No description available.

No description available.

No description available.

No description available.

A survey of the literature showed that nitrobenzene and related radical-anions could be obtained from a variety of electron transfer reactions. The possibility of using hydroxylarnine and N-substituted hydroxylamines as electron donors to aromatic mono nitro compounds, in oxygen free, aqueous, alkaline ethanol, was investigated. Five of the seven hydroxylamine compounds investigated, hydroxylarnine, N-methylhydroxylamine, N-phenylhydroxylamine, N-benzylhydroxylamine and N-benzhydrylhydroxylamine, were effective in transferring an electron to a wide range of nitro compounds, the radical-anions produced being detected by electron spin resonance spectroscopy. The spectra of these radicals were recorded, and hyperfine splitting constants measured and compared with values in the literature. g-Values of the radicals were also measured and it was found for the nitro radical-anions containing halogen substituents that this increased - Br &gt; Cl &gt; F &gt; H. The limit of the reducing power of the hydroxylamine compounds was reached with para-nitroanisole, E_1/4 = 0.957v, para-nitrophenol not being reduced by this method. An investigation into the mechanism of this reaction and its products then followed. The build-up and decay of the nitro radical-anions were followed using stopped flow techniques. The general pattern which emerged was that of a very rapid build-up of the radical, reaching the maximum intensity within a minute of mixing the reactants, followed by a very slow decay. This is characteristic of consecutive reactions in which a stationary state intermediate is being destroyed much more rapidly than it is being produced. It was found that: Initial rate &prop; [OH^-] [ArNO₂] [RNHOH] and that [Radical]_max &prop; [OH^-]^&frac12; [ArNO₂]^&frac12; [RNHOH]^&frac12; and the following scheme was proposed: RNHOH + OH^- ͍^k RNHO^- + H₂O</li> <li>RHHO^- + ArNO₂ &rarr;^k₁ RNHO^&bull; + ArNO^&bull;-₂</li> <li>2ArNO^&bull;-₂ &rarr;^k₂ products.</li> <li>2RNHO^&bull; &rarr;^fast products. This was verified by the evaluation of k₂ as a rate constant for the bimolecular disappearance of the nitro radical-anion, in a series of experiments with para-nitrobenzoic acid and a range of hydroxylamine compounds. k₂ = 6.5 &times; 10² l.mol.^-1 sec.^-1 This value for k₂ is higher than literature values of rate constants for the disproportionation of nitro radical-anions. This may be due to traces of oxygen, present in the current work, or to the fact that k₂ may not be the rate constant for a direct disproportionation reaction. The relative rates at which the various hydroxylamines will transfer an electron was found to be: MeNHOH &gt; PhCH₂NHOH &gt; Ph₂CHNHOH &gt; NH₂OH the sequence that would be expected from the electron releasing powers of the substituents on the nitrogen atom of the hydroxylamine group. Using a single hydroxylamine in reaction with a range of nitro compounds an approximate value of p = + 3.3 was obtained for the electron transfer reaction. This indicated that a low electron density at the site of reaction in the nitro compound would be favourable i.e. the rates with the following substituents would vary: p -CN &gt; p -COO^- &gt; p -OCH₃ With hydroxylamine itself, kinetic measurements were much less accurate, owing to a pseudo unimolecular gas evolution which appeared to come from the reactions: 2NH₂O^&bull; &rarr; (H₂NO)₂ &rarr; N₂ + 2H₂O From the mechanism that has been proposed for the general reaction, it would be expected that nitroso compounds would be among the products. However these compounds were not isolated. With hydroxylamine, itself, the reaction proceeded further via a diazotate and a diazonium salt to yield an azide. From N-alkylhydroxylamines, R₂CH.NHOH, the intermediate aliphatic nitroso compounds in part tautomerized to the oximes, R₂C=NOH, which have been isolated. The subsequent reactions of the aromatic nitroso compounds fall into two categories. In the reactions involving N-benzhydryl and N-phenylhydroxyl-amines, the intermediate nitroso compounds derived from the original nitro compound underwent a further one electron reduction, followed by dimerisation to give an azoxy compound. In the case of N-phenyl-hydroxylamine this was also the fate of the PhNO^&bull;- radicals, derived from the hydroxylamine. In the reactions involving N-alkylhydroxylamines, R-CH₂-NHOH, deep red products were isolated and, with difficulty, purified. These were ultimately identified as formazans (I), by comparison of a range of their physical properties with those of synthesized materials. [For the diagram omitted here, please consult the PDF.] The mechanism proposed to account for their formation from the intermediate nitroso compounds had to satisfy the following: In the reaction, variations in the nitro compound alters the Ar-group in I,</li> <li>In the reaction, variation in the hydroxylamine compound alters the R- group in I,</li> <li>The lack of formation of formazans with other hydroxylamines indicates that an active methylene group adjacent to the nitrogen atom in the hydroxylamine is essential. The mechanism proposed depended upon the cross combination of nitroso radical-anions derived from both reactants to give an unsymmetrical azoxy compound, R.CH₂N=N(O)Ar. Attack by base at the active methylene group resulted in the formation of formazan (I). Evidence to support this mechanism has been gained by synthesizing the intermediate unsymmetrical azoxy compound and allowing it to react with base, with the result that traces of the predicted formazan were detected.

