Adsorption and Electrooxidation of Carbon Monoxide on Silver

Received February 6, 1998. In Final Form: ... 6.5 kcal molr1.2 A clear indication that CO adsorbs on Ag in an 1 M H2SO4 electrolyte was obtained alrea...
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Langmuir 1998, 14, 6297-6306

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Adsorption and Electrooxidation of Carbon Monoxide on Silver G. Orozco, Ma. C. Pe´rez, A. Rinco´n, and C. Gutie´rrez* Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, C. Serrano, 119, 28006-Madrid, Spain Received February 6, 1998. In Final Form: August 12, 1998 The adsorption and electrooxidation of CO on polycrystalline Ag at pH 13, 9.2, 7, and 0.3 have been studied by cyclic voltammetry and Fourier transform infrared spectroscopy (FTIRS). At the four pH values the cyclic voltammograms (CVs) showed the formation and electroreduction of underpotential (UP) Ag oxides, i.e., oxides formed at potentials lower than the equilibrium potential of the Ag/Ag2O couple. Two pairs of peaks of UP oxides appeared at pH 13 and 9.2, but only one pair at pH 7 and 0.3. At pH 13 and 9.2 the current density of the anodic peak of the UP oxide appearing at the more positive potential increased if the solution was saturated with CO, and its peak potential increased (pH 13) or remained the same (pH 9.2), indicating the electrooxidation of adsorbed CO. Both at pH 7 and 0.3 the anodic peak in the presence of CO appeared at potentials 0.2 V more negative than that in the corresponding base electrolyte. Only at pH 13 did the CO adsorbed on Ag remain adsorbed after eliminating the dissolved CO by N2 bubbling, indicating that CO was strongly adsorbed. The FTIR spectra showed, at the four pH values, a band at 1970-2000 cm-1 assigned to CO linearly adsorbed on Ag and, at neutral and alkaline pH, a band at 1860-1900 cm-1 assigned to CO adsorbed in a bridge position. At pH 9.2, 7 and 0.3 there appeared a band at 2048 (pH 9.2 and 7) or 2112 (pH 0.3) cm-1 whose frequency did not change with potential and whose intensity increased with increasing potential and which was therefore assigned to CO adsorbed on an UP Ag oxide.

Introduction Although the adsorption of CO on Cu, Ag, and Au is weaker than adsorption on metals such as Ni or Pt, since there is less retrodonation of electronic density from the d orbitals of the former metals to the CO molecule, it has been established, on the basis of core and valence photoemission, X-ray absorption, and autoionization of core excited states, that gaseous CO does chemisorb on the IB metals, the adsorption strength decreasing in the order Cu(100) > Au(110) > Ag(110).1 This conclusion rests on two criteria: that the width of the π resonance of X-ray absorption, corresponding to core-to-bound excitation of either a C1s or O1s electron into the unoccupied part of the 2π*-d hybrid level, increases with increasing chemisorption energy, and that the relative intensities of the satellites in the C1s and O1s X-ray photoelectron spectra increase with decreasing CO-metal interaction. In agreement with this ordering of CO adsorption strength, the initial heat of adsorption of gaseous CO on Ag(111), calculated from isotherms in the range 66-123 K, is only 6.5 kcal mol-1.2 A clear indication that CO adsorbs on Ag in an 1 M H2SO4 electrolyte was obtained already in 1974 by Gossner and Po¨lzl,3 who found that CO poisoned hydrogen evolution on silver but, contrary to the above estimate for the gas phase, not on copper or gold. In these experiments a fresh, uncontaminated silver surface was produced by cutting in situ with a glass knife at a rate of 1-5 Hz the end of a silver wire embedded with Araldit in a Plexiglas tube, a simple yet efficient method that yielded very reproducible results, while the reproducibility obtained with recommended complex, tedious cleaning procedures was very poor. * Corresponding author: Phone: 34-915619400, ext 1327. FAX: 34915642431. E-mail: [email protected]. (1) Sandell, A.; Bennich, P.; Nilsson, A.; Hernna¨s, B.; Bjo¨rneholm, O.; Mårtensson, N. Surf. Sci. 1994, 310, 16. (2) McElhiney, G.; Papp, H.; Pritchard, J. Surf. Sci. 1976, 54, 617. (3) Gossner, K.; Po¨lzl, H. Z. Phys. Chem. (Frankfurt) 1974, 90, 164.

A very weak surface enhanced Raman spectroscopy (SERS) band at 2000 cm-1 of CO adsorbed on Ag was first reported in 1984, but it appeared only after roughening the electrode with an oxidation-reduction cycle in the 0.1 M KCl electrolyte.4 The band showed a Stark shift of 58 cm-1 V-1, which indicates that CO was indeed chemisorbed, since the usual explanation of the CO frequency increase with increasing potential is that the latter, by decreasing the metal Fermi level, decreases retrodonation from the metal to the antibonding 2π* orbital of CO and, consequently, increases the strength of the C-O bond. A weak SERS band at 1998 cm-1 has been reported for CO adsorbed on an (apparently unroughened) Ag electrode in 3.5 M KCl.5 A Fourier-tramsform infrared (FTIR) spectrum of CO adsorbed on a roughened Ag electrode in 0.1 M NaClO4 was first reported in 1988.6 The Ag electrode was previously cycled for 1 h between -0.90 and 0.60 V vs SCE under CO bubbling, with which the electrode became so rough that a strong band of linearly adsorbed CO at 2021 cm-1 was observed. An intense band whose frequency increased from 2016 to 2026 cm-1 with increasing potential was observed7 in FTIR spectra of CO adsorbed on uncycled underpotential-deposited (UPD) Ag on Pt in 0.1 M KCl. The high CO coverage on Ag was attributed to a higher activity for CO chemisorption of UPD Ag as compared with bulk Ag, this hypothesis being supported by the finding5 that the intensity of an IR band at 1995 cm-1 of CO linearly chemisorbed on an Ag film, 2.5 monolayers thick, electrodeposited onto Pt(110) decreased very much when the thickness of the Ag film was increased to 7 monolayers. (4) Mahoney, R. M.; Howard, M. W.; Cooney, R. P. J. Electroanal. Chem. 1984, 161, 163. (5) Oda, I.; Ogasawara, H.; Ito, M. Langmuir 1996, 12, 1094. (6) Ikezawa, Y.; Saito, H.; Matsubayashi, H.; Toda, G. J. Electroanal. Chem. 1988, 252, 395. (7) Furukawa, H.; Ajito, K.; Takahashi, M.; Ito, M. J. Electroanal. Chem. 1990, 280, 415.

