Influence of Cobalt Crystal Structures on Activation of Nitrogen Molecule: A First-Principles Study

Identification of the structure sensitivity of nitrogen molecule (N2) activation and ammonia synthesis on metal surfaces is important for the mechanistic understanding and rational design of more efficient catalysts. In the present work, density functional theory calculations together with microkinetic simulations were performed to study the influence of cobalt crystal structures including hexagonal close-packed (HCP) and face-centered cubic (FCC) on nitrogen molecule dissociation and ammonia synthesis. Molecular and dissociative adsorption energies of N2 as well as dissociation barriers are calculated for a total of ten cobalt surfaces. It is found that molecular adsorption energies on Co surfaces vary modestly on the order of 0.25 eV, whereas dissociative adsorption energies and the corresponding barriers vary considerably in magnitude by about 0.80 eV. First-principles microkinetic simulations show that HCP Co displays higher activity than FCC cobalt for nitrogen molecule dissociation and ammonia synthesis due to the higher intrinsic activity and density of active sites of HCP cobalt. Nitrogen molecule dissociation is the rate-determining step of ammonia synthesis due to the weak interaction between nitrogen and cobalt. The crystal phase sensitivity of nitrogen molecule dissociation on cobalt is compared with the dissociation of an isoelectronic molecule, carbon monoxide on cobalt, ruthenium, and nickel. This work provides valuable insights into nitrogen molecule dissociation and ammonia synthesis on cobalt catalysts with different crystal phases, and highlights the interplay between activated molecules and catalyst composition on the crystal phase sensitivity