| Popis: |
A rotating disk electrode was used to examine the steady state kinetics of O 2 reduction at oxide free Pt in alkaline solutions of various pH's. Two linear V -log i regions are observed. At low current densities, the ∂V/∂ log i slope is close to −60 mV; at high cd 's it is close to −120 mV. In the low cd region, the reaction order with respect to OH − is −1/2. In the high cd region, the reaction order is zero. In both regions the order with respect to O 2 is 1. The “unusual”, fractional reaction order of −1/2 cannot be accounted for in terms of simple electrode kinetics with Langmuirian conditions. It is suggested that in both regions the first electrochemical step is rate determining. In the low cd region, due to intermediate coverages with adsorbed oxygen species, Temkin conditions prevail and lead to the ∂ V /∂ log i slope of −60 mV and the “unusual” reaction order. At high cd 's, the coverage is low and the “usual” kinetics with ∂ V /∂ log i = −120 mV and zero reaction order with respect to OH − follow. |
