Earth-abundant based nanostructured catalysts for CO2 reduction

Our contemporary lifestyle has caused an extreme increase in CO₂ emissions into the atmosphere. Within this context and alternatively to conventional technologies, electrocatalytic and photocatalytic CO₂ reduction powered by renewable energies are emerging as sustainable solutions to attain the desired generation-consumption cycle for feedstocks and fuels. However, this ambitious target requires efficient and durable catalysts and photocatalysts based on inexpensive and earth-abundant elements to take a step towards industrial applications.

Our work in this thesis has focused on the development of nanostructured systems based on earth-abundant elements, specifically carbon dots (CDs) or semiconductor metal oxides. We exploited the properties of these two materials as intrinsic (photo)electrocatalysts or as photoactive supports for the iron (III) porphyrin CO₂ reduction molecular catalyst. On the one hand, we synthesised bare or transition metal-doped CDs by solvothermal synthesis and we faced the complexity of their purification. The obtained CDs had no intrinsic electrocatalytic activity and processed limited ligand capacity for metal ions, which overall restricted their use as electrocatalysts. Consequently, we moved to microwave-synthesised CDs rich in amino groups to which an active catalytic centre, the iron (III) porphyrin, was covalently anchored. We evaluated the photo- and photoelectrocatalytic activity of the hybrid in organic solvent, determining that CDs are not good photosensitisers for the iron (III) porphyrin but do improve its stability under photoelectrocatalytic conditions. Finally, we used TiO₂ in the form of a mesoporous layer as a support for two variants of the iron (III) porphyrin. Interestingly, the resulting TiO₂ photocathode showed 100% selectivity for the photoelectrochemical reduction of CO₂ to CO. Besides, we revealed by transient absorbance spectroscopic measurements that electron recombination processes limit the efficiency of the hybrids because of undesired recombination processes.

In conclusion, we emphasise that for any catalytic material, the fundamental understanding of the electron transfer processes occurring during the catalytic process is essential to establish the basis for fine-tuning and optimising the catalyst performance. Overall, we hope that this thesis will support future research on hybrid nanomaterials for emerging CO₂ reduction technologies.

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