In Fischer-Tropsch catalysis, hydrocarbons are produced from carbon monoxide and hydrogen gas with the use of cobalt or iron based catalysts. Iron based catalysts are especially useful to obtain short-chain olefins (C2-C4 alkenes). The mechanism of this reaction is rather complicated due to the balance between chain growth and two possible termination steps, forming either alkanes or alkenes. This is complicated further by the active phase being iron carbide (most important is the Hägg carbide, Fe5C2, but other carbide phases are present as well) and the need for promoters to obtain the right selectivity. Hence, this system is a challenge for computational modeling, however it is expected to increase the fundamental understanding of this reaction. Here, we focus on the effects of promoters, especially on the combination of Na and S. We have performed DFT calculations on the Fe5C2 (111) surface comparing the effects of different promoter geometries and counter ions on CO and H adsorption. We show that the main effect of the promoters is to donate charge to the catalyst surface, which changes the reactivity of iron atoms and carbon atoms in opposite directions.1 This supports the experimentally observed lower formation of methane and longer alkanes in favor of the desired alkenes. On top of that, the specific promoter geometry appears crucial to explain the especially good results for Na2S promoted systems. Calculations are ongoing to extend the work to the full reaction path towards methane, ethane, and ethene, to elucidate all subtleties of the Na2S promoter effect on this reaction. References J. Xie. J. Yang, A. Holmen, D. Chen, A.I. Dugulan, K.P. de Jong and M.J. Louwerse, submitted.

Modeling promoter effects in iron-based Fischer-Tropsch catalysis

251st National Meeting (2016)

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