Poisoning the Active Site of Electrochemical Reduction of Dioxygen on

The four electron electroreduction of dioxygen to water on the (2 × 2) Bi upd ... have been examined using scanning probe microscopy and X-ray scatte...
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Langmuir 2000, 16, 1397-1406

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Poisoning the Active Site of Electrochemical Reduction of Dioxygen on Metal Monolayer Modified Electrode Surfaces Ilwhan Oh,† Mary Ellen Biggin, and Andrew A. Gewirth* Department of Chemistry and the Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801 Received July 26, 1999. In Final Form: September 29, 1999 The four electron electroreduction of dioxygen to water on the (2 × 2) Bi upd adlattice on Au(111) has been studied by deliberately poisoning the adlattice with thiocyanate and ethanethiol during the course of electroreduction activity. The diminution in reduction activity was monitored using chronoamperometry. For SCN-, the drop in current could be modeled using a Langmuir kinetic expression yielding an adsorption rate constant of 1.1 × 104 s-1 M-1. The rate for ethanethiol could not be measured exactly but is approximately the same. STM images of the surface obtained following introduction of SCN- revealed a (4 × 4) adlattice, which was partially (6%) defected. The percentage of defects agreed well with the percentage of residual current found at long times (3%) leading us to associate these defects with sites of catalytic activity. STM images obtained from surfaces poisoned with ethanethiol revealed two lattices: a (8 × 8) structure which was unstable and a more stable (x57 × 3) structure which is consistent with an overlayer of thiols lying flat on the surface. IR studies of the SCN--poisoned surface showed that the SCN- was S-bound to the surface at almost the same energy as that expected from SCN- bound to a bare Au(111) surface. XPS measurements on emersed samples showed that Bi and S were present on the surface. Analysis of these data suggests that the site of dioxygen association with the (2 × 2) Bi unpoisoned surface is the uncoordinated Au atom in the (2 × 2) unit cell.

1. Introduction One of the most important reactions in the electrochemical environment is the electroreduction of dioxygen to water. This importance stems not only from the utility of this reaction in fuel cells and fuel cell devices but also because of its relevance to corrosion and other processes. Despite intensive effort extending over several decades, relatively little is understood concerning fundamental aspects of either dioxygen or peroxide reduction.1,2 In the acid form of oxygen reduction, the reaction can proceed in either a concerted four-electron process as shown in eq 1 or via two two-electron steps, as shown in

O2 + 4H+ + 4e- f 2H2O

(1)

O2 + 2H+ + 2e- f H2O2

(2)

H2O2 + 2H+ + 2e- f 2H2O

(3)

eqs 2 and 3. In the two-electron process, cleavage of the O-O bond appears to be the rate-limiting step. The oxygen electroreduction process is highly irreversible and occurs with a substantial overpotential on most surfaces. This overpotential is important in slowing rates of corrosion on materials such as iron but is detrimental in fuel cell applications because of the direct relationship between overpotential and cell inefficiency. Both Pt and Ag have * To whom correspondence should be addressed. Tel: 217-3338329. Fax: 217-333-2685. E-mail: [email protected]. † Present address: Electrochemistry Group, Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Kusong-dong, Yusong-gu Taejon, South Korea 305701. (1) Tarasevich, M. R.; Sadkowski, A.; Yeager, E. In Oxygen Electrochemistry; Conway, B. E., Bockris, J. O. M., Yeager, E., Kahn, S. U. M., White, R. E., Eds.; Plenum: New York, 1983; Vol. 7, pp 301-398. (2) Adzic, R. In Recent Advances in the Kinetics of Oxygen Reduction; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; pp 197242.

been used as cathode materials for fuel cell devices. However, the kinetics of O2 reduction on either surface are slow at overpotentials typically utilized in fuel cells. The cost of Pt, its susceptibility to interference through crossover in direct methanol fuel cell devices, and the slow kinetics have been primary motivators in the search for new cathode materials. There is no real understanding of the mechanism(s) associated with cleavage of the O-O bond, the absorption sites of dioxygen or peroxide on the electrode surface, or the origin of the overpotential for dioxygen reduction. One set of systems exhibiting catalytic activity for dioxygen and peroxide electroreduction are those formed by the underpotential deposition (upd) process.3-5 In the acid electrochemical environment, underpotentially deposited submonolayers of Pb, Tl, and Bi on Au(111) all enhance the rate of peroxide reduction relative to the bare Au surface. Although probably of limited practical utility, these upd systems provide ideal assemblages to test issues concerning the interplay between surface structure and electrocatalytic activity, particularly because the surface structures associated with this activity have been examined using scanning probe microscopy and X-ray scattering techniques. Our work with Bi6,7 and Pb8 showed that only certain structures were associated with this catalysis. Specifically for Bi, the surface evinced three different lattice structures6,7 in the upd region as shown in Figure (3) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, pp 125-271. (4) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1984; Vol. 13, pp 159-260. (5) Aramata, A. In Modern Aspects of Electrochemistry; Bockris, J., Ed.; Plenum Press: New York, 1997; Vol. 31, pp 181-250. (6) Chen, C.-H.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 5439. (7) Chen, C.-h.; Kepler, K. D.; Gewirth, A. A.; Ocko, B. M.; Wang, J. J. Phys. Chem. 1993, 97, 7290-7294. (8) Chen, C.-H.; Washburn, N.; Gewirth, A. A. J. Phys. Chem. 1993, 97, 9754-9760.

10.1021/la991005g CCC: $19.00 © 2000 American Chemical Society Published on Web 11/17/1999

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Figure 1. Schematic showing the three different structures observed in the upd region for Bi: (A) bare Au(111); (B) (2 × 2) structure; (C) (p × x3) structure. The arrows in the (2 × 2) Bi cell point to uncoordinated Au atoms.

