Journal of
Electroanalytical
Chemistry
Journal of Electroanalytical Chemistry 560 (2003) 135–141
www.elsevier.com/locate/jelechem
Accurate determination of the CO coverage at saturation
on a cyanide-modified Pt(1 1 1) electrode in cyanide-free 0.5 M H2SO4
Isabel Morales-Moreno 1, Angel Cuesta *, Claudio Gutierrez
Instituto de Quımica Fısica ‘‘Rocasolano’’, C.S.I.C., C. Serrano, 119, E-28006 Madrid, Spain
Received 13 May 2003; received in revised form 17 June 2003; accepted 8 July 2003
Abstract
We have used the CO charge-displacement method, in combination with a thermodynamic cycle, to obtain the double-layer
correction necessary to determine accurately, using the charge in the corresponding CO-stripping voltammograms, the maximum
amount of CO that can adsorb on a cyanide-modified Pt(1 1 1) electrode. The resulting CO coverage at saturation is hCO ¼ 0:25, and
corresponds to a mixed CN–CO adlayer where some Pt atoms are still free and consequently can adsorb hydrogen. The hydrogen
adsorption charge for the mixed adlayer, obtained from the corresponding cyclic voltammogram, agrees very well with that estimated from the CO and CN coverages, assuming that one hydrogen atom adsorbs on every free Pt atom. Taking into account these
data, we propose a structural model for the mixed CN–CO adlayer on Pt(1 1 1).
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Cyanide-modified Pt(1 1 1) electrode; Carbon monoxide; Adsorption; CO charge-displacement; CO stripping
1. Introduction
Although not so intensively studied as that of carbon
monoxide, the adsorption of cyanide on platinum electrodes has received some attention, partly stimulated by
the fact that the CN ion and the CO molecule are
isoelectronic. Hubbard and co-workers [1–3] showed by
LEED that ordered cyanide adlayers, which resist electrode emersion in ultrahigh vacuum (UHV), can be
formed on Pt(1 1 1) by immersion in cyanide containing
solution. A similar tactic was used by Korzeniewski and
co-workers [4,5] to obtain infrared spectra, in aqueous
solutions, of cyanide adlayers on polycrystalline and
low-index single-crystal Pt electrodes.
An ordered cyanide adlayer on a Pt(1 1 1) electrode
was observed in situ for the first time, using STM, by
Stuhlmann
p
p et al. [6]. They reported the observation of a
(2 3 2 3)R30° structure consisting of hexagonally
*
Corresponding author. Tel.: +34-91-561-9400; fax: +34-91-5642431.
E-mail address: [email protected] (A. Cuesta).
1
Present address: Departamento de Quımica Fısica, Universidad de
Murcia, Campus de Espinardo, E-30071 Espinardo, Murcia.
0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jelechem.2003.07.008
packed arrays, each containing six spots surrounding a
slightly more intense central one. Each spot was assumed to correspond to a CN group, which would yield
a coverage hCN ¼ 7=12. Similar STM images were obtained later by Kim et al. [7], but they could show quite
convincingly that the central spots in the hexagonal arrays did not correspond to CN, but to a co-adsorbed
cation (H3 Oþ , Naþ or Kþ , depending on the solution
composition and the electrode potential).
p
p The cyanide
coverage corresponding to the (2 3 2 3)R30° structure on Pt(1 1 1) corresponds, therefore, to hCN ¼ 0:5.
Cyanide adsorption on Pt(1 1 1), Pt(1 0 0) and
Pt(1 1 0) has been intensively studied by Huerta et al. [8–
11] in the last five years, using cyclic voltammetry (CV)
and spectroelectrochemical techniques. They have
shown that the cyanide adlayer on Pt(1 1 1) is remarkably stable [8,9], no change being observed in the CVs of
the cyanide-covered electrode upon repetitive cycling
between 0.06 and 1.1 V vs. RHE. Accordingly, as
pointed out by Huerta et al. [8], the cyanide-covered
Pt(1 1 1) electrode can be considered as a chemically
modified electrode.
