Foil, wire and similar coherent bodies of catalytic metal are provided with catalytically active surfaces by diffusing a chemically removable metal such as aluminum or zinc into those surfaces and then selectively dissolving out at least about a third of the removable metal. platinum wire screens activated in this way make effective exhaust catalysts for automotive engines.
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5. A stirrer propeller whose surface is nickel catalytically activated by first diffusing into it a chemically leachable alloying metal and then chemically leaching at least about a third of that leachable metal.
4. In the preparation of a highly active platinum catalyst, the improvement according to which aluminum is diffused into the platinum by activated pack diffusion, and the aluminum-containing platinum has at least about one-third of its aluminum leached out by a first treatment with aqueous caustic and a second treatment with aqueous acid.
1. A self-supported coherent structure essentially of metal selected from the class consisting of a platinum family metal and nickel having its surface in the form of a three-dimensional micro-fissured fragmentation into cells principally ranging from about 500 Angstroms to about 5000 Angstroms in size and produced by dissolving out from that surface a different metal introduced into that surface by activated pack diffusion.
2. The structure of
3. The structure of
6. A platinum catalyst in the form of a platinum screen produced by the process of
7. The structure of
8. The structure of
9. The structure of
10. A stack of catalytic screens in which each screen is made of platinum hardened with rhodium and having the structure of
11. The preparation of
12. The structure of
14. A self-supporting nickel web that has a visibly active pyrophoric surface in the form of a three-dimensional micro-fissured fragementation into cells principally ranging from about 500 Angstroms to about 5000 Angstroms in size, and produced by dissolving out from that surface a different metal introduced by low-temperature activated pack diffusion into the surface of a starting web not over about 50 mils thick. 15. The combination of
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This application is in part a continuation of applications Ser. No. 576,981, filed May 13, 1975, now U.S. Pat. No. 4,041,196, and Ser. No. 507,126 filed Sept. 18, 1974, now abandoned, each of which is in form among the platinum wires, inasmuch as this increases the cost-efficiency of the catalyst. A similar increase is obtained if the wires are made of palladium-platinum alloys containing 10 to 90% of platinum.
The platinum catalyst tends to also produce sulfuric acid in the exhaust arrangement of FIG. 1 Such sulfuric acid can be trapped by a porous canister of powdered BaO, CaO, CaCO3 or similar chemical placed in the exhaust pipe 16 downstream of the catalyst. Alternatively the sulfuric acid can be collected as by a sintered ceramic disc placed in an enlarged portion of the exhaust pipe so as to cause the exhaust to pass through the disc. The acid will then drain down into a ceramic holder where it can accumulate and from which it can be emptied periodically. Either of these embodiments can be arranged to provide a small by-pass path through which the exhaust can pass without obstruction so that carbon, rust and other solid material in that exhaust can find their way out. Alternatively the trapping members can be made readily removable so that the exhaust pipe can be frequently purged, or the exhaust pipe upstream of the trapping members can be made of material such as 18-8 stainless steel, that does not produce many loose particles of corrosion products.
The air pump 15 is not essential to the use of the present invention. Thus the engine can be operated with a very lean fuel mixture, as described for example in U.S. Pat. No. 3,447,516 granted June 3, 1969, and under these conditions the exhaust will contain a significant amount of oxygen which is not consumed by the principal combustion in the engine. This oxygen will then be used to burn up residual amounts of unburnt or partially burnt fuel ingredients present in the exhaust.
The active platinum surfaces of the present invention make very good igniters for hydrogen and the like, and can accordingly be used wherever such ignition is needed. One such use is to ignite the hydrogen which is passed through a diffusion coating retort as described and illustrated in U.S. Pat. No. 3,801,357, for the purpose of providing a hydrogen atmosphere for the diffusion coating. To this end it is convenient to have a small piece of platinum screen such as that of Example X held by a flexible arm adjacent the hydrogen outlet of such a furnace so it can be readily moved over to the outlet and effect ignition.
Similar ignition arrangements can be provided for igniting acetylene used in oxyacetylene welding, as well as lighting methane and similar gases in domestic gas use for cooking, hot water heating, and home heating in general, although for such uses the platinum surfaces of the present invention, or the gas, should be preheated.
