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Ethanol electro-oxidation on catalysts with TiO2 coated carbon nanotubes as support
Electrochemistry Communications 9 (2007) 1416–1421 www.elsevier.com/locate/elecom
Ethanol electro-oxidation on catalysts with TiO2 coated carbon nanotubes as support Huanqiao Song b
a,b
, Xinping Qiu
b,*
, Fushen Li a, Wentao Zhu b, Liquan Chen
b
a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 1000084, China
Received 13 December 2006; received in revised form 14 January 2007; accepted 26 January 2007 Available online 3 February 2007
Abstract Pt–TiO2/CNTs electrocatalysts for direct ethanol fuel cells (DEFCs) were prepared by sol–gel and ethylene glycol reduction method. XRD and TEM showed that the size of the Pt particles on TiO2/CNTs is 3.5–4 nm and with narrow particle size distribution. HRTEM revealed that a thin layer of uniform amorphous TiO2 on CNTs was formed and the faces of the Pt crystal on Pt–TiO2/CNTs catalysts were quite ‘‘rough’’ and ‘‘rounded’’ and some grain bounders and/or twins also appeared. The electrochemical studies using cyclic voltammetry (CV), chronoamperometry and CO stripping voltammetry indicate that Pt–TiO2/CNTs catalysts have higher electro-catalytic activity and CO-tolerance for ethanol oxidation than Pt/C (20 wt% Pt, E-TEK) and Pt/CNTs catalyst in acid. The Pt/TiO2 molar ratio was also optimized and proved that 1:1 was the best Pt/TiO2 molar ratio. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Pt–TiO2/CNTs; Electrocatalyst; Ethanol electro-oxidation; Direct ethanol fuel cells
1. Introduction The low kinetic of electro-oxidation of alcohol is still the main obstacle for the commercialization of direct alcohol fuel cells (DAFC) [1–5]. Recently some papers have reported that the addition of oxide is efficient to improve catalytic activity of platinum and its CO-tolerance for alcohol electro-oxidation. Qiu [6,7] and Vatistas et al. [8] have found that RuO2 can enhance the catalyst activity in acid solution for methanol electro-oxidation. Olivi [9] and Xin [10] have reported, respectively, that the addition of SnO2 can promote the catalyst activity for methanol and ethanol oxidation. ZrO2 [11], CeO2 [12] and MgO [13] were also studied and found they can improve the catalytic activity and CO-tolerance in alkaline solution for ethanol electrooxidation, but it is well known that alkaline aqueous solu*
Corresponding author. Tel./fax: +86 10 62794234. E-mail address: [email protected] (X. Qiu).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.01.048
tions are not stable for DAFC owing to the carbonation, and the Nafion membrane, which is very popular to use in DAFC, can only be used in acid. So it is better to find a new catalyst that not only can enhance the catalytic activity and CO-tolerance for ethanol electro-oxidation, but also can be used in acid. TiO2, as one of the semi conductive oxide, has been widely studied for its special photoelectric properties. In addition, TiO2 is very stable in acidic solution and it has been reported that TiO2 electrode as the support of Pt [14,15] or PtRu [16] has high catalytic activity and CO-tolerance for alcohol electro-oxidation because the interaction between Pt and TiO2. Herein, we prepared TiO2 coated carbon nanotubes (TiO2/CNTs) by a sol–gel method, and used them as catalyst support of platinum. The electrochemical activity for ethanol oxidation and CO-tolerance were investigated by using cyclic voltammetry (CV), chronoamperometry and CO stripping voltammetry at 25 °C in acid solutions.
