Stoichiometric magnetites

Since the spinel phase must be prepared at low temperatures (by hydrothermal synthesis or by careful oxidation of magnetite at a temperature T < 300 C, for example) it has been widely suspected that some incorporation of hydrogen is needed to stabilize it. However, Schrader and Buttner have shown that pure y-Fe203 does exist, and Coey et al. have been able to prepare Fe3 j04 in the compositional range 0 < x < 0.08 by quenching non-stoichiometric magnetite prepared at 1450 °C. There is no evidence that hydrogen is needed to stabilize the system. 

In general, as discussed earlier, the chemical properties of small particles may be different from the properties of the corresponding bulk samples. An investigation of this effect in oxidation reactions was made by Tops0e et al. (237) in the study of the room temperature oxidation of Fe304 particles 40 nm in size. The Mossbauer spectra of several partially oxidized samples and that of magnetite are shown in Fig. 37, in which it can be seen that oxidation is reflected in the ratio S of octahedral to tetrahedral spectral areas (see Section III, A, 2). Specifically, the value of S for stoichiometric magnetite... 

Structural Formula Poly[N-(2-aminotethyl)-3-aminopropyl]siloxane-coated non-stoichiometric magnetite... 

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. 

Magnetite possesses an inverse spinel structure with oxygen ions forming a face-centred cubic closely packed structure. The formula for describing Fe occupancy is (Fe " ) [Fe ", Fe ]04 where the parentheses ( ) stand for cations at tetrahedral sites while brackets [ ] denote cations at octahedral lattice sites. Stoichiometric magnetite has all available substitutional sites occupied by Fe and Fe ions. Non-stoichiometric magnetites also exist, with various numbers of available sites being either vacant or occupied by impurity ions. 

It is difficult to compare the preponderance of the three methods mentioned. The reasons are the difference of ratio between the investment cost and running cost because their relative income changes with the production capacity. Undoubtedly, the induction method is the preferred way to secure the best quality of catalyst, especially when the promoters are dispersed as molecular in the melt magnetite and the oxidation degree is close to the stoichiometric magnetite. 

Fig. 3.14 Spectra below the Verwey transition a spectrum of stoichiometric magnetite at 100 K with visible Fe lines (indicated by arrows), and b spectrum of non-stoichiometric magnetite Fe2.94404 at 100 K with the two typical Fe " and Fe " sextets...

Still shows the typical and Fe " sextets at 100 K which is below the normal Verwey transition for stoichiometric magnetite [35]. 

The spectrum (Figure 4.71) of sample 3.3 (corrosion products taken from SG42 pipelines at a low level) consists of just two magnetically split components, with the hyperfine parameters assigned to the A and B sites of near-stoichiometric magnetite (Fe304) and a relative area ratio = 1.85. 

We have seen above (Section 3.2) that precipitation of ferric ions in solution forms an oxyhydroxide that turns into goethite (a-FeOOH) or hematite (a-Fc203) through very different reaction mechanisms. In the presence of ferrous ions, ferric ions form a spinel oxide over a wide range of compositions. Alkalinization at pH > 9 of a Fe(ll)/Fe(lll) = 0.5 mixture forms stoichiometric magnetite Fe304 [102-104]. Small quantities of ferrous ions [Fe(lI)/Fe(III) >0.1] lead to a similar structure but containing vacancies [105,106]. 

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