The coherent bodies of the present invention can be provided in configurations other than described above. They can for instance be in t8e shape of baskets made of folded screen cloth spot welded to hold them in shape. Such baskets can be conveniently agitated in the body of liquid in which the desired catalytic reaction is to take place. Where a solid reactant takes part in the reaction, the solid can be held in such a basket.
It is preferred to start with catalyst metals that are very pure, although such metals containing as much as 2% of impurities and as much as 50% of alloying metals are usually also effective, particularly where the alloying metal is a promotor. Some metals are poisonous to the surface, an example being chromium in iron, and should be avoided. Catalytic metals other than those named above can also be prepared by the process of the present invention, and in addition other readily leached out alloying metals can be used in place of the aluminum. Magnesium, tin and cadmium are examples of other metals that are readily leached out with hydrochloric or nitric acid. When preparing catalytic iron, neither aluminum nor zinc is suitable for use as the diffusing metal to be leached out by an uninhibited acid like hydrochloric acid inasmuch as the iron would also be attacked by the leaching acid. The same difficulty is experienced with other catalytic metals such as nickel that dissolve in acids.
The catalytic surfaces of the present invention are porous but are more effective than porous surfaces produced by simple etching for example. Thus the surface of platinum etched in dilute aqua regia seems to only become roughened rather than porous and is not nearly as active as the surfaces of the present invention. Also the surfaces of the present invention seem to withstand high temperatures better, a characteristic of some importance in automotive exhaust use. A 24-hour exposure of the platinum catalyst surface of the present invention to 1300° F. in air will, for instance, not detract significantly from its activity.
The coherent platinum of the present invention has a striking structure illustrated in FIGS. 3, 4, and 5. In FIG. 5 the platinum coating is the micro-fissured layer 40, and as shown in this figure as well as in FIG. 4 the micro-fissured structure is three dimensional, extending throughout the depth of the layer. The overall structure is considered best described as fragmented into small cells, as well as fissured. These cells principally range from about 500 Angstroms to about 5000 Angstroms in size, that is in their largest dimensions.
The surface of a nickel catalyst, made pursuant to the present invention and examined after the surface explosion that takes place upon exposure to air, shows a similar structure, even though that surface takes on a brown color as a result of the explosion.
Platinum activated in the foregoing manners is very effective for all processes catalyzed by platinum metals. For example it works well in the decomposing of nitrogen oxides as for oxidizing carbon monoxide, hydrocarbons and ammonia. Thus a stream of nitrogen containing 2000 parts per million of higher nitrogen oxides as well as 1% CO, will show no significant nitrogen oxide content after passing through a 0.2 inch thick pile of platinum wire screens (60 mesh, 4 mil wire activated in accordance with Example IV) at essentially atmospheric pressure and an hourly space velocity of 125,000, when the gas stream before contacting the first screen has a temperature of about 230°C or higher. The hourly space velocity is the volume of gas flowing per hour divided by the gross volume occupied by the screens. Decreasing the contact time by reducing the number of screens diminishes the effectiveness slightly, but even with only 5 screens in place and at an hourly space velocity of 475,000, about half the nitrogen oxides is removed at 230°C, more than 75% is removed at 250°C, and over 90% removed at about 300°C The so-called "light-off" temperature for this treatment, with any number of screens, is about 200°C
The oxidation of CO in a similar stream of nitrogen containing only 1% CO and 1% O2 is even more effective. With the 19 screens and the same gas velocity no CO is detected in the outflow gas when the inflowing gas mixture is at a temperature as low as 210°C A temperature of about 255°C is needed for this result when 5 screens are used, that is when 14 of the 19 screens are removed. For this oxidation the light-off temperature is a little below 200°C
The foregoing screen material also exhibits a light-off temperature of 200°C for oxidizing propane in a 0.13% concentration in nitrogen containing 0.88% oxygen. Nevertheless with 19 screens about 80% conversion of the propane is effected at 230°C and about 96% at 310° C. The light-off temperature for burning methane in air, with the foregoing screens, is about 450° F.
