Origin of non-Faradayicity in electrochemical promotion of catalytic ethylene oxidation

The electrochemical promotion of catalytic C(2)H(4) oxidation has been investigated under low pressure conditions (p approximate to 10(-6)-10(-4) mbar) with a Pt film on yttrium stabilized zirconia (YSZ) as catalyst. All measurements were conducted with a UHV system with a differentially pumped quadruple mass spectrometer (QMS) for rate measurements and a photoelectron emission microscope (PEEM) for spatially resolved measurements. A pronounced rate hysteresis upon cyclic variation of p(C(2)H(4)) was observed under open circuit conditions which is attributed to carbonaceous CH(x) adlayer inhibiting O(2) adsorption and hence poisoning the reaction. Application of a positive potential causes a partial removal of the inhibiting CH(x) adlayer by spillover oxygen thus triggering a transition from thus poisoned state of the surface to an active state with reduced carbon coverage. The ignition effect and therefore also the electrochemical promotion effect are linked to the presence of a carbon adlayer on the surface. The non-Faradayicity which is observed in this reaction system is explained as an ignition effect.