Supported cobalt catalysts find their most widespread application in low-temperature Fischer-Tropsch synthesis (FTS), a process in which C-C bond-forming reactions produce long-chain hydrocarbon products from synthesis gas, a mixture of CO and H21. In today’s fossil fuel-based economy, synthesis gas is predominantly manufactured from natural gas or coal, where FTS adds value by converting gaseous (gas-to-liquids) or solid (coal-to-liquids) reactants into more valuable products, such as food-grade wax, lubricants and sulfur-free transportation fuels. The FTS process will continue to play a role in future energy scenarios: synthesis gas can be derived from any carbon-containing source, e.g. biomass or even CO2 may be used2. These renewable carbon sources offer an alternative route to produce a ‘syncrude’ that can, to a large part, replace petroleum as the principal feedstock of chemicals and the liquid fuels that power transportation modes (airplanes, ships, heavy vehicles) that cannot be readily replaced by fully electric alternatives. Insight into the molecular mechanism by which long-chain hydrocarbon species grow on the surface of the cobalt catalyst particle is of direct relevance to better understand the molecular origin of selectivity in FTS and may ultimately drive rational design of catalysts.
A large variety of chain growth mechanisms can be found in the literature, summarized in, e.g. refs. 3-5. Growth intermediates of different chain length co-exist on the active surface, and steady-state isotopic transient kinetic analysis studies reveal that their concentration is low6,7. Furthermore, they are surrounded by much larger quantities of co-adsorbates such as C1Hxad species8-10, COad7,9,10, Had10 and long-chain products11. This complexity makes it impossible to distinguish those few active surface species from other adsorbates by, e.g. in situ infrared (IR) absorption spectroscopy12. We instead use a model approach to study CxHy reactivity on a cobalt catalyst under conditions relevant to FTS. As shown hereafter, it is important to study how co-adsorbed Had and CO affect CxHy reactivity, since both adsorbates will be present on the surface under reaction conditions.
Water is a major by-product of FTS, and the high conversion levels reached lead to a water partial pressure that amounts to several bars during industrial operation. However, since chain growth also occurs under low conversion conditions where the H2O partial pressure is low, the presence of H2O does not appear to be essential to the chain growth mechanism and therefore it was omitted from our study. Moreover, surface science studies show that water adsorbs much weaker on Co(0001) than on both CO and hydrogen13 and the H2O surface coverage under reaction conditions is expected to be low even when the H2O partial pressure is comparable to that of CO and H2. An in situ X-ray absorption study of cobalt supported on a carbon nanofiber support shows that neither bulk oxidation nor substantial surface oxidation occurs on cobalt during FTS14. Furthermore, cobalt single crystals were found to be active for FTS11,15-18, and the turnover frequencies reported are similar to those found for supported catalysts. This confirms that metallic cobalt is the active phase for chain growth and that insights from single crystal studies are of direct relevance for fundamental understanding of FTS.
We here use a Co(0001) model catalyst to study how C2Hxad species react to form a new C-C bond under FTS-like conditions, that is, in the presence of co-adsorbed hydrogen and COad. Using this approach, we find that C-C bond formation is promoted by CO spectators, which stabilize the alkylidyne intermediate needed for this reaction. This finding can rationalize why CO promotes alkene dimerization on cobalt catalysts and reveals the hidden role of CO as promoter of chain growth during FTS on supported cobalt catalysts.