Adsorption of bisulfate and sulfate anions on a Pt(111) electrode

A modified radioactive labeling method was used to study bisulfate and sulf ate anion adsorption on a Pt(lll) electrode from aqueous HClO4/H2SO4 media. The highest surface concentration of the anion is 3.2 x 10(14) ions/cm(2), which corresponds to a coverage (packing density) of 0.21 +/- 0.01monolaye r. Overall, the coverage data confirm our previous results reported in J. E lectroanal. Chem. 1993, 348, 451. We found fast adsorption kinetics and a s emilogarithmic anion adsorption isotherm. In the "butterfly" electrode pote ntial range [that is, in the range where anomalous anion adsorption was fou nd on the Pt(111) electrode (Clavilier, J. J. Electroanal Chem. 1980, 107, 211)], the charge on the anion and, consistently, electrosorption valency v ary with the electrode potential, demonstrating progress in bisulfate disso ciation to sulfate, with the increase in potential. On the plateau of the s urface concentration-electrode potential plot, we conclude that the adsorba te is a partially discharged (or neutralized) sulfate anion, which displays a net charge of -1.7 e. We also conclude that the species coadsorbed with sulfate are predominantly water molecules rather than hydronium cations. A brief review of the theory of anion adsorption on metal surfaces is present ed and a model of sulfate-platinum bonding is proposed.