The role of surface sodium species in electrochemical promotion of catalysis

Abstract

Electrochemical Promotion of Catalysis (EPOC) studies the promotion of catalytic activity and selectivity by supplying promoting species from the electrolyte support to the catalyst surface via the application of an external electrical potential between the catalyst-electrode and a counter electrode also supported on the electrolyte. The effect has been observed for a wide range of catalytic systems, however, very little work exists on the role of impurities in EPOC, although their presence may affect the catalytic and electrocatalytic properties of a catalyst or may in fact is necessary for the promotion to occur. In order to systematically study the role of impurities in EPOC, a known type and amount of an impurity can be deposited in increasing concentration on a nominally ‘clean’ catalyst surface. In this work, the role of sodium addition to a platinum catalyst-film interfaced with an yttria-stabilised-zirconia (YSZ) dense membrane was studied under non-reactive conditions (oxygen charge transfer) and reactive conditions (ethylene oxidation and NO reduction by propene). It was found that sodium addition on the catalyst surface can significantly affect the oxygen charge transfer, catalytic and electrocatalytic properties of the Pt/YSZ system, however, there is no clear evidence that such species are necessary for the observation of EPOC. Electrical polarisation and sodium addition seem to a first approximation to have an additive effect as electronic promoter on the electrochemical promotion when there is low lateral interaction between the surface ions and insignificant sodium interaction with the reaction components. Ethylene oxidation reaction changed in behaviour from electrophilic at low sodium coverage (0.11%) and low to intermediate oxygen partial pressure (pO2 ≤ 3kPa) to electrophobic at high sodium coverage (65%) and under high oxygen partial pressures (pO2 = 8 kPa). In between the two sets of conditions, the reaction showed volcano-type behaviour depending on the coverage of sodium and gas phase oxygen partial pressure. The behavioural changes are more complicated for the NO reduction system as more reaction components are involved especially under high oxygen partial pressures.