Ethane 1,2-diamineborane (BH3NH2CH2CH2NH2BH3, EDAB hereafter) samples
have been synthesized by reacting ethylenediamine
dihydrochloride with sodium borohydride in tetrahydrofuran solution.
Structural and bonding properties of EDAB have been characterized
by liquid-state nuclear magnetic resonance, X-ray powder diffraction,
and vibrational spectroscopy (infrared and Raman). The thermolytic
decomposition of EDAB has been investigated by means of combined thermogravimetry,
differential thermal analysis, and mass spectrometry measurements,
both under vacuum and inert gas flow conditions. These experiments
allow the determination of the enthalpies and activation energies
of two hydrogen desorption stages below 520 K as well as the yields
and purity of the released gases. These results show that EDAB presents
a thermal stability,
both under vacuum and under inert gas flow, higher than that of its
parent counterparts methylamine borane (BH3NH2CH3) and ammonia borane (BH3NH3).
Contrary to those compounds, EDAB releases pure hydrogen when heated
under inert flow. In contrast, moderate fractions of diborane, residual
tetrahydrofuran, and volatile B–N–C–H species
are released when conducting the experiments under dynamic vacuum.
In situ temperature-programmed infrared spectroscopy measurements
using synchrotron radiation and operando Raman-mass spectrometry experiments
provide insight into the EDAB thermolysis reaction mechanism.
The occurrence of the structural phase transition of NH3BH3 dispersed in mesoporous silica was studied by anelastic spectroscopy and differential scanning calorimetry. Both measurements indicate that the structural phase transition is suppressed in the sample in which ammonia borane covers only the internal surface of the scaffold. Such a drastic change in the main features of this compound indicates that novel thermodynamic properties can be obtained by means of the fine dispersion of NH3BH3 at a monolayer level.
A systematic study of the dehydrogenation process of undoped and of catalyzed NaAlH4 by means of anelastic spectroscopy is presented. Evidence is reported of the formation of a highly mobile species during decomposition, which has been identified in off-stoichiometric AlH6-x units, giving rise to fast H vacancy local dynamics. The formation of such stoichiometry defects starts at temperatures much lower in Ti doped than in undoped samples, and concomitantly with the decomposition reaction. The catalyst atoms decrease the energy barrier to be overcome by H to break the bond, thus enhancing the kinetics of the chemical reactions and decreasing the temperature at which the dehydrogenation processes take place. The experimental data show that not all the hydrogen released by the formula units during the evolution of decomposition evolves out of the sample, but part of it remains in the lattice and migrates on a long-range scale within the sample. We identify, in this H mobilized population, the species which induces the fast tetragonal to monoclinic phase transformation accompanying decomposition. A partial spontaneous thermally activated regression of decomposition has also been observed by aging experiments. A model is proposed which accounts for the action of the Ti catalyst and for the atomistic mechanism of decomposition.
We report the first measurements of elastic modulus and energy dissipation in Ti-doped and undoped sodium aluminum hydride. It is shown that the chemical reactions that occur by varying the sample temperatures or by aging most sensitively affect the elastic constants, such that the modulus variations allow the time and temperature evolution of decomposition to be monitored. After a well-defined thermal treatment at 436 K, a thermally activated relaxation process appears at 70 K in the kilohertz range, denoting the existence of a new species, likely involving hydrogen, having a very high mobility, that is, 10(3) jumps/s at the peak temperature corresponding to a relaxation rate of about 10(11) s(-1) at room temperature. The activation energy of the process is 0.126 eV and the preexponential factor 7 x 10(-14) s, which is typical of point defect relaxation. The peak is very broad with respect to a single Debye process, indicating strong interaction or/and multiple jumping type of the mobile entity. The present data suggest that the models aiming at interpreting the decomposition reactions and kinetics should take into account the indicated point-defect dynamics and stoichiometry defects.
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