J. Phys. Chem. B 1997, 101, 9287-9291
9287
Electroadsorption and Electrooxidation of CO on Anodic Ni Oxide in Acidic CO-free Solution A. Cuesta and C. Gutie´ rrez* Instituto de Quı´mica Fı´sica “Rocasolano”, C. S. I. C., C. Serrano, 119, 28006-Madrid, Spain ReceiVed: May 29, 1997; In Final Form: July 23, 1997X
To check if in acidic media CO is desorbed as such, i.e., as molecular CO, from the surface of nickel during a positive linear potential sweep, we have carried out a Fourier-transform infrared spectroscopic study of this system. We have found that, in the absence of dissolved CO, in a positive linear potential sweep at 1 mV s-1 most, if not all, of the CO chemisorbed on Ni in CO-free 0.5 M Na2SO4 acidified to pH 3.0 is electrooxidized to CO2 instead of being electrodesorbed from the surface. Furthermore, we have found that a band at 2112 cm-1 appears at potentials near the passivation maximum of nickel. We have assigned this band to CO adsorbed on Ni oxide or on Ni0 sites perturbed by the oxidation of the neighboring Ni atoms. As far as we know, this is the first time that such a band has been found by FTIRS during the electrooxidation of a COcovered metal.
Introduction found1-3
We have recently that CO chemisorbs without dissociation on the surface of the ferrous metals (Fe, Co, and Ni) in aqueous solutions in the pH range 3-14 provided that the native oxide layer that forms on most metals upon contact with atmospheric oxygen has been removed by a cathodic cleaning of the electrode (15 min at -0.5 V vs RHE). Molecularly chemisorbed CO inhibited both metal electrooxidation and H2 evolution. It has been claimed,4-6 on the basis of cyclic voltammetry and differential electrochemical mass spectroscopy (DEMS) results, that CO chemisorbed on Ni in acidic and neutral media is desorbed intact as molecular CO at a rate roughly proportional to the anodic current according to
Ni(CO)ads + 2H2O f Ni(OH)2 + 2H+ +CO + 2e- (1) instead of being electrooxidized to CO2:
COads + H2O f CO2 + 2H+ + 2e-
(2)
In the following we will designate the first mechanism electrodesorption and the second one electrooxidation of chemisorbed CO. We have already shown3 by linear potential sweep (LPS)FTIR (SPAIRS) spectroscopy that CO2 appears simultaneously with the onset of the anodic current when the Ni electrode is in contact with a CO-saturated, pH 3.0, 0.5 M Na2SO4 solution. In this work, to test if the chemisorbed CO is electrooxidized to CO2 or electrodesorbed, we have eliminated dissolved CO, which could be an alternative source of CO2. Experimental Section Disks of 12 mm diameter were cut from a 1 mm thick Ni sheet, 99.998% pure, from Alpha-Johnson Matthey. They were polished with alumina down to a particle size of 0.05 µm and sonicated in Milli-Q water. * Corresponding author. Phone: 34-1-5619400. Fax: 34-1-5642431. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, October 15, 1997.
S1089-5647(97)01739-2 CCC: $14.00
For infrared measurements the Ni disk was glued to a 0.5 cm length of Pyrex tubing, connected by means of heat-shrink polyolefin tubing with an inner adhesive lining to another length of Pyrex tubing. This flexible connection makes possible a close, parallel fit between the electrode and the fluorite optical window. Electrical contact was effected with a copper cable soldered to the Ni disk. A homemade Teflon cell and Ag/AgCl (saturated KCl) reference electrode were used. The potentials are given vs the reversible hydrogen electrode in the same solution (RHE). The electrolyte was 0.5 M Na2SO4 whose pH was adjusted to 3.0 with 0.5 M H2SO4. Analytical grade reagents and Milli-Q water were used. The FTIR instrument was Model 1725-X from Perkin-Elmer and purged with CO2 and H2O-free air (Peak Scientific Equipment) at a rate of 16 L min-1. A PAR potentiostat, Model 362, was used. Before measurements the Ni electrode was cathodically cleaned from its native oxides by holding it at -0.5 V vs RHE for 15 min. Two programs of spectra acquisition were used: SW (square wave)-FTIRS, originally designated as subtractively normalized Fourier-transform infrared spectroscopy (SNIFTIRS) and LPSFTIRS, originally designated as single potential alteration infrared spectroscopy (SPAIRS). For obtaining SW-FTIR spectra, useful for increasing the S/N ratio of FTIR spectra of chemisorbed species, of chemisorbed CO we alternately accumulated 20 spectra at the reference potential and 20 spectra at the sample potential, each spectrum being the sum of 100 interferograms recorded in 45 s. A quantity of 20 cycles were carried out, i.e., 2000 spectra at each potential were collected. A bipolar band indicates a Stark shift of a chemisorbed species present at both reference and sample potentials. In LPS-FTIRS, useful for identifying the species that appear or disappear during a linear potential sweep, interferograms were continuously recorded during an LPS at 1 mV s-1, every 25 successive interferograms recorded over a potential interval of 10 mV being coadded into a spectrum. The LPS was started at the negative potential limit with the first spectrum being taken as reference. Differential spectra were calculated as ∆R/R ) (Rsample/Rref) - 1. With this convention a positive band indicates that a species present at the reference potential is consumed at © 1997 American Chemical Society
9288 J. Phys. Chem. B, Vol. 101, No. 45, 1997
Figure 1. SW-FTIR spectrum of CO chemisorbed on Ni in a 0.5 M Na2SO4, pH 3, CO-free solution (solid line) and in the same but COsaturated solution (dotted line). The reference and sample potentials were +0.085 and +0.285 V, respectively. We alternately collected 20 spectra at each potential, each spectrum being the sum of 100 interferograms.
