Method of reducing pyrophoricity of catalytic metals produced by diffusion coating

CATALYTIC METAL

II The present invention relates to the diffusion coating .

of metals .

1' Among the objects of the present invention is the j provision of novel coating techniques and compositions for use ¹ therewith, as well as novel coated products, all suitable for , commercial operations.

!.’ When a nickel surface of high catalytic activity is ' desired, the nickel is best subjected to diffusion aluminizing ן in a plurality of stages before the leaching. In the first diffu; sion stage the activity of the inwardly diffusing aluminum is kept low, as by using an aluminum diffusion pack containing 10 • to 40% aluminum, the balance alumina, with a 0.3/0 AICI3 energizer ¹ added. A diffusion treatment of 1050 to 1100°F for 20 to 30 hours with such a pack causes the formation on the nickel surface of an aluminized case in which essentially all its aluminum is in the form of N12AI3 intermetallic.

In a subsequent diffusion aluminizing stage the outer portion of the aluminized case is modified to convert the N12AI3 ! to a higher aluminide of nickel. The aluminizing pack of the ! i first stage can be used in a second stage conducted at a lower ί temperature, as for example from about 880 to about 920 F, for i 20 to 30 hours as an effective technique for converting the

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aluminide in the outermost portion of the case to higher j aluminide.

These two diffusion steps are the only ones needed, and nickel so treated and then leached with 5 to 50% aqueous caustic soda has an exceptionally good catalytic action in !hydrogenation reactions such as the hydrogenation of benzene or phenol or coal as in coal gasification. This double diffusion ;'is illustrated in the following example. I

EXAMPLE

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ί A six-foot square section of nickel screening is solvent degreased, placed in a previously aluminized steel retort con-ן taining a mix of 20% Al (40 microns) and 80% AI2O3 (325 mesh)ן 'with 1/2% AICI3 distributed throughout the mix. Diffusion coating Jis effected in H2 at 1050-1100°F for 25 hours to provide a 0.006 j Ij !¹,inch case of primarily N12AI3. The diffusion coating step is .| !then repeated using the same pack mixture as above but this time j !:at a temperature of 850 to 900°F for 30 hours to provide a pre¹dominately NiA13 1 mil thick coating on top of the aforementioned !

^! Ni2A13 coating. The resulting screen is washed, dried, and store^ ^! or leached. Treatment with 20% NaOH at room temperature primarily ! leaches out the Al from the NiA13 layer with minor effect on the 1; !

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N12AI3 layer. When the leaching action has subsided, the screen !is washed and immediately placed in a liquid such as cyclohexane | to prevent surface oxidation until ready for use. Poorly volatile :liquids such as glycerol, ethylene glycol, kerosene or triethanol-, ן amine, or congealable liquids such as melted wax can be used to j !protect the active surface while the screen is installed in the hydrogenating equipment and can then be washed off. Ihe double diffusion can

82׳2-9ז also be effected on sheet, foil or tubes, as well as on nickel that is plated on a support such as steel, with equally good results. In general at least about 1 mil of Ni2Al׳j case should ;

i be present in the leached product below the activated surface ;

layer left by the leaching. Also that activated surface layer ; should be at least about 0.5 mil deep.

| The second aluminizing stage can, if desired, be ! effected by merely extending the time during which the nickel ^! being treated cools down after the completion of the first stage. ! Thus the furnace heat can be turned on again during the initial i i cool-down.when the retort temperature reaches about 925°F, and

I i 1' can then hold the desired second stage temperature.

! ’ ; The improved activity resulting from the foregoing dual

I! diffusion is not noticeable when preparing activated platinum j by diffusing aluminum into it and then leaching. However acti!' vated.platinum prepared in this way is more stable and can be !! ^; exposed to air without loss of activity. Indeed when such acti- ! l. ;

I^I vated platinum is used as an igniter for hydrogen, a job it does | >' I i well at and even below room temperature, it is preferably kept !

: exposed to air when not doing any igniting. It can also be ! advantageously arranged to be held in a stream of a mixture'of air and hydrogen as that stream flows to a hydrogen-burning jet, I and in a location relatively close to but upstream from that jet, so that the active platinum assures the ignition of the stream yet remains out of the flame burning at the jet. In such arrangement the dual diffusion of the aluminum is not needed.

