Oxygen Reduction Reaction Activity for Strain-Controlled Pt-Based Model Alloy Catalysts: Surface Strains and Direct Electronic Effects Induced by Alloying Elements

Abstract

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 Moire 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...