Electrocatalytic Activity of Ru-Modified Pt(111) Electrodes toward CO

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J. Phys. Chem. B 1999, 103, 6968-6977

Electrocatalytic Activity of Ru-Modified Pt(111) Electrodes toward CO Oxidation W. F. Lin,* M. S. Zei, M. Eiswirth, and G. Ertl Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

T. Iwasita† and W. Vielstich† Institut fu¨ r Physik, UniVersita¨ t der Bundeswehr Mu¨ nchen, 85579 Neubiberg, Germany ReceiVed: March 30, 1999

The electrochemical deposition of Ru on Pt(111) electrodes has been investigated by electron diffraction, Auger spectroscopy, and cyclic voltammetry in a closed UHV transfer system. At small coverages Ru formed a monatomic commensurate layer, at higher coverage mostly small islands with a bilayer height were detected. When the Pt was almost completely covered by Ru, three-dimensional clusters developed. The island structure of Ru changed upon electrooxidation of CO, reflecting an enhanced mobility of Ru. Adsorption and electrooxidation of CO have been studied on such Ru-modified Pt(111) electrodes using cyclic voltammetry and in situ FTIR spectroscopy. Compared to the pure metals, the Ru-CO bond is weakened, the Pt-CO bond strengthened on the modified electrodes. The catalytic activity of the Ru/Pt(111) electrode toward CO adlayer oxidation is higher than that of pure Ru and a PtRu alloy (50:50). It is concluded that the electrooxidation of CO takes place preferentially at the Ru islands, while CO adsorbed on Pt migrates to them.

1. Introduction It was already known in the late 1960s1,2 that, for the electrooxidation of CO, PtRu alloys are better catalysts than pure platinum.3 The necessity of finding CO tolerant materials for hydrogen electrodes and PEM fuel cells has motivated renewed interest in these systems. The investigation of electrodeposits and alloy electrodes4-9 has been extended recently by the use of UHV-prepared alloy probes having surface compositions characterized by LEISS and AES.10-13 Most recent papers claim that a PtRu alloy (50:50) is the material with the highest catalytic activity for CO electrooxidation.7,8,11,12,14,15 However, this conclusion is valid only for adsorbed CO layers. Watanabe and Motoo4 studied the electrooxidation of CO in saturated solutions and reported stationary Tafel plots showing the lowest overpotential for pure Ru, which conclusion was also confirmed by our most recent work.16 On the other hand, several studies17-21 have used Pt single-crystal electrodes modified by Ru deposition as model electrocatalysts for methanol and CO oxidation. In situ scanning tunneling microscopy (STM) studies on Ru electrodeposits on Pt(111) have shown that a Ru overlayer consists of Ru islands with 2-5 nm diameters and monatomic heights.22,23 In situ X-ray surface diffraction (XRSD) on Ru deposits on Pt(100) has shown a commensurate (1 × 1) Ru adlayer on the substrate surface.24 For determining the Ru coverage on Pt electrodes, Watanabe and Motoo used a phenomenological approach that consists of integrating the current in the cyclic voltammogramm attributed to the formation of RuOH and relating this charge to the hydrogen desorption charge of the unmodified Pt.5 Chrzanowski and Wieckowski have used voltammetric data from the electrodeposition of Ru on Pt single-crystal electrodes for the determination of Ru * Corresponding author: fax, +49-30-84135106; e-mail, lin@ fhi-berlin.mpg.de. † Present address: Instituto de Quı´mica de Sa ˜ o Carlos (USP)-Caixa Postal 780, CEP, 13560-970, Sa˜o Carlos, Brazil.

coverage.20,25 Ex situ Auger electron spectroscopy (AES)20 and X-ray photoelectron spectroscopy (XPS)22,23 have also been applied to derive the amount of Ru on Pt(111) surfaces. In order to understand the mechanism of CO and methanol electrooxidation on PtRu catalysts, more work on the surface structure (including electronic structure) and the relationship between surface structure and reactivity of the model Ru/Pt single-crystal electrocatalysts are necessary. In this paper, we report first upon the surface structural characterization and the coverage determination of Ru-modified Pt(111) electrodes by ex situ low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). Then, we study the catalytic activity of the well-characterized Ru-modified Pt(111) electrodes toward CO electrooxidation by cyclic voltammetry and in situ FTIR spectroscopy. Finally, we discuss the results and present our current understanding of the structure-reactivity relationships of the model Ru/Pt(111) electrocatalysts and the related catalytic enhancement mechanism. 2. Experimental Millipore water (>18 MΩ), suprapure grade perchloric acid (Merck), and analytical grade ruthenium(III) chloride (Aldrich) or ruthenium(III) chloridhydrate (RuCl3‚xH2O) with 35.7% Ru (Degussa) were used for preparation of the solutions. Carbon monoxide 4.7 N (Messer Griesheim) was used. Nitrogen 5.0 N or Argon 5.0 N (Messer Griesheim) was used to deaerate the solution and to keep an air-free atmosphere over the solution during the measurements. All experiments were performed at room temperature and all potentials are given vs the reversible hydrogen electrode (RHE). 2.1. UHV (LEED/RHEED, AES) - Electrochemical Experiments. The experimental setup comprises a UHV system which consists of a main chamber (base pressure < 2 × 10-10 mbar) incorporating LEED, RHEED, and AES, an electro-

