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The data on the rate constants for ferrocene/ferrocenium (Fc/Fc + ) electron transfer in ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate ([bmim][BF 4 ]) are reported for a series of -S-(CH 2 ) n -Fc adlayers (n = 8…16) on polycrystalline gold. The values and trends are compared to the previously published data for similar series in aqueous medium, as well as to the results for ‘free’ Fc at -S-(CH 2 ) n − 1 -CH 3 modified gold in the same ionic liquid. For the latter case, the trend is opposite as compared to that in the available aqueous series (attached and ‘free’ ruthenium ammine complex): the ratio of rate constants for attached and ‘free’ reactants decreases with electrode-reactant distance in the former case, and increases in the latter. The presented comparison discovers the role of reactant and product work terms, which are essential for attached reactants as well because of conformational diversity. Corresponding contributions are estimated from molecular modeling and considered in terms of reaction volume.

Cerium redox couple is widely investigated in aqueous flow batteries due to its high redox potential. While mixed supporting electrolytes of methanesulfonic acid (MSA) and sulfuric acid have been adopted to increase the solubility and reversibility of the cerium redox reaction, the influence of such mixed ligands on the electron transfer is absent. Herein, the density functional theory is applied to study the activation energy of the electron transfer, which is in agreement with the reaction kinetics measured by electrochemical impedance spectroscopy. Cerium complexes with different ratios of methanesulfonate and sulfate ligands are assumed with optimized geometries. The standard redox potentials of these cerium complexes are calculated, which follows the same trend as the open circuit potentials of the cerium electrolyte. The exchange current densities of cerium redox couples in the electrolytes with different ratios of MSA and sulfuric acid are 0.16–0.28 mA cm−2 on glassy carbon electrode, qualitatively conforming the small variation of activation energies.

Outer-sphere electron transfer (ET) between optically active transition-metal complexes and either other transition-metal complexes or metalloproteins is a prototype reaction for kinetic chirality. Chirality as the ratio between bimolecular rate constants of two enantiomers mostly amounts to 1.05-1.2 with either the Λ or Δ form the more reactive, but the origin of chirality in ET parameters such as work terms, electronic transmission coefficient, and nuclear reorganization free energy has not been addressed. We report a study of ET between the Λ-/Δ-[Co(Ox)3](3-) pair (Ox = oxalate) and horse heart cytochrome c (cyt c). This choice is prompted by strong ion-pair formation that enables separation into inter-reactant interaction (chiral "recognition") and ET within the ion pair ("stereoselectivity"). Chiral selectivity was first addressed experimentally. Λ-[Co(Ox)3](3-) was found to be both the more strongly bound and faster reacting enantiomer expressed respectively by the ion-pair formation constant KX and ET rate constant kET(X) (X = Λ and Δ), with KΛ/KΔ and kET(Λ)/kET(Δ) both ≈1.1-1.2. rac-[Co(Ox)3](3-) behavior is intermediate between those of Λ- and Δ-[Co(Ox)3](3-). Chirality was next analyzed by quantum-mechanical ET theory combined with density functional theory and statistical mechanical computations. We also modeled the ion pair K(+)·[Co(Ox)3](3-) in order to address the influence of the solution ionic strength. The complex structure of cyt c meant that this reactant was represented solely by the heme group including the chiral axial ligands L-His and L-Met. Both singlet and triplet hemes as well as hemes with partially deprotonated propionic acid side groups were addressed. The computations showed that the most favorable inter-reactant configuration involved a narrow distance and orientation space very close to the contact distance, substantiating the notion of a reaction complex and the equivalence of the binding constant to a bimolecular reaction volume. The reaction is significantly diabatic even at these short inter-reactant distances, with electronic transmission coefficients κel(X) = 10(-3)-10(-2). The computations demonstrated chirality in both KX and κel(X) but no chirality in the reorganization and reaction free energy (driving force). As a result of subtle features in both KX and κel(X) chirality, the "operational" chirality κET(Λ)KΛ/κET(Δ)KΔ emerges larger than unity (1.1-1.2) from the molecular modeling as in the experimental data.

