Chloride Adsorption on Cu(111) Electrodes: Electrochemical Behavior and UHV Transfer Experiments

Abstract

The whole range of processes occurring at a single crystal Cu(l:l I ) elec- trode in a sulfuric acid electrolyte (5 mMI-hS04) between hydrogen evolution and copper dissolution has been studied using a combination of Cyclic Voltamrnetry (CV), in ~itu Scanning Tunneling Microscopy (STM), and in situ Fourier Trans- form Infrared Spectroscopy (FTIR) as well as ex situ characterization by means of Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). In particular , in sit u STM measurements with atomic resolution, carried out under various conditions, namely in the poten- tiostatic, potentiodynamic and quasi-spectroscopic mode, provide very detailed and direct information about the following interfacial properties and processes: the ad- sorpt ion and desorption kinetics of the sulfate anions; the structure of the sulfate adlayer and t he reconstruction of the copper surface underneath including drastic morphological changes of the electrode surface; the absolute adsorption geometry of individual so~- anions; the anodic copper corrosion and redeposition as well as the formation of cationic adsorbate (probably hydronium) layers in the regime of hydrogen evolution. Altogether the results demonstrate that already a relatively simple system such as Cu(lll) in sulfuric acid solution may exhibit a surprisingly high degree of complexity The adsorption of chloride on Cu(lll ) in dilu te HCl solutions was stud- ied by in situ STM and UHV transfer experiments. X-ray Photoelectron Spec- troscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS) and Ion Scattering Spectroscopy (ISS) were applied ex situ. The cyclic voltammogram features two peaks which were unequivocally assigned to the adsorption and desorption of chlo- ride. The transfer experiments are hampered by the presence of a large hydrogen evolution current. This faradaic process leads to the adsorption of an extra amount of chloride upon emersion from the electrolyte. This unwanted effect can be sup- pressed by the use of very dilute electrolytes or, even better , mixed electrolytes. The adsorbed chloride enhances the kinetics of the hydrogen evolution reaction. The in- fluence of chloride on the reaction rate is demonstrated by a detailed analysis of the voltammogram, using a simple model for the simulation of voltammograms