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Study on the Molecular Diffusion in Nano-porous Materials by Molecular Dynamics Simulation

Author: ZouGuiMin
Tutor: YangXiaoFeng
School: University of North
Course: Condensed Matter Physics
Keywords: molecular dynamics simulation molecular sieve carbon nanotube diffusion coefficient pore loading
CLC: TB383.1
Type: Master's thesis
Year: 2011
Downloads: 164
Quote: 1
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Abstract


Zeolites and carbon nanotubes are two important porous materials. Zeolites are excellent in gas storage and separation because of their ordered, uniform pores and high BET. Carbon nanotubes (CNTs), however, show quite different molecular transportation behavior due to the smooth and hydrophobic quasi-one-dimensional channels. The experiments have shown that water in CNTs has the diffusion coefficient being several order magnitude larger than originally anticipated. The study on the fast diffusion properties of molecuales inside carbon nanotubes are important both for the theoretical reason and their biological significance.Theoretical study, especially molecular dynamics, the molecular diffusion in these porous material can elucidate the microscopic mechanism being inaccessible from experimental measurments and such knowledge can help researcher to build robust and efficient application facilities. There are voluminous papers regarding works on molecular diffusion, yet we still have very limit understanding on some aspects like the reason why exists the fast diffusion in CNTs.In this dissertation, two of our works in following aspects will be reported:Firstly, we have reported the simulating results for several simple gases diffusing on zeolite ITQ-3. We explored the dependence of the molecular diffusion coefficients along Y and Z directions upon the loadings and found the anistropicty diffusion behavior led by its structure. Specifically, the diffusion coefficient in Z component shows an increase at first, then decease as the loading increasing. This is because the molecules need overcome a big energy barrier to jump from one adsorption site to its neighbor one. The diffusion coefficient in Y component, whereas, decrease monotonically with the loading increases due to the blockage only taking effect. Further, we have illustrated the potenital energy profile along the axis of pores in Z direction and demonstrated that the above distinct diffusion mechanisms are led clearly by lowering the diffusion activated energy.Secondly, we have simulated the diffusion of water on CNTs to examine the abnormal fast diffusion property. The results show that the motion of water molecles is ballistic with the behavior whose mean square displacement could be well predicted by Knudsen diffusion model. We have also explored the the influence of pore diameter on water microscopic configurations. As the diameter increase, the water molecules distribute more likely to be random untill the familiar cluster structure of bulk water appears.

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