A single crystal study of the silver-catalysed selective oxidation and total oxidation of ethylene

Abstract

Ethylene oxidation has been investigated on a well-characterised Ag(111) single crystal surface at pressures of up to 50 Torr. In the absence of promoters and moderators, chemisorbed atomic oxygen reacts with adsorbed ethylene to yield both ethylene oxide and (CO 2 + H 2O). Chemisorbed dioxygen, though present, appears to play no direct role in either of these reactions; the presence of subsurface oxygen is necessary for selective oxidation but not for total oxidation. Batch reactor studies yield rate parameters for both partial and total oxidation which are consistent with the values reported for conventional supported catalysts; selectivity decreases with increasing temperature, pressure, and ethylene coverage. Acetaldehyde, acetic acid, and oxalic acid are identified as reaction intermediates in the pathway to CO 2 formation. Results for the oxidation of C 2D 4 confirm these observations, and the observed kinetic isotope effect indicates that H-transfer rather than CC cleavage is rate-determining in the combustion of both ethylene and ethylene oxide. Possible reaction pathways and mechanisms are examined.