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High Active Carbon Supported PdAu Catalyst for Formic Acid Electrooxidation and Study of the Kinetics Yang Liu, Liwei Wang, Gang Wang, Chao Deng, Bing Wu, and Ying Gao* ProVince Key Laboratory for AdVanced Functionalized Materials and Excited State, College of Chemistry and Chemical Engineering, Harbin Normal UniVersity, Harbin 150025, China ReceiVed: June 22, 2010; ReVised Manuscript ReceiVed: NoVember 3, 2010
The promotion effect of Au in the catalyst of PdAu/C for formic acid oxidation is confirmed. As the ratio of Pd:Au in the catalyst is 3:1, the PdAu/C-3 catalyst has the largest electrochemical activity compared with that of catalysts with other ratios of Pd:Au. Tafel plots are employed to examine the charge-transfer kinetics of formic acid electrooxidation. The results suggest that the reaction kinetics is faster on PdAu/C than on Pd/C. The kinetic parameters such as the charge transfer parameter and the diffusion coefficient of formic acid electrooxidation on PdAu/C electrocatalysts are obtained under the quasi-steady-state conditions. The temperature dependence of the formic acid oxidation shows that the oxidation reaction is more sensitive to temperature at 0.1 and 0.7 V. 1. Introduction Recently the electrochemical oxidation of formic acid has been attracted more attention1-3 because a direct formic acid fuel cell has some advantages over a direct methanol fuel cell. Formic acid is nontoxic, nonflammable and low crossover effect through the Nafion membrane because of anodic repulsion between the Nafion and the partially dissociated form of formic acid.4,5 Pt and Pd metals are the most frequently employed catalyst materials for formic acid electrooxidation.6-12 Generally accepted mechanism on Pt-based electrodes is via a dual path, the direct path and indirect path. In the direct path, formic acid is directly oxidized to CO2 and in the indirect path, formic acid is first oxidized to form CO, an intermediate and then CO is oxidized to CO2.13-16 Previous studies have shown that the electrooxidation of formic acid at the Pt catalyst is mainly through the indirect pathway and Pt is easily poisoned by CO at lower potential. Recent researches show that Pt-modified Au nanoparticles can largely improve the activity toward formic acid oxidation and the reaction is mainly through the dehydrogenation or direct path.17-19 Pd is another important transition metal with high catalytic activity. In recent years, much attention has been focused on the high electrocatalytic activity for formic acid oxidation on Pd and Pd-based catalysts.7,11,20-22 The electrooxidation of formic acid at the Pd catalysts is mainly through the direct pathway. However, Pd as an anodic catalyst is instable compared with Pt and slow deactivation was found to reduce the oxidation current during the oxidation of formic acid on a Pd electrode. To improve the electrocatalytic performance and the stability of a Pd catalyst, the Pd-based binary metallic catalysts such as PdPt,23-26 PdSn,27-29 PdCo,30,31 and PdIr32 have been investigated. Au is another metal often used in the catalysts and Au in Pd catalyst can improve the activity and selectivity.33,34 He and co-workers28 found that PdAu/C exhibited high activity for ethanol oxidation in alkaline media and PdAu supported on MWCNTs showed better behavior for formic acid electrooxidation than PdAu/ MWCNTs.35 Zhou36 prepared core-shell AuPd nanoparticles * To whom correspondence should be addressed: E-mail: yinggao99@ 126.com. Phone:+86-451-88060853. Fax: +86-451-88060853.
