3390
Langmuir 1998, 14, 3390-3396
Study by Fourier Transform Infrared Spectroscopy of the Adsorption of Carbon Monoxide on a Cobalt Electrode at pH 3-14 A. Cuesta and C. Gutie´rrez* Instituto de Quı´mica Fı´sica “Rocasolano”, C.S.I.C., C. Serrano, 119, 28006 Madrid, Spain Received June 13, 1997. In Final Form: March 16, 1998 The adsorption and electrooxidation of CO on Co at four pH values: 14, 9.2, 6.8, and 3 have been studied by cyclic voltammetry and Fourier transform infrared spectroscopy. Over this wide pH range, 3-14, the main species chemisorbed on Co was linear CO. At pH 14 and 9.2, and possibly also at pH 6.9, a small amount of multibonded CO was also present. From the intensity of the infrared bands it was estimated that a sizable coverage of the Co surface with CO was reached, which is in agreement with the inhibition of cathodic H2 evolution and anodic Co electrooxidation observed in the presence of CO over the whole pH range 3-14. The frequency of CO stretching of linear CO chemisorbed on Co was, with good approximation, dependent only on the potential versus a pH-independent reference electrode, and not on the pH of the solution. This independence on the pH value was attributed to the exclusion of water and its ions from the inner part of the double layer by chemisorbed CO.
1. Introduction Carbon monoxide chemistry is of enormous technological importance.1 At the CO-gas interface CO tends to chemisorb dissociatively on the transition metals on the left-hand side of the periodic table and molecularly on those on the right-hand side. The boundary between these two behaviors probably lies between Fe and Co,1 and consequently much work has been directed to the chemisorption of gaseous CO on Fe, less effort having been devoted to Co. However, as far as we know no work has been reported on the chemisorption of CO on Fe and Co in an electrolyte, except for our previous one on Fe.2 A study of the electroadsorption and electrooxidation of CO on Co over the pH range 3-14 has been carried out, using cyclic voltammetry and in situ Fourier transform infrared spectroscopy. Although the results are similar to those of the previous study on Fe,2 significant differences emerge. 2. Experimental Section Co disks were cut from a 1.0-mm thick, 99.997% pure Co sheet from Alfa-Johnson Matthey. The disks for Fourier transform infrared spectroscopy (FTIRS) and cyclic voltammetry had diameters of 10 and 15 mm, respectively. The Co disks were polished with alumina of 5, 0.3, and 0.05 µm and sonicated in Milli-Q water. A commercial saturated calomel electrode and a homemade miniature Ag/AgCl, saturated KCl reference electrode were used for cyclic voltammetry and for the FTIRS experiments, respectively, but the potentials are always referred to the reversible hydrogen electrode. Two potential programs, linear potential sweep FTIRS (LPSFTIRS) and square wave FTIRS (SW-FTIRS), were used for obtaining IR spectra. In LPS-FTIRS interferograms are continuously recorded during an LPS at typically 1 mV s-1, each, for example, 25 successive interferograms being co-added 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 (1) Campuzano, D. In Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1990; Vol. 3A, Chapter 4. (2) Cuesta, A.; Gutie´rrez, C. J. Phys. Chem. 1996, 100, 12600.
