Influence of PtâSn System Nanostructure on the Electronic Conditions

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

The Influence of Pt-Sn System Nanostructure on the Electronic Condition of Pt Adsorption Surface Site Francielle Bortoloti, Nicolas Andrade Ishiki, Maria-Laura DellaCosta, Kleper Oliveira Rocha, and Antonio C.D. Angelo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01662 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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The Journal of Physical Chemistry

The Influence of Pt-Sn System Nanostructure on the Electronic Condition of Pt Adsorption Surface Site Bortoloti, F.; Ishiki, N. A.; Della-Costa, M.L.; Rocha, K.O.; Angelo, A.C.D*. Electrocatalysis Laboratory, Faculty of Science, UNESP, Bauru, BRAZIL.

Author to whom correspondence should be addressed: [email protected]

Abstract

This paper describes the synthesis and full characterization of Pt-Sn intermetallics with the same stoichiometry and similar particle size but distinct atomic arrangements (ordered intermetallic, ordinary alloy, and core-shell configuration). We propose that these distinct structures provide different electronic conditions to the noble metal surface adsorption site, which could influence the active catalytic species electrochemical potential. We prepared ordered intermetallic PtSn, ordinary PtSn alloy, and Sn@PtSn core-shell by three different methods. X-Ray diffraction (XRD) confirmed the chemical identity of the products; Energy Dispersive X-Ray Spectrometry (EDX) attested to a 1:1 Pt/Sn atomic ratio for all the materials. High Resolution Transmission Electron Microscopy (HR-TEM) showed that the mean particle size ranged from 2 to 3 nm for the ordered intermetallic and alloy structures and was about 4 nm for the core-shell material. Electron diffraction corroborated the chemical identity of the materials that was earlier suggested by the XRD measurements and evidenced the ordered intermetallic character of the shell in Sn@PtSn. In conclusion, the X-Ray absorption (XAS) technique with synchrotron radiation demonstrated that the different Pt-Sn structures afforded distinct electronic conditions for the same Pt surface adsorption site probably due to changes in the energy of the bond established between Pt and Sn in the investigated materials. The results obtained here will guide the development of (electro)catalysts and will aid understanding of (electro)catalytic processes for various purposes.

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Introduction

Surface science researchers have long sought to explain how the structure of heterogeneous catalysts determines their surface active site electronic density. Elucidating such relationship is crucial to the design of selective heterogeneous catalysts for several important reactions. The distance between surface active sites and their respective electronic densities should drive the proper reactant species structural adsorption (steric factor) and consequently weaken or strengthen the interaction between the surface site and the adsorbate, to improve the surface process. Earlier works have attempted to investigate how catalyst structure affects the surface active site electronic density. However, these attempts have usually failed due to the dated limitations of techniques like X-Ray Photoelectronic Spectroscopy, X-Ray Absorption Near Edge Spectroscopy, Extended X-Ray Absorption Fine Structure, and Transmission Electron Microscopy. The advent of nanoscience still has not provided all the answers regarding the relationship between catalyst structure and electronic density, and the phenomena that take place at the nanoparticle surface still need to be clarified. Many times, researchers misinterpret catalyst structure because particle size impacts a given surface reaction1-3. 4

Alayoglu and Eichorn reported a very interesting investigation on the synthesis, characterization and catalysis properties of different architectural core-shell, alloy and monometallic nanoparticles of the Pt-Rh system. The authors suggested the influence of the structures towards the CO and H2 oxidation reactions due to the alteration of the electronic structures of the surface metals and/or facilitate alternate reaction mechanism relative to pure Pt. Pen and Yang5 published a review where they reproduced the study conducted by V.R. Stamenkovic et al.6 on the effect of the surface crystallographic plane between Pt3Ni and Pt on the oxygen reduction reaction (ORR). The considerably higher Pt3Ni performance was ascribed to changes in the dband center and to decreased coverage with non-reactive (OH)ads. Nai et al.7 reported significant results concerning the selectivity of three types of Ni(OH)2 structures with respect to their electrochemical activity toward L-histidine. Even though the paper did not refer to energy device-related electrode materials, it pointed out that material structure influenced the electrode reaction. The Ni(OH)2 hexagonal hourglass-like structure performed the best, which was attributed to the cooperative mechanism promoted by the structure larger surface area and to the more frequent occurrence of internal structural disorder (stacking faults), which could generate more unstable