Very recently, Huerta et al. [12] have adsorbed CO on
a cyanide-modified Pt(1 1 1) electrode. They demon-
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I. Morales-Moreno et al. / Journal of Electroanalytical Chemistry 560 (2003) 135–141
strated that CO adsorption and subsequent stripping
provokes no distortion in the cyanide adlayer at all, and
determined the CO saturation coverage using coulometry. We report here a more accurate determination
(using the true, exact thermodynamic double-layer correction) of the CO coverage at saturation of a cyanidemodified Pt(1 1 1) electrode, and deduce some interesting
structural consequences regarding the mixed CN–CO
adlayer.
RHE. We measured the current flowing during the potentiostatic adsorption of CO and stopped the gas flow
when the current dropped to zero. Then, the solution
was purged with N2 for 15 min, in order to remove
traces of dissolved CO from the solution.
A home-made AgjAgCljKCl(sat) electrode was used
as the reference, but all potentials in the text are referred
to the reversible hydrogen electrode (RHE).
3. Results
2. Experimental
The working electrode was a cylindrical platinum
single crystal (4 mm in diameter, 5 mm long) purchased
from MaTecK (Germany), one of whose circular faces
had been oriented parallel to the (1 1 1) plane (miscut < 1°) and polished with alumina (final particle size
0.03 lm). The Pt(1 1 1) electrode was annealed in a hydrogen-air flame and, following a recently described
procedure [13], cooled in a CO + N2 atmosphere, protected with a droplet of ultrapure water (Milli-Q, 18 MX
cm, 3 ppb TOC) saturated with the cooling gas, and
transferred to the electrochemical cell containing 0.5 M
H2 SO4 (Merck Suprapur). After stripping of the protective CO adlayer formed during the cooling process,
the CV showed the characteristic features of a clean,
well-ordered Pt(1 1 1) surface (see Fig. 1, dashed line).
The electrode was then rinsed with ultrapure water and
immersed in a 0.1 M KCN (Merck p.a.) solution for
approximately 3 min, after which the electrode was
rinsed again and transferred to the electrochemical cell
containing cyanide-free 0.5 M H2 SO4 .
Saturated CO adlayers were formed potentiostatically
by blowing CO into the hanging meniscus while the
electrode potential was held at 0.215 or 0.615 V vs.
The solid line in Fig. 1 shows the CV for a cyanidemodified Pt(1 1 1) electrode in cyanide-free 0.5 M H2 SO4
(the dashed line corresponds to the clean Pt(1 1 1) surface, shown for comparison), which coincides with those
reported in the literature [8–12]. As mentioned in the
Introduction, it has been shown by means
ofp in situ
p
STM [6,7] that cyanide forms a (2 3 2 3)R30°
structure on Pt(1 1 1), with a coverage hCN ¼ 0:5 [7]. The
feature between 0.055 and 0.6 V corresponds to hydrogen adsorption on the free platinum sites. The voltammetric charge of hydrogen adsorption (double-layer
corrected) amounts to 86 lC cm2 , in very good
agreement with a previous report [9], and slightly higher
than that of 80 lC cm2 expected for the adsorption of
atomic hydrogen on two thirds of the Pt atoms left free
upon modification of the Pt(1 1 1) with cyanide. It has
been suggested that the peak at approximately 0.9 V in
the CV of cyanide-modified Pt(1 1 1) corresponds to a
process similar to that occurring on unmodified Pt(1 1 1)
in perchloric acid solutions at about 0.8 V [9], which is
thought to correspond to the adsorption of oxygenated
species and whose exact nature is unknown.
Fig. 2 shows the current transients recorded during
the potentiostatic adsorption of CO on a cyanide-mod-
Fig. 1. Cyclic voltammogram, at 50 mV s1 , of a cyanide-modifed
Pt(1 1 1) electrode in cyanide-free 0.5 M H2 SO4 (solid line). The cyclic
voltammogram of a clean, well-ordered Pt(1 1 1) electrode (dashed line)
has been included for comparison.
