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Langmuir 1996, 12, 1094-1097
Notes Carbon Monoxide Adsorption on Copper and Silver Electrodes during Carbon Dioxide Electroreduction Studied by Infrared Reflection Absorption Spectroscopy and Surface-Enhanced Raman Spectroscopy Ichiro Oda, Hirohito Ogasawara,† and Masatoki Ito* Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku Yokohama 223, Japan Received March 3, 1995. In Final Form: October 27, 1995
Introduction The electrocatalysis of CO2 has been investigated on several transition metal electrodes (Pt, Ni, Au, Ag, and Cu).1-3 A Cu electrode especially promotes hydrocarbon formation. Ag and Au electrodes, however, have no activity for hydrocarbon formation. Hori and co-workers1 have reached the conclusion that CO formation is the first elemental step of an electroreduction of CO2 on a Cu electrode. From this aspect, it is important to study the behavior of adsorbed CO on these surfaces. Several workers reported the vibrational spectroscopic results on the reaction of CO or CO2 with Cu surfaces under UHV.4,5 For the CO adsorbed on a Cu, Ag, or Au electrode surface, however, the vibrational results are insufficient.6-11 So far, little direct spectroscopic evidence for the formation of adsorbed CO on either a Cu or Ag electrode in the electroreduction of CO2 has been reported.12,13 In this report, the adsorbed species were studied during the course of CO2 electroreduction on both Cu and Ag electrodes by use of vibrational spectroscopies, infrared reflection absorption spectroscopy (IRAS) and surface-enhanced Raman spectroscopy (SERS). Prior to a CO2 electroreduction, an adsorption of CO on a Cu or Ag electrode was investigated, because CO is known to be an important intermediate in the CO2 electroreduction. Then, the vibrational evidence of CO formation from CO2 on a Cu or Ag electrode was exhibited. The species other than CO in CO2 reduction were also presented on a Cu electrode. † Present address: Surface Chemistry Laboratory, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-01, Japan.
(1) Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 1695. (2) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. J. Chem. Soc., Chem. Commun. 1988, 17. (3) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. J. Am. Chem. Soc. 1987, 109, 5022. (4) Taylor, P. A.; Rasmussen, P. B.; Chorkendorff, I. J. Vac. Sci. Technol., A 1992, 10, 2570. (5) Chadwick, D.; Zheng, K.; Pritchard, J. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 304. (6) Chang, S. C.; Hamelin, A.; Weaver, M. J. Surf. Sci. 1990, 239, L543. (7) Ikezawa, Y.; Saito, H.; Matsubayashi, H.; Toda, G. J. Electroanal. Chem. 1988, 252, 395. (8) Furukawa, H.; Takahashi, M.; Ito, M. J. Electroanal. Chem. 1990, 280, 415. (9) Leung, L.-H.; Weaver, M. J. J. Am. Chem. Soc. 1987, 109, 5113. (10) Leung, L.-H.; Weaver, M. J. Langmuir 1988, 4, 1076. (11) Westerhoff, B.; Holze, R. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 418. (12) Hori, Y.; Murata, A.; Tsukamoto, T.; Wakabe, H.; Koga, O.; Yamazaki, H. Electrochim. Acta 1994, 39, 2495. (13) Ichinohe, Y.; Wadayama, T.; Hotta, A. J. Raman Spectrosc. 1995, 26, 335.
