Boron-Doped Palladium Nanoparticles on Carbon Black as a Superior

Apr 20, 2009 - Phone: +86-21-55664050. ... Boosting Formate Production in Electrocatalytic CO2 Reduction over Wide Potential Window on Pd Surfaces...
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J. Phys. Chem. C 2009, 113, 8366–8372

Boron-Doped Palladium Nanoparticles on Carbon Black as a Superior Catalyst for Formic Acid Electro-oxidation Jin-Yi Wang,† Yong-Yin Kang,‡ Hui Yang,‡ and Wen-Bin Cai*,† Shanghai Key Laboratory for Molecular Catalysis and InnoVatiVe Materials and Department of Chemistry, Fudan UniVersity, Shanghai 200433, China, and Energy Science and Technology Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: March 25, 2009

Highly dispersed boron-doped palladium nanoparticles supported on carbon black (Pd-B/C) with high Pd loading (ca. 40 wt % Pd) are synthesized through an aqueous process using dimethylamine borane as the reducing agent. The as-prepared Pd-B/C catalyst shows extraordinary activity toward formic acid electrooxidation compared to that of a commercially available Pd/C catalyst and the one prepared by using NaBH4 as the reductant. Subsequent thermal treatment further enhances the durability of the electro-oxidation current on Pd-B/C, enabling this new material to be a promising anode catalyst for direct formic acid fuel cells. The superior performance of our Pd-B/C catalyst may arise from uniformly dispersed nanoparticles within optimal size ranges, the increase in surface-active sites, and the electronic modification effect of boron species. 1. Introduction Direct formic acid fuel cells (DFAFCs) have attracted great attention as a new generation of environment-friendly power sources with high operating power densities, which are particularly suitable for portable devices.1,2 The success of the DFAFCs largely depends on the design and preparation of highperformance anode catalysts. Due to the lower cost and higher catalysis toward formic acid electro-oxidation as compared to those of Pt-based nanoparticles,3 high-performance Pd-based nanoparticles are the focus of recent investigations.4-8 In contrast to the so-called “dual pathway mechanism” of the electrooxidation of formic acid on Pt surfaces,9-11 the direct oxidation pathway to CO2 is believed to be the predominant reaction on Pd surfaces without significant CO poisoning.12-14 Nevertheless, pure Pd nanoparticles-based catalysts possess a fatal drawback of slow deactivation during the electro-oxidation of formic acid, due to either the oxidation of Pd surfaces or the poisoning adsorption of some non-CO organic species.14,15 Therefore, developing new Pd-based electrocatalysts with higher activity and extended durability is crucial for its practical application in DFAFCs. It is well-known that the reactivity of a catalyst strongly depends on the electronic, structural, and geometric properties of metallic materials. To achieve the above goal, first, alloying16,17 or building core-shell structures18-20 with a second element is a most practicable way for appropriate modification of electronic and/or structural properties of Pd surfaces. In fact, all Pd-based binary catalysts reported so far are bimetallic in nature,16-20 except that a recent Pd/C catalyst prepared with sodium hypophosphite may incorporate some phosphorus composition.21 On one hand, theoretical prediction on the d-band center shift and/or surface segregation in the bimetallic structures agrees largely with a number of existing empirical data, and thus may guide the design of new efficient bimetallic catalysts for liquid * To whom correspondence should be addressed. E-mail: wbcai@ fudan.edu.cn. Phone: +86-21-55664050. Fax: +86-21-65641740. † Fudan University. ‡ Chinese Academy of Sciences.

