This is the submitted version of the journal article: Yu, Xiaoting; Luo, Zhishan; Zhang, Ting; [et al.]. «Stability of Pd3Pb nanocubes during electrocatalytic thanol oxidation». Chemistry of materials, Vol. 32, issue 5 (March 2020), p. 2044-2052. DOI 10.1021/acs.chemmater.9b05094 This version is available at https://ddd.uab.cat/record/236018 under the terms of the license Page 1 of 12 Chemistry of Materials 1 2 3 4 5 6 7 Stability of Pd3Pb Nanocubes during Electrocatalytic Ethanol Oxida- 8 tion 9 10 Xiaoting Yu a,b, Zhishan Luo c,d, Ting Zhang a,e, Pengyi Tang e, Junshan Li a,b, 11 12 Xiang Wang a,b, Jordi Llorca f, Jordi Arbiol e,g, Junfeng Liu h,*, Andreu Cabot a,g,* 13 14 a Catalonia Institute for Energy Research - IREC, Sant Adrià de Besòs, 08930 Barcelona, Spain 15 b Department of Electronic and Biomedical Engineering, Universitat de Barcelona, 08028 Barcelona, Spain 16 17 c Department of Chemistry, Southern University of Science and Technology (SUSTech),518055 Shenzhen, Guangdong, 18 P. R. China 19 d SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology (SUS- 20 Tech), 518055 Shenzhen, Guangdong, P. R. China 21 e Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Bar- 22 celona, Spain 23 24 f Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research Center in Multiscale 25 Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain 26 g ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain 27 h Institute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu University, 212013 Zhenjiang, 28 P.R. China 29 30 31 32 33 ABSTRACT: Intermetallic Pd3Pb nanocrystals with controlled size and cubic geometry exposing (100) facets are synthe- 34 sized and tested as electrocatalysts for ethanol oxidation in alkaline media. We observe the ethanol oxidation activity and 35 stability to be size-dependent. 10 nm Pd3Pb nanocrystals display the highest initial current densities, but after few hun- 36 dred cycles, the current density of smaller nanocrystals becomes much larger. All the catalysts exhibit a pronounced cur- 37 rent decay during the first 500 s of continuous operation, which is associated to the accumulation of strongly adsorbed 38 reaction intermediates, blocking reaction sites. These adsorbed species can be removed by cycling the catalysts or main- 39 taining them at slightly higher potentials for a short period of time to oxidize and later reduce the Pd surface. Such simple cleaning processes, that can be performed during operation breaks without cell disassembly, is sufficient to effectively 40 remove the poisoning species adsorbed on the surface and recover the electrocatalytic activity. 41 42 43 44 INTRODUCTION Pd is an excellent catalyst for oxidation of hydrogen and 45 Direct alcohol fuel cells are a particularly interesting liquid fuels,5 and provides the best performance in alka- 46 energy technology to power mobile applications.1,2 Being line-type direct ethanol fuel cells (DEFCs).6-8 In terms of 47 liquid at ambient temperature, low-molecular weight surface facets, Pd nanocrystals (NCs) with cubic geometry 48 alcohols are easy to handle and store, and provide high and offering (100) facets display electrocatalytic perfor- 49 energy densities, comparable to that of gasoline.3 While mance above that of octahedrons with (111) facets and 50 cells based on the simplest alcohol fuel, methanol, has rhombic dodecahedrons with (110) facets.9 However, the 51 been the most extensively investigated, disadvantages catalytic activity, selectivity, and especially stability of 52 such as membrane permeation, high volatility and toxici- pure Pd catalysts require further optimization for cost- 53 ty are limiting their deployment.1,4 Alternatively, ethanol effective commercialization.10 54 mitigates the methanol drawbacks and offers comparable Alloying Pd with additional elements is a common 55 electrochemical activity and energy density, with the strategy to further improve electrocatalytic 56 additional advantage of potential biosourcing. Thus the performances.11,12 Adatoms are commonly considered to 57 research for more active and cost-effective ethanol oxida- improve electrocatalytic characteristics through three 58 tion catalysts is a worth endeavor. basic mechanisms: i) electronic effect, when adatoms 59 alter the electronic nature of active atoms; ii) steric effect, 60 ACS Paragon Plus Environment Chemistry of Materials Page 2 of 12 when the adatom blocks neighboring catalyst site, pre- 6 nm Pd3Pb NCs. 46 mg Pd(acac)2, 11 mg PbO, 1 mL 1 venting formation of strongly-bound poison intermedi- OAc and 10 mL OAm were mixed in a 25 mL three-neck 2 ates; iii) bifunctional effect, when adatoms directly partic- flask connected to Schlenk line with magnetic stirring. 1 3 ipate in the catalytic oxidation of the fuel.13 Among the mL TOP was injected into the mixture under argon flow. 4 different elements able to promote Pd electrocatalytic The reaction mixture was purged with argon at 100 C for 5 performance, e.g. Pd2Sn,14,15 PdCu,16,17 Pd2Ru,18 PdAu,19 and 1h. Then the reaction temperature was increased to 200 6 PdNi,20,21 alloying Pd with Pb results in the highest EOR C in 10 min and kept at 200 C for 1 h before cooling 7 activities. Pb enhancement has been explained through a down to ambient temperature. The product was collected 8 combination of bifunctional mechanism and electronic by centrifugation at 4000 rpm and purified by 3 cycles of 9 effect. In the first direction, Pb plays a key role on the dispersion/precipitation with hexane and acetone. NCs 10 oxidation of absorbed intermediates, which is the limiting were dispersed in hexane until posterior application. 