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On the kinetics of oxygen reduction on platinum stepped surfaces in acidic media
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 564 (2004) 141–150 www.elsevier.com/locate/jelechem
On the kinetics of oxygen reduction on platinum stepped surfaces in acidic media ~a, E. Herrero, J.M. Feliu M.D. Maci a, J.M. Campin
*
Dpto. de Quımica Fısica, Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain Received 4 June 2003; received in revised form 1 September 2003; accepted 28 September 2003
Abstract Oxygen reduction on platinum single crystals has been studied in perchloric and sulfuric acid media using a hanging meniscus rotating disk electrode configuration. The surfaces studied belong to the ½0 1 1 zone and can be classified in two different series, i.e., surfaces with terraces with (1 1 1) symmetry and (1 0 0) steps and surfaces with (1 0 0) terraces and (1 1 1) steps. In sulfuric acid media, the highest catalytic activity is found for the Pt(2 1 1) electrode whereas the Pt(1 1 1) electrode exhibits an anomalously low j0 value. Conversely, the extrapolated value for the catalytic activity of Pt(1 1 1) obtained from the series of stepped surfaces having (1 1 1) terraces is much higher than that obtained experimentally. The comparison with the results obtained in perchloric acid solutions indicates that the anomalously low catalytic activity of the Pt(1 1 1) electrode in sulfuric acid solutions is related to the formation of an ordered adlayer of specifically adsorbed (bi)sulfate anions. For practical purposes, the effect of the step density and the (bi)sulfate specific adsorption is small, except for the Pt(1 1 1) electrode. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Oxygen reduction; Platinum single crystal; Platinum stepped surfaces; Anion adsorption; Long range effects
1. Introduction Oxygen reduction is one of the most important electrochemical reactions since it has multiple applications in a variety of research fields, ranging from energy conversion to corrosion science. Therefore, the understanding of the nature of the electrocatalytic process of oxygen reduction has been the subject of a vast number of papers. Although knowledge of the oxygen reaction process has advanced considerably in recent years [1–5], many aspects of the kinetics of this heterogeneous reaction are not fully understood. The nature and surface structure of the electrode is a fundamental aspect to consider when one or several intermediates can adsorb on the electrode surface. This is specially the case for oxygen reduction, in which four electrons are involved in the final reduction to water. The high number of electrons exchanged per oxygen molecule would imply the possible existence of several *
Corresponding author. Fax: +34-965-909301. E-mail address: [email protected] (J.M. Feliu).
0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.09.035
adsorption intermediates, whose interaction energies with the surface will depend on the metal and its surface structure. It is well known that platinum exhibits the highest catalytic activity for the 4 e pathway among the pure metals studied [4]. On this metal, the reaction is structure sensitive, the Pt(1 1 0) electrode being the most active basal surface in acidic media [6–11]. The activity of the electrode is also greatly affected by the presence of adsorbed anions. Thus, in perchloric acid solutions, the Pt(1 1 1) electrode is more active than the Pt(1 0 0) electrode, whereas in sulfuric acid solutions the situation is reversed. Moreover, the reaction is strongly inhibited when other strongly adsorbing anions, such as chloride or bromide, are present in the solution [12,13]. Therefore, the strength of the anion adsorption and its surface structure has always to be considered in order to understand better the effect of surface structure on the oxygen reduction reaction. Most of the studies of structure sensitive reactions deal with the comparison of the different low index surface electrodes. It is well known that these electrodes
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have different amounts of surface defects that contribute unexpectedly to the reaction kinetics. These defects play an important role in CO oxidation on Pt(1 1 1) electrodes [14]. On the other hand, anion adsorption shows especial characteristics when it occurs on Pt(1 1 1) wide terraces, as compared to the behavior observed for stepped surfaces with relatively narrow terraces [15]. In order to extrapolate the results obtained with well-defined basal planes to the smaller domains, such as those present in nanosized crystallites, it is necessary to perform studies in which the terrace size can be controlled. This can be achieved by studying the behavior of the structure sensitive reactions on stepped surface electrodes. The aim of this work is to study the oxygen reduction reaction on platinum single crystals belonging to the ½0 1 1 zone in two different supporting electrolytes, 0.1 M HClO4 and 0.5 M H2 SO4 . These surfaces can be classified in two different series, i.e., surfaces with terraces with (1 1 1) symmetry and (1 0 0) steps and surfaces with (1 0 0) terraces and (1 1 1) steps. The use of stepped surfaces allows the determination of the effect of the steps and the symmetry of the terrace on the oxygen reduction reaction. The extrapolation of the observed behavior for the stepped surface to the basal planes, i.e., Pt(1 1 1) and Pt(1 0 0), provides insight into the role of the small amount of defects always present in the real surfaces and also the effect of the long range order in the reactions.
2. Experimental Working electrodes were made from platinum single crystal beads, obtained by fusion and subsequent slow crystallization of a 99.999% platinum wire (Goodfellow Metals in all platinum samples). After careful cooling, the resulting single crystal beads were oriented, cut and polished following the procedure described in [16]. Prior to any experiment, the working electrodes were heated again for 10 s in a gas-oxygen flame, cooled in a reductive atmosphere (H2 + Ar), quenched in ultrapure water in equilibrium with this atmosphere and then transferred to the electrochemical cell. STM results indicate that the surfaces obtained after this treatment correspond to the nominal surfaces [17,18]. The stepped platinum electrode surfaces (2 mm diameter) used in this work were those located in the stereographic triangle between the (1 1 1) and (1 0 0) poles, which correspond to the crystallographic ½0 1 1 zone. These surfaces can be classified in two different series, i.e., surfaces with terraces with (1 1 1) symmetry and (1 0 0) steps and surfaces with (1 0 0) terraces and (1 1 1) steps. The Miller indices for the surfaces with (1 1 1) terraces are (n þ 1,n 1,n 1), which also can be denoted as Pt(S)[n(1 1 1) (1 0 0)] in the terrace-step
notation proposed by Lang et al. [19] (n is the number of atoms in the terrace). The surfaces with (1 0 0) terraces have Miller indexes (2n 1,1,1), which are equivalent to Pt(S)[n(1 0 0) (1 1 1)]. The Pt(3 1 1) surface is the turning point in the zone since it can be denoted as Pt(S)[2(1 0 0) (1 1 1)] or Pt(S)[2(1 1 1) (1 0 0)]. The voltammetric profiles of the surfaces employed in this work in 0.5 M H2 SO4 solution are shown in Fig. 1. A full description of the voltammetric behavior of the electrodes in this zone can be found elsewhere [18,20– 22]. The main voltammetric characteristics of the electrodes are the following. The sharp spike at 0.45 V present in the voltammetric profile of the Pt(1 1 1) electrode is associated with the order–disorder transition in the adsorbed (bi)sulfate layer. This peak disappears for the Pt(n þ 1,n 1,n 1) stepped surfaces [23,24], even for those having relatively wide (1 1 1) terraces. As the (1 0 0) step density increases from the Pt(1 1 1) electrode, the charge under the peak at 0.27 V, which corresponds to the adsorption states on this step [20], also increases, together with a diminution of the charge associated with hydrogen adsorption (between 0.06 and 0.35 V) and (bi)sulfate (between 0.35 and 0.6 V) on the (1 1 1) terraces. For the Pt(3 1 1) electrode, the peak has shifted to 0.28 V and comprises the maximum charge, as expected for the surface with the highest step density of the series. Moving towards the Pt(1 0 0) surface, a new signal centered at 0.36 V appears, which corresponds to the adsorption states on the (1 0 0) terrace. Conversely, the signal linked to hydrogen adsorption on the (1 1 1) step (between 0.06 and 0.2 V) diminishes [18]. Oxygen reduction experiments were carried out in a hanging meniscus rotating disk (HMRD) configuration [25] in an oxygen-saturated cell (oxygen pressure in the cell atmosphere is 1 atm). For this purpose, an electrode holder for the bead-type single crystal electrodes was
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Fig. 1. Voltammetric profile of the electrodes used in this work in 0.5 M H2 SO4 . Only the positive scan is shown. Scan rate: 50 mV s1 .
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Fig. 2. Comparison of the voltammetric profile of the Pt(1 0 0) electrode obtained at 2000 rpm (––) and 0 rpm (- - - -) in 0.5 M H2 SO4 . Scan rate: 50 mV s1 .
adapted to the Radiometer EDI10K rotor. The electrode was placed in the holder so that its surface was perpendicular to the rotation axis and centered (as much as possible) with respect to this axis. With this arrangement, the maximum rotation rate that can be achieved, without breaking the meniscus, is 4000 rpm. Since the solution cleanliness requirements for the HMRD experiments are more stringent due to the forced convention, special care was taken to assure it. For this purpose, the voltammetric profiles of the electrodes in a de-oxygenated test electrolyte were recorded under rotating conditions and compared to those obtained at 0 rpm, taken as the reference. Fig. 2 shows a typical example of the voltammetric profile of a Pt(1 0 0) electrode recorded in 0.5 M H2 SO4 . In the reported case, the negligible diminution of the peak at 0.36 V observed under rotating conditions is proof of the cleanliness (the charge of the overall voltammogram has diminished to less than 2%). Solutions displaying higher surface blockage were discarded. Experiments were carried out at room temperature, 20 °C, in a classical two-compartment electrochemical cell including a platinum counterelectrode and a reversible hydrogen electrode (RHE) as reference. Solutions were prepared with H2 SO4 (Aldrich, twice distilled) and perchloric acid (Merk suprapur) in ultra pure water. H2 , O2 and Ar (N50, Air Liquide) were also employed.
