Phase equilibria of binary clathrate hydrates of nitrogen+cyclopentane/cyclohexane/methyl cyclohexane and ethane+cyclopentane/cyclohexane/methyl cyclohexane

In this communication, we first report hydrate dissociation conditions for the nitrogen+cyclopentane, cyclohexane or methyl cyclohexane+water and ethane+cyclopentane, cyclohexane or methyl cyclohexane+water systems at various temperatures. The experimental data were generated using an isochoric pressure-search method. The hydrate dissociation data for the aforementioned systems along with the hydrate dissociation data for the methane, carbon dioxide or hydrogen sulfide+cyclopentane, cyclohexane or methyl cyclohexane+water systems collected from the literature are compared with the corresponding literature data in the absence of the aforementioned heavy hydrocarbons in order to study the hydrate promotion effects of cyclopentane, cyclohexane or methyl cyclohexane. It is shown that these effects on ethane simple hydrate are not considerable unlike the corresponding effects on nitrogen, methane, carbon dioxide and hydrogen sulfide simple hydrates.     

Thermodynamic and Raman spectroscopic studies on hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrates

Thermodynamic stability and hydrogen occupancy on the hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate have been investigated by means of phase equilibrium (pressure-temperature) measurements and Raman spectroscopic analyses for two mole fractions, 0.018 and 0.034 (stoichiometric for the cubic structure) of tetra-n-butyl ammonium fluoride aqueous solutions. In the case of higher concentration (0.034), the stability boundary curve of hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate locates at about 23 K higher temperature than that of hydrogen+tetrahydrofuran mixed gas hydrate. The storage capacity of hydrogen in the cubic structure for the hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate is smaller than that of hydrogen+tetrahydrofuran mixed gas hydrate. In the case of hydrate prepared from the lower concentration (0.018) of aqueous solution, the Raman spectra and phase behavior reveal that the cubic structure of semi-clathrate hydrate is changed to a different one at about 9 MPa and 299.2 K. The new structure can entrap larger amount of hydrogen than the cubic one. The stability boundary curve of hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate obtained in the aqueous solution of lower mole fraction (0.018) is shifted to slightly low-temperature or high-pressure side from that of higher mole fraction (0.034).     

Phase evolution,dielectric and impedance spectroscopic study of SrNb2O6 columbite phase

Strontium niobate SrNb₂O₆ has been synthesized by columbite solid-state reaction method and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and temperature as well as frequency dependence of dielectric and impedance study. XRD analysis indicates single phase formation of the compound with ∼180 nm crystallite size. Study of SEM micrographs pointed out that prepared material has good sinterability and enough density with homogeneous grain distribution. It was found that the magnitude of relative dielectric constant (?_r) was relatively high with low dielectric loss compared with reported columbite compounds. Impedance spectroscopy was used to characterize the electrical behavior of the compound. AC impedance spectrum results indicate that the relaxation mechanism of the material is temperature dependent and has bulk and grain boundary contribution in different temperature ranges.     

Separation and capture of carbon dioxide from CO2/H2 syngas mixture using semi-clathrate hydrates

The relation between anthropogenic emissions of CO₂ and its increased levels in the atmosphere with global warming and climate change has been well established and accepted. Major portion of carbon dioxide released to the atmosphere, originates from combustion of fossil fuels. Integrated gasification combined cycle (IGCC) offers a promising fossil fuel technology considered as a clean coal-based process for power generation particularly if accompanied by precombustion capture. The latter includes separation of carbon dioxide from a synthesis gas mixture containing 40 mol% CO₂ and 60 mol% H₂.A novel approach for capturing CO₂ from the above gas mixture is to use gas hydrate formation. This process is based on selective partition of CO₂ between hydrate phase and gas phase and has already been studied with promising results. However high-pressure requirement for hydrate formation is a major problem.We have used semiclathrate formation from tetrabutylammonium bromide (TBAB) to experimentally investigate CO₂ capture from a mixture containing 40.2 mol% of CO₂ and 59.8 mol% of H₂. The results shows that in one stage of gas hydrate formation and dissociation, CO₂ can be enriched from 40 mol% to 86 mol% while the concentration of CO₂ in equilibrium gas phase is reduced to 18%. While separation efficiency of processes based on hydrates and semi-clathrates are comparable, the presence of TBAB improves the operating conditions significantly. Furthermore, CO₂ concentration could be increased to 96 mol% by separating CO₂ in two stages.     

