J. Phys. Chem. C 2007, 111, 17305-17310
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Controllable Synthesis of Pd Nanocatalysts for Direct Formic Acid Fuel Cell (DFAFC) Application: From Pd Hollow Nanospheres to Pd Nanoparticles Junjie Ge, Wei Xing,* Xinzhong Xue, Changpeng Liu, Tianhong Lu, and Jianhui Liao State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China ReceiVed: May 14, 2007; In Final Form: July 9, 2007
The controllable synthesis of nanosized carbon-supported Pd catalysts through a surface replacement reaction (SRR) method is reported in this paper. Depending on the synthesis conditions the Pd can be formed on Co nanoparticles surface in hollow nanospheres or nanoparticles structures. Citrate anion acts as a stabilizer for the nanostructures, and protonation of the third carboxyl anion and hence the nanostructure and size of the resulting catalysts are controlled via the pH of the synthesis solution. Pd hollow nanospheres, containing smaller Pd nanoparticles, supported on carbon are formed under the condition of pH 9 reaction solution. Meanwhile, highly dispersed carbon-supported Pd nanoparticles can be formed with higher pH (pH g 10). All catalysts prepared through the SRR method show enhanced activities for the HCOOH electro-oxidation reaction compared to catalysts reduced by NaBH4.
1. Introduction
2. Experimental Section
Electrochemical oxidation of formic acid has attracted a lot of attention in the past few years1-4 due to the great potential of direct formic acid fuel cell (DFAFC) applications.5-10 Recently, noble Pd catalysts were found to possess superior performances in formic acid oxidation in DFAFCs compared with Pt-based catalysts. Considerable efforts were made to find controllable synthetic routes of Pd/C catalysts with designed microstructure, high activity, and enhanced performance for DFAFC applications.11-13 However, the Pd particle size in the Pd/C catalyst prepared with the general reduction method is difficult to control and usually larger than expected.14 Therefore, controllable synthesis of the supported Pd catalysts with designed sizes and nanostructures appears to be significantly desirable. Zhou and co-workers13 investigated the size effects on the electronic and catalytic properties of palladium nanoparticles, and the results revealed that the most active Pd catalyst in formic acid electro-oxidation is made of the smallest particles. Novel synthesis methods to yield designed nanostructures of electrocatalysts for organic molecules and dramatically enhanced catalyst activities were studied by different researchers.15-25 Pt hollow nanospheres synthesized with Co nanoparticles as templates and reducing agent had been reported previously,15 and the catalyst showed improved activity for the oxidation of methanol. In this paper, carbon-supported Pd hollow nanospheres and nanoparticles were prepared, respectively, through the surface replacement reaction between Co nanoparticles and H2PdCl4 with a variety of synthesis conditions. The catalytic activity, morphology, crystal phase structure, and particle size of the catalysts were characterized with cyclic voltammetry (CV), chronoamperometry (CA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The supported Pd catalysts structures and activities were discussed in terms of the pH value of the synthesis solutions.
2.1. Preparation of Supported Pd Hollow Nanospheres and Nanoparticles. All chemicals in this work were analytical grade and used without further purification. Distilled water (DI water) was used to prepare solutions. PdCl2 was dissolved in dilute hydrochloric acid solution to form H2PdCl4 aqueous solution. The Pd/C catalysts at 20 wt % were prepared in water solutions with different pHs. First, appropriate amounts of Vulcan XC-72R carbon black were dispersed ultrasonically in 100 mL of DI water and subsequently mixed with citric acid (0.6 mmol) solutions. The volume of the mixed solutions was adjusted to 500 mL by adding excess DI water. The solution pHs were adjusted to 9, 10, and 11 by adding NaOH aqueous solutions with mechanical stirring. The stirring solutions were purged with N2 to eliminate O2 from the solution. Pre-prepared NaBH4 solutions (0.6 mmol NaBH4 with 50 mL of DI water) were poured to the mixed solutions after N2 had purged for 30 min; the reaction solutions were mixed with CoCl2 solution (0.6 mmol CoCl2 dissolved in 50 mL of DI water) subsequently. After 1 h H2PdCl4 (0.12 mmol) aqueous solutions diluted in 100 mL of DI water were poured into the reaction solutions, and N2 purging was stopped after another hour. The solutions maintained stirring for another 24 h. Finally, the reaction solutions were filtered with filter paper, washed with excess DI water, and dried in air at room temperature. Through these processes 20 wt % Pd/C catalysts were synthesized for characterization. All reactions were conducted at room temperature except for the pH 11 solution. The reaction with pH 11 was conducted in a 40 °C water bath due to CoCl2 being hardly reduced at room temperature in the presence of citrate anion. 2.2. Preparation of Pd/C Catalyst Reduced by NaBH4. A 51.1 mg amount of carbon black was ultrasonically dispersed in 300 mL of DI water, then 2.4 mL of H2PdCl4 aqueous solution (0.12 mmol) was added, and the mixed solution was stirred. Subsequently, 45.5 mg of NaBH4 (1.2 mmol) dissolved with 50 mL of water was added slowly. After 12 h stirring, the solution was filtered with filter paper, washed with DI water,
* To whom correspondence should be addressed. Phone: +86-4315262223. Fax: +86-431-5262225. E-mail:
[email protected].
