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Title: Correlative tomography : three dimensional multiscale imaging and modelling of hierarchical porous materials
Author: Tariq, Farid
ISNI:       0000 0004 2695 3889
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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Heterogeneous catalyst based pellets typify a material where functionality is dependant on hierarchical pore structures spanning many orders of magnitude from nanometers up to tens of microns. The total activity, selectivity and lifetime of catalyst based pellets depends on the ability of molecules to flow through a large pellet bed (m), into the pellets (mm) and their pore structure (μm-nm) to/from the active sites. Three dimensional imaging techniques such as tomography allow for the direct characterisation and quantification of pore structures. However, the field of view in tomography decreases as resolution increases. This work circumvents this issue with multiscale tomography (MT) combining x-ray microtomography (XMT), dual beam focused ion beam tomography (DB-FIB) and electron tomography (ET) to probe porous pellet based catalysts. The results show MT as a viable method that offers new insights into the quantification and behaviour of pellet based catalysts across large length scales, all in three dimensions (3D), that no single tomographic technique can adequately capture. MT was successfully used in the characterising of pore sizes, distributions, structures and spatial relationships and this was compared to existing multiscale characterisation techniques to illustrate the new insights that can be obtained. The pore structures were meshed and modeled using MT data to provide results for understanding the transport properties scaled up from the nanometre length scale to the packed bed, through pellet based catalysts produced under different manufacturing conditions. The results show the very strong dependence on the calcining temperature which is important for designing better catalysts in future. The tomography data was also used to determine thermal/mechanical stresses at both a pellet and pellet bed level. Although many stresses are compressive; the packing of the pellets creates local tensile stresses and a potential cause for pellet failure through internal flaws at relatively low loads. In summary, multiscale tomography was demonstrated to be a viable method for obtaining new insights for the development of pellet based catalysts by both improved quantification and allows for the first time direct 3D multiscale simulation of transport and mechanical properties across multiple scales from nanometers to metres to catalyst pellets in beds.
Supervisor: McComb, David ; Lee, Peter Sponsor: Shell Global Solutions International B.V.
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral