Surface Stability of Pt3Ni Nanoparticulate Alloy Electrocatalysts in

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Surface stability of Pt3Ni nanoparticulate alloy electrocatalysts in hydrogen adsorption Hana Hoffmannova, Maki Okube, Valery Petrykin, Petr Krtil, Jonathan E. Mueller, and Timo Jacob Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401562t • Publication Date (Web): 01 Jul 2013 Downloaded from http://pubs.acs.org on July 6, 2013

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Surface stability of Pt3Ni nanoparticulate alloy electrocatalysts in hydrogen adsorption Hana Hoffmannova1, Maki Okube1+, Valery Petrykin1, Petr Krtil1*, Jonathan E. Mueller2, and Timo Jacob2*. 1

J. Heyrovský Institute of Physical Chemistry, Dolejškova 3, Prague 18223, Czech Republic 2

Institut für Elektrochemie, Universität Ulm, Albert-Einstein-Alee 47, D-89081 Ulm, Germany

Corresponding Author * [email protected]. *[email protected]

ACS Paragon Plus Environment

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ABSTRACT: Nanoparticles of Pt/Ni alloys represent state of the art electrocatalysts for fuel cell reactions. Density functional theory (DFT) based calculations along with in-situ X-ray absorption spectroscopy (XAS) data show that the surface structure of Pt3Ni nanoparticulate alloys is potential dependent during electrocatalytic reactions. Pt3Ni based electrocatalysts demonstrate preferential confinement of Ni to the subsurface when the electrode is polarized in the double layer region where the surface is free of specifically adsorbed species. Hydrogen adsorption triggers nickel segregation to the surface. This process is facilitated by a high local surface coverage of adsorbed hydrogen in the vicinity of the surface confined Ni due to an uneven distribution of the adsorbate(s) on the catalyst’s surface. The adsorption triggered surface segregation shows a non-monotonous dependence on the electrode potential and can be identified as a breathing of the catalyst as was proposed previously. The observed breathing behavior is relatively fast and proceeds on a timescale of 100-1000s.

KEYWORDS: Pt3Ni alloy · hydrogen adsorption · EXAFS · breathing catalysts.


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INTRODUCTION Power generation in fuel cell-based devices is among the most pressing issues stimulating electrocatalytic research in recent decades. Both experimental and theoretical approaches have clearly outlined the potential of nanoparticulate Pt-based catalysts in these applications, since they allow the catalytic activity to be maximized at both the anode and cathode 1. It has been repeatedly reported that alloying Pt with less noble metals in binary alloy systems (containing, e.g. Ni2−4, Co5,6 or Cr2) or in ternary alloys (adding either Fe or Cu to the previously listed binary alloys7) leads to significantly improved catalytic activity compared with pure Pt4. It has been concluded that Pt3Ni and Pt3Co alloys represent so far the most promising catalysts for the oxygen reduction reaction (ORR) in acidic media3,8,9. The presence of a second less noble metal complicates the analysis of the catalyst’s behavior. The enhancement of the catalytic activity of Pt-based alloys including those in Pt/Ni systems most likely results from conjoining various mechanisms such as the alloying-induced strain dband center shift10 or chemical modifications of the active reaction sites. Therefore, the nature of the catalytically active sites remains poorly understood. Furthermore, alloying has pronounced effects both on the chemical composition of the surface as well as on the topology of the prospective active sites11,12. A variety of Pt-based bimetallic alloys have been studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and medium energy ion scattering (MEIS).13,14 Regarding Pt/Ni at various bulk compositions, it was found that the system forms fcc solid solutions over the whole range of compositions.15 Concerning the ordering of the surface, Bardi et al. have shown that the similar Pt3Co(111) system first forms an ordered fcclike structure, which during the ultra-high vacuum (UHV) treatment changes into a disordered


