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Pd-M/C (M = Pd, Cu, Pt) Electrocatalysts for Oxygen Reduction Reaction in Alkaline Medium: Correlating the Electronic Structure with Activity Marcus Vinicius Castegnaro, Waldemir J. Paschoalino, Mauro R Fernandes, Benjamin Balke, Maria do Carmo Martins Alves, Edson A. Ticianelli, and Jonder Morais Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00098 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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Pd-M/C (M = Pd, Cu, Pt) Electrocatalysts for Oxygen Reduction Reaction in Alkaline Medium: Correlating the Electronic Structure with Activity Marcus V. Castegnaro a, Waldemir J. Paschoalino b, Mauro R. Fernandes b, Benjamin Balke c, Maria C. M. Alves d, Edson A. Ticianelli b, and Jonder Morais a* a

Electron Spectroscopy Lab (LEe-), Instituto de Física, Universidade Federal do Rio Grande do

Sul (UFRGS), Avenida Bento Gonçalves, 9500, 91501-970, Porto Alegre, RS, Brazil. b

Instituto de Química de São Carlos (USP), 13560-970, São Carlos, SP, Brazil.

c

Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität, 55099

Mainz, Germany. d

Instituto de Química, Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento

Gonçalves, 9500, 91501-970, Porto Alegre, RS, Brazil. Corresponding Author *[email protected]

ACS Paragon Plus Environment

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ABSTRACT

The increasing global needs for clean and renewable energy have fostered the design of new and highly efficient materials for fuel cells applications. In this work, Pd-M (M = Pd, Cu, Pt) and Pt nanoparticles were prepared by a green synthesis method. The carbon-supported nanoparticles were evaluated as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium. A comprehensive electronic and structural characterization of these materials was achieved using X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Their electrochemical properties were investigated by cyclic voltammetry, while their activities for the ORR were characterized using steady-state polarization experiments. The results revealed that the bimetallic nanoparticles consist of highly crystalline nanoalloys with size around 5 nm, in which the charge transfer involving Pd and M atoms affects the activity of the electrocatalysts. Additionally, the samples with higher ORR activity are those whose d-band center is closer to the Fermi level.

KEYWORDS: Oxygen reduction reaction; palladium; platinum; copper; nanoalloy.


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1 INTRODUCTION In order to meet the ever-growing world needs for energy, as well as to reduce the impacts of the atmospheric pollution related to the large use of fossil fuels, the development of clean and efficient energy sources has become one of the central aims in applied materials science. In this scenario, the fuel cells appear among the most promising electrical power sources for both stationary and mobile applications

1-5

. Currently, the major challenge is to make fuel cells

commercially available by improving performance and durability, and especially, by lowering costs for large-scale production. For instance, a PEMFC (proton exchange membrane fuel cell), which stands out among the various kinds of fuel cells for its low working temperature, high energy density and low environmental hazards 5,6, requires the use of expensive catalysts for both fuel oxidation reaction and oxygen reduction reaction (ORR). Regarding to the electrocatalysis of ORR, the most used materials are Pt based nanoparticles (NPs) 1-6. Besides Pt being a scarce and expensive metal, the Pt NPs are subjected to poisoning, coalescence and dissolution processes during their use in fuel cells

6,7

, thus limiting the

performance of such systems for ORR. Therefore, innovative approaches have been tried in the search of new and better catalysts. Recent efforts are focused on the synthetic procedures to obtain pure Pt or Pt-based multimetallic NPs with controlled shapes 3,6,9-14

and atomic arrangement

11-14

. Additionally, various metals

1-7

6,8

, sizes

9-11

, composition

and/or metal oxides

16

have

been tested as substitutes for Pt towards the catalysis of the ORR. Among them, Pd-based materials proved to be active for ORR in alkaline media 1-5,13,15-19. PdCu is one of the most widely investigated bimetallic Pd-based systems given its use in environmental catalysis, in processes such as NOx abatement

20,21

, CO oxidation 21,22 and water-

gas shift reaction23.


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In a recent work

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we combined X-ray photoelectron spectroscopy (XPS) and in situ X-ray

absorption near-edge structure (XANES) measurements to understand the reactivity of carbon supported Pd-Cu nanoalloys (PdxCu1−x (x = 0.7, 0.5 and 0.3) during heating under CO flux. In that case, it was observed that the alloying involves a significant charge transfer from Pd to Cu, which hinders the surface oxidation of the as-prepared samples. Additionally, the necessary temperature for the complete reduction under CO proved to be dependent on the NPs composition. So, the partial substitution of Pd for Cu atoms, apart from reducing the cost of the catalysts, strongly influences their catalytic properties. In a series of papers

24-26

, we have

investigated the reactivity of Pd-Pt NPs during sulfidation and reduction processes, elucidating the compositional and structural effects behind the resistance against sulfur poisoning of these materials in gas phase. Now, our objective is to expand our research to a more complex system in which the interaction of the metallic nanoparticles with O2 occurs in an aqueous basic medium. Accordingly, the ORR furnishes the ideal condition to evaluate if the previously observed charge transfer process influences the electrocatalytic performance of these nanoalloys. This paper reports on the synthetic methods to obtain Pd/C, PdCu/C and PdPt/C electrocatalysts and on their physical and electrochemical characterization. For that purpose, X-ray diffraction (XRD) and Xray absorption spectroscopy (XAS) probed the samples´ long and short range ordering, respectively. XPS evaluated the Pd core-level electronic structure and the catalysts´ valence band structure. Transmission electron microscopy (TEM), Rutherford backscattering spectroscopy (RBS) and energy-dispersive spectroscopy (EDS) were used to investigate, respectively, the samples´ morphology and composition. Additionally, cyclic voltammetry and steady-state polarization experiments were used to address their performance as electrocatalysts for the ORR.


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2 MATERIALS AND METHODS The carbon supported catalysts were obtained by adsorbing the colloidal solutions of different kinds of Pd-based NPs on Vulcan XC-72 (Cabot), following the previously published method 22, and aiming to achieve catalysts with 10 wt.% of metal loading. The different NPs were obtained by chemical reduction of the metal salts, using aqueous media at room temperature. For Pd/C, the colloidal solution was prepared by chemical reduction of 0.05 x 10-3 mol of PdCl2 (Vetec) by 0.17 x 10-3 mol of trisodium citrate (C6H5O7Na3.2H2O, Sigma-Aldrich) and 4.54 x 10-3 mol of L-ascorbic acid (C6H8O6, Vetec). After 30 minutes under stirring, a black colloidal suspension was obtained. The bimetallic NPs were prepared by simultaneously reducing the salts of both metals. For PdPt/C, a solution of 0.025 x 10-3 mol of PdCl2 and 0.025 x 10-3 mol of K2PtCl6 was reduced by 0.34 x 10-3 mol of trisodium citrate and 4.54 x 10-3 mol of L-ascorbic acid. For PdCu/C, the colloid was prepared by adding 0.11 x 10-3 mol of trisodium citrate and 2.72 x 10-3 mol of L-ascorbic acid to a solution containing 0.05 x 10-3 mol of PdCl2 and 0.05 x 10-3 mol of CuCl2.2H2O (Vetec). For both bimetallic samples black and stable colloids were obtained after 20 minutes under stirring. A Pt/C catalyst was also prepared in order to serve as a reference and compare with the Pd-based samples. The Pt NPs were prepared by reduction of 0.025 x 10-3 mol of K2PtCl6 (Vetec) by 0.17 x 10-3 mol of trisodium citrate and 4.54 x 10-3 mol of L-ascorbic acid. After 60 minutes under stirring, a black colloidal suspension was obtained. TEM analyses were performed in a JEOL JEM-1200 EX II microscope (CMM-UFRGS), working at an accelerating voltage of 100 kV. The samples for TEM analyses were prepared by placing a drop of an aqueous suspension of the catalysts on a copper grid coated with a Formvar film, which was dried in vacuum at room temperature. For each sample, TEM micrographs were used to determine mean size and the size distribution of the NPs. The high-resolution TEM