H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
2. Experimental 2.1. Preparation and characterization of Pt–TiO2/CNTs catalyst TiO2/CNTs catalyst was prepared using tetrabutyl titanate as precursors. First, tetrabutyl titanate was dissolved in ethanol with agitating to get a 10% solution, and then the solution was slowly dropped into acetic acid solution (which contained 10% acetic acid and 56% ethanol)to get the sol, diluted to 5% by adding the ethanol. The CNTs (MWNTs were used) were put into the diluted sol, agitating for 20 min, dried at 80 °C in an oil bath and then calcined at 600 °C for 2 h to get TiO2/CNTs. Pt–TiO2/CNTs catalysts were prepared by reducing of chloroplatinic acid with ethylene glycol on TiO2/CNTs powders. The amount of tetrabutyl titanate was controlled by the Pt/TiO2 molar ratio in the final catalyst. The nominal loading of Pt in the catalyst was 20 wt%. The morphology of the catalyst was observed by TEM (JEM-1200EX) at 100 kV for conventional and HRTEM (JEM-2010) equipped with an energy-dispersive X-ray detector (EDX) for high resolution imagine. X-ray diffraction (XRD) analysis was performed using the Rigaku X-ray diffractometer with Cu Ka-source. The 2h angular regions between 20° and 90° were explored at a scan rate of 6° min 1 with step of 0.02°. The elemental composition of the catalyst was investigated by energy-dispersive X-ray (OXFORD INCA 300) attached to scanning microscope (JSM-6301F). 2.2. Electrochemical assessment of catalysts and determination of ethanol oxidation activity The catalysts slurry was casted onto a gold electrode (1 cm in diameter) to determine their ethanol oxidation activity. The catalyst slurry was prepared by mixing the calalysts with distilled water and Nafion (20% Nafion and 80% ethylene glycol) solution under sonicate for 20 min. After casting, the catalysts were air-dried for 60 min at 80 °C. Electrochemical measurements were carried out in a three-electrode cell with Solartron workstation at room temperature. The gold electrode (1 cm in diameter) coated with catalyst ink was used as working electrode. A saturated calomel electrode (SCE) and Pt gauze were used as reference and counter electrodes, respectively. All electrode potentials in this paper were referred to the SCE. A solution of 1.0 M perchloric acid or 1.0 M ethanol + 1.0 M perchloric acid was used as electrolyte. All the reagents used were of analytical grade. The cyclic voltammetry data for ethanol electro-oxidation were recorded in the potential range of 0.2 to 1.0 V vs. SCE with a scan rate of 50 mV s 1 and the chronoamperometric curves were recorded at 0.45 V for 3600 s. The CO stripping voltammetry was measured in the potential range of 0.2 to 1.0 V with a scan rate of
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10 mV s 1. Before CO is adsorbed at 0.1 V, the catalyst was cycled in N2-saturated solution (the content of N2 in the used gas is 99.999%) until a well-established cyclic voltammogram was observed. After forming a saturated CO adlayer (holding at 0.1 V for 20 min in CO-saturated solution), the electrolyte was purged with N2 (20 min) again to remove the dissolved CO from the electrolyte solution. If there were no special indication, the testing result for Pt– TiO2/catalyst is 1:1 molar ratios of Pt and TiO2. 3. Results and discussion 3.1. Structure and morphology Fig. 1a and b shows the TEM images of Pt supported on TiO2/CNTs. Mean particle size of prepared Pt was estimated to be 3.5–4 nm with narrow particle size distribution. The characteristic XRD peaks of platinum in Fig. 2d can be indexed as the face-centered cubic phase and the particle size calculated from Pt (220) using Scherrer formula after background subtraction is about 3.7 nm, which is agreed well with the results of TEM observation. Comparing the XRD patterns of Pt/CNTs, Pt–TiO2/ CNTs, it is easy to find that there is no any shift in the diffraction peaks of platinum indicating that the addition of TiO2 has no effect on the crystalline lattice of platinum in Pt–TiO2/CNTs catalysts. It can also be found that there is no diffraction peak of TiO2 in the XRD patterns of TiO2/CNTs and Pt–TiO2/CNTs, which means that the prepared TiO2 is amorphous. This can be confirmed from high-resolution electron microscopy (HRTEM) and energy-dispersive X-ray (EDX) analyses. In Fig. 3a, a layer of uniform amorphous TiO2 on CNTs can be clearly seen and the thickness of the TiO2 is about 4–5 nm. The EDX analyses carried out on the amorphous layer proved the presence of TiO2. The HRTEM image, depicted in Fig. 3b, reveals that although the basic shape of Pt particles on TiO2/CNTs is face-centered cubic (as described in Fig. 2), the faces of the crystal are quite ‘‘rough’’ and ‘‘rounded’’, and some defects, such as grain bounders and/or twins have also appeared on the surface of Pt– TiO2/CNTs catalysts. From the above analysis, it appears that the uniform amorphous TiO2 layer and irregular surfaces of nano-platinum particles may play an important role in synergetic interaction between Pt and TiO2. 3.2. Ethanol electro-oxidation The typical cyclic voltammogram curves for ethanol electro-oxidation on Pt/CNTs, Pt–TiO2CNTs were shown in Fig. 4, where CV curves of Pt/C (20 wt% Pt, E-TEK) were also presented for comparison. It can be seen that ethanol oxidation began at approximately 0.51 V for Pt–TiO2/ CNTs and reached its current peak at about 0.8 V. On the reverse sweep, re-oxidation of ethanol began at approximately 0.78 V and reached a peak current density at around 0.68 V, after which strongly bonded surface inter-
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H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
Fig. 1. TEM image of Pt–TiO2/CNTs (1:1).
