Catalytic effects of metal submonolayers formed by faradaic adsorption on the anodic oxidation of ascorbic acid at a plantimum electrode

Abstract

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO4 were studied by linear sweep voltammetry. The anodic peak due to a two-electron oxidation of ascorbic acid shifts to the negative potential side on the addition of Bi3+. This indicates the accelerating effect of Bi3+ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb2+, Hg2+, Tl+, Ag+ and Sb3+, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as Mad) still remain on the electrode surface until the electrode potential goes up to and beyond the peak potential of the oxidation of ascorbic acid. On the other hand, metal ions forming no adsorbed layer on Pt, such as Co2+, Zn2+, Fe3+ and Ni2+, exhibit no catalytic effect. These facts suggest that the presence of a Mad on Pt is essential for the promotion of the anodic oxidation of ascorbic acid. However, there is a difference in the catalytic action among the Mad, for example, Cuad, Cdad, Inad, Snad and Moad display no catalytic action. The catalytic activity depends on the degree of surface coverage by the Mad. The maximal effect of the Mad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the Mad plays its major role in the removal of the adsorbed ascorbic acid occupying active sites on the electrode surface, and provides effective sites for the activation of adjacent water molecules. Furthermore, from the 13C NMR spectra for the oxidation products, the adsorbed water on the Mad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.