Study of anion adsorption on polycrystalline Pt by electrochemical quartz crystal microbalance

The changes observed on Pt surfaces during a potential incursion into the underpotentially deposited (UPD) hydrogen and double layer ranges (0.05 to 0.8 V versus HESS) were analyzed in perchloric, sulfuric, phosphoric and hydrochloric acid media. Surface occupation after hydrogen desorption was verified in terms of both mass variations and voltammetric charges measured by EQCM and cyclic voltammetry. Firstly, mass incorporation due to the adsorption of water molecules (approximately 39 ng cm-2, corresponding to a full monolayer of adsorbed water) replacing the UPD H atoms was observed in every case. The potential range associated with water adsorption varied from 0.05 V to a final value that depended on the strength of anion adsorption on Pt (0.4 V for ClO4- and 0.3 V for Cl-). Secondly, the mass incorporations in the potential region between 0.4 and 0.8 V were associated to adsorption of the corresponding hydrated anions, i.e., ClO4-·2H2O, HSO4-·2H2O, HPO42- and Cl·6H2O. Calculated anion coverage values varied from 7 (perchlorate) to 19% (phosphate) on the Pt surface.