J. Phys. Chem. C 2007, 111, 16613-16617
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Well-Dispersed PtAu Nanoparticles Loaded into Anodic Titania Nanotubes: A High Antipoison and Stable Catalyst System for Methanol Oxidation in Alkaline Media Lixia Yang, Wenyue Yang, and Qingyun Cai* State Key Laboratory of Chemo/Biosensing and Chemometrics, Department of Chemistry, Hunan UniVersity, Changsha 410082, People’s Republic of China ReceiVed: April 10, 2007; In Final Form: July 7, 2007
PtAu nanoparticles, ca. 20 nm in diameter, are electrochemically deposited inside the titania nanotubes (NTs) fabricated by anodization of titanium in fluoride-ion-containing electrolyte. The catalytic activities of the resulting architecture in the catalytic oxidation of methanol in sodium hydrate medium and sulfuric medium are investigated. FTIR spectroscopy in the spectra reflectance mode is used to elucidate the catalytic mechanism in alkaline media through identifying species adsorbed on the electrode surface during the reaction. The enhanced catalytic activity in alkaline media is attributed to the high affinity of the Au nanoparticles and titania NTs to the abundant -OH groups and the synergistic catalytic effects of platinum-gold (PtAu) nanoparticles.
1. Introduction The excellent catalytic activity of nanoarchitectured platinum for methanol oxidation makes this metal an ideal anode in direct methanol fuel cells (DMFCs).1-5 Pure platinum, however, is readily poisoned by CO-like intermediate species produced in methanol oxidation reaction (MOR). Considerable efforts have been devoted to the design and synthesis of antipoisoning Ptbased catalysts by alloying it with Ru, Sn, Ni, Os, Bi, W, Au, etc.6-13 In alloy catalysts Pt supplies the main sites for dehydrogenation of methanol and the second metal provides the sites for absorbing hydroxide (OH) and oxidizing CO-like species to carbonate. Recently, gold nanoparticles have been extensively investigated as a catalyst for CO oxidation since Haruta’s pioneering work.14,15 The catalytic activity of gold was remarkably relative to the size of the gold particle, preparation methods, and nature of the support.16,17 Supporting materials with large specific surface area are essential to achieving a high dispersion of catalyst nanoparticles and therefore a high catalytic activity. Titania NT arrays prepared by anodization, well known for its easy preparation, high orientiation and uniformity, and large surface area, are a promising functional material.18-21 The electrode fabricated by dispersing Pt/Ru nanoparticles on the top of the anodic TiO2 NT arrays was with a higher catalytic activity for MOR as compared with an electrode using compact TiO2 support.22 Catalytic oxidation of CO on pure Pt catalyst can proceed at lower potential in alkaline media than in acidic media23 due to the relatively higher surface OH coverage in the former media. However, the formed carbonates would clog polymer electrolyte membranes (e.g., Nafion) of fuel cells,24,25 limiting application of alkaline electrolytes in fuel cells. The studies on MOR were therefore mainly performed in acidic media. We noticed the work by the Wieckowski group on the investigation of MOR in alkaline media.25-27 They proposed a membraneless lamina flow-based micro fuel cell,25 achieving a high activity by removing the carbonates through a flowing stream. It is deduced that incorporation of appropriate supporting materials would help to fleetly deplete the carboxyl compounds and, furthermore,
Figure 1. Characterization of the AuPt/titania NTs: (a) SEM image of the titania NT array topology, (b) EDS analysis, and (c and d) TEM images of the titania NT inner view.
could not only simplify the manufacture process but also point out a new way for development of fuel cells. Hence, in this work, the catalytic properties of titania NT arrays functionalized with PtAu nanoparticles for MOR were investigated in alkaline media. The present results suggest a way for designing new catalysts for DMFCs. 2. Experimental Section Titanium foil (99.8%, 0.127 mm thick) was purchased from Aldrich (Milwaukee, WI). Sodium fluoride, sodium hydroxide, sulfuric acid, methanol, sodium hydrogen sulfate, hexachloroplatinic(IV) acid, and hydrogen tetrachloroaurate trihydrate of analytical reagent grade were purchased from commercial
10.1021/jp0727695 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007
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Figure 2. Cyclic voltammograms (a) and chronoamperometry curves (b) of methanol oxidation at (1) 0.51 µg of Au + 1.36 µg of Pt/cm2, (2) 1.36 µg of Pt/cm2, and (3) 0.51 µg of Au/cm2 decorated titania NT arrays.
