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Catalytic Influence of Pt Monolayer Islands on the Hydrogen Electrochemistry of Ru(0001) Studied by Ultrahigh Vacuum Scanning Tunneling Microscopy and Cyclic Voltammetry† H. Hoster, B. Richter,‡ and R. J. Behm* Department of Surface Chemistry and Catalysis, UniVersity of Ulm, D-89069 Ulm, Germany ReceiVed: June 4, 2004; In Final Form: July 18, 2004
The electrochemical behavior of well-defined Pt-modified Ru(0001) electrodes in perchloric acid solution was investigated by cyclic voltammetry. The Ru(0001) samples were prepared under ultrahigh vacuum conditions and modified by pseudomorphic Pt monolayer islands, which were produced by Pt vapor deposition. The resulting electrodes were quantitatively analyzed by high-resolution scanning tunneling microscopy. The data show that hydrogen adsorption on the strained Pt monolayer is very weak, much weaker than on bulk Pt, which agrees well with theoretical predictions and recent findings for gas-phase hydrogen adsorption. The appearance of a new pair of peaks and the simultaneous disappearance of the peaks characteristic for H adsorption/desorption on the unmodified Ru(0001) electrode in the cyclovoltammogram in the presence of already small amounts of Pt deposit is explained by a catalytic enhancement of the adsorption and desorption of hydrogen on the surrounding Ru substrate areas. It involves hydrogen adsorption on the Pt monolayer islands, followed by removal of the OHad species at the perimeter of the Pt islands by reaction with Had and finally spillover of Had to the surrounding Ru substrate in the cathodic scan or the reverse process in the anodic scan. Possible explanations for the physical origin of this novel effect are discussed.
Introduction Bimetallic electrodes and bimetallic electrode surfaces have attracted increasing interest in the last years because of the high activity of bimetallic catalysts for the electro-oxidation of small organic molecules or for the oxidation of CO-contaminated H 2rich feed gases and their potential for application in polymer electrolyte fuel cells (PEFC).1 Model systems for these catalysts include both metal electrodes covered by a monolayer or thicker film of a second metal and mixed surfaces with both species present in the surface layer, such as bulk alloy surfaces, surface alloys, or metal surfaces modified by small islands (often also denoted as “particles”) of a second metal. In general, these model surfaces are prepared by in situ electrodeposition or electroless deposition or bulk alloys electrodes are used.2 Another possibility for the in situ electrochemical preparation of well-defined surfaces alloys, via diffusion-controlled deposition of two metals, has recently been demonstrated.3 The variety of configurations that can be prepared this way, however, is limited, e.g., by the limited temperature range accessible during deposition. Therefore, in specific cases, ex situ preparation and possibly also characterization of bimetallic electrodes and their subsequent controlled transfer into an electrochemical environment offer decisive advantages in preparing specific desired configurations (for an overview, see ref 4). This is exploited and demonstrated in a combined ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and electrochemical study of bimetallic Pt/Ru(0001) surfaces. In the present paper, we will concentrate on Ru(0001) substrates modified by increasing amounts of small Pt monolayer islands. Results on PtRu(0001) †
Part of the special issue “Gerhard Ertl Festschrift”. * To whom correspondence may be addressed. E-mail: juergen.behm@ chemie.uni-ulm.de. ‡ Current address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138.
surface alloys with well-defined amounts and distributions of the two species in the surface layer will be reported later. PtRu is the presently preferred anode catalyst for methanol oxidation in the direct methanol fuel cell (DMFC) and for oxidation of CO-contaminated H2 feed gases in PEFCs,5,6 which led to numerous studies on PtRu model systems. In accordance with the routes described above, these were prepared, e.g., by electroless deposition of Ru on Pt(111) (ref 7 and subsequent studies in ref 8) or of Pt on Ru(0001) substrates.9-13 In addition, PtRu bulk alloys, cleaned and characterized ex situ under UHV conditions, were used in a number of studies.14-21 The morphology of the surfaces resulting from electroless deposition is characterized by a large number of small multilayer islands (“nanoparticles”), which depending on the deposition conditions are homogeneously distributed over the surface.8 Recently, ex situ deposition of Ru on Pt(111)20,22,23 and Pt(110)24,25 substrates was used to prepare better defined bimetallic PtRu electrodes. In the present study, we focus on the reverse system, on PtRu electrodes prepared by MBE growth of Pt on Ru(0001) substrates. By control of the deposition conditions and by application of subsequent annealing procedures, it is possible to prepare bimetallic surfaces with well-defined monolayer islands or Ru electrodes covered by a continuous Pt monolayer film. Both of these morphologies have not been accessible by electroless deposition. Because of their simpler geometry, these well-defined PtRu model systems are particularly interesting for fundamental studies of the reaction mechanism, since they allow a direct comparison with theoretical studies on the chemical properties of bimetallic PtRu surfaces.26-28 They also allow us to more easily discriminate between different effects responsible for the improved catalytic activity of these surfaces, such as the bifunctional mechanism,29 structural effects,30 or electronic strain effects.31
10.1021/jp047576l CCC: $27.50 © 2004 American Chemical Society Published on Web 08/28/2004
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Figure 2. Cyclic voltammogram of the surface shown in Figure 1: solid line, 110-1050 mV; dashed line, -150 to 1050 mV (scan rate 50 mV/s).
Figure 1. Large-area overview STM image of a freshly prepared clean Ru(0001) surface (Ut ) 0.65 V, It ) 0.56 nA, 126 nm × 126 nm). Inset, atomic resolution image recorded on a flat terrace (Ut ) 1 mV, It ) 56 nA, 5.6 nm × 5.6 nm).
