Article pubs.acs.org/cm
Enhanced Electrocatalytic Activity of Carbon-Supported Ordered Intermetallic Palladium−Lead (Pd3Pb) Nanoparticles toward Electrooxidation of Formic Acid Takao Gunji,*,† Seung Hyo Noh,‡ Toyokazu Tanabe,† Byungchan Han,‡ Chiao Yin Nien,† Takeo Ohsaka,† and Futoshi Matsumoto*,† †
Department of Material and Life Chemistry, Kanagawa University, 3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama-shi, Kanagawa 221-8686, Japan ‡ Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea S Supporting Information *
ABSTRACT: Nanosized ordered intermetallic Pd3Pb nanoparticles (NPs)/carbon black (CB) (1−8 nm), Pd3Pb NPs/CB, in which Pd3Pb has a Cu3Au-type structure and its NPs are supported on CB, were prepared by the polyol method under an air atmosphere and characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS). The XRD and XPS measurements confirmed the formation of ordered intermetallic Pd3Pb NPs with a super lattice phase, and the TEM and STEM images indicated a relatively uniform dispersion of Pd3Pb NPs on the CB surface with an average size of 4.3 nm and an atomic ratio (Pd:Pb) of 75.9:24.1. The surface of the as-prepared Pd3Pb NPs/CB was found to be covered with the Pb (and its oxide) layer and to possess actually no electrocatalysis for the electrooxidation of formic acid (FA). However, this “inactive” as-prepared Pd3Pb NPs/CB could be changed drastically to the “active” one with a high level of electrocatalysis by the electrochemical treatment using cyclic voltammetry, i.e., the pertinent electrooxidation of the Pb surface coating in a 0.1 M HClO4 aqueous solution. The atomic-resolution STEM measurements confirmed that the surface state of the “inactive” as-prepared Pd3Pb NPs/CB can be controlled by changing the number of potential scans employed in the electrochemical treatment. That is, when the potential scan number is suitably chosen, the surface covered with the Pb coating dissolves and becomes an active, ideal structure of Pd3Pb, and further scanning leads to a surface close to that of Pd NPs. The thus electrochemically treated ideal Pd3Pb NPs/CB possessed a largely higher level of electrocatalysis for the FA oxidation than Pd NPs/CB, which could be explained reasonably on the basis of the experimentally measured and/or theoretically calculated d-band center values of both catalysts and CO binding energies on them.
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INTRODUCTION In recent years, direct fuel cells (DFCs) have attracted a great deal of interest as promising energy conversion devices because of their high efficiency and high power density in the field of polymer electrolyte membrane fuel cells. Low-molecular weight organic molecules, such as formic acid (FA),1 methanol (MeOH),2 and ethanol (EtOH),3 are used as fuels for DFCs because a direct electrooxidation of these molecules is possible. In particular, FA has been receiving paramount attention as fuel for DFCs because of its high energy density (1740 Wh kg−1, 2086 Wh L−1) and because its fuel crossover is smaller than that of MeOH.1 In addition, the electrooxidation current density (and mass activity) of FA over palladium (Pd) nanoparticles (NPs) is higher than that of a platinum (Pt) electrocatalyst.4,5 Therefore, FA-based DFCs can be operated without using a Pt electrocatalyst. However, the electrocatalytic activity of Pd NPs is still insufficient. For the electrooxidation of FA, the following reaction mechanism is proposed:6,7 © 2017 American Chemical Society
HCOOH → CO2 + 2H+ + 2e−
(1)
HCOOH → CO + H 2O
(2)
CO + H 2O → CO2 + 2H+ + 2e−
(3)
Equation 1 is a direct pathway to produce CO2 by oxidizing FA. In the CO pathway (eqs 2 and 3), CO is produced as an intermediate by dehydration of FA (eq 2), and then CO is oxidized to CO2 (eq 3). When polycrystalline Pt is used as an electrocatalyst, the intermediate CO is adsorbed on the polycrystalline Pt surface, resulting in a lower electrocatalytic activity. In the case of a Pd electrocatalyst, Cai et al. reported that CO is formed by the electrochemical reduction of CO2 rather than the direct dehydration of FA on the Pd surface at potentials where adsorbed H species exist.8−10 Thus, developReceived: December 8, 2016 Revised: March 10, 2017 Published: March 10, 2017 2906
DOI: 10.1021/acs.chemmater.6b05191 Chem. Mater. 2017, 29, 2906−2913
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Chemistry of Materials
