A.R. Despić

Electrocatalysis by foreign metal monolayers: Oxidation of formic acid on platinum☆

Abstract It was demonstrated that some foreign metal monolayers formed by underpotential deposition have pronounced catalytic effects on the oxidation of formic acid on platinum. The explanation of these effects was sought within the framewor of existing data on the formic acid oxidation and the underpotential deposition. It was found that the catalytic effect of foreign metal monolayers originates in the decrease of hydrogen adsorption thus preventing the formation of the main poisoning species COH. At the same time these experiments confirm the previously postulated mechanism of formation of the poisoning species involving adsorbed hydrogen.

Electrochemical oxidation of formic acid at noble metals: Catalytic effects of foreign metal monolayers

Abstract The catalytic effects of a submonolayer of lead on noble metals have been shown and discussed in terms of a previously published model. A theoretical analysis of these effects is presented. It was demonstrated that foreign metal monolayers allow a determination of the true catalytic activity of electrodes in the case of self-poisoning reactions. A volcano-shaped curve for oxidation of formic acid on noble metal electrodes was obtained.

Characterization of electrochemically formed thin layers of binary alloys by linear sweep voltammetry

It has been demonstrated that linear sweep voltammetry can be used as anin situ technique for characterization of electrodeposited thin layers of binary alloys. The anodic dissolution characteristics of linear sweep voltammograms are very sensitive to the type of electrodeposited alloy. If both metals do not passivate in the investigated solution, eutectic type alloys are characterized by two sharp dissolution peaks, indicating no miscibility between components in the solid phase. From the ideal solid solutions, components of the alloy dissolve simultnaeously, while in the case of intermediate phases and intermetallic compounds each phase, or compound, has its own peak of dissolution.

The influence of adsorbing substances on the lead UPD onto (111) oriented silver single crystal surface. I

Abstract Effect of anions present in solution on the UPD of lead onto (111) oriented silver surface has been investigated. Linear sweep voltammetry was used. It was established that organic anions, such as acetate and citrate, shift the UPD phenomena towards the reversible potential of lead. Adsorption isotherms of lead indicate first order phase transition. Addition of molecular thiourea in solution, which is known to be strongly adsorbed, justified the assumption that the observed effects are due to the adsorbing properties of the anions.

Electrocatalysis by Foreign Metal Monolayers: Oxygen Reduction on Gold and Platinum*

Es wird gezeigt, daß monoatomare Schichten von Fremdmetallen, die bei Potentialen oberhalb des reversiblen Potentials dieser Metalle abgeschieden werden, deutliche katalytische Wirkung auf die Sauerstoffreduktion an Gold besitzen. Auch die Reduktion von Wasserstoffperoxyd wurde katalysiert. Eine Inhibition ergab sich für die Sauerstoffreduktion an Platin. Die katalytischen Effekte werden durch eine begünstigte Adsorption von Sauerstoff oder sauerstoffhaltigen Substanzen auf der fremdmetallbedeckten Schicht bedeutet. Einige andere mögliche Erklärungen werden ebenfalls herangezogen.

Identification of intermetallic compounds in electrodeposited copper-cadmium alloys by electrochemical techniques

Abstract Anodic sweep voltammetry was used to identify different intermetallic compounds and assess their amount in copper-cadmium alloys. Four compounds are found in thin (1–8 μm) electroplated alloy films: CuCd 3 , Cu 5 Cd 8 , Cu 4 Cd 3 and Cu 2 Cd. Their presence is reflected in four peaks recorded in the voltammograms, the fifth peak corresponding to dissolution of copper remaining after dissolution of cadmium. For quantitative assessment potential-step technique was also used, avoiding the problem of overlap of peaks. Indications were obtained that the first peak is a mixture of pure cadmium and the cadmium rich compound. Since changes in voltammograms with time of resting of the alloy in an inert atmosphere were found, they were ascribed to solid state reactions of formation of compounds richer in copper from the one richer in cadmium (or pure cadmium) and pure copper. Thermodynamics of alloy formation was considered and approximate values of the standard potentials and standard free energies of the compounds are evaluated.

Activation of aluminium in chloride containing solutions

Abstract Galvanostatic transients were recorded for anodic dissolution of high purity aluminium in neutral chloride solutions. All transients, except at very low current densities (below 50 μA cm −2 ), are characterized by an overshoot and a sudden decay of potential to the potential plateau of active anodic dissolution. Initial portions of the transients were analyzed for electrode capacitance and oxide growth. Sudden decay is interpreted in terms of adsorption of chloride and its effect on the structure and conductivity of the oxide film.

Kinetics of electrochemical oxidation of ammonia in alkaline solution

Abstract The kinetics of the anodic oxidation of ammonia in alkaline solutions at platinized platinum has been investigated by the galvanostatic method. The galvanostatic charging curves show three polarization levels. Times of transition between the second and third level are independent of ammonia concentration, as are also the exchange current densities, and the standard current densities at −300 mV(she) for the first and the second levels respectively. The second level showed Tafel behaviour with slopes of 40. The nature of the rest potential is discussed and some remarks about possible reaction mechanisms are made. It is suggested that the rest potential is a reversible hydrogen potential determined, by the atomic hydrogen produced by the chemical decomposition of ammonia at the electrode. It is also shown that no mechanism suggested in the literature can be accepted without further investigation. Raney nickel was also tested but meaningful results were not obtained.

A contribution to the knowledge of electrochemical deposition of W induced by Ni

Abstract It was found that codeposition of Ni and W from an ammonia and citrate-containing electrolyte, is very sensitive to changes in operating conditions. The Ni–W alloy deposit exhibited sensitivity of chemical composition and phase structure on the composition of the electrolyte, pH in particular, as well as on the current density (cd), hydrodynamic conditions and quantity (thickness) of the deposit. Thus, small changes in pH, which arise during the process of bath utilization, are found to change the relationships between peaks in anodic dissolution voltammograms reflecting the existence of different phases. The anodic linear sweep voltammograms (ALSV) of the alloys deposited at medium cds, pH lower than 8.4 and a high rate of rotation of the RDE contained two peaks. The first one could be ascribed to some tungsten oxide species while the appearance of the second one basically reflects the presence of solid solution of W in Ni, but also points to some other phases. The increase in pH, increase in cd, and the decrease in the rate of rotation of the RDE all lead to ALSV with a single peak with a peak potential shifted in the negative direction, i.e. towards the dissolution peak of pure Ni. Analysis confirmed a decrease of W content in that shift. Still a somewhat more noble behavior than that of Ni points out that some W still remains in the Ni.

Hydrogen evolution at Zn-Ni alloys

Zn-Ni alloys were electrodeposited from a borate-citrate and TEPA bath in potentiostatic and galvanostatic regimes. Potential of electrodeposition or current density were varied and chemical composition and phase structure were determined. It was found that both chemical composition and phase structure of alloys electrodeposited at the same potential or current density but in the two electrolytes were different. Differences were found even when one and the same chemical composition was achieved. Hydrogen evolution kinetics at the Zn-Ni alloys deposited from the two baths was investigated in alkaline solution (1 M NaOH). Substantial differences in the reaction rates at the alloys with the same chemical but different phase composition were found.