This thesis describes an investigation into the hydrogen storage properties of zeolites and high surface area carbons. In particular, zeolite X, zeolite Y, zeolite A, zeolite Rho, carbon nanotubes and activated carbons materials were studied. The zeolites encompass a range of different pore geometries and compositions. The aim was to investigate the potential of zeolites as an alternative to carbons in low-cost, safe hydrogen stores, particularly for large-scale stationary applications. Zeolites were synthesised by hydrothermal methods, and different cation-exchanged forms were prepared. through ion exchange from aqueous metal nitrate solutions. All hydrogen storage capacities were measured at room temperature or 77 K and a pressure range of 0 to 15 bar, using a constant pressure thermogravimetric analyser. The results show that, zeolites exhibit diverse behaviour with respect to hydrogen uptake, dependent on both the framework structure and the nature of the cations present. A major factor influencing uptake is the available void space: in zeolites A and Rho, pore blocking by large extraframework cations is a prominent phenomenon restricting hydrogen uptake, but in zeolites X and Y, blocking of supercages by exchangeable cations does not occur. In general, gravimetric uptake is affected by the weight of the zeolite since hydrogen uptakes in heavier zeolites were relatively lower. Volumetric hydrogen storage capacities show that zeolites are roughly twice as efficient in binding hydrogen than activated carbons. This study also suggests that, extraframework cations act as binding sites for hydrogen molecules. In both zeolites and carbons, hydrogen adsorption occurs by physisorption, and the adsorption - desorption process is fast and completely reversible. Preliminary adsorption theory analysis shows that, the hydrogen adsorption isotherms conform reasonably well to the Langmuir equation. For carbons, zeolite X and zeolite Y, hydrogen uptake relates closely with the BET surface area. Suitably ion-exchanged zeolites offer great promise as low-cost, safe hydrogen storage media for stationary applications. Further work, involving detailed characterisation, needs to be carried out to fully explore the hydrogen storage and engineering properties of these materials.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:404468 |
| Date | January 2004 |
| Creators | Langmi, Henrietta Wakuna |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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