Oxygen Reduction and Corrosion Kinetics on Phase‐Oxide‐Free Palladium and Silver Electrodes as a Function of Temperature in 85% Orthophosphoric Acid

Abstract

Oxygen reduction on oxide‐free palladium and silver surfaces has been studied as a function of temperature in purified 85% orthophosphoric acid. In each case, oxygen reduction is first order, and available evidence suggests that on both metals the reaction is the rate‐determining step. Experimental activation energies, extrapolated to the reversible oxygen electrode potential, are on palladium and about 26 kcal on silver. Activity of the palladium electrode is only slightly (< one decade) less than on platinum under the same conditions, whereas silver is even less active than gold. On palladium, as with platinum, the extrapolated activation energy includes a heat of adsorption term corresponding to the effect of Temkin‐type equilibrium adsorption of ‒O species derived from water oxidation, which are present on the electrode under the experimental conditions. This term may be eliminated by considering activation energies extrapolated to potentials at which ‒O adsorption is small, where an activation energy difference between silver and palladium on the order of 9.8 kcal occurs. This suggests that the intermediate is relatively strongly adsorbed on palladium and only weakly on silver, and accounts for the rate difference on the two metals.Rest potentials of the metals are corrosion mixed potentials and the metal dissolution reaction is fast in both cases so anodic processes are under Nernstian control. Palladium passivates at potentials greater than 900 mV HRE. No passivation of silver was observed.