Study by Fourier Transform Infrared Spectroscopy ... - ACS Publications

A. Cuesta and C. Gutiérrez*. Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, C. Serrano, 119, 28006-Madrid, Spain. Received June 13, 1997. ...
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Langmuir 1998, 14, 3397-3404

3397

Study by Fourier Transform Infrared Spectroscopy of the Adsorption of Carbon Monoxide on a Nickel Electrode at pH 3-14 A. Cuesta and C. Gutie´rrez* Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, C. Serrano, 119, 28006-Madrid, Spain Received June 13, 1997. In Final Form: March 23, 1998 We have studied the adsorption and electrooxidation of CO on Ni at four pH values: 14, 9.2, 6.9, and 3. Over this pH range bridge and linear (on-top) CO coexist on the Ni surface. The ratio of bridge to linear CO as estimated from IR data, assuming that the ratio of the molar absorption coefficient of linear CO to that of bridge CO is 2.5, increased with pH, from 1:1 at pH 3 to 8:1 at pH 14. From the intensity of the bands of chemisorbed CO and/or CO2 produced by its electrooxidation a complete CO coverage of the Ni surface at the four pH values was estimated. In a plot of the stretching frequency of linear CO on Ni vs the electrode potential referred to a pH-independent reference electrode the data for the pH values 3, 6.9, and 9.2 clustered around a straight line, this pH independence of the stretching frequency of linear CO being attributed to an exclusion by chemisorbed CO of water and its ions from the surface of the metal. For bridge CO, although the data for pH 6.9, and 9.2 also clustered around a common line in the CO frequency vs electrode potential plot, the data for pH 14 defined a straight line shifted vertically toward lower frequencies by about 20 cm-1. The Stark shift was 56 ( 6 cm-1 V-1 for linear CO at pH 3, 6.9, and 9.2 and 64, 56, and 79 cm-1 V-1 for bridge CO at pH 6.9, 9.2, and 14. An important consequence of the pH independence of the CO stretching frequency at a given potential vs a pH-independent reference electrode is that the typical frequencies for linear and bridge CO are about 50 cm-1 lower at pH 14 than at pH 0.

1. Introduction Although Ni and Pt are the metals on which the chemisorption of CO at the solid-gas interface has been most studied, there are very few Fourier transform infrared spectroscopy (FTIRS) studies of the adsorption of CO on Ni in an electrolytic medium. In a pH 6.9 phosphate buffer Hori et al.1 detected bridge and linear CO, while in 0.1 M KOH Zhao et al.2 found only one broad band at 1890 cm-1, assigned to bridge CO. In general, the adsorption of CO on the ferrous metals in electrolytic media, despite their potential use in electrocatalysis,3 has received little attention, probably because they are so readily oxidized that they are far from an ideal system for surface studies. However, we have already shown that if Fe and Co are held at a potential in the hydrogen evolution region in order to electroreduce their native oxides, the adsorption and electrooxidation of CO on Fe4 and Co5 can be successfully studied by cyclic voltammetry and FTIRS. Only linear CO was found on Fe, while on Co, besides linear CO, some bridge CO was also found at alkaline pH. The present work, which completes our study of the ferrous metals, has enabled us to establish that the tendency of CO to adsorb in the bridge position increases in the order Fe < Co < Ni. 2. Experimental Section Ni disks, 12 mm in diameter, were cut from a 1-mm thick sheet of 99.99% Ni from Alfa-Johnson Matthey. A commercial SCE and a homemade miniature Ag/AgCl, saturated KCl * Corresponding author. Phone: 34-1-5619400 ext 1327. Fax: 34-1-5642431. E-mail: [email protected]. (1) Hori, Y.; Koga, O.; Aramata, A.; Enyo, M. Bull. Chem. Soc. Jpn. 1992, 65, 3008. (2) Zhao, M.; Wang, K.; Scherson, D. A. J. Phys. Chem. 1993, 97, 4488. (3) Degner, D. Topics in Current Chemistry; Springer-Verlag: Berlin, 1988; Vol. 148, p 1. (4) Cuesta, A.; Gutie´rrez, C. J. Phys. Chem. 1996, 100, 12600. (5) Cuesta, A.; Gutie´rrez, C. Langmuir, submitted for publication.

