Formic Acid Oxidation on Shape-Controlled Pt Nanoparticles Studied

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Formic Acid Oxidation on Shape-Controlled Pt Nanoparticles Studied by Pulsed Voltammetry Vitali Grozovski, Jose´ Solla-Gulloń, Vıćtor Climent, Enrique Herrero,* and Juan M. Feliu Instituto de Electroquı´mica, UniVersidad de Alicante, Apdo. 99, E-03080 Alicante, Spain ReceiVed: May 24, 2010; ReVised Manuscript ReceiVed: July 5, 2010

Pulsed voltammetry was used to study formic acid electrooxidation on Pt nanoparticles with well-characterized surfaces. Polyoriented and preferential (100), (100-111), and (111) Pt nanoparticles were characterized and employed to evaluate the influence of the surface structure and shape of the Pt nanoparticles on this model electrochemical reaction. The results pointed out that, in agreement with fundamental studies with Pt single crystal electrodes, the surface structure of the electrodes plays an important role on the reactivity and kinetics of formic acid oxidation. Thus the electrocatalytic properties for this reaction strongly depend on the dominant structure on the surface of the nanoparticles, in particular on the presence of domains with (100) and (111) symmetry. Among the Pt nanoparticles studied, those containing (100) domains are clearly the most active toward formic acid electrooxidation via the active intermediate path, but also exhibit the highest poisoning rate. (111) Pt nanoparticles show the lowest CO formation constant in the series and a moderate reaction rate via the active intermediate reaction path. 1. Introduction The study of nanoparticles is, at present, a hot topic with strong projection in the fields of both catalysis and electrocatalysis.1 In the latter case, these studies have been carried out with two main objectives: (1) the understanding of fundamental aspects of electrochemical reactivity and (2) the development of new materials for practical applications, in particular, fuel cells. One of the most interesting questions in nanoparticle electrochemistry is to understand how the electrocatalytic activity is influenced by the surface structure and shape of the nanoparticles. In this regard, since the majority of electrocatalytic reactions are structure sensitive or site demanding,2-7 it is of outstanding importance to achieve an effective control of the crystalline surface structure of the nanoparticles for a wellfounded comparison of the electrocatalytic activity of different electrocatalysts. Regarding this issue, the influence of the surface structure/shape of the nanoparticles in reactions of relevance such as ammonia oxidation, oxygen reduction, and CO electrooxidation was extensively studied in our group in the past.8-13 Formic acid oxidation on platinum is considered a model reaction in organic fuel electrocatalysis. This is because the oxidation of formic acid is a simple structure sensitive process that involves only two electrons. The reaction occurs through a dual path mechanism, earlier summarized by Capon and Parsons,14,15 whose understanding could be transferred to more complicated cases of technological importance, such as methanol, ethanol, or ethylene glycol oxidation (which also involves a dual path mechanism16):

* To whom correspondence should be addressed. E-mail: [email protected].

As both paths are structure sensitive, the reaction rate strongly depends on the surface structure of the electrode. Adsorbed CO, produced through a dehydration reaction, was identified as the poison intermediate by infrared spectroscopy.17-20 Adsorbed formate has been detected also in IR experiments, and for that reason, it has been proposed as the active intermediate.21 However, the role of formate in the whole mechanism is still under discussion.22-24 Nowadays, direct oxidation of formic acid has also an increased interest for practical purposes, especially for the direct formic acid fuel cells.25 It is known that formic acid oxidation kinetics is strongly dominated by poison formation, which in turn depends on the surface structure. In this way, the almost negligible reactivity of the Pt(100) electrode in the positive going scan is a consequence of its fast poisoning.26 In the absence of poisoning, the intrinsic activity per platinum atom of the so-called active intermediate reaction path is as high as 100 molecules s-1.27 On the other hand, poison formation on the Pt(111) electrode is very small and probably associated to the presence of defects on the surface. However, the intrinsic activity of this surface for the direct oxidation is only 5 molecules/s, much smaller than that of the Pt(100) electrode.28 From the described behavior, it appears that a good catalytic activity for the active path is associated also with a high poisoning rate. However, the analysis of previous results indicates that the potential window and intermediates in the two paths are different, which suggests a different interaction on the formic acid molecule with the surface for each path.27,28 For that reason it is very important to determine the kinetic parameters of the reaction, in order to understand how the different properties of the surface affect it. In the case of the formic acid oxidation on shape-controlled Pt nanoparticles, a qualitative analysis of the voltammetric profiles pointed out that preferentially oriented nanoparticles possessing (100) domains are more prone to poison formation compared to the ones having (111) domains and that the electrocatalytic activity depended on the relative amounts of the different surface sites.29 It was also proposed that the different behavior for formic acid oxidation when the particle

