Electrochemical Behavior of Benzaldehyde on Polycrystalline

Gabriel A. Planes,† Eduardo Moran,‡ José Luis Rodriguez,§ César Barbero,† and. Elena Pastor*,§. Departamento de Quı´mica, Universidad Nacional de Rio ...
1 downloads 0 Views 196KB Size
Download PDF
Langmuir 2003, 19, 8899-8906

8899

Electrochemical Behavior of Benzaldehyde on Polycrystalline Platinum. An in Situ FTIR and DEMS Study Gabriel A. Planes,† Eduardo Moran,‡ Jose´ Luis Rodriguez,§ Ce´sar Barbero,† and Elena Pastor*,§ Departamento de Quı´mica, Universidad Nacional de Rio Cuarto, 5800 Rio Cuarto, Argentina, and Departamento de Quı´mica Fı´sica, Universidad de la Laguna, 38071 Tenerife, Spain Received April 14, 2003. In Final Form: July 15, 2003 The reactions of benzaldehyde at platinum in 0.1 M HClO4 were studied by means of differential electrochemical mass spectrometry (DEMS) and in situ Fourier transform infrared (FTIR) spectroscopy combined with cyclic voltammetry. With the application of DEMS, it was established that this compound oxidizes completely to CO2 at E > 0.60 V (RHE) and reduces to toluene, benzene, and cyclohexane in the potential region for hydrogen adsorption and H2 evolution. No partial or total hydrogenation of toluene was observed. In situ FTIR measurements show that the oxidation of benzaldehyde also produces adsorbed benzoate ions (band at 1376 cm-1) at about 0.60 V (RHE), which are protonated to benzoic acid (band at 1273 cm-1) upon desorption at E > 1.0 V (RHE). The assignment is confirmed by FTIR experiments with benzoic acid. Fragmentation of benzaldehyde during adsorption is confirmed by the presence of vibration bands at 2002 and 1807 cm-1 assigned to lineal and bridge bonded CO, respectively, and at 1240 cm-1 due to (COH)ad. The results are compared with previous studies on the reactivity of benzyl alcohol, benzoic acid, toluene, and benzene.

1. Introduction 1-3

The electrochemical behavior of benzyl alcohol, benzoic acid,4 benzene,2 and toluene2 has been previously studied at platinum electrodes in acidic media applying differential electrochemical mass spectrometry (DEMS). The aim of this research was to establish the influence of the substituent in the aromatic ring on the reactivity of the aromatic compound. All these molecules interact irreversibly at the platinum surface and oxidize to carbon dioxide. Applying the method of displacement with CO,1,2 it was established that benzyl alcohol dissociates, producing benzene, but also suffers hydrogenolysis in the hydrogen adsorption/desorption region of platinum, forming adsorbed toluene. The dissociation reaction produces adsorbed CO from the alcoholic group in addition to benzene from the aromatic ring. Partial and total hydrogenation compounds from benzene and toluene were detected simultaneously with the production of molecular hydrogen.2,3 On the other hand, CO cannot displace benzoic acid, and no hydrocarbons were detected from its reduction.4 However, partial desorption of the adlayer was observed during the cathodic potential scan down to the onset of hydrogen evolution. This result was justified assuming that benzoic acid desorbs without further hydrogenation but is not volatile enough to be detected by DEMS.

The purpose of the present work is to complete the series of aromatic molecules (alcohol-aldehyde-acid), analyzing the reactivity of benzaldehyde at platinum in acidic media. The results will be compared with the electrochemical behavior of benzene and toluene.2 As in refs 1-4, DEMS is applied for the detection of gaseous (e.g., CO2) and volatile (e.g., benzene) compounds produced during electrochemical reactions. Preliminary results5 suggested that other likely oxidation products of benzaldehyde (e.g., benzoic acid) could not be detected by DEMS. It has been shown in the literature that in situ Fourier transform infrared (FTIR) spectroscopy could render complementary information by detection of all products, including nonvolatile species.6 In that way, not only benzaldehyde oxidation and reduction but also the related adsorptiondesorption of benzoic acid could be investigated. Accordingly, both techniques are used in this paper to establish a general overview of the electrochemical reactions of benzaldehyde at platinum. 2. Experimental Section

* Author to whom correspondence should be addressed. Telephone: +34922318028. Fax: +34922318002. E-mail: [email protected]. † Universidad Nacional de Rio Cuarto. ‡ On leave from Departamento de Ciencias Ba ´ sicas, Universidad de Santiago del Estero, Santiago del Estero, Argentina. § Universidad de la Laguna.

Solutions were prepared from Millipore Milli-Q water and analytical-grade reagents. The working solutions were 1-10 mM benzaldehyde or 10 mM benzoic acid in the supporting electrolyte (0.1 M HClO4). All the experiments were performed at room temperature under an argon atmosphere. 2.1. DEMS Experiments. The DEMS system consists of an electrochemical cell directly attached to a vacuum chamber containing the quadrupole mass spectrometer. The working electrode was a porous platinum layer deposited on a Teflon membrane that interfaces the electrochemical cell and the vacuum components. The experimental setup allows the simultaneous detection of mass spectrometric cyclic voltammograms (MSCVs) for selected mass-to-charge ratios (m/z) and cyclic

(1) Rodrı´guez, J. L.; Souto, R. M.; Gonza´lez, S.; Pastor, E. Electrochim. Acta 1998, 44, 1415. (2) Rodrı´guez, J. L.; Pastor, E. Electrochim. Acta 2000, 45, 4279. (3) Souto, R. M.; Rodrı´guez, J. L.; Pastor, G.; Pastor, E. Electrochim. Acta 2000, 25, 1645. (4) Souto, R. M.; Rodrı´guez, J. L.; Ferna´ndez-Me´rida, L.; Pastor, E. Electrochim. Acta 2000, 494, 127.

(5) Planes, G.; Mora´n, E.; Rodrı´guez, J. L.; Pastor, E. Port. Electrochim. Acta 2001, 19, 377. (6) Pastor, E.; Iwasita, T. Electrochim. Acta 1994, 39, 531. Rodriguez, J. L.; Pastor, E.; Schmidt, V. M. J. Phys. Chem. B 1997, 101, 4567. Souto, R. M.; Rodrı´guez, J. L.; Pastor, E.; Iwasita, T. Langmuir 2000, 16, 8456.

10.1021/la034627h CCC: $25.00 © 2003 American Chemical Society Published on Web 09/11/2003

8900

Langmuir, Vol. 19, No. 21, 2003

voltammograms (CVs) at a scan rate of v ) 0.01 V s-1. Three m/z ratios can be simultaneously detected. The electrochemical cell allows for the solution exchange, holding the potential control on the working electrode. More details about the technique and the experimental procedures have been given elsewhere.1,7 The working electrodes were activated by potential cycling between the onset for hydrogen and oxygen evolution at 0.01 V s-1. The real area (18-20 cm2) was estimated from the hydrogen adsorption region.8 For bulk experiments, after electrode activation, the solution containing the organic compound was introduced in the cell under potential control in the double-layer region of the platinum electrode. Then, three potential cycles were recorded, and the second is shown in the figures. The adsorption of benzaldehyde was studied at several potentials, Ead, in the range 0.05 < Ead< 0.8 V. The complete experimental procedure was as follows: (I) Adsorption. After activation of the electrode, the potential was stopped during the anodic scan at Ead and the benzaldehyde solution was admitted to the cell. Faradaic current and ion current responses were recorded during adsorption. (II) Elimination of Bulk Benzaldehyde. After 5 min of adsorption, the solution was completely replaced by pure base electrolyte under potential control. To ensure complete electrolyte replacement, about 20 times the cell volume was allowed to flow trough. (III) Stripping the Adlayer. After steps I and II, the adsorbates were oxidized or reduced by potential scans. In displacement experiments of adsorbates with CO, after steps I and II, a CO saturated solution was admitted to the cell for 5 min. Faradaic current and ion current signals were recorded during displacement. Once the CO adsorption was completed, bulk CO was eliminated by rinsing the cell with base electrolyte as in step II. Then, step III was performed. 2.2. FTIR Spectroscopy Experiments. The FTIR spectrometer was a Bruker Vector 22 provided with a mercurycadmium telluride detector. Parallel (p) and perpendicular (s) polarized IR lights were employed. A glass cell with a 60 °C aF2 prism at its bottom was used. The working electrode was a Pt disk of 10-mm diameter. The cell and experimental arrangements were described previously.9 Spectra were acquired from the average of 128 scans obtained with 8-cm-1 resolution. The reflectance ratio R/R0 was calculated, where R and R0 are the reflectances measured at the sample and the reference potential, respectively. In this way, positive and negative bands represent, respectively, the loss and gain of species at the sampling potential. Bulk studies were performed in 10 mM organic solutions. Benzaldehyde adsorbates were isolated following the same flowcell procedure described for DEMS with a difference in step III, where a potential step instead of a potential scan was applied for the stripping of the adlayer. For both techniques, the counter electrode was a platinum wire or foil, and the electrochemical cell was completed with a reversible hydrogen electrode (RHE) prepared in the electrolyte solution. All the potentials in the text refer to this electrode.

3. Results and Discussion 3.1. DEMS. 3.1.1. Benzaldehyde in Solution. The CV for a platinum working electrode in a 5 mM benzaldehyde + 0.1 M HClO4 solution is given in Figure 1A (solid line). For the sake of comparison, the CV in the base electrolyte is shown in the same figure (dashed line). A partial blockage of the surface is deduced from the current in the 0.05-0.40 V potential range. During the positive-going potential scan, an electrooxidation current develops at E > 0.60 V, attaining a broad maximum at 1.31 V. In the reverse scan, the presence of an anodic contribution is confirmed, comparing the curves in the presence (solid (7) Bittins-Cattaneo, B.; Cattaneo, E.; Ko¨nigshoven, P.; Vielstich, W. In Electrochemical Chemistry: A Series of Advances; Bard, A., Ed.; Marcel Dekker: New York, 1991; Vol. 17, p 181. (8) Woods, R. In Electrochemical Chemistry: A Series of Advances; Bard, A., Ed.; Marcel Dekker: New York, 1976; Vol. 9, p 1. (9) Nart, F. C.; Iwasita, T.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1030.

Planes et al.

Figure 1. (A) CV for Pt in a 5 mM benzaldehyde + 0.1 M HClO4 solution and (B) MSCV for m/z ) 44 simultaneously recorded. The dashed line corresponds to the base electrolyte. v ) 0.01 V s-1; area ) 19.6 cm2.

line) and in the absence (dashed line) of benzaldehyde in the solution. The cathodic current at E < 0.10 V confirms the reduction of benzaldehyde in this region. In the potential ranges where the anodic currents are observed, only the mass signal for m/z ) 44 displays a potential-dependent signal (Figure 1B). From this result, it could be assumed that CO2 is the sole electrooxidation product from benzaldehyde (the signal for m/z ) 44 corresponds to the radical cation [CO2]•+). However, it was demonstrated that benzoic acid is not detected through DEMS,4 and, therefore, its formation cannot be excluded in these experiments. Three features are well established from the MSCV in Figure 1B: two in the positive-going scan centered at 0.90 and 1.35 V and the third one in the negative sweep at 0.75 V. Only the contribution at 1.35 V was defined in the CV in Figure 1A, showing the sensitivity of the MSCVs for these studies. A similar behavior was observed during the anodic stripping of benzyl alcohol1-3 and toluene.2 The anodic current during the reverse scan in Figure 1A cannot be justified by the small CO2 signal in Figure 2. This fact suggests the formation of a nonvolatile oxidation product in this potential region. Oxidation reactions will be analyzed later. The electroreduction products from benzaldehyde are summarized in Figure 2A-D. The signal for m/z ) 91 describes the formation of toluene ([C6H5CH2]+), whereas those for m/z ) 84 and 78 correspond to cyclohexane ([C6H12]•+) and benzene ([C6H6]•+), respectively. These masses show different potential dependencies. Thus, the onset for the detection of benzene during the reverse run is observed at 0.40 V, and the ion current attains a maximum at 0.07 V (Figure 2C). The production of toluene (Figure 2A) and cyclohexane (Figure 2B) occurs at E < 0.10 V in the same potential region for hydrogen evolution (Figure 2D). From these results, it is made clear that benzaldehyde interacts with platinum, at least in part, dissociatively producing benzene (m/z ) 78), which probably adsorbs at

Electrochemical Behavior of Benzaldehyde on Pt

Langmuir, Vol. 19, No. 21, 2003 8901

Figure 2. MSCVs for (A) m/z ) 91 (toluene); (B) m/z ) 84 (cyclohexane); (C) m/z ) 78 (benzene), and (D) m/z ) 2 (H2). Same conditions as those in Figure 1.

alcohol.1-3

the platinum surface as in the case of benzyl Adsorbed benzene desorbs as the potential runs negatively:

(C6H6)ad h (C6H6)sol

E < 0.40 V

(1)

Next, we consider the production of cyclohexane (m/z ) 84). The onset for its formation (Figure 2B) coincides with the decrease of the benzene signal in the potential range for H2 evolution. Accordingly, benzene can be regarded as being consumed through reaction with H2, yielding cyclohexane:

(C6H6)sol + 3H2 f (C6H12)sol

E < 0.10 V (2)

In contrast to benzyl alcohol, partially hydrogenated hydrocarbons were not detected during electroreduction of benzaldehyde. Finally, toluene is also detected from benzaldehyde at E < 0.10 V. In the case of benzyl alcohol, the onset potential for toluene was established at 0.20 V, that is, at more positive potentials than those of the onset for the production of molecular hydrogen. Then, the formation of toluene from the alcohol was explained through the hydrogenolysis of the molecule by reaction with Had.1-3 In the case of benzaldehyde, interaction with molecular hydrogen is necessary:

(C6H5CHO)sol + 2H2 f (C6H5CH3)ad + H2O E < 0.10 V (3) (C6H5CH3)ad h (C6H5CH3)sol

E < 0.10 V (4)

Hydrogenation of the aromatic ring is not achieved for toluene. This fact suggests that toluene produced potentiodynamically adsorbs in a very small amount, and the main process in this potential region is the reduction of bulk benzaldehyde molecules to the hydrocarbon in the solution; that is, under these conditions, reaction 4 is shifted to the right. Evidence for the presence of adsorbed benzene and toluene molecules at the Pt surface can be achieved

Figure 3. (A) CV and (B) MSCV (m/z ) 44) for adsorbates formed from benzaldehyde at different Ead’s in 0.1 M HClO4. The dotted line corresponds to the base electrolyte. v ) 0.01 V s-1; area ) 19.6 cm2.

applying a flow-cell procedure to study the electrochemical behavior of isolated adsorbed species and also from CO displacement experiments. 3.1.2. Benzaldehyde Adsorbates. Benzaldehyde adsorbates formed at selected Ead’s are oxidized to CO2, giving a signal for m/z ) 44 (Figure 3). Complete oxidation requires four potential cycles up to 1.50 V, indicating that oxidation and cracking of the adsorbed species occur with the formation of partially oxidized adspecies during each potential excursion up to 1.50 V. The same results have been reported for benzene2, toluene2, benzyl alcohol,1-3 and benzoic acid.4 Only the first cycle is shown in Figure 3 for the sake of clarity. Using the integration of the mass signal, it is possible to evaluate the dependence of adsorbate coverage with potential (Figure 4B), which corresponds closely to the one obtained from the oxidation charge (Figure 4A). In both cases, the four potential cycles were considered for the integration. Three different contributions can be distinguished during the first oxidation cycle in Figure 3: two during the positive-going potential scan in the ranges 0.50 < E < 0.90 V and 0.90 < E < 1.50 V, and the third centered at 0.70 V in the negative sweep. Although the relative contribution of these features depends on Ead, a constant value for the faradaic and CO2 charge densities is obtained between Ead ) 0.15 and Ead ) 0.75 V (see Figure 4). It should be noticed that the potential regions for these features coincide with those observed in the presence of benzaldehyde in the bulk of the solution. Moreover, the intensity of the mass signal for m/z ) 44 is of the same order of magnitude. All these results are arguments to ensure that the production of CO2 in Figure 1 is related exclusively with the oxidation of the residues. The presence of several contributions in the MSCVs for the oxidation of the adsorbates strongly suggests the presence of different species on the surface. The first one around 0.80 V appears in the potential region where the

8902

Langmuir, Vol. 19, No. 21, 2003

Planes et al.

Figure 5. Ion current transients for benzene (m/z ) 78) and toluene (m/z ) 91) during adsorption of CO on a Pt surface already covered by adsorbates formed from benzaldehyde at 0.07 (dashed line) and 0.20 (solid line) V. Figure 4. (A) Benzaldehyde adsorption curve obtained from the anodic charge in the CV related to the oxidation of the adsorbate. (B) Benzaldehyde adsorption curve obtained from the integration of the ion current for m/z ) 44 in the MSCV. Values correspond to the integration over four cycles.

oxidation of COad occurs in the same experimental conditions.2 The formation of this adsorbate is confirmed by FTIR spectroscopy (see the next section). The oxidation reaction can be written as follows:

COad + H2O f (CO2)sol + 2H+ + 2e0.50 < E < 0.90 V (5) On the other hand, the signals in the platinum oxide region, during the negative scan and further potential cycling, have also been observed for benzene2 and toluene,2 and, therefore, these compounds, if adsorbed, could be responsible for this behavior. A general equation for the oxidation process could be:

residues + xH2O f x′(CO2)sol + x′′H+ + x′′eE > 0.80 V (6) In a previous work,2 it was proved that both benzene and toluene are susceptible to CO displacement. On the basis of this, the corresponding experiments were performed after benzaldehyde adsorption, as can be seen in Figure 5. It has to be taken in mind that the mass signals recorded have to be due to species already adsorbed on the platinum surface at the selected Ead because no further reactions or rearrangements occur during CO adsorption. It is shown that benzene (m/z ) 78) and toluene (m/z ) 91) are formed, but their relative amounts depend on Ead: as the potential is set to more positive potentials, the amount of benzene increases opposite to that of toluene. Then, the presence of adsorbed benzene in reaction 1 and adsorbed toluene in reaction 4 is confirmed. More information can be achieved from the electroreduction of the residues. This reaction was studied cycling the potential between Ead and 0.05 V in the base electrolyte after adsorption and solution replacement (Figure 6). The

Figure 6. Reduction of adsorbates formed from benzaldehyde at Ead) 0.35 V. v ) 0.01 V s-1; area ) 19.6 cm2. (A) CV and (B-D) MSCVs for m/z ) 84 (cyclohexane), m/z ) 91 (toluene), and m/z ) 78 (benzene), respectively.

same products as those in the bulk (Figure 1) are detected. The intensities of the mass signals for cyclohexane and benzene are of the same order of magnitude as the ones observed with bulk benzaldehyde (Figure 2), but much lower for toluene. This fact indicates that benzaldehyde molecules from the solution are reduced, producing toluene, while only adsorbates give rise to benzene and cyclohexane. The oxidation of the residues remaining at the Pt surface after potential cycling in the hydrogen region is given in Figure 7. An oxidation peak centered at about 0.60 V develops after reduction, that is, in the COad region, whereas most of the adsorbate that contributes in the platinum oxide region was desorbed in the same process. CV and MSCV resemble those in Figure 3 obtained at Ead ) 0.07 V. From these results, it can be concluded that the

Electrochemical Behavior of Benzaldehyde on Pt

Langmuir, Vol. 19, No. 21, 2003 8903

Figure 7. (A) CV and (B) MSCV for m/z ) 44, for the direct oxidation of adsorbates from benzaldehyde (dashed line) and for the remaining species after the reduction process (solid line), in 0.1 M HClO4. The dotted line corresponds to the base electrolyte. v ) 0.01 V s-1; area ) 19.6 cm2.

fragmentation of the benzaldehyde molecule is not complete during adsorption. Accordingly, the first step for the adsorption process should be

(C6H5CHO)sol h (C6H5CHO)ad

(7)

following by the fragmentation

(C6H5CHO)ad f (C6H6)ad + COad

(8)

Reaction 7 takes place in the whole potential range of adsorption. However, the extent at which reaction 8 occurs depends strongly on the adsorption potential. This reaction 8 is responsible for the formation of (C6H6)ad, which is displaced with CO in Figure 5, and, therefore, in a first approach, it could be initially assumed that this reaction predominates as the potential is set to more positive values. On the other hand, according with Figure 7, this reaction seems to be favored during reduction of the residues, which appears to be a contradiction with the CO displacement results. However, it has to be considered that benzene desorbs as the negative charge of the surface increases (Figure 6D), and then, for potentials more negative than 0.40 V, part of the (C6H6)ad formed in reaction 8 directly goes in the bulk, as indicted in reaction 1, and cannot be detected in the CO displacement studies. Then, the correct conclusion is that the fragmentation of the molecule is favored as the potential is set to more negative values, probably due to the presence of Had, which favors the transposition of the H atom needed for reaction 8. The formation of cyclohexane from the adsorbates occurs as in reaction 2. The presence of adsorbed toluene only at Ead next to 0 V (Figure 5) implies the production of H2 at this Ead for the reaction of benzaldehyde molecules to take place and then desorbs (reactions 3 and 4). The small amounts of toluene from the adsorbates during the potential run down

Figure 8. In situ FTIR spectra obtained in a 10 mM benzaldehyde + 0.1 M HClO4 solution. The spectrum at 0.05 V was taken as the reference. Sample spectra were collected in steps of 0.10 V during the positive-going excursion up to 1.50 V and back to 0.05 V. For the sake of clarity, only the positive series is given. 128 scans; 8-cm-1 resolution; p-polarized light.

to 0 V (Figure 6) can be justified in a similar way:

(C6H5CHO)ad + 2H2 f (C6H5CH3)sol + H2O E < 0.10 V (3′) When compared with the results reported for benzyl alcohol,1-3 it is reasonable to assume that the initial adsorbed species is the aldehyde because this molecule does not suffer hydrogenolysis by reaction with Had, a process which can easily occur with the alcoholic group in benzyl alcohol. While DEMS is able to detect volatile products, it is not able to detect nonvolatile products such as carboxylic acids. To do that, in situ FTIR is used. 3.2. In Situ FTIR. 3.2.1. Benzaldehyde in Solution. A series of spectra acquired with p-polarized light in a 10 mM benzaldehyde + 0.1 M HClO4 solution (Figure 8) shows the production (negative bands) of CO2 (signal at 2345 cm-1 assigned to the stretching vibration of CO2) and benzoic acid (signal at 1715 cm-1 due to the carbonyl group). The broad feature at about 1647 cm-1 corresponds to the vibration of uncompensated water, and that at 1107 cm-1 is due to perchlorate ions. The bipolar band at approximately 2000 cm-1 and the positive signal at 1809 cm-1 are related to adsorbed species because they are not present in the spectra acquired with s-polarized light (Figure 9). These contributions, which are related to lineal and bridge bonded CO, respectively, confirm the presence of COad species previously proposed in reaction 8. The negative bands developed around 1386 and at 1273 cm-1 need further consideration to be assigned. First, these features do not develop together in the spectra and have to be related with different species. From the spectra in Figure 9, it can be concluded that the signal at 1273 cm-1 is a solution band (it is present in both s- and p-polarized spectra at the same wavenumber), whereas that at 1386 cm-1 is due to adsorbed species (it is apparent only in the p-polarized spectrum). On the other hand, the band at

8904

Langmuir, Vol. 19, No. 21, 2003

Figure 9. In situ FTIR spectra in a 10 mM benzaldehyde + 0.1 M HClO4 solution taken with s- and p-polarized light at (A) 1.00 V (RHE) and (B) 1.5 V (RHE). The spectrum at 0.05 V was taken as the reference. 128 scans; 8-cm-1 resolution.

Planes et al.

at 0.60 V. The band intensity shows a potential dependence, which peaks at a potential where the platinum oxide layer builds (forward excursion) or the oxide layer is reduced (backward excursion). Corrigan and Weaver11 studied the adsorption of benzoic acid on gold, using potential-dependent FTIR spectra, and observed a band at 1390 cm-1, which they assigned to the carboxylate (CO2-) symmetric stretching of benzoate ions adsorbed with both oxygens oriented toward the metal surface (C2v symmetry). They observed a potential dependence of the band intensity similar to that reported here, where the intensity increases until the onset of oxide layer formation. Therefore, the same orientation for the benzoate species can be assumed to explain the spectra in Figures 8 and 9. The COO asymmetric stretching for benzoate expected around 1550 cm-1 has the vibration dipole moment parallel to the surface and, in accordance with the surface selection rule, cannot be observed. The intensity of the band at about 1273 cm-1, assigned to the C-O stretching in benzoic acid, increases sharply with the potential at E > 0.80 V, just when the platinum oxide layer starts to be formed. It has been reported for other carboxylic acids13 that the acid dissociates upon adsorption even in acidic media because the anion interacts more strongly than the free acid with the metal surface. Then, it seems that benzaldehyde oxidation produces benzoate species, which become adsorbed on the surface until the oxide layer builds up and displaces the benzoate to the solution, where they are protonated to benzoic acid. The following reactions account for these processes:

(C6H5CHO)sol + H2O f (C6H5COO)ad + 3H+ + 3e0.60 < E < 1.50 V (9) (C6H5COO)ad h (C6H5COO-)sol 0.80 < E < 1.50 V (10) (C6H5COO-)sol + H+ h (C6H5COOH)sol

Figure 10. Band intensity of the vibrational signals in the in situ FTIR spectra of Figure 8. For the integration of the lineal CO band, spectra were recalculated with a reference at 1.50 V to obtain absolute bands. Potential dependence for the CO bridge could not be established because its band is affected by the water and carbonyl vibrations.

1273 cm-1 is observed in the spectrum of pure benzoic acid,10,11 whereas the spectral contribution at 1386 cm-1 is absent in the spectrum of benzoic acid but present in the spectrum of benzoate.12 Then, benzoic acid seems to be responsible for the former band and adsorbed benzoate for the latter. More information is obtained from the potential dependence of the band intensities (Figure 10). The onset for the spectral feature at about 1385 cm-1 is established (10) SDBS Web. http://www.aist.go.jp/RIODB/SDBS/ (accessed Feb 2003). (11) Corrigan, D. S.; Weaver, M. J. Langmuir 1988, 4, 599. (12) Li, H.-Q.; Roscoe, S. G.; Lipkowski, J. J. Electroanal. Chem. 1999, 478, 67.

(11)

Spectra obtained with benzoic acid in the solution (Figure 11A) confirm the previous assignment. The same feature around 1390 cm-1 associated with benzoate adspecies is observed in a 10 mM benzoic acid + 0.1 M HClO4 solution. Not only the position of this spectral signal but also the potential dependence for the integrated band (Figure 11B) are similar to those shown in Figure 10, with the exception of the presence of the anion adsorbed in the 0.20-0.60 V potential range. The explanation for this difference is obvious considering that the onset for benzaldehyde oxidation was established at 0.60 V. The difference between the potential dependence of the 1390 cm-1 band in Figure 11B for the backward and that for the forward potential excursions confirms the role of oxide layer buildup on the benzoate adsorption/desorption. Finally, the correlation between the consumption of adsorbed CO and the production of CO2 is established from Figure 10A,B. The decrease in the intensity for lineal CO is observed for E g 0.60 V just at the onset for CO2, in good accord with the DEMS results in Figure 1. This fact confirms the relation between the formation of CO2 and the oxidation of the adsorbed species. Moreover, an increasing production of CO2 can be observed at E > 1.10 V, where no more COad is present (Figure 10). This is the potential region for the oxidation of the aromatic ring,1-4 which probably adsorbs through the π system in a flat, (13) Iwasita, T.; Pastor, E. In Interfacial Electrochemistry: Theory, Experiments and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 353.

Electrochemical Behavior of Benzaldehyde on Pt

Figure 11. (A) In situ FTIR spectra for benzoic acid (10 mM benzoic acid + 0.1 M HClO4) and benzaldehyde (10 mM benzaldehyde + 0.1 M HClO4) at E ) 1.00 V (RHE). The spectrum at 0.05 V is taken as the reference. p-Polarized light; 128 scans; 8-cm-1 resolution. (B) Band intensity of the feature at 1390 cm-1 obtained from the in situ FTIR spectra of benzoic acid (10 mM benzoic acid + 0.1 M HClO4).

oriented position (then, the CdC stretching mode is IR inactive). The band for benzoate is first observed at 0.70 V, indicating that bulk benzaldehyde oxidation cannot occur until the adsorbed species start to be oxidized. 3.2.2. Benzaldehyde Adsorbates. Spectra for the oxidation of the adsorbates formed at Ead ) 0.20, 0.35, and 0.50 V, applying the flow-cell procedure described in the Experimental Section, are given in Figure 12. From these results, some general aspects can be pointed out: (i) The amount of COL (positive band at ca. 2000 cm-1) increases as Ead is set to more negative values. This observation is in good agreement with the previous assumption that the fragmentation of benzaldehyde molecules is favored with decreasing the potential. (ii) COL is already oxidized when a potential step to 0.80 V is performed, but its contribution to the CO2 signal is small. At 1.50 V, a strong increase in the production of the latter is observed. (iii) Although the water bending around 1640 cm-1 disturbs the spectra, a positive band at about 1680 cm-1 and a negative feature at about 1740-1744 cm-1 are detected in all the cases in the spectra taken at 1.50 V (only when Ead ) 0.07 V, not shown, are these features absent). The contribution around 1740 cm-1 could be related to the production of some amounts of benzoic acid. The wavenumber is shifted to higher values compared with bulk experiments in Figure 8, but first, it has to be considered that the position of this band in both spectra is affected by the presence of that at 1677-1688 cm-1. Moreover, in the case of very dilute solutions (which is the case because less than a monolayer of benzaldehyde molecules can desorb as benzoic acid), the acid is present