supported on carbon black, which showed improvement for formic acid oxidation relative to the Pd catalyst. Despite the progress for improving the behavior of the catalyst, much work remains to be done. In this work, we prepared PdAu/C by reducing palladium chloride and chloroauric acid at the same time and obtained high active and stable PdAu/C catalyst. The kinetic parameters obtained under the quasi-steady-state conditions at the PdAu/C catalyst electrode was also investigated. 2. Experimental Section The preparation method of carbon supported PdAu catalysts (PdAu/C) is similar to the method in ref 35. The PdAu/C catalyst were prepared by reducing palladium chloride and chloroauric acid simultaneously. The preparation of PdAu/C catalysts is briefly described as follows: 10 mL ethylene alcohol and 0.04 g the carbon black of pretreatment above were mixed and ultrasonicated for 1 h, followed by adding PdCl2 solution (3.987 g L-1 PdCl2) and HAuCl4 solution (4.972 g · L-1 HAuCl4) drop by drop. (1) for PdAu/C-1, 7.8 mL PdCl2 solution + 6.9 mL HAuCl4 solution; (2) for PdAu/C-2, 5.4 mL PdCl2 solution + 2.4 mL HAuCl4 solution; (3) for PdAu/C-3, 4.9 mL PdCl2 solution + 1.5 mL HAuCl4 solution; (4) for PdAu/C-4, 4.7 mL PdCl2 solution + 1.1 mL HAuCl4 solution; (5) for Pd/C, 4.2 mL PdCl2 solution; (6) for Au/C, 3.5 mL HAuCl4 solution. After that the pH value of mixture was adjusted to pH 8-9 and NaBH4 solution was added to the mixture for reducing the metals for 4 h. Finally, the slurry was filtered and dried under vacuum at 100 °C for 10 h. The preparation of Pd/C and PdAu/C catalyst electrode was as follows: The catalyst of 2.5 mg was mixed with 11.5 µL 5% Nafion solution, 10 µL 20% PTFE, and ethanol was ultrasonicated for 5 min. Then the slurry obtained was spread on a carbon paper and dried at the room temperature. The geometrical surface area of the electrode was 0.5 cm2. The Pd loading of the electrode was 1 mg cm-2. The electrochemical measurements were performed with a CHI650 electrochemical analyzer and a conventional threeelectrode electrochemical cell. A Pt wire was used as the counter electrode. The saturated Ag/AgCl electrode was used as the
10.1021/jp105779r 2010 American Chemical Society Published on Web 11/19/2010
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reference electrode. All of the potentials were quoted with respect to the saturated Ag/AgCl electrode. All of the solutions were prepared with triply distilled water. The solution was carefully purged with N2 (99.999%) prior to each experiment to avoid any O2 contamination in the electrolyte. The atomic ratio of Pd:Au in the catalysts was determined using an energy dispersive spectrometer (EDS, S-4800, Hitachi, Japan). The X-ray diffraction (XRD) analyses of the catalysts were carried out with an X-ray diffractometer (Rigaku D/MaxIIIA) using a Cu Ka source operated at 40 keV, with tube current at 150 mA. Transmission electron microscopic (TEM) measurement was performed with a Tecnais G2 Twins. Before TEM examination, the sample was first ultrasonicated in ethanol for 20 min and then deposited onto 3 mm Cu grids covered with a continuous carbon film. 3. Results and Discussion Part a of Figure 1 shows the cyclic voltammograms of Pd/C and different PdAu/C catalysts in 0.5 M H2SO4 solution and the inset is part of the curves which only shows the hydrogen desorption peaks of the catalysts. The Pd:Au atomic ratios of different catalysts are listed in Table 1. It can be observed that there is almost no desorption peaks of hydrogen at the Au/C electrode (insert curve f of Figure 1), illustrating that Au is not active for the adsorption and desorption of hydrogen. All of the catalysts except Au/C exhibited similar responses in hydrogen adsorption/desorption region and in the double layer charging region. However, the adsorption and desorption peak currents are higher at PdAu/C compared with that at Pd/C electrode suggesting that the corresponding electrochemical active surface areas of PdAu/C electrodes is larger than those of Pd/C. The electrochemical active surface of the catalysts, which evaluates by the Coulombic charge associated with hydrogen desorption, is listed in Table 1. The electrochemical active surface areas increase in the order of the Pd/C < PdAu/ C-1 < PdAu/C-4 < PdAu/C-2 < PdAu/C-3 catalyst. This result indicates that adding Au in Pd/C catalyst can increase the dispersion of Pd. Parts b and c of Figure 1 show the TEM images of the Pd/C and PdAu/C-3 catalysts. It can be observed from parts b and c of Figure 1 that the particles in the PdAu/C-3 catalyst are dispersed more well and uniformly than in the Pd/C catalyst. The XRD patterns for catalysts are shown in parts A and B of Figure 2. In part A of Figure 2, the first broad peak at 2θ ) 25° originates from Vulcan XC-72 carbon support for the catalysts. The 2θ values of the other three peaks, 39.9°, 46.3°, and 67.5° at Pd/C (curve f of part A of Figure 2) are reflections of the face-centered cubic crystal lattice of the Pd(111), Pd(200), and Pd(220) crystal faces and the 2θ values of 38.26°, 44.41°, 64.60°, and 77.66° at Au/C (curve a of part A of Figure 2) are reflections of the face-centered cubic crystal lattice of the Au(111), Au(200), Au(220), and Au(311) crystal faces. However, the 2θ values of the four diffraction peaks at PdAu/C-1 (curve b of part A of Figure 2) catalyst are 38.37°, 44.57°, 64.88°, and 77.78°, which correspond to the 2θ value of Au(111), Au(200), Au(220), and Au(311) respectively but they all shift to higher 2θ values and the peaks related to Pd are hardly observed. This indicates that the Pd atoms in PdAu/C-1 are dispersed into the Au lattice or the formation of the alloy is Pd in Au. To observe the shift of the diffraction peaks clearly, the shout-range (from 35.5° to 42.0°) XRD profiles of the different catalysts are shown in part B of Figure 2. It can be seen from part B of Figure 2, as the atomic ratio of the Pd:Au increases (curves c, d, and e of part A of Figure 2), the
Figure 1. (A) Cyclic voltammograms of Au/C, Pd/C and the different ratio of PdAu/C catalysts in 0.5 M H2SO4 solution at 25 °C, scan rate 10 mV · s-1. Insert: shows only the hydrogen desorption region. (B) and (C) are TEM images of Pd/C and PdAu/C-3 catalysts.
