M. F. Butman

Knudsen effusion mass spectrometric determination of the complex vapor composition of samarium, europium, and ytterbium bromides

RATIONALE The vaporization of Sm, Eu, and Yb tri- and dibromides is accompanied by decomposition and disproportionation reactions. These result in complex vapor compositions whose analysis is an intricate problem for experimentalists. Approaches have been developed to interpret mass spectra and accurately determine the vapor composition of thermally unstable compounds. METHODS A sector type magnet instrument was used. A combined ion source allowed the study of both the molecular and ionic vapor compositions in the electron ionization (EI) and the thermionic emission (TE) modes. The methodological approaches were based on a joint analysis of the ionization efficiency functions, the temperature and time dependences of the ion currents, and special mathematical data evaluation. RESULTS The vaporization of SmBr3 , YbBr3 , SmBr2 , EuBr2 , and YbBr2 was studied in the temperature range of 850-1300 K. An initial stage of incongruent vaporization was observed in the case of the tribromides, SmBr2 , and YbBr2 . This eventually changed to a congruent vaporization stage. Various neutral (Ln, Br, Br2 , LnBr, LnBr2 , LnBr3 , Ln2 Br4 , Ln2 Br5 , and Ln2 Br6 ) and charged (Br(-), LnBr3 (-), LnBr4 (-)) species were detected at different vaporization stages. CONCLUSIONS The quantitative vapor composition of Sm, Eu, and Yb tri- and dibromides was determined. It was found that only EuBr2 was stable in the studied temperature range. The developed approaches can be useful in the case of other thermally unstable compounds.

Formation enthalpies of molecules and negative ions of ytterbium bromides

A procedure for determining the formation enthalpies of LnXn (n = 1–3) molecules of thermally unstable lanthanide di- and trihalides that is based on measuring the equilibrium constants of reactions in Ln-X systems of various content and solving a system of thermochemical equations is suggested. The procedure is used to determine the enthalpies of formation ΔfH298o of molecules and negative ions found in the vapors of ytterbium bromides: YbBr (20 ± 3), YbBr2 (−135 ± 10), YbBr3 (−233 ± 12), YbBr3− (−615 ± 31), and YbBr4− (−766 ± 23) kJ/mol.

Thermodynamic parameters of vaporization of EuBr2

The vaporization process of europium dibromide was studied using high-temperature mass spectrometry. It was ascertained that saturated vapor in temperature range of 1049–1261 K was represented mainly by EuBr2 molecules; the fraction of dimer molecules Eu2Br4 was less than 1%. Heat capacities of solid and liquid europium dibromide, as well as the melting enthalpy were measured by means of differential scanning calorimetry in temperature range 300–1100 K; using these data thermodynamic functions for EuBr2 in condensed state were calculated. For all experimental data, including the literature data, thermodynamic characteristics of the vaporization of europium dibromide were determined using a unified set of thermodynamic functions according to the methods of the second and third laws of thermodynamics. The value of ΔsHo(298.15 K) = 354 ± 5 kJ/mol was recommended for the reaction of sublimation of EuBr2(cr.) = EuBr2.

Structural and textural properties of pillared montmorillonite at intercalation of large Al- and Al/Ce-polyhydroxocomplexes

The possibility of adjusting the separation of silicate layers in montmorillonite in a broad range of basal distances d001 (1.3–2.4 nm) at the intercalation of large (0.7–1.8 nm) polyhydroxocomplexes of aluminum ([Al13O4(OH)24(H2O)12]7+, [Al30O8(OH)56(H2O)24]18+) and aluminum/cerium synthesized through the combined hydrolysis of aluminum and cerium salts in a reactor under pressure is shown. The formation of polyhydroxocomplexes was controlled by the methods of 27Al NMR and photon correlation and fluorescent spectroscopy at different concentrations of Al3+ ions (2.5–5.1 M) in solution. Textural properties (specific surface area, total pore volume, and mesoporosity) and fractal dimensionality of the samples of intercalated montmorillonite obtained by annealing at 300°C have been determined using the method of low-temperature nitrogen adsorption–desorption.

Formation energies of molecules and anions of europium bromides

The content of saturated vapors above europium dibromide and Eu-EuBr2, Eu-Ba-BaBr2, EuBr2-LaBr3 systems is investigated by means of high-temperature mass-spectrometry in the electron ionization and thermoionic emission regimes. On the basis of the measured equilibrium constants for reactions with participation of molecules and negative ions, the enthalpies of formation ΔfH298° (kJ/mol) are determined using the method of the third law of thermodynamics: −59 ± 13 (EuBr), −349 ± 19 (EuBr2), and −861 ± 24 (EuBr3−).

The effect of preliminary delamination of silicate filler on its exfoliation degree in polyvinyl alcohol/montmorillonite nanocomposites

It is found that the preliminary delamination of the silicate filler allows one to achieve its high degree of exfoliation in the process of preparing polymer/layered silicate nanocomposites. It is shown using the example of various forms of montmorillonite, such as natural, intercalated with large [A113O4(OH)24(H2O)12]7+ Keggin ions, and pillared, that polyvinyl alcohol/montmorillonite nanocomposite films show much higher strength characteristics in the case of using a delaminated silicate component. According to the data of small angle X-ray diffraction studies, the almost complete exfoliation of silicate layers is observed, particularly for delaminated forms of montmorillonite. The thermal stability of the composites slightly decreases in comparison with pure PVA.

Composition of saturated vapor over Ytterbium bromides

The vaporization process of ytterbium di- and tribromide was studied using high-temperature mass spectrometry over the temperature range of 850 to 1300 K. It was ascertained that, at the early vaporization stages, the vapor contained molecules YbBr3, YbBr2, YbBr, Br2, Yb2Br2, Yb2Br3, Yb2Br4, Yb2Br5, Yb2Br6, and atoms Yb and Br. The partial pressures of all components of saturated vapor were calculated. It was found that vapor composition reflected the course of the reactions of decomposition of tribromide and disproportionation of dibromide in the condensed phase. It was concluded that vaporization of di- and tribromide was incongruent at the initial stages; vaporization of both agents acquired a congruent character with the Yb: Br = 1.0: 1.9±0.2 ratio with time.

Study of Molecular and Ionic Vapor Composition over CeI3 by Knudsen Effusion Mass Spectrometry

The molecular and ionic composition of vapor over cerium triiodide was studied by Knudsen effusion mass spectrometry. In the saturated vapor over CeI3 the monomer, dimer, and trimer molecules and the negative ions I−, , and were identified in the temperature range of 753–994 K. The partial pressures of CeI3, Ce2I6, and Ce3I9 were determined and the enthalpies of sublimation, (298.15 K) in kJ·mol−1, in the form of monomers (), dimers (), and trimers () were obtained by the second and third laws of thermodynamics. The enthalpy of formation, (298.15 K) in kJ·mol−1, of the CeI3 (), Ce2I6 (), and Ce3I9 () molecules and the () and () ions were calculated. The electron work function, = eV, for the CeI3 crystal was evaluated.

Photocatalytic and adsorption properties of TiO2-pillared montmorillonite obtained by hydrothermally activated intercalation of titanium polyhydroxo complexes

We report on a new approach for the synthesis of TiO2-pillared montmorillonite, where the pillars exhibit a high degree of crystallinity (nanocrystals) representing a mixture of anatase and rutile phases. The structures exhibit improved adsorption and photocatalytic activity as a result of hydrothermally activated intercalation of titanium polyhydroxo complexes (i.e., TiCl4 hydrolysis products) in a solution with a concentration close to the sol formation limit. The materials, produced at various annealing temperatures from the intercalated samples, were characterized by infrared spectroscopy, differential scanning calorimetry (DSC)/thermogravimetric analysis (TGA), X-ray diffraction, dynamic light scattering (DLS) measurements, and liquefied nitrogen adsorption/desorption. The photocatalytic activity of the TiO2-pillared materials was studied using the degradation of anionic (methyl orange, MO) and cationic (rhodamine B, RhB) dyes in water under UV irradiation. The combined effect of adsorption and photocatalysis resulted in removal of 100% MO and 97.5% RhB (with an initial concentration of 40 mg/L and a photocatalyst-sorbent concentration of 1 g/L) in about 100 minutes. The produced TiO2-pillared montmorillonite showed increased photocatalytic activity as compared to the commercially available photocatalyst Degussa P25.

Adsorption of anion and cation dyes onto pillared montmorillonite

Al2O3- and Al(Ce)2O3-pillared montmorillonite materials were synthesized using the intercalation of large-sized polyhydroxocomplexes of aluminum and aluminum/cerium in order to obtain adsorbents for removing the water-soluble anionic (acid red, AR) and cationic (methylene blue, MB) dyes. It was shown that the adsorption of dyes heavily depended upon the pH of the medium reaching a maximal adsorption capacity for AR at low pH and for MB at high pH. The distinction between the adsorption efficiencies for dyes observed in the materials with a different pillar structure was interpreted in the light of both the charge states (the ζ-potential) of tactoids in suspensions and texture properties of adsorbents investigated by small-angle X-ray diffraction and porosimetry. We used the classic models to describe adsorption: the kinetic pseudo-first-order and pseudo-second-order models and the equilibrium Langmuir and Freundlich equations.