Crystallographic and Morphological Sensitivity of N₂ Activation over Ruthenium

Ruthenium (Ru) serves as a promising catalyst for ammonia synthesis via the Haber-Bosch process, identification of the structure sensitivity to improve the activity of Ru is important but not fully explored yet. We present here density functional theory calculations combined with micro-kinetic simulations on nitrogen molecule activation, a crucial step in ammonia synthesis, over a variety of hexagonal close-packed (hcp) and face-center cubic (fcc) Ru facets. Hcp \left\ 21\overline 3 0 \right\ facet exhibits the highest activity toward N_2 dissociation in hcp Ru, followed by the (0001) monatomic step sites. The other hcp Ru facets have N_2 dissociation rates at least three orders lower. Fcc \211\ facet shows the best performance for N_2 activation in fcc Ru, followed by \311\, which indicates stepped surfaces make great contributions to the overall reactivity. Although hcp Ru \left\ 21\overline 3 0 \right\ facet and (0001) monatomic step sites have lower or comparable activation barriers compared with fcc Ru \211\ facet, fcc Ru is proposed to be more active than hcp Ru for N_2 conversion due to the exposure of the more favorable active sites over step surfaces in fcc Ru. This work provides new insights into the crystal structure sensitivity of N_2 activation for mechanistic understanding and rational design of ammonia synthesis over Ru catalysts.