the sample potential. Conversely, a negative band indicates that a new species, not present at the reference potential, is produced at the sample potential. We prefer to use the denominations SW-FTIRS and LPSFTIRS instead of the original ones of SNIFTIRS and SPAIRS, respectively, which in our opinion are less descriptive of the potential programs involved. Results and Discussion Electrooxidation of Chemisorbed CO. After a cathodic cleaning of the electrode at -0.50 V for 15 min, CO was bubbled through the solution at this potential for 15 min and then at -0.11 V for another 15 min. The cathodic current of H2 evolution decreased over the whole 30 min, indicating that CO adsorption progressed all the time. (The adsorption potentials were purposely low in order to avoid Ni oxidation.) The solution was then replaced by deoxygenated, CO-free solution (pressure-fed into the FTIR cell through a 1.5-mm outer diameter Teflon tube that reached the bottom of the cell) while the electrode potential was kept at -0.11 V. The potential was then increased to +0.2 V, which was the most positive potential at which no anodic current was observed, and the electrode was pushed against the fluorite window. This increase of the potential avoided the formation of hydrogen bubbles in the thin layer between the electrode and the window, which causes strong artifacts in the spectra. An SW-FTIR spectrum was taken between +0.085 V (reference) and +0.285 V (sample), both potentials being located in the region of negligible current preceding the anodic peak (see Figure 2D below). The spectrum showed bipolar bands at 2037 and 1896 cm-1, corresponding to linear (on-top) and bridge CO, respectively, chemisorbed on the Ni surface (solid line in Figure 1). The intensities of the bipolar bands of linear and bridge CO were 54% and 64% of those obtained in CO-saturated solution, respectively, which are also included in Figure 1 (dashed line). Selected spectra obtained by LPS-FTIRS are given in parts A-C of Figure 2, and the simultaneously recorded voltammo-
Cuesta and Gutie´rrez gram at 1 mV s-1 is shown as the solid line in Figure 2D, in which the potentials at which the most relevant changes in the spectra occur are indicated by arrows. Chemisorbed CO inhibits the electrooxidation of Ni, as can be seen by comparing the CVs in the presence of CO (solid line) and in the background electrolyte (dashed line) (Figure 2D). The reference spectrum was taken at 0.185 V. Two bipolar bands at about 2040 and 1910 cm-1, corresponding to CO chemisorbed in the linear and bridge forms, respectively, can be seen already at 0.385 V (spectrum a in Figure 2A and point a in Figure 2D). At 0.51 V, past the current maximum, bridge CO has already been completely electrooxidized, as shown by a positive band at 1911 cm-1 (spectrum d in Figure 2B and point d in Figure 2D). The oxidation of linear CO is not completed until +0.645 V at the end of the current maximum; at this potential its loss originates a positive band at 2028 cm-1 (spectrum f in Figure 2C and point f in Figure 2D). The bands of linear and bridge CO show the usual increase in frequency with potential, and they do not appear with s-polarized light, which means that the Ni atoms on which they are chemisorbed remain in contact with the Ni substrate, or at least that they remain as clusters unperturbed by the electrooxidation of the Ni substrate and are so near the surface of the Ni substrate as to be undetectable with s-polarized radiation. The disappearance of chemisorbed CO is due, at least partly, to its electrooxidation to CO2, as evidenced by the negative band at 2344 cm-1. The amount of CO2 produced, about 1 nmol cm-2, corresponds to about one-third of a monolayer of CO (see below). Band at 2112 cm-1. A negative band at 2112 cm-1 appears at 0.45 V at the current maximum (spectrum c in Figure 2A and point c in Figure 2D), reaches its