electrode were used for cyclic voltammetry and for FTIR spectroscopy, respectively, but the potentials are always referred to the reversible hydrogen electrode (RHE). Milli-Q water (Millipore, Bedford, MA) and analytical grade reagents were used. NaOH was R. P. Normapur grade from Prolabo. Before the voltammograms were recorded, the Ni electrode was held at -0.50 V for 15 min in order to reduce the native Ni oxides. The sweep rate was 50 mV s-1. A Perkin-Elmer FTIR instrument, model 1725-X, purged with 16 L min-1 of CO2- and H2O-free air from a Peak Scientific equipment was used. The IR beam was p-polarized with a Harrick wire grid polarizer. Two potential programs, Linear Potential Sweep FTIRS (LPS-FTIRS) and Square Wave FTIRS (SW-FTIRS), were used. In LPS-FTIRS, a technique suitable for irreversible systems, 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 simultaneously recorded voltammogram. The other procedure, SW-FTIRS, is more suitable for chemisorbed species and is based on the Stark shift of their IR bands, 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, 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. More details can be found in the previous works on the adsorption of CO on Fe4 and Co.5

3. Results 3.1. 1 M NaOH. (a) Cyclic Voltammetry. The first cyclic voltammogram (CV) of Ni in quiescent 1 M NaOH in N2 atmosphere (solid curve in Figure 1a) shows an anodic peak of Ni passivation at +0.35 V with a charge corresponding to the electrooxidation of about 3 layers of Ni to Ni(II). It has been shown by potential-modulated reflectance spectroscopy6 that in the pH region 1-14 NiO

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Figure 2. FTIR spectra of Ni in CO-saturated 1 M NaOH: (a) SW-FTIR spectra obtained with reference and sample potentials of -0.12 and +0.03 V, respectively; (b) LPS-FTIR spectra collected during a positive LPS from -0.11 to +0.79 V.

Figure 1. (a) First cyclic voltammogram (CV) at 50 mV s-1, between -0.13 and +0.83 V, of Ni in quiescent 1 M NaOH in N2 atmosphere, after electroreduction of native Ni oxides for 15 min at -0.50 V (solid curve). The dashed curve is a CV recorded under the same conditions but after oxide electroreduction CO was bubbled through the solution for 10 min at a potential Edos ) -0.03 V. (b) First two CVs recorded after saturating the electrolyte with CO and then bubbling N2 in order to eliminate the dissolved CO while still holding the potential at -0.03 V.

is present in the passivating layer, and the prevailing view, based on ellipsometric,7 XPS,8 and SERS9 results, (6) Larramona, G.; Gutie´rrez, C. J. Electrochem. Soc. 1990, 137, 428. (7) Paik, W.-k.; Szklarska-Smialowska, Z. Surf. Sci. 1980, 96, 401. (8) Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1977, 73, 327. (9) Melendres, C. A.; Pankuch, M., J. Electroanal. Chem. 1992, 333, 103.

is that the passivating film has a duplex structure, with an inner layer of NiO and an outer layer of Ni(OH)2. If CO is bubbled through the solution for 10 min at a dosing potential Edos ) -0.03 V, a subsequent CV shows (dashed curve in Figure 1a) that both hydrogen evolution and Ni electrooxidation are inhibited, undoubtedly due to the chemisorption of CO. There is a shoulder at +0.45 V, about the same potential of the peak in N2 and which therefore perhaps corresponds to the same process, although hindered by CO. When dissolved CO was eliminated by N2 bubbling through a solution previously saturated with CO at -0.03 V, the first CV (solid curve in Figure 1b) was similar to that in a CO-saturated solution, showing that the adsorption of CO was strong, and the second CV (dashed curve in Figure 1b) was similar to that obtained in a N2 atmosphere after 2-3 potential cycles, since the chemisorbed CO had been electrooxidized in the first positive sweep. (b) FTIRS. SW-FTIR spectra with reference and sample potentials of -0.12 and +0.03 V, respectively, were obtained. Only four spectra, of 10 scans each, were collected at each of the two potentials in order to avoid formation of H2 bubbles and electrooxidation of chemisorbed CO at the low and high potentials, respectively. Two bipolar bands appear in the SW-FTIR spectrum (Figure 2a), a larger one of bridge CO at 1858 cm-1 and a smaller one of linear CO at 1984 cm-1 (for assignment of the bands see Discussion), which disproves the claim2 that the stretching frequency of bridge CO (the only species detected in that study) chemisorbed on Ni in 1 M NaOH does not depend on the potential.

