Characterisation of Cobalt/Manganese Fischer-Tropsch Catalysts using In Situ Gas Cell Analytical Transmission Electron Microscopy

Abstract

The emergence of gas cell scanning transmission electron microscopy (STEM) has provided a platform for characterising the dynamic morphological, structural and chemical features of materials during exposure to gaseous environments at elevated temperatures. These developments have significant relevance to heterogeneous catalysis, where the adsorption/desorption processes of reactants and products can lead to dramatic changes in the dispersion and crystallographic nature of the system and, subsequently, the catalysts functionality. In this work gas cell STEM has been used to investigate the effect of an H2 reduction treatment on titania-supported, cobalt-based catalysts used industrially for Fischer-Tropsch (FT) synthesis of hydrocarbon fuels, including the impact of this treatment with the inclusion of a manganese promoter. Reduction treatments are an essential component of FT chemistry, as the starting calcined catalyst is typically in the form of cobalt oxide and is only catalytically active to FT synthesis when in its metallic form. In situ gas cell STEM observations, combined with energy dispersive x-ray spectroscopy (EDS) have demonstrated the dispersion-inducing effect of manganese and the consequence of its presence during the reduction process on the resulting particulate-formation of active cobalt catalyst. This work also established some key limitations in using gas cell STEM, specifically when performing electron energy loss spectroscopy (EELS). The requirement of membrane windows to encapsulate the specimen and gas within the vacuum of the microscope effectively increases the extent of plural scattering, making observations of specimen core-loss edges challenging. The situation is made worse when considering additional effects, such as high dispersion, support thickness and large background peaks from the windows which reduce edge peak intensities signals to levels comparable to the spectrum noise. STEM-EELS observations have determined the significant extent of scatting is from the membranes windows and revealed a temperature dependent relationship of scattering from the encapsulated gas, arising from a decrease in gas density due to membrane bowing.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Sarah Haigh (Supervisor) & Chris Hardacre (Supervisor)