Outermost surface structures and oxygen reduction reaction activities of Co/Pt(111) bimetallic systems fabricated using molecular beam epitaxy

We studied oxygen reduction reaction (ORR) activities for outermost surfaces of 0.3 nm thick Co deposited on Pt(111) (Co_{0.3 nm}/Pt(111)) bimetallic systems fabricated using molecular beam epitaxy at various Co deposition temperatures. Results show that Co_{0.3 nm}/Pt(111) fabricated at temperatures lower than 393 K displays extra low-energy electron diffraction (LEED) spots outside the integer ones, indicating incoherent epitaxial growth of Co. A new IR band that is attributed to linearly bonded carbon monoxide (CO) on the Pt site influenced by neighboring Co atoms emerges at 2052 cm^-1 for 333 K fabricated Co_{0.3 nm}/Pt(111), in addition to the CO-Pt and CO-Co bands. With increasing fabrication temperature, the new band shifts to higher frequencies and reaches 2082 cm^-1 for 773 K fabricated Co_{0.3 nm}/Pt(111), which has a diffuse (1× 1) LEED pattern. We evaluated the dependence of the deposition temperature on the lattice parameters of the Co_{0.3 nm}/Pt(111) and ascribed the band at 2082 cm^-1 to adsorbed CO on a Pt-enriched topmost surface having 6-fold symmetry. Although the incoherent epitaxial Co layer was unstable in 0.1 M HClO₄ aqueous solution, the Pt-enriched topmost surface is rather stable and the ORR activity is 10 times higher than that for clean Pt(111). The activities for Pt_{0.3 nm,0.6 nm}/Co_{0.3 nm}/Pt(111) artificial sandwich (superlattice) surfaces were also evaluated. The obtained results indicate that the Co atoms located at the second atomic layer strongly modify the electrocatalysis of the topmost surface.