Mechanism of the Dissociation and Electrooxidation of Ethanol and

Nov 11, 2008 - Stanley C. S. Lai, Steven E. F. Kleyn, Victor Rosca and Marc T. M. Koper* .... Annett Rabis , Paramaconi Rodriguez , and Thomas J. Schm...
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19080

J. Phys. Chem. C 2008, 112, 19080–19087

Mechanism of the Dissociation and Electrooxidation of Ethanol and Acetaldehyde on Platinum As Studied by SERS Stanley C. S. Lai, Steven E. F. Kleyn, Victor Rosca,† and Marc T. M. Koper* Leiden Institute of Chemistry, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands ReceiVed: August 17, 2008; ReVised Manuscript ReceiVed: September 26, 2008

The dissociation and electrooxidation of ethanol and acetaldehyde was studied in perchloric acid on platinum electrodes by employing Surface Enhanced Raman Spectroscopy (SERS). It is found that the scission of the carbon-carbon bond occurs readily at low potentials, leading to adsorbed C1 species. Besides adsorbed CO, chemisorbed CH has been observed for the first time as an adsorbed intermediate in the electrochemical oxidation of ethanol and acetaldehyde. In addition, it is found that the breaking of the C-C bond is easier in acetaldehyde than in ethanol. On the basis of the results obtained here, a reaction mechanism for the dissociation and oxidation of ethanol and acetaldehyde is suggested. 1. Introduction The increased concern in the last decades about the future availability of fossil fuels as the main energy source has spurred a renewed interest in the search for alternative energy sources. Low-temperature fuel cells, which directly convert the chemical energy of a fuel into electric energy, are generally considered one of the more promising technologies.1 Although the bestknown example of a fuel cell is based on hydrogen, practical difficulties in the production, storage, and transportation of hydrogen have barred a widespread adoption of hydrogen fuel cell application as an energy carrier. As an alternative to hydrogen, the direct electrochemical oxidation of small organic molecules, such as methanol,2-4 formic acid,5-7 ethylene glycol,8-11 dimethyl ether,12-14 and ethanol,15-18 have been investigated widely. In particular, ethanol has received considerable interest due to its low toxicity and because it can be produced in large quantities as a renewable biofuel from the fermentation of biomass. In addition, ethanol can serve as a model compound since it is, together with acetaldehyde, the smallest oxygenated organic molecule containing a carbon-carbon bond that needs to be broken to achieve total oxidation. Although the full oxidation of ethanol to carbon dioxide has a favorable thermodynamic potential of 0.08 V (vs RHE), the efficiency of direct ethanol fuel cells has as yet been severely limited by the formation of partial oxidation products containing an intact carbon-carbon bond and by the formation of strongly adsorbed intermediates (Figure 1). In particular, for the (high) concentrations of ethanol which will likely be employed in fuel cells, the major reaction products are found to be the acetaldehyde19,20 and acetic acid.19,21 These products not only decrease the total fuel cell efficiency but are also unwanted due to their polluting nature. Another (related) issue in the ethanol oxidation reaction is the formation of surface poisoning species. Adsorbed carbon monoxide has often been identified as the primary surface poisoning species by FTIR studies,17-19,22-25 although a second poisoning species originating from the CH3 group of the ethanol molecule, generally dubbed adsorbed CHx, has often been suggested but has not been observed direct* Corresponding author. E-mail: [email protected]. † Current address: Energy research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands.

Figure 1. Simplified schematic representation of the parallel oxidation pathways for ethanol on platinum.

