Potassium silanide (KSiH3): a reversible hydrogen storage material

KSi silicide can absorb hydrogen to directly form the ternary KSiH₃ hydride. The full structure of α‐KSiD₃, which has been solved by using neutron powder diffraction (NPD), shows an unusually short Si? D lengths of 1.47 Å. Through a combination of density functional theory (DFT) calculations and experimental methods, the thermodynamic and structural properties of the KSi/α‐KSiH₃ system are determined. This system is able to store 4.3 wt % of hydrogen reversibly within a good P–T window; a 0.1 M Pa hydrogen equilibrium pressure can be obtained at around 414 K. The DFT calculations and the measurements of hydrogen equilibrium pressures at different temperatures give similar values for the dehydrogenation enthalpy (≈23 kJ mol^?1 H₂) and entropy (≈54 J K^?1 mol^?1 H₂). Owing to its relatively high hydrogen storage capacity and its good thermodynamic values, this KSi/α‐KSiH₃ system is a promising candidate for reversible hydrogen storage.     

Chemical equilibria 6. Measurement of equilibrium constants for the dehydrogenation of 2-methylpropan-1-ol by a vapour-flow technique

Equilibrium constants for 2-methylpropan-1-ol + 2-methylpropanal + hydrogen have been calculated from measurements of the composition of mixtures formed by passing the vapour over a catalyst at several temperatures in the range 473 to 563 K. Equations relating the changes in enthalpy and entropy of the dehydrogenation reaction to temperature were derived from the equilibrium constants with the aid of heat capacities. By coupling these changes with other thermodynamic data, the standard enthalpy of formation and the standard entropy of 2-methylpropanal at 298.15 K were calculated to be ?(215.7 ± 1.3) kJ mol^?1 and (331.2 ± 1.7) J K^?1 mol^?1 respectively, in the gas state, and ?(247.3 ± 1.8) kJ mol^?1 and (238.3 ± 4.4) J K^?1 mol^?1 respectively, in the liquid state.     

Enthalpy of Phase Transitions and Heat Capacity of Stoichiometric Compounds in LaBr3-MBr Systems (M=K,Rb, Cs)

Molar enthalpies of solid-solid and solid-liquid phase transitions of the LaBr₃, K₂LaBr₅, Rb₂LaBr₅, Rb₃LaBr₆ and Cs₃LaBr₆ compounds were determined by differential scanning calorimetry. K₂LaBr₅ and Rb₂LaBr₅ exist at ambient temperature and melt congruently at 875 and 864 K, respectively, with corresponding enthalpies of 81.5 and 77.2 kJ mol^-1. Rb₃LaBr₆ and Cs₃LaBr₆ are the only 3:1 compounds existing in the investigated systems. The first one forms from RbBr and Rb₂LaBr₅ at 700 K with an enthalpy of 44.0 kJ mol^-1 and melts congruently at 940 K with an enthalpy of 46.7 kJ mol^-1. The second one exists at room temperature, undergoes a solid-solid phase transition at 725 K with an enthalpy of 9.0 kJ mol^-1 and melts congruently at 1013 K with an enthalpy of 57.6 kJ mol^-1. Two other compounds existing in the CsBr-based systems (Cs₂LaBr₅ and CsLa₂Br₇) decompose peritectically at 765 and 828 K, respectively. The heat capacities of the above compounds in the solid as well as in the liquid phase were determined by differential scanning calorimetry. A special method - 'step method' developed by SETARAM was applied in these measurements. The heat capacity experimental data were fitted by a polynomial temperature dependence. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogen sorption–desorption studies on ZrCo–hydrogen system

The ZrCo–H₂ system was investigated in this study owing to its importance as a suitable candidate material for storage, supply, and recovery of hydrogen isotopes. Desorption hydrogen pressure-composition isotherms were generated at six different temperatures in the range of 524–624 K. A van’t Hoff plot was constructed using the plateau pressure data of each pressure-composition isotherms and the thermodynamic parameters were calculated for the hydrogen desorption reaction of ZrCo hydride. The enthalpy and entropy change for the desorption of hydrogen were found to be 83.7 ± 3.9 kJ mol^?1 H₂ and 122 ± 4 J mol^?1 H₂ K^?1, respectively. Hydrogen absorption kinetics of ZrCo–H₂ system was studied at four different temperatures in the range of 544–603 K and the activation energy for the absorption of hydrogen by ZrCo was found to be 120 ± 5 kJ mol^?1 H₂ by fitting kinetic data into suitable kinetic model equation.     

