The role of chlorine promoters in catalytic ethylene epoxidation over the Ag(110) surface

Abstract

We have recently shown that the kinetics and selectivity of ethylene epoxidation over clean Ag(110) mimic identically the results over unpromoted, supported Ag catalysts, except for a 100-fold enhancement of the specific activity (per surface Ag atom) on Ag(110) [1,2]. In this report, we will discuss results in which we have modeled the role of chlorine promoters in this reaction by combining ultrahigh vacuum surface analysis (XPS, AES, LEED, TDS) before and after high-pressure (≈ 100 torr) kinetic measurements. In this way, we were able to correlate the reaction rate and selectivity not only with temperature and reactant pressures, but also with the coverages of atomically adsorbed oxygen and chlorine. In industrial catalysis, trace amounts of chlorinated organics are added to the feed stream to promote selectivity with, however, some decrease in the reaction rate. We have obtained these same results on Ag(110) by predosing atomically adsorbed chlorine. The major effect occurs between the p(2 × 1) ( θ Cl = 0.5) and the c(4 × 2) ( θ Cl = 0.75) structures, suggesting an ensemble rather than electronic effect. Chlorine coverages above 0.3 completely suppress the rate of dissociative O 2 adsorption and consequently the steady-state coverage of atomic oxygen under reaction conditions. Both ethylene oxide and CO 2 production utilize molecularly absorbed oxygen as the oxidizing agent. Atomically absorbed oxygen and chlorine play a similar role in the reaction by creating Ag δ+ sites for ethylene adsorption.