X-ray Raman Scattering for Bulk Chemical and Structural Insight into Green Carbon

X-ray Raman scattering (XRS) spectroscopy is an emerging inelastic scattering technique which uses hard X-rays to study the X-ray absorption edges of low-Z elements (e.g. C, N, O) in bulk. This study applies XRS spectroscopy to pyrolysis and hydrothermal carbons. These materials are thermochemically-produced carbon from renewable resources and represent a route for the sustainable production of carbon materials for many applications. Results confirm local structural differences between biomass-derived (Oak, Quercus Ilex) pyrolysis and hydrothermal carbon. In comparison with NEXAFS, XRS spectroscopy has been shown to be more resilient to experimental artefacts such as self-absorption. Density functional theory XRS calculations of potential structural sub-units confirm that hydrothermal carbon is a highly disordered carbon material formed principally of furan units linked by the α carbon atoms. Comparison of two pyrolysis temperatures (450 °C and 650 °C) shows the development of an increasingly condensed carbon structure. Based on our results, we have proposed a semi-quantitative route to pyrolysis condensation

Evidence for a core-shell structure of hydrothermal carbon

Hydrothermal carbonisation (HTC) has been demonstrated to be a sustainable thermochemical process, capable of producing functionalised carbon materials for a wide range of applications. In order to better apply such materials, the local chemistry and reaction pathways governing hydrothermal carbon growth must be understood. We report the use of scanning transmission X-ray microscopy (STXM) to observe chemical changes in the functionality of carbon between the interface and bulk regions of HTC. Spatially-resolved, element-specific X-ray photo-absorption spectra show the presence of differing local carbon chemistry between bulk “core” and interface “shell” regions of a glucose-derived hydrothermal carbon spherule. STXM provides direct evidence to suggest that mechanistic pathways differ between the core and shell of the hydrothermal carbon. In the shell region, at the water-carbon interface, more aldehyde and/or carboxylic species are suspected to provide a reactive interface for bridging reactions to occur with local furan-based monomers. In contrast, condensation reactions appear to dominate in the core, removing aryl-linking units between polyfuranic domains. The application of STXM to HTC presents opportunities for a more comprehensive understanding of the spatial distribution of carbon species within hydrothermal carbon, especially at the solvent-carbon interface

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