Insights into the Mechanism of High CO₂ Selectivity over Co₂C-Based Fischer-Tropsch to Olefins

The reasons for the too high CO₂ selectivity (near 50 C %) in Co₂C-based Fischer-Tropsch to olefins (FTO) are not clear. Water-gas-shift (WGS) side reaction is thought to be responsible for this, but in-depth research is lacking. Here, we revealed the mechanism of the too high CO₂ selectivity in Co₂C-based FTO. Kinetic studies showed that WGS reaction was more likely to occur than the hydrocarbon production reaction. In addition, a too severe WGS reaction would result in the formation of an H₂-enriched and CO-deficient environment on the catalyst surface, which was not favorable for the generation of light olefins. Catalyst characterization results proved that the severe WGS reaction could also lead to the transformation of prismatic Co₂C to spherical Co₂C. Theoretical calculations demonstrated that prismatic Co₂C (020) and Co₂C (101) facets were dominant crystal facets for both FTO and WGS reactions. Furthermore, in situ diffuse reflectance infrared Fourier transform spectroscopy experiments found that the H₂O produced in FTO completely participated in the WGS reaction. This clearly revealed why the CO₂ selectivity was close to 50 C % in FTO. Therefore, we believed that accelerating the desorption of H₂O on the catalyst surface and preventing its readsorption are the key elements for inhibiting CO₂ formation in FTO.