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J. Phys. Chem. C 2008, 112, 8944–8950
Infrared Reflection-Absorption Study of Carbon Monoxide Adsorption on Fe/Pt(111) Bimetallic Surfaces Toshimasa Wadayama,* Hiroshi Osano, Toshiaki Maeyama, Hirosato Yoshida, Koji Murakami, Naoki Todoroki, and Shogo Oda Department of Materials Science, Graduate School of Engineering, Tohoku UniVersity, Sendai 980-8579, Japan ReceiVed: December 26, 2007; ReVised Manuscript ReceiVed: March 12, 2008
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, Fex/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 Fex/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, Pty/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 Pty/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. 1. Introduction Fabrication of well-defined bimetallic alloy surfaces through vacuum deposition of a metal onto a different single-crystal metal substrate has been studied intensively for designing new materials with unique surface properties that are advantageous for practical applications and which are not attainable in a single metal.1–7 These approaches for constructing specific surface structures are crucial not only for investigating physical, chemical, and electronic properties of bimetallic alloy surfaces, but also for innovating industrial processes, particularly in the fields of magnetic devices and heterogeneous catalysis. For example, Pt-based bimetallic surfaces have been particularly subjected to scrutiny: precise control of Pt/Fe heterostructures is indispensable for developing novel magnetic-informationstorage devices.8–11 From a catalytic perspective, numerous studies of Pt-based alloys have been undertaken to develop highly efficient and low-noble-metal-content electrode materials for use in fuel cells;12–18 atomic arrangements and compositions of the outermost alloy surfaces remain as key issues for improving the catalytic activities for the electrode. Recent developments of core-shell structures of a platinum monolayer on non-noble-noble metal nanoparticles for the oxygen reduction reaction strongly indicate that specific bimetallic and ternary alloy surfaces determine the cathode side reaction.12–14 Further* To whom correspondence should be addressed. Phone: +81-22-2177319; fax: +81-22-217-7319; e-mail:
[email protected].
more, although carbon monoxide (CO) poisoning of the platinum electrode surface severely depresses the hydrogen dissociation reaction on the anode electrode, alloying of Pt with Fe, Ni, Co, Pd, Ru, etc., improves the electrode’s CO tolerance.15–21 In situ observations of molecular behavior on alloy surfaces in electrochemical systems are expected to shed some light on the mechanisms of electrode catalysis. Pioneering work has been conducted by Watanabe and co-workers on alloys formed by sputtering of Pt and X (X ) Fe, Ni, or Co),16,20 and by Liu et al. for arc-melting of Pt and X (X ) Ru, Os, or RuOs).22 For both, the electronic properties of the alloy films and molecular behavior of adsorbed CO were evaluated using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy, respectively. However, discussion of the mechanism is not straightforward because of uncertainties in the outermost surface structures, which are expected to strongly influence molecular behavior. For that reason, the relation between the alloy surface structure and molecular behavior remains to be elucidated. Because molecular characterizations of CO on well-defined Pt-based alloy surfaces can elucidate the CO tolerance, vibrational spectroscopic measurements of adsorbed CO on a well-defined bimetallic surface are expected to offer a deeper insight into CO tolerance mechanisms. To date, atomic arrangements of the single-crystal alloy of the Pt80Fe20(111) surface have been studied intensively; an almost pure Pt top layer can be generated through surface segregation of Pt atoms.23,24 Several studies have described the
