Carbon Monoxide Activation on Cobalt Carbide for Fischer–Tropsch Synthesis from First-Principles Theory

Cobalt carbide based catalyst shows a promising activity and selectivity in the direct conversion of syngas (a mixture of carbon monoxide and hydrogen molecules) toward oxygenates and lower olefin. A mechanistic understanding of the cobalt carbide structure as well as its intrinsic reactivity under Fischer–Tropsch reaction conditions is vital but remains controversial. On the basis of ab initio thermodynamics and density functional theory (DFT) calculations, we study here a number of the pristine Co2C surfaces with different orientations and compositions as well as their catalytic activity on CO direct dissociation. The corresponding phase diagram and equilibrium morphology of Co2C under a wide range of the chemical potential of carbon are constructed. Under a higher chemical potential of carbon (carbon-rich conditions), carbon-rich surfaces like (110) and (111) facets are preferentially exposed surfaces, whereas at a lower chemical potential of carbon, the stoichiometric surfaces like (011) facet could appear. Cobalt-rich surfaces such as (101) and (010) facets could be exposed only due to the kinetics hindrance under carbon-poor or hydrogen-rich conditions where the pristine bulk carbide is thermodynamically not stable. It is found that though CO adsorbs strongly on stoichiometric and carbon-rich Co2C surfaces, the barrier for subsequent CO direct dissociation is significantly high. The presence of the carbon vacancy could promote CO direct dissociation. However, the high energy cost to produce the carbon vacancy for instance via methanation limits its overall activity toward CO activation. Implications of the present results on the role of Co2C in direct conversion of syngas toward oxygenates and lower olefin are discussed along with available experiments.