Visiting CH₄ formation and C₁ + C₁ couplings to tune CH₄ selectivity on Fe surfaces

To tune CH₄ selectivity of Fe-based Fischer-Tropsch synthesis (FTS) in the initial stage is of prime scientific and industrial importance to further improve the catalyst performance. Herein, distribution of CH₄ selectivity on the metallic Fe nanoparticle is predicted by DFT calculations and micro-kinetics analysis about the competition between C₁ hydrogenations and C₁ + C₁ couplings on abundant Fe surfaces including Fe(1 0 0), Fe(1 1 0), Fe(1 1 1), Fe(2 1 1), and Fe(3 1 0). The results show that HCO mechanism (HCO → CH + O) is an available source of C₁ species apart from CO direct dissociation. These Fe surfaces exhibit high effective barriers for CH₄ formation, which is linearly correlated to the thermal stability of CH₂ species. However, carbon chain prolongation on the more stable surfaces greatly depends on the coupling of C and CH species. On the less stable Fe(1 1 1) surface, the CO + C coupling is the main route for chain prolongation. Utilizing the effective barrier difference between the CH₄ formation and the most feasible C₁ + C₁ coupling as a descriptor of CH₄ selectivity, it is quantified that CH₄ selectivity decreases in sequence of Fe(1 0 0) > Fe(2 1 1) > Fe(1 1 0) > Fe(3 1 0) > Fe(1 1 1). It is revealed that thermal stability of the CH₂ species and exposition of the Fe facets could play essential roles in tuning CH₄ selectivity. Trying to expand the area of Fe(2 1 1), Fe(3 1 0) and especially Fe(1 1 1) surfaces would greatly suppress CH₄ selectivity without a decrease of activity. This work provides new insights and design principles for the Fe-based FTS catalysts.