Pt(111) Pt(200)
c
Pt(220)
Pt(311)
d
indensity/a.u
c b a 20
30
40
50
60
70
80
90
2θ/degree
Fig. 2. XRD patterns of (a) CNTs; (b) TiO2/CNTs; (c) Pt/CNTs; (d) Pt– TiO2/CNTs (1:1).
mediates began to block the catalyst surface. The electrochemical behaviors of Pt/CNTs and Pt/C (E-TEK) were similar to that of Pt–TiO2/CNTs. However, the peak current density of Pt–TiO2/CNTs is significantly higher than others. The peak current density for Pt–TiO2/CNTs was about 640 mA mg 1Pt, while the peak current density for Pt/CNTs was only 440 mA mg 1Pt and Pt/C (E-TEK) was about 410 mA mg 1Pt. The forward scan peak potential of the ethanol electro-oxidation for the three catalysts were almost the same, but the onset potential (in the positive-going scan) of ethanol electro-oxidation on Pt–TiO2/ CNTs was only 0.37 V, for Pt/CNTs was 0.49 V and for Pt/C (20 wt% Pt, E-TEK) reached 0.51 V. These facts indicate that the Pt–TiO2/CNTs have higher catalytic activity and better stability for ethanol oxidation than Pt/CNTs and Pt/C (20 wt% Pt, E-TEK) because of the addition of TiO2. This can be confirmed further from the results of current density-time curves of ethanol oxidation shown in Fig. 5. The ethanol oxidation current density at Pt–TiO2/ CNTs electrode was higher than that at Pt/CNTs and Pt/ C (E-TEK) electrode though the current decay with time was observed for the three electrodes, which is consistent with the results of cyclic voltammogram.
The average size of platinum particle for Pt–TiO2/ CNTs, Pt/CNTs and Pt/C (20 wt% Pt, E-TEK) was calculated from Pt (2 2 0) in XRD patterns using Scherrer formula after background subtraction. The results were listed in Table 1. It can be seen that the average size for the three catalysts was almost identical, which indicates that the increase in the catalytic activities of ethanol oxidation on Pt–TiO2/CNTs is not due to the change of particle size. 3.3. CO stripping The CO stripping voltammogram curves were shown in Fig. 6, significant differences in the onset potential and peak potential for CO oxidation between the catalysts containing TiO2 and those of pure platinum were observed. The onset potential of CO oxidation on Pt– TiO2/CNTs was at.0.42 V, which was about 0.11 V lower than the measured on Pt/C (20 wt% Pt, E-TEK) and 0.10 V lower for Pt/CNTs electrode, thus illustrating the beneficial role of TiO2 for CO oxidation. The peak potential of CO oxidation on Pt–TiO2/CNTs was 0.51 V, which decreased 0.07 V and 0.08 V compared with 0.58 V for Pt/CNTs and 0.59 V for Pt/C. K.A. Friedrich et al. [17] have reported that the smaller platinum particles show more positive CO oxidation potentials in relation to polycrystalline Pt and large particles. As shown in Table 1, the three catalysts have almost same platinum particle size, which indicates that the more negative CO oxidation potential on Pt–TiO2/CNTs catalyst is not due to the different particle size. This fact shows that the Pt–TiO2/ CNTs electrode possesses a good electrocatalytic activity for CO oxidation. Additionally, the corrected surface areas of Pt, which were calculated for the charge of CO oxidation, for the Pt–TiO2/CNTs, Pt/CNTs and Pt/C catalyst are 552 cm2, 514 cms and 542 cm2 g 1, respectively, assuming that CO adsorption occurs only on Pt. This further proved that the high activity of Pt–TiO2/CNTs catalyst is not due to the great change of the corrected surface area for Pt.
H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
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Fig. 3. HRTEM image and EDX spectrum of Pt–TiO2/CNTs (1:1): (a) TiO2/CNTs; (b) Pt–TiO2/CNTs. The strong Cu-peak originate from the Cu supporting HRTEM grid.
3.4. Influence of Pt/TiO2 molar ratios To optimize the content of TiO2 in Pt–TiO2/CNT catalyst, a series of catalysts was prepared with different Pt/ TiO2 molar ratios, while maintaining the content of Pt in the catalysts at 20 wt%. CO stripping voltammogram were carried out to investigate the effect of molar ratios of Pt:
TiO2 in the catalysts on CO electro-oxidation. The results are presented in Fig. 7 including the peak potentials and corresponding peak current densities. It can be seen that all the peak potential shifts toward negative with the increase of TiO2 content until the molar ratio reached 1:1. Then the peak potential shifts toward the higher direction with the further increase of TiO2 content. The lowest
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H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
600
Pt/C(E-TEK)
Current Density (mA/mg Pt)
Current Density (mA/mg pt)
700
Pt/CNTs Pt-TiO2/CNTs(1:1)
500 400 300 200 100 0 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
potential (V) Fig. 4. Cyclic voltammogram curves for ethanol electro-oxidation in 1 M C2H5OH + 1 M HClO4 solutions with a scan rate of 50 mV s 1.
