| One of the major obstacles for hydrogen energy application is hydrogen storage tenologies, whose ultimate target is to develop reversible, safe and high H-content solid state hydrogen storage materials. In this thesis, a series of characterization methods, i.e. thermogravimetric analysis (TGA) and mass spectrometry (MS), X-ray diffraction (XRD), solid-state 11B NMR, Fourier transform infrared(FTIR), differential scanning calorimetry (DSC), and density functional theory(DFT) calculation are carried out to investigate the hydrogen release properties and machnism of amine metal cholorides/borohydrides. The main results are as follows:1. As for the MgCl2(NH3)/LiBH4 system, it was found that a new phase, namely, MgCl2(NH3)-LiBH4, to which the following dehydrogenation relates, is formed after ball milling. Judging from the reaction products, it is confirmed that MgCl2 is inclined to work as an ammonia carrier, and the ligand NH3, transferring from MgCl2, is able to combine with the LiBH4 to release H2 with a trace of ammonia at ca.240℃. With the increase of LiBH4 content in the mixture, the emission of ammonia was totally suppressed, and Mg(BH4)2 was produced by the decomposition reaction of MgCl2 with the excessive LiBH4 after the ligand NH3 was exhausted, resulting in an improved dehydrogenation in the whole system. As for the MgCl2(NH3)/NaBH4 system, no new phases are detected by XRD after ball milling. The MgCl2 works as a BH4- acceptor, and the ligand NH3 stays with Mg2+ to combine with the BH4-, which transfers from NaBH4 to Mg2+, resulting in a totally different decomposition route and thermal effects as compared with the MgCl2(NH3)/LiBH4 system. DSC results revealed that the decomposition of MgCl2(NH3)/LiBH4 presented an exothermic reaction with an enthalpy of-3.8 kJ mol-1 H2, while the MgCl2(NH3)/NaBH4 showed two apparent endothermic peaks associated with its two-step dehydrogenation with enthalpies of 8.6 and 2.2 kJ mol-1 H2, respectively. Moreover, the MS profiles of the MgCl2(NH3)/2NaBH4, with excessive BH4-, still released a trace of NH3, indicating that the NaBH4 is not so effective in suppressing the emission of NH3 as LiBH4 did.2. Zn(BH4)2·2NH3, a new ammine metal borohydride, has been synthesized via simply ball-milling a mixture of ZnCl2·2NH3/2LiBH4. Structure analysis shows that the subsequent complex has a monoclinic structure with unit-cell parameters ofα=6.392(4) A, b= 8.417(6) A, c=6.388(4) A andβ= 92.407(4)°and space group P21, in which Zn atoms coordinate with two BH4 groups and two NH3 groups. The interatomic distances reported herein show that Zn-H bonding in Zn(BH4)2·2NH3 is shorter than Ca-H bonds in Ca(BH4)2·2NH3 and Mg-H in Mg(BH4)2·2NH3. This reduced bond contact leads to an increase in the ionic character of H. This study is able to show a good correlation between the reduced M-H distance and enhanced dehydrogenation behavior of the hydride material. Dehydrogenation results showed that this novel compound is able to release 8.9 wt.% hydrogen below 115℃within 10 min without concomitant release of undesirable gases such as. ammonia and/or boranes.Thereby demonstrating the potential of Zn(BH4)2·2NH3 to be used as a solid hydrogen storage.3. Ammonia borane (AB) reversibly absorbs up to at least 6 equiv. NH3 forming the liquid AB(NH3)n (n=1-6)complexes at 0℃.. Reasonable structures for AB(NH3)n were identified via DFT calculations, which indicate that the strong classical hydrogen bond formed between the lone pair of NH3 and the-NH3 of AB is the driving force for the absorption of ammonia by AB. Employing the Van't Hoff equation, the enthalpy change (ΔH) for AB to absorb one NH3 was determined to be-2.24 kcal/mol which is in good agreement with the theoretical calculations. Other organic amines were screened to further confirm the role of the N lone pair; only DABCO formed a stable adduct and X-ray structural analysis showed that it was the DABCO-BH3 species. Finally, raman spectra were collected of AB(NH3)n and unique spectral features were discussed. |
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