| Summary: | 碩士 === 元智大學 === 化學工程與材料科學學系 === 100 === 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. 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 were to develop and investigate the synthesis methods, fine structural characterization, and capacity of hydrogen storage of MOFs using XRD, FE-SEM, TGA, BET, FTIR, 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 120 to 170℃. These MOFs were named as Ni2(BDC)2dabco, Cu2(BDC)2dabco, Co2(BDC)2dabco, and Zn2(BDC)2dabco having the particle size about 5~10, 3~9, 10~30, and 4~8 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 M2(BDC)2dabco, M2(BDC)2dabco, M2(BDC)2dabco, and M2(BDC)2dabco were 1,415, 923, 1,200, and 1,275 m2/g, respectively. N2 adsorption isotherms of M2(BDC)2dabco were type I and the distribution of pore diameter curves revealed that M2(BDC)2dabco were microporous and mesopores materials. The XRD patterns represented that M2(BDC)2dabco had well crystallinity. FTIR spectra exhibited vibrational bands in the usual region of 3,000~3,500 cm-1 for OH- group of these M2(BDC)2dabco. TGA curves showed that these M2(BDC)2dabco were stable around 200~400℃.
XANES/EXAFS spectroscopy was performed to identify the fine structures of Co2(BDC)2dabco and Zn2(BDC)2dabco. The XANES spectra indicated that the valencies of Co2(BDC)2dabco and Zn2(BDC)2dabco were Co(II) and Zn(II), respectively. The EXAFS data also revealed that Co2(BDC)2dabco and Zn2(BDC)2dabco have a first shell of Co-O and Zn-O bonding with bond distances of 2.03 and 2.01 Å, respectively. The coordination numbers of Co2(BDC)2dabco and Zn2(BDC)2dabco were 4 and 3, respectively. The hydrogen storage capacity of Ni2(BDC)2dabco, Cu2(BDC)2dabco, Co2(BDC)2dabco, and Zn2(BDC)2dabco were 0.26, 0.20, 0.23, and 0.25 wt%, respectively at 450 psig (30 atm) and room temperature measured using high-pressure thermogravimetric analyzer. The catalytic properties of Pt/AC and Pd/AC were studied for hydrogen spillover in M2(BDC)2dabco modified by 5 wt % of catalyst. The hydrogen adsorption capacity of modified Pd-AC/Ni2(BDC)2dabco was significantly enhanced up to 0.42 wt% by using the secondary spillover 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. At lower pressures, the adsorption heat of hydrogen onto M2(BDC)2dabco is 4~8 kJ/mol.
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