Synthesis and Durability of Highly Dispersed Platinum Nanoparticles

Oct 18, 2010 - by hydrogen gas, almost 100% platinum nanoparticle catalysts (Pt/MPC) were confined in the pores of MPC. Physical (XRD, HRTEM, and BET)...
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J. Phys. Chem. C 2010, 114, 19055–19061

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Synthesis and Durability of Highly Dispersed Platinum Nanoparticles Supported on Ordered Mesoporous Carbon and Their Electrocatalytic Properties for Ethanol Oxidation Ming-Hui Chen, Yan-Xia Jiang,* Shu-Ru Chen, Rui Huang, Jian-Long Lin, Sheng-Pei Chen, and Shi-Gang Sun* State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed: September 24, 2010

In this work, we develop a novel method to prepare a well-dispersed platinum precursor in the modified mesoporous carbon-46 (MPC) by using a simple melt-diffusion strategy. After the Pt precursor was reduced by hydrogen gas, almost 100% platinum nanoparticle catalysts (Pt/MPC) were confined in the pores of MPC. Physical (XRD, HRTEM, and BET), electrochemical, and in situ FTIR spectroscopic methods were used to investigate the properties of the Pt/MPC. The HRTEM image illustrates that Pt nanoparticles (NPs) of ca. 2-3 nm are loaded inside the small mesopores of the pore walls and NPs of ca. 5-6 nm are in the mesochannels of the MPC, and the Pt loading in Pt/MPC is 35 wt %. In comparison with the commercial Pt/C catalysts (40 wt %, Johnson Matthey), the Pt/MPC displays a high activity toward ethanol oxidation. The Pt/MPC exhibits also a high durability after a long-time potential scan: the loss of catalytic surface area is only 4.73% for Pt/MPC, whereas it is 14.75% for Pt/C. Moreover, in situ FTIR studies demonstrate that the Pt/MPC can promote the cleavage of the C-C bond of ethanol. 1. Introduction Ordered mesoporous carbon (OMC) has an ordered twodimensional hexagonal structure, a high specific surface area, and a large pore volume. Due to its multifunctionality, OMC has received much attention recently. It is potentially of great technological interest for the development of catalyst supports,1-4 electrode materials,5 and hydrogen-storage systems.6,7 In comparison with the traditional carbon supports, such as carbon black and activated carbon, the OMC has advantages in tuning the pore structure, pore size, and pore surface chemistry and allowing for a higher dispersion of platinum nanoparticles, which results in the improvement of the catalytic activity and stability.2 Ever since then, great efforts have been made to seek suitable OMC to be applied in fuel cells and to investigate the preferable method to embed the Pt nanoparticles. Shanahan et al. synthesized highly stable graphitic mesoporous carbon by heat-treating polymer-template mesoporous carbon at 2600 °C, and the electrochemical durability of it as a Pt catalyst support exhibits a greatly reduced loss of catalytic surface area compared with that of carbon black (Pt/XC-72).8 A conventional postsynthesis impregnation route and a one-pot synthesis approach were usually used to prepare the catalyst loaded in OMC. However, the postsynthesis method combining impregnation and following reduction in the liquid phase always leads Pt nanoparticles to appear both on the external surfaces and in the pores, thus, with a much wider size distribution. The one-pot synthesis process is too complex to be applied in large-scale. Joo et al. reported first the preparation of Pt nanoparticles loaded on ordered nanoporous arrays of carbon with the postsynthesis method and found that Pt nanoparticles were well-dispersed, and the diameter of Pt nanoparticles could be controlled by the nanopores of carbon.9 Liu et al. synthesized highly dispersed platinum * To whom correspondence should be addressed. Tel/Fax: 00-86-5922180181. E-mail: [email protected] (Y.-X.J.), [email protected] (S.G.S.).

