NON-FARADAIC ELECTROCHEMICAL MODIFICATION OF CATALYTIC ACTIVITY .10. ETHYLENE EPOXIDATION ON AG DEPOSITED ON STABILIZED ZRO2 IN THE PRESENCE OF CHLORINE MODERATORS

The effect of non-Faradaic electrochemical modification of catalytic a ctivity (NEMCA), or in situ controlled promotion was investigated duri ng ethylene epoxidation on Ag films deposited on yttria-stabilized zir conia at temperatures from 240 to 300 degrees C and 500 kPa total pres sure in the presence of chlorinated hydrocarbon moderators. It was fou nd that the rates of epoxidation and complex oxidation change by a fac tor of 230 as the catalyst potential and work function are varied by 0 .6 V and 0.6 eV, respectively. The change in the total reaction rate i s typically a factor of 100 higher than the rate I/2F of electrochemic al supply or removal of promoting oxide ions. The selectivity to ethyl ene oxide can be varied between 0 and 78% by varying the catalyst pote ntial and dichloroethane partial pressure. At low catalyst potentials acetaldehyde becomes the main product with a selectivity up to 55%. Th e observed behavior is discussed on the basis of previous NEMCA studie s and of the prevailing ideas about the mechanism of ethylene epoxidat ion. (C) 1996 Academic Press, Inc.