Electrode processes and their kinetic parameters in the reduction and oxidation of Cr(III) and Cr(II) in molten CsCl

The reduction of chromium ions in the CsCl-CrCl₃ melt in wide concentration and temperature ranges is reported. It is assumed that the melt contains polynuclear chromium complexes. The reduction of Cr(III) to Cr(II) is a reversible one-electron process. The electroreduction of divalent chromium to chromium metal is an irreversible process involving dinuclear and mononuclear chromium complex ions. The electron transfer rate constant has been calculated for various temperatures and chromium chloride concentrations in the salt melt. The activation energy of electron transfer and its dependence on the CrCl₃ concentration in the electrolyte have been determined for the Cr(III) ↔ Cr(II) process.     

Kinetics of nickel electroplating from methanesulfonate electrolyte

The overpotential of nickel ion electroreduction on the nickel and mercury electrodes is shown to increase in the following sequence of anions: ClO₄⁻,CH₃SO₃⁻, SO₄²⁻. On the nickel electrode, the overpotential of nickel evolution decreases as the pH^v increases from 1.5 to 4. This is associated with the increase in pH^s as the result of a parallel reaction of hydrogen evolution. It is shown that in contrast to mercury, the Tafel plots of the nickel electrode demonstrate a bend corresponding to the change in their slope from −0.044 to −0.132 V. This is accompanied by the lowering down of the reaction order in nickel ions from 2 to 1. A mechanism of nickel ion electroreduction that includes two parallel routes is proposed and substantiated by a model. In the low overpotential range, the predominant process is the electroreduction of nickel hydroxocomplexes, which is characterized by the strong dependence of the reaction rate on the potential and the concentration of electroactive species. For high overpotentials, the predominant process is the direct discharge of nickel aquacomplexes the rate of which depends weaker on the potential and the concentration of electroactive species.     

Theoretical analysis of catalytic current-potential curves: Part II. First-order regeneraration reactions coupled with dimeric product of the electrode reaction

Equations of the polarographic current-potential curves are derived for electrode reactions of the type 2A+ze^?k_s,α→B coupled by an irreversible chemical regeneration reaction B+C^k→2A+D. Analytical solutions based on a general treatment were derived, including reversible, irreversible and quasi-reversible electrode processes. The kinetic domain over which an irreversible following chemical reaction affects the half-wave potentials is defined.     

Electrochemical activity of solid solutions La1 -x Sr x CoO3 in the oxygen electroreduction reaction in carbonate melts

Solid solutions Lad_1-xSr_xCoO₃ are synthesized. An X-ray diffraction study reveals the formation of a rhombohedrically distorted perovskite-like phase. The kinetics and mechanism of the electroreduction of oxygen at the synthesized compounds in Li₂CO₃-K₂CO₃ (62: 38 mol %) and Na₂CO₃-Cs₂CO₃ (39:61 mol %) melts are studied by static coulometry and chronoamperometry. The exchange current grows with the strontium content in samples. The obtained apparent reaction orders of 0 and 0.14 by O₂ and CO₂ contradict all mechanisms offered earlier for the oxygen electroreduction at a gold electrode. A new mechanism is offered, in which the limiting step is the transfer of oxygen ions from the solid phase into the melt with corresponding changes in the electron subsystem of the electrode material. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

Electrode Processes in a Eutectic KCl–KF Melt Containing Gadolinium Oxide

The electroreduction of gadolinium from a eutectic KCl–KF melt containing 0.210–1.233 wt % Gd₂O₃ on molybdenum and glassy carbon electrodes is studied at 1008 K. The solubility of gadolinium oxide is determined. It is shown that the reduction of a gadolinium oxyfluoride complex proceeds reversibly, with a subsequent reversible chemical reaction. The gadolinium reduction on an iron electrode in a eutectic KCl–KF melt purified of unwanted oxide ion impurities yields an iron-gadolinium alloy.     

