U.S. patent application number 10/307841 was filed with the patent office on 2003-05-01 for method for preparation of catalytic material for selective oxidation and catalyst members thereof.
Invention is credited to Farrauto, Robert J., Korotkikh, Olga, McFarland, Andrew.
Application Number | 20030083196 10/307841 |
Document ID | / |
Family ID | 23552095 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030083196 |
Kind Code |
A1 |
Korotkikh, Olga ; et
al. |
May 1, 2003 |
Method for preparation of catalytic material for selective
oxidation and catalyst members thereof
Abstract
The invention pertains to the preparation and use of catalytic
materials and catalyst members for the selective oxidation of
carbon monoxide in a gas stream that contains hydrogen. One such
catalyst member may be produced by depositing by electric arc
spraying a metal feedstock onto a metal substrate to provide a
metal anchor layer on the substrate, and depositing a catalytic
material comprising platinum and iron dispersed on a refractory
inorganic oxide support material onto the metal substrate. The
catalytic material may optionally be produced by wetting the
support material, especially a particulate support material, with a
platinum group metal solution and iron solution and drying and
calcining the wetted support material in air at a temperature in
the range of from 200.degree. C. to 300.degree. C., preferably
using a solution containing bivalent platinum ion species. The
catalyst member may be used by flowing the gas stream therethrough
at a temperature at about 90.degree. C. with a O.sub.2:CO ratio of
about 1:1 and a space velocity of about 20,000/hr or,
alternatively, at a temperature of about 150.degree. C. with a
O.sub.2:CO ratio of about 1.5:1 and a space velocity of about
80,000/hr.
Inventors: |
Korotkikh, Olga; (Edison,
NJ) ; Farrauto, Robert J.; (Edison, NJ) ;
McFarland, Andrew; (Dunellen, NJ) |
Correspondence
Address: |
Chief Patent Counsel
Engelhard Corporation
101 Wood Avenue
P.O. Box 770
Iselin
NJ
08830
US
|
Family ID: |
23552095 |
Appl. No.: |
10/307841 |
Filed: |
December 2, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10307841 |
Dec 2, 2002 |
|
|
|
09392813 |
Sep 9, 1999 |
|
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Current U.S.
Class: |
502/326 ;
423/247; 502/327 |
Current CPC
Class: |
C01B 2203/044 20130101;
C01B 3/583 20130101; B01J 37/0225 20130101; B01D 53/864 20130101;
B01J 37/34 20130101; C01B 2203/047 20130101; B01J 23/8906 20130101;
B01J 37/0244 20130101 |
Class at
Publication: |
502/326 ;
502/327; 423/247 |
International
Class: |
B01J 023/70 |
Claims
What is claimed is:
1. A method of preparing a catalytic material, comprising wetting a
refractory inorganic oxide support material with a bivalent
platinum solution and iron solution and drying and calcining the
wetted support material under oxidizing conditions at a temperature
in the range of from 200.degree. C. up to, but not including,
300.degree. C.
2. A method of preparing a catalytic material, comprising wetting a
refractory inorganic oxide support material with a bivalent
platinum solution and iron solution and drying and calcining the
wetted support material under oxidizing conditions at a temperature
in the range from 200.degree. C. to 300.degree. C. wherein the
catalytic material is substantially free of at least one metal
selected from the group consisting of palladium, rhodium and
cerium.
3. The method of claim 1 wherein the support material is a powdered
support material.
4. The method of claim 3 wherein the support material comprises
powdered alumina.
5. The method of claim 1 wherein the support material comprises a
pelletized support material.
6. A method of preparing a catalyst member, comprising wetting a
monolith comprising a refractory material with a bivalent platinum
solution and iron solution and drying and calcining the wetted
monolith under oxidizing conditions at a temperature in the range
of from 200.degree. C. up to but not including 300.degree. C.
7. A method of preparing a catalyst member, comprising wetting a
monolith comprising a refractory material with a bivalent platinum
solution and iron solution and drying and calcining the wetted
monolith under oxidizing conditions at a temperature in the range
of from 200.degree. C. to 300.degree. C., wherein the catalyst
member is substantially free of at least one metal selected from
the group consisting of palladium, rhodium and cerium.
8. A catalytic material prepared by the method comprising wetting a
refractory inorganic oxide support material with a bivalent
platinum solution and an iron solution, and drying and calcining
the wetted support material under oxidizing conditions at a
temperature in the range of from 200.degree. C. up to but not
including 300.degree. C.
9. A catalytic material prepared by the method comprising wetting a
refractory inorganic oxide support material with a bivalent
platinum solution and an iron solution, and drying and calcining
the wetted support material under oxidizing conditions at a
temperature in the range of from 200.degree. C. to 300.degree. C.
wherein the catalytic material is substantially free of at least
one metal selected from the group consisting of palladium, platinum
and cerium.
10. The catalytic material of claim 8 or claim 9 wherein the
support material comprises a powdered support material and wherein
the catalytic species comprises platinum at a loading in the range
of about 3 to 5 weight percent and iron at a loading of about 0.3
weight percent of the catalytic material.
11. The catalytic material of claim 8 or claim 9 wherein the
support material comprises alumina.
12. The catalytic material of claim 8 or claim 9 wherein the
support material comprises pelletized support material.
13. A catalyst member prepared by the method comprising wetting a
monolith comprising a refractory material with a bivalent platinum
solution and an iron solution, and drying and calcining the wetted
monolith under oxidizing conditions at a temperature in the range
of from 200.degree. C. up to but not including 300.degree. C.
14. A catalyst member prepared by the method comprising wetting a
monolith comprising a refractory material with a bivalent platinum
solution and an iron solution, and drying and calcining the wetted
monolith under oxidizing conditions at a temperature in the range
of from 200.degree. C. to 300.degree. C. wherein the catalyst
member is substantially free of at least one metal selected from
the group consisting of palladium, rhodium and cerium.
15. A catalyst member comprising a carrier substrate having a
catalytic material thereon, wherein the catalytic material is
prepared by the method comprising wetting a refractory inorganic
oxide support material with a bivalent platinum solution and an
iron solution, and drying and calcining the wetted support material
under oxidizing conditions at a temperature in the range of from
200.degree. C. up to but not including 300.degree. C.
16. A catalyst member comprising a carrier substrate having a
catalytic material thereon, wherein the catalytic material is
prepared by the method comprising wetting a refractory inorganic
oxide support material with a bivalent platinum solution and an
iron solution, and drying and calcining the wetted support material
under oxidizing conditions at a temperature in the range of from
200.degree. C. to 300.degree. C. wherein the catalyst member is
substantially free of at least one metal selected from the group
consisting of palladium, rhodium and cerium.
17. The catalyst member of claim 15 or claim 16 wherein the support
material comprises a powdered support material.
18. The catalyst member of claim 17 wherein the support material
comprises alumina.
19. The catalyst member of claim 18 wherein the catalytic species
comprises platinum at a loading in the range of from about 3 to 5
percent and iron at a loading of about 0.3 percent by weight of the
catalytic material.
20. The catalyst member of claim 15 wherein the carrier substrate
comprises a tube.
21. The catalyst member of claim 15 or claim 16 comprising a
coating of refractory inorganic oxide material on the carrier as a
binder coat beneath the catalytic material.
22. The catalyst member of claim 15 or claim 16 wherein the carrier
substrate comprises a metal substrate having a surface layer of
metal oxide.
23. The catalyst member of claim 22 wherein the metal substrate is
calcined before the catalytic material is deposited thereon to
produce the surface layer of metal oxide.
24. The catalyst member of claim 22 wherein the metal substrate
comprises a foamed metal substrate.
25. The catalyst member of claim 23 wherein the metal substrate
comprises a foamed metal substrate.
26. A catalyst member comprising at least one tube mounted in a
housing defining two fluid flow paths therethrough, the at least
one tube having a catalytic material deposited thereon for exposure
to at least one fluid flow path.
27. The catalyst member of claim 26 wherein the catalytic material
is prepared by the method comprising wetting a refractory inorganic
oxide support material with a platinum group metal solution and an
iron solution, and drying and calcining the wetted support material
under oxidizing conditions at a temperature in the range of from
200.degree. C. up to but not including 300.degree. C.
28. The catalyst member of claim 26 wherein the catalytic material
is prepared by the method comprising wetting a refractory inorganic
oxide support material with a platinum group metal solution and an
iron solution, and drying and calcining the wetted support material
under oxidizing conditions at a temperature in the range of from
200.degree. C. to 300.degree. C. wherein the catalytic material is
substantially free of at least one metal selected from the group
consisting of palladium, rhodium and cerium.
29. The catalyst member of claim 27 or claim 28 wherein the
solution comprises bivalent platinum ion species.
30. The catalyst member of claim 36, claim 37 or claim 38
comprising at least one tube having a metal anchor layer on which
the catalytic material is deposited, the anchor layer being
thermally sprayed onto the tubes.
31. The catalyst member of claim 30 wherein the anchor layer is
electric arc sprayed onto at least one tube.
32. The catalyst member of claim 26 wherein the at least one tube
has a coating of a refractory inorganic oxide material on the
carrier as a binder coat beneath the catalytic material.
33. The catalyst member of claim 32 wherein the at least one tube
comprises a metal tube having a surface layer of metal oxide.
34. The catalyst member of claim 33 wherein the metal tube is
calcined before the catalytic material is deposited thereon to
produce the surface layer of metal oxide.
35. A method for oxidizing carbon monoxide in a gas stream
containing carbon monoxide, hydrogen and oxygen, comprising
contacting the gas stream at a temperature less than 300.degree. C.
with a catalytic material prepared by the method comprising wetting
a refractory inorganic oxide support material with a platinum group
metal solution and an iron solution, and drying and calcining the
wetted support material under oxidizing conditions at a temperature
in the range of from 200.degree. C. to 300.degree. C. to produce a
catalytic material comprising catalytic species dispersed on the
support material.
36. A method for oxidizing carbon monoxide in a gas stream
containing carbon monoxide, hydrogen and oxygen, comprising
contacting the gas stream at a temperature less than 300.degree. C.
with a catalyst member prepared by the method comprising wetting a
monolith with a platinum group metal solution and an iron solution,
and drying and calcining the wetted monolith under oxidizing
conditions at a temperature in the range of from 200.degree. C. to
300.degree. C.
37. The method of claim 35 or claim 36 wherein the solution
contains bivalent platinum ion species.
38. The method of claim 37 wherein the catalytic material comprises
platinum at a loading of from about 3 to 5 percent by weight of
platinum plus support material and iron at a loading of about 0.3
percent by weight of iron plus support material.
39. The method of claim 38 wherein the support material comprises
alumina.
40. The method of claim 37 comprising contacting the gas stream at
a temperature at about 90.degree. C. with a O.sub.2:CO ratio of
about 1:1 and a space velocity of about 20,000/hr.
41. The method of claim 35 or claim 36 comprising contacting the
gas stream with the catalytic material at a temperature of about
150.degree. C. with a O.sub.2: CO ratio of about 1.5:1 and a space
velocity of about 80,000/hr.
42. The method of claim 37 comprising operating under conditions in
which the temperature, gas velocity and O.sub.2:CO ratio are
related as follows: temperature=(90+x).degree. C.,
speed=(20,000+1,000x)VHSV and O.sub.2:CO ratio=(0.8+0.0033x):1,
where x.gtoreq.0.
43. A method for treating a gas containing carbon monoxide,
hydrogen and oxygen, comprising flowing the gas through a first
flow path in a catalyst member having at least two flow paths
therethrough with gas in the second flow path whereby to exchange
heat between the gases in the two flow paths.
44. The method of claim 43 wherein the catalytic material is
effective for the selective oxidation of carbon monoxide of the gas
in the first flow path.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to the preparation of catalytic
material for use in the selective oxidation of carbon monoxide, to
catalyst members comprising such materials and to the conditions of
their use. The invention finds utility in the preparation of
hydrogen-containing gas streams for use in fuel cells, which
generate power by the oxidation of hydrogen.
[0003] A known strategy for the use of fuel cells involves the
generation of hydrogen from carbonaceous fuels. Generally, this
process involves subjecting the fuel to desulfurization, steam
reforming and high- and low-temperature water-gas shift reactions.
The resulting gas stream comprises significant quantities of
hydrogen (H.sub.2), carbon dioxide (CO.sub.2), water (H.sub.2O) and
about 0.5% carbon monoxide (CO). The aforesaid quantity of CO is
greater than desired for fuel cell purposes, since CO is known to
poison the catalyst for the fuel cell reaction. It is therefore
necessary to remove some or all of the CO, e.g., by oxidizing it to
CO.sub.2, without removing the H.sub.2 needed to power the fuel
cell. The CO must be removed or reduced to a maximum of about 10
ppm. In a prior art process known under the trade name
SELECTOXO.TM., the product of the water-gas shift reactions is
stripped of CO in a catalytic selective oxidation process that
avoids oxidation of H.sub.2. The commercial SELECTOXO.TM. catalyst
involved comprises from 0.3 to 0.5% platinum and 0.03% iron
dispersed on alumina support tablets or pellets by wet impregnation
of the alumina with a solution of platinum and iron salts. The
SELECTOXO.TM. catalyst material was dried at not more than
125.degree. C. because it was expected that that catalyst would be
used at temperatures not higher than 125.degree. C. and that a
higher drying temperature would detrimentally affect the platinum.
The catalyzed alumina tablets are typically assembled into a bed
through which the feed stream is flowed.
[0004] 2. Related Art
[0005] U.S. Pat. No. 3,088,919 to Brown, Jr. et al, entitled
"Treatment Of Gases" and dated May 7, 1963, discloses a process for
the preferential oxidation of carbon monoxide in a
hydrogen-containing gas, in particular, ammonia synthesis gas.
According to the disclosed process, the gas is treated with water
and is then passed over a supported platinum catalyst (see col. 1,
lines 34-39). The platinum is loaded at from 0.01 to 5 weight
percent of the catalyst (col. 3, lines 25-27). The Patent states
that the selective oxidation process may be carried out at
temperatures from 60.degree. F. to 1200.degree. F. (15.degree. C.
to 650.degree. C.), preferably 200.degree. F. to 450.degree. F.
(93.degree. C. to 232.degree. C.) ( see col.3, lines 4-6). Example
III discloses a process for oxidizing carbon monoxide using a
platinum catalyst at a catalyst temperature bed in the range of
230.degree. C. to 500.degree. F. (110.degree. C. to 260.degree.
C.). The inlet gas contained 1.7 percent carbon monoxide (CO) and
the exit gas contained up to 4,000 parts per million CO. The
pressure may be from atmospheric to 300 psig (col. 3, line 7) and
the space velocity of the gas through the catalyst may be 100 to
25,000 ft.sup.3 gas/ft.sup.3 catalyst per hour, preferably 4,000 to
6,000 ft.sup.3 gas/ft.sup.3 catalyst per hour for a single-stage
operation (col. 3, lines 8-17).
[0006] U.S. Pat. No. 3,216,783 to Cohn, dated Nov. 9, 1965 and
entitled "Process For Selectively Removing Carbon Monoxide From
Hydrogen-Containing Gases", discloses the use of a supported
platinum catalyst containing from 0.01 to 5 weight percent platinum
on pelleted, powdered or granulated support material, for use in
oxidizing carbon monoxide (col. 1, lines 53-62) at a reaction
temperature in the range of 110.degree.C. to 200.degree. C. (col.
1, lines 35-42). The space velocity of the gas is in the range of
from 500 to 100,000 VHSV at 70.degree. F. (21.1.degree. C.).
[0007] U.S. Pat. No. 4,492,769 to Blanchard et al, dated Jan. 8,
1985 and entitled "Pollution Control Catalyst For Internal
Combustion Engine Exhaust System/Catalytic Converter and Process
For Its Preparation", discloses the preparation of certain
platinum-containing catalysts with calcining of 300.degree. C.
(Examples 2, 3, 4, 5).
[0008] U.S. Pat. 5,583,087 to Slotte, entitled "Method For
Impregnating Catalyst Support With Platinum", dated Dec. 10, 1996,
discloses a method for the preparation of a catalytic material that
may comprise platinum dispersed on alumina by a wet impregnation
technique. In accordance with the teachings of this Patent, a
solution of bivalent platinum is prepared, the bivalent platinum is
oxidized to Pt.sup.+4, e.g., by adding hydrogen peroxide or ozone,
and the Pt.sup.+4 solution is then impregnated into the support
material, preferably via chemisorption. The wetted support material
is then calcined at 275.degree. C. (see col. 3, lines 29-40).
[0009] U.S. Pat. 4,818,745 to Kolts, entitled "Catalyst For
Oxidation Of Carbon Monoxide And Process For Preparing The
Catalyst", dated Apr. 4, 1989, discloses a catalyst for the
oxidation of carbon monoxide under conditions suitable for laser
applications. The catalyst comprises platinum and/or palladium
dispersed on alumina via impregnation. The catalyst may contain
from 0.5 to 5 weight percent platinum and/or palladium (col. 4,
lines 22-29). Iron is also used in the catalyst at a loading of
from about 0.2 to 4 weight percent (col. 4, lines 54-57). The
preparation method includes drying and calcining the wetted support
material in two stages, first at temperatures of about 30.degree.
C. to about 200.degree. C. and then in the range of from about
300.degree. C. to about 700.degree. C. The material is then
subjected to reducing conditions by exposure to a reducing gas at a
temperature of about 550.degree. C. to 700.degree. C. The feed
gases for the carbon monoxide oxidation processes described therein
are substantially free of hydrogen.
[0010] U.S. Pat. No. 4,440,874 to Thompson entitled "Catalyst
Composition And Method For Its Manufacture", dated Apr. 3, 1984,
discloses a method for the preparation of a catalytic material used
for the purification of exhaust gases from internal combustion
engines. The catalytic material may comprise platinum and iron and
is prepared using a wet impregnation technique to deposit the
catalytic metals on an alumina support material. This Patent
illustrates drying and calcining the wetted support material at
450.degree. C. (col. 7, lines 57-61).
[0011] U.S. Pat. No. 4,749,671 to Saito et al entitled "Exhaust Gas
Cleaning Catalyst And Process For Production Thereof", dated Jun.
7, 1988, discloses a catalytic material useful for cleaning diesel
engine exhaust gases or other exhaust gases containing combustible
carbonaceous particles. The catalytic material may comprise
platinum and iron supported on alumina (see col. 3, lines 40-61).
The disclosed method of preparation involves impregnating alumina
pellets with a solution of the catalytic metals, drying and
calcining the pellets and then grinding them and forming them into
a slurry for coating on a carrier. In each example, the wetted
support material was dried and then calcined at temperatures of
500.degree. C. or 600.degree. C.
[0012] U.S. Pat. No. 4,621,071 to Blanchard et al, dated Nov. 4,
1996, entitled "Composite Catalyst For Treatment Of Vehicular
Exhaust Gases . . . ", discloses catalytic materials for the
treatment of vehicular exhaust gases. The disclosed materials may
comprise platinum and iron dispersed on a support material that may
be alumina. This Patent teaches that the support material is
impregnated with a solution containing the catalytic metals and is
then dried and calcined at a temperature of 300.degree. C. to
800.degree. C. (see col. 6, lines 53-63). The catalyst is then
"activated" by exposure to a reducing atmosphere at a temperature
between 200.degree. C. and 700.degree. C. (see col. 6, lines
64-68). In Examples 3 and 4 of this Patent, impregnated alumina
materials were dried at 150.degree. C. and then activated at
350.degree. C. In Example 6, an impregnated material was dried at
150.degree. C. and then calcined at 350.degree. C. in air (see col.
11, lines 13-20).
[0013] The following references address selective oxidation of
carbon monoxide: U.S. Pat. No. 3,631,073 to Cohn, dated Dec. 28,
1971; Canadian Patent 609,619 to Cohn, dated Nov. 29, 1960; Brown,
Jr. et al, "Purifying Hydrogen by Selective Oxidation of Carbon
Monoxide", 52 Industrial Engineering Chemistry, No. 10, Oct. 1960,
page 841; Anderson et al, "Removing Carbon Monoxide From Ammonia
Synthesis Gas", 53 Industrial Engineering Chemistry, No. 8, August
1961, page 645.
[0014] U.S. Pat. No. 5,204,302, issued Apr. 20, 1993 to I.V.
Gorynin et al, is entitled "Catalyst Composition and a Method For
Its Preparation" and is hereinbelow referred to as "the '302
Patent". The '302 Patent discloses a multi-layered catalyst
material supported on a metal substrate. The metal substrate
(column 4, lines 64-68) may be any thermally stable metal including
stainless steel and low alloy steel, the '302 Patent stating that,
regardless of which type of substrate is used, there is no
appreciable difference in the performance of the bonded layers. As
illustrated in FIG. 1 of the Patent and described at column 4, line
32 et seq, a flame spraying or plasma spraying apparatus (FIG. 2
and column 5, line 32 et seq) is used to apply an adhesive sublayer
12 to metal substrate 11, which is shown in solid cross section as
a dense (solid) plate-like structure. Adhesive sublayer 12 contains
a self-bonding intermetallic compound formed from any one of a
number of metal pairings, including aluminum and nickel, as
described at column 5, lines 1-6 of the '302 Patent. The high
temperature of the flame or plasma spray operation is said to
generate a diffusion layer (13 in FIG. 1) caused by diffusion of
material of substrate 11 and sublayer 12 across their interface
(column 4, lines 37-41). A catalytically active layer 14 (FIG. 1)
is sprayed atop the sublayer 12 and has a gradient composition with
an increasing content of catalytically active material as one
proceeds away from the interface (column 5, lines 7-24). The
catalytically active layer can be alumina, preferably
gamma-alumina, and may further include specified metal oxide
stabilizers such as CaO, Cr.sub.2O.sub.3, etc., and metal oxide
catalytic materials such as ZrO.sub.2, Ce.sub.2O.sub.3, etc. A
porous layer 18 (FIG. 1 and column 5, lines 25-32) contains some
catalytically active components and transition metal oxides as
decomposition products of pore forming compounds such as
MnCO.sub.3, Na.sub.2CO.sub.3, etc., which presumably form pores as
gases evolve from the carbonates or hydroxides (column 7, lines
40-45) as they thermally decompose (see column 7, lines 37-45). As
described at column 5, line 44 et seq and at column 7, line 37 et
seq, sublayer 12, catalytically active layer 14 and porous layer 18
may be applied by a continuous plasma spray operation in which
different ones of the powders 21, 28 and 33 (FIG. 2) are fed into
the plasma spray in a preselected sequence and at preselected
intervals. An optional activator coating 19 may be applied onto the
porous layer, preferably by magnetron sputtering (see column 4,
lines 56-63 and column 8, lines 24 et seq).
[0015] U.S. Pat. No. 4,027,367, issued Jun. 7, 1977 to H. S.
Rondeau, which is incorporated herein by reference, is entitled
"Spray Bonding of Nickel Aluminum and Nickel Titanium Alloys" and
is hereinbelow referred to as "the '367 Patent". The '367 Patent
discloses a method of electric arc spraying of self-bonding
materials, specifically, nickel aluminum alloys or nickel titanium
alloys, by feeding metal constituent wires into an electric arc
spray gun (column 1, lines 6-13). The '367 Patent mentions,
starting at column 1, line 25, combustion flame spray guns, e.g.,
guns feeding a mixture of oxygen and acetylene to melt a powder fed
into the flame. Such combustion flame spray guns are said to
operate at relatively low temperature and are often incapable of
spraying materials having melting points exceeding 5,000.degree. F.
(2,760.degree. C.). The '367 Patent also mentions (starting at
column 1, line 32) that plasma arc spray guns are the most
expensive type of thermal spray devices and produce much higher
temperatures than combustion-type flame spray guns, up to
approximately 30,000.degree. F. (16,649.degree. C.). It is further
pointed out in the '367 Patent that plasma arc spray guns require a
source of inert gas for the creation of plasma as well as extremely
accurate control of gas flow rate and electric power for proper
operation. In contrast, starting at column 1, line 39, electric arc
spray guns are stated to simply require a source of electric power
and a supply of compressed air or other gas to atomize and propel
the melted material in the arc to the substrate or target. The use
of electric arc spraying with a wire feed of nickel aluminum or
nickel titanium alloys onto suitable substrates, including smooth
steel and aluminum substrates is exemplified starting at column 5,
line 28, but no mention is made of open, porous or honeycomb-type
substrates, or ceramic substrates and there is no suggestion for
the use of the resulting articles as carriers for catalytic
materials.
[0016] U.S. Pat. No. 3,111,396 to Ball, dated Nov. 19, 1963
(hereinafter referred to as "the '396 Patent") and entitled "METHOD
OF MAKING A POROUS MATERIAL", discloses a method for making a
porous metal material or "metal foam". Essentially, the method
comprises forming a porous organic structure such as a mesh, cloth,
or a cured foam structure such as an open pore sponge, impregnating
the structure with a fluid suspension of powdered metal in a liquid
vehicle, and drying and heating the impregnated structure to remove
the liquid vehicle and then further heating the organic structure
to decompose it and to sinter the metal powder into a continuous
form. The resulting metallic structure, while not foamed during the
manufacturing process, is nevertheless described as foamed because
its ultimate structure resembles that of a foamed material.
[0017] SAE (Society of Automotive Engineers) Technical Paper
971032, entitled A New Catalyst Support Structure For Automotive
Catalytic Converters by Arun D. Jatkar, was presented at the
International Congress and Exposition, Detroit, Mich., Feb. 24-27,
1997. This Paper discloses the use of metal foams as a substrate
for automotive catalysts. The Paper describes the use of various
metal foams as catalyst substrates and notes that foams made of
pure nickel or nickel-chromium alloys were not successful as
substrates for automotive catalysts because of corrosion problems
encountered in the environment of an automotive exhaust catalyst.
Metal foams made from Fecralloy and ALFA-IV.RTM. ferritic stainless
steel powders were said to be successful, at least in preliminary
tests, for use as substrates for automotive catalysts. A ceramic
washcoat having a precious metal loading was deposited onto disks
of ALFA-IV.RTM. metal foam produced by Astro Met, Inc. The washcoat
comprised gamma-alumina and cerium oxide on which platinum and
rhodium in a ratio of 4:1 were dispersed to provide a loading of 40
grams of the precious metal per cubic foot of the foam-supported
catalyst. Such catalyzed substrates were said to be effective in
treating hydrocarbon emissions.
[0018] In an article entitled "Catalysts Based On Foam Metals",
published in Journal of Advanced Materials, 1994, 1(5) 471-476,
Pestryakov et al suggest the use of foamed metal as a carrier
substrate for catalytic materials for the catalytic neutralization
of exhaust gases of car engines. The use of an intermediate layer
of high surface area alumina between the metallic foam and the
catalytic material is recommended, by direct deposition on the foam
carrier. In addition to increasing the surface area of the
substrate, the alumina is also credited with protecting the surface
of the substrate against corrosion.
[0019] SAE Paper 962473 by Reck et al of EMITECH, GmbH, entitled
"Metallic Substrates and Hot Tubes For Catalytic Converters in
Passenger Cars, Two- and Three-Wheelers", addresses the use of
catalytic converters and hot tubes to treat the exhaust of scooters
and motorcycles, especially those having two-stroke engines.
[0020] A supplier of wire mesh carriers for catalytic materials
known as OptiCat offers for sale wire mesh comprising wire that has
been plasma spray coated to form a rough surface thereon to improve
the adherence of a catalytic material deposited thereon.
[0021] Prior art attempts to adhere catalytic materials to metallic
substrates include the use of ferrous alloys containing aluminum.
The alloy is formed into a substrate structure and is heat-treated
under oxidizing conditions. The aluminum oxidizes, forming whiskers
of alumina that project from the substrate surface and are believed
to provide anchors for catalytic materials. The use of other
alloying elements, e.g., hafnium, in ferrous metals for this
purpose is known to provide such whiskers upon oxidizing
treatment.
SUMMARY OF THE INVENTION
[0022] In one aspect, the present invention relates to a method of
preparing a catalytic material. The method comprises wetting a
refractory inorganic oxide support material with a bivalent
platinum solution and iron solution and drying and calcining the
wetted support material under oxidizing conditions at a temperature
in the range from 200.degree. C. up to but not including
300.degree. C. If the catalytic material is substantially free from
at least one of palladium, rhodium and cerium, the temperature
range may include 300.degree. C.
[0023] According to one aspect of the invention, the support
material may be a powdered support material such as powdered
alumina. Alternatively, the support material may comprise a
pelletized support material.
[0024] This invention also provides a method of preparing a
catalyst member comprising wetting a monolith comprising a
refractory material with a bivalent platinum solution and iron
solution and drying and calcining the wetted monolith under
conditions described above.
[0025] The invention also pertains to the catalytic material and
catalyst members produced by the methods described herein.
[0026] This invention further provides a catalyst member comprising
at least one tube mounted in a housing defining two fluid flow
paths therethrough, the at least one tube having a catalytic
material deposited thereon for exposure to at least one fluid flow
path. The catalytic material may be prepared by any one of the
methods described above.
[0027] This invention further provides a method for oxidizing
carbon monoxide in a gas stream containing carbon monoxide,
hydrogen and oxygen, comprising contacting the gas stream at a
temperature less than 300.degree. C. with a catalytic material or
catalyst member as described herein.
[0028] The method may comprise contacting the gas stream at a
temperature of about 90.degree. C. with a O.sub.2:CO ratio of about
1:1 and a space velocity of about 20,000/hr or, alternatively, at a
temperature of about 150.degree. C. with a O.sub.2:CO ratio of
about 1.5:1 and a space velocity of about 80,000/hr. More broadly
described, the operating conditions may be described as
(90+x).degree. C., (20,000+,1000x)VHSV and (0.8+0.0033x): 1O.sub.2
:CO ratio, where x is zero or greater, e.g., from 0 to 60.
[0029] A further method of this invention for treating a gas
containing carbon monoxide, hydrogen and oxygen comprises flowing
the gas through a first flow path in a catalyst member having at
least two flow paths therethrough with gas in the second flow path
whereby to exchange heat between the gases in the two flow
paths.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIGS. 1A-1D are photomicrographs of a foamed metal substrate
without an anchor layer deposited thereon, at magnifications of
38.times., 55.times., 152.times. and 436.times., respectively;
[0031] FIGS. 2A-2D are photomicrographs of a foamed metal substrate
having an anchor layer electric arc sprayed thereon, at
magnifications of 38.times., 55.times., 152.times. and 434.times.,
respectively;
[0032] FIGS. 2E-2G are photomicrographs of a cross section of a
flat metal substrate and an anchor layer electric arc sprayed
thereon, at magnifications of 500.times., 1.51k.times. and
2.98k.times.;
[0033] FIG. 2H is an elevation view of a perforated, tubular metal
substrate;
[0034] FIG. 2I is an elevation view of a catalyst member in
accordance with the present invention comprising the substrate of
FIG. 2H;
[0035] FIG. 2J is a schematic view of a wire mesh substrate having
an anchor layer sprayed thereon in accordance with the present
invention;
[0036] FIG. 3A is a schematic cross-sectional view of a metal
substrate having an anchor layer electric arc sprayed thereon
according to one embodiment of the present invention;
[0037] FIG. 3B is a schematic cross-sectional view of the substrate
of FIG. 3A after processing into a corrugated configuration and
being disposed upon another sprayed substrate;
[0038] FIG. 3C is a schematic cross-sectional view of the
substrates of FIG 1B after further processing to wind the
substrates to form a honeycomb;
[0039] FIG. 3D is a schematic process diagram illustrating the
manufacture of a catalyst member according to a particular
embodiment of the present invention;
[0040] FIG. 3E is a plan view illustrating a fragment of a skewed
corrugated strip used in the invention;
[0041] FIG. 3F is an enlarged fragmentary side profile of the
corrugated strip shown in FIG. 3E;
[0042] FIG. 3G is a perspective view illustrating a honeycomb
carrier core body formed by folding the strip shown in FIG. 3E;
[0043] FIG. 3H is an exploded perspective view depicting the
assembly of the core body with a jacket tube;
[0044] FIG. 31 is an enlarged fragmentary end view of the core body
shown in FIG. 3G;
[0045] FIG. 3J is an enlarged fragmentary end view, similar to FIG.
31, but illustrating the core body and jacket after assembly;
[0046] FIG. 3K is a fragmentary cross section illustrating a
preferred way of inserting the core body of the invention into a
jacket tube;
[0047] FIG. 3L is a cross section illustrating a swaging operation
of the assembled core body and jacket tube after assembly;
[0048] FIG. 3M is a plan view illustrating an alternative manner of
assembling the core body and jacket tube;
[0049] FIG. 3N is a plan view illustrating the core body and jacket
tube of FIG. 3M after assembly is completed;
[0050] FIG. 3P is a plan view illustrating the honeycomb carrier
body product of the invention;
[0051] FIG. 3Q is a side elevation of the carrier body illustrated
in FIG. 3P;
[0052] FIGS. 3R, 3S, 3T and 3U are plan views showing alternative
configurations of core bodies that may be formed by and used in the
present invention;
[0053] FIG. 3V is a schematic view of a catalyst member configured
as a heat exchanger in accordance with another embodiment of the
present invention;
[0054] FIG. 4 is a perspective view of a ceramic honeycomb
substrate having an anchor layer deposited on the smooth outer
surface thereof according to another embodiment of the
invention;
[0055] FIG. 5 is a schematic cross-sectional view of an exhaust gas
treatment apparatus including two foamed metal regions of different
densities according to the present invention;
[0056] FIG. 6 is a plot showing the conversion of CO and
consumption of O.sub.2 and O.sub.2 selectivity exhibited by
catalytic materials calcined alternatively in air or N.sub.2 and at
various temperatures;
[0057] FIG. 7 is a plot showing conversion of CO, consumption of
O.sub.2 and O.sub.2 selectivity for a feed stream tested at various
space velocities;
[0058] FIG. 8 is a plot showing conversion of CO, consumption of
O.sub.2 and O.sub.2 selectivity over a range of temperatures;
[0059] FIG. 9 is a plot showing conversion of CO, consumption of
O.sub.2 and O.sub.2 selectivity at a variety of O.sub.2:CO ratios
in gas streams tested at 20,000 VHSV and 90.degree. C.;
[0060] FIG. 10 is a plot similar to FIG. 9 for gas streams tested
at 80,000 VHSV and 90.degree. C.; and
[0061] FIG. 11 is a plot showing conversion of CO, consumption of
O.sub.2 and O.sub.2 selectivity at a variety of O.sub.2:CO ratios
in gas streams tested at 80,000 VHSV and 150.degree. C.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
THEREOF
[0062] One aspect of the present invention relates to the use of
particular catalysts useful for the selective oxidation of carbon
monoxide in an oxygen- and hydrogen-containing gas, to a method of
preparing such catalysts and to the products of the method. The
selective or preferential oxidation of carbon monoxide in a gas
stream containing a significant quantity of hydrogen is required in
various processes, e.g., for removing carbon monoxide from the gas
stream effluent from high- and low-temperature water-gas shift
reactions to produce a fuel cell feed stream. Such gas streams
typically contain at least 5 percent, preferably at least 10
percent, more preferably at least 20 percent, hydrogen by weight
and about 0.5 percent CO. The water-gas shift gas stream product is
typically contacted with a catalytic material prepared in
accordance with the present invention at a process temperature
lower than about 200.degree. C., often at low or ambient
temperature, e.g., 125.degree. C., or lower. The amount of CO in
the gas stream is reduced to not more than about 0.001 weight
percent, preferable to less than 10 parts per million (ppm).
Accordingly, the rate of CO conversion should be at least about
95%, preferably at least about 98%.
[0063] The Applicants have made the surprising discovery that
superior catalytic activity for the selective oxidation of carbon
monoxide can be obtained by using a catalyst comprising platinum
and iron that have been impregnated onto a support material or
monolith which was then dried and calcined under oxidizing
conditions, e.g., in air, in the temperature range of from
200.degree. C. to 300.degree. C. The prior art does not recognize
the advantage of the use of materials calcined in this range for
the selective oxidation processes described herein. The present
invention also relates to a method for the preparation of a
catalyst and catalytic material and to the products of the method.
The method comprises wetting a support material such as alumina (or
a monolith of such material) with platinum and iron in solution and
calcining the wetted material or monolith in oxidizing conditions,
e.g., in air, at temperatures in the range of from 200.degree. C.
up to, but not including, 300.degree. C. The prior art (e.g., U.S.
Pat. No. 4,492,769 (discussed above)) fails to suggest the use of
such material for the selective oxidation of carbon monoxide in a
hydrogen-containing gas stream, and it shows the use of catalytic
metals not necessarily found in the platinum- and iron-containing
catalyst of the present invention. Accordingly, the methods,
catalytic material and catalyst members according to the present
invention may optionally be substantially free of at least one, any
two or, optionally, all three metals selected from the group
consisting of palladium, rhodium and cerium. They may optionally be
free of any one or more of barium, copper and/or manganese. The
calcination temperature range can optionally be limited to the
non-inclusive range of from 200.degree. C up to, but not including,
300.degree. C. According to this aspect of the invention, the
platinum-containing catalytic material is not heated in air above
this temperature range during the calcination process or at any
time before it is placed in service. Preferably, the temperature of
the catalytic process for which the catalytic material is used does
not exceed the calcination temperature. In any event, the thermal
history of a platinum-containing catalytic product according to an
optional embodiment of this invention will not include exposure to
temperatures greater than 300.degree. C. after the platinum is
deposited thereon. Temperatures of 200.degree. C. or less are
permitted as long as calcination in the range between 200.degree.
C. to 300.degree. C. is realized and not exceeded.
[0064] In various embodiments of the practice of the invention,
suitable upper temperature limits to be optionally observed within
the preferred calcination temperature range (i.e., temperatures not
to be attained or exceeded) are 285.degree. C., 275.degree. C.,
265.degree. C. and 250.degree. C. Suitable lower temperature limits
optionally to be observed within the preferred calcination
temperature range (i.e., temperatures to be exceeded during
calcination) are 215.degree. C., 225.degree. C. and 235.degree. C.
This method aspect of the invention may be practiced in the
preparation of any form of catalytic material comprising catalytic
species dispersed on a support material, not just the catalytic
materials specifically described herein.
[0065] Still another aspect of the invention relates to the
discovery of optimum process conditions of temperature and flow
rate that yield superior conversion rates for the primary object in
the oxidation of CO without undue collateral oxidation of H.sub.2.
This aspect of the invention relates to temperature and flow rate
of the feed stream through the catalyst member for improved
oxidation of CO.
[0066] This invention also relates to the optional coating of the
catalytic materials described herein onto carrier substrates that
comprise an anchor layer as described herein for adhering the
catalytic material to the carrier. In addition, this invention
relates to the novel use of a catalyst member comprising a washcoat
of catalytic material applied to flow-through monoliths, e.g., to
honeycomb monoliths and/or foamed metal monoliths. The use of such
monoliths provides greater mechanical stability to the catalytic
material than the tablet or granule beds used in the prior art.
[0067] A catalytic material is prepared in accordance with one
aspect of this invention by dispersing compounds and/or complexes
of platinum and iron onto relatively inert support material. As
used herein, the term "compound", as in "platinum compound" or
"iron compound" means any compound, complex, or the like of
platinum or iron which, upon calcination or upon use of the
catalyst, decomposes or otherwise converts to a catalytically
active form, which is often, but not necessarily, an oxide. The
compounds or complexes may be dissolved or suspended in any liquid
which will wet or impregnate the support material, and which is
capable of being removed from the catalyst by volatilization or
decomposition upon heating and/or the application of a vacuum.
Generally, both from the point of view of economics and
environmental aspects, aqueous solutions of soluble compounds or
complexes are preferred. For example, suitable water-soluble
platinum compounds are chloroplatinic acid and amine solubilized
platinum hydroxide; suitable water-soluble iron compounds include
FeCl.sub.2, FeCl.sub.3, Fe.sub.2(SO.sub.4).sub.3,
Fe(NO.sub.3).sub.2, Fe(NO.sub.3).sub.3. The solution of catalytic
species impregnated into the pores of the bulk support particles of
the catalyst, i.e., the support particles are wetted with the
solution, and the wetted or impregnated material is dried and
calcined subject to the temperature limitations set forth herein,
to remove the liquid and bind the platinum group metal and iron
onto the support material. Wetting the support material with a
platinum group metal solution and an iron solution may comprise
wetting the support material with a solution containing both
platinum and iron compounds or with separate solutions, one
containing a platinum compound and one containing an iron compound.
The wetted support is then dried and calcined and the dissolved
platinum group metal and iron compounds are thus converted into
catalytically active forms. An analogous approach can be taken to
incorporate other components into the catalytic material. In
particular embodiments, the solution contains bivalent platinum
ions. For example, the solution may contain
Pt.sup.II(NH.sub.3).sub.4 Cl.sub.2 and may not be oxidized prior to
calcination. The optional use of bivalent platinum ions and of a
non-oxidized platinum solution both run contrary to the teaching of
U.S. Pat. No. 5,583,087 (described above).
[0068] Suitable support materials for the catalytic component
include alumina, silica, titania, silica-alumina,
alumino-silicates, aluminum-zirconium oxide, aluminum-chromium
oxide, etc. Such materials may be provided in various forms, but a
support material is preferably used in a particulate, high surface
area form. For example, gamma-alumina is preferred over
alpha-alumina. The support material and therefore the resulting
catalytic material are typically used in particulate form with
particles in the micrometer-sized range, e.g., 10 to 20 micrometers
in diameter, so that they can be formed into a slurry applied as a
washcoat onto a carrier member.
[0069] The loading of platinum on a particulate support material
should be in the range of from about 3 to 7 weight percent,
preferably about 5 weight percent. The iron loading will be roughly
proportional to the platinum loading at about six percent thereof,
e.g., in the range of from about 0.1 to 0.6 weight percent,
preferably about 0.3 weight percent.
[0070] Loadings of 3 to 5 weight percent platinum and 0.3 weight
percent iron on powdered alumina correspond to the platinum and
iron content in the surface layer of the prior art SELECTOXO.TM.
catalysts described above. The overall loadings of 0.3 to 0.5
weight percent platinum and 0.03 weight percent iron stated above
relative to the SELECTOXO.TM. catalysts reflect the fact that the
SELECTOXO.TM. tablets contain within their interiors substantial
quantities of alumina that are substantially free from catalytic
species and which do not have significant contact with feed stream
gases. The interior mass of alumina reduces the overall loading of
the catalytic species on the support material to about 0.3 to 0.5
weight percent platinum and about 0.03 weight percent iron. The
loading of the catalytic species in the active layer of the
tablets, however, is believed to be about 3 to 5 weight percent
platinum and 0.3 weight percent iron.
[0071] In optional but preferred embodiments, catalytic material
prepared in accordance with this invention is applied as a thin
layer, e.g., as a washcoat, onto a carrier member of high surface
area, which is believed to enhance contact between the gas stream
and the catalytic species. A high surface area carrier member
defines numerous apertures, pores, channels or similar structural
features that cause liquid and/or gas to flow therethrough in
turbulent or substantially non-laminar fashion and give the
substrate a high surface area per overall volume of the flow path
of the fluid through the substrate, e.g., features that create a
high mass transfer zone for the fluid therein. Open substrates may
be provided in a variety of forms and configurations, including
honeycomb-type monoliths, woven or non-woven mesh, wadded fibers,
foamed or otherwise reticulated or lattice-like three-dimensional
structures, etc. For gas phase, i.e., fluid phase, reactions, a
suitable carrier typically has a plurality of fluid-flow passages
extending therethrough from one face of the carrier to another for
fluid-flow therethrough. Optionally, a binder layer or etch coat
may be applied to the carrier substrate before the catalytic
material is coated onto the carrier. The etch coat, which may
comprise a refractory inorganic oxide powder, helps the catalytic
washcoat adhere to the carrier substrate and is particularly
helpful in adhering the washcoat to a smooth metal surface. The
etch coat may optionally comprise the same material used as the
support material of a particulate catalytic material. An etch coat
may not be needed, however, if the substrate surface is rough
(e.g., if it is thermally sprayed onto the substrate as described
below) or if it adheres well to the catalytic material. For
example, an etch coat would be optional for use on a ceramic
honeycomb monolith onto which a catalytic material comprising an
alumina support material is to be applied, since the alumina is
expected to adhere well to the ceramic material. Likewise, when the
carrier comprises an aluminum metal substrate, the aluminum
substrate may be calcined in air before the catalytic material is
applied thereto, to produce on the surface a layer of alumina to
which the catalytic material will adhere.
[0072] In one conventional carrier configuration that is commonly
used for gas phase reactions and is known as a "honeycomb"
monolith, the passages are typically essentially (but not
necessarily) straight from an inlet face to an outlet face of the
carrier and are defined by walls on which the catalytic material is
coated so that the gases flowing through the passages contact the
catalytic material. The flow passages of the carrier member may be
thin-walled channels which can be of any suitable cross-sectional
shape and size such as trapezoidal, rectangular, square,
sinusoidal, hexagonal, oval, or circular. Such structures may
contain from about 60 to about 1000 or more gas inlet openings
("cells") per square inch of cross section ("cpsi"), more typically
200 to 600 cpsi. Such a honeycomb-type carrier monolith may be
constructed from metallic substrates in various ways such as, e.g.,
by placing a corrugated metal sheet on a flat metal sheet and
winding the two sheets together about a mandrel. Alternatively,
they may be made of any suitable refractory materials such as
cordierite, cordierite-alpha-alumina, silicon nitride, zirconium
mullite, spodumene, alumina-silica magnesia, zirconium silicate,
sillimanite, magnesium silicates, zirconium oxide, petallite,
alpha-alumina and alumino-silicates. Typically, such materials are
extruded into a honeycomb configuration and then calcined, thus
forming passages defined by smooth interior cell walls and a smooth
outer surface or "skin".
[0073] Foamed metal may provide one species of open substrate for
use in the present invention. Methods for making foamed metal are
known in the art, as evidenced by U.S. Pat. No. 3,111,396,
discussed above, and the use of foamed metal as a carrier for a
catalytic material has been suggested in the art, as recognized
above by reference to SAE Technical Paper 971032 (cited above) and
to the journal article by Pestryakov et al (cited above). Briefly
described, a foamed metal substrate can be formed by a casting
process in which a mold is filled with a mixture of metal powder
and granules of an expendable, removable material. The sleeve and
the metal powder-removable granules mixture therein are sintered.
The metal powder forms a porous matrix about the removable
granules, which are burned away. The resulting foamed metal
substrate is then removed from the mold for finishing. Foamed metal
can be characterized in various ways, some of which relate to the
properties of the initial organic matrix about which the metal is
disposed. Some characteristics of foamed metal substrates
recognized in the art include cell size, density, free volume, and
specific surface area. For example, the surface area may be 1500
times that of a solid substrate having the same dimensions as the
foamed substrate. As mentioned by Pestryakov et al, foamed metal
substrates useful as carriers for catalyst members may have mean
cell diameters in the range of 0.5 to 5 mm, and they may have a
free volume of from about 80 to 98%, e.g., 3 to 15 percent of the
volume occupied by the foamed substrate may constitute metal. The
porosity of the substrate may range from 3 to 80 ppi, e.g., from 3
to 30 ppi or from 3 to 10 ppi or, alternatively, from 10 to 80 ppi.
In the illustrative range of 10 to 80 ppi, other characteristics
such as cells per square inch may range from 100 to 6400 and the
approximate web diameter may vary from 0.01 inch to 0.004 inch.
Such foams may have open-cell reticulated structures, based on a
reticulated/interconnected web precursor. They typically have
surface areas that increase with porosity in the range of from
about 700 square meters per cubic foot of foam (m.sup.2/ft.sup.3)
at about 10 ppi to 4000 m.sup.2/ft.sup.3 at about 60 ppi, etc.
Other suitable foamed metal substrates have surface areas ranging
from about 200 square feet per cubic foot of foamed metal
(ft.sup.2/ft.sup.3) at about 10 ppi to about 1900 ft.sup.2/ft.sup.3
at about 80 ppi. One such substrate has a specific weight of 500
g/m.sup.2 at a thickness of about 1.6+/-0.2 millimeters with a
porosity of 110 ppi. They may have volume densities in the range of
0.1 to 0.3 grams per cubic centimeter (g/cc). Foamed metal sheets
can be rolled, layered, etc., to build up a substrate of any
desired dimension.
[0074] Suitable foamed nickel with which the present invention may
be practiced is commercially available in extruded sheets about 1.6
millimeters (mm) thick. It may have tensile strengths of at least 3
kilograms per square centimeter (kg/cm.sup.2) in the machine
direction and 9 percent in the transverse direction. At thicknesses
of 1.3 to 2.5 mm, it may have specific weights in the range of 350
to 1000 g/m.sup.2 and a pore size of 60 to 110 pores per lineal
inch (ppi). One particular material has a specific weight of 500
g/m.sup.2 and 80 ppi.
[0075] One suitable foamed metal substrate for use with the present
invention had a density of about 6 percent. Foamed metal substrates
can be formed from a variety of metals, including iron, titanium,
tantalum, tungsten noble metals, common sinterable metals such as
copper, nickel, bronze, etc., aluminum, zirconium, etc., and
combinations and alloys thereof such as steel, stainless steel,
Hastalloy, Ni/Cr, Inconel (nickel/chromium/iron) and Monel
(nickel/copper).
[0076] Stainless steel foam is a good, low-cost alternative to
plate-like substrates and to more expensive alloy foams such as
Fecralloy (FeCrAl).
[0077] Pestryakov et al state that the specific surface area for
pure foam metals equals approximately 0.01 to 0.1 m.sup.2/g, but
that this is insufficient to produce active catalysts for a
majority of catalytic processes taking place in the kinetic region.
They therefore recommend increasing the specific surface area by
direct deposition on the foamed metal of gamma-alumina having a
surface area of 20 to 50 m.sup.2/g, although they state that low
surface area foamed metals may be used in high temperature external
diffusion processes. The present invention teaches instead the
thermal spraying such as electric arc spraying of a metal anchor
layer preferably comprising nickel aluminide onto the metal foam
substrate.
[0078] Another species of open substrate may be provided by woven
or non-woven wire mesh. A woven wire mesh substrate for use with
the present invention may be formed in any suitable weave, e.g.,
plain, twill, plain Dutch weave, twill Dutch weave, crocheting,
etc. Wire mesh is commonly available in weaves that leave from
about 18 to 78 percent open area, more typically, from about 30 to
70 percent open area. "Open area" is known in the art as a measure
of total mesh area that is open space. Mesh counts (the number of
openings in a lineal inch) for such materials vary from two per
inch by two per inch (2.times.2) to 635.times.635. The mesh may be
woven from wires comprising aluminum, brass, bronze, copper,
nickel, stainless steel, titanium, etc., and combinations and
alloys thereof. A non-woven wire mesh that can be used as an open
substrate in accordance with this invention may be made from the
same materials as woven mesh. A wire mesh substrate may comprise
one or more layers of wire mesh joined together by soldering,
welding or any other suitable method.
[0079] Any metal substrate used as a carrier monolith in the
practice of the present invention may optionally be pre-coated with
a binder layer of alumina or another refractory inorganic oxide
before the catalytic material is deposited thereon. As an
alternative to the binder layer, or optionally in addition thereto,
the metal monolith may be calcined in air to produce a surface
layer of metal oxide before the catalytic material is deposited
thereon. Employing a binder layer and pre-calcining the metal
substrate both help to improve the adherence of the catalytic
material to the monolith.
[0080] The use of catalyst members that comprise catalytic material
deposited upon carrier monoliths with such high cell or pore
densities as described above allows the use of smaller catalyst
members or beds than was previously practicable. In other words,
less physical space or volume is needed for a high cell or high
pore density catalyst member than was needed for a granular or
tableted catalyst bed that achieves the same degree of catalytic
activity.
[0081] Methods for applying a catalytic washcoat onto carrier
substrates, including both open substrates and dense substrates,
are well-known in the art.
[0082] When catalytic species are deposited onto a carrier,
especially onto an open substrate, the amounts of the catalytic
species and other components of the catalytic material are often
presented based on grams per volume basis, e.g., grams per cubic
foot (g/ft.sup.3) for platinum group metal components and grams per
cubic inch (g/in.sup.3) for support material and for the catalytic
material as a whole, as these measures accommodate different
gas-flow passage configurations in different carriers. In typical
embodiments, the loading of catalytic material on a flow-through
open carrier substrate for use according to the present invention
may be in the range of from about 1 to 3 g/in.sup.3 of the
catalytic material with the platinum and iron components therein
according to their relative weight percents as described above. The
finished catalyst member may be mounted in a metallic canister that
defines a gas inlet and a gas outlet and that facilitates flow of
the feedstream into contact with the catalyst.
[0083] In contrast to an open substrate, a dense substrate (or low
surface area substrate), such as a plate, tube, foil and the like,
on which the catalytic material of the invention may be deposited
as well, has a relatively small surface area per overall volume of
the flow path through the substrate regardless of whether it is
perforated or not, and do not substantially disrupt laminar flow
therethrough.
[0084] Another broad aspect of this invention pertains to the
preparation of a carrier for catalytic material by the thermal
spraying of a metal anchor layer onto any type of substrate.
Catalytic material may then be deposited on the carrier. In
particular, this broad aspect of the present invention pertains to
thermal spraying processes in general, including plasma spraying,
single wire plasma spraying, high velocity oxy-fuel spraying,
combustion wire and/or powder spraying, electric arc spraying,
etc., which have not previously been utilized for depositing anchor
layers onto open substrates. One reason that thermal spraying has
not been used in open substrates is the belief that to obtain good
results it is necessary that substantially all of the surface area
of a substrate to be sprayed had to be accessible in a line of
"sight" from the spray head and that open substrates have so much
surface area that is not accessible in this way, i.e., that open
substrates have such a high degree of surface area that is obscured
relative to a line of sight from a spray head, that satisfactory
spraying could not be achieved. The present invention reveals,
however, that open substrates can in fact be satisfactorily coated
using thermal spray methods.
[0085] Still another aspect of the present invention arises from a
discovery that electric arc spraying, e.g., twin wire arc spraying,
of a metal (which term, as used herein and in the claims, includes
mixtures of metals, including without limitation, metal alloys,
pseudoalloys, and other intermetallic combinations) onto a metal or
ceramic substrate yields a structure having unexpectedly superior
utility as a carrier for catalytic materials in the field of
catalyst members, regardless of whether the substrate is an open
substrate or a dense substrate. Twin wire arc spraying (encompassed
herein by the term "wire arc spraying" and by the broader term
"electric arc spraying") is a known process, as indicated by the
above reference to U.S. Pat. No. 4,027,367 which is incorporated
herein by reference. Briefly described, in the twin wire arc spray
process, two feedstock wires act as two consumable electrodes.
These wires are insulated from each other as they are fed to the
spray nozzle of a spray gun in a fashion similar to wire flame
guns. The wires meet in the center of a gas stream generated in the
nozzle. An electric arc is initiated between the wires, and the
current flowing through the wires causes their tips to melt. A
compressed atomizing gas, usually air, is directed through the
nozzle and across the arc zone, shearing off the molten droplets to
form a spray that is propelled onto the substrate. Only metal wire
feedstock can be used in an arc spray system because the feedstock
must be conductive. The high particle temperatures created by the
spray gun produce minute weld zones at the impact point on a
metallic substrate. As a result, such electric arc spray coatings
(sometimes referred to herein as "anchor layers") have good
cohesive strength and a very good adhesive bond to the
substrate.
[0086] The principal operating parameters in wire arc spraying
include the voltage and amperage for the arc, the compression of
the atomizing gas, the nozzle configuration and the stand-off from
the substrate. The voltage is generally in the range of from 18 to
40 volts, and is typically in the range of from 28 to 32 volts; the
current may be in the range of from about 100 to 400 amps. The
atomizing gas may be compressed to a pressure in the range of from
about 30 to 70 psi. The nozzle configuration (e.g., slot aperture
or cross aperture) and spray pattern vary in accordance with the
desired nature of the anchor layer or may be chosen to accommodate
the other parameters or the character of the substrate. A suitable
stand-off is generally in the range of from about 4 to 10 inches
from the substrate to the nozzle. Another operating parameter is
the spray rate for the feedstock, a typical example of which would
be 100 pounds per hour per 100 amps (4.5 kg/hr/100 amps). Still
another parameter is the coverage or feedstock consumption rate,
which may be, to give a particular example, 0.9 ounce per square
foot per 0.001 inch thickness of the anchor layer. (It is typical
to have a deposition efficiency of 70 percent (e.g., for spraying a
plate) or less.)
[0087] Electric arc spray coatings are usually harder to finish
(e.g., to grind down) and normally have higher spray rates than
coatings of other thermal spray processes. Dissimilar electrode
wires can be used to create an anchor layer containing a mixture of
two or more different metal materials, referred to as a
"pseudoalloy". Optionally, reactive gases can be used to atomize
the molten feedstock to effect changes in the composition or
properties of the applied anchor layer. On the other hand, it may
be advantageous to employ an inert gas or at least a gas that does
not contain oxygen or another oxidizing species. Oxygen, for
example, may cause oxidation on the surface of a metal substrate or
in the feedstock material and thus weaken the bond between the
anchor layer and the substrate.
[0088] Anchor layers of a variety of compositions can be deposited
on a substrate in accordance with the present invention by
utilizing, without limitation, feedstocks of the following metals
and metal mixtures: Ni, Ni/Al, Ni/Cr, Ni/Cr/Al/Y, Co/Cr,
Co/Cr/Al/Y, Co/Ni/Cr/Al/Y, Fe/Al, Fe/Cr, Fe/Cr/Al, Fe/Cr/Al/Y,
Fe/Ni/Al, Fe/Ni/Cr, 300 and 400 series stainless steels, and,
optionally, mixtures of two or more thereof. One specific example
of a metal useful for wire arc spraying onto a substrate in
accordance with the present invention is a nickel/aluminum alloy
that generally contains at least about 90% nickel and from about 3%
to 10% aluminum, preferably from about 4% to 6% aluminum by weight.
Such an alloy may contain minor proportions of other metals
referred to herein as "impurities" totaling not more than about 2%
of the alloy. A preferred specific feedstock alloy comprises about
95% nickel and 5% aluminum and may have a melting point of about
2642.degree. F. Some such impurities may be included in the alloy
for various purposes, e.g., as processing aids to facilitate the
wire arc spraying process or the formation of the anchor layer, or
to provide the anchor layer with favorable properties.
[0089] One aspect of the present invention derives from the
discovery that electric arc spraying a metal onto a metal substrate
yields an unexpectedly superior carrier for catalytic materials
relative to carriers having metal anchor layers applied thereto by
other methods. Catalytic materials have been seen to adhere better
to a carrier comprising an electric arc sprayed anchor layer than
to a carrier comprising a substrate without an intermediate layer
applied thereto and even better than to a carrier comprising a
substrate having a metal layer deposited thereon by plasma
spraying. Before the present invention, catalytic materials
disposed on metal substrates, with or without intermediate layers
between the substrate and the catalytic material, often did not
adhere sufficiently well to the substrate to provide a commercially
acceptable product. For example, a metal substrate having a metal
intermediate layer that was plasma-sprayed thereon and having a
catalytic material applied to the intermediate layer failed to
retain the catalytic material, which flaked off upon routine
handling, apparently due to a failure of the intermediate layer to
bond with the substrate. The catalytic material on other carriers
was seen to spall off upon normal use as automotive environmental
catalysts, apparently as a result of being subjected to a high gas
flow rate, to thermal cycling, to the eroding contact of high
temperature steam and other components of the exhaust gas stream,
vibrations, etc. The present invention therefore improves the
durability of catalyst members comprising catalytic materials
carried on carrier substrates by improving their durability. It
also permits the use of such catalyst members in positions upstream
from sensitive equipment that would be damaged by catalytic
material and/or anchor layer material that spall off prior art
catalyst members.
[0090] Surprisingly, the Applicants have discovered that electric
arc spraying (of which wire arc spraying is a particular
embodiment) of a metal onto a metal substrate results in a superior
bond between the resulting anchor layer and the substrate relative
to plasma spraying. An electric arc sprayed anchor layer is
believed to have at least two characteristics that distinguish it
from anchor layers applied by plasma spraying: a superior anchor
layer-metallic substrate interface bond and a highly irregular or
"rough" surface. It is believed that the anchor layer-metallic
substrate interface bond may be the result of diffusion between the
sprayed material and the metallic substrate that is achieved at
their interface despite the relatively low temperature at which
wire arc spraying is practiced. For example, the electric arc
temperature may be not more than 10,000.degree. F. In such case,
the temperature of the molten feedstock is expected to be at a
temperature of not more than about 5000.degree. F., preferably in
the range of 1000.degree. to 4000.degree. F., more preferably not
more than about 2000.degree. F. The low temperature is also
believed to be responsible for the especially uneven surface of the
anchor layer because the sprayed material cools on the substrate
(whether metal or ceramic) to its freezing temperature so quickly
that it does not flow significantly on the substrate surface and
therefore does not smooth out. Instead, it freezes into an
irregular surface configuration. Accordingly, the surface of the
anchor layer has a rough profile that provides a superior physical
anchor for catalytic components and materials disposed thereon. The
rough profile appears to be the result of "pillaring", the
formation of small, pillar-like structures resulting from the
sequential deposition and freezing of one molten drop of feedstock
material atop another.
[0091] An electric arc spray process can be used to produce an
anchor layer on a variety of substrates that may vary by their
composition and/or by their physical configuration. For example,
the substrate may be an open substrate or a dense substrate; it may
be in the form of a metal plate, tube, foil, wire, wire mesh, rigid
or malleable foamed metal, etc., ceramic structures, or a
combination of two or more thereof. It does not appear to be
important to match the sprayed metal to the metal of the
substrate.
