Abstract
The kinetics of the transformation of metallic Fe to the active Fe carbide phase at the start of the Fischer–Tropsch (FT) reaction were studied. The diffusion rates of C atoms going in or out of the lattice were determined using 13C-labeled synthesis gas in combination with measurements of the transient 12C and 13C contents in the carbide by temperature-programmed hydrogenation. In the initial 20 min, C diffuses rapidly into the lattice occupying thermodynamically very stable interstitial sites. The FT reaction starts already during these early stages of carburization. When reaching steady state, the diffusion rates of C in and out of the lattice converge and the FT reaction continues via two parallel reaction mechanisms. It appears that the two outer layers of the Fe carbide are involved in hydrocarbon formation via a slow Mars–Van Krevelen-like reaction contributing to ∼10% of the total activity, while the remainder of the activity stems from a fast Langmuir–Hinshelwood reaction occurring over a minor part of the catalyst surface.
Highlights
Since the invention of the Fischer−Tropsch (FT) process, Febased catalysts have been studied intensively.[1]
As active Fe catalysts typically contain Fe carbides as well Fe oxides and metallic Fe,[3] many studies have dealt with the changes in the catalyst composition during the FT reaction aiming at resolving the nature of the active phase.[4,5]
The CH4 formation peak maximum shifts to a higher temperature with increasing carburization time, pointing to a lower reactivity of C atoms in Fe carbide with increasing carburization degree
Summary
Since the invention of the Fischer−Tropsch (FT) process, Febased catalysts have been studied intensively.[1]. As active Fe catalysts typically contain Fe carbides as well Fe oxides and metallic Fe,[3] many studies have dealt with the changes in the catalyst composition during the FT reaction aiming at resolving the nature of the active phase.[4,5] Nowadays, Fe carbides are generally considered to be the active ingredient of Fe-based FT catalysts.[6−9] Among these, ε(′)-carbide, χ-Fe5C2, aconndditΘio-nFse.130C−13are typically formed under FT process. Because of their pyrophoric character, Fe carbides are commonly synthesized in situ prior to or during the FT reaction. While initially most C derived from CO dissociation is consumed by the carburization process, C hydrogenation to gaseous products becomes more dominant when the bulk of the active phase is saturated by C.14 the formation of adsorbed C species by CO dissociation is relevant to both carburization and FT reaction
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