“Cap-Pair” Effect as the Reason of the Catalytic Properties of Nicotinic Acid: Experimental and Theoretical Studies

The influence of nicotinic acid (NC) on the kinetics and the mechanism of electroreduction of Zn²⁺ ions in the acetate buffer (pH=6.0) was investigated using electrochemical methods (EIS, CV, SWV and DC). It was shown that the anions of NC catalyze the electrode reaction (cap-pair effect) by adsorbing on the surface of the mercury electrode. The catalytic activity of NC is due to its ability to form active NC−Zn²⁺ complexes on the electrode surface, facilitating the electron transfer process. However, no evidence of the formation of such complexes in the solution was found using classical molecular dynamics. Moreover, it was proved that the electroreduction of Zn²⁺ ions in the presence of NC is a two-stage process. The first stage involves the transfer of the first electron, preceded by the partial loss of the hydration shell by the Zn²⁺ ions and formation of the active complex. Moreover, it was shown that in the range of lower concentrations, c≤1.10⁻² mol.dm⁻³, the nicotinic acid shows weaker catalytic abilities than another form of vitamin B3 – nicotinamide. In the range of its higher concentrations, the nicotinic acid is a more effective catalyst for the electroreduction of Zn²⁺ ions.     

The Effect of Supporting Electrolyte Concentration on Zinc Electrodeposition Kinetics from Methimazole Solutions

The kinetic parameters of Zn²⁺ ion electroreduction in sodium perchlorate used as the supporting electrolyte on the mercury electrode in the presence of methimazole were determined using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and DC polarography. The two‐stage nature of this process was demonstrated. Both steps are catalysed by a methimazole. The size of the acceleration effect depends on the concentrations of methimazole and the supporting electrolyte. The acceleration of the electrode process involves the formation of active complexes between the depolarizer ions and methimazole on the electrode surface. These complexes facilitate the exchange of charge between the electrode and zinc ions during electroreduction process. The change of the hydrating sphere of the zinc ion is also important here. This in turn depends on its oxidation state and the concentration of the supporting electrolyte.     

Adsorption of Co2+ and Zn2+ on cryptomelane-type hydrous managese dioxide

Co²⁺ and Zn²⁺ ions are adsorbed on cryptomelane-type MnO₂ by exchange with surface protons and with structural ions (probably K⁺ and/or Mn²⁺) in the oxide. The latter sites are responsible for the much higher capacity to these cations, compared to Na⁺. At all pH values, two straight lines expressing the presence of mainly two groups of sites with distinctly different adsorption energies are located in the Langmuir plots for both Co²⁺ and Zn²⁺. The apparent capacities of the two groups increase with the increase of pH, indicating the involvement of protons in the adsorption process over the whole concentration range. The higher Co²⁺ capacity at relatively low pH, compared to the Zn²⁺ capacity, is probably due to a more exchange with the structural ions. Crytomelane type MnO₂ seems to be a quite heterogenous ion adsorbent whose adsorption sites could be approximated to two groups only.     

Electroreduction of carbon dioxide: Part II. The mechanism of reduction in aprotic solvents

The mechanism of electroreduction of carbon dioxide in aprotic solvents on mercury, lead, tin, indium and platinum is studied using the photoemission method and the method of stationary polarization curves. When comparing the data of photoemission and “dark” (polarization) measurements it was found that in the first Tafel region of the polarization curves the rate-determining step is the transfer of the second electron to (CO₂)^.?₂ anion-radicals formed as a result of the interaction of initially generated CO^.?₂ anion-radicals with adsorbed CO₂ molecules. In the second Tafel region the rate-determining step is the transfer of the first electron to an adsorbed CO₂ molecule. The peculiarities of electroreduction of carbon dioxide in aprotic solvents can be explained provided that the effect of potential on adsorption of CO₂ and anion-radicals and the effect of repulsion of negatively charged radicals are taken into account.     

On the Electrochemical Deposition and Dissolution of Divalent Metal Ions

The deposition of Cu²⁺ and Zn²⁺ from aqueous solution has been investigated by a combination of classical molecular dynamics, density functional theory, and a theory developed by the authors. For both cases, the reaction proceeds through two one‐electron steps. The monovalent ions can get close to the electrode surface without losing hydration energy, while the divalent ions, which have a stronger solvation sheath, cannot. The 4s orbital of Cu interacts strongly with the sp band and more weakly with the d band of the copper surface, while the Zn 4s orbital couples only to the sp band of Zn. At the equilibrium potential for the overall reaction, the energy of the intermediate Cu⁺ ion is only a little higher than that of the divalent ion, so that the first electron transfer can occur in an outer‐sphere mode. In contrast, the energy of the Zn⁺ ion lies too high for a simple outer‐sphere reaction to be favorable; in accord with experimental data this suggests that this step is affected by anions.     

