Fischer-Tropsch synthesis: Deuterium kinetic isotope study for hydrogenation of carbon oxides over cobalt and iron catalysts

The hydrogenation of CO₂ using Pt promoted Co/γ-Al ₂O₃ and doubly (Cu, K) promoted iron catalysts exhibits an inverse isotope effect (r^H/r^D < 1). The observed inverse isotope effect for hydrogenation of CO₂ shows that hydrogen addition to CO₂ should be involved in the kinetically relevant step. The systematic increase of inverse isotope effect with carbon number of products obtained during H₂-D₂-H₂ switching experiments suggests the possible existence of a common intermediate (CH _x O) for hydrogenation of CO₂ over both cobalt and iron FT catalysts. The magnitude of the inverse isotope effect is lower for CO₂ compared to CO hydrogenation under similar reaction conditions. The deuterium isotope effect does not provide a definite conclusion regarding the mechanism which CO ₂ hydrogenation follows (alkyl, enol, or alkylidine mechanisms).