ARTICLE pubs.acs.org/JPCC
Electrocatalytic Performance of Gold Nanoparticles Supported on Activated Carbon for Methanol Oxidation in Alkaline Solution Shaohui Yan, Shichao Zhang,* Ye Lin, and Guanrao Liu School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing, 100191, China ABSTRACT: Gold nanoparticles supported on activated carbon (Au/ C) are prepared by rapid reduction of AuCl4 on the surface of activated carbon with KBH4 in the presence of polyvinylpyrrolidone. Through the characterization of the transmission electron microscope, the Au nanoparticles (AuNPs) are highly well dispersed on the carbon support. Cyclic voltammetry, quasi-steady-state polarization, and electrochemical impedance spectroscopy methods are employed to investigate the catalytic activity of the Au/C catalyst for the methanol electro-oxidation (MEO) and the reaction kinetics. The results indicate that the Au/C catalyst shows good catalytic activity toward the MEO, and the weakly adsorbed OH on the surface of AuNPs has auxiliary catalysis for the MEO. On the basis of this crucially auxiliary catalysis, a novel mechanism of the rate-determining step of the MEO catalyzed by Au/C in alkaline solution is proposed.
1. INTRODUCTION Noble metal nanoparticles material has been a focus of intensive research in recent years due to their numerous applications, such as sensors,1,2 catalysts,35 photocatalysts,6 and electrocatalysts.710 Gold has long been recognized as a poorly active material.1114 When gold is dispersed as ultrafine particles, however, it exhibits highly active catalyst for CO oxidation,15 owing to its dramatically changing chemical properties at the nanoscale.3,1518 In general, the catalytic activity of Au nanoparticles (AuNPs) is closely associated with the methods of preparing AuNPs. It has been reported that a highly dispersed Au catalyst prepared by depositionprecipitation, gas-phase grafting, and liquid-phase grafting methods using an Al2O3, SiO2, or TiO2 support exhibits higher overall catalytic activities compared with that prepared by the impregnation method.14 Among the methods for the synthesis of nanogold catalysts, the chemical reduction process has been an effective one. Tsukuda and co-workers prepared colloidal gold nanoparticles stabilized by poly(N-vinyl-2pyrrolidone) (PVP), which are extremely active toward the carbon carbon bond formation of phenyboronic acid.19,20 The electrocatalytic activity of gold to methanol oxidation in aqueous solution also aroused great interest in recent years. Numerous relevant studies on gold catalysts were carried out with the main aim of using gold as an electrocatalyst in direct methanol fuel cells (DMFCs).2125 It is generally accepted that the catalytic activity of gold is poor compared with that of platinum and platinum-based alloys toward the methanol electro-oxidation (MEO), particularly in acidic medium. However, the activities of platinum and platinum-based alloy catalysts are often influenced by strongly chemisorbed carbonaceous intermediates, causing their catalytic activity to diminish gradually.26,27 On the contrary, the activity of Au in an alkaline medium is comparatively high,28 and no poisoning species forms during the process of the MEO.27,29,30 Therefore, Au and Au-based electrocatalysts have promising applications in DMFCs.2830 r 2011 American Chemical Society
The mechanism study of the MEO catalyzed by gold is one of the basic studies related to the application of gold catalysts. The generally accepted mechanism is as follows: In the low-potential range (before the formation of the compact gold oxide layer), the methanol molecules are mainly oxidized to yield formate with the exchange of four electrons, according to the reaction given below24,31,32 CH3 OH þ 5OH ¼ HCOO þ 4H2 O þ 4e
ð1Þ
In the high-potential range (after so-called “turnover”), methanol is oxidized to carbonates via a six-electron transfer reaction, as shown by eq 224,31,33 CH3 OH þ 8OH ¼ CO2 3 þ 6H2 O þ 6e
ð2Þ
Furthermore, it has been reported that the formation of the preoxidation species or the strong OH adsorption at the lowpotential range acts as key roles for the MEO.24,29,31 In this work, we mainly focus on the preparation of highly active nanogold catalysts and the research of the MEO mechanism on Au/C catalyst in the low-potential range. According to the results of cyclic voltammetry, steady-state polarization, and electrochemical impedance spectroscopy methods, the weakly adsorbed OH on the surface of AuNPs has auxiliary catalysis for the MEO.
