Activation of CO and Other Diatomics

We start this subsection with reaction paths for the activation of CO and subsequently extend this to other diatomic molecules. In a subsequent subsection we then advance ideas learned from diatomic molecules to slightly more complex molecules such as methane and ethane to examine C-H and C-C activation. 

In Fig. 3.20 we show that the change in metal affects the adsorption energy of adatoms much more so than that of adsorbed molecules. The thermodynamics for dissociative adsorption therefore become more imfavorable with increasing d-electron valence-bond occupation of the metal atoms in a row of the periodic table. This trend is a general result that is likely observed for all dissociation reactions. 

The reaction proceeds by stretching the CO bond as well as tilting the CO axis toward the surface. This helps to lower the 27t state and enhances the transfer of electrons from the metal into this state. This charge transfer of electrons into the 2it state (back-donation) weakens the CO bond and thus aids CO activation. 

Hammer and NprskovI isolated the transition states for various diatomic molecules such as CO, N2, and O2 and nicely demonstrated that they all show very similar structures whereby the adsorbate-adsobate bond is significantly stretched and the product framents can form fairly strong bonds with the surface. The transition states are all considered late (for a definition of a late transition state, see Chapter 2) and have comparable values of a of approximately 0.9. 

The Br0nsted-Evans-Polanyi relation applied to the CO dissociation reaction results in Table 3.4. The quantum-chemical result upon which this is based is the dissociation of CO over Ru(OOOl) with 2x2 coverage. 

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