be correlated with features in the voltammogram simultaneously recorded during the LPS-FTIRS experiment. The other procedure, SW-FTIRS, is based on the Stark shift of IR bands of chemisorbed species. 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 IR-active 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 experimental details are given in the work on the adsorption and electrooxidation of CO on Fe.2
3. Results 3.1. Cyclic Voltammetry. At the four pH values, before any cyclic voltammogram (CV) was recorded the potential was initially held at -0.50 V for 15 min in a N2 atmosphere in order to electroreduce the native oxides on Co. Subsequent CVs at 50 mV s-1 are shown in Figure 1. 3.1.1. Cyclic Voltammetry in 1 M NaOH. In quiescent 1 M NaOH in a N2 atmosphere the CV showed an anodic peak of Co passivation at +0.44 V, with a charge corresponding to the electrooxidation of about 43 monolayers of Co to Co(II) (solid curve in Figure 1a). Co(OH)2 has been unequivocally identified by its band at 3634 cm-1 among the electrooxidation products of Co in alkaline solution.3 If CO is bubbled through the solution for 10 min at a dosing potential Edos ) -0.23 V, the stationary cathodic current of H2 evolution at -0.33 V decreased from 0.9 to 0.05 mA cm-2. A subsequent CV shows (dashed curve in Figure 1a) that the anodic peak of Co electrooxidation is shifted positively, chemisorbed CO being the obvious candidate as the inhibiting species in both cases. If after CO was bubbled for 10 min at Edos ) -0.23 V N2 was bubbled in order to eliminate any dissolved CO from the solution, the first positive sweep is similar to that in the CO atmosphere (dotted curve in Figure 1), which confirms that the adsorption of CO is strong. As (3) Bewick, A.; Gutie´rrez, C.; Larramona, G. J. Electroanal. Chem. 1992, 333, 165.
S0743-7463(97)00628-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/13/1998
FTIRS of CO on Cobalt at pH 3-14
Langmuir, Vol. 14, No. 12, 1998 3391
(a)
(b)
(c)
(d)
Figure 1. Cyclic voltammograms (CVs) at 50 mV s-1 of a Co electrode, previously held at -0.50 V for 15 min in order to reduce the native Co oxides, under the following conditions: Solid curves, CVs in N2 atmosphere; dashed curves, CVs in CO-saturated electrolyte; dotted curves, CVs recorded after saturating the electrolyte with CO and then bubbling N2 in order to eliminate the dissolved CO. The four electrolytes were (a) 1 M NaOH; (b) 0.1 M Na2B4O7; (c) phosphate buffer, pH 6.9; (d) a 0.5 M Na2SO4 solution acidified with H2SO4 until its pH decreased to 3. The potential at which the Co electrode was held while CO gas was bubbled in the cell was, respectively, -0.23, -0.21, -0.14, and -0.50 V.
expected, the second CV (not shown) is very similar to that in a N2 atmosphere, indicating that the chemisorbed CO has been electrooxidized in the first positive sweep. A detailed discussion of the decrease with cycling of the first anodic peak of Co electrooxidation, and of its change in shape, can be found in ref 4. (4) Go´mez Meier, H.; Vilche, J. R.; Arvia, A. J. J. Electroanal. Chem. 1982, 134, 251.
3.1.2. Cyclic Voltammetry in 0.1 M Na2B4O7, pH 9.2. The CV of Co in 0.1 M Borax in a N2 atmosphere shows, in contrast with the anodic peak observed in 1 M NaOH, three anodic shoulders at +0.09, +0.33, and +0.59 V (solid curve in Figure 1b). The anodic charge is 6 times lower than that in 1 M NaOH, due to a lower solubility of Co(II) hydroxides at this lower pH. If CO is bubbled at Edos ) -0.21 V for 10 min, a subsequent CV shows a single anodic peak at +0.56 V
3392 Langmuir, Vol. 14, No. 12, 1998
Cuesta and Gutie´ rrez
(a)
(b)
(c)
(d)
Figure 2. SW-FTIR spectrum of Co in the following CO-saturated solutions: (a) 1 M NaOH; (b) 0.1 M Borax; (c) phosphate buffer, pH 6.9; (d) a 0.5 M Na2SO4 solution acidified with H2SO4 until its pH decreased to 3. The reference and sample potentials, respectively, were (a) -0.12 and +0.08 V; (b) +0.05 and +0.25 V; (c) -0.04 and +0.16 V; (d) -0.33 and -0.13 V.
