Direct Oxidation of Methanol on Pt Nanostructures Supported on

The electrochemical properties were examined using a CHI 760 dual channel electrochemical workstation (CH instruments, Inc.) with a three-electrode sy...
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9970

2008, 112, 9970–9975 Published on Web 06/18/2008

Direct Oxidation of Methanol on Pt Nanostructures Supported on Electrospun Nanofibers of Anatase Eric Formo,†,§ Zhenmeng Peng,‡,§ Eric Lee,† Xianmao Lu,† Hong Yang,*,‡ and Younan Xia*,† Department of Biomedical Engineering, Washington UniVersity, St. Louis, Missouri 63130, and Department of Chemical Engineering, UniVersity of Rochester, Rochester, New York 14627 ReceiVed: April 25, 2008; ReVised Manuscript ReceiVed: June 3, 2008

This letter reports an electrocatalytic study of electrospun anatase nanofibers decorated with Pt catalysts in the form of nanoparticles or nanowires. We decorated the surface of nanofibers with Pt through a polyol process, in which nanoparticles of 2-5 nm in size were formed with different densities depending on the reaction time. By adding Fe(III) ions to the polyol process, we also obtained Pt nanowires of ∼7 nm in diameter and up to 125 nm in length. We then studied the effects of both the coverage and morphology of the Pt nanostructures on the methanol oxidation reaction. Nanofibers with a submonolayer of Pt nanoparticles were found to display improved catalytic durability over commercial Pt/C as determined by chronoamperometry owing to a synergistic effect of the underlying anatase surface and the Pt nanostructures with well-defined facets. Improvement in catalytic activity and durability were also observed for Pt nanowires, indicating that the additional catalytic facets on the nanowires can enhance both catalytic ability and robustness. Introduction The use of noble metals in green technologies has garnered an increasing level of research interest, with notable examples including the use of Pt nanostructures in direct methanol fuel cells (DMFCs).1 For the standard DMFC device, Pt-based catalysts are employed as the anode because of their excellent performance in catalyzing the dehydrogenation of methanol, a key step in the direct oxidation of methanol to CO2.2 The active surface area of the catalyst can be greatly increased by reducing the size of the Pt nanoparticles and therefore creating a larger number of catalytically active centers for methanol oxidation reaction (MOR).3 The efficiency of MOR can also be improved by changing the morphology of the Pt nanostructures from nanoparticle to nanowire whose large side surface can provide additional catalytically active facets.4 It is well documented that the Pt nanostructures are vulnerable to poisoning from CO-like intermediates formed during the oxidation of methanol and may become catalytically inactive over time.5 To improve the COtolerance of Pt-based catalysts, various techniques have been developed, among which utilization of metal oxide supports including TiO26 and SnO27 has been shown to have great promise. The selection of a support for the Pt nanostructures is of key importance to both the catalytic activity and durability.8 In a conventional system, Pt is deposited on a conductive substrate such as carbon black or carbon nanotubes.9 Since Pt can increase the corrosion rate of carbon, this system is intrinsically limited in terms of life span and the catalytic surface area of the electrode may shrink with time.8 Titania is potentially useful * To whom correspondence should be addressed. E-mail: xia@ biomed.wustl.edu. † Washington University. ‡ University of Rochester. § These two authors contributed equally to this work.

10.1021/jp803763q CCC: $40.75

as a support for the deposition of Pt to be used in direct oxidation of methanol.10 The TiO2 support can lower the adsorption energy of CO intermediates, increasing the mobility of CO group on Pt nanostructures. In addition, adsorption of OH species (OHad) on TiO2 during methanol oxidation can facilitate the conversion of the catalytically poisonous CO intermediates into CO2, thereby improving the durability of the Pt catalysts.10 There are a number of methods for preparing supported Pt nanostructures, including the deposition of metal nanoparticles onto porous substrates via a gas-phase decomposition or reduction process, the in situ formation of metal nanoparticles on carbon materials, and the growth of Pt nanowires on ceramic particles or metal gauze.4,11 Electrospun nanofibers have proven to be efficient catalytic supports owing to the high porosity and large surface areas. The high porosity in a nonwoven mat of nanofibers enables direct growth of catalytic nanostructures. Recent success in using electrospun nanofibers as supports for catalytic metal nanoparticles has been demonstrated for a number of reactions.12 Previously, we have developed a method for depositing Pt nanostructures on the electrospun nanofibers of anatase through a simple polyol reduction.13 Using this method, the size and morphology of the nanostructures and their coating density on the nanofibers can all be readily controlled, offering an excellent system for the study of catalytic performance of Pt nanostructures supported on ceramic nanofibers. In this report, we present an electrochemical study of direct methanol oxidation using Pt-decorated anatase nanofibers. We examined and compared the catalytic activity and durability of Pt nanoparticles with varying sizes and coating densities, as well as those of Pt nanowires. Experimental Methods Fabrication of Anatase Nanofibers. The nanofibers were prepared by electrospinning a solution containing 3 mL of  2008 American Chemical Society

