Platinum Monolayer Electrocatalysts for O2 Reduction - American

J. Phys. Chem. B 2004, 108, 10955-10964

10955

Platinum Monolayer Electrocatalysts for O2 Reduction: Pt Monolayer on Pd(111) and on Carbon-Supported Pd Nanoparticles J. Zhang, Y. Mo, M. B. Vukmirovic, R. Klie, K. Sasaki, and R. R. Adzic* Materials Science Department, BrookhaVen National Laboratory, Upton, New York 11973-5000 ReceiVed: December 23, 2003; In Final Form: April 21, 2004

The kinetics of oxygen reduction was studied in acid solutions on Pt monolayers deposited on a Pd(111) surface and on carbon-supported Pd nanoparticles using the rotating disk-ring electrode technique. These electrocatalysts were prepared by a new method for depositing Pt monolayers involving the galvanic displacement by Pt of an underpotentially deposited Cu monolayer on a Pd substrate and characterized by scanning tunneling and transmission electron microscopies. The kinetics of O2 reduction shows a significant enhancement at Pt monolayers on Pd(111) and Pd nanoparticle surfaces in comparison with the reaction on Pt(111) and Pt nanoparticles. The four-electron reduction, with a first-charge transfer-rate determining step, is operative on both surfaces. The observed increase in the catalytic activity of Pt monolayer surfaces compared with Pt bulk and nanoparticle electrodes may reflect decreased formation of PtOH. An enhanced atomic scale surface roughness and low coordination of some atoms may contribute to the observed activity. The results illustrate that placing a Pt monolayer on a suitable metal nanoparticle substrate is an attractive way of designing better O2 reduction electrocatalysts. Also, by using this method the Pt content is reduced to very low levels. The Pt mass-specific activity of the Pt/Pd/C electrode is 5-8 times higher than that of the Pt/C electrocatalyst. The noble metal (Pt + Pd) mass-specific activity is two times higher than that of Pt/C.

Introduction Electrocatalytic oxygen reduction plays a major role in electrochemical energy conversion in fuel cells and metal-air batteries, and is equally important for corrosion processes. This reaction has remained the focus of considerable attention because of its complex kinetics and the need for better electrocatalysts for fuel cells.1-3 A further increase in their efficiency critically depends on improving the reaction’s kinetics. In addition, the platinum content of electrocatalysts must be lowered before fuel cells can be broadly applied. The major problem of the oxygen electrocatalysis is the slow kinetics of O2 reduction even on Ptsthe best available electrocatalyst. Particularly troubling is the large loss in potential of 0.3-0.4 V in the initial part of the polarization curves that is the source of a major decline in the fuel cell’s efficiency. A part of this polarization was attributed to the inhibition of O2 reduction caused by OH adsorption on Pt in the potential region 0.75-1 V.2,4 Alloying Pt with transition metals was reported to reduce PtOH formation5 and to produce some improvements in activity. Another problem of existing electrocatalyst technology is the high Pt loading in fuel cell cathodes. Attempts have been made to reduce Pt loadings by decreasing particle size. However, this method is not entirely satisfactory, and the eventual use of alloys would not significantly lower Pt loading since these alloys usually are rich in Pt.6,7 A promising approach to solving both problems is to design electrocatalysts having monolayer amounts of Pt on a surface of suitable metal nanoparticles. It might facilitate modification of the catalytic properties of Pt in the right direction, and, at the same time, achieve a considerable reduction in its loading. A monolayer can be deposited following our new process involving the redox displacement of an adlayer of a non-noble metal by a monolayer of a more noble metal. For example, we

