In order to elucidate the activation mechanism of iron based Fischer¿Tropsch catalysts, we performed in situ nanoscale structural and compositional analysis during carburization in CO atmosphere. We found that inside an ETEM at 20 Torr CO, hematite reduces while severe sintering occurs. This sintering is enhanced by low pressure conditions in combination with CO gas. The reduction inside the ETEM is so effective that metallic iron can be formed. During the process, about 50 nm sized iron carbide crystallites grow out of the iron (oxide) particles. Upon prolonged reaction, these newly formed crystallites reduce in size to about 5¿30 nm. In contrast to what has been described in the literature until now, no carbon deposits were found on the outer rim of these iron carbides at high temperature conditions. Note however that, in contrast to our measurements, previous studies involved catalysts that were cooled or passivated prior to (TEM) analysis.@en

This thesis focused on iron-based Fischer-Tropsch catalysts. It deals with different iron phases that might play a role in the catalytic process. As the activation step takes place inside the reactor (in-situ at high temperature and pressure), studying the catalyst activation process is not trivial. In addition, since the working catalysts are air sensitive, also post analysis on activated and used catalysts is challenging. As a result, the identity of the real active phase is still unknown. To gain more insight in Fischer-Tropsch catalysts, we used different techniques like (in-situ) TEM, TEM-EELS, ED and PXRD. TEM and ED are very suitable techniques since they provide very high spatial resolutions (in the Å range). Special TEM grids and TEM holders were designed for this research in order to mimic the activation conditions and / or avoid any exposure to air. Our results indeed showed that it is essential to avoid any exposure to air prior to analysis. In fact, we can question the results of some older publications where catalysts were exposed. In contrast to literature data published so far, we found no carbon deposits on the outer rim of the iron carbides at high temperature conditions. Overall, we showed that carbon surface layers can change, or even form, during exposure to air. We also found evidence for what in the literature has been designated as “hypothetical q¥-Fe2C”. We proposed a structure model for which the carbon content is higher than in the pure c-Fe5C2 carbide. More research is required to elucidate the exact nature of working iron-based Fischer-Tropsch catalysts. @en