Ludwig A. Kibler

Initial stages of Pd deposition on Au(hkl) Part I: Pd on Au(111)

Abstract The electrochemical deposition of palladium onto Au(111) from chloride-containing solutions was investigated by in-situ STM and cyclic voltammetry. These methods are complementary by yielding structural and thermodynamic information. For [PdCl 4 ] 2− adsorbed on Au(111), a distorted hexagonal structure was imaged, and a phase transition within this adsorbed layer was observed. This palladate anion was found to play a crucial role in Pd deposition and dissolution. Pd deposition starts by forming a pseudomorphic layer in the underpotential region, followed by the formation of a second Pd monolayer at overpotentials. By using stepped Au(111) surfaces, an assignment of voltammetric peaks to nucleation of Pd at steps and on terraces was achieved. It is generally observed that Pd nucleates at monoatomic high steps and grows two-dimensionally. The morphology of the overlayers changes with increasing Pd coverage from flat and well-ordered for the first two monolayers to increasingly rough and defect-rich, but retaining Pd(111) characteristics in cyclic voltammetry up to at least 10 monolayers.

In-situ STM characterisation of the surface morphology of platinum single crystal electrodes as a function of their preparation

Abstract A systematic study of the surface morphology of Pt(111) and Pt(100) electrodes as prepared by flame-annealing and cooling in different atmospheres (air, N 2 , H 2 +N 2 and CO+N 2 ) is presented. The electrodes were characterised by cyclic voltammetry and in-situ STM in 0.1 M H 2 SO 4 . Preliminary voltammetric results for Pt(110) are also shown. In this case, Cu upd served as a structure sensitive probe. It was observed that the presence of oxygen during cooling induces surface defects and leads to rough surfaces. Cooling in pure N 2 preserves the reconstructed Pt(100) and Pt(110) surfaces, while cooling in H 2 +N 2 or CO+N 2 lifts the reconstruction. The use of CO as the cooling gas turned out to be advantageous for the preparation of clean and well-ordered (1×1)-surfaces. The stability of reconstructed Pt surfaces in an electrochemical environment is discussed. For H 2 +N 2 -cooled Pt electrodes, a clear influence of the H 2 -concentration on the surface morphology was observed.

Initial stages of Pt deposition on Au(111) and Au(100)

Abstract The deposition of Pt onto unreconstructed Au(111) and Au(100) was studied with cyclic voltammetry and in-situ STM. The latter revealed that in [PtCl 4 ] 2− containing electrolytes, both surfaces are covered by an ordered adlayer of the complex. For the adsorbed [PtCl 4 ] 2− a slightly compressed (√7×√7) R19.1°-structure was assumed for Au(111) and a (3×√10) for Au(100). In both cases, a rather high overpotential for Pt deposition was observed, most probably due to the high stability of the [PtCl 4 ] 2− complex. Nucleation of Pt starts mainly at defects like step edges for low deposition rates and three-dimensional clusters are formed. Due to the high overpotential, some nuclei appear also on terraces at random sites. Higher coverages of Pt lead to a cauliflower like appearance. It is not possible to dissolve the platinum clusters at positive potentials without severely roughening the gold surface. The [PtCl 4 ] 2− complex is oxidized to the [PtCl 6 ] 2− complex at about 0.7 V, when metallic Pt is on the surface.

Influence of steps on the electrochemical oxidation of CO adlayers on Pd(111) and on Pd films electrodeposited onto Au(111)

Electrochemically deposited palladium monolayers on well-ordered and on vicinal Au(111) electrodes have been characterised in sulphuric acid solution by cyclic voltammetry. The electrochemical behaviour of these pseudomorphically grown palladium overlayers is compared with that of a well-ordered and a previously oxidised Pd(111) electrode for hydrogen adsorption, sulphate adsorption as well as for surface oxide formation. A strong impact of the surface defect density was observed for the latter reaction. The electrooxidation of irreversibly adsorbed carbon monoxide monolayers on the various palladium surfaces was studied by cyclic voltammetry and chronoamperometry. The oxidation kinetics are strongly dependent on the number of monoatomic high steps, that act as nucleation centres for adsorbing oxygen-containing species and allow for an anodic stripping of carbon monoxide at more negative potentials than oxidation on terraces. A tailing in the chronoamperometric transients at 0.65 V vs. SCE for the palladium surfaces suggests that diffusion of the reacting species on the terraces is rate-determining.

A method to prepare single crystal electrodes of reactive metals : Application to Pd(hkl)

A method has been developed to prepare single crystal electrodes of reactive metals by resistive heating. Instead of using a flame, the crystals are annealed in a controlled atmosphere by passing a high electric current through them. This method was tested with Au(111) and Pt(111) electrodes, for which the well-known cyclic voltammograms characteristic of a well-ordered surface were obtained. As a second step, the preparation of Pd(hkl) electrodes was addressed. The cyclic voltammograms for Cu upd on Pd(111), Pd(100) and Pd(110) electrodes, prepared by this method, are shown and compared with literature data.