Investigation of hydrogen desorption from CaSiH by means of calorimetric method

The present work deals with the study of the reaction of hydrogen desorption from the CaSiH_X hydride by means of the calorimetric method. The dehydrogenation of the CaSiH_X hydride was carried out at 548 K. For a calorimetric study, the installation composed of the differential heat-conducting Tian–Calvet type calorimeter connected with a conventional Sieverts-type apparatus was employed. Such installation permitted us to obtain simultaneously the P-X isotherms (P—equilibrium hydrogen pressure, X = H/CaSi) and variation of the partial molar enthalpies of the reaction of hydrogen desorption from CaSiH_X with the hydrogen concentration in the metallic matrix. It was ascertained that in the CaSi-H₂ system there was one region where values of the partial molar enthalpy of the reaction of hydrogen desorption from the CaSiH_X hydride remained constant. This means that formation of one hydride phase in the CaSi-H₂ system took place. The enthalpy and entropy values for the reaction of hydrogen desorption from the CaSiH_X in the plateau range are ΔH _des = 53.8 ± 1.2 kJ mol^?1 H₂ and ΔS _des = 94.2 ± 2.7 J mol^?1 H₂ K^?1 (ΔH _des and ΔS _des—the differential molar enthalpy and entropy desorption, respectively).     

The standard enthalpies of formation of cyclohexylamine and cyclohexylamine hydrochloride

The standard enthalpy of combustion of cyclohexylamine has been measured in an aneroid rotating-bomb calorimeter. The value ΔH_o^o(c-C₆H₁₁NH₂, 1) = ?(4071.3 ± 1.3) kJ mol^?1 yields the standard enthalpy of formation ΔH_f^o(c-C₆H₁₁NH₂, 1) = ?(147.7 ± 1.3) kJ mol^?1. The corresponding gas-phase standard enthalpy of formation for cyclohexylamine is ΔH_f^o(c-C₆H₁₁NH₂, g) = ?(104.9 ± 1.3) kJ mol^?1. The standard enthalpy of formation of cyclohexylamine hydrochloride, ΔH_f^o(c-C₆H₁₁NH₂·HCl, c) = ?(408.2 ± 1.5) kJ mol^?1, was derived by combining the measured enthalpy of solution of the salt in water, literature data, and the ΔH_c^o measured in this study. Comment is made on the thermochemical bond enthalpy H(CN).     

Differential scanning calorimetry of RbH2PO4 and CsH2PO4

The high-temperature phase behaviour of RbH₂PO₄ and CsH₂PO₄ have been studied. RbH₂PO₄ undergoes a single quasi-irreversible phase transition with an enthalpy of 4.665 kJ mol^?1. The transition is found to occur over the temperature range 86–111°C. CsH₂PO₄ undergoes two transitions at 149 and 230°C. The lower one is quasi-irreversible and has an enthalpy of 4.284 kJ mol^?1. The one at 230°C is reversible and has an enthalpy of 1.071 kJ mol^?1.     

Sorption and desorption properties of a CaH2/MgB2/CaF2 reactive hydride composite as potential hydrogen storage material

The hydrogenation behavior of 3CaH₂+4MgB₂+CaF₂ composite was studied by manometric measurements, powder X-ray diffraction, differential scanning calorimetry and attenuated total reflection infrared spectroscopy. The maximum observed quantity of hydrogen loaded in the composite was 7.0 wt%. X-ray diffraction showed the formation of Ca(BH₄)₂ and MgH₂ after hydrogenation. The activation energy for the dehydrogenation reaction was evaluated by DSC measurements and turns out to be 162±15 kJ mol⁻¹ H₂. This value decreases due to cycling to 116±5 kJ mol⁻¹ H₂ for the third dehydrogenation step. A decrease of ca. 25–50 °C in dehydrogenation temperature was observed with cycling. Due to its high capacity and reversibility, this composite is a promising candidate as a potential hydrogen storage material.     

Dynamic changes in composition of extracts of natural products as monitored by in situ NMR

