Electrochemistry Communications 6 (2004) 395–399

www.elsevier.com/locate/elecom

Potential control of the CO adsorption site on Pt(1 0 0) electrodes

Ana L�opez-Cudero, Angel Cuesta *, Claudio Guti�errez

Instituto de Qu�ımica F�ısica ‘‘Rocasolano’’, C. Serrano, 119, E-28006 Madrid, Spain

Received 5 February 2004; accepted 18 February 2004

Published online:

Abstract

A careful choice of the potential at which CO is adsorbed (dosing potential, Ed) on Pt(1 0 0) electrodes in 0.1 M H2SO4 allows to

control the formation of (i) CO adlayers that block all the adsorption sites on the surface (if Ed 6 0:40 V vs. RHE), (ii) CO adlayers

that only block adsorption on (1 0 0) terraces, allowing hydrogen adsorption on small islands (if 0.45 V vs. RHE 6 Ed 6 0.60 V vs.

RHE), or (iii) subsaturated CO adlayers that leave free adsorption sites of every kind (if Ed ¼ 0:65 V vs. RHE).

� 2004 Elsevier B.V. All rights reserved.

Keywords: Pt(1 0 0) electrode; Carbon monoxide; Chemisorption

1. Introduction

The adsorption and oxidation of CO on platinumsurfaces, both at the metal–gas and at the metal–elec-

trolyte interfaces, has been extensively studied since

more than half a century ago. However, and despite the

apparent simplicity of the system, new results are being

continuously reported.

At the metal–electrolyte interface, and thanks to the

seminal work by Clavilier et al. [1,2], studies with single

crystal platinum electrodes have become widespreadsince the early 80s. Most of them deal with surfaces

having (1 1 1) terraces, those using electrodes having

(1 0 0) or (1 1 0) oriented terraces being much more

scarce.

Like Au(1 0 0), the Pt(1 0 0) surface reconstructs

during the flame-annealing treatment, adopting a hex-

agonal structure (usually termed Pt(1 0 0)-hex) with a

surface atom density approximately 24% higher than theunreconstructed surface [3]. In the absence of adsorption

phenomena, the reconstructed Pt(1 0 0)-hex surface is

thermodynamically stable, and it survives if, after flame-

annealing, the crystal is allowed to slowly cool down in

an oxygen-free Ar or N2 atmosphere [4,5]. As first noted

by Al-Akl et al. [6], the Pt(1 0 0)-hex surface can also

* Corresponding author. Tel.: +34-91-5619400; fax: +34-91-5642431.

E-mail address: a.cuesta@iqfr.csic.es (A. Cuesta).

1388-2481/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2004.02.008

survive in contact with an electrolyte, but the recon-

struction is lifted upon scanning the electrode potential

into the hydrogen adsorption region [4]. As noted bothby Al-Akl et al. [4] and by Kibler et al. [5], adsorption of

oxygen on the initially reconstructed Pt(1 0 0)-hex sur-

face lifts the reconstruction and yields a rough, defect-

rich electrode surface.

The necessity of avoiding oxygen adsorption during

the cooling process after the flame-annealing treatment,

in order to prevent a roughening of the surface, was first

noted by Clavilier et al. [7]. Usually, H2 +N2 gas mix-tures are used, but N2 +CO atmospheres have been

shown to also serve this purpose [5]. A typical feature of

the surface morphology of Pt(1 0 0)-(1� 1) surfaces

prepared by flame-annealing and cooling in an atmo-

sphere containing a reducing and adsorbing gas, is the

presence on the (1 0 0) terraces of small, square islands

[5]. These islands accommodate the 24% excess atoms

present on the Pt(1 0 0)-hex surface, which are ejectedduring the lifting of the reconstruction. They have to be

taken into account when describing the behaviour of

Pt(1 0 0) electrodes, since they provide an extra amount

of ((1 1 1)-oriented) step sites and small (1 0 0) domains,

that could behave different to the large (1 0 0) terraces.