Isomerization and fragmentation of methylfuran ions and pyran ions in the gas phase

The mutual interconversion of the molecular ions [C₅H₆O]⁺ of 2-methylfuran (1), 3-methylfuran (2) and 4H-pyran (3) before fragmentation to [C₅H₅O]⁺ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energles and by a MNDO calculation of the minimum energy reaction path. The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1–3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions [C₅H₅O]⁺ arise by the loss of a H atom. This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions. Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an iostope effect k_H/k_D = 1.67–2.16 for metastable ions. In contrast, no H/D scrambling has been observed in deuterated 4H-pyran ions. However, the loss of a H atom from all metastable [C₅H₅O]⁺ ions gives rise to a flat-topped peak in the mass-analysed ion kinetic energy spectrum and a kinetic energy release (T₅₀) of 26 ± 1.5 kJ mol^?1. The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1 and 2 favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions. This agrees with the experimental results.     

Single Atoms of Iron on MoS2 Nanosheets for N2 Electroreduction into Ammonia

Efforts have been devoted to achieving a highly efficient artificial synthesis of ammonia (NH₃). Reported herein is a novel Fe-MoS₂ catalyst with Fe atomically dispersed onto MoS₂ nanosheets, imitating natural nitrogenase, to boost N₂ electroreduction into NH₃ at room temperature. The Fe-MoS₂ nanosheets exhibited a faradic efficiency of 18.8 % with a yield rate of 8.63 μg mg_cat.⁻¹ h⁻¹ for NH₃ at −0.3 V versus the reversible hydrogen electrode. The mechanism study revealed that the electroreduction of N₂ was promoted and the competing hydrogen evolution reaction was suppressed by decorating the edge sites of S in MoS₂ with the atomically dispersed Fe, resulting in high catalytic performance for the electroreduction of N₂ into NH₃. This work provides new ideas for the design of catalysts for N₂ electroreduction and strengthens the understanding about N₂ activation over Mo-based catalysts.     

Electrochemical behavior of the C60–γ-cyclodextrin inclusion complex (1:1) on HMDE in an aqueous solution

The electrochemical behavior of the water-soluble C₆₀–γ-CD (1:1) inclusion complex has been studied on the hanging mercury drop electrode. A one-electron reversible adsorptive electroreduction and three irreversible adsorptive electroreductions were detected by cyclic voltammetry. The amount of C₆₀–γ-CD adsorbed at saturation is 1.73 × 10⁻¹¹ mol/cm², the diffusion coefficient is 5.81 × 10⁻⁶ cm²/s, and the standard rate constant of the surface reaction k_s are 0.888 s⁻¹, 0.853 s⁻¹ and 1.032 s⁻¹, respectively.     

Overall Carbon-neutral Electrochemical Reduction of CO2 in Molten Salts using a Liquid Metal Sn Cathode

An overall carbon-neutral CO₂ electroreduction requires enhanced conversion efficiency and intensified functionality of CO₂-derived products to balance the carbon footprint from CO₂ electroreduction against fixed CO₂. A liquid Sn cathode is herein introduced into electrochemical reduction of CO₂ in molten salts to fabricate core–shell Sn−C spheres (Sn@C). An in situ generated Li₂SnO₃/C directs a self-template formation of Sn@C. Benefitting from the accelerated reaction kinetics from the liquid Sn cathode and the core–shell structure of Sn@C, a CO₂-fixation current efficiency higher than 84 % and a high reversible lithium-storage capacity of Sn@C are achieved. The versatility of this strategy is demonstrated by other low melting point metals, such as Zn and Bi. This process integrates energy-efficient CO₂ conversion and template-free fabrication of value-added metal-carbon, achieving an overall carbon-neutral electrochemical reduction of CO₂.     