0743-7463/96/2412-1094$12.00/0
Experimental Section IRAS and SERS measurements were conducted in a 0.05 M Na2SO4 or 1 M KCl aqueous solution at room temperature. These solutions were prepared with super special grade Na2SO4 or KCl (Suprapur, Merck) and ultrapure water (Milli-Q SP TOC system, Millipore Inc.). For infrared measurements Ag and Cu surfaces were prepared by use of an electrodeposition on a platinum electrode substrate. The Pt(110) electrode (8 mm in diameter and 2 mm in thickness) was immersed in a solution containing 0.01 M CuSO4 or AgClO4 until the desired amounts of Cu or Ag were coated, and the electrode was subsequently rinsed. The amounts of deposited metal were determined by oxidation current densities of Cu0 to Cu2+ or Ag0 to Ag+. CO or CO2 gas was introduced into solution by the bubbling technique. CO gas was normally introduced for 30 s at -0.6 V. CO2 gas was introduced for 5 min at -0.8 V where reduction current was observed. The IR cell was attached to a 1720X Fourier transform infrared spectrometer (Perkin-Elmer Inc.) with either a liquid nitrogencooled InSb or HgCdTe detector (Infrared Associates Inc.). The spectra were normally obtained by an accumulation of 64 scans with a 2 cm-1 resolution or 256 scans with an 8 cm-1 resolution. In subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) measurements,14,15 each spectrum was obtained by an accumulation of 32 scans with an 8 cm-1 resolution, and normalized spectra were obtained by coadding 256 scans at both reference and sample potentials, switching between the reference and sample potential a total of 8 times. The Raman spectrometer system used was a RAMAN ONE (Chromex Inc.) with a charge-couple device detector (Photometrics Ltd.) cooled to 105 K. The Raman excitation wavelength was 632.8 nm from a He-Ne laser (NEO ARK Inc.). Before SERS measurements, a working electrode was subjected to an oxidation-reduction cycle (ORC). The electrode potential was cycled from -0.2 to 0.38 V (Ag) or 0.7 V (Cu) and back to the desired voltage at a rate of 0.05 V/s. SER spectra were plotted out by subtracting each spectrum before ORC. All the electrode potentials in this paper were quoted against the standard hydrogen electrode (SHE).
Results and Discussion 1. Carbon Monoxide Adsorption on Copper and Silver Electrodes. Figure 1a shows Ag coverage dependent IRAS spectra. The Ag atom was electrochemically deposited on a Pt(110) substrate at -0.6 V in a 0.05 M Na2SO4 solution. Until θAg ) 0.5 ML (the θ value indicates a ratio of Ag atoms to substrate Pt atoms, ML ) monolayer), a linear CO (not shown in the figure) was selectively adsorbed on a ridge Pt atom along the [11h 0] direction.16-18 The absorption for the CO at 2046 cm-1 was strong and remarkably sharp. At θAg ) 2.5 and 7 ML, the linear CO on the ridge Pt atom was hardly seen since Ag atoms completely covered the Pt substrate. Instead, a linear CO adsorbed on a Ag atom at θAg ) 2.5 and 7 ML was observed at 1995 and 1980 cm-1, respectively. A recent report on an initial growth of Ag/Pt(110) under (14) Bewick, A.; Pons, S. B. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley-Heyden: London, 1985; Vol. 12, p 1. (15) Pons, S.; David, T.; Bewick, A. J. Electroanal. Chem. 1984, 160, 63. (16) Hoffman, P.; Bare, S. R.; King, D. A. Phys. Scr. 1983, T4, 118. (17) Watanabe, S.; Kinomoto, Y.; Kitamura, F.; Takahashi, M.; Ito, M. J. Electron Spectrosc. 1990, 54/55, 1205. (18) Ogasawara, H.; Inukai, J.; Ito, M. Chem. Phys. Lett. 1992, 198, 389.
© 1996 American Chemical Society
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Figure 1. IRAS spectra of (a) CO on a Ag-modified Pt(110) electrode in a 0.05 M Na2SO4 solution as a function of θAg, electrode potential -0.6 V/SHE, and (b) CO on a Cu-modified Pt(110) electrode in a 0.05 M Na2SO4 solution as a function of θCu, electrode potential -0.6 V/SHE. SER spectra of (c) CO on a Ag electrode in a 3.5 M KCl solution, electrode potential ) -0.6 V/SHE, and (d) CO on a Cu electrode in a 3.5 M KCl solution, electrode potential -0.6 V/SHE.