The prediction of energy loss properties and track structure effects of ionising radiation in materials is of importance to many areas of science, healthcare and technology, especially the nuclear industry. This study examines three different aspects of the electronic effects of ionising radiation on solid materials: the calculation of inelastic cross sections, the measurement of charge state fractions of ions through materials, and the radiolytic hydrogen produced from slurries under gamma irradiation. Predicting how ionising radiation will interact with matter often utilises collision cross sections for the interaction process. The electronic energy loss cross sections of ions in materials are predicted using a novel formalism requiring only the dipole oscillator strength distribution (DOSD) of the material of interest. DOSDs are constructed for silicon carbide and various oxides of interest to the nuclear industry. Electronic collision cross sections as well as average energy loss properties for incident protons, helium and other ions are calculated using the developed formalism. The formalism is shown to predict macroscopic energy loss properties well, especially at higher energies, suggesting the formalism is an acceptable simple yet elegant method for calculating electronic cross sections for use in Monte Carlo simulations of radiation track structures. The charge of an ion in a material affects the rate of energy loss during the passage through that material. The charge state fractions of lithium and helium ions in several metallic materials pertinent to the nuclear industry are measured and compared in order to improve the understanding of ion charge states in a radiation track structure. The new charge state fraction measurements show a clear dependence on material properties which appears to correlate with the ionisation potential of the material; however, a full understanding of the dependence is lacking. Radiolytic hydrogen production is of importance when considering the safety of spent nuclear material in cooling ponds and after disposal. One proposed clad coating for accident tolerant fuel currently under investigation is silicon carbide however its radiation chemistry is relatively unknown. The hydrogen produced from gamma-irradiated silicon carbide water slurries is investigated. The measured yield of H2 produced is greater than would be expected from a mixture rule. This excess production of H2 is believed to be due to transfer of energy from the solid ceramic to the aqueous phase by either low energy electrons or exciton dissociation at the water-carbide interface. These three areas of investigation are complementary aspects of the interaction of ionising radiation with solid material and add to the knowledge base necessary for an acceptable risk-based justification for sustainable energy production by nuclear fission power plants.

