Magnetite catalysts crystal structure

In the past century, it was commonly believed that a catalyst has the best activity when its chemical composition and crystal structure of the precursor are most similar to those of magnetite. The relationship between the activity and the ratio... 

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. 

Thereout, it is found that the catalyst has the highest activity among all the fused iron catalysts for ammonia synthesis when its chemical composition and crystal structure of the precmsor are those of wiistite (Fei xO)- It is called Fei xO or wiistite based ammonia sjmthesis catalysts, where the defect concentration x of iron ion is 0.04 experimental results break through the classical conclusion that lasted for more than 80 years, namely the catalyst has the best activity when its chemical composition and crystal structure of the precursor are most close to those of magnetite. It also provides a new approach for a novelcat-alytic system — wiistite Fei xO system for improving the performances of the fused iron catalysts. 

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. 

It can be readily understood that the structure of the oxide, from which the reduced catalyst is prepared, plays an important role for the properties of the catalyst. This dependence has been proved experimentally by the influence which the rate of cooling of the oxides of a given catalyst composition shows upon the catalytic properties of the reduced catalyst. This effect can be interpreted by considering that in the reduced catalyst the promoters are distributed all over the surface and that it is, of course, highly important how they are distributed. This distribution cannot be independent of the way in which the promoters are present in the oxidic state, whether in solid solution in the magnetite, as separate crystals or as amorphous glassy layers. 

Mechanism of the Promoter Effect. The action of the so-called structural promoters (stabilizers), such as A1203, is closely associated with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron - aluminum spinels [33], [289]-[291]. The solid solutions of Fe304 and the spinel FeAl204 have a miscibility gap below 850 °C... 

Matsui has found for an I.G. catalyst that the aluminum oxide at its surface is present in the form of ferrous aluminate which covers parts of the metal surface, consisting of a-iron. The same author prepared a small single crystal of iron coated by an oxide film and studied its structure by the transmission method of electron diffraction. The pattern shows, Fig. 14, that the (111) plane of magnetite is produced parallel to the (111) plane and also to the (110) plane of iron, suggesting that with the... 

The chemical composition, crystal phases and some structure parameters of Fei xO catalyst and magnetite based catalyst are listed in Tables 1.7-1.9. 

Pennock et al. investigated the catalysts before and after reduction by in situ TEM reduction techniques. They observed iron cyrstals and the pore structure of approximately 20 nm-30 nm and indicated that the pores in magnetite grow along with (111) face. The same morphological trends of all of iron crystals are found in certain fields, namely, the crystal (100) face of both iron and magnetite are paralleled, while that (010) face, (001) face of iron are paralleled with (Oil) face, (Oil) face of magnetite. 

Alumina is incorporated as a sohd solution of the iron aluminate spinel, hercynite, in the crystal lattice. The alumina concentration should be less than the solubility of alumina in magnetite. This corresponds to a maximiun content of about 3% alumina. Any excess of alumina does not go into solid solution, and leads to a reduction in catalytic activity, particitlarly when using catalysts promoted with alumina. The presence of alumina as a structural promoter also leads to the formation of wustite and stabihzes the reduced catalyst. Small amounts of magnesia can also dissolve into magnetite and act as a promoter. The calcium component exists in the form of ferrites or alirminates by neutrahzing acidic components—such as silica—and protects the potash that activates the catalyst. 

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