Preparation and spectroscopic properties of chlorofullerenes C60Cl24, C60Cl28, and C60Cl30

We report easy preparation of recently discovered highly chlorinated fullerenes T_h-C₆₀Cl₂₄, C₁-C₆₀Cl₂₈, and D_3d-C₆₀Cl₃₀ in high-temperature reactions of C₆₀ with PCl₅ and ICl. Formation and interconversion of chlorofullerenes was investigated in details for C₆₀-ICl system. C₆₀Cl₂₈ is the least stable chlorofullerene that undergoes rearrangement (accompanied by partial chlorine elimination) into more stable T_h-C₆₀Cl₂₄ under more drastic reaction conditions (increased temperature and time of chlorination). T_h-C₆₀Cl₂₄ yields D_3d-C₆₀Cl₃₀ at temperatures above 220 °C via a sequence of rearrangements and further addition of chlorine. In contrast to the fullerene reaction with ICl, interaction of C₆₀ with PCl₅ is very selective with respect to formation of C₆₀Cl₂₄ in a wide temperature range. Solid-state electronic (UV-Vis) and vibrational (IR) spectra of chlorinated fullerenes C₆₀Cl₂₄, C₆₀Cl₂₈, C₆₀Cl₃₀ and fluorinated fullerenes C₆₀F₁₈ and C₆₀F₃₆ were recorded in the spectral range between 30 and 45,000 cm⁻¹. Raman spectra were also acquired for all investigated compounds. Moreover, molecular geometry of the C₆₀Cl₂₄ and its theoretical IR-absorption spectrum were calculated using B3LYP/STO-3G chemistry model.     

Resistive,capacitive and conducting properties of Bi0.5Na0.5TiO3-BaTiO3 solid solution

In the present paper, the effect of addition of a small amount (8 wt%) of barium titanate (BT) on electrical properties of bismuth sodium titanate (BNT) forming a solid solution of a composition (0.92)(Bi_0.5Na_0.5TiO₃)+(0.08)(BaTiO₃) (BNT-BT-8) has mainly been reported. The solid solution of BNT-BT-8 was prepared by a cost effective and standard mixed-oxide method. Preliminary structural analysis using X-rays diffraction pattern and data showed the existence of two phases; orthorhombic (major) and tetragonal (minor impurity/secondary) phase. Analysis of scanning electron micrograph and energy dispersive spectrum of the pellet sample reveals the formation of high density with homogeneously distributed grains of varying dimension. The locations, phonon modes statistics, width and intensity of peaks of Raman spectra of BNT-BT-8 was analyzed by Raman spectroscopy and provided some data on molecular structure of the material. The effect of temperature and frequency on some ferroelectric characteristics of the material were studied. The frequency-temperature dependence of electrical characteristical such as impedance of the material was studied by impedance spectroscopy. The electric conductivity follows the Arrhenius equation and provided activation energy at different frequency. The dielectric and impedance spectroscopy suggest the existence of a non-Debye relaxation mechanism in the material.     

Nb2O5-supported WO3: a comparative study with WO3/Al2O3

WO₃/Nb₂O₅-supported samples prepared by impregnation are characterised by X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) at the W–L₃ absorption edge, as well as temperature programmed reduction (TPR) and FT-IR monitoring of pyridine adsorption. Results are compared with those obtained for WO₃/Al₂O₃ samples prepared in the same conditions, showing that niobia is able to disperse tungsta better than alumina does. Formation of a crystalline WO₃ needs larger tungsten contents on niobia than on alumina, since tungsten solution into niobia is easier than into alumina. Raman and XAS spectra recorded under ambient conditions suggest that similar WO_x species are formed on both supports at tungsten contents 0.5–1 theoretical monolayers; however, TPR results for the low tungsten loaded samples indicate that, when reduction starts (always at temperatures higher than 700 K under H₂/Ar flow) there is a larger concentration of tetrahedral [WO₄] species on alumina, than on niobia. Samples with low tungsten loading have been tested in isopropanol decomposition and ethylene oxidation, following both processes by FT-IR of adsorbed species up to 673 K. Results show that adsorption of ethylene on WO₃/Nb₂O₅ yields acetaldehyde and acetate at 473 K, while this adsorption is non-reactive either on the supports or on WO₃/Al₂O₃. Isopropanol adsorbs dissociatively on both supports, leading to acetone and propene formation on tungsta–niobia, but only propene on tungsta–alumina, probably due to the larger reducibility of the tungsten-containing phases.     

