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

Abstract

Abstract The selective oxidation of ethylene to ethylene epoxide (C 2 H 4 + 1 2 O 2 → C 2 H 4 O) 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 (θO) under steady-state reaction conditions. The oxygen adatoms are shown by LEED to exist in p(2 × 1) islands of local coverage θO = 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 θO while the rate of the side reaction leading to CO2 shows a sharp break in slope near θO = 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 CO2 formation. Another pathway for CO2 production via atomically adsorbed oxygen and carbon predominates at very low or very high θO. Oxygen adatoms are necessary in ethylene epoxide formation only for creating special ethylene adsorption sites (Agδ+). We were unable to produce observable amounts of ethylene epoxide by reacting ethylene gas with oxygen adatoms alone.