Note: Descriptions are shown in the official language in which they were submitted.
l~9lA-CAN ~08V~01
BACKG~Nn OF TIIE I~VENTION
This invention relates to a catalytic structure
and a process ~or r~roducing such structure. More par-
ticularly, it relates to the production of a catalytically
active structure which is especially suited for high
temperature use in a gaseous stream. In a specific em-
bodiment, the present invention is directed to the
preparation of catalysts comprised of an alloy consisting
essentially of Cr-Ni-Cu in a configuration which is highly
effective for the purification of automobile exhaust and
industrial waste gases.
It is well known that catalysts may be used in
a variety of forms in a catalytic bed, and that the cata-
lyst structural design plays a marked role in determining .
catalyfit suitability for a particular process. The inter-
relationships of the catalyst composition, de~ign and
process conditions are highly complex and often difficult
to isolate from each other. In many processes using fixed
bed catalysts, where large volumes of gaseous or liquid
streams are passed through the reactor, it is advantageous
to have a high surface area of the catalyst exposed
relative to the amount of catalyst so as to facilitate
contact of the reactants at the surface. The availability
of the catalyst surface is particularly important in
certain gaseous reactions, such as the purification of
automobile enyine exhaust streams and nitric acid plant
tail gas streams, where the gas may pass through or
adjacent to the catalyst at a very high linear velocity.
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In such processes, conventional particulate catalysts,
e.g., pellets, chips, cylinders, spheres, and the like,
have the disadvantage that they contribute to back-
pressure build-up in the system. Also, the particulate
catalysts are more subject to attrition than a catalyst
in which the movement is more restricted. In such types
of reactions a structural form which will permit free
passage of the reacting gases relative to the surface
exposed will greatly enhance the suitability of an active
catalytic material. Typical of the high surface area -
larger open area stationary catalyst structures are metal
gauze, woven metal screens, expanded metal mesh, honeycomb,
metal foam, and knitted metal mesh. These may be massive
catalyst structures or the catalyst may be deposited on
a support of suitable design. The supports which may be,
for example, of metal, ceramic, or glass, may participate
in the catalytic activity or may serve merely to present
the catalyst in a suitable structural pattern.
U.S. Patent No. 3,928,235 describes catalysts
which are useful for high temperature applications, par-
ticularly for the purification of automotive exhaust streams.
These catalysts are comprised of alloys containing at least
chromium and copper. The chromium-copper alloys are ef-
fective oxidation catalysts, e.g., for oxidizing CO to
CO2, and alloys of the chromium-nickel-copper are par-
tic~larly suitable for the removal of oxides of nitrogen
as well as carbon monoxide and hydrocarbons from such
streams. While the catalysts disclosed exhibit excellent
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activity and selectivity for many reactions, they are
expensive to fabricate into complex shapes by conven-
tional hot and cold working techniques because of
limited malleability at room temperature.
It is an object of the present lnvention to
provide a catalyst in a form which is especically suited
for reactions involving high velocity gas or liquid
flow reactants.
Another object is to provide a catalyst of
a structural design which has a high surface area and
large open area relative to the amount of catalyst. ~ -
A further object is to provide a catalyst
structure consisting of a coating of the catalytically
active material on a pre-formed structure which is
stable at high temperatures.
A still fuxther object is to provide a catalyst
structure comprising an alloy consisting essentially of
chromium, nickel and copper, the catalyst being developed
on a pre-formed metal mesh substrate, which is made of a
nickel- and chromium-containing alloy.
The objects and advantages will become apparent
from the following description.
In accordance with U.S. Patent No. 3,925,259
a coherent catalyst comprising an alloy of chromium and
copper is provided in a suitable structural form for
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permitting high rates of mass transfer and low resistance
` to flo~ by means of liquid-phase sintering of the alloy
prepared as a pre-alloyed powder. In a preferred em-
bodiment, the catalyst or precursor alloy thereof is
provided as a coherent adherent coating of nickel, chromium
and copper on a substrate of suitable design. The present
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application concerns a further method of fabricating
supported catalytic structures.