10.1021/jp104755b  2010 American Chemical Society Published on Web 07/23/2010

Formic Acid Oxidation on Pt Nanoparticles size decreased in the range from 9 to 2 nm was the result of a “catalytic ensemble effect”.30 An important enhancement of the oxidation reaction rate was observed as the particle size decreases below 4 nm. This was attributed to the lack of a Pt site ensemble requirement for the CO poisoning process, as has been observed for platinum single crystal electrodes.31 Finally, for rounded Pt nanoparticles obtained after potential cycling, with a particle size higher than 4 nm, the reaction rate for the formic acid electrooxidation is similar to that observed on polycrystalline Pt.30 In the present paper we report recent results about the influence of the surface structure of well-characterized Pt nanoparticles on formic acid oxidation studied by pulsed voltammetry. This technique, based on the pulsed voltammetry initially designed by Clavilier,26 is useful to study both paths independently and has been used to determine the reaction kinetics parameters through the active intermediate path and the dehydration step (the step that leads to the formation of CO) for the Pt nanoparticles. To avoid any size complications, the nanoparticles used in this work have mean sizes higher than 4 nm. The aim is to establish links between reactivity and kinetics previously established for Pt electrodes with welldefined surface structures27-29,32 and the reactivity and kinetics of Pt nanoparticles with some preferential two-dimensional surface domains using the pulsed voltammetry technique. These results are also of fundamental importance for the understanding of the elementary processes that take place in formic acid oxidation on platinum nanoparticles, and the different elements that play a role in the reaction, i.e., crystallographic orientation, anion, hydrogen, and water adsorption. 2. Experimental Section In this work, four types of Pt nanoparticles used in previous cases were employed.29,33 Polyoriented nanoparticles (further notated in text as (poly)Pt) were synthesized by the water-inoil method described in refs 34 and 35. Preferentially oriented nanoparticles (100)Pt, (111)Pt, and (100-111)Pt were prepared by using a colloidal method.33 This notation introduced by our investigation group is very useful to describe preferentially oriented nanoparticles according to the symmetry of the predominant sites while keeping it easy to distinguish from the Miller indices used for the notation of the single crystal surfaces. The particle size and morphology of all the nanoparticle samples were characterized by transmission electron microscopy (TEM). Nanoparticles were deposited on a circular flat gold collector, which was mechanically polished with alumina to eliminate the nanoparticles from previous experiments and flame annealed to remove any contamination from the surface. The cleaning of the nanoparticles after preparation is an essential step in the electrochemical characterization process. The procedures used for the electrochemical cleaning of the different Pt nanoparticles have been previously described.33-35 This step is crucial because the classical electrochemical procedure to decontaminate platinum by electrochemical cycling cannot be used as high upper potential limits should be avoided to preserve the surface order of the synthesized nanoparticles. Thus, nanoparticles should be cleaned with reasonable care if their surface sensitive properties have to be compared and analyzed. It should be remarked that the cleaning procedures avoid significant (electro)chemical adsorption of oxygen, thus preserving as much as possible the integrity of the surfaces. They involve, in the final step, a CO adsorption and stripping treatment, a process that does not perturb significantly the platinum surface order as observed with platinum single crystals.36 CO adsorption and stripping is likely

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13803 to lead to better-ordered surfaces since it facilitates the incorporation of the less coordinated atoms in positions with higher coordination number.13 For the sake of comparison, platinum single crystal electrodes have also been used, as described previously.27,28 Voltammograms were first recorded in the blank solution and afterward the electrode was transferred to the second cell where voltammetric and pulsed voltammetry experiments for formic acid oxidation were carried out. The active surface area of the Pt nanoparticles was determined from the blank voltammograms by measuring the charge involved in the so-called hydrogen adsorption region assuming 230 µC cm-2 for the total charge after the subtraction of the double layer charging contribution recorded in 0.5 M H2SO4 solution.37 Formic acid oxidation experiments were performed in a 0.5 M H2SO4 + 0.1 M HCOOH solution. All experiments were carried out at room temperature, 22 ( 2 °C, in classical two-compartment electrochemical cells deaerated by using Ar (N50, Air Liquide in all gases used), including a large platinum counter electrode and a reversible hydrogen (N50) electrode (RHE) as reference. Solutions were prepared from sulfuric acid (doubly distilled, Aldrich), formic acid (Merck suprapur), and ultrapure water from Elga. The cleanliness of the solutions was tested by the stability of the characteristic voltammetric features of well-defined single crystal electrodes. The potential program for the transients was generated with an arbitrary function generator (Rigol, DG3061A) used together with a potentiostat (eDAQ EA161) and a digital recorder (eDAQ, ED401) connected to a personal computer. 3. Results and Discussion 3.1. Characterization of Pt Nanoparticles. As described in the Experimental Section four different types of platinum nanoparticles were selected for this work. Figure 1 shows some representative TEM images of these Pt nanoparticles. Species obtained by the water-in-oil method show a quasispherical shape (Figure 1A) with a particle size of 4.0 ( 0.6 nm. These nanoparticles can be considered as representative of polyoriented, nonspecifically structured nanoparticles. On the other hand, nanoparticles prepared with the colloidal method give larger size species with preferential shapes as a function of the experimental synthetic conditions.32,33 A typical TEM image of the cubic Pt nanoparticles ((100)Pt) prepared with this method is shown in Figure 1B. As can be observed, a preferential cubic shape is obtained, which suggests the presence of a predominant (100) preferential surface structure. The particle size is 8.2 ( 1.6 nm. Figure 1C corresponds to the (100-111)Pt nanoparticles. As is clearly seen, the projected shape of the Pt nanoparticles is predominantly hexagonal suggesting the coexistence of (100) and (111) surface domains. The particle size of this sample is 11.5 ( 1.7 nm. Finally, Figure 1D shows a representative TEM image of the (111)Pt nanoparticles, where an important number of Pt nanoparticles with tetrahedral and octahedral shapes is observed. In any case, the shapes suggest the presence of a preferential (111) surface structure. The Pt particle size of this sample is 8.6 ( 1.4 nm. TEM data give valuable information about mean size and distribution of the nanoparticles as well as about its shape. However, it is very difficult to obtain the actual surface structure of the “average” nanoparticle. As has been reported, electrochemical characterization can be used to “in situ” determine the surface structure of the nanoparticles.33 Figure 2 shows the characteristic voltammetric profiles of the different Pt nanoparticles in 0.5 M sulfuric acid supporting electrolyte. The sharpness

