THE HISTORICAL DEVELOPMENT AND THEORY OF AMMONIA SYNTHESIS* A. MITTAS&
AND
w. FRANKENBURGER. I. G. LABORATORIES, OFPA", GERMANY
Among the numerous catalytic processes of industry, there is hardly one which has brought so much attention and profit to the study of catalysis as the ammonia synthesis. In a period of twenty years, it has advanced from a state of mere empiricism and fruitless speculation to one of scientific study and technical realization. This development has been described frequently in the past years.' The introductory researches by Nernst and the fundamental work of Haber in conjunction with the Radische Anilin und Soda Fabrik (now the I. G. F.) led to the working out of practical, large-scale methods. In this paper, the development is to be treated from the chemical side, and particularly from the point of view of catalysis. For the catalyst is the heart of every catalytic reaction, without whose heartbeat the reaction must st0p.l A method of investigation of ammonia catalysts, supported by conclusive experimental work, was first carried out by Nernst and by Haber. The work of Haber goes farther back in point of time, whereas, as far as the high pressure reaction is concerned, Nernst has precedence. In the first work done by Nernst and his associates with high pressure (190607) relatively inefficient catalysts were used; platinum foil, finely divided iron, and electrolytically deposited manganese being the only mateE rials appli~able.~ Similarly, Haber started with weakly active catalysts-finely divided iron, calcium, manganese, chromium, and nickel occupying his attention during 1904-1907. Of course, Haber did not rest content with these, and
* Delivered before the "Conference on Catalysis and the Mechanism of Chemical Reactions" a t the Dedication of the New Chemical Laboratory, Princeton University, September 28,1929. LBernth~en,Z. angw. Chem., 26, 10 (1913); Haber, Ibid., 1914, 473; Bosch, Natur. Monat., 8,867 (1920); Mittasch, Ibid., 1925,205; Mittasch, Ber.,59, 13 (1926); C.Miiller in Ullman, "Enzyklopadie der Techn. Chem.," 2nd edition, 1, pp. 363427. Thus we pass over the earlier efforts of Doebereiner (1823). Johnson (1881). and Woltereck (about 1900). They did not give a deep insight into the conditions under which ammonia catalysis can take place, and because of the state of the science, they could not. They could not give more than a hint of a qualitative source and, without exception, these hints as well as their experimental connections, were poor, or even completely without foundation. a The fallowing are the most important references leading to a scientific treatment of synthetic ammonia catalysis: Ramsay and Young, J. Ckem. Soc., 45, 88 (1881); E. Baur, Ber., 34, 2382 (1901); E. Baur, Z.anorg. Chem., 29, 305 (1901); Perman and Atkinson, Proc. Roy. Soc., 74, 110 (1904); Perman and Atkinson, &d., 76, 167 (1905). On the technical side, a patent of Le Chatelier's [French, 313,950 (1901)l is of interest.
