Temperature dependence of the COads oxidation process on Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0) electrodes

Journal ofElectroanalytical

Chemistry

Journal of Electroanalytical Chemistry 567 (2004) 139–149

www.elsevier.com/locate/jelechem

Temperature dependence of the COads oxidation process onPt(1 1 1), Pt(1 0 0), and Pt(1 1 0) electrodes

Enrique Herrero a, Bernab�e �Alvarez a, Juan M. Feliu a,*, Sonia Blais b,Zorana Radovic-Hrapovic b, Gregory Jerkiewicz c,*

a Departamento de Qu�ımica F�ısica, Universidad de Alicante, Apdo. 99, Alicante E-03080, Spainb D�epartement de Chimie, Universit�e de Sherbrooke, 2500 boul. Universit�e, Sherbrooke, QC., Canada J1K 2R1

c Department of Chemistry, Queen�s University, 90 Queens Crescent, Kingston, Ont., Canada K7L 3N6

Received 15 July 2003; received in revised form 4 December 2003; accepted 15 December 2003

Available online 28 February 2004

Abstract

The influence of temperature variation on the COads oxidative desorption at Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0) electrodes in 0.5 M

H2SO4 and 0.1 M HClO4 solutions is examined. A distinct shift of the COads stripping peak towards less-positive potentials is

observed as the temperature is increased from 273 to 333 K. Despite the displacement of the desorption peak towards lower po-

tentials, its current density and shape remain unaffected for the Pt(1 1 1) and Pt(1 0 0) electrodes, therefore indicating that the ox-

idation mechanism for the COads is not influenced by the temperature variation in this particular range. On the other hand, the

morphology of the peak on the Pt(1 1 0) electrode changes significantly with the temperature, as a result of the interaction with the

oxide formation. The activation energies for the oxidation processes in the two media have been evaluated through the dependence

of the peak potential and the voltammetric current of the peak with the temperature. The activation energy obtained in H2SO4 is ca.

15–25 kJmol�1 higher than that in HClO4, highlighting the role of the anions in the oxidation process.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Pt(1 0 0); Pt(1 1 1); CO overlayer; Electro-oxidation; Oxidative desorption; Temperature dependence; Surface kinetics; Activated complex;

Activation energy

1. Introduction

Carbon monoxide is a small and simple molecule that

chemisorbs strongly on platinum and other noble met-

als, whose adsorption behavior is structure-dependent

[1]. From the point of view of applied research, CO is animportant surface species because it poisons the noble

metal-based electrocatalysts used in low-temperature

fuel cells. The CO is formed as an intermediate in the

oxidation of methanol or is present in low concentra-

tions in the reformed gas used in fuel cells and therefore

its behavior on platinum electrodes is a subject of in-

tense research.

* Corresponding authors. Tel.: +1-613-533-6413; fax: +1-613-533-

6669 (Gregory Jerkiewicz), +34-965-909301 (Juan M. Feliu).

E-mail addresses: [email protected] (J.M. Feliu), grego-

[email protected] (G. Jerkiewicz).

0022-0728/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2003.12.019

It is well established that the oxidation of adsorbed

CO (COads) on platinum single-crystal electrodes occurs

at potentials above 0.75 V [2,3], potentials that are much

higher than the standard potential of the process (ca. 0.0

V). The COads electro-oxidative desorption requires the

presence of oxygen-containing species such as H2O orOH� depending on the pH of the aqueous electrolyte

and the reaction takes place according to the general

reaction scheme:

PtþH2O¢Pt–OHþHþ þ e�; ð1Þ

Pt–OHþ Pt–CO ! CO2Hþ þ e� þ 2Pt: ð2Þ

In this scheme, the exact nature of the species involved in

reaction (1) is still not clear, although research on ad-atom-modified Pt(1 1 1) electrode surfaces points to the

adsorbed OH as the species involved in the process

[4,5]. Reaction (2) occurs through a Langmuir–Hinshel-

wood (L–H)-type mechanism. The kinetics of the COads

mail to: [email protected]

140 E. Herrero et al. / Journal of Electroanalytical Chemistry 567 (2004) 139–149

oxidation depend on several factors, namely, the COads

coverage and surface structure, the mobility of the

COads on the electrode surface, the presence of defects on

the surface, and the anions present in the solution.

Of all these factors that affect the kinetics of COads

oxidation, the surface structure of the COads layers

on platinum single crystals has been extensively char-

acterized by electrochemical techniques [2,3,6–8], IR

spectroscopy [9–16], in situ STM [17–19], coupled UHV-

electrochemistry techniques [20,21], and in situ grazing

incident X-ray diffraction (GIXD) [22–24]. On the

Pt(1 1 1) electrode, three distinct ordered structures were

observed using STM, namely (2� 2)-HCO ¼ 0:75[17,18], (

p19�p

19) R23.4�-HCO ¼ 0:68 [17,18], and

(p7�p

7) R19.1�-HCO ¼ 0:56 [17] and confirmed by

GIDX [22–24]. The COads surface structure is also

known to be potential-dependent and at E > 0:5 V,

STM studies show that a structural transformation

takes place leading to formation of the (p19�p

19)

R23.4�-HCO ¼ 0:68 adlayer [17,18]. The transformation

from the (2� 2)-HCO ¼ 0:75 structure to the(p19�p

19) R23.4�-HCO ¼ 0:68 structure appears to

be associated with a partial oxidation of the more

compact layer and is related to the presence of surface

defects [16].

The kinetics of CO oxidation on Pt single-crystal

electrodes have been studied extensively in recent years

[6,25–32]. These studies have revealed that the CO oxi-

dation on Pt(1 1 1) and vicinal electrodes takes place ac-cording to the mean field L–H-type mechanism [29–31].

Since the oxidation process takes place at very localized

sites, where OHads and COads species can interact, the

mean field L–H-type mechanism is fulfilled only if CO

diffusion on the surface is fast [33]. It has also been ob-

served that defects play a critical role in the oxidation

process. An extrapolation of the rate constants for the

COads oxidation obtained with the Pt(1 1 1) vicinal elec-trodes to the ‘‘ideal’’ (defect-free) Pt(1 1 1) electrode in-

dicates that the oxidation on real Pt(1 1 1) electrodes

takes place mainly at the defect sites [31].