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Catalytically activating the surface of a metal foam, such as nickel foam described in U.S. Patent 3,111,396 is particularly desirable inasmuch as such a foam presents a very large (metal surface and does not have the fragility of thin foil or screening. The diffusion-coating treatment generally introduces into the surface of the metal being activated a relatively high concentration of an embrittling metal such as aluminum, and fragile !shapes such as foil and thin screening tend to break unless very [¹carefully handled during this treatment.

The catalytic activation of platinum and other metals i.

!«of the platinum family is preferably applied to very thin sub:strates in the interest of reducing the amount of such expensive Ί metals needed, and here also a metal foam shape as described in U. S. Patent 3,111,396 is desirable.

ji To minimize having thin nickel foil rendered too 'fragile the diffusion step can be carried out at minimum temperatures to reduce the depth of the diffusion case. Thus a 0.1 to 0.5 mil case can be provided on a nickel foil 1 to 2 mils thick by aluminizing at a temperature between 650 and 800 °F. The foil is increased in thickness by the aluminizing, so that a heavy aluminizing of a one-mil thick foil can yield a product 3 or more mils thick with each face having an aluminized case and both cases together occupying about 2.6 mils of that thickness.

ן The fragility of thin foils when diffusion coated, can 'also be offset by having the metal of the foil in the form of a .1 surface layer securely held on a carrier foil or sheet made of metal that does not have the objectionable fragility and can be subsequently separated from that surface layer. Thus a 5 mil j'thick aluminum sheet can have electroplated on one face a one mil !!thick layer of nickel or cobalt, using a preliminary flash coating jof chromium or zinc to get the nickel or cobalt to adherently !deposit. The final coated aluminum sheet is then subject to a 1120-hour diffusion treatment at 750 to 850 °F in a hydrogen-bathed i .atmosphere to cause the aluminum to diffuse most of the way through the nickel or cobalt layer, after which the resulting product is subjected to caustic leaching to dissolve the aluminum. This leaves a catalytically active nickel or cobalt foil that contains a residual small content of diffused-in aluminum and holds itself together adequately for normal handling. Although ipyrophoric, its catalytic activity is not as high as is available from a pack diffusion process without the aluminum carrier sheet, but it does a good job of catalyzing hydrogenation and other chemical reactions including ammonia oxidation and HCN formation. Of particular interest, a Ni or Co plating formed by electroplating in the above manner has a particularly low density thus limiting the amount of mass needed per unit volume of catalyst.

Both faces of the aluminum sheet can be given nickel or cobalt platings in which case the corners or edges of the plated sheet can be trimmed off to expose the aluminum substrate for leaching.

i A very effective zinc flash for the foregoing plating ;sequence is obtained by a 1/2 to 1 minute dip of the cleaned 'aluminum sheet at room temperature in the following solution:

NaOH 525 grams ; ZnO 100 grams

I ί FeCl₃H₂O 1 gram j NaKC4H₄O₆H₂O 10 grams ! Water to make one liter i i

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Platinum and other catalytic metals can similarly be converted to catalytic foil» and other Teachable metals Hke zinc can be used as the supporting foil 1n place of aluminium. The supporting layer can also be made by spraying molten aluminum or zinc on a surface of the foil that 1s to be activated.

Palladium 1s best catalytically activated with zinc. A single diffusion of about 1 to 30 milligrams of zinc per square centimeter of palladium surface» followed by leaching with strong, at least 10%, HC1 1n water at 95°C, produces from a 1 mil thick foil a very effective catalyst. A typical pack diffusion treatment for palladium 1s the embedding of a 52-mesh palladium screen the wires of which ate 4 mils thick, 1n a pack of 20% powdered zinc and 80% alumina, energized with 1/2% NH^Cl, and heating for 4 hours at 600°F 1n a hydrogen-bathed atmosphere. This effects a zinc pickup of 4 mg/cm^, essentially all of which 1s leached out by 20% HC1 1n 10 minutes. The resulting screen 1s pyrophoric when exposed to air.

