物理化学学报 >> 2020, Vol. 36 >> Issue (11): 1911044.doi: 10.3866/PKU.WHXB201911044

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基于静电效应的石墨烯纳米孔选择性渗透特性

孙成珍(), 周润峰, 白博峰   

  • 收稿日期:2019-11-25 录用日期:2019-12-16 发布日期:2019-12-20
  • 通讯作者: 孙成珍 E-mail:sun-cz@xjtu.edu.cn
  • 基金资助:
    国家自然科学基金(51876169);国家自然科学基金(51425603);热能动力技术重点实验室开放基金(TPL2017BB009)

Electrostatic Effect-based Selective Permeation Characteristics of Graphene Nanopores

Chengzhen Sun(), Runfeng Zhou, Bofeng Bai   

  • Received:2019-11-25 Accepted:2019-12-16 Published:2019-12-20
  • Contact: Chengzhen Sun E-mail:sun-cz@xjtu.edu.cn
  • Supported by:
    the National Natural Science Foundation of China(51876169);the National Natural Science Foundation of China(51425603);the General Open Project of Key Laboratory of Thermal Power Technology, China(TPL2017BB009)

摘要:

二维石墨烯纳米孔已被证明可作为一种可靠的分子筛,但仅仅依靠分子大小筛选效应很难实现混合气体分子的高选择性分离。本文采用分子动力学模拟方法研究表面电荷对石墨烯纳米孔分离CO2/N2混合分子选择性的影响规律,进而实现基于静电效应的石墨烯纳米孔分子选择性渗透,为提高石墨烯纳米孔的气体分离选择性提供一种可行的方法。模拟结果表明,表面施加负电荷后,随着负电荷密度的增加CO2分子的渗透率增加而N2分子的渗透率降低,石墨烯纳米孔展示出了CO2/N2的分离选择性;但是施加正电荷后,纳米孔的选择性几乎没有发生变化。静电效应引起的纳米孔选择性跟气体分子在带电石墨烯表面的不同吸附能力有关。当施加负电荷后,CO2分子的吸附能力增强,通过表面机制的渗透分子数增加,分子的平均渗透时间增加,总体渗透率提高;N2分子的渗透率由于CO2分子的抑制效应相应地随着负电荷的施加而降低。当施加正电荷后,CO2和N2分子的吸附能力都未发生明显的改变,CO2和N2分子的渗透率也未增加或减小,因此纳米孔没有展示出静电效应选择性。

关键词: 石墨烯纳米孔, 选择性渗透, 静电效应, 气体分子, 分子动力学, 分子筛

Abstract:

Two-dimensional graphene nanopores have proved to be a very effective molecular sieve with ultra-high molecular permeance due to the atomic thickness of graphene sheets. The mechanism of graphene nanopores for molecular sieving is generally the size-sieving effect of different molecules. However, high-selective molecular separation is difficult to realize based only on the size-sieving effect. Therefore, graphene nanopore-based membranes usually present high permeance but a moderate selectivity, such that the separation performance cannot far exceed those of traditional separation membranes. In this study, the effects of charges on graphene surfaces on the selective permeation of CO2/N2 mixtures through a graphene nanopore is studied using molecular dynamics simulations; its purpose to realize electrostatic effect-based selective molecular permeation through graphene nanopores and find a promising method to improve the selectivity of molecular separation. The simulation results show that graphene nanopores with negative charges have higher CO2 permeance and lower N2 permeance and, thus, present a high selectivity for the separation of the CO2/N2 mixtures. The graphene nanopore with positive charges, however, does not improve the selectivity. The electrostatic effect-based selectivity of graphene nanopores is related to the different molecular adsorption abilities on the graphene surface with charges. For negative charges, the adsorption ability of CO2 molecules increases and the number of permeated molecules via surface mechanism increases and the experience time during the permeation process also increases; ultimately the CO2 permeance increases with increasing the charge density. For the molecules permeated through the surface mechanism, they are firstly adsorbed onto the graphene surface and then diffuse to the pore region for the ultimate permeation; thus, their experience time is longer than that of the molecules permeated through a direct mechanism. Therefore, a longer experience time means a more significant contribution of the surface flux to the total flux. At high surface charge densities, the contribution of surface flux is dominated and thus the experience time is longer. For CO2 molecules, the permeation rates increase with increasing the surface charge density. Namely, a higher experience time corresponds to a higher permeation rate for CO2 molecules. A decrease of N2 permeance with increasing the charge density is correlated to the increasing CO2 permeance via the inhibition effects of non-permeating components on the permeation of permeating components. For positive charges, the adsorption abilities of CO2 and N2 molecules have no obvious variation with the charge density and their permeance is constant; therefore, the graphene nanopore still has no electrostatic effect-based selectivity.

Key words: Graphene nanopore, Selective permeation, Electrostatic effect, Gas molecule, Molecular dynamics, Molecular sieve

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