Article pubs.acs.org/cm
Ordered Pd2Ge Intermetallic Nanoparticles as Highly Efficient and Robust Catalyst for Ethanol Oxidation Sumanta Sarkar,† Rajkumar Jana,† Suchitra Prasad,‡ Umesh V. Waghmare,‡ Balamurugan Thapa,§ S. Sampath,§ and Sebastian C. Peter*,† †
New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, India Theoretical Science Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, India § Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560012, India ‡
S Supporting Information *
ABSTRACT: Pd2Ge nanoparticles were synthesized by superhydride reduction of K2PdCl4 and GeCl4. The syntheses were performed using a solvothermal method in the absence of surfactants, and the size of the nanoparticles was controlled by varying the reaction time. The powder X-ray diffraction (PXRD) and transmission electron microscopy data suggest that Pd2Ge nanoparticles were formed as an ordered intermetallic phase. In the crystal structure, Pd and Ge atoms occupy two different crystallographic positions with a vacancy in one of the Ge sites, which was proved by PXRD and energy-dispersive X-ray analysis. The catalyst is highly efficient for the electrochemical oxidation of ethanol and is stable up to the 250th cycle in alkaline medium. The electrochemical active surface area and current density values obtained, 1.41 cm2 and 4.1 mA cm−2, respectively, are superior to those of the commercial Pd on carbon. The experimentally observed data were interpreted in terms of the combined effect of adsorption energies of CH3CO and OH radical, d-band center model, and work function of the corresponding catalyst surfaces.
1. INTRODUCTION Fuel cells have emerged as one of the most prominent options for alternative energy conversion and have been applied for various electrochemical reactions. Currently, platinum on different supports1,2 and in the form alloys,3 bimetallic,4 and intermetallic compounds5 are the mostly used electrocatalysts in the low temperature fuel cells. A few examples are Pt3Pb nanoparticles showed high efficiency for formic acid oxidation,6 PtCu3 nanocages were proved to be highly efficient in methanol oxidation,7 and Pt−Ru bilayer core−shell nanoparticles are very good CO tolerant electrocatalyst.8 Direct alcohol fuel cells (DAFCs) have drawn great attention because they are considered as promising future power sources for electric vehicles and small portable electronics. On the basis of the electrolyte membrane used, DAFCs can be divided into two types: acid- and alkaline-membrane DAFCs. Over the past few decades, extensive attention has been paid to the acid-type DAFCs and significant progress has been made in their development. However, the commercialization of acid-type DAFCs still remains a challenge because, in acidic media, the expensive Pt or Pt-based electrocatalysts are usually required. In addition, the kinetics of alcohol oxidation on Pt-based electrocatalysts is rather sluggish in acidic media. Recently, increasing attention has been paid to the alkaline-type DAFCs because, in alkaline media, less expensive Pd-based catalysts have comparable or even better electrocatalytic activities than Pt-based catalysts for alcohol oxidation, especially for ethanol © XXXX American Chemical Society
oxidation. Xu et al. prepared a series of oxide nanocrystal promoted-Pd/C electrocatalysts which were markedly superior to those of Pt-based electrocatalysts in terms of alcohol oxidation activity and poison tolerance in alkaline media.9 Although Pd proved to be as a suitable electrocatalyst for ethanol oxidation reaction (EOR) in alkaline media, more effort is needed to further improve the electrocatalytic performance of Pd-based catalysts. Ethanol is relatively less studied than methanol; the reason could be because the C−C bond cleavage in ethanol requires very high dissociating energy (∼350 kJ/ mol).10 However, the use of ethanol is advantageous over methanol because it is less toxic, highly abundant, low cost, and results in 12 electrons per molecule when it is completely oxidized to CO2 and H2O. Some of the catalysts which have been used for electrochemical oxidation of ethanol are SnO2/ Pt−Sn alloys,11 Ni supported/Pt−Ru alloys,12 and Pt/C on other oxides like CeO2 and NiO9. In alkaline medium, it has been shown that the reaction catalyzed by Pt is sluggish as compared to Pd. Hence, Pd on various support materials along with its alloys has been highly studied during the past two decades. A few examples of this class are Pd on TiO2/C,13 Al2O3/C, VOx/C,14 CeO2/C,15 Co3O4/C,16 MnO2/C,15 NiO/ C, In2O3/C,17 CNTs (both SWCNT and MWCNT),16,18 and Received: September 10, 2015 Revised: October 10, 2015
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DOI: 10.1021/acs.chemmater.5b03546 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