10.1021/jp9910901 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/30/1999

Ru-Modified Pt(111) Electrodes chemical chamber (base pressure < 1.5 × 10-9 mbar), an electrochemical cell, and a closed sample transfer, and has been described previously.26-28 To facilitate experiments on Ru deposition and CO adsorption and electrooxidation in the present work, we have modified the electrochemical cell with two cells and used a flow-cell procedure which allowed to change electrolyte solutions under potential control and at an air-free atmosphere. RHEED was performed with an incident electron beam energy of 40 keV and at a grazing angle of 1° ∼ 2° to the surface. The RHEED electron beam also acts as the primary electron source for AES. This combination allows to measure RHEED and AES from the same surface region, thus correlating structure and coverage. A 150° hemispherical analyzer was used, and a high voltage of 2.6 kV was applied to the channeltron connected to a lock-in amplifier with a modulation voltage of 8 V (peak-to-peak voltage). The working electrode, a Pt(111) single-crystal disk of 8 mm diameter and 2 mm thickness, was mounted between tungsten wires which also served for resistive heating of the sample. The electrode surface was prepared by cycles of argon ion sputtering (5 × 10-5 mbar, room temperature and 600 °C) and oxygen treatment (3 × 10-7 mbar, 700 °C), until AES and LEED/ RHEED indicated the formation of a clean and ordered surface. This treatment was repeated after each electrochemical experiment. The electrode with a clean and ordered surface was transferred to the electrochemical chamber under UHV. The electrochemical chamber was brought to atmospheric pressure with 5.0 N argon gas. The electrochemical cell, filled with electrolyte solution in 5.0 N nitrogen atmosphere in the lower position, was placed into the electrochemical chamber through a gate valve. The electrode surface was then contacted to the solution via a meniscus configuration. Nitrogen overpressure efficiently kept air out of the transfer system at any phase of the experiments. After the electrochemical experiments, the electrode was tilted to a near vertical position to facilitate solution draining to the edge of the surface, and the drop was removed via a thin Teflon tubing attached to a syringe. Finally, the electrochemical cell (together with the tubing) was withdrawn from the electrochemical chamber to its original lower position. After closing the gate valve, the electrochemical chamber was evacuated to 10-7 mbar, first with a sorption pump for 2 min, and then with the turbo molecular pump (it took about 5 to 10 min). The sample was quickly transferred back into the main UHV chamber (about 1 min) for surface characterization at 10-9 mbar. The UHV-prepared Pt(111) surface exhibited its typical voltammogram in 0.1 M HClO4 when cycling between 0.05 and 0.85 V vs RHE.29 Deposition of Ru at open circuit potential (at about 0.85 V) was carried out with freshly prepared solutions of 0.01 to 5 mM RuCl3 in 0.1 M HClO4. A better control of the coverage was obtained using electrochemical deposition at constant potential (0.07 V to 0.85 V RHE) in 0.1 to 0.5 mM RuCl3 in 0.1 M HClO4 for 1 to 5 min. The Ru coverage was determined by AES from the intensity ratios IRu(200)/IPt(257) and IRu(200)/IPt(170) as well (the strong Pt signal at 237 eV overlaps with the Ru signal at 231 eV). As suggested by Watanabe and Motoo,5 the charge increase in the double-layer region of the cyclic voltammogram in the presence of Ru can be used as a measure of the coverage. We have integrated the charge between 0.35V and 0.6V vs RHE and found a linear correlation with the AES signal (as shown in Figure 2). CO adsorption was achieved by immersion of the electrode in 0.1 M HClO4 saturated with CO at a potential of 0.10 V for

J. Phys. Chem. B, Vol. 103, No. 33, 1999 6969

Figure 1. Cyclic voltammograms at unmodified and Ru-modified Pt(111) electrodes in 0.1 M HClO4 solution. (a) Pure Pt(111); (b-e) Ru-modified Pt(111) with various Ru surface coverage of (b) 0.2, (c) 0.6, (d) 0.75, and (e) 1. Sweep rate ) 50 mV s-1.