diffraction peaks of Pd(111), Pd(200), and Pd(220) appears. Conversely, diffraction peaks of pure Au in these catalysts are not shift to lower 2θ values but the peaks of Pd shifted to higher 2θ values indicating an enlargement of interatomic distance in the crystal (part B of Figure 2). This means that some Au atoms
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TABLE 1: ESA and the Mean Particle Size of Pd/C and the Differernt Ratio of PdAu/C Catalysts catalysts
atomic ratio of Pd:Au
ESA (m2/g)
da (nm)
Pd/C PdAu/C-1 PdAu/C-2 PdAu/C-3 PdAu/C-4
1:0 0.93:1 2.3:1 2.9:1 4.25:1
36.9 39.4 64.2 70.1 46.9
6.3 5.9 4.1 3.9 5.2
a
d, mean particle size of the catalysts was calculated using the Debye-Scherrer equation.
Figure 3. Cyclic voltammograms curves of Au/C, Pd/C and the different ratio of PdAu/C catalysts in 1.0 M HCOOH + 0.5 M H2SO4 solution at 25 °C, scan rate 10 mV · s-1.
Figure 4. Tafel plots for oxidation of 1.0 M HCOOH in 0.5 M H2SO4 solution on PdAu/C and Pd/C electrodes.
Figure 2. XRD patterns of the Pd/C and the different ratios of PdAu/C catalysts (a) Au/C, (b) PdAu/C-1, (c) PdAu/C-2, (d) PdAu/C-3, (e) PdAu/C-4, (f) Pd/C.
substitute for Pd atoms into the Pd lattice and form PdAu alloy. The results above exhibit that part of the Au atoms exist as Au metal and other part of the Au atoms exist in the lattice of Pd in the PdAu/C-2, PdAu/C-3, and PdAu/C-4 catalysts or the alloys formed in this catalysts is Au in Pd. Figure 3 shows the cyclic voltammograms of 1.0 M HCOOH in 0.5 M H2SO4 solution at Pd/C and PdAu/C catalyst electrodes at 25 °C. No anodic peak is evident at the Au/C catalyst electrode (curve f of Figure 3), indicating that the Au/C catalyst has no electrocatalytic activity for the oxidation of formic acid. It can be seen from Figure 3 the anodic peaks at Pd/C electrode are located at 0.16 and 0.63 V. The corresponding peak current densities are 98.8 and 71.2 mAcm-2. The main anodic peaks at PdAu/C electrodes are almost the same, 0.10 V and 0.63 V. The peak at the lower potential is about 60 mV more negative than that at the Pd/C electrode and current densities are different at the different PdAu/C electrodes but all higher than that of Pd/C. The oxidation currents of formic acid on the catalysts
decreases apparently at 0.65 V because the palladium oxide is formed at the potential (part A of Figure 1) and palladium oxide is not active for formic acid oxidation. This can also explain the markedly larger anodic currents during the negative going sweep when the palladium oxide is reduced to metal (Figure 3). The above results illustrate that adding Au into the catalyst can largely increase the current density of formic acid oxidation at the lower potential and shift the peak potential negatively compared with that of Pd/C. One reason of the activity improvement is that Pd in PdAu/C catalysts dispersed better than in Pd/C, another reason is the electronic effect. Metal Pd in the catalyst can gain d-electrons from the Au, which can weaken the adsorptive strengths of the reaction intermediates in formic acid oxidation.36 Figure 4 shows the Tafel plot for formic acid electrooxidation at Pd/C electrode in 1 M HCOOH + 0.5 M H2SO4 solution. The linear region of the Tafel plot stretches from -0.05 to 0.05 V. With further increasing potential, the Tafel lines become curved, indicating that the oxidation of formic acid is no longer a charge transfer control reaction. The Tafel slopes and intercept of formic acid oxidation on Pd/C and PdAu/C and related standard errors in the linear fit are listed in Table 2. It can be seen from the Table 2 that the linear fit errors are very small which indicate the relatively accurate results. The values of the Tafel slope decreased largely with the existence of Au in the catalysts and decreased gradually with the increasing ratio of Au in the catalysts. This means that the charge-transfer kinetics of formic acid electrooxidation on PdAu/C is faster than on Pd/ C. The intercept of the Tafel plot is related to the ln j0, j0 is the exchange current density. The intercepts for PdAu/C except for
