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Title: New directions in Ag-catalysed alkene epoxidation : molecular mechanisms and the genesis of realistic model catalysts
Author: Bird, D. P. C.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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Abstract:
Atomic force microscopy was used to study the genesis of realistic Ag/α-alumina model catalysts from an aqueous precursor (AgNO3), using conditions identical to those used in the synthesis of practical dispersed catalysts. The various stages in the evolution of the final catalyst from the initially deposited precursor were successfully imaged. It is found that the structure of the alumina substrate exerts a dramatic influence on the structure and stability of the silver deposit. On Al2O3 (0001), sintering in 1 bar oxygen at 780 K causes a quasi-2D network of metallic Ag to collapse to large well-formed hemispherical particles on the bare oxide surface. On Al2o3 (1120), precursor decomposition leads initially to the formation of quasi-1D Ag nanowires that decorate the faceted step edges and extend for many hundreds of nm. This is entirely different from the behaviour of vacuum deposited Ag. These nanowires are much more stable against sintering in 1 bar oxygen at 780 K. Their formation and stability suggests a particularly favourable wetting interaction between Ag and the (1102) plane of α-alumina. Styrene epoxidation on a well-defined silver surface was used as a model for heterogeneous ethene epoxidation in order to study the promoting or poisoning effects of SOx, NOx, Cs and Oa. Fast XPS, NEXAFS and TPR experiments were used to study the adsorption and subsequent epoxidation of styrene on Ag {100}. Styrene adsorbs on clean Ag {100} with a flat lying geometry at 200 K and a saturation first layer coverage of 0.3 ML. Styrene is epoxidised in the presence of chemisorbed atomic oxygen with a selectivity that increases with oxygen coverage (55% - 87%). It has been shown for the first time that chemisorbed atomic oxygen moderates the electronegativity of neighbouring Oa and directs the reaction towards epoxide formation. Cs has the opposite effect, suppressing epoxidation and causing 100% styrene combustion.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.596655  DOI: Not available
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