Stability of Pd 3 Pb Nanocubes during Electrocatalytic Ethanol Oxidation

: Intermetallic Pd 3 Pb nanocrystals with controlled size and cubic geometry exposing (100) facets are synthesized and tested as electrocatalysts for ethanol oxidation in alkaline media. We observe the ethanol oxidation activity and stability to be size-dependent. 10 nm Pd 3 Pb nanocrystals display the highest initial current densities, but after few hundred cycles, the current density of smaller nanocrystals becomes much larger. All the catalysts exhibit a pronounced current decay during the first 500 s of continuous operation, which is associated to the accumulation of strongly adsorbed reaction intermediates, blocking reaction sites. These adsorbed species can be removed by cycling the catalysts or maintaining


INTRODUCTION
Direct alcohol fuel cells are a particularly interesting energy technology to power mobile applications. 1,2Being liquid at ambient temperature, low-molecular weight alcohols are easy to handle and store, and provide high energy densities, comparable to that of gasoline. 3While cells based on the simplest alcohol fuel, methanol, has been the most extensively investigated, disadvantages such as membrane permeation, high volatility and toxicity are limiting their deployment. 1,4Alternatively, ethanol mitigates the methanol drawbacks and offers comparable electrochemical activity and energy density, with the additional advantage of potential biosourcing.Thus the research for more active and cost-effective ethanol oxidation catalysts is a worth endeavor.
7][8] In terms of surface facets, Pd nanocrystals (NCs) with cubic geometry and offering (100) facets display electrocatalytic performance above that of octahedrons with (111) facets and rhombic dodecahedrons with (110) facets. 9However, the catalytic activity, selectivity, and especially stability of pure Pd catalysts require further optimization for costeffective commercialization. 10loying Pd with additional elements is a common strategy to further improve electrocatalytic performances. 11,12Adatoms are commonly considered to improve electrocatalytic characteristics through three basic mechanisms: i) electronic effect, when adatoms alter the electronic nature of active atoms; ii) steric effect, when the adatom blocks neighboring catalyst site, preventing formation of strongly-bound poison intermediates; iii) bifunctional effect, when adatoms directly participate in the catalytic oxidation of the fuel. 13Among the different elements able to promote Pd electrocatalytic performance, e.g.Pd 2 Sn, 14,15 PdCu, 16,17 Pd 2 Ru, 18 PdAu, 19 and PdNi, 20,21 alloying Pd with Pb results in the highest EOR activities.Pb enhancement has been explained through a combination of bifunctional mechanism and electronic effect.3][24][25][26] In particular, the oxidation and stripping of intermediate acetyl, which strongly binds to the catalyst surface, is regarded as the rate-determining step of ethanol electrooxidation catalyzed by Pd in alkaline media. 10Pb can activate water at lower potentials than Pd to oxidize this adsorbed intermediate and liberate Pd active sites.Through this mechanism, the addition of Pb can improve poisoning tolerance of Pd, overall improving both activity and stability of the catalyst.Pb is also reported to help cleaving the C-C bond. 27Additionally, the introduction of Pb atoms into Pd can also promote ethanol oxidation by changing the electronic properties of Pd. 26 When Pd with small lattice constant (3.89 Å) is alloyed with Pb with large lattice constant (4.93 Å), the dband center of Pd shifts up, which improves the ethanol adsorption ability on the surface of Pd 3 Pb catalysts. 26,28ile most fundamental studies have been carried out on well-defined surfaces, real catalysts display a much wider phenomenology.Catalytic performance depends not only on the overall composition of the catalyst, but also on the catalyst crystal domain size and shape, the surface distribution of the elements and their reorganization during operation/cycling.In this regard, Pd 3 Pb NCs with a large variety of sizes and shapes, including flowerlike, 22 nanowire, 29 nanocubes, 30 nanoplates, 31 and tripods 32 have been reported for oxygen reduction [31][32][33] and oxidation of formic acid 34,35 and alcohols, 26,36 but showing a large variety of results.
Herein, intermetallic Pd 3 Pb NCs with well-defined cubic shape and sizes in the range from 6 nm to 10 nm were synthesized and supported on carbon black.The obtained catalysts were tested toward ethanol oxidation in alkaline media to compare the influence of NC parameters on the EOR electrocatalytic activity and stability.
