TRANSPORT AND KINETICS AT MICROHETEROGENEOUS ELECTRODES .5. THE METHYLVIOLOGEN PLATINUM SYSTEM

Results are reported for the reaction of methylviologen radical cation, MV** plus , with platinum colloidal particles, studied by stopped flow spectrophotometry. The rate of the reaction depends on the gaseous pretreatment of the particles. For particles reduced by hydrogen, the kinetics are usually first order with respect to MV** plus . The reaction is also first order in the concentration of platinum, and is inhibited in a first order manner by the product MV**2** plus . This inhibition suggests that MV**2** plus is adsorbed on the particle surfaces, and this has been confirmed by ac, ring-disc electrode studies on macroscopic platinum electrodes. At high concentrations of MV** plus some deviation from first order kinetics is observed. These results are all explained by a kinetic model in which either the desorption of MV**2** plus or the adsorption of MV** plus is the rate limiting process. The rate of consumption of MV** plus on an oxidized surface is an order of magnitude faster than that on the reduced surface.