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Low-Energy Electron Diffraction, X-ray Photoelectron Spectroscopy, and CO-Temperature-Programmed Desorption Characterization of Bimetallic Ruthenium-Platinum Surfaces Prepared by Chemical Vapor Deposition Abbas Lamouri,† Yosi Gofer,† Yu Luo,† Gary S. Chottiner,‡ and Daniel A. Scherson*,† Departments of Chemistry and Physics, Case Western ReserVe UniVersity, CleVeland, Ohio 44016 ReceiVed: NoVember 17, 2000; In Final Form: February 24, 2001
Low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and CO-temperatureprogrammed desorption (TPD) were used to characterize ruthenium-modified Pt(100) surfaces of very high purity and controlled stoichiometry prepared in ultrahigh vacuum (UHV) by irradiating Ru3(CO)12 films condensed on cold Pt substrates at 150 K with X-rays, and subsequent annealing at ca. 620 K. The presence of Ru on Pt(100) lifted the (5 × 20) reconstruction characteristic of the bare clean substrate; however, the reconstruction reappeared as the bimetallic surfaces were briefly annealed to ca. 900 K. Exposure of nonannealed Ru(θRu g 0.22)/Pt(100), where θRu represents the Ru coverage in monolayers, to large exposures of CO at ca. 200 K yielded smaller θCO, as well as TPD peaks with onset desorption temperatures, Tdes(CO), ca. 50 K lower than those observed for bare Pt(100). More strikingly, however, the CO-TPD spectra of CO-saturated Ru(θRu ) 0.42)/Pt(100), which had been briefly annealed to 900 K, displayed Tdes(CO) as low as 250 K, very similar to desorption temperatures reported for Pt-modified Ru(0001) by de Mongeot et al. (Surf. Sci. 1998, 411, 249).
Introduction Bimetallic Ru/Pt alloys, including those containing other metals such as Os and Ir, have emerged as the most active electrode materials for methanol oxidation in aqueous acidic electrolytes.1-7 Significant progress has been made over the past few years toward elucidating the nature of the active sites responsible for these electrocatalytic effects, owing primarily to the development of methods for the preparation and characterization of Ru/Pt surfaces of well-defined stoichiometry and roughness. Indeed, a number of procedures for producing such bimetallic surfaces have been reported, using bulk alloys2-3,5,7-10 or adsorption of Ru atoms onto bulk Pt2,11-22 by physical,2,11-12 chemical,2,13-15 or electrochemical means.16-22 As pointed out by Jarvi et al.,12 surfaces involving adsorbed Ru on Pt substrates prepared in ultrahigh vacuum (UHV) possess special advantages in that Ru coverages can be determined using conventional surface analytical techniques, such as Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Furthermore, specimens can be transferred to a high-pressure environment under highly controlled conditions, allowing direct correlations to be established between surface properties and electrochemical activity. This strategy was recently implemented by Jarvi et al.12 for the study of hydrogen adsorption and methanol oxidation in acid electrolytes on Pt(111) electrodes modified by electron beam deposited Ru in UHV. This work presents low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy, and temperature-programmed desorption (TPD) of adsorbed CO data for Ru-modified Pt(100) surfaces in UHV prepared by a versatile method involving X-ray irradiation of Ru3(CO)12 layers condensed on cold Pt substrates † ‡
Department of Chemistry. Department of Physics.