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Figure 1. TEM pictures of (A) polyoriented, (B) (100)Pt, (C) (100-111)Pt, and (D) (111)Pt nanoparticles.

and the symmetry of the voltammetric peaks are clear evidence of the surface cleanliness. Figure 2A shows the voltammogram obtained with the polyoriented nanoparticles. This voltammogram looks very similar to that reported for polycrystalline

platinum electrodes.38 The voltammogram shows the presence of adsorption states associated to (110) and (100) step sites at 0.12 and 0.26 V, respectively.33 Moreover, a very small shoulder around 0.32 V characteristic of short (100) terraces is detected.

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Figure 2. Voltammetric profiles for (A) polyoriented Pt, (B) (100)Pt, (C) (100-111)Pt, and (D) (111)Pt nanoparticles in 0.5 M H2SO4. Sweep rate: 50 mV s-1.

Also the unusual adsorption states typical of (111) ordered domains are visible around 0.5 V, although with a very low intensity. All these peaks can be observed with all the Pt nanoparticles used here, although with a different extent that reflects the different surface composition. Panels B, C, and D in Figure 2 show the voltammograms obtained for the preferentially oriented Pt nanoparticles where corresponding adsorption states are seen. Figure 2B corresponds to the cubic Pt nanoparticles. In comparison to the previous case, the main feature of the voltammogram is the sharpness of the peak at 0.26 V associated to (100) steps and also to the (100) terrace sites close to the steps. Also the well-marked state at 0.37 V, typical of wide (100) Pt terrace sites, is now much more evident than in the polyoriented surface, Figure 2A. Thus, the voltammetric profile clearly points out that these Pt nanoparticles have a (100) preferential surface structure and that a large part of these (100) sites are present at the surface as relatively wide (100) terraces, as suggested by the peak at 0.37 V, together with another one at 0.35 V, which also reflects the existence of shorter (100) terraces.33 Figure 2C shows voltammetric data corresponding to (100-111)Pt nanoparticles. The adsorption state around 0.5 V, which is characteristic of (111) ordered surface domains, is much more clearly marked than in the previous cases. This feature is directly related to the presence of bidimensionally ordered (111) domains. On the other hand, the sharp peaks at 0.12 and 0.26 V are also present with a similar intensity in the voltammetric profile. Similar sharp peaks are observed on stepped surfaces vicinal to Pt(111) single crystal with monatomic steps with (110) or (100) symmetry, respectively.39,40 Finally, well-marked adsorption states at 0.37 and 0.35 V are also observed pointing out that these nanoparticles also contain two-dimensional (100)

domains. Taking into account TEM data (Figure 1C) and voltammetric data it is clearly seen that the surface structure of this type of Pt nanoparticles is mainly formed by (100) and (111) surface domains, separated by “linear defect” sites with (100) and (110) symmetry. Finally, Figure 2D shows the voltammetric profile obtained with the (111)Pt nanoparticles characterized by the presence of preferential tetrahedral and octahedral shapes. In comparison with the previous graphs, the voltammogram presents two main features, a very sharp peak at 0.12 V and the largest and symmetrical contribution at 0.5 V. The first contribution is related to the presence of (110) surface sites and the second one to the presence of relatively large two-dimensionally ordered (111) Pt domains. It is also important to note the small contributions at 0.26 V due to (100) surface sites, likely as steps between the (111) ordered domains. The same can be said about two-dimensional (100) surface domains, located at the 0.35-0.37 V region, whose signal is very small. This practically negligible amount of (100) surface contributions makes it possible to observe a better reversibility in the characteristic adsorption states related to anion adsorption on the bidimensionally ordered (111) Pt domains, which was more difficult to distinguish in the previous samples due to the overlapping with the contribution coming from large (100) surface domains. The represented electrochemical data in Figure 2 as well as the TEM images in Figure 1 are in good agreement with nanoparticles reported in previous papers and prove the reproducibility of the methodology used in the experiments.29,32,33 The quantitative analysis of the site distribution for the different nanoparticles has been measured by using Bi and Ge irreversible adsorption as previously reported33,41,42 and the results have been presented in Table 1. In this table, the

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TABLE 1: Site Distribution on the Different Nanoparticle Samples Determined by Irreversibly Adsorbed Bi and Ge According to References 41 and 42 sample

