Bismuth and CO Coadsorption on Platinum Nanoparticles - The

Article pubs.acs.org/JPCC

Bismuth and CO Coadsorption on Platinum Nanoparticles Marta C. Figueiredo,*,† Marko Melander,‡ José Solla-Gullón,§ Tanja Kallio,† and Kari Laasonen‡ †

Research Group of Fuel Cells, Department of Chemistry, Aalto University, FI-00076 Aalto, Finland COMP Center of Excellence, Department of Chemistry, Aalto University, FI-00076 Aalto, Finland § Institute of Electrochemistry, University of Alicante, E-03080 Alicante, Spain ‡

S Supporting Information *

ABSTRACT: CO adsorption onto Pt- and Pt-based catalysts is a very relevant topic in electrocatalysis, and in particular, in fuel cells research. CO is known to be the main responsible of the poisoning of the catalysts and consequent decrease on performance of the fuel cells devices. In this paper, density functional theory (DFT) calculations and experiments were combined to access the effect of modifying Pt nanoparticles with Bi on CO adsorption. CO adsorption energies were calculated for Pt sites nearby Bi atoms in different type of structures: Pt clusters with Bi as surface adatom and Pt clusters with Bi as surface and subsurface dopant (alloys). The results show that, when compared with pure Pt, the adsorption energies for CO are lower on PtBi clusters in both adatom and surface alloy configurations. Subsurface PtBi alloys reveal higher adsorption energies for CO but these structures are energetically very unfavorable. On the basis of the calculations, a high degree of mobility of Bi on the surface was found in the presence of CO. These results suggest that the experimental differences between cyclic voltammogram before and after CO stripping can be due to a reorganization of the Bi layer on the catalyst when CO is coadsorbed. It was also experimentally observed, that CO oxidation peaks on the modified electrodes shift to higher potentials with increasing Bi coverage. These results suggest a higher effect of the decrease of CO coverages on the oxidation process than the decrease of the biding energies for Pt−CO in the presence of Bi.

1. INTRODUCTION

catalyst for the above-mentioned oxidation reactions) decreasing the catalysts performance. Modifying Pt with other metals, as alloys, intermetallic compounds or by irreversibly adsorbing adatoms, is a common strategy to increase its tolerance toward CO poisoning and simultaneously improving its catalytic properties. PtBi is one of these bimetallic systems that have been reported to be CO tolerant10,11 and that, at the same time, revealed higher catalytic activities for several oxidation reactions, e.g., formic acid,12,13 methanol, 14−16 glycerol,17 ethylene glycol,18 and ethanol.15,17,19,20 The enhancement on the catalytic activity has been mainly attributed to the lower poisoning of PtBi catalysts with CO. The effect of Bi on the CO adsorption on Pt surfaces has been reported in literature for intermetallic structures,21,22 surface coadsorption22,23 and bimetallic overlayers.24 For the intermetallic structures, Oana et al.21 found that the propensity for CO adsorption on Pt is drastically reduced of PtBi2 and PtBi surfaces (with respect to Pt) due to an increase of the Fermi level of the system (that determines the occupation of higherlying antibonding orbitals) induced by Bi. The results from Lin

The CO adsorption on metal surfaces is one of the most extensively studied subjects in electrocatalysis both experimentally1−4 and theoretically5−9 (these references are only a limited example within the available ones). The continuous interest in CO arises from two main reasons: due to its simplicity, CO is commonly used as “model fuel” for studying electrocatalysis of C1 molecules; and from the fact that it is one of the most important contaminants/poisons for anodic reactions in fuel cell. Fuel cells emerged in the last decades, as the most promising carbon free systems for energy conversion. Nevertheless, fuel cells development and introduction in the markets has been slowed down due, within other problems, CO poisoning of the catalysts. In hydrogen fuel cells (proton exchange membrane fuel cells − PEMFC) CO is one trace component in the hydrogen fuel which is produced from hydrocarbons reforming process. These traces of CO can adsorb on Pt blocking the active sites for other reactants thus significantly reducing the FC performance. Moreover, CO is formed in the electrocatalytic oxidation of small organic molecules and alcohols oxidations (used as fuels in direct alcohol and formic acid fuel cells) and also in these cases, CO adsorption causes substantial poisoning of the active sites on precious metal catalysts, particularly on Pt (the most used © 2014 American Chemical Society

Received: June 26, 2014 Revised: September 7, 2014 Published: September 9, 2014 23100

dx.doi.org/10.1021/jp5063513 | J. Phys. Chem. C 2014, 118, 23100−23110

The Journal of Physical Chemistry C

Article

et al.23 for the surface coadsorption of CO at Pt surfaces single crystals modified with Bi adatoms suggest that Bi adatom increases the extent of the dπ → 2π* back bonding between Pt and the adsorbed CO stabilizing the CO molecule at the surface. However, other authors22,24 report that Bi changes the electronic structure of the Pt by lowering its d-band center. This decreases the overlap between the d-band of the alloy and the adsorbate states, resulting in a weaker binding between Pt and CO. It was found that the CO adsorption in these surfaces is determined by the interaction of the platinum d states and 2π states of CO. From these reports, it is evident that the electronic effects due to the presence of Bi on the catalyst are not clear yet. With the aim of shedding some light into this subject, we report here density functional theory (DFT) calculations on the effect of Bi and CO coadsorption on Pt nanoparticles. The calculation were performed using simple models with Pt particles in a cuboctahedron shape with 147 atoms with Bi and CO adsorbed in different sites and adsorption geometries. One Bi atom and one CO molecule in the Pt sites nearby Bi were used in order to simulate medium-high coverages obtained experimentally, where due to obvious space limitations, CO will be close to Bi. Differences between Bi adatoms (adsorbed on the catalysts surface) and PtBi alloys (both Bi replacing one Pt atom at the surface and subsurface sites of the catalysts) were accessed, as well as the effect of these structures on the CO adsorption energies. The calculations were coupled with experiments on CO stripping at Pt nanoparticles modified with different Bi coverages. The results show that both Bi and CO prefer different adsorption sites when coadsorbed, and that there is a clear effect on CO adsorption energies induced by a decrease in the CO-Pt binging energies because of the presence of Bi. However, these binding energies are still significantly negative and it cannot be said that PtBi structures completely inhibit the CO adsorption. On the basis of the experimental results, the CO oxidation on the PtBi surfaces seems to have a higher dependence on geometric effects and, consequent lowering on the CO coverage than on the decrease of the CO−Pt biding energies due to Bi.

