Effect of increasing amount of steps on the potential of zero total charge of Pt(111) electrodes

Electrochimica Acta 45 (1999) 629 – 637 www.elsevier.nl/locate/electacta

Effect of increasing amount of steps on the potential of zero total charge of Pt(111) electrodes Vı´ctor Climent, Roberto Go´mez, Juan M. Feliu * Departament de Quı´mica Fı´sica, Uni6ersitat d’Alacant, E-03080 Alacant, Spain Received 2 September 1998; received in revised form 20 January 1999

Abstract This paper reports results on the interaction of CO with Pt(s)[n(111)×(111)] electrodes in perchloric acid medium. Charge displacement experiments allow the estimation of the potential of zero total charge (pztc) values for this series of stepped surfaces. The pztc values decrease linearly with the step density for n ]5, while for shorter terraces a plateau is observed. Knowledge of the pztc is a key point in the evaluation of the CO coverage by means of coulometric measurements. Using this method, we have calculated the CO surface concentration at saturation for a series of stepped surfaces. The resulting values are similar regardless of the step density, but they are significantly lower than those for the Pt(111) basal plane. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Potential of zero total charge (pztc); Pt(111) electrodes; Stepped surfaces

1. Introduction At the beginning of the 1980s, an important breakthrough took place in electrochemical surface science. The seminal works by Clavilier and co-workers [1,2] provided reliable and inexpensive methods for preparing and treating single crystal platinum electrodes. In the following years, many papers were published on the electrochemical behavior of Pt(hkl) single crystals. An impressive amount of information, obtained not only with electrochemical techniques but also with other in-situ and ex-situ techniques, has been gathered. However, one of the most important energetic parameters characterizing the interface, the potential of zero charge (pzc), remains basically unexplored in the case of platinum-group metals. Obviously, the pzc is defined as the value of the electrode potential at which the charge on the metal side of the electrode/electrolyte interface is zero. The importance of the pzc extends to all aspects of electrocatalysis and electron transfer phe* Corresponding author.

nomena. The reason for this lack of information stems not only from experimental difficulties, but also in the ambiguity of the electrode charge concept when adsorption processes involving charge transfer take place at the metal/electrolyte interface. This lack of definition was clarified by Frumkin by defining two types of electrode charge [3,4]: 1. the free charge, s, defined as the true free excess charge density on the metal surface. 2. the total charge, q, defined as the amount of electricity to be supplied to the electrode when its surface increases by unity with the concentration of the solution components remaining constant. It involves both the free charge and the charge transferred in reversible chemisorption. Obviously, in the case of an ideally polarizable electrode both concepts coincide since no charge transfer associated with chemisorption thereby occurs. However, in the case of the platinum-group metals both values are generally different at a given potential. This is the reason why we should distinguish two types of potentials of zero charge: the potential of zero free

0013-4686/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 2 4 1 - 8

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V. Climent et al. / Electrochimica Acta 45 (1999) 629–637

charge, Es = 0, and the potential of zero total charge (pztc), Eq = 0. It is important to point out that only the total charge is accessible from thermodynamical analysis of electrochemical measurements; the free charge is to be evaluated from an appropriate model of the interface. In reference to the determination of the total charge at different potentials, a suitable method (although approximate) for determining total charges is based on ‘charge displacement’ experiments at constant potential by adsorption of a neutral species to saturation [5–7]. They essentially consist in recording the transient current produced during CO dosing at a given constant potential. This current is ascribed to the desorption of the previously adsorbed species at the potential of the experiment. It is also worth noting that a different method has been proposed for determining ‘local’ potentials of zero total charges: the ‘local’ pztc is identified as the potential of maximum current for N2O reduction [8]. While the potentials of zero total charge have been determined by the charge displacement technique for the platinum and rhodium basal planes [7,9], no reports on the values for platinum stepped surfaces have appeared to date. It is, however, of fundamental interest, as it is known that step sites play an important role in catalysis and adsorption. It would be desirable to compare the variations of the electron work function and pzc upon the introduction of defects on the surface, since this can lead to a better understanding of the metal/solvent interactions. For this purpose, it is particularly fruitful to have an estimate of the dipole moment per step atom and to compare it with the one worked out in vacuum. In addition, this dipole moment is possibly linked to the enhanced catalytic activity (including adsorption) of these sites in terms of charge transfer from edge atoms. In this paper, we have determined Eq = 0 values for a series of Pt[n(111)×(111)] surfaces by the charge displacement technique. We have analyzed the results by comparing them with UHV data. On the other hand, a knowledge of Eq = 0 is also necessary to evaluate the correction needed to convert CO stripping voltammetric charges into coverages [10]. Since we get stripping voltammograms as a part of the experimental procedure of the CO displacement technique, we have dealt with the influence of steps on the CO saturation coverage obtained upon CO dosing during the charge displacement experiment.

