The dissertation is dedicated to the study of the mechanism of nanocomposites decomposition reactions based on magnesium and lithium borohydrides. Infiltration of magnesium borohydride from its solution, as well as testing of the infiltrated samples in terms of thermal decomposition were conducted for the first time. For γ-Mg(BH4)2 infiltrated into mesoporous silica, the possibility of the reversibility of the decomposition reaction at T ~673 K with the value of the hydrogen storage capacity(≥3 wt.% H2) was shown, which, if transferred into pure γ-Mg(BH4)2, is >9% wt.% H2. The thermal decomposition of α-Mg(BH4)2 infiltrated into mesoporous anatase occurred mainly within the region of 300-600 K. Analysis of the residual gases confirmed the release of pure gaseous hydrogen only at T <500 K, and of the mixture of gases (H2, B2H6 and CO) – at T> 500 K. The results of XPS for α-Mg(BH4)2 infiltrated into mesoporous anatase confirm the presence of boron atoms with 1s spin in B2O3 or H3BO3 compounds, as well as in anion [BH4]–. This means that at the depth up to 10 nm, along with magnesium borohydride, products of its chemical interaction with residual amounts of oxygen were present. For the anatase surface with nickel coating Ni and NiO signals were found using the XPS method, which confirms the possibility of oxidation of 20-nm nickel film in the open air. Presence of nickel nanoparticles on the mesoporous anatase surface influenced the kinetics of the decomposition reaction, and also some stages of the process. Using XAS spectroscopy it was proved for the first time that for all Mg(BH4)2–Niadd (Niadd = Ninano; NiCl2; NiF2; Ni3B) composites after their decomposition-resynthesis a new nickel compound was formed, with the local structure that is very similar to amorphous Ni3B. In addition, the results of 11B-NMR spectroscopy show that anion [B3H8]– is the main product of the decomposition of Mg(BH4)2–Niadd composites, and anion [B10H10]2– appeared to be the only intermediate the content of which increases after resynthesis reaction. Analysis of the behavior of cobalt compounds for Mg(BH4)2–Coadd (Coadd = CoF3; Co2B; CoCl2; Co3O4) composites during decomposition-resynthesis cycles using XAS method testified that such Coadd form a new chemical compound of cobalt, which resembles Co2B. A detailed analysis of the decomposition reaction of Mg(BH4)2–Х (Х = TiO2; MoO3 both nanopowders) composites made using DSC, TGA and TPD methods confirmed the essential influence of these oxides additives and their dispersity level on possibility of reduction of the Td value.
Infiltration process for 2LiBH4–MgH2 reactive hydride composite in resorcin formaldehyde or resorcin furfural aerogel was first applied through melt infiltration. Decomposition rate of this nanoconfined composite increase twice compare to the bulk analogue. For the first time acceleration of the hydrogen release rate 10-30 times, and the absorption of hydrogen 5 times for 2LiH–MgB2–0.1Tiadd (Tiadd = TiF4, TiO2, TiN, TiC) composites was experimentally confirmed. The highest influence on the kinetics of the reaction of interaction with gaseous hydrogen was found for 2LiH–MgB2–0.1TiO2 composite, in addition, its value of hydrogen storage capacity remained rather high, ~8.1 wt% H2 during five decomposition-resynthesis cycles. By methods of 1H, 11B and 19F NMR, 11B 3QMAS NMR and CP the existance of [BH4-xFx]– (0x4) tetrahedra in the products of such synthesis was experimentally proven. The increase of elementary cell period with the increase of fluorine content well correlated with the higher number of fluorine atoms in [BH4-xFx]– (0x4) tetrahedra. In practice it was shown that thermal decomposition of the milled 3KBH4–KBF4 mixture occurs at the value of T that is on ≈50 K lower than for pure KBF4 compound and ≈160 K – than for KBH4. It was found that for the triple LiH–LiF–MgB2 composite, the absorption-desorption of gaseous hydrogen is completely reversible with hydrogen storage capacity of ≈7.0 wt.% H2. The mechanism of hydrogen absorption reaction for such composite can be considered as overlapping of two processes, namely, absorption of hydrogen for binary 2LiH–MgB2 and 2LiF–MgB2 composites. Using 19F MAS NMR method for LiH–LiF–MgB2 composite after thermal treatment in hydrogen atmosphere the presence of LiBH4-xFx (0x4) type compounds was clearly confirmed for the first time.
