Single-crystal structure of a-Fe and the active sites

It has been extensively studied indicating that all of the surfaces of a-Fe are actually not of the same activity and only those surface sites called as active center are able to show the high activity. So-called active center means those certain sites with the specific atomic structure, arrangement, charge density, geometric morphology on the surface. 

After the fused iron catalysts with Fe304 or Fei xO as precursor are reduced, in which the oxygen anions are removed out and the iron cations form the a-Fe crystallites with body centered cubic lattice structures as shown in Fig. 3.76. Both 

XRD studies proved that all of the metals, beside a few exceptions, belong to three kinds of crystal structmes, namely, face centered cubic (F. C. C.), body centered cubic (B. C. C.) and hexagonal close packed structme (H. C. P.) as shown in Figs. 3.79, 3.80 and part of structural parameters are hsted in Table 3.31. 

Lattice structures of three kinds of crystal of metal (1) (b in graph is dense 

Crystals always tend to decrease the surface energy during their growth. Those crystal faces with low surface energy are quite easily exposed on the surface. The 

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