U.S. patent application number 09/805807 was filed with the patent office on 2002-09-19 for alkaline earth/transition metal lean nox catalyst.
Invention is credited to Fisher, Galen B., Hemingway, Mark, Kupe, Joachim, Labarge, William J..
Application Number | 20020132733 09/805807 |
Document ID | / |
Family ID | 25192554 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020132733 |
Kind Code |
A1 |
Labarge, William J. ; et
al. |
September 19, 2002 |
ALKALINE EARTH/TRANSITION METAL LEAN NOX CATALYST
Abstract
A catalyst for treating an exhaust gas stream comprising an
alkaline earth/transition metal oxide component in combination with
an alkaline earth/support component.
Inventors: |
Labarge, William J.; (Bay
City, MI) ; Hemingway, Mark; (Columbiaville, MI)
; Kupe, Joachim; (Davisburg, MI) ; Fisher, Galen
B.; (Bloomfield Hills, MI) |
Correspondence
Address: |
VINCENT A. CICHOSZ
DELPHI TECHNOLOGIES, INC.
Legal Staff, Mail Code: 480-414-420
P.O. Box 5052
Troy
MI
48007-5052
US
|
Family ID: |
25192554 |
Appl. No.: |
09/805807 |
Filed: |
March 13, 2001 |
Current U.S.
Class: |
502/325 ;
502/328; 502/340 |
Current CPC
Class: |
B01J 37/0246 20130101;
B01D 2255/2042 20130101; B01D 2255/2073 20130101; B01D 2255/20746
20130101; B01D 53/9422 20130101; B01J 23/34 20130101; B01D
2255/20753 20130101; B01J 37/0215 20130101; B01D 2255/2045
20130101; B01J 23/78 20130101; B01D 2251/208 20130101; B01J 23/02
20130101 |
Class at
Publication: |
502/325 ;
502/328; 502/340 |
International
Class: |
B01J 023/02; B01J
023/34 |
Claims
We claim:
1. A catalyst for treating an exhaust gas stream comprising an
alkaline earth/transition metal oxide component in combination with
an alkaline earth/support component
2. The catalyst of claim 1 wherein the alkaline earth/transition
metal oxide component comprises an alkaline earth in an amount
ranging from about 1 to about 50 weight percent
3. The catalyst of claim 2 wherein the alkaline earth/transition
metal oxide component comprises an alkaline earth in an amount
ranging from about 18 to about 36 weight percent.
4. The catalyst of claim 1 comprising at least about 14 weight
percent of an alkaline earth/transition metal oxide component in
combination with not more than about 81 weight percent of an
alkaline earth/support component.
5. The catalyst of claim 4 comprising at least about 21 weight
percent of an alkaline earth/transition metal oxide component in
combination with not more than about 76 weight percent of an
alkaline earth/support component.
6. The catalyst of claim 1 comprising about 28 weight percent of an
alkaline earth/transition metal oxide component in combination with
about 69 weight percent of an alkaline earth/support component.
7. The catalyst of claim 1 for treating an exhaust gas stream
comprising an alkaline earth/transition metal oxide component in
combination with an alkaline earth/support component and including
a ceramic binder in an amount greater than about 1 weight percent
and less than about 7 weight percent.
8. The catalyst of claim 7 for treating an exhaust gas stream
comprising an alkaline earth/transition metal oxide component in
combination with an alkaline earth/support component and including
a zirconium oxide binder in an amount greater than about 1 weight
percent and less than about 7 weight percent
9. The catalyst of claim 1 for treating an exhaust gas stream
comprising an alkaline earth/transition metal oxide component
wherein the alkaline earth is selected from the group consisting of
barium, strontium, calcium, and mixtures thereof.
10. The catalyst of claim 9. for treating an exhaust gas stream
comprising an alkaline earth/transition metal oxide component
wherein the alkaline earth is calcium.
11. The catalyst of claim 1 for treating an exhaust gas stream
comprising an alkaline earth/transition metal oxide component
wherein the transition metal is selected from the group consisting
of nickel, manganese, and cobalt.