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structure.16 This finally leads to segregation on the surface. For example, the Pt3Ni(111) surface, which is of particular interest in this paper, shows a Pt enriched surface followed by a damped oscillatory Pt/Ni composition profile in the first three surface layers.17 Despite the segregation, the surface relaxation is less than 2%.14 Though a complete abatement of Ni from the surface layer of Pt3Ni and a strong Ni enrichment in the 2nd layer was also reported for electrochemical conditions in the hydrogen UPD-region18, this observation seems rather counterintuitive since Ni ought to adsorb hydrogen more strongly than Pt19. This instability, whether intrinsic or adsorbate-induced, reportedly limits the practical application of Pt/Ni- and Pt/Co-based materials in catalytic processes7, 19-21. Theoretical studies mainly focused on the chemisorption behavior of Pt3Ni in comparison with pure Pt. The theoretical assertion relevant to both hydrogen evolution as well as oxygen reduction proposed fcc type three-fold positions containing two Pt and one Ni atom as the most probable adsorption sites. All theoretical predictions and rationalizations of the experimental behavior of Pt/Ni based alloy systems are based on the assumption that the alloy’s surface structure and composition do not change under operating conditions. This assumption, however, does not hold universally, as has been shown previously in heterogeneous catalysis22 [22] and in electrocatalysis related experiments23 [23] and as we will show using in-situ X-ray absorption spectroscopy (XAS). This paper presents evidence of the Pt3Ni alloy catalyst’s intrinsic instability based on the interpretation of voltammetry, in-situ X-ray absorption spectraand DFT calculations. The local structures refined from spectra acquired at conditions of hydrogen adsorption/evolution clearly outline the limited ability of conventional electrochemical as well as non-electrochemical techniques to assess the alloy surface composition. The local structure development based on the


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XAS refinement is compared with the results of quantum chemical calculations to rationalize the behavior of the catalyst and to outline the nature of the characteristic voltammeteric imprint of Pt containing alloys. EXPERIMENTAL SECTION The carbon supported 5 nm Pt3Ni nanoparticulate alloy samples were obtained from Johnson Matthey and characterized with X-ray diffraction (see Figs. 1S of the Supporting information) using a Bruker Advance 8 diffractometer and Cu Kα radiation. The electrodes employed in electrochemical experiments were cast on a 5 mm glassy carbon disc from a suspension containing 10 mg of the catalyst, 5.00 mL Millipore water, 4.95 mL isopropyl alcohol, and 50 µL of 5 wt.% Nafion® ionomer solution . The X-ray absorption spectra were collected at the CEMO beam line [Si(111) and Si(311) monochromators] of the Hasylab synchrotron (Deutsches Elektronen Synchrotron (DESY)). Xray absorption spectra were measured under conditions of the electrochemical experiments on electrodes containing approximately 5 mg of Pt3Ni attached on ca. 40 mg of graphite stabilized by Nafion (Aldrich, 98%). The data were acquired in transmission mode on Pt L3 edge (11534 eV) and in fluorescence mode on Ni K-edge (8333 eV). The fluorescence spectra were acquired using a 7-channel Ge detector. All data normalization, smoothing and background subtraction were performed in the IFEFFIT software package.

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A detailed description of the refinement

procedure is given in the supporting information. The voltammetric experiments were carried out in a single compartment cell with Pt auxiliary electrode and saturated calomel reference electrode (SCE). The applied potential was controlled using a PAR 263A potentiostat. The 0.05M H2SO4 solution used for electrochemical experiments was saturated with argon prior to the measurements. All potentials were measured and are quoted


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with respect to the saturated calomel electrode (SCE). Periodic density functional theory (DFT) calculations utilized the generalized gradient approximation (GGA) for exchange correlation energy developed by Perdew, Burke and Ernzerhof (PBE).25 The core electrons of Pt, Ni and O were replaced with angular momentum projected, norm-conserving pseudopotentials. Valence electrons were described using a doublezeta plus polarization basis set as implemented in the SeqQuest periodic DFT code.26

RESULTS AND DISCUSSION The electrochemical behavior of the PtNi alloys is reflected in Fig. 1. Reversible adsorption of hydrogen represents a prevailing process at potentials negative to 0 V (vs. SCE) in the absence of oxygen. An exposure to potentials positive to 0.55 V indicates surface oxidation, eventually leading to the formation of stable surface oxides, the reduction of which requires significant activation in the reduction scan of the voltammogram. The potential region between 0.0 V and ca. 0.5 V corresponds to a “double layer region,” where the alloy surface ought to be free of chemisorbed species and, therefore, electrocatalytically active, e.g. in oxygen reduction, which proceeds with strong preference for a 4-electron reaction pathway.4


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Figure 1: Cyclic voltammogram of a Nafion stabilized carbon supported nanocrystalline Pt3Ni electrode in Ar-saturated 0.05M H2SO4. Voltammogram was recorded at a polarization rate of 20 mV/s.