ly.16,17,19,21,23,25-29 For example, methane has been observed upon reduction of adsorbed ethanol26 and acetaldehyde27 in DEMS (differential electrochemical mass spectrometry) measurements. In addition, smaller amounts of CO2 were reported if the oxidation of the adsorbates was preceded by a “reductive” treatment.15,26 By the isotopic labeling of one of the carbon atoms, it was found that methane was formed from the CH3 group of the ethanol or acetaldehyde molecule. On the basis of these findings, both studies suggested CHx as an adsorbed intermediate, which can be reduced to methane or oxidized to a CO-like intermediate before subsequent oxidation to CO2. FTIR studies of ethanol oxidation in acidic media show similar results. Adsorbed CO is observed at relatively low potentials and originates from oxidation of the COH group of ethanol. At potentials above 0.3 V, an increase in the COads signal is observed.22 By employing isotopic labeling, it was found that this increase is due to the formation of CO from an adsorbed precursor, dubbed CHx, containing the carbon in the CH3 group of the ethanol molecule,17 although CHx has not been characterized. A number of studies report some νCH signals in FTIR after ethanol or acetaldehyde adsorption, but these low signals have been difficult to isolate from the noise23 and are commonly attributed to nondissociated ethanol16 or acetaldehyde.23 In addition to its elusiveness to spectroscopic detection, the electrochemical nature of CHx has also been a matter of debate. Both the DEMS26 and FTIR17 studies have suggested that the carbon fragment originating from the CH3 group of ethanol or acetaldehyde requires a higher potential to be removed from the surface than the adsorbed CO from the COH group. On the other hand, some studies report a voltammetric peak corresponding to the conversion of CHx to CO at a lower potential than the main CO oxidation peak.15,26 This suggests that the carbon fragment originating from the CH3 group cannot require a higher potential to be oxidized to CO2 than the carbon

10.1021/jp807350h CCC: $40.75  2008 American Chemical Society Published on Web 11/11/2008

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fragment from the COH group, since both are limited by the oxidation of adsorbed CO.26,15 In this context, the present study will employ surface enhanced Raman spectroscopy (SERS) to investigate the dissociation and the subsequent electrochemical oxidation of ethanol and acetaldehyde. Compared with infrared spectroscopy, SERS has two main advantages. First, SERS is based on visible light rather than infrared light, allowing the detection of vibration modes below 700 cm-1, a region that is difficult to access with infrared spectroscopy due to the limited transparency of most optical windows in this range, but is of interest since most metal-adsorbate vibrations lie in this region.30 Second, water, which is ubiquitous in most electrochemical systems, has a small Raman scattering cross-section, facilitating the collection and interpretation of spectra. By employing SERS, this study reports for the first time the spectroscopic detection of an adsorbed CHfragment as an intermediate in the dissociation and electrooxidation of ethanol and acetaldehyde. On the basis of these observations, we may further substantiate the general mechanism for ethanol oxidation on platinum electrodes that we suggested in our previous paper.15 2. Experimental Section Surface Enhanced Raman Spectroscopic (SERS) measurements were performed with a HR 800 spectrograph (Jobin Yvon) with a holographic grating of 600 gr mm-1. The confocal hole of the system was set at 100 µm. A CCD camera with 1024 × 256 pixels was used as detector. The excitation line was provided by a 20 mW HeNe laser at 632.8 nm. The laser beam was focused through an Olympus 50× microscope objective, which was not immersed in the electrolyte, into a 5 µm spot on the electrode surface. A notch filter was used to filter the SERS signal before reaching the sample. With this configuration, a resolution of 1.2 cm-1 was obtained. The working electrode in the SERS experiments was a gold disk 5 mm in diameter embedded in a PTFE shroud, which was mechanically polished with alumina (up to 0.3 µm), rinsed, and treated ultrasonically in ultrapure water before use. The gold electrode was roughened by applying a succession of 25 potential sweep oxidation and reduction cycles in 0.1 M KCl (Merck, pro analysis) from 1.25 to -0.25 V vs the Hg/Hg2Cl2 electrode.31 An ultrathin film of a few monolayers of platinum32 was deposited galvanostatically on the gold substrate from a 0.005 M H2PtCl6 (Sigma-Aldrich, ACS reagent) in 0.5 M Na2HPO4 (Merck, pro analysis) aqueous solution by applying a current of 0.4 mA cm-2 for 40 s.32 All measurements were carried out in conventional singlecompartment three-electrode glass cells. The electrochemical cell employed in the SERS measurements had an optical quartz window parallel to the electrode surface. The cell and all other glassware were cleaned by boiling in a mixture of concentrated nitric acid and sulfuric acid (ca. 1:1) followed by repeated boiling with ultrapure water (Millipore MilliQ A10 gradient, 18.2 MΩ cm, 2-4 ppb total organic content) before each experiment. In all experiments, a platinum wire was used as counter electrode, while a mercury-mercury sulfate electrode (MMSE: Hg/Hg2SO4/K2SO4) was employed as a reference electrode. All potentials reported here have been converted to the RHE scale in the same electrolyte. All solutions employed 0.1 M HClO4 as supporting electrolyte, prepared from concentrated perchloric acid (Merck, “Suprapur”) and ultrapure water. Ethanol (Merck, pro analysis) or acetaldehyde (Sigma-Aldrich, “ReagentPlus”) were adsorbed dissociatively in a separate cell from a 0.5 M solution in the