12. The catalyst of claim 11 for treating an exhaust gas stream
comprising an alkaline earth/transition metal oxide component
wherein the transition metal is cobalt.
13. The catalyst of claim 1 for treating an exhaust gas stream
comprising an alkaline earth/support component wherein the alkaline
earth is selected from the group consisting of barium, strontium,
calcium, and mixtures thereof.
14. The catalyst of claim 13 for treating an exhaust gas stream
comprising an alkaline earth/support component wherein the alkaline
earth is barium.
15. The catalyst of claim 1 for treating an exhaust gas stream
comprising an alkaline earth/support component wherein the support
is selected from the group consisting of zeolite, alumina, and
mixtures thereof.
16. The catalyst of claim 13 for treating an exhaust gas stream
comprising an alkaline earth/support component wherein the support
comprises a mixture of zeolite in a proportion ranging from about
55 to about 65 weight percent and alumina in a proportion ranging
from about 35 to about 45 weight percent.
17. A catalyst for treating an exhaust gas stream comprising more
than about 14 weight percent of an alkaline earth/transition metal
oxide component in combination with less than about 46 weight
percent of an alkaline earth/zeolite component and less than about
35 weight percent of an alkaline earth/alumina component and not
more than about 7 weight percent of a ceramic binder.
18. The catalyst of claim 17 for treating an exhaust gas stream
comprising more than about 21 weight percent of an alkaline
earth/transition metal oxide component in combination with less
than about 42 weight percent of an alkaline earth/zeolite component
and less than about 34 weight percent of an alkaline earth/alumina
component and not more than about 5 weight percent of a ceramic
binder.
19. The catalyst of claim 18 for treating an exhaust gas stream
comprising about 28 weight percent of an alkaline earth/transition
metal oxide component in combination with about 38 weight percent
of an alkaline earth/zeolite component and about 31 weight percent
of an alkaline earth/alumina component and about 3 weight percent
of a ceramic binder.
20. A catalyst for treating an exhaust gas stream comprising more
than about 14 weight percent of a calcium/cobalt oxide component in
combination with less than about 46 weight percent of a
barium/zeolite component and less than about 35 weight percent of a
barium/alumina component and not more than about 7 weight percent
of a zirconium oxide binder.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a catalyst for purifying
exhaust gases from an internal combustion engine. In particular, it
relates to a lean NO.sub.x catalyst.
[0002] It is well known in the art to use catalyst compositions,
including those commonly referred to as three-way conversion
("TWC") catalysts to treat the exhaust gases of internal combustion
engines. Such catalysts, containing precious metals like platinum,
palladium, and rhodium, have been found both to successfully
promote the oxidation of unburned hydrocarbons (HC) and carbon
monoxide (CO) and to promote the reduction of nitrogen oxides
(NO.sub.x) in exhaust gas, provided that the engine is operated
around balanced stoichiometry for combustion ("combustion
stoichiometry"; i.e., between about 14.7 and 14.4 air/fuel (A/F)
ratio).
[0003] However, fuel economy and global carbon dioxide (CO.sub.2)
emissions have made it desirable to operate engines under lean-burn
conditions, where the air-to-fuel ratio is somewhat greater than
combustion stoichiometry to realize a benefit in fuel economy.
Diesel and lean-burn gasoline engines generally operate under
highly oxidizing conditions (i.e., using much more air than is
necessary to burn the fuel), typically at air/fuel ratios greater
than 14.7 and generally between 19 and 35. Under these highly lean
conditions, typical three-way catalysts exhibit little activity
toward NO.sub.x reduction, as their reduction activity is
suppressed by the presence of excess oxygen.
[0004] The control of NO.sub.x emissions from vehicles is a
worldwide environmental problem. Lean-bum, high air-to-fuel ratio,
and diesel engines are certain to become more important in meeting
the mandated fuel economy requirements of next-generation vehicles.
Development of an effective and durable catalyst for controlling
NO.sub.x emissions under net oxidizing conditions accordingly is
critical.
