Hard-templating Synthesis of Mesoporous Pt-Based ... - CSJ Journals

doi:10.1246/cl.130054 Published on the web April 5, 2013

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Hard-templating Synthesis of Mesoporous Pt-Based Alloy Particles with Low Ni and Co Contents Prasannan Karthika,1,2,3,# Hamed Ataee-Esfahani,1,4,# Yu-Heng Deng,5 Kevin C.-W. Wu,*5 Natarajan Rajalakshmi,2 Kaveripatnam S. Dhathathreyan,2 Dakshinamoorthy Arivuoli,3 Katsuhiko Ariga,1 and Yusuke Yamauchi*1,4 1 World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044 2 Centre for Fuel Cell Technology, International Advanced Research Centre for Powder Metallurgy and New Materials, IITM Research Park, Chennai 600 113, India 3 Crystal Growth Centre, Anna University, Chennai 600 025, India 4 Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555 5 Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan (Received January 23, 2013; CL-130054; E-mail: [email protected], [email protected]) Pt-Based mesoporous bimetallic alloy particles with low Co and Ni contents were synthesized successfully through a hard-templating method and show great potential as effective catalysts for methanol oxidation reactions. Platinum (Pt) is a well-known catalyst that has high electrocatalytic activity, especially for methanol oxidation reactions. However, a pure Pt electrocatalyst is usually prone to poisoning by CO, since CO molecules can be adsorbed chemically onto the Pt surface and block its active sites.1 As a result, the Pt catalyst rapidly becomes inactive in the electrooxidation of methanol owing to the formation of PtCO species. The need for the noble metal Pt as a catalyst also limits the largescale application of this technology.2 Pt-Based alloys are frequently synthesized as alternatives to overcome these limitations, which rely on the fine tuning of the Pt electronic properties to improve its catalytic performance substantially and thus reduce the amount of noble metal needed.3 To date, the development of bimetallic catalysts has usually entailed a primary metal with a high catalytic activity and a secondary metal that can enhance the catalytic activity or prevent poisoning issues. The use of the second metal gives a notable improvement in the CO tolerance and electrochemical activity and decreases the costs associated with Pt. There has been considerable improvement in the search for Pt-based bimetallic electrocatalysts such as PtPd nanocomposites,4 Pt monolayers on a second metal,5 or Pt alloys with less expensive 3d transition metals.6 One important issue relating to the application of Pt and its alloys as catalysts is the amount used. It is essential that the consumption of Pt should be kept as low as possible without sacrificing catalytic performance. One strategy to overcome this problem is to create Pt with meso and nanoporous structures.7 In addition to the reduction in Pt consumption, the interconnected structure has additional benefits in improving the catalytic activities of some reactions that involve two or more reactants, because such networks can potentially have high surface areas and also supply enough absorption sites for all the reactants involved.7 Compared to their solid counterparts, porous structured metals and alloys show improved physical and chemical performances.7 Much effort has been devoted to the synthesis of meso and nanoporous metals or alloys. Several nanostructures have been prepared with various templates (soft8 and hard9 templates) or

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template-free systems.10 In particular, ordered mesoporous metals have garnered great interest owing to their outstanding physicochemical properties such as their high surface area, large pore volume, and variety of pore structures. Well-defined pore architectures with uniform sizes are expected to reduce masstransport limitations and are highly desirable for applications in catalysis and sensing. Very recently, we developed a novel hard-templating method for the preparation of uniform-sized mesoporous Pt and PtRu particles.11 It is of great interest to explore other mesoporous Pt particles containing inexpensive secondary metals such as Co and Ni for further improvement in the catalytic activity.6 Here, we synthesize ordered mesoporous Ptbased alloys containing low Co or Ni contents and perform preliminary electrochemical tests. In the experimental methods, mesoporous silica (SBA-15 with a 2D hexagonal structure, p6mm, or KIT-6 with a double gyroid structure, Ia3d) was first prepared according to our 11 previous reports. Mesoporous silica powders were immersed in mixed aqueous solutions containing metal sources. The mixtures were dried under vacuum conditions. Then, an aqueous solution of ascorbic acid, the reducing agent, was added dropwise on the powder. After the deposition of Pt, the samples were washed with hydrofluoric acid solution to remove the silica template and undeposited metal sources. The obtained mesoporous PtCo and PtNi alloys are abbreviated as x_PtCo and x_PtNi (where x represents the template used: “S” means SBA-15, while “K” means KIT-6). In both cases, the compositional ratios of the secondary metals were measured to be around 23 atom % through ICP analysis and elemental mapping. The shape of the obtained mesoporous Pt-based alloy was observed by field emission scanning electron microscopy (FESEM) (Figure 1a). The particle sizes were uniform with a distribution ranging from 100 to 150 nm. Low-angle XRD profiles of the original mesoporous silica particles and the obtained mesoporous alloy particles are shown in Figure 1b. The observed peaks can be assigned to the p6mm (for SBA-15 (for KIT-6 system) structures. The main peak system) and Ia3d positions of the obtained samples were exactly the same as those of the original mesoporous silica. This result shows that the replication of Pt retained the original mesostructural symmetry. Inside the particles of S_PtNi and S_PtCo, periodically arranged nanowires replicated from SBA-15 were observed clearly, as indicated by the arrow in Figures S112 and 2b. In the case of the KIT-6 system, sponge-like Pt nanostructures were observed

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Figure 1. (a) Low-magnification SEM images of S_PtCo and K_PtCo. (b) Low-angle XRD profiles of S_PtCo, S_PtNi, K_PtCo, and K_PtNi. SBA-15 and KIT-6 as the starting hardtemplates are also shown as references.