6 nm Pd 3 Pb NCs.46 mg Pd(acac) 2 , 11 mg PbO, 1 mL OAc and 10 mL OAm were mixed in a 25 mL three-neck flask connected to Schlenk line with magnetic stirring.1 mL TOP was injected into the mixture under argon flow.The reaction mixture was purged with argon at 100 C for 1h.Then the reaction temperature was increased to 200 C in 10 min and kept at 200 C for 1 h before cooling down to ambient temperature.The product was collected by centrifugation at 4000 rpm and purified by 3 cycles of dispersion/precipitation with hexane and acetone.NCs were dispersed in hexane until posterior application.
8 nm Pd 3 Pb NCs.46 mg Pd(acac) 2 , 11 mg PbO, 34 mg MAC, 1 mL OAc and 10 mL OAm were mixed in a 25 mL three-neck flask connected to Schlenk line with magnetic stirring.1 mL TOP was injected into the mixture under argon flow.The heating and purification steps were the same as for the synthesis of 6 nm Pd 3 Pb NCs.
10 nm Pd 3 Pb NCs.46 mg Pd(acac) 2 , 22 mg Pb(OAc) 4 , 34 mg MAC, 1 mL OAc and 10 mL OLA were mixed in a 25 mL three-neck flask connected to Schlenk line with magnetic stirring.1 mL TOP was injected into the mixture under argon flow.The reaction mixture was purged with argon at 100 C for 1h.Then the reaction temperature was increased to 300 C in 15 min and kept at 300 C for 1h before cooling down to ambient temperature.The heating and purification steps were the same as for the synthesis of 6 nm Pd 3 Pb NCs.
Carbonsupported Pd 3 Pb catalysts (Pd 3 Pb/C) were prepared following a reported method. 37Briefly, 5 mg of Pd 3 Pb NCs dispersed in 5 mL of hexane were mixed with 5 mg of Vulcan XC-72 carbon dispersed in EtOH.The mixture was sonicated for 1 h to support the Pd 3 Pb NCs on the carbon and the product was collected by centrifugation.To remove organic ligands on the NC surface, Pd 3 Pb/C was redispersed in a mixture of 10 mL EtOH and 1 mL acetic acid by sonication for 30 min.Then, Pd 3 Pb/C was washed with EtOH, dried naturally under ambient condition, and annealed at 250 C in argon atmosphere for 1 h.The final product was dispersed in isopropanol containing 5% Nafion to formulate the catalyst ink.The concentration of Pd 3 Pb in the catalyst was 1 mg/mL (2 mg of Pd 3 Pb/C dispersed in 1 mL isopropanol with 5 L 5% Nafion).
Electrochemical measurements.Electrochemical measurements were conducted with a BioLogic electrochemical workstation using a three-neck-type cell at room temperature.A Pt gauze was used as counter electrode and Hg/HgO (1 M KOH) as reference electrodes.The working electrode was a 5 mm diameter glassy carbon (GC) electrode with 3 L of drop-casted catalyst ink, i.e. 1.82 g of Pd.The precise amount of Pd loaded on the GC electrode, determined by inductively coupled plasma mass spectrometry (ICP-MS), was: 2.1 μg of Pd for 6 nm Pd 3 Pb/C, 1.95 μg for 8 nm Pd 3 Pb/C, and 2.5 μg for 10 nm Pd 3 Pb/C catalysts.Cyclic voltammetry (CV) curves were recorded at a scan rate of 50 mV s -1 in argon-saturated 0.5 M KOH or 0.5 M KOH + 0.5 M EtOH aqueous solutions.Chronoamperometry (CA) measurements were conducted at potential of -0.1 V vs. Hg/HgO for 7200 s in 0.5 M KOH + 0.5 M EtOH aqueous solution for stability evaluation.
Structural and Chemical Characterization.Size and shape of the Pd 3 Pb NCs were characterized by transmission electron microscopy (TEM) using a ZEISS LIBRA 120, operating at 120 kV.High-resolution TEM (HRTEM) studies were conducted using a field emission gun FEI Tecnai F20 microscope at 200 kV with a point-to-point resolution of 0.19 nm.Scanning electron microscopy (SEM) analyses were carried out in a ZEISS Auriga microscope with an energy dispersive X-ray spectroscopy (EDS) detector operating at 20 kV to assess composition.Powder X-ray diffraction (XRD) patterns were collected directly from the as-synthesized NPs dropped on Si (501) substrates on a Bruker AXS D8 Advance X-ray diffractometer with Ni-filtered (2 μm thickness) Cu Kα radiation (λ = 1.5406Å) operating at 40 kV and 40 mA.A LynxEye linear position-sensitive detector was used in reflection geometry.Characterization of the surface was done by X-ray photoelectron spectroscopy (XPS) on a SPECS system equipped with a XR50 source operating at 250 W and a Phoibos 150 MCD-9 detector.The pass energy of the hemispherical analyzer was set at 20 eV and the energy step of high-resolution spectra was set at 0.05 eV.The pressure in the analysis chamber was always below 10 -7 Pa.Binding energy (BE) values were referred to the adventitious C 1s peak at 284.8 eV.Data processing was performed with the CasaXPS software.ICP-MS analyses were carried out on an iCAP 6500 ICP emission spectrometer (Thermo) with samples prepared by digestion of nanocrystals in aqua regia overnight followed by dillution in MQ-Water (Milipore).