[0005] Recently, copper-ion exchanged zeolite catalysts have been
shown to be active for selective reduction of NO.sub.x by
hydrocarbons in the presence of excess oxygen. Platinum-ion
exchanged zeolite catalyst is also known to be active for NO.sub.x
reduction by hydrocarbons under lean conditions. However, this
catalytic activity is significant only in a narrow temperature
range around the lightoff temperature of hydrocarbon oxidation. All
the known lean-NO.sub.x catalysts reported in the literature tend
to lose their catalytic activity for NO.sub.x reduction when the
catalyst temperature reaches well above the lightoff temperature of
hydrocarbon oxidation. This narrow temperature window of the
lean-NO.sub.x catalysts is considered to be one of the major
technical obstacles, because it makes practical application of
these catalysts difficult for lean-bum gasoline or diesel
engines.). As an example, the Cu-zeolite catalysts deactivate
irreversibly if a certain temperature is exceeded. Catalyst
deactivation is accelerated by the presence of water vapor in the
stream and water vapor suppresses the NO reduction activity even at
lower temperatures. Also, sulfate formation at active catalyst
sites and on catalyst support materials causes deactivation.
Practical lean-NO.sub.x catalysts must overcome all three problems
simultaneously before they can be considered for commercial use. In
the case of sulfur poisoning, some gasoline can contain up to 1200
ppm of organo-sulfur compounds. Lean-NO.sub.x catalysts promote the
conversion of such compounds to SO.sub.2 and SO.sub.3 during
combustion. Such SO.sub.2 will adsorb onto the precious metal sites
at temperatures below 300.degree. C. and thereby inhibits the
catalytic conversions of CO, C.sub.xH.sub.y (hydrocarbons) and
NO.sub.x. At higher temperatures with an Al.sub.2O.sub.3 catalyst
carrier, SO.sub.2 is converted to SO.sub.3 to form a large-volume,
low-density material, Al.sub.2(SO.sub.4).sub.3, that alters the
catalyst surface area and leads to deactivation. In the prior art,
the primary solution to this problem has been to use fuels with low
sulfur contents.
[0006] Another alternative is to use catalysts that selectively
reduce NO.sub.x in the presence of a co-reductant, e.g., selective
catalytic reduction (SCR) using ammonia or urea as a co-reductant.
Selective catalytic reduction is based on the reaction of NO with
hydrocarbon species activated on the catalyst surface and the
subsequent reduction of NO.sub.x to N.sub.2. More than fifty such
SCR catalysts are conventionally known to exist. These include a
wide assortment of catalysts, some containing base metals or
precious metals that provide high activity. Unfortunately, just
solving the problem of catalyst activity in an oxygen-rich
environment is not enough for practical applications. Like most
heterogeneous catalytic processes, the SCR process is susceptible
to chemical and/or thermal deactivation. Many lean-NO.sub.x
catalysts are too susceptible to high temperatures, water vapor and
sulfur poisoning (from SO.sub.x).
[0007] Yet another viable alternative involves using co-existing
hydrocarbons in the exhaust of mobile lean-bum gasoline engines as
a co-reductant and is a more practical, cost-effective, and
environmentally sound approach. The search for effective and
durable non-selective catalytic reduction "NSCR" catalysts that
work with hydrocarbon co-reductant in oxygen-rich environments is a
high-priority issue in emissions control and the subject of intense
investigations by automobile and catalyst companies, and
universities, throughout the world.
[0008] A leading catalytic technology for removal of NO.sub.x from
lean-bum engine exhausts involves NO.sub.x storage reduction
catalysis, commonly called the "lean-NO.sub.x trap". The
lean-NO.sub.x trap technology can involve the catalytic oxidation
of NO to NO.sub.2 by catalytic metal components effective for such
oxidation, such as precious metals. However, in the lean NO.sub.x
trap, the formation of NO.sub.2 is followed by the formation of a
nitrate when the NO.sub.2 is adsorbed onto the catalyst surface.