The direct in situ NMR observation and quantification, based on the aldehyde –CH chemical shift region, of the inter‐conversion of secoiridoid derivatives due to temperature and solvent effects is demonstrated in complex extracts of natural products without prior isolation of the individual components. The equilibrium between the aldehyde hydrate form and the dialdehyde form of the oleuropein aglycon of an olive leaf aqueous extract in D₂O was shown to be temperature dependent. The resulting thermodynamic values of the Van't Hoff plot with ΔH^o = ?26.34 ± 1.00 kJ mol^?1 and TΔS° (298 K) = ?24.70 ± 1.00 kJ mol^?1 demonstrate a significant entropy term which nearly compensates the effect of enthalpy at room temperature. The equilibrium between the two diastereomeric hemiacetal forms and the dialdehyde form of the oleuropein 6‐O‐β‐d ‐glucopyranoside aglycon of an olive leaf aqueous extract in CD₃OD was also shown to be strongly temperature dependent again because of the significant entropy term (TΔS° (298 K) = ?26.50 ± 1.39 kJ mol^?1) compared with that of the enthalpy term (ΔH^o = ?36.64 ± 1.46 kJ mol^?1). This is the first demonstration of the significant role of the entropy parameter in determining the equilibrium of chemical transformations in complex mixtures of natural products due to solvent and temperature effects. Copyright © 2014 John Wiley & Sons, Ltd.     

Phase transitions of K<Subscript>2</Subscript>TaF<Subscript>7</Subscript> within 680–800°C

Three thermal effects on heating/cooling of K₂TaF₇ in the temperature interval of 680–800°C were investigated by the DSC method. The values determined for the enthalpy change of the individual processes are: Δ_transIIH_m(K₂TaF₇; 703°C) = 1.7(2) kJ mol⁻¹, Δ_transIH_m(K₂TaF₇; 746°C) = 19(1) kJ mol⁻¹ and Δ_transIIIH_m(K₂TaF₇; 771°C) = 13(1) kJ mol⁻¹. The first thermal effect was attributed to a solid-solid phase transition; the second to the incongruent melting of K₂TaF₇ and the third to mixing of two liquids. These findings are supported by in situ neutron powder diffraction experiments performed in the temperature interval of 654–794°C.      

Thermal studies on metal complexes of 5-nitroso-pyrimidine derivatives

The following Zn(II) complexes of deprotonated 6-amino-5-nitrosouracil (AH), 6-amino-3-methyl-5-nitrosouracil (BH) and 6-amino-1-methyl-5-nitrosouracil (CH) have been prepared and their thermal behaviour studied by TG and DSC techniques: ZnA₂(H₂O)₂, ZnB₂ · 4 H₂O, ZnC₂ · 4 H₂O and ZnC₂(H₂O)₂ · H₂O. The values of the dehydration enthalpy of the complexes are in the 31.3–76.5 kJ mol^?1 H₂O range and, except in the first complex, the dehydration processes take place in several steps. The pyrolysis of the complexes finishes between 540 and 725° C ZnO remaining as residue.     

Mass Spectrometric Investigations of Thermodynamic Properties of the RbCl/GdCl3 System

The vaporization of pure RbCl, GdCl₃, and RbCl‐GdCl₃ samples of different phase compositions was investigated in the temperature range between 666 K and 982 K by use of the Knudsen effusion mass spectrometry. The gaseous species RbCl, Rb₂Cl₂, GdCl₃, and RbGdCl₄ were identified in the equilibrium vapours and their partial pressures were determined. The enthalpy of dissociation of RbGdCl₄(g), Δ_dissH°(859 K) = 263.1 ± 7.7 kJ mol^—1, was evaluated by second law treatment of the equilibrium partial pressures. The thermodynamic activities of RbCl and GdCl₃ were obtained at 800 K in the two‐phase fields {Rb₃GdCl₆(s) + liquid} and {RbGd₂Cl₇(s) + GdCl₃(s)}. The Gibbs free energies of formation of the pseudo‐binary phases Rb₃GdCl₆(s), Δ_fG°(800 K) = —75.1 ± 2.5 kJ mol^—1 and RbGd₂Cl₇(s), Δ_fG°(800 K) = —40.6 ± 1.2 kJ mol^—1, were evaluated from the thermodynamic activities of the components. The results are compared with the available literature data.     

A theoretical study on the effect of intercalating sulfur atom and doping boron atom on the adsorption of hydrogen molecule on (10,0) single-walled carbon nanotubes

Adsorption of molecular hydrogen on single-walled carbon nanotube (SWCNT), sulfur-intercalated SWCNT (S-SWCNT), and boron-doped SWCNT (BSWCNT), have been studied by means of density functional theory (DFT). Two methods KMLYP and local density approximation (LDA) were used to calculate the binding energies. The most stable configuration of H₂ on the surface of pristine SWCNT was found to be on the top of a hexagonal at a distance of 3.54 Å in good agreement with the value of 3.44 Å reported by Han and Lee (Carbon, 2004, 42, 2169). KMLYP binding energies for the most stable configurations in cases of pristine SWCNT, S-SWCNT, and BSWCNT were found to be ?2.2 kJ mol^?1, ?3.5 kJ mol^?1, and ?3.5 kJ mol^?1, respectively, while LDA binding energies were found to be ?8.8 kJ mol^?1, ?9.7 kJ mol^?1, and ?4.1 kJ mol^?1, respectively. Increasing the polarizability of hydrogen molecule due to the presence of sulfur in sulfur intercalated SWCNT caused changes in the character of its bonding to sulfur atom and affected the binding energy. In H₂-BSWCNT system, stronger charge transfer caused stronger interaction between H₂ and BSWCNT to result a higher binding energy relative to the binding energy for H₂-SWCNT.     