Unique activity of Pd monomers: hydrogen evolution at AuPd(111) surface alloys

Well-defined Au/Pd(111) alloy films have been prepared on a Ru(0001) substrate by electrochemical metal deposition and subsequent heating up to 700 degrees C. The electrochemical behaviour of the 20 monolayers thick epitaxially-grown films is in excellent agreement with both equilibrium surface composition and distribution for Au/Pd alloys on Mo(110) as previously reported (D. W. Goodman et al., J. Phys. Chem., 2005, B109, 18535). The electrocatalytic activity of the AuPd(111) surface alloys was studied for the hydrogen evolution in 0.1 M H(2)SO(4) as a function of surface composition. Maximum activities were found for Pd fractions of 0.2 +/- 0.1, where the population of Pd atoms surrounded by Au has its maximum. These Pd monomers are found to be about 20 times more active than Pd atoms in the Pd overlayer.

Initial stages of palladium deposition on Au(h k l): Part III: Pd on Au(1 1 0)

The deposition of palladium onto the unreconstructed Au(1 1 0) surface was studied by cyclic voltammetry and in situ scanning tunnelling microscopy. An ordered adlayer of [PdCl4] 2� was imaged with atomicresolution on the bare Au(1 1 0) surface. Pd deposition starts at monoatomic high steps by forming a layer that grows onto the lower terrace. Coulometricdata point towards the deposition of approximately three monolayer equivalents in the Pd underpotential region. This high coverage and the presence of holes in the Au(1 1 0) surface after the complete anodic dissolution of the Pd deposit are explained by surface alloy formation. Furthermore, the palladium overlayers on Au(1 1 0) appear to be rather rough, because there is no strict layer-by-layer growth. Important aspects of the initial stages of palladium deposition on the three low-index Au surfaces are summarised and the influence of the crystallographic orientation of the substrate as well as the effect of different Pd film thicknesses on the electrochemical properties are briefly discussed. 2001 Published by Elsevier Science B.V.

The initial stages of rhodium deposition on Au(111)

Abstract The electrochemical deposition of rhodium on Au(111) was studied by means of in-situ STM and cyclic voltammetry. A phase transition within the [RhCl 6 ] 3− adlayer was found for Au(111) in 0.1 M H 2 SO 4 +10 −4 M RhCl 3 +10 −3 M HCl at 520 mV versus SCE, where a ( 13 × 13 )R13.9°-superstructure of [RhCl 6 ] 3− is formed on the gold substrate. Rhodium starts to deposit irreversibly around 200 mV versus SCE. First a rhodium bilayer grows, the electrochemical behaviour of which is similar to that of a well-ordered Rh(111) surface. On top of the second rhodium monolayer the formation of small, three-dimensional clusters is observed. The electrocatalytic properties of rhodium films with an average thickness up to ten monolayers were studied by using CO oxidation as a test reaction. The influence of the surface structure on the CO oxidation kinetics is discussed briefly.

Methanol Oxidation on Ru-Modified Preferentially Oriented Pt Electrodes in Acidic Medium

Abteilung Elektrochemie, Universita¨t Ulm, 89069 Ulm, GermanyPolycrystalline Pt electrodes have been faceted by fast periodic potential scans. The electrocatalytic behavior of preferentiallyoriented ‘‘Pt~111!’’, ‘‘Pt~110!’’, and ‘‘Pt ~100!’’ surfaces modified by Ru submonolayers was examined by cyclic voltammetry andchronoamperometry. The enhancement factor for methanol electro-oxidation depends on the surface crystallography and on the Rucoverage. The optimum Ru coverage at room temperature was found to be 30-40% of a monolayer for all electrodes under study.The catalytic activity of the Ru/‘‘Pt~hkl!’’, electrodes for methanol oxidation was compared with that of Ru/Pt~111!. The catalyticactivity at optimum Ru coverage decreases in the following order: ‘‘Pt~111!’’ . ‘‘Pt ~110!’’ . Pt~111! . ‘‘Pt ~100!’’. The reac-tivity of the Ru/‘‘Pt~hkl!’’ electrodes for methanol oxidation can be attributed to the high number of steps and defect sites on theirsurfaces.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1705660# All rights reserved.Manuscript submitted August 26, 2003; revised manuscript received November 26, 2003. Available electronically May 4, 2004.

Evidence for a change in valence state for tellurium adsorbed on a Pt(111) electrode

Information about structure and chemical composition of irreversibly deposited tellurium on Pt(111) in sulphuric acid solution has been gathered ex-situ by LEED and XPS as a function of emersion potential. From the chemical shift of the Te 3d5/2 level, a change in the valence state from Te(0) to Te(IV) with positive-going electrode potential is deduced, the total amount of Te on the surface remaining constant. XP spectra also reveal the presence of Te(II) on the surface, which may be a consequence of partial double layer collapse in the emersed state. The maximum coverage of Te(0) is around 0.4 ML, as derived by XPS and LEED. The coadsorption of anions accompanying the valence transition from Te(0) to Te(IV) has also been studied. It is demonstrated that the very first potential cycle in the region of the surface redox peak, i.e. between /0.65 and /0.45 V versus Ag/AgCl, brings about drastic changes in the adlayer properties. Although XPS has provided strong evidence for a change in the valence state of adsorbed Te on Pt(111), this system has demonstrated the limitations of the ex-situ approach to obtain electrochemical information. # 2002 Elsevier Science Ltd. All rights reserved.