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It is usually accepted that, as in the electrooxidation of many organic compounds, in the electrooxidation of both adsorbed and dissolved CO the necessary oxygen atoms proceed not from water, but from oxides or other oxygenated compounds formed by oxidation of the metal electrode. In 1986 Burke9 gave a further refinement to this model, postulating that in many organic electrooxidation reactions the necessary oxygen atoms are supplied by “incipient hydrous oxides”, whose concentration is so low that in many cases their formation does not produce appreciable currents. We introduce here the unequivocal term underpotential (UP) oxides, whose Gibbs energy of formation is, due to their interaction with the surface of the metal, lower than that of bulk oxides. Consequently, UP oxides are formed at potentials lower than the thermodynamic, equilibrium potential of the metal/metal oxide couple with the lowest equilibrium potential. Kita et al.8 observed some electrooxidation of CO on Ag in 1 M NaOH between 0.4 and 1.0 V vs the reversible hydrogen electrode (RHE) (in this work the potentials are always referred to the RHE unless otherwise specified), although they did not comment that the cyclic voltammogram (CV) in the 1 M NaOH background electrolyte showed three anodic peaks, with a total charge of about 200 µC cm-2, at 0.33, 0.58, and 0.86 V, which unequivocally correspond to UP oxides, since the equilibrium potential of the Ag/Ag2O couple is higher, 1.18 V vs RHE. Burke’s model applies here, since CO oxidation occurred in the same potential range as the two UP oxides that appeared at the higher potentials. In the present work we will show, both by cyclic voltammetry (CV) and FTIR spectroscopy, that CO adsorbs on Ag immersed in an electrolyte, over the pH range 0.313, without any previous potential cycling, i.e., that CO adsorbs on an unroughened Ag electrode. This adsorbed CO is electrooxidized to CO2 by a positive potential sweep. Experimental Section Disks of 5 and 12 mm in diameter for CV and FTIRS, respectively, were cut from a 1-mm thick sheet of 99.97% pure Ag from Advent. For CV a single-compartment cell was used, with a smooth Pt auxiliary electrode and a saturated calomel electrode (SCE) as reference. However, the potentials are given vs a reversible hydrogen electrode in the same solution (RHE). Ultrapure water was obtained from a Milli-RO + Milli-Q setup. Analytical grade reagents were used. In the first experiments the electrodes were cleaned by polishing with alumina down to 0.05 µm and sonication in water, but this method did not yield reproducible results. After being etched with a mixture of 5 volumes of 30% NH4OH and 3 volumes of 30% H2O2,10 the Ag electrodes lost their luster but retained the same electrochemical behavior of the polished electrodes, while the reproducibility of the CVs was greatly increased. However, especially in acidic media the currents decreased to about one-half if after the etching the Ag electrodes were sonicated in Milli-Q water, which we attribute to a dislodging by sonication of ultrafine Ag particles, produced during the etching, from the surface of the Ag electrodes. All CVs reported here were obtained with etched and sonicated Ag electrodes. A Perkin-Elmer FTIR instrument, model 1725-X, purged with CO2- and H2O-free air from a Peak Scientific equipment at a rate of 16 L min-1 was used. A Teflon cell with a flat fluorite window was built to fit in a Harrick specular reflectance accessory. The IR beam was p-polarized with a Harrick wire grid polarizer, and the resolution was 8 cm-1. Mechanically polished Ag electrodes were used for obtaining IR spectra, since the IR reflectivity of etched Ag was too low. (8) Kita, H.; Nakajima, H.; Hayashi, K. J. Electroanal. Chem. 1985, 190, 141. (9) Burke, L. D.; Cunnane, V. J. J. Electroanal. Chem. 1986, 210, 69. (10) Furtak, T. E.; Sass, J. K. Surf. Sci. 1978, 78, 591.

Orozco et al. Two potential programs, linear potential sweep FTIRS (LPSFTIRS) and square wave FTIRS (SW-FTIRS), were used. In LPS-FTIRS, a technique suitable for following the evolution with potential of an electrochemical system, interferograms are continuously recorded during an LPS at typically 1 mV s-1, each, e.g., 25 successive interferograms being coadded into a spectrum. The LPS was started at the negative potential limit, and the first spectrum was taken as reference. Normalized differential spectra, calculated as ∆R/R ) (Rsample /Rref) - 1, can be correlated with features in the voltammogram. The other procedure, SWFTIRS, is more suitable for detecting chemisorbed species which are stable over a potential range at least 0.2 V wide, and is based on the Stark shift of IR bands of chemisorbed species, typically 30-60 cm-1 V-1 for CO. Interferograms are alternately collected at two different potentials, after which all the spectra collected at each potential are added, and from the two spectra the normalized differential reflectance spectrum, showing the typical bipolar band(s) of chemisorbed species only, is obtained. With this procedure the artifacts produced by slow changes in the experimental conditions are minimized, and so a larger number of interferograms can be collected.