(dashed curve in Figure 1b). The anodic charge was somewhat higher than that in the absence of CO, due to the electrooxidation of chemisorbed, and possibly also dissolved, CO. The stationary cathodic current at the initial potential of -0.41 V was 0.65 and 0.09 mA cm-2 in the absence and presence, respectively, of CO in solution, due to inhibition of H2 evolution by chemisorbed CO. The same anodic peak still appears, although slightly decreased and shifted to +0.51 V, if N2 is bubbled in the solution in order to eliminate any dissolved CO while still holding the potential at -0.21 V (dotted curve in Figure 1b). 3.1.3. Cyclic Voltammetry in 0.05 M K2HPO4 + 0.05 M KH2PO4, pH 6.9. In a pH 6.9 phosphate buffer the first positive sweep (solid curve in Figure 1c) in a N2 atmosphere is similar to that in 1 M NaOH in that it shows only one anodic peak, at +0.29 V, although the anodic charge is much smaller. If CO is bubbled through the solution at Edos ) -0.14 V for 10 min, in a subsequent CV (dashed curve in Figure 1c) the anodic peak increases and is shifted positively to +0.52 V. The stationary cathodic current at the initial potential of -0.14 V was 0.09 mA cm-2 and practically zero in the absence and presence, respectively, of CO in solution. If N2 is bubbled for 15 min after a previous saturation with CO at Edos ) -0.14 V, the first positive sweep (dotted curve in Figure 1c) is very similar to that in a CO-saturated
solution. As was to be expected, the second positive sweep (not shown) is similar to that in a N2 atmosphere. 3.1.4. Cyclic Voltammetry in 0.5 M Na2SO4 + H2SO4, pH 3. A 0.5 M Na2SO4 solution was acidified with H2SO4 until its pH decreased to 3. In a N2 atmosphere the electrooxidation of Co starts at -0.08 V and then increases steeply and linearly with potential (solid curve in Figure 1d), but if the solution is previously saturated with CO at Edos ) -0.50 V, then the beginning of Co electrooxidation is shifted positively to +0.32 V (dashed curve in Figure 1d), due to inhibition by chemisorbed CO. 3.2. FTIRS Results. 3.2.1. FTIRS in 1 M NaOH. After electroreduction of the native Co oxides, CO was bubbled for 15 min at Edos ) -0.23 V with the Co electrode as separated from the fluorite window as possible, after which the CO bubbling was stopped and the electrode was pressed against the window. In the SW-FTIR spectrum with reference and sample potentials of -0.12 and +0.08 V, respectively, there appears a bipolar band at 1973 cm-1, in the linear CO region (Figure 2a) (see Discussion). The feature at 1810 cm-1 should be the negative peak of a very small bipolar band of multiply bonded CO (see Discussion), whose positive lobe is hidden by the noise. In the LPS-FTIR experiment the first spectrum (reference) was taken at 0.00 V. Up to +0.3 V the only feature is a bipolar band of linear CO (Figure 3a). At this potential, and coinciding with the appearance of an anodic current,
FTIRS of CO on Cobalt at pH 3-14
Langmuir, Vol. 14, No. 12, 1998 3393
Figure 3. LPS-FTIR spectra of Co in the following CO-saturated solutions: (a) 1 M NaOH; (b) 0.1 M Borax; (c) phosphate buffer, pH 6.9; (d) a 0.5 M Na2SO4 solution acidified with H2SO4 until its pH decreased to 3. The first spectrum (reference) was taken at (a-c) 0.00 V; (d) -0.06 V.