2 min. CO stripping voltammetry was then carried out in N2saturated 0.1 M HClO4. In some cases, the electrode surface was characterized again by LEED/RHEED and AES after the CO experiments to check for changes caused by CO adsorption and electrooxidation. 2.2 In Situ FTIR Measurements. A Pt(111) electrode having a surface area of 0.67 cm2 was used for the FTIR experiments. The single-crystal surface was flame annealed, cooled in an Ar + H2 mixture, and transferred to the electrochemical cell after protecting the surface with a droplet of water saturated with the cooling gases.29 The quality of the surfaces was tested by cyclic voltammetry in the supporting electrolyte using a meniscus configuration. The same voltammogram was obtained for the flame-annealed and the UHV-prepared Pt(111) surfaces. The same procedures for preparing Ru-modified surfaces and for CO adsorption as above were used. In this case, Ru-modified Pt(111) electrode surfaces were only characterized with cyclic voltammetry, and the Ru coverage was estimated using the linear correlation between the charge in the 0.35-0.6 V region and the AES signal. The CO adlayer on the modified surface was studied by in situ FTIR spectroscopy. After the IR measurements, the electrode surfaces were checked again by cyclic voltammetry in the supporting electrolyte, followed by CO adsorption and CO stripping voltammetry measurements. Then the Ru overlayer was removed, and the Pt(111) surface was flame-annealed again for the next series of experiments. In situ FTIR experiments were performed with a Bruker IFS 66 spectrometer equipped with a globar infrared source and a MCT detector. Parallel polarized light was obtained from a BaF2-supported Al grid polarizer. The spectroelectrochemical cell, fitted with a 60° prismatic CaF2 window, was designed as to allow electrolyte exchange under potential control.6 For each spectrum, 100 interferograms were collected at 8 cm-1 resolution during ca. 16 s. Reflectance spectra were evaluated as the ratio (R/R0) of a sample (R) and a reference (R0) spectrum.

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Figure 2. (A) Auger electron spectra of pure Pt(111) (a) and Ru-modified Pt(111) (b-f) electrodes after emersion from 0.1 M HClO4 solution. Ru coverage was indicated in the figure. (B) Correlation of the Ru coverages evaluated from the cyclic voltammograms with the AES intensity ratios IRu(200)/IPt(170).

3. Results 3.1. Surface Structure and Composition. Figure 1 shows cyclic voltammograms of unmodified and Ru-modified Pt(111) electrodes in 0.1 M HClO4 electrolyte solution. For the pure Pt(111) electrode, we observe the characteristic “butterfly” structure with a sharp maximum at 0.79 V.29,30 For the Rumodified Pt(111) electrodes, there are obvious changes in the voltammograms. Thus, with increasing amounts of Ru, the sharp maximum first decreases and then disappears and the charge in the double-layer region increases. The latter feature is sensitive to the amount of Ru and has been used to calibrate the Ru coverage in association with the AES measurement on the emersed electrode. Thus, as shown in Figure 2, a linear correlation is found for the Ru coverage evaluated by using the charge between 0.35 and 0.6 V vs RHE, and the AES intensity ratio IRu(200)/IPt(170). Figure 3 depicts LEED and RHEED patterns for Pt(111) modified with adsorbed Ru (θRu ) 0.25). Both LEED and RHEED patterns show no additional reflections, indicating that the Ru adlayer is commensurate (1 × 1) with the Pt(111) substrate surface. There is no intensity modulation along the

reflection streaks (00), (11) (see Figures 3b and 5b) which suggests the formation of a Ru adlayer with monatomic height (as characterized in Figure 5 with N ) 1). Assuming that the broadening of the reflection streaks stems from limited domain size, these can be estimated to g 7 nm. Similar results were obtained with Ru coverages of 0.15, 0.27, and 0.34. In particular, there was no evidence that higher layers are significantly populated up to θRu ≈0.4. Figure 4a shows the RHEED pattern for an electrochemical Ru deposit on Pt(111) with a Ru coverage of 0.64. Now wellseparated satellite reflections (as indicated by arrows) around the Pt(111) substrate beams become evident, signaling an additional lateral periodicity of around 2 nm. Furthermore, a Ru bilayer growth is found by analysis of the intensity profile of the satellite reflection (as characterized in Figure 5 with N ) 2). Some population of the third (and higher) layer may be present. The cyclic voltammetry for this type of surface was conducted in 0.1 M HClO4 electrolyte. It was found that after potential sweeping between 0.07 and 0.85 V RHE at 50 mV s-1 for 10 cycles, the satellite reflection of RHEED (see Figure 4b) still

Ru-Modified Pt(111) Electrodes


Figure 3. (a) LEED (66 eV) and (b) RHEED ([112] azimuth) patterns for a Ru deposit modified Pt(111) electrode surface with the Ru coverage of 0.25.

