Ethanol, Acetaldehyde and Acetic Acid Adsorption ... - ACS Publications

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J. Phys. Chem. C 2010, 114, 9850–9864

Ethanol, Acetaldehyde and Acetic Acid Adsorption/Electrooxidation on a Pt Thin Film Electrode under Continuous Electrolyte Flow: An in Situ ATR-FTIRS Flow Cell Study M. Heinen, Z. Jusys, and R. J. Behm* Institute of Surface Chemistry and Catalysis, Ulm UniVersity, D-89069 Ulm, Germany ReceiVed: February 16, 2010; ReVised Manuscript ReceiVed: May 3, 2010

The interaction of ethanol and its oxidative C2 derivatives acetaldehyde and acetic acid with a Pt thin film electrode in 0.5 M H2SO4 solution was investigated by in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration (ATR-FTIRS). Time-resolved spectro-electrochemical measurements were carried out under well-defined mass transport to/from the electrode in a thin-layer flow cell setup, allowing to in situ monitor the electrode|electrolyte interface and the formation/removal of adsorbed species in both potentiodynamic and potentiostatic mode. Spectro-electrochemical transients at constant electrode potentials upon electrolyte exchange were employed to identify adsorbed species and their temporal evolution, followed by subsequent stripping of the resulting adsorbates in the supporting electrolyte. Adsorption transient and stripping measurements performed at different constant potentials lead to the following conclusions. (i) Ethanol does not adsorb on Pt at potential below 0.15 V (RHE), whereas acetaldehyde decomposes to COad already at 0.06 V. Acetaldehyde decomposition proceeds via adsorbed acetyl species and the decomposition rate depends on the potential, having its maximum at 0.25 VRHE and being slow at 0.06 VRHE. (ii) At potentials between 0.3 and 0.5 VRHE, both ethanol and acetaldehyde adsorption result in COad and adsorbed acetyl species coexisting on the Pt surface. (iii) Stable acetyl species adsorbed in the double layer region are decomposed to COad and CHx,ad fragments when scanning the potential into the Hupd region. (iv) At potentials where COad is oxidized to CO2 and the ethanol oxidation current is increased, adsorbed acetate is observed. The latter species are in a fast adsorption-desorption equilibrium with acetic acid in the solution. 1. Introduction The ethanol oxidation reaction (EOR) is an important electrocatalytic reaction, in particular because of the potential application of ethanol as a liquid fuel in direct alcohol fuel cells (DAFC), being an attractive alternative to methanol due to the higher energy density and biocompatibility of ethanol vs methanol.1-7 The adsorption and (electro)oxidation of ethanol proceeds via a complex sequential reaction scheme,8 involving the formation of numerous adsorbed species and stable reaction products, as supported by spectroscopic techniques such as online differential electrochemical mass spectrometry (DEMS),7,9-15 in situ infrared reflection absorption spectroscopy (IRRAS),11-14,16-23 ex situ ultrahigh vacuum (UHV) surface-probing techniques,24-26 and gas chromatographic EOR product analysis.8,23,27,28 It is largely confirmed and generally accepted that the main oxidation products during ethanol oxidation are acetaldehyde, acetic acid, and CO2.8,23,28,29 The product yield depends on the temperature,1,15,30-32 ethanol concentration,1,6,15 electrode potential, and catalyst material.4,7,29,33-35 Because of the complex reaction situation, with ethanol, acetaldehyde and acetic acid interacting simultaneously with the Pt electrode, a detailed understanding of the interaction of the respective molecules with the electrode is a prerequisite for the mechanistic understanding of the EOR. This is topic of the present paper, where we followed the formation and removal of adsorbed species on a Pt film electrode in 0.5 M H2SO4 solution by in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration (ATR-FTIRS). The * To whom correspondence should be addressed. E-mail juergen.behm@ uni-ulm.de.

measurements were performed under well-defined mass transport to/from the electrode in a thin-layer flow cell setup. It is important to note that due to its high surface sensitivity and the combination with a flow cell, this technique allows us to follow adsorption and desorption processes with high time resolution, e.g., upon rapid electrolyte exchange from solutions containing the reactant molecules under study to pure base electrolyte and/ or vice versa. In the following, we will briefly summarize the major previous findings relevant for this topic, starting with the simplest case, the interaction of acetic acid with Pt, and proceeding via acetaldehyde to finally ethanol adsorption/ oxidation on this electrode. Studying acetic acid adsorption on Pt(100), Pt(110), and Pt(111) electrodes by FTIRS, Rodes et al. observed a characteristic band near 1420 cm-1 on all three surfaces, which they associated with adsorbed acetate species36 (see also ref 37). They related this to the symmetric C-O stretch vibration of a COO group, which was 2-fold coordinated to the surface. Similar findings were reported for acetate adsorption on polycrystalline Pt electrode, which was studied by combined potential dependent infrared spectroscopy (PDIRS) and radioactive tracer measurements.38 From the absence of the corresponding asymmetric vibration, the latter authors concluded on a C2V geometry with both carboxylate oxygen atoms oriented toward the metal surface, with the possible involvement of water molecules. For solutions containing 3-10 mmol of acetic acid, radiotracer data revealed a maximum coverage of adsorbed acetate of ∼0.27-0.37 monolayers (ML), where 1 ML corresponds to 1 adsorbed acetate species per Pt surface atom, which was achieved at ca. 0.75 V.38 In a recent paper by Berna et al., the temporal

10.1021/jp101441q  2010 American Chemical Society Published on Web 05/12/2010

In Situ ATR-FTIRS Flow Cell Study development of adsorbed acetate on gold single crystal electrodes was followed in potential step experiments, exploiting the high surface sensitivity of in situ ATR-FTIRS.39 These authors could show that acetate adsorption involves the desorption of initially adsorbed perchlorate anions and that the timedependent intensity of the acetate band fits to a Langmuir kinetics, which led them to the conclusion of random adsorption of acetate anions.39 For the interaction of acetaldehyde with a Pt electrode, COad was identified as an adsorbed species by IR spectroscopy,19,40-42 which led to the proposal of a parallel reaction mechanism. In one pathway, C-C bond breaking results in COad formation, which is further oxidized to CO2, and the other leads to CH3COOH formation.41-43 Besides adsorbed CO, additional weak signals at 1667-1697 cm-1 were reported, which were attributed to strongly bound species containing two C atoms, possibly η1-acetaldehyde and η1-acetyl.42 These findings agree with high resolution electron energy loss spectroscopy (HREELS) data for acetaldehyde adsorption on a Pt(755) surface performed under UHV conditions, where two bands at 1647 and 1667 cm-1 were assigned to the ν(CO) mode of surface-bound acetyl groups and η2(O)-CH3CHO species, respectively.44 Based on adsorbate stripping experiments, Rodes et al. proposed that the latter adsorbates do not act as intermediate during formation of acetic acid. In an ATR-FTIRS study, Shao and Adzic detected a band at 1620-1635 cm-1 during potentiodynamic acetaldehyde oxidation, which they also assigned to adsorbed acetaldehyde and/or acetyl species.45 In contrast to Rodes et al., these authors observed the formation of adsorbed acetate/acetic acid upon oxidation of adsorbed acetaldehyde adsorbates (“acetaldehyde stripping”), which led them to predict that these species represent a reaction intermediate on the pathway to acetic acid formation. Recently, the formation of CHx,ad species upon ethanol and acetaldehyde adsorption was detected by in situ surface enhanced Raman spectroscopy (SERS) by Lai et al.46 Potential dependent transients for the adsorption of acetaldehyde on Pt/C electrodes were performed by Wang et al., employing online DEMS.34 Based on a similar Faradaic current response for the oxidation of acetaldehyde adsorbate compared to the oxidation of a COad monolayer resulting from CO adsorption, they concluded that mainly COad is formed upon dissociative acetaldehyde adsorption.34 The temporal evolution of the adsorbed species upon adsorption/oxidation of acetaldehyde in an electrochemical environment has not been studied so far. Finally, extensive studies on the interaction of ethanol with Pt electrodes led to the following relevant results: Based on the previous IR spectroscopy studies, linearly and multiply bound COad is the dominant adsorbed species formed during ethanol adsorption, especially at potentials below 0.5 V.11,47,48 There is disagreement, however, on the presence and the nature of other (co)adsorbed species and their role in the mechanism of the ethanol oxidation, specifically on the question of whether C-C bond splitting occurs already during adsorption or during subsequent oxidation of the adsorbed intermediates. Willsau et al. concluded from DEMS experiments that ethanol adsorption on porous Pt at 0.3 V does not involve C-C bond splitting and that the latter occurs only at potentials above 0.7 V, together with the onset of adsorbate oxidation.10 Iwasita et al. and Pastor and co-workers reported that besides COad more than 60% of the adsorbates produced during ethanol adsorption at 0.3 V consist of C2 species for which they proposed structures such as Pt-OCH2-CH3, (Pt)2dCOH-CH3, and Pt-COCH3.29,33,49,50 These results are supported by a more recent paper by Wang et al.,34 reporting the desorption of methane and small amounts

J. Phys. Chem. C, Vol. 114, No. 21, 2010 9851 of ethane at low potentials, and additional adsorbed C2 species resulting upon adsorption of ethanol and acetaldehyde. In contrast to these results, Beden et al. concluded from their electrochemically modulated infrared reflectance spectroscopy (EMIRS) measurements that the Pt-surface at 0.18 V is mainly covered by COad, resulting from instantaneous bond breaking at low potentials.47 Schmiemann et al.51 and Gootzen et al.13 proposed that ethanol adsorption is mainly dissociative, leading to adsorbed CO and a CHx,ad fragments, where the latter can be desorbed as methane in the Hupd region. Based on DEMS experiments using 13C labeled ethanol, Bittins-Cattaneo et al.11 could demonstrate that CO2 is formed from both the methyl and the alcohol group of ethanol. Furthermore, they proposed that ethanol adsorption is mainly dissociative, resulting in COad and CHO as the main adsorbed species. A weak band at 1404-1412 cm-1 appearing in the EOR on different Pt single crystal electrodes (Pt(111), Pt(100), Pt(110), and Pt(335)) was assigned to adsorbed acetate.21,22,52,53 As mentioned above, the formation of CHx,ad species upon ethanol adsorption was detected by in situ surface enhanced Raman spectroscopy (SERS).46 Furthermore, employing ATR-SEIRAS for studying ethanol and acetaldehyde oxidation on a Pt thin-film electrode,45 Shao and Adzic found that acetate and CO adsorbates are formed during ethanol oxidation. The intensity of symmetric OCO stretch band of adsorbed acetate was found to correlate well with the voltammetric profile in the potential range between about 0.6 and 1.1 VRHE. A CdO stretch band related to adsorbed acetaldehyde and/or acetyl was also observed (1620-1635 cm-1). These compounds were interpreted as reaction intermediates, which further react to COad and acetic acid. Despite extensive electrochemical and spectro-electrochemical studies, a systematic investigation of the interaction of ethanol and its oxidative derivatives acetaldehyde and acetic acid with Pt as most commonly used electrode material is still missing. This is the topic of the present work. Following a brief description of the experimental setup and procedures, we will present our results, starting with potentiodynamic adsorption/ desorption and potentiostatic adsorption/desorption transients of acetic acid (section 3.1), followed by similar measurements on the acetaldehyde-Pt interaction (section 3.2) and finally on the adsorption/oxidation behavior of ethanol (section 3.3). We will focus on the detection of the adsorbed species and their potential dependent temporal evolution upon electrolyte exchange experiments. Finally, consequences of the resulting findings on the mechanistic understanding of the EOR are discussed. 2. Experimental Section 2.1. Experimental Setup. The adsorption/oxidation experiments were performed in a thin-layer flow cell described in detail earlier,54,55 with a cell volume of about 100 µL and a flow rate of about 50 µL s-1. This allows us to obtain a nearly completely exchange of the electrolyte within 3-4 s, by switching between two supply bottles, e.g., between base electrolyte (0.5 M H2SO4) and solutions containing the respective reactant (0.1 M solutions of the organic compounds in 0.5 M H2SO4). The Pt working electrode (exposed area ca. 1 cm2, roughness factor ca. 5) was prepared by electroless deposition of a thin Pt film on the flat surface of a Si prism, following a procedure described in ref 56; a Pt foil and a reversible hydrogen electrode (RHE) served as counter and reference electrode. IR spectra were acquired using a BioRad FTS-6000 spectrometer equipped with a MCT detector at a resolution of 4 cm-1, coadding 5 (25) interferograms for each spectrum (ca. 1(5) s per spectrum). The intensities are given by the absorbance, defined by log(R0/R),

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where R0 and R represent the reflected IR intensities at the reference and at the sample potential, respectively. This data processing results in spectra where peaks pointing up reflect an increased absorption and peaks pointing down a loss of absorption compared to the reference spectrum. All potentials in this paper are referenced against that of the RHE, and the experiments were performed at room temperature. 2.2. Experimental Protocol. For the potentiodynamic measurements, the Pt film was cleaned by continuous cycling between 0.06 and 1.3 V in base electrolyte. The potential was stopped at 0.06 V and a reference spectrum was acquired in 0.5 M H2SO4, just before changing the electrolyte to 0.1 M ethanol or 0.1 M acetaldehyde containing solution, respectively. After about 30 s adsorption at 0.06 V, potential cycling between 0.06 and 1.3 V was started. For the potentiodynamic measurements in 0.1 M acetic acid containing solution, the same experimental protocol was applied, except that, after cleaning the electrode in 0.5 M H2SO4, the potential was stopped at 0.2 V in the negative-going scan. The subsequent potential cycling was started in the negative-going direction. For the adsorption transient measurements, the electrode was cycled between 0.06 and 1.3 V in base electrolyte, and then the potential was stopped in the negative-going scan at the corresponding adsorption potential. A reference spectrum was recorded and subsequently the electrolyte was exchanged to the respective reactant containing electrolyte. Then the adsorption/ oxidation of reactant was followed for 300 s, before the electrolyte was exchanged again to pure 0.5 M H2SO4 solution. To identify subsequent changes in the adlayer composition, the ATR-IR spectra were recorded for another 120 s. After 5 min rinsing the cell with base electrolyte to remove any residues of ethanol from the solution, a potential scan was started (10 mV s-1), to identify adsorbed species formed on the surface during interaction with reactant solution and their reactive changes during the potential scan (“adsorbate stripping”). The potential scan went initially in the negative-going scan direction to identify reductive reaction/desorption processes (“reductive adsorbate stripping”) and then reverted to the positive-going scan direction (“oxidative adsorbate stripping”). 3. Results and Discussion 3.1. Adsorption/Oxidation of Acetic Acid. 3.1.1. Potentiodynamic Measurements. A representative base voltammogram (CV) of the Pt thin film electrode in 0.5 M H2SO4 (potential range from 0.06 to 1.3 V, scan rate 10 mV s-1) is shown in Figure 1a as a dotted line (open squares). It reproduces the typical features of polycrystalline Pt,57 confirming that the Pt film electrode has the same electrochemical properties as polycrystalline Pt. Furthermore, CVs of the Pt electrode in 0.5 M H2SO4 solution containing different concentrations of acetic acid (1 and 100 mM) are included in Figure 1a. These CVs reveal the following systematic changes with increasing acetic acid concentration: (i) The onset of Pt oxidation shifts to more positive potentials with increasing acetic acid concentration, accompanied by an increase in the oxidative current between 1 and 1.3 V in the positive-going scan, due to Pt oxide formation. (ii) The charge for Pt surface oxide reduction decreases with increasing acetic acid concentrations, in agreement with previous findings.38,58 The potential of the peak maximum for Pt reduction remains, however, at 0.8 V. (iii) The peak at more positive potential in the hydrogen underpotential deposition (Hupd) range shifts to lower potentials with increasing acetic acid concentration compared to base electrolyte (CV shown only for 0, 1, and 100 mM, see also ref 58), whereas the second (low potential)

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Figure 1. In situ ATR-IR spectro-electrochemical cyclic voltammetry measurements for acetic acid bulk oxidation on a Pt thin film electrode. (a) Faradaic current and integrated band intensities of (b) adsorbed (bi)sulfate (1240 and 1100 cm-1) and (c) adsorbed acetate (1410 cm-1) (see Figure 2), recorded in electrolytes with different acetic acid concentration: 0.5 M H2SO4 + 0.0 M CH3COOH (open squares), 0.5 M H2SO4 + 0.001 M CH3COOH (filled circles), and 0.5 M H2SO4 + 0.1 M CH3COOH (open triangles). Note: the potential was held constant at 0.2 V (see experimental) before the potential scan was started in the negative-going direction (room temperature; potential sweep rate 10 mV s-1, electrolyte flow rate 50 µL s-1, reference spectrum at 0.06 V in 0.5 M H2SO4).

Hupd peak remains almost unaffected by the presence of acetic acid. (iv) Similar trends are observed for the Hupd region in the positive-going scan, namely a hardly affected low potential peak and a shift to lower potentials for the oxidative desorption of hydrogen with increasing acetic acid concentrations in the higher potential peak (see also ref 58). Finally, (v) no additional current peak was found in the entire potential regime from 0.06 to 1.3 V. In total, these finding indicate that acetic acid adsorption does not cause a measurable formation of stable irreversibly adsorbed species. Comparable results were reported by other groups.38,58-61 Typical ATR-FTIR spectra recorded during the potential sweep are plotted in Figure 2. Besides IR signals arising from interfacial water (OH stretch vibration at 3450 cm-1 and OH deformation vibration at 1610 cm-1)56 and adsorbed (bi)sulfate species at 1230 and 1100 cm-1,62 an additional band centered around 1410 cm-1 is observed, whose wavenumber changes with potential from 1406 cm-1 at 0.15 V to 1413 cm-1 at 0.85 V. Following previous studies, this peak is assigned to the symmetric OCO stretch vibration of adsorbed acetate, which is bound to the surface via the two oxygen atoms.36,38,45,50 The small peak at 1350 cm-1, which always appears together with the peak at 1410 cm-1, was assigned to the in-plane bending vibrations of the CH3 group of the adsorbed acetate (δCH3).39,45,61 No other bands belonging to adsorbed acetic acid were observed, like for example the antisymmetric OCO stretch vibration (ca. 1550 cm-1), which indicates that these species, if present on

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Figure 2. Selected in situ ATR-IR spectra of a sequence recorded during the first cycle (a) and the second positive-going scan (b) in potentiodynamic flow-cell measurements in acetic acid containing solution (0.5 M H2SO4 + 0.1 M CH3COOH) on a Pt thin-film electrode (room temperature, potential scan rate 10 mV s-1, electrolyte flow rate 50 µL s-1, reference spectrum recorded at 1.3 V in the same electrolyte).

the surface, do not have a net dipole change perpendicular to the surface. Furthermore, no signals related to COad or other adsorbed species were detected on the Pt surface under these conditions, confirming earlier findings that oxidation or decomposition of acetic acid is inhibited under these conditions.60,61 In FITR spectroscopic studies on the adsorption of acetic acid, which were performed in an external reflection configuration, three additional IR bands were detected at 1710, 1380, and 1280 cm-1. These bands were assigned to the CdO stretch (νCdO), CH3 bend (δ(CH3), and C-O stretch (νC-O) vibrations, respectively, of acetic acid in solution.38 In the present ATR-FTIRS study, these features were only detected for solutions containing more than 100 mM of acetic acid, which is attributed to the low bulk sensitivity of the in situ ATR-FTIRS measurements (see Figure 6). The potential dependent integrated band intensities of adsorbed (bi)sulfate (1230 and 1100 cm-1) and acetate (1410 cm-1) in electrolytes with different concentrations of acetic acid (0, 1, and 100 mM) in 0.5 M H2SO4 are plotted in panels b and c of Figure 1, respectively. In 0.5 M H2SO4, the (bi)sulfate intensity increases in the positive-going scan from 0.15 to 0.8 V, reflecting an increasing coverage of adsorbed (bi)sulfate, and then decreases at higher potentials together with the oxidation of the Pt surface. After the onset of the Pt reduction peak in the negative-going scan, the (bi)sulfate intensity increases again, passes a maximum at about 0.65 V and then decreases continuously until it completely disappears in the Hupd region. Going to 1 mM acetic acid solution, the shape of the (bi)sulfate intensity/potential curve remains nearly identical except for a decrease in the (bi)sulfate intensity for potentials between 0.06 and 0.9 V. For 100 mM acetic acid solution, the (bi)sulfate intensity and hence (bi)sulfate adsorption is nearly completely suppressed in the entire potential region. The integrated band intensity versus potential plot for adsorbed acetate is shown in Figure 1c. The potential dependent adsorption of acetate, which is quite similar to the adsorption characteristics of sulfate, starts at about 0.15 V. However, the potential of the maximum intensity is shifted from 0.4-0.8 V for (bi)sulfate adsorption to 0.9 V in 1 and 10 mM acetic acid solution. Further details of the adsorption behavior of acetic acid at constant potential will be given in the following sections.

The results of the potentiodynamic electrochemical and in situ ATR-FTIR measurements in acetic acid containing electrolyte can be summarized as follows. (i) Acetic acid can not be oxidized at room temperature on a Pt electrode in the potential regime between 0.06 and 1.3 V, as seen from the absence of an additional current peak in the CVs for acetic acid containing electrolytes. (ii) Under the present reaction conditions, the C-C bond can not be broken and therefore acetic acid can not decompose to COad. Adsorbed acetate is the only acetic acid related adsorbate detected by ATR-FTIR in the potential range between 0.15 and 1.3 V in acetic acid containing electrolyte. (iii) Acetate adsorbs more strongly compared to (bi)sulfate, as evidenced by the absence of any (bi)sulfate correlated intensity in 100 mM acetic acid containing 0.5 M H2SO4 solution. The higher adsorption strength of adsorbed acetate results in a shift to higher potentials of the onset of Pt oxidation compared to that in a base CV, probably due to a similar shift for OH adsorption. Likewise, a shift to more negative potentials of the Hupd peak centered at 0.25 V is due to the competing adsorption of acetate. Due to the decreasing adsorption probability of negatively charged acetate with decreasing electrode potential (see Figure 1c), the Hupd peak centered at 0.1 V is hardly affected by the presence of acetic acid in the electrolyte. 3.1.2. Acetic Acid Adsorption Transients. Figure 3 shows the development of the integrated band intensity of adsorbed acetate with time, after changing from 0.5 M H2SO4 to 0.1 M acetic acid containing 0.5 M H2SO4 electrolyte and back at different, constant potentials. (Since the spectra closely resemble those in Figure 2 in their main features, these are not shown.) For potentials below 0.15 V, no adsorbed acetate can be observed at all, which we attribute to a Hupd blocking of the surface and weak electrostatic interaction between the negatively charged electrode and the negatively charged acetate anion. At E g 0.15 V, the band intensity of adsorbed acetate increases immediately after the electrolyte exchange, within about 3 s, to a steady-state value. In a similar way, it decreases very fast (about 3-4 s) when changing back to pure base electrolyte after about 280 s. The general shape of the adsorption transient of acetic acid does not change significantly when increasing the adsorption potential. The intensity, however, increases continu-

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Figure 3. Integrated intensity of adsorbed acetate during exchange of electrolyte from 0.5 M H2SO4 to 0.5 M H2SO4 + 0.1 M CH3COOH (t ) 12 s) and back (t ) 280 s) at different constant potentials (electrolyte flow rate 50 µL s-1).

ously with increasing potential up to 0.6 V. The difference in the integrated band intensity of adsorbed acetate after the electrolyte exchange to acetic acid free solution is attributed to slight deviations in the IR background before and after acetic acid adsorption at the respective adsorption potential. (Note: Transients at higher potentials were not measured, but the potential dependent intensity of adsorbed acetate is expected to follow the trends observed in the potentiodynamic measurements presented in the preceding section.) The kinetics of the acetate/acetic acid adsorption and desorption processes on Pt electrodes is expected to proceed on a similar time scale as (bi)sulfate adsorption/desorption, which was shown in recent potential step experiments on a Pt film electrode to occur at least on a millisecond time scale.63 Adsorption of acetate on gold single crystal electrodes was shown to occur on a similar time scale.39 Therefore, the kinetics of acetate adsorption/desorption, can not be determined from the present electrolyte exchange measurements, considering that the time required for exchanging about 99% of the electrolyte is about 3-4 s (cell volume ∼100 µL, flow rate ∼50 µL s-1). Hence, the measured intensity of adsorbed acetate always represents the steady-state value of the adsorption-desorption equilibrium of acetic acid. Subsequent adsorbate stripping experiments (not shown, since identical to the base CV), performed after the acetic acid adsorption transients, indicate that no stable adsorbates are formed during acetic acid adsorption at room temperature for potentials up to 0.6 V. In total, the adsorption transients of acetic acid have shown that (i) acetate adsorption/desorption is fast and (ii) acetate is only weakly adsorbed compared to COad and can easily be removed from the surface by desorption. No other stable adsorbates were detected. (iii) Acetate adsorption starts at E g 0.15 V and, based on the potentiodynamic data, reaches its highest steady-state coverage at about 0.95 V. It desorbs at E > 1.1 V upon PtO formation. 3.1.3. Concentration Dependence. Figure 4 illustrates the development of the integrated intensity of (a) adsorbed acetate and (b) displaced (bi)sulfate at 0.6 V with adsorption time for different concentrations of acetic acid between 1 and 100 mM. At very high acetic acid concentrations it is necessary to distinguish between adsorbed acetate and contributions from bulk acetic acid, which become significant for acetic acid concentrations above 0.5 M (see IR spectra below). This was done by removing the contributions from bulk acetic acid by

Figure 4. Integrated intensity of (a) adsorbed acetate and (b) (bi)sulfate during exchange of electrolyte from 0.5 M H2SO4 to 0.5 M H2SO4 + 1, 5, 10, 50, and 100 mM CH3COOH (t ) 0 s) and back (t ) 120 s) at 0.6 V (electrolyte flow rate 50 µL s-1).

taking the reference spectrum at 0.06 V in acetic acid containing solution, and assuming that at this low potential no adsorbed acetate is present on the Pt surface. The intensity of adsorbed acetate increases with increasing concentration of acetic acid. This trend is mirrored by that of the negative intensity for (bi)sulfate. The dependence of the acetate intensity, averaged from t ) 30 to 100 s, on the concentration of acetic acid is shown in Figure 5a. The linear correlation between acetate intensity, which is assumed to be proportional to the acetate coverage at these low concentrations, and the logarithm of the concentration obtained for acetic acid concentrations up to 0.5 M, is compatible with nondissociative Langmuir-type adsorption.39,64 At higher concentrations (>500 mM), the surface is apparently saturated with adsorbed acetate, with no further increase in the intensity. Similar trends are observed for the dependence of the wavenumber against the acetic acid concentration (Figure 5b). Here we find a rapid linear increase of the wavenumber with increasing bulk concentration, which levels off for concentrations of g50 mM of acetic acid in the bulk, supporting the linear relation between surface coverage and concentration. The spectra at high acetic acid concentrations allow us also to distinguish between adsorbed acetate and contributions from bulk acetic acid, which become detectable for solutions containing more than 0.5 M of acetic acid. Figure 6 shows IR spectra acquired at 0.6 V in solutions containing different concentrations of acetic acid in 0.5 M H2SO4. These spectra were referenced against a spectrum recorded at 0.6 V in 0.5 M H2SO4. They show both positive and negative peaks, equivalent to an increase or decrease of absorption compared to the background conditions (sulfuric acid solution at 0. 6 V). For 0.01 and 0.1 M acetic acid solution, we find an increase in the intensity of absorbed acetate (1410 cm-1) and a decrease in the signal related to interfacial water (3450 and 1600 cm-1). The negative bands in the spectral region between 1250 - 1100 cm-1 are attributed


Figure 5. (a) Integrated band intensity and (b) the wavenumber of adsorbed acetate plotted against the concentration of acetic acid in 0.5 M H2SO4 solution (E ) 0.6 V, electrolyte flow rate 50 µL s-1).

Figure 6. In situ ATR-IR spectra recorded at 0.6 V in 0.5 M H2SO4 + different concentrations of acetic acid as indicated in the figure (electrolyte flow rate 50 µL s-1, reference spectrum taken at 0.6 V in 0.5 M H2SO4).

to displaced (bi)sulfate.62,65 With increasing concentration of acetic acid, additional features appear in the IR spectra, which can be attributed to vibrations of acetic acid in the bulk electrolyte, namely a CdO stretch mode at around 1714 cm-1, a CH3 bending vibration at ∼1417 cm-1, and a C-O stretch mode at ∼1283 cm-1, in agreement with the literature.38 These date demonstrate the high surface/low bulk sensitivity of IR measurements performed in an ATR configuration.36 In total, the ATR-FTIRS results presented in this section have shown that for potentials between 0.15 and about 1.3 V, acetic acid can adsorb on a Pt electrode, as evidenced by an IR band at 1410 cm-1. In agreement with earlier reports, this band was assigned to adsorbed acetate bound perpendicularly to the


Figure 7. Potentiodynamic acetaldehyde bulk oxidation on a Pt thinfilm electrode in an ATR-IR spectro-electrochemical flow-cell: (a) Cyclic voltammogram (a) and integrated band intensities of (b) adsorbed COL and (c) adsorbed acetate (from Figure 8) (0.5 M H2SO4 + 0.1 M CH3CHO solution, potential sweep rate 10 mV s-1, electrolyte flow rate 50 µL s-1).

surface via two oxygen atoms. Typically for the adsorption of anions, the integrated band intensity of adsorbed acetate increases with potential up to about 0.95 V and decreases at higher potentials due to formation of PtO. Acetic acid can not be oxidized on the Pt electrode at room temperature, which was concluded from the absence of a Faradaic current. Based on the fast decrease of the integrated absorbance of adsorbed acetate species in electrolyte exchange experiments, these species are in a fast adsorption-desorption equilibrium with acetic acid in the solution. 3.2. Adsorption/Oxidation of Acetaldehyde. 3.2.1. Potentiodynamic Measurements. Similar measurements as presented above for the Pt-acetic acid interaction were performed also for acetaldehyde. The CV for the oxidation of 0.1 M acetaldehyde on a Pt film electrode in 0.5 M H2SO4 (first and second scan) is shown in Figure 7. The general shape of the CV largely reproduces the voltammetric features of acetaldehyde oxidation on polycrystalline Pt.42,45,66,67 Two oxidative current peaks appear in the positive-going scan, one centered at 0.9 V (peak I) with a small shoulder on the lower potential side and the other one starting at about 1.3 V (peak II). These peaks were attributed recently to the oxidation of acetaldehyde to acetic acid, since both Faradaic current peaks were not correlated with CO2 formation.66 On the basis of the finding that CO2 formation was only observed in a peak corresponding to the low potential shoulder of the first Faradaic current peak in the positive-going scan, which starts at 0.5 V and passes through a maximum at 0.78 V, the authors of the latter study attributed this shoulder to the oxidation of COad formed by dissociative acetaldehyde decomposition at lower potentials.66 In the first positive-going potential scan, the onset of the oxidation current is shifted by about 50 mV to more negative potentials compared to the second and subsequent positive-going scans, which can be explained

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Figure 8. Selected in situ ATR-IR spectra of a sequence recorded during the first cycle (a) and the second positive-going scan (b) in potentiodynamic flow-cell measurements of acetaldehyde bulk oxidation at a Pt thin-film electrode (0.5 M H2SO4 + 0.1 M CH3CHO solution, potential sweep rate 10 mV s-1, electrolyte flow rate 50 µL s-1, reference spectrum recorded at 1.3 V in the same electrolyte).

by the lower COad intensity/coverage in the first positive-going scan (see below). In the negative-going scan, we find a broad Faradaic current peak centered at 0.7 V (peak III). The Hupd peaks are largely suppressed, due to the presence of COad (see below). A series of selected ATR-IR spectra recorded during the voltammetric measurements are shown in Figure 8. The observed bands are attributed to linearly and multiply bound COad at 2080 and 1880 cm-140,42,45 as well as to adsorbed acetate (1410 cm-1, see section 3.1.1). The potential dependence of the COL and adsorbed acetate integrated band intensities are plotted in Figure 7, panels b and c, respectively. In the first positive-going, scan, the intensity of COL steeply increases from 0.1 to 0.3 V, followed by a slow increase from 0.3 to 0.7 V and a rather fast decrease to zero at more positive potentials. In the negative-going potential sweep, the COL band reappears at about 0.6 V and its intensity increases up to 0.06 V. This is followed by a further intensity increase in the second positivegoing sweep, up to 0.7 V, leading to a significantly higher COL integrated band intensity in the second positive-going scan than in the first one. At E > 0.7 V, the COL intensity decreases continuously due to its oxidation to CO2, reaching the zero level at about 0.85 V. In the first positive-going scan, the integrated band intensity of adsorbed acetate starts to increase at 0.7 V, passes through a maximum at about 1.0 V and then decreases at more positive potentials. The onset of acetate detection in the second positive-going scan is shifted by about 100 mV to more positive potentials, which is attributed to a similar shift of the COad intensity decrease in the second positive-going scan compared to the first positive-going scan. In the negative-going scan, the intensity of adsorbed acetate reappears at about 0.85 V, passes through a maximum at ∼0.65 V and then decreases until disappearing at potentials lower than 0.4 V. In addition to COad and adsorbed acetate, an additional IR band at around 1630 cm-1 is visible in the spectra in Figure 8. In this spectral region, signals for the deformation vibrations of interfacial water (1600 cm-1)56 and water coadsorbed with COad (1630 cm-1)56 are detected, which are accompanied by O-H stretch vibrations at higher wave numbers (∼3450 and 3550 cm-1), respectively. However, especially in the first

positive-going scan, the band at 1635 cm-1 is too large to solely originate from water coadsorbed with CO, when comparing its intensity to the intensity of the corresponding O-H stretch vibration at 3550 cm-1. Instead, it must be related to the formation of an additional surface species, due to adsorption of acetaldehyde. A similar band at 1620 - 1635 cm-1 was also detected by Shao and Adzic and attributed to adsorbed acetaldehyde/acetyl species.45 Based on measurements using CH3CD2OH, we will show in a forthcoming publication that the band is caused by adsorbed acetyl species rather than by adsorbed acetaldehyde. This is also in agreement with a recent findings from density functional theory calculations by Wang et al., who proposed adsorbed acetyl to be the spectroscopically detectable precursor for C-C bond breaking.68 Consequently, we will refer to these species as adsorbed acetyl in the following. The potential dependent integral absorbance in the spectral region between 1520 - 1720 cm-1 is plotted in Figure 8d. As mentioned before, besides adsorbed acetyl also interfacial water absorbs in this spectral region. Already before the electrolyte exchange to acetaldehyde containing solution, a positive value is obtained for the integrated intensity, which is attributed to an increase in the concentration of interfacial water at 0.06 V compared to the potential were the reference spectrum was acquired (1.3 V). After the electrolyte exchange at 0.06 V, the integrated intensity in the spectral region between 1520 and 1720 cm-1 increases due to the formation of adsorbed acetyl species (Figure 8d). In the first positive-going potential scan, the integrated intensity continuously decreases and reaches a value of zero at 1.3 V. In the negative-going scan, it increases for potentials below 0.8 V. Due to the potential dependent changes in the structure of the interfacial water, it is hardly possible to distinguish between the contributions from adsorbed acetyl and from interfacial water. These limitations can be circumvented in potentiostatic adsorption transients, which will be discussed in section 3.2.2. In summary, the potentiodynamic spectro-electrochemical data for acetaldehyde oxidation on a Pt film electrode in the present section show that (i) the adsorption/oxidation of acetaldehyde in 0.5 M H2SO4 solution is accompanied by the formation of adsorbed COL, COM, acetate species, and a species

In Situ ATR-FTIRS Flow Cell Study characterized by a peak at ∼1630 cm-1, which we associate with adsorbed acetyl species. (ii) The decrease of the COL intensity in the positive-going potential scan occurs in the same potential range as the increase in the Faradaic current peak, indicating that this peak is correlated with the oxidation of adsorbed CO. (iii) At potentials where the COad coverage is low due to COad oxidation, adsorbed acetate accumulates on the Pt surface, which results from the production of acetic acid. The shape of the intensity-potential profile for adsorbed acetate corresponds to the Faradaic current signal in the region of peak I, which supports this suggestion (indication of acetic acid formation by adsorbed acetate). It should be mentioned that there are significant differences between our results and the results reported by Shao and Adzic for the potentiodynamic oxidation of acetaldehyde.45 They reported that the COL intensity is already rather high prior to the first positive-going scan, while after the first negative-going scan, the COL intensity at 0.12 V is significantly lower.45 In contrast, in the present measurements, the COL intensity is rather low upon 30 s (initial waiting time, see experimental part) adsorption at 0.06 V (see also the acetaldehyde adsorption transients in the next section). Furthermore, after a potential excursion to anodic potentials, the COL intensity reaches significantly larger values at 0.06 V, in the first negative-going scan, compared to the value before the first positive-going scan. We attribute these differences to the different experimental conditions, as explained in the following. While we cycle between 0.06 and 1.3 V, the potential region was reduced to 0.12 and 1.22 V in the work of Shao and Adzic. The reduced potential window is expected to result in an accumulation of CHx,ad species, which in turn will slow down the COad formation in the negative-going potential scan in the latter study compared to our results. Furthermore, in our study the electrode was exposed to acetaldehyde at 0.06 V (for details see experimental), whereas Shao and Adzic chose 0.12 V. The faster COad formation at 0.12 compared to 0.06 V (see next section) explains the significantly higher COad intensity prior to the first positive-going scan in the paper by Shao and Adzic compared to our results. 3.2.2. Acetaldehyde Adsorption Transients. To exclude possible potential effects on the IR signature of the adsorbates and to further clarify the nature of the species related to the band at ∼1630 cm-1, the adsorption/oxidation of 0.1 M CH3CHO in 0.5 M H2SO4 and the temporal evolution of the IR bands were followed in adsorption transients at different, constant potentials. Figure 9 shows ATR-IR spectra measured 5 s after changing from 0.5 M H2SO4 to 0.1 M CH3CHO containing 0.5 M H2SO4 solution at different potentials, from 0.06 to 0.6 V. These spectra were chosen, since in the initial stages of acetaldehyde oxidation/adsorption, the coverage of strongly adsorbed COad is still small, which would otherwise displace weakly bound adsorbates. Therefore, the coverage of other adsorbates is higher, and their detection via in situ ATRFTIRS is more likely. Upon continuing acetaldehyde adsorption, the coverage of theses species indeed decreases, together with the build-up of COad (see also Figure 10). The reference spectrum for each spectrum was taken at the corresponding potential in pure base electrolyte. For adsorption at the lowest potential of 0.06 V, we see only one sharp band centered at 1637 cm-1 with a small shoulder at higher wave numbers. Going to higher adsorption potentials, this peak broadens and then turns into two overlapping bands centered at 1640 and 1668 cm-1, respectively, for adsorption at 0.6 V. This suggests that there are at least two different adsorbed species on the surface. In a previous study, η1(O)-


Figure 9. In situ ATR-IR spectra taken at different constant potentials 5 s after changing from 0.5 M H2SO4 to 0.1 M CH3COH (reference spectra taken in 0.5 M H2SO4 at the respective adsorption potential before changing to acetaldehyde containing electrolyte, acquisition rate 1 s per spectrum).

Figure 10. In situ ATR-IR spectro-electrochemical flow-cell transients for acetaldehyde adsorption on Pt thin film electrode at different, constant electrode potentials between 0.06 and 0.4 V: (a) chronoamperometric transients; (b) integrated absorption intensity transients for COL. At t ) 10 s electrolyte was switched from supporting electrolyte to 0.5 M H2SO4 + 0.1 M acetaldehyde containing solution and back to supporting electrolyte at t ) 280 s at a constant electrode potential (electrolyte flow rate 50 µL s-1, reference spectra were taken at corresponding potentials in acetaldehyde-free supporting electrolyte).

acetaldehyde adsorbed on Pt(111) under UHV conditions was correlated with a band at 1667 cm-1, whereas the band at 1647 cm-1 was assigned to an adsorbed η1(C)-acetyl species.44 In a later ATR-IR study on electrocatalytic ethanol oxidation on a Pt film electrode,45 the authors also reported two peaks in this wavenumber region, which they attributed to adsorbed acetaldehyde and acetyl species, respectively. (Adsorbed acetyl species

9858


on Pt were postulated by Iwasita et al. and Hitmi et al. but could not be identified for experimental reasons.8,12) Following the above assignment, the data in Figure 9 clearly demonstrate that at potentials E e 0.1 V the first adsorbed species contain an intact C-C bond and that COad develops only later. This observation can be explained either by a decomposition of the C2-species to COad or by a displacement of the adsorbed C2 species by COad which may have developed via other reaction pathways, e.g., instantaneous C-C bond breaking. The question of the temporal correlation between COad formation and adsorbed acetyl formation, which is important for clarifying the question whether the latter acts as precursor for COad formation, was explored by evaluating the temporal evolution of (a) the Faradaic currents and the integrated intensities of (b) COL and (c) adsorbed acetyl at different constant potentials (Fig. 10). The Faradaic current transients reproduce quite well the results reported by Wang et al. on a thin-film Pt/C catalyst electrode34 and therefore will not be discussed here in detail. In short, for adsorption at 0.06 V a negative steady-state current is measured, whereas at higher potentials, in the range between 0.1 and 0.4 V, the current first increases in the first few seconds to positive values and decays to zero within about 10 s after the electrolyte exchange to 0.1 M CH3CHO containing base electrolyte. (Note that the time scale for the current onset is shifted by about 10 s between subsequent traces for more clarity.) For all adsorption potentials shown, the integrated band intensity of COL (Figure 10b) increases continuously after changing to CH3CHO containing electrolyte (t ) 5 s), up to the end of the adsorption period after ∼280 s. For adsorption at 0.06 V, the increase of the COL intensity is rather slow and steady, whereas for higher adsorption potentials the initial COL intensity rises increasingly steeper and then levels off at longer times. This behavior becomes more pronounced with increasing potential. The final COL intensity after 280 s increases with adsorption potential, is highest for 0.2 V, and decreases again for higher adsorption potentials (0.3 and 0.4 V). After changing to acetaldehyde free base electrolyte at t ) 300 s (see vertical line in the figure), the COL intensity remains constant at 0.4 V, but increases further at lower potentials, although after few seconds the electrolyte is free of acetaldehyde. We tentatively attribute this additional increase, which appears already at 0.3 V and becomes increasingly pronounced for lower adsorption potentials (0.2 and 0.1 V), to decomposition of adsorbed C2 species to COad and CHx,ad species. This will be discussed in more detail in the following paragraph. The integrated intensity of the adsorbed acetyl species is plotted in Figure 10c. Different from the COad related intensity, the intensity of this band increases sharply upon electrolyte exchange and reaches its maximum value about 5 s after the electrolyte exchange to 0.1 M CH3CHO containing electrolyte. Subsequently, it continuously decreases with time. The final intensity is highest at 0.3 and 0.4 V, and decreases at lower adsorption potentials (I0.4V ≈ I0.3V > I0.2V > I0.1V ≈ I0.06V). When changing to acetaldehyde free solution (at t ) 300 s), the intensity of the adsorbed acetyl species remains constant at 0.4 and 0.3 V, decreases slightly for 0.2 V, and faster for 0.1 V, respectively. These findings can be explained either by a potential dependent desorption of the adsorbed acetaldehyde or by a potential dependent decomposition of adsorbed acetaldehyde to COad and (most likely) to CHx,ad.34,42 The latter interpretation is supported by the rather good (qualitative) correlation between the relative increase in COad intensity and the decrease in the intensity of the adsorbed acetyl species. At

Heinen et al. 0.5 and 0.6 V, the general trend for the integrated band intensity of adsorbed acetyl is similar to the other adsorption potentials. The values, however, are significantly smaller (see Figure 9; for clarity the data are not included in Figure 10). Taking the evolution of the COL intensity as an indicator for C-C bond breaking, this figure shows that C-C bond splitting on a “clean” Pt surface (initial slope of the COad evolution with time) depends strongly on the adsorption potential. It is fastest at potentials between 0.2 and 0.4 V, but rather slow for potentials of 0.1 and below. The increase of the rate for C-C bond breaking with increasing potential in the lowest potential regime can be explained either by electrochemical activation of C-C bond splitting or by the fact that at low potentials adsorbed hydrogen blocks the Pt sites, which are necessary for the further decomposition of adsorbed acetyl. The alternative explanation that acetaldehyde adsorption is hindered by adsorbed Hupd species, as it was concluded for ethanol adsorption/oxidation under these conditions, can be excluded due to the fast development of the adsorbed acetyl species at 0.06 V, which reaches its saturation coverage within the time constant for complete exchange of electrolyte in our setup (3-4 s). These measurements unambiguously confirm that compared to ethanol adsorption (see below), adsorption of acetaldehyde is very fast at 0.06 V. This indicates that the adsorption energy of adsorbed acetyl species is larger than that of the upd-hydrogen. However, the rate for C-C bond splitting of the adsorbed acetyl species is significantly lower at 0.06 V than at higher potentials. Hence, the rate limiting step for dissociative acetaldehyde adsorption (C-C bond breaking) at low potentials is not the initial adsorption step, which includes the loss of one hydrogen atom, but the subsequent dissociation of the adsorbed acetyl species. 3.2.3. Acetaldehyde Adsorbate Stripping. Additional information on the nature and amount of adsorbed species formed on the surface during the adsorption of acetaldehyde is obtained from adsorbate stripping experiments, by reactive removal of the stable adsorbates in base electrolyte after the adsorption transients (total time for rinsing after the adsorption was about 10 min). The Faradaic current traces (a) and the integrated intensities of the COL(b), COM (c), and adsorbed acetyl (d) bands are shown for two representative adsorption potentials of 0.2 and 0.4 V in Figure 11. The resulting charges in the main Faradaic current peak (0.45-0.88 V) and the related COad coverages are summarized for all adsorption potentials in Table 1. Starting with the negative-going scan, the IR intensities are approximately constant after adsorption at 0.2 V. After adsorption at 0.4 V, both the COL (b) and the COM (c) intensity start to increase at around 0.2 V, while that of adsorbed acetyl species decreases simultaneously. To exclude effects caused by potential induced changes in the composition of the COad layer (redistribution between COL and COM), which might also lead to an increased intensity, we can compare the COL and COM intensities measured after adsorption with those obtained at 0.4 V in the subsequent positive-going scan, after scanning through the Hupd region. Both COad related intensities are significantly higher than after adsorption at 0.4 V. Together with an increase in the wavenumber of the COL band, from 2051 cm-1 at 0.4 V prior to the potential excursion into the Hupd region to about 2053 cm-1 at 0.4 V after the first negative-going scan, this is unambiguous proof for an increase in the COad coverage during the scan through the Hupd region. This finding is tentatively explained as follows: upon adsorption of acetaldehyde at 0.4 V, adsorbed acetyl species are formed on the Pt surface (see previous section), which dissociate to



Figure 11. Acetaldehyde adsorbate stripping in base electrolyte after 5 min adsorption of acetaldehde at 0.2 and 0.4 V, respectively. (a) Faradaic current response, (b and c) integrated band intensities of COL and COM, respectively, and (d) integrated intensity of the peak at 1630 cm-1 (electrolyte: 0.5 M H2SO4, potential sweep rate 10 mV s-1).

TABLE 1: Charges for Acetaldehyde Adsorbate Stripping after Adsorption Transients (300 s) at the Potential Indicateda E/VRHE

0.06

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Q/mC θCO,rel.

0.58 0.36

0.89 0.56

1.08 0.68

1.22 0.78

1.18 0.75

1.15 0.72

1.14 0.72

1.15 0.72

a The calculated relative COad coverages are normalized to the saturation COad coverage obtained upon CO adsorption from a CO-saturated electrolyte.

COad and CHx,ad. The resulting CHx,ad species produced during adsorbed acetyl splitting can not be reduced to methane or ethane at potentials positive of the Hupd region.34,66 The accumulation of both species, COad and CHx,ad, increasingly blocks the Pt sites, which are necessary for further dissociation of adsorbed acetyl species. Hence, after the adsorption transient at 0.4 V, the decomposition of the adsorbed acetyl species still present on the surface is hindered by to the high coverage of COad and CHx,ad species. During the negative-going potential scan (see Figure 11), the CHx,ad species are reactively removed from the surface, by reduction to CH4 or by recombination to C2H6.34,66 This in turn produces free Pt sites, which allow the further decomposition of the remaining adsorbed acetyl species to COad and CHx,ad. Wang et al. reported that after adsorption of 0.1 M acetaldehyde on carbon supported Pt catalysts at 0.4 V, the amount of CO2 produced during oxidative stripping, is less when scanning directly positive after adsorption, than when first scanning into the Hupd region, followed by a positive-going scan.66 From the present IR results, this can be understood. We propose that upon scanning directly to positive potentials, the adsorbed acetyl species on the surface are largely oxidized to acetic acid at higher potentials rather than being decomposed to COad. In contrast, when scanning negatively first, decomposi-