Letters

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Figure 1. (A-C) TEM image of the anatase nanofiber after its surface had been decorated with Pt nanoparticles by immersing the sample in a polyol reduction bath for 3, 7, and 19 h, respectively. (D) TEM image of the 19-h sample after its surface had been further decorated with Pt nanowires in another polyol reduction bath containing Fe(III) for 13.5 h. The scale bars are 10 nm. The inset illustrates the morphology and coverage of Pt nanostructures on each surface.

titanium tetraisopropoxide (Aldrich), 2 mL of acetic acid, 0.3 g of poly(vinyl pyrrolidone) (PVP) (Mw ≈ 1.3 × 10,6 Aldrich), and 5 mL of ethanol. The solution was passed through a syringe with a 21 gauge stainless steel needle. The needle was electrified at 10 kV using a high-voltage DC supply (ES30P-5W, Gamma High Voltage Research Inc., Ormond Beach, FL). The solution was continuously supplied using a syringe pump (KDS-200, Stoelting, Wood Dale, IL) at a rate of 0.3 mL/h. The fibers were collected on a piece of grounded aluminum foil and left overnight in air to fully hydrolyze. The composite nanofibers were then calcinated in air at 510 °C for 6 h. XRD powder diffraction indicates that the final phase was anatase. The diameter of the fiber varied in the range of 150-400 nm. Preparation of Pt Catalysts. To decorate the anatase nanofibers with Pt nanoparticles, 4 mL of ethylene glycol (EG, J. T. Baker, Lot# A34B16) was initially injected into a 3-neck flask (fitted with a reflux condenser and a Teflon-coated stir bar) and heated in air at 110 °C for 30 min. A total of 10 mg of the anatase nanofibers were then added to the EG and heated for an additional 30 min. PVP (400 mM, 0.045 g, Mw ≈ 55 000, Aldrich) and H2PtCl6 (80 mM, 0.033 g, Aldrich) solutions were prepared separately in 2 mL of EG at room temperature. These two solutions, each 1 mL in volume, were then added simultaneously into the flask over a period of 1.5 min. The reaction was allowed to continue at 110 °C for 3, 7, and 19 h, respectively. The final samples were washed thoroughly with ethanol and water to remove EG and excess PVP. Growth of Pt Nanowires. In a typical procedure, 4 mL of EG was injected into a 3-neck flask (fitted with a reflux

condenser and a Teflon-coated stir bar) and heated in air at 110 °C for 30 min. A total of 100 µL of Pt-coated anatase nanofibers (from a solution of 10 mg dispersed in 6 mL of water) was then added to the EG along with 20 µL of 20 mM FeCl3 · 6H2O or FeCl2 · 6H2O (Aldrich) solution in EG. The solution was heated for an additional 30 min to remove any trace amount of water. PVP (400 mM, 0.045 g, Mw ≈ 55 000, Aldrich) and H2PtCl6 (80 mM, 0.033 g) solution were prepared separately in 2 mL of EG at room temperature. These two solutions, each of 1 mL in volume, were then added simultaneously into the flask over a period of 1.5 min. The reaction was allowed to proceed at 110 °C in air for 13.5 h. The final solution was transparent, decorated with some black precipitate. The anatase fibers coated with Pt nanowires were washed thoroughly with ethanol and water to remove EG and excess PVP. Preparation of Electrocatalysts. In a typical procedure, 0.05 mL of Pt/TiO2 in ethanol was dispersed in a mixture containing 0.45 mL of ethanol, 0.5 mL of water, and 5 µL of Nafion solution (5 wt %, Aldrich). This mixture was then sonicated for 20 min. The amount of Pt in Pt/TiO2 samples was determined using energy dispersive X-ray (EDX) analysis. The solid content of Pt/TiO2 in ethanol was quantified using a thermo gravimetric analyzer (TGA) from TA Instrument, Inc. (model SDT Q600). To prepare catalyst membrane, a specific volume of the above mixture of Pt/TiO2 was measured (to give 1.5 µg Pt) and dropcast onto a glassy carbon electrode (5 mm in diameter) and dried gently under air flow. The Pt/C catalyst (20 wt % Pt on Vulcan XC-72, E-TEK) was prepared using the same procedure

