Theoretical Insight into Core–Shell Preference for Bimetallic Pt‑M (M = Ru, Rh, Os, and Ir) Cluster and Its Electronic Structure

PtmMn (M = Ru, Rh, Os, and Ir; m+n = 38 and 55) clusters are systematically investigated using the DFT method. In an octahedral 38-atom cluster, core–shell structure M6@- Pt32 with M6 core and Pt32 shell is stable for Pt–Rh and Pt–Ir combinations but is not for Pt–Ru and Pt–Os combinations. In a 55-atom cluster, icosahedral M13@Pt42 structure is stable for all Pt-M combinations, indicating that a large cluster is more preferable to stabilizing the core–shell structure than a small cluster. The difference in cohesive energy (Ecoh) between M13 and Pt13 and the distortion energy {Edis(M13)} of M13 are parallel to the segregation energy (Eseg), indicating that these are important factors for stabilizing M13@Pt42. One more crucially important factor is the interaction energy (Eint) between M13 core and Pt42 shell, because Eint is parallel to Eseg and its absolute value is much larger than those of Edis(M13) and Edis(Pt42). The Eint depends on the energy gap between LUMO of M13 core and HOMO of Pt42 shell, indicating that LUMO energy of M13 and HOMO energy of Pt42 are good properties for understanding and predicting stability of core–shell structure. Pt atom is more positively charged in M13@Pt42 than in Pt55 and the HOMO energy of M13@Pt42 is higher than that of Pt55. The presence of these two contrary factors for O2 binding suggests that M13@Pt42 is not bad for O2 binding.