he was the first to set out experimentally in the search for new highly active materials. The results are well known. After indierent success with cerium, splendid catalysts were found in metallic osmium, and shortly after in uranium metal or carbide. These were far superior to the contact masses hitherto used in the ammonia synthesis.' In the following years, further development was carried out exclusively in the Badische Company, which had taken over Haber's investigations. Badische Investigations, 1910-12 Between 1910 and 1012, Bosch, Mittasch, and their co-workers (Stern, Wolf, etc.) discovered a whole series of strongly active catalysts, amons which industry was able to choose the most useful. Further developments in principle beyond these latter examples has not been forthcoming up to the present time. So, we distinguish two groups of contact masses. The first consists of catalysts allied to those found by Haber, and the second of those we found by new lines of inve~tigation.~ A. Pure Metals.-We write "pure" metals with a purpose, for the work in the Badische, which was the basis for our information, showed that the purity of the substance, i. e., the absence of certain poisons, has tlze greatest siqnificance in synthetic ammonia catalysts. The responsibility for poisoning of the contact mass was next associated with various nonmetals (S, Se, Te, As, P, etc.). The, effect was greatest in the case of sulfur, which can make the catalyst useless when present in mere traces (0.01 per cent or less). As a rule, this effect% not so great for phosphorus, etc. Furthermore, such easily reducible low-melting metals as lead and bismuth were recognized to be poisonous.' Of the pure metals, the following were thoroughly investigated. Iron. The pure metal by itself was found to be very poorly efficient, its activity being very good a t the start of a reaction, but falling off very rapidly thereafter. By particular methods of preparation, the activity of pure iron can be increased, but it never performs in a satisfactory manner over a long period of time. It was found that7 iron was not nearly so satisfactory as a metal which had not yet been investigated, namely, molybdenum. Molybdenum has the added advantage of being less sensitive to poisons.' As we discovered, i t was not necessary to start with metallic molybdenum, molybdic acid or ammonium molybdate taking its place in the contact chamber. Under the influence of nitrogen and hydrogen gases (at, say, 'German Patents 223,408 (0s). 229,126 (U). Both 1909. See also Haber, Z. angeu. Chem., 1914,473; Z. Elektrochem., 20,597 (1914); Natunuiss., 10,1041 (1922). 5 The whole development is described in detail in Ullman, 1, pp. 420, etc. 8 With regard to poisons, see German patents 254,344 and 263,612 (1910). German patents 246,377; 256,855; 259,702. German patent 246,377 (1910).
500°C. and 100 atmospheres pressure), the grains are gradually changed to a molybdenum nitride containing about 9 per cent nitrogen. This is accompanied by a parallel increase in the rate of formation of ammonia, the nitride being the catalytic agent. Tungsten behaves as a rather poor imitation of molyhden~m,~ and is without further significance. Iron, molybdenum, and tungsten have the great advantage of being reduced by hydrogen a t moderately high temperatures, so that a slight amount of oxygen, steam, CO, or Cot in the gas mixture does not spoil the catalyst. In other cases, the best yields were obtained with absolutely pure, dry gas, as had already been f o ~ n d . ' ~ The metals we have mentioned stand in contrast to those used earlier (Mn, U, Ce) which are irreversibly oxidized by traces of free or combined oxygen. "Irreversible" refers only to oxidation under the conditions of catalysis. Equivalent to the metals are the corresponding hydrides, nitrides, amides, carbides, cyanides, and oxides; in case we are dealing with easily reduced.metals. In the reaction chamber, the initial catalytic mass is chemically changed, and finally gives the same end product. Whether this will he a metal or a nitride, etc., cannot he predicted, and can only be determined by careful analysis of catalysts which have been used over a long period of time. In the case of iron, we always obtain the pure metal, no matter what the starting material may he. With adsorbed or dissolved gases, and ammonia, molybdenum forms a nitride. B. Activated Metals.