Figure 2. Voltammetric profiles of Au, Pt, and Pt deposited on an Au substrate in (a) 0.1 M HClO4 and (b) 0.1 M HClO4 + 0.5 M CH3CH2OH. The currents for the oxidation of ethanol on gold are multiplied by 20 for the sake of clarity. All voltammograms were recorded at a scan rate of 50 mV s-1.

supporting electrolyte at 0.10 V vs RHE for 15 min before transferring the electrode to the spectroelectrochemical cell. Deuterated ethanol (ethanol-d6, 99%) and acetaldehyde (acetaldehyde-d4, 99%) were obtained from Cambridge Isotope Laboratories. Argon (Linde Gas, 6.0) was used to deoxygenate all solutions. All measurements were performed at room temperature with a computer-controlled IviumStat potentiostat (Ivium Technologies). 3. Results and Discussions 3.1. Cyclic Voltammetry. To check the surface cleanliness and to verify the deposition of platinum, blank cyclic voltammograms were recorded at 50 mV s-1 for the gold electrode before and after platinum deposition. Typical voltammograms are shown in Figure 2a. The surface areas of the electrodes were determined by calculating the surface oxide reduction charge for the gold electrode and by calculating the charge in the hydrogen adsorption/desorption potential range for the bulk platinum and the platinum film electrodes. The blank voltammogram of the electrode after polishing displays the characteristic profile of a polycrystalline gold electrode: In the anodic sweep, a large double layer charging potential region with a low current is followed by the surface oxidation region above ca. 1.3 V. In the return sweep, a single surface reduction peak is found at 1.2 V. Finally, hydrogen evolution starts at ca. -0.05 V. After platinum deposition, the blank voltammogram of the working electrode strongly resembles the blank voltammogram of a “bulk” polycrystalline platinum electrode. Compared to the blank voltammogram of gold, there is a clear hydrogen adsorption/desorption potential region. Also, the hydrogen evolution onset is shifted 100 mV to more positive potentials. Distinct changes can also be observed in the surface oxidation region, which start at ca. 0.705 V, corresponding to the oxidation

19082 J. Phys. Chem. C, Vol. 112, No. 48, 2008 of surface platinum. The associated reduction peak is found at 0.75 V. Compared to a polished bulk platinum electrode, the platinum film electrode shows a slight difference at potentials above 1.0 V. Whereas a bulk platinum electrode shows a clear maximum at ca. 1.2 V, the platinum film electrode shows a continuously increasing current. This difference in the electrochemical response is likely related to the different surface structure of the platinum film electrode, which has a relatively high roughness factor (around 10). A higher roughness factor, the fact that platinum initially grows in islands rather than epitaxially, and a pronounced effect of the deposition procedure on the structure of the platinum deposits33 indicate that the platinum film electrode obtained by electrodeposition has more low coordination sites than a mechanically polished platinum electrode (roughness factor 1-2). In addition, there is still a small contribution of the gold support on the surface oxidation of the platinum film electrode, as can be seen by the small reduction peak at 1.2 V, indicating there are still some gold sites on the surface, even though the used procedure should produce “pinhole free” overlayers.32 However, since gold is not active in the electrocatalytic oxidation of ethanol and acetaldehyde in an acidic medium in the potential range under investigation,34 the presence of some gold sites was not expected to pose significant problems. Figure 2b shows typical voltammograms obtained for the electrooxidation of 0.5 M ethanol in 0.1 M HClO4 on a gold electrode, on a platinum film electrode, and on a “bulk” polycrystalline platinum electrode, sweeping initially positive from 0.1 to 1.2 V. It is clear that, in this potential range, gold shows very little activity for the oxidation of ethanol. Both platinum electrodes show the typical profile for the oxidation of small organic molecules,35 in which the surface is initially blocked by decomposition products, usually COads, in the anodic scan, up to 0.4-0.5 V, at which the adsorbates are oxidized. At higher potentials, the current decreases again due to surface oxidation,35,36 blocking the adsorption of the reactant. In the negative going sweep, after the surface oxides have been reduced, higher currents are obtained again, until the potential is too low to oxidize the adsorbates and the surface is blocked again. Although the shape of the voltammetric profiles is the same for both electrodes, the current densities for the platinum film electrode are somewhat higher. Since it is known from single-crystal studies that the ethanol oxidation rate is enhanced by low coordination sites,15,37 this is likely an effect of the different surface structure and the somewhat higher amount of low coordination sites for the platinum film electrode, similar to the small differences in surface oxidation behavior. Finally, a very low activity of gold in the electrooxidation of ethanol was observed. Therefore, based on the strong similarity between the voltammograms of the platinum film electrode and the bulk platinum electrode and on the low activity of gold in ethanol electrooxidation, we can conclude that the presence of some small pinholes in the platinum film does not significantly influence the electrocatalytic activity of the electrode. To investigate the electrochemical behavior of adsorbed ethanol and acetaldehyde fragments without interference from the bulk signal, stripping voltammograms were recorded. To record these stripping voltammograms, ethanol or acetaldehyde was adsorbed on the working electrode at 0.10 V for 15 min in a cell containing ethanol or acetaldehyde and subsequently introduced in a “clean” electrochemical cell, containing only supporting electrolyte. An adsorption time of 15 min was found to yield reproducible results: at shorter adsorption times the coverage was too low to obtain a useful signal-to-noise ratio,

