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The structure and reactivity of microporous and oxide catalysts

Lewis, Dewi Wyn; (1995) The structure and reactivity of microporous and oxide catalysts. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Microporous and metal oxide heterogeneous catalysts have been investigated using a range of computational techniques. The results of studies of the factors influencing the synthesis, structure and activity of these materials are presented here. A study of the interactions between zeolitic frameworks and organic templates has demonstrated how the efficacy of a template can be determined and an energetic rationalisation of templating ability is demonstrated. The location of templates within frameworks are found to be accurately determined and subtle differences in framework structure are rationalised in terms of the template used. We have been able to determine the templating action of bis-quaternary ammonium cations in the aluminophosphate DAF-1, results which have allowed the synthesis of new compositions of this material. We have further used these results to design a new template which will not form DAF-1 and which we propose as a termplate which may favour a new material. We demonstrate that such computer-aided design of templates can be used to assist the search for new materials. We have successfully modelled the local geometry of the iron and Bronsted acid site in Fe-ZSM5 using atomistic simulation techniques. A broad range of cation T sites are predicted to be occupied by iron, with T19 and T18 being the most energetically favourable. The accuracy of the calculations is demonstrated by the reproduction of the experimental EXAFS. We further propose improved models which better describe the local environment and which improve on the fit to experimental data. The effect of the inclusion of iron on the physical properties and catalytic activity is determined and compared to similar results for A1-ZSM5. The subtle differences between these two materials is reproduced. Iron incorporation modifies the pore dimensions, reducing the maximum pore dimension by 0.4A, an effect which can be correlated to experimental data on the selectivity of the material. Calculations on the interaction of the iron and the template, the first of their kind, have provided a model for understanding the experimental data on this material. The defect chemistry of the bulk and surface of the partial oxidation catalyst Li/MgO is investigated. The proposed active site, a lithium trapped oxygen hole, is found to be bound and segregated to the surface. Low coordinate surface sites are found to be more favourable than the (1 0 0) surface; in particular they stabilise untrapped oxygen holes. A mechanism for the improved catalytic performance of the material on doping with Cl- is presented. Cl- is found to be strongly surface segregated and to compete for the lithium with the oxygen holes, forming [LiCl] defect centres. These calculations also allow us to comment on the relative activity of the different sites and to draw further conclusions on the reaction mechanism. Quantum mechanical studies of the abstraction of hydrogen from methane by an oxygen hole in MgO demonstrate the importance of accurate treatments of lattice relaxation. We show that there is a critical influence on the results arising from the degree to which surface relaxation and the relaxation of the lattice in response to defect formation are included in the point charge arrays used to simulate the infinite lattice in the embedded cluster calculations. Reaction enthalpies and activation barriers are shown to be over-estimated and underestimated respectively if these effects are not considered.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The structure and reactivity of microporous and oxide catalysts
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Applied sciences; Microporous; Oxide catalysts
URI: https://discovery.ucl.ac.uk/id/eprint/10101791
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