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Langmuir 1997, 13, 6370-6374
Underpotential Deposition of Lead on Pt(111) in Perchloric Acid Solution: RRDPt(111)E Measurements B. N. Grgur, N. M. Markovic´,* and P. N. Ross, Jr. Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 Received July 2, 1997. In Final Form: October 9, 1997X Underpotential deposition (UPD) of lead (Pbupd) on Pt(111) in perchloric acid solution was studied using the rotation ring disk electrode (RRDPt(111)E) method for measurements of the Pb2+ ion-specific flux. The total amount of Pb deposited underpotentially is 0.62 ( 5% monolayer (1 monolayer ≡ 1 Pb per Pt) equivalent to a close-packed monolayer of fully discharged Pb adatoms. The electrosorption valence, γPb, of Pbupd is ≈ γPb ) 2, implying that two electrons per Pbupd adatom are exchanged through the interface. In this work we showed that the formation of this close-packed monolayer of Pbupd occurs through four distinctive voltammetric features: two major sharp peaks at ≈0.5 V (vs saturated calomel electrode) and 0.3 V and smaller reversible peaks at 0 V and -0.15 V. We concluded that the adsorption of OHad is shifted negatively by Pbupd adatoms, with the shift attributed to induced adsorption of OHad onto Pt atoms neighboring the Pbupd adatoms due to lowering of the local point of zero charge (pcz) by the Pbupd. Consequently, the pH dependence of the Pb UPD peak at 0.3 V appears entirely through a pH-dependent adsorption of OHad onto the Pt(111) surface modified by Pbupd adatoms. The pH independence of the peak at 0.5 V is consistent with the Pb2+ ion-flux measurements that the main process associated with this pseudocapacitance is Pb UPD. The smaller peaks observed at lower potentials are probably associated with a final deposition of Pbupd adatoms along step edges and compacting of adatoms on the (111)-terraces to form an ordered (3×x3) close-packed structure.
1. Introduction The underpotential deposition (UPD) of metal adatoms on platinum single crystals in aqueous electrolytes occupies an important position in the surface electrochemistry.1-13 Of various systems examined the Pb UPD on the Pt(111) surface has been of particular interest since the interpretation of processes with the formation of Pb UPD peaks is still controversial.1,2,10-12 The study by ElOmar and Durand1 demonstrated, for the first time, that the Pb UPD on Pt(111) in 0.1 M HClO4 proceeds as a stepwise process, with four clearly resolved reversible peaks. While three of them (A2-C2, A4-C4, A5-C5, in the notation of the authors, corresponding to the features at ca. 0.5, 0, and -0.15 V, respectively, in Figure 1) exhibited a positive shift with an increase of Pb2+ concentration, the position of one, at ca. 0.3 V in Figure 1 (the A3-C3 couple in ref 1), was found to be independent of the concentration of Pb2+ and strongly dependent on the pH of solution and/or on the presence of strongly adsorbed anion, such as Cl-.1 Closely following the El-Omar and Durand work, Clavilier et al. suggested that the A3-C3 * Corresponding author: phone, (510) 486-2956; fax, (510) 4865530; e-mail, NENAD@LBL. GOV. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Omar, F. El.; Durand, R. J. Electroanal. Chem. 1984, 178, 343. (2) Stickney, J. L.; Rosasco, S. D.; Hubbard, A. T. J. Electrochem. Soc. 1984, 131, 260. (3) Schardt, B. C.; Stickney, J. L.; Stern, D. A.; Wieckowski, A.; Zapien, D. C.; Hubbard, A. T. Surf. Sci. 1986, 175, 520 (4) Aberdam, D.; Traore, S.; Durand, R.; Faure, R. Surf. Sci. 1987, 180, 319. (5) Michaelis, R.; Zei, M. S.; Zhai, R. S.; Kolb, D. M. J. Electroanal. Chem. 1992, 339, 299. (6) White, J.; Abrua, H. D. J. Phys. Chem. 1990, 94, 894. (7) Leung, L.-W.; Gregg, T. W.; Goodman, D. W. Chem. Phys. Lett. 1992, 188, 467. (8) Markovic´, N. M.; Ross, P. N. Langmuir 1993, 9, 580. (9) Tidswell, I. M.; Lucas, C. A.; Markovic´, N. M.; Ross, P. N. Phys. Rev. B 1995, 51, 10205. (10) Clavilier, J.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1990, 293, 197. (11) Borup, R. L.; Sauer, D. E.; Stuve, E. M. Surf. Sci. 1993, 293, 10. (12) Adzic´, R. R; Wang, J.; Vitas, C. M; Ocko, B. M. Surf. Sci. 1993, 293, L876. (13) Lei, H.; Hattori; Kita, H. Electrochim. Acta 1993, 41, 1619.