2. EXPERIMENTAL SECTION Synthesis of Au/C Catalyst. The typical procedure for preparation of the Au/C catalyst is as follows: 67 mg of activated Received: September 12, 2010 Revised: March 6, 2011 Published: March 18, 2011 6986
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Figure 1. (a) TEM image of Au/C. (b) EDX spectrum of the boxed area marked in (a). (c) Magnified TEM image corresponding to the EDX spectrum. (d) Size distribution of the AuNPs in Au/C.
Figure 2. Schematic diagram of the synthesis of Au/C catalyst.
carbon (Vulcan XC-72R, Cabot Corp., USA) is dispersed in a 480 mL of HAuCl4 (0.3 mM) and 1.598 g of poly(N-vinyl-2pyrrolidone) (PVP, K-30, 40 Kda) mixed aqueous solution by ultrasonic agitation for 30 min. Subsequently, the mixture is adjusted to alkalescence (pH = 9.3) using 1 M ammonia. Afterward, the KBH4 solution (0.6 M, 25 mL) is rapidly added to the mixture under vigorous agitation. After stirring further for 24 h and aging for 24 h, the mixture is filtered. The filter residue is washed adequately using distilled water to remove the PVP on the surface of the AuNPs due to the fact that residual PVP would decrease the activity of AuNPs. Ultimately, the Au/C catalyst is obtained after drying in an oven at 60 °C for 5 h. Characterization of Au/C Catalyst. The morphology and chemical composition of the Au/C catalyst are investigated using a JEM 2100F transmission electron microscope (TEM, JEOL) equipped with an energy-dispersive X-ray analysis system (EDX, Phoenix, EDAX International Corporation). X-ray powder diffraction (XRD) data of Au/C is collected using a Bruker D8 advanced X-ray diffractometer using Cu KR radiation (λ = 1.5418 Å) at a step rate of 0.02°/s. The average Au loading of the catalyst is determined by Oxford INCA EDX equipped on an S-530 scanning
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Figure 3. (a) XRD pattern of Au/C. (b) EDX spectrum of Au/C.
electron microscope (SEM, HITACHI). The SEM sample is prepared by dispersing the Au/C catalyst on the surface of a copper column (Φ = 2 mm) set in a polytetrafluoroethylene (PTFE) substrate. Electrochemical Measurement of a Au/C Composite Film Coated Electrode. All electrochemical measurements are performed in a conventional three-electrode cell at ambient temperature (∼25 °C) using an IM6ex electrochemical workstation (ZAHNER, Germany). The fabrication of the working electrode is given below: 5 μL of catalyst ink, prepared by mixing 2.0 mg of Au/C catalyst, 950 μL of ethanol and 50 μL of Nafion solution (5 wt %, DuPont Corp., U.S.A.), is placed on a polished glassy carbon working electrode (Φ = 4 mm) using a microsyringe. A sheet of glassy carbon (0.5 cm2) and a mercury|mercury oxide electrode (Hg|HgO, 1.0 M KOH) are employed as counter and reference electrodes, respectively. Prior to electrochemical measurements, the electrolytes are deaerated by bubbling with highpurity nitrogen for 30 min.
3. RESULTS AND DISCUSSION Figure 1a shows the TEM image of Au/C, which indicates that the AuNPs are highly well dispersed on the carbon support. The EDX spectrum (Figure 1b) of the boxed range marked in Figure 1a reveals the chemical compositions (C and Au) of Au/C (Cu peaks are from the support grid). The magnified TEM image corresponding to this boxed area is given in Figure 1c. It shows that the size of the AuNPs ranges from 2 to 16 nm. The size distribution of the AuNPs in the Au/C catalyst is shown in the histogram in Figure 1d, which is obtained by counting more than 200 randomly chosen particles from TEM images of the Au/C catalyst. As shown in Figure 1d, the average diameter of the AuNPs in Au/C is around 6.7 nm. Generally, the size of the nanoparticles is determined by the relative rates between the nucleation and the particle growth.20 The possible process during which AuNPs grow on activated carbon under our preparation conditions is shown in Figure 2. In this process, the effect of the 6987
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Figure 4. (a) CVs on Au/C at a scan rate of 20 mV/s in deoxygenated 0.1 M KOH solutions with different concentrations of methanol. (b) Tafel plots for electro-oxidation of methanol on Au/C as a function of methanol concentration, at a scan rate of 0.2 mV/S. (c) CVs on Au/C at a scan rate of 20 mV/ s in deoxygenated solutions with 5 M CH3OH and different concentrations of KOH. (d) Tafel plots for electro-oxidation of methanol on Au/C as a function of KOH concentration, at a scan rate of 0.2 mV/S. (e) Plots of log(i) against log(CCH3OH) at different potential in Tafel range. (f) Plots of log(i) against the pH of the solution at different potentials in Tafel range. The variable in each figure, the concentration of methanol, the concentration of KOH, or the potential, is shown in the corresponding figure.
