Preparation of silica-coated Pt-Ni alloy nanoparticles using microemulsions and formation of carbon nanofibers by ethylene decomposition

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Abstract

Silica-coated Pt-Ni alloys were prepared using a water-in-oil-type microemulsion. The silica-coated Pt-Ni alloys prepared without thermal treatment decomposed ethylene to form nanocomposites of carbon nanofibers.

Introduction

Highly dispersed precious metal particles on supports have been shown to be active for various catalytic reactions such as oxygen reduction at the cathode or hydrogen oxidation at the anode in a proton exchange membrane fuel cell (PEMFC), and dehydrogenation of alkanes 1, 2. However, the active metal particles agglomerate easily at high temperatures because they are supported on the outer surface of the supports, which results in a decrease in catalytic activity. Thus, supported metal catalysts with high resistance to sintering at high temperatures are required.

We have previously studied the preparation of silica-coated metal catalysts using microemulsions 3, 4, 5, 6. Using these methods, metal particles such as Ni, Co and Pt can be uniformly covered with silica layers. The metal particles in these catalysts show high resistance to sintering at high temperatures because each particle is covered with silica layers. In our previous study, these silica-coated metal catalysts allowed the selective formation of carbon nanotubes (CNTs) or carbon nanofibers (CNFs) with uniform diameters through ethylene decomposition while the metal catalysts without silica-coating formed CNTs or CNFs with various diameters because the metal particles aggregated severely during ethylene decomposition at 973 K 4, 5. Thus, silica-coated metal catalysts are effective catalysts for the production of nanoscale carbon structures.

The catalytic performance of precious metals is frequently modified by the addition of other metal species. For example, the tolerance to CO and the durability of Pt electrocatalysts in PEMFCs are improved by the addition of metal species such as Ni, Co, Mo and Pd [7]. In addition, the activity of the Pd catalyst for the oxidative dehydrogenation of sodium lactate to pyruvate was improved by the addition of Te [8]. Recently, silica-coated Pt-Co alloys and Pt-Pd alloys covered with silica layers were prepared and used as catalysts for ethylene decomposition to form CNTs [6]. The structure of the graphene that forms the walls of the CNTs was different for the two catalysts. This result indicates that the nanoscale carbon structure formed by hydrocarbon decomposition was influenced by the type of metal species in the silica-coated metal alloys. Therefore, further research on the hydrocarbon decomposition of silica-coated alloys using other metal species is needed because it is possible to develop nanocomposites using the silica-coated metal alloy catalyst and nanoscale carbon structures.

Supported Ni is known to be an effective catalyst for hydrocarbon decomposition [9]. In this study, silica-coated Pt-Ni alloys were prepared using a microemulsion and they were used as catalysts for ethylene decomposition to form nanoscale carbon structures. The influence of silica-coated Pt-Ni alloy thermal treatment on the formation of the nanoscale carbon structures was also investigated.

Section snippets

Experimental

Silica-coated Pt-Ni alloys (denoted as coated Pt-Ni) were prepared in a water-in-oil-type microemulsion 3, 4, 5, 6. Mixed aqueous solutions of H2PtCl6 and Ni(NO3)2 were used for the preparation of coated Pt-Ni. The microemulsion system was prepared by adding aqueous solutions containing the metal cations described above into a surfactant solution in cyclohexane. Polyoxyethylene (n = 15) cetyl ether was used as a surfactant. Nanoparticles containing the metal species were formed by the addition

Results and discussion

Figure 1 shows TEM images for coated Pt-Ni prepared with or without thermal treatment. In both the TEM images, particles with diameters of ca. 40 nm were observed. Judging by the contrast of the TEM images, the particles are mainly composed of silica.

Small particles with diameters of a few nanometers were also observed in the TEM images. It should be noted that the small particles were not observed on the surfaces of the silica particles but in their bodies. Although the small particles grow a

Conclusions

Pt-Ni alloy particles covered with silica layers were prepared using microemulsion systems. The metal species in the coated Pt-Ni were present as alloys. Carbon nanofibers formed over the silica-coated Pt-Ni alloys prepared without thermal treatment during ethylene decomposition. These nanocomposites composed of coated Pt-Ni and the CNFs are useful in various fields because of the electronic and chemical properties of the CNFs as well as the silica-coated Pt-Ni alloy nanoparticles.

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Cited by (3)

  • Preparation of carbon-supported Pt catalysts covered with microporous silica layers using organosilanes: Sintering resistance and superior catalytic performance for cyclohexane dehydrogenation

    2012, Applied Catalysis A: General
    Citation Excerpt :

    In addition, the metal particles can react with reactant molecules through the silica layers since the silica layers that are wrapped around the metal particles have a porous structure, therefore, the silica-coated metal catalysts can be applied to catalytic reaction. For example, the coverage of Co, Ni and Pt metal particles with silica layers prevents the sintering of their metal particles during hydrocarbon decomposition, which results in the preferential formation of carbon nanotubes or nanofibers with uniform diameters, while metal catalysts that are not covered with silica layers form carbon nanotubes or nanofibers with various diameters because these metal particles are severely aggregated during hydrocarbon decomposition [10–14]. In addition, we prepared the carbon nanotube-supported Pt or Pd nanoparticles covered with silica layers [15–19] and this coverage of Pt or Pd nanoparticles with silica layers prevents the dissolution of Pt or Pd particles from the supports, resulting in the excellent durability for the oxygen reduction reaction under severe cathode conditions in proton-exchange-membrane fuel cells [15,18,19].

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