organic fuels.22,23 On the other hand, unfortunately, relevant information on metallic catalysts containing a second nonmetal element remains nearly unknown due to the lack of experimental data. Boron was a metalloid element widely used for modifying the physicochemical properties of a metal in functional material and catalysis studies.24,25 Boron atoms are much smaller than Pd atoms, which may incorporate into Pd-Pd interlattice spaces rather than substitute Pd atoms. Different from phosphorus, boron usually serves as the electron donator,26 leading to further shrinking of the size of boron atoms favoring their doping in the interstice of the palladium lattice. Hence, unique performances can be expected with B-doped Pd catalysts. B-doped Pd films have been grown by using RF (radio frequency) sputtering method under ultra-high vacuum conditions;26 however, this dry process is certainly not practicable for synthesizing catalysts with carbon black-supported B-doped Pd nanoparticles. However, in a wet chemistry process, the reducing agent NaBH4 has been proved not to be an effective source for doping boron.27 Therefore, effective incorporation of B into Pd nanoparticles should be tackled in a related synthesis. Second, the size control over Pd nanoparticles is important as well for achieving high-performance Pd-based catalysts.4-6 Generally, carbon-supported Pd nanoparticles of 4-7 nm with a large specific surface are most favorable in practical applications in DFAFCs.5,6 Too small particle sizes would lead to lower metal loading and severe aggregation, while catalysts with higher Pd loadings are preferred in working cells. However, wet chemistry synthesis through conventional aqueous reduction without stabilizing or complexing additives would usually yield Pd nanoparticles with sizes larger than 7 nm.28 As a result, previous investigations were mainly limited to the synthesis of Pd-based catalysts of lower metal loadings (e.g., 20 wt %) on carbon supports for good monodispersity.5,6,28 In a word, facile preparation of B-doped Pd/C catalyst with higher metal loading, optimized particle sizes, and high dispersion on carbon black has never been attempted, and thus is a demanding task in developing high-performance Pd-based novel catalysts for formic acid electro-oxidation. It is also noted that,

10.1021/jp900349g CCC: $40.75  2009 American Chemical Society Published on Web 04/20/2009

Boron-Doped Pd Nanoparticles on Carbon Black in nearly all the previous attempts to develop new Pd-based catalysts, the investigators used their homemade Pd/C catalysts for performance comparison to show the advantages of their newly developed catalysts. Obviously, using a commonly acceptable catalyst as reference sample will be more convincing for such comparison. The present work focuses on the facile synthesis and characterization of an efficient novel catalyst, i.e., carbonsupported B-doped Pd nanoparticles (simplified hereafter as Pd-B/C), aiming for its potential application in DFAFCs. The structural and compositional properties, as well as the electrocatalytic activity and durability of the as-prepared and afterannealed Pd-B/C samples, were characterized and compared with a commercial Pd/C catalyst and the one prepared by using NaBH4 as the reducing agent. The enhanced catalysis and durability for formic acid electro-oxidation was briefly discussed in terms of particle sizes, surface-active sites, and electronic modification effect. 2. Experimental Section 2.1. Catalysts Synthesis. The carbon-supported Pd-B alloy catalyst was prepared through an aqueous solution-phase synthesis,21 with dimethylamine borane (DMAB, (CH3)2NH · BH3) as the reducing agent: 0.05 g of NH4F, 0.25 g of H3BO3, and 6.3 mL of 0.045 mol · L-1 PdCl2 (containing 0.27 mol · L-1 NaCl) was mixed in 20 mL of H2O; such a mixture was kept under vigorous stirring with high-purity N2 bubbling for 10-15 min to form a transparent yellow solution. The pH was adjusted to about 8.5 using NH3 · H2O, yielding a colorless solution. Then, 45 mg of Vulcan XC-72 carbon was added to form a carbon slurry, which was further sonicated for 30 min. Under vigorous stirring, 20 mL of freshly prepared ice-cold 0.1 mol · L-1 DMAB aqueous solution was added dropwise into the slurry through a constant-flow pump at 0.5 mL · min-1. The reduction temperature was maintained by an ice-water bath for 2 h. The resulted suspension was further stirred at 30 °C for 2 h. Finally, it was filtered, washed with a copious amount of ultrapure Milli-Q water (>18 MΩ · cm), and vacuum-dried at 70 °C for 6 h. The solid powder obtained with 40 wt % Pd was noted as the Pd-B/C catalyst. A Pd/C catalyst was prepared through a procedure similar to that mentioned above; instead of DMAB, 20 mL of 0.1 mol · L-1 NaBH4 was added to the slurry as the reductant. The solid powder obtained was noted as Pd/C. The post thermal treatment on Pd-B/C or Pd/C catalysts was carried out at 120 °C under constant N2 flow for 2 h; the resulted samples were noted as Pd-B/C(120 °C) and Pd/C(120 °C), respectively. The commercial carbon-supported 40 wt % Pd catalyst purchased from BASF Fuel Cell Inc. (Lot No. F0301009), noted as Pd/C(BASF), was used as the reference sample for the structural and electrochemical characterization. 2.2. Materials Characterizations. The chemical compositions of Pd-based catalysts were analyzed by means of inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on an Hitachi P-4010 by determining the concentrations of dissolved Pd and B species from the catalyst powder using hot aqua regia of a known volume. The structures of these Pd-based catalysts were examined by X-ray diffraction (XRD) (D8 Advance X-ray Diffractometer) with Cu KR radiation from 10 to 90°. The morphology and size distribution of the nanoparticles were determined by transmission electron microscopy (TEM), which was performed on a JEOL JEM-2010 microscope. 2.3. Electrochemical Measurement. The electrocatalysis measurement was performed with a CHI 660B electrochemistry