Langmuir, Vol. 19, No. 21, 2003 8905

Figure 12. FTIR spectra for benzaldehyde adsorbed at different Ead’s. The reference spectrum was collected at Ead. p-Polarized light; 128 scans; 8-cm-1 resolution. Oxidation of the adsorbates was performed by potential steps from Ead to 0.80 and 1.50 V. Several spectra at 1.50 V were collected successively after the potential step, but only the first and the third are shown.

in the bulk as monomer instead of the dimers formed in more concentrated solutions, and a shift of about 40 cm-1 to higher wavenumbers is expected.14 On the other hand, the positive band at about 1680 cm-1 can be assigned to the CdC stretching of the aromatic ring,14 but to be allowed by the surface selection rule, the ring should be tilted on the surface. Then, it seems that benzaldehyde is adsorbed, at least in part, with a certain angle with the surface, and it is oxidized slowly at 1.50 V to the acid (the intensity of the upward band increases with time). Probably, adsorbed benzene molecules lie parallel to the surface and, therefore, cannot be detected, but in this position, the oxidation to CO2 is favored (adsorbed benzene is responsible for the increase in the intensity of the 2343 cm-1 signal observed at 1.50 V).

(C6H5CHO)ad + H2O f (C6H5COOH)sol + 2H+ + 2e0.80 < E < 1.50 V (12) The presence of a positive signal at 1240 cm-1 in the spectra for Ead ) 0.20 V, which is also observed for Ead ) 0.07 V (not shown), should be mentioned. Bands related to adsorbed species in this spectral region have been reported previously for different alcohols.15,16 At these potentials, it is established that θH > 0.50 and the formation of (COH)ad are possible in reaction 8:

(C6H5CHO)ad + Had f (C6H6)ad + (COH)ad E e 0.20 V (8′) 3.3. General Scheme. All the processes described previously on the basis of the experimental results can be summarized as shown in Scheme 1. The numbers in the

8906

Langmuir, Vol. 19, No. 21, 2003

Planes et al. Scheme 1

scheme correspond to the number for the corresponding reaction in the text. 4. Conclusions Spectroscopic studies on the reactivity of benzaldehyde at platinum have allowed the establishment of the reactions that take place in the different potential regions. Four processes can be distinguished: (1) Adsorption of Benzaldehyde. This reaction occurs in the 0.07-0.80 V potential range. (2) Fragmentation. This reaction occurs immediately after adsorption, but it is potential-dependent. As the potential is set to more negative values, this process is favored with the formation of adsorbed benzene and COad. Species (COH)ad was proposed to be present on the surface for Ead e 0.20 V. Benzene desorbs and goes into the bulk for E < 0.40 V. Similar behavior was observed previously for benzyl alcohol1-3 but not for benzoic acid4 because no benzene could be detected for the latter. (14) Socrates, G. In Infrared Characteristic Group Frequencies; Wiley: Chichester, 1980. (15) Xia, X. H.; Iwasita, T.; Ge, F.; Vielstich, W. Electrochim. Acta 1996, 41, 711. (16) Iwasita, T.; Pastor, E. Electrochim. Acta 1994, 39, 531.

(3) Hydrogenation. The evolution of H2 is necessary for hydrogenation, and, therefore, it occurs at E < 0.10 V. Benzene suffers complete addition to the double bonds and produces cyclohexane. Both (C6H5CHO)ad and benzaldehyde molecules in the bulk solution react with H2, yielding toluene. Hydrogenation of toluene has not been detected. The lack of hydrogenation of toluene and the need of H2 for this process are the main differences with the electrochemical behavior of benzyl alcohol1-3 (for the latter, hydrogenolysis is proposed to occur in the hydrogen adsorption/desorption region, and, accordingly, toluene is formed for E < 0.40 V). (4) Oxidation. Partial oxidation to adsorbed benzoate and finally to benzoic acid is observed for E > 0.60 V from benzaldehyde in the bulk solution. The formation of benzoate is confirmed from FTIR experiments of benzoic acid. All adsorbates suffer complete oxidation to CO2 for E > 0.50 V, except benzaldehyde adsorbs with the aromatic ring tilted on the surface, which produces benzoic acid. Acknowledgment. The authors thank the Gobierno de Canarias for financial support of this work (Project No. PI2001/023). LA034627H