MIL-53金屬-有機架構物之特性鑑定及儲氫機制之研究

Abstract

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.