Study by Fourier Transform Infrared Spectroscopy of the

The electroadsorption and electrooxidation of CO on Fe have been studied at the .... upon underpotential deposition on platinum surfaces in sulphate m...
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J. Phys. Chem. 1996, 100, 12600-12608

Study by Fourier Transform Infrared Spectroscopy of the Electroadsorption of CO on the Ferrous Metals. 1. Iron A. Cuesta and C. Gutie´ rrez* Instituto de Quı´mica Fı´sica “Rocasolano”, C.S.I.C., C. Serrano, 119, 28006-Madrid, Spain ReceiVed: February 16, 1996; In Final Form: April 26, 1996X

The electroadsorption and electrooxidation of CO on Fe have been studied at the following pH values: 14, 9.2, 6.9, and 3. In spite of the high oxidizability of Fe, a cathodic cleaning sufficed to produce an unoxidized surface on which CO could chemisorb. Over the whole pH range 3-14 chemisorbed CO was detected on the electrode surface as evidenced by an IR band at 1916-1993 cm-1, which was assigned to linear (on-top) CO. Its intensity was about 0.2%, from which a surface coverage of 5-10% was estimated. This low CO coverage inhibited both hydrogen evolution and the electrooxidation of Fe itself, which was interpreted by assuming that the cathodic cleaning reduced the native Fe oxides only over a small fraction of the surface and that CO chemisorption, H2 evolution, and start of Fe electrooxidation take place only on this small fraction of the surface.

Introduction The chemisorption of CO on transition metals occurs with or without dissociation, depending on their position, left- or right-hand side, respectively, of the periodic table. It has been said that the dividing line between these two behaviors is a diagonal line between Fe and Co, Tc and Ru, and W and Re, although this is known to be an oversimplification.1 Furthermore, since 1984 it is known that molecular adsorption on single crystals of some metals can occur either on a linear, on-top position with bonding by the conventional CO 5σ f metal electron donation and metal f CO 2π* back-bonding or with the CO molecule lying flat, “π-bonded” parallel to the metal surface.2 Also in this case Fe is a strong candidate as the border between on-top and flat molecular chemisorption of CO.3 As far as we know, the adsorption of CO on Fe in an electrolytic medium has not been studied, perhaps because of the difficulties of producing an oxide-free Fe surface, since this metal is so prone to corrosion. However, with a simple cathodic cleaning of the Fe electrode we have been able to observe molecular electroadsorption of CO on polycrystalline Fe over an extended pH range, 3 to 14. Experimental Section Disks of 12 mm diameter were cut from a 1 mm thick Fe sheet, 99.998% pure, from Alfa-Johnson Matthey. They were polished with alumina down to a particle size of 0.05 µm and sonicated in Milli-Q water. For voltammetric measurements an Fe disk was set in a polypropylene holder with a Viton O-ring, electrical contact being made with an aluminum cylinder pressed with a brass screw against the Fe disk. A one-compartment three-electrode cell was used. The electrolyte was deaerated by N2 bubbling under vigorous magnetic stirring. The reference electrode was a SCE, but all the potentials are given here Vs the reversible hydrogen electrode (RHE). A PAR Model 362 potentiostat and a YEW X-Y recorder were used for cyclic voltammetry. For infrared measurements the Fe disk was glued to a 5.5cm length of Pyrex tubing, connected by means of heat-shrink * Corresponding author: phone, 34-1-5619400 ext 1327; fax, 34-15642431; e-mail, [email protected]. X Abstract published in AdVance ACS Abstracts, July 1, 1996.

S0022-3654(96)00479-0 CCC: $12.00

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 window. Electrical contact was effected with a copper cable soldered to the Fe disk. A home-made Teflon cell and Ag/AgCl reference electrode were used. The FTIR instrument was Model 1725-X from Perkin-Elmer, purged with CO2- and H2O-free air from Peak Scientific Equipment at the rate of 16 L min-1. The potential of the Fourier transform infrared (FTIR) cell was controlled by a PAR potentiostat, Model 362, connected to an arbitrary wavefunction generator, Model DS345 from Stanford. Before any measurement was carried out, the Fe electrode was held at -0.5 V in order to reduce the native oxide layer formed on most metals, and obviously on base ones, upon contact with air. Infrared spectra were obtained with two different techniques. In order to follow the evolution of the system during a linear potential sweep (LPS), interferograms were continuously recorded during an LPS at 1 mV s-1, each 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, 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 the sample potential. Conversely, a negative band indicates that a new species, not present at the reference potential, is produced at the sample potential. A bipolar band indicates a Stark shift of a chemisorbed species present at both the reference and sample potentials. The signal-to-noise ratio of IR spectra of chemisorbed species can be increased if there is a potential range over which the electrode is ideally polarizable, i.e., over which no faradaic reactions occur, taking advantage of the Stark shift of the IR bands of the chemisorbed species, which produces bipolar bands in the potential-difference IR spectrum. Then a square-wave (SW) potential program, in which interferograms are alternately recorded at two different potentials, compensates out the artifacts produced by drifts in the temperature of the thin layer, its thickness, purge rate, etc. For obtaining SW-SNIFTIR spectra we alternately accumulated 20 spectra at the reference potential © 1996 American Chemical Society

