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

Abstract

Pt_<i>m</i>M_<i>n</i> (M = Ru, Rh, Os, and Ir; <i>m</i>+<i>n</i> = 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 (<i>E</i>_coh) between M₁₃ and Pt₁₃ and the distortion energy {<i>E</i>_dis(M₁₃)} of M₁₃ are parallel to the segregation energy (<i>E</i>_seg), indicating that these are important factors for stabilizing M₁₃@Pt₄₂. One more crucially important factor is the interaction energy (<i>E</i>_int) between M₁₃ core and Pt₄₂ shell, because <i>E</i>_int is parallel to <i>E</i>_seg and its absolute value is much larger than those of <i>E</i>_dis(M₁₃) and <i>E</i>_dis(Pt₄₂). The <i>E</i>_int 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