Details of the Pt growth process,32 surface alloy formation,33 and of the chemical properties for CO adsorption,32 H2 adsorption,34,35 and H2/CO coadsorption34 of these surfaces under UHV conditions have been extensively investigated in our laboratory. In the following, we will, after a brief description of the experimental equipment and procedures, first characterize the electrochemical behavior of the UHV-prepared Ru(0001) surface in perchloric acid solution and compare our data with previous results by El-Aziz and Kibler on an Ar-annealed Ru(0001) electrode.36 In the second section, we investigate the influence of increasing coverages of platinum monolayer islands, whose size, shape, and structure has been characterized by STM prior to the electrochemical measurements, on the cyclovoltammetric response of the Ru(0001) substrate. To elucidate the behavior of the Pt monolayer islands themselves, we also included an electrode that is fully covered by a Pt monolayer film. The strongly nonlinear effects introduced by even very small amounts of Pt are discussed in detail in the last chapter. Experimental Section The experiments were performed in a UHV electrochemistry (UHV/EC) transfer system that is based on an existing UHVSTM apparatus. It is equipped with a home-built pocket-size STM and facilities for sample preparation and characterization such as metal evaporators, Auger electron spectroscopy (AES), and a quadrupole mass spectrometer for residual gas analysis.32,37 Details of the transfer system itself will be given in a forthcoming publication.38 Sample Preparation. The preparation of the Ru(0001) crystal followed standard procedures, which are described in more detail in ref 34. After each electrochemical experiment, it was cleaned by 3-4 cycles of argon sputtering (0.5 kV Ar+ ions, 5 µA/ cm2, 10 min), followed by flash annealing to 1650 K. Remaining carbon impurities were removed by oxidation, involving oxygen adsorption (10 L O2 - 1 L ) 1.33 × 10-6 mbar‚s) and subsequent repeated flash annealing to 1650 K. The freshly prepared surface exhibits 50-200 nm wide and atomically flat terraces separated by monolayer steps (Figure 1). Contamination levels
are generally far below the AES detection limit of about 0.01 monolayers (ML); the high surface purity is confirmed by atomically resolved images such as the one in the inset in Figure 1. Platinum was evaporated from an electron beam evaporator (FOCUS EFM 3), with the substrate at 300 K and typical deposition rates of about 0.25 monolayers (ML)/min. The resulting coverage was determined by quantitative evaluation of STM images and by AES. For the transfer into the electrochemical cell, the surface was protected by an oxygen adlayer, which was created by O2 exposure at 300 K (10 L exposure). Prior to the sample transfer, the prechamber for the electrochemical measurements was vented with high-purity N2 (Messer Griesheim, 5.0). The adlayer can be reductively removed under potential control in the first voltammetric scan. This was confirmed by comparative experiments that showed that the oxygen adlayer has no influence on the electrochemical behavior after the first potential cycle, i.e., it is completely removed by this procedure. Electrochemical Measurements. All electrochemical measurements were performed in 0.1 M HClO4 (Merck, Suprapur) electrolyte. To avoid experimental artifacts arising from trace impurities of chloride,39 we performed test experiments in solutions with ppm contents of chloride, which allowed the clear identification of Cl--induced modifications in the CV and thereby to avoid these impurities. A platinized Pt wire enclosed in a glass capillary filled with the base electrolyte and partially surrounded by a H2 bubble served as reference electrode. All potentials are given versus that of the reversible hydrogen electrode (RHE). The electrochemical measurements were performed at room temperature. During the electrochemical experiments, the sample is pressed face down onto a miniaturized flow cell made of KEL-F.38 To avoid problems that can arise from the electrolyte contacting the possibly contaminated sample edges, the sample area in contact with the electrolyte was restricted by an elastic Kalrez O-ring pressed against the sample. Results 3.1. Ru(0001) Electrodes. We first characterized the electrochemical behavior of the clean Ru(0001) substrate. The cyclic voltammogram in Figure 2, which was recorded immediately after STM analysis and transfer into the electrochemical cell, closely resembles that of Ru(0001) samples prepared under UHV conditions40 or by inductive annealing in an argon stream.36 If the scan range is kept within 110-1050 mV (full
