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/gamma-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) and variable temperature in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We show that in this case over Pt/Co-gamma-Al2O3, under model conditions, H2 reacts with CO forming H–C=O bridges over [Co-miO2-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.