The chemisorption of hydrogen is often used in an analytical manner to estimate the surface Pt content by comparison of the charge observed for the hydrogen under-potential deposition on the studied material with that of single crystal Pt electrodes.12 The applicability of this approach has recently been challenged, since in the case of Pt/Ni alloy systems it is known that this HUPD approach consistently yields results, which conflict with data originating from, e.g. CO adsorption.27 In fact, the charge corresponding to HUPD is sensitive to the experimental time-scale and the electrode history as shown in Fig. 2.


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C

Figure 2: Voltammetric curves (a) and integrated charge (b) of the uderpotential deposition of hydrogen on Pt3Ni nanoparticulate electrode in 0.05 M H2SO4 recorded consecutively at various polarization rates (see Figure legend). Pane C shows a comparison of the last voltammogram recorded before polarization in HUPD region (blue curve) with first two cycles (green and red curves respectively) recorded after cycling the electrode in HUPD region for 2 hours. All voltammograms in part c were recorded at the polarization rate of 20 mV/s.

The integral of the charge in the HUPD region decreases with increasing time-scale (see Fig. 2). The observed HUPD electrode history dependence suggests profound changes in the alloy’s surface structure and composition taking part on a time-scale comparable with voltammeteric experiments. Also a comparison of volatammograms reflecting the electrochemical behavior of Pt3Ni catalyst before and after extensive polarization in HUPD region clearly shows that Ni gets accumulated at the catalysts surface and is oxidatively stripped during anodic polarization (see Fig. 2c). The electrode history dependence is shown in Figure 2S of the supporting information.


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The suggested structural changes are reflected in surface segregation triggered by hydrogen adsorption, which can be followed by in-situ X-ray absorption spectroscopy. EXAFS functions extracted from Ni K edge and Pt L3 X-ray absorption spectra of the Pt3Ni alloy electrodes polarized at constant potential are shown in Figure 4S of the supporting information. The local environment of Pt shows negligible dependence on the applied potential. This can be attributed to a large excess of subsurface (bulk-like) Pt, which is not affected by the electrocatalytic reactivity. The refinement results of the Ni EXAFS functions are summarized in Table 1S of the supporting information. The hydrogen adsorption affects the chemical composition of the surface (see Fig. 3).

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Figure 3 Average coordination number (CN) (a) and local chemical composition in the vicinity of Ni (b) for a Nafion stabilized nanoparticulate Pt3Ni electrode, polarized at various potentials in the HUPD region. Local chemical composition was calculated from independently refined Pt and Ni occupancy in the first coordination shell.


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The coordination number (CN) of Ni changes with electrode potential in the hydrogen adsorption evolution region. The average coordination number initially decreases from a value close to 12 characteristic for preferentially sub-surface accommodated Ni in the double layer region to a lower value of approximately 9 suggesting significant Ni confinement to the surface in the hydrogen adsorption region. The coordination number behavior is matched with complementary trends in local chemical composition (see Fig. 3B) when the sub-surface confinement of Ni results in apparent Ni clustering reflected in higher than average Ni content in the Ni environment The presented experimental data outline an intrinsic instability of Pt/Ni based catalysts in a relatively narrow range of electrode potentials. The recorded voltammetric and in-situ EXAFS based trends intuitively suggest stronger adsorption of hydrogen on Ni than on Pt, which has been predicted previously.18 The hydrogen adsorption triggered surface segregation processes take place on time scales of 102 to 103 s. The short time-scale along with relatively good reversibility allows for an identification of the observed behavior with the dynamic formation of active sites proposed previously.

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It needs to be noted that a Ni

confinement to the surface under reducing conditions was observed by Strasser in gas phase experiments23. Although this process is plausible it is not supported by our data since we were unable to track any formation of Ni-O bonds; also the oxidative nature of Ni stripping shown in Fig 2c contradicts this hypothesis. To rationalize the experimental studies we compared our results to quantum chemical simulations. DFT calculations reveal that in the absence of adsorbates, a Pt skin represents the thermodynamically most stable configuration favored by 0.33 eV per surface atom (see Fig. 4).