2. Experimental Single crystal electrodes were prepared from small (2 mm diameter) Pt beads obtained by melting Pt wires (99.99%) (0.5 mm in diameter). The facets present on

the surface of the beads were used to select the desired orientation within 93 min of arc. The electrodes were fixed, cut and polished as described previously [11]. Before each experiment the electrodes were flame annealed, quenched with water in equilibrium with a mixture of H2 + Ar [12], and polarized in the hydrogen evolution potential region ( −0.06 V) for 60 s. It was found that this treatment leads to sharper peaks around 0.1 V, characteristic of (110) steps [13]. Similarly, this treatment leads to Pt(110) electrode samples whose voltammetric profiles in sulfuric acid exhibit sharp voltammetric peaks [14]. Recently, it has been proposed that this voltammetric profile correspond to an unreconstructed Pt(110) surface [15]. Solutions were prepared from concentrated perchloric acid (Merck Suprapur) and Millipore Milli Q water. The solution purity, as well as the surface order and cleanliness, was tested by standard voltammetric procedures. The voltammetric profiles obtained for the different stepped surfaces are in qualitative agreement with those reported by Clavilier et al. [13]. However, cooling in the Ar+ H2 atmosphere leads to sharper profiles, as pointed out by Rodes [16]. The working solution was deaerated by bubbling Ar (Air liquide, N50) for 15 min. The reductive atmosphere characteristic of the electrode pretreatment consisted of a mixture of Ar with H2 (Air liquide, N50). Charge displacement experiments at constant potential were performed as described elsewhere [5] by dosing CO (Air liquide, N48). A wave signal generator (EG&G PARC 175), a potentiostat (Amel 551) and a recorder (Phillips PM 8133) X-Y-t were arranged in the conventional way. The cell was a conventional two-compartment glass cell with an additional inlet for dosing CO gas. Care was taken to avoid the presence of atmospheric oxygen in the vicinity of the meniscus. Potentials were measured against and are quoted versus the reversible hydrogen reference electrode (RHE). A coiled Pt wire immersed in the working electrolyte was used as counterelectrode. All experiments were performed at room temperature.

3. Results and discussion The data reported in this work have been obtained following an experimental protocol described elsewhere [5,6]. In summary, the procedure consists of the following steps: 1. the voltammetric profile for the electrode is recorded after the flame treatment as to check the quality of the surface and the purity of the working solution, 2. CO is dosed at constant potential and simultaneously the current is recorded; this step corresponds to the aforementioned ‘charge displacement’ technique, and

V. Climent et al. / Electrochimica Acta 45 (1999) 629–637

3. CO is stripped off the electrode voltammetrically and the profile of the resulting electrode surface is then recorded. As a criterion, the initial and final voltammetric profiles of the electrode should be virtually superimposable, thus ensuring that the initial and final states of the electrode are the same. Clearly, step (ii) may be performed at any desired constant potential negative to the onset of the CO oxidation process. The experimental determination of the charges displaced by CO facilitates the discussion of two different aspects of platinum single crystal electrochemistry. Firstly, the value of the pztc is estimated for a number of Pt stepped surfaces, together with that of the Pt(110) surface. As shown bellow, this determination is based in the identification of the pztc with the potential at which the displaced charge is zero. Secondly, the values for the CO saturation coverage on the same surfaces are determined. In fact, it is necessary to know Eq = 0 in order to calculate from the raw stripping charge the CO coverage [10]. We therefore divide this section into two parts along these lines.

