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

The storage of hydrogen is one of the main problems that needs to be solvedbefore hydrogen can become a real alternative to oil in mobile applications.Physisorption of hydrogen in an adsorbate is one of the possible solutions tothis problem.This thesis studies the adsorption of hydrogen in Single-Walled Carbon Nanotubes(SWNTs). Neutron scattering techniques are used to probe the possibleadsorption sites and the interaction between the hydrogen and the nanotubes atthose sites.First, the thesis studies the diffraction of SWNT bundles and the changesto the pattern when hydrogen is adsorbed. The results of a neutron diffractionexperiment performed at the LOQ diffractometer at ISIS are presented. Theexperiment measures the diffraction pattern of SWNT bundles at low temperaturewith different amounts of deuterium adsorbed. The results are compared tocomputer simulations in order to identify the changes in the diffraction patternand extract information about the adsorption sites. The experiments show a shiftof the lattice parameter as we increase the amount of deuterium. This increase isassociated with the hydrogen molecule pushing the nanotubes apart as they getadsorbed in the interstitial channels and in the grooves of the bundles.Secondly, the results of an inelastic neutron scattering experiment performedat the MARI spectrometer at ISIS are presented. The experiment measures thespectrum of the neutrons scattered from the hydrogen molecule when adsorbedin SWNTs at 20 K. The spectrum is measured at different concentrations ofhydrogen on the nanotubes. This experiment provides information about the Qdependence of the J = 0 to J = 1 rotational transition of the hydrogen molecule when adsorbed on the nanotubes. Information about the potential felt by themolecule in the adsorption sites can be extracted by comparing the spectrum ofthe free hydrogen molecule to the spectrum measured in this experiment.The results of the energy dependence show two different effects related tothe adsorption sites that are occupied by the hydrogen molecules. First, thereis a contribution originated from hydrogen molecules that are not perturbed intheir rotational motion, corresponding to adsorption on the external surface ofthe nanotube bundles. Second, there is a contribution from hydrogen moleculesadsorbed in a site that perturbs their rotational motion, producing a split intheir 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 theintensity of the split energy levels in the perturbed sites determine that the typeof site that is preferentially occupied is the groove of the bundles.For the momentum dependence, the results show the mean squared displacementof the adsorbed hydrogen molecules as a function of the amount of hydrogenin the sample. The hydrogen molecules are more tightly bound for all concentrationsthan the hydrogen molecules in solid hydrogen. Additionally, as the amountof hydrogen in the sample increases, the molecules interact more strongly amongthemselves and thus the mean squared displacement decreases.

Level dc:type.qualificationlevel

Doctoral (Level 8)

Year dc:date.issued

2008

Author dc:creator

• Garcia, JF

Language dc:language

en

Identifier

oai:salford-repository.worktribe.com:1337170

OAI identifier oai:identifier

oai:salford-repository.worktribe.com:1337170