ELECTROCHEMICAL PROMOTION

The catalytic activity and selectivity of metal films in contact with electrolytes, both solid state and aqueous, can be varied in situ in a very pronounced and reversible manner via electrical potential applic ation (typically +/-1-2 V) between the catalyst film and a counter ele ctrode also in contact with the electrolyte. Catalytic rates can thus be varied by up to a factor of 200. The induced steady-state change in catalytic rate is up to five orders of magnitude larger than the stea dy-state rate of electrochemically supplied ionic species from the ele ctrolyte onto the catalyst surface. This novel effect of electrochemic al promotion or non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) has been studied for over 45 catalytic reacti ons on Pt, Rh, Pd, Ag, Ni and IrO2 catalyst-electrodes using a variety of solid electrolytes (O2-, F-, Na+, H+ conductors) and more recently mixed ionic-electronic conductors and aqueous alkaline solutions. In addition to potential technological applications, this new effect allo ws for a systematic study of the role of promoters in heterogeneous ca talysis. In this paper the main features of electrochemical promotion are summarized and the origin of the effect is discussed on the basis of work function measurements and recent XPS, UPS, TPD, cyclic voltamm etric, STM and ab initio quantum mechanical investigations.