ammonia on the size and dispersion of AuNPs is crucial. On one hand, ammonia reacts with AuCl4 to form fulminating gold nanoparticles stabilized by PVP (PVP acts as a stabilizing agent in this process),34,35 which are partially deposited on the surface of the carbon support. When KBH4 is added in the mixture, the fulminating gold nanoparticles turn into Au nuclei immediately. During the 24 h of stirring, the Au nuclei on the surface of the activated carbon grow into AuNPs in situ, consuming a part of the Au nuclei that existed in the mixture, while, the rest of the Au nuclei directly deposit onto the carbon support after they turn into AuNPs capped by PVP in solution. The average size of the latter AuNPs is larger than that of the former AuNPs. This leads to the size distribution of AuNPs being rather broad. In general, the small particle tends to show better activity. The AuNPs mostly distribute below 10 nm; however, the effect of this large particle on the activity of the Au/C catalyst is rather slight. On the other hand, ammonia restrains the hydrolysis of KBH4 to ensure the high concentration of BH4 in solution, which increases the rates of nucleation and particle growth. The increase in the rates of nucleation and particle growth results in the small size and the good dispersion of the AuNPs in Au/C catalysts.
Figure 3a is the powder XRD pattern of Au/C. The peak at 2θ = 25.1° in the XRD pattern is the (002) plane of graphite (JCPDS, No. 74-2330); the peaks located at 2θ = 38.4, 44.5, 64.9, and 77.8° correspond to the (111), (200), (220), and (311) lattice planes of the face-centered cubic structure of gold (JCPDS, No. 4-0784), respectively. The mean size of the AuNPs in the catalyst is estimated to be about 7.2 nm by Scherrer’s equation20 according to the peak (220) revealed by Figure 3a, which is approximately in accordance with the size distribution data. The average Au loading of the catalyst is 25.9%, revealed by the EDX analysis. The corresponding EDX pattern is shown in Figure 3b (Cu and F peaks are from Cu and the PTFE substrate, respectively). The electrochemical behaviors of the Au/C electrode are investigated in deoxygenated 0.1 M KOH solutions with and without methanol. Figure 4a shows the cyclic voltammograms (CVs) on Au/C in 0.1 M KOH solutions with different concentrations of methanol at a scan rate of 20 mV/s. According to Figure 4a, the peak value of the anodic mass-specific current density enhances significantly with increasing methanol concentration and attains 48.6 mA 3 mg1 Au at 0.355 V in 0.1 M KOH and 5 M CH3OH mixed solution. The mass-specific current 6988
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Figure 5. Chronoamperograms on Au/C catalyst in 0.1 M KOH and 5 M methanol mixed solutions at 0.25, 0.3, and 0.35 V versus Hg|HgO. The potentials are displayed the figure.