maximum intensity (∆R/R ) 9 × 10-4) at +0.53 V about halfway down the current maximum (spectrum e in Figure 2B and point e in Figure 2D), and disappears at 0.66 V at the end of the current maximum (spectrum g in Figure 2C and point g in Figure 2D), which indicates that the corresponding species was not present at the reference potential. This band also appears with s polarization of the IR beam (Figure 3), which would indicate that it corresponds to a species in solution. Contrary to what could be expected, the band does not appear when CO is electrooxidized on Ni in CO-saturated solution. The frequency of this band is practically the same claimed7 to correspond to the electroadsorption of CO on copper in 0.1 M KClO4; a band at 2110 cm-1 was observed, the intensity of which increased with increasing potential up to the positive potential limit of 1.05 V vs RHE. With copper in 0.5 M NaClO4 or pH 6.9 phosphate buffer, we have found8 that in the presence of CO in solution in a positive sweep a band at 2112 cm-1 appears simultaneously with the anodic peak of Cu electrooxidation and disappears when Cu oxide is reduced in the cathodic peak, which unequivocally proves that the band is due to CO chemisorbed on an anodic Cu oxide. The band appears with both s and p polarization and therefore cannot correspond to CO chemisorbed on Cu metal. We believe that the band at 2112 cm-1 observed here corresponds to CO chemisorbed on anodic Ni oxide, since its intensity increases with increasing positive potential. Furthermore, the fact that the band is also observed with s polarization can be explained only if the perturbed metal atoms are on top of an insulating oxide layer. This would explain why the frequency of the band at 2112 cm-1 is unaffected by changes in the potential. It should be kept in mind that the anodic charge between +0.285 and +0.88 V, 35 mC cm-2, corresponds nearly exclusively to the electrooxidation of about 60 layers of Ni to a Ni(II) oxycompound, since only about 1 nmol cm-2 of CO2
CO on Ni Oxide
J. Phys. Chem. B, Vol. 101, No. 45, 1997 9289
Figure 2. (A-C) LPS-FTIR spectra with p polarization of CO chemisorbed on Ni in a 0.5 M Na2SO4, pH 3.0, CO-free solution. The spectra were collected during a linear potential sweep at 1 mV s-1, each spectrum being the sum of 25 interferograms. The reference spectrum was taken at +0.185 V. (D) Voltammogram recorded simultaneously with the LPS-FTIRS experiment (solid line). The potentials at which the most important changes in the spectra occur are indicated in the figure. The voltammogram in the background electrolyte is also included for comparison (dashed line).
(see below) was produced with the consumption of only 0.2 mC cm-2 (0.6% of the total charge). Actually, we cannot discriminate if the band at 2112 cm-1 is due to CO chemisorbed on anodic Ni oxide or to CO chemisorbed on unoxidized Ni0 atoms surrounded by Ni oxide, since in an FTIRS study9 of the adsorption of gaseous CO on NiO thin films grown on a Ni(110) substrate a broad, weak band at 2070-2110 cm-1, which disappeared upon heating to
250-320 K, was assigned to CO adsorbed on perturbed (not oxidized) Ni0 sites. A band at 2075 cm-1 in a similar study10 with a NiO film grown on Ni(111) was attributed to CO adsorbed on less oxidized Ni sites, but this band is neither related to the band at 2112 cm-1 detected here nor related to the broad band at 2070-2110 cm-1 in ref 8, since it was desorbed at very low temperatures, yielding in temperature-programmed desorption a broad peak between 125 and 160 K.
9290 J. Phys. Chem. B, Vol. 101, No. 45, 1997
Cuesta and Gutie´rrez
Figure 3. LPS-FTIR spectra with s polarization of CO chemisorbed on Ni in a 0.5 M Na2SO4, pH 3.0, CO-free solution. The spectra were collected during a linear potential sweep at 1 mV s-1, each spectrum being the sum of 25 interferograms. The reference spectrum was taken at +0.185 V.