tion of adsorbed acetyl prevails, and the resulting COad species are oxidized to CO2 at more positive potential, together with remaining CHx,ad species, which were not reduced in the Hupd region. 3.3. Adsorption/Oxidation of Ethanol. 3.3.1. Potentiodynamic Measurements. Having characterized the adsorption/ oxidation behavior of the possible reaction intermediates or side products during ethanol electrooxidation to CO2, acetaldehyde and acetic acid, we will now concentrate on the adsorption and electrooxidation of ethanol. This was investigated in the same way as done before for the other two species. ATR-IR spectra measured during potentiodynamic ethanol bulk oxidation are shown in Figure 12. Besides linearly and multiply bound COad at 2080 and 1850 cm-1, respectively, adsorbed acetate at 1410 cm-1 (see section 3.1.1) can be detected in the spectra. An additional band at around 1630 cm-1 was assigned to adsorbed acetyl species (see section 3.2 and ref 45). As discussed before (section 3.2.1), a clear assignment in this wavenumber region to adsorbed species resulting from ethanol adsorption is rather difficult due to the overlapping signals from interfacial water (negative band around 1600 cm-1) and water coadsorbed with COad (positive band around 1630 cm-1). In the next section (3.3.2), where we will discuss the adsorption transients for ethanol at different constant potentials starting from a “clean” Pt surface, it will be shown that under those conditions a similar IR band is formed, which is definitely not correlated with COad formation and hence not with COad induced changes in the structure of interfacial water. In analogy to acetaldehyde adsorption (see section 3.2.2), we assign this to adsorbed acetyl species. Therefore, a similar assignment is plausible also for the same band observed during potentiodynamic measurements. Figure 13a shows the first and the second potentiodynamic cycle recorded on the Pt thin film electrode in 0.5 M H2SO4 + 0.1 M ethanol. In general, the cyclic voltammogram for ethanol electrooxidation on the Pt film electrode reproduces the Faradaic current response for polycrystalline Pt published previously.8,15,18,34,45 In the positive-going scan, ethanol oxidation starts at about 0.4 V. Then the current passes through a maximum at 0.85 V (peak III), with a shoulder at lower potentials (peak II), and arises again at 1.1 V (peak IV). Significant differences between the first and the subsequent positive scans are (i) for the first scan, an additional current peak at about 0.3 V is found (peak I), (ii) the onset of the main peak is shifted to lower potentials, and (iii) the shoulder (peak II) is more pronounced. The negative-going scan shows one oxidative current peak at 0.7 V (peak V). The assignment of the Faradaic current peaks to specific surface processes will be given below, after the presentation of the spectroscopic results. The integrated intensities of linearly bound COad and of the band of adsorbed acetate at 1410 cm-1, plotted against the potential, are shown in Figure 13, panels b and c, respectively. At 0.06 V in the first positive-going scan, no adsorbed CO is detected, indicating that Hupd efficiently blocks ethanol dissociative adsorption. The build-up of COad, sets in at about 0.3 V, indicative of dissociative adsorption of ethanol on a Hupd-free Pt surface. The intensity of the COL band increases continuously up to about 0.6 V, before it drops down to zero for potentials above 0.9 V. In the negative-going scan, adsorbed CO can be detected for E < 0.6 V and the intensity increases down to 0.06 V. In the second positive-going scan, the intensity of COL is significantly higher compared to the first scan, but also decreases to zero at 0.9 V. The higher COad intensity/coverage in the second positive-going scan compared to the first positive-going scan can explain the shift in the onset of ethanol oxidation to

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Heinen et al.

Figure 12. Selected in situ ATR-IR spectra recorded during recorded during the first (a) and the second cycle (b) in potentiodynamic flow-cell measurements of ethanol bulk oxidation at a Pt thin-film electrode (0.5 M H2SO4 + 0.1 M C2H5OH solution, potential sweep rate 10 mV s-1, electrolyte flow rate 50 µL s-1, reference spectrum recorded at 1.3 V in the same electrolyte).

Figure 13. Potentiodynamic ethanol bulk oxidation on a Pt thin-film electrode in an ATR-IR spectro-electrochemical flow-cell: Cyclic voltammogram (a) and integrated band intensities of (b) adsorbed COL and (c) adsorbed acetate (from Figure 12) (0.5 M H2SO4 + 0.1 M C2H5OH solution, potential sweep rate 10 mV s-1, electrolyte flow rate 50 µL s-1).

higher potentials and the decrease of the current at lower potentials (peak II). When COad is removed oxidatively from the Pt surface, no more differences in the Faradaic current between the first and the second scan can be detected. Interestingly, the onset of the COad formation in the first positive-going scan correlates well with the oxidative Faradaic current peak I, suggesting that ethanol is initially oxidized to acetaldehyde (due to the lack of OHad species on Pt electrode at these potentials), which then decomposes to adsorbed CO. In the IR spectra,

however, adsorbed acetyl species can hardly be detected at 0.2 V. Most likely, this is due to a much larger rate constant for their decomposition/desorption compared to that for their formation, which results in a very low coverage of these species, below the detection limit of the present in situ ATR-FTIRS setup. Although a direct oxidation of ethanol to COad, without an adsorbed aldehyde intermediate, can not be excluded from the present data, it seems to be little likely since this would require four bonds to be broken in a concerted reaction step. Similar measurements, following the development of the integrated band intensity of COL during the potentiodynamic ethanol adsorption/oxidation (0.1 M ethanol, 0.1 M HClO4, first and second cycle), were also reported recently by Shao and Adzic.45 While our results agree with their findings in many aspects, there are also distinct differences compared to our results (see also section 3.2.1). First, we found the COL band intensity to be essentially zero before starting the first positivegoing scan, whereas in the study of Shao and Adzic the Pt surface is already covered by COad. Second, we found the COL band intensity to be significantly larger when approaching 0.06 V at the end of the first potential cycle (in the negative-going scan), while in the study by Shao and Adzic this was opposite. Similar differences between the results of Shao and Adzic compared to our results were already mentioned and discussed for the potentiodynamic oxidation of acetaldehyde and were tentatively related to the different experimental conditions in both studies (see section 3.2.1). The intensity of adsorbed acetate (Figure 13c) starts to increase sharply during the positive-going scan at about 0.7 V, passes through a maximum at 1.0 V, and then decreases again, reaching values close to the background level at ∼1.2 V. In the negative-going scan, the acetate intensity increases only at 0.8 V, passes again through a maximum at 0.6 V, and then decreases to the background level again, which is reached at 0.4 V. Similar results for the potential dependent appearance of adsorbed acetate upon ethanol oxidation were reported by Shao and Adzic.45 There is no difference between the first and subsequent positive-going scans. As already mentioned in section 3.1, adsorbed acetate shows the typical behavior of an anion, which can not be oxidized further under these conditions (room


Figure 14. In situ ATR-IR spectra taken at different constant potential 5s after changing from 0.5 M H2SO4 to 0.1 M CH3CH2OH (reference spectra taken at the respective adsorption potential in 0.5 M H2SO4 before adding organic species, acquisition rate 1 s per spectrum).

temperature). Since it is in reversible equilibrium with acetic acid in the bulk electrolyte, it can be used as indicator for the production of acetic acid. The shape of the integrated acetate intensity-potential profile fits quite well to that of the peaks III and V in the cyclic voltammogram of ethanol oxidation. Since the adsorbed acetate is desorbed and removed rapidly, its integrated band intensity qualitatively reflects the formation rate of acetic acid/acetate. Therefore the Faradaic current peaks can at least partly be attributed to the oxidation of ethanol to acetic acid. Shin et al.21 reported that the appearance of adsorbed acetic acid on Pt(111) and Pt(335) correlates with the drop in ethanol oxidation current, which contrasts our spectro-electrochemical flow cell data. We explain this apparent discrepancy by accumulation of acetic acid in the thin-layer gap and the decrease in ethanol concentration upon the EOR in a conventional FTIR configuration. These effects are excluded (or at least significantly weaker) under continuous electrolyte flow as present in the thinlayer flow cell measurements. In this case, adsorbed acetate can easily be washed away from the surface (see section 3.1.2) and thus does not act as irreversibly adsorbed poison for ethanol oxidation. However, adsorbed acetate blocks Pt sites required for the adsorption/oxidation of ethanol and thus is expected to slow down the ethanol oxidation reaction.52,53 In agreement with this idea, the Faradaic current for the electrooxidation of formic acid on a Pt film electrode was found to decrease, when acetic acid was added to the solution and adsorbed acetate was detected on the surface.69 3.3.2. Ethanol Adsorption Transients. Ethanol adsorption transients were recorded in a similar way as described before for acetic acid and acetaldehyde adsorption. ATR-IR spectra obtained about 3-5 s after the onset of ethanol adsorption are plotted in Figure 14. As discussed in section 3.2.2, under these conditions the COad coverage is still small ( 0.15 > 0.1 V. A direct correlation between COL intensity and COad coverage is not straightforward due to potential effects on the intensity and dipole-dipole coupling between adsorbed CO molecules;71-73 an experimentally derived correlation between COad coverage and COL intensity was derived for different potentials in ref 74. For the present experimental conditions (θ(COrel) ≈ 0.5, as calculated from subsequent stripping experiments (see next section), 0.1 V e E e 0.4 V), the COL intensity at t ) 280 s can be used as a qualitative measure of the COad coverage. A lower steady-state COad coverage at higher potentials (adsorption at 0.3, 0.35, and 0.4 V) compared to adsorption at 0.2 and 0.25 V was already seen for acetaldehyde adsorption (section 3.2.2). This was explained by the presence of additional coadsorbed C1 (CHx,ad75) and C2 (acetyl) species at the higher adsorption potentials (adsorption at 0.3, 0.35, and 0.4 V). The further decomposition

Heinen et al.

Figure 16. Ethanol adsorbate stripping in base electrolyte after 5 min adsorption of ethanol at 0.2 and 0.4 V, respectively, starting with a negative-going scan to 0.06 V, followed by a positive-going scan. (a) Faradaic current response and (b and c) integrated band intensities of COL and COM, respectively (0.5 M H2SO4 solution, potential scan rate 10 mV s-1; the reference spectrum was recorded at 0.2 and 0.4 V, respectively, in ethanol-free supporting electrolyte).

of the C2 species at 0.4 V was suppressed due to the lack of neighboring free Pt sites on the adsorbate covered Pt surface. A similar situation appears also for ethanol adsorption. Also here, adsorbed acetyl species are detected at 0.3 to 0.4 V after 5 min of ethanol adsorption, in addition to COad, whereas at 0.2 V (and below), adsorbed acetyl species can not be detected and the adsorbate layer consists mainly of adsorbed CO. Furthermore, CHx,ad species formed upon C-C bond splitting can desorb as methane at potentials below 0.2 V,12,34 leaving more Pt sites for the formation of COad. If these explanations were correct, the COad coverage should not change during a potential scan to lower potentials after adsorption at 0.2 V, whereas the COad coverage reached during ethanol adsorption at 0.4 V should increase further during a potential scan to lower potentials. This should occur even in the absence of ethanol in the solution, due to the decomposition of the remaining C2 species. Such kind of spectro-electrochemical adsorbate stripping experiments will be discussed in the following section. 3.3.3. Adsorbate Stripping. After the adsorption transients at different constant potentials the electrolyte was exchanged to 0.5 M H2SO4 and the cell was rinsed carefully with this solution to remove any residues of ethanol from the solution. During this procedure the respective adsorption potential was kept constant. A potential scan, starting in the negative-going scan direction, was then started at 10 mV s-1 to follow the potential dependent adlayer changes. The cyclic voltammograms for this adsorbate stripping experiment are plotted in Figure 16 for adsorption potentials of 0.2 and 0.4 V. For both potentials an oxidative current is measured in the positive-going scan between 0.45 and 0.9 V correlated to charges of about 1.01 and 0.97 mC for the adsorbate stripping experiments performed



TABLE 2: Charges for Ethanol Adsorbate Stripping after Adsorption Transients (300 s) at the Potential Indicateda E/VRHE

0.06

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Q/mC θCO,rel.

0 0

0 0

0.58 0.35

1.01 0.63

0.95 0.57

1.02 0.62

1.04 0.63

0.97 0.59

a The calculated relative COad coverages are normalized to the saturation COad coverage obtained upon CO adsorption from a CO-saturated electrolyte.

after ethanol adsorption at 0.2 and 0.4 V respectively. The charges for the other adsorption potentials are summarized in table 2. A similar Faradaic current response, accompanied by CO2 formation for ethanol adsorbate stripping, was reported by Wang et al. for ethanol adsorbate stripping on a thin-film Pt/C catalyst electrode,34 which was explained by oxidation of adsorbed CO as predominant adsorbate. The overall amount of CO2 measured by DEMS for adsorption potentials of 0.2 and 0.4 V was practically identical. However, the IR data discussed in the previous section have shown that the COad coverage after 5 min of ethanol adsorption/oxidation was higher for 0.2 V compared to 0.4 V. These apparently different results can be understood from the integrated intensity of COL during the adsorbate stripping experiment, which is plotted in Figure 16b. For an adsorption potential of 0.4 V, both COL and COM bands increase in the negative scan direction. This increase in intensity can be explained either by an increase of the COad coverage or by a potential-induced intensity change. The latter was also observed from adsorption of gas phase CO due to changes in the CO adlayer.76 To exclude the influence of the potential variation on the absorption band, which is present in the spectro-electrochemical cyclic voltammetry experiments (Figure 16), we compare the integrated band intensities for COL and COM after the adsorption with the corresponding intensities after the negative-going potential excursion at the same potential. This shows a clear increase in both intensities after scanning through the Hupd region, in agreement with an earlier report.49 These findings suggest that after ethanol adsorption at 0.4 V and rinsing the cell with ethanol free solution, the COad coverage increases further upon cycling in the Hupd region. In analogy to similar experiments and findings for the adsorption of acetaldehyde, this COad coverage increase is attributed to the decomposition of adsorbed acetyl species. Unfortunately, the intensity of the adsorbed acetyl species can not be evaluated properly in this experiment because of considerable potential dependent background changes in the wavenumber region between 1600 and 1700 cm-1, caused by changes in the structure of interfacial water,56 and is therefore not shown in Figure 16. The conclusion that decomposition of adsorbed acetyl species is responsible for the increase in the COad coverage is therefore based on the combined results for ethanol and acetaldehyde adsorption, where the latter showed similar trends, but a significantly higher intensity of adsorbed acetyl species and increase of the COad coverage in the negative-going scan. 4. Conclusions The interaction of acetic acid, acetaldehyde and ethanol with a Pt film electrode was studied by in situ ATR-FTIR spectroscopy in a flow cell configuration, making use of the possibility to monitor in situ the changes in the adsorbate layer during fast electrolyte exchange at constant potential. Acetic acid was found to adsorb on a Pt electrode as acetate, bound perpendicularly to the surface via two oxygen atoms. This was evidenced by an IR absorption band at 1410 cm-1. This band was ac-

companied by a shoulder at 1350 cm-1, attributed to the CH3 plane bending of absorbed acetate. These bands are clearly separable from bands related to acetic acid in the bulk electrolyte, which appear in ATR-FTIRS only at concentration of >0.1 M. The integrated band intensity of adsorbed acetate increases with potential up to about 0.95 V. At higher potentials, it decreases due to formation of PtO, very similar to the typical potential dependent adsorption of anions. Acetic acid is completely inactive for oxidation on a Pt electrode in the potential regime between 0.06 and 1.3 V under the present experimental conditions. Furthermore, adsorbed acetate species are in a fast adsorption-desorption equilibrium with acetic acid in the solution. This conclusion is based on the fast decrease of the integrated absorbance of adsorbed acetate in fast electrolyte exchange experiments. Adsorption/oxidation of acetaldehyde and ethanol results in the formation of adsorbed acetate (bands see above), linearly and multiply bound COad (bands at 2020-2070 and at 1800-1880 cm-1), and adsorbed acetyl species (band at 1630-1670 cm-1), respectively. The adsorption of acetaldehyde is very fast already at potentials in the Hupd potential region, which was evidenced by the rapid formation of adsorbed acetyl species even at 0.06 V. Hence, the adsorption energy of adsorbed acetyl species is larger than that of Hupd. However, the rate for C-C bond splitting of the adsorbed acetyl species is significantly lower at 0.06 V than at higher potentials. This was tentatively attributed to the presence of Hupd, which blocks Pt sites required for the decomposition of adsorbed acetyl species. Therefore, the rate limiting step for dissociative acetaldehyde adsorption (C-C bond breaking) at low potentials is not the initial adsorption step, which includes the loss of one hydrogen atom (adsorbed acetyl formation), but the subsequent dissociation (C-C bond breaking) of the adsorbed acetyl species. At higher potentials, the rate for acetyl decomposition increases, which results in a fast poisoning of the Pt surface due to the formation of COad and CHx,ad species. In contrast to the adsorption of acetaldehyde, ethanol adsorption is efficiently hindered by the presence of Hupd; neither COad nor any other ethanol related adsorbate could be detected via in situ ATR-FTIRS for adsorption at potentials of 0.1 V and below. With increasing potential, C-C bond breaking and COad formation sets in during ethanol adsorption/oxidation. Adsorbed acetyl formation is resolved in the initial stages of ethanol adsorption in the potential range of 0.3 - 0.6 V, with a maximum at 0.4 V. Likewise, the highest initial rates for C-C bond breaking and COad formation were found at 0.3 and 0.4 V, supporting that adsorbed acetyl species act as active intermediate for subsequent C-C bond splitting and formation of COad and CHx,ad. This leads to (almost) complete poisoning of the Pt surface by COad and CHx,ad species, e.g., within about 20 s at 0.3 V. Therefore, the rate-determining step for ethanol oxidation at potentials below 0.4 V on a Pt electrode (under steady state conditions) is not C-C bond splitting, but rather the removal of the poisoning COad and CHxad surface species. The oxidation of ethanol and acetaldehyde to acetic acid was evidenced via the detection of adsorbed acetate species. From the good agreement between the shape of the integrated acetate intensity - potential profile and the Faradaic current in the peaks III and V during potentiodynamic ethanol oxidation indicates that the Faradaic current peaks can at least partly be attributed to the oxidation of ethanol to acetic acid. On the other hand, adsorbed acetate species negatively affect the ethanol and acetaldehyde oxidation reaction via (reversible) site blocking.

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Ethanol, Acetaldehyde and Acetic Acid Adsorption ... - ACS Publications

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