U.S. patent application number 14/538321 was filed with the patent office on 2015-03-05 for zeiolite catalyst containing metal.
The applicant listed for this patent is Johnson Matthey Public Limited Company. Invention is credited to Todd Howard BALLINGER, Philip Gerald BLAKEMAN, Guy Richard CHANDLER, Hai-Ying CHEN, Julian Peter COX, Joseph Michael FEDEYKO, Alexander Nicholas Michael GREEN, Paul Richard PHILLIPS, Stuart David REID, Erich Conlan WEIGERT, James Alexander WYLIE.
Application Number | 20150064074 14/538321 |
Document ID | / |
Family ID | 45446178 |
Filed Date | 2015-03-05 |
United States Patent
Application |
20150064074 |
Kind Code |
A1 |
BALLINGER; Todd Howard ; et
al. |
March 5, 2015 |
ZEIOLITE CATALYST CONTAINING METAL
Abstract
A catalyst composition is provided wherein the composition
includes a zeolite having a non-phosphorous CHA crystal structure
and having a mean crystalline size of about 1 to about 5 microns;
and at least one non-aluminum base metal present in an amount
sufficient to achieve a NOx conversion of at least about 65% at a
temperature of at least 450.degree. C.
Inventors: |
BALLINGER; Todd Howard;
(Downingtown, PA) ; BLAKEMAN; Philip Gerald;
(Shanghai, CN) ; CHANDLER; Guy Richard;
(Cambridge, GB) ; CHEN; Hai-Ying; (Conshohocken,
PA) ; COX; Julian Peter; (Malvern, PA) ;
FEDEYKO; Joseph Michael; (Malvern, PA) ; GREEN;
Alexander Nicholas Michael; (Baldock, GB) ; PHILLIPS;
Paul Richard; (Royston, GB) ; REID; Stuart David;
(Cambourne, GB) ; WEIGERT; Erich Conlan;
(Morgantown, PA) ; WYLIE; James Alexander;
(Royston, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Johnson Matthey Public Limited Company |
London |
|
GB |
|
|
Family ID: |
45446178 |
Appl. No.: |
14/538321 |
Filed: |
November 11, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13966382 |
Aug 14, 2013 |
8906329 |
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14538321 |
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13310216 |
Dec 2, 2011 |
8535629 |
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13966382 |
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61419015 |
Dec 2, 2010 |
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61565774 |
Dec 1, 2011 |
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Current U.S.
Class: |
422/177 ; 502/60;
502/73; 502/75 |
Current CPC
Class: |
B01D 2251/2062 20130101;
B01D 53/9422 20130101; B01D 2258/012 20130101; B01J 29/7065
20130101; B01J 2229/42 20130101; B01J 29/763 20130101; B01D 53/8621
20130101; B01D 53/9413 20130101; B01D 2255/20761 20130101; B01D
2255/2065 20130101; B01D 2255/9202 20130101; B01J 37/0009 20130101;
B01D 2255/20738 20130101; B01J 2229/186 20130101; B01J 37/0246
20130101; B01D 2251/2067 20130101; B01J 35/023 20130101; B01J
37/0215 20130101; B01D 53/9418 20130101; B01D 53/565 20130101; B01D
2255/50 20130101 |
Class at
Publication: |
422/177 ; 502/60;
502/73; 502/75 |
International
Class: |
B01J 29/76 20060101
B01J029/76; B01D 53/86 20060101 B01D053/86; B01D 53/56 20060101
B01D053/56; B01J 29/70 20060101 B01J029/70 |
Claims
1. A catalyst composition comprising: a. a zeolite having a
non-phosphorous CHA crystal structure and having a mean crystalline
size of about 1 to about 5 microns; and b. at least one
non-aluminum base metal present in an amount sufficient to achieve
a NOx conversion of at least about 65% at a temperature of at least
450.degree. C.
2. The catalyst composition of claim 1, wherein the non-aluminum
base metal present in an amount sufficient to achieve a NOx
conversion of at least about 75% at a temperature of at least
550.degree. C.
3. The catalyst composition of claim 1, wherein the non-aluminum
base metal present in an amount sufficient to achieve a NOx
conversion of at least about 85% at a temperature of at least
650.degree. C.
4. The catalyst composition of 1, wherein the non-aluminum base
metal present in an amount sufficient to achieve a NOx conversion
of at least about 70 percent over at temperature range of
250-650.degree. C.
5. The catalyst composition of claim 1, wherein the NOx conversion
of at least about 80 percent and a N2 selectivity of at least about
85% over at temperature range of 250-650.degree. C.
6. The catalyst composition of claim 1, wherein non-aluminum base
metal is selected from Cr, Ce, Mn, Fe, Co, Ni and Cu.
7. The catalyst composition of claim 1, wherein non-aluminum base
metal is Cu.
8. The catalyst composition of claim 1, wherein the non-aluminum
base metal is included by ion exchanged.
9. The catalyst composition of claim 1, wherein the non-aluminum
base metal is included by incipient wetness.
10. The catalyst composition of claim 1, wherein the non-aluminum
base metal is included by isomorphous substitution.
11. The catalyst composition of claim 1, wherein the non-aluminum
base metal is included during zeolite synthesis.
12. The catalyst composition of claim 1, wherein the zeolite has a
silica-to-alumina ratio of about 10 to about 25.
13. A catalyst article comprising a substrate coated with a
catalyst composition according to claim 1.
14. The catalyst article of claim 12, wherein the substrate is a
flow-through honeycomb.
15. The catalyst article of claim 12, wherein the substrate is a
wall-flow filter.
16. The catalyst article of claim 12, further comprising an ammonia
oxidation catalyst.
17. The catalyst article of claim 15, wherein the catalyst of claim
1 and the ammonia oxidation catalyst are in series and the catalyst
of claim 1 is upstream of the ammonia oxidation catalyst.
18. A system for treating exhaust gas comprising: a. a catalyst
composition according to claim 1; b. an oxidation catalyst
comprising at least one platinum group metal.
19. The system of claim 18, wherein the platinum group metal is
palladium.
20. The system of claim 19, wherein the palladium is supported on a
zeolite.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/966,382, filed Aug. 14, 2013 which is a continuation of U.S.
application Ser. No. 13/310,216, filed Dec. 2, 2011, now U.S. Pat.
No. 8,535,629 which claims the priority benefit of U.S. Provisional
Application No. 61/419,015, filed Dec. 2, 2010, and U.S.