Fig. 2. Current transients recorded during the potentiostatic adsorption of CO on a cyanide-modified Pt(1 1 1) electrode in cyanide-free 0.5
M H2 SO4 at 0.215 V (solid line) and 0.615 V (dashed line).
I. Morales-Moreno et al. / Journal of Electroanalytical Chemistry 560 (2003) 135–141
ified Pt(1 1 1) electrode in cyanide-free 0.5 M H2 SO4 at
0.215 V (solid line) and 0.615 V (dashed line), in a CO
charge-displacement experiment as developed by Feliu
and co-workers [14–16]. The charges displaced when CO
is adsorbed at 0.215 and 0.615 V are 17 and )4 lC cm2 ,
respectively.
Fig. 3 shows the first (solid line) and second (dashed
line) CVs, starting at 0.615 V in the negative direction,
after CO adsorption on a cyanide-modified Pt(1 1 1)
electrode (the shape of the CV does not depend on the
potential at which CO was adsorbed). The first cycle
(solid line) corresponds to a cyanide-modified Pt(1 1 1)
electrode covered by a saturated CO adlayer, and the
second one (dashed line) to the same surface after COstripping, and is identical to the CV recorded before CO
adsorption (see solid line in Fig. 1). The mixed CN–CO
adlayer on Pt(1 1 1) is stable if the upper potential limit
is kept below 0.7 V. As reported previously [12] adsorption of CO does not block the adsorption of hydrogen on a CO-saturated, cyanide-modified Pt(1 1 1)
electrode completely. A surface process (anodic peak at
0.31 V; cathodic peak at 0.21 V) superimposed to a
current plateau can be observed in the hydrogen region
between 0.055 and 0.6 V. The hydrogen charge, without
double-layer correction, amounts to 80 lC cm2 , in very
good agreement with that reported previously [12]. This
value reduces to 58 lC cm2 after double-layer correction, which is very near the value of 60 lC cm2 expected for adsorption of 0.25 ML of hydrogen on
Pt(1 1 1).
If the positive potential limit of the scan is increased
to 1.1 V, CO stripping occurs in a peak at 0.99 V (Fig. 3,
solid line). Although, as reported by Huerta et al. [12]
and indicated above, the second cyclic voltammogram
overlaps perfectly that recorded before CO adsorption,
indicating that the initial state of the surface has been
137
completely restored, a (slight) distortion of the cyanide
adlayer is evident from the comparison of the two cyclic
voltammograms in Fig. 3: the CV of the initially COcovered electrode overlaps that of the CO-free surface
only when a potential of 0.78 V is reached in the negative scan after CO stripping. This is especially evident if
the cathodic peak at approximately 0.9 V, just after
stripping of the CO adlayer (Fig. 3, solid line), is compared with the same peak in the second cycle (Fig. 3,
dashed line). The same difference can be observed after a
careful comparison between Figs. 3a and b in [12], although no comment on this was made there. We do not
know what the nature of the distortion in the cyanide
adlayer produced by CO adsorption might be, but desorption of some CN ions can be discarded, since both
the shape of the CV and the charges associated with the
different processes match exactly those measured before
CO adsorption. In any case, as we will show below (see
Section 4.2), the charge associated with the reorganisation process taking place after CO adsorption must be
taken into account if the coverage by CO is to be determined accurately.
