Surface phase stability of PdAg core-shell nanoalloys in oxidizing atmospheres and its relevance to surface atomic charge

The early stage of oxidation on Pd₆@Ag₃₂ and Ag₆@Pd₃₂ core-shell nanoalloys are calculated by the surface phase stability diagrams using a first-principles atomistic thermodynamics method in oxidizing atmospheres. Ag₃₈ nanoparticle with Δμ_O = − 0.95 eV is more stable than Pd₃₈ nanoparticle with Δμ_O = − 1.3 eV. Unexpectedly, Pd₆@Ag₃₂ core-shell nanoalloy with Δμ_O = − 0.9 eV exhibits better surface phase stability than Ag₃₈ nanoparticle but Pd-segregated Pd₆@Ag₃₂ nanoalloys have lower stability than Ag₃₈ nanoparticle. Meanwhile, Ag₆@Pd₃₂ core-shell nanoalloy with Δμ_O = − 1.5 eV displays lower surface phase stability than Pd₃₈ nanoparticle but Ag-segregated Ag₆@Pd₃₂ nanoalloys show better surface phase stability than Pd₃₈ nanoparticle. Surface-segregated Pd₆@Ag₃₂ and Ag₆@Pd₃₂ core-shell nanoalloys with more surface Ag atoms tend to possess the higher surface phase stability. More interestingly, the order of surface phase stability follows the same trend of atomic charges, that is, the more negative charges corresponding to the lower surface phase stability. In addition, oxidation of PdAg nanoalloys can greatly change electronic structure and atomic charges and have diverse influences on catalytic properties. Unlike bulk PdAg alloys, the oxygen-induced Pd surface segregation in Pd₆@Ag₃₂ core-shell nanoalloy takes place under high oxygen coverage rather than vacuum and medium oxygen coverage. The high oxygen coverage upto 24 oxygen atoms can segregate Pd to outer shell in Pd₆@Ag₃₂ core-shell nanoalloy and restrain the segregation of Ag onto Ag₆@Pd₃₂ core-shell nanoalloy. Our results can provide useful information for designing PdAg-based catalyst materials with appropriate surface phase stability towards fuel cells.