Interlayer and surface Ir-modified Pt/Pd(111) model catalyst surfaces [Pt/Ir/Pd(111) and Ir/Pt/Pd(111)] were synthesized as surface structural models for third element-modified core-shell-type Pd@Pt catalysts by vacuum depositions of Ir and Pt on the Pd(111) substrate surface. The oxygen reduction reaction (ORR) properties (initial activity and electrochemical stabilities) were compared to non-Ir-modified Pt/Pd(111) and discussed on the basis of atomic structural observations of the near surface regions. ORR activities for both the non-Ir-modified and Ir-modified Pt/Pd(111) surfaces increased up to 500 potential cycles (PCs) of 0.6-1.0 V, which were likely caused by densification of the surface Pt(111) shell layers through dissolution of Pd atoms. The nonmodified Pt/Pd(111) surface deactivated monotonically from 500 to 5000 PCs. The interlayer Ir-modified Pt/Ir/Pd(111) surface exhibited improvements in ORR activity and durability. In fact, from over 500 to 5000 PCs, it outperformed the activity of surface Ir-modified Ir/Pt/Pd(111) in comparison. Furthermore, the Pt/Ir/Pd(111) showed fivefold activity enhancement at 1000 PCs and fourfold after 5000 PCs vs clean Pt(111). In contrast, ORR activity of Ir/Pt/Pd(111) remained almost constant from 500 PCs, with an approximately 3.5 times enhancement at 5000 PCs. Considering atomically resolved observations by scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy and surface chemical state analysis by X-ray photoelectron spectroscopy, the ORR behavior suggests that Ir located in the Pt(111) shell layers contributed to ORR activity enhancement via charge transfer between Ir and Pt surface atoms, while surface Ir oxides generated by PC loadings are correlated with ORR durability improvement. This study demonstrates an effective way to enhance ORR performances of Pt-based core-shell-type catalysts, that is, the third element Ir addition, on the basis of the enhancement scenario deduced by the atomically resolved structural evaluations during the PC loading process.
ASJC Scopus subject areas
- 化学 (全般)