Figure 4. (a) RHEED pattern for a Ru deposit modified Pt(111) electrode surface with the Ru coverage of 0.64; the satellite reflections are indicated by arrows. (b) RHEED pattern for the Ru/Pt(111) electrode after subjected to potential cycling between 0.07 and 0.85 V RHE at 50 mV s-1 for 10 cycles in 0.1 M HClO4. (c) RHEED pattern for the Ru/Pt(111) electrode after subjected to CO adsorption and electrooxidation (stripping, only one potential sweep up to 0.85 V in 0.1 M HClO4).

remains sharp, indicating that the structure of the Ru overlayer survives. However, after the surface was subjected to CO adsorption (at 0.1 V) and CO electrooxidation (only one potential sweep up to 0.85 V at 50 mV s-1 was applied), the

separation between the satellite and the Pt substrate reflection in RHEED is no longer visible as shown in Figure 4c. Thus, the Ru overlayer structure is affected by CO adsorption and electrooxidation, suggesting enhanced mobility of Ru. Measure-


Lin et al.

Figure 5. (A) The satellite scattering intensity measured along the surface normal (L) for (I) Figure 3b and (II) Figure 4a. (B) Calculated intensity profiles of various adlayers along the reciprocal lattice rod perpendicular to the surface (the intensity was calculated with an interference function ∼ sin2(πNL)/sin2(πL) where L ) integer number, N ) number of the adlayer).

Figure 6. RHEED pattern for a Ru deposit modified Pt(111) electrode surface with Ru coverage of 0.95.

ments with Ru coverages of 0.51 and 0.74 revealed analogous results. For Ru coverages above 0.80, 3D reflection spots became clearly discernible in the RHEED patterns (Figure 6). A mean crystallite size around 0.8 nm was deduced from the spot width (neglecting the contribution of distortion). From the separation of the reflections, a Ru-Ru distance of ∼0.27 nm was evaluated, which corresponds to the bulk Ru structure. 3.2. Reactivity and Electronic Properties toward CO Adsorption and Electrooxidation. 3.2.1. Cyclic Voltammetry Study. Figure 7 shows the cyclic voltammograms for CO electrooxidation on the unmodified and the Ru-modified Pt(111)

Figure 7. Cyclic voltammograms for the oxidative stripping of CO at unmodified (a) and Ru-modified (b-d) Pt(111) electrodes in 0.1 M HClO4 solution. Ru coverage: (a) 0, (b) 0.2, (c) 0.6, (d) 0.75; CO adsorption at 0.1 V. Sweep rate 10 mV s-1. Dotted line: second scan coinciding with the cyclic voltammogram of the clean (CO-free) electrode surfaces in the supporting electrolyte.

surfaces in 0.1 M HClO4 base electrolyte. CO adsorption was performed at 0.10 V in CO saturated solution, then the solution was changed with the N2-saturated base electrolyte under potential control. In Figure 7 only three representative voltammograms of Ru-modified surfaces are shown. Table 1 lists the CO stripping peak potentials and onset potentials as well as the Coulombic charge of CO oxidation on the complete set of surfaces studied, including polycrystalline Ru and a PtRu alloy (50:50) for comparison. As evident from Figure 7 and Table 1, Ru-modified Pt(111) surfaces exhibit a substantial electrocatalytic enhancement toward the oxidation of adsorbed CO, indicated by the shift of



TABLE 1: Summary of the Peak Potentials and Onset Potentials of CO Electrooxidation at the Clean and Ru-Modified Pt(111) Surfaces; Data from Polycrystalline Ru and a PtRu Alloy (50:50) also Included for Comparisona Ru/Pt(111) surfaces Ru coverage

Pt(111)

0.20

0.35

0.60

0.75

1

Rub pure metal

PtRub (50:50)

peak potential (mV/RHE) onset potentialc (mV/RHE) CO oxidation charge µC/cm2 CO2 IR bandd intensity (a.u.)