demonstrated that a monolayer of Cu deposited at underpotentials on Au could be replaced by a monolayer of Pt.8 We proved the success of this approach to designing electrocatalysts based on a monolayer of Pt on carbon-supported metal nanoparticles in preparing a submonolayer of Pt on Ru nanoparticles as an anode electrocatalyst for H2 or H2/CO oxidation.9 In this study, we describe O2 reduction on Pt monolayers on Pd(111) and carbon-supported Pd nanoparticle electrocatalysts, which is a part of our broader investigation of the catalytic properties of Pt monolayers on different noble-metal substrates. The O2 reduction on such Pt surfaces may help elucidate the role of the electronic factor in the kinetics of the O2 reaction since the other two factors, viz., the structural effects (local bonding geometry), and the ensemble effect, are expected to show minimal variation from one substrate to another. Experimental Procedures The working electrodes were Pd single crystals of 6 and 8 mm in diameter, obtained from Metal Crystals and Oxides, Cambridge, England. The Pd(111) surfaces were oriented to better than 0.2°. The crystals were polished with diamond pastes and alumina down to 0.05 µm and annealed by inductive heating in an argon atmosphere. Protected by a drop of ultrapure water, the crystal was mounted in the disk-interchangeable rotating disk-ring electrode (RRDE). The Pd/C and Pt/C electrocatalysts (10 and 20 wt %) were obtained from E-TEK, Somerset, NJ. The catalyst particles were deposited on a glassy carbon RRDE with a Pt ring (Pine Instruments, Grove City, PA). The electrochemical measurements were taken at room temperature. A leak-free Ag|AgCl, 3 M Cl- reference electrode was used with a double-junction reference chamber (Cypress, Lawrence, KS). All the potentials are given with respect to a reversible hydrogen electrode (RHE). A platinum flag was used as counter electrode. All measurements were performed at 25 °C.

10.1021/jp0379953 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/22/2004

10956 J. Phys. Chem. B, Vol. 108, No. 30, 2004

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Figure 1. (a) Voltammetry curves for the underpotential deposition of Cu on a Pd(111) surface (solid line) in 0.05 M H2SO4 with 50 mM Cu2+, and for a Pd(111) surface in the absence of Cu (dashed line); sweep rate 20 mV/s. (b) Voltammetry curves for a Pt monolayer on a Pd(111) surface (solid line) obtained by galvanic displacement of the Cu monolayer from Figure 1a, and for a Pd(111) surface (dashed line). The electrolyte solution is 0.05 M H2SO4 and the sweep rate is 20 mV/s.

Figure 2. STM images, (a) 250 nm × 250 nm, and (b) 125 nm × 125 nm, of the Pt monolayer deposited on a Pd(111) surface by galvanic displacement of a Cu monolayer deposited at underpotential. The electrode potential is 0.8 V in 0.1 M HClO4; the tunneling current is 1.24 nA.

The electrocatalysts were prepared by a new method for depositing Pt monolayers involving the galvanic displacement by Pt of an underpotentially deposited (upd) Cu monolayer8 on a Pd substrate. After the deposition of Cu from 50 mM CuSO4 in an 0.10 M H2SO4 solution, the Pd(111) electrode covered with this Cu monolayer was rinsed to remove Cu2+ from the solution film and placed into a 1.0 mM K2PtCl4 in 50 mM H2SO4 solution in an N2 atmosphere. After a 2-min immersion to completely replace Cu by Pt, the electrode was rinsed again. All these operations were carried out in a multi-compartment cell in a N2 atmosphere that prevents the oxidation of Cu adatoms in contact with O2. The Pt monolayer deposition on a Pd(111) surface was verified by voltammetry and by scanning tunneling microscopy (STM) using a Molecular Imaging Pico STM with a 300S scanner and a 300S Pico Bipotentiostat. The cell was made of Teflon, and STM tips were prepared from 80:20 Pt/Ir wire, insulated with Apiezon wax. Preparing an electrode with Pd nanoparticles involved dispersing a certain amount of Pd/C in water, and sonicating it for ca. 5-10 min to make a uniform suspension. Then, 5 µL of this suspension was placed on a glassy carbon disk electrode and dried in air. After depositing Pt, the electrode was covered

by 5 µL of a 4 µg/10 µL Nafion solution (diluted with water from 5% Nafion solution by Aldrich). Finally, the electrode was dried in air. We checked by Auger electron spectroscopy (AES) measurements that no Cu remained after its replacement by Pt. Therefore, we can assume that in this procedure a Cu monolayer is completely oxidatively desorbed, and Cu2+ ions are removed by rinsing. Transmission electron microscopy (TEM) measurements were performed using the JEOL-3000F STEM/TEM, equipped with a Schottky field-emission source operated at 300 keV, an ultrahigh-resolution objective lens pole piece, an energydispersive X-ray spectrometer, and a postcolumn Gatan imaging filter (GIF). Powder samples were dispersed on a Cu mesh grid coated with a lacey amorphous carbon-film. For quantitative analysis, all the images and diffraction patterns were recorded with a slow-scan CCD camera. In-situ X-ray absorption near-edge structure spectroscopy (XANES) measurements were conducted using an electrochemical cell designed for data acquisition in fluorescence transmission modes. The compartments for the working and counter electrodes were separated by a proton exchange membrane (Nafion 117, Du Pont Chemical Co., DE). The