In the present communication, we report how the

control of the potential at which CO is adsorbed can be

used to easily tune the CO adsorption site, allowing forselective blocking of the adsorption states appearing at

396 A. L�opez-Cudero et al. / Electrochemistry Communications 6 (2004) 395–399

E > 0:2 V vs. RHE, while leaving those at E < 0:2 V vs.RHE free. We also provide evidence that the voltam-

metric features at E < 0:2 V vs. RHE correspond to

adsorption processes occurring on top of small islands,

and not at (1 1 1)-oriented steps.

2. Experimental

The working electrode was a bead-type Pt single

crystal prepared according to the method developed by

Clavilier et al. [1], oriented and polished parallel to the

(1 0 0) plane (miscut< 30). Before each experiment, the

electrode was annealed in the flame of a Bunsen burner,

cooled down to room temperature in a H2 +N2 atmo-

sphere, and transferred to the electrochemical cell pro-

tected by a droplet of ultrapure water saturated with thecooling gas mixture.

A platinum wire was used as a counter electrode, and

a reversible hydrogen electrode (RHE), to which all the

potentials in the text are referred, was used as reference.

Saturated CO adlayers were formed potentiostatically

at different dosing potentials (Ed) by blowing pure CO

into the hanging meniscus. The solution was then

purged with N2 for 15 min, in order to remove traces ofdissolved CO from the solution and the cell atmosphere,

and a CO-stripping cyclic voltammogram (CV) at 50

mV s�1 was recorded.

The working solution (0.1 M H2SO4) was prepared

from concentrated H2SO4 (Merck Suprapure) and Milli-

Q water (18 MX cm, 3 ppb TOC). Nitrogen (N50), hy-

drogen (N50) and carbon monoxide (N47, aluminium

alloy cylinder) were supplied by Air Liquide.

Fig. 1. CO-stripping voltammograms at 50 mV s�1 (solid line) of a

Pt(1 0 0) electrode covered by a CO adlayer formed at Ed ¼ 0:30 V (a),

Ed ¼ 0:55 V (b), and Ed ¼ 0:65 V (c) in 0.1 M H2SO4. The dashed line

corresponds to the cyclic voltammogram, at 50 mVs�1, of a clean and

well-ordered Pt(1 0 0) electrode in the same solution. The insets in (a)

and (b) show the potential region between 0.05 and 0.8 V in an ex-

panded scale.

3. Results and discussion

Depending on the appearance of the hydrogen ad-

sorption region (between 0.05 and 0.45 V), three kinds

of CO-stripping CVs, shown in Fig. 1, have been found.

If Ed 6 0:40 V, all the adsorption sites in this regionare blocked by adsorbed CO (Fig. 1(a)). If 0.45

V 6 Ed 6 0.60 V, only the adsorption states at E > 0:20V, associated mainly to the concomitant adsorption

(desorption) of hydrogen and desorption (adsorption) of

(bi)sulphate on (1 0 0) terraces remain blocked, while

those adsorption states at E < 0:20 V, which have been

attributed to hydrogen adsorption/desorption on (1 1 1)-

step sites, are now free (Fig. 1(b)), the amount of chargein this potential region increasing with Ed. Another in-

teresting feature, occurring when CO is adsorbed at 0.50

or 0.55 V, and which can be better appreciated in the

inset in Fig. 1(b), is the appearance of a spike around 0.1

V. At Ed ¼ 0:65 V, a subsaturated CO adlayer leaving

free adsorption sites of both kinds is formed (Fig. 1(c)),

and, finally, if Ed P 0:70 V, no CO is left on the surface

after purging the solution with N2. We would like to

note here that, if Ed 6 0:25 V, a pre-peak in the potential

region between 0.45 and 0.60 V can be observed in the

CO-stripping voltammograms. Although we comment

shortly on the nature of this pre-peak below, a more

systematic and comprehensive study will be reported

separately [8]. In all cases a CV typical of a clean, well-ordered Pt(1 0 0)-(1� 1) electrode is recovered after

complete stripping of the CO adlayer.

