pubs.acs.org/Langmuir © 2009 American Chemical Society
Cyclic Voltammetry, FTIRS, and DEMS Study of the Electrooxidation of Carbon Monoxide, Formic Acid, and Methanol on Cyanide-Modified Pt(111) Electrodes Angel Cuesta,*,† Maria Escudero,† Barbora Lanova,‡ and Helmut Baltruschat‡ ‡
† Instituto de Quimica Fisica “Rocasolano”, CSIC, C. Serrano 119, E-28006 Madrid, Spain, and :: :: :: Institut fur Physikalische und Theoretische Chemie, Universitat Bonn, Romerstrasse 164, D-53117 Bonn, Germany
Received December 15, 2008. Revised Manuscript Received February 11, 2009 We have used cyanide-modified Pt(111) electrodes, in combination with cyclic voltammetry (CV ), Fourier transform infrared spectroscopy (FTIRS), and differential electrochemical mass spectrometry (DEMS), to investigate the oxidation of formic acid and methanol on Pt electrodes. Since CO is the poison intermediate formed during the oxidation of both formic acid and methanol, we have previously characterized the CO adlayer on cyanide-modified Pt(111) electrodes. Poison formation on cyanide-modified Pt(111) is nearly completely inhibited in the case of formic acid and methanol, the corresponding electro-oxidation reaction proceeding, hence, exclusively through the reactive intermediate pathway. These results suggest that, in the oxidation of formic acid and methanol, the formation of adsorbed CO would require the presence of at least three contiguous Pt atoms.
1. Introduction Carbon monoxide, formic acid, and methanol are without a doubt within the most studied molecules in electrocatalysis, due to their relevance in fuel-cell technology: formic acid and methanol are promising fuels (formic acid is, in addition, the simplest possible organic fuel and has, hence, often been considered as a model to elucidate the oxidation mechanism of more complex organic molecules), and carbon monoxide is a catalytic poison produced during the oxidation of organic molecules or present in hydrogen produced by methanol reforming. It is well known that on platinum electrodes both formic acid and methanol undergo parallel electrooxidation paths:1-8 a relatively fast one, usually known as the direct reaction pathway, and a very slow one, involving a strongly adsorbed intermediate acting as an electrocatalytic poison. The advent of in situ infrared reflectance spectroscopy at the beginning of the 1980s allowed the identification of CO as the poisoning intermediate,9,10 but the identification of adsorbed formate as the active intermediate in the direct reaction pathway, in both *To whom correspondence should be addressed. E-mail: a.cuesta@iqfr. csic.es. (1) Breiter, M. In Electrochemical Processes in Fuel Cells; Springer Verlag: Berlin, Germany, 1969. (2) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973, 45, 205. (3) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (4) Herrero, E.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. 1995 99, 10423. (5) Jarvi, T. D.; Stuve, E. M. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; pp 75-153. (6) Hamnett, A. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; pp 843-883. (7) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 117. (8) Iwasita, T. Electrochim. Acta 2002, 47, 3663. (9) Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K. J. Electroanal. Chem. 1981, 121, 343. (10) Beden, B.; Bewick, A.; Lamy, C. J. Electroanal. Chem. 1983, 148, 147. (11) Miki, A.; Ye, S.; Osawa, M. Chem. Commun. 2002, 1500. (12) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680.
6500
DOI: 10.1021/la8041154
the case of formic acid11 and methanol,12 has been possible only recently thanks to the high sensitivity of attenuated total reflection-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Investigation of structural effects in the oxidation of formic acid and methanol on platinum electrodes was made possible by single-crystal electrochemistry with well-defined model surfaces.13-16 The determination of the smallest number of surface atoms and their geometric arrangement, necessary for a given reaction to proceed,17 could help us to channel the formic acid or methanol electrooxidation through the direct reaction pathway, by fabricating a surface without sites suitable for the formation of adsorbed CO and with only those sites leading to the reactive intermediate. √ √ Cyanide adsorbs on Pt(111) electrodes adopting a (2 3 2 3) R30° structure, first detected, using low-energy electron diffraction (LEED), by Stickney et al.,18 and whose atomic, real space structure could be elucidated by in situ scanning tunneling microscopy (STM) by Stuhlmann et al.19 and later by Kim et al.20 The structure consists of hexagonally packed arrays, each containing six CN groups adsorbed on top on a hexagon of Pt atoms surrounding a free Pt atom 1). The cyanide √ (see Figure √ coverage corresponding to the (2 3 2 3)R30° structure on Pt(111) is θCN = 0.5.