10.1021/jp073666p CCC: $37.00 © 2007 American Chemical Society Published on Web 11/01/2007
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Figure 1. Formation mechanisms of Pd hollow nanospheres and nanoparticles: (a) Pd hollow nanosphere; (b) separated Pd nanoparticles in adjusted condition.
Figure 3. XRD patterns of synthesized Pd/C catalysts with initial solution pHs: 9 (b), 10 (c), and 11(d).
Figure 2. Schematic flow chart for preparation of Pd/C catalysts.
and then dried in air under room temperature. Through these processes, 20 wt % Pd/C catalyst reduced by NaBH4 was synthesized for characterization. 2.3. Physical Characterization. The X-ray diffraction (XRD) patterns from the catalysts were obtained using a Rigaku-D/ MAX-PC2500 X-ray diffractometer with the Cu KR (λ)1.5405 Å) radiation source operating at 40 KV and 200 mA. The transmission electron microscope (TEM) images were obtained using a JEOL 2010 TEM system operating at 200 KV. For TEM analysis, the carbon-supported catalyst powders were suspended in ethanol, and a drop of catalyst powder suspension was applied to a wholly amorphous carbon film supported on a 3 mm -diameter copper grid. 2.4. Electrochemical Measurements. The electrochemical formic acid oxidation activity of Pd/C electrodes was measured by CV with an EG & G 273 A potentiostat/galvanostat. A conventional, three-electrode cell in which the reference electrode was separated from the working and counter electrode by a Luggin capillary was employed for the electrochemical studies. Large surface area Pt gauze served as the counter electrode, and a Ag/AgCl electrode was used as the reference electrode. All potentials in this study were reported with respect to the Ag/AgCl electrode. The catalyst ink for electrode preparation was formed by 30 min of ultrasonic mixing of 5 mg catalyst powders with 1 mL of ethanol and 50 µL of Nafion solution (5
wt %, Aldrich). Subsequently, 10 µL of the catalyst ink was pipetted on a mirror-finished glassy carbon electrode with 4 mm diameter. Finally, the electrode was dried under room temperature for 20 min. The glassy carbon electrode was polished with a slurry of 0.5 and 0.03 µm alumina successively and washed ultrasonically in DI water prior to use. The apparent surface area of the glassy carbon electrode was 0.126 cm2; thus, the catalyst loading was 0.176 mg cm-2, and the metal loading was 0.0352 mg cm-2. The voltammetric experiments were performed in 0.5 M H2SO4 solution containing 0.5 M HCOOH at a scan rate of 20 mV s-1; N2 was purged for 20 min before starting the experiment. The potential range was from -0.2 to 0.8 V. The measurements were carried out at 25 °C. Electrochemical CO stripping voltammograms was measured by oxidation of preadsorbed CO (COad) in the 0.5 M H2SO4 solution at a scan rate of 20 mV s-1. CO was purged into the 0.5 M H2SO4 solution for 30 min to allow complete adsorption of CO onto the catalyst when the working electrode was kept at 100 mV vs Ag/AgCl electrode, and excess CO in the electrolyte was then purged out with N2 for 40 min. The amount of COad was evaluated by integration of the COad stripping peak, corrected for the electric double-layer capacitance. 3. Results and Discussion 3.1. Formation Mechanisms of the Pd Hollow Nanospheres and Separated Nanoparticles. Figure 1 a shows the formation mechanism of Pd hollow nanospheres. The CoCl2 reacted with NaBH4 to give Co nanoparticles with citrate anion as stabilizer.
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Figure 4. TEM images of 20 wt % Pd/C catalysts: (a) Pd/C catalyst reduced with NaBH4, Pd/C catalysts with initial solution pHs of 9 (b), 10 (c), and 11 (d), (e) unsupported Pd hollow nanospheres prepared with method in literature. The bars in the images indicate a 20 nm scale.
TABLE 1: Effect of Synthesis Solution pH on the Result Particle Size Analyzed from TEM initial pH 9 10 11
pH after add CoCl2
final pH
average particle size, nm
6.0 6.7 8.8
5.3 5.9 7.1
5.8 3.3 3.1
The Co nanoparticles were immediately oxidized to cobalt ions when H2PdCl4 was added to the solution, and the replacement reaction occurred rapidly. The Pd atoms nucleated and grew into small particles, eventually evolved into a thin shell around the cobalt nanoparticles, and the shell had an incomplete porous structure, as reported by Liang and co-workers in their synthesis of Pt hollow nanospheres.15 As can be seen in Figure 1a, the hollow nanospheres were composed of connected Pd nanoparticles. Therefore, separated nanoparticles could be formed with this method if effective techniques could stop the nanoparticles from connecting to each other. The pH of the synthesis solutions was a main factor considered in controlling the nanostructure and size of carbon-supported Pd catalysts since the stabilizing property of citric acid (CA) is directly related to the solution pH. The stabilizing property of CA is rendered by the three carboxyl anions of it. During the reducing process, the three carboxyl anions of CA adsorb on the metal particles and stabilize them through repulsive forces between negatively charged metal nanoparticles and prevent the particles from growth and agglomeration. Hence, citrate anion is believed to act as a better stabilizer compared to citric acid,26,27 and to what extent the protonation of the carboxyl anions occurs will undoubtedly affect the stabilizing ability of CA. Therefore, stabilized separated nanoparticles with smaller sizes will probably be prepared by improving the repulsive forces between negatively charged metal nanoparticles, which can be achieved by increasing the solution pH. Considering the factor mentioned above, separated Pd nanoparticles could be prepared in adjusted conditions within the SRR method as shown in Figure 1b. The catalysts structures and particle size will be discussed in terms of the pH in the following sections.
Figure 5. Histograms to TEM images shown in Figure 4b-d.
3.2. Synthesis of Pd Catalysts. A schematic flow chart for the Pd/C catalysts preparation via the SRR method is illustrated in Figure 2. The initial pHs were adjusted to 9, 10, and 11 to study the pH effects on the structure and size of the catalysts. In all cases the pHs decreased after addition of CoCl2, in which H+ was produced in the reaction between CoCl2 and NaBH4. The pHs dropped further after addition of H2PdCl4 by ionization of H2PdCl4, as shown in Table 1. It should be mentioned that the pHs dropped to low-water marks after addition of CoCl2 and H2PdCl4 before slight increases were observed, which were probably due to oxidation of Co nanoparticles by the remaining PdCl2 in the solutions. However, further increases in pHs could not affect the particle size and morphology since the Co nanoparticles and Pd nanoparticles had already formed. Hence, Table 1 only listed the pH values before increases. The molar ratio of cobalt and palladium was kept at 5:1. Experiments for the reduction of CoCl2 without addition of carbon black were conducted in parallel to observe the experiment phenomenon. No extra step was needed for removal of cobalt nanoparticles since the cobalt nanoparticles could redissolve in water after N2 bubbling ceased, as further proved by XRD analysis. The molar ratio between cobalt and NaBH4 was 1:1 since the
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Figure 8. Chronoamperometric curves for the catalysts prepared in the initial solution pHs: 9 (c), 10 (b), 11 (a).
Figure 6. Cyclic voltammograms at room temperature for formic acid oxidation on different Pd/C catalysts: (a) comparison of catalytic performance between Pd/C reduced by NaBH4 and Pd/C prepared in the initial pH 11 solution; (b) catalytic performance of catalysts prepared in the initial pH 9, 10, and 11 solutions.
Figure 7. Chronoamperometric curves for Pd/C catalysts: (a) Pd/C catalyst prepared in the initial pH 11 solution and (b) Pd/C catalyst reduced by NaBH4.
