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Potential-Step Chronocoulometric Investigation of the Surface Coverages of Coadsorbed Bi and Hydroxide on Au(111) Electrodes Brian K. Niece and Andrew A. Gewirth* Department of Chemistry, University of Illinois, 600 S. Mathews Avenue Urbana, Illinois 61801 Received December 15, 1995. In Final Form: July 16, 1996X Bi underpotentially deposited on Au(111) has been studied using potential-step chronocoulometry to determine the actual surface coverage of Bi. In the potential region where this system exhibits catalytic activity for the electroreduction of peroxide to water, the observed coverage is 0.25 monolayer (ML), which agrees well with the coverage of the reported (2 × 2) Bi overlayer observed by scanning probe microscopy in this region. At more cathodic potentials, the coverage increases to 0.67 ML. This coverage agrees with that expected based on the (p × x3) structure proposed from scanning tunneling microscopy, atomic force microscopy, and SXS measurements in this region. The electrosorption valency calculated based on these coverages is 3, indicating that the Bi is fully discharged on the surface. Potential-step chronocoulometry has been used at various pH values to determine the surface coverage of hydroxide anion in the presence of underpotentially deposited (upd) Bi. The coverage is negligible in the absence of upd Bi and at potentials where the Bi adlayer condenses. It rises to a peak of 0.17 ML in the region where the coverage is 0.25 ML, indicating that OH- is coadsorbed with the Bi.
1. Introduction Small molecule oxidation and reduction in fuel cells is one possible source of electrical energy which may make widespread use of electric vehicles practical. In addition, catalytic electroreduction of common wastes such as nitrogen oxides to ammonia and N2 has great potential as a method for conversion of pollutants to useful products. Monolayers of metals underpotentially deposited (upd)1 onto electrode surfaces have been found to act as catalysts in electrochemical systems.2 There is little understanding of the mechanism of this catalytic activity, however, because the structure of the modified electrodes is poorly understood.
H2O2 +2e- + 2H+ f 2H2O
(1)
Monolayers of Bi on gold electrodes have been found to act as catalysts in a number of electrochemical systems such as the Cr(III)/Cr(II) couple3 and the oxidation or reduction of small molecules such as ethylene glycol4 and 2,4-dinitrosoresorcinol and tetraoxime of 1-cyclohexene3,4,5,6-tetrone.5 The Bi upd system is of particular interest, however, because it catalyzes the two-electron reduction of hydrogen peroxide to water (eq 1).6,7 This reaction is crucial because it is often the limiting step in the reduction of O2 to water, which occurs in many potential fuel cell schemes. The peroxide reduction in this system is characterized by three potential regions in X Abstract published in Advance ACS Abstracts, September 15, 1996.
(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) Rodes, A.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1989, 271, 127-139. (4) El-Shafei, A. A.; Shabanah, H. M.; Moussa, M. N. H. J. Power Sources 1993, 46, 17-27. (5) Hasiotis, C.; Kokkinidis, G. Electrochim. Acta 1994, 39, 639644. (6) Adzic, R. R.; Despic, A. R. Z. Phys. Chem. (Munich) 1975, 98, 95-110. (7) Ju¨ttner, K. Electrochim. Acta 1986, 31, 917-927.