THE INVENTION
In accordance with the invention a composite
catalytic structure comprised of nickel, copper and
chromium and having a stratified catalytic surface material
developed as a coherent adherent layer on a substrate, said
substrate being a nickel- and chromium-containing alloy, is
formed by a method comprising: providing a copper-containing
coating on the substrate, said substrate alloy containing at
least about 10 weight % chromium and said coating being defi-
cient in nickel relative to the substrate, and heating said
coated substrate in a protective atmosphere to a temoerature
of at least the melting point of copper to form a surface
area material which is or is capable of being developed into
a stratified surface material and a chromium-rich sublayer.
Preferably, the surface layer is developed to con-
tain a nickel-copper alloy oP predetermined composition and
the maximum temperature employed is the freezing point of
such nickel-copper alloy.
By "protective atmosphere" is meant an atmosphere
which is non-oxidizing or preferably reducing with respect
to the substrate and coating. ~or example, the atmosphere
may be an inert gas such as argon or helium, or preferably
it is a reducing atmosphere such as dry hydrogen or an inert
gas containing, e.g. about 5~ hydrogen.
The surface area material formed in a protective
atmosphere ma~ alread~ be strati~ied. However, on being
treated in an atmosphere which is oxygenating with respect
thereto, the surface area material will develop into
stratified oxide layers and an oxidized chromium-rich sub-
layer. The oxidized chromium--rich sublaver consists essen-
~08V~O~
tially of at least one chromium-containing oxide, e.g.
Cr2O3 and/or a mixture of chromium-containing oxides which
may include nickel-chromium-oxides and/or copper-chromium-
oxides, depending on the alloy com~osition and treatment.
Any method may be used to apply the coating
material to the substrate. For example, one or more
layers of copper or copper alloys and/or other desired
elements may be deposited as a film, e.g. by electrolytic
or non-electrolytic plating, vapor de~osition, thermal
or chemical decomposition of an inorganic or organic com-
pound dissolved or susnended in a liquid medium, as a
paint, e.g. by deposition from a colloidal silica-containing
solution, or as a dry powder. In the embodiments using
powders, e.g. dry or slurried in a li~uid medium, mixtures
of elemental powder and/or alloys thereof may be employed.
As indicated, the coating composition mav contain
one or more components. However, an essential component
is copper. Nickel, chromium and any other elements desired
in the coating mav be present. But, the deposited coating
must be deficient in nickel ralative to the ultimate
copper-nickel surface alloy desired. In preferred embodi-
ments the coating is substantially only copper or pre-
dominantly copper.
In one embodiment, a copper-containing powder
is deposited on a substrate of a desired shape with the
aid of a fugitive binder. The binder is applied, e.g.,
by spraying, painting, dipping or the like, on a metal
pre-form, and pre-alloyed powder is applied to the coated
substrate. The substrate with the alloy powder thereon is
then heated in an atmosphere containing low effective
oxygen potentials to drive of the binder and perm-t
dissolution of nickel into the copper layer. The initial
temperature to which the coated structure is raised is at
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least the melting point of copper or sliyhtly in excess
thereof, and in a preferred temperature profile for treating
a powder deposit, the temperature of the coating (or coated
material) is raised slowly to about 650C, and then rapidly
to about the melting point of copper. Thereafter, the
temperature is raised slowly to the freezing point of the
desired nickel-copper alloy in the surface materlal. As
will be explained further below the temperature at which
the composition is heat treated plays a role in the compo-
sition and structure of the surface material.
Depending on the particular application, the
substrate must satisfy certain requirements of mechanical
strength and resistance to the environment. Preferably,
it will be readily formable into the desired shape. In
addition, the substrate contributes to the final surface
composition and stratified structure. Because the support,
as well as the coating, will contribute to the final sur-
face layer composition and thickness, there must be suffi-
- cient concentration of one of the desired alloying elements
present in the support to form the ultimate surface layer
quickly and without excess dilution of the layer with other
components of the support.
