Preparation of Pt nanoparticles supported on ordered mesoporous carbon FDU-15 for electrocatalytic oxidation of CO and methanol

Electrochimica Acta 67 (2012) 127–132

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Preparation of Pt nanoparticles supported on ordered mesoporous carbon FDU-15 for electrocatalytic oxidation of CO and methanol Dong-Hai Lin, Yan-Xia Jiang ∗ , Shu-Ru Chen, Sheng-Pei Chen, Shi-Gang Sun ∗,1 State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

a r t i c l e

i n f o

Article history: Received 8 September 2011 Received in revised form 29 January 2012 Accepted 4 February 2012 Available online 13 February 2012 Keywords: Mesoporous carbon FDU-15 Pt nanoparticles Electrocatalysis Methanol CO

a b s t r a c t In this work, we proposed an improved wet-chemical method to synthesize Platinum (Pt) nanoparticles supported on the mesoporous carbon (FDU-15). A well-dispersed Pt precursor in FDU-15 was first received by using a CH2 Cl2 solvent process and a simple melt-diffusion strategy. After the Pt precursor was reduced by formic acid without surfactant, almost 100% Pt nanoparticles were confined in the channels of FDU-15 (denoted as Pt/FDU-15). Physical (XRD, HRTEM and BET) and electrochemical (Cyclic voltammetry, COad stripping and chronoamperometry) methods were used to investigate the properties of Pt/FDU-15. Compared with the commercial catalyst Pt/C, the Pt/FDU-15 catalyst exhibits a much higher electrocatalytic activity toward CO and methanol oxidation. The origins of the high electrocatalytic activity of the Pt/FDU-15 were discussed. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Ordered mesoporous carbon (OMC) has attracted considerable attention in recent years because of their potential applications as catalyst supports [1–3], adsorbents [4], hydrogen storage materials [5,6] and supercapacitors [7–10]. Compared with the traditional carbon supports, such as carbon black and activated carbon, the advantages of OMC relies on tuning the pore structure, pore size, and pore surface chemistry. The OMC allows the diffusion of reactants and products inside the pores and the high dispersion of Pt nanoparticles, which results in a significant improvement of catalytic activity and stability [11]. The conventional OMC materials are mostly prepared by a hard template route using ordered mesoporous silicates such as SBA15 [1,12,13]. However, the key drawbacks of this hard template method are the extra steps required to prepare the scaffolds and the sacrificial use of not only the silica scaffolds themselves but also the surfactant templates; therefore, the process is expensive, complex, and time-consuming, and in turn, unsuitable for large-scale production and industrial applications [14]. For this reason great efforts have been made to find a way of directly synthesizing OMC materials. Zhao and co-workers [15] developed the organic–organic self-assembly approach to a family of mesoporous carbons such as

∗ Corresponding authors. E-mail addresses: [email protected] (Y.-X. Jiang), [email protected] (S.-G. Sun). 1 ISE member. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.02.007

two dimensional hexagonal mesoporous carbon FDU-15. FDU-15 shows excellent structural stability under severe oxidation treatments by acidic (NH4 )2 S2 O8 , HNO3 , and H2 O2 solutions. FDU-15 is much more stable than the mesostructural analogue CMK-3 carbon prepared by the nanocasting method [16]. The BET surface area of mesoporous carbon FDU-15 is 600 m2 /g and the total pore volume is 0.35 cm3 /g, respectively. The pore diameter is about 3.5 nm with a narrow distribution. It is well-known that Pt nanoparticles with average diameter of 3.0 nm show the best electrochemical activity towards methanol oxidation [17]. Thanks to the particular structure of mesoporous carbon FDU-15, the size of Pt particles can be controlled. Their diameters may reach exactly 3.0 nm as long as they are confined in the FDU-15. Studies on mesoporous FDU-15 have already been carried out for exploiting their potential as electrochemical capacitors [18], sensors [19,20] and hydrogen electrooxidation [21]. However, to the best of our knowledge, no work has been done so far to illustrate that Pt nanoparticles loaded in mesoporous carbon FDU-15 exhibit a higher catalytic activity toward CO and methanol electrooxidation than the commercial catalyst. In this paper, we present a novel wet-chemical method to prepare Pt nanoparticles confined in the mesoporous carbon FDU-15. This method can reduce the loss of precursors in the conventional wet-chemical process. The melt-diffusion strategy was first reported in preparing sulfur with high loading in the channel of OMC [22]. We have used the melt-diffusion method to prepare a well-dispersed Pt precursor in the modified mesoporous carbon46 (MPC) [3]. Moreover, Stucky’s group first used CH2 Cl2 solvent to induce the outer surface bound metal precursors to move into