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(HRTEM) analyses were performed in a JEOL JEM-3010 URP microscope (LNNano) working at 300 kV. For these measurements, copper grids coated with an ultrathin carbon film supported by a lacey carbon film were used. RBS analyses (Ion Implantation Lab-UFRGS) were done at 165° using He+ (2.0 MeV) as incident ions. For these measurements, the powders were pressed to prepare homogeneous pellets. The metal loadings and the Pd:M (M= Pt or Cu) ratios were calculated from the RBS data, using the heights of the signals related to metals and C in the spectra 27. The compositions were independently verified by EDS in a scanning electron microscope JEOL JIB-4500 (LCNUFRGS), working at an accelerating voltage of 15 kV. XRD experiments were conducted with a Siemens Diffraktometer D500. The mean crystallite sizes were estimated from the broadening of (111) and (200) Bragg peaks, following the Scherrer formula 28: λ

D = Bcosθ

Eq. 1

Where D is the mean crystallite size in Å, k is the dimensionless shape factor (taken as 0.9), λ is the X-rays wavelength (1.5418 Å), B is the peak FWHM in radians, and θ is the Bragg angle. The XAS spectra of the Pd-based samples were collected at the Pd K edge (24350 eV) at the XDS beamline

29

(LNLS). The spectra were acquired at room temperature and in transmission

mode, using three Argon filled ionization chambers. A Si (311) double-crystal monochromator and a toroidal Pt focusing mirror were used to acquire each spectrum in the range from 24200 eV to 25250 eV with a 2 eV step and 2 s/point. More than five spectra were collected in order to improve the signal-to-noise ratio. The XAS data were reduced and analysed using the IFEFFIT package

30

. The energy calibration was made using a standard Pd foil. The Extended X-ray

Absorption Fine Structure (EXAFS) signals χ(k) were extracted and then Fourier transformed


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using a Kaiser-Bessel window (∆k = 8.2 Å-1). The structural parameters were obtained by fitting the EXAFS data using the scattering amplitudes and phase shifts calculated with FEFF 9.6 31 for Pd-Cu, Pd-Pt and Pd clusters in fcc phases. It was also necessary to introduce a Pd-O scattering to better fit the Fourier transforms (FT) at short distances. During the fitting, the number of free parameters was less than 1/3 of the number of independent points in the region. The fitting led to R-factors lower than 0.0005 and to structural parameters similar to the theoretical models. The short range order and homogeneity of the bimetallic samples were based on the Cowley´s short range parameter 32, and calculated from the EXAFS structural parameters. This parameter was previously employed in EXAFS studies on bulk alloys 33 and, more recently, in papers about bimetallic NPs 34,35 (nanoalloys). Supposing a bimetallic AxB1-x alloy, the Cowley´s parameter, α, is defined as follows:

=1

−

Eq. 2

Where NAB is the partial coordination number (CN) of A atoms, i.e., the number of B atoms as first nearest neighbours of A atoms; NAM is the total CN of A, i.e., the number of first nearest metal neighbours of A atoms; and xB is the molar concentration of B atoms in the sample. Except for xB, all parameters in Eq. 2 can be obtained from the EXAFS analyses at one of the absorption edges of A-type atoms. The results obtained from the EXAFS analyses at the Pd K edge and from the RBS measurements were used to calculate the Cowley´s parameter. And this value was used as an indicative of the negative (α < 0) or positive (α > 0) tendency to form segregated or heterogeneous structures. A detailed discussion on the interpretation of possible α values for nanoalloys can be found in the paper of A. Frenkel and coworkers 35.


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For the bimetallic samples, XAS spectra were also collected at both Pt L3 (11564 eV) and Cu K (8979 eV) edges using the XAFS1 beamline 36 (LNLS). Details on the experimental setup and data analysis are specified in the Supporting Information. XPS was employed to probe the chemical environment of Pd atoms within the NPs hosted on the surface of the Pd-based electrocatalysts. The as-prepared samples were analysed with a PREVAC system equipped with a SCIENTA R4000 hemispherical electron analyser (pass energy of 30 eV) using monochromatized Al Kα radiation (450 W, 15 kV, 30 mA). The analyser’s energy calibration was performed using the Ag 3d5/2 peak (368.3 eV from a clean Ag foil. The C 1s peak (284.5 eV

35

37

), measured

) was used as an internal reference to correct

any charging effects. All peaks were adjusted with XPSPeak 4.1 using a Shirley background

38

and a Gaussian-Lorentzian sum functions (30% L-G). Each chemical component was adjusted as a doublet with the appropriated constrains on the spin-orbit-splitting and branching ratio values. Valence band photoelectron spectra were also collected using Al Kα radiation (150 W, 14 kV, 11 mA) in a SPECS system equipped with a Phoibos 150 1D-DLD hemispherical electron analyser adjusted at a pass energy of 20 eV, an energy step size of 0.1 eV and a acquisition time of 0.5 s. The analyser’s energy calibration was performed using the Au7/2 peak (84 eV 37), measured from a clean Au foil. Cyclic voltammetry was employed to characterize the electrocatalyst´s particle surface. Steady state polarization curves were used to evaluate the ORR kinetic parameters, for which the experiments were recorded at several rotation speeds in the range of potentials from 0.1 to -0.9 V Hg/HgO/OH-, using an AUTOLAB model bipotentiostat (PGSTAT30). All the experiments were conducted at room temperature (25 ± 1 °C) in a conventional glass cell. The counter electrode was a large area platinized platinum foil and a reversible mercury/mercury oxide in KOH


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(Hg/HgO/OH-) served as the reference electrode. The experiments were carried out in 1.0 mol L−1 KOH aqueous solutions, prepared using high purity reagents (Sigma-Aldrich) and water obtained from a Milli-Q (Millipore) system. Cyclic voltammetry experiments were conducted N2-saturated electrolyte solutions, while the ORR polarization measurements were conducted after saturation of the solution with O2. The working electrodes consisted of an ultra-thin layer of the Pd-M/C catalysts deposited over a pyrolitic graphite disk (0.196 cm2) of a rotating disk electrode, which was previously polished to a mirror-finish before each experiment. The catalyst layers were prepared with 14 µg metal.cm-2. The attachment of the catalyst particles onto the graphite disk was made by pipetting a diluted Nafion solution (5 wt.%, DuPont) on the electrode surface, and left drying at room conditions. After this, the electrode was immersed in oxygen free electrolyte solutions and cycled several times between -1.0 and 0.1 V Hg/HgO/OH- at 20 mV/s until a steady-state profile was reached.

3 RESULTS AND DISCUSSION The morphological aspects of the carbon supported NPs were probed by TEM and HRTEM. Figure 1a and 1b present representative TEM images and histograms of Pd/C and PdCu/C, respectively. In the TEM images, the metallic particles appear as small structures dispersed onto the support. The other two catalysts display similar morphologies. About 500 NPs from different micrographs were used to estimate the size distribution of the metal NPs. The histograms were fitted considering a Gaussian-Lorentzian sum function. Average particle sizes (summarized in Table 1) of 5.1 ± 1.3 nm, 4.2 ± 1.1 nm , 4.1 ± 1.1 nm and 5.0 ± 1.1 nm were obtained for Pd/C, PdCu/C, PdPt/C and Pt/C, respectively.


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Figure 1. Representative TEM micrographs and size distribution for the Pd/C (1a) and PdCu/C (1b). Selected HRTEM micrographs of Pd/C, PdCu/C, PdPt/C and Pt/C samples are shown in Figures 2a, 2b, 2c and 2d, respectively. In those images, the observed average morphology in Figure 1a and 1b is reproduced. Additionally, the HRTEM images indicate that the NPs are highly crystalline. The local lattice constants (a) were estimated from the fast Fourier transforms and from the direct measurement of the HRTEM fringes spacing. The obtained values (Table 1) for the monometallic samples are almost the same, and they agree with the expected lattice parameter for bulk Pd (3.8898 Å, from ICSD code: 76148) and Pt (3.9110 Å, from ICSD code: 64917) in fcc phases. The value that a assumes for the bimetallic samples are close to those expected for Pd-Cu (about 3.76 Å, from ICSD codes: 103082, 103085 and 166153) and Pd-Pt (about 3.90, from ICSD codes: 105564, 105655 and 648701) alloys.


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Figure 2. HRTEM micrographs: Pd/C (a), PdCu/C (b), PdPt/C (c) and Pt/C (d). EDS and RBS results in terms of the wt.% of metal anchored into carbon are presented in Table 1. The total metal loadings obtained from the EDS analysis are higher than those obtained by RBS, but both are close to the expected value (10 wt.%). The difference rests in the fact that EDS provides information from the surface of the samples, where the NPs are expected to be anchored

22

, while the RBS signal came from a deeper region

27

. As seen, all Pd-containing

catalysts presented real compositions that is very close to the nominal value of 10 wt.%, while for Pt/C the composition is somewhat smaller than the expected value, but still adequate for the present investigation. The atomic proportions of Pd:M (M= Pt or Cu), also included in Table 1, resulted very similar to the nominal value (1:1).


