Cationic catalysis and hidden negative differential resistance in reduction of radical anion of nitrobenzene

Well-known reduction of nitrobenzene in acetonitrile yields an anion radical, which is further reduced at more negative potentials. The reduction of anion radical is strongly influenced by the nature of tetraalkylammonium salts (tetramethyl- to tetraheptylammonium) used as supporting electrolytes. At low concentration of tetrahexylammonium hexafluorophosphate the anion radical reduction occurs at extremely negative potential, however, traces of alkali metal cations yield a remarkable acceleration effect. At potentials where alkali metal cations are reduced the acceleration ceases. As a result the current-potential curves show negative or hidden negative slope. Electrochemical impedance spectroscopy sensitively indicates the presence of the negative differential resistance (NDR) or the hidden negative differential resistance (HNDR). Oscillatory behavior based on this type of cationic catalysis is currently under investigation.     

Ionic association of Ce(IV)-decatungstate in the context of heteroatom reduction

The kinetics of single electron heteroatom reduction in Ce(IV)-decatungstate anion is demonstrated to be strongly dependent on the nature of the supporting electrolyte cation. DC polarography reveals an increase of both reduction rate at a positively charged mercury electrode and a limiting diffusion current in the Li⁺…Cs⁺ sequence. Both effects look anomalous in the framework of traditional concepts of electrode kinetics. To clarify the nature of these observations, ionic association of Ce(IV)-decatungstate with various alkali metal cations was addressed experimentally by means of the potentiometric technique. Finally, our self-consistent interpretation of various cation nature effects rests on a difference in the number of bound cations and their location around the polyanion. The ionic association is modeled by using the results of DFT calculations of the CeW₁₀ anion, non-local electrostatics and an extended version of the Bjerrum model.     

Kinetics of discharge of zinc at the dropping mercury electrode

The rate of the discharge reaction of zinc ions at the dropping mercury electrode is diminished if the concentration of the indifferent surface inactive electrolyte is increased, because of the decrease of the potential difference in the diffuse double layer. It diminishes also with increase of radius of the alkali-metal cations present in the electrolyte and with increase of charge of the cation of the electrolyte. A strong retarding effect is exhibited by tetraalkylammonium cations, which increases with the size of the alkyl group. The electrode reaction rate increases in presence of iodide anions, as manifested by formation of a new, more positive wave. In presence of iodide and tetraalkylammonium cations together the process is accelerated at more positive potentials and is retarded at more negative potentials.

All these phenomena are analogous to those observed with the hydrogen-evolution reaction at the mercury electrode, and are the opposite of those observed with anions of the persulphate type.

Abstract

La vitesse de décharge de Znl²⁺ sur électrode à mercure gouttante diminue quand s'accroît la concentration superficielle d'un électrolyte inactif, par suite de l'abaissement de la tension électrique dans la couche diffuse. Elle diminue aussi quand s'accroîssent le rayon on la charge des cations de l'électrolyte. Les cations tétraalkylammonium exercent un effet fortement retardateur, qui augmente avec l'encombrement du groupe alkyl. La vitesse s'élève par centre en présence d'anions I⁻. Si cet anion est accompagné d'un des cations précédents, le processus s'accélère aux tensions plus positives et se ralentit aux tensions plus négatives. Tons ces phénomènes, contrastant avec ceux manifestés avec les anions du type persulfate, sont comparables à ceux observés dans l'évolution d'hydrogène sur électrode de mercure.

Abstract

The influence shown by the composition of the electrode double layer on the electrode reaction rate of the zinc ion is quite similar to analogous effects observed with the hydrogen-evolution reaction at mercury electrodes. The decrease of the rate of the latter process with increase of the radius of alkali-metal cations, of the charge of the cation of the electrolyte and of the concentration of the electrolyte was described by Herasymenko and lendyk.²¹ The strong retardation of this process by quaternary cations and the acceleration by iodide was found by Yofa, Kabanov and co-workers.²² Tsa Chuan Sin and Yofa²³ observed the combined effect of iodide and tetraalkylammonium cations which at positive potentials accelerate the hydrogen evolution reaction slightly over the value in presence of iodide only, but retard it at more negative potentials.

From the similar features of both reactions it may be concluded that in spite of a different total reaction path, the rate-determining step is identical, i.e. the transfer of the charged particle from its hydration sheath to the electrode. It is expected that analogous phenomena will be observed with other cations which are not adsorbed too strongly at the electrode (cf. the influence of surface-active cations on the discharge of T1⁺ and Cd^{2+ 24,25}).

