This research investigates and closes a number of knowledge gaps in the safety of the indoor use of fuel cell and hydrogen (FCH) systems. New analytical models were developed and validated for steady state release and dispersion of hydrogen in an enclosure with one and two vents. Jet fire phenomena were studied for different combustion regimes, including well-ventilated and under-ventilated regimes, self-extinction, re-ignition and an external jet flame. To identify aims and objectives a broad literature review of theoretical, experimental and modelling work relating to hydrogen releases and jet fires has been completed. Based on knowledge gaps identified a model for passive ventilation of a sustained gaseous leak in an enclosure with one vent has been developed on the assumption of perfect mixing. The model predictions are compared against experimental helium concentration for both uniform and non-uniform helium-ail' mixtures. The criterion for mixture uniformity in an enclosure with one vent is suggested. Model equations derived for passive ventilation aided for the developments of engineering tools in the form of nomograms. These nomograms can be used to calculate the mass flow rate limit leading to 100% hydrogen concentration and calculation of steady state hydrogen concentration in the enclosure. In situations where simple analytical formulas are not applicable use of CFD is required. For this reason Numerical and physical requirements were formulated for simulations of light gas release and dispersion in one vent enclosure based on a parametric study and are acceptable from hydrogen safety engineering point of view. Hydrogen jet fire phenomenon requires numerical investigation therefore a study performed to identify general rules for hydrogen fire regimes in an enclosUl'e. Two modes of under-ventilated fires were observed and identified: external flame and self-extinction of the flame. The phenomenon of the hydrogen flame "re-ignition" by termination of hydrogen supply in an enclosure with one and two vents was reproduced in numerical experiments and recommendations are given. An equation is proposed foJ' the prediction of time when, two possible modes of under-ventilated regimes can occur: self-extinction or an external flame. The results of the current research have contributed to the European guidelines deliverable D 5.1 of the "Pre-normative research on safe indoor use of fuel cells and hydrogen systems" HyIndoor project (www.hyindoor.cu).

It is widely recognised that fossil fuels are finite, and alternatives should be investigated to secure future energy supplies. Much research is directed towards hydrogen as a fuel, but the gas is unmanageable without an effective storage and distribution strategy. This work investigates the Methylcyclohexane-Toluene-Hydrogen (MTH) system of hydrogen storage with a view to providing vehicular fuel or storing energy produced by intermittent producers. Stable liquid-hydrocarbon hydrogen storage enables hydrogen distribution using the existing fossil fuel network, eliminating the need to build a new fuel infrastructure. A literature survey is carried out covering the area of Liquid Organic Hydrogen Carriers (LOHCs). A study of the technoeconomic bottlenecks which would prevent the widespread use of the MTH system is conducted to direct the project research efforts, which reveals that the vehicular on-board dehydrogenation system must be reduced in size to be practical. Process intensification is attempted by dehydrogenating methylcyclohexane in the liquid-phase, which is experimentally demonstrated in this work (an original contribution). However, to be feasible for a vehicle, the liquid-phase dehydrogenation system demands a specific window of conditions, with hydrocarbon vapour pressure, enthalpy of reaction and equilibrium constant all being important factors. No window is possible to satisfy all conditions for the MTH system, which renders this vehicular system infeasible. Alternative liquid carriers are investigated to solve the problem, but no clear candidate carrier is found without using highly experimental and costly molecules. This leads to a new investigation of other applications for the MTH system. MCH for power to a Scottish whisky distillery is investigated, followed by an investment appraisal of the distillery system. The system is technically feasible but attracts a high capital expenditure (almost £16M) and operational cost (£2.4M annually) which is uncompetitive with alternative options such as biomass fuels. Finally, possible future work in the field of LOHC technology is considered.