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8367 workstation. The electrolyte was 0.5 mol · L-1 H2SO4 with or without 0.5 mol · L-1 formic acid. All electrolytes were deaerated with high-purity Ar prior to measurements, and Ar bubbling was maintained during the measurements. The working electrode was a thin layer of Nafion-impregnated catalyst cast on a glassy carbon rotating disk electrode (GC-RDE, 3 mm diameter, 0.07 cm2) prepared as follows: A catalyst slurry was first prepared by mixing 10 mL of C2H5OH and 10 mg of catalyst ultrasonically for 1 h. Then 1 mL of the slurry mixed with 120 µL of Nafion (5 wt %, Aldrich) was sonicated for another hour to obtain the catalyst ink; 5.5 µL of this ink was transferred onto a freshly polished GC-RDE by a pipet. Therefore, each electrode contained ca. 28 µg · cm-2 of Pd. An Au gauze was used as the counter electrode, and Hg/Hg2SO4 electrode served as the reference electrode. All potentials were referred to the saturated calomel electrode (SCE) in this work. The rotating speed of the GC-RDE during the chronoamperometry test was controlled by a speed regulator provided by Tacussel Electronics (France). All electrochemical experiments were carried out at 25 ( 1 °C. CO stripping voltammograms began with bubbling CO (>99.9% purity) over the working electrode at 0.0 V for 20 min. Subsequently, the dissolved CO was removed from the electrolyte by bubbling Ar for 40 min while maintaining the electrode potential at 0.0 V. Finally, the CO stripping voltammograms were obtained between -0.2 and +1.0 V at a scan rate of 10 mV · s-1. 3. Results and Discussion 3.1. Formation of the B-Doped Pd Nanoparticles. Dimethylamine borane (DMAB, (CH3)2NH · BH3) was used as the reducing agent and boron source in mild alkaline solutions for the synthesis of Pd-B/C catalyst. DMAB reacts with OH- to form BH3OH- according to eq 1, and BH3OH- is suggested to be the actual reducing species.29,30 Theoretically, it is possible for each BH3OH- molecule to provide up to six electrons to reduce three Pd(II) ions (eq 2); the codeposition of boron may proceed according to eq 3,30 leading to the formation of Pd-B material.