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TABLE 2: Tafel Slope of Pd/C and PdAu/C Catalysts for Formic Acid Oxidation and Related Standard Errors in Linear Fit
catalysts
Tafel slope (mV dec-1)
standard error of slope in linear fit
intercept (V)
standard error of intercept in linear fit
PdAu/C-1 PdAu/C-2 PdAu/C-3 PdAu/C-4 Pd/C
138 140 142 140 170
0.0037 0.0043 0.0044 0.0042 0.0044
-0.214 -0.234 -0.259 -0.233 -0.416
0.0061 0.0072 0.0078 0.0069 0.0072
PdAu/C-1 (Table 2) are larger than that for Pd/C indicating that the exchange current densities are greater on PdAu/C compared with Pd/C. These results indicate that Au in small ratio in the PdAu/C catalysts can accelerate the rate of formic acid oxidation on the electrode. Figure 5 shows the linear sweep voltammograms of 1.0 M HCOOH in 0.5 M H2SO4 solution on PdAu/C-3 electrode at different scan rates. It can be clearly seen that the peak at the lower potential shifts positively with the increasing of scan rate. This means that the rate-determining step of formic acid oxidation at lower potential on PdAu/C is an irreversible electrode process which is controlled by diffusion rate. The peak at 0.65 V was almost no shift with the increase of scan rate, which means that the reaction at this potential related to the species adsorbing at the surface of the catalyst. The previous studies have shown that the mechanism of formic acid electrooxidation on Pd at lower potential is mainly through the direct path. So the total reactions could be expressed as the following:37-39
HCOOH + Pd f Pd-COOH + H+ + e-
(1)
Pd-HCOO f Pd + CO2 + H+ + e-
(2)
Pd + H2O f Pd-OH + H+ + e-
(3)
Pd-OH + Pd-HCOO f 2Pd + CO2 + H2O
(4)
dφp 2.3RT ) (V) dlgν 2Rn′F
(5)
where n′ is the number of electrons transferred in the ratedetermining step and R is the charge transfer coefficient. The dependence of the peak potential on the logν is shown in Figure 6. The slope of the line is 137.9 mV dec-1. So the value of Rn′ can be obtained by eq 5. The calculated value of Rn′ is 0.21. The rate-determining electron transfer is one-electron-process, n′ ) 1, so R is 0.21. In the case of a completely irreversible reaction, the peak current in amperes is as the following.41
jp ) 2.99 × 105n(Rn′)1/2C∞D1/2V1/2
(6)
where jp is the peak current density measured in A cm-2, D is the diffusion coefficient in cm2s-1, C∞ is the formic acid concentration in the solution, in mol cm-3, ν is the sweep rate, in V s-1, and n is the number of electrons transferred in the total reactions. Figure 7 shows the curve of peak current density versus the square root of the scan rate. The linear relationship between the peak current density and the square root of scan rate confirmed that the oxidation of formic acid at PdAu/C-3 is a completely irreversible kinetic process. The slope of the line is 0.027. The value of diffusion coefficient, D, can be obtained from the slope of the lines in Figure 7 by using eq 6. The diffusion coefficient is a measurement of the charge-transport rate within the liquid film near the electrode surface. In this case, the value of D is 4.23 × 10-6 cm2s-1 which is larger than
At the lower potential, the oxidation of formic acid is through the path of reaction 1 and 2. The rate-determining step is the mass-transfer process40 or reaction 1. According to Brett,41 the peak potential values are proportional to the logν of scan rates as the following equation for the completely irreversible system.
Figure 6. Plot of the peak potential versus log ν for 1.0 M HCOOH in 0.5 M H2SO4 solution at the PdAu/C-3 catalyst electrode.
Figure 5. Linear sweep voltammograms of 1.0 M HCOOH + 0.50 M H2SO4 solution on PdAu/C-3 electrode at different scan rates.
