Structure-activity relation of Re nanoparticles

Abstract

By combining density functional theory and thermodynamic considerations we determined the equilibrium shape of Re particles in contact with O${}_{2}$ and N${}_{2}$ environments. At low and intermediate coverages oxygen adsorption has a minor influence on the particle shape, while nitrogen adsorption leads to the stabilization of spherically shaped polyhedra with a large contribution from atomically rough faces. At very high coverages of O and N particles are perfect prisms consisting of low-index faces. By comparing these results with the experimental data for the activity of different Re surfaces we propose that the experimentally measured high initial activity of polycrystalline Re for ammonia synthesis might be due to the N-induced formation of high-index faces with a high density of low-coordinated atoms (roughening). Moreover, the measured decrease in the activity of polycrystalline Re at saturation coverage might be because of the N-induced formation of close-packed faces (smoothing). Our calculations suggest that adsorbate-induced roughening or smoothing of catalytic particles on the atomic scale is capable of controlling the activity of the catalysts.