3.1. Determination of the Eq = 0 Let us consider the voltammogram for a platinum electrode in a typical test electrolyte. As long as the process associated to the adsorption of hydrogen and anions is reversible for the usual scan rates, there is a direct relation between the total charge as a function of the applied potential and the voltammetric profile. In fact, the voltammogram can be considered as the derivative of the total charge versus potential curve. Obviously, one cannot determine the q versus E curve from the voltammogram unless the integration constant is known. The charge displacement experiments provide an approximate value of the total charge at a given potential, i.e. a value for the integration constant. It is interesting to discuss to what extent the potential of zero displaced charge (Ed = 0) is a good estimation of the pztc. We will assume that CO is a neutral probe as has shown it is by performing the displacement experiment with adlayers of known stoichiometry [5] and also by using alternative probes [17]. With this assumption, the charge flowing during the displacement is equal to the difference between the total charge on the electrode before (qi) and after (qf) CO adsorption at saturation: qtrans =qf −qi

(1)

where qf and qi are given by the following expressions: qi = Cav(E) ×(E−Eq = 0)

(2)

qf =C%(E−Eq% = 0)

(3)

631

Cav(E) is the integral capacity calculated at a given potential as:

&

Cav(E) =

E

C·dE

Eq = 0

(4)

E−Eq = 0

C% and q% correspond to the capacity and charge for the saturated surface after the CO adsorption. The potential at which qf equals qi correspond to Ed = 0:



Eq = 0 = Ed = 0 1−



C% C% + Eq% = 0 Cav Cav

(5)

It should be stressed at this point that Ed = 0 will be a good estimate of the Eq = 0 only when Cav(Ed = 0) \ \ C% and Eq = 0 \ \ (C%/Cav)Eq% = 0. These conditions are fulfilled in our case. Let us consider as an example the case of Pt(111) electrode in 0.1 M HClO4 solution. The capacity for a full CO-covered electrode is constant and about 14 mF/cm2, whereas Cav is around 370 mF/cm2 for potentials near Ed = 0. The first condition is thus clearly satisfied. For checking the second one, we are in need of Eq% = 0. Although we do not know this value exactly, let us consider a maximum value for Eq% = 0 as high as 1.0 V versus RHE [18]. Even for this high value of Eq% = 0, and taking into account that Ed = 0 = 0.33 V versus RHE [7], the difference between Eq = 0 and Ed = 0 is about 25 mV. Therefore, in the following, we will consider that both values are the same and we will refer to them as Eq = 0. Therefore, taking into account that C% is very low compared to Cav we can neglect qf in Eq. (1) to get: qtrans $ − qi

(6)

From one value of qi and the integration of the voltammogramic current densities, it is easy to obtain the curve relating q to E. Therefore, just one value of charge displaced at a given potential is enough for estimating Eq = 0. We have performed displacement experiments at two different potentials: 0.10 and 0.33 V. The fact that the difference between the total charge density displaced at two different potentials and the charge density integrated under the voltammetric profile between the same potential limits are virtually equal supports the reliability of the procedure. A relative total charge versus potential curve was obtained by integrating the voltammetric profile and then it was fitted to the values of total charge obtained from displacement experiments by means of a least squares method. The reliability of the resulting Eq = 0 values for the series of stepped surfaces investigated may be considered to be within 10 mV. The experimental results obtained with different stepped surfaces are depicted in Fig. 1, which shows the voltammetric profiles for four of them. The integrated curve corresponding to the total charge value is also

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shown, as well as the point at which the total charge vanishes (Eq = 0). The values of displaced charge are also given (filled circles). The resulting values of Eq = 0 for the different surfaces are reported in Fig. 2: a linear drop of the Eq = 0 with the step density is obtained for n ]5 whereas for higher step densities the Eq = 0 values form a plateau. We have included Pt(110) in this series (highest step density point in Fig. 2) by considering this basal plane as a stepped surface denoted by 2(111) ×(111). Taking into account the latter point we realize that the Eq = 0 actually increases slightly at higher step (defect) densities, a situation similar to that obtained in the same case for gold [19]. One of the most significant results reported in this work is the linear decrease of the Eq = 0 values as the step density increases for surfaces Pt[n(111)×(111)] having n]5. Although for platinum, as far as we