two small negative bands appear, one of carbonate at ∼1400 cm-1 and another at ∼1650 cm-1 of water, both formed in the electrooxidation of CO in this alkaline medium:
CO + 4OH- f CO32- + 2H2O + 2e (The electrooxidation of Co would also produce water if the oxidation product is a Co oxide or oxyhydroxide, but not if it is a hydroxide.) The negative, very broad band centered at ∼2250 cm-1 is also due to water formation. The distortion due to the production of water is so strong that at +0.7 V the positive band indicating CO disappearance is only seen as a shoulder at ∼1973 cm-1. Obviously, this is the frequency at the reference potenctial of 0 V; the frequency at +0.6 V is 2010 cm-1. The simultaneously recorded LPS at 1 mV s-1 (not shown) showed, as the CV at 50 mV s-1, only an anodic peak (at 0.46 V), with a charge 3.7 times higher than that in the sweep at 50 mV s-1. As is well-known, the charge of a passivation peak decreases with the increasing sweep rate, since less time is needed to achieve the passivation potential. 3.2.2. FTIRS in 0.1 M Na2B4O7, pH 9.2. After electroreduction of the native Co oxides, CO was bubbled through the solution at Edos ) -0.21 V and then the electrode was pushed against the window. In the SW-FTIR spectrum with reference and sample potentials of +0.05 and +0.25 V, respectively, two bipolar bands appear (Figure 2b): a large one at 2009 cm-1 and a smaller one at 1850 cm-1, corresponding to linear and multibonded CO, respectively, with a preponderance of the former species.
In the LPS-FTIRS experiment, the first spectrum (reference) was taken at 0 V. Multibonded CO produces a bipolar band at 1839 cm-1 and linear CO an initially bipolar band which apparently remains so up to the highest potential used (Figure 3b). However, this is an artifact originated by the positive water band at 2200 cm-1, since in consecutive differential spectra (not shown), obtained by using as reference for each spectrum collected during the LPS the spectrum immediately preceding it, linear CO disappears at +0.90 V. The stretching frequency at 0 V is 1993 cm-1. The simultaneoulsy recorded LPS at 1 mV s-1 (not shown) showed a single anodic peak (at 0.49 V), as the CV at 50 mV s-1. The acidification due to the electrooxidation of Co is so high that the thin layer becomes neutral or even acidic, as indicated by the positive bands at 1650 and 2200 cm-1, both due to the consumption of water in the electrooxidation of CO and Co at neutral or acidic pH:
CO + H2O f CO2 + 2H+ + 2e Co + 2H2O f Co(OH)2 + 2H+ + 2e Two negative bands at 1411 and 1158 cm-1 signal an increase of the concentration of H3BO3 as a consequence of the acidification of the thin layer, as already reported for Fe.2 3.2.3. FTIRS in 0.05 M K2HPO4 + 0.05 M KH2PO4, pH 6.9. The reference and sample potentials for the SWFTIR spectrum were -0.04 and +0.16 V, respectively. The spectrum shows (Figure 2c) a bipolar band at 2019
3394 Langmuir, Vol. 14, No. 12, 1998
cm-1, corresponding to CO chemisorbed in the linear form. Very possibly the broad negative band at 1856 cm-1 is the negative lobe of a bipolar band of multiply bonded CO. The first spectrum (reference) for the LPS-FTIRS experiment was taken at 0 V (Figure 3c). As already reported,2 in phosphate buffer the acidification involved in the electrooxidation of CO and the metal electrode provokes a decrease in the concentration of HPO42- ions (positive band at 1095 cm-1) and an increase in those of H2PO4- and H3PO4 (negative bands at 1160 and 1035 cm-1). The acidification was so high that the anodic current during the LPS increased monotonically, without showing the passivation peak that appeared at 50 mV s-1 in a conventional cell (dashed curve in Figure 1c). At the end of the positive sweep the main concentration increase was that of H2PO4-, as is the case with Ni.5 However, with Fe the main concentration increase was that of H3PO4.2 This is in agreement with the much higher corrodability of Fe as compared with Co and