Surface Chemistry of CO on Pt(100)−Bimetallic Surfaces

Materials Sciences Division, Lawrence Berkeley National Laboratory, ... M.J Ball , C.A Lucas , N.M Marković , V Stamenković , P.N Ross .... Congratu...
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1998

Langmuir 2000, 16, 1998-2005

Surface Chemistry of CO on Pt(100)-Bimetallic Surfaces: Displacement Effects N. M. Markovic´,*,‡ B. N. Grgur,‡ C. A. Lucas,§ and P. N. Ross‡ Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, and Oliver Lodge Laboratory, Department of Physics, University of Liverpool, Liverpool L69 7ZE, United Kingdom Received March 4, 1999. In Final Form: August 10, 1999 The surface chemistry of coadsorbed CO and metal adatoms on the Pt(100) surface in acid electrolyte has been studied by means of rotating ring disk electrode (RRDE) measurements. These results showed that Cu and Pb adatoms are readily displaced from the Pt(100) surface by CO. The consequence of the observed displacement phenomenon is that the kinetics of the electrooxidation of CO on Pt(100) surfaces in Cu2+- (Pb2+-) containing solution is the same as in Cu2+- (Pb2+-) free solution.

1. Introduction The electrooxidation of CO and small organic molecules on metal electrodes has been investigated in considerable depth in the last 15 years.1-24 In these studies, substantial differences in catalytic activity were observed between the clean metal surface and the same metal modified by a submonolayer of metal adatoms that are deposited at * Corresponding author: phone (510) 486-2956; FAX (510) 4865530; e-mail [email protected]. ‡ University of California. § University of Liverpool. (1) Adzic, R. R. In Modern Aspects of Electrochemistry; Bockris, J. O’M., Conway, B. E., White, Eds.; Plenum: New York, 1990; Vol. 2. (2) Beden, B.; Bilmes, S.; Lamy, C.; Leger, J. M. J. Electroanal. Chem. 1983, 149, 295. (3) Kitamura, F.; Takeda, M.; Takahashi, M.; Ito, M. Chem. Phys. Lett. 1987, 143, 318. (4) Sun, S.; Clavilier, J.; Bewick, A. J. Electroanal. Chem. 1988, 240, 147. (5) Leung, L.-W.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. 1988, 92, 6985. (6) Zurawski, D.; Wasberg, M.; Wieckowski, A. J. Phys. Chem. 1988, 94, 2076. (7) Feliu, J. M.; Orts, J. M.; Fernandez-Vega, A.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1990, 296, 191. (8) Kinomoto, Y.; Watanabe, S.; Ito, M. Surf. Sci. 1991, 242, 538. (9) Weaver, M. J.; Chang, S.-C.; Leung, L.-W.; Jiang, X.; Rubel, M.; Szklarczyk, M.; Zurawski, D.; Wieckowski, A. J. Electroanal. Chem. 1992, 327, 247. (10) Clavilier, J.; Albalt, R.; Gomez, R.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1992, 330, 489. (11) Orts, J. M.; Fernandez-Vega, A.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1992, 327, 191. (12) Kita, H.; Narumi, H.; Ye, S.; Naohara, H. J. Appl. Electrochem. 1993, 23, 589. (13) Villegas, I.; Weaver, M. J. J. Phys. Chem. 1994, 101, 1648. (14) Couto, A.; Perez, M.; Rincon, A.; Gutierrez, C. J. Phys. Chem. 1996, 100, 19538. (15) (a) Wieckowski, A.; Rubel, M.; Gutirrez, C. J. Electroanal. Chem. 1995, 382, 97. (b) Palaikis, L.; Zurawski, D.; Hourani, M.; Wieckowski, A. J. Electroanal. Chem. 1988, 199, 183. (16) Kita, H.; Naohara, H.; Nakato, T.; Taguchi, S.; Aramata, A. J. Electroanal. Chem. 1995, 386, 197. (17) Hayden, B. E.; Murray, A. J.; Parsons, R.; Pegg, D. J. J. Electroanal. Chem. 1996, 409, 51. (18) Markovic´, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. Surf. Sci. 1997, 384, L 805. (19) Gomez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci. 1998, 410, 48. (20) Markovi, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. J. Phys. Chem. B 1999, 103, 487. (21) Lucas, C. A.; Markovi, N. M.; Ross, P. N. Surf. Sci. 1999, 425, L 381. (22) Marinkovic´, N. M.; Wang, J. X.; Marinkovic´, J. S.; Adzi, R. R. J. Phys. Chem. 1999, 103, 139, and references therein.

