Capillary condensation in MMS and pore structure characterization
Capillary condensation in MMS and pore structure characterization
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DOI：
10.1016/s1387-1811(01)00251-7
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发表时间：
2001-06-01
影响因子：
5.2
通讯作者：
Ravikovitch, PI
中科院分区：
文献类型：
--
作者：
Neimark, AV;Ravikovitch, PI
利用非局域密度泛函理论（NLDFT）研究了具有圆柱孔道的硅质介孔分子筛（MMS）中的毛细凝聚和脱附现象。将结果与基于开尔文 - 科恩（KC）方程和德贾金 - 布罗克霍夫 - 德比尔（DBdB）方程的宏观热力学方法进行了比较。我们表明：构成传统用于孔径分布分析的BJH方法基础的KC方程，即使在大至20 nm的孔中也是错误的。对于直径大于约7 nm的孔，具有一致确定的吸附层厚度（分离压力等温线）的DBdB方程是合理的。随着孔径减小，宏观理论变得不太准确，并且在小于约4 nm的孔中，NLDFT和DBdB的结果差异显著。发现NLDFT预测的吸附 - 脱附等温线与在孔径大于5 nm的MCM - 41型材料上的实验氮（77 K）和氩（87 K）等温线在定量上一致。由此，实验脱附分支对应于平衡毛细凝聚/蒸发转变。实验吸附分支对应于自发旋节线凝聚，它发生在吸附膜的稳定性极限处。已经开发了NLDFT方法用于从吸附和脱附等温线计算孔径分布。（C）2001 Elsevier Science B.V.保留所有权利。
Phenomena of capillary condensation and desorption in siliceous mesoporous molecular sieves (MMS) with cylindrical channels are studied by means of the non-local density functional theory (NLDFT). The results are compared with macroscopic thermodynamic approaches based on Kelvin-Cohan (KC) and Derjaguin-Broekhoff-de Beer (DBdB) equations. We show that: The KC equations. which constitute the basis of the traditional BJH method for the pore size distribution analysis, are in error even in pores as large as 20 nm. The DBdB equations with consistently determined thickness of the adsorbed layer (disjoining pressure isotherm) can be justified for pores wider than approximate to7 nm in diameter. As the pore size decreases. the macroscopic arguments become less accurate. and the NLDFT and DBdB results differ significantly in pores smaller than approximate to4 nm. The adsorption-desorption isotherms predicted by NLDFT are found to be in quantitative agreement with the experimental nitrogen (77 K) and argon (87 K) isotherms on MCM-41 type materials with pores larger than 5 nm. Therewith, the experimental desorption branch corresponds to the equilibrium capillary condensation/evaporation transition. The experimental adsorption branch corresponds to the spontaneous spinodal condensation, which occurs at the limit of stability of adsorption films, The NLDFT method has been developed for the calculation of pore size distributions from both the adsorption and desorption isotherms. (C) 2001 Elsevier Science B.V. All rights reserved.