The NO.sub.2 is thus "trapped", i.e., stored, on the catalyst
surface in the nitrate form and subsequently decomposed by
periodically operating the system under stoichiometrically
fuel-rich combustion conditions that effect a reduction of the
released NO.sub.x (nitrate) to N.sub.2.
[0009] The lean-NO.sub.x-trap technology has been limited to use
for low sulfur fuels because catalysts that are active for
converting NO to NO.sub.2 are also active in converting SO.sub.2 to
SO.sub.3. Lean NO.sub.x trap catalysts have shown serious
deactivation in the presence of SOX because, under oxygen-rich
conditions, SO.sub.x adsorbs more strongly on NO.sub.2 adsorption
sites than NO.sub.2, and the adsorbed SO.sub.x does not desorb
altogether even under fuel-rich conditions. Such presence of
SO.sub.3 leads to the formation of sulfuric acid and sulfates that
increase the particulates in the exhaust and poison the active
sites on the catalyst. Attempts with limited success to solve such
a problem have encompassed the use of selective SO.sub.x adsorbents
upstream of lean NO.sub.x trap adsorbents. Furthermore, catalytic
oxidation of NO to NO.sub.2 is limited in its temperature range.
Oxidation of NO to NO.sub.2 by a conventional Pt-based catalyst
maximizes at about 250.degree. C. and loses its efficiency below
about 100 degrees and above about 400 degrees. Thus, the search
continues in the development of systems that improve lean NO.sub.x
trap technology with respect to temperature and sulfur
considerations.
[0010] Another NO.sub.x removal technique comprises a non-thermal
plasma gas treatment of NO to produce NO.sub.2 which is then
combined with catalytic storage reduction treatment, e.g., a lean
NO.sub.x trap, to enhance NO.sub.x reduction in oxygen-rich vehicle
engine exhausts. In the lean NO.sub.x trap, the NO.sub.2 from the
plasma treatment is adsorbed on a nitrate-forming material, such as
an alkali material, and stored as a nitrate. An engine controller
periodically runs a brief fuel-rich condition to provide
hydrocarbons for a reaction that decomposes the stored nitrate into
benign products such as N.sub.2. By using a plasma, the lean
NO.sub.x trap catalyst can be implemented with known NO.sub.x
absorbers, and the catalyst may contain less or essentially no
precious metals, such as Pt, Pd and Rh, for reduction of the
nitrate to N.sub.2. Accordingly, an advantage is that a method for
NO.sub.x emission reduction is provided that is inexpensive and
reliable. The plasma-assisted lean NO.sub.x trap can allow the life
of precious metal lean NO.sub.x trap catalysts to be extended for
relatively inexpensive compliance to NO.sub.x emission reduction
laws. Furthermore, not only does the plasma-assisted lean NO.sub.x
trap process improve the activity, durability, and temperature
window of lean NO.sub.x trap catalysts, but it allows the
combustion of fuels containing relatively high sulfur contents with
a concomitant reduction of NO.sub.x, particularly in an oxygen-rich
vehicular environment.
[0011] What is needed in the art is an exhaust gas catalyst system
having improved durability, as well as effective NO.sub.x
management, over extended operating time. The present invention
overcomes many of the shortcomings of the prior art.
SUMMARY OF THE INVENTION
[0012] Diesel exhaust emissions have high levels of CO, NO.sub.x
and particulates. In addition, hydrocarbon is introduced into the
exhaust stream as a reducing agent. All of these exhaust gas stream
components must be reduced to meet emissions standards. The
particulates in the exhaust gas stream initially are trapped as
large particulates and converted to CO by the plasma reactor. The
NO.sub.x component of the exhaust gas stream is reduced on the
first catalyst bed, containing an occluding material such as barium
and a support such as a zeolite. However, the CO and hydrocarbon
levels also must be reduced. Now, according to the present
invention, an exhaust gas catalyst is provided to serve as a
clean-up type catalyst bed that will oxidize CO to CO.sub.2 and HC
to H.sub.2O and CO.sub.2, without re-oxidation of N.sub.2 to NO and
NO.sub.2, without re-oxidation of SO.sub.2 to SO.sub.3.
