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The main goal of this research project is to:  1) provide results that advance the understanding of the properties of adsorbates in MOFs for a wide range of temperatures and pressures, 2) assess existing materials and design not-yet synthesized materials for separation of CO₂ / H₂ mixtures and 3) establish a strong research program suitable for training of students and enriching the curriculum.

The very first step was to study of the force fields to be used for each adsorbate. We considered different models. For the H₂ molecule we adopted the Lennard Jones model, and for the CO₂ a model that uses point charges combined with LJ. For the Monte Carlo calculation we will also test a quadrupole-quadrupole form of the interaction.

Once the force fields were selected we found numerically the global minimum of the substrate-adsorbate potential interaction, for the MOF and also for carbon nanotube bundles and carbon nanohorns, to have a possible comparison. A team of two undergraduate students and three graduate students studied the interaction energy of CO₂ in MOFs, CNTs and CNHs. It was determined that the minimum energy configuration for one molecule corresponds to the molecule oriented axially along the groove between two CNTs, having an energy of -269 meV (for 1 nm-radius NTs). For two molecules, the energetically most favorable configuration corresponds to the case of   the second molecule in axial orientation with an energy of -220 meV. This value is relatively high, consistently with the hypothesis of enhancing the adsorption energy due to the CO₂-CO₂ interaction. For CO₂ in the CNHs, the energy is minimum in the axial orientation and has a value of -275 meV.  For the MOFs, the minimum potential energy was -29.6 meV and -323 meV for H₂ and CO₂ respectively. The large difference between the energies for H₂ and CO₂ suggest this material could be use for gas separation.  Also, the MOF provides higher adsorption energy than the carbons.

We designed a simplified model of the MOF, the JGMOF that consists of a cubic cell with metallic centers at the corners and several LJ/point charge points placed at the edges. The structure of the JGMOF (strength of the metal corners and points at the edges) is designed to reproduce the main features of the potential landscape with a reduced number of atoms in the unit cell.

We made important improvements in the simulation code. The code that we use for the simulations (GCMC) was originally developed and used in our group for many years to study the properties of simple spherical adsorbates in carbons.  In order to study CO₂ and H₂ in MOFs we needed to make important changes in the code. For example, because CO₂ is a linear molecule that has a finite quadrupole moment, the interatomic potential has to include the quadrupole-quadrupole interaction. Also, additional monte carlo steps are included, for the rotation of the molecule.  

A workshop on “Physical adsorption in nanostructured materials” was organized in the University of Missouri by a colleague, the PI served as co-chair of the organizing committee. Although the workshop was intended to take place at Howard University, we decided to join efforts with the U. Missouri that had significant funding available.

This research has a very important impact on the career of the PI and students participating.  Receiving this funding significantly serve the PI to realize her career goals, by for example: 1) improving the GCMC code to be suited for simulation of linear molecules, enabling future studies related to the interaction of carbon dioxide with materials with potential use for gas separation;  2) closely supervising the progress of three PhD students, one of whom is expected to defend his thesis by the end of 2012; 3) designing and offering a new course on “Molecular Simulations” in the department of Physics at Howard University; 4) advancing in a larger project of the PI: the investigation of adsorption of CO₂ in materials, aiming to help reduced the   impact of carbon excesive dioxide on the planet.