RESULTS AND DISCUSSION
Structural and chemical characterization of Pd 3 Pb NCs.1d).XRD analysis confirmed the cubic Pd 3 Pb intermetallic structure (JCPDS 01-089-2062, Figure 1e) and displayed the ratio of peak intensities I (200) /I (111) and thus the [100] faceting to increase with the NC size.EELS chemical composition maps demonstrated a homogeneous distribution of the two elements within each NC (Figure 1f) with a ratio Pd/Pb≈3, which matched well with results obtained from HRTEM, XRD, ICP and EDS analyses (Figure S2).ing agent.9][40] When no OAc was added to the precursor reaction mixture, NCs with irregular geometries and large size distributions were produced (Figure S3a).With the addition of small amounts of OAc, OAc/OAm=0.05 to 0.2 (v/v), Pd 3 Pb NCs with uniform cubic shapes were produced (Figures 1 and S3b, c).
Without TOP, the formed Pd-OAm complex decomposed at a low temperature, ca. 100 C, 41 yielding elemental Pd NPs, i.e. with no Pb alloying.TOP is known to strongly coordinate with Pd to form a Pd-TOP complex. 42he decomposition of this Pd complex, which is more stable than Pd-OAm, takes place at higher temperatures, 200 C, facilitating the simultaneous reduction of Pd and Pb precursors to form intermetallic Pd 3 Pb NCs.Thus, a large excess of TOP, TOP/Pd=15, was introduced in all batches.Besides playing a fundamental role in the control of the NC composition, TOP was also able to direct the NC shape.In the presence of too small amounts of TOP, irregular-shaped NCs were formed (Figure S3d), and when too larger amounts of TOP were added, instead of cubic NCs, nanoplates/nanorods were grown (Figures S3e  and S3f).
Finally, the introduction of MAC and the replacement of PbO by Pb(OAc) 4 resulted in larger NCs (Figures 1b,c).We hypothesize this effect to be associated to an increase of the precursor stability in the reaction mixture and a related reduction of the number of nucleation events 43,44 The PbO-MAC system reacted at 200 °C, while the Pb(OAc) 4 -MAC system required higher temperatures, up to 260 °C, to form Pd 3 Pb NCs.
Figure 2a shows the Pd 3d XPS spectrum of Pd 3 Pb NCs, displaying the Pd 3d 5/2 peak at 335.3 eV, i.e. shifted around 0.4 V with respect to Pd 0 (334.9eV). 45This binding energy shift was consistent with the presence of more electronegative Pb that effectively modified the electronic structure of Pd. 30 A second minor component, accounting for 10% of the Pd observed, was associated to Pd 2+ .This component was related to the binding of surface Pd with surfactant molecules and potentially to a slight surface oxidation of the NCs during manipulation and transportation before XPS measurements.
In the Pb 4f XPS spectrum, three 4f doublets were identified, corresponding to three Pb chemical states (Figure 2b).The main peaks at 141.6 eV (4f 5/2 ) and 136.7 eV (4f 7/2 ) were associated to Pb 0 , although were shifted -0.2 eV with respect to the reference value for elemental Pb 4f 7/2 (136.9 eV), 45 consistently with the electronic interaction between Pd and Pb atoms within the Pb 3 Pb alloy.A second component was located at 143.0 eV (4f 5/2 ) and 138.1 eV (4f 7/2 ), and it was assigned to an oxidized Pb state, Pb n+ .A third component appeared at even higher binding energies, 139.1 eV (4f 7/2 ) and 144.0 eV (4f 5/2 ), and it was associated with a Pb 2+ chemical state. 45The oxidized Pb components, which we will globally refer to as Pb x+ , were associated to the binding of Pb with oleic acid molecules on the NC surface and to the surface oxidation of the NCs during handling and transportation.The ratio Pb 0 /Pb x+ was 1.55, i.e. 40% of the Pb within the 2-3 nm-thick outer layer of the Pd 3 Pb NCs was oxidized.This percentage was significantly higher than that of Pd.Besides, the Pd/Pb surface ratio obtained by XPS was 1.19, much lower than the stoichiometric values deduced from EDS, ICP, XRD and HRTEM analyses.These results indicated a segregation of Pb atoms to the NC surface during the NC synthesis.The radial gradient of Pb could be also in part driven by the surface oxidation during manipulation and storage of the samples in ambient conditions, due to the lower oxidation potential of Pb when compared with Pd.

