Revealing the Details of the Surface Composition of Electrochemically

Article pubs.acs.org/JPCC

Revealing the Details of the Surface Composition of Electrochemically Prepared Au@Pd Core@Shell Nanoparticles with in Situ EXAFS Stephen W. T. Price, Jennifer M. Rhodes, Laura Calvillo, and Andrea E. Russell* Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom ABSTRACT: Carbon supported Au@Pd core@shell nanoparticles were prepared using two methods based on displacement of a Cu-under potential deposited (upd) layer; the standard method, in which a single Cu-upd layer is formed and then displaced, and a Cu-upd-mediated deposition method, wherein the Pd displaces the Cu-upd layer during the Cu deposition. The resulting materials were characterized using in situ extended X-ray absorption fine structure as a function of the applied potential in an electrochemical cell at both the Au L3 and Pd K absorption edges. This detailed structural analysis shows that the standard, single Cu-upd layer method results in the formation of Pd clusters or islands on the Au core rather than a complete monolayer shell, while the Cu-upd-mediated method produces a mixed/alloyed PdAu shell.

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Wang et al.21 for the preparation of Pd@Pt core@shell electrocatalysts. In that study Pt2+ was added in low concentration to the Cu-upd solution and then the carbon supported Pd electrode was cycled repeatedly in the Cu-upd region to repeatedly form and remove the Cu-upd layer. This allowed for diffusion-controlled deposition of the Pt at potentials below the bulk deposition potential. An increase in the Cu-upd current, corresponding to an increase in the surface area of the nanoparticles as the Pt was irreversibly deposited, was observed. This Cu-upd-mediated deposition method succeeded in creating a smoother surface layer than the standard upd method.

INTRODUCTION Au@Pd core@shell nanoparticle catalysts have attracted a great deal of recent attention as both heterogeneous catalysts1 and electrocatalysts. Examples in heterogeneous catalysis include the synthesis of peroxide1−5 and alcohol oxidation,6−10 while the applications in electrocatalysis are primarily focused on methanol and formic acid oxidation11 and on the oxygen reduction reaction.2,12,13 It is well recognized that the activity and selectivity of bimetallic catalysts depend on both the ligand effects14,15 of one metal atom on the other, which include dband center shifts, charge transfer, and strain and also on ensemble effects,16,17 which describe the more specific effects of particular arrangements of atoms, especially at the surface of the nanoparticle. Thus, much attention has focused on the careful preparation of optimal core@shell structures. Preparation of these bimetallic nanoparticles has followed several routes including intermittent microwave heating,12 colloidal synthesis,18,19 and electrodeposition methods.20−23 In this article, we focus on the electrodeposition approach, examining the effectiveness of two different methods of depositing a Pd shell onto a Au core, both of which use under potential deposited (upd) Cu. First, we investigated the standard displacement approach, whereby a sacrificial predeposited “template” Cu layer is displaced by Pd,20 and second a modified approach whereby Cu is employed as a mediator for Pd deposition onto the Au core.21 As we have previously reported,24 the upd of Cu does not occur as uniformly on nanoparticles as on single crystal surfaces, resulting both in incomplete Cu encapsulation and in the deposition of clusters of Cu on the Au core. We will show that these differences have an effect on the structure of the resulting Pd shell when the standard approach is used. The second, Cu-upd-mediated deposition, method has been previously been reported by © 2013 American Chemical Society

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EXPERIMENTAL SECTION The core@shell catalysts formed using both approaches were characterized in situ in the electrochemical environment using extended X-ray absorption fine structure (EXAFS). The EXAFS was collected at both the Au L3 and Pd K absorption edges, and the analysis presented here provides structural models of the different bimetallic structures formed by the two methods as a function of potential from the perspective of both the Au and Pd atoms. Catalyst Preparation. Au nanoparticles were prepared using the thiol encapsulation method of Brust et al.,25 which has become one of the standard preparation methods. To prepare the carbon supported electrocatalyst, the Au colloid dispersion and a suspension of Vulcan carbon XC-72R were combined in ethanol to yield a catalyst corresponding to 4 wt % Au/C. The Received: August 25, 2013 Revised: October 21, 2013 Published: October 21, 2013 24858