11 step in EOR.22-26 In particular, the oxidation and stripping 8 nm Pd3Pb NCs. 46 mg Pd(acac)2, 11 mg PbO, 34 mg 12 of intermediate acetyl, which strongly binds to the cata- MAC, 1 mL OAc and 10 mL OAm were mixed in a 25 mL 13 lyst surface, is regarded as the rate-determining step of three-neck flask connected to Schlenk line with magnetic 14 ethanol electrooxidation catalyzed by Pd in alkaline me- stirring. 1 mL TOP was injected into the mixture under 15 dia.10 Pb can activate water at lower potentials than Pd to argon flow. The heating and purification steps were the 16 oxidize this adsorbed intermediate and liberate Pd active same as for the synthesis of 6 nm Pd3Pb NCs. 17 sites. Through this mechanism, the addition of Pb can improve poisoning tolerance of Pd, overall improving 10 nm Pd3Pb NCs. 46 mg Pd(acac)2, 22 mg Pb(OAc)4, 18 34 mg MAC, 1 mL OAc and 10 mL OLA were mixed in a 25 both activity and stability of the catalyst. Pb is also re- 19 mL three-neck flask connected to Schlenk line with mag- ported to help cleaving the C-C bond.27 Additionally, the 20 netic stirring. 1 mL TOP was injected into the mixture introduction of Pb atoms into Pd can also promote etha- 21 under argon flow. The reaction mixture was purged with nol oxidation by changing the electronic properties of 22 argon at 100 C for 1h. Then the reaction temperature was Pd.26 When Pd with small lattice constant (3.89 Å) is al- 23 loyed with Pb with large lattice constant (4.93 Å), the d- increased to 300 C in 15 min and kept at 300 C for 1h 24 band center of Pd shifts up, which improves the ethanol before cooling down to ambient temperature. The heating 25 adsorption ability on the surface of Pd3Pb catalysts.26,28 and purification steps were the same as for the synthesis 26 of 6 nm Pd3Pb NCs. While most fundamental studies have been carried out 27 Carbon-supported Pd3Pb catalysts. Carbon- on well-defined surfaces, real catalysts display a much 28 supported Pd3Pb catalysts (Pd3Pb/C) were prepared fol- wider phenomenology. Catalytic performance depends 29 lowing a reported method.37 Briefly, 5 mg of Pd3Pb NCs not only on the overall composition of the catalyst, but 30 also on the catalyst crystal domain size and shape, the dispersed in 5 mL of hexane were mixed with 5 mg of 31 surface distribution of the elements and their reorganiza- Vulcan XC-72 carbon dispersed in EtOH. The mixture was 32 tion during operation/cycling. In this regard, Pd3Pb NCs sonicated for 1 h to support the Pd3Pb NCs on the carbon 33 with a large variety of sizes and shapes, including flower- and the product was collected by centrifugation. To re- 34 like,22 nanowire,29 nanocubes,30 nanoplates,31 and tripods32 move organic ligands on the NC surface, Pd3Pb/C was re- 35 have been reported for oxygen reduction31-33 and oxidation dispersed in a mixture of 10 mL EtOH and 1 mL acetic acid 36 of formic acid34,35 and alcohols,26,36 but showing a large by sonication for 30 min. Then, Pd3Pb/C was washed with 37 variety of results. EtOH, dried naturally under ambient condition, and an- 38 nealed at 250 C in argon atmosphere for 1 h. The final Herein, intermetallic Pd3Pb NCs with well-defined cu- 39 bic shape and sizes in the range from 6 nm to 10 nm were product was dispersed in isopropanol containing 5% 40 synthesized and supported on carbon black. The obtained Nafion to formulate the catalyst ink. The concentration of 41 Pd3Pb in the catalyst was 1 mg/mL (2 mg of Pd3Pb/C dis- catalysts were tested toward ethanol oxidation in alkaline 42 persed in 1 mL isopropanol with 5 L 5% Nafion). media to compare the influence of NC parameters on the 43 EOR electrocatalytic activity and stability. Electrochemical measurements. Electrochemical 44 measurements were conducted with a BioLogic electro- 45 chemical workstation using a three-neck-type cell at 46 EXPERIMENTAL SECTION room temperature. A Pt gauze was used as counter elec- 47 Chemicals. Palladium(II) acetylacetonate (Pd(acac)2, trode and Hg/HgO (1 M KOH) as reference electrodes. 48 99%), lead(II) oxide (PbO, 99%), lead(IV) acetate The working electrode was a 5 mm diameter glassy car- 49 (Pb(OAc)4, 99%), oleylamine (OAm, 80%), oleic acid bon (GC) electrode with 3 L of drop-casted catalyst ink, 50 (OAc, 90%), trioctylphosphine (TOP, 90%), methylamine i.e. 1.82 g of Pd. The precise amount of Pd loaded on the 51 hydrochloride (MAC, 98%), potassium hydroxide (KOH, GC electrode, determined by inductively coupled plasma 52 85%), Nafion (5 wt%) were all purchased from Sigma- mass spectrometry (ICP-MS), was: 2.1 μg of Pd for 6 nm 53 Aldrich. Hexane, ethanol (EtOH) and acetone were tech- Pd3Pb/C, 1.95 μg for 8 nm Pd3Pb/C, and 2.5 μg for 10 nm 54 nical grade, obtained from other company and used with- Pd3Pb/C catalysts. Cyclic voltammetry (CV) curves were 55 out further purification. recorded at a scan rate of 50 mV s-1 in argon-saturated 0.5 56 2 57 58 59 60 ACS Paragon Plus Environment Page 3 of 12 Chemistry of Materials M KOH or 0.5 M KOH + 0.5 M EtOH aqueous solutions. energy (BE) values were referred to the adventitious C 1s 1 Chronoamperometry (CA) measurements were conducted peak at 284.8 eV. Data processing was performed with the 2 at potential of -0.1 V vs. Hg/HgO for 7200 s in 0.5 M KOH CasaXPS software. ICP-MS analyses were carried out on 3 + 0.5 M EtOH aqueous solution for stability evaluation. an iCAP 6500 ICP emission spectrometer (Thermo) with 4 Structural and Chemical Characterization. Size and samples prepared by digestion of nanocrystals in aqua 5 shape of the Pd3Pb NCs were characterized by transmis- regia overnight followed by dillution in MQ-Water (Mili- 6 sion electron microscopy (TEM) using a ZEISS LIBRA 120, pore). 