Pt Monolayer Electrocatalysts for O2 Reduction

J. Phys. Chem. B, Vol. 108, No. 30, 2004 10957

Figure 3. TEM micrographs of Pd nanoparticles on a carbon Vulcan XC 72 substrate, supported on an amorphous carbon film: (a) Low magnification overview showing the spread of the Pd particles’ size distribution on the carbon substrate; (b) high-resolution image showing one-dimensional lattice fringes, corresponding to the (111) lattice planes in Pd.

experiments were carried out in beamline X-11A of the National Synchrotron Light Source (NSLS). A Si(111) double crystal monochromator was used. Harmonic rejection was accomplished using a harmonic rejection mirror. The Pt data were obtained by monitoring the transmission signal. The study was done at room temperature in 1.0 M HClO4. The kinetics of oxygen reduction was studied in acid solutions on Pt monolayers deposited on a Pd(111) surface and on carbonsupported Pd nanoparticles using the rotating disk-ring electrode technique. The reaction kinetics shows a significant enhancement at Pt monolayers on Pd(111) and Pd nanoparticle surfaces in comparison with the reaction on Pt(111) and Pt nanoparticles. The four-electron reduction, with a first-charge transfer-rate determining step, is operative on both surfaces. The observed increase in the catalytic activity of Pt monolayer surfaces compared with Pt bulk and nanoparticle electrodes may reflect decreased adsorption of PtOH. Results Pt Monolayers on Pd(111) and Pd Nanoparticles. Figure 1a shows the typical curve for the under-potential deposition (upd) of Cu on a Pd(111) surface, with a single peak at the potential of 0.50 V. The dashed line shows the curve for a Pd(111) surface in the absence of Cu ions in solution. Both curves agree with data in the literature.10 The electrode was held at the potential of 0.32 V and emersed at it from solution. The charge associated with the upd of Cu is 448 µC/cm2, which is close to 490 µC/cm2 needed for depositing a pseudomorphic monolayer of Cu on a Pd(111) surface. Figure 1b shows the voltammetry curves for the Pt monolayer on a Pd(111) surface (solid line) and that for a Pd(111) surface without Pt (dashed line). As expected, the deposition of the Pt monolayer causes a partial blocking of oxide formation on Pd(111) since Pt is oxidized at more positive potentials. Scanning tunneling microscopy (STM) was used for additional characterization of the Pt deposit. Figure 2 shows two images of the Pt monolayer deposit on a Pd(111) surface obtained at 0.8 V in a 0.1 M HClO4 solution. The deposit

Figure 4. Pd particle-size distribution obtained from the micrograph in Figure 3.

consists of interconnected Pt islands, with some holes of monoatomic depth. The steps are those of the Pd surface. A similar deposit was observed for a Pt monolayer on Au(111).8 The Pt islands are probably epitaxial with a Pd(111) surface, given the small mismatch between the Pt and Pd lattices. An interesting property of such a Pt monolayer is an increased atomic scale roughness and the low coordination of many atoms that are considered to have enhanced catalytic activity. Figure 3 shows the TEM images of Pd nanoparticles on a carbon Vulcan XC 72 substrate, while Figure 4 displays the Pd particle size distribution obtained from Figure 3a. The size distribution peaks at about 9 nm. Some of the larger particles appear to be agglomerates of several smaller ones. The atomically resolved image in Figure 3b shows Pd atomic rows with the row spacing corresponding to the (111) facets of the Pd nanoparticle.


Figure 5. (a) Voltammetry curves for the under-potential deposition of Cu on carbon-supported Pd nanoparticles on a glassy carbon disk (solid line) in 0.05 M H2SO4 with 50 mM Cu2+, and the curve for a Pd surface in the absence of Cu (dashed line), 10 nmol Pd (60.9 nmol/ cm2) on carbon electrode; sweep rate 50 mV/s. (b) Voltammetry curves for a Pt monolayer on Pd nanoparticles (solid line) obtained by galvanic displacement of the Cu monolayer from Figure 5a, and for Pd nanoparticles 10 nmol Pd (60.9 nmol/cm2) on carbon electrode (dashed line). The electrolyte solution is 0.05 M H2SO4 and the sweep rate 50 mV/s. The insert in Figure 5b depicts a proposed structural model for the electrocatalyst in the form of a Pd cubooctahedron with a twodimensional Pt adlayer on its surface. A full monolayer of Pt is shown only on one face for clarity.