dx.doi.org/10.1021/jp408493q | J. Phys. Chem. C 2013, 117, 24858−24865

The Journal of Physical Chemistry C

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mixture was sonicated for 30 min and refluxed at 80 °C under vigorous stirring for 10 h. The resulting powder was separated by filtration and then thoroughly washed with purified water, before being dried at 110 °C for 2 h. Electrode Preparation. The required amount of catalyst (approximately 60 mg) was finely ground and then dispersed in a small amount of deionized water (0.75 mL) and isopropanol (0.20 mL). Nafion (10.85 wt % solids in water) was added to the solution to give an ink with 30 wt % Nafion solids when dry. The mixture was sonicated for 20 min followed by mixing with a Fisher PowerGen 125 homogenizer. The resulting mixture was painted onto carbon paper (TGP-H-060), which was dried, weighed, and the process repeated until the desired loading of 0.07 mg cm−2 Au was obtained. The electrodes were then pressed at 177 °C and 1 bar for 3 min. The 1.32 cm2 circular button electrodes were cut from the sheet and hydrated by boiling in deionized water prior to use. Electrochemistry. An in situ electrochemical cell was used to collect XANES and EXAFS data of the Pd deposition on Au/ C as a function of potential. The working electrode (WE) was held in place by a Au wire contact, a Pt wire served as the counter electrode (CE), and the reference electrode (RE) was a mercury mercurous sulfate (Hg/Hg2SO4) electrode that was connected to the cell via a short length of tubing containing the electrolyte. The cell was controlled by a μAutolab type III potentiostat running the General Purpose Electrochemistry Software 4.9 (GPES). The electrolyte was purged with N2 and then pumped through the cell using a peristaltic pump. All potentials reported are referred to the Hg/HgSO4 reference electrode. Prior to deposition of the Cu-upd layer, the prepared electrodes were cycled in 0.5 M H2SO4 from −0.65 to 0.9 V, until subsequent voltammograms overlaid, typically requiring 50 cycles to clean the surfaces of any residual adsorbed thiol from the synthesis. Preparation of the Au@Pd catalyst electrode via the standard Cu-upd layer displacement method was conducted in the EXAFS cell. As in our previous study, 2 mM CuSO4 in 0.5 M H2SO4 was chosen to ensure there was enough Cu2+ in the volume of the solution in the cell to yield a full Cu monolayer on the Au nanoparticles.24 The Cu-upd layer was deposited by holding the potential at −0.455 V for 30 min. The galvanic displacement of the Cu-upd layer by Pd was then accomplished by first flushing the cell with 0.5 M H2SO4 and then with 5 mM K2PdCl4 in 0.5 M H2SO4, while maintaining the potential at −0.455 V. The cell was then flushed once again with 0.5 M H2SO4 to remove any residual Pd2+ ions from the solution and the potential held at 0.0 V for 15 min prior to collection of the EXAFS data at the Au L3 edge at 0.0, −0.455, −0.640, and −0.665 V. Pd K edge EXAFS spectra were then collected at the same four potentials. The Cu-upd-mediated Au@Pd catalyst was prepared in a separate electrochemical cell containing a N2 purged solution of 0.5 M H2SO4, 5 mM CuSO4, and 0.1 mM K2PdCl4 by cycling at 10 mV s−1 for 25 cycles between 0.0 and −0.455 V vs Hg/ HgSO4 before being removed and carefully rinsed with deionized water. During the preparation of the Cu-updmediated Au@Pd, the position of the Cu-upd peaks shifts to more positive potentials between the first and last scans as shown in Figure 1. Previous studies by Okada et al.26 have shown that Cu-upd occurs at more positive potentials on Pd than on Au and thus the results indicate that the surface becomes more Pd rich during the cycling. The EXAFS data

Figure 1. Cyclic voltammetry of the Cu-upd-mediated Pd deposition. The blue line corresponds to the Au/C core in 0.5 M H2SO4 before the addition of Cu and Pd. The black line is the first cycle following the addition of 5 mM CuSO4 and 0.1 mM K2PdCl4, and the red line corresponds to the 25th cycle. The scan rate was 10 mV s−1.

were then collected in the EXAFS cell in 0.5 M H2SO4 first at the Au L3 edge and second at the Pd K edge at 0.0, −0.455, −0.640, and −0.665 V. X-ray Absorption Measurements. X-ray absorption measurements were recorded at the Cu K edge (8979 eV), the Au L3 absorption edge (11919 eV), and the Pd K absorption edge (24350 eV) on beamline B18 at Diamond Light Source, which operated with a ring energy 3 GeV and at a current of 300 mA. The monochromator used Si(311) crystals operating in Quick EXAFS (QEXAFS) mode. Each spectra took 15 min to collect, and a total of three spectra were averaged for each potential hold. The in situ electrode measurements were collected in fluorescence mode at 298 K using a 9-element Ge detector. Calibration of the monochromator was carried out at both edges using Au and Pd foils before the in situ data collection of each sample. The stability of the beam ensured the energy scale did not drift during data collection. All EXAFS measurements were collected at fixed potentials, as described above. Data Analysis. The spectra were processed and analyzed using the programs Athena and Artemis,27 which implement the FEFF6 and IFEFFIT codes.28,29 The AUTOBK method30 was used to isolate the k-space EXAFS data from the raw data, and a theoretical EXAFS signal was constructed using FEFF6. Data was collected for a Au foil prior to the nanoparticle measurements to enable determination of the amplitude reduction factor. This was found to be 0.82 ± 0.03; all coordination numbers and subsequent results were corrected accordingly. To fit the Au L3 absorption edge data, the theoretical signal included single and multiple scattering contributions up to the third nearest neighbor in the Au facecentered cubic (fcc) structure with a k range of 3−16 Å−1 and an R range of 2−5.8 Å. The single scattering paths to the nearest three neighbors were [Auabs−Au1−Auabs], [Auabs−Au2− Auabs], and [Auabs−Au3−Auabs], where abs denotes the absorbing atom and the numeric subscripts identify Au the first three Au neighbor distances in the fcc structure (i.e., Au1 is first nearest neighbor, Au2 is second nearest neighbor. etc.). Also included were noncollinear multiple scattering paths [Auabs−Au1−Au1−Auabs], and [Auabs−Au3−Au1−Auabs]. The 24859