FTIR spectroscopy study of the electrochemical reduction of CO2 on various metal electrodes in methanol

The electrochemical reduction of CO₂ on Sn, Cu, Au, In, Ni, Ru and Pt electrodes in methanol containing 0.1 M sodium perchlorate was studied by cyclic voltammetry and in-situ FTIR spectroscopy. Dissolved CO₂ increases the cathodic current at potentials below −1.3 V vs. Ag|0.01 M Ag⁺ with Sn, Au, Cu, In and Ni electrodes. It is concluded from the FTIR spectra obtained that there is no reduction of CO₂ on any of the metals studied, and that the only reaction product detected by Fourier transform (FT) IR spectroscopy, i.e. CO²⁻₃, is formed by reaction of CO₂ with hydroxyl anions produced in the electroreduction of residual water.In order to identify the electroreduction products of CO₂ it was necessary to obtain the FTIR spectra of sodium oxalate and sodium carbonate in methanol. They were obtained by the electroreduction of oxalic acid and the alkalinization of CO₂-saturated methanol respectively. It could be proved that the electroreduction of carboxylic acids to carboxylate anions in organic solvents does not require either a H-chemisorbing metal electrode, or the presence of water in the solvent.     

Pulse radiolysis study of one-electron oxidation of thionine in aqueous solutions

One-electron oxidation of thionine has been studied using specific oxidizing radicals such as Cl⁻Tl(II) and N₃ generated by pulse radiolysis of aqueous solutions. The semioxidized thionine exhibited threepK’s indicating four conjugate acid-base forms. N₃ radicals were found to be less efficient in oxidizing thionine as compared to Cl ₂ ⁻ , Tl²⁺ and Tl(OH)⁺. The rate constants for electron abstraction from thionine by Cl ₂ ⁻ , Tl²⁺, Tl(OH)⁺, Tl(OH)₂ and N₃ were evaluated. The spectra of different protonated forms of semioxidized thionine and the extinction coefficients at λ_max are presented. Reaction of OH radicals with thionine gave transient products whose spectra and acid-base properties were different from those of semioxidized thionine. The rate constant for formation of the product transient agrees well with competition kinetic value for reaction of OH with thionine reported earlier.     

One-electron oxidation of toluidine blue. A pulse radiolysis study

The one-electron oxidation of toluidine blue by specific oxidising radicals such as Cl ₂ ⁻ , Tl(II), N₃, Br ₂ ⁻ etc. has been studied by employing the pulse radiolysis technique. The Br ₂ ⁻ radical was found to be less efficient in oxidising toluidine blue as compared to Cl ₂ ⁻ ., Tl⁺² and N₃. The semioxidised species exhibited only onepK _a indicating the presence of two conjugate acid-base forms whose spectral and kinetic features were evaluated. Reaction of OH radicals with the dye gave rise to a transient species which exhibited spectral and kinetic features different from that of the semioxidised species indicating that the mode of reaction of OH is different.     

The kinetics of chlorine evolution and reduction on glassy carbon in molten tetrachloroaluminate at 175°C

The chlorine electrode reaction on glassy carbon in sodium tetrachloroaluminate melt (AlCl₃+NaCl) with near equimolar compositions was investigated at 175°C with voltammetric techniques. The kinetic parameters (Tafel slope and exchange current density) measured as functions of chloride ion activity and partial pressure of chlorine, and the reaction orders with respect to Cl^? and Cl₂ have been collected extensively, being compared with the theoretical kinetic derivatives deduced from the rate equations solved under three different kinds of adsorption isotherms: Langmuir, non-activated Temkin and activated Temkin isotherms. All the evidence collected in this study indicates that the reaction mechanism for both evolution and dissolution of chlorine consists of a fast electron transfer (Cl^?→Cl_ad+e) followed (or preceded) by a slow Heyrovsky-type reaction (Cl^?+Cl_ad→Cl₂+e) on glassy carbon surfaces where the adsorbed intermediate obeys the activated Temkin isotherm. The exchange current density was found as 8.6±0.8 μA cm^?2 at 175°C in the melt of pCl=1.1 under an atmospheric pressure of Cl₂, and its electrode potential (E°_CΓ/Cl2) was determined as 2.182±0.005 V vs. Al.     