This thesis investigates the adsorption mechanisms and changes in activity of bilirubin oxidase from Myrothecium verrucaria immobilised to gold- and silica-modified surfaces. This enzyme is used as an efficient bioelectrocatalyst for the four-electron oxygen reduction in fuel cells. An electrochemical quartz crystal microbalance with dissipation monitoring (E–QCM–D) was used to show how applying a constant potential to bilirubin oxidase adsorbed to a carboxylate-terminated gold electrode resulted in activity loss attributed to structural rearrangement of the adsorbed enzyme layer. When a varying potential was applied, rapid enzyme deactivation occurred, with no mitigation of activity loss through covalent attachment to the electrode. The E-QCM-D was further used to observe how changing enzyme concentration affects the adsorption mechanics and catalytic activity of the adsorbed layer. An optimum concentration produced greatest activity and stability, with lower concentrations denaturing more readily, and higher concentrations adopting an unfavourable geometry for electron transfer. Surface functionality showed adsorption to hydrophobic methyl- terminated electrodes revealed a rigid layer with reduced catalytic activity. Ammonium terminated surfaces resisted denaturation, but misorientated the enzyme for efficient electrocatalysis. Increasing the chain length of the surface modifiers increased the enzyme–electrode distance; this decreased activity for the carboxylate surface and removed the activity for methyl- or ammonium-terminated surfaces. Dual polarisation interferometry further showed no enzyme denaturation when it was adsorbed to amine and sulfonic acid surfaces. Enzyme adsorption under an applied constant potential caused a decrease in both mass loading and activity when compared to open circuit potential adsorption. The presence of an applied potential did not cause increased layer denaturation, but changed the orientation of the enzyme in a position unfavourable for electron transfer. Lower applied potential give lower mass loadings, yet similar surface mechanics and activity per adsorbed enzyme. Chemical modification to pristine graphene showed targeted interactions with biomolecules such as proteins and fluorophores. This surface modification has the potential to be adapted towards adsorption of bilirubin oxidase for fuel cell catalysis and other electrochemical sensing applications. The observations in this thesis show how the E–QCM–D and DPI can provide a more expansive picture of the applicability of redox enzymes in fuel cell systems. Preventative steps need to be taken in order to maintain an enzyme’s structural integrity and in turn its catalytic competency. Without such provisions the observations above suggest that redox enzymes have a finite lifetime when under conditions approximating fuel cell systems.

Low-molecular-weight gelators (LMWGs) form a network via non-covalent interactions to immobilise the surrounding bulk solvent and form a gel. Whilst such gels are highly responsive and dynamic, they are often mechanically weak. In order to enhance the mechanical strength of such networks, the LMWG network can be supplemented with a second network formed from stronger polymer gelators (PGs) to yield a multi-component, multi-functional material – a hybrid gel. By using this multi-functionality, hybrid gels were made that could demonstrate the following: a) robustness yet responsiveness, b) spatial control over the formation of one network in the presence of another, and c) temporal control over the formation of one network in the presence of another. For the first aim, a pH-responsive LMWG (1,3:2,4-dibenzylidene-D-sorbitol dicarboxylic acid, DBS-CO2H) was combined with the robust PG agarose. The assembly of DBS-CO2H in the presence and absence of agarose was investigated by NMR and CD spectroscopies, whilst materials properties were examined by rheology. DBS-CO2H was found to retain its pH-responsive character, as was demonstrated by cycling the pH within the gel – whilst the DBS-CO2H network could be switched “on” or “off”, the robust agarose network remained intact. Following this, DBS-CO2H was combined with the photo-inducible PG poly(ethylene glycol) dimethacrylate (PEGDM). Spectroscopic methods and electron microscopy showed that the kinetics and morphology of DBS-CO2H assembly were impacted by the presence of PEGDM. The application of a mask during photoirradiation allowed patterning of the PEGDM network to form a material with two distinct, spatially-resolved regions, defined as a “multidomain gel”, achieving the second aim. The different domains had different properties with regards to the diffusion and release of dyes. DBS-CO2H was then combined with another pH-responsive LMWG (1,3:2,4-dibenzylidene-D-sorbitol-dicarbonyl-glycine, DBS-Gly). The two gelators showed a good degree of kinetic self-sorting, their self-assembly being triggered at different pHs. It was possible to use two proton sources – the slow hydrolysis of glucono-δ-lactone, and the more rapid photoacid generator diphenyliodonium nitrate – to achieve a two-step process of network formation. As the second step was UV-initiated, photopatterned multi-component gels were produced; these materials were both spatially and temporally resolved, achieving the third aim. Finally, the combination of DBS-CO2H, DBS-Gly and PEGDM into a three-gelator, multi-component hybrid hydrogel was investigated.