1. The first of these was the bare Au surface found at the most positive potentials. Just prior to bulk deposition the surface showed a (p × x3) structure with Φ ) 63-65% coverage. Neither of these is active for peroxide or dioxygen electroreduction. However, at intermediate potentials the surface evinced a (2 × 2) Bi structure with Φ ) 25% Bi coverage. This structure occurs only in the potential region where the surface is catalytically active, and we speculated that a heterobimetallic site was the actual site of dioxygen or peroxide association with the surface. Subsequently we showed that at least in the Bi upd system hydroxide was coadsorbed with Bi in the catalytically active (2 × 2) Bi (25% coverage) structure.9 The presence of hydroxide as the stabilizing anion in the Bi (2 × 2) upd structure led us to speculate that the hydroxide has a role in the electrocatalytic event as well. Such a role is not unimaginable. The special activity of Au(100) toward oxygen reduction in acid solution2 occurs in the potential region where OH is known to adsorb strongly on the surface. Hydroxide is also thought to play a role in biological dioxgen activation systems.10 Other authors have also examined the role of upd in mediating oxygen electroreduction activity. In the case of Pt, upd of Ag11 and Cu12 have been shown to switch the mechanism of electroreduction from the four electron to two-electron pathway. Analysis of the structure and coverage dependence of the upd adlayer suggested that the site of four-electron reduction activity was a bridge site part of which the upd poison blocks. A bimetallic site for dioxygen reduction has also been suggested for upd Pb8 on Au(111). Recently, Adzic and Wang used X-ray scattering measurements to propose that dioxygen electroreduction on Tl upd modified Au(111) surfaces led to oxidation of the Tl layer suggesting that Tl+ is involved in the reduction process.13 Whether this is a general result for upd systems remains to be seen. In this paper we report the results of measurements designed to elucidate the role of the particular configuration maintained in the Bi upd system that acts as a dioxygen reduction catalyst. In particular, we use anions that appear to replace the hydroxide whose presence in the Bi upd structure we inferred from electrochemical measurements. This anion replacement scheme has consequences with regard to both the structure and reactivity of the (2 × 2) Bi monolayer. 2. Experimental Section Electrochemical solutions were prepared from ultrapure water (Milli-Q UV plus, Millipore Inc., 18.2 MΩ cm) and Bi2O3 (Aldrich, 99.999%). The supporting electrolyte for Bi adsorption studies (9) Niece, B. K.; Gewirth, A. A. Langmuir 1996, 12, 4909-4913. (10) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem. Rev. 1996, 96, 2563-2605. (11) Adzic, R. R.; Wang, J. X. J. Serb. Chem. Soc. 1997, 62, 873. (12) Abe, T.; Swain, G. M.; Sashikata, K.; Itaya, K. J. Electroanal. Chem. 1995, 382, 73-83. (13) Wang, J. X.; Adzic, R. R. J. Phys. Chem. B 1999, in press.

Oh et al. was 0.1 M HClO4 (Baker, Ultrex II). The working electrode for cyclic voltammetric and chronocoulometric measurements was a Au(111) single crystal (Monocrystals Inc.) with a diameter of 1 cm and a nominal area of 0.785 cm2. The crystal had a roughness factor of 1.3 determined by integrating the current passed in the formation of the first layer of oxide in pure supporting electrolyte. The orientation of the crystal was confirmed with Laue´ backscattering. The crystal was annealed for 3 min in a hydrogen flame prior to use and quenched in ultrapure water. Oxide formation and stripping voltammetry of the surface in pure electrolyte were found to closely match that reported in the literature for Au(111).14 Voltammetric data were collected using a gold wire counter electrode and a saturated Hg/Hg2SO4 or AgCl reference electrode connected to the electrochemical cell via a capillary salt bridge. All potentials in this paper are reported relative to the normal hydrogen electrode. The solutions were purged with Ar prior to use, and an atmosphere of Ar was maintained in the cell during all electrochemical measurements. Potential control and sweeps were established using a Pine AFRDE-5 potentiostat. Voltammetric data were digitized and collected by computer using a Data Translation DT-2821 analogue I/O board and software written at the University of Illinois. Rotating disk electrode (RDE) measurements were obtained using a Pine model MSRX rotator equipped with a collet designed to hold the Au single crystal. STM images were obtained in constant current mode with Digital Instruments Nanoscope II or Nanoscope III instrumentation (Digital Instruments, Santa Barbara, CA) which was calibrated by imaging a highly ordered pyrolytic graphite (HOPG) surface in air. A mechanically cut Pt/Ir wire (Digital Instruments) coated with polyethylene was used as an STM tip. The working electrode was formed from Au evaporated onto glass and annealed following a published procedure15 while Pt and Au wires served as the reference and counter electrodes, respectively. Images were obtained in height mode and typically took 2 min to complete. Images are presented unfiltered except where noted. X-ray photoelectron spectroscopy (XPS) was performed on a Physical Electronics PHI 5400 XPS spectrometer equipped with a sample rotation stage. For this measurement, a Au(111) surface was prepared in solution using the poisoning protocol described below. The surface was then removed from solution, rinsed with purified water, and transferred through air to the spectrometer. Polarization modulation infrared spectra were collected on a Nicolet Magna-IR System 550 spectrometer. The instrument was modified by directing the IR beam into an external sample compartment. The IR light was focused with a 60° off-axis parabolic mirror onto the sample at an angle of incidence of 69° relative to the normal. Prior to the sample, p-polarized light was selected by a ZnSe polarizer and passed through a 74 kHz Hinds photoelastic modulator. Light was collected with a ZnSe lens and focused onto a liquid-nitrogen-cooled MCT-A detector. The detector signal was sent to a synchronous sampling demodulator (GWC Instruments, Madison, WI),16 which was used to obtain an average ((Ip + Is)/2) and difference (Ip - Is) signal. The electrochemical sample cell was made of Kel-F and glass. The Ag/AgCl reference electrode was connected to the cell via a capillary bridge, and the Au counter electrode was placed into the cell through a Teflon fitting. A CaF2 trapezoidal prism (Wilmad Glass, Buena, NJ) was attached to the cell by a metal holder and sealed with an O-ring. The Au(111) single crystal working electrode, held onto a glass plunger with suction, was pressed against the prism and maintained in tension against the surface with rubber bands. Spectra were collected with 4 cm-1 resolution. Twenty blocks of 10 average scans and 50 difference scans each were taken, and the 20 spectra were then averaged together. (14) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429-453. (15) Will, T.; Dietterle, M.; Kolb, D. M. In Nanoscale Probes of the Solid-Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; Kluwer: Dordrecht, The Netherlands, 1995; Vol. 228, p 137. (16) Barner, B. J.; Green, M. J.; Saez, E. I.; Corn, R. M. Anal. Chem. 1991, 63, 55.

