Herbert, Jennifer J.; (2021) Real space analysis of operando X-ray spectroscopy and scattering studies of Cobalt Fischer-Tropsch Synthesis catalysts. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

Size effects on Co-based Fischer Tropsch synthesis (FTS) catalysts are well known, however their evolution over time is less well interrogated. Three key issues related to particle size remain: the behaviour of small Co0 nanoparticles during FTS and their potential re-oxidation, the evolution and influence of the Co0 nanostructure (fcc/hcp/intermixed Co0), and the formation of metal-support interactions during FTS. Two synthesis methods were used to produce Co/SiO2 catalysts. The first experimental chapter concerns the application of an inverse micelle synthesis (IMS) method for preparing powder-supported FTS catalysts. The aim was to create a catalyst with tight particle size control in order to better study particle size effects, however this was limited by the difficulty in removing the surfactant. While the final two experimental chapters focus on operando characterization using synchrotron-based X-ray techniques for two different sized catalysts synthesized using a standard Schlenk technique; 3 and 11 nm which fall below and above the stable size range respectively. Novel Debye simulations using a unic model were applied to synchrotron-based X-Ray diffraction (XRD) data acquired from a working FTS catalyst. While the 11 nm particle size catalyst is stable, this analysis shows a small variation in the hcp/fcc/intermixed ratio over the first 12 h of FTS, possible related to a known drop in activity during the initial stages of FTS. The hcp fraction decreases by -5.03 %, fcc increase by 1.24 %, and the mixed block increase by 3.7 %. About 20 % of the initial sample is unreduced CoO which reduces very slightly under FTS conditions. The majority of the Co0 content in the 3 nm sample oxidizes in the first hour of exposure to FTS conditions, although some of the larger Co0 particles remain active - as observed by XRD. Additionally, no support phases are observed initially or being formed during FTS on either of the different average particle sized catalysts.

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