Magnetite ammonia catalysts Reduction

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. 

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 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. 

Since the realization of ammonia synthesis on the industrial scale in 1916 there have been no fundamental changes in the composition of the iron synthesis catalyst. Essentially, potassium carbonate, aluminum oxide, and small amounts of other promoters are fused with magnetite, followed by reduction in situ as described fully in Chapter 2,... 

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. 

Ammonia synthesis catalysts have traditionally been based on iron and have been made by the reduction of magnetite (Fe304). The difference between different commercially available products lies in optimized levels of metal oxide promoters that are included within the magnetite structure. These metal oxides promote activity and improve the thermal stability of the catalyst. Typical promoters are alumina (AI2O3X potassium oxide (K2O), and calcium oxide (CaO). The interactions between the many components in the catalyst can radically affect 1) the initial reducibility, 2) the level of catalyst activity that is achieved, 3) the long-term catalyst performance and 4) the long-term catalyst stability204. 

Thus, ammonia does not reduce magnetite at an appreciable rate at temperatures below 450°C., and it appeal s that at 450°C. and above, the reduction may be accomplished by decomposition products of ammonia rather than by ammonia itself. This contention is based on the fact that the reduction of fused catalysts with ammonia at 450°C. and 550°C. appeared to be an autocatalytic process that is, the rate of reduction increased with time in the initial part of the experiment. Reduction with hydrogen does not appear to be autocatalytic. It may be postulated that a-iron and nitride formed in the reduction are better catalysts for the ammonia decomposition than iron oxide. 

The following discussion concentrates mainly on the ammonia synthesis reaction over iron catalysts and refers only briefly to reactions with non-iron catalysts. Iron catalysts which are generally used until today in commercial production units are composed in unreduced form of iron oxides (mainly magnetite) and a few percent of Al, Ca, and K other elements such as Mg and Si may also be present in small amounts. Activation is usually accomplished in situ by reduction with synthesis gas. Prereduced catalysts are also commercially available. 

The subsequent reduction of the magnetite is of crucial importance to the quality of the catalyst. It is normally carried out with synthesis gas in the pressure reactor of the ammonia plant at not too high pressures (70 to 300 bar, depending on the plant type) and at temperatures between 350 and 400°C, whereupon highly porous a-iron is formed ... 

Different reduction procedures apply if the catalyst is prereduced or when a combination of prereduced and unreduced catalyst is used. Whereas reduction of the bulk magnetite catalyst goes on over days, the reduction of the superficial oxidic layer of the prereduced catalyst is facile and may be accomplished within approximately one day if solely prereduced catalyst is charged. Often the first bed is charged with prereduced catalyst to enable fast reduction and onset of the ammonia synthesis reaction, which thereby liberates heat to support the endothermic reduction in the remaining part of the bed. 

For the reduction process of magnetite a core-and-shell mechanism was proposed and tested in detail (8). Thereafter, a catalyst particle is continuously converted into a porous shell of metallic iron topotactically formed on a nonre-duced core by a propagating reaction front. It was, however, found that this simple concept has to be modified and that the reduction process is, in fact, rather complex in the presence of the promoters (9). In particular, it was concluded that the resulting ammonia-iron consists of Fe particles with a preferred (111) texture determined by the preferential migration of iron ions separated from each other by particles of the structural promoter. 

Under industrial conditions the nitrogen coverage is low (hence the theoretical description applies), but in the bulk phase the catalyst is nitrified, which causes a distorted crystal structure. Through suitable pretreatments, this structure has also been included in model experiment, however, it was not further characterized. After the required activation — a process now well understood — the industrial catalyst shows a distorted iron structure that has been demonstrated to be essential for its catalytic function. This distortion manifests itself as metastable plates in the (111) orientation, which are formed by the topotactic reduction of the magnetite precursor at extremely mild conditions, but also by stress states in the regularly orientated (100) regions of the ammonia iron. These stress states participate in the structme-sensitive activation of nitrogen, which is particularly efficient on the (111) faces, but also on different faces and on strain-induced step defects. These recent theoreticaf ° and experimental resuits require that the discussion about the role of the promoters (especially the potassimn), which is normally considered as closed, is reopened. 

The active catalyst for the ammonia synthesis is alpha iron with small amounts of oxidic additives.. .. The quality of the final catalyst is crucially influenced by the activation process which is the reduction of magnetite to metallic iron. It is important to minimise the concentration of the reaction product water which is a catalyst poison. ... 

The experiment was performed inside a UHV chamber with a dynamic atmosphere of 8 X 10" mbar hydrogen. The result is shown in Fig. 2.17, in which the water evolution detected by a QMS is compared to the evolution of the metallic character of the surface, as expressed by the intensity at the Fermi edge monitored by He I UPS (for the shape of the whole spectra, see Section 2.7). The water evolution curve indicates two steps in the reduction process, but only the second step leads to the formation of metallic iron in the region near the surface. This evidence provides further strong support for the suggested two-step nucleation mechanism found for the reduction of magnetite in its modified form, which is now suggested as a model for the activation of the ammonia synthesis catalyst. 

A commercially available ammonia synthesis catalyst is usually supplied with the iron phase in the form of magnetite, which first must be reduced to metallic iron before the catalyst is used. The reduction time is typically from three to five days, although the actual time required is dependent on the plant design and on limitations of equipment, such as the start-up heater. The general principles of reduction are outlined below. More detailed information to suit a specific plant can be obtained from catalyst suppliers. The principal factors governing a plant reduction are the water content of the circulating gas, the gas flowrate, the reduction pressure, and the reduction temperature. 

In its active state, an ammonia synthesis catalyst is a high-density, medium-surface area composite, which is produced by the reduction of promoted magnetite (see Chapter 2). With a resulting surface area of about 10 m g and a bulk density around 2.8 g cm", an approximate surface area of 2-3 x 10 m per m is available. On a microscopic scale this corresponds to an average crystallite size of about 20-30 nm. 

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