nanoparticles of ca. 2-3 nm with a one-pot synthesis approach using SBA-15, a kind of ordered mesoporous silica, as a hard template, and platinum acetylacetonate as the cofeeding carbon and Pt precursor.10 Pt nanoparticles loaded in OMC (Pt/OMC) exhibit a higher catalytic activity toward methanol electrooxidation than the commercial catalyst.9-12 To the best of our knowledge, no work shows that Pt/OMC has a high activity toward ethanol electrooxidation. The modified MPC-46 (MPC), a bimodal ordered mesoporous carbon, was synthesized according to the procedure reported by Zhao’s group.1 The surface area and the pore volume of MPC are much larger than those of other common OMC materials. In addition, the MPC has plenty of small mesopores (pore size ∼ 2.3 nm), which disperse randomly in the ordered mesopore wall and interconnect with the ordered hexagonal mesochannels.2 These small mesopores contribute up to 46% of the Brunauer-Emmett-Teller (BET) surface area and lead to a coarse surface in the mesochannels, thus favorable to embedding Pt nanoparticles. The catalytic activity and durability are two main issues of the application of catalyst. The preparation methods of catalysts have a great impact on the structure and size of catalysts and further affect the catalytic activity, electrical conductivity, and stability. In this work, we present a novel strategy, by using a simple melt-diffusion approach, to prepare the highly dispersed platinum nanoparticle catalysts. The melt-diffusion strategy was first reported in preparing sulfur with high loading in the channel of OMC.5 In our study, the precursor was first loaded in MPC by impregnation and then underwent a long-time melting process, in which the precursor was well-dispersed in MPC by capillary forces. After the precursor was reduced by hydrogen gas, Pt nanoparticles on MPC (35 wt % Pt) of ca. 2-3 nm on the pore walls and ca. 5-6 nm in the mesochannels were achieved. Scheme 1 shows the preparation processes for platinum nanoparticles supported on MPC by the melt-diffusion approach and the corresponding SEM image. Pt nanoparticles

10.1021/jp1091398  2010 American Chemical Society Published on Web 10/18/2010

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SCHEME 1: Illustration of the Preparation Processes for Platinum Nanoparticles Supported on MPC by the Melt-Diffusion Approach

cannot be observed in the SEM image, indicating that the Pt nanoparticles are totally inside the pores of the MPC. When the precursor is evaporated rapidly without undergoing the melt-diffusion approach, the Pt nanoparticles thus formed are partially in the external surface (shown in Figure S1, Supporting Information). The combination of the MPC with a high specific surface area and well-dispersed Pt nanoparticles makes the reaction interfaces of the three-phase boundary (catalystelectrolyte-reactant) maximizing. Accordingly, we investigate the electrocatalytic activity, stability, and selectivity of ethanol oxidation on Pt/MPC synthesized by the melt-diffusion strategy. 2. Experimental Section 2.1. Chemicals. A difunctional block copolymer surfactant terminating in primary hydroxyl groups, poly(propylene oxide)block-poly(ethylene oxide)-block-poly(propylene oxide) triblock copolymer Pluronic F127 (Mw ) 12 600, PEO106PPO70PEO106) was purchased from Sigma Corporation. Tetraethyl orthosilicate (TEOS), phenol, formalin solution (37 wt %), H2SO4 (GR), NaOH (AR), HCl (AR), H2PtCl6 (AR), and ethanol (AR) were purchased from China Medicine Shanghai Chemical Reagent Corp. All chemicals were used as received without any further purification. Millipore water (18.2 MΩ · cm) provided by a Milli-Q Labo apparatus (Nihon Millipore Ltd.) was used in all experiments. The Pt/XC-72 catalyst with 40 wt % total metal loading was obtained from the Johnson Matthey Corporation. Nafion solution (5%) was purchased from Dupont Corporation. 2.2. Synthesis of Ordered Mesoporous Carbon. The ordered mesoporous carbon (MPC) can be synthesized in a large quantity by the cooperative assembly process that takes place between the precursor of resol and silica species and the amphiphilic poly(alkylene oxide)-type triblock copolymers in alcohol. The resol precursor (Mw < 500) was prepared according to the literature method.1,13 In a typical procedure, 10 g of phenol was melted at 40-42 °C and mixed with 2.125 g of NaOH (20 wt %) aqueous solution for 10 min under stirring. A 17.7 g portion of formalin (37 wt %) was dripped into the above solution below 50 °C and further stirred for 1 h at 70-75 °C. The mixture was cooled to room temperature, and HCl solution was added to adjust the pH value to 6.0. Water was then removed by vacuum evaporation below 50 °C. The product thus formed was dissolved in ethanol (20 wt % ethanol solution). Mesoporous carbon was prepared by a modified method according to the literature.1 For clarity, 8 g of F127 was

