Engineering Thermodynamics, Process & Energy laboratory, Delft University of Technology , Leeghwaterstraat 39, 2628CB Delft, The Netherlands.
Catalysis Engineering, Chemical Engineering Department, Delft University of Technology , van der Maasweg 9, 2629HZ Delft, The Netherlands.
Langmuir. 2016 Dec 6;32(48):12664-12675. doi: 10.1021/acs.langmuir.6b03531. Epub 2016 Nov 17.
In this modeling study, the uses of nitrogen (77.3 K), probe molecule of choice for decades, and argon, opted as alternative in the 2015 IUPAC report on adsorptive characterization, as probe molecules for geometric surface area determination are compared. Graphene sheets possessing slit-shaped pores with varying size (width) are chosen as model porous solids, and different methods for the determination of specific surface areas are investigated. The BET method, which is the most commonly applied analysis, is compared to the Langmuir and relatively recently proposed ESW (excess sorption work) method. We show that either using argon or nitrogen as adsorptive, the physical meaningfulness of adsorption-derived surface areas highly depends on the pore size. When less than two full layers of adsorbate molecules can be formed within slitlike pores of a graphitic material (D < 5.8 Å for Ar/N), adsorption-derived surface areas are about half that of the geometric surface area. Between two and four layers (6.8 < D < 12.8 Å), adsorption surface areas can be significantly larger (up to 75%) than the geometric surface area because monolayer-multilayer formation and pore filling cannot be distinguished. For four or more layers of adsorbate molecules (D > 12.8 Å), adsorption-derived surface areas are comparable to their geometrically accessible counterparts. Note that for the Langmuir method this only holds if pore-filling effects are excluded during determination. This occurs in activated carbon materials as well. In the literature, this indistinguishability issue has been largely overlooked, and erroneous claims of materials with extremely large surface areas have been made. Both the BET and Langmuir areas, for D > 12.8 Å, correspond to geometric surface areas, whereas the ESW method yields significantly lower values. For the 6.8 Å < D < 12.8 Å range, all methods erroneously overestimate the specific surface area. For the energetically homogeneous graphene sheets, differences between argon and nitrogen for the assessment of surface areas are minor.
在这项建模研究中，我们比较了氮（77.3 K）和氩这两种被选择作为替代探针分子的物质，氮是几十年来首选的探针分子，氩则是在 2015 年 IUPAC 关于吸附特性的报告中被选为替代探针分子。我们选择具有不同尺寸（宽度）的狭缝形孔的石墨烯片作为模型多孔固体，研究了不同的比表面积测定方法。BET 方法是最常用的分析方法，我们将其与 Langmuir 方法和相对较新提出的 ESW（过量吸附功）方法进行了比较。我们发现，无论是使用氮气还是氩气作为吸附质，吸附衍生的比表面积的物理意义都高度依赖于孔径。当石墨材料的狭缝孔内不能形成少于两层的吸附质分子时（对于 Ar/N，D < 5.8 Å），吸附衍生的比表面积约为几何表面积的一半。在两层到四层之间（6.8 < D < 12.8 Å），吸附表面面积可以显著增大（高达 75%），超过几何表面积，因为无法区分单层-多层形成和孔填充。对于四层或更多层的吸附质分子（D > 12.8 Å），吸附衍生的比表面积与它们的几何可及的对应物相当。请注意，对于 Langmuir 方法，只有在确定过程中排除孔填充效应时才成立。这种情况在活性炭材料中也会发生。在文献中，这个无法区分的问题在很大程度上被忽视了，导致了对具有极高表面积的材料的错误断言。对于 D > 12.8 Å，BET 和 Langmuir 面积都对应于几何表面积，而 ESW 方法则产生显著较低的值。对于 6.8 Å < D < 12.8 Å 范围，所有方法都错误地高估了比表面积。对于能量均匀的石墨烯片，评估表面积时，氩气和氮气之间的差异较小。
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