Surface strain and electronic interactions (i.e., strain and ligand effects) play key roles in enhancing the oxygen reduction reaction (ORR) catalytic activity of Pt-based alloy catalysts. Herein, we evaluate the ORR activity enhancement factors for Pt(111)-shell layers on Pt25Ni75(111) single-crystal surfaces prepared by molecular beam epitaxy under ultrahigh vacuum (UHV). Scanning tunneling microscopy images of the pristine surfaces collected under UHV revealed periodic surface modulations, known as Moiré patterns, suggesting that the topmost Pt(111)-shell layers are compressively strained by the influence of the underlying Ni atoms. The correlation between the ORR activities and estimated strains for 3-ML- and 4-ML-thick Pt shells (where ML represents monolayer), each having -1.7% and -1.2% strained Pt-shells, correspond well to the strain-based theory predictions. On the other hand, a 2-ML-thick Pt shell, with -2.8% strain, exhibits a remarkable ORR activity enhancement, i.e., 25 times higher than the pristine Pt(111): the enhancement factor anomalously deviates from the value predicted by the strain-based theory. Therefore, the activity enhancement of the 2-ML-thick Pt sample can be ascribed to a ligand effect induced by the Ni atoms just below the topmost Pt(111)-shell layer. The results obtained in this study provide a fundamental insight into the ORR activity enhancement mechanisms of Pt-based electrocatalysts.
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