BET法适用于各种吸附剂的特点

V. Gun'ko
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引用次数: 1

摘要

对于各种吸附剂,特别是纳米多孔吸附剂,存在以氮为探针吸附物的brunauer - emmet - teller (BET)方法的适用性问题。因此,在p/p0 = 0.05-0.3, 0.06-0.22和更窄的压力范围内,对大量(约200个样品)本质上不同的吸附剂,如烟状氧化物(单个,二元和三元,初始和改性),多孔硅,活性炭和多孔聚合物,BET方法的氮吸附(a(p/p0))等温线进行了分析。采用无孔纳米颗粒聚集成微粒的石墨化炭黑env - carb作为标准吸附剂,其氮分子占据的标准面积为σm(N2) = 0.162 nm2。对于由非多孔纳米颗粒组成的初始纳米氧化物,σm = 0.162 nm2的标准值导致SBET值高估约10%,这是由于轻微极化的N2分子与极性表面官能团(如各种羟基)相互作用的非平行于表面取向。对于由低分子和高分子(线性、二维和三维聚合物和蛋白质)化合物修饰的纳米氧化物,σm = 0.162 nm2时SBET的高估可达30%,对于某些活性炭也是如此。对于具有纳米孔(半宽x或半径R < 1 nm)和窄介孔(1 nm < R < 3 nm)的吸附剂,单层和多层吸附的重叠(在σm = 0.162 nm2处SBET明显低估)和N2分子的不平行于表面取向(导致σm小于0.162 nm2)会导致标准化氮吸附等温线的位置(在BET范围内)不同于env - carb。它可以由BET常数cBET的正值或负值来表征。表明BET方法(以氮为探针)的不适用性或适用性的两个主要标准与cBET值和在BET范围内的还原吸附过程a´(1- p/p0)与p/p0在吸附剂中不一致,这些吸附剂不是纯纳米孔吸附剂,但它们与纯纳米孔或介孔/大孔吸附剂一致
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Features of BET method application to various adsorbents
For various adsorbents, especially nanoporous, there is an applicability problem of the Brunauer-Emmett-Teller (BET) method using nitrogen as a probe adsorbate. Therefore, the nitrogen adsorption (a(p/p0)) isotherms in several pressure ranges of the BET method at p/p0 = 0.05–0.3, 0.06–0.22, and narrower are analyzed for a large set (about 200 samples) of essentially different adsorbents such as fumed oxides (individual, binary and ternary, initial and modified), porous silicas, activated carbons and porous polymers. Graphitized carbon black ENVI–Carb composed of nonporous nanoparticles aggregated into microparticles is used as a standard adsorbent characterized by the standard area occupied by nitrogen molecule σm(N2) = 0.162 nm2. For initial nanooxides composed of nonporous nanoparticles, the standard value of σm = 0.162 nm2 results in the overestimation of the SBET values by ca. 10 % because of non-parallel-to-surface orientation of slightly polarized N2 molecules interacting with polar surface functionalities (e.g., various hydroxyls). For nanooxides modified by low- and high-molecular (linear, 2D and 3D polymers and proteins) compounds, the overestimation of SBET at σm = 0.162 nm2 could reach 30 %, as well as for some activated carbons. For adsorbents possessing nanopores (at half-width x or radius R < 1 nm) and narrow mesopores (1 nm < R < 3 nm), an overlap of monolayer and multilayer sorption (giving apparent underestimation of SBET at σm = 0.162 nm2) and non-parallel-to-surface orientation of the N2 molecules (causing σm lower than 0.162 nm2) could lead to various location of the normalized nitrogen adsorption isotherms (in the BET range) with respect to that for ENVI–Carb. It could be characterized by positive or negative values of the BET constant cBET. Two main criteria showing the inapplicability or applicability of the BET method (with nitrogen as a probe) related to the cBET values and a course of reduced adsorption a´(1- p/p0) vs. p/p0 in the BET range could not be in agreement for adsorbents, which are not pure nanoporous, but they are in agreement for pure nanoporous or meso/macroporous adsorbents
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