Model studies of ethylene epoxidation catalyzed by the Ag(110) surface

Abstract

The selective oxidation of ethylene to ethylene epoxide (C 2H 4 + 1 2 O 2 → C 2H 4O) over Ag is the simplest prototype for the entire class of kinetically-controlled, selective catalytic reactions. We have studied the mechanism of this reaction on a well-characterized Ag(110) surface by combining high-pressure kinetic measurements with ultrahigh vacuum surface analysis in a single apparatus. In a typical experiment, the surface cleanliness and order are established by AES and LEED; the sample is transferred into a microreactor where a steady-state reaction rate is established; and the sample is rapidly (17–45 s) transferred back into UHV for surface characterization (AES, LEED, XPS, TDS). In this way we ware able to develop a method for quantitatively measuring the coverage of atomically adsorbed oxygen (θ O) under steady-state reaction conditions. The oxygen adatoms are shown by LEED to exist in p(2 × 1) islands of local coverage θ O = 0.5. The effects of temperature and reactant pressures upon the rate and selectivity over Ag(110) agree perfectly with results on high-surface-area supported Ag catalysts, although the specific activity (per Ag surface atom) is some 100-fold higher on Ag(110). At constant temperature and ethylene pressure, the rate of ethylene epoxidation varies linearly with θ O while the rate of the side reaction leading to CO 2 shows a sharp break in slope near θ O = 0.02. This and our other data are interpreted by a mechanism involving molecularly adsorbed oxygen and adsorbed ethylene in a common rate-determining step for both ethylene epoxide and CO 2 formation. Another pathway for CO 2 production via atomically adsorbed oxygen and carbon predominates at very low or very high θ O. Oxygen adatoms are necessary in ethylene epoxide formation only for creating special ethylene adsorption sites (Ag δ+). We were unable to produce observable amounts of ethylene epoxide by reacting ethylene gas with oxygen adatoms alone.