U.S. patent application number 11/043613 was filed with the patent office on 2005-09-15 for zeolite-containing oxidation catalyst and method of use.
This patent application is currently assigned to ENGELHARD CORPORATION. Invention is credited to Adomaitis, John R., Deeba, Michel, Farrauto, Robert J., Voss, Kenneth E., Yavuz, Bulent O..
Application Number | 20050201916 11/043613 |
Document ID | / |
Family ID | 26715134 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050201916 |
Kind Code |
A1 |
Yavuz, Bulent O. ; et
al. |
September 15, 2005 |
Zeolite-containing oxidation catalyst and method of use
Abstract
Oxidation catalyst compositions for treating diesel exhaust
include ceria and, optionally, alumina, each having a surface area
of at least about 10 m.sup.2/g, and a zeolite, e.g., Beta zeolite.
Optionally, platinum may be included in the catalytic material,
preferably in amounts which are sufficient to promote some
gas-phase oxidation of carbon monoxide ("CO") and hydrocarbons
("HC") but which are limited to preclude excessive oxidation of
SO.sub.2 to SO.sub.3. Alternatively, palladium in any desired
amount may be included in the catalytic material. The zeolite is
optionally doped, e.g., ion-exchanged, with one or more of
hydrogen, a platinum group metal or other catalytic metals. The
catalyst compositions may be used in a method to treat diesel
engine exhaust by contacting the hot exhaust with the catalyst
composition to promote the oxidation of gas-phase CO and HC and of
the volatile organic fraction component of particulates in the
exhaust.
Inventors: |
Yavuz, Bulent O.;
(Plainfield, NJ) ; Voss, Kenneth E.; (Somerville,
NJ) ; Deeba, Michel; (North Brunswick, NJ) ;
Adomaitis, John R.; (Old Bridge, NJ) ; Farrauto,
Robert J.; (Westfield, NJ) |
Correspondence
Address: |
Chief Patent Counsel
Engelhard Corporation
101 Wood Avenue
P.O. Box 770
Iselin
NJ
08830-0770
US
|
Assignee: |
ENGELHARD CORPORATION
|
Family ID: |
26715134 |
Appl. No.: |
11/043613 |
Filed: |
January 26, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11043613 |
Jan 26, 2005 |
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09880375 |
Jun 13, 2001 |
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09880375 |
Jun 13, 2001 |
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08458052 |
Jun 1, 1995 |
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6274107 |
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08458052 |
Jun 1, 1995 |
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08255289 |
Jun 7, 1994 |
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6248684 |
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08255289 |
Jun 7, 1994 |
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08038378 |
Mar 29, 1993 |
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08038378 |
Mar 29, 1993 |
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07973461 |
Nov 19, 1992 |
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Current U.S.
Class: |
423/239.2 ;
502/64; 502/65; 502/66 |
Current CPC
Class: |
B01D 2255/50 20130101;
B01J 35/023 20130101; B01J 29/7615 20130101; B01D 2255/20723
20130101; B01D 2255/912 20130101; B01J 23/10 20130101; F01N
2510/0684 20130101; B01D 2255/102 20130101; F02B 3/06 20130101;
F01N 2510/063 20130101; Y02A 50/20 20180101; B01J 37/0215 20130101;
B01D 2255/206 20130101; B01D 53/9481 20130101; B01D 2255/9207
20130101; B01J 35/1019 20130101; B01J 23/63 20130101; B01D 53/864
20130101; B01J 35/1014 20130101; F01N 2510/06 20130101; Y02A
50/2341 20180101; B01J 29/7215 20130101; B01D 2255/20753 20130101;
B01D 2255/20761 20130101; F01N 2510/0682 20130101; B01J 35/0006
20130101; B01J 37/0244 20130101; B01J 37/0248 20130101; B01J
37/0246 20130101; B01D 53/944 20130101; B01D 2255/20738
20130101 |
Class at
Publication: |
423/239.2 ;
502/064; 502/065; 502/066 |
International
Class: |
B01J 029/06 |
Claims
1. A catalyst composition for treating a diesel engine exhaust
stream containing a volatile organic fraction comprises a
refractory carrier on which is disposed a coating of a catalytic
material comprising a catalytically effective amount of a
stabilized ceria having a BET surface area of at least about 10
m.sup.2/g and a catalytically effective amount of a zeolite.
2. The catalyst composition of claim 1 further including a
catalytically effective amount of alumina having a BET surface area
of at least about 10 m.sup.2/g.
3. (canceled)
4. (canceled)
5. The catalyst composition of claim 1 wherein the zeolite is
selected from the group consisting of Beta zeolite,
.gamma.-zeolite, pentasil, Mordenite, and mixtures thereof.
6. The catalyst composition of claim wherein the stabilized ceria
is stabilized with alumina.
7. The catalyst composition of claim 2 wherein the zeolite
comprises from about 10 to 90 percent by weight, the alumina
comprises from about 60 to percent by weight, and the ceria
comprises from about 60 to 5 percent by weight, of the combined
weight of the zeolite, the alumina and the ceria.
8. (canceled)
9. The catalyst composition of claim 1 wherein the zeolite is doped
with a catalytic moiety selected from the group consisting of one
or both of hydrogen, platinum, rhodium, palladium, ruthenium,
osmium, iridium, copper, iron, nickel, chromium and vanadium.
10. (canceled)
11. The catalyst composition of claim 5 wherein the zeolite is Beta
zeolite and the catalytic moiety comprises hydrogen.
12. (canceled)
13. (canceled)
14. The catalyst composition of claim 2 wherein the zeolite is
disposed in a discrete layer which is overlain by one or more
discrete layers containing the alumina and the ceria.
15. (canceled)
16. The catalyst composition of claim 1 wherein the refractory
carrier has a plurality of parallel exhaust flow passages extending
therethrough and defined by passage walls on which the catalytic
material is coated, and further comprising dispersed platinum
carried on the catalytic material in an amount of from about 0.1 to
about 60 g/ft.sup.3.
17. (canceled)
18. The catalyst composition of claim 16 wherein the refractory
carrier has a plurality of parallel exhaust flow passages extending
therethrough and defined by passage walls on which the catalytic
material is coated, and at least a catalytically effective amount
of the dispersed platinum is carried on the ceria.
19. The catalyst composition of claim 1 wherein the refractory
carrier has a plurality of parallel exhaust flow passages extending
therethrough and defined by passage walls on which the catalytic
material is coated, and further comprising dispersed palladium
carried on the catalytic material in a quantity of from about 0.1
to 200 g/.sup.3.
20. (canceled)
21. A method for treating a diesel engine exhaust stream containing
a volatile organic fraction comprises contacting the stream with a
catalyst composition under oxidizing conditions including a
temperature high enough to catalyze oxidation of at least some of
the volatile organic fraction, the catalyst composition comprising
a catalytically effective amount of a stabilized ceria having a BET
surface area of at least about 10 m.sup.2/g and a catalytically
effective amount of a zeolite.
22. The method of claim 21 wherein the catalyst composition further
comprises a catalytically effective amount of alumina having a BET
surface area of at least about 10 m.sup.2/g.
23. (canceled)
24. The method of claim 21 wherein the stabilized ceria is
stabilized with alumina.
25. The method of claim 21 wherein the zeolite is selected from the
group consisting of Beta zeolite, Y-zeolite, pentasil, Mordenite
and mixtures thereof.
26. (canceled)
27. (canceled)
28. The method of claim 21 wherein the zeolite is doped with a
catalytic moiety selected from the group consisting of one or both
of hydrogen, platinum, rhodium, palladium, ruthenium, osmium,
iridium, copper, iron, nickel, chromium and vanadium.
29. (canceled)
30. The method of claim 28 wherein the catalytic moiety comprises
and hydrogen.
31. (canceled)
32. (canceled)
33. (canceled)
34. The method of claim 21 wherein the refractory carrier has a
plurality of parallel exhaust stream flow passages extending
therethrough and defined by passage walls on which the catalytic
material is coated, and the catalyst material further comprises
dispersed platinum carried thereon in an amount of from about 0.1
to 60 g/ft.sup.3.
35. (canceled)
36. (canceled)
37. The method of claim 21 wherein the temperature of the exhaust
stream initially contacted with the catalyst composition is from
about 100.degree. C. to 800.degree. C.
38. The method of claim 21 wherein the refractory carrier has a
plurality of parallel exhaust stream flow passages extending
therethrough and defined by passage walls on which the catalytic
material is coated, and the catalytic material further comprises
dispersed palladium carried thereon in the amount of from about 0.1
to 200 g/ft.sup.3.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of application
Ser. No. 07/973,461, filed Nov. 19, 1992.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a catalyst composition and method
for the oxidation of oxidizeable components of diesel engine
exhaust, and more specifically to the treatment of such diesel
exhaust to reduce the content of particulates and other pollutants
discharged to the atmosphere.
[0004] 2. Background and Related Art
[0005] As is well-known, diesel engine exhaust is a heterogeneous
material which contains not only gaseous pollutants such as carbon
monoxide ("CO") and unburned hydrocarbons ("HC"), but also soot
particles which comprise both a dry, solid carbonaceous fraction
and a soluble organic fraction. The soluble organic fraction is
sometimes referred to as a volatile organic fraction ("VOF"), which
terminology will be used herein. The VOF may exist in diesel
exhaust either as a vapor or as an aerosol (fine droplets of liquid
condensate) depending on the temperature of the diesel exhaust.
