No description available.

No description available.

This project has involved the identification of photochemical and photo-thermolytic reaction cycles for the conversion and storage of solar energy in the form of chemical potential, using the photochemistry of simple organic ketones for the production of hydrogen from alcohols. Several approaches have been investigated which involved; (i) transfer of hydrogen from hydroxyalkyl radicals to aromatic compounds, (ii) photolysis of diphenylhydroxymethyl radicals and (iii) the thermolytic decomposition of pinacols. Irradiation of suitable benzophenone solutions with a 1kW Hg~Xe U.V.lamp generated hydrogen as a photolysis product (by excitation of the diphenylhydroxymethyl radical) and demonstrated that this can be achieved without employing dye lasers for specific photoexcitations. An attempt was made to improve the efficiency of hydrogen production in this system by the photochemical generation of hydroxy alkyl radicals in zeolite channels. Pyrolysis of pinacols under flash vacuum and static thermolysis conditions produced hydrogen and a ketone product. This decomposition is essentially the reverse of a photolytic reduction of the ketone, from which the pinacol may originally be formed, and therefore provides a completed photo-thermolytic cycle for the production of hydrogen from an organic solvent. Two mechanistic routes were proposed for the pinacol decomposition a six-centred molecular process and a mechanism involving free radical intermediates. Experimental evidence and UMINDO calculations were found to support the free radical route

Previous research on the precipitation and oxidation of lead sulphide has been reviewed especially in relation to thermodynamic and kinetic studies. Samples of lead sulphide have been precipitated from aqueous solutions of lead salts under varying conditions of pH, concentration, temperature and time of ageing. The precipitates were examined by optical and electron microscopy, X-ray diffraction and gas sorption analysis. Their crystallinity, particle size and phase composition have been correlated with the conditions of precipitation. The oxidation of selected samples of precipitated and commercial lead sulphide was investigated by thermal analysis and the above techniques. The phase composition of the oxidation products were studied under varying conditions of temperature, gas flow rate and oxygen content of air. For the low temperature oxidation (~500°C) the thickness of the lead sulphate product layer was estimated and attempts made to correlate this value with the kinetics of the oxidation. The stability of lead sulphate at high temperature (~900 °C) has been investigated. Sintering behaviour of the products in relation to their incomplete desulphurization was examined. Oxidation studies on lead sulphide were extended to lead-zinc ores with respect to the effect of moisture content and bentonite on sintering. This investigation was carried out on a sinter pallet using 22 kg mixes at ISP Research Department of Imperial Smelting Processes Limited, Avonmouth. The possible formation of lead sulphite as an intermediate in the low temperature oxidation of lead sulphide was investigated by studying the thermal stability of the sulphide under various gaseous atmospheres. These studies were extended to zinc sulphite in relation to previous studies on the oxidation of zinc sulphide . These researches form a part of a wider study in the oxidation of sulphides during shipment, storage and processing.

The investigation was limited mainly to NaCl.Mn++ and NaCl.Ca++, and was concerned with colour centres in single crystals of these materials. The crystals were grown by the Kyropoulis technique and optical absorption measurements were made with a grating spectrophotometer. The emphasis of the work was placed on the production of Z-bands in NaCl. Ca++ but the effects of optical and thermal bleaching and of quenching were investigated in all crystals. The results of the work on NaCl.Mn++ confirmed the model of Schneider and Caffyn (1955) in which the Mn++ ions and positive ion vacancies are deposited at dislocations and become more uniformly distributed after quenching. In particular, it was found that quenched crystals were luminescent after being X-rayed, indicating the presence of dispersed Mn++ ions; a similar result was found in NaCl .Ni++. A short study of the distribution of Mn++ in crystals was also made. It was found that Mn++ and Ni++ gave rise tocharacteristic bands in the ultra violet, a weakband at 275 Mµ with Mn++, a strong band at 247 mµ with Ni++. The presence of a band at 255 mµ in NaCl.Cu++ was confirmed. No evidence was obtained for the formation of Z-bands by Mn++, Ni++ or Cu++. Calcium was found to be much more soluble in NaCl than manganese, and enhanced the colourability proportionately less, pointing to a close relationship between solubility, mis-match, and the production of negative ion vacancies. Analysis of the F-band in coloured NaC1.Ca++ crystals indicated that Z- centres were formed by X-rays. Quenching enhanced the number of Z-centres, indicating a deposition at dislocations similar to that found for manganese. Evidence for the formation of both Z(_1)- and Z(_2)- centres by X-irradiation was found. In crystals containing a high proportion of Ca++ a band at 345 mp, which is probably to be associated with Ca++- positive ion vacancy complexes, was prominent. The results of an investigation of the effect of concentration of Ca++ on the growth of the F- and Z-bands indicated that a Ca++ ion introduced about ten negative ion vacancies into the crystal. On the basis of a statistical mechanical model of the formation of Z-centres it was inferred that of the Ca++ ions producing Z-centres no more than 2% were associated with positive ion vacancies, confirming the work of Etzel (l952). It was also inferred that the activation energy associated with the liberation of Ca++ ions from dislocations to form incipient Z-centres was 1.9 eV, a value which agreed well with an estimated value. Factors influencing the width of the F-band are also discussed.

