Structural Characterization and Hydrogen Desorption Properties of Metal Hydrides in Aluminum System
| Autor: | Wei Lu, 盧威 |
|---|---|
| Rok vydání: | 2011 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 99 In recent years, since the current use of fossil fuels and the demands for energy independence, resulted in the increasing concerns of environmental pollution. Hydrogen is an ideal fuel because of its zero or low pollutant emission during combustion.The most safely and potential method for the absorbing and desobing of hydrogen is to use the metal hydrides. Recently, substantial interest has developed in AlH3 for use as a source of hydrogen since it contains 10.1 wt% hydrogen. Therefore, the main objectives of present study were to develop and investigate the synthesis methods, fine structural characterization, and conditions to hydrogen desorption of metal hydrides using XRD, FE-SEM/EDS, ESCA, SSNMR, HPTGA and XANES/EXAFS techniques. The AlH3 polymorphs were synthesized using the organometallic methods. Because of AlH3 is extremely sensitive to the desolvating conditions, we can do the change of heating time and temperatures to synthesize and recognize the different polymorphs such as γ phase appearing as bundles of fused needles, and the α'' phase appearing as small multiple needles growing from single points to form fuzzy balls. We observed that 100 and 150 oC are the phase transformation temperatures, and it depends on not only temperature height, but also heating time. The NaAlH4 was modified with Ti ions to increase hydrogen desoption to 4.86 wt% and affect the bonding between central metal ion and molecular hydrogen depending on doped deferences. Ti particles would be tiny size by ball milling for 3 h. The structure of hydrides would be different since dissimilar temperatures of hydrogen desorption. When the formal oxidation state of Ti was zero, for instance, in Ti metal, the edge positions were at 4966.7 eV. After ball milling, the position of the Ti edge was being changed. When the desorption temperature was 475 oC, the local structure around Ti did not change significantly compared to that after desorption at 225 oC. However, XRD showed that a crystalline TiAl3 alloy was formed at this temperature. Thus, the TiAl3 clusters had agglomerated to a crystalline TiAl3 phase on going from 225 to 475 oC. In appearance to our research, the hydrogen desorpted experiments are processed between 200 to 250 oC, so there are chances to effect the bonding between Ti-Al and hydrogen. There are not so much different in chemical shifting, because of that parts of metal Ti, base on the results of SSNMR, are similar chemical shifts in 161.0, 70.4, 2.5 and -22.7 for 4 % Ti doping and 157.2, 70.4, 2.5 and -42.1 for 20 % Ti doping. It confirm that the same bonding positions in hydrogens. Therefore, the signal of one hydrogen bond would decrease with greater amounts of Ti doping, the Ti particles would get into the structure of hydrides causing some changes to arrangements of hydrogens. The attracted central ions adding doping metals Ti would make desorption difficult, for this reason, the amounts of doping would be more important than before. By substituting the kinetic equation, we can understand the mechanism of hydrogen desorptive reaction well, and predict that geometry of the growth without complicated instrument. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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