多元吸氫奈米複合材料合成、鑑定及其儲氫性質測試之研究

Synthesis and Characterization of Multi-Nanocomposites for Hydrogen Storage and Their Enhancements of Hydrogen Adesrption Capacities

羅立誠(Li-Cheng Luo)

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(2006年)

https://doi.org/10.6838/YZU.2006.00245

摘要

由於奈米碳管(CNTs)之特殊結構，使其在儲氫特性上有優異的表現，本實驗的主要目的在於摻雜不同儲氫金屬於奈米碳管與多種吸氫材料中，來探討其對於提升儲氫性能的影響。本實驗中使用的奈米碳管有四種，分別為以溶劑熱合成法搭配鈉與鉀為還原劑所製備之兩種奈米碳管(MWCNT-K及MWCNT-Na)、噴霧熱解法(MWCNT-SP)及電弧放電法(MWCNT-AD)所合成出來之奈米碳管；經場發掃描式電子顯微鏡(FE-SEM)及穿透式電子顯微鏡(TEM)測試結果為以鉀為還原劑所合成之MWCNTs-K管徑約為170 nm，管壁為多層石墨化結構，合成條件為285℃反應24 h；以鈉為還原劑所合成之MWCNTs-Na管徑約為130 nm，管壁為非晶質碳(amorphous carbon)所構成，合成條件為285℃反應18 h，並可得到碳雜質較少且純度較高之MWCNTs；另外再輔以X光粉末繞射儀(XRPD)與拉曼光譜(Raman spectroscopy)分析來驗證各種奈米碳管之反應機構及碳管結構之結晶性，結果顯示以MWCNT-AD＞MWCNT-SP＞MWCNT-Na＞MWCNT-K。
在本實驗中所使用之提升儲氫之材料可分為七大類，分別為鋁氫化鈉(NaAlH4)、氫化鎂(MgH2)、奈米碳管(carbon nanotube)、奈米鈦管(TiO2 nanotube)、沸石(zeolites)、金屬有機架構(metal-organic frameworks, MOFs)與氣凝膠碳材(carbon aerogel)等；在NaAlH4方面，先加入Ti來提升NaAlH4的吸放能力，再與奈米碳管做結合；在MgH2方面，則是先將Mg氫化成MgH2後，再加入Ti來提升MgH2的吸放氫能力後，再與奈米碳管做結合；而在其他四大類方面(MOF除外)，我們皆以化學還原法將鈀(Pd)摻雜於其中，而MOF則以研磨法將鈀與活性碳(Ac)加入其中後，再對其做TEM、FE-SEM/EDS、XRPD、Raman spectra、FT-IR、BET isotherm、TA-MS、ESCA、XANES與EXAFS等特性分析與儲氫量之測量；在本實驗中所使用之儲氫系統為程溫還原系統(TPR)，利用鈀在相同壓力下，低溫時之吸氫量高於高溫時之吸氫量之特性來進行吸/脫附氫之實驗，在所有的儲氫材料中，儲氫量以奈米碳管最佳，依序如下：CNT-AD-Pd (1.263 wt%)＞CNT-SP-Pd (1.135 wt%)＞Cu-MOF-Ac-Pd (1.038 wt%)＞Cu-MOF-Pd (0.739 wt%)＞carbon aerogel-A (0.465 wt%)＞carbon aerogel-B (0.209 wt%)，而鋁氫化鈉、氫化鎂、奈米鈦管、沸石皆小於0.10 wt%，效果不佳；其中奈米碳管雖然比表面積較低，但其本身具有吸氫官能基，故對於氫氣有極佳之吸附效果，再輔以金屬鈀之幫助，對於儲氫能力之提昇有極佳的效果；MOFs與carbon aerogel則除具有極高之比表面積與接近微孔級(d < 20 Å)之孔洞，且對於氫氣亦有極佳之吸附性質，並添加金屬鈀，故對於儲氫能力具有良好之提昇效果；而奈米鈦管在儲氫時會形成一種TiO2．xH2之化合物，但在0.1 atm下其中所能含之氫氣量極低，因此造成儲氫效果較低之結果；而沸石為高矽鋁比之中型孔洞材料，含金屬陽離子較少，故儲氫能力較低。