density can also be expressed as the current density normalized by the actual surface area (104 cm2 3 mg1 Au), which is estimated from the amounts of electricity involved in the reduction of the Au oxide monolayer.36,37 Figure 4b displays the Tafel plots for the MEO on Au/C, log(i) versus E, as a function of methanol concentration at a scan rate of 0.2 mV/s. The Tafel plot in the 0.1 M KOH solution without methanol (black squares in Figure 4b) has three linear ranges, which are 0.070.13, 0.130.3, and 0.30.5 V. This indicates that the electrochemical behaviors are different in these three ranges. In the first linear range (0.070.13 V) for the solution without methanol, the anodic current revealed in the CV on Au/C in the 0.1 M KOH solution without methanol (black curve in Figure 4a) increases slightly, which is assigned to the chemical adsorption of OH and the formation of preoxidation species on the surface of AuNPs.24,25,31,33 For instance, is formed on AuNPs according to eq 3, which is in AuOH(1λ) ads agreement with previous research of J. Lipkowski.38 ð1 λÞ
Au þ OH f AuOHads
þ λe
ð3Þ
where the ads and λ denote the chemical adsorbed species on AuNPs and the charge-transfer coefficient that varies between 0 and 1, respectively. In the second range (0.130.30 V) for the solution without methanol, surface oxidation processes occur; the surface of the AuNPs is oxidized to form gold oxide species with faradaic current flowing through the interface.39 These two ranges are called the ranges before the gold oxide monolayer formation in refs 24 and 29. In the third range (0.300.5 V) for the solution without methanol, the gold oxide monolayer is formed.31 According to the CV on Au/C in the 0.1 M KOH solution without methanol (black curve in Figure 4a), the gold oxide is reduced to form a cathodic current peak at 0.18 V during the negative scan.24,25,31,33 For the solutions with methanol, Tafel slopes have the same value, 285.5 mV 3 dec1, in the Tafel range (0.0250.3 V). The value of the transfer coefficient (Rn) calculated from the slope is about 0.21. This suggests that the first charge transfer is the rate-determining step.30,40 As revealed in Figure 4b, the top potential (0.3 V) of the Tafel range (0.0250.3 V) for the solutions with methanol is identical to the ending (0.3 V) of the second Tafel range (0.130.3 V) for the solution without methanol, implying that the influence of the OH with a different form on the MEO on Au/C is dissimilar. It
Figure 6. Complex-plane (Nyquist) impedance plots of the MEO on Au/C in the 0.1 M KOH and 5 M methanol mixed solution at different potentials. The potentials are given in the figures. Solid lines are representative simulations based on the corresponding equivalent circuits shown in Figure 7.
is interesting that, according to Figure 4b, the starting potential (0.025 V) of the Tafel range (0.0250.3 V) for the solutions with methanol is more negative than that (0.07 V) of the first Tafel range (0.070.13 V) for the solution without methanol. This indicates that the onset potential of the MEO is lower than the potential of the chemical adsorption of OH, which suggests that abundant activated OH for the MEO exists on the surface of AuNPs before the OH is chemisorbed on the surface of AuNPs. 6989
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Table 1. Sampling Data and Overall Reaction Orders (m) Estimated from Figure 6 for Methanol and OH at Different Potentials
0.1 M KOH
5 M CH3OH
0.05 V
0.1 V
0.15 V
0.2 V
0.25 V
0.1 M CH3OH
0.94119
0.76045
0.61726
0.4681
0.31545
0.2 M CH3OH
0.72268
0.55737
0.38984
0.23096
0.07642
0.5 M CH3OH
0.43112
0.25212
0.08712
0.07988
0.24688
1 M CH3OH
0.21112
0.02512
0.15188
0.32588
0.49488
2 M CH3OH
0.02288
0.21988
0.38888
0.56588
0.75088
5 M CH3OH
0.31588
0.51788
0.69388
0.86488
1.03088
overall reaction order
0.741
0.759
0.774
0.788
0.802
0.05 M KOH, pH = 12.7 0.1 M KOH, pH = 12.97
0.16888 0.31588
0.39388 0.51788
0.57888 0.69388
0.74788 0.86488
0.90688 1.03088
0.2 M KOH, pH = 13.24
0.50888
0.69788
0.87588
1.04988
1.21888
0.5 M KOH, pH = 13.59
0.67288
0.85188
1.03388
1.21388
1.37888
1 M KOH, pH = 13.87
0.83288
1.01788
1.20788
1.38588
1.53388
overall reaction order
0.567
0.533
0.539
0.548
0.540
Figure 7. Equivalent circuits for the electrochemical reaction on the Au/C in the 0.1 M KOH and 5 M methanol mixed solution at different potentials, (a) 50250, (b) 300400, and (c) 450600 mV.