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Figure 2. (a) k3 weighted Au L3 edge experimental data (black) and fit (red) and (b) the corresponding k3 weighted Fourier transforms for the standard Cu-upd prepared Au@Pd catalyst electrode in 0.5 M H2SO4 as a function of the applied potential.

Table 1. Structural Parameters Obtained by Fitting the Au L3 Edge Data Shown in Figure 2 for the Cu-upd Prepared Au@Pd Electrode V vs Hg/HgSO4

shell

+0.000

Au−Au1 Au−Au2 Au−Au3 Au−Au1 Au−Au2 Au−Au3 Au−Au1 Au−Au2 Au−Au3 Au−Au1 Au−Au2 Au−Au3

−0.455

−0.640

−0.665

N 9.6 3.5 21.4 9.1 2.4 19.2 9.3 2.2 19.5 9.2 3.5 21.8

± ± ± ± ± ± ± ± ± ± ± ±

R/Å 0.5 2.1 7.4 0.5 1.7 7.9 0.5 1.4 7.3 0.5 2.8 8.7

2.83 4.02 4.93 2.82 4.02 4.93 2.82 4.03 4.92 2.83 4.03 4.93

± ± ± ± ± ± ± ± ± ± ± ±

path degeneracy was left to vary as a fitting parameter to account for the under-coordination of the surface atoms. Bond length correction terms and mean square disorder parameters were constrained based on the photoelectron mean free path length, and a single photoelectron energy correction applied to all paths. The selection of paths chosen is similar to those used in the study of other nanoparticle systems.24,31,32 The Pd K absorption edge data was fit between 3 and 11 Å−1 in k space and 1.4−3.1 Å in R space, including the first nearest neighbors only. A Pd foil reference was measured and a S02 of 0.83 ± 0.06 was determined. Fitting models for Pd−Pd and Pd−Au were based on fcc Au/Pd alloys with the bond length varying between that of the Au and Pd lattices, and the ratio of the local Au:Pd coordination number being set to 11:1, 5:1, 2:1, 1:1, 1:2, 1:5, and 1:11. These empirical calculations of the input structure are consistent with previously published lattice parameters of gold−palladium alloys with varying atomic percentages.33,34 As with the Au fitting model, the path degeneracy was allowed to vary with the fit along with a bond length correction, mean square disorder, and a single photoelectron energy correction. The best fit to the Pd K edge data was obtained using a FEFF input of an alloy with Pd:Au atomic ratio of 5:1 with full Pd bond character. The Au L3 edge data was best fit by an fcc Au input. The addition of Pd into the structure and decreasing of the lattice parameter worsened the quality of fit for the Au−Au scattering paths.