Results 0.1 M NaOH. Cyclic Voltammetry. The first CV between -0.19 and 1.01 V at 50 mV s-1 of etched and sonicated Ag in quiescent 0.1 M NaOH after 10 min of N2 purging at -0.19 V usually shows ill-defined peaks, but after a few cycles the peaks become better defined and the current density increases up to a maximum, after which it decreases monotonically (Figure 1a). Two anodiccathodic processes appear in the CV. The first process is actually a convolution of a slow UP oxide formationreduction with anodic and cathodic peak potentials of 0.53 and 0.25 V, respectively, and a fast UP oxide formationreduction with the same anodic and cathodic peak potential, 0.40 V, only this fast surface process remaining after a few cycles. It is very interesting that the half-sum potential of the anodic and cathodic peaks of the slow process is the same as that of the fast process, 0.40 V, which indicates that the difference between the two processes is kinetic only. In other words, the same UP oxide is being formed and electroreduced, but at different rates. The second process has a half-sum potential of 0.79 V. The CV behavior was the same if the N2 purging was carried out at open circuit instead of at -0.19 V. If the upper potential limit is lowered to 0.61 V, so that only the first pair of peaks appears in the CV, the current density grows monotonically up to a maximum, but, quite interestingly, the process that remains is the slow one, i.e., that with a hysteresis between the anodic and cathodic peak potentials (Figure 1b). The simplest explanation of this behavior is that no restructuring of the Ag surface occurs with a positive potential limit of 0.61 V, with which the anodic and cathodic charges are 135 and 142 µC cm-2, respectively, corresponding to about 1 electron per Ag surface atom (Figure 1b). However, when the positive potential limit is increased to 1.01 V, with which the charge corresponds to about 2 electrons per surface Ag atom (Figure 1a), apparently a restructuring of the Ag surface occurs which, of the two surface processes at 0.40 V, favors the fast one. As said in the Introduction, Kita et al.8 reported three anodic peaks in the positive sweep of Ag in 1 M NaOH. Only in a few experiments did we find a third UP oxide formation-reduction, with a half-sum potential of 0.18 V, as shown in Figure 2a, in which consecutive CVs obtained at sweep rates ranging from 0.01 to 2 V s-1are plotted with adsorption capacity, i/(dV/dt) (which is independent of sweep rate for a surface process), as the vertical axis. The rather good superposition of the CVs

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Figure 1. (a) Successive cyclic voltammograms (CVs) at 50 mV s-1 from -0.19 to 1.01 V of etched and sonicated Ag in quiescent 0.1 M NaOH after 10 min of N2 bubbling at -0.19 V. The first CV is shown in dotted lines. The solid lines correspond, in order of decreasing current density, to the 4th, 10th, 15th, 20th, 25th, and 30th CV. (b) Same as (a), but with a smaller potential excursion, from -0.19 to 0.61 V. The current increases with cycling up to a maximum. The CVs shown are the 1st, 5th, 10th, 15th, 20th, 25th, and 30th. The arrows indicate the evolution of the CVs.

shows no trace of the reported11 lack of superposition of the adsorption capacities at 0.05 and 0.5 V s-1 of Ag(111) at pH 11, from which it was surmised that UP oxide formation was slow. The conventional plot of current densities vs sweep rate is shown in Figure 2b; it can be appreciated that this plot is far less sensitive for detecting deviations from an ideal surface process than a plot of CVs with adsorption capacity as the vertical axis. The slopes of the plots of the anodic and cathodic peak potentials of the three surface processes vs the sweep rate were nearly negligible up to at least 2 V s-1 (Figure 2c), which shows that formation and reduction of these UP oxides are fast processes, in apparent contradiction with the fact that for each process there is hysteresis between the anodic and cathodic peak potentials. The same behavior has been reported9 for the UP oxide that appears at 0.2 V on Au in 0.2 M Na2SO4, pH 11.7; tentative explanations for the simultaneous occurrence of hysteresis between the anodic and cathodic peaks, and the invariance of the peak potentials over a range of sweep rates spanning 2 orders of magnitude, were advanced. (11) Savinova, E. R.; Kraft, P.; Pettinger, B.; Doblhofer, K. J. Electroanal. Chem. 1997, 430, 47.

Figure 2. (a) CVs from -0.19 to 1.01 V of etched and sonicated Ag in quiescent 0.1 M NaOH at sweep rates ranging from 0.01 to 2 V s-1. Instead of the current density the adsorption capacity was chosen for the vertical axis, to show that the three anodiccathodic processes are surface processes. (b) Plot of anodic peak current densities vs sweep rate for the CVs in (a). (c) Plot of the peak potentials of the three anodic-cathodic processes in the CVs in (a) vs the logarithm of the sweep rate. Full and empty signs correspond to the anodic and cathodic peak potentials, respectively. The values of the slopes are shown against each straight line.