A possibility to synthesize heteronuclear compounds from aqueous solutions containing the Fe(II), Fe(III), Al(III), Cl− and OH− ions is explored. The XRF analysis, IR-spectroscopy, a model based on the potentiometric titration data, as well as molecular modeling and density functional theory are employed to determine the elemental and phase composition and the structure of synthesized compounds. The most probable forms of the heteronuclear complexes are proposed on the basis of a combined quantum chemical and spectroscopic analysis.

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-x Cu x /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-x Cu x /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.

Adsorption of a H2O molecule on Bi, Ga and Hg electrode surfaces is studied in the framework of cluster model at the density functional theory (DFT) level. At bismuth(111) single crystal plane the hollow site is energetically more preferable for the H2O adsorption (− 31.1 kJ mol− 1), while the adsorption at top site of Hg and Ga metal surfaces is confirmed to be energetically the most preferable (− 35.6 and − 24.7 kJ mol− 1, respectively). The calculations for Bi(111), Hg, and Ga are further extended to include the effect of external electrical field, and data analysis is completed with the help of the mean field approximation in order to model adsorbed water behaviour in the H2O molecules' bilayer. An associate of 13 H2O molecules is modelled in order to address the influence of lateral interactions in a water bilayer. The Ga surface is argued to be more hydrophilic than the Bi(111) and Hg surfaces. Despite the weaker adsorption energy of a single H2O molecule at the Ga surface, water molecules at the Ga/water interface are additionally stabilized by stronger hydrogen bonds. We stress the important role of the H2O bilayer at a metal electrode surface, which depends on the atomic corrugation of a metal surface.

We investigate oxygen reduction in alkaline solution with the focus on gold and silver electrodes. On the basis of theoretical considerations we propose a detailed mechanism, which we support by explicit calculations. In accordance with previous suggestions we conclude that the first step is an outer-sphere electron transfer to the oxygen molecule. The breaking of he oxygen-oxygen bond involves adsorption of OH on the electrode surface. This is much stronger on Au(100) than on Au(111), which explains why the former is a better catalyst. Fil: Ignaczak, Anna. University of Lodz; Polonia Fil: Nazmutdinov, Renat. Kazan National Research Technological University; Rusia Fil: Goduljan, Aleksej. Universitat Ulm; Alemania Fil: Moreira de Campos Pinto, Leandro. Universidade Federal do Mato Grosso do Sul; Brasil. Universitat Ulm; Alemania Fil: Juarez, Maria Fernanda. Universitat Ulm; Alemania Fil: Quaino, Paola Monica. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; Argentina Fil: Santos, Elizabeth del Carmen. Universitat Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Schmickler, Wolfgang. Universitat Ulm; Alemania

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 CeW 10 anion, non-local electrostatics and an extended version of the Bjerrum model.

The electrochemical reactivity of Co(III), Cr(III), and Ru(III) ammine complexes is discussed within the framework of a quantum chemical approach. The electronic structure of oxidized and reduced complex forms is addressed at the density functional theory level; intramolecular and solvent contributions to the Franck−Condon barrier, as well as electronic transmission coefficients are computed. Standard electrode potentials are estimated for certain redox pairs. The charge distribution in the reactants is employed to calculate electrostatic interactions with a mercury electrode surface and to make double layer corrections more realistic as compared to point/spherical reactant models. The model predictions are employed to explain the rate constant and transfer coefficient. Also discussed are trends in the available experimental data, including new original results on a comparative study of Co(III) and Cr(III) hexaammine complexes.