0.01 0.03 0.01 0.01 0.03 0.01 0.01 0.02 0.01 0.01 0.03 0.01

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σ2 × 104/ Å2

ΔE0/eV

Rf

± ± ± ± ± ± ± ± ± ± ± ±

5.0 ± 0.3

0.013

4.4 ± 0.3

0.020

4.0 ± 0.3

0.020

4.7 ± 0.3

0.020

99 147 161 93 110 150 96 86 158 95 145 161

3 60 29 4 56 33 4 38 39 4 78 35

RESULTS AND DISCUSSION The 4 wt % Au/C core catalyst was characterized ex situ and, as reported previously,24 had an approximate size of 2.5 nm and spherical shape (based on first and third coordination shell numbers and their ratio).35 After electrode preparation and cycling to remove the residual thiol, the particle size was found to increase to approximately 3.5 nm with the shape remaining spherical over the range of potentials measured. The absence of residual Cu in both the Au@Pd catalysts following the galvanic displacement by Pd was confirmed in situ by the lack of absorption at the Cu K edge (8979 eV). Au−Cu and Pd−Cu fitting models were also explored during the analysis, however, inclusion of Cu neighbors did not improve the fits, further supporting the removal of Cu from the catalysts. The EXAFS data and fitted parameters at the Au L3 edge for the Au@Pd catalyst prepared using the standard Cu-upd displacement method are reported in Figure 2 and Table 1, respectively. In agreement with our previous study, the Au L3 EXAFS is independent of the applied potential, indicating that there is no variation in the nanoparticle shape over the limited potential range explored in the study presented here. Inclusion of a first shell Au−Pd path did not improve the quality of the fit and was thus excluded, which is also consistent with the findings of our earlier study regarding a first shell Au−Cu contribution. A first coordination shell bond length contraction of 0.05 Å, relative to the bulk value, was observed and remained constant as the potential was decreased. This suggests that the 24860



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Figure 3. (a) k3 weighted Au L3 edge experimental data (black) and fit (red) and (b) the corresponding k3 weighted Fourier transforms for the Cuupd-mediated prepared Au@Pd catalyst electrode in 0.5 M H2SO4 as a function of the applied potential.

Table 2. Structural Parameters Obtained by Fitting the Au L3 Edge Data Shown in Figure 3 for the Cu-upd-Mediated Prepared Au@Pd Electrode V vs Hg/HgSO4

shell

+0.000

Au−Au1 Au−Au2 Au−Au3 Au−Au1 Au−Au2 Au−Au3 Au−Au1 Au−Au2 Au−Au3 Au−Au1 Au−Au2 Au−Au3

−0.455

−0.640

−0.665

N 8.9 2.3 20.8 9.0 1.8 17.7 8.7 4.7 20.5 8.6 2.4 13.8

± ± ± ± ± ± ± ± ± ± ± ±

R/Å 0.5 2.1 8.7 0.6 2.1 9.2 0.6 5.0 10.5 0.6 2.4 8.4

2.83 4.05 4.93 2.83 4.06 4.92 2.82 4.02 4.92 2.83 4.05 4.93

± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.03 0.02 0.01 0.03 0.02 0.01 0.05 0.02 0.01 0.03 0.02

σ2 × 104/ Å2

ΔE0/eV

Rf

± ± ± ± ± ± ± ± ± ± ± ±

4.1 ± 0.3

0.017

4.2 ± 0.4

0.021

4.5 ± 0.4

0.026

3.9 ± 0.4

0.022

85 107 173 88 92 157 86 191 167 87 103 141

3 63 39 5 72 45 4 130 47 5 68 46

Figure 4. (a) k3 weighted Pd K edge experimental data (black) and fit (red) and (b) the corresponding k3 weighted Fourier transforms for the Cuupd prepared Au@Pd catalyst electrode in 0.5 M H2SO4 as a function of the applied potential.

Au core is not affected by any effects of the applied potential on the Pd shell, as described below. The EXAFS data and fits for Au@Pd formed via Cu-updmediated Pd deposition at the Au L3 edge are reported in Figure 3 and Table 2, respectively. After the Cu-upd-mediated Pd deposition the Au coordination numbers are consistently smaller than those for the single Cu-upd sample, although the difference is within the error of the fits. With the exception of the third shell coordination number, which is lower at −0.665

V, the coordination numbers for each shell are constant within the error of the fitting model. The decrease in the third shell coordination number may be a consequence of perturbation of the Pd structure at this potential, as described below. The errors in the Au coordination numbers are slightly larger for the Cu-upd-mediated sample than for the single Cu-upd sample and this may reflect the presence of multiple coordination environments, for example, the Au environment of the core and a mixed Au and Pd environment in the shell. A 24861



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Table 3. Structural Parameters Obtained by Fitting the Pd K Edge Data Shown in Figure 4 for the Cu-upd Prepared Au@Pd Electrode V vs Hg/HgSO4 0.000 −0.455 −0.640 −0.655

