Carbon surface chemistry drives speciation and reactivity of cationic Fe species in CO2 activation: a HERFD-XANES and valence-to-core XES study

Abstract

While carbon-supported iron nanostructures are able to provide inexpensive frameworks where the dispersion of single-atom centres enables unique catalytic properties for carbon dioxide functionalization, detailed understanding of the structure of the transition metals is often prevented by the heterogeneous nature of the hosting C matrix and the variety of available sites, consequently hindering the understanding and development of CO₂ reduction chemistry. Herein, we report an experimental and computational spectroscopic investigation of few-layer graphene-based samples decorated with Fe atoms immobilised at the edges and in-plane defects of the graphene layers. We find that Fe–OH bound to N-terminated edge sites or in-plane defects of the graphene layers reacts with CO₂, forming bicarbonates. A similar reactivity is observed for Fe–OH bound to C-terminated edge sites, whereas Fe–OH coordinated to C-terminated in-plane defects remains unreactive towards CO₂. In stark contrast, FeN₄ sites in Fe–porphyrin present a direct, carbon-atom-mediated interaction with CO₂. These results provide insights into the local coordination environment of iron and its role in the reactivity towards CO₂ activation.