Ir15 (100) Cluster
N adsorbs at the top and bridge positions for all methods and for B97-D3, the hollow position as well; however, the bridge position is the most favorable across all methods. For NH, B3LYP-D3 does not converge to a bridge site, but B3LYP and B97-D3 do. B97-D3 shows an additional, less favorable adsorption at the top position. Both B3LYP and B3LYP-D3 converged to hollow sites located only at the edges of the surface. Due to edge effects on this hollow site, it is likely that bridge is the most favorable site for NH. This is in agreement with adsorption of monatomic O on Ir(100), which also has two unpaired electrons and adsorbs at the bridge position.39 NH2and NH3 have only one favorable site – bridge and top respectively.
Overall, all methods yield the same preferential adsorption sites on both the (100) and (111) surfaces, with the exception of NH on (100). B3LYP always gives the smallest binding energies, while its dispersion-corrected form gives larger binding energies and B97-D3 gives the largest binding energies.