potentials more positive than the potential of reversible deposition of the bulk phase, so-called “underpotential deposition” (UPD). Both pronounced catalytic effects and strong inhibitory effects of the UPD adatoms have been observed in the electrooxidation rates of methanol, formaldehyde, and formic acid on a platinum electrode. For example, while substantial enhancement of the formic acid oxidation was observed on Pbupd- (Biupd-) modified Pt(hkl) surfaces,23a,d the electrooxidation rates of the methanol oxidation were diminished on Cuupd- (Biupd-) modified Pt(hkl) surfaces.23c,d In contrast to the oxidation of small organic molecules, however, with the exception of a study by Chang and Weaver24 and recently by Marinkovic et al.,22 the surface chemistry of CO on UPDmodified Pt(hkl) surfaces is largely lacking. Given that the CO oxidation reaction is currently one of the most important in electrocatalysis and that adsorbed CO is a common intermediate in the electrooxidation of small organic molecules, it is apparent that the surface chemistry of CO on well-characterized UPD modified surfaces is a subject that should be explored in greater detail. This paper will focus on the study of the effects of Cuupd (Pbupd) adatoms on the electrooxidation of CO in a solution containing weakly adsorbing anions. We do not describe in detail the formation and characterization of Pt(100)Cuupd (Pbupd) bimetallic surfaces as these are discussed in a recent series of publications.25-28 The subject of this work is, therefore, limited to the interactions of the Pt(100)Cuupd (Pbupd) systems with CO dissolved in solution (denoted hereafter as COb) in order to correlate this information with the catalytic properties of the UPDmodified surfaces for the electrooxidation of COb. In this (23) See for example: (a) Clavilier, J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1988, 243, 419. (b) Haner, A.; Ross, P. N. J. Chem. Phys. 1991, 95, 3740. (c) Markovic, N.; Ross, P. N. J. Electroanal. Chem. 1992, 330, 499. (d) Chang, S.-C.; Ho, Y.; Weaver, M. Surf. Sci. 1992, 265, 81. (e) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Electroanal. Chem. 1993, 361, 269. (f) Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1994, 376, 151. (g) Wasmus, S.; Vielstich, W. J. Electroanal. Chem. 1993, 359, 175. (h) Baldauf, M.; Kolb, D. M. J. Chem. Phys. 1996, 100, 11475. (24) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1991, 241, 11. (25) Lucas, C. A.; Markovic´, N. M.; Ross, P. N. Phys. Rev. B 1998, 57, 13 184. (26) Markovic´, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. Electrochim. Acta 1998, 44, 1009. (27) Lucas, C. A.; Markovic´, N. M.; Ross, P. N. Langmuir 1998, 13, 5517. (28) Markovic´, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. J. Chem. Soc., Faraday Trans. 1998, 94, 3373.

10.1021/la990255u CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/1999

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study displacement of Cu and Pb metal adatoms from the Pt(100) surface by COb was observed. By utilizing rotating ring disk electrode (RRDE) measurements, we were able to monitor displacement of Cuupd and Pbupd layers by COb from the Pt(100) surface, even in the potential region where a full monolayer of the UPD metal is initially present in COb-free solution. Thus, there is neither enhancement nor inhibition of the COb electrooxidation kinetics by Cuupd or Pbupd adatoms on Pt(100). 2. Experimental Section 2.1. Electrochemical Measurements. The Pt(100) electrode (miscut ≈ 0.18°) was prepared by the flame annealing method in a hydrogen flame. Flame annealing was followed by cooling of the crystal in a closed quartz tube purged with argon. Following flame annealing and cooling in argon, with the surface protected by a drop of pure water, the crystal was placed face-down on a polypropylene film covered with a thin film of water. After removal of residual water from the edge of the crystal, the cylindrical sample was pressed into the disk position29,30 and immersed in solution under potential control. In RRDE experiments the UPD of Cu and UPD of Pb were conducted at relatively low Cu and Pb concentrations (8 × 10-5 M) in order to minimize unshielded ring currents at the rotation rate of 900 rpm used in this study. If either Cu2+ or Pb2+ is adsorbed onto the Pt(100) disk electrode, the surface coverage by Cuupd and Pbupd (θCu,Pb) can be assessed from ring currents in either potentiodynamic or potentiostatic measurements:30

∫

1 (ir - i∞r ) dE 1 v θCu,Pb ) Q AnN (i 1∫ ∞

θCu,Pb )

0

Q

r

- i∞r /N) dτ An

(1)

Figure 1. (a) CO stripping voltammetry on the Pt(100) surface in 0.1 M HClO4 solution free of COb (s) and base voltammetry of Pt(100) in 0.1 M HClO4 (- - -). (b) Potentiodynamic COb oxidation current densities on Pt(100) in 0.1 M HClO4 saturated with COb. Sweep rate 20 mV/s; rotation rate ω ) 900 rpm. distilled water. All potentials are referred to the saturated calomel electrode (SCE) at room temperature.

(2)

where ir∞ and ir are unshielded and shielded ring currents, respectively, N is the collection efficiency (N ) 0.2 ( 5%), v is the sweep rate, A denotes the disk area (0.283 cm2), n ) 2 is the number of electrons for Cu2+ (Pb2+) reduction to Cu0 (Pb0) on the ring electrode, and Q is the charge corresponding to monolayer (ML) formation (1 ML ) 1 adatom/Pt) of adatoms on Pt(100) based on the surface atomic density of Pt(100)-(1 × 1) (1.3 × 1015 atoms/cm2), and assuming two electron transfers per adsorbed adatom. The maximum coverage by Pb will be 0.45 V).28 The stripping voltammetry of COad shows that, upon sweeping of the potential positively from -0.2 V, the onset of COad oxidation in the Ar-purged solution commences at ≈-0.1 V, forming an almost featureless preoxidation wave over the potential region of ≈-0.1 to 0.35 V. This nearly featureless prewave oxidation region differs from that reported by other groups in 1 M HClO4,15a phosphate buffer solution,16 and sulfuric acid solution,31 where some small peaks were observed in the preoxidation potential region. We suggest that the shape of the