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8

protons. In a theoretical and experimental study, Liu et al. studied the CO tolerance of PtSn nanoparticles prepared as alloy, core-shell and intermetallic with same composition and size. Intermetallic was reported to be converted to PtSn@Pt through a successive potential cycling in CO-saturated H2SO4 solution. XRD measurements showed PtSn intermetallic core and the authors attested the superior CO-tolerance of the PtSn@Pt catalyst to the electronic effect while intermetallic was due to the 9

bifunctional mechanism. Santos et al. described a similar relationship between the ordered intermetallic PtSn d-band center of materials and their electrocatalytic activity toward the hydrogen oxidation reaction. More recently, Di Salvo et al.

10

studied the

thermal conversion of PtSn alloy phases to ordered intermetallic compounds and confirmed that the ordered intermetallic displayed enhanced stability as compared to the solid solution alloy. The authors also pointed out that the catalytic behavior of the alloys and their intermetallic phases could be quite different. Calle-Vallejo et al.

11

introduced the concept of “coordination-activity” plots to predict the geometric structure of the optimal active site for the ORR. Their hypothesis was based on the surface adsorption energies of the reactants (namely HO*).The authors stated that the nearest neighbors were important to the surface site electrocatalytic activity and concluded that selectivity unquestionably involved oxidation of H2 or small molecules at the Pt surface 12

adsorption sites. Zou et al. published studies on the ORR at non-ordered and ordered Pt-Ni and their tolerance to methanol. Ordered intermetallic Pt-Ni nanoparticles exhibited improved mass and specific activities for the ORR and methanol tolerance. The authors suggested that the stronger interaction between Pt-Ni within the ordered PtNi/C structure provided Pt and Ni with increased oxidation resistance in an ordered intermetallic phase, which may have boosted the ORR kinetics. Calvillo et al.13 published very recently the results obtained from the investigation of the nanostructures effects on the activity, stability and selectivity of PtSn ethanol oxidation catalysts. The authors synthesized PtSn nanoparticles with different atomic ratios and a Sn modified Pt catalyst. They indicated that the modification on the electronic environment of Pt in Pt-Sn alloys results in a weaker adsorption of intermediates, resulting in an incomplete oxidation to CO2. On the other hand, when the electronic structure is not modified, the amount of CO2 produced increases. The relatively few afore mentioned papers presented a slight and non-explicit connection between the structural disposition of adsorption sites in relation to each other or to foreign atoms and the activity of these adsorption sites toward a certain electrode reaction. Although the connection was sometimes faintly suggested, this



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subject was not adequately considered on the basis of an experimental approach that targeted a better understanding of the phenomenon. Establishing a relation between the geometric structure of materials and their electronic configuration is a challenging task for most scientists in the field of electrocatalysis. The surface adsorption site electrochemical potential can be described as: µa = µa + z. F. ∅ , e

(1)

C

where µa is the species electrochemical potential in the α phase; µa is the chemical e

C

potential of the same species in the same phase; z is the charge; F is the Faraday constant; and ø is the potential at the point where the species is located (and which can be later related to the electrode potential). Regarding a surface active site species, this species electrochemical potential can be seen as the surface site ability to be active for a given electrochemical process. The chemical potential µa depends on the species intrinsic nature and on the chemical environment in which it is inserted. µa = µO + χa , C

(2)

int

where µO represents the species intrinsic chemical ability (chemical nature), and χa is the species ability to interact with the environment. Deeper analysis of Equations (1) and (2) provides the relation of the active site chemical potential with the environment where it is located and therefore with its electrochemical potential. In this approach, we propose that the surface species of distinct material structures such as ordered intermetallics, alloy, and core-shell display different bonding energies, which would impact the electronic density of the noble metal surface adsorption sites and the surface species electrochemical potential. This is a vital characteristic for reactant, intermediate, and product adsorption if we consider that the adsorption energy of these species could be affected. Given this scenario, in this study we report the synthesis and full characterization of Pt-Sn nanoparticles with different structural configurations—solid solution alloy, ordered intermetallic phase, and core-shell structure—as an experimental approach to investigate how said configuration influences the surface adsorption site electronic state. We evaluate whether the distance (and consequently the level of interaction) between neighbor species impacts the surface adsorption site electronic state differently. We strongly believe that this approach will improve our understanding of the cooperative and/or synergistic action between neighbor surface atoms and thus


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contribute to the development of heterogeneous catalysts, catalysis theories, and (electro)catalyst design.