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the channels of SBA-15 [23]. Since the structure of FDU-15 was similar to that of SBA-15 and the property of FDU-15 was similar to MPC, both the CH2 Cl2 solvent process and the melt-diffusion approach were combined to make the precursors disperse uniformly in the channel of mesoporous carbon FDU-15. Then the H2 PtCl6 precursors were reduced by formic acid without surfactant (denoted as Pt/FDU-15). The results confirmed that the Pt/FDU-15 catalyst exhibits higher catalytic activity towards CO and methanol electrooxidation compared to the commercial Pt/C catalyst. 2. Experimental

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2.1. Preparation of ordered mesoporous carbon FDU-15 Mesoporous carbon FDU-15 was prepared according to the literature method [14]. 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, then stirred constantly for 10 min. A 17.7 g portion of formalin (37 wt%) was dripped into the above solution 20–50 ◦ 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 at 40–50 ◦ C. The product thus formed was dissolved in ethanol (20 wt% ethanol solution) named the resols’ ethanol solution (Mw < 500). 1.0 g F127 template (Mw = 12,600, PEO106 PPO70 PEO106 , Sigma–Aldrich) was dissolved in 20.0 g ethanol by stirring for 1 h. Later, 5.0 g of 20 wt% resols’ ethanol solution was added, and stirred for another 2 h to form a homogeneous solution. The mixture was transferred to several dishes. After ethanol was evaporated at room temperature for 8–14 h the residue was thermo-polymerized at 100 ◦ C for 24 h. The resulting solid was collected and ground into fine powder. Calcination treatment was carried out in a tubular furnace at 350 ◦ C for 3 h then at 600 ◦ C for 3 h under N2 flow to obtain mesoporous carbon FDU-15. 2.2. Preparation of catalysts A typical run for the catalyst Pt/FDU-15 was carried out as follows: 80 mg FDU-15 was mixed with 5.3 mL of 0.0193 M H2 PtCl6 acetone solution by ultrasonication to obtain uniform slurry. Power precursor was obtained after the solvents were evaporated at 50 ◦ C in a vacuum oven for 12 h. 20 mL of CH2 Cl2 solvent was then added to the dry powder to induce the metal precursors from the outer surface into the channels of FDU-15, followed by further vacuum-dried at 60 ◦ C, in which the precursors H2 PtCl6 could be well-dispersed uniformly in the inner channels by melt-diffusion strategy [3]. The precursor then solidified and shrank to form crystals. Finally, the precursor was refluxed at 90 ◦ C in 40 mL of 80% (v/v) formic acid for 24 h. The product was filtered, rinsed with Millipore water and dried under vacuum at 80 ◦ C overnight and denoted as Pt/FDU-15. 2.3. Physical characterization The powder samples were characterized by powder X-ray diffraction (XRD) using a Panalytical X’pert PRO diffractometer. The pore diameter, pore volume, and surface area of the samples were derived from the nitrogen sorption isotherm at 77 K using a Micromeritics TriStar 3000 system. Prior to measurements, the sample was evacuated at 393 K for 5 h. The Brunauer–Emmett–Teller (BET) method was used to measure the specific surface areas (SBET ). By using the Barrett–Joyner–Halenda (BJH) model, the pore volumes and pore size distributions were derived from the adsorption branches of isotherms. Transmission electron microscopy (TEM) patterns were obtained on FEI TecnaiF30 and JEM-2100 electron microscopy instruments. The loadings

0.5

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2θ /degree Fig. 1. Small-angle XRD patterns of (a) FDU-15, (b) Pt/FDU-15.