Electrochemical Reduction of Dioxygen

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3.1. Voltammetry and Catalytic Behavior. Figure 2A shows cyclic voltammetry obtained from a solution containing 0.5 mM Bi3+ and 0.1 M HClO4 on a Au(111) single crystal. The voltammetry shows the already reported upd of Bi onto Au(111).9 The first peaks, labeled C1 and A1 in the figure, correspond to the adsorption and desorption, respectively, of the (2 × 2) Bi upd adlayer. Following formation of this structure, the second upd peaks C2 and A2 correspond to transformation between the (2 × 2) and (p × x3) Bi upd adlayers. Bulk deposition occurs at 0.190 V on the scale shown in Figure 2A. The potential region between peaks C1 and C2 is associated with the presence of the (2 × 2) open adlayer structure which is known to catalyze the four-electron electroreduction of dioxygen to water17 according to the scheme given in eq 1. Figure 2B shows the RDE i-E response for a Au(111) surface immersed in 10 mM H2O2 + 0.5 mM Bi3+ + 0.1 M HClO4 and rotating at 400 rpm. At positive potentials, the electroreduction current is effectively zero, but as E is scanned toward more negative potentials, cathodic current begins to flow, which is due to the catalytic reduction of H2O2 by the Bi adlayer. The Bi system is assumed to be responsible for the electroreduction behavior because without Bi3+ in solution there is virtually no current until a potential of 0 V as shown by the dashed line in Figure 2B. At the onset of the Bi upd deposition wave (near 0.40 V) the catalytic current rises abruptly. At this point, the plot resembles the characteristic

sigmoidal curve obtained from a mass-transfer limited electrochemical reaction at an RDE.18 Around 0.35 V, the current begins to reach its maximum value and drops abruptly upon further negative potential excursion. The small residual current (ca. 5% of maximum) is due to the reduction of H2O2 by the (p × x3) Bi adlayer. If the potential sweep direction is reversed, the current again rises quickly at the onset of the (p × x3) to (2 × 2) structural change, demarcated by the peak at A2. The inset to Figure 2B shows the behavior of the maximum of the current as a function of rotation rate. The maximum current increases with the square root of the rotation rate for low rotation rates as is expected from the Levich equation.18 Above 900 rpm, however, the maximum current begins to saturate, because the rate of the electrode reaction is now rate-determining rather than mass-transfer-limited. The maximum rate of reduction was v ) 9.1 × 1015 s-1 cm-2 which corresponds to about 25 s-1 per site assuming the active site is the (2 × 2) adlattice and this completely covers the Au(111) crystal. As the rotation speed increases, the potential at which the maximum in electroreduction current occurs becomes more negative, along with the potential at which the current is inhibited. Since CVs in control experiments obtained using the rotating disk apparatus show no change with rotation speed, this result suggests that the more rapid flux of peroxide to the surface at high rotation rates affects the potential at which the structural transformation between the (2 × 2) and (p × x3) lattices occurs. The reasons for this change could be related to the possible displacement of the stabilizing hydroxide anion by peroxide. In the original measurements performed by Ju¨ttner with a stationary electrode17 the sigmoidal wave was absent and the peak was consequently much narrower (40 mV vs 100 mV seen here). 3.2. Poisoning by KSCN. Figure 3A shows the temporal response of the electroreduction current to the introduction of KSCN. Initially, we poised the potential of the electrode at 0.39 V which is in the middle of the first and the second upd peaks in a solution containing 10 mM H2O2 + 0.5 mM Bi3+ + 0.1 M HClO4 with a RDE rotation speed of 400 rpm. At this potential, peroxide reduction is catalyzed by the Bi upd adlayer. Initially, the current reaches a steady-state value, which depends on H2O2 concentration, RDE rotation speed, and electrode potential. With the electroreduction current at a value of ca. 3 mA, the variation of [H2O2] in solution due to H2O2 consumption is ca. 10-7 M s-1 for our experimental setup. Thus, the variation of [H2O2] is negligible for the time scale of the experiment (less than 10 min). While the electrocatalysis is occurring, a small amount of a 50 µM KSCN stock solution was injected into the slowly stirred solution using a syringe, resulting in a final SCNconcentration in the cell of 1.7 µM. The volume of the injected solution was about 5% of that of the original solution in the cell, so variations in [H2O2], [Bi3+], and [HClO4] following injection were small. Upon injection of KSCN, there is a small increase in the current, which might be due to the perturbation of convection by the injected solution or by the change in the meniscus between the crystal and the solution. The solution becomes homogeneous within 5 s after injection. Figure 3A shows that the current abruptly decreases in a few seconds after injection, the time scale for which depends on the amount of KSCN injected. However, the current does not decay entirely to zero; in Figure 3A about

(17) Sayed, S. M.; Juttner, K. Electrochim. Acta 1983, 28, 16351641.

(18) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley and Sons: New York, 1980.

Figure 2. (A) CV in 0.5 mM Bi3+ + 0.1 M HClO4 on a Au(111) single crystal. The scan rate is 5 mV/s. (B) RDE voltammogram in 10 mM H2O2 + 0.5 mM Bi3++ 0.1 M HClO4. The rotation speed is 400 rpm, and the scan rate is 5 mV/s. Dashed line: control experiment without Bi3+. Inset: plot of current as a function of rotation rate obtained at the maximum of electroreduction activity.

3. Results and Analysis

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Figure 4. RDE i-t response in 10 mM H2O2 + 0.5 mM Bi3+ + 0.1 M HClO4 on a Au(111) single crystal when the electrode is rotating at 400 rpm and the electrode potential is 0.39 V. A small volume of 5.4 mM ethanethiol was injected at the moment indicated by the arrow.

in Figure 3B, we find that τ relates to c as

τ ) 1/(ck) Figure 3. (A) RDE i-t response in 10 mM H2O2 + 0.5 mM Bi3+ + 0.1 M HClO4 on a Au(111) single crystal when the electrode is rotating at 400 rpm and the electrode potential is 0.39 V. A small volume of 50 µM KSCN was injected at the moment indicated by the arrow. (B) Linear relationship between the time constant of catalyst inhibition (τ) and the reciprocal of poison concentration (1/[KSCN]), as predicted by the first-order Langmuir kinetics.