-Under A, the workpf the Badische Company was described in relation to the previous more theoretical work. Now we will survey a series of experiments in which a new direction was taken. This work was the result of an inquiry as to whether metals, only slightly active as such, could be improved, by the addition of other substances, so that a satisfactory catalytic action would result." This work was brought about by our observation, somewhat earlier, of the advantageous action accompanying the formation of nitrides from metals or oxides. I n this way we were led to assume that added substances can work to advantage in the ammonia synthesis, as the latter can be linked with intermediate nilride formation. The result of determined efforts in this direction was complete fulfilment of our expectations, even German vatents 259,996. 260.756 (1911). In regard to the dimmation of oxygen, w e & m a n patent 25!3,8:1 (191 I ) . In the German patent litern. fl Thc nomenclature of this procrss is not unifurm. ture we find the words "Aktivierung." "Aktivator," "Aktivieren." corresponding to the Anglo-Saxon "promoter." In connection with the expressions used by Brade in homogeneous catalysis IZ. phys. Chenz., 37, 257 (1901)l we now find the terms "verstitrken." and "Verstirkerwirkung" applied [Schwab-Ergebn. d. Exakt. Natunuiss., W, 322, etc. (1928)l. Compare: Mittasch, Z. f. Elektrochem., 35, 96 (1929); Pietsch, Ibid., 35, 366 (1929). lo
though the hypothesis we had made concerning the mechanism of these mixtures proved to be in partial error in certain cases. Experiments were first made with metallic iron, and in a similar manner with nickel and cobalt, leading to the following resu~lts.'~ 1. Pure iron and similar metals are definitely improved by mixing with them metallic oxides, in particular, such difficultly reducible and high-melting oxides as Alz03and MgO. This is termed "simple activator." 2 . Pure iron and the like are also essentially improved in their activity by the presence of metals particularly active catalytically (mutual activation). So, it is advantageous to unite iron and its adsorbed hydrogen with such a metal as Mo, which adsorbs or combines with nitrogen.I3 3. Such a substance as copper is practically indifferent. Others, by themselves, or along with added materials, are strongly poisonous. (See section A,) In the simultaneous presence,of "promoters" and "poisons," particular cases result. According to the conditions, we may have anything from complete compensation of the poisons up to a total preponderance of the poisoning action. 4. Slightly or moderately active metals, particularly those which are difficultly reducible (easily oxidized) can be improved as ammonia catalysts just as Fe, Co, and Ni were improved-by the addition of metals. As a rule, combinations of metals which do not stand too close to one another in their chemical relations give the best results. 5. By the addition of third or hurth components, the catalytic activity can be increased still more. 4 typisal example is the mixture Fe ALOJ K,o.'~ So, in 1911, we could distinguish the following groups of ammonia catalysts:
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I. F e (Co, Ni) catalysts activated by oxides of other elements. Fe-ALOa Fe-MgO Fe-Cr20a,MnO, etc. Fe-Rare earths Fe-AI.0.-KxO, etc. Ca-Al.03, etc. 11. Combinations of Fe, Ni, Co, etc., with other metals. Co-Ma Pd-Mo Fe-Mo Ni-Mo Pd-W Ni-W Co-W Fe-W 1% German patents 249,447; 254,437; 258,146; 261,507; 262,823; 276,133; 286,430. American 1,068,967; 1,068,968; 1,068,969; 1,094,194; 1,118,625; 1,148,750; 1,152,930. These patents were all taken out between 1910 and 1912. l a Compare the particular example of Ti-Pt given by Hlavati-German patents 275,343 and 275,663. l4 Snuch mixtures were used very early: for example, in 1910, iron with 20 per cent aluminum nitrate and 10 per cent potassium carbonate was dissolved in nitric acid, evaporated, and the nitrates calcined. 2 per cent Al 1 per cent MgO 1 per cent KNOJ, November 1910: iron or iron aluminate of soda or calcium, in an oxidizing atmosphere.
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A N D THEORY OF A-ONIA SYNTHESIS Val. 6, No. 12 DEVELOPMENT
Fe-Li
Ni-Li
Fe-Sr
Fe-Sr
Co-Li Co-Na
Pt-Mo Pt-W
111. Combinations of metals with one another, where the particular object is to choose the combination so that the components are markedly diRcrent in their chemical properties. We give hcrc a list of cxamplcs in connection with the American patent 1,094,194 of the Badische (1910).15
Ba-Cr Ba-Mg Ba-0s Ca-Mn Ce-Mg Ce-Mo
Cr-Mg Cr-0s Cr-Zr Cr-Na K-V La-Mo Li-Mn Li-Cb
Li-V Mg-Mo Mg-Na Mg-U Mg-V Mn-0s
Mn-V Mo-Ta Mo-Zr Na-0s Th-0s U-Zn Y-Zr
W-Zr
It should he noticed that, in place of the metals alone, the corresponding hydrides, nitrides, amides, carbides, and cyanides may he wed. Furthermore, corresponding ternary mixtures can he used advantageously: Al-Cr-Na Al-Co-Na Be-Mn-Na Cr-Mn-Na
Cr-Mn-Na Cr-Ni-K Co-V-K Mo-Pd-K
Mo-Pt-U02 Ni-Mo-MnO etc.
