Eels-stem investigation of the formation of nano-zones in iron catalysts for Fischer-Tropsch synthesis

Abstract

This chapter studies Iron Fischer-Tropsch (Fe-FTS) catalyst particles using high resolution transmission electron microscopy (HR-TEM) and electron energy loss spectroscopy (EELS) as well as energy filtered TEM (EFTEM) with spectrum imaging spectral analysis and mapping capabilities. These studies have independently demonstrated the formation of various nanozones within iron catalyst particles. By this study's classification schema, nanozones comprises of nm-wide layers that host a dense array of either iron carbide or iron oxide crystallites and multiple carbon-rich nano-layers that occur both on the exterior of the Fe-FTS catalyst grains and also further inwards. The precursor catalyst material that underwent phase transformations leads to the formation of the reported nanozones during the FT-experiments was a precipitated unsupported iron oxide and the materials under investigation were derived from slurry-bed reactor runs that were sampled after exposure to increasing length of synthesis time. It discusses that the presence of a carbon-rich layer closer to the core was a surprise, while the amorphous carbon zone that occurs at the rim of the catalyst grains had already been known. The chapter explores the carbon-rich layer located closer to the core in fact has carbon EELS signatures that are different from those of the carbide carbon that is concentrated in the core region and also those of the exterior amorphous carbon layer. It suggests that the inner carbon nanozone was derived either from the carbide core after some phase transformations of the iron carbide crystallites that caused a spatial repositioning of extra carbon outside of the iron carbide nanozone, or that the carbon was independently deposited onto the inner core during the reactor runs.