On the mechanism of Cs promotion in ethylene epoxidation on Ag.

We have examined the mechanism by which Cs impacts the selectivity of ethylene epoxidation on silver. The main focus was analysis of promoter-intermediate and promoter-transition state interactions. We show that Cs enhances selectivity to EO by stabilizing the transition state involved in the formation of EO relative to the transition state that is involved in the combustion. It is found that adsorbed Cs induces significant electric fields over Ag(111). This effect can be understood in terms of simple dipole/dipole interactions where the transition state involved in the selective pathway has a favorable dipole orientation as compared to the transition state involved in combustion.