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

The early stage of oxidation on Pd-6@Ag-32 and Ag-6@Pd-32 core-shell nanoalloys are calculated by the surface phase stability diagrams using a first-principles atomistic thermodynamics method in oxidizing atmospheres. Ag-38 nanoparticle with Delta mu(O) = - 0.95 eV is more stable than Pd-38 nanoparticle with Delta mu(O) = - 1.3 eV. Unexpectedly, Pd-6@Ag-32 core-shell nanoalloy with Delta mu(O) = - 0.9 eV exhibits better surface phase stability than Ag-38 nanoparticle but Pd-segregated Pd-6@Ag-32 nanoalloys have lower stability than Ag-38 nanoparticle. Meanwhile, Ag-6@Pd-32 core-shell nanoalloy with Delta mu(O) = - 1.5 eV displays lower surface phase stability than Pd-38 nanoparticle but Ag-segregated Ag-6@Pd-32 nanoalloys show better surface phase stability than Pd-38 nanoparticle. Surface-segregated Pd-6@Ag-32 and Ag-6@Pd-32 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 Pd-6@Ag-32 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 Pd-6@Ag-32 core-shell nanoalloy and restrain the segregation of Ag onto Ag-6@Pd-32 core-shell nanoalloy. Our results can provide useful information for designing PdAg-based catalyst materials with appropriate surface phase stability towards fuel cells. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The early stage of oxidation on Pd-6@Ag-32 and Ag-6@Pd-32 core-shell nanoalloys is analyzed using first-principles atomistic thermodynamics, revealing that nanoalloys with more surface silver atoms tend to possess higher surface phase stability.