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voltammetry curve in this potential region depends on the delicate balance between the coverage of COad, OHad, and the third party (spectator species), viz., adsorbed anions from the supporting electrolyte. While anions of the supporting electrolytes have negligible effect on the adsorption of COad, the specific adsorption of anions indeed has a strong effect on OHad adsorption, blocking adsorption of sufficient OHad for CO oxidation reaction to proceed. In the preoxidation region, therefore, the surface coverage of OHad is strongly affected by the nature and the concentration of the acid anion. For example, in perchloric acid solutions the shape of the oxidation prewave is most likely determined by the concentration of Cl-, which is present as an impurity in even the most meticulously prepared perchloric acid solution.23c Because the Cl- level is adventitious in perchloric acid solutions, results in these solutions between different groups can be varied. Along the same lines, the difference in the shape of the prewave oxidation region (as well as in the position of the main COad stripping peak) observed in solution containing a high concentration of phosphate anions16 is presumably due to the different role these anions have in the blocking of OH adsorption. We obtained nearly the same voltammetry as reported in ref 16 when we used 0.5 H2SO4 as supporting electrolyte.31 Figure 1a shows that the prewave oxidation region is followed by the fast oxidation of COad, which in stripping voltammetry is characterized by the sharp peak at ≈0.4 V. The successive negative-going sweep and the second cycle trace accurately the base voltammetry of a clean Pt(100) surface. The results for the electrooxidation of gaseous COb dissolved in the bulk electrolyte (Figure 1b) show that the continuous supply of COb to the electrode surface causes the onset potential for the electrooxidation of COad to increase by ≈0.25 V. The potential shift in the oxidation of CO in solution containing COb, relative to COad oxidation, arises from the competition between COb and H2O for the free Pt sites that are created each time a COad molecule is oxidized. This is discussed in detail in refs 20 and 31. We note that the kinetics of dissolved COb on the stationary electrode14-16 is not comparable with results from our RRDE. Relatively large currents at low overpotentials on stationary electrodes arise from slow diffusion of COb through a relatively thick diffusion layer (ca. 0.05 cm), resulting in little or no readsorption of CO as COad is oxidized. In contrast, in the RRDE measurements, Figure 1b, hydrodynamic flow is imposed on the solution, giving well-defined and calculable transport of COb through the Nernst diffusion layer of thickness of only 0.005 cm, resulting in a high flux of CO to the electrode. The simple Langmuir-Hinshelwood model for the competitive adsorption of COad and OHad for free Pt sites (that are created each time COad is oxidized) can account for differences between the COb oxidation shown in Figure 1b and those shown in refs 14-16. With a more effective supply of COb to the Pt(100) electrode in the RRDE measurements, active platinum centers for the adsorption of OHad can easily be poisoned with COad, and consequently the activity of Pt electrode shifts negatively versus measurements on the stationary electrode. 3.2. UPD of Cu and Pb on the Pt(100) Surface. Potentiodynamic ring-disk curves of Pt(100) in 0.1 M HClO4 in the presence of 8 × 10-5 M Cu2+ are shown in Figure 2. Independent of the rotation rate, the voltammetry of Pt(100) in solution containing Cu2+ ions illustrates that deposition of Cu2+ occurs over a broad potential range (31) Markovic´, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. J. Phys. Chem. 1999, 103, 9616.

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Figure 2. (a) Potentiodynamic curves for Cu UPD on the Pt(100) disk electrode in a RRDE assembly at 900 rpm in 0.1 M HClO4 with 8 × 10-5 M Cu2+ (s) and base voltammetry of Pt(100) in 0.1 M HClO4 (- - -). (b) Potentiodynamic ring-shielding (collection) current with the ring being potentiostated at -0.275 V. Inset: Integrated charge for the stripping of Cuupd from the disk electrode, assessed from the ring potentiodynamic curve recorded in the positive sweep direction, QCud ) QCur/N. Sweep rate 50 mV/s; rotation rate ω ) 900 rpm.

without the formation of distinct peaks. In contrast, current peaks from the dissolution of the overpotential deposit (OPD; ECu2+/Cu0 ) 0.22 V vs SCE) and underpotential deposit (UPD) are clearly resolved in the stripping voltammetry, i.e., a sharp peak at ca. 0.1 V for Cu OPD is followed by a second Cu UPD peak at 0.6 V. This irreversible behavior supports our previous observation that, in the absence of relatively high concentration of bisulfate and halide anions, Cu deposition is a kinetically limited reaction.30 The maximum surface coverage of Cuupd estimated from the positive-going voltammetric sweep (Cuupd stripping) experiment in Figure 2 is ca. 400 ( 10% µC/cm2, yielding a Cu UPD coverage of ≈ 0.95 ( 10% ML; see inset. A typical voltammogram of the Pt(100) surface in 0.1 M HClO4 with 8 × 10-5 M Pb2+ is shown in Figure 3. In contrast to the UPD of Cu, and on account of the relatively fast kinetics of Pb2+ in perchloric acid solution,15 the initial deposition of Pb2+ is manifested as a broad peak at ca. 0.4 V. Completion of a close-packed monolayer of Pb, however, occurs in a wide potential region (-0.22 < E < 0.35 V), as illustrated by the RRDE measurements in Figure 3. On the reverse sweep, the initial slow stripping of Pbupd is followed by a relatively sharp stripping peak at ca. 0.5 V. Note that there is some asymmetry between the positive and negative sweep direction in the cyclic voltammetry of Figure 3. Considering that the shape and the position of Pb UPD peaks are strongly dependent on the sweep rate and/or concentration of Pb2+ in electrolyte, we have suggested that this asymmetry is due to a combination of these factors; for details see refs 27 and 28. As the kinetics of stripping of UPD Pb are relatively fast, even in perchloric acid solution, the resulting surface coverage of Pbupd on the Pt(100) disk electrode can be assessed from the ringcollection experiments, yielding a charge of 255 ( 5% µC/