The first CV of an etched and sonicated Ag electrode after 10 min of N2 bubbling at -0.19 V, and the first CV after 10 min of CO bubbling, also at -0.19 V, are shown as solid and dashed lines, respectively, in Figure 3a. It can be seen that the electrooxidation of Ag is inhibited by CO and that the more positive anodic peak increases very much and is shifted positively by 0.13 V. Both effects

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Figure 3. (a) First CV at 50 mV s-1 from -0.19 to 1.01 V of etched and sonicated Ag in quiescent 0.1 M NaOH, after 10 min of N2 bubbling (solid line), after 10 min of CO bubbling (dashed line), and after 10 min of CO bubbling followed by 5 min of N2 bubbling (dotted line), in all cases at -0.19 V. (b) Plot of the current density of the most positive anodic peak of etched and sonicated Ag in CO-saturated 0.1 M NaOH vs the sweep rate. (c) Plot of the peak potential (vs SCE) of the most positive anodic peak of etched and sonicated Ag in CO-saturated 0.1 M NaOH vs the logarithm of the sweep rate.

should be due to a poisoning of the Ag surface by chemisorbed CO, as already reported for Fe,12 Co,13 and (12) Cuesta, A.; Gutie´rrez, C. J. Phys. Chem. 1996, 100, 12600. (13) Cuesta, A.; Gutie´rrez, C. Langmuir 1998, 14, 3390.

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Ni.14 The total anodic charge in the positive sweep in N2 was 381 and 617 µC cm-2, i.e., 1.6 times higher, in CO. The peak current density of the most positive anodic peak of Ag in CO-saturated solution increases linearly with the sweep rate up to at least 2 V s-1, showing that this peak corresponds to a surface process (Figure 3b). A plot of the anodic peak potential vs the logarithm of the sweep rate yields a straight line with a slope, equal to the Tafel slope (since the process is a surface one), of 125 mV (Figure 3c), which suggests that the rate-determining step is the withdrawal of the first electron from the chemisorbed CO. Although the peak corresponds to the electrooxidation of both Ag and chemisorbed CO, it is clear that it is the electrooxidation of chemisorbed CO which controls the kinetics, since the peak potential of UP oxide formation is nearly unaffected by an increase of the sweep rate (Figure 2c). If after saturation of the solution with gaseous CO the CO in solution is eliminated by N2 bubbling, also at -0.19 V, a subsequent CV (dotted line in Figure 3a) remains similar to that in CO-saturated solution, but shifted toward more negative potentials, as is typically the case with noble metals.15 The total anodic charge in the positive sweep was 490 µC cm-2, about one-third higher than that in base electrolyte, indicating that the chemisorption of CO is strong enough as to be unaffected by N2 bubbling. FTIR Spectroscopy. Several SW-FTIR spectra of polished Ag in CO-saturated 0.1 M NaOH are shown in Figure 4a. The lower (reference) potential of the modulation was kept constant at 0.01 V, and the higher potential was increased from 0.21 to 0.81 V. Already with a modulation amplitude of 0.20 V two bipolar bands, typical of chemisorbed species, can be seen, with midfrequencies of 1968 and 1905 cm-1. For their assignment it is crucial to take into account that, with good approximation, for a given coverage the frequency of an IR band of a chemisorbed species depends on the potential vs a pHindependent reference electrode (e.g., an SCE) only and not on the pH of the solution, which has been attributed to the exclusion of water and its ions from the inner part of the double layer by chemisorbed CO.13,14 The independence of the stretching frequency of adsorbed CO on pH was first reported by Kunimatsu et al.,16 who found that the frequencies of CO chemisorbed on Au in 0.2 M NaOH and in 1 M HClO4 fell on the same straight line, spanning the ranges 1935-1985 and 1990-2050 cm-1, respectively, and with a slope of 64 cm-1 V-1 over a potential range of nearly 2 V. Since the potential range where chemisorbed CO is stable decreases by 60 mV (vs a pH-independent reference electrode) per pH unit increase, the typical frequencies for CO stretching will be about 0.06 V (pH unit)-1 × 14 pH units × 60 cm-1 V-1 ) 50 cm-1 lower at pH 14 than at pH 0. Therefore, the band at 1968 cm-1 corresponds to linearly chemisorbed CO and that at 1905 cm-1 to CO in the bridge position. The size of the bipolar bands in Figure 4a increases with increasing modulation amplitude up to a positive limit of 0.61 V, at which electrooxidation of the chemisorbed CO has not started yet. However, the bands decrease very much for a positive limit of 0.71 V and completely disappear for 0.81 V, when electrooxidation of chemisorbed CO has been completed. The adsorption of CO on Ag at this pH value, 13, is strong, since after displacing under controlled potential (14) Cuesta, A.; Gutie´rrez, C. Langmuir 1998, 14, 3397. (15) Caram, J. A.; Gutie´rrez, C. J. Electroanal. Chem. 1991, 305, 259. (16) Kunimatsu, K.; Aramata, A.; Nakajima, H.; Kita, H. J. Electroanal. Chem. 1986, 207, 293.

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Figure 5. First CV at 50 mV s-1 between -0.11 and 0.99 V of etched and sonicated Ag in 0.1 M borax, pH 9.2, after 10 min of N2 bubbling (solid line), after 10 min of CO bubbling (dashed line), and after 10 min of CO bubbling followed by 5 min of N2 bubbling (dotted line), in all cases at -0.11 V.