The pack diffusion to activate the catalyst can be effected In one step or 1n multiple diffusion steps under different diffusion conditions to vary the distribution of the d1ffused-1n metal. Regardless of how the diffusion 1s carried out, the catalytic activity of the leached product can be entirely blocked by Immersing the activated surface 1n an aqueous solution of a chromate such as an alkali metal, magnesium or ammonium chromate or chronic acid. A concentration of about 17. chromate ion is all that is needed to inhibit essentially all the cata- j lytic activity of nickel or platinum for example, after only a ; few secondscontact. Smaller concentrations, as low ai 0.17. '!

chromate ion, will also be effective if kept in contact for longer times.

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A catalyst so inhibited can be conveniently stored or shipped without being immersed in a protective liquid. Moreover the catalyst can subsequently be reactivated by merely treating it for about 2 minutes with 57. aqueous NaOH, preferably at 90°C.|

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Rinsing away this treating liquid will then leave the catalyst j with almost 811 of its original activity. j

The catalytic activity withstands intense evacuation, i although the activated surface gives off large quantities of | hydrogen under the influence of the lowered pressure. i

Activated nickel surfaces can be modified by dipping I for a. few seconds in aqueous hydrogen peroxide. A three volume percent H2O2 solution modifies the surface and turns it black so that it is no longer pyrophoric. The modified surface is however still catalytic and׳will for example show a strong decrease in overvoltage when used as a .cathode or anode in electrolyzing ; aqueous NaOH to generate gas. The activation of platinum or its alloys can be similarly modified by Immersion in aqueous H₂O₂ or in aqueous HC1 at least 207. strong.

A pyrophoric activated surface will retain’ its pyro- j phoricity for some time if kept covered by an inert fluid -- even

12.9.82 argon gas - or by a dried film of caustic soda or sodium silicate. Such drying is best conducted In an inert atmosphere.

The preserving effect of a wet or dried film of caustic soda 1$ greatly Improved when a little tin is dissolved in it, is described in p_atent 4,435,48! . Httle as 4 grams of SnClg-ZHgO, or about 2 grams of tin, in 500 cc of aqueous caustic, and preferably twice that much, is desirable.

Inert fluids used to cover and preserve the pyrophoric surface are those that are inert to active hydrogen and nickel. Thus saturated hydrocarbon liquids like n-heptane or any of the nonane isomers make very effective preserving films, and if they are kept from evaporating off as by sealing such a filmed pyrophoric nickel in an argon atmosphere, will preserve the pyro66770/2

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<td> 12.9.82</td><td> phoricity for months. Methyl ethyl ketone can also be used this way as a preserving film Inasmuch as it is sufficiently inert, but glycerol is unusually effective. Readily vaporizable preserving films, such as those having boiling points below about 125®C also have the advantage of permitting pyrophoric action to develop by merely exposing the filmed surface to the open air so that the film evaporates within 1 or 2 minutes. The presence of a liquid saturated hydrocarbon film preserves the pyrophoricity better than the mere sealing in an inert gas without such film. Preservation of pyrophoricity can also be accomplished by maintaining a blanket or atmosphere of hydrogen around the activated metal. For the production of large quantities of foil or ribbon, it is convenient to start with a coil of suitable material available in the market place. Thus a coil of 1 to 1.5 | mil thick nickel or iron foil about six inches wide can be prepared for diffusion coating by unwinding it, passing the foil as 1t unwinds through a cleaning station where it can be electrolytlcally cleaned as a bipolar electrode in aqueous caustic, or abrasively cleaned on both sides by an air-propelled stream of fine glass grit, and coiling up the cleaned foil with one or more</td>

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spacers that keep the adjacent coil turns spaced from each other.

A thin layer of powdered diffusion coating pack makes an effective spacer. .Only about 1/8 inch spacing is needed between turns, but the spacing can be as great as 1/4 inch if desired.