3. Results and discussion 3.1. Hydrodynamic behavior of the HMRD electrode Fig. 3 shows the voltammetric profiles for oxygen reduction at different rotation rates on Pt(1 1 1) elec-
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Fig. 3. Oxygen reduction current recorded on a HMRD Pt(1 1 1) electrode in 0.5 M H2 SO4 at different rotation rates. Scan rate: 50 mV s1 .
trodes in 0.5 M H2 SO4 . The curve shows the characteristic behavior reported previously [5–8,11,26]. The onset for oxygen reduction is 0.8 V, and the limiting current is obtained between 0.3 and 0.5 V. The main feature of these curves is the appearance of two current drops at potentials below 0.3 V, instead of the single drop reported previously [5]. The first current drop coincides with the beginning of hydrogen adsorption (0.3 V). At 0.15 V, the current measured is 82% of the limiting current at 500 rpm and 67% at 2500 rpm. Then, the current drops again at 0.13 V. At this potential, hydrogen coverage on the electrode surface, as determined by CO displacement measurements, corresponds to half a hydrogen monolayer. Ring-disk experiments indicated that this drop is related to the formation of H2 O2 [5,8]. The presence of adsorbed hydrogen on the electrode surface prevents the scission of the O–O bond in some cases, and therefore only two electrons are exchanged in the reduction process for these O2 molecules [5,6]. The observed dependence of the current drop on the rotation rate is related to the possibility of further reduction of the H2 O2 formed. Since the reduction of H2 O2 is not completely inhibited at these potentials, some of the newly formed H2 O2 molecules can be reduced to water. In these cases, four electrons will be exchanged per O2 molecule. For high rotation rates, these molecules will diffuse fast away from the surface. This fact will prevent any further reaction and a higher drop in the total current is observed. Another characteristic of the curves is the presence of the spike associated with the order/disorder transition in the (bi)sulfate adlayer at 0.45 V superimposed on the reduction current. This is additional proof of the cleanliness of the solutions employed and indicates that neither the electrode surface nor the adsorbed (bi)sulfate layer is perturbed by the oxygen reduction reaction. The
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presence of wide two-dimensional domains, as is evident from the presence of the spike, is related directly to the particular behavior of this electrode surface as compared to the stepped surface having narrow (1 1 1) terraces, as will be discussed below. In order to obtain kinetic parameters from the oxygen reduction reaction the hydrodynamic behavior of the system has to be characterized. A detailed study of the hydrodynamic properties of these electrodes has been carried out by Villulas et al. [25,27,28]. In this work, the limiting current of the electrode for a reduction process is described by jlim ¼ 0:62nFD2=3 m1=6 cb x1=2 ð1 kx1=2 Þ;
ð1Þ
where D is the diffusion coefficient, m, the kinematic viscosity, cb , the bulk concentration, x, the rotation rate and k is a parameter for the system that depends on the meniscus height and electrode diameter. This equation is a modification of the Levich equation, which describes the hydrodynamic behavior of the rotating disk electrode. The correction term ð1 kx1=2 Þ accounts for the reduction of the effective electrode radius promoted by the change in the flow direction in the electrode edges [27,28]. Since the electrodes employed in this work are smaller and the electrode arrangement is different, the first step is to verify whether Eq. (1) holds under the present experimental conditions. Using the values for D and cb for oxygen and m for a H2 SO4 solution at pH ¼ 0 [29], Eq. (1) yields: jlim =mA cm2 ¼ 0:339x1=2 þ k:
ð2Þ
The plot of the limiting current vs. x1=2 is shown in Fig. 4. The line fitted to the experimental data has zero intercept (within the experimental error) and a slope of 0.369 ± 0.07 mA cm2 s1=2 . The slope is very close to the
theoretical Levich slope given in Eq. (2). The zero intercept has also been obtained for all the electrodes employed in this work and in experiments using ferricyanide reduction as the test reaction, indicating that a negligible value of k is characteristic of the present configuration of the HMRD. It is also worth noting that the limiting current obtained for a given rotation depends slightly on the meniscus height; ca. a 5% difference in the limiting current can be found between different experiments for the same electrode, each one starting after the flame annealing step. Therefore, the hydrodynamic behavior of the HMRD electrodes in the present configuration is quite well described by the Levich equation, within reasonable experimental error. This result is in disagreement with those obtained in [26–28], where the zero intercept is observed only for low meniscus heights with the electrolyte wetting the walls of the electrode. The wetting of the electrode walls can be discarded in the present case and the accidental occurrence of this situation can be easily identified. For instance, significant currents are observed for the Pt(1 0 0) electrode between 0.1 and 0.2 V when electrolyte wets the electrode walls. As can be seen in Fig. 2, no signal from the walls is observed, and therefore, the differences have to be due to different experimental conditions. The main difference in the set up employed in this work is the use of small hemispherical electrodes for which the rotation axis may not be completely perpendicular to the electrode surface. Although the electrode surface is always centered as much as possible with respect to the rotation axis, a small eccentric rotation of the meniscus can never be avoided. This fact prevents the attainment of rotation rates higher than 4000 rpm, but probably improves the mass transfer process to the whole surface, thus diminishing k to negligible values. In order to study the kinetics of the oxygen reduction reaction, the kinetic current density that would have been obtained in the absence of mass transport limitations (jtrans ) has to be calculated. For a first order reaction, the following relationship is obtained: 1 1 1 ¼ þ : j jtrans jlim Solving for jtrans : j jlim : jtrans ¼ jlim j
ð3Þ
ð4Þ
If the system behaves according to the Levich equation, Eq. (3) can be rewritten as 1 1 1 ¼ þ : 2=3 j jtrans 0:62nFD m1=6 cb x1=2
Fig. 4. Plot of jlim vs. x1=2 for oxygen reduction on a Pt(1 1 1) electrode in 0.5 M H2 SO4 .
ð5Þ
jtrans can then be obtained from the intercept of the plot of 1=j vs. 1=x1=2 [30]. Fig. 5 displays the values of jtrans at different potentials calculated according to Eqs. (4) and (5). As can be seen, both procedures give identical re-
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Fig. 5. jtrans values vs. potential calculated for a Pt(1 1 1) electrode in 0.5 M H2 SO4 . ðjÞ Values calculated from the intercept of the plot of j1 vs. x1=2 , using Eq. (5). ðsÞ values calculated applying Eq. (4) to the curve at 3000 rpm from Fig. 3.
-0.8 -1.0 -1.2 0.0
sults for the calculation of jtrans , thus corroborating that the Levich equation describes the hydrodynamic behavior of the system. At potentials below 0.5 V, the accuracy of both methods diminishes significantly since the measured current value is very close to the limiting current and therefore the calculated values of jtrans are no longer reliable. 3.2. Voltammetric behavior of oxygen reduction on platinum electrodes on the [0 1 1] zone in sulfuric acid solutions Once the hydrodynamic behavior of the system has been established, oxygen reduction on the different electrodes can be studied systematically. Fig. 6 presents a selection of the curves for oxygen reduction on different electrodes in 0.5 M H2 SO4 . Owing to the small variation of the limiting current with the meniscus height, the curves have been normalized to the absolute value of the limiting current ðjjlim jÞ, for a better comparison between them. As can be seen, nearly all the curves for oxygen reduction have a very similar value for the onset of oxygen reduction. Two curves stand out from the rest: the Pt(1 1 1) and Pt(2 1 1) electrodes, which show the lowest and the highest catalytic activities, as will be discussed later. Another important characteristic of the curves is the diminution of the current density at negative potentials (below 0.3 V). In the two series of electrodes, Pt(1 1 1) and Pt(1 0 0) electrodes exhibit the highest drop in the current at negative potentials. As the step density increases, the onset of the current density drop is shifted toward more negative potentials and the total drop is
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Fig. 6. (a) Curves for oxygen reduction on selected Pt(h k l) electrodes of the ½0 1 1 zone in 0.5 M H2 SO4 at 1500 rpm. (b) Curves of the panel A normalized to jjlim j. Scan rate 50 mV s1 .