CV analysis.
Pd 3 Pb-based electrocatalysts were prepared by mixing colloidal Pd 3 Pb NCs with carbon black, 1:1 wt%.FTIR analyses confirmed the removal of organic ligands from the NC surface through the treatment with acetic acid and the posterior annealing at 250 C (Figure S4).Initial CV analyses were carried out at a sweep rate of 50 mV s -1 with an argon-saturated 0.5 M KOH solution.Figure 3a depicts the CV curves recorded for Pd 3 Pb/C catalysts with different NC sizes and for a commercial Pd/C catalyst tested as a reference.The same total amount of metal was used to prepare each of the tested electrodes, which translated in significantly lower amounts of Pd on the Pd 3 Pb/C catalysts than on the reference Pd/C.The current density peaks in the region between -0.6 V and -0.8 V vs. Hg/HgO were attributed to the adsorption (cathodic scan) and desorption (anodic scan) of hydrogen.The current density increase measured at ca. -0.2 V vs. Hg/HgO in the anodic scan was attributed to the formation of palladium oxide on the NC surface.Subsequently, a PdO reduction peak appeared at about -0.2 V vs. Hg/HgO during the negative scans.
Compared with Pd/C, Pd 3 Pb/C catalysts displayed less obvious peaks associated to hydrogen desorption and adsorption, owing to the lower amount of Pd on their surface.On the other hand, the intensity of the Pd oxide reduction peak clearly increased with the addition of Pb and when increasing the NC size.Besides, the maximum of this peak was shifted toward higher potentials on Pd 3 Pb/C catalysts compared to Pd/C.The distinct anodic and cathodic characteristics of Pd 3 Pb-and Pd-based catalysts were related to their different electronic structure involving partial electron donation from Pd to Pb sites, consistent with XPS results. 46e electrochemically active surface area (ECSA) of the catalysts was estimated from the coulombic charge for the reduction of PdO, i.e. from the area over the voltammetry curve in the PdO reduction peak region: 15

10
where Q PdO = 405 μC cm -2 was the charge value given for the reduction of a PdO monolayer, the coulombic charge Q was calculated by integrating the area of the PdO reduction peak, and Pd loading was the Pd mass on the working electrode.ECSA values for 10 nm Pd 3 Pb/C were larger than that of 8 nm and 6 nm.The observed increase of ECSA with the NC size was unintuitive taking into account the decrease of surface area of the NCs when increasing their size.The higher ECSAs measured for the larger particles must be thus indirectly related to their size through a decrease of the agglomeration when supported on CB, an improved crystallinity/faceting, and potential differences in surface composition.Besides, the ECSA was significantly larger for all Pd 3 Pb/C catalysts compared with Pd/C (14.9 m 2 g -1 ) in spite of the higher amount of exposed Pd on Pd/C.ECSA increased during the first cycles for all Pd 3 Pb/C catalysts, but not for commercial Pd/C catalyst.The ECSA increase with cycling on Pd 3 Pb/C could be related to a reorganization of the surface composition and potentially to a stripping of remaining surface ligands.After 30 CV cycles, the ECSA obtained from commercial Pd/C, and for 6 nm, 8 nm and 10 nm Pd 3 Pb/C catalysts were 14.9 m 2 g -1 , 16.4 m 2 g -1 , 18.9 m 2 g -1 , and 24.2 m 2 g -1 , respectively.The Pd utilization effectiveness was estimated considering that the active surface area for full utilization of 1 g of Pd would be 448 m 2 .Thus, the Pd utilization efficiencies of commercial Pd/C, and for 6 nm, 8 nm and 10 nm Pd 3 Pb/C catalysts were 3.3 %, 3.7 %, 4.2 %, and 5.4 %, respectively.