14782 J. Phys. Chem. B, Vol. 108, No. 38, 2004 line in Figure 2), the CV can be divided into two main parts, which are cathodic and anodic of 300 mV. The symmetric couple of peaks below 300 mV was initially attributed to underpotential deposited (upd) hydrogen.40-43 On the basis of the results of COad replacement experiments, El-Aziz and Kibler recently assigned these peaks to the reduction and, in the anodic scan, the readsorption of about 0.4 ML OHad.36 Above about 300 mV, the surface is covered by a monolayer of OHad. The formation of surface oxide takes place in two main steps, a broad wave between 450 and 700 mV, with a small pre-peak in front, followed by a large, flat, plateaulike region ranging up to the anodic limit of the scan at 1050 mV. The reduction of the oxidic species in the reverse, cathodic scan starts at somewhat lower potentials, below 550 mV, and results in a distinct peak, which under present conditions is centered around 470 mV, with a shoulder at potentials below 435 mV. For scans ranging to more cathodic potentials, in this case to -150 mV, the shape of the CV (dashed line in Figure 2) changes significantly in the region below ∼500 mV. At the onset of H2 evolution, a pronounced cathodic peak appears at -20 mV, which comprises a charge of about 280 ( 30 µC/cm2. The uncertainty in this value results from the superimposed hydrogen evolution. This peak was attributed to the reduction of about 0.6 ML adsorbed OH and the concomitant adsorption of 0.5 ML hydrogen.36 The expected charge for this process would be 1.1 × 250 ) 275 µC/cm2, in good agreement with the value of 280 µC/cm2 observed here. In the anodic scan, the reverse process takes place, but at significantly higher potentials, with a single peak centered around 240 mV (onset potential about 150 mV), followed by a constant current regime between 300 and 450 mV. It should be noted that around 160 mV the current is slightly lower in this case than in the CV obtained for a cathodic limit at 110 mV. The difference in charge between the two CVs in the potential range between 110 mV and 450 mV is about 280 ( 20 µC/cm2, equal to the charge in the cathodic peak at -20 mV. Assuming that the respective first reaction in the two processes taking place in the peaks at -20 mV (cathodic scan) and 240 mV (anodic scan) is rate limiting, the significant hysteresis between H adsorption and Had oxidation can result (i) from a kinetic inhibition of the preceding OH reduction in the cathodic scan, (ii) from a kinetically hindered H oxidation in the anodic scan, or (iii) from a combination of the two processes. In the absence of a kinetic barrier for Had oxidation, the high onset of Had oxidation would indicate a significant stabilization of the H adlayer, i.e., the existence of a H upd adlayer. On the basis of the existing data, it is not possible to distinguish between these possibilities. This will be discussed in more detail later. Also CO displacement experiments, as used in ref 36, would not give a clear answer, since a CO-induced displacement is also possible for a stable H adlayer if the adsorption energy of the COad species is higher than that of Had at the respective potential and if Had adsorption/desorption is reversible under these conditions. If, however, the late onset of the H oxidation process is caused by kinetic limitations, it would be conceivable that this process is accelerated by the presence of a noble metal with a high exchange current density for hydrogen on the surface, such as Pt or Pd. This is topic of the experiments in the following chapter. 3.2. Pt-Modified Ru(0001) Electrodes. For investigating the effect of well-defined amounts of Pt on the electrochenmical behavior of the Ru(0001) substrate, a number of Pt-modified Ru(0001) model surfaces with different Pt coverages, up to 1 ML, and different distributions of the Pt deposit were prepared
Hoster et al.
Figure 3. STM images of Ru(0001) substrates covered with monolayer Pt islands and cyclic voltammograms recorded on these surfaces (scan rate in all cases 50 mV/s). (a) STM image of a Ru(0001) surface with 0.03 ML Pt deposited at 300 K on a 0.5 ML Oad precovered Ru(0001) (Ut ) 0.9 V, It ) 0.56 nA, 142 nm × 142 nm). (b) CV of the surface in Figure 3a (scan range of 50-1070 mV). (c) STM image of a Ru(0001) surface with 0.15 ML Pt deposited at 300 K on a 0.5 ML Oad precovered Ru(0001), followed by annealing at 900 K for 30 s (inset, close-up of the area marked by the white rectangle, resolving a (2 × 2)O superstructure between the Pt islands) (Ut ) 1.2 V, It ) 1.8 nA, 135 nm × 135 nm; inset, Ut ) 0.7 V, It ) 1.8 nA, 14 nm × 14 nm). (d) CV of the surface in Figure 3c (scan range 50-750 mV). (e) STM image of a Ru(0001) surface with 0.51 ML Pt deposited on the Oadfree Ru(0001) substrate at 300 K, followed by brief annealing to 850 K (inset, atomic resolution image of a Pt island) (Ut ) 0.35 V, It ) 1 nA, 142 nm × 142 nm). (f) CV of the surface in Figure 3e (scan range 50-1050 mV). (g) STM image of a Ru(0001) surface with 0.8 ML Pt deposited on the Oad-free Ru(0001) substrate at 300 K, followed by annealing at 900 K for 30 s (Ut ) 0.5 V, It ) 0.56 nA, 142 nm × 142 nm). (h) CV of the surface shown in Figure 3g (scan range 50-1050 mV).
by vapor deposition under UHV conditions. For all coverages, this resulted in monolayer Pt islands, with the island diameter and island-island separation depending on the Pt coverage and the experimental conditions.32,44 Typical surfaces obtained this way are shown in the STM images in parts a, c, e, and g of Figure 3. For the lowest Pt coverage, 3% of a monolayer, Pt was deposited on an oxygen precovered surface (10 L O2) to increase the island density and therefore also the fraction of the Pt edge atoms (Figure 3a). Such effects had been reported for Au
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Figure 4. Plot of the charge in the cathodic peak between 110 and 60 mV vs the platinum coverage as determined by STM (data from Figure 3 and comparable measurements). See text for the explanation of the error bars.