Figure 1. Representative TEM image of the Pt nanoparticles used in this work (insert: HRTEM image).

HRTEM experiments have been carried out on a JEOL 3010 microscope (LaB6, Cs = 1.1 mm) operated at 300 kV, providing a point-to point resolution of 0.19 nm. The sample was obtained by placing a drop of the dispersed solution onto a Formvar covered copper grid and evaporating it in air at room temperature. b. Electrochemical Experiments. The experiments were performed, at room temperature, in three electrode electrochemical cell deaerated using nitrogen (AGA, 99.999%). The counter electrode was a platinum wire and a reversible hydrogen electrode (RHE) was used as reference connected to the cell through a Luggin capillary. The synthesized Pt nanoparticles were deposited on a polycrystalline gold disk electrode (5 mm diameter). The gold disk was previously cleaned with alumina suspension and ultrasounds bath, and cyclic voltammograms in the potential range 0.05 to 1.45 V were done to check the cleanness of the surface. Metal nanoparticles were transferred to a gold collector by depositing a drop (5 μL) of the nanoparticle water suspension on the surface of the gold disk. An argon atmosphere was used to evaporate the excess of water. The experiments were performed with a potentiostat/galvanostat PGSTAT100 Autolab system. The working electrode (nanoparticles deposited on the gold disk) was characterized by cyclic voltammetry in a solution of 0.1 M H2SO4 (Merck). The active surface area of the Pt nanoparticles was determined by the charge involved in the socalled hydrogen UPD region assuming 0.23 mC cm−2 for the total charge after the subtraction of the conventional current attributed to double-layer charging contribution.25 The Bi adsorption was performed by putting the working electrode in contact with an acidic solution of Bi salt (Bi3O2 from Sigma-Aldrich) at open circuit potential. The electrode was then, rinsed and placed in the electrochemical cell for voltammetric characterization. The adatom coverage on the particles was calculated according to ref 13. Briefly, the Bi coverage (θBi) is calculated from the blockage of the so-called hydrogen region H charge according to

2. METHODS 2.1. Experimental Section. a. Synthesis and Characterization of the Nanoparticles. The synthesis of the unsupported Pt nanoparticles has been performed using a similar methodology to that previously described.13 In brief, a 20 mL aqueous solution containing 2.5 × 10−4 M H2PtCl6 and 2.5 × 10−4 M trisodium citrate was prepared in a glass beaker at room temperature. Then, 0.6 mL of an ice-cold and freshly prepared 0.1 M NaBH4 solution was added to the solution under vigorous stirring. The stirring was slowed down after 30 s and the solution was keep unperturbed for the next 30 min. Finally, a NaOH pellet was directly added to the solution. The addition of NaOH results in the destabilization of the nanoparticles that subsequently precipitate. After complete precipitation, the sample was washed 3−4 times with ultrapure water. TEM experiments were performed with a JEOL, JEM 2010 microscope working at 200 kV. The sample for TEM analysis was obtained by placing a drop of the dispersed solution onto a Formvar-covered copper grid and evaporating it in air at room temperature. The mean size of the particles was estimated to be about 3.9 ± 0.7 nm.

θBi = 1 − θPt = 23101

(qPt 0 − qPt Bi) qPt 0

(1)



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Figure 2. Optimized structures of Pt147 nanoparticles. Circle numbers are location of different adsorption sites for Bi (A and C) and circle letter adsorption sites for CO (B and C) discussed in the text.

where qPt0 and qPtBi are the hydrogen adsorption charges for a clean electrode and bismuth modified electrode, respectively. The experimental setup for CO (AGA, 99.999%) stripping studies has been described in reference.26 Prior to CO adsorption, the voltammogram of the platinum nanoparticles electrodes in the test electrolyte (0.1 M H2SO4) was recorded. The potential was held constant at 0.1 V while a stream of CO flew into the cell for 5 min in order to achieve the saturation of the platinum surface by adsorbed CO. The excess CO was eliminated from the solution and cell atmosphere by flowing argon for 15 min, while keeping the electrode potential under control. Then, three voltammograms were recorded: (i) for the control of full surface blocking, in the lower potential range (0.06−0.4 V); (ii) for the electrochemical stripping and measurement of the amount of adsorbed CO; (iii) for monitoring the recovery of hydrogen adsorption capability of the CO-free surface. 2.2. Computational Details. All density functional theoretical (DFT) calculations were carried out using the CP2K/Quickstep software27 with mixed plane waves and Gaussian basis set to represent the wave functions. Shorter range molecularly optimized DZVP-MOLOPT-SR basis set28 was used for all elements. For the plane wave expansion the cutoff energy was 700 Ry. SCF criterion for the wave functions was set to 10−6. GTH pseudopotentials29,30 with 4, 6, 18, or 5 valence electrons were used for carbon, oxygen, platinum, and bismuth, respectively. The PBE functional31 was used to account for exchange-correlation effects. The nanoparticles were modeled in the gas phase using a periodic cubic cell with the edge length of 24 Å. Geometries and multiplicities for each PtBi particle were fully optimized within the unrestricted Kohn−Sham formalism without any symmetry requirements. Convergence threshold for geometry optimization was 2.0 × 10−2 eV/Å. Multiplicities were held constant during adsorption or adsorbate induced segregation calculations. The adsorption energy is defined as Eads = Ecluster+CO − Ecluster − ECO and negative adsorption energies correspond to stable adsorption. Segregation energies Eseg are reported with respect to the energy of most stable cluster of a given composition.

