Canonical Monte Carlo simulation of adsorption of O2 and N2 mixture on single walled carbon nanotube at different temperatures and pressures.

Adsorption of pure and mixtures of O(2) and N(2) on isolated single-walled carbon nanotube (SWCNT) have been investigated at the subcritical (77 K) and different supercritical (273, 293, and 313 K) temperatures for the pressure range between 1 and 31 MPa using (N,V,T) Monte Carlo simulation. Both O(2) and N(2) gravimetric storage capacity exhibit similar behaviors, gas adsorption is higher on outer surface of tube, compared to the inner surface. Results are consistent with the experimental adsorption measurements. All adsorption isotherms for pure and mixture of O(2) and N(2) are characterized by type I (Langmuir shape), indicating enhanced solid-fluid interactions. Comparative studies reveal that, under identical conditions, O(2) adsorption is higher than N(2) adsorption, due to the adsorbate structure. Excess amount of O(2) and N(2) adsorption reach to a maximum at each temperature and specified pressure which can be suggested an optimum pressure for O(2) and N(2) storage. In addition, adsorptions of O(2) and N(2) mixtures have been investigated in two different compositions: (i) an equimolar gas mixture and (ii) air composition. Also, selectivity of nanotube to adsorption of O(2) and N(2) gases has been calculated for air composition at ambient condition.

Effect of the adsorption of oxygen on electronic structures and geometrical parameters of armchair single-wall carbon nanotubes: a density functional study.

We investigated the effect of atomic and molecular oxygen adsorption on the geometries and electronic properties of small and large armchair single-walled carbon nanotubes (SWCNTs) by means of the density functional theory (DFT). We calculated the equilibrium geometries, energetics, and electronic properties of the nanotubes as well as the tube-molecules. Global indices such as electronic chemical potential and hardness were calculated using the Kopmann's theorem. Our investigation involved the physisorption of molecular oxygen, chemisorption of atomic oxygen, and formation of epoxide-like structures. The adsorption energies of the oxygen molecules physisorbed to different sites were determined by calculating the short-range interactions. The effect of the tube diameter on the stability of the tube-O(2) system was studied for the different sites. Also we considered the orientation of O(2) molecule during adsorption of O(2) molecule on the outer surface of tubes. Adsorption of oxygen atom on top of the carbon atom of the tubes was also considered. We found out that O atoms bind to the outer surface of the SWCNTs to give stable epoxide-like structures. The most stable epoxide-like structure on the outer surface of the nanotubes was the (4,4)-O system with a calculated adsorption energy of -2.944 eV.