EOR activity, CV analysis.
The electrocatalytic activity of Pd 3 Pb-based catalysts towards EOR was investigated by CV in a solution containing 0.5 M KOH and 0.5 M EtOH. Figure 4a and 4b display the CV curves of Pd 3 Pb/C and Pd/C catalysts after 30 and 900 cycles at a sweep rate of 50 mV s -1 , respectively.In the presence of EtOH, hydrogen absorptiondesorption peaks were suppressed due to the dominant adsorption of EtOH in the low-potential region.The positive scan peak related to the oxidation of freshly adsorbed alcohol started at -0.6 V and reached its maximum at -0.1 V.At higher voltages, the catalytic activity decreased due to the progressive oxidation of the Pd surface.The oxidation peak in the cathodic scan, associated with the oxidation of freshly adsorbed EtOH and of unreacted species adsorbed before or after Pd-O blocking, 47 was triggered by the reduction of the oxide layer grown in the preceding anodic scan. 48The peak current densities of Pd 3 Pb/C catalysts were much higher than those of Pd/C, and the onset potential for EtOH oxidation of Pd 3 Pb/C catalysts was more negative than for Pd/C, what suggested a more favorable EOR on the surface of Pd 3 Pb/C catalysts than on Pd/C.For comparison, Table S1 shows a resume of results reported in the literature on Pd 3 Pb NCs for EOR.
Figure 4c exhibits the forward oxidation peak current density for EOR as a function of cyclic scan number.During the first cycles the forward peak current density increased to later decrease gradually with the continued scanning.Such a general trend is often observed in electrocatalytic alcohol oxidation and is usually ascribed to a structural or surface reorganization of the catalyst. 49,50For Pd/C, the current density decay started at the 50 th cycle, dropping from 12.53 to 5.38 mA cm -2 after 900 cycles.A very similar evolution was measured from 10 nm Pd 3 Pb/C, with a progressive increase of current density up to approximately the 50 th cycle, yet with significantly higher current densities, from 21.82 to 11.72 mA cm -2 .6 nm and 8 nm Pd 3 Pb/C catalysts were characterized by a significantly different evolution of the forward peak current density with the cycle number.6 nm and 8 nm Pd 3 Pb/C catalysts displayed a progressive increase of the current density up to the 400 th -500 th cycle and a quasi-stabilization of the activity at current densities around 20 mA cm -2 with additional cycling.Thus, while 10 nm Pd 3 Pb/C displayed higher catalytic activity in the initial CV cycles, 6 nm and 8 nm Pd 3 Pb/C had much longer durability and higher activities after longer cycling.In terms of mass current density (Figure 4d), after 30 cycles 10 nm Pd 3 Pb/C catalyst showed the highest values up to 2.05 A mg -1 Pd , while the performance of 6 nm and 8 nm Pd 3 Pb/C was slightly lower than that of Pd/C.However, after 900 cycles, 8 nm Pd 3 Pb/C catalyst displayed much larger mass current densities up to 2.05 A mg -1 Pd .
-0.8 -0.The ratio of the maximum intensities measured for the forward and reverse peaks, I f /I r , of Pb 3 Pb/C catalysts also changed with the cycling number, decreasing for 6 nm Pd 3 Pb/C from 1.1 to 0.90, and 8 nm Pd 3 Pb, from 1.3 to 0.85, but increasing for 10 nm Pd 3 Pb, from 0.82 to 0.93 and increasing very moderately for Pd/C, from 0.90 to 0.91.This ratio is related to the reactivation efficiency, the higher the reverse peak, the higher the reactivation, which denoted an improvement of the reactivation with cycling for the smallest NCs and a degradation for the largest one. 48gure 4e shows the linear region of the Tafel plots obtained from the 30 th and 900 th CV curves in the range from −0.5 to -0.3 V vs. Hg/HgO for Pd 3 Pb/C and Pd/C catalysts.From the 30 th cycle, the Tafel slopes were: 162 mV dec −1 for Pd/C, 156 mV dec −1 for 8 nm, 145 mV dec −1 for 6 nm and 140 mV dec −1 for 10 nm.After 900 cycles, all the Tafel slopes increased: 203 mV dec −1 for Pd/C, 173 mV dec −1 for 6 nm, 164 mV dec −1 for 8 nm and 160 mV dec −1 for 10 nm.All Pd 3 Pb/C catalysts displayed lower Tafel slopes and thus faster charge-transfer kinetics than Pd/C.On the other hand, the increase of the Tafel slopes with the scan number revealed a performance decay for all the catalysts, being the 8 nm Pd 3 Pb/C catalyst the one showing the best stability.th and 900 th cycles, respectively.(f) CA measurements of 6 nm, 8 nm and 10 nm Pd 3 Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH aqueous solution at the potential of −0.1 V vs. Hg/HgO.Differences between Pd 3 Pb/C catalysts containing Pd 3 Pb NCs of different sizes may be related to several interconnected mechanisms: i) The higher activity of the 10 nm NCs had associated the generation of a higher amount of intermediates that may poison the catalyst surface more rapidly than in less active catalysts; ii) The oxidation and reduction of surface Pd during each scan and the development of channels of electron collection could rearrange the catalyst structure and NC surface. 51his process may be significantly different in NCs with different sizes; iii) Depending on the NC size, the extend of material oxidation and reduction could be significantly different.

