Chemisorption of oxygen, chlorine, hydrogen, hydroxide, and ethylene on silver clusters: A model for the olefin epoxidation reaction

We have examined various postulated pathways for the catalytic epoxidation of olefins by silver using ab initio quantum mechanical methods to study likely intermediates in this reaction. In particular, we predict preferred binding sites, geometries, vibrational frequencies, and binding energies for O, O₂, Cl, H, OH, and C₂H₄ on a cluster model for Ag aggregates present on supported catalysts. These calculations suggest the presence of two nearly degenerate states of chemisorbed atomic oxygen. The calculated binding energy for these surface oxides are 78-79 kcal/mol (experimental values are 77-78 kcal/mol). One surface oxide has the form of an oxyradical anion and is predicted to be selective for olefin epoxidation. The other surface oxide is a closed shell state expected to be less active and nonselective for olefin epoxidation. These results lead to a detailed mechanistic model that explains: (i) why C₂H₂ exhibits high selectivity for epoxide while higher olefins do not; (ii) the difference in activity per surface site between the (110) and (111) surfaces of Ag; and (iii) the role of both electropositive (e.g. Cs) and electronegative (e.g. Cl) promoters in increasing selectivity. A number of experiments are proposed which would test key points of this new mechanism.