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Bromide Adsorption on Pt(111): Adsorption Isotherm and Electrosorption Valency Deduced from RRDPt(111)E Measurements Hubert A. Gasteiger,† Nenad M. Markovic´, and Philip N. Ross, Jr.* Materials Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Received October 3, 1995. In Final Form: January 24, 1996X The rotating ring-disk electrode technique with a Pt(111) single crystal in the disk position is used to establish, for the first time, both the bromide adsorption isotherm and its electrosorption valency on Pt(111). In 0.1 M HClO4 with ≈10-4 M Br-, bromide adsorption begins at ca. -0.15 V (SCE) and reaches a coverage of ≈0.42 ML at 0.5 V; the electrosorption valency is essentially unity. By subtracting the bromide flux from the Pt(111)-disk current, we were able to deconvolute hydrogen adsorption/desorption processes in the presence of bromide. The major peak in the voltammetry at ca. -0.1 V is the simultaneous desorption/adsorption of hydrogen and adsorption/desorption of bromide. Furthermore, we demonstrated that the saturation coverage of adsorbed hydrogen on Pt(111) is ≈160 µC/cm2.
1. Introduction One of the successes of single-crystal electrochemistry has been the clear demonstration that the interaction of anions with platinum is a fundamental, omnipresent component in the physical description of the mechanisms governing a large variety of reactions on platinum electrodes. Two prominent examples are oxygen reduction and metal underpotential deposition (UPD). Thus, the adsorption of anions at positive electrode potentials impairs the oxygen reduction kinetics on platinum single crystal electrodes by blocking Pt surface sites (e.g., (bi)sulfate1 and bromide2 ). On the other hand, halide adsorption also strongly affects UPD processes, often enhancing the UPD kinetics, e.g., Cu UPD on Pt(111). A knowledge of both adsorption isotherm and electrosorption valency of the halide in the presence of Cu is crucial for the elucidation of the UPD process; consequently, the lack of this information has led to significant controversy in the field.3-6 In the present work, we will present new results on the adsorption behavior of bromide on Pt(111). One of the earliest measurements of bromide adsorption was carried out on polycrystalline platinum by Kazarinov et al.7 using the radiotracer method, but in light of later studies, the reported bromide coverages appear to be too small. Subsequently, Hyde et al.8 employed ellipsometry, showing qualitatively an optical signature of the bromide adsorption isotherm without determining actual coverages. Neither of the above studies succeeded in establishing a value of the electrosorption valency, γ. The only † Current address: Abt. fu ¨ r Oberfla¨chenchemie und Katalyse, Universita¨t Ulm, D-89069 Ulm, Germany. Phone: (731)-502-2901. FAX: (49)-731-502-2902. * Corresponding author. Lawrence Berkeley Laboratory, Mail Stop 2-100, Berkeley, CA 94720, USA. Phone: (510)-486-4793. FAX: (510)-486-5530. X Abstract published in Advance ACS Abstracts, March 1, 1996.
(1) Markovic´, N. M.; Gasteiger, H. A.; Ross, P. N., Jr. J. Phys. Chem. 1995, 99, 3411. (2) Markovic´, N. M.; Gasteiger, H. A.; Ross, P. N., Jr. In preparation. (3) Michaelis, R.; Zei, M. S.; Zhai, R. S.; Kolb, D. M. J. Electroanal. Chem. 1992, 339, 299. (4) Go´mez, R.; Feliu, J. M.; Abrun˜a, H. D. J. Phys. Chem. 1994, 98, 5514. (5) Markovic´, N. M.; Gasteiger, H. A.; Lucas, C. A.; Tidswell, I. M.; Ross, P. N. Surf. Sci. 1995, 335, 91. (6) Markovic´, N. M.; Gasteiger, H. A.; Ross, P. N., Jr. Langmuir 1995, 11, 4098. (7) Kazarinov, V. E.; Petrii, O. A.; Topolev, V. V.; Losev, A. V. Elektrokhimiya 1971, 7, 1365. (8) Hyde, P. J.; Maggiore, C. J.; Redondo, A.; Srinivasan, S.; Gottesfeld, S. J. Electroanal. Chem. 1985, 186, 267.