[0013] Pursuant to the present invention, a catalyst is provided
comprising an alkaline earth/transition metal oxide component and
an alkaline earth/support component.
[0014] The invented catalyst is designed not to be poisoned by
sulfur and not to convert gaseous SO.sub.2 to particulates formed
from SO.sub.3 and H.sub.20. The catalyst exhibits good activity at
relatively low temperatures. By inclusion of both a reducing
catalyst component and an oxidizing catalyst component the
bifunctional catalyst serves not increase the NO.sub.x or
particulate sulfate levels. Further, the catalyst is not
significantly affected by the high levels of H.sub.2O in a diesel
exhaust stream.
[0015] The above-described and other features and advantages of the
present invention will be appreciated and understood by those
skilled in the art from the following detailed description, and
appended claims.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0016] The catalyst generally comprises a calcined coating of an
alkaline earth/transition metal oxide, and an alkaline
earth/support oxide coated onto a chemically stable and thermally
insulating substrate, typically such as cordierite, mullite, and
the like. The support oxide generally may be a zeolite or an
aluminum oxide or a mixture of zeolite and aluminum oxide.
Preferably, the catalyst coating also includes a ceramic oxide
binder and/or stabilizer.
[0017] The alkaline earth/transition metal oxide catalyst component
preferably comprises an alkaline earth content of about 1 wt % to
about 50 wt %; about 18 wt % to about 36 wt % is particularly
preferred.
[0018] The alkaline earth/support catalyst component preferably is
not more than about 81 wt % of the total catalyst, and more
preferred at not more than about 76 wt %. Particularly preferred is
alkaline earth/support component at about 69 wt %. Preferably, the
transition metal/alkaline earth oxide catalyst component is at
least about 14 wt % and more preferred at least about 21 wt %.
Particularly preferred is transition metal/alkaline earth oxide at
about 28 wt %. The ceramic binder preferably is greater than about
1 wt % and not more than about 7 wt %; about 3 wt % is particularly
preferred.
[0019] A preferred calcined catalyst coating comprises less than
about 46 wt % alkaline earth/zeolite, less than about 35 wt %
alkaline earth/aluminum oxide, more than about 14 wt % transition
metal/alkaline earth oxides and not more than about 7 wt % ceramic
oxide binder. A more preferred calcined coating comprises less than
about 42 wt % alkaline earth/zeolite, less than about 34 wt %
alkaline earth/aluminum oxide, more than about 21 wt % transition
metal/alkaline earth oxide, and not more than about 5 wt % ceramic
oxide binder. A particularly preferred calcined coating comprises
about 38 wt % alkaline earth/zeolite, about 31 wt % alkaline
earth/aluminum oxide, about 28 wt % transition metal/alkaline earth
oxide and about 3 wt % ceramic oxide binder.
[0020] Any alkaline earth may be utilized in the alkaline
earth/transition metal component of the catalyst. Barium,
strontium, and/or calcium are the preferred alkaline earths;
strontium and calcium are more preferred; and, calcium is
particularly preferred.
[0021] Suitable calcium compounds for introducing the alkaline
earth include calcium succinate, calcium tartrate, calcium citrate,
calcium acetate, calcium carbonate, calcium hydroxide, calcium
oxylate, calcium oleate, calcium palmitate and calcium oxide.
Calcium acetate, calcium oxylate, and calcium citrate are
preferred. Calcium citrate is particularly preferred.
[0022] Suitable strontium compounds for introducing the alkaline
earth include strontium citrate, strontium acetate, strontium
carbonate, strontium hydroxide, strontium oxylate and strontium
oxide. Strontium acetate, strontium oxylate, and strontium citrate
are preferred. Strontium citrate is particularly preferred.
[0023] Suitable barium compounds for introducing the alkaline earth
include barium butyrate, barium formate, barium citrate, barium
acetate, barium oxylate, barium carbonate, barium hydroxide and
barium oxide. Barium acetate, barium oxylate, and barium citrate
are preferred. Barium citrate is particularly preferred.
