Comparative Study of Ethanol and Acetaldehyde Reactivities on

The electrochemical behavior of ethanol and acetaldehyde was studied at rhodium electrodes in ... due to the production of acetaldehyde and acetic aci...
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Langmuir 2002, 18, 763-772

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Comparative Study of Ethanol and Acetaldehyde Reactivities on Rhodium Electrodes in Acidic Media E. Me´ndez,† J. L. Rodrı´guez, M. C. Are´valo, and E. Pastor* Departamento de Quı´mica Fı´sica, Universidad de La Laguna, 38071, La Laguna, Tenerife, Spain Received May 21, 2001. In Final Form: September 10, 2001 The electrochemical behavior of ethanol and acetaldehyde was studied at rhodium electrodes in acid solutions using electrochemical mass spectrometry (DEMS). Acetaldehyde and CO2 were detected from bulk ethanol, whereas only CO2 was recorded from acetaldehyde. The production of acetic acid could not be confirmed with this method. The formation of methane was established from both molecules, which suggests that the fragmentation of the C-C chain occurs during the electroreduction and/or adsorption processes. Adsorbed species were isolated by applying a flow cell procedure. The residues produce CO2 and methane during oxidation and reduction reactions, respectively, the amount of these compounds depending on the adsorption potential. By use of isotopic labeled 12CH313CH2OH and 12CH313CHO, the contribution of each C atom in the adsorbate was distinguished. It was established that the CH3 group oxidizes at potentials within the O adsorption potential region of rhodium during the positive-going sweep and also at E < 0.60 V during the reverse scan. The CH2OH or CHO groups yield CO2 in the same potential region as COad. The influence of the anion on these reactions was established using sulfuric and perchloric acids as base electrolytes, being noticeable for ethanol adsorption. Compared to Pt, Rh electrodes exhibit an enhanced activity for deprotonation and C-C cleavage reactions.

1. Introduction The reactivity of ethanol on noble metals has been the subject of several investigations, most of them devoted to study the electrochemical behavior at Pt electrodes.1-15 This alcohol has been considered as a model for C2 organic substances in electrocatalytic studies as well as a target compound in fuel cells. The main problem in using ethanol as a fuel is the low efficiency toward the oxidation to CO2 due to the production of acetaldehyde and acetic acid and the formation of catalytic poisons blocking the active surface sites of the metal.1-15 COad was detected as an intermediate during the electrocatalytic oxidation of ethanol on Pt,2-8,10,11 though the presence of C2 adspecies was also demonstrated by * Corresponding author. E-mail: [email protected]. Fax: 34 922 318002. Tel: 34 922 318028. † On leave from Laboratorio de Electroquı´mica, Facultad de Ciencias, Universidad de la Repu´blica, C. P. 11400 Montevideo (Uruguay). (1) Willsau, J.; Heitbaum, J. J. Electroanal. Chem. 1985, 194, 27. (2) Beden, B.; Morin, M.-C.; Hahn, F., Lamy, C. J. Electroanal. Chem. 1987, 229, 353. (3) Bittins-Cattaneo, B.; Wilhelm, S.; Cattaneo, E.; Buschmann, H. W.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1210. (4) Iwasita, T.; Vielstich, W. J. Electroanal. Chem. 1988, 257, 319. (5) Iwasita, T.; Rasch, B.; Cattaneo, E.; Vielstich, W. Electrochim. Acta 1989, 8, 1073. (6) Pe´rez, J. M.; Beden, B.; Hahn, F.; Aldaz, A.; Lamy, C. J. Electroanal. Chem. 1989, 262, 251. (7) Hitmi, H.; Belgsir, E. M.; Le´ger, J.-M.; Lamy, C.; Lezna, R. O. Electrochim. Acta 1994, 39, 407. (8) Iwasita, T.; Pastor, E. Electrochim. Acta 1994, 39, 531. (9) Gootzen, J. F. E.; Visscher, W.; van Veen, J. A. R. Langmuir 1996, 12, 5076. (10) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem. 1989, 266, 317. (11) Chang, S.-C.; Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6013. (12) Cases, F.; Lo´pez-Atalaya, M.; Va´zquez, J. L.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1990, 278, 433. (13) Schmiemann, U.; Mu¨ller, U.; Baltruschat, H. Electrochim. Acta 1995, 40, 99. (14) Morin, M.-C.; Lamy, C.; Le´ger, J.-M.; Vasquez, J.-L.; Aldaz, A. J. Electroanal. Chem. 1990, 283, 287. (15) Snell, K. D.; Keenan, A. G. Electrochim. Acta 1982, 27, 1683.

infrared spectroscopy.2,6-8 CO2, acetaldehyde, and acetic acid were established as bulk oxidation products.3-5,10,11 The extent at which these reactions occur depends on the crystallographic orientation of the surface,11-14 the ethanol concentration,5-7 and the pH.14,15 During reduction, CH4 and CH3CH3 were observed.3,8,13 In the case of acetaldehyde, CO2 was the sole oxidation product for a 0.01 M solution on Pt,16 whereas both CO2 and acetic acid were obtained at higher acetaldehyde concentrations (0.1 and 1 M).16 Structural effects were established using single-crystal Pt electrodes.17,18 Methane3 and ethane19 were produced during potential cycling in the hydrogen region. Similar research on Rh is scarce. The electroadsorption of ethanol on Rh has been studied by cyclic voltammetry and potential-modulated reflectance spectroscopy (PMRS).20 The reaction products of bulk ethanol and the nature of the adspecies were evaluated from in situ FTIR spectroscopic data.21 It was found that ethanol adsorbs dissociatively yielding mainly COad. On the other hand, bulk ethanol undergoes oxidation to acetic acid and CO2, the latter being the main product from a 0.1 M ethanol solution. Acetaldehyde was proposed as a possible intermediate in the reaction to acetic acid, but its production was not confirmed from FTIR spectra.21 To our knowledge, no investigations on the reactivity of acetaldehyde on Rh have been published. The purpose of the present work is the study of the electrochemical reduction, oxidation, and adsorption of ethanol and acetaldehyde on Rh in acidic solutions, by (16) Iwasita, T.; Rasch, B. Electrochim. Acta 1990, 35, 989. (17) Rodrı´guez, J. L.; Pastor, E.; Xia, X. H.; Iwasita, T. Langmuir 2000, 16, 5479. (18) Cases, F.; Va´zquez, J. L.; Pe´rez, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1990, 281, 283. (19) Silva-Chong, J. A.; Me´ndez, E.; Are´valo, C.; Rodrı´guez, J. L.; Pastor, E. Port. Electrochim. Acta, submitted. (20) Caram, J. A.; Gutie´rrez, C. J. Electroanal. Chem. 1992, 336, 309. (21) de Tacconi, N. R.; Lezna, R. O.; Beden, B.; Hahn, F.; Lamy, C. J. Electroanal. Chem. 1994, 379, 329.

10.1021/la010747i CCC: $22.00 © 2002 American Chemical Society Published on Web 01/04/2002

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using conventional electrochemical techniques combined with differential electrochemical mass spectrometry (DEMS). Isotopic labeled compounds (12CH313CH2OH and 12 CH313CHO) allow one to establish the contribution of each C atom in these molecules to the processes mentioned above. The effect of the anion is also investigated using H2SO4 and HClO4 as base electrolytes. Results obtained in these conditions will be compared to those already published for Pt. 2. Experimental Section 2.1. Experimental Conditions. Solutions were prepared from Millipore-MilliQ* water and analytical grade reagents: H2SO4 and HClO4 (p.a., Merck), ethanol (99.8%, Merck), acetaldehyde (99.5%, Fluka), and RhCl3 hydrate (99.98%, Aldrich). Working solutions were 0.5/0.2 M ethanol and 0.1/0.05 M acetaldehyde prepared in the base electrolytes (0.05 M HClO4 and 0.5 M H2SO4). For HClO4, a lower concentration was chosen in order to minimize perchlorate ion reduction.22-24 Isotopic labeled 0.2 M ethanol (12CH313CH2OH from Cambridge Isotopes Laboratories, 13C 99%) and 0.05 M acetaldehyde (12CH313CHO from MSD Isotopes, 13C 90%) prepared in sulfuric acid as the base electrolyte were used without further purification. All experiments were carried out in electrochemical flow cells at room temperature under an Ar (99.998%) atmosphere. This gas was employed for the desoxygenation of the solutions. DEMS experiments were performed in a 2 cm3 flow cell, directly attached to the vacuum chamber of the mass spectrometer (Balzers QMG 112) with a Faraday cup detector. The working electrode was prepared by potentiostatic electrodeposition of Rh from a 3% RhCl3 + 1 M HCl solution onto an Au electrode sputtered on a microporous PTFE membrane (Scimat Ltd.; pore size, 17 µm; porosity, 50%). This membrane acts as the interface between the electrochemical cell and the ionization chamber of the mass spectrometer. Through this ensemble, cyclic voltammograms (CVs) and mass spectrometric cyclic voltammograms (MSCVs) for a preset mass to charge (m/z) ratio were simultaneously obtained at scan rates of 0.010 or 0.007 V s-1. Up to three ion currents for different m/z ratios could be simultaneously acquired. The real area of the working electrode25 varied in the range 15-40 cm2. Some cyclic voltammetric experiments were carried out using a bead-shaped polycrystalline Rh electrode of 0.05 cm2 real area as working electrode at a scan rate of 0.050 V s-1. The counter electrode was a rhodized Au wire, and a reversible hydrogen electrode (RHE) prepared with the suitable base electrolyte solution was employed as the reference electrode. All potentials in the text are given in the RHE scale. The electrochemical setup consists of a INELECSA PDC10+ potentiostat with a PARC Universal Programmer (mod. 175). The program WINDEMS was developed from INELECSA for the potentiostatic control and data acquisition from the DEMS system. 2.2. Experimental Procedures. Before each experiment, the working electrode was activated by potential cycling between 0.02 and 1.40 V at 0.05 V s-1. The absence of organic impurities in the electrochemical system was checked by recording the ion current for m/z ) 44 (corresponding to the molecular ion [12CO2]•+) in the base electrolyte until a potential-independent signal was obtained. 2.2.1. DEMS Calibration. Determination of K*. The DEMS equipment was calibrated as described in ref 26 using CO (99.999%) as a probe molecule. The electrochemical oxidation of CO yields CO2 in a single potential cycle with a 100% faradaic efficiency. The value of K*, the constant of the mass spectrometer, was calculated using the expression (22) Ahmadi, A.; Evans, R. W.; Attard, G. J. Electroanal. Chem. 1993, 350, 279. (23) Wasberg, M.; Hora´nyi, G. J. Electroanal. Chem. 1995, 385, 63. (24) Hora´nyi, G.; Wasberg, M. J. Electroanal. Chem. 1996, 404, 291. (25) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1971, 31, 29. (26) Wolter, O.; Heitbaum, J. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 2.

Me´ ndez et al. K* )

n QM A QI

where A is the faradaic efficiency of the reaction and n is the number of electrons involved in the reaction per molecule of the product (in this case, per molecule of CO2). QI and QM are the faradaic charge and the ion charge associated with the fragments of the product reaching the mass analyzer, respectively. Once K* is determined, n can be calculated for the adsorbed species obtained from ethanol and acetaldehyde (these residues oxidize also to CO2). Assuming that only CO2 is detected from the adsorbates, it can be considered that the faradaic efficiency is 100% in all cases. 2.2.2. Electrochemical Behavior of Bulk Ethanol and Acetaldehyde. After activation of the working electrode, the base electrolyte was replaced by the working solution. Then, the potential was cycled several times between 0.02 and 1.40 V, and the corresponding CVs and MSCVs for selected mass-to-charge ratios were recorded. The solution was replaced by fresh working solution prior to the acquisition of the MSCVs. The second potential cycle is shown in the figures. Baseline correction was performed for those m/z values in which a variation of the ground signal with time was observed. 2.2.3. Adsorption Experiments. Each adsorption experiment consisted of the following steps: (i) Once the electrode was activated in the base electrolyte, the potential was set to the adsorption potential (Ead), chosen in the range 0.07-0.55 V. After achieving a stationary current value, the working solution was injected and the current transient was simultaneously recorded during an adsorption time of 10 min. (ii) The working solution was replaced by the base electrolyte, while maintaining the potential at Ead (ca. 100 times the volume of the cell was used to ensure the total replacement of the organiccontaining solution). (iii) CVs and MSCVs for appropriate mass signals were recorded by sweeping the potential from Ead in the positive- or negative-going potential direction. Depending on the scan direction, the electrooxidation or the electroreduction processes were studied. The direct electrooxidation of the adsorbates was carried out by running the potential from Ead up to 1.40 V and further cycling in this potential range. The reduction of the residues was investigated by cycling the potential between 0.30 and 0.02 V, and then the anodic stripping of the adsorbates remaining at the surface after the reductive scans was also performed.

3. Results 3.1. Electrochemical Behavior of Bulk Ethanol and Acetaldehyde in Acidic Media. 3.1.1. Ethanol in Perchloric Acid Solution. The electrooxidation of 0.5 M ethanol on electrodeposited Rh in 0.05 M HClO4 gives rise to a single current peak located at 0.72 V in the positive run, with a hump at ca. 0.92 V (Figure 1A). A partial blockage of the surface is observed in the Had potential region. During the negative-going sweep, an anodic current is superimposed with the reduction of the oxide layer and the formation of Had, as is evidenced from the comparison between the CVs in the ethanol-containing (solid line) and base electrolyte (dashed line) solutions. Figure 1B-D shows the MSCVs corresponding to the ion currents for m/z ) 29, 44, and 15. Carbon dioxide, acetaldehyde, and acetic acid are the most probable oxidation products from ethanol.3-5,10,11 Acetaldehyde contributes to all the m/z ratios mentioned above ([CHO]+, [CH3CHO]•+, and [CH3]+ fragments for m/z ) 29, 44, and 15, respectively), whereas the radical cation [CO2]•+ is related to the mass signal for m/z ) 44. As both acetaldehyde and carbon dioxide contribute to the latter signal, it is necessary to perform a detailed analysis of the MSCVs for these three signals in order to establish the potential range for the production of each compound.

Ethanol and Acetaldehyde Reactivities on Rhodium

Figure 1. Electrochemical behavior of ethanol and acetaldehyde in 0.05 M HClO4 on a porous Rh electrode. (A) and (F) show (s) CVs for ethanol and acetaldehyde, respectively, and (‚ ‚ ‚) for the supporting electrolyte. (B)-(E) and (G)-(J) are the corresponding MSCVs recorded for the mass signals m/z ) 29 ([COH]+), 44 ([CO2]•+ and [CH3CHO]•+), 15 ([CH3]+), and 2 ([H2]•+) for ethanol and acetaldehyde solutions, respectively. A ) 22 cm2 for ethanol and 33 cm2 for acetaldehyde experiments; v ) 0.010 V s-1.

The mass signal for m/z ) 29 (Figure 1B) develops a broad peak with two contributions at 0.72 and 0.92 V in the positive sweep, while in the negative scan a third feature appears between 0.44 and 0.20 V. This mass-tocharge ratio is attributed exclusively to the [CHO]+ fragment of acetaldehyde. On the other hand, the ion current for m/z ) 44 (Figure 1C) is related to both acetaldehyde and CO2. This signal defines a peak at 0.70 V with a hump at 0.92 V in the positive run, whereas during the reverse scan two maxima are apparent at 0.32 and 0.12 V. Comparison between the MSCVs for m/z ) 44 and 29, taking in mind the fragmentation probabilities,27 allows one to establish the potential dependence for the production of these substances. Thus, the shoulder at 0.92 V and the signal in the 0.44-0.20 V potential range are assigned to acetaldehyde as they clearly appear in the MSCV for m/z ) 29. Finally, the peak at 0.72 V in the positive run and the feature at E < 0.20 V in the negative sweep correspond to CO2, as they are not visible in the signal for m/z ) 29. This assignment will be confirmed later with the adsorption experiments (only CO2 was detected from the adspecies). The production of acetaldehyde at E < 0.44 V during the reverse scan is explained in terms of the surface state in this potential region: once the oxidized surface starts to be reduced, free Rh sites become available for further reactions, with the consequent formation of the aldehyde from ethanol molecules in the bulk of the solution. Acetic acid is a weak acid, and the undissociated form prevails in acid solutions. This molecule is volatile enough to be detected (its detection can be confirmed by injecting (27) Atlas of Mass Spectral Data; Stenhagen, E., Abrahamsson, S., McLafferty, F. W., Eds.; Interscience: New York, 1969.

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acetic acid in the mass spectrometer), and the mass signal for m/z ) 60 can be used with this purpose. However, the ion current for m/z ) 60 displays no potential dependence; that is, acetic acid is not detected in these experimental conditions. This result can be justified not only by the low concentration due to the equilibrium in the solution but also by the high volatility of its precursor, the acetaldehyde, which is pumped in the vacuum chamber as soon as it is formed on the surface, avoiding further oxidation to the acid. The same effect has been previously described for the oxidation of other alcohols.28,29 The ion current for m/z ) 15 (Figure 1D) at E > 0.20 V develops the same features as that observed for m/z ) 29. Therefore, these contributions correspond to acetaldehyde. The signal at E < 0.20 V in the negative-going potential scan cannot be related to this compound (compare with Figure 1B) but is assigned to the formation of CH4. This hydrocarbon would be formed from the cleavage of ethanol molecules and further hydrogenation by Had present on the surface in this potential region. In accordance with the assignment described before, both reduction and oxidation reactions occur at potentials below 0.20 V during the reverse scan. Although these contributions appear in the so-called “hydrogen potential region”, for Rh the O adlayer reduction overlaps with the H electroadsorption region,30 and in this way, oxidation reactions are also feasible. The production of ethane during ethanol reduction was disregarded by following the signal for m/z ) 30 ([CH3CH3]•+). No potential dependence was observed in this MSCV, confirming the absence of this compound. The mass signal for m/z ) 2 (Figure 1E) shows the onset for H2 production at 0.10 V in both base electrolyte and working solutions. The presence of adsorbed residues is reflected in the decrease of the intensity of the signal, that is, in the amount of H2 evolved, due to the partial blockage of the surface. However, no shift is observed in the onset potential for its production. 3.1.2. Acetaldehyde in Perchloric Acid Solution. Acetaldehyde electrooxidation in HClO4 is characterized by a wide anodic feature located in the range 0.60-1.20 V (Figure 1F). A positive shift in the onset for O electroadsorption is noticeable during the positive-going potential scan. As in the case of ethanol, the hydrogen region is partially suppressed whereas an anodic contribution is present in the negative run. The same mass signals selected for ethanol were analyzed in this case, that is, m/z ) 29, 44, and 15. All of them are affected by acetaldehyde consumption, being more evident for m/z ) 29 (Figure 1G) at E > 0.65 V in the positive sweep and E < 0.50 V in the negative-going scan. However, the ion current for m/z ) 44 (Figure 1H) develops a well-defined peak at 0.73 V in the positive run which is undoubtedly assigned to CO2. The mass signal for m/z ) 15 (Figure 1I) shows an intense contribution during the negative-going potential scan for E < 0.32 V attaining a peak at 0.19 V. This feature is attributed, as in the case of ethanol reduction, to the production of CH4. Potential-independent MSCVs were recorded for m/z ) 60 and 30. The shape of the signal for m/z ) 2 (Figure 1J) is the same as for ethanol, in which the influence of the adsorbates is appreciated by the smaller amount of H2. 3.1.3. The Influence of Anion Adsorption: Electrochemical Behavior in Sulfuric Acid Solution. In (28) Pastor, E.; Wasmus, S.; Iwasita, T.; Are´valo, M. C.; Gonza´lez, S.; Arvia, A. J. J. Electroanal. Chem. 1993, 350, 97. (29) Schmidt, V. M.; Ianniello, R.; Pastor, E.; Gonza´lez, S. J. Phys. Chem. 1996, 45, 17901. (30) Jerkiewicz, G.; Borodzinski, J. J. Langmuir 1993, 9, 2202.

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Figure 3. Anodic stripping of the residues formed from (A) 0.5 M ethanol and (B) 0.1 M acetaldehyde, at selected Ead in the range 0.07 V e Ead e 0.55 V for tad ) 10 min, on smooth Rh in 0.05 M HClO4. A ) 0.05 cm2; v ) 0.050 V s-1.

Figure 2. Electrochemical behavior of ethanol in 0.5 M H2SO4 on a porous Rh electrode. (A): (s) CVs for 0.5 M ethanol and (‚ ‚ ‚) for the supporting electrolyte. (B)-(D): MSCVs recorded for the mass signals m/z ) 29 ([COH]+), 44 ([CO2]•+ and [CH3CHO]•+), and 15 ([CH3]+), respectively. A ) 40 cm2; v ) 0.010 V s-1.

