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Role of crystalline effects in electrocatalysis : mechanism andkinetics of carbon monoxide oxidation on stepped platinumelectrodesCitation for published version (APA):Lebedeva, N. P. (2002). Role of crystalline effects in electrocatalysis : mechanism and kinetics of carbonmonoxide oxidation on stepped platinum electrodes. Technische Universiteit Eindhoven.https://doi.org/10.6100/IR553166

DOI:10.6100/IR553166

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Role of crystalline defects inelectrocatalysis

Mechanism and kinetics of carbon monoxideoxidation on stepped platinum electrodes

Proefschrift

ter verkrijging van de graad van doctor aan deTechnische Universiteit Eindhoven, op gezag van de

Rector Magnificus, prof.dr. R.A. van Santen, voor eencommissie aangewezen door het College voor

Promoties in het openbaar te verdedigenop dinsdag 7 mei 2002 om 16.00 uur

door

Natalia Pavlovna Lebedeva

geboren te Novosibirsk, Rusland

Dit proefchrift is goedgekeurd door de promotoren:

prof.dr. R.A. van Santenenprof.dr. J.M. Feliu

Copromotor:dr. M.T.M. Koper

Printed by the Eindhoven University of Technology Press.

CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN

Lebedeva, Natalia P.

Role of crystalline defects in electrocatalysis. Mechanism and kineticsof carbon monoxide oxidation on stepped platinum electrodes / byNatalia P. Lebedeva. - Eindhoven : Techische Universiteit Eindhoven,2002.Proefschrift. - ISBN 90-386-2813-7NUGI 813Trefwoorden: elektrochemie / elektrokatalytische oxidatie / reactiekinetiek /koolmonoxide / gestapte platinaelektrodenSubject headings: electrochemistry / electrocatalytic oxidation / reactionkinetics / carbon monoxide / stepped platinum electrodes

The work described in this thesis has been carried out at Schuit Institute ofCatalysis, Laboratory of Inorganic Chemistry and Catalysis, Eindhoven Universityof Technology. Financial support was provided by a Spinoza grant of theNetherlands Foundation for Scientific Research (NWO).

To my parents, my sister’s family and Luis

Contents

Chapter 1. Introduction 1

Chapter 2. CO oxidation on stepped Pt[n(111)×(111)] electrodes: 13a voltammetric study

Chapter 3. The effect of the cooling atmosphere in the preparation of 29flame-annealed Pt(111) electrodes on CO adlayer oxidation

Chapter 4. Mechanism and kinetics of the electrochemical CO adlayer 37oxidation on Pt(111)

Chapter 5. Role of crystalline defects in electrocatalysis: 59mechanism and kinetics of CO adlayer oxidation onstepped platinum electrodes

Chapter 6. Role of crystalline defects in electrocatalysis: 85an infrared study of CO adsorption and oxidation onstepped platinum electrodes

Summary 113

Samenvatting 115

Resumen 117

Резюме 119

List of publications 121

Acknowledgment 122

Curriculum Vitæ 123

1

1Introduction

1.1. Electrocatalysis

The term electrocatalysis is often used to identify a scientific field at theinterface of catalysis and electrochemistry. Trasatti defined electrocatalysis ascatalysis of electrode reactions [1]. The catalytic effect in electrocatalysis can beachieved by the action of the electrode material or by the action of species insolution. In the former case, one deals with heterogeneous electrocatalysis and inthe latter with homogeneous electrocatalysis [1]. Similar to heterogeneouscatalysis, the effects of the electrode material and its structure on the rate and onthe mechanism of electrode reactions are the main topics of interest inheterogeneous electrocatalysis.

Heterogeneous catalysis and heterogeneous electrocatalysis are, however,not totally independent disciplines. It has been long realized that liquid phaseheterogeneous catalytic systems, where a charged interface is formed, are to beseen as electrochemical systems. Electrocatalytic measurements would be thusadvantageous to the studies in the liquid phase heterogeneous catalysis, since thesurface potential is known or/and controlled.

Advances in electrocatalysis are deeply connected to the understanding ofthe mechanism of electrode reactions. The mechanisms and kinetics ofelectrocatalytic reactions can be studied not only with the help of classicelectrochemical methods, such as voltammetric and transient techniques, but alsousing a number of in situ physical characterization techniques [2]. The latter

Chapter 1

2

techniques allow the investigation of a surface structure and to identify adsorbateand reaction products and their concentration. In situ scanning tunnelingmicroscopy (STM) and surface X-ray scattering (SXS) have proved to be powerfultechniques for the characterization of surface and adsorbate structures on theatomic level. Surface Enhanced Raman Spectroscopy (SERS) and InfraRedReflection-Absorption Spectroscopy (IRRAS) are widely used to determine thenature of the adsorbate and the structure of the adlayer at the electrode surface bythe adsorbate vibrational frequencies. Sum Frequency Generation (SFG) andSecond Harmonic Generation (SHG) represent a class of non-linear spectroscopictechniques and can be also used for surface and adsorbate characterization. Finally,Differential Electrochemical Mass Spectroscopy (DEMS) allows in situidentification of the type and amount of the gaseous products of an electrochemicalreaction.

Several areas in the field of electrocatalysis are of industrial interest: (a) thedevelopment of the alternative energy sources, i.e. fuel cells, (b) environmentalelectrocatalysis, (c) electrosynthesis.

Since the world’s fossil fuel deposits are limited, fuel cells, i.e. devicesconverting chemical energy directly into electrical energy, are often seen as one ofthe most promising alternative sources of energy [3-6]. The two best known typesof low temperature fuel cells are the hydrogen-oxygen and the direct methanol fuelcell. In these cases hydrogen or methanol are oxidized to protons or/and carbondioxide and oxygen is reduced to water:

H2 2 H+ + 2 e- (1.1)

CH3OH + H2O CO2 + 6 H+ + 6 e- (1.2)

O2 + 4 H+ + 4 e- 2 H2O (1.3)

The overall cell reactions are:

2 H2 + O2 2 H2O (1.4)

CH3OH + 3/2 O2 CO2 + 2 H2O (1.5)

Other small C1-C3 organic molecules were also proposed as a feedstock for fuelcells.

Since water and carbon dioxide are the main products of the electrocatalyticconversion of either hydrogen or methanol, the emission of noxious gases such asCO and NOx is greatly reduced. This makes fuel cells especially attractive from theenvironmental point of view.

Although some fuel cells have become commercially available, bothhydrogen and direct methanol fuel cells are not widely used at present. The major

Introduction

3

impediment in the development of these devices is the rapid deactivation of theelectrocatalyst by CO, which is present in the hydrogen feed stock as produced byreforming methanol or gasoline or results from the dehydrogenation of methanolon the electrode surface [4,5]. Therefore, the design of a more CO-tolerantelectrocatalyst is highly desirable.

Treatment of industrial waste streams containing metal ions, cyanides andeven organic pollutants are well-known examples of environmental electrocatalysis[1]. Other applications include production of ozone and hydrogen peroxide toreplace chlorine for water desinfection and chlorine dioxide for pulp and paperbleaching [1]. Attempts to design an electrocatalytic process for conversion oftoxic nitrogen-containing compounds, such as ammonia and NOx, into molecularnitrogen were also undertaken [1,7].

Several industrial processes of organic synthesis are electrocatalytic [8]. Thebest known one is the synthesis of adiponitrile from acrylonitrile developed byMonsanto in 1964. Production of gluconic acid from glucose, fluorinatedcompounds from hydrocarbons and aliphatic carboxylic acids, glyoxylic acid fromoxalic acid and succinic acid from maleic acid are a few other large-scale industrialprocesses [8]. The electrochemical fixation of carbon dioxide to produce higherhydrocarbons was also suggested [1]. At present, however, a poor selectivity of thiselectrocatalytic process is a major problem [1].

1.2. The concept of structure sensitivity

The concept of structure-sensitivity in heterogeneous catalysis is probably asold as the concept of active centers and can be found in the original paper of Taylorpublished in 1925 [9]. He wrote: “There will be all extremes between the case inwhich all the atoms in the surface are active and that in which relatively few are soactive” and “…the amount of surface which is catalytically active is determined bythe reaction catalyzed” [9], thus introducing a possible dependence of the activityof a catalyst on its surface structure.

Further numerous kinetic studies of different catalytic reactions lentexperimental support to this idea [10,11]. Initially, a structure sensitivity was mostfrequently observed for reactions catalyzed by supported metal particles, where thevariation of the reaction rate with the particle size or/and preparation method wasdetected [10]. Classic examples of heterogeneous catalytic reactions includeammonia synthesis on iron, ethane hydrogenolysis on nickel, hydrogenation andisomerization of various hydrocarbons on platinum [10,11]. However, the rate ofsome other reactions, such as hydrogenation of cyclohexene and cyclopropane onplatinum [11], was shown to be independent on the particle size or metal

Chapter 1

4

dispersion. This lead Boudart to the classification of the catalytic reactions intostructure-sensitive or demanding reactions and structure-insensitive or facilereactions [10]. The former were defined as reactions exhibiting a pronounceddependence of the rate on the surface structure of the catalyst. Otherwise, thereaction is called structure-insensitive.

In practice the suggested classification did not always appear clear-cut, sincefactors such as metal-support interactions, impurities or even mass-transferlimitations may alter the activity of a catalyst [10,11]. Because of suchcomplications, it is generally accepted that a safer test of structure sensitivity is theone preformed on large single crystals exposing well-defined crystallographicplanes of varying Miller indices [11,12]. The use of stepped single crystals,popularized by Somorjai [12], allows the systematic investigation of the influenceof surface defects (steps and kinks) on the reaction kinetics and mechanism.

Although the reason for the structure-sensitivity for a given reaction is oftenspecific to that reaction, recent studies of different elementary reactions on singlecrystals in combination with quantum chemical calculations revealed severalgeneral effects related to a structure sensitivity of a reaction [13]. Two main factors(although not completely independent), so-called “electronic” and “geometric”effects, have been pointed out [13]. The electronic effect originates from thechange of the local electronic properties with the surface structure of a catalyst. Ithas been shown that for transition metals the local average of the d-electronenergies largely suffices to describe the electronic effect [13]. The electronic effectis responsible for the variation in reactivity of different facets of a metal, providedthat the adsorbate geometry is the same. Geometric effects manifest themselves asa site preference of a given adsorbate on the surface with similar local electronicproperties. The simplest measure of the geometric effect is thus the coordinationnumber of the adsorbate with respect to surface atoms [13].

At steps and kinks the electronic effect may become very large [13,14]. Ahigher adsorption energy for many molecular and atomic adsorbates and loweractivation energies for molecular dissociation reactions are often observed at thosesites [13].

The geometric effect may, however, also become prominent at steps [13].Very low activation barriers for the dissociation of nitric oxide and molecularnitrogen on monoatomic steps on Ru(0001) were explained in terms of thestabilization of the transition states in the particular geometric arrangement at thebottom of the step [13 and references therein].

The concept of structure sensitivity was adopted in electrocatalysis with theincreasing interest to fuel cell reactions – oxidation of hydrogen and small organicmolecules and reduction of oxygen at metal electrodes [15]. Similarly to gas phaseheterogeneous catalysis, this concept has been initially utilized for explaining the

Introduction

5

variation of the reaction rate with particle size or preparation method for supportedmetal electrodes [6,16]. With the invention of the flame-annealing procedure byClavilier et al. in 1980 [17] studies of the electrocatalytic processes on well-definedsurfaces of metal single crystals became common [15]. A large amount ofexperimental evidence of structure sensitivity of various electrochemical reactionshas been acquired since then [15]. However, a molecular level understanding of thereasons of this phenomenon is still forthcoming.

1.3. Electrocatalytic oxidation of carbon monoxide on platinum

The electrocatalytic oxidation of carbon monoxide has been extensivelystudied in the past decades, since a better understanding of the mechanism andkinetics of the electrooxidation of carbon monoxide (CO) would be helpful fordesigning of a more efficient electrocatalyst for fuel cells. This relatively simplemodel reaction is also of fundamental interest for establishing a structure –reactivity relation in electrocatalysis.

A brief overview of the results and concepts reported previously in theliterature on this subject is presented in this section along with an outline of themain unresolved questions.

1.3.1. Adsorption of carbon monoxide on platinum

From the early experiments with polycrystalline Pt electrodes chemisorptionof CO on platinum at room temperature was concluded to be irreversible, i.e. nodesorption of CO was observed upon the removal of dissolved CO from theelectrolyte [18 and references therein]. Under electrochemical conditions CO wasshown to displace adsorbed hydrogen from the platinum surface [18,19]. On thebasis of coulometric measurements at least two kinds of species – atop and bridge-bound CO – were concluded to co-exist in the CO adlayer on polycrystallineplatinum [18].

It has been believed for a long time that highly compressed adlayers of CO,with CO coverage approaching unity, can be prepared on low-index surfaces oftransition metals under aqueous electrochemical conditions, while significantlylower CO coverages (from 0.5 to 0.71) were reported for the correspondingsystems in ultra high vacuum (UHV). It should be noted that in the former case COcoverage was often estimated from the charge transferred during the oxidativestripping of the adlayer in the voltammetric sweep or current transient. However,values of CO coverage, estimated from the integrated absorbance of the CO2 bandappearing at 2343 cm-1 as a result of adlayer oxidation, were close to the

Chapter 1

6

corresponding values for metal-ultrahigh vacuum interfaces. The apparentcontradiction between the voltammetric and IR spectrophotometric procedures forCO coverage estimation was recognized and extensively discussed in the literature[20].

With the help of charge displacement technique [21] the origin of thesystematic error in the conventional voltammetric evaluation of CO coverage wasidentified. It has been shown that the conventional voltammetric procedureoverestimates the CO coverage by taking into account the charge transferred due tothe anion re-adsorption during the CO adlayer oxidation. The reliable and accurateprocedure of the absolute CO coverage determination on Pt-group transition metalselectrodes requires subtraction of the “anion re-adsorption” charge from the totalcharge of CO adlayer oxidation [22]. It has been shown that saturation COcoverages in the UHV and aqueous electrochemical environments are largelysimilar for the low-index platinum, rhodium and iridium surfaces [22].

It is now well established that the most compressed adlayer of CO onPt(111) attainable under aqueous electrochemical conditions is p(2×2)-3CO with 2CO molecules occupying three-fold hollow sites and 1 CO molecule adsorbed atop;the maximum CO coverage is 0.75. This was convincingly demonstrated by in situscanning tunneling microscopy in combination with in situ infrared reflection-absorption spectroscopy [23]. A consistently close value of CO coverage for asaturated adlayer prepared under the same conditions can be obtained from thecoulometric analysis of voltammetric stripping profile of CO adlayer using theprocedure outlined in Ref. 22. The formation of the p(2×2)-3CO structure wasfurther supported by a study of the CO adlayer structure on Pt(111) with the help ofin situ surface X-ray scattering [24] and Second Harmonic Generation [25]. Thep(2×2)-3CO structure on Pt(111) is only formed when CO is dosed from the CO-saturated electrolyte and at electrode potentials below ca. 0.2 V vs. RHE. In allother circumstances as, for example, CO dosing from dilute CO solutions orthrough the meniscus from the gas phase, a less compressed adlayer withconsequently lower CO coverage is formed on Pt(111). The stability of p(2×2)-3CO structure was shown to depend on the pH and anion type of the electrolyte[26] as well as the amount of surface crystalline defects [27].

No ordered structures for CO adlayers on Pt(100) and Pt(110) were detected[26]. The infrared spectra of CO/Pt(100) system showed that the adlayer consists ofatop and bridge-bound CO molecules, while exclusively atop sites are occupied bychemisorbed CO on Pt(110) [28]. The maximum CO coverages for Pt(100) andPt(110) were estimated to be ca. 0.8 and 1.0, respectively [22].

Introducing periodic steps of either (100) or (110) orientation in the surfaceof Pt(111) disrupts the long-range order, necessary for the formation of p(2×2)-

Introduction

7

3CO structure [27], and leads to the formation of less compressed CO adlayerswith a saturation coverage of 0.65-0.7 [29]. The lowering of the CO packingdensity on Pt stepped surfaces at saturation is mirrored by a systematic replacementof a three-fold-hollow spectral band by a bridge feature, observed for surfaces withprogressively narrower (111) terraces [27]. In situ IRRAS investigations of COadsorption on Pt[n(111)×(100)] at sub-saturated CO coverages revealed that,similarly to that in UHV, this process is structure-sensitive [30]. At low coverageCO preferentially adsorbs on steps, while terraces become populated at increasedCO coverage [30]. Although the same qualitative effect is expected forPt[n(111)×(111)] surfaces, such experimental studies have not been reported yet.

Theoretical studies of CO adsorption on transition metals, particularlyplatinum, have been made to explain CO bonding at gas-solid interface [13,31].The donation – backdonation model, originally proposed by Blyholder [32], gives agood qualitative description of the Pt-CO interactions. Subsequent ab initiocalculations largely proved this model to be correct and essential for theunderstanding of CO chemisorption on metals [33,34]. Ab initio calculationsallowed quantitative analysis of the effect of external electric field on CO bindingenergy, C-O and Pt-C vibrational frequencies and intensities [34]. Co-adsorption ofCO and atomic oxygen has been also addressed [35]. The activation barrier and thepre-factor for the oxidation of chemisorbed CO with either atomic oxygen [36] orOH [37] have been computed using quantum chemical methods.

1.3.2. Oxidation of carbon monoxide on platinum

The electrochemical oxidation of CO on platinum has been studied quiteextensively over the past decades using different electrochemical and spectroscopicmethods. The achievements until 1990 in the understanding of this reaction havebeen reviewed by Beden et al. [31].

The “reactant-pair” mechanism for the electrochemical oxidation ofadsorbed CO in acidic solutions, originally proposed by Gilman [19], is nowgenerally accepted. This mechanism assumes a Langmuir-Hinshelwood typereaction between carbon monoxide and a surface oxygen-containing species,adsorbed on adjacent sites, to form CO2. The oxygen-containing species resultsfrom the oxidation of water at the electrode surface and is usually supposed to beOHads, although the exact nature of this species is still unknown. The overallreaction scheme is:

H2O + * OHads + H+ + e- (1.6)

Chapter 1

8

COads + OHads CO2 + H+ + e- + 2* (1.7)

where * denotes a free surface site.Most of the previous studies of CO electrochemical oxidation employed

linear sweep voltammetry [31]. The voltammetric electrooxidation of a CO adlayeron polycrystalline platinum as well as on low-index single crystal surfaces exhibitsmultiple current peaks, illustrating the complexity of the reaction [31]. The onsetpotential of CO adlayer oxidation has been shown to depend on the CO coverage,pH and anion type of the electrolyte, as well as the structure of the electrodesurface [26,31].

An increase in CO coverage results in a substantial positive shift of the onsetpotential for CO adlayer oxidation [31]. The effect can be quite pronouncedleading to an increase in the overpotential of CO adlayer oxidation on Pt(111) inperchloric acid from ca. 0.6 V at low CO coverage to ca. 0.9 V at saturation [31]. Acontinuous re-adsorption of CO from the solution also causes a substantial positiveshift of the onset potential for bulk CO oxidation compared to CO adlayeroxidation on Pt [38]. This behavior is expected for the Langmuir-Hinshelwoodmechanism Eqs.(1.6) and (1.7) with a competitive adsorption of the reactants if COhas a much higher affinity for the free surface sites than OH. However, theappearance of a new voltammetric feature, the so-called pre-wave, at significantlylower overpotentials compared to the main oxidative peak was reported for theoxidation of a saturated CO adlayer [39] and dissolved CO [26,38]. This processwas ascribed by Marković et al. to the oxidation of a weakly bound CO species,formed at high CO coverages due to the repulsive CO-CO lateral interactions[26,40]. The oxidation of the weakly bound CO in the pre-wave does not producemore Pt sites available for water adsorption [26,40] and results in an adlayerrelaxation into a strongly adsorbed state [26,40]. It is noteworthy that the reactionorder with respect to the partial pressure of CO in the pre-wave region is positive[26,40], while oxidation of the relaxed strongly adsorbed adlayer in the mainvoltammetric peak exhibits a clear negative reaction order with respect to CO (videsupra). Despite the difference in the apparent reaction orders, the mechanism of theoxidation of CO in both the prewave and the main voltammetric peak wassuggested to be of the Langmuir-Hinshelwood type [26,40]. However, we wouldlike to point out that at present still little is known about the initiation of the COoxidation reaction and this issue deserves further investigation.

Strongly adsorbing anions, such as halogens, phosphate and (bi)sulfate havean adverse effect on the reaction rate, since they inhibit oxidative electrosorption ofwater at the electrode surface [26]. The electrocatalytic oxidation of CO is stronglypromoted in alkaline solutions presumably due to the presence of reactive hydroxylspecies on the electrode surface at lower overpotentials [26]. Oxidation of a

Introduction

9

saturated CO adlayer in the presence of dissolved CO occurs at potentials as low asca. 0.2 V vs. RHE [26].

The electrocatalytic oxidation of a CO adlayer as well as of dissolved COwas shown to be a structure-sensitive process [26,31]. Among the low-index Ptsingle crystal surfaces the activity towards CO oxidation was shown to increase inthe sequence Pt(111)

Chapter 1

10

1.4. Scope of this thesis

The research described in the present thesis was focused on theelectrooxidation of adsorbed CO on platinum. An attempt to establish aquantitative correlation between the surface structure of the electrocatalysts and themechanism and kinetics of the reaction was undertaken. To achieve this theelectrochemical oxidation of CO on a number of stepped Pt single crystals from[1 1 0] and [0 1 1] zones was studied using linear sweep voltammetry,chronoamperometry and in situ infrared absorption-reflection spectroscopy. Theresults are presented in the following five chapters.

Chapter 2 reports a voltammetric study of the adsorption andelectrooxidation of adsorbed as well as dissolved CO on Pt[n(111)×(111)]electrodes. Both CO adsorption and oxidation reactions are shown to be structure-sensitive. The influence of step density and CO coverage on the reaction rate isqualitatively addressed.

The effect of the cooling atmosphere in the preparation of flame-annealedPt(111) is discussed in Chapter 3. It is shown that a higher amount of surfacecrystalline defects is formed when the annealed Pt(111) is cooled in the presence ofoxygen. The sensitivity of the saturated CO adlayer oxidation to the presence ofsurface defects on Pt(111) makes this reaction a sensitive, though qualitative,method of assessing the degree of crystalline order of Pt(111).

The study of the kinetics of the electrooxidation of saturated and sub-saturated CO adlayers on Pt(111) with the help of chronoamperometry is reportedin Chapter 4. A mathematical model to describe the reaction kinetics is developed.The apparent rate constant for electrochemical CO oxidation on Pt(111) and itspotential dependence are determined by fitting the experimental data with themodel. For sub-saturated CO coverages the overall picture is shown to be morecomplicated and remains to be understood.

The chronoamperometric study of the kinetics of the saturated CO adlayeroxidation was extended to a number of stepped Pt[n(111)×(111)] electrodes asdescribed in Chapter 5. The effect of crystalline defects on the rate of the reactionis analyzed quantitatively using the model described in Chapter 4. The reaction isshown to take place uniquely at steps. The apparent rate constant for theelectrochemical CO oxidation, the apparent intrinsic rate constant and its potentialdependence were determined. The minimal surface diffusion coefficient ofchemisorbed CO on (111) terraces is estimated.

Finally, we report a study of CO adsorption and oxidation on bothPt[n(111)×(111)] and Pt[n(111)×(100)] stepped surfaces using in situ infraredreflection-absorption spectroscopy in Chapter 6. Coverage dependent and potential

Introduction

11

dependent spectra of chemisorbed CO on these surfaces are reported. The structureof the active site of the electrocatalyst and the arrangement of the reacting speciesis proposed based on infrared spectra, acquired during CO adlayer oxidation.

References

[1] S. Trasatti, Int.J.Hydrogen Energy, 20 (1995) 835.[2] for recent overview see, for example: A. Wieckowski (Ed.), Interfacialelectrochemistry: theory, experiment, and applications. Marcel Dekker: New York, 1999.[3] G.-Q. Lu, A. Wieckowski, Curr.Opin.Colloid and Interface Sci. 5 (2000) 95.[4] A. Hamnett, Catal.Today 38 (1997) 445.[5] G. Hoogers, D. Thompsett, CatTech 3 (1999) 106.[6] T. Frelink, Ph.D. thesis, Eindhoven University of Technology, 1995, Chapter 1.[7] (a) J.F.E. Gootzen, Ph.D. thesis, Eindhoven University of Technology, 1997.(b) A.C.A. de Vooys, Ph.D. thesis, Eindhoven University of Technology, 2001.[8] M.M. Baizer, Recent and anticipated development of industrial organic electrochemicalsynthesis in Electrochemistry in Industry, New Directions, U.Landau, E.Yeager, D.Kortan(Eds.), Plenum Press: New York, 1982.[9] H.S. Taylor, Proc.Roy.Soc. London A 108 (1925) 105.[10] M. Boudart, Adv.Catal. 20 (1969) 153.[11] M. Boudart, G. Djéga-Mariadassou, Kinetics of heterogeneous catalytic reactions,Princeton University Press: Princeton, 1984, Chapter 5.[12] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, John Wiley&Sons:New York, 1994.[13] B. Hammer, J.K. Nørskov, Adv.Catal. 45 (2000) 71.[14] J. Tersoff, L.M. Falicov, Phys.Rev.B 24 (1981) 754.[15] J. Lipkowski, P.N. Ross (Eds.), Electrocatalysis, Wiley-VCH: New York, 1998.[16] See, for example, (a) O.A. Khazova, Yu.B. Vassiliev, V.S. Bagotzky, Elektrokhimiya6 (1970) 1367. (b) B.I. Podlovchenko, R.P. Petukhova, Elektrokhimiya 5 (1969) 380. (c)B.I. Podlovchenko, R.P. Petukhova, Elektrokhimiya 8 (1972) 899. (d) V.S. Tyurin,A.G. Pshenichnikov, R.Kh. Burshtein, Elektrokhimiya 7 (1971) 708.[17] J. Clavilier, R. Faure, G. Guinet, R. Durand, J.Electroanal.Chem. 107 (1980) 205.[18] M.W. Breiter, Electrochemical processes in fuel cells, Springer Verlag: New York,1969.[19] S. Gilman, J.Phys.Chem. 68 (1964) 70.[20] (a) J.M. Orts, A. Fernandez-Vega, J.M. Feliu, A. Aldaz, J. Clavilier,J.Electroanal.Chem. 327 (1992) 261. (b) M.J. Weaver, S.-C. Chang, L.-W.H. Leung,X. Jiang, M. Rubel, M. Szklarczyk, D. Zurawski, A. Wieckowski, J.Electroanal.Chem. 327(1992) 247.[21] a) J. Clavilier, R. Albalat, R. Gómez, J.M. Orts, J.M. Feliu, A. Aldaz,J.Electroanal.Chem. 330 (1992) 489. (b) J.M. Orts, R. Gómez, J.M. Feliu, A. Aldaz,J. Clavilier, Electrochim.Acta 39 (1994) 1519. (c) J.M. Feliu, J.M. Orts, R. Gómez,A. Aldaz, J. Clavilier, J.Electroanal.Chem. 372 (1994) 265.

Chapter 1

12

[22] R. Gómez, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf.Sci. 410 (1998) 48.[23] I. Villegas, M.J. Weaver, J.Chem.Phys. 101 (1994) 1648.[24] N.M. Marković, B.N. Grgur, C.A. Lucas, P.N. Ross, J.Phys.Chem. B 103 (1999) 487.[25] W. Akemann, K.A. Friedrich, U. Linke, U. Stimming, Surf.Sci. 402-404 (1998) 571.[26] N.M. Marković, P.N. Ross, Surf.Sci.Rep. 286 (2002) 1.[27] A. Rodes, R. Gómez, J.M. Feliu, M.J. Weaver, Langmuir 16 (2000) 811.[28] (a) S.-C. Chang, M.J. Weaver, J.Phys.Chem. 94 (1990) 5095. (b) S.-C. Chang, M.J.Weaver, Surf.Sci. 230 (1990) 222. (c) S.-C. Chang, M.J. Weaver, Surf.Sci. 238 (1990) 142.[29] (a) V. Climent, R. Gómez, J.M. Feliu, Electrochim.Acta 45 (1999) 629. (b) R. Gómez,V. Climent, J.M. Feliu, M.J. Weaver, J.Phys.Chem.B 104 (2000) 597.[30] (a) C.S. Kim, W.J. Tornquist, C. Korzeniewski, J.Phys.Chem. 97 (1993) 6484. (b)C.S. Kim, C. Korzeniewski, W.J. Tornquist, J.Chem.Phys. 100 (1994) 628. (c) C.S. Kim,C. Korzeniewski, Anal.Chem. 69 (1997) 2349.[31] B. Beden, C. Lamy, N.R. de Tacconi, A.J. Arvia, Electrochim.Acta 35 (1990) 691.[32] G. Blyholder, J.Phys.Chem. 68 (1964) 2772.[33] (a) F. Illas, S. Zurita, J. Rubio, A.M. Márquez, Phys.Rev.B 52 (1995) 12372. (b)F. Illas, S. Zurita, A.M. Márquez, J. Rubio, Surf.Sci. 376 (1997) 279. (c) H. Aizawa,S. Tsuneyuki, Surf.Sci. 399 (1998) L364.[34] (a) F. Illas, F. Mele, D. Curulla, A. Clotet, J.M. Ricart, Electrochim.Acta 44 (1998)1213. (b) D. Curulla, A. Clotet, J.M. Ricart, F. Illas, J.Phys.Chem. B 103 (1999) 5246. (c)M.T.M. Koper, R.A. van Santen, J.Electroanal.Chem. 476 (1999) 64. (d) D. Curulla,A. Clotet, J.M. Ricart, F. Illas, Electrochim.Acta 45 (1999) 639. (e) M. García-Hernández,D. Curulla, A. Clotet, F. Illas, J.Chem. Phys. 113 (2000) 364. (f) M.T.M. Koper, R.A. vanSanten, S.A. Wasileski, M.J. Weaver, J.Chem. Phys. 113 (2000) 4392. (g) S.A. Wasileski,M.T.M. Koper, M.J. Weaver, J.Phys.Chem.B 105 (2001) 3518. (i) S.A. Wasileski,M.T.M. Koper, M.J. Weaver, J.Chem. Phys. 115 (2001) 8193. (j) S.A. Wasileski,M.T.M. Koper, M.J. Weaver, J.Am.Chem.Soc., in press.[35] (a) K. Bleakly, P. Hu, J.Am.Chem.Soc., 121 (1999) 7644. (b) M. Lynch, P. Hu,Surf.Sci. 458 (2000) 1.[36] A. Eichler, Surf.Sci. 498 (2002) 314.[37] A.B. Anderson, E. Grantscharova, J.Phys.Chem. 99 (1995) 9143.[38] H.A. Gasteiger, N.M. Marković, P.N. Ross, Jr., J.Phys.Chem. 99 (1995) 8290.[39] J.M. Orts, A. Fernandez-Vega, J.M. Feliu, A. Aldaz, J. Clavilier, J.Electroanal.Chem.327 (1992) 261.[40] N.M. Marković, B.N. Grgur, C.A. Lucas, P.N. Ross, J.Phys.Chem.B 103 (1999) 487.[41] (a) B. Love, J. Lipkowski, ACS Symp.Ser. 378 (1988) 484. (b) E. Santos,E.P.M. Leiva, W. Vielstich, Electrochim.Acta 36 (1991) 555. (c) A.V. Petukhov,W. Akemann, K.A. Friedrich, U. Stimming, Surf.Sci. 402-404 (1998) 182. (d) M. Bergelin,E. Herrero, J.M. Feliu, M. Wasberg, J.Electroanal.Chem. 467 (1999) 74.[42] R. Gómez, J.M. Orts, J.M. Feliu, J. Clavilier, L.H. Klein, J.Electroanal.Chem. 432(1997) 1.[43] W. Schmickler, Interfacial Electrochemistry, Oxford University Press: New York,1996; Chapter 10.[44] C. McCallum, D. Pletcher, J.Electroanal.Chem. 70 (1976) 277.

13

2CO oxidation on stepped Pt[n(111)××××(111)]electrodes: a voltammetric study

Abstract

The oxidation of CO adlayers, formed by direct dosing from CO-saturated solution,and bulk CO has been studied on Pt [n (111)×(111)] single crystals in 0.5 M H2SO4. For thestepped Pt surfaces studied, CO is found to adsorb preferentially on the steps, blocking theelectrochemical hydrogen adsorption there. A pronounced effect of electrode surfacestructure on CO oxidation has been observed. The overpotential for the oxidation of asaturated CO adlayer, as well as of submonolayer CO coverages, is found to increase in thesequence Pt(553)

Chapter 2

14

2.1. Introduction

Studies of carbon monoxide oxidation on platinum electrodes are offundamental as well as practical interest. In fact, an understanding of themechanism of such a simple model reaction as CO electrooxidation on Pt singlecrystal electrodes may lead to a deeper insight into the relation between surfacestructure and reactivity in electrocatalysis. On the practical side, it is well-knownthat the major impediment in the development of hydrogen/oxygen or directmethanol fuel cells is the rapid deactivation of the anode in the presence of eventrace amounts of CO [1]. Thus, a molecular-level understanding of COelectrooxidation will be advantageous for both areas of electrochemical research.

The electrocatalytic oxidation of CO is believed to follow a Langmuir-Hinshelwood surface reaction between adsorbed CO molecules and oxygen-containing species [2-4], the latter species resulting from oxidative waterdecomposition. In acidic solutions the overall reaction mechanism is typicallyrepresented by two steps:

H2O + * OHads + H+ + e- (2.1)

COads + OHads CO2 + H+ + e- + 2* (2.2)

where * denotes a free surface site.One of the most effective approaches for obtaining molecular-level

knowledge about the reaction mechanism is to use atomically well-definedsurfaces, i.e. single crystals. The flame annealing technique used by Clavilier in1980 [5] gave the possibility to clean the Pt single crystals in an easy way. Later,the use of bead-type single crystal electrodes provided an opportunity to preparehigh-quality single crystals of any desired orientation, by simply regulating themiscut angle [6]. Higher index vicinal planes, composed of low index terraces,separated by step rows, are of particular interest for the study of structure-sensitivereactions, since the amount of active sites/steps can be changed in a controlledway.

Although CO adsorption on stepped Pt surfaces has been extensively studied[7-12], only a few papers on the oxidation of CO adlayers [13] and bulk CO [14]have been published. The reverse reaction, electrochemical CO2 reduction onstepped Pt surfaces, has also been investigated [15].

In this chapter CO adlayer oxidation on Pt(553)=Pt[5(111)×(111)],Pt(554)=Pt[10(111)×(111)] and Pt(111) is studied in a wide range of CO coveragesusing cyclic voltammetry. Bulk CO oxidation on these Pt surfaces is alsoexamined.

CO oxidation on stepped Pt[n(111)×(111)] electrodes: a voltammetric study

15

2.2. Experimental

Bead-type single crystals of Pt [n(111)×(111)] (or, equivalently,Pt [(n−1) (111)×(110)]) orientation - Pt(553) with n=5, Pt(554) n=10 and Pt(111),prepared according to the Clavilier method [6], were used in this study. Beforeeach experiment the single crystal electrode was flame annealed and cooled downto room temperature in H2+Ar atmosphere, after which it was transferred to the cellunder the protection of a droplet of deoxygenated water.

A platinum wire was used as a counter electrode and a saturatedHg/Hg2SO4/K2SO4 electrode, connected via Luggin capillary, as a reference.However, all potentials quoted were converted into the RHE scale.

A special glass cell, described elsewhere [16], contained a small movableglass spoon that allowed dosing of CO at open circuit potential from a saturatedCO solution without dissolving carbon monoxide in the working solution. The celland all glassware were cleaned by boiling in a mixture of concentrated nitric andsulfuric acid (1:1), followed by repeated boiling with ultra-pure water.

0.5 M H2SO4 working solution was prepared from concentrated H2SO4(Merck, “Suprapur”) and ultra-pure water (Millipore MilliQ gradient A10 system,18.2 MΩcm, 2 ppb total organic carbon). Argon (N50) was used to deoxygenate allsolutions and CO (N47) to prepare CO-saturated solution. For bulk CO oxidationstudies the working solution was saturated with CO, while the electrode potentialwas held at 0.1 V, after which potentiodynamic measurements were performed.

Measurements were performed at constant temperature (25°C) with the helpof a computer-controlled Autolab PGSTAT20 potentiostat.

CO coverages were calculated taking into account corrections for (bi)sulfateadsorption on Pt(hkl) in accordance with data, obtained by the charge displacementtechnique [17].

2.3. Results and discussion

2.3.1. Voltammetric profiles in CO-free electrolyte

Fig. 2.1 shows the cyclic voltammograms of Pt(111), Pt(554) and Pt(553)electrodes in 0.5 M H2SO4 electrolyte. Pt(111) exhibits the well-known sharp“butterfly” peak at 0.45 V, characteristic for a disorder-order phase transition in thesulfate adlayer [18,19]. Also, very few defects of (110) and (100) orientation canbe seen in the hydrogen adsorption/desorption region on Pt(111).

Chapter 2

16

Both stepped surfaces show sharp peaks at 0.125 V, which are due tohydrogen adsorption/desorption on (110) steps [20,21], and broad features atpotentials negative to 0.35 V and positive to 0.4 V due to hydrogen desorption andanion adsorption on terraces, respectively. All profiles are in good agreement withpreviously published data [20-22], indicating a high degree of surface crystallineorder and cleanliness of the system.

0.0 0.2 0.4 0.6 0.8 1.0

-100

-50

0

50

100(a)

Pt(111)

i / µ

A c

m-2

E / V vs. RHE

0.0 0.2 0.4 0.6 0.8 1.0-300

-150

0

150

300 (b)

Pt(554)

i /

µA c

m-2

E / V vs. RHE

0.0 0.2 0.4 0.6 0.8 1.0-300

-150

0

150

300 (c)

i / µ

A c

m-2

E / V vs. RHE

Pt(553)

0.8 0.9 1.0-8

-4

0

4

8 (d)

i / µ

A c

m-2

E / V vs. RHE

Figure 2.1. Cyclic voltammograms of Pt(111) (a), Pt(554) (b) and Pt(553) (c) in0.5 M H2SO4. Figure (d) shows the upper potential limits for Pt(111) (dashed line), Pt(554)(solid thin line) and Pt(553) (solid thick line). Sweep rate 50 mV/s, T=25°C.

It is now well established that extensive oxygen adsorption (both from gasphase and in electrochemical oxidation of water to produce surface oxygen-containing species) causes significant roughening of the surface, destroying thecrystalline order [23,24 and references therein]. Thus, from determining the upperpotential limit of electrode surface stability, one can obtain information, althoughindirect, about the incipient surface oxidation, since formation of OHads alwaysprecedes the formation of bulk oxide [25]. For the most open Pt(553) surface the


17

upper potential limit is as low as 0.9 V, for Pt(554) it is 0.95 V, whereas wateroxidation on Pt(111) takes place at potentials well above 1.00 V (Fig. 2.1d). Thisindicates that the overpotential for the formation of oxygen-containing species,which are required to oxidize CO, decreases and hence its concentration at thesurface at a given potential increases with increasing step density. A similar effectwas observed previously in UHV studies [26] and quantum chemical calculations[27,28], where steps were shown to be the most favorable adsorption sites foratomic oxygen.

Another important factor, influencing the onset of water oxidation, is thecleanliness of the system. The presence of impurities is known to retard theformation of oxygen-containing species [29]. In order to have some quantitativemeasure of the cleanliness of the system, we introduce a parameter Rhkl for eachPt(hkl) defined as the ratio of two peaks in the voltammogram, which arecharacteristic for the given plane and sensitive to the presence of contamination.R111 is determined as the current density of the spike at 0.45 V, which is verysensitive to the presence of contamination [20], divided by the current density ofthe plateau at 0.2 V. For the stepped surfaces R554 and R553 are defined as the ratioof the heights of hydrogen desorption peak at 0.125 V (contamination sensitive)and the peak current density of sulfate adsorption on terraces at 0.46 V and 0.54 V,respectively. Experiments with R111 lower than 2.5, R554 < 5.4 and R553 < 15.4 weresystematically discarded, since a shift of the oxidation of the CO adlayer towardshigher anodic potentials was observed in those cases.

2.3.2. CO adsorption

The voltammetric profile of Pt(554) in the presence of 0.094 ML of adsorbedCO is shown in Figure 2.2.

Since such a low coverage of CO is seen to block hydrogen adsorption onsteps almost completely, while influencing hydrogen adsorption on terraces onlyslightly, it can be concluded that at low coverages CO adsorption preferentiallytakes place on steps. A similar effect has been observed in the case of Pt(553). Ourexperimental results are consistent with UHV studies [26,30-33], quantumchemical calculations [34] and earlier studies of CO adsorption on Pt steppedsurfaces in electrochemical environment [7-10], where preferential CO adsorptionon steps has been observed.

Chapter 2

18

0.0 0.2 0.4 0.6 0.8 1.0-200

-100

0

100

200

i / µ

A c

m-2

E / V vs. RHE

Figure 2.2. Voltammetric profile of Pt(554) in the presence of 0.094 ML of CO (thick line),blank profile of the same electrode in 0.5 M H2SO4 (thin line), sweep rate 50 mV/s,T=25°C.

2.3.3. Saturated CO adlayer

Figure 2.3 shows voltammetric profiles of oxidation of saturated adlayers ofCO on Pt(553), Pt(554) and Pt(111). Each of three electrodes exhibits one mainanodic peak, occurring at 0.77 V for Pt(553), at 0.84 V for Pt(554) and at 0.94 Vfor Pt(111), (the experimental error of our measurements was always lower than0.01 V).

Figure 2.3 illustrates how a higher step density lowers the overpotential forCO adlayer oxidation, the difference between peak potentials for Pt(553) andPt(111) being as high as 0.17 V. This effect could be explained by the preferentialformation of oxygen-containing species at the step sites compared to the terracesites, even when electrode surface is pre-dosed with CO. On the other hand, thereis another factor, which could affect the CO stripping, namely the packing densityof CO adlayer. Indeed, saturated coverages decrease with the step density for thethree surfaces studied – 0.67 for Pt(111), 0.59 for Pt(554) and 0.55 for Pt(553),


19

assuming all surface Pt atoms are equally accessible for CO. This would indicatethat the adlayer packing density is lower on the stepped surfaces than on Pt(111).

0.6 0.7 0.8 0.9 1.00.0

0.5

1.0

1.5

2.0

Pt(553)

Pt(554)

Pt(111)i /

mA c

m-2

E / V vs. RHE

Figure 2.3. Oxidation of saturated CO adlayers on Pt(553) (thin solid line),Pt(554) (dashed line) and Pt(111) (thick solid line), sweep rate 50 mV/s, T=25°C.

In a recent careful study of CO adsorption on Pt(17 17 15) and Pt(17 15 15)using in situ infrared reflection-adsorption spectroscopy (IRRAS), Rodes and co-workers have also shown that the presence of even dilute steps lowers the saturatedCO coverage significantly, from 0.67 to 0.57 or so [11]. Saturated CO coverages of0.66, 0.60 and 0.65 for Pt(111), Pt(554) and Pt(553), respectively, were found inanother recent study of the effect of steps on the potential of zero total charge ofPt(111) electrodes [12]. The former two values are very close to those obtained inthis work, however the latter is much higher.

Hence, it seems that both higher concentration of oxygen-containing speciesat a given potential on stepped surfaces compared to Pt(111) and a lower COpacking density on stepped surfaces may lead to the decrease of the overpotentialfor saturated CO adlayer oxidation on stepped Pt surfaces. The distinction betweenthe two effects is made in the following section.

Chapter 2

20

2.3.4. Effect of CO coverage

To address further the effect of the CO packing density on the oxidationpeak potential, a number of CO coverages was prepared on all electrodes. A plot ofthe peak potential versus the CO coverage is shown in Figure 2.4. It can be seenimmediately from the Fig. 2.4 that multiple peaks in the CO strippingvoltammograms are observed for all the electrodes studied, the number of peaksdepending on the surface structure and the initial CO coverage. For a given COcoverage the peak potential decreases with increasing step density.

0,65

0,75

0,85

0,95

0,00 0,20 0,40 0,60 0,80

CO coverage

Ep

/ V v

s. R

HE

P t(553)Pt(554)Pt(111)

Figure 2.4. Effect of CO coverage on the peak potential of CO adlayer oxidation. Peakpotentials are determined at a sweep rate of 50 mV/s, the straight line is to guide the eye.

At low CO coverages (θCO


21

appears and the two peaks co-exist (see also typical voltammograms in Fig. 2.5b).The “critical” CO coverage, at which the second peak develops, depends on theelectrode orientation: it is 0.22 for Pt(111), 0.26 for Pt(554) and 0.39 for Pt(553).The charge under the “intermediate” peak gradually increases, while that under the“low-potential” peak decreases with increasing CO coverage. Finally, for Pt(553)only one “intermediate” peak at 0.77 V remains at saturated CO coverage (0.55).

0.4 0.6 0.8 1.00.00

0.02

0.04

0.06(a) Pt(554)

Pt(553)

Pt(111)

i / m

A c

m-2

E / V vs. RHE0.4 0.6 0.8 1.0

0.00

0.03

0.06

0.09

0.12(b)

Pt(553)

Pt(111)

Pt(554)

i / m

A c

m-2

E / V vs. RHE

0.6 0.7 0.8 0.9 1.00.0

0.2

0.4

0.6

0.8(c)

Pt(553) Pt(111)

Pt(554)

i / m

A c

m-2

E / V vs. RHE

Figure 2.5. CO oxidation profiles at different CO coverages on Pt(553) (thin solid line),Pt(554) (dashed line) and Pt(111) (thick solid line), sweep rate 50 mV/s, T=25°C. (a) θCO =0.13, note: CO coverage on Pt(111) is 0.11; (b) θCO = 0.28; (c) θCO = 0.56, note: COcoverage is 0.55 on Pt(553). At low CO coverages (a,b) the features due to residual sulfateadsorption on (111) terraces are visible prior to CO oxidation peaks.

Even higher CO coverages can be obtained for Pt(111) and Pt(554), whichleads to the disappearance of the “low potential” peak for Pt(554) at θCO>0.56, andto the emergence of a third “high potential” peak at 0.92-0.94 V on Pt(111) atθCO=0.55 (see typical CVs at θCO=0.56 in Fig 2.5c). When θCO on Pt(111) reaches

Chapter 2

22

0.61, both “low” and “intermediate” peaks disappear and only the “high potential”peak is observed up to saturated coverage. Similar to the case 0.2


23

references therein, 39]. On the basis of in situ IRRAS studies [39,40], ex situLEED [37] and related data for coadsorbed CO/H2O on Pt(111) in UHV [41] themultiplicity of voltammetric peaks has been explained in terms of formation of COislands with a higher local packing density. In a recent Monte Carlo simulationstudy of CO oxidation on Pt(111) [42], the emergence of the “high potential” peakin the cyclic voltammogram was attributed to the oxidation of dense CO islands,whereas the “low potential” peak corresponded to the “loose” adlayer oxidation.

For stepped surfaces, however, there was as yet no clear evidence of COislands formation. We observe two peaks for CO oxidation on Pt(554) and Pt(553),despite the fact that the (111) terraces appear too narrow, especially on Pt(553), forthe formation of large islands, which would be needed for the appearance of thehigh potential peak according to the model presented in Ref. 42 (vide supra). Acareful study of CO adlayer structure on Pt[n(111)×(111)] electrodes at low COcoverages, using, for instance, in situ IRRAS, may clarify the relation between thevoltammetry and possible island formation.

We would like to point out that although it is tempting to ascribe themultiplicity of CO oxidation peaks to a transition from a saturated CO structure toless and less compact structures during oxidation (for example, from p(2×2)-3COθCO=0.75 to (√19×√19)23.4°-13CO θCO=0.69 and then further to (√7×√7)19.1°-4CO θCO=0.57 or a disordered adlayer) this is in fact an incorrect or at leastincomplete explanation of the phenomenon. Such a model would always predictthe emergence of the low-potential peak at high initial coverages and the high-potential peak at low initial coverages (see Fig. 2.6), in contradiction with ourexperimental results. In fact, due to the repulsive lateral interactions between COmolecules in the adlayer, a saturated adlayer would be more unstable and henceeasier to oxidize to form a less compressed structure, a transition that would giverise to a low-potential peak. A simulation of peak multiplicity in voltammetry dueto a transition through different ordered adlayers is given in Fig. 2 of Ref. 43,where it is due to a transition from a (1×1) to a c(2×2) to a disordered adlayer on asquare (100) lattice. Even though for CO oxidation free sites are needed for theadsorption of the oxygen-containing reaction partner, this effect would explain whythe peak potential of a given peak decreases with decreasing initial CO coverage(as illustrated in Fig. 2.6), but not why at low coverages CO is oxidized at lowpotentials, as in Fig. 2.4. However, changes in the oxidation mechanism of highlycompact adlayers, from Langmuir-Hinshelwood to for instance Eley-Rideal, couldlead to a shift of the peak towards more positive potentials with increasingcoverages.

Chapter 2

24

High potential peak

Intermediate peak

Low potential peakPea

k po

tent

ial

CO coverage

Figure 2.6. Peak potential versus CO coverage plot, expected from the model discussed inthe text.

Unlike the majority of previous publications, in our experiments weobserved three peaks during CO adlayer oxidation on Pt(111). As shown inChapter 3 the number of oxidative peaks and their position depends crucially onthe surface crystalline order. The presence of even a low amount of defects,introduced via cooling the flame-annealed Pt(111) in air (not detectable from blankvoltammetric profiles), leads to appearance of only two peaks, corresponding to the“low” and “intermediate” peaks reported in this work. A similar effect of thecrystalline order of the (111) plane on CO adlayer oxidation has been observed forRh [44].

These observations could indeed indicate that CO packing density onPt(111) is higher than on stepped surfaces and it is local arrangement of COmolecules that determines the number and the position of the voltammetric peaksof CO adlayer oxidation. However, the detailed relation of particular bindinggeometry of CO, as well as its long-range structure, to the voltammetric peaks stillhas to be understood.

2.3.5. Bulk CO oxidation

Potentiodynamic curves of Pt(553), Pt(554) and Pt(111) in CO-saturated0.5 M H2SO4 solution are shown in Fig. 2.7. Bulk CO (COb) oxidation is found tobe a structure-sensitive process, since the onsets of dissolved CO oxidation aredifferent for the three electrodes studied (see also the insert of Fig. 2.7). Only a


25

subtle difference (ca. 10 mV) is observed between the two stepped surfacesstudied, but CO oxidation on Pt(111) takes place at noticeably higheroverpotentials. Our experimental result is in qualitative agreement with apreviously published comparison of Pt(997) to Pt(111) in perchloric acid [14].However, most probably due to inequality of experimental procedures, thedifference in the onset potentials of bulk CO oxidation on Pt(554) (its terrace beingonly 1 atom wider than on Pt(997)) and Pt(111) is only about 35 mV, which isalmost 3 times lower than observed in [14].

It is known from previous work on bulk CO oxidation on polycrystalline Pt[45] that a continuous readsorption of CO from the solution causes a substantialpositive shift of the onset potential for bulk CO oxidation compared to CO adlayeroxidation. This behavior is expected for the Langmuir-Hinshelwood mechanismEqs.(2.1) and (2.2) if CO has a much higher affinity for the free surface sites thanOH. The most considerable difference in the onsets of the CO oxidation isobserved for Pt(553), 0.18 V, whereas we find 0.12 V in case of Pt(554) and only0.07 V for Pt(111) (compare Figs. 2.3 and 2.7).

0.6 0.7 0.8 0.9 1.00

1

2

3

4

5

i / m

A c

m-2

E / V vs. RHE

0.90 0.92 0.940.0

0.1

0.2

Figure 2.7. Cyclic voltammograms of Pt(553) (thin solid line), Pt(554) (dashed line) andPt(111) (thick solid line) in CO-saturated 0.5 M H2SO4 solution, sweep rate 50 mV/s,T=25°C. The insert shows the onset of bulk CO oxidation.

