MEDA Engineering and Technical Services, Southfield, MI 48075, USA.
Phys Chem Chem Phys. 2012 Dec 28;14(48):16544-51. doi: 10.1039/c2cp40722a. Epub 2012 Jun 29.
The description of experimental gas adsorption data in terms of an accurate model is key to understand the adsorption mechanism and its limits. As a basic feature such a model should predict correctly the conditions under which saturation occurs. However, in the absence of bulk condensation properties for a supercritical adsorbate this matter remains open to discussions. In this study, the decreasing region of excess hydrogen adsorption isotherms measured down to 50 K is used to determine the adsorbed phase volume, density and pressure corresponding to saturation. The experimental method developed for these key measurements addresses the challenges of very low temperature adsorption measurements at high pressure. Therefore, the modifications specially made to a cryostat used in conjunction with a Sievert apparatus to reach high temperature stability (±10 mK) down to 40 K are presented. The approach is implemented on the novel nanoporous materials UMCM-1 and NOTT-112 over 50-77 K and 0-40 bar. The derived hydrogen saturation properties are found to be consistent with a Dubinin-Astakhov model. Importantly, the measured adsorbed hydrogen phase volume also compares well with the pore volume obtained from Ar porosimetry. The found saturation properties provide a physical basis to calculate consistent absolute adsorption isotherms and enthalpies, and to project the ultimate adsorption capacity of a conceptual material with a maximized specific surface area. The present findings provide additional evidence that the common view on supercritical adsorption, in which it is assumed that no liquid is formed and that the only possible mechanism involves monolayer coverage, does not hold in many nanoporous materials.
用准确的模型来描述实验气体吸附数据对于理解吸附机制及其极限至关重要。作为一个基本特征，这样的模型应该正确地预测达到饱和的条件。然而，在缺乏超临界吸附质的体凝结性质的情况下，这仍然存在争议。在这项研究中，使用测量至 50 K 的过量氢吸附等温线的下降区域来确定饱和时对应的吸附相体积、密度和压力。为这些关键测量开发的实验方法解决了在高压下进行非常低温度吸附测量的挑战。因此，本文介绍了为达到高温稳定性（±10 mK）而对与 Sievert 仪器一起使用的低温恒温器进行的特殊修改，温度范围为 40 K。该方法在新型纳米多孔材料 UMCM-1 和 NOTT-112 上实施，温度范围为 50-77 K，压力范围为 0-40 bar。得出的氢饱和性质与 Dubinin-Astakhov 模型一致。重要的是，测量得到的吸附氢相体积也与 Ar 孔隙率法得到的孔体积很好地吻合。所发现的饱和性质为计算一致的绝对吸附等温线和焓提供了物理基础，并为具有最大比表面积的概念材料的极限吸附容量提供了预测。本研究结果进一步证明了超临界吸附的常见观点，即假设不会形成液体，并且唯一可能的机制涉及单层覆盖，在许多纳米多孔材料中并不成立。