sources and used as supplied. Twice distilled water was used throughout the experiments. Prior to anodization, titanium ribbons were ultrasonically cleaned in acetone and ethanol. The cleaned titanium ribbon was anodized at 15 V in an electrolyte containing 0.1 M NaF and 0.5 M NaHSO4 at room temperature for 5 h in a two-electrode configuration with a platinum cathode. The resulting titania NTs were amorphous with a 90 nm pore diameter, 320 nm in length,21,28 with an efficient electrode area of 0.75 cm × 1 cm on each side. Au and Pt nanoparticles were deposited in sequence using chronopotentiometry at a current density of 2.5 × 10-5 A/s in a standard three-electrode configuration with a titania NT working electrode, a platinum wire auxiliary electrode, and an Ag/AgCl (saturated by KCl) reference electrode (CHI 660B; CH Instruments. Inc., Austin, TX) in 5 mM HAuCl4, or H2PtCl6 solution. The loading content was tuned by controlling the deposition duration. Cyclic voltammetry (CV) and chronoamperometry were performed to investigate the electrooxidation of methanol in a solution containing 2 M CH3OH and 0.5 M NaOH or in a solution containing 2 M CH3OH and 0.5 M H2SO4 at a scan rate of 100 mV/s. All chronoamperometric analyses were performed at -0.15 V versus Ag/AgCl in sodium hydrate medium. Fourier transform infrared (FTIR) spectra were collected with a FTIR spectrometer (Nicolet 5700, Thermo) equipped with a DTGS detector using the FT-80° grazing angle reflectance mode. The topology of the catalyst was characterized using a fieldemission scanning electron microscope (FE-SEM) operating at 5 kV (JSM 6700F; JEOL, Tokyo, Japan). An energy-dispersive X-ray (EDX) spectrometer fitted to the scanning electron microscope was used for elemental analysis. Transmission electron microscopy (TEM) images were obtained using a JEM 3010 (JEOL, Tokyo, Japan) operating at 300 kV.
Yang et al.
Figure 3. (a) Cyclic voltammograms of methanol oxidation at a 0.51 µg of Au + 1.18 µg of Pt/cm2 decorated titania NTs in (1) 0.5 M NaOH + 2 M CH3OH and (2) 0.5 M H2SO4 + 2 M CH3OH. (b) FTIR spectra for the species adsorbed on the electrode during MOR in sodium hydrate media (curve 1) and sulfuric acid media (curve 2).