Figure 4. FTIR spectroscopy of polished Ag in 0.1 M NaOH. (a) Square-wave FTIR (SW-FTIR) spectra between a constant reference potential of 0.01 V and an increasingly positive sample potential, from 0.21 to 0.81 V. (b) Linear potential sweep FTIR (LPS-FTIR) spectra taken during a positive LPS at 1 mV s-1, with the interferogram at -0.29 V as reference.

the CO-saturated with CO-free electrolyte (the smallness of our cell did not allow us to use N2 bubbling to eliminate the dissolved CO) the same two bands still appeared in SW-FTIR spectra (not shown), although their size decreased by about one-half. This is in perfect agreement with the CV results, in which the peak of CO electrooxidation remained after elimination of the dissolved CO by N2 bubbling (dotted CV in Figure 3a). Several LPS-FTIR spectra of the same system are shown in Figure 4b. They are severely distorted by a negative band at 1650 cm-1, due to the formation of water at this alkaline pH. A bipolar band of bridge CO appears at 1926 cm-1, while the band of linear CO at higher frequencies can barely be seen. A band of carbonate ion at 1410 cm-1 begins to appear at 0.62 V and reaches maximum intensity (∆R/R ) 0.73%) at 0.92 V. 0.1 M Borax. The first CV at 50 mV s-1 between -0.11 and 0.99 V of etched and sonicated Ag in quiescent 0.1 M borax after 10 min of N2 purging at -0.11 V shows an anodic and two cathodic peaks of UP oxides (the solid curve in Figure 5). The total anodic charge in the positive sweep was 163 µC cm-2. The current densities increase

slightly with cycling and then decrease, as in 1 M NaOH. The missing anodic peak was observed only after potential cycling of an etched, but not sonicated, Ag electrode. The first CV of an etched and sonicated Ag electrode after 10 min of CO bubbling at -0.11 V is given as the dashed curve in Figure 5. The total anodic charge in the positive sweep was 241 µC cm-2, 1.5 times higher than that in N2, but the main anodic peak potential remains constant, which suggests that at this pH there is no strong interaction between CO and Ag. If after CO saturation the dissolved CO is eliminated by N2 bubbling, also at -0.11 V, the first CV is the same as that in N2 (dotted line in Figure 5), which confirms that CO does not chemisorb on Ag in 0.1 M borax. The total anodic charge in the positive sweep is 156 µC cm-2, about the same as that in N2. FTIR Spectroscopy. Quite surprisingly, the SW-FTIR spectrum in the CO-stretching region of polished Ag in CO-saturated 0.1 M borax (Figure 6a) shows, besides the two bands that had already appeared in 0.1 M NaOH, namely, a band of linear CO at 1966 cm-1 and a band of bridge CO at 1884 cm-1, a new band at 2036 cm-1. The three bands were not observed with s-polarization (Figure 6b), which indicates that they correspond to adsorbed species. The negative bands of formation of carbonate at 1410 cm-1 and of bicarbonate at 1365 cm-1 indicate that there is migration of these anions in to and out of the thin electrolyte layer during the more and less positive potentials, respectively, of the square-wave modulation. The carbonate band did not appear in SW-FTIR spectra in 0.1 M NaOH (Figure 4a), probably because the migration of hydroxyl ions was much higher than that of carbonate, although obviously the carbonate band did appear in LPSFTIR spectra (Figure 4b). The negative band of increase of water concentration could be due to the hydration shells of the carbonate and bicarbonate anions. No CO bands remained in the SW-FTIR spectra after displacing the CO-saturated electrolyte with CO-free one, which shows, in agreement with the CV results, that at pH 9.2 CO does not chemisorb on Ag (spectra not shown). The same weakness of the adsorption of CO on Ag was observed at pH 7 and 0.3. In LPS-FTIR spectra of the same system (Figure 6c) the band at 2048 cm-1 shows the following characteristics: it is monopolar; in agreement with this, its frequency does not change with potential; and it increases with

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Figure 7. First cyclic voltammogram at 50 mV s-1 between -0.14 and 0.90 V of etched and sonicated Ag in 0.5 M NaClO4 after 10 min of N2 bubbling (solid line), after 10 min of CO bubbling (dashed line), and after 10 min of CO bubbling followed by 5 min of N2 bubbling (dotted line), in all cases at -0.14 V.

Figure 6. FTIR spectroscopy of polished Ag in 0.1 M borax, pH 9.2. (a) Square-wave FTIR (SW-FTIR) spectra between a constant reference potential of -0.11 V and an increasingly positive sample potential, from 0.09 to 0.89 V. (b) Square-wave FTIR (SW-FTIR) spectra with s- and p-polarized light between 0 and 0.40 V. (c) Linear potential sweep FTIR (LPS-FTIR) spectra taken during a positive LPS at 1 mV s-1, with the interferogram at -0.11 V as reference.