In order to provide a further mechanistic insight

into the COads oxidative desorption, we engaged in

temperature-dependent research on Pt(1 1 1) in 0.5 M

aqueous H2SO4. The only previous study of tempera-

ture effects on CO oxidation was performed for twoother systems, namely for Pt(1 1 1) in aqueous NaOH

with CO present in solution [27] and for Pt(1 1 1) in

aqueous H2SO4 again with CO in solution [28]. These

measurements allowed us to examine the T-dependence

of the COads surface coverage, the relation between the

peak potential and T , and subsequently to elucidate the

apparent activation energy of the process. The latter

value sheds light on the nature of the activated com-plex, and thus on the mechanism of the process. This

paper is a continuation of our previous research on the

Pt–COchem system.

We present new experimental results for CO ad-

sorption and oxidative desorption at the Pt(1 1 1),

Pt(1 0 0), and Pt(1 1 0) electrodes in aqueous H2SO4 and

HClO4. Comparative analysis of these results leads to an

assessment of the influence of the surface geometry,electrolyte nature, and concentration on the CO oxida-

tive stripping through the determination of kinetic pa-

rameters of the process.

2. Experimental

The Pt(1 1 1), Pt(1 1 0) and Pt(1 0 0) electrodes wereobtained from small (2 mm in diameter) single-crystal

beads according to the well-established procedure [34].

The crystals were oriented, cut, and polished with di-

amond paste down to 0.25 lm; then they were an-

nealed in a gas-oxygen flame for 30 min. Prior to each

experiment, the electrodes were flame-annealed and

cooled in an Ar+H2 atmosphere followed by attach-

ment of a water droplet [35]. Subsequently, a cyclicvoltammetry (CV) profile of the electrode was recorded

in 0.5 M H2SO4 or 0.1 M HClO4 at each temperature

in order to verify the surface order and the solution

cleanliness.

The chemisorption of CO was carried out at 0.1 V

(RHE) for 1 min in 0.5 M aqueous H2SO4 and 0.1 M

HClO4 solutions, respectively, containing CO. After-

wards, the excess CO present in solution (COsol) wasremoved by purging with Ar for ca. 10 min. The strip-

ping of COads was accomplished by the application of a

single, positive-going CV scan from 0.05 to 1.00 V. A

CV profile following the COads oxidative desorption was

also recorded and was compared to that preceding the

CO chemisorption. No difference was observed between

the initial profile prior to CO adsorption and that ob-

tained after CO oxidation, thus confirming that (i) allCOads had been completely oxidatively desorbed and (ii)

dissolved CO (COsol) had been effectively removed from

the electrolyte.

The temperature-dependent measurements were per-

formed by immersing the cell in a water bath (Haake),

with the temperature controlled to within 0.5 K. The

water level in the bath was maintained above that of the

electrolyte in the cell to ensure uniformity of the tem-perature distribution. The experiments were conducted

between 273 and 333 K with a 5� interval.All potentials were measured versus a reversible hy-

drogen electrode maintained at the same temperature as

the working electrode (RHE, T). In order to compare

the potentials for the COads oxidation at different tem-

peratures and to obtain the apparent rate constant, all

potentials were converted to a common standard scale,the standard hydrogen electrode at 298 K (SHE, 298 K).

This was achieved by using the following equations

[36,37]:

-100

-50

0

50

100

0.0 0.2 0.4 0.6 0.8 1.0

0.0 0.2 0.4 0.6 0.8 1.0

-200

-150

-100

-50

0

50

100

150

200

E/V (RHE)

j/µA

cm

-2

Pt(111)

E/V (RHE)

278 K 293 K 308 K

j/µA

cm

-2

Pt(100)

Fig. 1. Evolution of the initial voltammetric profile with temperature

for Pt(1 1 1) and Pt(1 0 0) electrodes in 0.5 M H2SO4. Scan rate 50

mV s�1.

E. Herrero et al. / Journal of Electroanalytical Chemistry 567 (2004) 139–149 141

ESHE;T ¼ ERHE;T � 0:021 V; ð3Þ

ESHE;298 ¼ ESHE;T þ ðT � 298ÞoE=oT ; ð4Þwhere oE=oT is the variation of the potential of the SHE

measured versus a SHE at 298 K with the temperature

and has a value of 8.4� 10�4 VK�1 [36].

The COads coverage is defined as the number of COads

molecules per number of platinum surface atoms. TheCOads coverage was determined from the total COads

stripping charge density (measured between 0.35 and

0.93 V) corrected for the double-layer charging and for

the charge density associated with the anion re-adsorp-

tion [3]. This correction is known to provide coverage

values that are in agreement with the coverage deter-

mined by other techniques [17,18,38]. As the anion ad-

sorption and double layer charging on the Pt(1 1 1) andPt(1 0 0) electrodes are only slightly affected by T vari-

ation [39,40], the charge density correction for the entire

T range was the same and it stood at 144 lCcm�2 for

the Pt(1 1 1) and 120 lCcm�2 for the Pt(1 0 0) electrodes,

respectively. The COads coverage was calculated as

follows:

HCO ¼qCO2q1e

; ð5Þ

where qCO is the corrected charge density for COads

stripping, 2 is the number of electrons exchanged in the

COads oxidative desorption, and q1e is the charge densityassociated with transfer of 1 electron per surface atom.

The values of q1e for Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0)electrodes are 241, 209, and 150 lCcm�2, respectively.

All solutions were prepared using ultra-pure water

(Nanopure, BrandsteadTM) and BDH Aristar grade

H2SO4 and Merck Suprapur HClO4. All gases were of

ultra-high purity (Ar, 99.999%; CO, 99.997%; and H,

99.999%, Air Products).

3. Results

Fig. 1 shows the initial CV profiles of the Pt(1 1 1) and

Pt(1 0 0) electrodes in 0.5 M H2SO4 at three different

temperatures that are representative of the overall ten-

dency. As can be seen, there are only minor changes in

the CV transients and they are in agreement with pre-

viously published results [39,41]. However, the totalcharge associated with the anion and hydrogen ad-

sorption on both electrodes remains the same.

