Energetics of the Underpotential Deposition of Hydrogen on Platinum Electrodes: II. Presence of Coadsorbed Sulfur

Abstract

Overlayers of chemisorbed sulfur, , on polycrystalline Pt can be formed spontaneously by Pt immersion in aqueous solution and they entirely suppress the underpotential deposition of hydrogen, UPD H. Auger electron spectroscopy and core‐level electron energy loss spectroscopy measurements indicate that is not present in an oxidized or reduced state; has a partial negative charge (well below unity) due to charge redistribution between the Pt substrate and the adsorbate. The layer of can be gradually removed from the Pt surface through its oxidative desorption under cyclic‐voltammetry conditions in clean electrolyte. The oxidative desorption was strongly affected by the temperature‐variation. The desorption under T‐control conditions resulted in formation of submonolayers of whose coverage could be controlled to within 5% of a complete overlayer. The UPD H on Pt free of follows the Temkin adsorption isotherm, while in the presence of having , the process follows a Frumkin‐type adsorption isotherm with the energy of later interactions being T‐dependent. The underpotential‐deposited H, , reveals enhanced surface mobility in the presence of than in its absence. The overlayer significantly decreases the surface bond energy. This behavior can be explained in terms of the “electronic blocking” effects which arise from direct electrostatic interaction between the adsorbates. © 1999 The Electrochemical Society. All rights reserved.