CO Adsorption on Fe

J. Phys. Chem., Vol. 100, No. 30, 1996 12601

and 20 spectra at the sample potential, each spectrum being the sum of 100 interferograms recorded in 45 s. Results 1. 1 M NaOH. (a) Cyclic Voltammetry. The first cyclic voltammogram of Fe in quiescent 1 M NaOH at 50 mV s-1 between -0.23 and +0.57 V after a cathodic cleaning at -0.5 V for 15 min shows two anodic peaks at +0.01 and +0.19 V and only one cathodic peak at -0.15 V (Figure 1a, dashed line). The anodic charge is 3.28 mC cm-2, which corresponds to the electrooxidation of about 4 monolayers of Fe to the Fe(III) state. In the second sweep a third anodic peak and its cathodic counterpart, which increase with cycling, appear at +0.32 and -0.03 V, respectively, as is well-known4 (the third CV is shown as a dotted line in Figure 1a). The species produced in the anodic peak are mainly FeOOH and some hematite and maghemite, as has been established both by UV-vis potentialmodulated reflectance spectroscopy5 and by UV-vis differential reflectance spectroscopy.6 If after a cathodic cleaning of the electrode CO is bubbled through the solution for 10 min at an admission potential Eadm ) -0.03 V, at which there is a small cathodic stationary current which ensures that the Fe electrode remains reduced, and then a cyclic voltammogram between -0.33 and +0.57 V is recorded, it can be seen in Figure 1a (solid line) that the hydrogen evolution is shifted by 0.12 V toward more negative potentials by CO, and similarly the electrooxidation of Fe is shifted by 0.16 V toward more positive potentials by CO. Both inhibiting effects should be due to chemisorbed CO, which is electrooxidized in a well-defined peak at +0.25 V, after which the negative sweep practically coincides (but for the inhibition of H2 evolution) with that recorded in N2 atmosphere. The charge in the positive sweep is 4.51 mC cm-2, i.e., 38% higher than in the absence of CO, due to the electrooxidation of chemisorbed CO. As will be seen below, the IR band of the produced CO32- is so small that very little, if any, dissolved CO was electrooxidized. In a similar experiment CO was bubbled through the solution at Eadm ) -0.03 V during 10 min, but then the CO in solution was eliminated by N2 bubbling with the potential still held at -0.03 V. The first CV (solid line in Figure 1b) is similar to that obtained in a CO-saturated solution, but the second one (dashed line) is similar to the third CV in N2 atmosphere. From these experiments it can be concluded that CO chemisorbs on an oxide-free Fe surface in 1 M NaOH, inhibiting both hydrogen evolution and Fe oxidation. (b) FTIRS. After a cathodic cleaning of the electrode, CO was bubbled in the cell during 15 min at Eadm ) -0.03 V with the electrode as separated from the fluorite window as possible, after which the CO bubbling was stopped and the electrode pushed against the fluorite window. In the SW-FTIR spectrum (Figure 2a) the reference and sample potentials were -0.12 and +0.08 V, respectively. The potential interval was so short because of the limitations imposed by hydrogen evolution and chemisorbed CO electrooxidation on the negative and positive limits, respectively. The spectrum shows a bipolar band at 1936 and 1870 cm-1, which could equally well be assigned to linear or bridge CO. However, the results obtained at the lower pH values have helped us to assign it to linear CO. In the LPS-FTIRS experiment (Figure 2b), the first spectrum (reference) was taken at -0.065 V. Up to +0.175 V, the only feature that can be seen in the spectra is a bipolar band at the same frequency of the SW-FTIRS band of linear CO. With increasing potential this bipolar band progressively loses its