In the case of the Pt(1 1 1) electrode, the CV features

associated with the anion adsorption/desorption (be-

tween 0.35 and 0.6 V) move towards more positive po-

tentials on the RHE scale. In fact, the onset for anion

adsorption remains at approximately the same potential

(ca. 0.33 V), but the final part of the adsorption process,which is characterized by the sharp spike associated with

the phase transition in the (bi)sulfate layer, moves to

more positive potentials. These results show that the

(bi)sulfate adsorption requires a wider potential window

to be completed at high temperatures, an indication that

the adsorption process is less favored at high tempera-

tures. On the other hand, the spike becomes sharperwith the temperature increase, thus indicating that the

kinetics of the phase transition from the disordered

(bi)sulfate adlayer to the ordered one become faster. The

effect on the hydrogen adsorption process on this elec-

trode has been discussed elsewhere [42,43].

In the case of the Pt(1 0 0) electrode, the peak at ca.

0.37 V, associated with the concurrently occurring hy-

drogen desorption and (bi)sulfate adsorption (or viceversa) processes [44], moves towards less-positive nega-

tive potentials upon T increase. This means that the

strength of the (bi)sulfate adsorption relative the to the

hydrogen adsorption process is higher as the tempera-

ture is raised. However, nothing can be said with respect

to the absolute values, since the respective signals

overlap due to the competitive nature of the processes. It


is conceivable that both processes become less favored

with the temperature increase, with H adsorption being

affected more.

In order to examine the kinetics of the CO oxidation

process in relation to the geometry of the Pt electrode, theCO adsorption and oxidation experiments were carried

out over a wide T range and at well-defined surfaces. In

Fig. 2, we showCOads oxidative desorption CV transients

in 0.5 M aqueous H2SO4 for the Pt(1 0 0) and Pt(1 1 1)

electrodes. In each case, the process is manifested by a

well-defined anodic peak. We observe that the COads

oxidation peak moves towards less positive potentials on

the RHE scale as the temperature is raised. In addition,the morphology of the oxidation peak remains un-

changed for temperatures up to 323 K, i.e., the peaks

have (within the reproducibility of the experiment) the

same maximum current density and full width at half

height. This fact indicates that there are no changes in the

oxidation mechanism within this temperature range.

It is also important to verify whether the initial con-

ditions for the oxidation experiments are the same forthe entire set of experiments. This is especially important

in the case of the Pt(1 1 1) electrode since three different

0.7 0.8 0.9 1.0

0

200

400

600

800

1000

1200

1400

0

200

400

600

800

1000

1200

1400

0.7 0.8 0.9 1.0

j/µA

cm

-2

E/V (RHE)

E/V (RHE)

j/µA

cm

-2

278 K 293 K 308 K

Pt(111)

Pt(100)

Fig. 2. Evolution of the voltammetric profile with temperature for the

COadsoxidation for Pt(1 1 1) and Pt(1 0 0) electrodes in 0.5 M H2SO4.

Scan rate 50 mV s�1.

ordered adlayers have been reported for COads at room

temperature (T ¼ 293–298 K), namely (2� 2) at

HCO ¼ 0:75 [17,18], (p19�p

19) R23.4� at HCO ¼ 0:68[17,18], and (

p7�p

7) R19.1� at HCO ¼ 0:56 [17]. The

stability of these different structures is likely to be af-fected by T . Therefore, if kinetic information is to be

extracted from the COads oxidative desorption tran-

sients, then they have to correspond to the same COads

overlayer having the same coverage. The CO coverage

was determined as a function of T using the CO oxi-

dation charge density and the methodology described in

Experimental (Fig. 3). For the Pt(1 1 1) and Pt(1 0 0)

electrodes, the total charge density is 460 lCcm�2. Thischarge has to be corrected for the anion re-adsorption as

CO becomes desorbed. In the case of the Pt(1 1 1) elec-

trode, the correction charge density amounts to 140

lCcm�2, which gives a values of qCO ¼ 320 lCcm�2.

Using Eq. (5), this value is found to correspond to

a CO coverage of 0.67, a value that agrees well with

the coverage obtained for the (p19�p

19) R23.4� ad-

layer observed by STM after the pre-oxidation wave orunder mild dosing conditions [17,18]. However, as T is

raised further (here 323 6 T 6 333 K), a steep dimi-

nution in the coverage is observed, indicating a change

in the stability of the COads adlayer, and the new

value of the COads coverage is 0.53� 0.02. For the

Pt(1 0 0), the correction amounts to 120 lCcm�2, which

results in qCO ¼ 340 lCcm�2 and a coverage value of

HCO ¼ 0:80, thus similar to the values reported in theliterature [21,45].

Different behavior is observed for the CO oxidation

process at the Pt(1 1 0) electrode, as can be seen in Fig. 4.

270 280 290 300 310 320 330 3400

100

200

300

400

500

q/µC

cm

-2

T/K

Fig. 3. Oxidation charge for the COads oxidation process versus the

temperature for the Pt(1 1 1) (closed symbols) and Pt(1 0 0) (open

symbols) electrodes prior to (squares) and after correction (circles) in

0.5 M H2SO4.

0.0 0.2 0.4 0.6 0.8 1.0

-400

-200

0

200

400

600

800

-400

-200

0

200

400

600

800

0.0 0.2 0.4 0.6 0.8 1.0

j/µA

cm

-2

E/V (RHE)

T=273 K

E/V (RHE)j/µ

A c

m-2

T=333 K

Fig. 4. Initial voltammetric profile for a Pt(1 1 0) electrode prior to CO

adsorption and COads stripping voltammograms after CO adsorption

at two different temperatures in 0.5 M H2SO4. Scan rate 50 mVs�1.


In this case, the oxidation of CO coincides with theonset of the oxidation of the Pt(1 1 0) surface. Since the

surface oxidation is temperature dependent, the co-ex-

istence of these two processes complicates the interpre-

tation of the COads oxidation transients. Specifically, the

morphology of the peak changes significantly with the

temperature, as shown in Fig. 4, indicating a very

complicated dependence on temperature. Since the effect

of the surface oxidation on the CO oxidative desorptionprocess is difficult to account for quantitatively, no ki-

netic studies will be performed for the Pt(1 1 0) electrode.

