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物理化学学报  2018, Vol. 34 Issue (10): 1136-1143    DOI: 10.3866/PKU.WHXB201801301
所属专题: 材料科学的分子模拟
论文     
气体分子在二维石墨烯纳米孔中的选择性渗透特性
孙成珍,白博峰*()
Selective Permeation of Gas Molecules through a Two-Dimensional Graphene Nanopore
Chengzhen SUN,Bofeng BAI*()
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摘要:

二维石墨烯纳米孔中气体分子的选择性渗透对多孔石墨烯分离膜非常重要。本文采用分子动力学方法研究了气体分子在氮氢修饰石墨烯纳米孔中的渗透特性,从分子的大小和结构、纳米孔的构型以及分子与石墨烯之间的作用强度等角度阐明了分子出现选择性渗透的原因。结果表明,不同分子的渗透率不同,即H2O>H2S>CO2>N2>CH4。渗透率跟分子的质量和直径以及分子在石墨烯表面上的吸附密度有关;根据气体分子动理学理论,渗透率跟分子质量成反比关系;而分子在石墨烯表面上的高吸附密度对渗透起促进作用。对于H2O和CH4分子,分子直径起主导作用;H2O分子直径最小,其渗透率最大;同理,CH4分子的渗透率最小。对于H2S和CO2分子,H2S分子的直径较大,但其与石墨烯之间的作用强度较大(吸附密度较高),导致渗透率较高;对于CO2和N2分子,CO2分子的直径较小,并且与石墨烯之间的作用强度较大,渗透率较高。同时发现,分子在纳米孔中的渗透使得其在石墨烯表面的密度分布极不均匀。纳米孔左右两侧的功能化氮原子使CH4分子容易从孔两侧区域穿过,而其它分子由于直径较小在纳米孔中心区域穿过的概率最大。分子与石墨烯之间的作用越强,导致分子在石墨烯表面区域内停留的时间越长,最终使其在渗透纳米孔的过程中所经历的时间越长。本文所采用的氮氢修饰石墨烯纳米孔中,分子渗透速率达到~10-3 mol·s-1·m-2·Pa-1,并且其它分子相对于CH4分子的选择性也很高,说明基于该类型纳米孔的多孔石墨烯分离膜在天然气处理等工业气体分离领域具有很好的应用前景。

关键词: 石墨烯纳米孔选择性渗透气体分子分子动力学    
Abstract:

Selective molecular permeation through two-dimensional nanopores is of great importance for nanoporous graphene membranes. In this study, we investigate the selective permeation characteristics of gas molecules through a nitrogen-and hydrogen-modified graphene nanopore using molecular dynamics simulations. We reveal the mechanisms of selective molecular permeation from the aspects of molecular size and structure, pore configuration, and interactions between gas molecules and graphene. The results show that the permeances of different molecules are different, and the following order is observed in our study: H2O > H2S > CO2 > N2 > CH4. Molecular permeance is related to the molecular size, mass, and molecular density on the graphene surface. The molecular permeation rate is inversely proportional to the molecular mass based on gas kinetic theory, while the molecular density on the graphene surface exerts a positive effect on molecular permeation. The permeance of H2O molecules is the highest owing to their smallest diameter, while the permeance of CH4 molecules is the lowest owing to their biggest diameter; in these cases, the molecular size is a dominating factor. For H2S and CO2 molecules, the diameters of H2S molecules are larger than those of CO2 molecules, but the interactions between H2S molecules and graphene are stronger, resulting in a stronger permeation ability of H2S molecules. Between CO2 and N2 molecules, CO2 molecules show higher permeation rates owing to smaller diameters and stronger interactions with graphene. The graphene surface also shows nonuniform molecular density distribution owing to molecular permeation through graphene nanopores. Because of the doped nitrogen atoms, the CH4 molecules prefer to permeate from the left and right sides of the graphene nanopore, while the other molecules prefer to permeate from the center of the nanopore owing to their small diameters. For the molecules that show stronger interactions with graphene, the molecular density on the graphene surface is higher; accordingly, the residence time on the graphene surface is longer and the experience time period during permeation is also longer. The mechanisms identified in this study can provide theoretical guidelines for the application of graphene-based membranes. In addition, the permeance of gas molecules in the graphene nanopore adopted in this study is on the order of 10-3 mol·s-1·m-2·Pa-1, and the selectivity of other molecules relative to CH4 molecules is also high, showing that the membranes based on this type of nanopore can be employed in natural gas processing and other separation industries.