Thermodynamics of Hydrogen Adsorption on Metal‐Organic Frameworks

Interaction between adsorbed hydrogen and the coordinatively unsaturated Mg²⁺ and Co²⁺ cationic centres in Mg‐MOF‐74 and Co‐MOF‐74, respectively, was studied by means of variable‐temperature infrared (VTIR) spectroscopy. Perturbation of the H₂ molecule by the cationic adsorbing centre renders the H? H stretching mode IR‐active at 4088 and 4043 cm^?1 for Mg‐MOF‐74 and Co‐MOF‐74, respectively. Simultaneous measurement of integrated IR absorbance and hydrogen equilibrium pressure for spectra taken over the temperature range of 79–95 K allowed standard adsorption enthalpy and entropy to be determined. Mg‐MOF‐74 showed ΔH⁰=?9.4 kJ mol^?1 and ΔS⁰=?120 J mol^?1 K^?1, whereas for Co‐MOF‐74 the corresponding values of ΔH⁰=?11.2 kJ mol^?1 and ΔS⁰=?130 J mol^?1 K^?1 were obtained. The observed positive correlation between standard adsorption enthalpy and entropy is discussed in the broader context of corresponding data for hydrogen adsorption on cation‐exchanged zeolites, with a focus on the resulting implications for hydrogen storage and delivering.     

The crystal polymorphs of metazachlor

Five crystal polymorphs of the herbicide metazachlor (MTZC) were characterized by means of hot stage microscopy, differential scanning calorimetry, IR- and Raman spectroscopy as well as X-ray powder diffractometry. Modification (mod.) I, II and III° can be crystallized from solvents and the melt, respectively, whereas the unstable mod. IV and V crystallize exclusively from the super-cooled melt. Based on the results of thermal analysis and solvent mediated transformation studies, the thermodynamic relationships among the polymorphic phases of metazachlor were evaluated and displayed in a semi-schematic energy/temperature-diagram. At room temperature, mod. III° (T _fus =76°C, Δ_fus H _III =26.6 kJ mol^-1) is the thermodynamically stable form, followed by mod. II (T _fus =80°C, Δ_fus H _II =23.0 kJ mol^-1) and mod. I (T _fus =83°C, Δ_fus H _II=19.7 kJ mol^-1). These forms are enantiotropically related showing thermodynamic transition points at ~55°C (T _{trs, III/II}), ~60°C (T _{trs, III/I}) and ~63°C (T _{trs, II/I}). Thus mod. I is the thermodynamically stable form above 63°C, mod. III° below 55°C and mod. II in a small window between these temperatures. Mod. IV (T _fus =72-74°C, Δ_fus H _II =18.7 kJ mol^-1) and mod. V (T _fus =65°C) are monotropically related to each other as well as to all other forms. The metastable mod. I and II show a high kinetic stability. They crystallize from solvents, and thus these forms can be present in commercial samples. Since metazachlor is used as an aqueous suspension, the use of the metastable forms is not advisable because of a potential transformation to mod. III°. This may result in problematic formulations, due to caking and aggregation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermodynamic properties of diosgenin determined by oxygen-bomb calorimetry and DSC

The combustion enthalpy of diosgenin was determined by oxygen-bomb calorimetry. The standard mole combustion enthalpy and the standard mole formation enthalpy have been calculated to be ?16098.68 and ?528.52 kJ mol^?1, respectively. Fusion enthalpy and melting temperature for diosgenin were also measured to be ?34.43 kJ mol^?1 and 212.33°C, respectively, according to differential scanning calorimetry (DSC) data. These studies can provide useful thermodynamic data for this compound.     