respectively. Pt NPs/CB and Pt−Ru NPs/CB (Pt loading of 20 wt %) were commercially available (E-TEK). Nafion (5 wt %; EW of 1100) was obtained from Sigma-Aldrich. Formic acid (98.0%), isopropanol (99.8%), and perchloric acid (60.0%) were purchased from Wako. Water was purified using a Millipore system (electrical resistivity of 18.2 MΩ cm at 25 °C). Preparation of Carbon-Supported Ordered Intermetallic Pd3Pb NPs/CB. The ordered intermetallic Pd3Pb NPs/CB were prepared by the polyol method under an air atmosphere. Commercially available 20 wt % Pd NPs/CB were used as the starting material. The 20 wt % Pd NPs/CB (0.040 g) and Pb(CH3COO)2 (0.074 mmol), which was used as a Pb precursor, were dissolved in 50 mL of ethylene glycol used as a solvent and reducing agent. The molar ratio of Pd to Pb was adjusted to be 3:1.1. The mixture was sonicated in a bath-type ultrasonicator. The crystal structure of Pd3Pb NPs/CB was controlled by microwave irradiation time. The ordered intermetallic Pd3Pb NPs/CB were obtained by irradiation with a microwave power of 300 W for 6 min. The obtained powder was washed three times using a centrifuge with MeOH and then dried under vacuum at room temperature overnight. Physical Characterization. The electrocatalysts were characterized by powder X-ray diffraction (pXRD) using a Rigaku Ultima III diffractometer, and diffraction patterns were collected at a scanning rate of 3° min−1. Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and elemental mapping profiles were performed on a 200 kV transmission electron microscope (JEMARM200F, JEOL) equipped with two aberration correctors (CEOS GmbH) for the image- and probe-forming lens systems, and an X-ray energy-dispersive spectrometer (JED-2300T, JEOL) was employed for compositional analysis. Both aberration correctors were optimized to obtain TEM and scanning transmission electron microscopy (STEM) resolutions of 1.3 and 1.1 Å, respectively. A probe convergence angle of 29 mrad and a high-angle annular dark field (HAADF) detector with an inner angle of >100 mrad were used for HAADF−STEM observation. The samples for TEM were prepared by dropping a methanol suspension of the Pd3Pb NPs/CB electrocatalyst powder onto a commercial TEM grid coated with a collodion film. X-ray photoelectron spectroscopy (XPS) measurements (JP-9010 MC, JEOL) were performed to examine the chemical states (Pd 3d and Pb 4f regions) of the electrocatalysts. Mg Kα radiation as an X-ray source with an anodic voltage (10 kV) and current (10 mA) was used for XPS measurements. All the XPS spectra of the samples were recorded with a takeoff angle of 45°. Hard X-ray photoemission spectroscopy (HAXPES) measurements were performed using an X-ray with a photon energy of 8.0 keV at BL47XU of SPring-8. The prepared catalyst powder was pasted on carbon tape. The sample was transferred into an ultra-high-vacuum (UHV) chamber equipped with an electron spectrometer. The binding energy of photoelectrons was referenced to the Fermi energy of an Au reference. Computational Details. To calculate total ground state energies, the Vienna ab initio simulation package (VASP)19 was utilized, and the generalized gradient approximation (GGA) was considered to deal with the exchange-correlation energy.20 The interaction potentials of the core electrons were substituted for the projector augmented wave (PAW) pseudopotential.21 Each atom was relaxed to obtain the optimized structure with a cutoff energy of 520 eV for the plane wave basis, keeping the fix of two bottom layers to retain the lattice constant of the bulk structure. A γ-point mesh of 5 × 5 × 1 k-points was used for the unit cell of the slab model. The global break condition of the ionic relaxation loop was 10−3 eV/unit cell in all the calculations. The tetrahedron method with Blö chl corrections was employed as implemented in VASP to calculate the density of states.22 Electrochemical Measurements. All the electrochemical measurements were taken with a VMP-3 potential workstation (EC-LAB) using a three-neck-type cell at room temperature. The ink of each electrocatalyst was prepared by mixing the commercially available catalyst or the electrocatalyst prepared in 2.2 (4.5 mg) with 0.995 mL of 2-propanol and 0.005 mL of Nafion. The glassy carbon (GC)
ment of FA-based DFCs to create an electrocatalyst that has a high tolerance for CO is strongly desired. To enhance the electrocatalytic activity of Pt, Pb was used as a secondary material for alloying with Pt to increase CO poisoning tolerance of Pt electrocatalysts.11,12 Recently, we have reported that ordered intermetallic PtPb efficiently electrocatalyzes the electrooxidation of FA, MeOH, and EtOH.13−15 In particular, the core−shell structure of bimetallic Pt3Pb has been found to possess an efficient electrocatalytic activity for the electrochemical oxidation of FA, MeOH, and EtOH.15,16 In addition, Hongsen and co-workers have found that when ordered intermetallic PtPb is used as an electrocatalyst for the electrooxidation of FA, adsorbed CO is not formed to such an extent that it poisons the surface, resulting in a high electrocatalytic activity toward the electrooxidation of FA.17 These indicate that the Pb atom