The understanding of the chemistry of technetium is of vital importance to the development of a geological disposal facility (GDF) for the storage of radioactive waste in the UK. This has led to the research that has been conducted to produce this thesis. As technetium has a highly mobile anionic species, the pertechnetate TcO4-, in general it is desirable to reduce this species to a sparingly soluble TcO2 species. This thesis includes studies conducted using both Tc(VII) and Tc(IV). Solubility studies have been conducted using Tc concentrations of 10-9, 10-10 and 10-11. These were conducted using ligands that are likely to be present such as Ethylenediaminetriacetic acid (EDTA), Nitriloacetic acid (NTA), picolinic acid, Iso-saccarinic acid (ISA), and an ISA analogue gluconic acid. Ligands were chosen that are likely to be present in Low Level Waste (LLW). They would become present in the GDF as they can be used in cleaning and decontamination proceses. These studies were conducted using electrolysis as a reduction technique both before and in the presence of, the anthropogenic complexing agents mentioned previously. It has been established that when the ligands were added to a technetium solution prior to the reduction process taking place there was an apparent inhibition of the full reduction of technetium in solution. Sorption studies of both oxidised and reduced technetium species have been conducted and the results have shown that there is apparent sorption of oxidised technetium to some of the solids that have been tested as well as technetium in a reduced oxidation state of Tc(IV). Solids that have been tested have been chosen to accommodate a broad sprectrum of solids which may be present in, or in close proximity to, a GDF in the UK. These experiments have been conducted after a review of relevant literature to ascertain the level of, if any, work that has been completed using technetium in a reduced form for both sorption and solubility studies. The methods that have been used involved the development of an electrochemical technique which allowed for the rapid and repeatable reduction of a pertechnetate solution electrochemically rather than using a more complicated system involving either tin or iron as a reducing agent.

No description available.

The temperature dependent hyperfine interactions of the Fe-57 nuclei in the hexagonal rare earth intermetallic compounds of the form RFe3 have been studied using the Mossbauer effect. The resultant spectra have been investigated using a model which takes into account(1) the local point symmetry of the three distinct iron sites and (2) the differing local dipolar fields in these compounds. From this analysis it is possible to conclude that (1) YFe3,GdFe2.9 TbFe3 and DyFe3 magnetize along the b-axis;(2) HoFe3 changes its direction of magnetization from an a-axis to a b-axis at 87oK;(3) ErFe3 magnetizes along the c-axis. In addition Mossbauer studies on the pseudo-binary compounds R(Fe1-xCoX3) and R(Fe1-xNix3) where O < X < 0.7 have also been carried out. It is found that the replacement of Fe by Co or Ni leads to pronounced effects in th FE-57 Mossbauer spectra, the lattice parameters and the ordering temperatures of these compounds. Similarities between the pure Fe-Co and Fe-Ni binary alloys and the pseudo-binary R(Fe1-xMx2) and R2(Fe1-xMx17) compounds, where M= Co or Ni, are also noted.

No description available.

The mantle is the most massive carbon storage reservoir on Earth. Interactions between surface and mantle reservoirs of carbon strongly govern atmosphelic chemistry and the habitability of our planet on geological timescales. The petrological and dynamical behaviour of carbon in mantle rocks promotes chemical differentiation and extraction of heat-producing elements through the migration of low-degree melts. Additionally carbonate melts mobilise hydrogen from nominally anhydrous minerals, which helps lower mantle viscosity and maintain active tectonics. I present four studies, each of which focusses on a different aspect of carbon's behaviour under high-pressure conditions. I study a suite of Brazilian diamonds and their mineral assemblages (Chapter 2). The isotopic signature of the host diamonds and mafic chemistry of their inclusion cargo reveals that they are fragments of recycled crust that has been exhumed from mid-mantle depths. In Chapter 3 I determine the melting curves of MgSi03- MgC03 and MgC03-CaC03, analogues of carbonated mantle material, between 15 and 80 GPa by performing experiments in the laser-heated diamond-anvil cell. Results from these experiments demonstrate that carbon-bearing lithologies will melt, potentially creating an isolated carbon reservoir, in the lower mantle. Chapter 4 presents multi anvil experiments that investigate the melting phase relations of carbonated MORB throughout the upper mantle. These reveal a solidus ledge that implies the transition zone can act as a carbon filter to downwelling material. This causes a narrow interval of melt production that could be geophysically observable and responsible for diamond formation. Finally, in chapter 5 I present experiments and geochemical modelling that tests whether diamond-hosted inclusion assemblages are compatible with a transition zone growth mechanism where slab-derived melts interact with ambient mantle material.