sulfuric acid media, most of the features described in HClO4 for both substances are observed. However, some differences should be pointed out. The inhibition of the hydrogen adsorption/desorption region in the CVs is lower than that observed in perchloric acid, especially in the case of ethanol (Figure 2A). This result suggests a competitive adsorption between ethanol and the anions present in the sulfuric acid solution, which are strongly adsorbed on rhodized rhodium in the potential region between 0.20 and 0.50 V.31,32 The MSCVs for m/z ) 29 (Figure 2B) and m/z ) 15 (Figure 2D) for E > 0.30 V show the electrooxidation of ethanol to acetaldehyde. The main difference with the electrochemical behavior in perchloric acid is the absence of the feature during the negative-going potential scan in the potential range 0.400.20 V. The ion current for m/z ) 44 (Figure 2C) displays a broader contribution (compare with Figure 1C) in the positive sweep. During the reverse scan, two features, which are not observed in the signals for m/z ) 15 and 29, overlapped at E < 0.40 V, indicating the yield of CO2. Finally, the production of CH4 from ethanol is assessed from the ratio m/z ) 15 at E < 0.20 V (Figure 2D, see inset). The maximum in the mass intensity is observed at the cathodic limit of the scan overlapping the production of H2; that is, no peak is defined in this mass signal in H2SO4, contrary to the behavior observed in perchloric acid (Figure 1D). In the case of acetaldehyde, the production of CH4 is partially masked by the consumption of acetaldehyde (not shown) and cannot be confirmed from bulk experiments. This fact implies a decrease in the yield of this hydrocarbon considering the high intensity observed in Figure 1I for (31) Hora´nyi, G.; Rizmayer, E. M. J. Electroanal. Chem. 1986, 201, 187. (32) Zelenay, P.; Hora´nyi, G.; Rhee, C. K.; Wieckowski, A. J. Electroanal. Chem. 1991, 300, 499.

m/z ) 15 in perchloric acid. As will be discussed later, the production of CH4 was fully confirmed from the adsorption experiments of acetaldehyde in the H2SO4 solution. 3.2. Reactions of the Adsorbates from Ethanol and Acetaldehyde. Both substances were adsorbed at selected potentials in the range 0.07 V e Ead e 0.55 V in steps of 0.10 V using perchloric and sulfuric acids as base electrolytes. CO2 and CH4 were the sole oxidation and reduction products, respectively. The ion current for m/z ) 44 was selected for the detection of CO2 during the anodic stripping of the residues, whereas the production of CH4 during the reduction cycles was followed through the mass signal for m/z ) 15. The ion current-E profiles for each m/z signal were integrated in order to obtain the corresponding ion charge density (σion). In the case of the ratio m/z ) 44, the first sweep was considered for this calculation, both the oxidation and the ion charge densities being negligible in the following cycles. For the mass charges measured in the reductive cycles, four potential cycles were considered. 3.2.1. Electrooxidation of the Adspecies. First, the adsorption experiments were performed using a beadshaped electrode. CVs for the anodic stripping of the residues in HClO4 can be seen in Figure 3. Two potential regions can be distinguished during the electrooxidation of the adsorbates: one extending from 0.40 to 0.80 V and the second at E > 0.80 V, the potential range at which the electrode is covered by the O adlayer. For Ead in the hydrogen region, an oxidation peak is recorded in the positive scan in the same potential region as for COad on Rh.33 This result is more evident for acetaldehyde adsorbates (see Figure 3B). As Ead is set to more positive values, the onset for the oxidation of the residues also shifts positively, and the relative contribution in the second potential range increases. The shape of the CVs for the stripping of the residues from both compounds is similar for Ead > 0.25 V, and therefore the nature of the adspecies seems to be similar at these adsorption potentials. This fact will be discussed later. The same experiments were repeated applying DEMS. Figures 4A,B and 5A,B show the CVs and MSCVs for the (33) Are´valo, C.; Gomis-Bas, C.; Hahn, F. Electrochim. Acta 1998, 44, 1369.

Ethanol and Acetaldehyde Reactivities on Rhodium

Figure 4. Anodic stripping of the adsorbates formed at (s) 0.15 V, (- - -) 0.25 V, and (- ‚ ‚ -) 0.45 V in 0.5 M H2SO4: from 0.2 M 12CH312CH2OH, (A) CVs and (B) MSCVs for 12CO2 (m/z ) 44), A ) 23 cm2; and from 0.2 M 12CH313CH2OH, (C) CVs and (D) MSCVs for 12CO2 (m/z ) 44) and 13CO2 (m/z ) 45). A ) 17 cm2; v ) 0.010 V s-1.

Figure 5. Anodic stripping of the adsorbates formed at (s) 0.07 V, (- - -) 0.25 V, and (- ‚ ‚ -) 0.45 V in 0.5 M H2SO4: from 0.05 M 12CH312CHO, (A) CVs and (B) MSCVs for 12CO2 (m/z ) 44), A ) 24 cm2; and from 0.05 M 12CH313CHO, (C) CVs and (D) MSCVs for 12CO2 (m/z ) 44) and 13CO2 (m/z ) 45). A ) 21 cm2; v ) 0.010 V s-1.

oxidation of ethanol and acetaldehyde adsorbates, respectively, formed at selected potentials in H2SO4. The mass signal for CO2 (m/z ) 44, Figures 4B and 5B) behaves parallel to the cyclic voltammograms (Figures 4A and 5A). This fact, in addition to the absence of a contribution to the signal for m/z ) 29, confirms that CO2 is the sole electrooxidation product. Two contributions are observed in the ion current during the positive scan, in the same potential regions described before from the CVs. However, during the negative scan a feature develops for E < 0.50 V, which is also attributed to the formation of CO2. In this potential region, the electrode surface is not completely free from oxygen,24,30 and probably the remaining residues

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Figure 6. (A) (s) CV for the anodic stripping of adsorbed CO formed at Ead ) 0.01 V in 0.5 M H2SO4 on a porous electrodeposited Rh for tad ) 10 min; (‚ ‚ ‚) CV for the supporting electrolyte. (B) MSCV for m/z ) 44 ([CO2]•+). A ) 36 cm2; v ) 0.010 V s-1.

