Potential-determining Reaction Research Articles

Beyond the thermodynamic volcano picture in the nitrogen reduction reaction over transition-metal oxides: Implications for materials screening

Electrocatalytic production of ammonia from dinitrogen is considered as a sustainable alternative to the energy-demanding and pollutive Haber-Bosch process. A promising class of materials for selective nitrogen reduction (NRR) corresponds to transition-metal oxides given that these electrodes do not show a high activity toward the competing hydrogen evolution reaction. So far, density functional theory calculations have been used to comprehend trends in a class of materials by using the concept of scaling relations and volcano plots. This thermodynamic theory pinpoints that either the formation of the *NNH adsorbate or the formation of ammonia are reconciled with the potential-determining reaction steps. Thus, the development of NRR catalyst has largely focused on the optimization of these two elementary processes. In the present contribution, overpotential and kinetic effects are factored into the volcano plot for the NRR over transition-metal oxides by making use of the recently introduced activity descriptor Gmax(η). It is illustrated that the thermodynamic volcano picture is too simplistic as the limiting reaction step may alter close to the volcano apex: there, particularly surface reactions may govern the reaction rate. In addition, it is demonstrated how to include the formation of hydrazine as a competing side reaction into the volcano plot, which is of importance for weak binding *NNH catalysts where the formation of hydrazine may compete with the formation of ammonia. Given that the outlined methodology in this manuscript is universal and not restricted to the class of transition-metal oxides, the presented kinetic volcano picture may contribute to the development of NRR catalysts for nitrogen fixation.

Use of Compleons for Environmentally Friendly Processes of Dissolution of Iron-Oxide Deposits

The article considers the influence of OEDF and EDTA on the rate of dissolution of magnetite with the varied conditions for experiment. It is shown that the effectiveness of action both nitrogen- and phosphorus-containing complexon comes to a change in the value of potential on the border of the division of phases oxide/solution. As a result of kinetic study nature of the potential-determining reaction is revealed, which is determined by the following equation: .

Contribution from the ion-exchange factor to the potential of a copper-containing electron-ion exchanger

The process of formation of the electrode potential of EI-21 electron-ion exchanger, composed of ultrafine copper particles and KU-23 sulfocationite, was studied. The potentials of a EI-21 powdery electrode with a platinum lead in copper(II) sulfate solutions of various concentrations (0.005–1.0 M) were measured using currentless-mode potentiometry. The potential of this electrode first shifted by 0.02–0.15 V in the negative direction with respect to a compact copper electrode, after which the shift eventually decreased to −0.010 ± 0.003 V. It was demonstrated that the time evolution of the potential is determined by the interplay of electron and ion exchange. When EI-21 is placed onto a platinum lead, the role of the potential-determining reaction passes from Cu2+ + e− ⇄ Cu+ to Cu2+ + 2e− ⇄ Cu. At the same time, H+-Cu2+ ion exchange gives rise to a change in the ratio of the concentration of copper(II) ions in the internal and external solutions. The Donnan potential, which arises at the boundary between the electron-ion exchanger and the external solution, maintains a high concentration of copper(II) ions in the internal solution, a factor that facilitates the recrystallization of the particle distributed over the bulk of the exchanger. The process of recrystallization slows down with time to such an extent that the electrode potential stops changing, remaining at a level close to the equilibrium potential of the Cu2+/Cu pair. It was concluded that the internal stability of the system makes the potential of the EI-21 electrode sensitive to the dispersity of the metal component and the concentration of potential-determining metal ions in the external solution.

Settling Peculiarities of Silver Sulfide Potential in Acetate Media

Peculiarities of the argentite (α-Ag2S) electrode potential settling in acetic acid-acetate solutions are revealed by electroless chronopotentiometry, voltammetry at low polarization, and measuring the rim (wetting) angle. The process rate and the rate constant for the chalcogenide dissolution in a pre-equilibrium state are calculated from the self-dissolution current and the potential transient, respectively. The zero charge potential is found; the pH effect on the above parameters is revealed. It is shown that the potential-determining reaction involves the metal, rather than sulfide, ions.