(13) Clavilier, J.; Parsons, R.; Durand, R.; Lamy, C.; Leger, J. M. J. Electroanal. Chem. 1981, 124, 321. (14) Adzic, R. R.; Tripkovic, A. V.; Vesovic, V. B. J. Electroanal. Chem. 1986, 204, 329. (15) Sun, S. G.; Clavilier, J. J. Electroanal. Chem. 1987, 236, 95. (16) Sun, S. G.; Clavilier, J.; Bewick, A. J. Electroanal. Chem. 1988, 240 147. (17) Maroun, F.; Ozanam, F.; Magnussen, O. M.; Behm, R. J. Science 2001, 293, 1811. (18) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985, 1, 66. (19) Stuhlmann, C.; Villegas, I.; Weaver, M. J. Chem. Phys. Lett. 1994 219, 319. (20) Kim, Y.-G.; Yau, S.-L.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 393.
Published on Web 4/28/2009
Langmuir 2009, 25(11), 6500–6507
Cuesta et al.
Article
√
√
Figure 1. Ball model of the (2 3 2 3)R30° structure adopted by the cyanide adlayer on Pt(111) electrodes, according to ref 20. Black balls correspond to Pt atoms, and white balls correspond to linearly chemisorbed CN groups.
Cyanide adsorption on Pt(111) has been intensively studied by Huerta et al.21-25 using cyclic voltammetry (CV) and spectroelectrochemical techniques. They have shown that the cyanide adlayer on Pt(111) is remarkably stable, with no change being observed in the CVs of the cyanide-covered electrode upon repetitive cycling between 0.06 and 1.10 V versus reversible hydrogen electrode (RHE). Accordingly, as pointed out by Huerta et al.,21 the cyanide-covered Pt(111) electrode can be considered as a chemically modified electrode, with CN groups acting as a third body, blocking those Pt atoms onto which they are adsorbed but leaving unaffected the CN-free Pt atoms onto which H, OH, CO,25,26 or NO27 can adsorb. One of us28 has recently shown that methanol molecules can also reach the free Pt atoms of a cyanide-modified Pt(111) electrode, which can therefore be used as a model surface to investigate the role of atomic ensembles in their electrooxidation. Here, we present a broader study, using CV, Fourier transform infrared spectroscopy (FTIRS), and differential electrochemical mass spectrometry (DEMS) of methanol and formic acid electro-oxidation on cyanide-modified Pt(111) electrodes. Since CO is the poison intermediate formed during the oxidation of both formic acid and methanol on platinum, we have also used these techniques to characterize the CO adlayer on cyanide-modified Pt(111) electrodes.
2. Experimental Section The working electrodes for CV and FTIRS were bead-type Pt single crystals (2 mm in diameter for CV and 4 mm in diameter for FTIR experiments) prepared according to the method developed by Clavilier et al.,29 oriented and polished parallel to the (111) plane (miscut 0.50 V,21,22,42 but a comparison to the CV of the cyanide-modified Pt(111) electrode (see the dashed line in Figure 2) strongly suggests that the high slope at E e 0.50 V must be due to the presence of adsorbed hydrogen within the CN adlayer. This is confirmed by the fact that the COL band of CO adsorbed on cyanide-modified Pt(111) (purple stars in Figure 4) also shows a higher Stark tuning rate (109 cm-1 V-1) at E e 0.50 V (coinciding with the potential region where there is hydrogen adsorption within the mixed CO-CN adlayer; see the solid line Figure 2) than at E g 0.50 V (40 cm-1 V-1), although in this case, the difference is smaller. We therefore think that, over the whole frequency range of 2085-2123 cm-1, the band of the cyanide-modified Pt(111) (Figure 3, left) corresponds to C-bound CN adsorbed on top on Pt(111),19,33,43 the higher slope at E e 0.50 V being due to the presence of co-adsorbed hydrogen. The spectra of the cyanide-modified Pt(111) electrode covered by a saturated CO adlayer show bands of CNL, COL, and COB, but the band of CNL appears at frequencies that, for E e 0.90 V, are approximately 20 cm-1 higher than those in the absence of adsorbed CO. This is better discerned in Figure 4 (compare black (42) Huerta, F.; Montilla, F.; Morallon, E.; Vazquez, J. L. Surf. Sci. 2006 600, 1221. (43) Paulissen, V. B.; Korzeniewski, C. J. Phys. Chem. 1992, 96, 4563.