[0006] Oxidation catalysts comprising a platinum group metal
dispersed on a refractory metal oxide support are known for use in
treating the exhaust of diesel engines in order to convert both HC
and CO gaseous pollutants and particulates, i.e., soot particles,
by catalyzing the oxidation of these pollutants to carbon dioxide
and water. One problem faced in the treatment of diesel engine
exhaust is presented by the presence of sulfur in diesel fuel. Upon
combustion, sulfur forms sulfur dioxide and the oxidation catalyst
catalyzes the SO.sub.2 to SO.sub.3 ("sulfates") with subsequent
formation of condensible sulfur compounds, such as sulfuric acid,
which condense upon, and thereby add to, the mass of particulates.
The sulfates also react with activated alumina supports to form
aluminum sulfates, which render activated alumina-containing
catalysts inactive. In this regard, see U.S. Pat. No. 4,171,289 at
column 1, line 39 et seq. Previous attempts to deal with the
sulfation problem include the incorporation of large amounts of
sulfate-resistant materials such as vanadium oxide into the support
coating, or the use of alternative support materials such as
.alpha.-alumina (alpha), silica and titania, which are
sulfate-resistant materials.
[0007] The prior art also shows an awareness of the use of
zeolites, including metal-doped zeolites, to treat diesel exhaust.
For example, U.S. Pat. No. 4,929,581 discloses a filter for diesel
exhaust, in which the exhaust is constrained to flow through the
catalyst walls to filter the soot particles. A catalyst comprising
a platinum group metal-doped zeolite is dispersed on the walls of
the filter to catalyze oxidation of the soot to unplug the
filter.
[0008] As is well-known in the art, catalysts used to treat the
exhaust of internal combustion engines are less effective during
periods of relatively low temperature operation, such as the
initial cold-start period of engine operation, because the engine
exhaust is not at a temperature sufficiently high for efficient
catalytic conversion of noxious components in the exhaust. To this
end, it is known in the art to include an adsorbent material, which
may be a zeolite, as part of a catalytic treatment system in order
to adsorb gaseous pollutants, usually hydrocarbons, and retain them
during the initial cold-start period. As the exhaust gas
temperature increases, the adsorbed hydrocarbons are driven from
the adsorbent and subjected to catalytic treatment at the higher
temperature. In this regard, see for example U.S. Pat. No.
5,125,231 which discloses (columns 5-6) the use of platinum group
metal-doped zeolites as low temperature hydrocarbon adsorbents as
well as oxidation catalysts.
SUMMARY OF THE INVENTION
[0009] Generally, in accordance with the present invention, there
is provided a catalyst composition and a method for oxidizing
oxidizeable components of diesel engine exhaust in which at least
some of a volatile organic fraction of the diesel exhaust is
converted to innocuous materials, and in which gaseous HC and CO
pollutants may also be similarly converted. The objectives of the
invention are attained by an oxidation catalyst comprising a
catalytic material comprising a mixture of high surface area ceria,
a zeolite and, optionally, a high surface area alumina. The
catalytic material optionally may carry a low loading of platinum
catalytic metal dispersed thereon or palladium catalytic metal
dispersed thereon. Alternatively, or in addition, the zeolite of
the catalyst composition may be doped, e.g., ion-exchanged, with a
catalytic moiety such as one or more of hydrogen ion, platinum,
copper, nickel, cobalt, iron, etc. The method of the invention is
attained by flowing a diesel engine exhaust, e.g., the exhaust of a
diesel-powered automobile or light truck, into contact under
oxidation reaction conditions with a catalyst composition as
described above.
[0010] Specifically, in accordance with the present invention there
is provided a catalyst composition for treating a diesel engine
exhaust stream containing a volatile organic fraction, which
composition comprises a refractory carrier on which is disposed a
coating of a catalytic material comprising a catalytically
effective amount of ceria and, optionally, a catalytically
effective-amount of alumina, each having a BET surface area of at
least about 10 m.sup.2/g, preferably a surface area of from about
25 m.sup.2/g to 200 m.sup.2/g, and a zeolite, for example, Beta
zeolite or a zeolite selected from the group consisting of
.gamma.-zeolite, pentasil (e.g., ZSM-5), Mordenite, and mixtures
thereof.
[0011] In one aspect of the present invention, the zeolite
comprises a three-dimensional zeolite characterized by pore
openings whose smallest cross-sectional dimension is at least about
5 Angstroms and having a silicon to aluminum atomic ratio ("Si:Ai
atomic ratio") of greater than 5, e.g., a Si:Al atomic ratio of
from about 5 to 400.
[0012] In another aspect of the invention, the zeolite comprises
from about 10 to 90, preferably from about 20 to 70, percent by
weight, the alumina comprises from about 60 to 5, preferably from
about 50 to 20, percent by weight, and the ceria comprise's from
about 60 to 5, preferably from about 50 to 20, percent by weight,
of the combined weight of the zeolite, the alumina and the
ceria.
[0013] Yet another aspect of the invention provides for the zeolite
to be doped with a catalytic moiety, e.g., ion-exchanged or
impregnated, with an ion or with a neutral metal-containing species
selected from the group consisting of one or more of hydrogen,
platinum, rhodium, palladium, ruthenium, osmium, iridium, copper,
iron, nickel, chromium and vanadium, preferably, one or both of
platinum and iron.
[0014] Still another aspect of the invention provides that the
refractory carrier has a plurality of parallel exhaust flow
passages extending therethrough and defined by passage walls on
which the catalytic material is coated, and further comprising
either dispersed platinum carried on the catalytic material in an
amount of from about 0.1 to about 60, e.g., 0.1 to 15, preferably
0.1 to 5, g/ft.sup.3 or dispersed palladium carried on the
catalytic material in a quantity of from about 0.1 to 200,
preferably 20 to 120, g/ft.sup.3.
[0015] In accordance with the method aspect of the present
invention, there is provided a method for treating a diesel engine
exhaust stream containing a volatile organic fraction, the method
comprising contacting the stream with any of the catalyst
compositions described above under oxidizing conditions including a
temperature high enough to catalyze oxidation of at least some of
the volatile organic fraction. For example, the temperature of the
exhaust stream initially contacted with the catalyst composition
may be from about 100.degree. C. to 800.degree. C.
DEFINITIONS
[0016] As used herein and in the claims, the following terms shall
have the indicated meanings.
[0017] The term "BET surface area" has its usual meaning of
referring to the Brunauer, Emmett, Teller method for determining
surface area by N.sub.2 adsorption. Unless otherwise specifically
stated; all references herein to the surface area of a ceria,
alumina or other component refer to the BET surface area.
[0018] The term "activated alumina" has its usual meaning of a high
BET surface area alumina, comprising primarily one or more of
.gamma.-, .theta.- and .delta.-aluminas (gamma, theta and
delta).
[0019] The term "catalytically effective amount" means that the
amount of material present is sufficient to affect the rate of
reaction of the oxidation of pollutants in the exhaust being
treated.
[0020] The term "inlet temperature" shall mean the temperature of
the exhaust, test gas or other stream being treated immediately
prior to initial contact of the exhaust, test gas or other stream
with the catalyst composition.
[0021] The term "doped" used to refer to a zeolite being doped with
a metal or hydrogen, and the terms "dope" or "doping" used in the
same context, means that the metal or hydrogen moiety is
incorporated within the pores of the zeolite, as distinguished from
being dispersed on the surface of the zeolite but not to any
significant degree within the pores of the zeolite. Doping of a
zeolite is preferably carried out by known ion-exchange techniques
in which a zeolite is repeatedly flushed with a solution containing
metal cations (or an acid to provide hydrogen ions), or the zeolite
pores are flooded with such solution. However, the defined terms
include any suitable technique for incorporating a catalytic
moiety, e.g., one or more metals as ions or neutral
metal-containing species or hydrogen ions, within the pores of the
zeolite, especially by exchange or replacement of cations of the
zeolite.
[0022] The term "washcoat" refers to a thin, adherent coating of a
material, such as the catalytic material of the present invention,
disposed on the walls forming the parallel gas flow passages of a
carrier, which is typically made of a refractory material such as
cordierite or other oxide or oxide mixture, or a stainless
steel.
[0023] Reference herein or in the claims to ceria or alumina being
in "bulk" form means that the ceria or alumina is present as
discrete particles (which may be, and usually are, of very small
size, e.g., 10 to 20 microns in diameter or even smaller) as
opposed to having been dispersed in solution form into another
component. For example, the thermal stabilization of ceria
particles (bulk ceria) with alumina as described in U.S. Pat. No.
4,714,694, results in the alumina being dispersed into the ceria
particles and does not provide the dispersed alumina in "bulk"
form, i.e., as discrete particles of alumina.
