PtRu Alloy Nanoparticles. 2. Chemical and Electrochemical Surface

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PtRu Alloy Nanoparticles II. Chemical and Electrochemical Surface Characterization for Methanol Oxidation Roksana Bavand, Qiliang Wei, Gaixia Zhang, Shuhui Sun, Arthur Yelon, and Edward Sacher J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04437 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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The Journal of Physical Chemistry

PtRu Alloy Nanoparticles II. Chemical and Electrochemical Surface Characterization for Methanol Oxidation R. Bavanda,b, Q. Weia,c, G. Zhangc, S. Sunc, A. Yelonb and E. Sacher*b a. These authors contributed equally. b. Département de Génie Physique, École Polytechnique, Montréal, Québec H3C 3A7, Canada. c. Institut National de la Recherche Scientifique-Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada.

ABSTRACT Platinum-ruthenium (PtRu) nanoparticles (NPs) were evaporatively deposited in a 1:1 mass ratio onto carbon paper, using three different orders of deposition: Pt deposited onto Ru, Ru deposited onto Pt, and both Pt and Ru deposited simultaneously. The three samples were further annealed at 650 oC for 1.5 h. A sample of Pt NPs on carbon paper was also prepared as a reference. All the deposits and the reference, (a total of seven samples) were characterized by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and electrochemical techniques, in order to investigate the relationship between their catalytic surface chemical properties and their electrocatalytic activities during the methanol oxidation reaction. The simultaneous deposition of Pt and Ru demonstrated higher electrocatalytic activity, as well as excellent chronoamperometric stability, compared to either sequential deposition. This can be attributed to the synergistic effects between Pt and Ru species at the surface. Annealing at 650 oC led to a reduction of the electrocatalytic oxidation peaks. This appears to be due to the deposition of surface hydrocarbons at high temperature, thereby blocking active catalysis sites on the NP surface, as well as to the decomposition of metal oxides, which occurs above 350 oC.

1 ACS Paragon Plus Environment


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1. INTRODUCTION Direct methanol fuel cells (DMFCs) have attracted much attention for portable power applications because of their distinct advantages of high energy density, low cost, safe storage, low pollution and the easy handling of methanol as a liquid fuel.1-2 The main reactions occurring within DMFCs are the cathodic oxygen reduction reaction (ORR) and the anodic methanol oxidation reaction (MOR). The major energy losses in the application of DMFCs are caused by the poor activity of the MOR/ORR catalysts and by methanol crossover to the cathode. Thus, the large scale commercialization of this promising device will require high electrocatalytic activity for both MOR and ORR. At the anode, platinum (Pt) has been widely studied as a MOR catalyst, but it suffers from the adsorption of CO-like carbonaceous material, poisoning the catalyst, and from the high cost of Pt.3 To minimize poisoning and reduce the cost, Pt-based alloys have been studied as effective MOR catalysts;4-7 among them, PtRu is considered to be one of the most promising.7-11 Using carbon paper for evenly dispersing Pt-based catalysts not only provides a convenient route to enhance their electrochemically active surface area but also decreases the total amount of noble metal used.3 However, we still lack a clear understanding of the relation between the chemical and electrochemical characteristics of the PtRu alloy nanoparticle (NP) surface. Here, we investigate the catalytic methanol oxidation activities of PtRu NPs deposited onto a carbon paper substrate, such as used in DMFCs. Except for replacing the original HOPG substrate with carbon paper, these are the same NPs characterized in our previous physicochemical study.12 Because catalysis occurs at the outer surface, we have employed XPS and TOF-SIMS, both characterization techniques capable of studying the surface composition and chemistry at the very outermost surface of the NPs. TEM was employed to study their formation, morphology and distribution, which are crucial catalysis parameters. We prepared PtRu NPs using three different orders of evaporative deposition: Pt deposited onto Ru, Ru deposited onto Pt, and both metals deposited simultaneously, and then followed their evolutions as a function of high temperature annealing. It was our purpose to investigate which method of deposition, and which annealing condition, produced the best catalyst, information which we intend to use in a follow-on study of electrocatalytic optimization.