at 220 °C for 24 and 36 h. The product was repeatedly washed several times with ethanol. 2.3. Powder X-ray Diffraction (PXRD). PXRD measurements were done at room temperature on a Rigaku Miniflex X-ray diffractometer with a Cu−Kα X-ray source (λ = 1.5406 Å), equipped with a position sensitive detector in the angular range of 30° ≤ 2θ ≤ 80° with the step size 0.02° and a scan rate of 0.5 s/step calibrated against corundum standards. The experimental patterns were compared to the pattern calculated from the single crystal structure refinement. 2.4. Elemental Analysis. Quantitative microanalysis on all the samples was performed with an FEI NOVA NANOSEM 600 instrument equipped with an EDAX instrument. Data were acquired with an accelerating voltage of 20 kV and a 100 s accumulation time. The EDAX analysis was performed using the P/B-ZAF standardless method (where Z = atomic no. correction factor, A = absorption correction factor, F = fluorescence factor, P/B = peak to background model) on selected spots and points. The Pd contents in Pd2Ge_24 and Pd2Ge_36 samples are 72% and 76% and the Ge contentsP/B are 28% and 24%, respectively. The EDAX spectra for both compounds are shown in Figure S1. On the basis of these values obtained from the EDAX data, the compositions of these two samples can be written in general as Pd(2+x)Ge(1−x), where x = 0.18 and 0.30, respectively, for 24 and 36 h samples. For the sake of simplicity, these two samples are designated, respectively, as Pd2Ge_24 and Pd2Ge_36 throughout this paper. 2.5. Transmission Electron Microscopy (TEM). TEM images and selected area electron diffraction (SAED) patterns were collected using a JEOL 200 TEM instrument. Samples for these measurements were prepared by sonicating the nanocrystalline powders in ethanol and dropping a small volume onto a carbon-coated copper grid. 2.6. X-ray Photoelectron Spectroscopy (XPS). XPS measurement was performed on an Omicron Nanotechnology spectrometer using a Mg−Kα (1253.6 eV) X-ray source with a relative composition detection better than 0.1%. 2.7. Electrochemical Studies. All the electrochemical measurements were performed on a CHI 660A electrochemical workstation at 25 °C. A three-electrode setup was used with a glassy carbon electrode (having diameter 3 mm) as working electrode, platinum wire as counter electrode, and Hg/HgO (MMO) as reference electrode. All the solutions were purged with nitrogen gas for 20 min prior to measurement. The catalyst ink was prepared by dispersing 5 mg of catalyst in 1 mL of mixed solvent solution (IPA:H2O = 1:1 v/v) and 10 μL of 1 wt % Nafion binder. The Nafion binder (Signa Aldrich, 5 wt %) was diluted to 1 wt % with isopropyl alcohol (IPA). A 20 μL aliquot of the catalyst ink was dropcasted onto a glassy carbon (GC) electrode and dried overnight at room temperature. Before depositing the catalyst, the GC was polished with 0.05 μm alumina slurry and washed several times with distilled water (16.2 mΩ), and IPA. Commercial Pd/C (10 wt %, Sigma-Aldrich) was used for comparison of activity with the Pd2Ge catalyst. The blank cyclic voltammetry (CV) measurement was carried out with 1 M KOH aqueous solution at a scan rate 50 mV/s. CV and chronoamperometry (CA) were performed in 1 M KOH/1 M EtOH electrolyte solution at a scan rate 50 mV/s. Linear sweep voltammetry (LSV) was recorded with a sweep rate of 1 mV/s at 1000 rpm in 1 M KOH/1 M EtOH electrolyte solution. Tafel plots (TP) were derived from LSV measurement. For all the catalysts (Pd2Ge_24, Pd2Ge_36, and commercial Pd/C), we have maintained the same loading of Pd. For Pd2Ge_24 and Pd2Ge_36, the loading of catalysts on the GC electrode (3 mm diameter) is 0.1 mg, in which the loading of Pd is 0.0745 mg. For commercial Pd/C, the loading of Pd/C on the GC electrode is 0.745 mg, in which the loading of Pd is kept the same as other catalysts (0.0745 mg). In effect, for all the three catalysts, the loading of active noble metal (Pd) per unit area is the same (1.05 mg/cm2). As the amount of Pd present on electrode is the same for all cases, ECSA data of all the catalysts and commercial Pd/C are comparable. 2.8. Computational Details. The first-principles calculations are performed on the density functional theory as implemented in the QUANTUM ESPRESSO package.30 The exchange-correlation energy
