U.S. patent application number 14/669446 was filed with the patent office on 2016-09-29 for catalyst with lanthamide-doped zirconia and methods of making.
This patent application is currently assigned to CLEAN DIESEL TECHNOLOGIES, INC.. The applicant listed for this patent is Stephen J. Golden, Randal Hatfield, Johnny T. Ngo, Jason D. Pless. Invention is credited to Stephen J. Golden, Randal Hatfield, Johnny T. Ngo, Jason D. Pless.
Application Number | 20160279611 14/669446 |
Document ID | / |
Family ID | 47668938 |
Filed Date | 2016-09-29 |
United States Patent
Application |
20160279611 |
Kind Code |
A1 |
Golden; Stephen J. ; et
al. |
September 29, 2016 |
Catalyst with lanthamide-doped zirconia and methods of making
Abstract
The invention generally relates to three-way catalysts and
catalyst formulations capable of simultaneously converting nitrogen
oxides, carbon monoxide, and hydrocarbons into less toxic
compounds. Such three-way catalyst formulations contain
ZrO.sub.2-based mixed-metal oxide support oxides doped with an
amount of lanthanide. Three-way catalyst formulations with the
support oxides of the present invention demonstrate higher
catalytic activity, efficiency and longevity than comparable
catalysts formulated with traditional support oxides.
Inventors: |
Golden; Stephen J.; (Santa
Barbara, CA) ; Hatfield; Randal; (Oxnard, CA)
; Pless; Jason D.; (Pottstown, PA) ; Ngo; Johnny
T.; (Oxnard, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Golden; Stephen J.
Hatfield; Randal
Pless; Jason D.
Ngo; Johnny T. |
Santa Barbara
Oxnard
Pottstown
Oxnard |
CA
CA
PA
CA |
US
US
US
US |
|
|
Assignee: |
CLEAN DIESEL TECHNOLOGIES,
INC.
Oxnard
CA
|
Family ID: |
47668938 |
Appl. No.: |
14/669446 |
Filed: |
March 26, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 2235/3224 20130101;
B01J 35/002 20130101; C04B 2235/3227 20130101; B01J 23/10 20130101;
B01J 37/038 20130101; C01P 2002/54 20130101; B01J 37/0036 20130101;
C01F 17/32 20200101; C04B 2235/3229 20130101; B01J 35/02 20130101;
C01G 25/02 20130101; B01J 2523/00 20130101; B01J 35/0006 20130101;
B01J 2523/00 20130101; C01G 25/00 20130101; C04B 2235/3205
20130101; B01J 37/0236 20130101; B01J 2523/3712 20130101; C04B
35/50 20130101; B01J 2523/3725 20130101; B01J 2523/3718 20130101;
B01J 2523/3712 20130101; B01J 2523/36 20130101; B01J 2523/48
20130101; B01J 2523/48 20130101; C04B 35/486 20130101; B01J
2523/3725 20130101; C01P 2002/72 20130101; B01J 23/464 20130101;
B01J 23/002 20130101; B01J 37/0201 20130101; B01J 2523/00 20130101;
B01J 35/023 20130101; C01P 2002/52 20130101; C04B 35/488 20130101;
B01J 37/0244 20130101; B01J 23/63 20130101 |
International
Class: |
B01J 23/46 20060101
B01J023/46; B01J 37/02 20060101 B01J037/02; B01J 23/10 20060101
B01J023/10; B01J 35/00 20060101 B01J035/00; B01J 35/02 20060101
B01J035/02 |
Claims
1. A support oxide comprising ZrO.sub.2 doped with an amount of
lanthanide.
2. The support oxide of claim 1, wherein said support oxide further
comprises Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3.
3. The support oxide of claim 1, wherein said lanthanide is La or
Pr.
4. The support oxide of claim 1, wherein the ZrO.sub.2 is doped
with between about 1-30% of said lanthanide.
5. The support oxide of claim 4, wherein the ZrO.sub.2 is doped
with between about 5-15% of said lanthanide.
6. The support oxide of claim 1, wherein said support oxide
comprises La.sub.X %Zr.sub.(1-X) %O.sub.2 and/or Pr.sub.X
%Zr.sub.(1-X) %O.sub.2.
7. The support oxide of claim 6, wherein said support oxide
comprises La.sub.5%Zr.sub.95%O.sub.2, Pr.sub.5%Zr.sub.95%O.sub.2,
La.sub.10%Zr.sub.90%O.sub.2, Pr.sub.10%Zr.sub.90%O.sub.2,
La.sub.15%Zr.sub.85%O.sub.2, Pr.sub.15%Zr.sub.85%O.sub.2, or
mixtures thereof.
8. The support oxide of claim 1, wherein said support oxide is in
the tetragonal phase.
9. The support oxide of claim 1, wherein said support oxide further
comprises an oxygen storage material (OSM).
10. A washcoat or overcoat comprising a support oxide, an oxygen
storage material (OSM) and a catalyst, wherein said support oxide
is according to claim 1.
11. The washcoat or overcoat of claim 10, wherein said catalyst is
a platinum group metal (PGM) catalyst.
12. The washcoat or overcoat of claim 11, wherein said PGM is Rh,
Pt, Pd, or a mixture thereof.
13. The washcoat or overcoat of claim 12, wherein said PGM is
Rh.
14. The washcoat or overcoat of claim 13, wherein said Rh is
present at 0.25% (by weight) of said washcoat.
15. The washcoat or overcoat of claim 10, wherein said support
oxide constitutes 1-100% (by weight) of said washcoat or
overcoat.
16. A washcoat or overcoat comprising: a) (i) 40% oxygen storage
material (OSM); (ii) 30% Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3;
and (iii) 30% Pr.sub.0.05Zr.sub.0.95O.sub.2; b) (i) 40% OSM; (ii)
30% Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3; and (iii) 30%
La.sub.0.05Zr.sub.0.95O.sub.2; c) (i) 40% OSM; 30% Al.sub.2O.sub.3
or La--Al.sub.2O.sub.3; and (iii) 30%
Pr.sub.0.10Zr.sub.0.90O.sub.2; d) (i) 40% OSM; 30% Al.sub.2O.sub.3
or La--Al.sub.2O.sub.3; and (iii) 30%
La.sub.0.10Zr.sub.0.90O.sub.2; e) (i) 40% OSM; 30% Al.sub.2O.sub.3
or La--Al.sub.2O.sub.3; and (iii) 30%
Pr.sub.0.15Zr.sub.0.85O.sub.2; f) (i) 40% OSM; 30% Al.sub.2O.sub.3
or La--Al.sub.2O.sub.3; and (iii) 30%
La.sub.0.15Zr.sub.0.85O.sub.2; g) (i) 40% OSM; and (ii) 60%
Pr.sub.0.05Zr.sub.0.95O.sub.2; h) (i) 40% OSM; and (ii) 60%
La.sub.0.05Zr.sub.0.95O.sub.2; i) (i) 40% OSM; and (ii) 60%
Pr.sub.0.10Zr.sub.0.90O.sub.2; j) (i) 40% OSM; and (ii) 60%
La.sub.0.10Zr.sub.0.90O.sub.2; k) (i) 40% OSM; and (ii) 60%
Pr.sub.0.15Zr.sub.0.85O.sub.2; or l) (i) 40% OSM; and (ii) 60%
La.sub.0.15Zr.sub.0.85O.sub.2.
17. A washcoat or overcoat according to claim 15, wherein said OSM
is: Ce.sub.1-a-b-c-dD.sub.aE.sub.bF.sub.cZr.sub.dO.sub.2, wherein:
a, b and c are, independently, 0-0.7; d is 0-0.9; and D, E and F
are, independently, selected from the group consisting of
lanthanides, alkaline earth metals and transition metals.
18. A washcoat or overcoat according to claim 17, wherein said OSM
is Ce.sub.0.3Nd.sub.0.05Pr.sub.0.05Zr.sub.0.6O.sub.2.
19. A catalyst system comprising: a substrate and a washcoat,
wherein said washcoat is according to claim 10.
20. A catalyst system comprising: a substrate, a washcoat, and an
overcoat, wherein said washcoat is according to claim 10; and said
overcoat comprises a support oxide, an oxygen storage material
(OSM), and a catalyst.
21. A catalyst system comprising: a substrate, a washcoat, and an
overcoat, wherein said washcoat comprises a support oxide, an
oxygen storage material (OSM) and a catalyst; and wherein said
overcoat is according to claim 10.
22. The catalyst system of claim 21, wherein said catalyst in said
overcoat is a platinum group metal (PGM) catalyst.
23. The catalyst system of claim 22, wherein said catalyst in said
overcoat is Rh, Pt, Pd, or a mixture thereof.
24. The catalyst system of claim 23, wherein said catalyst in said
overcoat is Pd.
25. The catalyst system of claim 24, wherein said Pd is present at
100 g/ft.sup.3.
26. The catalyst system of claim 21, wherein said catalyst in said
washcoat is a platinum group metal (PGM) catalyst.
27. The catalyst system of claim 26, wherein said catalyst in said
washcoat is Rh, Pt, Pd, or a mixture thereof.
28. The catalyst system of claim 27, wherein said catalyst in said
washcoat is Pd.
29. The catalyst of claim 28, wherein said Pd is present at 100
g/ft.sup.3.
30. The catalyst system of claim 19, wherein said catalyst system
is a three way conversion catalyst system.
31. The catalyst system of claim 19, wherein said catalyst system
exhibits improved gas flow when compared to catalyst systems
comprising only traditional support oxides.
32. The catalyst system of claim 19, wherein said catalyst system
exhibits improved light-off performance when compared to catalyst
systems comprising only traditional support oxides.
33. A method of making a catalyst system comprising: a) depositing
a washcoat according to claim 10 on a substrate; b) treating said
washcoat and substrate by calcination; and c) optionally
impregnating a platinum group metal (PGM) catalyst into said
washcoat, followed by drying and calcination.
34. A method of making a catalyst system comprising: a) depositing
a washcoat according to claim 10 on a substrate; b) treating said
washcoat and substrate by calcination; c) optionally impregnating a
platinum group metal (PGM) catalyst into said washcoat, followed by
drying and calcination; d) depositing an overcoat onto the
washcoat; wherein said overcoat comprises a support oxide, an
oxygen storage material and a catalyst; e) optionally impregnating
a PGM catalyst into said overcoat, followed by drying and
calcination.
35. A method of making a catalyst system comprising: a) depositing
a washcoat on a substrate, wherein said washcoat comprises a
support oxide, an oxygen storage material and a catalyst; b)
treating said washcoat and substrate by calcination; c) optionally
impregnating a platinum group metal (PGM) catalyst into said
washcoat, followed by drying and calcination; d) depositing an
overcoat according to claim 10 onto the washcoat; e) optionally
impregnating a PGM catalyst into said overcoat, followed by drying
and calcination.
36. (canceled)
37. (canceled)
38. (canceled)
39. (canceled)
40. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional
Application No. 61/521,831, filed Aug. 10, 2011, which is
incorporated herein by reference in its entirety.
TECHNICAL FIELD OF INVENTION
[0002] The invention generally relates to three-way catalysts and
catalyst formulations capable of simultaneously converting nitrogen
oxides, carbon monoxide, and hydrocarbons into less toxic
compounds. Such three-way catalyst formulations contain
ZrO.sub.2-based mixed-metal oxide support oxides doped with an
amount of lanthanide. Three-way catalyst formulations with the
support oxides of the present invention demonstrate higher
catalytic activity, efficiency and longevity than comparable
catalysts formulated with traditional support oxides.
BACKGROUND OF THE INVENTION
[0003] Nitrogen oxides, carbon monoxide, and hydrocarbons are toxic
and environmentally damaging byproducts found in the exhaust gas
from internal combustion engines. Methods of catalytically
converting nitrogen oxides, carbon monoxide, and hydrocarbons into
less harmful compounds include the simultaneous conversion of these
byproducts (i.e., "three-way conversion" or "TWC"). Specifically,
nitrogen oxides are converted to nitrogen and oxygen, carbon
monoxide is converted to carbon dioxide, and hydrocarbons are
converted to carbon dioxide and water.
[0004] It has generally been found that TWC increases catalytic
activity and efficiency and, thus, aids in meeting emission
standards for automobiles and other vehicles. In order to achieve
an efficient three-way conversion of the toxic components in the
exhaust gas, conventional TWC catalysts contain large quantities of
precious metals, such as Pd, Pt and Rh, dispersed on suitable oxide
carriers. Typically, conventional TWC catalysts use precious metal
catalysts at concentrations in the range of 30-300 g/ft.sup.3, with
Rh, being used in the range of 5-30 g/ft.sup.3.
[0005] Commonly used catalyst systems suffer from several
drawbacks. For example, commonly used TWC catalyst systems require
precious metal catalysts in order to efficiently carry out the TWC.
Such precious metals are expensive, can be inefficient, and have
been shown to degrade over time/use.
[0006] There have been several previous attempts at improving the
light-off performance of catalyst systems. Such attempts have tried
to address problems relating to inefficiency of precious metal
catalysts at lower temperatures and the degradation of such
catalysts as a result of exposure to high temperatures. For
example, some approaches utilize higher loadings of active precious
metal catalysts (e.g., Rh) with predictable increases in cost.
Other approaches have utilized substrate structures with a higher
channel density (and, thus, higher amounts of precious metal
catalyst). These approaches not only suffer from increases in cost,
but also from higher back pressure. The higher back pressure, which
is an artifact of the fact that the higher channel density
decreases the amount of space through which exhaust may pass,
results in an increase in fuel usage. A third approach has been to
use a dual TWC system. Such TWC systems comprise a first TWC
catalyst placed near the engine (i.e., a close coupled "CC"
catalyst), thus exposing it to the engine's heat exhaust and
allowing it to reach light-off temperature more quickly and a
second, larger, TWC catalyst placed further away from the engine
(e.g., under the floor of the vehicle) where there additional space
allows for the placement of larger TWC catalysts systems. While
such techniques lead to improved TWC catalyst efficiencies, they
tend to decrease the lifespan of at least the CC TWC catalyst by
exposing it to higher temperatures. In addition, CC TWC catalysts
suffer from increased poisoning of the precious metal catalysts by
virtue of their increased exposure to sulfur or phosphorous in
engine exhaust. Thus, there is a trade-off between increasing
catalyst efficiency at the expense of decreasing lifespan and,
thus, requiring the expensive replacement of TWC catalysts.
[0007] Other methods for improving light-off performance focused on
modifying the layout of the PGM catalysts in CC TWC catalysts. For
example, some methods place additional or extra PGM catalysts at
the front of the CC TWC catalysts as a further means of quickly
bringing catalysts to their light-off temperatures. As can be
expected, such catalyst designs suffer from the same drawbacks
discussed above--decreased lifespan by thermal degradation of the
catalyst and poisoning of the catalysts by virtue of the fact that
they are exposed to higher amounts of upstream exhaust--in addition
to the fact that they require increased amounts of expensive PGM
catalysts.
