| 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高達560及750 K，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方法促進其脫氫特性。在本研究中，LiBH4的脫氫特性將藉由添加不同的金屬(鈰(Ce)、鈷(Co)、錫(Sn))氯化物、氧化物以及碳支撐的金屬氧化物加以改質，而自行製備的碳支撐金屬氧化物用X光粉末繞射儀(XRD)和熱重分析儀(TGA)來鑑定其結構組成和金屬氧化物的承載量，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光近緣結構(XANES)來鑑定添加物脫氫前後之金屬氧化態的變化。由實驗結果可發現，LiBH4樣品的脫氫特性可以透過參雜33 wt%的各種添加物加以改善，其中含有金屬Co的添加物最能有效的降低LiBH4的動力學障礙，無論其為氯化物或氧化物的狀態。以添加氯化鈷和氧化鈷的樣品之主要脫氫溫度下降至564 和468 K且脫氫量分別可達14.5和15.5 wt%而有較佳的改質結果。添加金屬氯化物與氧化物於LiBH4中，確實能有效的改善其原本脫氫特性。其中金屬氯化物的添加，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，使得脫氫溫度能夠下降；而在添加金屬氧化物的系統中，則因為氧化還原反應而能有效的改善整體的脫氫特性。另一方面，對於碳支撐的金屬氧化物添加，除了碳支撐的氧化鈰樣品之外，其餘的樣品的起始脫氫溫度都能大幅的下降，但也造成脫氫量的降低，而其改質機制推測是LiBH4和金屬氧化物在球磨的過程中會透過碳材來增加其接觸表面積而形成一種複合材料使得脫氫溫度降低。此外，XANES圖譜結果顯示脫氫過程中金屬氯化物和LiBH4之間發生還原反應，從而促進了脫氫反應。LiBH4 is a potential hydrogen storage material and gains lots of interests recently due to its extremely high hydrogen capacity (18.4 wt%). However, the initial decomposition (Ti) and main dehydrogenation temperatures (Tm) of LiBH4 are as high as 560 and 750 K, respectively. In order to overcome the drawbacks, several approaches have been developed to modify the system thermodynamically or kinetically. In this study, the effects of metal chlorides (CeCl3, CoCl2 and SnCl2), metal oxides (CeO2, CoO and SnO2), and carbon supported metal oxides (CeO2/C, CoO/C and SnO2/C) additions on promoting the dehydrogenation properties of LiBH4 have been investigated. The structures and metal oxides loadings of as-prepared CeO2/C, CoO/C and SnO2/C additives are measured by X-ray diffraction (XRD) and thermal gravimetric analysis (TGA), respectively. Furthermore, the dehydrogenation behavior of the fresh and modified LiBH4 is analyzed by temperature programmed reduction (TPR) and temperature programmed desorption–mass spectrometer (TPD-MS). Besides, X-ray absorption near-edge structure (XANES) is applied to detect the oxidation states of additives before and after dehydrogenation.Based on the results, it can be observed that the dehydrogenation properties of the LiBH4 can be successfully improved by doping 33 wt% of metal chlorides, metal oxides and carbon supported metal oxides. Among the three different systems, the metal Co shows the best performance in reducing the reaction kinetic barrier of LiBH4 whether it is in chloride or oxide forms. CoCl2 and CoO doped samples have the Tm to 564 and 486 K with the capacity as 14.5 and 15.5 wt%, respectively. In terms of various metal (Ce, Co and Sn) chlorides and oxides modified LiBH4, the improvement of the dehydrogenation properties can both be observed. For the metal chlorides modified samples, the enhancement may be due to some ion exchange reactions and then formation of the unstable transition metal borohydrides during the heating process. On the other hand, for metal oxides doped samples, the promotion may be ascribed as the effects of reduction reactions during the decomposition processes. In terms of the LiBH4 modified by carbon supported metal oxides, although the Ti can dramatically reduce except LiBH4/CeO2(C)-2, their capacities also conspicuously reduce. It is speculated that the promotion effect is owing to the increased contact area and formation of composite material between metal oxides and LiBH4 during the ball-milling process. Moreover, the XANES results show the reduction reaction between metal chlorides and LiBH4 during the dehydrogenation process occurs, which promotes the dehydrogenation reaction. |