S0743-7463(95)01548-4 CCC: $12.00
which the catalytic activity is alternately off, on, and off again.8 There has been much interest in elucidating the structure of this modified electrode in order to determine the nature of the catalytic activity. The Bi on Au upd system has been studied by a variety of techniques. Voltammetric,9-11 surface conductivity,12 specular reflectance,13,14 and quartz crystal microbalance15 investigations have been performed on polycrystalline gold electrodes. These studies indicate a full monolayer of Bi forms with an electrosorption valency of 2.6-2.7. Bi3+ is also found to adsorb on the oxide-covered Au surface at more anodic potentials.10 Further work on the Au(111) face8 indicates that Bi adsorbs in multiple steps with only partial coverage in the catalytically active region, as shown in Figure 1. Atomic force microcopy (AFM),16 scanning tunneling microscopy (STM),17 and (SXS)17 results indicate that the surface exhibits three distinct structures in the underpotential region. At positive potentials, the structure is that of bare Au(111). In the catalytically active region, the structure is an open (2 × 2) Bi adlayer. As the potential is swept more negative to a region where the catalytic activity ceases, the structure is observed to condense to an incommensurate (p × x3) Bi adlayer. The availability of two different metal atom sites on the electrode surface to which the molecule can bind seems to be crucial to the catalysis of the H2O2 reduction reaction. A similar effect is observed in the Pb on Au(111) system, where the Pb adatoms form close-packed islands during (8) Sayed, S. M.; Ju¨ttner, K. Electrochim. Acta 1983, 28, 1635-1641. (9) Schmidt, E.; Gygax, H. R.; Cramer, Y. Helv. Chim. Acta 1970, 53, 649. (10) Cadle, S. H.; Bruckenstein, J. J. Electrochem. Soc. 1972, 119, 1166. (11) Salie, G.; Bartels, K. Electrochim. Acta 1994, 39, 1057-1065. (12) Romeo, F. M.; Tucceri, R. I.; Posadas, D. Surf. Sci. 1988, 203, 186-200. (13) Adzic, R.; Jovancicevic, V.; Podlavicky, M. Electrochim. Acta 1980, 25, 1143-1146. (14) Takamura, K.; Watanabe, F.; Takamura, T. Electrochim. Acta 1981, 26, 979-987. (15) Deakin, M. R.; Melroy, O. J. Electroanal. Chem. 1988, 239, 321331. (16) Chen, C-h. C.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 5439-5440. (17) Chen, C-h. C.; Kepler, K. D.; Gewirth, A. A.; Ocko, B. M.; Wang, J. J. Phys. Chem. 1993, 97, 7290-7294.
© 1996 American Chemical Society
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Figure 1. Cyclic voltammogram of Bi upd on Au(111) taken in 1.0 mM Bi3+ + 0.1 M HClO4. Scan rate was 5 mV/s. Insets are the previously proposed Bi overlayer structures in the three regions of interest. Shaded area is the catalytically active region.
Niece and Gewirth
Figure 2. Current-time curve recorded on Au(111) in a solution of 1.0 mM Bi3+ + 0.1 M HClO4 after the potential was stepped from Ei ) 0.22 V to Ef ) 0.76 V.
2. Experimental Section the early stages of upd.18,19 The catalytic activity is highest at the potential with the highest island density and drops off as the islands begin to coalesce into a complete monolayer.20,21 It is desirable to determine the factors which cause the Bi adlayer to remain open in the early stages of upd. Previous studies have indicated that the Bi adatoms may be only partially discharged on the electrode surface.11,15 The adlayer may therefore be held open by electrostatic repulsions between the adatoms. The Cu22 and Ag23 on Au(111) systems exhibit different structures in differing electrolytes, indicating that the anion may play a role in determining the adlayer structure. In both of these systems, the adlayer is open in the presence of a strongly adsorbing electrolyte such as sulfate, and more closely packed in a weakly adsorbing electrolyte such as perchlorate or acetate. It is also known that Bi is strongly associated with hydroxide anions even in acidic solution.24 The adlayer may contain coadsorbed hydroxide ions at low coverages which keep the Bi atoms separated. In this paper, we use the potential-step chronocoulometric analysis popularized by Lipkowski25 to determine the actual surface coverages of Bi and hydroxide in the upd region. This technique has been used to determine the surface concentration of individual surface species,26 as well as surface concentrations of both a metal and a coadsorbed anion.27 The surface coverage is used to estimate the electrosorption valency of Bi on Au(111). The data are also used to confirm the structures observed by AFM, STM, and SXS measurements. The surface coverages are also used to determine the extent of coadsorption of OH- with Bi at the electrode surface. (18) Green, M. P.; Hanson, K. J. Surf. Sci. Lett. 1991, 259, L743L749. (19) Tao, N. J.; Pan, J.; Li, Y.; Oden, P. I.; DeRose, J. A.; Lindsay, S. M. Surf. Sci. Lett. 1992, 271, L338-L344. (20) Ju¨ttner, K. Electrochim. Acta 1984, 29, 1597-1604. (21) Chen, C-h. C.; Washburn, N.; Gewirth, A. A. J. Phys. Chem. 1993, 97, 9754. (22) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183-186. Haiss, W.; Lackey, D.; Sass, J. K.; Meyer, H.; Nichols, R. J. Chem. Phys. Lett. 1992, 200, 343-349. Batina, N.; Will, T.; Kolb, D. M. Faraday Discuss. 1992, 94, 93-106. (23) Chen, C-h. C.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451-458. Ogaki, K.; Itaya, K. Electrochim. Acta 1995, 40, 1249-1257. (24) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd English ed.; National Association of Corrosion Engineers: Houston, TX, 1974; pp 534-539. (25) Shi, Z.; Lipkowski, J.; Gamboa, M.; Zelanay, P.; Wieckowski, A. J. Electroanal. Chem. 1994, 366, 317-326. (26) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 364, 289294. (27) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 365, 303309.