[0092] To illustrate the dramatic difference in the surface of an
anchor layer applied in accordance with the present invention as
compared to the surface of a metal substrate without the anchor
layer, reference is made herein to FIGS. 1A through 1D and, for
comparison thereto, FIGS. 2A through 2D. FIGS. 1A through 1D are
photomicrographs of a foamed metal substrate, taken at a variety of
magnification levels. These Figures show that the substrate has a
three-dimensional web-like structure having smooth surfaces. By
comparison, FIGS. 2A through 2D are photomicrographs of a foamed
metal substrate taken at corresponding magnification levels after
an anchor layer has been electric arc sprayed thereon. A visual
comparison of FIGS. 1A through 1D and the corresponding FIGS. 2A
through 2D illustrates the roughened surface that results from
electric arc spraying an anchor layer onto a substrate as taught
herein. FIGS. 2E, 2F and 2G show sections of a high temperature
steel plate substrate 100 and a nickel aluminide anchor layer 110
electric arc sprayed thereon, at magnifications of 500.times.,1.51
k.times. and 2.98 k.times., respectively. As is evident from these
Figures, the anchor layer 110 provides a highly irregular surface
on the substrate 100. Accordingly, the anchor layer 110 effectively
increases the surface area on which catalytic material may be
deposited on the carrier relative to a non-sprayed substrate and it
provides structural features such as crevices, nooks, etc., that
help prevent spalling of catalytic material from the anchor layer.
FIGS. 2E through 2G illustrate that the relatively low temperature
of the electric arc spray process deposits the metal feedstock for
the anchor layer on the substrate at a temperature that permits the
feedstock to freeze when it impinges upon the substrate rather than
remaining molten and flowing into a smoother configuration.
[0093] In another example of the practice of the present invention,
a perforated stainless steel tube substrate as shown in FIG. 2H was
electric arc sprayed with a nickel aluminide feedstock to deposit
an anchor layer thereon; a catalytic material can then be deposited
on the anchor layer. A sample of a resulting catalyst member is
shown in FIG. 21. The anchor layer will provide superior adhesion
of a catalytic material to the carrier when it is used to prepare a
catalyst member in accordance with the present invention. In an
alternative embodiment, a non-perforated tubular substrate may be
wire arc sprayed and coated with catalytic material. Such tubular
catalyst members may be assembled into a heat exchange device for
catalytically treating and exchanging heat with a gas stream.
Optionally, a flow-through catalyst member may be mounted within
the tubular catalyst member.
[0094] The strong bond of an anchor layer achieved by electric arc
spraying permits the resulting substrates to be mechanically
processed in various ways that reshape the substrate but that do
not diminish the mass of the substrate, i.e., they do not involve
cutting, grinding or other removal of substrate material. For
example, pliable (i.e., malleable and/or flexible) anchor
layer-coated substrates may be bent, compressed, folded, rolled,
woven, etc., after the anchor layer is deposited thereon, in
addition to or instead of being cut, ground, etc. As used herein
and in the claims, the term "reshape" is meant to encompass all
such processes that deform the substrate but do not reduce its mass
by cutting, grinding, etc. Thus, a wire arc-sprayed foil substrate
can be reshaped by being corrugated and rolled with a flat foil to
provide a corrugated foil honeycomb. A wire can be reshaped by
being sprayed and then woven with other wires to compose a mesh
that is used as a carrier for a catalytic material. Similarly, a
flat wire mesh substrate that has been wire arc sprayed in
accordance with this invention can then be reshaped by being curled
into a cylindrical configuration or by being formed into a
corrugated sheet that may optionally be combined with other
substrates to compose a carrier, or that may be used on its own. A
plural-layer wire mesh substrate with an anchor layer thereon that
can optionally be reshaped in these ways is shown in FIG. 2J.
Likewise, foamed metal having an anchor layer thereon may be
reshaped by being compressed to change its shape and/or density as
discussed herein. Such reshaping may occur before or even after
catalytic material is deposited on the foamed metal substrate. The
present invention permits the manufacture of carriers and/or
catalyst members that can easily be molded to fit within a
container for the catalyst member, e.g., in a canister specifically
designed to house a catalyst member, or in another portion of the
apparatus, e.g., in a gas stream flow pipe, a high mass transfer
area conduit, etc. For example, a flat, catalyzed wire mesh patch
prepared in accordance with the spraying and coating methods
described herein may be reshaped for installation in a pipe by
being rolled into a coiled configuration. Optionally, the substrate
may be resilient and may, upon insertion into a containing
structure, be allowed to unwind or otherwise relax from the
reshaping force to the extent that it bears against the interior
surface of the containing structure, thus conforming to the
structure.
[0095] One example of a substrate that has been reshaped after
having an anchor layer deposited thereon is seen in FIG. 3A, which
shows a metal substrate 100 that has been wire arc sprayed to
deposit an anchor layer 110' thereon. The sprayed substrate 111 may
then be corrugated and placed against a second, optionally sprayed
substrate 105, as shown in FIG. 3B. The two substrates may be
further processed by coiling them together as shown FIG. 3C to
compose a carrier 109 for catalytic material to be deposited
thereon. A process for producing a catalyst member from such a
carrier is shown in schematically in FIG. 3D, beginning with a flat
metal foil substrate 100 which is passed through a corrugation
station 210 to produce a corrugated foil substrate 100a. The
corrugated substrate 100a is passed through an electric arc
spraying station 212 comprising two electric arc spraying
apparatuses 212a, 212b, one for spraying each side of substrate
100a. Each apparatus comprises a pair of electrified feedstock
wires 212d and 212e which may comprise a nickel aluminide alloy or
other metal, and a spray gun 212c for atomizing the molten metal
formed by the electric charge passing between the electrode wires.
The spray gun sprays the molten metal feedstock onto the substrate.
Separately, a flat substrate 100' has an anchor layer electric arc
sprayed on both sides thereof in station 212'. The corrugated,
electric arc sprayed substrate 111 is disposed upon the flat
electric arc sprayed substrate 105 in step 214, and the two
substrates are wound (reshaped) and then secured together in step
216 to produce a metallic honeycomb carrier in a manner generally
known in the art. At coating station 218, the carrier 216a is
dipped in a bath 218a comprising a slurry of catalytic material. In
step 220, an air knife 220a is used to blow excess catalytic
material from the carrier. In a fixing step 222, the coated carrier
is placed in an oven 222a where it is dried and optionally but
preferably calcined preferably at temperatures in the range of
200.degree. C. to 300.degree. C. and not higher, to remove the
liquid portion of the slurry and to bind the catalytic material
onto the carrier, thus producing a catalyst member comprising
catalytic material deposited upon an electric arc sprayed carrier
substrate. The catalyst member may be incorporated into a fuel cell
system by being mounted in a body or canister for placement in the
fuel cell feed stream.
[0096] Metallic honeycomb carriers may be made according to a
method that makes use of a corrugated foil strip having opposite
side edges and corrugations oriented at an oblique angle to the
side edges. The foil strip is folded on lines perpendicular to the
side edges to provide a core body having fluid passages between
opposite ends and a shaped periphery defined by parallel outside
folds in the corrugated strip. The core body thus formed is
inserted into a jacket tube so that folds at the core body
periphery are in compressive contact with the jacket tube, and the
periphery of the core body is joined to the jacket tube. The method
for producing such carriers and the carrier resulting therefrom are
described in detail in U.S. patent application Ser. No. 08/728,641
filed Oct. 10, 1996, the disclosure of which is incorporated herein
by reference. Briefly restated, a preferred honeycomb carrier core
body may be formed from a corrugated foil strip in which
corrugations are oriented at an oblique angle to side edges of the
strip. An embodiment of such a foil strip is shown as a substrate
in FIGS. 3E and 3F of the drawings and generally designated by the
reference numeral 110.
[0097] The illustrated strip 110 is initially of an undefined or
continuous length and has opposite side edges 112 and 114 to
establish a strip width W which may be between 1 and 9 inches,
depending on the size of the core body to be formed. The strip 110
is "skew corrugated", that is, the corrugations 116 extend on
linear paths between the side edges 112 and 114, and are inclined
at an oblique angle .beta. with respect to the side edges. Ideally,
the angle .beta. is the same for all corrugations and is preferably
in a range of from 4.degree. to 15.degree.. In practice, the
oblique angle of individual corrugations may vary relative to
others of the corrugations, although the angles .beta. for all
corrugations will fall within the preferred range.
[0098] In FIG. 3F, the side profile of the foil strip 110 is shown
at an enlarged scale to reveal an exemplary shape and relative
dimensions of the corrugations 116. As shown, each corrugation 116
has a height h and pitch length l. The thickness of the foil
material from which the strip 110 is formed is designated by t.
[0099] In some applications, corrugations preferably have a height
h of from about 0.01 inch to about 0.15 inch, and a pitch length 1
of from about 0.02 inch to about 0.25 inch. The height and pitch
length of the corrugations determine cell density, that is, the
number of cells per unit of cross-sectional area in the converter
body, in accordance with equation (1):
c=cos .beta./h 1 (1)
[0100] Typically, the cell density c is expressed in cells per
square inch (cpsi) and, in some applications, may vary from about
50 cpsi to 1000 cpsi.
[0101] The foil strip 110 may be constructed from "ferritic"
stainless steel such as that described in U.S. Pat. No. 4,414,023
to Aggen. One usable ferritic stainless steel alloy contains 20%
chromium, 5% aluminum, and from 0.002% to 0.05% of at least one
rare earth metal selected from cerium, lanthanum, neodymium,
yttrium, and praseodymium, or a mixture of two or more of such rare
earth metals, balance iron and trace steel making impurities. A
ferritic stainless steel is commercially available from Allegheny
Ludlum Steel Co. under the trade designation "ALFA-IV.RTM.".
[0102] Another usable commercially available stainless steel metal
alloy is identified as Haynes 214 alloy. This alloy and other
useful nickeliferous alloys are described in U.S. Pat. No.
4,671,931 to Herchenroeder et al. These alloys are characterized by
high resistance to oxidation and high temperatures. A specific
example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron,
optionally trace amounts of one or more rare earth metals except
yttrium, 0.05% carbon, and steel making impurities. Still another
suitable alloy is Haynes 230 alloy, which contains 22% chromium,
14% tungsten, 2% molybdenum, 0.10% carbon, a trace amount of
lanthanum, balance nickel.
[0103] The ferritic stainless steels and the Haynes alloys 214 and
230, all of which are considered to be stainless steels, are
examples of high temperature resistive, oxidation resistant (or
corrosion resistant) metal alloys that are useful for use in making
the foil strip and core body sheet elements of the present
invention, as well as the multicellular honeycomb converter bodies
thereof. Suitable metal alloys must be able to withstand "high"
temperature, e.g., from 900.degree. C. to 1200.degree. C.
(1652.degree. F. to 2012.degree. F.) over prolonged periods.
[0104] Other high temperature resistive, oxidation resistant metal
alloys are known and may be used as well. For most applications,
and particularly automotive applications, these alloys are used as
"thin" metal or foil, that is, having a thickness of from about
0.001 inch to about 0.005 inch, and, preferably, from 0.0015 inch
to about 0.0037 inch.
[0105] In accordance with this aspect of the present invention, the
skew corrugated foil strip is folded on lines perpendicular to the
side edges thereof to provide a core body with a shaped periphery
defined principally by parallel outside folds in the corrugated
strip. In particular, the foil strip is reverse-folded in accordion
fashion on fold lines spaced at intervals selected to generate the
desired peripheral shape of the core body. The overlying adjacent
segments of the strip between the folds provide fluid passages
between the ends of the core body.
[0106] In FIG. 3E, exemplary parallel fold lines are designated by
the reference numerals 118, 119, 120, 121 and 122. These fold lines
are also shown to be spaced at increasing intervals, from right to
left in FIG. 3E, to generate part of a core body 125 having a
circular periphery as shown in FIG. 3G. Although the spacing of
fold lines in FIG. 3E is not precise and representative only, given
the height h of corrugations in the strip 110, folding that strip
to generate the circular periphery shown in FIG. 3G is easily
accomplished using known algorithms and computer controlled folding
apparatus, for example. As a result of the folding operation,
adjacent chord-like segments of the strip 110 extend between pairs
of outside folds 128 located at the core periphery 126. Also, the
corrugations 116 of adjacent strip segments cross each other in
non-nesting relation to provide a network of fluid passages between
the ends of the core body 125.
