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J. Phys. Chem. B 1998, 102, 818-823
Potential-Step Chronocoulometric and Quartz Crystal Microbalance Investigation of Underpotentially Deposited Tl on Au(111) Electrodes Brian K. Niece and Andrew A. Gewirth* Department of Chemistry, UniVersity of Illinois, 600 S. Mathews AVe., Urbana, Illinois 61801 ReceiVed: July 14, 1997; In Final Form: December 1, 1997
Tl underpotentially deposited (upd) on Au(111) has been studied using potential-step chronocoulometry and electrochemical quartz crystal microbalance to determine the actual coverage of Tl. At anodic potentials, Tl adsorbs to the surface at a coverage that slowly increases up to 0.55 ML. At more cathodic potentials, the coverage rises sharply to a plateau of 0.79 ML where the surface is covered with a close-packed Tl monolayer. The electrosorption valency calculated based on this coverage data indicates that the low coverage overlayer is incompletely discharged but that the overlayer undergoes further discharge during the voltammetric features associated with complete monolayer formation. Voltammetric experiments also indicate that the presence of hydroxide anions plays a role in stabilizing the charged low coverage phase. The implications of this structural and charge information for the catalytic activity of the Tl upd system are discussed.
1. Introduction There is currently much interest in small molecule redox reactions at electrode surfaces. These reactions have potential utility as energy sources in fuel cells, for electrosynthesis, and for electrochemical remediation of pollutants. One reaction of particular interest is the two-electron reduction of hydrogen peroxide to water according to eq 1.
H2O2 + 2e- + 2H+ f 2H2O
(1)
This reaction is important because it is often the limiting step in the reduction of molecular oxygen, which occurs as the cathode reaction in many potential fuel cell schemes. When this reaction is kinetically hindered, as it is on Au(111) electrodes, O2 reduction occurs only via a two-electron process to produce H2O2. On more active surfaces, however, O2 reduction occurs via a four-electron process, producing H2O. Underpotentially deposited1 monolayers and submonolayers of foreign metals on electrode surfaces have been found to catalyze a variety of small molecule redox reactions.2 The H2O2 and O2 reduction reaction is catalyzed by the presence of underpotentially deposited Bi,3,4 Tl,5 and Pb5 on gold electrodes. Until recently, however, there has been little insight available into the mechanism of this catalytic activity, due to a lack of structural information about the active sites on the electrode surface involved in the catalysis. The advent of the scanning probe microscopies (SPM) and their application to electrochemical systems6 along with electrochemical surface X-ray scattering (SXS) experiments has begun to provide information about the structures present on catalytically active electrodes surfaces. The Bi on Au upd system, for example, has been found to exhibit an open (2 × 2)-Bi adlattice in the potential region where the peroxide reduction reaction is catalyzed, suggesting that the presence of mixed metal sites is responsible for the enhanced reactivity.7 The Pb on Au upd system, similarly, shows maximal * To whom correspondence should be addressed.
catalytic activity at the potential where the electrode surface has its highest coverage of Pb islands, but before the islands coalesce into a complete monolayer.8 In situ STM investigations of another catalytically active upd system, Cd on Au(111), however, reveal linear structures with no obvious active site for the metal catalysis.9 In this system, more information is required about the species present at the electrode interface in order to elucidate the nature of the catalysis. In addition, it is unclear from the microscope studies what forces give rise to the specific structures exhibited in these systems. The presence of coadsorbed species or partially discharged metal adatoms could give rise to the open structures and influence the surface reactivity. To gather further information about the electrode/electrolyte interface in these systems, we have applied the potential-step chronocoulometric technique popularized by Lipkowski for determination of adsorbate coverages on solid electrodes.10 This technique makes use of the Gibbs adsorption isotherm (eq 2)
( )
-
∂γ ∂µi
) Γi
(2)
T,P,µj*i
which relates the surface concentration of any species at the interface, Γi, to the change in surface tension of the electrode, γ, with respect to the change in chemical potential, µi, of that species in the bulk solution. This allows for determination of the coverage of coadsorbed species in addition to the upd metal.11 This technique allows surface coverages to be determined without making assumptions about the number of electrons transferred to an individual adatom during the deposition (electrosorption valency), thus providing an accurate measurement of the surface coverage of the species of interest. The coverage can then be used with the surface charging data to calculate the actual electrosorption valency. We also report the results of electrochemical quartz crystal microbalance measurements we have made to confirm the surface coverages of the adsorbed species. These two techniques provide complementary information about the identity and quantity of species adsorbed to the electrode surface.
