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J. Phys. Chem. C 2008, 112, 17214–17218
Hydrogen Vanadate as an Effective Stabilizer of Pd Nanocatalysts for Formic Acid Electroxidation Junjie Ge,†,‡ Yuwei Zhang,†,‡ Changpeng Liu,† Tianhong Lu,† Jianhui Liao,†,‡ and Wei Xing*,† State Key Laboratory of Electro-analytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P.R. China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing, P.R. China ReceiVed: July 2, 2008; ReVised Manuscript ReceiVed: September 19, 2008
In this paper, a simple chemical reduction route is discussed that results in small size, uniform dispersion of Pd nanoparticles supported on carbon black. HVO42-, the tridentate oxoanion with its O-O distance of 2.76 Å, closely matching with the Pd-Pd distance (2.75 Å), is expected to be an effective stabilizer for Pd according ¨ zkar, S. Coord. Chem. ReV. 2004, 248, 135). to the lattice size-matching binding model (Finke, R. G.; O Because it has never been tested, HVO42- is exploited and found to be a very simple and effective stabilizer. TEM shows the formation of dispersed Pd nanoparticles supported on carbon with an average particle size of 4.8 nm and rather narrow size distribution. Adsorbed HVO42- can be completely removed after Pd reduction by simply washing with water, as indicated by EDX measurement. The Pd/C catalyst thus prepared shows greatly enhanced activities for the HCOOH electrooxidation reaction compared to catalyst commonly reduced by NaBH4 due to the size reduction of Pd and the uniform distribution. The chemical reduction route of Pd/C catalyst with HVO42- as stabilizer is very simple and has great potentials for mass-producing Pd/C and others noble metal catalysts. 1. Introduction
Overall:
The electrochemical oxidation of formic acid has attracted a lot of attention in the past few years1-4 due to the great potential prospect of direct formic acid fuel cell (DFAFC) in applications.5-10 Recently, noble Pd catalysts were found to possess superior performances compared to Pt-based catalysts in DFAFCs since the oxidation of formic acid occurred mainly through the direct pathway (eq 1) on Pd instead of a dual path way mechanism5 on Pt, in which formic acid oxidation occurred through both a direct pathway and a CO pathway (eq 25). Hence, considerable attentions are paying to new synthetic routes of Pd/C catalysts to gain controllable microstructure, high activity and enhanced performance for DFAFC applications.11-13 Zhou and co-workers14,15 investigated size effects in electronic and catalytic properties of palladium nanoparticles, and the result revealed that the most active Pd catalyst in formic acid electrooxidation is made of the smallest particles. However, the Pd particles size in the Pd/C catalyst prepared with the general reduction method is large.16 Therefore, the synthesis of the supported Pd catalysts with simple method and small sizes appears to be significantly desirable. +
-
HCOOH + CO2 f 2H + 2e
(1)
HCOOH + Pt f Pt-CO + H2O
(2)
Pt0+H2O f Pt-OH + H+ + e-
(3)
Pt-CO + Pt-OH f 2Pt0+CO2+H+ + e-
(4)
0
* To whom correspondence should be addressed. E-mail: xingwei@ ciac.jl.cn. Phone: 86-431-5262223. Fax: 86-431-5262223. † Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.
HCOOH f CO2+2H+ + 2e-
(5)
The preparation of nanocatalysts with small size usually requires the utilization of stabilizers.17 Organic stabilizers such as citric acid, polyvinyl pyrolidine, and different surfactants have been used.18-20 Once the precursors are reduced, the stabilizer molecules are preferably removed, as they otherwise hinder the access of the fuel to the catalyst sites. It has shown that the organic stabilizers can be removed by oxidative heat treatment.21 However, heat treatment results in the agglomeration of the particles and hence the lower catalyst activities. In this paper, a simple chemical reduction route is discussed that results in small size, uniform dispersion of Pd nanoparticles supported on carbon black. In this preparation method, HVO42-, the C3 symmetry oxoanion with its O-O distance of 2.76Å, was exploited as a stabilizer for the first time, and it showed excellent stabilizing ability to Pd nanoparticles by adsorbing on the particle surfaces. What’s more, the inorganic HVO42anion can be removed simply by washing with excessive water. The Pd/C catalyst synthesized by this method was with reduced size, high dispersion, and greatly enhanced catalytic performance for formic acid oxidation. 2. Experimental Section 2.1. Chemicals. All chemicals used in this work were analytical grade and used without further purification. Distilled water (DI water) was used to prepare solutions. PdCl2 was dissolved in the dilute solution of ammonia to prepare the aqueous solution of Pd(NH3)4Cl2. 2.2. Synthesis of Pd-C Catalysts. The Pd-C catalysts were prepared with NaBH4 as the reducing agent and HVO42- as the stabilizing agent. The typical procedure is presented in the following. 100 mg of carbon black was ultrasonically dispersed in 300 mL of water, and then the aqueous solution of 2.5 mL of Pd(NH3)4Cl2 (10.0 mg/mL, 0.24mmol Pd) and appropriate
10.1021/jp8057965 CCC: $40.75 2008 American Chemical Society Published on Web 10/15/2008
Hydrogen Vanadate as an Effective Stabilizer
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17215
Figure 2. EDX pattern of the Pd/C-A catalyst.
Figure 1. XRD patterns of synthesized (a) Pd/C-A catalyst and (b) Pd/C-B catalyst.
amounts of NH4VO3 (purchased from Shanghai regent company, China), in which the molecular ratio of Pd and NH4VO3 was kept to 1:2, was added. The pH of the mixed solutions was adjusted to 10 by ammonia solution with stirring and then heated to 90 °C in a water bath. Subsequently, 91 mg of NaBH4 (2.4mmol) dissolved in 100 mL of water was added slowly. After stirring for 12 h, the solution was filtered with filter paper, washed with excessive hot water to remove the remaining HVO42-, and then dried in air under room temperature. Through these processes, 20 wt % Pd/C catalyst was synthesized for characterizations and referred as Pd/C-A. For comparison, a Pd/C catalyst was prepared without the presence of NH4VO3 and referred as Pd/C-B. 2.3. Reduction of Hydrogen Vanadate with NaBH4 as the Reducing Agent. A total of 56 mg (0.48 mmol) of NH4VO3 was dissolved in 200 mL of water. Then an appropriate amount of ammonia was added to adjust the solution pH to 10. Subsequently, 91 mg of NaBH4 (2.4mmol) dissolved in 100 mL of water was added slowly. The color of the solution turned slowly from colorless to clear yellow and then to brown, indicating the reduction of hydrogen vanadate to vanadium in lower valence states. However, the solution color turned back to colorless after the complete decomposition of NaBH4 in the solution, indicated the regeneration of vanadium (V) in the solution through the oxidation of vanadium in lower valence states by O2 dissolved in water. 