- 學位論文
交織網狀金屬有機架構物之合成、特性鑑定及其儲氫能力之研究
Preparation, Characterization, and Hydrogen Storage Capacity of Isoreticular Metal-organic Frameworks
若您是本文的作者，可授權文章由華藝線上圖書館中協助推廣。
摘要
因氫氣已被證實為理想的攜能元素，應用在燃料電池深具潛力，但關鍵技術在儲氫的密度。優良儲氫材料應具備儲氫量大、重量輕、容易活化、吸放氫之溫度與壓力適當、吸放氫反應速率快、使用壽命長及成本低廉等優點。為達成此目標，安全的儲放氫氣變成為一個必要的標準。因此本論文的研究課題即是以微孔的金屬-有機架構物，又稱為無機-有機共價化合物(MOFs)當作吸附材，藉由鍵結力強的金屬-氧-碳原子形成之化合物具有高孔洞性、高比表面積、熱穩定性佳及氣體儲存的能力等特性之高性能特殊材料，提供更加適合的氫氣存放點，並開發新穎的IRMOFs儲氫材料，IRMOFs之合成方法、精細結構或特性及其儲氫能力，更進一步以XRD、FE-SEM、FTIR、TGA技術來分析鑑定。 實驗部份主要包括利用金屬(Zn)硝酸鹽類作為合成原料，提供之金屬配位，並可以連接相同或不同之有機配基；合成反應溫度範圍在100~120oC之間，並於溶劑狀態下反應。所合成之IRMOFs稱為IRMOF-1及IRMOF-8。FE-SEM分析結果顯示，顆粒大小分別為20~50 μm及5~8 μm；最初合成之產物會具有許多不純物質而不具孔洞性，為使其產生孔洞性及高比表面積，必須經過高溫煆燒或溶劑清洗之處理程式，可以清除孔洞中之有機物雜質而使孔洞顯現出來，IRMOF系列必須利用丙酮或其他有機溶劑將孔洞內之難揮發有機溶劑進行交換，再經由Ar帶走及在真空烘乾下去除後生成，才具有孔洞性。經過適當處理之IRMOF-1及IRMOF-8，比表面積分別為2,273及920 m2/g，從吸/脫附曲線判斷為Type I和Type Ⅳ，孔徑分佈圖顯示IRMOFs具有微孔材料之特性。當其如果再空氣中暴露過久會與空氣中的水氣吸收而造成孔洞容易被阻塞而造成比表面積下降。EDS分析指出，IRMOFs成分中含有C、O以及不同金屬的成份；XRD圖譜亦表示IRMOFs具有良好之結晶性；FTIR光譜得知MOFs於波長1,400~1,700 cm-1之C-O官能基及因為水氣造成3,000~3,500 cm-1處，而有譜線加寬之現象；TGA分析結果顯示IRMOFs具有較一般之有機化合物優異之熱穩定性，並可達到300~400oC。此外，亦利用X光吸收邊緣結構光譜(XANES)及延伸X光吸收細微結構光譜(EXAFS)，來進一步分析IRMOF-1和IRMOF-8的精細結構，由XANES分析指出IRMOF-1與IRMOF-8主要為Zn(II)的成份；EXAFS數據結果顯示IRMOF-1第一層之Zn-O鍵結之鍵長為1.94 Å，配位數為3.5。另以高壓熱重分析儀測量IRMOF-1和IRMOF-8在室溫及450 psig (30 atm)下儲氫量分別為0.09和0.145wt%。 為了研究及改進IRMOFs之儲氫能力，進一步合成金屬/活性碳(metal/AC)混合之IRMOFs。BET表面積分析結果得知，AC、acid-treatment AC、Pt/AC及Pd/AC表面積分別為1,039、1,108、739及882 m2/g，其中acid-treatment AC表面積最高。FE-SEM結果可知，Pt/AC及Pd/AC之顆粒大小分佈分別為2~3及5~10 nm；EDS亦顯示材料中具有Pt及Pd之成份。此外，亦利用XPS及XANES分析，價數近似零價元素Pt及Pd；EXAFS分析證實出Pt/AC及Pd/AC之第一層鍵結Pt-Pt及Pd-Pd鍵長分別為2.78及2.75 Å，因與周圍8個原子鍵結成面心立方結構，故其配位數為8。氫氣溢流法探討IRMOF-1和IRMOF-8與5%之Pt/AC、Pd/AC混合，IRMOF-1和IRMOF-8表面經由碳鍵結形成的碳橋在室溫及450 psig (30 atm)下的儲氫量可提升至0.23 wt%與0.36 wt%。此外，本研究亦以熱力學推導印證熱力學一致性，吸附熱其主要會影響低壓時的吸附行為，吸附熱的大小會從低吸附量慢慢往高吸附量遞減，可以發現IRMOF-1和IRMOF-8表面經由碳鍵結形成碳橋之吸附熱在低壓時吸附量可達到6.29 kJ/mol與8.4 kJ/mol。
並列摘要
Both from the point of view of global warming and from that of the inevitable exhaustion of Earth’s oil reserve, worldwide interest is focused on using a clean burning substitute such as hydrogen in place of fossil fuels. However the storage of hydrogen is one of the most important challenges impeding its practical application. Isoreticular metal–organic frameworks (IRMOFs) are a new emerging class of crystalline porous materials, displaying very low density, significant thermal stability, and very high surface area. They offer significant opportunities for hydrogen storage. Therefore, the main objectives were to develop and investigate the synthesis methods, fine structural characterization, and capacity of hydrogen storage of IRMOFs using XRD, FE-SEM/EDS, TEM, BET, TGA, ESCA, and XANES/EXAFS techniques. Experimentally, IRMOFs were synthesized with zinc nitrates in the presence of different solvents combined with organic linkers. Followed by refluxing the solution method was used to synthesize the IRMOFs with the reaction temperatures range from 100 to 120oC. These IRMOFs were named as IRMOF-1 and IRMOF-8 having the particle size about 20~50 and 5~8 μm, respectively identified by FE-SEM microphotos. Since as-synthesized IRMOFs contain many impurities, it may cause low porosity. Therefore the cleaning methods, such as optimum calcination temperatures