Changes in the electrocatalytic activity of Pt for the oxygen and hydrogen peroxide reduction reactions (ORR and HPRR, respectively) in an aqueous acidic electrolyte induced by the adsorption of bromide, as a model impurity, have been investigated using chronoamperometric techniques under forced convection. Experiments were carried out using a polycrystalline Pt|Pt rotating ring-disk electrode in O2-saturated 0.1 M HClO4 containing 10 μM KBr (800 ppb). Potential steps were applied to the Pt disk from Eo, at which Br- is fully desorbed, to more positive values, Estep, at which Br- undergoes adsorption. During this period, the currents both at the disk, idisk, and at the ring, iring, were monitored, with the ring polarized at a potential at which H2O2(aq) oxidation proceeds under diffusion-limited conditions. The results obtained, assuming the ORR does not interfere with Br- adsorption, made it possible to correlate idisk and iring with the coverage of adsorbed Br-, θBr -, and Estep. As evidenced from the data collected, significant drops in the diffusion-limited currents for O2 reduction induced by the presence of 10 μM KBr could only be observed for θBr - > 0.25 regardless of Estep. Similar measurements involving the same Pt(poly) rotating disk electrode performed in 1 mM H2O2 in deaerated 0.1 M HClO4 devoid of O2 displayed a similar trend. This behavior was found to be consistent with a simple blockage of surface-active sites by adsorbed bromide, as predicted by the attenuation model proposed by Levart.
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