smaller. As aforementioned, this drop in the current is related to the formation of H2 O2 [5,8], and therefore a diminution in the total number of electrons exchanged in the process of oxygen reduction. It appears that H2 O2 formation is smaller on electrodes having narrower terraces. This change in the oxidation mechanism can be related to hydrogen adsorption on the surfaces. In this potential region, in which the current drop is observed, oxygen must compete with hydrogen for the adsorption sites. A strong hydrogen adsorption and/or a high coverage hinder oxygen adsorption on the platinum surface. Moreover, two neighboring Pt sites are required to break the O–O bond in order to form H2 O as the final product. It seems that the O–O bond cannot be broken when all the neighboring sites to an adsorbed O2 molecule are blocked by strongly adsorbed species, yielding only H2 O2 as the final product. In the two series, Pt(1 1 1) and Pt(1 0 0) electrodes have the most positive onset for hydrogen adsorption (see Fig. 1), and the highest hydrogen coverage at a given potential [18,22]. Hydrogen adsorption on the step sites occurs at more negative potentials than in the terrace sites. In this way, hydrogen adsorption on the (1 1 1) steps occurs between 0.06 and 0.2 V [18], whereas on the (1 0 0) steps, the hydrogen adsorption peak is located at 0.27 V. Consequently, at a given potential, it will be more difficult to
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find two neighboring available sites for oxygen adsorption on the terrace than on the step, and therefore the Pt(1 1 1) and Pt(1 0 0) will exhibit the higher current drop at more positive potentials. This inhibition of the four-electron mechanism due to strongly adsorbed species has also been found for chloride on Pt(1 0 0) electrodes and bromide on Pt(1 1 1) and Pt(1 0 0) electrodes [12,13]. Other strongly adsorbed adatoms, such as silver, selenium and sulfur on polycrystalline platinum, inhibit the scission of the O–O bond, yielding only H2 O2 as the final product at high adatom coverages [31,32]. However, adsorbed (bi)sulfate does not prevent the scission of the O–O bond, although it is adsorbed on the electrode surface at potentials higher than 0.35 V on Pt(1 1 1) and Pt(1 0 0) electrodes [15,33]. When compared to chloride and bromide, (bi)sulfate adsorption is weaker, and therefore, the adsorbed (bi)sulfate anions can be more easily displaced by the incoming O2 molecules. Furthermore, the analysis of the STM images [23,24] and the comparison with the coverage values obtained by radiotracer techniques [34] and coulometry [15] for (bi)sulfate adsorption on Pt(1 1 1) electrodes indicate that water is coadsorbed on the (bi)sulfate adlayer. This fact suggests that the (bi)sulfate adlayer is quite open and flexible, and can accommodate the oxygen molecules adsorbed on two platinum sites to yield H2 O. The presence of the (bi)sulfate adsorbed on the electrode surface will then shift the curve for oxygen reduction toward more negative potential (as compared to that obtained in the absence of strongly adsorbed anions), since O2 should compete for the adsorption sites but it will not prevent the formation of H2 O. On the other hand, the particular properties of this adlayer on wide Pt(1 1 1) domains are clearly pointed out. Other effects, such as the possible diminution of the adsorption strength of the (bi)sulfate as the step density increases could also be possible. However, the relevant data in order to consider such effects are not available.
yields a different Tafel slope in this region [35]. The Tafel slopes obtained in all cases lie between 120 and 130 mV, which indicates that the rate determining step is the first electron transfer, assuming that O2 adsorption follows a Langmuir isotherm. Extrapolating the linear behavior in the Tafel region to the equilibrium potential for the O2 –H2 O system (which under the present conditions is 1.23 V vs. RHE), the values of j0 for the different electrodes can be obtained. In Fig. 7, j0 values have been plotted versus the surface angle with respect to the Pt(1 1 1) surface. Because of the voltammetric behavior, the Pt(1 1 1) electrode shows a much lower electrocatalytic activity than the rest of the electrodes, even when compared to electrodes with wide (1 1 1) terraces. The difference between the Pt(1 1 1) electrode and the surface with the widest (1 1 1) terraces, Pt(15,13,13) with 14 atom-wide (1 1 1) terraces, is much higher than that between this latter electrode and the rest of the different electrodes in the series. On the other hand, Pt(2 1 1) displays the highest catalytic activity of the surfaces studied, since the onset for oxygen reduction occurs at the most positive potentials. This surface, which has three atoms on the (1 1 1) terrace and a (1 0 0) step, is, according to these results, the most active surface structure. It is also worth mentioning that the electrocatalytic activity is not completely associated with the step density, since the surface with the highest step density, Pt(3 1 1), has a lower catalytic activity than that of the Pt(2 1 1) electrode. Interestingly, the higher catalytic activity of the Pt(2 1 1) electrode with respect to the Pt(1 1 1) electrode is in agreement with the computed interaction of O 2
3.3. Kinetics of oxygen reduction on platinum electrodes on the [0 1 1] zone in sulfuric acid solutions The values of jtrans as a function of potential have been obtained for all the curves using Eq. (4). The Tafel plots constructed with these values show a wide linear range (over 300 mV) (see for instance Fig. 3), which allows the determination of the values of the Tafel slope and exchange current density ðj0 Þ. In all cases, a single Tafel slope is obtained in the potential range studied (between 0.9 and 0.5 V). Unlike other studies where two Tafel slopes are obtained for some electrodes [6,8], oxidation of the surface has been avoided to prevent the disordering of the surface that takes place when the surface oxides are reduced. It is known that the presence of oxides alters the kinetics of oxygen reduction and
Fig. 7. Plot of j0 for oxygen reduction vs. the angle of the surface with respect to the Pt(1 1 1) surface in ðjÞ 0.5 M H2 SO4 and ðsÞ 0.1 M HClO4 .
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with the Pt(1 1 1) (0.7 eV) [36] and Pt(2 1 1) (0.9 eV) [37]. As aforementioned, a Tafel slope value of 120 mV indicates that the first electron transfer is the rate-determining step. For this reaction this step is probably: O 2 þ e O 2;ads
ð6Þ
The higher adsorption energy for the Pt(2 1 1) electrode will indicate that the reaction is favored on the Pt(2 1 1) electrode, provided that this higher adsorption energy does not prevent the further reaction of O 2 from proceeding. The ‘‘ideal’’ behavior of the Pt(1 1 1) and Pt(1 0 0) can be obtained by extrapolation to zero step density of the observed behavior for the stepped surfaces. When dealing with two different series of electrode surfaces, two representations have to be made for each series since the nature of the step is different for each series. If both representations have to be combined, the angle between a given surface and the Pt(1 1 1) electrode will be used alternatively, since it is proportional to the step density. In Fig. 7, the straight line fitted for the j0 values for the surfaces in the Pt(2n 1,1,1) series agrees well (within the experimental error) with the observed j0 value for the Pt(1 0 0) electrode. On the other hand, the extrapolated j0 value for the Pt(1 1 1) electrode is ca. one order of magnitude higher than that observed experimentally. The deviation of the actual values from the extrapolated behavior can have two different origins: (a) the presence of defects in the real surface and (b) long range effects not observed on the stepped surfaces. The effect of the presence of defects in the real surfaces has been shown, for instance, for CO oxidation [14] or formic acid dissociation [38] on Pt(1 1 1) electrodes. For CO oxidation on Pt(1 1 1) electrodes, the behavior extrapolated from the stepped surface would indicate that the Pt(1 1 1) electrode would be almost inactive for CO oxidation. Since the catalytic activity for CO oxidation on the small amount of defects present in a real Pt(1 1 1) electrode is orders of magnitude higher than that of the basal plane, the experimental rate for CO oxidation is dependent only on the number of defects on the surface, and is not related to the activity of the (1 1 1) sites. In this case, the extrapolated activity of the ‘‘ideal’’ Pt(1 1 1) surface is lower than that of the real surface, which is the opposite behavior from that observed here for the oxygen reduction reaction. Therefore, the deviation observed for the real Pt(1 1 1) surface cannot be due to the presence of defects. In this latter case, the step sites have higher catalytic activity for oxygen reduction than the basal planes, but the increase of the catalytic activity with the number of step sites is rather small. For this reason, the extrapolation of the observed behavior for the surfaces in the Pt(2n 1,1,1) series to the Pt(1 0 0) electrode is in agreement with the experimental result, despite the relatively high number of defects present in a real Pt(1 0 0) electrode [18,39].
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Since the presence of defects should lead to a higher j0 value for the real Pt(1 1 1) electrode, as compared to the extrapolated value, the deviation has to be attributed to the presence of long-range effects on the Pt(1 1 1) electrode not observed on the stepped surfaces. On Pt(1 1 1) electrodes, (bi)sulfate forms an ordered structure which is very sensitive to long-range order. The spike at 0.45 V, which is associated with the disorder– order transition, is observed only for terraces wider than 20 atoms. This disorder–order transition involves a significant increase of the (bi)sulfate coverage. If the coverage values obtained for stepped surfaces (for which the disorder–order transition does not take place) are extrapolated to the Pt(1 1 1) electrode, the real coverage values are 15% higher than those obtained from the extrapolation [40]. This 15% of additional (bi)sulfate species are adsorbed on the electrode upon ordering of the (bi)sulfate adlayer. It is well known that this spike is very sensitive to surface perturbations, such as small amounts of surface contamination of organic or inorganic adsorbed species, and this latter problem should be avoided in order to observe this behavior properly. As aforementioned, oxygen reduction takes place in this medium at potentials where (bi)sulfate is adsorbed on the electrode surface. For this reason, the coverage of the (bi)sulfate adlayer affects the behavior of the electrode, as has been previously reported [5,10,11]. The difference between the extrapolated and real j0 value will then indicate the effect of these additional (bi)sulfate species that are adsorbed upon ordering in the oxygen reduction reaction. Therefore, the low catalytic activity of the Pt(1 1 1) electrode with respect to the rest of the electrodes in sulfuric acid media is clearly related to the formation of the bidimensionally ordered adlayer, which is more stable than the disordered adlayers formed on the other electrodes, and inhibits the oxygen reduction on the Pt(1 1 1) electrode. 3.4. Kinetics of oxygen reduction on platinum electrodes on the [0 1 1] zone in perchloric acid solutions In order to prove the effect of the ordered (bi)sulfate adlayer in the oxygen reduction kinetics on the Pt(1 1 1) electrode, the reaction was also studied in 0.1 M HClO4 . Fig. 8 shows the voltammetric profiles in the HMRD configuration obtained for this medium. The first significant difference between the curves in sulfuric and perchloric acid are that the onset of O2 reduction has shifted more than 100 mV towards more positive potentials, as expected for a weakly adsorbing electrolyte. Another important difference is that no significant variation between the Pt(1 1 1) electrode and the other curves in its series are observed. In fact, all the curves in Pt(n þ 1,n 1,n 1) almost overlap in the same region. A similar situation is observed for the curves in the other series. However, the currents for the electrodes in the
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Pt(111) Pt(15,13,13) Pt(211) Pt(511) Pt(11,1,1) Pt(100)
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Fig. 9. Tafel slope values obtained for oxygen reduction in 0.1 M HClO4 on the different electrodes.
