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 {213 over bar 0} 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 {213 over bar 0} 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.