Figure 1. (a) First (dashed line) and third (dotted line) cyclic voltammograms (CVs) of Fe in quiescent 1 M NaOH at 50 mV s-1 between -0.23 and +0.57 V after a cathodic cleaning at -0.50 V for 15 min. The solid line is the first CV of Fe in 1 M NaOH, but after the cathodic cleaning CO was bubbled through the solution for 10 min at an admission potential Eadm ) -0.03 V, at which a small cathodic stationary current kept the Fe electrode reduced, and then a CV was recorded. (b) First (solid line) and second (dashed line) CVs of Fe in 1 M NaOH in which CO had been bubbled through the solution at Eadm ) -0.03 V during 10 min, after which the CO in solution was eliminated by N2 bubbling with the potential still held at -0.03 V.

bipolar character, becoming a positive band at 1916 cm-1 at 0.26 V, in the valley between the two peaks that appear in the CV at 1 mV s-1 in the FTIR cell (Figure 2c), which shows that chemisorbed CO is completely electrooxidized in the first peak. Obviously 1916 cm-1 is the stretching frequency of linear CO chemisorbed on Fe in 1 M NaOH at the reference potential of -0.065 V. The position of the CO band at the highest potential at which it could still be observed, taking as reference the

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Figure 2. FTIR spectra of Fe in CO-saturated 1 M NaOH. After a cathodic cleaning of the electrode, CO was bubbled in the cell during 15 min at Eadm ) -0.03 V with the electrode as separated from the fluorite window as possible, after which the CO bubbling was stopped and the electrode pushed against the fluorite window. (a) SW-FTIR spectrum of linearly chemisorbed CO, with reference and sample potentials of -0.12 and +0.08 V, respectively. (b) LPS-FTIR spectra obtained during a positive linear potential sweep at 1 mV s-1, the first spectrum (reference) being taken at -0.065 V. With increasing potential a bipolar band of linear CO evolves into a positive one at 1916 cm-1 as the chemisorbed CO is progressively electrooxidized. The positive bands at 1650 and 1410 cm-1 indicate the production of water and carbonate ion consequent to the electrooxidation of CO and Fe in alkaline media. (c) Positive sweep at 1 mV s-1 recorded during the LPS-FTIR experiment of (b).

spectrum at a potential at which chemisorbed CO had been completely electrooxidized, was 1950 cm-1 at 0.23 V. The anodic charge was 10.5 mC cm-2, which is 2.65 times higher than that in the sweep at 50 mV s-1 in a conventional cell, although the positive limit was 0.1 V lower in the former case. This anodic charge corresponds to an increase of the proton concentration of about 0.1 M, which would produce a negligible pH decrease. Two negative bands appear at 1650 cm-1 (from +0.175 V) and 1410 cm-1 (from +0.255 V), corresponding to an increase of water and carbonates, respectively, both produced by the electrooxidation of CO, which in alkaline media proceeds according to

CO + 4OH- f CO32- + 2H2O + 2e-

(1)

At alkaline pH the oxidation of Fe produces water as well, mainly through the reaction

Fe + 3OH- f FeOOH + H2O + 3e-

(2)

In the case of Ni in 1 M NaOH10 both the band of chemisorbed CO and that of water have a similar intensity to that observed here with Fe. However, the band of carbonate is about three times larger in the case of Ni, indicating that it is a better catalyst for the electrooxidation of dissolved CO than Fe. The band of water has about the same height for the two metals because the higher oxidizability (with the consequent production of water as indicated in reaction 2) of Fe, mostly to an electrocatalytically inactive oxyhydroxide, FeOOH, compensates its lower activity for dissolved CO electrooxidation.

