US2267753A - Process of preparing platinum alloy catalysts for gas reaction and the catalysts obtainable thereby - Google Patents

Description

Patented Dec. 30, 1941 PROCESS OF PREPARING PLATINUMALLOY CATALYSTS FOR GAS REACTION AND THE CATALYSTS OBTAINABLE THEREBY Konrad Ruthardt, Hanan, Germany, assignor to I. G. Farbenindustrle Aktiengesellschatt, Frankfort-on-the-Main, Germany No Drawing. Application January 6, 1939, Serial No.249,590. In Germany December 31, 1936 12 Claims.

The present invention relates to a process of preparing highly active platinum alloy catalysts and to the catalysts obtainable thereby.

Numerous methods have been suggested for the industrial oxidation of ammonia and for other gas reactions in which pressure, temperature and the arrangement of the apparatus vary. As catalysts there are preferably used platinum metals and alloys of diflerent platinum metals or of platinum metals with other metals suitable for this purpose. The reaction is rapidly started by platinum itself. In the case of other catalysts, for instance alloys of platinum and rhodium, the catalysts begin to work satisfactorily only after several days, as the "catalyst is activated by the reaction of the ammonia itself only after a certain time of this reaction, which activation is accompanied by, a roughening and sprouting of the surface. Alloys like the known alloys of platinum and ruthenium could hitherto not be used as-catalysts in the oxidation of ammonia, for no activation occurs withthem while in contact with the ammonia. By many other metals likewise the activation of the alloys is preof the other metals of the afore-said group tungsten may be present in an amount of 0.1 to per cent, if desired together with rhodium.

After having been recrystallized the alloys 0 the kind described are distinguished by an especially good catalytic action already at reaction temperatures which are essentially lower than those necessary for catalysts of platinum or of platinum and rhodium without obtaining a worse yield. Moreover, the said alloys have a good mechanical stability especially a good tensile strength, even in the heat. The recrystallized alloys are therefore particularly valuable for gas reactions, for they give higher yields than the alloys hitherto known and the losses in weight of the catalyst are smaller, owing to the low reactiontemperatures. The catalysts may not only be used for the oxidation of ammonia but also for "the synthesis of ammonia, the preparation of hydrocyanic acid from compounds containing carbon and nitrogen according to Andrussow as well as for hydrogenations and dehydrogenations" of hydrocarbons, tor the oxidation of alcohols so as to form aldehydes, and for other catalytic reactions.

The alloys mentioned could hitherto not b utilized in the industry on a large scale because they had the drawback of insufliciently causirw the reaction to begin. If, however, the alloys are r ystallized, the reaction begins after a rela-' ti ely short time; this is called: the catalyst is lighted. The period of time until the reaction of ammonia entirely sets in, when non-recrystalof all alloy which contain at least 50 per cent of platinum and at least one metal selected from the group comprising ruthenium, osmium, iridium, cobalt, nickel, iron, zirconium, thorium, vanadium, niobium, tantalum, chromium, molybdenum, manganese-and rhenium, each metal of the said group being presentin a quantity of at least 0.1" per cent. According to the present invention these alloys are recrystallizedbefore they are used as catalysts by heating them to such an extent that the formatlon of crystals may be detected by means cili -rays; It is necessary that the catalysts be m the'worked state in order to obtainthe desired recrystallization (see pages 102 through 104 .of The Principles of Physical .Metallurgy" by Doan, first edition,

In case the alloys contain platinum and ruthenium, rhodium may be added as a further lized catalysts of the aforesaid kind are used. is a multiple of the time necessary while using the same catalysts in the recrystallized state. In the case of the combustion of ammonia with the aid of recrystallized catalysts, for instance, the reaction sets in after short time so that even when the catalyst is hardly retarded. When the new catalysts are used the yields are very high. Besides, the new catalysts, especially those which besides platinum and ruthenium contain at least one further metal 01 the aforesaid group, are distinguished by iavorable mechanical properties and a high stability. This is surprising, for ternary or quaternary alloys generally are of lower mechanical strength thanibinary alloys.