deposition on bare and Oad precovered Ru(0001) surfaces.45,46 As mentioned above, the O adlayer can easily be removed electrochemically. For deposition under these conditions, the Pt island density is around 8 × 1011 cm-2, the average island size around 2.5 nm. Figure 3c shows a typical STM image of a surface with 0.15 ML Pt adlayer islands, equally obtained by deposition on an Oad precovered surface. In this case, the surface was annealed to 900 K for 30 s after platinum deposition in order to obtain straight island edges. In the atomically resolved image in the inset in Figure 3c, one can see the well-ordered (2 × 2) adlayer formed by the adsorbed oxygen and the hexagonal shape of the Pt islands. The island density and average island size obtained under these conditions are around 3.5 × 1012 cm-2 and around 3 nm, respectively. The actual size of the islands, corrected for tip effects, is indicated by the black hexagon on one of the islands. It was determined by setting a threshold value for the actual step position, which was chosen as the highest value still giving compact islands with straight edges. To illustrate the actual contribution of the tip-induced island broadening, we have artificially enhanced the contrast in the inset of Figure 3c by illuminating the surface from the lefthand side. This way, the small decrease in height at the actual position of the island edge becomes apparent. The following two model surfaces (Parts e and g of Figure 3) with Pt coverages of 0.51 and 0.85 ML, respectively, were prepared by deposition on Oad-free Ru(0001) surfaces, followed by annealing to 850 and 900 K, respectively. These procedures result in surfaces with extended monolayer Pt islands (Figure 3e) or in a Pt adlayer with vacancy islands (Figure 3g). Secondlayer Pt islands, which start to grow at high Pt coverages, are dissolved by the annealing step. The atomic structure of the pseudomorphic islands is resolved in the atomic resolution image in Figure 3e. The cyclovoltammetric data, which were recorded for each surface immediately after the STM analysis and the subsequent sample transfer, are shown in Parts b, d, f, and h of Figure 3. Starting with the 0.03 ML Pt-covered surface (Figure 3b), the main features of the cyclic voltammograms above ∼180 mV resemble those of the pure Ru(0001) substrate discussed above (Figure 2). In addition, however, we find a couple of intense peaks at lower potentials, centered around 100 mV. With increasing platinum coverage, these peaks decrease in amplitude, which will be further investigated in Figure 4. As expected also the characteristic Ru-related features at higher potentials decay with increasing Pt coverage. The contribution of the platinumcovered surface areas themselves to the total current is comparably small, independent of the Pt coverage. The latter effect will be discussed in more detail later, together with the
Figure 5. Comparison of the CVs for (a) pure Ru(0001) (see Figure 2) and (b) Ru(0001) modified by 0.03 ML of Pt (see Figure 3b). The peaks belonging to the replacement of OHad by Had and vice versa.
Figure 6. Cyclic voltammogram of Ru(0001) modified by 0.085 ML of Pt monolayer islands, scan range extended into the cathodic range (compare with Figure 2b) to 0 mV (solid line) and -150 mV (dashed line) (scan rate 50 mV/s). In contrast to pure Ru(0001), no additional voltammetric features appear below 60 mV apart from the evolution of hydrogen and its oxidation in the anodic back scan. Inset, same surface, enlarged view of the CV around 100 mV for 3 different cathodic turning points: 0 mV (dashed line); 60 mV (solid line); 130 mV (dotted line).
electrochemical behavior of a full pseudomorphic Pt layer on Ru(0001) (see Figure 8). A quantitative analysis of the net charge of the cathodic peak in the range between 110 mV and 60 mV vs the platinum coverage is shown in Figure 4. The error bars for the coverage reflect the uncertainty of the quantitative STM analysis and the small spatial inhomogeneities found by AES analysis. The uncertainty in the integrated charge arises from (i) possible variations of the wetted surface area, (ii) errors in the background subtraction due to the overlap of the sharp cathodic peak with the OH reduction peak at 180 mV, (iii) variations of the double-layer capacity with increasing platinum coverage, and (iv) charge contributions due to hydrogen adsorbed at the platinum islands. The latter effects become measurable for higher amounts of Pt (see also Figure 8). Since at this point we are interested only in the contribution from H adsorption on
14784 J. Phys. Chem. B, Vol. 108, No. 38, 2004
Figure 7. (a) CV (50-1050 mV) of Ru(0001) with 0.03 ML of Pt (see Figure 3a) (scan rate 10 mV/s). (b) Anodic charge transient attained by CO adsorption at 80 mV after having passed the cathodic peak. The charge of 121 µC/cm2 corresponds to ∼0.48 ML of displaced hydrogen. (c) Cathodic charge transient measured during CO-adsorption at 110 mV, before reaching the cathodic peak. The charge is equivalent to 0.55 ML of displaced OH (see text).
the Ru substrate, contributions from the Pt islands are treated here as undesired background. Clearly, the charge in these peaks decreases linearly with the platinum coverage, decaying to zero for a full Pt monolayer. The slope of a least-squares fit to these data points results in a value of about 290 ( 10 µC per cm2 of uncovered ruthenium. This is very close to the charge in the cathodic peak at -20 mV observed on pure Ru(0001), which was discussed above. On the basis of the good agreement between these charge values, we assign the couple of sharp peaks around 100 mV on the Pt-modified Ru(0001) surfaces to the same processes that generate the peaks observed on the bare Ru(0001) electrode at -20 mV in the cathodic scan and at 240 mV in the anodic scan, namely, to OH desorption and subsequent H adsorption on the Ru(0001) areas in the cathodic scan and to (oxidative) Had desorption followed by OH adsorption in the anodic scan, respectively. A second interesting observation in these CVs is that the hysteresis between these two peaks is significantly smaller than for the Pt-free surface, indicating that the kinetic barriers for the underlying processes are significantly reduced by the presence of the Pt adlayer islands. For a more quantitative comparison, we compare the CVs recorded on the Pt-free Ru-