Ozone is finding increasingly wider application across a range of industries, from semiconductor manufacture to water treatment. In principle, electrochemical ozone generation is capable of producing very high concentrations of ozone both in the gas phase and directly into solution, in contrast to Cold Corona Discharge, the present, most generally applied ozone technology. The aim of the work described in this thesis was to develop highly active and selective anodes for the generation of ozone based on Ni and Sb-doped SnO: Ni/Sb-SnO. 22 Each step of the synthesis of Ni/Sb-SnO-coated Ti mesh anodes was investigated in 2 detail and the electrodes so produced characterized by SEM and EDX, and their activity and selectivity determined using UV-Vis spectroscopy. Anodes with ozone current efficiencies of up to 50% in aqueous acidic electrolyte were developed, and efficiencies of ca. 30-40% were calculated routinely. However, such impressive efficiencies were derived only when operating the electrochemical cells in single pass mode. When the ozonated electrolyte was injected into the inlet of the electrochemical cell in order to generate gas phase ozone, efficiencies <10% were determined. Such an effect of ozone in solution inhibiting the ozone generating reaction, has not been determined with other electrocatalysts, and suggests that the mechanism of ozone evolution at Ni/Sb-SnO anodes is novel. This inhibiting effect of ozone was 2 investigated in detail and it was concluded that dissolved ozone was displacing a key intermediate or intermediates. Anode durability is a key issue, particularly when using aqueous acid. It was found that some anodes showed high stability, whilst others activated very quickly. When deactivation occurred was found to be due to physical loss of catalyst and/or etching. A key strategy with respect to the former challenge was to coat the Ti mesh with an Electro Deposited Inter Layer (EDIL), to protect the Ti from oxidation as this would lead to theformation of TiO and spalling of the catalyst layer. The methodology employed was 2 implemented initially by collaborators in Hong Kong University and based on precedent literature. In essence, etched Ti mesh was held at cathodic potentials in ethanolic solutions of SbCl and SnCl; this was reported to produce a protective EDIL containing 34 Sn and Sb. However, it was shown that, no Sn deposited, under the conditions employed. Furthermore, there was no evidence that the EDIL was effective with respect to durability and, indeed, the electrodeposition step introduced significant variability into the catalyst coating process. The reason why some catalysts were durable and others very short lived remains unclear; however, it was postulated that the former involved Ni as NiOOH. A strategy based on the addition of Au to prevent structural change of the SnO and/or 2 passivation of the SnO surface was investigated. Unfortunately, no beneficial effect (in 2 terms of durability, activity or ozone selectivity) was derived. 2 2 The scale up of the synthesis of the Ni/Sb-SnO anodes from 6.25 cm to 35.0 cm was 2 achieved successfully. Preliminary experiments with an industrial collaborator suggested the technology is transferable. Preliminary experiments using a prototype water/air cell gave very promising current efficiencies (up to 22%) and showed a major design flaw in the prototype in terms of the compression of the anode/Nafion/cathode membrane electrode assembly.

The main objective of this study is to investigate on the ability of a perovskite-based membrane reactor to produce hydrogen via simultaneous reforming and water splitting processes. Being able to perform such processes will confirm on the ability of the membrane system in performing an autothermal production of hydrogen. Initial experiments were conducted to evaluate the ability of two different types of hollow fibre membrane namely La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF6428) and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF5582) in permeating oxygen in three different inlet configurations. All of the experiments were conducted at 900oC. The LSCF6428 membrane gives lower oxygen permeation rate comparing to BSCF5582 when inert gas argon was used as the sweep gas on the shell side of the membrane. The oxygen permeation rate into the shell side of LSCF6428 membrane reactor was at 0.24μmolO.s-1 whereas for BSCF5582 was at 1.50μmol O.s-1. The trend is similar when the shell sides were fed with 5% methane and the lumen sides were fed with 10% oxygen. In these experiments, both membranes were stable enough to perform oxygen permeation up to more than 100 hours of operation. BSCF5582 membrane however shows instability in performing oxygen permeation when the lumen side was fed with 4% water and shell side was fed with 5% methane. BSCF5582 membrane was only able to perform oxygen permeation for less than two hours before showing substantial amount of leaks upon breaking. In contrast, the iii LSCF6428 membrane shows good stability in the same condition with the shell side oxygen permeation rate of 0.04±0.01μmolO.s-1. The experiment operating time lasted for more than 90 hours. Based on its stability in performing oxygen permeation in the combination of highly reducing and highly oxidising environment, the LSCF6428 membranes were chosen to perform the simultaneous methane reforming and water splitting process in a multiple-membrane based reactor. The results obtained from this experiment proved that simultaneous methane reforming and water splitting can be achieved using a membrane reactor.