3% of the initial current remains at long times (>400 s). As expected, the more dilute the poison in the solution, the longer it takes for the current to decay. This decay could be fit reasonably well to an exponential decay curve of the form

I ) Ae-t/τ + I∞

(4)

where A is an exponential factor, τ is the time constant of catalysis inhibition, and I∞ is the residual catalytic current. To examine the effect of KSCN concentration on the rate of catalyst inhibition, the above catalyst poisoning experiment was repeated with different [KSCN]. In Figure 3B, values of the time constant of catalysis inhibition (τ) are plotted against 1/[KSCN], and the resultant plot exhibits a linear relation. According to first-order Langmuir kinetics,19 the time evolution of the surface coverage (θ) of an adsorbate can be written as

θ ) 1 - e-ckt

(5)

where c is the concentration of the adsorbing species and k is the rate constant of adsorption. If we assume that the surface coverage of SCN- is proportional to the decrease in the peroxide electroreduction current, then

θ ) (I0 - I)/I0

(6)

where I0 is the initial steady-state current. From the plot (19) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; WileyInterscience: New York, 1990.

(7)

A least-squares fit of the data in Figure 3 gives the value of k, and it was measured to be 1.1 × 104 s-1 M-1 for the poisoning of (2 × 2) Bi upd adlayer by SCN-. An alternative but equivalent formulation of Langmuir kinetics is to plot 1/τ as a function of c. The slope of this plot gives k, and the intercept gives the desorption rate, kd. For our system, kd was small and negative, indicating that desorption of SCN- is not an important process. For comparison, measurements of the rate constants for the adsorption of octadecane thiols on Au(111) in different solvents gave values on the order of 2 × 103 s-1 M-1.20 3.3. Poisoning by Ethanethiol. To examine more fully the poisoning mechanism, we interrogated the effect of ethanethiol, CH3CH2SH, on the Bi-modified Au(111) surface. Thiols are well-known as strong adsorbates on Au surfaces,21 even when the adsorption is performed with the surface held at negative potentials. Ethanethiol was chosen as a poison over longer-chain alkane thiols, which are known to order more completely on Au surfaces,22 because ethanethiol is soluble in the solutions used here and the larger alkanes were not. Long-chain thiols have already been shown to replace more weakly coadsorbed anions in both Cu and Ag upd on Au(111) surfaces.23-26 Figure 4 shows the RDE i-t response from a solution containing 10 mM H2O2 + 0.5 mM Bi3+ + 0.1 M HClO4 on a Au(111) single crystal with RDE rotation speed of 400 rpm obtained at a potential of 0.39 V. We note that the initial current here is somewhat different from that shown in Figure 3, even though the conditions are ostensibly the same. This disparity probably relates to (20) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 33153322. (21) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (22) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-40. (23) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23-L25. (24) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208-5214. (25) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. Langmuir 1997, 13, 5215-5217. (26) Cavalleri, O.; Gilbert, S. E.; Kern, K. Surf. Sci. 1997, 377, 931936.

Electrochemical Reduction of Dioxygen

the steep slope of the i-E curve shown in Figure 2B at this potential. While the electrocatalysis was occurring, a small amount of a 5.4 mM ethanethiol stock solution was injected into the solution yielding a resulting ethanethiol concentration of 0.5 mM. Upon injection, the current abruptly decreases, the time scale for which is at least 2 orders of magnitude greater than that observed with much more dilute concentrations of SCN-. Since it takes approximately 5 s to mix the solutions together in our apparatus, we could not quantitate the rate of adsorption. However, from fitting the longer time part of the current decay to an exponential, a lower bound for k was determined to be ca. 3 × 103 M-1 s-1. This number is approximately the same order of magnitude as the rate typically quoted for long-chain thiol assembly onto Au(111) surfaces.20 The rate of assembly of the short-chain thiol on surfaces in the electrochemical environment has not been measured.27 3.3. STM Imaging. To elucidate the structure of Bi upd electrocatalyst poisoned by SCN- and ethanethiol, we performed STM measurements on these systems. We previously characterized the Bi upd system on Au(111) using both AFM and STM. To reproduce the structures responsible for the decay in electroreduction current described above, we held the electrode potential at 0.37 V and injected sufficient KSCN stock solution into the STM cell to yield a final SCN- concentration in the cell of 0.5 mM. Following injection of SCN-, the (2 × 2) Bi UPD adlayer disappears suddenly to be replaced after ca. 30 min by a new structure as shown in Figure 5A. The STM image shows a hexagonal array of spots, the appearance of which was observed to be quite uniform over the entire imagable area of the surface. The spots are composed of two components, a small bright area at the center and the surrounding diffuse part. The distance between the spots is 1.17 ( 0.07 nm, which is 4 times the interatomic distance of Au. Since the lattice vectors of this structure are aligned with those of the (2 × 2) adlattice, this structure must be a (4 × 4) adlattice and the ideal spot coverage then is Φ ) 0.0625 ML. The image in Figure 5A shows the presence of some defects in the lattice. As marked by arrows in Figure 5A, bright parts of some spots are missing in an irregular pattern. The density of these missing spots was measured to be ca. 5-6% of the total spot coverage. Figure 5B shows a profile of the STM image along the line marked in Figure 5A. The profile shows the periodic array of peaks associated with the bright spots in Figure 5A, the apparent height of which ranges up to 0.14 nm. The profile also shows the peak associated with the missing spot along the line in Figure 5A. The height of this peak is 0.07 nm. For reference, the length of an SCN- molecule is roughly 0.5 nm. Figure 6 shows the structures associated with poisoning of the (2 × 2) Bi upd structure with ethanethiol. As with the SCN-, the potential was first poised at 0.38 V in order to form the (2 × 2) structure. An aliquot of ethanethiol was then injected into the STM cell in order to yield a resultant ethanthiol concentration of ca. 0.5 mM. Following injection, the (2 × 2) Bi upd adlayer abruptly disappeared to be replaced after ca. 20 min by new features, a long-range view of which is shown in Figure 6A. Figure 6A shows a series of islands (H1) and terraces (H2), upon which a hexagonal corrugation is apparent, even at the long range shown. A one-dimensional profile of this image, taken along the line drawn in Figure 6A, is shown in Figure (27) Hagenstrom, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435-2443.