[0024] The abbreviation "TGA" stands for thermogravimetric
analysis, which is a measure of the weight change (e.g., weight
loss) of a sample as a function of temperature and/or time. The
abbreviation "DTA" stands for differential thermal analysis, which
is a measure of the amount of heat emitted (exotherm) or absorbed
(endotherm) by a sample as a function of temperature and/or
time.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIGS. 1-5 show graphically the results of the engine test of
the catalysts of Example 1, set forth numerically in TABLE I-B:
specifically;
[0026] FIG. 1 is a plot of gas-phase HC conversion for sample E-1
and comparative samples C-1, C-2 and C-3 as a function of catalyst
inlet temperature;
[0027] FIG. 2 is a plot of carbon monoxide conversion for samples
E-1 and C-3 as a function of catalyst inlet temperature;
[0028] FIG. 3 is a plot of VOF removal for samples E-1 and C-3 as a
function of catalyst inlet temperature;
[0029] FIG. 4 is a plot of the reduction of the total mass of
particulates (TMP) for samples E-1 and C-3 as a function of
catalyst inlet temperature;
[0030] FIG. 5 is a plot of SO.sub.3 and H.sub.2O emissions for
samples E-1 and C-3 and for untreated exhaust as a function of
catalyst inlet temperature;
[0031] FIGS. 6-9 are plots of hydrocarbon, SO.sub.2 and CO
conversion for samples E-2, E-3 and C-4 of Example 2:
specifically;
[0032] FIG. 6 is a plot of gas-phase hydrocarbon conversion as a
function of catalyst inlet temperature at 50,000 space velocity for
samples E-2, E-3 and C-4;
[0033] FIG. 7 is a plot of gas-phase hydrocarbon conversion as a
function of catalyst inlet temperature at 90,000 space velocity for
samples E-2, E-3 and C-4;
[0034] FIG. 8 is a plot of SO.sub.2 conversion as a function of
catalyst inlet temperature at 90,000 space velocity for samples
E-2, E-3 and C-4;
[0035] FIG. 9 is a plot of CO conversion as a function of catalyst
inlet temperature at 90,000 space velocity for samples E-2, E-3 and
C-4; and
[0036] FIG. 10 is an exemplary TGA/DTA plot of sample E-8 of
Example 5.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
THEREOF
[0037] The present invention provides an oxidation catalyst
composition which is effective for treating diesel engine exhaust,
particularly with regard to reducing the total particulates and HC
and CO content of the exhaust. The carbonaceous particulates
("soot") component of diesel engine exhaust is, as discussed above,
known to be comprised of relatively dry carbonaceous particulates
and a volatile organic fraction ("VOF") comprising high molecular
weight hydrocarbons resulting from unburned and partially burned
diesel fuel and lubricating oil. The VOF is present in the diesel
exhaust as either a vapor phase or a liquid phase, or both,
depending on the temperature of the exhaust. Generally, it is not
feasible to attempt to remove or treat the dry, solid carbonaceous
particulates component of the total particulates by catalytic
treatment, and it is the VOF component which can be most
effectively removed by conversion via utilization of an oxidation
catalyst. Therefore, in order to reduce the total particulates
discharged so as to meet present and impending Government
regulations concerning maximum allowable total particulates, the
volatile organic fraction, or at least a portion thereof, must be
oxidized to innocuous CO.sub.2 and H.sub.2O by being contacted with
an oxidation catalyst under suitable reaction conditions. The
required U.S. Government limits for 1991 on HC, CO, nitrogen oxides
("NO.sub.x") and total particulate emissions ("TPM") in diesel
engine exhaust have been largely met by suitable engine design
modifications. For 1994 the HC, CO and NO.sub.x limits remain
unchanged from 1991 standards but the upper limit on TPM will be
reduced from the 1991 level of 0.25 grams per horsepower-hour
("g/HP-hr") to 0.10 g/HP-hr. Although the oxidation catalysts of
the present invention, when employed as a diesel exhaust treatment
catalyst, are effective for effectuating a reduction in total
particulates, they are also capable, especially with the optional
addition of platinum or other catalytic metals as described below,
of providing the added-advantage of also oxidizing a portion of the
HC and CO contained in the gaseous component of the diesel engine
exhaust. When sulfur or sulfur compounds are present in the exhaust
in significant quantities, platinum is eliminated or used in
limited amounts so as not to promote the unwanted effect of
excessive oxidation of SO.sub.2 to SO.sub.3.
[0038] Further, the zeolite component of the present invention is
able to trap hydrocarbon molecules which might otherwise, during
periods when the exhaust gas is relatively cool, escape untreated
from the catalyst. It is believed that the trapped hydrocarbons are
either oxidized within the zeolite or released from the zeolite
only when the temperature of the catalyst composition is high
enough to effectively catalyze oxidation of the trapped
hydrocarbons, or both.
[0039] A basic and novel characteristic of the present invention is
believed to reside in a catalyst composition comprising the defined
combination of ceria, zeolite and, optionally, alumina, and one or
both of the optional doping of the zeolite and dispersal of the
catalytic metals, platinum or palladium as part of the composition
and in the use thereof to treat diesel exhaust streams.
[0040] As noted above, the bulk ceria and the bulk alumina may each
have a surface area of at least about 10 m.sup.2/g, for example, at
least about 20 m.sup.2/g. Typically, the bulk alumina may have a
surface area of from about 120 to 180 m.sup.2/g and the bulk ceria
may have a surface area of from about 70 to 150 m.sup.2/g.
[0041] The fact that a diesel oxidation catalyst composition which
may contain activated alumina as a major component thereof has
proven to be successful is in itself surprising, in view of the
consensus of the prior art that alumina, if used at all in diesel
oxidation catalysts, must be a low surface area alumina
(.alpha.-alumina) and/or be used in conjunction with
sulfate-resistant refractory metal oxides such as zirconia, titania
or silica. It has nonetheless been found that in accordance with
one aspect of the present invention, surprisingly, a catalyst
composition comprising a combination of high surface area ceria, a
suitable zeolite, and, optionally, high surface area alumina,
provides a catalytic material which effectively catalyzes the
oxidation of the volatile organic fraction so as to provide a
significant reduction in total particulates in diesel engine
exhaust and is capable of adsorbing and catalyzing the combustion
of gaseous hydrocarbons. It should be noted that the prior art
generally considers refractory base metal oxides used in diesel
oxidation catalysts to be merely supports for the dispersal thereon
of catalytically active metals such as platinum group metals. In
contrast, the present invention teaches that a catalytic material
comprising ceria and, optionally, alumina of sufficiently high
surface area (10 m.sup.2/g or higher) and zeolite, dispersed on a
suitable carrier, provides a durable and effective diesel oxidation
catalyst.
[0042] It has further been found that beneficial effects are
attained by the optional incorporation of platinum or palladium in
the catalyst composition, provided that in the case of platinum,
the platinum is present at loadings much lower than those
conventionally used in oxidation catalysts. If the catalytic metal
platinum or palladium is added to the catalytic composition, it
serves to catalyze the oxidation of gas-phase HC and CO pollutants
as an added benefit. However, such catalytic metal is not needed to
supplement the action of the ceria-zeolite or ceria-alumina-zeolite
catalytic material in reducing total particulate emissions. Neither
the platinum or palladium catalytic metal nor the metals or
hydrogen used to dope the zeolite appear to significantly affect
the rate of particulates conversion.
[0043] The Zeolite
[0044] The zeolite employed serves both to catalyze the oxidation
of VOF and to crack the larger VOF molecules and, during periods of
relatively low temperature operation, to trap gas-phase
hydrocarbons within the zeolite pores. If the zeolite has been
doped with one or more catalytic metals or hydrogen, the trapped
gas-phase hydrocarbons are brought into intimate contact with the
catalytically active cations which facilitates oxidation of the
hydrocarbons. In any case, the zeolite pores also serve to retain
some of the gas-phase hydrocarbons during start-up or other periods
when the catalyst is relatively cool and therefore less effective
in catalyzing oxidation reactions, and to release the hydrocarbons
only when the catalyst has been heated to higher temperatures. The
higher temperatures impart sufficient energy to the trapped
hydrocarbon molecules to enable them to escape the zeolite pores,
but also enhance oxidation of the hydrocarbons in contact with the
catalyst. The zeolite therefore serves not only as a catalyst for
VOF oxidation, but as a hydrocarbon filter which traps hydrocarbons
during periods of relatively low temperature and concomitant low
catalytic activity and retains them until they can be efficiently
oxidized by the catalyst during periods of relatively high
temperature.
[0045] The Carrier (Substrate)
[0046] The carrier used in this invention should be relatively
inert with respect to the catalytic composition dispersed thereon.
The preferred carriers are comprised of ceramic-like materials such
as cordierite, .alpha.-alumina, silicon nitride, zirconia, mullite,
spodumene, alumina-silica-magnesia or zirconium silicate, or of
refractory metals such as stainless steel. The carriers are
preferably of the type sometimes referred to as honeycomb or
monolithic carriers, comprising a unitary body, usually cylindrical
in configuration, having a plurality of fine, substantially
parallel gas flow passages extending therethrough and connecting
both end-faces of the carrier to provide a "flow-through" type of
carrier. Such monolithic carriers may contain up to about 700 or
more flow channels ("cells") per square inch of cross section,
although far fewer may be used. For example, the carrier may have
from about 7 to 600, more usually from about 200 to 400, cells per
square inch ("cpsi").
[0047] While this discussion and the following examples relate to
flow-through type carrier substrates, wall-flow carriers (filters)
may also be used. Wall-flow carriers are generally similar in
structure to flow-through carriers, with the distinction that each
channel is blocked at one end of the carrier body, with alternate
channels blocked at opposite end-faces. Wall-flow carrier
substrates and the support coatings deposited thereon are
necessarily porous, as the exhaust must pass through the walls of
the carrier in order to exit the carrier structure.