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LPS-FTIR spectra were collected during a positive LPS between -0.11 and +0.79 V, which only showed a peak at 0.42 V preceded by a shoulder at 0.23 V. At the lower potentials there appear two bipolar bands of bridge and linear CO (Figure 2b), which become monopolar, positive bands at 1834 and 1949 cm-1, respectively, at +0.41 V (the peak potential), once the electrooxidation of these two species has been completed. Obviously these frequencies are those at the reference potential of -0.11 V. Other workers2 detected only one broad band at 1890 cm-1 in 0.1 M KOH, which they assigned to bridge CO. We think that they did not detect linear CO for the same reason that they did not observe any variation in the stretching frequency of bridge CO: they started the positive sweep at a too positive potential, just before the electrooxidation of CO and Ni. A negative band at 1410 cm-1 corresponds to the formation of carbonate ions, and two negative bands at 1650 and 2200 cm-1 correspond to the formation of water, both products being formed in the electrooxidation of CO in alkaline media according to

CO + 4 OH- f CO32- + 2H2O + 2e although water is mainly formed by the electrooxidation of Ni

Ni + 2OH- f NiO + H2O + 2e At pH 9.2 and 6.9 the concomitant change of pH changes the concentrations of the buffering species, producing bands that severely distort those of chemisorbed CO, carbonate, and bicarbonate. It must be stressed that although these distortions of the baseline loom large, actually the change in pH of the 1 M NaOH electrolyte at the end of the LPS was very small: the total anodic charge was 4.10 mC cm-2, which, assuming a thickness of 5 µm for the thin electrolyte layer, would correspond to a decrease in the OH- concentration of 0.08 M, so that at the end of the LPS the pH decreased by only 0.04 units, from 14 to 13.96. 3.2. 0.1 M Na2B4O7, pH 9.2. (a) Cyclic Voltammetry. The first CV of Ni in 0.1 M Na2B4O7 in N2 atmosphere shows (solid curve in Figure 3a) an anodic peak at +0.40 V with a charge slightly lower than that in 1 M NaOH. If CO is bubbled at Edos ) -0.11 for 10 min, in the subsequent CV both hydrogen evolution and Ni electrooxidation are inhibited by chemisorbed CO, and the anodic peak is shifted positively to 0.65 V (dashed curve in Figure 3a). The adsorption of CO is strong, since the same anodic peak still appears, although slightly decreased and shifted negatively, in the first sweep if N2 is bubbled through the solution in order to eliminate any dissolved CO while still holding the potential at -0.11 V (solid curve in Figure 3b), while the second positive sweep (dashed curve in Figure 3b) is similar to that obtained in N2 after a few cycles, showing that chemisorbed CO has been electrooxidized in the first sweep. (b) FTIRS. At pH e 9.2 H2 evolution and chemisorbed CO electrooxidation did not proceed so readily as at pH 14, which allowed us to increase the potential interval and the number of scans in SW-FTIR spectra. The reference and sample potentials were -0.11 and +0.09 V, 20 spectra of 100 scans each being collected at each potential. The spectrum (Figure 4a) is similar to that in 1 M NaOH, showing two bipolar bands, a larger one of bridge CO at 1881 cm-1 and a smaller one of linear CO

Figure 3. (a) First cyclic voltammogram at 50 mV s-1 between -0.31 and +0.74 V of Ni in 0.1 M Na2B4O7 in N2 atmosphere, after electroreduction of native Ni oxides at -0.50 V (solid curve). The dashed curve is a CV recorded under the same conditions but after bubbling CO at Edos ) -0.11 V for 10 min. (b) First (solid curve) and second (dashed curve) CVs recorded after saturating the solution with CO, after which N2 was bubbled through the solution in order to eliminate the dissolved CO while still holding the potential at -0.11 V.

at 2013 cm-1. Although bridge CO continues to be the main species, its intensity with respect to linear CO has decreased. LPS-FTIR (Figure 4b) spectra were collected during a sweep from -0.11 to +0.67 V, which showed only an anodic peak at 0.46 V. The negative bands at 1411 and 1158 cm-1 are due to an increase of the concentration of boric acid as a consequence of the acidification produced by the electrooxidation of CO and Ni. Again, as in 1 M NaOH

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Figure 4. FTIR spectra of Ni in CO-saturated 0.1 M Borax: (a) SW-FTIR spectra with reference and sample potentials of -0.11 and +0.09 V; (b) LPS-FTIR spectra collected in a sweep from -0.11 to +0.67 V.