(111) ordered domains/%

(100) ordered domains/%

(poly)Pt (100)Pt (100-111)Pt (111)Pt

9 18 30 42

18 40 32 3

remaining fraction that does not belong to (100) and (111) ordered domains corresponds to (100) and (110) step and defect sites on the surface. These results are in agreement with the qualitative analysis of the voltammograms, since the voltammetric response in sulfuric acid depends on the site distribution of the nanoparticles. 3.2. Formic Acid Oxidation: Voltammetric Results. The voltammetric profiles for the different types of Pt nanoparticles oxidizing formic acid in sulfuric acid supporting electrolyte are represented in Figure 3. The upper potential limit has been adjusted to 0.90 V in all experiments to avoid surface perturbations due to electrochemical oxygen adsorption at high potentials.43,44 Due to the formation of CO at low potentials,27 which blocks the electrode surface for the direct reaction, currents observed in all curves (Figure 3) in the positive-going scan are smaller than those in the negative-going scan. The same effect of surface poisoning is known and well-documented on Pt single crystal electrodes.45 At potentials close to the upper limit, although CO is readily oxidized and the surface is free from CO, the direct oxidation of formic acid on Pt is inhibited by the formation of strongly bounded water structures (as was reported for the Pt single crystal electrodes46,47), so that quite low currents are recorded in this region. From that potential value, currents increase during the negative-going scan yielding a voltammetric peak at 0.50-0.55 V, with a high current reflecting the low amount of poisoning accumulated during the potential excursion from the upper potential limit to this potential region.26-28 At potentials below 0.5 V, currents decrease again,

Figure 3. Voltammetric profiles for formic acid oxidation on (A) polyoriented Pt, (B) (100)Pt, (C) (100-111)Pt, and (D) (111)Pt nanoparticles in 0.5 M H2SO4 + 0.1 M HCOOH. Sweep rate: 50 mV s-1.

Figure 4. Voltammetric profiles for formic acid oxidation on Pt(100), Pt(110), and Pt(111) electrodes in 0.5 M H2SO4 + 0.1 M CHOOH. Sweep rate: 50 mV s-1.

because of the formation of adsorbed CO through the dehydration step, which cannot be oxidized at these potentials, and/or the diminution of the rate constant for the direct oxidation. Some qualitative differences are readily observed on the formic acid oxidation voltammetric profiles. In the case of the polyoriented nanoparticles (Figure 3A) the voltammetric profile is similar to that typically associated with the polyoriented single crystal Pt electrodes.29 In the positive going sweep a first peak is observed at 0.5 V, where current is associated with the direct formic acid oxidation on the Pt sites nonblocked by CO. This peak is also observed in all the studied nanoparticles, indicating that there always exists a fraction of the surface sites (likely related to (111) symmetry domains) not poisoned by CO. It is important to note that currents in the positive scan presented here are higher than those in ref 29, not only due to the higher scanning rate (20 mV/s vs 50 mV/s), which gives less time for the poison formation as has been already proven for single crystal electrodes,48 but also to the different formic acid concentration (0.5 M vs 0.1 M), which also affects the CO formation rate. It has been reported that currents in the negativegoing scan for single crystal electrodes are independent of the scan rate, since the process is fully controlled by the kinetics of the transfer reaction, but the currents in the positive-going scan are affected by the scan rate, diminishing as the scan rate increases.49 When the reactivity of nanoparticles is compared to that of massive polycrystalline electrodes, polyoriented nanoparticles exhibit higher activity for formic acid oxidation.29 Preferentially oriented nanoparticles show a more complex voltammetric behavior when compared to (poly)Pt, although the general features of the voltammogram are maintained. To establish a good comparison with the reactivity observed for single crystal electrodes, the voltammograms of the three basal planes are shown in Figure 4. In general, the voltammograms obtained for the three basal planes, which nominally have infinite terraces, can be very different from that obtained for the polyoriented surface, and therefore the comparison with single crystal electrodes has to include stepped surfaces. However, for formic acid oxidation, the shape of the voltammograms is not significantly affected by the terrace length, so that the main features in the voltammograms are mainly dependent on the symmetry of the terrace.28 As can be seen in this figure, currents for the three electrodes are very different. The Pt(111) electrode

