Wiistite Solid Solutions

Interdiffusion coefficients in wiistite solid solutions containing BaO were measured by using a diffusion couple method at between 1073 and 1473K, in a controlled atmosphere for which the CO/CO2 ratio was equal to unity. It was found that the... 

Foster, P.K. Welch, A.J.F. (1956) Metal-oxide solid solutions. Part 1. Lattice constant and phase relationships in ferrous oxide (wiistite) and in solid solutions of ferrous oxide and manganous oxide. Trans. Faraday Soc. 52 1626-1635... 

Subsequent calculation of the high-temperature equilibrium of magnetite with wiistite, taking into account the values of the activity of the components of the solid solutions and the nonstoichiometry of the phases (Kurepin, 1975), made it possible to obtain the following equilibrium ... 

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 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. 

D In general, the formation of replaced solid solution induces the generation of defect because the lattice must be an electric charge neutral. For example, the introduction of trivalent iron ions to wiistite leads to the generation of cationic vacancy. If the other atoms are smaller, they can enter the crunode between lattice sites (called immersion solid solution). 

The above data are in an ideal system, in which many of factors are not taken into account under the actual conditions (generated solid solution, etc.), and it will change the above data. The curve shape of the wiistite reduction by H2 will change with the oxygen content i.e., the defect concentration y of iron in Fei j O (Fig. 5.12). 

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