Structural comparison of multi-walled carbon nanotubes produced from polypropylene and polystyrene waste plastics

Polypropylene and polystyrene were processed in a pyrolysis/catalytic reactor with a Ni-Fe/Al2O3 catalyst to produce carbon nanotubes (CNTs). A high yield of catalyst carbon deposits were produced; 33.5 g 100 g−1 polypropylene and 29.5 g 100 g−1 polystyrene and consisted of multi-walled carbon nanotubes (MWCNTs). X-ray diffraction (XRD) of the Ni-Fe/Al2O3 catalyst suggested the active metal was a Ni-Fe alloy which was confirmed using X-ray absorption near edge structure (XANES); extended X-ray absorption fine structure (EXAFS) analysis showed that the alloy was primarily FeNi2. Electron microscopy showed that the MWCNTs were entangled, several μm in length and ~50 nm in diameter comprising ~30 graphene layers. Optical Raman spectroscopy confirmed the carbons to be of high purity and crystallinity with polypropylene showing a higher degree of graphitisation and fewer defects compared to those produced from polystyrene. X-ray Raman scattering spectroscopy of the MWCNTS confirmed their graphitic carbon composition, but demonstrated poor alignment. Commercially produced MWCNTs showed a high degree of graphitisation, with less metal impurities and were of long length (several μm), straighter, smaller diameter (~10 nm) and with fewer number of graphene layers (~12) in the CNT wall compared with the plastic derived MWCNTs.