Institut Charles Gerhardt Montpellier, CNRS (UMR 5253), Université Montpellier 2, ENSCM, Université Montpellier 1, 8 rue de l'Ecole Normale, 34296 Montpellier Cedex 05, France.
Langmuir. 2013 Jun 25;29(25):7864-75. doi: 10.1021/la401228k. Epub 2013 Jun 11.
Adsorption and transport in hierarchical porous solids with micro- (~1 nm) and mesoporosities (>2 nm) are investigated by molecular simulation. Two models of hierarchical solids are considered: microporous materials in which mesopores are carved out (model A) and mesoporous materials in which microporous nanoparticles are inserted (model B). Adsorption isotherms for model A can be described as a linear combination of the adsorption isotherms for pure mesoporous and microporous solids. In contrast, adsorption in model B departs from adsorption in pure microporous and mesoporous solids; the inserted microporous particles act as defects, which help nucleate the liquid phase within the mesopore and shift capillary condensation toward lower pressures. As far as transport under a pressure gradient is concerned, the flux in hierarchical materials consisting of microporous solids in which mesopores are carved out obeys the Navier-Stokes equation so that Darcy's law is verified within the mesopore. Moreover, the flow in such materials is larger than in a single mesopore, due to the transfer between micropores and mesopores. This nonzero velocity at the mesopore surface implies that transport in such hierarchical materials involves slippage at the mesopore surface, although the adsorbate has a strong affinity for the surface. In contrast to model A, flux in model B is smaller than in a single mesopore, as the nanoparticles act as constrictions that hinder transport. By a subtle effect arising from fast transport in the mesopores, the presence of mesopores increases the number of molecules in the microporosity in hierarchical materials and, hence, decreases the flow in the micropores (due to mass conservation). As a result, we do not observe faster diffusion in the micropores of hierarchical materials upon flow but slower diffusion, which increases the contact time between the adsorbate and the surface of the microporosity.
采用分子模拟方法研究了具有微孔 (~1nm) 和介孔 (>2nm) 的分级多孔固体中的吸附和传输。考虑了两种分级固体模型：介孔中刻蚀出微孔的微孔材料（模型 A）和介孔中插入微孔纳米颗粒的介孔材料（模型 B）。模型 A 的吸附等温线可以表示为纯介孔和微孔固体吸附等温线的线性组合。相比之下，模型 B 中的吸附偏离了纯微孔和介孔固体的吸附；插入的微孔颗粒充当缺陷，有助于在介孔内成核液相并将毛细凝聚推向较低压力。就压力梯度下的传输而言，由介孔中刻蚀出微孔的微孔固体组成的分级材料中的通量符合纳维斯托克斯方程，因此在介孔内验证了达西定律。此外，由于微孔和介孔之间的传递，这种由微孔固体组成的分级材料中的流量大于单一介孔中的流量。由于在介孔表面存在非零速度，因此这种分级材料中的传输涉及到介孔表面的滑移，尽管吸附质与表面具有很强的亲和力。与模型 A 相反，由于纳米颗粒作为阻碍传输的限制，模型 B 的通量小于单一介孔中的通量。通过介孔中快速传输产生的微妙影响，介孔的存在增加了分级材料中微孔中的分子数量，从而减少了微孔中的流量（由于质量守恒）。因此，我们在分级材料的微孔中没有观察到更快的扩散，而是观察到较慢的扩散，这增加了吸附质与微孔表面之间的接触时间。
