We studied oxygen reduction reaction (ORR) activities for outermost surfaces of 0.3 nm thick Co deposited on Pt(111) (Co0.3 nm/Pt(111)) bimetallic systems fabricated using molecular beam epitaxy at various Co deposition temperatures. Results show that Co0.3 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 Co0.3 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 Co0.3 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 Co0.3 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 Pt0.3 nm,0.6 nm/Co0.3 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.

Infrared reflection−absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on sub-monolayer (ML)-thick to 1.0 ML-thick Fe deposited Pt(111) bimetallic surfaces, that is, Fe x /Pt(111) (x, Fe thickness in ML units), fabricated using molecular beam epitaxy at substrate temperatures of 343, 403, and 473 K. The 1.0 L CO exposure to a clean Pt(111) at room temperature yielded linearly bonded and bridge-bonded CO−Pt bands at 2093 and 1855 cm−1. The CO−Pt band intensities for the CO-exposed surfaces of the Fe x /Pt(111) decreased with increasing Fe thickness. The CO−Pt bands almost disappeared, and the bridge-bonded CO−Fe band at 1950 cm−1 dominated the spectra for the Fe1.0ML/Pt(111) deposited at 343 K. In addition, the Fe deposition brought about a new absorption band at around 2060 cm−1; this band is predominant for the Fe0.5ML/Pt(111) deposited at 473 K. The 1 ML-thick Fe deposition onto the 473 K Pt(111) engenders less-intense, rather broad absorption at 2050 cm−1, accompanied by a weak band attributable to bridge adsorption of CO on the surface Fe atoms. The IRRAS spectra for CO adsorption on the 0.6 nm and 0.3 nm-thick Pt grown on the Fe1.0ML/Pt(111), that is, Pt y /Fe1.0ML/Pt(111) (y, Pt thickness in nm units), respectively, showed single absorption bands at 2080 and 2070 cm−1. The reflection high-energy electron diffraction (RHEED) patterns for the Fe1.0ML/Pt(111) deposited at 343 K gave rise to new RHEED streaks, outside the original streaks, attributable to the substrate Pt(111). In contrast, for Pt y /Fe1.0ML/Pt(111) “sandwich” structures, the new streaks disappeared, leaving streaks that had slightly wider separation than that of the clean Pt(111). The Fe0.5ML/Pt(111) deposited at 473 K showed similar streaks to those of the Pt/Fe1.0ML/Pt(111). The temperature-programmed desorption (TPD) spectrum of adsorbed CO on the Fe0.5ML/Pt(111) deposited at 473 K revealed a 40% weaker and 10 K lower desorption signal than those for the clean Pt(111). We discuss the CO adsorption behavior on the well-defined Fe deposited Pt(111) bimetallic surfaces.

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