Alkali metal, chlorine and other promoters in the silver-catalysed selective oxidation of ethylene

Abstract

Ethylene oxidation over well characterised Ag(111) surfaces has been investigated by temperature-programmed reaction measurements and by differential batch reactor studies at pressures up to 50 Torr. The influence of chlorine predosing on catalytic activity indicates that a chemisorbed atomic oxygen species is responsible for both partial oxidation and complete oxidation to CO₂+ H₂O. This tends to be confirmed by experiments using N₂O as the oxidant, both with the single crystal speciemen and with a practical Ag–αAl₂O₃ catalyst in a flow microreactor. Dissolved oxygen, like adsorbed chlorine, is found to be a selectivity promoter. Adsorbed Cs also increases the rate of ethylene oxide production but can also positively influence the overall activity of the system. The results suggest that chlorine and dissolved oxygen promoters principally affect the primary chemistry, whereas the main effect of Cs is on the secondary chemistry (further oxidation of ethylene oxide). This view tends to be confirmed by temperature programmed reaction measurements and by direct experiments on the influence of CI and Cs on the isomerisation and combustion of ethylene oxide itself. It is suggested that these effects are primarily electronic in origin and a mechanism based on this view is presented. In the presence of Cs, both CO₂ and NO_x can act as selectivity promoters for the formation of ethylene oxide.