Pritchard, Jay Morgan; (2024) Manipulation of Fischer Tropsch Catalysts for Controlled Selectivity: Investigating the role of a Manganese Dopant. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Cobalt-based Fischer-Tropsch (FT) catalysts are widely used to convert syngas to long-chain hydrocarbons. Recently, significant increases in alcohol selectivity have been reported with high manganese doping. In this work, commercially relevant catalyst extrusions have been studied by synchrotron X-ray Diffraction Computed Tomography (XRD-CT) during catalyst activation and operation to understand the evolution of crystalline phases of cobalt nanoparticles supported on TiO2 with various manganese doping levels (up to 10 wt. %). Data processing pipelines have been developed to assist in the Rietveld refinement of large quantities of diffraction data to provide quantitative metrics for the spatially and temporally resolved diffraction results. A high pressure cell and testing methodology have been developed to conduct operando studies at relevant pressures (25 bar) and temperatures on an imaging beamline, with the potential to operate multiple samples simultaneously to maximise runtime and operation utilisation while removing inter-run variability. Since a large proportion of the Mn dopant tends to be highly disperse, a series of model catalyst systems were developed to evaluate different catalyst-dopant interactions and doping levels. A modified salt-mediated hydrothermal synthesis was used to produce Co3O4 nanoparticles which either comprised a Co-Mn spinel solid solution, manganese surface coated Co3O4 particles or physical mixtures of Co3O4 and Mn3O4. These systems were produced with various Co:Mn ratios to correspond to the commercially relevant extrusions, before being supported on TiO2. The resulting catalysts were characterised via XRD and STEM and tested for FT activity and selectivity to understand what type of interaction, and what ratios of Co:Mn, are required to produce significant quantities of alcohol products under typical FT conditions. The % Mn requirements for alcohol selectivity in the commercially relevant catalysts correspond to the model systems where the cobalt and manganese were closely interacting. The development of the model systems also provides effective reference materials for comparison of characterisation results with the commercial catalyst samples.

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