All the effects of the double-layer structure connected with the electrode reaction of the zinc ion are just the opposite of those observed with the reduction of anions of persulphate type.²⁰     

Reduction electrochimique du nitrobenzene,en milieu micellaire aqueux,a divers pH. Influence de la nature des tensio-actifs

The electrochemical reduction of nitrobenzene has been studied in the presence of some anionic, cationic and non-ionic surfactants using a glassy carbon electrode or a dropping mercury electrode (polarography). The results are depending upon the electrode nature, the surfactant polarity and the pH of the solutions.On glassy carbon electrode and with cationic surfactants, the reduction is always a one step process for the totality of the studied pH scale (2–12). With the anionic and non-ionic surfactants, the potential is shifted towards the more negative values for a one step process or for a two step process, according to the pH value.For the polarographical studies, the electrochemical reduction of nitrobenzene is generally retarded by the presence of monomer surfactant molecules adsorbed on the mercury electrode, principally in acid media. In micellar solutions, the CTAB decreases the protonation rate of the nitrobenzene radical anion and facilitates the reduction by a dianion way. The anionic and non-ionic surfactants increase the radical anion stability and slow down the rate of the radical anion reduction.     

Electrochemical reduction of 2,2′-dinitrobiphenyl-X type substances in nonaqueous and aprotic solvents

The oxidation-reduction behaviour of 2,2′-dinitrobiphenyl-X type compounds (X = O, S, NH, CH₂, CO) and of 2,2′-dinitrobiphenyl in non-aqueos acetonitrile and dimethylformamide and their mixtures with water was investigated by polarography and cyclic voltammetry at mercury drop electrodes. In combination with ESR-spectroscopy the reversible formation of a radical anion has been found in the primary one-electron step which is followed by a second one-electron uptake leading to a di-anion. According to the group X, to the water content and to the supporting electrolyte the second reduction step is or is not reversible. The ease of reduction decreases in the following sequence with changing X : CO > NH > S > O> - >CH₂(ie E_{$\text{1}\text{2}$} becomes more negative). According to the composition of the solution further 2 or 6 electrons are consumed at more negative potentials, giving to the 2-hydroxylamine-2′-nitro derivative or to the 2,2′-di-hydroxylamine compound, respectively. In solutions with more than approx 5–10% (by vol.) of water the reduction of the two nitro groups proceeds either in a single 8-electron step or in two 4-electron waves. In 90–94% acetonitrile solutions with alkali metal cation salts as supporting electrolytes a dip appears on the limiting current of the overall 8-electron reduction wave with the dme. The inhibition of the reduction process has been interpreted in terms of formation at the electrode of an insoluble precipitate of tri-ions resulting from the interaction between the di-anion and the alkali metal cations.     

Polarography of some anions in presence of aqueous organic solvent mixtures: Reduction of IO−3 ions in presence of various base electrolytes

The present studies treat with the reduction of IO^?₃ ions different types of base electrolytes at the dropping mercury electrode. All the measurements were done in 0.1 M base electrolytes at 30°C. The effect of various sodium and potassium halides, different cations (alkali and alkaline earth metal chloride), various alkali metal nitrates and some ammonium salts, on the polarographic characteristics of iodate ions, have been studied in details, iR measurements have also been carried out in presence of various oxy-anions and sodium salts of organic acids. In all cases the reduction was found to be diffusion controlled but irreversible. The kinetic parameters have been evaluated by Koutecky's method.     

On the electric double-layer structure at carbon electrode/organic electrolyte solution interface analyzed by ac impedance and electrochemical quartz-crystal microbalance responses

ac impedance and electrochemical quartz crystal microbalance (EQCM) techniques have been applied to analyze the structure of electric double-layer formed at carbon/organic electrolyte solution interface using a sputtered carbon electrode. The mass changes caused by electrochemical adsorption (accumulation) of ions have been estimated in the solutions of propylene carbonate (PC) dissolving tetrafluoroborate (BF₄⁻) salts of lithium (Li⁺), tetraethylammonium (TEA⁺) and tetra-n-butylammonium (TBA⁺) cations. The observed mass changes during the cathodic polarization in the solutions containing TEA⁺ and TBA⁺ were well consistent with those expected by the calculation based on mono-layer adsorption of the cations with giving the consideration to the surface roughness. On the other hand, the mass change observed in the solution containing Li⁺ salt showed that the solvation of Li⁺ with three or four molecules of PC would be the charge compensation species at the interface. Comparison of the quantity of the electricity passed during the EQCM measurements with that from theoretical calculation with simple Helmholtz-layer model revealed that the major part of the double-layer capacitance would be based on the electrostatic polarization of the solvent molecule directly adsorbed at the carbon surface.     

