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In the past century, it was commonly believed that a catalyst has the best activity when its chemical composition and crystal structure of the precursor are most similar to those of magnetite. The relationship between the activity and the ratio 

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. 

Country Date Type Chemical component/%wt Activity= /%NH3  

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 results mentioned above are in agreement with the Mossbauer spectroscopy of these samples shown in Fig. 1.11(a). The Mossbauer spectroscopy of magnetite based catalyst consists of two typical hexa-finger peaks of Fe304 as in Fig. 1.11(b), and that of Fei xO catalyst consists of one t3rpical diss3unmetrical double peak of Fei xO as Fig. 1.11(a) presents. 

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