ELECTROCATALYTIC REDUCTION OF IODATE AT PLATINUM-ELECTRODES IN 0.5 M H2SO4

Cyclic voltammetry at rotated disk electrodes was applied to a study of the electrocatalytic reduction of IO3- at Pt electrodes in 0.5 M H2SO4. Transient reduction of IO3- to I2,aq was observed to occur concomitantly with cathodic dissolution of surface oxide during the negative potential scan at a preanodized electrode. Furthermore, IO3- reduction at oxide-free Pt surfaces was determined to be very sensitive to the method applied for surface pretreatment. Hence, IO3-reduction at the oxide-free surfaces, previously concluded to occur by a homogeneously mediated process involving reduction of I2,aq generated by reaction of IO3- with I- in the diffusion layer, is now determined to have a strong electrocatalytic component. A mechanism is proposed in which IO3- is weakly associated to strongly adsorbed iodine (I0ads) on the oxide-free surface. This mechanism is very sensitive to the presence of adsorbed organic impurities originating from some cleaning materials. However, those impurities do not interfere with reduction of I2,aq, probably because of the stronger adsorption of I2 to the I0ads sites on the Pt surface.