hydrolysis rate of NaBH4 was slow in high pH environment and excess NaBH4 would affect the replacement reaction between cobalt and palladium. The molar ratio between cobalt and CA was maintained at 1:1. 3.3. XRD Characterization. Figure 3 shows the X-ray diffraction (XRD) patterns of the Pd/C catalysts prepared in different pH solutions. All peaks except for the one at about 25° could be indexed as the palladium face-centered cubic (fcc) phase based on the data of the JCPDS file,28 indicating formation of Pd nanocrystals by a replacement reaction with Co nanoparticles. The peaks at about 25° could be attributed to the XC72 carbon support. No cobalt nanocrystals peaks were found, indicating complete redissolving of cobalt into solution after N2 bubbling stopped. This result is also consistent with the
observations in the Experimental Section. Although the crystalline size of Pd can be calculated according to the Scherrer formula, the result thus obtained is influenced by many factors and may be misleading for ultrafine particles.30 Hence, catalyst particle size measurements were obtained based on TEM analysis29 rather than XRD calculations. 3.4. TEM Characterization. Figures 4 and 5 show the TEM images and corresponding size distribution histograms of the catalysts. As shown in Figure 4a, Pd hollow nanospheres, containing connected smaller nanoparticles, were synthesized according to the method provided in the literature.15 The TEM image shows that the centers of the spheres are brighter than the edges, indicating a hollow structure, with an average diameter of 40 nm of the hollow nanostructure and 8 nm of the Pd nanoparticles. Different from the synthetic method above, the nanostructures of supported Pd catalysts synthesized in this work can be changed from hollow nanospheres, containing connected smaller nanoparticles, to separated nanoparticles, depending on the synthesis solution pH. The effects of pH on Pd nanoparticles sizes are shown in Figure 4b-d. As shown in Figure 4b, with the pH 9 solution Pd hollow nanospheres, with an average size of 15 nm, containing smaller Pd nanoparticles were formed. With the increase of the solution pH (pH g 10) the hollow nanospheres disappeared; instead, highly dispersed carbon-supported Pd nanoparticles were formed (Figure 4c and d). The Pd transformation mechanisms from hollow nanospheres to separated nanoparticles can be ascribed to the following two factors: (i) the size of the Co nanoparticles decreased with the increase of the solution pH. In the preparations of the Pd/C catalysts, the initial pHs of the reaction solutions were 9, 10, and 11, respectively, and then the values dropped to 6.0, 6.7, and 8.8 after addition of CoCl2. Protonation of the third carboxyl group can happen at pHs around the pK3 of citric acid (pK3 ) 6.4), resulting in a variety of repulsive forces of the negatively charged metal nanoparticles in the above three solutions. Consequently, the Co nanoparticle size reduced with the increase of the solution pH, the size reduction of Co nanoparticle provided more growing points for palladium, less opportunities for Pd atoms reduced on a single Co nanoparticle, and then contributed to the synthesis of separated Pd nanoparticles without agglomeration. (ii) The stability of the Pd nanoparticles was enhanced in higher pH solutions. The final solution pHs were in the range where varying degrees of protonation occur, resulting in the differences in stability of Pd nanoparticles. Therefore, the nanostructures transformed from hollow nanospheres to separated nanoparticles with an increase in solution pH. The TEM images suggested that the size of the nanoparticles can also be controlled by varying the pH of the synthesis
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Figure 9. Cyclic voltammograms (CVs) for the oxidation of preadsorbed CO at different electrocatalysts in 0.5 M H2SO4. Dashed curves were CVs for these electrodes without COad. Sweep rate was 20 mV s-1.
solution. As shown in Figures 4 and 5, the size of the Pd nanoparticles decreased when the pH of the solution increased from 9 to 11. The mean particle sizes for the Pd/C catalysts prepared with the initial solution pHs of 9, 10, and 11 were 5.8 (referred to the diameter of Pd nanoparticles, not those Pd hollow nanospheres), 3.3, and 3.1 nm, respectively. The size reduction of Pd nanoparticles can also be attributed to the enhanced stability of Pd nanoparticles in high pH with a lower degree of protonation of citrate anions. For the initial pH 11 solution, the final pH was 7.1; an average particle size of 3.1 nm was obtained with a rather narrow size distribution, as revealed by the corresponding histogram (Figure 4d). With the condition of pH 11, the Pd particle size was expected to be the smallest since nearly no protonation occurred. The above results showed that the synthesis solution pH is a key factor that influences both catalyst nanostructure and particle size. The optimized pH for preparation of supported catalysts with a small size and uniform size distribution appears to be 11. As can be seen in Figure 4e for the TEM image of the catalyst reduced by NaBH4, the particle size was within the range of 3-30 nm with serious agglomeration. 