The metals Fe, Ni, Co, and Ru have the required FT activity for commercial appli­cation. Under practical operating conditions Ni produces too much CH4, while the available supply of Ru is insufficient for large scale application. This leaves only Fe and Co as viable catalysts [11].

Fe and Co are commercial catalysts for FT synthesis with CO2 — free syngas. Riedel et al. [16] compared the performance of Fe and Co catalysts in the mixtures of CO, CO2, and H2. With increasing CO2 and decreasing CO content in the feedgas, the product composition shifts from a mixture of mainly higher hydrocarbons to almost exclusively methane for Co catalyst, while Fe-based catalyst synthesizes the same hydrocarbon products from CO2/H2 as from CO/H2 syngas. Zhang et al. [17] also found that the CO2 hydrogenation products contained about 70% or more methane for supported cobalt.

These distinctions are partly attributed to that Fe catalyst is active for WGS reaction, but Co catalyst has no such activity. On Fe catalyst, CO2 can be hydrogenated into FT products by two steps as shown in (1) and (2): CO is converted from CO2 by reverse WGS reaction, then produced CO is further hydrogenated to hydrocar­bons [9, 11, 18]. In contrast, Co catalyst cannot convert CO2 into CO.

Another factor is the different prerequisites to achieve the kinetic regime of FT synthesis for Fe and Co catalysts. With Fe catalyst, the FT kinetic regime is gener­ated through the formation of stable FT sites via carbiding, and their selectivity is invariant against changes of reactant concentrations [19] . On the contrary, the FT regime can exist only at a sufficiently high CO partial pressure for Co catalyst [20]. Therefore, Co is not fit to hydrogenate CO2 in nature, while Fe is promiseful to FT synthesis from CO2-containing syngas.