Formic Acid Oxidation on Pt Nanoparticles shows the smallest currents, but the lower poisoning rate, as revealed by the small difference between positive and negativegoing scans. On the other hand, the Pt(100) electrodes exhibit much larger currents but a higher poisoning rate. Finally, the currents measured for the Pt(110) electrode are only significant at high potentials. By using these data, the activity of the different nanoparticles can be analyzed. The highest oxidation current in the positive scan direction is observed for the (111)Pt nanoparticles (Figure 3D), whereas the minimum current in the same scanning direction is obtained for (100)Pt nanoparticles, Figure 3B. These differences are in very good agreement with the behavior observed for single crystal electrodes since poisoning rates on the perfect Pt(111) electrode can be considered negligible, and increase with the step (or defect) density,28 whereas Pt(100) electrodes have a high poisoning rate in the potential region close to the potential of zero total charge (pztc).27 Considering the negative scan direction, two peaks are observed in the voltammetric signal of (100)Pt nanoparticles, where the peak at low potentials corresponds to the oxidation of formic acid on (100) terraces and that at higher potentials is associated with the oxidation on (110) sites or defect sites, as the behavior of this reaction on the platinum single crystal electrodes suggests. As shown in Figure 4, formic acid oxidation on the Pt(110) electrode has a peak at high potentials, at ca. 0.7 V, which coincides with the second peak observed in the voltammograms of the nanoparticles. In the case of (111)Pt nanoparticles, only one peak is observed in the negative scan direction, which should be linked to the (111) terraces, in which formic acid is oxidized at low potentials, Figure 3D. Finally, the voltammogram for the (100-111)Pt nanoparticles shows contributions from both (100) and (111) sites. The current density for the oxidation peak in the positive scan direction has an intermediate value when compared to the (100)Pt and (111)Pt nanoparticles. In the negative-going scan, the two peaks that can be observed are less pronounced than in the case of (100)Pt, but they are still easily distinguishable, Figure 3C. The hysteresis in the oxidation current densities between positive and negative scan directions points out the lower poisoning rate on (111) domains as compared to the (100) domains. Thus, (111) domains show quite high activity to oxidize formic acid through the direct reaction pathway in the positive sweep due to the absence of surface poison formation.28 In the case of (100) domains, a higher amount of poison from the dehydration reaction is produced and the surface of Pt is blocked for the direct oxidation reaction of formic acid by adsorbed CO. As aforementioned in the voltammetric characterization, some (110) surface sites bring their contribution elevating the current for the (111)Pt nanoparticles,50 as compared to the expected maximum current for a stepped (111) surface.28 The differences in the oxidation peak height in the negative scan direction are more directly connected with the surface activity through the direct path of formic acid oxidation. In the negative scan direction, the surface can be considered clean from CO at high potentials and nothing is blocking the formic acid molecules from accessing the Pt surface. This fact points out the importance of poison elimination prior to studying oxidation kinetics. Figure 5 shows various voltammetric curves recorded in a sequence in which the upper potential limit is increased. As can be clearly seen, currents in the negative-going scan are strongly dependent on the upper potential limit: the higher the value for the upper potential, the more surface is recovered from the CO poisoning and the higher the current is measured at 0.5 V for the direct oxidation reaction. The experiment in Figure 5


Figure 5. Voltammetric profiles for formic acid oxidation on (100)Pt nanoparticles with different upper potential limits in 0.5 M H2SO4 + 0.1 M CHOOH. Sweep rate: 50 mV s-1.

was conducted up to an upper potential limit of 0.95 V, but considering possible surface perturbations, the upper limit was routinely set at 0.90 V in both the voltammetric and kinetic studies. It is important to highlight that the effect of the upper potential limit is not observed in the 0.85-0.95 V window with the single crystal electrodes. At 50 mV s-1, CO is completely oxidized from the surface by using an upper potential limit of 0.85 V.27 This means that the oxidation rate of CO in the nanoparticles is slower than that in platinum single crystal electrodes. To analyze the difference with the behavior of single crystal electrodes, these results will be compared with the CO oxidation behavior in the nanoparticles. It should be mentioned that the nanoparticles used in this work are agglomerated, that is, they are very close to each other although they maintain their individual entity (they are not aggregated). The agglomeration has a significant effect catalyzing the CO oxidation reaction.13,51,52 The CO oxidation peak shifts toward negative values as the mean distance between nanoparticles is reduced. For agglomerate situations, the oxidation of a full CO monolayer on semispherical nanoparticles occurs between 0.7 and 0.8 V, whereas in isolated nanoparticles CO oxidation takes place above 0.8 V.53 The catalysis found for agglomerated nanoparticles has been explained as an interparticle mechanism in which the OH species required for CO oxidation are in a neighboring nanoparticle. On the other hand CO oxidation on preferentially oriented nanoparticles shows two peaks, one at low potentials and the second one at ca. 0.80 V, which is connected to the oxidation of CO on the (100) ordered domains.12 Nonetheless, CO molecules have been completely stripped from the surface of agglomerated nanoparticles at 0.85 V (irrespective of their shape). Additionally, it is also known that the CO stripping peak for CO layers formed from the dehydrogenation reaction of formic acid is oxidized at lower potentials than those corresponding to the oxidation of full monolayers, since the CO coverage in the first case is lower.54,55 All of these facts point out that an additional inhibition process is occurring in the presence of formic acid in solution, because CO stripping in the presence of formic acid is occurring at higher potentials. The whole series of voltammetric experiments (Figure 3) shows that (100) domains exhibit more poisoning in the studied potential window than (111) domains. Polyoriented nanoparticles show contributions from all crystallographic domains on their

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Figure 7. Selected transients recorded during the pulsed voltammetry for the (100)Pt and (111)Pt nanoparticles in 0.5 M H2SO4 + 0.1 M HCOOH. Only a few experimental points in the transients are shown. Figure 6. (A) Pulse program applied during the pulsed voltammetry to measure jθ)0 and kads for E > 0.3 V and (B) pulse program applied to measure kads for E < 0.3 V.

