T. Ya. Safonova

Effect of inorganic cations on the electroreduction of nitrate anions on Pt|Pt electrodes in sulfuric acid solutions

Abstract It is shown that modification of the surface of Pt|Pt electrodes by cations Ni 2+ , Co 2+ , Cd 2+ , and Ge 4+ accelerates the electroreduction of NO − 3 anions in sulfuric acid solutions in the potential region close to a complete coverage of the surface by H ads in the supporting electrolyte solution. The electrocatalytic effect of these surface modifiers increases in the following sequence: Ge>Ni>Co≈Cd and also depends on the conditions of adsorption of the cations and nitrate anions. It is shown that Ge adatoms promote the NO − 3 electroreduction most efficiently when they occupy only a part of the electrode surface, and the remaining part is occupied by adsorbed hydrogen. In the presence of Ni 2+ , Co 2+ , and Cd 2+ cations, ammonium ions are detected as the reaction product, while, in the presence of Ge adatoms, hydroxylamine is also found among the reaction products. Suppositions are made about the possible causes of the electrocatalytic effects observed.

Nanoheterogeneous electrocatalysts fabricated by palladizing of platinum

Abstract Palladium samples deposited on platinum at constant potentials of 0.02–0.07 V (the reversible hydrogen electrode, rhe) are studied by measuring potentiodynamic and charging curves. These samples are shown to exhibit an anomalous behavior throughout the whole region of reversibility, namely, the enhanced hydrogen contents in α- and β-phases of palladium hydride and the higher real surface areas. The parameters of the isotherms of the β-phase formation differ from those observed for usual palladium samples. The model analysis based on the defectiveness concept correlates the anomalously high hydrogen content and unusual isotherm parameters with the concentration of defective lattice regions.

Electrolytic palladium deposits: Dependence of adsorption properties and texture on the deposition potential

Specific features of the copper and oxygen adsorption on electrolytic palladium deposits obtained from chloride solutions at different deposition potentials are considered. It is shown that, in sulfuric acid solutions, the palladium dissolution, which accompanies the oxygen adsorption, obscures the adsorption behavior of the deposits with respect to oxygen. Assumptions are made about the predominant crystallographic orientation (100) of the surface of some palladium deposits being dependent on the deposition potential and about the presence of regions whose adsorption properties are anomalous with respect to copper. The assumptions are based on a comparative analysis of the copper adsorption data and X-ray diffraction patterns. In addition to sites of crystalline palladium, the deposits are found to have disordered areas as well.

Adsorption of polyethylene glycol on platinum electrode from acidic solutions

The effect of the molecular mass of polyethylene glycol (PEG) on its adsorption on platinized platinum from aqueous solutions of 0.5 M H2SO4 and 1 M HCl is studied using the methods of open-circuit potential shifts and voltammetry. In sulfuric acid solutions, the PEG adsorption is accompanied by dehydrogenation and hydrogenation processes, which probably involve the terminal groups of polymers. For PEG with the molecular mass of 600–40000, the established stationary surface states turn out to be close to one another. Anions Cl− inhibit the PEG adsorption and electrooxidation. The adsorption behavior of PEG samples studied substantially differs from that of ethylene glycol under comparable conditions.

Intensification of the Nitrate Anion Reduction on a Membrane Palladium Electrode

Polarization curves of electroreduction of nitrate anions in sulfuric acid aqueous solutions on a Pd/Pd palladium electrode with the opposite side kept at different constant potentials in the region of Pd–H α-phase formation are compared. The reduction rate of nitrate anions is shown to be substantially higher on a membrane electrode, provided the conditions of hydrogen supply from the membranes opposite side to the reaction zone are realized. This phenomenon is caused by the reduction of a chemisorbed intermediate (a certain N(III) form) by hydrogen diffused through the membrane. It is shown that the measurements of hydrogen diffusion currents through a membrane can be used in plotting hydrogen sorption isotherms in the Pd–H α-phase.

Hydrogen sorption and electrochemical properties of alloys: Systems Zr-Ti-Ni-V-Mn with laves phase structures

Using scanning electron microscopy and x-ray phase analysis techniques, it is shown that the C14-type Laves phase with a wide homogeneity reaching the AB2.3 stoichiometry is the main phase in the Zr0.5Ti0.5NiyV0.5Mnx alloys, where y = 0.8–1.4 and x = 0.1–1.7. With the increase in the nickel and manganese contents and in the stoichiometric ratio B/A, the hydrogen capacity decreases from 2 to 1.6 wt %, the equilibrium pressure of hydride phases increases, and the hydrides become less stable. The highest discharge capacity is reached for stoichiometric ratios AB1.7–2.3, where the maximum discharge capacity at a discharge current density of 100 mA/g is 300 (mA h)/g. Alloys that contain ferrovanadium in place of vanadium are also considered.

Structural peculiarities of anomalous electrolytic palladium deposits prepared under hydride formation conditions

The peculiarities of bulk and surface structures of electrolytic palladium deposits formed under hydride formation conditions are studied by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy techniques. For anomalous deposits with high hydrogen capacities, the evolution of the XRD behavior is traced as a function of thermal treatment. The material is shown to comprise two cubic phases with different lattice parameters.

Electrochemical Hydrogenation of Substituted α-Phenylvinylphosphonic Acids: General Characteristics of the Reaction Layer and Prediction of Preparative Electrolysis Conditions

The conditions for electrochemical hydrogenation and anodic oxidation of α-phenylvinylphosphonic acid and its 4-chloro- and 4-methyl-substituted derivatives on palladized platinum electrodes in aqueous sulfuric-acid solutions are found. It is shown that, in the system under study, the products of strong chemisorption are not the intermediates of the electrocatalytic hydrogenation. The reactant molecules are modeled by quantum-chemical methods in terms of the restricted Hartree–Fock approximation. The calculated charge distributions in reactant molecules are used for estimating the electrostatic components of works of approach and the orientation distributions in the reaction layer.

Texture of Electrolytic Platinum Deposited on Polycrystalline Gold

Platinum deposits of submicron thicknesses on polycrystalline gold substrates subjected to different thermal treatments and characterized by texture 〈200〉, 〈311〉, and 〈220〉 are characterized by methods of x-ray diffractometry and scanning electron microscopy. No deposits prepared in a potentiostatic mode exhibit a pronounced texture or a reliable correlation between a weak texture of a deposit and the substrate texture. Established is the difference of predominant directions of secondary nucleation–growth of deposit on neighboring grains of the substrate. The assumption that the deposit includes fragments the size of a few tens of micrometers with pronounced texture of different directions is substantiated.

Electrochemical Behavior of Intermetallic Compounds of the Type AB5 Synthesized during the Reduction of a Mixture of Oxides and Salts by Lithium Hydride

The intermetallic compounds LaNi4.5Co0.2Sn0.3, La0.9Zr0.1Ni4.5Al0.5, Mm0.8Ni4.7Co0.2Sn0.3, Mm0.8Ni4.7Co0.2Ge0.3, MmNi4.5Co0.2Ge0.3, and MmNi4.5Co0.2Sn0.3, which are synthesized by a new method, specifically, during a high-temperature reduction of a mixture of oxides and carbonates by lithium hydride, have high homogeneity and dispersion degree. According to electrochemical charge-discharge tests, electrodes of these compounds ensure a high discharge capacity (∼300 mA h g–1) as early as after the first 2–3 cycles and their behavior is stable and reversible.