Methods of Preparatign Since heterogeneous catalysis works best on contact masses with a large surface, the following forms of catalysts were first considered: (a) powders; (b) porous grains and lumps; (c) supported catalysts. The emphasis on (b) has heretofore been for practical reasons, since the advantages of easy manipulation of the contact masses and low resistance to the flow of gas, are also present for greater thicknesses. The application of supports (c), is of no advantage with cheap catalysts. Numerous methods of preparation had been considered, of which many examples may he found in the patent literature of 191Ck1912. The most important methods of preparation for mixed catalysts are: (a) Simple mixing of the components in powdered form, with suhsequent compression. ( b ) Preparation of compact oxide masses, which are then reduced in the contact chamber. This method is particularly useful for iron. The iron, along with the necessary additions, is melted in the presence Compare German patents 261,507 and 276,133.
of a stream of oxygen, an oxide resulting which closely resembles magnetite (FeaOJ chemically. After pulverizing, it may be put directly into the catalyst chamber.'% (c) Preparation of oxygen-containing compounds by ordinary precipitation from solution, by heating mixed nitrates, etc. (d) Soaking the catalyst A with a solution of the substance B. ( e ) The use of alloys of different sorts. (f) The use of complex compounds which are changed to the active catalyst by heating in the contact bomb. The complex ferrocyanides are particularly useful. They were first employed in December 1912 in our work. (Cerium ferrocyanide.)" With methods (c) to ff), we can easily add supports if desired. The Development from 1913-1923 In the foregoing we have described b r i d y the results which we obtained in the Badische in 191Cb1912 with regard to catalysts for the synthesis of ammonia. They are rather scattered in the patent literature and other places, so that a brief compilation may be welcome. In later work from other angles, the work of the Badische has been thoroughly verified. Nothing fundamentally new in connection with catalysts bas been found, so that we can refer merely to the description given in "Ullman," and the patenbreview it contains. As a matter of fact, in the various researches carried ou$ from 1913 to 1929 the emphasis has been placed not on the development of new catalysts, but on the scientific analysis of the new catalytic process. Thus, in England, France, and later most especially in the United States, efforts were initiated to penetrate more deeply into the mechanism of the synthetic ammonia process, and to elucidate the method in which the catalyst works. Work of this sort of a more theoretical orientation has been going on in the Badische from the beginning. But in industrial laboratories it has had to be in relatively narrow fields, because the object naturally had to be the employment of new resultsparticularly with mixed catalysts-in developing other technically important cases. So, with some interruption, the years 1913 to 1923 a t the Badische were filled with work concerning various other industrially important reactions, in connection with which valuable catalytic data were obtained. For example, we have catalytic hydrogenation of organic compounds by catalysts of the type AlzOa (Bosch, Mittasch, Krauch, Schneider), catalytic oxidation Ni
+
German patents 251,437; 263,612 (1910). These ferrocyanide contact agents were first studied exhaustively by the Farbenfabriken Leverkusen. See German patents 285,698 and 286,719 of 1914. Also Mittasch and Kuss, Z. j.Elektrochem., 34,159 (1928). '8 '7
of ammonia with FezOa-Bip08(Bosch, Mittasch, Beck), catalytic preparation of hydrogen according to the reaction
(Bosch and Wild); and the catalytic production of methanol from CO and Hzby ZnO-Cr203contact agents (Mittasch, Pier, and others).18 We had a relatively small amount of time to spend on other catalytic themes, and will not go into them here. As a matter of fact, a number of scientific results had been obtained by 1914 (unpublished for the most part), but a more intimate study of the theory of ammonia catalysis and ammonia-making catalysts could only be initiated some years ago. '8
See Ber., 59, 13, etc. (1926).