Thermodynamic modeling of CO2 solubility in aqueous solutions of NaCl and Na2SO4

A thermodynamic model based on the electrolyte NRTL activity coefficient equation and PC-SAFT equation-of-state is developed for CO₂ solubility in aqueous solutions of NaCl and Na₂SO₄ with temperature up to 473.15 K, pressure up to 150 MPa, and salt concentrations up to saturation. The Henry's constant parameters of CO₂ in H₂O and the characteristic volume parameters for CO₂ required for pressure correction of Henry's constant are identified from fitting the experimental gas solubility of CO₂ in pure water with temperature up to 473.15 K and pressure up to 150 MPa. The NRTL binary parameters for the CO₂-(Na⁺, Cl⁻) pair and the CO₂-(Na⁺, SO₄²⁻) pair are regressed against the experimental VLE data for the CO₂-NaCl-H₂O ternary system up to 373.15 K and 20 MPa and the CO₂-Na₂SO₄-H₂O ternary system up to 433.15 K and 13 MPa, respectively. Model calculations on solubility and heat of solution of CO₂ in pure water and aqueous solutions of NaCl and Na₂SO₄ are compared to the available experimental data of the CO₂-H₂O binary, CO₂-NaCl-H₂O ternary and CO₂-Na₂SO₄-H₂O ternary systems with excellent results.     

二氧化碳水合物浆在圆管中的流动特性

实验研究了固相分数为8.2%～23.1%的CO₂水合物浆在内径为8 mm的圆管中的流动特性。结果发现水合物浆在管内的流动压降随着流速的增加而增大。当流速低于0.60 m·s^-1时,浆体流变指数小于1,且随着固相体积分数的增大而减小,CO₂水合物浆为H-B流体,其表观黏度随着流速的增大而减小,呈剪切变稀特性。剪切速率为600 s^-1时,CO₂水合物浆的表观黏度为8.5～10.6 mPa·s。实验得到了CO₂水合物浆的流变特征参数及其流变方程,可为CO₂水合物浆的流动及其应用研究提供理论指导。     

Pressure-Induced Phase Transformation of HfO2

The pressure dependence of the Raman spectra of HfO₂ was measured by a micro-Raman technique using a singlecrystal specimen in the pressure range from 0 to 10 GPa at room temperature. The symmetry assignment of Raman bands of the monoclinic phase was experimentally accomplished from the polarization measurements for the single crystal. With increased pressure, a phase transformation for the monoclinic phase took place at 4.3 ± 0.3 GPa. Nineteen Raman bands were observed for the high-pressure phase. The spectral structure of the Raman bands for the high-pressure phase was similar with those reported previously for ZrO₂. The space group for the high-pressure phase of HfO₂ was determined as Pbcm , which was the same as that of the high-pressure phase for ZrO₂ on the basis of the number and the spectral structure of the Raman bands.     

High growth of SWNTs and MWNTs from C2H2 decomposition over Co-Mo/MgO catalysts

A series of MgO supported catalysts having Co and Mo metals 5-40 wt.% in a ratio of 1:1 was prepared by impregnation method. Carbon nanotubes (CNTs) were grown over the catalysts by decomposition of C₂H₂ at 800 °C for 30 min. It was found that 5 and 10 wt.% Co-Mo/MgO catalysts produced single-wall nanotubes (SWNTs), whereas 20, 30 and 40 wt.% Co-Mo/MgO catalysts produced multi-wall nanotubes (MWNTs). The catalyst Mo/MgO was inactive in growing CNTs. In Co-Mo/MgO catalysts, however Mo generated a favorable environment to grow SWNTs. The growth of SWNTs was strongly dependent on the formation of small clusters of cobalt, which may generate from the decomposition of CoMoO₄ species during the nanotube growth. MWNTs were produced over comparatively larger cobalt clusters generated from Co₃O₄ phase during the nanotube growth stage. The yields of SWNTs were about 6% and 27% over 5 and 10 wt.% Co-Mo/MgO catalysts, respectively. MWNTs yield (576%) was observed over 40 wt.% Co-Mo/MgO catalyst. Carbon yield (%) highly varied with acetylene concentration.     

Structural investigation of Li1−xNi0.5Co0.25Mn0.25O2 by in situ XAS and XRD measurements

A combination technique of in situ synchrotron X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) was employed to study the Li_1−xNi_0.5Co_0.25Mn_0.25O₂ cathode material for Li-ion battery. The Li/Li_1−xNi_0.5Co_0.25Mn_0.25O₂ cell with x = 0.82 charged to 4.5 V showed the first charge capacity of 225 mAh/g. The X-ray absorption near edge structure (XANES) indicated that the initial valences were +2/+3, +3 and +4 for Ni, Co and Mn, respectively. The main redox reaction during delithiation was achieved by Ni via the reaction Ni²⁺ → Ni³⁺ followed by Ni³⁺ → Ni⁴⁺. The oxidation states of Co and Mn remained Co³⁺ and Mn⁴⁺. The bond length of Ni-O decreased drastically, while the Co-O and Mn-O distances exhibited a slight change with the decrease of Li content in the electrode. It was further revealed that all the second shell metal-metal (Ni-M, Co-M and Mn-O) distances decreased due to the oxidation of metal ions. In situ XRD data showed that both a- and c-axes varied with different Li contents in this material system. At the beginning of charge, there was a contraction along the c-axis and a slight expansion along the a-axis. As x reached 0.57, the trend of the variation in c-axis was opposite. The changes of lattice parameters could be explained by the balance between ionic radius and the repulsive force of the layer-structured material.     