The activated OH cannot be the OH in the electrolyte due to its concentration that remains a constant all of the time. Therefore, this kind of actively adsorbed OH (OH a-ads) can only be the weakly adsorbed OH on the surface of AuNPs. After the potential (0.13 V) where the gold oxide species forms on the surface of AuNPs, the OH a-ads still plays a key role for the MEO based on the same kinetics exhibited in the whole Tafel range (0.0250.3 V) for the solutions with methanol. This also suggests that the chemical adsorption of OH and the formation of the gold oxide species have no impact on the amount of the OH a-ads. According to CVs on Au/C in the solutions with methanol in Figure 4a, the acceleration of the current density of the MEO decreases with the enhancement of the electrode potential above 0.3 V, resulting in the current density reaching a maximum value at about 0.355 V. This displays that the gold oxide monolayer on the surface of AuNPs restrains the MEO and that the formation of the gold oxide monolayer results in a decrease in the number of the OH a-ads. This is also approved by a new MEO peak that appeared at 0.14 V (Figure 4a) after gold oxide reduction during the reverse sweep.24,25,31,33 The reduction peak at about 0.1 V in Figure 4a is related to the desorption of OH on the surface of AuNPs, which exhibits the powerful adsorbability of the Au/C catalyst toward OH. The effect of the KOH concentration on the MEO on Au/C is investigated by varying the concentration of KOH from 0.01 to 1.0 M. Figure 4c displays the CVs on Au/C in solutions containing 5 M CH3OH and different amount of KOH at a scan
rate of 20 mV/s. With increasing KOH concentration, according to Figure 4c, the anodic mass-specific current density enhances significantly, and the potential of the peak current shifts negatively. The Tafel plots obtained in solutions with different KOH concentrations at a scan rate of 0.2 mV/s are shown in Figure 4d, which indicates that the onset potential of the MEO shifts negatively when the KOH concentration increases. These results demonstrate that the OH is beneficial to the MEO on the Au/C electrode. In addition, Figure 4d shows that the Tafel slopes in the corresponding Tafel range are almost same, showing that the kinetics of the MEO does not change with the increasing concentration of KOH. The enhancement of the catalytic activity for MEO in the high KOH concentration solution is caused by the increase in the amount of the OH a-ads. The overall reaction orders (m) of methanol and OH at a fixed electrode potential, E, can be estimated from the slope of the straight line obtained according to eq 4.41 ! ∂ ln i ∂ log i ¼ ¼m ð4Þ ∂ ln C E ∂ log C E
where C is the concentration of methanol or OH. In the case of OH, eq 4 could be translated into the following equation ! ! ∂ log i ∂ log i ∂ log i ¼ ¼ ¼m ∂ log COH ∂ð14 log CHþ Þ ∂pH E E
E
ð5Þ The linear relation for log(i) versus log(CCH3OH) is obtained over the methanol concentration range of 0.15 M, while the linear relation of log(i) versus pH is only obtained in the OH concentration range of 0.051 M. The reason that the linear relation is not obtained at the low OH concentrations would be the effect of the abundant H2O adsorbed on the surface of AuNPs, which influences the relationship between the concen trations of the OH a-ads on the surface of AuNPs and the OH in the solution. The plots of log(i) versus log(CCH3OH) and the pH at different potentials in the Tafel ranges are displayed in Figure 4e and f. The corresponding data are respectively taken from Figure 4b and 5b at potentials of 0.05, 0.10, 0.15, 0.20, and 0.25 V. The sampling data and the overall reaction orders (m) estimated from Figure 6 for methanol and OH at different potentials are listed in Table 1. The overall reaction orders for 6990
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Table 2. Values for All of the Parameters of Au/C Electrodes Determined by the Fitting the Experimental EIS Data Using Z-View Software Based on the Equivalent Circuits Shown in Figure 7 CPEct
CPEads
W
E/mV
Rs/Ω
T/μF
P
T/μF
P
50
76.77
111.36
0.9035
642.4
100
76.02
103.52
0.946
566.9
150 200
78.24 76.42
106.28 105.28
0.9316 0.9426
250
75.28
111.18
0.9363
300
73.68
164.2
0.8631
325
79.67
97.70
350
76.91
400
81.22
450
78.67
500 550 600
Rct/Ω
Rads/Ω
R
T
P
0.69
14108
11848
0.59
7994
9308
3834 2441
0.8 0.69
5336 2927
1489 1462
1596
0.5
1939 1481
1612 105.7
5.5446 104
0.1190
0.9365
2094
132.5
7.4616 104
0.1483
87.33
0.9605
2688
286
1.0662 103
0.1806
82.89
0.9862
2788
829
1.1914 103
0.1906
72.90
2368
1536
1.5391 104
0.1437
78.17 81.75
72.04 62.90
2136 1764
1872 2093
1.5518 104 1.5534 104
0.131 0.1207
82.73
61.71
1399
2361
1.5357 104
0.1037
Figure 8. Idealized scheme for the rate-determining step of the MEO on Au/C in alkaline solution. The “•” indicates an unstable radical.