in ordered intermetallic PtPb (and Pd3Pb) may play an essential role in the enhancement of not only electrocatalytic activity but also CO poisoning tolerance. Recently, Jana et al. have reported the synthesis of ordered intermetallic Pd3Pb nanocrystals and their morphology-dependent superior electrocatalytic activities toward oxidation of FA and ethanol.18 Thus, it is interesting to explore catalytic activity of nanosized ordered intermetallic Pd3Pb and correlation between the catalytic activity and surface structure of Pd3Pb NPs. This study aimed to prepare carbon-supported ordered intermetallic Pd3Pb NPs (Pd3Pb NPs/CB), in which Pd3Pb has a structure of the Cu3Au type (Pm3̅m; a = b = c = 0.4022 nm), via the polyol method under an air atmosphere without annealing. The as-prepared Pd3Pb NPs/CB shows only a poor electrocatalytic activity toward FA electrooxidation because its surface is covered with the inactive Pb and/or Pb oxide layer, which is formed by the segregation of Pb from the ordered intermetallic Pd3Pb and the subsequent oxidation. It is found that the inactive Pb and/or Pb oxide layer can be electrooxidized “suitably” in 0.1 M HClO4 by cyclic voltammetry up to 1.1 V [vs the reference hydrogen electrode (RHE)] to create a clean surface of Pd3Pb NPs with a high level of electrocatalysis; i.e., the electrocatalytic activity of the thus electro-treated Pd3Pb NPs/CB significantly depends on the degree of dissolution of Pb from the Pd3Pb. The thus-prepared ordered intermetallic Pd3Pb NPs/CB were characterized by using X-ray photoelectron spectroscopy (XPS), hard X-ray photoemission spectroscopy (HAXPES), scanning transmission electron microscopy (STEM), X-ray energy-dispersive spectrometry (STEM-EDS), and high-resolution transmission electron microscopy (HR-TEM). The electrocatalytic activity of Pd3Pb NPs/CB was compared with that of commercially available catalysts (i.e., Pt NPs/CB, PtRu NPs/CB, and Pd NPs/CB), demonstrating that suitably electro-treated ordered intermetallic Pd3Pb NPs/CB possess catalytic activity toward the electrooxidation of FA significantly higher than those of the other electrocatalysts; the catalytic activity decreases in the following order: Pd3Pb NPs/CB ≫ Pd NPs/CB ≫ PtRu NPs/ CB > Pt NPs/CB. The difference in electrocatalytic activity between Pd3Pb NPs/CB and Pd NPs/CB is explained successfully by theoretically calculating and/or experimentally obtaining the d-band centers and CO binding energies.
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EXPERIMENTAL SECTION
Materials. The Pb precursor, Pb(CH3COO)2·3H2O, and carbonsupported Pd NPs (Pd loading of 20 wt %) (Pd NPs/CB) were purchased from Wako Pure Chemicals Co. and Premetek Co., 2907
DOI: 10.1021/acs.chemmater.6b05191 Chem. Mater. 2017, 29, 2906−2913
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Chemistry of Materials electrode (diameter of 5 mm) was sonicated in water for 2 min, polished using 0.3 and 0.05 μm alumina (Extec and Buehler) slurries for 5 min, and sonicated for 10 min to remove any adsorbed impurities on the GC electrode with a 0.01 M NaOH solution. Five microliters of the prepared ink was dropped on the thus-prepared GC electrode and then air-dried. The catalyst-coated GC electrode, a Ag/AgCl/NaCl electrode (3 M), and a Pt wire were used as the working, reference, and counter electrodes, respectively. A 0.1 M HClO4 solution was purged with Ar gas for 15 min to remove any dissolved O2, and then the catalyst (Pd3Pb NPs/CB, Pd NPs/CB, Pt−Ru NPs/CB, or Pt NPs/CB)-coated GC electrode was activated by repeating the potential scan between 0.05 and 1.1 V (vs RHE) at a scan rate of 100 mV s−1 in a 0.1 M HClO4 solution under an Ar atmosphere. The electrode was then transferred into 0.1 M HClO4 and 0.5 M FA, and the anodic sweep was performed at 20 mV s−1 from −0.05 to 1.25 V (vs RHE). In every case, the anodic current was normalized on the basis of the weight of Pd or Pt coated on the electrode. CO stripping voltammograms were obtained in a 0.1 M HClO4 solution. The HClO4 solution into which catalyst-coated GC electrodes were soaked was purged with CO gas for 20 min while the electrode potential was kept at 0.05 V, and then it was purged with Ar gas for 20 min to remove dissolved CO. After that, CO stripping voltammograms were measured between 0.05 and 1.1 V at a scan rate of 20 mV s−1.
of a Cu3Au-type structure are successfully synthesized via irradiation for 6 min at a microwave power of 300 W. The typical TEM and HAADF−STEM images of Pd3Pb NPs/CB are shown in Figure 2. Pd3Pb NPs are relatively
Figure 2. (A) TEM and (B) atomic-resolution HAADF-STEM images of ordered intermetallic Pd3Pb NPs/CB. (C) HAADF−STEM image of Pd3Pb NPs and the corresponding mapping images. The inset of panel B shows the FFT pattern obtained from the HAADF−STEM image.