molecule were chosen to access the Bi effect and the trend on its influence on the CO adsorption. These configurations will represent medium-high coverages used experimentally, where Bi will cover a substantial number of Pt sites and CO will adsorb on the nearby those Bi occupied sites. Moreover, if we assume local coverages for each facet separately, the models used in the calculations can be considered as representative of the experimental conditions. The evaluation of Bi coverage in well-defined surfaces (single crystals) is well described in the literature32,33 and it is known to be dependent on the geometry of the site. For Pt(111) the maximum Bi coverage is 0.33 Bi/Pt while for Pt(100) 0.5 Bi/Pt.32 Taking these values, we can assume that the clusters computed in this paper have a local coverage of 0.11 Bi/Pt (0.33 Bi/Pt when the maximum coverage is normalized to 1) and 0.083 Bi/Pt on the (100) facet (0.18 when the maximum coverage is normalized to 1). Obviously, some effects are expected from different Bi concentrations and consequent Bi−Bi interactions on the CO adsorption energies, but these effects are not addressed in the present manuscript. The amount of possible adsorption sites and combinations on the coadsorption is enormous. However, only the most representative positions (with higher degeneracy, see Supporting Information) were computed with special attention, as mentioned before, for CO adsorption on the sites nearby the Bi adatom. The Bi adsorption was done, separately, on the two facet geometries with different adsorption sites (Figure 2A): atop, 3fold and hollow in which the Bi atom is bonded to 1, 3, or 4 Pt atoms, respectively. The adsorption energies for the optimized structures of Pt147 with Bi adatoms in different adsorption sites are presented in Table 1. These adsorption energies were obtained by subtracting the energy of Pt147 from the total energies of Pt147+ Bi1 clusters having as reference the most stable structure for this composition (eq 2). Ei , ads = Ei , total − Eref total

(2)

where Eref total is the total energy for Bi in the hollow site in the (100) facet−position 3 in Figure 2A. The results show that the most stable configuration for the Bi adsorption is the hollow site in the (100) facet (3 in Figure 2A). In our results none of the bridge adsorption sites were stable, independently of the site geometry. The energies for Bi adsorption onto Pt show that it will preferentially adsorb in a hollow site for the (100) and both on 3-fold (1 in Figure 2A) and atop (2 in Figure 2A) for the (111) facet. However, the obtained energies (Table 1) show that atop site is less favored

3. RESULTS 3.1. Bi as an Adatom. The calculations reported in this paper were performed using a Pt cluster of 147 atoms (Pt147) as model for simulation of Pt particles with sizes between 1.5 to 1.8 nm. The structures were optimized to a local minimum energy in cuboctahedron particle containing eight (111) and six (100) facets. Simplified systems with one Bi atom and one CO 23102



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The adsorption energies for Bi and CO coadsorption are reported in Table 1. The energies were calculated using two references: the called total reference in which all the energies are calculated having the same referencethe most stable configuration for Bi adsorption (in this case Bi adsorbed in the hollow site in the (100) facet, structure 3); and the relative reference in which the reference used is the cluster with the same Bi geometry (structure 1 for 1D and 1E, structure 2 for 2F and 2G, structure 3 for 3H and 3I). The values considering the total reference will give us a more realistic view on the energies of the cluster after interaction of Bi and CO at the surface, taking in account possible influence of, not only Bi on CO but also CO on Bi. In the case of the values obtained for the energies calculated with the relative reference, they will give the effect of the Bi on the CO binding energies on the surface. A quick look through the results shows that the coadsorption of Bi and CO on the Pt nanoparticle has a strong influence on the adsorption of the single compound. When CO is also adsorbed on the surface, Bi can be displaced to different adsorption site like for example in the structures 1D, 1E, 3H, and 3I. In the 1D, Bi is initially adsorbed on the (111) facet in the 3-fold site at the edge of the facet, but in the presence of CO, it moves to a hollow site in the neighbor (100) facet. In the case of the structure 1E the adsorption of Bi is still 3-fold after CO but Bi is displaced to an adsorption site further from CO. For the (100) facet, Bi adsorbed in both position 3H and 3I, is moved from the hollow site (here is stable in the absence of CO) to the 3-fold and bridge sites, respectively. These results reveal a high mobility of the Bi atom on the platinum surface due to repulsive interaction with CO. When the energies are compared with the most stable structure (3 in Figure 2) as reference (total reference) the most stable configurations are 1D and 3I with adsorption energies very close to each other (∼−2 eV) and they are both weaker than the CO adsorption in the absence of Bi. The adsorption of CO nearby the Bi in position 2 is very unlikely with adsorption energies ∼2 eV more positive than those of the other configurations. These results are not surprising if we take in account the high adsorption energies found for Bi adsorption by itself on atop position (position 2 in Figure 2). In summary, when the adsorption energies are compared using the same reference for all the structures the CO adsorption on Pt is always weaker in the presence of Bi, revealing the existence of an electronic effect due to the presence of the adatom on the surface. With respect to CO, its adsorption energies reveal to be dependent on the presence of Bi and on the adsorption site. Comparing the adsorption energies obtained with the respective cluster configuration as reference (relative reference), structure 2F present and increase of 0.1 eV on the adsorption energies when compared with the values obtained for CO adsorption at the (111) facet. For structures 1E and 2G, where CO is in a bridge site the adsorption energies decrease in comparison with CO in the same position without Bi. Nevertheless, the difference between these energies is rather small (0.33 eV maximum). Both of the adsorption sites on the (100) facet reveal a decrease of the adsorption energies for CO when it is coadsorbed with Bi of 1.33 eV for 3H and 0.8 eV for 3I. These results suggest that Bi has an electronic effect on the CO adsorption on Pt surfaces by weakening the Pt-CO binding energy. 3.2. Bi on the NanoparticlePtBi Surface Alloys. Calculations with the Bi incorporated on the nanoparticle

Table 1. Adsorption Energies for the Structures from Figure 2a

Calculated from for Bi adsorption Ei,ads = Ei,total − Eref total, for CO adsorption Ei,ads = Ei,total − EPt147 − ECO, for the co-adsorption Ei,ads = Ei,tot − Eref total (1/2/3) − ECO for the values with the relative reference and Ei,ads = Ei,tot − Eref total (100, 3) − ECO for energies with total reference. bNote: Bi moves from the hollow position at the (111) facet to hollow in (100). a

by almost 2.5 eV compared to Bi on the hollow site at the (100) facet. A similar procedure was followed for the CO adsorption on the Pt147 cluster. The single CO molecule adsorption energies were calculated for the (111) and (100) facets in different adsorption sites (Figure 2 B) and are reported (Table 1). The adsorption energies were calculated from the total energy for Pt147 + CO after subtraction of the Pt147 cluster and CO molecule energies according to Ei , ads = Ei , total − E Pt147 − ECO