Chapter 2

26

A higher shift of ca. 0.13 V of COb oxidation compared to saturated COadlayer oxidation on Pt(111) has been reported in Ref. 38, although the onsets ofbulk CO oxidation in this work and in [38] are equal - ca. 0.93 V vs. RHE. Themain difference is therefore in the potential of CO adlayer oxidation, which isobserved more anodically in this work, probably due to the lower amount ofsurface crystalline defects (see Chapter 3).

Similar to the adlayer oxidation, the structure sensitivity of bulk COoxidation, the overpotential increasing in the sequence Pt(553)


27

concentration of oxygen-containing species, needed to oxidise CO, is higher on thesurfaces with higher step density, and hence the rate of CO oxidation is enhanced.

References

[1] A. Hamnett, Catalysis Today 38 (1997) 445.[2] S. Gilman, J.Phys.Chem. 68 (1964) 70.[3] R. Parsons, T. Vandernoot, J.Electroanal.Chem. 257 (1988) 9.[4] M.T.M. Koper, A.P.J. Jansen, R.A. van Santen, J.J. Lukkien, P.A.J. Hilbers,J.Chem.Phys. 109 (1998) 6051.[5] J. Clavilier, R. Faure, G. Guinet, R. Durand, J.Electroanal.Chem. 107 (1980) 205.[6] J. Clavilier, D. Armand, S.G. Sun, M. Petit, J.Electroanal.Chem. 205 (1986) 267.[7] C.S. Kim, W.J. Tornquist, C. Korzeniewski, J.Phys.Chem. 97 (1993) 6484.[8] C.S. Kim, C. Korzeniewski, Anal.Chem. 69 (1997) 2349.[9] C.S. Kim, C. Korzeniewski, W.J. Tornquist, J.Chem.Phys. 100 (1994) 628.[10] H. Wang, R.G. Tobin, D.K. Lambert, J.Chem.Phys. 101 (1994) 4277.[11] A. Rodes, R. Gómez, J.M. Feliu, M.J. Weaver, Langmuir 16 (2000) 811.[12] V. Climent, R. Gómez, J.M. Feliu, Electrochim.Acta 45 (1999) 629.[13] H. Massong, S. Tillmann, T. Langkau, E.A. Abd El Meguid, H. Baltruschat,Electrochim.Acta 44 (1998) 1379.[14] W. Akemann, K.A. Friedrich, U. Linke, U. Stimming, Surf.Sci. 402-404 (1998) 571.[15] N. Hoshi, T. Suzuki, Y. Hori, Electrochim.Acta 41 (1996) 1647.[16] J.M. Feliu, J.M. Orts, A. Fernandez-Vega, A. Aldaz, J. Clavilier, J.Electroanal.Chem.296 (1990) 191.[17] R. Gómez, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf.Sci. 410 (1998) 48.[18] A.M. Funtikov, U. Stimming, R. Vogel, J.Electroanal.Chem. 428 (1997) 147.[19] M.T.M. Koper, J.J. Lukkien, J.Electroanal.Chem. 485 (2000) 161.[20] J. Clavilier, K. El Achi, A. Rodes, J.Electroanal.Chem. 272 (1989) 253.[21] J. Clavilier, A. Rodes, J.Electroanal.Chem. 348 (1993) 247.[22] J. Clavilier, K. El Achi, M. Petit, A. Rodes, M.A. Zamakhchari, J.Electroanal.Chem.295 (1990) 333.[23] (a) S. Motoo, N. Furuya, J.Electroanal.Chem. 172 (1984) 339. (b) K. Itaya,S. Sugawara, K. Sashikata, N. Furuya, J.Vac.Sci.Technol.A 8 (1990) 515.[24] A.A. Gewirth, B.K. Niece, Chem.Rev. 97 (1997) 1129.[25] B.E. Conway, Progress in Surface Science 49 (1995) 331.[26] J. Xu, J.T. Yates, Jr., J.Chem.Phys. 99 (1993) 725.[27] P.J. Feibelman, S. Esch, T. Michely, Phys.Rev.Lett. 77 (1996) 2257.[28] P.J. Feibelman, J. Hafner, G. Kresse, Phys.Rev.B, 58 (1998) 2179.[29] H. Angerstein-Kozlowska, in E. Yeager, J.O’M. Bockris, B.E. Conway,S. Sarangapani (Eds.), Comprehensive Treatise of Electrochemistry, Vol.9, Plenum Press:New York, 1984, p.15-61.[30] E. Hahn, A. Fricke, H. Roder, K. Kern, Surf.Sci. 297 (1993) 19.

Chapter 2

28

[31] B.E. Hayden, K. Kretzschmar, A.M. Bradshaw, R.G. Greenler, Surf.Sci. 149 (1985)394.[32] J.S. Luo, R.G. Tobin, D.K. Lambert, G.B. Fisher, C.L. Di Maggio, Surf.Sci. 274(1992) 53.[33] M.A. Henderson, A. Szabo, J.T. Yates, Jr., J.Chem.Phys. 91 (1989) 7245.[34] B. Hammer, O.H. Nielsen, J.K. Nørskov, Catal.Lett. 46 (1997) 31.[35] M. Bergelin, J.M. Feliu, M. Wasberg, Electrochim.Acta 44 (1998) 1069.[36] L. Palaikis, D. Zurawski, M. Hourani, A. Wieckowski, Surf.Sci. 199 (1988) 183.[37] D. Zurawski, M. Wasberg, A. Wieckowski, J.Phys.Chem. 94 (1990) 2076.[38] N.M. Marković, B.N. Grgur, C.A. Lucas, P.N. Ross, J.Phys.Chem.B 103 (1999) 487.[39] S.C. Chang, M.J. Weaver, J.Chem.Phys. 92 (1990) 4582.[40] M.J. Weaver, Surf.Sci. 437 (1999) 215.[41] F.T. Wagner, T.E. Moylan, S.J. Schmieg, Surf.Sci. 195 (1988) 403.[42] J.M. Orts, E. Louis, L.M. Sander, J.M. Feliu, A. Aldaz, J. Clavilier, Surf.Sci. 416(1998) 371.[43] M.T.M. Koper, A.P.J. Jansen, J.J. Lukkien, Electrochim.Acta 45 (1999) 645.[44] R. Gómez, J.M. Orts, J.M. Feliu, J. Clavilier, L.H. Klein, J.Electroanal.Chem. 432(1997) 1.[45] H.A. Gasteiger, N.M. Marković, P.N. Ross, Jr., J.Phys.Chem. 99 (1995) 8290.

29

3The effect of the cooling atmosphere in thepreparation of flame-annealed Pt(111)electrodes on CO adlayer oxidation

Abstract

The effect of the cooling atmosphere on the rate of CO adlayer oxidation on flame-annealed Pt(111) has been studied. Cooling of a flame-annealed Pt(111) electrode in airresults in a higher amount of crystalline defects compared to Pt(111) cooled in a hydrogen-argon stream. Although the blank profiles in 0.5 M H2SO4 of Pt(111), cooled in air andunder oxygen exclusion, are virtually identical, CO adlayer oxidation occurs at significantlylower overpotentials on the former one. Three voltammetric peaks are observed forsubsaturated CO adlayer oxidation on Pt(111), cooled in Ar+H2 mixture, while only twopeaks develop in case of a Pt(111) surface cooled in air. Random crystalline defects,introduced via cooling of a flame-annealed Pt(111) in air, enhance CO adlayer oxidation,but apparently also suppress the third high-potential peak observed on a quasi-perfect (111)surface. The high sensitivity of the saturated CO adlayer oxidation to the presence ofcrystalline defects on Pt(111) can hence be used as a straightforward, sensitive, thoughqualitative method to assess the degree of crystalline order of the electrode.

This chapter is published as N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van SantenElectrochem.Comm. 2 (2000) 487.

Chapter 3

30

3.1. Introduction

Since the invention of the flame annealing technique by Clavilier in 1980[1,2], this procedure has been widely used for the preparation of well-ordered andclean surfaces of metal single crystals.

The conditions under which the crystal is cooled down to room temperatureafter flame treatment have been shown to have a marked influence on thecrystalline order of the surface. Cooling under strict exclusion of oxygen, forinstance, in an argon-hydrogen stream, is required for Pt(100) [3] and stepped Ptsurfaces [4-7] in order to obtain a well-ordered surface. The presence of oxygen inthe cooling atmosphere has been shown to have an adverse effect on the surfaceorder also for Pt(111) [8]. However, in several other papers, Pt(111) has beenclaimed to be insensitive to the presence of oxygen in the cooling atmosphere [3,9],so that cooling Pt(111) in air has been widely used in different electrochemicalstudies [10-15].

In this chapter we demonstrate that the cooling procedure of a Pt(111)electrode has a pronounced effect on the rate of a model structure-sensitive reactionsuch as CO adlayer oxidation. In the presence of oxygen in the cooling atmospherea higher amount of crystalline defects is formed, which enhance the CO oxidationsignificantly and also affect important qualitative features of the strippingvoltammetry profile. We show that CO adlayer oxidation may be used asqualitative but very sensitive and straightforward method for assessing the degreeof crystalline surface order.

3.2. Experimental

A bead-type Pt (111) single crystal, prepared according to the Claviliermethod [16], was used in this study. Before each experiment the single-crystalelectrode was flame annealed. Two different atmospheres were employed forcooling the electrode down to room temperature: H2+Ar mixture (ca. 1:3) or air,after which the working electrode was transferred to the cell under the protection ofa droplet of ultra-pure water.

The electrochemical cell, chemicals and experimental procedures were asdescribed in Chapter 2.

The effect of the cooling atmosphere in the preparation of flame-annealed Pt(111)electrodes on CO adlayer oxidation

31


It is now well established that voltammetric profiles of Pt(hkl) in supportingelectrolyte (for instance, in 0.5 M H2SO4) can be considered as a fingerprint ofcrystalline structure of the Pt electrode. Very useful information can be derivedfrom the so-called blank cyclic voltammogram (CV) of Pt(hkl) electrodes: thecrystal orientation itself, i.e. the hkl indices, the amount and type of crystallinedefects, and also the degree of cleanliness of the system.

Blank voltammetric profiles of Pt(111) electrodes, cooled in air and inhydrogen-argon mixture, are shown in Figure 3.1. Both CVs agree well withpreviously published profiles, that would in general be an indication of a highquality of the crystal and a high degree of cleanliness. By comparing the twovirtually identical profiles in one and the same figure, one can clearly distinguishthat the amount of crystalline defects of (110) orientation (small peak at 0.125 V) ishigher for the electrode cooled in air.

