17366 Phys. Chem. Chem. Phys., 2013, 15, 17366--17373 This journal is c the Owner Societies 2013 Cite this: Phys. Chem. Chem. Phys., 2013, 15, 17366 To the pore and through the pore: thermodynamics and kinetics of helium in exotic cubic carbon polymorphs† Piotr Kowalczyk,* a Julong He, b Meng Hu, b Piotr A. Gauden, c Sylwester Furmaniak c and Artur P. Terzyk c Applying pore size analysis, Monte Carlo simulations, and transition state theory, we study the molecular sieving properties of recently discovered crystalline exotic cubic carbon allotropes (Hu et al., J. Phys. Chem. C, 2012, 116, 24233–24238) at 298 K and infinite dilution. The fcc-C 10 cubic carbon crystal shows unusual molecular sieving characteristics. The carbon cavities of the fcc-C 10 cubic carbon poly- morph (with an effective size of B3.5–4 Å) are kinetically closed to common gaseous contaminants of He fluid (including: Ne, Ar, H 2 , and CO). Because the sizes of nanowindows connecting carbon cavities are comparable with the effective size of a He atom (B2.556 Å), we predict a significant resistance to self-diffusion of the He in the fcc-C 10 crystal. Computed self-diffusion coefficients B1.3  10 6 – 1.3  10 7 cm 2 s 1 for He inside fcc-C 10 fall in the range characteristic of molecular diffusion in zeolites. Infrequent ‘‘jumps’’ of He atoms between neighboring carbon cavities and kinetic rejection of other gaseous particles indicate potential application of the fcc-C 10 carbon polymorph for kinetic molecular sieving of He near ambient temperatures. The theoretical results presented here are useful for correct interpretation of the pore volumes of carbon molecular sieves measured from helium porosimetry. I. Introduction In the last decade, novel exotic porous carbon nanostructures (such as carbon nanotubes, single-walled carbon nanohorns, graphene and graphitic nanoribbons, ordered porous carbons, wormlike nanotubes and graphitic nanofibers, stacked-cup carbon nanofibers, cubic carbon allotropes, carbon onions, carbyne networks, and others) were projected to be among the most useful materials for selective adsorption and separation of fluid mixtures. 1–16 Unique structures, distinctive nano-porosity, and outstanding mechanical and thermal properties of novel exotic carbon nanostructures provide opportunities beyond those of ordinary carbonaceous materials (i.e., amorphous porous carbons and molecular carbon sieves). 17–20 However, understanding of the impact of the internal nanopore size and morphology on the thermodynamic equilibrium and kinetics of confined particles (i.e., atoms and molecules) is essential for rational design of novel carbon and composite permselective membranes. 21 Helium is a rare gas with a wide range of applications (e.g., lasers, fluorescent light fixtures, medical imaging, cooling technologies, nuclear physics, diving technologies, and others). 22,23 Due to the small size of He atoms, the diffusion rate through materials is a very fast process. As with other noble gases, He is inert, that is, it does not normally form chemical compounds. 24 Therefore, it is not surprising that the separation and recovery of He from various natural and industrial resources is a challenging problem for both experimentalists and theoreticians. 22,23 Thorium oxide mineral (known as thorianite) 25 dissolved in strong nitric acid liberates He together with other gaseous contaminants (such as H 2 , CO 2 , and N 2 ). Similarly, other resources of He (e.g., natural gas, air, and waste gases of ammonia synthesis) 22,23 are composed of various small particles (such as CH 4 , CO, Ar, Ne, O 2 , and others) that need to be removed through purification processes. Industrial technologies currently being used for purification include combined cryogenic distillation and adsorption technologies ( e.g. , pressure-swing adsorption, vacuum pressure swing adsorption, and temperature swing adsorption). 23 The high cost and energy consumption of these separation processes has triggered a Nanochemistry Research Institute, Department of Chemistry, Curtin University of Technology, P.O. Box U1987, Perth, 6845 Western Australia, Australia. E-mail: Piotr.Kowalczyk@curtin.edu.au; Tel: +61 8 9266 7800 b State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China c Department of Chemistry, Physicochemistry of Carbon Materials Research Group, N. Copernicus University, Gagarin St. 7, 87-100 Torun, Poland † Electronic supplementary information (ESI) available: Comparison of the experimental equation of states for Ar, Ne, and CO fluids at 298 K with the ones computed from Monte Carlo simulations in the grand canonical ensemble. See DOI: 10.1039/c3cp52708e Received 28th June 2013, Accepted 19th August 2013 DOI: 10.1039/c3cp52708e www.rsc.org/pccp PCCP PAPER Published on 21 August 2013. Downloaded by Curtin University Library on 26/09/2013 07:29:58. View Article Online View Journal | View Issue