Gas Separation in Nanoporous Graphene from First Principle Calculations

Thanks to its single atom thickness and its mechanical strength, nanoporous graphene is currently being regarded as a promising candidate for efficient and reliable gas separation applications. Clearly the accurate energetic characterization of the penetration processes involving relevant gas-phase molecules is a fundamental prerequisite for any possible application. Here we evaluate permeation barriers and adsorption energies of the H$_2$O, CH$_4$, CO, CO$_2$, O$_2$, H$_2$ molecules, and of the Ar atom on two types of hydrogen saturated pores by means of ab initio simulations, based on the density functional theory (DFT), able to include dispersion corrections too. We find that, although the qualitative trend followed by the values of the permeation barriers of the considered molecules is independent on the adopted DFT functional, at a quantitative level the results are noticeably affected by the dispersion corrections and the chosen exchange contribution characterizing the different functionals, as well as by the allowed graphene sheet distortions. Interestingly, we observe that, due to the occurrence of non-trivial H-bond interactions with the pore-saturating H atoms, the permeation barrier of water remains low even considering a small-size pore. The barrier is further diminished when considering the interaction with a second water molecule on the opposite side of the pore. These observations, combined with the relatively strong binding of the water molecule with the defected surface, suggests that porous graphene could also represent a promising membrane for water filtration.