Competition of quantum effects in H$_2$ /D$_2$ sieving in carbon nanotubes

TL;DR

This study investigates quantum effects in hydrogen and deuterium separation within carbon nanotubes, revealing how resonances influence diffusion rates and challenge expected isotope separation behaviors at low temperatures.

Contribution

It provides a detailed quantum dynamical analysis of isotopologue diffusion in carbon nanotubes, highlighting the role of resonances in quantum sieving.

Findings

Resonances significantly affect diffusion channels.

Quantum effects alter isotope separation efficiency.

Inverse kinetic isotope effect is suppressed at low temperatures.

Abstract

Nanoporous materials have the potential to be used as \emph{molecular sieves} to separate chemical substances in a mixture via selective adsorption and kinetic sieving. The separation of isotopologues is also possible via the so-called \emph{quantum sieving} effect: the different effective size of isotopologues due to their different ZPE. Here we compare the diffusion rates of Hydrogen and Deuterium in (8,0) Single Walled Carbon Nanotubes obtained through quantum dynamics methods. The diffusion channels obtained present important contributions from resonances connecting the potential wells. These resonances, which are more important for H$_2$ than for D$_2$, increase the low-temperature diffusivity of both isotopologues, but prevent the inverse kinetic isotope effect reported for similar nanostructured systems.