E. Gileadi

The potential sweep method- a theoretical analysis

A comparison is made of the current/time curve obtained by the potential-sweep method and of the potential/time curve obtained galvanostatically. Limitations of the slow and of the fast potential-sweep technique, from the point of view of obtaining kinetic data or coverages in the case of complex reactions (eg electro-organic oxidations) are discussed. A detailed mathematical analysis of the method is carried out for a charge transfer reaction of the type A− → A + e0−. Three cases that depend on the displacement of the reaction from equilibrium are distinguished. Analytical expressions are derived for current as a function of potential, for peak current, and for the peak potential for the cases of quasi-equilibrium and the condition under which the reverse reaction may be neglected. For the case where both forward and reverse reactions have to be taken into account, only a numerical analysis of the dependence of current on potential is possible. The capacitance/potential plots (capacitance = current/sweep-rate) are displaced to more positive potentials with departure from equilibrium, and the capacitance maxima decrease and reach a steady value for considerable departure from equilibrium. The experimental results of Will and Knorr on the adsorption of hydrogen and oxygen on Pt using the potential-sweep method are in good agreement with the present theory.

A molecular theory of the charge dependence of competitive adsorption

Abstract The adsorption of neutral molecules from solution is calculated theoretically as a function of the field strength in the electric double layer. The basis of the treatment is molecular competition determined by the field interactions of solvent and solute at the interface. The general case is developed in which the solvent dipoles may have field and lateral interactions which differ for different orientations taken up as the double-layer field changes direction. It is suggested that lateral interactions between adsorbed solute molecules may be neglected. The theory is tested with data, obtained on mercury electrodes both from capacitance and electrocapillary measurements, on the adsorption of butanol and phenol as solutes from two different solvents, water and methanol. The change in adsorption behaviour when going from water to methanol, in particular with respect to the shape of the coverage/charge-density relationship and the position of the point of maximum adsorption, is explained consistently for the four system, using independently known data on molecular properties. It is shown that such an explanation is not possible with a recent theory proposed by Devanathan and Tilak.

The determination of the potential of zero charge on solid metals

Abstract An attempt has been made to evaluate several experimental methods by which the potentials of zero charge (pzc) of metals could be concordantly measured. The methods chosen involved measurements of the adsorption in the double layer of a neutral organic compound as a function of the concentration of the supporting electrolyte; double-layer capacitance as a function of potential; and static friction between surfaces in solution. The pzc for Pt containing a negligible amount of H is Eq=0 = 0·56 (±0·07) − 0·067 pH (3 methods). For Au, Eq=0 = 0·15 ± 0·02 (2 methods). For Ag, Eq=0 = −0·44 ± 0·02 (2 methods). For Ni, Eq=0 = −0·26 ± 0·06 (3 methods). Introduction of H into Pt shifts the pzc in the negative direction. Anodic and cathodic pulsing of Au and Ag electrodes produced surfaces which gave pzc values 0·2 and 0·4 V respectively more positive than those for unpulsed electrodes. Results reported earlier for Pt are for Pt containing dissolved H. The dissolved proton concentration, abnormally high near the surface, contributes significantly to the surface potential. The results obtained oil pulsing of silver and gold are for dermasorbed surfaces. The most satisfactory methods for the determination of the pzc are the capacitance, static friction, immersion, and tension vibration methods.

Dielectric Relaxation in the Electric Double Layer

There are two groups of phenomena that can cause frequency dispersion of capacity and resistivity at a perfectly polarizable interface. The first comprises artifacts which lead to the observation of an apparent frequency dispersion: Of these, penetration of the electrolyte to shielded parts of the electrode and the resulting transmission line phenomena are discussed. The second group comprises processes which lead to an inherent frequency dispersion of the double‐layer impedance: among these are, in particular, the relaxation phenomena of polar double‐layer constituents.Experimental results have been obtained for the interface: Hg (dropping) /6NHCl aq., in terms of parallel capacity, Cpd1, and resistivity, Rpd1, of the electric double layer as a function of frequency, v. These results are consistent with the occurrence of dielectric relaxation of the water dipoles adsorbed at the interface, and characterized by a certain distribution of relaxation times around τ0≃10−6 sec. The results are not consistent w...

A comparison of radiotracer and electrochemical methods for the measurement of the electrosorption of organic molecules

The electrosorption of benzene on platinized Pt has been reinvestigated by a radiotracer and three electrochemical methods. The results obtained by all four techniques are in good agreement in certain potential and temperature ranges. At higher positive potentials agreement between the three electrochemical methods is found but the radiotracer method yields higher coverage values. The discrepancy is attributed to the existence on the surface, in such potential ranges, of significant concentrations of intermediate radicals formed during the oxidation of benzene to CO2. The significance and reliability of results obtained by the various techniques for adsorption measurements show the necessity always to employ both radiotracer and electrochemical methods to the same system. From an analysis of the potential dependence of the total number of radioactive particles on the surface, and the apparent coverage (using the number of electrons which are assumed for the over-all reaction) given by the electrochemical methods, information concerning radicals present on the surface may be gained. The shape of the coverage/potential relation gives information as to the general nature of the species that predominate on the electrode surface. If radicals involved in the reaction predominate, the θ/V relation is unlikely to be parabolic: the observation of such a relation thus indicates the predominance of adsorbed solute. If there is good agreement between electrochemical and radioactive methods, this is confirmed. Use of both radiotracer and electrochemical techniques together for studying adsorption is particularly informative. The study of adsorption isotherms as a basis to electrode-kinetics-mechanism determination is important.

Electrosorption of uncharged molecules on solid electrodes

Abstract The theory of electrosorption of uncharged organic molecules on solid electrodes is reviewed. The fundamental differences between electrosorption and adsorption from the gas phase are pointed out. Experimental techniques for the study of electrosorption on solid electrodes are described and the limitation of the electrochemical methods are emphasized. Some results on the adsorption of n -decylamine and naphthalene on several metals at room. temperature and on the adsorption of ethylene on Pt over a range of temperatures are presented.

Electrochemical kinetics of parallel reactions

Abstract The electrochemical oxidation of complex molecules (e.g., hydrocarbons) sometimes involves branched reactions. Under these conditions, it is difficult to define a reversible potential and the coulombic efficiencies of product formation may then be potential-dependent. A number of cases of the kinetics of branched reactions of the type with step A → B rate-determining, are worked out. It is found that in most cases, the coulombic yields are potential-dependent over certain potential regions. The calculated variation of coulombic yields with potential is compared with results obtained in the anodic oxidation of some unsaturated hydrocarbons.