increasing positive potential, up to 0.91 V, and then remains constant up to the positive limit of the sweep, 1.08 V. With the help of these characteristics, completely unusual for CO chemisorbed on a metal, an assignment for this band will be proposed in the Discussion. Obviously the negative band of carbonate formation, clearly seen already at 0.30 V, is originated by the electrooxidation of CO adsorbed on metallic Ag. 0.5 M NaClO4. Cyclic Voltammetry. The first CV at 50 mV s-1 between -0.14 and 0.90 V of etched and sonicated Ag in quiescent 0.5 M NaClO4 after 10 min with N2 bubbling at -0.14 V shows an anodic-cathodic process with a half-sum potential of 0.63 V, although the anodic current begins from the start of the sweep and increases up to a plateau that precedes the anodic peak (solid line in Figure 7). The total anodic charge in the positive sweep was 192 µC cm-2. In the second CV the anodic peak decreases by one-half and is shifted negatively by 0.12 V, the decrease continuing in successive sweeps. The first CV of etched and sonicated Ag after 10 min at -0.14 V under CO bubbling shows, quite unexpectedly (dashed line in Figure 7), an anodic peak at a potential 0.18 V more negative than the peak in N2, and which should correspond to the electrooxidation of adsorbed CO. The hump at the same potential as the peak in N2 should correspond, as in N2, to the electrooxidation of Ag. The total anodic charge in the positive sweep was 163 µC cm-2, slightly lower than that in N2. If after CO saturation the dissolved CO is displaced by N2 bubbling, also at -0.14 V, the first CV (dotted line in Figure 7) is basically the same as in the background electrolyte, with the anodic peak shifted positively by 45 mV only. The disappearance of the peak at 0.58 V shows that CO does not chemisorb on Ag in NaClO4 solutions. The total anodic charge in the positive sweep was 233 µC cm-2, slightly higher than in N2. FTIR Spectroscopy. The SW-FTIR spectra of polished Ag in CO-saturated 0.5 M NaClO4 are very similar to those in 0.1 M borax, showing a band at 2036 cm-1 and two bands at 1963 and 1871 cm-1 of linear and bridge CO, respectively (Figure 8a). The negative band at 1100 cm-1 is due to an increase and decrease of the concentration of ClO4- ions in the thin electrolyte layer at the high and low potentials, respectively, of the square-wave modulation. The negative peak at 1637 cm-1 of increase of water concentration could be due to the hydration shell of the ClO4- ions.

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Figure 9. First CV at 50 mV s-1 between -0.24 and 0.51 V of etched and sonicated Ag in 0.5 M HClO4, pH 0.3, after 10 min of N2 bubbling (solid line), after 10 min of CO bubbling (dashed line), and after 10 min of CO bubbling followed by 5 min of N2 bubbling (dotted line), in all cases at -0.24 V.

Figure 8. FTIR spectroscopy of polished Ag in 0.5 M NaClO4. (a) Square-wave FTIR (SW-FTIR) spectra between a constant reference potential of -0.34 V and an increasingly positive sample potential, from 0.14 to 0.36 V. (b) Linear potential sweep FTIR (LPS-FTIR) spectra taken during a positive LPS at 1 mV s-1, with the interferogram at -0.30 V as reference.

The LPS-FTIR spectrum shows at the lower potentials two bipolar bands of linear and bridge CO, which become positive at higher potentials, once the electrooxidation of CO has been completed (Figure 8b). In the same way as in 0.1 M borax, the frequency of a third band at 2048 cm-1 does not change with potential. This band can be clearly appreciated already at 0.08 V, a potential at which the formation of an UP Ag oxide has just begun.

0.5 M HClO4. Cyclic Voltammetry. The first CV at 50 mV s-1 between -0.24 and 0.51 V of etched and sonicated Ag in quiescent 0.5 M HClO4 after 10 min of N2 bubbling at -0.24 V shows a peak at 0.38 V with a hump at 0.42 V (solid line in Figure 9). The total anodic charge was 582 µC cm-2. In the second CV the current density of the anodic peak decreases to 1/4 of that in the first CV and continues to decrease with further cycling. The first CV of etched and sonicated Ag after 10 min of CO bubbling at -0.24 V shows a peak at 0.18 V (dashed line in Figure 9), a potential 0.20 V more negative than the anodic peak in N2. The peak at 0.18 V should correspond exclusively to adsorbed CO, as confirmed by FTIRS (see below), since in N2 the anodic current starts at 0.20 V. As far as we know, 0.18 V is the lowest peak potential ever reported for the electrooxidation of adsorbed CO, this potential being only 0.28 V more positive than the equilibrium potential for the electrooxidation of dissolved CO to CO2, -0.10 V. (The lowest peak potential for the electrooxidation of dissolved CO, 0.29 V, has been reported for Au in 1 M NaOH, under conditions at which no CO was adsorbed on the electrode.8) The peak of Ag electrooxidation at 0.38 V becomes a plateau. The total anodic charge is 502 µC cm-2, slightly lower than that in N 2. If the solution is first saturated with CO and then the dissolved CO is displaced by N2 bubbling, also at -0.24 V, the first CV (dotted line in Figure 9) is similar to that in the background electrolyte, although slightly shifted by 30 mV to more positive potentials and with a charge of 683 µC cm-2, slightly higher than that in N2. The elimination by N2 bubbling of the peak at 0.18 V shows that this peak is due to weakly adsorbed CO, as confirmed by FTIRS (see below). The anodic current starts at 0.10 V, at the same potential as the foot of the anodic peak in the presence of CO in solution. FTIR Spectroscopy. In SW-FTIR spectra (Figure 10a) of polished Ag in CO saturated in 0.5 M HClO4 a bipolar band of linear CO near 2000 cm-1 appears already with a modulation amplitude of 0.10 V. Modulating between -0.14 and 0.26 V another band appears at 2115 cm-1. It is monopolar and negative, as the band at 2048 cm-1 found at pH 9.2 and 7.2 whose frequency did not change with potential. The LPS-FTIR spectra in Figure 10b show the bipolar band at about 2000 cm-1 of linear CO over the potential

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in which it remained up to the positive potential limit, 1.08 and 0.68 V, respectively. This behavior is probably related to the different nature of the UP oxides on which the CO is adsorbed, as evidenced by the CVs in N2 and by the fact that the linear dependence between the anodic peak potential of the UP oxide appearing at the most positive potential and the pH holds only for pH values higher than 7 (see Discussion below). Discussion

Figure 10. FTIR spectroscopy of polished Ag in 0.5 M HClO4, pH 0.3. (a) Square-wave FTIR (SW-FTIR) spectra between a constant reference potential of -0.14 V and an increasingly positive sample potential, from -0.04 to 0.26 V. (b) Linear potential sweep FTIR (LPS-FTIR) spectra taken during a positive LPS at 1 mV s-1, with the interferogram at -0.14 V as reference.