3.5. Electrochemical Characterization. Figure 6 shows the cyclic voltammograms of 0.5 M HCOOH in the 0.5 M H2SO4 solution on different electrodes at a scan rate of 20 mV s-1. The higher HCOOH oxidation current observed from CVs in Figure 6a suggests that Pd/C catalyst (pH 11) exhibited much better HCOOH electro-oxidation activity than the catalyst reduced by NaBH4. The peak current of Pd/C (pH 11) catalyst was 1.66 times the Pd/C catalyst reduced by NaBH4 contributed from the high surface areas due to the smaller particle size and uniform size dispersion, which is consistent with the TEM image shown in Figure 4d. Figure 6b shows CV curves of catalysts
prepared in the initial pH 9, 10, and 11 solutions. The peak currents moved to higher values with increasing pH, which was coincident with the TEM results showing that the particle size decreased. All three catalysts synthesized by the SRR route possessed higher electrochemical activity than the counterpart catalyst reduced by NaBH4. The Pd/C catalyst prepared in pH 11 solution had the highest catalytic activity within the SRR method. Figure 7shows the chronoamperometric curves of 0.5 M HCOOH in 0.5 M H2SO4 solution for Pd/C catalysts electrodes at 200 mV vs Ag/AgCl electrode. The currents for the two electrodes at 1000 s were 562.5 and 247.5 mA/mg, respectively. For comparison, results of chronoamperometric activities of the catalysts prepared in the pH 9, 10, and 11 initial solutions are shown in Figure 8. The currents for these three electrodes at 1000 s were 360, 487, and 563 mA/mg of metal, respectively, suggesting that the Pd/C catalyst synthesized in pH 11 solution had the best performance. Figure 9 shows cyclic voltammograms with (solid curve) and without (dashed curve) COad at Pd electrocatalyst in 0.5 M H2SO4. It was found that difference CV’s (solid curve) in Figure 9, which is the transition from a single, well-defined stripping peak at Pd/C catalyst reduced simply by NaBH4 to a broad, extended stripping peak at the Pd/C catalysts made within SRR method. It suggests that the catalysts made with the SRR method had a higher number of active surface metal sites.31 In particular, the CO stripping charges are 221, 329, 394, and 527 mC mg-1 metal for NaBH4 reduced catalyst, catalysts made in pH 9, 10, and 11 in sequence. The above data shows that the specific area for the pH 11 catalyst was 2.38 times that for the NaBH4 reduced catalyst, which is very consistent with the results from CA and CV. Hence, the higher activity of the catalysts made with the SRR method can be attributed to the higher surface area.
17310 J. Phys. Chem. C, Vol. 111, No. 46, 2007 4. Conclusions The controllable synthesis of carbon-supported Pd catalysts by the surface replacement reaction (SRR) method was introduced. Pd/C (20 wt %) catalysts with different structures were synthesized and studied. The nanostructures of Pd catalysts can be changed from hollow nanospheres, containing connected smaller nanoparticles, to separated nanoparticles depending on the synthesis conditions. Fformation of the catalysts involves CoCl2 reduction by NaBH4, through which the Co nanoparticles can be formed, followed by the surface replacement reaction between Co nanoparticles and H2PdCl4. Citrate anion acts as the stabilizer for the nanostructures and protonation of the third carboxyl anion, and hence, the nanostructure and size of the resulting catalysts is controlled via the pH of the synthesis solution. Pd hollow nanospheres, composed of smaller nanoparticles, supported on carbon were synthesized with pH 9 reaction solution; with the increase of the pH values, the hollow nanospheres disappeared, while highly dispersed carbon-supported Pd nanoparticles with small size and narrow size distribution were formed. Additionally, the size of the Pd nanoparticles also decreased with increasing solution pH. The catalysts synthesized in pH 11 solution with an average particle size of 3.1 nm possessed the highest catalytic activity. The catalytic activity measured by CV was 1.66 times the NaBH4reduced catalyst and 2.27 times measured by CA at 1000 s. Further experiments are in progress to synthesize bimetallic catalysts through the SRR method to enhance the Pd catalytic stability. Acknowledgment. This work was supported by the High Technology Research Program (863 program 2001AA323060) of the Science and Technology Ministry of China, the National Natural Science Foundation of China (20373068 and Key Project 20433060). References and Notes (1) Schmidt, T. J.; Behm, R. J.; Grgur, B. N.; Markovic, N. M.; Ross, P. N. Langmuir 2000, 16, 8159. (2) Levia, E.; Iwasita, T.; Herrero, E.; Feliu, J. M. Langmuir 1997, 13, 6287. (3) Kang, S.; Lee, J.; Lee, J. K.; Chung, S. Y.; Tak, Y. J. Phys. Chem. B 2006, 110, 7270.
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