Abstract
Facile C-C bond formation is essential to the formation of long hydrocarbon chains in Fischer-Tropsch synthesis. Various chain growth mechanisms have been proposed previously, but spectroscopic identification of surface intermediates involved in C-C bond formation is scarce. We here show that the high CO coverage typical of Fischer-Tropsch synthesis affects the reaction pathways of C2Hx adsorbates on a Co(0001) model catalyst and promote C-C bond formation. In-situ high resolution x-ray photoelectron spectroscopy shows that a high CO coverage promotes transformation of C2Hx adsorbates into the ethylidyne form, which subsequently dimerizes to 2-butyne. The observed reaction sequence provides a mechanistic explanation for CO-induced ethylene dimerization on supported cobalt catalysts. For Fischer-Tropsch synthesis we propose that C-C bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions.
Highlights
Facile C-C bond formation is essential to the formation of long hydrocarbon chains in FischerTropsch synthesis
The Fischer–Tropsch synthesis (FTS) process will continue to play a role in future energy scenarios: synthesis gas can be derived from any carboncontaining source, e.g. biomass or even CO2 may be used[2]
Synchrotron-based highresolution X-ray photoemission spectroscopy (XPS) was used as the primary tool to determine both nature and concentration of the CxHy surface intermediates that form at various stages in our experiments
Summary
Facile C-C bond formation is essential to the formation of long hydrocarbon chains in FischerTropsch synthesis. For Fischer-Tropsch synthesis we propose that C-C bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions. 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 low[6,7] They are surrounded by much larger quantities of co-adsorbates such as C1Hxad species[8,9,10], COad[7,9,10], Had[10] and long-chain products[11]. 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
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