the negative band at 1650 cm-1 indicates that water is formed in the electrooxidation of CO and Ni in alkaline media. The anodic charge was 2.86 mC cm-2, which corresponds to a final increase in the H+ concentration of 0.06 M, lower than those observed with Fe4 and Co5 (due to the lower corrodability of Ni), which explains why with Ni the water band at 1650 cm-1 is negative even at the most positive potentials, showing that the pH is still alkaline, and that consequently electrooxidation produces water instead of consuming it, as occurs at neutral and acidic pH. The initially bipolar bands of bridge and linear CO become positive after their electrooxidation has been completed at +0.58 V, their frequencies being 1865 and 1996 cm-1 CO, respectively, obviously at -0.11 V, the reference potential. At +0.24 V the bands appear at 1898 and 2027 cm-1, respectively. 3.3. 0.1 M K2HPO4 + 0.1 M KH2PO4, pH 6.9. (a) Cyclic Voltammetry. The first CV (solid curve in Figure 5a) of Ni in phosphate buffer in N2 atmosphere shows an anodic peak of Ni passivation at +0.34 V with a charge twice that in 1 M NaOH. If CO is bubbled through the solution at Edos ) -0.25 V for 10 min, a subsequent CV shows (dashed curve in Figure 5a) inhibition of both H2 evolution and Ni electrooxidation. If N2 is bubbled during 15 min after a previous saturation with CO at Edos ) -0.25 V, the first CV (solid curve in Figure 5b) is similar to that in CO-saturated solution, showing that the adsorption of CO is strong, but in the second CV (dashed curve in Figure 5b) the hydrogen evolution current increases and Ni electrooxidation starts already at +0.05 V, since chemisorbed CO has been electrooxidized already. (b) FTIRS. SW-FTIR spectra were obtained with reference and sample potentials of -0.11 and +0.09 V, respectively, 20 × 100 scans being collected at each

Figure 5. (a) First cyclic voltammogram at 50 mV s-1 between -0.25 V and +0.81 V of Ni in phosphate buffer, pH 6.9, in N2 atmosphere, after electroreduction of native Ni oxides at -0.50 V (solid curve). The dashed curve is a CV recorded under the same conditions but after CO was bubbled through the solution at Edos ) -0.25 V for 10 min. (b) First (solid curve) and second (dashed curve) CVs recorded in a solution first saturated with CO and then freed from dissolved CO by bubbling N2 for 15 min.

potential. The spectrum (Figure 6a) is similar to those at pH 14 and 9.2, with two bipolar bands, a larger one of bridge CO at 1885 cm-1 and a smaller one of linear CO at 2015 cm-1, respectively. These bands coincide with those observed by Hori et al.2 in the same phosphate buffer, pH 6.9, and which they assigned to bridge and linear CO. LPS-FTIR spectra (Figure 6b) were recorded during a sweep between -0.11 and +0.75 V, which showed only an anodic peak at 0.46 V. Initially bipolar and then mo-

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Figure 6. FTIR spectra of Ni in CO-saturated phosphate buffer, pH 6.9: (a) SW-FTIR spectra with reference and sample potentials of -0.11 and +0.09 V, respectively; (b) LPS-FTIR spectra recorded during a sweep from -0.11 to +0.75 V.

nopolar bands of bridge and linear CO appear at 1871 and 2013 cm-1, respectively, the change occurring at +0.57 V for bridge CO and at +0.59 V for linear CO, at which potentials CO has been completely oxidized. At +0.55 V the bands appear at 1903 and 2056 cm-1 for bridge and linear CO, respectively. The acidification produced by the electrooxidation of CO and Ni increases the concentration of H2PO4- (negative band at 1160 cm-1) and of H3PO4 (negative band at 1035 cm-1) at the expense of HPO42- (positive band at 1095 cm-1), as happened with Fe4 and Co.5 Although at the end of the sweep, and as happened with Co, the highest concentration increase was that of H2PO4-, the ratio ∆[H3PO4]/∆[H2PO4-] was higher for Co than for Ni, which indicates a stronger acidification with Co. As could be expected, Fe showed the highest acidification, and for it the highest concentration increase was that of H3PO4. These results agree with the relative corrodability of the three ferrous metals (Fe > Co > Ni). Due to the acidification, the electrooxidation of CO yields exclusively CO2 (the band of HCO3- at 1364 cm-1 is absent in the spectrum), instead of both CO2 and HCO3- as could be expected at pH 6.9, since the pK1 of carbonic acid is 6.4. The CO2 band at 2344 cm-1 begins to appear at 0.39 V, at the foot of the anodic peak, and its high intensity (∆R/R ) 4.8 × 10-3) indicates oxidation of a considerable amount of CO. 3.4. 0.5 M Na2SO4 + H2SO4, pH 3. (a) Cyclic Voltammetry. A 0.5 M Na2SO4 solution was acidified with H2SO4 until its pH decreased to 3. The first CV of Ni shows (solid curve in Figure 7a) two anodic peaks at +0.29 and +0.46 V with a total charge six times higher than that in 1 M NaOH, in which the formation of passivating oxyhydroxides is more favored. In a CO-saturated solution (Edos ) -0.6 V) the first CV shows (dashed curve in Figure 7a) inhibition of hydrogen

