Keating, JL; (2014) Elucidating structure over atomic length scales through advanced synchrotron-based methodologies. Doctoral thesis , UCL (University College London).

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Abstract

The work undertaken in this project has focused on developing methodologies to determine the atomic architecture of catalytic materials over short-, medium- and long-range structures. The main objective of the work has been to employ in situ techniques at synchrotron sources to enable development of new analytical methods which will provide a unique opportunity to characterise multi-phase systems. The reduction behaviour of a model cobalt Fisher-Tropsch catalyst was investigated using in situ combined XANES/EXAFS/XRD techniques where the sensitivity and selectivity of X-ray absorption spectroscopy (XAS) was exploited to obtain detailed structural information on the dynamic local structure. From these studies a new model on the structure of Co/Al2O3 catalyst in the reduced form has been proposed. Model metallic cobalt systems consisting of both HCP and FCC structures were characterised in detail using both pair distribution function (PDF) and XAS techniques. In-depth structural knowledge over the short-short and medium-range order has been garnered through multi-cluster modelling of EXAFS and PDF data providing new insights into the cobalt phase composition and phase transformation. In situ PDF studies were carried out on a Pd/Al2O3 model catalyst to determine, quantitatively, the phase composition at various temperatures during which PdO ↔ Pd transformations occur. Differential PDF analysis was successful in characterising the metal particles of interest despite the significant contribution from the disordered support phase. Supporting in situ XAS studies were undertaken and the findings were in good agreement with those from PDF. Finally, using low energy chlorine K-edge XAS, real Vehicle Emission Control (VEC) catalysts were studied in order to determine the speciation of chloroplatinate compounds within fresh and road aged catalysts. With a high sensitivity and selectivity, an investigation of this type demonstrated the advantages of performing XAS speciation studies at the chlorine K-edge.

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