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Figure 3. Blank voltammogram and stripping voltammograms of ethanol and acetaldehyde decomposition products on a platinum film deposited on a gold electrode. The voltammograms are recorded at 50 mV s-1.

whereas at longer adsorption times the distribution of the different adsorption products changes due to slow reactions of the surface species during adsorption, which leads to adsorbed CO slowly accumulating on the surface, even at 0.10 V.15 The voltammograms shown in Figure 3 contain three potential regions of interest. There is a broad feature between 0.1 and 0.4 V that can be attributed to the desorption of hydrogen on platinum sites not blocked by the adsorbed ethanol decomposition products. Comparing this region in the stripping voltammograms of ethanol and acetaldehyde, it can be seen that the hydrogen coverage is lower in the case of acetaldehyde adsorption, indicating a higher coverage of decomposition products. This reflects that C-C scission is faster in acetaldehyde compared to ethanol.15 At high potentials, there is a broad peak with a maximum at ca. 0.75 V. This peak can be attributed to the oxidation of adsorbed CO on platinum38,39 resulting from the dissociation of ethanol26 and acetaldehyde.27 Finally, there is a distinguishable shoulder or prepeak between 0.40 and 0.60 V. A peak has been observed in this potential region for the voltammetric stripping of adsorbed ethanol on polycrystalline platinum26 and single-crystal platinum electrodes15,40 and for the voltammetric oxidation of methane on platinum electrodes41 and was attributed to the irreversible oxidation of CHx to CO. 3.2. Surface Enhanced Raman Spectroscopy. 3.2.1. Ethanol. The SERS measurements were performed in a similar fashion to the stripping experiments described in the previous section. Ethanol dissociation products were adsorbed at 0.10 V on the Au-Pt electrode in a separate electrochemical cell, before being introduced to the spectroelectrochemical cell, which contained no dissolved ethanol. Instead of linearly increasing the potential, the potential was held constant for approximately 5 min during each Raman measurement and was increased in steps of 25 mV between subsequent recordings of the SERS spectra. This procedure allowed us to ensure that only strongly adsorbed ethanol (or acetaldehyde) fragments are present on the electrode surface at the start of each SERS experiment. Although numerous studies have aimed at the nature of these irreversibly adsorbed species, it is still under debate whether these adsorbates are mainly the result of dissociative adsorption, leading to C1 species,16,26,29,42 or a combination of dissociative and molecular adsorption.23,27,43 As will be discussed below, the SERS results predominantly show dissociative adsorption.

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Figure 5. Integrated Raman intensities for the bands associated with (a) the CHx fragments and (b) the CO fragment resulting from the dissociation of ethanol as a function of potential.

Figure 4. Surface enhanced Raman spectra of ethanol dissociation products on the Au-Pt electrode in 0.1 M HClO4 recorded at the indicated potentials. (a) Spectra recorded with increasing potential. (b) Effect of decreasing the potential to 0.10 V after the disappearance of the first peak.