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Table 1. Composition determined by RBS and SEM/EDS, mean size of particles determined by XRD and TEM and lattice parameter (a) determined by HRTEM and XRD. Metal loading (wt.%)

Pd:M ratio

Mean particle size (nm)

Sample

Lattice Parameter (Å)

RBS

EDS

RBS

EDS

TEM

XRD

HRTEM

XRD

Pd/C

9.0 ± 1.0

10.9 ± 1.3

-

-

5.1 ± 1.3

5.3 ± 1.1

3.91

3.90

PdCu/C

8.6 ± 0.9

10.2 ± 1.2

1.01

1.08

4.2 ± 1.1

4.2 ± 1.0

3.76

3.78

PdPt/C

8.8 ± 0.9

10.5 ± 1.2

1.34

1.40

4.1 ± 1.1

4.8 ± 1.0

3.92

3.91

Pt/C

7.5 ± 0.8

8.2 ± 1.0

-

-

5.0 ± 1.1

6.6 ± 1.0

3.92

3.90

Figure 3 shows the XRD patterns for the various electrocatalysts, which display the general pattern of a face centered cubic (fcc) structure. The lack of extra peaks signifies that no ordered phases of pure Pt or pure Cu were formed in the bimetallic samples. The observed Bragg peaks positions indicate that alloy phases of Pd-Cu and Pd-Pt are present in PdCu/C and PdPt/C samples. The resulting lattice parameters (a) for all electrocatalysts are included in Table 1 and the values agree with those extracted from the HRTEM analyses.


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Figure 3. XRD patterns of the investigated electrocatalysts. The observed broad reflections indicate that the catalysts are nanostructured materials with crystallite size below 100 nm. The average size of the crystallites calculated using the broadening of the (111) and (200) diffraction peaks are given in Table 1, together with the particle sizes obtained by TEM. It is noticeable that the sizes obtained by TEM and by XRD are similar. The EXAFS signals, χ(k), extracted from the Pd K edge XAS data, along with their FT´s, are displayed in Figures 4(a) and 4 (b), respectively. The experimental points are plotted with the continuous lines that correspond to the best fits. The resulting quantitative structural parameters are presented in Table 2.


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Figure 4. EXAFS analysis of Pd-based catalysts: (a) EXAFS χ(k) signals at the Pd K edge and (b) magnitude and imaginary parts of the Fourier transforms of each k3-weighted χ(k) signal. In (a) and (b), the grey circles represent the data and the red lines indicate the best fitted theoretical curves. In (b), the blue circles are the imaginary part of the FTs and the black lines are the best obtained adjusts. During the fitting procedure, the passive electron reduction factor (S02) was obtained from the fitting of a standard Pd foil. The value (0.8) was kept constant for all the fits. EXAFS DebyeWaller factor (σ2) values were between 0.005 and 0.007 for the nearest neighbors of the absorber. The path lengths were adjusted supposing an isotropic expansion/contraction by using ∆R = αReff, where Reff is the effective path length and α is the proportionality constant, whose obtained values were close to 1. Thus, the distances listed in Table 2 are closed to those from the theoretical models, and, for the bimetallic samples, they corroborate the formation of bimetallic alloys in the ordered fcc phases, as pointed out by the XRD results. In order to complete the structural analysis via EXAFS of the bimetallic samples, XAS data were also collected at the Pt


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L3 (for PdPt/C) and Cu K (for PdCu/C) edges. Those results and details on the experimental setup and data analysis procedures are available in the Supporting Information The observed reduction of the coordination number around Pd (NPd-M) and the enlarged local disorder (in terms of σ2) are noteworthy, as expected for nanosystems

35,36,39-42

. And, regarding

the bimetallic samples, one can observe in Table 2 that the coordination number of homometallic (Pd-Pd) and heterometallic (Pd-Pt or Pd-Cu) paths are slightly different, indicating that the Pd nearest neighbors consist mainly of Pd atoms, rather than its co-metals. As pointed before

35,36

,

that difference may lead to the assumption that the samples are composed by NPs with segregated structures, as, for example core-shell NPs. To elucidate that point, the Cowley´s short range parameter was calculated for bimetallic samples with Eq. 2, using the partial and total coordination numbers and the average compositions, which were obtained from RBS. The parameter values were close to 0, indicating that the atoms are randomly distributed within the NPs, forming Pd-based alloys rather than core-shell or other segregated structures. The Cowley parameter was also calculated using the coordination numbers extracted from the analysis of the EXAFS signals acquired at the Cu K edge (for PdCu/C) and Pt L3 edge (for PdPt/C). The values achieved (available as Supporting Information) corroborated the formation of Pd-M nanoalloys. Small contributions of Pd-O bonds were also observed for all Pd-M/C samples, and it was attributed to the partial surface oxidation, which was also observed in the Cu K and Pt L3 edges for the bimetallic samples.


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Table 2. Structural parameters extracted from the EXAFS analyses: coordination number (N), bond distance (R), EXAFS Debye-Waller factor (σ2) and total number of metal neighbours of Pd (NPd-M). For the bimetallic samples the Cowley parameter is also listed. Sample

Pair

N

R (Å)

σ2 (10-2 Å2 )

NPd-M

Pd foil

Pd-Pd Pd-O Pd-Pd Pd-O Pd-Pd Pd-Cu Pd-O Pd-Pd Pd-Pt

12 0.2 ± 0.1 9.8 ± 0.3 0.3 ± 0.2 5.0 ± 0.1 4.9 ± 0.2 0.1 ± 0.1 5.7 ± 0.3 4.2 ± 0.3

2.74 ± 0.02 2.02 ± 0.02 2.76 ± 0.02 2.03 ± 0.02 2.68 ± 0.02 2.68 ± 0.02 2.02 ± 0.02 2.75 ± 0.02 2.75 ± 0.02

0.47 ± 0.08 0.72 ± 0.02 0.51 ± 0.03 0.84 ± 0.04 0.51 ± 0.02 0.54 ± 0.04 0.68 ± 0.02 0.57 ± 0.03 0.59 ± 0.03

12

Cowley parameter -

9.8

-

9.9

0.006

9.9

0.006

Pd/C PdCu/C

PdPt/C

Figure 5 shows the Pd 3d region of the XPS spectra collected for the as-prepared Pd-M/C (M = Pd, Cu, Pt) electrocatalysts. It also shows the chemical components used for fitting the photoemission peaks. The BE, which are indicated in Figure 5, as well as the FWHM and areas of each component are summarized in Table S2, available as Electronic Supporting Information. In the two upper spectra of Figure 5, just the doublet Pd 3d5/2 and Pd 3d3/2 is present, while an additional peak with lower binding energy (BE) can be seen in the PdPt/C spectrum. That broad peak at 331 eV corresponds to the Pt 4d3/2 photoemission peak related to Pt-M (M = Pd, Pt) bonds 24,37. Three components were necessary to adjust the Pd 3d doublet. The component at BE = 336.4 eV were related to Pd-O bonds in PdO

22,24,37

and it indicates that the Pd atoms in the three

samples are partially oxidized. As expected, the co-metal present in the bimetallic samples also presented a partial oxidation similar to that observed in Pd 3d XPS region. The Pt 4f and Cu 2p3/2 regions of the XPS spectra collected respectively for samples PdPt/C and PdCu/C are available in the Supporting Information.


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The Pd-O component has the same BE and FWHM in all samples, given that it is related to Pd atoms in the same chemical environment. But, as can be seen in Figure 5, the BE of the Pd-M (Cu, Pd) component depends on the sample composition. For Pd/C, the most intense component, at BE = 335 eV, corresponds to Pd-Pd bonds 22,24,37. As previously reported

22,23,43-45

, the charge transfer involving Cu and Pd atoms in an alloy

phase affects the core-level of both metals. So, the observed core-level shift for the Pd-Cu component in PdCu/C photoemission data corroborates the formation of a Pd-Cu alloy. For PdPt/C, the presence of Pt around Pd has not changed the BE of the Pd 3d electrons, given that the charge transfer between Pd and Pt is not strong enough to modify the core-levels of the Pd atoms 23,43-45. Thus, although the structural analyses of PdPt/C stated the formation of a Pd-Pt alloy, the Pd 3d electrons in that sample have the same BE of the Pd 3d electrons in Pd/C. Finally, the component at higher energy is the previously reported

22,24,46

3d-electrons shake-up

satellite. Typically, such satellite presents BE values about 7 eV higher than the BE of the Pd 3d electrons in a metallic environment, as observed here for all samples.