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2. ExXPERIMENTAL 2.1.

Sample Preparation

Following our earlier studies13-14 of pure Ru and Pt NPs deposited onto HOPG, we prepared PtRu bimetallic materials on a carbon paper substrate, using the three different orders of deposition of the two metals employed in our investigation of NPs deposited onto HOPG:12 Pt was evaporated onto previously deposited Ru (deposit 1), the two metals were evaporated simultaneously (deposit 2), and Ru was evaporated on previously deposited Pt (deposit 3). All the evaporations were carried out in the preparation chamber of a VG ESCALab 3 MARK II XPS spectrometer (Thermo VG Scientific), at a pressure of < 3 × 10-8 Torr, using a Quad-EVC evaporator (Mantis Deposition, Ltd.) containing high purity Ru and Pt rod targets (American Elements) and a tungsten filament e-beam source. For all these deposits, we attempted to keep a 1:1 mass ratio, depositing 9.3 nm (20 µg cm-2) of Pt and 10.3 nm (13 µg cm-2) of Ru, by maintaining the deposition rate unchanged. The nominal thicknesses of both Ru and Pt were monitored using a quartz crystal microbalance placed near the sample. In addition to preparing samples at room temperature, all three deposits were annealed (VG Scientific Model 240 stage), for 1.5 h, at 650 °C. After annealing, they were cooled to room temperature in the preparation chamber, and kept in a vacuum oven until they were characterized. 2.2.

XPS Measurements and Data Analysis

After each deposition at room temperature and annealing step, the samples were transferred to the analysis chamber of the instrument, without exposure to atmosphere. In-situ XPS was performed in this chamber, at a base pressure of < 2 × 10−9 Torr, using non-monochromated Mg Kα radiation (1253.6 eV). High-resolution spectra were obtained at a perpendicular takeoff angle, using a pass energy of 20 eV (step size: 0.05 eV; step dwell time: 200 ms). The instrument resolution was 0.7 eV. Core level spectra were obtained for the Ru3d, Pt4f, C1s, and O1s electron emissions, and valence band (VB) spectra were obtained for the Ru4d, 5s and Pt 5d, 6s emissions. The core level spectra were discussed in our previous paper;12 the VB spectra are discussed here. After Shirley background removal, the component peaks were separated with the VG Avantage software, using mixed Gaussian-Lorentzian functions. The binding energy was calibrated by placing the principal C1s peak at 284.6 eV; this commonly used procedure adjusts the energy



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scale, precisely locating the binding energy positions of both core and valence spectra, as well as the position of the Fermi level. The peak widths employed in the component separations, given as full widths at half maxima (fwhm), were those found in our earlier studies.14-15 Relative concentrations were obtained from high resolution spectra, using sensitivity factors regularly confirmed with standard samples. 2.3.

TOF-SIMS Analysis

For TOF-SIMS and TEM, it was necessary to transfer samples from the XPS chamber to these instruments. These techniques were applied to as-prepared samples and to samples which had been annealed at 650 °C. Since TOF-SIMS is more highly surface sensitive than XPS, samples for these measurements were transported in a VG vacuum transfer device, at a pressure of ˂1 × 10-6 Torr, which could be coupled to both systems. TOF-SIMS samples were measured in an ION-TOF TOF-SIMS IV mass spectrometer, with a mono-isotopic Bi+ beam, mass resolution (M/∆M) ≥8000. Spectral mapping of the samples was performed over an area 50 µm × 50 µm, with 256 × 256 pixel resolution, under a beam voltage of 25 kV, a beam current of 2.0 nA in bunch mode, and a beam diameter of 0.34 µm. Spectra were obtained from three different sites on each of the samples. Due to surface charging, signals, especially in negative mode, were somewhat unstable. The two most abundant isotopes of each metal were considered. Ru fragmental yields were greater in positive mode, and Pt fragmental yields, in negative mode. The relative yields of these metals, which define their ease of fragmentation, were found to be in the ratio of ~ 3:2.

2.4.