N

shell Pd−Pd1 Pd−Au1 Pd−Pd1 Pd−Au1 Pd−Pd1 Pd−Au1 Pd−Pd1 Pd−Au1

5.1 5.3 5.9 5.1 5.4 5.7 4.8 7.3

± ± ± ± ± ± ± ±

R/Å 0.7 3.8 1.3 3.7 0.7 3.0 0.9 5.9

2.73 2.78 2.73 2.82 2.73 2.76 2.78 2.77

± ± ± ± ± ± ± ±

first shell Au−Pd scattering path can be fitted to the Au edge data with a coordination number of 0.5 ± 0.3, resulting in a moderately improved quality of fit, however the errors associated with the bond length in particular and in the disorder are such that the Au−Pd path was not included in the final fits reported. The EXAFS data and fitted parameters for the standard Cuupd Au@Pd catalyst at the Pd K edge are reported in Figure 4 and Table 3, respectively. The Pd−Pd to Pd−Au first shell coordination number ratio is approximately one to one, with a much greater error in the Pd−Au coordination number compared to that for Pd−Pd. Combined with the lack of Au−Pd coordination at the Au L3 edge, this supports the formation of a segregated Au/Pd structure, with Pd preferentially situated at the surface of the nanoparticles. The coordination numbers obtained for the Pd shell are larger than those previously reported for Cu for the initial Cu-upd on Au/ C.24 This result is attributed first to the fact that the Cu-upd deposition potential was chosen to be slightly more negative than in the previous study and second to the increased miscibility of the Pd and Au compared to Cu and Au. Equilibrium phase diagrams of Pd−Au alloys show that a solid solution will be formed over all atomic percentages, whereas the Cu−Au system forms separate phases between 40 and 90% Au.36 Pd is therefore much more likely to migrate into the surface of the Au core particle, thus increasing the Pd−Au coordination number. The movement of Pd into the surface will increase the number of Pd local environments and therefore increase the error associated with the Pd−Au coordination number. The increased variety of coordination environments also gives rise to the larger σ2 term observed and an increase in the error associated with in this fitting parameter. The Pd−Au distance obtained is intermediate between those of the Pd−Pd and Au−Au distance and was not found to change within error over the potential range investigated. In contrast, an increase in the Pd−Pd distance is observed at −0.665 V compared to the other potentials, as shown in Figure 5. This increase is accompanied by a corresponding increase in the disorder. The −0.665 V represents a potential at which H is absorbed into the Pd lattice, forming the metal hydride. The Pd−Pd distance obtained, 2.78 Å, is intermediate between the α-PdH phase (2.753 Å), and the β-PdH phase (2.846 Å).37,38 The change in Pd−Pd distance is directly observable in the increased frequency of the EXAFS oscillations (Figure 4a). There is also a change in the amplitude of the EXAFS, particularly at higher k values, brought about by phase cancellation of the Pd−Pd scattering with the Pd−Au scattering, which changes as a result of the H absorption. Thus, the decrease in the Au−Au third shell and Pd−Au coordination numbers observed at −0.665 V is attributed to a

0.01 0.03 0.01 0.07 0.01 0.04 0.01 0.04

σ2 × 104/ Å2

ΔE0/eV

Rf

± ± ± ± ± ± ± ±

5.96 ± 0.68

0.005

6.97 ± 1.01

0.019

5.52 ± 0.80

0.010

4.40 ± 1.17

0.018

54 187 64 209 56 213 78 224

6 121 11 157 7 161 12 154

Figure 5. Potential dependence of the Pd−Pd distance determined by fitting the Pd K edge EXAFS data for the standard Cu-upd Pd shell (black squares) and Cu-upd-mediated Pd shell (red circles) Au@Pd catalyst electrodes.

either phase cancellation or a real structural effect of the perturbation of the structure of the Pd shell brought about by H absorption. The extent of increase in the Pd−Pd distance at −0.665 V and lack of a Au−Pd first shell fitting path at the Au L3 edge, both indicate the formation of a Au core−Pd shell structure. However, the ratio of Pd−Pd to Pd−Au coordination numbers at the Pd K edge and the facile absorption of H, resulting in an expansion of the Pd−Pd distance, suggest that a complete monolayer shell has not been formed and, instead, the Pd has formed clusters on the surface of the Au core. Baldauf and Kolb previously reported that H absorption only occurs in Pd films that are thicker than 2 monolayers.39 Finally, the data indicate that there may be further migration of the Pd into a subsurface layer of the Au, particularly once H is absorbed as evidenced by the increased Pd−Au coordination number at −0.665 V. This increase is accompanied by a corresponding increase in the error which further reflects the increase in the number of Pd− Au first shell coordination environments being fitted using a single environment as the fitting parameter. Note that fitting this first shell Pd−Au path using a number of coordination environments would not be statistically justified within the Nyquist criterion, that is, the number of variables required would exceed the number of data points available for the fit.40 The fitted EXAFS parameters for the Cu-upd-mediated Au@ Pd catalyst at the Pd K edge are reported in Table 4 and the plots in Figure 6. The ratio of the Pd−Pd to Pd−Au coordination numbers is approximately 0.5 at all the potentials 24862



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Table 4. Structural Parameters Obtained by Fitting the Pd K Edge Data Shown in Figure 6 for the Cu-upd-Mediated Prepared Au@Pd Electrode V vs Hg/HgSO4 0.000 −0.455 −0.640 −0.655