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RESULTS AND DISCUSSION Figure 1 shows the pXRD profiles of the commercially available Pd NPs/CB and the Pd3Pb NPs/CB prepared. The diffraction
uniformly dispersed on the CB surface in the same manner as the commercially available Pd NPs/CB (see Figure S1). The average size of the Pd3Pb NPs is 4.3 nm, which is 1.2 nm larger than that of the Pd NPs. This difference in size is explained by considering that Pb ions are efficiently reduced on the Pd NPs by the polyol method using ethylene glycol as a reducing agent because Pd can function as a catalyst to reduce the Pb2+ ion. The crystal structure of Pd3Pb NPs/CB was examined on the basis of its atomic-resolution HAADF−STEM image and the corresponding fast-Fourier transformation (FFT) pattern. In the FFT pattern, the additional spots reflecting the super lattice could be observed (Figure S2). These spots were assigned to the super lattice reflection at the 100 and 110 planes in agreement with the pXRD data. Figure 2C shows a typical HAADF−STEM image of Pd3Pb NPs/CB and the corresponding compositional mapping profiles. From these data, it is obvious that the Pd and Pb atoms are uniformly dispersed in the prepared particles and the Pd:Pb atomic ratio is 75.9:24.1. In addition, the overview of elemental mapping in the HAADF−STEM image of Pd3Pb NPs/CB and the compositional mapping profiles are shown in Figure S3. These results demonstrate that the ordered intermetallic Pd3Pb NPs are formed over the whole surface of the CB. Figure 3 shows the cyclic voltammograms recorded continuously for the ordered intermetallic Pd3Pb NPs/CB at a scan rate of 100 mV s−1 between 0.05 and 1.1 V in 0.1 M HClO4. The dotted lines indicate the voltammograms at cycles 20, 30, and 40. At scan 10, the Pd3Pb NPs/CB do not show a clear response characteristic of the adsorption/desorption of hydrogen on the Pd surface between 0.05 and 0.4 V. On the other hand, the anodic peak corresponding to the oxidation of Pb metal in Pd3Pb was observed significantly at ∼0.92 V. This is not observed for the pure Pd NPs electrocatalyst (Figure S4) but is in the case of ordered intermetallic PtPb.23 The hydrogen adsorption/desorption peak current and the Pb oxidation current gradually increase and decrease, respectively, with an increasing number of potential scans. These results may be explained qualitatively as follows. The surface of the as-
Figure 1. (A) Comparison of the X-ray diffraction profiles of (a) commercially available Pd NPs/CB and (b) ordered intermetallic Pd3Pb NPs/CB. (B) Magnified presentation of typical (111) peaks. Asterisks denote the super lattice peaks of the Pd3Pb ordered intermetallic phase.
peak of hexagonal carbon from the 002 plane can be observed at 24.8° for both samples. The Pd NPs/CB give three diffraction peaks at 40.1°, 46.7°, and 68.1°, corresponding to the 111, 200, and 220 planes, respectively, and thus, it can be assigned as a face-centered cubic lattice (fcc) structure (Fm3̅m; a = 0.389 nm; ICDD 00-005-0681). Three main diffraction peaks can also be observed for the prepared ordered intermetallic Pd3Pb NPs/CB. The pXRD peaks for Pd3Pb NPs/CB significantly shift to a lower scattering angle when compared to those of pure Pd NPs/CB, as can be expected from the fact that the atomic radius of Pb is larger than that of a Pd atom. Furthermore, interestingly, Pd3Pb NPs/CB exhibited peaks characteristic of the super lattice phase at 22.1°, 31.4°, 50.7°, and 56.0°, which can be assigned to the reflections at the 100, 110, 210, and 211 planes, respectively, of the Cu3Au-type structure (Pm3̅m; a = b = c = 0.4022 nm; ICDD 03-065-3266). These results indicate that the ordered intermetallic Pd3Pb NPs 2908
DOI: 10.1021/acs.chemmater.6b05191 Chem. Mater. 2017, 29, 2906−2913
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Figure 3. Cyclic voltammograms obtained for ordered intermetallic Pd3Pb NPs/CB at scans 10, 20, 30, 40, 50, and 100 at 100 mV s−1 in a N2-saturated 0.1 M HClO4 aqueous solution. The potential was scanned continuously 100 times at 100 mV s−1 between 0.05 and 1.1 V.