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The hydrogen adsorption changes the energetics of the alloy leading eventually to Ni transport to the surface in accordance with the experiment. The overall segregation energy shows a pronounced dependence on the hydrogen surface coverage. In contrast to Pt skin materials which adsorb hydrogen at top positions the surface-confined Ni atoms direct the hydrogen adsorption into three-fold fcc sites, which conforms to the known adsorption modes of hydrogen on pure Pt(111) 30 and Ni(111) 31 . It should be noted, however, that the energy discrimination between top and fcc positions in hydrogen adsorption on Pt-Ni based surfaces is not significant (see Table 2S of the supporting information). The Ni transfer to the surface has to be sanctioned with at least 1.0 ML equivalent hydrogen coverage as illustrated in Fig. 4.


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Figure 4: Surface segregation energies in eV per 2×2 simulation supercell for Pt3Ni slabs with no adsorbates, ½ ML H, 1 ML H and 1½ ML H. Surface compositions are (left to right) 0%, 25%, 50%, 75% and 100% Ni. H atoms are white, Ni atoms are blue, O atoms are red, and Pt atoms are gray. Side views of slabs along the bottom illustrate Ni-Pt layer composition for various surface segregation states. Top views of slabs along the right side illustrate the most stable adsorbate configuration and metal surface composition for each coverage.

Formation of higher than one monolayer surface coverage strengthens the previously reported trend in which the overall energetics of the alloy surface state favors the surface composition combining presence of both Pt and Ni. The existence of either Pt or Ni skin arrangement is not, therefore, very probable since these arrangements are disfavored by 0.91 and 0.52 eV per atom, respectively. Therefore, our studies apparently contradict the SAXS behavior of P3Ni single


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crystals, which reportedly remain stable with Pt skin also in HUPD region.21 The presented calculations, however, focus on the thermodynamics, while a full investigation of the segregation processes would need to be complemented by taking into account kinetic aspects based on calculations of the actual diffusion processes. The DFT based segregation energies are apparently also in conflict with the amount of adsorbed hydrogen obtained in voltammetry, which is always smaller than 1.0 ML (see Fig. 2). This discrepancy can be resolved assuming a non-homogeneous distribution of the adsorbed hydrogen on the surface to achieve a local surface coverage higher than 1.0 ML. Such a situation is likely associated with a restriction of the adsorbed hydrogen surface mobility, which is rather high on Pt surfaces.32 Surface confined Ni atoms may be viewed as defects restricting the adsorbed hydrogen from attaining thermodynamically most stable adsorption sites, hence facilitating high local hydrogen coverage in its vicinity. The Ni atoms in turn also facilitate further Ni transfer to the catalyst’s surface. The apparent drop in surface Ni representation at potentials corresponding to the onset of bulk hydrogen evolution can be linked to a decrease in the total hydrogen coverage. The experimentally observed time-scale dependence of the HUPD coverage in voltammetric experiments, therefore, reflects the kinetics of the alloy’s breathing which is not so far reflected in the DFT-based adsorption energies describing the thermodynamic driving force for the process. The limited applicability of the voltammetry to assess alloys composition shown above for Pt3Ni and HUPD can be foreseen to hold generally for all alloyadsorbate systems if the final adsorbate structure is established via extensive surface transport. CONCLUSIONS Both the experimental as well as the theoretical data for the behavior of nanoparticulate Pt3Ni in HUPD bear general features of the catalytic behavior of most binary alloy systems. The transfer


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of the alloy’s constituents to/from the catalyst’s surface demonstrates the long term instability of the alloy’s surface in the presence of specifically adsorbed species. This leads to the dynamic formation of the active sites during catalytic operation (induced in this case by H-adsorption) which needs to be reflected in the rational design of catalysts. The in-situ XAS detected surface segregation processes, as suggested by our DFT simulations, may need to be facilitated by nonhomogeneous distributions of adsorbates on the real catalyst’s surface. The timescale of the catalysts surface breathing is likely to be of the order of 102 to 103 s, which is of technological importance.

ASSOCIATED CONTENT Analysis of the voltammetric data, summary of the EXAFS refinement and the hydrogen adsorption energies are available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Present Addresses † Present address: Materials and Structures Laboratory Tokyo Institute of Technology,4259 Nagatsuta, Midori, Yokohama 226-8503, Japan..