or washing several times with different solvents at different warm temperatures were effective and approved to improve higher specific surface area and porosity. The specific surface area of IRMOF-1 and IRMOF-8 were 2,273 and 920 m2/g, respectively. N2 adsorption isotherms of IRMOFs were type I and typeⅣ. The distribution of pore diameter curves revealed that IRMOFs were microporous materials. The XRD patterns represented that IRMOFs had well crystallinity after chemical treatment. EDS data indicated that IRMOFs consist of C, O elements and different kinds of metals. FTIR spectra exhibited vibrational bands in the usual region of 1,400~1,700 cm-1 for the carboxylic function and 3,000~3,500 cm-1 for OH- group of these IRMOFs. TGA curves showed that these IRMOFs were stable around 300~400oC. XANES/EXAFS spectroscopy was performed to identify the fine structures of IRMOF-1 and IRMOF-8. The XANES spectra indicated that the valence of IRMOF-1 and IRMOF-8 was Zn(II). The EXAFS data also revealed that IRMOF-1 had a first shell of Zn-O bonding with bond distance of 1.94 Å and the coordination number was 3.5. The hydrogen storage capacity of IRMOF-1 and IRMOF-8 were 0.09 and 0.145 wt%, respectively at 450 psig (30 atm) and room temperature measured using high-pressure thermogravimetric analyzer. In order to improve the hydrogen storage capacity of IRMOFs, metal/activated carbons were mixed with IRMOFs. FE-SEM microphotos of Pt/AC and Pd/AC indicated that the particle sizes were 2~5 and 5~10 nm, respectively. By using XPS and XANES spectra, it had found that both Pt and Pd species had zero valency. The EXAFS data revealed that Pt/AC and Pd/AC have a first shell of Pt-Pt and Pd-Pd bonding with bond distances of 2.78 and 2.75 Å, respectively. Coordination numbers of both nanoparticles were close to 8 with a FCC structure. The catalytic properties of Pt/AC and Pd/AC were studied for hydrogen spillover in IRMOF-1 and IRMOF-8 modified by 5 wt % of catalyst. The hydrogen adsorption capacity of modified IRMOF-1 and IRMOF-8 was significantly enhanced up to 0.23 and 0.36 wt% by using the secondary spillover by carbon bridges measured at 450 psig and room temperature. In addition, the adsorption thermodynamic of the data was also confirmed using thermodynamic equations for thermodynamic consistency. Under lower pressures, the adsorption heat is affected by adsorption behaviors. The adsorption heats decrease of increasing adsorption capacities. The adsorption heat of hydrogen onto modified IRMOF-1 and IRMOF-8 is 6.29 and 8.4 kJ/mol using the secondary spillover of carbon bridges under lower pressures.
參考文獻
1.Carter T. J. and Cornish L. A., Hydrogen in metals, Eng. fail. anal., 8(2), 113-121 (2001).
2.Ahmed S., Kumar R., and Krumpelt M., Fuel processing for fuel cell power systems, Fuel Cells Bulletin, 2(12), 4-7 (1999).
3.Babbir F. and Gómez T., Efficiency and economics of proton exchange membrane (PEM) fuel cells, Int. J. Hydrog. Energy, 22(10-11), 1027-1037 (1997).
7.Kalal P. J., Portable fuel cell system, U. S. Patent, 20040086755 (2004).
11.Rosi N. L., Eckert J., Eddaoudi M., Vodak D. T., Kim J., O’Keeffe M., and Yaghi O. M., Hydrogen storage in microporous metal-organic frameworks, Science, 300(5622), 1127-1129 (2003).