TEM Analysis

Since TEM is essentially a bulk measurement, it was not necessary to transfer samples under vacuum. TEM analysis was carried out in a JEOL JEM-2100F microscope, equipped with a LaB6 filament, operating at 200 kV and having its own energy dispersive X-ray analyzer (EDX, Phoenix). Samples were prepared, as we have done previously,14 by using a scalpel to scrape small pieces of the NP-containing carbon paper substrate onto a Cu TEM grid.

2.5.

Electrochemical Measurements

Electrochemical measurements of the samples were performed using an electrochemical workstation (Autolab PGSTAT302N), in a conventional three-electrode system, at ambient 4 ACS Paragon Plus Environment

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temperature, under a N2 atmosphere. The system included a Ag/AgCl reference electrode, a Pt wire counter-electrode, and a 1.0 cm x 0.5 cm rectangle of the carbon paper sample as the working electrode. In each case, the electrode surface was cleaned and activated: cyclic voltammetry (CV) was carried out, between -0.2 and +1.0 V vs. Ag/AgCl, in N2-purged 0.5 M H2SO4 solution, at a scan rate of 100 mV s-1, for 30 cycles, in order to obtain a stable CV curve. To investigate the activities of various catalysts toward the MOR, CV tests were carried out in 0.5 M H2SO4 with or without 1.0 M CH3OH (scan rate: 50 mV s-1, potential range: -0.2 to +1.0 V vs. Ag/AgCl). The catalyst activity was normalized by plotting the unit geometrical area of the catalysts, as well as the Pt content (calculated from our XPS results). Chronoamperometric tests, in a solution of 0.5 M H2SO4 + 1.0 M CH3OH, at a fixed potential of 0.4 V vs. Ag/AgCl, were performed for 1200 s to evaluate the stabilities of the catalysts.

3. RESULTS AND DISCUSSION 3.1.

Morphological Study of PtRu Alloy NPs

Figure 1 shows TEM photomicrographs of deposit 1 (Pt deposited on Ru) on carbon paper, before and after annealing at 650°C. The EDX spectra in Figure 2 confirm that the NPs contain comparable amounts of Pt and Ru. As in our previous Ru study on HOPG,14 the metals do not wet the carbon paper, causing the deposit to retract and form NPs. In addition, TOF-SIMS, HAADF/STEM and EELS on PtRu NPs,12 showed that all the samples begin to form alloys on deposition, due to the high heats of condensation of Pt and Ru (~5 and ~6 eV, respectively). In both photomicrographs, many NPs are in contact because of their high number density. However, there is no further aggregation on annealing. This is because both Pt and Ru are present at the substrate surface and, while pure Ru NPs diffuse across the surface,14 Pt NPs do not.13 That is, NPs containing Pt at their interface with the substrate will not diffuse and coalesce. This lack of diffusion indicates that the effective surface area of these electrocatalysts will not change on annealing.



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Figure 1. TEM photomicrographs of deposit 1 NPs on carbon paper: (a) as deposited (particle size, 4.9 ± 0.6 nm), and (b) annealed at 650°C (particle size, 5.1 ± 0.5 nm).


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Figure 2. EDX spectra of the PtRu NPs shown in Figure 1. The Cu peaks come from the support grid.

3.2.

Electrochemical Performance

The MOR activities of the samples were characterized by CV measurements (Figure S1) and the resulting voltammograms were treated by removing the background found for the samples without methanol; the results are shown in Figures 3, S2 and S3. Two anodic features (one in the positive scan direction, one in the negative scan direction), typical of methanol electro-oxidation, are observed for all samples. For the positive scan, this feature is complex, and may be resolved into two peaks, which we designate as peak 1 and peak 2. In fact, the methanol oxidation reaction is complicated and may well involve several processes.16 A detailed precise interpretation for those peaks has not yet been found. On deconvolution (Table 1), peak 1, for all samples, is found to have a maximum at 0.51±0.01 V, and peak 2, at 0.64±0.01 V. For the negative scan, the feature can be fit with a single peak, peak 3, which has a maximum at 0.46±0.02 V. We conclude that the same reactions are being observed for all the samples. The normalized areas of these peaks are presented and illustrated in Table 1 and Figures S4 and S5.



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Samples

Sample

Peak 1

description

maximum

Normalized Peak 1 Area (cm-2)

Peak 2 maximum

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Normalized

Peak 3

Normalized

Peak 2 Area

Maximu

Peak 3

m

Area (cm )

-2

(cm )

-2

9.3 nm (20 Deposit 1

2

µg/cm ) of Pt on

0.51 V

242

0.64 V

283

0.47 V

295

0.51 V

462

0.64 V

392

0.45 V

112

0.50 V

271

0.64 V

331

0.46 V

315

0.51 V

38

0.63 V

206

0.46 V

176

10.3 nm Ru 9.3 nm (20 2

µg/cm ) of Pt and Deposit 2

10.3 nm Ru deposited simultaneously

Deposit 3 Deposit 1 (annealed)

10.3 nm of Ru on 9.3 nm Pt Deposit 1 annealed


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Deposit 2

Deposit 2 annealed

0.50 V

28

0.62 V

9.31

0.46 V

3

Deposit 3 annealed

0.52 V

22

0.64 V

95.49

0.44 V

54

0.50 V

60

0.63 V

479

0.47 V

479

(annealed) Deposit 3 (annealed)

9.3 nm Pt (20 Deposit 4

2

µg/cm )

Figure 3. (a-b) CV plots of methanol oxidation for the various samples (a: before annealing; b: after annealing); (cd) deconvolutions of methanol oxidation peaks (b: positive scan and c: negative scan) for deposit 2.

Table 1. CV results for the samples.

The ratios of various peak areas are presented in Table 2 and Figure 4. On deconvolution, peak area changes are observed for the different oxidation processes, suggesting that the (peak 1 + peak 2)/peak 3 area ratio can be used to indicate the MOR efficiency of the catalyst; the larger the area of peak 1 (i.e., the oxidation happens at a more negative potential) and the smaller the area of peak 3 (i.e., the oxidation of the residue species), the more efficient the catalyst. A high value of (peak 1 + peak 2)/peak 3 is taken to indicate an increased methanol oxidation efficiency. Deposit 2 and its annealed counterpart exhibit the highest ratios, apparently caused by improved synergistic effects between Pt and Ru under simultaneous deposition. For comparison, Table S1 presents a summary of If/Ib values (If denotes the peak current density of the forward (positive) scan and Ib denotes the peak current density of the backward (negative) scan) from the recent literature, as well as that of our deposit 2, from which it can be seen that deposit 2 has a higher If/Ib ratio than those recently reported. (Whether or not the anodic peak, seen during the reverse scan, is related to the removal of incompletely oxidized carbonaceous species is still debatable2, 17-22

)

Although the value of the ratio for annealed deposit 2 is higher than that for the unannealed deposit, its peak areas are much smaller (Figure 3a, b and Table 1), and XPS12 reveals that annealing has resulted in the deposition of surface hydrocarbon. This deposit may block active catalytic sites on the NP surface. That study also showed that metal oxides begin to decompose above 350 ºC, and this may also play a role in reducing catalytic activity.



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Table 2. Various peak ratios of the samples. Area ratio of Samples

Sample

Area ratio of

description

Peak 1/Peak 3

Area ratio of

Area ratio of Peaks (1+2)/

Peak 2/Peak 3

Peak 1/Peak 2 Peak 3

9.3 nm (20 Deposit 1

2

0.82

0.96

1.78

0.86

4.12

3.50

7.62

1.18

0.86

1.05

1.91

0.82

Sample 1 annealed

0.22

1.17

1.39

0.18

Sample 3 annealed

9.17

2.99

12.44

3.01

Sample 2 annealed

0.41

1.78

2.18

0.23

0.13

1.00

1.13

0.13

µg/cm ) of Pt on 10.3 nm Ru 9.3 nm (20 2

µg/cm ) of Pt and Deposit 2

10.3 nm Ru deposited simultaneously

Deposit 3 Deposit 1

10.3 nm of Ru on 9.3 nm Pt

(annealed) Deposit 2 (annealed) Deposit 3 (annealed)

9.3 nm Pt (20 Deposit 4

2

µg/cm )

A high value of the peak 1/peak 2 area ratio indicates that more methanol oxidation occurs at a lower potential, which means less energy is consumed for the oxidation process. Table 2 shows that the peak 1/peak 2 ratio for deposit 2 is higher than those for deposits 1 and 3; this is also true for the annealed samples, showing that the simultaneous deposition Pt and Ru leads to better MOR efficiency than does sequential deposition. This supports our previous statement that simultaneous deposition achieves better synergistic effects. It is interesting to note that both the peak 1/peak 2 and the peaks (1+2)/peak 3 ratios are higher for all the PtRu samples than for pure Pt (Figure 4), confirming that the addition of Ru improves the MOR efficiency of Pt, with a lower oxidation potential and a higher methanol oxidation rate (i.e., a reduced formation of intermediate species).


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Figure 4. Peak ratios of the samples.

3.3.

Relative Atomic Concentrations in PtRu Alloy NPs

The MOR efficiency and CO tolerance rankings of the various samples, as discussed above, indicate that deposit 2, the simultaneous deposition of Pt and Ru, shows better catalytic activity than deposits 1 (Pt deposition onto Ru deposition) and 3 (Ru deposition onto Pt deposition). To help determine the reasons for this superior performance, the atomic fractions of all the atoms present at the surface are required. Table 3 lists the Ru, Pt, C, and O component fractions in deposits 1-3, both at room and elevated temperatures. These relative concentrations are obtained by XPS, performed on PtRu alloy NPs deposited onto HOPG, as discussed in our recent paper12. At both room temperature and 650°C, deposit 1 has the most surface Pt and the least surface Ru, while deposit 3 has the most surface Ru and the least surface Pt (see Table 3). Thus, the amounts of surface Pt and Ru in deposit 2 are always between those of deposits 1 and 3. This suggests that deposit 2 may have a favorable amount of surface Pt and Ru, which provides its superior performance. In addition, as presented in Table 3, deposit 2 contains the greatest amount of carbide (which forms only on Ru surfaces), and also contains the greatest amount of Ru oxide.



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This supports our contention that the amount and type of surface oxide participate in defining MOR performance.20 For all the samples, oxide decomposition occurs above 350 °C and the hydrocarbon layer thickness increases on annealing.12 As a result, as seen in Table 3, on annealing, the amounts of surface Pt and Ru decrease, due to the increased surface hydrocarbon layer. The hydrocarbon layer may also influence the amount of Ru carbide, which appears to correlate with MOR efficiency. Table 3. Component fractions of Ru, Pt, C, and O, in deposits 1-3, obtained from XPS data.

Deposit 1

Deposit 2

Deposit 3

RT

650°C

RT

650°C

RT

650°C

Atomic fraction of

Ru1

8

6.5

8.4

5.7

11.5

8.7

Ru

Ru2

1.5

1.3

1.7

1

2.1

1.3

Ru3

0.6

0.5

0.7

0.5

0.9

0.5

Total Ru

10.1

8.3

10.8

7.2

14.5

10.5

Atomic fraction of

Pt1

8.8

5.6

6.4

4.7

1.4

1.4

Pt

Pt2

1.9

1.2

1

0.5

0.4

0.3

Pt3

0.7

0.5

0.5

0.3

0.2

0.1

Total Pt

11.4

7.3

7.9

5.5

2

1.8

Atomic fraction of

O1

0.7

0.25

0.55

0.5

0.85

0.15

oxide

O2

2.6

1

1.7

0.85

O1+O2

3.15

1.5

2.55

1

(Metallic) O3

2.85

0.55

0.65

0.6

0.45

0.6

Total O

3.55

0.8

3.8

2.1

3

1.6

Atomic fraction of

C1

27.7

28.6

19.6

27

23.7

34

carbon

C6

15.7

20.4

21.8

20.5

22.8

19.6

73.2

81.7

76.9

84

80.1

85.8

(Carbide) Total C


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3.4.

TOF-SIMS Analysis

TOF-SIMS was used to investigate the chemical composition at the outer surface (≤1 nm) of PtRu NPs, where catalytic activity takes place. For this study, the samples were deposited onto HOPG, due to its highly ordered, chemically inert surface and ease of handling. Since TOFSIMS is not a quantitative technique, quantifying the amounts of the chemical species at the NPs surface is not possible. Figure 5 shows oxygen, carbon and hydrocarbon fragments for Ru, and Figure 6 shows those for Pt. The two most abundant isotopes of Ru and Pt were used in attributing each of the fragments in these figures, in order to confirm their presence. These fragments were seen in all deposits, at both room and elevated temperatures, although with different intensities. This technique indicates that, in addition to the metals, there are other species at the NPs surface, such as metallic oxides and partially oxidized hydrocarbons. The MOR results, when viewed from this perspective, indicate that these chemical species play some role in electrocatalysis. However, the amounts and types of surface oxides and the amounts of hydrocarbons and carbide, depend on which metal (Ru or Pt) predominates at the surface. This is why these three orders of deposition all have different surface chemistries, and each deposit, at room and elevated temperatures, shows a different catalytic activity.

Figure 5. Positive SIMS spectra of Ru-related fragments at room temperature of deposit 1.



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Figure 6. Negative SIMS spectra of Pt-related fragments at room temperature of deposit 1.

As shown in Figure S1, the C-V plots indicate that the electrochemically active surface area (ECSA) is reduced on annealing in all cases. For example, the ESCA of deposit 3 is reduced from 28.6 m2/gPt, before annealing (Figure S1c), to 13 m2/gPt, following annealing (Figure S1f). Similar reductions are found for the other deposits. This further supports our XPS results, supporting our contention that the deposition of surface hydrocarbons reduces the electrocatalytic oxidation peaks.

3.5.

Valence Band Study of PtRu Alloy NPs

In order to confirm whether a relationship exists between the VB and catalytic activity for these PtRu alloy NPs, the VB XPS spectra of all three deposits (deposited on HOPG) were studied. Basing ourselves on our previous studies of the valence bands of pure Ru and Pt deposited onto HOPG,14-15 in which both contain three peaks, we were able to separate the valence band spectra of all the deposits into six peaks; Ru4d3/2, Ru4d5/2, Ru5s, Pt5d3/2, Pt5d5/2, and Pt6s, at essentially the same binding energies found for the pure metals. All the binding energies, for the pure metals and the three deposits, are listed in Table 4. The spectral component intensities differed from those of the pure metals, and varied with the method of deposition and with annealing. As an example, Figure 7a shows the VB components of deposit 1, at room temperature.


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The evolution of the PtRu NP VB spectrum on annealing had not previously been studied. Figure 7b presents this evolution for deposit 1. There is no obvious shift of component peak positions, as a function of annealing. This was also found for deposits 2 and 3. It appears that annealing has no discernable effect on the VB peak positions of any of the deposits. It also may suggest that alloying has already started before annealing, so that no further variation occurs during annealing.

Table 4. Valence band peak components and their binding energies and fwhm values for pure Ru and Pt evaporated onto HOPG and deposits 1-3. Element Label Binding energy (eV) fwhm (eV) Ru5s

0.8

1.3

Ru4d5/2

2.3

2.7

Ru4d3/2

5.0

2.7

Pt6s

1.8

2.0

Pt5d5/2

4.4

3.7

Pt5d3/2

6.0

3.7

Ru5s

0.5

1.0

Ru4d5/2

2.2

2.1

Ru4d3/2

4.8

2.1

Pt6s

1.3

1.3

Pt5d5/2

3.4

3.1

Pt5d3/2

6.4

3.1

Ru5s

0.5

1.0

Ru4d5/2

2.7

2.7

Ru4d3/2

5.4

2.7

Pt6s

1.4

1.7

Pt5d5/2

4.0

2.6

Pt5d3/2

6.3

2.6

Ru5s

0.5

1.3

VB of pure Ru

VB of pure Pt

VB of deposit 1

VB of deposit 2



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VB of deposit 3

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Ru4d5/2

2.1

2.5

Ru4d3/2

4.8

2.5

Pt6s

1.5

1.2

Pt5d5/2

3.2

2.5

Pt5d3/2

6.6

2.5

Figure 7. (a) Schematic separation of the valence band spectrum for deposit 1, at room temperature, (b) Evolution of the valence band spectra for deposit 1, as a function of annealing temperature; 1 to 8 represent the samples at room temperature, 150, 250, 350, 450, 580, 720 and 770 °C, respectively.