Quantum chemical models are employed to get molecular level insight into the cationic catalysis phenomenon occurring at the peroxodisulfate electroreduction on mercury. A sharp increase of the reaction rate in cesium-containing solutions as compared with sodium containing media is considered, which is known to be one of the striking manifestations of cationic catalysis. The important feature under discussion is enhancement of the cationic catalysis with increasing the negative electrode charge. Special attention is paid to the ion pairing in both the solution bulk and the interfacial reaction layer. A conspicuous structural difference is predicted for Na + · S 2 O 8 2 - (solvent-separated ion pair) and Cs + · S 2 O 8 2 - (contact ion pair). The rough estimations of the bulk association constants do not afford to explain the experimentally observed effect even qualitatively. The partial contributions of S 2 O 8 2 - and ion pairs ( Na + · S 2 O 8 2 - or Cs + · S 2 O 8 2 - ) to the resulting current are compared with account of their molecular features which affect the work terms and transmission coefficients. The results of analysis are in satisfactory agreement with the experimentally observed trends. To check a possibility of an additional acceleration of the reaction due to local electrostatic effects, the specific adsorption of a cesium cation on the mercury surface was modeled as well. It is concluded that the partial contribution of ion pairs in the vicinity of metal/solution interface is higher at the more negative electrode charges. Local electrostatic interactions with a sodium cation are also involved in the model calculations. It has been demonstrated that the cationic catalysis results from a number of specific features characterizing cesium-containing electrolytes: (1) higher association degree in the solution bulk; (2) stronger overlap between the electrode wave functions and acceptor orbital of ion pairs; (3) the charge-induced shift of specifically adsorbed cations towards the electrode surface.

The mechanism of the Zn(II) reduction from acid aqueous solution at a metal electrode surface has been elucidated at a microscopic level. The Anderson–Newns model was employed in order to construct the adiabatic potential energy surfaces along the solvent coordinate for several reactions as a function of the electrode–reactant distance and the overpotential. A quantum chemical approach was employed to treat the coupling of the reactant to the metal electrode within a cluster model. The reduction of a [Zn(H2O)6]2+ complex was found to proceed in the adiabatic regime, the transfer of the first electron being rate-determining. Main attention is focused on effects of a qualitative nature, which are discussed in the light of available experimental data. The electrode charge excess and distance of maximal approach were found to affect significantly the Frank–Condon barriers of the reaction. An experimentally observed dependence of the rate constant of Zn(II) reduction upon the electrode material has been interpreted.

Adsorption of the halide ions X (X = Cl − , Br − , I − ) at a cadmium monocrystalline face (0 0 0 1) from aqueous solutions has been studied by the impedance spectroscopy. Analysis of the impedance data does not predict specific adsorption for Cl − and shows that the Gibbs adsorption energy increases from Br − to I − in accordance with the decrease of the hydration energy of anions. The adsorption of halide ions and their atomic forms on a Cd(0 0 0 1) electrode at the potential of zero charge (pzc) has been studied with the use of the cluster model. The quantum chemical calculations were performed at the DFT and the SCF levels involving the hydrated species X − (H 2 O) 3 and X − (H 2 O) 6 besides the bare anions. The effective distance-dependent solvation energy of halide ions constructed on the basis of molecular dynamics simulations was used to build the adsorption terms. For all three halides contact adsorption was found to be the most favourable state. The partial charge transfer (PCT) from the adsorbed species to the metal was analysed in terms of the Anderson–Newns model. A combined consideration of the solvation and PCT effects enables prediction of the experimentally observed metal–ion interaction order (Cl − − − ).

The fluoride ion adsorption from a gas phase on various faces of a single crystal of silver is studied by a density functional method within a cluster model for metal. The adsorption bond energy is found to increase in the series Ag(100) < Ag(111) < Ag(311) < Ag(110). A substantial structural and energetic heterogeneity of various adsorption sites is revealed. The results are utilized to simulate the electrochemical interface between individual faces of a single crystal of silver and aqueous solutions containing the fluoride ion. It is assumed that the adsorption potential may be represented as the sum of two contributions, one of which describes the metal–ion interaction and the other, the ion solvation energy. The plotted adsorption terms take into account partial degradation of the fluoride ion when adsorbed from an aqueous solution. Estimates of discreteness of the electrical double layer are presented. A conclusion on the maximum manifestation of specific adsorption of the fluoride ion for the Ag(100) face is made.

