The electrochemical promotion of the C2H4 + O-2 reaction over a bimetallic Pt/Ag catalyst (Pt:Ag ratio approximate to 1.5) interfaced to yttrium stabilized zirconia (YSZ) has been studied at 0.25 mbar and T = 650 K using X-ray photoelectron spectroscopy as in situ method. Applying a positive potential of 2 V causes a relative rate increase in the CO2 production up to 120%; the electrocatalytic promotion effect is non-Faradaic (Lambda approximate to 2). An electrochemical promotion is found to occur only at high enough p(C2H4) when a carbonaceous CHx layers builds up inhibiting O-2 adsorption. The CHx film is estimated to be about 5-6 layers thick. Only at low p(C2H4), the application of an electric potential causes a decrease in the carbon signal associated with a growth of the O1s signal at 529.3 eV. The latter species can be assigned to an electrochemically generated oxygen spillover species at Ag sites. (C) 2012 Elsevier Inc. All rights reserved.