4. Discussion

Kinetic information regarding the COads oxidative

desorption may be obtained by examining the change ofthe Ep vs. T relation [28]. The overall reaction taking

place at the working electrode is as follows:

Ptð111Þ–COþH2Oþ xA�

! Ptð111Þ–Ax þ CO2 þ 2Hþ þ ð2þ xÞe�; ð6Þ

where A is an anion present in solution that is ad-

sorbed on the electrode surface after CO oxidation.

One of the main objectives of this research is the de-

termination of the activation energy of this reaction. In

order to accomplish this objective, an oxidationmechanism for COads has to be proposed. In this

mechanism, there is an initiation process (precursor),

which may account for 2–3% of the overall charge-

density for process on Pt(1 1 1) electrodes [29], and the

main oxidation process, which can be modeled using a

mean-field Langmuir–Hinshelwood equation [29,30]. It

should be mentioned that the transients have also been

modeled with a modification of the nucleation andgrowth model proposed by Love and Lipkowski [25],

which includes an induction time for the oxidation [32].

However, since the mean field equation is simpler, we

have used this latter equation to model the kinetic

behavior. It is also known that the Tafel slope for the

oxidation desorption of CO whose surface coverage is

below 0.68 is ca. 70 mV/decade [6,25,29,30]. Using

these data, the following reaction mechanism can beproposed:

Pt–H2O¢k1

k�1

Pt–OHþHþ þ e� ðfastÞ; ð7Þ

Pt–COþ Pt–OH!k2 Pt–COOH–Pt ðrdsÞ; ð8Þ

Pt–COOH–Pt¢ 2Ptþ CO2 þHþ þ e� ðfastÞ; ð9Þ

Ptþ xA�¢Pt–xAx þ xe� ðfastÞ: ð10Þ

This mechanism satisfies both conditions found exper-

imentally: (i) the rate determining step (rds) is a

chemical reaction after the first electron transfer has

occurred, which should give a Tafel slope close to 60

mV and (ii) it implies the reaction between adsorbed

CO and OH. In the first step, the adsorbed watermolecule dissociates to produce the adsorbed OH

species. The presence of adsorbed water in the sup-

porting electrolyte, here 0.5 M sulfuric acid, at the

potentials at which CO is oxidized can be argued. As

aforementioned, (bi)sulfate anions are adsorbed at any

adsorption site that is not occupied by COads. How-

ever, the adsorbed (bi)sulfate does not preclude the

presence of water in the double layer. There is FTIRevidence pointing to water molecules being included in

the adsorbed layer [46]. Moreover, the structures found

by STM for the adsorbed (bi)sulfate in this potential

range has been interpreted as a water-(bi)sulfate layer

[47], an explanation that has been corroborated by

radiotracer [48] and chronocoulometry experiments

[49]. Therefore, the proposed first step is reasonable

under the present experimental conditions: water mol-ecules are adsorbed at the electrode together with the

(bi)sulfate anions. According to the mean field L–H-

type mechanism, in the second step the two adsorbed


species, COads and OHads, react to form an adsorbed

intermediate, COOHads. The final steps of the process

include the desorption of the COOHads intermediate

species to yield CO2 and the resulting re-adsorption of

anions.For this mechanism, the reaction rate kinetics are

given by the following equation [28]:

m ¼ � dhCOdt

¼ k1k2k�1

expF E � E0ð Þ

RT

� �hCOhH2O

aHþ; ð11Þ

where E is the applied potential, E0 is the standard po-

tential at 298 K for the COads oxidation reaction, and k1,k�1, and k2 are the standard rate constants for the steps

1, )1, and 2, respectively. In this case, hCO is defined as

hCO ¼ HCO

HCO;max

; ð12Þ

where HCO is the CO coverage defined as the number of

COads molecules per surface platinum atom and HCO;max

is the maximum CO coverage value for which all plati-

num adsorption sites have been completely covered. For

the present treatment and in agreement with previously

published data [21,45], we assume that the values of

HCO;max are 0.68 and 0.81 for the Pt(1 1 1) and Pt(1 0 0)

electrodes, respectively, under the experimental condi-

tions described above.

The standard rate constants can be written as afunction of a pre-exponential factor A and an activation

energy DH 6¼

m ¼ � dhCOdt

¼ A1A2

A�1

exp

� DH 6¼

1 þ DH 6¼2 � DH 6¼

�1

RT

!

� expF E � E0ð Þ

RT

� �hCOhH2O

aHþ: ð13Þ

Since the rate constants are referred to the standard

potential of the process, the activation energy will bealso referred to this potential. The current density is

then given by

j ¼ � qCOð þ qanionsÞdhCOdt

¼ qCO þ qanionsð ÞA1A2

A�1

exp

� DH 6¼

1 þ DH 6¼2 � DH 6¼

�1

RT

!

� expF E � E0ð Þ

RT

� �hCOhH2O

aHþ; ð14Þ

where qCO is the charge density required to oxidize the

CO layer and qanions is the charge density transferred in

the anion re-adsorption. The value (qCO þ qanions) is thecharge density determined through the integration of the

CO oxidation peak (without corrections). The peak

current density, jp can be described by the followingequation:

jp ¼qCO þ qanionsð ÞA1A2

A�1

exp

� DH 6¼

1 þ DH 6¼2 � DH 6¼

�1

RT

!