10.1021/jp712095w CCC: $40.75 2008 American Chemical Society Published on Web 05/21/2008
IRRAS Study for CO on Fe/Pt(111) surface structures of vacuum-deposited Fe on single-crystal Pt surfaces. For example, epitaxial growth of Fe on a Pt(100) surface was investigated by Hufnagel et al.9 Jerdev and Koel examined surface structures of as-deposited and Fe/Pt(111) annealed at 500 K.25 The surface structures of the Fe-deposited Pt(997) vicinal surfaces were described by Lee et al.8 and Cheng et al.10 Nevertheless, few studies have specifically examined the surface chemistry for well-defined Fe/Pt, except for highresolution electron energy loss spectroscopic (HR-EELS) results for CO adsorption on Pt20Fe20(111).26 Infrared reflection-absorption spectroscopy (IRRAS) is a powerful tool for studying adsorption of simple molecules onto well-defined metal surfaces. Its high sensitivity to adsorption sites and metal/molecule interactions supports investigation of the chemical properties of metal and alloy surfaces. Results of past studies have shown27–33 that CO is useful as a probe of surface lattice structures and electronic properties of single metals and bimetallic alloys. We have reported IRRAS results for CO adsorption on several bimetallic surfaces having periodic atomic structures.7,34–37 For the present study, we examine CO adsorption on the Fedeposited Pt(111) bimetallic surfaces (designated as Fex/ Pt(111),38 where x is the Fe thickness in ML units), fabricated under ultrahigh vacuum (UHV) conditions with substrate temperatures of 343, 403, and 473 K. We conducted IRRAS and temperature-programmed desorption (TPD) measurements for adsorbed CO on the surfaces. The obtained results demonstrate that alloying the deposited Fe with Pt(111) substrate generated an outermost Pt layer on the Fe atoms incorporated in the Pt lattice, thereby modifying CO adsorption behavior on the alloy surfaces. 2. Experimental Details of the experimental equipment used in this study have been described elsewhere.7,34–37Briefly, a Pt(111) crystal of less than 1° miscut was used as the substrate for Fe deposition. Repeated Ar+ sputtering and annealing at 1250 K under UHV conditions cleaned the Pt(111) surface. The cleanliness and crystallographic order of the Pt(111) substrate were verified using Auger electron spectroscopy (AES), reflection high-energy electron diffraction (RHEED), and low-energy electron diffraction (LEED). Using a Knudsen Cell, Fe of 99.999% purity was deposited onto the Pt(111) substrate surface. The Fe thickness in monolayer (ML) units on each Pt surface was estimated based on the deposition time and number of regular oscillations in the RHEED intensity observed during fcc-Fe epitaxial growth on the clean Cu(100) conducted in the same MBE system.7 The Fe deposition rate was approximately 0.3MLmin. Platinum of 99.99% purity was deposited using electron-beam evaporation onto the sample at room temperature (RT). The RHEED measurements were carried out using a 10 keV electron beam incident at 2° or less with respect to the surface. The diffraction images were analyzed quantitatively via detection of light emitted from the fluorescent screen using a computer-controlled CCD video camera and a data acquisition system (400; KSA). Exposure of CO to the resultant Fex/Pt(111) surfaces was carried out at approximately 7 × 10-10 Torr. The IRRAS spectra of adsorbed CO were recorded at 2 cm-1 resolution as an average of 300 scans using an FT-IR spectrophotometer (RS-2; Mattson Instruments) equipped with a liquid-N2-cooled HgCdTe detector. Each spectrum is presented here as a ratio with the spectrum recorded before CO exposure. The TPD spectra of CO on the resulting Fex/Pt(111) were recorded using a quadrupole mass spectrometer (RGA100; SRS). Heating of the sample was
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8945
Figure 1. IRRAS spectra for adsorbed CO on Fe1.0ML/Pt(111) as a function of CO exposure: clean Pt(111) (a), Fe deposited at 343 (b), 403 (c), and 473 K (d).