No description available.

This thesis presents work seeking to elucidate the active site and the mechanism of ozone generation at nickel and antimony-doped tin oxide (NATO) electrodes. To this end, tin oxide (TO, SnO2), antimony-doped tin oxide (ATO, Sb-SnO2) and nickel-antimony doped tin oxide (NATO, Ni/Sb-SnO2) nanopowders were prepared via a hydrothermal (HT) method and either left uncalcined or calcined at 300, 400 and 700 oC. The nanopowders were characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), measurement of surface area by the Brunauer Emmett Teller (BET) technique, thermogravimetric analysis (TGA), diffuse reflectance Fourier Transform Infra-Red spectroscopy (DRIFTS) and X-ray Photoelectron Spectroscopy (XPS). The electrochemical ozone activity and selectivity of the powders were also determined in 0.5M H2SO4 and compared to those of ceramic anodes prepared via conventional methods. All the nanopowders showed a single cassiterite phase with crystallite sizes that varied with composition and calcination temperature. The BET surface areas of the nanopowders decreased with increasing calcination temperature and also on doping with Sb and Ni. The BET surface areas in general were smaller than those calculated from XRD, suggesting the agglomeration of crystallites to form larger grains. Addition of Sb to undoped SnO2 resulted in a significant increase in the number of crystallites per grain. Co-doping with Ni initially caused a large reduction in the number of crystallite per grain, but not back to the undoped value, with additional Ni having little or no effect. The ozone activities and selectivity of the nanopowders were studied by UV-Vis spectroscopy in 0.5M H2SO4 by deposition onto Ti foil substrates and using a UV-Vis cuvette as the electrochemical cell. The data so obtained were compared to results using a ceramic Ni/Sb-SnO2 anode prepared via the conventional method. All the Ni/Sb-SnO2 nanopowders calcined at 400 oC were inactive with respect to ozone, whilst the Ni/Sb-SnO2 nanopowders calcined at 700 oC were all active, showing comparable current densities and ozone current efficiencies to those observed using the ceramic anodes. This was the first work to show ozone generated with high selectivity and activity at Ni/Sb-SnO2 nanopowders. Durability studies on a ceramic anode showed no change in ozone activity or selectivity over a 10 day period, supporting the results of earlier such studies in Newcastle and strongly suggesting that the Ni species responsible for ozone evolution at Ni/Sb-SnO2 is not located at the surface. A key aspect of the research programme was the study of undoped SnO2 calcined at 400 oC and 700 oC using BET, XRD, TGA-MS and in-situ variable temperature DRIFTS. BET showed the relatively high surface area and nanometer scale of the SnO2 particles, whilst XRD confirmed the nano dimension of the crystallites and showed only the cassiterite phase. TGA analysis indicated four temperature regions over which mass loss was observed. These and the in-situ DRIFTS studies revealed the existence of various forms of water associated with specific crystal facets of the SnO2, as well as existence of isolated O-H groups and adsorbed oxygen species. For the (100) facets, hydrogen bonding does not occur, and water absorption is less strong than for the (111) and (110) facets where hydrogen bonding does occur. On the (100) facets, the hydrogen atoms of the OH groups are located in cavities in the plane of the O atoms, and hence are unavailable for hydrogen bonding. In contrast, the H atoms on the (111) and (110) facets are available. The samples calcined at 700 oC showed significantly less adsorbed water than those calcined at 400 oC, and this could be attributed to lower coverage by OH on the former. The reversible uptake of oxygen was observed in the TGA studies, and this seeded the development of the final model. Electronic absorptions were also observed and the data rationalised in terms of the existence of both free electron absorptions, and absorptions from oxygen vacancy states. XPS of the Sb-containing nanopowders (i.e. Sb-SnO2 and Ni/Sb-SnO2) showed Sn in the +4 oxidation state, whilst Sb was present as both Sb(III) and Sb(V), and Ni as Ni(II) and Ni(III). Combining these studies with TGA-MS, it was shown that Sb(V) ions substitute for Sn(IV) in the lattice, with a preference for centrosymmetric coordination sites whilst the Sb(III) ions occur at the grain boundaries or surface. The Sb(V) ions confer electronic conductivity on the SnO2 whilst both Sb(III) and Ni are essential for O3 generation. The Ni occupies Sn(IV) sites in the subsurface region at concentrations below the detection limit of XPS. A model was postulated on the basis of the data, as well as a mechanism for ozone generation. The remediation of the Reactive Blue dye (RB50) in 0.5M H2SO4 was studies using both powder and ceramic anodes. Decolourization of RB50 solution was achieved within minutes of electrolysis, with COD and TOC removal of more than 80%. In addition to identifying a possible mechanism for ozone formation, the work reported in this thesis resulted in the production of active nanopowders which will allow the fabrication of high surface-area anodes with the potential to exceed the space-time yield of β-PbO2 anodes, permitting the application the Ni/Sb-SnO2 anodes in the treatment of real waters. A key aspect of the research programme was the study of undoped SnO2 calcined at 400 oC and 700 oC using BET, XRD, TGA-MS and in-situ variable temperature DRIFTS. BET showed the relatively high surface area and nanometer scale of the SnO2 particles, whilst XRD confirmed the nano dimension of the crystallites and showed only the cassiterite phase. TGA analysis indicated four temperature regions over which mass loss was observed. These and the in-situ DRIFTS studies revealed the existence of various forms of water associated with specific crystal facets of the SnO2, as well as existence of isolated O-H groups and adsorbed oxygen species. For the (100) facets, hydrogen bonding does not occur, and water absorption is less strong than for the (111) and (110) facets where hydrogen bonding does occur. On the (100) facets, the hydrogen atoms of the OH groups are located in cavities in the plane of the O atoms, and hence are unavailable for hydrogen bonding. In contrast, the H atoms on the (111) and (110) facets are available. The samples calcined at 700 oC showed significantly less adsorbed water than those calcined at 400 oC, and this could be attributed to lower coverage by OH on the former. The reversible uptake of oxygen was observed in the TGA studies, and this seeded the development of the final model. Electronic absorptions were also observed and the data rationalised in terms of the existence of both free electron absorptions, and absorptions from oxygen vacancy states. XPS of the Sb-containing nanopowders (i.e. Sb-SnO2 and Ni/Sb-SnO2) showed Sn in the +4 oxidation state, whilst Sb was present as both Sb(III) and Sb(V), and Ni as Ni(II) and Ni(III). Combining these studies with TGA-MS, it was shown that Sb(V) ions substitute for Sn(IV) in the lattice, with a preference for centrosymmetric coordination sites whilst the Sb(III) ions occur at the grain boundaries or surface. The Sb(V) ions confer electronic conductivity on the SnO2 whilst both Sb(III) and Ni are essential for O3 generation. The Ni occupies Sn(IV) sites in the subsurface region at concentrations below the detection limit of XPS. A model was postulated on the basis of the data, as well as a mechanism for ozone generation. The remediation of the Reactive Blue dye (RB50) in 0.5M H2SO4 was studies using both powder and ceramic anodes. Decolourization of RB50 solution was achieved within minutes of electrolysis, with COD and TOC removal of more than 80%. In addition to identifying a possible mechanism for ozone formation, the work reported in this thesis resulted in the production of active nanopowders which will allow the fabrication of high surface-area anodes with the potential to exceed the space-time yield of β-PbO2 anodes, permitting the application the Ni/Sb-SnO2 anodes in the treatment of real waters.