Figure 8 shows the evolution of the Ru and Pt VB component fwhm values, for the three deposits, as a function of annealing temperature. The variation of the fwhm value of a peak indicates changes in crystalline order. In all the deposits, it is only the Pt5d fwhm that shows abrupt changes with temperature; this indicates us that the Pt5d orbitals are greatly influenced in the alloying process, while the other orbitals appear to play only a minor role. Despite the fact that the variation of Pt5d is clearly seen in all deposits, the reason for this variation is unknown.


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Figure 8. Evolutions of fwhm of valence band components, (a) deposit 1, (b) deposit 2, and (c) deposit 3, as a function of annealing temperature.

Figure 9 compares the VB spectra of all the deposits, as well as those for pure Ru and Pt, both before and after annealing at about 715°C. The spectral shapes are all different, and their widths broaden with increasing annealing temperature. The changes in the spectra at elevated temperatures may signal chemical reactions that take place at the surface. Such broadening results in the displacements of the d-band centers of all three deposits to higher binding energies. As shown in Figure 9a, the shape of the spectrum of deposit 1, at room temperature, is similar to that of pure Pt because of the initial Pt coverage at the surface, although narrower than for pure Pt; at 715 °C, the shape is a combination of those of both pure Ru and Pt. It seems that when Pt predominantly covers the NP surface, the spectrum resembles that of Pt; when Ru covers the surface, as we shall see for deposit 3, the spectrum resembles that of Ru; when both Ru and Pt 17 ACS Paragon Plus Environment


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are present at the surface, as we shall see for deposit 2, the spectrum resembles a combination of both. That is, the VB spectrum may indeed, reflect the outer surface. Hence, for deposit 1, it appears that, on annealing, Ru diffuses through Pt and moves to the surface; as explained in detail in our earlier study.12 For deposit 2, the shapes of the spectra, before and after annealing, resemble a mixture of Ru and Pt, becoming broader for elevated temperatures. This surely indicates that both Ru and Pt exist at the surface. For deposit 3, the shape of the spectrum changes from one that, at room temperature, is identical to pure Ru, to one having a slightly different shape at elevated temperature, although still with a major Ru-like component, reflecting the interaction between Ru and diffusing Pt at the elevated temperatures, as confirmed in the TOF-SIMS results of our earlier study12. We conclude that the changes observed in Figure 9 are consistent with the results of the core level XPS data12. The variations in the VB shapes are accompanied by changes in the d-band centers of all the deposits; the d-band center is the energy that divides each VB band spectrum into two equal halves. Clearly, both at room temperature and at 715 ºC, the d-band center of deposit 1 lies at the highest binding energy (the farthest from the Fermi level, and the most stable) and deposit 3 lies at the lowest binding energy. The d-band center of deposit 2 remains in between, at both room and elevated temperatures, although the d-band centers of all deposits are shifted to higher binding energies on annealing. Given the similarities in VB spectral shapes discussed above, what we see appears to indicate what is at the surface. If this is so, the shift of the d-band center to higher binding energies should be beneficial for use in catalysis.23 However, the evidence we have so far accumulated in our study indicates that the changes in the d-band center do not play an important role in MOR performance. Rather, the chemical species present at the NP surface participate in defining MOR performance.


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Figure 9. Comparisons of the valence band spectra among pure Ru, pure Pt, each of them deposited separately onto HOPG, at room temperature and deposit 1, 2, and 3, along with their d-band centers, at (a) room temperature and (b) after annealing at 715 ºC.

3.6.