S1089-5647(97)02286-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/29/1998
Underpotentially Deposited Tl on Au(111) Electrodes
J. Phys. Chem. B, Vol. 102, No. 5, 1998 819 completely matched with the underlying Au lattice,28 in keeping with the X-ray measurements. Specular reflectance measurements have indicated that the Tl on Au monolayer forms through several stages of intermediate coverage.29 Surface conductivity measurements have indicated that the growth of the layer is consistent with an island growth mechanism.30 SXS measurements of Tl on Au(100) electrodes have found a c(p × 2) monolayer at negative potentials and a (2 × 2) adlattice at more positive potentials.31
Figure 1. Cyclic voltammograms recorded on Au(111) in 0.1 M HClO4 at a sweep rate of 10 mV/s: (s) 4.88 mM Tl+; (‚‚‚) 0 mM Tl+.
In the Bi on Au(111) system, these techniques revealed coadsorbed hydroxide on the electrode surface in the potential region where the open adlattice is observed.12 In the Cd on Au system, they revealed coadsorbed sulfate ions as well as significant residual positive charge on the deposited Cd adatoms.13 These results point to different factors controlling the upd adlayer structures and catalytic activity. In this paper we turn to the Tl on Au(111) system,14 which also catalyzes the hydrogen peroxide reduction reaction.5 Underpotential deposits of Tl on Au, Pt, Ag, Pd, and Ir have also been observed to enhance the redox activity of formic acid,15 hydrazine,16 nitro compounds,17 quinones,18 ethylene glycol,19 methanol,20 glucose,21 and Cr ions,22 many of which are interesting from a standpoint of remediation or energy production. Understanding of the mechanism of catalysis in this system could have important consequences for the development of these fields. STM23 and SXS24 studies have revealed an open Tl adlayer on Au(111) with stoichiometrically coadsorbed hydroxide at low coverages in alkaline solutions and a close-packed adlayer at high coverages. However, the structure of Tl on Au(111) in the catalytically active region in acidic solutions remains unclear. Figure 1 shows the cyclic voltammogram recorded on a Au(111) electrode in acid solution during the underpotential deposition of Tl. The current response is broadened relative to the current in pure electrolyte between 0.56 and -0.10 V, the region including the peaks labeled A and A′, due to the upd of Tl+. In this region, SXS studies in acid solution25 revealed a low coverage Tl adlayer. Two underpotential deposition peaks, labeled D1and D2, appear between -0.10 and -0.20 V on the cathodic scan, and two corresponding stripping peaks, labeled D1′ and D2′, appear in the same potential range during the anodic scan. In the region negative of -0.20 V, the SXS studies revealed a close-packed hexagonal structure with a lattice constant slightly larger than the gold substrate and lower activity for peroxide reduction than the low coverage phase. At potentials positive of peak A′, the voltammogram coincides with that recorded in pure electrolyte. The upturn at the anodic limit of potential indicates the onset of oxidation of Tl+ to Tl3+. It is also noteworthy that the hydrogen evolution reaction is almost completely suppressed in the presence of Tl, even at the negative limit of potential (-0.46 V vs NHE), an effect also observed on Pt electrodes.26 Other X-ray diffraction measurements on the Tl/Au(111) system have indicated that the Au-Tl separation is constant throughout the upd potential region.27 Optical second harmonic generation studies indicate that the Tl on Au overlayer is not
2. Experimental Section Electrochemical solutions were prepared from ultrapure water (Milli-Q UV plus, Millipore Inc., 18.2 MΩ cm) and TlNO3 (Aldrich, 99.999%). Supporting electrolyte for Tl adsorption studies was 0.1 M HClO4 (Baker, Ultrex II). For the pHdependent studies, solutions were made as above, and the pH was adjusted by addition of 0.5 M NaOH (Merck, Suprapure) in ultrapure water. 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).32 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 analog I/O board and software written at the University of Illinois. Potential-step data were collected similarly, using a potential step generated by the computer system and applied through the AFRDE-5. The sampling frequency for potential step data was 21 kHz. For the Tl adsorption study 16 solutions were used with concentrations of Tl ranging from 3.9 × 10-4 to 9.8 × 10-3 M. For the hydroxide adsorption experiments, 17 solutions were used with pH values ranging from 1.04 to 2.32. For each solution, current-time traces were recorded after stepping to Ef ) 1.09 V from Ei for Ei between -0.36 and 1.08 V at 10 mV intervals. The potential step experiments were performed in a thermally jacketed cell kept at a constant temperature of 25.0 °C. The solution was stirred during data acquisition to ensure that the system was at thermal equilibrium and had no concentration gradients. QCM data were collected with a Maxtek model PM710 electrochemical thickness monitor (Maxtek, Inc., Torrance, CA). The potential at the working electrode of the thickness monitor was controlled by the AFRDE-5 potentiostat, and the rear electrode of the quartz crystal was allowed to oscillate around the applied voltage. The measurements were made by immersing the QCM probe in a jacketed beaker kept at 25.0 °C. The solutions used for QCM experiments were purged with Ar prior to use, and data were collected in a glovebag to maintain an argon atmosphere, as continuous flow of argon over the solution surface produced noise in the oscillator response.