followed by thermal annealing. Particularly noteworthy were the results of CO-TPD measurements for saturation CO coverages on Ru/Pt(100) surfaces that had been annealed to ca. 900 K for a brief period of time. As will be shown, the onset of CO desorption for Ru(θRu ) 0.42)/Pt(100) was 250 K lower than that observed for bare Pt(100) under otherwise the same conditions, and similar to that reported by de Mongeot et al.23 for CO on Pt-modified Ru(0001). Experimental Section A. Instrumentation. All measurements were performed in a two-level UHV chamber described in an earlier publication.24 This system is equipped with rear-view LEED optics (PRI), a hemispherical electron energy analyzer (PHI, model 10-360), a dual (Mg/Al) anode X-ray source for XPS (PHI model 04-548), a 15 µm spot size 10 keV electron gun for AES, a sputter gun (PHI model 04-192), and a Dycor M200 M mass spectrometer for residual gas analysis and TPD. Samples were mounted on an XYZ manipulator with heating (>1500 K) and cooling capabilities (90 K). One of the ports was connected via a gate valve to an auxiliary chamber used for sample insertion and for electrochemical experiments. B. Preparation of Clean Ru/Pt Bimetallic Surfaces in UHV. The procedure implemented in this laboratory for producing Ru-modified Pt surfaces involves the use of Ru3(CO)12 as the metal carrier and, therefore, shares some similarities with the technique described by Sham et al.25,26 for depositing Ru layers on Cu(111) surfaces. This organometallic compound is an air-stable solid27 that sublimes at very slow rates in UHV at room temperature. As such, it provides ideal conditions for a highly controlled deposition, without the complexities associated with the synthesis and handling of Ru(CO)5, yet another precursor for Ru film deposition on solid substrates in UHV.28 For these experiments, a few Ru3(CO)12 crystals were placed
10.1021/jp004234g CCC: $20.00 © 2001 American Chemical Society Published on Web 06/09/2001
Ru-Pt Surfaces Prepared by CVD in a copper boat covered by a collimator with a 1 mm opening, attached, in turn, to the tip of an externally actuated magnetically coupled manipulator. When not in use, this entire assembly was stored in an auxiliary chamber under an inert gas atmosphere, connected via a gate valve to the main chamber. Ru3(CO)12 displayed no affinity for Pt at ca. 300 K; hence, all depositions were carried out with the substrates cooled to 150 K. After evacuating the auxiliary chamber to ca. 10-7 Torr, the Ru source assembly was transferred into the main chamber, placed in close proximity to the cold clean Pt surface (see below) for a specified period of time to condense a Ru3(CO)12 layer of the desired thickness, and later retracted and isolated into the auxiliary chamber, which was then filled with inert gas. Ruthenium-modified Pt surfaces were prepared by condensing Ru3(CO)12 onto clean Pt substrates at 150 K followed by exposure to X-rays (while allowing the specimen to warm to room temperature) and further brief annealing to 600 K. As will be discussed in detail in the following section, in the case of Pt(poly), it was found to be necessary to perform the last step in an oxygen atmosphere in order to produce bimetallic surfaces devoid of carbon and oxygen. Specimens produced in this fashion before characterization by TPD will be referred to as freshly prepared. Prior to Ru3(CO)12 condensation, Pt specimens, either a polycrystalline (poly) foil or a Pt(100) single crystal, were cleaned by Ar+-sputtering, followed by thermal annealing (1200 K) in an oxygen atmosphere (ca. 10-7 Torr). This procedure yielded surfaces displaying XPS features characteristic of clean Pt, with no evidence for carbon, oxygen, or Ru derived from previous experiments (see below). In the case of clean Pt(100), the LEED patterns were found to be consistent with the (5 × 20) reconstruction, as reported by other groups.29 Unless otherwise indicated, Ru was sputtered off Pt(poly) and Pt(100) surfaces following each set of CO-TPD scans to avoid alloy formation during the subsequent higher temperature annealing (see above), thus preserving the chemical integrity of the specimens. Results and Discussion A. X-ray Photoelectron Spectroscopy (XPS). Exposure of a clean Pt(poly) foil at 150 K to Ru3(CO)12 vapors for 30 min yielded XPS spectra recorded at that temperature, showing peaks attributed to Ru in a slightly oxidized state at ca. 282 (3d5/2) and 286 (3d3/2) eV, respectively (see curve a, upper panel, Figure 1) and features characteristic of metallic Pt (not shown here). Also evident in the Ru(3d) XPS spectra were large amounts of carbon at 285.5 and 287.5 eV (see insert, upper panel, Figure 1), and oxygen at ca. 534 eV (see curve a, lower panel, Figure 1). On the basis of the integrated peak intensities and atomic sensitivity factors (see Table 1), the relative concentrations of Ru:C:O were 1:2.7:3.2, and thus much closer to the stoichiometric values, i.e., 1:4:4, compared to those reported by Sham et al.26 Except for very minor C and O impurities, the XPS of condensed Ru3(CO)12 films on Pt(poly) heated to room temperature (not shown in this work) revealed no evidence for the presence of Ru, indicating that the metal carbonyl compound sublimes, presumably intact, from the Pt surface. This behavior is unlike that observed by Sham et al.26 for Ru3(CO)12 condensed on polycrystalline Cu, who found about one monolayer of Rucontaining species once the specimen was warmed to room temperature. A very different behavior was found for Ru3(CO)12/Pt surfaces condensed at 150 K for 30 min, and then irradiated with X-rays from the XPS source while the sample was slowly heated to
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Figure 1. X-ray photoelectron spectra in the Ru(3d) (upper panel) and O(1s) (lower panel) binding energy regions for a cold Pt(poly) foil (150 K) exposed to Ru3(CO)12 vapors for 30 min (curves a), and warmed to 330 K while being irradiated with X-rays (curves b). Curves c show the corresponding XPS spectra obtained after annealing the X-ray-irradiated specimen in an oxygen atmosphere (ca. 10-7 Torr) at 600 K. Contributions due to C(1s) and Ru(3d3/2) in curve a (upper panel) are shown in solid and dotted lines in the inset in that panel, respectively.
higher temperatures. In particular, Ru3(CO)12/Pt(poly) surfaces allowed to warm to ca. 330 K following such treatment, displayed clearly defined XPS peaks ascribed to Ru(3d) and O(1s) (see curves b in the upper and lower panels, Figure 1), signaling decomposition of the Ru3(CO)12 to yield species with significantly lower vapor pressure. As shown in curve b, upper panel, Figure 1 (see also Table 1), the Ru(3d) peaks for such X-ray-exposed Ru3(CO)12/Pt(poly) surfaces, were found to shift by ca. 1 eV toward lower binding energies, and the ratio of the Ru(3d5/2) to Ru(3d3/2) integrated areas was 0.67, in agreement with that expected for pure Ru (0.67)30 with small contributions due to carbon at 286.7 eV. See Table 1. Both C and O could be eliminated, however, by annealing these X-ray-irradiated surfaces at ca. 620 K in an O2 atmosphere (ca. 10-7 Torr) for 2-3 min, yielding no discernible peak in the region about 533 eV (see curve c, lower panel). Also noteworthy is the shift in the binding energies of the Ru(3d) features to values characteristic of metallic Ru, (Ru(3d3/2) ) 284 eV; Ru(3d5/2) ) 280 eV) (see Table 1). Of particular significance is the fact that in the case of bimetallic Ru/Pt(100) surfaces