(3)

As obtained for Bi adsorption, also for CO, there are only a few stable adsorption sites. For the (111) facets CO can either adsorb on a 3-fold (A in Figure 2B) or bridge (B in Figure 2B) configuration with an adsorption energy of −2.29 and −2.09 eV, respectively. At (100) facets the favored adsorption site is the bridge (C in Figure 2B) with adsorption energy of −2.84 eV. Comparing the adsorption in the 2 facets, the adsorption on (100) is stabilized by almost 0.6 eV in comparison with the (111) facets. For analyzing the influence of the coadsorption of both species, calculations using Pt147 + Bi1 + CO were performed for the different geometry of the facets. The different adsorption sites simulated for the coadsorption are presented in Figure 2C. 23103



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Figure 3. Optimized structures of Pt146 + Bi1 nanoparticles. Circle letter adsorption sites for CO discussed in the text.

cluster were also performed to access the differences between the Bi adsorbed on the Pt surface as adatom and the Bi in the metal cluster (alloy) in the CO coadsorption. Clusters where 1 Pt atom was replaced by Bi (Pt146 + Bi1), in the cuboctahedron particle, having Bi in different sites were optimized (Figure 3). Bi was placed on the center (4 in Figure 3A) and in the edge (5 in Figure 3B) of (111) facets and on the center (6 in Figure 3C) and on the edge (7 Figure 3D) of the (100) facets. The total energies for Pt146 + Bi1 clusters were calculated by Ei , cluster = Ei , total − E Pt147 + E Pt1

Table 2. Adsorption Energies for the Structures from Figure 3a

(4)

and EPt1 = EPt147/147, to take in account the same number of atoms in the cluster than for the Bi adatom adsorption on Pt147. The adsorption energies are reported in Table 2 and correspond to the difference from the cluster to the most stable one with the same composition (in this case Pt146 + Bi1 in position 7). On the basis of the cluster energies obtained, the most stable configurations is obtained when Bi is replacing a Pt atom on the edge of the facet (5 and 7 on Figure 3, parts B and D), for both (100) and (111). In any case, Bi on the edge of the (100) facet is still almost 0.3 eV more stable than in the same position of the (111) facet. Bi atoms in the center of the (111) sites are less stable with segregation energy of 1.27 eV more positive than that of the most stable configuration (edge (100)). For the CO adsorption energies onto these structures, a similar analysis for Bi as an adatom was done. The adsorption energies were calculated from Ei , ads = Ei , total − Ecluster , ref − ECO

(5) Calculated as for Bi adsorption Ei,cluster = Ei,total − EPt147 + EPt1 and EPt1 = EPt147/147; for the CO adsorption Ei,ads = Ei,tot − Eref,cluster (4/5/6/7) − ECO for the values with the relative reference and Ei,ads = Ei,tot − Eref,cluster(100,7) − ECO for energies with total reference. a

where the Ecluster,ref was either Ecluster,7 (most stable configuration for this composition) for the total reference or the cluster with the same Bi geometry (4 for 4J and 4K; 5 to 5L; 6 to 6 M and 6N and 7 for 7O, 7P, 7Q, and 7R) for the relative reference. As previously observed for the structures with Bi adatoms, CO adsorption energy is also sensitive to the presence of Bi on the Pt nanoparticle as a surface alloy. In the (111) facets, when Bi is replacing one Pt atom, the CO adsorption is always more stable (for all the adsorption geometries) than on the Pt147 nanoparticle. The configurations 4J, 4K, and 5L have adsorption energies more negatives than A and B. In this facet, CO adsorbed in bridge at the edge of the facet and Bi replacing the central Pt atom (4K) is the most stable configuration with an adsorption energy 0.36 eV more negative than CO 3-fold on pure Pt. For the CO on the atop position near the Bi atom on the edge of the facet (5L), the adsorption energy is very close to that obtained with a similar geometry in the adatom configuration 2F (−2.39 eV in the alloy and −2.35 eV for the adatom) and both are more stables that in the absence of Bi. However, the energy in 4K (−2.57

eV) is still lower than the one obtained for the (111) facet when Bi and CO are coadsorbed in the configuration 1E. In respect to the (100) facet, the values obtained for the alloy and the adatom configurations are clearly different, suggesting a strong dependence of the CO adsorption not only on the presence of Bi but also on the adsorption site in the nanoparticle. The incorporation of Bi on the cluster increases the adsorption energy for the CO adsorption in bridge configuration (6M, 6N, 7O, 7Q, and 7R) in comparison with the bridge adsorption in Pt with Bi adatom (3G). The configuration 7P, where CO adsorbs on top, is the only one that presents higher adsorption energy that the similar geometry with the dopant (3H). However, all the adsorption sites on the (100) facets for the alloy are less stable than on 23104



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on the (111) sublayer is only stabilized by 0.07 eV in comparison with Bi in the (100). Regarding CO adsorption energies, a significant increase on the adsorption is obtained when Bi is present in the sublayer of a (111) facet. When compared with CO adsorption on pristine Pt the adsorption energies for CO in these sublayer alloys is stabilized by more than 1 eV. The increase on the adsorption energy of CO was also observed for Bi on the surface alloy in (111) facets although, in this case, the increase was only of 0.5 eV. When compared with the adatom structure, the energies obtained for the alloy are always higher, whether the Bi is on the surface or in the alloy. For the (100) facet, the adsorption energy for CO on the neighbor Pt atoms seems to have a small influence from the sublayer Bi. Contrarily to the observed for Bi on the surface (both alloy and adatom) where CO is significantly less stable, the decrease of the energy is only 0.04 eV, when compared with pure Pt. 3.4. Comparison of the Different Structures with Bi. Comparison between the stability of the clusters containing Bi was done by evaluation of the segregation energies for the structures with the same number of atoms (147 Pt and 1 Bi). The segregation energies were calculated using as reference the most stale structure within the ones studied in this study (eq 6) and are presented in Table 5.

pure Pt nanoparticle with a decrease of energy from 0.33 to 1.1 eV. Looking to the adsorption energies obtained with the total reference we can see that the most stable structure is 7O from the (100) facet with Bi in the edge and CO bridge with an energy of −2.45 eV, followed by 5L on the (111) facet also with Bi on the edge but CO on atop position. The remaining structures present energies very similar around −2 eV, with exception of 4J which adsorption energy is lower than −1 eV. 3.3. Bi on the NanoparticlePtBi Sublayer Alloy. Two structures in which a Bi atom was placed in the sublayer (under the outmost Pt layer) of the Pt146 nanoparticle (Figure 4) were

Figure 4. Optimized structures of Pt146 + Bi1 sublayer nanoparticles. Circle letter adsorption sites for CO discussed in the text: (A) (111) facet and (B) (100) facet.