Langmuir 2009, 25(11), 6500–6507
Article
Figure 4. Dependence upon the electrode potential of the frequency of the CNL band of a cyanide-modified Pt(111) electrode in 0.1 M HClO4 (black squares), the CNL band of a CO-covered cyanide-modified Pt(111) electrode in 0.1 M HClO4 (red circles), the CNL band of a cyanide-modified Pt(111) electrode in 0.1 M HClO4 + 1 mM HCOOH (green triangles), the CNL band of a cyanide-modified Pt(111) electrode in 0.1 M HClO4 + 0.2 M HCOOH (blue inverted triangles), the CNL band of a cyanidemodified Pt(111) electrode in 0.1 M HClO4 + 0.2 M CH3OH (cyan diamonds), the COL band of CO adsorbed on cyanide-modified Pt(111) in CO-free 0.1 M HClO4 (purple stars), and the COL band of CO adsorbed on Pt(111) in CO-free 0.1 M HClO4 (yellow pentagons).
squares and red circles), which clearly shows that CO adsorption on the cyanide-modified Pt(111) electrode provokes an increase of the frequency of the CNL band, which slowly returns to its initial value above 0.90 V, during oxidation of adsorbed CO. Simultaneously, the rate of increase of the CNL stretching frequency with potential increases to 104 cm-1 V-1 in the presence of adsorbed CO, with the simplest explanation for this behavior being that CO adsorption on cyanide-modified Pt(111), compresses the CN adlayer without disturbing its structure. The compression of the CN adlayer would decrease the distance between CN groups, increasing their dipole-dipole coupling and, hence, their stretching frequency. Alternatively, CO and CN compete for electron density, which should also lead to an increased stretching frequency. A third interpretation, supported by the similitude between the CO spectra on unmodified and cyanide-modified Pt(111) electrodes, is that, upon CO adsorption, segregated CO and CN domains are formed, although in this case, it would be difficult to explain the recovery of the initial CV upon CO stripping, the higher frequency of the COB band on the cyanide-modified Pt(111) as compared to the unmodified Pt(111) surface, and the higher Stark tuning rate of the COL band observed in the case of the cyanide-modified Pt(111) electrode (see below). The plots of the COL band of CO adsorbed on cyanidemodified Pt(111) electrodes and the COL band of CO on unmodified Pt(111) electrodes intersect at 0.35 V, the Stark tuning rate for E e 0.50 V being more than twice as large for cyanidemodified Pt(111). This suggests that adsorbed cyanide affects the potential drop felt by the CO molecules. Alternatively, changes in the electronic density and/or structure of the electrode surface, brought about by the presence of adsorbed CN, might induce differences in the amount of electronic density back-donated to the 2π* orbital of the CO molecule and how this changes with potential. It has been shown that, from a theoretical perspective, DOI: 10.1021/la8041154
6503
Article
Cuesta et al.
Figure 6. Cyclic voltammograms at 50 mV s-1 of a cyanidemodified Pt(111) electrode in 0.1 M HClO4 + 1 mM HCOOH (solid line) and 0.1 M HClO4 + 0.2 M HCOOH (dashed line). The scan starts at 0.20 V in the negative direction.
Figure 5. Simultaneously recorded CO-stripping CV (solid line, top) and MSCV (m/z 44, bottom) at 10 mV s-1 for CO adsorbed on a cyanide-modified Pt(111) electrode in 0.1 M HClO4. The dashed line in the top panel corresponds to the second sweep and coincides with the CV of the CO-free cyanide-modified Pt(111) electrode in 0.1 M HClO4.