[0048] The Catalytic Material
[0049] The ceria-zeolite or ceria-alumina-zeolite catalytic
material may be prepared in the form of an aqueous slurry of ceria,
alumina (optional) and zeolite particles, the particles optionally
being impregnated with the platinum or palladium catalytic metal
component if one is to be dispersed on the catalytic material, and
the zeolite may optionally be doped, e.g., may be ion-exchanged
with selected cations. The slurry is then applied to the carrier,
dried and calcined to form a catalytic material coating
("washcoat") thereon. Typically, the ceria, optional alumina and
zeolite particles are mixed with water and an acidifier such as
acetic acid, nitric acid or sulfuric acid, and ball milled to a
desired particle size.
[0050] Dispersed Catalytic Metals: The optional platinum or
palladium catalytic metal component is, when used, dispersed onto
the ceria and zeolite particles or onto the ceria, alumina and
zeolite particles. In such case, the ceria, zeolite and optional
alumina act not only as catalysts in their own right but also as a
support for the optional platinum or palladium catalytic metal
component. (Such platinum or palladium dispersed onto the zeolite
is not to be confused with the optional metals--which may be, among
others, platinum and/or palladium--with which the zeolite may be
doped by ion-exchange or otherwise as discussed in more detail
below).
[0051] The dispersal of the platinum and/or palladium onto the
ceria, the optional alumina and the zeolite may be carried out
after the ceria-zeolite or ceria-alumina-zeolite catalytic material
is coated as a washcoat onto a suitable carrier, by impregnating
the coated carrier with a solution of a suitable platinum and/or
palladium compound, followed by drying and calcination. However,
the ceria particles or both the ceria and alumina particles may be
impregnated with a suitable platinum compound before the ceria or
ceria-alumina material (plus the zeolite) is applied to the
carrier. The latter technique reduces the amount of platinum and/or
palladium dispersed onto the outer surface of the zeolite
particles, thereby increasing the amount of catalytic material
dispersed on the high surface ceria or ceria-alumina relative to
the zeolite. In either case, the optional dispersed platinum or
palladium metal may be added to the ceria-zeolite or
ceria-alumina-zeolite catalytic material as, e.g., a solution of a
soluble platinum compound, the solution serving to impregnate the
ceria and optional alumina particles (or a coating thereof on the
carrier) and the zeolite, if present at this point of the process.
The impregnated components may then be dried and the platinum or
palladium fixed thereon. Fixing may be carried out by calcination
or by treatment with hydrogen sulfide or by other known means, to
render the metal in water-insoluble form. Suitable platinum
compounds for use in the foregoing process include potassium
platinum chloride, ammonium platinum thiocyanate, amine-solubilized
platinum hydroxide and chloroplatinic acid, as is well-known in the
art. During calcination, or at least during the initial phase of
use of the catalyst, such compounds, if present, are converted into
the catalytically active elemental platinum metal or its oxide.
Palladium nitrate or palladium analogs of the aforementioned
platinum compounds may be used to provide palladium.
[0052] Doped Metals or Hydrogen: The zeolite used may optionally be
doped either by conventional acid treatment to convert the zeolite
to the acid form or by conventional ion-exchange techniques to
exchange catalytically active metal cations for cations of the
zeolite, or by any other suitable technique which disposes the
catalytic metal or metals or hydrogen within the pores of the
zeolite. (Conventional acid treatment of a zeolite to convert it to
an acid [hydrogen] form of the zeolite is, for economy of
expression, referred to herein as doping the zeolite with
hydrogen.) For doping the zeolite with metals, conventional
ion-exchange techniques including repeated flushing with a solution
of suitable metal compounds may be carried out. However, other
doping techniques may be employed. For example, if the zeolite
pores are flooded with a solution of, e.g., tetraammine platinum
hydroxide or tetraammine platinum chloride, the platinum
dissociates as a cation, and this positively charged platinum ionic
species will adhere to negatively charged sites within the zeolite
pores, even if the conventional ion-exchange technique of repeated
exchanges of the solution with the zeolite is not carried out.
Other techniques are not excluded, as long as the metals are
dispersed throughout or within the zeolite pores. The conventional
practice of indicating the catalytic moiety used to dope a zeolite
by using it as a prefix for the zeolite is followed herein and in
the claims. Thus, "H-Mordenite" indicates the hydrogen form of
Mordenite, i.e., a hydrogen-doped Mordenite, "Fe-Beta" indicates an
iron-doped Beta zeolite, "Fe--Pt-Beta" indicates an iron and
platinum-doped Beta zeolite, etc.
[0053] Preparing the Catalyst Composition
[0054] Generally, a slurry of ceria particles, activated alumina
particles and zeolite particles will be deposited upon the carrier
substrate and dried and calcined to adhere the catalytic material
to the carrier and, when the dispersed platinum or palladium
compound is present, to revert the platinum or palladium compound
to the elemental metal or its oxide. The zeolite particles may
optionally have been doped with one or more metals, e.g., with a
combination of platinum and/or base metals, or acid-treated to dope
the zeolite with hydrogen, i.e., to provide an acid form of the
zeolite.
[0055] When the catalytic material or any component is applied to a
suitable honeycomb carrier, such as described above, the amount of
the component is conventionally expressed herein and in the claims
as weight of component per unit volume of catalyst, as this measure
accommodates the presence of different sizes of catalyst
composition voids provided by different carrier wall thicknesses,
gas flow passage dimensions, etc. Grams per cubic inch
("g/in.sup.3") units are used herein and in the claims to express
the quantity of relatively plentiful components such as the
ceria-alumina-zeolite catalytic material, and grams per cubic foot
("g/ft.sup.3") units are used to express the quantity of the
sparingly-used ingredients, such as the platinum metal. For typical
diesel exhaust applications, the ceria-zeolite or
ceria-alumina-zeolite catalytic material of the present invention
generally may comprise from about 0.25 to about 4.0 g/in.sup.3,
preferably from about 0.25 to about 3.0 g/in.sup.3 of the coated
carrier substrate. The catalytic material may optionally also
include from about 0.1 to 60, preferably from about 0.1 to 15
g/ft.sup.3 of dispersed platinum or from about 0.1 to 200,
preferably from about 20 to 120 g/ft.sup.3 of dispersed palladium.
The zeolite may optionally be doped with from about 0.1 to 50
g/ft.sup.3 of metal, e.g., from about 1 to 60 g/ft.sup.3 of
precious metal, and/or from about 0.1 to 200 g/ft.sup.3 of base
metal or the zeolite may be converted to its hydrogen form. The
conversion to hydrogen form may be a 5 to 100 percent hydrogen
exchange.
[0056] Without wishing to be bound by a particular theory,
applicants offer the following hypothesis to explain the superior
performance, when used to treat diesel engine exhaust, of the
ceria-zeolite or ceria-alumina-zeolite catalytic materials
according to this invention. It is believed that diesel exhaust
contains a significant proportion of gases or vapors which are
close to their dew point, i.e., close to condensing to a liquid,
and thereby adding to the VOF portion of the particulates at the
conditions obtaining in the exhaust pipe. These "potential
particulates" condense in the ceria-zeolite or
ceria-alumina-zeolite catalytic materials, their condensation being
enhanced by a capillary condensation effect, a known phenomenon in
which a capillary-like action facilitates condensation of oil
vapors to liquid phase. The small pore size of the high surface
area ceria or ceria and alumina components of the catalytic
material is believed to provide such capillary condensation action
for the VOF. Generally, the higher the surface area of the ceria
and optional alumina, the smaller is the pore size. As the exhaust
temperature increases during increased work loads imposed on the
diesel engine, the condensed hydrocarbon liquids (condensed VOF)
are desorbed from the ceria or ceria-alumina components of the
catalytic material and volatilize, at which time the catalytic
effect of the ceria or the ceria-alumina components of the
catalytic material, which provide numerous active sites, and the
cracking effect attained by contact of the VOF with the surface of
the zeolite, enhances gas-phase oxidation, i.e., combustion, and
cracking of the desorbed, re-volatilized hydrocarbon (VOF) vapors.
Even if a proportion of the vapors re-volatilized from the
condensate is not combusted, the cracking of heavy VOF components
to lighter hydrocarbons on the outer surface of the zeolite reduces
the total amount of condensibles, so that the total particulates
output from the diesel engine is concomitantly further reduced in
this latter regard, the ceria or ceria-alumina component of the
catalytic material is believed to act as a trap and a storage
medium for condensed or condensible VOF during relatively cool
phases of the exhaust, and volatilizes the VOF and cracks it during
relatively hot phases. The zeolite serves, during relatively cool
phases of the exhaust, to trap gas-phase hydrocarbon components and
retain them until temperatures high enough for the platinum or
other metal to oxidize at least some of the gas-phase HC are
attained. The porous nature of the ceria-zeolite or
ceria-alumina-zeolite catalytic material is also believed to
promote rapid diffusion of the VOF throughout the washcoat
structure, thereby facilitating relatively low temperature
gasification and oxidation of the VOF upon increases in temperature
of the catalyst during higher engine load (increased exhaust gas
temperature) cycles. The presence of sulfates does not
significantly adversely affect the capacity of the ceria-zeolite or
ceria-alumina-zeolite catalytic material to reduce particulate
emissions.