prepared by the same condensation/X-ray irradiation method, no oxygen was found to be necessary during annealing at ca. 620 K, to obtain a clean, i.e., carbon and oxygen free, Ru overlayer, as evidenced by the Ru(3d) XPS spectra shown in Figure 2 (see Table 1). The Ru source was calibrated using clean Pt(poly) substrates, yielding a linear relationship between the normalized Pt(4f) XPS signal, i.e., APt(4f)/[APt(4f) + ARu(3d)], where Ai is the area under each of the XPS feature i, expressed in terms of percent (see left ordinate, Figure 3) and the time of Ru deposition, tdep. These results are given in Figure 3, where the upper abscissa is
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TABLE 1: XPS Analysis of Vapor Deposited Layers of Ru3(CO)12 on Pt(poly) and Pt(100)a specimen
A
Pt (polycrystalline) B
C
Pt (100) D
Ru (3d5/2)b binding energy (eV) 282 281 280 280 1.55 atomic concn (%)c 0.14 0.20 1 1 fwhh (eV)d 1.42 1.68 1.56 1.19 C(1s) binding energy (eV) 287.5 285.5 286.7 284 0.205 atomic concn (%) 0.34 0.06 0.33 0.15 fwhh (eV) 1.33 0.78 1.77 1.31 O (1s) binding energy (eV) 534 533 0.63 atomic concn (%) 0.46 0.32 fwhh (eV) 2.07 2.33 a A: layer of Ru (CO) 3 12 obtained after 30 min deposition with the substrate kept at 150 K (see curve a, Figure 1). B: after warming specimen in A to 330 K, while irradiating with X-rays (see curve b, Figure 1). C: after annealing specimen in B to 600 K in a 10-7 Torr O2 atmosphere for a few minutes (curve c, Figure 1). D: layer of Ru3(CO)12 obtained after 10 min deposition at 150 K and subsequent annealing to 600 K for a few minutes in UHV (see Figure 2). b The relative areas under the Ru(3d3/2) and Ru(3d5/2) was found to be about 0.66 ( 1 for all spectra reported in this table. c Atomic concentration are defined as (Ai/Si)/Σj(Aj/Sj) where Ai is the integrated area under peak i, and Si the corresponding sensitivity factor for that specific orbital of the element. Values for Si obtained from ref 30 are shown under each of the peaks in column 1 of this table. d This parameter refers to the full width at half-height of the peak in question.
Figure 2. Ru(3d) XPS spectra for a Pt(100) surface exposed to Ru3(CO)12 vapors for 10 min at 150 K and then irradiated with X-rays recorded after the specimen had been allowed to warm to room temperature and heated to 600 K for a few seconds. Solid lines represent best asymmetric fits using both Gaussian and Lorentzian functions. The Ru coverage was estimated to be 0.91 ML.
expressed in terms of the number of Ru monolayers (ML) determined using the homogeneous attenuation model,31 assuming a thickness per Ru ML of 2.68 Å. On this basis, the rate of Ru deposition corresponds to ca. 0.05 ML/min, i.e., 23 min per Ru ML. X-ray-irradiated, oxygen-annealed Ru3(CO)12/Pt(poly) surfaces were found to yield constant integrated Ru/Pt XPS peak ratios from ca. 350 K up to about 900 K (see Figure 4). At higher temperatures, the Ru signal gradually decreased due most likely to migration of Ru into the Pt lattice, yielding for very high temperatures, i.e., >1200 K, a clean Pt surface. On the basis of the low-energy ion scattering (LEIS) data reported by
Figure 3. Plots of integrated area under the Pt 4f7/2 as a function of time of exposure of Pt(poly) foils to Ru3(CO)12 at 150 K. The Ru3(CO)12/Pt(poly) surfaces were irradiated with X-rays during XPS spectral acquisition, and then annealed in oxygen (10-7 Torr) at 600 K. Top abscissa are values for the Ru coverage obtained from the homogeneous attenuation model.
Figure 4. Relative Ru(3d)/Pt(4f) XPS signals of X-ray-irradiated Ru3(CO)12 deposited on Pt(poly) surfaces at 150 K and subsequently annealed in oxygen (ca. 10-7 Torr) at 600 K to remove carbon from the surface as a function of temperature.