Hoster et al. (0001) electrode and on the 0.03 ML Pt modified Ru(0001) electrode in Figure 5. Clearly, both the cathodic and the anodic peaks are shifted in the presence of the Pt adlayer islands, the cathodic peak to higher potentials, from -20 mV on the Ptfree electrode to 67 mV on the Pt-modified electrode, and the anodic peak to more negative potentials, from 240 to 135 mV, for going from the Pt-free to the Pt-modified electrode. By assumption that the respective first process is rate limiting in each scan direction on the Pt-free Ru(0001) electrode (see 3.1), this demonstrates that both OHad reduction and Had oxidation are kinetically hindered on that electrode and that the barriers for these processes are significantly reduced by the presence of the Pt monolayer islands. The dotted vertical line at 100 mV marks the point of intersection of the lines connecting the anodic and cathodic peaks of the CVs of the Pt-modified surfaces with the potential axis (see Figures 5 and 6). It indicates the onset potential for the OH/H adlayer exchange process in both scan directions. Since both in the cathodic as well as in the anodic scan the replacement of an OH adlayer by a H adlayer, or the reverse in the anodic scan, occurs in a very narrow potential range and considering the small hysteresis between forward and backward scans, the value of 100 mV must be close to the thermodynamic potential for the replacement of OHad by Had on the Pt-free Ru(0001) surface. If the above interpretation of the additional sharp peaks is true, we would expect that extending the CV to more negative potentials does not lead to additional peaks. This was tested for a 0.085 ML Pt adlayer modified Ru(0001) electrode (Figure 6). By use of the same cathodic scan limit as for the Pt-free Ru(0001) electrode in Figure 2b (-150 mV), one obtains a voltammogram dominated by a strong evolution of H2 and the oxidation of dissolved H2 in the anodic backscan (dashed line in Figure 6). The small peaks in the anodic scan represent the OHad-reduction/H-adsorption and Had-oxidation/OHad-adsorption peaks discussed above. No further structures are resolved in the cathodic region. The high H2 evolution current on the Pt adlayer island modified surface, which is much higher than for the Pt-free Ru(0001) electrode, demonstrates a pronounced kinetic effect of these islands also on H2 evolution, even for very low Pt coverages. For a cathodic limit of 0 mV, the CV (full line in Figure 6) closely resembles that obtained for the 0.03 ML Pt modified surface (Figures 3a and 5b). A closer examination of the effect of varying the cathodic limit of the CV is possible in the inset in Figure 6, which displays an enlarged view of the CVs of the 0.085 ML
Figure 8. (a) STM image of PtRu electrode after deposition of 1.08 ML Pt at 300 K on bare Ru(0001) and subsequent short annealing to 870 K (Ut ) 0.9 V, It ) 0.56 nA, 150 nm × 150 nm). (b) Solid line, cyclic voltammogram of the surface in (a), scan range 50-1050 mV; dashed line, CV of Pt(111) for comparison (scan rate 50 mV/s).
Catalytic Influence of Pt Monolayer Islands Pt-covered electrode between 0 and 300 mV with cathodic turning points of 0, 60, and 125 mV, respectively. As expected, the two sharp peaks are completely absent for a cathodic limit of 125 mV (dotted line). For the two other CVs, with cathodic limits at 60 and 0 mV, respectively, we again find the pronounced peaks, which on this electrode are centered at 65 mV in the cathodic scan and at 135 mV in the anodic scan. The onset potential of the anodic and the cathodic peak, respectively, was determined to 101 mV (dashed line). This value was reproduced with a precision better than (2 mV for different Pt coverages, electrolyte concentrations (0.01-0.1 M), and scan rates (2-50 mV/s). The height of the peaks, however, changes with the square root of the scan rate, analogous to the potentiodynamic behavior of many other electrochemical redox processes.47 This effect may be due to surface diffusion of hydrogen toward or away from the Pt islands, depending on the respective scan direction. This issue will be investigated in detail by systematically varying the spatial distribution of the Pt islands in a forthcoming study. To further verify that the peaks around 100 mV result from the same processes as the peaks at -20 mV and +240 mV on the unmodified Ru(0001) electrode (Figure 2, dashed line), we performed CO displacement experiments analogous to those in ref 36 at two different potentials on a 0.03 ML Pt-modified Ru(0001) electrode (parts b and c of Figure 7). Between the experiments, the electrochemical prechamber was thoroughly flushed with nitrogen to remove any gaseous CO. To clean the electrode surface and the electrochemical cell, the potential was cycled in the range 50-1050 mV under a continuous flux of fresh electrolyte until no more CO related effects were detectable in the voltammograms. In the first experiment, CO displacement of the adlayer was started after a potential excursion from 50 to 1050 mV and back to 110 mV (scan rate 10 mV/s), i.e., just before the onset of the large cathodic peak (Figure 7a). Right after starting the CO exposure, we observed a pronounced cathodic current peak, which decayed with time, over about 150 s (Figure 7c). The cathodic current must result from the displacement of anionic species. The second experiment was performed in a similar way. In this case, however, the cathodic backscan was stopped at 80 mV, right after having passed the large cathodic peak at 100 mV. This time, we observed an anodic current transient with a significantly larger peak current upon CO admission (Figure 7b). The shape of the cathodic transient recorded during CO adsorption at 110 mV (Figure 7c) is rather similar to that observed on Pt-free Ru(0001) in the same potential region.36 The integrated charge is 137 ( 14 µC/cm2, which would correspond to a CO-induced displacement of 0.55 ( 0.06 ML OHad. Also this value closely resembles that obtained on Ru(0001).36 The anodic current transient (Figure 7b) exhibits a much larger peak current, which decays, however, much more rapidly. Hence, the CO-induced displacement of the adlayer is much faster in this case than for OHad displacement. It also appears to be much faster than the same process on pure Ru(0001),36 while OHad displacement proceeds on a similar time scale for both surfaces, with or without Pt islands. The integrated charge is about 121 ( 12 µC/cm2, which following the above assignment would correspond to the displacement of 0.48 ( 0.05 ML Had, displaced via CO + Had f COad + H+ + e-. Also in this case, the transient behavior closely resembles that observed on Pt-free Ru(0001).36 (It should be noted that because of the much more cathodic position of the large cathodic peak on the Pt-free Ru(0001) surface the potential was scanned far into the hydrogen evolution region and then back to around 90
