Chlorine promotion of selective ethylene oxidation over Ag(110): Kinetics and mechanism

The selective oxidation of ethylene to ethylene epoxide (C₂H₄ + 1 2 O₂ → C₂H₄O) is catalyzed industrially with a supported Ag catalyst which is promoted for selectivity by adding trace amounts of chlorinated hydrocarbons to the reactant feed. It was recently shown that the role of the chlorine promoters can be modeled by adding chlorine adatoms to a clean Ag(110) surface, and observing how the reaction rates and selectivity vary with chlorine coverage (θ_Cl) (C. T. Campbell and M. T. Paffett, Appl. Surf. Sci. 19, 28 (1984); Proceedings 1983, MRS Symposium, Boston. A continuation of that work is presented in which the reaction kinetics as a function of θ_Cl have been carefully mapped. From these data a mechanism for promotion is developed. The experiments are performed in a special apparatus which allows rapid (< 20 s) transfer of the model Ag(110) catalyst between an ultrahigh vacuum chamber for surface preparation and analysis (XPS, AES, LEED, TDS) and a high-pressure (< 10 atm) microreactor for catalytic rate measurements. It was found that the steady-state reaction rates to produce both ethylene epoxide and CO₂ + H₂O have activation energies and orders in the reactant pressures which vary in characteristic ways with θ_Cl. These variations can be very nicely correlated with the electronic effects of θ_Cl, upon the desorption energies for molecularly adsorbed O₂ and ethylene. These have, however, no relationship to the large increase in selectivity appearing when θ_Cl > 0.4 (after nucleation of a p(2 × 1)-Cl structure). It is concluded that the selectivity increase is driven by an ensemble effect, whereby CO₂ production requires more free Ag sites than does epoxide production.