N

shell Pd−Pd1 Pd−Au1 Pd−Pd1 Pd−Au1 Pd−Pd1 Pd−Au1* Pd−Pd1 Pd−Au1

3.6 6.6 3.9 6.7 3.8 7.0 3.5 6.3

± ± ± ± ± ± ± ±

R/Å 0.8 3.3 1.0 4.3 1.2 0.3 1.0 2.5

2.72 2.77 2.72 2.76 2.73 2.78 2.74 2.76

± ± ± ± ± ± ± ±

0.01 0.03 0.01 0.03 0.01 0.04 0.02 0.03

σ2 × 104/ Å2

ΔE0/eV

Rf

± ± ± ± ± ± ± ±

5.96 ± 0.97

0.006

6.21 ± 1.17

0.007

6.14 ± 0.50

0.026

4.18 ± 1.38

0.012

52 158 60 175 60 178 82 131

10 71 12 96 5 18 24 55

*

A restraint was required for the Pd−Au1 path to keep the values of the amplitude and disorder within reasonable bounds. The restraint had no effect on the other parameters.

Figure 6. (a) k3 weighted Pd K edge experimental data (black) and fit (red) and (b) the corresponding k3 weighted Fourier transforms for the Cuupd-mediated prepared Au@Pd electrode in 0.5 M H2SO4 as a function of the applied potential.

Figure 7. Schematic of the cross sections of the predicted structures for each shell formation method and those determined from the EXAFS analysis. Gold circles correspond to Au atoms and gray to Pd atoms.

decreases to zero when the Au content exceeds approximately 70%. On the basis of the coordination numbers, the Pd environment is ca. 36% Pd and 64% Au.

investigated, which is half that obtained for the Cu-upd sample described above, indicating much more extensive mixing of the Pd and Au atoms in the surface of the particle. Further evidence for the increased mixing is provided by examining the decreased effect of H absorption at −0.655 V compared to that for the Cu-upd catalyst (smaller increase in the frequency of the EXAFS oscillations (Figure 6a) and the corresponding smaller increase in the Pd−Pd distance). Łukaszewski et al.41 have shown that the amount of H absorbed by Pd−Au alloys

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CONCLUSION On the basis of the potential dependent EXAFS analysis, the effects of hydrogen absorption on the structural parameters, in particular, we find that the structure formed by the two routes differ from those predicted. A schematic of the proposed 24863



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(7) Tiruvalam, R. C.; Pritchard, J. C.; Dimitratos, N.; Lopez-Sanchez, J. A.; Edwards, J. K.; Carley, A. F.; Hutchings, G. J.; Kiely, C. J. Aberration Corrected Analytical Electron Microscopy Studies of SolImmobilized Au + Pd, Au{Pd} and Pd{Au} Catalysts Used For Benzyl Alcohol Oxidation and Hydrogen Peroxide Production. Faraday Discuss. 2011, 152 (0), 63−86. (8) Balcha, T.; Strobl, J. R.; Fowler, C.; Dash, P.; Scott, R. W. J. Selective Aerobic Oxidation of Crotyl Alcohol Using AuPd Core− Shell Nanoparticles. ACS Catalysis 2011, 1 (5), 425−436. (9) Deplanche, K.; Mikheenko, I. P.; Bennett, J. A.; Merroun, M.; Mounzer, H.; Wood, J.; Macaskie, L. E. Selective Oxidation of BenzylAlcohol over Biomass-Supported Au/Pd Bioinorganic Catalysts. Top. Catal. 2011, 54 (16−18), 1110−1114. (10) Kim, D.; Lee, Y. W.; Lee, S. B.; Han, S. W. Convex Polyhedral Au@Pd Core−Shell Nanocrystals with High-Index Facets. Angew. Chem. -Int. Edit. 2012, 51 (1), 159−163. (11) Zhou, W.; Lee, J. Y. Highly Active Core−Shell Au@Pd Catalyst for Formic Acid Electrooxidation. Electrochem. Commun. 2007, 9 (7), 1725−1729. (12) Nie, M.; Shen, P. K.; Wei, Z. Nanocrystalline Tungsten Carbide Supported Au−Pd Electrocatalyst for Oxygen Reduction. J. Power Sources 2007, 167 (1), 69−73. (13) Kuai, L.; Yu, X.; Wang, S. Z.; Sang, Y.; Geng, B. Y. Au-Pd Alloy and Core−Shell Nanostructures: One-Pot Coreduction Preparation, Formation Mechanism, and Electrochemical Properties. Langmuir 2012, 28 (18), 7168−7173. (14) Rodriguez, J. A.; Goodman, D. W. The Nature of the MetalMetal Bond in Bimetallic Surfaces. Science 1992, 257 (5072), 897− 903. (15) Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Role of Strain And Ligand Effects in the Modification of the Electronic And Chemical Properties of Bimetallic Surfaces. Phys. Rev. Lett. 2004, 93, 156801. (16) Rodriguez, J. A. Physical and Chemical Properties of Bimetallic Surfaces. Surf. Sci. Rep. 1996, 24 (7−8), 225−287. (17) Maroun, F.; Ozanam, F.; Magnussen, O. M.; Behm, R. J. The Role of Atomic Ensembles in the Reactivity of Bimetallic Electrocatalysts. Science 2001, 293 (5536), 1811−1814. (18) Schmidt, T. J.; Jusys, Z.; Gasteiger, H. A.; Behm, R. J.; Endruschat, U.; Boennemann, H. On the CO Tolerance of Novel Colloidal PdAu/Carbon Electrocatalysts. J. Electroanal. Chem. 2001, 501 (1−2), 132−140. (19) Rybalka, K. V.; Tarasevich, M. R.; Grafov, B. M.; Bogdanovskaya, V. A.; Beketaeva, L. A.; Loubnin, E. N.; Kolbanovskii, Y. A. Binary Pt- and Pd-Based Electrocatalysts for Oxidation of Hydrogen With CO Admixtures. J. New Mater. Electrochem. Syst. 2007, 10 (2), 81−89. (20) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Metal Monolayer Deposition by Replacement of Metal Adlayers on Electrode Surfaces. Surf. Sci. 2001, 474 (1−3), L173−L179. (21) Wang, J. X.; Inada, H.; Wu, L.; Zhu, Y.; Choi, Y.; Liu, P.; Zhou, W.-P.; Adzic, R. R. Oxygen Reduction on Well-Defined Core−Shell Nanocatalysts: Particle Size, Facet, and Pt Shell Thickness Effects. J. Am. Chem. Soc. 2009, 131 (47), 17298−17302. (22) Simonov, A.; Pyrjaev, P.; Moroz, B.; Bukhtiyarov, V.; Parmon, V. Electrodeposited Pd Sub-Monolayers on Carbon-Supported Au Particles of Few Nanometers in Size: Electrocatalytic Activity for Hydrogen Oxidation and CO Tolerance Vs. Pd Coverage. Electrocatalysis 2012, 3 (2), 119−131. (23) Safavi, A.; Farjami, F. Electrochemical Design of Ultrathin Palladium Coated Gold Nanoparticles as Nanostructured Catalyst for Amperometric Detection of Formaldehyde. Electroanalysis. 2011, 23 (8), 1842−1848. (24) Price, S. W. T.; Speed, J. D.; Kannan, P.; Russell, A. E. Exploring the First Steps in Core-Shell Electrocatalyst Preparation: In Situ Characterization of the Underpotential Deposition of Cu on Supported Au Nanoparticles. J. Am. Chem. Soc. 2011, 133 (48), 19448−19458.