UHV supports the present result.19 While a CO adsorption on a Ag bulk electrode has hardly been reported, the band intensity in the present spectra was not so weak. The result suggests the existence of bimetal properties derived from the thin Ag film on a Pt(110) substrate. The fact was confirmed by the drastic decrease in intensity with increasing Ag adlayers. Therefore, the adsorption site of CO on a Ag electrode is quite limited (defect, etc.). The results on a Cu electrode differed from that on a Ag electrode. The IR spectra for CO on a Cu electrode in a 0.05 M Na2SO4 solution are presented in Figure 1b as a function of θCu. In contrast to the results on Ag, two kinds of linear CO on Cu appeared at ∼2100 and 1985 cm-1. There is a large body of literature concerning underpotential deposition (UPD) of Ag or Cu on Pt(hkl). No remaining free Pt sites exist in the Ag or Cu multilayers (θmetal > 1). Thus two kinds of absorptions are attributed to CO on a Cu surface. Similar to the results on a Ag, the intensities were reduced with the further increase of θCu. In our previous report,18 the bands at ca. 2000 and ca. 2100 cm-1 were assigned to CO adsorbed on terrace and adatom defect Cu atoms, respectively. The results agreed well with those for CO adsorbed on a Cu electrode by Westerhoff and Holze.11 They reported a single absorption at 2110 cm-1. Therefore, it is of importance to examine CO adsorption on a bulk Ag or Cu electrode, since the bands of CO in Figure 1a and b might still have an influence of the platinum substrate underneath. Figure 1c and d shows the SERS results of CO adsorbed on polycrystalline bulk Ag and Cu electrodes, respectively. Although the bands at 2084 and 1940 cm-1 in SERS are fairly low in frequency, compared with those in IRAS, the bands are definitely assignable to CO adsorbed on a Cu electrode. (19) Shern, C. S.; Chang, D. U.; Shyu, K. D.; Tsay, J. S.; Fu, T.-Y. Surf. Sci. 1994, 318, 262.
Figure 2. IRAS spectra of (a) CO on a Cu electrode in a 0.05 M Na2SO4 solution as a function of electrode potential, θCu ) 6 ML. SER spectra of (b) CO on a Cu electrode in a 3.5 M KCl solution as a function of electrode potential.
The electrode potential dependent IR spectra for CO on a Cu electrode in a 0.05 M Na2SO4 solution are presented in Figure 2a (the result on θCu ) 6 ML is shown). After an introduction of CO at -0.6 V, the electrode potential was subsequently swept stepwise in the positive direction. Two kinds of linear CO were again observed at each potential. With the anodic potential sweep, the band at ca. 2100 cm-1 shifted gradually to a higher wavenumber with the decrease in intensity. The band at ca. 2000 cm-1 also showed the remarkably large shift by 90 cm-1/V to higher wavenumbers. The corresponding SER spectra for CO on a polycrystalline Cu electrode in a 3.5 M KCl solution are shown in
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Figure 3. SNIFTIRS spectra of (a) CO2 reduction on a Cu electrode in a 0.05 M Na2SO4 solution, reference potential ) -0.3 V/SHE and θCu ) 5 ML, and (b) CO2 reduction on a Ag electrode in a 0.05 M Na2SO4 solution, reference potential ) -0.3 V/SHE ) θAg ) 5 ML.
Figure 2b. Except for the band appearance at lower wavenumbers by 50 cm-1 compared with that of IRAS, the spectral features were quite similar. The lower frequency CO band again exhibited the marked blue shift by 110 cm-1/V with an anode sweep. The electrode potential dependence of CO adsorbed on a Ag electrode was also studied by IRAS and SERS. CO on Ag yielded a single band at ca. 1950 cm-1. The band also shifted to higher wavenumbers with a positive potential sweep. On both Ag and Cu electrodes, we could not determine the coverage of adsorbed CO since the current due to CO oxidation was hardly detectable in the current-potential curve. The present results indicate that CO coverages at the Cu or Ag surface are remarkably small. 2. Carbon Dioxide Reduction at Copper and Silver Electrodes. The current-potential curve was examined for a Cu electrode in a 0.05 M Na2SO4 solution purged by CO2. The reduction current peak derived from CO2 was observed between -0.6 and -0.9 V with a total charge of 83.5 µC/cm2. Figure 3a shows SNIFTIR spectra during CO2 reduction on Cu. The reference potential for SNIFTIRS was chosen at -0.3 V where no reduction or oxidation currents were observed. Both spectra have a bipolar characteristic band ascribed to CO on Cu: an upward peak at ca. 2010 cm-1 and a downward peak at ca. 1980 cm-1. The bipolar character indicates that both reference and sample spectra have an absorption at ca. 2000 cm-1. The band coincided with CO adsorbed at ca. 2000 cm-1 as seen in the previous section. The existence of an upward peak shows that CO derived from CO2 at a negative sample potential did not desorb at the reference potential of -0.3 V. The higher frequency band of CO at ca. 2100 cm-1 was not detectable. While the adsorbed CO at ca. 2000 cm-1 showed a large frequency shift with the electrode potential change, the amount of the shift for the higher frequency band at ca. 2100 cm-1 was not significant. Therefore the spectral difference of the band around 2100 cm-1 upon the potential change became quite small, resulting in no signal in SNIFTIRS. Some characteristic bands were also seen in the region 1200-1800 cm-1. At -0.5 V, a band was observed at 1370 cm-1. At -0.7 V where reduction currents were observed in the current-
Figure 4. SER spectra of (a) CO2 reduction on a Cu electrode in a 3.5 M KCl solution and (b) CO2 reduction on a Ag electrode in a 3.5 M KCl solution.