Ni, leading to a stronger acidification in the first case. A bipolar band of linear CO appears already at 0.1 V (Figure 3c). Its frequency at the reference potential of 0.00 V is 2006 cm-1, corresponding to linear CO. This band remains bipolar up to the highest potential used, and its position and intensity remain constant from +0.4 V. We attribute this anomalous behavior to a large ohmic drop due to the formation of insoluble corrosion products. A very small negative band of CO2 at 2340 cm-1 indicates that the total amount of oxidized CO was much smaller than with Ni. Although the initial pH of the phosphate buffer was 6.8, the formation of H3PO4, whose pK1 is 2.2, indicates that the acidification was so strong that the CO2 produced was un-ionized (the pK1 of carbonic acid is 6.4). 3.2.4. FTIRS in 0.5 M Na2SO4 + H2SO4, pH 3. CO was bubbled at Edos ) -0.50 V. The reference and sample potentials for the SW-FTIR spectrum were -0.33 and -0.13 V, respectively. The spectrum shows a bipolar band at 1996 cm-1, again in the region of linearly chemisorbed CO (Figure 2d), but no band of multibonded CO. In the LPS-FTIRS experiment the first spectrum (reference) was taken at -0.06 V. There is a bipolar band of linear CO at ∼2015 cm-1 at the reference potential of -0.06 V (Figure 3d), which remains bipolar up to the highest potential used, as was the case in phosphate buffer, pH 6.9. A small negative shoulder of CO2 production appears at 2340 cm-1. A very broad, W-shaped band at 1650 cm-1 is the convolution of a positive, narrower band and a broader, negative band, both at the same frequency. The positive band indicates that water is consumed in the electrooxidation of CO and Co in this acidic medium. Indeed, according to Pourbaix6 at this pH the only stable species of Co is Co(OH2)62+, whose formation consumes water without increasing the acidity of the thin layer:
Co + 6H2O f Co(OH2)62+ + 2e The positive water band splits into two a very broad negative band at the same frequency, due to the production of [Co(OH2)6]2+ (all aquo complexes exhibit the three fundamental modes of the free water molecule7), as confirmed by the pink color of the solution. At this pH 3 (5) Cuesta, A.; Gutie´rrez, C. To be published. (6) Pourbaix, M. Atlas d’e´ quilibres e´ lectrochimiques a` 25 °C; GauthierVillars, E Ä d.: Paris, France, 1963. (7) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1978.
Cuesta and Gutie´ rrez
very broad, W-shaped bands at 1650 cm-1 can also be seen with iron and nickel, due to the production of [Fe(OH2)6]2+ and [Ni(OH2)6]2+, respectively.2,5 The negative band at 1141 cm-1 is due to the formation of a species such as Co2(SO4)3‚18H2O, in which the sulfate is the anion in an ionic compound, since the similar compound [Co(NH3)6]2(SO4)3‚5H2O has a very strong band at 1130-1140 cm-1.7 The formation of a blue precipitate on the electrode confirms the assignment of the negative band at 1141 cm-1 to a cobalt sulfate. Similar bands in iron2 and nickel5 can be attributed to the corresponding sulfates. The small positive band at 1100 cm-1 indicates that, as with Fe2 and Ni,5 free SO42- anions, which absorb at 1100 cm-1,7 have been consumed, precisely by the formation of sulfates of Fe, Co, and Ni. The negative band at around 1071 cm-1 is actually an artifact due to the convolution of the negative band at 1141 cm-1 of cobalt sulfate formation and the positive band at 1100 cm-1 of consumption of free sulfate ions. 4. Discussion 4.1. Coverage with Chemisorbed CO of the Co Electrode. Production of CO2. It is to be remarked that while only linear CO was detected on Fe over the pH range 3-14,2 in the case of Co multibonded CO was also present at pH 14 and 9.2, and possibly also at pH 6.9. However, the deformation of the IR spectra due to the electrooxidation of Co was so severe that only at pH 9.2, at which the electrooxidation of Co was lowest, could the intensities of the monopolar CO bands be estimated. They were ∆R/R ) 4 × 10-4 and 1.5 × 10-3 