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

Abstract

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 ΔμO = − 0.95 eV is more stable than Pd38 nanoparticle with ΔμO = − 1.3 eV. Unexpectedly, Pd6@Ag32 core-shell nanoalloy with ΔμO = − 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 ΔμO = − 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.