4. Discussion
4.1. Consistency of the experimental data
As shown by Climent et al. [17], the data from CO
charge-displacement experiments can be used to check
the consistency of the experimental results and to test the
cleanliness of the system. This is illustrated in the thermodynamic cycle in Scheme 1, from which it is clear that:
Qmð12Þ Q0mð12Þ ¼ Q1 Q2 :
ð1Þ
As we have discussed recently, in the case of a clean
Pt(1 1 1) surface Q0mð12Þ is much smaller than Qmð12Þ and,
hence, Eq. (1) can be simplified to
Qmð12Þ ¼ Q1 Q2 :
ð2Þ
Table 1 shows experimental data for Q1 , Q2 , Qmð12Þ and
Q0mð12Þ , for E1 ¼ 0:215 V and E2 ¼ 0:615 V. As can be
seen in Table 1, for the case of a cyanide-modified
Pt(1 1 1) electrode the difference between Qmð12Þ and
Q0mð12Þ is small, and so the unsimplified Eq. (1) must be
used. The values for Qmð12Þ Q0mð12Þ and Q1 Q2 are
also given in Table 1, demonstrating that the experimental data obey Eq. (1) within approximately 1%.
4.2. Determination of the CO coverage at saturation
Fig. 3. First (solid line) and second (dashed line) cyclic voltammogram
at 50 mV s1 of a cyanide-modified Pt(1 1 1) electrode initially covered
with a saturated CO adlayer in cyanide-free 0.5 M H2 SO4 .
As demonstrated by the Alicante group [17,18], in
order to calculate accurately the coverage by CO of
platinum and other noble metals electrodes using voltammetry, it is necessary to include the charge displaced
during the potentiostatic adsorption of CO in the dou-
138
I. Morales-Moreno et al. / Journal of Electroanalytical Chemistry 560 (2003) 135–141
Scheme 1. Thermodynamic cycle from which Eq. (1), used to check the consistency of the experimental results, can be deduced. r1 , surface charge
density on the cyanide-modified Pt(1 1 1) electrode at E1 ; r01 , surface charge density on the CO-covered cyanide-modified Pt(1 1 1) electrode at E1 ; r2 ,
surface charge density on the cyanide-modified Pt(1 1 1) electrode at E2 ; r02 , surface charge density on the CO-covered cyanide-modified Pt(1 1 1)
electrode at E2 ; Q1 , charge density displaced at E1 during the potentiostatic adsorption of CO; Q2 , charge density displaced at E2 during the potentiostatic absorption of CO; Qmð12Þ , charge density obtained by intergration of the voltammogram of cyanide-modified Pt(1 1 1) between E1 and E2 ;
Q0mð12Þ , charge density obtained by intergration of the voltammogram of CO-covered cyanide-modified Pt(1 1 1) between E1 and E2 .
Table 1
Experimentally measured charges displaced during the potentiostatic adsorption of CO at E1 and E2 on a cyanide-modified Pt(1 1 1) electrode, and
charges obtained by integration of the cyclic voltammograms of CO-free and CO-covered cyanide-modified Pt(1 1 1) between E1 and E2
Q1 (lC cm2 )
Q2 (lC cm2 )
(E1 ¼ 0:215 V)
(E2 ¼ 0:615 V)
17
)4
Qmð12Þ (lC cm2 )
Q0mð12Þ (lC cm2 )
Q1 Q2 (lC cm2 )
Qmð12Þ Q0mð12Þ
(lC cm2 )
89
66
22
23
Q1 , charge density displaced at E1 during the potentiostatic adsorption of CO on a cyanide-modified Pt(1 1 1) electrode; Q2 , charge density
displaced at E2 during the potentiostatic adsorption of CO on a cyanide-modified Pt(1 1 1) electrode; Qmð12Þ , charge density obtained by integration
of the voltammogram of a cyanide-modified Pt(1 1 1) electrode between E1 and E2 ; Qmð12Þ , charge density obtained by integration of the voltammogram of a CO-covered, cyanide-modified Pt(1 1 1) electrode between E1 and E2 .
