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.