� expF Ep � E0� �

RT

� �hH2O;phCO;p

aHþ; ð15Þ

where the subscript p indicates the values at the peak

potential, Ep. Solving the equation for Ep leads to

Ep � E0 ¼ RTF

lnjpA�1aHþ

qanions þ qCOð ÞA1A2hCO;p; hH2O;p

þ DH 6¼1 þ DH 6¼

2 � DH 6¼�1

F: ð16Þ

The first term in this equation depends on T and other

variables and the second one is a function of the acti-vation energy. All the variables in the first term can be

considered constant in the temperature range between

273 and 318 K because (qCO þ qanions) and jp remain

constant for both electrodes, as is evident from the data

shown in Fig. 2. Moreover, the integrated charge be-

tween the onset of COads oxidation and Ep is constant,

indicating that the CO coverage has a constant value at

the peak potential. Assuming that the water coverage isconstant at that peak potential, the relation of Ep � E0

vs. T should give a straight line and its intercept leads to

the determination of the activation energy. The standard

potential for the bulk CO/CO2 process in aqueous so-

lutions is )0.106 V [50]. In order to calculate the stan-

dard potential for the COads/CO2 process from this

value, the Gibbs energy of adsorption for CO on Pt is

required. Since this value is not known, only an estimateof its value can be made from the voltammetric behavior

of the CO/CO2 pair on platinum. COads is oxidized at a

very low rate at potentials higher than 0.3 V, and the

reduction of CO2 to COads is achieved only at potentials

below 0.25 V [51]. Therefore, the standard potential for

COads has to remain close to 0 V. Since the exact value of

E0 is not known, a value of 0 V has been used for

practical reasons, i.e., all the activation energies deter-mined in this work are referred to 0 V vs. SHE.

In Figs. 5 and 6, we show Ep vs. T for the Pt(1 0 0)

and Pt(1 1 1) electrodes in aqueous H2SO4 and HClO4.

We find that these relationships are linear, thus, con-

firming the validity of the hypothesis made initially;

namely jp, HCO, and HH2O were assumed to be constant.

We also observe a change in the Ep vs. T behavior at

323 K for Pt(1 1 1) that can be related to the change ofthe initial CO coverage. On the basis of the linear rela-

tionships presented in Figs. 5 and 6, we determined the

activation energy for the COads oxidation and found

that its value was 131� 2 kJmol�1 for Pt(1 1 1) and

139� 5 for Pt(1 0 0) (Table 1).

Another important issue that has to be considered in

the data analysis is how the adsorbed anions affect the

COads oxidation process. Although not explicitly con-sidered in Eq. (5), the presence of anions is known to

270 280 290 300 310 320 330 340

0.70

0.75

0.80

0.85 0.5 M H

2SO

4

0.1 M HClO4

E=1.26-0.00181T

Ep/V

(SH

E,2

98 K

)

T/K

E=1.44-0.00223T

Fig. 6. Ep vs. T for the Pt(1 0 0) electrode in 0.5 M H2SO4 and 0.1 M

HClO4.

260 270 280 290 300 310 320 330 340

0.68

0.70

0.72

0.74

0.76

0.78

0.80

0.82

0.84

0.86

0.88

0.90

E=1.36-0.00177T

0.5 M H2SO

4

0.1 M HClO4

E=1.15-0.00149T

Ep/V

(SH

E,2

98 K

)

T/K

Fig. 5. Ep vs. T for the Pt(1 1 1) electrode in 0.5 M H2SO4 and 0.1 M

HClO4.


affect this reaction. The adsorption process in the

electrochemical environment should be always re-

garded as competitive. Therefore, the presence of a

strongly adsorbed anion such as (bi)sulfate should

modify the rate constants k1 and k�1 since it modifies

the environment in which the reaction takes place. In

order to analyze this effect, the same experiments were

Table 1

Activation energies obtained for the CO oxidation process using the values

E

Pt(1 1 1) 0.1 M HClO4 1

0.5 M H2SO4 1

Pt(1 0 0) 0.1 M HClO4 1

0.5 M H2SO4 1

conducted in the absence of strongly adsorbing anions

(in 0.1 M HClO4). In the case of this electrolyte, the

CV behavior of the COads oxidation process is com-

parable to that depicted in Fig. 2 and the CO coverages

are the same as those obtained in 0.5 M H2SO4 forboth electrodes. The results obtained for this medium

are shown in Figs. 5 and 6 as open symbols. As can be

seen, in the case of HClO4, the peak potential values

are lower than those obtained in 0.5 M H2SO4. For the

Pt(1 1 1) electrode, the measurements were carried out

at a scan rate of 20 mV s�1 in order to avoid inter-

ference with the butterfly that appears in the same

potential range. For that reason, the peak potentialshave values ca. 100 mV lower with respect to the sul-

furic acid medium. If the experiments were carried out

at the same scan rate, then the peak potential would be

reduced by ca. 50 mV, a value comparable to that

obtained for the Pt(1 0 0) electrode. From the intercept,

we again determined activation energy values of

111� 5 kJmol�1 for the Pt(1 1 1) and 122� 5 kJmol�1

for the Pt(1 0 0) electrode. These values are ca. 20kJmol�1 lower than those obtained in the sulfuric acid

medium, thus giving a clear indication of the anionic

effect on the reaction kinetics.

The determination of the activation energy using the

peak potential may present several problems that could

lead to erroneous results: (1) although jp is fairly con-

stant, the real values are scattered around 10% of the

mean value (see Fig. 6); (2) jp and Ep are extremelysensitive to the initial conditions of the electrode and

even a small number of surface imperfections can result

in a slightly different value of the activation energy; and

(3) the activation energy is obtained from the intercept

at T ¼ 0 K, a temperature which is far away from the

working temperature. In order to check the validity of

previous results, a new approach will be used to calcu-

late the activation energies. In this approach, the entireCV profile corresponding to the CO oxidation process

will be used to calculate an apparent constant rate.

Then, through the analysis of changes of the constant

rate as a function of temperature, an apparent activation

energy can be obtained.

In order to obtain the apparent constant rate, the

shape of the voltammetric peak has to be simulated.

According to the model shown in Appendix A, thecurrent density of the CV peak is given by the following

formula:

EpðE 6¼;1Þ and kappðE 6¼;2Þ6¼;1 (kJmol�1) E 6¼;2 (kJmol�1)

11� 5 106� 2

31� 2 134� 2

22� 5 125� 6

39� 4 135� 4


j ¼ qCOð þ qanionsÞkapp exp FE

RT

� �exp � kappRT

F m exp FERT

� �þ b


F m exp FERT

� �þ b

� �þ 1

� �2ð17Þ

where kapp is the apparent rate constant (and is related to

k1, k�1, and k2), m the scan rate, and b is a parameter that

groups all the initial values for the process (Eini and

hCO;ini). This equation will be used to fit the CV peaks

using (qCO þ qanions), kapp, and b as parameters. As can be

seen, this method is not affected by the aforementioned

sources of error, since the initial conditions of the elec-

trode are parameters within the model and they do notrely on constant jp values.

