Cl. Buess-Herman

Oxygen reduction at platinum modified gold electrodes

The reduction of oxygen has been studied on polycrystalline gold electrodes modified by platinum deposited spontaneously from an aqueous K2PtCl6 solution via the displacement of copper or lead adlayers. The change in the surface composition and morphology has been checked by XPS, AES and AFM data. The kinetic results have shown that such electrodes may present a higher catalytic activity compared to bulk platinum electrodes during a scan where the potential is made more positive and is thus clearly expressed by an hysteresis in the CV curves. The displacement of copper and lead deposits gave similar amplitudes of the hysteresis but the modified electrodes obtained via a lead deposit present a better stability upon cycling in acid solutions. The observed behaviour can be correlated to the modification of the MOH formation and reduction on noble metals.

On the behaviour of molecules of the quinoline group at the water-mercury interface: Part III. The kinetics of the isoquinoline transition in the presence of an inert electrolyte

Abstract The reorientation transition, which involves two distinct monolayers of isoquinoline molecules, has been investigated by using the single-potential step and the double-potential step methods under various experimental conditions. The cathodic transients correspond to a liquid → solid transition. Their morphology and half-times have been measured at various initial and final potentials, and for several isoquinoline concentrations close to the saturation value. At small overvoltages, the reorientation process is determined by heterogeneous (“instantaneous”) nucleation, while progressive nucleation and growth prevail at higher potentials. Recourse to the double-potential step method affords an efficient way of assessing separately the influence of the overvoltage on the rate constants for nucleation and growth. Determination of the critical nucleus size and the activation energy for each of the two processes has been based on an adaptation of the Brandes theory developed for two-dimensional crystallization from a supersaturated vapour phase. The kinetics are markedly dependent on the initial state, which can be easily controlled by the potential and the bulk concentration. When the potential is stepped from the region where there is superadsorption, progressive nucleation is particularly fast, while starting from a partially depleted layer gives much slower transients with extensive tailing due to diffusional hindrance. The anodic transients show consonant characteristics which indicate that the initial film can be considered as a two-dimensional solid, and that boundary defects are acting as nucleation centres, during the solid → liquid transition triggered by the potential step.

Non-faradaic phase transitions in adsorbed organic layers: Part I. General Theory

Abstract non-faradaic phase transitions are fairly common in the field of organic adsorption in interfacial electrochemistry. According to circumstances, these transitions may reflect adsorption-desorption processes, displacement of one surfactant by another, or molecular rearrangement within a compact monolayer. In the present work, the different types of mechanisms by which these phase transitions may proceed are briefly described and the case of nucleation and growth is considered in detail. The nucleation kinetics are developed using the classical theory of nucleation and expressions for the rate of nucleation, the radius and the Gibbs energy of formation of the critical nucleus are derived in terms of the change in specific Gibbs energy in passing from one phase to another.Non-faradaic phase transitions are fairly common in the field of organic adsorption in interfacial electrochemistry. According to circumstances, these transitions may reflect adsorption-desorption processes, displacement of one surfactant by another, or molecular rearrangement within a compact monolayer. In the present work, the different types of mechanisms by which these phase transitions may proceed are briefly described and the case of nucleation and growth is considered in detail. The nucleation kinetics are developed using the classical theory of nucleation and expressions for the rate of nucleation, the radius and the Gibbs energy of formation of the critical nucleus are derived in terms of the change in specific Gibbs energy in passing from one phase to another.

On the behaviour of molecules of the quinoline group at the water—Mercury interface: Part II. Electrocapillary study of isoquinoline in solutions containing only an “inert” electrolyte

Abstract The present paper is the first of a series dedicated to the interfacial behaviour of several surfactants of the quinoline group, respectively in the absence and in the presence of coadsorbed anions. The present communication deals with the adsorption of quinoline (Q) itself at the mercury—water interface, in a medium such that coadsorption can be considered as negligible. Study of equilibrium adsorption has been based on measurements of interfacial tensions, charge densities and differential capacities as a function of the potential and the Q concentration. The experimental isotherms have been analysed with the help of Esin-Markov plots. In the range of sufficiently negative potentials, a compact film of invariant structure is observed, which remains stable up to a sharply defined cathodic desorption potential. At less negative potentials, the Esin-Markov plots, as well as the capacity—concentration—potential map, indicate that three different superficial structures may prevail, according to the domain of potential and concentration: o (1) water with dilute Q molecules lying flat; (2) a mixed aqueous layer which embodies both flat and standing Q molecules; (3) a film characterized by abnormally large values of the superficial excess, reflecting the presence of randomly distributed multilayered clusters. No clear evidence has been found for a stable compact layer of molecules lying flat. As can be expected, surface—pressure plots are non-congruent, in view of the variety of orientation and lateral forces involved with potential or concentration changes. Some data are presented which will be made fully explicit at a later stage (Parts IV and V of this series), where the various surfactants will be compared in terms of their molecular structure and dipole orientation.

Mechanism of electron transfer through monomolecular films of neutral organic species adsorbed at an electrode surface

The kinetics of reduction of Co(NH3)3+6, CrO2-4, S2O2-8 and O2 at a Hg electrode coated with monolayers of quinoline, iso-quinoline and 3-methyl-iso-quinoline have been investigated. The mechanism of electron transfer at the film-covered electrode depends on the structure of the superficial film. At a liquid-like film, the reacting ions are discharged both from inside and outside the film. Within the potential range where sold-like films are observed, it has been found that: (1) the rate of ETR reactions is slowed down by 6–7 orders of magnitude; (2) the decelerating effect of the film is distributed equally between changes in the probability of the electron transition and the energy of activation of ETR; (3) the logarithm of the reaction rate decreases linearly with the film thickness; (4) Tafel plots are linear and their slopes are slightly smaller than those observed for the bare electrode surface; and (5) the electronic transitions are probably elastic in character although distinction between elastic and resonance tunnelling is difficult for thin films.

On the two modes of condensation displayed by thiourea at the mercury-water interface

Abstract In the presence of “inert” electrolytes (sodium fluoride, sulfate, carbonate, phosphate) there exists a range of temperatures and thiourea (TU) concentrations (low and high respectively) where two distinct condensed TU monolayers, totally devoid of anions, are observed. The two relevant molecular structures which are suggested rest on the analysis of the following data: (1) E-T-[TU] phase diagrams derived from capacitance measurements, (2) superficial excesses and molecular areas, (3) charge densities, (4) inhibition power, and (5) kinetics displayed by the various phase transitions involved. Anions like ClO−4, NO−3, ClO−3 are only coadsorbed within the gas-like film of TU, with the result that the pure TU condensed regions are squeezed detectably. On the other hand, anions known to interact strongly with Hg (Cl−, Br−, I−, SCN−)_do coadsorb significantly at all potentials. However, pure TU films tend to reappear if the concentration of these anions is lowered sufficiently with respect to that of the surfactant.

Non-faradaic phase transitions in adsorbed organic layers: Part II. Experimental aspects

Abstract We propose in Part I of the present paper that electrocapillary processes which display discontinuous phase transitions and which are controlled by nucleation and growth mechanisms, can be sufficiently described by the classical theory of nucleation. This theory relates the nucleation rate to the change in the surface Gibbs energy gap Δγ#. However, the experimental verification of this theory raises number of methodological problems. These are now identified and then discussed in terms of the most suitable perturbation to be applied to the system. The recommended procedure involves two steps. Firstly, the measurement of a thermodynamic transition potential Eθ and secondly the analysis of certain charge density transients which are observed at overpotentials relative to Eθ. Depending on the circumstances (nature of the surfactant, type of phase change, programme imposed on the electrode, electrode size) the kinetics of the nucleation and growth of a new phase can be observed in three different transformation regimes: mononucleation, oligonucleation and polynucleation. Each of them requires different methodological and mathematical approaches. Furthermore, artefacts are commonly observed in each case and these may complicate the kinetic analysis. The two most common artefacts are faradaic interference and extraneous nucleation. Methods designed to cope with these are described. It is then shown that extraneous nucleations, far from being a nuisance, can in fact be turned to advantage to measure nucleation and growth rates separately.