J. Phys. Chem. B, Vol. 108, No. 38, 2004 14785 mV in the latter experiments.) The sum of the charges measured in the two displacement experiments (258 ( 25 µC/cm2) is practically equivalent to the charge in the cathodic peak in the voltammogram of the 0.03 ML Pt-modified electrode in the range between 110 and 80 mV (267 ( 26 µC/cm2). Finally, to better define the electrochemical contributions resulting from the Pt-covered areas, we investigated the electrochemical behavior of a Ru(0001) electrode that was (almost) completely covered by a Pt monolayer. Figure 8 shows the surface morphology (Figure 8a) and the cyclic voltammogram (Figure 8b) of a 1.08 ML Pt-covered Ru(0001) electrode. The Pt film was deposited at 300 K, followed by annealing to 870 K to attain a smooth surface. Apart from a few secondlayer islands and some small vacancy islands, the surface is covered by a closed, pseudomorphic Pt monolayer. Comparing the CV of this surface (full line in Figure 8b) with that of a Pt(111) electrode (dashed line in Figure 8b),21 we find higher pseudocapacitive currents in the double-layer region between 400 and 600 mV on the Pt-covered electrode, but significantly lower ones in the H upd region (70-350 mV) and in the potential range for reversible OH adsorption (600-800 mV). Actually, there is no real double-layer region in the CV of the Pt monolayer covered electrode. Furthermore, similar to Pt(111), the CV is symmetric up to 850 mV. Voltammograms of thicker films (g2 ML Pt) are much more similar to those of Pt(111), and the respective double-layer regions are smooth.48 The potential region, in which hydrogen adsorption takes place, is much narrower on the Pt monolayer covered surface than on Pt(111). H2 evolution equally sets in at about 80 mV, but the anodic limit of the H upd region is significantly lower, between 150 and 250 mV. Moreover, the related pseudocapacitive current densities are much smaller. Integrating the total charge between 70 and 250 mV, one yields about 31 µC/cm2 (without background subtraction), which would correspond to 0.12 ML as the upper limit for the H upd adlayer coverage. The much smaller Had coverage in comparison with bulk Pt(111) and the much more negative onset potential of H adsorption can be explained by a significantly weaker Pt-H bond, in agreement with experimental findings for gas-phase hydrogen adsorption on a Pt monolayer covered Ru(0001) surface34 and theoretical predictions for such surfaces based on density functional calculations.26,28,31,49,50 Finally it is important to note that the characteristic peaks at about 100 mV observed on the partly Pt covered Ru(0001) electrodes are completely absent on this surface. Also the characteristic butterfly peak observed for Pt(111) in the reversible OH region is not observed for the monolayer Pt covered surface, but reappears again for a bilayer Pt film.48 Discussion The presence of small Pt monolayer islands on a Ru(0001) electrode leads to distinct changes in the cyclovoltammetry of this electrode in HClO4 solution, in particular with respect to the adsorption and desorption of hydrogen. Already for very small amounts of Pt deposited, the characteristic cathodic peak at -20 mV and the anodic peak around 240 mV are replaced by a pair of peaks around 100 mV. The results presented in the preceding chapter lead to the following conclusions on the underlying processes and the physical origin: 1. The sharp cathodic peak between 100 and 80 mV observed for a Pt monolayer island modified Ru(0001) electrode in HClO4 solution results from the reduction of the OH adlayer on the bare Ru(0001) regions with a local coverage of 0.6 ML, followed by the adsorption of (locally) 0.5 ML Had on this
14786 J. Phys. Chem. B, Vol. 108, No. 38, 2004 surface. In the anodic peak between 100 and 150 mV, in turn, the hydrogen adlayer is oxidized to H+ and the OH adlayer is reformed again. Additional 0.4 ML OH (locally) are adsorbed/ desorbed in the peak between 100 and 250 mV (cathodic and anodic scan). Hence, the total amount of OHad, about 1 ML locally, is adsorbed and reductively desorbed in the potential range between 80 and 250 mV. In contrast, these processes range over a potential regime from -130 and +450 mV on the unmodified Ru(0001) surface, indicative of a significant positive potential shift of these processes in the cathodic scan and a negative potential shift in the anodic scan induced by the Pt monolayer islands. 2. The results on the bare Ru(0001) electrode and on the Pt monolayer island modified Ru(0001) electrode fully agree with and confirm the previous interpretation of the features in the Ru(0001) base voltammogram in perchloric acid solution.36 According to that study, the surface is covered by 1.0 ML OHad in the potential range between 250 and 300 mV in the cathodic scan. 0.4 ML OHad are reduced in a peak appearing below 250 mV in the cathodic scan; reduction of the remaining 0.6 ML OHad and subsequent adsorption of 0.5 ML H takes place in a sharp peak centered at -20 mV. The present data provide clear proof that the late onset for OH reduction and H adsorption on the Ru(0001) electrode in the cathodic scan around -20 mV is caused by kinetic limitations. Already very small amounts of Pt monolayer islands shift the onset potential for OH reduction/H adsorption to about 100 mV and thus lead to a considerable enhancement of the process. The same is true for Had oxidation and subsequent OH adsorption on Pt-free Ru(0001) in the anodic scan, between 150 and almost 300 mV, which is shifted to more cathodic potentials by the presence of the Pt monolayer islands. 