CA1078364A

CA1078364A - Catalysts comprising substrate, intermediate oxide layer and catalytic layer

Info

Publication number: CA1078364A
Application number: CA212,089A
Authority: CA
Inventors: Laurence A. Heathcote; Walter G. Bugden; Allin S. Pratt
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1973-10-24
Filing date: 1974-10-23
Publication date: 1980-05-27
Also published as: JPS5823138B2; IT1024675B; SE408137B; SE464798B; SE7413242L; DE2450664A1; FR2257336B1; FR2257336A1; JPS50112293A

Abstract

ABSTRACT OF THE DISCLOSURE
Catalysts and catalytic structures are provided comprising a substrate, an intermediate oxide layer and a catalytic layer. For example, the catalyst may comprise: (a) a metal substrate made from a heat and oxidation resistant alloy of iron including chromium (3-40) wt. %, aluminum (trace - 10) wt. %, cobalt (0 - 5.0) wt. %, nickel (trace -72) wt. % and carbon (0 - 0.5) wt. %, balance essentially iron, the metal substrate being extended on an assembly of at least two juxtaposed sheets made from the alloy with at least one sheet shaped so that when contacted with an adjacent sheet channels are formed therebetween for the passage of gaseous reactants through the substrate; (b) an aluminum oxide film on the surface of the extended metal substrate, the film being formed by oxidizing the surface of the metal substrate; (c) a high surface area aluminum coating disposed over the aluminum oxide film and keyed to the substrate through the film; and (d) a catalytic layer comprising at least one metal selected from the group consisting of ruthenium, rhodium, palladium, iridium, platinum, silver, gold, and alloys of at least one of such metals with at least one additional element comprising a base metal selected from the group consisting of aluminum, magnesium, chromium, molybdenum, tungsten, manganese, iron, rhenium, cobalt, titanium, vana-dium, copper, zinc, cadmium, indium, thorium, bismuth, tin, lead, anti-mony, a lanthanide or an actinide, the base metal being present in an amount from a trace to 25 wt. % on the high surface area alumina coating.
Such catalysts have been found to be useful for a wide variety of cata-lytic reactions, e.g. the catalytic oxidation of ammonia in the manufac-ture of nitric acid and the catalytic oxidation of organic compounds.

Description

Lo~7~3~

This invention relates to catalysts catalytic struc-tures comprising a substrate, an intermediate oxide layer and a catalytic layer, and to procedures for their manufacture.
The catalysts of various aspects of this invention may be us~d for a wide variety of catalytic reactions, e.g.
the catalytic oxidation of ammonia in the manufacture of nitric acid and the catalytic oxidation of organic compounds (e.g. methane, ethane and propylene) for heating purposes and carbon contain~ng compounds, e.g. carbon monoxide and for catalytic reduction, e.g. the catalytic reduction of oxides of nitrogen with a reducing fuel and for use in aut~-mobile exhaust control units. The catalysts of aspects of this invention may also be used in NOX Abatement units ad-a~ted to be attached to plants for the manufacture of nitric acid.
Many nitric acid plants use hig~ pressure processes in which the oxidat~on of ammonia is carried out in the presence , . .
of solid rhodium-platinum wire which has beon woven into a ' gauze. I-t is also highly advantageous to recover from the r ~- 20 tail gas plant, energy which can be utilised to supply power ~'' .
~- to the system. In some systems sufficient energy can be re--i covered for the process to be self-sustaining and even to provide additional power. This is nor~ally carried out as a catalytic process inlwhich the oxides of nitrogen are reacted !:' ... . .
with a gaseous reducing fuel. It is important that the cat-alyst used be highly active, exhibit a minimum re~istance to ` gas flow and catalyse the reaction at low initial reaction or ,,: ..
` ignition temperatures. The catalyst itself, should however ,:
still have reasonably high (e.g. 750-800 C) temperature sta- -bility.

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10'783~;~

Air pollution control by catalytic combustion ihposes a number of restrictions on the type of catalyst which may be used. In addit20n to a requirement that the catalyst should be active at low temperatures, it should be stable under both oxidising and reducing conditions. Further, when the catalyst is p~cked into a reactor it should be~t~
thermal and mechanical shock and clogging by dust particles.
It is an object of a broad aspect of the present in-vention to provide improved catalytic structures for uses as above described.

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-2a-"--`-` 1078364 which are alternative to the ones now in use.
An object of another aspect of this invention is to provide methods for the preparation of such catalysts.
According to one broad aspect of this invention, a catalyst is provided comprising: a metal substrate made from a heat and oxidation resistant alloy of iron including chromium (3 - 40) wt. %, aluminum (trace - 10) wt. %, cobalt (0 - 5.0) wt. %, nickel (trace - 72) wt. %, and carbon (0 - 0.5) wt. %, balance essentially iron, the metal sub-,. ..
strate being in ~éxtended form; (b) an aluminum oxide film on the surface of the extended metal substrate, the film being formed by oxidizing thesurface of the metal substrate; (c) a high surface area alumina coating disposed over the aluminum oxide film and keyed to the substrate through the film; and (d) a catalytic layer comprising at least one metal selected from the group consisting of ruthenium, rhodium, palladium, .:' `
iridium, platinum, silver, gold, and alloys of at least one of such metals with at least one additional element comprising a base metal selected from the group consisting of aluminum, magnesium, chromium, molybdenum, tungsten, manganese, iron, rhenium, cobalt, titanium, vana-dium, copper, zinc, cadmium, indium, thorium, bismuth, tin, lead, anti-mony, a lanthanide or an actinide, the base metal being present in an ~-~ amount from a trace to 25 wt. % on the high surface area alumina coating.
. ,~. . .
by another aspect of this invention, a catalyst is provided consisting essentially of: (a) a metal substrate having a high surface area and made from a heat and oxidation resistant alloy of iron including :
~` chromium (3 - 40) wt. %, aluminum (trace - 10) wt. %, cobalt (0 - 5.0) ., ~; wt. ~, nickel (trace - 72) wt. % and carbon (0 - 0.5) wt. %, balance : `;::' ! essentially iron, the metal substrate comprising an assembly of at least two juxtaposed sheets made from such alloy with at least one sheet shaped ; so that when contacted with an adjacent sheet channels are formed there-between for the passage of gaseous reactants through the substrate; (b) .: . .,., ~,- . :. :. : : ,... , ... ,, . : :.
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an aluminum oxide film on the surface of the substrate, the film being formed by oxidizing the surface of the substrate; (c) a layer of alumina applied to the sheets and calcined; and (d) a catalytic layer applied to the first layer, the second layer consisting essentially of one or more metals selected from the group consisting of ruthenium, rhodium, palladium, iridium, platinum, silver, gold, and alloys of at least one of such metals with at least one additional element comprising a base metal selected from the group consisting of aluminum, magnesium, chromium, molybdenum, tungsten, manganese, iron, rhenium, cobalt, titanium, vana-dium, copper, zinc, cadmium, indium, thorium, bismuth, tin, lead, anti-mony, a lanthanide or an actinide, the base metal being present in an amount from a trace to 25 wt. %.
By one variant, the metal substrate (a) is made from a heat and oxidation resistant alloy of iron including at least one of the ;` elements chromium (3 - 40) wt. %, aluminum (1 - 10) wt. %, cobalt (trace - 5.0) wt. %, nickel (trace - 72) wt. %, and carbon (trace - 0.5) wt. %.
By another variant, no more than 10 wt. % of the alloy of iron consists of one or more of the additional elements silicon, manganese, ~ 20 zirconium, copper, tungsten, vanadium, molybdenum, cerium, niobium, - tantalum and titanium.
`~ By variations thereof, not more than 5 wt. %, e.g., not more ~ than 3 wt. % of the alloy of iron consists of one or more of the addi-; tional elements.
By still another variant, the metal substrate (a) is made from a heat and oxidation resistant alloy of iron which comprises, apart from impurities, at least one of the elements Cr (3 - 30) wt. %, Al (l - lO) wt. %, C (a trace to 0.5) wt. %, Co (a trace to 5.0) wt. %, Ni (a trace to 15.0) wt. % and balance iron.
By another variant, the metal substrate (a) is made from a H ` 4 -,' 1~7~3364 heat and oxidation resistant alloy of iron which comprises, apart from impurities, at least one of the elements Cr (13 - 24) wt. %, Al (3.5 -6.5) wt. %, C (a trace - 0.1) wt. %, Co (0.2 - 0.8) wt. %, Ni (a trace -5.0) wt. %, balance iron.
- By a further variant9 the metal substrate (a) is made from a heat and oxidation resistant alloy which includes one or more of the additional elements: Si (0.2 - 0.9) wt. %, Mn (0.2 - 0.7) wt. %, Zr (0.01 - 0.20) wt. %, Cu (0.01 - 0.15) wt. %, Nb (0.01 - 0.3) wt. %, Ta (0.8 - 1.2) wt. %, Ti ~0.8 - 1.2) wt. %, Co (0.01 - 1.0) wt. %, Ca (0.01 - 0.5) wt. %, C (0.01 - 0.1) wt. %.
By yet another variant, the metal substrate (a) is made from a : ::
- heat and oxidation resistant alloy of iron which comprises 0.09 wt. % C, 22.60 wt. % Cr, 2.0 wt. % Co, 4.5 wt. % Al and balance iron.
By a still further variant, the metal substrate (a) is made ",,:
,~'r from a heat and oxidation resistant alloy of iron which comprises Cr 30 ' wt. %, Fe 65 wt. % and Al 5 wt. %.
By yet another variant, the metal substrate (a) is made from a . ~
heat and oxidation resistant alloy of iron which comprises C 0.08 wt. %, Cr 18.00 wt. %, Ni 10.00 wt. %, Ti 0.40 wt. % and balance iron.
By a further variant, the metal substrate (a) is made from a ^` heat and oxidation resistant alloy of iron which comprises Cr (10 - 40) ::
; wt. %, Al (1 - 9) ~t. %, Fe (51 - 88.9) wt. %, Ta (0.1 - 5) wt. %, Ce (0.01 - 1) wt. %.
By yet another variant, the metal substrate (a) is made from a `~ heat and oxidation resistant alloy of iron which comprises Cr (10 - 40) wt. %, Al (1 - 9) wt. %, Fe (51 - 88.9) wt. %, Ta (0.1 - 5) wt. %, Ce (0.01 - 1) wt. % and wherein Ce is partially replaced by Ca.
By other variants, the second catalytic layer may comprise a 7-1/2 wt. % Rh/Pt alloy; or may comprise a 10 wt. % Rh/Pt alloy; or may comprise a 20 - 50 wt. % Rh/Pt alloy; or may comprise a 1 - 50 wt. %

