An electrochemical quartz crystal microbalance study of the hydrogen underpotential deposition at a Pt electrode

The effect of various ions on the frequency response of a quartz supported platinum electrode in the hydrogen upd region was investigated. The experiments were carried out in alkaline solution, and in acid solutions with various concentrations of chloride or sulfate ions. Traces of specifically adsorbed ions, such as chloride ions, were readily detected with the EQCM. On the other hand, the effect of perchlorate and sulfate/bisulfate anions on the frequency response is quantitatively the same. From these results, it seems that the specific adsorption of sulfate/bisulfate anions on the Pt electrode cannot be considered as a possible cause of the observed frequency change. In addition, a large variation of the concentration of sulfate/bisulfate anions in solution, and hence at the outer Helmholtz plane, does not modify the amplitude of the frequency response. Whether or not the observed frequency shift with potential is due to hydrogen upd, water contact adsorption or change in the concentration of anions at the oHp, is further discussed. (C) 1999 Elsevier Science S.A. All rights reserved.