3. The data on the Pt monolayer island containing Ru(0001) electrodes are clear proof for the existence of a thermodynamically stable H adlayer on the Pt-free Ru(0001) electrode at potentials up to 100 mV, i.e., for the existence of a H upd species on that surface. In a limited potential region above 100 mV, Had is thermodynamically stable against H+ formation but is being displaced by more strongly adsorbing OH adspecies, as it had been discussed in general in ref 51. The principal existence of a H upd species and the extension of the H upd regime to potentials significantly higher than 100 mV agrees well with expectations based on the rather high Ru-H bonding energy (2.86 eV),52 which is even higher than that on Pt(111) (2.64 and 2.56 eV, respectively, based on experimental data53 and calculations27). 4. Hydrogen adsorption is extremely weak on the monolayer Pt covered Ru(0001) surface, much weaker than on Pt(111) or on Ru(0001), with a narrow H upd regime between 80 mV and at most 150 mV. Consequently, the steady-state Had coverage is very low, although it is not limited by kinetic effects such as on Ru(0001). The considerable reduction in adsorption energy on the Pt monolayer compared to bulk Pt is attributed to electronic strain effects, which are induced by the 2.5% compression of the Pt lattice in the pseudomorphic Pt islands/ layer26,28,31,49 and to modifications of the Pt electronic structure due to the underlying the Ru substrate.50 Structural effects, such as a stabilization of the H adlayer by steps, do not seem to play a major role for the electrochemistry of the bimetallic electrode. 5. Despite the weak interaction between hydrogen and the Pt monolayer covered Ru(0001) electrode, H2 evolution is much stronger on the Pt-modified than on the unmodified Ru(0001) surface. This effect can simply be explained in terms of the classic relationship between the bond strength of the metalhydrogen bond and the exchange current density for H2
Hoster et al. evolution.51,54 Because of its strong metal-hydrogen bond, Ru-H (2.86 eV)52 is located on the branch of the plot where the metal-hydrogen bond is too strong for maximum H2 evolution. Pt-H is located on the same branch, but because of the much weaker Pt-H bond (see above) it is much closer to the maximum current density. For a Ru(0001) electrode covered by a pseudomorphic Pt monolayer, where both TPD experiments34 and DFT calculations26,27 show that hydrogen bonding is again significantly weaker than on Pt(111), 2.34 eV vs 2.56 eV on Pt(111),26,27 the decrease in metal-hydrogen bond strength is sufficient to shift this electrode to the other side of the maximum of the volcano plot. The exchange current is expected to be of comparable magnitude as that on Pt(111). The observed strong H2 evolution despite of the weak H upd features on the Pt monolayer covered Ru(0001) electrode is therefore not surprising. These observations and conclusions lead to the following reaction model for the Pt monolayer island modified Ru(0001) electrode in perchloric acid solution: Similar to the unmodified Ru(0001) electrode, the free Ru(0001) areas are covered by a dense OH adlayer with a local coverage of 1 ML after removal of the surface oxide in the cathodic scan. This situation is reached around 300 mV. The first 0.4 ML OHad (local coverage) are reductively desorbed in the reversible peak between 250 and 100 mV. From the close similarity with the behavior of the unmodified Ru(0001) electrode, we conclude that this process proceeds directly, i.e., it is not catalyzed by the Pt monolayer islands. Desorption of OHad species from the Pt monolayer islands is negligible in the range of small Pt coverages. The situation is very different for the reductive desorption of the remaining OHad species and the subsequent adsorption of 0.5 ML hydrogen on the uncovered Ru areas surrounding the Pt islands in the sharp peak between 100 and 50 mV. Here we suggest the following mechanism: Between 250 and 100 mV, H adsorption starts on the Pt monolayer islands, while the surrounding Ru areas are still blocked by adsorbed OH species, and proceeds up to the (rather low) local saturation coverage. On the basis of the data in Figure 8, the OHad coverage on the Pt monolayer islands is negligible under these conditions. The Had can react with OHad species adsorbed on the Ru(0001) substrate right at the edge of the Pt monolayer islands. The resulting free sites can then be occupied by Had species, which either adsorb directly on the Ru(0001) substrate or via spillover from the Pt monolayer islands. Also in this case we favor the indirect process, via spill-over from the Pt islands, since the direct pathway should be possible also in the absence of the Pt monolayer islands, where it could be initiated by local fluctuations in the density of the protecting OH adlayer. Further reaction between Had and OHad on the Ru(0001) substrate can take place, e.g., upon Had diffusion on the Ru terrace toward the OHad covered areas. In that picture, the Pt islands act as nuclei for increasing islands of a H adlayer in the surroundings of an OH adlayer on the Ru(0001) areas. These processes lead to a rapid depletion of the OH adlayer on the Ru substrate and the simultaneous buildup of a H adlayer, until a local coverage of 0.5 ML is reached on the Pt-free Ru(0001) areas. The spill-over of Had species to the Ru terraces is energetically favorable because of the much higher H adsorption energy on Ru(0001) compared to H adsorption on the monolayer Pt islands.34 Since H adsorption on Pt is very fast, the steadystate coverage on the Pt monolayer islands can largely be maintained despite the Had loss due to Had spill-over. Surface diffusion of the Had species must be sufficiently fast so that
Catalytic Influence of Pt Monolayer Islands even on the 0.03 ML Pt-modified surface, with distances of about 25 nm between Pt monolayer islands, the reactive removal of OHad is fast on the time scale of the potential scan rate, resulting in a comparable peak width as obtained on surfaces with much smaller distances between Pt island edges. In total, the data clearly demonstrate that OHad removal does not proceed via a direct reaction with H+ or via desorption of OH- and subsequent reaction with H+ in the electrolyte, since these processes should not be affected by the presence of the Pt monolayer islands but via the indirect process outlined above. In the reverse, anodic scan, the peak reflecting Had oxidation and subsequent OH adsorption is anodically shifted compared to its counterpart in the cathodic scan. Interestingly, we find the same onset potential of about 100 mV for both the cathodic and the anodic processes. The presence of a hysteresis, although it is much smaller than in the absence of the Pt monolayer