Electrochemical reduction and kinetics of hydrogen peroxide on the rotating disk palladium-plated aluminum electrode modified by Prussian blue film as an improved electrocatalyst

This paper describes the use of an aluminum electrode plated by metallic palladium and modified by Prussian blue (PB/Pd-Al) in the electrocatalytic reduction of hydrogen peroxide (H₂O₂). The effect of pH on the electroreduction of H₂O₂ on the modified electrode is investigated and a simple irreversible reduction pathway is suggested. The electroreduction kinetics including transfer coefficient α, potential-dependent charge transfer rate constants k _f, and diffusion coefficient D are estimated by means of forced hydrodynamic voltammetry using a rotating disk PB/Pd-Al electrode. The mean values obtained for kinetics are 0.38, 10⁻² cm⁻¹, and 7.6 × 10⁻⁶ cm² s⁻¹, respectively. The long-term stability of the modifying layers on the Al substrate was studied.     

Differential pulse polarographic study of thiamine (vitamin B1) in neutral and acidic aqueous solutions

The differential pulse (dp) polarograms of thiamine in neutral aqueous solutions exhibited six peaks at low depolarizer concentration (⋦10⁻⁴ mol dm⁻³) and only three peaks at concentrations ≥10⁻³ mol dm⁻³. Only one of these was found to correspond to the diffusion-controlled reduction of this compound at the dme and this was shown to be an irreversible two-electron process. The kinetic parameters derived from the dp polarograms were found to be in good agreement with those calculated from classical polarograms and were:E _1/2=−1·261 Vvs SCE,an _a=0·54 andD≈3·5×10⁻⁶ cm² sec⁻¹ for 10⁻³ mol dm⁻³ thiamine in 0·1 mol dm⁻³ acetate buffer (pH 6·5). The reduction product has been identified as dihydrothiamine. The effect of pH on the dpp of thiamine was studied in the pH range 0–7. In the pH region 5·5 to 7·0 only one peak attributable to the B₁ ⁺ form of thiamine is present. In the pH region 3·5–5·5 another dpp peak attributable to the protonated form (B₁H²⁺) of thiamine was also observed. At pHs less than 3 only one peak was observed which could be attributed to the doubly protonated form (B₁ H₂ ³⁺) of thiamine. Surfactants like triton-X-100 and CTABr were found to inhibit the electroreduction of thiamine due to the strong adsorption of these compounds on the dme. Thiamine itself was found to have an inhibitory effect on its own electroreduction, although to a smaller extent.     

A study of samarium ions electroreduction at various electrodes in KCl-NaCl-CsCl melt at <Emphasis Type="Italic">T</Emphasis> = 823 K

Samarium ions electroreduction mechanism was studied on the supporting of KCl-NaCl-CsCl eutectic melt at 823 K on various electrodes (W, Mo, Al, Ni, Pt, and Ag). The diffusion coefficients of SmCl₆³⁻ chloride complexes and Sm³⁺ → Sm²⁺ heterogeneous recharge stage rate constant K _fh⁰ were calculated in KCl-NaCl-CsCl melt at T = 823 K.     

Hyperbranched Poly([1,2,3]-triazole-[1,3,5]-triazine)s: An Unusual High Degree of Branching as an Effect of a Polyaddition Kinetics

The results of the kinetic study of melt and solution polymerization at the 1,3-dipolar cycloaddition reaction of the AB₂ monomer – 2-azido-4,6-bis(prop-2-yn-1-yloxy)-[1,3,5]-triazine (ABPOT) are presented in this work as well as the results of the ¹³C-NMR characterization of the obtained hyperbranched poly([1,2,3]-triazole-[1,3,5]-triazine)s. It is established, that the first-shell substitution effect during polyaddition process and unusual high degree of branching (up to 0.9) of polymers synthesized in melt are held.     

Simulation of the kinetics of adenosine 5′-triphosphate hydrolysis catalyzed by the Cu2+ ion: The role of conformation and the catalytic effect of the OH− ion