Concerns about the impact of anthropogenic CO2 emissions on global climate change are necessitating the development of low-carbon technologies. Despite its energy requirements, carbon capture and sequestration (CCS) provides a promising approach to limiting carbon dioxide emissions, in addition to renewable energy and nuclear power. Due to the depth of technological experience and retrofitting capabilities associated with the process, post- combustion CO2 capture based on absorption in alkanolamine solutions is presently the most feasible technology available for mitigating the soaring levels of atmospheric CO2 and achieving the ambitious targets set by the Intergovernmental Panel for Climate Change (IPCC). Whilst the process of CO2 capture by alkanolamines involves a technologically mature process, it is still faced with numerous challenges, such as high energy requirements and operating costs, as well as operational difficulties. Monoethanolamine (MEA), the benchmark amine for the process, frequently becomes contaminated with degradation products and, when CO2-loaded, is corrosive to the carbon steel process equipment. Hence, this thesis aimed to determine the oxidation and reduction reaction kinetics and mechanisms of iron in aqueous MEA-CO2 systems as function of experimental variables, aiming to predict iron corrosion rates under process conditions. The behaviour of iron in aqueous MEA solutions was characterized by voltammetry with a rotating disc electrode (RDE) and an electrochemical quartz crystal microbalance (EQCM), as functions of temperature (25-80oC), CO2 loading (0-0.6 mol CO2 (mol amine)-1), pH (8.10- 12.55), MEA concentration (5-60 wt%) and oxygen concentration. Electrode potential-pH and activity-pH diagrams of iron-water-CO2 systems were used to assist with reaction assignments. The passive electrochemical behaviour of Fe in MEA at pH ca. 12 switched to active dissolution on loading the MEA with CO2, causing the pH to decrease to ca. 8. Analysis of the resulting kinetic data enabled corrosion rates to be predicted as functions of the experimental variables. Based on the proposed corrosion mechanisms from the voltammetric results, a mechanistic model was developed for the uniform corrosion of iron in CO2-loaded MEA systems, taking into account the CO2 absorption equilibria reactions and the electrochemical reactions at the iron | solution interface. Equilibrium concentrations of the amine species (RNH2, RNH3+, RNHCOO-), carbon(IV) species (HCO3-, CO32-) and hydrogen ions (H+) were calculated with a Kent-Eisenberg type model. The electrochemical reactions incorporated in the model were the anodic dissolution of iron and the cathodic reduction of H+, direct water reduction and the reduction of oxygen. The corrosion model was developed by simulating polarization curves based on the species concentrations and the transport limited current densities of the iron RDE defined by the Levich equation. In order to measure dissolved iron concentrations under typical CO2 absorption conditions, an electrochemical flow reactor was designed and fabricated from PTFE with an iron/steel anode, platinised titanium cathode and a cation-permeable Nafion membrane. Inductively coupled plasma optical emission spectrophotometry (ICP-OES) was used to determine FeII concentrations in the amine solution after potentiostatic electrolyses as a function of solution flow-rate, temperature, CO2 loading, pH, MEA concentration, oxygen content, steel type and amine type, enabling partial current densities leading to iron dissolution to be deconvoluted from measured current densities. Results from the voltammetric data from the RDE were used to propose a mechanism for the oxidation and reduction reactions occurring at the iron surface. During positive-going potential sweeps, the voltammograms were characterised by 3 anodic peaks corresponding to the anodic dissolution of Fe to FeII at low potentials, leading to iron carbonate formation if its solubility product was exceeded, and a passive region from the formation of adsorbed products such as iron(III) (hydr-)oxide at higher potentials. The cathodic reactions at the iron surface included reductions of H+, H2O and O2. Based on the thermodynamic predictions, the anodic dissolution of iron was assigned to the formation of a soluble FeII species. Using a flow reactor operated in batch recycle mode, constant potential electrolyses at a potential -0.6 V, within the anodic dissolution potential range, resulted in a dissolution charge yield of less than unity, which varied depending on the experimental conditions, implying the formation of insoluble iron species such as Fe(OH)2 or FeCO3. The mass transport behaviour of the flow reactor was characterised as a function of solution flow rate using the transport controlled reduction of hexacyanoferrate(III) ions at a platinised titanium electrode, resulting in a mass transport correlation for the reactor. A value of the diffusion coefficient for FeII from the literature enabled prediction of the conditions required for FeCO3 and Fe(OH)2 formation, based on measured fluxes and dissolved FeII concentrations in solution, from which the rate of FeCO3 formation could also be estimated. Kinetic analysis of the data from the RDE, EQCM, flow reactor and the corrosion model resulted in a complementary set of results. The corrosion behaviour of iron in aqueous MEA- CO2 solutions was sensitive to changes in the operating parameters; iron dissolution rates were enhanced by increasing temperature, CO2 loading, solution velocity and oxygen content. However, at high temperatures, high CO2 loading and high FeII concentration and lower solution velocity provided favourable conditions under which a protective FeCO3 layer could precipitate. Dissolution rates increased with concentration at lower concentrations of MEA (5- 40 wt%) and decreased with increasing concentration at higher concentrations (40-60 wt%), due to viscosity effects. Significantly lower corrosion rates were measured in the other commercially available solvents tested namely methyldiethanolamine (MDEA), 2-amino-2- methyl-1propanol (AMP) and aminoethylpiperazine (AEP). Carbon steel exhibited similar dissolution rates to those of iron, whereas those for stainless steel were significantly lower. From the comprehensive results on the oxidation and reduction kinetics of iron in benchmark MEA-CO2 systems, the effect of MEA concentrations on predicted iron corrosion rates brings into question the effectiveness of the concentration of MEA used in amine scrubbing being limited to 30 wt%, specifically to limit corrosion related issues. The results also indicated that MEA was the most 'aggressive' in corrosion behaviour, as other amines such as MDEA, AMP and AEP provided more promising alternatives, based on both their predicted corrosion behaviour and their reported efficiencies in the absorption process.