60 45 30 15 0 -15 60 45 30 15 0 -15 60 45 30 15 0 -15 -0.2
Pt-TiO2/CNTs
Pt/CNTs
Pt/C
0.0
0.2
0.4
0.6
0.8
1.0
Potential (v) Fig. 6. CO stripping voltammograms in 1 M HClO4 solutions with a scan rate of 10 mV s 1.
48 Peak current density
40
0.68
Potential (v)
44
36 0.64 32 0.60
28
Peak potential
24
0.56
20 0.52
16
0.48
Current Density (mA/mg Pt)
0.72
12 8
Fig. 5. Current density–time curves at 0.45 V for 3600 s at Pt–TiO2/CNTs (a), Pt/CNT (b) and Pt/C (c) electrode in 1 M C2H5OH + 1 M HClO4 solutions.
Table 1 Comparison of the as-prepared catalysts and the commercial Pt/C (20 wt% Pt, E-TEK) in terms of the average particle size, CO adsorption charges and specific surface area Catalyst
Pt/TiO2 (by molar ratio)
d (nm)
QH (mC/mg)
SS (cm2/mg)
Pt/C Pt/CNTs
1:0 1:0 2:1 1:1 1:2 1:3 1:4
3.2 4.0 3.8 3.7 4.0 4.2 4.8
116.1 108.2 109.5 114.0 113.6 110.1 110.9
552 514 520 542 540 523 527
Pt–TiO2/CNTs
peak potential was found when the molar ratio of Pt: TiO2 is 1:1 among the catalyst obtained. As seen from Fig. 2, there was no atomic interaction between Pt and TiO2. And according to Table 1, the shift of peak potential was not owing to the change of particle size. The negative shift of peak potential may relate with the synergetic interaction
1:0
2:1
1:1
1:2
1:3
1:4
Molar Ratios of Pt/TiO 2 Fig. 7. Peak potential and peak current density on the Pt–TiO2/CNTs electrode with different Pt/TiO2 molar ratios for CO oxidation in 1 M HClO4 solutions with a scan rate of 10 mV s 1.
between Pt and TiO2, if the Pt/TiO2 molar ratio was too small, the platinum particles may be insulated due to the low electron conductivity of TiO2. For this reason, the current density of CO oxidation on the catalyst containing TiO2 was lower than those of Pt/CNT and Pt/C. 4. Conclusions The results described in the presented work proved that Pt–TiO2/CNTs was a promising anode catalyst for ethanol electro-oxidation. The XRD and TEM showed that the prepared catalysts had narrow particle size distribution. The CV curves and chronoamperometric curves presented that the Pt–TiO2/CNTs catalyst had higher ethanol oxidation current density than Pt/CNTs and Pt/C (20 wt% Pt, E-TEK) and CO stripping voltammogram proved that the prepared catalyst had good CO-tolerance
H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
for ethanol electro-oxidation. And the lowest peak potential for CO electro-oxidation on Pt–TiO2/C catalysts is obtained when the molar ratio of 1:1. The higher electrochemical activity can attribute to that the CO poisoning of the platinum was being decreased by the presence of TiO2. Acknowledgements The authors appreciate the financial support of the State Key Basic Research Program of PRC (2002CB211803), the National Natural Science Foundation of China (90410002). References [1] X.M. Ren, P. Zelenay, S. Thomas, J. Davey, S. Gottesfeld, J. Power Sources 86 (2000) 111. [2] F. Maillard, E.R. Savinova, P.A. Simonov, V.I. Zaikovskii, U. Stimming, J. Phys. Chem. B 108 (2004) 17893. [3] Z. Jusys, R.J. Behm, J. Phys. Chem. B 105 (2001) 10874. [4] F. Maillard, G.Q. Lu, A. Wieckowski, U. Stimming, J. Phys. Chem. B 109 (2005) 16230.