measurement of γBr in the literature,9 by means of a titration method, suggested that it be less than one, viz., that bromide be adsorbed in anionic form. In contrast, a theoretical treatment of the electrosorption valency as reviewed by Conway10 would predict that γ deviate only marginally from a value of one, on the basis of the difference in electronegativity between Pt11 and Br12 which is ≈0.6 eV. The latter prediction is consistent with the experimental value of γ ) 1 for bromide on Au(111) reported recently by Shi et al.,13 with a difference in the electronegativity values of ≈0.5 eV. The first quantitative assessment of bromide adsorption on platinum single crystals by means of ex-situ LEED/ AES emersion experiments was reported by Salaita et al. for Pt(111)14 and by Baltruschat et al. for Pt[6(111)×(111)].15 On Pt(111) at pH 2 they observed (3×3) and (4×4) bromide overlayers depending on potential with an ideal bromide coverage, θBr, of ≈0.44 ML (monolayer based on Pt(111)); however, no attempt was made to evaluate γBr. In analogy, the same (3×3) bromide structure on Pt(111) is attained in ultrahigh vacuum (UHV) by dosing with either HBr16 or Br2.17 In a recent scanning tunneling microscopy (STM) study, Bittner et al.18 prepared bromide overlayers on low-index platinum single crystals by flameannealing and subsequent quenching in bromine vapor, an extension of the “iodine method” developed by Wieckowski et al.19 They assigned their STM image on Pt(111) attained in air to a (4×4) bromide overlayer, but no insitu STM measurements were reported to establish θBr versus potential. To our knowledge, the first in-situ measurements of bromide adsorption on Pt(111) come from (9) Lane, R. F.; Hubbard, A. T. J. Phys. Chem. 1975, 79, 808. (10) Conway, B. E. In Progress in Surface Science; Davison, S. G., Ed.; Pergamon Press: New York, 1984; Vol. 16, pp 1-138. (11) Emsley, J. The Elements; Clarendon Press: Oxford, 1989; p 140. (12) Lange’s Handbook of Chemistry; Dean, J. A., Ed.; McGraw-Hill Book Company: New York, 1973; pp 3-9. (13) Shi, Z.; Wu, S.; Lipkowski, J. Electrochim. Acta 1994, 40, 9. (14) Salaita, G. N.; Stern, D. A.; Lu, F.; Baltruschat, H.; Schardt, B. C.; Stickney, J. L.; Soriaga, M. P.; Frank, D. G.; Hubbard, A. T. Langmuir 1986, 2, 828. (15) Baltruschat, H.; Martinez, M.; Lewis, S. K.; Lu, F.; Song, D.; Stern, D. A.; Datta, A.; Hubbard, A. T. J. Electroanal. Chem. 1987, 217, 111. (16) Garwood, G. A.; Hubbard, A. T. Surf. Sci. 1981, 112, 281. (17) Bertel, E.; Schwaha, K.; Netzer, F. P. Surf. Sci. 1979, 83, 439. (18) Bittner, A. M.; Wintterlin, J.; Beran, B.; Ertl, G. Surf. Sci. 1995, 335, 291. (19) Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem. 1984, 23, 565.
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Langmuir, Vol. 12, No. 6, 1996 1415 sweep rate of 50 mV/s during the positive-going sweep (Bradsorption). Since mass transport does not affect the negativegoing voltammetric sweep (Br- desorption), the difference between 50 and 10 mV/s diminishes substantially. In ring-shielding experiments, the ring electrode is potentiostated such that Br- oxidation to Br2 occurs under diffusion control: E
r 1Br + e-; E° Br- 98 2 2 Br2/Br- ) 0.847 V (SCE, from ref 22) (1)
Thus, a ring potential of 1.08 V and the rather low Brconcentration are sufficiently large to produce a conversion of Br- to Br2 in excess of 99%, while being low enough to prevent further oxidation to HBrO22 and the formation of tribromide ions.22,23 In the absence of Br- adsorption/desorption on the disk electrode, the measured ring current represents the diffusionlimited conversion of Br- to Br2 and is referred to as the unshielded ring current, i∞r . Figure 1b shows a plot of (i∞r ) versus ω-0.5, yielding a straight line with a slope of Bring ) 1.42 rpm0.5/ µA and an intercept corresponding to a kinetic current exceeding 200 µA. Theoretically, Bring may be evaluated according to6,24
Figure 1. (a) Cyclic voltammograms of Pt(111) mounted in the disk position of the RRDE in 0.1 M HClO4 and 8 × 10-5 M Br-. In order to allow a comparison with the voltammetric response at 10 mV/s, currents at 50 mV/s were scaled by a factor of 1/5; the rotation rate in all cases was 900 rpm. (b) Steady-state diffusion-limited Br- reduction currents at the ring electrode (ring potential, Er ) 1.08 V; see eq 1) for rotation rates between 400 and 2500 rpm shown in a Levich-type plot of (i∞r )-1 versus ω-0.5. The straight line represents a leastsquares fit, yielding a slope of 1.42 rpm0.5/µA and an intercept of 4.32 × 10-3 µA-1.