As far as we know, the detection by IR spectroscopy of a CO band at frequencies higher than 2100 cm-1 during a positive linear sweep of a CO-covered metal had not been reported before. However, such a band has been unequivocally detected11 by surface-enhanced Raman spectroscopy (SERS) in CO-saturated 0.1 M HClO4 of one to three equivalent monolayers of rhodium and ruthenium electrodeposited onto gold. When the potential was increased to 0.5-0.7 V vs SCE, intense bands at 2110 and 2140 cm-1 appeared with Rh and Ru, respectively, which were assigned to CO bound to “oxidized”
surface sites defined as sites where the metal atoms had undergone formal oxidation and/or adjacent to sites where surface oxide had formed. The absence of these bands in FTIR spectra was attributed to a lower signal-to-noise ratio. Band of Carbon Dioxide. At 0.425 V, before the current maximum, a negative band of CO2 formation starts to appear at 2344 cm-1, is clearly visible at +0.45 V (the current maximum), and continues increasing during the positive sweep up to 0.66 V (end of the current maximum), in agreement with the fact that at this potential both bridge and linear CO and the
CO on Ni Oxide species originating the band at 2112 cm-1 have disappeared completely (parts A-C of Figure 2). Between 0.66 and 0.78 V the intensity of the CO2 band remains stable but decreases slightly between 0.78 and 0.88 V (spectrum h in Figure 2C and point h in Figure 2D), most likely because of diffusion out of the thin layer. In a similar experiment in a CO-saturated solution the CO2 band increased up to 0.88 V. The maximum intensity of the CO2 band (Figure 2C) is ∆R/R ) 0.005, which is 54% of that in a CO-saturated solution. Since the intensities of the bipolar bands of linear and bridge CO chemisorbed on Ni in a CO-free solution relative to those in a CO-saturated solution were very similar to this value, 54% and 64%, respectively, it is clear that, with a thin electrolyte layer, in CO-saturated solution little, if any, dissolved CO is electrooxidized on Ni. This was to be expected, since the solubility of CO in water is very low, 0.91 mM,12 and the thickness of the electrolyte layer is a few micrometers so that the amount of CO in this layer is about 0.5 nmol cm-2 only, corresponding to about 0.2 monolayers. To convert the height of the CO2 band into the concentration of CO2 in solution, two opposing influences on the peak height of solution species must be taken into account. On one hand, the path length traversed by the IR beam in the thin electrolyte layer between the electrode and the flat fluorite window is about 2.8 times its thickness, which would multiply by this amount the height of the CO2 peak compared with that obtained in a conventional transmission cell (obviously this is strictly true for a collimated beam only, which is not the case). On the other hand, the fraction of IR radiation reaching the detector, which has been reflected not at the electrode-electrolyte interface but at the air-fluorite window and window-electrolyte interface, is about 0.30 of the total energy reaching the detector (this fraction is easily determined as the ratio of the detector signal with the electrode separated from the window to the detector signal with the electrode pressed against the window), which would decrease the height of a solution peak to 0.70 of that measured in a transmission cell. (Obviously, with a prismatic window the spurious reflection at the air-window interface does not reach the detector.) Therefore, when a flat fluorite window is used, the values of ∆R/R for solution species must be divided by 2.8 × 0.70 ≈ 2 for converting them into the values that would be obtained in a transmission cell. A value of 1200 M-1 cm-1 13 has been given for the molar absorption coefficient (formerly known as extinction coefficient) of CO2 dissolved in acetonitrile. Leung and Weaver14 give 3.5 × 104 M-1 cm-2 as the effective integrated molar absorption coefficient of CO2 in aqueous solution with a ratio of the effective to the bulk integrated absorption coefficient of 2.5 ( 0.5. With these data and a value of 14.6 cm-1 for the “equivalent width” (value that multiplied by the absorption at the peak yields the integrated absorption) of the CO2 band, the molar absorption coefficient of CO2 in water would be 960 ( 192 M-1 cm-1, in good agreement with the value measured directly in acetonitrile with a transmission cell.13 By use of this latter value, it is calculated that ∆R/R ) 0.005/2 ) 0.0025 corresponds to a concentration of CO2 of about 1 nmol cm-2, which we assume to correspond to the amount of CO chemisorbed on Ni at the start of the experiment. Part of this CO is converted into CO chemisorbed on perturbed Ni0 atoms before being eventually oxidized to CO2. However, the high electrooxidation of Ni produces a serious distortion of the baseline, which makes it impossible to obtain an accurate potential dependence of the bands of CO2, of CO chemisorbed on Ni, and of CO chemisorbed on perturbed Ni0 atoms.
