Structure Sensitivity of the Ammonia Synthesis

The (111) surface can be considered a rough surface, since it exposes second-and third-layer atoms to reactant gases in contrast to the (110) surface which only exposes first-layer atoms. Work functions are related to the roughness of a surface and it is useful to quantify the corrugation of a plane in this way. The work functions of all the iron faces are not currently available but they are for tungsten, another bcc metal which also shows structure sensitivity for ammonia decomposi-tion. The order of decreasing work function (4 ) is as follows no 211 0100 0210 Open faces, like the (111) surface, have lower work functions 

The second possible explanation for the structure sensitivity involves the nature of the active sites. The (111) and (211) faces of iron are the only surfaces which expose C7 sites (iron atoms with seven nearest neighbors) to the reactant 

The reaction rates, in Fig. 4.2, show that the (211) face is almost as active as the (111) plane of iron, while Fe(210) is less active than Fe(lOO). The Fe(210) and Fe(lll) faces are open faces which expose second- and third-layer atoms. The Fe(211) face is more close packed, but it exposes C7 sites. If either surface roughness or a low work function were the important consideration for an active ammonia synthesis catalyst, then the Fe(210) would be expected to be the most active face. However, in marked contrast, Fe(lll) and Fe(211) faces are much more active, indicating that the presence of C7 sites is more important than surface roughness in an ammonia synthesis catalyst. 

The idea of C7 sites being the most active site in ammonia synthesis on iron has been suggested in the past. Dumesic et found that the turnover number 

Additional research, which supports the contention that highly coordinated surface sites are most active for ammonia synthesis, has been carried out on rhenium. In this study the rate of ammonia synthesis over the (0001), (lOlO), (1120), and (1121) faces were determined (Fig. 4.4). Here, as in the case of iron the face with the highest activity, the (1121) plane, is the surface which exposes the most highly coordinated sites. It is also shown in Fig. 4.4 that the (1121) face 

---

We have seen how studies on the various faces of Fe and Ni have contributed to the understanding of the structure sensitivity of the ammonia synthesis and of the hydrogenolysis of alkanes. Clearly, the effect of crystal faces, steps, and kinks on other reactions should be pursued. In some cases, kinetics can be measured at vacuum conditions, permitting the use of transient methods. The example of CO oxidation is striking (315). 

---