Spectroscopic analysis of carbon dioxide and nitrogen mixed gas hydrates in silica gel for CO2 separation

In this study solid-state NMR spectroscopy was used to identify structure and guest distribution of the mixed N₂ + CO₂ hydrates. These results show that it is possible to recover CO₂ from flue gas by forming a mixed hydrate that removes CO₂ preferentially from CO₂/N₂ gas mixture. Hydrate phase equilibria for the ternary CO₂–N₂–water system in silica gel pores were measured, which show that the three-phase H–L_w–V equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO₂ in the vapor phase decreased. ¹³C cross-polarization (CP) NMR spectra of the mixed hydrates at gas compositions of more than 10 mol% CO₂ with the balance N₂ identified that the crystal structure of mixed hydrates as structure I, and that the CO₂ molecules occupy mainly the abundant 5¹²6² cages. This makes it possible to achieve concentrations of more than 96 mol% CO₂ gas in the product after three cycles of hydrate formation and dissociation.     

Adsorption of CO2 from dry gases on MCM-41 silica at ambient temperature and high pressure. 2: Adsorption of CO2/N2, CO2/CH4 and CO2/H2 binary mixtures

Adsorption equilibrium capacity of CO₂, CH₄, N₂, H₂ and O₂ on periodic mesoporous MCM-41 silica was measured gravimetrically at room temperature and pressure up to 25 bar. The ideal adsorption solution theory (IAST) was validated and used for the prediction of CO₂/N₂, CO₂/CH₄, CO₂/H₂ binary mixture adsorption equilibria on MCM-41 using single components adsorption data. In all cases, MCM-41 showed preferential CO₂ adsorption in comparison to the other gases, in agreement with CO₂/N₂, CO₂/CH₄, CO₂/H₂ selectivity determined using IAST. In comparison to well known benchmark CO₂ adsorbents like activated carbons, zeolites and metal-organic frameworks (MOFs), MCM-41 showed good CO₂ separation performances from CO₂/N₂, CO₂/CH₄ and CO₂/H₂ binary mixtures at high pressure, via pressure swing adsorption by utilizing a medium pressure desorption process (PSA-H/M). The working CO₂ capacity of MCM-41 in the aforementioned binary mixtures using PSA-H/M is generally higher than 13X zeolite and comparable to different activated carbons.     

Infra-red and Raman spectroscopic study on the thermal stability and high temperature transformation of hydroazafullerene C59HN

Hydroazafullerene C₅₉HN was studied by vibrational infra-red and Raman spectroscopy and its thermal stability was examined. Fingerprints modes were identified and unambiguously differentiate it from bisazafullerene. At 700 K full transformation to bisazafullerene occurred, while an intermediate metastable phase was identified at 540 K showing different spectra where the splitting of most of the lines is strongly reduced.     

Formation of gyrolite during hydrothermal synthesis in the mixtures of CaO and amorphous SiO2 or quartz

Parameters of gyrolite hydrothermal synthesis were determined when primary mixtures consisting CaO and amorphous SiO₂·nH₂O or quartz and a sequence of intermediary compound formation were examined and explained. The molar ratio (C/S) of the primary mixtures was 0.66 (C—CaO; S—SiO₂) and the water/solid ratio (W/S) of the unstirred suspension was 10. Hydrothermal synthesis was carried out in saturated steam at 150, 175, and 200 °C temperatures. The duration of isothermal curing was 4, 16, 24, 32, 48, 72, and 168 h.Gyrolite does not form even after a week in the mixtures of CaO and amorphous SiO₂ at 150 °C temperature in saturated steam. Increase in the temperature positively affects the synthesis of this compound—pure gyrolite at 175 °C was obtained after 72 h at 200 °C—after 32 h. It should be noted that while synthesizing gyrolite, intermediary compounds C-S-H (I) and Z-phase are always formed. The mechanism of hydrothermal reactions and the sequence of compounds to be formed in the mixtures of CaO and quartz are totally different. Due to low quartz solubility rate at temperature range from 150 to 200 °C, neither Z-phase nor gyrolite forms even during 72 h of hydrothermal curing. In the beginning of the synthesis, α-C₂S hydrate prevails, which gradually recrystallizes into 1.13 nm tobermorite and xonotlite. Almost half of the quartz reacts during the first 4 h at 150 °C temperature, and the further decrease of its quantity depends much on the duration of hydrothermal curing. However, about 10% of the quartz does not react at all, when the C/S in the products approach approximately 0.8, stable calcium silicate hydrates—1.13 nm tobermorite and xonotlite—are formed. They are relatively stable. Experimentally obtained data and preconditions were approved by thermodynamic calculations.     