3. Results and Discussion 3.1. Nanoparticle Deposition. As previously reported in our work,29 in an externally applied electric field polarization of amorphous titania results in electron hopping in Ti3+-O-Ti4+ chains, producing an enhanced conductivity of the titania NTs that facilitates electrodepositon of Au and Pt nanoparticles into the NTs. The first deposited Au not only enhanced the conductivity of titania facilitating the sequent Pt electrodeposition but also would protect Pt sites from poisoning by COlike species in MOR. The SEM image, Figure 1a, shows a few nanoparticles on the titania NTs surface, and the corresponding EDS spectrum, Figure 1b, shows the presence of Pt and Au. The TEM images, Figure 1c and d, show uniformly distributed metal nanoparticles of about 20 nm in diameter inside the nanotubes, indicating that the Pt and Au nanoparticles are mainly deposited inside the NTs. The novel construction effectively holds noble metal nanoparticles on supports benefiting a stable catalytic activity. 3.2. Catalytic Properties. Figure 2a shows the CVs of MOR on PtAu/titania NTs, Pt/titania NTs, and Au/titania NTs in 2 M CH3OH + 0.5 M NaOH. The onset potential on MOR at PtAu/ titania was negatively shifted by 100 mV as compared with those on Pt/titania and Au/titania. Shown as curve 1 in Figure 2a, the peak current (Ip) at the titania NTs-loaded 0.51 µg of Au +1.36 µg of Pt is 3.25 times that on the 1.36 µg of Pt/titania NTs (curve 2) and hundreds times that on the 0.51 µg of Au-loaded titania NTs (curve 3), which indicates a strong synergistic catalytic function of Pt and Au. Figure 2b shows the current densities measured at -0.15 V for a period of 600 s on these electrodes in 2 M CH3OH + 0.5 M NaOH. The high initial density is due to the double-layer charging30 and abounding
Well-Dispersed PtAu Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16615
Figure 4. Cyclic voltammograms (a) and chronoamperometric curves (b) obtained on four electrodes loaded with (1) 0.325, (2) 0.65, (3) 1.36, and (4) 2.72 µg/cm2 of Pt, respectively, and a fixed Au loading of 0.51 µg/cm2. The peak current density of CVs is listed in Table 1.
active sites initially available for methanol activation. Current density drops sharply within the first few seconds as the Pt sites are deactivated by CO-like species formed in methanol decomposition.31,32 A pseudo-steady state is reached in approximately 500 s. The PtAu-modified titania NTs exhibit the highest oxidation current density. Nevertheless, the result from the acidic electrolyte shows a low activity. In Figure 3a the oxidation Ip in alkaline electrolyte is found to be 3 times that in acidic electrolyte. In order to investigate the difference of the catalytic activities in acidic and alkaline media, species absorbed on the electrode during the reaction were identified by FTIR spectroscopy with results given in Figure 3b. The band around 1623 cm-1 assigned to the δ(-OH) vibration of absorbed water33 is much weaker in acidic electrolyte than in alkaline electrolyte. The band at 3715 cm-1 assigned to the isolated Ti-OH groups is also much weaker in acidic electrolyte than in alkaline electrolyte. In fact, although the MOR on platinum has been widely investigated in recent decades, the actual mechanism containing all the reactions and kinetic parameters is still unclear. By referring to the literature6,23,32 and based on our experimental results, we prefer a parallel mechanism which has been applied to MOR by a lot of authors34-37
Step 1: CH3OH + Pt w Pt-COad + CO2
(1)
CO2 and CO-like species were formed by direct methanol oxidation. Pt sites were deactivated by CO-like species, resulting in the initial current decrease in the chronoamperometric test
Step 2: Pt-COad + Au w Au-COad + Pt
(2)
Figure 5. Cyclic voltammograms (a) and chronoamperometric curves (b) obtained on four electrodes loaded with (1) 0.255, (2) 0.51, (3) 1.02, and (4) 1.53 µg/cm2 of Au, respectively, and a fixed Pt loading of 2.72 µg/cm2. The peak current density of CVs is listed in Table 1.
Formation of Au-COad reactivated a majority of the Pt sites, resulting in an enhanced MOR Ip on PtAu-modified titania NT electrode shown as the CVs, where Au works as the synergistic catalyst in a bifunctional system. At the same time, abundant Au-OHad and Ti-OHad groups were formed
Ti + Au + OH- w Au-OHad + Ti-OHad + 2e- (3) And reaction happened between metal-COad and metal-OHad releasing CO2
Au-COad + Pt-COad + Au-OHad + Ti-OHad w CO2 + Pt + Au + 2e- + Ti (4) The above reactions are parallel with direct methanol oxidation, enhancing both the catalyst antipoison capability and the current density. In summary, Pt works as the main dehydrogenation site, and Au together with titania speeds up removal of CO-like species, inhibiting poisoning of Pt. Furthermore, the presence of abundant OH- in alkaline electrolytes helps to promote conversion of CO-like species to CO2, preventing the catalyst from being poisoned and producing a much higher activity for MOR in alkaline electrolyte as compared with that in acidic electrolyte. 3.3. Effect of PtAu Composition on Ccatalytic Properties. Figures 4 and 5 show the typical CV and chronoamperometric curves for MOR on electrodes with variable Pt and Au as shown in Table 1 in which the corresponding CV Ip density of unit weight of AuPt load is listed. While keeping the Au constant at 0.51 µg/cm2, the Ip is enhanced from 0.27 to 2.64 mA/cm2
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Yang et al. due to the poor conductivity of Al2O3. The higher conductivity of titania and the special nanotubular structure providing abundant active centers make anodic titania NT array an ideal support for DMFCs. 4. Conclusion
Figure 6. Cyclic voltammograms of MOR on (1) PtAu/titania NTs film and (2) PtAu/AAO nanoporous film.