[0057] Generally, other ingredients may be added to the catalyst
composition of the present invention such as conventional thermal
stabilizers for the alumina, e.g., rare earth metal oxides such as
ceria. Thermal stabilization of high surface area ceria and alumina
to militate against phase conversion of these oxides to less
catalytically effective low surface area forms is well-known in the
art, although thermal stabilization of alumina is not usually
needed for diesel exhaust service wherein exhaust temperatures are
typically lower than for gasoline-fueled engines. Such thermal
stabilizers may be incorporated into the bulk ceria or into the
optional bulk activated alumina, by impregnating the ceria (or
alumina) particles with, e.g., a solution of a soluble compound of
the stabilizer metal, for example, an aluminum nitrate solution in
the case of stabilizing bulk ceria. Such impregnation is then
followed by drying and calcining the impregnated ceria particles to
convert the aluminum nitrate impregnated therein into alumina, to
thermally stabilize the ceria. A suitable technique is shown in
U.S. Pat. No. 4,714,694 to C. Z. Wan et al (the disclosure of which
is incorporated by reference herein), in which ceria particles are
impregnated with a liquid dispersion of an aluminum compound, e.g.,
an aqueous solution of a soluble aluminum compound such as aluminum
nitrate, aluminum chloride, aluminum oxychloride, alumnium acetate,
etc. After drying and calcining the impregnated ceria in air at a
temperature of, e.g., from about 300.degree. C. to 600.degree. C.
for a period of 1/2 to 2 hours, the aluminum compound impregnated
into the ceria particles is converted into an effective thermal
stabilizer for the ceria, to provide an "alumina-stabilized
ceria".
[0058] In addition, the catalyst compositions of the invention may
contain other catalytic ingredients such as other base metal
promoters or the like. However, in one embodiment, the catalyst
composition of the present invention consists essentially only of
the high surface area ceria and the zeolite, and, optionally, the
high surface area alumina, preferably present in the weight
proportions given above, with or without thermal stabilizers
impregnated into the alumina and ceria and, optionally, including
palladium or limited amounts of platinum dispersed thereon, and
optionally, employing a doped zeolite as described above.
[0059] Examples A and B show typical methods of preparing
metal-doped zeolites which are useable as components of the
catalytic material of the present invention.
EXAMPLE A
Preparation of Fe-Beta Zeolite
[0060] To prepare a sample of iron-exchanged Beta zeolite (Fe-Beta
zeolite), 17 grams of iron [II] sulfate was dissolved in 800 ml
water. One hundred grams of Beta zeolite was added to the solution
which was then stirred for 1 hour at a temperature of 70.degree. C.
to 80.degree. C. The resulting slurry was then filtered and washed
with 2 liters of water, dried at 120.degree. C. and calcined at
540.degree. C. The resulting material comprised 1.65% by weight
iron. This technique was employed to prepare the Fe-Beta zeolite
catalysts used in the following Examples.
EXAMPLE B
Preparation of Pt--Fe-Beta Zeolite
[0061] A solution was prepared using 0.54 grams of tetraammine
platinum chloride in 500 ml water, to which 100 grams of Beta
zeolite was added. The mixture was stirred for 24 hours at room
temperature so that platinum ions replaced sodium ions in the
zeolite material. The slurry was then filtered and washed with 2
liters water, dried and calcined at 540.degree. C. The resulting
calcined, platinum ion-exchanged Beta zeolite was then
ion-exchanged with iron [II] sulfate by adding the zeolite to a
solution of iron [III] sulfate equivalent to 17 grams of iron [II]
sulfate in 800 ml water. The solution was allowed to stand for
about 1 hour and was then stirred at 70.degree. C. to 80.degree. C.
for 1 hour. The resulting slurry was then filtered, washed with
water, dried at 120.degree. C. and calcined at 540.degree. C. On a
dry basis, the Beta zeolite comprised 1.6% by weight iron and 0.25%
by weight platinum. This technique was employed to prepare the
Fe--Pt-Beta zeolite catalyst components used in the following
Examples.
EXAMPLE 1
[0062] A. A catalyst according to the present invention was
prepared by coating a honeycomb monolith with a catalytic material
comprised of a zeolite, bulk ceria and bulk alumina to provide 0.84
g/in.sup.3 .gamma.-alumina, 0.83 g/in.sup.3 alumina-stabilized
ceria and 0.83 g/in.sup.3 Fe-Beta zeolite. The honeycomb monolith
was a cordierite substrate measuring 9 inches in diameter by 6
inches long and having 4.00 cpsi. The catalyst material also
provided 2.5 g/ft.sup.3 of platinum, 80 percent by weight of which
was dispersed on the alumina, and 20 percent by weight of which was
dispersed on the ceria. This catalyst was designated E-1.
[0063] B. For comparison, three other catalysts were prepared to
provide a series of three otherwise identical compositions
containing a ceria-alumina catalytic material but no zeolite and,
in two cases, having platinum dispersed thereon. The platinum
loadings of these comparative catalysts were 0.0, 0.5 and 2.0
g/ft.sup.3 platinum. Each comparative catalyst comprised a
.gamma.-alumina undercoat at a loading of 1.0 g/in.sup.3 upon which
was coated a top coat layer comprised of 1.05 g/in.sup.3
.gamma.-alumina plus 0.90 g/in.sup.3 alumina-stabilized ceria. The
alumina-stabilized ceria contained 2.5 weight percent
Al.sub.2O.sub.3 based on the combined weight of bulk ceria and
stabilizing alumina dispersed therein. The catalysts were coated
onto a 9 inches diameter by 6 inches long, 400 cpsi cordierite
substrate. As used herein, "cpsi" is an abbreviation for cells per
square inch, denoting the number of gas flow passages per square
inch of face area of the substrate. The resulting catalyst samples
were designated as C-1 (0.0 g/ft.sup.3 platinum, aged 24 hours as
described below), C-2 (0.5 g/ft.sup.3 platinum, aged 25 hours) and
C-3 (2.0 g/ft.sup.3 platinum, aged 24 hours).
[0064] C. The four catalyst samples were conditioned prior to
evaluation using an aging cycle involving 20 minutes each at Modes
2,6 and 8 of the European Thirteen Mode Cycle Test Procedure (ECE
R.49 Thirteen Mode Cycle). This Test Procedure is set forth in the
Society of Automotive Engineers Publication, SAE Paoer #880715,
published at the International Congress and Exposition, Detroit,
Mich., Feb. 29 through Mar. 4, 1988, by Georgio M. Cornetti et al.
The disclosure of this SAE publication is incorporated by reference
herein. Prior to testing to develop the data of TABLE I-B and FIGS.
1-5, the three catalyst samples were aged 24 or 25 hours as
indicated above on a Cummins 6BT turbocharged diesel engine having
a 5.9 liter displacement and rated at 230 horsepower. For both
aging and test purposes, the engine was run with low sulfur fuel
(0.05 weight percent sulfur) under steady state conditions using
test modes selected from the aforesaid European Thirteen Mode Cycle
Test Procedure.
[0065] The engine conditions for the test modes along with average
(for five runs) catalyst inlet temperatures and baseline emissions
(of untreated engine exhaust) are shown in TABLE I-A.
1TABLE I-A Cummins 6BT 230 HP Turbocharged Diesel Engine, 5.9 Liter
Displacement, Steady State Conditions For Catalyst Tests Engine
Conditions Average Test % Catalyst Inlet Mode No. rpm Load Temp.
(.degree. C.) 1 803 Low 128 .+-. 16 2 1560 10 214 .+-. 3 10 2515 50
338 .+-. 4 4 1609 50 400 .+-. 4 6 1609 100 549 .+-. 5 8 2515 100
571 .+-. 2 Baseline Emissions - Untreated Exhaust Test Average
Emissions (g/bhp-hr).sup.1 Mode No. TPM VOF HC CO 1 -- 0.078 1.04
3.01 2 0.265 0.137 0.541 2.57 10 0.151 0.047 0.212 0.68 4 0.146
0.023 0.103 0.52 6 0.221 0.016 0.099 2.23 8 0.097 0.010 0.122 0.46
.sup.1grams per brake horsepower hour
[0066] The conversion activities of each of these catalysts was
evaluated in selected steady state modes of an ECE R.49 Thirteen
Mode Cycle Test Procedure using a 1991 Cummins 6BT (6 liter DI/TC)
engine after approximately 24 hours aging, as described above. The
exhaust inlet temperature and the conversion rate for the
conversion of VOF, TPM (total particulates), hydrocarbons and
carbon monoxide were measured for various modes of the test, and
are set-forth in the following TABLE I-B.
2TABLE I-B ECE Thirteen Mode Cycle Test DOC Pt Load. Cat. Inlet %
Removal Sample (g/ft.sup.3) Mode # Temp. (.degree. C.) VOF TPM HC
CO C-1 0.0 2 209 72 63 31 1 10 335 60 27 32 7 4 399 62 18 38 18 6
547 84 -40 44 27 8 572 79 -181 39 -4 C-2 0.5 2 215 60 45 27 6 10
343 58 28 41 63 6 549 91 -64 56 85 8 570 80 -201 62 45 C-3 2.0 1
127 56 52 37 -1 2 215 61 61 39 8 10 341 53 31 74 86 4 397 61 22 82
87 6 554 89 -60 78 95 8 572 79 -200 71 70 E-1 2.5 1 121 55 75 72 3
2 217 83 63 61 6 10 338 66 37 72 86 4 425 61 -1 81 87 6 552 90 -72
79 95 8 557 70 -172 74 77
[0067] The data of TABLE I-B are shown graphically in FIGS. 1-5 and
show that the catalyst according to the present invention (E-1)
exhibited high levels of gas-phase hydrocarbon removal in the
lowest temperature modes (modes 1 and 2) of the test (72% and 61%)
compared to the comparative samples under the same conditions
(e.g., 37% and 39% for catalyst C-3). At higher temperatures, e.g.,
340.degree. and above, gas-phase hydrocarbon conversion rates of
sample E-1 were comparable to sample C-3 but still markedly
superior to samples C-1 and C-2. The superior gas-phase hydrocarbon
conversion performance of sample E-1, especially at low temperature
regimes, is attributed to the Fe-Beta zeolite. Carbon monoxide
conversion was essentially the same for sample E-1 and the
comparative sample C-3, as will be readily appreciated from FIG. 2,
indicating that the addition of the iron-Beta zeolite did not
appear to have an effect on carbon monoxide conversion efficiency.