for multibonded and linear CO, respectively. In our experimental setup with a flat window the chemisorption of one monolayer of CO on Pt caused a reflectance decrease of ∆R/R ) 0.43-0.50%, a value comparable with that of Lu and Bewick.8 The extinction coefficient of bridge-bonded CO on Pt is between 2 and 2.5 times lower than that of linearly-bonded CO,9-11 and the extinction coefficient of multibonded CO should be comparable to that of bridge CO. In conclusion, it can be estimated that at least a sizable fraction of the Co surface sites are covered with CO. The molar absorption coefficient (formerly known as extinction coefficient) of dissolved CO2 has been given as 1200 M-1 cm-1.12 The amount of CO2 produced is estimated to be very low. 4.2. Dependence of the CO Stretching Frequency on pH. A plot of the stretching frequency of the band of linear CO as a function of the potential versus a pHindependent reference electrode is shown in Figure 4a. The points for pH 14 and 9.2 define straight lines with slopes of 64 ( 5 and 61 ( 3 cm-1 V-1, respectively. In the experiment at pH 9.2 the three points at the highest potentials were not included in the calculation, since their frequencies were lower than expected, perhaps because at these higher potentials electrooxidation of chemisorbed CO began to take place, and as is well-known, the frequency of chemisorbed CO increases linearly with coverage by up to 80 cm-1.13-16 The points for pH 6.9 were not included because the high Co electrooxidation severely (8) Lu, J.; Bewick, A. J. Electroanal. Chem. 1989, 270, 225. (9) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1989, 90, 7426. (10) Schweizer, E.; Persson, B. N. J.; Tu¨shaus, M.; Hoge, D.; Bradshaw, A. M. Surf. Sci. 1989, 213, 49. (11) Chang, S.-C.; Weaver, M. J. Surf. Sci 1991, 241, 11. (12) Mizen, M. B.; Wrighton, M. S. J. Electrochem. Soc. 1989, 136, 941. (13) Severson, M. K.; Russell, A.: Campbell, D.; Russell, J. W. Langmuir 1987, 3, 202.
FTIRS of CO on Cobalt at pH 3-14
(a)
Langmuir, Vol. 14, No. 12, 1998 3395
reference electrode, the dependence on pH of the CO stretching frequency, at the same potential versus RHE, should then be
(∂ν/∂pH)RHE ) (∂ν/∂ESHE) (∂ESHE/∂pH)RHE ≈ 63 × 0.06 ≈ 3.8 cm-1 (pH unit)-1
deformed the spectra. Kunimatsu et al.17 first reported the pH independence of CO frequency, for CO chemisorbed on Au. Since the stretching frequency of linear CO on Co depends only on the potential versus a pH-independent
Consequently, a plot of the stretching frequency at 0 V versus RHE of linear CO on Co (Figure 4b) shows a linear decrease, with a slope of 3.9 ( 0.4 cm-1 (pH unit)-1. In other words, the stretching frequency of linear CO decreases by about 55 cm-1 upon increasing the pH from 0 to 14. This has the important consequence that since the potential region, measured versus the RHE, of stability of CO chemisorbed on a given metal is nearly independent of pH, the frequency ranges at acidic pH (2020-2090 cm-1 for linear CO, 1900-1960 cm-1 for bridge CO, and 17501880 cm-1 for multiple CO) become then, at pH 14, 19602030 cm-1 for linear CO, 1840-1910 cm-1 for bridge CO, and 1690-1820 cm-1 for multiple CO. Chang et al.18 found that the CO stretching frequency of saturated CO adlayers on Pt(111) in four nonaqueous solvents was virtually independent of the solvating medium, a behavior which they attributed to the exclusion by chemisorbed CO of solvent molecules from the electrochemical inner layer. They also studied the influence of the cation size, using polycrystalline Pt19 and Pt(110).20 Two types of models, physical (first-order vibrational Stark effect of the double-layer electric field on the static dipole moment of the CO molecule) and chemical (changes in the electron back-donation from the filled metal orbitals to the antibonding 2π* orbital of the CO molecule) have been proposed for explaining the linear increase of the stretching frequency of chemisorbed CO with increasing potential at a given pH value. In a recent review Lambert21 concludes that a better model of the double layer is required for reaching a single consistent explanation of all the data. If the physical model is applicable, the observed pH independence of the CO stretching frequency would indicate that the point of zero charge versus a fixed reference electrode of a given CO-saturated metal is independent of pH, which is consistent with the mentioned hypothesis18 that a complete CO coverage excludes solvent molecules (in this case water and its ions) from the metal surface. 