Ei , seg = Ei , cluster − Ecluster , ref (where Ecluster , ref is the energy for structure 7)

(6)

From the results presented in Table 4, it can be observed that Bi on the (100) facet shows the lower segregation energies, meaning that this is the most favored site for Bi on a Pt nanoparticle. When comparing clusters with different structures, it is observed that the alloy with Bi on the surface is thermodynamically more stable than the Bi adatom or the alloy with subsurface Bi. In addition, for all the structures the preferred position for Bi is on the edge of the facets both on (100) or (111) geometry. It should be mentioned that the subsurface alloy cluster presents very high segregation energies (very unfavorable) when compared with the most stable configuration (Bi on the edge of the (100) facet as surface alloy). For sake of comparison, the energies for the different structures with CO investigated in this study are combined in Table 5. The adsorption energies were calculated as described previously for each of the structures. The results show the existence of an electronic effect on the CO adsorption on Pt due to the presence of Bi. With exception to the PtBi sublayer alloy structures, all the energies for the coadsorption are lower in the presence of Bi (less negative) than in the pure Pt. For the sublayer alloy, a significant increase on the adsorption energy is obtained in the (111) facet, however, as previously mentioned, these structures are very unlikely to occur (Table 4). 3.5. Experimental Results. CO adsorption and stripping experiments were performed with irreversible adsorbed Bi modified Pt nanoparticles to gain some insights on the effect the adatom on the CO oxidation reaction. This irreversible adsorption allows the adatom, Bi in the present case, to be deposited only on the Pt surface to a maximum of one monolayer of foreign metal without need of external potential control. The electrode can be then rinsed and transferred to the electrochemical cell that does not contain the corresponding

also computed, with the aim of comparing the influence of Bi position in the alloy on CO adsorption energy. The same procedure than that used for the surface alloy was followed: one Pt atom was replaced in the Pt cuboctahedron structure but in this case in the second layer of the nanoparticle (Pt146+Bi1,sub). The adsorption energies for the optimized structures with Bi on the (111) and (100) facets were obtained having as reference the most stable structure with this configuration (Bi on the (111) facet) and the results are presented in Table 3. The results show that almost no difference in the segregation energies are obtained for Bi in the two computed structures. Bi Table 3. Adsorption/Segregation Energies for the Structures from Figure 4a

Calculated as for Bi adsorption Ei,cluster = Ei,total − EPt147 + EPt1 and EPt1 = EPt147/147; for the CO adsorption Ei,ads = Ei,tot − Eref,cluster(8/9) − ECO for the values with the relative reference and Ei,ads = Ei,tot − Eref,cluster(111, 8) − ECO for energies with total reference. a

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Table 4. Segregation Energies for All the Structures Containing Bi Calculated as Ei,seg = Ei,cluster − Ecluster,ref segregation energies (eV) Bi as adatom

Bi surface alloy

1

2

3

4

5

6

7

adsorption site

3-fold

atop

hollow

center

edge

center

edge

(111) (100)

1.78

3.33

1.27

0.299 0.59

0

0.96

adsorption energies surface site (111)

Pt

3-fold bridge

−2.29 −2.09

atop (100)

bridge

atop

− −2.84

−

Pt + Bi adatom −1.17 0.65 −2.06 0.04 −1.51

−2.01

PtBi surface alloy

PtBi subsurface alloy

−0.87 −1.30

− −3.34

−2.09 −1.26 −1.94 −2.50 −1.73 −1.98 −1.89

3.49 3.41

with peaks at 0.13 and 0.27 V corresponding to hydrogen on the (110) and (100) sites, respectively; followed by the double layer region. The obtained CV perfectly resembles a polycrystalline Pt electrode and adsorption sites with different geometry are expected to exist.33 When Bi is adsorbed on the nanoparticles, the blank profile clearly changes. In the low potential range, the contributions at 0.13 and 0.27 V remarkably diminish, suggesting that the surface steps are blocked by Bi adatoms. In addition, a new peak at 0.62 V appears which is much more evident for high Bi coverage. This peak is well-known to be characteristic of the redox surface process of Bi adsorbed on the (111) terraces whereas the other contributions appear at potentials higher than 0.75 V.34 In this regards, it is worth noting that previous studies on stepped (111) electrodes33 showed that adatoms adsorb on terraces only when step sites have become fully blocked. However, contrarily to the observed for single crystal electrodes, in the case of the nanoparticles, the Bi adsorption on the (111) sites is visible before the full blockage of the steps (region at low potentials). This is due to the limited width of the (111) domains present at the surface of the Pt nanoparticles. CO stripping was performed in the Bi-modified electrodes and the obtained results are plotted in Figure 5B. In the pure Pt nanoparticles, the CO oxidation gives rise to multiple peaks closely associated with the surface sites distribution.33,35 It is possible to observe that the electrode surface is blocked by the presence of CO until around 0.4 V and between 0.4 and 0.6 V a “pre-wave” can be identified in the CV. This “pre-wave” has been described in the literature as CO oxidation at isolated defects and small ensembles.34 The main oxidation peaks occurs at 0.71 V and a smaller peak at 0.77 V is also present. The later contribution has been described as strongly dependent on the amount of ordered (100) domains.34 After Bi decoration, and for both surface coverages, the prewave is completely absent, the peak multiplicity disappears and the main CO oxidation peak shifts to higher potentials. However, for Bi coverages of 0.30, a small shoulder at 0.72 V is still observed in the main oxidation peak. When the Bi coverage is close to the monolayer this lower potential shoulder totally disappears. The blank voltammograms for the PtBi surfaces, before and after CO stripping, were compared and are presented in Figure 6. For both coverages accessed in this study, there are subtle differences in the CVs before and after CO adsorption/ stripping on the modified electrode. The most noticeable difference is the decrease of the currents associated with the Bi redox process at 0.63 V. Is worth to notice that, the potential limits on the experiment were kept within the stability window for Bi irreversible adsorption (>1 V vs RHE).32 For this reason no Bi dissolution from the surfaces is expected as an effect of the electrode potential. Moreover, the differences observed on the blank voltammograms showed that no significant decrease