From the values of kapp, the activation energy can be

calculated as (see Appendix A):

Rd ln kapp� �

dT�1ffi DH 6¼

1 þ DH 6¼2 � DH 6¼

�1: ð18Þ

Therefore, the slope of the representations of the

logðkappÞ vs. T�1 relation will be used to calculate the

activation energy.Fig. 7 shows the experimental CV transients and the

corresponding fitted curves using adjustable parameters

for COads oxidative desorption in 0.5 M H2SO4 for the

Pt(1 1 1) electrode at different temperatures. As can be

seen, the model reproduces the experimental CV tran-

sient well. A similar fitting procedure was applied to the

experimental results obtained for the Pt(1 1 1) in 0.1 M

HClO4 and for Pt(1 0 0) in both media, and a very goodagreement with the fitted data was found. It is important

to emphasize that this model is based on the fact that the

kinetics of CO oxidation at these electrodes follow the

0.76 0.78 0.80 0.82 0

0

200

400

600

800

1000

1200 293303 K

313 K

j/µA

cm

-2

E/V (S

Fig. 7. Comparison between the COads stripping peak (symbols) and the fitt

H2SO4. Scan rate 50 mVs�1.

mean field L–H-type mechanism. For the Pt(1 1 1) elec-

trode, the extrapolation of the results obtained with a

Pt(1 1 1) vicinal electrode for CO oxidation to the be-

havior of the Pt(1 1 1) electrode indicates that the oxi-

dation takes place mainly at the defect sites of thesurface [31]. In a process like this, in which the oxidation

process takes place at very localized sites, the mean field

L–H-type mechanism is fulfilled only if CO diffusion on

the surface is fast. The measurements performed in

UHV indicate a high mobility of CO on the Pt(1 1 1)

surface [52–55]. For the Pt(1 0 0) electrode, IR mea-

surements also indicate a high CO mobility [56]. The

good agreement of the fitted and experimental resultsalso supports the view of high mobility of COads on the

platinum surfaces.

The values of kapp vs T�1 obtained for the Pt(1 1 1)

and Pt(1 0 0) electrodes are shown in Figs. 8 and 9. As

can be seen, the rate constants in 0.1 H HClO4 are lower

than those obtained in 0.5 M H2SO4, as would be ex-

pected on the basis of the peak potentials of the CV

transients. It should be mentioned that the deviationsfrom linearity in Figs. 8 and 9 are smaller than those

observed in Figs. 5 and 6. Very small changes in the

initial coverage lead to significant changes in the peak

potential (>10 mV), increasing the errors and deviations

in Figs. 5 and 6. On the other hand, this second ap-

proach accounts for the influence of the initial CO

coverage in the voltammetric behavior since hCO;ini is

included in b as a fitting parameter, resulting in a moreaccurate determination of the activation energy.

The values of the activation energy obtained are,

within experimental error, the same as those obtained

with the previous approach (Table 1). As can be seen,

.84 0.86 0.88 0.90 0.92

283 K K

273 K

HE, 298 K)

ings obtained from Eq. (17) (lines) for the Pt(1 1 1) electrode in 0.5 M

0.0036 0.0034 0.0032 0.003010-16

10-15

10-14

10-13

10-12

10-11

10-10

10-9

10-8

0.5 M H2SO

4

0.1 M HClO4

k app/s

-1

T-1/K-1

Fig. 8. kapp vs. T�1 for for the Pt(1 1 1) electrode in 0.5 M H2SO4 and

0.1 M HClO4.

0.0038 0.0036 0.0034 0.003210-15

10-14

10-13

10-12

10-11

10-10 0.5 M H2SO

4

0.1 M HClO4

k app/s

-1

T-1/K-1

Fig. 9. kapp vs. T�1 for for the Pt(1 0 0) electrode in 0.5 M H2SO4 and

0.1 M HClO4.


there is a difference of ca. 15–20 kJmol�1 in the acti-

vation energy when bisulfate (a strongly adsorbed an-

ion) is present in the solution. In perchloric acid, the

activation energy shows a small dependence on the

electrode surface, being, for the Pt(1 1 1) electrode, ca. 10

kJmol�1 lower than that obtained for the Pt(1 0 0)electrode, indicating that the surfaces have different in-

teraction energies with the adsorbed species (COads and

COOHads). However, in the presence of sulfuric acid, the

activation energies are comparable, indicating that the

effect of the adsorbing anion has compensated these

differences in the interaction energies.

Density functional theory (DFT) has been used to

evaluate the activation energy of the COads reaction. For

step (8), the activation energy calculated is DH 6¼2 ¼ 0:44

eV [57]. Since the activation energy we have determined

in this work corresponds to DH 6¼1 þ DH 6¼

2 � DH 6¼�1 , the

value of DH 6¼1 � DH 6¼

�1 has to be estimated in order to

compare the experimental and theoretical results. From

basic kinetics, the difference between DG6¼1 � DG 6¼

�1 is

related to the equilibrium constant of the reaction. For

an electrochemical step this relation can be written as

DG6¼1 � DG 6¼

�1 ¼ F ðE01 � ErefÞ; ð19Þ

where E01 is the standard potential of this step and Eref is

the potential at which the activation energy is calcu-

lated. Since all the rate constants determined in this

manuscript are referred to the standard potential of the

overall reaction, Eref for Eq. (19) should be E0 whichhave been taken as very close to zero for practical rea-

sons. The value of E01 ¼ 0:57 V has been also estimated

by DFT, assuming that the entropy contributions to the

Gibbs energy are negligible [58] and therefore

DH 6¼1 � DH 6¼

�1 � DG6¼1 � DG6¼

�1 ¼ F ðE01 � ErefÞ

¼ 55 kJmol�1: ð20Þ

By adding the contributions, the total activation energy

calculated using the DFT data would be ca. 97 kJmol�1,

very close to the value determined experimentally.

Acknowledgements

An acknowledgment is made to the NSERC of

Canada (Discovery Grant) and Ministerio de Educacion

y Cultura, and DGES (Grant No. BQU2003-4029) for

support of this research project.