of Pt into the FDU-15 were determined by energy-dispersive Xray spectroscopy (EDS). The cyclic voltammetry (CV) studies were carried out with a CHI631C electrochemical work station (CH instruments, Inc.). 2.4. Electrochemical measurements Electrochemical experiments were carried out in a standard three-electrode cell at room temperature. The counter electrode was a Pt flake and the reference electrode was a saturated calomel electrode (SCE), and potentials reported in the present paper are those with respect to the SCE scale.The glassy carbon electrode (GC) was polished mechanically with sandpaper (6#) and alumina powder with sizes of 5, 1, and 0.3 ␮m and then washed with Millipore water in an ultrasonic bath. The Pt/FDU-15 was ultrasonically dispersed in Millipore water (1 mg of catalyst/mL) to form the catalyst ink, which was dropped on the GC surface and left to dry, upon which a drop of 0.5% (v/v) Nafion solution was dispersed to fix the catalyst particles on the surface. The electrocatalytic oxidation of preadsorbed carbon monoxide (COad ) was measured by COad stripping voltammetry at a scan rate of 50 mV s−1 . Before each measurement, the solution was deaerated by bubbling pure N2 gas to the cell for 20 min. The CO adsorbed on the Pt/FDU-15/GC electrode (denoted as COad -Pt/FDU-15/GC) was obtained by purging CO of high purity (99.95%) along with potential cyclic scanning between −0.2 V and 0.1 V for 20 min. The solution CO species were then removed by purging pure N2 gas while maintaining a constant voltage of −0.1 V, in which CO can be adsorbed steadily on the Pt/FDU-15/GC surface. The performance of the Pt/FDU-15/GC electrode for CO and methanol oxidation was generally evaluated at 50 mV s−1 in a solution of 0.5 M H2 SO4 and 1 M CH3 OH + 0.5 M H2 SO4 , respectively. Electrolyte solutions were prepared using Millipore water purified by a Milli-Q system with a resistivity of 18.2 M cm. All the experiments were carried out at room temperature. 3. Results and discussion Small-angle XRD diffraction is utilized to obtain the structural information of the samples. Fig. 1a shows a small-angle XRD pattern of the FDU-15, in which the strong (1 0 0) diffraction peak at 0.5–1.5 of 2 can be observed, illustrating that the FDU-15 has a high-quality mesostructure [14,15]. After the loading of Pt into the FDU-15, no significant spectral feature changes were observed for

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b (200) (220)

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a

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2θ / degree Fig. 2. Wide-angle XRD patterns of (a) JM Pt/C, (b) Pt/FDU-15.

Pt/FDU-15 at a low 2 angle, indicating that the ordered mesoporous structure remained unchanged after the incorporation of Pt nanoparticles [20]. This is different from the case of Pt/SBA-15 [24], because mesoporous silica SBA-15 contains lots of Si OH in the channels but the FDU-15 has a rigid structure. Furthermore, the intensity of the peaks (Fig. 1b) has decreased to a certain extent, while the full width at half-maximum (FWHM) of the peaks that is normalized by height has increased. These features are attributed to contraction of framework during the support treatment, implying that Pt nanoparticles have been introduced into the channel of FDU-15, which is similar to that reported in the literature [25]. The XRD patterns of the wide-angle XRD patterns of JM Pt/C and Pt/FDU-15 catalysts were compared in Fig. 2. One broad diffraction peak at about 26◦ comes from the (0 0 2) diffraction of graphite. The diffraction peaks at the Bragg angles of 39.5◦ , 46.2◦ , 67.6◦ and 81.7◦ were respectively indexed to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) reflections of face-centered cubic Pt nanocrystals by comparing with the Joint Committee on Powder Diffraction Standards (JCPDS card, No. 04-0802) [20]. Through calculation from the Scherrer equation based on the (2 2 0) reflection, the average Pt crystallite sizes for JM Pt/C and Pt/FDU-15 were 3.0 and 3.1 nm, respectively, which further confirms the confinement effect of FDU-15 in synthesizing uniform Pt nanoparticles catalyst. Fig. 3a shows the TEM