Key words: Graphene nanopore    Selective permeation    Gas molecules    Molecular dynamics
收稿日期: 2017-12-18 出版日期: 2018-01-30
中图分类号:  O647  
基金资助: 国家自然科学基金青年项目(51506166);国家杰出青年科学基金项目(51425603);中国博士后科学基金特别资助项目(2016T90915)
通讯作者: 白博峰     E-mail: bfbai@mail.xjtu.edu.cn
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引用本文:

孙成珍,白博峰. 气体分子在二维石墨烯纳米孔中的选择性渗透特性[J]. 物理化学学报, 2018, 34(10): 1136-1143.

Chengzhen SUN,Bofeng BAI. Selective Permeation of Gas Molecules through a Two-Dimensional Graphene Nanopore. Acta Physico-Chimica Sinca, 2018, 34(10): 1136-1143.

链接本文:

http://www.whxb.pku.edu.cn/CN/10.3866/PKU.WHXB201801301        http://www.whxb.pku.edu.cn/CN/Y2018/V34/I10/1136

Diameter/nm Relative molecular mass Molecular structure
CH4 0.380 16.04 spherical
CO2 0.330 44.01 linear
H2S 0.360 34.08 triangular
N2 0.364 28.01 linear
H2O 0.290 18.01 triangular
Table 1  Size (diameter and relative molecular mass) and structure of various gas molecules.
Fig 1  Schematic of simulation system and atomic view of molecules. (a) simulation system; (b–f) atomic views of CH4, CO2, H2S, N2 and H2O molecules.
Fig 2  Structure of two-dimensional graphene nanopore and charge distribution on the pore-rim atoms. (a) pore structure; (b) charge distribution (unit: e). Orange: N; Purple: H; Blue: C in graphene, color online.
ε/eV σ/nm Charge/e
CO218
C-C 2.424 × 10-3 0.2757 0.6512
C-O 4.101 × 10-3 0.2895
O-O 6.938 × 10-3 0.3033 -0.3256
H2S23
H-H 0.336 × 10-3 0.0980 0.124
H-S 2.691 × 10-3 0.2350
S-S 21.545 × 10-3 0.3720 -0.248
N224
N-N 3.126 × 10-3 0.3297 0
H2O25
H-H 1.999 × 10-3 0.0400 0.417
H-O 3.634 × 10-3 0.1775
O-O 6.611 × 10-3 0.3151 -0.834
Table 2  L-J potential parameters of molecules and atomic charges.
Er
10-2Kr/(eV·nm-2) r0/nm
N≡N (N2) 24 1.426 0.1112
C=O (CO2) 27 6.158 0.1160
H―S (H2S) 27 2.021 0.1365
H―O (H2O) 25 19.56 0.0957
E
Kθ/(eV·rad-2) θ0/(°)
O=C=O(CO2) 28 6.416 180
H―S―H (H2S) 29 1.110 91.5
H―O―H (H2O) 25 2.390 104.5
Table 3  Bond and angle Harmonic potential parameters of molecules.
Fig 3  Relationship between bidirectional molecular crossing number and timesteps.
Fig 4  Molecular probability density maps inside the nanopore during the permeation process. (a) CH4; (b) CO2; (c) H2S; (d) N2.
Fig 5  Molecular density distribution along the z-direction and on the graphene surface. (a) density distribution along the z-direction; (b) density distribution on the graphene surface.
Fig 6  Relationship between molecular number and residence time on the graphene surface.
Fig 7  Probability distribution for the time period during permeation process.
Fig 8  Gas permeance and selectivity relative to CH4 molecules.
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