, and having a higher affinity for a first
support particle compared to a second support particle. This would
allow for either of the first or second supports to always be drawn
closer to the substrate, resulting in the slurry segregating such
that one washcoat layer will always overlay the other. Any of the
PGM, supports, etc. from the above process may be applied in this
process.
Method of Treating Exhaust
[0052] The catalytic articles described herein may be used to treat
internal combustion exhaust. In one or more embodiments, the method
comprises contacting the exhaust gas stream of an internal
combustion engine (for example a stoichiometric gasoline engine)
with a catalytic article comprising an elongated substrate monolith
having a plurality of longitudinally extending passages, each
passage having at least a first surface and a second surface
opposite the first surface, the first and second surfaces coated
with at least a first coating and a second coating, wherein the
first coating comprises a first catalyst composition and overlies
the second coating on the first surface, the second coating
comprises a second catalyst composition and overlies the first
coating on the second surface, and wherein the first catalyst
composition and second catalyst composition have a difference in
surface charge. Any of the variations described above may be
applied to this method.
EXAMPLES
[0053] The following non-limiting examples shall serve to
illustrate the various embodiments of the present invention.
Example 1
[0054] A catalytic article was prepared. A single slurry was
prepared which featured Pd on ceria, Rh on Pr-doped zirconia and
Dispal.RTM. high purity dispersible alumina (binder) supports. The
charge differential between the two supports was 12
coulombs/m.sup.2 with Pd and Rh. The solid content of the slurry
was 30-40%. The slurry was used to coat on a cordierite substrate
via dip coating. FIGS. 2A-D show scanning electron microscope
images at different magnifications of a cross-section of the
substrate. The black areas represent the passage way, while the
very dark grey sections correspond to the walls of the monolith
channels. The light and medium grey areas show the washcoat.
Specifically, the light grey areas correspond to the ceria support
and the medium grey areas correspond to the Pr-doped zirconia
support. FIGS. 2B and 2D show the bottom two corners of a channel.
In FIG. 2B, the Pr-doped zirconia support (medium grey) is shown as
a layer directly over the substrate, with the ceria support (light
grey) overlying the Pr-doped zirconia support. However, in FIG. 2D,
the opposing wall shows the ceria support (light grey) as a layer
over the substrate, with the Pr-doped zirconia support (medium
grey) overlying the ceria support.
[0055] A catalytic article was prepared. A single slurry was
prepared which contained calcined 0.16% rhodium on a high density
ceria support, 1.4% palladium on alumina support and Dispal.RTM.
high purity dispersible alumina supports. The sample was generally
prepared as in Example 1. ICP analysis of the supernatant liquid of
the slurry after centrifugation showed no leached palladium or
rhodium. This shows that all platinum group metals were fixed on
the supports as designed.
[0056] The slurry was used to coat on a cordierite substrate and
was calcined at a temperature of 400-550.degree. C. for 0.5-3
hours. FIGS. 3A-D show transmission electron microscope images at
different magnifications of a cross-section of the substrate. The
black areas represent the passage way, while the medium dark grey
sections correspond to the walls of the monolith channels. The
light and dark grey areas show the washcoat. Specifically, the
light grey areas correspond to Rh/ceria and the medium grey areas
correspond to the Pd/alumina. FIGS. 3B and 3D show the right two
corners of a channel. In FIG. 3B, the Rh/ceria (dark grey) is shown
as a layer directly over the substrate, with the Rh/ceria (light
grey) overlying the Pr-doped zirconia support. However, in FIG. 3D,
the opposing wall shows the Rh/ceria (light grey) as a layer over
the substrate, with the Rh/ceria (dark grey) overlying the ceria
support.
[0057] Reference throughout this specification to "one embodiment,"
"certain embodiments," "one or more embodiments" or "an embodiment"
means that a particular feature, structure, material, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the invention. Thus, the
appearances of the phrases such as "in one or more embodiments,"
"in certain embodiments," "in one embodiment" or "in an embodiment"
in various places throughout this specification are not necessarily
referring to the same embodiment of the invention. Furthermore, the
particular features, structures, materials, or characteristics may
be combined in any suitable manner in one or more embodiments.
[0058] The invention has been described with specific reference to
the embodiments and modifications thereto described above. Further
modifications and alterations may occur to others upon reading and
understanding the specification. It is intended to include all such
modifications and alterations insofar as they come within the scope
of the invention.
* * * * *