The dehydrogenation reactions between LiH and organic amines for solid state hydrogen storage systems

Hydrogen is considered as an appealing alternative to fossil fuels in the pursuit of sustainable, secure and prosperous growth in the UK and abroad. However there exists a persisting bottleneck in the effective storage of hydrogen for mobile applications in order to facilitate a wide implementation of hydrogen fuel cells in the fossil fuel dependent transportation industry. To address this issue, new means of solid state chemical hydrogen storage are proposed in this thesis. This involves the coupling of LiH with three different organic amines: melamine, urea and dicyandiamide. In principle, thermodynamically favourable hydrogen release from these systems proceeds via the deprotonation of the protic N-H moieties by the hydridic metal hydride. Simultaneously hydrogen kinetics is expected to be enhanced over heavier hydrides by incorporating lithium ions in the proposed binary hydrogen storage systems. Whilst the concept has been successfully demonstrated by the results obtained in this work, it was observed that optimising the ball milling conditions is central in promoting hydrogen desorption in the proposed systems. The theoretical amount of 6.97 wt% by dry mass of hydrogen was released when heating a ball milled mixture of LiH and melamine (6:1 stoichiometry) to 320 °C. It was observed that ball milling introduces a disruption in the intermolecular hydrogen bonding network that exists in pristine melamine. This effect extends to a molecular level electron redistribution observed as a function of shifting IR bands. It was postulated that stable phases form during the first stages of dehydrogenation which contain the triazine skeleton. Dehydrogenation of this system yields a solid product Li2NCN, which has been rehydrogenated back to melamine via hydrolysis under weak acidic conditions. On the other hand, the LiH and urea system (4:1 stoichiometry) desorbed approximately 5.8 wt% of hydrogen, from the theoretical capacity of 8.78 wt% (dry mass), by 270 °C accompanied by undesirable ammonia and trace amount of water release. The thermal dehydrogenation proceeds via the formation of Li(HN(CO)NH2) at 104.5 °C; which then decomposes to LiOCN and unidentified phases containing C-N moieties by 230 °C. The final products are Li2NCN and Li2O (270 °C) with LiCN and Li2CO3 also detected under certain conditions. It was observed that ball milling can effectively supress ammonia formation. Furthermore results obtained from energetic ball milling experiments have indicated that the barrier to full dehydrogenation between LiH and urea is principally kinetic. Finally the dehydrogenation reaction between LiH and dicyandiamide system (4:1 stoichiometry) occurs through two distinct pathways dependent on the ball milling conditions. When ball milled at 450 RPM for 1 h, dehydrogenation proceeds alongside dicyandiamide condensation by 400 °C whilst at a slower milling speed of 400 RPM for 6h, decomposition occurs via a rapid gas desorption (H2 and NH3) at 85 °C accompanied by sample foaming. The reactant dicyandiamide can be generated by hydrolysis using the product Li2NCN.

Radiogenic isotopes in the Mediterranean Sea: water mass exchange and precessional variability during the Messinian

During the late Miocene, exchange between the Mediterranean Sea and Atlantic Ocean changed dramatically, culminating in the Messinian Salinity Crisis (MSC). Understanding Mediterranean-Atlantic exchange at that time could answer the enigmatic question of how so much salt built up within the Mediterranean, while furthering the development of a framework for future studies attempting to understand how changes may have impacted global thermohaline circulation. Due to their association with specific water masses at different scales, radiogenic Sr, Pb, and Nd isotope records were generated from various archives contained within marine deposits to endeavour to understand better late Miocene Mediterranean-Atlantic exchange. The archives used include foraminiferal calcite (Sr), fish teeth and bone (Nd), dispersed authigenic ferromanganese oxyhydroxides (Nd, Pb), and a ferromanganese crust (Pb). The primary focus is on sediments preserved at one end of the Betic corridor, a gateway that once connected the Mediterranean to the Atlantic through southern Spain, although other locations are investigated. The Betic gateway terminated within several marginal sub-basins before entering the Western Mediterranean; one of these is the Sorbas Basin, a well-studied location whose sediments have been astronomically tuned at high temporal resolution, providing the necessary age control for sub-precessional resolution records. Since the climatic history of the Mediterranean is strongly controlled by precessional changes in regional climate, the aim was to produce records at high (sub-precessional) temporal resolution, to be able to observe clearly any precessional cyclicity driven by regional climate which could be superimposed over longer trends. This goal was achieved for all records except the ferromanganese crust record. The 87Sr/86Sr isotope record (Ch. 3) shows precessional frequency excursions away from the global seawater curve. As precessional frequency oscillations are unexpected for this setting, a numerical box model was used to determine the mechanisms causing the excursions. To enable parameterisation of model variables, regional Sr characteristics, data from general circulation model HadCM3L, and new benthic foraminiferal assemblage data are employed. The model results imply that the Sorbas Basin likely had a positive hydrologic budget in the late Miocene, very different to that of today. Moreover, the model indicates that the mechanism controlling the Sr isotope ratio of Sorbas Basin seawater was not restriction, but a lack of density-driven exchange with the Mediterranean. Beyond improving our understanding of how marginal Mediterranean sub-basins may evolve different isotope signatures, these results have implications for astronomical tuning and stratigraphy in the region, findings which are crucial considering the geological and climatic history of the late Miocene Mediterranean is based entirely on marginal deposits. An improved estimate for the Nd isotope signature of late Miocene Mediterranean Outflow (MO) was determined by comparing Nd isotope signatures preserved in the deeper Alborán Sea at ODP Site 978 with literature data as well as the signature preserved in the Sorbas Basin (Ch. 4; -9.34 to -9.92 ± 0.37 εNd(t)). It was also inferred that it is unlikely that Nd isotopes can be used reliably to track changes in circulation within the shallow settings characteristic of the Mediterranean-Atlantic connections; this is significant in light of a recent publication documenting corridor closure using Nd isotopes. Both conclusions will prove useful for future studies attempting to understand changes in Mediterranean-Atlantic exchange. Excursions to high values, with precessional frequency, are also observed in the radiogenic Pb isotope record for the Sorbas Basin (Ch. 5). Widening the scope to include locations further away from the gateways, records were produced for late Miocene sections on Sicily and Northern Italy, and similar precessional frequency cyclicity was observed in the Pb isotope records for these sites as well. Comparing these records to proxies for Saharan dust and available whole rock data indicates that, while further analysis is necessary to draw strong conclusions, enhanced dust production during insolation minima may be driving the observed signal. These records also have implications for astronomical tuning; peaks in Pb isotope records driven by Saharan dust may be easier to connect directly to the insolation cycle, providing improved astronomical tuning points. Finally, a Pb isotope record derived using in-situ laser ablation performed on ferromanganese crust 3514-6 from the Lion Seamount, located west of Gibraltar within the MO plume, has provided evidence that plume depth shifted during the Pliocene. The record also suggests that Pb isotopes may not be a suitable proxy for changes in late Miocene Mediterranean-Atlantic exchange, since the Pb isotope signatures of regional water masses are too similar. To develop this record, the first published instance of laser ablation derived 230Thexcess measurements are combined with 10Be dating.