[0024] Any transition metal may be utilized. Nickel, manganese,
and/or cobalt are the preferred transition metals; manganese and
cobalt are more preferred; and, cobalt is particularly
preferred.
[0025] Suitable nickel compounds for introducing the transition
metal include nickel formate, nickel acetate, nickel citrate,
nickel nitrate, nickel carbonate, nickel hydroxide, nickel
isopropoxide, nickel oxylate, nickel tartrate and nickel amine
nitrate. Nickel acetate, nickel citrate, and nickel formate are
more preferred. Nickel formate is particularly preferred.
[0026] Suitable manganese compounds for introducing the transition
metal include manganese formate, manganese acetate, manganese
citrate, manganese nitrate, manganese carbonate, manganese
hydroxide, manganese isopropoxide, manganese oxylate, manganese
tartrate and manganese amine nitrate. Manganese acetate, manganese
citrate, and manganese formate are preferred. Manganese formate is
particularly preferred.
[0027] Suitable cobalt compounds for introducing the transition
metal include cobalt formate, cobalt acetate, cobalt citrate,
cobalt nitrate, cobalt carbonate, cobalt hydroxide, cobalt
isopropoxide, cobalt oxylate, cobalt tartrate and cobalt amine
nitrate. Cobalt acetate, cobalt citrate, and cobalt formate are
preferred. Cobalt formate is particularly preferred.
[0028] The surface area of the alkaline earth and transition metal
oxides may be stabilized with other binder oxides, such as oxides
of silicon, titanium, and/or zirconium. Titanium and zirconium are
preferred; zirconium is particularly preferred. The
binder/stabilizer preferably comprises not more than about 7 wt %;
more preferred at not more than about 5 wt %; and, particularly
preferred not more than about 3 wt %.
[0029] Suitable zirconium compounds for introduction of the surface
area stabilizer include zirconium acetylacetonate, zirconium
n-butoxide, zirconium nitrate, zirconium tetraamine nitrate,
zirconium tetraamine citrate, zirconium 2-ethylhexanoate, and
zirconium isopropoxide. Zirconium tetraamine citrate, zirconium
butoxide, and zirconium isopropoxide are preferred; zirconium
butoxide and zirconium isopropoxide are more preferred; and,
zirconium isopropoxide is particularly preferred.
[0030] Suitable titanium compounds for introduction of the surface
area stabilizer include titanium oxylate, titanium ethoxide,
titanium methoxide, titanium isopropoxide, zirconium n-butoxide.
Titanium butoxide, titanium ethoxide, and titanium isopropoxide are
preferred; titanium ethoxide and titanium isopropoxide are more
preferred; and, titanium isopropoxide is particularly
preferred.
[0031] Suitable silicon compounds for introduction of the surface
area stabilizer include silicon acetate, tetraethoxysilane,
tetramethoxysilane, vinyltrimethoxysilane,
ureidopropyltriethoxysilane and aminopropylsilanetriol.
Tetraethoxysilane, silicon acetate, and tetramethoxysilane are
preferred; silicon acetate and tetramethoxysilane are more
preferred; and, tetramethoxysilane is particularly preferred.
[0032] The catalyst of the present invention comprises an alkaline
earth/support component, wherein the support may be a zeolite
and/or alumina. The alumina fraction of the support may be present
in a proportion ranging from about 0 wt % to about 100 wt % of the
alkaline earth/support fraction. A particularly preferred admixture
comprises an alkaline earth/aluminum oxide fraction in a proportion
ranging from about 35 wt % to about 45 wt %, and an alkaline
earth/zeolite fraction in a proportion ranging from about 55 wt %
to about 65 wt %.