Electrochemical Oxidation of Disulfides Derived from Isocyanuric Acid

Anodic oxidation of linear and bicyclic disulfides derived from isocyanuric acid was studied by cyclic voltammetry and numerical simulation. The PM3 molecular modeling revealed three types of the highest occupied molecular orbitals determining the electrochemical reactivity of these compounds: n electrons of the sulfur atoms of the disulfide bridge in linear and N-Me- and N-Ph-substituted bicyclic disulfides, π electrons of the aromatic fragment in the disulfide with the N-2-(2-methoxyphenoxy)ethyl isocyanurate substituent, and n electrons of the sulfide sulfur atoms in the compound with the -SS- and -S- fragments in the aliphatic chain. Oxidation of the disulfides with the SS-localized highest occupied molecular orbital follows the scheme with reversible electron transfer, followed by a fast potential-determining second-order reaction and a slow current-determining first-order reaction (ECC mechanism). The isocyanurate heterocycle does not participate directly in redox transformations but stabilizes the electrochemically generated radical cations by the mechanism of transannular interaction.

A Superficial Diffusion Zone of a Solid as the Intermediate Correlating Partial Processes of the Anodic Dissolution of Intermetallic Phases

A quantitative description of the correlation between partial processes of anodic dissolution of intermetallic phases is proposed. It is based on the gradient energy of the superficial diffusion zone formed at an initial stage of dissolution and accompanying the electrode process at its following stages. The gradient energy incorporated in a certain manner into the energy of the potential-determining anodic reaction can either increase or decrease the phase's quasi-equilibrium potential compared with the similar potential of a pure potential-determining component.

The electrochemical oxidation of cyclic alkyl disulfide derivatives of isocyanuric acid

Bicyclic disulfide derivatives of isocyanuric acid—methyl, phenyl and 2-(2-methoxy-phenoxy)ethyl N-substituted derivatives of 8,9-dithia triaza-bicyclo[14.3.1]eicosane and 7,8-dithia triaza-bicyclo[12.3.1]octadecane and 18-methyl-5λ 4,8 λ 4,9,12-tetrathia-1,16,18-triaza-tricyclo[14.3.1.0 5,8]eicosane-17,19,20-trion were studied by cyclic voltammetry and PM3 level molecular modeling. The oxidation involves an electrochemically reversible electron transfer followed by fast potential-determining reaction of second kinetic order and a relatively slow current-determining reaction of first-order ( k=0.33–32.7 s −1). Molecular modeling has shown three different types of HOMOs in this reaction series and consequently different oxidation mechanisms. None of the frontier orbitals (HOMO and LUMO) is localized on the isocyanurate heterocycle so it does not intervene directly in redox transformations but it may participate in the stabilization of the positive charge in electrogenerated cation radicals by trans-annular interaction.

Properties of interface between platinum sponge and solid electrolyte Na5TbSi4O12 in an oxygen atmosphere at moderate temperatures: Effect of theplatinum grain size

The behavior of electrochemical cells Pt(sponge)/Na 5 TbSi 4 O 1 2 /Na 0 . 6 5 CoO 2 (cell I) and Pt(sponge)/Na 5 TbSi 4 O 1 2 /Pt(sponge) (cell II) is studied by, respectively, potentiometric and impedance methods in oxygen and argon atmospheres at 50-300°C. Dependence of potential of cell I on the oxygen concentration is affected by the grain size of platinum sponge and the temperature. At 50-200°C the coarse-grained platinum electrode in contact with solid electrolyte Na 5 TbSi 4 O 1 2 exhibits an oxygen function characterized by the potential-determining reaction O 2 (g) + H 2 O + 2e = HO - 2 + OH - . An impedance method shows the triple-phase boundary O 2 , Pt(sponge)/Na 5 TbSi 4 O 1 2 to be reversible with respect to oxygen. The reversibility is realized by minority charge carriers in the solid electrolyte, i.e. by oxygen ions.