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Figure 5. XPS spectra at the Pd 3d region for the as-prepared Pd-M/C (M = Pd, Cu, Pt) electrocatalysts. The open circles are the raw data, the black line are the best fits obtained using the chemical components (coloured lines). The most intense peak of each component´s doublet are identified in the graphs, and their BE are presented. The steady stated voltammetric profiles for the different materials are shown in Figure 6. In the Figure 6 (a), the curves show well-established features in the hydrogen and the oxide regions, as published for carbon-supported Pt and Pd in alkaline solutions 47,48. For Pt/C and Pd/C, the peaks in the Hupd region (-1.0 to -0.5 V vs Hg/HgO/OH-) are stable, reasonably well resolved and very similar to published data

47,48

. For PdPt/C, the voltammograms are stable and present mixed

features of Pd and Pt. For PdCu/C, the redox features at -0.2 to 0.0 V vs Hg/HgO/OH- are due to Cu oxi-reduction processes 49; these peaks progressively diminish until becoming very small and the voltammogram stabilizes (Figure 6(b)). Following interpretation of results for flat Cu electrodes 50,51, the small shoulder present in the beginning of the oxidation peak appearing at ca. -0.3 V (Figure 6(b)) may be assigned to the sequence of reactions:


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Cu + OH- = CuOH + e-

Eq. 3

2 CuOH = Cu2O + H2O,

Eq. 4

while the peak centered at ca. – 0.05 V refers mainly to the reactions Cu + 2 OH- = Cu(OH)2

Eq. 5

Cu(OH)2 = CuO + H2O

Eq. 6

Both, Cu2O and CuO are insoluble species, probably leading to the passivation of the Cu atoms in the PdCu alloy

50,51

. In the reverse scan, the peak centered at about -0.15 V represents some

small re-precipitation of Cu from soluble Cu+ and Cu2+ species, but it may also be related to PdO reduction

48

since its position coincides with reduction peak seen for Pd/C in Figure 6(a).

Although for Pd/C the currents involving oxidation of Pd in this potential range to form PdO is quite small, this process might be accelerated by the interactions of Pd with Cu. In any case, the leaching of Cu and/or re-precipitation of Pd is clearly evidenced by the increase of Pd hydrogen features observed at ca. -0.8 V. In summary, as seen by comparing the curves in Figures 6 (a), 6(b) and 6(c), the stable CV profile resembles that of Pd/C and is reproduced even after conducting all ORR experiments. However, a closer look at the results denotes a change in the peak position in the H region (the peak potential changed from the first to the final scan) and a persistent presence of small Cu redox features, probably indicating the formation of a stable core@shell (PdCu@Pd) structure. Further investigations on the structural evolution of PdCu and PdPt NPs will be carried out in the near future by in situ XAS measurements during the ORR. Results in Figure 6 (a) denotes a much smaller magnitude of CV currents for the Pt/C electrode, clearly evidencing a smaller electrochemical active area, in accordance with the smaller Pt/C wt.% presented by this material.


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Figure 6. (a) Stationary cyclic voltammograms obtained for the Pd/C, PdPt/C and Pt/C catalysts in 1.0 M KOH, 20 mVs-1. For Pt/C, the current scale was multiplied by 5 to enlarge the signal; Cyclic voltammograms obtained for the PdCu/C catalyst in 1.0 M KOH, 20 mVs-1: (b) results for the 1st to the 23rd cycles obtained before ORR investigation; (c) same after ORR investigation. Arrows indicate the effect of successive cyclings. The steady-state polarization curves for the ORR at several rotation speeds are illustrated in Figure 7 (a) for the PdPt/C catalyst. These results indicate that, for potentials smaller than -0.2 V vs Hg/HgO/OH-, the ORR is diffusion-controlled, and in the region between -0.1 and 0.0 V vs Hg/HgO/OH- it is under mixed kinetic-diffusion control. Similar behaviors were observed for all other materials. Comparisons of the steady-state polarization curves at 1600 rpm for the different materials are made in Figure 7(b). Results show that the ORR limiting currents assume somewhat larger values for the Pd/C and PdPt/C catalysts, consistent with a large number of electrons involved in the ORR. In the kinetic-diffusion controlled region, the polarization curves evidence a slight higher activity for these materials as compared to PdCu/C and Pt/C.


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Figure 7. (a) Steady-state polarization curves for the PdPt/C catalyst in 1.0 M KOH at several rotation speeds. T = 25 oC; (b) Steady-state polarization curves for the different Pd-M/C catalysts in 1.0 M KOH at 1600 rpm rotation speeds.

Figure 8(a) present Levich plots ( j vs ω1/2) obtained for all catalysts, where j is the measured current density and ω the rotation rate. Since in this work the electrodes were prepared using ultrathin layers of the catalyst materials and contained low coverages of Nafion, diffusion phenomena inside the layer and in the Nafion film can be neglected 47. In this way, if the ORR is assumed to be of first order regarding the oxygen concentration, the slope (B) of the Levich lines, given by Eq. 3, can be used to obtain the number of electrons involved in the ORR. = 0.2⁄ ν⁄

∗

Eq. 3

Eq. 3 is for ω in rpm, while n is the number of exchanged electrons per oxygen molecule in the ORR, F the Faraday constant, D the diffusion coefficient of oxygen in the electrolyte, ν the cinematic viscosity of the electrolyte, C* the bulk concentration of oxygen in the electrolyte.


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Figure 8. (a) Levich plots for the different Pd-M/C catalysts. E = 800 mV; (B) Mass-transport corrected Tafel plots for the ORR on the 1.0 M KOH solutions at 25°C. Currents are per unit of geometric area. Conversion to the RHE (reversible hydrogen electrode) scale was made taking 0 V vs. RHE = - 0.985 V vs Hg/HgO/OH-. From the values of physicochemical parameters (D, ν, and C*) obtained in the literature 48, the B term in the Levich equation is calculated as 0.088 mA cm-2 rpm-1/2 for n = 4 and for 1.0 M KOH solution at 25 oC. Using this B value and the slopes of the Levich plots (Fig. 8 (a)), the n values were obtained for all catalysts and the results are summarized in Table 3. These results evidence that the values of n are very close to 4 for Pd/C and PdPt/C and around 3.5 for Pt/C and PdCu/C. Previous works have reported that in alkaline media, electrodes formed by graphite and carbon catalyze the ORR through a two-electron transfer to adsorbed molecular oxygen (O2)ad 47,53, (O2)ads + H2O + 2 e- = HO2- +

OH-

Eq. 8

with the peroxide ions leaving the electrode surface and reaching the electrolyte. On the other hand, palladium and platinum (in absence of carbon) essentially promote the four electron pathway

47,48,53

, particularly at low electrolyte concentrations, involving further reduction of

adsorved (HO2-)ads species,


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(HO2-)ads + H2O + 2e- = 3 OH-

Eq. 9

or eventually involving a concerted transfer of 4e- directly to adsorbed molecular oxygen, [3 ver abaixo], (O2)ads + 2 H2O + 4 e- =

4 OH-

Eq. 10

For a flat Pt electrode, it was observed previously that reaction in (Eq. 9) is not so fast in concentrated KOH solutions (0.5 M to 8.0 M), so that part of the (HO2-) species can leave the Pt surface without reacting, and so resulting in an overall process involving less than 4 electrons per oxygen molecule

54

. However, when supported Pt/C catalysts are considered, the porosity and

roughness of the electrode layer may provide longer diffusion path in the catalytic layer, allowing the peroxide ions to reach another active site where it may react, so that the overall number of electrons may be higher than in flat smooth surfaces. Here, the number of electrons observed for Pt/C in 1.0 M KOH solutions is smaller than 4, and this may be in part due to difficulties of HO2- to remain adsorbed, thus providing a similar situation as for the flat electrode, as discussed above

54

. However, the small Pt content in this

catalyst may also favor participation of carbon, in parallel with Pt, in the electrocatalysis of the ORR, as observed previously for catalysts with different Pt/C ratios 53. In the case of PdCu/C, a weakening of the Pd2-O2 and/or Pd-HO2- adsorption bonds caused by interaction of Pd with Cu may also occur, enhancing the possibility for peroxide ion to be released to the electrolyte (Eq. 8). Finally, for Pd/C and PdPt/C, it is noted that reactions follow the desired 4 e- pathways (Eq. 10), evidencing balanced interactions of the catalysts with O2 and/or HO2- and very low participation of carbon in the catalysis of the reaction. Mass-transport corrected Tafel plots obtained with the current normalized by the geometric electrode area were employed to compare the activity for the ORR on the electrocatalysts, as shown in Figure 8(b), where the increase of the magnitude of the currents at a given electrode


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potential, as for example 0.9 V vs. RHE, evidences that the catalytic activity for the ORR increases in the sequence: Pt/C < PdCu/C < Pd/C ≈ PdPt/C. Figure 8(b) also evidences one linear region, with line slopes (Tafel slopes) in the range of 45-55 mVdec-1, as summarized in Table 3.