Figure 7. (a) First cyclic voltammogram at 50 mV s-1 between -0.60 V and +0.82 V of Ni in a 0.5 M Na2SO4 solution acidified with H2SO4 down to pH 3 in N2 atmosphere, after electroreduction of native Ni oxides at -0.50 V (solid curve). The dashed curve is the CV obtained after saturating the solution with CO at Edos ) -0.60 V. (b) First (solid curve) and second (dashed curve) CVs recorded after saturating the solution with CO and then eliminating the dissolved CO by N2 bubbling.

evolution and of Ni electrooxidation, with an anodic charge 7.7 times that in N2 atmosphere because chemisorbed CO shifts positively by 0.2 V the start of Ni electrooxidation. If N2 is bubbled during 15 min after a previous saturation with CO at Edos ) -0.60 V, the first positive sweep (solid curve in Figure 7b) is very similar to that in a CO-saturated solution, but in the second sweep, although Ni electrooxidation starts at approximately the same potential as in N2 atmosphere, the charge is 3.3 times

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4. Discussion

Figure 8. FTIR spectra of Ni in a CO-saturated 0.5 M Na2SO4 solution acidified with H2SO4 down to pH 3: (a) SW-FTIR spectrum with reference and sample potentials of -0.11 and +0.09 V, respectively; (b) LPS-FTIR spectra obtained during a sweep from -0.11 to +0.79 V.

that in N2 atmosphere, perhaps due to an increase in surface roughness by the first sweep. (b) FTIRS. The SW-FTIR spectrum with reference and sample potentials of -0.11 and +0.09 V and 20 × 100 scans at each potential is similar to those at higher pHs, with two bipolar bands of bridge and linear CO at 1900 and 2043 cm-1, respectively (Figure 8a). At this pH 3 the band of linear CO is larger than that of bridge CO, continuing the monotonic tendency of decreasing ratio of bridge to linear CO with decreasing pH. LPS-FTIR spectra (Figure 8b) were recorded during a sweep from -0.11 to +0.79 V, which only showed a peak at 0.46 V. They show, as at higher pHs, two initially bipolar bands of chemisorbed CO, which become two positive bands of bridge and linear CO at 1899 and 2023 cm-1, respectively, at +0.56 V, indicating that at this potential the chemisorbed CO has been completely oxidized. Obviously these are the stretching frequencies at the reference potential of -0.11 V. At +0.15 V the bands of bridge and linear CO appear at 1919 and 2048 cm-1, respectively. A very broad, W-shaped band appears at 1650 cm-1. Its origin is rather interesting, since it is a convolution of a very broad, negative band due to the formation of a Ni(H2O)62+ band (all aquo complexes exhibit the three fundamental modes of the free water molecule10) with a positive band of water consumption, precisely by the formation of the hexaaquonickel(II) complex. The same band appeared with Fe4 and Co.5 The band of CO2 at 2344 cm-1 first appears at 0.34 V, at the foot of the anodic peak. (10) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley: New York, 1978; p 227.

