CA1080201A

CA1080201A - Method of preparing a catalytic structure

Info

Publication number: CA1080201A
Application number: CA258,727A
Authority: CA
Inventors: Robert H. Kane
Original assignee: Vale Canada Ltd
Current assignee: Vale Canada Ltd
Priority date: 1975-09-12
Filing date: 1976-08-09
Publication date: 1980-06-24
Also published as: JPS5236594A; DE2641059A1; SE7610025L; ES451470A2; NL7609719A; BE846084R; FR2323441A2

Abstract

ABSTRACT OF THE DISCLOSURE
A process for producing a catalyst structure comprised of nickel, chromium and copper and having a stratified surface material formed as a coherent layer on a substrate, said substrate being a nickel- and chromium-containing alloy, comprising: providing a copper-containing coating on the substrate, said substrate containing at least about 10 weight percent chromium; and heating said coated substrate in a protective atmosphere to a temperature 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.
The catalyst is particularly useful in the purification of automobile exhaust and industrial waste streams.

Description

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

- 2 -t 10~ 01 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 . . .
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

- 3 -~08VZ01 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 1080~01 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-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 ~. . . .

-i 1080'~01 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 ~, : ': , , ~ ' , ': ' iO8l);~

: 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 . .

g . 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 lO~VZOl 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, *Trademark . . . . .
:, . ' ' , 1 0~Uv2 ~1 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 .::

i: :
: ' 10~Z~, 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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a catalyst structure comprised of nickel, chromium and copper and having a stratified surface material formed as a coherent layer on a substrate, said substrate being a nickel- and chromium-containing alloy, comprising: providing a copper-containing coating on the substrate, said substrate containing at least about 10 weight percent chromium and said coating being deficient in nickel relative to the substrate; and heating said coated substrate in a protective atmosphere to a tem-perature of at least the melting point of copper to develop a surface area material which is or is capable of being developed into a stratified surface material and a chromium-rich sublayer.

2. A process as described in claim 1 wherein the protective atmosphere is selected from at least one member of the group consisting of inert gases and dry hydrogen.

3. A process as described in claim 1 wherein a nickel-copper alloy of predetermined composition is formed in the surface area material and the maximum temperature of the heat treatment is the freezing point of such nickel-copper alloy.

4. A process as described in claim 1 wherein the developed surface area material has a predetermined nickel-copper composition and wherein the copper-containing coating has a composition which is selected from at least one member of the group consisting of elemental copper, and an alloy consisting essentially of nickel and copper, said alloy being nickel deficient relative to the predetermined composition.

5. A process as described in claim 1 wherein the substrate is a nickel-chromium alloy consisting essentially, by weight, of about 10 to about 50% chromium and about 50 to about 80% nickel.

6. A process as described in claim 1 wherein the heat treatment is effected in the range of from about 1083°C to about the solidus temperature of the developed surface area material.

7. A process as described in claim 6 wherein heating is effected rapidly to a temperature of about 1083°C and thereafter heating to the maximum temperature is effected at a rate sufficiently slow to minimize sur-face tension effects and to produce a more uniformly planar surface area material.

8. A process as described in claim 1 wherein after the heat treatment in the protective atmosphere, the resultant material is treated in an atmosphere which is oxygenating with respect to the resultant surface area material to develop stratified oxide layers and a chromium-oxide containing sublayer.

9. A process as described in claim 1 wherein the substrate is an open metal mesh in a configuration selected from expanded metal mesh, woven wire mesh, knitted metal mesh, gauze, honeycomb, and metal foam.

10. A process as described in claim 1 wherein the developed stratified surface material and chromium-rich layer has a total thickness of about 0.3 to about 5 mils.

11. A process as described in claim 1 wherein the substrate is an alloy comprising about 20% to about 30%
chromium.