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Thesis (M. Phil.)--University of Hong Kong, 1991.

No description available.

No description available.

Iron oxides and their metal- doped variants of formulation M/Fe₃O₄, M/y-Fe₂O₃, M/a-Fe₂O₃ have been prepared by the calcination of solids formed by the boiling of precipitates under reflux and by hydrothermal processing. Titanium-, tin-, ruthenium- and magnesium- doped variants of y-Fe₂O₃ have been subjected to special investigation and the influence of the dopants on the surface area, particle size, reduction properties, stability to conversion to a-Fe₂O₃- related structures, and oxidation- reduction cycling has been examined. Titanium- doped y-Fe₂O₃ prepared by the calcination of a solid formed by the addition of base to salt and boiling the precipitate under reflux was formed with a smaller particle size and higher surface area than the other counterparts. These properties remained superior to those of the other materials when Ti/y-Fe₂O₃ was subjected to a three-fold reductionoxidation cycle. The stabilisation of a spinel- related y-Fe₂O₃ structure with respect to thermally induced conversion to a corundum- related a-Fe₂O₃ phase was related to the presence of titanium as opposed to particle size or surface area effects. Impregnation of Ti/y-Fe₂O₃ with palladium lowered the temperatures at which the iron oxide was reduced as a result of the adsorption and dissociation of hydrogen on palladium which facilitates more facile reduction of iron in iron oxide. The oxygen storage capacity and the performance of palladium- impregnated Ti/y-Fe₂O₃ for the oxidation of carbon monoxide to carbon dioxide was good. Although generally similar behaviour was observed in y-Fe₂O₃ doped with tin, ruthenium and magnesium, and in palladium impregnated variants, Ti/y-Fe₂O₃ and palladium impregnated Ti/y-Fe₂O₃ showed the most favourable properties in terms of their potential use as oxygen-storage components in automobile exhaust catalysts. A comparison of the results with those recorded from materials derived from Fe₃O₄ and a-Fe₂O₃ confirmed the superior properties of Ti/y-Fe₂O₃ and palladium impregnated Ti/y-Fe₂O₃ for use as catalysts in automobile exhaust systems.

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Olefin metathesis is arguably the most versatile catalytic route yet developed for the assembly of carbon-carbon bonds. Metathesis methodologies are attractive from both synthetic and ecological standpoints, because they employ unactivated double bonds. This reduces the total number of synthetic steps, and the associated generation of chemical wastes. The drive to deploy olefin metathesis in highly demanding contexts, including pharmaceutical manufacturing and chemical biology, puts severe pressure on catalyst lifetime and productivity. Understanding the relevant decomposition pathways is critical to achieve essential performance goals, and to enable informed catalyst redesign. This thesis work expands on significant prior advances that identified and quantified critical decomposition pathways for ruthenium catalysts stabilized by N-heterocyclic carbene (NHC) ligands. Because pristine catalyst materials are essential for mechanistic study, it focuses first on methods aimed at improving efficiency and purity in catalyst synthesis. Merrifield iodide resins were shown to function as efficient, selective phosphine scavengers in the production of clean second-generation catalysts from PCy3- stabilized precursors. The thesis then turns to mechanistic examination of decomposition pathways that underlie success and failure for leading NHC catalysts, for comparison with a new family of catalysts stabilized by cyclic (alkyl)(amino) carbene (CAAC) ligands. These represent the first in-depth mechanistic studies of the CAAC catalysts, which have attracted much attention for their breakthrough productivities in challenging metathesis reactions. The remarkable productivity of the CAAC catalysts is shown to originate in their resistance to decomposition of the key metallacyclobutane intermediate via b-elimination, and (to a lesser extent) in their resistance to attack by nucleophiles and Bronsted bases. Importantly, however, they are more susceptible to bimolecular decomposition. The latter behaviour, as well as their resistance to b-elimination, is traced to the strong trans influence of the CAACs relative to NHC ligands. This insight significantly advances our understanding of the fundamental properties governing both productivity and decomposition. Finally, two new catalysts are developed, building on the principle that nucleophilic stabilizing ligands should be avoided in the precatalysts. In the first of these complexes, an o-dianiline ligand is employed to stabilize the precatalyst. This flexible, H-bonding chelate serves the further purpose of accelerating macrocyclization of flexible dienes that bear polar functionalities. As its H-bonding capacity also increases its sensitivity to trace water, however, an alternative catalyst architecture was pursued. The latter consists of a dimer bearing bulky Ru-indenylidene centers, in which a dative bond from a bridging chloride affords the fifth ligand essential to stabilize the precatalyst.

No description available.

No description available.

No description available.