It should be noted that when the gas velocities are smaller the various catalytic conversions are more complete and can be effected with fewer screens. Thus even a single platinum screen activated in accordance with the present invention is a very effective exhaust clean-up device for automotive engine exhaust systems when the engine is operating at low speeds or idling. Also for higher or even maximum engine operating speeds a single activated platinum screen of the present invention is very desirably combined with other exhaust catalyst devices, such as a silica-supported film of platinum-palladium alloy, to improve the clean-up effectiveness. Because of the low light-off temperatures of the catalysts of the present invention it is preferred to have the platinum screen of the present invention at the upstream end of the catalytic combination. Such a single screen can have a mesh of from about 10 to about 300 wires per inch, with the wires varying in thickness from about one mil or less to about 10 mils.
The wires can have a square or other non-circular cross-section to thus increase their surface area. Also the wires need not all be identical. A single screen can have wires of different thickness, different cross-section, and different composition. Palladium is a desirable addition to a platinum exhaust catalyst and some of the wires can be of palladium or palladium-platinum alloy. Rhodium is also helpful as a platinum alloying ingredient for strengthening purposes and some or all of the platinum wires can be alloyed with 10 to 30% rhodium before activation. Iridium is also suitable for the same purposes and in the same proportions. Platinum is also strengthened with dispersed oxide such as zirconium oxide, as described by G. L. Selman et al in Platinum Metals Review, April 1974, pp. 2-13, and such dispersion-strengthened platinum also makes very active catalyst when activated pursuant to the present invention. Both the rhodium-alloyed platinum and the dispersion-strengthened platinum are given the coherent three-dimensional micro-fissured fragmentation structure having fragmentation cells principally ranging from about 500 Angstroms to about 5000 Angstroms in size, as described above. It should also be noted that the activating action, particularly for the platinum-rhodium alloys, is improved when using leaching solutions at high temperatures, such as under boiling conditions at atmospheric pressure.
All of the active platinums described above also contain pores less than 100 Angstroms in width and in a density of 108 or more, generally 109 or more, pores per square centimeter of gross surface. The gross surface is the outermost surface of the mass of platinum measured with a ruler, ignoring all the fissures and fragmentations. Where the platinum is in the form of a wire having a circular cross-section, its gross surface is the product of the wire length and pi times its diameter. The pores are more clearly shown in the FIGS. 6 and 7.
The platinum of FIG. 6 was prepared as described in Example IV, the original platinum being a 60-mesh screen of wires having a circular cross-section and 4 mils in diameter. The platinum of FIG. 7 was made from another portion of the same original screen using the processing procedure of Example XI.
The activated strengthened platinums of the present invention are also particularly well suited for catalyzing large-scale HCN production from ammonia-air-hydrocarbon combustion as described in U.S. Pat. No. 3,215,495 for example, where catalytic platinum screens can be as much as six or more feet in diameter and are operated at temperatures over 1100° C. Because the screens of the present invention are so highly active, the ignition of the HCN-forming gas stream occurs rapidly without necessitating an induction period for the thermal roughening of the surface as is the practice in the prior art. It is thus practical to use one or more fresh screens of pure platinum or platinum alloyed with 10% rhodium activated pursuant to the present invention as the first or top screen, combined with one or more unactivated or used screens downstream of it to make an effective catalytic screen combination giving high HCN yields right from the start.
The activated platinum of the present invention, either pure or hardened as above with rhodium or by dispersion-strenghening, is also very effective for oxidizing ammonia to nitric acid as described in Kirk-Othmer Encyclopedia of Chemical Technology, 2nd edition, Vol. 13, pp. 802-807, as well as Chemical Week, Vol. 98, pp. 85ff (Feb. 19, 1966), and U.S. Pat. No. 3,873,673, and ignites a hydrogen stream in air at as well as below room temperature. These catalysts make very effective igniters for hydrogen-oxygen rocket jets in space vehicles. Hydrogen peroxide jets for such vehicles are also improved by these catalysts, and they ignite a stream of propane in air at temperatures as low as 240° F.
The platinum or nickel catalysts of the present invention can also be used in the methanation of coal or carbon dioxode by hydrogen. Thus coal can be reacted with steam to form carbon monoxide and the resulting carbon-monoxide-containing product reacted with excess hydrogen by passing a mixture of these materials through a set of nickel screens activated pursuant to the present invention.