S0743-7463(97)00699-9 CCC: $14.00
couple is a surface process associated with a redox reaction between the irreversibly adsorbed lead and OH species.10 The ex-situ x-ray photoelectron spectroscopy (XPS) data by Borup et al.11 provides some direct spectroscopic evidence that both Pb0 and Pb2+ are adsorbed onto the Pt(111) surface, supporting the proposal of surface redox behavior. In addition, Borup et al. suggested that a mechanism for the potential-dependent adsorption of Pb2+ must include some form of codeposited anionic species, such as Oad or OHad. The possible interaction of Pbupd and OHad has also been discussed by Adzic´ et al..12 According to this group, the reversible couple producing the peaks at ca. 0.5 V is associated with OHad formation, and the potential dependence of the peak position on the Pb2+ concentration in solution is due to the interaction between OHad and Pbupd. A total coverage of θPb ) 0.66 was inferred from the diffraction pattern for Pbupd adatoms in a rectangular (3×x3) unit cell, obtained by in-situ surface X-ray spectroscopy (SXS), and also from real-space images with scanning tunneling microscopy (STM).12 The authors pointed out, however, that there is a considerable discrepancy in coverage calculated from the charge assessed from the voltammetry and that obtained from their SXS and STM work, which is not understood. In this Letter, we present results of ion-specific flux measurements during Pb UPD on Pt(111) in 0.1 M HClO4 obtained by using the rotation ring disk electrode (RRDPt(111)E) method. We report an independent quantitative assessment of the Pbupd surface coverage in 0.1 M HClO4 acid which explains the apparent discrepancy between the coverage of Pb adatoms estimated from cyclic voltammetry and from SXS and STM measurements. 2. Experimental Section The Pt(111) crystal (0.283 cm2) for RRDPt(111)E experiments was prepared ex-vacuo by a flame-annealing method;14 i.e., after annealing in an hydrogen-oxygen flame and cooling in a stream of argon (hydrogen) to room temperature, the crystal was pressed into the disk position in the arbor of the RRDPt(111)E.15 (14) Markovic´, N.; Hanson, M.; McDougall, G.; Yeager, E. J. Electroanal. Chem. 1986, 214, 155. (15) Markovic´, N. M.; Gasteiger, H. A.; Ross, P. N. Langmuir 1995, 11, 4098.
© 1997 American Chemical Society
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Figure 1. (a) Cyclic voltammograms of Pt(111) mounted in the disk position of the RRDE in 0.1 M HClO4 and 5 × 10-5 M Pb2+: rotation rate, 900 rpm; sweep rate, 50 mV/s. (b) Ring electrode currents recorded with the ring being potentiostated at -0.59 V. RRDPt(111)E experiments were conducted at relatively low lead concentrations (5 × 10-5 M) in order to minimize the unshielded ring currents (i.e., the amount of Pb deposited on the ring) at the rotation rate of 900 rpm used in this study. If Pb is adsorbed onto the Pt(111) disk, the surface coverage by lead (θPb) can be assessed from the shielding currents on the ring in both potentiodynamic and potentiostatic measurements15 The lead coverage was based on the surface atomic density of Pt(111)(1.53 × 1015) (atoms/cm2), and θPb ) 1 monolayer would correspond to one completely discharged adatom per platinum atom, corresponding to a total charge of 304 µC/cm2 for a full monolayer (ML). Thus, the maximum coverage of Pbupd if the Pb adatoms form a close-packed layer having the atomic dimensions of bulk Pb is 0.63 ML. The supporting electrolyte was 0.1 M HClO4 (J. T. Baker, Ultrex), and lead was added to the solution as Pb(ClO4)2 (Aldrich, Puratronic). All potentials are referred to the saturated calomel electrode (SCE) at a room temperature. Data from the Pine Instruments bipotentiostat (Model AFRDE4) were acquired digitally on an IBM PC using LabView for Windows.
