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

Abstract

The selective oxidation of ethylene to ethylene epoxide (C 2H 4 + 1 2 O 2 → C 2H 4O) 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 CO 2 + H 2O 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 O 2 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 CO 2 production requires more free Ag sites than does epoxide production.