Figure 7. Plot of the peak current density versus the square root of scan rates for 1.0 M HCOOH in 0.5 M H2SO4 solution at the PdAu/ C-3 catalyst electrode.
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Figure 8. Linear sweep voltammograms of 1.0 M HCOOH + 0.50 M H2SO4 solution on PdAu/C-3 electrode at different temperatures, at the scan rate of 2 mV · s-1.
Figure 9. Arrhenius plots of log j versus 1/T at different potentials for the oxidation of formic acid on the PdAu/C-3 electrode.
TABLE 3: Activition Energy of Formic Acid Oxidation on PdAu/C-3 at Different Potential and Related Standard Errors and Standard Deviations of the Linear Fit potential V activition energy standard error in standard deviation (vs. Ag/AgCl) (kJ mol-1) linear fit of the fit 0.1 0.2 0.3 0.4 0.5 0.6 0.7
18.8 15.9 15.0 16.4 18.3 20.1 26.8
1.67 1.66 1.58 1.75 1.82 1.91 2.12
0.044 0.044 0.042 0.054 0.078 0.082 0.091
that on Pd/C (1.47 × 10-7 cm2s-1)42 in the solution of 0.5 M HCOOH + 0.5 M H2SO4. We further investigated the temperature dependence of the formic acid oxidation on a PdAu/C-3 electrode in the temperature range of 20-60 °C at very low scan rate (2 mV/s) which can be thought of as a quasi-steady state (Figure 8). With elevated temperature, the activities are enhanced, as indicated by the increase in oxidation current. Especially for the peak around 0.7 V, the peak current increases much faster with increasing temperature, also indicating the different reaction mechanism for formic acid oxidation at this higher potential. Figure 9 shows the Arrhenius plots for the current densities of formic acid oxidation reaction at various potentials. The apparent activation energies, the standard errors and the standard deviation of the linear fit are listed in Table 3. The values of the standard deviation of the linear fit are lower than 0.1, which means that log i and 1/T have relatively good linear relationship. The standard errors are 1.67-2.12 in the potential range 0.1-0.7 V, as shown in Table 3. The result shows that the activation
Figure 10. Dependence of HCOOH oxidation current density on HCOOH concentration.
energies decreased with increasing potential at 0.1-0.3 V and then increased at 0.4-0.7 V. This result means that the activity of formic acid oxidation on the PdAu/C-3 electrode was more sensitive to temperature at 0.1 and 0.7 V. It is because that the mechanisms for formic acid oxidation at about 0.1 and 0.7 V are different. At the lower potential, the oxidation of formic acid is through the path of reaction 1 and 2 and at the relatively higher potential, the oxidation is through the path of reaction 3 and 4.38 The influence of the formic acid concentration was investigated in 0.5 M H2SO4 with 0.1-4 M HCOOH at PdAu/C-3 (Figure 10). The reaction rate increased with the increase of HCOOH concentration up to 4 M. Further increasing the concentration of HCOOH, the reaction rate no longer increased significantly. The slope of the curve of log j vs logCHCOOH (Figure 10) is 1.05. Therefore, the reaction order of reaction 1 is about 1.0 at the PdAu/C-3 catalyst electrode. 4. Conclusions In the present study, the promotion effect of Au in the AuPd/C catalyst is investigated. The results in this paper show that the activity of PdAu/C-3 catalyst is largely improved compared with Pd/C though the Au/C has no activity for formic acid oxidation. The oxidation of formic acid on a PdAu/C electrode is a completely irreversible kinetic process at lower potential. The lower values of the Tafel slope on PdAu/C electrodes indicate the relatively fast kinetics compared with that on Pd/C. The charge transfer parameter (R) and diffusion coefficient of formic acid (D), obtained in the present work are 0.21 and 4.23 × 10-6 cm2s-1, respectively. Oxidation of formic acid increased with the increasing HCOOH concentration with the reaction order of 1.0. Formic acid oxidation on a PdAu/C-3 electrode was sensitive to temperature at relatively high potential because the activation energy was significantly increased with the increase of potential. Acknowledgment. The authors are grateful for the financial support of the National Natural Science Foundation of China(No.20573029, 50902041), Natural Science Foundation of Heilongjiang Province (B200905) of China, Innovation special fund of Harbin Science and Technology Bureau of China (2010RFXXG018), The Youth Fund of Harbin Science and Technology Bureau of China (2007RFQXG059) and The Youth Found of Harbin Normal University (KGB200812). References and Notes (1) Zhu, Y. M.; Khan, Z.; Masel, R. I. J. Power Sources 2005, 139, 15.
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