C. Buess-Herman

Electroreduction of Carbon Dioxide on Copper-Based Electrodes: Activity of Copper Single Crystals and Copper–Gold Alloys

The activity of copper single-crystal and copper–gold alloyed (Au1Cu99, Au10Cu90, Au20Cu80, Au50Cu50) electrodes for CO2 electroreduction has been investigated in aqueous phosphate buffer solutions by cyclic voltammetry and potentiostatic long-term electrolysis. In addition to H2, four species were identified in the gaseous phase by chromatography after electrolyses: CO, CH4, C2H4 and traces of C2H6. At copper electrodes, the major carbon-containing products are CO and CH4. Analysis of the data indicates that the fraction of methane increases in the order Cu (poly) < Cu (100) < Cu (111), whereas a concomitant decrease of the carbon monoxide fraction is observed. The selectivity of low-index faces towards methane formation is consistent with a mechanism involving CO(ads) and H(ads). Analytical information obtained after potentiostatic electrolyses at copper–gold alloys shows that the CO production increases markedly with the Au content, while the fraction of CH4 diminishes. Amongst all the examined electrodes, the Au50Cu50 alloy appears to be the most efficient substrate for the conversion of CO2 into carbon-containing gaseous products.

High performance gold-supported platinum electrocatalyst for oxygen reduction

High performance gold-supported Pt electrocatalyst for the reduction of oxygen was prepared by replacing Cu adlayers, deposited potentiostatically on Au, with Pt at open-circuit potential in a 0.1 M HCl solution containing K2PtCl6. Auger Electron Spectroscopy and Atomic Force Microscopy reveal the surface modification. The kinetics of oxygen reduction on this platinum modified electrode was studied by the rotating-disc electrode technique. The activity of the electrode is lower than the activity of a smooth Pt electrode in the negative potential scan, but it is significantly higher in the positive scan.

Electrochemical nucleation and growth of rhodium on gold substrates

Abstract The mechanism of rhodium adlayer formation on the (100) face of a gold single crystal and on a polycrystalline gold electrode has been studied using cyclic voltammetry and chronoamperometry. The voltammogram shows the presence of two peaks associated with the deposition of rhodium from the Na 3 RhCl 6 salt in 0.1 M HClO 4 . The results obtained on the (100) face of a gold single crystal indicate that the two monolayers of rhodium can be deposited before the nucleation and growth of the three-dimensional (3D) rhodium clusters. The experimental results described in this paper show that the bulk rhodium electrodeposition on a gold electrode includes a nucleation process combined with a diffusion-controlled growth of the stable rhodium crystals. The nucleation kinetics is compared for the 3D electrodeposition on the 〈100〉 face of a single crystal gold electrode and on a polycrystalline gold electrode.

Synthesis and characterisation of chromium carbides

Abstract This paper presents the synthesis and the characterisation of various chromium carbide compounds. Thin Cr 23 C 6 films were deposited by reactive sputtering while Cr 7 C 3 films were formed by the carburisation of chromium films in a CH 4 /H 2 atmosphere. Cr x C y powders were synthesised from various precursors (Cr, CrN, Cr 2 O 3 ) by reaction with CH 4 /H 2 at high temperature. The samples were characterised by AES, XRD and electron diffraction. The effects of the experimental parameters (gas composition, temperature, reaction time) on the purity, the phase formed and the composition of the product of reaction are examined and discussed.

AES–XPS study of chromium carbides and chromium iron carbides

Abstract The nature of chromium rich carbides which precipitate at grain boundaries in steels is still not perfectly understood. We performed a multitechnique approach on model chromium carbide and chromium–iron carbide samples: Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), and High Energy Electron Diffraction (HEED) were used to characterise the samples. Significant chemical shifts were observed for the Cr, Fe and C XPS peaks in the M 7 C 3 compound (M stands for metal), indicating unambiguously that the compound formed is a mixed iron–chromium carbide.

On the occurrence of two-dimensional phase transitions of uridine at the aqueous solution-gold single crystal interface

Abstract The adsorption of uridine from 0.1 M KClO4 on gold (111) and gold (210) electrodes has been examined with as main objective the detection of 2D condensed phases. The formation of an organized layer of uridine at Au(111) is supported by the occurrence of sharp spikes in the i-E and C-E curves, the influence of the surfactant concentration and the temperature on the extent of the phase, the presence of a marked hysteresis and the associated kinetics which are controlled by a nucleation and growth mechanism.

Chemical effects in the Auger spectrum of copper–oxygen compounds

Abstract AES and XPS are two powerful techniques for the determination of the composition of a surface. The nature of the chemical binding of an element is usually studied by XPS. However, the Auger peaks also contain theoretical information about the chemical environment of the elements. As mostly one or two valence levels are involved in the emission of the Auger electron, a careful analysis of the Auger lines should allow to reveal the changes in the electron densities in the electronic levels due to a change in the chemical environment. However, for metals (with a high number of electrons) the chemical effects are mostly weak, hidden or hard to interpret. In order to try to understand better the nature of the chemical effects in AES of metals, a systematic study of well-defined copper compounds is performed. Cu, CuO, Cu2O, CuCO3 and copper acetate standard samples were prepared and characterised. The copper LMM lines were analysed. The energy shifts, change in relative intensities and change in FWHM were studied as a function of increasing oxygen environment. We show that the energy shift and the relative intensities of the Auger peak are good fingerprints of the oxidation state, and that the width of the Cu L3M4,5M4,5 peak can be related to the width of the valence band.

On the dissolution kinetics of two-dimensional uridine layers adsorbed on gold single-crystal electrodes

The kinetics of dissolution of a two-dimensional chemisorbed layer made of uridine molecules adsorbed onto gold single-crystal electrodes have been studied by the potential step technique. Current transients have been recorded after single potential steps aimed at triggering the dissolution of the uridine chemisorbed monolayer formed on Au(111), Au(13,13,12) and Au(554). Irrespective of the electrode surface all the transients have satisfactorily been described by a model that combines a desorption step with a simultaneous hole nucleation and growth process. On the basis of a model considering an exponential law of nucleation and a surface diffusion-controlled growth, it is found that an increase of the step density of the electrode surface by affecting the ordering of the organic layer enhances markedly the rate of formation of holes in the film but slows down the expansion of the holes. The influence of the temperature and of the surfactant concentration on the nucleation and growth parameters is also reported.

Electrochemical quartz crystal microbalance and spectroelectrochemical studies of the influence of cations on the formation and electroactivity of poly(N-vinylcarbazole) films in acetonitrile solutions

Films of poly(N-vinylcarbazole) (PVCz) were prepared and studied in the presence of different cations using spectroelectrochemical and electrochemical quartz crystal microbalance methods. Evidence for the effect of the cation on both formation and redox behaviour has been obtained. The experimental results have been interpreted by considering the presence of different types of ionic species inside the polymers layer (free anion, screened anion, bound anion and cation).

Effect of the crystallographic orientation of gold single-crystal electrodes on the occurrence of 2D phase transitions in adsorbed organic monolayers

The influence of the surface structure of a gold electrode on the occurrence of 2D phase transitions in adlayers of uracil and uridine has been studied by cyclic voltammetry. Au(111) surfaces with different step densities have been prepared by cutting the single crystals between 2 and 15.8° of their (111) axis. Results confirm that flat surfaces are a prerequisite for the formation of a 2D condensed physisorbed film.