The substrates according to this invention are
made of nickel- and chromium-containing alloys. The alloy
provides at least a part of the nickel to form the active
layer with the applied copper, and chromium serves several
functions. First, it provides mechanical strength and oxida-
tion resistance at elevated temperature; second, it can be a
minor component of the catalyst alloy layer; third, when
nickel diffuses outward to form the active layer, the sub-
layer is enriched in chromium because chromium will not
readily alloy with the initially copper-rich surface layer -
this enriched layer, when oxidized, can form a chromium-
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1080;201
oxide~containing isubscale to enhance catalyst durability.
Iron may also be present in the support alloy as well, but
it must be cautloned that iron does not dissolve as readily
in copper as nickel and the iron present must not diminish
the nickel concentration so as to inhibit the depletion of
the non-chromium portion of the alloy thereby preventing
the formation of the required chromium-rich sublayer. Like-
wise, such metals as molybdenum, niobium and tungsten may
also be present.
- 10 If homogeneous structures are to be produced,
advantageously, only the elements to be present in the
coating - less the coating element(s) - are incorporated
in the substrate, and these are present in a suitable ratio. -
In general, the nickel-chromium-containing
alloys of the substrate should contain from at least about
10~ to about 50% chromium. (All percentage compositions '
1 herein are on a weight basis.~ The nickel content should be
-l at least that sufficient to provide the nickel needed to
furnish the desired nickel-copper composition in the surface
layer and to deplete the nickel content with respect to
chromium so that a suitable chromium-rich layer will form,
but without depleting the nickel content of the substrate
undesirably. It will be appreciated that this is a function
of the thickness of the coating and substrate as well as
the composition of both.
Exemplary alloys for the substrate are nickel-
chromium, nickel-chromium-copper and nickel-chromium-iron
; alloys consisting essentially of, by weight about 10% to
about 50% chromium and about 50% to about 80% nickel, e.g.,
80Ni-20Cr, 70Ni-30Cr, 50Ni-50Cr, 62Ni-28Cr-lOFe. There
,. .
are many commercially available alloys which are satisfactory
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for this purpose. For example, INCONEL* alloys 600, 601, 671
& 690, and INCOLOY* alloy 800, are among the Ni-Cr alloys
suitable. (These alloys are products of The International
Nickel Company, Inc. New York, N.Y.) Tophet A* and Tophet 30*
(Ni-Cr alloys, products of Wilber B. Driver Co., Newark,
N.J.) are also suitable. It will be noted that these
suitable alloys contain minor amounts of elements in addi-
tion to the nickel and chromium without harmful effects.
Composition selection will depend at least in part on the
final composition desired and the coating deposited in
addition to the physical and chemical requirements of the
final substrate.
, The thickness of the coating depends in part on
the composition of the coating applied. It will be noted
that thickness can be easily controlled with certain coating
techniques. For example, in electrodeposition, the thickness
of the coating is readily adjusted to form films of, e.g.,
~- from about 1/2 to about 5 mils thickness. In general,
stratified surface areas including the chromium-rich layer
; 20 which have a thickness of 0.3 to 5 mils are satisfactory. If
the surface area material developed in the heat treatment in
a protective atmosphere is too thick, upon oxidation the
partial pressure of oxygen at the chromium-enriched sublayer
may be too small to form the desired chromium-containing-
oxide layer, and nitridation of the unprotected support and
failure,-will result. If copper is applied too thickly and
; liquid~phase treated then the diffusion zone becomes too
broad and chromium enrichment in the sublayer will be diffused,
with a reduced possibility of forming the chromium-containing
~ 30 oxide layer. If the initial copper layer is too thin, then
;~ only a small amount of nickel diffusion from the support need
occur to satisfy the desired final compositional requirement.
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*Trademarks
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: The small amount of chromium enrichment will occur may be
smoothed out quickly by solid state diffusion and thus will
be lost. In ~eneral, this last restriction on the thickness
is removed if the substrate size is of the order of the
coating thickness. If a homogeneous structure is desired,
coating thickness is fixed by a materials balance at a
given substrate composition and thickness.
