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Persistent URL http://purl.org/net/epubs/work/67426185
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Record Id 67426185
Title Hydrogen isotope separation using porous materials
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Abstract Hydrogen (H) and its stable heavy isotopes deuterium (D) and tritium (T) play important roles in clean energy supply, from direct combustion to future energy development (nuclear fusion). H is a globally abundant green resource, D is recognised as an outstanding moderator in heavy-water nuclear reactors with good moderation and low absorption cross-section, and both D and T are vital components in the fuel cycle of fusion reactors. However, D and T only naturally occur in a mixture of the protium isotope at an extremely low percentage. To maximise the concentration of the isotopes, purification or separation becomes necessary. To achieve a higher separation efficiency of hydrogen isotopes, adsorption separation based on porous materials was investigated in this thesis using porous materials such as porous organic cages (POCs) and trapdoor zeolites. This thesis begins with a review of the state-of-the-art of H2 and D2 separation (Chapter 1), discussing the alloys used in the nuclear industry followed by the porous materials that meet the requirements of industry with stable structures and thermal responses. Pd-based alloys are still used in the nuclear industry for H2 separation although they are limited by their functional periods, high material cost and high operation temperatures. The industry is therefore seeking to replace these alloys. Porous materials reported recently with high structural stability for H2 isotope separation through physisorption were introduced in the following section. Apart from isotope separation, some candidates that can solve another pressing industrial challenge (the storage issue of T2 in the gas phase) were also examined for future applications. This section further suggests stable porous materials for both hydrogen isotope separation and storage. As indicated in the review, the nanometre-scale pores in POCs had previously been found to be beneficial for the adsorptive separation of hydrogen isotopes based on kinetic quantum sieving (KQS). However, instead of focusing on the difference in quantities adsorbed of the different isotopes as had been done in the past, this research looked in depth at the dynamics of the sorption process to separate H2 and D2 with more practical operational temperatures. Herein, Chapter 2 included the methodologies used in candidate synthesis and characterisation. Following that, in Chapter 3, various neutron techniques were selected to study the data collection and analysis. Neutron Compton scattering (NCS) and quasi-elastic neutron scattering (QENS) showed that the kinetic difference of the isotopes was greater at 77 K compared to 50 K based on momentum distribution and diffusion coefficients. The kinetic difference demonstrated on H2 and D2 by QENS proved that D2 became localised in the pores and behaved differently from the two-phased jumping diffusion behaviour of the H2. However, limitations in material stability are a barrier to the application of organic materials. Thus, another class of thermally controllable porous candidates (trapdoor zeolites) for H2 isotope separation with higher thermal and structural stability are explored. Two families of trapdoor zeolites with two different framework types (merlinoite (MER) and chabazite (CHA)) were analysed to understand the influence of the door-keeping cation (K+, Rb+, Cs+) and framework structure (MER, CHA) on the thermal behaviour and gas uptakes (second data chapter, Chapter 4)). Before applying the candidates to hydrogen isotope separation, an understanding of the trapdoor principle and gaseous behaviour was essential. Various techniques were applied in this section to help the characterisation. Simultaneous thermal analysis (STA) showed the effect of the size of the cations on thermal behaviour. Breakthrough experiments for CO2 (to simulate the potential behaviour of D2) and H2 were measured below the threshold temperature (300 K) and above the threshold temperature (348 K) to demonstrate the thermal dependence of adsorbing guest molecules like CO2 was different for MER and CHA. In-situ powder x-ray diffraction (PXRD) with H2 dosing further indicated the interaction between H2 and the frameworks was more significant above the threshold temperature (348 K), although the higher temperatures limit the adsorption amount. To complete the research on the H2 and D2 behaviour in the zeolites system, the final study not only tested the threshold temperature of H2 but also focused on both kinetic and quantitative analyses of the candidates with different trapdoor layouts (Chapter 5). Both XPS and FTIR results confirmed that the synthesised candidates had identical Si/Al ratios and chemical bondings, but the coherent bonding environment was different in MER and CHA frameworks. The measurement of the threshold temperature of H2 on all six candidates (KCHA, RbCHA, CsCHA, KMER, RbMER and CsMER) indicated the influence of the cation’s size in the thermal response under a gas environment. The preference of D2 was noticed during adsorption and with faster diffusion compared to H2 at that temperature, suggesting the heaviest cation Cs+ performed the best blockage of the functional window with the highest D2/H2 ratio at around 1.89 at 0.9 bar at 77 K. However, the adsorption capacity was also an important factor of the porous materials. The size of the heavier cation limited the sorption capacities, with the sorption amount being halved when the cation changed from K+ to Cs+. Therefore, the thesis suggests that a fully reversible process by breakthrough or temperature swing using KMER or RbCHA materials would be more commercially applicable compared to the current H2 and D2 separation methods.
Organisation ISIS , ISIS-VESUVIO , STFC
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Language English (EN)
Type Details URI(s) Local file(s) Year
Thesis PhD, University of Bristol, 2024. https://research-…ng-porous-materials/ 2024