(CH3)2NH · BH3 + OH- f (CH3)2NH + BH3OH- (1) BH3OH- + 3Pd(II) + 2H2O f 3Pd + B(OH)3 + 5H+ (2) (CH3)2NH + 2BH3 + H+ f (CH3)2NH2+ + 2B + 3H2 (3) 3.2. Materials Characterizations. 3.2.1. Physicochemical Characterizations. The average Pd and B atomic percentages for Pd-B/C (through DMAB reduction) and Pd/C (through NaBH4 reduction) catalysts were evaluated by ICP-AES analysis. Both catalysts contained about 38 wt % Pd, nearly the same as the preset values, indicating that Pd2+ can be totally reduced to form metallic Pd in our synthesis. The atomic percentages of boron in Pd-B/C and Pd/C nanoparticles were 6.3 and 2.9 at.%, respectively, indicating that DMAB was a more effective boron source than NaBH4 using such preparation procedure. Figure 1 shows the XRD patterns of the freshly prepared Pdbased catalysts (Pd-B/C, Pd/C) and their counterparts (Pd-B/ C(120 °C), Pd/C(120 °C)) obtained after the thermal treatment, and the commercial Pd/C(BASF) catalyst. Four peaks corresponding to Pd(111), (200), (220), and (311) were characteristics of the Pd face-centered cubic (fcc) phase (2θ ) 40.1°, 46.7°, 68.2°, and 82.2°, respectively. They are adapted from PDF#65-6174 and indicated by the vertical lines in Figure 1). Both Pd/C(BASF) and Pd/C showed the typical diffraction angles matched well with the standard lines. By contrast, the peaks

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Figure 1. XRD patterns of the Pd-based catalysts before and after heat treatment at 120 °C. The vertical lines correspond to the Pd facecentered cubic phase (PDF-#65-6174).

for Pd-B/C shifted to lower 2θ values, indicating an enlargement in the Pd-Pd interatomic distance. The dilation of the lattice constant can be attributed to a structural change, or the incorporation of small atoms into the Pd lattice. It has been reported that the diffraction angles of Pd nanoparticles may shift to lower 2θ values accompanied by a significant broadening of the Pd(111) peak for ultrafine nanoparticles with a mean size smaller than 3 nm.31 Nevertheless, the full-width at halfmaximum (fwhm) values for our nanoparticles are fairly close, i.e., ca. 1.8, 1.9, and 1.6 for Pd-B/C, Pd/C(BASF), and Pd/C, respectively. TEM images (vide infra) also show that the mean particle size of Pd-B/C is even slightly larger than that of Pd/ C(BASF). Therefore, the effect of boron doping should mainly be responsible for the observed peak shift of Pd-B/C in Figure 1, in accordance with the ICP-AES analysis. The same trend in XRD patterns was observed on the Pd-B thin films prepared by RF sputtering when the atomic percentage of boron was Pd-B/C (120 °C) > Pd/C (120 °C) > Pd/C(BASF) > Pd/C, as tabulated in Table 1. The highest peak currents were observed on Pd-B/C and Pd-B/C(120 °C), nearly twice that on Pd/C(BASF) or Pd/C electrode catalysts, and their corresponding peak potentials are located at ca. 0.18 V, 20 mV more negative than that for the Pd/C catalysts (marked in Figure 4 with vertical dashed lines). Given that the size difference between Pd nanoparticles is insufficient to account for this large gap, the boron-doping effect is assumed to be more relevant. According to Hammer and Norskov’s d-band center model,44,45 the interstitial incorporation of a metalloid element into a metal lattice should cause the downshift of the d-band center of the

Figure 5. Cyclic voltammograms for formic acid oxidation on various Pd-based catalysts-coated GC-RDEs in 0.5 mol · L-1 formic acid and 0.5 mol · L-1 H2SO4 at a scan rate of 50 mV · s-1. ω ) 0 rpm.

