Laser-activated voltammetry.: Mechanism of aqueous iodide oxidation at platinum electrodes:: Theory and experiment

The mechanism of iodide oxidation on platinum electrodes is investigated using laser-activated voltammetry under both channel flow and no flow conditions together with independent in situ atomic force microscopy (AFM) measurements. Laser activation using a 10 Hz pulsed Nd:YAG 532 nm laser is shown to remove bulk iodine from the electrode surface so that under sustained pulsed irradiation a steady-state surface evolves at which the iodide oxidation can be reproducibly studied. When the concentration and flow rate dependence of the voltammetric wave shape are modeled, the mechanism is shown to be the following: 2I(-) (ag) - 2e(-) reversible arrow I-2 (ag) (i); I-2 (ag) + I- (aq) reversible arrow I-3(-) (ag) (ii); I-2 (aq) I-2 (s) (iii); I- (ag) + I-2 (s) --> I-3(-) (ag) (iv), where the formal redox potential for reaction i is 0.358 V vs SCE in 0.1 M H2SO4, the equilibrium constant for reaction ii is 580 M-1 with a forward rate constant of 1 x 10(5) mol(-1) cm(3) s(-1), the solubility of I-2 (reaction ii) is 1.85 x 10(-3) M, and the heterogeneous rate constant for reaction iv is 1.6 x 10-3 cm s(-1).