Understanding the Effect of Au in Au–Pd Bimetallic Nanocrystals on

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Understanding the Effect of Au in Au−Pd Bimetallic Nanocrystals on the Electrocatalysis of the Methanol Oxidation Reaction Cameron H. W. Kelly,† Tania M. Benedetti,*,† Ali Alinezhad,† Wolfgang Schuhmann,‡ J. Justin Gooding,†,§,¶ and Richard D. Tilley*,†,§,∥ †

School of Chemistry, §Australian Centre for NanoMedicine, ∥Electron Microscope Unit, Mark Wainwright Analytical Centre, and ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, University of New South Wales, Sydney, NSW 2052, Australia ‡ Analytical ChemistryCenter for Electrochemical Sciences (CES), Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany

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S Supporting Information *

ABSTRACT: Pd or Pt alloyed with a secondary metal are the typical catalysts at the anode for the direct oxidation of methanol. The secondary metal is employed to diminish deactivation commonly ascribed to CO poisoning. Here we investigate the origin of the improved performance of Au−Pd core−shell and alloy nanocrystals as electrocatalysts for the methanol oxidation reaction (MOR), relative to Pd alone. Monodisperse Au−Pd core− shell nanocrystals were synthesized using H2 as a mild reducing agent followed by annealing under a 5% H2 atmosphere to produce the Au−Pd alloys. The nanocrystals were characterized using high-resolution electron microscopy to confirm their structures. The core−shell and alloy nanocrystals showed an improvement in specific activity with respect to pure Pd nanocrystals. Importantly, the stability was also improved by the inclusion of Au for both nanocrystals, being 2.7× higher for the alloy than for the core−shell after 30 min, while the activity is completely lost for the Pd nanocrystals within 10 min. We show that there is no evidence of CO formation for any of the Pd-based catalysts in an alkaline environment. The origin of the improvement in terms of both activity and stability results from positive shifts in the PdO formation/reduction potential caused by the presence of Au, which results in more Pd sites available for the MOR.

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INTRODUCTION

Pd-based MOR catalysts show good activity for MOR in alkaline electrolytes. The MOR is generally faster in alkaline electrolytes due to faster CO stripping. Issues related to solid carbonate/bicarbonate precipitation in alkaline electrolyte in a fuel cell can be mitigated by using alkaline anion exchange membranes instead of liquid electrolytes.12 However, in contrast to Pt, Pd oxidation to form PdO occurs at similar potentials as the MOR, which, together with CO poisoning, can also affect the reaction performance. The purpose of this work is to explore how the MOR on Pd-based electrocatalysts can be improved by the introduction of a second metal into their structure. We have chosen Au−Pd nanocrystals as a model system for the following reasons: (1) MOR on PtAu alloy predominantly follows the formate pathway; thus, CO poisoning should be minimized. (2) CO and HCOOH oxidation pathways on Au−Pd core−shell nanocrystals are dependent on the Pd shell thickness.14 The latter observation suggests Au can affect the catalysis even when not exposed at the surface, although the mechanism for such an effect is not completely understood.

Due to the easy storage of methanol as liquid fuel and its high energy density, the direct methanol fuel cell (DMFC) is an attractive source of clean energy for portable and transport applications.1−5 PtM alloy nanocrystals have been widely studied as catalysts for the methanol oxidation reaction (MOR),6,7 with M being a second metal such as Ru or Au.8 The reason for using two metals is that with pure Pt, poor performance due to CO poisoning is observed. Thus, the additional metal is used to reduce the CO poisoning on catalysts used for MOR and enhance catalytic performance. The conventionally discussed mechanism of MOR at the surface of such PtM catalysts was proposed by Watanabe9 for PtRu and states that the second metal enables the oxidation of the CO intermediates from the Pt surface by providing OH species and hence preventing the poisoning. Recently, an alternative mechanism has been proposed suggesting that, although CO is still bound to the Pt surface, the poisoning is not important with the reaction occurring via the formation of formate as the intermediate with OH binding at M and CH3OH binding at PtM sites.10 In both mechanisms, both metals are required to be available at the catalyst surface. In light of the success of Pt and Pt-based alloys, it is surprising that Pd and Pd-based alloys are far less studied.11 © XXXX American Chemical Society

