Nanocarbons for Energy Conversion: Supramolecular Approaches by Naotoshi Nakashima

Author:Naotoshi Nakashima
Language: eng
Format: epub, pdf
ISBN: 9783319929170
Publisher: Springer International Publishing

12.5 Polymer-Functionalized Carbon Nanotubes as the Supports for Transition Metal Oxide and Phosphate Catalysts

In addition to oxidation, wrapping the CNTs with a conducting polymer is also a convenient way to functionalize the inert outer surfaces of CNTs. One of the most widely used conducting polymers is polyaniline (PANI) and its derivatives that possess a high electrical conductivity and environmental stability due to their π-conjugated structures [17]. Moreover, PANIs have numerous amino groups as anchor sites for metal ions. Therefore, PANI-wrapped MWNTs are frequently reported as a promising composite support for precious metal nanoparticles [18–22]. The typical preparation procedure of PANI/MWNTs is as follows. MWNTs are oxidized by HNO3 to form binding sites on their surfaces. Then, the oxidized CNTs are dissolved in an organic solvent, such as CH2Cl2 together with excess aniline. Subsequently, PANI is formed by an efficient interfacial polymerization and wraps the whole surfaces of the CNTs [22].

In recent years, a great effort have been made to deposit non-precious metal and metal oxides on PANI-wrapped CNTs. Hu et al. deposited iron and cobalt nanoparticles on such PANI/MWNT support for ORR [23, 24]. Aniline monomer, CNTs, and citric acid pretreated metal chlorides are mixed and ultrasonicated in an organic solvent to which NaBH4 is added, then ultrasonicated in an ice bath. During this process, the formation of PANI and iron nanoparticles occurred simultaneously. It is proposed that a two-step redox process occurs for the coordination of metal and aniline. First, metal cations oxidize the polymer unit (aniline), and then reduced to metallic nuclei, which subsequently bind to imine nitrogen in aniline be oxidized to cation in situ, leading to the formation of the radical cation segments [25].

The typical resultant catalyst, Fe-PANI/CNT(+CA), is shown in Fig. 12.10. The CNTs were covered with a PANI film (see Fig. 12.10a, b), and uniform Fe nanoparticles with 2–4-nm diameters was coated on the PANI/CNTs homogeneously (see Fig. 12.10c–e). The crystal phase of the nanoparticles was confirmed to be iron. However, the X-ray photoelectron spectroscopy (XPS) revealed that the Fe atoms at the surfaces were bivalent and trivalent, i.e., a core-shell structure of the Fe nanoparticles was formed.

Fig. 12.10a TEM of Fe-PANI/CNT(+CA); b HRTEM of Fe-PANI/CNT(+CA); c TEM of Fe-PANI/CNT(−CA); d STEM of Fe-PANI/CNT(+CA).

Reproduction from [23] with permission of Elsevier

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