[0008] Thus, there is a need for catalyst formulations which have
increased conversion efficiencies without requiring additional
amounts of precious metals.
SUMMARY OF THE INVENTION
[0009] In some embodiments, the present invention relates to a
support oxide comprising ZrO.sub.2 doped with an amount of
lanthanide. In additional embodiments, the support oxide further
comprises Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3. In some
embodiments, the lanthanide used is La or Pr. Such support oxides
are generally referred to herein as La--ZrO.sub.2-based mixed-metal
oxide support oxides (MMOSOs). The amount of lanthanide present in
the support oxide can vary. For example, the ZrO.sub.2 support
oxide may be doped with between about 1-30% of the lanthanide. In
some embodiments, the ZrO.sub.2 support oxide is doped with between
about 5-15% of the lanthanide. In other embodiments, the ZrO.sub.2
support oxide is doped with about 5%, 10% or 15% of the
lanthanide.
[0010] In particular embodiments, the support oxide comprises
La.sub.X %Zr.sub.(1-X) %O.sub.2 and/or Pr.sub.X %Zr.sub.(1-X)
%O.sub.2. In other particular embodiments, the support oxide
comprises La.sub.5%Zr.sub.95%O.sub.2, Pr.sub.5%Zr.sub.95%O.sub.2,
La.sub.10%Zr.sub.90%O.sub.2, Pr.sub.10%Zr.sub.90%O.sub.2,
La.sub.15%Zr.sub.85%O.sub.2, Pr.sub.15%Zr.sub.85%O.sub.2, or
mixtures thereof.
[0011] The support oxides of the present invention are stabilized
in the tetragonal phase. Accordingly, the present invention refers
to support oxides wherein the tetragonal phase is stabilized.
[0012] In some embodiments, the support oxides further comprise an
oxygen storage material (OSM). Suitable OSMs include those of the
general formula
Ce.sub.1-a-b-c-dD.sub.aE.sub.bF.sub.cZr.sub.dO.sub.2, wherein a, b
and c are, independently, 0-0.7; d is 0-0.9; and D, E and F are,
independently, selected from the group consisting of lanthanides,
alkaline earth metals and transition metals. In a particular
embodiment, the OSM is Ce.sub.0.3Nd.sub.0.05Pr.sub.0.05
Zr.sub.0.6O.sub.2.
[0013] In another aspect, the present invention relates to
washcoats comprising La--ZrO.sub.2-based MMOSOs, an OSM and a
catalyst. In some embodiments, the catalyst is a platinum group
metal (PGM) catalyst. Examples of suitable PGM catalysts include
Rh, Pt, Pd, or mixtures thereof. In particular embodiments, the PGM
is Rh. In additional particular embodiments, the Rh is present at
0.25% (by weight) of the washcoat.
[0014] The present invention similarly relates to overcoats
comprising La--ZrO.sub.2-based MMOSOs, an OSM and a catalyst. In
some embodiments, the catalyst is a platinum group metal (PGM)
catalyst. Examples of suitable PGM catalysts include Rh, Pt, Pd, or
mixtures thereof. In particular embodiments, the PGM is Rh. In
additional particular embodiments, the Rh is present at 0.25% (by
weight) of the overcoat.
[0015] The washcoats and overcoats of the present invention may
constitute varying amounts of the La--ZrO.sub.2-based MMOSOs. For
example, the La--ZrO.sub.2-based MMOSO may constitute 1-100% (by
weight) of the washcoat or overcoat. In some embodiments, the
La--ZrO.sub.2-based MMOSO constitutes 40-80%, 45-75%, 50-70% or
55-65% (by weight) of the washcoat or overcoat. In other
embodiments, the La--ZrO.sub.2-based MMOSO constitutes 60% (by
weight) of the washcoat or overcoat.
[0016] Moreover, the washcoats and overcoats of the present
invention may constitute varying amounts of OSMs. For example, in
some embodiments, the OSM constitutes 30-50% (by weight) of the
washcoat or overcoat. In other embodiments, the OSM constitutes 40%
(by weight) of the washcoat or overcoat.
[0017] In particular embodiments, the washcoat or overcoat
comprises about:
[0018] a) (i) 40% oxygen storage material (OSM); (ii) 30%
Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3; and (iii) 30%
Pr.sub.0.05Zr.sub.0.95O.sub.2;
[0019] b) (i) 40% OSM; (ii) 30% Al.sub.2O.sub.3 or
La--Al.sub.2O.sub.3; and (iii) 30%
La.sub.0.05Zr.sub.0.95O.sub.2;
[0020] c) (i) 40% OSM; 30% Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3;
and (iii) 30% Pr.sub.0.10Zr.sub.0.90O.sub.2;
[0021] d) (i) 40% OSM; 30% Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3;
and (iii) 30% La.sub.0.10Zr.sub.0.90O.sub.2;
[0022] e) (i) 40% OSM; 30% Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3;
and (iii) 30% Pr.sub.0.15Zr.sub.0.85O.sub.2;
[0023] f) (i) 40% OSM; 30% Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3;
and (iii) 30% La.sub.0.15Zr.sub.0.85O.sub.2;
[0024] g) (i) 40% OSM; and (ii) 60%
Pr.sub.0.05Zr.sub.0.95O.sub.2;
[0025] h) (i) 40% OSM; and (ii) 60%
La.sub.0.05Zr.sub.0.95O.sub.2;
[0026] i) (i) 40% OSM; and (ii) 60%
Pr.sub.0.10Zr.sub.0.90O.sub.2;
[0027] j) (i) 40% OSM; and (ii) 60%
La.sub.0.10Zr.sub.0.90O.sub.2;
[0028] k) (i) 40% OSM; and (ii) 60% Pr.sub.0.15Zr.sub.0.85O.sub.2;
or
[0029] l) (i) 40% OSM; and (ii) 60%
La.sub.0.15Zr.sub.0.85O.sub.2.
[0030] In another aspect, the present invention relates to catalyst
systems comprising a substrate and a washcoat, wherein the washcoat
comprises a La--ZrO.sub.2-based MMOSO, as described above. In
addition, the present invention relates to catalyst systems
comprising a substrate, a washcoat, and an overcoat, wherein the
washcoat comprises a La--ZrO.sub.2-based MMOSO, as described above,
and the overcoat comprises a support oxide, OSM, and a catalyst. In
such embodiments, the catalyst in the overcoat may be Rh, Pt, Pd,
or a mixture thereof, preferably Pd. In specific embodiments, the
catalyst in the overcoat is Pd and is present at 100
g/ft.sup.3.
[0031] The present invention further relates to catalyst systems
comprising a substrate, a washcoat, and an overcoat, wherein the
washcoat comprises a support oxide, an OSM and a catalyst, and the
overcoat comprises a La--ZrO.sub.2-based MMOSO, as described above.
In such embodiments, the catalyst in the washcoat may be Rh, Pt,
Pd, or a mixture thereof, preferably Pd. In specific embodiments,
the catalyst in the washcoat is Pd and is present at 100
g/ft.sup.3.
[0032] The catalyst systems of the present invention may be TWC
catalyst systems. In addition, the catalyst systems may improve gas
flow and/or light-off performance when compared to catalyst systems
comprising only traditional support oxides.
[0033] The present invention also relates to methods of making the
catalyst systems described above. For example, the present
invention relates to a method of making a catalyst system
comprising: a) depositing a washcoat comprising a
La--ZrO.sub.2-based MMOSO described above on a substrate; b)
treating the washcoat and substrate by calcination; and c)
optionally impregnating a PGM catalyst into the washcoat, followed
by drying and calcination.
[0034] The present invention also relates to a method of making a
catalyst system comprising: a) depositing a washcoat comprising a
La--ZrO.sub.2-based MMOSO described above on a substrate; b)
treating the washcoat and substrate by calcination; c) optionally
impregnating a PGM catalyst into the washcoat, followed by drying
and calcination; d) depositing an overcoat onto the washcoat,
wherein the overcoat comprises a support oxide, an OSM and a
catalyst; and e) optionally impregnating a PGM catalyst into the
overcoat, followed by drying and calcination.
[0035] In addition, the present invention relates to a method of
making a catalyst system comprising: a) depositing a washcoat on a
substrate, wherein the washcoat comprises a support oxide, an OSM
and a catalyst; b) treating the washcoat and substrate by
calcination; c) optionally impregnating a PGM catalyst into the
washcoat, followed by drying and calcination; d) depositing an
overcoat comprising La--ZrO.sub.2-based MMOSO described above onto
the washcoat; and e) optionally impregnating a PGM catalyst into
the overcoat, followed by drying and calcination.
[0036] In each of the above methods, the washcoat (and overcoat,
where applicable) may be deposited as a slurry. In such
embodiments, the PGM catalyst may be preloaded onto the slurry.
Such preloaded PGM catalysts may be in the form of a nitrate,
acetate or chloride salt.
[0037] In embodiments wherein the PGM catalyst is impregnated onto
a washcoat (or overcoat, where applicable) the PGM catalyst may be
impregnated as an aqueous solution. PGM catalysts may be in the
form of a nitrate, acetate or chloride salt.
[0038] The present invention also relates to methods of reducing
toxic exhaust gas emissions comprising contacting gas emissions
with the catalyst systems described above. In addition, the present
invention refers to methods of increasing oxygen flow through a
catalytic system by stabilizing the phase (in, for example, the
tetragonal phase) of the support oxide present in the catalyst
system. In some embodiments, the catalyst system used in these
methods comprises a La--ZrO.sub.2-based MMOSO described above. In
other embodiments, the catalyst system used in these methods is a
catalyst system described above.
[0039] In addition, the present invention relates to methods of
improving the lifetime of a catalyst system comprising a PGM
catalyst by: a) reducing the amount of PGM catalyst deactivated
during the aging of the catalyst system; b) increasing the amount
of metallic PGM initially present in the catalyst system; or c)
both a) and b). In some embodiments, the catalyst system used in
these methods comprises a La--ZrO.sub.2-based MMOSO described
above. In other embodiments, the catalyst system used in these
methods is a catalyst system described above. In some embodiments
the amount of Rh(0) and/or Rh(III) as Rh.sub.2O.sub.3 initially
present in the catalyst system is increased. In other embodiments,
the amount of Rh(0) which is converted to Rh(III) as
Rh.sub.2O.sub.3 or Rh(III)-MMO during aging of the catalyst system
is decreased. In a particular embodiment, the amount of Rh(0) which
is converted to Rh(III)-MMO during aging of the catalyst system is
decreased.
[0040] The present invention further relates to methods of
improving the conversion of a) nitrogen oxides to nitrogen and
oxygen; b) hydrocarbons to carbon dioxide and water; or c) both a)
and b), present in exhaust gas emissions, by utilizing
La--ZrO.sub.2-based MMOSOs described above in the washcoat,
optional overcoat, or both. The present invention also relates to
methods of improving the light-off performance of a catalyst system
by utilizing La--ZrO.sub.2-based MMOSOs described above in the
washcoat, optional overcoat, or both. Moreover, the present
invention relates to methods of reducing the amount of PGM present
in a catalyst system while maintaining catalyst efficiency by
utilizing La--ZrO.sub.2-based MMOSOs described above in the
washcoat, optional overcoat, or both. In particular embodiments of
each of these methods, the PGM is Rh.
[0041] Methods of TWC of gas emissions comprising contacting gas
emissions with the catalyst systems described above are also
contemplated by the present invention.
[0042] In some embodiments, the present invention relates to
catalytic convertor systems comprising the catalyst system
described above. Such catalytic convertor systems may comprise two
or more catalytic converters. In some embodiments, the catalytic
convertor system comprises a close coupled catalytic converter.
BRIEF DESCRIPTION OF THE FIGURES
[0043] FIG. 1 is a schematic representation of a TWC catalyst
comprising (1) a substrate, and (2) a washcoat containing at least
one metal catalyst, wherein the washcoat is supported by the
substrate.
[0044] FIG. 2 is a schematic representation of a TWC catalyst
comprising (1) a substrate, (2) a washcoat containing at least one
metal catalyst, wherein the washcoat is supported by the substrate,
and (3) an overcoat containing at least one metal catalyst, wherein
the overcoat is supported by the washcoat.
[0045] FIG. 3 is a schematic representation of a TWC catalyst
comprising (1) a substrate, (2) a washcoat containing at least one
metal catalyst, wherein the washcoat is supported by the substrate,
and (3) an overcoat which is free of metal catalyst.
[0046] FIG. 4 is a schematic representation of a TWC catalyst
comprising (1) a substrate, (2) a washcoat which is free of metal
catalyst and (3) an overcoat containing at least one metal
catalyst, wherein the overcoat is supported by the washcoat.
[0047] FIG. 5 shows an X-ray powder diffraction plot illustrating
the relative amounts of tetragonal vs monoclinic phase in
ZrO.sub.2-based MMOSO doped with 5%, 10% or 15% Pr. The tested
MMOSO had not been aged. It is noted that the tetragonal phase is
the only phase detected in MMOSOs doped with 10% and 15% Pr.
[0048] FIG. 6 Shows an X-ray powder diffraction plot illustrating
the relative amounts of tetragonal vs monoclinic phase in
ZrO.sub.2-based MMOSO doped with 5%, 10% or 15% Pr after aging at
1000.degree. C. for 20 hours. The data indicates that increasing
the Pr content increases the stability of the tetragonal phase to
aging.
[0049] FIG. 7 is a diagram illustrating the steric hindrance
presented by Pr.sup.3+ cations. Oxygen mobility occurs via a
"hopping" mechanism. That is, the oxygen anion "hops" to a
neighboring vacant site which is usually present due to lattice
defects or the fact that two Pr.sup.3+ atoms are adjacent to each
other. The presence of too many Pr.sup.3+ cations makes it
difficult for relatively large oxygen anion to migrate through the
lattice.
[0050] FIG. 8 is a diagram illustrating that the inter-domain
boundary between monoclinic and tetragonal facilitates gas
diffusion and plays a role as a gas diffusion pathway. Thus,
stabilization of the tetragonal phase by Pr doping leads to faster
and easier diffusion of oxygen through the catalyst structure.
[0051] FIG. 9 is a plot of lattice parameter versus Pr content of
the MMOSO. The data indicate a linear relationship between the
amount of Pr introduced into the support oxide and the lattice
parameters. The fact that Pr affects the lattice parameters in this
respect signifies that a solid solution exists between Pr and Zr
rather separate phases.
[0052] FIG. 10 is a plot of the rate of H.sub.2 absorption and of
cumulative hydrogen absorption vs temperature for the 550.degree.
C. TPO-TPR cycle. As the amount of Pr increased, the major Rh and
Rh-MMO reduction peaks (A+B) shifted to lower temperatures,
indicating lower light-off temperatures. It is noted that the 5% Pr
and 10% Pr loaded samples had the highest total H.sub.2
absorption.