know, there are no previous data along these lines, Lecoeur et al. [19] have determined the value of the pzc for a variety of gold stepped surfaces. They found that the pzc (in this case, NaF was used as a test electrolyte and Es = 0 = Eq = 0 because of the lack of specific adsorption with charge transfer) becomes more negative as the surface density of monoatomic steps increases. More specifically, a linear relation between pzc and step density was reported for small step densities. In this respect, our results are unexpectedly similar. Moreover, in the case of gold, Lecoeur et al. [19] compared their results to those of vacuum on the same type of stepped surfaces. They employed the classical relationship in the form proposed by Trasatti that relates the values for the work function and the pzc [20]: E M(hkl) =F M(hkl)/e-[dx M(hkl) +g sM(hkl)(dipoles)]s=0+K s s (7)

Fig. 1. Voltammetric profiles for four different stepped surfaces in 0.1 M HClO4. Sweep rate: 50 mV/s. The integrated curve that gives the value of the total charge at each potential is also shown. ( ) Experimental values obtained for the total charge through CO displacement; () Eq = 0.

V. Climent et al. / Electrochimica Acta 45 (1999) 629–637 (100) (111) E (hkl) s = 0-E s = 0 = 4pnm

Fig. 2. Potential of zero total charge for a series of Pt(s)[n(111) ×(111)] electrodes as a function of the step density in 0.1 M HClO4.

and F M(hkl) are the pzc and the electron where E M(hkl) s work function of a metallic single crystal with a definite and orientation, respectively, while dx M(hkl) s s gM(hkl)(dipoles) are the metal superficial electronic structure modification and the orientation of water dipoles resulting from the aqueous solution contact, respectively. K is the potential drop at the reference electrode (absolute potential of the reference electrode). Based on this equation, they made an analysis of the E M(hkl) s variations parallel to that made for F M(hkl). In fact, there are several reports on the influence of steps [21,22], adatoms [23] or defects caused by Ar sputtering [24] on the work function values of several well-ordered metal surfaces. For example, in the cases of platinum and gold, the deliberate introduction of defects in the form of steps [21,22] causes a linear decrease in the work function values at least for small step densities. This has been related to the increasing presence of surface dipoles. In this kind of studies, a surface dipole moment associated with defect sites is usually calculated by using the Helmholtz equation from a plot of the work function against the step density. In fact, the surface dipole per unit of length along the step direction is obtained or, equivalently, the dipole moment per step atom. Lecoeur et al. [25] have also employed the Helmholtz formula under electrochemical conditions for stepped surfaces with (111) terraces and (100) steps:

633

(8)

for calculating the dipole moment of each step, m (100), where n denotes the step density. In trying to extend these methods and procedures to the case of platinum electrodes, one problem emerge. In fact, as we pointed out in Section 1, two types of charge values are to be distinguished for platinum: namely, the free and the total charge. Both values coincide in the absence of charge transfer processes, i.e. electrodes that do not adsorb hydrogen, like gold and silver, in contact with electrolytes which do not lead to specific adsorption. Obviously, this is not the case for platinum. The question that arises is whether there is a direct relation between the work function and the pztc. It does not seem that such a relation holds in all cases. For instance, in the case of Rh(111) and Rh(100), the existing difference in work functions is not reflected in changes in the Eq = 0 of both samples, which are approximately equal in sulfuric acid medium [9]. For Rh electrodes in H2SO4 test solutions, the pztc is mainly determined by the adsorption of H and (bi)sulfate. However, in an early attempt to relate the pzc to the electron work function made by Trasatti in the late 1970s, the platinum-group metals were included [26]. The pzc values for the latter were taken from results of Eyring and co-workers in neutral solution [27] by assuming that the metal surfaces were free of adsorbed gases. However, as pointed out later by Frumkin et al. [4], this does not seem to be correct, since the surface renewal without the supply of electricity from outside can at best lead to the disappearance of the total, but not the free charge. Nevertheless, the potential of zero free charge appears to be more appropriate for making comparisons with vacuum. Indeed, Trasatti’s relation (Eq. (7)) is based on the evaluation of the various components contributing to the potential difference in both interfaces. Only the free charge corresponds to the actual charge (in the physical sense) at the metallic side of the interface and can contribute to the build-up of a potential difference comparable to that found in metal/vacuum interfaces. Furthermore, from our viewpoint, true comparison between both interfaces can be only done if we consider a hydrated, partially hydrogen-covered Pt(111) surface in UHV, these data being not available at present. Fortunately, the variation in work function caused by the adsorption of hydrogen is similar for Pt(111) and vicinal stepped surfaces [28,29]. In this way, for Pt(111), adsorption of H to saturation causes a decrease of 230 mV in the work function, whereas for Pt(s)[9(111)×(111)] surface the decrease is around 350 mV. The plot of Eq = 0 versus step density reported in Fig. 2 shows a behavior similar to what it has been reported for F for the same series of surfaces [21]. Provided that