our laboratory, in a recent study by Lucas et al.20 using surface X-ray scattering (SXS). Above ≈0 V (SCE) an ordered hexagonal overlayer was observed, undergoing compression with increasing potential; this overlayer is mostly incommensurate and is commensurate (3×3) (θBr, ideal ≈ 0.44 ML) only at one potential (≈0.05 V). In spite of being a powerful in-situ method, SXS can neither yield a measure of γ nor provide a complete bromide adsorption isotherm, since only well-ordered structures can be detected. In the following we will present a new in-situ approach for the assessment of both the bromide adsorption isotherm and its electrosorption valency on Pt(111): a rotating ringdisk electrode with a Pt(111) single crystal in the disk position (RRDPt(111)E). We will also show how the bromide adsorption isotherm may then be used to deconvolute the hydrogen adsorption/desorption process from the overall current response in the presence of Br-. 2. Experimental Section The Pt(111) crystal (0.283 cm2) was prepared by flameannealing and mounted in the disk position of an insertable rotating ring-disk electrode assembly (Pine Instruments). Subsequently, it was transferred into a standard electrochemical cell and immersed into 0.1 M HClO4 (EM Science Suprapure, made with triply pyrodistilled water) with 8 × 10-5 M Br- (KBr, Fluka Microselect) under potential control at ≈0 V. All potentials are referred to the saturated calomel electrode, SCE, separated by an electrolyte bridge. For further details see ref 1. The cyclic voltammogram of Pt(111) in 0.1 M HClO4 with 8 × 10-5 M Br- under a rotation rate of 900 rpm (Figure 1a) is in excellent agreement with the data in the literature14,21 and clearly demonstrates the high quality of the Pt(111) surface in the RRDPt(111)E configuration. On account of the rather low Brconcentration, mass transport resistances can be discerned at a (20) Lucas, C. A.; Markovic´, N. M.; Ross, P. N., Jr. Surf. Sci., in press. (21) Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. Electrochim. Acta 1995, 39, 1519.
i∞r ) 0.62β2/3nFAc0D2/3ν-1/6ω1/2 )
1 1/2 ω Bring
(2)
where β is a constant based on the ring and disk radii (β ) 0.89),25 n is the number of electrons (n ) 1), A denotes the disk area (0.283 cm2), D is the diffusivity of Br- (D ) 2.08 × 10-5 cm2/s26 ), ν is the viscosity of the electrolyte (ν ) 9.13 × 10-3 cm2/s5), ω is the rotation rate in radians/s (1 rpm ) π/30 radians/s), and c0 is the Br- bulk concentration. With the above values, the calculated slope, Bring ) 1.49 rpm0.5/µA, is in good agreement with its measured value. In the following we will make use of the ring-shielding properties of the RRDE to assess the mass flux of Br- from and to the Pt(111) disk electrode.25 If Br- is adsorbed at the Pt(111) disk, its concentration in the vicinity of the ring electrode is being reduced so that the ring current, ir, decreases from its unshielded value and vice versa. By means of the collection efficiency, N, of the RRDE assembly (N ) 0.22 ( 5%1,6), deviations of ir from i∞r are quantitatively related to changes of θBr on the Pt(111) disk in either potentiodynamic or potentiostatic experiments:
∫
1 (ir - i∞r ) dE 1 v ∆θBr ) AnN 240 µC/cm2
∫ (i ∞
1 ∆θBr ) 240 µC/cm2
0
r(t)
- i∞r ) dτ
AnN
(3)
(4)
where v refers to the sweep rate and 240 µC/cm2 is the ideal one-electron surface charge density of Pt(111) (1.50 × 1015 atoms/ cm2). Equations 3 and 4 constitute the basis for the below assessment of the bromide adsorption isotherm and its electrosorption valency.