structures is shown in Figure 7. The difference is partly as a result of the incomplete/imperfect monolayer formation following Cu-upd as discussed previously24 but also a result of the miscibility Au and Pd. The Cu-upd-mediated deposition method as reported by Wang et al.21 was previously found to form more distinct core@shell structures for deposition of Pt on to a Pd core. However, Pt and Pd are less miscible than Pd and Au and this is likely to result in a better core@shell structure for the Pd@Pt system. The results presented thus show that the standard Cu-upd method results in the formation of a very thin and nonuniform Pd layer onto the Au core with likely formation of Pd islands/clusters, as evidenced by the effects of hydrogen absorption, which indicates the presence of three-dimensional Pd rich regions. In contrast, the Cu-updmediated method leads to a Pd−Au alloyed shell on the surface of the Au core. Such detailed structural information would not be as readily obtained using ex situ characterization methods such as TEM or XPS, as exposure to air during drying out of the catalyst electrode structure would be likely to result in oxidation of the surface, which for Au@Pd is likely to result in Pd being drawn to the surface of the particle. Thus, the study presented here provides an excellent illustration of the need for in situ characterization to determine the structures of such complex bimetallic nanoparticles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Giannantonio Cibin, Dr. Andy Dent, and Dr. Stephen Parry at Diamond Light Source for the excellent beamline support, and Professor Patrick Hendra at Ventacon for the design and construction of the in situ cell. This work has been made possible by the financial support of the University of Southampton. L.C. acknowledges support via an EU Marie Curie Intra-European Fellowship under contract no. FP7-PEOPLE-2010-IEF-272632 and SWTP support under and EU FP7 contract POWAIR.

