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

Garcia, JF 2008, Neutron scattering studies and simulations of hydrogen adsorption in single-walled carbon nanotubes , PhD thesis, University of Salford.

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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.

Item Type: Thesis (PhD)
Contributors: Ross, K (Supervisor)
Schools: Schools > School of Computing, Science and Engineering
Depositing User: Institutional Repository
Date Deposited: 17 Aug 2021 09:53
Last Modified: 27 Aug 2021 21:57
URI: http://usir.salford.ac.uk/id/eprint/61590

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