The overall VOF removal performance of sample E-1 was slightly
better than any of the comparative samples. FIG. 3 graphically
illustrates VOF conversion efficiency of catalyst E-1 and
comparative sample C-3.
EXAMPLE 2
Catalyst Comprising Fe-Beta Zeolite and Platinum on Alumina and
Ceria
[0068] A.(1) A catalyst component comprising platinum dispersed on
Fe-Beta zeolite and on ceria was prepared as follows. Two hundred
grams of Fe-Beta zeolite containing 1.65% by weight iron was made
into a water-based slurry and milled for 3 hours. The pH was
adjusted to 4.5 by the addition of monoethanolamine. An
amine-solubilized platinum hydroxide solution was added to the
slurry, followed by acetic acid to precipitate the platinum, which
constituted 0.029% by weight. The slurry was further milled to
reduce the particle size so that 90 percent of the particles have a
diameter of 12 microns or less.
[0069] (2) Separately, a similar platinum salt solution was slowly
added to 650 grams of ceria having a BET surface area of 143
m.sup.2/g that had previously been impregnated with 2.5% by weight
alumina stabilizer (basis, weight of ceria plus alumina stabilizer)
by a conventional technique of impregnating the ceria with a
solution of an alumina precursor and drying and calcining the
impregnated ceria. Sufficient platinum salt solution was added to
deposit 0.029% platinum onto the ceria, by weight of ceria plus
platinum. The platinum was fixed onto the ceria by adding 5 cc of
acetic acid per 100 grams of solids to the Pt/ceria mixture. The
Pt/ceria mixture was milled in water so that 90% of the particles
have a diameter of 12 microns or less to produce a slurry having
47.8% solids.
[0070] (3) Portions of the two slurries of parts (1) and (2) were
mechanically mixed to produce a slurry comprising about 50% ceria,
50% Fe-Beta zeolite and 0.029% platinum, all by weight (dry basis).
A portion of the mixed slurry was dried overnight at 100.degree. C.
in air and then calcined in air at 450.degree. C. for 1 hour. A
portion of the dried, calcined powder was made into a slurry and
coated onto a cordierite honeycomb 1.5 inches in diameter, 3 inches
long and having 400 cpsi, at a loading of 1.0 g/in.sup.3. This
washcoat loading yielded 0.5 g/ft.sup.3 of platinum. The coated
monolith was dried and calcined and designated E-2.
[0071] B. (1) A catalyst was prepared from a slurry comprising
three types of particles, platinum-bearing ceria particles,
platinum-bearing alumina particles and Fe-Beta zeolite particles,
i.e., Beta zeolite ion-exchanged with iron. These particles were
prepared as separate slurries before being mixed together.
[0072] (2) The slurry of platinum-bearing ceria particles was
prepared by impregnating ceria particles having a BET surface area
of 143 m.sup.2/g with a platinum salt solution as described above
in part A(1) to obtain a platinum metal loading of 0.035% upon
calcination. The resulting particles were milled to an average
particle size of less than 12 microns.
[0073] (3) The slurry of platinum-bearing alumina particles was
prepared by impregnating alumina having a BET surface area of 150
m.sup.2/g with a platinum salt solution to yield 0.138% by weight
platinum metal upon calcination. The particles were milled to an
average particle size less than 12 microns.
[0074] (4) A slurry of Fe-Beta zeolite particles was prepared by
milling the particles to an average particle size of less than 12
microns.
[0075] (5) The foregoing three slurries were mechanically mixed to
form a composite slurry yielding 33.18% ceria, 33.18% Fe-Beta
zeolite, 33.58% alumina and 0.058% platinum by weight (dry basis).
The composite slurry was coated onto an oval-shaped honeycomb
monolith having cross section dimensions of 3.18.times.6.68 inches,
a length of 3.15 inches and 400 cpsi, at a total loading of 2.5
g/in.sup.3, consisting of 0.84 g/in.sup.3 alumina, 0.83 g/in.sup.3
ceria, 0.83 g/in.sup.3 Fe-Beta zeolite and 0.0014 g/in.sup.3
(equivalent to 2.4 g/ft.sup.3) platinum. The catalyst-coated
honeycomb carrier was dried in air at 100.degree. C. and then
calcined at 450.degree. C. for 2 hours. A sample core 3 inches long
and 1.5 inches in diameter was drilled from the calcined honeycomb
and designated E-3.
[0076] (6) Fe-Beta zeolite having 1.65 weight percent iron was made
into an aqueous slurry and milled so that about 90 percent of the
particles had a diameter of 12 microns or less. The slurry was then
coated onto a carrier monolith 1.5 inches in diameter and 3.0
inches in length having 400 cpsi. The coated carrier was calcined
at 450.degree. C. in air for 1 hour and had a washcoat loading of
1.0 g/in.sup.3 (dry basis). This sample was designated C-4.
[0077] C. The catalysts designated E-2, E-3 and C-4 were tested in
a laboratory diagnostic reactor for comparison. The inlet stream of
the diagnostic reactor comprised 200 ppm heptane measured as
C.sub.1, 4.5% CO.sub.2, 50 ppm SO.sub.2, 1000 ppm NO, 200 ppm CO,
10% O.sub.2, 10% H.sub.2O, balance N.sub.2. The inlet temperatures
of the test gas stream to the catalyst were 275, 350, 425 and
500.degree. C.
[0078] The results of the diagnostic laboratory reactor tests as
set forth in FIGS. 6-9 show that the catalysts of the present
invention, E-2 and E-3, exhibited substantially better gas phase HC
activity at a space velocity ("sv") of 50,000/hr and better HC
activity at sv=90,000/hr than did the catalyst containing only
Fe-Beta zeolite (C-4). Furthermore, E-2 gave better results than
E-3 despite having a lower Pt loading (0.5 vs. 2.5 g/ft.sup.3). It
is also clear that the catalysts of the present invention gave much
better CO conversion activity than did the catalyst containing only
Fe-Beta zeolite, the latter exhibiting net negative CO conversion
due to partial oxidation of HC to form CO in an amount to give a
higher concentration in catalyst outlet stream than in the inlet
stream. Finally, the catalysts of the present invention exhibit
very low SO.sub.2 oxidation levels due to the low Pt loading,
indicating control of sulfate-make by the catalyst which would
otherwise contribute to particulate emissions. As used herein and
throughout this application space velocity ("sv") has the usual
meaning of the volumes of the exhaust stream or test gas, measured
at standard conditions of temperature and pressure, passing through
the volume of the catalyst composition (the dimensional volume of
the coated honeycomb monolith) per hour.
EXAMPLE 3
Catalyst Comprising Pt/CeO.sub.2, Pt/Al.sub.2O.sub.3 and Fe-Beta
Zeolite
[0079] A. A catalyst according to the present invention was
prepared by coating two differently configured honeycomb carriers
with identical loadings of the same catalytic material. One
honeycomb carrier to be tested on an Audi 100 automobile as
described below measured 5.66 inches in diameter by 6 inches long
and had 400 cpsi and was designated an "A-type carrier". A second
honeycomb carrier to be tested on a Mercedes Benz 200D automobile
as described below was of oval configuration and measured 3.03
inches by 5.78 inches (minor and major axes of the oval face) by 6
inches long and had 200 cpsi. This carrier was designated an
"M-type carrier". Both the A-type carrier and the M-type carrier
were coated with 2.5 g/in.sup.3 of a washcoat comprising 33% by
weight alumina-stabilized ceria and 34% by weight .gamma.-alumina
with platinum dispersed on both the ceria and the alumina as
described above in Example 2. The washcoat further comprised 33% by
weight Fe-Beta zeolite having 1.65% iron by weight. The washcoat
yielded a platinum loading of 2.5 g/ft.sup.3 platinum distributed
equally on the ceria and alumina; there was no platinum on the
Fe-Beta zeolite. These catalysts were each designated E-4.
[0080] B. A second pair of catalysts according to the present
invention was prepared as described in part A above, i.e., one was
prepared using an A-type carrier and one using an M-type carrier,
except that the platinum loading was 10 g/ft.sup.3 dispersed
equally on the ceria and alumina, with no platinum on the Fe-Beta
zeolite. These catalysts were each designated E-5.
[0081] C. (1) A comparative catalyst was prepared by coating an
M-type carrier with 2.5 g/in.sup.3 of a catalyst washcoat
consisting of 46% by weight alumina-stabilized ceria having a BET
surface area of 143 m.sup.2/g and 54% by weight .gamma.-alumina
having a BET surface area of 150 m.sup.2/g. The catalyst was
prepared by separately impregnating bulk alumina and bulk ceria
with an amine-solubilized platinum hydroxide solution,
precipitating the platinum with acetic acid and milling the
particles to a size of 90 percent of the particles having a
diameter of 12 microns or less. The two slurries were then blended
to provide a 50% solids slurry which was used to coat the honeycomb
carrier. The washcoat yielded a platinum loading of 2.0 g/ft.sup.3
equally dispersed on the ceria and alumina. This catalyst was
designated comparative sample C-X.