J. Phys. Chem. B, Vol. 101, No. 45, 1997 9291 Ni is the metal with the smallest atomic radius, 0.124 nm, and an interatomic distance in fcc faces of about 0.249 nm, shorter than the minimum distance of 0.285 nm between adjoining CO molecules in a compact monolayer.15 Therefore, the maximum coverage of CO on Ni(100) is smaller than unity, about 0.67,16 corresponding to 1.8 nmol cm-2. Assuming a roughness factor (the ratio of the real to the geometric surface) of the polished Ni electrode of about 1.5, a monolayer of CO would correspond to about 2.7 nmol cm-2. Despite the high CO2 production, the possibility that some CO is desorbed as such cannot be discounted, since the molar absorption coefficient at 2137 cm-1 of CO dissolved in methanol is only about 20 M-1 cm-1,17 and desorption of a monolayer (about 2.7 nmol cm-2) of CO would produce a peak of ∆R/R ) 10-4, which is on the order of the noise in our experimental setup. Comparison with Results Obtained by DEMS. The above result is in disagreement with those obtained by DEMS in COsaturated acidic and neutral media in which mostly CO was unequivocally identified during a positive linear sweep of a Ni electrode.6 In 0.005 M H2SO4 the amount of electrodesorbed CO was 6 nmol cm-2 of the geometric area, this amount being 53 times larger than that of CO2. The simplest explanation of this disagreement would be that since DEMS was carried out with a gold-sputtered porous Teflon film on which Ni was electrodeposited for 15 min, the catalytic properties of this electrodeposited Ni film would not be the same as those of a smooth Ni electrode. In fact, it is surprising that the charge of the anodic peak of Ni passivation in 0.005 M H2SO4 of the DEMS electrode in a linear potential sweep at 10 mV s-1 was about 40 mC cm-2 of the geometric surface, which is about the same as that measured by us for a smooth Ni electrode. The amount of CO2 detected in the DEMS experiments was very low, about 0.1 nmol cm-2, compared with the 10-times higher CO2 production found here for a smooth Ni electrode. Probably an FTIRS study of this system would explain its anomalous behavior. Acknowledgment. This work was carried out with the help of the Spanish DGICYT under Project PB93-046. A.C. acknowledges a scholarship from the Spanish Ministry of Education and Science. References and Notes (1) Cuesta, A.; Gutie´rrez, C. J. Phys. Chem. 1996, 100, 12600. (2) Cuesta, A.; Gutie´rrez, C. Langmuir, submitted. (3) Cuesta, A.; Gutie´rrez, C. Langmuir, submitted. (4) Castro Luna, A. M.; Arvia, A. J. J. Appl. Electrochem. 1991, 21, 435. (5) Zinola, C. F.; Castro Luna, A. M. Corros. Sci. 1995, 37, 1919. (6) Zinola, C. F.; Vasini, E. J.; Mu¨ller, U.; Baltruschat, H.; Arvia, A. J. J. Electroanal. Chem. 1996, 415, 165. (7) Westerhoff, B.; Holze, R. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 418. (8) Cuesta, A.; Gutie´rrez, C. To be published. (9) Sanders, H. E.; Gardner, P.; King, D. A.; Morris, M. A. Surf. Sci. 1994, 304, 159. (10) Yoshinobu, J.; Ballinger, T. H.; Xu, Z.; Ja¨nsch, H. J.; Zaki, M. I.; Xu, J.; Yates, J. T., Jr. Surf. Sci. 1991, 255, 295. (11) Leung, L.-W. H.; Weaver, M. J. Langmuir 1988, 4, 1076. (12) Stephen, H.; Stephen, T. Solubilities of Inorganic and Organic Compounds; Pergamon: Oxford, 1963. (13) Mizen, M. B.; Wrighton, M. S. J. Electrochem. Soc. 1989, 136, 941. (14) Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem. 1988, 92, 4019. (15) van Hove, M. A.; Koestner, R. J.; Frost, J. C.; Somorjai, G. A. Surf. Sci. 1983, 129, 482. (16) Lauterbach, J.; Wittmann, M.; Ku¨ppers, J. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 326. (17) Ortiz, R.; Ma´rquez, O. P.; Ma´rquez, J.; Gutie´rrez, C. J. Electroanal. Chem. 1995, 390, 99.