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rhodium, 0.01 - 25 wt. % base metal, balance platinum.

By another variant, the layer of alumina comprises calcined . O
crystal aggregates having a diameter of 20 - lOOA and a pore diameter of 40A.
By a further variant, the juxtaposed sheets are helically wound together to form a metallic monolith.
By yet another variant, one sheet is corrugated and another, adjacent sheet is plain.
By a further variant, the two separate thin metal sheets have a thickness within the range 0.0015 to 0.0045 inches.
By one variation thereof, the sheets are corrugated of such dimensions that the wound up structure has between 200 and 800 cells per ' ' ''!
~' - square inch when considered in cross section.

By another variation thereof, the catalyst has a substrate - ; composed of metal sheets of approximate thickness 0.002 inches assembled .:,'`' to form a structure having approximately 400 cells per square inch when considered in cross section and a surface to volume ratio of approximate-.. ,:.
ly 1200 sq. ft. per cubic foot.
,: .
- ~y another aspect of this invention, a method is provided for making a catalyst consisting essentially of a substrate having a high surface area and made from a heat and oxidation resistant alloy of iron including chromium (3 - 40) wt. %, aluminum (trace - 10) wt. %, cobalt (0 - 5.0) wt. %, nickel (trace - 72) wt. %, and carbon (0 - 0.5) wt. %, balance essentially iron, the substrate comprising at least two juxta-posed sheets made from such alloy with at least one sheet shaped so that when contacted with an adjacent sheet channels are formed therebetween for the passage of gaseous reactants through the substrate, such method comprising the steps of: (a) treating the substrate assembly to oxidize its surface, thereby to provide an aluminum oxide film thereon; (b) applying a first layer containing alumina to the sheets; (c) calcining . ~ .
~t ~: - 6 -1~78364 .
the sheets of (b) so that the first layer is bonded to the underlying sheet and serves to bond adjacent and contacting sheets together; and (d) applying a second and catalytic layer to the first layer, the second -layer consisting of one or more metals selected from the group consisting of ruthenium, rhodium, palladium, iridium, platinum, silver, gold, and alloys of at least one of such metals with at least one additional metal comprising a base metal selected from the group consisting of aluminum, magnesium, chromium, molybdenum, tungsten, manganese, iron, rhenium, ; cobalt, titanium, vanadium, copper, zinc, cadmium, indium, thorium, bis-muth, tin, lead, antimony, a lanthanide or an actinide, the base metal ; being present in an amount from a trace to 25 wt. %.
By a variant thereof, the juxtaposed sheets are helically wound together to form a metallic monolith.
By another variant, one sheet is corrugated and another, adja-cent sheet is plain.
By a further variant, the two separate thin metal sheets have a thickness within the range of 0.0015 to 0.0045 inches.
By a variation thereof, the sheets are corrugated of such -~ dimensions that the wound up structure has between 200 and 800 cells per square inch when considered in cross section.
By another variation, the substrate is composed of metal sheets of approximate thickness 0.002 inches assembled to form a structure having approximately 400 cells per square inch when considered in cross section and a surface to volume ratio of approximately 1200 sq. ft. per cubic foot.
By yet another vacia~lt, the alumina is calcined to form chi and gamma alumina.
By another variant of this invention, the extended metal sub-strate (a) is made from a heat and oxidation resistant alloy containing nickel and/or chromium having an aggregate nickel and/or chromium content Y ~ 7 -- . .: ;. - . , . : :
. I . . ! . , , , ' ~ i 1C~783~;4 of greater than 20 wt. %.
By a variant thereof, the catalyst includes one or more of the additional elements Si (0.2 - 0.9) wt. %, Mn (0.2 - 0.7) wt. %, Zr (0.01 - 0.20) wt. %, Cu (0.01 - 0.15) wt. %, Nb (0.01 - 0.3) wt. %, Ta (0.8 0 1.2) wt. %, Ti (0.8 - 1.2) wt. %, Co (0.01 - 1.0) wt. %, Ca (0.01 - 0.5) wt. %, C (0.01 - 0.1) wt. %.
By a variation thereof, the alloy of nickel and chromium com-prises not less than 72 wt. % nickel, not more than 0.15 wt. % carbon, not more than 1.00 wt. % manganese, 8.00% iron, not more than 0.015 wt. %
sulphur, not more than 0.50 wt. % silicon, not more than 0.50 wt. %
- copper and 15.50 wt. % chromium.
By another variation, the alloy of nickel and chromium con~
:
~ prises 60.50 wt. % nickel, ~3 wt. % chromium, 14.10 wt. % iron, 1.35 wt.
: . ~
% aluminum, not more than 0.50 wt. % manganese, not more than 0.25 wt. %
~;~ copper, not more than 0.25 wt. % silicon, not more than 0.50 wt. %
..':
carbon and not more than 0.007 wt. % sulphur.
By a further variation, the alloy of nickel and chromium com-~; prises 32.50 wt. % nickel, not more than 21 wt. % chromium, not more than 44 wt. % iron, not more than 0.10 wt. % carbon, not more than 1.50 wt. % manganese, not re than 0.15 wt. % sulphur, and not more than 0.75 wt. % copper.
By yet another variation, the alloy of nickel and chromium comprises 20.0 wt. % chromium, not more than 77.5 wt. % nickel, 1.0 wt. %
iron, not more than 1.0 wt. % silicon, 0.4 wt. % manganese and not more than 0.1 wt. % carbon.
By another variant, the catalyst consists essentially of (a) a substrate comprising an assembly of at least two juxtaposed sheets made from an alloy with one sheet being corrugated and the other sheet being plain, so that channels are formed therebetween for the passage of gaseous reactants through the substrate, the juxtaposed sheets being ~ 7a -:~ : , . . .

