Formation Mechanism of the Co2C Nanoprisms Studied with the CoCe System in the Fischer–Tropsch to Olefin Reaction

Cobalt carbide (Co2C) nanoprisms are efficient for olefin production via Fischer–Tropsch to olefin (FTO) reaction. However, these Co2C nanoprisms can only be prepared from the given CoMn composite oxide precursor with a sodium promoter and the formation mechanism remains ambiguous up to now. In the present work, the Ce-stabilized spherical Co nanoparticles were synthesized and were used as a precursor to study the formation mechanism of the active phase. The experimental results and theoretical analysis showed that Ce promoters were favorable to provide oxygen to the dangling bonds of the surface Co sites. The coordination of the surface Co sites and the Ce promoters reduced the surface energy of the Co2C nanostructures, resulting in different growth rates of the Co2C crystal in various orientations. Such effects facilitated the formation of the Co2C with specific exposed facets. Combining the abovementioned findings with the different morphologies of the intermediates, a seven-step formation process of Co2C nanoprisms was proposed. As a result, it exhibited that the transition from the Co2C nanospheres to the Co2C nanoprisms was enhanced by increasing the Ce content, leading to a decrease in the CH4 and paraffin selectivities and an increase in the C2–4= selectivity and olefin/paraffin ratio.