Displacement of CO on Pt(100)-Bimetallic Surfaces

Figure 3. (a) Potentiodynamic curves for Cu UPD on the Pt(100) disk electrode in a RRDE assembly at 900 rpm in 0.1 M HClO4 with 8 × 10-5 M Pb2+ (s) and base voltammetry of Pt(100) in 0.1 M HClO4 (- - -). (b) Potentiodynamic ring-shielding current with the ring being potentiostated at -0.6 V. Inset: Integrated charge for the stripping of Pbupd from the disk electrode, assessed from the ring potentiodynamic curve recorded in the positive sweep direction, QPbd ) QPbr/N. Sweep rate 50 mV/s; rotation rate ω ) 900 rpm.

cm2, which corresponds to a Pb coverage of ∼0.62 ( 5% ML; see inset of Figure 3 and ref 28. Given the difference in atomic size between Pb and Pt, a full monolayer of Pb on the Pt surface would correspond to a coverage of 0.63 ML. Thus, close to the Nernst potential for the Pb deposition (EPb2+/Pb0 ) -0.27 V vs SCE), a full monolayer of Pb adatoms is formed on the Pt(100) surface. 3.3. Electrooxidation of COb at the Pt(100)-Cuupd and Pt(100)-Pbupd Surfaces. By use of either shielding or collection properties of the RRDE, it is possible to monitor the flux of Cu2+ and Pb2+ to (from) the Pt(100) electrode during the electrooxidation of COb on the Pt(100)-Cuupd (Pbupd) electrode. This type of experiments makes it possible to study both the influence of COad on the interaction of Cuupd and Pbupd with the platinum surface atoms and the effects of the surface coverage by Cuupd and Pbupd on the kinetics of the electrooxidation of COb. 3.3.1. Electrooxidation of COb on the Pt(100)-Cuupd Surface. Direct information regarding the effects of Cu UPD on the kinetics of CO oxidation on the Pt(100) electrode was obtained by measuring the potentiodynamic curves for the electrooxidation of gaseous COb dissolved in the bulk electrolyte containing 8 × 10-5 M Cu2+ (Figure 4a). In this experiment, a full monolayer of Cuupd was formed by holding the potential at ≈0.2 V for 3 min before CO was introduced into the cell. The solution was then purged with CO for 5 min while the electrode potential was held at 0.2 V. Upon sweeping the potential positively from 0.2 V, the onset of COb oxidation on the “Pt(100)Cuupd” surface commences at ca. 0.3 V, i.e., at the same

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Figure 4. (a) Potentiodynamic curves for the electrooxidation of COb on the Pt(100) disk electrode in 0.1 M HClO4 saturated with COb (- - -) and potentiodynamic curves for the electrooxidation of COb on the Pt(100) disk electrode in 0.1 M HClO4 saturated with COb and containing 8 × 10-5 M Cu2+ (s). (b) Ring current corresponding to the change in the Cu2+ flux from ir∞ during COb oxidation on the disk electrode. Inset: Ringcurrent transients for displacement of Cuupd with COb at a disk potential of -0.1 V. Ring potential -0.275 V; sweep rate 10 mV/s; rotation rate ω ) 900 rpm.

potential that COb oxidation occurs on the clean Pt(100) surface in solution free of Cu2+. When first observed, we considered this a surprising result (!). In subsequent sweeps (-0.4 < E < 0.7 V), polarization curves for the electrooxidation of COb were almost identical with the electrooxidation of COb on a bare Pt(100) surface; see Figure 4a. This implies that Cuupd adatoms may be displaced from the Pt(100) surface by COb, and consequently COb oxidation occurs on an unmodified Pt(100) surface even in electrolyte containing Cu2+ ions. An alternative explanation is that the electronic properties of 1 ML of Cuupd on the platinum substrate are significantly different from those of a bulk Cu electrode surface. To resolve this issue, we used either the ring-shielding or the ring collection properties of the RRDE to measure the change in the surface coverage by Cuupd during the electrooxidation of COb on the Pt(100) disk electrode in solution with Cu2+. The effect of COb on Cu deposition becomes apparent in the voltammetry of Figure 4b, with two major distinguished features. First, in the presence of COb, no change in the ir∞ was observed in the Cu UPD potential region (0.2 V < E < 0.7 V), implying that in a solution containing COb, UPD of Cu is completely inhibited by COad. Second, if the disk potential is swept in the Cu OPD potential range, ≈-0.4 > E > 0.1 V, then the associated deposition/desorption of Cu is mirrored by an increase of the ring current above its unshielded value, indicating that multilayer deposition of Cu can still take place in the presence of COb, Figure 4b. Additional information regarding the displacement of Cuupd from Pt(100) by COb can be deduced from potentiostatic ring-collection experiments.30 A typical current versus time response for displacement of Cuupd by COb at 0.2 V is shown in the inset of Figure 4. Displacement of Cuupd by COb from Pt(100) is evident from the positive

