Oxygen reduction reaction activities for Pt-enriched Co/Pt(111), Co/Pt(100), and Co/Pt(110) model catalyst surfaces prepared by molecular beam epitaxy

Oxygen reduction reaction (ORR) activities for 0.3 nm thick Co deposited Pt(111), Pt(100), and Pt(110) surfaces prepared by molecular beam epitaxy (MBE) were investigated after the samples were transferred from ultra-high vacuum (UHV) to an electrochemical system without being exposed to air. The low-energy electron diffraction and IR reflection-absorption spectroscopic results for adsorbed carbon monoxide indicated that Co deposition on the low-index single-crystal Pt substrates at 753 K-773 K led to Pt-enrichment at the topmost surfaces through surface segregation of the substrate Pt atoms. The ORR activity was evaluated in O ₂-saturated 0.1 M HClO ₄ at 0.9 V vs. reversible hydrogen electrode (RHE) for the as-prepared Pt-enriched Co _{0.3 nm}/Pt(111), Co _{0.3 nm}/(100), and Co _{0.3 nm}/(110) surfaces; the respective surfaces exhibited about 10, 2, and 1.5 times the ORR activity relative to their corresponding clean surfaces. When 1000 potential cycles between 0.6 V and 1.0 V were applied, the activities for the Pt-enriched Co _{0.3 nm}/Pt(111) and Co _{0.3 nm}/Pt(110) surfaces decreased and came close to that for as-cleaned Pt(110). These results show that the ORR activity enhancement for Pt-Co alloy surfaces significantly depends upon the atomic arrangement of the Pt-enriched topmost surfaces and the underlying Co atoms.