prepared Pd3Pb NPs/CB is covered with the Pb (and/or its oxide) layer, which inhibits hydrogen adsorption/desorption on the Pd surface, but the continuous potential cycling between 0.05 and 1.1 V electrochemically oxidizes such a Pb (and/or its oxide) layer. Consequently with an increase in the number of potential scans, the anodic peak current corresponding to Pb oxidation decreases gradually while the peak current corresponding to the adsorption/desorption of hydrogen increases gradually; i.e., a clear surface of Pd3Pb NPs appears accordingly. This different dependence of the hydrogen adsorption/ desorption peak current and the Pb oxidation current upon the number of potential scans, i.e., the fact that the Pb oxidation continues to occur also after scan 50 and its current decreases gradually with potential scan up to scan 100 and the CV response characteristic of Pb oxide layer formation and its reduction is observed, may suggest that Pb atoms of the Pd3Pb lattice are gradually segregated into the surface because the surface energy of Pb is smaller than that of Pd and they are oxidized electrochemically, and as the number of Pb atoms in the Pd3Pb decreases gradually and finally (around scan 100), the surface region of the Pd3Pb NPs would be close to that of the Pd NPs actually. Very interestingly, such a change in the surface and inner structure of the Pd3Pb NPs is closely reflected in the difference in the observed electrocatalytic activity of the Pd3Pb NPs/CB when they are electrochemically treated using different numbers of potential scans, as a result of an essential difference in the electrocatalysis of the ordered intermetallic Pd3Pb NPs and Pd NPs for the FA oxidation (i.e., the Pd3Pb NPs are superior in the electrocatalysis to the Pd NPs), as mentioned below. Such an electrochemical (cyclic voltammetric) oxidation treatment of the as-prepared Pd3Pb NPs/CB will hereafter be denoted simply as “electrochemical treatment”. Figure 4 shows the HR-TEM images of the Pd3Pb NP before and after the electrochemical treatment shown in Figure 3. Clear lattice fringes can be recognized in both TEM images. The FFT pattern of the as-prepared ordered intermetallic Pd3Pb NP (the inset in Figure 4A) suggests that it is a completely atomically ordered intermetallic compound. The essentially similar FFT pattern showing super lattice spots was also obtained for the electrochemically treated Pd3Pb NPs, as shown in the inset in Figure 4B. These results indicate that the Pd3Pb NPs/CB are kept as atomically ordered intermetallic compounds without a significant change in particle size even after the electrochemical treatment (see Figure S5). Panels C and D of Figure 4 show the XPS profiles of the ordered
Figure 4. HR-TEM images of (A) as-prepared ordered intermetallic Pd3Pb NPs and (B) Pd3Pb NPs after 50 cycles of electrochemical treatment. XPS profiles for (a) as-prepared ordered intermetallic Pd3Pb NPs/CB and (b) Pd3Pb NPs/CB after 50 cycles of electrochemical treatment in the (C) Pd 3d and (D) Pb 4f regions. Insets in panels A and B show FFT patterns obtained from HR-TEM images.
intermetallic Pd3Pb NPs/CB before and after the electrochemical treatment. All the XPS data were calibrated using the binding energy of the C 1s peak, which was found to be 284.5 eV (Figure S6). In the Pd region, irrespective of the electrochemical treatment, Pd 3d5/2 and 3d3/2 peaks were observed at 334.6 and 339.9 eV, respectively, which can be assigned to Pd (0),24 and the oxidized states were also observed. The Pd (0) peak in the 3d5/2 region of the ordered intermetallic Pd3Pb NPs/CB, 335.1 eV, is by 0.5 eV higher than the binding energy of commercially available Pd NPs/CB. This shift can be explained as the change in the electronic state of Pd (0) caused by the electronic interaction between pure Pd and Pb atoms, in other words, the formation of ordered intermetallic Pd3Pb NPs on the CB surface. In addition, the Pd 3d5/2 peak position for the electrochemically treated Pd3Pb NPs/CB was coincident with that of the as-prepared Pd3Pb NPs/CB, indicating that the ordered intermetallic structure of Pd3Pb is kept even after the electrochemical treatment. The atomic ratios of Pd and Pb were calculated by the following equations. Pd atomic ratio (%) = [SPdAPd /(SPdAPd + SPbAPb)] × 100 (4)
Pb atomic ratio (%) = [SPbAPb /(SPdAPd + SPbAPb)] × 100 (5)
where S and A are sensitivity factors of Pd and Pb and peak areas of Pd 3d and Pb 4f regions, respectively. The obtained results are listed in Table S1. The atomic ratio of Pb in the Pd3Pb NPs/CB is 39.8%, which is significantly larger than that (25%) expected for Pd3Pb, and after the etching treatment is 20.1%, which is close to 25% (Figure S7). This result indicates that an excess of Pb atoms exists on the surface of the asprepared Pd3Pb NPs/CB. In addition, the atomic ratio of Pb in 2909