both explanations are different ways to describe the same phenomena.44 In order to confirm the value of θCO = 0.25 previously determined by one of us,26 we recorded simultaneously COstripping CVs and mass spectrometric cyclic voltammograms (MSCVs) of m/z 44 (Figure 5) and used the value of Qf* determined from the MSCV to calculate θCO using eq 2. We obtained a value of 0.24 ( 0.03, in excellent agreement with the previously reported value. 3.2. Formic Acid Oxidation on Cyanide-Modified Pt (111) Electrodes. It is well-established that formic acid electrooxidation on platinum electrodes follows a dual path mechanism,2 in which a direct pathway through a reactive intermediate (recently shown to be adsorbed formate)11 competes with a slow reaction pathway, in which the poisoning species, adsorbed CO,10 is formed by dehydration of formic acid. It has also been shown that the dehydration leading to poison formation is strongly dependent on the Pt surface structure, being clearly faster for Pt(100) and Pt(110) than for Pt(111) electrodes.45 Figure 6 shows the CVs, starting at 0.20 V in the negative direction, of a cyanide-modified Pt(111) electrode in 0.1 M HClO4 + 1 mM HCOOH (solid line) and 0.1 M HClO4 + 0.2 M HCOOH (dashed line). The first clear difference with the CV in the presence of formic acid of an unmodified Pt(111) electrode is that the hydrogen adsorption region (E < 0.50 V) is unaffected by HCOOH, indicating that, contrary to what happens with unmodified Pt(111) electrodes, the formation of a poison or any other adsorbed species does not occur in this potential region. Formic acid oxidation starts around 0.50 V, a potential at which nearly no adsorbed hydrogen remains on the surface. Taking into account the lack of hysteresis between the positive and negative sweeps in the presence of 0.2 M HCOOH (not observable in 1 mM HCOOH because (44) Lambert, D. K. Electrochim. Acta 1996, 41, 623. (45) Iwasita, T.; Xia, X.; Herrero, E.; Liess, H.-D. Langmuir 1996, 12, 4260.
6504
DOI: 10.1021/la8041154
of the very low oxidation currents), as compared to the unmodified Pt(111) electrode, the amount of adsorbed CO formed on the cyanide-modified Pt(111) electrode must be very small, if any. Another interesting feature in the CVs in Figure 6 is the sharp current increase of formic acid oxidation at 0.90 V in the negative sweep, coinciding with OH desorption. This is a clear indication that OH is a poison for formic acid oxidation, in agreement with the hypothesis put forward by Miki et al.,11 according to which adsorbed formate is the intermediate in the reactive pathway of formic acid oxidation: adsorbed formate already contains two oxygen atoms, and adsorbed OH most likely blocks adjacent vacant sites necessary for decomposition of adsorbed formate to CO2. Figure 7 shows a series of FTIR spectra at increasing potentials recorded during formic acid oxidation on a cyanide-modified Pt(111) electrode in 0.1 M HClO4 + 0.2 M HCOOH and 0.1 M HClO4 + 1 mM HCOOH. The spectra in the frequency region between 2500 and 2200 cm-1 were recorded using the spectrum at 0.10 V as a reference, to monitor the potential at which CO2 starts to appear, while the spectra in the frequency region between 2200 and 1800 cm-1 were recorded using the spectrum at 1.30 V as a reference, to obtain positive bands for both CN and CO and, thus, to better monitor the possible appearance of the latter. The spectra in Figure 7 confirm the assumptions made above based on the voltammetric results in Figure 6: formic acid electrooxidation, indicated by the positive CO2 band at 2343 cm-1, starts between 0.50 and 0.60 V in 0.2 M HCOOH and around 0.50 V in 1 mM HCOOH, and no band corresponding to adsorbed CO (poison intermediate) emerges above the noise level, confirming that adsorbed CO is not formed at all during formic acid oxidation on cyanidemodified Pt(111) electrodes. Additional information can be gained from the dependence of the frequency of the CNL band on the electrode potential in formic-acid-containing solutions, shown in Figure 4. In both 1 mM (green triangles in Figure 4) and 0.2 M HCOOH (blue inverted triangles in Figure 4) solutions, the CNL stretching frequency increases as compared to the case in HCOOH-free solution, although the spectra in Figure 3 clearly show that no adsorbed CO forms on the electrode surface during formic acid electrooxidation. The increase in 1 mM HCOOH solution is similar to that observed for a cyanide-modified Pt(111) electrode covered by a saturated CO adlayer, while in 0.2 M HCOOH solutions, the increase is clearly larger. Since CO cannot be the reason for this increase, it must be concluded that, in formic-acid-containing solutions, there is another adsorbate on the surface of the cyanide-modified Pt(111) electrode. This adsorbed species must be the active intermediate in the direct Langmuir 2009, 25(11), 6500–6507
Cuesta et al.
