IN-SITU CONTROLLED PROMOTION OF CATALYST SURFACES - NON-FARADAIC ELECTROCHEMICAL MODIFICATION OF CATALYTIC ACTIVITY

The in situ electrochemical supply of promoters (ionic species), via c urrent and voltage application (typically +/- 1-2 V) on metal catalyst films, interfaced with solid electrolytes, can cause dramatic and rev ersible modifications in catalytic activity and product selectivity. T he catalytic activity can be enhanced by up to a factor of 100, and th e induced change in the catalytic rate is strongly non-faradaic, i.e., up to five orders of magnitude larger than the steady-state rate of e lectrochemical supply of ionic species from the solid electrolyte onto the catalytic surface. This novel phenomenon has been termed electroc hemical promotion or in situ controlled promotion or Non Faradaic Elec trochemical Modification of Catalytic Activity (NEMCA) and has been de monstrated for a large variety of catalytic reactions (over 40), catal ysts Pt, Ag, Rh, Pd, Ni, and IrO2), and solid electrolytes (O2-, F-, N a+, H+ conductors), and more recently for aqueous alkaline solutions. In this paper, we show explicitly, on the basis of XPS, TPD, and cycli c voltammetry, that NEMCA is due to the electrochemically controlled b ack-spillover of promoting species onto the catalytic surface and the concomitant change in catalyst work function and surface chemisorptive properties. This in situ controlled promotion of catalysts underlines the role of the electronic factor in catalysis and allows for a syste matic study of the role of promoters.