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Figure 5. (A) 11.3 nm × 11.3 nm in-situ STM image of the (4 × 4) adlayer obtained following addition of KSCN to the (2 × 2) Bi upd structure to make a 0.5 mM solution in 0.5 mM Bi3+ + 0.1 M HClO4 on Au(111). Missing dots in the image are marked by arrows. E ) 0.36 V, Ebias ) -0.39 V, Itip ) 2 nA, and scan rate ) 20.3 Hz. (B) STM profile along the line indicated in (A). The missing dot in the image is marked by an arrow.

6B. The section profile shows that the island with the hexagonal corrugation is approximately 0.13 nm above the corresponding terrace, while the terraces themselves are separated by 0.27 nm. This latter number is close to that expected for an atomic step on an Au(111) surface (0.24 nm). However, the former is too small to be anything but a step induced by molecular association with the surface. A high-resolution image of the island features shown in Figure 6A is shown in Figure 7A. This 26 nm by 26 nm image shows a honeycomblike structure of holes surrounded by six dots. The spacing between the holes is 2.35 ( 0.08 nm which is eight times the interatomic spacing of Au. We thus assign this structure to an (8 × 8) lattice. The ideal coverage of an (8 × 8) primitive unit cell on a (111) surface is Φ ) 0.015, which is the hole coverage. However, since six spots surround each hole, the actual spot coverage is actually twice this or 3%. The honeycomb island is so fragile that it becomes more and more disordered as scanning is repeated in the same area. Figure 7B shows a cross section along the line indicated in Figure 7A. This profile is taken along the next-nearest neighbor direction. Each minimum of the corrugation corresponds to the dark spot in Figure 7A and each maximum to the bright spots. The height difference between the maximum and the minimum (A) is 0.07 nm. The maximum is split at its center, which clearly shows the two neighboring bright dots in Figure 7A. The distance between the two neighboring dots (B) is 1.4 nm, which agrees well with the calculated value of 1.36 nm for (8 × 8) honeycomb. Finally, the depth of the pit between the two neighboring dots (C) is 0.01 nm. For reference, the

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Figure 8. 10 nm × 10 nm in-situ STM image of the terrace area (H2) in Figure 6A, which shows a (x57 × 3) ordered structure. A unit cell is indicated by the box. Ebias ) 20 mV, Itip ) 1 nA, E ) 0.38 V, and scan rate ) 27.1 Hz.

Figure 6. (A) 138 nm × 138 nm in-situ STM image of the (2 × 2) Bi upd adlayer following addition of ethanethiol to make a 0.5 mM solution in 0.5 mM Bi3+ + 0.1 M HClO4 on Au(111). The regions with a honeycomb island (H1) can be distinguished from the terrace area (H2). E ) 0.38 V, Ebias ) 0.37 V, Itip ) 3 nA, and scan rate ) 10.2 Hz. (B) STM profile along the line indicated in (A). Heights of the honeycomb island (H1) and terrace (H2) are measured to be 0.15 and 0.15 nm, respectively.

Figure 7. (A) 26 nm × 26 nm in-situ STM image of the (8 × 8) honeycomb islands (H1) seen in Figure 6A. E ) 0.38 V, Ebias ) 0.37 V, Itip ) 2 nA, scan rate ) 10.2 Hz, and measurement taken in constant-height mode. (B) Height profile along the line indicated in (A). The two neighboring dots are clearly resolved. The image in panel A was Fourier-filtered before profiling.

length of ethanethiol moleculesnot including the thiol protonsis about 0.5 nm.

A completely different structure was observed in the terrace areas of the image in Figure 6A. This structure, shown in Figure 8, exhibits a unit cell with a periodicity of 0.87 ( 0.04 nm in the short direction and 2.2 ( 0.3 nm at 60° to this vector in the long direction. This first spacing is three times the Au(111) spacing. The second spacing is x57 times the Au(111) lattice. The unit cell is a (x57 × 3) structure. Because this lattice is a superset of the underlying 3-fold periodicity of the Au surface, different domains of the structure should be rotated with respect to each other by 60°. Indeed, at the lower left part of the image, a phase boundary with the two phases rotated by 60° is observed. This rotation is consistent with that expected for the (x57 × 3) lattice. In contrast to the (8 × 8) structure described above, the (x57 × 3) lattice was stable to repetitive imaging. The lattice is not simple. As shown in Figure 8, the structure consists of an oblong feature approximately 1.0 nm long closely associated at one end with a single dot. Another row consists of dimers of spots. Although the long feature is almost exactly the length expected for two ethanethiol molecules associated via a disulfide linkage, there is no easy assignment available for the three spots in the unit cell. 3.4. Infrared Studies of the Poisoned Surface. We used polarization modulation infrared spectroscopy to characterize more fully the poisoned electrocatalyst surface. The Au(111) single crystal was immersed initially into a solution containing 0.5 mM Bi3+ + 0.1 M HClO4, and the potential was swept to 0.39 V in order to form the (2 × 2) Bi adlayer and provide a background spectrum. At this point the crystal was withdrawn several millimeters from the CaF2 prism and a sufficient amount of KSCN to form a 1 mM final concentration was injected into the cell. After the solution was allowed to equilibrate, the crystal was again pressed against the CaF2 prism and a spectrum obtained. Figure 9 shows the FTIR spectrum of the region between 2000 and 2200 cm-1 obtained after ratioing the spectrum obtained at 0.39 V with KSCN in solution against a background taken at the same potential without KSCN in solution. The spectrum shows a single peak centered at 2100 cm-1 exhibiting a full width at half-maximum (fwhm) of 30 cm-1. There is no evidence for any intensity at 2068 cm-1, which is the frequency exhibited by the CN

Electrochemical Reduction of Dioxygen

Figure 9. Polarization modulation FTIR spectrum obtained from a Au(111) crystal poised in the (2 × 2) Bi region obtained following addition of SCN-. The peak at 2100 cm-1 is associated with SCN- on the surface.

