Evolution and reduction of oxygen on oxidized platinum in 85% orthophosphoric acid

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Kinetics of electrode oxygen evolution and dissolution on oxidized smooth platinum surfaces in 85% orthophosphoric acid have been studied in fused silica apparatus over a wide range of temperatures (25.1°–136.1°C). Anodic and cathodic Tafel slopes were approximately 2 RT/F , although they did not exhibit the theoretical temperature-dependence. Because of the instability of the oxide film, cathodic Tafel slopes could only be reproducibly obtained by reanodizing between each point. Provided special precautions are taken, it is shown that oxide reduction does not interfere with oxygen reduction, except at the extreme limit of the potential range attainable on the oxidized surface. In 85% orthophosphoric acid, this limit is about 800 mV at 25.1°C, rising to 900 mV at higher temperatures. The oxygen reduction reaction involves a low oxygen reaction order. As anodic and cathodic slopes intersect close to the reversible potential for the four-electron reactions, it is reasonable to suppose that the same rate-determining step is involved throughout the anodic and cathodic range on the oxidized surface. The mechanism is analyzed assuming Langmuir conditions, and it is concluded that a complex process involving two parallel mechanisms is in accord with the data. The rate-determining step and the mechanism in alkaline solution are discussed. Activation energy at the reversible potential was determined as 13.1±0.6 kcal.