Received: June 6, 2018 Revised: August 30, 2018

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DOI: 10.1021/acs.jpcc.8b05407 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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ICP-OES Characterization. The carbon-supported nanocrystals were digested in aqua regia for 1 h at 80 °C to dissolve the metals and then analyzed using an Optima7300DV-ICPOES PerkinElmer instrument to give the composition and weight percentage of the metals in carbon. TEM Characterization. TEM samples were prepared by dropping a dilute suspension of nanocrystals in toluene (or isopropanol for the carbon-supported particles) onto carbon coated copper grids and allowed to dry. Low-resolution TEM analysis was performed on a FEI Tecnai G2 20 TEM at an accelerating voltage of 200 kV. STEM imaging and EDX elemental analysis were performed on a JEOL JEM-F200 (200 kV, cold field emission gun) equipped with an annular darkfield (ADF) detector and a JEOL windowless 100 mm2 silicon drift X-ray detector. Electrochemical Characterization. The catalyst inks were prepared as 2.5 mg mL−1 dispersions in 3:1 v/v H2O/ isopropanol and 1 μL of Nafion solution (Nafion 117, Aldrich, ∼5%). All electrochemical measurements were performed using N2-purged 1 mol L−1 KOH electrolyte prepared in MilliQ water with or without addition of 1 mol L−1 methanol. The measurements were carried out using a μAUTOLABIII potentiostat controlled with NOVA software. Platinum mesh and 3 mol L−1 Ag|AgCl|NaCl were used as the counter and reference electrodes, respectively. The reference electrode was separated from the main solution with a fritted glass tube double-junction to protect it against the OH ions and avoid chloride contamination. An aliquot of 15 μL of the catalyst ink (37.5 μg of catalyst) was deposited on a glassy carbon surface (0.196 cm2) as the working electrode. The electrochemical surface area (ECSA) was calculated by integration of the PdO reduction peak during cyclic voltammetry in the absence of methanol to give a charge that was divided by the specific charge of 424 and 411 μC cm−2 for Au−Pd core−shell and alloy nanocrystals and Pd nanocrystals, respectively.

We have synthesized Au−Pd core−shell nanocrystals and annealed them at mild conditions to produce Au−Pd alloys. Our electrochemical results show that there is no evidence of CO formation as an intermediate during MOR when Pd-based nanocrystals are used in alkaline electrolyte. The results further show the improved activity and stability caused by the presence of Au is due to changes in the potential for the formation/reduction of PdO.

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EXPERIMENTAL METHODS Synthesis of Au−Pd Core−Shell Nanocrystals. The Au−Pd core−shell nanocrystals were synthesized in a Fischer− Porter bottle using a method adapted from the literature15 to scale up the Au seed synthesis. Synthesis of Gold Seeds. Gold chloride trihydrate (78.8 mg, 0.2 mmol, Sigma-Aldrich, >99.9%) and hexadecylamine (1.073 g, 2 mmol, Sigma-Aldrich, 90%) were dissolved in 4 mL of toluene in a 20 mL sample vial. The solution was transferred to a 15 mL Fischer−Porter bottle, and a stir bar was added. The bottle was then sealed with bivalves and repeatedly evacuated and filled with 1 bar hydrogen gas. The bottle was then filled with 3 bar hydrogen gas, immersed in an oil bath at 70 °C for 24 h under stirring. The gas was released, and the nanocrystal suspension was washed with toluene and methanol via centrifugation at 4000 rpm. The product was then collected in glass vials and dried. Palladium Coating of Gold Seeds. To synthesize core− shell particles, palladium acetylacetonate (30.4 mg, 0.1 mmol, Sigma-Aldrich, 99%) and hexadecylamine (0.24 g, 1 mmol, 90%) were dissolved in 1 mL of toluene. Gold seeds synthesized above were added to give an Au/Pd ratio of 1:2 and a total toluene volume of 2 mL. The solution was transferred to a 15 mL Fischer−Porter bottle and sealed with bivalves and prepared as above. The bottle was filled with 1 bar hydrogen gas and immersed in an oil bath at 55 °C and left to react for 2 h. The hydrogen was released and the nanocrystal suspension was washed with equal volumes of toluene and methanol via centrifugation at 4000 rpm for 10 min and then dried under a flow of nitrogen. Nanocrystal Loading on Carbon Support. The core− shell nanocrystals were suspended in hexane and mixed with carbon powder (Vulcan XC-72Cabot) into a 20 mL vial to give a 5 wt % loading of the nanocrystals. Four to five drops of oleylamine were added to aid with the nanocrystal dispersion, and the mixture was sonicated for 2 h until the hexane became clear. The particles were then left to settle, the clear supernatant was removed, and the supported particles were dried using a rotary evaporator. Catalysts were then thermally treated at 200 °C under constant airflow for 5 h to remove surfactants. Annealing To Obtain the Au−Pd Alloy Nanocrystals. The Au−Pd core−shell nanocrystals supported on carbon were placed into an alumina combustion boat in a stainless steel holder and sealed inside a tube furnace under a constant flow of 5% H2/N2. The furnace was heated up to 600 °C over 30 min with the sample at the end of the furnace, after which the holder was moved into the center of the furnace and heated for 60 min, then moved back to the edge of the furnace for fast cooling to minimize nanocrystal sintering. Samples were left to cool under the reducing atmosphere until they were under 200 °C, and then, they were removed and stored in a sample vial.