Article
Figure 7. FTIR spectra at increasing potentials of a cyanide-modified Pt(111) electrode in 0.1 M HClO4 + 0.2 M HCOOH (left) and 0.1 M HClO4 + 1 mM HCOOH (right). The spectra in the frequency region between 2500 and 2200 cm-1 were calculated using the spectrum at 0.10 V as a reference, while the spectra in the frequency region between 2200 and 1800 cm-1 were calculated using the spectrum at 1.30 V as a reference.
reaction pathway, which, as shown by Osawa and co-workers, must be adsorbed formate,11,46,47 which, most likely due to the insufficient signal-to-noise ratio, remained undetected by us. Since adsorbed formate must be in equilibrium with formic acid in the solution, its coverage will be higher the more concentrated the solution, thus explaining the larger increase in the stretching frequency of CNL in 0.2 M HCOOH as compared to 1 mM HCOOH. Our voltammetric and spectroscopic results show that, as in the case of methanol,28 by blocking with CN some of the atoms of the Pt(111) surface, dehydration of formic acid to yield adsorbed CO can be suppressed, forcing the oxidation of formic acid to proceed through the reactive pathway. This indicates that dehydration of formic acid is a site-demanding reaction that requires a minimum number of atoms, as previously suggested.48,49 As in the case of methanol, our results suggest that this process requires the simultaneous interaction of the molecule with three contiguous platinum atoms. We discuss now the effect of adsorbed hydrogen on the electrooxidation of formic acid on Pt(111). In this case, the CV in HCOOH-containing solutions overlaps that in HCOOH-free (46) Samjeske, G.; Miki, A.; Ye, S.; Osawa, M. J. Phys. Chem. B 2005, 109, 23509. (47) Samjeske, G.; Miki, A.; Ye, S.; Osawa, M. J. Phys. Chem. B 2006, 110, 16559. (48) Llorca, M. J.; Herrero, E.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1994, 373, 217. (49) Chang, S.-C.; Ho, Y.; Weaver, M. J. Surf. Sci. 1992, 265, 81.
Langmuir 2009, 25(11), 6500–6507
solutions in the hydrogen adsorption region, indicating that neither CO nor any other HCOOH-derived species adsorb on the surface within this potential region. This is in contradiction with the FTIR data, which, contrary to what happens in the case of methanol (see below), show an increase in the stretching frequency of CNL in 0.2 M and 1 mM HCOOH even at potentials within the hydrogen adsorption region. This apparent contradiction can only be explained by assuming that adsorbed formate does not block the sites for hydrogen adsorption on cyanidemodified Pt(111). Figure 8 shows the CV and the simultaneously recorded MSCV of m/z 44 of cyanide-modified Pt(111) in 0.1 M HClO4 + 1 mM HCOOH. The low concentration of formic acid used is due to the fact that, with higher concentrations (0.1 or 0.01 M), a continuous increase in the intensity of the m/z 44 signal was observed, which made the quantitative analysis of the DEMS results difficult. The red line in the top panel of Figure 8 is the Faradaic current corresponding exclusively to CO2 formation, as calculated from the ion current for m/z 44 shown in the middle panel of Figure 8 using eq 1. As expected (CO2 is the only possible product in formic acid oxidation), the current efficiency for CO2 formation is 100%. The differences between the current in the CV and the Faradaic current calculated from the MSCV are due to the contribution of the double-layer charging and OH adsorption-desorption process to the current in the CV. The results in Figure 8 do not provide any insight regarding the formation of CO during formic acid oxidation on DOI: 10.1021/la8041154
6505
Article
Cuesta et al.
Figure 8. CV (black line, top) and MSCV (bottom) recorded simultaneously during the oxidation of formic acid on a cyanidemodified Pt(111) electrode. The solution was 0.1 M HClO4 + 1 mM HCOOH. Scan rate: 10 mV s-1. The red line in the top panel corresponds to the Faradaic current due to CO2 formation, as calculated from the ion current for m/z 44 in the MSCV in the bottom panel.