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.

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.

The effect of the ligand surrounding on the character and degree of the inner-sphere reorganization during electroreduction of aqua, aquahydroxy, hydroxy, and Edta complexes of zinc(II) is studied in the framework of a quantum-chemical approach. It is established that the most substantial intramolecular reorganization occurs during the transfer of the first electron onto [Zn(Edta)]2–. The solvent's reorganization energy is estimated.

An approach is suggested in order to investigate the mechanism of interfacial electron-transfer processes with reagents of nonspherical form and complicated charge distribution. Chelate chromium(III) ethylenediaminetetraacetate (EDTA) complexes are regarded as a good model system. A series of SCF quantum chemical calculations at the ZINDO/1 level were performed for the systems Cr(EDTA)(H2O)n- (n = 0, 1, 2, 3, and 4). A quinquedentate complex Cr(EDTA)H2O- was found to be energetically more favorable compared to Cr(EDTA)-. The interaction of Cr(EDTA)- with a cadmium electrode (modeled as a 19-atomic planar cluster) was investigated as well. The analysis of both the potential energy surfaces and the partial charge transfer allows the explanation of the absence of “specific” interaction between Cr(EDTA)- complexes and mercury-like metals observed experimentally. A way to estimate the inner-sphere contribution (Ein) to reorganization energy is proposed. The inner-sphere asymmetry (nonequality of Ein values for...

PtCo alloys have long been known to possess higher activity in the electrochemical oxygen reduction reaction compared to pure Pt. This work addresses the influence of the chemical order on the electrocatalytic activity of PtCo alloy, the phenomenon, which has been already documented in the literature but yet poorly understood. Single crystalline PtCo(0 01) alloy films were epitaxially grown on Mg(0 0 1) substrates and used as model electrodes to study the influence of the chemical order on the electrochemical oxygen reduction reaction. Electrodes with close Pt:Co atomic ratios in the bulk and near surface region show grossly different electrocatalytic activities against the chemical order, the ordered L1(0) phase being ca. 7 times more active than the disordered fcc A1 structure. Electronic structure calculations and quantum mechanical theory of electron transfer are utilized to provide a rational for this remarkable phenomenon. (C) 2013 Elsevier Ltd. All rights reserved.

Self-consistent confirmations of a high negative value of the redox potential of the [Co(CN)6]3–/[Co(CN)6]4– system are demonstrated, which come from quantum-chemical calculations, electrochemical data and spectroscopic data.

The two faces of gold: the reduction of oxygen on gold electrodes in alkaline solutions has been investigated theoretically. The most favorable reaction leads directly to adsorbed O(2)(-), but the activation energy for a two-step pathway, in which the first step is an outer-sphere electron transfer to give solvated O(2)(-), is only slightly higher. d-band catalysis, which dominates oxygen reduction in acid media, plays no role. The reason why the reaction is slow in acid media is also explained.

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.

The two-step electrochemical reduction of tetrachloro-1,2-benzoquinone (chloranil), 2-methyl-1,2-benzoquinone (toluquinone), and 9,10-anthraquinone in two room-temperature ionic liquids is addressed by means of voltammetry on a platinum electrode. For the subsequent quinone/radical anion (Q/Q(•-)) and radical anion/dianion (Q(•-)/Q(2-)) redox reactions, the experimental data on formal potentials in 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF(4)]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF(6)]) and literature data for the same reactants in various aprotic molecular solvents are considered in the framework of a common potential sequence (Fc(+)/Fc scale) and compared with solvation energies computed at various levels. For the Q/Q(•-) couple, the agreement appeared to be satisfactory when solvation is described at the polarized continuum model (PCM) level. In contrast, for the Q(•-)/Q(2-) couple, the account for specific solvation at the molecular level is crucial.

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.