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Figure 5. (a) Potentiodynamic curve (second sweep) for the electrooxidation of COb on the Pt(100) disk electrode in 0.1 M HClO4 solution containing 8 × 10-5 Pb2+. (b) Potentiodynamic curve (first sweep) for the electrooxidation of COb on the Pt(100) disk electrode in 0.1 M HClO4 containing both Pb2+ and COb (s). The Pb2+ ion-specific currents (ISC) corresponding to the change in the Pb2+ flux from ir∞ in the first positive sweep (the shaded area for the positive currents) and in the subsequent negative sweep (the shaded area for the negative currents) during COb oxidation on the disk electrode are shown. Inset: Ring shielding current transients during stripping of a Pbupd layer at 0.62 V after stepping from 0 V in COb-free solution (s) and in COb-containing solution after 2 min (- - -) and after 7 min (‚‚‚) of holding the disk potential at 0 V. Ring potential -0.65 V; sweep rate 20 mV/s; rotation rate 900 rpm.

deviation of the ring current transient from ir∞. The amount of Cuupd displaced by COad is obtained simply by integrating the ring-current transient, as we have shown in ref 30. In this case, the total charge passing the interface during the displacement of Cuupd is ≈395 ( 10% µC/cm2, corresponding to a surface coverage of ≈0.92 ( 10% ML of Cuupd. This result confirms that Cuupd can be almost completely displaced from the Pt(100) surface by COb, which, in turn, suggests that COb oxidation indeed takes place on an essentially bare Pt(100) even in electrolyte containing Cu2+. 3.3.2. Electrooxidation of COb on the Pt(100)-Pbupd surface. Similarly to the Pt(100)-Cuupd system, by holding the potential at ≈-0.22 V for 3 min, a full monolayer of Pbupd is formed on the Pt(100) surface in COb-free solution. The solution containing 8 × 10-5 M Pb2+ was then purged with COb for 7 min before the potential was scanned in the positive direction. Comparison of the data in Figure 5 indicates that at potentials more positive than ca. 0.35 V the current for the electrooxidation of COb on the Pt(100)-Pbupd electrode recorded in the first positive sweep (Figure 5b) is almost doubled relative to the current for the electrooxidation of COb on the Pt(100)-Pbupd electrode recorded in the second positive sweep (Figure 5a). This difference in the kinetics of COb oxidation appears to be related to the sweep-dependent Pbupd surface coverage in the presence of COb. Consistent with the Pt(111)-Cuupd system, it is reasonable to propose that substantial Pbupd displacement is induced by COb. The amount of displaced

Markovic´ et al.

Pbupd can be quantitatively obtained from either the ringshielding or the ring-collection properties of the RRDE, as shown in Figure 5. In these experiments, while the disk current was monitored, the ring electrode was potentiostated at a potential for deposition of solutionphase Pb2+ onto the ring electrode at the diffusioncontrolled rate (-0.65 V), Figure 5b. The qualitative correspondence between ring and disk currents may be evaluated quantitatively in terms of the ion-specific currents (ISC), e.g., id ) ir/N, thus giving the surface coverage by Pbupd (θPb) during COb oxidation according to eq 1; for details see ref 28. Starting from -0.22 V, and sweeping the potential positively, the maximum charge passing through the interface for Pb2+ stripping in a solution containing COb is ca. 100 ( 5% µC/cm2 (the shaded area in Figure 5b), corresponding to θPb ) 0.37 ( 5% ML. The difference between the Coulombic charge evaluated from the stripping charge of Pbupd in COb-free solution (255 ( 5% µC/cm2 in Figure 3) and in a solution saturated with the COb is, therefore, of the order of 155 ( 5% µC/ cm2. Under these experimental conditions, i.e., purging the solution with COb with the potential held at -0.22 V for ca. 7 min, it is apparent that the Pbupd layer is partially displaced from Pt(100) by COb. Some insight into the kinetics of Pbupd displacement can be obtained by monitoring the ring current transients as the disk potential is stepped from the initial potential, where the Pt(100) surface is covered by a COad-Pbupd adlayer (-0.22 V), to the final potential, at which the surface is free of Pbupd (0.62 V). Such results are shown in the inset of Figure 5. The solid line corresponds to the ring current transients for Pbupd stripping in solution free of COb. In this case the stripping of a full monolayer of Pbupd (≈265 ( 5% µC/cm2) is evident from the positive deviation of the ring-current transients from ir∞. The measurement was repeated after the Pbupd layer had been exposed to the COb for 2 min and then 7 min, as shown in Figure 5. The amount of Pbupd that is coadsorbed with COad decreases from ≈0.38 ( 5% ML to ≈0.23 ( 5% ML. It should be noted that the maximum displacement of Pbupd by COb at -0.22 V was observed after 15 min, at which point ≈0.07 ( 5% ML of Pbupd remained on the surface. Having analyzed the results for the displacement of a closely packed monolayer of Pbupd by COb, we now examine the effects of Pb2+ on the Pt(100)-COad layer that is formed on the surface free of Pbupd. This is possible because, at the end of the first positive sweep (0.6 V), Pbupd is completely replaced with COad. The information is contained in the potentiodynamic ring-shielding (collection) experiments illustrated in Figure 5b. The ISC curve indicates a maximum coverage of 0.04 ( 5% ML (the shaded area for the negative currents) at the negative potential limit. On the basis of these data one can conclude that after stripping of the Pbupd layer at 0.6 V, Pb UPD is completely blocked by COad. As a consequence, in a subsequent positive sweep polarization curves for COb oxidation were identical with the curves obtained on the unmodified Pt(100) surface; see Figures 5a and 1. To get further insight into the competition between COb and Pb2+ for Pt surface sites, the stability of the Pt(100)Pbupd system was tested in solution with the same concentration of Pb2+ but at “lower” pressures of COb. To create these conditions, after the formation of the full Pb monolayer at -0.22 V in solution purged with Ar, the continuous flow of Ar was replaced with CO for 15 min. to form the Pt(100)-Pbupd-COad adlayer, and then the CO was replaced again with Ar. To get COb-free solution it was necessary to purge the solution with Ar for ∼30