The results in Fig. 1 clearly show that it is possible to

very easily control the CO adsorption site on the surface

of Pt(1 0 0) electrodes varying Ed, blocking only the

adsorption sites at E > 0:20 V if 0.45 V 6 Ed 6 0.60 V.

A. L�opez-Cudero et al. / Electrochemistry Communications 6 (2004) 395–399 397

The processes occurring at E < 0:20 V have been at-tributed to hydrogen adsorption on (1 1 1)-step sites [9–

11]. Accordingly, our results would suggest that it is

possible to adsorb CO on the terraces of a Pt(1 0 0)

electrode, leaving the steps that separate these terraces

free. However, and although Domke et al. [11] clearly

demonstrated that anion adsorption does not play any

role in the processes occurring at E < 0:20 V, its as-

signment to adsorption on (1 1 1)-oriented steps leavessome questions unanswered. As noted by Domke et al.,

the charge corresponding to hydrogen adsorption on

(1 0 0) terraces is about 15% less than the theoretical

value of 208 lCcm�2, corresponding to one electron per

site. Furthermore, they also noted that the charge

measured in the potential region below 0.20 V is much

larger (about 14% of that corresponding to one electron

per site on a (1 0 0) terrace) than that expected for (1 1 1)steps in a well-ordered Pt(1 0 0) surface. Based on our

results, we present here an alternative assignment of the

adsorption processes at E < 0:20 V, that takes into ac-

count the presence on the surface of the Pt(1 0 0)-(1� 1)

surface of small islands, that appeared during the lifting

of the (hex) reconstruction in the cooling step of the

preparation process.

A question that immediately arises when examiningthe results in Fig. 1, is whether, during adsorption at

potentials between 0.45 and 0.60 V, oxidation of the CO

molecules arriving at the sites that give rise to the pro-

cess occurring at E < 0:20 V in the CV of the clean

Pt(1 0 0) surface is taking place. The fact that, when CO

is adsorbed at Ed 6 0:25 V, a pre-peak between 0.45 and

0.60 V appears in the subsequent CO-stripping CV

suggests that it is indeed so. To confirm this, we per-formed the following experiment, illustrated in Fig. 2:

(i) CO was adsorbed at Ed ¼ 0:10 V; (ii) then we re-

corded a CO-stripping CV (dashed line in Fig. 2), with

Fig. 2. Cyclic voltammogram, between 0.05 and 0.6 V, of a Pt(1 0 0)

electrode covered by a CO adlayer formed at Ed ¼ 0:10 V (dashed

line). The solid line corresponds to the subsequent CO-stripping vol-

tammogram. Scan rate, 50 mV s�1. The inset shows, in an expanded

scale, the first and second sweep (dashed line), between 0.05 and 0.6 V,

of a Pt(1 0 0) electrode covered by a CO adlayer formed at Ed ¼ 0:10 V.

The solid line in the inset corresponds to the voltammogram between

0.05 and 0.75 V obtained after complete stripping of the CO adlayer.

the upper potential limit at 0.60 V (exactly after the pre-peak); and, (iii) finally, a CV with an upper potential

limit of 0.90 V, in order to oxidise the remaining CO,

was recorded (solid line in Fig. 2). It can be clearly seen

in Fig. 2 that, after oxidation of some CO in the pre-

peak region, the features associated with adsorption/

desorption processes on the (1 0 0) terraces remain ab-

sent, while those giving rise to a current at E < 0:20 V

have become active.The main conclusion that can be reached from the

results presented above is that the CO molecules oxi-

dised in the pre-peak appearing between 0.45 and 0.60 V

in Fig. 2 are those adsorbed on the very same sites that

give rise to the adsorption process appearing at E < 0:20V. Accordingly, during adsorption of CO at 0.45

V 6 Ed 6 0.60 V, the CO molecules reaching these sites

will be oxidised. Since the charge in the region below0.20 V increases when Ed is increased from 0.45 to 0.60

V, it can be concluded that the fraction of these sites that

will remain free of CO after the adsorption process de-

pends on Ed, being maximum at Ed ¼ 0:60 V.