Kinetics of Active Anodic Dissolution of Copper in Bicarbonate Media

The behavior of copper in the active state during the anodic polarization in bicarbonate media is studied. It is shown that the anodic ionization of copper represents a two-stage process with univalent copper as the intermediate. The presence of the latter was confirmed by the RRDE method. The diffusion of bivalent copper is the rate-determining stage, and the equilibrium between copper and its bivalent ions is the potential-determining reaction. It is shown that the conventional method of preparation of bicarbonate solutions for experiments is inadequate; hence, the corresponding conclusions that had formerly been drawn on the oxidation of copper to univalent state are incorrect.

The potential-determining reaction of electrogenerated cation radicals of diphenylselenide: dimerization versus disproportionation

The potential-determining reaction of cation radicals formed upon electrochemical oxidation of diphenylselenide is an orbital-controlled disproportionation both in the presence and in the absence of specially added nucleophiles (water, benzene, toluene, pyridine). The active species of ensuing bulk reactions leading to stable products was shown to be a dication of diphenylselenide which reacts with the starting molecule or with an appropriate external nucleophile.

Stereoelectronic effects in the reactivity of electrogenerated cation radicals of arylselenides

The role of stereoelectronic effects in the electrochemical oxidation of arylselenides Se-substituted by a trimethylsilylmethyl group and those with substituents bearing a carbonyl group have been considered. Although the HOMO of these compounds is formed of p z -type electrons of the ArSe moiety, the predominant contribution from the lone pair of Se makes the heteroatom more susceptible to direct electronic effects than to effects transmitted through the aromatic ring. The σ-p hyperconjugation and the interaction through space were shown to lower the charge localization on the reaction center and to stabilize cation radicals of these compounds, thus changing the control of the potential-determining reaction and promoting second order reactions.

Ion-selective electrodes - istory and conclusions

The working mechanism of ion-selective electrodes has been investigated. The contradictions of theories deduced from so-called analogous phenomena are discussed in detail. Based on experiments made by the author’s research group the conclusion was drawn that only the surface (the active groups located there) takes part in the potential-determining reaction and the bulk resistance of the membrane plays a role only in the selection of the instrument used for potential measurement.

Field-effect in electrochemical reactivity of alkylarylselenides

The anchimeric assistance of some substituents bearing the carbonyl group alters the nature of the potential-determining reaction of the cation radicals of alkylarylselenides,—instead of a charge-controlled fragmentation of the first order characteristic for these kinds of particles, the orbital-controlled second order disproportionation of dimerization take place.

Metal-Metal Oxide and Metal Oxide Electrodes as pH Sensors.

The pH is one of the most important parameters for char-acterizing the chemical properties of an aqueous solution. The glass eiectrode is by far the most commonly used pH sensor. The determination of pH in special situations, e.g., in vivo applications where the fragility of the glass electrode is a draw-back, requires pH sensors that can easily be miniaturized and built into physically rugged sleeves. Also, for other applications where the volume of solution is very restricted miniaturization of pH sensors is very important. The glass electrode does not respond properly to pH in some corrosive environments (e.g., in hydrogen fluoride [HF] solutions). At present, among alternatives to the hydrogen-selective glass electrode group of sensors, potentiometric metal-metal oxide pH sensors respond to pH, ideally due to an equilibrium involving the metal and its oxide. In the case of metal oxide electrodes, the metal is not involved in the potential-determining reaction. This distinction is of course not clear in many cases because the mechanism giving the pH response is not always unequivocal.

Electrically free-standing IrO itx thin film electrodes for high temprature, corrosive environment pH sensing

In this paper we show that electrodes prepared by d.c. reactive sputtering of iridum oxide on to alumina substrates have utility for pH sensing in high temperature/corrosive aqueous environments. The electrode potential pH response, resulting from a potential-determining reaction between protons from solution and the film, is also transmitted along the film, the alumina rod being an inactive support. With this structure interferences resulting from electrochemical reaction between ions from solution and a substrate metal are precluded, and film peeling due to corrosive attack of such a support-metal is also reduced. We report results of measurements in concentrated acids at temperatures T‘ 100°C and atmospheric pressure, and in dilute aqueous systems at 100°‘ T ‘ 200°C and high pressure in an autoclave.

