Magnetite structure

Pure, crystalline, thin films of magnetite (111) can be grown on AI2O3 (001) or MgO(OOOl) substrates by oxygen plasma assisted, molecular beam epitaxy the substrate temperature should not exceed 250 °C in order to avoid interfacial reactions and diffusion of Mg in the magnetite structure (Kim et al., 1997). 

If precipitation is carried out at ca. 90 °C while air is passed into the mixture at ca. pH > 7, black iron oxide pigments with a magnetite structure and a good tinting strength are obtained when the reaction is stopped at a Fe0 Fe203 ratio of ca. 1 1. 

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. 

Although they differ in detail, it may be accepted that the basic unit of the cluster is a tetrahedron with one interstitial iron (most likely Fe3+ [52, 53] surrounded by four vacancies on the nearest octahedral site, which is found locally in the magnetite structure. The wiistite structure is then understood to have these unit tetrahedra arranged in some ordered manner. From this point of view, the measurements suggesting three phases separated by second- or higher-order transitions within the wiistite phase [22, 22a, 78] can be interpreted as successive loss of different types of order as the temperature is raised or the number of the unit tetrahedra decreases (the reduction proceeds). However, no definite conclusions have yet been drawn and indeed, the existence of these three subphases is still disputed [19, 20, 23, 24, 28]. 

The effects of solid state alterations of the magnetite structure on the catalytic activity for WGS provide additional insight into the nature of the active sites. While gravimetric and chemisorptive studies provided a chemical picture of the active sites, a geometric or crystallographic description was lacking. Solid state probes of the active sites have supplied information on this aspect of the mechanism. 

Sharp current minimums in Figure 2 for magnetite (400 mV) and ilmenite (-40 mV) correspond to conditions in which no current is applied and the rates for the anodic and cathodic half cell reactions are equivalent and equal to equation 1. Polarization of the magnetite electrode to potentials less than 400 mV results in the dominance of the reductive dissolution of magnetite as described by equation 6. This reaction consumes electrons by reducing ferric atoms in the magnetite structure and releasing Fe(II) to solution. 

A small particle size is essential to reduce diffusional limitation. The unreduced catalyst consists predominantly of magnetite, Fe304 containing minor amounts of wustite, FeO, or in some cases hematite, Fe203. In the boundaries between the iron domains are crystalline and amorphous phases containing mainly oxides of potassium and calcium, both of which are too large to enter the magnetite structure. The unreduced catalyst is virtually non-porous with a typical... 

Magnetite structure is an inverse spinel with a face center cubic unit based on 32 O2- ions with a regularly cubic dose packed along the [111] direction. There are eight formula units per unit cell. Magnetite differs from other iron oxides in that it contains both divalent and trivalent iron. Its formula is written as Fe > [Fe Fe in]04and the brackets denote octahedral sites, tetrahedral Fe spans are directed antiparallel to octahedral Fe 3+ and Fe 2+ spins so... 

One of the most important aspects of nanoparticles in biomedical applications is their surface functionalization in order to improve their biocompatibility with biological entities, and Fourier infrared spectroscopy (FTIR) is very useful technique that provides information about iron oxides in their ground electronic state, and when this material is bonding with a polymeric coating provides information about mechanism of functionalized magnetic nanopartides. This technique is widely used in characterization nanopartides due to its simplicity and availability. In magnetite structure it provides information about the excitation of vibration or rotation of the trivalent and divalent iron cations and allows knowing the occupied sites when the divalent iron is replaced with other cations. 

Wustite is a nonstoichiometric microheterogeneous solid. Its idealized structure is rather similar to the magnetite structure. Both lattices are built from a cubic, close packed, oxygen lattice with the iron ions located only in octahedral voids for wustite, and in both octahedral and tetrahedral voids for magnetite. Wustite is thermodynamically metastable below 833 K but can be easily supercooled to room temperature. The composition of the material is variable and always deficient in iron Fei j,0 with x varying from 4.5% to 11%. Electroneutrality is preserved by the presence of Fe ions. These ions and the holes in the iron sublattice are not statistically distributed throughout the material but form a variety of clusters centered around the ferric ions placed in tetrahedral interstitial sites. ... 

Figure 3.30 HRTEM of a wear particle, (a) Maghemite nanoparticle observed under a [1, —1, 0] azimuth. Distances measured on the calculated diffractogram d = 4.85 nm ) are in good agreement with the theoretical magnetite structure observed under a [1, -1, 0] azimuth, (b) Intact carbon onions are observed...

Figure 4.8 A clinographic view of the magnetite structure showing the two kinds of coordination polyhedra tetrahedral and octahedral. See the text for details.

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