Electrocatalytic oxidation of acetaldehyde on Pt alloy electrodes

Electrochimica Acta 49 (2004) 2077–2083

Electrocatalytic oxidation of acetaldehyde on Pt alloy electrodes

K.B. Kokoha,∗, F. Hahna,1, E.M. Belgsira,1, C. Lamya,1, A.R. de Andradeb, P. Olivib,A.J. Motheoc,1, G. Tremiliosi-Filhoc,1

a Equipe Electrocatalyse, UMR 6503, CNRS—Université de Poitiers, 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, Franceb Departamento de Quimica da Faculdade de Filosofia, Ciˆencias e Letras de Ribeirão Preto, Universidade de São Paulo,

Av. Bandeirantes, Caixa Postal 3900, Ribeirão Preto 14040-901, SP, Brazilc Instituto de Qu´ımica de São Carlos, Universidade de São Paulo, Caixa Postal 780, São Carlos 13560-970, SP, Brazil

Received 10 October 2003; received in revised form 6 November 2003; accepted 9 November 2003

Abstract

Reaction intermediates occurring during the oxidation of acetaldehyde were investigated by in situ infrared reflectance spectroscopy(SPAIRS and SNIFTIRS techniques). These measurements, showing that acetaldehyde was transformed into CO and CO2 successively,allowed us to choose a suitable potential program to oxidize electrocatalytically the reactant on binary and ternary platinum alloy electrodeswith two compositions (Pt/Os and Pt/Ru/Os). IR results were useful to set a potential pulse program which allowed to adsorb dissociativelythe organic compound. The analysis of the electrolysis products was performed by high performance liquid chromatography (HPLC). Aceticacid, formic acid and carbon dioxide were determined as the main oxidation products of acetaldehyde.© 2003 Published by Elsevier Ltd.

Keywords:Electrocatalytic oxidation; Adsorption; Acetaldehyde; Platinum alloy electrode; In situ infrared reflectance spectroscopy

1. Introduction

Acetaldehyde was oxidized to CO2 on different platinumalloy electrodes in the over voltage potential region. Unsuc-cessfully, the sole reaction product we obtained was aceticacid, which is known to be hard to convert into carbon diox-ide. In order to break the C–C bond, a special potential pro-gram to oxidize acetaldehyde to CO2, has been applied andfor this process, potentials were chosen from cyclic voltam-metry and in situ infra red spectroscopic data.

Electrochemical oxidation of acetaldehyde continues to bean interesting topics because it arouses great investigationsin different catalytic application areas. It is an intermediateproduct in ethanol electrooxidation and in the degradationof lots of polluting organic compounds. In spite of manystudies, the knowledge on the oxidation of acetaldehyde re-mains far from complete. Its direct conversion into carbondioxide would avoid the formation of acetic acid, which iswell known to be a refractory molecule. Some papers haveshown that acetaldehyde can be oxidized to acetic acid and

∗ Corresponding author. Fax:+ 33-549453580.E-mail address:[email protected] (K.B. Kokoh).1 ISE member.

CO2 on platinum electrodes[1–7]. Spectroelectrochemicalresults revealed that the final product CO2 results from theoxidation of adsorbed carbon monoxide. The formation ofthe latter seemed to come either from the oxidation of aceticacid or from the dissociative adsorption of acetaldehyde[4].Pastor et al. showed by a differential electrochemical massspectroscopy (DEMS) study that acetaldehyde was oxidizedon platinum and rhodium electrodes to acetic acid and CO2[8,9]. Moreover, they identified other reaction products suchas methane and ethane, two gaseous organic compounds thatare not soluble in aqueous medium.

Although platinum is known to be the best electrocatalyst,it is known that its activity is improved by the modificationof its surface with transition metals such as Ru, Sn, Mo, etc.Wieckowski et al. reported that Pt alloyed with osmium canalso constitute a promising catalyst for the electrochemicaloxidation of small molecules[10–12]. When depositingRu and Os in submonolayer amounts on Pt(1 1 1) singlecrystals, they showed that these electrodes exhibited highersurface activity at lower potentials. This paper will focus onthe electro-oxidation of acetaldehyde on binary and ternaryphases of platinum alloys, Pt/Os and Pt/Ru/Os, respectively.IR spectroscopic investigations contribute to choose, inthe present work, the oxidation potential during long-term

0013-4686/$ – see front matter © 2003 Published by Elsevier Ltd.doi:10.1016/j.electacta.2003.11.015

2078 K.B. Kokoh et al. / Electrochimica Acta 49 (2004) 2077–2083

electrolyses in order to succeed to transform acetaldehydeinto carbon dioxide.

2. Experimental

The alloy electrodes were prepared in an arc-meltedfurnace under argon atmosphere (1.3 × 10−5 bar) [13].Single phase polycrystalline Pt-based Ru and Os alloyswere obtained. The basic atomic compositions for Pt/Ru/Os(70:20:10) and Pt/Os (80:20) were evaluated by EDX mi-croanalyses[14]. XRD patterns (with Cu K� ray) confirmedthe fcc single phase structure. The electrode surface waspolished using fine grade paper followed by alumina powderup to 0.05�m. The electrodes were then electrochemicallyactivated by 50 voltammetric cycles in 0.5 M H2SO4 from0.05 to 1.5 V versus RHE. Between the experiments theelectrodes were kept in a 0.5 M H2SO4 solution.Fig. 1 rep-resents the voltammograms of the alloy electrodes in 0.5 MH2SO4. In general, the voltammograms exhibit the profilefor the platinum electrode with the oxide formation regionshowing the overlay of the osmium and ruthenium oxidesformation. The adsorption/desorption hydrogen region al-lowed to calculate the true surface area of the electrodes

Fig. 1. Voltammograms of Pt/Os (a) and Pt/Ru/Os (b) electrodes at50 mV s−1 in a 0.5 M H2SO4 supporting electrolyte.

which are APtOs = 0.60 cm2 and APtRuOs = 0.67 cm2,respectively, when the entire disk was immersed in the so-lution. The geometric surface areas were, respectively, 0.44and 0.36 cm2. In these calculations were used the knownH-adsorption charge for bare Pt substrate and additionallyit was assumed that the organic electroactive species adsorbonly on the available platinum atoms. These assumptions arereasonable considering that the organic adsorption do notoccurs on osmium and ruthenium surface atoms, thus onlythe platinum atoms are effective in the adsorption process.

The solutions were prepared from ultra-pure water (Milli-pore Milli-Q System), and from Merck “Suprapur” reagents(HClO4, acetaldehyde, acetic acid and Na2CO3). Beforeeach experiment, the solutions were deaerated with ultrapure nitrogen (U Quality from L’Air Liquide) and a nitrogenstream was maintained over the solution during the mea-surements. All the experiments were carried out at roomtemperature (20± 2 ◦C).

2.1. Electrochemical set-up

Cyclic voltammetry was used for investigating the reac-tivity of acetaldehyde on Pt/Os and Pt/Ru/Os electrodes.Voltammetry measurements were performed in an undivideddouble-wall thermostated glass cell. Its volume is 11 cm3.The experimental set-up consisted of a waveform genera-tor (Wenking Model VSG72, Bank Electronik), a WenkingModel LB81 potentiostat and a X–Y recorder (Linseis LY17100). The counter electrode was a glassy carbon plate anda saturated mercury–mercurous sulphate electrode (MSE)served as reference electrode.

The electrolysis equipment was composed of a potentio-stat (PAR Model 362) monitored by a microcomputer. Thecurrent intensity versus time was followed on a Kipp & Zo-nen BD 40 X-t recorder, whereas the quantity of electricitywas measured directly by a coulometer (Wenking EVI 80).Each electrolysis was carried out using a potential programconsisting of three potential plateaus (Fig. 2). The appliedpotential program can be chosen according to the shape ofthe voltammogram. For instance, with the alloy electrodes, a

Fig. 2. Potential program used for long-term electrolysis of acetaldehyde.

K.B. Kokoh et al. / Electrochimica Acta 49 (2004) 2077–2083 2079

Fig. 3. Voltammograms of Pt/Os (a) and Pt/Ru/Os (b) electrodes at50 mV s−1 in 0.1 M HClO4 and in presence of 50 mM acetaldehyde.

potential plateau in the hydrogen region (0.25 V versus RHE)allows to reduce the possible passivating oxides formedduring the previous cleaning step (at 1.65 V versus RHE)and to adsorb the organic electroreactive species. Then, theelectro-oxidation potential plateau is chosen in the ascent ofthe oxidation peak (1.15 V versus RHE, seeFig. 3). Then athird potential plateau is often necessary to reactivate in situthe electrode surface by clearing out the poisoning speciesformed during the previous oxidation step. The time scaleof each applied potential plateau is optimized in order tohave a maximum electrolysis yield. In the present case, af-ter adsorbing the organic molecule atEads= 0.25 V versusRHE for 1 s, the oxidation reaction was performed at a po-tential plateau,Eox (1.15 V versus RHE) for 12 s. The thirdpotential pulse (Edes) was set at 1.65 V versus RHE for 0.2 s.This sequential unit program was repeated for several hoursthroughout the electrolysis and it is necessary to limit theeffect of the electrode deactivation. Potentials higher than1.65 V versus RHE (the surface reactivation potential) can-not be applied to avoid dealloying and surface rougheningas was observed in the course of this investigation.

In both voltammetry and electrolysis studies, the electrodepotentials indicated in the text are given in the reversiblehydrogen electrode (RHE) scale.

2.2. Spectroelectrochemical investigations

The Fourier transform IR spectrometer used was a BrukerIFS 66v, with the sample compartment modified to allowthe beam to be reflected on the electrode surface with anincidence angle of 65◦, after passing through the IR window(CaF2) of a conventional thin layer spectroelectrochemicalcell. The beam path was under vacuum and a liquid N2cooled HgCdTe detector (Infrared Associates) was used.

Using the single potential alteration infrared reflectancespectroscopy (SPAIRS) technique[15–18], reflectivitieswere recorded at 50 mV intervals during the first voltam-metric scan at a low sweep rate (1 mV s−1). Each spec-trum resulted from the co-addition of 128 interferograms.Data acquisition required 20 s, i.e. over ca. 20 mV. Spectrawere calculated as�R/R = (RE2 − RE1)/RE1, where the“reference” spectrum,RE1, was that recorded at the initialpotential in the acetaldehyde adsorption region.

Similarly, using the subtractively normalized interfa-cial Fourier transform infrared reflectance spectroscopy(SNIFTIRS) method[4,19,20], reflectivities were obtainedat two electrode potentialsE1 and E2 (the frequency ofpotential modulation was 0.025 Hz and 128 interferogramswere collected before the Fourier transform) and co-added30 times at each potential. Final spectra were normalizedas �R/R = (RE2 − RE1)/RE1. For the SNIFTIRS andSPAIRS calculations,E2 > E1, so that, a positive ab-sorption band indicates the consumption of species and anegative absorption band means the production of species.

Both techniques allowed the detection of adsorbed speciesand reaction products at the electrode surface. The measure-ments were performed withp polarization of the beam inorder to identify the adsorbed intermediates and the prod-ucts accumulated close to the electrode surface.

2.3. Chromatographic analysis

Analysis of the reaction products was carried out duringthe electrolysis by high performance liquid chromatogra-phy (HPLC). This apparatus was composed of an isocraticpump (Knauer HPLC Pump 64) and an ion-exchange col-umn (HPX-87H, from Bio-Rad). The eluent was a dilutesolution of sulphuric acid (3.33 mM H2SO4) at a flow rateof 0.6 cm3 min−1. After separation at room temperature,the electrolysis products were successively detected withan UV detector (Applied Biosystems 785A) working atλ = 210 nm, and a refractometer (Spectra-Physics SP8430).Chromatograms were recorded on a two-channel integrator(D 2500 Merck). The nature of the organic compounds wasdetermined by comparing their retention times to those forpure reference products under the same analysis conditions.During electrolysis, carbon dioxide formed was trapped ina bubble cell containing 1.0 M NaOH. At the end of theexperiment, nitrogen was bubbled in the cell to removeall the CO2 towards the trap cell. As explained previously,the obtained carbonate was analyzed quantitatively by


comparison with Na2CO3 reference prepared under the sameconditions.

3. Results and discussion

3.1. Behaviour of acetaldehyde in aqueous medium

Acetaldehyde is a molecule, which is very volatile. Itsboiling point is 21◦C. However, all the solutions were pre-pared by taking the necessary volume of acetaldehyde di-rectly from the freezer compartment.

In acid medium (0.1 M HClO4), acetaldehyde polymer-izes into paracetaldehyde as follows[21]:

In alkaline medium (0.1 M NaOH), acetaldehyde can un-dergo two kinds of chemical reactions. The first one is theCannizzaro reaction which yields a carboxylate and an al-cohol:

2CH3CHO+ OH− → CH3COO− + CH3CH2OH

As the molecule of acetaldehyde has aα hydrogen, themain reaction in alkaline medium is aldolization[21,22]:

The analysis of the solution by chromatography showedthat acetaldehyde was converted to paracetaldehyde inacid medium, and to acetate, 3-hydroxybutanal and3-hydroxybutanoate in alkaline medium.

This chemical behaviour of acetaldehyde in differentelectrolytes, as monitored by HPLC, incited us to carryout electrochemical studies in 0.1 M HClO4, where a lowconcentration of acetaldehyde was followed without deter-mining paracetaldehyde.

3.2. Electrochemical oxidation of acetaldehydein acidic medium

3.2.1. Cyclic voltammetryCyclic voltammetry was carried out in 0.1 M HClO4 to

show the reactivity of acetaldehyde on alloy electrodes.

The voltammograms were always recorded during the firstvoltammetric cycle in order to allow comparison and to avoidconsumption of the organic molecule.

A cyclic voltammogram of a Pt/Os alloy electrode in0.1 M HClO4 + 50 mM acetaldehyde is shown inFig. 3a.After the hydrogen adsorption region which is greatly de-creased by acetaldehyde adsorption, the oxidation of thismolecule begins at ca 0.5 V versus RHE. During the pos-itive potential sweep two oxidation peaks A (atEAmax =0.85 V versus RHE and B (atEBmax = 1.22 V versus RHE)are observed. They occur in a potential range where theM(OH)ads (M: Pt and Os) species are formed. Conversely,during the negative potential sweep, a small oxidationpeak C takes place after the electroreduction of the oxidespecies.

Fig. 3b represents the voltammogram of the Pt/Ru/Oselectrode in 0.1 M HClO4+50 mM acetaldehyde. The sameoxidation peaks observed as those with the previous elec-trode are obtained at the same potentials. But the presenceof the third metal (ruthenium) contributes to increase thecurrent intensity by a factor of 2.

3.2.2. Infra red spectroscopic studyTo understand the process of the electrocatalytic oxida-

tion of acetaldehyde on Pt/Os and Pt/Ru/Os electrodes, theadsorbed intermediates and the final reaction products weredetermined both by SPAIRS and SNIFTIRS techniques. Ref-erence spectra of acetaldehyde were obtained by transmis-sion spectroscopy using a dilute solution (10 mM) in 0.2 MHClO4.

3.2.2.1. SPAIR spectroscopy.During the SPAIRS experi-ments, the cyclic voltammograms (CV) showing the adsorp-tion and oxidation of acetaldehyde in acid medium on Pt/Osand Pt/Ru/Os at a scan rate of 1 mV s−1 are quite similar tothe CV presented inFig. 3. During this slow scan rate SPAIRspectra are accumulated at a given potential (every 50 mV).

Fig. 4 shows SPAIR spectra for Pt/Os in the (1000–3000 cm−1) spectral range, from 100 mV at the top to550 mV at the bottom. The reference reflectivity used for thecalculation of�R/R is taken at 0 mV. Two main absorptionbands are observed. A first one, always present in the series,around 1650–1700 cm−1. This complex band is attributedto interfacial water (1640 cm−1) and to acetaldehyde orother species (stretching modeνCO from a carbonyl grouparound 1700–1720 cm−1). The positive lobe of the band


Fig. 4. SPAIR spectra of the species resulting from the adsorption andoxidation of 10 mM acetaldehyde in 0.2 M HClO4 on a Pt/Os electrode atvarious potentials (top to bottom: (a) 100, (b) 150, (c) 200, (d) 250, (e)300, (f) 350, (g) 400, (h) 450, (i) 500 and (j) 550 mV vs. RHE),RERef

taken at 50 mV/RHE.

corresponding to interfacial water is more intense at lowpotentials before the formation of the negative lobe corre-sponding to some adsorbed species containing a carbonylfunction. This negative lobe appears at about 250 mV. Asmall shift in frequency of this band can be observed andshows both the adsorption of these species and also the ox-idation of the aldehyde to carboxylic acid. The second one,at 2345 cm−1, appearing about the middle of the series,corresponds to the reaction product CO2. The potential ofthe beginning of CO2 production is around 350 mV. Someother weak bands can be seen at about 2950 cm−1 for νCHof CH3, at around 1111 cm−1 for the perchlorate anionsand around 2050 cm−1 for the poisoning species CO. Thislatter one will be discussed from SNIFTIR spectra whichtechnique is more suitable to follow adsorbed species.

The equivalent series of SPAIR spectra for Pt/Ru/Os isshown inFig. 5. The interfacial water band is very intenseand may hide the carbonyl band. The band at 2345 cm−1 isalso observed but at lower potentials than on Pt/Os, sinceCO2 is detected on Pt/Ru/Os at about 300 mV, that is to say50 mV more negative.

3.2.2.2. SNIFTIR spectroscopy.With the SNIFTIRS tech-nique, spectra were recorded in the whole potential range(adsorption and oxidation range, from 100 to 1000 mV) ata constant potential modulation (�E = 300 mV) with ppolarization, as given inFigs. 6 and 7. In the adsorptionrange, at low potentials (from 100 to 400 mV), the COband, hardly observed on SPAIR spectra, appears strongly

Fig. 5. SPAIR spectra of the species resulting from the adsorption andoxidation of 10 mM acetaldehyde in 0.2 M HClO4 on a Pt/Ru/Os electrodeat various potentials (top to bottom: (a) 100, (b) 150, (c) 200, (d) 250, (e)300, (f) 350, (g) 400, (h) 450, (i) 500 and (j) 550 mV vs. RHE),RERef

taken at 50 mV/RHE.

as a bipolar band at 2060 cm−1. The poisoning species COL(linearly bonded CO) is present at the first modulation andthis shows that the adsorption step of acetaldehyde on thesetwo catalysts is dissociative. The intensity of this absorption

Fig. 6. SNIFTIR spectra of the species resulting from the adsorp-tion and oxidation of 10 mM acetaldehyde in 0.2 M HClO4 on aPt/Os electrode at various potential modulations vs. RHE (top tobottom: (100–400 mV), (200–500 mV), (300–600 mV), (400–700 mV),(500–800 mV), (600–900 mV) and (700–1000 mV)).


Fig. 7. SNIFTIR spectra of the species resulting from the adsorp-tion and oxidation of 10 mM acetaldehyde in 0.2 M HClO4 on aPt/Ru/Os electrode at various potential modulations vs. RHE (top tobottom: (100–400 mV), (200–500 mV), (300–600 mV), (400–700 mV),(500–800 mV), (600–900 mV) and (700–1000 mV)).

band increases with the mean electrode potential until(200–500 mV), then decreases when the potential is highenough to have oxidation process. The comparison of the

two figures shows that Pt/Os seems more poisoned by COLthan Pt/Ru/Os. Moreover, the absorption band is situatedaround 2050–2070 cm−1 in the case of Pt/Os and around2040–2060 cm−1 in the case of Pt/Ru/Os electrode. Thisnegative shift of 10 cm−1 may be due to both the electroniceffect and the coverage of COL. The Stark tuning rate wascalculated to be around 30 cm−1 for both electrodes andconfirms the adsorption phenomena of COL. The CO2 bandis more or less easy to observe since CO2 desorbs and dif-fuses into solution, so that its band can be cancelled by thecalculation of�R/R. These SNIFTIR spectra show princi-pally the effect of the third metal, i.e. Ru, which enhancesthe catalytic properties of the electrode. In fuel cell area, Ruis also well known to increase the electrocatalytic activityof platinum[23].

3.3. Oxidation of acetaldehyde

Despite the small current densities obtained at the Pt al-loy electrodes, long-term electrolyses were realized on Pt/Osand Pt/Ru/Os using the three-potential plateau program de-scribed inFig. 2. The final charge was obtained by the sumof the contribution of the charges related to the adsorptionstep (Qads), to the oxidation step (Qox) and to the desorp-tion step (Qdes). The potential plateauEox was set at 1.15 Vversus RHE and the electrolyses were conducted for 22 h.HPLC analyses of the bulk solution at the end of electroly-sis showed that acetaldehyde was oxidized mainly to aceticacid (seeTable 1).

According to the composition of the electrode, it can benoticed that the conversion into carbon dioxide is quite dif-ferent. Indeed, on a binary alloy electrode, traces of CO2 aredetected (its small amount is hard to be quantified by HPLC).However, the presence of ruthenium in a ternary alloy elec-trode led the oxidation of acetaldehyde to the formation ofCO2 in a great amount. This different behaviour of bothelectrodes can be explained by a synergetic effect due to thepresence of this third metal near the Pt sites. So it contributes,during the adsorption of acetaldehyde onto Pt, to much moredissociation of the molecule. And then, the fast formationof ruthenium oxides allows the oxygenated species from ad-sorbed acetaldehyde to be oxidized to carbon dioxide.

If we take into account the previous chromatographic andspectroscopic results and those obtained in the literature[4,9], it can be assumed that the oxidation of acetaldehydeoccurs according to the following processes:

Acetaldehyde is oxidized to acetic acid by the sole in-teraction of the carbonyl group with the electrode surface.The second assumption is that the dissociative adsorption ofacetaldehyde produces two adsorbates species: CH3ads andCHOads. The latest undergoes different oxidation steps toproduce carbon dioxide. On the other hand, CH3 speciesformed by the breaking of the C–C bond can combine withadsorbed hydrogen during the adsorption potential plateauto give CH4 or a recombination between two CH3 speciescan give rise to the formation of ethane. These two gaseousproducts were identified by Pastor and coworkers[9] byDEMS during the oxidation of acetaldehyde on platinumand rhodium electrodes. Oxidation of CH3 intermediate toCO2 is also possible but we do not have any indication thatsupports this assumption.


Table 1Distribution and concentration of the oxidation products formed after long-term electrolysis of 50 mM acetaldehyde in 0.1 M HClO4, on platinum alloyelectrodes

Electrode Potential ofoxidation

Conversion ofacetaldehyde (%)

Oxidation products Quantity ofelectricity (Qexp) (C)

Pt/Os 1.15 V vs. RHE 76 Acetic acid (11 mM), formic acid (1.5 mM), CO2 (traces) 55.3Pt/Ru/Os 1.15 V vs. RHE 68 Acetic acid (10 mM), formic acid (1.6 mM), CO2 (3.3 mM) 59.4

The previous assumptions can explain the differences be-tween the mass balance and the experimental quantity ofelectricity involved during the reaction since the chromato-graphic analysis allows us to monitor only the species inthe aqueous solution and the CO2 trapped in the NaOHsolution.

4. Conclusions

It has been shown in this paper that the application of athree plateaus potential program with an adsorption potentialplateau gains new insights on the electrochemical oxidationof acetaldehyde on Pt-based ruthenium and osmium alloyelectrodes. In situ reflectance infrared spectroscopy togetherwith the long-term electrolysis allowed us to show that thismolecule can be transformed, over acetic acid to carbondioxide. The analysis of the bulk solution by chromatogra-phy completed well the determination of the reaction prod-ucts. Although the oxidation process generates by-productswhich are adsorbed at the electrodes surface, SPAIRS andSNIFTIRS techniques allowed us to show that acetaldehydecan be firstly dissociated then adsorbed as carbon monox-ide species, which in its turn is oxidized to CO2. Otherintermediate species are probably present during this elec-trochemical process. According to the composition of theelectrode, it has been observed that the presence of ruthe-nium leads to less poisoning by CO and faster oxidationto CO2.

Acknowledgements

This work was mainly done under the framework of a col-laboration programme between the USP (University of SãoPaulo) and the COFECUB (Comité Français d’Evaluationde la Coopération Universitaire avec le Brésil) under grantno 79/01.

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Electrocatalytic oxidation of acetaldehyde on Pt alloy electrodes