[0082] (2) Catalyst pairs respectively designated C-5 and C-6 were
prepared as described for catalyst C-X, i.e., two C-5 samples, one
on each of an A-type carrier and an M-type carrier and two
C-6-samples, one on each of an A-type and an M-type carrier, except
that the platinum loadings were 5.0 g/ft.sup.3 on the C-5 samples
and 10 g/ft.sup.3 on the C-6 samples.
[0083] (3) A commercial catalyst material comprised a washcoat of
85-90% by weight alumina, 5-7% by weight vanadium oxide
(V.sub.2O.sub.5) and 1-2% by weight platinum coated onto a
honeycomb carrier measuring 5.66 inches in diameter by 6 inches
long and having 400 cpsi. The catalyst had 60 g/ft.sup.3 of
catalytic material. This catalyst was designated C-7.
[0084] D. The catalysts of parts A, B and C were tested for
hydrocarbon conversion, CO activity and SO.sub.2 conversion using
the exhaust from an Audi 100 automobile having a 5 cylinder, 2.5
liter, direct injected/turbocharged engine with intercooling and
exhaust gas recycle and a Mercedes Benz 200D automobile having a 4
cylinder, 2.0 liter, indirect injected/naturally aspirated
engine.
[0085] The tests were conducted using a chassis dynamometer and
emissions measurement instrumentation and techniques for total
gas-phase hydrocarbons (HC), carbon monoxide (CO, nitrogen oxides
NO.sub.x) and total particulates (TPM).
[0086] The emissions evaluation test used was the European
transient test known as Cycle "A". This test consists of two parts.
The first, the ECE part, is characterized by lower loads and cooler
exhaust temperatures than the second part, the EUDC (Extra Urban
Driving Cycle). Emissions are measured for each of these parts of
the test and the results are weighed and combined to give emissions
for the overall Cycle "A" test. Emissions are expressed in
grams/kilometer (g/km).
[0087] The results of the Cycle A tests are set forth in TABLE II-A
and TABLE II-B.
3TABLE II-A Test Results for Audi 100 Diesel Automobile Pt Carbon
Test Cata- Load. Hydrocarbons Monoxide Particulates Part lyst
g/ft.sup.3 g/km.sup.1 /%.sup.2 g/km.sup.1 /%.sup.2 g/km.sup.1
/%.sup.2 ECE None 0.95 -- 3.05 -- 0.236 -- C-5 5.0 0.71 25 3.05 0
0.088 62.7 C-6 10.0 0.67 29 3.05 0 0.069 70.8 E-4 2.5 0.18 81 3.05
0 0.083 64.8 E-5 10.0 0.19 80 3.05 0 0.068 71.2 C-7 40.0+ 0.46 52
2.59 15.1 0.082 65.3 EUDC None 0.13 -- 0.44 -- 0.114 -- C-5 5.0
0.09 30.8 0.30 32 0.065 43 C-6 10.0 0.09 30.8 0.30 32 0.048 57.9
E-4 2.5 0.104 20.0 0.41 7 0.06 47.4 E-5 10.0 0.061 53.1 0.16 64
0.05 56.1 C-7 40.0+ 0.017 86.9 0.02 95 0.088 22.8 CYCLE None 0.428
-- 1.4 -- 0.148 -- "A" C-5 5.0 0.320 25.2 1.3 7.1 0.073 50.7 C-6
10.0 0.300 29.9 1.3 7.1 0.056 62.2 E-4 2.5 0.132 69.2 1.37 2.1
0.068 54.1 E-5 10.0 0.107 75.0 1.21 13.6 0.057 61.5 C-7 40.0+ 0.178
58.4 0.96 31.4 0.085 42.6 .sup.1Emissions in exhaust. .sup.2Percent
of emissions converted to innocuous substances by catalytic
treatment.
[0088]
4TABLE II-B Test Results for Mercedes Benz 200D Diesel Automobile
Pt Carbon Test Cata- Load. Hydrocarbons Monoxide Particulates Part
lyst g/ft.sup.3 g/km.sup.1 /%.sup.2 g/km.sup.1 /%.sup.2 g/km.sup.1
/%.sup.2 ECE None 0.07 -- 0.9 -- 0.125 -- C-X 2.0 0.07 0 0.88 2.2
0.09 28 C-5 5.0 0.07 0 0.86 4.4 0.09 28 C-6 10.0 0.065 7.1 0.75 17
0.09 28 E-4 2.5 0.008 89 0.9 0 0.095 24 E-5 10.0 0.006 91 0.8 11
0.095 24 EUDC None 0.031 -- 0.31 -- 0.097 -- C-X 2.0 0.02 35 0.17
45 0.071 27 C-5 5.0 0.006 81 0.1 68 0.06 38 C-6 10.0 0.006 81 0.08
74 0.064 34 E-4 2.5 0.017 45 0.21 32 0.064 34 E-5 10.0 0.004 87 0.1
68 0.07 28 CYCLE None 0.045 -- 0.52 -- 0.107 -- "A" C-X 2.0 0.038
16 0.43 17 0.078 27 C-5 5.0 0.029 36 0.33 37 0.071 34 C-6 10.0
0.027 40 0.32 38 0.073 32 E-4 2.5 0.014 69 0.46 12 0.075 30 E-5
10.0 0.005 89 0.35 33 0.078 27 .sup.1Emissions in exhaust.
.sup.2Percent of emissions converted to innocuous substances by
catalytic treatment.
[0089] From the data in the foregoing TABLES II-A and II-B it can
be seen that samples E-4 and E-5 according to the present invention
gave higher gas-phase hydrocarbon conversion (about 80%) in the ECE
part of the test for the Audi 100 automobile than comparative
catalysts C-5, C-6 having comparable platinum loadings but without
Fe-Beta zeolite, and without significant gain in SO.sub.2
oxidation. Further, catalysts E-4 and E-5 according to the present
invention gave better hydrocarbon conversion performance than the
commercial catalyst C-7, which has a much higher platinum
loading.
[0090] In the hotter EUDC part of the test, on the Audi 100
automobile, catalyst samples of the present invention (E-4, E-5)
exhibited superior particulates removal compared to the comparative
commercial catalyst C-7, as did the comparative catalysts C-S and
C-6. This is probably due to the high degree of SO.sub.2 oxidation
caused by the high platinum loading of the commercial catalyst, the
resulting sulfate make adding to the mass of the particulates.
Overall, the catalysts according to the present invention (E-4,
E-5) gave better gas-phase hydrocarbon conversion and particulates
removal than the comparative commercial catalyst (C-7) and the
comparative catalysts (C-5, C-6).
[0091] The overall performance of the catalysts for Cycle A was
slightly better than the comparative samples.
[0092] The results for the ECE portion of the tests on the Mercedes
Benz automobile also show that catalysts according to the invention
(E-4, E-5) had superior low temperature gas-phase hydrocarbon
conversion while maintaining acceptable particulates conversion. It
is noted that all the catalysts except E-4 exhibited significant
improvement in CO conversion, in comparison to the Audi 100 test.
The improvement is attributed to the difference in exhaust
temperature between the respective vehicles (on average 10.degree.
C. higher for Mercedes than for Audi), highlighting the temperature
sensitivity of CO conversion activity. The overall Cycle A results
for the catalysts of the invention are substantially better than
the comparative samples due to the better overall hydrocarbon
conversion with comparable CO and particulates conversion. The
better total particulates conversion rates exhibited for the Audi
automobile relative to the Mercedes automobile can be attributed to
the lower VOF content of the Mercedes exhaust as compared to the
Audi exhaust.
EXAMPLE 4
Examples of Ceria-Alumina with Pt-Beta Zeolite and Pt--Fe-Beta
Zeolite
[0093] A. Platinum ion-exchanged Beta zeolite was prepared by
stirring 100 grams of Beta zeolite powder into a water solution
containing 0.85 grams tetraammine platinum [II] chloride in 500 ml
water. The resulting slurry was stirred and allowed to stand for 24
hours, and then filtered, washed with 1 liter of water, dried
overnight at 100.degree. C. and calcined at 540.degree. C. for 2
hours. The resulting Pt-Beta zeolite material contained 0.48%
platinum by weight.
[0094] A catalyst slurry was prepared comprising 33% by weight (dry
basis) of the Pt-Beta zeolite and 67% (dry basis) of a mixture of
equal parts by weight of ceria having a surface area of 143
m.sup.2/g and .gamma.-alumina having a surface area of 150
m.sup.2/g. The slurry was coated onto a honeycomb carrier to
provide a loading of 2 g/in.sup.3 using 10% by weight additional
.gamma.-alumina as a binder. The coated honeycomb was exposed to a
gaseous mixture of 10% steam in air at 400.degree. C. for 4 hours.
This catalyst contained 10g/ft.sup.3 Pt and was designated E-6.