Synthesis and Properties of [Ba(CHZ)(BT)(H2O)2]n as a New Energetic Coordination Compound

In order to enhance the thermal stability of the barium salt of 5,5′‐bistetrazole (H₂BT), carbohydrazide (CHZ) was used to build [Ba(CHZ)(BT)(H₂O)₂]_n as a new energetic coordination compound by using a simple aqueous solution method. It was characterized by FT‐IR spectroscopy, elemental analysis, and single‐crystal X‐ray diffraction. The crystal belongs to the monoclinic P2₁/c space group [a = 8.6827(18) Å, b = 17.945(4) Å, c = 7.2525 Å, β = 94.395(2)°, V = 1126.7(4) ų, and ρ = 2.356 g · cm^–3]. The Ba^II cation is ten‐coordinated with one BT^2–, two shared carbohydrazides, and four shared water molecules. The thermal stabilities were investigated by differential scanning calorimetry (DSC) and thermal gravity analysis (TGA). The dehyration temperature (T_dehydro) is at 187 °C, whereas the decomposition temperature (T_d) is 432 °C. Non‐isothermal reaction kinetics parameters were calculated by Kissinger's method and Ozawa's method to work out E_K = 155.2 kJ · mol^–1, lgA_K = 9.25, and E_O = 158.8 kJ · mol^–1. The values of thermodynamic parameters, the peak temperature (while β → 0) (T_p0 = 674.85 K), the critical temperature of thermal explosion (T_b = 700.5 K), the free energy of activation (ΔG^≠ = 194.6 kJ · mol^–1), the entropy of activation (ΔS^≠ = –66.7 J · mol^–1), and the enthalpy of activation (ΔH^≠ = 149.6 kJ · mol^–1) were obtained. Additionally, the enthalpy of formation was calculated with density functional theory (DFT), obtaining Δ_fH°₂₉₈ ≈ 1962.6 kJ · mol^–1. Finally, the sensitivities toward impact and friction were assessed according to relevant methods. The result indicates the compound as an insensitive energetic material.     

Effects of alkali metal ions on the thermal decomposition of ammonium heptamolybdate tetrahydrate

The thermal decomposition of pure ammonium heptamolybdate tetrahydrate (AHMT), and doped with Li⁺, Na⁺ and K⁺ ions was investigated using thermogravimetry, differential thermal analysis, infrared and X-ray diffraction techniques. Results obtained revealed that the decomposition of AHMT proceeded in three decomposition stages in which both NH₃ and H₂O were released in all stages. The presence of 0.5 mol % alkali metal ions enhances the formation of the intermediateb (NH₄)₂MO₇O₂₂·2H₂O while the decomposition of this intermediate into MoO₃ is slightly affected in the presence of all dopant concentrations used. The infrared absorption spectra of the thermal products of AHMT treated with 10 mol% alkali metal ions (AMI) at 350°C indicated a reduction of some Mo⁶⁺ ions. By heating of AHMT above 500°C in presence of 5 or 10 mol % of AMI, a solid-solid interaction between alkali metal oxides and MoO₃ giving rise to well crystallized alkali metal molybdates. finally the activation energies accompanied various decomposition stages were calculated.     

Thermodynamics of the second-stage dissociation of n-(2-acetamido)iminodiacetic acid in water from 5 to 55°c

The thermodynamic second dissociation constants of the protonated form of N-(2-acetamido)iminodiacetic acid were determined at 12 temperatures from 5–55°C by measurement of the electromotive force using a cell without liquid junction, with hydrogen and silver—silver bromide electrodes. At 25°C, pK₂is 6.844. The standard changes in Gibbs energy, enthalpy, entropy and heat capacity were derived from the change of the pK₂ values with temperature. At 25°C, ΔG° = 9335 cal mol^-1, ΔH° = 2928 cal mol^-1, ΔS^o = -21.5 cal K^-1 mol^-1, and ΔC°_p = -34 cal K^-1 mol^-1. The results are interpreted and compared with those of structurally related compounds.     

Kinetics of D,L–Lactide Polymerization Initiated with Zirconium Acetylacetonate

The kinetic of D,L-lactide polymerization in presence of biocompatible zirconium acetylacetonate initiator was studied by differential scanning calorimetry in isothermal mode at various temperatures and initiator concentrations. The enthalpy of D,L-lactide polymerization measured directly in DSC cell was found to be ΔH=−17.8±1.4 kJ mol⁻¹. Kinetic curves of D,L-lactide polymerization and propagation rate constants were determined for polymerization with zirconium acetylacetonate at concentrations of 250–1000 ppm and temperature of 160–220 °C. Using model or reversible polymerization the following kinetic and thermodynamic parameters were calculated: activation energy E_a=44.51±5.35 kJ mol⁻¹, preexponential constant lnA=15.47±1.38, entropy of polymerization ΔS=−25.14 J mol⁻¹ K⁻¹. The effect of reaction conditions on the molecular weight of poly(D,L-lactide) was shown.