Neutron scattering studies and simulations of hydrogen adsorption in single-walled carbon nanotubes

Garcia, JF

Abstract

The storage of hydrogen is one of the main problems that needs to be solved
before hydrogen can become a real alternative to oil in mobile applications.
Physisorption of hydrogen in an adsorbate is one of the possible solutions to
this problem.
This thesis studies the adsorption of hydrogen in Single-Walled Carbon Nanotubes
(SWNTs). Neutron scattering techniques are used to probe the possible
adsorption sites and the interaction between the hydrogen and the nanotubes at
those sites.
First, the thesis studies the diffraction of SWNT bundles and the changes
to the pattern when hydrogen is adsorbed. The results of a neutron diffraction
experiment performed at the LOQ diffractometer at ISIS are presented. The
experiment measures the diffraction pattern of SWNT bundles at low temperature
with different amounts of deuterium adsorbed. The results are compared to
computer simulations in order to identify the changes in the diffraction pattern
and extract information about the adsorption sites. The experiments show a shift
of the lattice parameter as we increase the amount of deuterium. This increase is
associated with the hydrogen molecule pushing the nanotubes apart as they get
adsorbed in the interstitial channels and in the grooves of the bundles.
Secondly, the results of an inelastic neutron scattering experiment performed
at the MARI spectrometer at ISIS are presented. The experiment measures the
spectrum of the neutrons scattered from the hydrogen molecule when adsorbed
in SWNTs at 20 K. The spectrum is measured at different concentrations of
hydrogen on the nanotubes. This experiment provides information about the Q
dependence of the J = 0 to J = 1 rotational transition of the hydrogen molecule when adsorbed on the nanotubes. Information about the potential felt by the
molecule in the adsorption sites can be extracted by comparing the spectrum of
the free hydrogen molecule to the spectrum measured in this experiment.
The results of the energy dependence show two different effects related to
the adsorption sites that are occupied by the hydrogen molecules. First, there
is a contribution originated from hydrogen molecules that are not perturbed in
their rotational motion, corresponding to adsorption on the external surface of
the nanotube bundles. Second, there is a contribution from hydrogen molecules
adsorbed in a site that perturbs their rotational motion, producing a split in
their energy levels. There are two types of site that can produce this effect:
the interstitial channels and the grooves of the bundles. The ratio between the
intensity of the split energy levels in the perturbed sites determine that the type
of site that is preferentially occupied is the groove of the bundles.
For the momentum dependence, the results show the mean squared displacement
of the adsorbed hydrogen molecules as a function of the amount of hydrogen
in the sample. The hydrogen molecules are more tightly bound for all concentrations
than the hydrogen molecules in solid hydrogen. Additionally, as the amount
of hydrogen in the sample increases, the molecules interact more strongly among
themselves and thus the mean squared displacement decreases.

• Thesis Type: Thesis
• Deposit Date: Aug 17, 2021
• Award Date: Aug 1, 2008