performed using thermal radiation emitted from an infrared heating system (GVH198; Thermo Rico Co. Ltd.). For TPD measurements, the sample was oriented to face a 3 mm diameter aperture in a stainless steel tube surrounding the ionization space of the mass spectrometer. All TPD spectra were recorded with a heating rate of 2.5 K/s and were background-subtracted. The binding energies of Pt and Fe for the Pt(111) and Fe0.5ML/Pt(111) deposited at 473 K substrates were measured using an XPS (Theta Probe; Thermo VG Scientific) apparatus attached in another UHV chamber. 3. Results and Discussion 3.1. IRRAS Spectra for Adsorbed CO on Fe/Pt(111) Bimetallic Surfaces. Figure 1 portrays IRRAS spectral changes for CO adsorption on the Fe1.0ML/Pt(111) fabricated at various substrate temperatures as a function of CO exposures (1 L ) 1.0 × 10-6 Torr · s). For comparison, the spectral changes for the clean Pt(111) are also presented in panel a. On the clean Pt(111), the bands appear at 2093 and 1855 cm-1 and increase in intensity with increasing CO exposure. The strong band at 2093 cm-1 with full width at half-maximum (fwhm) of 7 cm-1 dominates the IRRAS spectrum for adsorbed CO at 1.01 L. The band can be assigned to CO on the on-top site of Pt(111), as shown schematically in panel a. The weak absorption at 1855 cm-1 in Figure 1a (indicated by an asterisk) is ascribable to the bridge-bonded CO on the Pt(111).39,40 The 343 K deposited Fe1.0ML/Pt(111) (Figure 1b) brings about a band at 1955 cm-1, concomitant with the near disappearance of the bands attributable to CO-Pt(111). We carefully investigated CO adsorption behaviors on the fcc-Fe surfaces fabricated on the Cu single-crystal surfaces,7,34 in which the band attributable to CO-Fe having an fcc structure gave rise to the IRRAS bands near 1950 cm-1. Consequently, the 1955 cm-1 band for the spectrum of the 1.0 L CO-exposure can be assigned to the bridge CO-Fe bonds, as shown schematically in the bottom of panel b. The result for the 343 K deposited Fe1.0ML/ Pt(111) suggests that the Pt(111) surface is almost covered by the deposited 1 ML-thick Fe. Actually, as depicted in Figure 3, the RHEED pattern for the corresponding surface of Figure 1b showed new streaks, outside the original streaks, that were equivalent to the lattice parameter for the fcc-Fe, suggesting that Fe grows epitaxially on the 343 K Pt(111).37 With increasing substrate temperature, the new absorption feature at around 2060 cm-1 becomes prominent; the band at 2050 cm-1 dominates the spectra for the Fe1.0ML/Pt(111) fabricated at 473 K (Figure
8946 J. Phys. Chem. C, Vol. 112, No. 24, 2008
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Figure 2. IRRAS spectra of saturated CO on the Fe0.25ML, Fe0.5ML, and Fe1.0ML/Pt(111) fabricated at Pt(111) substrate temperatures of 343 (a), 403 (b), and 473 (c). Dashed lines: peak frequency for linear-bonded CO on the clean Pt(111). Inset pictures: side view surface models for the corresponding Fex/Pt(111).42
1d). Therefore, the band at around 2060 cm-1 might have arisen from the specific surface structure generated by the depositions, in particular for the higher substrate temperatures of Pt(111). To date, surface structural investigations have been reported for the Pt80Fe20(111) by Beccat et al.,23 for Fe-deposited Pt(111) by Jerdev and Koel,25 and for Pt50Ni50(111) and Pt78Ni22(111) by Gauthier et al.41 Their results show that annealing of the alloys created topmost surface layers of pure Pt atoms; surface segregation of the substrate Pt atoms occurred. The Pt atoms’ segregation might be related to the emergence of the new IRRAS band at around 2060 cm-1 because the band becomes predominant with increasing substrate temperature of Pt(111). At higher substrate temperatures, incorporation of incoming Fe into the substrate Pt lattice might be activated, thereby forming the outermost surface structure that gives rise to the 2060 cm-1 band. To discuss the origin of the 2060 cm-1 band in greater detail, we conducted IRRAS measurements