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Functional Materials for Energy

Heavier D2 moves much faster than H2 at low temperatures


Quasi-elastic neutron scattering data show that deuterium becomes mobile at lower temperature (16 K; red symbols) than hydrogen (22 K; blue symbols). Mobility is shown by broadening of the scattering line when neutrons collide with moving molecules.

Quantum confinement effects in carbon nanopores cause deuterium to diffuse as much as 76 times faster than the much lighter hydrogen at low temperatures. This surprising experimental result suggests that selectivity levels for separation of hydrogen isotopes unobtainable by other methods can be reached by using quantum properties of light molecules. At extremely low temperatures, light particles tend to be less localized, so the “cloud” of lighter H2 molecules is always larger than that of D2. Thus, “bulkier” hydrogen moves much slower than “slimmer” deuterium in narrow carbon nanopores, as shown by energy transferred in collisions with a neutron flux. This first time ever experimental demonstration of highly selective quantum sieving was made possible by the BASIS spectrometer at the Spallation Neutron Source (SNS).

C. I. Contescu, H. Zhang, R. J. Olsen, E. Mamontov, J. R. Morris, and N. C. Gallego, “Isotope effect on adsorbed quantum phases:  diffusion of H2 and D2 in nanoporous carbon,” Phys. Rev. Lett. 110, 236102 (2013).

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