Figure 4a shows a series of typical SERS spectra of adsorbed ethanol fragments in perchloric acid at selected potentials. Three spectral regions were found to exhibit significant changes upon increasing the potential, namely the low-frequency region, showing two bands at about 425 cm-1 and about 500 cm-1, a region with a broad peak around 2000 cm-1, and a region with an initially small but significant peak centered at 2880 cm-1. In addition to these bands, a peak at 934 cm-1 and a broad feature around 1600 cm-1 were observed, corresponding to the asymmetric stretching mode of the perchlorate anion45,46 and the bending mode of water,45 respectively. The bands around 500 and 2000 cm-1 are characteristic for CO linearly adsorbed on the Pt surface, being the Pt-CO stretch (νPt-CO) and CO stretch (νC-O), respectively.25,31,45,46 Compared to the case of an adlayer obtained directly from the adsorption of CO, the frequency of the CO band is lower while the frequency of the Pt-C band is higher. This difference can be

explained by a lower CO coverage on the surface, since the dissociation of ethanol is slow and may require a certain ensemble of sites to occur, similar to the dehydrogenation of formic acid47 and methanol33 to CO. It should be noted that no bands at 1800-1900 cm-1 are observed, suggesting that no bridge bound CO46 is formed during the adsorption of ethanol. In addition, although the voltammetry showed some residual gold sites, there is no feature for CO on gold sites,46 substantiating that the residual gold sites do not play a significant role in the dissociation of ethanol. The proximity of the band around the 425 cm-1 band to the 500 cm-1 band alludes to the existence of another Pt-C vibration. Although the position of this band corresponds closely to the Pt-C stretch of bridge-bonded CO,46 the lack of a bridging CO stretch feature in the higher wavenumber region suggests that this feature is probably related to another adsorbed species. Furthermore, the ∼2880 cm-1 vibrational energy band is close to the C-H stretch vibrations on Pt(111) under ultrahigh vacuum (UHV) conditions.48 Figure 4a also shows that both features at 425 and 2880 cm-1 disappear at potentials above 0.45 V. Since decreasing the potential to 0.10 V afterward does not lead to the recovery of these features (Figure 4b), it can be concluded that the disappearance of these bands is related to an irreversible chemical process. In addition, Figure 4b also shows that increasing the potential to 0.45 V again yields a spectrum that is essentially similar to the spectrum recorded at 0.45 V before decreasing the potential to 0.10 V, suggesting that the excursion to 0.10 V did not cause any irreversible chemical or physical changes. On the basis of these findings, we attribute these features to the Pt-C stretch (νPt-CH) and the C-H stretch (νC-H), respectively, of adsorbed CHx fragments on the Pt surface, as will be discussed in detail below. Figure 5 show the normalized integrated intensities of the bands shown in Figure 4 as a function of potential. Upon increasing the potential after adsorption at 0.10 V, the bands at 425 and 2880 cm-1 show a linear decrease in intensity until they completely disappear at 0.45 V. Their strong correlation

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Figure 6. Surface enhanced Raman spectra of acetaldehyde dissociation products on an Au-Pt electrode in 0.1 M HClO4 recorded at the indicated potentials.

Figure 7. Integrated Raman intensities for the bands associated with (a) the CHx fragments and (b) the CO fragment resulting from the dissociation of acetaldehyde as a function of potential.

suggests that both bands originate from the same species. At the same time, the bands associated with adsorbed CO showed increasing Raman intensities up to 0.45 V, which is especially noticeable for the CO stretch feature around 2000 cm-1. Further increases in the potential led to a decrease of the intensities of the CO features between 0.50 and 0.70 V. At potentials above 0.70 V, a spectrum corresponding to a bare platinum surface was recovered (Figure 4). Cyclic voltammograms recorded after such a series of SERS experiments were very similar to blank cyclic voltammograms recorded at the start of the experiments, indicating the absence of any adsorbed species. 3.2.2. Acetaldehyde. It has been suggested that the main species in which the carbon-carbon bond breaking occurs is acetaldehyde, formed as a partial oxidation product, rather than ethanol, both under ultrahigh vacuum conditions49 and under electrochemical conditions.15 Therefore, similar oxidative stripping SERS experiments were performed on adsorbed acetaldehyde. SERS spectra of acetaldehyde adsorbates at different potentials are shown in Figure 6. It can be seen that these spectra are very similar to the spectra of ethanol adsorbates (Figure 4). More specifically, SERS features can be observed at almost the same positions (425, 500, 2000, and 2880 cm-1), suggesting that adsorption of ethanol and acetaldehyde leads to the same adsorbed surface species, suggested to be CHx and CO. There are, however, two main differences between the SERS spectra for the decomposition products of acetaldehyde. The positions of the SERS features of the acetaldehyde adsorbates are shifted by a few wavenumbers compared to those of ethanol at the same potential. Furthermore, unlike ethanol, small features related to adsorbed CO from acetaldehyde can still be distinguished at 0.65 V. Both changes in the SERS spectra can be related to a higher surface coverage of adsorbed species resulting from the decomposition of acetaldehyde as compared to ethanol, in agreement with our previous results.15 The prolonged existence