Appendix A

The current for the CO oxidation process can becalculated using

j ¼ � qCOð þ qanionsÞdhCOdt

: ðA:1Þ

The value dhCO=dt is defined according to the kinetics

law governing the process. In this case, the mean field

Langmuir–Hinshelwood equation replicates the experi-

mental behavior observed in potentiostatic CO oxida-tion experiments [27,30]. According to the detailed

mechanism proposed, the reaction rate can be written as

(Eq. (13))

m ¼ � dhCOdt

¼ k1k2

k�1

expF E � E0ð Þ

RT

� �hCOhH2O

aHþ: ðA:2Þ


The water coverage is not known, but it will be pro-

portional to the fraction of the electrode surface not

covered by CO

hH2O ¼ K 1ð � hCOÞ; ðA:3Þ

K being the proportionality constant. This proportion-

ality constant is very dependent on the anion present in

the solution, i.e., the stronger the anion adsorption, the

lower is the proportionality constant obtained. Addi-

tionally, in a voltammetric experiment, the applied po-

tential depends on the time according to

E ¼ Eini þ mt; ðA:4Þ

where Eini is the initial potential and m the scan rate.

Substituting (A.3) and (A.4) in (A.2) yields

m ¼ � dhCOdt

¼ k1k2k�1

K expF Eini þ mt � E0ð Þ

RT

� �hCO 1� hCOð Þ

aHþ

¼ kapp expF Eini þ mtð Þ

RT

� �hCO 1ð � hCOÞ; ðA:5Þ

where the terms (k1; k2; k�1; K; aH3Oþ and E0) in Eq.

(A.5) for a single experiment have been grouped in kappand is equal to

kapp ¼k1k2k�1

KaH3O

þexp

�� FRT

E0

�: ðA:6Þ

In Eq. (A.5), dhCO=dt is a function of t and hCO which

in turn is also function of t. In order to obtain the

current, an explicit relationship between dhCO=dt and thas to be obtained. For this reason, this equation has to

be integrated, solved for hCO and differentiated again toobtain dhCO=dt as a function only of t. The initial con-

ditions for the integration are a CO coverage value of

hCO;ini at t ¼ 0. Thus,Z hCO

hCO;ini

dhCOhCO 1� hCOð Þ ¼ �

Z t

0

kappRTF m

expFRT

Einið�

þ mtÞ�dt:

ðA:7ÞSolving for hCO yields

hCO ¼ 1� 1

exp � kappRTF m exp F

RT Eini þ mtð Þ� �

þ b� �

þ 1;

ðA:8Þwhere b has a value of

b ¼ kappRTF m

expFEini

RT

� �þ ln

hCO;ini

1� hCO;ini

ðA:9Þ

and groups all the initial values (Eini and hCO;ini) for a

given experiment. Under the present conditions of theexperiments, where Eini ¼ 0:06 V (RHE), the term

kappðRT =F mÞ expðFEini=RT Þ is negligible with respect to

lnððhCO;iniÞ=ð1� hCO;iniÞÞ and therefore, b depends only

on the initial CO coverage. Substituting Eq. (A.8) in Eq.

(A.1) gives

j ¼ qCOð þ qanionsÞ

�kapp exp F




þ b� �



þ b� �

þ 1� �2 :

ðA:10Þ

Since E ¼ Eini þ mt, the current as a function of the

electrode potential is

j ¼ qCOð þ qanionsÞ

�kapp exp FE

RT


F m exp FERT

� �þ b


F m exp FERT

� �þ b

� �þ 1

� �2 :

ðA:11Þ

This expression will be used to fit the voltammetric pro-

files obtained for CO oxidation, where ðqCO þ qanionsÞ,kapp, and b are the fitting parameters.

The values obtained from kapp will be used to estimate

the activation energy of the process. The activation en-

ergy is defined as

DH 6¼ ¼ Rd lnðkÞdT�1

; ðA:12Þ

where k is the rate constant of the process. In this case,

kapp contains, aside from the rate constants of the pro-

cess, other terms. The differentiation of lnðkappÞ with

respect to T�1 gives

Rd ln kapp� �

dT�1¼ R

d ln k1ð ÞdT�1

þ Rd ln k2ð ÞdT�1

� Rd ln k�1ð ÞdT�1

þ Rd ln Kð ÞdT �1

þ Rd ln aH3O

þ� �dT�1

þ FE0

¼ DH 6¼1 þ DH 6¼

2 � DH 6¼�1 þ R

d ln Kð ÞdT�1

þ Rd ln aH3O

þ� �dT�1

þ FE0: ðA:13Þ

Since E0 is close to 0 for the CO oxidation process and

the variations of lnðKÞ and lnðaH3OþÞ with respect to T�1

can be considered negligible in this temperature range,

Rd ln kapp� �

dT�1ffi DH 6¼

1 þ DH 6¼2 � DH 6¼

�1 ðA:14Þ

Therefore, the activation energy of the process can be

obtained from the slope of the representation of

logðkappÞ vs. T�1.

References

[1] S.-C. Chang, L.-W.H. Leung, M.J. Weaver, J. Phys. Chem. 93

(1989) 5341.


[2] J.M. Feliu, J.M. Orts, A. Fern�andez-Vega, A. Aldaz, J. Clavilier,

J. Electroanal. Chem. 296 (1990) 191.

[3] J.M. Orts, A. Fern�andez-Vega, J.M. Feliu, A. Aldaz, J. Clavilier,


[4] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, J. Phys.

Chem. 98 (1994) 617.

[5] E. Herrero, A. Rodes, J.M. P�e rez, J.M. Feliu, A. Aldaz, J.

Electroanal. Chem. 393 (1995) 87.

[6] E. Santos, E.P.M. Leiva, W. Vielstich, Electrochim. Acta 36

(1991) 555.

[7] N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van Santen,

Electrochem. Comm. 2 (2000) 487.

[8] N.P. Lebedeva, M.T.M. Koper, E. Herrero, J.M. Feliu, R.A. van

Santen, J. Electroanal. Chem. 487 (2000) 37.