Electrochemical solutions were prepared from ultrapure water (Milli-Q UV plus, Millipore Inc., 18.2 MΩ cm) and Bi2O3 (Aldrich, 99.9%). Supporting electrolyte was 0.1 M HClO4 (Merck, Suprapure, 1 s) that the charge could not be accurately separated from faradaic background processes, limiting the pH range which could be used to values between 1 and 2.65. The charge density at the electrode surface in representative pH solutions is presented in Figure 8. The steps again correspond to the steps in the voltammetry. These curves are then integrated from negative to positive potentials to give the electrocapillary curve shown in Figure 9. The value of ∆γ at the bottom integration limit from the Bi data is used as the integration constant. These curves coincide at each end, but vary with pH in the catalytically active region, indicating that there is a species adsorbed in that potential region. Differentiation of the surface tension in these solutions with respect to RT ln([OH-]) gives the surface concentration of hydroxide which is presented in Figure 10. The concentration is zero at more positive potentials, indicating that hydroxide is not adsorbed. It rises to a peak of 2.4 × 1014 at 0.43 V in the catalytically active region. This corresponds to a coverage of 0.17 monolayer (ML) and
indicates that there is a significant amount of OHcoadsorbed with the Bi in this potential region. At more negative potentials, the concentration again falls back to near zero, leveling off at about 0.33 V. This indicates that the anion is no longer present at the interface with the Bi. 4. Discussion 4.1. Bi Coverage. The coverage of Bi rises steeply in the catalytically active region, passing through 0.25 ML 0.42 V. This coverage is the same as that calculated from the structure observed with AFM, STM, and SXS at the same potential. This provides confirmation for the identification of this structure and is more evidence that the open structure is in fact the one responsible for catalysis of the hydrogen peroxide reduction reaction. At more negative potentials just prior to the onset of bulk Bi reduction, the coverage rises steadily until it reaches a limiting value of 0.67 ML. In the same potential region STM and AFM images revealed an incommensurate close-packed rectangular (p × x3) Bi adlattice. SXS measurements revealed that the overlayer in this region compresses as the potential is decreased, changing from 0.62 to 0.65 ML. This observation is in agreement with the results presented here in which the coverages rise steadily prior to the onset of bulk deposition without reaching a plateau. The final coverage observed is also in agreement with the previously proposed structure of the Bi adlayer.
Surface Coverage of Bi on Au
The electrosorption valency calculated from this work is roughly 3 throughout the Bi upd potential region. The large variation between 0.4 and 0.5 V corresponds to the location of the large Bi upd peak and likely arises from an amplification of the variation in the charge data when dividing by the coverage which has a very steep slope at this point. At potentials more negative than this, the charge remains between 2.8 and 3.0. As indicated by the error bars, these data are not statistically different from other estimates of electrosorption valency for Bi on polycrystalline Au as 2.6-2.7.13,15 The value observed is close to 3, however, and indicates that the Bi adatoms are significantly discharged on the electrode surface. While nothing is known about the polarization of the electrons within the Bi-Au bond, the electrons have left the electrode and are more closely associated with the Bi. The Bi atoms therefore have no net charge which could hold open the adlayer. 4.2. Hydroxide Coverage. The coverage of hydroxide observed at potentials greater than 0.65 V is essentially zero. The electrode surface consists of bare Au(111) in this range. Hydroxide is known not to strongly associate with bare gold in acidic solution at potentials negative of the onset of oxidation at about 1.2 V and is not expected to be present on the surface. As the potential approaches the Bi upd region, the OHcoverage begins to rise as well, reaching a peak of 0.17 ML at 0.43 V, the potential where the (2 × 2) Bi adlayer is observed by scanning probe microscopy (SPM) and 0.25 ML Bi coverage is observed by the potential-step method. There is a negative charge density at the electrode surface in this potential region, and hydroxide does not adsorb in the absence of Bi. The presence of hydroxide must be due to