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ACKNOWLEDGMENT This work was supported by the Marie Curie Initial Training Network ELCAT (Project No. 214936) of the European Commission. The synchrotron measurement time was provided by the Hasylab facility of the Deutsches Elektronen Synchrotron - Project No. 11734.). JEM gratefully acknowledges financial support from the Alexander von Humboldt foundation. REFERENCES [1] van der Vliet, D.; Wang, C.; Debe, M.; Atanasoski, R.; Marković, , N.M.; Stamenković, V. R.;

Platinum-alloy Nanostructured Thin Film Catalysts For The Oxygen Reduction

Reaction, Electrochim. Acta 2011, 56, 8695-8699. [2] Mukerjee, S.; Srinivasan, S.; Soriaga, M.P.; McBreen, J.; Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction An In Situ XANES and EXAFS Investigation, J. Electrochem. Soc. 1995, 142, 1409-1422. [3] Stamenković, V.R.; Schmidt, T.J.; Ross, P. N.; Marković, N.M.; Surface Composition Effects in Electrocatalysis: Kinetics of Oxygen Reduction on Well-defined Pt3Ni and Pt3Co Alloy Surfaces, J. Phys. Chem. B 2002, 106, 11970-11979. [4] Stamenković, V.R.; Fowler, B.; Mun, B.S.; Wang, G.F., Ross, P.N.; Lucas, C.A.; Marković, N.M.; Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability, Science 2007, 315, 493-497. [5] Koh, S.; Toney, M.F.; Strasser, P.; Activity-stability Relationships of Ordered and Disordered Alloy Phases of Pt3Co Electrocatalysts for the Oxygen Reduction Reaction (ORR), Electrochim. Acta 2007, 52, 2765-2774.


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[6] Lima, F.H.B; Salgado, J.R.C.; Gonzalez, E.R.; Ticianelli, E.A.; Electrocatalytic Properties of PtCo/C and PtNi/C Alloys for the Oxygen Reduction Reaction in Alkaline Solution, J. Electrochem. Soc. 2007, 154, A369-A375. [7] Mani, P.; Srivastava, R.; Strasser, P.; Dealloyed Binary PtM3 (M = Cu, Co, Ni) and Ternary ;PtNi3M (M = Cu, Co, Fe, Cr) Electrocatalysts for the Oxygen Reduction Reaction: Performance in Polymer Electrolyte Membrane Fuel Cells, J. Power Sources, 2011, 196, 666-673. [8] Greeley, J.; Stephens, I.E.L.; Bondarenko, A.S.; Johansson, T.; Hansen, H.A.; Jaramillo, T.F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J.K.; Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts, Nature Chem. 2009, 1, 552-556. [9] Nørskov, J.K.; Bligaard, T.; Rossmeisl, J.; Christensen, C.H.; Towards the Computational Design of Solid Catalysts, Nature Chem. 2009, 1, 37-46. [10] Rossmeisl, J.; Karlberg, G.S.; Jaramillo, T.; Nørskov, J.K.; Steady State Oxygen Reduction and Cyclic Voltammetry, Faraday Discussions 2008, 140, 337-346. [11] Paulus, U.A.; Wokaun, A.; Scherer, G.; Schmidt, T.J.; Stamenković, V.R..; Marković, N.M.; Ross, P.N.; Oxygen Reduction on High Surface Area Pt-based Alloy Catalysts in Comparison to Well Defined Smooth Bulk Alloy Electrodes, Electrochim. Acta, 2002, 47, 3787-3798. [12] Mavrikakis, M.; Hammer, B.; Nørskov, J.K. Effect of Strain on the Reactivity of Metal Surfaces, Phys. Rev. Lett. 1998, 81, 2819-2822. [13] Vasiliev, M.A.; Surface Effects of Ordering in Binary Alloys, J. Phys. D 1997, 30, 30373070.