2.4. Physical Characterizations. The X-ray diffraction (XRD) patterns of the catalysts were obtained using a RigakuD/MAX-PC2500 X-ray diffractometer (Japan) with the Cu KR (λ ) 1.5405 Å) as a radiation source operating at 40 kV and 200 mA. The transmission electron microscope (TEM) images were obtained by using a JEOL 2010 TEM system operating at 200 kV. For TEM analysis, the carbon supported catalysts powders were suspended in ethanol, and a drop of catalysts powder suspension was cast to a wholly amorphous carbon film supported on a 3 mm-diameter copper grid. The metal loading of the catalyst prepared was measured using energy dispersive X-ray analysis (EDX) in a scanning electron microscope (JEOL JAX-840). 2.5. Electrochemical Measurements. Electrochemical oxidation activity of formic acid on Pd-C electrodes were measured by CV experiments with an EG&G model 273 A potentiostat/galvanostat (USA). A conventional cell with three electrodes was employed for the electrochemical measurements, in which the reference electrode was connected to the working electrode by a Luggin capillary. A Pt plate with large surface area served as the counter electrode and a Ag/AgCl electrode
was used as the reference electrode. All of the potentials in this study were reported with respect to the Ag/AgCl electrode. The catalysts ink for electrode preparation was formed by 30 min ultrasonic mixing of 5 mg catalysts powders with 1 mL of ethanol and 50 µL of Nafion solution (5wt%, Aldrich USA). Subsequently, 10 µL of the catalysts 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 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 catalysts loading was 0.176 mg cm-2, and the metal Pd loading was 0.0352 mg cm-2. The voltammetric experiments were performed in 0.1 M H2SO4 solution containing 0.5 M HCOOH at a rate of 20 mV s-1, and N2 was purged for 20 min before starting the experiment. The potential range was from -0.2 to +0.8 V, and the measurements were carried out at 20 °C. Chronoamperometric curves for the catalysts were measured for 3600 s with the fixed potential of 0.3 V vs Ag/AgCl electrode in the 0.5 M H2SO4 + 0.1 M HCOOH solution. Electrochemical CO stripping voltammograms was measured by oxidation of preadsorbed CO (COad) in the 0.5 M H2SO4 solution. CO was purged into the 0.5 M H2SO4 solution for 30 min to allow the complete adsorption of CO onto the catalysts 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. 3. Results and Discussions 3.1. Stabilization Mechanism of Pd Nanoparticles by HVO42-. Nanoparticles stabilization seems to involve (i) charge stabilization by the surface adsorbed anions such as polyoxoanions, chloride and citrate plus (ii) steric stabilization by the presence of polymers such as the often used poly(vinylpyrrolidone).22 In the charge stabilization, the particles are kinetically stabilized through electrostatically repulsive forces between the particles. The resultant, often anionic particles electrostatically repel each other, thereby providing the particles with Coulombic (“charge repulsion”) kinetic stabilization against further growth and agglomeration.22,23 According to the tridentate, C3 symmetry, lattice size-matching binding model developed by Saim ¨ zkar and Richard G. Finke, two fundamental important insights O to the formation and stabilization of nanoclusters are provided: (i) the premier anionic stabilizers of transition-metal (0) nanoclusters present a tridentate, facial array of oxygen atoms for coordination to the metal (0) surface, and (ii) the preferred tridentate oxoanion stabilizers of nanoclusters are those that have the best match between the ligand O-O and surface M-M distances.24 HPO42-, meeting the demands required above, was found an effective anion for the stabilizing of Ir recently.25 HVO42-, the tridentate oxoanion with 2.76Å O-O distance
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Ge et al.
Figure 3. (a) TEM image and the corresponding size distribution histogram of Pd/C-A; (b) TEM image of Pd/C-B.
