CO2 hydrogenation to hydrocarbons over Co and Fe-based Fischer-Tropsch catalysts

The performances of representative Co-based and Fe-based Fischer-Tropsch catalysts have been comparatively investigated in the hydrogenation of CO and CO2. Over an un-promoted Co/γ-Al2O3 catalyst, CO2 is easily hydrogenated and its conversion rate is even faster than that of CO; however, the selectivities of the two processes are extremely different, with methane largely dominating the product distribution in the case of CO2 hydrogenation and long-chain hydrocarbons dominating the products pool during CO hydrogenation. As opposite to cobalt, CO2 hydrogenation rate over K-promoted 100Fe/10Zn/1Cu (at/at) catalysts is slower than that of CO, but the products are dominated by middle distillates when CO2 replaces CO in the feed. Such behaviors depend on the different adsorption strengths of CO and CO2, which affect the H/C atomic ratio on the catalyst surface. In the case of Fe-based catalyst, we have also found that the catalytic sites active in the chain growth process (iron carbides) are transformed into sites active in the hydrogenation reactions (iron oxides/reduced iron centers) at low CO partial pressures. Potassium has a key role in promoting the stability of chain growth sites, thus decreasing the secondary reactions of olefins.