Influence of carbon deposits on Fe-carbide for the Fischer-Tropsch reaction

A well-known observation in the Fe-catalyzed Fischer-Tropsch (FT) reaction is that Fe-carbide catalysts deactivate due to carbon deposition. In comparison, the influence of such carbon deposits on the product distribution has been less widely reported. The present work investigates the relationship between the kinetics of the FT reaction for a carburized Raney-Fe model catalyst and the composition of the catalytic surface with a focus on carbon deposits. The deposition of carbon during the ongoing FT reaction at low pressure decreases the FT activity and increases the CH4 selectivity. Steady-state transient isotopic kinetic analysis (SSITKA) at low pressure shows that the buildup of C deposits affects fast CO conversion sites on the Fe-carbide surface, mainly responsible for CC coupling via a Langmuir-Hinshelwood mechanism, more than slow sites that mainly produce CH4 via a Mars-Van Krevelen mechanism. Hindrance of migration of chain-growth monomers by C deposits decreases the amount of longer hydrocarbons. Nevertheless, SSITKA shows that C2 products form faster, which is likely since their formation increasingly depends on monomers formed at proximate CO dissociation sites when the amount of carbon deposits increases. Carbon deposition can be suppressed and reversed by increasing the H2/CO ratio at constant pressure or by increasing the total pressure at constant H2/CO ratio. As such, the FT performance can be stabilized by operating at elevated pressure, even if initially the reaction was started at low pressure. Nevertheless, prolonged operation at low pressure leads to a less reactive character of the C deposits that cannot be removed anymore during high-pressure operation.