the alloys PdNi,19 PdPt,20 PdAu,21 and PdRu.22 It is experimentally proven that the presence of another metal or a support highly enhances the catalytic activity of Pd nanoparticles by the synergistic effect between Pd and the support. Apart from the above-mentioned systems, several catalysts have been reported in the recent literature for the electrooxidation of ethanol applying different strategies to improve both the activity and the durability of the catalyst. For example, both Pt−Sn23 and Pd−Sn24 based alloy nanoparticles are considered as superior catalysts in ethanol electrooxidation because of their high activity and durability in acidic and alkaline media, respectively, but these two classes of compounds do not form ordered intermetallic phases under the normal reaction conditions. In the same manner, TaPt3 ordered intermetallic nanoparticles25 were reported to show far better activity than Pt3Sn nanoparticles. However, the ordered phase of TaPt3 was only obtained after the postsynthetic heating at a very high temperature (1000 °C). PtRh alloy nanocubes supported on graphene26 were shown to have high activity toward ethanol electrooxidation; however, both the constituents are costly. PdAu nanowires27 were synthesized in the presence of Br− and PVP as a shape directing and stabilizing agent. The presence of an additive on the catalyst surface is detrimental to its catalytic activity as these polymer molecules significantly reduce the effective coverage of ethanol molecules. Motivated by the incredible catalytic activity of Pd−Sn based alloy nanoparticles, we have synthesized the ordered intermetallic compound Pd2Ge in nanodimension by a modified polyol method at 220 °C without using any sort of additives or postsynthetic heating. The use of intermetallic nanoparticles are advantageous over alloys or bimetallics owing to their ordered structure that provides (i) a uniform surrounding of the active sites, and hence, (ii) the number and the distance between the neighboring sites can easily be known both experimentally and theoretically. The compound Pd2Ge crystallizes in the Fe2P type crystal structure with two each independent crystallographic positions for Pd and Ge atoms.28 We studied electrochemical oxidation of ethanol in alkaline medium using Pd2Ge as a catalyst synthesized over a reaction times of 24 h (Pd2Ge_24) and 36 h (Pd2Ge_36) of synthesis. The order of activity toward electrochemical oxidation of ethanol is Pd2Ge_36 > Pd2Ge_24 > Pd/C. The presence of Ge deficiency in the Pd2Ge_24 sample showed an immense effect on the catalytic activity of the compound owing to the localized defects developed as deficiency. However, the increase in the catalytic activity for the Pd2Ge_36 sample was due to the dealloying effect with the evolution of a small amount of Pd, which outweighed the presence of localized defects in the Pd2Ge_24 sample.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Potassium tetrachloropalladate (K2PdCl4), tetraethylene glycol (TEG), and superhydride (Li(Et3BH)) were purchased from Sigma-Aldrich. Germanium29 chloride (GeCl4) and ethylene glycol were purchased from Alfa Aesar. All materials were used as purchased, and all air-sensitive samples were handled inside an Arfilled glovebox (H2O, O2 < 0.1 ppm). 2.2. Synthesis. Pd2Ge nanoparticles were synthesized by a solvothermal method. In a typical solvothermal reaction, 0.2 mmol of K2PdCl4, 0.1 mmol of GeCl4 and 0.8 mL of superhydride solution (reducing agent) were mixed in 18 mL of TEG with vigorous stirring and loaded in a 23 mL Teflon-lined autoclave. The autoclave was kept B
DOI: 10.1021/acs.chemmater.5b03546 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials of electrons is treated with a local density approximation with the Perdew−Zunger parametrized form.31 We employ periodic boundary conditions with 4 × 4 and 3 × 3 supercells of Pd and Pd2Ge surfaces, respectively, and include a vacuum of about 14 Å to keep interaction between periodic images low. We have used a kinetic energy cutoff of 35 Ry to truncate the plane-wave basis. We use uniform meshes of 3 × 3 × 1 and 3 × 3 × 2 k-points for Pd and Pd2Ge surfaces, respectively, in sampling integration over the Brillouin zone. We use different orientations of OH and CH3CO on the surface and relax the system until the Hellmann−Feynman forces on each atom are less than 0.03 eV/Å. We have considered Pd2Ge_24 as a Ge-deficient surface and Pd2Ge_36 as a pristine Pd2Ge phase throughout our calculation.
Figure 2. Comparison of PXRD patterns of Pd2Ge nanoparticles synthesized by a solvothermal technique at 220 °C after 24 and 36 h with simulated powder pattern from bulk Pd2Ge intermetallic compound and Pd. The inset shows the enlarged plot showing the relative shift of Pd2Ge_24 compared to the bulk compound.
3. RESULTS AND DISCUSSIONS 3.1. Structure and Ordering. 3.1.1. XRD Analysis. A typical representation of the unit cell of the Pd2Ge crystal structure is shown in Figure 1a. The coordination environments
48 and 72 h yielded some unknown impurity phases (Figure S2) and hence were not studied for their electrochemical activity. 3.1.2. TEM Analysis. Figure 3 shows the TEM images for Pd2Ge_24. The sample mostly contains wormlike one-dimen-
Figure 1. (a) Crystal structure of Pd2Ge. The white atom represents a vacant site at the Ge position for the sample Pd2Ge_24. Coordination spheres of (b) Pd1 and (c) Pd2. Figure 3. TEM images of Pd2Ge nanoparticles showing the growth from ultrasmall nanoparticles (