:, ., .. :, ~:.

1C~78364 helically wound togethe~ to form a metallic monolith, the alloy having the composition, in weight percert, carbon (0.09); chromium (22.60);
cobalt (2.0); aluminum (4.5); and iron, balance; (b) an aluminum oxide film on the surface thereof, being produced by thermal oxidation; (c) a catalytic layer washcoat of alpha aluminum trihydrate formed by con-tacting the surface thereof with sodium aluminate solution containing excess aluminum and caustic soda at 80C. and then firing, the catalytic :: .~: ., layer being metallized with a 7-1/2% Rh/Pt alloy to give a metal loading of 30 g/ft .
By variants thereof, the washcoat loading may be 3.17 g/in3;
or 1.48 g/in3, or 0.7 g/in3.
" .
By another variant, the catalyst was formed by impregnating ....
; the metal substrate with a slurry of a pre-activated alumina trihydrate ` and an alumina monohydrate having a solid:liquid ratio between 25 and 50% and a pH less than 7, drying the substrate in warm air and firing at 450C. for 2 hours to form chi and gamma alumina in an adherent coating of up to 0.002 in. thick in the metal substrate.
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By a variation thereof, the catalyst includes a loaded washcoat ~ by the step of immersing the support in a solution of a water-soluble ; 20 inorganic salt or salts of the platinum group metals.
By a further variant, the solution is of chloroplatinic acid, - rhodium trichloride and nickel chloride.
By another variant, the loading is with 7-1/2% Rh/Pt.
By another aspect, the process involves using the catalyst which comprises one of the steps of (a) manufacturing nitric acid by the catalytic oxidation of ammonia; or (b) chemically reducing an oxide of nitrogen with a gaseous reducing fuel in the presence of the catalyst.
The oxygen in the first layer may be present as an oxide selected from the group consisting of alumina, silica, titania, zirconia, hafnia, thoria, beryllia, magnesia, calcium oxide, barium oxide, chromia, boria, scandium oxide~ yttrium oxide, and oxides of the lanthanides.

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, : : :: : :: ::, . ::: .:, :: . :
. - ~ : -1C~78364 Alternatively, the oxygen in the first layer is present as an oxygen containing anion selected from the group consisting of chromate, phos-phate, silicate and nitrate. Preferably it is an aluminum oxide film.
The base metal alloyed to the platinum metal and included in the second and catalytic layer may be aluminum, magnesium, chromium, ., molybdenum, tungsten, manganese, iron, rhenium, cobalt, titanium, vana-; dium, copper, zinc, cadmium, iridium, thorium, bismuth, tin, lead, anti-mony, a lanthanide or an actinide, present in an amount of from a trace to 25 wt. %.
One particular catalyst according to an aspect of the present ; invention comprises an extended metal substrate containing 3 - 40 wt. %
i~ chromium, 1 - 10 wt. % aluminum, a trace to 3 wt. % of one or more -; of the additional elements and the balance, apart from impurities, iron, and in which the second and catalytic layer comprises a metal selected from the group consisting of ruthenium, rhodium, palladium, iridium, ; platinum, silver, gold, or an alloy containing at least 10% by weight of one of those metals with at least one of the base metals.
The catalytic layer comprising one or more of the platinum group metals listed above or an alloy containing at least one of the platinum group metals is preferably deposited upon the oxide coating or film. It may be deposited in a form which directly possesses cataly-tic activity or in a form which is capable of being rendered catalyti-cally active by subsequent treatment.' Catalytic structures according to aspects of the present invention are extremely robust and effective in catalytic reactions under the stated reaction conditions.
Moreover, it has been found that in some reaction environments, e.g., automobile exhaust purification, the metal substrate on the cata-lyst of aspects of this invention causes the catalyst to reach running temperature much more quickly and therefore considerably more efficiently than conventional catalysts, e.g., those having a ceramic substrate.

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`: ~078364 In this specification, the term "extended metal substrate" means a metallic substrate which has been mechanically deformed in such a way ~- that, when compared with a plain non-deformed substrate, a very much ,~
increased surface area is obtained. Typically the increased surface area may be achieved by corrugating or folding in a former and winding up a flat roll and a corrugated foil together into a tube having a ~ spiral cross section.
-i In the accompanying drawing, the single Figure is a photograph of one variant of a metallic substrate for use in providing the catalyst structure of an aspect of this invention.
In a preferred embodiment of the present invention, the metallic substrate which is employed in the catalytic structure is first crimped, corrugated, folded, indented and/or perforated in such a way that a very much extended surface area is produced. Such a surface area is normally much greater than that obtained with a ceramic honeycomb ; or with particulate catalyst supports for the same given volume. An example of a metallic substrate made in accordance with these teachings comprises a roll of corrugated sheet of a heat resisting alloy inter-leaved with a non-corrugated sheet of such alloy to form a metallic monolith, is shown in the attached drawing. Alternatively, two corru-gated sheets may be used with the corrugations in each sheet parallel with each other or at an angle to each other. A coated substrate is then provided with a firmly adherent oxide coating which is porous and ~ 7d _ :

~0783f~4 ` - sbsorbent and ha~ a high surface area and which acts as the carrier for - the second catalytically active layer containing one or more of the platinum group metals as herein defined.
The heat resistant alloys comprising the extended metal sub-~trate are preferably alloys having a minimum nickel plus chromium con-tent of 20~ by weight. Typical alloys which therefore may be-used for ; the extended metal substrate are high nickel and chromium stainless steels and proprietary products, e.g., those known by the Trade Marks of "INCONEL" 600 and "INCONEL" 601.
It is preferred to provide the metallic substrate with a first firmly adherent oxide layer in an essentially two stage process. In the first stage the metallic substrate is thermally oxidized to provide a thin first oxide layer which acts as a key. It is preferred to carry out thermal oxidation by maintaining the formed metallic substrate at from 1000 - 1200C. in air or moist cracked ammonia vapour for 1 hour.
The higher temperature is required with very oxidation-resistant alloys, e.g., those high chromium stainless steel alloys known as KANTHAL
(registered Trade Mark) and the moist hydrogen atmosphere is preferred with alloys having a high Ni content.
The adherent oxygen-containing or oxide film may be produced by any one of several known procedures~including chemical techniques.
The film must be of sufficient thickness to provide adequate absorptive capacity for retaining the catalytically active alloy comprising one or more of the platinum group metals. The film is preferably from 0.004 to 0.001 inches thick.
Where aluminum is present in the alloy forming the extended metal substrate, the oxide film may be produced by treating the aluminum-containing surface with a solution of an alkaline carbonate, usually a sodium carbonate chromate solution. The film may be produced by the anodic oxidation of the metal surface whereby the metal is made the - i~