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1.0
Fig. 8. (a) Curves for oxygen reduction on selected Pt(h k l) electrodes of the ½0 1 1 zone in 0.1 M HClO4 at 2000 rpm. (b) Curves of the panel A normalized to jjlim j. Scan rate: 50 mV s1 .
Pt(n þ 1, n 1, n 1) series are significantly higher than those obtained for the Pt(2n 1,1,1) at a given potential. The values of jtrans obtained using Eq. (4) were used to calculate the values of j0 and the Tafel slope for each electrode. For this medium, the linear region is observed between 0.6–0.7 and 0.9 V. Unlike the behavior observed in 0.5 M H2 SO4 , where the Tafel slope was 120 mV for the whole zone, the values for the Tafel slopes range between 90 and 120 mV (Fig. 9). The values of 90 mV for the Tafel slope are obtained for the electrodes in the Pt(n þ 1,n 1,n 1) series. However, similar low slope values have been obtained in basic media, and these results have been interpreted taking into account that OH adsorption takes place in the same potential region where the Tafel slope was calculated [35]. A qualitative treatment was proposed in that work in which it was assumed that the electrode sites covered by OH do not contribute to the reduction reaction. For the Pt(n þ 1,n 1,n 1) electrodes, OH adsorption also takes place between 0.6 and 0.9 V, which can also interfere with the reduction reaction. For these cases, the current density for oxygen reduction can be defined as [35]: aF jtrans ¼ 4Fkcb ð1 hA Þ exp E ; ð7Þ RT
where k is the kinetic constant, a, the electron transfer coefficient and hA is the OH coverage of the electrode (or any other anion strongly adsorbed on the electrode surface). This equation represents only a qualitative approach since the OH covered sites should also contribute to the reduction process, but probably with a much lower rate constant. Then, the experimental Tafel slope for Eq. (7) is defined as dE 1 ¼ mTafel ¼ : ð8Þ RT d logjjj þ 1 dhA 2:3aF
1hA dE
As can be seen, the experimental Tafel slope contains an extra term related to the variation of the OH coverage in this region in addition to the term that contains the electron transfer coefficient. Since the value of dhA =dE is positive, the experimental values of the Tafel slope are lower than those found when hA is constant. Therefore, for the electrodes in the Pt(n þ 1,n 1, n 1) series, the lower Tafel slopes in HClO4 are not due to a different kinetic mechanism, but to the interference of the OH adsorption/desorption on the oxygen reduction reaction. If the contribution for the adsorbed OH species were eliminated, the Tafel slope values would have been similar to those found in sulfuric acid solutions. This effect is not observed in sulfuric acid solutions since the (bi)sulfate coverage is nearly constant at potentials above 0.6 V [40] and therefore the term dhA =dE is negligible in the region where Tafel slopes are determined. The values of j0 for this medium were calculated using the experimental Tafel slope values. In an exact treatment, the effect of the OH would have to be evaluated in order to calculate the expected currents for zero OH coverage and then extrapolate these values to the equilibrium potential. Since the model in Eq. (7) repre-
M.D. Macia et al. / Journal of Electroanalytical Chemistry 564 (2004) 141–150
sents only a qualitative approximation to the effect of OH adsorption in the current, the experimental currents and Tafel slopes, without further corrections, were used. Therefore, the real j0 values for the electrodes in the Pt(n þ 1,n 1,n 1) series would be higher than those calculated neglecting the OH effects. The j0 values obtained in 0.1 M HClO4 are plotted in Fig. 7. As expected, the exchange current densities are higher that those obtained in 0.5 M H2 SO4 , highlighting the effect of the adsorbing anions in the overall kinetics. For this supporting electrolyte, the catalytic activity of the stepped surfaces is higher that that for the basal planes; however, the difference between them is rather small. Another important characteristic of the results is that the extrapolated values for the Pt(1 1 1) and Pt(1 0 0) electrodes agree well with the experimental values, thus confirming that the unusual low activity of the Pt(1 1 1) electrode is due to the formation of an ordered (bi)sulfate adlayer. The small difference in the catalytic activity for the stepped surfaces in comparison to the basal planes is rather unexpected, since UHV measurements indicate that oxygen is preferentially adsorbed on the step sites [41]. In fact, the dissociative adsorption process of oxygen on Pt(1 1 1) and Pt(5 3 3) is controlled by the presence of defects or steps [42]. A direct extrapolation of the UHV data would have suggested a significantly higher catalytic activity for the step sites in comparison with the basal planes. As aforementioned, in the electrochemical environments, the adsorption process is always competitive in nature and O2 adsorption must compete for the adsorption sites with other species present in the aqueous solution. Owing to the Smoluchowski effect, the step sites will have a lower local work function as compared to the terrace sites. Using the relationship between the pzc (potential of zero charge) and the work function [43], the step sites would have a lower local pzc than the terrace sites. In the case of surfaces with Pt(1 1 1) terraces, such a relationship between the pzc and the work function has been observed [22]. Therefore, at a given electrode potential, anion adsorption will be stronger on the step sites, as compared to the terrace sites. For instance, the much higher catalytic activity of the step sites for the CO oxidation reaction has been associated with the adsorption of OH on these sites [14]. The adsorbed CO molecules on the terrace will diffuse until they reach a step site covered by OH, where they react to yield CO2 . Therefore, it can be proposed that the step sites are already covered by OH, and oxygen must displace the adsorbed OH to react on these sites. The higher adsorption energy for O2 on the step sites is then counterbalanced with a higher adsorption strength of the OH species on these sites, resulting in a lower catalytic activity than the expected value from UHV experiments.
149
Acknowledgements This work was supported by the Spanish DGI through Project no. BQU2003-0429 and by the European Union Framework V Growth Programme, CLETEPEG project, Contract no. G5RD-CT-200100463.
References [1] M.R. Tarasevich, A. Sadkowsky, E. Yeager, in: B. Conway, J.OÕM. Bockris, E. Yeager, S.U.M. Khan, R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, vol. 7, Plenum Press, New York, 1983, pp. 301–398. [2] D.J. Schiffrin, in: D. Pletcher (Ed.), Electrochemistry Specialist Periodical Report, vol. 8, The Royal Society of Chemistry, Burlington House, London, 1983, pp. 126–170. [3] K. Kinoshita, Electrochemical Oxygen Technology, Wiley, New York, 1992. [4] R. Adzic, in: J. Lipkowsky, P.N. Ross (Eds.), Electrocatalysis, VCH Publishers, New York, 1998, pp. 197–242. [5] N.M. Markovic, P.N. Ross, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Theory, Experiments and Applications, Marcel Dekker, New York, 1998, p. 197. [6] N.M. Markovic, H.A. Gasteiger, P.N. Ross, J. Phys. Chem. 99 (1995) 3411. [7] N.M. Markovic, R.R. Adzic, B.D. Cahan, E.B. Yeager, J. Electroanal. Chem. 377 (1994) 249. [8] N.M. Markovic, H. Gasteiger, P.N. Ross, J. Electrochem. Soc. 144 (1997) 1591. [9] B.N. Grgur, N.M. Markovic, P.N. Ross, Can. J. Chem. 75 (1997) 1465. [10] F. El Kadiri, R. Faure, R. Durand, J. Electroanal. Chem. 301 (1991) 177. [11] H. Kita, H.-W. Lei, Y. Gao, J. Electroanal. Chem. 379 (1994) 407. [12] N.M. Markovic, H.A. Gasteiger, B.N. Grgur, P.N. Ross, J. Electroanal. Chem. 467 (1997) 157. [13] V. Stamenkovic, N.M. Markovic, P.N. Ross, J. Electroanal. Chem. 500 (2001) 44. [14] N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van Santen, J. Phys. Chem. B 106 (2002) 12938. [15] E. Herrero, J. Mostany, J.M. Feliu, J. Lipkowski, J. Electroanal. Chem. 534 (2002) 79. [16] J. Clavilier, D. Armand, S.G. Sun, M. Petit, J. Electroanal. Chem. 205 (1986) 267. [17] E. Herrero, J.M. Orts, A. Aldaz, J.M. Feliu, Surf. Sci. 444 (1999) 259. [18] K. Domke, E. Herrero, A. Rodes, J.M. Feliu, J. Electroanal. Chem. 552 (2003) 115. [19] B. Lang, R.W. Joyner, G.A. Somorjai, Surf. Sci. 30 (1972) 454. [20] A. Rodes, K. El Achi, M.A. Zamakchari, J. Clavilier, J. Electroanal. Chem. 284 (1990) 245. [21] A. Rodes, M.A. Zamakhchari, K. El Achi, J. Clavilier, J. Electroanal. Chem. 305 (1991) 115. [22] R. Gomez, V. Climent, J.M. Feliu, M.J. Weaver, J. Phys. Chem. B 104 (2000) 597. [23] K. Itaya, Prog. Surf. Sci. 58 (1998) 121. [24] A.M. Funtikov, U. Linke, U. Stimming, R. Vogel, Surf. Sci. 324 (1995) L343. [25] B.D. Cahan, H.M. Villullas, J. Electroanal. Chem. 307 (1991) 263. [26] J. Perez, H.M. Villullas, E.R. Gonzalez, J. Electroanal. Chem. 435 (1997) 179. [27] H.M Villullas, M. Lopez Teijelo, J. Electroanal. Chem. 384 (1995) 25.
