Electrochemical Reactivity of Ethanol on Porous Pt and PtRu

Electrochemical oxidation/reduction reactions of ethanol in 1 M HClO4 were studied on porous Pt, Pt0.92Ru0.08, Pt0.85Ru0.15, and Ru under potentiodyna...
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J. Phys. Chem. 1996, 100, 17901-17908

17901

Electrochemical Reactivity of Ethanol on Porous Pt and PtRu: Oxidation/Reduction Reactions in 1 M HClO4 Volkmar M. Schmidt* and Remo Ianniello Institut fu¨ r EnergieVerfahrenstechnik (IEV), Forschungszentrum Ju¨ lich GmbH, D-52425 Ju¨ lich, Germany

Elena Pastor and Sergio Gonza´ lez Departamento de Quı´mica Fı´sica, UniVersidad de La Laguna, 38204 Tenerife, Spain ReceiVed: June 11, 1996; In Final Form: August 28, 1996X

Electrochemical oxidation/reduction reactions of ethanol in 1 M HClO4 were studied on porous Pt, Pt0.92Ru0.08, Pt0.85Ru0.15, and Ru under potentiodynamic conditions. The electrodes with defined bulk and surface compositions were made by electrodeposition on porous Au substrates. Cyclic voltammetry in combination with on-line mass spectrometry (DEMS) was employed to correlate faradaic currents with ion currents associated to reaction products. The formation of both CO2 and ethanal was unequivocally identified during ethanol oxidation on PtRu and Pt by using d3-ethanol. On pure Ru neither faradaic currents were observed nor oxidation/reduction products could be detected by mass spectrometry. The reduction of ethanol on Pt and PtRu to methane and ethane was found in the potential range of hydrogen adsorption. The yields of oxidation and reduction products and the corresponding onset potentials for their formation are greatly influenced by the Ru content. Alloys with high Ru content exhibit a lower reaction rate for ethanol electrooxidation. On the other hand, the selectivity for ethanal production is significantly enhanced for PtRu as compared to pure Pt.

1. Introduction The reactivity of ethanol on noble metal electrodes has been the subject of many spectroscopic investigations during the past years. For ethanol oxidation on Pt in acid solution ethanal, acetic acid and CO2 have been identified as reaction products by means of in-situ Fourier transform infrared spectroscopy (FTIRS)1,2 and on-line differential electrochemical mass spectrometry (DEMS).3,4 The latter method has also revealed the formation of methane4,5 and ethane5 during ethanol reduction. A comparative study on ethanol oxidation at Rh and Ir electrodes by in-situ FTIRS has shown that on Ir the major product is acetic acid whereas CO2 is the main product on Rh.6 In this connection, two interesting aspects arise with respect to the electrocatalysis of ethanol oxidation. For possible applications in a fuel cell, ethanol should be oxidized completely using the maximum energy content of the fuel. Then, the electrocatalyst should have a high activity for CO2 formation with a low overpotential for the overall reaction. On the other hand, if the partial oxidation of ethanol to ethanal or acetic acid is desired, a selective catalyst should be chosen which inhibits the break of the C-C bond of the ethanol molecule during the oxidation reaction. PtRu alloys are currently being regarded as electrocatalysts with enhanced activity for the oxidation of small organic molecules to CO2 such as CO7-9 and methanol.10,11 However, no systematic investigations are known in the literature dealing with the Ru influence in defined PtRu alloys on the reactivity of ethanol. Recently, Savinell et al.12 studied the oxidation of ethanol in a polymer electrolyte fuel cell using Pt and PtRu as anode catalysts in the temperature range between 150 and 190 °C. Depending on the water to ethanol ratio in the feed gas of the cell, they found a relative ethanal yield between 60% and 80% and CO2 as the second oxidation product. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, October 1, 1996.

S0022-3654(96)01702-9 CCC: $12.00

The purpose of our studies concerning the electrochemical reactivity of ethanol is to obtain a deeper insight into the different oxidation and reduction reaction pathways with respect to the Ru influence in defined PtRu alloy electrodes. Different questions arise in this context; i.e., what are the reaction products being formed during ethanol oxidation/reduction, and how are the product distributions as a function of the electrode composition? In order to make a product analysis, cyclic voltammetry was combined with on-line mass spectrometry (DEMS). This method allows the investigation of electrochemical reactions on porous electrodes which could be model systems for technical electrodes. In the present work the oxidation/reduction reactions of ethanol at the electrode/electrolyte interface in perchloric acid solution are being described, whereas surface reactions of adsorbed intermediates coming from ethanol will be discussed in a forthcoming publication.13 2. Experimental Section 2.1. Electrodes. Working electrodes used for on-line mass spectrometry were prepared by potentiostatic deposition of Pt, Ru, or Pt/Ru codeposition onto porous Au substrates. The substrate layer (thickness 50 nm) was sputtered onto a hydrophobic PTFE membrane (Scimat Ltd., average thickness 60 µm, mean pore size 0.17 µm, 50% porosity). The electrodeposition of Pt and/or Ru was performed at 0.05 V vs RHE for 30 min. By varying the concentrations of H2PtCl6 and RuCl3 in the solution, alloy layers with different compositions were produced. The chemical composition of the solutions used for the codeposition was obtained from total X-ray reflection fluorescence spectroscopy (TXRF).14 The bulk composition of the alloys was characterized using energy dispersive analysis of X-ray (EDAX). In addition, the Pt to Ru ratio on the surface was determined by X-ray photoelectron spectroscopy (XPS) according to the procedure described in ref 15. Results obtained from these analysis are summarized in Table 1. The Pt content at the surface was found to be higher compared to the bulk © 1996 American Chemical Society

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Schmidt et al.

TABLE 1: Pt to Ru Ratios (in atomic %) Determined by TXRF in the Deposition Solution, EDAX for the Bulk Composition of the Deposited Alloy, and XPS for the Surface Composition of the Alloy Pt:Ru TXRF (solution) EDAX (bulk alloy) XPS (alloy surface) denoted as

94:6 83:17 92:8 Pt0.92Ru0.08

63:37 67:33 85:15 Pt0.85Ru0.15

concentration of the porous electrode. More detailed information on the preparation and characterization of electrodeposited PtRu electrodes is discussed elsewhere.15,16 The alloys studied are denoted as Pt0.92Ru0.08 and Pt0.85Ru0.15 according to XPS analysis of the surfaces (see Table 1). For each experiment freshly prepared electrodes were employed. The electrodes were made in the same electrochemical cell which was used for the DEMS experiments. In this way, possible contamination effects during transfer could be avoided. The geometric area of the electrodes was 0.95 cm2. The real surface area of the Pt electrodes varied in the range 50-350 cm2 estimated from integrating the current in the potential region of hydrogen adsorption.17 On the other hand, the electrochemical active surface area of the PtRu electrodes cannot be calculated in the same way as for Pt. This is due to the fact that two competing processes occur in this potential region, that is, adsorption/desorption of hydrogen predominantly occurring on Pt sites and the formation of Ru-OH species.18 Consequently, a quantitative comparison of current densities obtained with different electrodes cannot be made. Before each experiment, the electrodes were subjected to a potential cycling program in the range 0.05-0.80 V for 30 min with a scan rate of dU/dt ) 0.10 V s-1. The upper potential of the PtRu electrodes was limited to values below 0.80 V in order to avoid irreversible oxidation of Ru surface atoms.18 For reasons of comparison, the studies on PtRu and pure Pt were performed in the potential range between 0 and 0.80 V with dU/dt ) 0.01 V s-1. A Pt wire served as counter electrode. A reversible hydrogen electrode (RHE) in 1 M HClO4 was employed as reference electrode. All experiments were performed at room temperature. 2.2. Chemicals. Solutions were prepared with MilliporeMilliQ water and analytical grade chemicals: HClO4 (Merck), CH3CH2OH (Riedel-de-Haen), CD3CH2OH (D 98%, Cambridge Isotope Laboratories), H2PtCl6 (Aldrich), and RuCl3 (Aldrich). The experiments were performed using 0.01 M CH3CH2OH or 0.1 M CD3CH2OH in 1 M HClO4. All solutions were degassed by purging with argon (5.0, Messer-Griessheim). 2.3. On-Line MS Technique. The DEMS technique allows the on-line mass spectrometric detection of volatile and gaseous species formed in electrochemical reactions.19 The experiments were performed in a glass flow cell containing approximately 2 mL of electrolyte solution. The cell is directly attached to the vacuum system. The details of the equipment used in this work are described elsewhere.11 A quadrupole mass spectrometer (Leybold Heraeus, Quadruvac Q 200) is combined with a potentiostat (EG&G, Model 362). The electron energy for electron impact ionization was 70 eV. By comparing literature data,20 mass spectrometric signals (m/z values) can be assigned to typical fragments of reaction products. Simultaneously with cyclic voltammograms (CVs), ion currents of representative m/z signals of reaction products were recorded as a function of the electrode potential, thus obtaining mass spectrometric cyclic voltammograms (MSCVs). For each electrode studied the ion currents of typical m/z signals were recorded under the same

Figure 1. CV for 0.01 M CH3CH2OH in 1 M HClO4 on Pt (top) and MSCVs for m/z ) 30 (ethane) and m/z ) 15 (methane); dU/dt ) 0.01 V s-1; electrochemically active surface area ) 58 cm2.

mass spectrometric conditions, and relative ratios of ion currents were determined. Thus, a calibration of the mass spectrometer for getting absolute ion currents is not necessary. 3. Results 3.1. Reactivity of Ethanol on Pt. To evaluate the reactivity of ethanol on PtxRuy alloys with respect to Pt, the electrochemical reactions of ethanol were first studied on a pure Pt electrode that was prepared in the same way as the PtRu alloys. Figure 1 (top) shows the voltammetric profile for 0.01 M CH3CH2OH in 1 M HClO4. The onset potential for ethanol electrooxidation is about 0.42 V, and during the positive-going potential scan the current increases to 0.80 V. The formation of reaction products during electroreduction of ethanol can be followed by the ion current increase for m/z ) 15 and m/z ) 30 at potentials below 0.30 V (Figure 1). The signal for m/z ) 30 in this potential region is assigned to the radical cation [C2H6]•+, indicating the formation of ethane.5,21 According to the fragmentation probability of ethane under electron impact ionization conditions,20,21 the signal for the [CH3]+ fragment (m/z ) 15) should be 5 times smaller than the signal of the molecular peak (M•+) of ethane (m/z ) 30). However, the ion current for m/z ) 15 is 10 times higher. This leads to the conclusion that the ion current for m/z ) 15 should be assigned to the [M-H]+ fragment of methane (M ) CH4). The integration of the ion currents for m/z ) 15 and 30 for U < 0.30 V leads to charges of 2.0 × 10-13 and 3 × 10-14 C, respectively. From these values a methane to ethane ratio of 2 was calculated after correcting the fragmentation probability of ethane and methane.20,21 This methane to ethane ratio is much lower than that reported for the reduction of adsorbed ethanol on a sputtered Pt electrode, yielding a value of 6.5 In principle, a methane to ethane ratio of 6:1 obtained for adsorbate reactions in an ethanol-free solution5 cannot be directly compared with the experimental condition in which ethanol is dissolved in the electrolyte solution as in the present case. Furthermore, the

Reactivity of Ethanol on Porous Pt and PtRu different morphology of the sputtered and the electrodeposited electrodes may cause this difference in the yield ratios. The ion currents for m/z ) 15 and 30 show different potential dependence: in the negative-going potential scan the signal for m/z ) 15 (methane) exhibits a maximum at 0.08 V, and its production is much smaller in the positive-going potential scan. On the other hand, ethane production (m/z ) 30) increases in the negative-going scan between 0.20 and 0 V, and the ion current in the positive-going scan exceeds those in the negative going scan (see Figure 1). These observations suggest that the potential dependence of the m/z ) 15 and 30 signals should be associated with different precursor species. The increase of the current for m/z ) 15 at U > 0.30 V in the positive-going scan in Figure 1 must be related to the formation of an oxidation product. It is known from literature that CH3CHO and CO2 were identified by DEMS as products of the electrooxidation of CH3CH2OH.3 Thus, the potentialdependent ion current for m/z ) 15 in this potential region can be assigned to the [CH3]+ fragment of ethanal. The formation of ethanal can also be followed by the potential dependence of the m/z ) 29 signal that corresponds to the typical aldehyde fragment [COH]+. The signal for m/z ) 44 can be assigned to [CO2]•+. Ethanal, however, may also have a contribution to this signal, namely, its molecular peak [CH3CHO]•+.20 In order to distinguish both molecules, a solution of 0.1 M CD3CH2OH in 1 M HClO4 was employed. Figure 2 shows the CV and simultaneously recorded MSCVs for m/z ) 44 and m/z ) 47. Under these experimental conditions, the ratio m/z ) 44 is clearly associated with the production of CO2 whereas the signal for m/z ) 47 corresponds to the molecular peak of CD3CHO. The onset potential for m/z ) 47 (aldehyde formation) is 0.43 V whereas the m/z ) 44 signal (CO2 production) starts to increase at about 0.50 V. During the positive-going scan the representative MSCVs for both species increases up to 0.80 V. In the negative-going scan the ion current for m/z ) 44 (CO2) decreases sharply whereas the signal for m/z ) 47 (CD3CHO) has a maximum at 0.74 V. The relative yields for CO2 and ethanal formation can be estimated by integrating the ion current for m/z ) 44 and 47 in Figure 2 for a complete potential cycle. Values of 2.2 × 10-13 and 2.6 × 10-13 C are obtained for m/z ) 44 and 47, respectively. According to the fragmentation probabilities of CO2 and CH3CHO,20 80% of the CO2 molecules produced are detected in the mass spectrometer as [CO2]•+ at m/z ) 44, whereas for ethanal only 25% are expected for the molecular peak M•+ at m/z ) 47. Therefore, when the integrated ion currents of m/z ) 44 and 47 were corrected in this way, a CH3CHO to CO2 ratio of 3.7 was calculated. The onset potentials, integrated ion currents, and yield ratios for the reduction and oxidation products are summarized in Tables 2 and 3. 3.2. Reactivity of Ethanol on Pt0.92Ru0.08. Figure 3 shows the CV for Pt0.92Ru0.08 in 0.01 M CH3CH2OH/1 M HClO4 and the MSCVs for m/z ) 15 and 30, indicating the formation of reduction products at U < 0.30 V comparable to pure Pt (see Figure 1). The integration of the ion currents in this potental range leads to an ion charge of 5 × 10-14 C for m/z ) 30 (ethane) and to 1.6 × 10-13 C for m/z ) 15 (methane). As shown in Table 2, the ratio CH4/C2H4 is 1, which is by a factor of 2 lower than on Pt. The CV in 0.1 M CD3CH2OH solution on Pt0.92Ru0.08 is shown in Figure 4. The difference in the voltammetric profile compared to the 0.01 M CH3CH2OH solution (Figure 3) can be explained by the higher concentration of the deuterated ethanol solution. The signal for m/z ) 47 in the MSCV (Figure

J. Phys. Chem., Vol. 100, No. 45, 1996 17903

Figure 2. CV for 0.1 M CD3CH2OH in 1 M HClO4 on Pt (top) and MSCVs for m/z ) 47 (d3-ethanal) and m/z ) 44 (CO2); dU/dt ) 0.01 V s-1; electrochemically active surface area ) 52 cm2.

TABLE 2: Onset Potentials (Uonset), Integrated Ion Currents (QI), and Yield Ratios for the Electroreduction Products Detected from a 0.01 M CH3CH2OH/1 M HClO4 Solution m/z ) 15 electrode Pt Pt0.92Ru0.08 Pt0.85Ru0.15 Ru

m/z ) 30

Uonset (V)a

QI (1014 C)b

Uonset (V)c

QI (1014 C)d

CH4:C2H6e

0.31 0.30 0.19

20 16 9 f

0.20 0.19 0.12

3 5 1

2 1 3.5

a Onset potential of the ion current for m/z ) 15 (assigned to methane). b Integrated ion current for m/z ) 15 (assigned to methane) at U < 0.30 V during one potential cycle. c Onset potential of the ion current for m/z ) 30 (assigned to ethane). d Integrated ion current for m/z ) 30 (assigned to ethane) at U < 0.30 V during one potential cycle. e Yield ratio CH4:C2H6 (corrected by the fragmentation probabilities).20 f No reactivity.

TABLE 3: Onset Potentials (Uonset), Integrated Ion Currents (QI), and Yield Ratios for the Electrooxidation Products Detected from a 0.1 M CD3CH2OH/1 M HClO4 Solution m/z ) 44 electrode Pt Pt0.92Ru0.08 Pt0.85Ru0.15 Ru

m/z ) 47

Uonset (V)a

QI (1014 C)b

Uonset (V)c

QI (1014 C)d

CD3CH2OH:CO2e

0.50 0.33 0.25

22 18 12 f

0.43 0.25 0.15

26 28 19

3.7 4.8 5.1

a Onset potential of the ion current for m/z ) 44 (assigned to CO ). 2 Integrated ion current for m/z ) 44 (assigned to CO2) during one potential cycle. c Onset potential of the ion current for m/z ) 47 (assigned to ethanal). d Integrated ion current for m/z ) 47 (assigned to ethanal) during one potential cycle. e Yield ratio CD3CH2OH:CO2 (corrected by the fragmentation probabilities).20 f No reactivity. b

4, bottom) has an onset potential for ethanal formation at 0.25 V in the positive scan with a maximum of the current at 0.60 V in the negative-going scan. The production of CO2 commences at 0.33 V. This is 0.17 V more negative compared with

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Figure 3. CV for 0.01 M CH3CH2OH in 1 M HClO4 on Pt0.92Ru0.08 (top) and MSCVs for m/z ) 30 (ethane) and m/z ) 15 (methane); dU/ dt ) 0.01 V s-1.

Figure 5. CV for 0.01 M CH3CH2OH in 1 M HClO4 on Pt0.85Ru0.15 (top) and MSCVs for m/z ) 30 (ethane) and m/z ) 15 (methane); dU/ dt ) 0.01 V s-1.

Figure 4. CV for 0.1 M CD3CH2OH in 1 M HClO4 on Pt0.92Ru0.08 and MSCVs for m/z ) 47 (d3-ethanal) and m/z ) 44 (CO2); dU/dt ) 0.01 V s-1.

Figure 6. CV for 0.1 M CD3CH2OH in 1 M HClO4 on Pt0.85Ru0.15 (top) and MSCVs for m/z ) 47 (d3-ethanal) and m/z ) 44 (CO2); dU/ dt ) 0.01 V s-1.

pure Pt (see Table 2). After integration of the ion currents for m/z ) 44 and 47 (1.8 × 10-13 and 2.8 × 10-13 C, respectively) the corrected CH3CHO to CO2 ratio is 4.8, i.e., 1.3 times higher than for Pt (see Table 3). 3.3. Reactivity of Ethanol on Pt0.85Ru0.15. Pt0.85Ru0.15 reveals a lower current in the CV (Figure 5, top) as compared to Pt and Pt0.92Ru0.08 (Figures 1 and 3). For Pt0.85Ru0.15 the production of methane (9 × 10-14 C for m/z ) 15) is increased in relation to that of ethane (1 × 10-14 C for m/z ) 30).

Consequently, a methane to ethane ratio of 3.5 is calculated (Table 2). The onset potentials for m/z ) 15 and 30 are shifted by about 0.12 and 0.08 V, respectively, to more negative potentials compared to pure Pt. The voltammetric profile and the MSCVs for m/z ) 44 and 47 obtained with d3-ethanol (see Figure 6) show a negative shift compared to pure Pt (Figure 2) and Pt0.92Ru0.08 (Figure 4). On the Pt0.85Ru0.15 the onset potentials are 0.15 V for ethanal formation and 0.25 V for CO2. The ion currents for both

Reactivity of Ethanol on Porous Pt and PtRu products increase up to ca. 0.70 V parallel to the current in the CV exhibiting a maximum at 0.72 V. The shape of the curve for m/z ) 47 is different from the corresponding curves measured with the electrodes discussed before. In the negativegoing scan the signal diminishes at first and increases again at U < 0.70 V. The integrated ion currents for m/z ) 44 and 47 during a potential cycle are 1.2 × 10-13 and 1.9 × 10-13 C, respectively. The maximum in the ethanal production is observed at 0.55 V in the negative-going scan. For Pt0.85Ru0.15, the ethanal to CO2 ratio is 5.1 (see Table 3). 3.4. Reactivity of Ethanol on Ru. No electrooxidation/ reduction reactions of ethanol are observed on pure Ru electrodes in 0.01 and 0.1 M ethanol solutions. This observation agrees with experimental results obtained with CH3OH on Ru.22 3.5. Potentiodynamic Adsorption of Ethanol on Unpoisoned Electrode Surfaces. The experimental findings presented so far show that the Ru content in a PtRu alloy influences the oxidation and reduction reactions of ethanol. As known from literature, the adsorption of intermediates during ethanol oxidation poisons the electrode surface.3 In this respect the question arises whether Ru in a PtRu alloy influences the adsorption of these ethanol intermediates as well. The behavior of the different electrodes with respect to the formation of adsorbed residues can be studied by the following adsorption experiments: Procedure A. After setting the potential to 0.05 V in pure 1 M HClO4, the electrolyte was replaced by 1 M HClO4/0.01 M CH3CH2OH while holding the electrode potential at 0.05 V for several minutes. Then the solution was completely replaced again by pure 1 M HClO4, and a positive-going potential scan was started at 0.05 V up to 0.8 V. It was found that no anodic current was detected in the CV for all electrodes studied, and no signals associated with oxidation products were found by mass spectrometry. This observation provides evidence that ethanol does not react with an electrode surface at a potential of 0.05 V. Procedure B. The electrode was previously cycled in pure perchloric acid solution between 0 and 0.80 V. Then pure 1 M HClO4 was replaced by 1 M HClO4/0.01 M CH3CH2OH at a constant potential of 0.05 V. Finally, a positive-going potential scan up to 0.8 V was started recording three potential cycles simultaneously with mass spectrometric analysis. According to procedure B, an anodic current peak at 0.31 V appears during the first scan for a Pt electrode (Figure 7, top). Parallel to this anodic current in the CV between 0.05 and 0.50 V no ion currents for m/z ) 29 (Figure 7, bottom) or for m/z ) 44 (not shown) are visible in the MSCVs. This indicates that no products of a possible oxidation process of ethanol to CO2 or CH3CHO were formed. Therefore, the peak at 0.31 V is interpreted as an electrochemical surface reaction of ethanol on the Pt surface, leading to the formation of adsorbed intermediates. The electrooxidation of dissolved ethanol to ethanal commences at 0.40 V in the first positive-going scan as clearly visible in the MSCV for m/z ) 29 (Figure 7). A similar potential dependence was found for m/z ) 44 (CO2). During the second cycle the adsorption current peak in the CV at 0.31 V is absent (Figure 7, top, dashed line), the current in the CV is smaller than in the first cycle, and the onset potential of the anodic current is at U > 0.50 V, which is a shift of approximately 0.07 V to more positive potentials compared to the first cycle (Figure 7). A positive shift between the second and the first cycle is also observed in the MSCVs for the signal m/z ) 44 (not shown in the figure) and for m/z ) 29 (Figure 7, bottom). In the third and following potential cycles reproducible

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Figure 7. Top: first (s) and second (- - -) potential scan (dU/dt ) 0.01 V s-1) for a Pt electrode in 0.01 M CH3CH2OH/1 M HClO4 after replacing the supporting electrolyte with an ethanol containing solution at U ) 0.05 V; (‚‚‚) clean Pt surface (surface area ) 346 cm2). Bottom: corresponding MSCVs for m/z ) 29 (ethanal).

CVs and corresponding MSCVs were obtained. Thus, the oxidation of dissolved ethanol to ethanal and CO2 is evidently influenced by the presence of adsorbed intermediates which act as poisons for ethanol oxidation. Similar experiments according procedure B were performed with the PtRu alloys. As in the case of Pt, an adsorption peak in the first positive-going scan was observed on a surface which was not poisoned by previously adsorbed intermediates. The CV and MSCV for m/z ) 29 on Pt0.92Ru0.08 are given in Figure 8. The peak potential in the CV is at 0.25 V. This is a negative shift of about 0.06 V compared to pure Pt (see Figure 7). The positive shift in the anodic current in the positive-going scan between the first and second cycle is 0.12 V. The same shift is observed in the MSCV for m/z ) 29 (Figure 8). For Pt0.85Ru0.15 (not shown) the adsorption peak is much smaller and broader, which agrees with the lower reactivity of this alloy electrode, thus rendering the determination of the peak potential more difficult. However, a negative shift of the adsorption peak with respect to Pt0.92Ru0.08 was established. These experimental findings demonstrate that also the formation of adsorbed ethanol intermediates is greatly influenced by the presence of Ru surface atoms. The catalytic activity of PtRu electrodes with respect to oxidation/reduction reactions of these adsorbed intermediates in ethanol-free electrolyte solution is going to be discussed in more detail elsewhere.13 4. Discussion The interaction of dissolved ethanol with the electrode/ electrolyte interface followed by electrochemical reactions at the surface includes a sequence of elementary steps that can depend on the applied electrode potential. These processes are the transport of ethanol to the reaction layer, adsorption and possible surface diffusion, electron transfer reactions accompanied by bond breaking and molecular rearrangements of

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Schmidt et al. in (2b) suggests that a residue remains adsorbed on the surface after reduction of (R)ad**. Ethane can also be produced through the following reaction:

Figure 8. Top: first (s) and second (- - -) potential scan (dU/dt ) 0.01 V s-1) of a Pt0.92Ru0.08 electrode in 0.01 M CH3CH2OH/1 M HClO4 after replacing the supporting electrolyte with an ethanol containing solution at U ) 0.05 V; (‚‚‚) clean Pt0.92Ru0.08 surface. Bottom: corresponding MSCVs for m/z ) 29 (ethanal).

adsorbed intermediates, and the desorption of the products into the electrolyte. Regarding the electrochemical reactivity of ethanol on Pt and PtRu alloys, the discussion is focused on the influence of Ru in PtRu alloys with respect to the product distribution for oxidation or reduction reactions of ethanol under potentiodynamic conditions. In the case of ethanol on pure Pt, methane and ethane were detected by DEMS as reduction products whereas CO2 and ethanal are identified as oxidation products.4,5 In order to discuss the Ru influence on the electroreduction of ethanol on PtRu alloys in the present study, the methane to ethane ratios during reduction are compared. In this way, relative product distributions can be obtained. Table 2 shows the results for the different electrodes studied. Due to the fact that reduction occurs at U < 0.30 V, the process can be described as a strong interaction of ethanol with the electrode/electrolyte interface forming different adsorbed intermediates that react with adsorbed hydrogen atoms as follows:

CH3CH2OH f (R)ad*

(1a)

(R)ad* + (H)ad f CH3CH3 + H2O

(1b)

CH3CH2OH f (R)ad**

(2a)

(R)ad** + (H)ad f CH4 + (R)ad′

(2b)

Adsorbed species such as (R)ad* and (R)ad** should have different structures. From a combined FTIR and DEMS study on ethanol adsorption at polycrystalline Pt electrode,5 it is known that both adsorbed intermediates proposed in (1a) and (2a) conserve the C-C structure, and the cleavage of the C-C bond in reaction 2b should occur during the reduction process. (R)ad′

CH3CH2OH a [CH3CH2OH]ad

(3a)

[CH3CH2OH]ad + 2(H)ad f CH3CH3 + H2O

(3b)

In this respect reaction 3a denotes a reversible interaction of ethanol with the electrode/electrolyte interface, assuming that no chemical bonding of ethanol to the surface takes place. This interpretation is corroborated by performing an electrolyte exchange experiment which reveals that this species is removed from the surface (for further details see ref 13). The formation of ethane in reaction 3b can be explained by the reaction of the OH group of ethanol with two Pt sites on which hydrogen is chemisorbed. According to the reaction sequences (1) and (3), no fission of the C-C bond occurs whereas in (2) the C-C bond must be broken. Since on pure Ru no reaction products could be detected, it can be assumed that these reduction reactions on PtRu alloys occur only on Pt sites. During the reduction of ethanol in the negative-going potential scan different methane to ethane ratios are obtained depending on the surface concentration of Ru. The experimental data summarized in Table 2 suggest that Pt0.92Ru0.08 favors reactions 1 and/or 3 in comparison to Pt. In contrast, on Pt0.85Ru0.15 the relative yield of methane was found to be much higher than on Pt. It should be mentioned that, for the reduction of adsorbed ethanol intermediates in ethanol-free electrolyte,13 the methane to ethane ratios are different from those presented in Table 2. Thus, for the chemisorbed species (R)ad* and (R)ad** methane to ethane ratios of 13, 9, and 5 were found for Pt, Pt0.92Ru0.08, and Pt0.85Ru0.15, respectively.13 These values are much higher than those in Table 2. This difference can be explained by assuming that [CH3CH2OH]ad formed in reaction 3 is unstable in ethanol-free solution. Considering the values of the methane to ethane ratios for the adsorbed ethanol intermediates,12 it can be concluded that the Pt0.92Ru0.08 electrode favors reaction 3 compared with Pt. The opposite tendency is found for the Pt0.85Ru0.15 alloy. A possible explanation for this experimental finding is the decrease of available adsorption sites with increasing Ru content: ethanol requires several Pt sites during the chemisorption although the adsorbed species finally occupies only one site.5 A similar decrease of the reaction rate with increasing Ru content in PtRu was reported for methanol adsorption on smooth PtRu alloys.10 However, [CH3CH2OH]ad can interact via the OH group of ethanol with one (H)ad on Pt. The presence of 8% Ru atoms on the PtRu surface probably inhibits at least partially reaction sequences (1) and (2) due to the decrease of active Pt adsorption sites, and therefore, reaction 3 is favored on Pt0.92Ru0.08 compared to pure Pt. A further increase of the Ru content on the PtRu surface to 15% also causes an inhibition of reaction 3, leading to a relatively high methane to ethane ratio (see Table 2). The oxidation products of ethanol dissolved in perchloric acid are CO2 and CH3CHO as identified by mass spectrometry. No evidence for acetic acid formation was found by means of DEMS that seems to contradict FTIR data1 showing vibration modes for the acid. However, this observation can be explained by the high volatility of ethanal. Under the experimental DEMS conditions ethanal is removed from the reaction layer by evaporating into the vacuum chamber of the mass spectrometer. Thus, the further oxidation of ethanal to acetic acid cannot take place. Similar effects were observed for propanol oxidation on Pt.23

Reactivity of Ethanol on Porous Pt and PtRu

J. Phys. Chem., Vol. 100, No. 45, 1996 17907

The formation of CO2 implies the fission of the C-C bond in the ethanol molecule and the complete oxidation reaction can be written as follows:

CH3CH2OH f (R)ad + H2O

8 CO2 (R)ad 9 + -H , -e

(4a)

Pt + H2O f Pt-OH + H+ + e-

Uad(H2O) ≈ 0.7 V (6)

(4b)

Ru + H2O f Ru-OH + H+ + e-

Uad(H2O) ≈ 0.2 V (7)

On the other hand, the oxidation of ethanol to ethanal implies the conservation of the C-C bond:

CH3CH2OH a [CH3CH2OH]ad

(5a)

[CH3CH2OH]ad f CH3CHO + 2H+ + 2e-

(5b)

In reaction 4 (R)ad denotes chemisorbed species including those denoted as (R)ad* and (R)ad** and other adsorbates that cannot be reduced according to reactions 1 and 2.5 Only CO2 was observed during oxidation of adsorbed intermediates in ethanolfree solution.5,13 Therefore, the formation of ethanal must be related to species such as [CH3CH2OH]ad that are the same as those described in reaction 3. Both oxidation products were found on pure Pt and PtRu. These results are in harmony with previous DEMS studies on Pt and with the dual path mechanism proposed for ethanol oxidation in acid solution.3 According to reactions 4 and 5, CO2 is produced through chemisorbed intermediates whereas ethanal is formed through weakly adsorbed ethanol. A general mechanism including oxidation and reduction reactions of dissolved ethanol can be sketched as follows: SCHEME 1 oxidation

CO2

(R)ad reduction

CH3CH3, CH4

(CH3CH2OH)sol oxidation

CH3CHO

[CH3CH2OH]ad reduction

Based on cyclic voltammetry, the adsorption of oxygen-like species on Pt commences at about 0.7 V while the adsorption of water on Ru occurs at potentials as low as 0.2 V:18

CH3CH3

Since no reactivity was observed on Ru, it is concluded that both reaction pathways proposed for the oxidation in Scheme 1 require an interaction of ethanol with an active metal surface. Therefore, the adsorption step should occur on Pt sites as in the reduction reactions (see above). The formation of chemisorbed species such as (R)ad on Pt and PtRu (see Scheme 1) can be followed by potentiodynamic adsorption experiments in the first positive-going potential scan as described previously (see Figures 7 and 8). The observation that on Ru neither strongly nor weakly adsorbed ethanol intermediates are found can be explained by the fact that at potentials between 0.20 and 0.40 V, at which ethanol adsorption occurs, Ru is partially covered with oxygen-containing species.18,24 Thus, the desorption energy for such species will not be overcompensated by the heat of adsorption for ethanol. This explanation can be confirmed by considering UHV results on the adsorption of methanol on Ru (0001), revealing no adsorption when the surface is covered with a monolayer of oxygen.25 The onset potential for CO2 and ethanal follows the sequence Pt0.85Ru0.15 < Pt0.92Ru0.08 < Pt with a total negative shift between Pt0.85Ru0.15 and pure Pt of 0.25 V and 0.28 V, respectively (see Table 3). Comparable negative shifts for oxidation reactions on PtRu alloys were found for CO8,9 and methanol.10,11 The enhanced reactivity of PtRu is explained in terms of the lower oxidation potential for H2O adsorption on Ru compared to Pt.

According to the bifunctional mechanism,26 ethanol interacts on Pt sites forming strongly adsorbed intermediates such as (R)ad while Ru sites are covered by oxygen containing species. This situation on a PtRu alloy leads to the oxidation of strongly adsorbed intermediates at lower potentials than on pure Pt. The overall reaction can be written as

8 xPt + Ru + CO2 Ptx (R)ad + Ru-OH 9 + -e , -H

(8)

In eq 8 the stoichiometry of the chemisorbed species was not taken into consideration. Principally, one or two CO2 molecules can be formed, depending on the structure of the adsorbate. However, it should be noticed that the negative shifts of the onset potentials for CO2 and ethanal formation with increasing Ru content in the alloy are accompanied by a decrease of the reaction rate for ethanol oxidation. The currents in the CV related to ethanol reactions of Pt0.92Ru0.08 and Pt0.85Ru0.15 (see Figures 3 and 5) are lower compared to pure Pt (Figure 1). It should be mentioned that the electroreactivity of ethanol on alloys having higher Ru contents could not be studied due to the low electrochemical currents in the CV. On the other hand, it was found that in the presence of Ru the ethanal to CO2 ratio is higher than on pure Pt (see Table 3). Regarding the composition of Pt0.92Ru0.08 and Pt0.85Ru0.15 the Ru concentration on the latter is higher by a factor of 2. This leads to an alloy surface with fewer active Pt sites which are available for the formation of strongly adsorbed intermediates according to reaction 4. Consequently, the high relative ethanal yield can be explained by a relativly lower reaction rate for CO2 production and the occurrence of reaction 5, thus favoring the formation of the partially oxidized product on the Ru-rich alloy. In principle, the selectivity of a catalyst is high when only one type of product is formed although the reaction may occur along several reaction pathways. Regarding the experimental results obtained by means of on-line mass spectrometry, it should be emphasized that a total quantitative product distribution of ethanol electrooxidation cannot be established. This is due to the fact that the on-line mass spectromectric product analysis is restricted to volatile and gaseous products. Therefore, a distinct fractional selectivity for one product with respect to all possible products formed cannot be determined. However, by integrating the mass spectrometric ion currents of relevant m/z signals associated with the reaction products, valuable assertions on the relative selectivitiy can be made. In this connection, the higher ethanal to CO2 ratio on PtRu in comparison to Pt can be rationalized by a higher selectivity of PtRu for the formation of partially oxidized products over the total oxidation of ethanol to CO2 (see Scheme 1). 5. Conclusions Porous Pt, Ru, and PtRu electrodes with defined bulk and surface compositions were made by potentiostatic deposition on porous Au substrates. The comparative study regarding the electrochemical reactivity of ethanol on Pt0.92Ru0.08, Pt0.85Ru0.15, Pt, and Ru shows that pure Ru displays no reactivity. On the other electrodes methane and ethane are detected as reduction

