Surface Electrocatalysis on Gold

Because gold displays very weak chemisorbing properties, the activated chemisorption model of electrocatalysis is assumed to be inapplicable in the case of this metal in aqueous media. The alternative, which is well established in the chemically modified electrode and redox sensor area, is the interfacial cyclic redox mediator model which, in the case of gold in aqueous media,is sometimes referred to as the incipient hydrous oxide/adatom mediator (IHOAM) [18,33] model. In the case of the Group 11 metals the mediator systems are unusual in that their redox transitions involve couples with nonequilibrium (or metastable) reduced and oxidized states.

A low coverage, surface-bonded, interfacial redox mediator system, M*/M(OH)n(n−z)−, is assumed to undergo a rapid, quasi-reversible redox transition at a potential Es which is well within the double layer region. In some cases this basic mediator (premonolayer redox) response is not evident (the active state surface coverage being too low) under dc voltammetry conditions. However, several options are available to locate such “hidden” mediator transitions, e.g.,

1. More sensitive analytical techniques, e.g., FT-ac voltammetry [12] or ac impedance may be used.

2. The surface may be deliberately disrupted or superactivated by severe thermal or cathodic pretreatment. Severe thermal activation results in strong promotion of at least one, and probably two, premonolayer oxidation responses in the case of gold in acid at E<0.6 V.

3. The onset of incipient oxidation, curve a , provides the interfacial mediator which triggers the oxidation of a dissolved reluctant, Red(aq). Thus, the electrocatalytic oxidation response, curve b , has an onset/termination potential coinciding with the interfacial mediator transition potential, and the former may be used to locate the latter (the same argument applies to an electrocatalytic reduction response, curve c.

4. The potential of the maximum of the multilayer hydrous oxide reduction peak, curve d, often coincides with (and thus indicates the value of) the interfacial mediator transition potential. Such correlations have been discussed for gold, platinum, and palladium in acid solution and for copper in base.