Provisional Application No. 61/565,774, filed Dec. 1, 2011, all of
which are incorporated herein by reference.
BACKGROUND
[0002] A.) Field of Use
[0003] The present invention relates to catalysts, systems, and
methods that are useful for treating an exhaust gas which occurs as
a result of combustion of hydrocarbon fuel, and particularly
exhaust gas containing nitrogen oxides, such as an exhaust gas
produced by diesel engines.
[0004] B.) Description of Related Art
[0005] The largest portions of most combustion exhaust gases
contain relatively benign nitrogen (N.sub.2), water vapor
(H.sub.2O), and carbon dioxide (CO.sub.2); but the exhaust gas also
contains in relatively small part noxious and/or toxic substances,
such as carbon monoxide (CO) from incomplete combustion,
hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NO.sub.x)
from excessive combustion temperatures, and particulate matter
(mostly soot). To mitigate the environmental impact of exhaust gas
released into the atmosphere, it is desirable to eliminate or
reduce the amount of these undesirable components, preferably by a
process that, in turn, does not generate other noxious or toxic
substances.
[0006] One of the most burdensome components to remove from a
vehicular exhaust gas is NO.sub.x, which includes nitric oxide
(NO), nitrogen dioxide (NO.sub.2), and nitrous oxide (N.sub.2O).
The reduction of NO.sub.x to N.sub.2 in a lean burn exhaust gas,
such as that created by diesel engines, is particularly problematic
because the exhaust gas contains enough oxygen to favor oxidative
reactions instead of reduction. NO can be reduced in a diesel
exhaust gas, however, by a process commonly known as Selective
Catalytic Reduction (SCR). An SCR process involves the conversion
of NO.sub.x, in the presence of a catalyst and with the aid of a
reducing agent, into elemental nitrogen (N.sub.2) and water. In an
SCR process, a gaseous reductant such as ammonia is added to an
exhaust gas stream prior to contacting the exhaust gas with the SCR
catalyst. The reductant is absorbed onto the catalyst and the NO
reduction reaction takes place as the gases pass through or over
the catalyzed substrate. The chemical equation for stoichiometric
SCR reactions using ammonia is:
2NO+4NH.sub.3+2O.sub.2.fwdarw.3N.sub.2+6H.sub.2O
2NO.sub.2+4NH.sub.3+O.sub.2.fwdarw.3N.sub.2+6H.sub.2O
NO+NO.sub.2+2NH.sub.3.fwdarw.2N.sub.2+3H.sub.2O
[0007] Known SCR catalysts include zeolites and other molecular
sieves. Molecular sieves are microporous crystalline solids with
well-defined structures and generally contain silicon, aluminum and
oxygen in their framework and can also contain cations within their
pores. A defining feature of a molecular sieve is its crystalline
or pseudo-crystalline structure which is formed by molecular
tetrahedral cells interconnected in a regular and/or repeating
manner to form a framework. Unique molecular sieve frameworks
recognized by the International Zeolite Association (IZA) Structure
Commission are assigned a three-letter code to designate the
framework type. Examples of molecular sieve frameworks that are
known SCR catalysts include Framework Type Codes CHA (chabazite),
BEA (beta), and MOR (mordenite).
[0008] Some molecular sieves have a three-dimensional molecular
framework that arises from the orientation of their interconnected
cells. The cells of these molecular sieves typically have volumes
on the order of a few cubic nanometers and cell openings (also
referred to as "pores" or "apertures") on the order of a few
angstroms in diameter. The cells can be defined by the ring size of
their pores, where, for example, the term "8-ring" refers to a
closed loop that is built from 8 tetrahedrally coordinated silicon
(or aluminum) atoms and 8 oxygen atoms. In certain zeolites, the
cell pores are aligned within the framework to create one or more
channels which extend through the framework, thus creating a
mechanism to restrict the ingress or passage of different molecular
or ionic species through the molecular sieve, based on the relative
sizes of the channels and molecular or ionic species. The size and
shape of molecular sieves affect their catalytic activity in part
because they exert a steric influence on the reactants, controlling
the access of reactants and products. For example, small molecules,
such as NOx, can typically pass into and out of the cells and/or
can diffuse through the channels of a small-pore molecular sieve
(i.e., those having framework with a maximum ring size of eight
tetrahedral atoms), whereas larger molecules, such as long chain
hydrocarbons, cannot. Moreover, partial or total dehydration of a
molecular sieve can results in a crystal structure interlaced with
channels of molecular dimensions.
[0009] Molecular sieves having a small pore framework, i.e.,
containing a maximum ring size of 8, have been found to be
particularly useful in SCR applications. Small pore molecular
sieves include those having the following crystalline structure
types: CHA, LEV, ERI, and AEI. Specific aluminosilicates and
silico-aluminophosphates examples of molecular sieves having the
CHA framework include SAPO-34, AIPO-34, and SSZ-13.
[0010] Zeolites are aluminosilicate molecular sieves having a
crystalline framework of interconnected alumina and silica, in
particular, cross-linked alumina and silica via a sharing of oxygen
atoms, and thus can be characterized by its silica-to-alumina ratio
(SAR). In general, as a zeolite's SAR increases, the zeolite
becomes more hydrothermal stability. Since the temperature of an
exhaust gas exiting a mobile lean-burn engine, such as a diesel
engine, is often 500 to 650.degree. C. or higher and typically
contains water vapor, hydrothermal stability is an important
consideration in designing an SCR catalyst.
[0011] While zeolites per se often have catalytic properties, their
SCR catalytic performance may be improved in certain environments
by a cationic exchange wherein a portion of ionic species existing
on the surface or within the framework is replaced by metal
cations, such Cu.sup.2+. That is, a zeolite's SCR performance can
be promoted by loosely holding one or more metal ions to the
molecular sieve's framework.
[0012] It is also desirable for an SCR catalyst to have high
catalytic activity at low operating temperatures. At low operating
temperatures, for example below 400.degree. C., a higher metal
loading on a molecular sieve results in higher SCR activity.