On the behaviour of molecules of the quinoline group at the water-mercury interface: Part IV B. The kinetics of two-dimensional phase transitions displayed by 3-methyl-isoquinoline

Abstract Adsorption of 3-methyl-isoquinoline (3-MeisoQ) at the mercury-water interface involves three distinct kinetically controlled processes: o (1) The liquid-solid (LS) transition, which is characterized by a conspicuous step in the charge-potential curves, presents a hysteresis region which, in the absence of heterogeneous nucleants, exceeds 60 mV. The true equilibrium transition potential has been determined by double-potential-step experiments devised in a way such that there is stable coexistence at the electrode of both the L and S phases. The L→S process is controlled by a nucleation-growth-collision mechanism. Methods for the independent evaluation of the rate of nucleation and that of growth have been used. The rate constants obtained in this way are in good agreement with the combined set derived from the Avrami equation for progressive nucleation. The reverse process S→L, which is considerably faster appears to originate from defect lines at the collision boundaries between the separate plates which form the solid patchwork. (2) A qualitatively similar behaviour characterizes the adsorptiondesorption (GS) process which occurs at very negative potentials and is partially influenced by mass transfer from the bulk phase. (3) The solid phase, which prevails in the middle range of the potential scale, shows indications of a limited slow pre-melting process, at the immediate proximity of the GS transition. With the exception of the latter phenomenon, which is specific to 3-MeisoQ, and provided that the concentrations are normalized with respect to the saturation value, 3-MeisoQ adsorption kinetics does not differ sharply from that of isoQ, although significant differences demonstrate that the presence of the methyl group weakens the stability of the flat and tilted molecular orientations, which are observed in the most positive range of charge densities.

Phase transition processes displayed by coumarin monolayers at the water−mercury interphase

Abstract At low temperatures and high concentrations, coumarin was found to form a quasi-solid monolayer at the merucy electrode, in addition to the two structurally different monolayers previously reported. This new condensed layer is characterized by a very low (3.5 μF cm−2) potential-independent double layer capacitance. The effects of potential, temperature, coumarin concentration and background electrolyte on the adsorption behaviour are presented and analyzed. It is shown that the condensed layer can be formed from either of the other two adjoining layers by a nucleation and growth mechanism. The related growth rates and line tensions have been calculated. The most probable structures for the three layers are proposed.

On the behaviour of molecules of the quinoline group at the water-mercury interface: Part IV A. Electrocapillary study of 3-methyl-isoquinoline

Abstract The adsorption of 3-methyl-isoquinoline at the mercury-water interface involves a number of rate-determining factors (local depletion uncompensated by mass transfer, influence of the diffuse-layer potential, slow kinetics) which has limited the present study to the most concentrated solutions. In that range, the major traits of the behaviour of 3-MeisoQ and isoQ are comparable at least qualitatively (rearrangement and desorption transitions, condensed layers, existence of a quasi-solid two-dimensional structure of invariant properties at negative potentials). Differences result from the steric hindrance of the methyl group on all adsorption modes, except that which involves the perpendicular-high orientation (with nitrogen and the methyl group facing the solution side of the interface). It appears that, as is the case for aliphatic alcohols or acids among others, meaningful comparison must be based on relative concentrations normalized with respect to the saturation value. The dependence of the rearrangement transition potential on surfactant concentration and ionic strength indicates that the inner potential difference is the most suitable independent variable.