4.1. Extent of CO Coverage. Production of CO2. To estimate the coverage by chemisorbed CO, we have assumed that the value of 0.43-0.50% obtained in our experimental setup with a flat window for a monolayer of CO on Pt also applies to a monolayer of CO on Ni. The ratio of the molar absorption coefficient of linear CO to that of bridge CO has been given as 2.7 for c(4 × 2)CO on Pt(111) (0.5 coverage) in UHV,11 as 2.3 for CO chemisorbed on Rh(111) in 0.1 M HClO4,12 and as 1.73.3, increasing with coverage, for CO chemisorbed on Pt(111) in 0.1 M HClO4.13 We have taken a mean value of 2.5 for the said ratio, which should be taken as a rough approximation only, since there can be intensity transfer as a consequence of dipole-dipole coupling between bands at close frequencies. Actually, Villegas and Weaver14 explained in this way the discrepancy between their IRRAS data of CO chemisorbed on Pt(111) (where the linear CO band is more intense than that of bridge CO) and the adlayer structure of CO chemisorbed on Pt(111) deduced from their STM images (according to which the main species was bridge CO). We could measure the intensities in LPS-FTIR spectra of the unipolar, positive CO bands (i.e. at potentials at which oxidation of chemisorbed CO was complete) only at pH 6.9 and 9.2, since at lower pHs electrooxidation of Ni produced a severe distortion of the bands, and at higher pHs the bands of chemisorbed CO shifted to lower frequencies (see below), so much so that the large band of water production at 1650 cm-1 severely distorted the band of bridge CO. However, the anodic peak in the LPS voltammograms recorded concurrently with the FTIR spectra appeared at the same potential vs RHE, 0.46 V, at the four pH values, which indicates that up to the anodic peak the pH changes in the thin electrolyte layer were not large. A value of 1200 M-1 cm-1 15 was taken for the molar absorption coefficient (formerly known as extinction coefficient) of dissolved CO2. The intensity of the band of CO2 could be measured only at pH 3 and 6.9, since the pK1 of carbonic acid is 6.37. At pH 9.2 the negative band of HCO3- formation at 1365 cm-1 is swamped by the large negative band of H3BO3 formation at 1411 cm-1, due to the acidification of the medium. At pH 14 there is a large negative band of CO32- formation, but we could not find in the literature the value of the molar absorption coefficient of carbonate in aqueous solution. The maximum coverage of CO on Ni(100) is 0.67,16 which corresponds to 1.8 nmol cm-2, 2.7 nmol cm-2 corresponding to a typical roughness factor of 1.5. The amount of CO in a monolayer is about 5 times higher than that in the thin electrolyte layer, which is about 0.5 nmol cm-2 only, since the solubility of CO in water is 0.91 mM,17 and the electrolyte layer is only a few micrometers thick. At pH 3 the amount of CO2 produced was estimated to correspond to the oxidation of a CO monolayer. At pH 6.9 the intensity of the band of linear CO was 7 × 10-4, and that of bridge CO was 6 × 10-4, from which a nearly (11) Schweizer, E.; Persson, B. N. J.; Tu¨shaus, M.; Hoge, D.; Bradshaw, A. M. Surf. Sci. 1989, 213, 49. (12) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1989, 90, 7426. (13) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1991, 241, 11. (14) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648. (15) Mizen, M. B.; Wrighton, M. S. J. Electrochem. Soc. 1989, 136, 941. (16) Lauterbach, J.; Wittmann, M.; Ku¨ppers, J. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 326. (17) Stephen, H.; Stephen, H. Solubilities of Inorganic and Organic Compounds; Pergamon: Oxford, U.K., 1963.

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Table 1. Ratio of the Coverage of Bridge CO to that of Linear CO Chemisorbed on Ni at 0.0 V vs RHE, at Four pH Valuesa COB/COL

pH 3 1:1

pH 6.9 4:1

pH 9.2 5:1

pH 14 8:1

a The CO coverages were estimated from the intensities in SWFTIR spectra of the bipolar bands of bridge and linear CO, using a modulation amplitude of 0.20 V.

complete coverage was also estimated, in agreement with the estimated amount of CO2 produced. Finally, at pH 9.2 the intensity of the band of linear CO was 2.8 × 10-4, while that of the band of bridge CO was very high, 1.4 × 10-3, also indicating that most of the Ni surface atoms are covered with bridge CO. It is interesting that in the cases of Fe4 and Co5 the amount of CO2 produced was much smaller than with Ni. This is attributed to the strong electrooxidation of Fe and Co, with formation of an insulating oxide layer which would impede the electrooxidation of most of the chemisorbed CO, which would remain on metal clusters sitting on top of the oxide layer. 4.2. Dependence of the Ratio of Bridge to Linear CO on the pH. As said above, in LPS-FTIR spectra we could measure the intensities of the positive bands of disappearance of chemisorbed CO only at pH 6.9 and 9.2, due to the distortion by Ni electrooxidation of the spectra at pH 3 and 14. Therefore, for estimating the relative coverages of bridge and linear CO at the four pH values we have used the intensities of their bipolar bands in SWFTIR spectra obtained with reference and sample potentials of -0.11 and +0.09 V vs RHE, respectively, both in the double layer region, as can be seen in Figure 1a, 3a, 5a, and 7a (in 1 M NaOH the potentials used were -0.12 and +0.03 V, respectively, to avoid electrooxidation of chemisorbed CO, but the bipolar band intensity was scaled to the same modulation amplitude, 0.20 V, used at the other three pH values). A value of 2.5, which is only a rough approximation due to the possibility of intensity transfer between bridge and linear CO, as explained above, was taken for the ratio of the molar absorption coefficient of linear CO to that of bridge CO. As seen in Table 1, for the four pH values used here the ratio of the coverages of bridge to linear CO increases with pH, so much so that while at pH 3 the coverages of bridge and linear CO are about equal, at pH 14 the former is 8 times larger than the latter. A result similar to ours, but over a more limited pH range, has been reported for CO chemisorbed on polycrystalline Pt:18 the intensity of the band of bridge CO was higher in 0.5 M Na2SO4 or 0.5 M K2SO4 than in 0.5 M HClO4 or in 1 M H2SO4. This was attributed to the more negative potential in neutral as compared with an acidic electrolyte. It is well-known that at a given pH the fraction of CO chemisorbed in the bridge form tends to increase with the negative charge on the metal. So, for CO chemisorbed on Pt in 0.5 M H2SO4 bridge CO can be observed if Edos ) 0.05 V vs SHE but not if Edos ) 0.40 V vs SHE.19 Using both FTIRS and STM, it has been shown that the ratio of bridge to linear CO on Rh(111) in 0.1 M NaClO4 increased reversibly from 1:2 (in a (2 × 2)-3CO unit cell) to 3:1 (in a (3x3rect)-4CO unit cell) when the potential was decreased from -0.1 V to -0.3 V vs SCE.20 It has also (18) Tornquist, W.; Guillaume, F.; Griffin, G. L. Langmuir 1987, 3, 477. (19) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G., II; Philpott, M. R. Langmuir, 1986, 2, 464. (20) Yau, S.-L.; Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Am. Chem. Soc. 1991, 113, 6049.

