Pore network model of low-temperature nitrogen adsorption-desorption in mesoporous materials

doi:10.11949/0438-1157.20221406

Abstract

Abstract:

Characterization and analysis of mesoporous structure are essential for the development of mesoporous materials, and one of the most commonly used mesoporous characterization methods is low-temperature nitrogen adsorption-desorption. However, the current analytical model used in the nitrogen adsorption-desorption method is still based on the assumption of parallel pores, which cannot describe the pore blocking phenomenon during desorption and obtain important pore structural parameters such as pore connectivity. In this work, a pore network model for low-temperature nitrogen adsorption-desorption is developed, which can be used to analyze the effect of mesoporous structure on the isothermal adsorption-desorption behavior of nitrogen. By comparing experimental data and simulation results of nitrogen adsorption-desorption in alumina materials, the established pore network model is proved can well describe the low-temperature nitrogen adsorption-desorption behavior in mesoporous materials. The simulation results show that when the average pore size is small, the partial pressure of capillary condensation is low, the pore blocking effect of liquid nitrogen is significant, and thus the range and area of the hysteresis loop of nitrogen adsorption-desorption curve are larger; when the pore size distribution is wide, the numbers of both small and large pores are larger, the capillary condensation and pore blocking effects are significant, and thus the area of the hysteresis loop is larger. The pore connectivity does not affect the adsorption process, but significantly affects the desorption process by changing the pore blocking effect, and the worse the connectivity, the stronger the pore blocking effect. This work indicates that pore blocking should be accounted for in method of nitrogen adsorption-desorption as pore blocking importantly affects the adsorption process, and also provides a pore network model for mesoporous structure analysis.

Key words: mesoporous materials, pore network model, nitrogen adsorption-desorption, pore size distribution, pore connectivity

摘要：

介孔结构的表征和分析对于介孔材料的开发至关重要，其中低温氮气吸脱附法是最常用的介孔表征方法之一。然而，目前氮气吸脱附法采用的分析模型仍基于平行孔假设，无法描述脱附过程堵孔现象，以及获取孔道连通性等重要孔结构信息。本文建立了低温氮气吸脱附的孔道网络模型，用于分析介孔结构对氮气等温吸脱附行为的影响。通过对比氧化铝材料的氮气吸脱附实验数据和模拟结果，证实了建立的孔道网络模型能很好地描述介孔材料中低温氮气吸脱附行为。模拟结果表明平均孔径较小时，毛细凝聚分压低，液氮堵孔效应显著，氮气吸脱附曲线回滞环的范围和面积较大；孔径分布较宽时，小孔和大孔数量均较多，毛细凝聚和堵孔效应显著，回滞环面积较大；孔道连通性不会影响吸附过程，但会通过改变堵孔效应显著影响脱附过程，连通性越差，堵孔效应越强。证实了堵孔效应对氮气脱附过程影响显著，因而氮气吸脱附法需要考虑堵孔效应，建立的孔道网络模型也可为介孔结构分析提供合理的模型工具。

关键词: 介孔材料, 孔道网络模型, 氮气吸脱附, 孔径分布, 孔道连通性

CLC Number:

TQ 018

Jinlin MENG, Yu WANG, Qunfeng ZHANG, Guanghua YE, Xinggui ZHOU. Pore network model of low-temperature nitrogen adsorption-desorption in mesoporous materials[J]. CIESC Journal, 2023, 74(2): 893-903.

孟金琳, 汪宇, 张群锋, 叶光华, 周兴贵. 介孔材料低温氮气吸脱附的孔道网络模型[J]. 化工学报, 2023, 74(2): 893-903.

Figures/Tables 12

Fig.1 Illustration of pore blocking during nitrogen desorption

Fig.2 Illustration of a two-dimensional pore network with a pore connectivity of 4

Fig.3 Algorithm for solving the pore network model

Table 1 Model parameters and variables used in this work

参数或变量	数值
氮气吸脱附温度(T)/K	77
氮气临界温度(T_c)/K	126
液氮密度(ρ)/( kg/m³)	807
吸脱附相对压力区间(P/P₀)	0.10~0.95
平均孔径(d_p)/nm	5~20
孔径分布标准偏差(σ)	0.2~0.8
孔道连通性(Z)	4~8

Fig.4 Comparison of nitrogen adsorption-desorption isotherms obtained from experiments and pore network model

Fig.5 Effect of pore network size on the nitrogen adsorption-desorption isotherms(Z = 4, dp = 5 nm, σ = 0.5)

Fig.6 Effect of average pore diameter on the nitrogen adsorption-desorption isotherms(Z = 4, σ = 0.5)

Fig.7 Liquid nitrogen distribution in pore networks with different average pore diameter at P/P0 = 0.60 during desorption

Fig.8 Effect of standard deviation on the nitrogen adsorption-desorption isotherms(Z = 4, dp =10 nm)

Fig.9 Liquid nitrogen distribution in pore networks with different standard deviation at P/P0 = 0.60 during desorption

Fig.10 Effect of pore connectivity on the nitrogen adsorption-desorption isotherms(σ = 0.5， dp = 10 nm)

Fig.11 Liquid nitrogen distribution in pore networks with different pore connectivity at P/P0 = 0.60 during desorption

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