inspection of the voltammetry in Figure 1a reveals that formation of a monolayer of lead in the solution containing Pb2+ indeed occurs through four distinctive voltammetric features: in the “butterfly” potential region two major sharp peaks appeared at ca. 0.5 and 0.3 V, and in the hydrogen region a minor set of reversible peaks were recorded at 0.0 and -0.15 V. While monitoring the Pb UPD on the disk electrode, the ring electrode was potentiostated at -0.6 V, i.e., at a potential at which a solution phase of Pb2+ is deposited onto the ring electrode at the diffusion-controlled rate. When there is no deposition of Pb2+ onto the disk, this current is referred to as the unshielded ring current, ir∞. Starting from the positive potential limit, the ring current above 0.6 V is close to ir∞, Figure 1b, indicating that the UPD of lead does not take place above 0.6 V and that the Pt(111) surface is free of Pbupd. As the Pt(111)-disk potential is swept across the Pb UPD region, the associated deposition of lead is demonstrated by the concomitant decrease of the ring current below its unshielded value, Figure 1b. It is important to note, however, that two sharp peaks recorded at the disk electrode, first at ≈0.5 V and second at ≈0.3 V, are not perfectly mirrored by the ring shielding current, establishing qualitatively that the Pb UPD on the disk electrode is accompanied by some other process. Following the second voltammetric peak at ≈0.3 V, the ring current does not return to ir∞, indicating that additional deposition of Pb2+ occurs between 0.3 and 0.0 V. As the disk electrode is swept across the hydrogen potential region, the two small Pb UPD peaks are mirrored by the corresponding change in the ring current from ir∞. These RRDPt(111)E analyses clearly indicate that all voltammetric features recorded at the disk electrode are related to the UPD of Pb. The same observation is made for the subsequent positive sweep, as shown in Figure 1b. The surface coverage (θPb) can be assessed from the potentiodynamic ring measurements based on eq 1:
∫
1 (ir - ir∞) dE 1 v θPb ) Q AnN
(1)
3.1. Potentiodynamic Experiments. The effect of the Pb UPD on the voltammetric features of Pt(111) recorded in 0.1 M HClO4 is easily observed simply by comparing the voltammetry of Pt(111) in solution with and without Pb2+, as shown in Figure 1a. The cyclic voltammograms of Pt(111) in 0.1 M HClO4 free of lead, and in the presence of 5 × 10-5 M Pb2+, are in excellent agreement with the data in the literature1,10,12 and clearly demonstrate the high quality of the Pt(111) surface in the RRDPt(111)E configuration. The cyclic voltammogram for Pt(111) in 0.1 M HClO4 shows a characteristic broad hydrogen adsorption wave, -0.22 < E < 0.1 V, and the so-called “butterfly” peak at 0.3-0.6 V, which corresponds to adsorption of ≈0.33 ML (≈80 µC/cm2) of hydroxyl species,16 from hereafter designated as OHad. A close
where ir∞ and ir are unshielded and shielded ring currents, N is the collection efficiency (N ) 0.18 ( 5%17 ), v refers to the sweep rate, n ) 2 is the number of electrons for Pb2+ reduction to Pb0 and Q is the charge corresponding to monolayer formation of the Pbupd adatoms on Pt(111). The maximum Pb UPD surface coverage on Pt(111) in 0.1 M HClO4, assessed from the potentiodynamic ring measurements over the potential region -0.22 < E < 0.6 V, is 0.48 ML, which is significantly smaller than the maximum surface coverage by Pbupd of 0.63 ML for the close-packed Pb monolayer. It appears that in the potentiodynamic experiments an equilibrium surface coverage is not achieved at the relatively fast sweep rate employed in these measurements (50 mV/s), as will be addressed in section 3.2. Our interest in this section is to establish qualitatively the true nature of Pb UPD voltammetry peaks on Pt(111) in perchloric acid, because the processes associated with these peaks are uncertain. In order to resolve the processes occurring in the four Pb UPD voltammetric peaks recorded on the Pt(111) disk electrode, the ion-specific partial currents (ISCs) for Pb UPD in 0.1 M HClO4 were calculated from the ring shielding currents (e.g., id ) ir/N). To avoid possible mass transport resistance only the ISC curves from the positivegoing sweep direction (dissolution) are used. These are
(16) Gasteiger, H. A.; Markovic´, N. M.; Ross, P. N. Langmuir 1996, 12, 1414.