820 J. Phys. Chem. B, Vol. 102, No. 5, 1998
Niece and Gewirth
Figure 2. Current-time curve recorded on Au(111) in a solution of 4.88 mM Tl+ + 0.1 M HClO4 after the potential was stepped from Ei ) -0.36 V to Ef ) 1.09 V.
Figure 3. Surface charge density on Au(111) calculated from a family of current-time curves for solutions of [Tl+] + 0.1 M HClO4. Ef ) 1.09 V.
Quartz QCM oscillator crystals with Au electrode surfaces over a chromium adhesion layer and a fundamental frequency of 5 MHz were purchased from Maxtek, Inc. The crystals were prepared for use by evaporation of an additional 900 Å of Au onto the working electrode face in a vacuum at a temperature of 200 °C, to limit Cr leakage from the electrode, and to provide a pseudo-(111) morphology on the surface for comparison with data taken on single-crystal electrodes. The surfaces thus prepared were observed with the STM to have flat terraces of about 100 Å in diameter on the surface. In addition, cyclic voltammograms recorded in the double-layer region in SO42electrolyte showed capactive features similar to those observed on Au(111) single-crystal electrodes. The voltammetry of gold oxide formation was unattainable, due to the oxidation of Cr from the adhesion layer at anodic potentials. Figure 4. Electrocapillary curves on Au(111) calculated from surface charge density for solutions of [Tl+] + 0.1 M HClO4.
3. Results 3.1. Thallium Chronocoulometry. The CV of Tl upd on Au(111) shown in Figure 1 reveals that the current response of the electrode is identical in solutions with and without added Tl+ at potentials greater than 0.56 V. This indicates that the Tl+ ions are desorbed from the surface in this potential region and that the surface state is the same in both solutions. Stepping the potential of a Au(111) electrode immersed in Tl+ solution from a potential, Ei ) -0.36 V, where Tl is known to be adsorbed to the electrode, to a potential, Ef ) 1.09 V, where Tl is known to desorb from the surface, results in the dissolution of Tl from the electrode surface. A current vs time trace recorded upon such a potential step is shown in Figure 2. The current shows a sharp initial spike due to the rearrangement of solvent ions in the double layer, with a wider shoulder produced by the oxidation of deposited Tl, which occurs more slowly. Integration of the current with respect to time yields the total charge passed during the potential step. This charge is then divided by the working electrode surface area to determine ∆σEi, the change in surface charge density due to the desorption. The change in surface charge density when stepping from the potential of zero charge (pzc) in pure supporting electrolyte to Ef can then be used to calculate σM, the charge density on the electrode surface, according to eq 3.
σM ) ∆σpzc - ∆σEi
(3)
A series of such potential steps can be used to map the surface charge density throughout the upd potential region. The surface
charge density at representative Tl+ concentrations is shown in Figure 3. At Tl+ concentrations lower than 0.39 mM, the Tl adlayer was ineffective at preventing H+ reduction at the negative potential limit; therefore, a somewhat smaller concentration range (0.39-9.76 mM) was used here than in previous studies. Taking the electrons in the electrode as species i in eq 2 indicates that integration of σM from positive to negative potentials for a given concentration of Tl+ gives the change in surface tension at the electrode, ∆γ, relative to the positive limit of integration, as indicated in eq 4.