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ordered phase in the [100] zone. On the other hand, the surface region did not indicate bright dots, meaning that it is formed by Pd atoms because Pb atoms on the surface region are dissolved by the electrochemical treatment (Figure S10). The STEM−EDS mapping profile obtained after the electrochemical treatment shows a region-dependent distribution of Pd (green) and Pb (red) in a NP, suggesting the formation of Pd shell structure during the electrochemical treatment of the ordered intermetallic Pd3Pb (Figure 5F). The atomic ratios of Pd and Pb are 91.7:8.3 and 83.1:16.9 in the surface and inner regions, respectively (Figure S11). These results clearly demonstrate that the ordered intermetallic Pd3Pb core with a Pd-enriched surface (i.e., Pd3Pb core−Pd shell structure) is formed by the electrochemical treatment. Figure 6 demonstrates the largely different electrocatalytic activity toward the electrooxidation of FA of the commercially
the ordered intermetallic Pd3Pb NPs/CB subjected to 50 electrochemical cycles is 15.0%, showing that after the electrochemical treatment the surface amount of Pb is reduced compared with that before the treatment; i.e., a selective dissolution of Pb takes place by this oxidative treatment, which also means that a segregation of Pb from the inner lattice to the surface occurs as mentioned above. Figure 5 shows the atomic-resolution HAADF−STEM images and STEM−EDS line profiles of the ordered
Figure 5. (A−C) Atomic-resolution HAADF−STEM images and (D− F) line profiles of (A and D) as-prepared, (B and E) 50-cycle, and (C and F) 100-cycle electrochemically treated ordered intermetallic Pd3Pb NPs. The insets of panels D−F show HAADF−STEM images of asprepared, 50-cycle, and 100-cycle electrochemically treated ordered intermetallic Pd3Pb NPs, respectively. Scale bars are 10 nm.
intermetallic Pd3Pb NPs before and after the electrochemical treatment. The atomic number of Pb is larger than that of Pd, and thus, Pb shows more bright dots in HAADF−STEM images (i.e., z-contrast). The more detailed structural analysis and EDS mapping profiles before and after the electrochemical treatment are shown in Figures S8−S10. Before the electrochemical treatment, Pd and Pb are relatively uniformly dispersed in the NPs. In addition, Pb or Pb oxide can be also observed on the surface of the NPs (Figure S8), in agreement with Figure 4D. The EDS line profiles were taken to estimate the presence of Pb and Pd atoms at the surface and inner regions of the as-prepared and electrochemically treated ordered intermetallic Pd3Pb NPs. In the case of the as-prepared Pd3Pb NPs, the atomic ratio of Pb to Pd at the surface region is approximately 1:1; i.e., the ratio of Pb is higher than that expected for Pd3Pb, because Pb atoms are segregated to the surface and undergo a passivation by oxygen, forming Pb oxide, as expected from the XPS analysis (Figure 5D). On the other hand, the Pd3Pb NPs electrochemically treated for 50 cycles are observed as the Pd3Pb itself because the Pb or Pb oxide on the surface of ordered intermetallic Pd3Pb is dissolved in 0.1 M HClO4 by the electrochemical treatment (Figure 5B,E). Moreover, in the case of the Pd3Pb NPs electrochemically treated for 100 cycle, the EDS line profile taken across the NP confirmed the presence of a Pd-enriched layer of ∼1 nm (Pd of five layers) on the surface. Figure S10 shows the typical expanded HAADF−STEM images and FFT patterns, which indicate the surface and inner regions, respectively. The inner region indicates clear bright dots of Pb atoms (Figure 5C). From the FFT pattern of the inner region, the ordered structure formed by Pd and Pb atoms can be indexed as the
Figure 6. (A) Anodic sweep voltammograms for FA electrooxidation over (a) ordered intermetallic Pd3Pb NPs/CB electrochemically treated for 50 cycles, (b) commercially available 20 wt % Pd NPs/CB, (c) Pt NPs/CB, and (d) Pt−Ru NPs/CB in 0.1 M HClO4 and 0.5 M FA at a scan rate of 20 mV s−1 at 1600 rpm. All the oxidation currents were normalized on the basis of the weight of Pd or Pt. (B) CO stripping voltammograms obtained with (a) ordered intermetallic Pd3Pb NPs/CB electrochemically treated for 50 cycles and (b) Pd NPs/CB at 20 mV s−1 in 0.1 M HClO4. (C) Cross-sectional images (in the [111] zone) of the surface of ordered intermetallic Pd3Pb NPs/ CB before and after the electrochemical treatment, and CO adsorption (poisoning) on ordered intermetallic Pd3Pb NPs. Times signs represent no bonding of CO with the ordered intermetallic Pd3Pb surface. The inset of panel A shows a comparison of the peak currents.
