On the origin of high activity of hcp metals for ammonia synthesis

Abstract

Structure and activity of nanoparticles of hexagonal close-packed (hcp) metals are studied using first-principles calculations. Results show that, in contact with a nitrogen environment, high-index {132} facets are formed on hcp metal nanoparticles. Nitrogen molecules dissociate easily at kink sites on these high-index facets (activation barriers of <0.2 eV). Analysis of the site blocking effect and adsorption energies on {132} facets explains the order of activity of hcp metals for ammonia synthesis: Re < Os < Ru. Our results indicate that the high activity of hcp metals for ammonia synthesis is due to the N-induced formation of {132} facets with high activity for the dissociation of nitrogen molecules. However, quite different behavior for adsorption of dissociated N atoms leads to distinctive activity of hcp metals.