cyanide-modified Pt(111) electrodes. For this reason, after performing the experiment in Figure 8, we stopped the potential at 0.2 V, exchanged the solution containing 1 mM HCOOH for a solution containing only 0.1 M HClO4, and recorded a CV and MSCV. The CV coincided with that recorded before the introduction of formic acid into the cell, and no current at m/z 44 could be detected in the simultaneous MSCV, confirming that no CO had been formed during the previous CV in the presence of formic acid. 3.3. Methanol Oxidation on Cyanide-Modified Pt(111) Electrodes. Due to the interest of direct methanol fuel cells as power sources for mobile devices, the oxidation of methanol on Pt electrodes has been extensively studied.6,50-53 It is now well accepted that the oxidation of methanol to CO2 follows, similar to that of formic acid, a dual path mechanism,3 in which CO is the poisoning species9 and formate is involved in the reactive pathway.12 In addition, formaldehyde and formic acid have been identified as products of the incomplete oxidation of methanol on Pt electrodes.30,54-59 Using a flow-through cell for DEMS, it was found that the current efficiency for CO2 formation was independent of the flow rate and it was concluded that, under forced (50) Williams, K. R.; Burstein, G. T. Catal. Today 1997, 38, 401. (51) Burstein, G. T.; Barnett, C. J.; Kucernak, A. R.; Williams, K. R. Catal. Today 1997, 38, 425. (52) Hamnett, A. Catal. Today 1997, 38, 445. (53) Beden, B.; Lamy, C.; Leger, J.-M. In Modern Aspects of Electrochemistry; Bockris, J. O’M., Conway, B. E., White, R. E., Eds.; Plenum: New York, 1992; Vol. 22, Chapter 2, pp 97-264. (54) Ota, K.-I; Nakagawa, Y.; Takahashi, M. J. Electroanal. Chem. 1984 179, 179. (55) Belgsir, E. M.; Huser, H.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1987, 225, 281. (56) Korzeniewski, C.; Childers, C. L. J. Phys. Chem. B 1998, 102, 489. (57) Wang, H.; Wingender, C.; Baltruschat, H.; Lopez, M.; Reetz, M. T. J. Electroanal. Chem. 2001, 509, 163. (58) Batista, E. A.; Malpass, G. R. P.; Motheo, A. J.; Iwasita, T. Electrochem. Commun. 2003, 5, 843. (59) Batista, E. A.; Malpass, G. R. P.; Motheo, A. J.; Iwasita, T. J. Electroanal. Chem. 2004, 571, 273.
6506
DOI: 10.1021/la8041154
Figure 9. CV (black line, top) and MSCV for m/z 44 (middle) and m/z 60 (bottom) recorded simultaneously during the oxidation of methanol on a cyanide-modified Pt(111) electrode. The solution was 0.1 M HClO4 + 0.1 M CH3OH. Scan rate: 10 mV s-1. The red line in the top panel corresponds to the Faradaic current due to CO2 formation, as calculated from the ion current for m/z 44 in the MSCV in the middle panel.
convection, CO2 is only formed via adsorbed CO, whereas soluble intermediates (formic acid and formaldehyde) are transported away from the electrode before they can further react.30 Recently, one of us has reported a study on the oxidation of methanol on cyanide-modified Pt(111) electrodes.28 Briefly, it was concluded there, with a basis on voltammetric and FTIRS results, that, on cyanide-modified Pt(111) electrodes, no CO is formed during methanol electrooxidation, which, hence, proceeds exclusively through the non-CO pathway. From this fact, it was deduced that methanol dehydrogenation to CO on Pt requires the presence on the surface of groups of at least three contiguous atoms. Figure 9 shows the CV and the MSCV for m/z 44 and m/z 60 of cyanide-modified Pt(111) in 0.1 M HClO4 + 0.1 M CH3OH. The red line in the top panel of Figure 9 is the Faradaic current corresponding exclusively to CO2 formation, as calculated from the ion current for m/z 44 shown in the middle panel of Figure 9 using eq 1. The most important features in Figure 9 are the lack of ion current for m/z 60 (bottom panel in Figure 9) and the low current efficiency (ca. 5-8%) of CO2 formation, as compared to 20% on an unmodified Pt(111) electrode (albeit in sulfuric acid).30 The small amount of CO2 detected must correspond to the oxidation of adsorbed carbon monoxide, which probably forms on defect areas of the cyanide adalyer, with this CO being most likely responsible for the very small band of adsorbed CO detected with FTIRS during the electrooxidation of methanol on cyanidemodified Pt(111) under the same conditions.28 The very low Langmuir 2009, 25(11), 6500–6507
Cuesta et al.
