Iron catalyst crystalline, reaction rates

The direct comparison of the catalytic activity and selectivity of surfaces with different orientations provides information abont the influence of the atomic structure. This has been well described (for example, see [15]). It is well established that catalytic reactions may depend on the atomic structure of the surface (i.e., they are structure sensitive). A classic example of a catalytic reaction that is sensitive to the atomic sUucture of the catalyst s surface is ammonia synthesis on iron surfaces [15], The (111) and (211) surfaces of iron exhibit a significantly higher reaction rate than the (100), (210), and (110) faces. This structural effect has been ascribed to C7 sites (i.e., Fe atoms with a coordination number of 7, or number of nearest neighbors), which exist only on the (111) and (211) surfaces. Now, what if the structure of the catalytic surface during the reaction differs substantially from the initially pure, well-defined crystalline metal surface For example, depending on the gas pressure (i.e., the chemical potential) new structures may become stable (see Sect. 8.2.2). Or what if only a small percentage of uncontrolled or varying defects and steps completely dominate the activity In the remainder of this chapter, these questions will be addressed. 

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