Electrocatalytic reduction of chromate(VI) ions in alkaline media on Pt modified by underpotential deposition of heavy metals

Abstract

The electrocatalytic reduction of Cr(VI) ions in alkaline media was investigated on a Pt electrode and Pt modified by underpotential deposition (upd) of Bi ions. It was shown that the electroreduction of Cr(VI) on smooth Pt is significantly inhibited and such inhibition was attributed mainly to adsorption phenomena and to the formation of insoluble Cr(OH) 3. The observed irregularities were affected by the concentration of OH − ions, since an excess of OH − causes the dissolution of the deposited Cr(OH) 3 by forming the soluble complex Cr(OH) − 4. The upd monolayers catalyse remarkably the reduction of Cr(VI) ions, shifting the peak potential towards less negative potentials by more than 200 mV. The currents obtained on Pt/Bi upd were also enhanced compared to those obtained on smooth Pt. The electrocatalytic activity of Pt/Bi upd was evaluated quantitatively by inspecting the Tafel slopes determined for sufficiently high concentrations of OH −. The observed catalytic effects could be interpreted in terms of a restriction of adsorption phenomena and an acceleration of the dissolution of the insoluble product.