dissolved in a mixture solution (40 g of ethanol and 5 g of 0.2 M HCl) and stirred for 1 h at 40 °C to obtain a clear solution. Next, 10.4 g of TEOS and 25 g of the 20 wt % resol ethanolic solution were added in sequence. After being stirred for 5 h, the mixture was transferred into dishes. It took 5-8 h at room temperature to evaporate the ethanol and then 24 h at 100 °C in an oven to thermopolymerize. The as-made products were scraped from the dishes and ground into fine powders. The carbon-silica nanocomposites was obtained by calcining the as-made products to remove the F127 and let the resols carbonize in a tubular furnace at 350 °C for 3 h and at 900 °C for 2 h under N2 flow. The mesoporous carbon (MPC) was formed by immersing the carbon-silica nanocomposites in 10 wt % HF solutions for 24 h to remove the silicas. 2.3. Synthesis of Pt/MPC. In a typical procedure, 0.2 g of MPC was mixed with 15 mL of 0.039 M hexachloroplatinic acid (H2PtCl6) ethanol solution. The mixture was ultrasonicated before the solvents were evaporated. The MPC containing H2PtCl6 was typically heated overnight in a blast oven at 80 °C. The melting point of H2PtCl6 is 60 °C, and the melting H2PtCl6 was diffused into mesopores and small mesopores inside the pore walls due to the capillary force. The precursor then solidifies and shrinks to form crystals and was denoted as H2PtCl6/MPC. Finally, the precursor was reduced at 400 °C in forming gas (10 vol % H2 in nitrogen) for 4 h, and the thus formed catalyst was named as (Pt/MPC). 2.4. Catalyst Characterization. The X-ray diffraction (XRD) measurements were taken on a Rigaku Dmax-3C diffractometer using Cu KR radiation (40 kV, 30 mA, λ is 0.15408 nm). The Brunauer-Emmett-Teller (BET) method was used to measure the specific surface areas (SBET). By using the Barrett-JoynerHalenda (BJH) model, the pore volumes and pore size distributions were derived from the adsorption branches of isotherms. The surface structure of the MPC and Pt/MPC were characterized by means of scanning electron microscopy (SEM, Hitachi S-4800 SEM) and transmission electron microscopy (TEM, TECNAI F30, JEM-2100). The loadings of Pt into the MPC were determined by energy-dispersive X-ray spectroscopy (EDS). The size distribution of the Pt nanoparticles was calculated by 200 particles for each sample. The glassy carbon electrode (GC) was polished mechanically with sand paper (6#) and alumina powder with sizes of 5, 1, and 0.3 µm and then washed with Millipore water in an ultrasonic bath. Pt/MPC was ultrasonically dispersed in Millipore

Highly Dispersed Platinum NPs Supported on OMC water (1 mg of catalyst/mL) to form the catalyst ink, which was dropped on the GC and left to dry. Nafion was used to form a thin layer on the surface of Pt/MPC. The 3.2 nm commercial Pt/C (from JM Inc. Somerset, New Jersey) electrode (Pt/C) was prepared by the same method for comparison. The electrochemical experiments were carried out in a three-electrode electrochemical cell connected to a PAR 263A potentiostat (EG&G) with a platinum foil counter electrode. All electrode potentials were quoted versus the saturated calomel electrode (SCE) except the stability test of samples, in which the potential was quoted against the reversible hydrogen electrode (RHE). The solution was deaerated by bubbling high-purity N2 for 20 min before measurements, and this atmosphere was maintained by a flow of N2 gas over it during the experiment. All experiments were carried out at room temperature. The working electrodes were cleaned by means of potential scans (50 mV s-1) between the onset of hydrogen and oxygen evolution in 0.1 M H2SO4, until a stationary cyclic voltammogram (CV) was obtained, before the CV and potential step curves of ethanol oxidation were recorded in 0.1 M HClO4. Electrochemical in situ FTIR reflection spectroscopy measurements were conducted on a Nexus 870 spectrometer (Nicolet) equipped with a liquidnitrogen-cooled MCT-A detector. A CaF2 disk was used as the IR window, and an IR cell with a thin layer configuration between the electrode and the IR window was approached by pushing the electrode against the window before FTIR measurement.14 In situ FTIR spectra were collected using both singlepotential alteration FTIR spectroscopy (SPAFTIRS) and timeresolved SPAFTIR spectroscopy (TRFTIR) procedures. The resulting spectra were reported as the relative change in reflectivity and calculated as follows

