Mikael Wasberg

Oxidation of CO adlayers on Pt(111) at low potentials: an impinging jet study in H2SO4 electrolyte with mathematical modeling of the current transients ☆

The constant potential oxidation of adlayers of CO on a bead type Pt(111) electrode created by the impinging jet flow technique has been studied in 0.5 and 0.05 mol dm−3 H2SO4 electrolyte. Washing procedures were developed which allowed work to be carried out for long periods of time without contamination problems. At low potentials (<680 mV vs. RHE) in 0.5 mol dm−3 H2SO4 the compact adlayers were slowly oxidized in a process which first involved a constant current plateau followed by a single symmetric peak. The total oxidation charges of the transients corresponded to that of a compact CO adlayer and the peak charges to that calculated from data reported in the literature for a looser adlayer structure. In 0.05 mol dm−3 H2SO4 the adlayers oxidize much faster and the oxidation process is more complex. The current−time traces for the stronger electrolyte were described well by an equation especially derived for the oxidation kinetics under the particular conditions used in this work. Values for a potential dependent rate constant for the oxidation were determined by means of curve fitting.

Electrochemical detection of microcystins, cyanobacterial peptide hepatotoxins, following high-performance liquid chromatography

A novel amperometric HPLC detection method for the cyanobacterial (blue-green algal) peptide toxins microcystin-LR, -YR and -RR was developed. Purified microcystins and cyanobacterial extracts were chromatographed using an internal surface reversed-phase column with acetate- and phosphate-based mobile phase systems. Electrochemical oxidation reactions at 1.20 V vs. Ag/AgCl (glassy carbon working electrode) were show to originate in arginine and tyrosine residues of microcystins.

The reduction of ClO4− ions on Rh electrodes

The literature data and the most important factors relevant to the reduction of perchloric acid at Rh electrodes (smooth polycrystalline, single-crystal faces and rhodized systems) are surveyed. The consequences of model assumptions are analysed for rhodized electrodes. The central role of some adsorption processes was demonstrated using a combined voltammetric and impinging jet method. The experimental results obtained confirm the assumption that the reduction process, which involves several consecutive steps, takes place exclusively on the surface of the electrode and Cl− ions are the main products. The latter species are responsible for the well-known self-inhibition phenomena.

Voltammetric and impinging jet study of the behaviour of rhodized electrodes in acidic perchlorate solutions

The reduction of ClO−4 ions in acid media was studied at rhodized electrodes (roughness factor values of about 100–200) by voltammetric and hydrodynamic (impinging jet) methods. The effect of HClO4 concentration and the influence of pH (at a fixed ionic strength) on the shape of the voltammetric curve was investigated and interpreted. The inhibition of the reduction process by adsorbing anions, first of all by Cl− ions produced by reduction, was demonstrated. It has been stated that reliable information concerning the laws governing the reduction process cannot be obtained from voltammetric measurements alone. As an additional tool, a version of the impinging jet technique was used to find a relationship between the potential E of the rhodized electrode and the reduction rate I. A characteristic, almost symmetric, |I| versus E curve going through a maximum was obtained from the experimental data. Reduction mechanisms were suggested for the interpretation of the experimental results. The reaction rate versus potential curves calculated on the basis of model assumptions were compared with the experimental data.

The impinging jet flow method in interfacial electrochemistry: an application to bead-type electrodes

Abstract The details of an impinging jet flow system to be used for interfacial electrochemical studies at smooth bead-type electrodes are described. The 0.7-ml min−1 electrolyte flow was propelled by hydrostatic pressure and accurately controlled by a computer-controlled rotating valve. Carbon monoxide dissolved in 1 mol dm−3 sulfuric acid was dosed onto a polycrystalline platinum surface in a carrier flow of pure electrolyte. The dosing performance of the flow cell was evaluated by electrooxidation of bulk and adsorbed CO, and the effect of dosing potential on the stripping voltammogram and on the charge displacement behaviour was investigated and discussed.

The harmony of EQCM and radiotracer sorption results obtained in the course of voltammetric study of a rhodized electrode

Abstract Results of coupled voltammetric and EQCM studies were compared with those of radiotracer (voltradiometric) investigations of anion adsorption (HSO4−, Cl−) at a rhodized electrode. The coincidence of the position of the peaks of voltammetric, differential voltradiometric and df/dE vs. E curves (E = potential, f = frequency) indicates that the EQCM measurements furnish reliable information on the anion specific adsorption.