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Figure 3. CVs of anatase nanofibers whose surfaces are coated with (A) varying densities of Pt nanoparticles and (B) Pt nanowires. The CVs were recorded with 0.5 M H2SO4 as the electrolyte and at a scan rate of 50 mV/s in (A) Pt nanoparticles and 20 mV/s for (B) Pt nanowires.

Figure 2. HRTEM images of (A) Pt nanoparticles after 7 h of deposition on anatase nanofibers and (B) Pt nanowires. The scale bars is 1 nm.

as that for the Pt/TiO2 samples except that the sonication time was 10 min and the loading amount of Pt was 1 µg. Characterization. The morphology of the Pt-decorated nanofibers was characterized using transmission electron microscopy (TEM). TEM samples were prepared by drop casting a dispersion of the nanofibers onto carbon-coated copper grids (Formvar/Carbon, 200 mesh, Ted Pella). TEM images were acquired using a Philips CM100 or a Hitachi H-7500 microscope operated at 100 kV. High-resolution TEM was performed using a JEOL JEM-2100F operated at 200 kV. Electrochemical Measurements. The electrochemical properties were examined using a CHI 760 dual channel electrochemical workstation (CH instruments, Inc.) with a threeelectrode system, which consists of a rotating-disk working electrode with the sample on it, a Pt wire counter electrode, and a hydrogen reference electrode (Gaskatel, HydroFlex).

Hydrogen adsorption-desorption cyclic voltammograms (CVs) were recorded in an argon-protected 0.5 M sulfuric acid aqueous solution. The solution was purged by Ar first to deplete dissolved O2. The region for hydrogen adsorption (0.05-0.4 V vs RHE on the backward potential scan) was used to estimate the electrochemical active surface areas (ECSAs). For CVs and chronoamperometry of methanol oxidation reaction, an air-free aqueous solution containing 0.5 M methanol and 0.5 M H2SO4 was used. Results and Discussion Fabrication and Characterization of Nanofibers. Figure 1A-C shows TEM images of anatase nanofibers after they had been placed in the polyol reduction bath for 3, 7, and 19 h, respectively. After 3 h of reaction, Pt nanoparticles of ∼2 nm in diameter were deposited on the surface of the nanofiber as a submonolayer (Figure 1A, designated as NP-3 h). The nanoparticles were more or less evenly distributed across the surface of each fiber without aggregation, offering high level exposure of the nanoparticle surface. Once the reaction had progressed for 7 h, the density of Pt nanoparticles was increased, along with an enlargement in average diameter to ∼5 nm (Figure 1B, NP-7 h). Also, these 5 nm nanoparticles began to form a more densely packed array close to a monolayer, as illustrated in the

Letters

Figure 4. (A) CVs for MOR on Pt nanoparticles that were deposited on anatase nanofibers in different periods of time. The CVs were recorded at a scan rate of 50 mV/s, with a mixture of 0.5 M MeOH and 0.5 M H2SO4 (serving as the electrolyte). (B) Chronoamperometry curves recorded at 0.85 V for various samples. The measurement condition is as follows: 0.5 M MeOH, 0.5 M H2SO4, set potential of 0.85 V (vs RHE), and pretreatment before tests at 0 V for 30 s.

inset of Figure 1B. Although there are more Pt nanoparticles on the nanofiber, the exposed Pt surface area is expected to decrease due to aggregation of the particles. Further increase in reaction time to 19 h resulted in a sheath of densely packed Pt nanoparticles on the anatase nanofibers (Figure 1C, NP-19 h). The close proximity of the nanoparticles to one another, as illustrated in the inset of Figure 1C, shows a further reduction in the amount of exposed Pt on the surface. As noted previously, the anatase nanofibers with a dense coating of Pt nanoparticles (sample NP-19 h) can be used as a template to grow Pt nanowires.13 The Pt nanowires produced using this method ranged from 50 to 125 nm in length (Figure 1D, NW-19 h). It should also be noted that for samples NP-3 h and NP-7 h some of the anatase surface was not covered by Pt, whereas for NP19 h and NW-19 h the surface of each anatase nanofiber was completely covered with Pt. Further investigation of the differing Pt nanostructure shows that there is a varying surface morphology. The crystal structures of the Pt nanoparticles and nanowires were analyzed using HRTEM (Figure 2). The nanoparticles deposited on the anatase fibers showed smooth surfaces. It has been reported that for Pt nanoparticles with smooth surfaces, all three low-index facets, including {100}, {110}, and {111}