ble-layer correction. We have recently shown [19] that
the true double-layer correction can be deduced from a
simple thermodynamic cycle. The cycle necessary to
calculate the coverage by CO of a cyanide-modified
Pt(1 1 1) electrode from the charge under the peak in the
corresponding CO-stripping voltammogram is shown in
Scheme 2, from which it follows that:
QCO
net
oxidation
stripping
¼ QCO
DQ
total
adsorption
1 1Þ–CN
QCO
DQ ¼ QPtð1
ox
initial
ð3Þ
where DQ is the double-layer correction. We would like
adsorption
to stress here that QCO
will be different from zero
initial
even in the absence of specific adsorption, since it arises
Scheme 2. Cycle from which the true, exact thermodynamic double-layer correction, necessary for the determination of CO coverages, can be deduced. rinitial , surface charge density on the cyanide-modified Pt(1 1 1) electrode at Einitial ; r0initial , surface charge density on the CO-covered cyanideadsorption
, charge density
modified Pt(1 1 1) electrode at Einitial ; rfinal , surface charge density on the cyanide-modified Pt(1 1 1) electrode at Efinal ; QCO
initial
Ptð1 1 1Þ–CN
displaced at Einitial by the potentiostatic absorption of CO; Qox
, charge density obtained by integration of the voltammogram of CO-free,
stripping
cyanide-modified Pt(1 1 1) between Einitial and Efinal ; QCO
, charge density obtained by integration of the CO-stripping voltammogram between
total
oxidation
Einitial and Efinal ; QCO
,
charge
density
corresponding
exclusively to the faradaic oxidation of the CO adlayer; DQ, double-layer correction.
net
I. Morales-Moreno et al. / Journal of Electroanalytical Chemistry 560 (2003) 135–141
from the difference in the surface charge density, at a
given potential, before and after CO adsorption, provoked by the change in the point of zero charge (pzc)
and in the double-layer capacity consequent upon CO
adsorption.
Due to the thermodynamic nature of the double-layer
correction, care must be exercised when choosing Efinal ,
since the state of the surface at this potential after CO
stripping must be the same as before CO adsorption. As
mentioned above, it can be concluded from the COstripping CV that the state of the surface at Eþ (the
positive limit of the potential scan) just after CO stripping is not the same as the state at Eþ of the surface
before CO adsorption and, hence, choosing Eþ as Efinal
would introduce an error in the CO coverage. This can
be avoided by choosing as Efinal a potential in the negative scan at which the current just after CO stripping
exactly follows that of the CO-free surface, and taking
Ptð1 1 1Þ–CN
stripping
QCO
and Qox
as the charge integrated, in
total
the corresponding CVs, between Einitial and Eþ in the
positive scan plus the charge integrated between Eþ and
Efinal in the negative scan.
Ptð1 1 1Þ–CN
stripping
Experimental values for QCO
and Qox
,
total
using Efinal ¼ 0:715 V in the negative scan, obtained from
CO-stripping CVs of CO-saturated, cyanide-modified
Pt(1 1 1) electrodes and from CVs of CO-free, cyanidemodified Pt(1 1 1) electrodes, respectively, are given in
Table 2 for two initial potentials (Einitial ¼ 0:215 V and
Einitial ¼ 0:615 V), together with the experimental values
adsorption
oxidation
for QCO
and the resulting values for QCO
net
initial
and hCO . The CO coverage at saturation is hCO ¼ 0:25,
and therefore 25% of the surface Pt atoms of the COsaturated, cyanide-modified Pt(1 1 1) electrode remain
uncovered, in very good agreement with the charge
(double-layer corrected) of 58 lC cm2 measured for
hydrogen adsorption-desorption on a CO-saturated,
cyanide-modified Pt(1 1 1) electrode (see above).
Regarding the distortion of the cyanide adlayer upon
CO adsorption, we believe that it must correspond to a
compression of the cyanide adlayer without modification of its geometrical arrangement, relaxation of the
adlayer back to its initial state occurring upon CO
stripping and desorption of the surface oxygenated
species in the subsequent negative scan.
139
p
p
Fig. 4 shows a ball model of the (2 3 2 3)R30°
structure of the cyanide adlayer on Pt(1 1 1) and of the
only two possible structures for the mixed CO–CN adlayer with hCO ¼ 0:25. In the model structure in
Fig. 4(B), the centre of the hexagon formed by the CN
groups is left free, and the sites between hexagons surrounded by four CN groups are occupied by a CO
molecule, while in the model structure in Fig. 4(C) the
centre of the hexagons and the sites between hexagons
surrounded by three CN groups are occupied by a CO
molecule. Several arguments favour the structure in
Fig. 4(B) over that in Fig. 4(C):
1. Steric factors make adsorption of a CO molecule in
the centre of the hexagons formed by the CN groups
unlikely.