Manganese dioxide electrode—VI. Thermodynamic evidence for the presence of inactive Mn(III) in oxyhydroxides derived from γ-MnO 2

The potentials of oxyhydroxides derived from γ-MnO 2 have been measured vs degree of reduction for r(in MnOOH r ) in the range 0.08–0.16. From these data the activities of MnO 2 and MnOOH in the solid solution have been determined. Application of the Gibbs—Duhem equation reveals that for a single phase the slopes of ln a MnO 2 vs ln (1 − r) and ln a MnOOH vs ln r must be identical. It is shown that the Gibbs—Duhem relationship is violated unless some of the Mn(III) initially present in the acid-washed γ-MnO 2 is considered inactive towards the potential-determining reaction. The quantity of inactive Mn(III) is probably equal to, or a little greater than, 0.6 m mol g −1 corresponding to MnOOH 0.06: this is similar to the amount of Mn(III) which is not removed by moderate acid-washing. This finding has an important bearing on the development of any satisfactory theory for the potential of oxyhydroxides derived from γ-MnO 2 particularly at low degrees of reduction.

Processes of the activation of passive iron by thiocyanate ion

The activation of passive iron has been examined in neutral or weak alkaline solutions containing thiocyanate ions. The passivation of iron was carried out potentiodynamically, and the specimen potential was allowed to decay under open-circuit conditions. The residual passive film was then subjected to the cathodic reduction in order to estimate its thickness. The spontaneous potential decay was very much influenced by the addition of thiocyanate ions. The duration of the potential arrest was shortened with increasing concentration of thiocyanate ions. The potential-determining reaction and the role of the thiocyanate ion are discussed.

Processes of the activation of passive iron by thiocyanate ion

The activation of passive iron has been examined in neutral or weak alkaline solutions containing thiocyanate ions. The passivation of iron was carried out potentiodynamically, and the specimen potential was allowed to decay under open-circuit conditions. The residual passive film was then subjected to the cathodic reduction in order to estimate its thickness. The spontaneous potential decay was very much influenced by the addition of thiocyanate ions. The duration of the potential arrest was shortened with increasing concentration of thiocyanate ions. The potential-determining reaction and the role of the thiocyanate ion are discussed.

Electrode kinetics by analysis of polarographic current-time curves

The method, which enables the evaluation of the kinetic parameters of electrode reactions by analysing the polarographic current-time curve, is presented. As examples of the application of this method, the electrode reactions of the simple zinc ions and of the zinc-acetate complex ions were experimentally examined. For the former case the current-time curves were measured as a function of the electrode potential and the perchlorate concentration. It was found that the standard rate constant, corrected both for the increase of the activity coefficient of the zinc ion and for the Frumkin double-layer effect, has a constant value over the concentration range of perchlorate 0.6–4.0 M. For the latter case the current-time curves in the solution with constant ionic strength (4 M) are measured as a function of the electrode potential and of the acetate concentration. It was concluded that the potential-determining reaction of zinc-acetate complexes is referred only to the species Zn(Ac)2 over a range of acetate concentrations larger than 0.5 M.

Electrode kinetics by analysis of polarographic current-time curves

The method, which enables the evaluation of the kinetic parameters of electrode reactions by analysing the polarographic current-time curve, is presented. As examples of the application of this method, the electrode reactions of the simple zinc ions and of the zinc-acetate complex ions were experimentally examined. For the former case the current-time curves were measured as a function of the electrode potential and the perchlorate concentration. It was found that the standard rate constant, corrected both for the increase of the activity coefficient of the zinc ion and for the Frumkin double-layer effect, has a constant value over the concentration range of perchlorate 0.6–4.0 M. For the latter case the current-time curves in the solution with constant ionic strength (4 M) are measured as a function of the electrode potential and of the acetate concentration. It was concluded that the potential-determining reaction of zinc-acetate complexes is referred only to the species Zn(Ac)2 over a range of acetate concentrations larger than 0.5 M.