EOR activity, CA analysis.
The stability of the catalyst performance was further investigated by CA. Figure 4f displays CA results obtained at -0.1 V vs. Hg/HgO in a 0.5 M KOH + 0.5 M EtOH aqueous solution.All the catalysts exhibited a pronounced current decay during the first 500 s.This rapid activity decay, characteristic of EOR catalyst, including Pd-based catalysts, remains as a major drawback toward the commercialization of DEFCs.While all the Pd 3 Pb/C catalysts showed enhanced EOR activity and better stability than Pd/C catalyst, 10 nm Pd 3 Pb/C catalysts displayed a more abrupt decrease in current densities than 6 nm and especially 8 nm Pd 3 Pb/C catalysts.Replacing the electrolyte by a fresh solution did not provide a significant increase in current density, demonstrating that the EtOH depletion was not the reason behind the large current density decrease.Instead, we succeeded in reactivating our Pd 3 Pb/C and Pd/C catalysts by either cycling few times the applied potential (Figure 5) or by slightly increasing the electrode potential for a short time (Figure S5). Figure 5 displays the current density decay during periods of 1000 s for 10 nm, 8 nm, 6 nm Pd 3 Pb/C catalysts and Pd/C catalysts.In between these 1000 s periods, 3 CV cycles in the range from -0.9 to 0.3 V vs. Hg/HgO with a rate of 50 mV s -1 for a total time of 144 s were applied.10 nm Pd 3 Pb/C catalyst reached the highest current densities after each reactivation.Figure S5 displays the current densities evolution during periods of 1000 s and reactivation by applying a voltage of 0.3 V vs. Hg/HgO for 100 s, i.e. by oxidizing and later reducing the Pd surface.The current density decay could be fitted considering at least two exponential decays, one with a characteristic time in the range of ca.10-30 s, and a second one with a longer characteristic time up to 500 s (Figure S6 and Table S2).We systematically observed that the larger the NCs, the lower the contribution of the fast exponential decay.Besides, an increase of the reaction temperature up to 50 C allowed to slightly increase the catalyst activity and extend at the same time its stability (Figure S7 and Table S3).
In alkaline media, EtOH is dehydrogenated into adsorbed acetyl, which is further oxidized to acetate by hydroxide species. 10,52 he oxidation of the acetyl to acetic acid by adsorbed hydroxyl is regarded as the ratedetermining step, while the stripping of the acetic acid in the form of acetate ions in alkaline solution is very rapid. 10cetaldehyde and reaction products such as CH and C may block additional sites, although breaking of the C-C bond is considered to have a minor contribution on the obtained current density. 52,53cling contributes to clean the Pd surface through different mechanism: i)the oxidation of the acetyl to acetate is favored by oxidative potentials; 10 besides ii) the oxidation of the catalyst surface may contribute to the stripping of adsorbed molecules.In this process the concentration and surface distribution of Pd and Pb, the overall surface area and the exposed facets play important roles.During EOR and during cycling all of these parameters may change.During the electrochemical reaction, Pd and Pb most probably redistribute within each particle as evidenced below, and even within the surface as noted by Gunji et al. 54 Changes of surface composition modifies the equilibrium coverage of hydroxyl and adsorbed acetyl, thus changing the overall current density. 10Besides, possible changes of particle geometry modify the exposed facets and thus the exposed distribution of Pb and Pd sites.

