The use of fuel cells for carrying out oxidation reactions with cogeneration of electrical power and chemicals led, upon cofeeding oxygen and fuel at the anode, to the discovery of the effect of non-Faradaic electrochemical modification of catalytic activity or electrochemical promotion of catalysis. This phenomenon has been studied already for more than 70 catalytic reactions, including oxidations, reductions and isomerizations and using a variety of metal catalysts, and solid electrolytes. In this work we summarize the main features of electrochemical promotion and discuss critically its currently accepted sacrificial promoter mechanism which involves electrochemically controlled migration (spillover-backspillover) of promoting species from the electrolyte to the catalytically active metal-gas interface. It is shown that the spillover ionic species (e.g., O(delta-), Na(delta+)) form an overall neutral double layer at the catalyst-gas interface which alters the catalyst work function and the binding energies of coadsorbed reactants and intermediates, thus causing very pronounced and reversible alterations in the catalytic activation energy and catalytic rate and selectivity. Recent efforts for the practical utilization of electrochemical promotion are also briefly discussed.

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The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion was investigated for the model catalytic reaction of C 2 H 4 oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement q is up to 400 for thinner (0.2 lm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 lm) films. The Faradaic efficiency L was found to increase moderately with increasing film thickness and to be described semiquantitatively by the ratio 2Fr o /I 0 , where r o is the unpromoted rate and I 0 is the exchange current of the catalyst-electrolyte interface. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O 2-species, which causes electrochemical promotion.

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion of catalysis (EPOC or NEMCA effect) was investigated for the model catalytic reaction of C 2 H 4 oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement q is up to 400 for thinner (0.2 lm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 lm) films. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O 2) species which causes electrochemical promotion.