N. A. Rogozhnikov

Kinetics of electrochemical processes in the system : silver/cyanide solutions

The kinetics and mechanism of electrode reactions of silver in cyanide solutions have been studied. The results of these studies have been compared with the data on cyanide ion adsorption on silver. The values of derivatives of the current with respect to the concentrations of cyanide ions and silver have been obtained both from the direct measurements and on the basis of model calculations taking account of the influence of cyanide and complex ion adsorption and the structure of electric double layers on the rate of electrochemical reaction. Candidates for the limiting stage have been considered.

Adsorption of S2− and HS− ions on the (111) face of coinage metals: A quantum-chemical study

The interactions of S2− and HS− ions with gold, silver, and copper were studied by density functional theory using the cluster model of the metal surface. The geometrical and energy characteristics of the interactions of these ions with the surface metal atoms were evaluated. The S2− ions form stronger chemical bonds with the surface metal atoms than HS− ions. A significant charge transfer from anion to metal occurs during the adsorption. The adsorbability increased in the series Ag < Cu < Au for both anions. The HS− ion showed greater ability to be transferred to the surface during the electrochemical adsorption due to the strong hydration of the S2− ion. In alkaline media, however, the dissociation of the adsorbed HS− leads to its conversion to S2−.

Effect of Alkali Metal Cations on the EDL Structure at a Gold Electrode in Alkali Solutions

Effect of the nature of cations of alkali metals on the EDL structure during adsorption of hydroxide ions at a gold electrode is considered. It is shown that the reason for such an effect can be the pulling of cations into the dense layer resulting from the formation of associates of cations and specifically adsorbed anions OH–. Parameters that reflect this phenomenon are estimated quantitatively.

Effect of thallium(I) ions on anodic dissolution of gold in alkali—cyanide solutions at negative potentials

The effect of thallium(I) ions on the rate of anodic dissolution of gold in cyanide solutions is studied by measuring the potential and time dependences of the current. The extreme nature of the dependences is attributed to a maximum catalytic effect, which is presumably connected with reaching a certain ratio between the surface coverages by thallium adatoms and cyanide adions, and to a complicated potential dependence of the process rate constant in the presence of thallium adatoms

Adsorption of complex silver cyanides on Ag(111). Quantum chemical consideration

The interaction of AgCN molecules and Ag(CN)2−, Ag(CN)32−, Ag(CN)43− ions with the silver surface is studied based on the cluster model of the metal surface by quantum chemistry methods. The geometrical and energy parameters of the interaction of these species with the metal surface are assessed. As regards the strength of their chemical bond with the surface, these compounds form the following series: Ag(CN)2− < Ag(CN)32− < AgCN < Ag(CN)43−. The surface activity of silver-containing species is compared with regard to the solvent effect. It is found that Ag(CN)2− and Ag(CN)32− anions exhibit close adsorbabilities on silver. Molecules AgCN are not accumulated on the surface because of their very low content in solution. The adsorption of Ag(CN)43− is hindered due to a considerable value of degradation energy of this three-charged ion. In the adsorbed state, the ions Ag(CN)2− and Ag(CN)32− represent stable compounds displaying no surface dissociation to yield compounds with the smaller coordination numbers.

Structure of electrical double layer at gold electrode in cyanide solution

The potential distribution in electrical double layer is calculated, basing on the data on the electrode charge and cyanide-ion adsorption at the gold electrode. It is shown that the integral capacitances of regions in the dense layer are not unambiguous functions of the electrode potential or charge per se, but depend also on the amount of specifically adsorbed ions Γ. A function is proposed for the describing of the Γ dependence of the dense layer integral capacitances.

The microbalance determination of kinetic parameters of anodic dissolution of gold in thiocarbamide electrolytes in the presence of sulfide ions

As shown by quartz-crystal microbalance measurements, in the potential range from 0.0 to 0.55 V (NHE) sulfide ions adsorbed on the gold electrode surface accelerate the electrode reaction of anodic dissolution of gold in acidic thiocarbamide solutions. The microbalance determination of kinetic parameters at a constant electrode surface coverage with sulfide ions includes a special procedure developed for the determination of the gold dissolution rate. The conditions (the potential range and the potential scan rate) of independence of the dissolution rate from the diffusion limitations associated with the ligand delivery is determined. Under these conditions, the polarization curve is shown to be linear on semilogarithmic coordinates and correspond to the Tafel equation. In this potential range, the transfer coefficient alpha and the reaction order with respect to the ligand p are determined at a constant electrode surface coverage thetas with adsorbed sulfide ions. It is shown that with the transition from the surface coverage with sulfide ions thetas = 0.1 to thetas = 0.8, the transfer coefficient alpha changes from 0.25 to 0.55, the exchange current (i0) changes from 10-5 to 5 times 10-5 A/cm2, and the effective reaction order p with respect to the ligand changes from 0.2 to 1.3. The mentioned changes are associated not only with the acceleration of gold dissolution in the presence of chemisorbed sulfide ions but also with the changeover in the mechanism of this process. Quartz-crystal microbalance data on the gold dissolution rate qualitatively agree with the results of voltammetric measurements of a renewable gold electrode. A possible version of explanation of the catalytic effect of sulfide ion adsorption on the gold dissolution is put forward.

Model description of specific adsorption of two anions on electrodes

Simultaneous specific adsorption of two anions on the electrode surface is considered. Calculations involve the use of the Frumkin adsorption isotherm for each anion and a model that describes the electric double layer structure based on one or two Helmholtz planes. Dependences of the ion adsorption on both the electrode potential and the solution composition are calculated for different electrode charges, ion sizes, and capacitance characteristics of the electric double layer.

Mechanism of metals’ electrodeposition from complex electrolytes allowing for the anions’ adsorption: Effect of the position of ligands and complex ions in the compact layer on the reaction orders

Effect of the relative position in the compact layer of specifically adsorbed electroactive complex ion and complexing ligand on the magnitude of current is considered. Effect of parameters that characterize the adsorption of such ions and the electrical double layer structure on the dependence of current on the solution composition and effective reaction orders is analyzed.

The effect of sulphide ions on anode dissolution of gold in thiocarbamide electrolytes

The introduction of sodium sulphide as a microaddition (about 10/sup 5/ mol/I) into acidic solutions of thiocarbamide causes a sharp acceleration of the anode dissolution of gold. The acceleration enhances with an increase in the concentration of Na/sub 2/S (c), and with an increase in the time of contact between the electrode and solution (/spl Delta/t) before measurements; it decreases after the polarization curve passes through its maximum at E /spl cong/ 0.5 V. Using the procedure of electrode surface renewal by cutting off a thin metal layer, we studied the regularities of gold dissolution in the solution containing 0.1 M thiocarbamide and 0.5 M H/sub 2/SO/sub 4/, for given c and /spl Delta/t values. An explanation of the discovered regularities is proposed, assuming that the dissolution process is catalyzed by sulphide ions adsorbed on the electrode.