[0033] Any type zeolite may be used; preferred zeolites include X
type zeolite, Y type zeolite, and/or ZSM-5 type zeolite. A ZSM-5
zeolite with pores 4-5 A (angstroms) is preferred and an X zeolite
is more preferred and a Y-type zeolite with pores 7-8 A is greatly
preferred. A zeolite surface area of at least about 300
m.sup.2/gram is preferred, at least about 400 m.sup.2/g is more
preferred, and a surface area of at least about 500 m.sup.2/g is
particularly preferred. The preferred zeolite average particle size
is less than about 0.9 microns; more preferred are zeolite
particles of an average size less than about 0.6 microns; and
particularly preferred are zeolites having an average particle size
less than about 0.3 microns. The zeolite particles preferably
feature average pore sizes ranging from about 4 to about 10
angstroms ("A"), with average pore sizes ranging from about 7 to 8
A particularly preferred. Preferably, the zeolite has a Na.sub.2O
content of at least about 3 wt %; a content of at least about 8 wt
% is more preferred; and a content of at least about 13 wt % is
particularly preferred.
[0034] The aluminum oxide particles providing the matrix comprise
agglomerations of small psuedocrystalline alumina particles
typically of about 0.3 microns or less. The agglomerations
preferably are larger than about 10 microns and less than about 30
microns. The alumina matrix is mixed with a Ba-zeolite catalyst
component, typically featuring a particulate size of about 0.3
microns. In general, the smaller the zeolite particle, the more
hydrothermally stable the zeolite becomes, thereby improving its
long-term activity. Pursuant to the invention, the zeolite active
catalysts preferably are dispersed throughout the alumina
matrix.
[0035] Preferably, the alumina has a surface area of at least about
150 m.sup.2/gram; a surface area of at least about 200 m.sup.2/g is
more preferred; and, a surface area of at least about 250 m.sup.2/g
is particularly preferred. An average alumina pore size of at least
about 40 A is preferred; a pore size of at least about 60 A is more
preferred; and, a pore size of at least about 80 A is particularly
preferred. Specific acidity ranging from about 50 moles
(millimoles) n-butylamine/m.sup.2.times.10.sup.-4 to about 500
mmoles n-butylamine/m.sup.2.times.10.sup.-4 is preferred; a
specific acidity of about 350 mmoles
n-butlyamine/m.sup.2.times.10.sup.-4 is particularly preferred.
[0036] The zeolite catalyst component preferably comprises an
alkaline earth content of about 18 wt % to about 36 wt %; about 24
wt % to about 30 wt % is particularly preferred. The aluminum oxide
catalyst component preferably comprises an alkaline earth content
of about 18 wt % to about 36 wt %; about 24 wt % to about 30 wt %
is particularly preferred.
[0037] Any alkaline earth element may be used as the active
catalyst. For example, the active catalyst element may comprise
calcium, strontium, and/or barium. The use of barium is
particularly preferred. Tests have indicated that calcium-doped
catalysts generally convert about 30% NO.sub.x to N.sub.2;
strontium-doped catalysts generally convert about 50% NO.sub.x to
N.sub.2; barium-doped catalysts generally convert about 70%
NO.sub.x to N.sub.2. Based on a desire in the industry to optimize
conversion of NO.sub.x to N.sub.2 at about 90% or better, barium is
the particularly preferred occluding catalyst material.
[0038] Accordingly, Ba is the preferred occluding catalyst for both
the zeolite and the alumina components of the present catalyst. The
alumina matrix features enhanced trapping efficiency if it is doped
with a material such as barium. In specific, nitrogen species can
neutralize catalytically active sites. Barium is the most robust
alkaline earth element for resistance to nitrogen poisoning. The
alumina matrix provides sacrificial sites for nitrogen poisoning,
thus precluding large polycyclic nitrogen compounds from entering
and poisoning the small pores of the zeolite catalyst component.
The zeolite catalyst component preferably comprises a barium
content of about 18 wt % to about 36 wt %; about 24 wt % to about
30 wt % is particularly preferred. The alumina matrix catalyst
component preferably has a barium content of at least about 14 wt
%; at least about 21 wt % is more preferred; and, at least about 28
wt % is particularly preferred. Exhaust deposits of oil derived
"glassy" compounds such as calcium phosphate and zinc phosphate can
greatly reduce diffusion. High levels of alkaline earths, such as
barium, prevent formation of these diffusion limiting barriers.
