Internal Concentration Gradients of Guest Molecules in Nanoporous Host Materials: Measurement and Microscopic Analysis

详细信息    查看全文

• 作者：Pavel Kortunov ; Lars Heinke ; Sergey Vasenkov ; Christian Chmelik ; Dhananjai B. Shah ; Jö ; rg Kä ; rger ; Rainer A. Rakoczy ; Yvonne Traa ; Jens Weitkamp
• 刊名：Journal of Physical Chemistry B
• 出版年：2006
• 出版时间：November 30, 2006
• 年：2006
• 卷：110
• 期：47
• 页码：23821 - 23828
• 全文大小：678K
• 年卷期：v.110,no.47(November 30, 2006)
• ISSN：1520-5207

文摘

Evolution of internal concentration profiles of methanol in 2-D pore structure of ferrierite crystal was measuredin the pressure range of 0 to 80 mbar with the help of the recently developed interference microscopy technique.The measured profiles showed that both a surface barrier and internal diffusion controlled the kinetics ofadsorption/desorption. Furthermore, they indicated that in the main part of the crystal, the z-directional 10-ring channels were not accessible to methanol and that the transport of methanol mainly occurred via 8-ringy-directional channels. The roof-like part of the crystal was almost instantaneously filled/emptied duringadsorption/desorption, indicating accessible 10-ring channels in this section. The measured profiles wereanalyzed microscopically with the direct application of Fick's second law, and the transport diffusivity ofmethanol in ferrierite was determined as a function of adsorbed phase concentration. The transport diffusivityvaried by more than 2 orders of magnitude over the investigated pressure range. Transport diffusivities,calculated from measured profiles from small and large pressure step changes, were all found to be consistent.Simulated concentration profiles obtained from the solution of Fick's second law with the calculated functionaldependence of diffusivities on concentration compared very well with the measured concentration profiles,indicating validity and consistency of the measured data and the calculated diffusivities. The results indicatethe importance of measuring the evolution of concentration profiles as this information is vital in determining(1) the direction of internal transport, (2) the presence of internal structural defects, and (3) surface/internaltransport barriers. Such detailed information is available neither from common macroscopic methods since,they measure changes in macroscopic properties and use model assumptions to predict the concentrationprofiles inside, nor from microscopic methods, since they only provide information on average displacementof diffusing molecules.