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Figure 5. (A) CVs for MOR on NW-19 h and Pt/C. The CVs were recorded at a scan rate of 50 mV/s and a mixture of 0.5 M MeOH and 0.5 M H2SO4 (serving as the electrolyte). (B) Chronoamperometry curves recorded at 0.85 V for various samples. The measurement condition is the same as that used in Figure 4B.

can be exposed.14 Pt nanowires showed growth direction of 〈111〉, indicating that the exposed low-index facets on the side surface could be mainly {110}. Examination of Electrochemical Performance. We analyzed electrochemical properties of the Pt nanostructures using cyclic voltammetry. Figure 3A shows the CVs recorded with a solution of 0.5 M H2SO4 in the absence of methanol. The CV curves showed the standard hydrogen adsorption potentials of 0.12 and 0.23 V (vs RHE) for the {110} facet of Pt.15 The variation in the shapes of these curves suggest different catalytic facet exposure on the surfaces of the varying nanostructures. ECSAs of the Pt nanoparticle-coated nanofibers were estimated to be 56.2, 27.5, and 16 m2/g Pt for samples NP-3 h, NP-7 h, and NP-19 h, respectively, indicating that anatase nanofibers with a less dense coating of Pt nanoparticles had a higher active surface area. For the samples obtained at longer reaction times, size growth and aggregation of the metal particles reduced the specific surface area and limited the access of the reactants to the catalytically active sites. Figure 3B shows the CV for sample NW-19 h at a scan rate of 20 mV/s. The ECSA of the Pt nanowire-decorated fibers expanded from 16 to 17.6 m2/g Pt for sample NP-19 h, as would be expected with the larger amount of catalytic Pt in the system.

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Figure 6. (A-C) TEM images of the anatase nanofibers whose surfaces were decorated with Pt nanoparticles (NP-3 h, NP-7 h, and NP-19 h) and Pt NWs, followed by the chronoamperometry tests. The scale bars are 10 nm.

Methanol Oxidation. Figure 4A shows the CVs recorded in a methanol solution for anatase nanofibers decorated with Pt nanoparticle at different coating densities. The peak area current density was 0.583, 0.558, and 0.470 mA/cm2 for sample NP-3 h, NP-7 h, and NP-19 h, respectively. The peak current density decreased with respect to longer deposition time and thus higher density of Pt coating, indicating that NP-3 h was the preeminent catalyst for MOR among all three nanoparticle-decorated nanofibers. The decrease of current density was mainly caused by the increase of particle size and by the reduction in surface exposure of the anatase support. For the decoration of nanofibers with nanoparticles, longer reaction time resulted in aggregation of Pt nanoparticles and larger particle sizes, which in turn lowered the catalytic activities of the particles.16 The higher current density of NP-3 h than the Pt/C can be partially attributed to the effect of the anatase support. During the electrochemical process, OH species can be generated on the anatase surface. It is well-known that OHad can assist the removal of CO-like intermediates from the Pt nanoparticle surface and reduce the poisoning of the Pt catalysts.11 The catalytic stability of the Ptdecorated nanofibers was examined using chronoamperometry. Figure 4B shows the chronoamperometry curves recorded at 0.85 V in a solution containing both CH3OH and H2SO4. Compared with commercial Pt/C, sample NP-3 h exhibited slower current decay over time, indicating higher tolerance to CO-like intermediates than Pt/C. However, the current decay with time for samples NP-7 h and NP-19 h was close to commercial Pt/C; thus, no stability gain was achieved for these two samples. In contrast to sample NP-3 h which showed high level exposure of anatase surface, the coating densities of Pt nanoparticles for NP-7 h and NP-19 h were so high that the

