On the electroreduction mechanism of Cr(VI) aqueous solutions on iron and copper cathodes

The electroreductive behaviour of CrO3 + H2SO4 aqueous solution is studied on poly-crystalline iron and copper cathodes. The electrochemical results have been also compared with XPS, AFM and SEM findings collected “ex situ” on emersed electrodes and an electrochemical mechanism featuring adsorbed Cr(IV) and Cr(III) species is proposed. In the case of iron a main role is played by the passive state of the electrodic surface caused by the presence of a protective mixed iron/chromium oxide layer and by its electrochemical dissolution. This passive to active transition of the electrodic surface and the occurrence of an irreversible reductive current peak, in a potential range where the deposition of metallic chromium does not yet occur, suggests that the electrodic surface plays an active role in the electrochemical process. A major difference is found between Fe and Cu cathodes, concerning the kinetic mechanism of formation of adsorbed species.