of CO features at high potentials implies that there is a larger quantity of adsorbates to be oxidized. Furthermore, since it is well-known that SERS frequencies of adsorbates are a function of the nature of the adsorbed species, the morphology of the electrode surface, the electrochemical potential (Stark tuning), and the surface coverage (through the dipole-dipole coupling),50 a higher CO surface coverage would be in correspondence with the shift of the CO stretch frequency to higher wavenumbers and the shift of the Pt-CO stretch to lower wavenumbers. Similar conclusions have been drawn in the literature regarding the CO surface coverage by comparing the SERS frequencies of adsorbed CO obtained from a saturated CO adlayer obtained by the direct adsorption of CO and from the CO adlayer obtained by formic acid adsorption.47 The higher surface coverage of adsorbates from the decomposition of acetaldehyde compared to ethanol decomposition could indicate that acetaldehyde is a necessary intermediate in the carbon-carbon bond breaking in ethanol. This explanation is in agreement with UHV49,51 and theoretical studies,52,53 which report that ethanolic carbon-carbon bond breaking occurs after the dehydrogenation of ethanol to acetaldehyde. Figure 7 depicts the integrated Raman intensities of the acetaldehyde adsorbate features as a function of potential. Qualitatively, the results are very similar to Figure 5: features assigned to a CHx fragment show a decreasing intensity with increasing potential, until they disappear around 0.45 V, while the intensities for the CO features roughly reach a maximum at that potential. 3.2.3. Deuterated Compounds. To gain further insight into the chemical nature of the adsorbates, the same SERS measurements were performed with fully deuterated ethanol (ethanold6, C2D5OD) and acetaldehyde (acetaldehyde-d4, C2D4O). Since the vibrational frequency of a chemical bond is inversely related to the mass of the constituent groups, employing deuterated compounds leads to changes in the Raman shifts assigned to hydrogen-containing fragments, such as CHx. The SERS spectra

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Figure 8. Surface enhanced Raman spectra of deuterated ethanol dissociation products on an Au-Pt electrode in 0.1 M HClO4 recorded at the indicated potentials.

Figure 10. Position of the bands associated with (a) the CHx fragments and (b) the CO fragment resulting from the dissociation of ethanol (closed symbols) and ethanol-d6 (open symbols) as a function of potential.

Figure 9. Surface enhanced Raman spectra of deuterated acetaldehyde dissociation products on an Au-Pt electrode in 0.1 M HClO4 recorded at the indicated potentials.

for ethanol adsorbates and acetaldehyde adsorbates are shown in Figures 8 and Figure 9, respectively. Similar to the SERS spectra of the nondeuterated compounds, four main Raman features are observed. The features at 500 and 2000 cm-1 are still present and can again be assigned to linearly adsorbed CO. This assignment is further supported by the fact that these features do no shift upon deuteration, indicating that it is a nonprotonated fragment, suggesting that the C1 fragment orig-

inating from the alcohol (or aldehyde) group is CO and not COH or CHO. In addition to these CO features, Raman bands around 400 and 2080 cm-1 are observed, while the features around 425 and 2880 cm-1 have disappeared. The band at 2080 cm-1 can be assigned to the typical frequency range of a carbon-deuterium stretching (νC-D) mode,54 replacing the νC-H mode of the nondeuterated compounds at 2880 cm-1. Therefore, we relate the Raman band at 400 cm-1 to the original band at 425 cm-1, which was previously assigned to the Pt-C stretch of a CHx fragment. The change in the vibrational frequency indicates a protonated fragment and further substantiates the initial assignment of the band at 425 cm-1 to adsorbed CHx rather than to bridge bonded CO. In addition, Figures 8 and 9 also show that no residual band remains at 425 cm-1, suggesting that deuterium/proton exchange between the adsorbed CHx species and the electrolyte is negligible on the time scale of the experiments. The comparison between the Raman bands of nondeuterated and deuterated ethanol and acetaldehyde is further clarified in Figures 10 and 11, which show the potential dependence of the Raman band positions of the deuterated and nondeuterated compounds. It can clearly be seen that significant shifts occur in features assigned to an adsorbed CHx fragment, whereas for the CO species no changes or, in the case of acetaldehyde, only small changes in comparison to the nonisotopically labeled compounds are observed. On the basis of the shift in the vibrational frequency of the platinum-carbon bond, it is possible to estimate the number