Figure 9. Stretching frequency of linear CO at pH 3, 6.9, and 9.2 and of bridge CO at pH 6.9, 9.2, and 14, as a function of electrode potential vs a pH-independent reference electrode.

been found that for Ni(111) in UHV linear CO disappears in the presence of coadsorbed potassium, which charges Ni negatively, as evidenced by its work function decrease.21 Therefore, we conclude that the increase of the ratio of the amount of bridge to linear CO with increasing pH indicates that the potential of zero charge (pzc) of COcovered Ni decreases with pH by less than 59 mV/pH unit and possibly does not change at all (see next paragraph), whereas the potential region at which chemisorbed CO is stable shows a Nernstian dependence on pH, and consequently at alkaline pH this region lies at more negative potentials vs the pzc than at acidic pH. With these results for Ni a comparison can now be made of the type of binding of CO on the three ferrous metals. While on Fe4 only linear CO was observed over the whole pH range 3-14, on Co,5 on which linear CO was also observed over the pH range 3-14, some bridge CO did also appear at pH 14 and 9.2, while on Ni both linear and bridge CO were present over the whole pH range 3-14. This monotonic trend of increasing binding of CO in the bridge position with increasing ferrous metal atomic number is interesting. However, it cannot be studied if this trend also applies to the same single-crystal face (which would be more amenable to theoretical studies), since the three metals have different crystal lattices (bodycentered cubic, hexagonal, and face-centered cubic for Fe, Co, and Ni, respectively). 4.3. Dependence of the CO Stretching Frequency on the pH. In a plot of the CO stretching frequency in LPS-FTIR spectra vs the electrode potential measured against a pH-independent reference electrode the data for linear CO at pH 3, 6.9 and 9.2 cluster around a straight line (Figure 9) (the data for pH 14 were not included because the small band of linear CO was distorted by the large water band at 2200 cm-1), and the same occurs for bridge CO at pH 6.9, and 9.2. The constancy, and even (21) Uram, K. G.; Ng, L.; Folman, M.; Yates, J. T., Jr. J. Chem. Phys. 1986, 84, 2891.