TABLE 1: Pt/Au Composition-Dependent Peak Current Density (IP) of the PtAu/titania Electrode for MOR in Alkaline Medium; Corresponding CV and Chronoamperometric Curves are Shown in Figures 4 and 5 electrode Figure 4
Figure 5
Pt/µg 1 2 3 4 1 2 3 4
cm-2
0.325 0.65 1.36 2.72 2.72 2.72 2.72 2.72
Au/µg
cm-2
0.51 0.51 0.51 0.51 0.255 0.51 1.02 1.53
IP/mA‚cm ‚(mg AuPt) -2
-1
334 429 853 816 473 816 468 306
with increasing Pt load from 0.325 to 2.72 µg/cm2 as shown in Figure 4a. The corresponding time-dependent current density shown in Figure 4b exhibits accordant results, but electrode 3 shows the slowest decreasing rate in current density in the initial 450 s, indicating the highest catalytic activity among these four electrodes. The current density of unit the catalyst weight shown in Table 1 confirms this conclusion. Electrode 3 shows the highest current density of 853 mA‚cm-2‚(mg of AuPt)-1. While keeping the Pt load constant at 2.72 µg/cm2, the maximum Ip was achieved at a Au load of 0.51 µg/cm2 as shown in Figure 5. The stable current density shown in Figures 4b and 5b indicates that catalysts with stable activities are achieved. The optimal Au and Pt loads are therefore 0.51 µg of Au + 1.36 µg of Pt/cm2. Overload of Au or Pt would reduce the active sites, decreasing catalytic activity. Improving the conductivity of the titania support and reducing the catalyst nanoparticle size would enhance the peak current density since a conductive support (e.g., carbon) favors electron transferring and nanoparticles with smaller size offer more reactive centers, achieving a higher peak current density.6 We are attempting to enhance the titania conductivity by doping other elements and reducing the PtAu nanoparticles size through variable modifying techniques. With the advantages of a simple fabricating process of PtAu/titania system and its antipoison property in alkaline media, this functional material would be a promising practical catalyst for alkaline fuel cells. In order to investigate the function of the titania NTs in the catalytic system nanoporous anodic aluminum oxide (AAO) films were fabricated to replace the titania NTs as the support. The AAO films were prepared by anodic oxidation of a pure aluminum plate (0.25 mm thick) in a 0.3 M oxalic acid electrolyte under a constant direct current voltage of 40 V at 17 °C for 1 h, achieving AAO film of ∼400 nm in thickness and ∼60 nm in pore diameter. Both the alumina and titania films were decorated with the optimum PtAu composition. Poor activities are observed at AAO electrode as shown in Figure 6