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currents (TDC),28 recorded on the positive-going sweep direction, and the ISC is shown in the inset of Figure 6. The maximum charge passing through the interface for Pb2+ stripping is QISC ≈ 185 ( 5% µC/cm2, corresponding to a coverage of Pb adatoms of θPb ≈ 0.45 ( 5% ML. The difference between the Coulombic charge evaluated from the integration of the total disk current (QTDS ≈ 450 µC/ cm2) and the ring current (QISC ≈ 185 µC/cm2) is ∼265 µC/cm2. Thus, the sharp peak observed at ≈0.4 V in Figure 6a is related to the stripping of Pbupd adatoms with simultaneous electrooxidation of COad, confirming that when there is a “reduced pressure” of COb, COad can partially be displaced from the Pt(100) surface by Pb2+. 4. Discussion

Figure 6. (a) Potentiodynamic stripping curves of the PbupdCOb adlayer that is formed on the Pt(100) disk electrode during the replacement of COb with Ar from the 0.1 M HClO4 containing Pb2+; first sweep (s) and subsequent sweeps (- - -). (b) Corresponding ring-shielding (collection) experiment. Inset: Comparison of the Pb2+ ion-specific partial currents (shaded area) with the total disk current (TDC) for the stripping of the PbupdCOb layer formed on the Pt(100) disk electrode during the replacement of COb with Ar from the 0.1 M HClO4 containing Pb2+. Ring potential -0.65 V; sweep rate 20 mV/s; rotation rate 900 rpm.

min. During the replacement of COb with Ar, the concentration of COb was continuously changing, e.g, from a maximum value at time zero (ca. 10-3 M) to COb-free solution after 30 min. Thus, a “lower” pressure of CO refers to a less than saturation but unknown COb solution concentration. Exposing the Pt(100)-Pbupd-COad surface to a dilute COb solution while keeping the Pb2+ concentration constant may lead to displacement of COad with Pbupd and to formation of the Pbupd-COad adlayer with a different ratio of Pbupd to COad. To establish the amount of Pbupd that is readsorbed during the replacement of COb with Ar we use either the ring-shielding or the ringcollection properties of the RRDE. Figure 6b shows that as the Pt(100) disk potential is swept between -0.22 V< E < 0.65 V, the associated stripping of lead is manifested by the concomitant increase in the ring current above its unshielded value. Note that Pb stripping commences at 0.0 V and that the sharp peak recorded at the disk electrode at ≈0.4 V is not perfectly mirrored by the ring-shielding current, establishing qualitatively that stripping of UPD Pb from the disk electrode is accompanied by some other process. Following the sharp peak the ring current returns to ir∞, indicating that that stripping of UPD Pb from the disk electrode does not take place at potentials above 0.6 V. The successive negative-going sweep and the second cycle traced accurately the voltammetry for Pb UPD, as evident from comparison of Figures 6a and 3. The qualitative correspondence between ring and disk currents during the first sweep can be evaluated quantitatively in terms of ion-specific partial currents (ISC); to avoid any mass transport resistance the Pb equilibrium coverage as a function of potential was extracted from the positivegoing sweep. The resulting relationship between total disk