The high effectiveness of the catalysts of the present invention in igniting hydrogen makes them well suited for igniting the fuel in the combustion chambers of hydrogen engines, and for combatting flame-out conditions. For this purpose the hydrogen nozzles in the combustion chambers can have their tips made of the activated platinum of the present invention in tubular form for example. The activated platinum can additionally or alternatively be formed as portions of the combustion chambers or cans.
Other specific processes in which the activated platinum metals of the present invention are effective include polymerization and cracking of hydrocarbons, the combination of H2 with Cl2 to make HCl, and the oxidation of ethanol vapors to acetic acid.
The catalysts of the present invention are also particularly suited for catalytic or so-called "flameless" burning to provide heat. The burning of propane, for example, in this way can be actuated by electrical ignition to bring to operating temperature a wire or small screen of the activated platinum of the present invention, after which the wire or screen will maintain the combustion. Methane and even city gas can also be catalytically burned in this way, although a somewhat higher ignition temperature is needed for such gases.
Alloying the platinum of the present invention with palladium in amounts up to 30% also provides the highly effective catalysis of the present invention in the foregoing uses. The platinum-palladium alloys as well as the platinum-rhodium alloys readily receive a diffusion of aluminum or related metals, and aqueous caustic soda or caustic potash, or hydrochloric acid either by itself or as a follow-up treatment, dissolves out the metal that diffuses in. Where the catalysts of the present invention are used at high temperatures, e.g. at 700°C or higher, it is preferred to leach out as much as possible of the metal that is duffused in. To this end it is helpful to use more than one leaching step and to alternate caustic and acid leaches.
Another typical activation treatment is as follows:
A group of circular 60 mesh 4 mil platinum wire screens 3 feet in diameter is loaded into a 6 inch deep diffusion retort made of plain carbon steel by first pouring into the retort a one-half inch layer of aluminizing pack, then placing a screen over that layer, pouring an additional 1/4 inch layer of pack over the screen, placing a second screen over the last layer, and repeating the alternate 1/4 inch pack and screen layers until 18 screens are in the retort. An extra 1 inch top layer of pack is applied over the packed stack and AlCl3 energizer then sprinkled over that top layer. The pack is a mixture of 20 weight percent powdered aluminum and 80 weight percent powdered alumina, and the energizer is applied in an amount 0.5% by weight of the foregoing mixture. A plurality of such loaded retorts are then placed within a tall outer retort through which hydrogen can be flushed as shown and described in U.S. Pat. No. 3,801,357, and the assembly heated in a furnace so that the contents of the retorts reach 800°-850° F. where they are held for 10 hours, after which the furnace is shut off and the retorts permitted to cool to 400° F., the flushing gas stream stopped, the retorts opened and unloaded. The coated screens thus made are then dropped into a 20 weight percent solution of caustic soda in water where they are kept overnight. After about 16 hours in such solution all traces of bubbling are dissipated, the solution is back to room temperature, and the screens are then removed, washed with water and ready for use.
The described screens can also be further subjected to acid leaching as by dropping them in 37% aqueous HCl held at 70°C and kept there for about 1 minute, followed by washing with water. Such a final acid leaching is also helpful when applied after the caustic leaching of Examples IV, V, VI and VII, as well as whenever a catalyst is prepared from a metal of the platinum family (platinum, palladium, rhodium, iridium and ruthenium). It is not helpful when preparing a catalyst from a metal that is readily attacked by the acid. A particularly desirable technique is to leach with caustic until no more leaching takes place, and then to apply the acid final leach with HCl or H2 SO4.
Large very active screens, such as those 6 feet in diameter, are readily made by securing together as by patch weaving, a plurality of smaller screens.
Instead of activating the platinum metal in the form of screens, it can be activated in the form of ordinary wire, as by diffusion coating loose coils of such wire packed in the foregoing diffusion coating pack in the same retorts, or by submerging such coils in a body of molten aluminum or zinc for about 8 hours, keeping the molten metal just above its melting point. The diffusion coated coils are then leached to dissolve out the aluminum or zinc. The thus-activated wire can then be woven into ordinary screens or coiled into flat spirals and braced with a few extra cross wires woven through the turns of the spiral.
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.
Baldi, Alfonso L., Damiano, Victor V.
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