| 其他摘要 | In this dissertation, the hydrogen adsorption properties on zeolites at supercritical conditions were investigated experimentally and theoretically. The main contents of the study were: the measurements of supercritical adsorption isotherms of hydrogen on five commercial microporous zoelite adsorbents of CaA, NaA, KA, NaX and ZSM-5, using a self-designed high-pressure adsorption apparatus, at 77K, 195K and room temperature (293K) and pressures up to 7MPa. The porous structures of the adsorbents were comprehensively characterized by using various technologies of 77K nitrogen adsorption, small angle X-ray scattering (SAXS), X-ray diffraction (XRD) and thermogravity analysis (TGA). The adsorption amount and the heat of adsorption of hydrogen on the zeolites at above critical temperatures were determined. The mechanisms of hydrogen adsorption on the zeolites under supercritical conditions were extensively studied by using classic adsorption theories, adsorption potential theory and lattice density function theory (LDFT). The main conclusions of the study are as follows:
1. The comparisons of the characterizations between nitrogen adsorption at 77K and SAXS measurements indicate that the pore diameters obtained from SAXS are smaller than that from nitrogen adsorption at 77K, whereas the specific surface areas are similar for zeolites NaX, CaA and ZSM-5. The zeolites NaA and KA with smaller pore diameters can be only characterized by SAXS. The sizes of crystal grains for zeolites determined by SAXS are comparable to that by XRD.
2. All hydrogen adsorption isotherms of the zeolites are basically Type I. The highest gravimetric hydrogen storage capacity of 2.55 wt.% is obtained for zeolite NaX at 77 K/4 MPa. The influence of cations on the hydrogen adsorption can be neglected for zeolite NaX with larger pore diameter and ZSM-5 without metal cations in its framework. However, for zeolites A (including NaA, CaA and KA) with smaller pore diameters, the influences of cations on hydrogen adsorption are evident. Furthermore, the isosteric heats of adsorption show decrease with the increase of the adsorption amounts of hydrogen on zeolites, and the adsorption amount was proportional to the specific surface area and microporous volume of the zeolites.
3. The hydrogen adsorption isotherms at supercritical conditions can be described by Toth and Freundlich-Langmuir equations. However, the saturated adsorption capacity is dependent on temperatures, which is inconsistent with its physical meaning. Based on the of Polanyi adsorption potential theory, the generalized characteristic function was derived in accordance with experimental results. It is shown that the generalized characteristic function can describe supercritical adsorption of hydrogen on zeolites in this study.
4. The lattice density function theory (LDFT) models, based on Ono-Kondo equation, were derived in this work. The models were fitted to the isotherms of hydrogen adsorption at experimental conditions. The results show that the four derived LDFT models can satisfactorily describe experimental data at low-temperature and high-pressure conditions. However, considering the structures of the zeolites under study, the LDFT model with cubic lattice are the most appropriate one for description of the adsorption isotherms of hydrogen.
5. The diffusion of hydrogen molecules in zeolites pores can be properly described by pore diffusion model. It is shown that the mechanism of hydrogen diffusion is a behavior of thermodynamic activation. The diffusion coefficients decrease with increase of pressures but increase with increase of the size of pores. These observations are attributed to the decrease in the mean free paths of the hydrogen molecules as pressure increases. The mechanism of diffusion for hydrogen molecules in zeolite pores is transition diffusion, but the diffusion coefficients are lower than that in commonly known pores, which are attributed to the pullback of the diffusing hydrogen molecules to the adsorption sites and adsorbed molecules. |
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