(17) Brisard, G. M.; Zenati, E.; Gasteiger, H. A.; Markovic´, N. M.; Ross, P. N. Langmuir 1995, 11, 2223.
3. Results and Discussion
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Figure 2. (a) Comparison of the Pb2+ ion specific partial current (ISC) with the total disk current (TDS) for the Pb UPD in 0.1 M HClO4. (b) Integrated charges for the stripping of Pbupd from the disk electrode, assessed from ring and disk potentiodynamic curves in Figure 1.
then compared to the total disk currents (TDC) on the same sweep in Figure 2a. The ion-specific charge (QISC) is calculated from the ion-specific current and compared to the total charge in Figure 2b. The curves in Figure 2 clearly show that there is another process in addition to Pb deposition that occurs in the potential region between ≈0.2 and 0.5 V, which contributes a charge of ≈95 µC/ cm2. An obvious candidate is anion adsorption, specifically OHad adsorption/formation (H2O being the source of OHad in acid solution versus OH- ion in basic solution), since perchlorate ion is only weakly adsorbed on Pt,8 according to Pb Pb Pb Pt Pt Pt Pt
+ H2O
Pb Pb OH Pt Pt Pt Pt
+ H+ + Pb2+ + 3e–
(2)
While we cannot prove that this process is OHad formation simultaneously with Pb stripping (on the positive going sweep), we suggest that this is the most reasonable assignment, based on indirect evidence. Figure 3 shows plots of the charge versus potential (Q vs E) curve for OHad formation on Pt(111) in the supporting electrolyte in the absence of Pb, and the difference curve (∆Q) for QTDC - QISC, the difference in charge between Pb deposition and the total charge. The amount of charge is the same in both cases, and the shape of the “isotherm” is similar, which is made more evident by shifting the QOH curve negatively by about 0.1 V. Physically, this shift of -0.1 V would correspond to a stabilization of the OHad state by the presence of co-adsorbed Pb. We note that the pH dependence of the peak position at 0.3 V (A3-C3 in ref 1) reported in ref 1 would then arise from the pH dependent adsorption of OHad onto the Pt(111) surface modified by Pbupd adatoms. We also note the previous hypothesis by Adzic´ et. al.,12 that the stripping of Pbupd adatoms from the Pt(111) surface in perchloric acid is completed at ca. 0.4 V and, consequently, that the peak at ca. >0.5 V is associated exclusively with PtOH formation, is clearly contradicted by our RRDPt(111)E ion-flux measurements. The Pb ISC results clearly indicate that Pb UPD is the primary process at 0.5 ( 0.05 V. An alternative process to anion adsorption for the pseudocapacitance at ca. 0.3 V is a surface redox process
Figure 3. Charge potential curves for (open squares) OHad on Pt(111) in Pb-free solution, (solid squares) the simulated curve for OHad in the presence of Pb2+ in solution, and (c) the difference curve between the total charge (QTDC) and the integrated Pb2+ ion-specific flux (QISC).