(∂E∂γ)
-
T,P,µTi
) σM
(4)
The integration constant for this integration is unknown, but can be ignored, as it is the same at all concentrations and will go to zero when the surface tension is differentiated. The relative surface tension values can be plotted vs the electrode potential to give the electrocapillary curves shown in Figure 4. The electrocapillary curves are identical at high potentials where the Tl+ is not adsorbed to the electrode surface. At lower potentials, where Tl undergoes adsorption and finally upd, the surface tension drops more rapidly as the solution concentration of Tl+ increases, as is expected for the adsorption of Tl to the electrode. Equation 5 shows that differentiation of the suface tension with respect to the chemical potential of Tl+ in the solution
Underpotentially Deposited Tl on Au(111) Electrodes
J. Phys. Chem. B, Vol. 102, No. 5, 1998 821
Figure 5. Surface concentration of Tl on Au(111) calculated from electrocapillary curves at 4.88 mM Tl+ + 0.1 M HClO4.
will yield the surface concentration of Tl, ΓTl.
( )
-
∂γ ∂µTl
) ΓTl
(5)
T,P,E
Differentiation of the calculated ∆γ values with respect to RT ln([Tl+]) gives the surface coverages plotted in Figure 5. The coverage is zero at the positive limit where no Tl is believed to be adsorbed to the surface. It rises slowly between 0.94 V and peak D1 to a coverage of 7.6 × 1014 Tl/cm2. The coverage then rises quickly through peaks D1 and D2 to a value of 1.1 × 1015 Tl/cm2 where it levels off prior to the onset of bulk Tl reduction. Subtraction of the charge passed during the same potential steps in pure supporting electrolyte from the charge passed during Tl desorption should yield the charge due to the removal of the Tl upd layer at each potential. Dividing this value by the calculated Tl surface coverage gives the charge passed per Tl adatom. However, extensive hydrogen ion reduction occurs while holding the electrode at potentials more negative than -0.16 V in the absence of Tl. Some of the resulting hydrogen atoms are oxidized on the subsequent positive potential step, yielding a larger background charging current than that for double-layer rearrangement alone and hence a smaller adlayer charge. The resulting numbers are therefore not indicative of the actual discharge state of the Tl adatoms. The charge is low through the initial adsorption region and then rises during the final upd step to a higher value. 3.2. Hydroxide Chronocoulometry. We attempted to perform a similar procedure to determine the surface coverage of hydroxide in the presence of underpotentially deposited Tl. In this procedure, the potential is again stepped from more negative potentials to 1.09 V. This potential was chosen because OH- does not adsorb to Au electrodes at this potential in acid solutions. The potential steps are repeated in solutions of various pH values. This allows the surface charge density and electrocapillary curves to be calculated. Differentiation of the surface tension with respect to RT ln([OH-]) yields the surface concentration of hydroxide. The calculated surface coverage of OH- is zero at positive potentials and becomes negative as the potential is lowered, which of course is unphysical. Further investigation yielded the cyclic voltammograms shown in Figure 6a which compares CVs for Tl upd at two different pH values. The relative amount of Tl desorption in the two upd peaks shifts toward the more positive peak D1′ as the OH- concentration increases. The charge passed in peak D1′ increases from 24 to 30 µC, while the charge in peak D2′ deacreases from 36 to 33
Figure 6. (a, top) Cyclic voltammograms recorded on Au(111) in 4.88 mM Tl+ + 0.1 M ClO4- at a sweep rate of 10 mV/s: (s) pH ) 1.04; (‚‚‚) pH ) 2.17. (b, bottom) Cyclic voltammogram recorded on Au(111) in 1.0 mM Pb2+ + 0.1 M HClO4 at a sweep rate of 20 mV/s.