Structural and chemical evolution.
HRTEM and HAADF-STEM analysis of the Pd 3 Pb/C catalysts after cycling in 0.5 M KOH and 0.5 M KOH + 0.5 M EtOH aqueous solutions showed the NCs to maintain a high crystallinity and to conserve the Pd 3 Pb intermetallic phase (Figure 6).However, extensive TEM analysis of the catalyst after cycling showed the cubic shape of the NCs to be partially lost for a significant number of NCs (Figures S7-13).
XPS analyses of the Pd 3 Pb catalyst after cycling in 0.5 M KOH and in 0.5 M KOH + 0.5 M EtOH aqueous solutions showed a notable increase of the surface Pd/Pb atomic ratio, from the initial Pd/Pb=1.19obtained from Pd 3 Pb NCs to Pd/Pb=3.09 of the Pd 3 Pb/C catalyst after cycling in a KOH + EtOH aqueous solution (Figure S8, Table S4).This decrease of the relative Pb surface concentration could be related to a partial dissolution of Pb, a phase segregation, or a redistribution of the elements within each NC.EDS analysis showed the total Pd/Pb ratio to slightly increase after cycling in a 0.5 M KOH + 0.5 M EtOH aqueous solution, from Pd/Pb=3.03 to Pd/Pb=3.42, or after cycling in KOH, Pd/Pb=3.77.This increase must be assigned to a leaching of Pb atoms from the Pd 3 Pb alloy to the electrolyte.However, the amount of leached Pb, 12 % after cycling in a KOH + EtOH aqueous solution, does not fully account for the much larger variations obtained on the surface composition.Besides, STEM-EDS analysis of the catalyst after 30 CV cycles in 0.5 M KOH aqueous solution (Figure S9) and EELS analysis after cycling in KOH + EtOH aqueous solutions (Figure 6) displayed no phase segregation, being both elements homogeneously distributed within each NC and with the same ratio from particle to particle.
These results pointed out at a reorganization of the two elements within the Pd 3 Pb NCs during cycling in a KOH + EtOH aqueous solution, involving an outward/inward diffusion of Pd/Pb to equilibrate the stoichiometry of the NCs surface, which was highly Pb-rich in the assynthesized materials.
A partial surface reorganization of the NC surface took place already during their use to formulate the catalyst, and could be related with the removal of organic ligands that were preferentially bond to Pb atoms.Additional decrease of the Pb surface concentration was observed during cycling in KOH (Table S4, Scheme S1).This decrease was partially correlated with EDS results that showed an important Pb leaching during KOH cycling that could account for the increase of the Pd/Pb ration obtained after this process.
However, we believe that a significant part of the elemental restructuration was driven by the oxidation/reduction of Pd.During cycling, only Pd was oxidized and reduced, while surface Pb atoms remained in an oxidized chemical state.In a previous work, we showed how cobalt and copper can reorganize by interfusion during oxidation and reduction processes.During oxidation Co diffused outward to form a CuO-Co 3 O 4 hollow structures, while during reduction Cu diffused inward to form Cu@Co 3 O 4 core-shell particles. 55In the present case we believe a similar process takes place.The initial NC surface is enriched with Pb due to the larger affinity of this element toward organic ligands and oxygen.Once removed the organics, Pd diffuses outwards/inwards during cycling, driven by the oxidation/reduction processes.Since the largest increase of the surface Pd/Pb ratio was obtained after cycling in the presence of EtOH, we need to assume that the higher affinity of Pd to EtOH may also play an important role in the surface restructuration.

CONCLUSIONS
In summary, we report a simple approach to produce intermetallic Pd 3 Pb NCs with well-defined cubic geometry and average sizes in the range from 6 nm to 10 nm.Compared with commercial Pd/C catalyst, Pd 3 Pb/C catalysts presented improved EOR electrocatalytic activities and stabilities.10 nm Pd 3 Pb/C catalyst provided the largest initial current densities, but smaller NCs were able to reach higher current densities after extended cycling.All the catalysts exhibited a pronounced current decay during the first 500 s of continuous EOR operation, which was associated to the accumulation of strongly adsorbed reaction intermediates and the related blockage of reaction sites.The catalyst could be reactivated by simple cycling, but this reactivation was also size dependent.
Such simple cleaning processes, that can be performed during operation breaks without cell disassembly, was sufficient to effectively remove the poisoning species adsorbed on the surface and recover the electrocatalytic activity.We finally demonstrated that during cycling a major redistribution of the elements within the NCs took place, driven by the different affinity of Pb and Pd towards oxygen and possibly ethanol, and the electrochemical oxidation/reduction of Pd.

