Ir15 (111) Cluster
N and NH adsorb at top, hexagonal close-packed, and face-centered cubic positions using all methods – B3LYP, B3LYP-D3, and B97-D3. The HCP position is most favored, closely followed by FCC. This is very similar to the theoretical findings of Krekelberg et al., whose study used periodic DFT methods PW91 and RPBE.38 They found that HCP and FCC were equally favorable. NH2 adsorbs at the bridge position for all methods and the top position for B3LYP-D3 and B97-D3, with the bridge position being slightly more favorable. NH3 also adsorbs at top and bridge positions for all methods, and the top position is most favorable. This agrees with Krekelberg’s theoretical findings, but is in conflict with an experiment that found adsorption of NH3 at FCC/HCP.38 Although for most of the molecules in this study, more than one adsorption site is found possible, none of the methods could find a minimum at the HCP or FCC position for NH3.