Sun Chengzhen, Boutilier Michael S H, Au Harold, Poesio Pietro, Bai Bofeng, Karnik Rohit, Hadjiconstantinou Nicolas G
Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
Langmuir. 2014 Jan 21;30(2):675-82. doi: 10.1021/la403969g. Epub 2014 Jan 8.
We present an investigation of molecular permeation of gases through nanoporous graphene membranes via molecular dynamics simulations; four different gases are investigated, namely helium, hydrogen, nitrogen, and methane. We show that in addition to the direct (gas-kinetic) flux of molecules crossing from the bulk phase on one side of the graphene to the bulk phase on the other side, for gases that adsorb onto the graphene, significant contribution to the flux across the membrane comes from a surface mechanism by which molecules cross after being adsorbed onto the graphene surface. Our results quantify the relative contribution of the bulk and surface mechanisms and show that the direct flux can be described reasonably accurately using kinetic theory, provided the latter is appropriately modified assuming steric molecule-pore interactions, with gas molecules behaving as hard spheres of known kinetic diameters. The surface flux is negligible for gases that do not adsorb onto graphene (e.g., He and H2), while for gases that adsorb (e.g., CH4 and N2) it can be on the order of the direct flux or larger. Our results identify a nanopore geometry that is permeable to hydrogen and helium, is significantly less permeable to nitrogen, and is essentially impermeable to methane, thus validating previous suggestions that nanoporous graphene membranes can be used for gas separation. We also show that molecular permeation is strongly affected by pore functionalization; this observation may be sufficient to explain the large discrepancy between simulated and experimentally measured transport rates through nanoporous graphene membranes.
我们通过分子动力学模拟对气体在纳米多孔石墨烯膜中的分子渗透进行了研究；研究了四种不同的气体，即氦气、氢气、氮气和甲烷。我们表明，除了分子从石墨烯一侧的体相直接穿过到另一侧体相的（气体动力学）通量外，对于吸附在石墨烯上的气体，膜通量的显著贡献来自一种表面机制，即分子在吸附到石墨烯表面后穿过。我们的结果量化了体相和表面机制的相对贡献，并表明如果考虑空间分子 - 孔相互作用对动力学理论进行适当修正，将气体分子视为具有已知动力学直径的硬球，那么直接通量可以用动力学理论相当准确地描述。对于不吸附在石墨烯上的气体（如氦气和氢气），表面通量可忽略不计，而对于吸附的气体（如甲烷和氮气），表面通量可能与直接通量相当或更大。我们的结果确定了一种纳米孔几何结构，它对氢气和氦气是可渗透的，对氮气的渗透性显著降低，对甲烷基本不可渗透，从而验证了之前关于纳米多孔石墨烯膜可用于气体分离的建议。我们还表明，分子渗透受到孔功能化的强烈影响；这一观察结果可能足以解释通过纳米多孔石墨烯膜的模拟传输速率与实验测量传输速率之间的巨大差异。
