Magnetite ammonia catalysts

The method of reduction influences the properties of ammonia catalysts. A generally appropriate reduction schedule cannot be prescribed because different types of catalysts call for different reduction procedures to reach their most active state. It has previously been mentioned that the promoters used in ammonia catalysts have a retarding effect on the reduction. According to the author s experience, oxides of the alkaline earth metals, especially CaO, make the catalysts especially difficult to reduce. As will be remembered these oxides enter the magnetite matrix readily. 

In principle, metals or metal alloys are suitable as ammonia catalysts, above all those from the transition-metal group [233] (Table 14). Metals or metal compounds for which the chemisorption energy of nitrogen is neither too high nor too low show the greatest effectiveness (Figs. 14, 15), [234], [235], but only the magnetite-based catalyst proved suitable for industrial use. 

Table 16 gives a composition survey of commercial ammonia catalysts in the years 1964-1966. The principal component of oxidic catalysts is more or less stoichiometric magnetite, Fe304, which transforms after reduction into the catalytically active form of a-iron. 

The term ammonia catalyst commonly refers to the oxidic form consisting of magnetite and oxidic promoters. In fact this is only the catalyst precursor which is... 

In 1986, Zhejiang University of Technology made an important breakthrough on iron catalyst, invented a novel Fei j 0 based catalyst system.In 1992, the first Fei a 0 based catalyst (A301) at low temperatures and pressures was successfully developed, which was superior to the best magnetite-based catalysts in the world. In 1998, they further developed ZA-5 catalyst, and the running temperature was further decreased, which established the technical foundation for low pressure ammonia synthesis process. 

Table 1.10 shows a comparison between the wiistite based and the magnetite based catalyst. It is shown that the wiistite (Fei xO) based catalyst is a new generation of ammonia synthesis catalyst that is completely different from the magnetite (Fe304) based catalyst (including Fe-Co catalyst) in the chemical composition, crystal structure, physical-chemical property, and producing principle etc. 

From Fig. 1.13, it can be seen that the activity of Fei xO based catalyst is much higher than those of the magnetite based catalysts. For example, ammonia concentration reached about 19.15% over the Fei xO based catalyst imder 15 MPa,... 

In the temperature range of 400°C-460°C, typical of a modern low-pressure ammonia synthesis unit, the reaction rate of Fei xO based catalyst is, on the average, 70% higher than that of the magnetite-based catalyst. ... 

The activity of Fei xO based catalyst at different pressures is shown in Fig. 1.14. It can be seen that the activity of Fei xO based catalyst is the highest at both high and low pressures. This kind of catalyst could be used in a wide range of operating pressures. The reaction pressure may be decreased by more than 3.5 MPa as compared to the magnetite based catalyst with the same ammonia yield. 

The basic technical characteristics of the wiistite-based catalyst (A301 and ZA-5) for ammonia synthesis are high activity at low-temperatme, and easy reduction. The following results could be obtained by comparison with the magnetite-based catalyst under the same conditions. 

The discovery of the Fei xO based catalyst with wiistite structure was a breakthrough of the classical theory on the oxide precursor and the activity is higher than the best magnetite-based catalysts in the world. This invention indicates an essential improvement on the research for ammonia synthesis catalysts since 1913. It is a chance for further progress of iron catalyst and aroused much attention of the scientists in the field. At present, the Fei xO based catalyst has been widely used in industry. 

Fortunately, since 1960s, the author has discerned the development of ammonia synthesis industry in China, and has joined in the research of Fes04-based, Fes04-cobalt-based, Fei xO-based and ruthenium-based catalysts. The author and his co-workers have first invented a novel generation of Fei-xO-based catalysts which is more active than the best magnetite-based catalysts in the world, and have developed successfully a series of new catalysts such as AllO-2, A301 and ZA-5 etc that are widely used in industry. 

Figure 2.7. Section of the iron-oxygen phase diagram m denotes magnetite, h hematite, and w wustite a and y stand for these modifications of iron metal. The dashed line describes the path of synthesis of the ammonia catalyst in the phase diagram.