Gasteiger et al.,3 the time required for bimetallic Ru/Pt surfaces to achieve full equilibration at 1100 K is about 10-15 min. In view of the low temperatures (650-700 K) and relatively short times (ca. 5 min) involved in the procedure described in the present work, the Ru deposits in our case may be assumed to be on the surface, rather than within the first few atomic Pt layers. B. Low-Energy Electron Diffraction (LEED) . Panel A of Figure 5 shows a LEED pattern for the (5 × 20) reconstruction of clean Pt(100) prior to Ru modification. Dosing Pt(100) with submonolayer amounts of metallic Ru, using the methods described in this work, however, lifted the reconstruction, yielding a (1 × 1) pattern (see panel B, Figure 5, where θRu ) 0.29). Brief thermal annealing of such bimetallic surfaces at
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Figure 5. Low-energy electron diffraction patterns of clean Pt(100) (A) before (5 × 20) reconstruction and (B) after deposition of a Ru layer (Ru3(CO)12 exposure for 2.5 min) using the method described in this work (see text for details) recorded at room temperature. The superimposed (5 × 20) and (1 × 1) LEED patterns in C were obtained after annealing the surface in (B) to ca. 900 K. The conditions employed for recording LEED patterns were as follows. Panel A: 89 eV, 5 kV, 0.3 mA. Panel B: 132 eV, 5 kV, 0.3 mA. Panel C: 135 eV, 5 kV, 0.3 mA.
Figure 6. Temperature-programmed desorption (TPD) m/e ) 28 curves for Pt(100) exposed to specified doses of gas-phase CO in langmuirs. Heating rate: 5 K/s.
900 K restored partially the (5 × 20) pattern (see panel C), suggesting an agglomeration of Ru to form discrete islands on an otherwise significantly reconstructed Pt(100) surface. C. Temperature-Programmed Desorption (TPD) of Adsorbed CO. The TPD of CO adsorbed on Pt(100) as a function of CO exposure using a heating rate of 5 K/s, yielded results in agreement with those reported by other workers (see Figure 6).32 In particular, at CO saturation coverages, obtained for CO exposures larger than ca. 30 langmuirs, the CO(m/e ) 28)-TPD spectra exhibited three peaks in the range 400-600 K. The reproducibility of CO-TPD experiments involving saturation exposure CO, i.e., >30 langmuirs, was estimated from the results of a series of consecutive runs involving in each case different exposures, e.g., 30, 40, 60, and 100 langmuirs. The actual magnitude of the error was calculated from the standard deviation of the integral under the CO TPD curve Ii, i.e., σ )
xΣi(Ii-Ih)2/n-1
normalized by the absolute CO coverage following saturation exposure on Pt(100) reported in the literature, i.e., 0.77, yielding a value of 0.025. Figure 7 compares TPD runs obtained for two CO exposures,
Figure 7. CO (m/e ) 28) TPD curves for 0.1 langmuir CO (panel A) and 30 langmuirs CO (panel B) exposure for Pt(100) at 200 K for freshly prepared Ru/Pt surfaces, i.e., Ru3(CO)12 deposition at 150 K, followed by X-ray irradiation until the temperature reached 330 K and then annealed to 620 K for a few minutes, at the specified Ru coverages. Curves obtained for bare Pt(100) are also shown for comparison. Heating rate: 5 K/s.
i.e., 0.1 langmuir (panel A) and 30 langmuirs (panel B), for freshly prepared Ru/Pt(100) for θRu ) 0.22, 0.42, and 0.62 ML, with the corresponding curves for bare Pt(100). A number of interesting observations can be made on the basis of these results. Specifically, for small θCO (panel A), the highest temperature peak observed on bare Pt(100) virtually disappears for 0.26 e θRu e 0.92. In fact, the CO-TPD curves for these bimetallic surfaces are virtually identical to those obtained on bare Ru(0001) under very similar conditions,33 i.e., a single desorption peak centered at ca. 485 K. For saturation CO coverages (panel B), the onset temperatures of CO desorption, Tdes, are ca. 50 K lower than those observed on bare Pt(100) and, although largely distorted, the overall shapes of the COTPD traces are also somewhat reminiscent of those observed on bare Ru(0001). Furthermore, the saturation values of θCO for θRu ) 0.22 and 0.42 estimated from the areas under the CO-TPD curves were found to be about half of those observed for bare Pt(100) (see Figure 7). The most remarkable effects of adsorbed Ru on Pt(100) on the CO bonding energetics were found for measurements in which a series of CO-TPD data were
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Figure 8. CO (m/e ) 28) TPD curves for 30 langmuirs CO exposure of Ru(0.42 ML) (panel A) and Ru(0.22 ML) (panel B) on Pt(100) at 200 K. Curves were obtained in sequence starting with an initially freshly prepared Ru/Pt surface (curve a). Heating rats: 5 K/s.