involving an irreversibly adsorbed state of Pb2+ ion (with some anionic ligands, possibly OH), as suggested by others groups,10-12 albeit with somewhat different details. In this process, because the Pb2+ ion is irreversibly adsorbed, there is no flux of Pb2+ to/from the disk electrode during the redox process, consistent with our RRDE data. While we cannot rule out this process as a possibility, we conducted what we felt was a “titration” experiment for OHad. We had shown previously16 that in the presence of Br anions OHad formation on Pt(111) in perchloric acid is completely suppressed in the potential region of interest here, e.g., -0.2 to 0.6 V. In addition, because the change of the muss flux of Br- to and from the Pt(111) disk electrode during Pb UPD onto the Pt(111) is well-defined,18 the physical model of the Pbupd-OHad interaction on the Pt(111) could be developed simply by following PbupdBrad interaction on the Pt(111) surface. The details of the analyses for the Br- ISC were given previously.16,19 Figure 4 shows the ion specific curves (ISCs) on the positive going sweep for both Pb2+ and Br- ion flux during Pb UPD on Pt(111) in the presence of Br- in solution. Note that the Pb ISC still has a four-peaked structure, but now the Pb ISC and the TDC closely follow one another, with the Br ISC, corresponding to Br- adsorption during Pb stripping, indicating an essentially potential-independent rate of adsorption of Br-. The sum of the Pb ISC and the Br ISC essentially completely account for the TDC and the total charge passing the disk electrode interface; i.e., there is no OHad process during Pb UPD in the presence of Br-. Consequently, the position of all Pb UPD peaks onto Pt(111) in Figure 4 has been found to be a pH independent in a solution containing Br-. The stabilization of anionic adsorbates by co-adsorbed UPD metal adatoms has been observed previously. We have referred to this previously8 as “induced adsorption” of anions by UPD metals and have shown how this induced adsorption could be discussed in the framework of the local work function concept and/or the local pzc concept.8,19 In this framework, applied to the present system, the local pzc of Pt atoms near the Pb UPD adatoms is negative with respect to the pzc of the clean Pt surface, creating a local dipole that attracts anionic species to the positive end of the dipole, i.e., inducing adsorption of OH onto the (18) Markovic´, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. J. Electroanal. Chem., in press. (19) Ross, P. N. J. Chim. Phys. 1991, 88, 1353.
Letters
Figure 4. (a) Comparison of the Pb2+ and Br- ion specific partial currents (ISC) with the total disk current (TDS) for the Pb UPD in 0.1 M HClO4 + 5 × 10-5 M Pb2+ Pb2+ containing 8 × 10-5 M Br-. (b) Integrated charges for the stripping of Pbupd and for the adsorption of Br- at the disk electrode assessed from the ion specific partial currents (ISC) and the total disk current (TDC).
Pt sites neighboring the Pb UPD adatoms at a lower potential than the adsorption on the Pb-free surface, see eq 2. 3.2. Potentiostatic Measurements. A true equilibrium isotherm for Pb UPD on Pt(111) is more easily and reliably obtained from potentiostatic (vs potentiodynamic) ring-shielding experiments, as we have done for the Cu UPD on the Pt(111) surface.15 Figure 5 shows disk and ring current transients as the disk potential is stepped from an initial potential (Ei ) 0.65 V) where the surface is free of Pbupd, to the final potential in the Pb UPD potential region (curves 1-5 are representative). Even though many more potentials were used than shown here, these transients demonstrate all of important characteristics of the process during the Pb UPD. As the disk potential is stepped into the Pb UPD potential region (steps labeled 1 and 2), the disk current transients reflect that initial deposition of Pb is under conditions of mixed diffusion and kinetic control, with currents well below the diffusion-limiting current for Pb2+ at this solution concentration and rotation rate (id∞ ≈ 17 µA). The deposition of Pb is evident from the positive deviation in the ring current transients from ir∞. Stepping the potential more negatively (steps labeled 3-5), the current is first below and then subsequently rises near to id∞, although the ring current transient clearly demonstrated that the Pb2+ ion flux is at the diffusion-limited rate in the same potential region (and thus the disk current should be equal to id∞ throughout this potential region). A very similar disk current transient was observed previously for Pt(111) in Br- solution16 and can be attributed to the same phenomenon, hydrogen adsorption/desorption. Since hydrogen adsorption is a much faster process than Pb deposition and is not significantly diffusion-limited, the Pt(111) surface will be covered with an equilibrium coverage of adsorbed hydrogen immediately following the potential step originating from Ei (on a time scale of milliseconds). As a consequence, in order to complete the first monolayer, Pb2+ has to displace the initially adsorbed hydrogen, resulting in an anodic partial current which causes the total disk current to have a plateau below id∞.