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[5] M. Arenz, K.J.J. Mayrhofer, V. Stamenkovic, B.B. Blizanac, T. Tomoyuki, P.N. Ross, N.M. Markovic, J. Am. Chem. Soc. 127 (2005) 6819. [6] Zh.G. Chen, X.P. Qiu, B. Lu, Sh.Ch. Zhang, W.T. Zhu, L.Q. Chen, Electrochem. Commun. 7 (2005) 593. [7] L. Cao, F. Scheiba, Ch. Roth, F. Schweiger, C. Cremers, U. Stimming, H. Fuess, L.Q. Chen, W.T. Zhu, X.P. Qiu, Angew. Chem. Int. Ed. 45 (2006) 5315. [8] H.B. Suffredini, V. Tricoli, L.A. Avaca, N. Vatistas, Electrochem.Commun. 6 (2004) 1025. [9] A.L. Santos, D. Profeti, P. Olivi, Electrochim. Acta 50 (2005) 2615. [10] L.H. Jiang, G.Q. Sun, Zh.H. Zhou, Sh.G. Sun, Q. Wang, Sh.Y. Yan, H.Q. Li, J. Tian, J.S. Guo, B. Zhou, Q. Xin, J. Phys. Chem. B 109 (2005) 8774. [11] Y.X. Bai, J.J. Wu, J.Y. Xi, J.Sh. Wang, W.T. Zhu, L.Q. Chen, X.P. Qiu, Electrochem. Commun. 7 (2005) 1087. [12] Ch.W. Xu, P.K. Shen, J. Power Sources 142 (2005) 27. [13] Ch.W. Xu, P.K. Shen, X.H.e. Ji, R. Zeng, Y.L. Liu, Electrochem. Commun. 7 (2005) 1305. [14] B.E. Hayden, D.V. Malevich, D. Pletcher, Electrochem. Commun. 3 (2001) 395. [15] J.H. Liu, C.B. Yu, Chem. J. Chin. U. 24 (2003) 2263. [16] K. Drew, G. Girishkumar, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 109 (2005) 11851. [17] K.A. Friedrich, F. Henglein, U. Stimming, W. Unkauf, Electrochim. Acta 45 (2000) 3283.
Ethanol electro-oxidation on catalysts with TiO2 coated carbon nanotubes as support Huanqiao Song b
a,b
, Xinping Qiu
b,*
, Fushen Li a, Wentao Zhu b, Liquan Chen
b
a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 1000084, China
Received 13 December 2006; received in revised form 14 January 2007; accepted 26 January 2007 Available online 3 February 2007
Abstract Pt–TiO2/CNTs electrocatalysts for direct ethanol fuel cells (DEFCs) were prepared by sol–gel and ethylene glycol reduction method. XRD and TEM showed that the size of the Pt particles on TiO2/CNTs is 3.5–4 nm and with narrow particle size distribution. HRTEM revealed that a thin layer of uniform amorphous TiO2 on CNTs was formed and the faces of the Pt crystal on Pt–TiO2/CNTs catalysts were quite ‘‘rough’’ and ‘‘rounded’’ and some grain bounders and/or twins also appeared. The electrochemical studies using cyclic voltammetry (CV), chronoamperometry and CO stripping voltammetry indicate that Pt–TiO2/CNTs catalysts have higher electro-catalytic activity and CO-tolerance for ethanol oxidation than Pt/C (20 wt% Pt, E-TEK) and Pt/CNTs catalyst in acid. The Pt/TiO2 molar ratio was also optimized and proved that 1:1 was the best Pt/TiO2 molar ratio. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Pt–TiO2/CNTs; Electrocatalyst; Ethanol electro-oxidation; Direct ethanol fuel cells
1. Introduction The low kinetic of electro-oxidation of alcohol is still the main obstacle for the commercialization of direct alcohol fuel cells (DAFC) [1–5]. Recently some papers have reported that the addition of oxide is efficient to improve catalytic activity of platinum and its CO-tolerance for alcohol electro-oxidation. Qiu [6,7] and Vatistas et al. [8] have found that RuO2 can enhance the catalyst activity in acid solution for methanol electro-oxidation. Olivi [9] and Xin [10] have reported, respectively, that the addition of SnO2 can promote the catalyst activity for methanol and ethanol oxidation. ZrO2 [11], CeO2 [12] and MgO [13] were also studied and found they can improve the catalytic activity and CO-tolerance in alkaline solution for ethanol electrooxidation, but it is well known that alkaline aqueous solu*
Corresponding author. Tel./fax: +86 10 62794234. E-mail address: [email protected] (X. Qiu).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.01.048
tions are not stable for DAFC owing to the carbonation, and the Nafion membrane, which is very popular to use in DAFC, can only be used in acid. So it is better to find a new catalyst that not only can enhance the catalytic activity and CO-tolerance for ethanol electro-oxidation, but also can be used in acid. TiO2, as one of the semi conductive oxide, has been widely studied for its special photoelectric properties. In addition, TiO2 is very stable in acidic solution and it has been reported that TiO2 electrode as the support of Pt [14,15] or PtRu [16] has high catalytic activity and CO-tolerance for alcohol electro-oxidation because the interaction between Pt and TiO2. Herein, we prepared TiO2 coated carbon nanotubes (TiO2/CNTs) by a sol–gel method, and used them as catalyst support of platinum. The electrochemical activity for ethanol oxidation and CO-tolerance were investigated by using cyclic voltammetry (CV), chronoamperometry and CO stripping voltammetry at 25 °C in acid solutions.