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RESULTS AND DISCUSSION A low-resolution transmission electron microscope (TEM) image of the Au seed nanocrystals is shown in Figure 1a. After coating with palladium, the particles maintained an icosahedral shaped Au−Pd core−shell as shown in Figure 1b. To convert the Au−Pd core−shell nanoparticle into Au−Pd alloy nanoparticles, the Au−Pd core−shell was loaded on a carbon support and thermally treated (Figure 1c). TEM images of carbon-supported Au and Au−Pd core−shell nanocrystals are shown in Figure S1. As Pd and Au can mutually diffuse at the nanoscale under high temperatures,16 by annealing the carbonsupported core−shell nanocrystals, alloy nanocrystals were formed with similar size and composition to that of the Au−Pd core−shell nanoparticles before annealing. Size analysis was performed on more than 100 particles for the Au cores, Au−Pd core−shell, and Au−Pd alloy nanoparticles (Figure 1d−f) to give sizes of 8.9 ± 0.7, 11.1 ± 1.1, and 11.5 ± 1.5 nm, respectively. The nanoparticles were analyzed by high-angle annular darkfield-scanning transmission electron microscopy (HAADFSTEM), Au−Pd nanocrystals before (Figures 2a and S2) and after (Figure 2b) annealing and examined by energy dispersive X-ray spectroscopy (EDX) mapping in STEM (Figure 2c−f). Contrast differences in the core and shell indicates a Au−Pd core−shell nanocrystal morphology with the presence of highatomic-mass Au giving bright contrast in the core and lowatomic-mass Pd giving darker contrast in the shell. The Pd B

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both the Au and Pd atoms are randomly arranged in the nanocrystals, confirming the alloy formation. The activity for the MOR for both core−shell and alloy Au− Pd nanocrystals, as well as for Pd nanocrystals17 was first assessed by cyclic voltammetry in alkaline electrolyte containing methanol. The potentiodynamic curves presented in Figure 3a for Au−Pd core−shell (blue), Au−Pd alloy (red),

Figure 1. Low resolution TEM images of (a) 8.9 ± 0.7 nm Au seed, (b) 11.1 ± 1.1 nm Au−Pd core−shell, and (c) 11.5 ± 1.5 nm carbonsupported AuPd alloy nanocrystals with (d−f) corresponding histograms. Scale bar = 20 nm (a,b), 50 nm (c). Figure 3. (a) Cyclic voltammograms carried out in 1 mol L−1 KOH + 1 mol L−1 MeOH at 50 mV s−1; (b) charges obtained from the integrations of If (qf), Ib (qb) and the ratio qf/qb; (c) zoom in on region in the cathodic scan between 0.75 and 0.9 V showing the beginning of the backscan oxidation peaks. Legend: AuPd core−shell (blue), alloy (red), and Pd (black) nanocrystals.

shell is also confirmed by EDX mapping in STEM (Figure 2c,e). The Au core is completely covered by an uneven Pd shell with no evidence of Au atoms present at the surface. In contrast, the STEM image of Au−Pd alloy nanocrystals (Figure 2b) shows no observable contrast difference. This indicates that particles no longer have their core−shell structure as expected due to the high-temperature annealing. The EDX mapping of these nanocrystals (Figure 2d,f) shows

and Pd (black) nanocrystals display the currents normalized by the ECSA. The voltammograms show two oxidation peaks, one

Figure 2. HAADF-STEM images of Au−Pd (a) core−shell and (b) alloy nanocrystals with corresponding STEM−EDX mapping of (c,d) Au and (e,f) Pd of a cluster of prepared nanocrystals. Scale bar = 20 nm. C

