Νέοι Ηλεκτροχημικά Ενισχυόμενοι Αντιδραστήρες Και Κυψέλες Καυσίμου

Abstract

Electrochemical promotion, or non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA), was discovered at the beginning of 1980s by Vayenas and Stoukides, when the first nonfaradaic enhancement in heterogeneous catalysis was reported for the case of ethylene epoxidation on Ag electrodes deposited on a ceramic O 2conductor, via electrical potential application between the catalyst and a counter electrode.Since then Electrochemical Promotion of Catalysis has been proven to be a general phenomenon at the interface of Catalysis and Electrochemistry. During the last ten years its importance has grown on the academic level and more than seventeen groups around the world have made important contributions in this area and this number is reasonably expected to grow further as the phenomenon of electrochemical promotion has very recently been found to be intimately related not only to chemical (classical) promotion and spillover, but also to the "heart" of industrial catalysis, i.e. metal-support interactions of classical supported catalysts.The effect of Electrochemical Promotion is due to an electrochemically induced and controlled migration (backspillover) of ions (O δin the case of YSZ, Na δ+ in the case of β"-Al 2 O 3 ) from the solid electrolyte onto the gas-exposed, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal and forming dipoles ([Ο δ--δ + ] or [Na δ+ -δ -]) that migrate and form an effective electrochemical double layer on the entire gas-exposed catalyst surface, change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner.The change in catalytic activity or selectivity of the active phase with varying catalyst support is usually termed Metal-Support Interaction (MSI). Metal-support interactions can influence in a very pronounced way the catalytic and chemisorptive properties of metal and metal oxide catalysts. An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers which are transported from the carrier to the metal particles are oxygen ions which diffuse to the surface of the metal particles thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters.Recent studies have shown that, at least for the cases of ZrO 2 -, TiO 2 -and CeO 2 -based supports, the mechanism of the metal-support interaction is identical with that of NEMCA when using YSZ. Small metal crystallites on these supports are covered during catalytic reactions with backspillover promoting O 2species which are Λ times less reactive than normally chemisorbed O and which are continuously replenished in the O 2--or mixed-conducting support by gaseous O 2 . Overall, in both cases O 2backspillover to the catalyst surface i...