Influence of the Mixed Adsorption Layer of 1‐Butanol/Toluidine Isomers on the Two Step Electroreduction of Zinc(II) Ions

Studies of mixed adsorption layers with respect to their influence on kinetics of Zn²⁺ ions reduction indicate dynamics of this process in the presence of inhibitor and accelerating substances. This effect can be seen as a much greater increase of standard constant rates compared with a similar increase without the inhibitor. 1‐Butanol was used as an inhibitor in the studies and p‐toluidine and m‐toluidine as accelerating substances. The adsorption measurements show that in the range of Zn²⁺ ion reduction potentials in the solutions containing 1‐butanol and the definite isomer of toluidine, toluidine plays a dominant role in establishing equilibrium in the mixed adsorption layer. The obtained values of the true standard constant rates of the transfer of the first electron k_s1^t and the second electron k_s2^t indicate that in each of the studied systems the stage of the first electron transfer is more strongly accelerated compared with the stage of the second electron transfer. The linear character of the dependence k_s1^t and k_s2^t in the function of the surface excess and in the function of 1‐butanol concentration indicates that active complexes are formed for each case of the mixed adsorption layer.     

生物小分子的电化学分析研究 Ⅲ.维生素B_2电化学还原反应的表征

用电化学、光谱电化学和顺磁共振谱等方法对维生素B_2在粗热解石墨电极上的电化学还原机理进行了表征。结果表明维生素B_2在粗热解石墨电极上的还原为2e、2H~+过程,它先经1e、1H~+过程还原生成自由电子定域在N(5)上的自由基中间体,然后再经1e,1H~+过程还原为二氢核黄素。     

Switching transition states with changing electrode potential: Zn(II)/Zn electrodeposition on glassy carbon in the N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid

The mechanisms for the electrodeposition and stripping of Zn²⁺|Zn in the N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid are investigated via cyclic voltammetry. Analysis showed that the deposition of Zn onto a bulk Zn surface occurred via a two-electron process, with the first electron transfer being rate determining. The electrodissolution was found to occur via a potential-dependent mechanism with the first electron transfer being rate determining near the formal potential, while an intermediate chemical step became rate determining at more positive potentials.     

The removal of heavy metal cations by natural zeolites

In this study, the adsorption behavior of natural (clinoptilolite) zeolites with respect to Co²⁺, Cu²⁺, Zn²⁺, and Mn²⁺ has been studied in order to consider its application to purity metal finishing wastewaters. The batch method has been employed, using metal concentrations in solution ranging from 100 to 400 mg/l. The percentage adsorption and distribution coefficients (K_d) were determined for the adsorption system as a function of sorbate concentration. In the ion exchange evaluation part of the study, it is determined that in every concentration range, adsorption ratios of clinoptilolite metal cations match to Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich (DKR) adsorption isotherm data, adding to that every cation exchange capacity metals has been calculated. It was found that the adsorption phenomena depend on charge density and hydrated ion diameter. According to the equilibrium studies, the selectivity sequence can be given as Co²⁺ > Cu²⁺ > Zn²⁺ > Mn²⁺. These results show that natural zeolites hold great potential to remove cationic heavy metal species from industrial wastewater.     

Removal of zinc metal ion (Zn) from its aqueous solution by kaolin clay mineral: A kinetic and equilibrium study

A laboratory batch study has been performed to study the effect of various physic-chemical factors such as initial metal ion concentration, solution pH, and amount of adsorbent, contact time and temperature on the adsorption characteristics of zinc (Zn²⁺) metal ions onto kaolin. It has been found that the amount of adsorption of zinc metal ion increases with initial metal ion concentration, contact time, solution pH but decreases with the amount of adsorbent and temperature of the system. Kinetic experiments clearly indicate that adsorption of zinc metal ion (Zn²⁺) on kaolin is a two steps process: a very rapid adsorption of zinc metal ion to the external surface is followed by possible slow decreasing intra-particle diffusion in the interior of the adsorbent which has also been confirmed by intra-particle diffusion model. The equilibrium time is found to be in the order of 60 min. Overall the kinetic studies showed that the zinc adsorption process followed pseudo-second-order kinetics among pseudo-first-order and intra-particle diffusion model. The different kinetic parameters including rate constant are determined at different initial metal ion concentration, solution pH, amount of adsorbent and temperature respectively. The equilibrium adsorption results are analyzed by both Langmuir and Freundlich models to determine the mechanistic parameters associated with the adsorption process. The value of separation factor, R_L from Langmuir equation also gives an indication of favorable adsorption. Finally thermodynamic parameters are determined at three different temperatures and it has been found that the adsorption process is exothermic due to negative ΔH° accompanied by decrease in entropy change and Gibbs free energy change (ΔG°).     

