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Title: Site specific characterisation of hydrocracking catalysts using nanoanalytical electron microscopy
Author: Husain, Sehban
ISNI:       0000 0004 2682 1632
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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During use, carbonaceous material or ‘coke’ can deposit on catalysts resulting in decreased activity and lifetime. In this thesis, the results of investigations into the structure and distribution of coke, on hydrocracking catalysts, are reported. The material consists of zeolite Y, alumina binder as well as tungsten and nickel sulfide. An extensive investigation regarding the preparation of the catalysts for electron microscopy was carried out. It was established that microtoming produced specimen damage and hence regions of porosity, zeolite and alumina binder were difficult to identify. Single beam and dual beam focused ion beam (FIB) milling produced intact specimens and the spatial distribution of the catalysts was maintained, although thinner specimens were obtained using the latter technique. Energy-dispersive X-ray (EDX) mapping identified gallium and platinum as artefacts in specimens that had been prepared by a single beam FIB system. In addition, argon ion beam milling was used and this technique produced large regions of thin material. Energy-filtered transmission electron microscopy (EFTEM) was employed to reveal the distribution of carbon in the catalyst. Carbon was identified on alumina binder, zeolite grains and meso-/macro-pores, although the distribution of carbon was generally not uniform as it is determined by the density and strength of acid sites, geometry of pores and the proximity of metal sulfide crystallites. All of these factors, especially pores size and shape, vary in the catalysts. Coke is thought to consist of polyaromatic hydrocarbons (PAHs). Electron energy-loss spectroscopy (EELS), of selected PAH standards, was performed to obtain the electron energy-loss near edge structure (ELNES) of carbon. In addition, the ELNES of four PAHs was modelled using multiple scatter calculations. EELS of the catalysts revealed that PAHs are present on zeolitic components but ELNES was not identified on the alumina binder. This is possibly because alumina contains larger pores than zeolite Y; therefore larger molecules can diffuse into the alumina structure, which increases the chemical variety of the coke species as the molecules are not sterically impeded.
Supervisor: McComb, David ; Haswell, Ralph Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral