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Matériaux fonctionnels carbonés
(9) Production(s) de l'année 2020
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Effect of low frequency phonons on structural properties of ZIFs with SOD topology 
Auteur(s): Formalik F., Fischer Michael, Rogacka J., Firlej L., Kuchta Bogdan
(Article) Publié:
Microporous And Mesoporous Materials, vol. 304 p.109132 (2020)
Texte intégral en Openaccess : 
Ref HAL: hal-02116566_v1
DOI: 10.1016/j.micromeso.2018.09.033
WoS: WOS:000546913300003
Exporter : BibTex | endNote
3 Citations
Résumé: The structural transformations of periodic structures are very often initiated by the dynamicalfluctuation of theequilibrium structure. The natural mechanical excitations in crystals are called phonons. If the energy of thesefluctuations is low, they can easily be transformed into static deformations which define new structural prop-erties of the materials. This is the case in so called gate opening transformations which modify the structure andthe adsorptive properties of porous solids. Using the example of three SOD-type zeolitic imidazolate frameworks(ZIFs) containing linker molecules with different substituents, we show that analysis of low-frequency phononsobtained from density-functional theory (DFT) calculations allows one to model the observed gate opening andto understand the microscopic mechanism of this structural transformation.
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How dense is the gas adsorbed in nanopores?
Auteur(s): Firlej L., Rogacka J., Formalik F., Kuchta Bogdan
(Article) Publié:
Microporous And Mesoporous Materials, vol. 304 p.109240 (2020)
Texte intégral en Openaccess :
Ref HAL: hal-02116565_v1
DOI: 10.1016/j.micromeso.2018.12.024
WoS: WOS:000546913300007
Exporter : BibTex | endNote
1 Citation
Résumé: Unlike macroscopic objects, any system of nanometric size shows characteristics that strongly depend on its sizeand geometric form. It is mainly because the major part of atoms (or molecules) of nano-object is located at itssurface, and their cohesive energy is smaller than for the atoms in the bulk. Here we show that when a fluid isconfined in nano-volume, delimited by non-attractive pore walls, its density is heterogeneous, in particular closeto the pore wall, and, on average, smaller than the density of bulk fluid. This effect progressively weakens whenthe pore size increases, and totally disappears for pores larger than 5 nm. The reported observation has nontrivialinfluence on evaluation of total and excess amount of fluid adsorbed in nanopores, as these quantities aretraditionally calculated assuming the known – and homogeneous –density of the bulk fluid. Additionally, wepropose a new method of the estimations of the accessible pore volume, based on the analysis of the density ofconfined fluid. The right estimation of both: pore volume and gas density is essential for quantitative interpretationof experimental adsorption isotherms: evaluation of pore size distribution and of the adsorbed amount.Although we analyze these problems taking an example of hydrogen at 77 K, our conclusions are general andapply to any fluid confined in nanopores.
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