Study on complexation adsorption behavior of dibenzo-18-crown-6 immobilized on CPVA microspheres for metal ions

Dibenzo-18-crown-6 (DBC) was immobilized on crosslinked polyvinyl alcohol (CPVA) microspheres, resulting in polymer-supported crown ether DBC–CPVA. The complexation adsorption behaviors of DBC–CPVA microspheres towards diverse metal ions were investigated. The experimental results show that among alkali metal ions, the complexation adsorption ability of DBC–CPVA for K⁺ ion is the strongest, and crown ether-metal complex in 1:1 ratio is formed, exhibiting a high adsorption capacity. The adsorption capacities of alkali metal ions on DBC–CPVA are in the order: K⁺ ? Na⁺ > LI⁺ > Rb⁺ > Cs⁺. Among several divalent metal ions, DBC–CPVA exhibits stronger adsorption ability towards Zn²⁺ and Co²⁺ ions, and a “sandwich”-type complex is formed probably in a molar ratio of 2:1 between the immobilized DBC and Zn²⁺ ion as well as between the immobilized DBC and Co²⁺ ion. The adsorption capacities of the several divalent metal ions on DBC–CPVA are in the order: Zn²⁺ > Co²⁺ ? Cd²⁺ > Cu²⁺ > Ni²⁺ > Pb²⁺. The complexation adsorption is exothermic physical physisorption process, and raising temperature leads to the decrease of the adsorption capacity. At the same time, the entropy during the complexation adsorption decreases, so the adsorption process is driven by the decrease of enthalpy.     

Molecular Description of the Persulfate Ion Reduction on a Mercury Electrode

The polarization curves for the S₂O₈ ^2- electroreduction on a mercury electrode at high overvoltages and various concentrations of a surface-inactive supporting electrolyte are modeled within modern theory of charge transfer in polar media and quantum-chemical approaches. Based on an analysis of the reactant adsorption in terms of a cluster model, the conclusion is drawn that the persulfate ion is localized in the diffuse part of EDL. When calculated the current, it was assumed that the transfer of the first electron, accompanied by the bond cleavage, is the limiting stage of the total two-electron process. The integration if performed over the entire electron spectrum of the metallic electrode and an attempt is made to account for electrostatic and solvation effects on a molecular level. It is shown that the experimentally-studied overvoltage interval corresponds to the occurrence of the process near the activationless region. The increase in the current at high negative charges of the surface is due to an increase in the reaction layer thickness. This effect arises from a change in the ratio between contributions made by the reactants at the distance of closest approach and the species farther away.     

Electroreduction of some pyridine carboxamides on carbon electrodes in aqueous solutions

The electroreductions of the NAD⁺ model compounds nicotinamide (I), N₁-methyl nicotinamide (II), N′-methyl nicotinamide (III) and isonicotinamide (IV) on carbon electrodes have been studied in aqueous media in the pH range 0–12 by linear-sweep cyclic voltammetry (Scheme 1, I-IV). Logarithmic analyses of the reduction peaks were performed by computing the convolution of the current with time as a function of the potential. On the basis of the experimental results it was concluded that the irreversibility of the electron transfers increased when a glassy carbon electrode was used, and this irreversibility being more marked when a plastic formed carbon electrode was employed. The reduction processes occurred with more difficulty on carbon electrodes than on mercury electrodes. Both the reduction and the reoxidation (when occurred) processes changed with respect to those observed on mercury electrodes, being irreversible electron transfers the rate-determining steps in most cases. Thus, for compounds I, II and III at pH < 2 the reductions occurred by the uptake of two electrons and two H⁺ ions, and the rate determining step was found to be the first one-electron transfer, for I and III, and the irreversible second electron transfer, preceded by the uptake of an H+ ion, for II. At pH>3 the processes consisted of electrodimerization reactions, preceded by the protonation of the heterocyclic nitrogen in cases I and III. The second electron transfer of the electroreduction of IV always appeared irreversible, in contrast with that found for mercury electrodes.     

Mechanism of simultaneous removal of Ca2+, Ni2+, Pb2+ and Al3+ ions from aqueous solutions using Purolite® S930 ion exchange resin

The sorption mechanism of Ca²⁺, Ni²⁺, Pb²⁺, and Al³⁺on ion exchange resin S930 has been studied as a function of pH in both mono-component and quaternary systems at ion ratio 1:1:1:1. The equilibrium of ion exchange process in mono-component systems has been followed as an adsorption process and it was tested for Langmuir and Freundlich isotherm equations. The selectivity coefficient of the resin for these cations varied in different manners depending on the pH range. The structural characteristics of pores in the initial resin and the resin after contact with synthetic solutions at pH 3.03 and 3.95 have been investigated by using SEM and EDAX techniques.     