Figure 5a shows the upd of Cu on Pd nanoparticles (solid line), and the base curve for Pd (dashed line). Several peaks are seen positive to the onset of the Cu bulk deposition at 0.21 V; their associated charge is 2.1 mC/cm2. This charge was used to calculate the effective Pd surface area for Pt deposition that will be available for O2 reduction (vide infra). The geometric electrode surface area was used in this plot, as well as for the other figures in this work. Figure 5b shows the voltammetry curve for the Pt monolayer on Pd nanoparticles obtained by displacing the Cu monolayer from Figure 5a. It depicts a characteristic hydrogen adsorption/ desorption region and the PtOH formation and reduction. As in the case of a Pd(111) surface (Figure 1 b), the deposition of a Pt monolayer shifts PtOH formation/reduction to more positive potentials in comparison with the process on Pd nanoparticles. The insert in Figure 5b is a proposed structural model for the electrocatalyst in the form of a Pd cubooctahedron with a twodimensional Pt adlayer on its surface. A full Pt monolayer is shown only on one face for the sake of clarity.

Zhang et al.

Figure 6. Polarization curves obtained with a rotating disk-ring electrode for O2 reduction on a Pt monolayer on a Pd(111) electrode surface in 0.1 M HClO4 solution. Rotation rates are indicated in the graph; sweep rate 20 mV/s; ring potential 1.27 V; ring and disk areas are 0.126 and 0.283 cm2, respectively; collection efficiency 24%.

Oxygen Reduction on a Pt Monolayer Deposited on a Pd(111) Surface. Figure 6 shows rotating disk-ring electrode measurements of the O2 reduction reaction on the Pt monolayer on a Pd(111) electrode surface in a 0.1 M HClO4 solution. The activity of this surface is considerable, as indicated by the very positive potential at the onset of O2 reduction (0.95-1 V), a half-wave potential of 0.838 V, and the lack of ring currents in the kinetic region. Measurable ring currents are observed only in the potential region of diffusion control, which is unsuitable for analyzing the disk-ring measurements. Since no peroxide is detected at the ring electrode in the kinetic region, on the basis of these measurements it is impossible to conclude whether the reaction involves a four-electron reduction in a direct or series pathway.2 The Koutecky-Levich plots at different potentials show a linear dependence at all potentials (Figure 7). The linearity and the parallelism of these plots are usually taken to indicate firstorder kinetics with respect to molecular oxygen, although this criterion is not very specific.11 From the Koutecky-Levich equation

1 1 1 ) + j nFkcO2 Bω1/2

(1)

where j is current density, F is the Faraday constant, k is the reaction rate constant, cO2 is the concentration of dissolved O2, B is a constant, and ω is the rotation rate, we can calculate from the intercepts of the 1/j axis at 1/ω1/2 ) 0 the kinetic currents of O2 reduction. From the slopes of the KouteckyLevich plots, i.e., the constant B, the number of electrons



Figure 7. The Koutecky-Levich plots at different potentials obtained from the data in Figure 6. The electrode potentials are indicated in the graph. The insert shows the Tafel plot obtained from the kinetic currents jk obtained from these plots.

exchanged in the reduction of an oxygen molecule can be obtained. The experimental value of B ) 0.0451 mA rpm-1/2, evaluated from Figure 7, agrees well with the calculated value of B′ ) 0.044 mA rpm-1/2. The calculation was performed for a four-electron reduction using published data for O2 solubility (1.26 × 10-3 mol-1),12 the solution’s viscosity (1.009 × 10-2 cm2 s-1),13 and oxygen diffusivity (1.93 × 10-5 cm2 s-1).13 A four-electron reduction of O2 is in agreement with negligible currents for H2O2 oxidation on the ring electrode. The Tafel plot obtained from the kinetic currents jk is given as the insert in Figure 7. The plot is linear with a slope of ∼-90 mV/dec above 0.65 V. The slope of -118 mV, usually observed for bulk Pt at high current densities, indicates the first electrontransfer rate-determining step. It appears likely that the intrinsic slope for O2 reduction on Pt/Pd(111) surface is -118 mV/dec, signifying that the first electron exchange is the rate-determining step on this surface, as on Pt (vide infra). Figure 8 compares the kinetics of O2 reduction on Pd(111), Pt(111), and Pt/Pd(111) in a 0.1 M HClO4 solution. The curve for Pt(111) is from reference 13. The activity of the Pt(111) and Pt/Pd(111) surfaces is considerably larger than that of Pd(111). The small increase in the O2 reduction kinetics on the Pt/Pd(111) surface compared with that on Pt(111) is surprising since the latter’s surface, along with Pt(110), is the most active electrocatalyst in HClO4 solutions. Although small, this improvement is of considerable importance because it shows that,