Table 3. Levich slopes (B), number of electrons per O2 molecule, and Tafel slopes of the ORR for the different catalysts in 1.0 M KOH. Material Pt/C PdPt/C PdCu/C Pd/C

B (mA cm-2 rpm1/2) 0.076 0.092 0.078 0.090

Number of electrons 3.4 4.1 3.5 4.1

Tafel slope (mV/dec) 47 55 46 55

Two Tafel regions, with slopes of 60 and 120 mV for low and high overpotentials, have been reported for the ORR on smooth polycrystalline Pt or Pd electrodes, either in acid or alkaline solutions

47

. The 60 mV slope appears at low overpotentials, where the adsorbed oxygen

coverage of the catalyst surface is high and described by the Temkin isotherm, while the 120 mV slope appears at higher overpotentials where the oxygen coverage is small and described by the Langmuir isotherm 47. In the present investigation, Figure 8(b) evidences the appearance of only the first region in the Tafel lines, because of the anticipation of the oxygen diffusion effects, while Table 3 shows that the Tafel slopes are in the range of 45-55 mV dec-1 for the several PdM/C catalysts. In the case of Pt/C and PdCu/C the values of Tafel slopes are appreciably smaller than the expected 60 mV indicating, as pointed above, an important contribution from carbon in the electrocatalysis of the reaction at the smaller electrode potentials observed for such catalysts (see Figure 8(b)). So, the smaller slopes appear because, in alkaline media at such potentials, the carbon support can promote the ORR by a two-electron reduction process and showing a Tafel


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slope of 40 mV

53

. It should be observed that the lower activity of PdCu/C for the ORR

compared to Pd, cannot be assigned to a smaller active area, as demonstrated by the cyclic voltammograms in Figure 6. Therefore, this could be a consequence of the charge transfer between Pd and Cu, which disfavors the overall rate of the ORR. Aiming to investigate the effect of the valence band electronic structure in the reactivity, the valence electrons photoemission spectra were collected. The spectra are presented in Figure 9(a), and the direct comparison between them indicates clear changes in the d-bands of the metals due to the alloying.


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Figure 9. (a) Valence band photoemission spectra for the as-prepared Pd-M/C (M = Pd, Cu, Pt) and Pt/C electrocatalysts. (b) Relation between the Levich slopes and the d-band center (measured relative to the Fermi level) for the electrocatalysts.

The activity of metal-based catalysts is generally associated with the metal d-band: center position, width and filling 18,55-59. It has been predicted by Hammer and Norskov [50,51] that as far from the Fermi level is a metal d-band center, weaker is the interaction with several types of adsorbates. And, some authors

57-59

have verified this for the adsorption of molecular oxygen

over metallic surfaces. Nonetheless, it does not imply that as close to the Fermi level is the dband of given metal, lower is its reactivity for ORR 18,55,58. Another factor that plays a major role in the reactivity of a catalyst is the energy barrier for the elementary reactions involved in the electrocatalytic process

55,58

. As one might expect, an ideal surface to catalyze the ORR would

present both a low energy barrier for the dissociative adsorption of O2 and a weak interaction with the products of the reaction. Many metals, as Co, Mo, Cu, have low barriers for the O2 dissociation, but they bind oxygen so strongly that their overall reactivity for ORR is low. On the other hand, oxygen binds weakly on noble metals but the reactivity of these surfaces is low due to the high barrier for the dissociative adsorption 18,55,58. Thus, the reactivity for ORR of a surface is given by the compromise between its reaction barrier and the strength of its interaction with oxygen. And, since these two properties are related to the electronic structure of the surface

18,58

, a correlation between the electronic structure and

reactivity of real catalysts is desirable. In the present work, the Levich slopes (B) obtained for all samples were plotted against the samples´ d-band center measured relative to the Fermi level (Figure 9(b)). It can be seen that the samples with higher B values (higher activity) are those whose d-band centers are closer to the Fermi level. Further deepening of understanding on this


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relationship between the d-band configuration and the reactivity for ORR will be reached in the future by varying the composition of the carbon-supported Pd-Pt and Pd-Cu NPs.

4 CONCLUSIONS The ORR in alkaline electrolyte was investigated using Pd-M/C (M= Pd, Cu, Pt) as electrocatalysts, which were submitted to a very detailed physical-chemical pre-characterization. Their catalytic activities were higher than those presented by the Pt/C sample, used as reference. Particularly, the Pd/C and PdPt/C samples showed improved reactivity in comparison with both Pt/C and PdCu/C cases. The XPS analysis indicated that the PdCu/C sample presented a significant chemical shift in the B.E. of Pd 3d photoelectron peaks, while no equivalent shift was observed for the PdPt/C XPS analysis. Additionally, the changes in valence band structure induced by the different compositions proved to affect the activity. In fact, the samples with higher activity, presented dband centers closer to the Fermi level than those with lower activity. Based on the results above, we deduced that the larger strength of the charge transfer mechanism induced by the partial substitution of Pd by Cu atoms contributed to lower its catalytic performance, when compared to the monometallic Pd/C. In opposition, the PdPt/C case showed no detriment of the catalytic performance in comparison with Pd/C. Further investigations will be carried out in order to evaluate different Pd-M alloys, as well as to perform in situ XAS analysis during ORR reaction.


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ACKNOWLEDGMENT This work was funded by CAPES, CNPq, FAPESP (Proc. No. 2013/16939-7), LNLS (XAFS115318 and XAFS1-19020) and LNNano (TEM-HR-14037 and TEM-HR 15294 proposals). M. V. Castegnaro thanks CAPES and CAPES/DAAD/PROBRAL for his PhD fellowships.

ASSOCIATED CONTENT Supporting Information. Table S1 summarizing the results of the XPS analyses. Cu 2p and Pt 4f XPS spectra for PdCu/C and PdPt/C. Complementary XAS data and experimental details.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources CAPES, CAPES/DAAD/PROBRAL, CNPq, FAPESP, LNLS, LNNano


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REFERENCES 1 Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energ. 2011, 88 (4), 981-1007. 2 Lucia, U. Overview on Fuel Cells. Renew. Sust. Energ. Rev. 2014, 30, 164-169. 3 Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486 (7401), 43-51. 4 Ge, X.; Sumboja, A.; Wuu, D.; An, T.; Li, B.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Liu, Z. Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts. ACS Catal. 2015, 5 (8), 4643-4667. 5 Sharaf, O. Z.; Orhan, M. F. An Overview of Fuel Cell Technology: Fundamentals and Applications. Renew. Sust. Energ. Rev. 2014, 32, 810-853. 6 Guo, S.; Zhang, S.; Sun, S. Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2013, 52 (33), 8526-8544. 7 Wu, G.; Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 2013, 46 (8), 1878-1889. 8 Xu, D.; Bliznakov, S.; Liu, Z.; Fang, J.; Dimitrov, N. Composition-Dependent Electrocatalytic Activity of Pt-Cu Nanocube Catalysts for Formic Acid Oxidation. Angew. Chem. Int. Ed. 2010, 49 (7), 1282-1285.