Table 5. Summary of the CO Adsorption Energies for All the Structures Used in the Simulations CO adsorption site

Bi sublayer alloy

− −2.81

−

ion of the deposited element, which remains irreversibly adsorbed on the surface.32 This method allows the preparation of stable and reproducible surfaces with well-defined surface coverages (see Methods) thus providing excellent opportunities to compare the theoretical calculations here performed with experimental results concerning Pt clusters with Bi adatoms. First, after the deposition of the nanoparticles onto the gold collector, blank CVs in the electrolyte solution (0.1 M H2SO4) were obtained for three samples: pure Pt, Pt−Bi with a surface coverage of 0.39 and Pt−Bi with surface coverage near saturation (θ = 0.85). These voltammograms are presented in Figure 5A. As previously described,25,34 in the voltammetric profile obtained with pure Pt, several features can be identified: the hydrogen adsorption/desorption region from 0.05 to 0.4 V

Figure 5. Cyclic voltammograms for Pt, PtBi (θ = 0.39), and PtBi (θ = 0.85) (A) in the supporting electrolyte solution, 0.5 M H2SO4 at 50 mV/s and (B) for stripping one monolayer of CO at 20 mV/s. 23106



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addition, the computational results reveal that more favorable energies are obtained for the (100) sites, where Bi should preferentially adsorb. Experimentally, this effect can also be observed because Bi adsorption on the (111) sites only occurs when the defects and (100) sites have been already partially blocked. This effect has also been observed experimentally by others.34 Bi in the sublayer alloy is, within all the computed structures calculated this study, the most unstable with an increase of more than 3 eV in energy when compared with the studied surface alloys. In the case of CO adsorption, several studies had been presented in literature mainly dealing with CO adsorption in well-defined bulk Pt surfaces.5,7−9 In our computational results it was found that, CO adsorption on 3-fold sites at (111) facets is favored in comparison with other adsorption sites and that CO atop is very unlikely. Experimentally it has been reported also that CO adsorbed in atop position is favored for low coverages and 3-fold CO is only observed for high coverages.1 This discrepancy in the experimental and DFT results is well documented in the literature and is known as “CO/Pt(111) puzzle”36 because due to some qualitative errors that the low energy site for CO adsorption on Pt(111) in DFT is not the atop site, but rather the 3-fold. When looking into the results for the (100) facet, the calculations show that CO adsorbed in bridge configuration is the only stable (and also the most stable for the different facets geometry) as also reported previously.5 Nevertheless, it is worth to mention that CO adsorption has shown a strong dependence on the facets geometry and adsorption site. The adsorption energies for CO on Pt in our calculations are between −2 and −2.8 eV. These values are considerably higher than those previously reported in literature both for experimental and computational studies (∼1.5 eV).6,24,37 However, the present study deals with the adsorption of a single molecule (opposite to other reports) and it is wellknown that the average biding energies of CO on Pt decrease with increasing coverage due to the existence of string lateral repulsions between CO molecules.6 The Pt−C distances (Supporting Information) are 1.9−2.1 Å depending on the adsorption site geometry and are in good agreement with the values reported in the literature for the same adsorption sites.6 Regarding the systems with both CO and Bi, the coadsorption of these compounds on the Pt nanoparticles revealed strong influence on the adsorption of the single compound. For example, Bi adatoms became very mobile on the surface in the presence of CO due to repulsive interactions. Bi adatoms can be moved to different adsorption sites from those where it is stable when alone on the surface. The distance between the two compounds is maximized in the coadsorption state in order to minimize the repulsive interactions. Moreover, CO adsorption is always weaker in the presence of Bi clearly suggesting changes on the electronic properties of Pt due to the second metal. An overall look into the results obtained for CO adsorption on the Bi containing structures shows similar effects of the Bi when it is on the surface (as adatom or surface alloy). The adsorption energies are lowered than in pure Pt on both cases and are between −1.8 to −2.5 eV depending on the adsorption sites. CO adsorption on (100) sites (in both compositions) is shown to be more sensitive to the presence of Bi with energy losses of ∼1 eV. In other hand, the CO energies for (111) adsorption sites are similar in all structures. Nevertheless, even

Figure 6. Blank voltammograms in the supporting electrolyte (0.5 M H2SO4) for the PtBi electrodes before and after CO adsorption/ stripping at 50 mV/s.

on the hydrogen adsorption/desorption region exists, thus indicating that a rather similar amount of Pt is blocked with Bi and that the same amount of Bi remains on the surface. A closer look into the hydrogen peaks at low potentials shows that the peaks at 0.13 V (associated with the presence of (110) defects on the particle) increases in current while the peaks at 0.27 V show a small decrease. These effects can be related with a certain Bi mobility and the change on the adsorption site of this molecule due to simultaneous adsorption with CO.