Figure 8. Comparison of the polarization curves for O2 reduction kinetics on Pd(111), Pt(111), and Pt/Pd(111) in a 0.1 M HClO4 solution. The curve for Pt(111) is taken from reference 13. The sweep rate for Pd(111) and Pt/Pd(111) is 20 mV/s, while for Pt(111) it is 50 mV/s.

with a suitable support, it is possible to devise a very active electrocatalyst with only a monolayer amount of Pt whose activity can surpass the activity of bulk Pt. Recent studies of O2 reduction on Pt3Ni and Pt3Co alloys6,7,14,15 may also indicate


Figure 9. Polarization curves obtained with a rotating disk-ring electrode for O2 reduction on a Pt monolayer on carbon-supported Pd nanoparticles in 0.1 M HClO4 solution. Rotation rates are indicated in the graph; sweep rate 10 mV/s; ring potential 1.27 V; ring and disk areas are 0.037 and 0.164 cm2, respectively; collection efficiency 22%.

a possibility to reduce Pt to the top layers, although the “Pt skin”, formed upon dissolution of Ni from the surface layers in these electrocatalysts, is usually several layers thick and supported by Pt-rich alloys. Oxygen Reduction at a Pt Monolayer on CarbonSupported Pd Nanoparticles. Figure 9 displays the rotating disk-ring polarization curves for a monolayer of Pt on carbonsupported Pd nanoparticles. The ring currents are negligible indicating a complete four-electron reduction of O2 on the disk electrode. The electrode consists of a monolayer of Pt deposited on 20 nmol of Pd/C placed on glassy carbon rotating disk electrode. Using the average size of 9 nm for the Pd particles determined by TEM measurements, the ratio of surface atoms to the total number of atoms was estimated as 15% based on Benfield’s calculation16 using an icosahedral particle model. Therefore, for the sample with 20 nmol Pd on carbon, the amount of Pt deposited by replacing the Cu monolayer on the Pd surface is approximately 3 nmol (18 nmol/cm2), or 3.4 µgPt/cm2. This estimate can be verified by calculating the charge associated with putting a Cu monolayer on to Pd nanoparticles. From this charge, the surface area accessible for the electrochemical reaction is obtained, which is exactly what is needed for this analysis. The charge for depositing a Cu monolayer, after correcting for the double layer charging of Pd, is 2.1 mC/cm2 for 10 nmol of Pd on the disk. Assuming there is a one-to-one ratio between the Cu and Pd atoms in a pseudomorphic adlayer, this charge indicates that Cu is deposited on 10.5 nmol/cm2 of Pd surface atoms. This number is in good agreement with the model’s calculation (18/2 ) 9 nmol/cm2) given above. This method of determining the real surface area