2. The compression of the cyanide adlayer suggested by
the CO-stripping CV can be easily explained by the
model in Fig. 4(B), where the formation of a hexagon
of CO molecules surrounding the hexagon of CN
groups would cause a compression of the latter. On
the other hand, the presence of a CO molecule in
the centre of the CN hexagons, as in the model in
Fig. 4(C), would make this compression difficult, if
not impossible.
3. The CO molecule in the centre of the CN hexagons,
as in the model in Fig. 4(C), would be especially difficult to oxidise, since oxygenated species cannot nucleate on their nearest neighbour sites, which are
blocked by CN. However, there is no indication in
the CO-stripping voltammogram of a hindered oxidation of some CO molecules.
4. In the hydrogen adsorption–desorption region, the
CV of a cyanide-modified Pt(1 1 1) electrode covered
by a saturated CO adlayer consists of two contributions: (i) a nernstian process giving rise to a current
a plateau, and (ii) a non-nernstian process giving rise
to a pair of peaks at 0.31 V (anodic) and 0.21 V (cathodic). We believe that the non-nernstian process
corresponds to the sterically hindered adsorption of
hydrogen on the centre of the compressed CN hexagons, although the charge under these peaks (14 lC
cm2 ) is considerably smaller than that of the 20
lC cm2 expected for adsorption of one hydrogen
atom on every central platinum atom.
Table 2
Calculated net faradaic charges for oxidation of a saturated CO adlayer on a cyanide-modified Pt(1 1 1) electrode, and experimentally measured
Ptð1 1 1Þ–CN
stripping
adsorption
oxidation
charges from which they have been obtained: QCO
¼ QCO
Qox
þ QCO
net
total
initial
Einitial ðVÞ
vs. RHE
adsorption
QCO
initial
(lC cm2 )
stripping
QCO
total
(lC cm2 )
1 1Þ–CN
QPtð1
ox
(lC cm2 )
oxidation
QCO
net
(lC cm2 )
hCO
0.215
0.615
17
)4
233
160
130
38
120
118
0.25
0.246
adsorption
, charge density displaced at Einitial by the
The upper limit of integration, Efinal , was chosen to be 0.715 V in the negative scan (see text). QCO
initial
Ptð1 1 1Þ–CN
stripping
potentiostatic adsorption of CO; QCO
,
charge
under
the
CO-stripping
peak,
integrated
between
Einitial and Efinal ; Qox
, charge density
total
CO oxidation
obtained by integration of the voltammogram of CO-free, cyanide-modified Pt(1 1 1) between Einitial and Efinal ; Qnet
, calculated net faradaic
charge corresponding exclusively to the oxidation of the CO adlayer.
140
I. Morales-Moreno et al. / Journal of Electroanalytical Chemistry 560 (2003) 135–141
p
p
Fig. 4. Ball model of the (2 3 2 3)R30° structure of the cyanide
adlayer on Pt(1 1 1) (A) and of the only two possible structures for the
mixed CO–CN adlayer with hCO ¼ 0:25.
Future work using in situ STM and IRRAS is necessary in order to confirm the compression of the cyanide adlayer and the formation of the structure in
Fig. 4(B) upon CO adsorption on a cyanide-modified
Pt(1 1 1) electrode.
