Abstract
A detailed examination of the electrochemical formation and reduction of surface oxides on platinum crystallites shows a sensitivity for the oxidation rate at constant potential and the mechanism of the oxide reduction on the platinum crystallite size. In the crystallite regions under consideration, the percentage of Pt atoms located in the surfaces of the crystallites changes from 45% at 92 m 2/g (30 Å dia.) to 4% at 10 m 2/g (280 Å dia.) so that very small crystallites exhibit surface physical properties different from bulk metal surfaces. This is reflected in the changes in electrocatalytic behavior shown here. The oxide reduction mechanisms show two separate “Tafel” slopes of 30 mV and 55 mV, dependent on the oxide coverage. Transitions between the two mechanisms as a function of oxide coverage are linearly dependent upon the percentage of platinum atoms located in the surfaces of the crystallites. Electrocatalytic reaction parameters obtained at bulk platinum metal electrodes may not be applied to high surface area platinum electrocatalysts.
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