We study numerically the adsorption of a mixture of CH4, CO2, and N2 at room temperature on a substrate composed of graphene flakes. Our study is based on Molecular Dynamics and Monte Carlo simulations. Methane is considered a spherical molecule, carbon dioxide and nitrogen are represented as linear rigid bodies, and the substrate is rigid and fixed. Our simulations of N2 at 77 K and CO2 at 273 K on the substrate are compatible with the experimental isotherms in activated carbons. We consider different concentrations for the mixtures and show that the substrate is adequate for the separation and adsorption of CO2. On average, CO2 is adsorbed 6.5 more than the other components in a ternary mixture CO2/CH4/N2. The rate of adsorption of CO2 is higher than CH4 and N2 in all the mixtures studied.

We study numerically the adsorption of a mixture of CO$_2$ and CH$_4$ on a graphite substrate covered by graphene nanoribbons (NRs). The NRs are flat and parallel to the graphite surface, at a variable distance ranging from 6 \r{A} to 14 \r{A}. We show that the NRs-graphite substrate acts as an effective filter for CO$_2$. Our study is based on Molecular Dynamics (MD) simulations. Methane is considered a spherical molecule, and carbon dioxide is represented as a linear rigid body. Graphite is modeled as a continuous material, while the NRs are approached atomistically. We observe that when the NRs are placed 6 \r{A} above the graphite surface, methane is blocked out, while CO$_2$ molecules can diffuse and be collected in between the NRs and the graphite surface. Consequently, the selectivity of CO$_2$ is extremely high. We also observe that the initial rate of adsorption of CO$_2$ is much higher than CH$_4$. Overall we show that the filter can be optimized by controlling the gap between NRs and the NRs-graphite separation.

We report our results on the adsorption of noble gases such as argon, krypton and xenon on a graphene sheet, using Grand Canonical Monte Carlo (GCMC) simulations. We calculated the two-dimensional gas-liquid critical temperature for each adsorbate, resulting in fair agreement with theoretical predictions and experimental values of gases on graphite. We determined the different phases of the monolayers and constructed the phase diagrams. We found two-dimensional incommensurate solid phases for krypton, argon and xenon, and a two-dimensional commensurate solid phase for krypton.

In this paper, we report the results of Monte Carlo simulations of the adsorption of neon, argon, methane and carbon dioxide in carbon nanohorns. We model the nanohorns as an array of carbon cones and obtained adsorption isotherms and isosteric heats. The main sites of adsorption are inside the cones and in the interstices between three cones. We also calculated the selectivity of carbon dioxide/methane, finding that nanohorns are a suitable substrate for gas separation. Our simulations are compared to available experimental data.

In this paper, we show that Krypton atoms form a commensurate solid (CS) phase with a fractional coverage of one krypton atom per every four carbons on zigzag carbon nanotubes. This is a unique phase, different from the \(\sqrt{3} \times \sqrt{3}\)R30\(^\circ \) CS monolayer formed on graphite, which has a lower coverage of one krypton atom per every six carbons. Our prediction disagrees with experiments that observe in nanotubes the same solid structure found on graphite. In order to address this discrepancy, we simulated adsorption of Kr on zigzag and armchair single-walled carbon nanotubes with radii ranging from 4.7 to 28.83 Å. Our simulations confirm that the CS of coverage 1/4 forms on medium-sized zigzag nanotubes. We also found the 1/6-coverage solid on graphene, which represents the infinite-radius limit of a nanotube. Our findings are key to experiments of adsorption on nanotubes where the interpretation and justification of the results are based on the monolayer coverage, such as mass or conductance isotherms measurements.

The NSF-funded REU summer program in the Department of Physics & Astronomy at Howard University provided cutting-edge research opportunities in Computational Nanophysics, Experimental Nanophysics, Laser Spectroscopy, Atmospheric Physics and Superstring Theory to six undergraduate students recruited from across the U.S. The REU students were engaged in challenging research projects under the supervision of seasoned mentors across a variety of stimulating physics sub-disciplines that included: (1) computation-intensive surface nanophysics of condensed phase systems focused on the adsorption of gases in Metal-Organic Frameworks (MOFs); (2) experimental measurements using light scattering techniques on gels and polymers in the condensed phase; (3) experimental laser spectroscopy with special emphasis on Raman spectral measurements on tungsten oxide nanolayer deposited on a silicon substrate; (4) observation-based and modeling-intensive atmospheric physics project for developing better understanding of wind lidar performance under various aerosol/cloud loading and relative humidity scenarios in the U.S. and regional ozone and aerosols modeling and analysis of data recorded in West Africa; and (5) a cross-disciplinary project involving quantum theory, supersymmetry, graph theory, encryption, and super-commutative algebra and algebraic geometry with applications to string theory. Each student learned a multitude of relevant techniques related to their research projects, with the vision of teaching and nurturing knowledge, both theoretical and experimental, that will be useful throughout their academic careers both in their major discipline and in interdisciplinary research as a whole. Raman Spectroscopy, 3D Physics Modeling, Monte Carlo Simulations, Algebraic Geometry and Graph Theory are some of the techniques that the students learned that illustrate the importance of physics research in general and have wide-ranging applications in interdisciplinary studies. In addition, the students participated in field trips to the University of Maryland (visit coinciding with NanoDay), Georgetown University (cleanroom tour and research presentations), NASA Goddard Space Flight Center (visit coinciding with Science Jamboree Day), and Smithsonian Museums (coinciding with evening fireworks viewing on the Mall on July 4). The REU students gave midterm and final research presentations and submitted a research paper in refereed journal format at the end of their internship. A comprehensive assessment of the REU program was conducted by an independent project evaluator.

Recent theoretical and simulation studies (Lueking et al. Phys Rev B 75:195425, 2007; Kim et al. J Phys Chem 115:7249–7257, 2011) on the adsorption of Kr on suspended nanotubes yielded different commensurate phases at submonolayer coverage than those found in a pioneering experiment (Wang et al. Science 327:552–555, 2010). This controversy between calculations and experiments is yet to be resolved. One of the tentative explanations of the apparent discrepancy is the possibly different chirality as the chirality of the nanotubes used in the experiment is not known. To address the question on chirality, we calculated the adsorption potential of krypton atoms on two sets of single wall carbon nanotubes of same radii with distinct chiralities. We found novel symmetries of the adsorption sites on a nanotube, which systematically vary depending on its chirality with an unexpected, yet intuitive delicacy. The same approach is equally feasible for other gases (Ar, Xe, CH\(_{4}\), etc.). The results of classical grand canonical Monte Carlo simulations confirm the predicted behavior of adsorption phases.

Using the method of Grand Canonical Monte Carlo we have computed the adsorption of CO2 and CH4 in MOFs with a periodic cubic structure. We used a model of the MOF that allows systematic variations in the charge distribution, size and LJ parameters. We estimated the selectivity of CO2 over CH4 for different temperatures in MOFs with various sizes and charge distributions. The results show that inserting dipoles at the corners of the MOF's unit cell would increase the selectivity of CO2; on the other hand adding quadrupoles to the structure is ineffective. The size of the cell strongly affects the adsorption of CO2 and selectivity: compressing the cell in only 10 % significantly increases the selectivity; expanding the cell by 20 % reduces it. Regarding thermal effects, we estimated that the selectivity drops from 250 to 2 when the temperature rises from 120 K to 300 K. Although this model is inspired by the IRMOF-1, which has a cubic unit cell, it can be adapted to represent other MOFs with noncubic structures by modifying the geometry accordingly. This work implies that MOFs suit gas separation.