3. Results and Discussion 3.1. Potentiodynamic Shielding Experiments. Figure 2 shows a ring-shielding experiment on Pt(111) in 0.1 M HClO4 with 8 × 10-5 M Br-at 50 mV/s (Er ) 1.08 (22) Standard Potentials in Aqueous Solution; Bard, A. J., Parsons, R., Jordan, J., Eds.; Marcel Dekker, Inc.: New York, 1985. (23) Conway, B. E.; Phillips, Y.; Qian S. Y. J. Chem. Soc., Faraday Trans. 1995, 91, 283. (24) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley and Sons: New York, 1980. (25) Albery, W.; Hitchman, M. Ring-Disc Electrodes; Oxford University Press: New York, 1971. (26) Newman, J. Electrochemical Systems; Prentice-Hall: Englewood Cliffs, NJ, 1973.
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Figure 2. Potentiodynamic ring-shielding experiment with a RRDPt(111)E in 0.1 M HClO4 and 8 × 10-5 M Br- at 50 mV/s and 900 rpm. (a) (s) Cyclic voltammogram on the Pt(111)-disk; (- - -) base voltammogram without bromide in solution under otherwise identical conditions for comparison. (b) Ring-shielding currents at a ring potential of 1.08 V; the value for the unshielded ring current, i∞r , is 19.36 µA. (c) Bromide adsorption isotherm in monolayers evaluated for the negative-going sweep (according to eq 3).
V), the significance of which we will now exemplify for the positive-going sweep. Starting from the negative potential limit, the ring current below approximately -0.15 V equals i∞r (Figure 2b), indicating the absence of bromide adsorption on the Pt(111) disk. This could similarly be inferred from the correspondence of the disk voltammograms with and without bromide in solution (Figure 2a) and is in agreement with the threshold potential for bromide adsorption found in ex-situ experiments.14 As the Pt(111)-disk potential is swept across the first voltammetric peak (Figure 2a), the associated adsorption of bromide is demonstrated by the concomitant decrease of the ring current below its unshielded value (Figure 2b). The same observation is made for the subsequent, more positive peak, which clearly relates this process to further bromide adsorption on the Pt(111)-disk. Following these two characteristic voltammetric peaks, the Pt(111)-disk current diminishes to a double-layer-like structure above ≈0.2 V. At the same time, the ring current remains below i∞r until the positive potential limit is reached, manifesting the continuous adsorption of bromide on the Pt(111)-disk, consistent with the potential-induced compression of the bromide adlayer on Pt(111) observed by SXS.20 The qualitative correspondence between ring and disk currents may be evaluated quantitatively in terms of θBr according to eq 3; to avoid the mass transport resistances addressed in section 2, the bromide adsorption isotherm will be extracted from the negative-going sweep. The resulting θBr versus disk potential is shown in Figure 2c, yielding a maximum coverage of ≈0.44 ML at the positive potential limit. At 0.2 V, θBr is ≈0.38 ( 0.02 ML, slightly smaller than the value of 0.47 ML inferred from in-plane
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Figure 3. Potentiodynamic ring-shielding experiment with a RRDPt(111)E in 0.1 M HClO4 and 8 × 10-5 M Br- at 50 mV/s and 600 rpm. Some of the transient ring and disk currents are shown for potential steps from an initial potential of -0.2 V to a final potential of -0.10 V (curve 1, for clarity, only the ring response is shown), -0.05 V (curve 2), +0.03 V (curve 3), +0.10 V (curve 4), +0.40 V (curve 5), and +0.55 V (curve 6). Top: Ring-shielding currents at a ring potential of 1.08 V; the value for the unshielded ring current, i∞r , is 16.2 µA. Bottom: Disk current transients; the diffusion-limited disk current, i∞d , inferred from eq 5 is indicated. Inset: Bromide adsorption isotherm based on eq 4; the cyclic voltammogram at the Pt(111) disk at 10 mV/s is shown for comparison.