However, the achievable metal loading is often dependent on the
quantity of exchange sites in the molecular sieve, which in turn is
dependent upon the material's SAR. In general, molecular sieves
with low SAR allow for the highest metal loadings, thus leading to
a conflict between the need for high catalytic activity and high
hydrothermal stability which is achieved by a relatively higher SAR
value. Moreover, high copper-loaded catalysts do not perform as
well at high temperatures (e.g., >450.degree. C.). For example,
loading an aluminosilicate having a CHA framework with large
amounts of copper (i.e., copper-to-aluminum atomic ratio of
>0.25) can result in significant NH.sub.3 oxidation at
temperatures over 450.degree. C., resulting in low selectivity to
N.sub.2. This shortcoming is particularly acute under filter
regeneration conditions which involves exposing the catalyst to
temperatures above 650.degree. C.
[0013] Another important consideration in designing an SCR catalyst
for mobile application is the performance consistency of the
catalyst. For example, it is desirable for a fresh catalyst to
produce a similar level of NOx conversion to the same catalyst
after it has aged.
[0014] Accordingly, there remains a need for SCR catalysts that
offer improved performance over existing SCR materials.
SUMMARY OF THE INVENTION
[0015] Applicants have discovered that certain zeolites having a
chabazite (CHA) crystal structure can be loaded with relatively low
amounts of promoter metal, such as copper, to provide good
conversion at high temperatures, while still retaining good
selectivity to NO. More particularly, the present invention
utilizes and/or embodies the surprising discovery that certain
large crystal zeolites having a CHA framework and a relatively low
SAR can be loaded with relatively low amounts of catalytically
active metals and still provide good NO.sub.x conversion over a
broad temperature range while improving selectivity for N.sub.2 at
high temperatures (e.g., >about 450.degree. C.). The synergistic
effect between one or more of crystal size, copper exchange level,
and SAR was heretofore unknown and unexpected.
[0016] Applicants have also discovered that high concentrations of
cerium can be incorporated into such metal promoted zeolites to
improve the material's hydrothermal stability, low temperature
catalytic performance, and/or consistency in catalytic performance
between the fresh and aged states of the catalyst. For example,
certain embodiments of the invention utilize the surprising
discovery that the addition of high concentrations of Ce to a fully
formulated copper-promoted, low SAR CHA zeolite improves the
catalyst's hydrothermal durability compared to similar
metal-promoted, low SAR aluminosilicates without Ce. Also
surprising is the fact that this improved performance is not
observed when Ce is added to similar metal promoted zeolites having
a higher SAR or higher promoter metal concentration.
[0017] Accordingly, an aspect of the present invention provides a
catalyst composition comprising (a) a zeolite material having a CHA
framework that contains silicon and aluminum and having a
silica-to-alumina mole ratio (SAR) of about 10 to about 25, and
preferably a mean crystal size of at least about 0.5 .mu.m; and (b)
an extra-framework promoter metal (M) disposed in said zeolite
material as free and/or exchanged metal, wherein the
extra-framework promoter metal is selected from the group
consisting of copper, iron, and mixtures thereof, and is present in
a promoter metal-to-aluminum atomic ratio (M:Al) of about 0.10 to
about 0.24 based on the framework aluminum. In certain embodiments,
such catalyst further comprises at least about 1 weight percent Ce,
based on the total weight of the zeolite.
[0018] In another aspect of the invention, provided is a
catalytically active washcoat comprising (a) a metal promoted
zeolite material having a CHA framework that contains silicon and
aluminum and having a silica-to-alumina mole ratio (SAR) of about
10 to about 25 and preferably having a mean crystal size of at
least about 0.5 .mu.m; wherein the zeolite is promoted with an
extra-framework promoter metal (M) selected from the group
consisting of copper, iron, and mixtures thereof, and wherein the
extra-framework promoter metal is present in a promoter
metal-to-aluminum atomic ratio (M:Al) of about 0.10 to about 0.24
based on the framework aluminum; and (b) one or more stabilizers
and/or binders, wherein the metal promoted zeolite and the one or
more stabilizers and/or binders are present together in a
slurry.
[0019] In yet another aspect of the invention, provided is a method
for reducing NO.sub.x in an exhaust gas comprising (a) contacting
an exhaust gas derived from a lean-burn combustion process and
containing NO.sub.x with a catalyst composition comprising (i) a
zeolite material having a CHA framework that contains silicon and
aluminum and having a silica-to-alumina mole ratio (SAR) of about
10 to about 25 and preferably having a mean crystal size of at
least about 0.5 .mu.m; and (ii) an extra-framework promoter metal
(M) disposed in said zeolite material as free and/or exchanged
metal, wherein the extra-framework promoter metal is selected from
the group consisting of copper, iron, and mixtures thereof, and is
present in a promoter metal-to-aluminum atomic ratio (M:Al) of
about 0.10 to about 0.24 based on the framework aluminum; and (b)
converting at a portion of said NO.sub.x to N.sub.2 and
H.sub.2O.
BRIEF DESCRIPTION OF THE DRAWING
[0020] FIG. 1 is a graphical depiction of data regarding NO.sub.x
conversion capacity of (1) a Cu-SSZ-13 catalyst having low copper
loading according to an embodiment of the invention and (2) a
comparative material having high copper loading; and
[0021] FIG. 2 is a bar graph showing data on NO.sub.x conversion of
various catalysts of the invention that contain Ce and also
comparative examples of other catalyst materials.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0022] In a preferred embodiment, the invention is directed to a
catalyst for improving environmental air quality, particularly for
improving exhaust gas emissions generated by diesel and other lean
burn engines. Exhaust gas emissions are improved, at least in part,
by reducing NO.sub.x and/or NH.sub.3 slip concentrations lean burn
exhaust gas over a broad operational temperature range. Useful
catalysts are those that selectively reduce NO.sub.x and/or oxidize
ammonia in an oxidative environment (i.e., an SCR catalyst and/or
AMOX catalyst).
[0023] According to a preferred embodiment, provided is a catalyst
composition comprising a zeolite material having a CHA framework
and a silica-to-alumina mole ratio (SAR) of about 10 to about 25,
and preferably having a mean crystalline size of about 0.5 to about
5 microns; and containing at least one a non-aluminum promoter
metal (M) present in said zeolite material in a promoter metal to
aluminum ratio (M:Al) of about 0.10 to about 0.24.
[0024] Zeolites of the present invention are aluminosilicates
having a crystalline or pseudo crystalline structure and may
include framework metals other than aluminum (i.e.,
metal-substituted), but do not include silico-aluminophosphates
(SAPOs). As used herein, the term "metal substituted" with respect
to a zeolite means a framework having one or more aluminum or
silicon framework atoms replaced by a substituting metal. In
contrast, the term "metal exchanged" means a zeolite having
extra-framework metal ions. Examples of metals suitable as
substituting metals include copper and iron.