[0053] FIG. 11 is a plot of the rate of H.sub.2 absorption vs
temperature for the 900.degree. C. TPO-TPR cycle. The 10% Pr loaded
sample exhibited the highest total H.sub.2 absorption.
[0054] FIG. 12 is a plot of cumulative H.sub.2 absorption vs
temperature for the 900.degree. C. TPO-TPR cycle. The 10% Pr loaded
sample exhibited the highest total H.sub.2 absorption.
[0055] FIG. 13 is a plot of the rate of H.sub.2 absorption vs
temperature for the 1000.degree. C. XHFC aging. At 25.degree. C.,
H.sub.2 absorption can be ranked as follows: (15% Pr)=(10%
Pr)>(5% Pr)>(0% Pr). The 10% Pr loaded sample had the highest
total H.sub.2 absorption.
[0056] FIG. 14 is a plot of the rate of H.sub.2 absorption and of
cumulative hydrogen absorption versus temperature for the
1000.degree. C. XHFC aging. At 25.degree. C., H.sub.2 absorption
can be ranked as follows: (15% Pr)=(10% Pr)>(5% Pr)>(0% Pr).
The 10% Pr loaded sample had the highest total H.sub.2
absorption.
[0057] FIG. 15 is a flowchart illustrating the generation of an
overcoat containing a Pr--ZrO.sub.2-based MMOSO.
[0058] FIG. 16 is a flowchart illustrating the steps of the
IWCP.
DETAILED DESCRIPTION OF THE INVENTION
[0059] In order that the invention herein described may be fully
understood, the following detailed description is set forth.
[0060] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as those commonly understood by
one of ordinary skill in the art to which this invention belongs.
Although methods and materials similar or equivalent to those
described herein can be used in the practice or testing of the
present invention, suitable methods and materials are described
below. The materials, methods and examples are illustrative only,
and are not intended to be limiting. All publications, patents and
other documents mentioned herein are incorporated by reference in
their entirety.
[0061] Throughout this specification, the word "comprise" or
variations such as "comprises" or "comprising" will be understood
to imply the inclusion of a stated integer or groups of integers
but not the exclusion of any other integer or group of
integers.
[0062] In order to further define the invention, the following
terms and definitions are provided herein.
DEFINITIONS
[0063] The term "catalyst system" refers to any system comprising a
catalyst such as a PGM catalyst. In some embodiments, the catalyst
system comprises a substrate, a washcoat, and optionally an
overcoat. Examples of catalyst systems are depicted in FIGS.
1-4.
[0064] The term "close-coupled catalyst" or "CC catalyst" refers
to, for example, a catalytic converter which is placed close to the
engine so as to be exposed to the heat generated by operation of
the engine. Such CC catalysts may be TWC catalysts.
[0065] The term "Ce-containing mixed metal oxide" refers to
materials based on a fluorite structure and containing Ce, Zr and,
typically, several lanthanide metals. Typical examples are
expressed in terms of the relative quantity of Ce and Zr (Ce-rich
or Zr-rich) and the nature of the lanthanide dopants at the 1-10%
level typically.
[0066] The term "conversion efficiency" refers to the percentage of
emissions passing through the catalyst that are converted to their
target compounds.
[0067] The term "coupled with" refers to a relationship (e.g.,
functional or structural) between components of a catalyst system
(e.g., the relationship between the washcoat and the substrate
and/or overcoat, or the relationship between the overcoat and the
washcoat). In some embodiments, components which are coupled to
each other are in direct contact with each other (e.g., the
washcoat may be in direct contact with and, thus, coupled with the
substrate). In other cases, components which are coupled to each
other are coupled via additional component(s) (e.g., an overcoat is
coupled to the substrate via the washcoat).
[0068] The term "high-surface area alumina" refers to aluminum
oxides that have a high specific surface area--i.e., a high surface
area per unit weight. High surface area aluminas typically have
crystal structures designated as gamma, delta or theta.
[0069] The term "high-temperature conditions" refers to engine
conditions wherein hot exhaust gas passes through a catalyst. Such
exhaust gas is typically in excess of 800.degree. C., and in
extreme circumstances, in excess of 1000.degree. C.
[0070] The term "Lanthanide group of elements" refers to the
elements La, Pr, Sm, Nd, Pm, Gd, Eu, Tb, Dy, Ho, Er, Tm, Yb and
Lu.
[0071] The term "Ln-doped Zirconia" refers to an oxide comprising
zirconium and an amount of dopant from the Lanthanide group of
elements, where Ln denotes any of the lanthanide group.
[0072] The term "light-off temperature" refers to the temperature
at which a catalyst is able to convert 50% of the emissions passing
through the catalyst (e.g., nitrogen oxides, carbon monoxide and
unburnt hydrocarbons) to their target compounds (e.g., nitrogen and
oxygen, carbon dioxide, and carbon dioxide and water,
respectively).
[0073] The term "metallic Rh" refers to the element Rh in its
metallic state "Rh(0)." The term "Rh(I)" refers to mono-valent
Rhodium (metallic Rh with one electron removed). The term "Rh(III)"
refers to tri-valent Rhodium (metallic Rh with three electrons
removed). Rh(III) is the stable form of Rh found in, e.g.,
Rh.sub.2O.sub.3.
[0074] The term "mixed metal oxide" refers to an oxide, wherein the
cation positions in the oxide's crystal structure can be occupied
by a variety of cations. Such cations may be selected from one or a
variety of lanthanides. In particular embodiments, the mixed metal
oxide cations are a mixture of either Pr and Zr, or La and Zr. The
term "multiphase catalyst" or "MPC" refers to a catalyst
represented by the general formula CeyLn1-xAx+sMOz. Such catalysts
are described in, e.g., U.S. Pat. No. 7,641,875, which is
incorporated herein in its entirety.
[0075] The term "overcoat" refers to a coating comprising one or
more oxide solids that are coupled with a substrate and a washcoat.
The oxide solids in the overcoat may be support oxides, one or more
catalyst oxides, or a mixture of support oxides and catalyst
oxides.
[0076] The term "oxygen storage material" or "OSM" refers to a
composition which supplies oxygen to rich exhaust and takes up
oxygen from lean exhaust, thus buffering a catalyst system against
the fluctuating supply of oxygen. OSMs increase catalyst
efficiency. Oxygen storage materials may be present in the washcoat
and/or the overcoat of a catalyst system.
[0077] The term "platinum group metal" or "PGM" refers to the
following six elements: ruthenium (Ru), rhodium (Rh), palladium
(Pd), osmium (Os), iridium (Ir), and platinum (Pt).
[0078] The term "poisoning" or "catalyst poisoning" refers to the
inactivation of a catalyst by virtue of its exposure to lead or
phosphorous in, for example, engine exhaust.
[0079] The term "solid solution" refers to the doping of a metal
either onto the crystallographic site of a host material, or in
between crystallographic sites of a host material. Such solid
solutions are composed of a single homogenous phase. The solid
solution has the same crystallographic type or structure as the
un-doped host material. Typically the lattice parameters of the
solid solution increase or decrease with increasing dopant amount.
Whether or not an increase or decrease in lattice parameters occurs
depends on whether the doping cation is smaller or larger than the
host cations (in addition to other specific chemical and
crystallographic factors).
[0080] The term "stoichiometric point" or "stoichiometric ratio"
refers to a particular air-fuel ratio (i.e., the ratio of air to
fuel present in an engine during combustion). An engine operates at
the stoichiometric point when exactly enough air is present in the
fuel mixture to burn all of the fuel present.
[0081] The term "stabilized alumina" refers to alumina wherein
modifiers are added to retard undesired phase transitions of the
alumina from, for example, the gamma phase to the alpha phase, when
the alumina is exposed to elevated temperatures. Such modifiers aid
in stabilizing the surface area of the alumina. Alumina is exposed
to high temperatures during formation of the catalyst system and
during operation of the catalyst system (e.g., when it is exposed
to exhaust gas). The modifiers or thermal stabilizers may include,
for example, one or more modifiers or stabilizers selected from,
but not limited to, rare earth oxides, silicon oxides, oxides of
Group IVB metals (e.g., zirconium, hafnium, or titanium) and
alkaline earth oxides. For example, lanthanide nitrate and/or
strontium nitrate may be added to washcoats and/or overcoats (in,
e.g., support oxides) as a modifier for the alumina. The lanthanide
nitrate solution may contain a single lanthanide nitrate (e.g.,
lanthanum nitrate), or the solution may contain a mixture of
lanthanide nitrates. Heating or calcining the lanthanide nitrate
and/or strontium nitrate forms lanthanide oxide (Ln.sub.2O.sub.3)
and/or strontium oxide.
[0082] The term "substrate" refers to any material known in the art
for supporting a catalyst. Substrates can be of any shape or
configuration that yields a sufficient surface area for the deposit
of the washcoat and/or overcoat. Examples of suitable
configurations for substrates include, but are not limited to,
honeycomb, pellet, and bead configurations. Substrates can be made
of a variety of materials including, but not limited to alumina,
cordierite, ceramic and metal.
[0083] The term "three-way conversion catalyst" or "TWC catalyst"
refers to a catalyst that simultaneously a) reduces nitrogen oxides
to nitrogen and oxygen; b) oxidizes carbon monoxide to carbon
dioxide; and c) oxidizes unburnt hydrocarbons to carbon dioxide and
water. Typically, TWC catalysts require the use of precious metals
such as platinum group metals.
[0084] The term "washcoat" refers to a coating comprising one or
more oxide solids that is coupled to a substrate or solid support
structure. The oxide solids in the washcoat may be support oxides,
one or more catalyst oxides, or a mixture of support oxides and
catalyst oxides.
Catalyst Systems
[0085] Catalyst systems in, for example, catalytic converters may
be used in conjunction with an internal combustion engine. Such
catalyst systems may be TWC catalysts. In light of the expense
associated with Rh-containing catalyst systems, there remains a
need for catalyst systems with improved Rh efficiencies and Rh
longevity. Thus, in one aspect, the present invention provides
catalyst system components which improve Rh efficiencies and Rh
longevity.
[0086] The catalyst systems (including TWC catalyst systems) of the
present invention may have a variety of architectures. TWC catalyst
systems typically comprise (1) a substrate, (2) a washcoat
supported by the substrate, and (3) an optional overcoat supported
by the washcoat (see, FIGS. 1-4). For example, the TWC catalyst
systems of the present invention may comprise (1) a substrate, and
(2) a washcoat containing at least one catalyst, wherein the
washcoat is supported by the substrate (see, FIG. 1). The catalyst
systems of the present invention may also comprise (1) a substrate,
(2) a washcoat containing at least one catalyst, wherein the
washcoat is supported by the substrate, and (3) an overcoat
containing at least one catalyst, wherein the overcoat is supported
by the washcoat (see, FIG. 2). The catalyst systems of the present
invention may also comprise (1) a substrate, (2) a washcoat
containing at least one catalyst, wherein the washcoat is supported
by the substrate, and (3) an overcoat which is relatively free of
catalyst, preferably at least 95%, 99%, or at least 99.99% free of
catalyst completely free (see, FIG. 3). Further, the catalyst
systems of the present invention may comprise (1) a substrate, (2)
a washcoat which is relatively free of catalyst, preferably at
least 95%, 99%, or at least 99.99% free of catalyst, and (3) an
overcoat containing at least one catalyst, wherein the overcoat is
supported by the washcoat (see, FIG. 4).
[0087] Catalyst systems are typically present in two locations in
automobile engines. For example, an automobile may contain two
catalytic converters: 1) a close-coupled catalyst ("CC catalyst")
placed near the engine; and 2) a larger catalyst placed, for
example, under the floor of the vehicle where there is more room
("underfloor catalyst" of "UF catalyst"). CC catalysts are placed
near the engine so they are exposed to the heat generated by
operation of the engine. This heat allows the CC catalyst to more
quickly reach its light-off temperature and, thus, more quickly
reach its maximum efficiency. The catalyst systems of the present
invention may be used in either CC or UF catalysts. In some
embodiments, the catalysts systems of the present invention improve
the lifetime and efficiency of CC catalysts containing Rh, even in
light of the exposure of CC catalysts to elevated engine exhaust
temperatures.
Improved Catalyst Systems
[0088] One aspect of the present invention is the provision of
catalyst system components with improved Rh efficiencies and Rh
longevity. Such components may be used in the washcoat and/or
overcoats of catalyst systems. Specifically, the present invention
provides support oxides for using in washcoats and/or overcoats
which improve Rh efficiencies and Rh longevity. In particular
embodiments, the support oxides are comprised of alumina doped with
an amount of lanthanide such as praseodymium (Pr) or Lanthanum
(La).
Lanthanide Doped Support Oxides
[0089] The mixed-metal oxide support oxides (MMOSOs) of the present
invention improve the catalyst properties of, for example, TWC
catalysts as described herein. Specifically, the MMOSOs of the
present invention demonstrate higher catalytic activity, efficiency
and longevity than comparable catalysts formulated with traditional
support oxides. For example, the MMOSOs of the present invention
improve the light-off temperature, stability/lifetime of the
precious metal catalysts such as Rh, and oxygen flow in catalyst
systems. In particular embodiments, the MMOSOs of the present
invention improve the light-off temperature and stability/lifetime
of Rh catalysts present in TWC catalyst systems.
[0090] In one aspect, the present invention refers to improved
support oxides for use in catalyst systems. The support oxides of
the present invention are MMOSOs such as, for example,
ZrO.sub.2-based MMOSOs. It has been found that doping
ZrO.sub.2-based MMOSOs with an amount of lanthanide yields support
oxides with the improved properties discussed herein. In
particular, the support oxides of the present invention have been
doped with an amount of lanthanide to yield Ln-ZrO.sub.2-based
MMOSOs. In general, the doped ZrO.sub.2-based MMOSOs of the
catalyst systems are of the following formula:
Ln.sub.X %Zr.sub.(1-X) %O.sub.2, [0091] wherein "X"=the amount (%
by weight) of Ln cation present in the catalyst; and [0092]
"1-X"=the amount by weight of Zr cation present in the
catalyst.
[0093] Any lanthanide (e.g., La, Pr, Sm, Nd, Pm, Gd, Eu, Tb, Dy,
Ho, Er, Tm, Yb or Lu) may be used in the MMOSOs of the present
invention. In a particular embodiment, the lanthanide used is
either La or Pr. In some embodiments, the catalyst systems comprise
a La--ZrO.sub.2-based MMOSO, a Pr--ZrO.sub.2-based MMOSO, or both.
Thus, in one embodiment, the catalyst systems comprise an amount,
as described herein, of Pr.sub.X %Zr.sub.(1-X) %O.sub.2. In another
embodiment, the catalyst systems comprise an amount, as described
herein, of La.sub.X %Zr.sub.(1-X) %O.sub.2. In yet another
embodiment, the catalyst systems comprise an amount, as described
herein, of Pr.sub.X %Zr.sub.(1-X) %O.sub.2 and La.sub.X
%Zr.sub.(1-X) %O.sub.2.
