Abstract
Understanding the chain growth mechanism is of vital importance for the development of catalysts with enhanced selectivity towards long-chain products in cobalt-catalyzed Fischer-Tropsch synthesis. Herein, we discriminate various C1 + C1 coupling reactions by theoretical calculations and kinetic isotope experiments. CHx(x=0−3), CO, HCO, COH, and HCOH are considered as the chain growth monomer respectively, and 24 possible coupling reactions are first investigated by theoretical calculations. Eight possible C1 + C1 coupling reactions are suggested to be energetically favorable because of the relative low reaction barriers. Moreover, five pathways are excluded where the C1 monomers show low thermodynamic stability. Effective chain propagation rates are calculated by deconvoluting from reaction rates of products, and an inverse kinetic isotope effect of the C1 + C1 coupling reaction is observed. The theoretical kinetic isotope effect of CO + CH2 is inverse, which is consistent with the experimental observation. Thus, the CO + CH2 pathway, owing to the relatively lower barrier, the high thermodynamic stability, and the inverse kinetic isotope effect, is suggested to be a favorable pathway.
Highlights
Fischer-Tropsch synthesis (FTS) is an alternative process to produce clean liquid fuels by converting syngas [1,2,3,4]
The thermodynamic stability of the reactants was analyzed at two different reaction pressures
The chain growth monomers are more stable at a high pressure
Summary
Fischer-Tropsch synthesis (FTS) is an alternative process to produce clean liquid fuels by converting syngas [1,2,3,4]. Cobalt-based catalysts, owing to the low water-gas shift activity and the remarkable stability, are favorable catalysts, for the synthesis of long-chain hydrocarbons from the synthesis gas with an H2 /CO ratio of about 2 [5,6,7]. Understanding the reaction mechanisms of chain propagation and termination of cobalt-catalyzed FTS is crucial to achieve a high selectivity of long-chain hydrocarbons. The carbide mechanism was originally proposed by Fischer and Tropsch [16], suggesting that CO adsorbs dissociatively on the catalyst surface and the surface carbon is subsequently hydrogenated to form a methylene (CH2 ) group, which is considered as a chain propagation monomer. An assumption was automatically made in their analysis, where the reactions of CH2 with O and OH were ignored
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