JOURNAL OF CATALYSIS 160, 205–213 (1996) ARTICLE NO. 0139 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 Ch. Karavasilis, S. Bebelis, andC. G. Vayenas Department of Chemical Engineering, University of Patras, GR-26500 Patras, Greece Received May 8, 1995; revised November 20, 1995; accepted January 18, 1996 The effect of non-Faradaic electrochemical modification of cata- lytic activity, or in situ controlled promotion, was investigated dur- ing ethylene epoxidation on Ag films deposited on β ′′ -Al 2 O 3 , a Na + conductor, at temperatures 240 to 280 ◦ C and 500 kPa total pres- sure in the presence of chlorinated hydrocarbon moderators. The β ′′ -Al 2 O 3 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 without affecting significantly the rate of complete oxidation. The promotion index of sodium for ethylene epoxida- tion is up to 40. Maximum selectivity to ethylene oxide (88%) is obtained for θ 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 promotion studies and the pre- vailing ideas about the mechanism of ethylene epoxidation. c 1996 Academic Press, Inc. INTRODUCTION The Ag-catalyzed epoxidation of ethylene, a reaction of great technological importance, is one of the most challeng- ing and thoroughly studied catalytic systems (1–4). Exten- sive research has been carried out during the past 30 years aimed at both enhancing the industrial selectivity to ethy- lene oxide and improving the fundamental understanding of the underlying catalytic chemistry. Work prior to 1987 has been reviewed by van Santen and Kuipers (4). Most industrial reactors operate currently with a selec- tivity in excess of 80% (4). The catalyst consists of reduced Ag particles dispersed on α-Al 2 O 3 . Alkalis are added to the catalyst as promoters and chlorinated hydrocarbons are added at the ppm level to the feed as moderators. The role of promoters and moderators is to a significant extent syn- ergistic and has been studied intensively in recent years (5– 12). Despite substantial progress in this area no unanimous conclusions have yet been reached. Regarding the catalytically active surface oxygen species, it is currently generally accepted that it is atomic oxygen (4), as shown conclusively by the work of Lambert and co- workers in the early and mid-1980s (13, 14). The presence of subsurface oxygen (15–17) is a necessary condition for obtaining high selectivity to ethylene oxide (4, 13, 18), while molecularly adsorbed oxygen has been shown to be a spec- tator species (4, 13, 14, 19). Product selectivity is strongly influenced by the binding state of atomic oxygen (4, 13). Weakly bound, electrophilic oxygen leads to epoxide by re- action with the π -orbitals of chemisorbed ethylene (4, 13, 20–22). Strongly bound, bridging, oxygen attacks preferen- tially C–H bonds and leads to nonselective oxidation (4, 13, 20–22). The effect of promoters and moderators on the coverage and binding state of adsorbed atomic oxygen is still the focal point of numerous studies (4–12). The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) (23) has been described in recent years for more than 30 catalytic reactions on Pt, Pd, Rh, Ag, Au, IrO 2 , and Ni catalysts interfaced with O 2− , Na + ,F − , and H + conducting solid electrolytes (24–37) and more recently with aqueous solutions (38). The NEMCA literature has been reviewed recently (34–37). The effect has been shown to be due to an electrochemically induced and controlled backspillover of promoting species (O δ− , Na δ + , etc.) from the solid electrolyte onto the catalyst sur- face (25, 34, 39) as recently confirmed by XPS (39). The NEMCA-induced reversible change in catalytic rate is up to 3 × 10 5 higher than the rate of ion supply (24) and up to 100 times larger than the open-circuit rate (27). More recently even higher rate enhancement values have been obtained for the NO reduction by ethylene (31). The im- portance of NEMCA in catalysis (40) and electrochemistry (41) has been discussed recently. Previous NEMCA studies utilizing β ′′ -Al 2 O 3 , a Na + con- ductor, as the solid electrolyte and promoter donor, were limited to Pt catalysts (28, 29, 31, 33). The present work is the first NEMCA study utilizing β ′′ -Al 2 O 3 to promote a catalytic reaction on Ag. A useful parameter for quantify- ing NEMCA and the role of promoters is the promotion index P i (27, 29) defined from P i = r /r o θ i , [1] 205 0021-9517/96 $18.00 Copyright c  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.