Quantum nature of adsorbed hydrogen on single-wall carbon nanohorns

Abstract

We have measured N2 adsorption isotherms on single-wall carbon nanohorns (SWNHs) over the temperature range of 77–92 K, and isosteric heat of adsorption, q st, were determined. Adsorption measurements for SWNHs have been also done for for H2 at 20 K, and for H2 and D2 at 77 K, respectively. We have performed grand canonical Monte Carlo (GCMC) simulations of N2, H2, and D2 for single-wall carbon nanotube (SWNT) models to compare with the experimental data. Simulated N2 adsorption isotherm on a SWNT bundle model is in reasonably good agreement with the experimental isotherm on the SWNH assembly over a wide range of pressures at 77 K; however, simulated q st-values for the SWNT bundle suggest that the SWNH particles are incompletely arranged in the SWNH assembly. Simulated endohedral isotherm of classical H2 inside an isolated SWNT model at 20 K showed that a density of adsorbed H2 in the internal space of the SWNH particle is quite smaller than that of classical H2 because of large quantum effects. In simulating H2 and D2 adsorption isotherms on the model SWNT bundle at 77 K, GCMC simulations based on the Feynman-Hibbs (FH) effective potential were applied to introduce quantum effects to the statistical properties generated by the classical Lennard-Jones (LJ) potential. We found that an adsorption ratio of H2 to D2 on the SWNT bundle from the FH-GCMC simulations is less than 0.91 depending on adsorption pressure; this is because the potential field of the SWNT bundle for H2 is relatively weaker than that for D2 even at 77 K, due to the wide quantum spreading of a H2 molecule.