[0039] Suitable barium sources for preparation of a barium-alumina
component include barium nitrate, barium acetate, barium hydroxide,
barium ethoxide, barium isopropoxide, and/or barium
2-ethylhexanoate. Barium acetate, barium isopropoxide, and barium
2-ethylhexanoate are preferred. Barium 2-ethylhexanoate is
particularly preferred.
[0040] The following examples are provided to further describe the
invention. The examples are intended to be illustrative in nature
and are not to be construed as limiting the scope of the
invention.
EXAMPLE I
Preparation of Ba-Cobalt Component
[0041] About 550 grams barium formate and 2090 grams cobalt formate
are ball milled for 4 hours. The ball milled material is then
calcined at 500.degree. C. for 4 hours.
Preparation of a Ba-Zeolite Component
[0042] About 1000 grams Y-Zeolite with a Na.sub.2O content of 13 wt
% is mixed with a saturated barium-nitrate solution and heated to
80.degree. C. for 4 hours. The solids are filtered then washed then
calcined at 740.degree. C. for 2 hours.
Preparation of Catalyst Monolith
[0043] About 280 grams barium-cobalt oxide, 700 grams Ba-Zeolite,
88 grams zirconium acetate and 1150 grams 0.1 N ammonium hydroxide
are well mixed. Ceramic monoliths containing 600 cells/in.sup.3 are
washcoated with the mixture then calcined at 500.degree. C. for 4
hours.
EXAMPLE II
Preparation of Ba-Cobalt Component
[0044] About 550 grams barium formate and 2090 grams cobalt formate
are ball milled for 4 hours. The ball milled material is then
calcined at 500.degree. C. for 4 hours.
Preparation of a Ba-Alumina Component
[0045] About 1000 grams gamma alumina with a surface area of at
least 150 m.sup.2/gram with is mixed with a saturated
barium-formate solution and heated to 80.degree. C. for 4 hours.
The solids are filtered then washed then calcined at 500.degree. C.
for 2 hours.
Preparation of Catalyst Monolith
[0046] About 280 grams barium-cobalt oxide, 700 grams Ba-aluminum
oxide, 88 grams zirconium acetate and 1150 grams 0.1 N ammonium
hydroxide are well mixed. Ceramic monoliths containing 600
cells/in.sup.3 are washcoated with the mixture then calcined at
500.degree. C. for 4 hours.
EXAMPLE III
Preparation of Ba-Cobalt Component
[0047] About 550 grams barium formate and 2090 grams cobalt formate
are ball milled for 4 hours. The ball milled material is then
calcined at 500.degree. C. for 4 hours.
Preparation of a Ba-Zeolite Component
[0048] About 1000 grams Y-Zeolite with a Na.sub.2O content of 13 wt
% is mixed with a saturated barium-nitrate solution and heated to
80.degree. C. for 4 hours. The solids are filtered then washed then
calcined at 740.degree. C. for 2 hours.
Preparation of a Ba-Alumina Component
[0049] About 1000 grams gamma alumina with a surface area of at
least 150 m.sup.2/gram with is mixed with a saturated
barium-formate solution and heated to 80.degree. C. for 4 hours.
The solids are filtered then washed then calcined at 500.degree. C.
for 2 hours.
Preparation of Catalyst Monolith
[0050] About 280 grams barium-cobalt oxide, 420 grams Ba-Zeolite,
280 grams Ba-aluminum oxide, 88 grams zirconium acetate and 1150
grams 0.1 N ammonium hydroxide are well mixed. Ceramic monoliths
containing 600 cells/in.sup.3 are washcoated with the mixture then
calcined at 500.degree. C. for 4 hours.
[0051] While preferred embodiments have been shown and described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustration only, and such illustrations and
embodiments as have been disclosed herein are not to be construed
as limiting to the claims.
* * * * *