R(E1) - R(E2) ∆R ) R R(E2) where R(E1) and R(E2) are single-beam spectra obtained by Fourier transform processing of co-added and averaged interferograms collected at E1 and E2. 3. Results and Discussion 3.1. Physicochemical Properties of Pt Nanoparticles Supported on MPC. Small-angle XRD diffraction is utilized to obtain the structural information of the samples. Figure 1 shows the small-angle XRD patterns of MPC, in which the strong 100 diffraction peak at 0.5-1.5 of 2θ can be observed, illustrating that the MPC has a high-quality mesostructure. The inset is the TEM image of MPC, which shows the typical stripelike image and further confirms that MPC presents an ordered 2-D hexagonal arrangement of the mesopores. The accurate structural information of MPC inspected by the small-angle X-ray scatter (SAXS) is shown in Figure S2 (Supporting Information), in which the strong 100 diffraction peak at a lower angle and weak 110 and 200 diffraction peaks at higher angles can be observed clearly. The metal dispersion in the catalyst is of great significance for the catalytic performance. Here, the metal dispersion was investigated on the basis of mesoporous carbon. TEM images of Pt/MPC (Figure 2a) clearly show that the Pt nanoparticles with high loading are uniformly dispersed throughout the sample, and the typical striplike image of Pt/MPC is similar to that of its parent MPC with the 2-D hexagonal mesostructure. These results clearly indicate that the MPC matrix is stable and highly ordered after the impregnation, drying, and reduction of

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Figure 1. Small-angle XRD patterns of the mesoporous carbon (MPC). The inset is the TEM image of MPC.

the precursor. The coarse and out-of-order small mesopores in the mesopore wall contain abundant anchor sites for Pt nanoparticles, which lead to the highly dispersed Pt nanoparticles throughout both mesochannels and small mesopores in the ordered mesopore walls after reduction of the precursor (see Scheme 1). The spatial limitation effect of the mesopores confines the growth of Pt nanoparticles and increases the amount of Pt nanoparticles with low coordination sites. The common impregnation method leads to Pt nanoparticles with a random distribution, which resulted in the majority of the Pt particles being present on the external surfaces of mesopores rather than in the pore channels (see Figure S1, Supporting Information). The size of the Pt nanoparticles was measured from 200 nanoparticles in the TEM image of Pt/MPC, and the value is ca. 2-3 nm in the small mesopores on the pore wall and 5-6 nm in the mesochannels, respectively. The size distribution histogram is shown in Figure 2b. The average size of the Pt nanoparticles was also estimated from the large-angle XRD patterns (shown in Figure S3, Supporting Information) using the Scherrer equation (D ) kλ/(β cos θ)). The value is 4.1 nm, which is the average size of all Pt nanoparticles of 2-3 and 5-6 nm. This can clearly support our assignment of the Pt nanoparticles: the 2-3 nm nanoparticles to the pore walls and the 5-6 nm particles to the mesochannels. The loading of Pt is measured by energy-dispersive X-ray spectroscop (EDS), and the average loading of Pt in Pt/MPC measured in different places is 34.24% ( 2.23%. Therefore, this melt-diffusion strategy of the precursor presented here has the advantage of extremely uniform Pt loading and a relatively narrow Pt nanoparticle size distribution. Figure 3 depicts the N2 sorption isotherms and the corresponding BJH pore size distributions of the MPC and Pt/MPC. Their textural parameters are summarized in Table 1. The isotherms of MPC and Pt/MPC exhibit the characteristic typeIV curves with well-defined hysteresis loops. The significant nitrogen uptake under the relative pressures of 0.4-0.8 (Figure 3a) indicates the uniform mesopores with a size ranging from 3 to 7 nm. Figure 3b displays the existence of bimodal pore, small mesopores of ca. 2.3 nm in the pore wall and mesochannels of ca. 5.6 nm in the channel, respectively. The coarse and out-of-order small mesopores of ca. 2.3 nm are produced in the ordered mesopore wall owing to the elimination of silica. They play the role of fixing the Pt nanoparticles and forming

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Figure 2. (a) TEM image of Pt/MPC. (b) Particle size distribution of Pt/MPC.

Figure 3. (a) Nitrogen sorption isotherms and (b) pore size distribution curves of the MPC and Pt/MPC. Sorption isotherm data herein were corrected based on the mass of carbon in the composite.

