B. I. Podlovchenko

The behaviour of platinized-platinum and platinum-ruthenium electrodes in methanol solutions

Abstract Measurements of the curves showing the dependence of the shift in the Pt/Pt electrode potential, after the introduction of methanol, upon pH of the solution and the potential at the moment of introduction of methanol, lead to the conclusion that the potentials of a Pt/Pt electrode, in methanol solutions, are determined by the adsorbed hydrogen. The presence of adsorbed hydrogen has been established from the charging curves and the potentiostatic curves of a platinum electrode measured after its contact with methanol. The analysis of these curves shows, also, that under definite conditions a predominant chemisorption of particles with the composition HCO occurs on platinized platinum in methanol solutions. The kinetics of methanol oxidation under non-steady-state and steady-state conditions have been studied and the difference in the mechanisms of the process on the bare platinum surface and on that covered with a stationary layer of chemisorbed substance, shown. The influence of pH of the solution, the dependence of the rate upon the potential and the kinetic characteristics of electro-oxidation of the chemisorption products, are used as possible criteria for the elucidation of the mechanism of the process. A hypothesis has been advanced, that on a bare surface the rate of the process is determined by dehydrogenation of the original alcohol molecules and under steadystate conditions-by the oxidation of the products of their chemisorption. In acid solutions the latter process may proceed by way of interaction with OH radicals. Some possible schemes of the oxidation process in alkaline solutions have been discussed. Composite electrolytic deposits of platinum and ruthenium, and of palladium and ruthenium with a small percentage of ruthenium, have been found to be extremely catalytically active in the reaction of methanol electro-oxidation.

The behaviour of a platinized-platinum electrode in solutions of alcohols containing more than one carbon atom, aldehydes and formic acid

According to electrochemical measurements and the analysis of gases evolved when a Pt/Pt electrode is immersed in solutions of saturated alcohols and aldehydes containing more than one carbon atom, processes of dehydrogenation, hydrogenation and self-hydrogenation of the original substances and their decomposition products (mainly along the C1—C2 bond) occur on the electrode surface. A steady concentration of Hads on the electrode surface, which determines the final potential, is established and maintained due to the above processes. The outgassed Pt/Pt surface exerts a stronger destructive action upon these substances than the surface covered with Hads.

Electroreduction of carbon dioxide on palladium electrodes at potentials higher than the reversible hydrogen potential

The electroreduction of CO2 was observed on dispersed electrolytic deposits of palladium at potentials more positive than the reversible hydrogen potential (pH 8–10) with the HCOO − current yield close to 100%. It was assumed that adsorbed hydrogen atoms take part in the slow stage of the electroreduction of HCO3− (CO2) to HCOO−.

Preparation and electrocatalytic properties of platinum microparticles incorporated into polyvinylpyridine and Nafion films

The deposition of platinum in Nafion (NF) and polyvinylpyridine (PVP) films (d=1 μ m) was carried out from 5 mM Na2PtCl4+0.5 M H2SO4 and 5 mM Na2PtCl4+0.05 M H2SO4+0.5 M Na2SO4 solutions potentiostatically (at 0.1 V vs RHE). The possibility is shown of preparing ultradispersed deposits with a particle diameter of ∼3 nm. The rate of hydrogen evolution on Pt deposits in NF and PVP films in the range of small overvoltage (η<30 mV) is limited by the removal of hydrogen from the external surface of the electrode. For Pt/NF/GC electrodes this process occurs at smaller overvoltage values than that on a smooth Pt surface. The higher hydrophobicity of the Pt/NF surface probably facilitates the formation of bubbles and reduces hydrogen supersaturation in the near-electrode layer. The electrocatalytic behavior of platinum deposits in NF and PVP was studied in the reaction of methanol and formic acid oxidation. The role of the dimensional effect and metal particles interaction with the polymeric matrix in electrocatalytic processes was demonstrated. In the Pt/PVP/GC system, the first effect is more pronounced, and in the Pt/NF/GC system the second effect is predominant.

Effect of the nature of platinum group metals on the formation and properties of adatom silver coatings

Abstract A comparison was made of the behaviour of Ag adatoms on electrolytic depositions (e.d.) of Pt, Pd, Rh and Ir. It was established that on the substrates studied the adsorption of Ag at “underpotentials” proceeds chiefly electrochemically in a irreversible way and that the adsorbed Ag atoms can displace the oxygen previously adsorbed. It was found that on e.d.s of Rh and Ir, unlike the case of Pt and Pd, the formation of the second Ag ads layer starts before the monolayer is completed. Correlations between the potentials of electrodesorption of silver adatoms ( E des r ), the work function W e of platinum metals and the potentials of zero free charge E pcz were examined. It was noted that there is an influence of the oxygen absorbed on the electrode on the E des r values of the Ag adatoms.