關鍵字

儲氫 ； 奈米碳管 ； 合金 ； 金屬氫化物 ； 奈米鈀 ； 奈米鈦管 ； 沸石 ； 金屬有機架構 ； 氣凝膠碳材 ； 吸附

並列摘要

The main objectives of this study were to investigate the influence of hydrogen storage properties when the hydrogen storage metals mixed with CNTs and other different hydrogen storage materials. There are four different types of multiwall CNTs (MWCNTs) used in this study, such as MWCNT-K, MWCNT-Na, MWCNT-SP, and MWCNT-AD. MWCNT-K and MWCNT-Na produced from the catalytic-assembly benzene-thermal routes to MWCNTs by reduction of hexachlorobenzene by metallic K or Na in the presence of Co/Ni catalyst precursors at moderate temperatures of 503-623 K, MWCNT-SP produced from spray pyrolysis method (SP), and MWCNT-AD produced from arc discharege method (AD). The MWCNTs of well-graphited walls were obtained with reductive K metals of catalytic hexachlorobenzene-thermal routes at 558 K for 24 h. Similarly, the amorphous MWCNTs with fewer impurities were also formed from the reductive Na metals of hexachlorobenzene-thermal catalytic pathways at lower temperature of 558 K for 18 h. The diameter of MWCNT-K and MWCNT-Na is 170 and 130 nm respectively measured by FE-SEM and TEM images. In addition, the fine structures and crystalline properties of MWCNTs were further identified by XRPD and Raman spectra. The result shows that the comparison of stronger well-graphited structures is MWCNT-AD > MWCNT-SP > MWCNT-Na > MWCNT-K in series.
Experimentally, seven kinds of materials were used to enhance the hydrogen storage properties, such as sodium Aluminium hydride (NaAlH4), magnesium hydride (MgH2), CNTs, titanium dioxide nanotube (TiO2 nanotube), zeolites, metal-organic frameworks, and carbon aerogel. In NaAlH4/CNTs, doped Ti species can enhance the adsorption/desorption capacities. Similarly, magnesium species in MgH2/CNTs were been hydrogenated to magnesium hydride and then were doped Ti species to enhance the adsorption/desorption abilities. Furthermore, other four kinds of adsorptive materials (excluding MOFs), palladium species were mixed with the adsorptive materials by a chemical reduction method. MOFs were also mixed with palladium and activated carbon by physical grinding method. In order to appraise their characteristics and hydrogen storage capacities, the materials were identified by TEM, FE-SEM/EDS, XRPD, Raman spectra, FTIR, BET isotherm, TGA/MS, XPS, XANES/EXAFS, and temperature program reducrtion system (TRP). Utilization of the hydrogen storage characteristics of Pd species in low temperatures more than high temperatures at the same pressure, the hydrogen storage capacities in TPR system were calculated. In all adsorptive materials, CNTs have the best hydrogen storage capacity and the others are compared as following: CNT-AD-Pd (1.263 wt%) > CNT-SP-Pd (1.135 wt%) > Cu-MOF-Ac-Pd (1.038 wt%) > Cu-MOF-Pd (0.739 wt%) > Carbon aerogel-A (0.465 wt%) > Carbon aerogel-B (0.209 wt%) in series, but there are no effect by adding NaAlH4, MgH2, TiO2 nanotube, zeolite species. Although the low specific surface area of the CNTs, but they have excellecnt physical absorption properties for hydrogen storage and been enhanced by doping Pd species. MOFs and carbon aerogel both have high specific surface area, nearly microporous (d < 20Å) structures, excellecnt physical absorption property for hydrogen, and been enhanced by doping Pd species. TiO2 nanotubes were produced clusters of TiO2．xH2, where x ≦ 1.5, and x is very small causes the less hydrogen storage capacity of TiO2 nanotubes at 0.1 atm. In addition, the mesopore structure zeolites have fewer cations in framework, lower hydrogen storage capacities than other materials were observed.

並列關鍵字

Hydrogen storage ； Carbon nanotubes ； Adsorption ； Metal hydride ； Palladium ； Titanium dioxide nanotube ； Zeolites ； Metal-organic frameworks ； Carbon aerogel

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