| 摘要: | 硼氫化鋰(LiBH4)由於具有相當高的理論氫氣儲存量(18.4 wt%)，而成為一具有潛力的儲氫材料。然而，其起始及主要的脫氫都落在比較高的溫度，因此目前許多的研究皆致力於使用動力學或者熱力學的改質促進其脫氫特性。在本研究中，LiBH4的脫氫特性將藉由添加不同的鈀(Pd)、鈷(Co)化合物，或是將其與氨基化鋰(LiNH2)以球磨混合的方式製成Li-B-N-H的四元複合物加以改質，並以程式溫控還原系統(TPR)及程式溫控脫氫質譜儀(TPD-MS)加以分析樣品的脫氫性質、以X光粉末繞射儀(XRD)來鑑定樣品脫氫反應後的相變化。 由實驗結果可發現，LiBH4樣品的脫氫特性可以透過添加33 wt% Pd-Co/C而改善，其中以添加Pd25Co75/C的樣品能最有效的將脫氫溫度下降至523 K且脫氫量可達10.5 wt%而有較佳的改質結果。此外，從實驗結果亦發現當添加物中的Co含量增加時，樣品的起始脫氫溫度將逐漸下降，而其脫氫含量則有漸漸增加的趨勢。若進而改變Pd-Co/C添加物在樣品中的含量達50 wt%時，則以Pd50改質的系統有較Pd75或Pd25優異的脫氫特性，其起始及主要的脫氫溫度可有效降低至533及639 K，且擁有10 wt%的脫氫量。另外，對於添加33 wt%的Pd-Co/C加以改質的LiBH4+2LiNH2雙元系統，則由於Pd50改質後的樣品其起始脫氫溫度最能夠大幅的從523 K下降至396 K且脫氫量可達9.5 wt%，而具有最佳的脫氫特性。 另一方面，添加Pd及Co金屬氯化物與氫氧化物於LiBH4及雙元系統中，亦可有效的改善其原本脫氫特性。然而，此二種添加物對於系統的改質機制卻不相同。金屬氯化物的添加，會使系統產生一些離子交換反應，進而在反應過程中產生不穩定的金屬硼氫化物，使得脫氫溫度因此而大幅下降；而在添加金屬氫氧化物的系統中，則因為部分的水解以及氧化還原反應而能有效的改善整體的脫氫特性。另一方面，對於雙元系統而言，雖然添加Pd及Co金屬氯化物與氫氧化物同樣能有效的下降氫氣脫附的溫度，但添加金屬氫氧化物系統的脫氫量卻也大幅的降低。因此，在雙元系統中添加金屬氯化物能較添加金屬氫氧化物擁有更好的脫氫特性。 LiBH4 is a potential hydrogen storage material and gains lots of interests recently due to the extremely high hydrogen capacity (18.4 wt%). However, the initial decomposition temperature (Ti) and main dehydrogenation temperature (Tm) of LiBH4 are as high as 567 and 754 K, respectively. In order to overcome the drawbacks, there are several approaches developed to modify the system thermodynamically or kinetically. In this study, LiBH4 is modified by various additives or mixing with LiNH2 to form a new Li-B-N-H quaternary hydride by ball-milling process. Besides, their dehydrogenation properties are analyzed through temperature programmed reduction (TPR) and temperature programmed dehydrogenation-mass spectrometers (TPD-MS), and the phase structures of the systems are characterized by the X-ray powder diffraction (XRD) method. Based on the results, it can be observed that the dehydrogenation properties of the LiBH4 can be successfully improved by doping 33 wt% of Pd-Co/C additives, and among the three different samples, Pd25Co75/C doped sample shows the optimal enhancement in promoting the dehydrogenation properties of LiBH4¬ by reducing the Ti to 523 K with the capacity as 10.5 wt%. Besides, it is found out that when the Co content in the additives increases, the Tis gradually decrease and capacities gently increase. Moreover, for the system modified by various amounts of Pd-Co/C, the results reveal that when the system is modified by 50 wt% of Pd-Co/C, Pd50Co50/C doped sample has better performance than Pd75Co25/C and Pd25Co75/C doped samples, which Ti and Tm can decrease to 533 and 639 K with 10 wt% of hydrogen desorbed. On the other hand, for 33 wt% of Pd-Co/C modified LiBH4+2LiNH2 binary system, the sample doped with Pd50Co50/C shows the effective modification, and the Ti is dramatically reduced from 523 K of the pristine binary system to 396 K and the capacity is 9.5 wt%. In terms of various metal (Pd and Co) chlorides and hydroxides modified LiBH4 and binary systems, the improvement of the dehydrogenation properties can both be observed. However, the reasons of the enhancements by metal chlorides and hydroxides may be different. For LiBH4 systems, the metal chlorides modified samples may have some ion exchange reactions and then form the unstable transition metal borohydrides during the heating process, thus the dehydrogenation properties can be enhanced. However, for metal hydroxides doped samples, the enhancement may be ascribed as the combinational effects of hydrolysis and redox reactions during the decomposition processes. On the other hand, for the metal (Pd and Co) chlorides and hydroxides modified binary systems, although the Tis and Tms can both significantly decrease to lower temperature ranges, the capacities of the samples modified by metal hydroxides also conspicuously reduce. Therefore, metal chlorides modified binary samples shows the better performance in improving the dehydrogenation properties than metal hydroxides. |