7 operating at 120 kV. High-resolution TEM (HRTEM) stud- RESULTS AND DISCUSSION 8 ies were conducted using a field emission gun FEI Tecnai Structural and chemical characterization of Pd3Pb 9 F20 microscope at 200 kV with a point-to-point resolu- NCs. 10 tion of 0.19 nm. Scanning electron microscopy (SEM) Figure 1 displays TEM micrographs of the NCs obtained 11 analyses were carried out in a ZEISS Auriga microscope from the above-detailed procedure. The Pb precursor and 12 with an energy dispersive X-ray spectroscopy (EDS) de- the amount of MAC were adjusted to produce NCs with 13 tector operating at 20 kV to assess composition. Powder different sizes (see experimental section for details, Fig- 14 X-ray diffraction (XRD) patterns were collected directly ures 1 and S1). HRTEM analysis showed the NCs to have a 15 from the as-synthesized NPs dropped on Si (501) sub- crystallographic structure compatible with that of cubic 16 strates on a Bruker AXS D8 Advance X-ray diffractometer Pd3Pb, [Pm-3m]-space group 221, and to be strongly facet- 17 with Ni-filtered (2 μm thickness) Cu Kα radiation (λ = ed in the [100] directions (Figure 1d). XRD analysis con- 18 1.5406 Å) operating at 40 kV and 40 mA. A LynxEye linear firmed the cubic Pd3Pb intermetallic structure (JCPDS 01- 19 position-sensitive detector was used in reflection geome- 089-2062, Figure 1e) and displayed the ratio of peak in- 20 try. Characterization of the surface was done by X-ray tensities I(200)/I(111) and thus the [100] faceting to increase 21 photoelectron spectroscopy (XPS) on a SPECS system with the NC size. EELS chemical composition maps 22 equipped with a XR50 source operating at 250 W and a demonstrated a homogeneous distribution of the two 23 Phoibos 150 MCD-9 detector. The pass energy of the hem- elements within each NC (Figure 1f) with a ratio Pd/Pb≈3, 24 ispherical analyzer was set at 20 eV and the energy step of which matched well with results obtained from HRTEM, high-resolution spectra was set at 0.05 eV. The pressure in 25 XRD, ICP and EDS analyses (Figure S2). the analysis chamber was always below 10-7 Pa. Binding 26 27 28 a b c 29 30 31 32 33 34 35 36 100 nm 100 nm 100 nm 37 38 d e Pd3Pb 10 nm f Pd 39 40 Intensity (a.u.) 41 Pd3Pb 8 nm 42 2 nm 2 nm 100 nm 10 nm Pd3Pb 6 nm 43 Pb Pd-Pb (010) 44 111 Pd3Pb 89-2062 (110) 200 45 110 210 211 220 311 300 310 46 (100) 30 40 50 60 70 80 47 2Theta (degree) 10 nm 10 nm [001] Pd3Pb 48 49 Figure 1. (a-c) TEM micrographs of 6 nm (a), 8 nm (b) and 10 nm Pd 3Pb NCs (c). (d) HRTEM, zoomed HRTEM detail after fre- 50 quency filtering with spot masks, power spectrum (FFT), and crystallographic unit cell (yellow for Pd and blue for Pb) of the 6 51 nm Pd3Pb NC. HRTEM analysis showed this NC to have a lattice parameter a=b=c=0.4035 nm and α=β＝γ＝90° as visualized 52 along the [001] direction of the Pd3Pb cubic crystal structure. (e) XRD patterns of 6 nm, 8 nm and 10 nm Pd 3Pb NCs. (f) ADF- STEM image and EELS composition maps from the white square region for Pd, Pb and Pd-Pb of 6 nm Pd3Pb NCs. 53 54 OAm, OAc and TOP were all necessary components of cubic Pd3Pb NCs. OAm played two roles, as coordinating 55 the precursor reaction mixture to produce highly faceted solvent forming Pd-OAm complexes, and as mild reduc- 56 3 57 58 59 60 ACS Paragon Plus Environment Chemistry of Materials Page 4 of 12 ing agent. OAc also played two roles, dissolving PbO to the binding of Pb with oleic acid molecules on the NC 1 form Pb oleate and directing the shape of the growing surface and to the surface oxidation of the NCs during 2 NCs by selective surface binding.38-40 When no OAc was handling and transportation. The ratio Pb0/Pbx+ was 3 added to the precursor reaction mixture, NCs with irregu- 1.55, i.e. 40% of the Pb within the 2-3 nm-thick outer layer 4 lar geometries and large size distributions were produced of the Pd3Pb NCs was oxidized. This percentage was sig- 5 (Figure S3a). With the addition of small amounts of OAc, nificantly higher than that of Pd. Besides, the Pd/Pb sur- 6 OAc/OAm=0.05 to 0.2 (v/v), Pd3Pb NCs with uniform face ratio obtained by XPS was 1.19, much lower than the 7 cubic shapes were produced (Figures 1 and S3b, c). stoichiometric values deduced from EDS, ICP, XRD and 8 Without TOP, the formed Pd-OAm complex decom- HRTEM analyses. These results indicated a segregation of 9 posed at a low temperature, ca. 100 C,41 yielding ele- Pb atoms to the NC surface during the NC synthesis. The 10 mental Pd NPs, i.e. with no Pb alloying. TOP is known to radial gradient of Pb could be also in part driven by the 11 strongly coordinate with Pd to form a Pd-TOP complex.42 surface oxidation during manipulation and storage of the 12 The decomposition of this Pd complex, which is more samples in ambient conditions, due to the lower oxida- 13 stable than Pd-OAm, takes place at higher temperatures, tion potential of Pb when compared with Pd. 14 200 C, facilitating the simultaneous reduction of Pd and a Pd 3d3/2 Pd 3d5/2 b Pb 4f5/2 Pb 4f7/2 15 Pb precursors to form intermetallic Pd3Pb NCs. Thus, a Intensity (a.u.) 16 Intensity (a.u.) large excess of TOP, TOP/Pd=15, was introduced in all 17 batches. Besides playing a fundamental role in the control 18 of the NC composition, TOP was also able to direct the 19 NC shape. In the presence of too small amounts of TOP, 20 irregular-shaped NCs were formed (Figure S3d), and 21 when too larger amounts of TOP were added, instead of 22 cubic NCs, nanoplates/nanorods were grown (Figures S3e and S3f). 