Anion effects and the mechanism of Cu UPD on Pt(111): X-ray and electrochemical studies

Abstract

We propose a mechanism for the underpotential deposition of Cu on Pt(111) in 0.1M H 2SO 4 in the presence and absence of halides. The mechanism is based on recent results from electrochemical and in situ surface EXAFS and X-ray standing wave (XSW) studies of Cu UPD on Pt on 0.1M H 2SO 4 and in the presence and absence of halides with emphasis on Cl −, Br − and I −. EXAFS data were obtained in the presence and absence of Cl − at a potential of +0.1 V corresponding to a coverage of approximately 0.75 ML. In the absence of chloride, the data were consistent with the presence of an incompletely discharged copper adlayer. The copper—copper bond distance was found to be 2.85Å. In the presence of chloride the X-ray data suggest the presence of a fully discharged copper layer and the Cu Cu bond distance, 2.59Å, approached the bulk copper value (2.56Å). In the presence of chloride, no oxygen is present as a backscatterer in the plane of the Cu Cu adlayer. However, oxygen (from either bisulfate or water) is present as a backscatterer in the absence of chloride giving rise to a copper—oxygen bond distance of 2.16Å. It appears that the chloride acts as a protective overlayer precluding oxygen (from either solvent or electrolyte) adsorption. Qualitatively similar results were obtained for Cu UPD on an iodine treated Pt surface. In addition, XSW data for this last system suggest the presence of electrochemically inactive (solvated) copper ions in the vicinity of but not in contact with the platinum surface which we describe as representing a “pre-adsorbed” state. From electrochemical studies at low (10 μM) copper concentrations, we find that the underpotential deposition of submonolayer amounts of copper induces an enhanced adsorption of chloride and bromide on Pt(111) that is reflected in exceedingly sharp voltammetric peaks that are transient in nature. The adsorbed anions are believed to be in contact with the platinum surface and in the vicinity of the electrodeposited copper. We ascribe this enhanced adsorption to the formation of a CuX adlayer on the surface of the electrode. These exceedingly sharp peaks increase in amplitude for copper coverages of up to half a monolayer and subsequently decrease so that upon completion of the copper monolayer they are absent. Based on these findings and on previous investigations, we propose a mechanism for the underpotential deposition of copper on Pt(111) which involves a “pre-adsorbed” state, the transient formation of a CuAn (An −x =anion) adlayer which in turn gives rise to a copper monolayer which itself is covered by an anion-adlayer.