Surface structure of activated iron catalyst

The surface structure of the ammonia synthesis catalysts continues to be investigated. The network structure composed of a-Fe crystallites shows a very large internal surface. Due to the nonuniform practical catalyst surfaces, there are many terraces on the surface. These terraces are also composed of the different steps in the highest ranges of 0.5-10 nm and to be cascaded step-by-step, which are called as ledges. 

In these terraces, there are also a lot of defects present. Therefore, there are the various types of surface sites present on the catalyst surfaces, which are illustrated in Fig. 3.68. Those surface sites include the kink, step, point defect and the surface atom such as the bonding part of atoms adsorbed and vacancies etc. All these surface sites are very active, even though the equilibrium concentrations at the melting points are far less than 1% of the mono-molecular layer. They play an important role in the migration of atoms on the surface. The physical and chemical features of these surface sites are active. The rates of redox and chemical reactions 

Therefore, the chemical reaction rates and product distribution are only the average values in any experiment, because it is the sum of the effect of the various surface sites, for which such kind of concepts are very important for the interpretations of reaction mechanisms. For instances, if the density of smface kink site is rather high and the reaction rate is far more than the other smface site, then the experimental results are mainly contributed by the kink. In practice, it is difficult to clarify the related elementary chemical procedures of the each of surface sites. 

In order to interpret the active surface structures of iron catalysts, numerous models for catalyst morphology have been postulated.Schlogl suggested that there are iron lumps in the catalyst meshes, which are isolated by the oxide promoters. An individual piece of iron crystallites are accumulated loosely in the iron lumps and are commonly viewed as the iron (111) face. 

Pennock et al. investigated the catalysts before and after reduction by in situ TEM reduction techniques. They observed iron cyrstals and the pore structure of approximately 20 nm-30 nm and indicated that the pores in magnetite grow along with (111) face. The same morphological trends of all of iron crystals are found in certain fields, namely, the crystal (100) face of both iron and magnetite are paralleled, while that (010) face, (001) face of iron are paralleled with (Oil) face, (Oil) face of magnetite. 

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