Voltammetric and EQCM investigation of conductive films formed by electroreduction of Ru(L)(CO)2Cl2 compounds

Abstract Electroreduction of trans (Cl) and cis (Cl)Ru(L)(CO) 2 Cl 2 (L=bpy=2,2′-bipyridine, dmbpy=4,4′-dimethyl-2,2′-bipyridine) in CH 3 CN and DMSO resulted in electrode-adhering films and the reactions were studied by voltammetry and the electrochemical quartz crystal microbalance. These methods were complemented by hydrodynamic rotating ring-disc and quartz crystal impedance methods. The deposition was strongly influenced by the type of solvent, polarisation mode and electrolyte stirring, but for all compounds, rapid charge transfer was found. For monomers with trans (Cl)Ru(bpy)(CO) 2 Cl 2 , the two-electron reduction and Ru 0 Ru 0  polymerisation process suggested in the literature was confirmed. Some adsorption of CH 3 CN was also indicated and for potentiostatic deposition in static solution the impedance measurements indicated increased film viscoelasticity. Depositions with cis (Cl)Ru(bpy)(CO) 2 Cl 2 were more complex and the rate declined gradually in static solution. In stirred solution a straightforward polymer formation took place. For trans (Cl)Ru(dmbpy)(CO) 2 Cl 2 the literature mechanism could not be confirmed and large deviations were observed during the early stages of deposition. The trans (Cl) dimer, [Ru(bpy)(CO) 2 Cl] 2 displayed a lower current efficiency during film formation than the monomer and no simple deposition mechanism could be suggested. For the trans (Cl)[Ru(dmbpy)(CO) 2 Cl] 2 dimer a straightforward deposition was found. The films could be removed by electrooxidation and mass/charge data show that the films break up in large fragments. The reduction and deposition process for trans (Cl) monomers (L=bpy, dmbpy) was light sensitive and the mass of films deposited during cyclic voltammetry (CV) was doubled when the electrode was irradiated.

Comparative radiotracer study of the adsorption of Cl−, HSO−4(SO2−4) and H2PO−4 anions on rhodized electrodes

Abstract The adsorption of 32 P labelled phosphoric acid, 35 S labelled sulphuric acid and 36 Cl labelled hydrochloric acid from 2 × 10 −2 mol dm −3 HClO 4 supporting electrode was studied at rhodized electrodes (roughness factor values about 150) by a coupled electrochemical and radiotracer technique. A special attempt is made to avoid the problems caused by the reduction of ions. It has been stated that the adsorption behaviour of H 2 PO − 4 and HSO − 4 (SO 2− 4 ) ions is very similar and the potential dependence of the adsorption can be described by a simple relationship reported previously (G. Horanyi and E.M. Rizmayer, J. Electroanal. Chem., 198 (1986) 379; 201 (1986) 187), while the relationship for Cl − ions can be conceived as composed of at least two simple functions describing the adsorption of HSO − 4 (SO 2− 4 ) ions, with two different sets of parameters.

The reversibility and overlapping of hydrogen and oxygen adsorption in the “double-layer” region of rhodized electrodes in H2SO4 solutions (the decomposition of water molecules on rhodium surfaces)

Abstract The voltammetric behaviour of rhodized electrodes (roughness factor values, 15–25) was studied in 1 mol dm −3 H 2 SO 4 solution in the potential range from 225 to 500 mV/RHE. It was shown that, at appropriately chosen sweep rates and at high current-measuring sensitivities, characteristic voltammograms with a singular point (minimum) could be obtained. Addition of strongly adsorbing chloride ions significantly modifies the picture: the singular point disappears and double-layer-like behaviour can be observed. The phenomena observed are treated and explained in terms of simultaneous adsorption of H and OH species.

Isopotential point : a singularity in the voltammetric behaviour of platinum metal electrodes characterising adsorption competition

It is well known from the literature (see Refs. l-4 and literature cited therein) that the voltammetric behaviour of platinum metals in acidic medium, the shape of cyclic voltammetric curves, depends strongly upon the nature and the concentration of the anions present in the system which is studied. All these phenomena could and should be ascribed to the adsorption competition between anions and hydrogen. It is a general rule that the stronger the adsorbability of the anion, the more negative the potential of the current peak(s) characterising the hydrogen adsorption. Despite the detailed and far-reaching studies about the influence of various anions on the hydrogen adsorption there seem to be some gaps in our knowledge concerning the question about how the adsorption competition of two anions, which are present simultaneously in a system, is reflected by the shape of the voltammetric curves and by the position of the hydrogen adsorption peaks. We have found that the voltammetric curves taken at various stages of the adsorption competition of two anions have a common crossing point, the isopotential point, the position of which could be informative regarding the relative strength of the adsorption of the two species and may serve as a reference point for the estimation of the extent of their adsorption. In this note some, at first sight, very surprising, preliminary results are reported proving the physical reality of the occurrence of an isopotential point for a host of voltamrnetric curves obtained under suitably chosen experimental conditions in an appropriately chosen system. For the experimental study a rhodium black electrode was selected as an adsorbent using HF as supporting electrolyte and Clions as the second partner in the overall adsorption competition process.