H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
2. Experimental 2.1. Preparation and characterization of Pt–TiO2/CNTs catalyst TiO2/CNTs catalyst was prepared using tetrabutyl titanate as precursors. First, tetrabutyl titanate was dissolved in ethanol with agitating to get a 10% solution, and then the solution was slowly dropped into acetic acid solution (which contained 10% acetic acid and 56% ethanol)to get the sol, diluted to 5% by adding the ethanol. The CNTs (MWNTs were used) were put into the diluted sol, agitating for 20 min, dried at 80 °C in an oil bath and then calcined at 600 °C for 2 h to get TiO2/CNTs. Pt–TiO2/CNTs catalysts were prepared by reducing of chloroplatinic acid with ethylene glycol on TiO2/CNTs powders. The amount of tetrabutyl titanate was controlled by the Pt/TiO2 molar ratio in the final catalyst. The nominal loading of Pt in the catalyst was 20 wt%. The morphology of the catalyst was observed by TEM (JEM-1200EX) at 100 kV for conventional and HRTEM (JEM-2010) equipped with an energy-dispersive X-ray detector (EDX) for high resolution imagine. X-ray diffraction (XRD) analysis was performed using the Rigaku X-ray diffractometer with Cu Ka-source. The 2h angular regions between 20° and 90° were explored at a scan rate of 6° min 1 with step of 0.02°. The elemental composition of the catalyst was investigated by energy-dispersive X-ray (OXFORD INCA 300) attached to scanning microscope (JSM-6301F). 2.2. Electrochemical assessment of catalysts and determination of ethanol oxidation activity The catalysts slurry was casted onto a gold electrode (1 cm in diameter) to determine their ethanol oxidation activity. The catalyst slurry was prepared by mixing the calalysts with distilled water and Nafion (20% Nafion and 80% ethylene glycol) solution under sonicate for 20 min. After casting, the catalysts were air-dried for 60 min at 80 °C. Electrochemical measurements were carried out in a three-electrode cell with Solartron workstation at room temperature. The gold electrode (1 cm in diameter) coated with catalyst ink was used as working electrode. A saturated calomel electrode (SCE) and Pt gauze were used as reference and counter electrodes, respectively. All electrode potentials in this paper were referred to the SCE. A solution of 1.0 M perchloric acid or 1.0 M ethanol + 1.0 M perchloric acid was used as electrolyte. All the reagents used were of analytical grade. The cyclic voltammetry data for ethanol electro-oxidation were recorded in the potential range of 0.2 to 1.0 V vs. SCE with a scan rate of 50 mV s 1 and the chronoamperometric curves were recorded at 0.45 V for 3600 s. The CO stripping voltammetry was measured in the potential range of 0.2 to 1.0 V with a scan rate of
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10 mV s 1. Before CO is adsorbed at 0.1 V, the catalyst was cycled in N2-saturated solution (the content of N2 in the used gas is 99.999%) until a well-established cyclic voltammogram was observed. After forming a saturated CO adlayer (holding at 0.1 V for 20 min in CO-saturated solution), the electrolyte was purged with N2 (20 min) again to remove the dissolved CO from the electrolyte solution. If there were no special indication, the testing result for Pt– TiO2/catalyst is 1:1 molar ratios of Pt and TiO2. 3. Results and discussion 3.1. Structure and morphology Fig. 1a and b shows the TEM images of Pt supported on TiO2/CNTs. Mean particle size of prepared Pt was estimated to be 3.5–4 nm with narrow particle size distribution. The characteristic XRD peaks of platinum in Fig. 2d can be indexed as the face-centered cubic phase and the particle size calculated from Pt (220) using Scherrer formula after background subtraction is about 3.7 nm, which is agreed well with the results of TEM observation. Comparing the XRD patterns of Pt/CNTs, Pt–TiO2/ CNTs, it is easy to find that there is no any shift in the diffraction peaks of platinum indicating that the addition of TiO2 has no effect on the crystalline lattice of platinum in Pt–TiO2/CNTs catalysts. It can also be found that there is no diffraction peak of TiO2 in the XRD patterns of TiO2/CNTs and Pt–TiO2/CNTs, which means that the prepared TiO2 is amorphous. This can be confirmed from high-resolution electron microscopy (HRTEM) and energy-dispersive X-ray (EDX) analyses. In Fig. 3a, a layer of uniform amorphous TiO2 on CNTs can be clearly seen and the thickness of the TiO2 is about 4–5 nm. The EDX analyses carried out on the amorphous layer proved the presence of TiO2. The HRTEM image, depicted in Fig. 3b, reveals that although the basic shape of Pt particles on TiO2/CNTs is face-centered cubic (as described in Fig. 2), the faces of the crystal are quite ‘‘rough’’ and ‘‘rounded’’, and some defects, such as grain bounders and/or twins have also appeared on the surface of Pt– TiO2/CNTs catalysts. From the above analysis, it appears that the uniform amorphous TiO2 layer and irregular surfaces of nano-platinum particles may play an important role in synergetic interaction between Pt and TiO2. 3.2. Ethanol electro-oxidation The typical cyclic voltammogram curves for ethanol electro-oxidation on Pt/CNTs, Pt–TiO2CNTs were shown in Fig. 4, where CV curves of Pt/C (20 wt% Pt, E-TEK) were also presented for comparison. It can be seen that ethanol oxidation began at approximately 0.51 V for Pt–TiO2/ CNTs and reached its current peak at about 0.8 V. On the reverse sweep, re-oxidation of ethanol began at approximately 0.78 V and reached a peak current density at around 0.68 V, after which strongly bonded surface inter-
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H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
Fig. 1. TEM image of Pt–TiO2/CNTs (1:1).
