K.N. Semenenko

The alkali metals in graphite matrixes-new aspects of metallic state chemistry

Abstract Graphite-alkaline metal systems (lithium, potassium, rubidium, cesium) have been investigated under high-pressure conditions. New graphite intercalation compounds of C4M composition with alkali metal contents twice as high as in those obtained with traditional methods without applying high pressure have been produced. The solid-phase postintercalation reactions of potassium and rubidium in C8 M under 0.2–0.3 GPa pressure have been investigated. The compressibility of different composition intercalation compounds obtained was determined under 2.5 GPa pressure by the volume method. The increase in metal content in layers has been shown to lead to reduced compound compressibility. The phase transformation in C8 K under −1.2 GPa pressure, associated with the metal compaction in the intervals between graphite layers, has been investigated.

The choice of oxidizers for graphite hydrogenosulfate chemical synthesis

Abstract The features of chemical oxidation of graphite in H2SO4 are discussed. Measurements of currentless potentials of K2Cr2O7, KMnO4, (NH4)2S2O8 and complex salt Ce(SO4)2·4(NH4)2SO4·2H2O solutions in 96% H2SO4 were carried out. Correlation of the redox potentials of oxidizers to the stage number of graphite hydrogenosulfate (GHS) is established. It is proposed to use the standard redox potential scale to choose the oxidizers for the formation of a given stage GHS. On the basis of this approach, numerous oxidizers for GHS formation are considered. For the first time, ozone and Ce(IY) sulfate were applied for GHS production. The stage 1 GHS is obtained in accordance with standard potentials of these oxidizers.

SODIUM GRAPHITE SYSTEM AT HIGH-PRESSURES

Abstract Sodium intercalation into graphite is investigated ‘ in situ ’ at high pressures, the volumetric and DTA techniques being used. Synthesis conditions of the highly saturated compounds (composition close to C 2.5–3.0 Na) are determined. It is shown that sodium intercalation into graphite at 40-kbar pressure takes place after metal melting, the reaction being complete within several minutes. In addition, the volumetric method establishes the possibility of this reaction at lower pressures (∼ 20 kbar) and heating for 5–10 h.

Chemical synthesis of graphite hydrogenosulfate: Calorimetry and potentiometry studies

In the present work dynamics of sulfuric acid intercalation into graphite by chemical oxidation were investigated by means of the calorimeter and potentiometer techniques. Phase composition of intermediate and end-products of the reaction was investigated by X-ray diffraction.

Intercalation in the graphite-rubidium system under high pressure

Abstract Results of investigations on the graphite-rubidium system under high pressure (up to 25 kbar) are presented. In this system new graphite intercalation compounds of C 4.3−6.0 Rb compositions have been obtained. These compounds have been shown to be unstable under normal conditions and to decompose to rubidium and less metal-saturated compounds when the pressure is reduced. The reaction is reversible - rubidium can be again intercalated into C 8 Rb under high pressure at room temperature.

Physicochemistry and crystallochemistry of intermetallic hydrides containing rare earths and transition metals

Various aspects of the interaction of hydrogen with intermetallic compounds belonging to the homologous series RT5, RT3 and RT2 (R ≡ rare earth metal or Sc; T ≡ Fe, Co, Ni) are described. The reactions of 150 compounds were studied experimentally to demonstrate that the direction of the reaction depends on the initial structure and on the nature of the R and T atoms. The interactions were shown to result in so-called hydrides of the intermetallic compounds in which the initial metal matrix either remains intact or increases in volume without a significant atomic rearrangement. X-ray structure studies and complete structure determinations of the hydride phases using neutron diffraction methods were carried out to investigate the principal rules governing the variations in hydride structures and properties. The insertion of hydrogen atoms into interstitial sites of the matrix lattice occurs at sites in an environment in which the lengths of the M-M bonds exceed the sum of the corresponding metal radii. The hydrogenolysis reaction which results in the formation of a binary hydride and the elimination of a transition metal was shown to be favoured thermodynamically. The mechanism of hydrogenolysis was studied.

Interaction mechanism of hydrogen with the CaCu5-type crystal structure intermetallic compounds

Abstract The methodological technique of the ‘calorimetry titration’ using differential calorimetry was applied for the first time to study the interaction mechanism of hydrogen with metallic phase in ‘IMC-H2’ systems, where the IMC-LaNi, Cu. It is shown, that the process of formation α- and β-hydriding phases in such systems proceeds by different mechanisms. The data obtained are well correlated with the results of the isotherms for the ‘IMC-H2’ systems.

Electronic structure of Mg2FeH6 and Mg2CoH5 hydrides and binding role of hydrogen in them

Participation of d and p symmetry valence electrons of iron and cobalt in metal-hydrogen bonding in the Mg2FeH6 and Mg2CoH5 hydrides has been investigated on the basis of X-ray emission spectroscopy data. An additional charge acquired by iron and cobalt atoms in the hydrides has been measured and the role played by hydrogen atoms as a binding component in the investigated hydrides has been explained.

Vibrational spectra of superdense lithium graphite intercalation compounds

IR investigations of lithium graphite intercalation compounds with different lithium in-plane density (LiC6, LiC4, LiC2) were carried out. The spectra interpretation was done on the basis of normal coordinate analysis for model structures. LiC2 and LiC4 spectra exhibit the bands corresponding to a complex skeleton stretching in Li7 or LiC6 clusters including equilateral triangles of metal atoms at short distances which shows that covalence exists between Li atoms in the intercalated layer and that LiC2 decomposition proceeds keeping these clusters in LiCx structure for x < 4 at least.

Investigation of materials based on intermetallic compounds of the CeNi3-CeCo3 system and a polymeric binding material

A study has been made of electrode materials based on intermetallic compounds (IMC) of the CeNi3CeCo3 series and a polymeric binding material, polytetrafluoroethylene (PTFE). It was shown that the nature of the electrode material interaction with hydrogen is similar to that of an individual IMC. The dissociation pressures of the hydride phases of CeNi3, CeNi2Co and CeNiCo2 at 293–333 K were established, as well as the main thermodynamic constants (δH, δS) characterizing the invariant equilibrium trihydrides ⤦ monohydrides in the investigated systems. Using electrochemical extraction, the formal hydrogen diffusion coefficients in IMCs and PTFE-based materials were evaluated. A study was made of the electrocatalytic activity of IMC-based materials of the CeNi3CeCo3 series in hydrogen-evolution reactions in alkaline solution.