344 342 340 338 336 334 146 144 142 140 138 136 23 Binding energy (eV) Binding energy (eV) 24 Finally, the introduction of MAC and the replacement 25 of PbO by Pb(OAc)4 resulted in larger NCs (Figures 1b,c). Figure 2. High resolution XPS spectra of 6 nm Pd3Pb NCs in 26 We hypothesize this effect to be associated to an increase the regions (a) Pd 3d and (b) Pb 4f. 27 of the precursor stability in the reaction mixture and a 28 related reduction of the number of nucleation events 43,44 CV analysis. 29 The PbO-MAC system reacted at 200 °C, while the Pd3Pb-based electrocatalysts were prepared by mixing 30 Pb(OAc)4-MAC system required higher temperatures, up colloidal Pd3Pb NCs with carbon black, 1:1 wt%. FTIR 31 to 260 °C, to form Pd3Pb NCs. analyses confirmed the removal of organic ligands from 32 Figure 2a shows the Pd 3d XPS spectrum of Pd3Pb NCs, the NC surface through the treatment with acetic acid 33 displaying the Pd 3d5/2 peak at 335.3 eV, i.e. shifted around and the posterior annealing at 250 C (Figure S4). Initial 34 0.4 V with respect to Pd0 (334.9 eV).45 This binding energy CV analyses were carried out at a sweep rate of 50 mV s-1 35 shift was consistent with the presence of more electro- with an argon-saturated 0.5 M KOH solution. Figure 3a 36 negative Pb that effectively modified the electronic struc- depicts the CV curves recorded for Pd3Pb/C catalysts with 37 ture of Pd.30 A second minor component, accounting for different NC sizes and for a commercial Pd/C catalyst 38 10% of the Pd observed, was associated to Pd2+. This com- tested as a reference. The same total amount of metal was 39 ponent was related to the binding of surface Pd with sur- used to prepare each of the tested electrodes, which 40 factant molecules and potentially to a slight surface oxi- translated in significantly lower amounts of Pd on the 41 dation of the NCs during manipulation and transporta- Pd3Pb/C catalysts than on the reference Pd/C. 42 tion before XPS measurements. The current density peaks in the region between -0.6 V 43 In the Pb 4f XPS spectrum, three 4f doublets were iden- and -0.8 V vs. Hg/HgO were attributed to the adsorption 44 tified, corresponding to three Pb chemical states (Figure (cathodic scan) and desorption (anodic scan) of hydro- 45 2b). The main peaks at 141.6 eV (4f5/2) and 136.7 eV (4f7/2) gen. The current density increase measured at ca. -0.2 V 46 were associated to Pb0, although were shifted -0.2 eV with vs. Hg/HgO in the anodic scan was attributed to the for- 47 respect to the reference value for elemental Pb 4f7/2 (136.9 mation of palladium oxide on the NC surface. Subse- 48 eV),45 consistently with the electronic interaction between quently, a PdO reduction peak appeared at about -0.2 V 49 Pd and Pb atoms within the Pb3Pb alloy. A second com- vs. Hg/HgO during the negative scans. ponent was located at 143.0 eV (4f5/2) and 138.1 eV (4f7/2), 50 Compared with Pd/C, Pd3Pb/C catalysts displayed less and it was assigned to an oxidized Pb state, Pbn+. A third 51 obvious peaks associated to hydrogen desorption and component appeared at even higher binding energies, 52 adsorption, owing to the lower amount of Pd on their 139.1 eV (4f7/2) and 144.0 eV (4f5/2), and it was associated 53 surface. On the other hand, the intensity of the Pd oxide with a Pb2+ chemical state.45 The oxidized Pb components, 54 reduction peak clearly increased with the addition of Pb which we will globally refer to as Pbx+, were associated to 55 and when increasing the NC size. Besides, the maximum 56 4 57 58 59 60 ACS Paragon Plus Environment Page 5 of 12 Chemistry of Materials of this peak was shifted toward higher potentials on 10 nm Pd3Pb/C catalysts were 3.3 %, 3.7 %, 4.2 %, and 5.4 1 Pd3Pb/C catalysts compared to Pd/C. The distinct anodic %, respectively. 2 and cathodic characteristics of Pd3Pb- and Pd-based cata- 3 lysts were related to their different electronic structure EOR activity, CV analysis. 