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image of Pt/FDU-15 sample viewed along the (1 1 0) direction. For comparison, TEM image of FDU-15 is shown in the inset to the left bottom of Fig. 3a. The stripes represent the channel of FDU-15 and the black points in the stripe represent the Pt nanoparticles. The Pt nanoparticles with an average diameter of 3.1 nm are uniformly dispersed in the ordered mesoporous matrix of FDU-15. It can be seen that almost 100% Pt nanoparticles are confined in the channels of FDU-15. The average size of nanoparticles which is measured in the TEM image is the statistics of the average of a large number of particles. So we enlarged the size of the TEM image, and used Image J software to count 172 nanoparticles to determine the average size of Pt nanoparticles. The average size of the Pt nanoparticles is 3.1 nm with a standard deviation of 0.9 nm, as illustrated by the size distribution histogram shown in Fig. 3b. This measurement is consistent with the XRD results. The loading of Pt is measured by EDS, and the average loading of Pt in Pt/FDU-15 measured in different points of electrode surface is 16.8%. In the conventional wet-chemical process some of the precursors will inevitably diffuse in the solution by concentration diffusion, which results in the homogeneous nucleation of precursors in the solution rather than heterogeneous nucleation in inert carrier. It is still a challenge to avoid a leakage of precursors from the channel of template in the wet-chemical method. So in most published works, reduction of the precursors was conducted by a dry-chemical process to avoid homogeneous nucleation in the solution. In our method, the precursors were first loaded into the FDU-15 by impregnation, after that CH2 Cl2 solvent were used to induce the metal precursors from the outer surface to move into the channels of the FDU-15, and then followed by a long-time melting process. The precursors were diffused into the channel of FDU-15 by capillary forces. The channel of the FDU-15 significantly reduces the loss of the precursors. Nitrogen adsorption/desorption isotherms for FDU-15, Pt/FDU15 are found to be the typical IV isotherm curves with distinct hysteresis loops (Fig. 4). Clear capillary condensation occurs at a relative pressure (P/P0 ) of 0.4–0.7 indicating a narrow pore size distribution. This suggests that the host FDU-15 mesoporous structure was still maintained during the modification process. Their textural parameters are summarized in Table 1. After the incorporation of Pt nanoparticles, the values of BET surface area (SBET ) and the total pore volume (Vt ) of Pt/FDU-15 decrease from 600 to 423 m2 /g and from 0.35 to 0.31 cm3 /g, respectively, with a slight decrease of the average pore size (DBJH ) from 3.5 to 3.2 nm (Table 1). All these data could be attributed to the Pt nanoparticles that have been confined inside the channel of the FDU-15 [3,26].

Fig. 3. (a) TEM image of Pt/FDU-15 and enlarged image (inset in left top), and inset to the left bottom is the TEM image of FDU-15, all viewed along the (1 1 0) direction of the carbon material, (b) Particle size distribution of Pt/FDU-15.

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240

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Fig. 4. Nitrogen sorption isotherms and the inset corresponding pore size distribution curves of (a) FDU-15 and (b) Pt/FDU-15. Table 1 Structural and Textural Parameters of the FDU-15 and Pt/FDU-15. Sample

SBET (m2 /g)

Vt (cm3 /g)

DBJH (nm)

FDU-15 Pt/FDU-15

600 423

0.35 0.31

3.5 3.2

SBET , BET specific surface area; Vt , total pore volume; DBJH , pore diameter calculated using BJH method.

The cyclic voltammograms (CVs) of JM Pt/C and Pt/FDU-15 catalysts in 0.5 M H2 SO4 solutions are shown in Fig. 5. The shapes of the CVs are the typical profiles of polycrystalline Pt electrode, and the observed hydrogen adsorption/desorption peaks in the potential region from −0.2 to 0.0 V were due to the presence of different Pt facets [27]. Moreover, hydrogen evolution occurs on −0.2 V on Pt/FDU-15, and the onset potential of hydrogen evolution on the Pt/FDU-15 is 30 mV positive-shifts than Pt/C, which is similar with the Pt tetrahexahedral electrodes enclosed by high-index facets [28]. This indicates that the Pt/FDU-15 catalyst exhibits a much higher electrocatalytic activity for the hydrogen evolution than that on the Pt/C catalyst. In the electrochemical experiments the weight of Pt on the GC electrode was the same in the two samples, and we used currents per unit area, i.e. current density to

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E vs. SCE / V

Relative pressure(P/P0)

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1.4

E vs. SCE / V Fig. 5. Cyclic voltammograms of (a) JM Pt/C/GC and (b) Pt/FDU-15/GC electrode in 0.5 M H2 SO4 solutions, sweep rate 50 mV s−1 .