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[14] Lundberg, M.; Surface Segregation and Relaxation Calculated by the Embedded Atom Method – Application to Face Related Segregation on Platinum-Nickel Alloys, Phys. Rev. B 1987, 36, 4692- 4699. [15] Hansen, M.; Anderko, K. in Constitution of Binary Alloys; McGraw-Hill: New York, 1958. [16] Bardi, U.; Beard, B.C.; Ross, P.N.; CO Chemisorption on the [111] and [100] Oriented Single Crystal Surfaces of the Alloy CoPt3, J. Catal. 1990, 124, 22-29. [17] Gauthier, Y.; Joly, Y.; Baudoing, R.; Rundgren, D., Surface-Sandwich Segregation on Nondilute Bimetallic Alloys – Pt50Ni50 and Pt78Ni22 Probed by Low Energy Electron Diffraction, Phys. Rev. B, 1985, 31, 6216-6218. [18] Fowler, B.; Lucas, C.A.; Omer, A. ; Wang, G.; Stamenković, V.R.; Marković, N.M.; Segregation and Stability at Pt3Ni(111) Surfaces and Pt75Ni25 Nanoparticles, Electrochim. Acta 2008, 53, ;6076-6080. [19] Nørskov, J.K. ; Bligaard, T.; Logadottir, A.; Kitchin, J.R.; Chen, J. G.; Pandelov, S.; Trends in the Exchange Current for Hydrogen Evolution, J Electrochem. Soc. 2005, 152, J23-J26. [20] Wang, C. ; Chi, M.F.; Wang, G. F.; van der Vliet, D.; Li, D.G.; More, K.; Wang, H.H. ; Schlueter, J.A.; Marković, N.M.; Stamenković, V.R.; Correlation Between Surface Chemistry and Electrocatalytic Properties of Monodisperse PtxNi1-x Nanoparticles, Adv. Funct. Mat. 2011, 21, 147-152. [21] Stamenković, V.R.; Mun, B.S.; Mayrhofer, K.J.J.; Ross, P.N.; Marković, N.M.; Effect of Surface Composition on Electronic Structure, Stability, and Electrocatalytic properties of PtTransition Metal Alloys: Pt-skin versus Pt-skeleton Surfaces, J. Am. Chem. Soc. 2006, 128, 8813-8819.


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[22] Bandarenka, A.S.; Varela, A.S.; Karamad, M.; Calle Vallejo, F.; Bech, L., Perez Alonso, F.J.; Rossmeisl, J.; Stephens, I.E.L.; Chorkedorff, I.; Design of an Active Site towards Optimal Electrocatalysis: Overlayers, Surface Alloys and Near Surface Alloys of Cu/Pt(111), Angew. Chem. Int. Ed., 2012, 21 11845-11848. [23] Cui, C.; Ahmadi, M.; Behafarid, F.; Gan, L.; Neumann, M.; Heggen, M.; Cuenya, B.R.; Strasser, P.; Shape‐selected bimetallic nanoparticle electrocatalysts: evolution of their atomic‐scale structure, chemical composition, and electrochemical reactivity under various chemical environments . Faraday Disc., 2013: DOI: 10.1039/C3FD20159G. [24] Newville, M.; IFEFFIT: Interactive EXAFS Analysis and FEFF Fitting, J. Syn. Rad. 2001, 8, 322-324. [25] Schultz, P.A.; SeqQuest, Sandia National Labs, Albuquerque, NM, http://dft.sandia.gov/Quest/. [26] Perdew, J.P.; Burke, K.; Ernzerhof, M.; Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 1996. 77, 3865-3868. [27] Stamenković, V.R.; Marković, N.M. in Handbook of the Fuel Cells, Fundamentals, Technology and Applications [W.Vielstich, H. Gasteiger and H. Yokosawa Eds.], J Wiley, Chichester, 2009, p. 18. [28] Liu, X.; Frank, B.; Zhang, W.; Cotter, T.P.; Schlogl, R.; Su, D.S.; Angew. Chem. Int. Ed, 2011, 50, 3318-3322 [29] Venkatachalam, S.; Jacob, T.; Hydrogen Adsorption on Pd-containing Au(111) Bimetallic Surfaces, Phys. Chem. Chem. Phys. 2009, 11, 3269-3270. [30] Jacob, T.; Merinov, B.V.; Goddard, W.A.; Chemisorption of Atomic Oxygen on Pt(111) and Pt/Ni(111) Surfaces, Phys. Lett. 2004, 385, 374.


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[31] Jacob, T.; Goddard, W.A.; Adsorption of Atomic H and O on the (111) Surface of Pt3Ni Alloys, J. Phys. Chem. B 2004, 108, 8311-8323. [32] Zhan, D.P.; Velmurugan, J.; Mirkin, M.V., Adsorption/Desorption of Hydrogen on Pt Nanoelectrodes: Evidence of Surface Diffusion and Spillover, J Am. Chem. Soc. 2009, 131, 14756-14760.


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Langmuir

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Table of contents

Surface stability of Pt3Ni nanoparticulate alloy electrocatalysts in hydrogen adsorption

Hana Hoffmannova, Maki Okube, Valery Petrykin, Petr Krtil, Jonathan E. Mueller and Timo Jacob


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Surface Stability of Pt3Ni Nanoparticulate Alloy Electrocatalysts in

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