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Figure 11. Position of the bands associated with (a) the CHx fragments and (b) the CO fragment resulting from the dissociation of acetaldehyde (closed symbols) and acetaldehyde-d4 (open symbols) as a function of potential.

of hydrogen atoms of the adsorbed CHx fragments. By approximating the platinum-carbon bond with a harmonic oscillator and considering the CHx species as a rigid block, the observed frequency shifts (from 426 to 404 cm-1 for ethanol and from 421 to 398 cm-1 for acetaldehyde, averaged over the full potential range) correspond to a value of 1.6 hydrogen atoms per fragment, suggesting the fragment to be either CH or CH2. Molecular adsorption of species with an intact carbon-carbon band can be dismissed based on these shifts, since the relatively smaller change in mass upon deuteration would result in smaller shifts. In addition, previous studies observed only CO2 as the oxidation product of adsorbed ethanol after electrolyte exchange, suggesting that only adsorbed C1 species are bound sufficiently strong to “survive” electrolyte exhange.43 It should, however, be noted that this simple approximation does not take the internal vibrations of the fragment into account. Some degree of coupling between the platinum-carbon stretch and internal carbonhydrogen vibrations is likely to occur, which would result in

Lai et al. lower wavenumbers compared to the case without vibrational coupling and as a result overestimates the mass of the fragment. Therefore, due to this effect, the calculated value of 1.6 hydrogen atoms per fragment would suggest the fragment to be CH rather than CH2 in this case. Methylidyne (CH) has also been observed to be the stable carbohydrate fragment resulting from the dissociation of methane on platinum under UHV conditions.55-58 In these studies, methane is dehydrogenated to methyl (CH3) at low temperatures. By increasing the temperature, CH3 is converted to CH, with CH2 as a probable short-lived intermediate. These findings are in agreement with DFT calculations showing that of the various CHx species adsorbed on platinum, CH can be considered a thermodynamic sink.59 Calculations also show that adsorbed CH3 is a metastable state, with an activation barrier of dehydrogenation to CH2 of 820 meV. Once formed, CH2 is rapidly dehydrogenated to CH, with a barrier of only 140 meV.60 In addition, DFT calculations52,53,61 and UHV studies51 on the decomposition of ethanol on platinum show that the most favored pathway for carbon-carbon bond breaking is initiated by the dehydrogenation of ethanol to acetaldehyde and subsequently to an adsorbed CHCO-species, which decomposes to give adsorbed CO and adsorbed CH. Although direct carbon-carbon bond breaking in ethanol cannot be excluded, from comparison with previous literature it is likely that acetaldehyde is an intermediate species in carbon-carbon bond breaking. 3.3. Mechanistic Implications. By combining our results with previous literature, we propose the reaction scheme shown in Figure 12 for the full oxidation of ethanol (and acetaldehyde) to carbon dioxide. Upon adsorption at 0.10 V, strongly adsorbed species are formed, which can also be deduced from the inhibition of hydrogen adsorption sites (Figure 3). Interpretation of the SERS spectra shows that these adsorbates consists of C1 species, CO and CH, indicating that bond breaking can already occur at potentials as low as 0.10 V. In addition, no features due to strongly adsorbed molecular species were observed. Upon increasing the potential, the intensities of SERS features corresponding to CH slowly decrease until disappearing at 0.45 V, while the intensities of the CO features increase in the same potential range, indicating the slow oxidation of CH to CO. The intensities of the CO features decrease between 0.50 and 0.70 V, corresponding to the oxidation of CO. These potentials are in reasonable correspondence with the features in the stripping voltammograms (Figure 3), taken into consideration that the SERS experiments are potentiostatic, with about 5 min between subsequent measurements, while voltammetry is a potentiodynamic method. By comparing the SERS spectra of the products of ethanol and acetaldehyde decomposition, it can be concluded that the coverage of C1 species resulting from acetaldehyde is higher than that from ethanol. This finding suggests that acetaldehyde is most likely an (short-lived) intermediate species in ethanolic carbon-carbon bond breaking at low potentials, in agreement with aforementioned DFT and UHV studies. However, since strongly adsorbed species are present after adsorbing ethanol

Figure 12. Schematic representation of the proposed mechanism for the dissociation and oxidation of ethanol and acetaldehyde to carbon dioxide.