[0025] Suitable zeolites have a CHA crystalline structure. The
distinction between zeolite type materials, such as naturally
occurring (i.e. mineral) chabazite, and isotypes within the same
Framework Type Code is not merely arbitrary, but reflects
differences in the properties between the materials, which may in
turn lead to differences in activity in the method of the present
invention. Zeolites for use in the present application include
natural and synthetic zeolites, but are preferably synthetic
zeolites because these zeolites have more uniform SAR, crystallite
size, and crystallite morphology, and have fewer and less
concentrated impurities (e.g. alkaline earth metals). Specific
zeolites having the CHA structure that are useful in the present
invention include, but are not limited to, SSZ-13, LZ-218, Linde D,
Linde R, Phi, and ZK-14, with SSZ-13 being preferred.
[0026] Preferred zeolites having a CHA crystal structure do not
have an appreciable amount of phosphorous in their framework. That
is, the zeolite CHA frameworks of the present invention do not have
phosphorous as a regular repeating unit and/or do not have an
amount of phosphorous that would affect the basic physical and/or
chemical properties of the material, particularly with respect to
the material's capacity to selectively reduce NO.sub.x over a broad
temperature range. Accordingly, non-phosphorous CHA crystal
structure may include crystalline structures having a de minimus
amount of phosphorous.
[0027] Zeolites with application in the present invention can
include those that have been treated to improve hydrothermal
stability. Conventional methods of improving hydrothermal stability
include: (i) dealumination by steaming and acid extraction using an
acid or complexing agent e.g. (EDTA--ethylenediaminetetracetic
acid); treatment with acid and/or complexing agent; treatment with
a gaseous stream of SiCl.sub.4 (replaces Al in the zeolite
framework with Si); and (ii) cation exchange--use of multi-valent
cations such as lanthanum (La). Other methods, such as the use of
phosphorous containing compounds, are not necessary due to the
synergistic effect of combining low copper loading on a CHA zeolite
having relatively low SAR and relative large mean crystal size.
[0028] In preferred embodiments, the catalyst composition comprises
molecular sieve crystals having a mean crystal size of greater than
about 0.5 .mu.m, preferably between about 0.5 and about 15 .mu.m,
such as about 0.5 to about 5 .mu.m, about 0.7 to about 5 .mu.m,
about 1 to about 5 .mu.m, about 1.5 to about 5.0 .mu.m, about 1.5
to about 4.0 .mu.m, about 2 to about 5 .mu.m, or about 1 .mu.m to
about 10 .mu.m. The crystals in the catalyst composition can be
individual crystals, agglomeration of crystals, or a combination of
both, provided that agglomeration of crystals have a mean particle
size that is preferably less than about 15 .mu.m, more preferably
less than about 10 .mu.m, and even more preferably less than about
5 .mu.m. The lower limit on the mean particle size of the
agglomeration is the composition's mean individual crystal
size.
[0029] Crystal size (also referred to herein as the crystal
diameter) is the length of one edge of a face of the crystal. For
example, the morphology of chabazite crystals is characterized by
rhombohedral (but approximately cubic) faces wherein each edge of
the face is approximately the same length. Direct measurement of
the crystal size can be performed using microscopy methods, such as
SEM and TEM. For example, measurement by SEM involves examining the
morphology of materials at high magnifications (typically
1000.times. to 10,000.times.). The SEM method can be performed by
distributing a representative portion of the zeolite powder on a
suitable mount such that individual particles are reasonably evenly
spread out across the field of view at 1000.times. to 10,000.times.
magnification. From this population, a statistically significant
sample of random individual crystals (e.g., 50-200) are examined
and the longest dimensions of the individual crystals parallel to
the horizontal line of a straight edge are measured and recorded.
(Particles that are clearly large polycrystalline aggregates should
not be included the measurements.) Based on these measurements, the
arithmetic mean of the sample crystal sizes is calculated.
[0030] Particle size of an agglomeration of crystals can be
determined in a similar manner except that instead of measuring the
edge of a face of an individual crystal, the length of the longest
side of an agglomeration is measured. Other techniques for
determining mean particle size, such as laser diffraction and
scattering can also be used.
[0031] As used herein, the term "mean" with respect to crystal or
particle size is intended to represent the arithmetic mean of a
statistically significant sample of the population. For example, a
catalyst comprising molecular sieve crystals having a mean crystal
size of about 0.5 to about 5.0 .mu.m is catalyst having a
population of the molecular sieve crystals, wherein a statistically
significant sample of the population (e.g., 50 crystals) would
produce an arithmetic mean within the range of about 0.5 to about
5.0 .mu.m.
[0032] In addition to the mean crystal size, catalyst compositions
preferably have a majority of the crystal sizes are greater than
about 0.5 .mu.m, preferably between about 0.5 and about 15 .mu.m,
such as about 0.5 to about 5 .mu.m, about 0.7 to about 5 .mu.m,
about 1 to about 5 .mu.m, about 1.5 to about 5.0 .mu.m, about 1.5
to about 4.0 .mu.m, about 2 to about 5 .mu.m, or about 1 .mu.m to
about 10 .mu.m. Preferably, the first and third quartile of the
sample of crystals sizes is greater than about 0.5 .mu.m,
preferably between about 0.5 and about 15 .mu.m, such as about 0.5
to about 5 .mu.m, about 0.7 to about 5 .mu.m, about 1 to about 5
.mu.m, about 1.5 to about 5.0 .mu.m, about 1.5 to about 4.0 .mu.m,
about 2 to about 5 .mu.m, or about 1 .mu.m to about 10 .mu.m. As
used herein, the term "first quartile" means the value below which
one quarter of the elements are located. For example, the first
quartile of a sample of forty crystal sizes is the size of the
tenth crystal when the forty crystal sizes are arranged in order
from smallest to largest. Similarly, the term "third quartile"
means that value below which three quarters of the elements are
located.
[0033] Preferred CHA zeolites have a mole ratio of
silica-to-alumina about 10 to about 25, more preferably from about
14 to about 18, and even more preferably from about 15 to about 17.