[0094] The ZrO.sub.2-based MMOSO can be doped with varying amounts
of a lanthanide, such as La or Pr, to yield a Ln-ZrO.sub.2-based
MMOSO. For example, the ZrO.sub.2-MMOSO may be doped with about 1%,
2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%,
17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29% or
30% of a lanthanide. In some embodiments, the ZrO.sub.2-MMOSO is
doped with about 5%, 10%, 15%, 20%, 25% or 30% of a lanthanide. In
other embodiments, the ZrO.sub.2-MMOSO is doped with about 1-5%,
1-10%, 1-15%, 1-20%, 1-25%, 1-30%, 5-10%, 5-15%, 5-20%, or 10-15%
of a lanthanide.
[0095] The ZrO.sub.2-based MMOSO can also be doped with up to about
1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%,
16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%,
29% or 30% of a lanthanide, such as La or Pr, to yield a
Ln-ZrO.sub.2-based MMOSO. In some embodiments, the ZrO.sub.2-based
MMOSO is doped with up to about 5%, 10%, 15%, 20%, 25% or 30% of a
lanthanide. In other embodiments, the ZrO.sub.2-based MMOSO is
doped with up to about 1-5%, 1-10%, 1-15%, 1-20%, 1-25%, 1-30%,
5-10%, 5-15%, 5-20%, or 10-15% of a lanthanide.
[0096] In particular embodiments, the ZrO.sub.2-based MMOSO is
doped with about 5%, 10% or 15% a lanthanide, such as La or Pr, to
yield a Ln-ZrO.sub.2-based MMOSO. In other embodiments, the
ZrO.sub.2-based MMOSO is doped with 5%, 10% or 15% a lanthanide,
such as La or Pr. In one particular embodiment, the ZrO.sub.2-based
MMOSO is doped with 10% of a lanthanide. For example, the
Ln-ZrO.sub.2-based MMOSO may be: Pr.sub.5%Zr.sub.95%O.sub.2;
Pr.sub.10%Zr.sub.90%O.sub.2; Pr.sub.15%Zr.sub.85%O.sub.2;
La.sub.5%Zr.sub.95%O.sub.2; La.sub.10%Zr.sub.90%O.sub.2; or
La.sub.15%Zr.sub.85%O.sub.2.
Improved Catalyst Stability/Lifetime
[0097] Traditional catalyst systems typically utilize a high
surface area "transition" alumina (as a support oxide) and a
Ce-containing mixed metal oxide (as an OSM). The high surface area
alumina enables extensive dispersal of metal catalysts, such as Rh,
allowing for effective access of the exhaust to the catalyst sites.
However, traditional catalyst systems suffer from drawbacks
stemming from the interaction of the Rh catalyst with the alumina
support oxide under the high-temperature conditions commonly found
in modern automobile engines.
[0098] The reaction of Rh catalysts with other components in the
catalyst system (e.g., the support oxide) can negatively impact the
efficiency of Rh-catalyzed reactions in a variety of conditions
including, for example, high temperature and high engine exhaust
flow rate. The species of Rh most active for NO.sub.x conversion to
nitrogen gas is the metallic or Rh(0) state. During exposure of
catalyst systems to high temperature exhaust, however, the Rh(0)
state is oxidized to a stable, and less active, Rh(III) state
(i.e., Rh.sub.2O.sub.3). The less active Rh(III) state in the
Rh.sub.2O.sub.3 state can be converted back to Rh(0) via the
oxidation cycle employed during use of the catalyst. That is,
Rh(III) present in the Rh.sub.2O.sub.3 state may be reduced to
Rh(0) through a reversible redox cycle. However, the presence of
the less active Rh(III) state reduces the efficiency of the
catalyst system.
[0099] In some instances, Rh(0) reacts with the alumina or ceria to
form a mixed oxide with Rh(III), resulting in a very stable and
inactive Rh(III) mixed metal oxide state (Rh(III)-MMO). Rh(III)-MMO
cannot be reduced to Rh(0) through a reversible redox cycle. Thus,
Rh present in the Rh(III)-MMO state and is essentially passivated
or removed from the catalytic cycle. Such conversions are described
in, for example, in Yao H. C., Jaspar, S and Shelef. M, J. Catal.,
50, p 407 (1977) and Yao H. C. and Gandhi H. S., J. Catal., 61, p
547 (1980), each of which is incorporated herein by reference in
its entirety.
[0100] Rh(0) can also be negatively affected when it interacts with
the Ce-containing mixed metal oxide (the OSM). For example, during
exposure to engine exhaust, Rh(0) is coated to or associated with
the Ce-containing mixed metal oxide. Thus, even though the
Ce-containing mixed metal oxides are beneficial because they
stabilize the tetragonal phase (discussed herein), their tendency
to oxidize Rh(0) detracts from their usefulness.
[0101] The overall effect of conversion of Rh(0) to either the
Rh.sub.2O.sub.3 or Rh(III)-MMO state is a decrease in the amount of
active Rh(0) available to act as a catalyst. The decrease in active
Rh(0) is exacerbated by the fact that the efficiency of Rh-mediated
catalysis is typically lower at high temperatures (e.g. at or above
800.degree. C.), which are typical of high-speed engine conditions.
For these reasons, traditional catalyst systems lead to an eventual
reduction in the ability of Rh(0) to aid in NO.sub.x conversion to
nitrogen gas.
[0102] The present invention addresses the Rh conversion problem by
reducing the aging or degradation of the Rh in the TWC or TWC
system caused by high-temperature conditions. This is accomplished
by using the support oxides (Ln-ZrO.sub.2-based MMOSOs) described
herein. The support oxides of the present invention allow the Rh to
stay more predominantly in the Rh(0) state.
[0103] Without being bound by a particular theory, it is believed
that the Ln-ZrO.sub.2-based MMOSOs of the present invention reduce
oxidation of Rh(0) for several reasons. First, the
Ln-ZrO.sub.2-based MMOSOs have properties which allow them to store
oxygen from exhaust gas and, in effect, sequester it from the Rh(0)
catalyst. This reduces the opportunity for unwanted oxidation of
Rh(0) to less desired Rh(III) states.
[0104] In addition, the Zr present in traditional support oxides
has a tendency to coagulate after exposure to hot exhaust. The
additional space between cations in the Ln-ZrO.sub.2-based MMOSOs
(e.g., Pr and Zr) furnished by the presence of the Ln allows for
easier and freer diffusion of oxygen throughout the catalyst system
(see, Example 8 and FIGS. 5-9).
[0105] Moreover, traditional support oxides such as alumina form a
mixed phase Rh aluminate (locally) which essentially traps Rh in
the Rh(III) state--a phenomenon which does not appreciably take
place with Ln-ZrO.sub.2-based MMOSOs.
[0106] Finally, Rh present in catalyst systems with traditional
support oxides becomes buried within the support oxide after
exposure of the catalyst system to high temperatures (i.e., aging).
Burying the Rh has the unwanted effect of sequestering it from
exhaust gases, thereby reducing the ability of the catalyst sites
to participate in conversion of the exhaust.
[0107] When catalyst systems of the present invention comprising a
Ln-ZrO.sub.2-based MMOSO, such as Pr--ZrO.sub.2-based MMOSO, are
formulated with a certain amount of Rh, it has been found that such
catalyst systems contain higher amounts of initial Rh(0)
immediately after formulation (i.e., fresh catalyst systems) when
compared to catalyst systems comprising traditional support oxides,
such as La--Al.sub.2O.sub.3, formulated with the same amount of
starting Rh (see Example 5). Specifically, fresh catalyst systems
comprising Rh and a Pr--ZrO.sub.2-based MMOSO (i.e., freshly
synthesized and not aged through use) comprise more Rh(0) and
reversible Rh.sub.2O.sub.3 than fresh catalyst systems comprising
fresh Rh and La--Al.sub.2O.sub.3, even though the same amount of Rh
was used during synthesis of the catalyst system. Interestingly,
even aged catalyst systems comprising Rh and a Pr--ZrO.sub.2-based
MMOSO (i.e., aged under normal use conditions) comprise more Rh(0)
and reversible Rh.sub.2O.sub.3 than fresh catalysts comprising
fresh Rh and La--Al.sub.2O.sub.3 (see Example 5).
[0108] After standard aging, none of the Rh(III)-MMO state is
observed in either the fresh or aged Rh/Pr--ZrO.sub.2-based MMOSO
catalysts. Comparatively, the fresh Rh/La--Al.sub.2O.sub.3 catalyst
contains a measurable amount of the Rh(III)-MMO state and the aged
Rh/La--Al.sub.2O.sub.3 catalyst contains significant amounts of the
Rh(III)-MMO state (see Example 5).
Improved Light-Off Temperature
[0109] The light-off temperature of catalyst systems tends to rise
(i.e., worsen) after exposure of the catalyst system to high
temperatures. When catalyst systems are present in, for example,
catalytic converters, such compositions routinely operate under
high-temperature conditions. Accordingly, catalyst systems with
improved light-off temperatures are extremely useful in extending
the lifetime of the catalyst. Thus, in one aspect, the present
invention refers to catalyst systems with improved light-off
performance, even after high-temperature aging.
[0110] As discussed above, there have been several previous
attempts at improving the light-off performance of catalyst systems
including: utilizing higher loadings of active precious metal
catalysts (e.g., Rh); utilizing substrate structures with a higher
channel density (and, thus, higher amounts of precious metal
catalyst); utilizing a dual TWC system comprising a first close
coupled catalyst and a second, larger, TWC catalyst placed further
away from the engine; and modifying the layout of the PGM catalysts
in CC TWC catalysts. As also discussed above, such approaches
suffer from drawbacks including: increased cost due to use of
higher amounts of precious metal catalysts; increase poisoning of
precious metal catalysts; increased degradation of precious metal
catalysts due to exposure to higher temperatures; and decreases in
exhaust and oxygen flow due to tighter packing of the catalysts
systems.
[0111] The Ln-ZrO.sub.2-based MMOSO catalyst systems of the present
invention (such as a La--ZrO.sub.2-based or Pr--ZrO.sub.2-based
MMOSOs), have an improved (i.e., lower) light-off temperature with
respect to Rh catalyzed NO.sub.x conversion. Thus, the catalyst
systems of the present invention are able to function efficiently
without unnecessarily exposing the catalysts to high exhaust
temperatures and without needing to increase the amount of precious
metal catalyst used. For example, catalyst systems of the present
invention comprising a washcoat and/or overcoat comprising
La.sub.0.10Zr.sub.0.90O.sub.2 or Pr.sub.0.10Zr.sub.0.90O.sub.2
exhibited lower light-off temperatures when compared to catalyst
systems comprising 10% La--Al.sub.2O.sub.3 (see Examples 3 and 6
and Table 5).
[0112] The improved (lower) light-off temperature is observed after
typical high-temperature in-use conditions. The lower light-off
temperatures are especially useful in improving catalyst efficiency
when an engine is first engaged (i.e., a "cold start") because the
catalytic converter is still at ambient temperature and is, thus,
in a cold state.
Improved Conversion of Emission Byproducts
[0113] The catalyst systems of the present invention also exhibit
improved NO.sub.x and hydrocarbon conversion efficiency in
high-temperature conditions. Such improvements are typically
observed in the context of Rh-catalyzed NO.sub.x and hydrocarbon
conversion to nitrogen and CO.sub.2/water, respectively
Improvements in NO.sub.x and hydrocarbon conversion aid in
designing vehicles which meet strict emissions standards.
[0114] In particular, catalyst systems of the present invention
which comprise a Ln-ZrO.sub.2-based MMOSO exhibit improved
Rh-catalyzed NO.sub.x conversion at standard and high engine
operating temperatures as measured by both the Federal Test
Procedure ("FTP"; standard operating temperature) and US06 (high
operating temperature) protocols. TWC catalyst systems with
washcoats comprising a) 40% OSM/30% La-Al.sub.2O.sub.3/30%
Pr.sub.0.10Zr.sub.0.90O.sub.2; or b) 40% OSM/60%
Pr.sub.0.10Zr.sub.0.90O.sub.2 exhibited improved Rh-catalyzed
NO.sub.x conversion when compared to catalyst systems comprising
40% OSM/60% La--Al.sub.2O.sub.3 (see Example 4 and Tables 2-3).
[0115] Further, catalyst systems with washcoats comprising a) 40%
OSM/30% La-Al.sub.2O.sub.3/30% Pr.sub.0.10Zr.sub.0.90O.sub.2; or b)
40% OSM/60% Pr.sub.0.10Zr.sub.0.90O.sub.2 exhibit improved
Rh-catalyzed non-methane hydrocarbon conversion when compared to
TWC catalyst systems comprising 40% OSM/60% La--Al.sub.2O.sub.3
(see Example 4 and Tables 2-3).
Improved Washcoat/Overcoat Structure
[0116] The Ln-ZrO.sub.2-based MMOSO of the present invention
improve the overall structure of the washcoats or overcoats in
which they are located. Without being bound by any particular
theory, it is believed that the presence of the lanthanide in the
ZrO.sub.2 structure stabilizes the tetragonal or cubic phases of
ZrO.sub.2 with respect to the monoclinic phase. It is believed that
these tetragonal or cubic phases of ZrO.sub.2 are arranged in a
manner which permits faster and easier diffusion of oxygen through
the catalyst structure enhancing the activity of the catalyst (see,
Example 8 and FIGS. 5-8). In addition, the Ln, such as Pr or La,
present in the Ln-ZrO.sub.2-based MMOSO is present as solid
solution--i.e., a solid solution of Ln exists with the Zr (see,
Example 8 and FIG. 9). Because solid solutions are composed of a
single homogenous phase, such an arrangement also permits faster
and easier diffusion of oxygen through the catalyst structure.
[0117] The effect of ease of oxygen diffusion appears to be
mitigated as the amount of lanthanide present in the
Ln-ZrO.sub.2-based MMOSO increases beyond a certain point. For
example, it has been found that doping of ZrO.sub.2 with 5%, 10% or
15% of Pr has the effect of stabilizing the ZrO.sub.2 support oxide
in the tetragonal or cubic phases. However, the larger Pr.sup.3+
cations (when compared to the Zr.sup.4+ cations) may act as steric
barriers to oxygen diffusion as the amount of Pr increases.
Catalyst Systems Comprising Ln-ZrO.sub.2-Based MMOSOs
Overview
[0118] The catalyst systems (including TWC catalyst systems) of the
present invention may have a variety of architectures. For example,
a catalytic converter system present in an automobile may contain
both a CC catalyst and an UF catalyst, wherein the CC catalyst is
placed closer to the engine in comparison to the UF catalyst.