4 involving partial electron donation from Pd to Pb sites, 5 consistent with XPS results.46 The electrocatalytic activity of Pd3Pb-based catalysts 6 towards EOR was investigated by CV in a solution con- The electrochemically active surface area (ECSA) of the 7 taining 0.5 M KOH and 0.5 M EtOH. Figure 4a and 4b catalysts was estimated from the coulombic charge for the 8 display the CV curves of Pd3Pb/C and Pd/C catalysts after reduction of PdO, i.e. from the area over the voltammetry 9 30 and 900 cycles at a sweep rate of 50 mV s-1, respective- curve in the PdO reduction peak region:15 10 ly. In the presence of EtOH, hydrogen absorption- Q ( C  cm 2 ) desorption peaks were suppressed due to the dominant 11 ECSA  12 QPdO ( C  cm )  Pd loading(mg  cm 2 ) 10 2 adsorption of EtOH in the low-potential region. The posi- Pd (1) tive scan peak related to the oxidation of freshly adsorbed 13 where QPdO = 405 μC cm-2 was the charge value given alcohol started at -0.6 V and reached its maximum at -0.1 14 for the reduction of a PdO monolayer, the coulombic V. At higher voltages, the catalytic activity decreased due 15 charge Q was calculated by integrating the area of the to the progressive oxidation of the Pd surface. The oxida- 16 PdO reduction peak, and Pdloading was the Pd mass on the tion peak in the cathodic scan, associated with the oxida- 17 working electrode. ECSA values for 10 nm Pd3Pb/C were tion of freshly adsorbed EtOH and of unreacted species 18 larger than that of 8 nm and 6 nm. The observed increase adsorbed before or after Pd–O blocking,47 was triggered 19 of ECSA with the NC size was unintuitive taking into by the reduction of the oxide layer grown in the preceding 20 account the decrease of surface area of the NCs when anodic scan.48 The peak current densities of Pd3Pb/C 21 increasing their size. The higher ECSAs measured for the catalysts were much higher than those of Pd/C, and the 22 larger particles must be thus indirectly related to their onset potential for EtOH oxidation of Pd3Pb/C catalysts 23 size through a decrease of the agglomeration when sup- was more negative than for Pd/C, what suggested a more 24 ported on CB, an improved crystallinity/faceting, and favorable EOR on the surface of Pd3Pb/C catalysts than on 25 potential differences in surface composition. Besides, the Pd/C. For comparison, Table S1 shows a resume of results 26 ECSA was significantly larger for all Pd3Pb/C catalysts reported in the literature on Pd3Pb NCs for EOR. 27 compared with Pd/C (14.9 m2 g-1) in spite of the higher Figure 4c exhibits the forward oxidation peak current 28 amount of exposed Pd on Pd/C. ECSA increased during density for EOR as a function of cyclic scan number. Dur- 29 the first cycles for all Pd3Pb/C catalysts, but not for com- ing the first cycles the forward peak current density in- 30 mercial Pd/C catalyst. The ECSA increase with cycling on creased to later decrease gradually with the continued 31 Pd3Pb/C could be related to a reorganization of the sur- scanning. Such a general trend is often observed in elec- 32 face composition and potentially to a stripping of remain- trocatalytic alcohol oxidation and is usually ascribed to a 33 ing surface ligands. structural or surface reorganization of the catalyst.49,50 For 34 a Pd/C, the current density decay started at the 50th cycle, 35 0.1 b 35 dropping from 12.53 to 5.38 mA cm-2 after 900 cycles. A 30 36 10 nm Pd3Pb/C very similar evolution was measured from 10 nm Pd3Pb/C, ECSA (m2 gPd-1) 0.0 Current (mA) 37 Pd/C 25 8 nm Pd3Pb/C with a progressive increase of current density up to ap- 38 -0.1 6 nm Pd3Pb/C proximately the 50th cycle, yet with significantly higher 20 39 8 nm Pd3Pb/C 6 nm Pd3Pb/C current densities, from 21.82 to 11.72 mA cm-2. 6 nm and 8 -0.2 40 10 nm Pd3Pb/C 15 nm Pd3Pb/C catalysts were characterized by a significant- 41 -0.3 10 Pd/C ly different evolution of the forward peak current density 42 -0.8 -0.4 0.0 0.4 0 10 20 30 40 50 with the cycle number. 6 nm and 8 nm Pd3Pb/C catalysts 43 Potential vs Hg/HgO (V) Cycle number displayed a progressive increase of the current density up 44 to the 400th-500th cycle and a quasi-stabilization of the 45 Figure 3. (a) CV curves of 6 nm, 8 nm, 10 nm Pd 3Pb/C and activity at current densities around 20 mA cm-2 with addi- 46 Pd/C catalysts. (b) ECSA values of 6 nm, 8 nm, 10 nm tional cycling. Thus, while 10 nm Pd3Pb/C displayed high- 47 Pd3Pb/C and Pd/C catalysts at different CV cycles. er catalytic activity in the initial CV cycles, 6 nm and 8 nm Pd3Pb/C had much longer durability and higher activ- 48 ities after longer cycling. In terms of mass current density 49 After 30 CV cycles, the ECSA obtained from commercial (Figure 4d), after 30 cycles 10 nm Pd3Pb/C catalyst showed 50 Pd/C, and for 6 nm, 8 nm and 10 nm Pd3Pb/C catalysts the highest values up to 2.05 A mg-1Pd, while the perfor- 51 were 14.9 m2 g-1, 16.4 m2 g-1, 18.9 m2 g-1, and 24.2 m2 g-1, mance of 6 nm and 8 nm Pd3Pb/C was slightly lower than 52 respectively. The Pd utilization effectiveness was estimat- that of Pd/C. However, after 900 cycles, 8 nm Pd3Pb/C 53 ed considering that the active surface area for full utiliza- catalyst displayed much larger mass current densities up 54 tion of 1 g of Pd would be 448 m2. Thus, the Pd utilization to 2.05 A mg-1Pd. 55 efficiencies of commercial Pd/C, and for 6 nm, 8 nm and 56 5 57 58 59 60 ACS Paragon Plus Environment Chemistry of Materials Page 6 of 12 The ratio of the maximum intensities measured for the from −0.5 to -0.3 V vs. Hg/HgO for Pd3Pb/C and Pd/C 1 forward and reverse peaks, If/Ir, of Pb3Pb/C catalysts also catalysts. From the 30th cycle, the Tafel slopes were: 162 2 changed with the cycling number, decreasing for 6 nm mV dec−1 for Pd/C, 156 mV dec−1 for 8 nm, 145 mV dec−1 for 3 Pd3Pb/C from 1.1 to 0.90, and 8 nm Pd3Pb, from 1.3 to 0.85, 6 nm and 140 mV dec−1 for 10 nm. After 900 cycles, all the 4 but increasing for 10 nm Pd3Pb, from 0.82 to 0.93 and Tafel slopes increased: 203 mV dec−1 for Pd/C, 173 mV dec−1 5 increasing very moderately for Pd/C, from 0.90 to 0.91. for 6 nm, 164 mV dec−1 for 8 nm and 160 mV dec−1 for 10 6 This ratio is related to the reactivation efficiency, the nm. All Pd3Pb/C catalysts displayed lower Tafel slopes 7 higher the reverse peak, the higher the reactivation, and thus faster charge-transfer kinetics than Pd/C. On the 8 which denoted an improvement of the reactivation with other hand, the increase of the Tafel slopes with the scan 9 cycling for the smallest NCs and a degradation for the number revealed a performance decay for all the catalysts, 10 largest one.48 being the 8 nm Pd3Pb/C catalyst the one showing the best 11 Figure 4e shows the linear region of the Tafel plots ob- stability. 