PASSIVATION OF ANODIC REACTIONS
NAVAL RESEARCH LAB WASHINGTON DC
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Extending the Wagner definition of passivity to anodic reactions of chemical species on inert electrodes, passivation of a Pt electrode for the hydrogen oxidation reaction can be explained as being caused by small amounts fraction of a monolayer of adsorbed anions. Anion adsorption can effectively reduce the active electrode area, but the major passivation effect is the poisoning of catalytic sites, which retards the catalytic reaction so that in most of the region where passivation occurs the reaction rate is controlled by the increased free energy of activation with increased potential. The change in sign from positive to negative of the kinetic relation dEdlog i can be explained as being caused by adsorption of a small number of anions and is limited when the active sits are essentially saturated. Dermasorbed oxygen atoms in addition to sulfate ions can have a strong effect on the catalytic activity of Pt for the hydrogen oxidation reaction. This concept may be extended to other retarded anodic reactions and to the passivation of iron in particular.
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