are oxidized by the OHad not desorbed yet. The ion charge observed at E < 0.50 V increases as the contribution of the residue in the O electroadsorption region develops. The peak position in the MSCV for CO2 recorded at Ead ) 0.07 V for acetaldehyde was practically the same as that obtained for CO by applying the same experimental procedure (Figure 6), suggesting that the main residue formed at this Ead is COad. The formation of CO2 at E < 0.20 V is also observed from COad (see inset in Figure 6B). Calibration of the mass spectrometer allows the calculation of n, confirming the formation of this adsorbate (see the discussion section). Results with the bead-shaped (Figure 3) and the electrodeposited Rh electrodes used for DEMS (Figures 4 and 5) are compared in order to establish the influence of the surface structure or its roughness. The values for the electrooxidation charge densities (σox) are given as a function of Ead in Figures 7A,B and 8A,B in perchloric (Figure 7) and sulfuric acid (Figure 8). These values were obtained by subtracting the charge used to form an oxide layer in the 0.40-1.40 V potential range (using the CV recorded just before the adsorption experiment) from the charge established for the electrooxidation of the adsorbates. The same values of σox were obtained for both types of electrodes within the experimental error ( 0.80 V) and a third one in the reverse sweep at E < 0.50 V. The latter contribution is only apparent in the signal for m/z ) 44 and therefore has to be related exclusively to the oxidation of the CH3 group. During the positive scan, the production of 12CO2 and 13CO2 is detected in both potential regions. However, the onset potential for the signal for m/z ) 44 is positively shifted (30-50 mV) with respect to the signal for m/z ) 45, the former depicting a broader feature. The integration of the mass signals lets us compare the amount of each group in the adsorbed layer (Table 1). σion for m/z ) 45 is always higher than that for m/z ) 44, confirming that part of the CH3 in the original C2 molecule is lost during adsorption. This difference becomes higher as Ead is set to more negative values. Thus, the ratio (m/z ) 45)/(m/z ) 44) decreases as the adsorption potential increases. For acetaldehyde adsorbed at 0.07 V, only a very small signal can be related with the oxidation of the methyl group (Figure 5D). In the case of ethanol residues, the ratio is approximately 1 for Ead ) 0.25 and 0.45 V. 3.2.4. Calculation of n. Further information on the nature of the adsorbates can be obtained from the calculation of n, the number of electrons per CO2 molecule, obtained during oxidation (see section 2.2.1). These data are given in Table 2. The error in these calculations was estimated to be about (0.2, due to the uncertainty in the calculation of faradaic oxidation charge.34 However, in general n increases with Ead from ca. 2 to a maximum value of 4.7 for acetaldehyde and 5.5 for ethanol. Results will be discussed in section 4.3. (34) Schmiemann, U.; Jusys, Z.; Baltruschat, H. Electrochim. Acta 1994, 39, 561.

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The values of n for the adsorbates remaining at the surface after potential cycling in the hydrogen region (not given in the table) are in all cases smaller than the corresponding data for the direct electrooxidation and vary between 2 and 3. 4. Discussion 4.1. Summary of Experimental Findings. For ethanol: 1. Bulk ethanol oxidizes to acetaldehyde at E > 0.30 V during the positive-going potential sweep and in the 0.440.20 V potential range during the reverse scan. 2. The sole electrooxidation product from the adsorbates is CO2. Three contributions can be distinguished: during the positive run, the contribution in the 0.40-0.80 V potential region and that at E > 0.80 V, and in the negative sweep, the feature at E < 0.20 V. 3. The amount of adsorbed species is lower for ethanol than for acetaldehyde residues, especially for Ead < 0.45 V. 4. From isotopic labeled experiments, the yield of CO2 in the negative scan is assigned to the oxidation of the methyl group in the ethanol molecule. During the positive run, the signal in the range 0.40 < E < 0.80 V is mainly due to the alcoholic group, whereas at higher potentials it is related to the oxidation of the CH3. The ratio σ45/σ44 approximates to 1 at Ead ) 0.25 and 0.45 V, whereas Ead ) 0.15 V yields 3.2. The latter value implies that part of the methyl group in the original ethanol molecule is lost during the adsorption process. 5. Methane is the only detectable reduction product from both bulk (at E < 0.20) and adsorbed ethanol (at E < 0.10 V). 6. In the sulfuric acid solution, the amount of ethanol adsorbed decreases when compared with the experimental results obtained with perchloric acid as the base electrolyte. 7. The number of transferred electrons per CO2 molecule varies from 2.2 to 5.5. These values are higher in sulfuric acid (except for Ead ) 0.55 V). 8. The production of methane and the values of n around 2 suggest the formation of CO-like species. For acetaldehyde: 1. Only CO2 is detected with DEMS during electrooxidation. The same potential regions as for ethanol can be established. However, the contribution in the range 0.400.80 V is much more important in this case. 2. As for ethanol, the yield of CO2 in the negative scan is assigned to the oxidation of the methyl group in the acetaldehyde molecule, according to the results with the isotopic labeled compounds. In a similar way, the contribution between 0.40 and 0.80 V is related to the COH group, and that at E > 0.80 V, to the oxidation of the CH3. The ratio σ45/σ44 is always higher than 1, though it diminishes as Ead is set to more positive values. At Ead ) 0.07 V, almost all methyl groups are lost during adsorption, and therefore CO seems to be the adsorbed species. 3. Large amounts of methane are detected from bulk acetaldehyde in perchloric acid solution at E < 0.35 V. From the adsorbates, methane is formed at E < 0.10 V, σCH4 increasing with Ead. 4. The influence of the presence of sulfuric anions is less significant than for ethanol. 5. The values of n vary from 2.1 to 4.7, being higher in sulfuric acid than in the HClO4 solution. 4.2. Bulk Reactions. As indicated in the experimental results summarized before, the oxidation of ethanol to acetaldehyde occurs at E > 0.30 V:

Me´ ndez et al.

