The Promotion Effect in Heterogeneous Catalysis: the Role of Cl on the Epoxidation of Ethylene over Ag

We report a direct in situ observation of a promoter modifying the active sites of a heterogeneous catalyst by near ambient pressure X-ray photoelectron spectroscopy. For Ag catalysts under ethylene epoxidation, the increase in selectivity promoted by Cl arises from changes in the balance between electrophilic and nucleophilic oxygen species.