The powdered spacing layer can be retained in place as : ףby winding a continuous length of a twisted pair of wires into I the recoiling so that the twisted wires extend along an edge of l. :

the foil and keep the powder from spilling out that edge, yet

H provides an adequate number of small gaps with respect to the

I; foil surface it engages, to permit the diffusion coating atmos- I ! phere to penetrate. Two separate lengths of twisted wires can ׳ ! be used, one length at each edge of the foil. j j! The wires can be made of the same metal as the foil, or׳ ! they can be made of aluminum or of plastic that is decomposed | { j : and driven off during the diffusion coating treatment. Acrylic 1 ; plastics are particularly suitable for this purpose. Such twisted wires can be wound into the coil as spacers without the ׳ layer of diffusion coating pack, and the pack then vibrated into the spaces by standing the space-wound coil by its edge on a

I' י !' plate, covering its top with powdered diffusion coating pack, 1 and subjecting the plate to vibration to cause the powder to work its way into the spaces between turns.

Alternatively the spacing layer of diffusion coating ¹ pack can be applied without supplemental devices for keeping the ׳ layer from spilling out prematurely. To this end the layer can be applied as a wet but non-flowing paste mixture or gel extruded i

onto the foil. A small content, such as 1/2% of a thixotropic polymer such as polyacrylic acid or mannan gum, added as a stirred, solution in a readily vaporized solvent such as methyl ethyl ketone, to a standard diffusion coating mixture, does a good job

I of producing an extrudable gelled mixture.

The rewound foil with its spaced turns can then be packed in additional powder pack in a diffusion coating retort

1! and subjected to the diffusion coating treatment. The rewound j

J; mass can be kept from unwinding during handling, as by wrapping |i

P lengths of the twisted wire or plastic around it, and securing

1’ ij the ends of these lengths together.

j' After the diffusion coating is completed, the treated ;

li. ' coil is removed from the retort and the pack particles can be!

I,‘ removed from between the turns. Because the foil is generallyן

Ij ΐί very brittle at this stage of the treatment, care is needed to j ! keep from damaging it. The coil can have its outer wrap removed, j :I stood‘by its side edge on a screen, and subjected to vibration to vibrate the pack particles down from between the coil turns | ן,I and through the screen. Without uncoiling, the coil is then I lowered into a leaching bath, such as 10 to 20% aqueous NaOH, to j dissolve out the diffused-in metal.,

When the leaching is completed the foil is no longer ; j objectionably brittle, and it can be manipulated even thotlh as much as 1/3 of the diffused-in metal has not been leached out. ן I ' However it must be protected against exposure to air or oxygen, j j until ready for use. The coiled foil can for example be kept wet ־ j! with a layer of nonane or triethanolamine or methylethylketone j |! or acetone or glycerine and then slit and chopped to reduce it j h to individual rectangular foils of a convenient size such asj |j 1/2 by 1 inch. A quantity of such individual foils or similarly ! prepared discs can then be packed in a container for pyrophoric!

׳ use elsewhere, as discussed above in connection with the flares.

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The nonane prevents premature pyrophoric action, but when a very thin layer is exposed to air it will evaporate away i after about 1/4 minute so that such action will then commence.

Foils that are aluminized on both faces tend to be les! brittle than foils having twice as much aluminizing on one face. Thus an iron or nickel foil originally one mil thick can be 31 urn־}ηΐzed to yield 0.7 mil thick aluminized cases on each surface with about 0.6 mil unaluminized case, and will withstand i' flexing much better than a corresponding foil aluminized on only one face to form a case 7/10 the final thickness.

i Instead of slitting the final coil completely apart, '1 the coil can be run through a perforating machine that cuts interrupted slits through it, longitudinally as well as transversely, leaving only a few unperforated points holding the ' individual small foils together. The thus-perforated material can then be rewound into a tight coil and packed in a container. Such a container can be fitted with an explosive charge as noted above, so that the coil can be expelled when desired, and the charge can be sufficiently strong to tear the coil into its individual perforated pieces.

The perforating treatment can be applied before the diffusion coating, instead of after. Regardless of the manner of preparation, the leached foil can be coated with picric acid, ji _t ׳' sodium picrate, triethanolamine or other material that increases ii u .

.ן its pyrophoric action, i!

!׳ The foil need not be a pure metal, but can be an alloy such as an alloy containing up to 4% chromium or up to 40% copper, and/or can have a laminated construction. An iron foil can for example be plated with a very thin layer, about 0.1 to i,

I! 0.5 mil thick, of nickel on one or both surfaces. On the other hand, a suitable foil can be made of low alloy steels or alloys of iron and nickel containing 1% to 99% of either metal. A little carbon dispersed in or alloyed with the metal, as little as 0.1 to 17־ or as much as 57־ is helpful, as is a small content of other ingredients such as magnesium that are easily ignited.