(calculated by DFT using the B3LYP hybrid functional combined with the 6-31G(d,p) basis set) and similar structure to phosphate could be a good stabilizer for Pd since it matches closely with the Pd-Pd distance (2.75 Å in the bulk and with little change atop the particles26-28). Vanadium in oxidation state V forms vanadate and vanadate derivatives in aqueous solution.29 NH4VO3 was used in experiment and HVO42- was formed in the aqueous solution at pH 10 according to eqs 6-8.29 HVO42- was expected to adsorb on the surface of Pd, with its C3 symmetry structure and closely matching O-O distance to Pd, thus stabilize the particles from further growth and agglomeration during the reaction.30
H3VO4+H2O f H2VO4-+H3O+
pKa ) 3.5
(6)
H2VO4-+H2O f HVO42-+H3O+
pKa ) 7.8
(7)
pKa ) 12.7
(8)
HVO42-+OH- f VO43-+H2O
The redox activity of hydrogen vanadate in the pH 10 aqueous solution was investigated with the addition of NaBH4 solution. The color of the solution turned slowly from colorless to brown with the addition of NaBH4, which indicated the reduction of hydrogen vanadate and the formation of vanadium in lower valence states. However, all redox potentials between V (V) and V (II, III, IV) are bellow -0.6 V vs NHE in the pH 10 solution and V (II, III, IV) are all unstable in the pH 10 solution,29 therefore, the regeneration of hydrogen vanadate occurred rapidly, as indicated by the solution color turned back to colorless, after NaBH4 has decomposed completely. Although the experiment result shows the reduction of hydrogen vanadate in the pH 10 solution, the redox property of hydrogen vanadate will not affect its stabilizing ability due to the reason shows below. During the experiment, NaBH4 was added slowly to the mixed solution containing both Pd2+ and HVO42-, hence, excessive amount of Pd2+ and HVO42- presented in the solution compared to NaBH4. The addition of NaBH4 to the solution may first cause the reduction of both Pd2+ and HVO42-, while, the instable vanadium in lower valence states will soon react with Pd2+ to yield hydrogen vanadate stabilized Pd (0) nanoparticles. The above process equals to the selective reduction of Pd2+ during the reaction with hydrogen vanadate acts only as the stabilizer. Hence, the redox character of hydrogen vanadate will not affect its stabilizing ability toward Pd nanoparticles, as further been proved by the XRD, TEM, and
Figure 4. Cyclic voltammograms at room temperature for formic acid oxidation on Pd/C-A catalysts (solid line) and the Pd/C-B catalyst (dashed line).
Figure 5. Current density-time dependence measured by CA in 0.1 M H2SO4 + 0.5 M HCOOH for Pd catalysts at 0.3 V (vs Ag/AgCl): (a) Pd/C-A, (b) Pd/C-B.
the electrochemical results which showed both small particle size and excellent catalytic activity of the resulting Pd/C catalyst. 3.2. XRD Characterization. Figure 1 shows the X-ray diffraction (XRD) patterns of the Pd/C-A catalyst prepared with HVO42- as stabilizer and the commonly NaBH4 reduced Pd/ C-B catalyst. 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,31 indicating the formation of Pd nanocrystals through the reduction process. The peaks at about 25° could be attributed to the XC-72 carbon support. Through the comparison of the XRD patterns for the two catalysts, it can be clearly observed that the Pd/C-A catalyst
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Figure 6. Cyclic voltammograms (CVs) for the oxidation of preadsorbed CO at Pd/C-A (a) and Pd/C-B (b) electrocatalysts in 0.5 M H2SO4. Dashed curves were CVs for these electrodes without COad. Sweep rate was 20 mV s-1.
was with smaller particle size due to the obviously broader B(2θ) value. However, the precise calculation of crystalline size according to the Scherrer formula is suspected to be influenced by many factors and misleading for Ultrafine particles.32 Hence, the catalyst particle size measurements were obtained based on TEM analysis16 rather than XRD calculations. 3.3. EDX Characterization. A Typical EDX spectrum for the Pd/C-A catalyst is shown in Figure 2. The mass percentage of Pd was 19.04%, almost the same as the content of Pd in the precursor, indicating the complete reduction of Pd through the preparation method. Trace of vanadium was not present, which means that HVO42- was completely removed from the Pd particle surfaces through simply washing with excessive water. 