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

Abstract

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO 4 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 Bi 3+. This indicates the accelerating effect of Bi 3+ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb 2+, Hg 2+, Tl +, Ag + and Sb 3+, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as M ad) 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 Co 2+, Zn 2+, Fe 3+ and Ni 2+, exhibit no catalytic effect. These facts suggest that the presence of a M ad 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 M ad, for example, Cu ad, Cd ad, In ad, Sn ad and Mo ad display no catalytic action. The catalytic activity depends on the degree of surface coverage by the M ad. The maximal effect of the M ad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the M ad 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 M ad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.