The pyrophoric metal web can be in the form of a screen. Thus, a 100 mesh screen of 10 mil thick nickel wire can have 38 milligrams of aluminum diffused into it per square centimeter of wire surface, which after 1 to 1 1/2 hours leaching with boiling 20% UaOH in water, becomes highly pyrophoric and merely left to dry will undergo a violent explosion.

A readily ignited metal such as titanium or magnesium in foil or wire from can also be given a nickel coating, as by rolling, and the nickel coating then rendered pyrophoric. To ignite the ignitable metal the coating should be at least about 0.4 mil thick.

The pyrophoric metals, regardless of how they are produced, can be plated as by electrolytic coatings from alkaline cyanide baths? of zinc, cadmium, silver, tin or copper, without significantly diminishing their pyrophoricity. Such coatings that are over about 0.2 mil thick will significantly reduce the pyrophoric action.

Pl at-ΐmim activated by the aluminum diffusion and leaching, still contains a significant amount of the aluminum. Even when heated to high temperatures, such as 1800 to 2300°F for days or weeks, that residual aluminum remains in the outermost 0.3 mil or so of the metal. This platinum wire so treated shows greater useful life at very high temperatures, e.g. 1800°F or higher. The increased life appears to be a characteristic of platinum that is alloyed with from about 0 to about 5% aluminum

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I ׳ i land has been heated to from 1800 to 2300°F for at least a week. This improvement is most significant for very fragile wires such j as those not over about 10 mils thick. Thus wires less than 4 mils thick when used as catalytic screens in gaseous reactions, are subjected to vibratory flexure by the flow of reacting gases, | !as well as attack by naturally occurring catalyst poisons swept along by those gases. The aluminum-containing platinum stands up much better in such service than aluminum-free platinum.

Platinum freshly activated by aluminum diffusion and ! !leaching, has a characteristic fissured surface illustrated in j U.S.Patent 4,154,705 and it would not be expected that longer mechanical life could be obtained with such a surface. However ^!after days of service at extremely high temperatures, the fissures become less pronounced, but the wire remains rough.

I: The presence of up to about 15% of other platinum i metals alloyed with the platinum and the aluminum, does not i; I significantly reduce the life-lengthening effect of the aluminum.

!’ Thus a platinum-rhodium alloy screen made of 3 mil thick wire i ^! containing 10% rhodium, that is further alloyed with 2% aluminum, :

L 1 also shows a markedly long service life at 1800 F and higher. j Similar results are obtained when iridium is substituted for some or all of the rhodium.

h Instead of introducing the aluminum into the platinum . I ’by diffusion, it can more inexpensively be introduced by melt !alloying, for example at the same time rhodium is alloyed with : !the platinum. Wire drawn from the resulting alloy does not have !the very high catalytic activity of platinum activated by surface !diffusion and leaching as described above, but it is still

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!strongly catalytic and it also has the long service life.

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When the aluminum is introduced into a platinum screen '!

I by low-temperature diffusion, that is diffusion below about 900°F; the aluminum introduction is generally very slight or completely ' j absent at locations such as wire crossover points where one wire ; touches another and at least partially blocks the diffusing action.

':Although little or no aluminum is located at those points, the : resulting screen performs even better than one which has been ' I ' :

!; subjected to heavy aluminizing that reaches 1007. of its surface, j !; . . . ¹A similar improvement can be obtained on heavily aluminized screens 'by following the caustic leach with an acid treatment, HC1 for !i r ו i (example, that leaches out extra amounts of aluminum. | i

The improved service life can be obtained when the : aluminum-carrying platinum is subjected to the preliminary heat ן : treatment in air, in a vacuum, in nitrogen, hydrogen or in ¹ ' catalytically reacting gases. It appears that 1800 to 2300°F temperature for the minimum of one week is the key to long life.