` 1~78364 , anode i~ an electrolytic solution. In anodizing aluminum-containing sur-faces, a 15~ sulphuric acid solution is commonly employed as the electro-lyte but other acid electrolytes, e.g., chromic acid, oxalic acid, phosphor-ic acid and sometimes boric acid may be used. The oxide film in catalysts according to which aspects of this invention relates is deliberately posi-;; tioned and does not include the relatively thin natural oxide films which sometimes occur on metal surfaces which have been exposed to the atmosphere.
One method of forming an alumina layer on these alloys which do ~-- not contain sufficient aluminum to form their one alumina layer upon oxi-dation is the use of a procedure known by the Trade Mark of CALORISING.
This involves the vapour deposition of an aluminum coating followed by anodizing or heating in an oxygen-containing gas. Alternative coatings, e.g., chromate, phosphate, silica or silicate or zirconia may all be de-posited by known methods. The many different techniques for the prepara-tion of a high surface area catalytically active refractory metal oxide wash coat containing one or more of the refractory metal oxides which confer beneficial properties as regard ageing and inertness to platinum group metals at high temperature under oxidizing and reducing conditions ~s well known in the art and in the interests of brevity need not be reproduced here.
A preferred adherent oxide coating deposited upon the extended metal substrate is alumina.
One method for the deposition of hydrous alumina is proposed in United States Patent No. 2,406,420 issued August 27, 1946 to H.B. Weiser .
et al. Any convenient aluminum compound e.g. alkali metal aluminates and aluminum salts may be used as the starting material. Either acidic or basic precipitants are used, depending upon the character of the starting material. Suitable acidic precipitants are ammonium chloride, ammonium sulphate, ammonium nitrate, ~ _ 7 f _ ...... . . ..

1(~78364 - hydrochloric acid, nitric acid, etc. Suitable basic precipitants are ammonium hydroxide, sodium hydroxide, hexa-methylene tetramine, etc.
One method is to precipitate the hydrous alumina from an alkali metal aluminate solution containing excess aluminum and alkali metal hydroxide directly on to the extended metal substrates forming ~ part of the catalyst of an aspect of the present invention. If the ; aluminate solution is maintained at a temperature of 60 - 85C., a film or coating of alpha alumina trihydrate (Gibbsite) is deposited. Subse-quent heating at 150 - 180C. converts the tri-hydrate to the monohydrate and subsequent heating at 540C. converts the monohydrate to gamma alumina without loss of the very high surface area coating which is pro-duced by this method. The high surface area results from the formation of hexagonal crystal aggregates of approximate size 8x8x20 microns.
Micropores of size of 40A diameter are present in the hexagonal crystal aggregates but appear to play no part in the catalytic activity of the structure. The high surface area is demonstrated (in Example 12) by resistance to lead and phosphorous poisoning after deposition of catalytic metal.
The heat-and-oxidation resistant alloys comprising the extended metal substrate used in the catalysts of aspects of the present invention comprise alloys of iron, chromium and aluminum in which the elements cobalt, nickel and carbon are preferably also present. Ranges, apart from .

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11~783~;4 : ` i trace elPments and impurities, for each element which may be present in ` the alloy are as follows;
.., ,,' . % ' .w/w . .
Cr 3 - 30 . Al 1 - 10 ...
~ 0 - 0.5 Co 0 ~ 5.0 - Ni 0 - 15.0 ~:
Fe Balance ' Preferred ranges for the above mentioned ele~ents are:
, % w/w Cr 13 - 24 Al 3,5 - 6.5 ~' , ' ' `' `

.

10783~4 .`
, . , . - % WtW
. _ ' C O - 0.1 Co 0.2 - 0.8 Ni o - 5 o Fe Balance I
: Typical quantities of trace elements which may be present in the ~: alloy to improve strength, oxidation-resistance and heat-resistance are:
% WIW
Si 0.2 - 0.9 Mn 0.2 - 0.7 Zr 0.01 - 0.20 ~ Cu 0.01 - 0.15 :~ Nb 0.01 - 0.3 Ta 0.8 - 1.2 Ti 0.8 - 1.2 Co 0.01 - 1.0 : Ca 0.01 - 0.5 - C 0.01 - 0.1 Specific heat-and-oxidation-resistant alloys suitable for use in carrying out the present invention are given in the following examples:

Kanthal D (Registered Trate Mark): a proprietary product supplied by the Kanthal Company of Sweden, and having the composition:

.

-: :

.

.. :: -. - . :,. ,: , :: : . . ... :: : .. : ;, ...

\\

, , % W/W
C o.~s -. Cr 22,60 Co 2.0 . .
Al 4 5 Fe Balance Megapyr (Registered Trade Mark): supplied by Telcon Metals Ltd.
and having the composition:
% W/W
.
Cr 30 Fe 65 Al 5 Stainless Steel 316 (one of the standard range of austenitic stainless steels):
:`
% W/W
.
C 0.08 .
Cr 18.00 `20 Ni lO.00 Ti 0 40 Fe Balance : EXAMPLE 4 The following range of alloys is supplied by the Kanthal Company of Sweden:

. . . . , . ,, .. : ... ~ ::
., : . . .. ~
:. . : .: : :
:: : . : .: :: , , ~: : . :: ,.: : ,, i :
% w/w Cr 10 - 40 Al 1 - 9 ; Fe 51 - 88.9 Ta 0.1- 5 Co 0 . 1- 1 In the above composition Ta may be partially replaced by Ti, and Ce may be partially replaced by Ca.
Optionally the following constituents may be present to impDOve the mechanical strength and oxidation resistance of the alloys:
% w/w Mn 0 - 2 Si - 0 - 2 :~ ~ 0'5 Nb 0 - 2 Mo 0 - 1.5 Th 0 - 0.2 Zr 0.01- 1, and P and A may be present as impurities.
Examples in thi~ range are as follows:
:` . ( 1 ) .96 . . .. .w./w ,~ Cr 20 - 30 Al 3.5 - 6 Ta 2,5 ~ Ce 0.15 - Fe Balance ~`

: 107836~ i Y
Cr 20 - 30 ~l 3 5 6 ~ .
~,. .I. 2 . 5 Ta . . 0.1 C~
Co 0.~ - 5 Fe ~3al ance Cr 3.5 - 1 Al 5 - . 9 . . . .
~b 1 . 5 ~r 0,15 - Ta o.i - 5 Fe i~alallce ( i v j 1~ W/~r Cr 10 - 15 hl 3 _ 5 5 . . - . . ' 1 . Th 0.1 Ta 0.1 - 5 .', ~,~ ' . 0.01 - 1 Fe i3alance .. ~ .......

, . , .

10783~;4 ~` Immaculate 5 tregistered Trade Mark): a proprietary product supplied by Firth Vickers Ltd.
% w/w ~ Cr 23.0 : - Ni 21.0 C 0.1 Fe Balance 10 Inconel 600 (registered Trade Mark): a proprietary product supplied by Henry Wiggin Ltd.
%W/W!
Ni not less than - 72.00 - . C not more than - 0.15 Mn not more than - 1.00 Fe 8.00 S not more than - 0.015 Si not more than - 0.50 Cu not more than - 0.50 Cr 15.50 ~ Inconel 601 ~registered Trade Mark) ,~ ~ % w/w ~, Ni 60.50 Cr 23.00 Fe 14.10 Al 1.35 ;

::

--~` 10783~4 Mn not more than - 0.50 Cu not more than ~ 0.25 Si not more than - 0.25 C not more than - 0.50 ;~
S not more than - 0.007 Inconley 800 (Registered Trade Mark); a proprietary product supplied by Henry Wiggin Ltd.
% W/W
Ni 32.50 C not more than - 0.10 Mn not more than - 1.50 S not more than - 0.15 Cu not more than - 0.75 Cr not more than - 21,00 ~:
Fe Balance : EXAMPLE 8 ., ~ Brightray S (Registered Trade Mark); a proprietary ., .~- product supplied by Henry Uiggin Ltd.
~ ~ W/W
- Cr 20.0 ::~ Fe 1.0 Si not ore than - 1.0 Mn not more than - 0.4 C not more than - O,1 Ni Balance ,~ .
-14_ 1~78364 , .

