Ammonia synthesis iron/cobalt

Ammonia production from natural gas includes the following processes desulfurization of the feedstock primary and secondary reforming carbon monoxide shift conversion and removal of carbon dioxide, which can be used for urea manufacture methanation and ammonia synthesis. Catalysts used in the process may include cobalt, molybdenum, nickel, iron oxide/chromium oxide, copper oxide/zinc oxide, and iron. 

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

Equation (305) describes the ammonia synthesis rate not only on iron catalysts, but also over molybdenum catalyst (105), tungsten (106), cobalt (95), nickel (96), and other metals (107). Equation (300) describes ammonia decomposition on various metals (provided that there is enough H2 in the gas phase). 

An experimental study of kinetics of ammonia synthesis on iron (101), cobalt, and nickel (96) catalysts, at ammonia concentrations much lower than that at equilibrium, showed that at pressures of the order of 1 atm the second of these possibilities is realized.7 When far from equilibrium, the... 

Flowing ammonia has been applied to the synthesis of the nitrides of iron, cobalt, and nickel compounds with very low enthalpies of formation (Table 2). Again, isothermal conditions were employed. Ammonia is preferable over the use of molecular nitrogen to nitride... 

Niobium and cobalt clusters exhibit size-sensitive reactions with nitrogen with a reactivity pattern similar to that observed for hydrogen. The reactivity of rhodium clusters (n = 1-12) toward N2 has also been studied. In this case the atoms through the tetramer appear to be inert, with reactivity turning on at Rhj. Maximum reactivity occurs at Rh7, and subsequently drops off by roughly a factor of 2 in going from Rh, to Rh,. Iron clusters appear to be nearly unreactive toward N2. Attempts to induce low-pressure ammonia synthesis on gas-phase iron clusters indicate that hydrogenated iron clusters Fe H are also unreactive toward N2. ... 

Among catalysts derived from intermetallic compounds, titanium-iron systems have received some attention. The precise course of reactions involved is not clear. For example, it is claimed that the catalyst derived from a suitably activated TiFe intermetallic phase is TiN + Fe. The TiN is said to react with molecular hydro-gen. On the other hand, in a series of patents on Fe-Ti systems, covering a range of iron-to-titanium ratios, with or without addition transition elements, it is quite clearly regarded that the titanium is capable of forming hydrides. Whatever the mechanism, such systems appear capable of promoting ammonia synthesis in commercial yields at 300 °C, 80 atm, while some are even claimed to be active at 125 °C and 1 atm. Rare earth metals, in combination with iron, ruthenium, or cobalt, can also function as catalysts. Again, the rare earth metals seem to be... 

Ammonia synthesis catalysts containing iron, potassium, and zirconium oxides, and cobalt and magnesium ferrites. P. D. Rabina, V. S. Komarov, and M. D. Efros. SU 539601 (1977). 

Ammonia synthesis catalysts of increased low-temperature activity, prepared from iron and potassium oxides, cobalt ferrite, and calcium aluminate. V. S. Komarov, P. D. Rabina, and L. M. Dmitrenko. SU 598632 (1978). 

Ammonia synthesis catalyst prepared by firing an iron and cobalt hydroxide mixture containing a magnesium compound, fusing, and treating with potassium hydroxide solution. V. S. Komarov, M. D. Evros, and P. D. Rabina. SU 818646 (1981). 

Ammonia synthesis catalyst comprising alkali metal, alkaline earth metal, iron, or cobalt hexacyanocobaltate or hexacyanoruthenate. M. M. Johnson, D. C. Tabler, and G. P. Nowack (Phillips Petroleum Co.). US 4309311 (1982). 

Iron catalyst for ammonia synthesis containing alumina, cobalt, and an alkali promoter and a method of producing the catalyst. J. R. Jennings (Imperial Chemical Industries Ltd). EP 174079 (1986) US 4668657 (1987). 

The first promising catalyst was introduced by 1931 and contained a high proportion of nickel oxide supported on a mixture of thoria and kieselguhr. The convention widely used at the time was to describe composition as 100 parts nickel, 18 parts thoria, 100 parts kieselguhr. Catalysts made with cobalt rather than nickel were more effective but could not be considered eommercially at that time because cobalt was not available in suffieiently large quantities. The same problem had, of course, faced Haber and Boseh in the replaeement of osmium by iron oxide for the ammonia synthesis eatalyst. 

Molybdenum In its pure form, without additions, it is the most efficient catalyst of all the easily obtainable and reducible substances, and it is less easily poisoned than iron. It catalyzes in another way than iron, insofar as it forms analytically easily detectable amounts of metal nitrides (about 9% nitrogen content) during its catalytic action, whereas iron does not form, under synthesis conditions, analytically detectable quantities of a nitride. In this respect, molybdenum resembles tungsten, manganese and uranium which all form nitrides during their operation, as ammonia catalysts. Molybdenum is clearly promoted by nickel, cobalt and iron, but not by oxides such as alumina. Alkali metals can act favorably on molybdenum, but oxides of the alkali metals are harmful. Efficiency, as pure molybdenum, 1.5%, promoted up to 4% ammonia. 

We now encounter a semantic problem of considerable size. It has been recognised for a very long time that the activity of metal catalysts can be helped by the presence of quite small amounts of substances that of themselves have no or little activity. This concept first achieved prominence in the development of iron catalysts for ammonia catalysts, and of iron and cobalt catalysts for Fischer-Tropsch synthesis, and the term promoter was applied to these substances. They were of two kinds (i) structural promoters such as alumina, which acted as grain stabilisers and prevented metal particle sintering and (ii) electronic promoters such as potassium that entered the metallic phase and actually enhanced its activity. In these cases the metal is the major component, so that the catalyst is a promoted metal rather than a supported metal. 

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