Oxametallacycle intermediates on clean and Cs-promoted Ag(111) surfaces

We have performed density functional theory calculations for a model cluster representative of the [111] face of Ag to study the effect of Cs and Cs+ on a proposed oxametallacycle intermediate of ethylene epoxidation. A systematic study of the binding energies of O, Cs, and Cs+ on the symmetry sites of the Ag(111) surface was performed by varying the basis sets and cluster sizes. We find that ethylene binds to an oxygen atom on an Ag cluster to make a bound -O-C2H4 species. In the presence of Cs or Cs+, we find a bound oxametallocycle. In both Cs-promoted cases, this oxametallocycle is, within the errors of our calculations, isoenergetic with respect to the desorption of ethylene with the O atom remaining bound to the promoted Ag cluster. Atomic charge calculations were performed revealing that Ca is an electron donor for the oxametallacycle without actually being a part of the ring. In fact, we find that the presence of the electron-donating Cs atom repels the oxametallacycle intermediate.