Μελέτη προσρόφησης υδρογόνου σε νέα νανοδομημένα υλικά

Abstract

In the present PhD thesis we have studied the hydrogen storage performance of IsoReticular Metal-Organic Frameworks (IRMOF) and Covalent-Organic Frameworks (COF), using various theoretical methods and computational techniques. IRMOF and COF materials have been proposed as hydrogen storage materials from the first time that they were synthesized, due to their unique physicochemical and structural properties. Firstly we studied IRMOF materials for hydrogen storage. Preliminary results had shown that hydrogen was interacting with these materials by weak van der Waals interactions. It was proposed that the enhancement of the interaction strength would lead to a better hydrogen storage performance of these materials. One way to do that is the introduction of point charges into the material, which can be done by decorating the pore with Li atoms. Li atoms can be incorporated either by interacting with the aromatic systems of the structures or by the formation of –SO3Li and –OLi groups. We have studied the last two cases by using first principles computational methods and we have found that the interaction strength was enhanced in the modified IRMOF materials by a factor of three. Further investigation by performing GCMC simulations showed that there was also an enhancement of the total hydrogen storage abilities of these materials both at cryogenic and room temperature. Next we have evaluated the hydrogen storage abilities of the COF family of materials. We were focused on 3-dimensional COF materials (3D-COF), since these material had superior physicochemical characteristics than other COF that made them more promising for hydrogen storage. We have performed first principles calculations on these materials to study the interaction of hydrogen with the individual building blocks that these materials were made from. We found that hydrogen interacted with the material by weak van der Waals and dispersion forces, where the strength of these interactions was similar to those of IRMOF. GCMC simulations showed that these materials had exceptional hydrogen storage capacities, which can be explained by the low density of these materials and the high surface and the pore volume that they preserve. The best of these materials was COF-105, which reach 20 %wt at 77K and 100bar, which was by far the best performed known material in the field. In order to increase the hydrogen storage ability of these materials, we have introduced –OLi groups in the structure of the best performed COF-105. We found an enhancement in both the interactions of the hydrogen with the material and of the total hydrogen amount that could be stored in these materials. Finally, we have designed new 3DCOF materials based on ctn network and by using the connectivity of COF-102. These new COF were firstly optimized by molecular mechanics method with the use of an appropriate modified MM3 forcefield and then their storage abilities were investigated by GCMC simulations. We found that the best of the materials that we designed could exhibit 27 %wt at 77K and 100bar, well above the best performed COF-105.