Dichromate reduction on gold and platinum electrodes in aqueous acid solutions

The reduction of dichromate, or Cr(VI), species in acid solution at noble metal electrodes is an intriguing example of electrocatalytic behaviour as, contrary to conventional expectation in this area, reaction occurs rapidly at a much lower overpotential on gold as compared with platinum. Variation of current/voltage behaviour with the nature of the electrode material is a clear indication of electrocatalysis and the trend outlined here for these two metals was confirmed (as will be reported shortly) for nitrous acid reduction under similar conditions, ie the observed trend is a property of the electrode systems and not the solution species. The onset (negative sweep)/termination (positive sweep) for rapid Cr(VI) reduction, at ca. 0.95 V (RHE) for gold and ca. 0.2 V (RHE) for platinum, may be rationalized on the basis of the hydrous oxide (or premonolayer oxidation) electrochemistry of these two metals. At the potentials quoted here the interfaces in question are not totally deactivated; below these potential values the metals are active with regard to electrocatalytic reduction, whereas above these values they are active with regard to electrocatalytic oxidation, of dissolved solution species. It is assumed that a redox transition at the above potentials causes a change in the interfacial mediators at low coverage active sites on the electrode surface. The nature of the solution anion (ClO4-, HSO4-, H2PO4-) had little effect on the reaction in the case of gold-but complications were observed with platinum. Copyright (C) 1996 Elsevier Science Ltd