TABLE 1: Structural and Textural Parameters of the MPC and Pt/MPC sample

SBET (m2/g)

V (cm3/g)

small mesopore size (nm)

mesopore size (nm)

MPC Pt/MPC

1943.1 1192.3

1.73 0.94

2.3 2.3

5.6 5.1

the interconnection between the channels. In addition, after Pt nanoparticles are embeded, the BET surface areas and the pore volumes of Pt/MPC decrease sharply from 1943.1 to 1192.3 m2/g and from 1.73 to 0.94 cm3/g, respectively. This result, combining with pore size distributions of Figure 3 and the data of TEM Figure 2, illustrates that Pt nanoparticles disperse uniformly both in the small mesopores and in the mesochannels. 3.2. Durability of Pt/MPC. The electrochemical active surface areas are calculated by QH/210 µC · cm-2, where QH is the charge of hydrogen adsorption by integration of the CV of the Pt/MPC or Pt/C electrode recorded in pure electrolyte solutions, and the value, 210 µC · cm-2, is taken for a monolayer hydrogen adsorption on a smooth polycrystalline Pt electrode.15 The oxidation current has been normalized to the electrochemical active surface area of Pt/MPC or Pt/C so that the current density (j) can be directly used to compare the catalytic activity of different samples. The durability of Pt/MPC was estimated using an accelerated stability test by the circular scanning in the potential region, which caused a surface oxidation/reduction reaction of Pt in the clean electrolyte solution. The surface reaction involves the formation of PtOH and PtO derived from the oxidation of water

that causes the dissolution of Pt via the Pt2+ oxidation state.16 We conducted the test by applying potential sweeps from 0.6 to 1.1 V (vs RHE) at the rate of 50 mV · s-1 in an O2-saturated 0.1 M HClO4 solution at room temperature. The solid lines in Figure 4a,b are the CVs of Pt/MPC and Pt/C in 0.1 M HClO4 solution, which is consistent with those of a polycrystalline Pt. The adsorption and desorption peaks of hydrogen and oxygen were, respectively, observed in the potential ranges of 0.0-0.35 V (vs RHE) and 0.6-1.1 V (vs RHE). It is noticed that a pair of current peaks at 0.4-0.7 V (vs RHE) and the large doublelayer current appear on Pt/MPC because of the high specific surface area of MPC. The dotted lines in Figure 4a,b show the CVs of Pt/MPC and Pt/C in 0.1 M HClO4 solution after 4000 cycles for the Pt/MPC and Pt/C catalysts. The loss of electroactive surface area was evaluated by calculating the difference of the charge associated with H adsorption before and after 4000 potential cycles. The Pt/C suffers a 14.75% loss in catalytic surface area, whereas for MPC/Pt, only 4.73% of the original Pt surface area is lost, indicating that MPC could potentially provide much higher durability than Vulcan XC-72. 3.3. Electrocatalytic Properties of Pt/MPC for Ethanol Oxidation. The steady-state CVs of ethanol oxidation on Pt/ MPC and Pt/C catalysts in 0.1 M HClO4 with the upper limit of potential at 1.25 V are presented in Figure 5. MPC has a characteristic of a high specific surface area, which widens the double layer. However, such an effect has been calibrated in the result, and details are shown in Figure S4 (Supporting Information). The shapes of the CVs are the typical profiles of ethanol oxidation on a Pt electrode. The currents of adsorption

Highly Dispersed Platinum NPs Supported on OMC

Figure 4. Cyclic voltammograms in 0.1 M HClO4 solution before and after 4000 cycles for Pt/MPC (a) and Pt/C (b).

Figure 5. Cyclic voltammograms of ethanol oxidation on Pt/MPC and Pt/C in 0.1 M HClO4 + 0.1 M ethanol.

and desorption of hydrogen were inhibited on Pt/MPC and Pt/C due to adsorption of dissociated species of ethanol. The onset potential of ethanol oxidation on Pt/MPC is at ca. 0.30 V, manifesting a negative shift of ca. 40 mV with respect to that on Pt/C. The peak potentials are at 0.61 and 0.63 V, with a current density of 0.69 and 0.64 mA cm-2 on Pt/MPC and Pt/ C, respectively. To investigate the electrocatalytic activity of the different catalysts, the potentials in the kinetic controlled regime were selected. Figure 6a depicts the potential dependence of the steady-state current density recorded at 60 s for the ethanol oxidation, and the corresponding j-t curves are shown in Figure S5 (Supporting Information). The current densities of ethanol oxidation on Pt/MPC are higher than that on the Pt/C catalyst at the kinetic controlled range from 0.05 to 0.40 V. In general, the current density of technical interest is the value more than 0.2 mA cm-2. The potential of ethanol oxidation shifts

