Possible importance of adions in both the binding of Lewis bases and the electrocatalytic oxidation of dissolved organics at gold electrodes in aqueous media

An investigation of the cyclic voltammetry behaviour of gold in aqueous sulphate media (pH = 11.7), containing dissolved organic or inorganic bases, confirmed the existence of reversible redox transitions at ca. 0 V (RHE). These results were attributed to the formation of Au(I) complexes, this unusual oxidation state of gold being stabilized by a Lewis acid/base reaction between the cation and the lone pair electrons of the base. The influence of such factors as base concentration, sweep limits, sweep-rate, and the pK_a of the conjugate acid of the base, on the voltammetric profile was investigated. The results were discussed in terms of the importance of adatom and adcomplex species at the metal/solution interface. A novel theory of electrocatalysis of electrooxidation reactions at noble metal anode surfaces was outlined. Such reactions usually commenced (positive sweep) and terminated (negative sweep) at potentials where metal/incipient hydrous Au(III) oxide transition occured at the metal/solution interface and the electrooxidation of solution species at more positive potential was explained in terms of a cyclic interfacial redox scheme as postulated earlier for oxide electrode systems. The complexity of the voltammetric response for such reactions, i.e. the apperance of peaks rather than a flat plateau, was attributed to changes in the density or character of the reactive hydroxy species on the metal surface with both time and potential. For certain species, e.g. CO and aldehydes, the onset of oxidation on gold in bases commenced at a lower potential, in the region of the Au(0)/Au(I) transition: an interfacial redox cycle, based on the latter process, was assumed to be involved in these cases.