17908 J. Phys. Chem., Vol. 100, No. 45, 1996 products using on-line mass spectrometry. Using CH3CH2OH and CD3CH2OH mass spectrometry enables the discrimination of CO2 and ethanal formation together with the determination of their relative yields during oxidation. Pt turns out to be the best electrocatalyst for the total oxidation of ethanol to CO2. As expected regarding the bifunctional mechanism, a negative shift of the oxidation potential is observed in the presence of Ru on the alloy surface. On the other hand, the formation of chemisorbed species coming from dissolved ethanol is partially inhibited by the presence of Ru. This favors the oxidation pathway through weakly adsorbed species, and therefore the selectivity for ethanal production was found to be higher compared to that for pure Pt. Acknowledgment. The authors are grateful to E. Wallura, E. M. Ko¨nig, and W. Krumpen (KFA) for the TXRF and EDAX analysis of the alloys. We acknowledge B. Wohlmann and F. Richarz (University of Bonn) for the XPS measurements of the alloy surfaces. Several discussions with U. Stimming (KFA Ju¨lich) have been helpful in preparing this manuscript. E. Pastor thanks the Deutscher Akadamischer Austauschdienst for a grant during her stay in Ju¨lich. This cooperation was supported by Ministerium fu¨r Bildung, Forschung und Technologie through Forschungszentrum Karlsruhe/Internationales Bu¨ro and Ministerio de Educacio´n y Ciencia (Acciones Integradas HispanoAlemanas 1995, AI95-31/109-A). References and Notes (1) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem. 1989, 266, 317. (2) Iwasita, T.; Rasch, B.; Cattaneo, E.; Vielstich, W. Electrochim. Acta 1989, 34, 1073. (3) Willsau, J.; Heitbaum, J. J. Electroanal. Chem. 1985, 194, 27. (4) Bittins-Cattaneo, B.; Wilhelm, S.; Cattaneo, E.; Buschmann, H. W.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1210.

Schmidt et al. (5) Iwasita, T.; Pastor, E. Electrochim. Acta 1994, 39, 531. (6) de Tacconi, N. R.; Lezna, R. O.; Beden, B.; Hahn, F.; Lamy, C. J. Electroanal. Chem. 1994, 379, 329. (7) Ross, P. N.; Kinoshita, K.; Scarpellino, A. J.; Stonehart, P. J. Electroanal. Chem. 1975, 63, 97. (8) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (9) Ianniello, R.; Schmidt, V. M.; Stimming, U.; Stumper, J.; Wallau, A. Electrochim. Acta 1994, 39, 1863. (10) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (11) Ianniello R.; Schmidt, V. M. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 83. (12) Wang, J.; Wasmus, S.; Savinell, R. F. J. Electrochem. Soc. 1995, 142, 4218. (13) Ianniello, R.; Pastor, E.; Schmidt, V. M. J. Phys. Chem., submitted. (14) Klockenka¨mper, R.; Knoth, J.; Prange, A.; Schwenke, H. Anal. Chem. 1992, 64, 1115 (15) Richarz, F.; Wohlmann, B.; Vogel, U.; Hoffschulz, H.; Wandelt, K. Surf. Sci. 1995, 335, 361. (16) Ianniello, R.; Wohlmann, B.; Schmidt, V. M.; Wandelt, K.; Stimming, U. To be published. (17) Biegler, T.; Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1971, 29, 269. (18) Ticannelli, E.; Berry, J. G.; Paffet, M. T.; Gottesfeld, S. J. Electroanal. Chem. 1989, 258, 61. (19) Wolter, O.; Heitbaum, J.; Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 2. (20) Stenhagen, E., Abrahamsson, S., McLafferty, F. W., Eds. Atlas of Mass Spectral Data; Interscience Publisher: New York, 1971. (21) Pastor E.; Iwasita, T. Electrochim. Acta 1994, 39, 547. (22) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. Electrochim. Acta 1994, 39, 1825. (23) Pastor, E.; Wasmus, S.; Iwasita, T.; Are´valo, M. C.; Gonza´lez, S.; Arvia, A. J. J. Electroanal. Chem. 1993, 350, 97. (24) Hadzi-Jordanov, S.; Angerstein-Kozlowska, A.; Vukovic, M.; Conway, B. E. J. Electrochem. Soc. 1978, 125, 1471. (25) Hrbek, J.; De Paola, R.; Hoffmann, F. M. Surf. Sci. 1986, 166, 361. (26) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267.

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