ASSOCIATED CONTENT
Supporting Information.This material is available free of charge via the Internet at http://pubs.acs.org."Characterization (TEM images, EDS, XPS), electrochemistry and scheme.

Figure 1
Figure 1 displays TEM micrographs of the NCs obtained from the above-detailed procedure.The Pb precursor and the amount of MAC were adjusted to produce NCs with different sizes (see experimental section for details, Figures 1 and S1).HRTEM analysis showed the NCs to have a crystallographic structure compatible with that of cubic Pd 3 Pb, [Pm-3m]-space group 221, and to be strongly faceted in the [100] directions (Figure1d).XRD analysis confirmed the cubic Pd 3 Pb intermetallic structure (JCPDS 01-089-2062, Figure1e) and displayed the ratio of peak intensities I (200) /I (111) and thus the [100] faceting to increase with the NC size.EELS chemical composition maps demonstrated a homogeneous distribution of the two elements within each NC (Figure1f) with a ratio Pd/Pb≈3, which matched well with results obtained from HRTEM, XRD, ICP and EDS analyses (FigureS2).

Figure 1 .
Figure 1.(a-c) TEM micrographs of 6 nm (a), 8 nm (b) and 10 nm Pd 3 Pb NCs (c).(d) HRTEM, zoomed HRTEM detail after frequency filtering with spot masks, power spectrum (FFT), and crystallographic unit cell (yellow for Pd and blue for Pb) of the 6 nm Pd 3 Pb NC.HRTEM analysis showed this NC to have a lattice parameter a=b=c=0.4035nm and α=β＝γ＝90° as visualized along the [001] direction of the Pd 3 Pb cubic crystal structure.(e) XRD patterns of 6 nm, 8 nm and 10 nm Pd 3 Pb NCs.(f) ADF-STEM image and EELS composition maps from the white square region for Pd, Pb and Pd-Pb of 6 nm Pd 3 Pb NCs.OAm, OAc and TOP were all necessary components of the precursor reaction mixture to produce highly faceted

Figure 2 .
Figure 2. High resolution XPS spectra of 6 nm Pd 3 Pb NCs in the regions (a) Pd 3d and (b) Pb 4f.

Figure 4 .
Figure 4. (a) 30 th CV curves of 6 nm, 8 nm and 10 nm Pd 3 Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH aqueous solution.(b) 900 th CV curves of 6 nm, 8 nm and 10 nm Pd 3 Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH aqueous solution.(c) Current density of 6 nm, 8 nm, 10 nm Pd 3 Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH solution at potential of -0.1 V vs. Hg/HgO for different CV cycles.(d) Mass peak current density of 6 nm, 8 nm, 10 nm Pd 3 Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH aqueous solution from 30 th and 900 th CV cycles.(e) Tafel plots of 6 nm, 8 nm, 10 nm Pd 3 Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH aqueous solution from 30 th and 900 th cycles, respectively.(f) CA measurements of 6 nm, 8 nm and 10 nm Pd 3 Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH aqueous solution at the potential of −0.1 V vs. Hg/HgO.

Figure 5 .
Figure 5. CA curves of 10 nm (a), 8 nm (b), 6 nm (c) Pd 3 Pb/C and Pd/C (d) catalysts.Three CV cycles were conducted to reactivate the catalyst every 1000 s.

Figure 6 .
Figure 6.(a) TEM and (b) HAADF-STEM micrographs of 6 nm Pd 3 Pb/C catalyst.HAADF-EDS mappings of (c) Pd and (d) Pb from 6 nm Pd 3 Pb/C catalyst.(e) TEM and (f) HAADF-STEM micrographs of 6 nm Pd 3 Pb/C catalyst after 30 CV cycles in a 0.5 KOH + 0.5 M EtOH aqueous solution.HAADF-EDS mappings of (g) Pd and (h) Pb from 6 nm Pd 3 Pb/C catalyst after 30 CV cycles in a 0.5 KOH + 0.5 M EtOH aqueous solution.All scale bars are 50 nm.