CO Adsorption on Fe 2. 0.1 M Na2Ba4O7, pH 9.2. (a) Cyclic Voltammetry. The CV at 50 mV s-1 between -0.35 and +0.68 V of Fe in 0.1 M Na2B4O7 in N2 atmosphere after a cathodic cleaning at -0.50 V shows in the positive sweep an anodic shoulder at +0.07 V after which the current increases slightly with potential (dashed line in Figure 3a). In the negative sweep there appear a very flat cathodic maximum at +0.15 V and a peak at -0.33 V. The anodic charge is 2.94 mC cm-2. If after the cathodic cleaning at -0.50 V CO is bubbled in the cell at Eadm ) -0.11 V during 10 min, in the subsequent CV H2 evolution is strongly inhibited, and the rise of the anodic current is shifted toward more positive potentials by 0.25 V (solid line in Figure 3a). Both inhibiting effects are due to the presence of chemisorbed CO, which is electrooxidized in a welldefined peak at 0.31 V, after which the negative sweep approximately coincides with that in N2 atmosphere. The anodic charge is 3.18 mC cm-2, only 8% higher than in N2 atmosphere. If in a similar experiment N2 is bubbled through the solution in order to eliminate dissolved CO while still holding the potential at -0.11 V, the first CV shows (solid line in Figure 3b) an anodic peak at +0.21 V with shoulder at +0.40 V. The second CV (dashed line) is similar to that in N2 atmosphere. These experiments show that CO chemisorbs on an oxidefree Fe surface also at pH 9.2, poisoning both H2 evolution and the electrooxidation of Fe. (b) FTIRS. CO was bubbled through the solution at Eadm ) -0.11 V after the cathodic cleaning of the electrode. In the SW-FTIR spectrum with reference and sample potentials of -0.18 and +0.02 V, respectively, there appears a bipolar band at 1983 and 1947 cm-1, corresponding to CO chemisorbed in the linear form (Figure 4a). In the LPS-FTIRS experiment the first spectrum (reference) was taken at -0.11 V. An initially bipolar band becomes a positive band at 1972 cm-1 at 0.36 V (Figure 4b), in the valley between the two peaks in the simultaneously recorded CV (Figure 4c), indicating the complete electrooxidation of linear CO in the first voltammetric peak. Its intensity is very small, ∆R/R ) 0.2%, as compared with the value for CO-saturated polycrystalline Pt, 4.3%.7 Therefore it can be estimated that the CO coverage is of 5-10%. The frequency of the CO band increased up to 1989 cm-1 at 0.33 V, the highest potential at which it was still observed. We have found no quantitative IR absorption data for CO chemisorbed on a flat Fe surface, which could allow us to estimate more confidently the CO coverage on the Fe electrode. Older data correspond to transmission experiments with thin Fe films deposited on fluorite at low temperatures, known to consist of islands, since CO chemisorbed on thicker, homogeneous Fe films showed no IR absorption, due to the orthogonality of the CO dipole with the electric field of the normalincidence IR beam.8 Modern data with Fe single crystals have been obtained by electron energy loss spectroscopy (EELS), in which band intensities are given in arbitrary units. The charge in the anodic sweep was 7.15 mC cm-2, which corresponds to an increase in the proton concentration of 0.07 M. This would produce a considerable decrease of the pH in the thin layer, since the borax concentration was only 0.1 M, due to its limited solubility. This acidification is clearly apparent in the IR spectra (Figure 4b). So, an initially negative band at 1650 cm-1 indicates that water is being produced, as is typical of the oxidation of CO and Fe in alkaline media (reactions 1 and 2). However, with increasing potential the water band decreases down to zero and then becomes positive. This interesting reversal of behavior is due to the acidification of the thin layer, since in neutral and acidic media water is

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Figure 3. (a) First cyclic voltammogram (dashed line) at 50 mV s-1 between -0.36 and +0.68 V, of Fe in 0.1 M Na2B4O7, pH 9.2, in N2 atmosphere after a cathodic cleaning at -0.50 V. The solid line shows the effect of bubbling CO in the cell at Eadm ) -0.11 V during 10 min after the cathodic cleaning at -0.50 V CO. (b) First (solid line) and second (dashed line) CVs of Fe in 0.1 M borax in which CO had been bubbled at Eadm ) -0.11 V for 10 min, after which N2 was bubbled through the solution in order to eliminate dissolved CO while still holding the potential at -0.11 V.

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Figure 4. FTIR spectra of Fe in CO-saturated 0.1 M Na2B4O7, pH 9.2. CO was bubbled through the solution at Eadm ) -0.11 V after a cathodic cleaning of the electrode. (a) SW-FTIR spectra of linear CO, with reference and sample potentials of -0.18 and +0.02 V, respectively. (b) LPS-FTIR spectra recorded during a positive linear potential sweep at 1 mV s-1, the first spectrum (reference) being taken at -0.11 V. The positive band at 1972 cm-1 corresponds to linear CO. The initially negative and then positive band at 1650 cm-1 indicates that initially water is produced at pH 9.2 but later is consumed as the medium is acidified by the electrooxidation of CO and Fe in alkaline media. The two negative bands at 1411 and 1158 cm-1 are due to the increase of the concentration of boric acid produced by the acidification of the medium. (c) Positive sweep recorded during the LPS-FTIR experiment of (b). (d) Dependence on potential of the frequency of the absorption band of CO chemisorbed on Fe at pH 9.2 (two sets of results), pH 14, and pH 6.9.