The new catalysts are obtainediby heating the alloys inquestion above the recrystallization 3 temperature, for instance in an-electric iurnace.-

The recrystallization must be continued until crystals have been distinctly formed.

constituent. Besides platinum and at least one 55. In the preparation of the catalysts by heating xchanged the production it is not suflicie'nt that the recrystallization has just set in.

The alloys have generally to be heated for at least one hour at a temperature of at least 900 C. but, according to the kind of the alloy, a longer period and a higher temperature may also be necessary. The progress of the recrystallization may be observed by means of X-rays. The process may be performed in the presence of protective gases such as nitrogen.

It has been found that the said alloys recrystallized in this manner are especially useful as catalysts. This is remarkable because the known facts indicate that the catalytic efilcacy would be decreased by too strong a development of the crystals. By this heating and a recrystallization any prevention of catalytic force of the alloys is removed.

For alloys with platinum and very small quantities of rhodium an annealing at 2000 F. (about 1100 C.) and a subsequent treatment with acid has already been suggested. These alloys of platinum and rhodium without any further additions do not necessitate an annealing because the alloys only after' a relatively short time are spontaneously activated in contact with the ammonia to be oxidized whereby their surface is roughening and sprouting. The new alloys according to the present invention, however, which contain other constituents are not or only very slowly roughened at the surface and activated by the action of the gas mixture containing ammonia.

The aforesaid activation of the, usual catalysts (so-called formation) which is occasionally observed at raised temperatures on applying these,

catalysts for gas reactions cannot be compared with the recrystallization according to this in vention. The.formation which always occurs during the reaction to be influenced by the catalyst is only very slow and generally requires days and weeks; it is combined with a deformation of the catalysts mammillary or clustered structures sprouting from the surface of the contact.

. The formation depends on the chemical process 1 the recrystallization and, furthermore, merely relatively low temperatures are applied which are not suflicient for the formation. By the fact that during the recrystallization and the further use of the surface of the contact, the temperature of use which is relatively lower than that of non-recrystallized contacts, is roughened only to a small extent, the danger of losses in metal caused by the current of hot gases is much smaller than in thecase of contacts roughened by the formation over which contacts the gases stream at high temperatures.

The advantage of the new contacts, therefore, lies in the fact that the gas reactions are actuated in a relatively short time, that a better guarantee is given for the starting of the reaction and that the yield attained is better, even at a relatively low temperature, then in the case of non-recrystallized catalysts. A further advantage is that, owing to theralpid lightening of the contacts, the temperature of the metal is tion, which in the case of the known contacts was inevitable and caused by the irregular distribution of temperature, may hardly occur.

Without this previous recrystallization the alloys start the reaction either not at all or insufficiently and therefore they could hitherto not be applied. The new alloys have quite another behaviour than the known alloys, for instance of platinum and rhodium, which start gas reactions relatively rapidly already without a thermal pre-treatment. The essential feature of the present invention is therefore: 1) the selection of such alloys as have a particularly good catalytic efiicacy which, however, in the nonpretreated state of the catalyst is still latent; (2) the exciting of this eificacy by the recrystallization. The combination of these two features constitutes the new invention. The methods known from. the use of platinum-rhodium alloys are not applicable for the new catalysts mentioned above, but this special group of alloys necessitates recrystallization in order to become efiective.

The catalysts of the recrystallized alloys herein described are applied in the form of foraminous screens, i. e, in the form of gauze, sieves, spirals, perforated plates and the like. They are likewise advantageously used in combination with other catalysts, for instance with catalysts of tion thereto; All the catalysts shown in the table are recrystallized by heating them for one hour to a temperature of 980 C. to 1020 C. in

an atmosphere of hydrogen.

rather uniform at all parts so that a deforma- The table shows also that in the oxidation of ammonia the catalystsof the invention, 1. e. the above-named alloys in the recrystallized state, have the same or a better yield than the catalysts hitherto usual. The recrystallized catalysts, therein, have the further advantage that sumcient yields are already attained at about 650 C. whereas with application of the known catalysts the same high yields are only obtained at a temperature of about 800 C. to 900 C. or more.