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in CO saturated electrolyte. The positive scans of the cyclic voltammograms under the different conditions (CO stripping and the MOR in electrolyte saturated with N2 and CO) in Figure 4 show that, for all samples, CO stripping (open circles

in the forward and one in the backward scan at around 0.85 and 0.74 V, respectively, as commonly observed for the MOR. The specific activities, corresponding to the maximum currents in the forward scan normalized by the ECSA (Supporting Information, Table S1), are 1.65, 1.84, and 2.81 mA cm−3 for the Pd, Au−Pd core−shell, and Au−Pd alloy nanocrystals, respectively. The activity of the pure Au nanoparticles is negligible when compared with that of the the Pd-based catalysts (Figure S3). The specific activity of the Au−Pd nanocrystals is comparable to that of the PtRu-based nanocrystals, such as the recently reported nanowires and nanorods,7 which are currently the nanocrystal catalysts for MOR with the highest reported specific activities. The increased activities of the Au−Pd nanocrystals with respect to Pd nanocrystals show that the presence of Au improves the MOR performance. Analysis of the voltammograms in Figure 3a shows that the forward oxidation peak is narrow and centered around 0.85 V. Note there was no evidence of a secondary peak at more positive potentials commonly related to oxidation of residual intermediate species.7 The ratio qf/qb (the charge from forward and backward peaks, respectively) is lower for the alloy nanocrystal than for the other samples (Figure 3b). This ratio is generally assumed to be inversely proportional to CO poisoning, where the backward peak is ascribed to CO oxidation.18,19 However, given the fact that, for the alloy nanocrystals, the presence of Au at the surface means there is less Pd at the surface to be poisoned and the activity is higher, it would be expected that the qf/qb would be higher for the alloy nanocrystals than for the core−shell and Pd nanocrystals. The fact that this is not the case indicates that the backward peak is likely due to oxidation of freshly adsorbed methanol20,21 and not of adsorbed CO. These observations suggest that, in contrast to Pt-based catalysts, MOR on Pd in an alkaline environment is not affected by CO poisoning. The reason for no evidence of CO poisoning even in the absence of Au at the surface of the catalyst (as is the case of both Pd and core−shell nanocrystals) can be explained by recent theoretical studies showing that participation of the OH− ions from the electrolyte6,22 leads to high OH surface coverage at the Pd sites in an alkaline environment, resulting in a low activation barrier for OH adsorption and subsequent COOHads formation13 instead of the pathway where CO is the intermediate. Interestingly, the Au−Pd core−shell nanocrystals also presented improved MOR performance when compared with the Pd nanocrystals, although at a lower extent than the alloy nanocrystals. As the Pd shells are uneven, being very thin at some regions of the nanocrystals, this may be due to an electronic effect caused by the Au core or because Au is partially exposed. This observation agrees with the trends in the qb and qf/qb (Figure 3b), since an earlier reactivation of the surface for further methanol oxidation in the backward scan will result in higher qb. A closer inspection of the onset region of the backward oxidation peak (Figure 3c) reveals that there is a positive shift in the potentials for the core−shell and alloy nanocrystals as compared with pure Pd of 19 and 68 mV, respectively (measured at a current density of 0.05 mA cm−2 dashed line in Figure 3c). This result shows that the potential at which the surface is reactivated and becomes available for further oxidation of methanol is affected by the presence of gold. The hypothesis that there is no formation of CO on the Pdbased catalysts was further studied by CO stripping and MOR

Figure 4. Positive potential scan of cyclic voltammograms of (a) alloy (red), (b) AuPd core−shell (blue), and (c) Pd (black) nanocrystals. CO stripping in the absence of methanol (open circles), MOR in N2 saturated electrolyte (full circles), and MOR in CO saturated electrolyte (full line).

in Figure 4) occurs at similar potentials to those observed for the oxidation of methanol under N2 in the forward scan (full circles). The similarity in oxidation potential could indicate that any CO intermediate is oxidized together with methanol in the forward scan, resulting in no further oxidation at more positive potentials in the backward scan. However, the MOR peak in the forward scan is shifted to more positive potentials when the electrolyte is saturated with CO (full line). In such a case, the methanol oxidation starts immediately after the CO oxidation, as shown by both the CO stripping scan and the presence of a shoulder on the MOR peak. This indicates that CO is not produced as intermediate, and its presence negatively affects the MOR, differently to what was previously observed with gold catalysts.23 The origin of the higher activity in the presence of Au (and early reactivation of the Pd surface as shown in Figure 3c) can D