[0119] Both CC and/or UF catalysts typically comprise (1) a
substrate, (2) a washcoat supported by the substrate, and (3) an
optional overcoat supported by the washcoat. In particular
embodiments, the CC and/or UF catalyst comprises (1) a substrate,
(2) a washcoat supported by the substrate, and (3) an overcoat
supported by the washcoat. In some embodiments of the present
invention, the catalyst systems comprise CC and UF catalysts
comprising a Ln-ZrO.sub.2-based MMOSO. The Ln-ZrO.sub.2-based MMOSO
may be present in either the washcoat, the overcoat, or both of
either the CC catalyst, UF catalyst, or both.
[0120] In particular embodiments, the catalyst systems, whether
present in the CC or UF catalysts, comprise (1) a substrate, (2) a
washcoat, wherein the washcoat is supported by the substrate, and
(3) an overcoat comprising a Ln-ZrO.sub.2-based MMOSO, wherein the
overcoat is supported by the washcoat. In other embodiments, the
catalyst systems, whether present in the CC or UF catalysts,
comprise (1) a substrate, (2) a washcoat comprising a
Ln-ZrO.sub.2-based MMOSO, wherein the washcoat is supported by the
substrate, and (3) an overcoat, wherein the overcoat is supported
by the washcoat. In yet other embodiments, the catalyst systems,
whether present in the CC or UF catalysts, comprise (1) a
substrate, (2) a washcoat comprising a Ln-ZrO.sub.2-based MMOSO,
wherein the washcoat is supported by the substrate, and (3) an
overcoat comprising a Ln-ZrO.sub.2-based MMOSO, wherein the
overcoat is supported by the washcoat.
Substrates
[0121] A variety of materials are appropriate as substrates for the
present invention. For example, the substrate may be a refractive
material, a ceramic substrate, a honeycomb structure, a metallic
substrate, a ceramic foam, a metallic foam, a reticulated foam, or
suitable combinations, where the substrate has a plurality of
channels and at least the required porosity. As is known in the
art, the number of channels present may vary depending upon the
substrate used. It is preferred that the substrate offer a
three-dimensional support structure.
[0122] The substrate may be in the form of beads or pellets. In
such embodiments, the beads or pellets may be formed from, for
example, alumina, silica alumina, silica, titania, mixtures
thereof, or any suitable material. In a particular embodiment, the
substrate may be a honeycomb substrate, for example a ceramic
honeycomb substrate or a metal honeycomb substrate. The ceramic
honeycomb substrate may be formed from, for example, sillimanite,
zirconia, petalite, spodumene (lithium aluminum silicate),
magnesium silicates, mullite, alumina, cordierite, other
alumino-silicate materials, silicon carbide, aluminum nitride, or
combinations thereof. Other ceramic substrates would be apparent to
one of ordinary skill in the art.
[0123] In embodiments wherein the substrate is a metal honeycomb
substrate, the metal may be, for example, a heat-resistant base
metal alloy, particularly an alloy in which iron is a substantial
or major component. In addition, metal substrate surface may be
oxidized at elevated temperatures (e.g., above about 1000.degree.
C.) to improve the corrosion resistance of the alloy by forming an
oxide layer on the surface of the alloy. This oxide layer on the
surface of the alloy may also enhance the adherence of a washcoat
to the surface of the monolith substrate.
[0124] In one embodiment, the substrate may be a monolithic carrier
having a plurality of fine, parallel flow passages extending
through the monolith. Such passages may be of any suitable
cross-sectional shape and/or size. For example, such passages may
be trapezoidal, rectangular, square, sinusoidal, hexagonal, oval,
or circular, although other shapes are also suitable. The monolith
may contain from about 9 to about 1200 or more gas inlet openings
or passages per square inch of cross section, although fewer
passages may be used.
Washcoats and Overcoats
[0125] The washcoats and overcoats of the catalyst systems of the
present invention typically comprise, inter alia, a metal catalyst,
an OSM, and a support oxide--each of which is described herein. In
some embodiments, the washcoats may further comprise additives
which aid in retarding metal catalyst poisoning.
[0126] One aspect of the present invention is the replacement of an
amount of the traditional support oxide typically present in the
overcoat or washcoat by an amount of Ln-ZrO.sub.2-based MMOSO, such
as a La--ZrO.sub.2-based MMOSO or Pr--ZrO.sub.2-based MMOSO. In
some embodiments, an amount of the traditional support oxide
typically present in the overcoat is replaced with an amount of
Ln-ZrO.sub.2-based MMOSO, as described herein. In some embodiments,
an amount of the traditional support oxide typically present in the
washcoat is replaced with an amount of Ln-ZrO.sub.2-based MMOSO, as
described herein. In other embodiments, an amount of the
traditional support oxide typically present in the both the
washcoat and overcoat is replaced with an amount of
Ln-ZrO.sub.2-based MMOSO, as described herein.
[0127] In some embodiments the washcoats and overcoats of the
catalyst systems of the present invention can be used in
conjunction with an OSM made by the Improved Wet Chemical Process
(IWCP) (see, Example 10 and FIG. 16) or the High Temperature
Process (HTP) (see, Example 11).
Metal Catalysts
[0128] The metal catalysts present in the catalyst systems of the
invention are typically present in the washcoat and/or overcoat (if
one is present). Metal catalysts useful for the present invention
include PGM, zirconia, alumina or lanthanide catalysts. The
washcoat and overcoat may contain the same metal catalyst or
different metal catalysts. In addition, the washcoat and overcoat
may contain the same combination of metal catalysts (e.g., both
contain metal catalysts "A" and "B") or different combinations of
catalysts (e.g., the washcoat contains metal catalysts "A" and "B"
while the substrate contains metal catalysts "C" and "D").
[0129] In some embodiments, the metal catalysts used are PGM
catalysts--i.e., Ru, Rh, Pd, Os, Ir, Pt, or combinations thereof.
In some embodiments, the metal catalysts used are Rh, Pd, Pt, or
combinations thereof.
[0130] In particular, Rh plays a critical role in the TWC process
with respect to the conversion of nitrogen oxides (NO.sub.x) to
nitrogen and oxygen, and of hydrocarbons to carbon dioxide and
water. As emissions standards tighten (specifically NO.sub.x
emissions) the tendency is to use higher amounts of expensive Rh
metal oxides in catalysts in order to meet such standards. This is
especially problematic for catalytic converters used in engines
wherein NO.sub.x emissions (so-called engine-out emissions) are
particularly high. Further, in order for new vehicles to meet CARB
(California Air Resources Board) and EPA (Environmental Protection
Agency) tailpipe regulations, NO.sub.x conversion efficiencies must
generally be in excess of 95% and, in some cases, higher than
99%.
[0131] Accordingly, in one aspect, the present invention provides
catalyst systems comprising Rh and a Ln-ZrO.sub.2-based MMOSO. In
one embodiment, the washcoat of a catalyst system comprises Rh and
a Ln-ZrO.sub.2-based MMOSO. In another embodiment, the overcoat of
a catalyst system comprises Rh and a Ln-ZrO.sub.2-based MMOSO. In
yet another embodiment, the washcoat and overcoat of a catalyst
system comprises Rh and a Ln-ZrO.sub.2-based MMOSO. In some
embodiments, the catalyst systems of the present invention are
present in TWC catalysts. In other embodiments, the catalyst
systems of the present invention are present in catalytic
converters.
Oxygen Storage Materials
[0132] Catalytic converters may be exposed to exhaust that is
either rich (contains a high amount of unburnt fuel compared to
oxygen) or lean (contains a low amount of unburnt fuel compared to
oxygen). Oxygen storage materials (OSMs) supply oxygen to rich
exhaust and take up oxygen from lean exhaust, buffering the
catalyst systems against the fluctuating supply of oxygen and, in
turn, increasing catalyst efficiency. Thus, oxygen storage
materials present in, for example, TWC catalyst systems, allow the
conversion efficiency of the catalyst system to remain relatively
constant even in the face of varying inlet air/fuel ratios. OSMs
may be comprised of zirconia, lanthanides, alkaline earth metals,
transition metals, cerium oxide materials, or mixtures thereof. The
use of cerium oxide in catalytic converters is described in
"Critical Topics in Exhaust Gas Treatment" (Research Studies Press
Ltd, Baldock, Hertfordshire, England, 2000), which is incorporated
herein by reference in its entirety.
[0133] In some embodiments the OSM has a composition according to
the formula:
Ce.sub.1-a-b-c-dD.sub.aE.sub.bF.sub.cZr.sub.dO.sub.2,
wherein: [0134] a, b and c may be, independently, 0-0.7; [0135] d
may be 0-0.9; and [0136] D, E and F may be, independently, selected
from the group consisting of lanthanides, alkaline earth metals and
transition metals.
[0137] In some embodiments, D is Nd and a may be 0.01-0.20,
0.01-0.15, 0.01-0.10, 0.02-0.08, 0.03-0.07, or 0.04-0.06. In a
particular embodiment, D is Nd and a=0.05.
[0138] In some embodiments, E is Pr and b may be 0.01-0.20,
0.01-0.15, 0.01-0.10, 0.02-0.08, 0.03-0.07, or 0.04-0.06. In a
particular embodiment, E is Pr and b=0.05.
[0139] In some embodiments, c=0.
[0140] In some embodiments, d may be 0.2-0.8, 0.3-0.7, 0.4-0.7,
0.5-0.7, or 0.55-0.65. In a particular embodiment, d=0.6.
[0141] In a particular embodiment, the OSM of the present invention
has the following formula:
Ce.sub.0.3Nd.sub.0.05Pr.sub.0.05Zr.sub.0.6O.sub.2.
[0142] In some embodiments, the OSM constitutes about 1%, 2%, 3%,
4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%,
18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%,
31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%,
44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%,
57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%
70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%,
83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%,
96%, 97%, 98%, 99% or 100% of the total weight of the washcoat
and/or overcoat. In one embodiment, the OSM constitutes about
20-60% of the total weight of the washcoat and/or overcoat. In
another embodiment, the OSM constitutes about 30-50% of the total
weight of the washcoat and/or overcoat. In yet another embodiment,
the OSM constitutes about 39-41%, 38-42%, 37-43%, 36-44%, or 35-45%
of the total weight of the washcoat and/or overcoat. In one
embodiment, the OSM constitutes about 40%, 80% or 100% of the total
weight of the washcoat and/or overcoat. In one embodiment, the OSM
constitutes 40%, 80% or 100% of the total weight of the washcoat
and/or overcoat.
[0143] In some embodiments, the OSM constitutes up to about 1%, 2%,
3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%,
18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%,
31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%,
44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%,
57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%
70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%,
83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%,
96%, 97%, 98%, 99% or 100% of the total weight of the washcoat
and/or overcoat. In one embodiment, the OSM constitutes up to about
20-60% of the total weight of the washcoat and/or overcoat. In
another embodiment, the OSM constitutes up to about 30-50% of the
total weight of the washcoat and/or overcoat. In yet another
embodiment, the OSM constitutes up to about 39-41%, 38-42%, 37-43%,
36-44%, or 35-45% of the total weight of the washcoat and/or
overcoat. In one embodiment, the OSM constitutes up to about 40%,
80% or 100% of the total weight of the washcoat and/or overcoat. In
one embodiment, the OSM constitutes up to 40%, 80% or 100% of the
total weight of the washcoat and/or overcoat.
Support Oxides
[0144] Support oxides are, generally, porous solid oxides which are
used to provide a high surface area which aids in oxygen
distribution and exposure of catalysts to reactants such as
NO.sub.x, CO, and hydrocarbons. Support oxides are normally stable
at high temperatures as well as at a range of reducing and
oxidizing conditions. Metal catalysts present in the washcoat,
overcoat (if one is present), or both, are typically supported by
support oxides.
[0145] The amount of support oxide present in a catalyst system may
vary depending on where in the system the support oxide is present.
In some embodiments, the washcoat and overcoat (if one is present)
of a catalyst system may contain the same amount of support oxide.
In other embodiments, the washcoat and overcoat (if one is present)
of a catalyst system may contain different amounts of support
oxide.
[0146] Compounds used in traditional support oxides include, but
are not limited to, gamma-alumina, ceria-based powders, or any
mixture of titania, silica, alumina (transition and alpha-phase),
ceria, zirconia, Ce.sub.1-.alpha.Zr.sub..alpha.O.sub.2, and any
possible doped ceria formulations. A transition phase is a
meta-stable phase of alumina (beta, gamma, theta, delta) that
transforms to the stable alpha-alumina with sufficient time and
temperature. In a preferred embodiment, the support oxide is
alumina.
[0147] Modifiers may optionally be added to the alumina to retard
undesired phase transitions of the alumina from the gamma phase to
the alpha phase when the alumina is exposed to elevated
temperatures--i.e., to stabilize the alumina. Examples of suitable
modifiers (or thermal stabilizers) include, for example, rare earth
oxides, silicon oxides, oxides of Group IVB metals (zirconium,
hafnium, or titanium), alkaline earth oxides, or combinations
thereof. Alumina is typically utilized in the washcoat as a high
surface area carrier solid or support and is referred to as "gamma
alumina" or "activated alumina." Suitable alumina compositions
generally have a BET (Brunauer, Emmett and Teller) surface area of
60 m.sup.2/g or more and, often, about 200 m.sup.2/g or more.
[0148] Specific examples of suitable stabilizing agents include
lanthanide oxides (Ln.sub.2O.sub.3) and/or strontium oxide (SrO).
Such lanthanide- and strontium-based stabilizing agents are
typically added to support oxides (e.g., alumina) as a solution of
lanthanide nitrate, strontium nitrate, or mixtures thereof. Heating
or calcining the lanthanide nitrate and/or strontium nitrate then
forms the desired oxide. A particular example of a useful
stabilized alumina is La--Al.sub.2O.sub.3.
[0149] In one aspect, the present invention improves upon catalyst
systems by replacing an amount of the traditional alumina support
oxide with an amount of the Ln-ZrO.sub.2-based MMOSOs of the
present invention. In some embodiments, the Ln-ZrO.sub.2-based
MMOSOs of the present invention are utilized in the washcoat of the
catalyst systems. In other embodiments, the Ln-ZrO.sub.2-based
MMOSOs are utilized in the overcoat of the catalyst system, if an
overcoat is present. In yet other embodiments, the
Ln-ZrO.sub.2-based MMOSOs are in utilized both the washcoat and the
overcoat of the catalyst system, if an overcoat is present. In some
embodiments, the Ln-ZrO.sub.2-based MMOSOs is utilized in a layer
wherein Rh is present,
[0150] Any of the metal catalysts described above may be used in
combination with the Ln-ZrO.sub.2-based MMOSOs. As noted, the
support oxides of the present invention have a particularly
beneficial impact on Rh lifetime and efficiency. However, metal
catalysts such Rh, Pd, Pt, or combinations thereof, may be used in
catalyst systems comprising Ln-ZrO.sub.2-based MMOSOs.