anatase surface was barely exposed. The lower catalytic durability for NP-7 h and NP-19 h than NP-3 h indicates that the synergistic effect of Pt/TiO2 to slow the poisoning rate of the catalyst can only be achieved in the presence of exposed anatase surface. The morphology of the Pt nanostructures also plays an important role in the catalytic activity and stability for MOR. Figure 5A shows the CV for anatase nanofibers decorated with Pt nanowires (sample NW-19 h) in a methanol solution. Compared to the nanoparticle-decorated sample NP-19 h, the CV curve of NW-19 h showed little difference for the on-set and peak potentials, indicating the same reaction pathway for both nanowire and nanoparticle-based catalysts. The primary difference between the samples resides in the peak current densities. The area peak current density of NW-19 h was 0.686 mA/cm2, which was 45.6% higher than that of NP-19 h, 14.3% higher than commercial Pt/C, which gave a peak current density of 0.6 mA/cm2, and was 17.7% higher than NP-3 h, which gave the highest value of the NP samples that we had generated. This indicates higher electrocatalytic activity for Pt nanowiredecorated nanofibers in the oxidation of methanol in comparison to the nanoparticle-decorated nanofibers and Pt/C. The variation in catalytic activities can also be attributed to different exposed facets of Pt nanostructures. It is known that the rate of methanol oxidation is highest on the Pt {110} among the low-index surfaces when using H2SO4 as the electrolyte.17 The more exposed {110} surface on Pt nanowires than nanoparticles would lead to higher activity. A slight positive shift of the peak for the NW-19 h also occurred when in comparison with Pt/C, which was probably due to different support used for these two samples. Enhanced durability in catalyzing oxidation of metha-

Letters nol was also observed for NW-19 h in comparison with Pt/C, as revealed from the current decay curves in Figure 5B. Although less anatase surface was exposed for sample NW-19 h, its catalytic performance was comparable with that of NP-3 h, the most durable catalyst among all three nanoparticledecorated nanofibers. This result indicates that, instead of providing a TiO2 support, we can achieve improved durability by changing the morphology of the Pt nanostructures from nanoparticle to nanowire. To confirm the effect of morphology of Pt nanostructures on durability, we obtained the TEM images of the samples post the accelerated durability test (Figure 6). The nanoparticles decorated on the nanofibers especially those with high coating densities (NP-7 h and NP-19 h) experienced an obvious growth during the durability test in which the nanoparticle grew to roughly 10 nm, leading to a loss of active surface and thus activity. NW-19 h, however, exhibited little change in terms of morphology, indicating a small loss of their electrochemical active surface and thus enhanced durability. Conclusion We have fabricated anatase nanofibers by electrospinning and then decorated their surfaces with Pt nanoparticles of 2-5 nm in size and varying densities of surface coverage, as well as Pt nanowires of ∼5 nm in diameter. These supported Pt nanostructures have proven to be effective electrocatalysts for direct methanol oxidation. As compared with a commercial Pt/C catalyst, we have observed both improved electrochemical activity and durability for the Pt nanowires and the Pt nanoparticles with a submonolayer coverage. The methodology described in this article offers a simple and versatile route to the functionalization of electrospun nanofibers with noble-metal nanostructures for use in fuel cell technologies. Acknowledgment. This work was supported in part by a grant from the NSF (DMR-0451788, Y.X.), an I-CARES grant from Washington University in St. Louis (Y.X.), a CAREER Award from the NSF (DMR-0449849, H.Y.), and a PRF grant from the ACS (42446-G10, H.Y.). References and Notes (1) (a) Wasmus, S.; Kuver, A. J. Electroanal. Chem. 1999, 461, 14. (b) Liu, H. S.; Song, C. J.; Zhang, L.; Zhang, J. J.; Wang, H. J.; Wilkinson, D. P. J. Power Sources 2006, 155, 95. (c) Teng, X. W.; Liang, X. Y.; Maksimuk, S.; Yang, H. Small 2006, 2, 249. (2) (a) Liang, Z. X.; Zhao, T. S. J. Phys. Chem. C 2007, 111, 8128. (b) Wang, J. N.; Zhao, Y. Z.; Niu, J. J. J. Mater. Chem. 2007, 17, 2251. (c) Mustain, W.; Kim, H.; Prakash, S.; Stark, J.; Osborn, T.; Kohl, P. A. Electrochem. Solid State Lett. 2007, 10, B210. (d) Hamnett, A. Cat. Today 1997, 38, 445. (3) Hepel, M.; Dela, I.; Hepel, T.; Luo, J.; Zhong, C. J. Electrochim. Acta 2007, 52, 5529.

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