Bimetallic surfaces formed through vacuum-deposition of one metal onto a metal substrate have been widely investigated, because such surfaces give us effective templates for discussion on catalytic activities of the alloys. Although Pt is known as an effective electrode for the fuel cell, its catalytic activity is seriously depressed by a very less-amount of carbon monoxide (CO) included in hydrogen sources. In contrast, alloying of Pt with Fe, Rh etc. improves CO tolerance of the electrode surface. The synthesis and characterization of new CO tolerant surface alloys should be directly linked to developments of inexpensive electrode materials for the fuel cell. In this study, CO adsorption behaviors on various-thick Fe deposited Pt(111) surfaces are discussed by infrared reflection absorption spectroscopy (IRRAS).
A Pt(111) crystal surface was cleaned by repeated Ar+ sputtering and annealing at 1500K under UHV condition. Surface cleanliness and crystallographic order were verified with RHEED, LEED and AES. Fe was deposited onto the Pt(111) surface at 340K using a Knudsen-cell. The resulting surfaces were exposed to ca. 7x10-10 Torr CO. The IRRAS spectra were recorded with 2 cm-1 resolution as an average of 300 scans using an FT-IR spectrometer equipped with a HgCdTe detector.
CO exposure to the clean Pt(111) surface lead to linear- and bridge-bonded CO-Pt bands at 2090 and 1850 cm-1. The intensities of the bands abruptly decreased with increasing Fe thickness from 0 to 0.5-monolayer (ML). For the 1ML-thick-Fe/Pt surface, 1950 cm-1 band ascribable to bridge-bonded CO-Fe dominated the spectra, accompanied by a new absorption at ca. 2050 cm-1. The result suggests that the Pt surface is almost covered by the deposited 1ML-thick Fe atoms. The IRRAS spectrum for CO adsorption on the 0.3nm-thick-Pt-covered 1ML-Fe/Pt surface showed a single absorption at 2070 cm-1, suggesting that the new band may be assigned to CO adsorption on the outermost Pt atoms formed through the depositing Fe diffusion into the Pt substrate. On the basis of these results, influence of the deposited Fe on CO adsorption behavior on the Pt sites is discussed.
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