CH3CH2OH f CH3CHO + 2H+ + 2e-

(1)

Although the production of acetic acid could not be established from DEMS experiments, FTIR spectra have shown the presence of this compound at E g 0.65 V (but this technique did not confirm the formation of acetaldehyde).21 In accordance with the explanation given before, it seems that the acid is formed from acetaldehyde through a successive reaction process: ethanol f acetaldehyde f acetic acid. Then, this reaction can be written as

CH3CHO + H2O f CH3COOH + 2H+ + 2e- (2) Differences observed between DEMS and FTIRS results can be explained by considering the experimental setup for these methodologies. A thin layer configuration is necessary for infrared spectroscopy. In this situation, acetaldehyde remains confined in the thin layer, and further oxidation to acetic acid is favored. In DEMS experiments, acetaldehyde is evacuated into the vacuum chamber as soon as it is formed, avoiding (at least in part) the formation of acetic acid. Therefore, it can be concluded that these techniques provide complementary information. Reactions 1 and 2 can be rewritten considering the formation of the hydroxide adlayer at the Rh surface for E > 0.40 V,30

H2O f (OH)ad + H+ + eand then the interaction of the organic molecules:

CH3CH2OH + 2(OH)ad f CH3CHO + 2H2O (1′) CH3CHO + (OH)ad f CH3COOH + H+ + e- (2′) Parallel to this process, both ethanol and acetaldehyde adsorb yielding finally CO2 as electrooxidation product:

CH3CH2OH/CH3CHO f (R)ad f CO2

(I)

The nature of the residue (R)ad will be considered next, and this equation will be rewritten. However, at this statement of the discussion, the fragmentation of the molecule during adsorption has to be also proposed in order to justify the large amounts of methane detected with the organic compounds present in the bulk of the solution. A general equation could be

CH3CH2OH/CH3CHO + (H)ad f CH4 + (C1)ad (II) where (C1)ad denotes an adsorbate containing only one C atom. For bulk acetaldehyde in perchloric acid solution, this reaction is established to occur at potentials as positive as 0.35 V during the negative excursion of the potential cycling, that is, just in the limit of the hydrogen adsorption region. 4.3. On the Nature of the Adspecies. It was demonstrated that the adsorbates oxidize only to CO2. Valuable information on the nature of the adspecies is obtained from n data given in Table 2, for the electrooxidation to carbon dioxide of the residues formed at each adsorption potential. Thus, the overall trend in the values of n gives us a first approximation to the transformations of the adspecies with Ead. In general, the number of electrons per CO2 increases with Ead from ca. 2 to 5.5. Let us now consider the different species that could support these values. For ethanol, the complete oxidation to CO2 yields n ) 6, and therefore this is the maximum

Ethanol and Acetaldehyde Reactivities on Rhodium

Langmuir, Vol. 18, No. 3, 2002 771 Scheme 1

value we can obtain for the adsorbates from this molecule:

CH3CH2OH + 3H2O f 2CO2 + 12H+ + 12e-

n ) 12/2 ) 6

However, deprotonation of ethanol could occur during adsorption (eq I) with the formation of C2 species A-C: +

CH3CH2OH f (CH3CH2O)ad+ H + e (A)

-

(3)

(CH3CH2O)ad f (CH3CHO)ad + H+ + e(B)

(4)

(CH3CHO)ad f (CH3CO)ad + H+ + e(C)

(5)

Cleavage of the C-C bond in adsorbate C produces two C1 adspecies:

(CH3CO)ad f (CH3)ad + (CO)ad

(6)

The DEMS technique does not allow one to distinguish between (CH3CO)ad or (CH3)ad + (CO)ad, because the value of n is the same. However, reaction 6 is proposed taking into account that (CO)ad has been undoubtedly established from FTIR spectra.21 The anodic stripping of these residues is given by the following equations:

(CH3CH2O)ad + 3H2O f 2CO2 + 11H+ + 11e(A) n ) 11/2 ) 5.5 (7) (CH3CHO)ad + 3H2O f 2CO2 + 10H+ + 10e(B) n ) 10/2 ) 5 (8) (CH3CO)ad + 3H2O f 2CO2 + 9H+ + 9e(C) n ) 9/2 ) 4.5 (9) As is shown, n diminishes from (A) to (C). Reactions involving adspecies B and C can be also proposed for acetaldehyde. The same n is obtained for the oxidation of the adsorbates formed in reactions 5 and 6. These species justify the values of n for ethanol at Ead ) 0.55 V and also for acetaldehyde in sulfuric acid at the same potential. However, values of about 2 are observed for the latter at Ead ) 0.07 V. This fact can be explained by assuming that fragmentation of the C2 molecules occurs in this potential region yielding methane which does not

adsorb (eq II). In this case, the (C1)ad species is assumed to be (CO)ad (see reaction 6), which also oxidizes to CO2:

(CO)ad + H2O f CO2 + 2H+ + 2e-

n ) 2/1 ) 2 (10)

