The adsorption of perchlorate, sulfate, selenate and water on Au(111)-textured electrodes from aqueous solutions: Simultaneous voltammetric, optical and microgravimetric studies

Abstract

The adsorption of perchlorate, ClO4−(aq), sulfate, SO42−(aq) and selenate, SeO42−(aq) on Au(111)-textured single crystals from aqueous acidic solutions was examined as a function of the applied potential, E, by simultaneous voltammetry, normal incidence differential reflectance spectroscopy, ΔR/R, and electrochemical quartz crystal microbalance, EQCM, techniques. In particular, ΔR/R vs E data collected in pure 0.1 M HClO4 or the same electrolyte containing either 1 mM Na2SO4 or 1 mM Na2SeO4 could be quantitatively accounted for by using a simple model introduced in similar studies reported earlier in this laboratory. This model regards the ΔR/R response as arising from a sum of contributions from bare and adsorbate covered areas of the surface, which are, in each case, linear functions of the applied potential. Adsorption isotherms for ClO4−(aq), and SO42−(aq) on Au(111) from the literature allowed fitting of the model to data obtained in 1 mM Na2SO4 in 0.1 M HClO4 solutions. The two oxyanions were found to co-adsorb over a wide potential range, with SO42−(aq) fully displacing ClO4−(aq) for E > 1.25 V vs RHE. Analogous ΔR/R measurements involving 1 mM Na2SeO4 in 0.1 M HClO4 solutions combined with the model allowed calculation of the coverage of the two oxyanions as a function of E. The experimental isotherms enabled calculation of the mass densities for the adsorbed oxyanions, ρth, yielding values virtually identical to those derived from EQCM experiments, ρexp, for data recorded in the range ca. E < 0.73 V vs RHE in all three solutions. At higher potentials, however, ρexp were significantly larger than those predicted by the optical measurements, a behavior consistent with water co-adsorption.