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the plot of Es = 0 versus step density is not available, we will discuss on its general trends. In a first attempt we estimated the Es = 0 for a Pt(111) in 0.1 M HClO4 to be around 0.10 V versus RHE [7]. A more refined correction already mentioned was published later [18] and the Es = 0 value for Pt(111) was estimated to be 0.30 V versus RHE. Then, if we take into account the charge carried by the CO/Pt(111) electrode resulting from the charge displacement we deduce that Es = 0 BEq = 0 by only 60 mV. If the data necessary for the correction (work function values for hydrated CO/Pt(111) samples) were available for platinum stepped surfaces we could roughly estimate Es = 0 from the corresponding Eq = 0 values. Then it would be possible to compare directly with UHV results and analyze the influence of the steps in a more straightforward way. Unfortunately, these data are not available for stepped surfaces at present. In any case, the values of Eq = 0 and Es = 0 estimated for Pt(111) electrodes may be just compared with those experimentally determined for platinized platinum [3]. In a summarizing paper, Frumkin and Petrii [3] reported that the Eq = 0 for platinized platinum in a 0.3 M HF +0.12 M KF solution is located at 0.38 V versus RHE, whereas the corresponding Es = 0 is 0.33 V versus RHE. As it may be seen the differences between both potentials for both kinds of platinum surfaces are of the same sign and not too high, despite the fact that the degree of order for these surfaces is completely different: a flame treated Pt(111) electrode is a mostly ordered platinum surface, whereas platinized platinum electrodes should correspond to a fully disordered surface on a microscopic level. Based on these results it is reasonable to suggest that the Es = 0 of the stepped surfaces used in this work are slightly more negative than the corresponding Eq = 0 values. We could assume that the difference between Eq = 0 and Es = 0 is similar to that estimated for Pt(111) and platinized platinum. In other words, it is plausible to assume that the Es = 0 versus step density curve is linear as that corresponding to the Eq = 0 and that both are approximately parallel. If this is so and following the procedure of Lecoeur et al. [25], we could estimate the value for the dipole moment associated to each atom along the step. Subsequently, a value around 0.14 D/atom is obtained in this manner. It is very instructive to compare the latter value with that calculated from the work function behavior for stepped platinum surfaces of the same type. The UHV value is around 0.6 D/atom [21,22]. Thus, there is a large difference between them. It could be argued that the adsorption of hydrogen under electrochemical conditions is one of the reasons. However, it does not seem to be of primary importance since:these stepped surfaces are not thought to have hydrogen adsorbed at the steps at the pztc; and the influence of adsorbed hydrogen on the work function for this type of Pt stepped

surfaces is quite limited, especially for low and moderate H coverages [29]. The adsorption of water around the steps probably causes a reduction of its associated dipole moment, as pointed out previously [19] in the case of Au[n(111)× (100)] stepped surfaces. However, and in spite of the fact that our estimation for the dipole moment at the step can be considered just as a first rough assessment, the difference between the dipole moment values for both environments is much larger than that estimated for gold. More work is in progress to elucidate this point by working in sulfuric acid solutions and by extending our results to electrodes with (100) steps. On the other hand, it is also remarkable that both the pztc versus step density and the DF versus step density for these Pt[n(111)×(111)] surfaces form a plateau for high step densities [21]. In both cases there exists a saturation in the diminution of the pztc and F. The behavior of F has been explained by considering the interaction among surface dipole moments [23]. There is a collateral question that deserves a short discussion. In previous publications [6,9], we pointed out that one of the virtues of the charge displacement experiments was to help discriminate between the adsorption of cations and the desorption of anions. Both processes give current of the same sign in voltammetry (negative). For positive current transients (negative total charges), the main process involved is clearly: PtH+ CO“ PtCO+ H+ + e −

