Magnetite catalysts spinel structure

Following correct reduction, the iron oxide/chromium oxide catalyst consists largely of magnetite, which has an inverse spinel structure (231) in which the... 

Less frequently, diffraction patterns such as those seen in Fig. 2.30b were obtained, typically from areas rich in promoter oxides. They arise from the spinel structure of magnetite/hercynite particles, with little long-range order. Such particles can only diffract X-rays very poorly and may well remain undetected in a phase analysis by XRD. The existence of a disordered phase has also been mentioned in Ref. 2. The poor crystallinity of the promoter oxides is in full agreement with their genesis as a product of exsolution from the oxide precurser, which has been described in the preceding sections. The dispersion of this poorly ordered phase was found to be much higher in catalysts 2, 3, and 5 than in catalyst 1, where this phase formed seams within the activated material. 

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... 

Additional information on the structure of the magnetite phase comes from the study of catalyst models, in particular of (Fe, Al) solid solutions. Mossbauer spectroscopic studies of Fe304 [47] and of unstochiometric Fe-spinels [44] have been reported. 

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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