potential curve, other bands appeared at 1542 and 1290 cm-1. The bands at 1370 and 1542 cm-1 were also observed in a 0.05 M Na2CO3 solution. Thus, these bands were assignable to the vibration of CO32- produced at the electrode. Figure 3b shows the SNIFTIR spectra for a Ag electrode surface. At -0.5 V, no absorption was observed. At -0.7 V, a downward peak appeared at 1954 cm-1. The band was ascribed to a linear CO derived from CO2 on Ag. In contrast to the results on a Cu electrode surface, no extra absorption was observed in the lower wavenumber region. The SER spectra of CO2 at Cu and Ag polycrystalline electrodes in a 3.5 M KCl solution obtained with 632.8 nm laser excitation are shown as a function of electrode potential in Figure 4a and b, respectively. In Figure 4a (on Cu), the bands observed were 1595 and 283 cm-1 at
Notes
a more positive potential than -1.0 V. The assignments of those bands are straightforward: H2O bending (δHOH) of adsorbed water and Cu-Cl stretching (νCuCl) of adsorbed chloride anion. The spectrum at -1.4 V indicates an appearance of 2000 and ∼360 cm-1 bands at the expense of the 280 cm-1 band. The bands are also assignable to C-O stretching (νCO) and Cu-CO stretching (νCuC). Therefore, CO2 reduction to CO started at this potential, along with the desorption of Cl- on the surface. The electroreduction developed at the further negative potential of -1.6 V. The SERS intensities were much reduced during the negative potential sweep and almost extinguished at around -1.8 V due to a loss of SERS active sites. We see at least two different CO bands, a sharper band at ∼2082 cm-1 and broad one at 2037-1998 cm-1. The latter band showed a large frequency shift with the potential sweep. The spectral features are quite similar to those of IRAS in spite of the different treatment of the surfaces and the electrolytes used. The band at 1054 cm-1 can be assignable to a totally symmetric stretching of CO32produced at the surface during the electroreduction. The result on a Ag electrode in Figure 4b was more simple: the bands at 1608 and 249 cm-1 at -0.6 V are δHOH of water and νAgCl, respectively. The reduction started
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at -1.0 V, where the band arose at 1940 cm-1 (νCO) and 520-420 cm-1 (νAgC(CO)). At the more negative potential, the bands increased in intensities with lower frequency shifts. Therefore the behavior of adsorbed CO during CO2 reduction on Cu and Ag electrodes is quite different. On a Cu electrode surface, CO was not desorbed between -0.7 and -0.3 V. However, on a Ag electrode surface, CO was desorbed by the positive potential sweep from -0.7 to -0.5 V. Conclusions Two kinds of linear CO at ca. 2100 and ca. 2000 cm-1 are observed on a Cu electrode, while on a Ag electrode a linear CO was located at ca. 2000 cm-1. On both Ag and Cu electrodes, the band at ca. 2000 cm-1 showed the large frequency shift with the potential. The electroreduction of CO2 produced linear CO on both Cu and Ag electrodes. The desorption of CO was seen above ca. -0.5 V on a Ag electrode. CO32- species were also observed at a Cu electrode. LA950167J