Fig. 6. Cyclic voltammograms for CO oxidation on (a) JM Pt/C/GC, (b) Pt/FDU-15/GC electrode in 0.5 M H2 SO4 solution, 50 mV s−1 .

compare the electrocatalytic activities of methanol oxidation and CO oxidation on the two samples. The current densities have been normalized to the electrochemical active surface area, so that the current density (j) can be directly used to compare the catalytic activity of different samples. The electrochemical active surface areas are usually calculated by QH /210 ␮C cm−2 , where QH is the charge of hydrogen adsorption by integration of the CV of the Pt/FDU-15 or Pt/C electrode recorded in pure electrolyte solutions, and the value, 210 ␮C cm−2 , is taken for a monolayer hydrogen adsorption on a smooth polycrystalline Pt electrode [29]. Herein, the method to evaluate electrochemical active surface area has a little modified due to hydrogen evolution reaction occurring. We can see from Fig. 5 the reduction current in the region of 0.0 to −0.2 V on Pt/FDU-15 contains hydrogen adsorption current and following hydrogen evolution current. Whereas the oxidation current in the same region consists of the hydrogen desorption current. So it is not suitable to use the charge of the hydrogen desorption or the charge of hydrogen adsorption of both samples to calculate the electroactive surface areas. In this case, the electrochemical active surface area is evaluate by the same formula (QH /210 ␮C cm−2 ), where QH is one half of the sum of oxidation charge and reduction charge measured by integration of the CV curve of Pt/FDU-15 in the region of 0.0 to −0.2 V. The electro-active surface area is a little larger than the actual active surface area due to Pt/FDU-15 containing a hydrogen evolution charge; as a result the actual catalytic activity per unit area is a little larger than that of the manuscript. It is noteworthy that the current density of oxygen species in the Pt/FDU-15 is larger than that in the JM Pt/C, which will contribute to oxidation of CO and methanol. The voltammograms corresponding to the COad oxidation (COad stripping) on the samples JM Pt/C and Pt/FDU-15 catalysts are depicted in Fig. 6. The hydrogen adsorption–desorption current is suppressed when the electrode surface is covered with adsorbed CO. COad oxidation on Pt/FDU-15 gave two oxidation peaks, a small peak and a main peak. The onset potential in the small peak started at 0.05 V, and peak climbed to the maximum 34 ␮A cm−2 at 0.21 V. Most CO oxidation happened in the main peak. In comparison with COad oxidation on single crystal planes [30], the small peak may come from oxidation of CO adsorbed on some particular sites on the Pt nanoparticles confined in the FDU-15, which are similar to those active sites on Pt(1 1 1) and Pt(1 0 0). The main peak potential of COad oxidation on Pt/FDU-15 was at 0.47 V, whereas the value was at 0.59 V on JM Pt/C. It is evident that the main peak potential of COad oxidation has been shifted negatively 120 mV on Pt/FDU-15/GC, indicating that the

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2.5

I backward

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I forward b a

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E vs. SCE / V Fig. 7. Cyclic voltammograms of (a) JM Pt/C/GC, (b) Pt/FDU-15/GC electrode in 1 M CH3 OH + 0.5 M H2 SO4 solution.