stretch of free SCN-. In separate measurements at 0.6 V, outside of the upd region, the IR spectrum revealed a CN stretch of 2130 cm-1, which is the frequency expected for SCN- adsorbed on a Au(111) surface.28 We were unable to obtain spectra in the C-H stretch region for the ethanethiol-modified surface. However, we can routinely obtain high-quality spectra from C18 thiol modified Au surfaces immersed in acid solution exhibiting both the methyl and methylene stretches. This result suggests that either the ethanethiol is too sparsely present on the surface or that the C-H stretches are not oriented appropriately normal to the electrode surface to be apparent in the infrared spectra from the surface. Infrared studies of shorter chain thiols on Au(111)29 show that monolayers of this material are typically less crystalline and more defected than those formed by longer chain thiols. We also note that a reasonable interpretation of the STM images has the thiols lying flat on the surface; although hydrocarbons lying flat on surfaces30 can yield reasonable IR intensities in UHV, this may not be the case for these very short thiols immersed in solution. 3.5. Electrochemical Studies of the Poisoned Surface. We used voltammetric methods to characterize the poisoned electrocatalyst more fully. For the SCNpoison, we first prepared the poisoned surface on a Au(111) crystal by scanning the potential to 0.39 V in solution containing 0.5 mM Bi3+ + 0.1 M HClO4. After holding the potential at this value, we injected a small volume of a 10 mM KSCN stock solution into the electrochemical cell in order to achieve a resulting [SCN-] of 1 mM. After holding at 0.39 V for 5 min, we scanned the potential to positive values, and the resultant linear sweep voltammogram is given in Figure 10A. As shown in Figure 10A, a single desorption peak is observed with a peak maximum of 0.41 V, which is shifted by 12 mV from the normal (2 × 2) Bi desorption peak potential. The broad desorption peak centered at 0.52 V in the normal Bi upd voltammetry is not observed in SCN- poisoned case. The formal potential shift by the change in [Bi3+] due to dilution by the KSCN solution injected into the cell is expected to be about 1 (28) Ong, T. H.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1993, 97, 12047-12050. (29) Anderson, M. R.; Gatin, M. Langmuir 1994, 10, 1638-1641. (30) Manner, W. L.; Bishop, A. R.; Girolami, G. S.; Nuzzo, R. G. J. Phys. Chem. B 1998, 102, 8816-8824.

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Figure 10. (A) Linear sweep voltammogram of (2 × 2) Bi upd adlayer which was poisoned by SCN- to make a final concentration of 1 mM in 0.5 mM Bi3+ + 0.1 M HClO4. Scan rate: 5 mV/s. Dashed line: normal Bi upd desorption peaks plotted for comparison. (B) Cyclic voltammogram in 1 mM KSCN + 0.45 mM Bi3+ + 0.09 M HClO4 on Au(111). Scan rate: 5 mV/s. Dashed: cyclic voltammogram of Bi upd absent KSCN.

mV, so this concentration change is not the likely source of the change in desorption potential. The charge for this peak integrated from the baseline is 56 µC, which is significantly smaller than the 140 µC measured on the same crystal for desorption of the (2 × 2) Bi adlayer absent SCN-. This suggests that addition of SCN- has occasioned removal of ca. 60% of the Bi from the surface. Voltammetry for Bi upd in solutions containing SCNgives additional information concerning the effect of this anion. Figure 10B shows voltammetry in a solution containing 1 mM KSCN + 0.45 mM Bi3+ + 0.09 M HClO4 on a Au(111) single crystal. Relative to Bi upd absent SCN- the presence of this anion gives rise to an additional peak in the upd region. Comparison of parts A and B of Figure 9 shows that the position and integrated charge of the final upd desorption peak are nearly identical. This means that the structure and stability of the features derived from the introduction of SCN- are thermodynamic rather than kinetic in origin and that the protocol useds designed to possibly trap a kinetic intermediatesis unnecessary for SCN-. The position and the shape of the peaks were quite reproducible with subsequent potential scans. The position and charges associated with each desorption peak in the voltammetry were the following: A1, 0.41 V, 56 µC; A2, 0.38 V, 4.2 µC; A3, 0.34 V, 38 µC. Figure 11 shows the CV obtained from a solution containing 0.45 mM ethanethiol + 0.45 mM Bi3+ + 0.09 M HClO4 on Au(111). The peaks at 0.20 V and at 0.49 V correspond to the adsorption and desorption of Bi, respectively. Just negative of the adsorption peak, Bi bulk deposition begins to occur and the small desorption peak at 0.22 V is due to the dissolution of the bulk Bi deposit. There are additional anodic peaks at 0.40, 0.45, and 0.48 V, the first one of which corresponds well with the upd peak of Bi absent thiols in solution (shown as the dashed line in Figure 11). These additional peaks become more distinct as potential scan is repeated, probably because Bi deposition and/or scanning the potential damages thiol layer. Compared to the CV of normal Bi upd, the adsorption peak is shifted to quite negative values and the desorption peak is shifted to positive values. The integrated charge of the desorption peak is 180 µC, which is comparable to

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Figure 11. Cyclic voltammogram in 0.45 mM ethanethiol + 0.45 mM Bi3+ + 0.09 M HClO4 on Au(111). Scan rate: 10 mV/s. Dashed: cyclic voltammogram of Bi upd for comparison.

