Trends in Dissociative Adsorption

Identifying the transition state and the associated energy barrier is essential for understanding the course of a reaction. Of course, details of the shape of the potential material, e.g. steric hindrance and entropic effects, may impede the system from crossing the barrier. The barrier energy (which is not very different from the activa- 

If we restrict ourselves to the late transition metals the trends will, as for the CO chemisorption energy, be dominated by the interaction of the antibonding orbital with the d band and the leading term is 

a high-lying d band ( 2jt- d is small) is favorable for a stronger interaction and consequently a lower barrier for chemisorption. This explains why CO carmot be dissociated on Cu and why the reactivity increases on going to the left in the transition series. However, there is more to it. 

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

4 Transition States and the Effect of Coverage Ethylene Hydrogenation 

---