Durability Test of PtRu Alloy NPs

Lastly, the durabilities of these samples were further investigated by chronoamperometry, at +0.4 V vs. Ag/AgCl, as shown in Figure 10. The potential value of +0.4 V was chosen because it is where methanol oxidation has just begun (Figure 3) with a relatively negative voltage value. While all the catalysts showed rapid current decays, probably caused by the formation of intermediate species during the MOR process, deposit 2 maintained a much lower current decay, over time, again appearing to benefit from synergistic effects. Our results indicate that the simultaneous deposition of Pt and Ru, deposit 2, shows the best MOR performance of all the samples that were tested.



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Figure 10. Normalized chronoamperometry curves of the samples in 0.5 M H2SO4 solution containing 1.0 M CH3OH, for 1200 s at 0.4 V vs. Ag/AgCl.

4. CONCLUSION Three types of PtRu alloy, deposited on carbon paper, have been successfully prepared using three different orders of deposition: Pt deposited onto Ru, the two metals deposited simultaneously, and Ru deposited onto Pt. All the samples were annealed at 650 oC, for 1.5 h, to determine their MOR performances on annealing. The deconvolution of the MOR spectra into their component peaks indicates that: (i) simultaneous sample deposition gives the best MOR catalytic performance (high efficiency and a durable stability), probably due to an optimum amount of surface Pt and Ru; (ii) on annealing, the areas of the MOR peaks become much smaller, probably due to the reduction of surface Pt and Ru caused by a greater deposition of surface hydrocarbon layer that blocks the active catalysis sites on the NP surface and/or the decomposition of surface metallic oxide; (iii) the changes in the d-band center of the PtRu catalysts do not play an important role in the MOR performance; rather, the chemical species present at the NP surface participate in defining MOR performance. Our analysis implies that Pt and Ru deposited simultaneously onto carbon paper could provide a very promising anode 20 ACS Paragon Plus Environment

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catalyst for high-performance MOR. This work also provides insights into the correlation between the surface chemical characterization and the electrochemical performance of PtRu alloy NPs in the MOR. It is our intention to use these results as the basis for further work on MOR catalysts.

ASSOCIATED CONTENT Supporting Information CV curves of the samples; CV peak deconvolutions; a plot of peak areas; a comparison table of If/Ib ratios from the literature.

AUTHOR INFORMATION CORRESPONDING AUTHOR *

Telephone: 514-340-4711, extension 4858; email: [email protected]

ORCID Edward Sacher: 0000-0002-3427-4136 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS The authors are grateful for financial support from the Natural Sciences and Engineering Research Council of Canada and the Fonds de la Recherche du Québec, Nature et Technologies (FRQNT). Q.W. thanks the China Scholarship Council and FRQNT for research fellowships.

REFERENCES 1. Ganesan, R.; Lee, J. S. Tungsten Carbide Microspheres as a Noble-Metal-Economic Electrocatalyst for Methanol Oxidation. Angew. Chem., Int. Ed. 2005, 44, 6557-6560. 2. Zhang, K., et al. A Highly Active, Stable and Synergistic Pt Nanoparticles/Mo2c Nanotube Catalyst for Methanol Electro-Oxidation. NPG Asia Mater. 2015, 7, e153. 3. Wang, Y.-S.; Yang, S.-Y.; Li, S.-M.; Tien, H.-W.; Hsiao, S.-T.; Liao, W.-H.; Liu, C.-H.; Chang, K.-H.; Ma, C.-C. M.; Hu, C.-C. Three-Dimensionally Porous Graphene–Carbon Nanotube Composite-Supported Ptru Catalysts with an Ultrahigh Electrocatalytic Activity for Methanol Oxidation. Electrochim. Acta 2013, 87, 261-269.



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23. Yang, Z.; Pedireddy, S.; Lee, H. K.; Liu, Y.; Tjiu, W. W.; Phang, I. Y.; Ling, X. Y. Manipulating the dBand Electronic Structure of Platinum-Functionalized Nanoporous Gold Bowls: Synergistic Intermetallic Interactions Enhance Catalysis. Chem. Mater. 2016, 28, 5080-5086.



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PtRu Alloy Nanoparticles. 2. Chemical and Electrochemical Surface

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