Stainless Steel No: 309 % _ W/W
Cr 22 - 24 Ni 12 - 15 C not more than - 0.2 Mn not more than - 2.0 - Si not re than - 1.0 Fe Balance Stainless Steel No: 310 (Supplied by the British Steel Corporation) W/W
Cr 24 - 26 (prefera-bly 25) Ni 10 -. 22 " ;~ ~
C not more than - - 0.25 n O.1 ~: Mn not more than - 2.0 " 1.8 Si not more than - 2.0 " 1.5 . Pe Balance Balance : EXAMPLE 11 _ Stainless Steel No: 321 ~ % _ W/W
Cr 17 - 19 Ni 9 - 12 C 0.8 Mn not more than - 2.0 Si not more than - 1.0 Ti not more than - 0.4 Fe Balance - , ,,. ~., .. .... :. :

EX~PLE 12 '~
Armco 18SR supplied by the Armco Corporation % _ _ _ w/w - Cr 18 Al 2 - Ti 0 4 si C 0.05 Fe Balance . ''' ' `' Generally speaking, in the Fe-Cr-Al system, there are very few ferritè steels having less than 12-12.5% by weight Cr which are satisfactory. Also the - aluminum content for alloys which function satisfactorily is rarely outside the range 1-6% by weight.
.
. . ~ .
The metal substrate used in the catalyst of aspects of the present invention is deformed so as to present a large surface area to the . , .
vapour phase reactants whose reaction is to be catalysed. This may be carried out, for example, by first producing the alloy in thin sheet form by rolling, corrugating by passage through fluted forming rolls, spirally winding the thin corrugated sheet so that a round sieve-like body is formed, the openings of which correspond to the depths of the grooves in the said corrugated sheet. The two sheets may be wound simultaneously, one of the , sheets being corrugated and the other plain, corrugated to a different con-figuration when compared with the first mentioned corrugated sheet or with the corrugations of the sheets having the same or a different corrugation configuration and with the corrugations in the respective sheets disposed at angles to each other. Where a flat and a corrugated sheet in strip form is used, the leading edges thereof are spot-welded together and strips of a certain length are fed into a device which rolls the strips between three sprung rollers to form a spirally wound collar-like or generally annular-shaped module as shownSin the accompanying single figure in the drawings.

- 1~78364 Extended metal substrates used in the catalyst of aspects of the present invention possess 1-1/2 - 3 times the surface to volume ratio that can be obtained with a ceramic honeycomb substrate. The normal substrate is 600 - 700 sq. ft. per cubic foot of substrate.
Examples are the products EX 20 (registered Trade Mark Corning) a ceramic honeycomb substrate which has a surface area of 576 sq. ft./ft ;
and Grace 400 (registered Trade Mark) which has a surface area of 780 sq. ft./ft.3 for 0.003" thick Kanthal D ~registered Trade Mark) sheet and 2000 sq.ft./ft.3 for 0.002 thick Kanthal D (registered Trade Mark) sheet.
It is preferred to use foil of thickness between 0.0015 and 0.0045 inch and more preferably of thickness 0.002 inch corrugated and assembled to form a structure having approximately ~00 cells per square inch when considered in cross section. A preferred range of cell sizes is 200 - 800 cells per square inch. Suitable surface to volume ratios are 1200 sq. ft. per cubic foot with 400 cells per sq. inch and 2000 sq.
ft. per cubic foot with 800 cells per square inch.
An alternative method of forming the module comprising spot-welding the two strips of foil to a stainléss steel tube of, for example, 20 1/~ inch outside diameter, which aids winding and module formation.
In yet another method, the two strips of foil to be simultane-ously wound are spot-welded together along their leading edges and then wound upon a hollow cone-shaped former, thereafter using friction rollers and slides in the cone, the strip is wound up and is forced into a retaining collar or shell in one operation.
Where a retaining collar or shell is not used, it is desirable to weld successive convolutions of the module by forming one or more radially extending runs of weld on the ends of the module. Other methods of securing successive convolutions or at least the outermost convolu-tions by stitching or tack-welding may be employed in order to prevent ' 't`' ~- 17 -.. : - : .- ~

`` -` 1~'783~

the spir~lly wound modules from~unwinding.
After forming the modules into finished units, they are pre~erably cleaned before the formation of the adherent oxygen containing film or coating, and this is preferably carried out by a vapour degreasing technique with a halogen containing solvent, e.g., trichloroethylene followed ..... .

-17a-~ 10783~
. by washing in acetone and drying.
- ~XAMPLE 13 In this example Gibbsite was deposited on thermally oxidised Kanthal D modules as shown in the accompanying single figure of the drawing . by contact with sodium aluminate solution containing excess aluminum and caustic soda at 80"C. After firing the ~7ashcoat ,'; ' .

.
- , .

, .

.
~ .
~ 18 ~
r 107836~
`~ was found to be almost pure gamma alumina. The washcoat was metallized with a 7-1/2% Rh/Pt alloy using known techniques to give a metal loading - of 30 g/ft of volume of the whole catalyst structure. Such structures have been found to have better poison resistance than washcoated ceramics and unwashcoated metallic substrates. This is demonstrated by results obtained from Qimulated load and phosphorous trials on catalyst structures with metal and ceramic substrates:
(a) Simulated lead poisoning trials The test exhaust gas contained:
.
Hydrocarbons 1000 ppm CO 2.5%
~:
NOX 1000 ppm Pb 0,5 gm/hr balance N2 Washcoat : Gamma alumina Metallized with : 7-1/2% Rh/Pt :"
-~ Substrate Washcoat Metal Hydrocarbon conversion efficiency load~ng load~ng after ~ g/in g/ft 1 hour 3 hours 7 hours ::
Kanthal D 3.17 30 79% 69% 52%
(registered Trade Mark) :
~anthal D 1.48 30 74% 63% 56%
(registered Trade Mark) Kanth?l D 0.7 30 67% 48% 36%
(registered Trade Mark) _ _ Ceramic Ex 20 2.0 30 56-75 40-54 27-39 ~registered Trade Mark) (Corning~
(range of results) ...

`'- ' ' '.:. :; ' . ' ' ,. : , ,: ' . ., .'. :; ' . "' '' -, '' ,. ; ' ;,'' '' ',, . ' ': ' ' ' . ' : ' ,: ' . ' '. , ''' '.; '' . ,, , ' , ' ' "