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PdO reduction, and a decrease in activity within the time of the experiment is observed for all samples. For the Pd nanocrystals, the activity is nearly lost after only 10 min at the applied potential, while for the Au−Pd nanocrystals, the activity is still retained after 30 min, being 2.7 times higher for the alloy than for the core−shell nanocrystals. Furthermore, if the potential is held at 0.47 V to reduce the PdO back to Pd, the activity is recovered (Figure S5). These results suggest that the stability for the MOR is related to the extent of PdO formation at the surface of the nanocrystals, with Au affecting the potential for the formation/ reduction of PdO rather than its participation in a bifunctional mechanism.

be rationalized by analyzing the PdO reduction peaks in the cyclic voltammograms of the different samples in the absence of methanol (Figure 5). The anodic limit of the voltammo-

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CONCLUSION We have examined the origin of improved MOR performance in alkaline electrolytes caused by the inclusion of Au into Pd nanocrystal structure. The results show that there is no evidence of CO poisoning at the surface of Pd during the MOR, which is ascribed to high OH surface coverage at the Pd sites in alkaline environment. Both activity and stability are improved when Au is present, being more prominent for the alloy nanocrystals than for the core−shell particles. The origin of the improved activity can be mainly ascribed to the presence of Au that facilitates the reduction of PdO back to Pd, making the surface available at more positive potentials for further methanol oxidation.

Figure 5. Cathodic scan of cyclic voltammograms in absence of methanol of Pd (black), Au−Pd core−shell (blue), Au−Pd alloy (red), and Au (gray) nanocrystals.

grams was set to 1.65 V to avoid Pd dissolution or segregation24,25 (Figure S4). For the Pd nanocrystals, a single reduction peak at 0.66 V corresponding to PdO reduction is observed. The peak for the core−shell nanocrystals is shifted by 10 mV to 0.67 V, and a small broad peak centered at 0.8 V is also observed. For the alloy nanocrystals, the peak at 0.8 V is the dominant peak, although the peak at 0.67 V is also present. The presence of two reduction peaks in the Au−Pd nanocrystals shows that the properties of the PdO surrounded by Au are different from the PdO surrounded by other Pd atoms,26 and the presence of Au facilitates the reduction of PdO back to Pd, making the surface available at more positive potentials for further methanol oxidation as observed in Figure 3c. Thus, the origin of the improved activity caused by the presence of Au is actually related to changes in the redox properties of the formation/reduction of PdO, resulting in higher availability of Pd sites for MOR. The effect of Au on the stability of the catalyst for the MOR was studied by chronoamperometry (Figure 6). In all cases, the applied potential of 0.88 V is more positive than the onset of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05407. Low resolution images of Au−Pd core−shell nanocrystals on carbon, pure Pd on carbon; dark-field STEM image of Au−Pd core−shell nanocrystals; table of calculated ECSA of Au−Pd core−shell, alloy and pure Pd; wide window CV scans of Au−Pd core−shell, alloy and pure Pd nanocrystals; MOR CV of Au seed nanocrystals; and chronoamperograms of Au−Pd alloy nanocrystals after regeneration of catalyst (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Phone: +61 (2) 9385 4435 (RDT) *E-mail: [email protected]; Phone: +61 (2) 9385 4435 (TMB) ORCID

Wolfgang Schuhmann: 0000-0003-2916-5223 J. Justin Gooding: 0000-0002-5398-0597 Richard D. Tilley: 0000-0003-2097-063X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the UNSW Mark Wainwright Analytical Centre, including facilities supported by AMMRF in the Electron Microscope Unit. We acknowledge funding from the ARC Centre of Excellence in Convergent Bio-Nano Science and Technology (JJG, CE140100036), an ARC Laureate Fellowship (JJG, FL150100060) program, and the

Figure 6. Chronoamperograms of Au−Pd core−shell (blue), Au−Pd alloy (red), and Pd (black) nanocrystals. Potential was held at 0.88 V for 30 min. Electrolyte: 1 mol L−1 KOH + 1 mol L−1 MeOH. E

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Deutsche Forschungsgemeinschaft (DFG) in the framework of the cluster of excellence RESOLV (WS, EXC1069).

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