The influence of tetramethylthiourea on a two-step Zn2+ ion electroreduction in concentrated NaClO4 solutions

Abstract  

A two-step Zn²⁺ ion reduction was estimated at the dropping mercury electrode in 2, 3, and 4 mol dm⁻³ NaClO₄ with the addition of tetramethylthiourea using an impedance method at wide potential and frequency ranges. With increasing tetramethylthiourea and NaClO₄ concentrations the k_{\texts 1} ^t k_{{{\text{s}}_{ 1} }}^{t} and k_{\texts 2} ^t k_{{{\text{s}}_{ 2} }}^{t} values increased, achieving their maximum values at the highest concentrations of tetramethylthiourea in 3 and 4 mol dm⁻³ NaClO₄ solutions. The catalytic effect of tetramethylthiourea at its lowest employed concentration increased with increasing NaClO₄ concentration.     

Role of Ion exchange in permeation processes

The single ion transport of transition metal ions like Cu²⁺, Co²⁺, Ni²⁺ and Zn²⁺ were carried out through the H⁺ and alkali metal ion forms of Nafion membrane. These studies showed that the ion exchange selectivity coefficient of the permeating ion had an effect on its transport process. It was found that the diffusion coefficient values (D) were directly proportional to the selectivity coefficient (K). This shows that the initial stage of permeation is governed by ion exchange process (effect of K on D).     

Electrochemical Investigation of Binding of Heavy Metal Ions to Turkish Lignites

Adsorption and desorption of Cu²⁺, Pb²⁺, Cd²⁺, Ni²⁺ and Zn²⁺ ions on samples of lignites (young brown coal) from three areas in the vicinity of Konya (Anatolia, Turkey) were followed using the polarographic method of analysis. This method enables the determination of free metal ions in suspensions containing both small and colloidal particles of lignite. Effects of pH, nature of the metal ion, and origin of the lignite on its adsorption capacity were followed. Binding is only between 5 and 30% reversible, indicating that ion exchange is not the predominant factor. The role of the size and shape of cavities inside pulverized lignite and of the functional groups inside these cavities was considered.     

Electroreduction of Bi(III) Ions at a Cyclically Renewable Liquid Silver Amalgam Film Electrode in the Presence of Methionine

The catalytic influence of methionine (Mt) on the electroreduction of Bi(III) ions on the novel, cyclically renewable liquid silver amalgam film electrode (R–AgLAFE) in a non-complexing electrolyte solution was examined. The presence of methionine leads to a multistep reaction mechanism, where the transfer of the first electron is the rate limiting step, which is the subject of catalytic augmentation. The catalytic activity of methionine is a consequence of its ability to remove water molecules from the bismuth ion coordination sphere, as well as to form active complexes on the electrode surface, facilitating the electron transfer process.     

Kinetics and Mechanism of Zn(II) Ions Electroreduction Catalyzed by Organic Compounds

The paper describes the results of the studies of organic substances catalytic activity on Zn(II) ions electroreduction on mercury in perchlorate solutions. Zn(II) ions electroreduction in the presence of catalyzing substance proceeds in two one‐electron stages. The first electron transfer is the stage determining the process rate. All catalyzing substances increase the rate of first electron transfer. The acceleration effect is connected with the stability of active complexes formed on the electrode surface. The rate of the second electron transfer depends mainly on the adsorption of the catalyzing substance on the electrode surface – of the surface excess and the structure of the adsorption layer. Hence the second electron transfer can be inhibited or catalyzed. The mechanism of the organic substance catalytic activity is also given.     

Coupled radiotracer and voltammetric study of the adsorption of 1-hydroxy-ethane-1,1-diphosphonic acid on polycrystalline gold

Coupled application of a version of the in-situ radiotracer ‘foil’ method and voltammetry provided information on the time-, potential-, concentration- and pH-dependent adsorption of 1-hydroxy-ethane-1,1-diphosphonic acid (HEDP) on a polycrystalline gold electrode, and on the effect of Zn²⁺ ions on the adsorption phenomena. Adsorption processes on the oxide-free surface of gold were observed to be potential-dependent in the potential range 0.05–1.00 V (versus RHE), while formation and irreversible accumulation of oxidation products of HEDP could be detected at E>1.00 V. The relative adsorption strength of HEDP (its dissociation and/or oxidation products) was found to be higher on an oxide-free gold surface than on an oxide-covered one. The surface excess of HEDP increased with increasing pH. Addition of Zn²⁺ ions to the solution exerted a substantial effect on the HEDP accumulation. Namely, significant differences in the surface coverage, as well as in the kinetics and mechanism of HEDP adsorption could be detected in the potential regions below and above E=0.2 V. Reduction of Zn(II) species at E≤0.1 V is probably coupled with the induced adsorption of HEDP on an Au electrode, leading to the formation of a polymolecular HEDP–Zn surface complex layer.