0.0 0.2 0.4 0.6 0.8 1.0

-100

-50

0

50

100

i / µ

A c

m-2

E / V vs. RHE

Figure 3.1. Cyclic voltammograms of Pt(111) cooled in argon-hydrogen atmosphere (solidline) and in air (dashed line), 0.5 M H2SO4, sweep rate 50 mV/s.

The mean current densities of the peak at 0.125 V for Pt(111) cooled in airand in argon-hydrogen stream are 38.0 ± 2.4 µAcm-2 and 33.3 ± 1.0 µAcm-2,respectively. The standard deviation of the mean value was calculated from 10

Chapter 3

32

measurements on one-and-the-same electrode. Performing a statistical analysis ofthe data one can indeed distinguish between these two values, but we would like topoint out that this was only possible because all our measurements fulfilled thecleanliness criteria for Pt(111), defined in Chapter 2. The height of the peak at0.125 V due to hydrogen adsorption on (110) defects decreases drastically in thepresence of traces of contamination, making the two profiles identical andindistinguishable from a statistical point of view. Note also, that standard deviationof the peak current density for the Pt(111) cooled in air is higher than for thePt(111) cooled under oxygen exclusion, reflecting the fact that the formation ofdefects in the former case is difficult to control.

As a consequence of higher amount of defects on the surface of Pt(111)cooled in air, water oxidation occurs at lower overpotentials (see Fig. 3.1). Notethat the so-called “butterfly”, which is characteristic for a disorder-order phasetransition in the sulfate adlayer [18,19] and is sensitive to the long-range order [6],is not noticeably affected by the presence of the higher amount of defects onPt(111) cooled in air. This would suggest that the amount of defects is too low toperturb the ordering of the sulfate adlayer and the defects are distributed randomlyon the surface, since the shape of the “butterfly” is known to be different in case ofthe Pt surfaces with periodic, even dilute, steps [4,6]. Our results are consistentwith the previous work of Motoo and Furuya [8] and very recent observations ofKolb’s group, who showed using in situ STM, that, indeed, cooling down ofPt(111) in air after flame annealing leads to a roughening of the surface, while theuse of hydrogen as cooling atmosphere yields essentially atomically flat surfaces[20].

CO adlayer oxidation turns out to be much more sensitive to the presence ofcrystalline defects on Pt(111) surface than the electrochemical processes takingplace in supporting electrolyte (see Figures 3.2 and 3.3). There are two maindifferences between CO oxidation on Pt(111) cooled in air and in Ar+H2 mixture.First, CO oxidation takes place at lower overpotentials on Pt(111) electrodescooled in air at any CO coverage (Fig. 3.2). Second, the number of peaks, observedon the stripping voltammograms of subsaturated CO adlayers oxidation is differentfor the Pt(111) electrode, cooled in the two different atmospheres (Fig. 3.3).

The first point can be clearly illustrated taking the oxidation of a saturatedCO adlayer as an example. The voltammetric peak on Pt(111), cooled in air, isobserved at 0.859±0.015 V, which is about 0.08 V lower compared to0.936±0.004 V of Pt(111) cooled in Ar+H2 mixture (see Fig. 3.2a) (the standarddeviation was, again, calculated from 10 independent measurements on the sameelectrode).


33

0.7 0.8 0.9 1.00.0

0.4

0.8

1.2 (a)

i / m

A c

m-2

E / V vs. RHE

0.45 0.60 0.75 0.900.00

0.03

0.06

0.09 (b)

i / m

A c

m-2

E / V vs. RHE

Figure 3.2. Oxidation of CO adlayers on Pt(111) cooled in argon-hydrogen atmosphere(solid line) and on Pt(111) cooled in air (dashed line); (a) saturated CO adlayer, (b)subsaturated CO adlayer, θCO = 0.18 (solid line) and θCO = 0.19 (dashed line), sweep rate50 mV/s.

0.65

0.75

0.85

0.95

0.00 0.20 0.40 0.60 0.80

CO coverage

Ep /

V vs

. RH

E

Figure 3.3. Effect of CO coverage on the peak potential of CO adlayer oxidation. Pt(111)cooled in argon-hydrogen atmosphere (open diamonds), Pt(111) cooled in air (filleddiamonds). Peak potentials are determined at a sweep rate of 50 mV/s, the straight linesare the least-squares fit of the data.

Chapter 3

34

In several earlier publications the peak potential for the oxidation of asaturated CO adlayer was also reported to be at about 0.85 V vs. RHE[10,14,15,21]. In all of those studies the Pt(111) electrode was either cooled in air[10,14,15] or had a noticeable amount of defects as can be seen from thecorresponding blank CV [21].

Unlike the majority of previous publications, in our experiments weobserved three voltammetric peaks for the oxidation of subsaturated CO adlayerson Pt(111), cooled in Ar+H2 mixture, while only two peaks develop in case ofPt(111), cooled in air (see Fig. 3.3). At low CO coverages (θCO


35

of the peak potential is lower and the difference between the mean values of theCO oxidation peak potential is higher compared to current densities due to thehydrogen adsorption on (110) defects. This allows us to conclude that it isexperimentally easier and more reliable to assess the crystalline order of thePt(111) surface using the oxidation of a saturated CO adlayer as a test.

3.4. Conclusions

In this chapter we demonstrated the significant effect of the coolingatmosphere on the rate of CO adlayer oxidation on a flame-annealed Pt(111)electrode, a reaction which is known to be highly structure-sensitive.

Cooling of a flame-annealed Pt(111) electrode in air results in a higheramount of crystalline defects compared to Pt(111) cooled in hydrogen-argonstream. Although the blank profiles in 0.5 M H2SO4 of Pt(111), cooled in air andunder oxygen exclusion, are virtually identical, CO adlayer oxidation occurs atsignificantly lower overpotentials on the former one at all CO coverages studied.Furthermore, three voltammetric peaks are observed for the oxidation ofsubsaturated CO adlayers on Pt(111) cooled in Ar+H2 mixture, while only twopeaks develop in the case of Pt(111) cooled in air.

Both effects are explained in terms of the formation of oxygen-containingspecies on random crystalline defects, introduced via cooling of a flame-annealedPt(111) in air, at lower overpotentials compared to the ordered (111) regions.

The high sensitivity of CO adlayer oxidation to the presence of crystallinedefects on Pt(111) hence provides a qualitative but straightforward method toassess the degree of crystalline order of the electrode, and demonstratesconvincingly the importance of the proper cooling atmosphere for obtaining aquasi-defect-free Pt(111) surface.

References

[1] J. Clavilier, R. Faure, G. Guinet, R. Durand, J.Electroanal.Chem. 107 (1980) 205.[2] J. Clavilier, J.Electroanal.Chem. 107 (1980) 211.[3] See for example, J. Clavilier, K. El Achi, M. Petit, A. Rodes, M.A. Zamakhchari,J.Electroanal.Chem. 295 (1990) 333.[4] S. Motoo, N. Furuya, Ber.Bunsenges.Phys.Chem. 91 (1987) 457.[5] A. Rodes, K. El Achi, M.A. Zamakhchari, J. Clavilier, J.Electroanal.Chem. 284 (1990)245.[6] E. Herrero, J.M. Orts, A. Aldaz, J.M. Feliu, Surf.Sci. 440 (1999) 259.

Chapter 3

36

[7] R. Gómez, V. Climent, J.M. Feliu, M.J. Weaver, J.Phys.Chem.B 104 (2000) 597.[8] S. Motoo, N. Furuya, J.Electroanal.Chem. 172 (1984) 339.[9] J. Clavilier, D. Armand, B.L. Wu, J.Electroanal.Chem. 135 (1982) 159.[10] J.M. Feliu, J.M. Orts, A. Fernandez-Vega, A. Aldaz, J. Clavilier, J.Electroanal.Chem.296 (1990) 191.[11] F.C. Nart, T. Iwasita, M. Weber, Ber.Bunsenges.Phys.Chem. 97 (1993) 737.[12] F.C. Nart, T. Iwasita, M. Weber, Electrochim.Acta 39 (1994) 961.[13] B.E. Conway, J. Barber, S. Morin, Electrochim.Acta 44 (1998) 1109.[14] M. Bergelin, J.M. Feliu, M. Wasberg, Electrochim.Acta 44 (1998) 1069.[15] J.M. Orts, A. Fernandez-Vega, J.M. Feliu, A. Aldaz, J. Clavilier, J.Electroanal.Chem.327 (1992) 261.[16] J. Clavilier, D. Armand, S.G. Sun, M. Petit, J.Electroanal.Chem. 205 (1986) 267.[17] R. Gómez, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf.Sci. 410 (1998) 48.[18] A.M. Funtikov, U. Stimming, R. Vogel, J.Electroanal.Chem. 428 (1997) 147.[19] M.T.M. Koper, J.J. Lukkien, J.Electroanal.Chem. 485 (2000) 161.[20] L.A. Kibler, A. Cuesta, M. Kleinert and D.M. Kolb, J.Electroanal.Chem. 484 (2000)73.[21] M.J. Weaver, S.-C. Chang, L.-W.H. Leung, X. Jiang, M. Rubel, M. Szklarczyk,D. Zurawski, A. Wieckowski, J.Electroanal.Chem. 327 (1992) 247.[22] A.V. Petukhov, W. Akemann, K.A. Friedrich, U. Stimming, Surf.Sci. 402-404 (1998)182.[23] R. Gómez, J.M. Orts, J.M. Feliu, J. Clavilier, L.H. Klein, J.Electroanal.Chem. 432(1997) 1.

37

4Mechanism and kinetics of the electrochemicalCO adlayer oxidation on Pt(111)

Abstract

The electrochemical oxidation of saturated and sub-saturated CO adlayers onPt(111) in 0.5 M H2SO4 has been studied using chronoamperometry. For the saturated COcoverage the oxidation is initiated by an apparently zeroth-order process of removing 2-3%of the adlayer, followed by the main oxidation process, which is shown to be of theLangmuir-Hinshelwood type with a competitive adsorption of the two reactants, CO andOH. The Langmuir-Hinshelwood kinetics can be modeled using the mean-fieldapproximation, which implies fast diffusion of adsorbed CO on the Pt(111) surface underelectrochemical conditions. The apparent rate constant for the electrochemical COoxidation and its potential dependence are determined by a fitting of the experimental datawith the mean-field model. For sub-saturated CO coverages the overall picture is shown tobe more complicated and remains to be understood.

This chapter is published as N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van SantenJ.Electroanal.Chem., 2002, in press.