The hydrolysis kinetics of the dimeric complex (CuATP^2? · OH₂)₂ {D} up to ≈40% ATP conversion at 25°C, pH 5.7–7.8, and [Cu · ATP]₀ = (2.07 ± 0.03) × 10^?3 mol/l is analyzed by numerical simulation. CuADP^? + P_i (P_i is an inorganic phosphate) form from DOH^?, and the latter forms rapidly from D. The abstraction of H⁺ from the coordinated H₂O molecule is an irreversible reaction involving an OH^? ion from the medium. The maximum possible DOH^? concentration at a given pH is reached at the initial stage of hydrolysis (0.3–6.0 min after the initiation of hydrolysis). CuADP^? + P_i form from D via two consecutive irreversible steps. The ADP buildup rate in the process is determined by the reversible conformational transformation of DOH^? resulting in a pentacovalent intermediate (IntK). OH^? ions from the medium are involved both in IntK formation and in the reverse reaction and are a hydrolysis inhibitor. AMP forms from the intermediate IntK₃, which forms reversibly from DOH^?, OH^? ions from the medium being involved in the forward and reverse reactions. This is followed by irreversible (AMPH)^? formation involving H₃O⁺ ions from the medium. The rate and equilibrium constants are determined for the formation and decomposition of hydrolysis intermediates. The concentrations of the intermediates are plotted versus time for various pH values. The structures of the intermediates are suggested. The causes of a peak appearing in the initial ADP formation rate versus pH curve are analyzed.     

Thermal decomposition of 1,3,3-trinitroazetidine in the gas phase, solution, and melt

1,3,3-Trinitroazetidine (TNAZ) was synthesized using the alternative approach based on the transformation of 3-oximino-1-(p-toluenesulfonyl)azetidine in the reaction with nitric acid through intermediate pseudonitrol. The thermal decomposition of TNAZ in the gas phase, melt and m-dinitrobenzene solution in a wide concentration range (5–80%) was studied by manometry, volumetry, thermogravimetry, IR spectroscopy, and mass spectrometry. In the gas phase in the temperature range from 170 to 220°C the thermal decomposition proceeds according to the first-order kinetic law with the activation energy 40.5 kcal mol^?1 and pre-exponential factor 10^15.0 s^?1. The major gaseous reaction products are N₂, NO, NO₂, CO₂, H₂O, and nitroacetaldehyde, and trace amounts of CO and HCN are formed. The rate-determining step of the process is the homolytic cleavage of the N-NO₂ bond in the TNAZ molecule. In melt at 170–210 °C the thermal decomposition proceeds with the pronounced self-acceleration and the maximum reaction rates are observed at conversions 53.9–67.4%. The solid decomposition products accelerate the reaction. It is most likely that the autocatalysis of TNAZ decomposition in the liquid phase is due to the autocatalytic decomposition of 1-nitroso-3,3-dinitroazetidine, which is formed by the thermal decomposition of TNAZ. In m-dinitrobenzene TNAZ also decomposes with self-acceleration. The higher the concentration in the solution, the more pronounced the self-acceleration. Additives of picric acid moderately accelerate the thermal decomposition of TNAZ, whereas hexamethylenetetraamine additives exert a strong acceleration.     

Co(III) ammine electroreduction reactions as kinetic probes of double layer structure: The reduction of fluoropentaammine cobalt(III) at varying ionic strengths

The electroreduction rate of fluoropentaammine cobalt(III) was studied in a variety of single electrolytes of varying ionic strengths at the mercury-aqueous interface in order to assess the experimental double layer effects in the presence of anion specific adsorption in comparison with the predictions of the coupled Gouy-Chapman-Stern-Frumkin (GCSF) theory. The net charge densities in the inner layer region determined from the experimental rate data using the GCSF model were usually in good agreement with the corresponding literature values that were determined from equilibrium double layer data over a range of ionic strengths (μ=0.01 to 1.0 M) and electrode charge densities (q^m～0–15μC cm^?2) in NaF, KPF₆, KCl, NaN₃, KNO₃ and NaClO₄ electrolytes. Large discrepancies between these kinetic and equilibrium results were observed in concentrated Na₂SO₄ electrolytes which were ascribed to the effects of ion-pairing between Co(NH₃)₅F²⁺ and SO₄^2?. The relative success of the simple GCSF model for this and other Co(III) ammine reduction reactions is compared and contrasted with the corresponding behavior of other electrode reactions that have been studied previously, and possible reasons for the behavioral simplicity of the present systems are suggested. The suitability of Co(III) ammine electroreduction reactions as kinetic probes of the double layer structure at solid electrode-aqueous interfaces is noted.