Dmitrii V. Glukhov

Contemporary understanding of the peroxodisulfate reduction at a mercury electrode

Specific aspects of the kinetics of anion electroreduction at high overvoltages are addressed by various traditional procedures to treat experimental data. The expansion of the Frumkin correction concept is proposed in terms of the reaction volume which increases with increasing negative electrode charge for anionic reactants. A molecular level approach based on quantum chemical calculations of the work terms and the electrode–reactant electronic coupling is employed for calculations of the reaction volume for the case of peroxodisulfate S2O82− and the ion pair Na+·S2O82−, the species which are to be considered as possible reactants in Na2S2O8 solutions with sodium salts as supporting electrolytes. Estimates of the partial rate constants for the anion and ion pair are reported as well. New experimental data are presented for solutions of equal ionic strength and equal degree of association but with different total reactant concentration. These data confirm the possibility of local ion pair formation at low negative charge. In parallel the previously ignored problem of correction for mass transport limitations is discussed for a system with two simultaneously discharging species of various charges. It follows from this reconsideration that previous data cannot be interpreted for sure as the absence of an ion pair contribution to the total current. The problem of separation of contributions from corresponding parallel steps to the total current is discussed, and the fast formation of an ion pair preceding the electron transfer was found to be rather probable.

Molecular Description of the Persulfate Ion Reduction on a Mercury Electrode

The polarization curves for the S2O82- 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.

Frumkin Correction: Microscopic View

Approaches to perfecting analysis of psi-prime effects, based on modeling the reaction layer of electrochemical reactions and allowing for real molecular structure of reactants and products, are considered. Conditions of applicability of the traditional slow-discharge theory relations to systems with reactants of complicated structure are analyzed in a wide overvoltage interval. Special attention is paid to the problem of accounting for nonuniformity of charge distributions in particles of reactants and products. It is shown that it becomes essential to account for the dielectricity of the effective cavity used for modeling the particles, provided the particles are sufficiently large.

Activationless Reduction of the Hexacyanoferrate Anion on a Mercury Electrode

Polarization curves for the electroreduction of [Fe(CN)6]3– on a mercury electrode in solutions containing different amounts of surface-inactive supporting cations are simulated on the basis of modern theory of charge transfer in polar media combined with quantum-chemical approaches. The conclusions about an activationless nature of the process in the overvoltage range 1.2 to 2.0 V accessible experimentally, which were drawn from the results of earlier calculations made within simpler models, are confirmed. The maximum contribution to the current is shown to be made by energy levels of metal that lie considerably (up to 1 eV) lower than the Fermi level. To establish the reasons for the anomalous behavior of the current in the activationless region at high overvoltages, the effect various factors sensitive to the electrode charge exert on the model curves at high negative charges of the electrode surface is analyzed. In connection with this, the stability of the results of a calculation of the electron overlap metal/reactant to a model of the interface is considered. The plausibility of the model proposed for the reaction layer and the approaches used for computing the activation energy and the preexponential factor is corroborated by good agreement between the temperature effect found for the region of a minimum current and its experimental value.

On the effect of Cu on the activity of carbon supported Ni nanoparticles for hydrogen electrode reactions in alkaline medium

Effects of adding varied amounts of copper to carbon-supported nickel particles on their structure, composition and electrocatalytic activity for the hydrogen oxidation and evolution reactions in alkaline medium have been explored. Ni1-xCux/C catalysts were prepared by the incipient wetness impregnation. Comprehensive characterization of the catalysts included X-ray powder diffraction, X-ray photoelectron spectroscopic, transmission electron microscopic and cyclic voltammetric analyses, while atomistic Monte Carlo simulations have been undertaken to obtain further insights into the structure of the bimetallic NiCu nanoparticles. We found that compared to monometallic Ni, NiCu nanoparticles show lower propensity towards oxidation under ambient conditions. Furthermore, we report that adding Cu allows increasing the surface-weighted electrocatalytic activity, and the specific surface area of Ni1-xCux/C electrodes, both contributing to a ca four-fold enhancement of the mass-weighted activity. The nature of the synergistic interactions between Ni and Cu is proposed on the basis of the analysis of experimental data and Monte Carlo structural modelling results.

Medium and Interfacial Effects in the Multistep Reduction of Binuclear Complexes with Robson-Type Ligand

We present a combined experimental and computational approach to the modeling and prediction of reactivity in multistep processes of heterogeneous electron transfer. The approach is illustrated by the study of a Robson-type binuclear complex (-Cu(II)-Cu(II)-) undergoing four-electron reduction in aqueous media and water-acetonitrile mixtures. The observed effects of solvent, pH, buffer capacity, and supporting electrolyte are discussed in the framework of a general reaction scheme involving two main routes; one of them includes protonation of intermediate species. The main three problems are addressed on the basis of modern charge transfer theory: (1) the effect of the nature of reactant and intermediate species (protonated/deprotonated, bare or associated with supporting anion/solvent molecule) on the standard redox potential, the electronic transmission coefficient, and the intramolecular reorganization; (2) possible effect of protonation on the shape of the reaction free energy surfaces which are built using the Anderson Hamiltonian; (3) electron transfer across an adsorbed chloride anion. Quantum chemical calculations were performed at the density functional theory level.

Interaction of Chloride Anion and Chlorine Atom with Indium and Gallium Surfaces: A Quantum-chemical Study

The adsorption of the chloride ion and chlorine atom on clusters simulating the surface of the (111) and (001) faces of the crystal lattice of indium and liquid gallium are calculated using the Hartree–Fock–Roothaan and density functional (B3LYP) quantum-chemical methods. The energy of adsorption of chloride ions from a gas phase at these faces increases in the following series: bridge≈hollow < on-top positions and equals to ∼179 kJ mol–1 for In(111) and ∼183 kJ mol–1 for gallium in the on-top position. Both metals exhibit similarity in the formation of bonds between their surface atoms and the adsorbate. The adsorbate charge does not depend on the adsorbed form (chloride ion or chlorine atom) and equals ∼ 0.5e. Parameter of a virial adsorption isotherm is estimated with allowance for coulombic interactions near the metal/electrolyte interface.

Specific Molecular Features of Potassium-Containing Cryolite Melts

Experimental in situ Raman spectra are presented for potassium, sodium and mixed fluoroaluminate melts. More acidic melts with cryolite ratio (CR) below 2 are accented. The cation-dependent ratio of integral band intensities is assigned to specific interactions of potassium cations with low coordinated fluoroaluminates. For alumina containing melts, the effect of potassium on the stoichiometry of predominating oxofluoroaluminate anion is observed indirectly. Density functional theory is employed to investigate the behavior of different fluoroaluminates (including dimers) and their associates formed by alkali metal cations.