methanol and OH are around 0.75 and 0.55, respectively, which suggests that the MEO mechanism on the Au/C catalyst at all of the Tafel ranges remains unchanged with increasing methanol (or OH) concentration. The durability of the Au/C catalyst is measured in the 0.1 M KOH and 5 M methanol mixed solution at 0.25, 0.3, and 0.35 V, respectively. The corresponding chronoamperograms are presented in Figure 5, which indicates that the ratios of the current densities obtained at 0.25, 0.3, and 0.35 V at 60 min to those at 20 min are 66, 64, and 48%, respectively. The decline in current density at 0.35 V is far larger than that at 0.25 or 0.3 V, suggesting that the gold oxide monolayer formed above 0.3 V restrains the MEO seriously. Electrochemical impedance spectroscopy (EIS) is widely applied to analyze the kinetics of the electro-oxidation of small organic molecules.4246 To further elaborate on the mechanism of the MEO on Au/C, the EIS measurements are performed at different potentials in the 0.1 M KOH and 5 M methanol mixed solution. Figure 6 displays the Nyquist complex-plane impedance diagrams of Au/C recorded from 100 kHz to 10 mHz. As shown in Figure 6a, when the electrode potential is lower than 0.25 V, the diameter of the impedance arcs decreases with the enhancement of the electrode potential. This implies that the chargetransfer resistance of the MEO decreases with potential and that the state of the Au/C electrode (the OH a-ads is abundant on the surface of AuNPs) remains unchanged before the gold oxide monolayer appears at the surface of AuNPs. The relevant equivalent circuit is depicted in Figure 7a, where Rs represents the solution resistance; CPEct and Rct stand for the constant-
Cdl/μF
phase element and charge-transfer resistance corresponding to the MEO, respectively; CPEads and Rads are, respectively, the constantphase element and the resistance of OH adsorption on the surface of AuNPs. In the potential range of 0.30.4 V, as displayed in Figure 6b, the diameter of the impedance arcs increases with increasing electrode potential, showing that the MEO is hindered above 0.3 V. This suggests that the amount of the OH a-ads decreases gradually while the gold oxide monolayer forms on the surface of AuNPs. In Figure 6b, a straight line with a slope of nearly 45° appears in the low-frequency region, which is due to the Warburg diffusion impedance. The equivalent circuit of the electrode corresponding to Figure 6b can be expressed as in Figure 7b, where CPEct, Rct, and W are the constant-phase element, chargetransfer resistance of the MEO, and the Warburg’s diffusion element, respectively. Above 0.45 V, as displayed in Figure 6c, the diameter of the impedance arcs decreases with the increase of the electrode potential. What’s more, the anodic current drops dramatically when the potential exceeds 0.45 V (Figure 4a). Thus, it can be concluded that the main electrochemical reaction at a potential higher than 0.45 V is the formation of the compact gold oxide layer. Figure 7c portrays the equivalent circuit of the electrode in the potential range of 0.450.6 V, where Cdl, Rct, and W are the double-layer capacitance, the charge-transfer resistance corresponding to the formation of the compact gold oxide monolayer, and the Warburg’s diffusion element, respectively. The values for all of the parameters of Au/C electrodes determined by fitting the experimental EIS data using Z-view software based on the equivalent circuits shown in Figure 7 are listed in Table 2. According to Figure 6a and b and the fitting data listed in Table 2, the minimum charge-transfer resistance of the MEO is obtained at around 0.3 V. This is coincident with the potential (0.3 V) where the state of OH varies from the gold oxide species to the gold oxide monolayer, again proving that formation of the gold oxide monolayer restricts the catalytic activity of Au for the MEO. On the whole, the catalytic activity of the Au/C catalyst for the MEO in the low-potential range (0.0250.4 V, before the formation of the compact gold oxide layer) is related to the amount of OH a-ads. Combined with the fact that the onset potential of the MEO shifts negatively with the increasing concentration of methanol (or KOH), the rate-determining step 6991
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(the first charge transfer step) of the MEO can be described as shown below • CH3 OH þ OH a-ads f CH 2 OH þ H2 O þ e
ð6Þ
where the a-ads and • represent activated adsorbed species and an unstable radical, respectively. The reaction mechanism of the ratedetermining step for the MEO in alkaline solution is depicted in Figure 8. Initially, a hydrogen bond is created between an oxygen atom in the OH a-ads and a hydrogen atom in the methyl of the methanol. Then, a hydrogen atom, while losing an electron, bonds with an OH a-ads to yield a H2O adsorbed on Au/C.