Zhang et al. of a Pt monolayer on Pd may be more reliable than measuring H adsorption because of possible interference of H absorption/ adsorption on Pd. A very high activity of a monolayer of Pt on Pd nanoparticles is indicated by a half-wave potential of 0.853 V and by negligible ring-electrode currents. The Koutecky-Levich plots obtained from the data in Figure 9 are shown in Figure 10. For a Pt/Pd(111) surface, a set of parallel straight lines at different potentials is obtained. The Tafel slope of -96 mV/dec fits the polarization curve obtained from the plot of log jk vs E, given as an insert to Figure 10. This slope is comparable to the slope of -90 mV obtained for a Pt/Pd(111) surface discussed above. The size of the Pd nanoparticles (9 nm) used to prepare the electrodes with a Pt monolayer is considerably larger than the optimal size of Pt particles for O2 reduction, i.e., 4 nm according to the study of Peucket et al.17 A reduction in the size of the Pd particles below 9 nm could increase the activity of this bimetallic surface, and the amount of 20 nmol of Pd could be decreased while increasing the number of active Pt sites. Figure 11 graphs O2 reduction on Pd and Pt nanoparticles and on a Pt monolayer on Pd nanoparticles (two different loadings). Comparing the activity of the Pt/C electrocatalyst with an average particle size of 3.1 nm with that of the Pt/Pd/C electrocatalyst of 9 nm is not an adequate assessment because of their different surface areas, but this does not affect our main conclusion. The activity of the Pt monolayer on Pd nanoparticles (10 nmol) is much higher than that of Pd nanoparticles (10 nmol) as indicated by a shift of the half-wave potential by 120 mV to positive values. More importantly, the activity of this surface is somewhat higher (25 mV in half-wave potential) than that of Pt nanoparticles (10 nmol). For the Pd loading of 10 nmol, or 6.4 µgPd/cm2, the amount of Pt in the monolayer on this surface is 1.5 nmol, or 1.7 µgPt/cm2. The half-wave potential for this electrode is 0.838 V. It is important to note that the activity of this surface is higher than that of 10 nmol (12 µgPt/cm2) of Pt nanoparticles, despite the fact that the Pd nanoparticles are 9 nm and the Pt nanoparticles are 3 nm, and the former have a smaller real surface area. The electrode consisting of a Pt monolayer on 20 nmol Pd had the highest activity, mainly due to the increased Pt surface area (Figure 11). The higher activity of Pt monolayers electrocatalysts compared with those of Pt and Pd indicate a synergetic effect, which is particularly interesting since the activity of the Pt/Pd surface surpasses that of Pt nanoparticles with seven times larger loading. In addition to the polarization curves, a useful way of comparing the activities of various electrocatalysts is by their mass-specific activities. Figure 12a shows the Pt mass-specific activity of the three electrodes containing Pt expressed as the current at 0.85 and 0.80 V divided by the Pt mass. The electrodes having a Pt monolayer have 5-8 times higher activity than the electrode with Pt nanoparticles. This finding underlines the importance of the monolayer-level electrocatalysts that can reduce the amount of Pt in the fuel cell’s electrode down to very low levels. If the total metal content is taken into account, i.e., the mass of Pt and Pd, the plot shows still significantly higher activity of the Pt monolayer electrocatalysts (Figure 12b). Discussion The catalytic properties of bimetallic surfaces consisting of metal monolayers on metal single-crystal surfaces have been extensively studied in an ultrahigh vacuum (UHV) environment18 and to a lesser extent in electrochemical systems.19-22 In many cases, the formation of a surface metal-metal bond



Figure 10. The Koutecky-Levich plots at different potentials obtained from the data in Figure 9. The electrode potentials are indicated in the graph. The insert shows the Tafel plot obtained from the kinetic currents jk obtained from these plots.

produced large changes in the electronic properties of the metal adlayer, and pronounced differences were observed in the reactivity of some transition metal monolayers on different substrates.18 Some interpretations of these effects include shifts of core levels due to a charge transfer between monolayer and the substrate18 or changes in the density of states near the Fermi level.23 Meitzner et al.’s earlier work on bimetallic alloys and nanoparticle catalysts linked trends in the reactivity to d-band occupancy and electronegativity,24 which also was used to interpret the electrocatalytic activities of several alloys for the O2 reduction reaction.25 An increased 5d vacancy of Pt, as a result of its interaction with a substrate metal, is believed to increase the interaction of O2 and Pt, thereby enhancing the catalytic activity of a Pt “skin” (several layers) on a PtNi, or PtFe alloy.26 Stamenkovic et al.14,15 ascribed the enhanced activity of a Pt “skin” in these alloys to a smaller formation of PtOH on that surface. A more recent description of the activity of metal monolayers was proposed by Nørskov and co-workers27 based on density functional theory calculations. According to them, the reactivity scales well with shifts in the center of the d-band for strained crystals and metal overlayers. Some experimental support has been reported. For example, the electronic factors in bimetallic systems showed a parallelism between the change of adsorption energy and the d-band center shifts for CO and H2.28 The data

for a Pt submonolayer on Ru nanoparticles29 and Ru(0001),30 where Pt is likely to be compressed (4% difference in lattice constants), indicate a reduced adsorption of CO in comparison with Pt. Our results, presented above, show increased activities of Pt monolayers on Pd substrates for O2 reduction, surpassing the catalytic activity of Pt in both the single crystal and nanoparticle forms. The possible factors that determine such activity may include a mismatch in the lattice constants between the Pt monolayer and the Pd substrate and the changes in the d-band properties of Pt caused by its interaction with Pd. A mismatch between the lattice constants is small, only 0.8%. Consequently, it may generate a very small compressive strain in a pseudomorphic Pt monolayer on Pd. Given the small difference in the lattice constants of Pt and Pd, and the fact that the Pt monolayer is not entirely pseudomorphic, a decrease in activity produced by compression probably is negligible. The effect of the d-band filling of the substrate is also expected to be slight since the fractional filling of the d-bands of Pd and Pt is the same. However, their interaction can be expected to result in some charge redistribution through hybridization of the states from each atom. The DFT calculations show a small increase of activity for a pseudomorphic monolayer of Pt on a Pd(111) surface compared with Pt(111).27 For the inverted system, i.e., a Pd monolayer on a Pt(111) surface, O2 reduction activity


Zhang et al.