4.3. Use of the CO charge-displacement method to obtain
surface charge density–potential curves and potentials of
zero total charge (pztc)
Surface charge density–potential curves can be obtained from the CVs and the CO charge-displacement
measurements by the procedure developed by the
Alicante group [17, and references therein], in which
the charge displaced at a given potential by CO adsorption is subtracted from the voltammetric
charge obtained by integrating the current–potential
curve:
Z E
j jj
r¼
dE QðE Þ;
ð4Þ
m
E
where r is the surface charge density at potential E, j is
the voltammetric current density, m is the scan rate, and
QðE Þ is the charge displaced during CO adsorption at
potential E . This procedure assumes that the charge
displaced at a given potential by the potentiostatic
adsorption of CO corresponds to the total charge on
the CO-free electrode surface at that potential. However, as already pointed out by Weaver [20], and as
recently discussed by us using a thermodynamic cycle
[19], the charge density displaced by CO adsorption
corresponds actually to the difference between the
charge densities of the CO-free and the CO-covered
surfaces at the potential at which CO is adsorbed.
Accordingly, the error incurred when calculating surface charge densities on clean Pt electrodes with this
procedure will be small and similar to the experimental
error in the hydrogen and oxide regions, where the
charge densities of the CO-free Pt surface are of
the order of hundreds of lC cm2 . On the contrary, the
error can be appreciable in the so-called ‘‘double-layer
region’’, where surface charge densities are much
smaller. In any case, and as also demonstrated by
Weaver [20], the error incurred when estimating the
potential of zero total charge (pztc) of clean Pt electrodes using this procedure is small, about 25 mV, due
to the fact that the pztc for Pt is located within the
potential region where hydrogen adsorption occurs.
The large adsorption capacitance values associated
with this process cause that even a significant error in
the charge density of the CO-free Pt surface leads to
only a small uncertainty in the pztc.
The arguments above can be used to demonstrate
that the CO charge-displacement method cannot be used
to determine the surface charge density–potential curve
and the pztc on a cyanide-modified Pt(1 1 1) electrode.
The data presented here allow us to assume that at 0.1 V
both the CO-free and the CO-covered cyanide-modified
Pt(1 1 1) electrodes are covered by a saturated hydrogen
adlayer, this corresponding to approximately 1/3 ML in
the case of a CO-free, cyanide-modified Pt(1 1 1) electrode, and to 0.25 ML in the case of a CO-covered,
cyanide-modified Pt(1 1 1) electrode. It is reasonable to
expect that the pztc >0.1 V in both cases and, hence, the
charge density, at 0.1 V, on the CO-free, cyanide-modified Pt(1 1 1) electrode must be r1 < 80 lC cm2 , and
that on the CO-covered cyanide-modified Pt(1 1 1) electrode r01 < 60 lC cm2 . Obviously, the condition that
r1 r01 is not fulfilled in this case: a CO charge-displacement experiment at 0.1 V would yield a charge of
approximately 20 lC cm2 , and if this were assumed to
correspond to the surface charge density on the cyanidemodified Pt(1 1 1) electrode at 0.1 V, the error incurred
would be of 75%!
This demonstrates that care must be exercised when
using the CO charge-displacement method to obtain
surface charge density–potential curves and pztcs, since
the method is applicable only if certain conditions are
fulfilled.
5. Conclusions
We have shown that thermodynamic cycles, in combination with the CO charge-displacement method, are
necessary in order to determine the CO coverage using
I. Morales-Moreno et al. / Journal of Electroanalytical Chemistry 560 (2003) 135–141
CO-stripping voltammograms. Furthermore, thermodynamic cycles allow us to visualise the basis of the CO
charge-displacement method, and therefore its limitations when used to determine surface charge densities
and the pztc, providing an unsimplified, general equation for testing the consistency of the experimental results.
The coverage by CO of a cyanide-modified Pt(1 1 1)
electrode can be accurately determined with the true,
exact thermodynamic double-layer correction, which
includes the charge displaced upon potentiostatic adsorption of CO. At saturation, hCO ¼ 0:25. We have also
proposed a model for the structure of the mixed CN–CO
adlayer on Pt(1 1 1).
Acknowledgements
This work was carried out with the help of the
Spanish DGI under Project BQU2001-0207. I.M.-M.
gratefully acknowledges a fellowship from the CSIC.
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