12 tained from the 30th and 900th CV curves in the range 13 a 25 b 25 c 25 30th 900th 14 Current density (mA cm-2) 8 nm Pd3Pb/C Current density (mA cm-2) Current density (mA cm-2) 20 20 8 nm Pd3Pb/C 20 15 10 nm Pd3Pb/C 6 nm Pd3Pb/C 16 15 15 6 nm Pd3Pb/C 15 6 nm Pd3Pb/C 17 10 10 10 nm Pd3Pb/C 10 10 nm Pd3Pb/C 18 Pd/C 5 19 5 8 nm Pd3Pb/C Pd/C 5 20 0 Pd/C 0 0 21 -0.8 -0.4 0.0 0.4 -0.8 -0.4 0.0 0.4 0 200 400 600 800 1000 22 Potential vs. Hg/HgO (V) Potential vs. Hg/HgO (V) Cycle number 23 24 d 3.0 30th e 30th 900th f 20 m) Current density (mA cm-2) 25 8n Potential vs. Hg/HgO (V) Current density (A mgPd ) -1 2.5 900th unit: mV dec 4( -1 -0.35 16 ) 15 10 nm Pd3Pb/C Pd 26 2.0 m) 20 3( 8n 27 1.5 1 56( m) 10 -0.40 6n 28 7 3 ( m) 1 0n 8 nm Pd3Pb/C (1 29 1.0 ) 6 nm Pd3Pb/C (Pd 160 5 2 m) ) 16 nm 30 0.5 -0.45 45( 6n 0 (10 Pd/C 1 14 31 0.0 0 32 Pd/C 6 nm 8 nm 10 nm -0.4 -0.3 -0.2 -0.1 0.0 0.1 0 2000 4000 6000 -2 33 Log (mA cm ) Time (s) 34 th Figure 4. (a) 30 CV curves of 6 nm, 8 nm and 10 nm Pd3Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH aqueous solution. th 35 (b) 900 CV curves of 6 nm, 8 nm and 10 nm Pd3Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH aqueous solution. (c) 36 Current density of 6 nm, 8 nm, 10 nm Pd3Pb/C and Pd/C catalysts in 0.5 M KOH + 0.5 M EtOH solution at potential of -0.1 V vs. 37 Hg/HgO for different CV cycles. (d) Mass peak current density of 6 nm, 8 nm, 10 nm Pd3Pb/C and Pd/C catalysts in 0.5 M KOH + th th 38 0.5 M EtOH aqueous solution from 30 and 900 CV cycles. (e) Tafel plots of 6 nm, 8 nm, 10 nm Pd3Pb/C and Pd/C catalysts in th th 0.5 M KOH + 0.5 M EtOH aqueous solution from 30 and 900 cycles, respectively. (f) CA measurements of 6 nm, 8 nm and 10 39 nm Pd3Pb/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. 40 41 42 Differences between Pd3Pb/C catalysts containing The stability of the catalyst performance was further in- 43 Pd3Pb NCs of different sizes may be related to several vestigated by CA. Figure 4f displays CA results obtained at 44 interconnected mechanisms: i) The higher activity of the -0.1 V vs. Hg/HgO in a 0.5 M KOH + 0.5 M EtOH aqueous 45 10 nm NCs had associated the generation of a higher solution. All the catalysts exhibited a pronounced current 46 amount of intermediates that may poison the catalyst decay during the first 500 s. This rapid activity decay, 47 surface more rapidly than in less active catalysts; ii) The characteristic of EOR catalyst, including Pd-based cata- 48 oxidation and reduction of surface Pd during each scan lysts, remains as a major drawback toward the commer- 49 and the development of channels of electron collection cialization of DEFCs. While all the Pd3Pb/C catalysts 50 could rearrange the catalyst structure and NC surface.51 showed enhanced EOR activity and better stability than 51 This process may be significantly different in NCs with Pd/C catalyst, 10 nm Pd3Pb/C catalysts displayed a more different sizes; iii) Depending on the NC size, the extend abrupt decrease in current densities than 6 nm and espe- 52 of material oxidation and reduction could be significantly cially 8 nm Pd3Pb/C catalysts. Replacing the electrolyte by 53 different. a fresh solution did not provide a significant increase in 54 EOR activity, CA analysis. current density, demonstrating that the EtOH depletion 55 56 6 57 58 59 60 ACS Paragon Plus Environment Page 7 of 12 Chemistry of Materials was not the reason behind the large current density de- a 30 b 30 10 nm Pd3Pb/C 8 nm Pd3Pb/C 1 Current density (mA cm-2) crease. Instead, we succeeded in reactivating our Pd3Pb/C Current density (mA cm-2) 2 and Pd/C catalysts by either cycling few times the applied 20 20 3 potential (Figure 5) or by slightly increasing the electrode 4 potential for a short time (Figure S5). Figure 5 displays the 5 current density decay during periods of 1000 s for 10 nm, 8 10 10 6 nm, 6 nm Pd3Pb/C catalysts and Pd/C catalysts. In be- 7 tween these 1000 s periods, 3 CV cycles in the range from 0 0 8 -0.9 to 0.3 V vs. Hg/HgO with a rate of 50 mV s-1 for a 0 1000 2000 3000 4000 0 1000 2000 3000 4000 Time (s) 9 total time of 144 s were applied. 10 nm Pd3Pb/C catalyst Time (s) c 30 d 30 10 reached the highest current densities after each reactiva- 6 nm Pd3Pb/C Pd/C Current density (mA cm-2) Current density (mA cm-2) 11 tion. Figure S5 displays the current densities evolution 12 during periods of 1000 s and reactivation by applying a 20 20 13 voltage of 0.3 V vs. Hg/HgO for 100 s, i.e. by oxidizing and 14 later reducing the Pd surface. The current density decay 10 10 15 could be fitted considering at least two exponential de- 16 cays, one with a characteristic time in the range of ca. 10- 17 30 s, and a second one with a longer characteristic time 0 0 0 1000 2000 3000 4000 0 1000 2000 3000 4000 up to 500 s (Figure S6 and Table S2). We systematically 18 Time (s) Time (s) observed that the larger the NCs, the lower the contribu- 19 Figure 5. CA curves of 10 nm (a), 8 nm (b), 6 nm (c) Pd 3Pb/C tion of the fast exponential decay. Besides, an increase of 20 and Pd/C (d) catalysts. Three CV cycles were conducted to the reaction temperature up to 50 C allowed to slightly 21 reactivate the catalyst every 1000 s. increase the catalyst activity and extend at the same time 22 its stability (Figure S7 and Table S3). 