Electrooxidation of Ethanol and Acetaldehyde on Pt at potentials as low as 0.10 V, while acetaldehyde formation on platinum,20,43 direct C-C bond breaking within the ethanol molecule cannot be excluded. 4. Conclusions In this paper, we have presented the results of a Surface Enhanced Raman Spectroscopy study on the dissociation and the electrocatalytic oxidation of ethanol and acetaldehyde on platinum electrodes. For both ethanol and acetaldehyde, adsorbed C1 species were observed, indicating that carbon-carbon scission can occur at low potentials. In agreement with previous spectroscopic studies on the dissociation of ethanol and acetaldehyde, adsorbed CO was found as one of the dissociation products. Furthermore, an additional carbon fragment was observed. By employing isotopically labeled compounds, this fragment was identified as CH, which is the first direct observation of the CHx species often suggested as an ethanol/ acetaldehyde decomposition product. This CH fragment can be oxidized to CO at low potentials and subsequently oxidized to CO2. Furthermore, it was found that the quantity of adsorbed species was higher for acetaldehyde than for ethanol oxidation, suggesting that carbon-carbon bond breaking occurs more readily in acetaldehyde. On the basis of these findings, a reaction mechanism is proposed for the dissociation of ethanol and acetaldehyde and subsequent oxidation of the resulting fragments. Acknowledgment. The research program of MTM Koper is supported by a VICI grant from The Netherlands Organisation for Scientific Research (NWO). References and Notes (1) Handbook of Fuel Cells: Fundamentals, Technology and Applications; John Wiley & Sons: Chichester, England, 2003; Vols. 1-4. (2) Housmans, T. H. M.; Wonders, A. H.; Koper, M. T. M. J. Phys. Chem. B 2006, 110, 10021–10031. (3) Wang, H.; Baltruschat, H. J. Phys. Chem. C 2007, 111, 7038–7048. (4) Herrero, E.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. 1995, 99, 10423–10424. (5) Iwasita, T.; Xia, X. H.; Herrero, E.; Liess, H. D. Langmuir 1996, 12, 4260–4265. (6) Vidal-Iglesias, F. J.; Solla-Gullon, J.; Herrero, E.; Aldaz, A.; Feliu, J. M. J. Appl. Electrochem. 2006, 36, 1207–1214. (7) Tripkovic, D.; Stevanovic, S.; Tripkovic, A.; Kowal, A.; Jovanovic, V. M. J. Electrochem. Soc. 2008, 155, B281-B289. (8) Wang, H.; Zhao, Y.; Jusys, Z.; Behm, R. J. J. Power Sources 2006, 155, 33–46. (9) de Lima, R. B.; Paganin, V.; Iwasita, T.; Vielstich, W. Electrochim. Acta 2003, 49, 85–91. (10) Dailey, A.; Shin, J.; Korzeniewski, C. Electrochim. Acta 1998, 44, 1147–1152. (11) Wang, H.; Jusys, Z.; Behm, R. J. J. Electroanal. Chem. 2006, 595, 23–36. (12) Kerangueven, G.; Coutanceau, C.; Sibert, E.; Leger, J. M.; Lamy, C. J. Power Sources 2006, 157, 318–324. (13) Heo, P.; Nagao, M.; Sano, M.; Hibino, T. J. Electrochem. Soc. 2008, 155, B92–B95. (14) Muller, J. T.; Urban, P. M.; Holderich, W. F.; Colbow, K. M.; Zhang, J.; Wilkinson, D. P. J. Electrochem. Soc. 2000, 147, 4058–4060. (15) Lai, S. C. S.; Koper, M. T. M. Faraday Discuss. 2009, 140, 339– 416. (16) Iwasita, T.; Pastor, E. Electrochim. Acta 1994, 39, 531–537. (17) Shin, J.; Tornquist, W. J.; Korzeniewski, C.; Hoaglund, C. S. Surf. Sci. 1996, 364, 122. (18) Del Colle, V.; Berna, A.; Tremiliosi, G.; Herrero, E.; Feliu, J. M. Phys. Chem. Chem. Phys. 2008, 10, 3766–3773. (19) Chang, S. C.; Leung, L. W. H.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6013–6021. (20) Camara, G. A.; Iwasita, T. J. Electroanal. Chem. 2005, 578, 315– 321.

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