The composition of the surface area material is,
ideally, determined by the treatment temperature, which ex-
ceeds the melting point of the lowest melting coating com-
ponent, and the final composition of the surface will be that
- having the treatment temperature as a solidus. In practice,
this may not occur, especially if the coating is thic~,
because diffusion in the liquid layer may not be sufficient
to maintain a uniform composition.
In thicker coatings, the nickel concentration will
normally not be uniform initially; however, the composites
are to be treated in oxidizing atmospheres to form the active,
stratified oxide layers, and such initial coating inhomogen-
ity will enhance the formation of the desired surface struc-
ture. This desired structure is a layer of CuO above a
.
mixed Cuo-NiO layer, all separated from the support by a
subscale of chromium oxides. In instances where a uniform,
homogeneous coating is desired, further heat treatment in the
solid state can be used.
The formation of the sublayers of chromium oxide(s)
.. . .
; will depend, as indicated above, on substrate and coating
.; , .compositions, coating thickness, and treatment period and
temperature.
It was noted above that in heat treating the coating,
preferabl~ the temperature is raiced relatively rapidly to at
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. 108V;~Ol
least the melting point of copper, i.e. about 1083C, apart
from consideration of the evolution of volatile materials,
e.g., in the binder used for powder coatings. Thereafter,
i.e. above 1083C, the rate of temperature rise to the desired
maximum is minimized, or the liquid layer may draw together by
capillarity. The temperature rise above the melting point of
copper is regulated to achieve a more uniformly planar
surface area material, to control the shape of the inter-
face, and to minimize surface tension effects. Rates of
temperature increase at above 1083C are suitably about 5 to
100C/min. For processing powder which is deposited with
a binder, below about 650Cj the rate of temperature increase
is slow, e.g., suitably less than about 150C to about 300C;
; minute, to permit the volatile agents to escape withoutdisturbing the coating. Suitably, the heat treatment is
held for a period of about 10 and 120 seconds at about
maximum temperature. If homogeneous structures are desired
the treatment periods are adjusted accordingly. In thin
sections, of course, ho~ogeneity is achieved more readily.
Some precautions must be observed, however, in
processing the materials. For example, the treatment temp-
erature must be above the melting temperature of the lowest
melting coating component, but limits can be present on the
maximum allowable temperature. In making homogeneous
materials by reaction of the liquid layer with the solid
support the treatment temperature must not exceed the solidus
of the ultimate homogeneous alloy to be formed or general
~ melting occurs. By way of illustration, a sample of 5 x 7
- mil ~ 20Cr expanded metal coated with 1/2 mil of electro-
deposited copper and exposed to a temperature of about 1204C
(2200F), melted.
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It will be noted that in the embodiment in which
the coating is applied as a metal powder, additional agents
may be used to effect a suitable coating. For example, graphite,
e.g. as micron-sized particles dispersed in a volatile hy~ro-
carbon, or a polymeric material, e.g. an acrylic lacquer, may be
sprayed on the powder metal coating. This will serve to hold
the powder in place during development on the substrate.
The following examples are intended to illustrate
the process of the present invention, but it is not in-
tended that this invention be limited to the specific em-
bodiments shown therein.
EXAMPLE I
A sample of 30 mil thick foil of INCONEL alloy
671, containing approximately 48% chromium, was coated
with Krylon* spray (a rubber cement product of the Borden
Co.) and dipped in copper powder to form a layer about
1 mil thick. The composite was sintered in argon for 1
minute at 1149C (2100F). Microexamination showed a
surface layer composed of a copper-rich matrix and a
dispersion of a chromium-containing second phase. A
second sample prepared by heating in argon quickly to
1093C (2000F), slowly to 1149C (2100F), and then
holding for 2 minutes in air at 815C (1500F). The sur-
face was found to be CuO with a sublayer of Nio and CuO,
and a chromite and a continuous, protective Cr2O3 sub-
layer formed below the copper and nickel oxide layer.
This is a desirable microstructure for durable, active NOX
reduction catalysts.
