Resolving atomic and electronic structures in complex organic catalysts using electron energy loss spectroscopy

The synthetic strategies applied nowadays to improve the performances of catalytic materials, such as heterojunction formation, heteroatom doping, and reduction of active catalytic species to single atoms, introduce structural complexities that cannot be assessed by standard spectroscopic techniques, making the detailed characterisation of these materials challenging. In this thesis, I explored the application of Electron Energy Loss Spectroscopy (EELS) to characterize atomic and electronic structures of complex organic catalysts at the nanoscale and related it to the materials' activity in specific catalytic reactions. The organic nature of these materials adds additional complexity due to the high sensitivity of soft materials to electron beams. I addressed these challenges by combining low-electron dose imaging techniques with the analysis of EELS spectra, particularly Extended Electron Energy Loss Fine Structure (EXELFS), which enabled us to resolve local atomic arrangements around specific elements. Two case studies have been performed: (1) analysis of a heterojunction between two structurally and chemically similar carbon nitride phases; (2) analysis of Ce single-atom catalysts within an amorphous carbon nitride matrix. In both cases, a specific redistribution of charge densities was a key factor that influenced the catalytic behaviour. In particular, our analysis of EELS spectra from carbon nitride heterojunction showed an enhanced exciton signal, a higher proportion of N defect sites, and a distinctive local ordering around N compared to the constituent phases. In the case of the single-atom catalyst, I was able to show the complexity of the obtained samples by resolving three distinct local coordination environments of Ce, which correspond to single atoms, clusters, and nanoparticles. The single atoms were found to be coordinated by 6 N atoms with the electron density shifted towards Ce active sites. Our approach provides nanoscale insights into the crystal and electronic structures of complex organic catalysts, complementing and resolving the ambiguities arising from the use of socalled bulk characterisation techniques. The proposed analysis can be applied in a standard transmission electron microscope equipped with an EELS spectrometer and thus can be broadly applied in different laboratories for the analysis of organic materials at the nanoscale.