Zuzana Kováčová

Application of the ac admittance technique to double-layer studies on polycrystalline gold electrodes

Abstract A detailed examination of the dependence of the ac admittance of a cell containing a polycrystalline gold electrode has been made in the double-layer region as a function of dc potential, ac frequency and electrode history. It is shown that the interfacial impedance of a gold electrode with a carefully prepared surface can be treated under these circumstances as a constant phase element when it is in contact with an aqueous solution containing 0.05 M KClO 4 . Analysis of the frequency dependence of the cell impedance gives the surface inhomogeneity parameter n which turns out to be very close to unity. Although the electrode surface is only slightly inhomogeneous on a microscopic scale, a very large frequency dispersion of the impedance is observed experimentally. A method of estimating the true specific capacity of the electrode is presented, and conditions for carrying out the experiments in a reproducible manner are discussed.

Oxidation of cysteine, cysteinesulfinic acid and cysteic acid on a polycrystalline gold electrode

Abstract The mechanism electro-oxidation of cysteine, cysteinesulfinic acid and cysteic acid in perchloric acid solutions has been studied using cyclic voltammetry. All the compounds investigated have been found to be chemisorbed on a polycrystalline gold electrode, and oxidized with four, two or one electron respectively. The water molecule coadsorbed with cysteine participates in the cysteine one-electron oxidation with a lower energy requirement, whereas bulk water is involved in multiple electron transfer reactions at the completely covered electrode surface. A strong inhibiting effect of chloride anions on the cysteine oxidation has been observed at the partially covered electrode surface. The effect of the adsorbate surface coverage on the reaction mechanism in anodically catalysed reactions involving oxygen transfer is discussed.

Adsorption study of cysteine, N-acetylcysteamine, cysteinesulfinic acid and cysteic acid on a polycrystalline gold electrode

Abstract Capacity-potential data for the adsorption of cysteine and other organosulfur compounds on polycrystalline gold are reported. Cysteine, N-acetylcysteamine and cysteinesulfinic acid form monolayers on gold through the thiol group and significantly lower the interfacial capacity in the double-layer region. However, cysteic acid forms multilayers under the same conditions. The surface excess of the compounds at full monolayer coverage has been estimated and compared with the data for the adsorption of simple n-alkylthiols. It is argued that the packing of the cysteine and related compounds is more compact because of strong electrostatic interactions.

SNIFTIRS studies of the electrochemical double layer: Part II. Au(111) electrode in solutions with specifically adsorbed nitrate ions

Abstract In-situ IR reflection spectroscopy has been used to probe the structure of the double layer on an Au(111) electrode in solutions containing specifically adsorbed nitrate ions. The adsorbed nitrate ion is found to be bonded to the gold surface through one of its oxygen atoms. Both adsorbed nitrate ions, as well as those in the solution are found to possess C 2 v symmetry. Nitrate ions in the double layer form contact ion-pairs with hydronium ions, possibly with an extra water molecule incorporated in the ion-pair moiety. The extent of ion-pairing is found to depend on the applied potential. An indication of the onset of water chemisorption on the gold surface is also found with the present technique. On the basis of combined ATR and SNIFTIRS (subtractively normalized interfacial Fourier transform IR spectroscopy) experiments it is shown that the width of the deconvoluted nitrate band can be used to estimate the vicinity of the potential of zero charge (pzc). It was found that the pzc in 0.01 M HNO 3 is close to that observed in 0.01 M HClO 4 , and that it shifts by ca. −100 mV per pH unit due to the specific adsorption of nitrate.

SNIFTIRS studies of the double layer at the metal/solution interface. Part 1. Single crystal gold electrodes in aqueous perchloric acid

In situ infrared reflection spectroscopy has been used to probe the concentration change of the perchlorate anion in the electrical double layer at low-index single crystal gold electrodes. Cyclic voltammetry was used to estimate the amount of anionic charge in the diffuse layer, assuming the absence of specific adsorption near the potential of zero charge (pzc). A correlation between the integrated intensity of the perchlorate band at 1109 cm−1 and the surface excess of the anion in the diffuse layer estimated by the Gouy-Chapman theory was found for σm < ¦10 μCcm−2¦. At higher positive charge densities on the metal, systematic deviations were observed. A strategy to determine the pzc at the higher concentration of perchloric acid (1 M), which is not available from electrochemical techniques, is presented. This involves the previous determination of the effective extinction coefficient of the perchlorate anion in the double layer. This parameter was determined from the analysis of integrated IR bands at a lower concentration (0.1 M), for which the cyclic voltammogram provides the value of the pzc. Based on these results it is shown that SNIFTIRS data can be used to confirm the location of the pzc in both dilute and concentrated solutions under circumstances where more traditional techniques cannot be used.

Studies of the Electrochemical Double Layer at (100) Au in Perchloric Acid Solutions by In Situ Infrared Spectroscopy

In situ infrared reflection spectroscopy has been used to probe the concentration change of the perchlorate anion in the electrical double layer at (100) Au. The results obtained show clearly that there is a correlation between the integrated intensity of the perchlorate band determined in a subtractively normalized interfacial Fourier transform infrared spectroscopic (SNIFTIRS) experiment and the surface excess of the anion in the diffuse layer predicted by the Gouy-Chapman theory. An expression that correlates the above two quantities is derived.

Numerical simulation of desorption transients at electrodes on the basis of non-linear adsorption isotherms

Numerical simulation of desorption transients on the basis of the non-linear Frumkin isotherm is presented. The differential equations which describe the boundary value problem are solved by using the orthogonal collocation technique with expansion of the simulation space with respect to time. The analysis is focused on the desorption process undergone by a modified electrode when it is transferred to a solution containing only electrolyte and a potential step is applied. Three different cases for the kinetics of the desorption/adsorption step are considered: irreversible, quasi-reversible and reversible. In the irreversible case it is shown that the shape of the chronoamperogram is governed by the difference between the interaction parameters involving adsorbate with adsorbate, and activated complex with adsorbate. There is a critical value of this difference above which a maximum in the i–t curve occurs. In the presence of the maximum a transformation of the chronoamperograms is proposed which allows one to define a unique curve regardless of the value of the kinetic and interaction parameters. In the quasi-reversible case, for certain values of the rate constants, an initial decay of the current before the appearance of the maximum is predicted. Under certain conditions for high coverage (θ≈ 1) two maxima are observed. The effect of the potential on the logarithm of the maximum current and the time at which the maximum occurs is considered. At a more negative potential a limiting linear relationship between both quantities and electrode potential is predicted.

Double‐Layer Effects Associated with Surface Reconstruction on a Au (100) Electrode

Double-layer effects for the reduction of Co(NH 3 ) 6 3+ and Fe(H 3 O) 6 3+ have been examined at a freshly annealed Au(100) electrode in aqueous HClO 4 solutions. Marked changes in the reduction current are observed after the electrode is cycled through a positive potential at which surface reconstruction is lifted. The observed effects are attributed to the large shift in the potential of zero charge which accompanies reconstruction, and to changes in electrode area

The role of the metal in determining electrosorption valency in the case of strong molecular adsorption at the electrode|solution interface

Abstract : The electrosorption valency for the adsorption of parabonic acid has been determined on single crystal gold electrodes and mercury near the point of zero charge. A new technique to perform the measurement using cyclic voltammetry is described. The electrosorption valency was found to be a linear function both of the potential of zero charge and of the work function of the metal. (AN)