Chapter 4

38

4.1. Introduction

The electrochemical oxidation of CO on platinum has been studied quiteextensively over the past decades using different electrochemical and spectroscopicmethods. The achievements until 1990 in the understanding of this reaction havebeen reviewed by Beden et al. [1].

The “reactant-pair” mechanism for the electrochemical oxidation ofadsorbed CO in acidic solutions, originally proposed by Gilman [2], is nowgenerally accepted. As discussed in Chapter 2 this mechanism assumes aLangmuir-Hinshelwood type reaction between carbon monoxide and a surfaceoxygen-containing species, adsorbed on adjacent sites, to form CO2. The oxygen-containing species results from the oxidation of water at the electrode surface andis usually supposed to be OHads, although the exact nature of this species is stillunknown. The overall reaction scheme is:

H2O + * OHads + H+ + e- (4.1)

COads + OHads CO2 + H+ + e- + 2* (4.2)

where * denotes a free surface site.The rate of reaction (4.2) depends markedly on the surface diffusion rate of thereactants, which in turn determines the spatial distribution of the reactants on thesurface. Analytical expressions for the overall reaction rate can be derived for twolimiting cases known as the “mean-field approximation” and the “nucleation andgrowth” model.

One of the assumptions of the mean-field approximation is that reactants areperfectly mixed on the surface and the reaction rate is proportional to the averagecoverages of the reaction partners [3]. This assumption is reasonable if the surfacediffusion is much faster than the reaction itself.

In the nucleation and growth model, the reacting species are consideredimmobile. In the case of a surface pre-dosed with a saturated adlayer of CO, theadsorption of OH proceeds via “nucleation” at certain places, for instance, atdefects in the CO adlayer. The reaction takes place only at the interface betweenthe two reacting phases, causing the formation and growth of islands. The timedependence of the nucleation determines whether the instantaneous or theprogressive nucleation and growth mode operates [4], i.e. whether the number ofnucleation centers is constant or increases with time.

For diffusion rates comparable with the reaction rate, a full numericalsolution of the kinetic equations is required. In this case, one either resorts to so-called Dynamic Monte Carlo simulations [5-7] or to a differential-equationapproach involving a more advanced approximation than the mean-field model [8].

Mechanism and kinetics of the electrochemical CO adlayer oxidation on Pt(111)

39

From the experimental point of view, the most suitable technique for aquantitative study of the reaction kinetics of CO adlayer electrooxidation ischronoamperometry or potential-step experiments, which have been employed in anumber of studies [2,7,9-14]. Despite these efforts there is still a lack of consensusregarding the most adequate kinetic model for describing the electrooxidation ofadsorbed CO. Some authors have found that the reaction kinetics can be treatedwithin the mean-field approximation [13], while others have adhered to thenucleation and growth model to describe the experimental data [9,11].

The present Chapter reports a chronoamperometric study of the kinetics ofthe electrochemical oxidation of CO adlayers on Pt(111). Our prime aim is tocompare the mean-field and nucleation and growth description of the currenttransients, and to provide quantitative kinetic parameters of the reaction, includingtheir potential dependence.

4.2. Experimental

A bead-type Pt(111) single crystal, prepared according to the Claviliermethod [15], was used in this study. Before each experiment it was flame annealedand cooled down to room temperature in H2+Ar atmosphere, after which it wastransferred to the cell under the protection of a droplet of deoxygenated water.

A platinum wire was used as a counter electrode and a saturatedHg/Hg2SO4/K2SO4 electrode, connected via Luggin capillary, as a reference.However, all potentials quoted were converted to the RHE scale.

The conventional glass cell and other glassware were cleaned by boiling in amixture of concentrated nitric and sulfuric acid (1:1), followed by boiling withultra-pure water several times.

A 0.5 M H2SO4 working solution was prepared from concentrated H2SO4(Merck, “Suprapur”) and ultra-pure water (Millipore MilliQ gradient A10 system,18.2 MΩcm, 2 ppb total organic carbon). Argon (N50) was used to deoxygenate allsolutions and CO (N47) to dose CO.

Adsorption of CO was performed from a flow of ca. 15 % of CO in Ar overthe solution, while the working electrode was kept in the meniscus mode at 0.1 V.After the formation of a saturated adlayer of CO, as indicated by the drop of thedisplacement current to zero [16], Ar was bubbled through the solution for at least15 minutes to remove traces of dissolved CO, while the working electrode was keptat 0.1 V in the bulk of the solution. Then the working electrode was brought backto the meniscus mode and the oxidation of CO adlayer was initiated by stepping thepotential to Estep > 0.7 V, with the current transient being recorded simultaneously.After the current had dropped to zero in the transient, a blank profile of the

Chapter 4

40

working electrode was recorded to check for a possible readsorption of CO orcontamination. Normally, no changes in the blank profile of Pt(111) after CO wasoxidized off were observed, and a current transient was accepted for further dataprocessing only if the cleanliness parameter R111 (defined in Chapter 2) was higherthan 2.5. Submonolayer CO coverages were prepared following the sameprocedure, except that the CO flow was stopped before the displacement currentdropped to zero.

Measurements were performed at constant temperature (25°C) with the helpof a computer-controlled Autolab PGSTAT20 potentiostat.

The CO and hydrogen coverages were calculated taking into accountcorrections for (bi)sulfate adsorption on Pt(hkl) electrode in accordance with data,obtained by the charge displacement technique [17].


A typical cyclic voltammogram of Pt(111) electrode in 0.5 M H2SO4electrolyte is shown in Figure 4.1. The profile is in good agreement with previouslypublished data [18,19], confirming the surface crystalline order of the workingelectrode and the cleanliness of the system. Only a small amount of defects of(110) orientation is evidenced by the hump at 0.125 V.

0.0 0.2 0.4 0.6 0.8 1.0-150

-100

-50

0

50

100

i /

µA c

m-2

E / V vs. RHE

Figure 4.1. A cyclic voltammogram of Pt(111) in 0.5 M H2SO4, sweep rate 50 mV/s,T=25°C.


41

4.3.1. Co-adsorption of CO and hydrogen

The importance of studying the electrochemical co-adsorption of CO andhydrogen was realized in the very first attempts to understand the reactionmechanism of the CO electrooxidation [20]. Since the precursor for bothelectroadsorbed hydrogen and oxygen-containing species is water, the hydrogenUPD coverage can be an estimate for the number of the free surface sites availablefor the oxidation of water to form oxygen-containing species. The experimentalrelation between the hydrogen coverage and the CO coverage in the co-adsorbedadlayer is shown in the Figure 4.2. Our results, which are in agreement with earlierstudies [20,21], demonstrate that there is a competition between water and COmolecules for surface sites.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

CO

cove

rage

Hydrogen coverage

Figure 4.2. Relationship between the coverages of hydrogen and CO in the adlayer of thetwo co-adsorbed species as measured from cyclic voltammetry. 0.5 M H2SO4, T=25°C.Solid line is a least-squares fit of the data for θCO

Chapter 4

42

reduction to form adsorbed UPD hydrogen is completely blocked, but the COuptake can still be increased and a number of CO coverages can be formed in therange between 0.65 ML and saturation in agreement with previously reported data[21].

Two-dimensional long-range order has been shown to be essential for theformation of these compact CO adlayers on Pt(111) [22]. When the long-rangeorder of the surface is broken by regular (110) steps, the saturation coverage of COon (111) terraces is only 0.5 ML [22], that coincides with the saturation coverageof CO on Pt(111) under UHV conditions [23]. Interestingly, maximum COcoverage extrapolated from the linear fit in Fig. 4.2 is also 0.5 ML, which seems toindicate a particularly stable adlayer. It is also apparent from Fig. 4.2 that long-range order starts to play a role at CO coverages higher than ca. 0.3 ML. Webelieve that this interesting point deserves further investigation.

The (2×2)-3CO adlayer with 0.75 ML coverage, which has been reported byVillegas and Weaver [24] and Marković et al. [25], is not formed under ourexperimental conditions, most likely because of the relatively low CO partialpressure during CO dosing.

Since the microscopic structure of the adlayer of the co-adsorbed species canhardly be derived from our macroscopic measurements, we restrict the discussionof this point to the postulation of the competitive adsorption of the two species.

4.3.2. Oxidation of saturated CO adlayer

The current – time profiles for the oxidation of a saturated adlayer of COobtained in the potential step experiments for a number of final potentials areshown in Figure 4.3. The charge under the transient is found to be independent ofthe step potential and corresponded to CO coverage of 0.64±0.02 ML under ourexperimental conditions. All the transients have a similar shape: after charging ofthe double layer (a high current at short times) there is a small pre-peak followedby a current plateau and a main symmetric peak as shown in Fig. 4.3.

The distinction of two main regions in the current transient - current plateauand a main peak - may seem somewhat arbitrary at first. However, from theanalysis of the current transient it is clear that while in the region of the main peakthe reaction is a second-order process with respect to CO coverage, the apparentreaction order in the current plateau region with respect to adsorbed CO is eitherzero, as described by Eq. 4.3

i ∝ kdt

d CO −=θ (4.3)


43

or “quasi zero”, as described by Eq. 4.4,

i ∝ COCO kdt

d θθ −= , with θCO ≈ const (4.4)

This suggests that the reaction mechanism in the plateau region is different fromthat of the main peak. Therefore, we will discuss this process first beforediscussing the main oxidation peak.

0 100 200 300 400

0

10

20

jih

g

t / s

0 10 20

0

100

200

300

fe

d

i / µ

A c

m-2

0 1 2

0

1000

2000

3000

c

b

a

0 20 40 60

0

5

10

15

20

25

main peak

plateau

pre-peaki /

µA c

m-2

t / s

Figure 4.3. Current transients of the oxidation of saturated CO adlayers on Pt(111), steppotential is (a) 0.955 V, (b) 0.93 V, (c) 0.905 V, (d) 0.88 V, (e) 0.855 V, (f) 0.83 V, (g)0.805 V, (h) 0.78 V, (i) 0.755 V, (j) 0.73 V. Enlarged transient (g) is shown to indicate thepre-peak, current plateau and the main peak.

The current in the plateau region is found to increase with increasing the steppotential, the plot of the logarithm of the current versus the step potential beinglinear with a slope of 81±4 mV/dec (Figure 4.4). As estimated from the integrationof the current in region of the pre-peak and the plateau, only 2-3% of the initial COcoverage is oxidized in this region.

It can be seen from Fig. 4.2 that at CO coverages of about 0.64 ML there arestill free sites on the surface, although only very few, ca. 0.005 ML, available forwater adsorption. Thus, the Langmuir-Hinshelwood mechanism

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