surface but this does not improve their catalytic activity compared to preferentially oriented nanoparticles. According to voltammetric data of (100-111)Pt nanoparticles, the behavior includes the effect of both the (100) and (111) ordered domains on its voltammetric profile, which is also in good agreement with a previous study.29 This behavior should not have been observed if the reaction rate would have been controlled by transport and reflects the kinetic limitations due to the structure sensitive nature of the formic acid oxidation reaction. 3.3. Formic Acid Oxidation: Chronoamperometric Behavior, Determination of the Intrinsic Oxidation, and Poisoning Rates. To obtain reaction rates, the reaction conditions have to be controlled and known from the experimental point of view. For that reason, we have used pulsed voltammetry where the pulse sequence is programmed to guarantee reproducible initial conditions for the reaction.26-28 In the modified approach used here, the potential during the measuring interval is kept constant. The upper pulse potential, as well as the upper limit of the sweep, should be low enough to avoid surface oxidation, in order to preserve the electrode integrity, but high enough to readily oxidize the adsorbed CO molecule during the pulse time. For that reason, 0.90 V has been chosen in this work as upper pulse potential for all Pt nanoparticle species. Two different programs used in this work are shown in Figure 6. For the first one (Figure 6A) the duration of the steps was 1 s, both for the step at the measuring potential and that at 0.90 V to strip the adsorbed CO molecules. For the second one (Figure 6B), discussed in more detail below, the measuring step was preceded by an accumulation step of different duration. The effect of the pulse duration and upper potential limit was carefully tested to ensure that CO has been completely stripped

from the surface. In this case, no significant differences were observed when using 0.9 and 0.95 V as the upper potential. Therefore, 0.9 V was selected as pulse and upper potential to better preserve the surface structure of the nanoparticles. Owing to the well-documented relationship between the voltammetry of platinum electrodes and surface structure, a cyclic voltammogram was recorded before and after the pulse sequence to ensure that the voltammetric profile of the electrode had not suffered any change during the experiments. Typical current density transients recorded for the studied nanoparticle samples during the pulsed voltammetry experiment are shown in Figure 7. After a pulse to 0.90 V, during which all the adsorbed CO molecules have been oxidized to CO2, the surfaces are initially free of adsorbed CO at t ) 0. Thus, the current density at t ) 0 reflects the activity of the electrode through the active intermediate reaction path in the absence of poison. However, this current is not directly measurable since double layer charging processes take place at short times. It should be mentioned that the double layer charging currents in the present working conditions are negligible for t > 10 ms. Some of the transients show a decay with time at t > 10 ms, being this decay associated with the diminution of active area by formation and accumulation of adsorbed CO. Other transients show constant currents, which means that the active area is not changing during the time of the experiment, that is, the CO formation is negligible, and therefore, the activity for the direct formic acid oxidation remains constant. This interpretation assumes that the rate of the reaction through the active intermediate is only a function of the electrode potential under the present experimental conditions. From a molecular point of view, this means that the interaction of the formic acid molecule with the adsorbed CO is negligible and does not affect the kinetics. Thus, the transients can be used to measure the rate through the active intermediate path, using currents at t ) 0,



Figure 8. Values of jθ)0 obtained from the analysis of the pulsed voltammetry transients for the different Pt nanoparticles in 0.5 M H2SO4 + 0.1 M HCOOH.

Figure 9. Values of kads obtained from the analysis of the pulsed voltammetry transients for the different Pt nanoparticles in 0.5 M H2SO4 + 0.1 M HCOOH.

and also the rate of the dehydration step by quantification of the decay rate. It should be stressed that diffusion effects on the currents should only be observed for currents above 10 mA cm-2,48 and for that reason, those effects can be neglected in the present conditions. To extract the kinetic constants from the oxidation transients, the previously used model has been again applied to the transients.27,28 This method has been previously tested with single crystal electrodes. In this model, the transient current is:

those obtained in cyclic voltammetry. When the catalytic activity at 0.45 V is compared for the four types of nanoparticles, the activity order is the following:

j ) jθ)0

(

1 1 + kadst(p - 1)

)

1/p-1

(1)

where jθ)0 is the intrinsic activity of the electrode, that is, the current through the direct path that would have been obtained in the absence of poison on the surface, kads is the CO formation rate (poisoning rate), and p is the number of contiguous free sites required to form CO from formic acid. When the poisoning rate is very low, p cannot be accurately determined. In these cases, the p value was fixed to 2, which is an average of the experimental values determined in reliable conditions for single crystal electrodes. Equation 1 was used to fit all the transients and the values of jθ)0, kads, and p were obtained as a function of the potential. The values of p for the transients with non-negligible kads values are always between 2 and 2.3, which indicates that the dehydration step requires two reaction sites to take place. This range of values is the same as that obtained for single crystal electrodes, which demonstrates that the mechanism of poison formation is the same in all the surfaces. The values of intrinsic activity, jθ)0 (Figure 8), are independent of the sweep direction and can be compared with the current densities measured by voltammetry in the positive- and negative-going sweep. As can be seen, the values of jθ)0 and the voltammetric currents have qualitatively similar behavior for potentials above 0.3 V. These data, together with the negligible kads values above 0.45 V (Figure 9), indicate that CO is not being formed at those potentials. In fact, the decay in the transients for potentials above 0.5 V is very small for all nanoparticle samples. However, for potentials between 0.3 and 0.45 V, jθ)0 values are significantly higher than the voltammetric currents, as a result of the accumulation of poison on the electrode surface in the classical voltammetric experiment. When jθ)0 values are compared for the different nanoparticle species (Figure 8) the results are quantitatively different than