A catalyst for MOR was constructed by electrochemically depositing PtAu nanoparticle inside highly oriented anodic titania nanotubes which possess a large specific surface area. The abundant Au-OH and Ti-OH bonds formed in alkaline medium help to convert CO-like species to carbonates and inhibit catalyst poisoning, producing a high activity. The maximum electrocatalytic activity was achieved with a loading of 0.51 µg of Au + 1.36 µg of Pt/cm2. The high antipoisoning capability and stable catalytic activity of the PtAu-modified titania NTs suggest a way to design new catalyst for DMFCs with high activity and good performance. Acknowledgment. Funding for this work by the National Science Foundation of China under grant no. 20475016 is gratefully acknowledged. References and Notes (1) Hearth, M. P.; Hards, G. A. Platinum Met. ReV. 1996, 40, 150. (2) Arico, A. S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 133. (3) Mcnicol, B. O.; Rand, D. A. J.; Williams, K. R. J. Power Sources 1999, 83, 15. (4) Bastide, B.; Enjalbert, R.; Fuess, H.; Galy, J. Solid State Chem. 2000, 2, 545. (5) Hamnett, A. Catal. Today 1997, 38, 445. (6) Luo, J. P.; Njoki, N.; Lin, Y.; Mott, D.; Wang, L. Y.; Zhong, C. J. Langmuir 2006, 22, 2892. (7) Holstein, W. L.; Rosenfeld, H. D. J. Phys. Chem. B 2005, 109, 2176. (8) Lin, Y.; Cui, X.; Yen, C. H.; Wai, C. M. Langmuir 2005, 21, 11474. (9) Rivera, E. C.; Volpe, D. J.; Alden, L.; Lind, C.; Downie, C.; Alvarez, T. V.; Angelo, A. C. D.; DiSalvo, F. J.; Abruna, H. D. J. Am. Chem. Soc. 2004, 126, 4043. (10) Park, K.-W.; Chli, J.-H.; Sung, Y.-E. J. Phys. Chem. B 2003, 107, 5851. (11) Huang, J.; Yang, H.; Huang, Q.; Tang, Y.; Lu, T.; Akinsb, D. L. J. Electrochem. Soc. A 2004, 151, 1810. (12) Umeda, M.; Ojima, H.; Mohamdi, M.; Uchida, I. J. Power Sources 2004, 136, 10. (13) Liu, H.; Gaigneaux, E. C.; Imoto, H.; Shido, T.; Iwasawa, Y. J. Phys. Chem. B 2000, 104, 2033. (14) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (15) Haruta, M. Chem. Rec. 2003, 3, 75. (16) Grunwaldt, D.; Maciejewski, M.; Becker, O. S.; Fabrizioli, P.; Baiker, A. J. Catal. 1999, 186, 458. (17) Schubert, M. M.; Hackenberg, S.; Van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. J. Catal. 2001, 197, 113. (18) Gong, D. W.; Grimes, C. A.; Varghese, O. K.; Hu, W. C.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331. (19) Varghese, O. K.; Gong, D. W.; Paulose, M.; Grimes, C. A. J. Mater. Res. 2003, 18, 156. (20) Mor, G. K; Varghese, O. K.; Paulose, M.; Mukherjee, N.; Grimes, C. A. J. Mater. Res. 2003, 18, 2588. (21) Cai, Q. Y.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2005, 20, 230. (22) Macak, J. M.; Barczuk, P. J.; Tsuchiya, H.; Nowakowska, M. Z.; Ghicov, A.; Chojak, M.; Bauer, S.; Virtanen, S.; Kulesza, P. J.; Schmuki, P. Electrochem. Commun. 2005, 7, 1417. (23) Markovic, N. M.; Schmidt, T. J.; Grgur, B. N.; Gasteiger, H. A.; Behm, R. J.; Ross, P. N. J. Phys. Chem. B 1999, 103, 8568. (24) Mclean, G.; Niet, T.; Richard, S. P. Int. J. Hydrogen Energy 2002, 127, 507. (25) Choban, E. R.; Spendelow, J. S.; Gancs, L.; Wieckowski, A.; Kenis, P. J. A. Electrochim. Acta 2005, 50, 5390. (26) Spendelow, J. S.; Goodpaster, J. D.; Kenis, P. J. A.; Wieckowski, A. J. Phys. Chem. B 2006, 110, 9545. (27) Spendelow, J. S.; Lu, G. Q.; Kenis, P. J. A.; Wieckowski, A. J. Electroanal. Chem. 2004, 568, 215. (28) Yang, L. X.; Cai, Q. Y. Inorg. Chem. 2006, 45, 9616.
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