for the Fex/Pt(111) fabricated at high substrate temperatures, the results of which are summarized in Figure 2. The inset images show side-view models of ideal structures for the corresponding “surfaces”.42 For the spectrum of the 343 K deposited Fe0.25ML/Pt(111) (Figure 2a), the most intense band appears at 2090 cm-1, in addition to the bridge-bonded CO-Fe band at 1955 cm-1. For the 403 K deposited Fe0.25ML/Pt(111) (Figure 2b), a band is positioned at 2057 cm-1, in addition to the two bands described above. The 2065 cm-1 band dominates the spectrum for the 473 K deposited Fe0.25ML/Pt(111) (Figure 2c). At 0.25 ML of Fe, the incoming Fe is expected to be unable to completely cover the substrate Pt(111) (the model in Figure 2a). Indeed, the absorption bands at 2090 and 1855 cm-1, which are ascribable to the CO adsorption on the pure Pt(111), are located on the spectra for the Fe0.25ML/Pt(111), irrespective of the deposition temperatures. Therefore, the prominent absorptions at 2090 and 1955 cm-1 for the 343 K deposited Fe0.25ML/Pt(111) can be assigned to the linear-bonded CO-Pt and bridge-bonded CO-Fe, respectively. The intensity of the bridge-bonded CO-Fe band decreases markedly for the 473 K Fe-deposited Fe0.25ML/Pt(111). A similar trend in deposition temperature for the IRRAS band intensity is apparent for the Fe0.5ML/Pt(111). It is noteworthy that the 0.5 ML-thick Fe deposition on the 473 K Pt(111) generates the most intense band at 2060 cm-1, with a fwhm value of 9 cm-1; the sharp absorption band seems to indicate
Figure 3. IRRAS spectra of saturated CO on: (a) clean Pt(111); (b) 1 ML Fe deposited at 343 K on (a); (c) 0.6 nm-thick Pt; and (d) 0.3 nm-thick Pt deposited at RT on (b); and (e) 0.5 ML Fe deposited at 473 K on (a). Dashed line: peak frequency for linear-bonded CO on the clean Pt(111). Inset pictures: side view models for adsorbed CO on the corresponding Fex/Pt(111) and Pty/Fex/Pt(111).42 Right side: RHEED patterns corrected by electron beam incidence from the λ direction for the substrates.
that the 0.5 ML-thick Fe deposition creates a rather homogeneous outermost surface structure. The 1 ML-thick Fe deposition on the 343 K Pt(111) brings about the bridge-bonded CO-Fe band at 1932 cm-1, accompanied by weak and broad absorption at 2050 cm-1. With increased deposition temperature, the band at 2050 cm-1 comes to dominate the spectra, although the bridge-bonded CO-Fe band remains on the spectrum of the 473 K deposited Fe1.0ML/Pt(111) (Figure 2c). The results presented in Figure 2 suggest that the outermost Pt layer, the Pt “skin” depicted schematically in Figure 2c, is formed through Pt surface segregation by gaining substrate thermal energy during Fe deposition; the adsorbed CO on the Pt skin probably creates the new band near 2060 cm-1. 3.2. Lattice Parameter and C-O Stretch Frequency for Fe/Pt(111) Bimetallic Surfaces. Because of the underlying Fe atoms, the Pt skin formed through the segregation of the substrate Pt atoms might have electronic and chemical properties that differ greatly from those of the clean Pt(111). Such a perturbation is expected to cause strong modification of vibrational properties of adsorbates. Indeed, pioneering work conducted by Rodriguez and Goodman5,6,43 revealed that the C-O stretch vibration is sensitive to the polarizability of the pseudomorphic admetal layers on single-crystal substrates. To precisely verify the influence of the Fe atoms on the C-O peak frequency for the Fex/Pt(111), Pty/Fe1.0ML/Pt(111)38 “sandwich” structures were fabricated through 0.3 nm-thick and 0.6 nmthick Pt depositions on the Fe1.0ML/Pt(111). We carried out RHEED and IRRAS measurements for the surfaces that were produced, the results of which are depicted in Figure 3. The CO-saturated IRRAS spectra are presented for the clean Pt(111) (a), Fe1.0ML/Pt(111) (b), Pt0.6nm/Fe1.0ML/Pt(111) (c), and Pt0.3nm/ Fe1.0ML/Pt(111) (d). The spectrum for the 473 K deposited Fe0.5ML/Pt(111) is also shown in panel e. The RHEED patterns for the corresponding surfaces are presented on the right-hand-