The silica-to-alumina ratio of zeolites may be determined by
conventional analysis. This ratio is meant to represent, as closely
as possible, the ratio in the rigid atomic framework of the zeolite
crystal and to exclude silicon or aluminum in the binder or, in
cationic or other form, within the channels. It will be appreciated
that it may be extremely difficult to directly measure the
silica-to-alumina ratio of zeolite after it has been combined with
a binder material. Accordingly, the silica-to-alumina ratio has
been expressed hereinabove in term of the silica-to-alumina ratio
of the parent zeolite, i.e., the zeolite used to prepare the
catalyst, as measured prior to the combination of this zeolite with
the other catalyst components.
[0034] CHA zeolites, particular SSZ-13, having a low SAR and large
mean crystal size are commercially available. Alternatively, these
materials can be synthesized by known processes in the art, such as
those described in WO 2010/043981 (which is incorporated herein by
reference) and WO 2010/074040 (which is incorporated herein by
reference), or D. W. Fickel, et al., "Copper Coordination in
Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD",
J Phys. Chem., 114, p.1633-40 (2010), which demonstrates the
synthesis of a copper-loaded SSZ-13 having an SAR of 12.
[0035] Preferably, the catalyst composition comprises at least one
extra-framework metal to improve (i.e., promote) the catalytic
performance and/or thermal stability of the material. As used
herein, an "extra-framework metal" is one that resides within the
molecular sieve and/or on at least a portion of the molecular sieve
surface, does not include aluminum, and does not include atoms
constituting the framework of the molecular sieve. The
extra-framework metal can be added to the molecular sieve via any
known technique such as ion exchange, impregnation, isomorphous
substitution, etc. Extra-framework metals may be of any of the
recognized catalytically active metals that are used in the
catalyst industry to form metal-exchanged molecular sieves. In one
embodiment, at least one extra-framework metal is used in
conjunction with the molecular sieve to increase the catalyst's
performance. Preferred extra-framework metals are selected from the
group consisting of copper, nickel, zinc, iron, tin, tungsten,
molybdenum, cobalt, bismuth, titanium, zirconium, antimony,
manganese, chromium, vanadium, niobium, ruthenium, rhodium,
palladium, gold, silver, indium, platinum, iridium, rhenium, and
mixtures thereof. More preferred extra-framework metals include
those selected from the group consisting of chromium, manganese,
iron, cobalt, nickel, and copper, and mixtures thereof. Preferably,
at least one of the extra-framework metals is copper. Other
preferred extra-framework metals include iron, particularly in
combination with copper. For embodiments in which the
aluminosilicate has a CHA framework, the preferred promoter is
copper.
[0036] In certain embodiments, the promoter metal loading is about
0.1 to about 10 wt % based on the total weight of the molecular
sieve, for example from about 0.5 wt % to about 5 wt %, from about
0.5 to about 1 wt %, and from about 2 to about 5 wt %. In certain
embodiments, the promoter metal (M), preferably copper, is present
in the aluminosilicate zeolite in an amount to produce a M:Al
atomic ratio of about 0.17 to about 0.24, preferably about 0.22 to
about 0.24, particularly when the aluminosilicate zeolite has an
SAR of about 15 to about 20. As used herein, the M:Al ratio is
based on the relative amount of M to framework Al in the
corresponding zeolite. In certain embodiments that included
exchanged copper, the copper is present in an amount from about 80
to about 120 g/ft.sup.3 of zeolite or washcoat loading, including
for example about 86 to about 94 g/ft.sup.3, or about 92 to about
94 g/ft.sup.3.
[0037] The type and concentration of the transmission metal can
vary according to the host molecular sieve and the application.
[0038] In one example, a metal-exchanged molecular sieve is created
by blending the molecular sieve into a solution containing soluble
precursors of the catalytically active metal. The pH of the
solution may be adjusted to induce precipitation of the
catalytically active cations onto or within the molecular sieve
structure. For example, in a preferred embodiment a chabazite is
immersed in a solution containing copper nitrate for a time
sufficient to allow incorporation of the catalytically active
copper cations into the molecular sieve structure by ion exchange.
Un-exchanged copper ions are precipitated out. Depending on the
application, a portion of the un-exchanged ions can remain in the
molecular sieve material as free copper. The metal-exchanged
molecular sieve may then be washed, dried and calcined. When iron
and/or copper is used as the metal cation, the metal content of the
catalytic material by weight preferably comprises from about 0.1 to
about 10 percent by weight, more preferably from about 0.5 to about
10 percent by weight, for example about 1 to about 5 percent by
weight or about 2 to about 3 percent by weight, based on the weight
of the zeolite.
[0039] In another embodiment of the invention, the amount of
promoter metal, such as copper, in the catalyst is not particularly
limited provided that the catalyst can achieve a NO.sub.x
conversion of at least about 65%, preferably at least about 75%,
and more preferably at least about 85%, at a temperature of at
least about 450.degree. C., more preferably a temperature of at
least about 550.degree. C., and even more preferably a temperature
of at least about 650.degree. C. Preferably, the conversion at each
of these temperature ranges is at least about 70%, more preferably
80%, and even more preferably 90% of the conversion capacity of the
catalyst when the catalyst is operating at a temperature of
250.degree. C. Preferably, the catalyst can achieve 80% conversion
with a selectivity for N.sub.2 of at least about 85% at one or more
of these temperature ranges.
[0040] Generally, ion exchange of the catalytic metal cation into
or on the molecular sieve may be carried out at room temperature or
at a temperature up to about 80.degree. C. over a period of about 1
to 24 hours at a pH of about 7. The resulting catalytic molecular
sieve material is preferably dried at about 100 to 120.degree.
overnight and calcined at a temperature of at least about
500.degree. C.
[0041] In certain embodiments, the metal promoted zeolite catalysts
of the present invention also contain a relatively large amount of
Ce. In certain embodiments, the zeolite, preferably a CHA
aluminosilicate, has an SAR of less than 20, preferably about 15 to
about 18, and is promoted with a metal, preferably copper and
preferably in a copper:aluminum atomic ratio of about 0.17 to about
0.24, and also contains Ce in a concentration of greater than about
1 weight percent, preferably greater than about 1.35 weight
percent, more preferably 1.35 to 13.5 weight percent, based on the
total weight of the zeolite. Such Ce-containing catalysts are more
durable compared to structurally similar catalysts, such as other
CHA zeolites having a higher SAR, particularly those with higher
loadings of promoter metals.