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[28] H.M Villullas, M. Lopez Teijelo, J. Electroanal. Chem. 385 (1995) 39. [29] K.L. Hsueh, E.R. Gonzalez, S. Srinivasan, Electrochim. Acta 28 (1983) 697. [30] A. Frumkin, G. Teodoradse, Z. Electrokhem. 62 (1958) 251. [31] R.R. Adzic, J.X. Wang, J. Phys. Chem. B 102 (1998) 8988. [32] Y. Mo, D.A. Scherson, J. Electrochem. Soc. 150 (2003) E39. [33] M.E. Gamboa-Aldeco, E. Herrero, P.S. Zelenay, A. Wieckowski, J. Electroanal. Chem. 348 (1993) 451. [34] A. Kolics, A. Wieckowski, J. Phys. Chem. B 105 (2001) 2588. [35] N.M. Markovic, H.A. Gasteiger, P.N. Ross, J. Phys. Chem. 100 (1996) 6715. [36] M. Tachiya, J. Phys. Chem. 93 (1989) 7050.
[37] A. Eicher, J. Hafner, Phys. Rev. Lett. 79 (1997) 4481. [38] M.D. Macia, E. Herrero, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 500 (2001) 498. [39] A. Al-Akl, G. Attard, R. Price, B. Timothy, Phys. Chem. Chem. Phys. 3 (2001) 3261. [40] J. Mostany, E. Herrero, J.M. Feliu, J. Lipkowski, J. Phys. Chem. B 106 (2002) 12787. [41] P.J. Feibelman, S. Esch, T. Michely, Phys. Rev. Lett. 77 (1996) 2257. [42] A.T. Gee, B.E. Hayden, J. Chem. Phys. 113 (2000) 10333. [43] S. Trasatti, E. Lust, in: R.E. White, J.OÕM. Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, vol. 33, Plenum Press, New York, 1999, pp. 1–215.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 564 (2004) 141–150 www.elsevier.com/locate/jelechem
On the kinetics of oxygen reduction on platinum stepped surfaces in acidic media ~a, E. Herrero, J.M. Feliu M.D. Maci a, J.M. Campin
*
Dpto. de Quımica Fısica, Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain Received 4 June 2003; received in revised form 1 September 2003; accepted 28 September 2003
Abstract Oxygen reduction on platinum single crystals has been studied in perchloric and sulfuric acid media using a hanging meniscus rotating disk electrode configuration. The surfaces studied belong to the ½0 1 1 zone and can be classified in two different series, i.e., surfaces with terraces with (1 1 1) symmetry and (1 0 0) steps and surfaces with (1 0 0) terraces and (1 1 1) steps. In sulfuric acid media, the highest catalytic activity is found for the Pt(2 1 1) electrode whereas the Pt(1 1 1) electrode exhibits an anomalously low j0 value. Conversely, the extrapolated value for the catalytic activity of Pt(1 1 1) obtained from the series of stepped surfaces having (1 1 1) terraces is much higher than that obtained experimentally. The comparison with the results obtained in perchloric acid solutions indicates that the anomalously low catalytic activity of the Pt(1 1 1) electrode in sulfuric acid solutions is related to the formation of an ordered adlayer of specifically adsorbed (bi)sulfate anions. For practical purposes, the effect of the step density and the (bi)sulfate specific adsorption is small, except for the Pt(1 1 1) electrode. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Oxygen reduction; Platinum single crystal; Platinum stepped surfaces; Anion adsorption; Long range effects
1. Introduction Oxygen reduction is one of the most important electrochemical reactions since it has multiple applications in a variety of research fields, ranging from energy conversion to corrosion science. Therefore, the understanding of the nature of the electrocatalytic process of oxygen reduction has been the subject of a vast number of papers. Although knowledge of the oxygen reaction process has advanced considerably in recent years [1–5], many aspects of the kinetics of this heterogeneous reaction are not fully understood. The nature and surface structure of the electrode is a fundamental aspect to consider when one or several intermediates can adsorb on the electrode surface. This is specially the case for oxygen reduction, in which four electrons are involved in the final reduction to water. The high number of electrons exchanged per oxygen molecule would imply the possible existence of several *
Corresponding author. Fax: +34-965-909301. E-mail address: [email protected] (J.M. Feliu).
0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.09.035
adsorption intermediates, whose interaction energies with the surface will depend on the metal and its surface structure. It is well known that platinum exhibits the highest catalytic activity for the 4 e pathway among the pure metals studied [4]. On this metal, the reaction is structure sensitive, the Pt(1 1 0) electrode being the most active basal surface in acidic media [6–11]. The activity of the electrode is also greatly affected by the presence of adsorbed anions. Thus, in perchloric acid solutions, the Pt(1 1 1) electrode is more active than the Pt(1 0 0) electrode, whereas in sulfuric acid solutions the situation is reversed. Moreover, the reaction is strongly inhibited when other strongly adsorbing anions, such as chloride or bromide, are present in the solution [12,13]. Therefore, the strength of the anion adsorption and its surface structure has always to be considered in order to understand better the effect of surface structure on the oxygen reduction reaction. Most of the studies of structure sensitive reactions deal with the comparison of the different low index surface electrodes. It is well known that these electrodes
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have different amounts of surface defects that contribute unexpectedly to the reaction kinetics. These defects play an important role in CO oxidation on Pt(1 1 1) electrodes [14]. On the other hand, anion adsorption shows especial characteristics when it occurs on Pt(1 1 1) wide terraces, as compared to the behavior observed for stepped surfaces with relatively narrow terraces [15]. In order to extrapolate the results obtained with well-defined basal planes to the smaller domains, such as those present in nanosized crystallites, it is necessary to perform studies in which the terrace size can be controlled. This can be achieved by studying the behavior of the structure sensitive reactions on stepped surface electrodes. The aim of this work is to study the oxygen reduction reaction on platinum single crystals belonging to the ½0 1 1 zone in two different supporting electrolytes, 0.1 M HClO4 and 0.5 M H2 SO4 . These surfaces can be classified in two different series, i.e., surfaces with terraces with (1 1 1) symmetry and (1 0 0) steps and surfaces with (1 0 0) terraces and (1 1 1) steps. The use of stepped surfaces allows the determination of the effect of the steps and the symmetry of the terrace on the oxygen reduction reaction. The extrapolation of the observed behavior for the stepped surface to the basal planes, i.e., Pt(1 1 1) and Pt(1 0 0), provides insight into the role of the small amount of defects always present in the real surfaces and also the effect of the long range order in the reactions.
2. Experimental Working electrodes were made from platinum single crystal beads, obtained by fusion and subsequent slow crystallization of a 99.999% platinum wire (Goodfellow Metals in all platinum samples). After careful cooling, the resulting single crystal beads were oriented, cut and polished following the procedure described in [16]. Prior to any experiment, the working electrodes were heated again for 10 s in a gas-oxygen flame, cooled in a reductive atmosphere (H2 + Ar), quenched in ultrapure water in equilibrium with this atmosphere and then transferred to the electrochemical cell. STM results indicate that the surfaces obtained after this treatment correspond to the nominal surfaces [17,18]. The stepped platinum electrode surfaces (2 mm diameter) used in this work were those located in the stereographic triangle between the (1 1 1) and (1 0 0) poles, which correspond to the crystallographic ½0 1 1 zone. These surfaces can be classified in two different series, i.e., surfaces with terraces with (1 1 1) symmetry and (1 0 0) steps and surfaces with (1 0 0) terraces and (1 1 1) steps. The Miller indices for the surfaces with (1 1 1) terraces are (n þ 1,n 1,n 1), which also can be denoted as Pt(S)[n(1 1 1) (1 0 0)] in the terrace-step
notation proposed by Lang et al. [19] (n is the number of atoms in the terrace). The surfaces with (1 0 0) terraces have Miller indexes (2n 1,1,1), which are equivalent to Pt(S)[n(1 0 0) (1 1 1)]. The Pt(3 1 1) surface is the turning point in the zone since it can be denoted as Pt(S)[2(1 0 0) (1 1 1)] or Pt(S)[2(1 1 1) (1 0 0)]. The voltammetric profiles of the surfaces employed in this work in 0.5 M H2 SO4 solution are shown in Fig. 1. A full description of the voltammetric behavior of the electrodes in this zone can be found elsewhere [18,20– 22]. The main voltammetric characteristics of the electrodes are the following. The sharp spike at 0.45 V present in the voltammetric profile of the Pt(1 1 1) electrode is associated with the order–disorder transition in the adsorbed (bi)sulfate layer. This peak disappears for the Pt(n þ 1,n 1,n 1) stepped surfaces [23,24], even for those having relatively wide (1 1 1) terraces. As the (1 0 0) step density increases from the Pt(1 1 1) electrode, the charge under the peak at 0.27 V, which corresponds to the adsorption states on this step [20], also increases, together with a diminution of the charge associated with hydrogen adsorption (between 0.06 and 0.35 V) and (bi)sulfate (between 0.35 and 0.6 V) on the (1 1 1) terraces. For the Pt(3 1 1) electrode, the peak has shifted to 0.28 V and comprises the maximum charge, as expected for the surface with the highest step density of the series. Moving towards the Pt(1 0 0) surface, a new signal centered at 0.36 V appears, which corresponds to the adsorption states on the (1 0 0) terrace. Conversely, the signal linked to hydrogen adsorption on the (1 1 1) step (between 0.06 and 0.2 V) diminishes [18]. Oxygen reduction experiments were carried out in a hanging meniscus rotating disk (HMRD) configuration [25] in an oxygen-saturated cell (oxygen pressure in the cell atmosphere is 1 atm). For this purpose, an electrode holder for the bead-type single crystal electrodes was
500 400
300
200 100 0 Pt(100) Pt(39,1,1) Pt(11,1,1) Pt(711) Pt(511) Pt(311) Pt(211) Pt(533) Pt(755) Pt(544) Pt(15,13,13) Pt(111)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Fig. 1. Voltammetric profile of the electrodes used in this work in 0.5 M H2 SO4 . Only the positive scan is shown. Scan rate: 50 mV s1 .
