Article pubs.acs.org/JPCC
Asymmetrically Decorated, Doped Porous Graphene As an Effective Membrane for Hydrogen Isotope Separation Marlies Hankel,† Yan Jiao,†,‡ Aijun Du,† Stephen K. Gray,§ and Sean C. Smith*,∥ †
Centre for Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072, Australia ‡ School of Chemical Engineering, The University of Queensland, QLD 4072 Brisbane, Australia § Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States ∥ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *
ABSTRACT: We propose a new route to hydrogen isotope separation which exploits the quantum sieving effect in the context of transmission through asymmetrically decorated, doped porous graphenes. Selectivities of D2 over H2 as well as rate constants are calculated based on ab initio interaction potentials for passage through pure and nitrogen functionalized porous graphene. One-sided dressing of the membrane with metal provides the critical asymmetry needed for an energetically favorable pathway.
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INTRODUCTION The separation of isotopic mixtures, such as D2 from H2, is a difficult and energy intensive process requiring special techniques. Existing centrifugation and cryogenic distillation methods are both costly and cumbersome. Recent work1−4 demonstrated that the heavier isotope D2 diffuses significantly faster than H2 at sufficiently low temperature in narrow pore nanomaterials, raising the possibility of kinetic separation as a competitive option for isotope separation. Microporous materials can be used to separate a mixture of molecules based either on their size, shape, or differences in chemical affinity. For example, molecules that are physically larger than the pores would be excluded resulting in selective absorption of smaller molecules. Traditionally, this kind of molecular sieving has not been considered for the separation of different isotopes as in a classical description different isotopic species of the same molecule have identical size and shape and adsorption properties. However in quantum sieving the separation is not based on size but on the difference in mass and associated differences in the quantum levels of the lighter and heavier molecules confined in the pore. This concept was first proposed by Beenakker et al.,5 and its intriguing effects have since been the focus of several theoretical1,2,5−21 and experimental3,22−24 studies. Molecular dynamics1 and experimental24 studies confirmed the effect of quantum sieving in isotope separation. Bhatia and co-workers1 investigated the kinetic molecular sieving approach for isotope separation based on materials such as zeolites and carbon molecular sieves (CMS). The study showed that high transport selectivity for D2 (exceeding 20) can be achieved at low temperatures (30−60 K) even when the equilibrium © 2012 American Chemical Society
selectivity is significantly lower, opening the door for practical isotope separation using membranes. Indeed, independent support for this approach has subsequently been obtained by Chu et al.4 and Zhao et al.3 Their results demonstrating faster desorption of D2 compared to H2 at 77 K are consistent with the prediction of ref 1 of faster diffusion of D2 compared to H2 below 94 K in zeolite of essentially the same critical pore size. At lower temperatures the effect is even more dramatic.2 Similar findings have been reported by Bhatia and co-workers24 in a recent combined experimental and theoretical study using quasielastic neutron scattering and molecular dynamics. The quantum kinetic sieving of the hydrogen isotopes has been observed in Takeda 3 Å CMS for temperatures