Experimental Synthesis Ordered intermetallic PtSn nanoparticles were prepared by following the modified 14

polyol method originally proposed by Cable and Schaak . Vulcan XC-72 Carbon black (Cabot®) nanoparticle support was previously heated in an oven at 400°C for 4h under Argon (White Martins, 4.8) atmosphere. Stoichiometric amounts (20% m/m, metal to Carbon black) of hexachloroplatinic acid (Fluka, ~38%Pt), Tin(II) Chloride(Merck, p.a.), and

the

dispersing

agent

Polyvinylpyrrolydone

(PVP,

Sigma

Aldrich)

were

homogenized with Carbon black in Tetraethyleneglycol (TEG, Merck, 98%) with the aid of an ultrasonic bath. Oleic acid (Sigma-Aldrich, 99%) and Oleylamine (Aldrich, 70%) were also added to the homogenized solution to disperse the particles better. Under N2 (White Martins 5.0) atmosphere, the mixture was refluxed at 260°C for 8 h; Sodium Borohydride (Sigma Aldrich, 98%) was slowly added as complementary reducing agent. The product was exhaustively washed with Acetone (Quimex, p.a.) and deionized water (Synergy-UV Ultrapure Water, 18 MΩ); a centrifuge (Excelsa R Model 206 MP) was employed between the washings. The material was pre-dried at 60°C for 24h and heated at 300°C for 1h under Argon (White Martins, 4.9) atmosphere to eliminate organic impurities that remained on the material. The PtSn (1:1) alloy was obtained via the microemulsion method proposed by 15

Eriksson and co-workers . Stoichiometric Tin(II) Chloride amounts were added to a freshly prepared Carbon black dispersion in deionized water. The organic solution was prepared by solubilizing Sodium Dioctylsulfosuccinate (Sigma-Aldrich, 89%) in nheptane (Merck, 98%). After the two solutions were mixed under vigorous stirring and N2 bubbling, Sodium Borohydride (Sigma-Aldrich, 98%) and Hexachloroplatinic acid (Fluka, ~38% Pt) were added. The emulsion was kept under stirring and N2 bubbling at room temperature for 12h. The washing and drying processes were the same as the processes described for the ordered intermetallic PtSn material. To obtain the Sn@PtSn core-shell material, the method used to prepare the ordered intermetallic PtSn nanoparticles was modified as follows. Tin(II) Chloride was dissolved in a freshly prepared Carbon black, Sodium Hydroxide, Oleylamine, and Oleic acid dispersion in TEG under vigorous stirring and N2 bubbling.


Sodium


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Borohydride was added after 30 min, and the mixture was heated to 120°C. Thirty minutes later, Hexachloroplatinic acid was added. The system was kept under reflux at 140°C for 30 min. The washing and drying processes were the same as the ones described previously for the ordered intermetallic PtSn material.

Characterization The chemical and structural identification of the synthesized materials was accomplished by powder X-ray diffraction (XRD) on a Rigaku MiniFlex 600 Diffractometer operating at 40kV and 15mA with Cu cathode (λ=1.5406 A). The diffractograms were registered for 2θ values ranging from 20° to 90° at a scan rate of 10°/min; the scan step was 0.04°. Energy Dispersive Spectrometry (EDS) was used to determine the mean atomic composition of the materials. The X-Ray beam was focused on five different points of the sample surface with the aid of the EDS detector (Oxford Instruments) operating at 15kV and work distance varying from 10 to 8 mm, coupled to a Zeiss Electronic Microscope Model LS-15. High-resolution transmission electron (HR-TEM) micrographs were taken by using a 2

FEI TECNAI G F20 transmission electron microscope operating at 200kV and with a HRTEM Gatan camera. The samples were prepared by depositing a thin layer of the material on a carbon grid. X-Ray Near Edge Absorption Spectroscopy (XANES) at the Pt L3-edge (11,564 eV) was performed in the D06A-DXAS beamline of the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil). The D06A-DXAS is a dispersive beamline equipped with a focusing curved Si (111) monochromator operating in the Bragg mode, which selects the X-ray energy bandwidth (11,400–12,000 eV), and with a 1152 × 1242 (500 × 900) pixel CCD solid-state detector, which converts X-rays into visible light for spectral analysis. The samples were obtained as self-supporting pellets containing a paste pressed onto a carbon hydrophobic tissue. The paste was prepared with 50 mg of the sample, 2 mL of Isopropanol (Sigma Aldrich), and 29µL of Nafion (Aldrich, 117 solution). Potential - resolved XANES spectra were acquired in situ in an acrylic cell designed for the transmission data acquisition mode. This cell employed Pt gauze as counter electrode and a Reversible Hydrogen Electrode (RHE) as reference. The electrolytic solution was KOH (Merck, 85%) 0.5 mol.L

-1


at room temperature. The

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working electrode potential was applied and monitored with an Autolab 302N Galvanostat/Potentiostat.

Results and discussion Figure 1 illustrates the series of powder X-ray diffractograms recorded for all the materials synthesized herein. The diffractograms also present the main peaks (the 16

most intense ones) of each material as obtained from the Crystmet Data File . The diffractograms of

the prepared materials matched the

standard

diffractograms, which attested to their identity and to the success of the syntheses. In particular, Figure 1A displays the diffractogram for PtSn ordered intermetallic where it is possible to verify the excellent match between the experimental peaks and data found in the Database16 confirming the material as ordered intermetallic PtSn in hexagonal structure. The data the PtSn alloy X-ray profile (Figure 1B) exhibited displaced peaks—at 39º, 46º, 67º, 81º, and 85º, attributed to the planes (111), (200), (220), (311), and (222), respectively—as compared to the pure Pt X-ray profile. The former profile resembled the pure Pt face-centered cubic structure profile. Such displacement is expected for alloys where the second metal is incorporated into the metal solvent crystalline net. In the present case, Sn had larger atomic radius (1.58 Å) as compared to Pt (1.39 Å). This difference shifted the diffraction peaks to smaller angles due to crystalline cell expansion17. The core-shell material X-ray diffraction pattern (presented in Figure 1C) confirmed the presence of peaks related to ordered intermetallic Pt-Sn at 30°, 42°, 44°, 62°, and 80°, which corresponded to the planes (101), (102), (110), (202), and (212), respectively. Peaks referring to pure Pt also emerged at 40°, 68°, 81°, and 86°, assigned to the planes (111), (220), (311), and (222), respectively. These peaks suggested that a shell corresponding to the ordered intermetallic material emerged. We proposed this ordered intermetallic formation on the basis of the temperature employed in the syntheses, which is known to favor a more stable structure (ordered intermetallic). This point will be clarified with the aid of the other characterization techniques used here.



A)

B)

C)

Figure 1: Powder X-Ray diffractograms of the (A) ordered intermetallic PtSn; (B) PtSn alloy; and (C) core-shell Sn@PtSn materials. Black lines represent the experimentally obtained profile; blue and red lines correspond to the standard profiles of the materials registered in the Crystmet database.


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If we consider the Pt-Sn alloy and the Bragg equation (Equation 3) depicted below, we can calculate the α lattice cubic parameter for the diffraction peak corresponding to the plane (220)18-19: α exp

√2.