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143
0 0 rpm
-2 500 rpm 1000 rpm
-4
1500 rpm 2000 rpm 2500 rpm
-6
3000 rpm
0.0
Fig. 2. Comparison of the voltammetric profile of the Pt(1 0 0) electrode obtained at 2000 rpm (––) and 0 rpm (- - - -) in 0.5 M H2 SO4 . Scan rate: 50 mV s1 .
adapted to the Radiometer EDI10K rotor. The electrode was placed in the holder so that its surface was perpendicular to the rotation axis and centered (as much as possible) with respect to this axis. With this arrangement, the maximum rotation rate that can be achieved, without breaking the meniscus, is 4000 rpm. Since the solution cleanliness requirements for the HMRD experiments are more stringent due to the forced convention, special care was taken to assure it. For this purpose, the voltammetric profiles of the electrodes in a de-oxygenated test electrolyte were recorded under rotating conditions and compared to those obtained at 0 rpm, taken as the reference. Fig. 2 shows a typical example of the voltammetric profile of a Pt(1 0 0) electrode recorded in 0.5 M H2 SO4 . In the reported case, the negligible diminution of the peak at 0.36 V observed under rotating conditions is proof of the cleanliness (the charge of the overall voltammogram has diminished to less than 2%). Solutions displaying higher surface blockage were discarded. Experiments were carried out at room temperature, 20 °C, in a classical two-compartment electrochemical cell including a platinum counterelectrode and a reversible hydrogen electrode (RHE) as reference. Solutions were prepared with H2 SO4 (Aldrich, twice distilled) and perchloric acid (Merk suprapur) in ultra pure water. H2 , O2 and Ar (N50, Air Liquide) were also employed.
3. Results and discussion 3.1. Hydrodynamic behavior of the HMRD electrode Fig. 3 shows the voltammetric profiles for oxygen reduction at different rotation rates on Pt(1 1 1) elec-
0.2
0.4
0.6
0.8
1.0
Fig. 3. Oxygen reduction current recorded on a HMRD Pt(1 1 1) electrode in 0.5 M H2 SO4 at different rotation rates. Scan rate: 50 mV s1 .
trodes in 0.5 M H2 SO4 . The curve shows the characteristic behavior reported previously [5–8,11,26]. The onset for oxygen reduction is 0.8 V, and the limiting current is obtained between 0.3 and 0.5 V. The main feature of these curves is the appearance of two current drops at potentials below 0.3 V, instead of the single drop reported previously [5]. The first current drop coincides with the beginning of hydrogen adsorption (0.3 V). At 0.15 V, the current measured is 82% of the limiting current at 500 rpm and 67% at 2500 rpm. Then, the current drops again at 0.13 V. At this potential, hydrogen coverage on the electrode surface, as determined by CO displacement measurements, corresponds to half a hydrogen monolayer. Ring-disk experiments indicated that this drop is related to the formation of H2 O2 [5,8]. The presence of adsorbed hydrogen on the electrode surface prevents the scission of the O–O bond in some cases, and therefore only two electrons are exchanged in the reduction process for these O2 molecules [5,6]. The observed dependence of the current drop on the rotation rate is related to the possibility of further reduction of the H2 O2 formed. Since the reduction of H2 O2 is not completely inhibited at these potentials, some of the newly formed H2 O2 molecules can be reduced to water. In these cases, four electrons will be exchanged per O2 molecule. For high rotation rates, these molecules will diffuse fast away from the surface. This fact will prevent any further reaction and a higher drop in the total current is observed. Another characteristic of the curves is the presence of the spike associated with the order/disorder transition in the (bi)sulfate adlayer at 0.45 V superimposed on the reduction current. This is additional proof of the cleanliness of the solutions employed and indicates that neither the electrode surface nor the adsorbed (bi)sulfate layer is perturbed by the oxygen reduction reaction. The
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presence of wide two-dimensional domains, as is evident from the presence of the spike, is related directly to the particular behavior of this electrode surface as compared to the stepped surface having narrow (1 1 1) terraces, as will be discussed below. In order to obtain kinetic parameters from the oxygen reduction reaction the hydrodynamic behavior of the system has to be characterized. A detailed study of the hydrodynamic properties of these electrodes has been carried out by Villulas et al. [25,27,28]. In this work, the limiting current of the electrode for a reduction process is described by jlim ¼ 0:62nFD2=3 m1=6 cb x1=2 ð1 kx1=2 Þ;
ð1Þ
where D is the diffusion coefficient, m, the kinematic viscosity, cb , the bulk concentration, x, the rotation rate and k is a parameter for the system that depends on the meniscus height and electrode diameter. This equation is a modification of the Levich equation, which describes the hydrodynamic behavior of the rotating disk electrode. The correction term ð1 kx1=2 Þ accounts for the reduction of the effective electrode radius promoted by the change in the flow direction in the electrode edges [27,28]. Since the electrodes employed in this work are smaller and the electrode arrangement is different, the first step is to verify whether Eq. (1) holds under the present experimental conditions. Using the values for D and cb for oxygen and m for a H2 SO4 solution at pH ¼ 0 [29], Eq. (1) yields: jlim =mA cm2 ¼ 0:339x1=2 þ k:
ð2Þ
The plot of the limiting current vs. x1=2 is shown in Fig. 4. The line fitted to the experimental data has zero intercept (within the experimental error) and a slope of 0.369 ± 0.07 mA cm2 s1=2 . The slope is very close to the
theoretical Levich slope given in Eq. (2). The zero intercept has also been obtained for all the electrodes employed in this work and in experiments using ferricyanide reduction as the test reaction, indicating that a negligible value of k is characteristic of the present configuration of the HMRD. It is also worth noting that the limiting current obtained for a given rotation depends slightly on the meniscus height; ca. a 5% difference in the limiting current can be found between different experiments for the same electrode, each one starting after the flame annealing step. Therefore, the hydrodynamic behavior of the HMRD electrodes in the present configuration is quite well described by the Levich equation, within reasonable experimental error. This result is in disagreement with those obtained in [26–28], where the zero intercept is observed only for low meniscus heights with the electrolyte wetting the walls of the electrode. The wetting of the electrode walls can be discarded in the present case and the accidental occurrence of this situation can be easily identified. For instance, significant currents are observed for the Pt(1 0 0) electrode between 0.1 and 0.2 V when electrolyte wets the electrode walls. As can be seen in Fig. 2, no signal from the walls is observed, and therefore, the differences have to be due to different experimental conditions. The main difference in the set up employed in this work is the use of small hemispherical electrodes for which the rotation axis may not be completely perpendicular to the electrode surface. Although the electrode surface is always centered as much as possible with respect to the rotation axis, a small eccentric rotation of the meniscus can never be avoided. This fact prevents the attainment of rotation rates higher than 4000 rpm, but probably improves the mass transfer process to the whole surface, thus diminishing k to negligible values. In order to study the kinetics of the oxygen reduction reaction, the kinetic current density that would have been obtained in the absence of mass transport limitations (jtrans ) has to be calculated. For a first order reaction, the following relationship is obtained: 1 1 1 ¼ þ : j jtrans jlim Solving for jtrans : j jlim : jtrans ¼ jlim j
ð3Þ
ð4Þ
If the system behaves according to the Levich equation, Eq. (3) can be rewritten as 1 1 1 ¼ þ : 2=3 j jtrans 0:62nFD m1=6 cb x1=2
Fig. 4. Plot of jlim vs. x1=2 for oxygen reduction on a Pt(1 1 1) electrode in 0.5 M H2 SO4 .
ð5Þ
jtrans can then be obtained from the intercept of the plot of 1=j vs. 1=x1=2 [30]. Fig. 5 displays the values of jtrans at different potentials calculated according to Eqs. (4) and (5). As can be seen, both procedures give identical re-
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0.0
100
0.2
0.4
145
0.6
0.8
1.0
0.8
1.0
0
(a) -1
10
-2 -3
1
-4 -5
0.1
-6 Pt(111) Pt(15,13,13) Pt(211) Pt(511) Pt(11,1,1) Pt(100)
-7 0.0
0.01
(b) -0.2
0.4
0.6
0.8
1.0
-0.4 -0.6
Fig. 5. jtrans values vs. potential calculated for a Pt(1 1 1) electrode in 0.5 M H2 SO4 . ðjÞ Values calculated from the intercept of the plot of j1 vs. x1=2 , using Eq. (5). ðsÞ values calculated applying Eq. (4) to the curve at 3000 rpm from Fig. 3.