Figure 11. Comparison of polarization curves for O2 reduction on Pd (10 nmol) and Pt (10 nmol) nanoparticles, and a Pt monolayer on Pd nanoparticles (10 and 20 nmol Pd). The electrode geometric area is 0.164 cm2.

decreased in acid solution,31 but there was an increase in alkaline electrolytes.20 The Tafel slope of about -90 mV/dec probably reflects a different state and coverage of PtOH on the Pt/Pd(111) surface in comparison with bulk Pt or Pt(111). Slopes of -59 and -118 mV/dec were observed for polycrystalline Pt1,2 surfaces at low and high current densities in nonadsorbing electrolytes, respectively. Markovic et al. recently reported a single slope of -75 mV for Pt(111) in HClO4 solution.13 Several authors have discussed the role of PtOH in determining a change of the Tafel slope. In the sixties, Damjanovic and his colleagues attributed the “low” Tafel slope at high potentials in HClO4 solutions to the Temkin conditions for the adsorption of reaction intermediates, mainly PtOH.32 Several workers proposed that PtOH is not derived from the reduction of O2, but rather from the reaction of H2O with Pt, and that causes inhibition of O2 reduction.33-35 Albu and Anderson36,37 suggested that the cathodic transfer coefficient, β, may vary with potential from about 0.5 to near 1.0, causing changes in the Tafel slope from -118 to -59 mV with increasing potential. This proposition, however, does not explain why the Tafel slope is independent of the potential for O2 reduction on Pt(111) in H2SO4 solutions. Markovic et al.13 proposed a model for the kinetic current, which assumes that the adsorbed OH can alter the adsorption energy of the O2 reduction intermediates, thus having an energetic effect on the kinetics of O2 reduction and on the Tafel slope in addition to blocking the Pt sites. Recently, we demonstrated that, in addition to site blocking, PtOH had a negative electronic effect on O2-reduction kinetics.38

To quantitatively evaluate these effects of the PtOH species, the following equation was used:38

jk(E) ) - j /0(1 - γOHθOH(E))m exp(2.303(E - E0 OHθOH(E))/b*) (2) where j /0 and b* are the intrinsic exchange current and Tafel slope for an adsorbate-free Pt surface. A (1 - γθ)m term accounts for the geometric site-blocking effect, while the electronic effect is described by a coverage-dependent potential shift through the exponential term, “-θ”, and m is the number of Pt sites involved in the rate-determining step. From eq 2, it follows that a decrease in the coverage of PtOH can enhance O2 reduction kinetics. This possibility also was discussed for a Pt “skin” on a Pt3Ni support.14 Figure 13 shows that the formation of PtOH on the Pt monolayer on Pd nanoparticles is considerably smaller than on the Pt nanoparticles’ surfaces, which might be partly responsible for the observed enhanced O2-reduction kinetics of the Pt monolayer electrocatalyst. In addition to this effect, an enhanced atomic scale surface roughness and low coordination of a considerable number of surface atoms may contribute to the observed activity. Strong evidence of delayed oxidation of a Pt monolayer on Pd nanoparticles in comparison with the oxidation of Pt nanoparticles was obtained from in-situ XANES measurements as a function of potential. Figure 14a shows the Pt L3 edge spectra obtained with the Pt/Pd electrocatalysts at four different potentials. Only at the highest potentials is there an increase in



Figure 12. Pt mass-specific activities of Pt (10 nmol) nanoparticles, and a Pt monolayer (1.3 and 2.4 nmol) on Pd nanoparticles (10 and 20 nmol Pd) expressed as a current at 0.8 and 0.85 V. Lower panel shows the total metal (Pt + Pd) mass-specific activity.

Figure 14. XANES spectra obtained with the Pt/Pd (a) and Pt/C (b) electrocatalysts at four different potentials in 1 M HClO4. (c) A comparison of the change of the absorption peak as a function of potential for Pt/Pd/C and Pt/C.