The phenomenon of deactivation of the synthesis catalyst has been recognized since the beginning of the industrial synthesis. Quite early on, compounds such as water and hydrogen sulfide were recognized as poisons, and short lifetimes, in the order of years, were usual. In modem plants, lifetimes of 10-20 years are not uncommon. This has been achieved largely by an improved purification system for the synthesis gas and an optimization of the catalyst production procedure, yielding a more stable and active magnetite, which is still the ammonia catalyst used exclusively in the industry today. 

The catalytic activity of an ammonia catalyst may be reduced in the presence of certain chemical compounds, referred to as poisons. These may be gaseous, occurring as minor components of the synthesis gas, or as solids introduced to the catalyst during the manufacturing process, as impurities in the natural magnetite from which the catalyst is made. The latter will not be dealt with here, since they are already covered in Chapter 2. 

Alkali-promoted Ru-based catalysts are expected to become the second generation NHs synthesis catalysts [1]. In 1992 the 600 ton/day Ocelot Ammonia Plant started to produce NH3 with promoted Ru catalysts supported on carbon based on the Kellogg Advanced Ammonia Process (KAAP) [2]. The Ru-based catalysts permit milder operating conditions compared with the magnetite-based systems, such as low synthesis pressure (70 -105 bars compared with 150 - 300 bars) and lower synthesis temperatures, while maintaining higher conversion than a conventional system [3]. 

An encouraging surprise came finally as Wolf introduced into the catalytic test oven, on November 6, 1909, a sample of magnetite from Gallivare (Sweden) which had stood for years on our shelf. With this substance as a catalyst, the exit gas contained for a relatively long period of operation, about 3% of ammonia compared with an average of 1% which was obtained heretofore over extended periods. What was the cause for this unexpected favorable effect ... 

At first this new assumption seemed confirmed by the enhanced activities of catalysts which were obtained by pressing loose powders of iron oxide into dense tablets. Soon, however, it was found that magnetites of another origin than the Swedish specimen yielded no ammonia, or only negligible amounts, in spite of the dense structure of their reduction products. 

In October, 1910, the author had established the fact that certain magnetites as well as synthetic iron catalysts can be as effective as the uranium carbide catalyst which, in the hands of F. Haber, had proved to be of outstanding activity. At 500°C. and at a pressure of 100 atmospheres and a gas velocity of 50 liters per hour, 5 volume % of ammonia were obtained in the exit gases with 2 g. of a magnetite and 4.5% with a synthetic iron catalyst, the latter being operated at 550°C. 

It is implicit in reaction 9.4 that the equilibrium yield of ammonia is favored by high pressures and low temperatures (Table 9.1). However, compromises must be made, as the capital cost of high pressure equipment is high and the rate of reaction at low temperatures is slow, even when a catalyst is used. In practice, Haber plants are usually operated at 80 to 350 bars and at 400 to 540 °C, and several passes are made through the converter. The catalyst (Section 6.2) is typically finely divided iron (supplied as magnetite, Fe304 which is reduced by the H2) with a KOH promoter on a support of refractory metallic oxide. The upper temperature limit is set by the tendency of the catalyst to sinter above 540 °C. To increase the yield, the gases may be cooled as they approach equilibrium. 

Table 5.1 shows an application of XPS to the study of the promoted iron catalyst used in the Haber synthesis of ammonia. The sizes of the various electron intensity peaks allows a modest level of quantitative analysis. This catalyst is prepared by sintering an iron oxide, such as magnetite (Fe304) with small amounts of potassium nitrate, calcium carbonate, aluminium oxide and other trace elements at about 1900 K. The unreduced solid produced on cooling is a mixture of oxides. On exposure to the nitrogen-hydrogen reactant gas mixture in the Haber process, the catalyst is converted to its operative, reduced form containing metallic iron. As shown in Table 5.1, the elemental components of the catalyst exhibit surface enrichment or depletion, and the extent of this differs between unreduced and reduced forms. 

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