[0151] The amount of traditional alumina support oxide in the
washcoat and/or overcoat replaced with an amount of
Ln-ZrO.sub.2-based MMOSO, such as La-ZrO.sub.2-based MMOSO or
Pr--ZrO.sub.2-based MMOSO, can vary. Thus, in one embodiment, about
1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%,
16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%,
29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%,
42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%,
55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%,
68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%,
81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%,
94%, 95%, 96%, 97%, 98%, 99% or 100% of the traditional alumina is
replaced with a corresponding amount of Ln-ZrO.sub.2-based MMOSO.
In other embodiments, about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%,
90% or 100% of the traditional alumina is replaced with a
corresponding amount of Ln-ZrO.sub.2-based MMOSO. In yet other
embodiments, about 10%, 50% or 100% of the traditional alumina is
replaced with a corresponding amount of Ln-ZrO.sub.2-based MMOSO.
In a particular embodiment, about 50% or about 100% of the
traditional alumina is replaced with a corresponding amount of
Ln-ZrO.sub.2-based MMOSO. In another particular embodiment, 50% or
100% of the traditional alumina is replaced with a corresponding
amount of Ln-ZrO.sub.2-based MMOSO.
[0152] In some embodiments, about 10-90%, 20-80%, 30-70%, 40-60%,
45-55%, 80-100%, 90-100%, or 95-100% of the traditional alumina is
replaced with a corresponding amount of Ln-ZrO.sub.2-based MMOSO.
In other embodiments, about 49-51%, 48-52%, 47-53%, 46-54%,
91-100%, 92-100%, 93-100%, 94-100%, 95-100%, 96-100%, 97-100%,
98-100%, or 99-100% of the traditional alumina is replaced with a
corresponding amount of Ln-ZrO.sub.2-based MMOSO.
[0153] In some embodiments, up to about 1%, 2%, 3%, 4%, 5%, 6%, 7%,
8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%,
22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%,
35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%,
48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%,
61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%,
74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%,
87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or
100% of the traditional alumina is replaced with a corresponding
amount of Ln-ZrO.sub.2-based MMOSO. In other embodiments, up to
about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100% of the
traditional alumina is replaced with a corresponding amount of
Ln-ZrO.sub.2-based MMOSO. In yet other embodiments, up to about
10%, 50% or 100% of the traditional alumina is replaced with a
corresponding amount of Ln-ZrO.sub.2-based MMOSO. In a particular
embodiment, up to about 50% or about 100% of the traditional
alumina is replaced with a corresponding amount of
Ln-ZrO.sub.2-based MMOSO. In another particular embodiment, up to
50% or 100% of the traditional alumina is replaced with a
corresponding amount of Ln-ZrO.sub.2-based MMOSO.
[0154] In some embodiments, up to about 10-90%, 20-80%, 30-70%,
40-60%, 45-55%, 80-100%, 90-100%, or 95-100% of the traditional
alumina is replaced with a corresponding amount of
Ln-ZrO.sub.2-based MMOSO. In other embodiments, up to about 49-51%,
48-52%, 47-53%, 46-54%, 91-100%, 92-100%, 93-100%, 94-100%,
95-100%, 96-100%, 97-100%, 98-100%, or 99-100% of the traditional
alumina is replaced with a corresponding amount of
Ln-ZrO.sub.2-based MMOSO.
[0155] The Ln-ZrO.sub.2-based MMOSO of the catalyst systems of the
present invention, such as La--ZrO.sub.2-based MMOSO or
Pr--ZrO.sub.2-based MMOSO, may constitute about 1%, 2%, 3%, 4%, 5%,
6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%,
20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%,
33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%,
46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%,
59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%,
72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%,
85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%,
98% or 99% of the washcoat, and/or the overcoat (if one is present)
by weight. In some embodiments, the Ln-ZrO.sub.2-based MMOSO
constitutes about 5-60%, 10-50%, 20-40%, 20-80%, 40-80%, or 50-70%
of the washcoat and/or the overcoat (if one is present), by weight.
In other embodiments, the Ln-ZrO.sub.2-based MMOSO constitutes
about 29-31%, 28-32%, 27-33%, 26-34%, 25-35%, 59-61%, 58-62%,
57-63%, 56-64% or 55-65% of the washcoat. In other embodiments, the
Ln-ZrO.sub.2-based MMOSO constitutes about 30% or 60% of the
washcoat. In one embodiment, the Ln-ZrO.sub.2-based MMOSO of
constitutes 30% or 60% of the washcoat.
[0156] The Ln-ZrO.sub.2-based MMOSO of the catalyst systems of the
present invention such as La--ZrO.sub.2-based MMOSO or
Pr--ZrO.sub.2-based MMOSO, may also constitute up to about 1%, 2%,
3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%,
18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%,
31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%,
44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%,
57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%,
70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%,
83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%,
96%, 97%, 98% or 99% of the washcoat, and/or the overcoat (if one
is present) by weight. In some embodiments, the Ln-ZrO.sub.2-based
MMOSO constitutes up to about 5-60%, 10-50%, 20-40%, 20-80%,
40-80%, or 50-70% of the washcoat and/or the overcoat (if one is
present), by weight. In other embodiments, the Ln-ZrO.sub.2-based
MMOSO constitutes up to about 29-31%, 28-32%, 27-33%, 26-34%,
25-35%, 59-61%, 58-62%, 57-63%, 56-64% or 55-65% of the washcoat.
In other embodiments, the Ln-ZrO.sub.2-based MMOSO constitutes up
to about 30% or 60% of the washcoat. In one embodiment, the
Ln-ZrO.sub.2-based MMOSO of constitutes up to 30% or 60% of the
washcoat.
Additives
[0157] The washcoats and overcoats of the catalyst systems of the
present invention may contain additives which aid in retarding the
poisoning of precious metal catalysts by phosphorus and sulfur.
Consumption of engine lubricants results in the generation of
phosphorus and, in turn, the poisoning and deactivation of precious
metal catalysts. Thus, additives such as calcium, barium,
lanthanides and/or cerium may be added to the washcoats and/or
overcoats (if present) as a means of retarding the poisoning
process. In some embodiments, the additive is CaCO.sub.3,
La.sub.2O.sub.3 or BaCO.sub.3. In a particular embodiment, the
additive is BaCO.sub.3. It is noted that, upon exposure to exhaust
containing, for example phosphorous, CaCO.sub.3, La.sub.2O.sub.3
and BaCO.sub.3 are converted to Ca.sub.3(PO.sub.4).sub.2,
LaPO.sub.4 and Ba.sub.3(PO.sub.4).sub.2, respectively.
[0158] Amounts of such additives suitable for use in catalyst
systems are well known in the art.
Catalyst Systems Comprising Lanthanide Doped Support Oxides
[0159] Accordingly, the washcoats and/or overcoats of the present
invention comprise, inter alia, by weight: a % of OSM; a % of
traditional support oxide, and a % of Ln-ZrO.sub.2-based MMOSO. In
particular embodiments, the catalyst systems of the present
invention comprise, by weight: a % of Ce-based OSM; a % of
Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3; and a % of Pr.sub.X
%Zr.sub.(1-X) %O.sub.2, as described herein. In some embodiments,
the catalyst systems of the present invention comprise, by weight:
40% of Ce-based OSM; 30% Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3;
and 30% of Pr.sub.X %Zr.sub.(1-X) %O.sub.2, as described herein. In
other embodiments, the catalyst systems of the present invention
comprise, by weight: 40% of Ce-based OSM; and 60% of Pr.sub.X
%Zr.sub.(1-X) %O.sub.2, as described herein. In a particular
embodiment, the catalyst systems of the present invention comprise:
40% (Ce.sub.0.3Nd.sub.0.05Pr.sub.0.05 Zr.sub.0.6O.sub.2); 30%
(La--Al.sub.2O.sub.3); and 30% (Pr.sub.X %Zr.sub.(1-X) %O.sub.2)
(i.e., 50% of the traditional alumina is replaced with Pr.sub.X
%Zr.sub.(1-X) %O.sub.2). In another particular embodiment, the
catalyst systems of the present invention comprise: 40%
(Ce.sub.0.3Ncl.sub.0.05Pr.sub.0.05 Zr.sub.0.6O.sub.2); and 60% of
Pr.sub.X %Zr.sub.(1-X) %O.sub.2 (i.e., 100% of the traditional
alumina is replaced with Pr.sub.X %Zr.sub.(1-X) %O.sub.2). As
discussed herein, in such embodiments, X can be, for example, 5%,
10% or 15%. In addition, as discussed herein, such OSM/support
oxides may be in the washcoat, overcoat, or both.
[0160] In other particular embodiments, the catalyst systems of the
present invention comprise, by weight: % of Ce-based OSM; a % of
Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3; and a % of La.sub.X
%Zr.sub.(1-X) %O.sub.2, as described herein. In some embodiments,
the catalyst systems of the present invention comprise, by weight:
40% of Ce-based OSM; 30% Al.sub.2O.sub.3 or La--Al.sub.2O.sub.3;
and 30% of La.sub.X %Zr.sub.(1-X) %O.sub.2, as described herein. In
other embodiments, the catalyst systems of the present invention
comprise, by weight: 40% of Ce-based OSM; and 60% of La.sub.X
%Zr.sub.(1-X) %O.sub.2, as described herein. In a particular
embodiment, the catalyst systems of the present invention comprise:
40% (Ce.sub.0.3Nd.sub.0.05 La.sub.0.05 Zr.sub.0.6O.sub.2); 30%
(La--Al.sub.2O.sub.3); and 30% (La.sub.X %Zr.sub.(1-X) %O.sub.2)
(i.e., 50% of the traditional alumina is replaced with La.sub.X
%Zr.sub.(1-X) %O.sub.2). In another particular embodiment, the
catalyst systems of the present invention comprise: 40%
(Ce.sub.0.3Nd.sub.0.05 La.sub.0.05 Zr.sub.0.6O.sub.2); and 60% of
La.sub.X %Zr.sub.(1-X) %O.sub.2 (i.e., 100% of the traditional
alumina is replaced with La.sub.X %Zr.sub.(1-X) %O.sub.2). As
discussed herein, in such embodiments, X can be, for example, 5%,
10% or 15%. In addition, as discussed herein, such OSM/support
oxides may be in the washcoat, overcoat, or both.
[0161] In a particular embodiment the catalyst system comprises a
washcoat and an overcoat as follows. The washcoat, loaded at 180
g/L, comprises La--Al.sub.2O.sub.3 and an OSM (30% CeO.sub.2, 60%
ZrO.sub.2, 5% Nd.sub.2O.sub.3 and 5% Pr.sub.6O.sub.11) in a ratio
of 1.5:1 (by weight). The washcoat is impregnated with Pd to give a
final Pd loading in the washcoat of about 139.3 g/ft.sup.3. Barium
is also impregnated into the washcoat to give a Pd:Ba ratio of 1:6.
The overcoat (loaded onto the washcoat at 60 g/L) comprises
(Pr.sub.0.10Zr.sub.0.90O.sub.2):(30% CeO.sub.2, 60% ZrO.sub.2, 5%
Nd.sub.2O.sub.3, 5% Pr.sub.6O.sub.11) in a ratio of 1.5:1 (by
weight) with a Rh loading of 10.71 g/ft.sup.3.
Methods of Making Catalysts
Methods of Making LnZrO.sub.2-Based Catalysts
[0162] In yet another aspect, the present invention relates to
methods of making the Ln-ZrO.sub.2-based MMOSOs disclosed
herein.
[0163] Washcoat and overcoats comprising Ln-ZrO.sub.2-based MMOSOs
can generally be made using the techniques exemplified in Example
2.
[0164] In addition, catalyst systems comprising Ln-ZrO.sub.2-based
MMOSOs, such as Pr--ZrO.sub.2-based MMOSOs or La--ZrO.sub.2-based
MMOSOs, can be made as follows. For example, a catalyst system
which comprises a substrate and washcoat can be generated by: a)
depositing a washcoat comprising a Ln-ZrO.sub.2-based MMOSO, an OSM
and a metal catalyst on a substrate; and b) treating the washcoat
and substrate by drying and calcination. Alternatively, a catalyst
system which comprises a substrate and washcoat can be generated
by: a) depositing a washcoat comprising a Ln-ZrO.sub.2-based MMOSO
and an OSM on a substrate; b) treating the washcoat and substrate
by calcination; and c) impregnating a metal catalyst into the
washcoat, followed by drying and calcination.
[0165] In some embodiments, the catalyst system comprises a
substrate, a washcoat and an overcoat. Such catalyst systems can be
generated by: a) depositing a washcoat comprising a
Ln-ZrO.sub.2-based MMOSO, an OSM and a metal catalyst on a
substrate; b) treating the washcoat and substrate by calcination;
c) depositing an overcoat onto the washcoat, wherein the overcoat
comprises a support oxide, an OSM and a metal catalyst, followed by
drying and calcination. Alternatively, the catalyst system can be
generated by: a) depositing a washcoat comprising a
Ln-ZrO.sub.2-based MMOSO and an OSM on a substrate; b) treating the
washcoat and substrate by calcination; c) impregnating a metal
catalyst into the washcoat; d) depositing an overcoat onto the
washcoat, wherein the overcoat comprises a support oxide, an OSM
and a metal catalyst; and e) impregnating a metal catalyst into the
overcoat, followed by drying and calcination. The same methods are
suitable for generating catalyst systems wherein the washcoat
comprises a support oxide, an OSM and a catalyst and the overcoat
comprises a Ln-ZrO.sub.2-based MMOSO, an OSM and a catalyst. In
addition, the same methods can be used to generate catalyst systems
wherein both the washcoat and overcoat comprise a
Ln-ZrO.sub.2-based MMOSO, an OSM and a catalyst.
[0166] In embodiments wherein the metal catalyst is mixed with the
washcoat and/or overcoat (typically present as a slurry) or
impregnated into an overcoat and/or washcoat, the metal catalyst
may be added in the form of a nitrate, acetate or chloride salt. In
embodiments wherein the metal catalyst is impregnated into an
overcoat and/or washcoat, the metal catalyst may be impregnated as
an aqueous solution.
Methods of Utilizing Catalysts of the Present Invention
[0167] Catalyst systems comprising the Ln-ZrO.sub.2-based MMOSOs of
the present invention, such as Pr--ZrO.sub.2-based MMOSO or
La--ZrO.sub.2-based MMOSO, are useful for a variety of purposes. As
discussed herein, the Ln-ZrO.sub.2-based MMOSOs may be used in
catalytic converter systems present in, for example,
automobiles.
[0168] In some embodiments, catalyst systems comprising
Ln-ZrO.sub.2-based MMOSOs, such as Pr--ZrO.sub.2-based MMOSO or
La--ZrO.sub.2-based MMOSO, are used to reduce toxic exhaust gas
emissions. Accordingly, the present invention envisions a method of
reducing toxic gas emissions comprising contacting the gas
emissions with catalyst systems comprising Ln-ZrO.sub.2-based
MMOSOs. The present invention also refers to a method of reducing
toxic gas emissions by utilizing catalyst systems comprising
Ln-ZrO.sub.2-based MMOSOs.