Pyrophorically activated nickel and iron also retain , i some of the aluminum or zinc or the like used to activate them, 1 I even though these activating metals are leached out with strong ן hot aqueous caustic for 12 hours or more. When such leached pyrophoric products are dipped in 1 to 5% aqueous H₂O₂ for about : 1/4 to about 1 hour, their pyrophoricity is sharply reduced or entirely eliminated. Notwithstanding such reduction the products ΐ’ί are still strongly catalytic. A nickel screen having forty 10 mil wires per inch, and diffusion aluminized at 950 F for 16 ׳ ' hours followed by 12 hours of leaching in 207. aqueous NaOH at ! 180° ׳F with a final 30 minute dip in 27. aqueous H₂0₂, shows a very l. ¹ l^! low overvoltage as a cathode in electrolyzing water containing !1 i a little dissolved KOH. Although the freshly leached screen is ! ' ' not pyrophoric, after two to four weeks electrolyzing the water, ¹ i i it tends to become pyr<jfl|poric. At this point a three-hour dip j in 160° F tap water will eliminate the pyrophoricity. >

Although the screen pyrophoricity tends to increase while I it is used as an electrolysis electrode, its catalytic effectiveness tends to very gradually drop during such use. After several years of such use, the screens are desirably re-activated by subjecting them to another aluminizing treatment followed by another leaching out of aluminum. The activated surface can be y removed beforehand by prolonged exposure to dilute aqueous I mineral acid.J

In some cases the leached activated or re-activated 1 i screens retain a little pyrophoricity after the Η2θ2 treatment,J even though that treatment is with 5% H2O2 for 81^χ hours. This, retention is more pronounced the lower the aluminizing temperature, so long as the aluminum pick-up effected by the aluminizing is at ! least about 8, preferably over 20, milligrams per square centimeter of the screen's wire surface. The foregoing dip of the j 1 H20£-treated screen in hot water for at least about 30 minutes or the H202 treatment after such dip, effectively eliminates the I residual tendency to pyrophoricity, even when the aluminizing j temperature is as low as 750°F; . j

The water for the final dip can be slightly acidulated, in which case the dip temperature can be reduced and still effect the desired pyrophoricity elimination. At a pH as low as 5, the lowest dip temperature should be about 145°F. A little ;

acetic acid added to the water reduces its pH to about 3.5 or a little lower, and reduces the minimum dip temperature to about 40°F. The same temperature reduction is provided with stronger , acids such as sulfuric and hydrochloric acids, although they ! bring the pH down to about 1 or less. 1

The higher the dip temperature, the shorter the dip time needed. Dips as long as 12 hours are extremely effective but when the water used in the dip is at or above about 170°F, adequate results are obtained by a dip of about 1 hour. There is a substantial amount of gassing as well as aluminum extraction during the dip, particularly when the dip water is close to its boiling point, and care should be taken to permit the elimination ן and release of gas from the dip water.

I Highly prolonged dips appear to effect removal of all residual diffused-in aluminum, leaving a nickel product that is particularly desirable for a water-electrolyzing cathode.

Although the water dip is highly effective for eliminatmg pyrophoricity of activated nickel or iron that has been or is to be treated with it does not eliminate or even significantly reduce the pyrophoricity of these activated metals without the H0_2־ treatment. Applying the H₂O₂ treatment after the water dip is just as effective for pyrophoricity elimination as applying the treatment followed by the water dip.

Activated nickel made by diffusion zincizing followed ! by leaching with an acid such as sulfuric or hydrochloric acid [ tends to have little or no pyrophoricity, although it is catalytic ! and has a very low overvoltage when used as cathode or anode in the electrolysis of water. Nickel activated this way need only be given the above treatment to assure the complete absence of pyrophoricity.

Raney nickel and iron powder also can have their pyrophoricity eliminated by the foregoing combination of H 0

2 treatment and water dip ן The electrolyzing of water with the catalytic nickel ! electrodes of the present invention is a highly desirable and

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very effective operation. It yields inexpensive gaseous hydrogen ן in large quantities, and the hydrogen is advantageously reacted I! with carbon monoxide to form methane and/or methanol. Along

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1' with hydrogen, gaseous oxygen is also produced by the electroi lyzing of the water. Moreover such catalytic nickel has an p extremely long life in such use.

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Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.