Some hints regarding the nature of these sites can be

obtained by analysing the amount of CO oxidised in the

pre-peak. As first demonstrated by the Alicante group

[12], the CO coverage can be accurately determined fromCO-stripping voltammograms after subtraction of the

true, exact thermodynamic double layer correction

[13,14], obtained from CO-charge displacement experi-

ments [12,15]. We have carried out a systematic study of

the influence of Ed on the final CO coverage of Pt(1 0 0),

to be reported elsewhere [8], which has provided us with

some relevant data in relation with the present discus-

sion. According to these data, the CO coverage forEd ¼ 0:10 V corresponds to 0.78 ML. After oxidation of

some CO in the pre-peak (dashed line in Fig. 2), the CO

remaining on the surface, as calculated from the sub-

sequent CO-stripping voltammogram (solid line in

Fig. 2), corresponds to 0.62 ML, indicating that 0.16

ML (approximately 20% of the CO initially present on

the surface) were oxidised in the pre-peak. This is ob-

viously too large a value for the oxidation of CO ad-sorbed on (1 1 1) step sites. Direct integration of the

voltammetric charge under the pre-peak (dashed line in

Fig. 2) yields 0.17 ML, suggesting a negligible contri-

bution of double layer charging to the charge under the

pre-peak.

An alternative explanation is that these sites corre-

spond to the platinum atoms in the small, square is-

lands, present on the terraces and formed upon lifting ofthe (hex) reconstruction. If the islands are small enough,

a significant fraction of their atoms will be in the

neighbourhood of (1 1 1) step sites, thus changing their

adsorption properties. As noted above, the Pt(1 0 0)-hex

surface contains approximately 24% more atoms than

the Pt(1 0 0)-(1� 1) structure. If all these atoms were

forming islands after lifting of the reconstruction during

398 A. L�opez-Cudero et al. / Electrochemistry Communications 6 (2004) 395–399

the cooling step in a hydrogen-containing atmosphere,the islands should cover 24% of the surface. However,

during the cooling step, larger islands will grow at the

expense of smaller ones (Ostwald ripening [16–19]), large

enough islands being expected to behave like terraces.

Furthermore, terrace steps can act as a sink for small

islands and for platinum adatoms in their neighbour-

hood, as indeed found by Kibler et al. [5], thus de-

creasing the surface fraction covered by islands.According to our interpretation, around 15–20% of the

surface would be covered by square platinum islands

small enough to have a significant fraction of their at-

oms affected by the neighbourhood of a step edge. Hy-

drogen adsorption on these sites would be responsible

for the current observed at E < 0:20 V in the CVs of

clean Pt(1 0 0) electrodes. This interpretation provides

an explanation to the fact that, as reported by Domkeet al. [11], the charge corresponding to hydrogen ad-

sorption on (1 0 0) surfaces is about 15% less than ex-

pected, and that the charge measured in the potential

region below 0.20 V is about 14% of that corresponding

to one electron per site on a (1 0 0) terrace.

The appearance of a spike around 0.1 V in the CV of

CO adlayers formed at 0.50 or 0.55 V (see. Fig. 1(b))

deserves further discussion. It has been shown that an-ion adsorption does not play a significant role in this

potential region [11], and so sulphate adsorption/de-

sorption can be discarded as being responsible for this

spike. This is confirmed by the fact that we could also

observe the spike in experiments performed in perchloric

acid solutions. A spike around 0.1 V has also been ob-

served by Al-Akl et al. [4] and by Wakisaka et al. [20]

with initially reconstructed Pt(1 0 0) electrodes, uponelectrochemically lifting the reconstruction by scanning

the electrode potential into the hydrogen adsorption

region. Despite the difference in sulphate concentration,

1 mM [20] vs. 0.1 M [4,21], both groups observed the

spike at the same potential, again suggesting that sul-

phate adsorption/desorption is not responsible for it. Al-

Akl et al. [21] also observed a spike around 0.1 V with

stepped surfaces containing (1 0 0)-oriented terraces and(1 1 1)-oriented steps, although only with terraces more

than three atoms wide. Since surfaces with (1 0 0) ter-

races smaller than three atoms wide do not give rise to

(hex) reconstructions, it is obvious that the spike found

by Al-Akl et al. and by Wakisaka et al. must be related

to the surface morphology resulting after lifting of the

(hex) reconstruction by hydrogen electroadsorption [21].