[0169] As discussed herein, catalyst systems comprising
Ln-ZrO.sub.2-based MMOSOs, such as Pr--ZrO.sub.2-based MMOSO or
La--ZrO.sub.2-based MMOSO, exhibit increased oxygen flow when
compared to catalyst systems comprising traditional support oxides.
Thus, the present invention also refers to a method of increasing
oxygen flow through a catalytic system by stabilizing the phase of
the support oxide present in the system, preferably stabilizing the
tetragonal phase. The tetragonal phase can be stabilized by using
Ln-ZrO.sub.2-based MMOSOs in catalyst systems.
[0170] Catalyst systems comprising Ln-ZrO.sub.2-based MMOSOs, such
as Pr--ZrO.sub.2-based MMOSO or La--ZrO.sub.2-based MMOSO, also
exhibit improved catalyst lifetime--particularly Rh lifetime. Thus,
the present invention contemplates methods of improving the
lifetime of a catalyst system comprising a metal catalyst such as
Rh by utilizing a washcoat, overcoat, or both comprising a
Ln-ZrO.sub.2-based MMOSO in a catalyst system. Such methods
include: a) reducing the amount of metal catalyst deactivated
during the aging of the catalyst system; b) increasing the amount
of metallic catalyst (e.g., Rh(0)) initially present in the
catalyst system; or c) both a) and b). In addition, such methods
include: a) increasing the amount of Rh(0) and/or Rh(III) as
Rh.sub.2O.sub.3 initially present in the catalyst system; or b)
decreasing the amount of Rh(0) which is converted to Rh(III) as
Rh.sub.2O.sub.3 or Rh(III)-MMO during aging of the catalyst
system.
[0171] Catalyst systems comprising Ln-ZrO.sub.2-based MMOSOs, such
as Pr--ZrO.sub.2-based MMOSO or La--ZrO.sub.2-based MMOSO, can also
be used in methods of improving TWC. For example, the present
invention includes methods of improving TWC of gas emissions
comprising contacting the gas emissions with catalyst systems
comprising Ln-ZrO.sub.2-based MMOSOs. The present invention also
refers to methods of improving TWC of gas emissions by utilizing
catalyst systems comprising Ln-ZrO.sub.2-based MMOSOs.
[0172] These and other embodiments of the invention may be further
illustrated in the following non-limiting Examples.
EXAMPLES
Example 1
Generation of Washcoats and Overcoats Comprising Traditional
Support Oxides
[0173] The following is a representative protocol for the
production of washcoats and overcoats comprising traditional
support oxides. Such washcoats and overcoats can be used in
combination with washcoats and overcoats comprising the
Ln-ZrO.sub.2-based MMOSOs of the present invention. The protocols
in this Example represent standard techniques known in the art
(see, for example, U.S. Pat. No. 7,641,875).
[0174] Traditional washcoats were generated as follows. A slurry
comprising the OSM, alumina powder and lanthanide nitrate solution
(commercially available as lanthanum nitrate product code 5248 from
Molycorp, Inc., Mountain Pass, Calif.) in deionized water was
generated. The slurry was then milled in a Szegvari Type IS
Atrittor until the rheology was suitable for coating the support. A
cordierite honeycomb support was dipped into the slurry. Excess
slurry was blown from the support with an air jet. The support was
dried in flowing air at room temperature, was heat-treated in air
at about 150.degree. C., and was calcined at 750.degree. C. for 4
hours to yield a MPC composition.
[0175] Traditional overcoats were generated using the process
described in Example 2, except that La--Al.sub.2O.sub.3 was used
instead of 10% Pr--ZrO.sub.2-based MMOSO.
Example 2
Generation of Overcoats Comprising Doped ZrO.sub.2-Based MMOSOs
[0176] A 10% Pr--ZrO.sub.2-based MMOSO overcoat was generated using
the following procedure. This procedure can generally be used to
generate overcoats containing Ln-ZrO.sub.2-based MMOSOs.
[0177] A 10% Pr--ZrO.sub.2-based
MMOSO/Ce.sub.0.3Zr.sub.0.6Nd.sub.0.05Y.sub.0.05O.sub.2 (OSM) (1.5:1
ratio) overcoat slurry containing 38% solid (by weight) was
generated as follows. The appropriate amount of 10% Pr--ZrO.sub.2,
Ce.sub.0.3Zr.sub.0.6Nd.sub.0.05Y.sub.0.05O.sub.2, and de-ionized
water were weighed out in separate containers. The 10%
Pr--ZrO.sub.2 and Ce.sub.0.3Zr.sub.0.6Nd.sub.0.05Y.sub.0.05O.sub.2
were weighed out in a 1.5:1 ratio. Acetic acid (0.5% relative to
the solids) was weighed and then added to the de-ionized water
container. The above reactants were then combined into an
attrition-mill as follows: 1) 75-80% of the de-ionized water and
acetic acid solution was added; 2) the 10% Pr--ZrO.sub.2 and the
Ce.sub.0.3Zr.sub.0.6Nd.sub.0.05Y.sub.0.05O.sub.2 were added; 3) the
remaining de-ionized water and acetic acid solution was added. The
resulting slurry was then milled until homogenous. Once the 10%
Pr--ZrO.sub.2-based MMOSO/OSM slurry particle size reached d(50)
5.+-.0.5 (4.5 target) micrometers, the milled slurry was dropped
into a container and the final pH and % solids were recorded.
[0178] The resulting 10% Pr--ZrO.sub.2-based
MMOSO/Ce.sub.0.3Zr.sub.0.6Nd.sub.0.05Y.sub.0.05O.sub.2 milled
slurry was then metalized with Rh as follows. The milled slurry was
mixed with a high shear mixer. The solid (%) content of the milled
slurry was then measured in a moisture balance and the initial pH
recorded. The appropriate amounts of 10% Pr--ZrO.sub.2-based
MMOSO/Ce.sub.0.3Zr.sub.0.6Nd.sub.0.05Y.sub.0.05O.sub.2 milled
slurry, Rh(NO.sub.3).sub.3 solution, and de-ionized water were then
measured in separate containers. To generate a concentration of 20
g/ft.sup.3 Rh in the overcoat, an Rh slurry concentration of 1.177%
(by weight) was required. Using the high sheer mixer, the
Rh(NO.sub.3).sub.3 solution was added to the 10%
Pr--ZrO.sub.2-based MMOSO/OSM milled slurry and mixed until
homogenous. The pH of the resulting slurry was then recorded. The
resulting slurry was adjusted to a pH of 6.4 using ammonium
hydroxide. The previously weighed de-ionized water was then added
to the metalized slurry and the pH was confirmed to remain at a pH
of 6.4 (note that, if the pH was not at 6.4, it would have been
adjusted to pH 6.4). The final viscosity range was 75-200 cp@60 rpm
(generally, the target viscosity is 120 cp@60 rpm). The slurry was
again mixed until homogenous and the final pH and % solids were
recorded. The metalized slurry was then used to coat the
appropriate substrate.
[0179] A schematic representation of the process is presented as
FIG. 15.
Example 3
Evaluation of the Effect of Doped ZrO-Based MMOSOs on NO Light-Off
Temperature
[0180] The performance of the TWC catalyst systems containing
Ln-ZrO.sub.2-based MMOSOs was evaluated by testing the catalysts on
a synthetic gas flow reactor that closely simulates the engine-out
gas chemistry and conditions (e.g., temperature) found in
vehicles.
[0181] The catalyst systems containing 0.25% Rh (by weight) at a
coating of oxide at 125 g/L were tested (see Table 1) on a washcoat
comprising an OSM made by either the IWCP or HTP process. A
representative procedure for making an OSM by the IWCP process is
described in Example 10. A representative procedure for making an
OSM by the HTP process is described in Example 11.
TABLE-US-00001 TABLE 1 Catalyst System (oxide used) Process
NO.sub.x T50 (.degree. C.) a) 10% La--Al.sub.2O.sub.3 HTP 284.8 b)
Nd.sub.0.05Pr.sub.0.05Ce.sub.0.30Zr.sub.0.60O.sub.2 HTP 301.8 IWCP
292.4 c) La.sub.0.10Zr.sub.0.90O.sub.2 HTP 273.8 d)
Pr.sub.0.10Zr.sub.0.90O.sub.2 HTP 274.7 IWCP 262.2
[0182] The catalyst formulations were aged by exposure to slightly
rich exhaust for 20 hours at 1000.degree. C. and a space velocity
of 12,000 hr.sup.-1. Temperature: 1000.degree. C.; Duration: 20
hours. The effects of the Ln-ZrO.sub.2-based MMOSOs on Rh catalytic
performance in the TWC process were then evaluated. Table 1
demonstrates the effect that varied Ln-ZrO.sub.2-based MMOSOs had
on light-off temperatures after aging.
[0183] As can be seen from Table 1, Ln-doped ZrO.sub.2 support
oxides showed a significant decrease in light-off temperature
compared to the traditional alumina-based support oxide.
Example 4
Evaluation of the Effect on NO.sub.x and Hydrocarbon Conversion of
Ln-ZrO.sub.2-Based MMOSOs
[0184] The performance of TWC catalysts containing
Ln-ZrO.sub.2-based MMOSOs was also evaluated through vehicle
testing.
[0185] Catalyst coatings were synthesized on conventional
cordierite substrates and assembled into a system using a
close-coupled catalyst (CC) and an underfloor (UF) catalyst. Each
catalyst system had a one-liter volume capacity. The substrate
contained 400 cells/in.sup.2 and a wall thickness of 3.5 mm
[0186] Each test system utilized the same washcoat containing Pd
(concentration 100 g/ft.sup.3 for both the CC and UF catalyst) and
La--Al.sub.2O.sub.3 as a support oxide.
[0187] In addition, each test system utilized Rh in the overcoat
(concentration 20 g/ft.sup.3 for the CC catalyst and 8.3 g/ft.sup.3
for the UF catalyst). However, the support oxide of the overcoat
was varied to enable a direct comparison of the effects of the
support oxide on Rh function in the catalyst systems. Testing
focused on performance after accelerated engine aging cycles that
have been established to simulate in-use vehicle aging
approximating 150,000 miles of driving. The aging cycle comprised
exposing the catalyst systems to 950.degree. C. for 200 hrs. Two
different controlled drive cycles were used to measure the tailpipe
emissions: a) the Federal Test Procedure (see Table 2); and b) the
high-speed cycle known as the US06 (see Table 3).
TABLE-US-00002 TABLE 2 Federal Test Procedure TWC Vehicle
Performance With Three Overcoat Compositions. Federal Test
Procedure Tailpipe Emissions Non-Methane Overcoat Composition
NO.sub.x (g/mile) HC (g/mile) Standard 40% OSM/60%
La--Al.sub.2O.sub.3 0.010 0.0067 Doping 40% OSM/30%
La--Al.sub.2O.sub.3/ 0.0085 0.0065 30%
Pr.sub.0.10Zr.sub.0.90O.sub.2 40% OSM/ 0.0088 0.0066 60%
Pr.sub.0.10Zr.sub.0.90O.sub.2
TABLE-US-00003 TABLE 3 US06 TWC vehicle Performance With Three
Overcoat Compositions US06 Tailpipe Emissions Non-Methane Overcoat
Composition NO.sub.x (g/mile) HC (g/mile) Standard 40% OSM/60%
Al.sub.2O.sub.3 0.018 0.042 Doping 40% OSM/30% Al.sub.2O.sub.3/
0.019 0.038 30% Pr.sub.0.10Zr.sub.0.90O.sub.2 40% OSM/ 0.011 0.034
60% Pr.sub.0.10Zr.sub.0.90O.sub.2
[0188] The data from Table 2 shows the significant reduction in
NO.sub.x emissions with catalyst systems comprising
Pr.sub.0.10Zr.sub.0.90O.sub.2 support oxides. Table 3 shows that
the decrease of NO.sub.x emissions is even more significant in the
case of the high-speed, high-temperature US06 cycle for the full
substitution case. Table 3 also shows a significant reduction in
hydrocarbon emissions with catalyst systems comprising
Pr.sub.0.10Zr.sub.0.90O.sub.2 support oxides.
Example 5
Evaluation of the Effect on Rh State of Ln-ZrO.sub.2-Based MMOSOs
Using X-Ray Photoelectron Spectroscopy
[0189] As discussed herein, the ability of the Rh catalyst to
participate in the catalytic cycle depends on the state of
Rh--i.e., Rh(0) vs Rh(III) as Rh.sub.2O.sub.3 vs Rh(III)-MMO. X-ray
Photoelectron Spectroscopy (XPS) was used to assess the relative
proportion of these three Rh states in fresh (as-made) and aged TWC
catalyst systems comprising different Ln-ZrO.sub.2-based MMOSOs.
The XPS technique measures changes in binding energy of the Rh 3d
electrons. Changes in oxidation state of Rh have a significant
effect on binding energy--thus, shifts in binding energy can be
assigned to changes in Rh oxidation state. The chemical environment
of the Rh can also have a major impact on binding energy. For
example, a higher binding energy is indicative of a Rh interaction
with the support oxide--i.e., it is an indication that Rh(0) has
interacted with the support oxide so as to form either Rh(III) as
Rh.sub.2O.sub.3 or Rh(III)-MMO.
[0190] Detailed XPS scans were conducted on a Kratos Axis Ultra XPS
system with an Al(mono) X-Ray source with the following
characteristics: @270 W; Pass energy=20 eV; Step: 0.05 eV; Dwell
time: 0.3 Second; Sweep: 5 times; Binding energy: 318-298 eV;
Charge neutralizer: On.
[0191] Supported Rh systems were calibrated using the corresponding
support and carbon tape (e.g. Zr 2p, O is and C 1s). A Rh metal
film was used as a reference to verify the calibration (Rh Foil
(0.25 mm thickness, 99.9%, Aldrich) and a Rh.sub.2O.sub.3 oxide was
used on the XPS system for the oxide reference (Rh.sub.2O.sub.3;
Powder, 99.8%, Aldrich).
[0192] The standard catalyst system tested was 5% Rh (by weight) on
10% La--Al.sub.2O.sub.3. The Rh levels present in the standard
catalysts were compared to catalyst systems comprising 5% Rh (by
weight) on Pr.sub.0.10Zr.sub.0.90O.sub.2MMOSOs.
[0193] The test samples were evaluated as follows: [0194] 1. After
a 2% H.sub.2 treatment for 1 hour at 150.degree. C., all samples
were cooled to room temperature in the 2% H.sub.2 gas flow. The
samples where then immediately transferred into a glove bag with
10% H.sub.2--Ar. [0195] 2. In the glove bag filled with 10%
H.sub.2--Ar, the samples were sealed in a gas-tight XPS sample
holder. [0196] 3. The XPS sample holder was transferred into the
preparation chamber of the analyzer and held until the pressure was
lower than 1.0.times.10.sup.-6 torr before transfer to the XPS
chamber for measurement.
