A New Approach to the Mechanism of Fischer-Tropsch Syntheses Arising from Gas Phase NMR and Mass Spectrometry

We used (CO)-C-13 labeling to show that gas-phase NMR spectroscopy and mass spectrometry are simple tools for mechanistic investigations of the Fischer-Tropsch (FT) reaction. Thus, monodisperse Fe nanoparticles (NPs) react with syngas to form monodisperse iron carbide (FeCx) NPs. As expected, the heating of C-13-labeled monodisperse FeCx NPs under H-2 results in the desorption of the carbide carbons as (CH4)-C-13 and, interestingly, restores the initial Fe NPs in terms of size and dispersity. The (FeCx)-C-13 NPs catalyze the hydrogenation of (CO)-C-12 at 210 degrees C to yield only C-12-labeled FT products, which evidences the absence of the incorporation of FeCx carbon atoms in the products. In addition, this approach shows for the first time that the formation of (CH4)-C-13 at 250 degrees C does not result from direct carbide hydrogenation but from an intermediate step that involves a reaction between (FeCx)-C-13 and H2O to give (CO2)-C-13, which is subsequently hydrogenated. These results rule out the involvement of FeCx carbon atoms in the chain growth process under our conditions.