714 500 ∼435 0.22

524 350 ∼465 0.236

518 350 ∼480 0.236

515 350 ∼525 0.250

497 350 ∼530 0.256

545 400 ∼540

570 400 ∼520 0.257

500 350 ∼500 0.241

a Conditions 10 mV s-1, 0.1 M HClO4 electrolyte. b CV curves have been reported previously;16 peak potential of 580 mV for Ru and 480 mV for PtRu (50:50) were obtained in 0.5 M H2SO4 with 20 mV/s.11 c Data taken from IR results. d CO2 signal in IR data corresponding to the complete oxidation of CO adlayer.

both the CO electrooxidation peak potential and the onset potential to more negative values. Increasing the amounts of Ru on Pt(111) surface decreases the peak potential of the CO oxidation, and the Ru-modified Pt(111) electrode with a Ru surface coverage of 0.75 yields the lowest CO oxidation peak potential of 497 mV, which is 217 mV more negative than that for the clean Pt(111) electrode. It is well known that for PtRu alloys the lowest CO stripping peak potential is obtained with a surface composition of 50 at. % Ru.7,8,11,12,14-16 The CO stripping peak potential of the Ru-modified Pt(111) electrode with an optimum Ru coverage of 0.75 is comparable to that of the best PtRu alloy (50:50) electrodes7,11,14-16 (see Table 1). The small difference between the Ru/Pt(111) (θRu ) 0.75) and the PtRu alloy (50 at. %) may be attributed to the different surface structures, as evidenced by IR data that CO adsorbed on PtRu alloys gives only one C-O stretch band6,8,16 while, on the Ru/Pt(111) surface, there are two distinct C-O bands corresponding to CO on Pt and on Ru, respectively (see below). To determine the coverage of adsorbed CO on the Rumodified Pt(111) surfaces, the Coulombic charge of CO oxidation on the modified surfaces was evaluated and the values (corrected for double-layer charging and oxide/hydroxide formation)31 are listed in Table 1. The charge data indicate that practically a monolayer of CO is adsorbed also on the modified surfaces. This is supported by the fact that complete blocking of the H-desorption current is observed in the CO stripping voltammetry. 3.2.2. In Situ FTIR Study. For the in situ FTIR experiments, a series of spectra were taken, after CO adsorption at 0.1 V, at subsequently applied potentials in the range 0.1-0.8 V at 50 mV intervals. Figures 8-10 show the spectra for adsorbed CO and for the oxidation product CO2 (in the electrolyte) on clean Pt(111) and two representative Ru-modified Pt(111) (with a low and a high Ru coverage) electrodes. For comparison, the same measurements were performed on a pure polycrystalline Ru electrode; the spectra are shown in Figure 11. The spectra for CO2 (on the left side of the figures) were evaluated against a reference spectrum at 0.1 V, where CO2 was absent; the increasing intensities reflect the formation of CO2 as the potential is stepped to successively higher potentials. The CO spectra (on the right side of the figures) were evaluated taking a spectrum at 0.8 V as a reference, a potential at which CO is completely oxidized, and thus the intensities of the bands are proportional to the CO coverage at the respective potential. The spectra on the clean Pt(111) surface (Figure 8) show two typical potential-dependent CO absorption bands around 2070 and 1830 cm-1, which were assigned to the linear-bonded (ontop, COL) and 2-fold bridge coordination (COB) species,32,33 respectively, in good agreement with the literature data.34,35 For the Ru-modified surfaces (Figures 9 and 10), an additional new band around 2000 cm-1 appears, which was assigned to CO linearly bonded to Ru surface atoms.16 As expected, this band increases with increasing Ru coverage while the bands of both

Figure 8. In situ FTIR spectra showing bands for CO adsorbed on a Pt(111) electrode and for the CO2 produced during the CO stripping in 0.1 M HClO4. The CO adlayer was formed at 0.1 V. The potential was positively changed from 0.1 V onward in 50 mV steps. The sample potentials are indicated in the corresponding spectra. The CO2 region (band at 2341 cm-1), was calculated with the reference spectrum taken at 0.1 V, a potential where CO2 is not formed. For the CO region the reference spectrum was one taken at 0.8 V, i.e., a potential where COads was completely oxidized.

COL and COB on the Pt(111) substrate decrease. The measurements with a Ru electrode (Figure 11) show a band around 2005 cm-1 which is attributed to the linear-bonded CO on Ru. On the Ru-modified Pt(111) surfaces with Ru coverage of 0.6 (not shown) or higher (Figure 10, θRu ) 0.75), we found a significant red shift (∼ -10 cm-1) for the COL band on Pt(111) substrate


Figure 9. In situ FTIR spectra for CO adsorbed on a Ru-modified Pt(111) electrode with Ru coverage of 0.2 and for the CO2 produced during the CO stripping in 0.1 M HClO4. Sample potential as indicated. The spectra were obtained and computed as described in Figure 8.