IRRAS Study for CO on Fe/Pt(111) side of Figure 3. The inset model pictures portray side-views for adsorbed CO on the corresponding surfaces.42 Although the RHEED pattern for the 343 K deposited Fe1.0ML/ Pt(111) (b) shows new streaks (indicated by arrows) outside the original streaks for the substrate Pt(111), the separation of the new streaks corresponds to the lattice parameter for fccFe(111).37 In contrast, additional deposition of the 0.3 nm-thick and 0.6 nm-thick Pt on the Fe1.0ML/Pt(111) (panels c and d) cause the disappearance of the new streaks, leaving streaks having slightly wider separations than those for the clean Pt(111) (a). The RHEED patterns show that Pt grows epitaxially on the Fe1.0ML/Pt(111) at RT. Additional aspects of surface structures will be presented in later discussions. In Figure 3, the bands appearing at 2070 and 2080 cm-1 on the Pt0.3nm/Fe1.0ML/Pt(111) and Pt0.6nm/Fe1.0ML/Pt(111) are clearly visible; with increasing topmost Pt thickness, the frequencies of the C-O stretch of adsorbed CO come close to the value for the clean Pt(111) (2093 cm-1). The interatomic distance of Pt is 0.278 nm. Therefore, the 0.3 nm-thick and 0.6 nm-thick depositions of Pt would correspond to ca. 1-2 ML-thick Pt layers on the Fe1.0ML/Pt(111) (shown schematically in the inset models). The underlying 1 ML-thick Fe is expected to modify the overlaid Pt atoms electronically as well as chemically. Watanabe and co-workers16 conducted FT-IR-ATR investigations for the CO adsorption on the Pt-based alloy films formed by Ar-sputtering of Pt and Fe. Their study showed 10-40 cm-1 red-shifts in peak frequencies of the adsorbed CO bands on the PtFe alloy film compared to those on the Pt film. The lower frequencies in the C-O stretch are expected to result from the influence of the underlying Fe epitaxial layer. The Pt-Fe phase diagram shows that alloys form disordered solid solutions with the fcc lattice (γ phase) at sufficiently high temperatures. At temperatures below 1400 K, depending upon atomic compositions, two ordered structures are present: Pt3Fe(Fe3Pt), in which ordering Fe(Pt) atoms might occupy any of four equivalent sites (L12; γ3 phase); and PtFe, in which Pt and Fe atoms occupy alternating closely packed planes (L10; γ2 phase). As presented in Figure 2c, the 473 K deposited Fe0.5ML/Pt(111) shows the most enhanced and sharp (fwhm ) 9 cm-1) absorption band at 2060 cm-1, which is 33 cm-1 lower than that for the clean Pt(111). A structural investigation for the annealed Fe/Pt(111) revealed that the diffusion of the incoming Fe atoms was limited in the near-surface region less than 800 K. The 0.5 ML-thick Fe deposition might correspond to alloy compositions of “Pt75Fe25” if we assume that the incoming Fe atoms into the Pt(111) lattice remain within the topmost two layers of the 473 K deposited Fe0.5ML/Pt(111). Considering the ordered structures of Pt-Fe alloy, the Fe0.5ML/ Pt(111) might have a surface structure resembling that of Pt3Fe (γ3). The surface segregation of Pt for the Pt80Fe20(111) surface has been intensively studied.23,24 The concentration profile of Pt decreases monotonously with depth, in contrast to PtxNi1-x(111) surfaces, for which the concentration oscillates around the bulk value.41 In the Pt80Fe20 ordered alloy, two kinds of Pt atoms are present at the first layer of the 111 plane; the Pt atoms bonded to three Pt atoms in the second layer and the atoms have two Pt atoms and one Fe atom as nearest neighbors in the second layer.23,26 The two kinds of surface Pt atoms probably yield the two IRRAS bands because of CO adsorption. However, the 473 K deposited Fe0.5ML/Pt(111) shows a very sharp single absorption band at 2060 cm-1. The LEED pattern for the Pt80Fe20(111) reveals a 2 × 2 pattern that might be attributable to surface reconstruction on top of a disordered