[0107] The crossings of corrugations establish contact points
between adjacent strip segments, and serve to provide support for
the individual foil segments or layers in a direction perpendicular
to the chords on which they lie. The number of contact points
between each strip segment or layer in the core body 125,
therefore, represents a parameter contributing to strength and
durability of the core body 125 in the completed honeycomb carrier
body in which it is used. It is preferred that each corrugation in
one strip segment or layer cross with corrugations in an adjacent
layer at least 6 contact points, more preferably, 8 contact points.
The number of contact points Np is dependent on the width W of the
strip 110, the angle .beta. of the skewed corrugations, and the
pitch length l of the corrugations in equation (2):
Np=2W sin.beta./I (2)
[0108] After the core body is folded and assembled to the
configuration shown in FIG. 3G, for example, it is temporarily
secured such as by tying a string or placing a rubber band or other
ligature about the periphery thereof. The periphery 126 of the core
body 125, particularly the outside folds 128, are cleaned to reveal
a clean metallic surface at each of the outside folds 128. All
coating materials applied to the strip 110 are removed by the
cleaning from at least the outside folds 128. The cleaning may be
accomplished, for example, by grit blasting the surfaces on the
periphery of the core body 125, using aluminum oxide particles in a
high velocity stream of compressed air. Silicon carbide grit also
may be used. Other cleaning methods may be used to remove coating
and other foreign materials from the periphery of the folded core
body 125. For example, the periphery of the core body may be
scraped or abraded with an assortment of well-known tools, such as
files, abrasive stones, wire wheels and the like. Also, it is
within the scope of the invention to provide a clean metal surface
at the folds by masking the fold lines prior to coating.
[0109] After assembly and cleaning as described, the folded core
body is inserted into a jacket tube so that folds at the core body
periphery are in contact, preferably under compression, with the
interior of the jacket tube, and the periphery of the core body is
joined to the jacket tube.
[0110] In the illustrated embodiment and as depicted in FIG. 3H,
the core body 125 is inserted axially into a jacket tube 130 of
cylindrical configuration to complement the exterior shape of the
core body 125. The jacket tube 130 has an interior surface 132 and
is formed preferably of stainless steel having a thickness of from
about 0.03 inch to about 0.08 inch, preferably 0.04 inch to 0.06
inch. Prior to insertion, the interior surface 132 of the jacket
tube 130 is coated with a brazing alloy such as AMDRY Alloy 770,
0.002 inch in thickness. Alternatively, and as illustrated in FIG.
3H, the core body 125 may be wrapped in a brazing foil 134 as a way
of providing a layer of brazing alloy between the outer periphery
126 of the core body 125 and the interior surface 132 of the jacket
tube 130.
[0111] It is important that a sufficient number of the outside
folds 128 at the periphery 126 of the core body 125 be in contact
with the interior surface 132 of the jacket tube 130 to ensure a
secure joining of the folds 128 to the interior surface 132 of the
jacket. Such contact is preferably achieved by compressing the core
body 125 to reduce its diameter approximately one to three percent.
The reason for this compression and accompanied reduction in
diameter of the core body 125 may be appreciated from the
illustrations in FIGS. 3I and 3J of the drawings.
[0112] As shown in FIG. 31, adjacent layers or segments of the
corrugated strip, designated 110a and 110b, are joined at the
intended periphery 126 by fold lines 128a, 128b and 128c. Because
of imperfections in the folding of the foil strip 110 with
presently known folding equipment, it is not possible for the folds
128 to lie precisely on the intended periphery 126 of the core body
125. Thus, and as shown in FIG. 3I, the fold 128a lies outside of
the intended periphery 126, the fold 128b lies outside the intended
periphery 126 and the fold 128c lies within the intended periphery
126. As shown in FIG. 3J, after the core body 125 is inserted into
the jacket 130 and is placed under compression against the inner
surface 133 on which the brazing alloy 134 is located, the folds
128a and 128b are compressed to be strained or deformed inwardly so
that all three folds firmly contact the brazing alloy 134. It
should be understood that illustration in FIGS. 3I and 3J is for
purposes of explanation only and that in practice, the respective
folds 128 at the periphery of the core body, as folded, will
deviate randomly from the intended periphery or that which
complements the inner surface 132 of the jacket tube 130.
[0113] A preferred way of inserting the core body 125 into the
jacket tube 130 is depicted in FIG. 3K. As shown, the jacket tube
130 is mounted on a pedestal 136 and fitted at its upper end with
an annular tapered die 138 having a frustoconical inner surface 139
which converges downwardly to an inside diameter equal to the
inside diameter of the jacket tube 130. A ram 140 is used to force
the core body 125 through the tapered inner surface 139 of the die
138 so that as the core body enters the jacket tube 125 it is
compressed to reduce the diameter of the core body periphery 126 by
the approximately one to three percent indicated above.
[0114] From the illustration in FIG. 3K, it will be understood that
the exterior periphery of the core body 125 is swaged upon
insertion into the jacket tube 130 and thereafter retained in
compressive contact with the interior surface 132 of the jacket
tube. Alternatively, the core body 125 may be inserted into the
jacket tube 130 without compression on insertion as depicted in
FIG. 3F. The periphery of the jacket tube 130 is then reduced by
swaging the exterior of the jacket tube using a die 138a as shown
in FIG. 3L. After reducing the peripheral diameter of the jacket
tube 130 in this manner, the core body is placed under compressive
contact with the jacket tube 130.
[0115] A still further alternative to attainment of a compressive
loading of the core body periphery 126 against the interior of the
jacket tube 130 is to insert the core body 125 into the jacket tube
while it is expanded and before it is closed by welding or brazing.
This embodiment is illustrated in FIGS. 3M and 3N of the drawings.
After insertion of the core body 125 with the jacket tube 130a
opened as shown in FIG. 3M, the open jacket tube is then compressed
radially against the core body 125 to be closed along its length.
The previously open slit is then joined by brazing or welding to
secure the compressed core body 125.
[0116] The compressive loading of the core body periphery and the
inner surface of the jacket tube against each other after the core
body is inserted into the jacket tube, as described with reference
to FIGS. 3L, 3M and 3N, offers a facility for mechanically joining
the periphery 126 of the core body 125 to the interior of the
jacket tube 130. For example, the inside surface 132 of the jacket
tube 130 may be roughened by various forms and shapes of surface
irregularities, such as peripheral striations, threads, barbs,
relieved coating materials, and the like, so that when the jacket
tube is compressed against the inserted core, a mechanical
retention of the core body 125 within the jacket tube 130 is
effected. Such a mechanical retention may be combined with a bond
typified by brazing and, in some instances, may be used as a
substitute for brazing. Thus, the term "join" is used herein to
characterize the connection of the core body periphery to the
jacket tube and is intended to encompass mechanical and bonding
connections, as well as a combination of both.
[0117] To braze the folds 128 at the periphery of the core body to
the inside surface 132 of the jacket tube 130 the compressed
assembly of the core body and jacket tube preferably is put in a
chamber. Air is evacuated and the chamber is backfilled with a
non-oxidizing gas, preferably an inert gas such as argon. Also, a
vacuum can be used without a gas backfill, as long as the remaining
atmosphere is non-oxidizing. Also in the chamber is an induction
coil which extends around the jacket tube with about an eighth to a
quarter inch clearance between the coil and the jacket tube. When
the induction coil is energized, it heats the jacket tube and the
outer folds of the foil by induction with a very localized heating
effect, melting the brazing metal between the periphery of the core
body and the jacket tube. The outside folds of the core body do not
have the coating on them so they braze nicely to the interior
surface of the jacket tube.
[0118] The aforementioned method provides a honeycomb carrier body
having a metal jacket, a core body having a length between opposite
ends and a periphery defined by folds in a corrugated strip, the
interior surface of the jacket engaging the periphery of the core
body to be in contact with all folds at the periphery, and a bond
between the periphery of the core body and the interior surface of
the jacket.
[0119] In an embodiment illustrated in FIGS. 3P and 3Q, a core body
125 of circular cross section is secured under compression within a
jacket tube 130 and also by a bond 134, preferably of brazing
material, between the outer periphery of the core body 125 and the
interior of the jacket tube 130. As shown in FIG. 3Q, the jacket
tube is of a length slightly larger than that of the core body 125
so that the ends of the core body are recessed into the ends of the
jacket tube 130.
[0120] Because of the facility offered by the method of forming the
core by selected fold spacing intervals along a continuous
corrugated strip, configurations other than the circular
cylindrical configuration shown in FIGS. 3P and 3Q can be attained.
Thus, in FIG. 3R, a polygonal, more particularly, a hexagonal, end
configuration of a core body 125a is illustrated. In FIG. 3S, an
elliptical end profile of core body 125b is shown in which the
layers of corrugated foil extend across the minor axis of the
ellipse. A variation of the elliptical end profile shown in FIG. 3S
is illustrated in FIG. 3T. Thus, in FIG. 3T, the end profile of the
core body 125c is oblong or "racetrack" shaped. Finally, in FIG.
3U, a core body 125d is illustrated as having a rectangular end
profile. In each of the embodiments illustrated in FIGS. 3P-3U, the
exterior configuration of the honeycomb carrier body is an erect
parallelepiped, that is, the peripheral surfaces of the core body
are generated by straight lines parallel with each other and also
parallel with the central axis of the core body.
[0121] An anchor layer deposited on a substrate as taught herein
can provide some rigidity to an excessively ductile or malleable
metal substrate, it can provide a roughened surface on which a
catalytic material may be deposited, and it can seal the surface of
a metal substrate and thus protect the substrate against surface
oxidation during use. As mentioned above, the ability to
tenaciously adhere a catalytic material to a metal substrate as
provided herein may also permit structural modification of a
catalyst member as required to conform to the physical constraints
imposed by canisters or other features of the exhaust gas treatment
apparatus in which the catalyst member is mounted, without
significant loss of catalytic material therefrom.
[0122] As mentioned above, various deposition methods for
depositing catalytic species onto a carrier substrate are known in
the art. These include, for example, disposing the catalytic
material in a liquid vehicle to form a slurry and wetting the
carrier substrate with the slurry by dipping the carrier into the
slurry (as mentioned elsewhere herein, e.g., with reference to FIG.
3D), spraying the slurry onto the carrier, etc. Alternatively, the
catalytic species may be dissolved in a solvent and the solvent may
then be wetted onto the surface of the carrier substrate and
thereafter removed to leave the catalytic species, or a precursor
thereof, on the carrier substrate. Optionally, the carrier
substrate may comprise a support material either as a pre-coat
layer thereon or because the carrier is formed from support
material. The procedure for removing the liquid or solvent may
entail heating the wetted carrier subject to the temperature
limitations set forth above. Each such method of applying the
catalytic species onto the carrier constitutes a separate step in
the manufacturing process relative to the thermal spray application
of the anchor layer, and their use therefore provides a distinction
to the teaching of U.S. Pat. No. 5,204,302 (discussed above) in
which the same plasma spray process for applying an undercoat is
used to apply the catalyst. This invention can therefore optionally
be described as electric arc spraying an anchor layer on a
substrate, discontinuing the spraying of that substrate and then
depositing a catalytic material thereon. Other methods for
depositing catalytic species onto a carrier member are known and
may be used as well, including chemical vapor deposition.
[0123] The wire arc spraying technique of the present invention can
be used to apply an anchor layer to the smooth interior surfaces of
the gas-flow passages formed in a honeycomb-type ceramic carrier,
as well as on the front face thereof, to provide a superior surface
on which to deposit catalytic material and to increase the
turbulence of the gas flowing through the catalyst member and thus
increase the catalytic activity. In addition, the anchor layer may
be deposited on the smooth exterior surface of the substrate to
facilitate mounting the substrate in a canister, as described
herein. Other flow-through-type carriers are known as well, e.g.,
porous foamed metal, wire mesh, etc., in which cases the gas-flow
passages may be non-linear, irregular or reticulated. In many such
embodiments, the inlet and outlet faces of the carrier are defined
simply as the surfaces through which the fluid enters or leaves the
carrier, respectively. A flow-through catalyst member is typically
mounted in a body such as a canister to guide fluid flow through
the carrier.
[0124] As stated above, a catalyst member according to the present
invention may be formed from any one or more of the metallic
substrates described above, e.g., corrugated, rolled sheet metal,
metal foil, wire mesh, foamed metal, etc. In one particular
embodiment illustrated in FIG. 4, catalyst member 14' comprises a
catalytic material deposited on a rolled foamed metal substrate
that has the optional nickel-aluminide anchor layer applied thereto
as described above. Catalyst member 14' is mounted in a canister 15
(FIG. 5) to provide a catalytic device. Canister 15 guides exhaust
gas first into an inlet face 14a' of catalyst member 14', then
through the catalyst member 14' and into contact with the catalytic
material thereon, and out the outlet face 14b' and then out the
outlet 15b of the canister, as indicated by the arrows.