3.4. TEM Characterization. Figure 3a shows the TEM images and the corresponding size distribution histogram of the Pd/C-A catalyst. As can be seen, dispersed Pd nanoparticles supported on carbon were formed with an average particle size of 4.8nm and rather narrow size distribution, as revealed by size distribution histogram in the inset. For the Pd/C-B catalyst, shown in Figure 3b, the particle size was within the range of 3-30 nm with serious agglomeration in comparison. The size reduction and uniform distribution of Pd nanoparticles indicates the excellent stabilizing ability of HVO42- to Pd nanoparticles. The facilely prepared catalyst with small particle size accompany with the simple removement of the stabilizer made the method possible for mass preparation of Pd/C catalysts. What’s more, HVO42- could also be used as stabilizer for other transition metal nanoparticles such as Fe, Co, Ni, Ru, Rh, Ir, Re, Os, and Pt with similar M-M distance to that of Pd.25 3.5. Electrochemical Characterizations. Figure 4 shows the cyclic voltammograms of 0.5 M HCOOH in the 0.1 M H2SO4 solution on different electrodes at a scan rate of 20mV s-1. The higher HCOOH oxidation current observed from CVs suggests that Pd/C-A catalyst exhibited much better electrooxidation activity of HCOOH than that of Pd/C-B reduced by NaBH4. The peak current of Pd/C-A catalyst was 2.6 times that of the Pd/C-B catalyst and it could be attributed to the high surface areas due to the smaller particle size and uniform size dispersion, which was consistent with the TEM image shown in Figure 3. Figure 5 shows the chronoamperometric curves for the two electrodes at 300 mV. The currents for Pd/C-A and Pd/C-B at 3600 s were 150 and 20.2 mA mg-1 Pd, respectively. The above data shows distinctive enhancement both in catalytic activity and stability of Pd/C-A contributed from the utilization of HVO42- as stabilizer. Figure 6 shows cyclic voltammograms with (solid curve) and without (dashed curve) COad at Pd electrocatalyst in 0.5 M H2SO4. It was found that the stripping peak area at Pd/C-A catalyst was much larger than that of the Pd/C-B catalyst, suggested a higher number of active surface sites of Pd/C-A catalyst compared to Pd/C-B.33 In particular, the CO stripping charges were 221 and 642 mC mg-1 Pd for
Pd/C-A and Pd/C-B catalyst, respectively. Thus, the specific area of Pd/C-A catalyst was improved by a factor of 2.9 compared to that of Pd/C-B catalyst, which was well consistent with a 2.6 times in peak current measured from CV and further proves the function of HVO42- in size reduction of Pd. 4. Conclusions The Pd nanocatalyst with small size was prepared by using HVO42- as stabilizer and the catalyst activity was greatly improved for formic acid electroxidation. HVO42-, the tridentate oxoanion with its C3 symmetry structure and the O-O distance closely matching with Pd-Pd distance, was expected to be an excellent stabilizer for Pd nanoparticles. As never been used, HVO42- was exploited and found to be a very simple, effective, but previously unrecognized stabilizer for the stabilization of Pd nanoparticles. Dispersed Pd nanoparticles supported on carbon black were obtained with average size of 4.8nm and narrow size distribution. The adsorbed HVO42- on the Pd particles after the preparation could be simply removed by washing with excessive water. The catalyst showed distinctly good performance for formic acid oxidation compared to the Pd/C catalyst prepared by common method. Especially, the enhancement on its activity led a 2.6 times advantage in the current density. The stability was also dramatically improved. Both the improvement in catalytic activity and stability could be attributed to the small size and the uniform distribution of Pd nanocatalyst with HVO42- as stabilizer. The facile preparation, the effective stabilizing of Pd nanocatalyst by HVO42-, and the simply remove of HVO42- make the method possible to mass-produce highly active Pd/C catalyst for DFAFC. What’s more, HVO42- can also be used for the synthesis of other transition metal catalysts. Acknowledgment. This work was supported by High Technology Research Program (863 program 2001AA323060, 2003AA517060, and 2006AA03Z224) of 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. (4) Lovic′, J. D.; Tripkovic′, A. V.; Gojkovic′, S. Lj.; Popovic′, K. Dj.; Tripkovic′, D. V.; Olszewski, P.; Kowal, A. J. Electroanal. Chem. 2005, 581, 294. (5) Rice, C.; Ha, S.; Masel, R. I.; Waszczuk, P. J. Power Sources 2002, 111, 83. (6) Rice, C.; Ha, S.; Masel, R. I.; Wieckowski, A. J. Power Sources 2003, 115, 229.
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JP8057965