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8947 domain or to the substrate ordered γ3 phase itself, covered by some less-ordered Pt outermost layers.23 According to ion scattering investigations for the vicinal surface structures of the Fe/Pt(111) by Jerdev and Koel,25 the Fe deposition onto the Pt(111) generates outermost layers composed of pure Pt atoms, and the Fe0.5ML/Pt(111) after annealing to 750-850 K produced a diffuse 2 × 2 LEED pattern. In the present study, however, the contrast of 1 × 1 LEED pattern for the clean Pt(111) remained nearly unchanged by the 0.5 ML-thick Fe deposition on Pt(111) at 473 K (not shown here). In addition, the RHEED pattern for the Fe0.5ML/Pt(111) surface reveals no substreaks, as depicted in Figure 3e. The IRRAS and RHEED results apparently indicate a slight disorder of the second layer Fe atoms compared to that obtained by Jerdev and Koel25 because of the low “annealing” temperature. Regarding the 473 K deposited Fe1.0ML/Pt(111) showing a CO-Pt band at 2050 cm-1 with a fwhm value of 15 cm-1 (Figures 1d and 22c), under the assumption of the limited diffusion of the incoming Fe atoms, the 473 K deposition of 1 ML-thick Fe might be parallel to the alloy composition of “Pt50Fe50” (γ2). The γ2 lattice (fcc) structure comprises alternating close-packed planes of Pt and Fe atoms. The top surface of the Pt550Fe50(111) is expected to include both Pt and Fe atoms if the “Pt50Fe50 alloy” grows epitaxially on the substrate of Pt(111) (fcc). Consequently, the Fe1.0ML/Pt(111) might generate the IRRAS bands because of both the linearly bonded CO-Pt and bridge-bonded CO-Fe. Considering the results described above, the band at 2060 cm-1 shown for the Fe-deposited surfaces might be ascribed to CO adsorbed onto the outermost Pt layer (Pt “skin”) on the incoming Fe atoms formed through the segregation of substrate Pt atoms, as shown schematically in the model of Figure 3e). The arriving Fe atoms eject substrate Pt to form a cluster at the outermost growing surface. The clusters diffuse at the surface through the thermal energy of the sample; they are finally trapped at the step edge of the original surface to generate the outermost Pt layer. Scanning tunneling microscopic (STM) observations for Co growth on Pt(111)44 reveal misfit dislocations in the Fe layer might also contribute to the formation of the Pt skin. Actually, in the Cu-Pd surface alloy, the topmost Cu layer “cap-layer” was formed after annealing the Pd deposited Cu(100).45 Whichever is true, the band at around 2060 cm-1 should probably be assigned to CO adsorption on the Pt outermost layer, although a much smaller amount of Fe atoms at the outermost surface cannot be ruled out. The lattice spacing parallel to the surface atomic rows can be estimated from the RHEED streak separations.46 From the RHEED patterns depicted in Figure 3, the lattice parameters for the Pt0.6nm/Fe1ML/Pt(111), Pt0.6nm/Fe1ML/Pt(111), and Fe0.5ML/ Pt(111) are estimated as 0.391, 0.389, and 0.387 nm, respectively. In contrast, the corresponding value for the clean Pt(111) is 0.399 nm. The lattice parameters versus the Pt thickness are presented at the bottom of Figure 4. The results indicate that the lattice parameters for the Fex/Pt(111) and Pty/Fex/Pt(111) are narrower than that for the clean Pt(111). Incorporation of the incoming Fe atoms into the substrate might reduce the lattice parameters because the atomic radius for Fe (0.126 nm) is smaller than that for Pt (0.139 nm). The top panel of Figure 4 shows the peak frequency dependence of adsorbed CO on the Pt thickness; the CO vibrational property seems to correlate with the lattice parameter. 3.3. Frequency and Intensity of C-O Stretch Vibration and CO-TPD. As described above, the 473 K deposited Fe0.5ML/ Pt(111) generates a rather homogeneous outermost Pt surface
8948 J. Phys. Chem. C, Vol. 112, No. 24, 2008
Figure 4. The C-O stretch frequencies (top) and lattice parameters evaluated from the distance between RHEED streaks (bottom) vs Pt thickness on the Fe1.0ML/Pt(111). Corresponding values for the clean Pt(111) (solid lines) and Fe0.5ML/Pt(111) fabricated at 473 K (dashed lines) are also shown.