(100)Pt > (100-111)Pt > (111)Pt > (poly)Pt Pt nanoparticles with (100) domains show the highest intrinsic activity for the whole group. This trend was already observed in classical voltammetry, although the difference between the currents measured here is significantly higher. In the voltammetric experiment, the current at 0.5 V for the (100)Pt nanoparticles is almost twice that obtained for (poly)Pt nanoparticles, whereas jθ)0 values for (100)Pt nanoparticles are three times higher. This fact indicates that the higher activity for the direct reaction is also associated to a higher activity for poison formation. It should be stressed that voltammetric currents for formic acid (and in general for any molecule that is oxidized with a double path mechanism involving CO) are the result of a complicate balance of the kinetics of both reaction paths and their dependence with the potential. For that reason, the differences observed with classical voltammetry are not following exactly either jθ)0 or kads value changes (vide infra). The observed intrinsic activity for the different nanoparticles is the weighted average of the behavior of the different domains in the nanoparticle. Thus (100) domains have the highest catalytic activity, whereas (111) domains have much lower catalytic activity and (110) sites (whose behavior can be assimilated to the edges of the nanoparticles) have significant activity only at high potentials.28 Thus, the observed order in jθ)0 values can be easily explained. Primarily the activity through the active intermediate reaction path is dependent, at low potentials, on the ratio of (100) ordered domains present on the surface (see Table 1). This explains why (100)Pt nanoparticles have the highest catalytic activity and (100-111)Pt nanoparticles are second to them. When the fraction of (100) domains is not very large, the activity at low potentials is dominated by the ratio of (111) domains. On the other hand, the polyoriented nanoparticles, where ordered domains are almost absent from the surface, have the lowest intrinsic activity of all studied nanoparticles. These nanoparticles have a higher ratio of (110) sites in all samples. (110) ordered domains have high activity only at potentials above 0.6 V. These differences show the importance of the surface integrity and the amount of surface sites with a given symmetry in the oxidation kinetics of formic acid on platinum nanoparticles. It is also important to note that

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the potential corresponding to maximum intrinsic activity (EM ) 0.39 V) of (100-111)Pt is situated between EM ) 0.37 V for (100)Pt and EM ) 0.49 V for (111)Pt, showing mixed contributions of both (111) and (100) domains. Polyoriented Pt nanoparticles have this maximum at 0.41 V, also a consequence of the presence of all possible surface structure configurations. The results obtained here for the activity for formic acid oxidation for nanoparticles with different shapes are also in good agreement with the observed behavior of this reaction in nanoparticles with different sizes, where a diminution of the activity is observed when the size of the nanoparticle is below 4 nm.30 For nanoparticles, the diminution of size leads to a diminution of the ratio of the ordered domains present on the surface and to an increase of the ratio of edge sites, independently of the shape of the nanoparticle. The studies here demonstrate that edge (or defect) sites are the least active at low potentials for the oxidation of formic acid. Therefore, the diminution of size leads to a diminution of the activity of the nanoparticle. It is evident that poison formation strongly affects the direct oxidation of formic acid. Poisoning rates were obtained from kads values. For single crystal electrodes, it has been shown that poisoning rates were significant at potentials below 0.5 V. However, the currents in the transients are only significant above 0.3 V. This fact implies that reliable kads values can only be obtained above 0.3 V using those transients, because kads is measured using the decay in the current. To obtain kads values for E < 0.3 V, a different pulse sequence was devised (Figure 6B). In this new pulse sequence the potential is stepped down for a given time at the potential where kads is to be measured after the pulse at the upper potential to completely oxidize the adsorbed CO molecules.28 To evaluate the amount of CO accumulated at this potential, the potential is stepped to 0.5 V and the current at t ) 0 is used. This potential was chosen since current at this potential is significant and the poisoning rate is almost negligible, as can be seen in Figure 7. kads values can be obtained by fitting with eq 1 the plot of j values at 0.5 V vs time at the measuring potential. Figure 9 shows the kads values measured with use of both step routines for the different Pt nanoparticles. Qualitatively the curves are quite similar, but they have quantitative key differences which are discussed in the following. (100)Pt nanoparticles have a broad potential region where significant dehydration rates are measured, also they have the highest values as compared to the other nanoparticles. Two well-defined peaks are situated at 0.15 and 0.35 V, representing steps and terraces, respectively, as will be discussed below. (100-111)Pt nanoparticles also show these same two peaks but the second peak at 0.35 V has lower kads values as compared with (100)Pt nanoparticles. (111)Pt nanoparticles have only one well-defined maximum at 0.17 V, showing very little activity for the dehydration step in the 0.30-0.40 V region. Finally, polyoriented nanoparticles have two maxima in the same positions as described for (100)Pt but with a lower poison formation rate constant. In summary, the nanoparticles studies show that there are two main regions where CO formation rates are significant. To understand the origin of these features, these results will be compared to those obtained with stepped surfaces. Poison formation occurs normally on the stepped surfaces in a narrow region, the potential of which depends on the nature of the surface site. Thus, surfaces with (100) terraces show very high kads values with a peak centered at ca. 0.37-0.39 V. On the other hand, a maximum poisoning rate is observed at 0.15 V for (111) vicinal surfaces containing monatomic (110) steps. The surfaces with (100) steps give a maximum activity for CO

