Abstract
Quasi-elastic neutron scattering was used for characterization of dynamics of molecular hydrogen confined in narrow nanopores of two activated carbon materials: a carbon derived from polyfurfuryl alcohol and an ultramicroporous carbon. Fast, but incomplete ortho-para conversion was observed at 10 K, suggesting that scattering originates from the fraction of unconverted ortho isomer which is rotation-hindered because of confinement in nanopores. Hydrogen molecules entrapped in narrow nanopores (<7 ) were immobile below 22-25 K. Mobility increased rapidly with temperature above this threshold, which is higher than the melting point of bulk hydrogen (13.9 K). Diffusion obeyed fixed-jump length mechanism, indistinguishable between 2D and 3D processes. Thermal activation of diffusion was characterized between ∼22 and 37 K, and structure-dependent differences were found between the two carbons. Activation energy of diffusion was higher than that of bulk solid hydrogen. Classical notions of liquid and solid do not longer apply for H 2 confined in narrow nanopores.
| Original language | English |
|---|---|
| Pages (from-to) | 1071-1082 |
| Number of pages | 12 |
| Journal | Carbon |
| Volume | 50 |
| Issue number | 3 |
| DOIs | |
| State | Published - Mar 2012 |
Funding
Research supported by the Materials Science and Engineering Division, Office of Basic Energy Sciences, US Department of Energy. QENS experiments were conducted at Oak Ridge National Laboratory (ORNL) Spallation Neutron Source (SNS) which is supported by the Scientific User Facility Division, Office of Basic Energy Sciences, US Department of Energy. ORNL is managed by UT-Battelle, LLC under contract DE-AC0500OR22725 for the US Department of Energy. The authors acknowledge stimulating discussions with Dr. Takeshi Egami (ORNL and University of Tennessee Joint Institute for Neutron Research) and Dr. Fred Baker (ORNL) for kindly supplying the UMC carbon. Two authors acknowledge their present (D.S.) and past (V.V.B.) appointments under ORNL post-doctoral associate program administered jointly by Oak Ridge Institute for Science and Education (ORISE), Oak Ridge Associated Universities (ORAU) and ORNL. This submission was supported by a contractor of the United States Government under Contract DE-AC05-00OR22725 with the United States Department of Energy. The United States Government retains, and the publisher, by accepting this submission for publication acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this submission, or allow others to do so, for United States Government purposes.