with respect to pure Pt(111), and a blue shift (∼ +6 cm-1) for the COL band at the Ru adatoms/adlayer with respect to pure Ru, as shown in Figure 12. These results demonstrate clearly that a significant electronic effect exists on Ru-modified Pt(111). A linear shift (“Stark tuning”) of the C-O stretch frequency with electrode potential is obtained for all surfaces (see Figure 12b) in the potential region before the onset of COad oxidation. This reflects that the electric field in the inner Helmholtz layer contributes dominantly to the shift of C-O frequency, while COad coverage and binding geometry are basically constant. The value of dνCO/dE (slope) for COL on pure Pt(111) and Ru is 36 and 48 cm-1 V-1, respectively. On the modified surface a value of 39 and 43 cm-1 V-1 is measured for COL at Pt substrate and at Ru overlayer, respectively. The various slopes may reflect different potential-dependent surface bondings (e.g., metal-2π* back-donation).36 To recheck the total CO coverage on the Ru-modified and unmodified Pt(111) surfaces, the measurements were performed by directly stepping the potential from 0.1 to 0.8 V to record the corresponding CO2 signal from the complete oxidation of the adsorbed CO layer at 0.8 V. The CO2 signal was measured at each surface by taking several spectra (100 scans each, 16 s) to ensure complete COad oxidation and to control any diffusion

Lin et al.

Figure 10. In situ FTIR spectra for CO adsorbed on a Ru-modified Pt(111) electrode with Ru coverage of 0.75 and for the CO2 produced during the CO stripping in 0.1 M HClO4. Sample potential as indicated. The spectra were obtained and computed as described in Figure 8.

effect of CO2. Already the first spectrum at 0.8 V exhibited the total loss of CO, the CO2 intensity remaining unchanged in the two subsequent spectra. The integrated CO2 band intensity correlates well with the above CO stripping charge data and confirms that almost a monolayer of CO was adsorbed on all surfaces measured. With increasing coverage the values for the Ru-modified Pt(111) surfaces approach those of polycrystalline Ru (Table 1). Another interesting finding for the Ru-modified surfaces is that when the potential was stepped upward to 400 mV and higher, both the band of CO on Ru and on Pt decrease simultaneously and concomitantly the CO2 band (from the solution) grows (see Figures 9 and 10). This indicates that the electrooxidation of CO on Ru and on Pt starts at the same potential. It could be interesting to test and compare the optimum Rumodified Pt(111) electrode with pure Pt(111), pure Ru, and PtRu alloy (50:50, the best activity for oxidation of adsorbed CO) electrodes with respect to the rate of CO oxidation near the onset of CO2 formation (a potential region relevant for fuel cells). For this purpose, saturated CO adlayers were prepared by adsorbing CO at 0.1 V. After a potential step to 0.45 V, a series


Figure 11. In situ FTIR spectra for CO adsorbed on a Ru electrode and for the CO2 produced during the CO stripping in 0.1 M HClO4. Sample potential as indicated. The spectra were obtained and computed as described in Figure 8.

of spectra were recorded in order to observe the change in the CO and CO2 band intensities with time. The spectra were computed against a reference spectrum obtained at 0.8 V, where the surface becomes free from adsorbed CO. The CO band intensities vs time are presented in Figure 13A, CO2 in Figure 13B. As expected, the CO band intensity at Pt(111) does not change, since the onset potential for CO oxidation on Pt(111) at room-temperature lies above 0.45 V. For the other surfaces an oxidative removal of CO was observed, the rate being the highest at the Ru/Pt(111), followed by the PtRu alloy and pure Ru. The same experiment was also performed at 0.35 V, CO2 formation at the Ru-modified Pt(111) sets in after 2 min, at the PtRu alloy after 3 min, while no oxidation was observed at pure Ru even after 10 min. The onset potentials for CO oxidation listed in Table 1 were obtained in this way. Thus, although the Ru-modified surface and the PtRu alloy hardly differ in the peak potential (∼0.5 V), the reaction is significantly faster on the former (at 0.45 V). 4. Discussion The electrodeposition of Ru on Pt(111) does not occur via layer-by-layer (Frank-van der Merwe)37 growth (as is typical


Figure 12. (A) Comparison of three IR spectra for linear bonded CO (COL) on a pure Pt(111), pure Ru, and a Ru-modified Pt(111) with Ru coverage of 0.75 at a given potential of 200 mV vs RHE. (B) Comparison of C-O stretch wavenumber for the COL on the Pt(111) (a), the Ru-modified Pt(111) (b, c: (b) for COL on Pt site and (c) for COL on Ru site), and the Ru (d) electrodes at various potentials. The data were derived from the IR spectra in Figures 8, 10, and 11 at the potentials before CO oxidation. Experimental conditions as described in Figure 8.