*Trademark
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EXAMI'LE I I
Samples of 80Ni-20Cr alloy in the form of ex-
panded metal mesh made from 5 mil foil were sprayed with
Krylon* and dipped in copper powder, -200 mesh. Samples
were treated in dry argon; sample 1 was slowly heated to
1121C (2050~F), sample 2 was heated slowly~to 1193C
(2180F) and held for 3 minutes. Both samples were ex-
- amined by electron microbeam probe and were tested for
NOX reduction activity. Sample 1 had a diffusion zone
about 50 microns thick, consisting of about 75~ copper,
22% nickel, and 1.3% chromium. Below the reacted layer
chromium enrichment was detected by microanalysis and
metallography. Sample 2 showed a broader diffusion zone
and a contlnuously decreasing copper content from surface
to inside, and a greater nickel content in the diffusion
zone, about 25 to 32%. Chromium enrichment was also
detected, the subsurface layer containing up to 31%
chromium. Both samples were tested for catalytic activity
without surface oxidation. During testing in synthetic
exhaust of variable 2 content, catalytic activity developed
quickly in both materials. Characteristic of higher nickel
NOX catalyst alloys, sample 2 became active sooner and had
less NH3 formation tendencies. The activity of both samples
was judged very good.
EXAMPLE I I I
Samples of 80-20 nickel-chromium expanded metal
mesh, having a web cross section of 5 by 7 mils, were
- electroplated with copper 0.0005 inch thick, with the ob-
jective of forming a homogeneous alloy comprised of
Ni-27Cu-15Cr. A sample was heated to 1093DC (2000F) and
the temperature was raised to 1121C (2050E) over 6
minutes. A further rise to 1149C (2100F) over 2 minutes,
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then to 1177C ~2150F) over 2 minutes, and a hold at
1177C (2150F) for 15 minutes resulted in formation of a
nearly homogeneous, single phase material, with a while
metallic surface coloring. While an etched microsection
showed some evidence of copper enrichment at the surface,
it was minor. Some additional time at temperature would
have essentially homogenized the sample. As shown in the
aforementioned U.S. applications, a stratified surface
.: ,
layer will develop in this alloy upon oxidation treatment.
As described in the aforementioned U.S. Patent ~o.
3,928,235 to develop a durable, active, stratified catalytic
surface, the precursor alloy, preferably in the form of
a shaped material, is heated in an atmosphere which is
oxygenating with respect to the alloy. Suitably the atmo-
sphere contains free oxygen, for example, the atmosphere may
be air, air containing up to about 10 percent moisture, or a
nitrogen stream containing oxygen. However, it has been
found that the active catalyst may be developed ~n-situ, e.g.,
in the exhaust stream itself where the NO, CO2 or H2O may
supply the oxygen needed to form the combination of oxides
and/or metal at the surface of the alloy which is active and
resistant to further oxidation. Thus, although stoichiomet-
rically reducing in respect to the CO, the stream may be
oxidizing with respect to constituents of the precursor
alloys. The temperature for developing the catalytic sur-
face may range from about 1100 to about 1900F. For
preliminary oxidation alloys may suitably be treated in
air at a temperature in the range of about 1500 to 1700F
for a period of about 2 to 30 hours.
Accordingly, the process of the present invention
enables for relatively simple fabrication of catalysts
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suitable for ~x reduction. It will be obvious that the
final composition can be adjusted by adding nickel or
chromium as electroplated layers onto commercially available
alloys such as those mentioned above. Thus, the substrate
need not be specially made to meet end composition require~
ments.
For convenience, and not for the purpose of
limitation, the catalyst of the present invention has
been described mainly with respect to the treatment of
automobile exhaust to remove nitrogen oxides. The cata-
lysts are also useful, for example, as oxidation catalysts,
e.g. for oxidation of hydrocarbons and carbon monoxide in
the presence of, e.g., air and/or H2O. This will include
such reactions as steam reforming and water-gas shift. They
are also catalytic for the formation of ammonia in the ab-
sence of alr.
Although the present invention has been described
in conjunction with preferred embodiments, it is to be
understood that modifications and variations may be resorted
to without departing from the spirit and scope of the in-
vention, as those skilled in the art will readily understand.
Such modifications and variations are considered to be within
the purview and scope of the invention and appended claims.
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