Hydrido/chloroaluminium alkoxides and metal (Al, Ge) alkoxides-synthesis, characterization and applications for preparation of novel hydrogen storage nano-materials

Abstract

In first part of this work, efforts are made to prepare structurally characterized molecular compounds of the general formulae [HnAl(OR)3-n]x (n = 2 {1, 2, 5}, 1 {3, 6} or 0 {4, 7}; x = 2 {5, 6 7}, 4 {4}, 5 {1} or ∞ {2, 3}; R = cHex {1-4} or cHexMe 1 {5-7}), [ClAl(H)(OR)]2 (R = cHexMe 1 {8}), [ClnAl(OR)3-n]2 (n = 1 {9} or 2 {10}; R = cHexMe 1 {9-10}), [Ge(OR)2]2 (R = cHex {11} or cHexMe 1 {12}, iPr {13}); [HAl(OR)(NR')]2 (R = tBu {14}, cHexMe 1 {15; R' = SiMe3 {14-15}). The molecular compounds 1, 2, 4, 5, 7, 8 - 12, 14 and 15 are obtained as crystalline powders and characterized by single crystal X-ray analysis as well as 1H, 13C NMR and IR spectroscopy. Crystals suitable for X-ray analysis could not be grown for compound 3, 6 and 13. However, these compounds were characterized by NMR and IR spectroscopy. Molecular compounds containing Al as central metal atoms were also characterized by 27Al NMR spectroscopy. The elemental compositions of all compounds were determined by CHN analysis as well as by other complexometric titration techniques wherever suitable. In the second part of this thesis, the results of hydrogen adsorption studies with Al/Al2O3-composite nano-wires (NWs) and Ni/Al2O3-composite nano-powders (NPs) using a modified Sievert apparatus are presented. Al/Al2O3-composites were prepared as thin films by the decomposition of a molecular precursor, [H2Al(OR)]2, both upon steel as well as glass substrates in a cold wall CVD reactor. Ni/Al2O3-composites were prepared as powders by the decomposition of Ni(acac)2 under a steady stream of [H2Al(OR)]2 in the same CVD set up, using an adapted sample holder. Both types of materials, Al/Al2O3 NW and Ni/Al2O3 NP, were characterized by XRD, SEM, TEM, EDXS, SAED and IR spectroscopy. Volumetric hydrogen storage measurements with variable temperature and pressure showed a maximum hydrogen uptake of 6.5(±0.2)wt.% with Al/Al2O3-composite NWs. The adsorption kinetics was followed by DSC and IR spectroscopy. The adsorption of hydrogen at the Al/Al2O3-composite NWs is controlled by pure physisorption at temperatures below 100°C while above 100°C, desorption occurs as a result of increasing temperature.