available Pt NPs/CB, Pt−Ru NPs/CB, and Pd NPs/CB and the electrochemically treated ordered intermetallic Pd3Pb NPs/ CB. For the Pt NPs/CB, two oxidation peaks are observed at 0.52 and 1.0 V (vs RHE), in which the first and second oxidation peaks correspond to the direct oxidation pathway of FA to CO2 and the CO oxidation pathway, respectively.6 The CO formed by the dehydration of FA adsorbs on the Pt surface, resulting in a decrease in its electrocatalytic activity. Therefore, Pt is not a suitable electrocatalyst for the electrooxidation of FA. The Pt−Ru NPs/CB gave one smaller oxidation peak at ∼200 mV positive potential compared with the Pt NPs/CB. On the other hand, the FA electrooxidation on the Pd NPs/CB and Pd3Pb NPs/CB exhibits a high mass activity. In the Pd NPs/ CB, two oxidation peaks were observed at 0.72 and 0.89 V (vs RHE). In this case, the first peak corresponding to the direct pathway is dominant and the CO oxidation peak is observed as 2910
DOI: 10.1021/acs.chemmater.6b05191 Chem. Mater. 2017, 29, 2906−2913
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Chemistry of Materials a shoulder.25 The highest mass activity is obtained for the electrochemically treated Pd3Pb NPs/CB. In addition, it should be noted here that the electrocatalytic activity of the Pd3Pb NPs/CB is much higher than that of the Pd NPs/CB, even though the average particle size of the former is larger than that of the latter (as mentioned previously). Intermetallic Pd3Pb nanocrystals with flowerlike and interconnected network-type morphology have been also found to possess far superior electrocatalytic activity toward the FA oxidation over the Pd NPs/CB.18 The current density for FA oxidation is also found to be larger at the Pd3Pb NPs/CB than at the Pd NPs/CB (Figure S12), in which the current densities were calculated using the real surface areas of both catalysts estimated on the basis of the assumption of 212 μC cm−2,26 using the amount of electric charge for hydrogen adsorption on the Pd surface. The mass activity of the Pd3Pb NPs/CB at 0.55 V is ∼9 times higher than that of the pure Pt NPs/CB. In addition, the peak current at the Pd3Pb NPs/CB is twice that at the Pd NPs/CB. Most importantly, the as-prepared [i.e., Pb (and/or its oxide) layercoated] ordered intermetallic Pd3Pb NPs/CB does not show an electrocatalytic activity for the FA oxidation, but the electrochemically treated one with an “ideal” Pd3Pb surface as mentioned above possesses a high level of electrocatalysis (Figure S13). As shown in Figure S13, it should be noted that the electrocatalysis for FA electrooxidation significantly depends on the number of potential scans employed for the electrochemical treatment; i.e., the level of electrocatalysis, which is largely higher than that of the Pd NPs, is attained by the potential scan of approximately 30−100 cycles, and in addition, from the viewpoint of the anodic peak potential and current for the FA oxidation, it seems to change with the number of potential scans even in this range. Further detailed study of the correlation between the surface and inner structures of the electrochemically treated ordered intermetallic Pd3Pb NPs and the electrocatalysis is currently under way. Figure 6B presents the CO stripping voltammograms at the Pd NPs and the electrochemically treated ordered intermetallic Pd3Pb NPs/CB, in which both catalysts were soaked in a COsaturated 0.1 M HClO4 solution for 10 min and then transferred into an Ar-saturated 0.1 M HClO4 solution and the stripping voltammograms were measured. The onset potential of CO oxidation at the Pd3Pb NPs/CB is, 0.80 V, significantly more negative than that (0.87 V) at the Pd NPs/ CB. In addition, the peak current for CO oxidation at the Pd3Pb NPs/CB is half that at the Pd NPs/CB, indicating less adsorption of CO and therefore a higher CO tolerance of the Pd3Pb NPs/CB when compared with those of the Pd NPs/CB. The electrocatalytic activity (and CO tolerance) of a catalyst is well-known to strongly depend on its crystal structure, electronic state, and domain size.27 The enhanced electrocatalytic activity toward the electrooxidation of FA and the high CO tolerance attained at the electrochemically treated Pd3Pb NPs/CB may be explained on the basis of its ordered intermetallic crystal structure of a Cu3Au type. The Pd−Pd bond length in the [200] zone of the ordered intermetallic Pd3Pb NPs/CB was calculated from the pXRD profiles to be ∼7 pm larger, and the cell volume is ∼1.1 times larger, compared with those of the Pd NPs/CB. According to the poisoning mechanism on the Pt surface, two or three neighboring sites are required to chemically adsorb CO on its surface (so-called ensemble effect or third-body effect).28−30 In the case presented here, the CO triple (i.e., strongest) bond formation on the Pd surface seems impossible because the
lattice size of the cubic structure of Pd is regularly expanded by the formation of ordered intermetallic Pd3Pb.31 Figure 7 shows the experimentally measured and density functional theory-calculated d-band centers of Pd NPs/CB and
Figure 7. (A) Projected density of states (Projected DOS) of (a) Pd (111) and (b) Pd3Pb (111) catalysts (left) and their structures (right). (B) Experimentally measured (■) and DFT-calculated (●) d-band centers for the Pd and ordered intermetallic Pd3Pb structures. CO binding energies (red circles) of Pd (111) and Pd3Pb (111) were plotted with respect to the right axis. The insets show a schematic illustration of the CO admolecules on the Pd NPs (left) and ordered intermetallic Pd3Pb NPs (right).