consumed through reactions such as

CO + H2O f CO2 + 2H+ + 2e-

(3)

Fe + 2H2O f FeOOH + 2H+ + 2e-

(4)

and

while the opposite behavior obtains in alkaline media, in which water is produced at the expense of hydroxyl ions through reactions such as (1) and (2). The change of sign of the water band occurs in the descending flank of the first peak of the LPS (Figure 4c), which shows that the change of sign is simply related to the amount of anodic charge passed and not to the onset of a different reaction. The same change of sign of the water band was observed in the absence of CO, which shows that electrooxidation of CO is

negligible as compared with that of Fe, as was to be expected from this base metal. No band of bicarbonate (at 1364 cm-1) is seen, since it would be swamped by the large band of boric acid increase at 1411 cm-1. Two negative bands at 1411 and 1158 cm-1 correspond to an increase of the concentration of boric acid, due to the acidification of the thin layer:

H2BO3- + H+ f H3BO3

(5)

Conversely, there is a positive band at 1354 cm-1 corresponding to a decrease of the concentration of H2BO3-. Another, broad positive band of borate decrease at about 1141 cm-1 is split into two by the negative band of boric acid increase at 1158 cm-1, the convolution of the two bands yielding a W-shaped band. The same boric acid/borate bands are seen in the absence

CO Adsorption on Fe of CO, showing again that reaction 3 is negligible as compared to reaction 4. In this borax medium, pH 9.2, the band of chemisorbed CO was observed over a fairly wide potential interval, 0.4 V, as can be seen in Figure 4d, where results at pH 14 and pH 6.9 are also included. There was a difference of 5-10 cm-1 between duplicate experiments in borax, probably due to differences in CO coverage. The plot of CO stretching frequency vs potential is a smooth curve, which makes it impossible to calculate the Stark shift. It is possible that the curvature is due to a decrease of the CO coverage with increasing potential, although the lack of a good base line did not allow us to measure the intensity of the CO band with the precision required to establish if there was such a decrease of the CO coverage. At the lower potentials the Stark shift would be much higher than that for linear CO chemisorbed on lowindex Pt and Rh single crystal faces.9 3. 0.05 M K2HPO4 + 0.05 M KH2PO4, pH 6.9. (a) Cyclic Voltammetry. The CV at 50 mV s-1 between -0.54 and +0.67 V of iron in phosphate buffer in N2 atmosphere, after a cathodic cleaning at -0.54 V, shows an anodic peak at +0.05 V and three cathodic peaks at +0.22, -0.32, and -0.46 V (dashed line in Figure 5a). The anodic charge between the zero crossing and the valley at 0.33 V is 3.12 mC cm-2. If, after the cathodic cleaning at -0.54 V, CO is bubbled through the solution at this potential during 10 min, after which the potential is increased to -0.34 V and the cathodic current is allowed to become stationary, a subsequent CV (solid line in Figure 5b) shows a strong inhibition of H2 evolution and a shift of the rise of the anodic current by 0.30 V toward more positive potentials, both effects being due to the presence of chemisorbed CO, whose electrooxidation produces a well-defined peak at 0.35 V. The charge of the anodic peak (between the zero crossing and the valley at 0.48 V) is 5.30 mC cm-2, 70% higher than in N2 atmosphere. The anodic peak at +0.22 V in the negative sweep indicates hysteresis. A similar experiment to the above one was carried out, but afterwards N2 was bubbled in the cell during 15 min in order to eliminate dissolved CO. The first CV (solid line in Figure 5b) is similar to, but smaller than, that obtained in a COsaturated solution. The second CV (dashed line) does not show the passivation observed in the first sweep in the absence of CO (dashed line in Figure 5a). Again, these voltammetric experiments show that also at pH 6.9 CO chemisorbs on iron, inhibiting both H2 evolution and Fe electrooxidation. (b) FTIRS. The reference and sample potentials for the SWFTIR spectrum were -0.26 and -0.06 V, respectively. The spectrum shows a bipolar band at 1984 and 1949 cm-1, clearly in the linear CO region (Figure 6a). The cell current recorded during the LPS-FTIR experiment increased in an approximately linear way. The absence of a passivation peak was undoubtedly due to the acidification of the thin layer, since the anodic charge was 115 mC cm-2, corresponding to an increase of the proton concentration of 1.15 M. On the contrary, the CV (not shown) at 1 mV s-1 in a conventional cell showed a well-defined passivation peak, as at 50 mV s-1. The first spectrum (reference) for the LPS-FTIRS experiment was taken at -0.18 V. A positive CO band appears at 1985 cm-1, this being the stretching frequency for linear CO chemisorbed on iron in phosphate buffer at the reference potential of -0.18 V. Its intensity could not be determined, since the band was severely deformed by the neighboring bands of water at higher frequency and H3PO4 at lower frequency.