Correlation between specific adsorption and corrosion inhibition. Adsorption of 1-butylpyridinium bromide

A correlation between specific adsorption and corrosion inhibition was made on the basis of calculations of the degree of coverage at zero charge on the electrode and corrosion rates. 1-butylpyridinium bromide (BPB) was choosen as a model compound for these studies. The electrical double-layer parameters were determined at constant charge on a mercury electrode, from values of the interfacial capacity and surface tension obtained experimentally from aqueous solutions of BPB at 25° C. The characteristics of the salt as a corrosion inhibitor were studied through Tafel plots and weight-loss measurements using mild steel plates immersed in H₂SO₄ solutions. Analysis of the results lead to the conclusion that the salt adsorbs intensely on the metal surface, forming a complete monolayer of BP⁺ ions in 0.1 mol dm^–3 solutions and multilayers for higher concentrations. The results of this work indicate that for high coverages there is a good correlation between the amount of BP⁺ specifically adsorbed on mercury and the inhibition of the corrosion process on mild steel. That type of compound acts mainly as an anodic inhibitor in acid media by the formation of protective layers on the metal surface.     

Electroreduction of Ce3+ at mercury electrode in aqueous media

In the polarographic study fo Ce³⁺ reduction in aqueous medium two waves have been found when Ce³⁺ concentration was larger than 0.001 M. The first of these displays a pseudo-prewave behaviour which can be explained by assuming that Ce metal is adsorbed on the mercury surface. When the electrode surface is completely covered, the reduction process is inhibited and a second wave appears at more negative potentials. LSV studies on the hanging mercury drop electrode confirm these results. The irreversibility of the electrodic reaction is established and the kinetic parameters of the process are calculated.     

Comportement électrochimique des chlorures de di et trinéophylétain dans divers milieux

The chlorides of di and trineophyltin are reducible on a mercury drop electrode. The steric hindrance of neophyl group displays the one-electron wave of chlorotrineophyltin towards negative potentials; the occurrence of a bond between the adsorbed radical and mercury accelerates the electrochemical reaction rate. According to the media, dichlorodineophyltin is reduced along two two-electron, two one-electron or one two-electron steps, the last is followed by the reaction between the depolarizer and dineophyltin; these various alternatives which depend on the interactions between mercury and the reduction product have been discussed. A polarographic determination has been proposed for these two compounds.     

Anwendung der coulostatischen methode zur bestimmung der doppelschichtkapazität einer bleielektrode

The coulostatic method used in determining the double-layer capacitance of the mercury electrode was applied for measurements on a lead electrode in sodium sulfate solutions. The curves: differential capacitance of the double-layer electrode potential are shown. As a result, the sulfate ions do not adsorb on the lead. The double-layer capacitance values in 10⁻³ N Na₂SO₄ at different potentials of the lead electrode have been calculated.

The theoretical results have been compared with the experimental data. The application of Grahame's model for the double-layer structure in the case of lead electrodes has been confirmed.     

Proton-Ionizable Crown Compounds: Transport of Alkali and Alkaline Earth Cations Using Proton-Ionizable Triazolo Macrocycles

Abstract

The macrocycle-mediated flaxes of the alkali and alkaline earth metal cations have been determined in a H₂O-CH₂Cl₂-H₂O bulk liquid membrane system. Water-insoluble proton-ionizable macrocycles of the triazolo type were used. The proton-ionizable feature allows the coupling of cation transport to reverse H⁺ transport. This feature offers promise for the effective separation and/or concentration of alkali metal ions with the metal transport being driven by a pH gradient. A counter anion in the source phase is not co-transported. Transport of the alkali cations only occurred when the source phase pH was greater than the aqueous pK_a value for the carriers. Transport increased regularly with increasing source phase pH. Transport of alkaline earth cations from neutral pH source phases was minimal. The alkali cation selectivity order was K⁺ > Rb⁺ > Cs⁺ > Na⁺ > Li⁺ for the l8-crown-6 sized macrocycles, while little selectivity was observed with the 15-crown-5 sized macro-cycle.     

Reduction electrochimique des cations complexes formes par dissolution dans le N,N-dimethylformamide des sels de chrome du type CrX3, 6H2O (X = Cl,Br, ClO4)

The electrochemical reduction of different cations formed by the solubilization of hexahydrated chromium salts in DMF was studied by polarography and cyclic voltammetry on stationary mercury dropping electrode, with tetrabutylammonium tetrafluoroborate 0.2 M as an indifferent electrolyte.The nature of the cations formed by water—DMF ligand exchange in the chromium complexation sphere depends on the nature of X(X = Cl, Br, ClO₄).The reduction potentials and some electrochemicals kinetics parameters of [Cr(DMF)₆]³⁺, [Cr(H₂O)₆]³⁺, [Cr(DMF)₅Br]²⁺, [Cr(DMF)₄Cl₂]⁺ and [Cr(DMF)₃Cl₃] were determined as well as exchange reactions between these electroactive species which may be simultaneously present either in solution or at the electrode.     