This adsorbate explains the values of n at low Ead. As the adsorption potential is set to more positive values, the amount of Had diminishes, and the CH3 group remains adsorbed. Scheme 1 summarizes reactions 1-10 described above. Solid and dashed boxes denote the original molecules and the detected products, respectively. The asterisk (*) indicates that these species were detected in FTIR studies.21 Similarities in the electrochemical behavior for the adsorbates from both acetaldehyde and ethanol seem to confirm that (CH3CO)ad (or its dissociated form) could be a common intermediate. Obviously, the change from the adsorption region where (CO)ad prevails to Ead values where (A)-(C) are dominant is not dramatic, and a mixture of adspecies should be formed at intermediate potentials. Potential cycling in the hydrogen region produces the desorption of methane from species C after fragmentation or directly from adsorbed CH3, (CO)ad remaining at the Rh surface. The reduction of these species yielding CH4 has been proposed from adsorbed ethanol at Pt.8 The values of n are, in general, higher for ethanol than for acetaldehyde adsorbates. Assuming that there is a relationship between these values and the dissociation of the molecule yielding methane, it could be concluded that the rupture of the molecule is more difficult in the case of ethanol. However, a second effect should be considered. The coverage of the surface by ethanol adsorbates is low for Ead < 0.45 V. This implies that ethanol cannot easily displace adsorbed hydrogen for its adsorption, that is, the adsorption is hindered by the presence of other adsorbed species. By increasing the concentration of ethanol to 3 M, Caram et al.20 observed a peak during the anodic stripping of the adsorbate formed at 0.07 V (that they assigned to chemisorbed CO), similar to that obtained in the present paper for acetaldehyde (see Figure 5). This result suggests that the fragmentation of the molecule is possible once adsorbed. In the case of ethanol, the adsorption process is slow and it is favored by an increase of bulk ethanol concentration. 4.4. Effect of the Anion in the Adsorption of Ethanol and Acetaldehyde. Perchlorate anions are considered not to adsorb on Rh, but decomposition yielding Cl-, which adsorbs strongly, occurs on this metal.22-24 In the present studies, a low HClO4 concentration (0.05 M) was used to prevent such a reaction.24 Thus, the studies

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using sulfuric acid as the base electrolyte allow one to establish the influence of the presence of strongly adsorbed anions on the electroreactivity of ethanol and acetaldehyde. These anions show the maximum adsorption in the 0.15-0.45 V potential range.31,32 Therefore, in this region the overall reaction given in eq I can be described as

CH3CH2OH/CH3CHO + (SO42-/HSO4-)ad f (R)ad + SO42-/HSO4- (III) For ethanol, it was proved that the adsorption is partially impeded by the presence of Had. Therefore, in the presence of adsorbed anions it is expected that a real competition occurs between the anions and alcohol molecules. This fact is reflected in the decrease of the oxidation charge associated to the residues as compared to that obtained in HClO4 solution. The same result was observed for the adsorption of CO2 on rhodium.33 In the case of acetaldehyde, the trend to adsorb is higher and process III is displaced to the right. For this compound, only slight differences at Ead < 0.30 V are observed comparing both electrolytes. Finally, the influence of anion adsorption on the values of n is briefly considered. In Table 2, it is shown that the values of n are smaller in perchloric acid, suggesting that the presence of adsorbed anions interferes in the dissociation process. 4.5. Comparison with the Reactivity on Pt. The electrochemical behavior of ethanol on Pt electrodes has been extensively studied by voltammetric and spectroelectrochemical techniques. During bulk electrooxidation, acetaldehyde, acetic acid, and CO2 are formed, as shown by EMIRS2,6,7 and SNIFTIRS.4,5 The relative amount of acetaldehyde produced in comparison to acetic acid depends on the bulk ethanol concentration.5,6 As the concentration of ethanol was increased, acetaldehyde

Me´ ndez et al.

predominated as the final product over acetic acid, probably due to the blockage of active surface sites that impeded the formation of O adlayers on the surface.5 With DEMS,1 only CO2 and acetaldehyde were detected. The same result is described in the present paper. However, FTIRS studies have shown that for Rh only the bands for CO2 and acetic acid appear in the spectra,20 suggesting that final oxidation to the acid is favored on this metal. Adsorbed CO was identified from infrared studies,2,4-8 but also other adspecies maintaining the C2 chain were proposed at Pt.2-8 A combined DEMS and FTIR study has established the composition of the adlayer formed upon adsorption of ethanol at Ead ) 0.35 V once removed from the solution.8 CO, COCH3 (acetyl), CH3CH2O (ethoxy), and CH3COH (species containing an OH group) were proposed as adsorbed intermediates.8 Similar combined studies are not available for acetaldehyde. Methane and ethane were observed during reduction of ethanol and acetaldehyde residues at Pt applying DEMS.8,19 (COCH3)ad and (CH3COH)ad, respectively, were considered responsible for these reactions. Thus, the adsorbed acetyl moiety dissociates and reduces in the negative-going scan to CH4 by Had, while (CO)ad is formed. The main difference with Rh is that only methane is detected with this metal. This result let us conclude that deprotonation and cleavage of the C-C bond are favored when Rh is used as the electrocatalyst. Acknowledgment. The authors thank the Direccio´n General de Investigacio´n (Project No. PB98-0434) and the Gobierno Auto´nomo de Canarias (Project No. PI1999/070) for financial support of this work. E.M. acknowledges Universidad de la Repu´blica (Uruguay), PEDECIBA, and Intercampus Programs for the fellowships during the stays at the University of La Laguna. LA010747I