(9)

However, in the case of negative current transients (positive total charges), it is less obvious to establish the nature of the adsorbed species implied. Given that ClO− 4 does not seem to adsorb specifically, (and thus, with charge transfer), it is more probable that oxygenated species (denoted by OH) coming from water adsorption are responsible for the negative current transients: PtOH+CO + e − “ PtCO+OH −

(10)

Taking into account that the higher the step density, the more positive the total charge is at a given potential, the adsorption of oxygenated species would occur preferentially at the steps.

3.2. CO co6erages from stripping charges It has been pointed out that the knowledge of the value of the Eq = 0 enables an accurate evaluation of the ‘double layer’ correction to be applied to the CO stripping charge and thus the determination of the faradaic CO oxidation charge [10]. When the CO is stripped off, the electrode recovers its state of charge, as indicated by the recuperation of the voltammetric profile characteristic of the clean electrode immediately after the stripping. Thus, a correc-

V. Climent et al. / Electrochimica Acta 45 (1999) 629–637

635

Table 1 CO stripping charges and coverages for platinum stepped surfaces n

Step density (cm−1×10−6)

q RAW CO

q0.8 V

q NET CO

uCO

20 14 10 7 5 4 3 2

0.00 2.15 3.12 4.45 6.56 9.59 12.46 17.80 31.15

496a 443a 392 396 377 389 399 395 384

178b 161b 119 118 103 107 114 104 100

318 283 273 277 274 281 285 290 284

0.66 0.60 0.58 0.60 0.61 0.65 0.68 0.76 0.96

a b

Upper potential limit taken as 0.9 V Correction at 0.9 V

tion is needed, since the charge density under the peak corresponding to the CO oxidation includes also the charge due to the reformation of the electrochemical double layer for the clean electrode. In order to obtain the charge for CO oxidation, the latter contribution should be subtracted [10,30]. This contribution is easily obtained from the integration of the voltammetric current for the clean surface between Eq = 0 and the potential chosen as upper limit for the integration of the CO stripping peak. Table 1 summarizes the results obtained with this series of stepped surfaces. Coverages are determined by normalizing the CO stripping charges to the corresponding surface atomic density in the following way: uCO =

q NET CO 2qPt(hkl)

From Table 1, it is evident that the values for the CO stripping charge do not depend much on the orientation of the stepped surface; however, q NET CO for Pt(111) is significantly higher. This is observed clearly in Fig. 3 where the CO stripping charge is plotted versus the step density. From Fig. 4, it may be seen that a linear relation exists between CO coverage and step density. The linear plot obtained for the CO coverage as a function of the step density partly reflects that we are assuming a decreasing number of outermost Pt atoms with the step

(11)

corresponds to the charge under the CO where q NET CO stripping peak corrected following the previous guidelines and qPt(hkl) represents the atomic density expressed in electrical units [13]. It is obtained for a Pt[n(111)× (111)] surface from the formula:

<

qPt(hkl) =qterr +qstep

=

2e cos b

3d

+

2

2e cos b

3d 2

1−



4

3 n−

2 3

< = 1

2 n− 3

=

=

 

2e cos b n−1

3d 2 n− 2 3

(12)

where b is the angle existing between the plane of the surface and that corresponding to the (111) terrace, qterr and qstep denote the atomic density of terrace and step sites, respectively, e is the electron charge and d the Pt atomic diameter. See Refs. [13,16] for further details.

Fig. 3. Net CO stripping charge for saturated layers as a function of the step density.