29

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Page 30 of 44

9 Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chem. Int. Ed. 2006, 45 (18), 2897-2901. 10 Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem. Int. Ed. 2009, 48 (1), 60-103. 11 Leontyev, I. N.; Belenov, S. V.; Guterman, V. E.; Haghi-Ashtiani, P.; Shaganov, A. P.; Dkhil, B. Catalytic Activity of Carbon-Supported Pt Nanoelectrocatalysts. Why Reducing the Size of Pt Nanoparticles is Not Always Beneficial. J. Phys. Chem. C 2011, 115 (13), 5429-5434. 12 Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J. Nanostructured Pt-alloy Electrocatalysts for PEM Fuel Cell Oxygen Reduction Reaction. Chem. Soc. Rev. 2010, 39 (6), 2184-2202. 13 Wang, D.; Xin, H. L.; Wang, H.; Yu, Y.; Rus, E.; Muller, D. A.; DiSalvo, F. J.; Abruna, H. D. Facile Synthesis of Carbon-Supported Pd-Co Core-Shell Nanoparticles as Oxygen Reduction Electrocatalysts and Their Enhanced Activity and Stability with Monolayer Pt Decoration. Chem. Mater. 2012, 24 (12), 2274-2281. 14 Yang, J.; Chen, X.; Yang, X.; Ying, J. Y. Stabilization and Compressive Strain Effect of AuCu Core on Pt Shell for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5 (10), 8976-8981. 15 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 (2), 666-673.


30

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16 Zhang, Z.; Liu, J.; Gu, J.; Su, L.; Cheng, L. An Overview of Metal Oxide Materials as Electrocatalysts and Supports for Polymer Electrolyte Fuel Cells. Energy Environ. Sci. 2014, 7 (8), 2535-2558. 17 Wang, L.; Zhai, J.-J.; Jiang, K.; Wang, J.-Q.; Cai, W.-B. Pd-Cu/C Electrocatalysts Synthesized by One-Pot Polyol Reduction Toward Formic Acid Oxidation: Structural Characterization and Electrocatalytic Performance. Int. J. Hydrogen Energy 2015, 40 (4), 17261734. 18 Tang, W. J.; Henkelman, G. Charge Redistribution in Core-Shell Nanoparticles to Promote Oxygen Reduction. J. Chem. Phys. 2009, 130 (19), 6. 19 Xu, Y.; Ruban, A. V.; Mavrikakis, M. Adsorption and Dissociation of O2 on Pt-Co and PtFe Alloys. J. Am. Chem. Soc. 2004, 126 (14), 4717-4725 20 Guy, K. A.; Xu, H.; Yang, J. C.; Werth, C. J.; Shapley, J. R. Catalytic Nitrate and Nitrite Reduction with Pd-Cu/PVP Colloids in Water: Composition, Structure, and Reactivity Correlations. J. Phys. Chem. C 2009, 113 (19). 21 Fernandez-Garcia, M.; Martinez-Arias, A.; Belver, C.; Anderson, J. A.; Conesa, J. C.; Soria, J. Behavior of Palladium-Copper Catalysts for CO and NO Elimination. J. Catal. 2000, 190 (2), 387-395. 22 Castegnaro, M. V.; Gorgeski, A.; Balke, B.; Alves, M. C. M.; Morais, J. Charge Transfer Effects on the Chemical Reactivity of PdxCu1-x Nanoalloys. Nanoscale 2016, 8 (1), 641-647.


31

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Page 32 of 44

23 Tierney, H. L.; Baber, A. E.; Sykes, E. C. H. Atomic-Scale Imaging and Electronic Structure Determination of Catalytic Sites on Pd/Cu Near Surface Alloys. J. Phys. Chem. C 2009, 113 (17). 24 Bernardi, F.; Traverse, A.; Olivi, L.; Alves, M. C. M.; Morais, J. Correlating Sulfur Reactivity of PtxPd1-x Nanoparticles with a Bimetallic Interaction Effect. J. Phys. Chem. C 2011, 115 (25), 12243-12249. 25 Bernardi, F.; Alves, M. C. M.; Traverse, A.; Silva, D. O.; Scheeren, C. W.; Dupont, J.; Morais, J. Monitoring Atomic Rearrangement in Pt(x)Pd(1-x) (x=1, 0.7, or 0.5) Nanoparticles Driven by Reduction and Sulfidation Processes. J. Phys. Chem. C 2009, 113 (10), 3909-3916. 26 Boita, J.; Bernardi, F.; Castegnaro, M. V.; Nicolao, L.; Alves, M. C. M.; Morais, J. Reversible Sulfidation of Pt0.3Pd0.7 Nanoparticles Investigated by in Situ Time-Resolved XAS. J. Phys. Chem. C 2014, 118 (10), 5538-5544. 27 Stedile, F. C.; dos Santos, J. H. Z. Analysis and Characterization of Real Catalysts Using Ion Beam Analysis. Nucl. Instrum. Meth. B 1998, 136, 1259-1266. 28 Weibel, A.; Bouchet, R.; Boulc'h, F.; Knauth, P. The Big Problem of Small Particles: A Comparison of Methods for Determination of Particle Size in Nanocrystalline Anatase Powders. Chem. Mater. 2005, 17 (9), 2378-2385. 29 Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537-541. 30 Rehr, J. J.; Kas, J. J.; Vila, F. D.; Prange, M. P.; Jorissen, K. Parameter-free calculations of X-ray spectra with FEFF9. Phys. Chem. Chem. Phys. 2010, 12 (21), 5503-5513.


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31 Cowley, J. M. Short-Range and Long-Range Order Parameters in Disordered Solid Solutions. Phys. Rev. 1960, 120 (5), 1648-1657. 32 Frenkel, A. I.; Machavariani, V. S.; Rubshtein, A.; Rosenberg, Y.; Voronel, A.; Stern, E. A. Local Structure of Disordered Au-Cu and Au-Ag Alloys. Phys. Rev. B 2000, 62 (14), 9364-9371. 33 Frenkel, A. I.; Wang, Q.; Sanchez, S. I.; Small, M. W.; Nuzzo, R. G. Short Range Order in Bimetallic Nanoalloys: An Extended X-ray Absorption Fine Structure Study. J. Chem. Phys. 2013, 138 (6). 34 Frenkel, A. I. Applications of Extended X-ray Absorption Fine-Structure Spectroscopy to Studies of Bimetallic Nanoparticle Catalysts. Chem. Soc. Rev. 2012, 41 (24), 8163-8178. 35 Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. in: Chastain, J. (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation: Eden Prairie, MN, 1992. 36 Tougaard, S.; Jansson, C. Comparison of Validity and Consistency of Methods for Quantitative XPS Peak Analysis. Surf. Interface Anal., 1993, 20 (13), 1013-1046. 37 Rehr, J. J.; Albers, R. C. Theoretical Approaches to X-ray Absorption Fine Structure. Rev. Mod. Phys. 2000, 72 (3), 621-654. 38 Koningsberger, D. C.; Mojet, B. L.; van Dorssen, G. E.; Ramaker, D. E. XAFS Spectroscopy; Fundamental Principles and Data Analysis. Top. Catal. 2000, 10 (3-4), 143-155. 39 Alayoglu, S.; Zavalij, P.; Eichhorn, B.; Wang, Q.; Frenkel, A. I.; Chupas, P. Structural and Architectural Evaluation of Bimetallic Nanoparticles: A Case Study of Pt-Ru Core-Shell and Alloy Nanoparticles. ACS Nano 2009, 3 (10).


