Universality of Transition State Scaling Relations

A guideline for identifying surface geometries with low-lying lines is to find surfaces where the two fragments of the reaction both can be stabilized without too many shared metal atoms. In Chapters 8 and 12, we will discuss electronic structure effects in more detail. 

The slopes of the relations are very similar and close to one, showing that the transition states of the reactants considered indeed are very final state like. The intercepts are different, and this difference identifies the structure dependence of the relations showing that for a given value of the dissociative chemisorption energy, over a range of relevant energies, the stepped surfaces have barriers that are much less than on the close-packed surfaces. 

FIGURE 6.9 Linear transition state scaling relations for the dissociation of a number of simple diatomic molecules. It is clear from the plots that for a given surface geometry, aU the data cluster around the same universal line. Adapted from Nprskov et al. (2002). (See insert for color representation of the figure.) 

We note that there are exceptions to these scaling relations, especially when molecules with weak interatomic bonds are considered such as in the dissociation of H. However, all these deviations can be understood in terms of general models within electfonic band structure theory. 

It is important to stress that even though deviations from the linear behavior are seen, these correlations are sufficient to describe tfends in reactivity, as we shall see in the next chapter. We also note that a higher accuracy can be obtained if one considers one specific reaction only. 

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