-0.8 -1.0 -1.2 0.0
sults for the calculation of jtrans , thus corroborating that the Levich equation describes the hydrodynamic behavior of the system. At potentials below 0.5 V, the accuracy of both methods diminishes significantly since the measured current value is very close to the limiting current and therefore the calculated values of jtrans are no longer reliable. 3.2. Voltammetric behavior of oxygen reduction on platinum electrodes on the [0 1 1] zone in sulfuric acid solutions Once the hydrodynamic behavior of the system has been established, oxygen reduction on the different electrodes can be studied systematically. Fig. 6 presents a selection of the curves for oxygen reduction on different electrodes in 0.5 M H2 SO4 . Owing to the small variation of the limiting current with the meniscus height, the curves have been normalized to the absolute value of the limiting current ðjjlim jÞ, for a better comparison between them. As can be seen, nearly all the curves for oxygen reduction have a very similar value for the onset of oxygen reduction. Two curves stand out from the rest: the Pt(1 1 1) and Pt(2 1 1) electrodes, which show the lowest and the highest catalytic activities, as will be discussed later. Another important characteristic of the curves is the diminution of the current density at negative potentials (below 0.3 V). In the two series of electrodes, Pt(1 1 1) and Pt(1 0 0) electrodes exhibit the highest drop in the current at negative potentials. As the step density increases, the onset of the current density drop is shifted toward more negative potentials and the total drop is
0.2
0.4
0.6
Fig. 6. (a) Curves for oxygen reduction on selected Pt(h k l) electrodes of the ½0 1 1 zone in 0.5 M H2 SO4 at 1500 rpm. (b) Curves of the panel A normalized to jjlim j. Scan rate 50 mV s1 .
smaller. As aforementioned, this drop in the current is related to the formation of H2 O2 [5,8], and therefore a diminution in the total number of electrons exchanged in the process of oxygen reduction. It appears that H2 O2 formation is smaller on electrodes having narrower terraces. This change in the oxidation mechanism can be related to hydrogen adsorption on the surfaces. In this potential region, in which the current drop is observed, oxygen must compete with hydrogen for the adsorption sites. A strong hydrogen adsorption and/or a high coverage hinder oxygen adsorption on the platinum surface. Moreover, two neighboring Pt sites are required to break the O–O bond in order to form H2 O as the final product. It seems that the O–O bond cannot be broken when all the neighboring sites to an adsorbed O2 molecule are blocked by strongly adsorbed species, yielding only H2 O2 as the final product. In the two series, Pt(1 1 1) and Pt(1 0 0) electrodes have the most positive onset for hydrogen adsorption (see Fig. 1), and the highest hydrogen coverage at a given potential [18,22]. Hydrogen adsorption on the step sites occurs at more negative potentials than in the terrace sites. In this way, hydrogen adsorption on the (1 1 1) steps occurs between 0.06 and 0.2 V [18], whereas on the (1 0 0) steps, the hydrogen adsorption peak is located at 0.27 V. Consequently, at a given potential, it will be more difficult to
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find two neighboring available sites for oxygen adsorption on the terrace than on the step, and therefore the Pt(1 1 1) and Pt(1 0 0) will exhibit the higher current drop at more positive potentials. This inhibition of the four-electron mechanism due to strongly adsorbed species has also been found for chloride on Pt(1 0 0) electrodes and bromide on Pt(1 1 1) and Pt(1 0 0) electrodes [12,13]. Other strongly adsorbed adatoms, such as silver, selenium and sulfur on polycrystalline platinum, inhibit the scission of the O–O bond, yielding only H2 O2 as the final product at high adatom coverages [31,32]. However, adsorbed (bi)sulfate does not prevent the scission of the O–O bond, although it is adsorbed on the electrode surface at potentials higher than 0.35 V on Pt(1 1 1) and Pt(1 0 0) electrodes [15,33]. When compared to chloride and bromide, (bi)sulfate adsorption is weaker, and therefore, the adsorbed (bi)sulfate anions can be more easily displaced by the incoming O2 molecules. Furthermore, the analysis of the STM images [23,24] and the comparison with the coverage values obtained by radiotracer techniques [34] and coulometry [15] for (bi)sulfate adsorption on Pt(1 1 1) electrodes indicate that water is coadsorbed on the (bi)sulfate adlayer. This fact suggests that the (bi)sulfate adlayer is quite open and flexible, and can accommodate the oxygen molecules adsorbed on two platinum sites to yield H2 O. The presence of the (bi)sulfate adsorbed on the electrode surface will then shift the curve for oxygen reduction toward more negative potential (as compared to that obtained in the absence of strongly adsorbed anions), since O2 should compete for the adsorption sites but it will not prevent the formation of H2 O. On the other hand, the particular properties of this adlayer on wide Pt(1 1 1) domains are clearly pointed out. Other effects, such as the possible diminution of the adsorption strength of the (bi)sulfate as the step density increases could also be possible. However, the relevant data in order to consider such effects are not available.
yields a different Tafel slope in this region [35]. The Tafel slopes obtained in all cases lie between 120 and 130 mV, which indicates that the rate determining step is the first electron transfer, assuming that O2 adsorption follows a Langmuir isotherm. Extrapolating the linear behavior in the Tafel region to the equilibrium potential for the O2 –H2 O system (which under the present conditions is 1.23 V vs. RHE), the values of j0 for the different electrodes can be obtained. In Fig. 7, j0 values have been plotted versus the surface angle with respect to the Pt(1 1 1) surface. Because of the voltammetric behavior, the Pt(1 1 1) electrode shows a much lower electrocatalytic activity than the rest of the electrodes, even when compared to electrodes with wide (1 1 1) terraces. The difference between the Pt(1 1 1) electrode and the surface with the widest (1 1 1) terraces, Pt(15,13,13) with 14 atom-wide (1 1 1) terraces, is much higher than that between this latter electrode and the rest of the different electrodes in the series. On the other hand, Pt(2 1 1) displays the highest catalytic activity of the surfaces studied, since the onset for oxygen reduction occurs at the most positive potentials. This surface, which has three atoms on the (1 1 1) terrace and a (1 0 0) step, is, according to these results, the most active surface structure. It is also worth mentioning that the electrocatalytic activity is not completely associated with the step density, since the surface with the highest step density, Pt(3 1 1), has a lower catalytic activity than that of the Pt(2 1 1) electrode. Interestingly, the higher catalytic activity of the Pt(2 1 1) electrode with respect to the Pt(1 1 1) electrode is in agreement with the computed interaction of O 2
3.3. Kinetics of oxygen reduction on platinum electrodes on the [0 1 1] zone in sulfuric acid solutions The values of jtrans as a function of potential have been obtained for all the curves using Eq. (4). The Tafel plots constructed with these values show a wide linear range (over 300 mV) (see for instance Fig. 3), which allows the determination of the values of the Tafel slope and exchange current density ðj0 Þ. In all cases, a single Tafel slope is obtained in the potential range studied (between 0.9 and 0.5 V). Unlike other studies where two Tafel slopes are obtained for some electrodes [6,8], oxidation of the surface has been avoided to prevent the disordering of the surface that takes place when the surface oxides are reduced. It is known that the presence of oxides alters the kinetics of oxygen reduction and
Fig. 7. Plot of j0 for oxygen reduction vs. the angle of the surface with respect to the Pt(1 1 1) surface in ðjÞ 0.5 M H2 SO4 and ðsÞ 0.1 M HClO4 .
M.D. Macia et al. / Journal of Electroanalytical Chemistry 564 (2004) 141–150
with the Pt(1 1 1) (0.7 eV) [36] and Pt(2 1 1) (0.9 eV) [37]. As aforementioned, a Tafel slope value of 120 mV indicates that the first electron transfer is the rate-determining step. For this reaction this step is probably: O 2 þ e O 2;ads
ð6Þ
The higher adsorption energy for the Pt(2 1 1) electrode will indicate that the reaction is favored on the Pt(2 1 1) electrode, provided that this higher adsorption energy does not prevent the further reaction of O 2 from proceeding. The ‘‘ideal’’ behavior of the Pt(1 1 1) and Pt(1 0 0) can be obtained by extrapolation to zero step density of the observed behavior for the stepped surfaces. When dealing with two different series of electrode surfaces, two representations have to be made for each series since the nature of the step is different for each series. If both representations have to be combined, the angle between a given surface and the Pt(1 1 1) electrode will be used alternatively, since it is proportional to the step density. In Fig. 7, the straight line fitted for the j0 values for the surfaces in the Pt(2n 1,1,1) series agrees well (within the experimental error) with the observed j0 value for the Pt(1 0 0) electrode. On the other hand, the extrapolated j0 value for the Pt(1 1 1) electrode is ca. one order of magnitude higher than that observed experimentally. The deviation of the actual values from the extrapolated behavior can have two different origins: (a) the presence of defects in the real surface and (b) long range effects not observed on the stepped surfaces. The effect of the presence of defects in the real surfaces has been shown, for instance, for CO oxidation [14] or formic acid dissociation [38] on Pt(1 1 1) electrodes. For CO oxidation on Pt(1 1 1) electrodes, the behavior extrapolated from the stepped surface would indicate that the Pt(1 1 1) electrode would be almost inactive for CO oxidation. Since the catalytic activity for CO oxidation on the small amount of defects present in a real Pt(1 1 1) electrode is orders of magnitude higher than that of the basal plane, the experimental rate for CO oxidation is dependent only on the number of defects on the surface, and is not related to the activity of the (1 1 1) sites. In this case, the extrapolated activity of the ‘‘ideal’’ Pt(1 1 1) surface is lower than that of the real surface, which is the opposite behavior from that observed here for the oxygen reduction reaction. Therefore, the deviation observed for the real Pt(1 1 1) surface cannot be due to the presence of defects. In this latter case, the step sites have higher catalytic activity for oxygen reduction than the basal planes, but the increase of the catalytic activity with the number of step sites is rather small. For this reason, the extrapolation of the observed behavior for the surfaces in the Pt(2n 1,1,1) series to the Pt(1 0 0) electrode is in agreement with the experimental result, despite the relatively high number of defects present in a real Pt(1 0 0) electrode [18,39].