4. CONCLUSIONS The catalyst of Au/C is prepared by a simple method and applied successfully for the MEO. The peak value of the anodic mass-specific current density on this catalyst in the 0.1 M KOH and 5 M CH3OH mixed solution reaches 48.6 mA 3 mg1 Au at 0.355 V, showing its high activity to the MEO in alkaline solutions. On the basis of the results of electrochemical measurements on Au/C in alkaline solutions with and without methanol, the OH a-ads on the surface of AuNPs has auxiliary catalysis for the MEO in alkaline. Moreover, a novel mechanism of the rate-determining step for the MEO on the Au/C catalyst is proposed, concerning with the auxiliary catalysis of the OH a-ads. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86 10 82338148. Fax: þ86 10 82339319. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by the Nature and Science Foundation of Beijing (2051001) and the National Basic Research Program of China (2007CB936502). Our sincere thanks go to Prof. Dingguo Xia and Dr. Xiang Li for their helpful discussions. ’ LIST OF SYMBOLS AuNP gold nanoparticles Au/C gold nanoparticles supported on activated carbon MEO methanol electro-oxidation DMFCs direct methanol fuel cells PVP poly(N-vinyl-2-pyrrolidone) Hg|HgO mercury|mercury oxide electrode TEM transmission electron microscope EDX energy dispersive X-ray analysis XRD X-ray powder diffraction SEM scanning electron microscope PTFE polytetrafluoroethylene CVs cyclic voltammograms ads chemical adsorbed species λ charge-transfer coefficient OH a-ads actively adsorbed OH m overall reaction orders EIS electrochemical impedance spectroscopy CPEct constant-phase element corresponding to chargetransfer process charge-transfer resistance Rct CPEads constant-phase element corresponding to adsorption process
Rads W a-ads •
resistance of OH adsorption Warburg’s diffusion element activated adsorbed species an unstable radical
’ REFERENCES (1) Nehl, C. L.; Liao, H.; Hafner, J. H. Nano Lett. 2006, 6, 683. (2) Pierrat, S.; Zins, I.; Breivogel, A.; So1nnichsen, C. Nano Lett. 2007, 7, 259. (3) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (4) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (5) Hughes, M. D.; Xu, Y. J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2005, 437, 1132. (6) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (7) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (8) Liao, H. G.; Jiang, Y. X.; Zhou, Z. Y.; Chen, S. P.; Sun, S. G. Angew. Chem., Int. Ed. 2008, 47, 9100. (9) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Science 2007, 315, 220. (10) Lim, B.; Jiang, M.