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40 Srabionyan, V. V.; Bugaev, A. L.; Pryadchenko, V. V.; Avakyan, L. A.; Van Bokhoven, J. A.; Bugaev, L. A. EXAFS Study of Size Dependence of Atomic Structure in Palladium Nanoparticles. J. Phys. Chem. Solids 2014, 75 (4), 470-476. 41 Olovsson, W.; Bech, L.; Andersen, T. H.; Li, Z.; Hoffmann, S. V.; Johansson, B.; Abrikosov, I. A.; Onsgaard, J. Core-Level Shifts for Two- and Three-Dimensional Bimetallic PdxCu1-x and PdxAg1-x Alloys on Ru(0001). Phys. Rev. B 2005, 72 (7). 42 Kleiman, G. G.; Landers, R. Energy Shifts and Electronic Structure Changes in Alloys: An Unfulfilled Promise? J. Electron. Spectrosc. Relat. Phenom. 1998, 88, 435 43 Sengar, S. K.; Mehta, B. R.; Gupta, G. Charge Transfer, Lattice Distortion, and Quantum Confinement Effects in Pd, Cu, and Pd-Cu Nanoparticles; Size and Alloying Induced Modifications in Binding Energy. Appl. Phys. Lett. 2011, 98(19), article number: 193115 44 Martensson, N.; Nyholm, R. Chemical-Shift Effects and Origin of the Pd 3d Core-Level Satellite in CuPd Alloys. Phys. Rev. Lett. 1980, 45 (9), 754-757. 45 Lima, F.H.B.; Ticianelli, E. A. Oxygen Electrocatalysis on Ultra-Thin Porous Coating Rotating Ring/Disk Platinum and Platinum–Cobalt Electrodes in Alkaline Media. Electrochim. Acta 2004, 49, 4091 46 Jiang, L.; Hsu, A.; Chu, D.; Chen, R. Size-Dependent Activity of Palladium Nanoparticles for Oxygen Electroreduction in Alkaline Solutions J. Electrochem. Soc. 2009, 156(6), B643B649. 47 Habekost, A. Experimental Investigations of Alkaline Silver-zinc and Copper-zinc Batteries. World J. Chem. Educ. 2016, 4, 4-12


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48 Qiao, J.; Xu, L.; Ding L.; Shi, P.; Zhang, L.; Baker, R.; Zhang, J. Effect of KOH Concentration on the Oxygen Reduction Kinetics Catalyzed by Heat-Treated Co-Pyridine/C Electrocatalysts Int. J. Electrochem. Sci. 2013, 8,1189-1208. 49 Perez, J.; Gonzalez, E. R.; Ticianelli, E. A. Oxygen Electrocatalysis on Thin Porous Coating Rotating Platinum Electrodes. Electrochim. Acta 1998, 44, 1329 50 Hammer, B.; Norskov, J.K. Electronic Factors Determining the Reactivity of Metal Surfaces, Surf. Science 1995, 343, 211-220. 51 Hammer, B.; Norskov, J.K. Theoretical Surface Science and Catalysis - Calculations and Concepts, Adv. Catal. 2000, 45, 71-129. 52 Greeley, J.; Nørskov, J. K.; Mavrikakis, M. Electronic Structure and Catalysis on Metal Surfaces, Annu. Rev. Phys. Chem. 2002, 53, 319-348. 53 Xu, Y.; Ruban, A. V.; Mavrikakis, M. Adsorption and Dissociation of O2 on Pt−Co and Pt−Fe Alloys, J. Am. Chem. Soc. 2004, 126, 4717. 54 Greeley, J.; Mavrikakis, M. Alloy Catalysts Designed From First Principles, Nature Mater.2004, 3, 810-815. 1 Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energ. 2011, 88 (4), 981-1007. 2 Lucia, U. Overview on Fuel Cells. Renew. Sust. Energ. Rev. 2014, 30, 164-169. 3 Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486 (7401), 43-51.


35

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 44

4 Ge, X.; Sumboja, A.; Wuu, D.; An, T.; Li, B.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Liu, Z. Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts. ACS Catal. 2015, 5 (8), 4643-4667. 5 Sharaf, O. Z.; Orhan, M. F. An Overview of Fuel Cell Technology: Fundamentals and Applications. Renew. Sust. Energ. Rev. 2014, 32, 810-853. 6 Guo, S.; Zhang, S.; Sun, S. Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2013, 52 (33), 8526-8544. 7 Wu, G.; Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 2013, 46 (8), 1878-1889. 8 Xu, D.; Bliznakov, S.; Liu, Z.; Fang, J.; Dimitrov, N. Composition-Dependent Electrocatalytic Activity of Pt-Cu Nanocube Catalysts for Formic Acid Oxidation. Angew. Chem. Int. Ed. 2010, 49 (7), 1282-1285. 9 Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chem. Int. Ed. 2006, 45 (18), 2897-2901. 10 Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem. Int. Ed. 2009, 48 (1), 60-103. 11 Leontyev, I. N.; Belenov, S. V.; Guterman, V. E.; Haghi-Ashtiani, P.; Shaganov, A. P.; Dkhil, B. Catalytic Activity of Carbon-Supported Pt Nanoelectrocatalysts. Why Reducing the Size of Pt Nanoparticles is Not Always Beneficial. J. Phys. Chem. C 2011, 115 (13), 5429-5434.


36

Page 37 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

12 Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J. Nanostructured Pt-alloy Electrocatalysts for PEM Fuel Cell Oxygen Reduction Reaction. Chem. Soc. Rev. 2010, 39 (6), 2184-2202. 13 Wang, D.; Xin, H. L.; Wang, H.; Yu, Y.; Rus, E.; Muller, D. A.; DiSalvo, F. J.; Abruna, H. D. Facile Synthesis of Carbon-Supported Pd-Co Core-Shell Nanoparticles as Oxygen Reduction Electrocatalysts and Their Enhanced Activity and Stability with Monolayer Pt Decoration. Chem. Mater. 2012, 24 (12), 2274-2281. 14 Yang, J.; Chen, X.; Yang, X.; Ying, J. Y. Stabilization and Compressive Strain Effect of AuCu Core on Pt Shell for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5 (10), 8976-8981. 15 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 (2), 666-673. 16 Zhang, Z.; Liu, J.; Gu, J.; Su, L.; Cheng, L. An Overview of Metal Oxide Materials as Electrocatalysts and Supports for Polymer Electrolyte Fuel Cells. Energy Environ. Sci. 2014, 7 (8), 2535-2558. 17 Wang, L.; Zhai, J.-J.; Jiang, K.; Wang, J.-Q.; Cai, W.-B. Pd-Cu/C Electrocatalysts Synthesized by One-Pot Polyol Reduction Toward Formic Acid Oxidation: Structural Characterization and Electrocatalytic Performance. Int. J. Hydrogen Energy 2015, 40 (4), 17261734. 18 Tang, W. J.; Henkelman, G. Charge Redistribution in Core-Shell Nanoparticles to Promote Oxygen Reduction. J. Chem. Phys. 2009, 130 (19), 6, doi: http://dx.doi.org/10.1063/1.3134684.


37

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 44

19 Xu, Y.; Ruban, A. V.; Mavrikakis, M. Adsorption and Dissociation of O2 on Pt-Co and PtFe Alloys. J. Am. Chem. Soc. 2004, 126 (14), 4717-4725 20 Guy, K. A.; Xu, H.; Yang, J. C.; Werth, C. J.; Shapley, J. R. Catalytic Nitrate and Nitrite Reduction with Pd-Cu/PVP Colloids in Water: Composition, Structure, and Reactivity Correlations. J. Phys. Chem. C 2009, 113 (19), 8177-8185. 21 Fernandez-Garcia, M.; Martinez-Arias, A.; Belver, C.; Anderson, J. A.; Conesa, J. C.; Soria, J. Behavior of Palladium-Copper Catalysts for CO and NO Elimination. J. Catal. 2000, 190 (2), 387-395. 22 Castegnaro, M. V.; Gorgeski, A.; Balke, B.; Alves, M. C. M.; Morais, J. Charge Transfer Effects on the Chemical Reactivity of PdxCu1-x Nanoalloys. Nanoscale 2016, 8 (1), 641-647. 23 Tierney, H. L.; Baber, A. E.; Sykes, E. C. H. Atomic-Scale Imaging and Electronic Structure Determination of Catalytic Sites on Pd/Cu Near Surface Alloys. J. Phys. Chem. C 2009, 113 (17), 7246-7250. 24 Bernardi, F.; Traverse, A.; Olivi, L.; Alves, M. C. M.; Morais, J. Correlating Sulfur Reactivity of PtxPd1-x Nanoparticles with a Bimetallic Interaction Effect. J. Phys. Chem. C 2011, 115 (25), 12243-12249. 25 Bernardi, F.; Alves, M. C. M.; Traverse, A.; Silva, D. O.; Scheeren, C. W.; Dupont, J.; Morais, J. Monitoring Atomic Rearrangement in Pt(x)Pd(1-x) (x=1, 0.7, or 0.5) Nanoparticles Driven by Reduction and Sulfidation Processes. J. Phys. Chem. C 2009, 113 (10), 3909-3916.


