AVS2001 Session EC-MoP: Poster Session
Monday, October 29, 2001 5:30 PM in Room 134/135
Monday Afternoon
Time Period MoP Sessions | Topic EC Sessions | Time Periods | Topics | AVS2001 Schedule
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EC-MoP-1 Ex situ and In situ Investigations of Dilute Perchloric Acid on Cu(111)
M. Duisberg, P. Wahl, B. Obliers, J. Hommrich, P. Broekmann, K. Wandelt (University of Bonn, Germany) In electrochemistry perchloric acid is assumed to be a prototype for an inert electrolyte. This statement, however, turns out to be not correct for copper electrode surfaces. In this study we present combined in-situ and ex-situ mesurements illustrating the surface electrochemistry of a Cu(111) electrode exposed to a dilute perchloric acid electrolyte. The initial cyclic voltammogram (CV) of an UHV prepared Cu(111) single crystal in 10 mM HClO4 shows a reversible pair of peaks at 180 mV vs RHE. After a few cycles this CV changes into a CV identical to that directly obtained after an electropolishing preparation procedure which does not show this reversible pair of peaks. Additional to this fast change there is a long time instability of the CV. A prolonged cycling of the potential leads to the formation of a cathodic peak at -220 mV and a smaller anodic peak at 190 mV. Additionally the onset of hydrogen evolution is shifted towards cathodic potentials by 60 mV in comparison to the initial CV. In-situ STM measurements of Cu(111) in perchloric acid reveal a completely disordered adlayer which is present at anodic electrode potentials. The bare copper surface is not seen under these conditions as one would expect for an inert electrolyte. Ex-situ XPS and LEIS measurements indicate the presence of copper oxide species and chloride as well. The origin of this species at the surface will be discussed. At cathodic potentials near the onset of hydrogen evolution a highly ordered superstructure is observed by STM. This adlayer remains stable also under massive hydrogen evolution. Under similar conditions identical adlayers were also found in other acidic elctrolytes like sulfuric acid or hydrobromic acid, and are attributed to ordered hydronium/water layers. |
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EC-MoP-2 Cadmium Underpotential Deposition on Cu(111): A Combined In Situ and Ex Situ Study
J. Hommrich, Z. Park, C. Stuhlmann, K. Wandelt (University of Bonn, Germany) The underpotential deposition (UPD) of Cd on the Cu(111) surface was studied through ex situ (XPS, LEIS, LEED)1 and in situ (CV, EC-STM) methods. Cd was deposited from an aqueous solution containing 10-4M CdCl2/10-2M HCl. After deposition of a complete UPD-layer a hexagonal adlayer with a 3.8 Å lattice spacing was observed by EC-STM. This superstructure shows an additional long-range periodicity (Moiré-pattern). Two rotational domains of this Moiré-pattern can be distinguished which is in agreement with ex situ LEED measurements. XPS and LEIS measurements led to the conclusion that this rather open structure (the Cd-Cd distance in bulk Cadmium is 2.98 Å) is stabilised by an adsorbed Cl overlayer. Furthermore the initial growth of the UPD-film seen by STM is discussed: In the beginning of the UPD the growth of 2D Cd islands can be monitored by STM. The observed height of these islands and the change of the contour of the substrate steps suggest that embedding of Cd into the upper substrate terraces takes place. Polarisation experiments revealed that the current-potential desorption spectra of UPD-films depend on the polarisation time. The potential of the sample was held in the UPD regime at -370 mV vs. RHE between 1 to 90 minutes. Subsequent anodic stripping of these UPD films led to the observation of an increasing charge transfer as a function of polarisation time accompanied by a change in the form of the desorption peak. In addition a roughening of the surface was observed by STM during the anodic stripping. . |
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EC-MoP-3 Microscopic Structure and Reactivity of Alloyed AuPd(111) Electrode Surfaces
F. Maroun (Universite P & M Curie, France); F. Ozanam (CNRS-Ecole Polytechnique, France); O. Magnussen, R.J. Behm (University Ulm, Germany) The electrochemical and electrocatalytic properties of defined atomic Pd ensembles in a PdAu(111) surface alloy electrode, serving as model system for bimetallic electrocatalyts, were evaluated by a combination of in-situ microscopic, IR spectroscopic and electrochemical measurements, using H and CO adsorption/ oxidation as model reactions. PdAu electrodes with atomically flat surfaces and variable stoichiometry were prepared in a controlled way by electrodeposition on Au(111) under diffusion-limited conditions. The amount and distribution of Pd surface atoms was determined by atomic resolution in-situ scanning tunneling microscopy with chemical contrast. The electrochemical/catalytic properties of individual types of Pd ensembles were determined by comparison with spectroscopic and voltammetric data. This way, the critical ensembles for CO and H adsorption were identified as Pd monomers and dimers, respectively. The specific chemical properties of these ensembles as compared to bulk Pd and Pd thin-film electrodes provide a microscopic basis for understanding the distinct catalytic properties of PdAu alloys observed at the macroscopic scale. |