Magnetite substituting cations

Substitution of a large range of cations can be readily induced in magnetite and maghemite because tetrahedral as well as octahedral positions are available. Sidhu et al. 

Ferrite compounds with the inverse spinel structure are similar to magnetite, with different ions substituting for the iron atoms. As with FeO (cf. Figure 6.62), the oxygen ions have no permanent magnetic moment. Tetrahedral sites in the FCC oxygen array are occupied by half of the trivalent cations, and octahedral sites are occupied equally by divalent cations and the remaining trivalent cations. 

The solid state structure of magnetite, a spinel(2.), contains iron cations in two different oxidation states (Fe " and Fe ) and in two lattice sites of different coordination (octahedral and tetrahedral) therefore, the catalytic surface of this material may be expected to provide a variety of possible sites capable of acting as adsorption or reaction centers. Also, it has been demonstrated that substitution of other cations for iron can significantly alter the catalytic activity for WGS (4,5). 

It has been shown that the addition of lead to a chromia-promoted magnetite WGS catalyst enhances the activity for WGS (4 ), A study of the solid state changes which occur upon this substitution was made to probe the active sites..of the catalyst. Through a combination of oxidation studies, Mossbauer spectroscopy, and X-ray diffraction line broadening, a model for the. catalyst was developed. It was concluded that Pb was present as Pb " at tetrahedral sites. The Pb substitution resulted in the expansion of the tetrahedral sites, contraction of the octahedral sites, and the oxidation of some Fe to Fe. The resulting octahedral cations became more covalent in nature, and since the octahedral cations have been reported to be the active sites for CO oxidation over ferrites... 

In another study, the effect of silica incorporation into the Fe O lattice was studied (5,49,50) A 20% Fe O on silica catalyst was prepared using conventional techniques. Ir was found that while direct oxidation of the catalyst at 800 K produced the expected a -catalyst was previously reduced in CO/CO2 to produce magnetite, then subsequent oxidation resulted in the formation of y-Fe203 Figure 10 shows MOssbauer spectra of this catalyst after various thermal treatments. In these spectra, the central doublets were demonstrated to be a result of small iron oxide particles which were superparamagnetic at the conditions where the spectrum was recorded. The suppression of the y - oOg to a -Fe203 transition is characteristic of the substitution of roreign cations into the mag-... 

Additionally, it was proposed that reaction 19 occurred only in the outermost 3-4 atomic layers of the magnetite crystallites. The MOssbauer spectrum of the catalyst in the reduced form agreed with this substitution. The spectral parameters of the tetrahedral cations were unaffected by the substitution, whereas the isomer shift and magnetic hyperfine field of the octahedral cations decreased. Also, the line width of the octahedral cations increased relative to an unsubstituted catalyst. Finally, the spectral area ratio of the iron cations in the tetrahedral to octahedral sublattices decreased. 

In closing, it is important to note that the CO/CO2 adsorption technique effectively titrates the active sites for WGS on magnetite catalysts which differ in activity by over an order of magnitude. Nitric oxide on the other hand titrates all of the surface cation sites and is unaffected by Si-substitution. Indeed, NO is known to chemisorb strongly on iron oxides and may even be able to reconstruct the surface. Thus, the combined use of NO and CO/CO2 adsorption provides information about the total magnetite surface area and fraction of the magnetite surface which is active for the WGS reaction. 

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. 

Pig. 3.48 Simplified scheme showing Fe +-Al + cationic substitution in magnetite ... 

The second difference between wiistite and magnetite precmsors is the roles of MgO. In Fei xO based catalysts, it is not only the cationic substitution of Fe + with Mg + ions, but also the fact that MgO can form a complete solid solution with FeO in the ranges of 0%-100%, and be well-dispersed into the catalyst precursor (Fei xO). This, to a certain extent, compensates the roles of AI2O3. 

Magnetite, Fe304, crystallizes in the inverse spinel-structure (cubic), with Fe " " in the tetrahedral sites and both Fe " " and Fe " " in the octahedral sites (space group Fd3m). Many cationic substitutions occur. It is an important iron ore and occurs in many geological environments, as aggregates, veinlets, and inclusions. 

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