J. Phys. Chem., Vol. 100, No. 30, 1996 12605

Figure 5. (a) First cyclic voltammogram (dashed line) at 50 mV s-1 between -0.54 and +0.67 V of Fe in phosphate buffer, pH 6.9, in N2 atmosphere, after a cathodic cleaning at -0.54 V. If, after the cathodic cleaning at -0.54 V, CO was bubbled through the solution at this potential during 10 min, after which the potential was increased to -0.34 V and the cathodic current was allowed to become stationary, the subsequent CV between -0.35 and +0.67 V shows considerable changes (solid line). (b) First (solid line) and second (dashed line) CVs of Fe in phosphate buffer in which CO had been bubbled through the solution at Eadm ) -0.54 V during 10 min, after which the CO in solution was eliminated by N2 bubbling with the potential still held at -0.54 V.

The frequency of the CO band at the highest potential, 0.33 V, at which it could still be observed was 1998 cm-1.

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Figure 7. First cyclic voltammogram (dashed line) of Fe in N2saturated Na2SO4 + H2SO4, pH 3, after a cathodic cleaning at -0.50 V. Corrosion of Fe started at -0.14 V and then increased at a high rate. The solid line shows the first CV after bubbling CO through the solution at Eadm ) -0.50 V after the cathodic cleaning.

Figure 6. FTIR spectra of Fe in CO-saturated phosphate buffer, pH 6.9. (a) SW-FTIR spectra of linear CO, with reference and sample potentials of -0.26 and -0.06 V, respectively. (b) LPS-FTIR spectra recorded during a positive LPS at 1 mV s-1. The first spectrum (reference) was taken at -0.182 V. The positive CO band at 1985 cm-1 is due to linear CO. The two negative bands at 1035 and 1900 cm-1 correspond to an increase of the concentration of H3PO4 and the negative band at 1160 cm-1 to an increase of the concentration of H2PO4-. Correspondingly, a positive band at 1095 cm-1 indicates that the concentration of HPO42- has decreased. These artifacts are due to the acidification produced mainly by the electrooxidation of iron, since the very small negative band of CO2 indicates that very little, if any, dissolved CO was electrooxidized. The positive band at 1650 cm-1 and a broad, also positive, band around 2100 cm-1 indicate that water is consumed in the electrooxidation of Fe in this neutral medium.

The strong acidification of the thin layer is clearly reflected in the IR spectra (Figure 6b). Two negative bands, a sharp one at 1035 cm-1 and a broad one at 1900 cm-1, correspond to an increase of the concentration of H3PO4, and the negative band at 1160 cm-1 indicates that the concentration of H2PO4has also increased. Correspondingly, a positive band at 1095 cm-1 indicates that the concentration of HPO42- has decreased. These artifacts are obviously due to the acidification of the thin layer of solution between the electrode and the fluorite window produced mainly by the electrooxidation of iron, since the very small negative band of CO2 at 2344 cm-1 indicates that, besides the chemisorbed CO, very little, if any, dissolved CO was electrooxidized.