3404 Langmuir, Vol. 14, No. 12, 1998

decrease, of the frequency at the higher potentials is due to a progressive oxidation of the chemisorbed CO, since it is well-known that the stretching frequency of a given type of chemisorbed CO decreases by up to 100 cm-1 with decreasing coverage.22 So, at pH 3 oxidation of linear CO commences at 0.10 V, at pH 6.9 oxidation of linear and bridge CO commences at -0.41 and -0.50 V, respectively, at pH 9.2 oxidation of linear and bridge CO commences at -0.58 and -0.47 V, respectively, and at pH 14 oxidation of bridge CO commences at -0.86 V (all these potentials vs Ag/AgCl). The same pH-independent linear increase of CO frequency with potential was already reported for CO chemisorbed on Co5 and had been found by Kunimatsu et al.23 for CO chemisorbed on gold in alkaline and acidic medium. The Stark shift was 56 ( 6 cm-1 V-1 for linear CO at pH 3, 6.9, and 9.2 and 64, 56, and 79 cm-1 V-1 for bridge CO at pH 6.9, 9.2, and 14, these values being very similar to that of 64 ( 3 cm-1 V-1 for linear CO chemisorbed on Co5 and to that of 64 cm-1 V-1 reported by Kunimatsu et al.23 for CO chemisorbed on Au. With basis on the results of Chang et al.24 and Roth and Weaver,25 in our work on the adsorption of CO on Co we concluded5 that the pH independence of the CO stretching frequency at a given potential vs a pH-independent reference electrode is due to the exclusion by chemisorbed CO of water and its ions from the metal surface. 4.4. Influence of pH on the Assignment of the Bands of CO Chemisorbed on Ni. In LPS-FTIR spectra two positive bands of CO chemisorbed on Ni at the reference potential of -0.11 V vs RHE appear, their frequency decreasing upon increasing the pH from 3 to 14, that of the higher-frequency band from 2023 cm-1 to1949 cm-1 and that of the lower-frequency band from 1899 to 1834 cm-1. However, for assignment of the bands we prefer to use the half-sum frequency of the bipolar bands of SW-FTIRS, which are nearly unaffected by the severe distortion by Ni electrooxidation of the LPS-FTIR spectra. Upon increase of the pH from 3 to 14 the frequency of the higher-frequency band decreases from 2043 to 1984 cm-1 and that of the lower-frequency band from 1900 to 1858 cm-1. (22) Jiang, X.; Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7453. (23) Kunimatsu, K.; Aramata, A.; Nakajima, H.; Kita, H. J. Electroanal. Chem. 1986, 207, 293. (24) Chang, S.-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5378. (25) Roth, J. D.; Weaver, M. J. Langmuir 1992, 8, 1451.

Cuesta and Gutie´ rrez

For a correct assignment of CO frequencies at alkaline pH it must be taken into account that in general the stability region of chemisorbed CO decreases by 60 mV per pH unit, which, together with the 60 cm-1 V-1 dependence of CO frequency on the potential (section 4.3), means that the frequencies of chemisorbed CO at pH 14 will be about 0.06 V (pH unit)-1 × 14 pH units × 60 cm-1 V-1 ) 50 cm-1 lower than at pH 0. Effectively, the measured decreases were 59 and 42 cm-1 for linear and bridge CO, respectively, upon increasing the pH from 3 to 14. The typical frequency ranges for acidic medium26 (2020-2090 cm-1 for linear CO, 1900-1960 cm-1 for bridge CO, and 1750-1880 cm-1 for multibonded CO) become, at pH 14, 1960-2030 cm-1 for linear CO, 1850-1910 cm-1 for bridge CO, and 1700-1830 cm-1 for multiple CO. It is clear then that the higher-frequency band observed here corresponds to linear CO and the lower-frequency band to bridge CO. In phosphate buffer, pH 6.9, Hori et al.1 also detected bridge and linear CO. Zhao et al.2 detected in 0.1 M KOH at +0.22 VRHE only one broad band at 1890 cm-1, which they assigned to bridge CO. 4.5. Discrepancy between CO Site Assignment by Infrared Spectroscopy and by Photoelectron Diffraction. Despite the generally accepted correlation between the CO stretching frequency and the type of bonding of chemisorbed CO, it has been claimed,27 with basis on photoelectron diffraction results, that in the chemisorption system Ni(111)c(4 × 2)-CO the CO occupies hollow sites, although IR spectroscopy shows a band clearly assignable to bridge CO. Although obviously there can be considerable differences in the CO adsorption behavior of single-crystal and polycrystalline metals, it is well possible that also in this case the IR band assigned to bridge CO could correspond to CO adsorbed on a hollow site. Acknowledgment. This work was carried out with the help of the Spanish DGICYT under Project PB930146. A.C. gratefully acknowledges a scholarship from the Spanish Ministry of Education and Science. LA9706274 (26) Beden, B.; Lamy, C.; de Tacconi, N. R.; Arvı´a, A. J. Electrochim. Acta 1990, 35, 691. (27) Davila, M. E.; Asensio, M. C.; Woodruff, D. P.; Schindler, K.-M.; Hofmann, Ph.; Weiss, K.-U.; Dippel, R.; Gardner, P.; Fritzsche, V.; Bradshaw, A. M.; Conesa, J. C.; Gonza´lez-Elipe, A. R. Surf. Sci. 1994, 311, 337.