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Science 2009, 324, 1302. (11) Hutchings, G. J.; Haruta, M. Appl. Catal., A 2005, 291, 2. (12) Okumura, M.; Tanaka, K.; Ueda, A.; Haruta, M. Solid State Ionics 1997, 95, 143. (13) Haruta, M.; Date, M. Appl. Catal., A 2001, 222, 427. (14) Okumura, M.; Akita, T.; Haruta, M. Catal. Today 2002, 74, 265. (15) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405. (16) Haruta, M. Nature 2005, 437, 1098. (17) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (18) Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374. (19) Haruta, M. Angew. Chem., Int. Ed. 2007, 46, 7154. (20) Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. Langmuir 2004, 20, 11293. (21) Zhong, C. J.; Maye, M. M. Adv. Mater. 2001, 13, 1507. (22) Luo, J.; Jones, V. W.; Maye, M. M.; Han, L.; Kariuki, N. N.; Zhong, C. J. J. Am. Chem. Soc. 2002, 124, 13988. (23) Maye, M. M.; Luo, J.; Lin, Y.; Engelhard, M. H.; Hepel, M.; Zhong, C. J. Langmuir 2003, 19, 125. (24) Borkowska, Z.; Tymosiak-Zielinska, A.; Shul, G. Electrochim. Acta 2004, 49, 1209. (25) Avramov-Ivic, M.; Jovanovic, V.; Vlajnic, G.; Popic, J. J. Electroanal. Chem. 1997, 423, 119. (26) Kua, J.; Goddard, W. A. J. Am. Chem. Soc. 1999, 121, 10928. (27) Burke, L. D.; Collins, J. A.; Horgan, M. A.; Hurley, L. M.; O’Mullane, A. P. Electrochim. Acta 2000, 45, 4127. (28) Avramov-Ivic, M.; Strbac, S.; Mitrovic, V. Electrochim. Acta 2001, 46, 3175. (29) Borkowska, Z.; Tymosiak-Zielinska, A.; Nowakowski., R. Electrochim. Acta 2004, 49, 2613. (30) Luo, J.; Njoki, P. N.; Lin, Y.; Mott, D.; Wang, L.; Zhong, C. J. Langmuir 2006, 22, 2892. (31) Zhang, J.; Liu, P.; Ma, H.; Ding, Y. J. Phys. Chem. C 2007, 111, 10382. (32) Tremiliosi-Filho, G.; Gonzalez, E. R.; Motheo, A. J.; Belgsir, E. M.; Leger, J. -M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 31. (33) Assiongbon, K. A.; Roy, D. Surf. Sci. 2005, 594, 99. (34) Nijhuis, T. A.; Visser, T.; Weckhuysen, B. M. J. Phys. Chem. B 2005, 109, 19309. (35) Damin, A.; Usseglio, S.; Agostini, G.; Bordiga, S.; Zecchina, A. J. Phys. Chem. C 2008, 112, 4932. (36) Tateishi, N.; Nishimura, K.; Yahikozawa, K.; Nakagawa, M.; Yamada, M.; Takasu, Y. J. Electroanal. Chem. 1993, 352, 243. 6992
dx.doi.org/10.1021/jp1086834 |J. Phys. Chem. C 2011, 115, 6986–6993
The Journal of Physical Chemistry C
ARTICLE
(37) Yahikozawa, K.; Nishimura, K.; Kumazawa, M.; Tateishi, N.; Takasu, Y.; Yasuda, K.; Matsuda, Y. Electrochim. Acta 1992, 37, 453. (38) Chen, A.; Lipkowski, J. J. Phys. Chem. B 1999, 103, 682. (39) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1. (40) Kowal, A.; Li, M.; Shao, M.; Sasaki, K.; Vukmirovic, M. B.; Zhang, J. N.; Marinkovic, S.; Liu, P.; Frenke, A. I.; Adzic, R. R. Nat. Mater. 2009, 8, 325. (41) Otomo, J.; Nishida, S.; Takahashi, H.; Nagamoto, H. J. Electroanal. Chem. 2008, 615, 84. (42) Lee, E. P.; Peng, Z.; Chen, W.; Chen, S.; Yang, H.; Xia, Y. ACS Nano 2008, 2, 2167. (43) Yu, E. H.; Scott, K.; Reeve, R. W. J. Electroanal. Chem. 2003, 547, 17. (44) Chen, W.; Kim, J.; Sun, S.; Chen, S. Langmuir 2007, 23, 11303. (45) Danaee, I.; Jafarian, M.; Forouzandeh, F.; Gobal, F.; Mahjani, M. G. J. Phys. Chem. B 2008, 112, 15933. (46) Chen, W.; Kim, J.; Xu, L. P.; Sun, S.; Chen, S. J. Phys. Chem. C 2007, 111, 13452.
6993
dx.doi.org/10.1021/jp1086834 |J. Phys. Chem. C 2011, 115, 6986–6993
