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MMS Electrochemistry Of GOLD

The MMS electrochemistry of gold is concerned with the behavior of unstable, high-energy, protruding surface gold atoms that undergo oxidation to yield a more dispersed (low-density), hydrated, largely amorphous, hydrous () gold oxide (a review of metal hydrous oxide electrochemistry was published earlier). Multilayer hydrous oxide deposits may be produced on gold in aqueous media by subjecting the electrode to either severe dc polarization , e.g., 3 min at 2.2 V, or to repetitive potential cycling using appropriate upper and lower limits [26,27], e.g., 0.90–2.40 V at 50 V s−1 for 1200 cycles (the efficiency of the oxide growth reaction is dependent on many factors,including the solution pH).

Thick deposits are not produced in a single sweep because the precursor state (MMS gold atoms) exists at the metal surface only at a very low coverage. In the dc procedure oxygen gas evolution occurs quite vigorously initially at an oxide–coated gold surface,and oxide formation apparently occurs as a side reaction, i.e., the gas evolution involves repetitive formation and decomposition of an unstable higher oxide (probably AuO2), and the resulting disturbance of the oxide film leads to a gradual accumulation of an outer, porous, gold oxide deposit. In the potential cycling procedure each cycle results in formation and reduction of an oxide film. Again, there is a side reaction involved, reduction of the place-exchanged oxide in the negative sweep leads to the formation of some MMS atoms which, in the next positive sweep, are converted to oxide species. It is important that, at the lower limit used for oxide growth, virtually all of the, but none of the, oxide is reduced; in this manner the oxide deposit can attain multilayer coverage.

Gold hydrous oxide is not particularly stable; according to Pourbaix’s thermodynamic data it should undergo reduction to the metal in acid solution at ca. 1.4 V. However, in practice oxide reduction occurs only under conditions of considerable cathodic overpotential and invariably at a lower potential than the peak for the reduction of the thin inner oxide film at the same gold surface (after multilayer oxide growth the layer configuration at the interface is usually gold/ oxide/ oxide/aqueous solution; there may be some degree of intermingling of the oxide and aqueous phase). The oxide reduction responses in the negative sweeps are often complicated by the presence of several oxide reduction peaks [27,29], plus the fact that (even in terms of the RHE scale) the  oxide reduction peaks tend to occur at a lower potential in base as compared to acid; this effect is highlighted by the observation, of a sharp oxide reduction peak at E≈ –0.2 V in the case of gold in base. This type of behavior is not confined to gold. Platinum [30,31] and iridium  oxides also show similar behavior.