Article
coverage of chemisorbed CO formed upon dehydrogenation of methanol on cyanide-modified Pt(111) is further confirmed by the coincidence (within experimental error), at E 0.8 V, of the frequencies of the CNL band of cyanide-modified Pt(111) in 0.1 M HClO4 + 0.2 M CH3OH (cyan diamonds in Figure 4) and 0.1 M HClO4 (black squares), while in the presence of a saturated CO adlayer, the CNL band of cyanide-modified Pt(111) electrodes appears at frequencies ca. 10 cm-1 higher (red circles in Figure 4). Interestingly, a small increase in the frequency of the CNL band in the presence of methanol can only be observed at potentials between 0.5 and 0.8 V (see Figure 4), coinciding exactly with the potential region where a very small band of adsorbed CO could be detected with FTIRS during the electrooxidation of methanol on cyanide-modified Pt(111) under the same conditions.28 We note that the fact that the current efficiency for CO2 formation is of ca. 5-8% does not indicate that this is the percentage of the electrode surface covered by defects in the cyanide adlayer, because a very small amount of defects could have a large effect. The DEMS results in Figure 9 show that, on the cyanidemodified Pt(111) electrode, the methanol electrooxidation reaction proceeds (nearly) exclusively through the non-CO pathway, which is known to include the formation of formaldehyde and formic acid in solution as partially oxidized products.60 In this sense, the lack of signal at m/z 60 (methylformate) appears striking. This suggests that the oxidation does not proceed beyond formaldehyde, which, at the flow rate used of 5 μL s-1, would be washed away from the electrode surface, thus avoiding formic acid (and, obviously, CO2) formation, although it is also possible that the methylformate concentration in the solution is simply below the detection limit. More experiments, involving changes in the flow rate and the methanol concentration, are needed to clarify this point.
obtained, in excellent agreement with the value of θCO = 0.25 determined in a previous work.26 Following a previous work on the electrooxidation of methanol on cyanide-modified Pt(111), we have used CV, FTIRS, and DEMS to study the electrooxidation of formic acid on this surface. As in the case of methanol,28 our results suggest that CO formation during formic acid electrooxidation is inhibited on the cyanide-modified Pt(111) surface, with the reaction proceeding (nearly) exclusively through the non-CO pathway. Our results also indicate that at least three contiguous atoms are necessary for dehydration of formic acid to adsorbed CO on Pt, with direct electrooxidation to CO2 without passing through adsorbed CO being possible on a smaller atomic ensemble, in perfect agreement with recent theoretical calculations by Neurock et al.61 In future work, we will study the dependence of the reaction rate on the formic acid concentration, using both DEMS and classical electrochemical techniques (CV and chronoamperometry), to obtain the formic acid reaction order for the non-CO pathway. We have also used DEMS to follow the electrooxidation of methanol on cyanide-modified Pt(111) electrodes, which had been previously studied using CV and FTIRS.28 Our new results confirm that CO formation is negligible. Apparently, at the flow rate used of 5 μL s-1, once formaldehyde is formed, it is washed away from the electrode surface and cannot react further to formic acid and CO2. To confirm this, future work must involve the use of different flow rates. As in the case of formic acid, we will also investigate, using both DEMS and classical electrochemical techniques, the dependence of the reaction rate on the methanol concentration, aiming at determining the methanol reaction order for the non-CO pathway. In this case, it will be possible to compare the value obtained to those obtained from the Faradaic current (0.4) and the formation rate of CO2 as measured by DEMS (0.14) with Pt(poly) electrodes.57
4. Conclusions and Perspectives We have used FTIRS to characterize CO adlayers on cyanidemodified Pt(111) electrodes. The electrooxidation of CO adsorbed on cyanide-modified Pt(111) electrodes was also followed using DEMS, from which a value of θCO = 0.24 ( 03 was
Acknowledgment. This work was carried out with the help of the Spanish DGI (Ministerio de Educacion y Ciencia) under CTQ2006-02109. A.C. gratefully acknowledges funding from DFG-CSIC for a short stay at the University of Bonn. M.E. acknowledges a FPI Fellowship from the Spanish Ministry for Education and Science.
(60) Lai, S. C. S.; Lebedeva, N. P.; Housmans, T. H. M.; Koper, M. T. M. Top. Catal. 2007, 46, 320.
Langmuir 2009, 25(11), 6500–6507
(61) Neurock, M.; Janik, M.; Wieckowski, A. Faraday Discuss. 2008, 140, 363.
DOI: 10.1021/la8041154
6507