23 Structural and chemical evolution. 24 In alkaline media, EtOH is dehydrogenated into ad- sorbed acetyl, which is further oxidized to acetate by HRTEM and HAADF-STEM analysis of the Pd3Pb/C 25 26 hydroxide species.10, 52 The oxidation of the acetyl to acetic catalysts after cycling in 0.5 M KOH and 0.5 M KOH + 0.5 acid by adsorbed hydroxyl is regarded as the rate- M EtOH aqueous solutions showed the NCs to maintain a 27 high crystallinity and to conserve the Pd3Pb intermetallic determining step, while the stripping of the acetic acid in 28 the form of acetate ions in alkaline solution is very rapid.10 phase (Figure 6). However, extensive TEM analysis of the 29 catalyst after cycling showed the cubic shape of the NCs Acetaldehyde and reaction products such as CH and C 30 to be partially lost for a significant number of NCs (Fig- may block additional sites, although breaking of the C-C 31 bond is considered to have a minor contribution on the ures S7-13). 32 obtained current density.52,53 XPS analyses of the Pd3Pb catalyst after cycling in 0.5 M 33 Cycling contributes to clean the Pd surface through dif- KOH and in 0.5 M KOH + 0.5 M EtOH aqueous solutions 34 showed a notable increase of the surface Pd/Pb atomic ferent mechanism: i)the oxidation of the acetyl to acetate 35 is favored by oxidative potentials;10 besides ii) the oxida- ratio, from the initial Pd/Pb=1.19 obtained from Pd3Pb 36 NCs to Pd/Pb=3.09 of the Pd3Pb/C catalyst after cycling in tion of the catalyst surface may contribute to the strip- 37 a KOH + EtOH aqueous solution (Figure S8, Table S4). ping of adsorbed molecules. In this process the concen- 38 This decrease of the relative Pb surface concentration tration and surface distribution of Pd and Pb, the overall 39 surface area and the exposed facets play important roles. could be related to a partial dissolution of Pb, a phase 40 During EOR and during cycling all of these parameters segregation, or a redistribution of the elements within 41 may change. During the electrochemical reaction, Pd and each NC. 42 Pb most probably redistribute within each particle as EDS analysis showed the total Pd/Pb ratio to slightly 43 evidenced below, and even within the surface as noted by increase after cycling in a 0.5 M KOH + 0.5 M EtOH 44 Gunji et al.54 Changes of surface composition modifies the aqueous solution, from Pd/Pb=3.03 to Pd/Pb=3.42, or 45 equilibrium coverage of hydroxyl and adsorbed acetyl, after cycling in KOH, Pd/Pb=3.77. This increase must be 46 thus changing the overall current density.10 Besides, pos- assigned to a leaching of Pb atoms from the Pd3Pb alloy to 47 sible changes of particle geometry modify the exposed the electrolyte. However, the amount of leached Pb, 12 % 48 facets and thus the exposed distribution of Pb and Pd after cycling in a KOH + EtOH aqueous solution, does not 49 sites. fully account for the much larger variations obtained on 50 the surface composition. Besides, STEM-EDS analysis of 51 the catalyst after 30 CV cycles in 0.5 M KOH aqueous 52 solution (Figure S9) and EELS analysis after cycling in 53 KOH + EtOH aqueous solutions (Figure 6) displayed no 54 phase segregation, being both elements homogeneously 55 56 7 57 58 59 60 ACS Paragon Plus Environment Chemistry of Materials Page 8 of 12 distributed within each NC and with the same ratio from However, we believe that a significant part of the ele- 1 particle to particle. mental restructuration was driven by the oxida- 2 These results pointed out at a reorganization of the two tion/reduction of Pd. During cycling, only Pd was oxi- 3 elements within the Pd3Pb NCs during cycling in a KOH + dized and reduced, while surface Pb atoms remained in 4 EtOH aqueous solution, involving an outward/inward an oxidized chemical state. In a previous work, we 5 diffusion of Pd/Pb to equilibrate the stoichiometry of the showed how cobalt and copper can reorganize by interfu- 6 NCs surface, which was highly Pb-rich in the as- sion during oxidation and reduction processes. During 7 synthesized materials. oxidation Co diffused outward to form a CuO-Co3O4 hol- 8 low structures, while during reduction Cu diffused inward A partial surface reorganization of the NC surface took 9 to form Cu@Co3O4 core-shell particles.55 In the present place already during their use to formulate the catalyst, 10 and could be related with the removal of organic ligands case we believe a similar process takes place. The initial 11 that were preferentially bond to Pb atoms. Additional NC surface is enriched with Pb due to the larger affinity of 12 this element toward organic ligands and oxygen. Once decrease of the Pb surface concentration was observed 13 removed the organics, Pd diffuses outwards/inwards dur- during cycling in KOH (Table S4, Scheme S1). This de- 14 ing cycling, driven by the oxidation/reduction processes. crease was partially correlated with EDS results that 15 Since the largest increase of the surface Pd/Pb ratio was showed an important Pb leaching during KOH cycling 16 obtained after cycling in the presence of EtOH, we need that could account for the increase of the Pd/Pb ration 17 to assume that the higher affinity of Pd to EtOH may also obtained after this process. play an important role in the surface restructuration. 