Isotopic Exchange Study on the Kinetics of Fe Carburization and the Mechanism of the Fischer-Tropsch Reaction
Jiachun Chai (Eindhoven University of Technology)
Robert Pestman (Eindhoven University of Technology)
Wei Chen (Eindhoven University of Technology)
Noortje Donkervoet (Eindhoven University of Technology)
Achim Iulian Iulian Dugulan (TU Delft - RID/TS/Instrumenten groep)
Zhuowu Men (National Institute of Clean-and-Low-Carbon Energy, Shenhua NICE, Future Science and Technology City, Changping District, Beijing 102211, People's Republic of China)
Peng Wang (Eindhoven University of Technology, National Institute of Clean-and-Low-Carbon Energy, Shenhua NICE, Future Science and Technology City, Changping District, Beijing 102211, People's Republic of China)
Emiel J. Hensen (Eindhoven University of Technology)
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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.
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