range 0.04-0.58 V and a monopolar band at 2112 cm-1 from 0.13 to 0.49 V. It is very interesting that a negative band of CO2 formation appears already at 0.04 V, this being probably the lowest potential at which electrooxidation of adsorbed CO has ever been observed. This potential is only 0.14 V more positive than the standard equilibrium potential for electrooxidation of dissolved CO to CO2, -0.10 V. The band at 2112 cm-1 disappears at 0.58 V, in strong contrast with its behavior in borax and in 0.5 M NaClO4,

UP Oxides of Ag. Here we have found that, over the whole pH range 0.3-13, UP oxides form on polished as well as on etched Ag at potentials well below the reversible potential of the Ag/Ag2O couple, 1.18 V. Actually already in 1966 two UP oxides with half-sum potentials of 0.26 and 0.70 V had been found in CVs at 0.1 V s-1 of Ag in 1 M KOH, after a previous electroreduction of the Ag native oxides at -1.5 mA cm-2.17 In 1980 Burstein and Newman18 also observed two anodic-cathodic processes using the scratched electrode method, in which a surface completely free from contamination is created in situ and under potential control by pressing a diamond stylus against an electrode rotated at typically 100 Hz during 0.2-1.5 ms, and the current transient is recorded. With Ag in 1 M KOH they observed an anodic transient of about 150 µC cm-2 between 0.5 and 0.8 V, which increased to about 300 µC cm-2 between 0.9 and 1.1 V (as said above, the equilibrium potential of the Ag/Ag2O couple is 1.18 V). In agreement with this, in CVs at 100 V s-1 they observed two anodic-cathodic processes, with half-sum potentials of 0.38 and 0.88 V. The total anodic charge was 300 µC cm-2. The two UP silver oxides have been tentatively identified by SERS as AgOH and AgO.11 Their formation and electroreduction are fast, since the corresponding peak potentials remain practically constant up to a sweep rate of at least 2 V s-1 (Figure 2c). It is very surprising that in most works on the electrooxidation of Ag in alkaline media electrooxidation of Ag is reported19 to begin only at 1.1 V, i.e., precisely at the equilibrium potential of the Ag/Ag2O couple, 1.18 V. This commonly reported CV shows two anodic prepeaks at 1.18 and 1.24 V, followed by two large peaks at 1.40 and 1.65 V. However, as shown in this work no special cleaning procedures are required for observing the anodiccathodic processes of formation and reduction, respectively, of UP oxides on silver over the pH range 0.3-13. The plot of the peak potential (vs SCE) of the UP oxide appearing at the most positive potential vs pH (Figure 11) shows that the points for pH 7, 9.2, and 13 define a straight line with a slope of 32 mV per pH unit. The point for pH 0.3 deviates from this line, which is not surprising since the CV at pH 0.3 does not show the two anodiccathodic processes observed at pH g 7. A dependence of 35 mV per pH unit over the pH range 7.2-10.8 has also been reported for the UP oxide on Au(111) and Au(110) appearing at less positive potentials.20 As first recognized by Burke,21 in order to justify a slope different from 60 mV per pH unit the participation of adsorbed ionic species must be invoked. A possible reaction with the required (17) Shumilova, N. A.; Zhutaeva, G. V.; Tarasevich, M. P. Electrochim. Acta 1966, 11, 967. (18) Burstein, G. T.; Newman, R. C. Electrochim. Acta 1980, 25, 1009. (19) Droog, J. M. M.; Alderliesten, P. T.; Bootsma, G. A. J. Electroanal. Chem. 1979, 99, 173. (20) Huong, G. N. V.; Hinnen, C.; Lecoeur, J. J. Electroanal. Chem. 1980, 106, 185. (21) Burke, L. D.; Lyons, M. E. In Modern Aspects of Electrochemistry; White, R. E., Bockris, J. O’M., Conway, B. E., Eds.; Plenum Press: New York, 1986; Vol. 18, Chapter 4.

CO on Silver

Figure 11. Plot of the anodic peak potential (vs SCE) of the underpotential (UP) oxide on Ag appearing at the most positive potential vs pH.

slope of 30 mV per pH unit is

AgOH+ads + 2e- a Ag + OHElectrooxidation of CO on Unroughened Ag. At pH 13 the potential range over which the CO adsorbed on Ag is electrooxidized is slightly positive of the most positive peak of UP oxidation of Ag (Figure 3a), and at pH 9.2 it is about the same (Figure 5), which supports the necessity of UP oxides for the electrooxidation of adsorbates to occur. Quite surprisingly, both at pH 7 (Figure 7) and pH 0.3 (Figure 9) the single peak in the CV is 0.2 V more negative in CO-saturated solution than the peak of UP Ag electrooxidation in N2, which runs contrary to the usual behavior, in which the anodic peak in the presence of CO appears at a potential more positive than, or at least equal to, that of electrooxidation of the metal in base electrolyte. However, the participation of a small (pH 7, Figure 7) or very small (pH 0.3, Figure 9) coverage of UP oxides in the electrooxidation of CO at neutral and acidic pH, respectively, cannot be excluded. At these pH values the interaction of CO with Ag is so weak that CO can be flushed away from the Ag surface by N2 bubbling (dotted CVs in Figures 7 and 9), and probably the energy of interaction of CO with the Ag surface atoms is, although certainly very low, just right for catalyzing the electrooxidation of adsorbed CO. Adsorption of CO on Unroughened Ag. The FTIR spectra here reported unequivocally show that over the pH range 0.3-13 CO does adsorb on Ag which had not been subjected to any previous oxidation-reduction cycle. CO chemisorbed in the linear form (band at 2000-1970 cm-1) appears over the whole pH range, while bridge CO (band at 1900-1860 cm-1) appears at neutral or alkaline pH only. This is in agreement with the finding that the ratio of bridge to linear CO increases with pH for all metals on which CO adsorbs in both forms.14 As far as we know, an IR band for CO chemisorbed on Ag in a bridge position had not been reported before. It is very interesting that only at pH 13 is the adsorption of CO on Ag strong enough as to be unaffected by N2 bubbling, i.e., that only in the presence of OH- ions CO chemisorbs on Ag. This behavior is in contrast with that reported for Au, on which although much less CO was adsorbed in acidic than in alkaline media, in neither case was the adsorbed CO displaced by N2 bubbling.22,23 As said in the Introduction, it has been estimated1 that the adsorption strength of CO decreases in the order Cu > Au

