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

The mechanism of iodide oxidation on platinum electrodes is investigated us ing laser-activated voltammetry under both channel flow and no flow conditi ons together with independent in situ atomic force microscopy (AFM) measure ments. Laser activation using a 10 Hz pulsed Nd:YAG 532 nm laser is shown t o remove bulk iodine from the electrode surface so that under sustained pul sed irradiation a steady-state surface evolves at which the iodide oxidatio n can be reproducibly studied. When the concentration and flow rate depende nce of the voltammetric wave shape are modeled, the mechanism is shown to b e 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- (a g) + I-2 (s) --> I-3(-) (ag) (iv), where the formal redox potential for rea ction i is 0.358 V vs SCE in 0.1 M H2SO4, the equilibrium constant for reac tion 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 hete rogeneous rate constant for reaction iv is 1.6 x 10-3 cm s(-1).