TABLE-US-00004 [0196] TABLE 4 XPS Data Showing Relative Proportions
of Rh Forms Present (Based on Oxidation States) in
La--Al.sub.2O.sub.3 and Pr--ZrO.sub.2 MMOSO Compositions % Rh(III)
(Rh-M-Ox % Rh(III) mixed Catalyst system % Rh(0) (Rh.sub.2O.sub.3)
oxide) Standard Fresh Rh/La--Al.sub.2O.sub.3 52 43 5 Aged
Rh/La--Al.sub.2O.sub.3 42 7 42 Doping Fresh
Rh/Pr.sub.0.10Zr.sub.0.90O.sub.2 78 22 0 MMOSO Aged
Rh/Pr.sub.0.10Zr.sub.0.90O.sub.2 68 32 0 MMOSO
[0197] The relative proportions of Rh forms present in the catalyst
systems were measured: a) after freshly preparing the catalysts;
and b) after 20 hours of aging at 900.degree. C. The data from
Table 4 demonstrates several benefits of Rh/Pr--ZrO.sub.2-based
MMOSO catalysts when compared to their Rh/La--Al.sub.2O.sub.3
counterparts. First, TWC catalyst systems comprising
Rh/Pr--ZrO.sub.2-based MMOSOs contain higher amounts of Rh(0) when
compared to TWC catalyst systems comprising La--Al.sub.2O.sub.3.
Specifically, when compared to catalysts containing
Rh/La--Al.sub.2O.sub.3, Rh/Pr--ZrO.sub.2-based MMOSO catalysts
contain higher amounts of Rh(0) when they are initially formed, and
these catalysts are better able to maintain Rh(0) during the aging
process. In addition, when TWC catalyst systems comprising
Rh/Pr--ZrO.sub.2-based MMOSOs are aged, they are able to retain
essentially all the Rh as either Rh(0) or Rh(III) as reversible
Rh.sub.2O.sub.3. Interestingly, not only is the vast majority of Rh
present in the Rh/Pr--ZrO.sub.2-based MMOSO compositions maintained
as Rh(0) (Rh(0):Rh(III) as Rh.sub.2O.sub.3=68:32), but the relative
population of Rh(0):Rh(III) as Rh.sub.2O.sub.3 is only moderately
impacted by the aging process. However, when catalysts comprising
Rh/La--Al.sub.2O.sub.3 are exposed to the same aging process, both
the Rh(0) and Rh(III) as reversible Rh.sub.2O.sub.3 initially
present in the composition are converted to the inactive and
irreversibly oxidized Rh(III)-MMO state in significant quantities.
Note that none of the inactive and irreversible Rh(III)-MMO state
was observed in either the fresh or aged Rh/Pr--ZrO.sub.2-based
MMOSO catalysts.
Example 6
Evaluation of the Effect on Catalyst Efficiency of Pr Doping of
ZrO.sub.2-Based MMOSOs
[0198] The effect of the amount of Pr doping of the ZrO.sub.2-based
MMOSO on catalyst efficiency was evaluated. In each of these
experiments, the same catalyst was used (single layer of 13
g/ft.sup.3 Rh) while the Pr content of the Pr--ZrO.sub.2-based
MMOSO was varied. Specifically, the effect of Pr doping on the T90
temperature was evaluated. The T90 temperature is the temperature
at which the catalyst is capable of converting 90% of the passing
exhaust compound (e.g., NO.sub.x or hydrocarbon) after the catalyst
has been aged at 1000.degree. C. for 10 hours.
TABLE-US-00005 TABLE 5 Efficiency of Catalyst systems Comprising
Pr--ZrO.sub.2-based MMOSOs Catalyst system NO T90 HC T90 Standard
Support oxide: 40% OSM/ 375.0 401.5 60% Al.sub.2O.sub.3 40% OSM/60%
X = 0.05 367.2 387.8 Pr.sub.xZr.sub.1-xO.sub.2 X = 0.10 360.1 385.1
X = 0.15 357.1 388.1
[0199] The data from Table 5 show that catalyst systems comprising
Pr--ZrO.sub.2-based MMOSO exhibit lower light-off temperatures with
respect to NO and hydrocarbon conversion. Thus, catalyst systems
comprising Pr--ZrO.sub.2-based MMOSOs are able to operate
efficiently at lower temperatures when compared to the relative to
a reference alumina catalyst.
Example 7
Evaluation of Amount of Reducible Rh in Ln-ZrO.sub.2-Based MMOSOs
Using Hydrogen Temperature-Programmed Reduction and Hydrogen
Chemisorption
[0200] Hydrogen Temperature-Programmed Reduction (H.sub.2-TRP) and
Hydrogen Chemisorption were used to evaluate the amount of
reducible Rh (i.e., R(0) or Rh(III) as Rh.sub.2O.sub.3) present in
Ln-ZrO.sub.2-based MMOSOs.
[0201] Samples were tested as follows using an AutoChem II
2920.
[0202] H.sub.2-TPR Portion of Testing [0203] 1) Samples were
exposed to a carrier gas (i.e., an atmosphere) of 20% O.sub.2--Ar.
The temperature was raised at a rate of 20.degree. C./min to a
final temperature of 300.degree. C. The temperature was maintained
at 300.degree. C. for 1 hour. [0204] 2) Samples were cooled to
40.degree. C. in 20% O.sub.2--Ar. [0205] 3) The carrier gas was
changed to 100% Ar and the sample was maintained until the thermal
conductivity detector (TCD) signal stabilized. [0206] 4) The
samples were cooled to -50.degree. C. in Ar. The temperature was
maintained for 5 minutes. [0207] 5) The carrier gas was change to
10% H.sub.2--Ar and maintained at -50.degree. C. for 15 minutes.
[0208] 6) TPR was carried out in 10% H.sub.2--Ar, wherein the
temperature was increased from -50.degree. C. to 550.degree. C. at
a rate of 20.degree. C./min. The temperature was then maintained at
550.degree. C. for 30 minutes.
[0209] H.sub.2-chemisorption Portion of Testing [0210] 7) Post TPR,
the carrier gas was changed to Ar and the temperature was
maintained at 550.degree. C. for 30 minutes. [0211] 8) The
temperature was then reduced to 40.degree. C. (with Ar as the
carrier gas). [0212] 9) The temperature was further cooled to
-70.degree. C. in Ar and maintained until the TCD signal
stabilized. [0213] 10) Chemisorption was carried out wherein 10%
H.sub.2--Ar Pulses were administered until saturation at
-70.degree. C. in Ar was observed.
[0214] The TPO protocol consisted of exposing the catalyst to
oxygen at the stated temperature. The XHFC protocol consisted of
high-temperature aging at the stated temperature in a fuel-cut gas
chemistry (56 seconds stoichiometric, 4 s A/F=20) at a space
velocity of 100,000 hr.sup.-1.
[0215] The results of the testing are listed in Table 6.
TABLE-US-00006 TABLE 6 Rh Properties After H.sub.2-chemisorption
Reducible Rh Avg. Rh Surface Area Rh Dispersion Particle Size*
Sample Type of Testing (m.sup.2/g Rh) (H/Rh, %) (nm) 0.6%
Rh/ZrO.sub.2 550.degree. C. TPO 182 41.3% 2.66 900.degree. C. TPO
104 23.7% 4.64 1000.degree. C. XHFC 1.20 0.27% 403 (20 hours) 0.6%
Rh/ 550.degree. C. TPO 204 46.4% 2.37 Pr.sub.0.5Zr.sub.0.95O.sub.2
MMOSO 900.degree. C. TPO 122 27.7% 3.96 1000.degree. C. XHFC 6.98
1.59% 69.3 (20 hours) 0.6% Rh/ 550.degree. C. TPO 242 55.0% 2.00
Pr.sub.0.10Zr.sub.0.90O.sub.2 MMOSO 900.degree. C. TPO 130 29.4%
3.73 1000.degree. C. XHFC 12.6 2.87% 38.3 (20 hours) 0.6% Rh/
550.degree. C. TPO 236 53.5% 2.05 Pr.sub.0.15Zr.sub.0.85O.sub.2
MMOSO 900.degree. C. TPO 110 25.1% 4.38 1000.degree. C. XHFC 13.0
2.95% 37.3 (20 hours) *Calculated based on complete Rh
reduction
[0216] The results of the above testing are illustrated in FIGS.
10-14. As can be seen by the data in those Figures, on the whole,
catalyst systems comprising Pr.sub.0.10Zr.sub.0.90O.sub.2 MMOSO
(10% Pr) as an Rh support exhibited the highest total H.sub.2
absorption capacity. This suggests that such systems exhibit good
oxygen storage capacity and good surface-redox active properties.
In addition, increasing the Pr-content increased stability against
Rh sintering. Moreover, the Pr.sub.0.10Zr.sub.0.90O.sub.2 MMOSO
(10% Pr) samples contained nearly twice the amount of H.sub.2
accessible Rh surface after XHFC aging compared to the
Pr.sub.0.10Zr.sub.0.90O.sub.2 MMOSO (5% Pr) samples.
Example 8
Effect of Pr do s in of ZrO.sub.2-Based MMOSOs on Catalyst
Structure
[0217] The effect of the amount of Pr doping of the ZrO.sub.2-based
MMOSO on the overall structure of the washcoats or overcoats in
which they are located was also tested. X-ray diffraction (XRD) was
used to determine the amount of tetragonal vs monoclinic phase
present in catalyst systems comprising various amounts of Pr doped
onto ZrO.sub.2-based MMOSO. The results are displayed in FIGS. 5
and 6.
[0218] XRD data was recorded on a Rigaku Mini Flex with
accelerating voltage=30 kV; electron beam current=15 mA; dwell
time=1.2 seconds; scan increment=0.02.degree. 2.THETA.; with
diffractometer optics: nickel filter on detector; K.sub..alpha.2
striping; and a scan range=10-70.degree. 2.THETA..
[0219] Increasing the amount of Pr doping above 5% lead to an
increase in the stability of the tetragonal phase. This phenomenon
is beneficial because the tetragonal phase of ZrO.sub.2 is arranged
in a manner which permits faster and easier diffusion of oxygen
(i.e., oxygen motility) through the catalyst structure enhancing
the activity of the catalyst. In addition, Pr present in the
Ln-ZrO.sub.2-based MMOSO is present as solid solution and, thus,
the MMOSO is composed of a single homogenous phase. This also
allows for faster and easier diffusion of oxygen through the
catalyst structure (see FIGS. 7 and 8).
[0220] When considering phase stability and H.sub.2 absorption
capacity, 10% Pr doping yields optimal catalyst properties. As
discussed above, increasing doping above 10% means that a large
amount of larger Pr.sup.3+ cations (when compared to the Zr.sup.4+
cations) are present in the catalyst system. The Pr.sup.3+ cations
may act as steric barriers to oxygen diffusion as the amount of Pr
increases. This phenomenon is illustrated in FIG. 7.
Example 9
Effect of Pr Doping of ZrO.sub.2-Based MMOSOs on Oxygen Storage
Capacity
[0221] The effect of the amount of Pr doping of the ZrO.sub.2-based
MMOSO on the oxygen storage capacity (OSC) of washcoats containing
Pr--ZrO.sub.2-based MMOSOs was evaluated. OSC is typically measured
by exposing a sample to either lean or rich air/fuel mixtures. In
such environments, the sample must either absorb O.sub.2 from the
exhaust stream (e.g., in lean air/fuel mixture environments) or
release O.sub.2 (e.g., in rich air/fuel mixture environments) in
order to maintain efficient catalysis of exhaust compounds. The
amount of time for which a sample can buffer the lean/rich air/fuel
mixture is one way to quantify the OSC of a sample. This time is
usually referred to as the "delay time"--i.e., the amount of time
that it takes for a perturbation in the air/fuel mixture to
manifest itself as a change in O.sub.2 levels within the catalyst
environment. The delay time can also be measured by the amount of
time that it takes for a perturbation in the air/fuel mixture to
manifest itself as a change in CO levels within the catalyst
environment. Thus, the longer the delay time, the better the OSC of
a sample.
[0222] As can be seen by the data in Table 7, increasing the amount
of Pr present in the MMOSOs has a positive effect on the OSC of the
washcoat. The washcoat comprised: 120 g/L, 9.6M Ba Impregnation (12
g/ft.sup.3 Pd); OSM:support oxide (1:1.5) (12.7 g/ft.sup.3 Rh).
TABLE-US-00007 TABLE 7 OSC of Pr--ZrO.sub.2-based MMOSOs O.sub.2
delay time at CO delay time at Support Oxide in Washcoat
575.degree. C. (seconds) 575.degree. C. (seconds) ZrO.sub.2 16.52
9.98 5% Pr--ZrO.sub.2 15.52 7.92 10% Pr--ZrO.sub.2 20.02 12.17 15%
Pr--ZrO.sub.2 20.29 13.47
Example 10
Generation of OSM Using the Improved Wet Chemical Process
(IWCP)
[0223] In a representative IWCP procedure, Pd(NO.sub.3).sub.2 was
added to an aqueous slurry of milled OSM (30% CeO.sub.2, 60%
ZrO.sub.2, 5% Nd.sub.2O.sub.3 and 5% Pr.sub.6O.sub.11).
Tetraethylammonium hydroxide was then added to generate the
IWCP-OSM slurry.
[0224] Separately, La--Al.sub.2O.sub.3 was milled with acetic acid
at a pH of .about.6.0. BaCO.sub.3 was then added to the milled
La--Al.sub.2O.sub.3 and stirred for approximately 5 minutes. The
La--Al.sub.2O.sub.3/BaCO.sub.3 mixture was then added to IWCP-OSM
slurry and the resulting composition was coated on to the washcoat
which was calcined to generate the Pd-OSM IWCP containing catalyst
composition (see, FIG. 16).
Example 11
Generation of OSM Using the High Temperature Process (HTP)
[0225] Oxygen storage materials generated using the High
Temperature Process (HTP) contain a metal catalyst (e.g., Pd) in a
solid solution with the OSM (in this case a Ce-containing mixed
metal oxide). Thus, OSMs generated using the HTP contain a metal
catalyst which is evenly dispersed throughout the OSM and the
surface of the OSM.
[0226] The HTP entails first mixing a Pd chemical precursor and an
oxide OSM, and then spraying the mixture into a hot furnace.
Typically, the temperature of the furnace is between 300.degree. C.
and 500.degree. C. and the temperature of the hot zone of the
furnace is greater than 500.degree. C. In a representative
experiment, the HTP OSM was generated by 1) co-milling (30%
CeO.sub.2, 60% ZrO.sub.2, 5% Nd.sub.2O.sub.3 and 5%
Pr.sub.6O.sub.11) and a Pd(NO.sub.3).sub.2; and 2) spraying the
resulting mixture of into a furnace.
* * * * *