SXS experiments at the same potential.20 This discrepancy is most likely due to the fact that the domain size of the ordered bromide overlayer found by SXS (≈150 Å) is substantially smaller than the average terrace size of the Pt(111) crystal (≈400 Å), such that the ideal coverage based on the superstructure geometry will overpredict the actual coverage which is measured by the RRDPt(111)E technique. Even though the major adsorption wave coincides with the more negative voltammetric peak, approximately 0.05 ML of bromide are being adsorbed during the second peak, clearly discernible by the inflection point of the isotherm at ≈0.02 V. Originally, in ex-situ measurements, this feature was attributed to a structural transition from a (4×4) (negative of the peak) to a (3×3) bromide adlayer concomitant with a postulated change in θBr of 0.007 ML.14 This is inconsistent with the data in Figure 2c as well as with our in-situ SXS experiments where an ordered structure was observed only at potentials positive of the second voltammetric peak. Therefore, we assign this voltammetric feature to a change in θBr producing the bromide coverage necessary for the transition from a disordered to an ordered phase. 3.2. Potentiostatic Shielding Experiments. In spite of the fast kinetics of bromide desorption, a true equilibrium isotherm of θBr versus potential must be deduced from potentiostatic ring-shielding experiments. Figure 3 shows the transient ring (top) and disk (bottom) currents as the disk potential is stepped from -0.2 V to successively more positive potentials in 0.1 M HClO4 with
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8 × 10-5 M Br- at 600 rpm. The experimental value of 16.2 µA for i∞r may be used to evaluate the diffusionlimited disk current, i∞d , for Br- adsorption:5
i∞d ) β-2/3i∞r
(5)
yielding i∞d ) 17.5 µA, which is marked in Figure 3. Stepping to -0.10 V (curve no. 1), a small negative deviation from i∞r is observed on the ring, indicating the adsorption of ≈0.05 ML of bromide on the Pt(111)-disk, as evaluated from eq 4 and shown in the inset of Figure 3 (the voltammogram in the inset is meant to provide a frame of reference). The corresponding disk transient is not shown for clarity but is analogous in nature to the one for a potential step to -0.05 V (curve no. 2): initially, a current spike is observed due to the fast desorption of adsorbed hydrogen (Hads) until its equilibrium coverage at the higher potential is reached (inferred from the voltammogram in bromide-free solution, Figure 2a); subsequently, however, the disk current remains above i∞d , indicating that the continuing adsorption of Br- on the Pt(111)-disk forces additional desorption of Hads on account of competitive adsorption. Curve no. 3 shows a potential step to +0.03 V, where hydrogen is almost completely desorbed from Pt(111) in bromide-free electrolyte (Figure 2a) such that an essentially unperturbed diffusion-limited bromide adsorption plateau can be observed on the disk, accompanied by the development of a current plateau on the ring electrode, referred to as the fully shielded ring current, ifs r . Similar observations hold for the potential step to +0.10 V, and the fully shielded ring current of 12.3 µA is in excellent agreement with its theoretical prediction:25 ∞ -2/3 ) ) 12.3 µA ifs r ) ir (1 - Nβ
(6)
As the disk potential is stepped into the potential region of anomalous adsorption27 (J0.3 V, Figure 2a), curve no. 5 at +0.40 V, the desorption of adsorbed species in the anomolous feature upon bromide adsorption produces a superposition of cathodic currents. If adsorbed hydrogen, as suggested,27-29 were the species desorbing in this region, a potential step positive of the anomalous feature (curve no. 6 to +0.55 V) would result in the immediate desorption of hydrogen followed by a diffusion-limited bromide adsorption current plateau on the disk contrary to the transient curve no. 6. The only hypothesis consistent with our experimental data is that the anomalous feature is related to the adsorption of oxygen-containing species (OHads), as proposed in refs 30 and 31, leading to cathodic currents as they are being displaced in the course of bromide adsorption. Even though many more potential-step experiments were conducted, the above examples demonstrate all of the observed characteristics. It may be noted that the initial oscillatory perturbation at the ring currents is produced by pH fluctuations due to the desorption of Hads such that they disappear if a less negative initial potential is chosen. The bromide adsorption isotherm derived from potentiostatic ring-shielding measurements is shown in the inset of Figure 3 and agrees to within 10 mV with the (27) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211. (28) Molina, F. V.; Parsons, R. J. Chim. Phys. 1991, 88, 1339. (29) Lynch, M. L.; Barner, B. J.; Corn, R. M. J. Electroanal. Chem. 1991, 300, 447. (30) Ross, P. N., Jr. In Electrochemical Surface Science; Soriaga, M. P., Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988; Vol. 378, p 37. (31) Markovic´, N.; Ross, P. N., Jr. J. Electroanal. Chem. 1992, 330, 499.
