Transport and kinetics at microheterogeneous electrodes: Part V. The methylviologen/platinum system |
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Authors: | WJohn Albery Philip N Bartlett Andrew J McMahon |
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Institution: | Department of Chemistry, Imperial College, London SW7 2AY Great Britain |
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Abstract: | Results are reported for the reaction of methylviologen radical cation, MV+ 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+. The reaction is also first order in the concentration of platinum, and is inhibited in a first order manner by the product MV2+. This inhibition suggests that MV2+ 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+ some deviation from first order kinetics is observed. These results are all explained by a kinetic model in which either the desorption of MV2+ or the adsorption of MV+ is the rate limiting process. The rate of consumption of MV+ on an oxidised surface is an order of magnitude faster than that on the reduced surface. Ring—disc studies show that this is because the MV+ is not producing H2 but is reducing the surface oxide. The results are shown to fit a simple model which takes into account this titration of the oxide layer. The model also explains why the rate on partially oxidised surfaces will appear to have an order greater than one in Pt]. |
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