Contribution a l'etude des effets catalytiques exerces par les composes soufres a l'electrode a gouttes de mercure. Cas de la thiouree

The influence of thiourea and some of its N-alkyl-derivatives on the reduction of various metallic cations (Ni²⁺, Co²⁺, Co²⁺) has been studied in neutral medium at the dropping mercury electrode. The polarographic curves exhibit kinetic prewaves reaching a 2^?_e amplitude at high concentration. In addition fairly large extra-currents can be detected at verynegative potentials. By resorting to classical and pulse polarography and to macro-electrolysis, it has been demonstrated that these currents are related to both the electroreduction of thiourea with production of sulphide and cyanide ions and catalytic discharge of water itself. The reaction mechanism is discussed in terms of the structure of the organic compound and the properties of the electrode metal surface, in connection with similar phenomena observed in the presence of thiocyanate ion (reduction of the thione group) and sulfur proteins (catalytic hydrogen evolution).     

Electroreduction of Diheptylviologen in a Self-Assembled Phospholipid Monolayer on Mercury and its Role as an Electron Transfer Mediator

The electroreduction mechanism of diheptylviologen (V²⁺) across a self-assembled dioleoylphosphatidylcholine (DOPC) monolayer supported on a mercury electrode has been investigated in aqueous 0.1 M KCl by the chronocoulometric technique. V²⁺ is reduced in two consecutive charge steps, 2V²⁺ + 3e → V⁺₂ and V⁺₂ + e → 2V⁰, involving 3/2 and 1/2 of a faraday per mole of reactant, respectively. Both charge steps are controlled by the rate of the elementary coupling step V⁺ + V⁰ → V⁺₂. V²⁺ acts as an electron transfer mediator from ferric and ferricyanide ions dissolved in the aqueous solution to the mercury surface across the DOPC monolayer, which is impermeable to these metal ions in the absence of V²⁺. The electron transfer mediation is more effective along the first charge step, 2V²⁺ + 3e → V⁺₂, and increases with the V²⁺ concentration in the lipid monolayer less than proportionally, thus pointing to the radical cation V⁺ as the most effective electron carrier from the mercury surface to the metal ions.     

Polypyrrole-based electrode coatings switchable electrochemically between the anion- and cation-exchanger states

Dodecylsulphate was incorporated into a polypyrrole matrix during anodic polymerization of pyrrole. The concentration was adjusted such that in the oxidized film about one half of the polypyrrole cationic groups was charge compensated by the sulphate groups from the incorporated dodecylsulphate, the second half by anions from the electrolyte. Upon electrochemical oxidation/reduction, the film is reversibly “switched” between a cation-exchanger (at negative electrode potentials), and an anion exchanger (at more positive potentials) state. The different states of the film are characterized by electrochemical, EDAX, and in particular by Volta-potential measurements. The Donnan potentials determined with the latter techniques as a function of electrode potential and electrolyte concentration were in good agreement with calculated values.     

Influence of proton-donors on the reduction mechanism of diacetyl on dme

The effect of the phosphoric and acetic acids concentration on the reduction mechanism of diacetyl on a mercury electrode is studied. Reaction orders with respect to these proton donors at potentials corresponding to the foot of the polarographic wave indicate that the acid species are liable to protonate the free radical resulting from the first electron transfer.     

In situ metal doping during modified anodization synthesis of Nb2O5 with enhanced photoelectrochemical water splitting

A new technique of in situ doping of alkali metal (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) in Nb₂O₅ was showcased by the modified anodization of Nb foils at high frequency, negative‐to‐positive pulsed voltage. At the optimized dopant concentration and synthesis condition, the doped‐Nb₂O₅ shows twofold enhancement in photoelectrochemical water splitting efficiencies compared with the undoped Nb₂O₅ electrode, as a result of improved charge carrier density and enhanced surface charge transfer. © 2015 American Institute of Chemical Engineers AIChE J, 62: 352–358, 2016     

Die differentielle Doppelschichtkapazität an der Phasengrenze Quecksilberanorganischer Elektrolyt—II. Das Kapazitätsminimum

In the negative range of charge, the differential double-layer capacitance at the interface of mercury and an inorganic electrolyte solution shows a characteristic minimum. This minimum is independent of the nature of the ion, of the concentration of electrolyte and of the temperature. However, the solvent strongly influences the capacitance of the minimum. The metal charge at which the minimum occurs is uninfluenced only by the temperature. Changes of the concentration of the electrolyte as well as of the nature of the anion are able to cause a shift of the minimum.