[0125] As mentioned above, this invention is not limited to the use
of powdered, i.e., particulate, support materials or powdered or
particulate catalytic materials. The platinum and iron catalytic
species may be dispersed onto various forms of support materials
other than particulate support materials, including, for example,
pelletized material as described above for the SELECTOXO.TM.
catalyst, or directly onto a flow-through carrier monolith, e.g., a
monolith formed from alumina or another refractory material
mentioned above. Accordingly, a catalytic material prepared in
accordance with the method of the present invention can be prepared
by dispersing the platinum and iron catalytic species onto, e.g.,
pelletized support material such as pelletized alumina (i.e.,
alumina tablets). The wetted tablets may be comparable in platinum
and iron loading to the prior art SELECTOXO.TM. catalyst described
above, except that they are dried and calcined according to the
method aspect of this invention. Alternatively, the catalytic
species may be dispersed onto a monolith to produce a catalyst
member by various methods, e.g., by spraying a solution of
compounds of the catalytic species onto the monolith or by
immersing the monolith into the solution. (These techniques can
also be used to disperse the catalytic species onto particulate
support materials.) The wetted monolith is then calcined in
accordance with the present invention.
[0126] Catalytic material may also be coated onto tubular carrier
monoliths, e.g., onto aluminum or other metal tubing. One or more
such catalyst members can be assembled to form a heat exchanger
that is able to simultaneously oxidize carbon monoxide from the gas
stream and to exchange heat from the gas stream with another gas
stream. The tubes may have a cylindrical configuration, but not
necessarily so; other configurations for the tubes will work as
well, as is understood in the art. One embodiment of such a device
is shown schematically in FIG. 3V. Catalyst member 150 comprises a
plurality of tubular catalyst members 152 mounted in a housing 154.
The tubular catalyst members comprise calcined aluminum tubes
coated with catalytic material as described above. Tubes 152 extend
between and through panels 154a and 154b of housing 154. Housing
154 further comprises plenums 156a and 156b that cooperate with
panels 154a and 154b to define a gas flow path through the interior
of tubes 152. First inlet 158a and first outlet 158b therefore
facilitate gas flow through a first flow path in member 150.
Housing 154 also comprises panels such as panels 160a and 160b that
cooperate with panels 154a to enclose a space about the exterior of
tubes 152. The enclosed space is not open to the interior of tubes
152. Second inlet 162a and second outlet 162b in housing 154
therefore permit gas flow through a second flow path through member
150. The tubes are made of metal or another heat-conductive
material so that gas flowing through one flow path will exchange
heat with gas in the second flow path, as is well-understood in the
art.
[0127] In alternative embodiments, a catalyst member comprising a
metal foam monolith can be mounted in one flow path of a heat
exchanger or it may be joined to a heat sink. Heat exchange and
heat sink embodiments are particularly useful in view of the
temperature sensitivity of the catalytic activity as discussed
below in Example 4 because they provide a means of influencing the
thermal conditions under which the exothermic CO oxidation reaction
is catalyzed.
EXAMPLE 1
[0128] Six steel wire mesh substrates and a 100 cpsi metal
honeycomb were each wire arc-sprayed using nickel aluminide wire as
the anchor layer feedstock. The nickel aluminide wire had a
diameter of {fraction (1/16)} inch (1.59 millimeters (mm)). The
molten nickel aluminide alloy was sprayed at 11 lbs/hr with a gas
pressure of 70 psi to deposit an anchor layer on the substrates at
a stand-off of 6 inches. The spraying process on the 100 cpsi
monolith successfully deposited an anchor coat in the interior
gas-flow passages of the monolith.
[0129] One of the wire mesh substrates was subjected to temperature
cycles in air at from about 100.degree. C. to 1000.degree. C. for
15 hours. After the temperature cycling, the mesh was examined and
compared to a reference, and no difference between the surfaces of
the two samples was noticed. A second wire mesh substrate was
cycled for three hours from room temperature to about 930.degree.
C. by heating in the flame of a Bunsen burner for about 6 seconds
per cycle. Again, upon comparison to a reference, no difference in
the surface of the anchor layers was seen. Catalytic material was
applied to each of the samples and excellent adhesion was seen in
all cases.
EXAMPLE 2
[0130] A particulate catalytic material was prepared by dispersing
platinum and iron catalytic components onto a particulate support
material comprising alumina in a wet impregnation process generally
as described above, in which the platinum was dissolved in a
solution in the form of bivalent cations. Platinum was dispersed on
the support material at a loading of 5 weight percent. The iron was
dispersed on the support material at a loading of 0.3 weight
percent. After the support material was impregnated with the
solution containing the platinum and iron, the wetted support
material was dried and then calcined at a temperature that was
within but that did not exceed the range of between 200.degree. C.
and 300.degree. C. A slurry comprising equal weights of catalytic
material and deionized water was prepared and was ball milled for 5
hours to achieve an average particle size in the slurry of about
9.2 micrometers.
[0131] Cordierite honeycomb carrier members were prepared by
washing with a mixture of equal quantities of acetic acid and
deionized water, and were then dried at 150.degree. C. and coated
with catalytic material. The coated carrier members were dried at
150.degree. C. to produce finished catalyst members.
[0132] The catalytic material was observed to adhere more securely
to the foamed metal substrates with the nickel-aluminide anchor
layer than to the cordierite honeycomb carrier members.
[0133] Several other catalyst members were prepared using a
metallic parallel channel carrier monolith onto which an etch coat
or bottom coat of alumina was applied at a target loading of 0.5
g/in.sup.3 before applying the top coat comprising the platinum and
iron catalytic material. In each case, the etch coat was dried and
calcined onto the carrier substrate at 500.degree. C. but the
catalytic material was calcined at 200.degree. C. The top coat was
applied at loadings in the range of 1.67 to 1.82 g/in.sup.3.
[0134] Metallic foam substrates characterized variously as having
20 or 40 pores per linear inch (ppi), each with a nickel-aluminide
anchor layer thereon were coated with two layers of the platinum-
and iron-containing catalytic material described above for a total
dry weight gain ranging from 1.1 g/in.sup.3 to 2.22 g/in.sup.3. The
20 and 40 ppi catalyst members were subjected to testing in a
laboratory reactor that provided a simulated feed gas from a
gasoline autothermal reformer. The reactor provided three catalytic
zones separated by glass beads for temperature control. The
conditions simulated the second reactor (polishing) of a
two-reactor system in which the inlet to the catalyst comprised 390
parts per million carbon monoxide. The catalyst members were found
to be effective for the conversion of carbon monoxide to carbon
dioxide in the presence of excess oxygen. After a few hours of
testing the catalytic members became deactivated as a result of
flooding of the reactor beds. Some spalling of the catalytic
material from the first catalyst member was observed.
EXAMPLE 3
[0135] A particulate alumina support material was impregnated with
a platinum- and iron-containing solution in which the platinum was
in the form of a bivalent cation, as described above. The platinum
was dispersed onto the alumina at a loading of 5 weight percent
(dry basis) and the iron was dispersed on the alumina at a loading
of 0.3 weight percent. The impregnated material was ground to a
particle size of 40 to 60 micrometers and was rendered in a slurry
with similarly sized quartz particles at a catalyst:quartz ratio of
1:2. The influence of calcination temperature in air, an
oxygen-containing atmosphere, and under a nitrogen blanket, i.e.,
an inert atmosphere, was performed by calcining samples of the
wetted alumina material for two hours at temperatures ranging from
200.degree. C. to 500.degree. C. The resulting catalytic materials
were then tested to evaluate their relative performance for the
selective conversion of carbon monoxide in a gas stream containing
hydrogen and oxygen. The catalytic materials were tested in
quantities of 1.6 grams and a density of 0.72 g/cm.sup.3. The test
gas feed stream was flowed through each material sample at
90.degree. C. at a volume hourly space velocity of 120,000 per
hour, for a total flow rate of 4 liters per minute. The test gas
contained an initial carbon monoxide concentration of 1,000 ppm,
20% H.sub.2, 10% H.sub.2O and sufficient oxygen to provide a
molecular O.sub.2 :CO ratio of 0.5:1 (i.e., stoichiometric
quantities). Carbon monoxide conversion was determined from
measurements of the amount of CO consumed and of CO.sub.2 generated
and was determined by an average of the data from those
measurements. The measured error of the carbon dioxide analyzer was
3 to 8 percent and the measured error of the carbon monoxide
analyzer was 1 to 3 percent. The measurement error for the oxygen
analyzer was 1 to 3 percent. Selectivity data regarding oxygen
consumption for conversion of carbon monoxide relative to oxidation
of hydrogen was calculated by using data from analyzers measuring
oxygen and carbon dioxide levels.
[0136] The results of the various measurements are depicted in FIG.
6. These results show that catalytic material prepared in a method
in which the calcination temperature is between 200.degree. C. and
300.degree. C. in an oxidizing atmosphere provides superior overall
performance for the oxidation of carbon monoxide and superior
selectivity for the conversion of carbon monoxide relative to the
conversion of hydrogen. In particular, calcination in air between
200.degree. C. and 300.degree. C. will yield a carbon monoxide
conversion rate of about 76 percent, which is higher than rates
obtained by materials calcined in air at temperatures below
200.degree. C. and above 300.degree. C., and higher than those
calcined at the same temperatures in an inert atmosphere.
Furthermore, the data show that catalytic material calcined in air
between 200.degree. C. and 300.degree. C. will generate a minimum
of unwanted oxidation of hydrogen, at a level of about 22 percent.
Materials calcined in an inert atmosphere are less selective than
materials calcined in air, i.e., they cause a significantly greater
rate of unwanted oxidation of hydrogen relative to the conversion
rate for carbon monoxide. The data thus show that unexpectedly
superior performance can be obtained from selective oxidation
catalytic materials calcined between 200.degree. C. and 300.degree.
C. in air.
EXAMPLE 4
[0137] A catalyst member was prepared in accordance with the
present invention by applying onto a cordierite honeycomb monolith
a catalytic material comprising 5 percent platinum and 0.3 percent
iron dispersed on alumina. The honeycomb monolith was configured to
have 400 cpsi and carried a catalytic washcoat loading of 1.44
g/in.sup.3. The resulting catalyst member was tested under a
variety of test conditions with a test gas containing 1,000 ppm CO,
20 percent H.sub.2 and 10 percent H.sub.2O.
[0138] Tests were run at various temperatures, volume hourly space
velocities and oxygen content. A summary of the pertinent test gas
compositions and testing conditions is set forth in the following
TABLE.
1TABLE I Test Gas Volume Hourly Figure in Which Test Temp.
O.sub.2:CO Space Velocity (VHSV) Results are Shown I 90.degree. C.
0.5:1 (varied) FIG. 7 II (varied) 0.5:1 80,000/hr FIG. 8 III
90.degree. C. (varied) 20,000/hr FIG. 9 IV 90.degree. C. (varied)
80,000/hr V 150.degree. C. (varied) 80,000/hr
[0139] The oxygen consumption rate and rate of carbon monoxide
conversion were measured in each test and the results are set forth
in the accompanying figures. FIG. 7 shows that CO oxidation
decreases with increasing volume hourly space velocity while FIG. 8
shows the CO oxidation increases with rising temperature. FIGS. 9,
10 and 11 show that CO oxidation increases as the 0.sub.2 :CO mole
ratio increases. FIG. 9 shows that adequate carbon monoxide
conversion was attained at low temperature (e.g., 90.degree. C.),
low space velocity (e.g., 20,000/hr), and at a greater than
stoichiometric O.sub.2 :CO ratio, i.e., around 0.8:1. FIG. 11 shows
that similar results were attained at 150.degree. C., 80,000/hr
VHSV and a O.sub.2 :CO ratio of at least 1:1. The Applicants have
surmised that the operating conditions that result in adequate
conversion of carbon monoxide by the catalyst described above can
be achieved under operating conditions described as (90+x).degree.
C., (20,000+1,000x)VHSV and (0.8+0.0033x):1 O.sub.2 :CO ratio,
where x is a non-negative number, e.g., it is equal to or greater
than zero. For example, it may vary from 0 to 60.
[0140] While the invention has been described in detail with
reference to particular embodiments thereof, it will be apparent
that upon a reading and understanding of the foregoing, numerous
alterations to the described embodiments will occur to those of
ordinary skill in the art and it is intended to include such
alterations within the scope of the appended claims.
* * * * *