A. Frumkin

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.

Potentials of zero total and zero free charge of platinum group metals

Some fundamental definitions and relations of the thermodynamic theory of the platinum electrode are considered. The notions of the potential of zero total charge (pztc) of the first and second kind are discussed. It is shown that in the presence of excess of surface-inactive ions, the potential of zero total charge of the second kind can be regarded as the potential of zero free charge (pzfc). The pzfc is an analogue of the pzc of metals not adsorbing hydrogen and oxygen. The methods of determination of the pztc and the pzfc of platinum metals, of their dependence on solution pH and salt concentration are considered, as well as the dependence of the pzfc on surface coverage with adsorbed hydrogen. Tables listing the pztc and the pzfc of platinum metals are given. The conditional nature of the notions pzfc and the charge of the double layer in the presence of strong chemisorption of ions is demonstrated and the difference between the formal and the true coefficients of charge transfer is emphasized. The notion of the charging curves and electrocapillary curves of the first and second kind is introduced and the methods of their finding are discussed. The electrocapillary curves of the first and second kind are given for platinum in solutions of different composition.

Potentials of Zero Charge

The notion of the potential of zero charge (pzc) and the relevant term were introduced 50 years ago.(1) Later, the pzc was proved to be an important electrochemical characteristic of metal and to play a major role in electrocapillary and electrokinetic phenomena, electric double-layer structure, adsorption of ions and neutral organic molecules on the electrode, wetting phenomena, physico-chemical mechanics of solids, photoemission of electrons from metal into solution, and in electrochemical kinetics. The introduction of the notion of pzc led to solution of the Volta problem and to rigorous interpretation of the attempts to measure or calculate the “absolute” electrode potential. All this testifies to the fundamental nature of the notion of pzc.

The congruence of the adsorption isotherm with respect to the electrode potential or charge and the choice of an independent electric variable

Summary In a strictly thermodynamic approach to the investigation of the adsorption isotherm, the choice of an electric variable is of no fundamental importance and is determined by considerations of expediency or convenience. This is not the case, however, when the thermodynamic approach to the investigation of adsorption is supplemented by the assumption of the congruence of the adsorption isotherm with respect to the electrode potential or charge. These assumptions are only compatible provided the double-layer capacity in the supporting electrolyte solution, C0, does not differ from the capacity, C′, at complete surface coverage with adsorbed substance. If C0 markedly exceeds C′ as, for example, in the case of adsorption of aliphatic compounds on mercury, the assumption of the congruence of the adsorption isotherm with respect to the electrode potential is in better agreement with the experimental data. Moreover, this assumption corresponds to a clear and consistent physical picture of the surface layer in the presence of adsorbed molecules of organic substance (the model of two parallel capacitors).

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.

Potentials of zero charge, interaction of metals with water and adsorption of organic substances—I. Potentials of zero charge and hydrophilicity of metals

The most reliable values of the potentials1 of zero charge for metals not adsorbing hydrogen are obtained from the position of the minimum on the differential capacity-potential curves. These data are confirmed by the scrape method and by electron photoemission measurements. However, as it was first found from the comparison of the behaviour of gallium and mercury, at the same potentials referred to pz the adsorption behaviour of various electrodes with respect to the simplest aliphatic surfactants—aliphatic alcohols—differs. This was explained by preferential water chemisorption at electrodes of the gallium type. The determination of the position of the differential capacity curve of the amyl alcohol desorption peak relative to pzc gives a semi-quantitative estimate of the hydrophilicity of various metals, inreasing in the sequence Hg < Bi < Sn < Pb < Cd < In < Ga. The potential at which the differential capacity of the dense layer starts to increase with decreasing negative charge shifts to more negative values relative to pzc in a similar sequence. The obtained results are compared with Trasattis data. The case of antimony requires further investigation.

The electroreduction of the S2O82− anion

Summary It has been shown that extrapolating the dependence of the i, φ curves for the electroreduction of the S2O82− anion in the presence of 0.05 M KCl on the r.p.m. of a rotating electrode under turbulent stirring conditions, an i, φ curve undistorted by concentration polarization at positive surface charges can be obtained. The corrected Tafel plots (c.T.p.) for the electroreduction of the S2O82− anion at antimony, amalgamated copper, bismuth, tin, lead, cadmium and indium rotating disc electrodes have been compared. The c.T.p. depend on the cation of the supporting electrolyte, but are practically independent of the nature of the electrode metal if the value −2 is ascribed to the charge of the reacting particle in the double layer. The coincidence of the c.T.p. shows that the work function does not appear in the equations of electrochemical kinetics and that, at least for the S2O82− anion, the slow penetration (dynamic ψ1 effect) does not markedly affect the i, φ dependence in the measurable section of the polarisation curve. No coincidence of the c.T.p. obtained with different metals is observed if the effective charge of the reacting particle in the double layer is equated with −1. The implications of this result are discussed. Additional measurements are required in the case of the Fe(CN)63− anion.

The notion of the electrode charge and the Lippmann equation

Summary The notion of the charge has been considered for the case of an ideally polarized electrode. It has been proved necessary to distinguish between the total charge, which figures in thermodynamic relations, and the free charge, which can be determined only in terms of a certain electric double-layer model. A definition of the total charge is given, equally applicable to ideally polarized and reversible electrodes, as the amount of electricity to be supplied to the electrode to keep the electrode potential constant when its surface is increased by unity and the composition of the bulk phases of the system is maintained constant. The total charge thus determined satisfies in all cases the Lippmann equation. Expressions are given for the Lippmann equation for reversible redox systems, as exemplified by platinum-hydrogen and amalgam-thallium electrodes. It has been shown that in such systems two kinds of electrocapillary curves can be obtained, depending on which chemical potential is held constant: that of the oxidised or that of the reduced component. The number of characteristic electrocapillary curves in the general case of a reversible redox system has been shown to be equal to that of the independent variables in the Nernst equation expressing the conditions of electrochemical equilibrium of the system. The results obtained have been used for the interpretation of the electrocapillary dependences observed under polarographic conditions.

On the role of polarity of adsorbate and solvent molecules in adsorption of organic substances on electrodes

Summary The role of the orientation of adsorbed dipoles of water and organic substance at the electrode/solution interface is discussed. It is shown that the charge corresponding to maximum adsorption on mercury of different aliphatic compounds does not remain constant but varies within rather wide limits depending on the polarity of adsorbate molecules. This proves the importance of the interaction of adsorbed organic dipoles with the double layer field in the adsorption of organic substances on electrodes. The displacement of adsorbed organic molecules by water molecules at sufficient large electrode charges is determined by the ratio of the capacities in the absence of adsorbate and at complete coverage of the electrode surface with it, rather than by the ratio of the effective dipole moments of these molecules per unit surface.

On the charge transfer in the process of adsorption at the electrode/solution interface: Remarks on the paper “experimental determination and interpretation of the electrosorption valency γ” by J. Schultze and K. Vetter

A critical analysis shows that the approach to the estimation of the charge transfer in the process of adsorption at the electrode/solution interface used by Vetter and Schultze in the general case is not justified without introduction of non-thermodynamic models. This approach can prove useful in the investigation of the electric double layer within the limits of applicability of Grahames model. A number of cases can however be cited where the approach of Schultze and Vetter is inapplicable. The conclusions of Schultze and Vetter regarding the adsorption of hydrogen at platinum are apparently based on some misunderstanding. In this case the thermodynamic approach6, 23 should be used. It would be more correct to call the quantity γ “the formal coefficient of charge transfer”, rather than “the electrosorption valency”.