G. Horányi

Recent developments in the application of the radiotracer method to the investigation of adsorption and electrocatalytic phenomena

Abstract A brief survey is given on the present state of the tracer adsorption studies. The main trends in the investigations are illustrated through examples taken from the results reported in the seventies. The following main topics are considered: 1. Developments in the improvement of the experimental technique. Different versions of experimental apparatus are presented discussing briefly the drawbacks and advantages of the methods used. 2. Ion adsorption studies. Examples for the application of the tracer method to the study of simultaneous adsorption processes are presented. 3. Adsorption of organic species. Problems connected with the equilibrium properties and the interpretation of the experimental results are summarized. Efforts made for the determination of the nature of simple chemisorbed species are reviewed. 4. Coupling of polarization and tracer adsorption studies. The possibility of simultaneous measurement of adsorption and reaction rate in the course of electrocatalytic processes is discussed. The illustrative examples refer mostly to platinum electrodes. Results obtained in the case of other electrodes are cited.

Some problems of the kinetics of the oxidation of organic compounds at oxide-covered nickel electrodes

Summary Problems of the kinetics of the oxidation of organic compounds at oxide-covered nickel electrodes have been studied. The role of the oxide in the reaction has been examined in the light of some new experimental data. The existence of the equilibrium between the lower and higher oxide in the course of the reaction has been questioned.

Investigation of adsorption phenomena on platinized platinum electrodes by tracer methods

Summary A method has been developed for the investigation of adsorption phenomena on platinum electrodes and Pt-powder by tracer techniques, using isotopes emitting soft β-radiation. The effect of the parameters of the apparatus on the practical application of the method has been analysed in detail, and the applicability of the method has been presented by an experimental test.

Investigation of adsorption phenomena on platinized platinum electrodes by tracer methods: II. The potential dependence of anion adsorption

Summary The potential dependence of Cl− and HSO4−-ion adsorption was investigated on platinized Pt electrodes in 1 M HClO4 supporting electrolyte solution by a tracer method. It has been established that there is a marked difference between experimental results obtained with freshly prepared electrodes and with electrodes previously used. In the case of freshly prepared or regenerated electrodes, characteristic sections can be distinguished in the potential dependence of adsorption in a wide concentration range. We have arrived at the conclusion that the dependence of adsorption on the concentration and electrode potential can be expressed as the product of two functions each of one variable (concentration or electrode potential).

Some interesting aspects of the catalytic and electrocatalytic reduction of perchlorate ions

The problem of the reduction of perchlorate ions is discussed in the light of recent results bearing in mind the perchlorate environmental contamination challenge and recent problems concerning the electrochemical stability of ClO4− ions. The catalytic and electrocatalytic transformations are compared in the case of WC and Pt electrodes (catalysts). Various methods for the detection of the occurrence of the reduction process are presented (radiotracer technique, potential step method and voltammetry). It is shown that the interpretation of the results of studies of corrosion processes of Al, Zn, Cu, Ni and Fe carried out in the presence of ClO4− ions cannot be considered as adequate without taking into consideration the reduction of ClO4− ions and all previous work should be revisited on the basis of the new findings. Some elements of the mechanistic aspects of the complicated reduction processes are discussed.

Some problems connected with the study and evaluation of the effect of pH and electrolyte concentration on the behaviour of polyaniline film electrodes

Abstract The effect of the concentration of various supporting electrolytes (acid and salt) on the cyclic voltammetric behaviour of polyaniline film electrodes has been studied in aqueous solutions. The role of junction potential and the ionic activity in voltammetric response was analysed. Conditions for the reliable evaluation of experimental data are suggested.

Radiotracer study of the adsorption of Cl− and HSO4− ions on a porous gold electrode and on underpotential deposited metals on gold

Abstract Adsorption of Cl − and HSO 4 − ions was investigated on porous gold electrodes by a radiotracer method in the presence of 1 mol dm −3 HClO 4 supporting electrolyte. In the potential range 0–1300 mV (on the RHE scale) a continuous increase of Cl − adsorption may be observed. However, in the case of HSO 4 − ions a measurable may be found only above 300 mV. In both cases a decrease of the adsorption occurs at potentials where the formation of the oxide layer starts (above 1200 mV). Underpotential deposition of Cd 2+ , Cu 2+ , Ag + results in a significant increase of the anion adsorption.

Electrocatalytic reduction of NO2− and NO3− ions at a platinized platinum electrode in alkaline medium

Abstract The electrocatalytic reduction of NO−3 and NO−2 ions was studied at platinized platinum electrodes in the presence of 1 mol dm−3 NaOH supporting electrolyte. Steady state polarization curves were taken by point-by-point potentiostatic measurements at different concentrations in the potential range 0–400 mV (on RHE scale). In both cases a sharp maximum appears on the polarization curves at low potentials (100–250 mV). In the case of NO2− ions a less pronounced second maximum may be observed at more positive potentials. The position of the maxima depends on the concentration of the reacting species. It is assumed that the polarization behaviour observed is connected with competitive adsorption phenomena.

Role of adsorption phenomena in the electrocatalytic reduction of nitric acid at a platinized platinum electrode

Abstract The electrocatalytic reduction of nitric acid was studied at a platinized platinum electrode in the presence of different supporting electrolytes. It has been found that at low HNO3 concentrations the polarization behaviour, the shape of the polarization curves and the reduction rate depend significantly upon the supporting electrolyte. (For instance, one maximum in HClO4, two maxima in H2SO4.) With increasing nitric acid concentrations the differences in the character of the polarization curves gradually disappear. The phenomena observed can be explained by the role of competitive adsorption between nitric acid and ions of the supporting electrolyte. Strongly adsorbing chloride ions exert a very pronounced influence on the reduction rate and induce periodical phenomena under certain experimental conditions. The adsorption of chloride ions was studied by a radiotracer method. Results of this study confirm the view on the importance of adsorption competition in the polarization behaviour. On the basis of simple adsorption models an attempt was made to explain the phenomena observed.

Electrochemical and adsorption behaviour of ethylene glycol and its oxidative derivatives at platinum electrodes: Part II. Electrocatalytic transformations under steady-state experimental conditions at a platinized platinum electrode in acid medium

Abstract Chemical transformations occurring in the course of electrocatalytic processes of ethylene glycol and its derivatives at a platinized platinum electrode are discussed. Both oxidation and reduction processes are considered. A reaction sequence leading to different intermediates in the course of the oxidation of ethylene glycol is formulated. Results obtained from adsorption and steady-state electrocatalytic studies are compared in order to throw light on the role of different types of adsorbed species in the electrocatalytic transformations.