Reduction of wiistite

First of all we shall describe the process of reduction of wiistite to iron [11]. We shall discuss the possible morphologies which may develop. Let us assume that our starting material is completely compact oxide with planar surfaces. The oxide is exposed to a reducing CO/CO2 or H2/H2O gas atmosphere. The ratio CO/CO2 is chosen so that the free energy change /I reaction 

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The conversion to iron is determined by the thermodynamics of the reduction of wiistite. At the temperature of industrial interest, this means a H2/H2O ratio of at least 2. Below this ratio, metallic iron cannot be formed. 

FIG. 3.16. Combined surface reaction and solid state diffusion in the partial reduction of wiistite. 

However, a high activation energy cannot be unambiguously interpreted as resulting from control by the chemical step(s) in the reaction sequence. The reduction of wiistite or magnetite to iron frequently results in a dense iron layer [10]. The transport of matter across such a dense iron layer is by solid state diffusion, a process with a high activation energy. 

Figures 8.5 and 8.6 from the work of Kohl and Engell [11] illustrate this point. Figure 8.5 is a plot of the reduction of wiistite at 800°C in hydrogen...

A small amount of Cr could be incorporated in wiistite at 1350 °C (Bogdandy Engell, 1971) and MgO and MnO were completely miscible with FeO the mixed phases are important in the reduction of iron ores. Wiistite can be doped with small amounts of Mn, Mg, Ca and <10 g kg Si or Al to promote reduction (Moukassi et al., 1984). In green rust Fe has been replaced by Ni" (Refait Genin, 1997) and by Mg (Refait et al. 2001). 

Fe(OH)2 exists as hexagonal plates as do the green rusts (Feitknecht Keller, 1950 Bernal et ak, 1959). The basic morphology of wiistite is cubic, but this compound is frequently obtained as very irregular particles. It is formed as irregular rounded crystals 20-100 (xm across by reduction of hematite with H2/H2O at 800 °C (Moukassi et al., 1984). 

The reduction of hematite with H2 at 387-610 °C has been followed in situ using TEM and an environmental cell (Rau et al., 1987). The reduction reaction started at nudeation sites on the edge of the sample and as the reaction proceeded, a particle showed four reaction zones consisting of umeacted hematite, lamellar magnetite, porous magnetite and finally porous iron (the temperature was too low for wiistite). 

D.A. (1993) Rock magnetic criteria for the detection of biogenic magnetite. Earth Planetary Sci. Letters 120 283-300 Moukassi, M. Gougeon, M. Steinmetz, P. Dupre, B. Gleitzer, C. (1984) Hydrogen reduction of single crystals of wiistite doped with Mg, Mn, Ca, A1 and Si. Met. Trans. 15B 383-390... 

St. John, D.H. Mathews, S.P. Hayes, P.C. (1984a) Establishment of product morphology during the initial stages of wiistite reduction. Met. Trans. 15B 709-717... 

For stoichiometric reasons, the hydrogen consumption for the reduction of haematite into wiistite is only about half of the hydrogen used for reduction of the wiistite into metallic iron. As a consequence of these thermod5mamic and stoichiometric restraints, only roughly 50% of the hydrogen (1/3+1/2- l/3=l/2) introduced at the bottom of the shaft furnace is used for the reduction process, whereas the rest is found -diluted with steam - in the top gas [495]. 

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. 

TPR patterns of the Fei xO based catalyst and the magnetite based catalyst are shown in Fig. 1.19. It can be seen that the reduction peak of wiistite catalyst is shifted towards lower temperatures, thus confirming the advantage of this catalyst as to shorter reduction period in the industrial reactor. This result is in agreement with the better reducibility of wiistite with respect to magnetite. ... 

Easy reduction. Its intrinsic reduction rate is 4.5 times that of the magnetite-based catalyst. Reduction temperatme of wiistite-based catalyst is lower by about 80°C 100°C than that of the magnetite-based catalyst, and the terminative... 

Table 1.15 Characteristic parameters of wiistite- and magnetite-based cataiysts during preparation and reduction...

In summary, it seems that the structure-sensitivity and the surface reconstruction induced by nitrogen are due to the availability or to the creation of more active sites that induce C7 atoms. The authors found that the activity increase of wiistite-based catalyst, after reduction, also showed that there are more active sites including C7 atom. ... 

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 existence of the corresponding amounts of Fe + in wiistite structures not only ensures the electric charge neutrality in the lattice, but also provides the extremely favorable conditions for the migrations of Fe + along the cavity, as well as for the transferences of the electrons (Fe + O Fe +). This is the structural factor for the extremely easy reduction of Fei xO by Hg and for Fei xO to be doped and modified with the promoters. 

Chemical reaction. The main component of wiistite-based catalyst such as A301, ZA-5 etc. is Fei xO. The reduction reaction is as follows ... 

Fe-FeO, Fe0-Fe304 is in a state of direct contact with each other during the reduction of magnetite while the outer layer of wiistite interacts with the reductant gas. For example,... 

The apparent isotropic particle size of the iron particles was estimated from the full width at half maximum (FWHM) using the Scherrer equation to be ca. 80 nm as shown in Fig. 7.35. Whether the iron nuclei are formed by thermal decomposition of wiistite or by reduction of magnetite can be decided from the observation that ammonia iron exhibits a strong texture. Microscopic studies showed the iron particles to be textured in the [111] direction. Unfortunately, the Fe (111) line is not detectable for symmetry reasons. But the texture is characterized by smaller (hOO)/ hkO) ratios than calculated from the structure factors of isotropic a-iron. The fact that the (200) (110) ratio of the initially formed detectable iron particles is identical with the theoretical value of 0.2 unambiguously indicates the formation of... 

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]. 

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]. 

Depending upon the reducing power of the gas, magnetite can be reduced either to wustite or to iron as end product. During a reduction to wustite, a non-porous compact reaction layer has been observed [48]. Local thermodynamic equilibrium was not attained at the wiistite/gas phase boundary. This indicates that the phase boundary reaction which occurs there has a high reaction resistance. 

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. 

The basic technical characteristics of the wiistite-based catalyst (A301 and ZA-5) for ammonia synthesis are high activity at low-temperatme, and easy reduction. The following results could be obtained by comparison with the magnetite-based catalyst under the same conditions. 

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