Selectivity in the epoxy:lawn of ethylene over Ag/alpha-Al2O3 catalysts has been shown to be a function of the Ag-O bond strength The weaker the Ag-O bond is the more selective it is Promotion by Cl atoms which are held under the surface of the Ag is effected by their weakening the Ag-O bond Promotion by Cs is effected by its being adsorbed on the Ag sites which form the stronger Ag-O bond so preventing the formation of the unselective Ag-O bond The reaction proceeds by the ethylene adsorbing on surface O atoms forming two types of Ag-O-CH2-CH2 intermediates whose fate is a function of the Ag-O bond strength The Ag-O-CH2-CH2 intermediate formed on the area of the Ag surface having the weaker Ag-O bond (the Ag(111) surface) has a greater probability of cyclising to form EO than being involved in unselective H abstraction reactions Conversely the Ag-O-CH2-CH2 intermediate formed on the area of the Ag surface having the stronger Ag-O bond (a stepped surface) will have a greater probability of being involved in unselective H abstraction reactions by the more electronegative surface O atoms than cyclising to form EO The unselecnve reaction on both surfaces is a collective reaction of the sea of surface O atoms forming H bonds of different strength to the adsorbed intermediate The same adsorbed O atom is therefore involved in both the selective and unselective reactions the extent of which it is involved in each is determined by the Ag-O bond strength This is an entirely new concept in the partial oxidation of ethylene to ethylene oxide (C) 2010 Elsevier B V All rights reserved