IN-SITU CONTROLLED PROMOTION OF CATALYST SURFACES VIA NEMCA - THE EFFECT OF NA ON THE AG-CATALYZED ETHYLENE EPOXIDATION IN THE PRESENCE OF CHLORINE MODERATORS

The effect of non-Faradaic electrochemical modification of catalytic a ctivity, or in situ controlled promotion, was investigated during ethy lene epoxidation on Ag films deposited on beta ''-Al2O3, a Na+ conduct or, at temperatures 240 to 280 degrees C and 500 kPa total pressure in the presence of chlorinated hydrocarbon moderators. The beta ''-Al2O3 support permits controlled and reversible potentiostatic introduction of various levels of sodium on the Ag catalyst surface. It was found that sodium coverages up to 0.03 enhance the rate of epoxidation witho ut affecting significantly the rate of complete oxidation. The promoti on index of sodium for ethylene epoxidation is up to 40. Maximum selec tivity to ethylene oxide (88%) is obtained for theta(Na) = 0.03 and 1 ppm dichloroethane. The promoting and synergistic action of sodium and chlorine is discussed on the basis of previous in situ controlled pro motion studies and the prevailing ideas about the mechanism of ethylen e epoxidation. (C) 1996 Academic Press, Inc.