IN-SITU CONTROLLED PROMOTION OF CATALYST SURFACES VIA SOLID ELECTROLYTES - THE NEMCA EFFECT

The catalytic activity and selectivity of metal films interfaced with solid electrolytes can be varied in situ in a dramatic and reversible manner by applying currents or voltages (typically +/-1-2 V) between t he catalyst film and a counter electrode also deposited on the solid e lectrolyte. Catalytic rates can thus be varied by up to a factor of 20 0. The induced steady-state change in catalytic rate is up to five ord ers of magnitude larger than the steady-state rate of electrochemical supply of ionic species from the solid electrolyte onto the catalyst s urface. This novel effect termed Non-Faradaic Electrochemical modifica tion of catalytic activity (NEMCA) has been studied for over 40 cataly tic reactions on Pt, Ph, Pd, Ag, Ni and IrO2 catalysts using a variety of solid electrolytes (O2-, F-, Na+, H+ conductors) and also, more re cently, in aqueous alkaline solutions. Besides potential technological applications, this new effect allows for a systematic study of the ro le of promoters in heterogeneous catalysis. In this paper the main fea tures of in situ controlled promotion are reviewed and the origin of t he effect is discussed on the basis of recent XPS, TPD and cyclic volt ammetric investigations.