Structural investigation of mechanically activated nanocrystalline BaTiO3 powders

In this article, in order to obtain tetragonal nanocrystalline BaTiO₃, structural investigations of mechanically activated BaTiO₃ powder have been performed. A mercury porosimetry analysis and scanning electron microscopy method have been applied for determination of the specific pore volume, porosity and microstructure morphology of the samples. The lattice vibration spectra of nonactivated and activated powders, their phase composition, lattice microstrains and the mean size of coherently diffracting domains were examined by Raman spectroscopy and the X-ray powder diffraction method. The average crystal structure of obtained nanocrystalline powders, estimated from X-ray diffraction data, gave evidence of retained, but slightly sustained tetragonality of powders, even for particles as small as ∼30 nm. Raman spectroscopy also gave clear evidence for local tetragonal symmetries, in particular through the presence of a band at ∼307 cm⁻¹.     

Influences of carbon structure on the reactivities of lignite char reacting with CO2 and NO

Raw and demineralized lignite samples were pyrolyzed from 773 to 1673 K to generate chars. The chars were characterized with Raman spectroscopy for the structure evolution. The reactivities of the chars reacting with CO₂ and NO were measured with thermogravimetric analysis. The derived reactivity indexes were correlated with the treatment temperature and the Raman structural parameters to demonstrate the applicability of Raman spectroscopy for evaluation of the reactivities of char CO₂ gasification and char-NO reaction. It was found that char microstructure evolution with the treatment temperature could be represented by Raman band area ratios. I_D1/I_G and I_G/I_ALL represented the evolution of the ordered carbon structure while the combination of I_D3/(I_G + I_D2 + I_D3) reflected the evolution of the amorphous carbon structure of the lignite chars with increasing the treatment temperature from 773 to 1673 K. Reactivity indexes of the demineralized chars reacting with both CO₂ and NO were found to increase with increasing the treatment temperature, implying that the structure ordering did result in the losses of the reactivities. Higher reactivities of the non-demineralized chars indicated the catalytic role of inorganic matter in the reactions with both gases. I_D1/I_G and I_G/I_ALL had good linear correlations with the reactivities particularly of the demineralized chars if considering the structure evolution behaviors at lower and higher temperatures, respectively. I_D3/(I_G + I_D2 + I_D3) was found to have fairly good linear correlations with the reactivity indexes of the lignite chars generated over the whole temperature range.     

Electrochemical and spectroscopic characterization of lithium titanate spinel Li4Ti5O12

Herein we describe electrochemical and spectroscopic properties of lithium titanate spinel as well as an easy method based on colorimetry to determine the lithium content of electrodes containing lithium titanate spinel as active material. Raman microspectrometry measurements have been performed to follow lithium insertion into and extraction from the active material, respectively. The Raman signals display a pronounced fading of intensity already at low levels of lithium intercalation and disappear at a SOC higher than ∼10%. However, the colorimetric method can be used up to a SOC of 50%.     

Hydrothermal transformation of the calcium aluminum oxide hydrates CaAl2O4·10H2O and Ca2Al2O5·8H2O to Ca3Al2(OH)12 investigated by in situ synchrotron X-ray powder diffraction

The hydrothermal transformation of calcium aluminate hydrates were investigated by in situ synchrotron X-ray powder diffraction in the temperature range 25 to 170 °C. This technique allowed the study of the detailed reaction mechanism and identification of intermediate phases. The material CaAl₂O₄·10H₂O converted to Ca₃Al₂(OH)₁₂ and amorphous aluminum hydroxide. Ca₂Al₂O₅·8H₂O transformed via the intermediate phase Ca₄Al₂O₇·13H₂O to Ca₃Al₂(OH)₁₂ and gibbsite, Al(OH)₃. The phase Ca₄Al₂O₇·19H₂O reacted via the same intermediate phase to Ca₃Al₂(OH)₁₂ and mainly amorphous aluminum hydroxide. The powder pattern of the intermediate phase is reported.