120 µC for the (2 × 2) Bi desorption. This voltammetry is qualitatively similar to that reported by others for Cu upd on Au(111) in the presence of thiols23,25,31 evincing the delayed metal stripping peak. The additional peaks found at potentials more negative from those associated with the main stripping peak appears to be unique to the Bi system, however. Electrochemical measurements obtained when the surface was prepared by first forming the (2 × 2) Bi lattice and then adding ethanethiol were not reproducible. In some trials, the stripping voltammetry exhibited a peak at 0.40 V and another peak at 0.49 V as described above. However, in other trials, we saw only a broad anodic wave with no particular structure. The different voltammetries may relate to differences in order and structure of the thiol layer on the Bi adlattice; we were not able to control for the (8 × 8) or (x57 × 3) structures. 3.6. X-ray Photoelectron Spectroscopy. We performed X-ray photoelectron spectroscopy (XPS) measurements on emersed samples in order to obtain more information on the elemental composition of the poisoned catalyst surface. The poisoned surface was prepared in the same fashion as for the IR or electrochemical studies but was then emersed from solution and transferred through air to the spectrometer. Figure 12A shows XPS from a SCN--modified Bi upd surface. The portion of the spectrum shown exhibits the Bi 4f and the S 2p peaks. The Bi 4f7/2 peak is at 159.3 eV, which is shifted by 2.3 eV from the pure Bi metal peak position of 157.0 eV. This positive shift is consistent with oxidation of the Bi adlayer. The origin of this oxidation could either be association of SCN anion with the Bi or it could also be due to the formation of Bi2O3, especially as there is an O1s peak in the spectrum. Between the two Bi 4f peaks lies the S 2p peaks. The noise level in the spectrum is so high that S 2p1/2 and S 2p3/2 peaks are not resolved. Figure 12B shows that the XPS response for ethanethiol-(2 × 2) Bi-Au(111) is similar to the SCN- case. As in the SCN- case, the Bi 4f7/2 peak is at 159.3 eV, which is shifted by 2.3 eV from the pure Bi metal peak position of 157.0 eV. The Bi 4f7/2 peak position coincides with that for Bi2S3, and the lack of oxygen signal in this XPS spectrum precludes assigning this to the isoenergetic Bi2O3. Between the two Bi 4f peaks lies the S 2p1/2 and S 2p3/2 peaks which are resolved in this spectrum. The S (31) Zamborini, F. P.; Campbell, J. K.; Crooks, R. M. Langmuir 1998, 14, 640-647.

Figure 12. (A) Ex-situ X-ray photoelectron spectrum of (2 × 2) Bi-Au(111) which is poisoned by 10 µM SCN-. The detector is at a 45° angle of incidence from the surface normal. (B) Exsitu X-ray photoelectron spectrum of (2 × 2) Bi-Au(111) which is poisoned by 0.5 mM ethanethiol. The detector is at the surface normal.

2p3/2 is at 162.2 eV, which is shifted by -1.8 eV from the pure S element peak position of 164.0 eV. The position of this peak is identical to that seen both with thiolates adsorbed on bare Au surfaces22,32 and with that found from longer chain thiolates on surfaces first modified with Cu upd.24 This shift means that S atom in ethanethiol is likely reduced (or more electronegative) than elemental S.22 4. Discussion The images and spectra shown in this paper speak to significant changes in both the structure and reactivity of the (2 × 2) Bi additive upon addition of either of the two anions used in this work. These changes provide illumination to the mechanism of dioxygen or peroxide electroreduction as it occurs in the presence of this adlattice. 4.1. Structural Changes. Addition of SCN- to the (2 × 2) Bi adlayer gives rise to a (4 × 4) structure evincing 10% Bi coverage on the surface as measured by coulometry assuming that Bi is fully discharged. This 10% figure is somewhat higher than the spot coverage of 6% measured in the STM image shown in Figure 6A. We tentatively assign each spot to SCN-. AFM and STM images of Cu upd in the presence of sulfate reveal structures which are now known to be due to the anion.33,34 However, definitive assignment of the structure must await more detailed X-ray scattering measurements. IR measurements show that SCN- is indeed associated with the surface and that the SCN- is likely S bound and oriented nearly vertical (32) Laibinis, P. E.; Whitedsides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, T. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (33) Blum, L.; Huckaby, D. A. J. Electroanal. Chem. 1994, 375, 6977. (34) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R. Phys. Rev. Lett. 1995, 75, 4472-4475.

Electrochemical Reduction of Dioxygen

to the surface. The energy of the CN stretch is very close to that expected for SCN- bound directly to Au. It is now well understood that the structure of nonclose-packed upd adlayers is controlled by anion coadsorption. In what is perhaps the best understood case, sulfate templates the subsequent deposition of Cu on Au(111) forming a 2/3 coverage (x3 × x3)R30 structure in the upd region.35 We have reported that anion size is also a factor in monolayer structure with larger anions giving rise to more open adlattices.36 As anions associate more strongly with the electrode surface, the position of the upd peak shifts to more negative potentials, in part because the anion must be displaced or interrupted in order for the metal adatom to approach the surface. As an example with halide as the anion, the stronger the halide adsorption to the surface, the larger the negative shift of the upd peak.3 The anion coadsorbed with Bi in the (2 × 2) structure is hydroxide. This hydroxide must be relatively weakly bound to the upd adlattice in the pH ) 0 solutions typically used to form the Bi adlayer. The STM images and the coulometry show that incorporation of SCN- yields a different structure which incorporates less Bi. SCN- is a stronger ligand for Au than is OH-, and the shift of the most positive upd peak some 40 mV more negative attests to this increased strength. The SCN--modified surface also exhibits two other upd peaks at still more negative potentials. The SCN- likely gives rise to different upd structures evincing less Bi on the surface for several reasons. First, the stronger association of SCN- with Au means that the upd activity will occur at more negative potentials. The most prominent upd peak occurs at 320 mV vs NHE. This means that most of the Bi could have desorbed by the time the last upd peak is approached. Second, SCN- is larger than OH- and so takes up more space. This too could produce a more dilute Bi surface coverage. Introduction of ethanethiol to the Bi (2 × 2) adlattice also changes the structure of the surface. Unlike the simple structure observed with SCN- the structures observed with the thiol are complex and the (8 × 8) structure seems unstable with respect to repeated scanning with the tip. Analysis of the spacings observed for the (x57 × 3) lattice in the presence of Bi suggests that this phase exhibits ethanethiol lying flat on the Au(111) surface because the oblong units seen in the unit cell are of the correct length for two ethanethiols to be lying flat on the surface with a weak association between the S headgroups. This model would be similar to the lying down phase reported by Poirier and Scoles for longer alkanethiols on Au(111).37,38 Ethanethiol forms ordered arrays on Au(111) surfaces in the absence of upd Bi as well. Kolb and co-workers examined the structure of ethanethiol on Au(111) in the absence of Bi.27 They found two different structures on the surface: a (p × x3) phase in common with that already observed for longer-chain alkanes39 and a (4 × 3) phase which was unique to ethanethiol. Neither of these are similar to the (x57 × 3) lattice found in this work although the (p × x3) structure could be a superset of this. The orientation of the ethanethiols with respect to the Au(111) surface was not determined. On the basis of (35) Huckaby, D. A.; Blum, L. Langmuir 1995, 11, 4583. (36) Chen, C.-H.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451-458. (37) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737-2746. (38) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (39) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383-3386.