4. DISCUSSION As previously mentioned, the main objective of this work was to identify trends in the effect of Bi on CO adsorption on Pt nanoparticles combining experiments and calculation with simplified models. A Pt cluster with 147 atoms in a cuboctahedron nanoparticle was taken as model, and, after proper optimization, one Bi atom and/or one CO molecule were added to the structures in different adsorption sites and/ or facets with (100) and (111) geometries. Effects due to the atoms/molecule concentrations and Bi−Bi interactions or CO−CO interaction are not addressed here. The most relevant configurations, with CO placed in the Pt atom nearby the Bi were accessed. The obtained results show that, on the (111) facet and when Bi is an adatom on the surface, the most favored adsorption site is the 3-fold as observed previously for Pt(111) single crystal surfaces.22 Concerning the (100) facets, comparison with other works are not possible because Bi adsorption on this sites geometry has not been reported neither for nanoparticles or single crystals. For Bi as a surface alloy, the most stable structures are those where Bi is on the edge of the facet. Bi has a larger atomic radii (1.70 Å) than Pt (1.39 Å) and when replacing one Pt atom in the cluster by one Bi, it introduces strain in the particles, so it is not surprising that the most stable site for Bi is on the edge of the facets where the induced stress is lower. These results are in good agreement with the behavior observed in the experimental results (Figure 4A). The Bi adsorption preferentially blocks the voltammetric peaks at lower potentials 0.13 and 0.26 V corresponding to the (110) defects (from the edges of the (111) sites) and (100) sites, respectively, which present higher adsorption energies due to their lower coordination number. In 23107



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adsorption sites but also to facets where it is more stable. This suggests that the differences on the CVs are due to the capability of CO to increase Bi mobility and reorganization at the surface. Bi mobility and reorganization on Pt surfaces has also been shown experimentally for the coadsorption of Bi and NO.39 On the basis of the results reported in this manuscript, both computational and experimental, it can be said that the presence of Bi on Pt nanoparticles has a significant effect on the properties of the bare catalyst. Clearly there is a higher tolerance of these bimetallic catalysts to CO poisoning due to the synergy of the electronic and geometric effects. However, despite the decrease of the CO binding energies in the Pt sites nearby Bi, the adsorption is still possible and this effect cannot explain by itself the poison tolerance of the catalysts. Definitely, the geometric effect generated by the presence of Bi that acts as a site blocker for CO adsorption and by repulsive interactions, generates a “free” CO region.

that adsorption energies are lower for nanoparticles with Bi it cannot be said that CO adsorption is unfavorable in all the surface sites. In comparison to other reports, 21,22,24 where lower adsorption energies for CO at PtBi catalysts were found, the decrease on the binding energy due to the presence of Bi in the Pt nanoparticle from the calculations in this paper is not so pronounced. In any case no stabilization was found as reported by Wen-Feng et al.23 However, as referred above, even for 1 atom and 1 molecule the calculations show that the optimized structures are those that maximize the distance between CO and Bi. It was also observed (results not shown) that CO does not adsorb on Bi atoms, due to repulsive interactions. UHV studies from Paffet et al.38 show also, an increase on the CO− CO repulsions due to the repulsive interaction between Bi and CO. Also, for intermetallic PtBi compounds,12 it was shown that the drop in the affinity of CO for PtBi is a consequence of the increase on the Pt−Pt distances. This expansion makes it very difficult for CO to bind in 3-fold and bridge sites decreasing the binding energies. The CO stripping experiments obtained with the Bi modified Pt nanoparticles show that an increase the adatom coverage on the Pt surface shifts the CO main oxidation peak to higher potentials together with a disappearance of the peak multiplicity. This voltammetric behavior (shift to higher oxidation potentials) has been attributed, mainly to the blockage of the defect sites (that are the most catalytic for CO oxidation) by the adatom.25,34 No enhancement on the oxidation reaction is experimentally observed due to the weakening of the CO binding to Pt during Bi coadsorption. The electro-oxidation of CO from the Pt or Pt alloyed surfaces requires both the activation of water to form oxygen or hydroxyl species and the subsequent coupling of these species with adsorbed carbon monoxide to form the oxidized product (CO2 or COOH), which is rapidly oxidized to CO2). The reaction mechanism follows a Langmuir−Hinshelwood, where the first step is the water activation on a free surface site leading to a surface adsorbed OH (* denotes a free Pt surface site): H 2O+* → OHads + H+ + e−

5. CONCLUSIONS In this paper density functional theory (DFT) calculations and experiments were combined to access the effect of Pt nanoparticles modification with Bi on CO adsorption. CO adsorption energies were calculated for Pt sites nearby Bi atoms for structures with Bi as an adatom and as surface and subsurface alloy. Bi on (100) facets is always more favored for all clusters studied, presenting lower energy difference for the most stable configuration. Comparing the segregation energies for Bi on the Pt cluster with different structures, it is observed that Bi in the alloy as surface dopant is thermodynamically more favored than Bi adatoms or Bi alloy in sublayer. Within the surface alloys investigated, the most stable structures are those where Bi is on the edge of the facet. The results show the existence of an electronic effect on the CO adsorption on Pt due to the presence of Bi. With exception to the PtBi sublayer alloy structures, all the energies for the coadsorption are lower in the presence of Bi (less negative) than in the pure Pt. For the sublayer alloy, a significant increase on the adsorption energy is obtained in the (111) facet, however, when compared with the others this structures are very unlikely to occurs. The experimental results have demonstrated that the increasing of Bi coverage on the Pt surface moves the CO oxidation peak to higher potentials due to the blockage, by Bi adsorption, of the most active sites for CO oxidation (defect sites). The electronic effects on the CO−Pt binding in the presence of Bi obtained computationally showed that the adsorption energies for CO are not inhibitive, although they are lower than in pristine Pt. On the basis of these findings, it can be suggested that the major contribution for the CV response is the blockage of the active sites for CO oxidation by Bi and the decrease of the CO coverage. It was observed, based on the simulations, that CO adsorption results in the mobility of Bi to different adsorption sites and geometries. It was seen that in order to achieve more stable adsorption sites CO can displace Bi not only to other adsorption sites but also to facets where it is more stable. This mobility can explain the differences between the cyclic voltammograms of the modified surfaces before and after CO stripping where different intensity on the H adsorption peaks for different surface sites was found after CO stripping. In summary, both computational and experimental results show that the presence of Bi on Pt nanoparticles has a

(7)

The surface bonded OH is the oxygen donor reacting with the surface bonded CO to form CO2. COads + OHads → CO2 + H+ + e− + 2*

(8)