recorded by cooling and reexposing the same Ru/Pt(100) surface to 30 langmuirs of CO, following each TPD scan. A series of such curves recorded in sequence for θRu ) 0.42 (a-e) and 0.22 (a-d) are shown in panels A and B, Figure 8, respectively, where curves a represent the TPD curves obtained for the freshly prepared Ru/Pt(100) at the prescribed Ru coverage. As shown in panel A, Ru(θRu ) 0.42 ML)/Pt(100) surfaces heated to 900 K following the first TPD scan showed a new CO desorption peak with an onset desorption temperature Tdes < 300 K, i.e., ca. 100 K lower than that found for bare Pt(100) under the same conditions. Very similar downshifts in desorption temperatures have been reported for Pt-modified Ru(0001) surfaces by de Mongeot et al.,23 who combined CO-TPD with STM in UHV to monitor the nature the Pt deposits. However, such a marked decrease in the onset CO desorption temperature, was not found for Ru(θRu ) 0.22 ML)/Pt(100) surfaces using a similar protocol (Panel B, Figure 8). Instead, these surfaces yielded after the first scan, a behavior similar to bare Pt(100) (see curve b, in that panel). On this basis, it seems likely that the unique effects induced by brief annealing of Ru(θRu ) 0.42 ML)/Pt(100) to 900 K are due to atomic rearrangements on the surface, which may include formation of Ru islands on the reconstructed Pt(100). Although somewhat speculative, these data suggest that a higher θRu, e.g., 0.42, favors island formation and thus depresses the rates of Ru migration into the lattice. However, regardless of the values of θRu, the CO-TPD obtained after several scans was very similar to that found for Pt(100). This observation is consistent with the reappearance of LEED spots attributed to the (5 × 20) reconstruction, shown for Pt(θRu ) 0.54)/Pt(100) surfaces obtained after a similar series of COTPD experiments had been completed (see panel B, Figure 9), not found for the freshly prepared bimetallic surface (panel A in this figure). Also considered was the possibility of subsurface Ru as being responsible for the appearance of the much weaker CO binding energy peak; however, it would be very difficult to explain why a surface with half the amount of Ru would not display the same effect after identical heat treatment (see panel B, Figure 8). Further studies of the type reported by de Mongeot et al. will certainly be required to elucidate in more detail the structural
Figure 9. Low-energy electron diffraction patterns for Pt(100) following fresh Ru deposition (panel A) and after several CO TPD scans up to 900 K (panel B). The Ru coverage was 0.54 ML.
and electronic factors underlying the strong destabilization of CO bonding on these bimetallic surfaces. Summary Ru-modified Pt(100) single-crystal surfaces prepared via X-ray irradiation of condensed Ru3(CO)12 layers at cryogenic temperatures and subsequent annealing at ca. 620 K have been examined by XPS, LEED, and CO-TPD. Inspection of COTPD data revealed a new weakly adsorbed state of CO for COsaturated Ru(θRu ) 0.42 ML)/Pt(100) upon brief annealing to 900 K with a peak onset temperature ca. 100 K lower than bare Pt(100). This effect is not unlike that observed by de Mongeot et al. for Pt-modified Ru(100) surfaces, who used in addition to TPD, STM in UHV. In analogy with the proposals made by that group, annealing would promote in our case formation of Ru islands, which would be responsible for the formation of the new adsorption sites with much lower CO binding energies. Acknowledgment. This work was supported by DARPA.
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