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REFERENCES

(1) Gao, F.; Goodman, D. W. Pd-Au Bimetallic Catalysts: Understanding Alloy Effects From Planar Models and Supported Nanoparticles. Chem. Soc. Rev. 2012, 41 (24), 8009−8020. (2) Dursun, Z.; Ulubay, S.; Gelmez, B.; Ertas, F. N. Electrocatalytic Reduction of Oxygen on a Pd Ad-Layer Modified Au(111) Electrode in Alkaline Solution. Catal. Lett. 2009, 132 (1−2), 127−132. (3) Edwards, J. K.; Hutchings, G. J. Palladium and Gold-Palladium Catalysts for the Direct Synthesis of Hydrogen Peroxide. Angew. Chem., Int. Ed. 2008, 47 (48), 9192−9198. (4) Edwards, J. K.; Ntainjua, E.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Direct Synthesis of H2O2 from H2 and O2 over Gold, Palladium, and Gold-Palladium Catalysts Supported on AcidPretreated TiO2. Angew. Chem., Int. Ed. 2009, 48 (45), 8512−8515. (5) Solsona, B. E.; Edwards, J. K.; Landon, P.; Carley, A. F.; Herzing, A.; Kiely, C. J.; Hutchings, G. J. Direct Synthesis of Hydrogen Peroxide From H2 and O2 Using Al2O3 Supported Au-Pd Catalysts. Chem. Mater. 2006, 18 (11), 2689−2695. (6) Jiang, H.-L.; Xu, Q. Recent Progress in Synergistic Catalysis Over Heterometallic Nanoparticles. J. Mater. Chem. 2011, 21 (36), 13705− 13725. 24864



Article

(25) Brust, M.; Walker, M.; Bethell, D.; Schriffin, D. J.; Whyman, R. Synthesis of Thiol-derivatised Gold Nanoparticles in a Two-phase Liquid-Liquid System. J. Chem. Soc., Chem. Commun. 1994, 801−802. (26) Okada, J.; Inukai, J.; Itaya, K. Underpotential and Bulk Deposition of Copper on Pd(111) in Sulfuric Acid Solution Studied by In Situ Scanning Tunneling Microscopy. Phys. Chem. Chem. Phys. 2001, 3 (16), 3297−3302. (27) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-ray Absorption Spectroscopy Using IFEFFIT. J. Synch. Rad. 2005, 12, 537−541. (28) Rehr, J. J.; Albers, R. C.; Zabinsky, S. I. High-Order MultipleScattering Calculations of X-ray-Absorption Fine Structure. Phys. Rev. Lett. 1992, 69 (23), 3397. (29) Newville, M. EXAFS Analysis Using FEFF and FEFFIT. J. Synchrotron. Radiat. 2001, 8 (2), 96−100. (30) Newville, M.; Liviņ ̅ s,̌ P.; Yacoby, Y.; Rehr, J. J.; Stern, E. A. Phys. Rev. B 1993, 47 (21), 14126. (31) Frenkel, A. Solving the Structure of Nanoparticles by MultipleScattering EXAFS Analysis. J. Synchrotron. Radiat. 1999, 6 (3), 293− 295. (32) Roldan Cuenya, B.; Croy, J. R.; Mostafa, S.; Behafarid, F.; Li, L.; Zhang, Z.; Yang, J. C.; Wang, Q.; Frenkel, A. I. Solving the Structure of Size-Selected Pt Nanocatalysts Synthesized by Inverse Micelle Encapsulation. J. Am. Chem. Soc. 2010, 132 (25), 8747−8756. (33) Maeland, A.; Flanagan, T. B. Lattice Spacings of Gold-Palladium Alloys. Can. J. Phys. 1964, 42 (11), 2364−2366. (34) Lee, A. F.; Baddeley, C. J.; Hardacre, C.; Ormerod, R. M.; Lambert, R. M.; Schmid, G.; West, H. J. Phys. Chem. 1995, 99 (16), 6096−6102. (35) Jentys, A. Estimation of Mean Size and Shape of Small Metal Particles by EXAFS. Phys. Chem. Chem. Phys. 1999, 1 (17), 4059− 4063. (36) Smithells, C. J. Smithells Metals Reference Handbook, 7th ed.; Hartnolls Ltd: Bodmin, 1992. (37) Flanagan, T. B.; Oates, W. A. The Palladium-Hydrogen System. Annu. Rev. Mater. Sci. 1991, 21, 269−304. (38) Aben, P. C.; Burgers, W. G. Surface Structure and Electrochemical Potential of Palladium While Absorbing Hydrogen in Aqueous Solution. T. Faraday Soc. 1962, 58, 1989−1992. (39) Baldauf, M.; Kolb, D. M. A Hydrogen Adsorption and Absorption Study with Ultrathin Pd Overlayers on Au(111) and Au(100). Electrochim. Acta 1993, 38 (15), 2145−2153. (40) Stern, E. A. Number of Relevant Independent Points in X-ray Absorption Fine-Structure Spectra. Phys. Rev. B 1993, 48 (13), 9825− 9827. (41) Łukaszewski, M.; Kuśmierczyk, K.; Kotowski, J.; Siwek, H.; Czerwiński, A. Electrosorption of Hydrogen into Palladium-Gold Alloys. J. Solid State Electrochem. 2003, 7 (2), 69−76.

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