[0042] Preferably, the cerium concentration in the catalyst
material is present in a concentration of at least about 1 weight
percent, based on the total weight of the zeolite. Examples of
preferred concentrations include at least about 2.5 weight percent,
at least about 5 weight percent, at least about 8 weight percent,
at least about 10 weight percent, about 1.35 to about 13.5 weight
percent, about 2.7 to about 13.5 weight percent, about 2.7 to about
8.1 weight percent, about 2 to about 4 weight percent, about 2 to
about 9.5 weight percent, and about 5 to about 9.5 weight percent,
based on the total weight of the zeolite. For most of these ranges,
the improvement in catalyst performance correlates directly to the
concentration of Ce in the catalyst. These ranges are particularly
preferred for copper promoted aluminosilicates having a CHA
framework, such as SSZ-13, with an SAR of about 10 to about 25,
about 20 to about 25, about 15 to about 20, or about 16 to about
18, and more preferably for such embodiments, wherein the copper is
present in a copper-to-aluminum ratio of about 0.17 to about
0.24.
[0043] In certain embodiments, the cerium concentration in the
catalyst material is about 50 to about 550 g/ft.sup.3. Other ranges
of Ce include: above 100 g/ft.sup.3, above 200 g/ft.sup.3, above
300 g/ft.sup.3, above 400 g/ft.sup.3, above 500 g/ft.sup.3, from
about 75 to about 350 g/ft.sup.3, from about 100 to about 300
g/ft.sup.3, and from about 100 to about 250 g/ft.sup.3.
[0044] In certain embodiments, the concentration of Ce exceeds the
theoretical maximum amount available for exchange on the
metal-promoted zeolite. Accordingly, in some embodiments, Ce is
present in more than one form, such as Ce ions, monomeric ceria,
oligomeric ceria, and combinations thereof, provided that said
oligomeric ceria has a mean crystal size of less than 5 .mu.m, for
example less than 1 .mu.m, about 10 nm to about 1 .mu.m, about 100
nm to about 1 .mu.m, about 500 nm to about 1 .mu.m, about 10 to
about 500 nm, about 100 to about 500 nm, and about 10 to about 100
nm. As used herein, the term "monomeric ceria" means CeO.sub.2 as
individual molecules or moieties residing freely on and/or in the
zeolite or weakly bonded to the zeolite. As used herein, the term
"oligomeric ceria" means nanocrystalline CeO.sub.2 residing freely
on and/or in the zeolite or weakly bonded to the zeolite.
[0045] For embodiments in which the catalyst is part of a washcoat
composition, the washcoat may further comprise binder containing Ce
or ceria. For such embodiments, the Ce containing particles in the
binder are significantly larger than the Ce containing particles in
the catalyst.
[0046] Cerium is preferably incorporated into a zeolite containing
a promoting metal. For example, in a preferred embodiment, an
aluminosilicate having a CHA framework undergoes a copper exchange
process prior to being impregnated by Ce. An exemplary Ce
impregnation process involves adding Ce nitrate to a copper
promoted zeolite via a conventional incipient wetness
technique.
[0047] The zeolite catalyst for use in the present invention can be
in the form of a washcoat, preferably a washcoat that is suitable
for coating a substrate, such as a metal or ceramic flow through
monolith substrate or a filtering substrate, including for example
a wall-flow filter or sintered metal or partial filter.
Accordingly, another aspect of the invention is a washcoat
comprising a catalyst component as described herein. In addition
the catalyst component, washcoat compositions can further comprise
a binder selected from the group consisting of alumina, silica,
(non zeolite) silica-alumina, naturally occurring clays, TiO.sub.2,
ZrO.sub.2, and SnO.sub.2.
[0048] In one embodiment, provided is a substrate upon which the
zeolite catalyst is deposited.
[0049] Preferred substrates for use in mobile application are
monoliths having a so-called honeycomb geometry which comprises a
plurality of adjacent, parallel channels, each channel typically
having a square cross-sectional area. The honeycomb shape provide a
large catalytic surface with minimal overall size and pressure
drop. The zeolite catalyst can be deposited on a flow-through
monolith substrate (e.g., a honeycomb monolithic catalyst support
structure with many small, parallel channels running axially
through the entire part) or filter monolith substrate such as a
wall-flow filter, etc. In another embodiment, the zeolite catalyst
is formed into an extruded-type catalyst. Preferably, the zeolite
catalyst is coated on a substrate in an amount sufficient to reduce
the NOx contained in an exhaust gas stream flowing through the
substrate. In certain embodiments, at least a portion of the
substrate may also contain a platinum group metal, such as platinum
(Pt), to oxidize ammonia in the exhaust gas stream.
[0050] Preferably, the molecular sieve catalyst is embodied in or
on a substrate in an amount sufficient to reduce the NO.sub.x
contained in an exhaust gas stream flowing through the substrate.
In certain embodiments, at least a portion of the substrate may
also contain an oxidation catalyst, such as a platinum group metal
(e.g. platinum), to oxidize ammonia in the exhaust gas stream or
perform other functions such as conversion of CO into CO.sub.2.
[0051] The catalytic zeolites described herein can promote the
reaction of a reductant, preferably ammonia, with nitrogen oxides
to selectively form elemental nitrogen (N.sub.2) and water
(H.sub.2O) vis-a-vis the competing reaction of oxygen and ammonia.
In one embodiment, the catalyst can be formulated to favor the
reduction of nitrogen oxides with ammonia (i.e., and SCR catalyst).
In another embodiment, the catalyst can be formulated to favor the
oxidation of ammonia with oxygen (i.e., an ammonia oxidation (AMOX)
catalyst). In yet another embodiment, an SCR catalyst and an AMOX
catalyst are used in series, wherein both catalyst comprise the
metal containing zeolite described herein, and wherein the SCR
catalyst is upstream of the AMOX catalyst. In certain embodiments,
the AMOX catalyst is disposed as a top layer on an oxidative
under-layer, wherein the under-layer comprises a platinum group
metal (PGM) catalyst or a non-PGM catalyst. Preferably, the AMOX
catalyst is disposed on a high surface area support, including but
not limited to alumina. In certain embodiments, the AMOX catalyst
is applied to a substrate, preferably substrates that are designed
to provide large contact surface with minimal backpressure, such as
flow-through metallic or cordierite honeycombs. For example, a
preferred substrate has between about 25 and about 300 cells per
square inch (CPSI) to ensure low backpressure. Achieving low
backpressure is particularly important to minimize the AMOX
catalyst's effect on the low-pressure EGR performance. The AMOX
catalyst can be applied to the substrate as a washcoat, preferably
to achieve a loading of about 0.3 to 2.3 g/in.sup.3. To provide
further NO.sub.x conversion, the front part of the substrate can be
coated with just SCR coating, and the rear coated with SCR and an
NH.sub.3 oxidation catalyst which can further include Pt or Pt/Pd
on an alumina support.