Figure 4. Deconvolution of potentiodynamic hydrogen adsorption/desorption currents from bromide desorption/adsorption on Pt(111) in 0.1 M HClO4 with 8 × 10-5 M Br- at 50 mV/s, based on the RRDPt(111)E data shown in Figure 2: (- - -) Pt(111) in the presence of Br-; (- - -) voltammogram of Pt(111) without Br-; (s) potentiodynamic hydrogen adsorption/desorption in the presence of Br-, extracted via eq 9 using γBr ) 0.95. Inset: Qtd is the disk charging curve in the presence of Br- (without double-layer correction); QBr d is the charge contribution to the disk due to bromide adsorption/desorption based on eq 8.
potentiodynamic data of Figure 2c, manifesting the fast kinetics of bromide adsorption/desorption. Dividing the Coulombic charge passed through the Pt(111)-disk by 240θBr µC/cm2 (extracted from ir via eq 3) will yield γBr if no other simultaneous processes occur on the disk. Following the above discussion, this condition is satisfied for the potential range between curves 3 and 4, for which neither bromide-induced hydrogen desorption nor the butterfly feature interferes. The resulting bromide electrosorption valency is γBr ) 0.95 ( 0.05 (note that the integration of the disk current below ≈0.25 s was carried out by assuming diffusion-limited bromide adsorption in this region, i.e., iBr ) i∞d ) 17.5 µA). An independent evaluation of γBr is based on the difference between i∞r and ifs r:
γ)
i∞d (i∞r - ifs r )/N
) 0.99
(7)
in good agreement (within our experimental error). Interestingly, a very similar value of γBr was previously found for the adsorption of Br- on a polycrystalline electrode,32 suggesting that the electrosorption valency of Br- on Pt surfaces is not a structurally sensitive quantity. Finally, having assessed both θBr and γBr, we can now deconvolute the adsorption/desorption of hydrogen on Pt(111) in the presence of bromide. (32) Nowak, D. M.; Conway, B. E. J. Chem. Soc., Faraday Trans. 1981, 77, 2341.
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3.3. Compression of Hads in the Presence of Br-. The voltammetry of Pt(111) in 0.1 M HClO4 with and without 8 × 10-5 M Br- at 50 mV/s and 900 rpm from Figure 2a is redrawn in Figure 4. The inset shows the total Coulombic charge of the Pt(111)-disk in bromide solution, Qtd, as well as the charge contribution which is due only to bromide adsorption/desorption, QBr d , extracted from the ring-shielding experiment (Figure 2b and c) according to 2 QBr d ) 240θBrγBr µC/cm
same potential with respect to the reversible hydrogen electrode in both HClO4 and KOH,31 it must be related to the adsorption of oxygen-containing species (OHads). We can now deconvolute hydrogen adsorption/desorption currents, iH, from the total disk currents on Pt(111) in the presence of bromide by subtracting the bromide flux measured in the ring-shielding experiments (Figure 2) from the disk current according to
(i∞r - ir)γBr iH ) id N
(8)
At the negative potential limit, no bromide is adsorbed on the Pt(111)-disk, such that a saturated monolayer of Hads must exist, identical to Pt(111) in bromide-free electrolyte. Sweeping positively, hydrogen desorbs as bromide is being adsorbed, finally leading to a saturation coverage of bromide above 0.5 V. At this potential, QBr d is ≈100 µC/ cm2, similar to the bromide charge contribution evaluated by measurements at various pH14 and smaller than the value reported in ref 21. More importantly, the difference 2 between Qtd and QBr d amounts to 170 µC/cm (inset of Figure 4) or to ≈160 µC/cm2 if corrected by a double-layer capacitance of ≈20 µF/cm2. Therefore, a saturated coverage of Hads on Pt(111) corresponds to the value associated with “weakly” adsorbed hydrogen on Pt(111) in the absence of bromide, a quite substantial proof that the anomalous feature is not adsorbed hydrogen as assumed originally27-29 and in agreement with CO displacement experiments.21 Furthermore, since the anomalous feature occurs at the
(9)
The resulting potentiodynamic curve is shown in Figure 4 (solid line) and demonstrates the reduced potential window of stability of Hads on Pt(111) in the presence of Br- due to competitive adsorption. Acknowledgment. The authors are greatly indebted to the invaluable assistance of Lee Johnson, who oriented, cut, and polished the crystal used in this study. Furthermore, he and Frank Zucca were assuring that the rotator kept rotating, not a small task for which we would like to express our thanks. This work was supported by the Office of Energy Research, Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. Department of Energy under contract no. DE-AC0376SF00098. LA950826S