Grozovski et al. formation at ca. 0.25 V, whereas (111) terraces show no activity for poison formation.28 It should be mentioned that the maximum activity for the poison formation coincides in all cases with the local pztc of the site.27 This means that the dehydration reaction to yield CO only occurs in a potential region where hydrogen and anion coverage are low. This fact implies that the adsorption of anions or hydrogen inhibits poison formation. As can be seen, the observations with the stepped surfaces are in good agreement with the results obtained here. Therefore, the peak at 0.15-0.20 V in these curves should be associated with the presence of (110) sites on the nanoparticles, and the peak at 0.37 V corresponds to the formation of CO on the (100) ordered domains. In fact, the rate constant values at 0.37 V are proportional to the ratio of (100) ordered domains present on the surface (see Table 1). Additionally, the peak at 0.37 V presents a shoulder at lower potentials, which should be associated to the (100) defects, as has been observed with stepped surfaces having (100) terraces.27 These results clearly indicate that CO formation in nanoparticles follows the same rules as those observed on single crystal electrodes, that is, the dehydration step takes place in the potential region where the local coverage of hydrogen and anion is low, that is, close to the local pztc. It is important to highlight here the relationship between the poison formation and the pztc for both nanoparticle and massive electrodes. The mean pztc for a surface is a weighted average of the local potential of zero total charge of the different sites present in the surface. For that reason, it depends on the particle size.56 However, the effect of the surface structure in the pztc changes is even more important.37 In fact, the dependence on the particle size can also be considered as a consequence of the surface structure changes when the size diminishes. As the size of the nanoparticle diminishes, the ratio of edge sites on the nanoparticle surface increases. Since these sites have a lower pztc, the mean pztc values diminish, as observed with stepped surfaces.57 It should be stressed that the effect of the surface structure on the mean pztc values is significantly more important than that of the size effects.37 Being the poison formation reaction of platinum dependent on the local potential of zero total charge, the optimization of the catalyst shape should include not only size optimization but also shape optimization. As has been shown, the reaction rates for the dehydration step are significantly affected by the poison formation for nanoparticles of similar size. The combined effect of the poison formation rate and the current through the direct oxidation path in the experimental behavior can be analyzed by measuring the currents in the pulsed voltammetry transients at 1 s (jt)1s), and could be used to qualitatively evaluate the performance of the nanoparticles under working conditions. Although the plot of jt)1s vs E (Figure 10) may look similar to that of jt)0, important and significant differences can be observed. First of all, the difference in activity after 1 s between (111)Pt and (100)Pt nanoparticles is now much smaller. In fact, at potentials lower than 0.4 V, currents for the (111)Pt nanoparticles are the highest among all the samples. The difference originates with the low poisoning rate for the (111)Pt nanoparticles in that potential region. Although (111)Pt nanoparticles have lower catalytic activity, in the long term they perform better due to the very low poisoning rate, especially at potentials above 0.3 V. On the other hand, (poly)Pt nanoparticles still have the lowest activity, since they combine a poor activity through the active intermediate and a moderate-high poisoning rate.


Figure 10. Currents at t ) 1 s measured in the pulsed voltammetry transients for the different Pt nanoparticles in 0.5 M H2SO4 + 0.1 M HCOOH.

4. Conclusions Formic acid oxidation was studied on polyoriented and preferential (100)Pt, (100-111)Pt, and (111)Pt nanoparticles with pulsed voltammetry. The results pointed out that the surface structure of the nanoparticles plays an important role in the reactivity and the electrocatalytic properties are strongly depend on the surface structure and shape of nanoparticles, in a similar way to what was established from fundamental studies with Pt single crystal electrodes. The voltammetric curves can be qualitatively analyzed from the reactivity properties shown by the single crystal electrodes with basal orientations, which can be taken as model surfaces. On the other hand, kinetic data obtained from the pulsed voltammetry experiment give good insight on the dual path mechanism of the formic acid oxidation reaction on Pt nanoparticles. Different nanoparticles exhibit a weighted electrocatalytic activity, depending on the relative amount of the surface sites. Among the Pt nanoparticles studied, those containing (100) domains are clearly the most active toward formic acid electrooxidation via the active intermediate reaction path, but also exhibit the highest poisoning rate. (111)Pt nanoparticles alone show the lowest CO formation rate constant in the series and a moderate reaction rate via the active intermediate. (100-111)Pt nanoparticles show also a significant activity to the oxidation of formic acid via the active intermediate path but also have high poison formation as seen in both kinetic and voltammetric data. These findings open the possibility of better understanding of formic acid oxidation kinetics on Pt nanoparticles and provide information for designing new and better electrocatalytic materials by using Pt nanoparticles of various shapes, as previously shown with Pt single crystal electrodes. Acknowledgment. This work has been financially supported by the MICINN-FEDER (Spain) (project CTQ2006-04071/ BQU) and Generalitat Valenciana (project PROMETEO/2009/ 045). References and Notes (1) Wieckowski, A.; Savinova, E. R.; Vayenas, C. G. Catalysis and electrocatalysis at nanoparticle surfaces; Marcel Dekker: New York, 2003. (2) Adzic, R. In Modern Aspects of Electrochemistry; Plenum Press: New York, 1990; Vol. 21, p 163. (3) Markovic, N. M.; Ross, P. N. In Interfacial Electrochemistry, Theory, Experiments and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1998; p 821. (4) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 121.

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Formic Acid Oxidation on Shape-Controlled Pt Nanoparticles Studied

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