Recent progress made in our laboratory in the field of electrochemical promotion of heterogeneous catalytic gas reactions is reviewed. The phenomenon consists of electrochemical polarization of the catalyst/solid electrolyte interface resulting in a dramatic, non-Faradaic modification of the catalytic reaction rate. Two main aspects have been addressed, the mechanistic interpretation of the phenomenon and the development of bipolar cell configurations suitable for practical application. Fundamental studies were made using single-pellet type electrochemical cells with yttria-stabilized zirconia (YSZ) solid electrolyte. Promotion of the reduction of NO by propylene over Rh/YSZ catalyst under oxidative conditions at 300ºC was found to be reversible over an active, essentially metallic, rhodium but irreversible over a deactivated, presumably partially oxidized, catalyst. It was shown that anodic current application with negligible power consumption might efficiently assist the recovery of accidentally lost catalytic activity of rhodium. In situ cyclic voltammetry of IrO2/YSZ catalysts showing a linear relation between voltammetric charge and combustion rate of ethylene suggested that promotion of IrO2 was ruled by the charge stored on it. In situ measurements of work function over RuO2/YSZ catalysts established a clear relation between catalyst work function and surface concentration of chemisorbed oxygen interpreted by formation of surface dipoles between adsorbed species and adsorption sites. Transient behaviour of the work function under and after polarization was closely related to the evolution of the combustion rate of ethylene supporting the idea of promoting oxygen species spreading out over the gas-exposed catalyst surface. On the application side, two new cell designs were developed, a ring-shaped and a multiple-channel configuration both operated in bipolar polarization mode. The current bypass of the ring-shaped cell determined from current-voltage curves was very low whereas in the multiple-voltage curves was very low whereas in the multiple-channel configuration both the current bypass and the cell voltage were much higher. Feasibility of electrochemical promotion was successfully demonstrated with both configurations. Promotion of the reduction of NO by propylene in ring-shaped bipolar Rh/YSZ cells and that of the combustion of ethylene in bipolar RuO2/YSZ cells of both configurations were non-Faradaic even at high open-circuit conversions. Realization of efficient bipolar cell configurations for electrochemical promotion is very promising in view of future applications in dispersed catalytic systems.
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