[0095] B. Beta zeolite was ion-exchanged with iron and then with
platinum as generally described in Example B to yield a Beta
zeolite comprising 1.65% by weight iron and 0.5% by weight
platinum. The resulting Pt--Fe-Beta zeolite material was made into
a slurry comprising 33% by weight (dry basis) Pt--Fe-Beta zeolite
and 67% (dry basis) of a mixture of equal parts by weight of ceria
having a surface area of 143 m.sup.2/g and .gamma.-alumina having a
surface area of 150 m.sup.2/g. The slurry was coated onto a
honeycomb carrier at a loading of 2 g/in.sup.3 using additional
alumina as a binder. This catalyst contained 10 g/ft.sup.3 Pt and
was designated E-7.
[0096] C. Catalysts E-6 and E-7 were tested in a diagnostic reactor
through which a test stream identical to that described in part C.
of Example 2 was passed. The heptane, carbon monoxide and SO.sub.2
conversion rates were noted at the inlet temperatures of the test
stream set forth below in TABLE III, in which the results are
tabulated.
5 TABLE III Conversion % Heptane at .degree. C. % CO at .degree. C.
% SO.sub.2 at .degree. C. Cat. 150 200 275 350 150 200 275 350 150
200 275 350 E-6 3.3 3.5 13 93 1.5 7 97 100 2.6 0 25 52 E-7 4.0 2.0
8.6 88 0 43 100 100 9.0 2 18 53
[0097] The data of TABLE III show both catalysts E-6 and E-7 were
effective for the conversion of heptane and carbon monoxide, and
that sample E-7 exhibits superior carbon monoxide conversion at
200.degree. C. and above relative to sample E-6, while providing
comparable performance with respect to heptane conversion and
SO.sub.2 oxidation.
[0098] Lube Oil Combustion Test and Test Results
[0099] In many diesel engines the VOF in the diesel exhaust
consists mainly of diesel lube oil which has been swept from the
cylinder walls and comes through valve guides and turbocharger
seals. A laboratory test was used to evaluate the relative
performance of the catalyst powders of Example 5 below for burning
diesel lube oil, as a model of the ability of the corresponding
catalyst to catalyze the oxidation of the VOF in diesel engine
exhaust. This test allows for the relative ranking of catalyst
materials for their effectiveness of burning lube oil via the
interactions between the catalyst and lube oil as they are heated
together in air.
[0100] Thus, a catalyst powder sample is mixed uniformly with a
measured amount of lube oil. The mixture of catalyst and lube oil
(about 10-30 mg) is placed into the quartz sample pan of a
simultaneous TGA/DTA instrument (Thermal Sciences STA 1500) and
heated in flowing air using a standard heating ramp (20.degree.
C./min.) from ambient temperature to 1000.degree. C.
[0101] The collected data, cumulative weight loss (TGA) and heat
evolution (DTA) as a function of temperature, are normalized for
the weight of catalyst sample and the amount of lube oil present.
The total weight loss (TGA) measured is made up of water loss which
occurs at about 100.degree. C. or less and lube oil loss either by
volatilization or by combustion. The water loss occurs in a
discrete step and can be thus differentiated from the lube oil
losses. The exotherm (DTA Peak) is a measure of the lube oil loss
due to combustion of the lube oil. These data are used to calculate
a DTA Peak Area (uv-sec/mg-catalyst sample/mg-lube oil) which is
used as the key measure of the catalyst's ability to catalyze the
combustion of VOF (lube oil) in this test. Tests have been
conducted in which catalyst powders were used to catalyze the
combustion of lubricating oil and the same catalytic powders were
used to prepare catalysts supported on a substrate. Tests of such
catalysts on diesel engine exhausts showed good correlation between
the performance of the catalyst powder in the laboratory TGA and
DTA tests and the performance of the corresponding catalyst in the
engine test.
EXAMPLE 5
TGA/DTA Lube Oil Combustion Test FCC Cat (Ceria/Zeolite)
[0102] A. A catalyst material according to the present invention
designated E-8 comprised a mixture of 50% by weight bulk ceria
having a BET surface area of 143 m.sup.2/g and 50% by weight
ZSM-5H-zeolite by weight of the mixture. The catalyst was prepared
by mixing the ceria and ZSM-5 powders, drying overnight at
100.degree. C. and calcining at 450.degree. C. for three hours. The
catalyst material was wetted with a quantity of Cummins SAE 15 W/40
Premium blue diesel engine lube oil equal to 4% of the weight of
the zeolite, and was then placed in a TGA/DTA analyzer. The results
are set forth in the attached FIG. 10.
[0103] B. A series of seven other catalyst materials was prepared
as generally described in Part A above, and were designated E-9
through E-15, respectively. The first three of these, E-9, E-10 and
E-11, comprised mixtures of ceria having a surface area of 143
m.sup.2/g on which platinum had been dispersed by the incipient
wetness method to provide 0.5 percent platinum thereon by weight of
ceria plus platinum (Pt/Ceria) with H-Mordenite, Fe-Beta zeolite
and H-ZSM-5, respectively. In each case, equal weights of Pt/ceria
and zeolite were used. Samples E-12, E-13, E-14 and E-15 all
comprised mixtures of zeolites, ceria and alumina, in which
mixtures of equal weights' of ceria plus alumina comprised 67% of
the weight of the catalytic material, the zeolite (including doped
metals) accounting for 33% by weight of the catalytic material. In
these samples, the zeolites were doped by ion-exchange with 0.6
percent platinum by weight of the zeolite. The zeolites in these
examples were Pt-ZSM-5, Pt--Y-zeolite, Pt--Fe-Beta zeolite and
Pt--H-Beta zeolite, respectively.
[0104] The ability of each of these sample materials to catalyze
the combustion of the lube oil was tested using thermogravimetric
analysis and differential temperature analysis (TGA/DTA)
techniques. In TGA/DTA studies, the heat absorbed or released and
the weight lost by a sample as a function of temperature are
recorded simultaneously, and can be used to evaluate the capability
of the material to catalyze the combustion of the lube oil. Since,
as discussed above, lube oil constitutes a significant portion of
the VOF particulates in diesel engine exhaust, the results of the
TGA/DTA study of lube oil-doped material reflects the ability of
the material to combust the VOF particulates. The amount of lube
oil added to the material (10% by weight) is far in excess of the
amount of VOF hydrocarbons that a diesel catalyst would ordinarily
be exposed to in the time frame of this test, but this quantity is
used so that reproducible results may be obtained from this test.
Each oil-wetted sample was placed in a quartz pan sample holder
located inside an STA/1500 Simultaneous Thermoanalyzer and heated
in air at a rate of 20.degree. C./min. from room temperature to
1000.degree. C. The sample weights varied between 30 and 10
milligrams. For comparison, a like quantity of lube oil was placed
in the analysis chamber so that the TGA/DTA curves could be
corrected for water loss and vaporization.
[0105] In FIG. 10, the ordinate on the left-hand side of the Figure
relates to the lower curve and shows changes in the weight of the
sample of oil-wetted material relative to temperature. The ordinate
on the right-hand side of the Figure relates to the upper curve and
shows the heat released from the sample per milligram of sample
(catalyst plus lube oil) at various temperatures. The quantity of
heat is measured by a thermocouple and is expressed in microvolts
per milligram of catalyst plus lube oil. The distinct rise in heat
release at about 200.degree. C. indicates the commencement of
catalytic oxidation of the wetted oil, as does the loss in weight
of the sample. One indication of the catalytic activity of the test
material is the area under the DTA plot, referred to in TABLE IV as
the DTA area. The DTA area is obtained by integrating the area
under the curve and dividing by the lube oil weight loss as
measured by TGA. The units are thus measured in microvolt-seconds
(generated by a thermocouple in response to the temperature change)
per milligram of catalyst sample per milligram of lube oil. The
units are abbreviated as "uv-s/mg/mg" in TABLE IV. Generally, the
more catalytically active the material, the greater is the DTA
area: that is, the greater is the heat release generated by
combustion of the lube oil as compared to the total amount of
normalized lube oil loss (resulting from combustion plus
volatilization). The following TABLE IV shows the DTA area for all
the samples tested in this example.
6TABLE IV DTA Area Sample (Percents By Weight) uv-s/mg/mg E-8 (50%
CeO.sub.2 plus 50% ZSM-5) 5,856 E-9 (50% Pt/CeO.sub.2 plus 50%
H-Mordenite) 10,100 E-10 (50% Pt/CeO.sub.2 plus 50% Fe-Beta Z)
14,800 E-11 (50% Pt/CeO.sub.2 plus 50% H-ZSM-5) 16,700 E-12 (67%
[Al.sub.2O.sub.3/CeO.sub.2]* plus 33% Pt-ZSM-5) 12,975 E-13 (67%
[Al.sub.2O.sub.3/CeO.sub.2]* plus 33% Pt-Y-Z) 6,990 E-14 (67%
[Al.sub.2O.sub.3/CeO.sub.2]* plus 33% Pt--Fe-Beta Z) 10,200 E-15
(67% [Al.sub.2O.sub.3/CeO.sub.2]* plus 33% Pt--H-Beta Z) 10,630 *50
Al.sub.2O.sub.3 plus 50% CeO.sub.2
[0106] The data of TABLE IV show that a variety of zeolites can be
combined with ceria or ceria and alumina to yield materials that
are catalytically active for the combustion of lube oil. Such
catalytic activity, as discussed above, is indicative of the
ability to reduce the VOF content of diesel exhaust.
[0107] While the invention has been described in detail with
respect to specific preferred embodiments thereof it will be
appreciated that variations thereto may be made which nonetheless
lie within the scope of the invention and the appended claims.
* * * * *