`10'7~3~

(b) Simulated phosphorous poisoning tr~ls Washcoat : Gamma alumina t Metallized with : 7-1/2~ RhtPt _ .
~-~ Substrate Washcoat ~Metal~. Hydrocarbon conversion efficiency - loading load~g~,after ~ g/in3 g/ft~ 1 hour 3 hours 7 hours . .
Kanthal D 2.5 30 98~ 97% 97%
; (registered Trade Mark) " .
Kanthal D 1.8 30 96.5~ 95.0% 93%
(registered Trade Mark~
Ceramic ~~ ~ - ~- -~ - ~- `
Ex 20 2.0 3~ 85% 76.5~ 59%
(registered Trade Mark) (Corning) . . _ . . . _ Instead of Pb, the test exhaust gas contained 1000 ppm in P in the form of 1~ tributylphosphate/toluene in air having a space velocity of 100,000 hr 1.
A washcoat loading is preferred which is within the range of 5 - 30~ by weight of the metallic monolith substrate.
A suitable loading of A1203 on Kanthal D (registered Trade Mark) having 400 cells per square inch is 10~ by wei~ht. The surface area of the alumina is 50 - 500 square meters per gram of alumina.
The aluminate method of deposition of alumina, described above, gives a surface area of from 120 - 160 square per gram of alumina.
An alternative preferred method for the deposition of an adherent alumina washcoat on the metallic substrate is to prepare a siurry of a pre-activated Gibbsite (alumina trihydrate) and an alumine monohydrate having a solid:liquid ratio of between 25 and 50% and a pH less than 7 and usi~ this to impregnate the shaped substrate by complete immersion. The exact strength of the slurry used twhich may be determined by trial and error) should be sufficient to produce an alumina 1(~7836~
washcoat of the required thickness. The substrate is then allowed to dry in warm air and finally fired for 2 hours at 450C. to form chi and gamma alumina in adherent coating up to 0.002 in. thick on the metallic substrate. Crystal aggregates of diameter 3 - 7 microns are produced having micropores of approximately the same size, i.e., 40A in diamet~r.
A further alternative method of deposition of an adherent alumina washcoat on the metallic substrate is to use a slurry of alpha alumina monohydrate. After firing at 450C. gamme alumina is formed having a surface area between 180 and 300 square meters per gram. Gamma alumina is added to alpha alumina monohydrate at the slurrying stage before firing in order to form a thixotropic mixture. Crystallite or crystal aggregates of 20 - 100A diameter are formed. Micropore diameters remain the same at 40A.
Suitable proprietary alumina trihydrates (Gibbsite) are "FRF
80" (registered Trade Mark) supplied by British Aluminium Chemicals Ltd.
and "C 333" (registered Trade Mark) supplied by Aluminum Company of America. Suitable alumina monohydrates (Boehmite) are "SOL-GE~ ALUMINA"
(registered Trade Mark) supplied by the United Kingdom Atomic Energy Authority. "DISPAL M" (registered Trade Mark~ supplied by Conoco and "CONDEA F" (registered Trade Mark) supplied by the Condea Group. Gibb-site is added to "SOL-GEL" (registered Trade Mark) alumina, which is microcrystalline Boehmite, at the slurrying stage in order to form a thixotropic mixture.
Optionally, one or more of the oxides titania, zirconia, hafnia and thoria may be present in the alumina for the purpose of pro-viding additional stabilization of the intermediate oxide (washcoat) layer as described in British Patent No. 1,401,022. If desired, rare earth oxides, alkaline earth oxides and alkali metal oxides may be used.
Many of the aluminum-containing metallic substrates used in ; 30 - 21 _ . :

: , ...

- : , . .

1(~78364 - - the catalyst of aspects of the present invention have the property of oxidizing "inwards". That is to say it is believed that a factor con-tributory to the success of the catalysts of aspects of the present invention is the fact that the extended metal substrate itself, which forms part of the catalytic structure of aspects of the present inven-- - tion has a tendency to oxidize under very strong oxidizing conditions in such a way that the first layer of adherent oxide film does not tend to grow over or cover the outermost layer of the alloy-containing platinum group metal, silver or gold.
Impregnation or deposition of the alloy of platinum group metals, silver and gold and optionally base metal upon the first oxygen containing adherent layer may be accomplished by known methods of deposition of catalytically active metals on washcoats or other supports, e.g. if a high surface area refractory metal oxide is the adherent oxygen containing film, the support may be immersed in a solution of water-soluble inorganic salt or salts of platinum group metal, e.g.
platinum, rhodium and nickel. In this case chloroplatinic acid, rhodium trichloride and nickel chloride would be used.
It is preferred that the composition of the outermost layer of catalystic metal consist of platinum group metals or alloys of the platinum group metals with each other and with base metals. Particularly preferred combinations are 7-1/2~ Rh/Pt for ~OX abatement, i.e., reduc-tion of oxides of nitrogen with a gaseous reducing fuel in nitric acid tail gas purification.
Particularly preferred alloys of platinum, rhodium and base ' '' ' ~ ' ~
~ 30 ... ..

~ 22 _ -: . . :. . . . :.. .:: ;:: . : ~ .

~1;)'~83~

metal which in the catalyst of aspects of the present invention are suitable in the outermost catalytic metal alloy layer of the catalyst structure are alloys containing:
w/w Rh 1 ~ 50 Base Metal 0.01 - 25 Pt Balance in which the base metal is selected from the group consisting of Al, Mg, Cr, Mo, W, Mn, Fe, Co, Ni, Ti, V, Th, U, Cu, Ag, Zn, Cd, Hg, In, Tl, Bi, Sn, Pb, the lanthanides and the actinides. This combinatinos is useful for the oxidation and reduction reactions and also in the steam reforming of naphtha and naphthadistillates.
The base metals mentioned above, however, are also useful in other platinum group metal ~lloy combinations and also, when employing catalytic structures according to aspects of the present invention in which the outermost catalytic metal layer contains silver or : .

.~-r ` or gold. Ruthenium is use ~ ~ ~O~ ~6 ~ecomposition of NOX in automobile exhaust purification.
EXA~'LE 14 A three inch wide piece of Kanthal D (registered Trade Mark) three-thousandths of an inch thick had fifty -thousandths corrugations formed in it by the method described above and was formed into a spiral - module. It was oxidized for one hour at 1200C. to form an alumina layer adherent upon the substrate. A 7-1/2% rhodium platinum alloy was then deposited upon the alunina and the catalytic structure was sub-~ected to oxidation trials using a space velocity of an automobile englne exhaust of 80,000 hours 1. The initial hydrocarbon conversion was 73%, after 100 hours it was 66%. The initial light off was 265C., after 100 hours it was 320C.
EXA~PLE 15 A corrugated spiral module formed as described above from a one inch piece of 321 stainless steel was calorized to form an alumanum - ; coating which was then oxidized for one hour at 1000C. and a loading of 25 grams per cubic foot of 7-1/2% rhodium platinum was placed on the oxide coated substrate. Using a space velocity of 240 thousand hours 1 the following results were obtained:
Initial hydrocarbon conversion was 72%, after 24 hours 77%;
initial light off 230C.; after 24 hours 290C.

A one inch piece of 316 stainless steel was calorized and oxidized for one hour at 1200C.~ using the same loading of rhodi~m -, platinum alloy and space velocity as described in Example 14 the following results were obtained:
'~ Initial hydrocarbon conversion was 76%, after initial light , off at 195C.; after 24 hours light off was at 290C.
. .~

, . . _ . _ . : . , ~ ' " , ,, ', , , , ' " ' .

~ 1078364 Using foil 0.002" thick made of (a) stainless steel 310SS
supplied by the British Steel Corporation and (b) Lanthal D (registered Trade Mark) supplied by the Kanthal Company of Sweden two cylindrical motules were made up 3" long, 2" in diameter having 800 cells per square inch (cross section). A high surface area alumina washcoat was deposited on both by the aluminate method described above and 40 gm/cu.
ft. platinum was deposited upon the alumina by known techniques.
Both units were then engine tested using low-lead fuel accor-ding to specification of the Environmental Protection Agency of theUnited States.

Hydrocarbon Carbon Monoxide Conversion_ Conversion Kanthal D (registered 90.8% 99.2%
Trade Mark) (100 hours) 310 SS 79,4% 90.3%
(200 hours~ -E ~YPLE 18 In this example a ceramic monolith is compared with a Kanthal D tregistered Trade Mark) module both of which have a high loading FRF
80 (registered Trade Mark) /Sol-Gel (registered Trade Mark) alumina washcoat with a 100% Pt catalytic layer and also included is a Kanthal (registered Trade Mark) module having a high loading hydrolytic alumina taluminate method) washcoating and also h~aving a 100% Pt :.~
catalytic layer.

.

`:

, .

-, - ~, , . . . .

