鋁系金屬氫化物之特性鑑定及其氫氣脫附特性之研究

Structural Characterization and Hydrogen Desorption Properties of Metal Hydrides in Aluminum System

盧威(Wei Lu)

指導教授 ： 林錕松

元智大學/工程學院/化學工程與材料科學學系/碩士(2011年)

https://doi.org/10.6838/YZU.2011.00168

摘要

近年來伴隨著溫室效應、環保意識抬頭及原油蘊藏量的潛在危機，零溫室氣體排放的氫能源相關技術發展與研究早已變得刻不容緩。其中以金屬氫化物做為氫氣能源的吸附和釋放，為目前最安全和最有潛力的方法。金屬氫化物中的氫化鋁作為一種理論儲氫量達10.1 wt.%的儲氫材料一直以來受到矚目，本研究便是以此為目標，並佐以市售之氫化物，期望以金屬摻雜方式改善其放氫條件，符合未來之應用，亦藉由XRD、FE-SEM/EDS、ESCA、SSNMR、HPTGA或XANES/EXAFS等貴重儀器來進一步的分析確認其變化。
本研究於氬氣環境下製備反應物與醚類之混和溶液，經沉澱法反應數分鐘，後使用抽氣過濾純化，再以真空烘箱80 °C、數小時來去除反應用之醚類溶劑，最後，得所求之氫化鋁(Aluminum hydride, alane)；並藉由其對溫度敏感特性，以加溫方式得到其過渡態結構，標明針狀的γ態與其團聚狀的α'態，實驗發現100至140度有其晶相轉變溫度(Phase transformation temp.)，晶相的轉變不只與溫度影響有關外並與加熱時間相關。另外， NaAlH4利用金屬 Ti的摻雜可提升有效放氫量，使放氫量提升至4.86 wt%左右，Ti粒子因為經過長時間(3 h)球磨而形成細小的摻雜，其與氫化物之結構會因為分解放氫的溫度變化而有所不同，Ti的吸收邊緣能量也會因為其氧化態的產生而受到影響，當氧化態尚未生成時，此一時間，Ti的邊緣能量為4966.7 eV，然而在進行球磨之後，氧化態也隨之形成，能量範圍也有所偏移，伴隨放氫溫度的上升，文獻中也提到達225 oC時， XRD分析中的Al訊號峰會有所偏移，那是因為Ti-Al合金的出現造成，而在225至475 oC一溫度範圍內，Ti會從Al表面逐漸往內部移動，但在此溫度區間內，即便是不同的放氫溫度，Ti的周遭鍵結是不會像低溫時有那麼大的影響。本實驗進行多是以200-250 oC進行氫氣的脫附實驗，故有機會存在Ti-Al與氫的鍵結影響，其中的部分金屬Ti，隨著摻雜比例逐漸提高，觀察SSNMR結果，其化學位移為摻雜4 %時的161.0、70.4、2.5和-22.7，與達到較大摻雜比例時20 % 的157.2、70.4、2.5和-42.1，偏移位置並無明顯不同，確認為同樣的H鍵結位置，另外，其中一H鍵訊號，伴隨摻雜比例上升，而在強度上有所減弱，推知可能是Ti金屬進入氫化物結構中，與之造成H原子排列產生些微改變，而與之吸引的中心金屬加上了來自Ti的額外吸引力，使放氫變得不易，因此，摻雜量的多寡便變得相當重要。此外，導入動力學方程式，可以讓我們有效了解放氫反應，其過程中所行反應之相位，而不必藉由複雜儀器的量測。

關鍵字

金屬氫化物 ； 儲氫量 ； 氫化鋁 ； 晶相轉變

並列摘要

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.

並列關鍵字

Metal hydride ； hydrogen storage ； aluminum hydride ； phase transformation

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