Iron oxide wiistite

Iron(II) oxide (wiistite, Section 4.6) also ranges in composition, from Fe0.88O to Fe0.95O stoichiometric FeO is not encountered in practice. As an extreme case, 5-TiO, an NaCl-type solid previously mentioned as exhibiting gross Schottky defects even when stoichiometric, ranges widely in composition ... 

The minor iron oxides are Fe(OH)2, FeO (wiistite) P-Fc203, 8-Fc203 and the high pressure form of FeOOH. The latter three compounds have not been found in nature. 

Certain individual steps during the adsorption or chemisorption of the reactant gas can also be rate-determining and can give rise to a linear rate law. In the simplest case, if diffusion in the reaction product and transport across the phase boundary are both sufficiently fast, then the supply of adsorbed gas molecules or gas atoms could become rate-controlling. This situation is most likely if the gas pressure and the adsorption coefficients are both low. Another example which belongs to the class of reactions discussed in this section is the oxidation of iron to wiistite in a CO2/CO gas mixture when the thickness of the product layer is small (7 == 700 °C, Ax < 10 cm). The slowest elementary step is the dissociation of CO2 into CO and adsorbed 0 atoms which are then rapidly incorporated into the wiistite lattice [18, 19]. 

It is important to stress that Mossbauer spectroscopy did not detect any divalent iron in all the samples measured. In a study using analytical transmission electron microscopy, the authors suggested that iron within the ferritin core in pathological SN and HP was present mainly as mixed ferric-ferrous iron oxides (magnetite-like or wiistite) and not as ferrihydrite that is the main mineral in control brain ferritin [31 ]. Mossbauer spectra of magnetite or wiistite differ from ferrihydrite spectra, but only ferrihydrite-like spectra were observed by MS in all pathological tissues. If other phases were present, they could be there only in minor quantities. 

Nowadays, six different crystalline forms of iron oxide are recognized (see Fig. 18.1) [2,10]. In iron oxides, iron can be in only trivalent (Fe ) state, only divalent (Fe ) state, or in both divalent and trivalent state. FeO (a black material mineralogically known as wiistite) contains only divalent iron and is very frequently nonstoichiometric with oxygen... 

In 1986, Liu et at found that the iron catalyst with wiistite as the precursor has extremely high ammonia synthesis activity and rapid reduction rate, which led to the invention of wiistite (Fei xO) based catalyst for ammonia synthesis. The relationship between the activity and the iron oxides (Fe304, FeO and Fe203) and their mixtures in the precursor were studied systematically, and a hump type curve was found between the activity and the ratio (Fe +/Fe +). It was speculated that the monophase of iron oxide phase in the precursor is an essential condition for high activity of the catalyst and a uniform distribution of iron oxide phase and promoters is a key to make a better performance of catalyst. The hump type curve was interpreted by the ratio of phase compositions in the precursor, that is, the activity change of the fused iron catalyst depends essentially on the molecule ratio of different iron oxides but not on the atomic ratio of Fe + and Fe +, or Fe +/Fe +, in the precursor under certain promoters. Thus we found that Fei xO based catalyst with wiistite phase structure (Fei xO, 0.04 < x < 0.10) for ammonia synthesis has the highest activity among all the fused iron catalysts for ammonia synthesis. 

X-ray diffraction analysis of the Fei xO catalyst before reduction shows that only wiistite is present in the XRD spectrum which shows only three Fei xO peaks (I/Ig = 36, 100 and 38, 29 = 42.18°, 49.10°, and 71.90°, respectively) as illustrated in Fig. 1.10(a), while the Fe304 phase disappears completely, though it is expected to exists according to chemistry when Fe +/Fe < oo. It is due to the fact that Fe + in the samples does not compose an independent magnetite phase, but dissolves into the wiistite phase non-stoichiometrically. This indicates that, when Fe +/Fe is higher than about 3.5, iron oxides transfer to the non-stoichiometric ones with iron cation defects, namely wiistite phase expressed as Fei xO, where x is the defect concentrations of the Fe + iron cations. From a solid-chemistry viewpoint, Fei xO is a solid solution of Fe2 03 and FeO, therefore x value may be calculated by chemical analysis. 

Iron which shows +2 and +3 chemical valences reacts with O2 to form three kinds of oxides chemically Fe203, Fe304 and FeO. These oxides are called as hematite, magnetite and wiistite in crystallography, respectively. Table 3.1 gives the characteristics of various iron oxides. Figure 3.1 illustrates the crystal structure of FeoOa, Fe304 and FeO. 

In other words, the activity of fused iron catalysts with iron oxides as a precusor relates to not only the content of FeO, but also, more importantly, to its crystal structure of wiistite. When the Fe +/Fe + ratio is smaller than one, although the content of FeO increases the activity decreases, because the crystal structure of wiistite is not yet formed. When the Fe +/Fe + ratio is smaller than 3.15 where the catalyst precursor begins to come to an incomplete structure of wiistite, the activity increases and surpasses strikingly that of the traditional catalyst with Fe +/Fe + at about 0.5. After the Fe +/Fe + reaches five, catalyst precursor forms a complete wiistite structure, while the fused iron catalysts shows its highest activities. Both the activity and reduction behavior are enhanced significantly compared to that of the traditional catalysts. 

Among all catalysts with the iron oxides and their mixtures as precursor studied, Fei xO based catalyst with nonstoichiometric and wiistite structure has the fastest reduction rate and lowest reduction temperature. In a wiistite structure, large amounts of defects are iron ions, which enable the diffusion of Fe in oxide lattices, and will be preferable to electron transferences. This is the structural factor for the easy reduction of Fei xO based catalysts. 

The contents of the mixed structural promoters in iron oxides can determine the surface features, while the iron oxide precursors can influence the catalyst surface area, in order to perform the catalytic activity. Prom this point of view, the most optimum compositions of the structural promoters for wiistite catalysts are not identical to that of those magnetite catalysts. 

The reduction performance of catalyst is closely related with the composition of its precursor in hydrogen flow. As mentioned earlier, this is due to the different reduction mechanisms for catalysts with different precursors. All precursors of iron oxide such as Fe304, Fei xO and their mixture are possible for fused iron catalysts, while the sequence of the reduction rate as well as the reduction temperature is Fei xO > Fe304 > mixture. Apparently, the catalysts with non-stoichiometric Fei xO with wiistite structure as precursors have the fastest reduction rate and the lowest reduction temperature. As mentioned before, the defects of iron ions in lattice of Fei xO has serious impact on its reduction properties. It can be seen from Fig. 5.13 that the reduction process is faster and more complete when the amount of the defects is larger in wiistite. 

If the product forms a nonporous layer around the grain then the reactant must be transported by solid state diffusion. Then may become very large and the overall process may be controlled by the slow solid state diffusion. This has been observed experimentally in the reduction of iron oxides at temperatures where iron forms a dense layer around the wiistite grains [43]. 

Wiistite is an iron(II) phase with the composition Fej 0(s), with the value of X being relatively small. The pure iron oxide, FeO(s), does not exist (Lemire et al., 2013). In addition, wiistite is unstable with respect to metallic iron and magnetite at temperatures below about 840 K (Lemire etal., 2013). As such, thermodynamic data for the phase are not considered in this review. 

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