µC. The increase in charge in peak D1′ can be explained, at least in part, by increased OH- adsorption at the higher pH. The lower charge in peak D2′, however, must be due to a decrease in Tl desorption in that peak. The Gibbs adsorption relation is only accurate when the Gibbs excesses of the species of interest are not influenced by a change in the chemical potential of other elements of the solution. Since the surface coverage of Tl changes with the OH- concentration, the OHadsorption measurements are meaningless. 3.3. Quartz Crystal Microbalance. The frequency change of a quartz crystal oscillator during Tl underpotential deposition is shown in Figure 7, along with the current recorded at the working electrode during deposition. The voltammogram recorded during the QCM measurements exhibits the relative upd peak heights characteristic of higher pH solutions, in that peak D1′ is larger than peak D2′. This is likely due to the poor ability of the glovebag and cell used in the QCM experiments to exclude O2, which is reduced to OH- at such negative potentials. The oscillator frequency is highest at 0.50 V. At negative potentials, the frequency begins to drop with the onset of Tl adsorption and reaches a minimum at the cathodic potential limit negative of peaks D1 and D2. Upon reversing the potential sweep, the frequency rises again, returning to its initial value at 0.70 V. At potentials positive of the pzc, the frequency goes down slightly, presumably due to the adsorption of solvent molecules and ClO4- ions from the electrolyte. The total frequency change in the upd potential region is observed to be -17.5 Hz. The slow kinetics of adsorption prevent the determination of an intermediate frequency change
822 J. Phys. Chem. B, Vol. 102, No. 5, 1998
Figure 7. Frequency (s) and current (‚‚‚) response of a Au QCM oscillator electrode in 4.88 mM Tl+ + 0.1 M HClO4 at a scan rate of 50 mV/s.
prior to the onset of the two upd peaks. AT cut quartz crystals with a fundamental frequency of 5 MHz have a mass coefficient of -17.7 ng/(cm2 Hz). This indicates an effective mass change at the interface of 310 ng/cm2. 4. Discussion 4.1. Thallium Coverage. The chronocoulometric measurements (Figure 5) indicate that there is negligible coverage of Tl on the Au electrode surface at the positive limit of potentials investigated, just prior to the onset of Tl+ oxidation. The Tl coverage rises through the region where Tl cations are believed to adsorb on the electrode surface to a concentration of 7.6 × 1014 Tl/cm2 at -0.10 V. This corresponds to a coverage of 0.55 ML. It then rises sharply through the deposition peaks D1 and D2 to a plateau with a concentration of 1.1 × 1015 Tl/ cm2 in the region negative of the upd peaks. This corresponds to a coverage of 0.79 ML. SXS measurements on this system in acid solution were not able to determine the structure in the low coverage region.25 At high coverages, they revealed a rotated hexagonal overlayer with a coverage of 0.74 ML. The coverage of 0.79 ML measured by chronocoulometry is consistent with this structure, which is essentially a close-packed hexagonal monolayer of Tl on the slightly smaller gold substrate. The interfacial mass change of 310 ng/cm2 measured with the QCM is about 12% smaller than the theoretical mass change expected based on the observed structure (351 ng/cm2). This discrepancy may be explained by a continued excess of ionic species in solution near the charged electrode surface at the positive limit of the potential region which lowers the oscillator frequency and results in a lower apparent mass change for the Tl desorption. A similar effect has been seen in QCM studies of Pb desorption where the observed mass change is about 15% lower than expected.33 The calculated charge passed per Tl adatom cannot be taken as an accurate estimate of the actual charge, due to the oxidation of H atoms produced during the equilibration step. However, the data indicate that there are two distinctly different charge states of the adsorbed Tl. Previous electrochemical34 studies have indicated that Tl undergoes a transition from partially to fully discharged during formation of the high coverage structure. The cyclic voltammogram of the underpotential deposition of Pb2+ on Au(111),35,36 shown in Figure 6b, is strikingly similar to that of Tl. Pb adsorbs to the Au electrode over a wide potential range at positive potentials. STM studies of the adsorbed Pb in this region indicate that deposition begins with step edges and island nucleation on the Au terraces.37 The low