Please note, that the surfaces obtained after lifting thereconstruction in the gas phase by introducing a red-hot

single crystal in a hydrogen-containing atmosphere, and

those obtained by electrochemically lifting the recon-

struction upon hydrogen adsorption on the surface of a

reconstructed Pt(1 0 0) electrode are completely differ-

ent. In the former case, both cyclic voltammetry [4,5,20]

and STM [5] reveal the formation of a well-ordered

Pt(1 0 0)-(1� 1) surface, containing square islands about10–20 nm in size. In the latter case, on the contrary,

rough surfaces are obtained, as revealed by a large peak

at approximately 0.30 V vs. RHE and a small, broad

peak at around 0.37 V vs. RHE in the corresponding

CV, indicative of the presence of a large amount of

(1 1 1) steps and narrow (1 0 0) terraces [4]. STM images

of these surfaces reveal the presence of many irregularly

shaped islands smaller than 2–5 nm [20]. This is inperfect agreement with a previous estimation by Kibler

et al. [5], who, taking into account the quadratic increase

of the number of atoms in the islands, and the linear

increase of the number of atoms at the edges of the is-

lands, with increasing island size, suggested that only for

islands smaller than seven atoms square would the

number of step atoms (24) be comparable with that of

terrace atoms (25) (in [5], due to an error, 7 nm insteadof 7 atoms was written).

Both before and after CO adsorption and stripping,

the CVs of our Pt(1 0 0) electrodes reveal a clean, well-

ordered Pt(1 0 0)-(1� 1) surface, with large terraces and

a small amount of (1 1 1)-oriented steps (see dashed line

in the insets of Fig. 1(a) and (b) and solid line in the inset

of Fig. 2). In our case, the spike at 0.1 V is not as large as

in the case of electrochemically generated unrecon-structed Pt(1 0 0) surfaces [4,20,21], and only appears if

CO is adsorbed at Ed ¼ 0:50 or 0.55 V. We cannot give a

definitive and clear explanation to this phenomenon at

this stage, but it must correspond to hydrogen adsorp-

tion and must be due to the appearance, under these

conditions, of very small domains similar to those oc-

curring when the (hex) reconstruction is lifted by elec-

trochemical adsorption of hydrogen. These very smalldomains might appear upon partial oxidation of the CO

adsorbed on top of the 10–20 nm big islands present on

the surface.

4. Conclusions

We have shown that, with a careful choice of thedosing potential, the nature of the sites onto which CO

is adsorbed at a Pt(1 0 0) electrode can be controlled. If

Ed 6 0:40 V, all the adsorption sites on the surface are

blocked, while for Ed between 0.45 and 0.60 V only the

terrace sites are blocked, those states appearing at

E < 0:20 V in the CV of a clean Pt(1 0 0) electrode re-

maining (partially) free.

Our results also show that the sites occupied by theCO molecules oxidised in the pre-peak appearing in CO-

stripping voltammograms of CO adlayers formed at

Ed 6 0:25 V, and those giving rise to a hydrogen ad-

sorption current at E < 0:20 V in the CV of a clean

Pt(1 0 0) electrode, must be the same. Taking into ac-

count that the charges associated to hydrogen adsorp-

tion and CO oxidation on these sites are clearly larger

A. L�opez-Cudero et al. / Electrochemistry Communications 6 (2004) 395–399 399

than those expected for (1 1 1) step sites, we have pro-posed that they might indeed correspond to platinum

atoms in small, square islands formed during lifting of

the reconstruction.

Acknowledgements

This work was carried out with the help of Project

BQU 2000-1496 from the Direcci�on General de Inves-

tigaci�on of the Spanish Ministry of Science and Tech-

nology. A.L.-C. acknowledges an FPI fellowship from

the Spanish Ministry of Science and Technology.

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