Efficient adsorption of super greenhouse gas (tetrafluoromethane) in carbon nanotubes.

Light membranes composed of single-walled carbon nanotubes (SWNTs) can serve as efficient nanoscale vessels for encapsulation of tetrafluoromethane at 300 K and operating external pressure of 1 bar. We use grand canonical Monte Carlo simulation for modeling of CF4 encapsulation at 300 K and pressures up to 2 bar. We find that the amount of adsorbed CF4 strongly depends on the pore size in nanotubes; at 1 bar the most efficient nanotubes for volumetric storage have size R = 0.68 nm. This size corresponds to the (10,10) armchair nanotubes produced nowadays in large quantities. For mass storage (i.e., weight %) the most efficient nanotubes have size R = 1.02 nm corresponding to (15,15) armchair nanotubes. They are better adsorbents than currently used activated carbons and zeolites, reaching approximately equal to 2.4 mol kg(-1) of CF4, whereas, the best activated carbon Carbosieve G molecular sieve can adsorb 1.7 mol kg(-1) of CF4 at 300 K and 1 bar. We demonstrate that the high enthalpy of adsorption cannot be used as an only measure of storage efficiency. The optimal balance between the binding energy (i.e., enthalpy of adsorption) and space available for the accommodation of molecules (i.e., presence of inaccessible pore volume) is a key for encapsulation of van der Walls molecules. Our systematic computational study gives the clear direction in the timely problem of control emission of CF4 and other perfluorocarbons into atmosphere.