for underpotential deposition, at least for the first layer). Rather, the higher layers become populated as soon as they become available through a significant coverage in the respective lower layer. Thus, the second layer is already visible in electron diffraction when only half of the Pt atoms are covered, the 3D reflections at coverages above 0.80. At low coverage (θRu e 0.4), the reflection streaks in RHEED (Figure 3b) are quite sharp and no additional reflections besides the substrate beams ((11) and (-1-1)) can be detected; therefore, Ru is considered to form a commensurate layer on the Pt(111) surface. It is worthy to point out that because the difference in the interatomic distance between Pt (Pt-Pt: 0.2774 nm) and Ru (Ru-Ru: 0.2704 nm) is only 2%, whether a commensurate or an incommensurate Ru adlayer forms on Pt(111) can be determined neither by LEED due to the limited coherence length nor by STM due to limited lateral resolution.


Lin et al. CO electrooxidation: the satellites vanish (Figure 4c), reflecting a loss of domain ordering, e.g., an enhanced mobility of Ru adatoms. Note that surface structure changes have recently also been observed by in situ XRD with a PtRu/SiO2 catalyst during CO oxidation in the gas phase at 220 °C.39 Although it is impossible to distinguish a layer-plus-island (Stranski-Krastanov)37 from an island (Volmer-Weber)37 growth mode by LEED or RHEED, the IR spectra (Figure 10) show clearly two distinct CO stretch bands corresponding to Pt-CO and Ru-CO, respectively, which does support the island (Volmer-Weber) growth mode. At θRu above 0.8, 3D growth was clearly observed (Figure 6). The Ru deposit modified Pt(111) with Ru coverage of 0.75 represents the optimum composition with respect to CO electrooxidation. As pointed out above, the IR data of CO adsorbed on this surface show COL on both Ru and Pt, suggesting that “patches” of pure Ru or Pt surface properties are still present. This is in contrast to PtRu alloys, where only one CO band was detected8,16,40 and was attributed to a homogeneous distribution of Ru and Pt atoms at alloy surfaces.11,40 The electrooxidation of preadsorbed CO is known to proceed via a 2-step (Langmuir-Hinshelwood) mechanism,11 namely electrosorption of OH followed by the reaction

H2O + * f OHad + H+ + e-

(1)

OHad + COad f 2* + CO2 + H+ + e-

(2)

(* denotes an empty site)

Figure 13. Change of the IR band intensity for adsorbed CO (A) and CO2 (B) on different electrodes at a constant potential of 0.45 V in the electrolyte: Pt(111) electrode; Ru-modified Pt(111) with Ru coverage of 0.75; Ru and PtRu(50:50) alloy electrodes. The respective surface was saturated with CO at 0.1 V after which, CO was eliminated from the electrolyte by nitrogen bubbling. The decrease in CO2 intensity after about 2 min is due to diffusion to the surrounding electrolyte.

With increasing Ru coverage (0.4 < θRu e 0.75), bilayer island growth was determined by the intensity profile analysis of the satellite reflection streak (Figures 4a and 5). The origin of the satellite reflections (Figure 4b) is not quite clear. It may be due to the lateral periodicity (around 2 nm) of domain walls separating the hcp and fcc stacking regions. Actually similar stacking faults have been observed by STM for Ag/Pt(111) with a mean separation of domain walls of 3.7 nm.38 Another possible origin may be due to a sinusoidal displacement wave of the Ru adlayer with a period around 2 nm. On the other hand, the rightside satellite reflection bs of the (11) beam b11 could be due to an incommensurate Ru layer. Since the ratio |bs|/|b11| of 1.07 is much larger than that of the bulk Pt-Pt to Ru-Ru distance (1.02), this would correspond to a Ru layer compressed by 5% with respect to its hcp layer. Although such a compression is possible, the left-hand satellite would then have to be attributed to double diffraction (2b11 - bs), so that its intensity should be much weaker than that of the right-hand satellite. In contrast, no intensity difference is visible in the RHEED patterns. At present we cannot decide between the possibilities mentioned above. We do know that the structure formed is not stable during