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Figure 5. RRDPt(111)E potential stepping experiments in 0.1 M HClO4 and 5 × 10-5 M Pb2+: (a) Disk current transients during Pb UPD; (b) simultaneous ring-shielding currents. Insert: Adsorption isotherm for the Pb UPD, assessed from the ring current transients during Pb deposition on the disk electrode: Initial potential (Ei) 0.6 V.
It is obvious, therefore, that the surface coverage by Pb UPD cannot be obtained from disk transients. The surface coverage by Pb adatoms on the Pt(111) disk electrode, however, can easily be obtained by integrating the ring current transients15
1 θPb ) Q
∫0∞(ir∞ - ir(t)) dτ AnN
(3)
The resulting adsorption isotherm of Pb on Pt(111) in 0.1 M HClO4 is shown in the insert of Figure 5, with the voltammetry included as a cross reference. The total charge passing the interface during the formation of the Pb monolayer on Pt(111) is ≈300 µC/cm2 ( 5%. Assuming that two electrons are discharged in the UPD of Pb, this charge corresponds to a total Pb surface coverage, of θPbmax ≈ 0.625 ( 5% ML Recall that 0.63 Pb per Pt is a closepacked monolayer with respect to the Pb(111) atomic density. Thus, our coverage results are in a very good agreement with the previous surface X-ray diffraction12,18 and scanning tunneling microscopy work,12 e.g., that in pure perchloric acid at negative potentials the Pb monolayer formed a (3×x3) rectangular structure with four Pb adatoms in the unit cell. We have shown15 that the Pb electrosorption valency can be obtained without assumptions about the deposition based on the difference between the unshielded ring current, ir∞, and the fully shielded ring current, irfs
γPb )
2id∞ (ir∞- irfs)/N
×2
(4)
A value of γPb ≈ 2 is consistent with our supposition above that during Pb UPD two electrons per Pb adatom are exchanged through the interface.
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Finally, we offer an interpretation of the four stages in the UPD of Pb on the Pt(111) surface in perchloric acid. We suggest that nucleation of Pbupd adatoms begins at the (111)-terrace sites, because the step sites are blocked by strongly adsorbed OHad and/or Clad (trace amounts of Cl- anions are always present as an impurity in HClO420). This initial deposition occurs without desorption of the more weakly bound form of OHad on the terrace sites (which we have referred to previously as “reversibly adsorbed” OH21), and thus the first peak corresponds primarily to the deposition of Pb in the presence of OHad. The second stage is desorption of the co-adsorbed OHad concurrent with deposition of Pb on the terrace sites, the result being terraces completely covered by Pb adatoms in a disordered
structure. The completion of the monolayer, and the formation of an ordered close-packed (3×x3), occurs in the final two stages, corresponding to the two small peaks in Figure 1 near 0 and -0.15 V. Both are order/disorder transitions produced by the desorption/reduction of the strongly bound anions concurrent with Pb deposition at the step sites and on the terraces adjacent to the steps, the latter producing the between “squeezing” of the Pb adatoms on the (111) terraces into the ordered structure. The appearance of two peaks or two potentials for essentially the same process corresponds to the two different types of step sites (and step edges) that occur on the Pt(111) face, (111)×(100) and (111)×(111).22
(20) Markovic´, N. M.; Ross, P. N. J. Electroanal. Chem. 1992, 330, 499. (21) Markovic´, N. M.; Gasteiger, H. A.; Ross, P. N. J. Phys. Chem. 1995, 99, 3411. Markovic´, N.; Gasteiger, H.; Ross, P. N. J. Electrochem. Soc. 1997, 144, 1591. (22) Itaya, K.; Surgawara, S.; Sashikata, K.; Furuya, N. J. Vac. Sci. Technol., A 1990, 8, 515.
Acknowledgment. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. LA970699V