In the case of Ni in the same medium,10 the CO2 band is about eight times higher than that in Fe, indicating a much higher electrocatalytic activity of Ni for dissolved CO electrooxidation. On the contrary, the intensities of the negative bands at 1035 cm-1 (formation of H3PO4) and 1160 cm-1 (H2PO4- increase) were lower in Ni than in Fe, and consequently the intensity of the positive band at 1095 cm-1 (HPO42- decrease) was also lower in Ni than in Fe, i.e., acidification of the thin layer was smaller in Ni, in agreement with its higher corrosion resistance. The presence of H3PO4, whose first ionization constant is 2.14,11 confirms a strong acidification of the thin layer. A positive band at 1650 cm-1 and a broad, also positive, band around 2100 cm-1 indicate that water is disappearing, as is typical of electrooxidation processes in neutral and acidic media (see reactions 3 and 4 above). 4. Na2SO4 + H2SO4, pH 3. (a) Cyclic Voltammetry. Sulfuric acid was added to a 0.5 M Na2SO4 solution until its pH decreased to 3. In this solution, deaerated by N2 bubbling, the CV (dashed line in Figure 7) shows that electrooxidation of Fe started at -0.14 V, and from 0.23 V increased linearly with potential, with an equivalent resistance of only 3.2 Ω cm2. This ohmic behavior is possibly due to current limitation by the ohmic drop in the electrolyte. The anodic charge, 507 mC cm-2, indicated heavy corrosion. After CO was bubbled through the solution at Eadm ) -0.50 V, hydrogen evolution was inhibited, and the electrooxidation of Fe started at a 0.20 V more positive potential, which decreased the anodic charge by 14% (solid line in Figure 7). (b) FTIRS. CO was bubbled at Eadm ) -0.5 V. The reference and sample potentials for the SW-FTIR spectrum were -0.43 and -0.23 V, respectively. The spectrum shows (Figure

CO Adsorption on Fe

J. Phys. Chem., Vol. 100, No. 30, 1996 12607 be acidified if the corrosion product is Fe2+ or Fe3+ ion, or a precipitate of them with an unprotonated anion such as SO42-. The positive band at 1650 cm-1 and the broad positive band around 2100 cm-1 indicate that water is consumed, probably as hydration water of the iron ions or of the precipitate, since it was shown above that the thin layer was not acidified, and consequently water was not consumed in the electrooxidation of Fe. Discussion

Figure 8. FTIR spectra of Fe in Na2SO4 + H2SO4, pH 3, saturated with CO at Eadm ) -0.50 V. (a) SW-FTIR spectra of linear CO, with reference and sample potentials of -0.43 and -0.23 V, respectively; (b) LPS-FTIR spectra recorded during a positive LPS at 1 mV s-1. The positive band at 1978 cm-1 corresponds to linear CO. A negative band at 1141 cm-1 of a green FeSO4 precipitate is split into two by a positive band at 1100 cm-1, due to a decrease of the concentration of SO42- ion, precisely by precipitation as FeSO4. Water consumption is evidenced by the positive band at 1650 cm-1 and the broad positive band at about 2100 cm-1.

8a) a bipolar band at 1992 and 1965 cm-1, clearly indicating that CO is chemisorbed in the linear form. In the LPS-FTIR spectrum (Figure 8b) there is a positive band at 1978 cm-1, this being therefore the stretching frequency of linear CO chemisorbed on Fe at pH 3 at the reference potential of -0.31 V. A very small negative band at 2344 cm-1 indicates the production of a very small amount of CO2, indicating again that Fe is far less active than Ni for the electrooxidation of dissolved CO. A negative band at about 1100 cm-1 of a green FeSO4 precipitate is split into two at 1141 and 1071 cm-1 by a positive band, also at 1100 cm-1, due to a decrease of the concentration of free SO42- ions in solution, precisely due to precipitation as FeSO4. This is confirmed by the absence of a negative band at 1195 cm-1 of HSO4- concentration increase which should appear if the loss of SO42- ions were due to acidification of the medium. It should be noted that in spite of the large anodic charge, 1630 mC cm-2, the medium will not