Washcoat Dimensions Effective Effective ~ frontal vol.
- area ..... _ Ceramic FRF 80 (registered Trade 4" x 6" 11.1 sq.in.66.4 cu.in.
Mark)/Sol Gel (registered Trade Mark) Kanthal D " " " " " " 10.75 " " 64.7 " "
(registered Trade Mark) Kanthal D Hydrolytic " 11.4 " " 66.5 " i' ~registered (Aluminate) Trade Mark~
Results o The results are sum weighted for automobile engine operation under different conditions:
Hydrocarbon conversion Carbon monoxide conversion 0 hrs 100 hrs 200 hrs 300 hrs 0 hrs 100 hrs 200 hrs 300hrs -Ceramic +92.9 76.677.0 75.1 96.1 89.7 94.6 98.2 FRF 80(regis-tered Trade Mark/Sol Gel (registered Trade Mark) Kanthal D87.8 71.875.3 63.0 88.5 90.5 79.5 80.0 (regis~ered Trade Mark) +
FRF 80 (regis-j tered Trade Mark)/Sol Gel ;~ (registered Trade Mark) ., Kanthal D92.3 79.380.2 75.0 95.4 87.6 91.1 90.0 ~registered i Trade Mark) +
Hydrolytic ' alumina ,, ~
These results show good comparability of metal supports with ceramic substrates both as regards performance and stability.
.:

:.
~`~- 30 . .

; , ,', ; ' , , , ~ ~ ' .. , ,- .' . :: ', , '.,: ';; , ;, :`

Claims

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

1. A catalyst consisting essentially of:
(a) a metal substrate made from a heat and oxidation resistant alloy of iron including chromium (3-40) wt. %, aluminum (trace-10) wt. %, cobalt (O -5.0) wt. %, nickel (trace-72) wt. % and carbon (0-0.5) wt. %, balance essentially iron, said metal being in extended form;
(b) an aluminum oxide film on the surface of said extended metal substrate, said film being formed by oxidizing the surface of said metal substrate;
(c) a high surface area alumina coating disposed over said aluminum oxide film and keyed to said substrate through said film; and (d) a catalytic layer comprising at least one metal selected from the group consisting of ruthenium, rhodium, palladium, iridium, platinum, silver, gold, and alloys of at least one of said metals with at least one additional element comprising a base metal selected from the group consisting of aluminum, magnesium, chromium, molybdenum, tungsten, manganese, iron, rhenium, cobalt, titanium, vanadium, copper, zinc, cadmium, indium, thorium, bismuth, tin, lead, antimony, a lanthanide or an actinide, said base metal being present in an amount from a trace to 25 wt. %, on said high surface area alumina coating.

2. A catalyst consisting essentially of:
(a) a metal substrate having a high surface area and made from a heat and oxidation resistant alloy of iron including chromium (3-40) wt. %, aluminum (trace-10) wt. %, cobalt (0-5.0) wt. %, nickel (trace-72) wt. % and carbon (0-0.5) wt. %, balance essentially iron, said metal substrate comprising an assembly of at least two juxtaposed sheets made from said alloy with at least one sheet shaped so that when contacted with an adjacent sheet channels are formed therebetween for the passage of gaseous reactants through the substrate;
(b) an aluminum oxide film on the surface of said substrate, said film being formed by oxidizing the surface of said substrate;
(c) a layer of alumina applied to the said sheets and calcined; and (d) a catalytic layer applied to said first layer, said second layer consisting essentially of one or more metals selected from the group consisting of ruthenium, rhodium, palladium, iridium, platinum, silver, gold, and alloys of at least one of said metals with at least one additional base metal selected from the group consisting of aluminum, magnesium, chromium, molybdenum, tungsten, manganese, iron, rhenium, cobalt, titanium, vanadium, copper, zinc, cadmium, indium, thorium, bismuth, tin, lead, antimony, a lanthanide or an actinide, said base metal being present in an amount from a trace to 25 wt. %.

3. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy of iron including at least one of the elements chromium (3-40) wt. %, aluminum (1-10) wt. %, cogalt (trace-5.0) wt. %, nickel (trace-72) wt. %, and carbon (trace-0.5) wt. %.

4. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy in which not more than 10 wt. % of the alloy of iron consists of one or more of the addi-tional elements silicon, manganese, zirconium, copper, tungsten, vanadium, molybdenum, cerium, niobium, tantalum and titanium.

5. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy in which not more than 5 wt. % of the alloy of iron consists of one or more of the said additional elements.

6. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy in which not more than 3 wt. % of the alloy of iron consists of one or more of the said additional elements.

7. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy of iron which comprises, apart from impurities, at least one of the elements Cr (3 -30) wt. %, Al (1 - 10) wt. %, C (a trace to 0.5) wt. %, Co (a trace to 5.0) wt. T, Ni (a trace to 15.0) wt. % and balance iron.

8. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy of iron which comprises, apart from impurities, at least one of the elements Cr (13 -24) wt. %, Al (3.5 - 6.5) wt. %, C (a trace - 0.1) wt. %, Co (0.2 - 0.8) wt. %, Ni (a trace - 5.0) wt. %, balance iron.

9. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy which includes one or more of the additional elements: Si (0.2 - 0.9) wt. %, Mn (0.2 - 0.7) wt. %, Zr (0.01 - 0.20) wt. %, Cu (0.01 - 0.15) wt. %, Nb (0.01 - 0.3) wt. %, Ta (0.8 - 1.2) wt. %, Ti (0.8 - 1.2) wt. %, Co (0.01 - 1.0) wt. %, Ca (0.01 - 0.5) wt. %, C (0.01 - 0.1) wt. %.

10. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy of iron which comprises 0.09 wt. % C, 22.60 wt. % Cr, 2.0 wt. % Co, 4.5 wt. % Al and balance iron.

11. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy of iron which comprises Cr 30 wt. %, Fe 65 wt. % and al 5 wt. %.

12. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy of iron which comprises C 0.08 wt. %, Cr 18.00 wt. %, Ni 10.00 wt. %, Ti 0.40 wt. %
and balance iron.

13. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy or iron which comprises Cr (10 - 40) wt. %, Al (1 - 9) wt. %, Fe (51 - 88.9) wt. %, Ta (0.1 - 5) wt. %, Ce (0.01 - 1) wt. %.

14. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy of iron which comprises Cr (10 - 40) wt. %, Al (1 - 9) wt. %, Fe (51 - 88.9) wt. %, Ta (0.1 - 5) wt. %, Ce (0.01 - 1) wt. % and wherein Ce is partially replaced by Ca.

15. The catalyst of claims 1 or 2 wherein said catalytic layer comprises a 7-1/2 wt. % Rh/Pt alloy.

16. The catalyst of claims 1 or 2 wherein said catalytic layer comprises a 10 wt. % Rh/Pt alloy.

17. The catalyst of claims 1 or 2 wherein said catalytic layer comprises a 20 - 50 wt. % Rh/Pt alloy.

18. The catalyst of claims 1 or 2 wherein said catalytic layer comprises a 1 - 50 wt. % rhodium, 0.01 - 25 wt. % base metal, and balance platinum.

19. The catalyst of claims 1 or 2 wherein said layer of alumina comprises calcined crystal aggregates having a diameter of 20 - 100.ANG. and a pore diameter of 40.ANG..

20. The catalyst of claim 2 wherein the said juxtaposed sheets are helically wound together to form a metallic monolith.

21. The catalyst of claim 2 wherein one said sheet is corru-gated and another, adjacent sheet is plain.

22. The catalyst of claim 2 in which said two separate thin metal sheets have a thickness within the range 0.0015 to 0.0045 inches.

23. The catalyst of claim 22 in which said sheets are corru-gated of such dimensions that the wound up structure has between 200 and 800 cells per square inch when considered in cross section.

24. The catalyst of claim 23 which has a substrate composed of metal sheets of approximate thickness 0.002 inches assembled to form a structure having approximately 400 cells per square inch when con-sidered in cross section and a surface to volume ratio of approximately 1200 sq. ft. per cubic foot.