metal due to the covalent bonding between the metal d states and the p states of B. It was demonstrated by Kibler et al. that a suitable downshift of the d-band center of Pd increased the electrocatalysis of formic acid on a Pd surface, yielding a negative shift of the electro-oxidation onset potential and/or higher electro-oxidation current at a given potential.23 Indeed, this is the case for the Pd-B/C electrocatalyst. The heat treatment was beneficial for enhancing the catalytic activity or durability of as-prepared catalysts. A nearly 30% increase in the oxidation peak current was observed on Pd/C catalysts prepared through NaBH4 reduction, which may be attributable to improved crystallinity of Pd nanoparticles subject to postsynthesis treatment.6 Although a slight decrease of the oxidation peak current was seen on Pd-B/C(120 °C) in the forward potential scan, no suppression or deactivation of the oxidation was found in the negative scan, indicative of a much better durability. The stability of Pd-based electrocatalyts is extremely important for their real applications in DFAFCs. The long-term activity and durability of the Pd-based catalysts were further assessed by chronoamperometry test (i-t curve), conducted on catalyst-coated GC-RDEs at 1000 rpm with the potential fixed at either 0.2 V (Figure 6) or 0.1 V (Figure 7). In contrast to most of the previous tests, in the current work, rotating disk electrodes were adopted for better control of mass transport and for prevention to a large extent of the accumulation of small CO2 bubbles on catalyst surfaces.46 The current density at the end of each test was listed in Table 1. Although the electrooxidation current on Pd-B/C dropped quickly at the beginning, it was still higher than that on Pd/C at the end of both tests. With the heat treatment at 120 °C, the long-term electrocatalytic activity on Pd/C or Pd-B/C catalyst was improved (curves b vs a, curves e vs d in Figures 6 and 7), especially for the former, which is probably due to the improvement in the crystalline

Boron-Doped Pd Nanoparticles on Carbon Black

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8371 work indicates that Pd-B/C is a promising anode catalyst material for practical applications in DFAFCs. Acknowledgment. This work is supported by NSFC (No. 20673027, 20833005, and 20873031) and STCSM (No. 08JC1402000 and 08DZ2270500). References and Notes

Figure 6. i-t curves measured for various catalyst-coated GC-RDEs at 0.2 V vs SCE in 0.5 mol · L-1 formic acid and 0.5 mol · L-1 H2SO4. ω ) 1000 rpm.

Figure 7. i-t curves measured for various catalyst-coated GC-RDEs at 0.1 V vs SCE in 0.5 mol · L-1 formic acid and 0.5 mol · L-1 H2SO4. ω ) 1000 rpm.

degree of nanoparticles.6 Notably, Pd-B and Pd-B/C(120 °C) showed superior activity and durability as compared to their Pd/C counterparts. The electro-oxidation current at 0.1 V on Pd-B/C(120 °C) electrode at 4000 s is 2.5-6.5 times as high as that on the others. The reason for the much better long-term electrocatalytic activity of Pd-B/C catalysts is not very clear right now; one possible explanation is that the lowering of the d-band center of Pd in Pd-B/C within a suitable range helps retard the gradual accumulation of poisoning species on Pdactive sites. Further investigations will aim to understand the origins of the superior performance of Pd-B/C electrocatalyts and to explore the technological aspects for their application in DFAFCs. 4. Conclusions In summary, we synthesized a novel efficient catalyst, i.e., boron-doped Pd nanoparticles supported on carbon XC-72, for formic acid electro-oxidation. Highly dispersed Pd-B nanoparticles on C at a loading of ca. 40 wt % Pd can be readily obtained with a narrow size distribution. Distinctive enhancement in activity toward the electrocatalytic oxidation of formic acid was found on such material; a post thermal treatment at 120 °C of as-prepared Pd-B catalyst further improved the longterm electrocatalysis. The unique electrocatalytic performance of Pd-B/C electrocatalyst may be attributed to (i) uniformly dispersed nanoparticles within optimal size range, (ii) the increase in surface-active sites, and (iii) the electronic modification effect of boron species to Pd nanoparticles. The current

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