The mechanism of the Fischer–Tropsch reaction over supported cobalt catalysts

Natural gas and coal are converted to fuels by the Fischer–Tropsch reaction, i.e., by reacting CO with H2, the currently accepted mechanism involving surface carbide formation. We have monitored the adsorbed species and their evolution during Fischer–Tropsch reaction on the commercial catalyst Pt,Co/γ-Al2O3 (as a standard), on Co/alumina-pillared montmorillonite (Co,Al-EFW) and its beidellite analogue (both are tri-octahedral smectite clays, but beidellite possesses tetrahedrally coordinated Al in the sheets) via optical diffuse reflectance (DRS 1) and variable temperature in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS 2). We show that in this case over Pt/Co-γ-Al2O3, under model conditions, H2 reacts with CO forming H–CO bridges over [Co-μO2-Co] units; this transition state moiety also involves adjacent matrix Al–O–CO units. Subsequent stages provide differing oxygenate species via concerted acid–base reactions. The early stages of the Fischer–Tropsch synthesis thus do not involve surface carbide species and surface oxygenates are generated via concerted reaction of support surface Al-OH with the above HCO-bridging binuclear Co2+ unit.

Using in situ DRIFTS spectroscopy backed up by visible/u.v. spectra it is demonstrated that the first stage of the Fischer–Tropsch synthesis on Pt,Co/γ-Al2O3, and Co/alumina-pillared clay catalysts does not involve formation of surface carbides. Instead, H2 reacts with CO forming a transition state in which H–CO bridges [Co-μO2-Co] units. Further, CO concurrently reacts with Al-OH groups of the alumina surface and this transition state moiety involves adjacent Al–O–CO units thus formed, to give – overall – a concerted reaction.Figure optionsDownload high-quality image (75 K)Download as PowerPoint slideResearch highlights▶ Early stages of Fischer–Tropsch synthesis (FTS) do not involve surface carbide species. ▶ Surface species from FTS are bonded Co-carbonyls, hydrocarbons, and oxygenates. ▶ H–CO bridges over [Co-μO2-Co] units are transition states of FTS.