the strong attraction between Bi and OH-, and is likely the reason that the Bi adlayer remains open at large underpotentials. In fact, solvated Bi has been found to form clusters with hydroxide even in the presence of 0.95 M HClO4.29 We observed no substantial difference in the upd voltammetry of Bi3+ in the presence of sulfate, as is seen in the cases of Ag and Cu. The strong association between Bi and OH- may preclude the presence of SO42in the adlattice and so render the structure insensitive to the presence of sulfate. The ratio between OH- and Bi coverage in the (2 × 2) structure region is 1:1.5. This is intermediate between a structure where OH- is adsorbed atop each Bi and one where the OH- is bridging between two Bi. Other possible structures for Bi and OH- include those where two OHmoieties are clustered above three Bi atoms in the (2 × 2) lattice. The OH- structure may also be fluxional. Further insight into the exact structure will have to await more definitive vibrational spectroscopic data. It has been observed that activity of a gold surface for oxygen reduction increases as the ease of formation of the AuOHads layer increases.30 The Au(100) and Au(110) surfaces, which more readily adsorb OH- than Au(111) even in acid electrolyte, are found to be considerably more (29) Levy, H. A.; Danford, M. D.; Agron, P. A. J. Chem. Phys. 1959, 31, 1458-1461. (30) Adzic, R. R. Proceedings of the Workshop on Structural Effects in Electrocatalysis and Oxygen Electrochemistry; Scherson, D., Tryk, D., Daroux, M., Xing, X., Eds.; The Electrochemical Society, Inc.: Pennington, NJ, 1992; Vol. 92-11, pp 419-433.
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active for oxygen reduction. The higher affinity of the (2 × 2)-Bi adlayer for OH- adsorption may play a similar role in this catalysis by lowering the energy of the adsorbed intermediate oxygen species. The presence of OH- in the catalytically active potential region may also indicate that OH- plays a role in the electrocatalysis itself. Finally, as the potential is swept more negative, the coverage of OH- continues to drop, reaching a steady state value near zero at 0.33 V. The hydroxide may be squeezed out by the condensing Bi adlayer as the driving force for Bi deposition increases. In addition, as the potential decreases, the Coulombic repulsion between the electrode surface and the anions will increase. 5. Conclusions This work confirms the (2 × 2) Bi adlayer observed by STM, AFM, and SXS in the catalytically active potential region of the Au(111)/Bi3+ upd system. The 25% surface coverage of Bi observed matches that calculated for the (2 × 2) structure. This work also indicates that there is 17% of a monolayer of hydroxide coadsorbed with the metal atoms in the early stages of upd where the surface structure is an open one. There have been numerous observations of the influence of anions on upd adlayer structure. The strong attraction between Bi and OHdominates the formation of Bi upd layers even in acidic media where the OH- is a minor constituent. The ready availability of this anion in aqueous systems, in turn, renders the adlayer structure insensitive to the identity of the supporting electrolyte. The coverages observed at more cathodic potentials in this work also confirm those observed by STM, AFM, and SXS measurements. The rectangular, incommensurate (p × x3) Bi adlayer observed by SXS compresses slightly with potential from 0.62 to 0.65 ML. The coverage we observe also rises through this potential region, reaching a limiting value of 0.67 ML. This study indicates that the structure of the Bi upd adlayer on Au(111) is controlled by the presence of an anion just as the Cu and Ag adlayers are. There is still much to be learned about the nature of the H2O2 reduction catalysis in this system. Quartz crystal microbalance (QCM) studies are underway in our group to confirm the coverages observed electrochemically. In addition, molecular poisons will be studied in this system by QCM and in situ IR to elucidate the nature of the interaction of the catalytic active site with small molecules. Acknowledgment. We thank Antoinette Hamelin for helpful discussions and advice during her visit to Illinois in December 1993. B.K.N. acknowledges a Department of Education fellowship administered by the University of Illinois. This work was funded by the Department of Energy through the Materials Research Laboratory at the University of Illinois (DE-FG02-91ER45349). 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. LA9515480