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> Ag, which would explain the larger influence of pH on the adsorption of CO on Ag as compared with Au, since the stronger the CO adsorption bond, the less the CO coverage will be affected by changes in pH, temperature, pressure, etc. The IR band which appears at 2048 cm-1 at pH 13 and 9.2 but at 2112 cm-1 at pH 0.3 shows characteristics that are completely unusual for CO chemisorbed on a metal. As can be seen in the LPS-FTIR spectra of Ag in 0.1 M borax (Figure 6c), the band is monopolar and therefore should not correspond to CO chemisorbed on a metal, which in all known cases gives rise to a bipolar band, due to the Stark shift. In agreement with this, the band frequency does not change with potential; i.e., it does not show the Stark shift typical of a species chemisorbed on a metal. Furthermore, the band intensity increases with increasing positive potential up to 0.91 V and then remains constant up to the positive limit of the sweep, 1.08 V, at which the CO chemisorbed on Ag metal should have been completely electrooxidized, as indicated by the disappearance of the bands of linear and bridge CO and by the CV in Figure 5. These characteristics can easily be explained if the band at 2048-2112 cm-1 is assigned to CO chemisorbed on an UP Ag oxide, whose surface concentration increases with potential, and consequently so would the amount of CO chemisorbed on it. Therefore the Ag oxide on which the CO is adsorbed cannot be a residual native Ag oxide formed during air exposure and not electroreduced during the initial 10 min at -0.11 V. It can be envisaged that changes in potential do not appreciably affect the bonding of CO to the UP Ag oxide, and consequently the frequency of CO stretching of the chemisorbed CO would be unaffected by changes in the electrode potential. The appearance of the CO band at 2048-2112 cm-1 in SW-FTIR spectra means that some amount of the UP Ag oxide is formed at the higher potential of the square-wave modulation and electroreduced at the lower potential, CO becoming adsorbed and desorbed, respectively, on this UP Ag oxide at the positive and negative potentials of the square wave. A similar behavior to that of CO adsorbed on Ag in 0.1 M borax has been recently reported5 for CO adsorbed on a 6 monolayer thick Cu electrode, electrodeposited on Pt(110), in 0.05 M Na2SO4. The LPS-FTIR spectra showed two bands: one at about 2000 cm-1, whose frequency increased by 55 cm-1 upon increasing the potential by 1 V, and the other one at about 2110 cm-1, whose frequency increased by only 3 cm-1 over a potential range of 0.5 V, which were assigned to CO adsorbed on terrace and adatom Cu atoms, respectively.24 In a review on the adsorption of gaseous CO on Cu, Hollins25 concludes that the single band above 2110 cm-1 observed for supported copper is due to an imperfect reduction of the copper oxide, i.e., to CO adsorbed on some sort of Cu oxide, while a single band in the range 2100-2110 cm-1 is observed for CO adsorbed on evaporated, i.e., oxide-free, copper films. Therefore, it could be argued that the band at 2110 cm-1 observed for CO/Cu/Pt(110) was due to CO adsorbed on a Cu oxide. Despite the above arguments, the possibility that the band at 2048-2112 cm-1 of CO adsorbed on Ag at pH (22) Nakajima, H.; Kita, H.; Kunimatsu, K.; Aramata, A. J. Electroanal. Chem. 1986, 201, 175. (23) Kunimatsu, K.; Aramata, A.; Nakajima, H.; Kita, K. J. Electroanal. Chem. 1986, 207, 293. (24) Ogasawara, H.; Inukai, J.; Ito, M. Chem. Phys. Lett. 1992, 198, 389. (25) Hollins, P. Surf. Sci. Rep. 1992, 16, 51.

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9.2-0.3 is due to CO linearly adsorbed on a Ag atom in a step or other less coordinated position on the metal surface cannot be ruled out, since the frequency of adsorbed CO increases with decreasing coordination number of the metal atom within the metal surface, although typically the shift is small, 10-20 cm-1.25 However, the constancy of the CO stretching frequency, irrespective of the electrode potential, would be hard to explain for CO adsorbed on a metal, since, as said in the Introduction, changes in the electrode potential (i.e., the Fermi level) of a metal will

Orozco et al.

affect the extent of retrodonation of electronic density from the metal to the antibonding 2π* orbital of CO chemisorbed on it and, consequently, will change the frequency of the stretching vibration of chemisorbed CO. Acknowledgment. This work was carried out with the help of the Spanish DGICYT under Project PB93-046. LA980157T