4.1. Displacement of UPD states by COb. The irreversible adsorption of COad on Pt(111), Pt(110), and Pt(100) and the electrooxidative removal of COad from platinum single-crystal surfaces has been extensively investigated in both COb-free solution and in solution saturated with gaseous COb.20,31 As shown in Figure 1 and discussed in detail elsewhere,33 the formation of a close-packed monolayer of COad at -0.2 V is accompanied by complete desorption of both Hupd and specifically adsorbed anions (e.g., HSO4-, OH-, Cl-, Br-). Following the UHV terminology,34 this event is usually called displacement, indicating that the weakly adsorbed species (in this case Hupd and anions) are displaced from the surface by the strongly adsorbed species (COad). Weaver and coworkers suggested that although the surface potential characterizing COad-saturated aqueous electrochemical interfaces tends to be substantially lower than the corresponding metal-UHV system, the interaction of COad with Pt surfaces in solution is, in general, the same as in UHV.35 Consequently, a structurally similar CO adlayer on Pt(111) has been observed by STM in these two environments (see ref 19). Recently, the Pt(100)-CO system has also been investigated in aqueous electrolytes by SXS measurements.31 These results showed that, although no structures of COad with long-range order were formed on Pt(100) in acid solutions, the adsorption of COad at -0.2 V caused an expansion of the surface Pt atomic layer, attributed to the displacement of Hupd by COad. At -0.2 V in solution free of COb, where the surface is covered by a monolayer of hydrogen, the topmost Pt atomic layer was expanded by ≈2.5% of the lattice spacing away from the second layer. This expansion increased to ≈ 4% when the Hupd was completely displaced by a full monolayer of COad.31 The difference in relaxation of the Pt(100) surface covered with Hupd and COad probably arises from the difference in the adsorbate-metal bonding, the Pt(100)COad interaction being much stronger than the Pt(100)Hupd interaction, as we recently discussed for the Pt(111)COad system.20 We next review the chemistry of Cu and Pb adlayers on Pt(100) before turning to the displacement of Cuupd and Pbupd by COb from the Pt(100) surface. A close-packed monolayer of fully discharged Cuupd and Pbupd is formed on the Pt(100) electrode close to the Nernst potential for bulk metal deposition; see Figures 2 and 3 and refs 2528. Cuupd is deposited at underpotentials in metallic islands (32) Tidswell, I. M.; Lucas, C. A.; Markovic´, N. M.; Ross, P. N. Phys. Rev. B 1995, 51, 10205. (33) Orts, J. M.; Gomez, R.; Feliu, M.; Aldaz A.; Clavilier, J. J. Electrochim. Acta 1995, 39, 1519. (34) White, J. M.; Akhter, S. CRC Critical Reviews in Solid State and Materials Sciences; CRC Press Inc.: Boca Raton, FL, 1988; Vol. 14, pp 131-173. (35) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391.

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until, at the Nernst potential, it forms a uniform metallic monolayer having the Pt lattice constant, i.e., a pseudomorphic monolayer. However, due to the mismatch in atomic size, the Pbupd on Pt(100) goes through a potentialdependent disorder-order-disorder transition. An initially disordered layer of Pbupd at low coverage forms a c(2 × 2) adlayer at θPb,upd ) 0.5 ML until, at the Nernst potential, the Pbupd adatoms move out of registry with the substrate to form a disordered close-packed monolayer (θPb,upd ) 0.62 ML).27,28

Scheme 1 2+

(1) Cu (Pbupd) + 2e- ) Cuupd(Pbupd) ∆G1 ) -nF∆Ep - ∆GMe2+/Me (2) COb ) COad

∆G2 ≈ -∆Had

(3) Cuupd(Pbupd) + COb ) COad + Cu2+(Pb2+) + 2e∆G3 ) ∆G2 - ∆G1 According to our RRDE results here, these close-packed UPD metal adlayers can be displaced from the Pt(100) surface either completely or partially by COb; see Figures 4-6. The displacement of Cu and Pb adatoms by CO is caused by a stronger binding energy of COad on Pt(100), compared to the binding energy of Cuupd and Pbupd on the same substrate. Therefore, assuming that the potential difference between a bulk-phase stripping peak and the most positive UPD stripping peak can be considered as a qualitative measure for the binding energy of UPD adatoms [so-called underpotential shift (∆Ep)36], one can calculate the Gibbs energy change (∆G3) for the displacement process (3) from the set of component steps shown in Scheme 1. For the Pt(100)-Cuupd (Pbupd) systems, ∆G1 is calculated on the basis of ∆Ep ) 0.51 V (from Figure 2 at 0.5 ML coverage), n ) 2, and ∆GCu2+/Cu ) +38.5 kJ/mol at 10-4 M Cu2+. For Pt(100)-Pbupd system, ∆G1 is calculated on the basis of n ) 2 and the underpotential shift value assessed from voltammetry recorded in solution containing a relatively high concentration of Pb2+, e.g., from ref 37, ∆Ep ) 0.63 V, and ∆GPb2+/Pb ) -58 kJ/mol at 10-4 M Pb2+.38 The difference in the Gibbs energy for UPD formation (∆G1 ) -136 ( 5 kJ/mol for Cu UPD and ∆G1 ) -64 ( 5 kJ/mol for Pb UPD) and heat of adsorption of CO (∆G2 ≈ -150 ( 5 kJ/mol)39 is ∆G3 ≈ -14 ( 5 kJ/mol and ∆G3 ≈ -86 ( 5 kJ/mol for Cu and Pb UPD systems, respectively.40 We note that in ref 31 in the same way we estimated that the Gibbs energy change for displacement of Hupd by COad from Pt(100) was estimated to be close to -90 ( 10 kJ/mol.41 On the basis of this simple thermo(36) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (37) Aberdam, D.; Traore, S.; Durand, R.; Faure, R. Surf. Sci. 1987, 64, 319. (38) Because the standard electrode potential for the Pb2+/Pb0 reaction is significantly lower than for the Cu2+/Cu0 reaction (e.g., by 0.466 V), a bulk-phase stripping peak for Pb cannot be observed for a low Pb2+ concentration used in our RRDE experiments. Note that ∆Ep is independent of the metal ion concentration. (39) Thiel, P. A.; Behm, R. A.; Norton, P. R.; Ertl, G. J. Chem. Phys. 1983, 78, 7458. (40) Given that CO adsorption on Pt(hkl) at near ambient temperature is essentially an irreversible process, it is impossible to evaluate unambiguously thermodynamic functions in aqueous electrolytes. Therefore, we estimate ∆G2 at the Pt(100)-(1 × 1)-COad liquid interface to be ≈150 ( 5 kJ/mol, which is equal to the CO binding energy determined in the TDS data for Pt(100)-(1 × 1)-COad gas interface. (41) The best estimate of the thermodynamic functions for the Hupd state on Pt(100) may be obtained from the underpotential shift of ∆Ep ≈ 0.36 V, corresponding to θHupd ) 0.5 ML referred to the potential of the 1/2H2/H+ electrode in the same solution at the same temperature.