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Since the presence of defects should lead to a higher j0 value for the real Pt(1 1 1) electrode, as compared to the extrapolated value, the deviation has to be attributed to the presence of long-range effects on the Pt(1 1 1) electrode not observed on the stepped surfaces. On Pt(1 1 1) electrodes, (bi)sulfate forms an ordered structure which is very sensitive to long-range order. The spike at 0.45 V, which is associated with the disorder– order transition, is observed only for terraces wider than 20 atoms. This disorder–order transition involves a significant increase of the (bi)sulfate coverage. If the coverage values obtained for stepped surfaces (for which the disorder–order transition does not take place) are extrapolated to the Pt(1 1 1) electrode, the real coverage values are 15% higher than those obtained from the extrapolation [40]. This 15% of additional (bi)sulfate species are adsorbed on the electrode upon ordering of the (bi)sulfate adlayer. It is well known that this spike is very sensitive to surface perturbations, such as small amounts of surface contamination of organic or inorganic adsorbed species, and this latter problem should be avoided in order to observe this behavior properly. As aforementioned, oxygen reduction takes place in this medium at potentials where (bi)sulfate is adsorbed on the electrode surface. For this reason, the coverage of the (bi)sulfate adlayer affects the behavior of the electrode, as has been previously reported [5,10,11]. The difference between the extrapolated and real j0 value will then indicate the effect of these additional (bi)sulfate species that are adsorbed upon ordering in the oxygen reduction reaction. Therefore, the low catalytic activity of the Pt(1 1 1) electrode with respect to the rest of the electrodes in sulfuric acid media is clearly related to the formation of the bidimensionally ordered adlayer, which is more stable than the disordered adlayers formed on the other electrodes, and inhibits the oxygen reduction on the Pt(1 1 1) electrode. 3.4. Kinetics of oxygen reduction on platinum electrodes on the [0 1 1] zone in perchloric acid solutions In order to prove the effect of the ordered (bi)sulfate adlayer in the oxygen reduction kinetics on the Pt(1 1 1) electrode, the reaction was also studied in 0.1 M HClO4 . Fig. 8 shows the voltammetric profiles in the HMRD configuration obtained for this medium. The first significant difference between the curves in sulfuric and perchloric acid are that the onset of O2 reduction has shifted more than 100 mV towards more positive potentials, as expected for a weakly adsorbing electrolyte. Another important difference is that no significant variation between the Pt(1 1 1) electrode and the other curves in its series are observed. In fact, all the curves in Pt(n þ 1,n 1,n 1) almost overlap in the same region. A similar situation is observed for the curves in the other series. However, the currents for the electrodes in the
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0.0
0.2
0.4
0.6
0.8
1.0
0 -1
(a)
-2 -3 -4 -5 -6 -7 -8
Pt(111) Pt(15,13,13) Pt(211) Pt(511) Pt(11,1,1) Pt(100)
0.0
(b) -0.2 -0.4 -0.6
Fig. 9. Tafel slope values obtained for oxygen reduction in 0.1 M HClO4 on the different electrodes.
-0.8 -1.0 -1.2 0.0
0.2
0.4
0.6
0.8
1.0
Fig. 8. (a) Curves for oxygen reduction on selected Pt(h k l) electrodes of the ½0 1 1 zone in 0.1 M HClO4 at 2000 rpm. (b) Curves of the panel A normalized to jjlim j. Scan rate: 50 mV s1 .
Pt(n þ 1, n 1, n 1) series are significantly higher than those obtained for the Pt(2n 1,1,1) at a given potential. The values of jtrans obtained using Eq. (4) were used to calculate the values of j0 and the Tafel slope for each electrode. For this medium, the linear region is observed between 0.6–0.7 and 0.9 V. Unlike the behavior observed in 0.5 M H2 SO4 , where the Tafel slope was 120 mV for the whole zone, the values for the Tafel slopes range between 90 and 120 mV (Fig. 9). The values of 90 mV for the Tafel slope are obtained for the electrodes in the Pt(n þ 1,n 1,n 1) series. However, similar low slope values have been obtained in basic media, and these results have been interpreted taking into account that OH adsorption takes place in the same potential region where the Tafel slope was calculated [35]. A qualitative treatment was proposed in that work in which it was assumed that the electrode sites covered by OH do not contribute to the reduction reaction. For the Pt(n þ 1,n 1,n 1) electrodes, OH adsorption also takes place between 0.6 and 0.9 V, which can also interfere with the reduction reaction. For these cases, the current density for oxygen reduction can be defined as [35]: aF jtrans ¼ 4Fkcb ð1 hA Þ exp E ; ð7Þ RT
where k is the kinetic constant, a, the electron transfer coefficient and hA is the OH coverage of the electrode (or any other anion strongly adsorbed on the electrode surface). This equation represents only a qualitative approach since the OH covered sites should also contribute to the reduction process, but probably with a much lower rate constant. Then, the experimental Tafel slope for Eq. (7) is defined as dE 1 ¼ mTafel ¼ : ð8Þ RT d logjjj þ 1 dhA 2:3aF
1hA dE
As can be seen, the experimental Tafel slope contains an extra term related to the variation of the OH coverage in this region in addition to the term that contains the electron transfer coefficient. Since the value of dhA =dE is positive, the experimental values of the Tafel slope are lower than those found when hA is constant. Therefore, for the electrodes in the Pt(n þ 1,n 1, n 1) series, the lower Tafel slopes in HClO4 are not due to a different kinetic mechanism, but to the interference of the OH adsorption/desorption on the oxygen reduction reaction. If the contribution for the adsorbed OH species were eliminated, the Tafel slope values would have been similar to those found in sulfuric acid solutions. This effect is not observed in sulfuric acid solutions since the (bi)sulfate coverage is nearly constant at potentials above 0.6 V [40] and therefore the term dhA =dE is negligible in the region where Tafel slopes are determined. The values of j0 for this medium were calculated using the experimental Tafel slope values. In an exact treatment, the effect of the OH would have to be evaluated in order to calculate the expected currents for zero OH coverage and then extrapolate these values to the equilibrium potential. Since the model in Eq. (7) repre-
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sents only a qualitative approximation to the effect of OH adsorption in the current, the experimental currents and Tafel slopes, without further corrections, were used. Therefore, the real j0 values for the electrodes in the Pt(n þ 1,n 1,n 1) series would be higher than those calculated neglecting the OH effects. The j0 values obtained in 0.1 M HClO4 are plotted in Fig. 7. As expected, the exchange current densities are higher that those obtained in 0.5 M H2 SO4 , highlighting the effect of the adsorbing anions in the overall kinetics. For this supporting electrolyte, the catalytic activity of the stepped surfaces is higher that that for the basal planes; however, the difference between them is rather small. Another important characteristic of the results is that the extrapolated values for the Pt(1 1 1) and Pt(1 0 0) electrodes agree well with the experimental values, thus confirming that the unusual low activity of the Pt(1 1 1) electrode is due to the formation of an ordered (bi)sulfate adlayer. The small difference in the catalytic activity for the stepped surfaces in comparison to the basal planes is rather unexpected, since UHV measurements indicate that oxygen is preferentially adsorbed on the step sites [41]. In fact, the dissociative adsorption process of oxygen on Pt(1 1 1) and Pt(5 3 3) is controlled by the presence of defects or steps [42]. A direct extrapolation of the UHV data would have suggested a significantly higher catalytic activity for the step sites in comparison with the basal planes. As aforementioned, in the electrochemical environments, the adsorption process is always competitive in nature and O2 adsorption must compete for the adsorption sites with other species present in the aqueous solution. Owing to the Smoluchowski effect, the step sites will have a lower local work function as compared to the terrace sites. Using the relationship between the pzc (potential of zero charge) and the work function [43], the step sites would have a lower local pzc than the terrace sites. In the case of surfaces with Pt(1 1 1) terraces, such a relationship between the pzc and the work function has been observed [22]. Therefore, at a given electrode potential, anion adsorption will be stronger on the step sites, as compared to the terrace sites. For instance, the much higher catalytic activity of the step sites for the CO oxidation reaction has been associated with the adsorption of OH on these sites [14]. The adsorbed CO molecules on the terrace will diffuse until they reach a step site covered by OH, where they react to yield CO2 . Therefore, it can be proposed that the step sites are already covered by OH, and oxygen must displace the adsorbed OH to react on these sites. The higher adsorption energy for O2 on the step sites is then counterbalanced with a higher adsorption strength of the OH species on these sites, resulting in a lower catalytic activity than the expected value from UHV experiments.
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Acknowledgements This work was supported by the Spanish DGI through Project no. BQU2003-0429 and by the European Union Framework V Growth Programme, CLETEPEG project, Contract no. G5RD-CT-200100463.
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