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Speed-up of thermal decomposition of magnesium hydride due to uniaxial pressing, including pressing in presence of nickel catalyst, was studied by barometry, SEM, DSC, XRD methods, and using mathematical modeling. Pressing even with no catalyst is shown to hasten the hydrogen desorption. The most probable reason for this is the formation of multiple defects in crystal lattice. They can serve as nucleation centers: metal nuclei significantly hasten hydrogen desorption. Besides, metal magnesium not converted to the hydride during the synthesis stage can possibly appear at surface. Adding nickel powder before pressing hastens dehydriding process even more. Comparing hydrogen evolution curves for samples with different amount of nickel allowed to propose the probable mechanism of hydrogen evolution. It is based on description of reactions of hydrogen desorption and hydride decomposition and of the metal-hydride phase morphology change due to these reactions. We develop a mathematical model in form of ordinary differential equations that fits the experimental data well. The model is based on conservation and symmetry assumptions. The fitting allowed to evaluate rate parameters of hydride phase decomposition.

We investigated the mechanism of activation of α-AlH 3 powder decomposition by irradiation with ultraviolet (UV) light using barometry, photoluminescence (PL), cathode luminescence (CL) and X-ray diffraction (XRD) methods. Exposing the samples under a mercury lamp through the filters we have shown that only the line, corresponding to the energy of 4.88 eV, leads to activation, whereas the spectral lines of lower energy are ineffective. XRD shows that after continued UV activation the aluminum clusters with metallic properties are formed on the surface of the AlH 3 powder particles at room temperature. The analysis of results of performed calculations of the absorption spectrum of the perfect crystal α-AlH 3 using DFT method showed that hydrogen vacancies are involved in activation by UV light. Studying of dynamics of CL and PL spectra at different UV expositions permitted to suggest the following mechanism of activation. Absorption of a photon implies that vacancy captures an electron from a neighboring hydrogen atom. Hydrogen atom then can leave its regular position in the lattice and form a new vacancy next to the first. Clustering of vacancies occurs. Continued UV activation is accompanied by merging of vacancies, which deprives the aluminum atoms of hydrogen bonds connecting them. Thus the aluminum clusters with metallic properties are formed.