[0052] According to another aspect of the invention, provided is a
method for the reduction of NOx compounds or oxidation of NH.sub.3
in a gas, which comprises contacting the gas with a catalyst
composition described herein for the catalytic reduction of
NO.sub.x compounds for a time sufficient to reduce the level of
NO.sub.x compounds in the gas. In one embodiment, nitrogen oxides
are reduced with the reducing agent at a temperature of at least
100.degree. C. In another embodiment, the nitrogen oxides are
reduced with the reducing agent at a temperature from about
150.degree. C. to 750.degree. C. In a particular embodiment, the
temperature range is from 175 to 550.degree. C. In another
embodiment, the temperature range is from 175 to 400.degree. C. In
yet another embodiment, the temperature range is 450 to 900.degree.
C., preferably 500 to 750.degree. C., 500 to 650.degree. C., 450 to
550.degree. C., or 650 to 850.degree. C. Embodiments utilizing
temperatures greater than 450.degree. C. are particularly useful
for treating exhaust gases from a heavy and light duty diesel
engine that is equipped with an exhaust system comprising
(optionally catalyzed) diesel particulate filters which are
regenerated actively, e.g. by injecting hydrocarbon into the
exhaust system upstream of the filter, wherein the zeolite catalyst
for use in the present invention is located downstream of the
filter. In other embodiments, the zeolite SCR catalyst is
incorporated on a filter substrate. Methods of the present
invention may comprise one or more of the following steps: (a)
accumulating and/or combusting soot that is in contact with the
inlet of a catalytic filter; (b) introducing a nitrogenous reducing
agent into the exhaust gas stream prior to contacting the catalytic
filter, preferably with no intervening catalytic steps involving
the treatment of NO.sub.x and the reductant; (c) generating
NH.sub.3 over a NO.sub.x adsorber catalyst, and preferably using
such NH.sub.3 as a reductant in a downstream SCR reaction; (d)
contacting the exhaust gas stream with a DOC to oxidize hydrocarbon
based soluble organic fraction (SOF) and/or carbon monoxide into
CO.sub.2, and/or oxidize NO into NO.sub.2, which in turn, may be
used to oxidize particulate matter in particulate filter; and/or
reduce the particulate matter (PM) in the exhaust gas; (e)
contacting the exhaust gas with one or more flow-through SCR
catalyst device(s) in the presence of a reducing agent to reduce
the NOx concentration in the exhaust gas; and (f) contacting the
exhaust gas with an AMOX catalyst, preferably downstream of the SCR
catalyst to oxidize most, if not all, of the ammonia prior to
emitting the exhaust gas into the atmosphere or passing the exhaust
gas through a recirculation loop prior to exhaust gas
entering/re-entering the engine.
[0053] The reductant (also known as a reducing agent) for SCR
processes broadly means any compound that promotes the reduction of
NOx in an exhaust gas. Examples of reductants useful in the present
invention include ammonia, hydrazine or any suitable ammonia
precursor, such as urea ((NH.sub.2).sub.2CO), ammonium carbonate,
ammonium carbamate, ammonium hydrogen carbonate or ammonium
formate, and hydrocarbons such as diesel fuel, and the like.
Particularly preferred reductant, are nitrogen based, with ammonia
being particularly preferred.
[0054] In another embodiment, all or at least a portion of the
nitrogen-based reductant, particularly NH.sub.3, can be supplied by
a NO.sub.x adsorber catalyst (NAC), a lean NO.sub.x trap (LNT), or
a NO.sub.x storage/reduction catalyst (NSRC), disposed upstream of
the SCR catalyst, e.g., a SCR catalyst of the present invention
disposed on a wall-flow filter. NAC components useful in the
present invention include a catalyst combination of a basic
material (such as alkali metal, alkaline earth metal or a rare
earth metal, including oxides of alkali metals, oxides of alkaline
earth metals, and combinations thereof), and a precious metal (such
as platinum), and optionally a reduction catalyst component, such
as rhodium. Specific types of basic material useful in the NAC
include cesium oxide, potassium oxide, magnesium oxide, sodium
oxide, calcium oxide, strontium oxide, barium oxide, and
combinations thereof. The precious metal is preferably present at
about 10 to about 200 g/ft.sup.3, such as 20 to 60 g/ft.sup.3.
Alternatively, the precious metal of the catalyst is characterized
by the average concentration which may be from about 40 to about
100 grams/ft.sup.3.
[0055] Under certain conditions, during the periodically rich
regeneration events, NH.sub.3 may be generated over a NO.sub.x
adsorber catalyst. The SCR catalyst downstream of the NO.sub.x
adsorber catalyst may improve the overall system NO.sub.x reduction
efficiency. In the combined system, the SCR catalyst is capable of
storing the released NH.sub.3 from the NAC catalyst during rich
regeneration events and utilizes the stored NH.sub.3 to selectively
reduce some or all of the NO.sub.x that slips through the NAC
catalyst during the normal lean operation conditions.
[0056] The method can be performed on a gas derived from a
combustion process, such as from an internal combustion engine
(whether mobile or stationary), a gas turbine and coal or oil fired
power plants. The method may also be used to treat gas from
industrial processes such as refining, from refinery heaters and
boilers, furnaces, the chemical processing industry, coke ovens,
municipal waste plants and incinerators, etc. In a particular
embodiment, the method is used for treating exhaust gas from a
vehicular lean burn internal combustion engine, such as a diesel
engine, a lean-burn gasoline engine or an engine powered by liquid
petroleum gas or natural gas.
[0057] According to a further aspect, the invention provides an
exhaust system for a vehicular lean burn internal combustion
engine, which system comprising a conduit for carrying a flowing
exhaust gas, a source of nitrogenous reductant, a zeolite catalyst
described herein. The system can include a controller for the
metering the nitrogenous reductant into the flowing exhaust gas
only when it is determined that the zeolite catalyst is capable of
catalyzing NO.sub.x reduction at or above a desired efficiency,
such as at above 100.degree. C., above 150.degree. C. or above
175.degree. C. The determination by the control means can be
assisted by one or more suitable sensor inputs indicative of a
condition of the engine selected from the group consisting of:
exhaust gas temperature, catalyst bed temperature, accelerator
position, mass flow of exhaust gas in the system, manifold vacuum,
ignition timing, engine speed, lambda value of the exhaust gas, the
quantity of fuel injected in the engine, the position of the
exhaust gas recirculation (EGR) valve and thereby the amount of EGR
and boost pressure.