Pt(111) Pt(200)
c
Pt(220)
Pt(311)
d
indensity/a.u
c b a 20
30
40
50
60
70
80
90
2θ/degree
Fig. 2. XRD patterns of (a) CNTs; (b) TiO2/CNTs; (c) Pt/CNTs; (d) Pt– TiO2/CNTs (1:1).
mediates began to block the catalyst surface. The electrochemical behaviors of Pt/CNTs and Pt/C (E-TEK) were similar to that of Pt–TiO2/CNTs. However, the peak current density of Pt–TiO2/CNTs is significantly higher than others. The peak current density for Pt–TiO2/CNTs was about 640 mA mg 1Pt, while the peak current density for Pt/CNTs was only 440 mA mg 1Pt and Pt/C (E-TEK) was about 410 mA mg 1Pt. The forward scan peak potential of the ethanol electro-oxidation for the three catalysts were almost the same, but the onset potential (in the positive-going scan) of ethanol electro-oxidation on Pt–TiO2/ CNTs was only 0.37 V, for Pt/CNTs was 0.49 V and for Pt/C (20 wt% Pt, E-TEK) reached 0.51 V. These facts indicate that the Pt–TiO2/CNTs have higher catalytic activity and better stability for ethanol oxidation than Pt/CNTs and Pt/C (20 wt% Pt, E-TEK) because of the addition of TiO2. This can be confirmed further from the results of current density-time curves of ethanol oxidation shown in Fig. 5. The ethanol oxidation current density at Pt–TiO2/ CNTs electrode was higher than that at Pt/CNTs and Pt/ C (E-TEK) electrode though the current decay with time was observed for the three electrodes, which is consistent with the results of cyclic voltammogram.
The average size of platinum particle for Pt–TiO2/ CNTs, Pt/CNTs and Pt/C (20 wt% Pt, E-TEK) was calculated from Pt (2 2 0) in XRD patterns using Scherrer formula after background subtraction. The results were listed in Table 1. It can be seen that the average size for the three catalysts was almost identical, which indicates that the increase in the catalytic activities of ethanol oxidation on Pt–TiO2/CNTs is not due to the change of particle size. 3.3. CO stripping The CO stripping voltammogram curves were shown in Fig. 6, significant differences in the onset potential and peak potential for CO oxidation between the catalysts containing TiO2 and those of pure platinum were observed. The onset potential of CO oxidation on Pt– TiO2/CNTs was at.0.42 V, which was about 0.11 V lower than the measured on Pt/C (20 wt% Pt, E-TEK) and 0.10 V lower for Pt/CNTs electrode, thus illustrating the beneficial role of TiO2 for CO oxidation. The peak potential of CO oxidation on Pt–TiO2/CNTs was 0.51 V, which decreased 0.07 V and 0.08 V compared with 0.58 V for Pt/CNTs and 0.59 V for Pt/C. K.A. Friedrich et al. [17] have reported that the smaller platinum particles show more positive CO oxidation potentials in relation to polycrystalline Pt and large particles. As shown in Table 1, the three catalysts have almost same platinum particle size, which indicates that the more negative CO oxidation potential on Pt–TiO2/CNTs catalyst is not due to the different particle size. This fact shows that the Pt–TiO2/ CNTs electrode possesses a good electrocatalytic activity for CO oxidation. Additionally, the corrected surface areas of Pt, which were calculated for the charge of CO oxidation, for the Pt–TiO2/CNTs, Pt/CNTs and Pt/C catalyst are 552 cm2, 514 cms and 542 cm2 g 1, respectively, assuming that CO adsorption occurs only on Pt. This further proved that the high activity of Pt–TiO2/CNTs catalyst is not due to the great change of the corrected surface area for Pt.
H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
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Fig. 3. HRTEM image and EDX spectrum of Pt–TiO2/CNTs (1:1): (a) TiO2/CNTs; (b) Pt–TiO2/CNTs. The strong Cu-peak originate from the Cu supporting HRTEM grid.
