Application of the hydrous oxide mediation model of electrocatalysis to reactions at noble metal anodes in base—II. Au, Ru, and Ir

Abstract

The oxidation of four compounds, hydrazine, the formate anion, formaldehyde and methanol, was investigated, using cyclic voltammetry techniques, at polycrystalline Au, Ir and Ru metal anodes in base (the behaviour at three other metals, Pt, Pd and Rh, in base was described in Part I of this series). The results were discussed in terms of the incipient hydrous oxide mediation model of electrocatalysis. In the case of Au it was demonstrated that the onset (anodic sweep)—termination (cathodic sweep) potential for different compounds varied—but the values noted coincided with earlier reported minor peaks for gold in base that corresponded to adatom—incipiet hydrous oxide transitions at quite low coverage sites on the surface. The behaviour of Ir and Ru was complicated by the fact that the potential ranges for hydrogen adsorption and monolayer oxide formation (both of which can exert an inhibiting influence) overlap, ie the region where the adatom—incipient hydrous oxide transition can occur without complications is extremely limited in the case of these two metals. Hydrous oxide transitions may be observed with the latter, but these tend to be oxide—oxide, rather than adatom-oxide reactions. For electrocatalytically demanding reactions involving an interfacial redox cycle the adatom—incipient hydrous oxide couples seem preferable as the reduced state (the adatom) is then more likely to enhance the activation of the dissolved reactant via chemisorption effects.