Figure 2. (a) High-magnification SEM images of (a-1) K_PtCo and (a-2) K_PtNi replicated from KIT-6. (b) High-angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) image and elemental mapping of S_PtNi replicated from SBA-15. (Figure 2a). From the N2 gas adsorptiondesorption isotherms, the surface areas were calculated to be around 42 (for S_PtCo), 40 (for S_PtNi), 40 (for K_PtCo), and 38 m2 g¹1 (for K_PtNi). The average pore sizes were found to be around 3.1 (for S_PtCo and S_PtNi) and 2.8 nm (for K_PtCo and K_PtNi). The mesopore size of 3.0 nm is almost the same as the wall thickness of the original mesoporous silica SBA-15 template.11 Similarly, the mesopore size of 2.8 nm is in agreement with the wall thickness of the original KIT-6 template.11 For further characterization of the crystal structure, the edge part of the synthesized Pt was observed by using high-resolution TEM (Figure S212). As seen in the TEM image, lattice fringes were clearly observed, and the distance between the fringes was measured to be around 0.23 nm, which corresponds to the (111) plane of a face-centered cubic ( fcc) Pt crystal. The elemental mapping revealed that both Pt and the secondary metal were distributed evenly over the entire area (Figure 2b). The crystal structures were further verified through wide-angle X-ray diffraction (XRD). The XRD peaks can be indexed to the Chem. Lett. 2013, 42, 447449

Figure 3. Mass-normalized cyclic voltammogram for methanol oxidation in H2SO4 (0.5 M) containing CH3OH (0.5 M) (Scan rate 50 mV s¹1) ((a) Commercially available Pt black, (b) S_Pt, (c) K_PtNi, (d) S_PtNi, (e) K_PtCo, and (f) S_PtCo). For clarity only forward scans are shown here. A summary of massnormalized activity is shown as an inset figure. (111), (200), (220), (311), and (222) diffractions of the fcc structure (Figure S312). The peak positions were not changed after alloying, which involved low contents of Ni or Co. It is thus proven that the obtained samples possess well-developed crystalline structures. Electrochemical methanol oxidation was measured using the obtained samples to investigate the potential use of these bimetallic mesoporous materials as electrocatalysts. The electrocatalytic performances were recorded in aqueous solution containing sulfuric acid (0.5 M) and methanol (0.5 M) at room temperature. As shown in Figure 3, mesoporous alloy catalysts exhibit better electrocatalytic performances than S_Pt (mesoporous Pt replicated from SBA-15) and commercially available Pt black. For the methanol oxidation reaction, an efficient catalyst exhibits a lower onset potential and a strong peak current density. The onset potentials of the mesoporous PtCo and Pt Ni samples were lower than those of S_Pt and Pt black (Figure S4A12). In this case, alloying with Co or Ni modifies the electronic properties of Pt and helps to enhance the electrocatalytic activity on the bimetallic catalyst surface.13 As shown in the inset of Figure 3, the mass-normalized current densities of S_PtCo and S_PtNi were approximately double that of S_Pt and eight times higher than that of Pt black. The long-term stability was also evaluated through chronoamperometric measurements under a constant potential of 0.6 V for 3000 s (Figure S4B12). It was found that the anodic currents of the mesoporous PtNi and PtCo alloy samples were considerably higher than those of mesoporous Pt and Pt black over the reaction time period. These results further confirm the good tolerance to intermediate species and high catalytic activity of our mesoporous bimetallic alloys. The performance of the catalysts followed the order of mesoporous PtCo > mesoporous PtNi > mesoporous Pt > Pt black. Because of the high surface area of the mesoporous structure, a significantly enhanced catalytic activity of Pt-based alloys toward the electrooxidation of methanol was realized. Compared to K_PtNi and K_PtCo (replicated from KIT-6), S_PtNi and S_PtCo (replicated from SBA-15) show good performances as electro-



449 catalysts. The same phenomena were observed in our mesoporous Pt and PtRu alloys reported previously.11 The 2D hexagonal structure with straight channels may be suitable for methanol diffusion, rather than the complicated inverse double gyroid structure. In conclusion, we have successfully synthesized mesoporous Pt particles with low contents of the secondary metals (Co and Ni) by using SBA-15 and KIT-6 as hard templates. The electrochemical performances and stabilities of our materials toward methanol oxidation were greatly improved relative to those of Pt black and mesoporous Pt. Our materials are good candidates for highly active fuel-cell catalysts. This synthetic strategy can be extended to the preparation of other mesoporous Pt-based alloys. Further control of the compositions, shapes, and mesoporous architectures will be important for tailoring their electrochemical properties in the future.

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