Dealing with carbon dioxide waste is an on-going societal and technological challenge. One attractive proposition is to chemically convert waste carbon dioxide into useful chemical products. One possible route is to combine two well-known chemical processes, reverse water gas shift and Fischer-Tropsch synthesis, to make a catalyst capable of converting carbon dioxide directly into hydrocarbons. Iron nanoparticles supported on carbon nanotubes (CNT) have shown promise in the Fischer-Tropsch process. In this thesis, iron nanoparticles supported on carbon nanotubes (Fe@CNT) are shown to be effective catalysts for the coupled reverse water gas shift and Fischer-Tropsch reactions. Controlled oxidation of synthesised CNT can remove the graphitic shell from residual iron nanoparticles, activating them for catalysis. This process removes the need for expensive purification of CNT prior to use. Carbon nanotube powders generated in this way are difficult to handle, and could be difficult to scale-up. A method has been developed to grow long, aligned carbon nanotubes on a commercial cordierite monolith support, which has potential for scale up. The developed method does not require pre-treatment of the monolith prior to CNT synthesis. Using the same oxidation method these Fe@CNTs-monoliths have been demonstrated to act as catalysts for carbon dioxide conversion. The monolithic catalysts demonstrate improved mass transfer capabilities, leading to higher activities for the monolithic catalyst over a similar powder catalyst.