Inhibition by Chemisorbed CO of H2 Evolution and Fe Electrooxidation. It is clear that bubbling CO in the cell produces a negative shift of the H2 evolution current and a positive shift of the anodic peak of Fe passivation. The inhibition of both processes should be originated by chemisorbed CO, showing that a cathodic pretreatment is enough to reduce, at least partially, the native oxides on the Fe electrode and provide a clean metallic surface on which CO can chemisorb. The question emerges of why a small coverage of CO is so efficient in both cases. The simplest answer is that actually only a small fraction of the native oxides on the Fe surface is reduced by the cathodic pretreatment and that it is precisely this small fraction the only active area on which H2 evolution, the start of Fe electrooxidation, and CO chemisorption takes place. In the simpler case of Pt, Kita et al.12 have found that chemisorbed CO inhibits H2 evolution on Pt and that the apparent exchange current density is proportional to the fraction of free surface, which means that chemisorbed CO has a blocking effect only and that H2 evolution takes place at sites randomly distributed over the Pt surface. Probably this situation also applies in our case, with the difference that even in the absence of chemisorbed CO H2 evolution would take place on the oxide-free Fe surface only. Assignment of the Band of CO Chemisorbed on Fe. A peak at 1916-1993 cm-1 of chemisorbed CO is observed in the FTIR spectra of the electrode for the whole pH range studied, 3-14. On the contrary, experiments in which a stream of CO gas was passed over the freshly polished Fe electrode in the FTIR cell in the absence of solution failed to reveal any band of chemisorbed CO, which proves that the Fe surface was covered by a native oxide layer which impeded the chemisorption of CO. The frequency of the observed band of chemisorbed CO varied over the range of 1916-1993 cm-1. The first value was that at pH 14, the CO band appearing at or above 1972 cm-1 at pH e 9.2. This shift of the frequency of the CO band toward lower frequencies with increasing pH is in agreement with the linear correlation of the CO frequency with the electrode potential vs SHE found by Kunimatsu et al.13 for CO on Au in 0.2 M NaOH and in 1 M HClO4: all the points fell in a straight line with a slope of 64 cm-1 V-1 over the range 1935-2050 cm-1. Beden et al.14 give the following ranges for the frequency of CO chemisorbed on noble metals in contact with an aqueous solution: 2020-2090 cm-1 for linear CO, 1900-1960 cm-1 for bridge CO, and 1750-1880 cm-1 for multiply-bonded CO. Although the pH was not specified, nearly all the studies have been carried out in acidic media. Certainly these frequency ranges are not to be taken as absolute, since other authors give different values. In any case, if these values for noble metals were also valid for the ferrous metals, the observed frequency range for CO chemisorbed on Fe at pH e 9.2, 1972-1993 cm-1, would fall in the no man’s land between bridge and linear CO, which renders difficult its assignment.

12608 J. Phys. Chem., Vol. 100, No. 30, 1996 As far as we know no IR spectra have been reported before for CO chemisorbed on Fe in an electrolytic environment, and therefore we must resort to results for the solid/gas interface. Sheppard and Nguyen,15 after reviewing the results of gaseous CO chemisorption on evaporated Fe films and on supported Fe particles, conclude that the two absorption bands that appear near 2020 and 1950 cm-1, the latter frequently being the dominant one, should be assigned to linear and bridge CO, respectively. It is well-known that the spectra at the solid/liquid interface are shifted several tens of cm-1 toward lower wavenumbers relative to the spectra at the solid/gas interface,16 and consequently it can be safely assumed that the CO band observed here at 1972-1993 cm-1 corresponds to CO linearly chemisorbed on Fe. 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) Muscat, J. P.; Newns, D. M. Prog. Surf. Sci. 1978, 9, 1. (2) Shinn, N. D.; Madey, T. E. Phys. ReV. Lett. 1984, 53, 2481.

Cuesta and Gutie´rrez (3) Maruyama, T.; Sakisaka, Y.; Kato, H.; Aiura, Y.; Yanashima, H. Surf. Sci. 1994, 304, 281. (4) Burke, L. D.; Murphy, O. J. J. Electroanal. Chem. 1980, 109, 379. (5) Larramona, G.; Gutie´rrez, C. J. Electrochem. Soc. 1989, 136, 2171. (6) Gutie´rrez, C.; Beden, B. J. Electroanal. Chem. 1990, 293, 253. (7) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G., II; Philpott, M. R. Langmuir 1986, 2, 464. (8) Baker, F. S.; Bradshaw, A. M.; Pritchard, J.; Sykes, K. W. Surf. Sci. 1968, 12, 426. (9) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (10) Cuesta, A.; Gutie´rrez, C. To be published. (11) Albert, A.; Serjeant, E. P. The determination of ionization constants. A laboratory manual; Chapman and Hall: London, 1984. (12) Kita, H.; Ye, S.; Sugimura, K. J. Electroanal. Chem. 1991, 297, 283. (13) Kunimatsu, K.; Aramata, A.; Nakajima, H.; Kita, H. J. Electroanal. Chem. 1986, 207, 293. (14) Beden, B.; Lamy, C.; de Tacconi, N. R.; Arvı´a, A. J. Electrochim. Acta 1990, 35, 691. (15) Sheppard, N.; Nguyen, T. T. AdVances in Infrared and Raman Spectrosocpy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 5, Chapter 2. (16) Beden, B.; Lamy, C. Spectroelectrochemistry. Theory and Practice; Gale, R. J., Ed.; Plenum Press: New York, 1988; Chapter 5.

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