The use of galvanic displacement in synthesizing Pt(Cu) catalysts with the core-shell structure

The formation of a Pt(Cu) bimetallic catalyst on the carbon support by galvanic displacement of copper electrodeposits with platinum (PtCl62− as the displacing agent) is systematically studied. Composition, structure, and electrocatalytic properties of samples corresponding to different stages of copper displacement are analyzed. For substantially long displacement times, the formation of stable Pt(Cu)st particles with the atomic ratio Pt: Cu ≈ 7: 3 is observed. The Pt(Cu)st/C electrodes are shown to be close to the Pt/C electrode as regards the adsorption of hydrogen and copper atoms and the specific activity in methanol oxidation (with 0.5 M H2SO4 as the supporting electrolyte). Such electrocatalytic behavior of Pt(Cu)st particles makes it possible to infer the formation of the “core(Pt, Cu)-shell(Pt)” structure, as confirmed by the XPS data.

Electrocatalytic Behavior of a Palladium–Polyaniline System Obtained by Electrodepositing Palladium into a Preliminarily Formed Polyaniline Film

The electrochemical behavior of Pd–PAN systems, where PAN stands for polyaniline, is studied. The systems are formed by electrodepositing palladium onto a PAN–GC electrode (GC, for glassy carbon). It is shown by atomic force microscopy that some palladium particles are deposited directly on the PAN surface even at potentials of low electroconductivity of PAN. Properties of Pd–PAN coatings formed by this technique are compared to those of Pd–PAN systems formed by cycling the potential of a GC electrode in a PdSO4+ aniline solution, which were studied earlier. Redox characteristics of PAN and the degrees of the promotion of the electrocatalytic activity in the HCOOH oxidation reaction differ in the two systems. The differences are attributed to specific features inherent in the formation of the mixed coating, in particular, to differences in the degree of contact of palladium crystallites with PAN. The PAN degradation in the Pd–PAN systems is shown to be linked mainly to the PAN oxidation, which is catalyzed by the incorporated palladium particles.

Specific features of hydrogen sorption by palladium dispersed forms at α-phase potentials

Abstract A method using foreign metal adatoms is proposed for distinguishing adsorbed and dissolved hydrogen in palladium electrodes. It is shown that the solubility of hydrogen in the α phase of different dispersed palladium samples is higher than that in smooth Pd and increases as the dispersity increases. On the basis of the Bucur model, a quantitative estimate is made of the structural defectiveness of the palladium dispersed forms investigated.

A correlation between the changes in total charge and open circuit potential of a hydrogen electrode during the adsorption of neutral particles and the formation of adatoms from ions

Abstract Analytical descriptions are given for the charges passed through the hydrogen electrode (at E ads =const) and the shifts of its open circuit potential, which are observed during the strong adsorption of neutral particles and ions with the formation of adatoms. The integral values of transients of current and those of open circuit potential are explained by using the concept of electrode total charge. The interrelation between transients of current and open circuit potential is analyzed. To check experimentally the relationships obtained, the adsorption measurements on a platinum electrode in solutions of iodine and iodide anions are mainly used (0.5 M H 2 SO 4 as the supporting electrolyte). The theoretical conclusions are shown to be in adequate agreement with the experimental results.

The formation and behavior of Ag adsorbates at Pd electrodes and their influence on electrocatalytic oxidation of formic acid at Pd

Abstract The electrochemical behavior of Ag ions at smooth Pd electrodes and Pd/Pt deposits is investigated. At adsorption potentials, which are more anodic than the reversible potential of the Ag + /Ag electrode, an underpotential deposition of Ag + ions occur. We can distinguished two types of Ag ad-atoms at Pd. The first type of Ag is irreversibly adsorbed. The second, which exists only in the presence of Ag ions in the solution, is reversibly adsorbed. The influenced of various coverages of Ag ad-atoms at smooth Pd and Pd/Pt deposits on the electrocatalytical oxidation of formic acid was investigated. Even small coverages of Ag ad-atoms lower the rate of formic acid oxidation. With higher coverages this inhibiting influence grows continously.