=

(3)

sene

According to the literature19, the alloy atomic proportion can be determined by Equation (4), whereas the Vergard Law (Equation 5) can help to estimate the relative amount of Sn in the alloy (alloying degree). MSn

χ

M

(4)

k

Sn=

χSn

(l-χSn )(

Where

χ Sn

Sn

,

(5)

)nom

represents the Sn atomic proportion in the alloy; M indicates the metal (Pt);

αM corresponds to the cell parameter for Pt; k is a constant value of 0.035219; and (Sn/M)nom is the Sn alloying degree in the Pt cell. For the Pt-Sn alloy, the experimental α value was 3.992 Å. This value was higher than the α value obtained for pure Pt (3.923 Å), which indicated that Sn addition expanded the Pt crystalline cell. This finding reinforced the earlier mentioned Pt crystalline cell expansion provoked by Sn atom addition to the Pt crystalline matrix. The alloying degree was 24.4%, and the Sn atomic proportion in the alloy was 0.20. Sn 19

oxide formation seemed to be the main reason for the low alloying degree . We accomplished EDS for different samples of the synthesized materials, at five distinct points. All the materials contained the Pt and Sn elements at a 1:1 ratio, which enabled us to investigate the Pt atom neighborhood influence without having to worry about stoichiometry. Table 1 lists the results we obtained after we analyzed all the prepared materials.



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Table 1: Nominal and experimental Pt/Sn ratio in the prepared materials as calculated from EDS measurements.

Atomic ratio Pt-Sn material Nominal

EDS

Pt

Sn

Pt

Sn

Ordered intermetallic

1

1

1.0

1.1

Alloy

1

1

1.0

1.1

Core-shell

1

1

1.0

1.1

The nominal and experimental ratios matched and pointed to a 1:1 atomic proportion for all the synthesized materials. Figure 2 shows the micrographs recorded with the HR-TEM technique for all the synthesized materials. In general, the micrographs attested to very good nanoparticle dispersion on the Carbon black surface. The particles were spherical, and the histograms in Figure 2 revealed a mean radius of about 2 to 3 nm for the Pt-Sn alloy and for the ordered intermetallic Pt-Sn. The Sn@PtSn core-shell nanoparticle had twice this diameter. As stated previously, the fact that the particles presented roughly the same particle size allowed us to examine the Pt site electronic condition without the influence of this parameter. The larger core-shell particle size was certainly a consequence of the synthesis methodology, which involved particle core formation followed by shell deposition. However, we believe such difference in particle size did not affect our investigation into the surface Pt site electronic condition.


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C)

Figure 3: Electron diffraction images recorded for the nanoparticles in the (A) ordered intermetallic PtSn; (B) PtSn alloy; and (C) core-shell Sn@PtSn nanomaterials.

In Figure 3, which contains electron diffraction images, we were able to measure the interplanar distances recorded as a result of the beam electron diffraction. The interplanar distance calculated for Figure 3A was 0.22 nm and was related to the ordered intermetallic Pt-Sn hexagonal structure plane (102) (Crystmet ID:17292, d = 2,16 Å). As for the PtSn alloy (Figure 3B), the interplanar distance was calculated as 0.33 nm, which could not be ascribed to any plane of the PtSn alloy cubic structure. The largest diffraction peak obtained for the Pt matrix corresponds to the plane (111), for which d=0.229 nm. This apparent discrepancy should be due to crystalline cell expansion resulting from addition of the larger Sn atom into the Pt matrix. Analysis of Figure 3C produced two different interplanar distance values: 0.19 nm for the diffraction in the particle center and which referred to the Sn plane (211) (ID:503272, d = 1,908 Å), and 0.22 nm for the surrounding part of the particle, which corresponded to the ordered intermetallic PtSn plane (102) (ID: 17292). These findings confirmed that the core-shell structure Sn@PtSn was formed and reinforced the X-Ray diffraction data, which suggested that the ordered intermetallic PtSn shell was product. It is not surprising the formation of the ordered intermetallic PtSn as shell in the nanoparticle since Liu et al.8 and DeSario and DiSalvo10 have already noticed the formation of that very stable phase from other PtSn phases as a result of potential cycling in COsaturated H2SO4 solution and heat evolution, respectively. Therefore, in the present paper it was observed the same phenomenon for the Sn@PtSn material and with agreement with the above mentioned studies and it can be proposed that the material