Figure 5. Changes in the IR peak frequency and peak intensity with CO exposure to clean Pt(111) (O) and 473 K deposited Fe0.5ML/Pt(111) (9). The TPD spectra of 0.1 L-CO exposed surfaces are shown on the right.
layer and stabilizes the incoming Fe in the second surface layer. Figure 5 presents intensity and frequency changes of the adsorbed CO band for the clean (O) and 473 K deposited Fe0.5ML/Pt(111) (9). The difference in peak frequency decreases from 37 cm-1 at 0.1 L to 33 cm-1 at 1.0 L, which might result from the increasing dipole-dipole coupling interaction between adsorbed CO molecules, as is commonly argued in CO adsorption on metal surfaces.31,33 At 0.1 L CO exposure (dashed
Wadayama et al. line in Figure 5), the peak intensity for the Fe0.5ML/Pt(111) is ca. 60% of that for the clean Pt(111), although the peak intensity for the Fe0.5ML/Pt(111) is close to that for the clean Pt(111) at ca. 1.0 L. The results indicate that the alloying of Pt with Fe strongly influences the C-O stretch property, particularly at low coverage of CO. The IRRAS band intensity includes factors not only for the amounts of adsorbates but also for the orientation of the adsorbates.33 Furthermore, in a bimetallic surface system comprising an admetal layer on a different metal substrate, the charge distribution in the admetal layer metal-CO units can render CO molecules bonded to the metal substrate invisible in the IR spectrum through the so-called “screening effect”.6,43 Therefore, the IRRAS intensity of adsorbed CO is not an appropriate index for the CO coverage. In contrast, the COTPD signal intensity is linked directly to the coverage. Therefore, the TPD spectra of adsorbed CO on the clean Pt (111) and 473 K deposited Fe0.5ML/Pt(111) are recorded. Those results are portrayed on the right side of Figure 5. The CO exposures for the surfaces are fixed at 0.1 L because the IRRAS intensity for the Fe0.5ML/Pt(111) is ca. 60% weaker than that for the clean Pt(111). As the figure shows, the integrated TPD signal for the Fe/Pt surface is about 60% of that for Pt(111), which is in agreement with the IRRAS CO band intensity ratio of the surfaces. The results suggest a lower sticking probability for the Fe0.5ML/Pt(111). Furthermore, the desorption peak for Pt(111) is located at 442 K. Hayden and Bradshaw reported a similar desorption temperature for the clean Pt(111).39 In contrast, the Fe/Pt(111) surface yields the desorption peak at 430 K. Under approximation of Redhead’s method,47 the activation energies for CO desorption from both surfaces are estimated as 118 kJ/mol (Pt(111)) and 115 kJ/mol (Fe0.5ML/ Pt(111)), suggesting that the CO-metal bond strength is weaker for the Fe0.5ML/Pt(111). A similar trend in activation energy for CO desorption has been reported by Atli et al. for the clean Pt(111) and Pt80Fe20(111).26 3.4. C-O Stretch Vibration on the Pt Outermost Layer. Rodriguez and Goodman demonstrated that a strong correlation between changes in CO desorption temperature, that is, CO-metal bond strength and chemical shifts in the surface core-level binding energies for supported metal monolayers.5,6,43 The results are rationalized theoretically using the model describing the interaction between the metal d states and the CO 2π* and 5 σ states, renormalized by the metal sp continuum.48 Jerdev and Koel25 and Watanabe and co-workers16 reported positive shifts in binding energy of Pt 4f band by alloying of Pt with Fe. Actually, the Pt 4f band for the 473 K deposited Fe0.5ML/ Pt(111) shifts to the higher binding energy side, indicating alloying of Fe and Pt.49 The positive binding energy shift by the 473 K deposited Fe0.5ML/Pt(111) indicates that the surface electronic density is reduced by the alloying of Pt with incoming Fe atoms, thereby decreasing the π back-donation from the alloy to the adsorbed CO molecules. In the Blyholder model,50 decreasing the π back-donation engenders a blue shift in the peak frequency of the C-O stretch band. However, we saw no blue shift, but rather a red shift in the peak frequency. Therefore, the peak frequency red shift caused by alloying cannot be explained using the simple back-donation scheme. The IRRAS investigation on the pseudomorphic Cu monolayer on the Pt(111) surface43 showed that the strength of the Cu-CO bond, the amount of π back-donation, and the C-O stretch frequency increase (or decrease) simultaneously. The authors conclude that an increase in the π back-donation cannot reproduce a reduction in the C-O stretch frequency when the behavior of CO on
