Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer–Tropsch Reaction: Evidence of a Two-Site Reaction Model

Abstract

One of the well-known observations in the Fischer–Tropsch (FT) reaction is that the CH4 selectivity for cobalt catalysts is always higher than the value expected on the basis of the Anderson–Schulz–Flory (ASF) distribution. Depositing graphitic carbon on a cobalt catalyst strongly suppresses this non-ASF CH4, while the formation of higher hydrocarbons is much less affected. Carbon was laid down on the cobalt catalyst via the Boudouard reaction. We provide evidence that the amorphous carbon does not influence the FT reaction, as it can be easily hydrogenated under reaction conditions. Graphitic carbon is rapidly formed and cannot be removed. This unreactive form of carbon is located on terrace sites and mainly decreases the CO conversion by limiting CH4 formation. Despite nearly unchanged higher hydrocarbon yield, the presence of graphitic carbon enhances the chain-growth probability and strongly suppresses olefin hydrogenation. We demonstrate that graphitic carbon will slowly deposit on the cobalt catalysts during CO hydrogenation, thereby influencing CO conversion and the FT product distribution in a way similar to that for predeposited graphitic carbon. We also demonstrate that the buildup of graphitic carbon by 13CO increases the rate of C–C coupling during the 12C3H6 hydrogenation reaction, whose products follow an ASF-type product distribution of the FT reaction. We explain these results by a two-site model on the basis of insights into structure sensitivity of the underlying reaction steps in the FT mechanism: carbon formed on step-edge sites is involved in chain growth or can migrate to terrace sites, where it is rapidly hydrogenated to CH4. The primary olefinic FT products are predominantly hydrogenated on terrace sites. Covering the terraces by graphitic carbon increases the residence time of CHx intermediates, in line with decreased CH4 selectivity and increased chain-growth rate.

Cobalt catalysts on carbon-based materials for Fischer-Tropsch synthesis: a review

“…However, the fixed-bed reactor is subjected to local overheating of the catalyst surface that may lead to a fast deactivation and to high methane selectivity [309]. This behavior was clearly observed in FTS using fixed-bed reactors for carbon supported cobalt catalyst [312]. In this context, the development of novel FTS catalysts with high thermal conductivity for efficient heat transfer is a key challenge in FTS.…”

“…Since the FTS is a rather complex reaction, this approach does not account for the mechanisms of formation of different reaction products [41,59]. In order to overcome this problem, the new approaches consist in using microkinetics [65][66][67], DFT calculations [68,69], or to combine DFT calculations and microkinetics in order to elucidate the reaction mechanism [70]. Yang et al [70] combined DFT, transient, and steady-state kinetic modeling to elucidate the reaction mechanism in FTS using a 20%Co/CNTox catalyst to collect the experimental data.…”

Cobalt@Silica Core‐Shell Catalysts for Hydrogenation of CO/CO2 Mixtures to Methane

“…[17,18] Miao et al [19] reviewed possible mechanisms during CO and CO 2 methanation focusing mainly on Ni and Ru catalysts. Chen et al [20] reported that amorphous carbon formed on the cobalt surface during methanation can easily be removed under reaction conditions, whereas graphitic carbon is deposited irreversibly. Furthermore, they suggest that CO is more active in methanation than CO 2 .…”

“…In addition, carbon formation plays an important role. Chen et al [20] reported that amorphous carbon formed on the cobalt surface during methanation can easily be removed under reaction conditions, whereas graphitic carbon is deposited irreversibly. This inactive carbon is mainly located on the terrace sites of the cobalt and decreases the CH 4 selectivity because of blocking the favored sites for CH x hydrogenation.…”

Use of Plasma-Synthesized Nano-Catalysts for CO Hydrogenation in Low-Temperature Fischer–Tropsch Synthesis: Effect of Catalyst Pre-Treatment

“…Today, carbon-supported catalysts are receiving attention, especially in regard to their application in the production of synthetic automobile fuels through the Fischer–Tropsch synthesis (FTS) process [ 12 ]. Some authors have indicated that the presence of graphitic carbon in such catalysts seems to enhance the hydrocarbon chain-growth probability [ 13 ]. The methods used to prepare these FTS catalysts vary greatly, from incipient wetness impregnation [ 14 ] to micro-emulsion [ 15 ], sol-gel [ 16 ], or colloidal synthesis, coupled with the chemical reduction of the metal salts [ 17 ].…”