[0058] In a particular embodiment, metering is controlled in
response to the quantity of nitrogen oxides in the exhaust gas
determined either directly (using a suitable NOx sensor) or
indirectly, such as using pre-correlated look-up tables or
maps--stored in the control means--correlating any one or more of
the abovementioned inputs indicative of a condition of the engine
with predicted NO.sub.x content of the exhaust gas. The metering of
the nitrogenous reductant can be arranged such that 60% to 200% of
theoretical ammonia is present in exhaust gas entering the SCR
catalyst calculated at 1:1 NH.sub.3/NO and 4:3 NH.sub.3/NO.sub.2.
The control means can comprise a pre-programmed processor such as
an electronic control unit (ECU).
[0059] In a further embodiment, an oxidation catalyst for oxidizing
nitrogen monoxide in the exhaust gas to nitrogen dioxide can be
located upstream of a point of metering the nitrogenous reductant
into the exhaust gas. In one embodiment, the oxidation catalyst is
adapted to yield a gas stream entering the SCR zeolite catalyst
having a ratio of NO to NO.sub.2 of from about 4:1 to about 1:3 by
volume, e.g. at an exhaust gas temperature at oxidation catalyst
inlet of 250.degree. C. to 450.degree. C. The oxidation catalyst
can include at least one platinum group metal (or some combination
of these), such as platinum, palladium, or rhodium, coated on a
flow-through monolith substrate. In one embodiment, the at least
one platinum group metal is platinum, palladium or a combination of
both platinum and palladium. The platinum group metal can be
supported on a high surface area washcoat component such as
alumina, a zeolite such as an aluminosilicate zeolite, silica,
non-zeolite silica alumina, ceria, zirconia, titania or a mixed or
composite oxide containing both ceria and zirconia.
[0060] In a further embodiment, a suitable filter substrate is
located between the oxidation catalyst and the SCR catalyst. Filter
substrates can be selected from any of those mentioned above, e.g.
wall flow filters. Where the filter is catalyzed, e.g. with an
oxidation catalyst of the kind discussed above, preferably the
point of metering nitrogenous reductant is located between the
filter and the zeolite catalyst. Alternatively, if the filter is
uncatalyzed, the means for metering nitrogenous reductant can be
located between the oxidation catalyst and the filter.
[0061] In a further embodiment, the zeolite catalyst for use in the
present invention is coated on a filter located downstream of the
oxidation catalyst. Where the filter includes the zeolite catalyst
for use in the present invention, the point of metering the
nitrogenous reductant is preferably located between the oxidation
catalyst and the filter.
[0062] In a further aspect, there is provided a vehicular lean-burn
engine comprising an exhaust system according to the present
invention. The vehicular lean burn internal combustion engine can
be a diesel engine, a lean-burn gasoline engine or an engine
powered by liquid petroleum gas or natural gas.
EXAMPLES
Example 1
[0063] A zeolite sample was prepared having CHA framework (isotype
SSZ-13) and an SAR of about 17. The sample was loaded with copper
to produce a catalyst material having a Cu:Al atomic ration of
about 0.20. Following aging at about 550.degree. C. for about 72
hours, the catalyst was exposed to a simulated diesel engine
exhaust gas that was combined with ammonia to produce a stream
having an ammonia to NO.sub.x ratio (ANR) of 1 and a space velocity
of 50,000 per hour. The catalyst's capacity for NO.sub.x conversion
was determined at temperatures ranging from 200.degree. C. to
550.degree. C.
Comparative Example 1
[0064] For comparison, a similar SSZ-13 zeolite was prepared, but
instead of being loaded with a low amount of copper, the
comparative material was loaded with enough copper to yield a Cu:Al
atomic ratio >0.44. The comparative material was exposed to a
similar exhaust gas stream under similar conditions. The
comparative material's capacity for NO.sub.x conversion was
determined at temperatures ranging from 200.degree. C. to
550.degree. C.
[0065] It was found that at temperatures above 350.degree. C., the
low loaded catalyst shows significant improvements in NO.sub.x
conversion.
Example 2
[0066] An aluminosilicate having a CHA framework (isotype SSZ-13)
having an SAR of 17 (zeolite A) and containing 2.4 weight percent
of exchanged copper (based on total weight of zeolite) was
impregnated with Ce nitrate using an incipient wetness technique
and then washcoated on a substrate to produce a catalyst sample
having 75 g/ft.sup.3 of Ce (1.35 weight percent Ce, based on total
zeolite weight). The same technique was repeated to produced
catalyst samples having the 96 g/ft.sup.3 of Ce, 119 g/ft.sup.3 of
Ce, 188 g/ft.sup.3 of Ce, and 285 g/ft.sup.3 of Ce. Each of these
samples was hydrothermally aged at 800.degree. C. in 10% H.sub.2O
for five hours. These samples were then analyzed to determine their
capacity for NOx conversion in an NH.sub.3 SCR process at
200.degree. C. and at 500.degree. C., wherein the NH.sub.3 SCR
process is tuned to allow 20 ppm ammonia slip. The results of this
analysis are provided in FIG. 2.
Comparative Examples 2 & 3
[0067] Zeolite A, without Ce impregnation, was analyzed to
determine its capacity for NOx conversion in an NH.sub.3 SCR
process at 200.degree. C. and at 500.degree. C., wherein the
NH.sub.3 SCR process is tuned to allow 20 ppm ammonia slip. The
results of this analysis are provided in FIG. 1.
[0068] An aluminosilicate having a CHA framework (isotype SSZ-13)
having an SAR of 25 and containing 3.3 weight percent of exchanged
copper (without Ce impregnation) was analyzed to determine its
capacity for NOx conversion in an NH.sub.3 SCR process at
200.degree. C. and at 500.degree. C., wherein the NH.sub.3 SCR
process is tuned to allow 20 ppm ammonia slip. The results of this
analysis are provided in FIG. 2.
[0069] The results of these tests demonstrate that low SAR,
copper-promoted zeolites that are impregnated with Ce have superior
hydrothermal durability.
* * * * *