On pure Pt the first step is rate determining, i.e., the overall process starts as soon as OH adsorption sets in.41,42 The oxidation of the CO adsorbate layer is considered to initiate from nucleation sites which allow OH adsorption and to proceed via island growth.32,33 In contrast, on pure Ru the second step seems to be rate determining. The adsorption of O-containing species has been reported already for potentials down to 0.2 V vs RHE43,44 well below the CO oxidation peak. This is also confirmed by our AES measurements on Ru/Pt(111) and pure Pt(111) electrodes emersed from the electrolyte at 0.35 V vs RHE where a significant O Auger signal was detected for the former but not for the latter. Moreover, FTIR studies combined with voltammetry showed a current due to OH adsorption before any decrease in CO coverage could be detected,16 i.e., for a certain potential region OH and CO can coexist on the surface. Only for higher potential step (2) sets in. Consequently, the difference between Pt and Ru is essentially caused by the higher affinity of Ru toward OH adsorption, but the coexistence of adsorbed OH and CO might also be attributed to the tighter Ru-CO bond. Since the work function of Pt(111) (5.93 eV)45 is substantially higher than that of Ru (4.71 eV),46 one would expect a significant electron transfer from the Ru to neighboring Pt atoms. Such an electronic effect should weaken the Ru-CO bond (because of decreased back-donation) and strengthen the PtCO bond (increased back-donation). This effect is indeed clearly visible in the IR spectra of the modified surfaces (Figure 12), showing decreased (increased) back-donation on Ru (Pt), i.e., increased (decreased) νCO at Ru (Pt). Assuming that the σ-acceptor properties of the metals have not significantly changed,47-49 the IR data can be taken as a measure of the overall M-CO bond strength. The fact that CO electrooxidation on Ru/Pt(111) occurs at substantially lower potential than on pure Ru becomes thus understandable: CO is more weakly

Ru-Modified Pt(111) Electrodes bonded and hence more reactive than on pure Ru; the reaction with adsorbed OH therefore sets in at lower potential. (The reactivity of OHad should also be influenced by the electron density, but we have no experimental evidence for this at present). Only one CO stripping peak was obtained on Ru/Pt(111). Indeed it is unlikely that electrooxidation of CO takes place on the Pt substrate, for lack of OHad at the potentials in question. Rather, it is assumed that processes (1) and (2) take place preferably on Ru patches, the complete mechanism including CO surface diffusion50 as an additional step. If it is assumed that the Ru-CO bond is of at least comparable strength as PtCO (Figure 12 suggests it is even stronger), CO will be able to migrate to the Ru. This is supported by recent UHV STM observations of the preferential CO adsorption at Ru areas on a bimetallic Pt/Ru(0001) surface,50 where TPD data show a higher adsorption energy on the Ru areas, which was however, lower than on pure Ru(0001)50 in agreement with the present IR data. Although the onset and peak potential for CO oxidation are practically the same for the optimum Ru/Pt(111) surface and the PtRu alloy, the former exhibits a higher catalytic activity (Figure 13), which may be due to the presence of Ru islands serving as active sites. 5. Conclusions The electrodeposition of Ru on Pt(111) forms a monatomic commensurate layer at low coverages, while at higher coverages the epitaxial growth is changed to the Volmer-Weber growth mode, i.e., higher layers are populated before the first layer is completed. Because of electronic effects and structural properties (presence of Ru islands), the electrocatalytic activity of Rumodified Pt(111) surfaces toward CO oxidation is substantially higher than that of the pure metals and even slightly better than that of PtRu alloys. Acknowledgment. W.F.L. acknowledges the Max-PlanckGesellschaft (MPG) and the Alexander von Humboldt-Stiftung (AvH) for a Research Fellowship. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. Stimulating discussions with Professor A. Wieckowski are also appreciated. References and Notes (1) Niedrach, L. W.; McKee, D. W.; Paynter, J.; Danzig, I. F. Electrochem. Technol. 1967, 5, 318. (2) McKee, D. W.; Scarpellino, A. J. Electrochem. Technol. 1968, 6, 101. (3) Ross, P. N.; Kinoshita, K.; Scarpellino, A. J.; Stonehart, P. J. Electroanal. Chem. 1975, 63, 97. (4) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 275. (5) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (6) Iwasita, T.; Nart, F. C.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1030. (7) Iwasita, T.; Dalbeck, R.; Pastor, E.; Xia, X. Electrochim. Acta 1994, 39, 1817. (8) Ianniello, R.; Schmidt, V. M.; Stimming, U.; Stumper, J.; Wallau, A. Electrochim. Acta 1994, 39, 1863. (9) Krausa, M.; Vielstich, W. J. Electroanal. Chem. 1994, 379, 307. (10) Gasteiger, H. A.; Ross, P. N.; Cairns, E. J. Surf. Sci. 1993, 293, 67. (11) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (12) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. Electrochim. Acta 1994, 39, 1825. (13) Markovic, N.; Gasteiger, H. A.; Ross, P. N.; Jiang, X.; Villegas, I.; Weaver, M. J. Electrochim. Acta 1995, 40, 91. (14) Kabbabi, A.; Faure, R.; Durand, R.; Beden, B.; Hahn, F.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 41.

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Electrocatalytic Activity of Ru-Modified Pt(111) Electrodes toward CO

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