3.4. Influence of Pt/TiO2 molar ratios To optimize the content of TiO2 in Pt–TiO2/CNT catalyst, a series of catalysts was prepared with different Pt/ TiO2 molar ratios, while maintaining the content of Pt in the catalysts at 20 wt%. CO stripping voltammogram were carried out to investigate the effect of molar ratios of Pt:
TiO2 in the catalysts on CO electro-oxidation. The results are presented in Fig. 7 including the peak potentials and corresponding peak current densities. It can be seen that all the peak potential shifts toward negative with the increase of TiO2 content until the molar ratio reached 1:1. Then the peak potential shifts toward the higher direction with the further increase of TiO2 content. The lowest
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H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
600
Pt/C(E-TEK)
Current Density (mA/mg Pt)
Current Density (mA/mg pt)
700
Pt/CNTs Pt-TiO2/CNTs(1:1)
500 400 300 200 100 0 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
potential (V) Fig. 4. Cyclic voltammogram curves for ethanol electro-oxidation in 1 M C2H5OH + 1 M HClO4 solutions with a scan rate of 50 mV s 1.
60 45 30 15 0 -15 60 45 30 15 0 -15 60 45 30 15 0 -15 -0.2
Pt-TiO2/CNTs
Pt/CNTs
Pt/C
0.0
0.2
0.4
0.6
0.8
1.0
Potential (v) Fig. 6. CO stripping voltammograms in 1 M HClO4 solutions with a scan rate of 10 mV s 1.
48 Peak current density
40
0.68
Potential (v)
44
36 0.64 32 0.60
28
Peak potential
24
0.56
20 0.52
16
0.48
Current Density (mA/mg Pt)
0.72
12 8
Fig. 5. Current density–time curves at 0.45 V for 3600 s at Pt–TiO2/CNTs (a), Pt/CNT (b) and Pt/C (c) electrode in 1 M C2H5OH + 1 M HClO4 solutions.
Table 1 Comparison of the as-prepared catalysts and the commercial Pt/C (20 wt% Pt, E-TEK) in terms of the average particle size, CO adsorption charges and specific surface area Catalyst
Pt/TiO2 (by molar ratio)
d (nm)
QH (mC/mg)
SS (cm2/mg)
Pt/C Pt/CNTs
1:0 1:0 2:1 1:1 1:2 1:3 1:4
3.2 4.0 3.8 3.7 4.0 4.2 4.8
116.1 108.2 109.5 114.0 113.6 110.1 110.9
552 514 520 542 540 523 527
Pt–TiO2/CNTs
peak potential was found when the molar ratio of Pt: TiO2 is 1:1 among the catalyst obtained. As seen from Fig. 2, there was no atomic interaction between Pt and TiO2. And according to Table 1, the shift of peak potential was not owing to the change of particle size. The negative shift of peak potential may relate with the synergetic interaction
1:0
2:1
1:1
1:2
1:3
1:4
Molar Ratios of Pt/TiO 2 Fig. 7. Peak potential and peak current density on the Pt–TiO2/CNTs electrode with different Pt/TiO2 molar ratios for CO oxidation in 1 M HClO4 solutions with a scan rate of 10 mV s 1.
between Pt and TiO2, if the Pt/TiO2 molar ratio was too small, the platinum particles may be insulated due to the low electron conductivity of TiO2. For this reason, the current density of CO oxidation on the catalyst containing TiO2 was lower than those of Pt/CNT and Pt/C. 4. Conclusions The results described in the presented work proved that Pt–TiO2/CNTs was a promising anode catalyst for ethanol electro-oxidation. The XRD and TEM showed that the prepared catalysts had narrow particle size distribution. The CV curves and chronoamperometric curves presented that the Pt–TiO2/CNTs catalyst had higher ethanol oxidation current density than Pt/CNTs and Pt/C (20 wt% Pt, E-TEK) and CO stripping voltammogram proved that the prepared catalyst had good CO-tolerance
H. Song et al. / Electrochemistry Communications 9 (2007) 1416–1421
for ethanol electro-oxidation. And the lowest peak potential for CO electro-oxidation on Pt–TiO2/C catalysts is obtained when the molar ratio of 1:1. The higher electrochemical activity can attribute to that the CO poisoning of the platinum was being decreased by the presence of TiO2. Acknowledgements The authors appreciate the financial support of the State Key Basic Research Program of PRC (2002CB211803), the National Natural Science Foundation of China (90410002). References [1] X.M. Ren, P. Zelenay, S. Thomas, J. Davey, S. Gottesfeld, J. Power Sources 86 (2000) 111. [2] F. Maillard, E.R. Savinova, P.A. Simonov, V.I. Zaikovskii, U. Stimming, J. Phys. Chem. B 108 (2004) 17893. [3] Z. Jusys, R.J. Behm, J. Phys. Chem. B 105 (2001) 10874. [4] F. Maillard, G.Q. Lu, A. Wieckowski, U. Stimming, J. Phys. Chem. B 109 (2005) 16230.
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