Markovic´ et al.

dynamic analysis, it appears that at the surface coverage of Θupd ≈ 0.5 ML the absolute value of ∆G3 may serve as an analytical tool to establish a hierarchy of bond strengths with Pt(100). Our examples clearly show that the bond strength with Pt(100) increases in the order Pt(100)(H2O)ad , Hupd < Pbupd < Cuupd < COad. We note that the above analysis neglects variation of the heat of adsorption between the substrate-adsorbates with coverage that might be induced through COad-COad, adatom-COad, and adatom-adatom interactions. Thus, the above analysis applies only to the initial displacement of Cuupd (Pbupd) by COad (we have used the initial heat of adsorption of COad) and not to the complete displacement. Depending on the change of ∆G1 and ∆G2 values during the displacement process (e.g., |∆G2| value decreases with increasing coverage by COad, thus causing a continuous decrease in |∆G3|), in the vicinity of the Me2+/Me Nernst potential the UPD state may be displaced either completely (as in the case of the Cuupd adlayer) or only partially (as in the case of the Pbupd adlayer) by COb. There may also be kinetic inhibition of the displacement reaction that may prevent complete displacement. The consequences of the observed displacement phenomena for the electrooxidation of COb are obvious. With the exception of the oxidation of COb on the Pt(100)Pbupd surface recorded in the very first potential sweep, there is no difference between COb oxidation on Pt(100) in Cu2+- (Pb2+-) free solution and in solution containing metal ions (Figures 4 and 5). In the case of Cu2+, the electrocatalysis of COb takes place on an unmodified Pt(100) surface, because Cuupd is completely displaced by COb from the Pt(100) surface even at the Nernst potential (Figure 4). Given that at the Nernst potential Pbupd is only partially displaced by COb, during the first positive sweep a small enhancement in the kinetics of COb oxidation is observed. Although the database is extremely limited, we suggest that desorption of Pbupd is first accompanied by simultaneous adsorption of COad and nucleation of OHad on COad-free sites and then by the electrooxidative removal of CO through the LangmuirHinshelwood-type reaction.42 The complete stripping of Pbupd at 0.6 V, and the formation of a Pbupd-free Pt(100)COad layer in the subsequent negative sweep, results in the same catalytic activity for COb oxidation as observed in Pb2+-free solution. Recently, it has been found that an Ag monolayer deposited on Pt had properties that facilitated both the adsorption and the oxidation of COb, e.g., oxidation currents for COb were observed before the onset of silver dissolution.22 Since the adsorption of CO cannot be achieved on bulk Ag(111), the authors proposed that the Ag monolayer had unusual behavior for CO electrocatalysis. The same type of thermochemical analysis we used here for the Pt-Cuupd and Pt-Pbupd systems indicates the binding energy of Ag adatoms with the Pt substrates is weaker than the Pt-Cuupd binding energy, e.g., based on ∆Ep ) 0.4 V and n ) 1, and ∆GAg+/Ag ) +54 kJ/mol at 10-4 M Ag+, ∆G1 for UPD of Ag on Pt is -92 kJ/mol and consequently ∆G3 ) -60 kJ/mol. It appears likely that the kinetic effect reported in ref 22 is related to the displacement of the Ag monolayer by COb from the Pt(111) surface rather than to a change in the electronic properties of the Ag monolayer. It is apparent, therefore, that investigation of the effects of UPD adatoms on the electrocatalysis of COb should be done with extreme caution. (42) Ertl, G. Chemistry and Physics of Chemical Surfaces; Vanselow, R., England, W., Eds.; CRC Press Inc.: Boca Raton, FL, 1982; Vol. 2, pp 21-28.

Displacement of CO on Pt(100)-Bimetallic Surfaces

5. Conclusions RRDE results for the surface (electro)chemistry of COad on Pt(100) electrode surfaces modified by Cuupd and Pbupd have shown that COb can easily displace metal adatoms from the platinum surface. We suggest that displacement of Cu and Pb adatoms by CO is caused by the stronger binding of COad on Pt(100) compared to the binding of Cuupd and Pbupd on the same substrate. By use of the underpotential shift, ∆Ep, as a measure for the binding energy of UPD adatoms, the calculated change in the Gibbs energy (∆G3) for the displacement of Hupd, Cuupd, and Pbupd by COad may be used to establish the hierarchy of bond strengths with Pt(100): Pt(100)-(H2O)ad , Hupd < Pbupd

Langmuir, Vol. 16, No. 4, 2000 2005

< Cuupd < COad. The consequence of the observed displacement phenomenon is that the kinetics of the electrooxidation of COb on Pt (100) in Cu2+- (Pb2+-) containing solution is the same as in Cu2+- (Pb2+-) free solution. Acknowledgment. This work was supported jointly by the Office of Basic Energy Sciences and Office of Advanced Automotive Transportation Technologies, Electric and Hybrid Propulsion Division of the U.S. Department of Energy under Contract DE-AC03-76SF00098. LA990255U