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coulometric measurements, Yoneyama and co-workers25 suggested that thiols template the adsorption of Cu in the Cu upd system in the same way that sulfate did. In particular, they suggested that Cu was arranged in a hexagonal array around each thiol. The (x57 × 3) lattice we observe here is inconsistent with a hexagonal array of Bi adatoms, and the details of the unit cell we observe suggest that the interaction between the Bi and the thiol must be more complex than that found with Cu. 4.2. Reactivity. In addition to the structural changes discussed above, introduction of ethanethiol or KSCN also has a dramatic effect on the reactivity of peroxide or dioxygen at the (2 × 2) Bi adlayer. The changes in reactivity provide some insight into the role of the (2 × 2) Bi structure in the catalysis of the electroreduction of these species. In the case of the thiol, the residual reduction current at 0.4 V following introduction of the thiol rapidly becomes essentially zero. This suggests that the surface is totally passivated because of complete coverage of this surface by a thiol adlayer. The barrier properties of thiol adlayers have been well documented,32 and even short-chain thiols are understood to form ordered, blocking arrays on Au surfaces. STM images reveal a surface with a dramatically altered structure, with no discernible remnants of the original (2 × 2) structure. The presence of an apparently fluid or weakly held (8 × 8) adlattice may be responsible for the facile poisoning of the surface. This weakly held layer can easily diffuse on the surface, rendering passivated any remaining catalytically active sites. The conversion to the (x57 × 3) layer occurs at a time scale (ca. 30 min) far slower than that for catalysis poisoning. Introduction of SCN- also gives rise to a substantial reduction in the current associated with electroreduction of peroxide. This reduction is also associated with a structural change on the surface. The kinetics for poisoning appears to follow a simple Langmuirian model, which does not allow for interaction between sites on the surface. These kinetics are somewhat surprising, since the kinetics of adsorption of many other species, especially anions in the electrochemical environment, follow more complex behavior.40,41 However, the Langmuiran kinetics do suggest that each (2 × 2) Bi unit cell is a unique site for peroxide reduction activity and the kinetics suggest that there is no cooperativity between sites. We note the distinction between surface poisoning studied here and anion assembly on the surface which may occur at a different time scale and with a different set of kinetics. Unlike the thiol case, the surface is not totally passivated following SCN- introduction. The chronoamperometry shows that on the order of 4% of the electroreduction current remains at long times following introduction of the poison. Examination of STM images obtained following SCN introduction reveals a (4 × 4) adlayer. However, approximately 6% of the protrusions associated with the SCN appear to be missing in the image. The correspondence between this number and the residual current suggests that the defects in the (4 × 4) adlayer are themselves the site of the catalysis. These sites are likely sites where SCN- has not adsorbed leaving a remnant of the (2 × 2) Bi overlayer. The exact structure of this defect site is of course unknown. Because the spectroscopy of SCN- with Bi is so close to that reported for SCN- on bare Au(111), we assign the site of SCN- adsorption to the bare Au surface. The protrusions in the STM image of the SCN--modified surface are likely the SCN- adsorbate. (40) Ocko, B. M.; Wang, J. X.; Wandlowski, T. Phys. Rev. Lett. 1997, 79, 1511-1514. (41) Fukuda, T.; Aramata, A. J. Electroanal. Chem. 1997, 440, 153161.

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These results suggest that S is bound to a site on the Au(111) surface. This site is likely not Bi (which should be electropositive relative to Au) but rather the Au surface itself. The similarity in νCN frequencies for SCN- on Au(111) and the Bi (2 × 2) system speak to this likelihood. There is one exposed Au atom in the (2 × 2) Bi unit cell as shown in Figure 1. The above suggests that this may be the site of initial adsorption of SCN-. The considerations above suggest that one of the roles of the Bi adlayer is to impart special properties to the lone exposed Au atom in the Bi unit cell. The Bi may make the Au atom somewhat more electronegative and make this the site wherein additional electron density is pumped into the O-O bond. The dioxygen or peroxide replaces the OH- on the surface at the bare Au site. We do not have any information at this time concerning participation of the Bi adatoms on the surface. The dioxygen may also associate with the Bi forming a heterobimetallic type of site. Information of this type will have to await further X-ray scattering and spectroscopic measurements. 5. Conclusion In conclusion, we have shown that both SCN- and ethanethiol act to poison the (2 × 2) Bi upd adlattice toward dioxygen or peroxide reduction activity. This poisoning is associated with dramatic structural changes attending the surface. Analysis of the poisoned surface suggests that the site of poisoning for SCN is the lone Au open site in

Oh et al.

the (2 × 2) Bi adlattice. The SCN- then acts to restructure the surface. Ethanethiol quickly forms an overlayer on the surface that at longer times then restructures to make a stable adlayer. Both anions poison the surface by blocking sites for peroxide adsorption; in cases where individual sites in the unit cell are not blocked reactions can occur. Examination of structural changes attending deliberate poisoning of surfaces involved in electrocatalytic events is one way to understand the active sites of these processes. A poison with a weaker interaction with the Au surface, such as OCN-, might perturb the (2 × 2) Bi adlattice less and provide a clearer picture of the way in which dioxygen associates with the active site. These experiments are in progress. Acknowledgment. We thank R. T. Haasch of the Center for Microanalysis of Materials in the Frederick Seitz Materials Research Laboratory at the University of Illinois for his invaluable assistance in XPS data acquisition and interpretation. I.O. gratefully acknowledges a grant from the Rotary International Foundation. XPS spectra were obtained in the Center for the Microanalysis of Materials which is funded by the Department of Energy (Grant DEFG02-91ER45439) through the University of Illinois Frederick Seitz Materials Research Laboratory. This work was funded by the NSF (CHE-98-20828) which is gratefully acknowledged. LA991005G