4.3. Assignment of the Bands of CO Chemisorbed on Co. At the four pH values used here a main band appeared in the spectra of Co in CO-saturated solution, its frequency at 0.0 V versus RHE decreasing with increasing pH, from 2015 cm-1 at pH 3 to 1973 cm-1 at pH 14. These frequencies are within the range for linear CO at acidic and alkaline pH, respectively. At pH 14, 9.2, and maybe also 6.9, a band so small that it can be clearly seen in SW-FTIR spectra, but not in LPSFTIR spectra, appears at lower frequencies, between 1810 and 1856 cm-1. Its assignment is difficult, since it appears at alkaline pH only, at which the observed frequencies are intermediate between those of bridge CO (1840-1910 cm-1 at pH 14) and of multiple CO (1690-1820 cm-1 at pH 14). In their review concerning the solid-gas interface (in which, as is well-known,22,23 the frequencies of chemisorbed CO are several tens of cm-1 higher than at the solid-
(14) Tornquist, W.; Guillaume, F.; Griffin, G. L. Langmuir 1987, 3, 477. (15) Leung, L.-W. H.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. 1988, 92, 6985. (16) Chang, S.-C-; Weaver, M. J. Surf. Sci. 1990, 230, 222. (17) Kunimatsu, K.; Aramata, A.; Nakajima, H.; Kita, H. J. Electroanal. Chem. 1986, 207, 293.
(18) Chang, S.-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5378. (19) Roth, J. D.; Weaver, M. J. Langmuir 1992, 8, 1451. (20) Jiang, X.; Weaver, M. J. Surf. Sci. 1992, 275, 237. (21) Lambert, D. K. Electrochim. Acta 1996, 41, 623. (22) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (23) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391.
(b)
Figure 4. (a) Stretching frequency of linear CO as a function of the electrode potential vs a pH-independent reference electrode for pH 14, 9.2, and 3. The slopes for the first two pH values are shown in the figure; for pH 9.2 the three points at the highest potentials were not included in the calculation of the slope (see text). (b) Stretching frequency of linear CO, at 0.00 V vs RHE, as a function of pH. The slope is 3.9 ( 0.4 cm-1 (pH unit)-1.
3396 Langmuir, Vol. 14, No. 12, 1998
electrolyte interface) Sheppard and Nguyen24 assign frequencies between 1815 and 1880 cm-1 to CO multibonded on evaporated Co films. Gopalakrishnan and Viswanathan25 obtained, for CO chemisorbed on Co powder at room temperature, two bands at 1990 and 1800 cm-1, which they assigned to linear and bridge CO, respectively. Rao et al.26 found, using EELS, and at high CO coverage of a polycrystalline Co foil, three bands at 1871, 1975, and 2072 cm-1 at 300 K. They assigned the bands at 1967-1975 and 2040-2072 cm-1 to bridge and (24) Sheppard, N.; Nguyen, N. T. Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 5, Chapter 2. (25) Gopalakrishnan, R.; Viswanathan, B. J. Colloid Interface Sci. 1984, 102, 370. (26) Rao, C. N. R.; Vishnu Kamath, P.; Prabhakaran, K.; Hedge, M. S. Can. J. Chem. 1985, 63, 1780.
Cuesta and Gutie´ rrez
linear CO, respectively. Marzouk et al.27 observed five Raman bands between 2130 and 2030 cm-1, assigned to linear CO; five bands between 2010 and 1840 cm-1, assigned to bridge CO; and three bands between 1830 and 1740 cm-1, assigned to multibonded CO. With the exception of ref 25, all these data support the assignment of the band at 1810-1856 cm-1 to multibonded CO. 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. LA970628W (27) Marzouk, H. A.; Bradley, E. D.; Arunkumar, K. A. Spectrosc. Lett. 1985, 18, 189.
