Multiscale X-ray Approaches Towards the Structure and Function of Pyrolysis and Hydrothermal Carbons

A paradigm shift is required to reduce current dependence on finite fossil resources and to remain within planetary boundaries. In response, a 21st century bioeconomy, where materials, chemicals and fuels are produced sustainably from biomass, is being developed. Pyrolysis and Hydrothermal Carbonisation (HTC) represent two biorefinery approaches to produce high-value carbon devices from biomass. To realise their potential, a mechanistic understanding of these carbonisation processes is required. This thesis utilises synchrotron X-ray science to study the structure and function of pyrolysis and hydrothermal carbon.

Scanning transmission X-ray and electron microscopy provide experimental evidence for a core-shell structure of carbohydrate-derived hydrothermal carbon. Mechanistic pathways differ between the core and shell of hydrothermal spherical carbons. Near-edge X-ray absorption spectra (NEXAFS) show that, at the water-carbon interface, carboxylic species drive growth. In the core, condensation dominates, removing linking units between polyfuranic domains.

X-ray Raman scattering spectroscopy (XRSS) is presented as a novel tool to study the C K-edge of sustainable carbons in bulk and in-situ. Experiment and density functional theory demonstrate that biomass-derived hydrothermal carbon is composed of a furanic local structural motif linked at the alpha-carbon. Two pyrolysis carbons produced at 450 °C and 650 °C are used to demonstrate XRSS as a route to a semi-quantitative measurement of aromatic condensation and highlight the advantages of XRSS over NEXAFS for these materials.