MIL-53金屬-有機架構物之特性鑑定及儲氫機制之研究

Structural Characterization and Hydrogen-storage Mechanism of MIL-53 Metal-Organic Framework

凃牧廷(Mu-Ting Tu)

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.2011.00188

摘要

由於全球暖化及地球石油儲藏量即將告竭，世界各研究單位正在積極開發新型乾淨之氫能源，以代替石化燃料，但因儲存的問題，使得氫氣的應用性受到相當大的限制。金屬有機架構物(MOFs)是將無機金屬與有機配基搭配鍵結而成的多孔性化合物，具有低密度、高熱穩定性、高比表面積等特性，故具有高氫氣儲存的潛力。因此，本研究之主要目的乃探討MOFs之合成方法、微細結構特性鑑定及其儲氫機制，亦藉由XRD、FE-SEM/EDS、TEM、BET、TGA、ESCA、high pressure TGA或XANES/EXAFS等貴重儀器來進一步的分析確認其成長過程及機制。
實驗部份主要包括利用不同的金屬硝酸鹽類作為合成原料，提供之金屬配位，並可以連接相同之有機配基；合成反應溫度範圍在200~220oC之間，並於溶劑狀態下反應。所合成之MOFs稱為MIL-53(Cr)。FE-SEM分析結果顯示，顆粒大小分別為0.5~0.7 μm；最初合成之產物會因含有不純物而不具孔洞性，為了使其產生孔洞性及高比表面積，必須經過高溫煅燒或溶劑清洗之處理程序，以清除孔洞中之有機物雜質而形成孔洞。經過上述處理之MIL-53(Cr)，可量測出其表面積分別為1015 m2/g，並由吸脫附曲線可知MOFs大多為Type Ι，表示孔洞直徑呈中孔和微孔的分布。XRD圖譜亦表示MOFs經過化學處理後具有良好之結晶性；EDS分析指出，MOFs成分中含有C、O及Cr的成份；XRD圖譜亦表示MOFs具有良好之結晶性；FTIR光譜得知MOFs於波長1400~1700 cm-1之C-O官能基及因為水氣造成3000~3500 cm-1處，而有譜線加寬之現象；TGA分析結果顯示MOFs具有較一般之有機化合物優異之熱穩定性，並可達到200~400oC。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MIL-53(Cr)的精細結構，由XANES分析指出MIL-53(Cr)主要為Cr(III)的成份；EXAFS數據結果顯示MIL-53(Cr)第一層之Cr-O鍵結之鍵長為1.96 A，配位數為5.4。另以高壓熱重分析儀測量MIL-53(Cr)在室溫及450 psig (30 atm)下儲氫量分別為0.462 wt%。
為了研究及改進MOFs之儲氫能力，進一步合成金屬/活性碳(metal/AC)混合之MOFs。HR-TEM結果可知，Pt/AC及Pd/AC之顆粒大小分佈分別為2~5及5~10 nm；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 A，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MIL-53(Cr)與5%之Pt/AC、Pd/AC混合，MIL-53(Cr)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0.612 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MIL-53(Cr)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51 kJ/mol。

關鍵字

金屬-有機架構物 ； MIL-53(Cr) ； 儲氫能力 ； 高比表面積 ； 溢流法 ； 熱力學

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. Metal–organic frameworks (MOFs) 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 of present study were to develop and investigate the synthesis methods, fine structural characterization, and capacity of hydrogen-storage mechanism of MOFs using XRD, FE-SEM/EDS, TEM, BET, TGA, ESCA, high pressure TGA, and XANES/EXAFS techniques.
Experimentally, MOFs were synthesized with different metal nitrates in the presence of different solvents combined with organic linkers. The solvothermal method was used to synthesize the MOFs with the reaction temperatures range from 200 to 220oC. These MOFs was named as MIL-53(Cr) having the particle size about 0.5~0.7 μm, respectively identified by FE-SEM microphotos. Since as-synthesized MOFs 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 MIL-53(Cr) was 1015 m2/g. N2 adsorption isotherms of MOFs were type I and the distribution of pore diameter curves revealed that MOFs were microporous and mesopores materials. The XRD patterns represented that MOFs had well crystallinity after chemical treatment. The EDS data indicated that MOFs consist of C or O elements and different kinds of metals. The FTIR spectra exhibited vibrational bands in the usual region of 1400~1700 cm-1 for the carboxylic function and 3000~3500 cm-1 for OH- group of these MOFs. TGA curves showed that these MOFs were stable around 200~400oC. XANES/EXAFS spectroscopy was performed to identify the fine structures of MIL-53(Cr). The XANES spectra indicated that the valence of MIL-53(Cr) was Cr(III). The EXAFS data also revealed that MIL-53(Cr) had a first shell of Cr-O bonding with bond distance of 1.96 A and the coordination number was 5.4. The hydrogen storage capacity of MIL-53(Cr) was 0.462 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 MOFs, metal/activated carbons were mixed with MOFs. 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 A, 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 MIL-53(Cr) modified by 5 wt % of catalyst. The hydrogen adsorption capacity of modified MIL-53(Cr) was significantly enhanced up to 0.612 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 MIL-53(Cr) is < 51 kJ/mol using the secondary spillover of carbon bridges under lower pressures.

並列關鍵字

High Specific Surface Area ； Spillover ； Thermodynamic consistency ； Metal-organic Framework ； MIL-53(Cr) ； Hydrogen Storage Capacity

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