18 19 a b c Pd d Pb 20 21 22 23 24 25 26 27 e f g Pd h Pb 28 29 30 31 32 33 34 35 Figure 6. (a) TEM and (b) HAADF-STEM micrographs of 6 nm Pd3Pb/C catalyst. HAADF-EDS mappings of (c) Pd and (d) Pb 36 from 6 nm Pd3Pb/C catalyst. (e) TEM and (f) HAADF-STEM micrographs of 6 nm Pd3Pb/C catalyst after 30 CV cycles in a 0.5 37 KOH + 0.5 M EtOH aqueous solution. HAADF-EDS mappings of (g) Pd and (h) Pb from 6 nm Pd3Pb/C catalyst after 30 CV cycles 38 in a 0.5 KOH + 0.5 M EtOH aqueous solution. All scale bars are 50 nm. 39 40 41 CONCLUSIONS Such simple cleaning processes, that can be performed 42 In summary, we report a simple approach to produce during operation breaks without cell disassembly, was 43 intermetallic Pd3Pb NCs with well-defined cubic geometry sufficient to effectively remove the poisoning species 44 and average sizes in the range from 6 nm to 10 nm. Com- adsorbed on the surface and recover the electrocatalytic 45 pared with commercial Pd/C catalyst, Pd3Pb/C catalysts activity. We finally demonstrated that during cycling a 46 presented improved EOR electrocatalytic activities and major redistribution of the elements within the NCs took 47 stabilities. 10 nm Pd3Pb/C catalyst provided the largest place, driven by the different affinity of Pb and Pd to- wards oxygen and possibly ethanol, and the electrochemi- 48 initial current densities, but smaller NCs were able to cal oxidation/reduction of Pd. 49 reach higher current densities after extended cycling. All 50 the catalysts exhibited a pronounced current decay dur- ing the first 500 s of continuous EOR operation, which ASSOCIATED CONTENT 51 was associated to the accumulation of strongly adsorbed Supporting Information. 52 reaction intermediates and the related blockage of reac- This material is available free of charge via the Internet at 53 http://pubs.acs.org.” Characterization (TEM images, EDS, 54 tion sites. The catalyst could be reactivated by simple cycling, but this reactivation was also size dependent. XPS), electrochemistry and scheme. 55 56 8 57 58 59 60 ACS Paragon Plus Environment Page 9 of 12 Chemistry of Materials AUTHOR INFORMATION Electrochim. Acta 2006, 52, 1087–1091. 1 https://doi.org/10.1016/j.electacta.2006.07.008. 2 Corresponding Author (8) Ma, L.; Chu, D.; Chen, R. Comparison of Ethanol Elec- 3 * E-mail: jliu@ujs.edu.cn (J. L.) tro-Oxidation on Pt/C and Pd/C Catalysts in Alkaline Media. Int. J. Hydrogen Energy 2012, 37, 11185–11194. 4 * E-mail: acabot@irec.cat (A. C.) https://doi.org/10.1016/j.ijhydene.2012.04.132. 5 ORCID (9) Ma, X. Y.; Chen, Y.; Wang, H.; Li, Q. X.; Lin, W. F.; Cai, 6 W. B. Electrocatalytic Oxidation of Ethanol and Ethylene Glycol Xiaoting Yu: 0000-0003-0457-4047 7 Junfeng Liu: 0000-0003-3164-6472 on Cubic, Octahedral and Rhombic Dodecahedral Palladium 8 Andreu Cabot: 0000-0002-7533-3251 Nanocrystals. Chem. Commun. 2018, 54, 2562–2565. 9 https://doi.org/10.1039/c7cc08793d. Author Contributions (10) Liang, Z. X.; Zhao, T. S.; Xu, J. B.; Zhu, L. D. Mecha- 10 nism Study of the Ethanol Oxidation Reaction on Palladium in 11 The manuscript was written through contributions of all Alkaline Media. Electrochim. Acta 2009, 54, 2203–2208. 12 authors. https://doi.org/10.1016/j.electacta.2008.10.034. 13 Notes (11) Bianchini, C.; Shen, P. K. Palladium-Based Electrocata- 14 The authors declare no competing financial interest. lysts for Alcohol Oxidation in Half Cells and in Direct Alcohol 15 Fuel Cells. Chem. Rev. 2009, 109, 4183–4206. 16 ACKNOWLEDGMENT https://doi.org/10.1021/cr9000995. (12) Iqbal, M.; Kaneti, Y. V.; Kim, J.; Yuliarto, B.; Kang, Y.- 17 This work was supported by the European Regional Devel- M.; Bando, Y.; Sugahara, Y.; Yamauch, Y. Chemical Design of 18 opment Funds and by the Spanish Ministerio de Economía y Palladium-Based Nanoarchitectures for Catalytic Applications. 19 Competitividad through the project SEHTOP, ENE2016- Small 2019, 1804378. https://doi.org/10.1002/smll.201804378. 20 77798-C4-3-R and ENE2017-85087-C3. X. Yu thanks the Chi- (13) Sadiki, A.; Vo, P.; Hu, S.; Copenhaver, T. S.; Scudiero, 21 na Scholarship Council for the scholarship support. Authors L.; Ha, S.; Haan, J. L. Increased Electrochemical Oxidation Rate 22 acknowledge funding from Generalitat de Catalunya 2017 of Alcohols in Alkaline Media on Palladium Surfaces Electro- SGR 327 and 2017 SGR 1246. ICN2 acknowledges support chemically Modified by Antimony, Lead, and Tin. Electrochim. 23 from the Severo Ochoa Programme (MINECO, Grant no. Acta 2014, 139, 302–307. 24 https://doi.org/10.1016/j.electacta.2014.07.019. SEV-2017-0706) and is funded by the CERCA Pro- 25 (14) Luo, Z.; Lu, J.; Flox, C.; Nafria, R.; Genç, A.; Arbiol, J.; gramme/Generalitat de Catalunya. J. Llorca is a Serra Húnter 26 Fellow and is grateful to ICREA Academia program and to Llorca, J.; Ibáñez, M.; Morante, J. R.; Cabot, A. Pd2Sn [010] Nano- 27 MINECO/FEDER grant RTI2018-093996-B-C31 and GC 2017 rods as a Highly Active and Stable Ethanol Oxidation Catalyst. 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