IRRAS Study for CO on Fe/Pt(111) different metal surfaces is compared. In contrast to the simple model proposed by Blyholder,50 the explanation for the red shift in C-O stretch frequency is complicated. Theoretical studies51,52 of the vibration frequency shift on metals and oxides show that the frequency shifts are induced from the combination phenomena of π back-donation, the interaction between the CO dipole moment, and the positive charge on the metal center, in addition to the repulsion produced when the CO stretches in the presence of the rigid surface to which it is bonded. As portrayed in Figure 2c, the 473 K deposited Fe0.5ML/ Pt(111) yields remarkable single absorption at 2060 cm-1, which might result from linear-bonded CO on the Pt skin, as opposed to the clean Pt(111) at which both linearly bonded and bridgebonded CO-Pt bands were located in the spectra (Figure 1a). Disappearance of the bridge-bonded CO-Pt band for the 473 K deposited Fe0.5ML/Pt(111) surface might also be related to modification in the electronic structure of the outermost surface of the Fe0.5ML/Pt(111). Indeed, Watanabe and co-workers reported CO adsorption on the Pt alloy electrode surfaces in electrochemical systems:16 their FTIR-ATR investigations revealed that alloying of Pt causes less intense bridge adsorption of CO on the surfaces. They inferred that weaker back-donation of d electrons at the alloy electrode engenders the less intense bridge-bonded CO band. Furthermore, the HR-EELS investigation made by Atli et al.26 showed that the on-top CO is the prevalent adsorbed species for the Pt80Fe20(111) surface. Figure 3 shows that the Pt overlayers grown epitaxially on the Fe1.0ML/Pt(111) yield single absorption bands at 2070 cm-1 (0.3 nm Pt) and 2080 cm-1 (0.6 nm Pt). In contrast, the 473 K deposited Fe0.5ML/Pt(111) (Figures 2 and 3) generate the C-O stretch bands positioned at lower peak frequencies than those for the Pt epitaxial overlayers, with the single absorption band positions at 2060 cm-1 for the Fe0.5ML/Pt(111) and at the 2050 cm-1 for the Fe1.0ML/Pt(111). The greater red shift of the adsorbed CO bands for the 473 K deposited Fex/Pt(111) is expected to correspond to the enhanced alloying of the incoming Fe with the substrate Pt atoms compared to those for the Pt epitaxial overlayers grown at RT. 4. Summary For adsorbed CO on the Fe0-1.0ML/Pt(111) bimetallic surfaces, IRRAS and TPD measurements were conducted. The 0.5 MLthick Fe deposition onto clean Pt(111) generates an outermost Pt layer (Pt skin) with electronic properties and an atomic structure that are distinct from those of Pt(111). The linearly bonded C-O stretch on the outermost Pt layer produces an IRRAS band that is ca. 30-40 cm-1 lower than that on the Pt(111). The IRRAS band intensity of adsorbed CO and the TPD signal of adsorbed CO on the Fe0.5ML/Pt(111) surface were 40% weaker than those for the Pt(111). All results obtained in this study strongly suggest that the sticking probability of CO on the Fex/Pt111) surface is lower than that for the clean Pt(111), particularly at lower CO coverage. The CO tolerance on the PtFe alloy is expected to correlate with the outermost Pt layer on the underlying Fe atoms. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research (B), Japan Society for the Promotion of Science. One author (T.W.) expresses his sincere appreciation to the Iketani Science and Technology Foundation for a financial support of this work. The author would also like to thank the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
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