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molecules per terrace sites coincides with the coverage resulting from the extrapolation to step density zero in Fig. 4 (u extr CO ). Therefore, the charge corresponding to the CO stripping off the terraces could be estimated as: q terr CO =

2e cos b

3d 2

<

1−

=

  4

2 3 n− 3

·2u extr CO

(14)

The difference between q NET and q terr CO CO will correstep spond to the value of q CO . If we accept that the CO coverage at the steps is constant, q step CO will follow the expression: q step CO =

Fig. 4. Plot of the total CO coverage vs. the step density.

:

2e cos b

1

3d

2 n− 3

2

;

·2u step CO

(15)

the term between the parenthesis being the step density [13,16]. The plot of the charges of CO at the steps versus the step density should therefore be a straight line passing through the origin as observed in Fig. 5. From the slope of this line, a CO coverage value of 0.99 0.1 is found for the step sites, including the basal plane Pt(110). In conclusion, it seems that CO molecules distribute equally over the terraces of all these surfaces, except in the case of the basal plane Pt(111) which accommodates a significantly higher coverage. In fact, both

density. In any case it is put forward that the CO coverage on Pt(111) (0.66 as obtained experimentally) is markedly higher than the extrapolation of this linear plot (0.55). Interestingly, this extrapolated coverage agrees quite well with the one deduced from STM measurements on CO adlayers imaged in absence of CO in solution [31], which are probably less compact than those obtained under other experimental conditions. This result indicates that 2D long range order is important to achieve more compact CO adlayers. Even for terraces as large as 20 atoms wide the effect is clearly shown. A similar effect of long range order may be observed for the adsorption of (bi)sulfate anions [13,16]. Finally, we will try to obtain information on the CO coverage both at the terraces and at the steps. Obviously, from macroscopic information, it is not possible to obtain microscopic information unless some assumptions are made. In the case of stepped surfaces we can divide the net stripping charge into two contributions, namely, the charge assigned to the terraces, q terr CO , due to CO molecules bound to Pt atoms in the terraces and q step CO , associated to CO molecules at the steps: step terr q NET CO =q CO +q CO

(13)

In this way we may try to arbitrarily split the CO stripping charges into two contributions, one belonging to terraces and the other to the step sites. As an assumption we may consider that the number of CO

Fig. 5. Calculated charges corresponding to the CO adsorbed on the steps (see text) as a function of the step density.

V. Climent et al. / Electrochimica Acta 45 (1999) 629–637

terrace and step sites would exhibit a different but uniform coverage, being around 0.55 for terraces and around 0.9 for steps.

4. Conclusions We have applied the charge displacement technique to a series of Pt(S)[n(111)×(111)] Pt(S)[(n− 1)(111)× (110)] surfaces. In this way, we have determined the different values for the pztc. There is a linear drop in the Eq = 0 as the step density increases, at least for terraces not too short. Although there are no theoretical relationships between both magnitudes, we have analyzed this behavior by assuming that the variation of the Eq = 0 with the step density parallels that of the Es = 0. Then the linear drop in pztc has been interpreted as a result of the appearance of surface dipoles at the steps. In addition from the slope of the pztc versus step density we have derived a value for the dipole moment assigned to each atom at the step. It happens to be much smaller than that deduced for the same surfaces in UHV conditions. We have suggested that this difference is due to the reorientation of the water dipoles adsorbed at the steps, also playing a minor role the adsorption of hydrogen. The Eq = 0 values determined for the stepped surfaces serve to obtain the correction to be applied to the raw CO stripping charge in order to work out the CO coverage values. In has been found that the deliberate introduction of a small density of steps brings about a diminution in the coverage of the saturated CO adlayer. In this way, whereas the saturation coverage resulting from the CO charge displacement for a Pt(111) electrode is around 0.66, the extrapolated coverage value for step density zero would be 0.55. The coverage estimated for step sites is about 0.9, markedly higher than that of terraces.

Acknowledgements Financial support of DGES through contract PB960409 is greatly acknowledged. One of us (V.C.) is grateful to the CEC of Generalitat Valenciana for the award of a Ph.D. grant. We greatly appreciate the comments of J. Lipkowski and M.J. Weaver during the preparation of the revised version of this manuscript.

References [1] J. Clavilier, J. Electroanal. Chem. 107 (1980) 211.

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