The proper explanation of the experimental results obtained for the CO oxidation over PtBi catalysts will require innumerous calculations on water dissociation, as well as the calculations of the barriers for reaction between the adsorbed species to form CO2 and they are out of the scope of the present manuscript. However, from voltammetric results it is possible to suggest that the major contribution for the CV response (shift of the oxidation peaks to higher potentials) is the blockage of the active sites for CO adsorption observed by the decrease of the CO coverage6 (from 0.83 for pure Pt to 0.12 on PtBi with higher coverage). These results support the idea that PtBi catalysts are less affected by CO poisoning. Experimentally, it was also found that the cyclic voltammograms of PtBi electrodes are different before and after CO stripping. The observed differences can be explained in terms of the high mobility observed in the simulations of Bi and CO coadsorption. It was seen that in order to achieve more stable adsorption sites CO can displace Bi not only to other 23108



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(14) Herrero, E.; Fernández-Vega, A.; Feliu, J. M.; Aldaz, A. Poison Formation Reaction from Formic Acid and Methanol on Pt(111) Electrodes Modified by Irreversibly Adsorbed Bi and As. J. Electroanal. Chem. 1993, 350, 73−88. (15) Huang, Y.; Cai, J.; Guo, Y. Facile Synthesis of a Bi-Modified PtRu Catalyst for Methanol and Ethanol Electro-Oxidation in Alkaline Medium. Int. J. Hydrogen Energy 2013, 38, 3250−3256. (16) Figueiredo, M. C.; Arán-Ais, R. M.; Feliu, J. M.; Kontturi, K.; Kallio, T. Pt Catalysts Modified with Bi: Enhancement of the Catalytic Activity for Alcohol Oxidation in Alkaline Media. J. Catal. 2014, 312, 78−86. (17) Nie, R.; Liang, D.; Shen, L.; Gao, J.; Chen, P.; Hou, Z. Selective oxidation of glycerol with oxygen with oxygen in base-free solution over MWCNTs supported PtSb alloy nanoparticles. Appl. Catal., B 2014, 127, 212. (18) Demarconnay, L.; Brimaud, S.; Coutanceau, C.; Léger, J. Ethylene Glycol Electrooxidation in Alkaline Medium at MultiMetallic Pt Based Catalysts. J. Electroanal. Chem. 2007, 601, 169−180. (19) Tusi, M. M.; Polanco, N. S. O.; da Silva, S. G.; Spinacé, E. V.; Neto, A. O. The High Activity of PtBi/C Electrocatalysts for Ethanol Electro-Oxidation in Alkaline Medium. Electrochem. Commun. 2011, 13, 143−146. (20) Figueiredo, M. C.; Santasalo-Aarnio, A.; Vidal-Iglesias, F. J.; Solla-Gullón, J.; Feliu, J. M.; Kontturi, K.; Kallio, T. Tailoring Properties of Platinum Supported Catalysts by Irreversible Adsorbed Adatoms Toward Ethanol Oxidation for Direct Ethanol Fuel Cells. Appl. Catal. B: Environ. 2013, 140−141, 378−385. (21) Oana, M.; Hoffmann, R.; Abruña, H. D.; DiSalvo, F. J. Adsorption of CO on PtBi2 and PtBi Surfaces. Surf. Sci. 2005, 574, 1− 16. (22) Kapur, N.; Shan, B.; Hyun, J.; Wang, L.; Yang, S.; Nicholas, J.; Cho, K. First-Principles Study of CO Oxidation on Bismuth-Promoted Pt(111) Surfaces. Mol. Simul. 2011, 37, 648. (23) Wen-Feng, L.; Shi-Gang, S.; Zhao-Wu, T. Investigations of Coadsorption of Carbon Monoxide with S Or Bi Adatoms at a Platinum Electrode by in-Situ FTIR Spectroscopy and Quantum Chemistry Analysis. J. Electroanal. Chem. 1994, 364, 1−7. (24) Pasti, I.; Mentus, S. DFT Study of Adsorption of Hydrogen and Carbon Monoxide on PtxBi1-X/Pt(111) Bimetallic Overlayers: Correlation to Surface Electronic Properties. Phys. Chem. Chem. Phys. 2009, 11, 6225. (25) Chen, Q.; Solla-Gullón, J.; Sun, S.; Feliu, J. M. The Potential of Zero Total Charge of Pt Nanoparticles and Polycrystalline Electrodes with Different Surface Structure: The Role of Anion Adsorption in Fundamental Electrocatalysis. Electrochim. Acta 2010, 55, 7982−7994. (26) Arán-Ais, R. M.; Figueiredo, M. C.; Vidal-Iglesias, F. J.; Climent, V.; Herrero, E.; Feliu, J. M. On the Behavior of the Pt(1 0 0) and Vicinal Surfaces in Alkaline Media. Electrochim. Acta 2011, 58, 184− 192. (27) VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. Quickstep: Fast and Accurate Density Functional Calculations using a Mixed Gaussian and Plane Waves Approach. Comput. Phys. Commun. 2005, 167, 103−128. (28) VandeVondele, J.; Hutter, J. Gaussian Basis Sets for Accurate Calculations on Molecular Systems in Gas and Condensed Phases. J. Chem. Phys. 2007, 127. (29) Goedecker, S.; Teter, M.; Hutter, J. Separable Dual-Space Gaussian Pseudopotentials. Phys. Rev. B 1996, 54, 1703. (30) Krack, M. Pseudopotentials for H to Kr Optimized for GradientCorrected Exchange-Correlation Functionals. Theor. Chem. Acc. 2005, 114, 145−152. (31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation made Simple. 1996, . (32) Clavilier, J.; Feliu, J. M.; Aldaz, A. An Irreversible Structure Sensitive Adsorption Step in Bismuth Underpotential Deposition at Platinum Electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1988, 243, 419−433. (33) Chen, Q.; Berna, A.; Climent, V.; Sun, S.; Feliu, J. M. Specific Reactivity of Step Sites Towards CO Adsorption and Oxidation on

significant effect on the properties of the catalyst. The higher tolerance of these bimetallic catalysts to CO poisoning is due to the synergy of the electronic and geometric effects, where the geometric effects resulting from the repulsive interactions between CO and Bi creating “CO free” sites play a key role on the catalytic effect.

■

ASSOCIATED CONTENT

S Supporting Information *

Relative concentrations of the clusters studied, bond distances, and CO coverages. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(M.C.F.) Telephone: +358503435167. Fax:+358947022580. E-mail: marta.figueiredo@aalto.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Aalto University is acknowledged. REFERENCES

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