This thesis is an expansion on previous work using the photostop technique for the production of near-zero velocity atoms and molecules. The goal is to produce stopped SH molecules and trap them in a permanent magnetic trap and the aim of this project was to construct a new experimental apparatus to accomplish this. During initial tests of the apparatus, the Rayleigh scattering cross-section of N2 was measured to provide a reference point for future experiments. The uncertainty and systematic errors in the measurements was such that denitive quantitative results of this were not be obtained at this stage. The emerging technique of cavity-enhanced laser-induced uorescence (CELIF) was used to perform absolute number density measurements of a molecular beam of SO2. CELIF was then applied to measuring the photostop of SD/SH. This showed that CELIF would not have the required sensitivity to measure the trapped SD/SH molecules due to issues of stray light from the lasers. As a result of this we elected to use resonance-enhanced multi-photon ionisation (REMPI) as an alternative. We devised and constructed a novel ion extraction system for use in performing REMPI, which was based on a time-of- ight mass spectroscopy system, but utilising the magnets themselves as electrodes, as well as some ion lensing components. This was initially tested using Xe, showing a strong signal and good mass resolution. Using this, the photostop of SH and S was measured showing that the detection apparatus is able to distinguish signal over a range of 9 orders of magnitude. However, despite this sensitivity, the trapping of these stopped molecules could not initially be demonstrated as the signal from these stopped molecules was obscured by signal from the inadvertent dissociation of the background parent molecules by the probe laser. More recent measurements in the group have directly addressed this issue with background subtraction and the results have now demonstrated the trapping of SH. Signicant headway has been made in the demonstration of the trapping of SH produced by photostop. From the results produced using REMPI the detection limit has improved signicantly over the prior experiments and very recent measurements have successfully demonstrated the trapping of SH.