Pt/FDU-15/GC electrode possesses a much higher electrocatalytic activity for COad oxidation. The fact that the small oxidation peak could not be observed on JM Pt/C indicates further the Pt nanoparticles confined in the FDU-15 may have more active surface sites, thus they exhibit a higher catalytic activity than the JM Pt/C does. Moreover, the oxidation of COad occurs in a wide potential region lying between 0.05 V and 0.8 V, which is due to CO can be adsorbed on different Pt sites. Besides the primary hexagonal cylindrical channel, the FDU-15 still possesses a lot of micropores or microchannels that have put up bridges between the primary channels [31]. When Pt nanoparticles are embedded in the above different locations, they will also ultimately result in different oxidation for COad in a large range of potentials. The COad oxidation current disappears after one potential cyclic scanning, indicating that COad can be easily oxidized on a Pt/FDU-15/GC electrode. This may be attributed also to the FDU-15 possessing mesoporous channels, the CO2 derived from the oxidation of COad can diffuse easily from the FDU-15 to the bulk solution. This is different from the microporous zeolite, in which the restriction of the supercage window in zeolite has resulted in the difficulty of COad oxidation [32,33]. The cyclic voltammograms of the two samples in 0.5 M H2 SO4 containing 1 M CH3 OH are shown in Fig. 7. The shapes of the CVs are the typical profiles of methanol oxidation on a Pt electrode. The currents of adsorption and desorption of hydrogen were inhibited on the Pt/FDU-15 and the Pt/C due to adsorption of dissociated species of methanol. The Pt/FDU-15 catalyst shows a significantly improved CH3 OH oxidation activity compared to the Pt/C catalyst. The current density of the methanol oxidation for Pt/FDU-15 reaches a peak value of 2.04 mA cm−2 (Iforward ), which is much higher than that of Pt/C (1.16 mA cm−2 ). The ratio of Iforward to Ibackward of the Pt/FDU-15 which can be used to describe the catalyst tolerance to carbonaceous species (such as CO) accumulation [34], also reaches 0.98, while the ratio of Pt/C reaches 0.74. The higher ratio of Iforward /Ibackward indicating better oxidation of methanol to carbon dioxide during the anodic scan and less accumulation of carbonaceous residues on the catalyst surface. To investigate the electrocatalytic activity of the different catalysts, the potentials in the kinetic controlled regime were selected. In general, the current density of technical interest is the value larger than 0.2 mA cm−2 . The potentials of methanol oxidation are 0.41 V and 0.45 V at 0.2 mA cm−2 on the Pt/FDU-15 and the Pt/C catalyst, respectively. The potential of methanol oxidation shifts negatively ca. 40 mV at 0.2 mA cm−2 on Pt/FDU-15, compared with that of the Pt/C catalyst. The initial oxidation potential of methanol (taken, for example, as

Fig. 8. Chronoamperometric curves for (a) JM Pt/C, (b) Pt/FDU-15 catalyst for the electro-oxidation of methanol in 1 M CH3 OH + 0.5 M H2 SO4 solution at 0.45 V.

the potential at which the current density is 10% that of the current at the peak) is a suitable parameter to compare the overpotential necessary for methanol oxidation in each one of these two materials. The potential at which the current density is 10% of the current at the peak is 0.41 V and 0.42 V on Pt/FDU-15 and Pt/C catalyst, respectively. The overpotential necessary for methanol oxidation on the Pt/FDU-15 shifts negatively ca.10 mV than that of the Pt/C catalyst. All the results show that the Pt/FDU-15 is more tolerant to poisoning by CO and a better catalyst for methanol oxidation than that of the Pt/C catalyst. Fig. 8 shows a comparison of current density of CH3 OH oxidation at 0.45 V of the samples. The oxidation current density obtained on Pt/FDU-15 particles is larger than that on the Pt/C catalyst. The stability of the samples was also been seen through chronoamperometry by applying the procedure reported in our previous work [29]. The current is decreased sharply in the first few seconds and then reaches a steady-state current gradually. It can be seen that the current density of methanol oxidation on Pt/FDU15/GC is always larger than that on Pt/C/GC, evidencing that the Pt/FDU-15 exhibits a much enhanced catalytic activity per unit surface area for the oxidation of methanol. 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 [35]. The structure of mesoporous carbon FDU-15 makes the reactive species more accessible and further facilitates the mass transmission. Hence, the Pt nanoparticles in mesopores can be utilized efficiently during the reaction. Moreover, both the CH2 Cl2 solvent process and the melt-diffusion strategy make the precursors disperse uniformly in the channel. Pt nanoparticles are confined in mesopores in FDU-15, which can overcome sintering through Ostwald ripening processes. In the present study, Pt/FDU-15 was prepared via the reduction of hexachloroplatinic acid (H2 PtCl6 ) by formic acid (HCOOH) without surfactant. The framework of FDU-15 thus confined the growth of Pt nanoparticles, which may increase the amount of Pt nanoparticles with low coordination sites and could be confirmed by the fact that the Pt/FDU-15 possesses a higher current density of oxygen adsorption species. The high performance of the Pt/FDU-15 could be therefore attributed to all of the above-mentioned observations. 4. Conclusions In summary, mesoporous carbon FDU-15 was synthesized and used as individual nanoscale reactors; both a CH2 Cl2 solvent process and a melt-diffusion strategy were applied to load the precursor. Highly dispersed Pt nanoparticles of uniform size of 3.1 nm

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