ordered intermetallic Pd3Pb NPs/CB. The enhancement of the electrocatalytic activity toward FA oxidation and the CO tolerance at the ordered intermetallic Pd3Pb NPs/CB may be explained by modification of the electronic states of Pd. A density functional theory (DFT) study is commonly performed to elucidate the interaction between CO and catalysts. In our calculations, the lattice constant of bulk Pd3Pb was estimated to be 0.410 nm, indicating a deviation of only 2% from the experimental value, 0.4035 nm. It implies the structure of the utilized modeling is close to the experimentally observed one. The values of the experimentally measured d-band center of the Pd NPs/CB and the electrochemically treated Pd3Pb NPs/CB are −2.92 and −3.11 eV, respectively (Figure S10), which were calculated from the HAXPES profiles in the valence band region (Figure S14). The DFT calculations also showed a lower d-band center value of the Pd3Pb NPs/CB, though slightly (by 0.05 eV), compared with that of the Pd NPs/CB. Abe and coworkers have discussed the relationship between electronic states of Pt-based ordered intermetallic compounds and CO adsorption energy using their d-band center values:32−34 Ptbased ordered intermetallic compounds, such as Pt3Ti, Pt3Ta, and Pt3Hf, have d-band center values lower than that of pure Pt because few electrons are donated by the transition metal element (Ti, Ta, and Hf) to Pt, and only the bonding states are occupied. Consequently, the CO adsorption energies on the transition metal surfaces are lower than that on the pure Pt surface.34 CO binding energies on the surfaces of Pd and Pd3Pb were calculated to elucidate the interaction between CO and them, 2911
DOI: 10.1021/acs.chemmater.6b05191 Chem. Mater. 2017, 29, 2906−2913
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Chemistry of Materials Notes
and the results are shown in Figure 7B, in which the CO binding energy was calculated according to the following equation.35,36 E bCO = −E(catal CO) − E(catal) − E(CO)
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was financially supported by the Iketani Science and Technology Foundation of Japan. This work was also supported partially by the National Institute for Materials Science (NIMS) microstructural characterization platform as a program of the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors are grateful to Dr. Eiji Ikenaga and Dr. Akira Yasui for their help with HAXPES measurements at BL47XU of SPring-8 (Proposal 2016A1668).
(6)
where ECO b is the CO binding energy on a catalyst. The more positive ECO is, the higher the binding energy. E(catalCO), b E(catal), and E(CO) are the ground state energies of a catalyst with CO, a catalyst without CO, and a CO molecule, respectively. Pd3Pb (111) has a CO binding energy (1.05 eV) that is lower than that of Pd (111) (1.29 eV), in agreement with the expectation from these d-band center values mentioned above. This may suggest that the present ordered intermetallic Pd3Pb NPs/CB are more tolerant toward CO adsorption than the Pd NPs/CB are, resulting in a more significantly enhanced FA electrooxidation on the Pd3Pb NPs/CB.
■
■
CONCLUSIONS The carbon-supported Pd-based ordered intermetallic Pd3Pb NP catalyst for the electrooxidation of FA was successfully prepared by the polyol method using ethylene glycol as the reducing agent under an air atmosphere. The pXRD peaks of Pd3Pb significantly shifted to a lower scattering angle compared to those of the Pd NPs/CB because of the increased lattice size due to the doping of Pb, as its atomic size is larger than that of Pd. Moreover, super lattice peaks could be observed for Pd3Pb, indicating its ordered intermetallic structure. The Pd3Pb surface with a high electrocatalytic activity for the FA electrooxidation could be created successfully by the electrochemical treatment using cyclic voltammetry, i.e., the pertinent electrooxidation of the Pb surface coating. The thus electrochemically treated Pd3Pb NPs/CB were found to possess a level of electrocatalysis significantly higher than that of the Pd NPs/CB, which could be reasonably explained on the basis of their d-band center values and CO binding energies. The Pd3Pb NPs/CB have a dband center value lower than that of the Pd NPs/CB, suggesting a weaker CO adsorption in agreement with a smaller CO binding energy calculated for the former. A more detailed study to clarify the correlation between the surface and inner structures of the ordered intermetallic Pd3Pb NPs and the electrocatalysis for FA electrooxidation is now in progress.
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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.chemmater.6b05191. Characterization (TEM images, STEM−EDS, and analysis of XPS) and electrochemistry (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Seung Hyo Noh: 0000-0002-7491-4038 Futoshi Matsumoto: 0000-0001-6808-6531 Author Contributions
S.H.N. and T.T. contributed equally to this work. 2912
DOI: 10.1021/acs.chemmater.6b05191 Chem. Mater. 2017, 29, 2906−2913
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Chemistry of Materials
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