Surface Science Study of Selective Ethylene Epoxidation Catalyzed by the Ag(110) Surface: Structural Sensitivity

摘要

The selective oxidation of ethylene to ethylene epoxide (C sub 2 H sub 4 + 1/2 O sub 2 implies C sub 2 H sub 4 O) over Ag is the simplest example of kinetically-controlled, selective heterogeneous catalysis. We have studied the steady-state kinetics and selectivity of this reaction for the first time on a clean, well-characterized Ag(110) surface by using a special apparatus which allows rapid (approx. 20 s) transfer between a high-pressure catalytic microreactor and an ultrahigh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The effects of temperature and reactant pressures upon the rate and selectivity are virtually identical on Ag(110) and supported, high-surface-area Ag catalysts. The absolute specific rate (per Ag surface atom) is however some 100-fold higher for Ag(110) than for high-surface-area catalysts. This is related to the well-known structural sensitivity of this reaction. It is postulated that a small percentage of (110) planes (or (110)-like sites) are responsible for most of the catalytic activity of high-surface-area catalysts. The high activity of the (110) plane is attributed to its high sticking probability for dissociative oxygen adsorption, since the rate of ethylene epoxidation is shown in a related work to be proportional to the coverage of atomically adsorbed oxygen at constant temperature and ethylene pressure. (ERA citation 08:056152)