No description available.

Gas-liquid flow may be characterised in terms of the gas void fraction, α. This is an important variable in two-phase flow, used in predicting the occurrence of flow regimes, and the associated pressure drop, and mass and heat transfer. The gas void fraction transitions in a two-phase flow system from uniform bubble flow (homogeneous) to churn-turbulent bubble flow (heterogeneous) in an open tube bubble column (OTBC) and an annular gap bubble column (AGBC) have been investigated using a vertical column with an internal diameter of 0.102 m, containing a range of concentric inner tubes which formed an annular gap; the inner tubes had diameter ratios from 0.25 - 0.69. Gas (air) superficial velocities in the range 0.014-0.200 m/s were studied. Tap water and aqueous solutions of ethanol and isopropanol, with concentrations in the range 8 - 300 ppm by mass, were used as the working liquids. Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in the OTBC and the AGBC, when operated at the same gas superficial velocity using a porous sparger. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (i) the presence of the inner tube causes large bubbles to form near the sparger, which destabilize the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles. Moreover, (ii) the shape of the void fraction profiles changes with gap geometry, which affects the distribution parameter in the drift flux model. Radial profiles of the local void fraction were obtained using a two- and four-point conductivity probe, and were cross-sectionally averaged to give mean values that were within 12% of the volume-averaged gas void fractions obtained from changes in aerated level. The presence of alcohol inhibited the coalescence between the bubbles, and consequently increased the mean gas void fraction at a given gas superficial velocity in both the open tube and the annular gap bubble columns. This effect also extended the range of homogeneous bubbly flow and delayed the transition to heterogeneous flow. Moreover, isopropanol results gave slightly higher mean void fractions compared to those for ethanol at the same mass fraction, due to their increased carbon chain length. It was shown that the void fraction profiles in the annular gap bubble column were far from uniform, leading to lower mean void fractions than were obtained in an open tube for the same gas superficial velocity and liquid composition. The chord length measurements in the OTBC for both the tap water and alcohol solutions exhibited two trends with respect to increasing j_g: (i) at low j_g, in the homogeneous flow, an increasing function was obtained; and (ii) with further increase in j_g, a reduction in the chord length was observed. In the presence of the orifice, the results concerning mean chord lengths show a decreasing function of the bubble size with increasing j_g; this was visually demonstrated using photographs. For the AGBC, the chord lengths obtained from the conductivity probe offered evidence of the bubble size decreasing as j_g increased in the heterogeneous regime, which agreed with the findings of the OTBC. This was also confirmed using the results obtained from photographs. A novel approach for bubble size transformation was implemented to process the conductivity probe measurements. An analytical method was used as a forward transform to predict the chord length distribution from the bubble size distribution and an optimisation approach was applied as a backward transform method to obtain the bubble size distribution from the chord length distribution. The challenge was to consider a variable aspect ratio, φ, for the bubble shape, which depended on their size. The model gave excellent and reasonable predictions for the bubble sizes as their trends were identical to the trend of the chord length, and to the bubble size obtained from photographs.

Chapter 1 - Introduction In this Chapter brief introductions to both excited states and time-resolved spectroscopy are provided. The majority of the work described in this Thesis has utilised time-resolved infrared (TRIR) spectroscopy on an ultrafast time-scale, and as such, the details of this approach are described. Chapter 2 - Utilising TRIR spectroscopy to probe the photophysics of Re and Ru dppz complexes The photophysics of several Re and Ru complexes containing the diimine ligand dppz have been investigated using ps and ns TRIR, in the v(CO) and 1600-1300 cm-I spectral regions. From the spectra in the < 1600 cm-I region it has been possible to differentiate between the types of excited states formed following photolysis for both previously assigned and novel complexes.