38

Page 39 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

26 Boita, J.; Bernardi, F.; Castegnaro, M. V.; Nicolao, L.; Alves, M. C. M.; Morais, J. Reversible Sulfidation of Pt0.3Pd0.7 Nanoparticles Investigated by in Situ Time-Resolved XAS. J. Phys. Chem. C 2014, 118 (10), 5538-5544. 27 Stedile, F. C.; dos Santos, J. H. Z. Analysis and Characterization of Real Catalysts Using Ion Beam Analysis. Nucl. Instrum. Meth. B 1998, 136, 1259-1266. 28 Weibel, A.; Bouchet, R.; Boulc'h, F.; Knauth, P. The Big Problem of Small Particles: A Comparison of Methods for Determination of Particle Size in Nanocrystalline Anatase Powders. Chem. Mater. 2005, 17 (9), 2378-2385. 29 Lima, F. A.; Saleta, M. E.; Pagliuca, R. J. S.; Eleoterio, M. A.; Reis, R. D.; Fonseca Júnior, J.; Meyer, B.; Bittar, E. M.; Souza-Neto, N. M.; Granado, E. XDS: a flexible beamline for X-ray diffraction and spectroscopy at the Brazilian synchrotron. J. Synchrotron Radiat. 2016, 23, 1538–1549. 30 Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537-541. 31 Rehr, J. J.; Kas, J. J.; Vila, F. D.; Prange, M. P.; Jorissen, K. Parameter-free calculations of X-ray spectra with FEFF9. Phys. Chem. Chem. Phys. 2010, 12 (21), 5503-5513. 32 Cowley, J. M. Short-Range and Long-Range Order Parameters in Disordered Solid Solutions. Phys. Rev. 1960, 120 (5), 1648-1657. 33 Frenkel, A. I.; Machavariani, V. S.; Rubshtein, A.; Rosenberg, Y.; Voronel, A.; Stern, E. A. Local Structure of Disordered Au-Cu and Au-Ag Alloys. Phys. Rev. B 2000, 62 (14), 9364-9371.


39

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 44

34 Frenkel, A. I.; Wang, Q.; Sanchez, S. I.; Small, M. W.; Nuzzo, R. G. Short Range Order in Bimetallic Nanoalloys: An Extended X-ray Absorption Fine Structure Study. J. Chem. Phys. 2013, 138 (6), doi: http://dx.doi.org/10.1063/1.4790509. 35 Frenkel, A. I. Applications of Extended X-ray Absorption Fine-Structure Spectroscopy to Studies of Bimetallic Nanoparticle Catalysts. Chem. Soc. Rev. 2012, 41 (24), 8163-8178. 36 Tolentino, H. C. N.; Ramos, A. Y.; Alves, M. C. M.; Barrea, R. A.; Tamura, E.; Cezar, J. C.; Watanabe, N. A 2.3 to 25 keV XAS Beamline at LNLS. J. Synchrotron Radiat. 2001, 8, 1040−1046. 37 Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. in: Chastain, J. (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation: Eden Prairie, MN, 1992. 38 Tougaard, S.; Jansson, C. Comparison of Validity and Consistency of Methods for Quantitative XPS Peak Analysis. Surf. Interface Anal., 1993, 20 (13), 1013-1046. 39 Rehr, J. J.; Albers, R. C. Theoretical Approaches to X-ray Absorption Fine Structure. Rev. Mod. Phys. 2000, 72 (3), 621-654. 40 Koningsberger, D. C.; Mojet, B. L.; van Dorssen, G. E.; Ramaker, D. E. XAFS Spectroscopy; Fundamental Principles and Data Analysis. Top. Catal. 2000, 10 (3-4), 143-155. 41 Alayoglu, S.; Zavalij, P.; Eichhorn, B.; Wang, Q.; Frenkel, A. I.; Chupas, P. Structural and Architectural Evaluation of Bimetallic Nanoparticles: A Case Study of Pt-Ru Core-Shell and Alloy Nanoparticles. ACS Nano 2009, 3 (10), 3127-3137.


40

Page 41 of 44

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42 Srabionyan, V. V.; Bugaev, A. L.; Pryadchenko, V. V.; Avakyan, L. A.; Van Bokhoven, J. A.; Bugaev, L. A. EXAFS Study of Size Dependence of Atomic Structure in Palladium Nanoparticles. J. Phys. Chem. Solids 2014, 75 (4), 470-476. 43 Olovsson, W.; Bech, L.; Andersen, T. H.; Li, Z.; Hoffmann, S. V.; Johansson, B.; Abrikosov, I. A.; Onsgaard, J. Core-Level Shifts for Two- and Three-Dimensional Bimetallic PdxCu1-x

and

PdxAg1-x

Alloys

on

Ru(0001).

Phys.

Rev.

B

2005,

72

(7),

DOI:https://doi.org/10.1103/PhysRevB.72.075444. 44 Kleiman, G. G.; Landers, R. Energy Shifts and Electronic Structure Changes in Alloys: An Unfulfilled Promise? J. Electron. Spectrosc. Relat. Phenom. 1998, 88, 435 45 Sengar, S. K.; Mehta, B. R.; Gupta, G. Charge Transfer, Lattice Distortion, and Quantum Confinement Effects in Pd, Cu, and Pd-Cu Nanoparticles; Size and Alloying Induced Modifications in Binding Energy. Appl. Phys. Lett. 2011, 98(19), article number: 193115 46 Martensson, N.; Nyholm, R. Chemical-Shift Effects and Origin of the Pd 3d Core-Level Satellite in CuPd Alloys. Phys. Rev. Lett. 1980, 45 (9), 754-757. 47 Lima, F.H.B.; Ticianelli, E. A. Oxygen Electrocatalysis on Ultra-Thin Porous Coating Rotating Ring/Disk Platinum and Platinum–Cobalt Electrodes in Alkaline Media. Electrochim. Acta 2004, 49, 4091. 48 Jiang, L.; Hsu, A.; Chu, D.; Chen, R. Size-Dependent Activity of Palladium Nanoparticles for Oxygen Electroreduction in Alkaline Solutions J. Electrochem. Soc. 2009, 156(6), B643B649.


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Page 42 of 44

49 Habekost, A. Experimental Investigations of Alkaline Silver-zinc and Copper-zinc Batteries. World J. Chem. Educ. 2016, 4, 4-12 50 Ambrose, J; Barradas, R. G.; Shoesmith, D. W. Investigations of Copper in Aqueous Alkaline Solutions by Cyclic Voltammetry, J. Electroanal. Chem. 1973, 47, 47-64. 51 ABD El Haleem, S. M.; Ateya B. G. Cyclic Voltammetry of Copper in Sodium Hydroxide Solutions, J. Electroanal. Chem. 1981, 117, 309-319. 52 Qiao, J.; Xu, L.; Ding L.; Shi, P.; Zhang, L.; Baker, R.; Zhang, J. Effect of KOH Concentration on the Oxygen Reduction Kinetics Catalyzed by Heat-Treated Co-Pyridine/C Electrocatalysts Int. J. Electrochem. Sci. 2013, 8,1189-1208. 53 Perez, J.; Gonzalez, E. R.; Ticianelli, E. A. Oxygen Electrocatalysis on Thin Porous Coating Rotating Platinum Electrodes. Electrochim. Acta 1998, 44, 1329 54 Yan, W-Y.; Zheng S-L.; Jin W.; Peng, Z.; Wang, S-N.; Du, N.; Zhang, Y. The Influence of KOH Concentration, Oxygen Partial Pressure and Temperature on the Oxygen Reduction Reaction at Pt Electrodes, J. Electroanal. Chem. 2015, 741, 100-108. 55 Hammer, B.; Norskov, J.K. Electronic Factors Determining the Reactivity of Metal Surfaces, Surf. Science 1995, 343, 211-220. 56 Hammer, B.; Norskov, J.K. Theoretical Surface Science and Catalysis - Calculations and Concepts, Adv. Catal. 2000, 45, 71-129. 57 Greeley, J.; Nørskov, J. K.; Mavrikakis, M. Electronic Structure and Catalysis on Metal Surfaces, Annu. Rev. Phys. Chem. 2002, 53, 319-348.


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58 Xu, Y.; Ruban, A. V.; Mavrikakis, M. Adsorption and Dissociation of O2 on Pt−Co and Pt−Fe Alloys, J. Am. Chem. Soc. 2004, 126, 4717. 59 Greeley, J.; Mavrikakis, M. Alloy Catalysts Designed From First Principles, Nature Mater.2004, 3, 810-815.

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