25. A method for making a catalyst consisting essentially of a substrate having a high surface area and made from a heat and oxida-tion resistant alloy of iron including chromium (3 - 40) wt. %, aluminum (trace - 10) wt. %, cobalt (0 - 5.0) wt. %, nickel (trace - 72) wt. %, and carbon (0 - 0.5) wt. %, balance essentially iron, the substrate com-prising at least two juxtaposed sheets made from said alloy with at least one sheet shaped so that when contacted with an adjacent sheet channels are formed therebetween for the passage of gaseous reactants through the substrate, such method comprising the steps of:
(a) treating said substrate assembly to oxidize its surface, thereby to provide an aluminum oxide film thereon;
(b) applying a first layer containing alumina to the said sheets;
(c) calcining the said sheets of (b) so that said first layer is bonded to the underlying said sheet and serves to bond adjacent and contacting said sheets together; and (d) applying a second and catalytic layer to said first layer, said second layer consisting of one or more metals selected from the group consisting of ruthenium, rhodium, palladium, iridium, platinum, silver, gold, and alloys of at least one of said metal with at least one additional metal comprising a base metal selected from the group consis-ting of aluminum, magnesium, chromium, molybdenum, tungsten, manganese, iron, rhenium, cobalt, titanium, vanadium, copper, zinc, cadmium, indium, thorium, bismuth, tin, lead, antimony, a lanthanide or an actinide, said base metal being present in an amount from a trace to 25 wt. %.

26. The method of claim 25 wherein the said juxtaposed sheets are helically wound together to form a metallic monolith.

27. The method of claim 25 wherein one sheet is corrugated and another, adjacent sheet is plain.

28. The method of claim 25 in which the two separate thin metal sheets have a thickness within the range of 0.0015 to 0.0045 inches.

29. The method of claim 28 in which said sheets are corrugated and are of such dimensions that the wound up structure has between 200 and 800 cells per square inch when considered in cross section.

30. The method of claim 29 which has a substrate composed of metal sheets of approximate thickness 0.002 inches assembled to form a structure having approximately 400 cells per square inch when considered in cross section and a surface to volume ratio of approximately 1200 sq. ft. per cubic foot.

31. The method of claim 25 wherein said alumina is calcined to form chi and gamma alumina.

32. The catalyst of claims 1 or 2 wherein said metal substrate (a) is made from a heat and oxidation resistant alloy contain-ing nickel and/or chromium having an aggregate nickel and/or chromium content of greater than 20 wt. %.

33. The catalyst of claim 32 including one or more of the additional elements Si (0.2 - 0.9) wt. %, Mn (0.2 - 0.7) wt. %, Zr (0.01 - 0.20) wt. %, Cu (0.01 - 0.15) wt. %, Nb (0.01 - 0.3) wt. %, Ta (0.8 - 1.2) wt. %, Ti (0.8 - 1.2) wt. %, Co (0.01 - 1.0) wt. %, Ca (0.01 - 0.5) wt. %, C (0.01 - 0.1) wt. %.

34. The catalyst of claims 32 or 33 wherein said alloy of nickel and chromium comprises not less than 72 wt. %, not more than 0.15 wt. % carbon, not more than 1.00 wt. % manganese, 8.00% iron, not more than 0.015 wt. % sulphur, not more than 0.50 wt. % silicon, not more than 0.50 wt. % copper and 15.50 wt. % chromium.

35. The catalyst of claims 32 or 33 wherein said alloy of nickel and chromium comprises 60.50 wt. % nickel, 23 wt. % chromium, 14.10 wt. % iron, 1.35 wt. % aluminum, not more than 0.50 wt. % manganese, not more than 0.25 wt. % copper, not more than 0.25 wt. % silicon, not more than 0.50 wt. % carbon and not more than 0.007 wt. % sulphur.

36. The catalyst of claims 32 or 33 wherein said alloy of nickel and chromium comprises 32.50 wt. % nickel, not more than 21 wt. %
chromium, not re than 44 wt. % iron, not more than 0.10 wt. % carbon, not more than 1.50 wt. % manganese, not more than 0.15 wt.% sulphur, and not more than 0.75 wt. % copper.

37. The catalyst of claims 32 or 33 wherein said alloy of nickel and chromium comprises 20.0 wt. % chromium, not more than 77.5 wt. % nickel, 1.0 wt. % iron, not more than 1.0 wt. % silicon, 0.4 wt. %
manganese, and not more than 0.1 wt. % carbon.

38. A catalyst consisting essentially of (a) a substrate comprising an assembly of at least two juxtaposed sheets made from an alloy with one sheet being corrugated and the other sheet being plain, so that channels are formed therebetween for the passage of gaseous reactants through the substrate, the juxta-posed sheets being helically wound together to form a metallic monolith, the alloy having the composition,in weight percent, carbon (0.09);
chromium (22.60); cobalt (2.0); aluminum (4.5); and iron, balance;
(b) an aluminum oxide film on the surface thereof, being produced by thermal oxidation;
(c) a catalytic layer washcoat of alpha aluminum trihydrate formed by contacting the surface thereof with sodium aluminate solution containing excess aluminum and caustic soda at 80°C. and then firing, the catalytic layer being metallized with a 7-1/2% Rh/Pt alloy to give a metal loading of 30 g/ft3.

39. The catalyst of claim 38 wherein the washcoat loading is 3.17 g/in3.

40. The catalyst of claim 38 wherein the washcoat loading is 1.48 g/in3.

41. The catalyst of claim 38 wherein the washcoat loading is 0.7 g/in3.

42. The catalyst of claims 1 or 2 which has been formed by impregnating the metal substrate wit,h a slurry of a pre-activated alumina trihydrate and an alumina monohydrate having a solid:liquid ratio between 25 and 50% and a pH less than 7, drying the substrate in warm air and firing at 450°C. for 2 hours to form chi and gamma alumina in an adherent coatlng of up to 0.002 in. thick on the metal substrate.

43. The catalyst of claims 1 or 2 which has been formed by impregnating the metal substrate with a slurry of a pre-activated alumina trihydrate and an alumina monohydrate having a solid:liquid ratio between 25 and 50% and a pH less than 7, drying the substrate in warm air and firing at 450°C. for 2 hours to form chi and gamma alumina in an adherent coating of up to 0.002 in. thick on the metal substrate, inclu-din8 a loaded washcoat by the step of immersing the support in a solu-tion of a water-soluble inorganic salt or salts of the platinum group metals.

44. The catalyst of claims 1 or 2 which has been formed by impregnating the metal substrate with a slurry of a pre-activated alumina trihydrate and an alumina monohydrate having a solid:liquid ratio between 25 and 50% and a pH less than 7, drying the substrate in warm air and firing at 450°C. for 2 hours to form chl and gamma alumina in an adherent coating of up to 0.002 in. thick on the metal substrate, inclu-ding a loaded washcoat by the step of immersing the support in a solu-tion of a water-soluble inorganic salt or salts of the platinum group metals , wherein said solution is of chloroplatinic acid, rhodium tri-chloride and nickel chloride.

45. The catalyst of claims 1 or 2 which has been formed by impregnating the metal substrate with a slurry of a pre-activated alumina trihydrate and an alumina monohydrate having a solid:liquid ratio between 25 and 50% and a pH less than 7, drying the substrate in warm air and fir-ing at 450°C. for 2 hours to form chi and gamma alumina in an adherent coating of up to 0.002 in, thick on the metal substrate, including a loaded washcoat by the step of immersing the support in a solution of a water-soluble inorganic salt or salts of the platinum group metals, wherein said loading is with 7-1/2% Rh/Pt.

46. A process for using the catalyst of claims 1 or 2 which comr prises one of the steps of:
(a) catalytically oxidizing nitric acid and ammonia in the presence of the catalyst:
or (b) chemically reducing an oxide of nitrogen with a gaseous reducing fuel in the presence of the catalyst.