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Figure 6. Comparison of the catalytic activity of ethanol oxidation on Pt/MPC and Pt/C catalysts per unit Pt surface area in 0.1 M HClO4 + 0.1 M ethanol. (a) Potential-dependent steady-state current density (recorded at 60 s). (b) Transient current density curves at 0.3 V during 1800 s.

negatively ca. 43 mV at 0.2 mA cm-2 on Pt/MPC, comparing with that of the Pt/C catalyst. Figure 6b compares the transient current density of ethanol oxidation at 0.4 V at 1800 s on Pt/ MPC and Pt/C catalysts at room temperature, in which the current density on Pt/MPC is always higher than that on the Pt/C catalyst. The above results demonstrate obviously that the Pt/MPC exhibits a much enhanced catalytic activity and stability per unit surface area for the oxidation of ethanol. Conventional carbon supports, such as carbon black, possess a wide pore distribution containing a high proportion of micropores (less than 2 nm). The transport of charge and mass are restrained, and the catalyst utilization is thus reduced.17 As a comparison, for MPC, the small mesopores in the ordered mesopore wall interconnects with mesopores. The structure of MPC makes the reactive species more accessible and further facilitates the mass transmission. Hence, the Pt nanoparticles even in small mesopores can also be utilized during the reaction. Moreover, Pt nanoparticles are confined in both mesopores in MPC, which can overcome sintering through Ostwald ripening processes. The continuous and interpenetrated carbon components on MPC enhance the electrical conductivity, and the large BET surface area enlarged the area of the three-phase boundary in catalyst-electrolyte-reactant. The melt-diffusion strategy makes the precursor disperse uniformly, resulting in more efficient utilization of the Pt nanoparticles. The framework of MPC confined the growth of Pt nanoparticles, which may increase the amount of Pt nanoparticles with low coordination sites. The high performance of the Pt/MPC can be attributed to all of the above-mentioned observations. Electrochemical in situ FTIR spectroscopic studies were carried out to evaluate the catalytic activity of Pt/MPC for ethanol oxidation. Little dissociated species were formed at -0.25 V during a short time; the potential can be safely used as a reference. Figure 7 shows SPAFTIR spectra of ethanol oxidation obtained for Pt/MPC and

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Figure 7. In situ SPAFTIR spectra of ethanol oxidation on Pt/MPC and Pt/C at 0.6 V in 0.1 M ethanol and 0.1 M HClO4 solutions. The reference potential was -0.25 V (SCE).

TABLE 2: Assignments of IR Bands in the FTIR Spectra of Ethanol Oxidation in Figures 6 and 7 wavenumbers (cm-1) 1044 (upward) 1110 1281 1370 1387 ∼1718 ∼2050 2343