Moreover, the table shows that some of the alloys (Nos. 1, 3, 6 and 7) are distinguished by a high stability in the heat at the most various temperatures, even after a continual glowing. The kind of the gaseous atmosphere in which the glowing is performed is not important herein.

Yield in the oxidation of ammonia at No. Composition of the alloys a reaction Remarks temperature of about 650 0.

Percent l 96% Pt+3% Rh+l% Ru 95. 6 Especially high italzility in the ca 2 97% Pt+2% Os+l% Ru... 94.0

3 95.5% Pt+2%'-Ru-H.5% 96. 1 Finely crystal- Ir+1% Rh. line-especially high stability in the heat. 4 99% Pt+l% 0s 95. 6 5 96.5% Pt+2% Ru+1.5% Ir. 96. 0 6 97% Pt+2% Ru+l% Os 96.4 Finely crystal- I in ed is t i nguished by an especially high stability in the heat. 7 975% Pt+2% Ru+0.5%. 95. 9 Do.

1'. 8 96.5% Pt+0.5% Ru+3% W 92. l

The following examples serve to illustrate that the catalysts of the new alloys become completely active only when recrystallized:

(l) Mixtures of air and ammonia are caused to react at a relatively low temperature (about 750 C.) and under conditions which procure oxidation of 90.2-90.4 per cent of the ammonia, when platinum is used as a catalyst. when nets of an alloy of platinum. and 2 per cent of ruthenium are used which has been recrystallized in an electric furnace by heating for several hours to 1000 C., the yields amount to 94.5 per cent, whereas in the case of non-treated alloys of platinum and ruthenium 80 per cent at best of the ammonia is oxidized. The crystals formed may be detected in the recrystallized alloy of platinum and ruthenium by means of X-rays. The nets made of this recrystallized alloy have been found to be particularly stable.

(2) .When using for the reaction described in 'Example 1 a net of an alloy of platinum, 2 per cent of ruthenium and 1 per cent of rhodium practically no combustion of ammonia could be obtained without a longer activating pre-treatment. With the same net treated at 1000 C. a yield of 96.4 per cent was obtained.

The present application is a continuation-inpart of my application Serial No. 133,821.

I claim:

1. A process of preparing highly active platinum alloy catalysts for gas reactions from worked alloys which are normally. practically inefiective as catalysts vfor said gas reactions, said alloys containing at least 50% of platinum and at least .1% of a metal selected from the group consist ing of ruthenium, osmium, iridium, zirconium, thorium, vanadium, niobium, tantalum, chromium, molybdenum, manganese and rhenium which comprises heating said alloys prior to contact with said gases to a temperature of at least about 900 C. but below the melting point of the alloy for a period of at least about one hour, the time being greater the lower the temperature, andvice versa,and the time and temperature being so correlated that recrystallization of the alloys may be detected to a substantial extent by means of X-rays,

2. A process of preparing highly active platinum alloy catalysts for gas reactions from worked alloys which are normally practically inefiective as catalysts for'said gas reactions, said alloys containing at least 50 of platinum, at least .l% of ruthenium and at least .1% of a metal selected from the group consisting of rhodium, osmium, iridium, cobalt, nickel, iron, zirconium, thorium, vanadium, niobium, tantalum, chromium, molybdenum, manganese and rhenium, which comprises heating said alloys prior to contact with said gases to a temperature of at least about 900 C. but below the melting point of the alloy for a period of at least about one hour, the time being greater the lower the temperature, and vice versa, and the time and temperature being so correlated that recrystallization of the alloys may'be detected to a substantial extent by means of X-rays.

