Diffusion in confinement: kinetic simulations of self- and collective diffusion behavior of adsorbed gases
Diffusion in confinement: kinetic simulations of self- and collective diffusion behavior of adsorbed gases
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DOI：
10.1039/c2cp41147d
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发表时间：
2012-01-01
影响因子：
3.3
通讯作者：
Smit, Berend
中科院分区：
文献类型：
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作者：
Abouelnasr, Mahmoud K. F.;Smit, Berend
利用分子动力学（MD）、蒙特卡罗（MC）、Bennett-Chandler （BC）和动力学蒙特卡罗（kMC）等一系列分子模拟技术，研究了不同浓度下吸附甲烷、氦和异丁烷在沸石框架LTA、MFI、AFI和SAS中的自扩散和集体扩散行为。本文有三个主要结论。(1)建立了吸附质在两个大笼间运动的新模型，建立了两个不等载荷笼间重交系数的混合规律。根据这一混合规律所作的预测在定量上与显式模拟相一致。(2)提出了一种新的动态修正过渡态理论方法来解析计算自扩散特性，明确地考虑了纳米级浓度波动。这种方法被证明在定量上与以前的方法一致，但它特别适合于可以模拟集体扩散行为的kMC模拟。(3)在低、中负荷条件下，LTA的自扩散和集体扩散行为一致，而在高浓度条件下则相反。在吸附质包装方案的变化被证明是导致这种分歧，这一特点是复制在一个kMC模拟，明确地模拟这种行为。这些现象在MFI沸石中的异丁烷中得到了进一步的研究，其中MD结果显示了自扩散和集体扩散行为的分离，kMC模拟再现了这一现象。
The self- and collective-diffusion behaviors of adsorbed methane, helium, and isobutane in zeolite frameworks LTA, MFI, AFI, and SAS were examined at various concentrations using a range of molecular simulation techniques including Molecular Dynamics (MD), Monte Carlo (MC), Bennett-Chandler (BC), and kinetic Monte Carlo (kMC). This paper has three main results. (1) A novel model for the process of adsorbate movement between two large cages was created, allowing the formulation of a mixing rule for the re-crossing coefficient between two cages of unequal loading. The predictions from this mixing rule were found to agree quantitatively with explicit simulations. (2) A new approach to the dynamically corrected Transition State Theory method to analytically calculate self-diffusion properties was developed, explicitly accounting for nanoscale fluctuations in concentration. This approach was demonstrated to quantitatively agree with previous methods, but is uniquely suited to be adapted to a kMC simulation that can simulate the collective-diffusion behavior. (3) While at low and moderate loadings the self- and collective-diffusion behaviors in LTA are observed to coincide, at higher concentrations they diverge. A change in the adsorbate packing scheme was shown to cause this divergence, a trait which is replicated in a kMC simulation that explicitly models this behavior. These phenomena were further investigated for isobutane in zeolite MFI, where MD results showed a separation in self- and collective-diffusion behavior that was reproduced with kMC simulations.