Niece and Gewirth coverage Pb adlayer at positive potentials is also only partially discharged.35 At more cathodic potentials, the lead undergoes deposition in two sharp, closely spaced peaks. The high coverage Pb adlayer observed at potentials negative of these peaks is a close-packed adlayer exhibiting a moire´ pattern that was attributed to the slight mismatch and rotation between the Pb and Au lattices.38 This structure is nearly identical to the high coverage structure observed by SXS at cathodic potentials in the Tl system.25 On the basis of these similarities, we propose that the initial adsorption of Tl on Au(111) at anodic potentials occurs in a manner similar to that seen in the Pb system. In this model, partially discharged Tl+ atoms adsorb on the Au electrode at potentials positive of upd peaks D1 and D2. This adlayer consists of Tl adsorbed at step edges and in close-packed islands which increase in coverage as the potential is lowered. STM images revealed similar behavior in the upd of both Pb and Tl on Ag(111) electrodes.39 In that system, the upd metal forms a close-packed adlayer at low coverages which forms in areas away from step edges. QCM data of Pb upd on Au indicate that coverage of adatoms in the low coverage region is about 0.33 ML,33 which is smaller than the 0.55 ML observed here. The lack of regular order in these islands would prevent observation of this structure by SXS. The Tl adlayer then undergoes further deposition in peaks D1 and D2 to form the rotated close-packed hexagonal adlayer observed by SXS. 4.2. Effect of Hydroxide on Thallium Upd. The CVs of Tl upd on Au(111) in solutions of varying pH (Figure 6a) indicate that more charge is passed in the more positive peak as the concentration of hydroxide in the solution is increased. Theoretical studies40 have suggested that the two closely spaced peaks in the Pb upd voltammogram arise from the discharge of the adsorbed Pb adlayer followed closely by the deposition of more Pb atoms to form the complete monolayer once the electrostatic repulsion by the partially discharged adlayer is removed. Stabilization of the partially charged islands by coadsorbed hydroxide would allow a higher initial coverage of adsorbed Tl to be attained in the anodic potential region at higher pH. Discharge of this adsorbed layer would then result in a higher current during the more anodic upd peak at higher OHconcentrations. We also note that Tl+ and Pb2+ are isoelectronic, and can be expected to behave similarly in the region prior to full discharge of the adatoms. Hydroxide has also been found to be coadsorbed with underpotentially deposited Bi on Au(111) electrodes.12 Bi has a high affinity for OH- ions, and the anions are coadsorbed within the Bi adlayer, resulting in an open adlattice. Tl+, however, is a strong base with little affinity for the hydroxide ion. The adsorbed Tl islands are therefore close-packed. The Tl islands would, however, attract hydroxide anions from the solution in order to stabilize the residual positive charge. 4.3. Catalysis by Tl Upd Adlayers. The current studies have been performed in the absence of the electroactive species O2 or H2O2 in the electrolyte. SPM studies on the catalytically active Bi and Pb upd adlayers, however, have revealed that there is no change in the surface coverage or structure when the electroactive species is added to these systems.7,8 The activity of the Tl adlayer for peroxide reduction is highest in the low coverage region at more anodic potentials where we propose the adlayer consists of Tl islands. This is the same region where O2 reduction switches from a two-electron to a four-electron process. Pb upd adlayers, similarly, exhibit a higher catalytic activity in the region where the surface has the highest coverage of separate Pb islands with maximal island edge area.8 The
Underpotentially Deposited Tl on Au(111) Electrodes highest catalytic activity in the Bi on Au(111) system occurs in the region where the surface exhibits an open (2 × 2) adlattice. Adzic has also pointed out the appearance of a low coverage catalytic Tl structure in both the acidic and alkaline environments.25 In all of these structures there are a maximal number of mixed adatom-substrate sites available in which the reacting molecule can bind. The polarization resulting from binding to metals of different electronegativities may ease charge injection into the peroxide molecule and thus enhance the rate of reduction. 5. Conclusions As has been seen in the Cu, Bi, and Cd systems, the presence of an anion has a direct influence on the formation of Tl upd adlayers. In this case, coadsorbed hydroxide anions stabilize the positively charged layer of Tl islands present at anodic potentials. The electrochemical and structural features of the Tl upd system are similar to the Pb system, and the structure of the low coverage Tl phase is likely similar to that of Pb. The partial monolayer present at anodic potentials may account for the catalytic activity which is lost when the adlayer condenses to a complete monolayer. Acknowledgment. This work was funded by the Department of Energy through the Materials Research Laboratory at the University of Illinois (DE-FG02-91ER45349). B.K.N. acknowledges a Department of Education fellowship administered by the University of Illinois. X-ray characterization of Au electrodes was carried out in the Center for Microanalysis of Materials, University of Illinois, which is supported by the U.S. Department of Energy under Grant DEFG02-91-ER45439. References and Notes (1) 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. (2) 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. (3) Adzic, R. R.; Despic, A. R. Z. Phys. Chem. (Munich) 1975, 98, 95-110. (4) Ju¨ttner, K. Electrochim. Acta 1986, 31, 917-927. (5) Kokkinidis, G.; Sazou, D. J. Electroanal. Chem. 1986, 199, 165176. (6) Gewirth, A. A.; Niece, B. K. Chem. ReV. (Washington, D.C.) 1997, 97, 1129-1162. (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.
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