3. ,A process of preparing highly active platinum alloy catalysts for the reactions from worked alloys which are normally practically ineffective as catalysts for said gas reactions, said alloys containing at least 50% of platinum, .1 to 30% of tungsten and at least .1% of a metal selected from the group consisting of ruthenium,

rhodium, osmium, iridium, zirconium, thorium, vanadium, niobium, tantalum, chromium, molybdenum, manganese and rhenium, which comsaid gases to a temperature of at leastabout 900 C. but below the melting point of the alloy for a period of at least about one hour, the time being greater the lower the temperature, and vice versa, and the time and temperature being so correlated that recrystallization of the alloys may be detected to a substantial extent-bymeans of X- rays.

4. A process of preparing a catalyst for gas reactions from a worked alloy not normally pracrtically effective as a catalyst for said gas reactions, said alloy consisting of 95.5% of platinum,

2% of ruthenium, 1.5% of iridium and "1% of rhodium, which comprises heating said alloy prior to contact with said gases to a temperature of at least about 900 C. but below the melting point of the alloy for a period of at least about one hour, the time being greater the lower the temperature, and vice versa, and the time and temperature being so correlated that recrystallization of the alloy may be detected to a substantial extent by means of X-rays.

5. A process of preparing a catalyst for gas reactions from a worked alloy not normally practically efiective as a catalyst for said gas reactions, said alloy consisting of 96.5% of platinum, 2% of ruthenium and 1.5% of iridium, which comprises heating said alloy prior to contact with said gases to atemperature of at least about 900 C. but below the melting point of the alloy for a period of at least about one hour, the time being greater the lower the temperature, and vice versa, and the time and temperature being so correlated that recrystallization of the alloy may be detected to a substantial extent by means of X-rays.

6. A process of preparing a catalyst for gas reactions from a worked alloy not normally practically effective as a catalyst for said gas reactions, said alloy consisting of 97% of platinum, 2% of ruthenium and 1% of osmium, which comprises heating said alloy prior to contact with said gases to a temperature of at least about 900 C. but below the melting point of the alloy for a period of at least about one hour, the time being greater the lower the temperature, and vice versa, and the time and temperature being so correlated that recrystallization of the alloy may be detected to a substantial extent by means of X-rays.

7. A catalyst for gas reactions containing worked alloys not normally practically effective as catalysts for said reactions, said alloys containing at least 50% of platinum and at least .1% of a metal selected from the group consisting of.

ruthenium, osmium, iridium, zirconium, thorium,-

ing recrystallized according to the method of claim 1.

9. A catalyst for gas reactions containing worked alloys not normally practically eflective as catalysts for said reactions, said alloys conprises heating said alloys prior to contact with,

taining atleast 50% of platinum, .1 to 30% of tungsten and at least .1% of a metal selected from the group consisting of ruthenium, rhodium, osmium, iridium, zirconium, thorium, 'vana dium, niobium, tantalum, chromium, molybdenum, manganese and rhenium, said alloys being recrystallized according to the method'of claim 1.

10. A catalyst for gas reactions in the form of a foraminous screen containing worked alloys not normally practically effective as catalysts for said reactions, said alloys containing at least 50% of platinum and at least .1% of a metal selected from the group consisting of ruthenium, osmium, iridium, zirconium, thorium, vanadium, niobium, tantalum, manganese and rhenium, said alloys being recrystallized by the method of claim 1.

chromium, molybdenum,

said reactions, said alloys containing at least 50% of platinum, at least .1% of ruthenium and at least .1% of a metal selected from the group consisting of rhodium, osmium, iridium, cobalt, nickel, iron, zirconium, vanadium, niobium, tan.- talum, chromium, molybdenum, manganese and rhenium, said alloys being recrystallized according to the method of claim 1.

12. A catalyst for gas reactions in the form of a foraminous screen containing worked alloys-