assignments νC-O νClO4νC-O δsCH3 δasCH3 νCdO νCO νasOdCdO

CH3CH2OH ClO4CH3COOH CH3CHO CH3COOH CH3COOH or CH3CHO adsorbed CO (COad) CO2

Pt/C catalysts in 0.1 M HClO4 + 0.1 M CH3CH2OH solution with E1 at 0.60 V and E2 at -0.25 V.18 Each single-beam spectrum was co-added with 640 interferograms at a spectral resolution of 8 cm-1. The negative-going band close to 2343 cm-1 is assigned to the asymmetrical stretch vibration of the CO2 species in solution, which indicates the complete oxidation of ethanol. Another negative-going band at ca. 2050 cm-1 is assigned to the IR absorption of linearly adsorbed CO (COL), produced by the dissociative adsorption of ethanol on the electrocatalyst surface. In addition, the negative-going bands at 1718 cm-1 are assigned to the stretching vibration of the CdO bond, and the two others at 1387 and 1370 cm-1 attributed to the symmetric deformation vibration of the CH3 band in acetaldehyde and acetic acid were also observed, which was due to the partial oxidation of ethanol to acetic acid. The welldefined negative-going band at 1281 cm-1 is the characteristic absorption of C-O stretching in acetic acid, which is usually employed for quantitative analysis of acetic acid. The positivegoing band at 1044 cm-1 is attributed to the C-O stretching vibration of ethanol, representing the consumption of ethanol by oxidation. The detailed assignment of IR bands is listed in Table 2. The integrated intensity ratio of the CO2 band and the C-O band in the acetic acid band is used to reflect the cleavage of the C-C bond for ethanol oxidation. Obviously, more CO2 and less acetic acid are formed on Pt/MPC than on the Pt/C catalyst for ethanol oxidation. The integrated intensities of the CO2 band at 2343 cm-1 and acetic acid at 1280 cm-1 are 0.2033 and 0.2156, respectively, on Pt/MPC catalysts; corresponding values are 0.1323 and 0.2902 on the Pt/C sample. The ratio of band intensities of CO2 to acetic acid on Pt/MPC is nearly double that on Pt/C (0.65 vs 0.39), indicating that the cleavage of the C-C bond is more efficient on the Pt/MPC catalyst than on Pt/C. A series of time-resolved SPAFTIR spectra of Pt/MPC and Pt/C are shown in Figure 8. The sequence spectra were collected with an interval of 11.5 s after the potential stepped from -0.25 to 0.60 V. The time-resolved SPAFTIR spectra clearly show the development of the curves. The intensities of the CO2 band

Figure 8. Series of in situ time-resolved SPAFTIR spectra of ethanol oxidation recorded for the time from 11.5 to 115.0 s on Pt/MPC (a) and Pt/C (b). Other conditions are the same as those for Figure 6.

Figure 9. Intensity ratios of the CO2 band (2343 cm-1) and the C-O band (1280 cm-1) increase rapidly.

at 2343 cm-1 and the C-O band at 1280 cm-1 increase rapidly with the collecting time. The CO2 band and the C-O band appear after 11.5 and 46 s on Pt/MPC, and the corresponding values are 23 and 34.5 s on Pt/C catalyst. The variation of peak high for the CO2 band and the C-O band with time is shown in Figure 9, in which high CO2 band intensity and low C-O band intensity always can be seen on Pt/MPC. The in situ FTIR results reveal distinctly that, compared with Pt/C, the Pt/MPC catalyst has enhanced reactivity for breaking the C-C bond in ethanol; as a consequence, Pt/MPC exhibits a higher selectivity for the complete oxidation of ethanol to CO2. 4. Conclusions MPC is used as individual nanoscale reactors, and a novel melt-diffusion strategy is applied to load the precursor. Highly dispersed Pt nanoparticles confined in the small mesopores of the pore wall and mesopores of the channel (Pt/MPC) are synthesized after the precursor is reduced by hydrogen gas. The Pt/MPC catalyst exhibits a much enhanced catalytic activity and stability for the oxidation of ethanol. In addition, Pt/MPC

Highly Dispersed Platinum NPs Supported on OMC catalysts have a higher reactivity for breaking the C-C bond in ethanol and a better selectivity for the complete oxidation of ethanol to CO2. The improved electrocatalytic behavior may be attributed to three aspects: (1) MPC has a large specific surface area and pore volume, as well as the interpenetrated small mesopores in pores wall, which facilitates the uniform dispersion of Pt confined in MPC and the mass transmission of reaction species. (2) The small mesopores and mesochannels limit the growth of Pt nanoparticles, which result in the formation of Pt nanoparticles with low coordination sites. (3) The continuous carbon components in MPC enhance the electrical conductivity and the utilization of the Pt nanoparticles. Therefore, the use of an ordered mesoporous carbon carrier is promising for potential applications for fuel cells, and the melt-diffusion strategy is a preferable approach to load the precursor. Acknowledgment. This study was supported by grants from the National Natural Science Foundation of China (Grant Nos. 20833005, 20873116, and 60936003) and State Key Laboratory of Supramolecular Structure and Materials of Jilin University, SKLSSM200910. We thank Prof. D. Y. Zhao (Chemistry Department, Fudan University, People’s Republic of China) for providing the opportunity for one of the authors to learn how to synthesize MPC. Supporting Information Available: Figures S1-S5 are the SEM of platinum nanoparticles supported on MPC by the directly impregnate-evaporation approach, SAXS pattern of the MPC, large-angle XRD pattern of the Pt/MPC, cyclic voltammogram of MPC and Pt/MPC, and plots of j-t transients for the oxidation of ethanol. This material is available free of charge via the Internet at http://pubs.acs.org.

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