Alkali Promotion of $N_2$ Dissociation over Ru(0001)

Using self-consistent density functional calculations, we show that adsorbed Na and Cs lower the barrier for dissociation of ${\mathrm{N}}_2$ on Ru(0001). Since ${\mathrm{N}}_2$ dissociation is a crucial step in the ammonia synthesis reaction, we explain in this way the experimental observation that alkali metals promote the ammonia synthesis reaction over Ru catalysts. We also show that the origin of this effect is predominantly a direct electrostatic attraction between the adsorbed alkali atoms and the dissociating molecule.