[9] B. Beden, S. Bilmes, C. Lamy, J.M. Leger, J. Electroanal. Chem.

149 (1983) 295.

[10] N. Furuya, S. Motoo, K. Kunimatsu, J. Electroanal. Chem. 239

(1988) 347.

[11] S.-C. Chang, M.J. Weaver, Surf. Sci. 223 (1989) 142.

[12] F. Kitamura, M. Takahashi, M. Ito, Surf. Sci. 223 (1989) 493.


[14] N.P. Lebedeva, A. Rodes, J.M. Feliu, M.T.M. Koper, R.A. van

Santen, J. Phys. Chem. B 106 (2002) 9863.

[15] W. Akemann, K.A. Friedrich, U. Stimming, J. Chem. Phys. 113

(2000) 6864.

[16] A. Rodes, R. G�omez, J.M. Feliu, M.J. Weaver, Langmuir 16

(2000) 811.

[17] I. Villegas, M.J. Weaver, J. Chem. Phys. 101 (1994) 1648.

[18] I. Oda, J. Inukai, M. Ito, Chem. Phys. Lett. 203 (1993) 99.

[19] I. Villegas, M.J. Weaver, J. Electroanal. Chem. 373 (1994) 245.

[20] D. Zurawski, M. Wasberg, A. Wieckowski, J. Phys. Chem. 94

(1990) 2076.

[21] C.K. Rhee, J.M. Feliu, E. Herrero, P. Mrozek, A. Wieckowski, J.

Phys. Chem. 97 (1993) 9730.

[22] C.A. Lucas, N.M. Markovic, P.N. Ross, Surf. Sci. 425 (1999)

L381.

[23] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, J. Phys.

Chem. B 103 (1999) 487.

[24] H. You, Abstracts of Papers, 225th ACS National Meeting, New

Orleans, 2003, COLL-488.

[25] B. Love, J. Lipkowski, ACS Symp. Ser. 378 (1988) 484.

[26] L. Palaikis, D. Zurawski, M. Hourani, A. Wieckowski, Surf. Sci.

199 (1988) 183.

[27] N.M. Markovic, T.J. Schmidt, B.N. Grgur, H.A. Gasteiger, R.J.

Behm, P.N. Ross, J. Phys. Chem. B 103 (1999) 8568.

[28] E. Herrero, J.M. Feliu, S. Blais, Z. Radovic-Hrapovic, G.

Jerkiewicz, Langmuir 16 (2000) 4779.

[29] M. Bergelin, E. Herrero, J.M. Feliu, M. Wasberg, J. Electroanal.

Chem. 467 (1999) 74.

[30] N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van Santen, J.

Electroanal. Chem. 524 (2002) 242.

[31] N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van Santen, J.

Phys. Chem. B 106 (2002) 12938.

[32] B. Pozniak, Y. Mo, D.A. Scherson, Faraday Discuss. 121 (2002)

313.

[33] A.V. Petukhov, W. Akemann, K.A. Friedrich, U. Stimming, Surf.

Sci. 402 (1998) 182.

[34] J. Clavilier, D. Armand, S.G. Sun, M. Petit, J. Electroanal. Chem.

205 (1986) 267.

[35] A. Rodes, K. El Achi, Z. Zamakhchari, J. Clavilier, J. Electroanal.

Chem. 284 (1990) 245.

[36] B.E. Conway, H. Angerstein-Kozlowska, W.B.A. Sharp, J. Chem.

Soc., Faraday Trans. I 74 (1978) 1373.

[37] S. Blais, G. Jerkiewicz, E. Herrero, J.M. Feliu, Langmuir 17

(2001) 3030.

[38] R. G�omez, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf. Sci. 410

(1998) 48.

[39] A. Zolfaghari, G. Jerkiewicz, J. Electroanal. Chem. 422 (1997) 1.

[40] A. Zolfaghari, G. Jerkiewicz, in: G. Jerkiewicz, P. Marcus (Eds.),

Electrochemical Surface Science of Hydrogen Adsorption and

Absorption, The Electrochemical Society, Pennington, NJ, 1997,

p. 133.

[41] A. Zolfaghari, G. Jerkiewicz, J. Electroanal. Chem. 420 (1997) 11.

[42] G. Jerkiewicz, Prog. Surf. Sci. 57 (1998) 137.

[43] A. Zolfaghari, G. Jerkiewicz, J. Electroanal. Chem. 467 (1999)

177.

[44] M.E. Gamboa-Aldeco, E. Herrero, P.S. Zelenay, A. Wieckowski,


[45] R. G�omez, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf. Sci. 410

(1998) 48.

[46] P.W. Faguy, N. Marinkovic, R.R. Adzic, Langmuir 12 (1996)

243.

[47] A.M. Funtikov, U. Stimming, R. Vogel, J. Electroanal. Chem. 428

(1997) 147.

[48] A. Kolics, A. Wieckowski, J. Phys. Chem. B 105 (2001) 2588.

[49] E. Herrero, J. Mostany, J.M. Feliu, J. Lipkowski, J. Electroanal.

Chem. 534 (2002) 79.

[50] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous

Solutions, IUPAC (Marcel Dekker), New York, 1985.

[51] N. Hoshi, E. Sato, Y. Hori, J. Electroanal. Chem. 540 (2003) 105.

[52] J.E. Reutt-Robey, D.J. Doren, Y.J. Chabal, S.B. Christman,

Phys. Rev. Lett. 61 (1988) 2778.

[53] J.E. Reutt-Robey, Y.J. Chabal, D.J. Doren, S.B. Christman, J.

Vac. Sci. Technol. A 7 (1989) 2227.

[54] J.E. Reutt-Robey, D.J. Doren, Y.J. Chabal, S.B. Christman, J.

Chem. Phys. 93 (1990) 9113.

[55] J. Ma, X. Xiao, N.J. DiNardo, M.M.T. Loy, Phys. Rev. B 58

(1998) 4877.


[57] A.B. Anderson, N.M. Neshev, J. Electrochem. Soc. 149 (2002)

E383.

[58] A.B. Anderson, T.V. Albu, J. Electrochem. Soc. 147 (2000) 4229.

Documents

Temperature dependence of the COads oxidation process on Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0) electrodes