The role of potassium promoter in surface carbon hydrogenation on Hägg carbide surfaces

• Surface carbon hydrogenation on (1 1 1), (1 0 0) and (1 1 0) of Fe5C2 was computed.
• Hydrogenation ability of surface carbon atoms has the order of (1 1 1) > (1 0 0) > (1 1 0).
• K2O promotion on CH4 formation is short-ranged and has electronic origin.

The role of K2O promoter on CH4 formation from surface carbon hydrogenation on the representative (1 1 1), (1 0 0) and (1 1 0) facets of the Fe5C2 phase has been computed. On the basis of the most favored hydrogenation routes, the (1 1 1) facet can be most easily hydrogenated, followed by the (1 0 0) facet, while the (1 1 0) facet has the highest effective barrier and can be hardly hydrogenated. K2O adsorption can stabilize or destabilize surface CHx species and affect the effective barriers. Under K2O adsorption, the effective hydrogenation barrier on the (1 1 1) facet can be slightly lowered, while that on the (1 0 0) and (1 1 0) facets can be increased strongly. It is found that the K2O promotion is shorted-ranged and electronic. Compared with the clean surfaces, the surface carbon atoms and adsorbed hydrogen atoms are stabilized or destabilized under K2O adsorption as they become more or less negatively charged.

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