Figure 13. Voltammetry curves for a Pt monolayer on Pd nanoparticles (solid line) obtained by galvanic displacement of the Cu monolayer from Figure 5a, and for Pt nanoparticles containing 10 nmol Pt on a carbon electrode (dashed line). The electrolyte solution is 0.05 M H2SO4 and the sweep rate 50 mV/s.

the intensity of white line as a consequence of the PtOH formation causing a depletion of Pt’s d-band.25 The increase in the intensity of the white line for the Pt/C electrocatalyst commences at considerably less positive potentials (Figure 14,

parts b and c). This shows that the oxidation of a Pt monolayer on palladium substrate requires higher potentials than platinum nanoparticles on a carbon substrate. A comparison of the spectra at 0.47 V shows a negligible change in the electronic properties of a Pt monolayer on Pd in comparison with Pt/C (Figure 14c). No effect of the potential change is observed below 1 V. This corroborates the above proposition of the role of PdOH in suppressing a PtOH formation. Conclusions The kinetics of oxygen reduction was studied in acid solutions on Pt monolayers on a Pd(111) surface and carbon-supported Pd nanoparticles using the rotating disk-ring electrode technique. Platinum monolayers deposited on Pd surfaces can be very active electrocatalysts for O2 reduction. The kinetics of O2 reduction shows a small but significant enhancement with Pt monolayers on Pd(111) and Pd nanoparticle surfaces in com-

10964 J. Phys. Chem. B, Vol. 108, No. 30, 2004 parison with the reaction on Pt(111) and Pt nanoparticles. The four-electron reduction, with a first-charge transfer-rate determining step, is operative on both Pt/Pd(111) and Pt/Pd/C surfaces, with a very small amount of H2O2 detected on the ring electrode in the hydrogen-adsorption potential region. The observed rise in the catalytic activity of Pt monolayer surfaces compared with Pt bulk and nanoparticle electrodes may be partly caused by decreased PtOH adsorption. The results presented above illustrate that placing a Pt monolayer on nanoparticles of a suitable metal substrate is an attractive way of designing improved O2 reduction electrocatalysts, and of reducing Pt loadings in fuel-cell electrodes. Our new method for the controlled deposition of a metal monolayer, which involves a galvanic displacement of an upd metal monolayer, offers a unique way of depositing a metal monolayer on carbon-supported metal nanoparticles in a surface-limited reaction. This cannot be achieved using either metal vapor deposition in UHV or chemical vapor deposition. The fact that it is not producing pseudomorphic monolayers is not a drawback, but rather a useful feature of this method. A disordered monolayer, having imperfections and low-coordination atoms, is likely to be more active than a pseudomorphic or a uniform monolayer. An additional support for this view comes from surface-enhanced Raman spectroscopy (SERS) measurements with a Pt layer on Au prepared using the upd monolayer displacement method. The best SERS spectra reported so far from Pt surfaces were obtained with it, which indicates a high reactivity of such Pt monolayers.39 Further work utilizing this approach seems quite promising for both reducing the loading of noble metals, and increasing the activity of fuel-cell catalysts. Acknowledgment. The U.S. Department of Energy, Divisions of Chemical and Material Sciences supports this work, under Contract No. DE-AC02-98CH10886. References and Notes (1) Tarasevich, M. R.; Sadkowski,: Yeager, E. In ComprehensiVe Treatise of Electrochemistry, Vol. 7; Conway, B. E., Bockris, J. O’M., Yeager, E., Khan, S. U. M., White, R. E., Eds.; Plenum Press: New York, 1983; pp 301-398. (2) Adzic, R. R. In Frontiers in Electrochemistry, Vol. 5, Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1998; p 197. (3) Gottesfeld, S.; Zawodzinski, T. A. In AdVances in Electrochemical Science and Engineering; Alkire, R. C., Kolb, D. M., Eds.; Wiley-VCH: Weinham, 1997; Vol. 5. (4) Anderson, A. Electrochim. Acta 2002, 47, 3759. (5) Mukerjee, S.; Srinivasan, S.; Soriaga, M.; McBreen, J. J. Phys. Chem. 1995, 99, 4577. (6) Toda, T.; Igarashi, H.; Watanabe, M. J. Electroanal. Chem. 1999, 460, 258.

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Platinum Monolayer Electrocatalysts for O2 Reduction - American

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