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NP synthesized was the core shell formed by a Sn core surrounded by an ordered intermetallic PtSn shell obtained probably due to the thermal treatment procedure experimented in this paper, which presented shifts in the X-Ray diffraction profile peaks due to mass effects. Figure 4 displays the XANES spectra recorded for each nanostructured material in the Pt L3 edge for an electrode potential of 0.4V. At this electrode potential, we did not detect any faradaic processes that could transform the surface of any of the investigated materials. Consequently, we were able to compare the d-band occupancy of the materials disregarding any influence of the surface oxidation state. The white lines recorded for the ordered intermetallic PtSn and for the PtSn alloy became significantly less intense as compared to commercially available pure Pt nanoparticles (E-TEK, size 2.820 - 3.021 nm) , which evidenced increased Pt 5d-band occupancy as a result of the Sn electron donor effect toward the Pt site. Mukerjee and McBreen22 reported the same effect when they investigated Sn addition to Pt in alloys. These authors verified that Sn incorporation to the Pt cell partially filled the Pt d-band and increased the Pt-Pt bond distance. In a similar study, Shukla and co-workers23 stated that Pt atoms were more electronegative than Sn atoms, so the former atoms removed electrons from Sn, to polarize the Pt-Sn bonds. On the other hand, the Sn@PtSn material had more intense white line as compared to Pt, which suggested that the Pt sites at the Sn@PtSn surface had lower electronic density than the corresponding sites in the other investigated materials. These results confirmed that materials with the same chemical identity, stoichiometry, and particle size could have distinct electronic density on the noble metal depending on the structural arrangement of the atoms around the platinum atom adsorption site. Such a phenomenon could facilitate

our

understanding

of

the

structure–electronic

state–catalyst

electrocatalyst performance relationship toward a given heterogeneous process.


and


Figure 4: XANES Pt L3 edge spectra for all the prepared Pt-Sn nanomaterials at a potential of 0.4V.

Figure 5: Influence of the electrode potential on the white line intensity recorded for the Pt L3 edge of the prepared Pt-Sn nanomaterials in KOH 0.5 mol.L-1 at room temperature.


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Figure 5 shows how the white line intensity behaved as a function of the applied electrode potential in alkaline solution. The electronic condition of the materials remained constant along the whole potential window, which demonstrated the Pt atom electronic condition in each material as a function of the applied potential. However, there is a significant difference among the electronic condition of the adsorption surface site (Pt) in the studied materials. The Pt white line intensified slightly for the Sn@PtSn material at more positive potentials, which will be the object of our next study on the electrocatalytic activity of this material as anode for the oxidation of small organic fuel molecules and hydrogen. On the basis of the reported results, for the same chemical identity and composition, distinct structural arrangement of the atoms in a material provides the material surface with different electronic conditions regarding the adsorption site (platinum in the present case). We propose that the energy involved in the Pt-Sn interaction, which is different for the distinct structures, is the main factor influencing the electronic density around the atoms. Because for every heterogeneous process the adsorption step usually assumes a definite role in reactant transformation to products and crucially depends on the surface adsorption site electronic condition, the Pt-Sn interaction feature will strongly influence the heterogeneous process.

Recalling

Equation (2), the term χa , which represents the neighborhood influence on the surface adsorption site chemical potential, heavily depends on the energy involved in the interaction between the species.

Conclusion We have synthesized nanostructured Pt-Sn materials as ordered intermetallic, ordinary alloy and core-shell structural arrangements at a 1:1 Pt:Sn ratio, with mean size ranging from 2 to 4 nm. We have also characterized the electronic density of the surface adsorption site (Pt). The way atoms are interconnected (geometric structure and corresponding bonding energy) influences the Pt adsorption site electronic condition, as expected. This condition will certainly affect how reactants, intermediates, and products adsorb onto the active electrode site during the electrode reaction. The performance of these materials during fuel oxidation reactions in proton exchange membrane (PEM) fuel cells are currently being assayed to confirm the influence of the structural arrangement of the atoms on the electrochemical activity of the materials.



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Acknowledgements Authors are grateful to Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) for the financial support of this work (#2013/05634-8). F. Bortoloti thanks to the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (Capes) and N.A. Ishiki and M.L.F. Della-Costa thank to the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) for their respectives fellowships.

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