islands, indicates that kinetic limitations are still present in at least one of the two processes, cathodic H adsorption/OH removal and/or anodic Had oxidation/OH adsorption. In that picture, the anodic shift of the Had oxidation state compared to its cathodic counterpart may be due to a stabilization of the H adlayer on the Ru(0001) electrode, i.e., the formation of a H upd layer, or result from kinetic limitations in the H oxidation process, as it had been discussed for the Pt-free Ru(0001) substrate (see above). A conclusive decision for one of these possibilities is not possible from the present data. The present data provide clear proof, however, for a kinetically limited Had oxidation process on the Pt-free Ru(0001) electrode, since on that surface the onset for H oxidation occurs at even more anodic potentials. A last point to be discussed is the fact that in previous electrochemical studies on Ru(0001) electrodes modified by (potential controlled) platinum electrodeposition13 or by spontaneously adsorbed platinum the effects discussed in this paper have not been observed.10,11 This can be explained by two different effects, either by the different electrolyte used in the latter studies, H2SO4 solution rather than HClO4 solution in refs 10 and 11, and/or by the significant differences in the morphology of these samples.10,11,13 In sulfuric acid solution, the platinum-induced features may be suppressed due to specific adsorption of bisulfate anions. In fact, the pronounced shift of the peaks associated with the removal/reformation of the high coverage OH adlayer on Ru(0001), which are symmetric in HClO4 solution but exhibit a pronounced hysteresis in sulfuric acid electrolyte, with a distinct peak in the cathodic scan and a broad charge distribution in the anodic scan, are clear evidence for such effects at higher potentials (100-250 mV).55 Second, both for Pt electrodeposition and for spontaneous Pt deposition 3D aggregates (“nanoparticles”) with heights between a few layers and 3-6 nm, depending on the exact deposition conditions, were reported to form already in the low sub-monolayer Pt-coverage regime. Considering the rapid change to a more Pt-like electrochemical behavior of thicker (2-4 ML) found in our measurements,48 the driving force for Had spill-over from Pt islands to the Ru areas is much less or even absent for the 3D Pt aggregates. Therefore, we expect the different morphologies obtained for gas phase deposition and electrodeposition to be mainly responsible for the different electrochemical behavior of the Pt-modified Ru(0001) electrodes in the present and previous studies. Finally we want to compare our results for Pt modified Ru(0001) electrodes with the closely related case of oxidation reactions on Ru-modified platinum electrodes. In the latter case, OHad formed on the ruthenium deposit is assumed to act as
J. Phys. Chem. B, Vol. 108, No. 38, 2004 14787 promotor for the oxidation of adsorbed CO or small organic molecules.8,56-58 Both cases can be explained by a bifunctional mechanism, with either the Pt islands enhancing the adsorption of hydrogen or the Ru deposits supporting the formation of OHad, via dissociative adsorption of H2O. The mechanism and the underlying physical origin for the kinetic acceleration, however, are strongly different in the two cases. For Rumodified Pt electrodes, the inherent higher activity of Ru for OH adsorption is the dominant contribution, while for Pt modified Ru electrodes, the catalytic effect of the Pt monolayer islands dominates. The latter is linked to the electronic modifications of the strained Pt monolayer islands, which via the resulting, much lower H adsorption energy allow for a rapid spill-over of the adsorbed hydrogen on the Pt-free Ru areas. This type of catalytic effect has not been reported so far. Conclusions Combining UHV-STM and electrochemical measurements in a new UHV-STM/EC transfer system, we have shown that already small amounts of Pt monolayer islands on a Ru(0001) electrode surface lead to a significant acceleration of the kinetically limited OHad removal and H adsorption in perchloric acid in the cathodic scan and also of the reverse processes in the anodic scan compared to the Pt-free, unmodified Ru(0001) surface. This kinetic acceleration is explained by a novel effect, by a catalytic action of the Pt monolayer islands, which acts as an additional channel for H adsorption and reactive OHad removal on the surrounding Ru areas via spill-over of Had from the Pt islands to the Ru(0001) terraces at potentials where the Ru substrate is still blocked by a 0.6 ML OH adlayer. The driving force for this spill-over process is the significantly lower Had adsorption energy on the strained, pseudomorphic Pt monolayer islands compared to bulk Pt and also to Ru, which in agreement with theoretical predictions and experimental findings for gas-phase adsorption is attributed to electronic strain effects and electronic contributions from the Ru-Pt interface. H adsorption on the Pt monolayer islands is fast, which explains the rapid H adsorption and also the much higher exchange current density, evidenced by the much stronger H2 evolution, on the Pt-modified electrode compared to Pt-free Ru(0001). These energetic modifications of the Pt deposit and the facile Had spillover to the Pt-free areas are the main reason for the strong catalytic influence of even very small amounts of platinum monolayers on the interaction of a Ru(0001) substrate with hydrogen. Finally the data are clear proof of the existence of a thermodynamically stable H upd adlayer up to 100 mV on Ptfree Ru(0001); in a limited potential region above 100 mV, Had is thermodynamically stable against H+ formation but is being displaced by more strongly adsorbing OH adspecies. Acknowledgment. This work was supported by the Landesstiftung Baden-Wu¨rttemberg via the network “Functional Nanostructures” (project B1). We gratefully acknowledge discussions with L. Kibler, A.M. El-Aziz, and Z. Jusys. References and Notes (1) Lamy, C.; Le´ger, J.-M.; Srinivasan, S. In Modern Aspects of Electrochemistry; Bockris, J. O., Conway, B., White, R. E., Eds.; Kluwer Academic/Plenum Publishers: New York, 2001; Vol. 34. (2) Ross, P. N. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998. (3) Maroun, F.; Ozanam, F.; Magnussen, O. M.; Behm, R. J. Science 2001, 293, 1811.
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